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Frustrated Lewis Pair vs Metal−Carbon σ‑Bond Insertion Chemistry at an o‑Phenylene-Bridged Cp2Zr+/PPh2 System Zhongbao Jian, Constantin G. Daniliuc, Gerald Kehr, and Gerhard Erker* Organisch-Chemisches Institut, Westfälische Wilhelms-Universität Münster, Corrensstraße 40, 48149 Münster, Germany S Supporting Information *

ABSTRACT: Methyl anion abstraction from (odiphenylphosphino)phenyl(methyl)zirconocene by trityl tetrakis(pentafluorophenyl)borate gives the o-phenylenebridged Zr+/P system 10. It behaves toward a variety of reagents as a typical Zr+/P frustrated Lewis pair (FLP). It undergoes cooperative 1,4-addition reactions to some chalcone derivatives and adds in a 1,2-fashion to a variety of organic carbonyls and to several heterocumulenes. The reactive Zr−C σ bond of the FLP 10 remains intact in these reactions. Complex 10 splits dihydrogen, but subsequently the Zr−C σ bond is protonolytically cleaved in this case. Only a few special reagents, among them carbon monoxide, undergo the usual insertion reaction into the Zr−C(aryl) σ-bond of the Zr+/P system 10.

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CO2, NO, SO2),11 olefins, alkynes, conjugated olefinic compounds, organic carbonyls, etc. The system 1 served as a template for CO reduction and ultimately even for the unique head to head coupling of CO and NO.12 Wass et al. had described the FLP chemistry of a variety of intramolecular Zr+/PR2 FLPs, which showed a number of similar reactions with H2, OCR2, CH2CH2, HCCR, CO2, etc.13,14 Some reactions were slightly different in detail, but overall there was a marked similarity between the chemistry of the pure main group element containing FLPs and the group 4 transition metal/phosphane hybrid systems. The Zr+/PR2 FLP compounds 3 and 4 contained an inert Zr−O linkage. We have recently described two Zr+/PPh2 FLP systems 5 and 6 that contained Zr−C bonds and, thus, featured a slightly more Lewis acidic (although considerably more sensitive) metallocene cation.15,16 We have now prepared an o-phenylenebridged Cp2Zr+/PPh2 FLP and found it to undergo a variety of typical frustrated Lewis pair reactions, despite the fact that it contained an active metal−C(sp2) σ bond. Mostly the FLP chemistry prevailed in this system toward a variety of reagents, but in a few cases we could observe the alternative typical reaction pathway of the metal σ-aryl moiety. This will be described and discussed in this account.

INTRODUCTION Small-molecule binding and/or activation by frustrated Lewis pairs (FLPs) has seen quite some development in the past decade.1 Mostly, phosphane (or amine)/borane pairs have been involved,2 but lately there is an increasing tendency toward the use of other Lewis acid functionalities, e.g. aluminum compounds3 or strongly electrophilic halogenated phosphonium Lewis acids.4 Although FLP chemistry had been a domain of main group element chemistry, there are some noteworthy developments using transition-metal-derived Lewis acidic functionalities. Probably, most visible in this area are the FLPs that contain the cationic group 4 metallocene Lewis acid building blocks,5,6 but group 4 metal systems with other ligands7 and FLPs with Lewis acid compounds derived from other metal atoms have also been reported.8,9 Intramolecular Lewis acid/Lewis base combinations attached at suitable organic backbones have considerably contributed to the rapid development of frustrated Lewis pair chemistry. A noteworthy example from our group is the ethylene-bridged P/ B FLP 1 (and its derivatives; see Scheme 1).10 It is an active H2-splitting reagent but also was shown to add a variety of small molecules, such as e.g. fundamental element oxides (CO,

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Scheme 1

RESULTS AND DISCUSSION Formation of the Zr+/P FLP 10. The new o-phenylenebridged zirconocene cation/phosphane Lewis pair was prepared starting from Cp2Zr(CH3)Cl (7), which in turn was obtained by treatment of the μ-oxo-metallocene complex (Cp2ZrCl)2(μO) with trimethylaluminum according to a literature Received: October 31, 2016

© XXXX American Chemical Society

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DOI: 10.1021/acs.organomet.6b00828 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

The cation shows a 31P NMR resonance at δ −26.6 and the sharp NMR signals of the pair of symmetry-equivalent Cp ligands at zirconium at δ 6.45 (1H, relative 10 H intensity)/δ 116.7 (13C). The 1H NMR resonances of the o-phenylene moiety were located at δ 7.72 (5-H), 7.48 (3H), 7.42 (4-H), and 7.19 (2-H) (corresponding 13C NMR signals: δ 193.8 (1C), 138.6 (2-C), 132.88 (3-C), 129.3 (4-C), 132.83 (5-C), and 114.6 (6-C)). The [B(C6F5)4]− anion shows characteristic heteronuclear magnetic resonance signals at δ −16.7 (11B) and δ −133.1, −163.7, and −167.6 (19F), respectively. Wass et al. had reported the formation of a rather unstable THF adduct of 3 which subsequently opened the THF ligand. In the case of the Cp* derivative the THF ring opening was fast.13b Complex 10 formed the stable THF adduct 11,23 which we isolated in 89% yield. Single crystals of complex 11 suitable for an X-ray crystal structure analysis were obtained from a dichloromethane solution layered with cyclopentane. In the crystal the THF ligand is found coordinated to zirconium in the σ-ligand plane at the front side of the bent metallocene wedge. It occupies one of the lateral positions. The central position is taken by the phosphane moiety, which has a contact with the metal atom inside the THF-stabilized bent metallocene cation moiety (see Figure 2).

procedure.17 Compound 7 was treated with the Ph2Psubstituted aryllithium reagent 8.18 Workup of the reaction mixture furnished the metallocene complex 9 as colorless crystals in 73% yield (see Scheme 2). The X-ray crystal Scheme 2

structure analysis showed all three coordination sites at the front of the bent metallocene wedge being occupied.19,20 The internal −PPh2 donor was found occupying the central position in the σ-ligand plane21 flanked by the Zr1−C22 σ-aryl and the Zr1−C1 σ-alkyl bonds (see Figure 1). In solution (benzene-d6)

Figure 1. Molecular structure of complex 9 (thermal ellipsoids are shown with 15% probability). Selected bond lengths (Å) and angles (deg): Zr1−Ct (Cp centroid), 2.200 and 2.215; Zr1−C22, 2.382(3); Zr1−C1, 2.372(3); Zr1−P1, 2.885; P1−C21, 1.796(3); C1−Zr1− C22, 131.4(1); Zr1−C22−C21, 113.5(2); C22−C21−P1, 105.7(2); C21−P1−Zr1, 83.1(1).

Figure 2. Molecular structure of complex 11 (only the cation is depicted; thermal ellipsoids are shown with 15% probability). Selected bond lengths (Å) and angles (deg): Zr1−C22, 2.357(5); Zr1−O1, 2.388(3); Zr1−P1, 2.818; P1−C21, 1.793(5); C21−C22, 1.398(6); O1−Zr1−C22, 135.5(1); O1−Zr1−P1, 76.8; O1−Zr1−C22−C21, −11.0(4); O1−Zr1−P1−C21, −179.6(2); Zr1−C22−C21−P1, 8.0(3).

complex 9 features 1H/13C NMR signals at δ 5.53/δ 107.7 of the Cp rings. It shows six separate C(aryl) 13C NMR resonances of the o-phenylene moiety (Zr−C(aryl) at δ 192.3) and the typical 1H/13C NMR signals of the [Zr]−CH3 methyl group at δ 0.19 (d, JPH = 16.6 Hz) and δ 15.1 (d, JPC = 7.5 Hz). The 31P NMR signal of complex 9 is at δ −34.5. For further details see the Supporting Information. Abstraction of the methyl anion equivalent from zirconium w a s e ff e c t e d by t r e a t m e n t w i t h t r i t y l t et r a k i s (pentafluorophenyl)borate.6,22 The reaction was carried out in bromobenzene solution at room temperature (5 min). Workup, including removal of the Ph3C−CH3 byproduct by washing with pentane, gave the salt 10 in 85% yield (see Scheme 2).

In solution (CD2Cl2) we monitored the 1H NMR Cp signal at δ 6.15 and the CH signals of the o-phenylene unit, partially overlapping with the −PPh2 signals. The 31P NMR resonance was found at δ −40.9 and we also found the typical NMR features of the [B(C6F5)4]− anion. 1,2-FLP Additions to Carbonyl Compounds. Wass’ Cp*-containing FLP 3 was reported to react with acetone by zirconium enolate formation.13b Main group element frustrated Lewis pairs often undergo 1,2-addition reactions to the CO functionality of ketones and aldehydes. This is especially typical for many intramolecular P/B FLPs.24 The Cp2Zr+/PPh2 system 10 shows a similar behavior toward a variety of organic carbonyl compounds. B


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Organometallics The Zr+/P FLP 10 was generated in situ by treatment of the neutral precursor 9 with the trityl salt in bromobenzene (room temperature, 5 min) as described above. Then a 2-fold excess of acetone was added. After 5 min, pentane was added and the product precipitated as a pale yellow oil. Workup gave the Zr+/ P acetone addition product 12a as a colorless solid in 86% yield (Scheme 3). It was characterized by C, H elemental analysis, by Scheme 3

Figure 3. View of the acetone addition product 12a (the cation and anion are depicted; thermal ellipsoids are shown with 30% probability).

81% yield. The complex shows a single 1H NMR Cp singlet and a slightly broadened singlet of the introduced formaldehyde derived [O]CH2 moiety. The corresponding 13C NMR resonance was located at δ 67.2 (d) with a 1JPC coupling constant of 64.4 Hz (see Scheme 3; for further details concerning the spectroscopic characterization of 12b and the subsequently described carbonyl addition products, see Table 2 and the Supporting Information). The addition reaction of the Zr+/P FLP 10 to the CO group of benzaldehyde, p-chlorobenzaldehyde, and ferrocene (Fc) carbaldehyde proceeded analogously and gave the heterocycles 12c−e in good yield. These complexes show spectra similar to those of 12b, except that we observe the 1 H/13C NMR signal pairs of diastereotopic Cp ligands at zirconium in each case due to the formation of a chiral carbon center as the result of the phosphane addition to the carbonyl carbon atom of these prochiral reagents. 4-Phenylbutynone could in principle undergo a 1,2- or a 1,4addition of the reactive Zr+/P pair.15,25 Under our typical reaction conditions (room temperature, 5 min) we observed the 1,2-carbonyl addition product 12f, which we isolated as a solid in 84% yield. The 13C NMR spectrum clearly showed the presence of the free phenylacetylide substituent (PhCC−: δ 93.4 (3JPC = 9.5 Hz), 87.2 (2JPC = 5.1 Hz)) at the former carbonyl carbon atom (δ 78.3, 1JPC = 70.4 Hz) that is now found bonded to the phosphorus atom. The presence of the newly formed carbon chiral center makes the Cp ligands at zirconium as well as the phenyl substituents at P pairwise diastereotopic, and we recorded the respective pairs of NMR resonances of these respective groups (for details see Table 2 and the Supporting Information). Complex 12f was also characterized by X-ray diffraction (see Figure 4 and Table 1). The structure features a six-membered heterocyclic benzannulated core that contains the metal atom (Zr1), the −O1−C1− unit derived from the ynone reagent, and the phosphorus atom (P1). It shows a distorted-half-chair conformation. The former carbonyl carbon atom C1 bears a methyl and the phenylacetylide substituent (C1−C2, 1.481(2) Å; C2−C3, 1.197(2) Å; C1−C2−C3, 174.0(2)°; C2−C3−C51, 179.1(2)°). trans-Cinnamaldehyde also undergoes a 1,2-carbonyl addition reaction with the Zr+/P FLP 10. Under our typical reaction conditions, the α,β-unsaturated aldehyde reacted within

Table 1. Selected Structural Data of the 1,2-Carbonyl Addition Products 12a,f,ga R1 R2 Zr1−C(Cp)b Zr1−O1 Zr1−C22 P1−C21 O1−C1 C1−P1 Zr1−O1−C1 O1−C1−P1 C1−P1−C21 O1−Zr1−C22 Zr1−O1−C1−P1 O1−C1−P1−C21 Zr1−C22−C21−P1

12a

12f

12g

CH3 CH3 2.526(3) 1.969(2) 2.337(3) 1.812(3) 1.397(4) 1.877(3) 146.1(2) 103.7(2) 109.7(1) 85.9(1) 57.1(4) −53.2(2) −5.8(4)

CH3 CCPh 2.506(2) 1.993(1) 2.343(1) 1.801(1) 1.396(2) 1.874(1) 140.1(8) 103.1(9) 108.7(6) 86.9(4) 64.2(1) −62.1(1) −11.6(2)

CH3 CHCHPh 2.511(6) 1.999(4) 2.328(5) 1.800(5) 1.393(6) 1.851(5) 130.2(3) 104.1(3) 106.8(2) 88.5(2) −75.3(4) 68.5(4) 9.1(7)

a Bond lengths in Å and angles in deg. bAverage over all these Zr1− C(Cp) distances.

X-ray diffraction (see Table 1 and Figure 3), and by NMR spectroscopy (see Table 2). The product 12a shows the 1H NMR CH3 signal (relative 6H intensity) of the newly incorporated acetone moiety at δ 1.74. It shows coupling with the adjacent phosphorus atom (3JPH = 15.6 Hz). The adjacent quaternary acetone derived carbon atom shows a 13C NMR resonance at δ 83.1 (d, 1JPC = 56.9 Hz). The X-ray crystal structure analysis of complex 12a confirms the Zr+/P addition to the carbonyl function of the acetone reagent. It shows the newly formed central benzannulated sixmembered heterocycle featuring the new Zr1−O1 and P1−C1 linkages. The six-membered ring shows a rather flat distorted half-chair conformation. In the crystal we also observe the crystallographically independent [B(C6F5)4]− counteranion (see Figure 3). We then reacted the Zr+/P FLP 10 (in situ generated) with a small series of aldehydes. Treatment of 10 with paraformaldehyde (room temperature, 20 min) resulted in the formation of the heterocyclic addition product 12b, which we isolated in C


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Organometallics Table 2. Selected NMR Data of the Compounds 12a−ga

a

12a

12b

12c

R1, R2

CH3, CH3

H, H

H, Ph

CpA CpB R1

6.11 6.11 1.74

6.14 6.14 5.36

6.41 6.21 6.64

CpA CpB Zr−C(aryl) P−C(aryl) O−C(R1,R2)

114.4 114.4 186.2 123.2 83.1

114.4 114.4 186.2 122.0 67.2

114.5 114.4 186.5 124.1 82.9

31.0

25.0

28.0

12d H, (p-Cl)Ph 1 H NMR 6.40 6.19 6.58 13 C NMR 114.6 114.5 186.4 123.8 82.3 31 P NMR 27.6

12e

12f

12g

H, Fc

CH3, CCPh

H, CHCHPh

6.47 6.17 6.32

6.28 6.10 2.05

6.37 6.11 6.26

114.4 114.3 186.3 123.8 82.1

115.0 114.6 186.3 123.1 78.3

114.6 114.5 186.1 123.9 80.9

25.9

29.6

24.2

In CD2Cl2; chemical shifts in ppm, δ scale.

Figure 5. View of the trans-cinnamaldehyde 1,2-addition product 12g (only the cation is depicted; thermal ellipsoids are shown with 30% probability). Figure 4. Molecular structure of complex 12f (only the cation is depicted; thermal ellipsoids are shown with 50% probability).

Scheme 4

minutes with 10 to give the 1,2-carbonyl addition product 12g, which we isolated as a white solid in 87% yield. Single crystals suitable for an X-ray crystal structure analysis were obtained at room temperature from layering a solution in dichloromethane with cyclopentane. In the crystal state complex 12g shows the typical benzannulated six-membered heterocyclic core (see Figure 5 and Table 1) with a trans-styryl substituent at carbon atom C1 (C2−C3, 1.322(8) Å; Zr1−O1−C1, 130.2(3)°). In solution the cation of complex 12g shows the 1H NMR signals of two diastereotopic Cp ligands at zirconium. The trans-CHCH− substituent moiety gives rise to 1H NMR resonances at δ 6.50 and 6.07 (3JHH = 18.6 Hz) and a 31P NMR signal at δ 24.2. 1,4-Enone Addition. Main group element FLPs often undergo 1,4-addition reactions not only to conjugated dienes, enynes, or diynes26 but also to α,β-unsaturated carbonyl compounds.25,27 In addition, this can sometimes be observed for transition metal derived frustrated Lewis pair systems.15,28 We have reacted the Zr+/P FLP 10 with chalcone. The reaction again proceeds rapidly at room temperature in bromobenzene solution, and we isolated the 1,4-addition product 13a as a white solid in 85% yield (see Scheme 4).

Complex 13a shows a 31P NMR resonance at δ 24.2. It features the 1H/13C NMR singlets of the pair of diastereotopic Cp ligands at zirconium (1H, δ 6.43, 5.97; 13C, δ 114.4, 113.9). We locate the C(sp3)−H NMR signal of the newly introduced −OC(Ph)CH−CH(aryl)− unit at δ 5.89 (2JPH = 20.4 Hz) and the 1H NMR resonance of the adjacent C(sp2)−H at δ 5.46 (3JPH = 5.4 Hz) (with corresponding 13C NMR features at δ 49.3 (1JPC = 38.7 Hz), 97.9 (2JPC = 8.2 Hz), and 163.9 (O−C, 3 JPC = 8.6 Hz)). We carried out the analogous reaction with p-chlorochalcone under our typical reaction conditions, and we isolated the 1,4addition product 13b in 88% yield. In this case we obtained D


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structure analysis (for further details and the depicted structure, see the Supporting Information). Reactions with Heterocumulenes. P/B FLPs are known to add either to the CO30 or (in some rare cases) to the C NR bond31 of isocyanates. The Zr+/P complex 10 (in situ generated under our typical conditions) shows FLP behavior toward phenyl isocyanate and adds to the CO bond. We isolated the 1,2-CO addition product 15a in 80% yield and characterized it by C, H, N elemental analysis, by NMR spectroscopy, and by X-ray diffraction (single crystals were obtained from a two-layer CH2Cl2/cyclopentane mixture at room temperature) (see Scheme 6).

single crystals that were suitable for a characterization of the product by X-ray diffraction (see Figure 6).

Scheme 6

Figure 6. Molecular structure of the Zr+/P 1,4-enone addition product 13b (one cation of two found in the asymmetric unit is depicted; thermal ellipsoids are shown with 30% probability). Selected bond lengths (Å) and angles (deg): Zr1−C22, 2.350(5); Zr1−O1, 1.965(3); O1−C3, 1.344(5); C3−C2, 1.335(6); C2−C1, 1.494(6); C1−P1, 1.891(4); Zr1−O1−C3, 153.2(3); O1−C3−C2, 119.8(4); C3−C2− C1, 125.0(4).

The X-ray crystal structure analysis has confirmed the 1,4addition reaction of 10 to the enone. We find an eightmembered heterocyclic framework that has the Zr1−C22− C21−P1 unit oriented in an almost coplanar fashion. The metal atom has formed a bond to the carbonyl oxygen atom, and the phosphorus nucleophile has added to the “Michael position” of the conjugated enone. The O1−C3−C2−C1 unit is also oriented in an almost coplanar fashion. The angle between these two planar frameworks is 33°. The former carbonyl carbon atom (C3) bears the phenyl substituent and carbon atom (C1) has the p-chlorophenyl substituent attached to it. In solution (CD2Cl2) complex 13b features a 31P NMR resonance at δ 25.5 and 1H and 13C NMR signals similar to those of 13a (e.g., 13b: δ 6.42, 5.96 (1H, Cp); δ 184.7 (2JPC = 32.4 Hz, Zr−Caryl), 163.9 (O−C, 3JPC = 9.0 Hz); for further details see the Supporting Information). Reaction with an Alkyl Halide. We have also treated the in situ generated Zr+/P system 10 with tert-butyl chloride. This resulted in the liberation of isobutene and addition of H− Cl13b,29 to the Zr+/P pair to form the respective Zr−Cl/PH+ product 14 (see Scheme 5). Complex 14 shows a sharp 1H NMR Cp singlet at δ 6.25 (in CD2Cl2, 13C δ 115.6) and the phosphonium PH+ doublet δ 8.78 (1JPH = 505.8 Hz, 31P δ 7.6). Complex 14 was isolated as a pale yellow solid in 83% yield. The compound was also characterized by a X-ray crystal

In the crystal complex 15a shows the typical six-membered heterocyclic core that is characteristic for the CO addition products of 10. The ring features a distorted half-chair conformation. The exocyclic CN−Ph unit shows a short bond length in the CN double-bond range. It possess an E configuration and has the plane of the phenyl substituent at nitrogen rotated in an almost perpendicular fashion to the σ plane of the adjacent CN moiety (see Figure 7). In dichloromethane-d2 solution complex 15a shows a sharp 1 H NMR Cp singlet of 10 H relative intensity at δ 6.13 (corresponding 13C NMR Cp resonance at δ 116.2). The N C−O 13C NMR carbon resonance occurs at δ 152.0. It shows a large phosphorus coupling constant of 1JPC = 152.8 Hz (31P NMR signal at δ 18.5). We also reacted the in situ generated Zr+/P FLP 10 with phenyl isothiocyanate under similar conditions and isolated the 1,2-thiocarbonyl addition product 15b as a yellow solid in 82% yield. The reaction is slightly slower than that of 10 with phenyl isocyanate. Complex 15b was also characterized by X-ray diffraction; the structure is depicted in the Supporting Information. The structure is analogous to that of complex 15a. Complex 15b has NMR data similar to those of 15a (15b: δ 6.27/115.2 (1H/13C of Cp), δ 186.5 (2JPC = 30.0 Hz, Zr− Caryl)). It shows the characteristic 13C NMR resonance of the Zr−SCN unit at δ 159.6 with the typical large phosphorus coupling constant of 1JPC = 120.0 Hz (31P NMR resonance at δ 27.4). We obtained evidence that the Zr+/P FLP 10 even reacted with carbon dioxide in the same way: namely, by 1,2-carbonyl addition to generate 16. However, we could not isolate the CO2 adduct because it proved to be unstable in the absence of a CO2 atmosphere. Our attempt to remove the solvent from the reaction mixture inevitably also resulted in CO2 cleavage with

Scheme 5

E


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Organometallics

bond chemistry. This is indeed the case. We had reacted the system 10 with a variety of reagents that could have attacked the active Zr−C(sp2) σ bond but did not. However, with a few reagents we saw typical zirconium σ-hydrocarbyl insertion chemistry. This was the case in the reaction of 10 with carbodiimides. We reacted the in situ generated zirconium cation complex 10 in bromobenzene solution with 1 molar equiv of dicyclohexylcarbodiimide. The reaction was complete within minutes at room temperature, and we obtained the insertion product 18a initially as a red oil, which could be solidified during the workup process (see Scheme 7). It was isolated in a ca. 70 mg amount in 87% yield. It was characterized by C, H, N elemental analysis, by NMR spectroscopy, and by X-ray diffraction. Scheme 7

Figure 7. View of the phenyl isocyanate addition product to the Zr+/P FLP 10 (only the cation of 15a is depicted; thermal ellipsoids are shown with 50% probability). Selected bond lengths (Å) and angles (deg): Zr1−O1, 2.041(2); O1−C1, 1.324(3); C1−N1, 1.269(3); C1− P1, 1.833(2); C22−Zr1−O1, 88.8(1); Zr1−O1−C1, 131.1(1); C1− N1−C51, 119.5(2).

The X-ray crystal structure analysis of complex 18a revealed that the dicyclohexylcarbodiimide reagent had not undergone the Zr+/P FLP addition reaction but inserted into the Zr− C(aryl) σ bond. The Zr−C(sp2) σ bond was ruptured, and we found the −C6H4−PPh2 residue attached via a new carbon− carbon σ-bond (C1−C22: 1.506(10) Å) at the former central carbodiimide carbon atom. The resulting amidinato ligand is found symmetrically κ2 coordinated to zirconium (Zr1−N1/ N2, 2.195(4) Å; C1−N1, 1.344(7) Å; C1−N2, 1.328(8) Å; N1−C1−N2, 115.1(4)°) (see Figure 8). The 13C NMR spectrum of complex 18a in CD2Cl2 solution shows the absence of the typical Zr−C(aryl) resonance. Instead we have monitored six 13C NMR signals of the o-phenylene moiety in a range between δ 136.8 and 128.1, all with distinct JPC coupling constants (for details see the Supporting Information). Complex 18a shows the 1H/13C NMR signals

re-formation of the starting material 10. However, we could spectroscopically characterize the CO2 addition product 16 from in situ experiments. Complex 16 shows a 13C NMR Zr− OC(O)−P signal originating from the carbon dioxide reagent at δ 162.4 with the typical large 1JPC = 116.5 Hz coupling constant. The corresponding 31P NMR signal was observed at δ 12.1, and we have monitored the 1H/13C NMR Cp signals at δ 6.35/116.7, respectively. The 13C NMR Zr− Caryl resonance of complex 16 occurs at δ 186.6 (2JPC = 29.0 Hz). Although we could not isolate the FLP CO2 addition product 16, we were able to trap it by the reaction with Piers’ borane [HB(C6F5)2].32 We could show that compound 10 itself did not react with Piers’ borane under our typical reaction conditions (see the Supporting Information for details). However, when we exposed a 1:1 mixture of the Zr+/P FLP 10 and HB(C6F5)2 to CO2, we isolated the Zr+/P-bonded CO2 reduction product 17 in 86% yield. The 1H NMR spectrum of complex 17 shows the signal of the newly introduced [P]− OCH−O[B] moiety at δ 6.85 with a 2JPH coupling constant of 26.0 Hz (corresponding 13C δ 101.8). Because of this newly introduced center of chirality complex 17 contains a pair of diastereotopic Cp ligands at zirconium (1H, δ 6.10, 6.08; 13C, δ 115.5, 114.9 (in CD2Cl2)). The 31P NMR resonance of 17 occurs at δ 24.3 and the cation of complex 17 now contains a −OB(C6F5)2 moiety (11B: δ 42.8; 19F: −129.9 (o), −145.4 (p), −159.2 (m), Δδ19Fm,p = 13.8 ppm). In addition we have also monitored the typical 11B and 19F NMR resonances of the [B(C 6F 5) 4]− counteranion (for further details see the Supporting Information). We could not reduce compound 16 with 9-BBN. Insertion Reactions. The o-phenylene-bridged Zr+/P system 10 has shown a number of typical frustrated Lewis pair reactions as described above. However, the system still contains an active Zr−C(sp2) σ-bond and, consequently, could principally be able to show some specific group 4 metal−carbon

Figure 8. Projection of the molecular structure of the carbodiimide insertion product 18a (only the cation is depicted; thermal ellipsoids are shown with 15% probability). F


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Organometallics

which is a typical chemical shift value of an alkenyl zirconium complex. The carbonyl carbon 13C NMR resonance was located at δ 195.8, as expected for a Zr+ Lewis acid coordinated carbonyl functionality. Complex 23 shows a 31P NMR resonance at δ 0.5. Complex 23 was also characterized by X-ray diffraction. The X-ray crystal structure analysis showed that carbon monoxide had been inserted into the metal−carbon bond of the Zr− C6H4PPh2 unit of complex 10 and then the resulting η2-acyl zirconocene moiety had inserted phenylacetylene to form the metallacyclic core structure of complex 23. The carbonyl oxygen is strongly coordinated to the metal center, while the −PPh2 group is free. The phenylacetylene insertion had been regioselective, positioning its phenyl substituent at the position α to the metal center (see Figure 9). The cation and anion are well separated in the solid state.

of a pair of diastereotopic Cp ligands at zirconium. The amidinato carbon 13C NMR resonance occurs at δ 158.9 (NCN), and we have located the 31P NMR resonance of complex 18a at δ −16.1. The reaction of 10 with diisopropylcarbodiimide gave a similar result (see Scheme 7). We isolated complex 18b, the product of heterocumulene insertion into the Zr−C(aryl) σ bond, as a red solid in 83% yield. Complex 18b shows a 31P NMR signal at δ −15.9 and the 13C NMR amidinato carbon resonance at δ 158.9 (NCN). Complex 18b also shows the 1 H/13C NMR signals of a pair of diastereotopic Cp ligands. We monitored a single 1H/13C NMR resonance of the isopropyl CH unit but the corresponding signals of pairs of isopropyl CH3 groups of these substituents, which indicates an axially prochiral structure of complex 18b in solution analogous to that we had found in 18a (for details of the NMR analysis, see the Supporting Information). Complex 18b turned out to be stable in solution at 80 °C for a prolonged time (3 h); it did not convert to the respective FLP addition product. Similarly, the isocyanate FLP addition product 15a (see above) did not interconvert with a respective insertion isomer under similar conditions. These two reaction branches seem to be well separated from each other in these systems. Jordan et al. had shown by NMR spectroscopy that the alkenyl zirconium cation 19 sequentially inserted CO and an acetylene to form 21 (via 20).33 Our o-phenylene-bridged Zr+/ P system 10 reacts similarly. Exposure of the in situ generated complex 10 to CO (1.5 bar, room temperature, 10 min) rapidly gave the CO insertion product 22 (see Scheme 8). Complex 22 Scheme 8

Figure 9. Molecular structure of complex 23 (only the cation is depicted; thermal ellipsoids are shown with 15% probability). Selected bond lengths (Å) and angles (deg): Zr1−O1, 2.135(3); O1−C9, 1.280(5); C8−C9, 1.424(6); C7−C8, 1.359(6); C7−Zr1, 2.319(5); C7−Zr1−O1, 74.1(1); Zr1−O1−C9, 119.3(3); O1−C9−C8−C7, 3.7(7); ∑P1CCC, 306.9.

was isolated in 80% yield. It was characterized by C, H elemental analysis and by NMR spectroscopy. In CD2Cl2 solution it features a 13C NMR carbonyl resonance at δ 302.7, which is very typical for a η2-acyl metallocene type structure.21,34,35 The signal shows a JPC coupling constant of 16.9 Hz, in contrast to only negligible JPC coupling in the related complex 23 (see below). Therefore, we assume some Zr···P contact in complex 22 (the 31P NMR resonance of 22 is at δ 42.8). The acyl zirconium complex 22 shows sharp 1H/13C NMR signals of the Cp ligands at zirconium (δ 5.80/δ 110.6). The IR (CO) band of 22 was located at ν 1643 cm−1. Complex 22 was then treated with phenylacetylene (in dichloromethane solution, room temperature, 12 h). Workup gave the alkyne insertion product 23 in 82% yield. It shows 1 H/13C NMR signals at δ 6.50 (10 H relative intensity)/δ 117.7. The 13C NMR feature of the newly introduced  C(sp2)−[Zr] carbon atom at zirconium occurs at δ 256.8,

Reaction with Dihydrogen. The CO/PhCCH coupling product 23 reacted slowly with dihydrogen (CD2Cl2, 1.5 bar, room temperature, 72 h). Workup gave the product 26 in 85% yield (see Scheme 9). We assume a pathway which proceeds by means of protonolytic cleavage of the reactive Zr−C bond of the starting material to generate the enone/[Cp2Zr−H]+ pair 25, which then directly gives the observed enone reduction product 26. The actual dihydrogen splitting reaction may have made use of the Zr+/P frustrated Lewis pair to initiate the sequence. Complex 26 shows single 1H and 13C NMR Cp resonances. The 31P NMR resonance occurs at δ 32.4. We have located 13C NMR signals of the newly formed Zr−enolate moiety at δ 158.0 (O−C) and δ 108.1 (CH−CH2−), respectively. The corresponding olefinic 1H NMR resonance occurs at δ G


Organometallics

Article

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CONCLUSIONS Frustrated Lewis pair chemistry rightfully is regarded by many as a domain of main group element chemistry. Truly enough inter- and intramolecular combinations of bulky main group element derived Lewis acids and bases have led to some spectacular developments in small molecule binding and activation, but the use of transition metal derived Lewis acids, especially of the group 4 d elements, has opened a niche in this overall development of some increasing importance and attention. This was initiated by early work by Stephan and by Wass et al., who mostly used oxygen-stabilized cationic systems. Now this and preceding work increasingly shows that even zirconocene cations that bear σ-hydrocarbyl groups can be employed in metal-containing FLP systems. This was not clear from the beginning, since the Zr−C σ bonds are very reactive and have been known to undergo cleavage or insertion reactions with a great variety of organic (and inorganic) reagents. Our work has shown more and more that this typical behavior is changed and eventually is overcome by having intramolecular Lewis base functionalities cooperate with the strongly electrophilic [Zr]−C-containing metal cation Lewis acid. Our present study shows that a variety of π reagents undergo clean 1,2- or 1,4-addition reactions to the Zr+/P FLP system (10), although for many of them an isolated reaction with the Zr−aryl moiety would have been an alternative. Only in very few specific examples did we actually observe the “normal” insertion chemistry into the Zr−C σ bond, which emphasizes the extraordinary situation of the majority of the cooperative Zr+/P reactions observed.

Scheme 9

5.73 as a triplet with 3JHH = 7.8 Hz, and the corresponding signal of the adjacent methylene group is at δ 3.36 (d). We have also reacted complex 23 with D2 in an in situ NMR experiment and observed the 1H/2H NMR spectra of the respective product 26-D2. In the 1H NMR spectrum the −CH2− signal was missing and the adjacent enolate CH− resonance was now only a slightly broadened singlet. In the 2H NMR spectrum we have observed the corresponding −CD2− Ph signal (for details see the Supporting Information). The Zr+/P FLP 10 reacts in a related way with H2. Complex 10 was exposed to H2 (1.5 bar) at −30 °C and then for 10 min at room temperature in CD2Cl2 (see Scheme 10). The NMR

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Scheme 10

EXPERIMENTAL SECTION

For general information and the spectroscopic and structural data of these new compounds see the Supporting Information. Some of the products were not obtained completely pure, as judged from the results of the elemental analysis and the NMR spectra (for details see the Supporting Information). Preparation of Complex 9. Under an argon atmosphere, Cp 2 ZrMeCl (544 mg, 2 mmol) and (2-lithiophenyl)diphenylphosphane (536 mg, 2 mmol) were mixed in toluene (5 mL) at −30 °C. The mixture was stirred at −30 °C for 5 min and then for a further 1 h at room temperature. The resulting mixture was filtered by syringe. Then pentane (5 mL) was added to the colorless filtrate, which was stored at −30 °C overnight to give colorless crystals. The crystals were collected, washed with pentane twice, and dried in vacuo to give compound 9. Yield: 727 mg, 73%. Crystals of complex 9 suitable for an X-ray crystal structure analysis were obtained from a solution of complex 9 in toluene and pentane at −35 °C. Anal. Calcd for C29H27PZr (497.7 g mol−1): C, 69.98; H, 5.47. Found: C, 70.14; H, 5.30. Preparation of Complex 10. Complex 9 (100 mg, 0.2 mmol) and [Ph3C][B(C6F5)4] (184 mg, 0.2 mmol) were mixed in bromobenzene (3 mL) at room temperature. After 5 min the mixture was added dropwise to pentane (10 mL) with vigorous stirring. Then the solvent was decanted to give an orange oil, which was washed with pentane (3 × 3 mL) with vigorous stirring to finally give compound 10. Yield: 197 mg, 85%. Anal. Calcd for C52H24PBF20Zr (1161.7 g mol−1): C, 53.76; H, 2.08. Found: C, 53.94; H, 2.21. Preparation of Complex 11. Complex 9 (30 mg, 0.06 mmol) and [Ph3C][B(C6F5)4] (55.5 mg, 0.06 mmol) were mixed in bromobenzene (1 mL) at room temperature. After 5 min an excess of tetrahydrofuran (8.6 mg, 0.12 mmol) was added. After it was stirred for a further 5 min, the resulting mixture was added dropwise to pentane (5 mL) with vigorous stirring. Then the solvent was decanted and the obtained pale yellow oil was washed with pentane (3 × 3 mL) with vigorous stirring to finally give a white solid. Yield: 66 mg, 89%. Crystals suitable for an X-ray crystal structure analysis were obtained

spectrum showed the in situ generation of the PPh3-stabilized hydrido zirconocene cation dimer 29. It shows a single Cp 1H NMR resonance (1H, δ 5.95) and a four-line Zr2H2 1H NMR signal (AA′ section of a AA′XX′ H/H′/P/P′ system; δ −2.10). The system shows a 31P{1H} NMR resonance at δ 28.1. The Zr2H2 1H NMR resonance was absent in the analogous product 29-D2 obtained from the 10/D2 reaction. In the 2H NMR spectrum we monitored the respective broad Zr2D2 resonance (for details see the Supporting Information). We assume H2 splitting by the Zr+/P FLP to generate the intermediate 27, which contains a Brønsted acidic [P]H+ phosphonium moiety in the vicinity of the reactive Zr−C(aryl) σ bond. The protonolytic cleavage directs the reaction toward the formation of the eventually observed product 29. Complex 29 is rather sensitive. In the dichloromethane solution it reacted further to give Cp2ZrCl2 within ca. 2 days (for details see the Supporting Information).15a H


Article

Organometallics

Preparation of Complex 15b. The procedure for the preparation of complex 15b is similar to that described for the preparation of complex 15a. For details see the Supporting Information. 15b: Anal. Calcd for C59H29PBNSF20Zr (1296.9 g mol−1): C, 54.64; H, 2.25; N, 1.08. Found: C, 55.61; H, 2.75; N, 1.03. In Situ Generation of Complex 16. In a Young tube a solution of complex 10 (58.1 mg, 0.05 mmol) in CD2Cl2 (1 mL) was degassed at −78 °C and then exposed to a CO2 atmosphere (2 bar) at −30 °C. The reaction mixture was kept at less than 10 °C for 10 min. Then the reaction was characterized by NMR experiments at 10 °C. (Note: complex 16 converted back to complex 10 after the CO2 atmosphere was released.) Preparation of Complex 17. In a Schlenk tube a solution of complex 10 (34.7 mg, 0.03 mmol) and HB(C6F5)2 (11.4 mg, 0.03 mmol) in CH2Cl2 (2 mL) was degassed at −78 °C and then exposed to a CO2 atmosphere (2 bar) at −30 °C. The reaction mixture was kept at less than 10 °C and stirred for 30 min. After the resulting mixture was added dropwise to pentane (5 mL) with vigorous stirring, the solvent was decanted to give a pale yellow oil, which was washed with pentane (3 × 3 mL) with vigorous stirring to finally give a pale yellow solid. Yield: 40 mg, 86%. Anal. Calcd for C65H25PB2O2F30Zr (1551.7 g mol−1): C, 50.31; H, 1.62. Found: C, 51.27; H, 2.01. Preparation of Complex 18a. Complex 9 (30.0 mg, 0.06 mmol) and [Ph3C][B(C6F5)4] (55.5 mg, 0.06 mmol) were mixed in bromobenzene (1 mL) at room temperature. After 5 min N,N′dicyclohexylcarbodiimide (12.4 mg, 0.06 mmol) was added. After it was stirred for a further 5 min the resulting mixture was added dropwise to pentane (5 mL) with vigorous stirring. Then the solvent was decanted and the obtained red oil was washed with pentane (3 × 3 mL) with vigorous stirring to finally give a red solid. Yield: 71 mg, 87%. Crystals suitable for an X-ray crystal structure analysis were obtained from a two-layer procedure using CH2Cl2/cyclopentane at room temperature. Anal. Calcd for C65H46PBN2F20Zr (1368.1 g mol−1): C, 57.07; H, 3.39; N, 2.05. Found: C, 57.65; H, 3.48; N, 1.54. Preparation of Complex 18b. The procedure for the preparation of complex 18b was similar to that described for the preparation of complex 18a. For details see the Supporting Information. 18b: Anal. Calcd for C59H38PBN2F20Zr (1287.9 g mol−1): C, 55.02; H, 2.97; N, 2.18. Found: C, 55.83; H, 3.23; N, 2.00. Preparation of Complex 22. In a Schlenk tube a solution of complex 9 (30.0 mg, 0.06 mmol) and [Ph3C][B(C6F5)4] (55.5 mg, 0.06 mmol) in bromobenzene (1 mL) was evacuated and then exposed to a CO atmosphere (1.5 bar) at room temperature. After it was stirred for 10 min, the resulting red mixture was added dropwise to pentane (5 mL) with vigorous stirring. Then the solvent was decanted to give a red oil, which was washed with pentane (3 × 3 mL) with vigorous stirring to finally give a red solid. Yield: 57 mg, 80%. Anal. Calcd for C53H24PBOF20Zr (1189.8 g mol−1): C, 53.51; H, 2.03. Found: C, 53.74; H, 1.98. IR: 1643 cm−1 (CO). Preparation of Complex 23. Complex 22 (47.6 mg, 0.04 mmol) and phenylacetylene (4.1 mg, 0.04 mmol) were mixed in dichloromethane (2 mL) at room temperature and stirred for 12 h. After the resulting deep red solution was added dropwise to pentane (5 mL) with vigorous stirring, the solvent was decanted to give a red oil, which was washed with pentane (3 × 3 mL) with vigorous stirring to finally give a red solid. Yield: 57 mg, 82%. Crystals suitable for an X-ray crystal structure analysis were obtained from a two-layer procedure using CH2Cl2/cyclopentane at room temperature. Anal. Calcd for C61H30PBOF20Zr (1291.9 g mol−1): C, 56.71; H, 2.34. Found: C, 57.42; H, 2.86. Preparation of Complex 26. In a Young tube a solution of complex 23 (51.7 mg, 0.04 mmol) in CD2Cl2 (2 mL) was evacuated at room temperature and then exposed to a H2 atmosphere (1.5 bar). After 72 h the starting material disappeared (monitored by NMR spectroscopy). Then the reaction mixture was added dropwise to pentane (5 mL) with vigorous stirring. Subsequently the solvent was decanted to give a light red oil, which was washed with pentane (3 × 3 mL) with vigorous stirring to finally give a light red solid. Yield: 44 mg, 85%. Anal. Calcd for C61H32PBOF20Zr (1293.9 g mol−1): C, 56.62; H, 2.49. Found: C, 57.16; H, 2.93.

from a two-layer procedure using CH2Cl2/cyclopentane at room temperature. Anal. Calcd for C56H32PBOF20Zr (1233.9 g mol−1): C, 54.51; H, 2.61. Found: C, 55.11; H, 2.60. Preparation of Complex 12a. Complex 9 (30 mg, 0.06 mmol) and [Ph3C][B(C6F5)4] (55.5 mg, 0.06 mmol) were mixed in bromobenzene (1 mL) at room temperature. After 5 min an excess of acetone (7 mg, 0.12 mmol) was added. After it was stirred for a further 5 min, the resulting mixture was dropwise added to pentane (5 mL) with vigorous stirring. Then the solvent was decanted, and the obtained pale yellow oil was washed with pentane (3 × 3 mL) under vigorous stirring to finally give a white solid. Yield: 63 mg, 86%. Crystals suitable for an X-ray crystal structure analysis were obtained from a two-layer procedure using CH2Cl2/cyclopentane at room temperature. Anal. Calcd for C55H30PBOF20Zr (1219.8 g mol−1): C, 54.16; H, 2.48. Found: C, 53.59; H, 2.47. Preparation of Complexes 12b−g. The procedures for the preparation of complexes 12b−g are similar to that described for the preparation of complex 12a. 12b: Anal. Calcd for C53H26PBOF20Zr (1191.8 g mol−1): C, 53.42; H, 2.20. Found: C, 52.99; H, 2.14. 12c: Anal. Calcd for C59H30PBOF20Zr (1267.9 g mol−1): C, 55.89; H, 2.39. Found: C, 54.40; H, 2.13. 12d: Anal. Calcd for C59H29PBOClF20Zr (1302.3 g mol−1): C, 54.41; H, 2.24. Found: C, 53.76; H, 2.34. 12e: Anal. Calcd for C63H34PBOF20FeZr (1375.8 g mol−1): C, 55.00; H, 2.49. Found: C, 53.81; H, 2.31. 12f: Anal. Calcd for C62H32PBOF20Zr (1305.9 g mol−1): C, 57.02; H, 2.47. Found: C, 56.79; H, 2.71. 12g: Anal. Calcd for C61H32PBOF20Zr (1293.9 g mol−1): C, 56.62; H, 2.49. Found: C, 57.24; H, 2.52. Preparation of Complex 13a. Complex 9 (30 mg, 0.06 mmol) and [Ph3C][B(C6F5)4] (55.5 mg, 0.06 mmol) were mixed in bromobenzene (1 mL) at room temperature. After 5 min 1 equiv of chalcone (12.5 mg, 0.06 mmol) was added. After it was stirred for a further 5 min the resulting mixture was added dropwise to pentane (5 mL) with vigorous stirring. Then the solvent was decanted and the obtained pale yellow oil was washed with pentane (3 × 3 mL) with vigorous stirring to finally give a white solid. Yield: 70 mg, 85%. Anal. Calcd for C67H36PBOF20Zr (1370.0 g mol−1): C, 58.74; H, 2.65. Found: C, 58.69; H, 2.73. Preparation of Complex 13b. The procedure for the preparation of complex 13b is similar to that described for the preparation of complex 13a. For details see the Supporting Information. 13b: Anal. Calcd for C67H35PBOClF20Zr (1404.4 g mol−1): C, 57.30; H, 2.51. Found: C, 55.94; H, 2.65. Preparation of Complex 14. Complex 9 (30 mg, 0.06 mmol) and [Ph3C][B(C6F5)4] (55.5 mg, 0.06 mmol) were mixed in bromobenzene (1 mL) at room temperature. After 5 min 1 equiv of tert-butyl chloride (5.6 mg, 0.06 mmol) was added. After it was stirred for a further 5 min the resulting mixture was added dropwise to pentane (5 mL) with vigorous stirring. Then the solvent was decanted and the obtained yellow oil was washed with pentane (3 × 3 mL) with vigorous stirring to finally give a pale yellow solid. Yield: 60 mg, 83%. Crystals suitable for an X-ray crystal structure analysis were obtained from a two-layer procedure using CH2Cl2/cyclopentane at room temperature. Anal. Calcd for C52H25PBClF20Zr (1198.2 g mol−1): C, 52.13; H, 2.10. Found: C, 52.68; H, 2.02. Preparation of Complex 15a. Caution! Phenyl isocyanate is toxic and must be handled with due care. Complex 9 (30 mg, 0.06 mmol) and [Ph3C][B(C6F5)4] (55.5 mg, 0.06 mmol) were mixed in bromobenzene (1 mL) at room temperature. After 5 min 1 equiv of phenyl isocyanate (7.2 mg, 0.06 mmol) was added. After it was stirred for a further 5 min, the resulting mixture was added dropwise to pentane (5 mL) with vigorous stirring. Then the solvent was decanted and the obtained pale yellow oil was washed with pentane (3 × 3 mL) with vigorous stirring to give a white solid. Yield: 61 mg, 80%. Crystals suitable for an X-ray crystal structure analysis were obtained from a two-layer procedure using CH2Cl2/cyclopentane at room temperature. Anal. Calcd for C59H29PBNOF20Zr (1280.9 g mol−1): C, 55.33; H, 2.28; N, 1.09. Found: C, 54.84; H, 2.20; N, 1.01. I


Article

Organometallics In Situ Generation of Complex 26-D2. In a Young tube a solution of complex 23 (25.9 mg, 0.02 mmol) in CD2Cl2 (or CH2Cl2) (1 mL) was evacuated at room temperature and then exposed to a D2 atmosphere (1.5 bar). After 72 h the starting material had disappeared (monitored by NMR spectroscopy) and the obtained reaction mixture was characterized by NMR experiments. In Situ Generation of Complex 29. In a Young tube a solution of complex 10 (11.6 mg, 0.01 mmol) in CD2Cl2 (0.5 mL) was evacuated at −75 °C and then exposed to a H2 atmosphere (1.5 bar) at ca. −30 °C. After 10 min at room temperature the resulting mixture turned from pale yellow to colorless. Then the reaction mixture was characterized by NMR experiments. In Situ Generation of Complex 29-D2. In a Young tube a solution of complex 10 (11.6 mg, 0.01 mmol) in CD2Cl2 (or CH2Cl2) (0.5 mL) was evacuated at −75 °C and then exposed to a D2 atmosphere (1.5 bar) at ca. −30 °C, which slowly gave a solution. After 10 min at room temperature the resulting solution turned from pale yellow to colorless. Then the reaction mixture was characterized by NMR experiments.

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A.; Green, M.; Townsend, N. S.; Wang, K.; Holmes, A. J.; Duckett, S. B.; McGrady, J. E.; Stephan, D. W. J. Am. Chem. Soc. 2014, 136, 13453−13457. (c) vom Stein, T.; Peréz, M.; Dobrovetsky, R.; Winkelhaus, D.; Caputo, C. B.; Stephan, D. W. Angew. Chem., Int. Ed. 2015, 54, 10178−10182. (5) Flynn, S. R.; Wass, D. F. ACS Catal. 2013, 3, 2574−2581. (6) (a) Jordan, R. F.; Dasher, W. E.; Echols, S. F. J. Am. Chem. Soc. 1986, 108, 1718−1719. (b) Yang, X.; Stern, C. L.; Marks, T. J. J. Am. Chem. Soc. 1991, 113, 3623−3625. (c) Yang, X.; Stern, C. L.; Marks, T. J. J. Am. Chem. Soc. 1994, 116, 10015−10031. (d) Bochmann, M.; Lancaster, S. J. Organometallics 1994, 13, 2235−2243. (e) Brintzinger, H. H.; Fischer, D.; Mülhaupt, R.; Rieger, B.; Waymouth, R. M. Angew. Chem., Int. Ed. Engl. 1995, 34, 1143−1170. (f) Bosch, B. E.; Erker, G.; Fröhlich, R.; Meyer, O. Organometallics 1997, 16, 5449−5456. (7) Sgro, M. J.; Stephan, D. W. Chem. Commun. 2013, 49, 2610− 2612. (8) (a) Jessop, P. G.; Morris, R. H. Coord. Chem. Rev. 1992, 121, 155−284. (b) Jia, G.; Lau, C.-P. Coord. Chem. Rev. 1999, 190−192, 83−108. (c) Miller, A. J. M.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem. Soc. 2008, 130, 11874−11875. (d) Rakowski DuBois, M.; DuBois, D. L. Chem. Soc. Rev. 2009, 38, 62−72. (e) Liu, T.; Chen, S.; O′Hagan, M. J.; DuBois, M. R.; Bullock, R. M.; DuBois, D. L. J. Am. Chem. Soc. 2012, 134, 6257−6272. (f) Sgro, M. J.; Stephan, D. W. Angew. Chem., Int. Ed. 2012, 51, 11343−11345. (g) Hulley, E. B.; Welch, K. D.; Appel, A. M.; DuBois, D. L.; Bullock, R. M. J. Am. Chem. Soc. 2013, 135, 11736−11739. (h) Liu, T.; DuBois, D. L.; Bullock, R. M. Nat. Chem. 2013, 5, 228−233. (i) Jiang, Y.; Blacque, O.; Fox, T.; Berke, H. J. Am. Chem. Soc. 2013, 135, 7751−7760. (9) (a) Shvo, Y.; Czarkie, D.; Rahamim, Y.; Chodosh, D. F. J. Am. Chem. Soc. 1986, 108, 7400−7402. (b) Doucet, H.; Ohkuma, T.; Murata, K.; Yokozawa, T.; Kozawa, M.; Katayama, E.; England, A. F.; Ikariya, T.; Noyori, R. Angew. Chem., Int. Ed. 1998, 37, 1703−1707. (c) Noyori, R.; Ohkuma, T. Angew. Chem., Int. Ed. 2001, 40, 40−73. (d) Casey, C. P.; Guan, H. J. Am. Chem. Soc. 2009, 131, 2499−2507. (e) Lagaditis, P. O.; Lough, A. J.; Morris, R. H. J. Am. Chem. Soc. 2011, 133, 9662−9665. (10) Spies, P.; Erker, G.; Kehr, G.; Bergander, K.; Fröhlich, R.; Grimme, S.; Stephan, D. W. Chem. Commun. 2007, 5072−5074. (11) Stephan, D. W.; Erker, G. Chem. Sci. 2014, 5, 2625−2641. (12) Ye, K.-Y.; Kehr, G.; Daniliuc, C. G.; Liu, L.; Grimme, S.; Erker, G. Angew. Chem., Int. Ed. 2016, 55, 9216−9219. (13) (a) Chapman, A. M.; Haddow, M. F.; Wass, D. F. J. Am. Chem. Soc. 2011, 133, 8826−8829. (b) Chapman, A. M.; Haddow, M. F.; Wass, D. F. J. Am. Chem. Soc. 2011, 133, 18463−18478. (c) Chapman, A. M.; Haddow, M. F.; Wass, D. F. Eur. J. Inorg. Chem. 2012, 2012, 1546−1554. (d) Wass, D. F.; Chapman, A. M. Top. Curr. Chem. 2013, 334, 261−280. (e) Chapman, A. M.; Flynn, S. R.; Wass, D. F. Inorg. Chem. 2016, 55, 1017−1021. (14) For related intermolecular examples, see e.g.: (a) Neu, R. C.; Otten, E.; Lough, A.; Stephan, D. W. Chem. Sci. 2011, 2, 170−176. (b) Metters, O. J.; Forrest, S. J. K.; Sparkes, H. A.; Manners, I.; Wass, D. F. J. Am. Chem. Soc. 2016, 138, 1994−2003. (c) Flynn, S. R.; Metters, O. J.; Manners, I.; Wass, D. F. Organometallics 2016, 35, 847− 850. (15) (a) Xu, X.; Kehr, G.; Daniliuc, C. G.; Erker, G. J. Am. Chem. Soc. 2013, 135, 6465−6476. (b) Xu, X.; Kehr, G.; Daniliuc, C. G.; Erker, G. Angew. Chem., Int. Ed. 2013, 52, 13629−13632. (c) Xu, X.; Kehr, G.; Daniliuc, C. G.; Erker, G. J. Am. Chem. Soc. 2014, 136, 12431−12443. (d) Normand, A. T.; Richard, P.; Balan, C.; Daniliuc, C. G.; Kehr, G.; Erker, G.; Le Gendre, P. J. Am. Chem. Soc. 2015, 137, 10796−10808. (16) (a) Xu, X.; Fröhlich, R.; Daniliuc, C. G.; Kehr, G.; Erker, G. Chem. Commun. 2012, 48, 6109−6111. (b) Xu, X.; Kehr, G.; Daniliuc, C. G.; Erker, G. Organometallics 2013, 32, 7306−7311. (c) Fromel, S.; Kehr, G.; Fröhlich, R.; Daniliuc, C. G.; Erker, G. Dalton Trans. 2013, 42, 14531−14536. (d) Xu, X.; Kehr, G.; Daniliuc, C. G.; Erker, G. Organometallics 2015, 34, 2655−2661. (e) Xu, X.; Kehr, G.; Daniliuc, C. G.; Erker, G. J. Am. Chem. Soc. 2015, 137, 4550−4557. (f) Normand, A. T.; Daniliuc, C. G.; Wibbeling, B.; Kehr, G.; Le Gendre, P.; Erker, G. Chem. - Eur. J. 2016, 22, 4285−4293.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.6b00828. Experimental and analytical details and crystallographic data (PDF) Crystallographic data (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail for G.E.: [email protected]. ORCID

Gerhard Erker: 0000-0003-2488-3699 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the European Research Council is gratefully acknowledged. REFERENCES

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Article

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