Functional Short-Bite Ligands: Synthesis, Coordination Chemistry, and

Review pubs.acs.org/CR

Functional Short-Bite Ligands: Synthesis, Coordination Chemistry, and Applications of N‑Functionalized Bis(diaryl/ dialkylphosphino)amine-type Ligands Christophe Fliedel,*,†,‡ Alessio Ghisolfi,*,†,§ and Pierre Braunstein*,† †

Laboratoire de Chimie de Coordination, Institut de Chimie (UMR 7177 CNRS), Université de Strasbourg, 4 Rue Blaise Pascal, CS 90032, 67081 Strasbourg, France ‡ Laboratoire de Chimie de Coordination (LCC), CNRS-UPR 8241, 205 Route de Narbonne, Toulouse F-31077 Cedex 4, France ABSTRACT: The aim of this review is to highlight how the diversity generated by Nsubstitution in the well-known short-bite ligand bis(diphenylphosphino)amine (DPPA) allows a fine-tuning of the ligand properties and offers a considerable scope for tailoring the properties and applications of their corresponding metal complexes. The various Nsubstituents include nitrogen-, oxygen-, phosphorus-, sulfur-, halogen-, and silicon-based functionalities and directly N-bound metals. Multiple DPPA-type ligands linked through an organic spacer and N-functionalized DRPA-type ligands, in which the PPh2 substituents are replaced by PR2 (R = alkyl, benzyl) groups, are also discussed. Owing to the considerable diversity of N-functionalized DPPA-type ligands available, the applications of their monoand polynuclear metal complexes are very diverse and range from homogeneous catalysis with well-defined or in situ generated (pre)catalysts to heterogeneous catalysis and materials science. In particular, sustained interest for DPPA-type ligands has been motivated, at least in part, by their ability to promote selective ethylene tri- or tetramerization in combination with chromium. Ligands and metal complexes where the N-substituent is a pure hydrocarbon group (as opposed to N-functionalization) are outside the scope of this review. However, when possible, a comparison between the catalytic performances of N-functionalized systems with those of their N-substituted analogs will be provided.

CONTENTS 1. Introduction 2. Nitrogen-Based Functionalizations 2.1. Alkyl- and Aryl-amino Groups 2.2. Direct N-Amino Functionalization (Hydrazines-Derived DPPA-type Ligands) 2.3. Nitrile Group 2.4. N-Heterocyclic Pendant Tails 2.5. Other N-Containing Functional Groups 3. Oxygen-Based Functionalizations 3.1. Ether Groups 3.1.1. Furan Derivatives 3.1.2. Alkyl and Aryl Mono-Ether Moieties 3.1.3. Aryl Bis-Ether Moieties 3.1.4. Other N-Functional Groups Containing an R−O−R′ Moiety 3.2. Ester Groups 3.2.1. Aryl Ester Moieties 3.2.2. Alkyl Ester Moieties 3.3. Other O-Containing Functional Groups 4. Phosphorus-Based Functionalizations 4.1. Phosphino Groups 4.2. Other P-Containing Functional Groups 5. Sulfur-Based Functionalizations 5.1. Thiophene Derivatives

© 2016 American Chemical Society

5.2. Alkyl and Aryl Thioether Groups 5.3. Other S-Containing Functional Groups 6. Halogen Functionalizations 7. Silicon-Based Functionalizations 7.1. N-(Alkyl/aryl)-alkoxysilyl or N-(Alkyl/aryl)alkyl/alkoxysilyl Derivatives 7.2. N-Trialkyl/arylsilyl (Direct N−Si Functionalization) 8. Poly-bis(diarylphosphino)amine Derivatives 8.1. Pure Alkyl Spacer 8.2. Aryl or Benzyl Spacer 8.3. Heteroatom-Containing Spacer 9. N-Functionalized Bis(dialkyl/dibenzylphosphino) amine Derivatives 10. N-Functionalizations with Other p-Block Elements (Al, Ga, and Sn) 11. N-Metal Functionalizations 11.1. Group 1 Metal N-Functionalization in Homometallic Complexes 11.2. Group 1 Metal N-Functionalization in Heterometallic Complexes 11.3. Group 2 Metal N-Functionalization

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Chemical Reviews 11.4. Group 3 Metal and Lanthanide N-Functionalization 11.4.1. Homoleptic Tris(DPPA-type-P,N) 11.4.2. Heteroleptic Bis(DPPA-type-P,N) 11.4.3. Heteroleptic Mono(DPPA-type-P,N) 11.4.4. Sandwich and Half-Sandwich RareEarth-Metal Complexes of (DPPAtype-P,N) Ligands 11.4.5. Heteroleptic Rare-Earth-Metal Complexes of (DPPA-type-P,N) Ligands Chelated by a Second Ligand 11.4.6. Mixed Potassium/Rare-Earth-Metal Heterometallic Complexes Incorporating a Deprotonated DPPA Ligand with an N−Metal Bond 11.5. Group 4 Metal N-Functionalization 11.6. Groups 10−12 Metal N-Functionalization 12. Miscellaneous 13. Applications in Homogeneous Catalysis 13.1. Chromium-Catalyzed Ethylene Oligomerization 13.1.1. Catalytic Systems Involving “PolyDPPA-type Ligands” 13.1.2. Catalytic Systems Involving NitrogenBased N-Functionalized DPPA-type Ligands 13.1.3. Catalytic Systems Involving Ether, Thioether, and Silyl Ether N-Functionalized DPPA-type Ligands 13.1.4. Catalytic Systems Involving DPPA-type Ligands N-Functionalized with Substituted Aryls 13.1.5. Catalytic Systems Involving Other NFunctionalized DPPA-type Ligands 13.2. Nickel-Catalyzed Olefin Poly/Oligomerization 13.3. Other Applications in (Co)Polymerization 13.4. Cross-Coupling Reactions 13.5. Transfer Hydrogenation of Ketones 13.6. Other Catalytic Applications 13.6.1. Hydrogenation of Alkenes 13.6.2. Hydroxylation of Arylboronic Acids to Phenols 13.6.3. Addition of β-Diketones to 1-Alkynes 13.6.4. Anti-Markovnikov Addition of Secondary Amines to Aromatic 1-Alkynes 13.6.5. Isomerization of Terminal Alkenes 14. Applications in Materials Science and Heterogeneous Catalysis 15. Conclusion Author Information Corresponding Authors Present Address Notes Biographies Acknowledgments Dedication Abbreviations Used References Note Added in Proof

Review

1. INTRODUCTION Short-bite ligands such as bis(diphenylphosphino)amine (DPPA) or bis(diphenylphosphino)methane (DPPM) and the N-substituted derivatives of DPPA continue to attract much attention in molecular chemistry owing to the diversity of their coordination modes to metals (monodentate, bridging, chelating), which results in a rich chemistry for the neutral ligands, e.g., Ph2PNHPPh2, as well as their deprotonated forms, e.g., (Ph2PNPPh2)− (Chart 1).1−4 Almost 50 years after the

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Chart 1. (Top) DPPA, DPPM, and an Example of a NSubstituted DPPA-type Ligand, DPPA-Me; (Middle) the Deprotonated Forms of DPPA and DPPM; and (Bottom) the Possible Coordination Modes of Such N-Substituted/ Functionalized DPPA-type Ligands, Resulting in Mono- or Polynuclear Complexesa

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a

M, M′ = metal center or metal complex; R′ = pure hydrocarbyl group (not covered here), heteroatom-containing group, metal center, or metal complex.

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synthesis of DPPA was first described, both by Schmitz-Du Mont and colleagues and by Noth and Meinel,5,6 new perspectives are regularly opened, often as a result of unexpected findings. N-Metallation of DPPA by KNH2 in liquid ammonia was first suggested by Schmitz-Du Mont and co-workers,5 and this approach can be used to coordinate the amido nitrogen atom directly to a metal center, an additional possibility to that offered by the coordination of the phosphorus donors (see section 11). Furthermore, an additional donor group can be introduced by functionalization of the nitrogen atom of the DPPA. The resulting N-functionalized DPPA-type ligands may form polynuclear assemblies involving coordination of this additional donor group, but the latter can also act as a labile donor, able to stabilize reactive species, via intra- or intermolecular interactions, e.g., during a catalytic cycle.7,8 Finally, if a suitable N-functionalized tail is used, new possibilities arise for the anchoring of organometallic species on solid matrices or on metal surfaces (see section 14). Although other methods were reported (see below), most Nfunctionalized DPPA-type ligands are accessible in good yield from the corresponding amine, by a classical aminolysis reaction (Scheme 1). This review provides a survey of N-functionalized DPPA-type ligands, in which the N-substituent contains at least one typical nitrogen-, oxygen-, phosphorus-, sulfur-, halogen-, or silicon-

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A more recent review by Fei and Dyson, published in 2005, summarizes the chemistry of phosphinoamides and related compounds, including the synthesis of DPPA-derived phosphinoamides and their reactivity toward main group elements and transition metals.19 The (coordination) chemistry of aminophosphines or P−N ligands has generally given rise to much more coverage in the literature, particularly in terms of reviews, than DPPA-type ligands, although the latter also belong to this class of ligands and sometimes display unique properties.20,21

Scheme 1. General Aminolysis Reaction for the Formation of N-Substituted DPPA-type Ligandsa

a

R = pure hydrocarbyl group, heteroatom-containing group.

based functionality, another DPPA-type donor, or a metal directly N-bound. Purely hydrocarbyl-type (aliphatic or aromatic) N-substituents will be outside the scope of this review. One section is dedicated to N-functionalized PNP ligands in which the P-substituents are alkyl or benzyl groups (DRPA-type ligands), instead of the classical phenyl rings, because of their specific properties. We will summarize the various access available to such N-functionalized DPPA-type ligands, their use for the formation of mono- or polynuclear metal complexes or clusters and the reactivity or photophysical properties of these complexes. The applications of the metal complexes containing such modified DPPA-type ligands in homogeneous and heterogeneous catalysis or materials science will also be detailed. In particular, the unique ability of DPPAtype ligands to promote ethylene tetramerization when associated with chromium was demonstrated at Sasol, and this early success has provided much incentive to better understand the chemistry of these ligands.9,10 Shortly after, it was demonstrated that under appropriate conditions, those systems were able to produce 1-hexene with high selectivity.11,12 The performances of DPPA-type ligands in Crcatalyzed selective oligomerization have been nicely reviewed over the last decade.13−17 It remains however unclear at this stage why this type of ligand offer such advantages over closely related ligand systems, like DPPM and its derivatives. This example can be used to illustrate the considerable impact on reactivity that relatively minor differences between two ligands as similar as DPPA and DPPM can bring about. A systematic comparison between DPPA- and DPPM-type complexes is however outside the scope of the present review. This review will also highlight that the ligands themselves can exhibit original reactivity, leading i.a. to P migration reactions that open the way to various ligand architectures. Interested readers are invited to consult the review of Witt and Roesky, in which the authors summarized the chemistry of transition and main group metals in cyclic phosphazanes until 1994. It included the chelating and bridging behavior of DXPA, deprotonated DXPA, and N-substituted or N-functionalized DXPA-type ligands [X = F, Cl, Ph, OMe, OPh, NHPh, NH(iPr), Chart 2].18

2. NITROGEN-BASED FUNCTIONALIZATIONS Very diverse nitrogen-containing chemical functions have been used to develop original N-functionalized DPPA-type ligands. Although no direct interaction between the N atom of these donor functions and the metal center(s) could be evidenced in their metal complexes, the catalytic properties of the latter have sometimes been found to be affected by the N-functionalization. Furthermore, the presence of specific N-hydrazide or Npyridyl groups in these DPPA-type ligands can induce original reactivity. 2.1. Alkyl- and Aryl-amino Groups

A series of DPPA-type ligands bearing an amino group as Nsubstituent have been recently reported, and the authors took advantage of the wide range of amine substituents available and their possible combinations in NR1R2R3, with R1 ≠ R2 ≠ R3, to develop poly-DPPA ligands or to fine-tune the steric and/or electronic properties of the resulting N-functionalized DPPAtype ligands. One objective is to examine the influence of the N-functional group on the activity and/or selectivity of the resulting metal complexes in catalytic reactions. Kim and colleagues pointed out the relevance of pendant amine donor functions to the design of chromium-based complexes for catalysis.22 Typically, tertiary amines are used, separated from the DPPA moiety by an aliphatic spacer (Scheme 2). During the formation of the precatalyst, the dangling amine is not involved in the complexation of the metal center, but once the catalytic process is started, the additional binding site on the nitrogen may interact with the active species. In particular, it can behave as a hemilabile stabilizing agent7,8 or modify the energy and/or the nature of the transition states in catalytic processes. Thus, such functionalization converts the simple bidentate DPPA in a potentially hemilabile, tridentate, mixed-donor ligand, and the nature of the spacer, its length and flexibility, represent critical aspects. In particular, it was found that when compared to the PNP ligand 13 (see section 2.2), the length of the spacer in molecules 1−6

Chart 2. Various Classes of Transition and Main Group Metal Complexes with DXPA-type Ligands Forming Cyclic Phosphazanes Reviewed by Witt and Roesky in 1994a 18

a

M = metal center or metal complex; X = F, Cl, Ph, OMe, OPh, NHPh, NH(i-Pr); R′ = pure hydrocarbon group, heteroatom-containing group, metal center or metal complex. 9239

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Scheme 2. DPPA-type Ligands N-Functionalized with an Amino Group

commercially available tris(2-aminoethyl)amine (Scheme 2). Baysal and colleagues investigated the coordination chemistry of ligand 7 toward group 10 metals and were able to prepare two trinuclear complexes of Pd(II) (9) and Pt(II) (10) by reaction of ligand 7 with 3 equiv of the corresponding [MCl2(COD)] precursor (Scheme 3). The 31P NMR spectrum of complex 9 exhibits a singlet signal at 30.7 ppm, while 10 displayed a singlet at 16.9 ppm flanked by satellites (1JP,Pt = 3400 Hz) due to coupling of the phosphorus atoms with the 195 Pt center (spin = 1/2) (Table 1). These NMR data were in agreement with the proposed structures, which unfortunately could not be confirmed by single-crystal X-ray diffraction studies. The authors studied compound 9 in the Heck and Suzuki cross-coupling reactions (see section 13.4), whereas Jiang and colleagues focused their interest on the in situ development of a new (pre)catalyst for the Cr-based tetramerization of ethylene (see section 13.1).25 Interestingly, both groups came to the same conclusion that the connection of multiple DPPA moieties, based on a tertiary aminoethyl linker, constitutes an efficient, versatile, and inexpensive access to functional complexes for different aims. In 2011, Biricik and colleagues26 described the synthesis of ligand 8 (Scheme 2) with a bulky N-substituent, starting from 4-aminodiphenylamine. This ligand was reacted with 1 equiv of [MCl2(COD)] (M = Pd, Pt) to produce the corresponding complexes 11 and 12, respectively (Scheme 3). The characteristic 31P NMR data recorded (Table 1) included for complex 11 a singlet at 35.9 ppm and, for 12, a singlet at 21.2 ppm flanked by satellites (1JP,Pt = 3336 Hz), against a chemical shift of 69.3 ppm for the free ligand. The Pd(II) complex was evaluated in the Suzuki and Heck cross-coupling reactions, and the results are discussed in section 13.4. This study concluded that the airstable complex 11 shows a good reactivity and do not require an induction period in catalysis.

strongly influences the selectivity of their complexes in catalytic ethylene tetramerization. The Cr complexes were not isolated nor fully characterized but generated in situ. Wasserscheid, McGuinness, and colleagues recently published a comparative study of the activity/selectivity in Cr-catalyzed ethylene tri- and tetramerization of several phosphine-based ligands, including of the N-functionalized DPPA-type, such as ligand 2 (section 13.1).23 The 31P NMR chemical shifts of the free ligands were found in the typical range for N-alkyl-functionalized DPPAderivatives, between 61.9 and 63.7 ppm, and are detailed in Table 1. The catalytic performances of the corresponding metal complexes will be discussed in section 13.1. Table 1. 31P NMR Data of the Ligands 1−8 and Their Metal Complexes 9−12 ligand 1 2 3 4 5 6 7 8 a

δ, ppm (mult) 63.7 61.9 63.2 63.0 62.9 62.5 62.0 69.3

(s)a (s)a (s)a (s)a (s)a (s)a (s)a (s)a

ref

complex

M

22 22 22 22 22 22 24 26

9 10 11 12

Pd Pt Pd Pt

δ, ppm (mult) 30.7 16.9 35.9 21.2

(s)b (s)b (s)a (s)a

1

JP,Pt, Hz

3400 3336

ref 24 24 26 26

2.2. Direct N-Amino Functionalization (Hydrazines-Derived DPPA-type Ligands)

As reported by Faught in 197627 (13, Scheme 4) and more recently by Kornev and colleagues28−31 (14−16, Schemes 4 and 5), the standard aminolysis reaction applied to hydrazinetype precursors of the general formula RR′N−NH2, leads to a series of N-functionalized DPPA-type ligands called diphosphinohydrazides, which display a direct N−N single bond. While ligand 13 exhibits a 31P NMR singlet resonance at 48.3 ppm, which is high-field-shifted compared to N-alkyl-functionalized DPPA-type ligands, the PNP fragment of compounds 14−16 resonates at lower field, between 65.9 and 77.8 ppm (Table 2). The structural parameters of these ligands are similar to each

In CDCl3. bIn DMSO-d6.

In 2008, Baysal and colleagues24 and Jiang and colleagues25 developed at the same time, but following different reaction conditions, the new hexadentate phosphinoamine ligand N{CH2CH2N(PPh2)2}3 (7) by an aminolysis reaction of the

Scheme 3. Synthesis of Pd(II) (9 and 11) and Pt(II) (10 and 12) Complexes from Ligands 7 and 8, Respectively

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Scheme 4. (Top) Diphosphinohydrazides 13−16 and Rearrangement after Deprotonation Leading to the Diphosphazenide Group in 14−, 15a−, and 15b−; (Middle) Formation of Complex 13·Fe by CO Substitution Reaction; and (Bottom) Mixed PMe2/PPh2 N-Hydrazino Ligand 13′ and Its Pt(II) Complex 17

Scheme 5. Different PPh2 Migration Reactions Observed with 16a

other, although the P−N−P angle is smaller and the N−N bond length is longer in 15a compared to 16, which might be attributed to the steric hindrance of the t-Bu group (Table 2). Deprotonation with LiN(SiMe3)2 of the derivatives 14 and 15a,b, containing one N−H group, gave rise to a molecular rearrangement and the migratory insertion of a PPh2 group into the N−N bond, with formation of the diphosphazenide products 14−, 15a−, and 15b− (Scheme 4). Formally, this rearrangement is associated with a redox process during which a three-coordinate P(III) becomes a four-coordinate P(V) atom, while the coordination number of one N atom is reduced from 3 to 2. The authors assumed that this process is influenced by the nature of the substituent on the nitrogen (NHR1) atom. A high electron density on the NHR1 atom causes an elongation of the N−N bond and its subsequent cleavage, which allows the insertion of the PPh2 group (through several postulated intermediates).28−30 The structurally characterized complex [FeCp(CO)(13)]I (13·Fe) was prepared by reaction between the PNP ligand and [FeCp(CO)2I] in THF; its 31P NMR spectrum exhibits a singlet at 123.3 ppm (Table 2).32 Similarly, the pentacoordinate Pt(II) complex [PtCl(13′)2]Cl (17), containing two chelating ligands 13′, analogous to 13 but with one PPh2 and one PMe2 group (ligand 13′ also belongs to section 9, but its structural analogy to 13 led us to discuss it here), could be isolated and structurally characterized (Scheme 4 and Table 2).33 The family of diphosphinohydrazides was recently extended by Wasserscheid, McGuinness, and colleagues, who prepared ligands 15c−15e in a similar manner and investigated their potential in Cr-catalyzed ethylene tri- and tetramerization (section 13.1).23 The use of the 8-quinolylhydrazine-based DPPA derivative 16 (Scheme 5) allowed Kornev and colleagues to produce three types of phosphine migration products, by selective formation of new P−C (A), P−N (B), or P−P (C) bonds, depending on

Conditions for transformations: (A) toluene or pyridine, 130 °C, 1 h, reduced pressure (16a), or LiN(SiMe3)2, toluene/Et2O, rt (16b); (B) 0.5 equiv of ZnI2, THF, rt, along with formation of 1 equiv of 16·HI; (C) H+; (D) 1 equiv of n-BuLi, followed by addition of 0.5 equiv of ZnI2, THF, rt, 3 h. a

the reaction conditions (Scheme 5).29 The rearranged products 16a and 16b (transformation A) were obtained by heating a toluene (or pyridine) solution of ligand 16 at 130 °C for 1 h under vacuum, or by reaction with LiN(SiMe3)2 at room temperature, respectively. Reaction of 16 with 0.5 equiv of ZnI2 led to transformation B and the formation of the dinuclear complex 16c, along with 1 equiv of 16·HI. Finally, as observed for other DPPA-type ligands attached in the ortho position of a pyridine ring (see section 2.4), protonation of ligand 16 led to transformation C. All the PPh2 migration reactions are represented in Scheme 5. The resulting ligands 16a−d are no longer of the DPPA-type and involve a P(V) atom, and for this reason they are not further detailed in this review.29,31,34 However, the authors could synthesize the coordination compound 16·Zn by deprotonation of the DPPA-type ligand 16 and reaction with ZnI2 (Scheme 5, D). A contrario to the “classical” coordination compounds with N-functionalized DPPA-type ligands, the metal center in 16·Zn is bis-chelated by the two nitrogen donor atoms of the N-tail, but none of the P atoms was involved in metal coordination. Selected bond lengths, angles, and spectroscopic data of the DPPA-type ligands 13−16 and complexes 13·Fe, 16·Zn and 17 (except the products resulting from PPh2 migration) are listed in Table 2. 2.3. Nitrile Group

In 2006, Dyson and colleagues reported the synthesis of the group 10 metal complexes 20−23, supported by a m- or pcyanoaryl-functionalized DPPA-type ligand, resulting from the 9241

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Table 2. 31P NMR Data and Characteristic Structural Parameters of the Ligands 13−16 and the Metal Complexes 13·Fe, 16·Zn and 17 bond length, Å δ, ppm (mult) 13 13·Fe 14 15a 15b 15c 15d 15e 16 16·Zn 17(Pt)

48.3 (s)a 123.3 (s)g 77.8 (d)b 47.1 (t) 71.0 (s)c 70.5 (s)a 65.9 (s)d 69.6 (s)a 68.8 (s)a 70.7 (s)a 71.1 (br)f multipleg

angle, deg

av P−M

av P−N

N−N

2.192

1.715 1.712 1.720

1.449(4) 1.432(9) 1.437(3)

1.720

1.729 1.722 1.705

2.297

P−M−P

P−N−P

ref

73.20(9)

126.3(5) 99.5(3) 124.8(1)

22, 27 32 28

1.445(1)

120.4(1)

1.418(1) 1.436e 1.425e

124.5(6) 124.1e 103.8e

31 34 23 23 23 29, 31 29 33

71.45e

a

In CDCl3. bIn C6D6; the signal at 77.8 ppm is attributed to the P−N−P fragment and that at 47.1 ppm to the P−N−H one (JP,P = 9 Hz). cIn toluene-d8. dIn C6D6. eAverage value. fIn THF-d8. gDeuterated solvent not communicated.

stable in this form. In a previous study, Dyson and colleagues investigated the impact of the reaction conditions on the products selectivity, i.e. aminophosphine (RNHPPh2), diphosphinoamine [RN(PPh2)2], or imino(bisphosphine) (RN PPh2−PPh2), for several aniline precursors, including cyanoanilines and halogenated aniline derivatives. The DPPA-type ligands 18 and 19, isomers of the imino(bisphosphine) 18′ and 19′, could finally be selectively produced using a 1:2:2 molar ratio of the corresponding anilines, PPh2Cl, and NEt3 in dichloromethane, while the o-CN, o-CF3, and perfluoroaniline derivatives could never be isolated (Scheme 6).36

rearrangement of the corresponding imino(bisphosphine) precursors 18′ and 19′ upon reaction with [MCl2(COD)] (M = Pd, Pt) (Scheme 6).35 In the solid-state, the metal center Scheme 6. (Left) DPPA-type Ligands 18 and 19 Featuring a m- or p-Cyano Phenyl Group as N-Substituent and (Right) Synthesis of Complexes 20−23, Starting from the Corresponding Imino(Bisphosphine) Precursors 18′ and 19′

2.4. N-Heterocyclic Pendant Tails

In 2003, Dyson and colleagues reported the synthesis of ligand 24, obtained by the classical aminolysis reaction of 2,6diaminopyridine (Scheme 7).37 This is a potentially pentadentate ligand based on the association of two DPPA-type units linked through a pyridine substituted in its 2 and 6 positions. Owing to the presence of numerous donor groups, such ligands allow the synthesis of polynuclear metal complexes and macrocyclic ring systems (see also section 8 for poly-DPPA derivatives). Thus, 24 was reacted with 2 equiv of [Mo(CO)4(NBD)] and 4 equiv of [AuCl(THT)] to form the dinuclear and tetranuclear neutral complexes 25 and 26, respectively (Scheme 7).37,38 The molecular structure of the metal complex 25 was determined by XRD in the solid-state and revealed that both metal centers adopt an octahedral coordination geometry. Probably induced by the large van der Waals radius of the Mo(0) center, the P−N−P angles in complex 25 [103.6(2)° and 103.4(2)°] are in the upper range

of all the resulting complexes presents a square-planar coordination geometry, with the DPPA-type ligand acting as a chelate. Typical 31P NMR resonances were observed for the PdCl2 [36.1 (20) and 37.8 (22) ppm] and PtCl2 [22.4 (21) and 23.7 (23) ppm] complexes of these N-aryl-substituted DPPA ligands. 31P NMR data and selected bond lengths and angles are listed in Table 3. This reaction represents a new approach to the synthesis of chelating and bis-chelating transition-metal complexes, since not all bis(diarylphosphino)amines can be obtained using the standard aminolysis reaction and/or are

Table 3. 31P NMR Data and Characteristic Structural Parameters of the Ligands 18, 19 and the Complexes 20−23 bond length, Å δ, ppm (mult) 18 19 20 21 22 23 a

68.5 69.5 36.1 22.4 37.8 23.7

(s)a (s)a (s)b (s)a (s)b (s)a

1

JP,Pt, Hz

3343 3350

angle, deg

P−M

P−N

P−N−P

2.221(8) 2.201(3) 2.219(3)c 2.212(2)c

1.728c 1.746(2) 1.715(2) 1.733(1) 1.706(8)c 1.713(6)c

114.5(1) 112.7(2) 100.1(2) 98.7(7) 100.2(2) 99.9(1)

P−M−P

ref

72.62(4) 73.38(1) 72.29(5) 72.77(3)

36 36 35 35 35 35

In CDCl3. bIn CD2Cl2. cAverage value. 9242

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27] was only reported in 2014 as part of a ligand screening study in Cr-catalyzed selective ethylene oligomerization.23 Very recently, Dyson and colleagues have extended their studies to the phosphorylation of 2,3-, 3,4-, and 2,5diaminopyridines with PPh2Cl, and while complete phosphorylation and formation of the bis-DPPA derivative was achieved for the latter (28, Scheme 8), another outcome was observed

Scheme 7. Reactivity of Ligand 24 toward Metal Precursors, Giving 25 and 26, and Protons, Giving 24·HBF4

Scheme 8. Phosphorylation Reactions of Various Diaminopyridines

of the values found in N-functionalized DPPA-supported metal chelate complexes and the P−M bond lengths are quite long (av 2.483 Å, Table 4). In further studies, Dyson and colleagues reported that protonation of ligands with DPPA-type moieties attached to pyridine at the ortho-positions, as in ligand 24, quantitatively afforded the corresponding imino(bisphosphine) species.39 Monitoring by 31P NMR spectroscopy of the reaction between ligand 24 and 1 equiv of HBF4 showed the disappearance of the initial signal at 59.0 ppm and the concomitant appearance of two doublets centered at 17.2 and −20 ppm, with a 1JP,P value of 227 Hz, indicative of the imino(bisphosphine) structure of 24·HBF4 (Scheme 7). Deprotonation of the pyridinium salt 24·HBF4 regenerated quantitatively the starting DPPA-type structure. 31P NMR data and main structural parameters of the latter compounds are detailed in Table 4. Interestingly, the “simpler” mono-DPPA ligand derived from 2-aminopyridine [(Ph2P)2N(2-pyridine),

for the former two.40 The reaction between chlorodiphenylphosphine and 3,4-diaminopyridine, in a 3:1 ratio, yielded the c o r r e s p o n d i n g 3 - b i s ( d i p h e n y lp h o s p h in o ) a m i n e - 4 (aminophosphine)pyridine intermediate 29, which further reacted with another equivalent of PPh2Cl to give compound 30, probably via an unstable mixed 3-bis(diphenylphosphino)amine-4-{imino(bisphosphine)}pyridine intermediate (Scheme 8). In contrast, in the case of the 2,3-diaminopyridine, the bisaminophosphine intermediate 31 could be isolated by reaction

Table 4. 31P NMR Data and Characteristic Structural Parameters of the Ligands 24, 27−29, 32, 36, and 42a,b and the Metal Complexes 25, 26, 33, and 37−41 av bond length, Å M

δ, ppm (mult)

24 25 26 27 28 29 32

− Mo Au − − − −

59.5 (s) 95.0 (s) 84.0 (s) 59.3 (s) 72 (s), 61 (s)c 64 (s),c,d 31 (s)e 63 (s)c,d 19 and −17, 1JP,P = 266 Hzf

33 36 37 38 39 40 41 42a,b

Cr − Pd Pt Ni Fe2 Fe2 −

a

62.6 (s) 34.2 (s) 20.2 (s) 1JP,Pt = 3308 Hz 52.3 (s) 121.9 (s) 120.0 (s) 64 (s),d 27 (s)g

angle, deg

P−N

P−M

P−N−P

P−M−P

1.726

2.483

103.5b

66.0b

1.737 1.731

120.4(1) 120.5(1)

1.748

2.413

117.1b

1.705 1.717 1.692

2.228 2.208 2.122

100.2(1) 99.4(4) 97.7(2)

1.727

2.202

119.4(2)

71.85(2) 71.49(4) 73.80(5)

ref 37 37 38 23 40 40 40 41 42 42 42 43 44 44 45

a

In CDCl3. bAverage value. cIn CD2Cl2. dCorresponds to the P−N−P fragment. eCorresponds to the P−N−H fragment. fCorresponds to the N PPh2−PPh2 fragment. gCorresponds to the P−N−H fragment of 42b. 9243

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Scheme 9. Cr(III) Complex 33 and Its Reactivity toward Aluminum Reagents

with 2 equiv of PPh2Cl, while the reaction of an additional 2 equiv of chlorodiphenylphosphine led to the formation of the mixed N,N-bis(diphenylphosphanyl)-2-{(1,1,2,2-tetraphenyl1λ5-diphosphanylidene)amino}pyridin-3-amine (32, Scheme 8). The structure of compounds 29, 30, and 32 was confirmed by XRD. Selected structural parameters, and 31P NMR data of the PNP moieties of ligands 28, 29, and 32 are listed in Table 4. Gambarotta and colleagues recently synthesized the chromium trichloride complex 33, by reaction between ligand 24 and [CrCl3(THF)3] in THF at rt (Scheme 9).41 Its solidstate structure was determined by XRD and established that the ligand acts as a P,N,P-pincer involving the central pyridine donor and one P-donor of each DPPA-fragment of the ligand, the second P atom remaining uncoordinated (Table 4). With the aim to produce single-component catalysts for the oligomerization of ethylene, the authors reacted complex 33 with an excess of aluminum activators (AlEt3 or AlEt2Cl), and they isolated two isostructural Cr(II) complexes (34 and 35), characterized by XRD, which resulted from a multiple attack of the activator at both the ligand and the metal center. The steps included (i) loss of the uncoordinated phosphine group at each nitrogen atom, (ii) reduction of Cr(III) to Cr(II), (iii) protonation of one of the two nitrogen atoms, and (iv) formation of one PEtPh2 molecule, which was found coordinated to the Cr center. All three complexes were evaluated in catalytic oligomerization of ethylene, and the results will be discussed in section 13.1. Biricik and colleagues reported the synthesis of a DPPA-type ligand functionalized at the nitrogen atom by a 2-picolyl group (36) and studied its coordination chemistry toward Pd(II) and Pt(II) (Scheme 10).42 Ligand 36 was prepared by classical aminolysis of 2-picolylamine with 2 equiv of PPh2Cl in the presence of a base, and its 31P NMR spectrum displays a single resonance at 62.6 ppm. This ligand readily reacted with [MCl2(COD)] (M = Pd, Pt) to afford the corresponding P,Pcomplexes 37 and 38, respectively (Scheme 10). Their 31P NMR spectra exhibited typical singlet resonances at 34.2 (37) and 20.2 ppm (1JP,Pt = 3308 Hz) (38), and their typical squareplanar coordination geometry was confirmed by XRD studies. The P−N and P−M bond lengths and P−M−P bite angle are in the typical range for group 10 dihalide complexes with DPPA-type ligands (Table 4). Complex 37 was found to be an active catalyst in C−C cross-coupling reactions (see section 13.4), while 38 showed a negligible activity (2% conversion) in the hydroformylation of 1-octene in the presence of SnCl2 as a cocatalyst.46 The reaction of ligand 36 with [NiBr2(DME)] allowed Gao, Wu, and their colleagues to synthesize and isolate the squareplanar complex 39 (Scheme 10) with a chelating PNP ligand.43 The structural parameters of 39 are similar to those in its Pd(II) and Pt(II) analogues (Table 4). Complex 39 was found to be diamagnetic, due to retention of its square-planar coordination geometry in solution, and this allowed identi-

Scheme 10. Synthesis of Dihalide Group 10 Complexes 37− 39 and Diiron Complexes 40 and 41 Supported by Ligand 36a

a

L2 = COD for Pd and Pt, and DME for Ni.

fication of its 31P NMR singlet resonance at 52.3 ppm (Table 4). Complex 39 was used as precatalyst in ethylene oligomerization, and its performances will be discussed in section 13.2. To evaluate the effect of the N-substituent on the catalytic process, the authors also attempted to isolate an analog of 39 with a longer spacer between the aromatic ring and the PNP moiety, i.e., a CH 2CH 2 group instead of CH2 . Unfortunately, this ligand could not be isolated, due to its rapid oxidation during purification by column chromatography in air. In 2009, Liu and colleagues described the use of ligand 36 to model the diiron subunit center of the H-cluster of the enzyme [Fe−Fe]-hydrogenase.44 In nature, this active site is formed by two low oxidation state iron atoms, bridged by three dithiolate ligands and coordinated with three carbonyl and two cyanide ligands, with an additional water molecule or a hydroxide ion. As shown in Scheme 10, the authors used ligand 36 to displace two CO ligands from the precursor and, at the same time, introduce an internal base (pyridyl substituent), whose presence is known to produce dynamic interactions, e.g., H transfer from internal base to the diiron center.44 The authors were thus able to isolate and characterize the Fe−Fe complexes 40 and 41 and describe their protonation and redox behavior using IR spectroscopy, cyclic voltammetry, and digital simulation techniques. The electronic and structural similarity between 40 and 41 explains their very similar 31P NMR spectra, with a singlet resonance at 121.9 and 120.0 ppm, respectively. The solid-state molecular structure of complex 41 was confirmed by XRD studies, and the P−M bond lengths were found in the range of the other structurally characterized metal 9244

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DPPA-type ligand 45 (Scheme 12).43 Due to the short spacer between the PNP moiety and the additional donor group, this

complexes of ligand 36, although the P−N−P angle was larger [119.4(2)°] because, in this case, the PNP ligand acts as a bridge [Fe−Fe 2.481(11) Å] instead of a chelate (Table 4). In bioinorganic chemistry, metal complexes able to form complementary hydrogen bonds represent an ever-increasing important class of compounds for the development of biochemically active molecules. With the aim to prepare compounds with potential antiviral or anticancer activity, such as the targeted N9 -(N2 ′ -diphenylphosphinoaminoethyl)adenine, Woollins and colleagues reported the functionalized DPPA-type ligands 42a,b, as part of a mixture of compounds that resulted from the aminolysis of a functionalized adenine derivative (Figure 1).45 Since compounds 42a,b were side

Scheme 12. N-Furfuryl-Functionalized DPPA-type Ligand 45 and Its Group 10 Metal Complexes 46−49a

a

L2 = COD for Pd and Pt, and DME for Ni.

type of functionality is not expected to interact intramolecularly with the metal center coordinated to the DPPA fragment, as in scorpionate-type ligands; however, it may play a role during a catalytic process by stabilizing active and/or low-valent species in an intermolecular fashion. The 31P NMR spectrum of ligand 45 exhibits a single peak at 63.2 ppm (Table 5). With the aim to develop a new precatalyst for ethylene oligomerization, the authors synthesized the Ni(II) complex 46 by reaction of ligand 45 with [NiBr2(DME)] in a 1:1 molar ratio (Scheme 12). Complex 46 is diamagnetic in solution, which allowed the recording of its 31P NMR spectrum (singlet at 52.4 ppm, Table 5). The solid-state structure of 46 was confirmed by XRD analysis, revealing a Ni(II) center in a square-planar coordination geometry, forming a four-membered chelate ring (NiP2N) with ligand 45. The values of the P−N (av 1.694 Å) and P−M (av 2.128 Å) bond lengths and P−N−P [97.83(11)°] and P−M−P [73.74(3)°] bond angles (Table 5) are consistent with those in other [NiX2(PNP)] complexes (e.g., 39 in Table 4). As observed for the N-picolyl derivatives (see above), the short spacer between the PNP fragment and the additional donor does not lead to intramolecular coordination of the latter, either in solution or in the solid state, although intermolecular coordination could have been conceivable. The catalytic performances of complex 46 in ethylene oligomerization, with MAO or AlEtCl2 as cocatalyst, will be discussed in section 13.2. More recently, Aydemir and colleagues reported the synthesis of a variety of other Group 10 metal complexes supported by ligand 45. Reactions of an equimolar amount of the latter with [MX2L2] (M = Pd, Pt; X = Cl, Me, L2 = COD, DME) precursors led to the corresponding complexes [MX2(45)] (47, M = Pd, X = Cl; 48, M = Pt, X = Cl; 49, M = Pt, X = Me) (Scheme 12).51 The dichlorido complexes exhibited typical singlet resonances in their 31P NMR spectra at 33.2 ppm for 47 and at 19.3 ppm with 1JP,Pt = 3301 Hz for 48 (Table 5). The nature of the X ligands in [MX2(PNP)]-type complexes significantly affects their 31P NMR data, as illustrated with complex 49, which is an analog of 48 with X = Me instead of Cl, for which the PNP fragment resonates as a singlet signal at 50.6 ppm (with 1JP,Pt = 1546 Hz) (Table 5). The expected square-planar coordination geometry around the metal center, as well as the chelating coordination mode of the DPPA-type ligand, could be confirmed in the solid state by XRD studies on complexes 47 and 48. The P−N [av 1.695 Å (47) and 1.698 Å (48)] and P−M [av 2.221 Å (47) and 2.203 Å (48)] bond lengths and P−N−P [100.5(3)° (47) and 100.3(3)° (48)] and P−M−P [72.3(1)° (47) and 72.8(1)° (48)] bond angles were

Figure 1. Adenine-based DPPA-type ligands 42a,b.

products of the reaction, they were not isolated; however, their P NMR data were provided and the PNP fragment resonates as a singlet at ca. 64 ppm (Table 4). The coordination chemistry of 42a,b was not investigated. 31

2.5. Other N-Containing Functional Groups

Recently, Kappes, Fuhr, Roesky, and co-workers described the synthesis of the N-functionalized DPPA-type ligand 43 (Scheme 11), the diazenyl analog of the amino derivative 8 Scheme 11. DPPA-type Ligand 43 Functionalized with a Diazenyl Function and Its Dinuclear Au(I) Complex 44

(see Scheme 2).47 Ligand 43 exhibits a single 31P NMR resonance at 69.0 ppm (in CDCl3), which is consistent with the value found for 8 (69.3 ppm in CDCl3, Table 1). Reaction of 43 with 2 equiv of [AuCl(THT)] afforded the corresponding dinuclear complex 44 (Scheme 11), characterized by a 31P NMR singlet resonance at 88.3 ppm (CDCl3). In the solidstate, complex 44 crystallizes in the monoclinic space group P21/n, and the azo function adopts an E configuration. The Au(I) centers were found in a typically linear coordination geometry with P−N and P−Au bond lengths in the usual ranges (av 1.704 and 2.235 Å, respectively), while the P−N−P angle is quite large [123.4(4)°], due to the bridging mode adopted by the ligand, which supports an aurophilic interaction [Au···Au 3.0095(7) Å].48−50 Other examples of DPPA-type ligands integrating Nmorpholinyl (93) or N-2-(methoxymethyl)pyrrolidyl (94) groups, i.e., mixed N- and O-donor function, have been reported and are discussed in section 3.1.

3. OXYGEN-BASED FUNCTIONALIZATIONS 3.1. Ether Groups

3.1.1. Furan Derivatives. Gao, Wu, and colleagues reported in 2009 the synthesis of the N-furfuryl-functionalized 9245

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Table 5. 31P NMR Data and Characteristic Structural Parameters of the Ligand 45 and the Metal Complexes 46−52 av bond length, Å M 45 46 47 48 49 50 51 52 a

− Ni Pd Pt Pt Rh Rh Ru

δ, ppm (mult) 63.2 52.4 33.2 19.3 50.6 69.0 70.5 92.0

1

JP,M, Hz

angle, deg

P−N

P−M

P−N−P

P−M−P

1.694 1.695 1.698

2.128 2.221 2.203

97.83(1) 100.5(3) 100.3(3)

73.74(3) 72.3(1) 72.8(1)

a

(s) (s)a (s)b (s)b (s)b (d)a (d)a (s)a

3301 1546 120 120

ref 43 43 51 51 51 52 53 53

In CDCl3. bIn DMSO-d6.

and 70.5 ppm, respectively, each with a 1JP,Rh coupling constant of 120 Hz. In contrast, the Ru(II) complex 52 exhibited a singlet signal at 92.0 ppm for the equivalent P atoms of the Nfunctionalized ligand 45 (Table 5). All these Rh and Ru complexes were evaluated in catalytic transfer hydrogenation of ketones (section 13.5).201 3.1.2. Alkyl and Aryl Mono-Ether Moieties. Zhu, Lin, and colleagues prepared the N-aryl ether DPPA-type ligand [(Ph2P)2N(p-OMe)C6H4] (53) by the classical aminolysis reaction of p-methoxyaniline (Scheme 14).54 A ligand analogous to 53, in which the p-OMe substituent is replaced by a p-NO2 group (54), was also reported, and both were used as supporting ligands in Cr-catalyzed ethylene oligomerization, as part of a study of the influence of N- and P-substituents on the activity/selectivity of the catalytic system (section 13.1).55 Ligands 53 and 54 exhibit a singlet resonance at 70.0 and 69.7 ppm in their 31P NMR spectrum, respectively. The reaction between 53 and NiCl2 afforded the distorted-square-planar (XRD evidence) complex 55Ni in moderate yield, which presents a characteristic 31P NMR singlet at 47.6 ppm (Scheme 14 and Table 6). Combined with MAO as cocatalyst, this complex was highly active in the catalytic vinyl polymerization of norbornene (see section 13.3). Smith and colleagues described the formation of the Pd(II) complex 55Pd, an analog of 55Ni, by reaction of ligand 53 with [PdCl2(COD)] (Scheme 14).46 In solution, complex 55Pd displayed a 31P NMR singlet at 70.1 ppm (in CDCl3). This low-field chemical shift, when compared, for example, to the value of 34.1 ppm (in CD2Cl2) for [PdCl2{(PPh2)2NPh}],56 was assumed to result from the electron-withdrawing inductive effect of the aryl ether ring. Recently, the molybdenum dinitrogen complex 55Mo was prepared, along with a series of analogous complexes with Nsubstituted (with hydrocarbyl groups) DPPA-type ligands, by addition of an excess of magnesium turnings to a mixture of ligand 53 and [MoCl3 (THF)3 ] in toluene under a N 2

found to be in the range of analogous group 10 metal complexes (see e.g., Table 4). Complex 47 has been evaluated as catalyst in Suzuki cross-coupling reactions, and the results will be discussed in section 13.4. Aydemir and colleagues also reported the synthesis of Rh(I), Rh(III), and Ru(II) complexes with ligand 45 (Scheme 13). Scheme 13. Synthesis of Rh(I), Rh(III), and Ru(II) Complexes of Ligand 45

While the latter reacted with [Rh(COD)2]BF4 in a 2:1 ligand/ metal ratio to give the monocationic Rh(I) bis-chelate complex 50,52 the monochelate half-sandwich Rh(III) (51) and Ru(II) (52) complexes were obtained by reacting ligand 45 with [RhCl(μ-Cl)Cp*]2 and [RuClCp*(COD)] in 1:1 ligand/metal ratio, respectively (Scheme 13).53 The Rh complexes 50 and 51 exhibit typical 31P NMR doublet resonances, due to Rh−P coupling, centered at 69.0

Scheme 14. Synthesis of Dinitrogen Molybdenum Complex 55Mo and Group 10 Metal Complexes 55Ni and 55Pd of Ligand 53

9246

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Table 6. 31P NMR Data and Characteristic Structural Parameters of the Ligands 53 and 60−63 and the Metal Complexes 54, 55, 57−59, and 64−74 av bond length, Å M

δ, ppm (mult)

53 54 55Ni 55Pd 56a 56b 57 58

− − Ni Pd Au Au Au Au

59 60a 60b 61 62 63 64 68 69 70 71 72 73

Au − − − − − Cr Pd Pt Pt Mo Ru Cu

70.0 (s)a 69.7 (s)b 47.6 (s)a 70.1 (s)a 85.9 (s)c 102.2 (s)a 93.7 (s)c 87.9 (dd)d 89.2 (dd)d 83.2 (s)e 64.6 (s)a 63.6 (s)d 63.1 (s)a 65.5 (s)a 59.9 (s)a

Au

74 a

37.9 22.6 52.0 92.4 77.7 89.1

(s)a (s)a (s)a (s)a (s)f (br)g

JP,P, Hz

1

JP,Pt, Hz

P−N

1.712

P−M

2.118

angle, deg P−N−P

96.3(2)

P−M−P

73.99(5)

113 113

1.705 1.707

2.456 2.214

105.0(8) 99.3(2)

71.99(5)

1.729 1.734 1.718

2.499 2.290 2.314

103.4(3) 101.2(4) 106.8(2) 107.1(2)

65.78(2) 69.84(9) 73.19(4) 73.06(4)

3343 1607

84.9 (s)a

ref 54 55 54 46 61 63 61 62 62 64 23 64 64 64 64 65 65 65 65 65 65 65

b

In CDCl3. Deuterated solvent not reported. cIn DMSO-d6. dIn CD2Cl2. eIn acetone-d6. fIn CDCl3/MeOH. gIn CDCl3 for the PNP moiety, and PF6‑ anion at δ = −144 ppm.

atmosphere.57 Using FT-IR spectroscopy and DFT calculations, the authors demonstrated that the nature of the Nsubstituent of the DPPA-type ligand, which imposes its orientation with respect to the PMoP coordination plane (coplanar or orthogonal), controls the properties of the coordinated N2 ligands. Complex 55Mo was also found to convert dinitrogen to silylamine with low yields (turnover number of 1.03) by treatment with excess trimethylsilyl chloride and sodium metal under a dinitrogen atmosphere (THF, rt, 24 h). Yam and colleagues, who thoroughly investigated the photophysical properties of polynuclear d10 metal complexes supported by short-bite diphosphine ligands over the last 20 years,58−60 reported a series of specifically designed polynuclear Au(I) complexes (56−59, Scheme 15) expected to be of interest for this purpose.61,62 The synthesis of the ureacontaining complex 57 was readily achieved by reaction of complex 56a with 2 equiv of HS(C6H4)NHC(O)NHPh in the presence of an excess of triethylamine. It was characterized by 1 H and 31P NMR and ESI-MS spectroscopic techniques and EA (Table 6).61 The dinuclear alkynyl Au(I) analog 56b was prepared by addition of ligand 53 (0.5 equiv) to a dichloromethane solution of [Au(CCPh)2]∞ (1 equiv), and the resulting complex displayed photoluminescence in both 77 K glass and in the solid state.63 The emissive state was tentatively considered to derive from triplet states of [π(ArC C) → π*(PNP)] ligand-to-ligand charge transfer character, with mixing of a [π(ArCC) → 6s/6p(Au)] ligand-to-metal charge transfer character. A red shift of the emission energy was observed for 56b relative to its N-(n-Pr) analog, which was attributed to the presence of the less-electron-rich N-(p-

Scheme 15. Au(I) Complexes with Ligand 53, Studied for Their Photophysical Properties

OMe)C6H4 group. The preparation of decanuclear (supported by four ligands 53) and hexanuclear (supported by three ligands 53) complexes 58 and 59, respectively, produced simultaneously and subsequently separated by repeated recrystallization and mechanical separation, was achieved by bubbling H2S through a suspension of 56a in a mixture of EtOH and pyridine, followed by anion metathesis with LiClO4· 9247

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3H2O.62 The latter complexes exhibit a polynuclear core with Au···Au interactions supported by the PNP skeleton and bridging S atoms, but this connectivity could only be demonstrated by XRD for analogous complexes with N-(pMe)C6H4 and N-Ph groups. In the case of the decanuclear complex 58, two Au centers, which are not coordinated to any phosphine, are found in the center of the cluster. Selenido analogues of the sulfido complexes with various Nsubstituents were prepared and their photophysical properties studied, in particular with DPPA-type ligands functionalized with an aryl−halogen N-function (see section 6). The differences in nuclearities (hexa- vs decanuclear) and chalcogenido ligands (S vs Se) in this series of complexes gave rise to different emission properties. The complexes exhibited intense green and/or orange phosphorescence, which have been tentatively considered to originate from the excited states derived from the phosphine-centered intraligand transition and a metal-centered (ds/dp) mixed with ligand-tometal−metal charge transfer (E → Au) transition, respectively, upon photoexcitation in the solid state or in solution. These luminescence properties were nearly insensitive to the nature of the N-substituent (alkyl, aryl, 53, or p-halogen derivatives 226, 237, and 238; section 6) on the DPPA-type ligands. Bercaw and colleagues studied the influence of the ligand tail in various R-OMe-functionalized DPPA-type ligands on the chromium-catalyzed tri- and tetramerization of ethylene (60a and 61−63; Scheme 16).64 These ligands, as well as the later

presence of MAO as cocatalyst, the Cr(III)(PNP) complexes 64−67 were efficient catalysts for the selective tri- and tetramerization of ethylene (see section 13.1). Smith and colleagues extensively studied the coordination chemistry of the N-functionalized DPPA-type ligand 62, toward Mo(0), Ru(II), divalent group 10, and monovalent group 11 (d10) metal centers (Scheme 17).65 Equimolar reactions between ligand 62 and [MX2(COD)] precursor complexes (M = Pd, Pt; X = Cl, Me) afforded the corresponding [MX2(62)] complexes 68−70 (Scheme 17). While the Pd(II) complex 68 exhibited a singlet resonance in its 31P NMR spectrum at 37.9 ppm, both Pt(II) complexes 69 and 70 displayed a single peak flanked by satellites, due to P−195Pt coupling, at 22.6 (1JP,Pt = 3343 Hz) and 52.0 ppm (1JP,Pt = 1607 Hz), respectively (Table 6). The solid-state molecular structure of complex 68 was determined by XRD analysis and confirmed the distorted square-planar geometry around the Pd(II) center, expected from the bite angle imposed by the chelating DPPAtype ligand (Table 6). Reaction of ligand 62 with 1 equiv of [Mo(CO)4(NBD)] resulted in the displacement of the labile NBD ligand and the formation of the octahedral Mo(0) complex 71, in which the ligand acts as a chelate, occupying two equatorial coordination sites, while four CO ligands complete the metal coordination sphere. The 31P NMR spectrum of complex 71 contains a single signal at 92.4 ppm. The neutral Ru(II) bis-chelate complex 72 was obtained from [Ru(p-Cym)(μ-Cl)Cl]2 (89% yield) or [RuCl2(DMSO)4] (33% yield), in a 2:1 ligand/metal molar ratio. Complex 72 has been characterized in the solid state by single crystal XRD analysis and presents an octahedral Ru(II) center bis-chelated by two mutually trans PNP ligands 62, and two axial chlorines complete the metal coordination sphere. The reaction between ligand 62 (2 equiv) and [Cu(NCMe)4]PF6 afforded the monocationic bis-chelate complex (73), in which the Cu(I) center is in a tetrahedral coordination environment, as deduced from XRD studies. The 31 P NMR spectrum of 73 exhibited a broad signal (ω1/2 = 110 Hz) centered at 89.1 ppm. Another d10 metal complex of ligand 62 was obtained by reaction of the latter with 2 equiv of [AuCl(THT)]. The proposed dinuclear structure of complex 74, with two AuCl units bridged by one ligand 62, was supported by the classical downfield shift of its 31P NMR singlet signal at 84.9 ppm, and the observation of the Au−Cl stretching vibration at 326 cm−1 in the FT-IR spectrum. 3.1.3. Aryl Bis-Ether Moieties. Biricik and colleagues studied the coordination chemistry, toward d8 and d10 metal centers, of a series of new N-aryl-(bis-ether)-functionalized DPPA-type ligands of general formula (Ph2P)2N−C6H3−R [R = 3,5-OMe (75), 2,5-OMe (76), 2,4-OMe (77), 3,4-OMe (78)] prepared via the classical aminolysis of the respective commercially available dimethoxyanilines (Scheme 18).66 The significant difference in chemical shifts observed in the 31P NMR spectra of the free ligand [68.6 ppm (75), 64.7 ppm (76), 66.1 ppm (77), and 71.1 ppm (78)] illustrates the influence of the substituents position in such N-functionalized DPPA-type ligands (Table 7). An equimolar reaction between ligands 75−78 and [MCl2(COD)] (M = Pt, Pd) afforded the corresponding platinum and palladium cis-dichloride complexes 79−82 and 83−86, respectively (Scheme 18). The 31P NMR spectra of the Pt(II) complexes 79−82 exhibit each a singlet resonance between 20.0 and 25.3 ppm, flanked by typical 1JP,Pt satellites (3229−3414 Hz), and those of the Pd(II) complexes 83−86 display a single peak in the range 34.1−38.2 ppm

Scheme 16. N-Ether-Functionalized DPPA-type Ligands 60− 63 and Synthesis of Their Corresponding Cr(III) Complexes 64−67a

a

R′ refers to the N-substituent of the corresponding ligand.

reported 60b,23 were obtained by the classical aminolysis route and exhibited 31P NMR singlet resonances in the region 59.9− 65.5 ppm (Table 6). The electron-withdrawing effect of the substituents and their position explains the difference in chemical shift between ligands 62 (65.5 ppm) and 53 (70.0 ppm, described above), which are ortho- and para-substituted isomers. The reactions of ligands 60a and 61−63 with [CrCl3(THF)3] led to the formation of the corresponding chlorido-bridged dinuclear Cr(III) complexes 64−67, on the basis of the structure of 64 established by XRD studies. The metal center has an octahedral coordination geometry, with the chelating PNP ligand occupying one apical and one equatorial site, resulting in a strong distortion away from a regular octahedron (Clapical−Cr−Papical < 180° and Table 6). In the 9248

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Scheme 17. Coordination Chemistry of the N-(o-Methoxy)aryl-Functionalized Ligand 62 toward Various Metal Centers

78 in 1:2 ratio, respectively (Scheme 18). Similar 31P NMR spectra were recorded for these four complexes, with a typical singlet resonance in the range 87.6−89.0 ppm (Table 7) and a septet at ca. −144 ppm for the counterion PF6‑. These data are in agreement with the tetrahedral coordination geometry already observed in complex 73 (see above). In a recent study, Biricik et al. have shown that the reaction of the DPPA derivatives 75−77 with aldehydes (2 equiv) led, after P−N bond cleavage, to the formation of their respective insertion products of general formula (Ph2PO)CH(R)NH[x,y-(MeO)2C6H3], (x,y = 3,5; 2,5; 2,4; R = H, Me, Ph, C6H4OMe-p, C6H4Cl-p).67 3.1.4. Other N-Functional Groups Containing an R− O−R′ Moiety. Kayan and colleagues reported the synthesis of the new N-methoxyaryl-functionalized bis-DPPA-type ligand 91 and its corresponding dinuclear Pd(II) and Pt(II) complexes 92Pd and 92Pt, respectively.68 These were obtained by the ligand substitution reaction between 1 equiv of the tetraphosphine ligand 91 and 2 equiv of the corresponding [MCl2(COD)] (M = Pd, Pt) precursor (Scheme 19).68 The four equivalent P atoms of ligand 91 give rise to a 31P NMR singlet peak at 68.8 ppm. The high symmetry of the ligand is maintained in both dinuclear Pd2 and Pt2 complexes, which exhibit singlet resonances at 41.4 ppm for 92Pd and at 19.5 ppm with 195Pt satellites (1JP,Pt = 3110 Hz) for the Pt complex 92Pt (Table 8). Both complexes were evaluated in the catalytic Suzuki cross-coupling reaction of aryl bromides and phenylboronic acid, but only the Pd derivative 92Pd showed a good activity (see section 13.4). Blann and colleagues studied the influence of the ligand structure, i.e., the nature of the N-pendant group, on the selectivity of the Cr(III)-catalyzed ethylene tetramerization reaction, and one of the ligands used contained a morpholinyl moiety (93, Figure 2) and was characterized in 31P NMR by a singlet signal at 62.6 ppm (Table 8).69 Metalation of ligand 93 was performed in situ by addition of 0.33 equiv of [Cr(acac)3], and the mixture was then combined with MAO and used in catalytic ethylene oligomerization (see section 13.1). The crystal structure of (S)-l-[bis(diphenylphosphino)amino]-2-(methoxymethyl)pyrrolidine (94), a chiral DPPAtype ligand presenting a mixed N- and O-functional group, was

Scheme 18. Synthesis of d8 (79−86) and d10 (87−90) Metal Complexes of Ligands 75−78

(Table 7). The expected distorted square-planar coordination geometry around the d8 metal centers, along with the chelating coordination mode of the PNP ligands, was established by XRD studies. Relevant structural parameters are reported in Table 7. The [PdCl2(PNP)] complexes 83−86 were evaluated in catalytic Heck and Suzuki coupling reactions of aryl bromides with styrene and phenylboronic acid, respectively (see section 13.4). Bis-chelated monocationic Cu(I) complexes 87−90 were synthesized by reaction of [Cu(NCMe)4]PF6 with ligands 75− 9249

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Table 7. 31P NMR Data and Characteristic Structural Parameters of the Ligands 75−78 and the Pt(II), Pd(II), and Cu(I) Metal Complexes 79−82, 83−86, and 87−90, Respectively av bond length, Å δ, ppm (mult)

M 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 a

− − − − Pt Pt Pt Pt Pd Pd Pd Pd Cu Cu Cu Cu

68.6 64.7 66.1 71.1 20.0 22.9 22.5 25.3 34.1 38.2 37.9 34.9 87.6 88.8 89.0 87.6

1

JP,Pt, Hz

(s)a (s)a (s)a (s)a (s)a (s)a (s)a (s)a (s)a (s)a (s)a (s)a (s)b (s)b (s)b (s)b

P−N

3414 3344 3229 3325

P−M

angle, deg P−N−P

P−M−P

1.707

2.202

99.6(1)

72.59(3)

1.717

2.201

98.7(3)

72.54(8)

1.712

2.221

99.4(2)

71.9(5)

1.719

2.215

98.8(3)

72.20(6)

ref 66 66 66 66 66 66 66 66 66 66 66 66 66 66 66 66

In CDCl3. bIn CDCl3 for the PNP moiety, and PF6‑ anion at δ = −144 ppm.

Scheme 19. Ligand 91 and Its Pd(II) (92Pd) and Pt(II) (92Pt) Complexes

To diversify the library of phosphine-based ligands applied to the selective Cr-catalyzed ethylene oligomerization (see section 13.1), Wasserscheid, McGuinness, and colleagues reported two DPPA-type ligands N-functionalized with an alkyl- and an aryltrimethylsilyl ether (TMS) group (95 and 96, respectively, Scheme 20), which were initially prepared to access the Scheme 20. N-Alkyl- and N-Aryltrimethylsilyl EtherFunctionalized DPPA-type Ligands (95, 96) and Reactivity toward TiCl4 (97) and MeOH (98)

Table 8. 31P NMR Data of the Ligands 91 and 93−96 and the Metal Complexes 92Pd, 92Pt, and 97 δ, ppm (mult) 91 92Pd 92Pt 93 94 95 96 97 a

68.8 41.4 19.5 62.6

1

JP,Pt, Hz

a

(s) (s)a (s)a (s)a

b

63.0 (s)c 62.8 (s)c 61.4 (s)c

3110

ref 68 68 68 69 70 23 23 23

In CDCl3. bOnly characterized in the solid state by XRD. cIn CD2Cl2.

corresponding alcohol derivatives.23 Since conventional methods failed to deprotect the alcohol function of ligands 95 and 96, the authors attempted to cleave the silyl ether by reaction with TiCl4, but surprisingly, they isolated the TiCl4 adduct 97 instead of the phenolic species. It was isolated in excellent yield and characterized by several spectroscopic methods (Table 8); chelation of the PNP moiety to the Ti center was unambiguously established by XRD. While the P−N bond lengths (av 1.716 Å) were in the range of those found in similar late transition-metal complexes (see Tables 6 and 7), the P−Ti bonds were quite long (av. 2.635 Å) and the P−Ti−P bite angle very small (63.2°).23 Noteworthy, when a methanol solution of ligand 96 was refluxed, facile deprotection of the trimethylsilyl

Figure 2. Ligands 93 and 94 bearing morpholinyl and 2(methoxymethyl)pyrrolidyl functional groups, respectively.

also reported in the late 1990s (Figure 2).70 The structure shows a P−N−P angle of 124.04(9)° and P−N bond lengths of 1.722(2) and 1.717(2) Å with a quasiplanar geometry around the PNP atom, a common feature in all, however few, reported examples of free N-functionalized DPPA-type ligands (e.g., ligand 141, section 4.1). 9250

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centers, i.e., P−Au−Cl in 100 and P−Au−P in 101, with the bridging ligand 99 supporting Au···Au aurophilic interactions [Au−Au 3.0593(7) and 2.798(2) Å in 100 and 101, respectively].48−50 Selected NMR data and structural parameters of compounds 99−101 are reported in Table 9. Smith and colleagues reported the synthesis of two other DPPA-type ligands of general formula Ph2PN(R)PPh2 [R = C6H4(3-CO2Me) (102), C6H4(4-CO2Me) (103)] (Scheme 22), which carry on their nitrogen atom a mono-ester-aryl function.72 These diphosphine ligands exhibit a single 31P NMR resonance at 69.0 (102) and 68.5 (103) ppm, very close to that of the bis-ester derivative 99 (see above and Table 9).71 The authors investigated the coordination chemistry of ligands 102 and 103, along with that of ligand 99, toward the group 10 metal centers of [MX2(COD)] (M = Pt, Pd; X = Cl, Br, I) (Scheme 22). The difference in ligand substitution and/or in the nature of the halogen in the PtX2 (X = Cl, Br, I) complexes 104−108 only slightly affected the 31P NMR chemical shift of the equivalent P atoms of the ligands (17.4−22.5 ppm) and the P−Pt coupling constants (3066 < 1JPt,P < 3339 Hz). However, the overall trend of reduction in the magnitude of the Pt−P magnetic coupling from chloride (1JPt,P = 3339 Hz, 104) to bromide (1JPt,P = 3260 Hz, 107) and iodide (1JPt,P = 3066 Hz, 108) complexes is consistent with the increasing trans influence of the halide in the order Cl < Br < I (Table 9). The PdCl2 complex 109, analogous to 104, was obtained from [PdCl2(COD)], and its 31P NMR spectrum exhibits a typical singlet resonance at 36.1 ppm. Complexes 105 and 108 were characterized in the solid-state by XRD analysis, which confirmed the expected distorted square-planar coordination geometry around the Pt(II) center, with the PNP ligand acting as a chelate. The authors observed that complexes 104 and 109 reacted with MeOH at room temperature, leading to clean and selective cleavage of one P−N bond, to afford complexes 110 and 111, respectively (Scheme 22).72 The NMR and structural data of these monophosphine complexes will not be further discussed here. 3.2.2. Alkyl Ester Moieties. Beck and colleagues investigated the coordination chemistry of the optically active bis(diphenylphosphino)-α-(S)-amino acid methyl ester derivatives 112−115,73 of general formula (Ph2P)2NCH(R)CO2Me [R = H, 112; R = Me (S), 113; R = Bn (S), 114; R = i-Pr (S), 115] toward piano-stool derivatives [(η5-C5H5)MCl(CO)3] (M

ether occurred and was followed by a rapid intramolecular reaction to generate a monocyclic hydrophosphorane (98). 3.2. Ester Groups

3.2.1. Aryl Ester Moieties. Roesky and colleagues reported the synthesis of dimethyl 5-[N,N-bis(diphenylphosphanyl)amino]isophthalate (99), a DPPA-type ligand functionalized on the N atom by a bis-ester-aryl group, resulting from the aminolysis reaction of dimethyl 5-aminoisophthalate (Scheme 21).71 Ligand 99 was characterized in the solid-state by XRD Scheme 21. Synthesis of Au(I) Complexes 100 and 101 from Ligand 99

analysis and selected data are given in Table 9. Depending of the metal precursor and the stoichiometry used, two types of Au(I) complexes could be obtained. Reaction between ligand 99 and 2 equiv of [AuCl(THT)] afforded the neutral dinuclear complex 100, characterized in 31P NMR by a low-field shift of the P resonance to 87.8 (s) ppm (vs 69.1 ppm for the free ligand 99, Table 9). The equimolar reaction between ligand 99 and the cationic Au(I) precursor [Au(THT)2]ClO4 resulted in the formation of the dicationic complex 101, which displayed a similar low-field shift of the 31P NMR resonance of the PNP moiety to 97.9 ppm (Table 9). Both dinuclear Au(I) complexes were characterized in the solid-state by XRD studies and revealed a typical linear coordination geometry for the Au(I)

Table 9. 31P NMR Data and Characteristic Structural Parameters of the Ligands 99, 102, and 103 and the Au(I) (100, 101), Pt(II) (104−108), and Pd(II) (109) Complexesa av bond length, Å M 99 100 101 102 103 104 105 106 107 108 109 a

− Au Au − − Pt Pt Pt Pt Pt Pd

δ, ppm (mult) 69.1 87.8 97.9 69.0 68.5 22.5 21.4 21.3 21.3 17.4 36.1

(s) (s) (s) (s) (s) (s) (s) (s) (s) (s) (s)

1

JP,Pt, Hz

3339 3334 3330 3260 3066

P−N

angle, deg

P−M

P−N−P

1.730 1.711 1.675

2.227 2.234

115.6(7) 118.6(5) 130.1(1)

P−M−P

1.711

2.200

99.1(5)

72.68(1)

1.712

2.210

100.15(2)

72.89(4)

ref 71 71 71 72 72 72 72 72 72 72 72

Spectra recorded in CDCl3. 9251

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Scheme 22. Synthesis of the Pt(II) and Pd(II) Complexes 104−109 of the Ester-Functionalized Ligands 99, 102, and 103 and Reactivity toward MeOHa

R −R3 refer to the substituents on the N-aryl group of the corresponding ligand.

a 1

= Mo, W) and examined the reactivity of the resulting complexes (Scheme 23).74 Initial reactions between these chiral

ligands and the metal precursors led to the cationic Mo(II) (116−119) and W(II) (120, 121) complexes of general formula [(η5-C5H5)M(CO)2(L)]Cl (L = 112−115) resulting from displacement of one carbonyl and one chlorido ligands. The Mo complexes 116 and 118 and the W complexes 120 and 121 were characterized in solution by 31P NMR spectroscopy, and for all of them a sharp singlet signal was observed. Their chemical shift much depends on the nature of the metal center, with values of 95.2 (116) and 101.8 (118) ppm for the molybdenum derivatives and of 63.0 (120) and 65.9 (121) ppm for the tungsten complexes (Table 10). An anion metathesis reaction performed with the tungsten derivatives 120 and 121 and KPF6 led to the corresponding salts 122 and 123 (Scheme 23). Providing energy (hν or ΔT) to complexes 116−121 allowed the elimination of one CO ligand and the reinsertion of the chloride counterion in the metal coordination sphere, leading to the neutral complexes 124−129 (Scheme 23). This complete series of neutral complexes displayed two diastereoisomers, RMSC and SMSC, which could easily be distinguished by 1H and 31P NMR. The detailed list of the 31P NMR data is provided in Table 10. Noteworthy, the tungsten PF6 salts 122 and 123 do not exhibit further reactivity, even after exposure to higher temperature or more intense light. In the course of a study on model complexes for the active site of [NiFe]-hydrogenase, Song and colleagues reported the synthesis of a series of heterodinuclear Ni/Mn complexes

Scheme 23. Synthesis of Piano-Stool Mo(II) (116−119 and 124−127) and W(II) (120−123, 128, 129) Complexes of the Chiral Ligands 112−115a

a

R′ refers to the N-substituent in the corresponding ligand.

Table 10. 31P NMR Data of the Ligands 112−115 and the Cationic Mo(II) (116, 118) and W(II) (120, 121) and Neutral Mo(II) (124−127) and W(II) (128) Complexes

b

M

δ, ppm (mult)

112 113 114 115 116 118 120 121 124

− − − − Mo Mo W W Mo

125 RMoSc

Mo

66.1 (s)b 55.5 (s)b 56.5 (s)b 56.9 (s)b 95.2 (s)c 101.8 (s)b 63.0 (s)c 65.9 (s)b 103.3 (d)c 128.2 (d)c 101.6 (d)c 124.7 (d)c

2

JP,P, Hz

ref

118

73 73 73 73 74 74 74 74 74

114

74

d

125 SMoSc 126 RMoSc 126 SMoSc 127 RMoSc 127 SMoSc 128

M

δ, ppm(mult)

Mo

102.0 (d)c 123.9 (d)c 106.6 (d)b 130.7 (d)b 107.0 (d)b 128.8 (d)b 104.8 (d)c 128.2 (d)c 105.9 (d)c 122.9 (d)c 73.7 (d)c 92.7 (d)c

Mo Mo Mo Mo W

2

JP,P, Hz

ref

116

74

118

74

118

74

118

74

124

74

77

74

In CH2Cl2. cIn 80% CDCl3/20% C6D6. dA 1JW,P coupling constant of 214.4 Hz was reported. 9252

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Scheme 24. Mononuclear Ni(II) Complexes 131 and 132 with the N-Ester-Functionalized Ligand 130 and Corresponding Dinuclear Ni/Mn Complexes 133 and 134, Studied as Models for the Active Site of [NiFe]-Hydrogenases

supported by two types of PN(R)P ligands [R = CH2CO2Et (130) or CH2C6H4Me-p] (Scheme 24).75 The Ni/Mn complexes 133 and 134 were obtained in a three-step procedure consisting of (i) coordination of the DPPA-type ligand to a NiCl2 center from its hexahydrate precursor [NiCl2· 6H2O], giving complex 131, followed by (ii) a thiolysis reaction using EtSH in the presence of excess NEt3, affording 132, and finally (iii) the reaction between the dithiolate complex 132 and an equimolar amount of [MnX(CO)5] (X = Cl or Br), leading after CO substitution to the triply-bridged complexes 133 (X = Cl) and 134 (X = Br, Scheme 24). The ligand 130, the Ni(II) complexes 131 and 132, and the heterodinuclear Ni/ Mn complexes 133 and 134 were characterized by multinuclear NMR and IR spectroscopic techniques and EA, and also XRD for 130 and 134 (Table 11). Electrochemical studies of the Ni/

Scheme 25. Post-Metalation Functionalization of DPPA with Benzoyl Chloride, to Form Complexes 138−140

required prior metalation; the formation of LiN(PPh2)2 and further reactivity are described in section 11.

4. PHOSPHORUS-BASED FUNCTIONALIZATIONS 4.1. Phosphino Groups

31

The chemistry of N-phosphino-functionalized DPPA-type ligands started with the investigations of Ellermann and colleagues in the mid-1980s and the synthesis of the tris(diphenylphosphino)amine ligand 141 (Scheme 26).77

Table 11. P NMR Data and Characteristic Structural Parameters of the Ligand 130 and the Ni(II) (131, 132) and Dinuclear Ni/Mn (133, 134) Complexes av bond length, Å δ, ppm (mult) 130 131 132 133 134 a

64.4 46.7 62.9 46.8 54.3

a

(s) (s)b (s)b (s)b (s)b

P−N

P−M

1.719

1.707

angle, deg P−N−P

P−M−P

ref

73.87(5)

75 75 75 75 75

124.17(7)

2.169

99.6(2)

Scheme 26. Access to the Tris(diphenylphosphino)amine Ligand 141

In DMSO-d6. bIn CDCl3.

The latter was obtained in low yield by treatment of DPPA with n-BuLi (1 equiv), followed by addition of an equimolar amount of PPh2Cl, and exhibited a 31P NMR singlet resonance at δ 59.2 ppm. The solid-state structure of ligand 141 was confirmed by XRD a few years later, the characteristic structural parameters of which are reported in Table 12,78 and the authors highlighted a (quasi-)planar NP3 fragment, as earlier discovered for N(PF2)3 by electron diffraction in the gas phase.79 Although 141 was obtained in low yield, it remains an important finding, since previous attempts to synthesize it led instead to the isolation of its formula isomer [(C6H5)2P−P(C6H5)2N−P− (C6H5)2].80 The authors were also able to produce ligand 141 by a postmetalation procedure, involving deprotonation/lithiation of [M(CO4)(DPPA)] (M = Cr, 135H; M = Mo, 136H; M = W, 137H), followed by reaction with an equimolar amount of PPh 2 Cl, which afford ed t he correspon din g t ris(diphenylphosphino)amine complexes [M(CO4){(Ph2P)2N(PPh2)}] (M = Cr, 142; M = Mo, 143; M = W, 144; Scheme 27).76 The pattern of the 31P NMR spectrum of the Mo(0) species 143 corresponded to an A2X spin system, with one doublet centered at 102.9 ppm for the two equivalent P atoms coordinated to the metal center (PNP chelate moiety), coupled (2JP,P = 22 Hz) to the free phosphorus donor, which appeared

Mn complexes were performed and one quasi-reversible reduction wave in the range from −1.21 to −1.26 V and one irreversible reduction wave between −1.51 and −1.58 V, respectively, were observed. The complexes with the N(CH2C6H4Me-p)-substituted ligand were evaluated as electrocatalysts for the HOAc proton reduction to dihydrogen. 3.3. Other O-Containing Functional Groups

Ellermann and colleagues described an elegant access to metal carbonyl complexes supported by N-functionalized DPPA-type ligands by direct reaction between benzoyl chloride and the preformed metalated and lithiated [M(CO4)(Ph2P)2NLi] (M = Cr, 135Li; M = Mo, 136Li; M = W 137Li) intermediates, generated in situ by treatment of their corresponding DPPA derivatives [M(CO4){(Ph2P)2NH}] (M = Cr, 135H; M = Mo, 136H; M = W, 137H) with n-BuLi (Scheme 25).76 The resulting complexes [M(CO4){(Ph2P)2NC(O)Ph}] (M = Cr, 138; M = Mo, 139; M = W, 140) were characterized by 31P NMR spectroscopy and displayed singlet resonances at 119.5 ppm (in CD2Cl2) and 93.4 ppm (in 80% CH2Cl2/20% acetone-d6), for the Cr(0) and the Mo(0) derivatives, respectively, while the W(0) complex exhibited a singlet at 70.7 ppm (in 80% CH2Cl2/20% acetone-d6) flanked by two satellites (1JW,P = 222 Hz). The authors assumed that this DPPA functionalization 9253

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Table 12. 31P NMR Data and Characteristic Structural Parameters of the Ligands 141, 150, and 155−156 and the Metal Complexes 143−149, 151, and 152 δ, ppm (mult)

a

M

PNP

141 142 143 144 145 146 147 148

− Cr Mo W Ag Au Cr, Au Cr, Au

149 150 151

Ru − Pd

59.2 (s)a 126.7 (d)a 102.9 (d)a 77.1 (br)a,b 51.7 (dm)c 67.2 (s)d 137.6 (d)e 135.7 (d)f 96.9 (d) 61.2 (d)g 60.6 (s)g 74.1 (dd)h 56.2 (d)

152 155

Pd −

156

−

other P

bond length, Å x

JP,P, Hz

72.0 (t) 76.0 (t) 75.7 (br)

23 22 22

72.0 (t) 70.1 (tt)

23 22, 22+5 5 38

51.9 (t) −21.8 (s) 19.0 (s)

P−N

44.7 64.2 44.8 64.1

(d) (d) (d)g (d)

angle, deg P−N−P

1.738l

120.12l

1.717l

124.9(5)

P−M−P

2.256l

124.4(3)

67 1.736(3)i 1.636(4)j

67

123.5(2)

ref 77, 78 77 76 76 81 82 82 82 83 84 84

ndk ndk 1.719l

g

P−M

92.15(2)

84 85 85

In CD2Cl2. b1JP,W = 194 Hz. cIn DMF-d7; 1JP,Ag = 449 Hz. dIn DMSO/acetone-d6 (4/1). eIn acetone-d6. fIn CH2Cl2/acetone-d6 (4/1). gIn CDCl3. In DMSO-d6; see assignment in the text. iN−PPh2 fragment. jN−P+Ph2CH2 fragment. kNot determined. lAverage value.

h

Scheme 27. Post-Metalation Functionalization of DPPA with Diphenylchlorophosphine, to Form Complexes 142−144

Scheme 28. Synthesis of Trinuclear Ag(I) (145) and Au(I) (146) Complexes with Ligand 141

Scheme 29. Synthesis of Di- (147) and Trinuclear (148) Heterometallic Complexes Supported by Ligand 141 as a triplet at 76.0 ppm, similarly to the situation in the Cr analog 142 (Table 12). The spectrum of the W(0) analog 144 corresponded to a strongly coupled A2B system, but the resonances were not well-resolved, and therefore, only a partial description was possible: the A-group of signals was centered at 77.1 ppm, while the B group was centered at 75.7 ppm. The authors could also deduce a 2JP,P coupling constant of 22 Hz, which is in accordance with that measured in 143, and a 1JW,P coupling constant of 194 Hz (Table 12). Ellermann and colleagues further examined the potential of the tris(diphenylphosphino)amine ligand 141 as assembling ligand for the formation of (hetero)polynuclear complexes.81,82 Homometallic trinuclear d10 metal complexes (Ag 145 and Au 146) were readily accessible by reacting the tridentate ligand 141 with 3 equiv of AgCl or [AuCl(CO)], respectively, and both complexes exhibited a 31P NMR singlet at δ 51.7 (dm, 1 JAg,P = 449 Hz) and 67.2 ppm, respectively, confirming that each phosphorus donor binds to one MCl (M = Ag, Au) moiety (Scheme 28).81,82 The synthesis of di- and trinuclear heterobimetallic complexes was also achieved, starting from the Cr complex 142. Reaction of the latter with 1 equiv of [AuCl(CO)] led to the dinuclear complex 147, which reacted further with in situ generated 135Li to yield the trinuclear complex 148 (Scheme 29).82 All these compounds were isolated and characterized by multinuclear NMR, IR, and Raman spectroscopic techniques, EA, and MS (Table 12). As

expected, complex 147 exhibits in its 31P NMR spectrum one doublet at δ 137.6 (2JP,P = 23 Hz) for the two P atoms coordinated to the Cr center and one triplet at δ 72.0 (2JP,P = 23 Hz) for the P atom bound to the AuCl moiety. The spectrum of the trinuclear complex 148 contains three signals at δ 135.7 (d, 2JP1,P2 = 22 Hz, for P1), 96.9 (d, 3JP2,P3 = 5 Hz, for 9254

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P3), and 70.1 (tt, 2JP1,P2 = 22 and 3JP2,P3 = 5 Hz, for P2) ppm (Scheme 29 and Table 12). Lorenz and colleagues isolated a piano-stool Ru(II) complex (149, Figure 3, Table 12) presenting a direct N-P

[PdCl2(P,P)] complex 151, in which the metal center is chelated by one PPh2 moiety of the PNP (P1Ph2) fragment and the P3Ph2 substituent (Scheme 30). In its 31P NMR spectrum, complex 151 exhibited three inequivalent phosphane signals with different multiplicities (JP,P coupling not provided) at δ 74.1 (dd, P1), 56.2 (d, P2), and 19.0 (s, P3) ppm (Table 12). Single crystals suitable for XRD analysis were grown from a 1:1 MeCN/hexane mixture of 151 over a period of 2 weeks, and their study revealed the structure of the dinuclear palladium(II) complex 152, in which an additional PdCl2(NCMe) moiety is coordinated to the P2Ph2 group of ligand 150 (Scheme 30). This transformation was assumed to be the consequence of the slow crystallization process in a coordinating solvent (MeCN). Both Pd(II) centers in the dinuclear complex 152 displayed a typical distorted square-planar coordination geometry, with usual bond lengths and angles (Table 12). The monopalladium complex 151 was active for the oxidative hydroxylation of arylboronic acids to phenols and the oxidative coupling of arylboronic acids (see section 13.6).

Figure 3. Piano-stool Ru(II) complex with an N-PPh2NH functionalized DPPA-type ligand.

functionalization of the DPPA derivative ligand [namely (Ph2P)2N−PPh2NH], which resulted from the reaction of [RuCpCl(PPh3)2] with DPPA in a 1:1 molar ratio.83 Complex 149 was initially formed as a minor product (25%, yellow-green precipitate), along with the expected [RuCpCl(DPPA)] complex, and the authors assumed that it resulted from (i) a nucleophilic attack from one phosphine of a free DPPA on the electrophilic N−H moiety of a metal-coordinated DPPA, (ii) easy formation and elimination of a PPh2H molecule (NMR evidence) via P−N cleavage, and (iii) coupling of the complex fragment with the generated iminophosphoranyl group. Their proposed mechanism was supported by the observations that the proportion of 149 increased with the amount of DPPA used and that [RuCpCl(DPPA)] could be converted to 149 by treatment with DPPA. Bhaduri, Lahiri, and colleagues reported the synthesis of ligand 150, which features a PNP moiety functionalized on the N atom with a dangling −CH2CH2PPh2 group (Scheme 30).84

4.2. Other P-Containing Functional Groups

Aladzheva and colleagues reported an easy access to the 1,2azaphophanolanes species 153−156, by the one-pot reaction between the corresponding 3-halopropylamine hydrohalides and 2 equiv of PPh2Cl in the presence of an excess of NEt3 to scavenge the HCl formed (Scheme 31).85 Although these ligands do not possess an additional P-donor function, they belong to N-functionalized DPPA-type ligands, involving a P atom (PR4+ moiety). The iodide 155 and the perchlorate 156 derivatives could be isolated in good yield (>80%), and the authors provided their complete FT-IR and 1H and 31P NMR spectroscopic characterization. In particular, the 31P NMR spectrum of 155 exhibited two doublets centered at 44.7 and 64.2 ppm, coupled with a 2JP,P of 67 Hz; 156 displayed a similar pattern with two doublets centered at 44.8 and 64.1 ppm and a 2 JP,P of 67 Hz. Compound 156 was characterized in the solid state by single-crystal XRD analysis, which revealed notably different P−N bond lengths, N−PPh2 1.736(3) vs N− P+Ph2CH2 1.636(4) Å, and the P−N−P angle of 123.5(2)° is in the usual range [vs 124.9(5)° for 150, Table 12].

Scheme 30. N-Alkylphosphino-Functionalized DPPA-type Ligand 150 and Its Mono- and Dipalladium(II) Complexes 151 and 152, Respectively

5. SULFUR-BASED FUNCTIONALIZATIONS 5.1. Thiophene Derivatives

Gao, Wu, and colleagues reported the synthesis of two DPPAtype ligands functionalized with N-methyl-2-thiophene (157) Scheme 31. Synthesis of Modified DPPA-type Ligands 153− 156

The 31P NMR spectrum of this N-alkylphosphine-functionalized DPPA derivative presents two singlets: one at 60.6 ppm for the two equivalent P atoms (P1 and P2) of the PNP moiety and one at −21.8 ppm, for the terminal PPh2 group (P3) (Table 12). The solid-state molecular structure of ligand 150 was determined by XRD analysis, and characteristic bond lengths and angles are reported in Table 12. An equimolar reaction of the triphosphine ligand 150 with [PdCl2(COD)] afforded the 9255

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and N-ethyl-2-thiophene (158) pendant groups (Scheme 32).43 Both ligands exhibited a singlet resonance in their 31P NMR

When the cationic [Rh(COD)2]BF4 precursor was reacted with 2 equiv of ligand 157, the monocationic tetrahedral complex 163 was generated, which displays a typical doublet centered at 68.9 ppm with a 1JRh,P of 120 Hz in its 31P NMR spectrum (Scheme 33, Table 13).52 Baysal and colleagues also synthesized the half-sandwich mono-cationic Rh(III) (164) and neutral Ru(II) (165) complexes with the chelating ligand 157, by reaction of the latter with 0.5 equiv of [RhCp*(μ-Cl)Cl]2 or 1 equiv of [RuClCp*(COD)], respectively.53 Both complexes 164 and 165 were characterized in solution by 31P NMR spectroscopy and exhibit a doublet centered at 71.4 ppm with a 1JRh,P coupling constant of 120 Hz and a singlet at 92.6 ppm (Scheme 33, Table 13), respectively.

Scheme 32. N-Thiophene-Functionalized DPPA-type Ligands 157 and 158 and Their NiBr2 Complexes 159 and 160

spectra at 62.7 and at 63.2 ppm, respectively (Table 13). Reaction of the PNP ligands 157 and 158 with [NiBr2(DME)], in a 1:1 molar ratio, afforded the neutral, diamagnetic, and square-planar complexes 159 and 160, respectively, as deduced from their 31P NMR [52.4 (s) and 48.8 (s) ppm, respectively] data and single-crystal XRD analysis (Scheme 32 and Table 13). All main structural parameters were found to be in the range of other [NiX2(PNP)] complexes. Both nickel(II) complexes are active catalysts in ethylene oligomerization, after prior activation with AlEt2Cl as cocatalyst (see section 13.2). In the early 2010’s, Aydemir, Baysal, and colleagues intensively studied the coordination chemistry of ligand 157 toward ruthenium and rhodium precursors with the aim to produce new catalysts for the transfer hydrogenation of aromatic ketones (see section 13.5).52,53,86,87 The reaction of the N-thiophene-functionalized DPPA-type ligand 157 with [RuCl(μ-Cl)(η6-p-Cym)]2 in a 4:1 ligand/metal ratio afforded the neutral complex 161, in which the Ru(II) center has an octahedral coordination geometry, bis-chelated by two PNP ligands 157 in trans position and occupying the basal positions, while two chlorine ligands complete the metal coordination sphere in the apical positions (XRD evidence, Scheme 33 and Table 13). The 31P NMR spectrum of complex 161 contains a singlet at 78.0 ppm for the four equivalent P atoms. The crystallization process of a solution of 161 also afforded few crystals of its cis isomer 161′ suitable for XRD studies, and selected bond distances and angles are reported in Table 13. Reaction of ligand 157 with an equimolar amount of [RuCl(μCl)(η6-p-Cym)]2 led to the hexacoordinated, monocationic chelate complex 162, which exhibits a characteristic singlet resonance at 88.2 ppm in its 31P NMR spectrum (Scheme 33).

5.2. Alkyl and Aryl Thioether Groups

Smith and colleagues described the synthesis of the novel octadentate ligand 1,2,4,5-{(Ph2P)2NCH2CH2SCH2}4C6H2 (166), which consists of four independent DPPA units, each connected to a phenyl ring by a thioether-based spacer (CH2CH2SCH2) (Scheme 34).88 Its 31P NMR spectrum exhibited a singlet resonance at 101.3 ppm and confirmed the equivalence of the eight P atoms. The authors synthesized the tetranuclear Mo(0) complex 167 by reacting the octopodal ligand 166 with 4 equiv of [Mo(CO)4(NBD)]. The resulting four equivalent [Mo(CO)4(PNP)] fragments in 167 give rise to a sharp singlet at 95.0 ppm in 31P NMR (Table 14). Our group further studied the coordination chemistry of the tetra-DPPA-type ligand 166, with the objective of stabilizing mixed Co/Pt molecular clusters.89 The tetranuclear Pt(II) complex 168 was first synthesized by reaction of 166 with 4 equiv of [PtCl2(COD)] (Scheme 34). Its 31P NMR spectrum contains a singlet at 17.3 ppm, flanked by 195Pt satellites (1JPt,P = 3298 Hz), while its 195Pt NMR spectrum displays, as expected, a triplet resonance at −4028 ppm (1JP,Pt = 3298 Hz, Table 14). Complex 168 was then reacted with 8 equiv of Na[Co(CO)4] to afford the polynuclear mixed-metal Pt/Co complex 169. Its multinuclear (31P, 195Pt) NMR spectroscopic analyses were consistent with the presence of two isomers (169a,b), in which four PNP moieties stabilize four Co2Pt heterotrinuclear clusters in a bridging (169a) or chelating (169b) mode (Scheme 34). The 31P NMR spectrum of a solution of complex 169 exhibited a broad signal at 100.0 ppm for the Co-bound P atom and a doublet centered at 71.0 ppm (2JP,P = 23 Hz), flanked by 195Pt satellites (1JP,Pt = 3629 Hz),

Table 13. 31P NMR Data and Characteristic Structural Parameters of the Ligands 157 and 158 and the Metal Complexes 159− 165 av bond length, Å M 157 158 159 160 161 161′ 162 163 164 165 a

− − Ni Ni Ru Ru Ru Rh Rh Ru

δ, ppm (mult) 62.7 63.2 52.4 48.8 78.0

(s) (s) (s) (s) (s)

88.2 68.9 71.4 92.6

(s) (d) (d) (s)

a

1

JP,Rh, Hz

P−N

1.685 1.701 1.718 1.726 120 120

P−M

2.126 2.122 2.338 2.309

angle, deg P−N−P

98.0(2) 96.4(1) 101.5(3) 100.4b

P−M−P

73.47(6) 73.39(3) 69.39(6) 70.65b

ref 43 43 43 43 86, 87 86, 87 86, 87 52 53 53

Spectra recorded in CDCl3. bAverage value. 9256

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Scheme 33. Ruthenium and Rhodium Complexes with the N-Thiophene-Functionalized DPPA-type Ligand 157a

a

Reagents: (i) 0.25 equiv of [RuCl(μ-Cl)(η6-p-Cym)]2, (ii) 1 equiv of [RuCl(μ-Cl)(η6-p-Cym)]2, (iii) 0.5 equiv of [Rh(COD)2]BF4, (iv) 0.5 equiv of [RhCp*(μ-Cl)Cl]2, and (v) 1 equiv of [RuClCp*(COD)].

Scheme 34. Octopodal N-Thioether-Functionalized DPPA-type Ligand 166 and Its Polynuclear Metal Complexes (167−171)

and a singlet with 195Pt satellites at 56.0 ppm (1JP,Pt = 3010 Hz), corresponding to the Pt-bound P atom in the bridged (169a) and chelate (169b) forms, respectively. Accordingly, the 195Pt NMR spectrum presented a broad doublet at −4129 ppm (1JP,Pt = 3629 Hz) and a triplet at −4330 ppm (1JP,Pt = 3010 Hz) for isomers 169a and 169b, respectively (Table 14). As an alternative route to building 166-supported clusters, Braunstein and colleagues reacted [Co3(μ3-CCl)(CO)9] with ligand 166, in a 4:1 ratio, respectively, which afforded the desired “cluster of clusters” 170. The latter presented a singlet signal in 31P NMR at 109 ppm, for the eight equivalent P atoms (Table 16).89 The solid-state structure of 170 was established by XRD analysis, and each PNP fragment bridges one edge of each Co3 triangle, which is capped by one μ3-CCl group (selected bond distances and angles are listed in Table 14). While the reaction of ligand 166 with [Co4(CO)12] afforded a mostly insoluble material, possibly a polymeric species, a

higher selectivity in the displacement of CO ligands was achieved with the precursor [Co4(CO)10(μ-DPPY)] (Y = A or M).89 Thus, the tetra-(Co4) “cluster of clusters” 171a,b were produced selectively in which each DPPA fragment of ligand 166 bridges one edge of each Co4(CO)8 tetrahedron (XRD evidence for 171a, Table 14). The quadrupolar nature of the Co center was responsible for a broadening of the NMR signals, which prevented their complete assignment. Mastrorilli, Braunstein and colleagues described the synthesis of two N-thioether-functionalized DPPA-type ligands 172 and 173, both characterized by a typical 31P NMR singlet resonance at 62.9 ppm (Scheme 35, Table 14).90 Since the corresponding amines were not commercially available, these precursors have been synthesized by derivatization of 2-aminoethanethiol with the relevant alkyl halide, followed by classical aminolysis. The product of the equimolar reaction between ligands 172 and 173 with [PtCl2(COD)] was found to depend on the 9257

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Table 14. 31P NMR Data and Characteristic Structural Parameters of the Ligands 166, 172, and 173 and the Metal Complexes and Clusters 167−171, 174−183, and 188−191 bond length, Å M

δ, ppm (mult)

166 167 168 169a

− Mo Pt Co2, Pt

169b 170 171a 172 173 174 175

Co2, Pt Co3 Co4 − − Pt Pt2

101.3 (s)b 95.0 (s)b 17.3 (br)b 100.0 (br)b 71.0 (d)b 56.0 (s)b 109.0 (s)b

176 177 178a

Pt Pt Co2, Pt

178b

Co2, Pt

179a

Co2, Pt

179b 180

Co2, Pt Mo, Pt

181

Mo, Pt

182

W, Pt

183

W, Pt

188a

Mo2, Pt

188b 189a

Mo2, Pt Mo2, Pt

189b 190a

Mo2, Pt W2, Pt

190b 191a

W2, Pt W2, Pt

191b

W2, Pt

62.9 62.9 29.7 65.6 42.4 17.8 17.7 99.5 70.3 55.2

(s)b (s)b (s)b (AA′)b (BB′)b (s)b (s)b (br)c (d)c (s)c

98.4 (br)c 69.5 (d)c 54.3 (s)c 29.3 (d)d,e 53.9 (d)d,f 29.2 (d)d,e 53.8 (d)d,f 27.5 (d)d,e 51.7 (d)d,f 28.0 (d)d,e 52.3 (d)d,f 122.1 (m)g,h 84.6 (m) 61.1g 121.7 (m)g,h 84.8 (m) 61.0g 87.1 (m)g,h 77.4 (m) 47.3g 85.2 (m)g,h 75.8 (m) 45.5g

2

JP,P, Hz

av P−N

av P−M

angle, deg M−M′

P−N−P

P−M−P

ref 88 88 89 89

23 1.702 1.711

2.181 2.173

1.706

2.207

1.706

2.239

2.466a 2.489a

89 89 89 90 90 90 90

117.96a 118.79a

99.34(1)

72.06(3)

90 90 90

100.2(2)

71.53(3)

90

26 2.5511(7) Co−Co 2.5426(5) Co−Pt

90 28 27

90 91

28

91

27

91

27

91

45

91

46

91 91

60

91 91

61

91 91 91

a

Average value. bIn CDCl3. cIn CH2Cl2. dIn CD3CN, two different 1JP,Pt were also observed. eTrans to Cl. fTrans to M(CO)3Cp. gIn THF-d8; 1JP,Pt and/or 1JP,W were also observed. hP-M(CO)3Cp.

mode of addition of the ligand to the metal precursor. Rapid addition of a solution of ligand 172 to a solution of [PtCl2(COD)] only afforded the dicationic and bis-chelated [Pt(172)2]Cl2 (174) species, along with 1 equiv of unreacted metal precursor (Scheme 35). Complex 174 was characterized in solution by multinuclear NMR analysis: its 31P NMR spectrum displayed a characteristic singlet signal at 29.7 ppm flanked by Pt−P satellites (1JPt,P = 2373 Hz), while the 195Pt NMR spectrum confirmed the bis-chelated nature of the complex with the presence of a quintet at −4383 ppm with a 1 JP,Pt of 2373 Hz (Table 14). The absence in the FT-IR

spectrum of 174 of absorption bands corresponding to Pt−Cl stretching vibrations confirmed its ionic nature. In dichloromethane solution, complex 174 slowly isomerizes into the bridged, dinuclear complex 175 (Scheme 35). 31P NMR spectra of an aged solution showed the progressive disappearance of the signals relative to 174 with a concomitant growing of those corresponding to an AA′XX′ spin system ascribable to 175, where A (65.6 ppm) corresponds to the P atom of the bridging ligands and X (42.4 ppm) to that of the chelating ligands. The observation of a triplet of triplets signal centered at −4682 ppm with 1JP(A),Pt of 2698 Hz and 1JP(X),Pt of 1908 Hz in the 195Pt 9258

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Scheme 35. Reactivity of the N-Thioether-Functionalized DPPA-type Ligands 172 and 173 toward [PtCl2(COD)] (174−177) and PNP-Supported Mixed-Metal Pt/Co Molecular Clusters (178 and 179)a

a Conditions: (i) 1 equiv of [PtCl2(COD)] added dropwise over 2 h and (ii) rapid addition of 1 equiv of [PtCl2(COD)]. R′ refers to the Nsubstituent of the corresponding ligand.

Scheme 36. Solvent-Induced Difference of Reactivity of PtCl2 Complexes 176 and 177 Containing the N-ThioetherFunctionalized DPPA-type Ligands 172 and 173, Respectivelya

a

R and R′ refer to the S- or N-substituent of the corresponding ligand, respectively.

example, [PtCl2(DPPA-Me)] (9.2%) and [Pt(DPPA-Me)2]2+ (14.6%). These data suggest that the structure of the thermodynamically stable complex 175 (presenting ligand 172 in both chelating and bridging modes) results from a compromise between the strong tendency of ligand 172 to act as a chelate, demonstrated by the inevitable formation of 174 (rapid addition) and 176 (slow addition), and the high ring strain imposed by the chelation. Reaction of the PtCl2 complexes 176 and 177 with 2 equiv of [NaCo(CO)4] led to the formation of the heterotrinuclear Co2Pt clusters 178 and 179, respectively (Scheme 35). Both clusters present a dynamic behavior in solution, due to an equilibrium between a bridging (178a and 179a) and a chelating (178b and 179b) coordination mode of the ligands. The 31P NMR spectra of the bridging-type (a) isomers presented broad signals at 99.5 (178a) and 98.4 (179a) ppm, corresponding to the Co-bound P atom, and a higher-field doublet resonance at 70.3 (2JP,P of 26 Hz, 178a) and 69.5 (2JP,P = 28 Hz, 179a) ppm, respectively, flanked by satellites corresponding to the Pt-bounded P atom, with 1JPt,P of 3604

NMR spectrum of 175 was also in agreement with the proposed structure (Table 14). However, when a solution of ligand 172 or 173 was added dropwise, over a period of at least 2 h, to an equimolar amount of [PtCl2(COD)], the desired [PtCl2(PNP)] complexes 176 and 177 were isolated, respectively (Scheme 35). Multinuclear NMR spectroscopic data were fully consistent with their proposed structure, especially the 31P NMR singlet resonances at 17.8 (176) and 17.7 (177) ppm and the 195Pt NMR triplet signals centered at −4037 (176) and −4038 (177) ppm, in each case flanked by satellites with 1JPt,P of 3304 (176) and 3308 (177) Hz (Table 14). Complex 176 was also characterized in the solid state by single-crystal XRD analysis and exhibited the typical distorted square-planar coordination geometry around the metal center, with ligand 172 acting as a chelate, and the bond distances and angles were found in the typical range (Table 14). According to the calculations, complex 176 presented a % PCC (percentage pyramidal character) of 17.7%, which indicates a higher ring strain in the latter compared to, for 9259

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Scheme 37. Synthesis of Ni(II) (197 and 198) and Cr(III) (193−195) Complexes with the N-Thioether-Functionalized DPPAtype Ligands 192 and 196

Table 15. 31P NMR Data and Characteristic Structural Parameters of the Ligands 192 and 196 and the Metal Complexes and Clusters 194, 195, 197−202, 206−209, 212, 213, 215, 216, 218, and 220 bond length, Å M

a

192 194 195 196 197 198 199

− Cr Cr − Ni Ni Ag

200

Ag

201 202

Ag Ag

206 207 208 209 212 213 215 216 218 220

Pd Pd Ni Ni Fe Fe Fe Co Co Co

δ, ppm (mult) 63.5 (s)

a

69.4 42.9 45.4 86.6 84.3 82.1

(s)a (s)b (s)a (t)c (br)c (t)c

89.8 88.0 86.3 98.1 77.7 74.8 48.4 55.5 60.8 69.5

(t)b (br)b (t)b (2 d)b (br q)c (br q)c (s)a (s)a (s)b (s)b

av P−N 1.719 1.692 1.699 1.730 16.91 1.712 1.707

av P−M

angle, deg M−M′

2.480 2.485 2.122 2.119 2.444

1.676

2.424

1.721 1.710

2.509 2.441

1.703 1.711 1.701 1.708 1.707 1.711 1.722 1.704 1.695 1.716

2.304 2.301 2.189 2.184 2.628 2.614 2.224 2.246 2.246 2.231

2.9778(3) block A 3.1184(6) block B 2.933(3) blocks A and B 3.088d

P−N−P 110.7(3) 106.7(4) 105.7(2) 120.8(1) 97.59(1) 96.70(2) 116.5d

P−M−P 66.37(9) 66.01(5) 73.69(3) 74.20(6)

126.2d

ref 92 92 92 94 93 94 96

97

109.8(3) 122.4d

68.51(6)

97 96

101.48(9) 102.2(2) 99.1(2) 100.02(9) 111.7(3) 111.0(1) 98.25d 100.8d 101.7d 99.8d

68.80(2) 70.77(4) 72.4(6) 73.62(2) 64.55(5) 64.98(3) 71.67d 71.51d 71.49d 72.07d

102 102 103 103 104 104 104 105 105 105

In CDCl3. bIn CD2Cl2. cIn acetone-d6. dAverage value.

−4333 (1JP,Pt = 3015 Hz) for isomer b. The ratio between the a and b isomers was found to depend on the temperature and the nature of the solvent. Attempts to crystallize a solution of complex 178 led to suitable crystals for XRD analysis, which established the structure of the chelate isomer 178b. Characteristic P−N and P−Pt bond lengths and P−N−P and P−M−P angles were similar to those in the mononuclear precursor complex 176 (Table 14). Mastrorilli, Braunstein, and colleagues further investigated the reactivity of complexes 176 and 177 toward the group 6

(178a) and 3604 (179a) Hz. In contrast, the chelating-type (b) isomers presented only one singlet resonance at 55.2 (178b) and 54.3 (179b) ppm, with satellites characterized by 1JPt,P coupling constants of 3019 (178b) and 3015 (179b) Hz (Table 14). In their 195Pt NMR spectra, the isomers 178a,b exhibit a doublet of doublets at −4120 ppm (1JP,Pt = 3589 Hz, 2+3JP,Pt = 78 Hz) and a triplet at −4325 (1JP,Pt = 3006 Hz), respectively. Similar patterns were observed for the couple 179a,b, which displayed a double of doublets resonance at −4136 ppm (1JP,Pt = 3604 Hz, 2+3JP,Pt = 80 Hz) for isomer a and a triplet signal at 9260

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found on the apical sites. Selected bond lengths and angles are reported in Table 15. For their studies on the selective Cr-catalyzed ethylene teramerization, Weng, Hor, and colleague did not use the isolated Cr(III) complexes, but the in situ mixture of [Cr(acac)3] with 2 equiv of the PNP ligand and 440 equiv of MAO (see section 13.1).92 Also targeting potential (pre)catalysts for the selective oligomerization of ethylene, Fliedel, Braunstein, and colleagues prepared the Ni(II) complexes 197 and 198 with the Nalkylthioether-functionalized DPPA-type ligand 192, developed by Weng, Hor, and colleague (see above), and the Narylthioether derivative 196, respectively (Scheme 37).93,94 The latter ligand was characterized in solution, by its 31P NMR single resonance at 69.4 ppm, and in the solid state. Its crystal structure presented some original features compared to other structurally characterized N-functionalized DPPA-type ligands, especially the orientation of the P lone pairs and of the Ph rings. Both nickel dichloride complexes were obtained by reaction between [NiCl2(DME)] and the corresponding PNP ligand, in a 1:1 molar ratio. As deduced from XRD analysis, the [NiCl2(PNP)] complexes 197 and 198 present in the solidstate the usual distorted square-planar coordination geometry (see selected bond lengths and angles in Table 15). These complexes are diamagnetic in solution, with singlet resonances in their 31P NMR spectra at 42.9 (197) and 45.4 (198) ppm. After activation with AlEtCl2, these complexes were found to be active catalysts for the selective oligomerization of ethylene (see section 13.2). Motivated by the synthesis of polynuclear complexes that display d10−d10 interactions,50,95 Braunstein and colleagues studied the coordination chemistry of ligands 192 and 196 toward Ag(I) ions, a versatile metal center.96,97 Over the last 2 decades, growing interest was dedicated to the study (experimentally and computationally) of the argentophilic interactions, responsible for bringing two or more Ag(I) centers in close proximity through suitable ligand design, because they lead to original and diverse structures and properties (notably, photophysical).98−100 Equimolar reactions of these ligands with AgBF4 afforded the corresponding dicationic, dinuclear complexes 199 and 200 (Scheme 38).

metals carbonylmetalates, i.e., Na[M(CO)3Cp] (M = Mo, W, Scheme 36).91 The reaction between the PtCl2 complexes 176 or 177 and 2.2 equiv of [NaM(CO)3Cp] (M = Mo, W) in THF surprisingly led to phosphanido-bridged trinuclear complexes (184−187), where a phenyl group from the PNP motif has migrated to Pt, along with a small amount of the expected clusters 188−191 (analogous to 178 and 179). The latter clusters were characterized by 31P NMR and typical patterns were observed, i.e., a singlet for 188b−191b with 1JP,Pt satellites and two multiplets for 188a−191a with 2JP,P and 1JP,Pt couplings and additional 1JP,W couplings for 190a and 191a (Table 14). In contrast, when the reaction was carried out in MeCN instead of THF, it stopped at the monosubstitution step, even in the presence of an excess of Na[M(CO)3Cp] (M = Mo, W), leading to the Pt−M dinuclear complexes 180−183. The latter exhibit well-resolved 31P NMR spectra in which each P atom is seen as chemically and magnetically different, leading for both to doublets (27 < 2JP,P < 28 Hz) flanked by 195Pt satellites with 1 JP,Pt values strongly influenced by the nature of the trans ligand (Cl ≈ 3550 Hz, M(CO)3Cp ≈ 2600 Hz, Table 14). Complexes 180−183 are presumably intermediates in the formation of 184−187 and the very broad 31P NMR signal observed for the P atom trans to the M(CO)3Cp moiety in THF suggests its partial dissociation, favoring the formation of 184−187 in this solvent. However, the comparative reaction of [PtCl2{(Ph2P)N(n-decyl)-P,P}] with 2 equiv of Na[Mo(CO)3Cp] in the presence of excess di-n-butyl sulfide led to a similar outcome as when performed in the absence of sulfide (complex similar to 180) and not to a rearranged complex (similar to 184), indicating the requirement for the sulfur donor atom to be “within” the N-tail to allow this P−C bond activation.91 Mechanistic pathways were suggested for the different transformations observed experimentally that were supported by DFT calculations.91 Weng, Hor, and colleague described the synthesis of the N[3-(methylthio)propyl]-substituted DPPA-type ligand 192, along with other N-functionalized PNP ligands (see section 9) (Scheme 37).92 Ligand 192 was prepared by the classical aminolysis reaction of the corresponding amine and exhibits a characteristic 31P NMR single resonance at 63.5 ppm. Its solidstate molecular structure was determined by XRD analysis and revealed P−N bond lengths and a PNP angle in the range found in the few other DPPA-type ligands structurally characterized (Table 15). An equimolar reaction between ligand 192 and [CrCl3(THF)3] led to the displacement of the labile THF ligands, and the formation of the neutral dinuclear complex 193 was assumed, in which two [CrCl2(192)] moieties are connected through two bridging chlorines (Scheme 37).92 The authors assumed that the formation of a halogen-bridged complex with stronger donors, such as four Cl and two P, was preferred over the coordination of the weak thioether donor, leading to the formation of higher nuclearity or polymeric complexes. Furthermore, the dinuclear complex 193 could be readily converted into the nitrile adducts 194 and 195 by cleavage of the μ-Cl bridges upon dissolution in the coordinating MeCN and EtCN solvents, respectively (Scheme 37). The solid-state molecular structures of complexes 194 and 195 were found to be very similar, with the Cr(III) centers in a distorted octahedral coordination geometry, the PNP ligand acting as a chelate and occupying two equatorial coordination sites. The other two equatorial sites were occupied by two chlorine atoms, while the nitrile ligand and the last Cl were

Scheme 38. Equimolar Reaction of AgBF4 with the NThioether-Functionalized DPPA-type Ligands 192 and 196

Their 31P NMR spectra are similar, with two triplets (82.1 and 86.6 ppm for 199, and 86.3 and 89.8 ppm for 200) flanking a central, broad signal (84.3 ppm for 199, and 88.0 ppm for 200). The complexity of this spectrum was thought to originate from an AMM′ spin system (A = 31P, M = 107Ag, M′ = 109Ag) with the expected mixture of isotopomers in the proportions 26.9:49.9:23.2 corresponding to (107Ag)2, 2 × (107Ag)(109Ag), and (109Ag)2, respectively, which is consistent with the presence in solution of dinuclear entities. However, the broad 31P and 1H NMR signals may suggest a dynamic behavior in solution, involving an equilibrium between species in which the ligand(s) switches from a bridging to a chelating coordination mode with 9261

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Scheme 39. Stepwise Formation of the Trinuclear Ag(I) Complexes 202 and 205, Involving the Intermediate Complexes 200, 201 and 203, 204, Respectively

Figure 4. 1D and 2D coordination polymers formed in the solid state by complexes 199 (A) and 200 (B), respectively.

concomitant coordination of the additional thioether donor from the N-substituent.96 The reaction between ligand 196 and AgBF4 in CH2Cl2 afforded a mixture of products, and these results triggered a series of experiments that are detailed below (Scheme 39). Complex 199 crystallized only in the presence of MeCN, a coordinating solvent, and a linear coordination polymer is formed in the solid-state (Figure 4A), in which the repeat unit is composed of tw o d inuclear centros ymmetric [Ag2(192)2(NCMe)2] subunits: “block A” and “block B”. Each A block is linked to two adjacent B blocks through the thioether functions of its 192 supporting ligands, while in the B blocks, the thioether functions of the ligands remain uncoordinated. This results in two different coordination geometries around the Ag centers. If one ignores the Ag···Ag metallophilic interactions, the Ag(I) ions of block A are in a strongly distorted trigonal planar coordination geometry [N−

Ag−P angles of 114.42(9)° and 106.73(9)° and P−Ag−P angle of 138.80(3)°] composed of two phosphine donors from two different bridging 192 ligands and one acetonitrile molecule. In the very similar block B, each Ag(I) center has a distorted trigonal-pyramidal coordination geometry, formed again by two P donors and one MeCN molecule, but is further ligated by one Sthioether donor of an adjacent A subunit [angles: N−Ag−P, 14.09(9)° and 106.81(9)°; P−Ag−P, 138.80(3)°; and N−Ag− S, 85.18(9)°]. In each block (A and B), the argentophilic interactions are supported by the bridging 192 ligands [2.9776(5) and 3.1184(6) Å for blocks A and B, respectively], and the resulting positive charges are balanced by two BF4‑ anions, which do not interact with the metal centers.96 Complex 200 crystallized as a 2D coordination polymer (XRD evidence, Figure 4B), which contains a linear array of blocks A and B, similar to those described for 199; however, in this case the coordinating MeCN solvent was replaced in block 9262

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Scheme 40. Bis-Chelate Dicationic Complexes 206−209 of Ligands 192 and 196 and Their Reactivity

analogous to 196, with an ether replacing the thioether function (Scheme 39). Very recently, our group also reported a series of bis-chelate dicationic group 10 metal complexes of ligands 192 and 196 (Scheme 40). While the Pd(II) derivatives 206 and 207 were used as linkers between Janus Au-coated silica microspheres to allow a simple and visual assessment of the anchoring of Sfunctionalized compounds on metal surfaces (see section 14),102 the Ni(II) species 208 and 209 were found to undergo CH2Cl2 activation in the presence of Zn(0) as reducing agent under mild conditions.103 The 31P NMR spectra of the resulting complexes 210 and 211 clearly showed the dissymetrization of the PNP ligands with retention of one P(III) and formation of one P(V) center (phosphonium ylide); this ligand class is outside the scope of this review and will therefore not be detailed. Characteristic NMR and structural data of complexes 206−209 are summarized in Table 15. Braunstein and colleagues also investigated the coordination chemistry of ligands 192 and 196 to determine if they could lead to polynuclear arrangements of paramagnetic centers, for example, via the coordination of the additional S-donor, and therefore result in interesting magnetic behavior. Reactions between ligands 192 and 196 and FeCl2 followed by recrystallization yielded complex 212 as a coordination polymer in which 192 acts simultaneously as a P,P-pseudochelate and a (P,P),S-bridge, while complex 213 has a chlorido-bridged dinuclear structure in which 196 acts only as a P,Ppseudochelate, as established by XRD analyses (Scheme 41 and Table 15).104 Since these complexes were obtained under strictly similar synthetic and crystallization conditions, the structural difference observed could only be ascribed to the

A by the dangling aromatic thioether tail of an adjacent block B unit, resulting in the 2D arrangement. Another structural difference with complex 199 is that in each subunit of the solidstate structure of 200, one BF4 counterion was found in interaction, through one of its F atoms, with one Ag(I) ion. Therefore, in each A or B block, one Ag(I) center is found in a distorted trigonal-planar geometry (P, P, S ligands) and another in a distorted trigonal-pyramidal environment (P, P, S, F ligands).97 Braunstein and Ghisolfi observed initially that the equimolar reaction of ligand 196 with AgBF4 in CH2Cl2 did not afford pure 200 (ca. 90%), as found in acetone, but that an additional species (ca. 10%) was present which resulted from Cl− abstraction from the solvent, which has been observed before (see below and Scheme 39).96 The formation of the minor species and possible intermediates were rationalized by a series of experiments that allowed the assignment of the different 31P NMR resonances observed (Scheme 39).97 All the complexes were characterized in solution, by multinuclear NMR spectroscopy techniques (31P NMR being the most helpful), and in the solid state by XRD studies, and selected spectroscopic data and bond distances and angles are reported in Table 15. First, reaction of ligand 196 with 0.5 equiv of AgBF4 in acetone afforded the tetrahedral bis-chelated cationic complex 201, which displayed in its 31P NMR spectrum the expected two doublets centered at 98.1 ppm, resulting from the superposition of the P−Ag couplings with the isotopomers involving 107Ag and 109Ag (1J(107)Ag,P = 222 Hz, 1 J(109)Ag,P = 252 Hz) (Scheme 39). The values of these coupling constants were consistent with a gyromagnetic ratio γ(109Ag)/ γ(107Ag) = 1.15. Further reaction of complex 201 with 0.5 equiv of AgBF4 in acetone resulted in the formation of the dinuclear dicationic complex 200 (see above). Finally, reacting this complex with 2/3 equiv of NEt4Cl in acetone allowed the quantitative formation of the trinuclear complex 202, which was initially obtained as a byproduct in CH2Cl2. Its 31P NMR spectrum exhibited two broad quadruplets centered at 77.7 and 74.8 ppm. A complex analogous to 202, in which three Ag(I) centers are connected by supporting DPPM ligands to form a triangular array and by μ3-capping bromide ligands, was reported in the late 1970s by Schmidbaur and colleagues.101 The molecular structure of complex 202 was confirmed by single-crystal XRD analysis and confirmed the formation of a nearly equilateral triangle of distorted tetrahedral Ag(I) cations of which each edge is supported by one bridging ligand 196. The observed Ag···Ag separations (av 3.088 Å) are compatible with the existence of argentophilic interactions. The coordination sphere of the Ag(I) metal centers is completed by two triply bridging chloride ligands, resulting in a trigonalbipyramidal Ag3(μ3-Cl)2 core, while one BF4‑ anion balances the resulting positive charge of the cluster (Scheme 39 and Table 15).96,97 Similar reactivity was observed using ligand 53,

Scheme 41. Influence of the Nature of the N-Thioether Group of Ligands 192 and 196 on Their Resulting FeCl2 Complexes 212 and 213 and Bis-Chelated Hexacoordinate Fe(II) Complexes 214 and 215, Respectively

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Scheme 42. (Left) Dinuclear Co(II) Complexes Resulting from the Reaction of Ligands 192 and 196 with CoCl2 (216 and 217) and (Right) Mononuclear Co(II) Complexes 218−221 Synthesized as Potential Constituting “Fragments” of the Dinuclear Structures

different spacer between the N atom and the −SMe group. In both Fe(II) complexes, one Fe−P bond was found to be unusually long [2.798(2) and 2.762(1) Å for 212 and 213, respectively], and a theoretical analysis was performed [including a series of Fe complexes of general formula [FeCl2(SR2){R21PN(R2)PR23}] (R = H, Me; R1, R2, and R3 = H, Me, Ph)] to unravel the electronic or steric reasons for this difference. The former factors nicely explain the observed structures, and steric reasons were further ruled out by the structural analysis in the solid-state of the bis-chelated complex 215, which displays usual and equivalent Fe−P bond lengths (Table 15). All the Fe(II) complexes were characterized by FTIR, MS, EA, and XRD for 212, 213, and 215, and magnetic susceptibility studies concluded that 212 behaves as an isolated high-spin Fe(II) mononuclear complex, while significant intraand intermolecular ferromagnetic interactions were evidenced for 213 at low temperatures. As an extension of these studies, our group also investigated the coordination chemistry of ligands 192 and 196 toward CoCl2 and could isolate complexes 216 and 217, respectively (Scheme 42).105 While the structure of 216 could be unambiguously determined by XRD analysis, this was not the case for 217 and its structure remained uncertain, in particular when considering the possible structural differences exemplified by 212 and 213 (see above). Complex 216 was shown by XRD to be a dinuclear zwitterion containing one CoCl moiety bischelated by two ligands 192 and one CoCl3 fragment coordinated by the S atom of a thioether function (Table 15). The authors attempted to synthesize different “molecular fragments” possibly constituting 216 and 217 and record their signatures by various physical methods to reproduce as closely as possible the structure of 217. The methodology was first successfully validated with 216 of known structure. Thus, the analytical data of 216 were well accounted for by a superimposition of those of the pentacoordinated complex [CoCl(192) 2 ]PF 6 (218) and the tetrahedral complex [CoCl3(H192′)] (219) (192′ = NH2(CH2)3SMe, Scheme 42 and Table 15). In a similar vein, the complexes [CoCl(196)2]PF6 (220) and [H196′]2[CoCl4] (221) [196′ = NH2(pC6H4)SMe] were synthesized and unambiguously characterized, and all FT-IR, MS, UV−vis, EPR, and XRD techniques as well as magnetic and DFT studies led to the conclusion that the

solid-state structure of 217 is best described as zwitterionic (comparable to 216). 5.3. Other S-Containing Functional Groups

Olivier-Bourbigou, Reek, Breuil and colleagues reported the synthesis of nickel dibromide complexes of a series of DPPAtype ligands directly functionalized on their N atom by a sulfonyl group (Scheme 43).106 The formation of these Scheme 43. Metal-Mediated Rearrangement of 222 and 223 to Afford Complexes 224 and 225, Respectively

compounds occurs via a metal-induced rearrangement, i.e., breakage of the P−P bond, transforming the corresponding imino(bisphosphine) ligands to symmetrical metalated diphosphine species. Access to nonsymmetrical and P-alkyl derivatives was also achieved and will be further discussed in section 9. The imino(bisphosphine) proligands 222 and 223 were obtained in moderate to good yield (68−79%) by reaction between their corresponding sulfonamides and 2 equiv of PPh2Cl in the presence of NEt3 as HCl scavenger. Their reactions with [NiBr2(DME)] in a 1:1 molar ratio gave the square-planar nickel complexes 224 and 225 of the corresponding symmetric DPPA-type ligands (Scheme 43). While the ligand precursors exhibited in 31P NMR spectroscopy two sets of signals for the chemically different PV and PIII atoms [19.7−19.5 ppm (PV) and −17.9 to −18.7 ppm (PIII), with 1JP(III),P(V) coupling (∼280 Hz)], both [NiBr2(P,P)] complexes afforded similar 31P NMR spectra, which contained one singlet for the two equivalent PIII atoms at 65.5 and 64.1 ppm for 224 and 225, respectively. Complex 224 and the nonsymmetrical complexes 328 and 329 (section 9) could also be characterized in the solid state by single-crystal X-ray diffraction, which established the distorted square-planar arrangement around the metal center and the 9264

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Scheme 44. Ligands 226 and 227 and Their Mono- (228−231), Di- (232), and Tetranuclear (233−236) Metal Complexes

Table 16. 31P NMR Data and Characteristic Structural Parameters of the Ligands 226, 227, 244, and 248−257 and the Metal Complexes and Clusters 228−236, 239−243, and 245−247 bond length, Å

a

M

δ, ppm (mult)

226 227 228 229 230 231 232 233

− − Pd Pd Pt Cu Au Co4

234

Co4

235

Co4

236 239

Pd Au

240

Au

241

Au

242

Au

243 244 245 246 247

Au − Ru Ru Ru

70.8 (s)a 69.8 (s)a 35.8 (s)a 36.3 (s)a 23.0 (s)a,b 60.9 (s)a 89.4 (s)a 116.6 (br s)ac 97.1 (br s)a,c 112.0 (br s)a,c 97.4 (br s)a,c 112.2 (br s)a,c 96.7 (br s)a,c 63.7 (s)a,c 87.4 (dd)d 89.4 87.9 (dd)d 89.4 88.1 (dd)d 89.5 85.6 (dd)d 87.1 78.3 (s)e 68.6 (s)f 88.8 (s)f 89.9 (s)f 87.0 (s)f

In CDCl3.

b1

JP,P, Hz

av P−N

av P−M

angle, deg M−M

P−N−P

P−M−P

ref

1.716 1.700

2.203 2.222

1.717 1.729

2.257 2.188

3.0402(2) 2.494g

118.4(3) 117.7(1)

107 107 107 107 107 107 107 107

1.729

2.186

2.507g

115.2(2)

107

99.0(2) 100.6(5)

72.66(5) 72.5(1)

107

108

107 62

108

62

108

62

114

62 62 108 108 108 108

JP,Pt = 3337 Hz. cValue for the DPPA-type ligand only. dIn CD2Cl2. eIn acetone-d6; fIn C6D6. gAverage value.

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parameters were found in the range of the previously reported (DPPA-type)-supported Co4 cluster 171a (Table 16 vs Table 14).89 Finally, to evaluate the reactivity of the C−Br function present in the ligand tail and its potential use in cross-coupling reactions, 2 equiv of the Pd(II) complex 228 were reacted with 3 equiv of the Pd(0) complex [Pd(DBA)2] in the presence of 2 equiv of DPPE, as a stabilizing ligand, with the objective of mimicking the oxidative insertion of Pd(0) into the carbon− bromide bond. This reaction allowed the partial characterization (unstable in solution) of the tetranuclear complex 236, which was composed of a dinuclear Pd(I)−Pd(I) entity supported by two bridging ligands 226 in which two Pd(cisDPPE) fragments have inserted into the C−Br bond (Scheme 44). This complex was characterized by 31P NMR spectroscopy and exhibits a singlet resonance at 63.7 ppm for the two equivalent P atoms of the bridging ligand and two doublets at 67.6 and 39.0 ppm, with a 2+3JP,P coupling constant of 26 Hz, assigned to the P atoms of the DPPE (Table 16). Ligand 226 and its p-Cl (237) and p-F (238) analogues were used by Yam and colleagues for the formation of deca- (239− 242) and hexanuclear Au clusters (243, Figure 5 and Table 16

small bite angle of the DPPA-type ligand [P−Ni−P, 75.54(2)°]. In general, the main structural parameters of complex 224 [Ni−P (av), 2.125 Å; N−P (av), 1.735 Å; P−N− P, 97.17(9)°] were found to be close to those in similar NiBr2 complexes of N-functionalized DPPA-type ligands (e.g., complexes 39 and 46, Tables 4 and 5, respectively). Complex 225, as well as the nonsymmetrical and P-alkyl derivatives (see section 9), was evaluated in the catalytic oligomerization of ethylene, in the presence of an excess of MAO as cocatalyst, and the results will be discussed in section 13.2.

6. HALOGEN FUNCTIONALIZATIONS Braunstein and colleagues reported the synthesis, via the classical aminolysis reaction, of two N-bromo-aryl-funtionalized DPPA-type ligands of the type Ph2PN(Ar)PPh2 (226, Ar = pBrC6H4; 227, Ar = p-BrC6H4−C6H4) and studied their coordination chemistry toward group 10 and 11 mononuclear metal precursors and Co(0) molecular clusters (Scheme 44).107 Ligands 226 and 227 each exhibited a characteristic 31P NMR singlet resonance at 70.8 and 69.8 ppm, respectively. As expected, their reaction with [MCl2(COD)] (M = Pd, Pt), in a 1:1 molar ratio, led to the formation of the corresponding neutral, square-planar complexes [MCl2(226)] (M = Pd, 228; M = Pt, 230) and [PdCl2(227)] (229) (Scheme 44). The 31P NMR spectra of the Pd complexes 228 and 229 presented a singlet at 35.8 and 36.3 ppm, while that of the Pt derivative 230 displayed a singlet at 23.0 ppm flanked by 195Pt satellites (1JPt,P = 3337 Hz, Table 16). The solid-state structures of 228 and 229 were established by XRD analysis and the characteristic bond distances and angles are similar to those in related [PdCl2(PNP)] complexes (Table 16). The reactions of ligand 226 with the d10 complexes [Cu(NCMe)4]PF6 and [AuCl(THT)], in a 1:2 and 2:1 metal/ligand ratio, respectively, afforded [Cu(226)2]PF6 (231) and [(AuCl)2(μ-226)] (232) (Scheme 44). They were characterized in solution by 31P NMR and exhibited typical singlet resonances at 60.9 and 89.4 ppm, respectively. The solid-state molecular structure of 232 was confirmed by XRD studies and is similar to that of other DPPAtype supported [(AuCl)2(μ-P,P)] complexes (Table 16). In view of our continuing interest in the chemistry of molecular metal clusters (see sections 5.2 and 7.1), we studied the possibility to stabilize low-valent cobalt carbonyl clusters with the new N-bromo-aryl-functionalized DPPA-type ligands 226 and 227. Reaction of the latter with 1 equiv of the tetrahedral clusters [Co4(CO)10(μ-DPPY)] (Y = A or M) afforded the corresponding complexes [Co4(CO)10(μ-DPPY)(μ-226 or 227)] (233−235), after silica gel column chromatography (Scheme 44).107 These complexes exhibit characteristic and similar broad 31P NMR resonances (Co quadrupole broadened): (i) one signal for the P atom of the DPPA-type ligand occupying the apical position of the Co4 tetrahedron at 116.6 (P226/apical, 233), 112.0 (P226/apical, 234), and 112.2 (P227/apical, 235) ppm; (ii) one signal for the P atom of the DPPA-type ligand coordinated to a basal position of the Co4 tetrahedron at 97.1 (P226/basal, 233), 97.4 (P226/basal, 234), and 96.7 (P227/basal, 235) ppm; and (iii) signals corresponding to the DPPY (Y = A, M) coordinated to the two other basal Co centers at 74.0 ppm (2PDPPA, 233), 28.8 and 25.2 ppm (2PDPPM, 234), and 28.5 and 24.8 ppm (2PDPPM, 235). The solid-state structures of 233 and 234 were determined by XRD analysis and revealed comparable structures, even if the nature of the DPPY (Y = A, M) ligand was found to influence slightly the CO arrangement in the latter.107 The main structural

Figure 5. Au(I) clusters 239−243 containing N-aryl-halide DPPAtype ligands 226, 237, and 238.

for 31P NMR data). Their photophysical properties were studied and compared to those of analogous clusters supported by N-alkyl-, N-aryl-, or N-ether-functionalized DPPA-type ligands (see 58 and 59 in section 3.1).62,109 Lau and colleagues prepared a DPPA-type ligand functionalized on the nitrogen atom by a p-trifluoromethyl-aryl substituent (244) via the classical aminolysis route and studied its coordination toward [RuCl(Tp)(PPh3)2] and the reactivity of the resulting species (Scheme 45).108 The ligand 244 exhibits a characteristic 31P NMR singlet signal at 68.6 ppm (Table 16). Reaction of the latter with 1 equiv of [RuCl(Tp)(PPh3)2] led to the displacement of both PPh3 ligands and chelation of the metal by ligand 244, to form the neutral complex 245, which displays a 31P NMR singlet at 88.8 ppm. This complex cleanly reacted with 1 equiv of AgOTf to lead (i) to the neutral complex 246, in which OTf replaces the Cl ligand, when the reaction was performed in THF, or (ii) to the cationic complex 247, in which one MeCN molecule is coordinated to the Ru(II) center, when the reaction was performed in a THF/MeCN mixture (Scheme 45). The 31P NMR chemical shift of coordinated 244 in these compounds (89.9 ppm for 246 and 87.0 ppm for 247, both as singlets) was not much affected by comparison with complex 245 (88.8 ppm, Table 16). Complexes 246 and 247 were evaluated for the 9266

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corresponding metal complexes isolated and characterized, while in situ generated compounds and materials preparation, characterization, and use will be discussed in sections 13 and 14. The second part of this section will highlight compounds presenting a direct N-SiMe3 functionalization, accessible via deprotonation and functionalization of DPPA.

Scheme 45. Ligand 244 and Its Corresponding Neutral (245 and 246) and Cationic (247) Tp-Supported Ru(II) Complexes

7.1. N-(Alkyl/aryl)-alkoxysilyl or N-(Alkyl/aryl)-alkyl/alkoxysilyl Derivatives

The design of this class of ligands consists of connecting a DPPA moiety to an alkoxy- or mixed alkyl/alkoxy-silyl group through a flexible aliphatic (258, 259 and 262) or rigid aromatic (261) spacer or a spacer containing an additional donor (260, Figure 7). Until now, only five examples of such

Figure 7. N-Functionalized DPPA-type ligands integrating a siliconbased group 258−262.

ligands were reported, and they were obtained by aminolysis of the corresponding amines. They exhibit 31P NMR singlet resonances in the usual range (62.0−68.6 ppm, Table 17), which are mostly affected by the nature of the spacer (cf., 261). The crystal structure of 261 was determined by XRD studies and revealed P−N bond lengths [1.732(3) and 1.720(3) Å] and a P−N−P angle [113.64(18)°] comparable to those in the other rare examples of free N-functionalized DPPA-type ligand.111 Braunstein, Schmid, and colleagues reported the synthesis of the ionic Pd(II) complexes [Pd(DMBA)(PNP)]Cl (263 and 264), from the reaction of 258 and 262 with 0.5 equiv of the dinuclear complex [Pd(μ-Cl)(DMBA)]2, respectively (Scheme 46).112 Both 263 and 264 exhibited in their 31P NMR spectra two characteristic doublets, for the nonequivalent P atoms, at 57.8 and 63.8 (2JP,P = 57 Hz) and 53.1 and 64.0 (2JP,P = 58 Hz) ppm, respectively. Neutral PdCl2 (265) and PtCl2 (266) complexes with ligand 259 were accessible in high yields by ligand displacement from the corresponding [MCl2(COD)] (M = Pd, Pt) precursors, and the resulting complexes exhibited typical 31P NMR singlet resonances at 30.8 and 16.3 ppm (1JP,Pt = 3293 Hz).113 Furthermore, reaction between excess 262 and the DPPA-stabilized Co4 cluster [Co4(CO)10(μ-DPPA)] resulted in the selective displacement of two labile CO ligands and formation of the metal cluster 267, which exhibits a characteristic (see sections 5.2 and 6) 31P NMR pattern composed of three broad signals at 72.1 (2P, Pdppa−Cobasal), 92.0 (1P, P262−Cobasal), and 103.1 (1P, P262−Coapical) ppm (Table 17).112 Two opposite Co−Co edges of the metal tetrahedron are spanned by the short-bite ligands. Similarly, the reaction of ligand 259 with [Co4(CO)10(μDPPM)] resulted in the formation of the metal cluster 268, analogous to 267, which was characterized in solution [31P NMR δ 28.1 (br) and 100.5 (br) ppm] and in the solid state (Scheme 46 and Table 17). The bond lengths and angles involving the N-functionalized DPPA-type ligand were similar

catalytic addition of β-diketones to 1-alkynes, and complex 246, performing the best, was tested toward various substrates and also in the anti-Markovnikov addition of secondary amines to aromatic 1-alkynes (see section 13.6). Jiang and colleagues reported a series of N-(halo)arylfunctionalized DPPA-type ligands 248−257 (Figure 6), which

Figure 6. Mono- (248−253) and bis-halogenated (254−257) DPPAtype ligands.

in combination with [Cr(acac)3] and MAO as cocatalyst afforded highly active systems for the catalytic selective tetramerization of ethylene (see section 13.1).110 The Nfunctionalized DPPA-type ligands were obtained via the classical aminolysis route in moderate to good yields and characterized by 1H NMR, MS, and EA, and the Cr(III) complexes were prepared in situ, but no additional data were provided.

7. SILICON-BASED FUNCTIONALIZATIONS The N-functionalization of DPPA-type ligands with N−(alky/ aryl)−Si(OR)3 or −SiR2(OR′) (R and R′ = Me or Et) groups was essentially investigated with the aim of anchoring metal complexes or clusters into porous supports, for the formation of potential heterogeneous catalysts. The first part of this section will detail the ligands developed for this purpose and the 9267

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Table 17. 31P NMR Data (of the N-functionalized DPPA-type ligand, free or coordinated) and Characteristic Structural Parameters of the Ligands 258, 259, 261, and 262, the Metal Complexes 263−266, 272−276, and 284, and the Clusters 267− 270 and 277, 279, 282, and 283 av bond length, Å M

a

258 259 261 262 263

− − − − Pd

264

Pd

265 266 267

Pd Pt Co

268

Co

269 270a/b 272 273 274 275 276 277 279 282

Co Co Ni Ni Ni Ni Pd Co2, Pd2 Co, Pd2 Co, Pd2

283

Co, Pd2

284

Ni

δ, ppm (mult) 62.2 (s)a 63.1 (s)b/62.0 (s)c 68.6 (s)b 62.2 (s)a 57.8 (d)a 63.8 (d)a 53.1 (d)a 64.0 (d)a 30.8 (s)a 16.3 (s)a 92.0 (br)a 103.1 (br)a 28.1 (br)a 100.5 (br)a 92.8 (br, w1/2 166 Hz)a 81.0 (br, w1/2 337 Hz)a 38.3 (s)d 40.0 (s)/47.8 (s)d 91.0b and 91.4c 96.0 (s)b 79.1 (s)b 101.6, 76.7, 71.7, 64.4b,e 99.1,b 64.8, 58.7e 101.9 (m)b 62.3 (dm)b 79.5 (m)b 45.4 (m)b 42.8 (s)a

P−N

P−M

1.726

angle, deg P−N−P

P−M−P

ref 112 111 111 112 112

113.64(18)

112 113 113 112 1.702

1.710

2.186

2.218

114.9(2)

102.12(15)

114

73.68(3)

114 114 111 111 111 111 115 115 115 116 116

1.692

2.129

97.47(5)

73.39(5)

117

In CDCl3. bIn acetone-d6. cIn C6D6. dPolycrystalline sample. eSee the text for details.

Scheme 46. Metal Complexes and Clusters Supported by NFunctionalized DPPA-type Ligands Containing Alkoxysilyl or Alkoxy/Alkylsilyl Groups and Their Reactivity

reaction between [Co4(CO)12] and ligand 259 afforded the corresponding Co4 cluster 269 with one ligand 259 bridging one edge of the Co4 tetrahedron (Scheme 47). Reaction of this stabilized cluster with a terminal alkyne led to the formation of the butterfly cluster 270a. A strong shift of the broad 31P NMR resonance was observed when going from 269 (92.8 br, w1/2 166 Hz) to 270a (81.0 br, w1/2 337 Hz, Table 17). Cluster Scheme 47. Synthesis of the Co4 Butterfly Clusters 270a,b Containing the Ligand 259 and a Terminal Alkyne

to those in related structurally characterized Co4 clusters (see sections 5.2 and 6).114 The authors also showed that the 9268

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270b, an isomer of 270a exhibiting the same 31P NMR spectrum, could be obtained by the reverse sequence of reactions, where [Co4(CO)12] was first reacted with PhCCH to give the butterfly cluster 271, to which 259 was added to give the final product (Scheme 47). Blümel and colleagues obtained the [NiCl2(PNP)] complexes 272 and 273 from the equimolar reaction of ligands 259 and 261 with [NiCl2Py4], respectively (Scheme 46).111 Their diamagnetic nature, indicative of a square-planar coordination geometry, allowed their characterization by 31P CP/MAS NMR, as polycrystalline samples because of their insolubility in common organic solvents, and they exhibited singlet resonances at 38.3 and 40.0 ppm (Table 17). Furthermore, reactions of these complexes with CO(g), in the presence of metallic Zn (powder), resulted in the reduction of the Ni(II) center to Ni(0) and replacement of both chlorido ligands by two CO molecules, affording complexes 274 and 275, respectively. Alternatively, the latter could be obtained by reaction of the free ligands 259 and 261 with the Ni(0) precursor [Ni(COD)2], followed by treatment with CO(g); however, the first method gave better yields and did not require the use of the air-sensitive Ni(0) precursor. As a result of the increased electron density at the Ni(0) centers in 274 and 275 compared to Ni(II) in 272 and 273, a downfield shift of the P resonances to 91.0 ppm (274) and 96.0 ppm (275) was observed by 31P NMR (Table 17). The dicarbonylnickel(0) complex 274 could be characterized in the solid state by XRD analysis, which revealed a distorted tetrahedral coordination geometry around the metal center, with the PNP ligand acting as a chelate and both CO ligands completing the coordination sphere of the complex. The main structural features (P−N, P− Ni bond lengths and P−N−P, P−Ni−P angles) of the complex are in the range found for other group 10 metal complexes (Table 17). Further examples were provided where DPPA-type ligands, functionalized on the nitrogen atom by a Si-containing pendant group, are able to stabilize low-valent organometallic species or heterometallic clusters.115,116 In particular, the reaction between ligand 259 and a mixture of [Pd2(DBA)3]·CHCl3 and [PdCl2(NCPh)2] precursors in a 2:0.5:1 molar ratio, respectively, led by redox conproportionation to the dinuclear Pd(I) complex 276, in which the Pd−Pd bond is bridged by two ligands 259 (Scheme 48).115 Its 31P NMR spectrum contains a singlet for the four equivalent P atoms at 79.1 ppm (Table 17). The dinuclear complex 276 was reacted with 2 equiv of Na[Co(CO)4] to afford in good yield the trinuclear cluster 277, in which one ligand 259 bridges the Pd−Pd edge of the triangle and the other a Pd−Co edge (Scheme 48). In this cluster, each P atom gave rise to a distinct signal. The most low field shifted P resonance at 101.6 ppm (dm, 3JP1,P4 = 194 Hz) was attributed to P4, and the other resonances were found to be closer to each other: P1 at 76.7 ppm (ddd, 3JP1,P4 = 194 Hz, 3JP1,P2 = 149 Hz, 3JP1,P3 = 16 Hz), P3 at 71.7 ppm (m, 3JP3,P2 = 53 Hz, 3JP3,P1 = 16 Hz), and finally P2 at 64.4 ppm (ddd, 3 JP2,P3 = 53 Hz, 3JP2,P4 = 21 Hz) (Table 17). A related Pd2Co cluster, in which each edge of the triangle is bridged by an N-functionalized ligand 258, was prepared by treatment of the tris-DPPA derivative 278 with excess KH, to deprotonate the NH of the DPPA ligands, followed by reaction with I(CH2)3Si(OMe)3 (279, Scheme 49).115 This cluster exhibited only three 31P NMR multiplets, at 99.1 ppm, for the P atoms coordinated to the Co center, and at 64.8 and 58.7 ppm, for the four remaining P centers bound to Pd (Table 17).

Scheme 48. Synthesis of the Dinuclear Pd(I) Complex 276, Supported by Bridging Ligands 259, and Formation of the Tetranuclear Mixed-Metal Pd2Co2 Cluster 277

The reaction between ligand 259 and 1 equiv of the tetranuclear mixed-metal clusters [Co2Pd2(μ3-CO)2(CO)5(μDPPY)2] [Y = M (280) or A (281)] in the presence of 1 equiv of [NH4][PF6] afforded the trinuclear mixed-metal clusters 282 and 283, respectively, which contain two types of bridging, short-bite ligands (Scheme 49).116 The 31P NMR spectrum of 282 exhibited for the coordinated ligand 259 one multiplet at 101.9 ppm for the P atom linked to the Co center and one doublet of multiplets at 62.3 ppm for the Pd-bound P atom, while the signals corresponding to the DPPM ligands were found at 26.2 (m, P−Co) and −7.2 (m, 3 P−Pd) ppm (Table 17). A similar pattern was observed for the DPPA-containing cluster 283, with multiplets at 79.5 (P−Co) and 45.4 (P−Pd) ppm for the functionalized PNP ligand (Table 17). As already mentioned in section 3.1, Blann and colleagues investigated the influence of the nature of the N-pendant group in DPPA-type ligands in the Cr(III)-catalyzed ethylene tetramerization reaction. Among the large series of ligands evaluated, 258 was evaluated in a tertiary catalytic mixture (258/[Cr(acac)3]/MAO = 1:0.33:300, section 13.1).69 Very recently, Svoboda and co-workers reported the formation of a NiCl2 complex (284, Scheme 46) by reaction between ligand 258 and [NiCl2(PPh3)2], which was isolated and characterized by spectroscopic and XRD techniques. It exhibited comparable spectral and structural features to those previously described for NiX2 complexes containing DPPA-type ligands (vide supra and Table 17).117 Complex 284 was then successfully immobilized into a SBA-15 matrix and evaluated in the catalytic Kumada coupling reaction (see section 14). 7.2. N-Trialkyl/arylsilyl (Direct N−Si Functionalization)

The first report mentioning the synthesis of ligand 285a, a DPPA-type ligand directly N-functionalized with the trimethylsilyl group, came from Schmidbaur and colleagues in the late 1970s.118 The two-steps procedure involved (i) deprotonation of the acidic N-H proton of DPPA using n-BuLi and (ii) reaction with SiClMe3 of the resulting N-Li derivative, used 9269

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Scheme 49. Functionalization of Trinuclear Mixed-Metal Clusters with Pendant Si(OR)3 Groups via Two Routes, Leading to Clusters 279 and 282−283

Scheme 50. Synthesis of Ligands 285 Starting from DPPA and Formation of the Cr(III), Ni(0), and Pt(II) Complexes 286−288

accessible by a classical ligand substitution reaction using [Ni(COD)2] and [PtMe2(COD)] as precursors, respectively (Scheme 50).120 Complexes 287 and 288 exhibited characteristic 31P NMR (C6D6) signals, i.e., one singlet at 82.8 ppm for 287 and one singlet at 35.0 ppm flanked by 195Pt satellites (1JP−Pt = 770 Hz) for 288. The solid-state structure of 288 was established by X-ray diffraction and confirmed the anticipated square-planar ligand arrangement around the Pt(II) center, with typical Pt−P and P−N bond lengths [2.268(1)−2.279(1) and 1.732(3)−1.735(3) Å, respectively] and P−Pt−P and P− N−P angles [70.37(4)° and 98.17(2)°, respectively]. The Si atom was found to be 0.601(3) Å out of the coordination mean plane [P2Pt(Me)2], whereas, in general, the C atom from the N-substituent/function of DPPA-type ligands is in the P−N−P plane.

without further purification and characterization, affording ligand 285a as a white crystalline material (Scheme 50).118 The authors widened the synthetic scope by varying the nature of the organosilyl chloride (SiClRR′2 for R = R′ = Et; R = Me, R′ = Ph) used and reported compounds 285b,c (Scheme 50).80 A similar procedure was followed more recently by Blann and colleagues to further characterize ligand 285a [31P NMR δ 57.8 (br s)] and synthesize its CrCl3 complex 286, presumably a dimer, by reaction with [CrCl3(THF)3] (Scheme 50). The performances of 286 in selective ethylene oligomerization were compared to those of an analogous CrCl3 complex supported by the N-(t-Bu)-substituted DPPA-type ligand and of the tertiary catalytic mixture (285a/[Cr(acac) 3 ]/MMAO = 3:2.5:300, section 13.1).69 Braunstein and colleagues also observed the formation of 285a when they attempted to oxidize the deprotonated and lithiated DPPA Li[N(PPh2)2] with trimethylsilyl azide (SiN3Me3), by substitution of the Li by the SiMe3 group, along with the formation of LiN3 (FT-IR evidence).119 More recently, Bochmann and colleagues showed that the Ni(0) (287) and Pt(II) (288) complexes of ligand 285a were

8. POLY-BIS(DIARYLPHOSPHINO)AMINE DERIVATIVES 8.1. Pure Alkyl Spacer

This section deals with compounds resulting from the assembling of two or more DPPA moieties through an organic 9270

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Table 18. 31P NMR Data and Characteristic Structural Parameters of the Ligands 289, 290, 292, and 296−299 and the Metal Complexes 293−295 and 300−307 av bond length, Å M − − − Ni Pd Pt − − − − Mo Mo

289 290 292 293 294 295 296 297 298 299 300 301

Mo Pd Pt Pd Ni Rh

302 303 304 305 306 307 a

δ, ppm(mult) 61.3 62.1 49.0 54.1 35.9 16.5 70.2 68.4 68.1 60.6 95.2 97.0 96.7 95.8 34.9 20.5 55.9 37.7 63.8

(s)a (s) (s) (s)a (s)a (s)a (s)a (s)a (s)a (s)a (s)a (s)a (s)a (s)a (s)a (s)a (s)d (s)d (d)e

1

JP‑M, Hz

P−N

P−M

angle, deg P−N−P

P−M−P

3392 1.732 1.726

b

114.0(1) 122.5(1)

1.713

2.499 2.506

103.5c 103.93(9)

65.6c 65.14(1)

1.706 1.723

2.209 2.117

99.9c 96.2c

72.47c 74.55c

3421

180.7

ref 121 69 69 122 122 122 88 68 37, 126 88 88 37 126 88 68 68 127 127 127

b

In CDCl3. Not given, due to the poor XRD data set. cAverage value. dIn CD2Cl2. eIn THF-d8.

spacer. The latter can be aliphatic or aromatic, a pure hydrocarbon group, or it can incorporate a chemical function. Characteristic NMR data and structural parameters of the compounds described in this section are summarized in Table 18. The examples reported to date of poly-DPPA-type ligands, obtained via the aminolysis route, in which the DPPA moieties are linked through a linear or cyclic aliphatic spacer, are depicted in Figure 8. Ligand 289 exhibits a singlet in its 31P

Scheme 51. Dichloride Complexes of the Group 10 Metals with the Bis-DPPA-type Ligand 289

around a cyclohexyl (291),125 or a 4,4′-methylene-bis-cyclohexyl spacer (292),69 respectively, were also reported and characterized by EA and 1H and/or 31P NMR (Figure 8). These ligands were used in a ternary catalytic mixture of ligand/Cr(III)/MAO for selective ethylene tetramerization (see section 13.1). 8.2. Aryl or Benzyl Spacer

A series of bis-DPPA-type ligands, in which both P,P-donor sets are directly linked to a phenylene spacer, were reported in the early 2000s (Figure 9). While the two PNP fragments of ligand

Figure 8. Poly-DPPA-type ligands linked through a pure aliphatic spacer.

NMR spectrum at 61.3 ppm,121 which is in the range of other DPPA-type ligands substituted by an aliphatic group (see below). Group 10 dichloride complexes with this ligand were then synthesized and characterized by EA and spectroscopic techniques (Scheme 51). All the complexes exhibit an upfieldshifted signal in their 31P NMR spectra compared to the free ligand, at 54.1, 35.9, and 16.5 (JPt−P = 3392 Hz) ppm for the Ni (293), Pd (294), and Pt (295) complexes, respectively.122 The Pd complex 294 was then evaluated in both Suzuki and Heck coupling reactions (see section 13.4). Ligands similar to 289, with a longer hexyl spacer (290),123,124 a tris- and a bis-DPPA-type system arranged

Figure 9. Bis-DPPA-type ligands linked through an aromatic or benzylic spacer.

296 synthesized by Smith and colleagues are in 1,4-positions,88 those constituting ligand 298, reported independently by Dyson and colleagues37 and Hey-Hawkins and colleagues126 are in 1,3-positions. More recently, the bis-DPPA-type ligand 297, the biphenyl analog of ligand 296, was reported by Kayan and colleagues.68 Variations of the P-substituents were also investigated (see section 9). The bis-DPPA-type ligands 9271

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296−298 all display a single 31P NMR resonance for their four equivalent P atoms between δ 68.1 and 70.2 ppm, while an upfield shift to 60.6 (s) ppm was observed when going from a phenylene to a 1,4-dimethylbenzene spacer, i.e., from ligand 296 to 299 (Figure 9).88 This observation illustrates how minor changes in the bis(diphenylphosphino)amine-type [and bis(dialkyphosphino)amine-type; see section 9] ligand architecture can affect the electronic properties of the P donors (Table 18). The structure of ligand 297, determined by XRD studies, shows Ci symmetry, with a center of inversion at the midpoint of the C−C bond of the biphenyl moiety. Both phenyl rings constituting the biphenyl spacer are coplanar, while the PNP fragments are nearly perpendicular to the biphenyl spacer (98.3°). In the structure of ligand 298, a C2 axis goes through the C2 and C5 carbon atoms of the phenylene spacer, and in contrast to 297, the lone pairs on the P atoms of each PNP unit are pointing in opposite directions. While the P−N bond lengths in 297 and 298 are close, a large difference in the P− N−P angles [114.0(1)° and 122.5(1)° for 297 and 298, respectively] was found (Table 18). The molybdenum(0) tetracarbonyl in complexes 300, 301, 302 with ligands 296, 298, and 299, respectively, were readily accessible in good yields (>85%) by reacting the free tetraphosphine ligands with 2 equiv of [Mo(CO)4L2] (L2 = η4-NBD or (NCEt)2, Scheme 52).37,126,88 All three complexes

and in section 3.1, Table 6, respectively). In complex 302 (the poor quality of the XRD data recorded for 301 did not allow a detailed discussion), the Mo−C(O) bond lengths of the apical carbonyl ligands (av 2.042 Å) are slightly longer than those trans to the phosphine donors (av 2.000 Å), as expected from the different trans influence of the donor atoms. The fourmembered Mo−P−N−P rings have a butterfly-type structure with a dihedral angle of 15.7°. Dihalide complexes of group 10 metals (303 and 304) with the tetraphosphine ligands 297 and 298 were synthesized in good yields (72−93%) by reaction between the bis-DPPA-type ligands and metal precursors of the type [MX2L2] [X = Cl, Br; L2 = DME, COD, (NCMe)2] in a 1:2 ligand/metal ratio (Scheme 53).68,127 All these d8 complexes were characterized Scheme 53. Dihalide Complexes of Group 10 Metals (303− 306) with the Bis-DPPA-type Ligands 297 and 298

Scheme 52. Synthesis of Mo(0) Tetracarbonyl Complexes (300−302) with Ligands 296, 298, and 299, Respectively [L2 = η4-NBD or (NCEt)2]

by EA, FT-IR, and multinuclear NMR techniques, with a characteristic 31P NMR singlet resonance [34.9 (303), 20.5 (304), 55.9 (305), 37.7 (306) ppm], and in the case of the Pt(II) complex 304, this singlet is flanked by satellites exhibiting a 1JP−Pt coupling of 3421 Hz (Table 18). Complexes 305 and 306 were further characterized in the solid state by XRD studies, which confirmed the square-planar arrangement around the Pd(II) and Ni(II) metal centers, as well as the cischelating mode of each PNP moiety. The nature of the metal (Pd vs Ni) does not induce significant differences in the bond lengths (P−Pd, 2.209 Å; P−Ni, 2.117 Å) and angles [av P−N− P, 99.9° (305, Pd), 96.2° (306, Ni); av P−M−P, 72.47° (305, Pd), 74.55° (306, Ni)] around them (Table 18). These metrical data remain in the range of those found for PdCl2 and NiBr2 complexes with other N-functionalized DPPA-type ligands (e.g., complexes 37 and 39, section 2.4, Table 4). Hey-Hawkins and colleagues also reported the formation in 73% yield of the tetrarhodium(I) complex 307 from the reaction between ligand 298 and 2 equiv of the dinuclear complex [Rh(μ-Cl)(COD)]2 (Figure 10).127 It was characterized by EA, FT-IR, ESI-MS, and multinuclear NMR techniques. While the 1H and 13C NMR spectra confirmed the presence of the COD ligands on the Rh center that is not directly linked to the phosphine donors, its 31P NMR spectrum exhibited a characteristic doublet at 63.8 ppm with a 1JP,Rh

exhibit in 31P NMR one singlet signal, reflecting the equivalence of the four P atoms of the ligands, in the range 95.2−97.0 ppm in CDCl3 (Table 18). In the case of complex 301, 13C NMR signals at δ 217.9 (dd, 2JP,C = 10 Hz, 2JP,C = 26 Hz) and 210.7 (t, 2JP,C = 8 Hz) were assigned to the CO ligands, in the trans and cis positions to the P donors, respectively. The carbonyl ligands in complexes 300−302 gave rise to characteristic νCO absorption bands in the region 2020−1876 cm−1, typical for a cis-Mo(CO)4 moiety (local symmetry C2v). Complexes 301 and 302 were also characterized in the solid state by XRD studies, which confirmed, in both cases, the dinuclear nature of the complex, as well as the chelating mode of both PNP moieties. The structural parameters around the metal center are very similar in complexes 301 and 302, in terms of Mo−P bond lengths (av 2.499 and 2.506 Å, respectively) and N−P−N [103.3(8)° and 103.6(8)° for 301 and 103.93(9)° for 302] and P−Mo−P [65.7(2)° and 65.5(2)° for 301 and 65.14(1)° for 302] angles (Table 18). These metrical data and 31P NMR chemical shifts are in the range found for [Mo(CO)4(PNP)] complexes containing other N-functionalized DPPA-type ligands (e.g., complexes 25 and 71, in section 2.4, Table 4

Figure 10. Tetrarhodium(I) complex with the bis-DPPA-type ligand 298. 9272

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Table 19. Poly-bis(diarylphosphino)amine Derivatives 7, 24, 91, and 166, in Which the Spacer Contains a Heteroatom

route ii). Both approaches open the way to a large variety Nfunctionalized DRPA-type ligands. Using the “Grignard route”, Balakrishna and colleagues prepared the bis-DRPA-type ligand 308, analogous to the bisDPPA-type ligand 296, in 73% yield by reaction between C6H4{(NPCl2)2}2 and 8.2 equiv of benzylmagnesium chloride, and they studied its reactivity toward various metal centers, i.e., Mo(0), Ru(II), Pd(II), Pt(II), Au(I), and Cu(I) (Scheme 55).128 Compared to 296 (δ 70.2 ppm), an upfield shift was observed in the 31P NMR spectrum of ligand 308 (δ 60.7 ppm), resulting from the insertion of a CH2 group between the P atom and the phenyl rings (Table 20). The free tetraphosphine ligand could also be characterized in the solid state by XRD studies, which revealed a centrosymmetric structure with the central phenyl ring nearly perpendicular to the P−N−P moieties, the two planes forming an angle of 80.5(1)°. The two P−N bond lengths of 1.726(1) and 1.724(1) Å and the P−N−P angle of 125.16(6)° (Table 20) are in the range found for related N-functionalized DPPA-type ligands (e.g., ligands 16, 99, 141, 150, 192, and 196; sections 2.2, 3.2, 4.1, and 5.2 and Tables 2, 9, 12, and 15, respectively). Reaction of ligand 308 with the Mo(0) precursor [Mo(CO)4{(CH2)5NH}2] in a 1:2 ligand/metal molar ratio afforded the neutral, dinuclear complex 309 containing two cis-Mo(CO)4 moieties. A 31P NMR singlet resonance is observed at 97.9 ppm, with a coordination shift of 37.2 ppm compared to the free ligand (Table 20), and characteristic νCO absorption bands occur in the region 2020−1820 cm−1. The reaction between the ligand 308 and 2 equiv of the dinuclear ruthenium(II) complex [Ru(p-Cym)(μ-Cl)Cl]2 led to the formation of the trichlorido-bridged tetranuclear complex 310, which exhibits a 31P NMR singlet at 78.8 ppm and the 1 H NMR resonances of the p-cymene ligand. The authors extended their study of the coordination chemistry of ligand 308 with the synthesis of d8 metal complexes [311 (Pd), 312 (Pt)], by reaction of 2 equiv of [MCl2(COD)] (M = Pd, Pt) with 1 equiv of the free tetraphosphine ligand. Both complexes displayed a 31P NMR singlet at 46.3 and 31.6 ppm, respectively, with satellites exhibiting a 1JPt,P coupling of 3336 Hz in the case of the Pt(II) complex (Table 20). The nature of the Cu(I) complexes of ligand 308 was found to depend on the molar ratio of the metal precursor [Cu(NCMe) 4 ]BF 4 used. Proceeding with a 1:2 ligand/metal ratio afforded the dicationic dinuclear complex 313, in which each PNP donor set is ciscoordinated to a Cu(NCMe)2 moiety, as confirmed by the

coupling of 180.7 Hz (Table 18), in the range found for bischelated, cationic Rh(I) complexes with other N-functionalized DPPA-type ligands (e.g., complexes 50 and 163, sections 3.1 and 5.1, Tables 5 and 13, respectively). Complex 307 was subsequently evaluated in catalytic alkene hydrogenation (see section 13.6). The well-defined complex 303 was evaluated in Suzuki coupling reactions of aryl bromides with phenylboronic acid, while a mixture of ligand 297 and [Pd(OAc)2] (1:2 ratio) was studied in copolymerization of CO and olefins (see section 13.3). 8.3. Heteroatom-Containing Spacer

The poly-bis(diphenylphosphino)amine derivatives in which the spacer contains a heteroatom (N for amine or pyridine derivatives) or is functionalized by another chemical function (−OR, −SR′) have been discussed in the related sections. Table 19 summarizes these ligands, the sections in which they are discussed, and the corresponding references.

9. N-FUNCTIONALIZED BIS(DIALKYL/DIBENZYLPHOSPHINO)AMINE DERIVATIVES The versatility of the aminolysis reaction route for the formation of DPPA-type ligands (Scheme 1) allows one not only to vary the ligand N-functionality by choosing the appropriate amine but also to tune the basicity of the phosphine donors, by the replacement of the phenyl substituents by alkyl or benzyl groups, using the appropriate chlorophosphine reagent (PR2Cl instead of PPh2Cl, R = alkyl) (Scheme 54, route i). An alternative route consists of the reaction of a bis(dichlorophosphino)amine [R′-N(PCl2)2] with a Grignard reagent (RMgCl) to form the corresponding bis(dialkyl/dibenzylphosphino)amine derivative (Scheme 54, Scheme 54. “Modified” Aminolysis Reaction (Route i) and “Grignard Route” (Route ii) for the Formation of NFunctionalized DRPA-type Ligandsa

a

R = alkyl or benzyl, R′ = pure hydrocarbon group, heteroatomcontaining group. 9273

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Scheme 55. Bis-DRPA-type Ligand 308 and Its Metal Complexes 309−315

Table 20. 31P NMR Data and Characteristic Structural Parameters of the Ligands 308 and 316−323 and the Metal Complexes 309−315 and 327−329 av bond length, Å M

δ, ppm (mult)

323

− Mo Ru Pd Pt Cu Cu Au − − − − − − − − −

327 328

Ni Ni

329

Ni

60.7 (s)a 97.9 (s)a 78.8 (s)a 46.3 (s)a 31.6 (s)c 79.1 (br)a 84.0 (s)c 92.7 (s)c 58.55 (s)a 58.61 (s)a 58.52 (s)a 58.36 (s)a 58.55 (s)a 58.41 (s)a 122.4 (br)d 23.8 (t)d 89.7 (s)d −20.1 (s)d 111.3 (s)e 117.1 (d)e 61.2 (d)e 108.2 (d)e 60.3 (d)e

308 309 310 311 312 313 314 315 316 317 318 319 320 321 322

a

JP‑M /nJP−P, Hz

1

P−N 1.726(1) 1.715

angle, deg

P−M

P−N−P

P−M−P

2.522

125.16(6) 105.56(7)

65.57(1)

1.710

2.202

98.10b

71.68b

1.716

2.308

107.10(9)

73.48(1)

1

JP−Pt = 3336

ref 128 128 128 128 128 128 128 128 92 92 92 92 92 92 129

4

JP−P = 2.5 1.758

2

JP−P 2 JP−P 2 JP−P 2 JP−P

= = = =

120 121 119 119

1.740 1.748

115.80(7)

2.119(1) 2.128(1) 2.119(5) 2.140(5)

Ph i-Pr Ph Cy

129

97.37(15)

75.93(4)

106 106

97.16(8)

75.98(2)

106

In CDCl3. bAverage value. cIn d6-DMSO; dIn benzene-d6; see the text for attribution. eIn CD2Cl2.

presence of νCN absorption bands in the FT-IR spectrum of 313. In contrast, a 1:1 molar ratio led to the cationic coordination polymer 314, in which all Cu(I) centers are bischelated by two diphosphine donor sets from two different ligands. Both complexes 313 and 314 exhibit in their respective 31 P NMR spectra one broad singlet, at 79.1 and 84.0 ppm, respectively (Table 20). The tetranuclear Au(I) complex 315 was also accessible by reaction of the tetraphosphine ligand 308 with 4 equiv of [AuCl(SMe2)] and exhibits in its 31P NMR

spectrum a characteristic singlet resonance at 92.7 ppm (Table 20). The Mo(0), Pd(II), and Cu(I) complexes 309, 311, and 313, respectively, were further characterized in the solid state by XRD analysis which confirmed in each case the cis-chelating coordination mode of both P,P donor sets of ligand 308. Even though the three metal centers adopt different coordination geometries, octahedral for Mo(0) in 309, square-planar for Pd(II) in 311, and tetrahedral for Cu(I) in 313, the largest deviation from regular geometry originates from the small bite 9274

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angle imposed by the chelating PNP ligand 308, with P−M−P angles ranging from 65.57(1)° to 73.48(2)° (Table 20). The P−N bond lengths are nearly equivalent in all three complexes, while the P−M bond lengths and P−N−P and P−M−P (M = Mo, Pd, Cu) angles depend on the nature of the metal center (Table 20) but remain in the range found for similar complexes with N-functionalized DPPA-type ligands (vide infra). Weng, Hor, and colleague reported the synthesis, via the “modified” aminolysis reaction (Scheme 54, route i), and characterization (multinuclear NMR techniques and FAB-MS) of a series of bis(diethylphosphino)amine derivatives with additional N-donor functionalities of the types ether (316, 317), thioether (318−320), and pyridyl (321), ranging from weak to moderate basicity (Figure 11, Table 20).92 These

moieties and the terminal C-PCy2 group, respectively. Its structural analysis confirmed the presence of a CC triple bond [1.201(2) Å], and typical P−N bond lengths (av 1.758 Å) and P−N−P angle [115.80(7)°] were observed (Table 20). The authors also highlighted the rather short C(sp)−P bond length [1.757(2) Å] compared to C(sp3)−P (av 1.857 Å) and the nearly trigonal planar environment around the N atom (sum of the angles around 359.4°). Olivier-Bourbigou, Reek, Breuil and colleagues reported the synthesis of the NiBr2 complexes 327−329 containing Nsulfonyl-functionalized DRR′PA-type (R, R′ = i-Pr, Cy, Ph) symmetric (R = R′) and nonsymmetric (R ≠ R′) ligands, from a coordination-induced rearrangement of the imino(bisphosphine) proligands 324−326 (Scheme 57), respectively, Scheme 57. Synthesis of the NiBr2 Complexes 327−329 with N-Sulfonyl-Functionalized DPPA-type Ligands from a Coordination-Induced Rearrangement of the Proligands 324−326, Respectively

Figure 11. N-Functionalized bis(diethylphosphino)amine ligands 316−321 used in combination with a Cr(III) source and MAO for the selective oligomerization of ethylene.92

ligands have been further used, in combination with 0.5 equiv of [Cr(acac)3] or [CrCl3(THF)3] and MAO (440 equiv) as cocatalyst, in selective oligomerization of ethylene, without further characterization (see section 13.1). Erker and colleagues examined the reaction of in situ deprotonated acetonitrile with different chlorophosphines, and while they isolated the C-functionalized DPPM-type ligands [NC−CH(PAr2)2] (Ar = Ph, Mes) by reaction with the corresponding chloroarylphosphines, a completely different outcome was observed when chloroalkylphosphine reagents, e.g. PCl(t-Bu)2 and PClCy2, were used: formation of the trisphosphine DPPA-type derivatives 322 and 323, respectively, was observed (Scheme 56).129 Compound 322 was formed in

as observed for their DPPA-type analogues (see section 5.3).106 The complexes were obtained in 54−61% yields and were characterized by EA or FAB-MS and multinuclear NMR techniques. As expected, the 31P NMR spectrum of the symmetric complex 327 exhibits only one singlet at δ 111.3 ppm, while the nonsymmetrical complexes 328 and 329 display two doublets centered at δ 117.1 and 61.2 ppm (2JP,P = 120 Hz) and δ 108.2 and 60.3 ppm (2JP,P = 119 Hz), respectively (Table 20). Both asymmetric complexes 328 and 329 could be characterized in the solid-state by XRD analysis, and their structures were found to be very similar to that of the symmetric DPPA-type analog 224 (see section 5.3), but with a higher asymmetry of the P−Ni bonds, the longest ones involving, surprisingly, the more basic alkyl-phosphines (Table 20). The authors performed additional experiments to gain further insight into the mechanism of the rearrangement, and they suggested the occurrence of a homolytic cleavage of the P−P bond with formation of PBrR2 (Br originating from the NiBr2 precursor) and R1-SO2−NHPR2, followed by an intermolecular process. From the stability and reactivity of the [NiBr2(P,P′)] complexes, it was concluded that the P−P bond is likely cleaved in P-alkyl-based systems and that the P− N bond, as in the “classical” DPPA-type systems, may also be broken. Complexes 327−329 were active in selective dimerization of ethylene, and these results are discussed in section 13.2.

Scheme 56. Synthesis of the Tris-Phosphine DPPA-type Compounds 322 and 323a

a

Conditions: (i) for R = t-Bu, molar ratio 1:2:2; (ii) for R = Cy, molar ratio 1:1:1.

ca. 10% yield by reaction between acetonitrile, n-BuLi, and PCl(t-Bu)2 in a 1:2:2 molar ratio, along with another P,Nheterocyclic product that was not isolated. The proposed structure of 322 is based on its spectroscopic data [31P NMR δ 122.4 (N-P2), 23.8 (C-P, t, 4JP,P = 2.5 Hz) ppm] and by comparison with its crystallographically characterized cyclohexyl analog 323 (see below). The latter was isolated in low yield (13%) from an equimolar (1:1:1) reaction of MeCN, nBuLi, and PClCy2 and exhibits two singlets in 31P NMR at 89.7 and −20.1 ppm, assigned to the two equivalent N-PCy2

10. N-FUNCTIONALIZATIONS WITH OTHER p-BLOCK ELEMENTS (Al, Ga, AND Sn) In this section, we examine DPPA-type derivatives presenting an N-functionalization with any p-block element that was not discussed in the previous sections, i.e. N, O, P, S, X (X = halogen) or Si functional groups (sections 2−7). 9275

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N-functionalization of DPPA with a group 13 element was first reported in the mid-1960s, by Clemens and colleagues, with the dimeric complex [Et2AlN(PPh2)2]2 (330), which resulted from the reaction between DPPA and AlEt3, the latter acting both as a base and a metal source (Scheme 58).130 This

[Ni(COD)2] to afford the diamagnetic Ni(0) complex 336 (Scheme 59). It was characterized in solution by a 31P NMR singlet at 83.7 ppm (C6D6) and its X-ray structure revealed in the solid state a distorted tetrahedral coordination environment around the Ni center, with Ni−P and P−N bond lengths in the range of 2.1532(2)−2.177(2) and 1.691(5)−1.724 Å and P− Ni−P and P−N−P angles of 73.22(7)° and 97.6(3)°, respectively. Like in the case of the Pt(II) complex 288 with the N-SiMe3-functionalized DPPA derivative (Scheme 50, section 7), the Sn atom lies out of the P−N−P plane by 0.653(2) Å. Complex 336 was also accessible by a postmetalation functionalization route, involving first coordination of the Ni(COD) moiety to Li[N(PPh2)2], followed by reaction with SnClMe3.

Scheme 58. Synthesis of N-Functionalized DPPA-type Species with Group 13 (330−333) Elements by Deprotonation of DPPA and Their Reactivity

11. N-METAL FUNCTIONALIZATIONS An overview of the structurally characterized compounds resulting from the reaction between DPPA and group 1 metal precursors and their reactivity toward main group elements and transition metals was produced by Fei and Dyson in 2005.19 Here we will only briefly mention them, because they now represent “standard reagents”, but we will focus on subsequent works in this area.

strategy was extended to other group 13 precursors, i.e., AlMe3, GaEt3, GaMe3, by Schmidbaur and colleagues nearly 20 years later, who isolated the respective [R2MN(PPh2)2]2 (M = Al, Ga) complexes (331−333, Scheme 58).131 In the 31P NMR spectra, the nonequivalent P atoms gave rise to two broad singlets at 49 and 43 ppm (C6D6) and 51 and 42 ppm (THFd8) for the Al derivatives 330 and 331, respectively, and two doublets at 56 and 51 ppm (C6D6/THF-d8) with a JP,P of 8 Hz for 333. In contrast, complex 332 exhibited a single resonance at 66 ppm (C6D6), and this led the authors to suggest that for the Ga complexes 332 and 333, a symmetrical P,P-coordinated isomer with an eight-membered ring was part of an equilibrium in solution. The six-membered metallocyclic structure of 331 was confirmed in the solid state by XRD analysis, and different P−N bond lengths were found for those involving coordinated [1.676(5) and 1.687(6) Å] and uncoordinated [1.729(6) and 1.732(6) Å] P atoms, while the P−Al and N−Al bond lengths were similar [2.526(3) and 2.543(3), and 1.895(6) and 1.893(6) Å, respectively]. The P−N−P bond angles were quite large [113.6(3)°−114.4(3)°], owing to the coordination of only one P donor, which limits the constrains. By lowering the temperature of the reaction between DPPA and AlMe3, from 70 to 20 °C, Braunstein and colleagues were able to detect by 31P NMR the clean formation of the adduct [(Me3Al)2· HN(PPh2)2].119 Hasselbring and Braunstein also reported that complex 331 reacted with the oxidizing agent N3SiMe3, leading after heating and N2 liberation to complex 334 [31P NMR (CDCl3 or C6D6) δ 40.6 and 34.0 (2JP,P = 100 Hz) ppm], via a phosphazide intermediate (Scheme 58).132 Bochmann and colleagues reported the N-functionalization of DPPA by a trimethylstannyl group (335) by reaction between the deprotonated and lithiated DPPA Li[N(PPh2)2] (337) with SnClMe3.120 The product exhibited a 31P NMR singlet at 61.2 ppm (C6D6) and was further reacted with

11.1. Group 1 Metal N-Functionalization in Homometallic Complexes

The chemistry of N-metal-functionalized DPPA-type ligands was initiated with the “simple” reagent Li[N(PPh2)2] (337) obtained by deprotonation of the DPPA with n-BuLi, acting as a strong base and metal source (Scheme 60).118,133 Schmidbaur Scheme 60. Deprotonation of DPPA Using n-BuLi To Form 337 and Its Reactivitya

a

Cases where solely P,P-metalation occurs are not covered in this review (see ref 18).

Scheme 59. Synthetic Paths for the Ni(0) Complex 336 Supported by the N-SnMe3-Functionalized DPPA-type Ligand 335

9276

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temperature, with a barrier of 8.1 kcal·mol−1, resulting in a single 31P NMR resonance at 53.6 ppm. This phenomenon can be slowed down by decreasing the temperature; coalescence was reached at −80 and −100 °C, and a “frozen” conformer was observed, which exhibits two signals at 53.2 and 48.0 ppm (Table 21). The observation of two resonances (δ 53.5 and 45.3 ppm) in the room temperature solid-state 31P CP-MAS spectrum of 339 suggests that the activation barriers for both P−N bond rotation and lithium “switching” are much higher in the solid-state, as would be expected. Both solution and solidstate 6Li NMR spectra only presented a single, sharp resonance in the 30 to −110 °C temperature range, which indicated the presence of only one species. Ab initio calculations confirmed the preference for the Li to bridge a P−N bond, while the η3(P,N,P) bonding corresponds to a transition state for the “switching” of the Li from one P atom to the other.134 The P,Pcoordination, without interaction with N, and formation of a four-membered cyclic phosphazane, as observed for transition metals,18 were ruled out. Shortly afterwards, Ellermann and colleagues reported the synthesis of the N,N,N′,N″,N″-pentamethyldiethylentriamineco-coordinated potassium (PMDTA)K[N(PPh2)2] (340)137 and sodium (PMDTA)Na[N(PPh2)2] (341) derivatives (Figure 12),138 using KOt-Bu and NaH as bases, respectively. Both compounds were characterized (i) in solution by multinuclear NMR techniques with typical 31P NMR singlet resonances at 63.3 and 54.2 ppm, respectively, and (ii) in the solid-state by XRD studies, which revealed for the Na (341) and Li (339) derivatives a similar coordination geometry, with the N,N,N′ donors from PMDTA replacing the three THF ligands. No P coordination was observed in the K derivative (340), which exhibits K···Cphenyl interactions (Table 21). Using a crown ether (12-crown-4) instead of the PMDTA ligand [1:1:1 (12-crown-4)/NaH/DPPA molar ratio] allowed the authors to isolate the [{NaN(PPh2)2}2(12-crown-4)5] (342) derivative; its formula was deduced from NMR and mass spectroscopy studies (Table 21).138 Complex 341 was reacted with tellurium powder in the presence of TMEDA, and in the resulting mixed Na/Te polynuclear complex, the deprotonated DPPA ligands were found in a monoanionic P,P-bridging form, with no more N-functionalization.19,139 More recently, to avoid the formation of LiCl or the presence of coligands, such as PMDTA, for further use of [M− N(PPh2)2] (M = alkali metal) reagents in the synthesis of lanthanide complexes (see below), Roesky and colleagues treated DPPA with KH in THF at reflux to form [(THF)nK− N(PPh2)2] (n = 1.25 or 1.5) (343).140 The latter cocrystallized in two forms (343a,b), both as coordination polymers. In complex 343a, two types of coordination geometries were found for the K+ centers, two-coordinated by two neighboring [N(PPh2)2]‑ ligands (through the N atom), with some additional π-interactions, and six-coordinated, with (P,P)2-bischelation by two [N(PPh2)2]‑ ligands and further ligation by two THF molecules (Figure 13). In contrast, in complex 343b, all K centers are four-coordinated by one THF molecule, one N from one [N(PPh2)2]− ligand and a P,P-chelate from another ligand (Figure 13). The 31P NMR spectrum of 343 contains a single peak, at 58.6 ppm, which is consistent with other group 1 metal complexes of deprotonated DPPA, as described in Table 21, which also provides characteristic metrical data. The deprotonation of DPPA using heavier group 1 metal precursors, i.e., RbOt-Bu and CsOt-Bu, was investigated by Ellerman and colleagues.141 The equimolar reaction of DPPA,

and colleagues did not isolate this compound but reacted it in situ with SiClMe3 to generate the silylated derivative [Me3Si− N(PPh2)2] (285a; see Scheme 50 in section 7).118 Shortly afterwards, Ellermann and Lietz isolated the deprotonated and metalated complex 337 (characterized by 1H NMR, FT-IR, and EA), and the authors synthesized the deuterated analog of DPPA 338 by reaction of the in situ generated 337 with methanol-d4 (Scheme 60).133 We have seen in the previous sections that 337 could be employed for the formation of different R−N(PPh2)2 compounds, with, for example, R = SiMe3. The relatively facile deprotonation of the acidic N−H of DPPA, and more generally of DXPA derivatives (see the Introduction), opens the way to the synthesis of various P,Pcoordinated metal complexes containing this monoanionic bidentate ligand (see Chart 2).18 However, here we will only focus on compounds in which a metal center is linked to the deprotonated nitrogen, what we call “N-metal functionalization”, or to the nitrogen and a phosphorus atom (P,N-chelates). In the second half of the 1990s, von Ragué Schleyer and colleagues and Nieger and colleagues independently reported the solution and solid-state structures of complex 337, which consists of THF-solvated lithium bis(diphenylphosphino)amide {(THF)3Li[N(PPh2)2]} (339, Figure 12).134,135 The latter

Figure 12. Views of the crystal structures of deprotonated DPPA derivatives 339−341 and 346 N-functionalized by group 1 metals.

crystallizes as a monomer in THF, with the Li center in a distorted trigonal-bipyramidal coordination geometry, surrounded by one P and the N atom from the deprotonated DPPA and three O atoms from the coordinated THF molecules. Significant structural differences result from the deprotonation and metalation of DPPA in 339,136 with shorter P−N [av 1.672 Å (339) vs 1.692(2) Å (DPPA)] and longer P− C bond lengths [av 1.854 Å (339) vs av 1.830 Å (DPPA)] and a wider P−N−P angle [124.7(3)° (339) vs 118.9(2)° (DPPA)] (Table 21). The structure of 339 is retained in solution and a 31 P NMR study suggested a rapid rotation about the P−N bond and a “switch” of the Li cation from one P to the other at room 9277

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Table 21. 31P NMR Data and Characteristic Structural Parameters of the DPPA-type Derivatives 339−343 and 346, NFunctionalized by Group 1 Metals bond length, Å M

δ, ppm (mult)

av P−N

P−M

N−M

P−N−P angle, deg

ref

339

Li

53.6 (s)a,b 53.2 (s); 48.0 (s)a,c

1.672

2.964(9)

2.030(1)

124.7(3)

134

339d 340 341 342 343a 343b 346

Li K Na Na K K Cs

1.660 1.666 1.670

− − 2.962(1)

2.022(9) 2.814(2) 2.349(2)

125.0(2) 114.96(1) 121.57(8)

1.660 1.666 1.643

3.408g 3.428g −

2.752g 2.749g 3.514(4)

112.20g 114.54g 113.4(3)

135 137 138 138 140 140 141

63.3 54.2 45.1 58.6

(s)a (s)a (s, br)e (s)e,f

59.0 (s)e

In benzene-d6. At 30 °C. At −100 °C. dObtained by treatment of triphenylsilyldiphenylphosphinoamide with t-BuLi in THF at 195 K. eIn THFd8. fSpectrum recorded for a solution of 343. gAverage value. a

b

c

DPPA-type ligand is functionalized by a group 1 metal while the P donors are coordinated to another class of metals. In the presence of multiple N-functionalizations, i.e. by a group 1 metal (typically Li, Na, K) and another metal, within the same molecule, the compounds will be discussed in the respective section of the heavier metal (typically groups 2−4 and rareearth metals, sections 11.3−11.5). Kirin and Roesky reported the post-metalation functionalization of DPPA by the reaction between [M(CO)4(DPPA)] (347a and 348a) and excess KH in THF, leading to the nearly quantitative formation of the corresponding group 6 tetracarbonyl complexes 347 (Cr) and 348 (W) containing N-K(THF)x-functionalized DPPA-type ligands (Scheme 61).142 Figure 13. Two forms observed in the solid state (343a,b) of the potassium complex 343, formed upon deprotonation of DPPA with KH in THF.

Scheme 61. Post-Metalation N-Functionalization by Alkali Metals of Group 6 Tetracarbonyl Complexes

RbOt-Bu, and PMDTA in toluene afforded [RbN(PPh2)2] (344a) and, after recrystallization from THF, [(THF)0.5RbN(PPh2)2] (344b), which were both characterized by various spectrometric and spectroscopic techniques [31P NMR δ = 59.2 (s) and 60.2 (s) ppm, for 344a and 344b, respectively], but the atom connectivity could not be determined (no XRD evidence). In contrast, when the authors used 18-crown-6 as coligand instead of PMDTA, the reaction between RbOt-Bu or CsOt-Bu and DPPA afforded complexes [(18-crown-6)Rb− N(PPh2)2] (345) and [(18-crown-6)Cs−N(PPh2)2] (346), respectively. Both complexes could be structurally characterized, and interestingly, while the Rb(I) cation was P,P-chelated by one [N(PPh2)2]− ligand and one crown ether coligand, the Cs(I) cation was coordinated by the 18-crown-6 coligand and the deprotonated DPPA, through the N atom and πinteractions with one phenyl ring (Figure 12). Characteristic spectroscopic and structural details are given in Table 21. The complexes reported in this section are of special interest when considered as “metal precursors” for the synthesis of other metal complexes, e.g., group 3 and 4 metals and lanthanides (see following sections). Sometimes heteroleptic complexes containing the group 1 metal from the precursor are formed, and these complexes will be discussed in the section dedicated to the heavier metals.

The 31P NMR spectra of 347 and 348 exhibited a singlet for the two equivalent P atoms, significantly upfield shifted (ca. 38 ppm) compared to their DPPA precursors 347a and 348a (Table 22). The solid-state structures of 347 and 348, established by XRD analysis, confirmed the octahedral coordination geometry around the group 6 metal and the P,P-chelating mode of the DPPA-type ligands, which occupy two basal sites (Table 22). In these structures, the K cation interacts with the N amido atom of one anion, molecules of

11.2. Group 1 Metal N-Functionalization in Heterometallic Complexes

This subsection summarizes the few examples available of heteropolynuclear complexes in which the N atom of the 9278

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Table 22. 31P NMR Data and Characteristic Structural Parameters of the Heterometallic Complexes of N-K- or N-LiFunctionalized DPPA-type Ligands 347−350a av bond length, Å M 347 348 349 350 a

δ, ppm (mult.)

K, Cr K, W Li, Ni Li, Pt

58.7 8.07 66.2 29.8

(s) (s) (s) (s)

1

JP,M, Hz 169 740

av angle, deg

P−N

P−M

N−M′

P−N−P

P−M−P

ref

1.669 1.666 1.689 1.689

2.383 2.530 2.196 2.288

2.711(2) 2.736(3) 2.012(4) 2.031(1)

97.30(9) 99.62(1) 95.46(9) 95.8(2)

63.43(2) 60.37(3) 69.36(2) 66.43(5)

142 142 120 120

Spectra recorded in THF-d8.

THF, and the oxygen of a carbonyl from a neighboring molecule, leading to the formation of infinite arrangements in the solid-state, as shown for 347 in Scheme 61 [K−N = 2.711(2) for 347 and 2.736(3) for 348 Å; K−OCO = 2.649(2) for 347 and 2.702(3) for 348 Å]. The formation of group 10 metal complexes of N-Li(THF)3functionalized DPPA-type ligands was also achieved by Bochmann and colleagues, who reacted the pre-functionalized ligand 339 with a Ni(0) or Pt(II) metal precursor and isolated the corresponding P,P-chelate complexes 349 and 350 (Scheme 62).120 Their 31P NMR spectra exhibit the expected

Scheme 63. Reaction of the Complex 343 with Iodide Precursors of Group 2 Metals Leading to the Ca(II), Sr(II), and Ba(II) Complexes 351, 352, and 353, Respectively

Scheme 62. Synthesis of Group 10 Metal Complexes (349 and 350) of the N-Li(THF)3-Functionalized DPPA-type Ligand 339

below), suggesting the occurrence of a dynamic behavior in solution, as in the case of complex 339 (see Table 21). Therefore, the authors performed variable-temperature 31P NMR studies on the Sr(II) and mixed Ba(II)/K(I) complexes 352 and 353, respectively, which revealed at low temperature two sets of signals for the phosphorus atoms, but the low solubility of the compounds at this temperature only led to broad, unresolved singlets (Table 23).143 In the solid state, the structures of the Ca(II) and Sr(II) complexes are similar, with two chelating η2-{N(PPh2)2}− ligands and three THF molecules coordinated to the metal center, resulting in seven-coordinated metal ions. Characteristic structural data are reported in Table 23. In the solid-state structure of the coordination polymer 353, each repeat unit is composed by (i) one seven-coordinated Ba(II) center surrounded by three η2-{N(PPh2)2}− ligands and one THF molecule, resulting in a [(THF)Ba-{N(PPh2)2}3]− anionic complex, and (ii) one potassium cation coordinated to the N of one {N(PPh2)2}− ligand and to two phenyls via η2−π interactions, while π coordination with one phenyl of a neighboring unit results in the formation of the coordination polymer chain (Scheme 63). In complex 353, two {N(PPh2)2}− ligands are η2-coordinated to barium through the N and one P atom, whereas the third {N(PPh2)2}− ligand chelates the barium via both P atoms. With the objective to convert CO2 into higher-valued products, such as organic isocyanates and carbodiimides,

singlet, flanked in the case of the Pt(II) complex by 195Pt satellites, and the proposed structures were confirmed in the solid state by XRD analysis (Table 22). Interestingly, the position of the Li ion relative to the coordination plane was significantly affected by the nature of the metal center and/or its coordination geometry [i.e., tetrahedral and square planar for Ni(0) and Pt(II), respectively] with displacements of 0.415(3)° and −1.314(11)° for 349 and 350, respectively. 11.3. Group 2 Metal N-Functionalization

Starting from complex 343 as the source of deprotonated DPPA, Roesky and colleagues synthesized various group 2 complexes containing a N-metal-functionalized DPPA.143,144 While the reaction with CaI2 and SrI2, in a 2:1 molar ratio, afforded the corresponding group 2 mononuclear P,N-bischelated complexes 351 and 352, a similar reaction with BaI2 led to the formation of a coordination polymer, [{(Ph2P)2N}2Ba(THF){(Ph2P)2N}K]n (353), involving one Ba(II) and one K(I) center in the repeat unit, along with three DPPA− ligands (Scheme 63). In solution at room temperature (or 50 °C for 353), all complexes exhibit a sharp singlet in their 31 P NMR spectrum (Table 23), indicating the chemical equivalence of the P atoms of the ligands in solution. However, in the solid state, the phosphorus atoms are nonequivalent (see 9279

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Table 23. 31P NMR Data and Characteristic Structural Parameters of the N-Metal-Functionalized DPPA-type Derivatives 351− 353 and 355 av bond length, Å δ, ppm (mult)

M

a

351 352

Ca Sr

353

K, Ba

355

Ca

43.2 48.0 ∼49 51.6 56.9 42.9

b

c

P−N

(s)a (s)b,c (s); ∼43 (s)b,d (s)b,e (s); 44.8 (s)b,g (s)b

P−M−N

ref

1.674 1.683

2.913 3.061

2.473 2.607

120.9 119.22

34.45 32.99

144 143

f

f

f

f

f

143

N−M

2.418(3)h

119.1(2)h

35.21(7)h

146

2.890(1)h

1.678 d

av angle, deg

P−M

e

P−N−P

f

In benzene-d6. In THF-d8. At room temperature. At 173 K. At 323 K. The poor quality of the refinement of the XRD data recorded for 353 does not allow one to discuss its bond lengths and angles. gAt 193 K. hNot average value.

Scheme 64. N-Functionalization of DPPA-type Ligands with Rare-Earth Metals through the Synthesis of Homoleptic Complexes 356−362

Kemp and colleagues studied the reactivity of the strontium(II) complex [Sr{N(PPh2)2}2(THF)3] (352) toward CO2. This resulted in the formation of [Sr 6 {O 2 CN(PPh 2 ) 2 } 6 {N(CO2)3}2](THF)7 (354), through first the insertion of CO2 into the Sr−N bond of N(PPh2)2 ligands and then the reaction of N(PPh2)2 ligands with CO2, leading to the cleavage of N− PPh2 bonds and formation of [N(CO2)3]3− ligands.145 The Sr6 framework was suitable for the fixation of 12 mol equiv of CO2, through the formation of two [N(CO2)3]3− and six [O2CN(PPh2)2]− ligands, the latter being formally a “N-(CO2)functionalized” DPPA-type ligand, exhibiting a single resonance in its solid-state 31P NMR spectrum at δ 36.5 ppm for all (nearly) equivalent P atoms. Roesky and colleagues reported the synthesis of the heteroleptic aminotroponiminate/DPPA− Ca(II) complex 355 by reaction of CaI2, the potassium aminotroponiminate derivative [K{(i-Pr)2ATI}], and 343 in a 1:1:1 molar ratio.146 The solid-state structure of 355 was established by XRD and revealed a six-coordinated Ca(II) center chelated by one N,N[(i-Pr)2ATI] and one N,P-deprotonated DPPA ligand, while the second P donor remains uncoordinated, and the metal coordination sphere is completed by two THF molecules (Figure 14). A dynamic behavior in solution at room

heavier LuCl3 precursor, led to the formation of the sevencoordinate [Lu{N(PPh2)2}3(THF)-P,N] (363).140 The solidstate structures of all complexes were established by singlecrystal XRD, and due to the similar ionic radii of the lanthanides, 356−362 are isostructural (Table 24). Three η2[N(PPh2)2]− ligands are symmetrically coordinated to the Ln center in a near-trigonal-prismatic geometry, through the N and one P atom, while the second phosphorus remains dangling. The structure of 363 contains also three P,N-ligands of this type; however, a seventh THF ligand was found coordinated to the Lu center. Within this series of complexes, as the ionic radius of the metal increases, the Ln−N and Ln−P distances become longer (Table 24). The P−Ln−N angles are rather small, and neither P−Ln−N nor P−N−P angle varies much. However, a significant difference was observed in the P−N bond lengths between those involving the coordinated P atom (closer to N) and the free phosphine (Table 24). Complexes 356−359 and 363 were characterized in solution by multinuclear NMR, including 31P NMR, where single resonances confirmed the chemical equivalence of the P atoms in solution. The resonance was upfield-shifted compared to that of 343 [58.6 (s) in THF-d8, section 11.1, Table 21). However, the solid-state structures indicate nonequivalent P atoms in 356− 363, and the low-temperature 31P NMR (173 K, THF-d8) of 357 revealed the presence of two broad singlets (solubility issues prevented the observation of any hyperfine structure), in accordance with the X-ray structures. The dysprosium complex 361 exhibited a slow magnetic relaxation, which is likely to be related to the individual ion anisotropy of the Dy(III) center.148 Complexes 356−362 were

Figure 14. Structure the aminotroponiminate calcium complex 355.

temperature accounted for the observation of a single resonance at 42.9 ppm for the two nonequivalent P atoms. Table 23 summarizes the characteristic 31P NMR and structural data of complex 355. Attempts to synthesize the analogous Sr(II) complex of 355 remained unsuccessful. 11.4. Group 3 Metal and Lanthanide N-Functionalization

11.4.1. Homoleptic Tris(DPPA-type-P,N). The homoleptic rare-earth metal complexes of deprotonated DPPA 356− 362, which present a N-Ln functionality, were readily obtained by reaction between 343 and the corresponding anhydrous trichloride precursor (LnCl3) of the desired rare-earth metal in a 3:1 molar ratio (Scheme 64, route a, for Y, Gd, Dy, and Er) or by reacting 3 equiv of DPPA with 1 equiv of the tris-amido precursors [Ln{N(Si(Me3)2}3], acting as a base and a metal source (Scheme 64, route b, for Y, La, Nd, and Sm).140,147,148 Similar reactions to route a described in Scheme 64, with the 9280

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Table 24. 31P NMR Data and Characteristic Structural Parameters of the N-Metal-Functionalized DPPA-type Derivatives 356−365 and 367−383e bond length, Å M 356 357 358 359 360 361 362 363 364 365 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 383

Y La Nd Sm Gd Dy Er Lu Eu Yb Y Sm Er Yb Sm Yb Sm La Sm Y Y La Nd Yb La Eu Yb Yb

δ, ppm (mult) 42.0 44.6 44.6 62.7

(d)a (s)a (s)a (br)a

1

JP‑Ln, Hz 6.9

43.8 (s)a 44.9 (s)b 39.5 (d)a 38.0 (s)a

5.3

56.6 (s)a 58.0 42.5 43.0 38.1 41.2 41.3

(br)a (s)a (s)b (d)b (d)a,c (s)a

10.1 7.7

43.1 (s)a,c 46.7 (s)b,c 49.2 (d)a,d

angle, deg

N−Pcoord

N−Pfree

P−Ln

N−Ln

P−N−P

P−Ln−N

ref

1.676 1.661 1.675 1.673 1.673 1.678 1.674 1.682 1.648 1.666 1.688(6) 1.680(2) 1.688(7) 1.681(2) 1.660(3) 1.668

1.715 1.707 1.713 1.720 1.716 1.717 1.714 1.711 1.696 1.696 1.689(6) 1.700(2) 1.707(6) 1.705(3) 1.69.2(4) 1.723

2.783 2.951 2.880 2.861 2.821 2.794 2.762 2.758 3.018 2.990 2.856(2) 2.902(6) 2.828(8) 2.828(8) 2.928(1) 2.750

2.274 2.424 2.365 2.344 2.316 2.282 2.257 2.279 2.584 2.454 2.345(6) 2.3894(2) 2.295(2) 2.295(2) 2.430(3) 2.253

120.0 122.6 121.9 121.2 121.0 120.3 119.9 125.8 122.5 121.39 119.0(4) 120.1(8) 118.9(4) 118.8(1) 131.4(2) 124.3

37.00 34.27 35.55 35.79 36.35 36.90 37.30 35.51 33.06 33.85 36.23(1) 35.38(3) 36.66(2) 36.46(6) 34.55(8) 37.29

1.662(2) 1.653(5)

1.693(3) 1.697(5)

3.005(7) 2.906(2)

2.501(2) 2.421(5)

125.2(1) 124.2(3)

33.58(6) 34.67(1)

1.686(3) 1.673(4) 1.668(3) 1.696(4) 1.678(2) 1.660(3) 1.668(3)

1.714(3) 1.718(4) 1.714(3) 1.721(4) 1.719(2) 1.689(4) 1.703(3)

2.878(2) 3.053(1) 2.875(1) 2.840(2) 2.974(5) 3.065(1) 2.950(1)

2.316(4) 2.448(3) 2.343(3) 2.268(4) 2.503(2) 2.527(4) 2.412(3)

118.3(2) 122.7(2) 124.2(2) 117.8(2) 120.0(9) 121.2(2) 119.0(2)

35.85(8) 33.13(8) 35.46(8) 36.7(1) 34.33(3) 32.77(9) 34.43(8)

140 140 140 147 147 147 140 140 143 144 149 149 149 149 150 151 151 149 149 149 152 152 152 152 152 153 153 153

62

In THF-d8. bIn benzene-d6. cValues for the PNP ligand, the NPCPN resonates at δ 20.6 (d, 2JP,Y = 7.7 Hz), 19.3 (s), 19.9 (s), and 17.2 (s) ppm for 377, 378, 381 and 383, respectively. dValue for the PNP ligand, the NPCPN resonates at δ 19.8 ppm (d, 2JP,Yb= 53 Hz). eBond lengths and angles without standard deviations in parentheses are average values. a

evaluated for the ring-opening polymerization of ε-caprolactone (section 13.3).140 11.4.2. Heteroleptic Bis(DPPA-type-P,N). Following a similar method to that described in route a of Scheme 64, but using 2 equiv of 343 and diiodide precursors of lanthanide metals in THF, Roesky and colleagues synthesized the heteroleptic complexes 364 and 365 in which a sevencoordinate Ln(II) center (Ln = Eu, Yb) is bis-chelated by two deprotonated DPPA ligands and three THF molecules complete the coordination sphere.143,144 Similarities between 364, 365 and 356, 362 (see above) were noted (Table 24). In the case of complex 365, for which a 31P NMR spectrum could be recorded, a dynamic behavior in solution was observed at room temperature, leading to a single resonance for the equivalent P atoms, while the X-ray structure shows two nonequivalent phosphino groups in the solid state (a similar observation was made in the previous paragraph for the homoleptic complexes 356−359 and 363, Table 24). In the course of their studies on the formation of halfsandwich complexes of rare-earth metals presenting a N-metal functionalization of DPPA-type ligands, Gamer and Roesky isolated after recrystallization complex 366, resulting from the reaction of a slight excess of 343 (optimized conditions) with YbCl3.151 Although the connectivity around the sevencoordinated, distorted-pentagonal-bipyramidal Yb(III) center

could be established (Scheme 65), the limited quality of the XRD data did not allow further discussion. 11.4.3. Heteroleptic Mono(DPPA-type-P,N). When the reaction described in route a of Scheme 64 between 343 and the rare-earth trichloride precursors was conducted in a 1:1 molar ratio, the corresponding dichloride P,N-Ln complexes 367−370 were isolated.149 The solid-state structures of these isostructural complexes [similar ionic radii of the Ln(III) Scheme 65. N-Functionalization of DPPA-type Ligands with Lanthanides via the Synthesis of Complexes 364−366

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obviously nonequivalent P atoms, a single resonance was observed in the 31P NMR spectrum of 371, which was accounted for by a dynamic behavior in solution. If the ratio between the KCp* (or NaCp) and the “metalloligand” 343 is reversed, i.e., 1:2 instead of 2:1, the half-sandwich lanthanides complexes 372 and 373 were obtained in moderate yields instead of 371 (Figure 16).151 The expected structure was confirmed by XRD analysis for 372, with a distorted square-pyramidal coordination geometry around the Yb(III) center, the Cp* ligand occupying the apical position and both chelating P,N-(DPPA-type) ligands arranged in an anti-fashion and constituting the base of the pyramid. Characteristic structural data for 372 are summarized in Table 24, and the P−Yb and N−Yb bond lengths are significantly shorter than those in the other Yb(III) complexes 365 and 370, and the P−N−P angle is wider. A similar structure was observed for the Sm(III) derivative 373, however, with the presence of an additional THF molecule as ligand, but no satisfactory refinement of the X-ray data was possible. The four P atoms, chemically equivalent at room temperature on the NMR scale, resonate in 373 as a broad singlet at δ 58.0 ppm, in the same range as for complex 359 (Table 24). A series of four-legged piano-stool rare-earth-metal complexes (374−376) of the dianions derived from cyclooctatetraene and 1,4-bis(trimethylsilyl)cyclooctatetraene, displaying DPPA-type ligands with a N-Ln functionality, were also accessible via the three routes described in Scheme 66.149 After recrystallization from a THF/pentane mixture, the complexes were isolated as enantiomerically pure compounds, as deduced by XRD analysis. The lanthanide(III) metal was 12-coordinated (if the dianion derived from η8-cyclooctatetraene is considered as an octadentate ligand), forming a square-pyramidal polyhedron with one deprotonated P,N-chelating ligand and two THF molecules forming the base and the cycle occupying the apical position. The characteristic bond lengths and angles involving the DPPA-type ligand and the metal center in 374− 376 are in the range of those found for other La(III) (357) and Sm(III) (359, 368) containing N-Ln DPPA-type ligands (Table 24). 31P NMR single resonances for 374 and 375 were also found at room temperature in the expected region, as well as the doublet at δ 38.1 ppm recorded for 376 and originating from a 1JP−Y coupling of 10.1 Hz, similarly to complexes 356 and 367 (Table 24). 11.4.5. Heteroleptic Rare-Earth-Metal Complexes of (DPPA-type-P,N) Ligands Chelated by a Second Ligand. The rare-earth-metal chloride complexes 377−380 chelated by

centers], revealed a metal center surrounded by seven ligands in a pentagonal-bipyramidal geometry, with one basal chelating P,N-ligand, two apical chlorides, and three THF molecules occupying the last three basal positions (Figure 15). Character-

Figure 15. Heteroleptic mono(DPPA-type-P,N) lanthanide dichloride complexes 367−370.

istic structural parameters involving the metal and the Nfunctionalized DPPA-type ligand and spectroscopic data for 367 and 368 are closely related to those of their bis- and trisP,N-chelated derivatives (see above and Table 24). 11.4.4. Sandwich and Half-Sandwich Rare-Earth-Metal Complexes of (DPPA-type-P,N) Ligands. The bis-cyclopentadienyl complex of Sm(III) 371 containing a η2-chelating deprotonated DPPA ligand was successfully synthesized in a one-pot procedure starting from SmCl3 and 343 followed by addition of KC5Me5 (1:1:2 molar ratio), or by the prior synthesis of the K[Cp2SmCl2] intermediate (Figure 16).150 The

Figure 16. Sandwich and half-sandwich (DPPA-type-P,N) lanthanide complexes 371−373.

solid-state structure of 371, determined by XRD, revealed a pseudo-four-fold coordination sphere of the ligands around the Sm(III) center, with a cis-P,N ligand that exhibits significantly longer P−Sm and N−Sm bond lengths and a wider PNP angle than in complexes 359 and 368 (Table 24). The second phosphine ligand remains uncoordinated, and despite these

Scheme 66. Synthesis of Piano-Stool Cyclooctatetraene-Derived Complexes of Rare-Earth-Metal Complexes with N-LnFunctionalized DPPA-type Ligands

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Scheme 67. Synthesis of Di- and Trivalent Rare-Earth-Metal Complexes P,N-Supported by a Deprotonated DPPA Ligand and a Second Chelating Liganda

a

Route e allows the reduction of the Yb(III) complex 380 to its Yb(II) counterpart 383. The orange dots signify Ln(III) metal centers and the blue dots Ln(II) ones.

from 122.8(2)° for 380 to 138.1(2)° for 378. Concerning the deprotonated DPPA ligand, it behaves in each structure as a P,N-chelating ligand, through the nitrogen and one phosphorus atom, the second one remaining dangling (Scheme 67). Similar observations were made for 377−380 as for the aforementioned group 3 metal complexes and lanthanides about the bond distances (e.g., N−Pcoord < N−Pfree) and angles (small variations) involving the DPPA-type ligand and the metal centers (Table 24). It is, however, interesting to notice that as the ionic radius of the metal increases, the P−Ln−N angle becomes more acute. Roesky, Glanz, and colleagues also reported the synthesis of the amido derivative 381 by reaction of 378 with KNPh2 in a 1:1 molar ratio (Scheme 67, route d).152 THF adducts of divalent complexes of europium (382) and ytterbium (383) supported by these N,N- and N,P-chelating ligands were also accessible. In the case of the Eu(II) species 382, the reaction was performed between a divalent iodo-bridged dinuclear (Scheme 67, route f)153 or a mononuclear cationic precursor (Scheme 67, route g)154 and 2 or 1 equiv of the potassium complex of deprotonated DPPA 343, respectively. The synthesis of the Yb(II) complex 383 was also possible by this latter procedure (Scheme 67, route g)153 or by an elegant route involving the reduction of the Yb(III) precursor 380 by elemental potassium in THF (Scheme 67, route e).153 Complexes 381−383 were structurally characterized by XRD analysis and adopt an arrangement similar to that of the chloride derivatives 377−380 (see above and Table 24).

one deprotonated DPPA ligand through one phosphine and the nitrogen donor and the bulky N,N-[CH(PPh2NSiMe3)2]− ligand were reported by Roesky, Glanz, and colleagues.152 These compounds could be synthesized following three different routes (a−c, Scheme 67). Route a consists of the treatment of the aforementioned dichloride complex 367 with the potassium salt of the bidentate N,N-[CH(PPh2NSiMe3)2]− ligand, leading to complex 377. Route b remains the most convenient and versatile synthetic route (it worked for all LnCl3 studied), since it is a one-pot reaction in which the potassium methanide K[CH(PPh2NSiMe3)2] is treated with anhydrous rare-earth metal trichlorides and 343 in a 1:1:1 molar ratio, affording complexes 377−380. Finally, route c involves the preformed and well-established [LnCl2{CH(PPh2NSiMe3)2}]2 (Ln = Y and Yb) complexes, which after treatment with 2 equiv of 343 lead to the corresponding complexes 377 and 380. The solid-state structures of 377−380 were established by XRD analysis, and in this case, an influence of the ionic radius of the rare-earth-metal ion was observed. Complexes 377 and 380, with a metal center presenting the smaller ionic radius, were found to be isostructural, while 378 and 379 crystallize in a different space group or with different unit cell metrics (Table 24). The influence of the nature of the metal was not limited to these parameters but significantly affected the conformation of the six-membered metallacycle (N−P−C−P−N−Ln), with, for example, (i) a displacement of the Ln center out of the N2P2 plane by only 0.233 Å for 378 but by 1.362 Å for 380 and (ii) a value of the P−C−P angle of the supporting ligand ranging 9283

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Figure 17. Lanthanide−potassium wheels integrating N-Ln and/or N-K functionalization of DPPA-type ligands.155

Scheme 68. Synthesis of Complex 387 by NFunctionalization of DPPA-type Ligands with a Group 4 Metal (Ti)

The heteroleptic complexes 377−380 were evaluated in the polymerization of ε-caprolactone and methyl methacrylate (MMA) (section 13.3).152 11.4.6. Mixed Potassium/Rare-Earth-Metal Heterometallic Complexes Incorporating a Deprotonated DPPA Ligand with an N−Metal Bond. The one-pot reaction between ErCl3 or YbCl3 and metalloligand 343, KNPh2, and NaCp in a 1:2:2:1 molar ratio in THF led to the corresponding polynuclear heterometallic complexes 384 and 385, for which the isostructural solid-state structures could be determined by XRD analysis (Figure 17, left).155 The structures can be viewed as a ring composed by two [K(THF)2][(η 5-C5H5)Ln(NPh2)2N(PPh2)2] moieties, in which the lanthanide metal center is surrounded by four ligands, and two [KN(PPh2)2(THF)2] units. These structures exhibit therefore DPPA-type ligands with N-Ln and N-K functionalities. The “supramolecular” Ln/K wheel structure is assembled through π interactions between the NDPPA-K and P,PDPPA-K moieties and the Cp and the phenyl rings of the neighboring DPPA-type or NPh2 ligands. Using samarium trichloride as precursor and under the same reaction conditions, however later optimized, the authors isolated a coordination polymer of general formula [K{(η5C5H5)Sm(NPh2)2N(PPh2)2}]n (386) in moderate yield, as revealed by XRD analysis (Figure 17, right).155 In complex 386, which can be seen as an octanuclear Sm4K4 wheel-shaped structure, the four samarium subunits are linked together through π interactions between the K and the Cp and the phenyl rings of the neighboring NPh2 ligands. In contrast to complexes 384 and 385, only N-Sm functionalization was observed for the DPPA ligands in 386.

of the signal at 47.0 ppm decreased while that of the two doublets increased, with the concomitant apparition of small additional signals, assigned to other monochelated and unsymmetrical species. Treatment of 387 with MAO at room temperature, expected to form the corresponding cationic Ti− Me derivative, led to a 31P NMR spectrum composed of four doublets (δ ≈ 30 and −12 ppm, 2JP,P = 278 Hz), supporting the exclusive formation of a C2-symmetric cis-octahedral complex, which led the authors to assume that the two doublets observed in the spectrum of the dichloride complex could only be attributed to the conformation 387b. A combination of complex 387 and MAO was evaluated in polymerization of propylene or isomerization of alkene substrates (sections 13.3 and 13.6, respectively). Reactions between the N-potassium or N-lithium DPPA-type ligands (343 and 339, respectively) and group 4 metallocene dichlorides (Zr and Hf) led to the formation of the corresponding amido-phosphine derivatives [MCp2Cl(P,N)]

11.5. Group 4 Metal N-Functionalization

The reaction of TiCl4 with 2 equiv of the deprotonated DPPA precursor 337 afforded the bis-amido complex [TiCl2{N(PPh2)2}2] (387, Scheme 68), which exhibits at room temperature a major 31P NMR signal at 47.0 ppm (90%), corresponding to complex 387a of tetrahedral symmetry, and a minor set of two small doublets at 5.0 and −10.0 ppm (10%), corresponding to a C2-symmetric octahedral configuration (387b) or to a complex with higher symmetry (387c,d, Scheme 68 and Table 25).156 By lowering the temperature, the intensity 9284

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Table 25. 31P NMR Data and Characteristic Structural Parameters of the N-Metal-Functionalized DPPA-type Derivatives 387− 389 av bond length, Å

a

M

δ, ppm (mult)

387a 387b 388

Ti Ti Zr

389

Hf

47.0 (s)a 5.0 (d), −10.0 (d)a 62.2 (br), −3.8 (br)a 61.2 (br), −4.5 (br)b 60.3, −4.7a,c

N−Pcoord

N−Pfree

P−M

av angle, deg N−M

P−N−P

P−M−N

ref 156 156 157 158 158

1.642(5)

1.708(5)

2.69(2)

2.249(5)

124.2(3)

38.1(1)

1.653(2)

1.708(2)

2.625(6)

2.208(2)

123.7(1)

38.8(5)

In THF-d8. bIn benzene-d6. cMultiplicity not determined.

(388 and 389) presenting a group 4 metal N-functionalization of the DPPA-type ligand (Scheme 69).157,158 Both complexes

Scheme 70. Synthesis of Tetranuclear Complexes (392−396) of DPPA-type Ligands N-Functionalized with d10 Ions

Scheme 69. Group 4 Metallocene Complexes 388 and 389 of Deprotonated DPPA

exhibit in their 31P NMR spectra two resonances for the nonequivalent P atoms of the DPPA-type ligand at δ 62.2 and −3.8 ppm for 388 and 60.3 and −4.7 ppm for 389, which contrast with the dynamic behavior in solution observed for the group 3 metal complexes of the DPPA− ligand, for which a unique signal for both P atoms was observed (section 11.4, Table 24 vs Table 25). The molecular structure of both group 4 metallocenes was found by XRD to be isostructural and confirmed the P,N-chelation of the DPPA-type ligands. All characteristic structural parameters in 388 and 389 follow a similar trend as for their group 3 metal analogues, especially with different P−N bond lengths depending on whether the P atom is coordinated or not (Table 25). Subsequent reaction of complex 388 with 0.5 equiv of elemental magnesium led to the reduction of the Zr(IV) to Zr(III) with concomitant abstraction of the Cl ligand and formation of the [ZrCp2(DPPA−)-P,P] complex exhibiting a four-membered metallacycle. The analogous Ti(III) complex could be obtained by reaction between the N-Li proligand 339 and 0.5 equiv of [TiCp2Cl]2 or by deprotonation of DPPA in a preformed [TiCp2Cl(DPPA)-P] complex.158

More recently, Knorr and colleagues studied the reactivity of bis(diphenylphosphino)amine-bridged heterobimetallic iron− platinum μ-aminocarbyne complexes and isolated the heterotrimetallic complexes 398−403, which resulted from the clean reaction of the zwitterionic precursor complex 397161 with [ML′]X (M = Au, Cu; L′ = PPh3, AsPh3; X = CF3SO3, NO3) or MeHgCl reagents (Scheme 71).162 Complexes 398−403 exhibit typical and similar patterns in their 31P NMR spectra, owing to the nonequivalent P atoms of the DPPA-type ligand and numerous JP,P and JP,Pt couplings. This results for 398 in doublets of doublets of doublets (ddd) at δ 105.6 (P1), 87.0 (P2) ppm for the DPPA-type ligand (Scheme 71, Table 26), and its spectrum contains also a signal at 34.8 (ddd, P3) ppm for the Pt−PPh3 phosphine and at 32.3 (q) ppm for the Au− PPh3 phosphine. The structures proposed for complexes 398− 403 were based on those determined in the solid-state by XRD analysis for 398 and 399. Both structures are very similar, with a Fe−Pt bond of 2.4932(6) Å (398) and 2.4943(13) Å (399) supported by the bridging N-Au−PPh3-functionalized DPPAtype ligand (Table 27). Reaction of diethylzinc with DPPA or HN(PPh2){P(i-Pr)2} in a 1:1 molar ratio led to the formation of the dinuclear sixmembered boat-like Zn2N2P2 metallacycle 404 and 405, respectively, as deduced from XRD analysis (Scheme 72).163

11.6. Groups 10−12 Metal N-Functionalization

Back in the late 1980s, Usón and colleagues reported that deprotonation of DPPA with [AuCl(CH2PR3)] or [Ag(CH2PR3)]ClO4 yielded the neutral, cyclic complexes [N(Ph2PMPPh2)2N] [M = Au (390), Ag (391), Scheme 70] that react further with [AuX(THT)] (X = C6F5 or Cl) or [Ag(OClO3)(PPh3)] to give the tetranuclear derivatives 392−396 with direct N−Au (392−394) or N−Ag bonds (395, 396) (Scheme 70 and Table 26).159,160 XRD studies confirmed the solid-state structure of the tetranuclear complex 395, with a nearly linear coordination geometry for the Au(I) centers [P−Au−P, 174.8(2)°], while that of the Ag(I) centers is much away from linear [N−Ag−P, 164.3(2)°], maybe due to the weak interaction with the ClO4 conterion (Table 27). The P−Au and N−Ag bond lengths and Au···Au interaction (2.873 Å) are as expected (Table 27). 9285

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Table 26. 31P NMR Data of the N-Metal-Functionalized DPPA-type Derivatives 392, 394−396, and 398−406 δ PPh2, ppm (mult) 392 394 395 396 398 399 400 401 402 403 404 405 406 a

δ PPh3, ppm

1

1

J109Ag−P, Hz

J107Ag−P, Hz

JAg‑PPh2, Hz

JAg‑PPh3, Hz

a

81.2 (s) 87.7 (dd)a 72.3 (br s)a 18.3 (dd) 68.1 (dm)a 17.7 (dm) 2+3 δ P1, ppm (mult) δ P2, ppm (mult) JP1−P2, Hz a

105.6 (ddd) 106.4 (ddd)a 101.9 (dd)a 105.1 (dd)a 102.5 (dd)a 109.0 (br d)a δ PPh2, ppm (mult)

87.0 87.7 83.7 86.3 80.7 84.9

(ddd) 115 (ddd) 125 (dd) 121 (dd) 129 (dd) 102 (br d) 102 δ Pi‑Pr2, ppm (mult)

49.7 (s)c 52.3 (s)c

288 728 2

253 630

JP2−P3, Hz

7 13 5 12 11 14 δ P1, ppm

3

JP1−P3, Hz 13 15 12 12 9 nde δ P2, ppm

524b JPt−P2, Hz

1

643b 1

JPt−P3, Hz

2689 3808 2774 3577 2819 3819 2754 3568 2834 3813 3584 nde δ P3, ppm

2+3

JPt−P1, Hz

64 60 69 nde 70 nde δ P4, ppm

71.4 (s)c 23.2c,d

37.1c,d

53.2c,d

ref 160 159 160 159 ref

66.0c,d

162 162 162 162 162 162 163 163 163

In CDCl3. bAverage value. cIn benzene-d6. dP1, 2JP,P = 94.0 and 31.2 Hz; P2, 2JP,P = 31.2 Hz; P3, J = 6.2 Hz; P4, 2JP,P = 94.0 Hz. eNot detected.

Table 27. Characteristic Structural Parameters of the N-Metal-Functionalized DPPA-type Derivatives 395, 398, 399, and 404−407 bond length, Å 395

398 399 404 405 406a

407 a

P−N

P−Au

1.653b

2.322b

2.239(4) 2.570(11) bond length, Å

N−PFe(Zn)

N−PPt

N−Pfree

1.664(3) 1.664(3) 1.661b 1.672b 1.679(2)

1.681(4) 1.681(4)

P−N

N−Pt

P−Pt1

P−Pt2

P−Pt3

N−Pt1−P

P−N−P

ref

2.096(3)

2.621(9)

2.371(1)

2.289(9)

38.62(8)

143.2(2)

164

1.649

b

N−Ag

angle, deg

1.715b 1.704b 1.719(2) bond length, Å

Ag−OClO3

P−Au−P

N−Ag−P

P−N−P

ref

174.8(2)

164.3(2)

nd

160

P−Fe(Zn)

P−Pt

N−M

2.258(1) 2.258(2) 2.449b 2.439b 2.416(7)

2.293(1) 2.293(1)

2.053(4) 2.054(4) 1.988b 1.979b 1.941(2)

P−N−P angle, deg 122.1(2) 122.2(2) 117.7b 120.8b 119.0(1) angle, deg

ref 162 162 163 163 163

Only the data related to the N-Zn DPPA-type ligands are provided. bAverage value.

Scheme 71. N-Functionalization of DPPA-type Ligands with a Group 11 or 12 Metal

In the latter structure, the short-bite ligands act as P,N-bridges between two Zn centers, while the second phosphine donor remains uncoordinated. The Zn−N bond lengths [1.976(3)− 1.993(3) Å] are in the expected range for EtZn-amido complexes, which suggests a strong covalent character for the Zn−N bond, while the N−P bond lengths are consistent with a contribution of the NP form (see Table 27 for characteristic structural parameters). Complexes 404 and 405 gave rise to one singlet at 49.7 ppm for the two equivalent P atoms and to two singlets at 52.3 ppm (PPh2) and 71.4 ppm {P(i-Pr)2}, respectively (Table 26). Complex 405 was further reacted with CO2 (1 atm) and the precipitate formed exhibited in IR spectroscopy a band at 1634 cm−1 attributed to an inserted CO2 molecule. The complexity of the 1H and 13C NMR spectra revealed a loss of symmetry in the reaction product, as also observed in the 31P NMR spectrum, which shows four different phosphorus environments (Table 26). The connectivity in complex 406 was unambiguously determined by XRD analysis and revealed that one of the two DPPA-type ligands present in 405 retained its P,N-bridging mode, while the CO2 molecule inserted into the Zn−Pi‑Pr bond with one oxygen bridging both Zn centers (Scheme 72, Table 27). 9286

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Scheme 72. Synthesis of Zn(II) Complexes Presenting a N-Zn-Functionalized DPPA-type Ligand and Their Reactivity toward CO2

In a recent work, Fortuño and Mastrorilli and colleagues investigated the reactivity of diphenylphosphanido-bridged diand trinuclear platinum (Pt2III and PtII,Pt2III, respectively) complexes toward nucleophiles.164 Along their studies, the authors observed that the reaction between complex [(C6F5)2PtIII(μ-PPh2)2PtIII(μ-PPh2)2PtII(C6F5)2] and NaN3 (as N3− nucleophile source) led to the coordination of the nucleophile selectively at the central Pt(III) center, and the PPh2/N3− reductive coupling yields the trinuclear complex [NBu4][Pt3II(μ3-Ph2PNPPh2)(μ-PPh2)2(C6F5)4] (407·NBu4), in which a newly formed deprotonated DPPA moiety bridges three Pt(II) centers, as determined by X-ray crystallography (for the PPh3Me salt) and represented in Figure 18. The

Figure 19. Heterobimetallic complex 408 with a metalloligand based on N-functionalization of the DPPA-type ligand, evaluated in ethylene (co)polymerization.

oligomerization processes, such as the Shell Higher Olefin Process (SHOP), typically lead to Schultz−Flory or Poisson distributions of LAOs,166,167 specific tri- and tetramerization catalysts (predominantly based on chromium) have been developed and given rise to many reviews in the recent years.13−17,168,169 The selective ethylene oligomerization, under homogeneous conditions, remains an ongoing and attractive field of research, as demonstrated by the high number of dedicated publications (for 2015, see, for example, refs 170−173), and continuing efforts to rationalize the role/effect of the cocatalyst(s) (nature, amount, etc.)174,175 and to gain more insight into the reaction mechanism are still in progress.15,176−179 Particularly notable are the construction of trimerization plants by CPChem, Axens, and Mitsui,180 and a tetramerization plant by Sasol.10,180 The Linde group also announced in 2015 an intention to commercialize a process based upon PNPNH.181−187 In view of the considerable interest for these oligomers, it is not surprising that other companies are also active in the field.14 The proposed mechanisms for chromium-catalyzed selective ethylene oligomerization appear to depend upon the oxidation state of the active chromium species generated by reaction with the aluminum-based cocatalyst. With the objective to gain a deeper insight and substantiate mechanistic proposals, several investigations have dealt with the isolation and characterization of catalytically active Cr/Al species resulting from the reaction of the chromium precatalyst with the aluminum activator.188,189 13.1.1. Catalytic Systems Involving “Poly-DPPA-type Ligands”. Jiang and colleagues studied the influence of several parameters, i.e., the reaction temperature, reaction pressure, chromium(III) source, Cr/Al(MAO) molar ratio, and ligand structure, on the catalytic activity and product selectivity in a series of poly-DPPA-type ligands (section 8). Comparing the performances of the bis-DPPA-type ligands 289, 290, and 296, as part of the ligand/Cr(III)/MAO catalytic mixture, the authors could highlight that the nature of the linker between the two DPPA moieties strongly affects the activity of the system.123,124 Under similar conditions, ligand 296 with a rigid

Figure 18. Trinuclear Pt(II) complex 407 presenting a DPPA-type ligand with N-Pt functionalization.

authors highlighted the dynamic behavior of complex 407 in solution and reported 31P NMR values of δ (acetone-d6) 128.4 (1JPt,P = 3045 Hz, 2JP,P = 70 Hz) and 31.7 (1JPt,P = 1690, 1JPt′,P = 510 Hz, 2JP,P = 70 Hz) ppm for the two P atoms constituting the DPPA-type ligand.

12. MISCELLANEOUS Along their studies to evaluate a potential cooperative effect in heterobimetallic complexes applied to the polymerization of olefins, Delferro, Marks and colleagues reported complex 408, in which the [CrCl3(PNP)] (PNP = DPPA-type ligand) moiety, well-known in the field of selective ethylene oligomerization, was associated with a [(η5-indenyl-1-{Me2Si(t-BuN)})TiCl2] moiety through the N atom of the DPPA-type ligand (Figure 19).165 Complex 408 was characterized by elemental analysis and MALDI-TOF spectrometry and compared to its monometallic derivatives and [CrCl3(SNS)] (Sasol-type catalyst) analogues in the (co)polymerization of ethylene (section 13.1). 13. APPLICATIONS IN HOMOGENEOUS CATALYSIS 13.1. Chromium-Catalyzed Ethylene Oligomerization

Intensive efforts have been made over the last 2 decades to develop catalytic systems able to selectively oligomerize ethylene for the production of linear α-olefins (LAOs). Among the latter, short-chain LAOs, i.e., 1-butene, 1-hexene, and 1-octene, are of greatest interest for the production of linear, low-density polyethylene. While conventional ethylene 9287

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Table 28. Selected Results in the Cr-Catalyzed Ethylene Oligomerization with Poly-DPPA-type Ligands, Compared to the Related N-Substituted Mono-DPPA-type Ligands entry a

1 2a 3a 4d 5e 6e 7f 8f 9g 10g,k 11j

ligand

activity, g/(g Cr·h)

C6 b

1-C6c

C8 b

1-C8c

Cr/lig./Al

ref

289 290 296 289 290 N-ethyl 292 N-Cy 7 7 291

10 200 31 700 40 600 17 100 662 000 1 020 000 1 760 000 2 150 000 42 900 35 600 181 700

22.1 27.5 18.4 17.2 18.3 17.5 17.6 19.4 29.0h 25.2h 25.0h

83.3 87.1 61.2 79.7 39.2 40.7 75.5 75.0

65.6 63.1 62.3 72.5 56.6 63.0 69.4 68.3 47.6i 43.9i 58.8i

96.7 99.3 98.1 97.9 99.6 97.3 99.0 99.0

1/1/100 1/1/100 1/1/100 1/1/180 2/1/300 1/1.2/300 2/1/300 1/1.2/300 3/1/300 3/1/300 3/1/900

124 124 124 123 69 69 69 69 25 25 125

Precursor, [CrCl3(THF)3]; Al = MAO; toluene; reaction time, 30 min; temp, 40 °C; pressure, 10 bar. bWeight percent of the oligomeric mixture. Percentage of 1-olefin within the corresponding fraction. dPrecursor, [CrCl3(THF)3]; Al = MAO; toluene; time, 30 min; temp, 60 °C; pressure, 30 bar. ePrecursor, [Cr(acac)3]; Al = MMAO; methylcyclohexane; time, 30 min; temp, 60 °C; pressure, 45 bar. fPrecursor, [Cr(acac)3]; Al = MMAO; methylcyclohexane; time, 17 min; temp, 60 °C; pressure, 45 bar. gPrecursor, [CrCl3(THF)3]; Al = MAO; cyclohexane; time, 30 min; temp, 60 °C; pressure, 30 bar. hOverall selectivity in 1-hexene (wt %). iOverall selectivity in 1-octene (wt %). jPrecursor, [Cr(acac)3]; Al = MAO; cyclohexane; time, 30 min; temp, 40 °C; pressure, 40 bar. k[(CrCl3)3(7)] preformed. a c

Table 29. Relevant Results in the Cr-Catalyzed Ethylene Oligomerization with Nitrogen-Based N-Functionalized DPPA-type Ligands, and Their Related N-Substituted DPPA-type Ligands entry a

1 2a 3a 4d 5d 6d 7e 8e 9f

ligand

activity, g/(g Cr·h)

C6 b

1-C6c

C8 b

1-C8c

Cr/lig./Al

ref

15c 15e 15b 2 4 N-(i-Pr) 2 N-(i-Pr) 93

4590 2310 1130 15390 10760 4370 9580 38300 12900

11.1 18.6 32.8 23.7 21.9 28.4 23.5 20.0 19.2

90.4 98.4 97.2 64.8 64.6 80.6 50.2 70.5 55.7

17.6 12.2 13.4 51.5 61.2 62.2 56.8 72.4 45.4

85.7 60.3 66.9 96.0 94.0 97.3 94.5 97.2 96.4

1/1.75/70 1/1.75/70 1/1.75/70 1/2/600 1/2/600 1/2/600 1/2/600 1/2/600 1/3/300

23 23 23 22 22 22 22 22 69

Precursor, [CrCl3(THF)3]; Al = AlEt3; cyclohexane; reaction time, 30 min; temp, 60 °C; pressure, 50 bar. bWeight percent of the oligomeric mixture. cPercentage of 1-olefin within the corresponding fraction. dPrecursor, [CrCl3(THF)3]; Al = MAO; toluene; time, 1 h; temp, 60 °C; pressure: 30 bar. ePrecursor, [Cr(acac)3]; Al = MAO; toluene; time, 1 h; temp, 30 °C; pressure, 30 bar. fPrecursor, [Cr(acac)3]; Al = MAO; toluene; time, 30 min; temp, 65 °C; pressure, 30 bar. a

and aromatic spacer exhibited an activity 30% higher than that of ligand 290, which has a long and flexible aliphatic chain, and four times higher than that of the N-ethyl-linked bis-DPPA ligand 289 (Table 28, entries 1−3). Similar results were obtained when [Cr(acac) 3 ] was used instead of [CrCl3(THF)3]. The reaction conditions were found to affect both the activity and the selectivity of the process, and after optimization of the system, the use of ligand 289 in combination with [CrCl3(THF)3] and 180 equiv of MAO at 60 °C under 3 MPa of ethylene led to a good selectivity for the formation of 1-octene (Table 28, entry 4). Researchers from Sasol Technology also evaluated a large library of DPPA-type ligands for the Cr-catalyzed selective tetramerization of ethylene (see below), including the two bisDPPA-type ligands 290 and 292, with a hexyl or a 4,4′methylene-bis-cyclohexyl spacer, respectively.69 Both ligands led to slightly lower activities than their mono-DPPA derivatives (N-hexyl and N-cyclohexyl, respectively), however, with similar selectivities (Table 28 entries 5 and 6, and 7 and 8, respectively). The well-defined Cr(III) and mixed Cr(II)/Al(III) precatalysts (33−35, section 2.4) of the bis-DPPA-type ligand 24, in which the PNP moieties are linked through the 2,6-positions of a pyridyl ring, displayed rather similar activities [(59−70) × 103

g/(g Cr·h); 500 equiv of MAO; toluene; time, 30 min; temp, 80 °C; pressure, 40 bar] in ethylene oligomerization, affording a substantial amount of polyethylene (PE) and oligomers in the range C4−C18 in similar proportions (5−20 wt %).41 The authors proposed a possible ring expansion mechanism or the involvement of several active species to account for this unusual distribution. Jiang and colleagues also investigated the Cr-catalyzed selective tetramerization of ethylene with the triple-site DPPA derivatives 7 and 291, and again they observed that the catalytic performances were dependent on the reaction temperature, ethylene pressure, and Al/Cr and ligand/Cr ratios.25,125 The optimized reaction conditions were not the same for both systems, and while a higher temperature (60 °C vs 40 °C) was preferable for the acyclic ligand 7, a beneficial effect of increasing the Al/Cr ratio was observed for the cyclic ligand 291 (900 vs 300). The best results are reported in Table 28, entries 9 and 11 for 7 and 291, respectively. Interestingly, when the catalysis was performed with the preformed complex [(CrCl3)3(7)], instead of generated in situ, both the activity and selectivity were lower, which was attributed to its very low solubility (Table 28, entry 10 vs 9). 13.1.2. Catalytic Systems Involving Nitrogen-Based NFunctionalized DPPA-type Ligands. The influence of 9288

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Table 30. Relevant Results in the Cr-Catalyzed Ethylene Oligomerization with Ether, Thioether, and Silyl Ether NFunctionalized DPPA-type Ligands and Their Related N-Substituted DPPA-type Ligands entry a

1 2a 3a 4d 5d 6a entry 7e 8f 9g 10g 11g 12h

complex 67 67 N-CH2(o-Et)C6H4 67 N-CH2(o-Et)C6H4 67 ligand 258 N-pentyl 320 192 316 N-Me; P-Et2

activity, g/(g Cr·h)

C6b

1-C6c

C8b

1-C8c

P, bar

ref 64 64 64 64 64 64 ref 69 69 92 92 92 9

1080 7840 7060 8140 4600 28270 activity, g/(g Cr·h)

62 44 40

93 90 91

24 33 34

93 93 93

30 C6b

83 1-C6c

34 C8b

93 1-C8c

1.01 4.05 4.05 4.05 (5 h) 4.05 (5 h) 8.51 Cr/lig./Al

42500 43600 14660 18600 20340 4400

23.5 24.9 26.7 9.2 17.3 16.8

54.7 54.7 nci nci nci 64.6

55.2 58.1 55.5 33.3 35.3 45.2

96.4 96.8 98.5 97.0 97.8 97.4

1/3/300 1/3/300 1/2/440 1/2/440 1/2/440 1/2/300

Al = MAO (300 equiv); chlorobenzene; reaction time, 90 min; temp, 25 °C. bWeight percent of the oligomeric mixture. cPercentage of 1-olefin within the corresponding fraction. dAl = MAO (300 equiv); chlorobenzene; time, 5 h; temp, 25 °C. ePrecursor, [Cr(acac)3]; Al = MAO; toluene; time, 30 min; temp, 65 °C; pressure, 30 bar. fPrecursor, [CrCl3(THF)3]; Al = MAO; toluene; time, 1 h, temp, 65 °C; pressure, 30 bar. gPrecursor, [Cr(acac)3]; Al = MAO; toluene; time, 3 h; temp, 80 °C; pressure, 30 bar. hPrecursor, [Cr(acac)3]; Al = MAO; toluene; time, 30 min; temp, 45 °C; pressure, 45 bar. iNot calculated. a

formation of α-olefins (Table 29, entries 7 and 8).22 However, ligand 2 showed a higher temperature stability, maintaining its activity and selectivity up to 90 °C, while the activity of the i-Prsubstituted PNP ligand drastically dropped and the preference toward tetramerization was lost. The DPPA-type ligand 93, integrating a N-morpholinyl (mixed N- and O-donor) functional group, was evaluated in combination with [Cr(acac)3] and MAO and exhibited performances in the range of that of ligand 2 under similar reaction conditions (Table 29, entry 9 vs 7).69 DPPA- and DRPA-type (R = Et) ligands N-functionalized with a 2-pyridyl (27) or 2-ethylpyridyl (321) moiety, respectively, were also evaluated in combination with [CrCl3(THF)3] and an aluminum-based activator (AlEt3 for 27, MAO for 321) in ethylene poly/oligomerization, and while ligand 27 afforded 70 wt % PE (Cr/lig./AlEt3 1:1.75:70; cyclohexane; time, 30 min; temp, 60 °C; pressure, 50 bar),23 ligand 321 afforded mostly a liquid material (78 wt %), however, without significant selectivity (best selectivity, C8 19.3 wt %, 1-C8 96.2 wt %; conditions, Cr/lig./MAO 1:2:440, toluene, 3 h, 80 °C, 30 bar).92 13.1.3. Catalytic Systems Involving Ether, Thioether, and Silyl Ether N-Functionalized DPPA-type Ligands. In contrast to most of the studies in this field, which are based on multicomponent catalytic mixtures and in situ generation of the Cr (pre)catalyst, Bercaw and colleagues evaluated the welldefined [CrCl3(PNP)]2 complexes 64−67 with N-etherfunctionalized DPPA-type ligands (60a and 61−63, section 3.1).64 These catalysts were found to be highly active, stable, and selective for the trimerization and tetramerization of ethylene under mild conditions (temp, 25 °C; pressure, 1 bar). The most active and selective catalyst was that bearing the NCH2(o-OMe)C6H4-functionalized DPPA-type ligand (compd 67, lig. 63), the productivity (in gproduct/g Cr) and/or activity of which could be improved further by extending the reaction time, increasing the ethylene pressure, or changing the reaction solvent (C6H5Cl vs toluene, Table 30, entries 1, 2, 4, and 6). However, the selectivity toward 1-hexene dropped from 57.7 to 24.9 wt % (Table 30, entry 1 vs 6). Most interesting was the direct comparison of the performances of complex 67 with the

structural parameters of hydrazine derivatives of DPPA (13 and 15a−e, section 2.2) was recently investigated on the Crcatalyzed selective oligomerization of ethylene. Under the reaction conditions studied by Kim and colleagues, ligand 13 (N-NMe2) associated with [CrCl3(THF)3] (Cr/ligand/MAO = 1:2:60; toluene; time, 1 h; temp, 30 or 60 °C; pressure, 30 bar) led to a conventional ethylene oligomerization with Schultz− Flory distribution.22 Wasserscheid, McGuinness, and colleagues showed that the increase in steric hindrance from ligand 15c (N-NHMe) to 15a,b and 15d,e (N-NHR, R = t-Bu, Ph, Cy, iPr, respectively) induced a switch of selectivity from predominantly 1-octene to 1-hexene (Table 29, entries 1 and 2 for 15c and 15e, respectively, as representative examples).23 It should also be noticed that ligands 15a and 15c−e produced a large amount of polyethylene (47−63 wt %) and a significant proportion of higher oligomers (≥C10, 7−18 wt %). The catalytic system involving the aromatic substituted hydrazine group (15b) was found to be the most selective, however, toward trimerization vs tetramerization for most of the Cr/ (PNP)/Al systems, with an overall 1-hexene selectivity of 32 wt %, but it also exhibited the lowest activity (Table 29, entry 3). Under the same reaction conditions as described for 15a−e (see above), a nearly similar selectivity toward 1-hexene (28 wt %) was observed for 2, when an ethyl chain was introduced between the PNP moiety and the additional nitrogen donor [N-C2H4N(i-Pr)2, 2], as for the phenyl hydrazine derivative (NNHPh, 15b).23 The N-alkylamine (N-RNR′2) DPPA-type ligands 1−6 were evaluated in combination with [CrCl3(THF)3] and MAO for the selective oligomerization of ethylene, and with ligands 2 and 4, the selectivity toward the formation of 1-hexene and 1-octene was in the range of the benchmark N-(i-Pr)-substituted DPPA-type ligand evaluated under the same conditions (Table 29, entries 4 and 5 vs 6).22 Albeit the proportion of 1-hexene within the C6 fraction was higher for the latter, ligands 2 and 4 performed 2−3.5 times better at 60 °C. Noteworthy, ligand 6, produced under nearly similar conditions, led quasi-exclusively to polyethylene (>95 wt %). With [Cr(acac)3], the N-(i-Pr)-substituted DPPA-type ligand displayed a much higher activity than ligand 2 along with a higher preference toward ethylene tetramerization and the 9289

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Table 31. Relevant Results in the Cr-Catalyzed Ethylene Oligomerization with N-Functionalized DPPA-type Ligands of Substituted Aryls and Their Related N-Substituted DPPA-type Ligands entry

ligand

activity, g/(g Cr·h)

C6b

1-C6c

C8b

1-C8c

PEb

time, min

ref

a

N-Ph N-(t-Bu)C6H4 53 54 248 249 250 251 252 253 254 255 256 257 N-(3,5-Me2)C6H3

765 900 1 147 200 932 800 385 900 48 460 21 730 18 850 17 880 26 730 27 500 29 240 26 350 44 420 36 350 95 770

16.6 17.7 14.8 22.1 14.7e 19.8e 19.2e 19.5e 27.6e 24.2e 18.8e 23.5e 13.7e 20.3e 13.2e

54.2 53.5 53.5 40.2

61.8 62.3 53.3 58.2 64.8f 33.7f 31.6f 29.7f 33.8f 47.5f 57.5f 62.5f 62.7f 66.6f 71.0f

97.1 96.6 96.9 96.7

3.3 1.9 16.2 4.6 6.1 19.7 29.6 31.7 24.9 19.7 1.3 1.5 4.7 0.7 1.5

12 9 13 26 30 30 30 30 30 30 30 30 30 30 30

55 55 55 55 110 110 110 110 110 110 110 110 110 110 190

1 2a 3a 4a 5d 6d 7d 8d 9d 10d 11d 12d 13d 14d 15d

Precursor, [Cr(acac)3]; Al = MMAO; Cr/ligand/Al 1:1:480; methylcyclohexane; temp, 60 °C; pressure, 50 bar. bWeight percent of the oligomeric mixture. cPercentage of 1-olefin within the corresponding fraction. d[Cr(acac)3], Al = MAO, Cr/ligand/Al 1:1:300, cyclohexane, temp.: 60 °C, pressure: 30 bar. eOverall selectivity in 1-hexene (wt %). fOverall selectivity in 1-octene (wt %). a

13.1.4. Catalytic Systems Involving DPPA-type Ligands N-Functionalized with Substituted Aryls. Killian and colleagues studied the influence of the N-aryl substitution/ functionalization of a series of DPPA-type ligands in the Crcatalyzed tri- and tetramerization of ethylene, and in the context of this review, a comparison between the N-Ph and N-{p-(tBu)C6H4}-substituted DPPA-type ligands and the N-{p(MeO)C6H4} (53) and N-{p-(O2N)C6H4} (54, section 3.1) functionalized analogues highlighted a non-negligible influence of the electronic factors on activity and selectivity (Table 31, entries 1−4).55 The authors observed that the introduction of electron-rich substituents on the para-position of the aryl, i.e., tBu, led to a much higher activity and lower PE formation than the N-Ph ligand, but ligand 53, with the p-MeO group did not allow such improvements. This was attributed to possible side reactions of the OMe group with the aluminoxane-based activator (Table 31, entries 1−3). In contrast, the introduction of the electron-withdrawing nitro group (54) only slightly affected the activity of the catalytic system, but the selectivity toward the formation of C6 and C8 products strongly suffered from this modification, and a concomitant increase of the PE fraction was observed (Table 31, entry 4). Jiang and colleagues studied the catalytic performances of a series of halogen-substituted aryl N-functionalized DPPA-type ligands (248−257, section 6) as part of a tertiary catalytic system involving [Cr(acac)3] and MAO.110 After optimization of the reaction conditions with ligand 248, the authors could evidence the influence of the halogen substitution on the ligand under otherwise similar conditions (Table 31, entries 5−14). The steric bulk and electronegativity of the ortho-substituents was found to strongly affect the catalytic performances of the system; indeed, upon going from fluorine (248) to chlorine (249), a decrease of activity and selectivity toward the formation of 1-octene was observed (Table 31, entry 5 vs 6). This trend was also seen when going to the bulkier o-Br and o-I substituents [Table 31, entry 7 (250) and 8 (251)]. Concerning the meta-substituted ligands 252 and 253, both the catalytic activity and selectivity toward 1-octene were improved by an increase of the relative steric hindrance (Table 31, entry 9 vs 10). In the series of the dichloro-substituted

[CrCl3(PNP)]2 analog containing the benchmark ligands N-(iPr) and N-CH 2 (o-Et)C 6 H 4 (without additional donor function), which demonstrated an increased catalyst stability for the N-functionalized derivative, resulting in a higher activity/productivity (Table 30, entry 2 vs 3 and 4 vs 5). The DPPA-type ligands presenting an ether or silylether group as N-functionality (60b and 95, 96, respectively, section 3.1) did not lead to any catalytic activity under the conditions studied by Wasserscheid, McGuinness, and colleagues ([CrCl3(THF)3], ligand, 1.75 equiv; AlEt3, 70 equiv; cyclohexane; time, 30 min; temp, 60 °C; pressure, 50 bar).23 When compared to its N-pentyl analog, the N-alkoxysilylfunctionalized ligand 258 (section 7) did not improve either activity or selectivity in the Cr-catalyzed ethylene oligomerization (Table 30, entries 7 and 8). However, this functionality is designed for an anchoring of the (pre)catalysts into mesoporous materials to facilitate catalyst recycling, and in this respect, the non-interference (negatively) of the alkoxysilyl group constitutes a positive result.69 Among the different N-ether and N-thioether DRPA-type ligands [316, 317 (section 9, N-ether, R = Et) and 318−320 (section 9, N-thioether, R = Et), and 192 (section 5.2, Nthioether, R = Ph), respectively] evaluated by Weng, Hor, and colleague as catalysts for selective ethylene tetramerization, the N-thioether derivatives (318−320) exhibited a higher selectivity (33.3−55.5 wt %) toward octene formation than their N-ether (316 and 317) counterparts (35.3−37.5 wt %), along with a reduced formation of PE (4.9−24.3 wt % vs 26.4− 26.9 wt %, for the P-Et2 derivatives, Table 30).92 Interestingly, ligand 192, with PPh2 moieties, analogous to the most effective ligand 320 with PEt2 donors, showed a very different behavior, leading to a lower selectivity toward tetramerization (Table 30, entries 9 and 10) and forming 62.4 wt % PE (vs 4.9 wt %). Moreover, ligand 320 could be compared to a non-functionalized analog, with an N-Me substitution (also with P-Et2 groups), which was used under nearly similar reaction conditions.9 Ligand 320 led to better performances in terms of activity and selectivity toward ethylene tetramerization (Table 30, entry 9 vs 12). 9290

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Table 32. Relevant Results in the Ni-Catalyzed Ethylene Oligomerization with N-Functionalized DPPA-type Ligands, and Their Related N-Substituted DPPA-type Ligands entry

complex

activity, g/(g Ni·h)

C4 b

1-C4c

C6 b

1-C6c

Al

Al/Ni

ref

a

N-benzyl 46 159 160 39 39 160 197 198 197 198 225 328 DPPE DiPrPE

4790 5180 5430 3490 6080 11650 4400 3100 12900 40400 20000 14000 81000 5000 2000

76.9 87.4 84.6 88.3 78.8 98.5 95.9 77 58 70 79 92.4 71.7 93.2 79.1

48.9 61.8 34.7 58.9 22.5 3.4 100 16 9 16 26 57.7 20.9 38.0 67.7

23.1 12.6 15.4 11.7 21.2 1.5 4.1 21 36 26 20 6.6 17.9 5.3 15.3

13.2 24.3 7.8 27.8 8.6 1.3 94.3 nci nci nci nci nci nci nci nci

MAO MAO MAO MAO MAO AlEt2Cl MAO AlEtCl2 AlEtCl2 AlEtCl2 AlEtCl2 MAO MAO MAO MAO

400 400 400 400 400 200 400 3 3 40 40 300 300 300 300

43 43 43 43 43 43 43 94 94 94 94 106 106 106 106

1 2a 3a 4a 5a 6a 7d 8e 9e 10e 11e 12f 13g 14f 15f

Toluene; temp, 20 °C; time, 30 min; pressure, 1 bar. bWeight percent of the oligomeric mixture. cPercentage of 1-olefin within the corresponding fraction. dToluene; temp, −40 °C; pressure, 1 bar. eChlorobenzene; temp, 28−30 °C; time, 35 min; pressure, 10 bar. fToluene; temp, 45 °C; time, 1 h; pressure, 30 bar. gToluene; temp, 45 °C; time, 45 min; pressure, 1 bar. iNot calculated. a

%),191 the heterobimetallic (PNP)Cr/Ti complex 400 reported by Delferro, Marks, and colleagues yielded a nearly 50:50 mixture of PE and oligomers (0.58 vs 0.52 g, respectively), and within the latter, linear C4−C22 α-olefins (58.5 wt % of 1hexene in the oligomeric fraction) were detected (reaction conditions: 400/MAO 1:500, toluene, 80 °C, 8.1 bar).165

ligands 254−257, those presenting ortho-substituents, 254 and 255, led to the worst activities, which is consistent with the behavior observed for the monosubstituted derivatives (see above). However, steric factors hardly explain the differences in activity/selectivity observed for the 3,5- (256) and 3,4disubstituted (257) ligands, which led to the highest activity and the highest 1-octene selectivity, respectively among this series of halogen-substituted DPPA-type ligands (Table 31, entries 13 and 14). The authors concluded that steric factors clearly affect both the activity and the selectivity within the ligand family studied. However, to determine if electronic factors also affect the performances of N-aryl-substituted/ functionalized DPPA-type ligands in the Cr-catalyzed ethylene oligomerization, the authors compared the X-substituted ligands 248, 249, 252, 253, and 256 (X = F, Cl, Br) to their Me-substituted analogues and found that the increase of electron density on the benzene ring (Me substitution) improved both activity and selectivity toward the formation of 1-octene.110 As a representative example of this trend, the performances of the N-(3,5-Me2)C6H3-substituted ligand are reported in Table 31 (entry 15) for direct comparison with its N-(3,5-Cl2)C6H3 analog (256, Table 31, entry 13). 13.1.5. Catalytic Systems Involving Other N-Functionalized DPPA-type Ligands. Blann and colleagues provided a direct comparison between the [CrCl3(PNP)]2 complex 286 (section 7) of the N-SiMe3-functionalized DPPA-type ligand 285a and its N-(t-Bu) analog, because the in situ procedure could not be applied due to the possible instability of ligand 285a in the presence of the organoaluminum cocatalyst.69 Under similar conditions, both complexes exhibited similar activities [2.07−2.25 kg/(g Cr·h)]. In terms of selectivity, 286 was slightly more selective toward tetramerization [65.7 wt % (97.7% of α) vs 59.8 wt % (99.6% of α)], but the N-(t-Bu) derivative was much more selective toward the formation of 1hexene [19.5 wt % (56.1% of α) vs 30.7 wt % (93.9% of α)]. Complex 286 also produced a considerable amount of C6 cyclic products (41.6 vs 6.1 wt %), while both complexes only afforded a small amount of PE. While the PNP/Cr system was reported to achieve among the highest selectivity for the formation of 1-hexene (>97 wt

13.2. Nickel-Catalyzed Olefin Poly/Oligomerization

The effect of the N-substitution on the catalytic performances in a series of [NiBr2(PNP)] complexes (39, 2-picolyl, section 2.4; 46, furfuryl, section 3.1; 159, 160, 2-methyl- and 2ethylthiophene, section 5.1) in ethylene oligomerization was investigated by Gao, Wu, and colleagues, and all complexes were found to be highly active and yielded mainly butene and a small amount of hexene (Table 32).43 The authors observed an increase of activity in relation with the increase of basicity of the N-functionalized pendant group, i.e., N-benzyl < furfuryl (46) < 2-methylthiophene (159) < 2-picolyl (39), but such a trend could not be evidenced for the selectivity (Table 32, entries 1− 5). However, an interesting point was the different selectivity observed between the 2-methylthiophene (159) and 2ethylthiophene (160) NiBr2 complexes, with an increase of α-selectivity (1-C4, 34.7 vs 58.9 wt %; 1-C6, 7.8 vs 27.8 wt %) upon lengthening of the spacer between the PNP moiety and the additional S donor (Table 32, entry 3 vs 4). These observations clearly illustrate how subtle changes on the Npendant group can affect the catalytic activity of this class of compounds and, moreover, the beneficial effect of the introduction of an additional donor. The catalytic performances of these systems were also affected by the reaction conditions, since (i) the catalytic activity and selectivity toward the formation of butene (lower selectivity to 1-butene) for complex 39 could be improved by using AlEt2Cl as cocatalyst instead of MAO (Table 32, entry 6 vs 5) and (ii) an impressively selective catalytic system for the formation of 1-butene (95.9 wt %) was achieved when using complex 160 at −40 °C (Table 32, entry 7). Fliedel, Braunstein, and colleagues aimed at determining if the nature of the spacer between the PNP moiety and the sulfur atom in the [NiCl2(PNP)] complexes 197 and 198 obtained from the N-alkyl- and N-aryl-thioether ligands 192 and 196 9291

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(section 5.2), respectively, influences their catalytic performances in the selective oligomerization of ethylene.94 It was found that even the C4 and α-selectivity of complex 197 were not affected by the Ni/Al molar ratio (C4, 70−77 wt %; α, 16− 17 wt %), while those of complex 198 varied much more (C4, 58−79 wt %; α, 4−26 wt %, Table 32). Also better activities were observed for the N-aryl complex 198 than for the N-alkyl complex 197 at lower AlEtCl2/Ni ratios (3−10 equiv), but the opposite trend was observed at higher AlEtCl2/Ni ratio, with, for example, an activity for 197 twice that of 198 using 40 equiv of the Al cocatalyst (Table 32, entries 8−11). The influence of the chelate ring-size was also evaluated by comparing the catalytic performances of the complexes [NiCl2(P,P)] (fourmembered metallacyle) with those of the monosulfide derivatives [NiCl2(P,PS)] (five-membered metallacyle); however, systems involving a P(V) atom are outside the scope of the present review. Both series of [NiBr2(PNP)] complexes (225 and 327, and 328, 329) of the corresponding symmetrical N−(PR2)2 and unsymmetrical (R′2P)−N−(PR2) N-sulfonyl DRPA-type ligands (R and R′ = Ph, Cy, i-Pr) were evaluated in ethylene oligomerization with MAO as activator and compared to the [NiCl2(DPPE)] (DPPE = Ph2PC2H4PPh2) and [NiBr2(DiPrPE)] [DiPrPE = (i-Pr)2PC2H4P(i-Pr)2] systems (Table 32).106 Along this series, the nature of the P-substituents was found to have a greater impact of the catalytic performances than the variation of the para-substituent of the N-sulfonyl functionality. As a result, the bis-PPh2 derivative 225 exhibited an activity at least 5 times lower than that of the bisP(i-Pr)2 (327) or the unsymmetrical P(i-Pr)2/PPh2 (328) and PCy2/PPh2 (329) analogues, however, with a much higher selectivity toward dimerization [C4, 92.4 wt % (225) vs 68.1− 71.7 wt % (327−329)] and the formation of α-butene [1-C4, 57.7 wt % (225) vs 20.9−22.3 wt % (327−329)]. The complexes [NiCl2(DPPE)] and [NiBr2(DiPrPE)] exhibited C4selectivities similar to those of the bis-PPh2 derivative 225 and complexes 327−329, respectively, however, with 3−40 times lower activities. The complex [NiCl2(53)] containing an N-(p-OMe)aryl functionalization was found to polymerize norbornene (NBE) in the presence of MAO as cocatalyst.54 The activity and control of the process were found to depend on the concentration of the catalyst, the Ni/Al molar ratio, and the reaction temperature. The best results (activity, 9 850 g/g Ni· h−1; Mn, 573 kg·mol−1; Mw/Mn, 2.48) were obtained with [Ni] = 0.32 mmol·L−1 and 500 equiv of MAO at 0 °C.

blocks (13C NMR evidence). The GPC-measured molecular weights (Mn = 104 470 g·mol−1 for an activity of 4190 gpolymer/ mol Ti·h) and polydispersity (1.57 < Mw/Mn < 1.65) of the polymer support the involvement of a single-site catalyst. The authors observed that both the increase of the MAO/Ti molar ratio (up to 1 000) and of the reaction temperature (ranging from 0 to 60 °C) improved the activity of the catalyst.156 The homoleptic [Ln{N(PPh2)2}3] 356−358 and 362 and the heteroleptic rare-earth-metal complexes 377−381 were evaluated in the polymerization of ε-caprolactone (CL), and the former series of complexes exhibited a very high activity, converting up to 150 equiv of CL at room temperature within 1 min to polycaprolactone (PCL), with a relatively good control of the process [Mn(exp)/Mn(theor), 1.06−1.55; 1.12 < Mw/Mn < 1.83], and noticeably, the nature of the metal center only slightly affected the catalytic behavior.140 In contrast, within the series of heteroleptic [LnX(N,N)(P,NDPPA‑type)] complexes 377−381, the catalytic activities were dependent on the ionic radius of the metal center. The lanthanum (378) and neodymium (379) complexes gave high conversions within 5 min (>80 °C conversion of 307 equiv of CL), whereas longer reaction times were needed for the yttrium (377) and ytterbium (380) analogs (5 min, 5-12%; 120 min, 58−94% of ≈300 equiv of CL). The mismatch between the experimental molecular weight of the polymer and their expected values and relatively broad PDIs (1.75 < Mw/Mn < 4.4) suggests only a moderate control of the process.152 The [LnX(N,N)(P,NDPPA‑type)] complexes 377−381 were also tested in methyl methacrylate (MMA) polymerization, and again, the ionic radius of the metal center was found to impact their catalytic performances.152 While all the tests could be carried out in toluene for complexes 378−381, the yttrium complex 377 was only active in THF, giving rise to an illdefined, but syndiotactically enriched, material at low temperature (−78 °C) and in the presence of 9 equiv of AlMe3. The lanthanum complex 378 was the only one exhibiting some activity without the need of any cocatalyst; however, the use of 6−9 equiv of AlEt3 considerably improved its activity (conversion of 1406 vs 113 equiv of MMA at −78 °C). The use of 9 equiv of AlEt3 and 6 equiv of AlMe3 with complexes 379 and 380 (toluene, −78 °C), respectively, also led to efficient catalysts for the production of high molecular weight and syndiotactically enriched (rr ≥ 70) polymers with higher Mn values than expected but relatively narrow polydispersity (Mw/Mn < 1.4).

13.3. Other Applications in (Co)Polymerization

Carbon−carbon bond formation through metal-catalyzed crosscoupling reactions stands as a powerful tool in organic synthesis and is now routinely employed in both academia and industry. For this purpose, palladium remains the most studied transition metal, and while the use of N-heterocyclic carbene (NHC) ligands has attracted increasing attention over the last 2 decades,192,193 phosphines still occupy a dominant place as ancillary ligands.194 This section will summarize the activity of Pd-based catalysts of N-functionalized DPPA-type ligands and tentatively highlight the influence of the nature of the Nfunctional group. The results obtained with complexes 37 (N-2-picolyl), 47 (N-2-furfuryl), and the N-benzyl-substituted DPPA-type ligand allowed a direct evaluation of the influence of an additional function on such ligands in C−C coupling reactions.42,51 The presence of an N- or O-heterocycle was not significantly

13.4. Cross-Coupling Reactions

A mixture of the bis-DPPA-type ligand 298 and 2 equiv of [Pd(OAc)2] in the presence of trifluoroacetic acid (4 equiv) and excess ethanol was evaluated in the copolymerization of CO and ethylene or 2,5-ethylidenenorbornene (ENB); the resulting polyketones represent a innovative class of low-cost thermoplastics.127 The copolymerization of ethylene/CO by the in situ generated Pd catalyst resulted after 1 h in the formation of a polymer exhibiting a molecular weight of 780 g· mol−1 and a polydispersity of 1.38 (conditions: CH2Cl2, 40 bar of CO/C2H4 1:1, 90 °C). In contrast, the ENB/CO copolymerization led to a mixture of tri- and tetramers after 6 h reaction (conditions: CH2Cl2, 40 bar of CO, 90 °C). The Ti(IV) complex 387 was applied, in combination with MAO, in the polymerization of propylene, and it led to an elastomeric polypropylene containing atactic and isotactic 9292

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Table 33. Catalytic Performances of the Pd(II) Complexes of N-Functionalized DPPA-type Ligands, and Their Related NSubstituted DPPA-type Ligands in Suzuki Coupling Reactionsa

% yield

a

entry

complex

R=H

1 2 3 4 5 6 7 8 9 10

37 47 N-benzyl 11 N-fluorene 83 92·Pd 303 294 9

88 84b 90 74 76 93 78 87 99 62

R = Me

R = OMe

R = CHO

R = COMe

temp, °C

time, h

ref

54c

94 61c 89

74 96 86 93 90 88 96 90 99 98

94 96 93 92 89 95 93 98 98 95

80 60 80 80 80 80 80 80 80 70

2 0.6 2 2 2 2 1.5 1.5 1 1

42 51 42 26 26 66 68 68 122 24

89 63 61 54 60

73 71 57 71

Ar−Br, 1 equiv; Ar−B(OH)2, 1.5 equiv; Cs2CO3, 2 equiv; Pd cat., 1 mol %; dioxane. bTime, 30 min. cTime, 1 h.

Table 34. Catalytic Performances of the Pd(II) Complexes of N-Functionalized DPPA-type Ligands, and Their Related NSubstituted DPPA-type Ligands in Heck Coupling Reactionsa

% yield

a

entry

complex

R=H

R = Me

R = OMe

R = CHO

R = COMe

temp, ° C

time, h

ref

1 2c 3 4c 5c 6 7c 8c

37 47 N-benzyl 11 N-fluorene 83 294 9

31 75d 55 79 73 30 59 68

9 50e 17

37b 55e 85b

37 54 60

65 48 55

39 92 84 94 95 76 93 96

41 94 95 94 92 65 92 91

80 100 80 120 120 80 110 100

6 0.25 6 1.5 1.5 6 1 1

42 51 42 26 26 66 122 24

Ar−Br, 1 equiv; styrene, 1.5 equiv; K2CO3, 2 equiv; Pd cat., 1 mol %; dioxane. bTime, 16 h. cIn DMF. dTime, 30 min, eTime, 1 h.

Table 35. Catalytic Performances of the Ru(II) (entries 1−4), Rh(III) (entries 5 and 6), and Rh(I) (entries 7 and 8) Complexes of N-Functionalized DPPA-type Ligands, in Catalytic Transfer Hydrogenation Reactionsa

% conversion/TOF, h−1b entry

compd

R=H

c

52 165 161 162 51 164 50 163

99/17 98/16

1 2c 3d 4e 5 6 7 8 a

97/582 99/33 98/33 98/98 99/99

R = p-F

99/99 99/594 98/49 99/50

R = p-Cl

98/98 98/588 98/33 98/33

R = p-Br

98/98 97/582 98/33 99/33

R = p-OMe

95/95 95/570 98/5 99/5

R = o-F

94/16 96/24

R = o-Br

96/24 98/33

R = o-MeO

ref

97/97 96/576 99/10 97/10

53 53 87 86 53 53 52 52

Ar−C(O)Me, 100 equiv; i-PrOH (solvent), excess; NaOH, 5 equiv; cat., 1 equiv; temp, 82 °C. bTOF = (mol product/mol cat.) × h−1. cTime, 6 h. Time, 1 h. eTime, 10 min.

d

beneficial, neither in Suzuki nor Heck coupling reactions, whereas the (N-2-picolyl)-functionalization (37) was detrimental in the Heck reaction of all studied substrates (Tables 33 and 34, entries 1−3). A comparison between complex 11 (N-4diphenylamine) and the N-2-fluorene derivative could not evidence beneficial effects in Suzuki and Heck coupling

reactions resulting from the introduction of an arylamine on the N-tail of DPPA-type ligands (Tables 33 and 34, entries 4 and 5), and as expected, the best yields were obtained for electron-deficient aryl bromides (R = CHO and COMe).26 A series of PdCl2 complexes of mono- and dimethoxyphenyl mono-DPPA- (83−86), dimethoxybiphenyl bis-DPPA-type 9293

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(92·Pd) and biphenyl bis-DPPA-type ligands (303) provided another point of comparison of different functionalities, and while complexes 83−86 exhibited very similar performances, the bis(PNP)PdCl2 complex, in which each DPPA-type unit coordinates one PdCl2 moiety, was found to be slightly less efficient toward the more electron-rich substrates (R = H, Me, OMe, Table 33, entries 6−8).66,68 Nevertheless, these results clearly showed that the introduction of two OMe groups on the biaryl linker of bis-DPPA ligands has no effect on their performances in Suzuki coupling reactions.68 Comparison between the catalytic performances of the bis- and tris-DPPAtype (PdCl2)2 and (PdCl2)3 complexes 294 and 9, respectively, showed that no significant improvement of the activity resulted from the presence of one more catalytic site (Table 33, entries 9 and 10 and Table 34, entries 7 and 8).24,122 However, a higher yield was obtained for R = H with the bis-PdCl2(PNP) complex 294 than for the tris-PdCl2(PNP) complex 9 (99 vs 62%), but the opposite trend was observed for R = Me (71 vs 57%) in Suzuki coupling under nearly similar reaction conditions. This difference of activity was less pronounced in Heck coupling. The nickel dichloride complex 284 was evaluated in Kumada coupling reaction as homogeneous catalyst or immobilized onto SBA-15, and these results are discussed in the following section 14.117

constants, from various arylboronic acids (18 examples, yields 45−95%) under an oxidizing (O2) environment (CHCl3; bubbling O2; cat., 1 mol %; NEt3, 30 mol %; rt; time, 24 h).84 Mixtures of phenol and coupling products were obtained when the reaction was carried out in more polar solvents (e.g., EtOH, DMF). The authors performed a series of experiments that led to the suggestion that the initial formation of a palladium− peroxo species was involved in the reaction mechanism and that the phenol products resulted from reductive elimination in a cis-[Pd(Ar)(OH)(P,P)] intermediate. 13.6.3. Addition of β-Diketones to 1-Alkynes. In a preliminary test involving various phosphine Ru complexes, the complex 246 ([N-(p-CF3)C6H4] functionalization, section 6) was found to be the most efficient for the addition of acetylacetone to phenylacetylene, exhibiting a conversion 22% higher than the N-(n-Bu)-substituted analog (Scheme 73).108

13.5. Transfer Hydrogenation of Ketones

a

Scheme 73. Ru-Catalyzed Addition of β-Diketones to 1Alkynesa

Conditions: cat., 0.005 mmol; diketone, 1.25 mmol; alkyne, 1.5 mmol; temp, 120 °C; time, 3−24 h.

Catalytic transfer hydrogenation via a hydrogen donor reagent, typically primary and secondary alcohols or formic acid and its salts, constitutes a valuable alternative method to catalytic hydrogenation by molecular dihydrogen because of the reduced risks and costs associated. Catalytic transfer hydrogenation generally involves late transition metals such as Rh, Ru, and Ir. Aydemir, Baysal and colleagues observed no influence of the N-functional group [furan (52) vs thiophene (165) derivatives, Table 35, entry 1 vs 2] in the transfer hydrogenation of ketones mediated by Ru(II) complexes;53 however, the nature of the coligands in [RuCl(Cp*)(157)] (165), [RuCl2(157)2] (161), or [RuCl(p-Cym)(157)] (162) clearly influenced the activity of the complex (Table 35, entries 2−4).86,87,201 Complex 162 was the most active catalyst, reaching turnover frequencies (TOFs) of nearly 600 h−1 and allowing a wide range of functionalization of the acetophenone substrate.86 The catalytic performances of the Rh(III) complexes 51 and 164, which exhibited TOF values twice those of their Ru(II) analogues (52 and 165), were also evaluated toward various substituted acetophenone derivatives (Table 35, entries 5 and 6).53 Under similar conditions, the bis-chelate cationic [Rh(PNP)2]BF4 rhodium(I) complexes 50 and 163 afforded quantitatively phenylethanol from acetophenone with TOFs that were 3 times higher (Table 35, entries 7 and 8). However, no difference of activity was observed between the systems involving the N-furfuryl (45) and N-2-methylthiophene (157) ligands.

The reaction was tolerant to several variations even on the alkyne (electron-rich and -poor aryl-, benzylalkynes, etc.) and the β-diketone substrates (13 examples, yields 69−93%), and an intramolecular addition was also accessible, but with a higher catalyst loading (33 mol %) and moderate yield (20% after 40 h reaction). 13.6.4. Anti-Markovnikov Addition of Secondary Amines to Aromatic 1-Alkynes. The ruthenium complex 246 ([N-(p-CF3)C6H4] functionalization section 6), already active in addition of β-diketones to 1-alkynes (see above), was also found to be effective in hydroamination of terminal alkynes with secondary amines (only) and led exclusively to the trans anti-Markovnikov products (Scheme 74).108 Scheme 74. Ru-Catalyzed Hydroamination of 1-Alkynes with Secondary Aminesa

a

Conditions: cat., 0.01 mmol; amine, 1 mmol; alkyne, 1.2 mmol; temp, 120 °C; time, 24 h.

13.6.5. Isomerization of Terminal Alkenes. The titanium complex 387 (section 11.5) and the mixture 387/ MAO (1:400) were found to isomerize 1-octene into trans-2octene (95%) and to isomerize allylbenzene into transmethylstyrene (77% in 16 h) in toluene at 80 °C, respectively.156 The treatment of trans-2-octene with the 387/MAO (1:400) mixture resulted in slow isomerization to cis-2-octene (conversion up to 5% after 16 h at 80 °C), with traces of oligomeric compounds, indicating that the process is in equilibrium.

13.6. Other Catalytic Applications

13.6.1. Hydrogenation of Alkenes. The tetra-Rh(I) complex 307 (section 8.2) was shown to catalyze the hydrogenation of styrene to afford quantitatively ethylbenzene (THF, 800 kPa H2 pressure, 48 h, rt).127 13.6.2. Hydroxylation of Arylboronic Acids to Phenols. The PdCl2 complex 151 (section 4.1), with a dangling PPh2 moiety, was found to catalyze the formation of phenols as the only products in solvents with low dielectric 9294

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Scheme 75. Strategies for the Anchoring of Metal Complexes, Clusters, or Colloids inside the Nanopores of Alumina Membranes

14. APPLICATIONS IN MATERIALS SCIENCE AND HETEROGENEOUS CATALYSIS As mentioned earlier, specific N-functional groups on DPPAtype ligands can allow the anchoring of the resulting ligands and/or metal complexes onto a mesoporous matrix or a metal surface. In 2000, Braunstein, Schmid, and colleagues reported complementary strategies for the anchoring of molecular palladium complexes, of cobalt or platinum clusters, or of gold colloids inside the nanopores of alumina membranes. Strategy A (Scheme 75) consisted of the condensation of the alkoxysilyl group of the metal P,P-coordinated functionalized DPPA ligands (Ph 2 P) 2 N(CH 2 ) 3 Si(OMe) 3 (258) and (Ph2P)2N(CH2)4SiMe2(OMe) (262) (Figure 7, section 7) with the hydroxy groups covering the pores of an alumina membrane and was compared with approach B consisting of derivatizing first the pore walls with the functional ligands, followed by reaction of the supported ligand with metal complexes, clusters, or colloids.112 Specific advantages that one strategy may have were illustrated in the case of the cluster [Co4(CO)8(μ-DPPA){μP,P-(Ph2P)2N(CH2)4SiMe2(OMe)}], which has a too limited solubility and is possibly also too bulky for a successful application of strategy A. However, its anchoring was

successfully achieved by using the second method B, which consisted of derivatizing first the pore walls (Scheme 75). Following strategy B, platinum clusters (diameter ca. 2 nm) and gold colloids (diameter ca. 13 nm) were successfully immobilized after passing their solution through the functionalized membrane pores.112 The successful anchoring of cluster [Co4(CO)10(μ-DPPA)] into the pores of a mesoporous silica matrix of the type SBA-15, that has been derivatized by (Ph2P)2N(CH2)3Si(OMe)3 (258) according to strategy B, led to an organometallic hybrid mesoporous silica, the thermal treatment of which led to pure nanocrystalline Co2P particles.195 The positive confinement effect brought about by the ordered matrix was evidenced by comparison with the particles obtained in a silica xerogel; those synthesized into the SAB-15 matrix were more regular in spatial repartition, size (ca. 60 Å in diameter), and shape. For comparison, the cluster [Co4(CO)10(μ-DPPA)] was inserted in the pores of nonfunctionalized SBA-15 by means of wet impregnation. Upon thermal activation at 650 °C, Co2P nanoparticles were obtained outside the pores and almost no nanoparticles could be observed in the pores. At 800 °C, Co2P nanoparticles were observed both inside and outside the pores. Their characterization was performed by microscopy in conventional and 9295

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of the medium. The heterogeneous catalyst could be recycled up to three times without visible loss of activity (42% < conversion < 45%), while it drops after the fourth (35%) and fifth (23%) runs, under more diluted conditions. Catalyst leaching was evaluated after each of the first four catalytic cycles, and the amount of Ni released represented about 13, 13, 10, and 6% (ca. 40% in total), respectively, of the initial amount of Ni used, without apparent loss of activity, which suggested that both the leached and anchored Ni species retained their activity. Treatment of Al2O3 with ligand (Ph2P)2N(CH2)3Si(OMe)3 (258) in benzene at 85 °C overnight led to the corresponding supported ligand 258−{Al2O3}, which was further applied to the catalytic Suzuki coupling reaction in combination with Pd(OAc)2.199 Under the optimized conditions [258−{Al2O3}, 0.0492 mmol; Pd(OAc)2, 0.025 mmol; K3PO4·3H2O, 3 mmol; ArB(OH)2, 1.5 mmol; Ar′Br, 1 mmol; toluene; temp, 65 °C; time, 8 h) this system converted various aryl bromides and boronic acids into the coupling products, in moderate to excellent yields, and the in situ generated catalyst could be recovered and reused four times, but with a loss of activity from 99% (first run) yield to 79% (third run) and 70% (fifth run). Following the stepwise strategy B described by Braunstein, Schmid, and colleagues (see above and Scheme 75),112 Wang and colleagues anchored a NiBr2 catalyst onto silica, which was further evaluated in the Biginelli reaction (Scheme 77).200 The reaction products of a series of aromatic aldehydes, ethyl acetoacetate, and (thio)urea were obtained in good to excellent yields, and the silica-supported nickel catalysts could be recovered and recycled up to five times without significant loss of activity (yields 95−89%). The authors proposed a reaction mechanism involving as the first step, a nickelcatalyzed condensation of the (thio)urea with the aldehyde, which dehydrates to an N-acylimine intermediate. Subsequent attack of the enol form of the β-keto ester to the latter generates an open chain ureide, which readily cyclizes to form the Biginelli product. Braunstein, Doudin, and colleagues reported that the bischelated dicationic Pd(II) complexes 206 and 207 of the Nthioether-functionalized DPPA-type ligand 192 and 196 respectively, can be used as metalloligand to pair Janus-Aucoated silica microspheres, through stable S···Au interactions (Figure 20, top).102 The resulting assemblies, in which the mutual orientation of the spheres’ gold coating is due to their connection by the rigid metalloligand, can be assessed with optical microscopy (Figure 20, bottom left), and the pairing of the microspheres was further established by scanning electron microscopy (SEM, Figure 20, bottom right). For comparison, the concept of visual assessment of molecular interconnects was extended to a series of difunctional molecules, e.g. dithiols and

scanning modes, electron tomography, energy-dispersive X-ray spectroscopy, and magnetic measurements.196 The influence of the Co/P ratio on the nature of the nanoparticles was examined by thermal treatment of bulk samples of the clusters [Co4(CO)10(μ-DPPA)] (Co/P = 2) and [Co4(CO)8(μ-DPPA)2] (Co/P = 1) used as single-source precursors. Whereas the former led to Co2P only, the latter gave a mixture of CoP (major) and Co2P.197 Through elegant studies by 31P CP/MAS spectroscopy, Blümel and colleagues studied the immobilization of the isolated Ni(0) dicarbonyl complex 274 containing the Nalkoxysilyl DPPA-type ligand 259 onto neutral and rigorously dried alumina and observed the exclusive formation of the supported catalyst 274−{Al2O3}.111 No uncomplexed phosphine nor substantial amount of side products were detected, and the procedure was then applied to the analogous complex 275 containing ligand 261, in which the flexible n-propyl linker between the PNP moiety and the alkoxysilyl group of 259 has been replaced by a rigid aromatic phenyl. A contrario, all attempts to immobilize the Ni(0) complexes 274 and 275 onto silica instead of alumina were unsuccessful. A series of experiments, consisting of mixing the N-alkoxysilyl DPPAtype ligands 259 and 261 or simpler model compounds with silica allowed the authors to point out a series of side reactions leading to the degradation of the PNP ligand. However, the supported catalysts were tested for the cyclotrimerization of phenylacetylene, and complex 274−{Al2O3} performed favorably in comparison with analogous complexes of alkoxysilylphosphine ligands with other linkers.198 More recently, the NiCl2 complex 284 containing the Nalkoxysilyl DPPA-type ligand 258 could be anchored onto SBA15 molecular sieves, affording the potential heterogeneous catalyst 284−{SBA} for the Kumada coupling reaction (Scheme 76), as confirmed by 31P MAS NMR analysis [δ = Scheme 76. Kumada Coupling Reaction Performed by Homogeneous 284 and Immobilized 284−{SBA} Ni Complexes of N-Functionalized DPPA-type Ligandsa

a

Conditions: cat., 1 mol %; 1-iodo-4-tert-butylbenzene, 1 mmol; ptolylMgBr, 1.2 mmol; THF, 3 or 0.8 mL; time, 1 h; rt.

41.8 ppm (284−{SBA}) vs 42.8 ppm (284)].117 Under similar reaction conditions, the heterogeneous system gave lower substrate conversion than the homogeneous catalyst (42 vs 79%), however, with slightly higher selectivity, i.e., a lower amount of the homocoupling products. Noteworthy, with the supported catalyst 284−{SBA}, the conversion could be increased to 64% by a three-fold increase of the concentration

Scheme 77. Biginelli Reaction Catalyzed by the Supported Ni Catalyst 401a

a

Conditions: cat., 0.15 mmol; aromatic aldehyde, 1 mmol; urea or thiourea, 1 mmol; EtOH, 5 mL; time, 5 h; temp, reflux. 9296

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or in homogeneous catalysis have rather rarely been offered but often remain speculative, in particular when this function does not interact directly with the metal center(s) in the groundstate structures. However, when this function is anchored onto an oxide or a metal surface, its contribution to the development of hybrid catalysts or of functional layers becomes obvious. From the range of functional DPPA-type ligands already available, the synthetic strategies that can be readily employed to increase their diversity, and the numerous fields of applications of their mono- and polynuclear metal complexes, one can anticipate promising and exciting further developments in chemistry and at the interface with other disciplines in the years to come. We hope that this review will contribute to enhance the interest of the scientific community in this topic.

AUTHOR INFORMATION Corresponding Authors

*C.F. e-mail: christophe.fl[email protected]. *A.G. e-mail: aghisolfi@unistra.fr. *P.B. e-mail: [email protected]. Figure 20. (Top) Square-planar Pd(II) complex with chelating Nthioether-functionalized DPPA-type ligands (192 or 196) used to pair Janus-Au-coated silica microspheres, through stable S···Au interactions. (Bottom left) Janus spheres functionalized with complex 207, after CH2Cl2 washing, showing agglomerates of microspheres. (Bottom right) SEM micrographs of partially gold coated microspheres treated with a solution of 207. The spheres are mutually connected through their Au hemispheres. Scale bars are 10 μm (left) and 200 nm (right). (Reproduced with permission from ref 102. Copyright Wiley-VCH Verlag GmbH & Co, KGaA).

Present Address §

Groupe Photocatalyse et Photoconversion, Institut de Chimie et des Procédés pour l’Energie, l’Environnement et la Santé (ICPEES), UMR 7515 CNRS/Université de Strasbourg, 25, Rue Becquerel, 67087 Strasbourg Cedex, France. Notes

The authors declare no competing financial interest. Biographies Christophe Fliedel obtained his Ph.D. from the University of Strasbourg (Strasbourg, France) in 2010 under the supervision of Dr. Pierre Braunstein, working on S-functionalized N-heterocyclic carbene (NHC) and diphosphine DPPA-type ligands and their metal complexes. He then moved to the New University of Lisbon (Lisbon, Portugal), where he was awarded a postdoctoral fellowship from the Fundaçaõ para a Ciência e a Tecnologia (FCT, Portugal) in 2011, to conduct research focused on the synthesis of highly Lewis acidic complexes and their use in ring opening polymerization of cyclic esters, in the groups of Prof. Teresa Avilés (Universidade Nova de Lisboa) and Dr. Samuel Dagorne (Université de Strasbourg). Early in 2016, he joined the group of Prof. Rinaldo Poli at the Laboratoire de Chimie de Coordination (LCC) in Toulouse, France, as a CNRS researcher. His research interests are mainly focused on organometallic and coordination chemistry and controlled radical polymerization.

diamines, but the metalloligand was found to be the best choice in the series.

15. CONCLUSION Encouraged by the long-standing interest of the chemical community in diphosphine ligands, in particular of the shortbite type, we have attempted to provide a comprehensive review on DPPA-type ligands and their various applications in coordination/organometallic chemistry, catalysis, and materials sciences, to highlight the considerable developments that have occurred during the past few years. The diversity of chemical functions that have been attached to the nitrogen atom of DPPA turns the precursor diphosphine into potentially multidentate ligands able to bridge and/or chelate metal centers and to generate complexes with very diverse and often unprecedented structures, endowed with interesting reactivity, catalytic properties, and applications, in which the anchoring ability of the function connected to the DPPA nitrogen atom opens new possibilities in surface and materials sciences. Although the development of functionalized DPPA-type ligands has been mostly motivated by the desire to attach the corresponding donor function to metal centers while the chelating or bridging ability of the diphosphine backbone is retained, examples have emerged where this additional donor group is actually not observed to bind to metal centers but is capable of significantly affecting its stoichiometric or catalytic reactivity. Such conclusions can be reached after a careful comparison with related DPPA complexes and between molecular systems that only differ by the nature of the functionality attached to the N atom of DPPA. Explanations for the role of this additional function in molecular rearrangements

Alessio Ghisolfi was born in 1985 in Cremona, Italy. In 2010, he received his B.S. degree in general and inorganic chemistry from the University of Parma (Parma, Italy), under the supervision of Profs. Antonio Tiripicchio and Daniele A. Cauzzi. He then joined the Coordination Chemistry Laboratory of the University of Strasbourg (Strasbourg, France) directed by Dr. Pierre Braunstein, where he obtained his Ph.D. in 2014 on the applications of functional diphosphine ligands and quinonoid zwitterions in coordination chemistry and surface functionalization. He continued his research on organometallic chemistry in this laboratory as a post-doc. After a short industrial experience, he became a post-doc early in 2016 in the Photocatalysis and Photoconversion Group of the Institut de Chimie et des Procédés pour l’Energie, l’Environnement et la Santé (ICPEES), in Strasbourg, under the direction of Dr. Valérie Keller-Spitzer. His research interests include chemical synthesis, homogeneous and heterogeneous catalysis, and surface functionalization. 9297

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NBE PCC% PE PMDTA p-Cym ROP rt THT Ts XRD NOTE:

Pierre Braunstein obtained his Dr. Ing. and Doctorat d’Etat from the Université Louis Pasteur (ULP) in Strasbourg, France. He spent the academic years 1971−1972 as a post-doctoral fellow at University College London (with Profs. R. S. Nyholm and R. J. H. Clark) and 1974−1975 as a A. von Humboldt fellow at the Technical University of Munich (with Prof. E. O. Fischer). He remained within the CNRS, at the University of Strasbourg, for his whole carreer and became director of research exceptional class. He is now emeritus CNRS research director and professeur conventionné of the University of Strasbourg. His main research interests lie in the inorganic and organometallic chemistry of the transition and main group elements [(co)authoring over 550 scientific publications and review articles] and include, for example, the synthesis and coordination/organometallic chemistry of (hetero)dinuclear and cluster complexes, heterofunctional phosphine and NHC ligands, and quinonoid zwitterions and the study of hemilabile metal−ligand systems. Applications range from homogeneous catalysis to nanosciences. He has received numerous national and international awards and is member of various academies, including the French Académie of Sciences and the German National Academy of Sciences Leopoldina and is head of the Chemistry Division of the European Academy of Sciences.

norbornene percentage of pyramidal character polyethylene N,N,N′,N″,N″-pentamethyldiethylentriamine p-cymene [1-methyl-4-(1-methylethyl)benzene] ring-opening polymerization room temperature tetrahydrothiophene tosyl X-ray diffraction Unless otherwise specified, 31P and 13C refer to decoupled 31P{1H} and 13C{1H} NMR experiments.

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ACKNOWLEDGMENTS We are grateful to the CNRS, the Ministère de la Recherche (Paris), the DFH/UFA (International Research Training Group 532-GRK532, Ph.D. grant to A.G.), and the Fundaçaõ para a Ciência e Tecnologia (FCT) (fellowship SFRH/BPD/ 73253/2010 to C.F.) for funding. DEDICATION Dedicated to Prof. em. Dr. Max Herberhold, on the occasion of his 80th birthday (14 July 2016), with our most sincere congratulations and best wishes. ABBREVIATIONS USED acac acetylacetonate av average Bu butyl COD 1,5-cyclooctadiene Cp* 1,2,3,4,5-pentamethylcyclopentadiene CP-MAS cross-polarization magic-angle spinning 12-crown-4 1,4,7,10-tetraoxacyclododecane 18-crown-6 1,4,7,10,13,16-hexaoxacyclooctadecane DiPrPE 1,2-bis(diisopropylphosphino)ethane DME 1,2-dimethoxyethane DMSO dimethyl sulfoxide DPPA bis(diphenylphosphino)amine DPPA-Me bis(diphenylphosphino)(N-methyl)amine DPPE 1,2-bis(diphenylphosphino)ethane DPPM bis(diphenylphosphino)methane EA elemental analysis ENB 2,5-ethylidenenorbornene ESI-MS electrospray ionisation mass spectrometry FAB-MS fast atom bombardment mass spectrometry FT-IR Fourier transform infrared spectroscopy GPC gel permeation chromatography LAO linear α-olefin MALDI-TOF matrix-assisted laser desorption/ionization MAO methylaluminoxane MMA methyl methacrylate MMAO modified methylaluminoxane NBD 2,5-norbornadiene 9298

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