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Probing the Influence of Phosphine Substituents on the Donor and Catalytic Properties of Phosphinoferrocene Carboxamides: A Combined Experimental and Theoretical Study Jiří Schulz,† Petr Vosáhlo,† Filip Uhlík,‡ Ivana Císařová,† and Petr Štěpnička*,† †

Department of Inorganic Chemistry and ‡Department of Physical and Macromolecular Chemistry, Faculty of Science, Charles University; Hlavova 2030, 128 40 Prague, Czech Republic S Supporting Information *

ABSTRACT: The stereoelectronic influence of phosphine substituents on the coordination and catalytic properties of phosphinoferrocene carboxamides was studied for the model compounds R2PfcCONHMe (1a−d), where fc = ferrocene-1,1′-diyl and R = i-Pr (a), t-Bu (b), cyclohexyl (Cy; c), Ph (d), using experimental and DFT-computed parameters. The electronic parameters were examined via 1JSeP coupling constants determined for R2P(Se)fcCONHMe (6a−d) and CO stretching frequencies of the Rh(I) complexes trans-[RhCl(CO)(1-κP)2] (7a−d); the steric properties of 1a−d were assessed through Tolman’s ligand cone angles (θ) and solid angles (Ω). Generally, a very good agreement between the calculated and experimental values was observed. Whereas the donor ability of the amidophosphines was found to increase from 1d through 1a,c to 1b, the trends in steric demand suggested by the two parameters differed, reflecting the different spatial properties of the phosphine substituents. In situ NMR studies and catalytic tests on the Suzuki−Miyaura cross-coupling of 4-bromoanisole with a bicyclic 4-tolylborate to give 4-methyl-4′-methoxybiphenyl using [Pd(η2:η2-cod)(η2ma)] (cod = cycloocta-1,5-diene, ma = maleic anhydride) as a Pd(0) precursor revealed that different Pd-1 species (precatalysts) were formed from different ligands and participated in the reaction. Specifically, the bulky and electron-rich donor 1b favored the formation of [Pd(1b)(ma)], while the remaining ligands provided the corresponding bis-phosphine complexes [Pd(1)2(ma)]. The best results in terms of catalyst longevity and efficacy were observed for ligands 1a,c.

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INTRODUCTION Phosphinocarboxylic amides1,2 are versatile ligands for coordination chemistry and catalysis. Their attractiveness results mainly from their hybrid-donor character3 and modular structures, which allow for fine tuning to the intended purpose. With these characteristics in mind, we extended our earlier studies focused on the coordination and catalytic properties of 1′-(diphenylphosphino)ferrocene-1-carboxylic acid (Hdpf)4,5 also to its corresponding amides (Chart 1).

approach provided a library of chiral donors whose catalytic properties were assessed in Pd-catalyzed allylic alkylations9 and in Cu-catalyzed conjugate additions of diethylzinc to chalcones.10 To further expand the scope of accessible ligands, we have recently prepared 1′-(diphenylphosphino)-1-(aminomethyl)ferrocene and converted this amine to phosphinoferrocene ureas11 and an amidosulfonate,12 which were also catalytically evaluated. However, we felt that the inherently modular structures of these phosphinoferrocene carboxamides are amenable to further modifications, mainly at the phosphine moiety, which can be regarded the primary biding site for (catalytically active) soft metal ions, whereas the amide moiety is used as a defined linker suitable for the introduction of functional fragments and can further serve as a secondary donor site. Indeed, attempts to modify phosphinoferrocene donors13 by changing their phosphine substituents are well documented. However, systematic structural variations have mostly concerned dppf analogues, [Fe(η5-C5H4PR2)2]14 (dppf = 1,1′bis(diphenylphosphino)ferrocene), and chiral donors of the Josiphos family introduced by Togni et al.15 With the goal of expanding the family of phosphinoferrocene carboxamide donors by variation of their phosphine parts, we decided to first evaluate the influence of the phosphine

Chart 1. Formal Design of Phosphinoferrocene Carboxamide Donorsa

a

FG = functional group.

Using amidation reactions,6 Hdpf was converted to a series of functional phosphinoferrocenes possessing hydrophilic substituents that proved to be suitable for catalysis in polar reaction media7 as well as amides bearing additional donor groups that were shown to coordinate as trans-chelating donors.8 Starting from suitable amines, including amino acid esters, and Hdpf or its planar-chiral analogues, an analogous © XXXX American Chemical Society

Received: March 8, 2017

A

DOI: 10.1021/acs.organomet.7b00181 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

prepared by the amidation of Hdpf4 with methylamine as described earlier16 and, for the sake of comparison, was further converted to the corresponding BH3 adduct 5d by reaction with BH3·SMe2. Amides 1a−c were isolated as orange solids and could be stored nearly indefinitely under a protective atmosphere. In solution, however, they were prone to decomposition, particularly when the phosphine moiety is electron rich (amide 1b appears to be the least stable). The amides and all synthetic intermediates were characterized by NMR and IR spectroscopy, electrospray ionization (ESI) mass spectrometry, and elemental analysis. Whereas the NMR spectra reflected changes at the phosphine substituents (not only through the 31 P NMR shifts but also via JPC coupling constants), manipulations of the polar parts of the molecules were clearly manifested in the IR spectra. The IR spectra of carboxylic acids 4a−c showed νCO vibrations near 1700 cm−1, and amides 1a− c and 5a−c displayed pairs of amide bands at approximately 1630 and 1560 cm−1 (amide I and II). The associated 13C NMR signals (CO) were observed at δC ca. 177 (acids) and 170 (amides). In addition, the molecular structures of 4a−c and amides 1c and 5c were determined by X-ray diffraction analysis. The structures of the acids and selected geometric data are presented in Figure 1 and Table 1, respectively.

substituents on the stereoelectronic and catalytic properties of these hybrid ligands. To this end, we prepared and studied a series of compounds bearing different substituents on the phosphorus atom, akin to the recently described Hdpf-amide 1d16 (Chart 2). Chart 2. Phosphinoferrocene Carboxamides Used in This Comparative Study

In this contribution, four series of compounds, i.e., amides 1a−d, their borane adducts and phosphine selenides, and Rh(I) complexes of the type [RhCl(CO)(1-κP)2], are characterized through a combination of spectroscopic methods and cyclic voltammetry. The collected results are analyzed in terms of changes in steric and electronic properties, and the observed trends are correlated with the results of density functional theory (DFT) computations and model catalytic experiments with the Suzuki−Miyaura cross-coupling reaction.

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RESULTS AND DISCUSSION Synthesis and Characterization of the Ligands. Phosphinoamide donors 1a−c were prepared from 1,1′dibromoferrocene (2), as depicted in Scheme 1. This Scheme 1. Synthesis of Amidophosphines 1a−ca

a

Legend: R = i-Pr (a), t-Bu (b), cyclohexyl (c); EDC = 1-ethyl-3-[3(dimethylamino)propyl]carbodiimide; HOBt = 1-hydroxybenzotriazole; DABCO = 1,4-diazabicyclo[2.2.2]octane.

Figure 1. Molecular structures of P-protected phosphinocarboxylic acids 4a−c (note: the conventional displacement ellipsoid plots are available in the Supporting Information).

dibromide was sequentially functionalized17,18 (via phosphinylation and carboxylation) to afford acids 4a−c, whose oxidation-sensitive phosphine moieties were protected in the form of stable BH3 adducts. Acids 4a−c were subsequently reacted with methylamine in the presence of peptide coupling agents6 to provide P-protected amides 5a−c. In the last step, the borane protecting group19 was removed by treatment with 1,4-diazabicyclo[2.2.2]octane (DABCO)20 to give target amides 1a−c. To complete the series, compound 1d was

Generally, the molecular structures of 4a−c resemble that of Hdpf.4 The ferrocene units adopt their usual geometries with similar Fe−C distances and tilt angles not exceeding 6°. However, the compounds differ in the mutual orientation of the substituents at the ferrocene unit.21 In the case of 4b, the ferrocene unit moiety has an anticlinal eclipsed conformation, whereas those in 4a,c assume conformations intermediate between synclinal eclipsed and anticlinal staggered. This B


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Organometallics Table 1. Selected Geometric Parameters for 4a−ca

Table 2. Selected Geometric Data for 1c and 5ca

param

4a

4b

4c

Fe−Cg1 Fe−Cg2 ∠Cp1,Cp2 τ C11O1 C11−O2 O1C11−O2 φb P−B P−C6 P−C12 P−Cn [n]

1.6562(9) 1.6563(9) 5.6(1) −99.6(1) 1.252(2) 1.297(2) 123.6(2) 0.9(2) 1.922(2) 1.796(2) 1.829(2) 1.840(2) [15]

1.6475(7) 1.6504(6) 5.16(9) −143.6(1) 1.237(2) 1.310(2) 123.1(1) 7.4(2) 1.940(2) 1.809(1) 1.877(2) 1.888(1) [16]

1.6576(8) 1.6556(8) 3.6(1) 93.4(1) 1.260(2) 1.281(2) 124.4(1) 4.5(2) 1.926(2) 1.799(2) 1.839(2) 1.838(2) [18]

1c

a

In Tables 1−4, bond distances are given in Å and angles in deg. Definitions: Cp1 and Cp2 are the cyclopentadienyl rings C(1−5) and C(6−10), respectively; Cg1 and Cg2 denote their respective centroids; τ is the torsion angle C1−Cg1−Cg2−C6. bφ is the dihedral angle subtended by the carboxyl plane {C11,O1,O2} and the plane of cyclopentadienyl ring Cp1.

5c

param

mol 1

mol 2

mol 1

mol 2

Fe−Cg1 Fe−Cg2 ∠Cp1,Cp2 τ CO C−N OC−N φb P−B P−Cm6 P−Cn2 P−Cn8

1.653(4) 1.654(4) 3.4(5) −140.0(6) 1.210(9) 1.341(9) 122.5(7) 10.4(8) n.a. 1.825(8) 1.866(9) 1.875(8)

1.658(4) 1.651(4) 3.0(5) 154.3(6) 1.239(9) 1.33(1) 123.1(7) 12.0(8) n.a. 1.840(8) 1.845(8) 1.871(8)

1.6568(7) 1.6479(8) 4.4(1) 159.1(1) 1.242(2) 1.334(2) 122.3(2) 5.9(2) 1.923(2) 1.797(2) 1.840(2) 1.836(2)

1.6527(7) 1.6479(7) 4.47(9) −141.9(1) 1.234(2) 1.341(2) 121.8(2) 8.9(2) 1.922(2) 1.800(2) 1.836(2) 1.838(2)

a All parameters are defined as for the parent acids 4 (see Table 1). m/ n = void/1 for molecule 1, and 5/6 for molecule 2. n.a. = not applicable. bφ is the dihedral angle subtended by the amide plane {C,O,N} and cyclopentadienyl ring Cp1.

chains via intermolecular N−H···OC hydrogen bonds between alternating molecules 1 and 2 with N1···O2/N2···O1 distances of 2.929(8)/2.923(7) and 2.902(2)/2.934(2) Å for 1c and 5c, respectively. Interatomic distances and angles determined for 1c and 5c (Table 2) generally parallel those reported previously for amide 1d.16 Differences between the structures of 1c and 5c are expectedly found in their phosphine groups. The P−C bonds in the borane adduct are slightly, but statistically significantly, shorter than in the free amide. This shortening is accounted for by an electron density shift from the substituents to phosphorus upon donation of the phosphorus lone pair (P→BH3). Moreover, the replacement of the lone electron pair with a BH3 fragment is associated with an opening of the C−P−C angles, which range from 100 to 102° in 1c and from 105 to 108° in 5c. As evidenced by ring puckering parameters,26 the cyclohexyl substituents in the structures of 1c and 5c adopt chair conformations with the pivotal P−C bonds in equatorial positions.27 Synthesis and Characterization of P-Selenides and Rh(I) Complexes from Amides 1a−d. To compare their properties, amides 1 were further converted to the corresponding phosphine selenides (6) and to the Rh(I) complexes trans[RhCl(CO)(1-κP)2] (7). The selenides were obtained in good yields upon reacting phosphinoamides 1 with potassium selenocyanate28 and isolated as air-stable orange crystalline solids (Scheme 2). Compounds 6 gave rise to signals typical of the ferrocene unit and its substituents in their NMR spectra and to characteristic amide bands in the IR spectra. The oxidation of the phosphine moieties resulted in a shift of the 31 P NMR signals to a lower field and an appearance of 77Se satellites.

difference is ascribed to the increased steric bulk of the phosphine moiety in 4b, which diverts the carboxyl moiety to a more distant position.22 The carboxyl groups are slightly tilted with respect to their parent cyclopentadienyl ring (see parameter φ in Table 1). The maximum departure from a coplanar arrangement is observed for 4b, which bears the bulkiest phosphine moiety. In the crystal state, molecules 4a−c assemble into dimers23 via double O−H···O hydrogen bridges located around inversion centers.24 Amide 1c and borane adduct 5c25 crystallize with two independent molecules in their structures. Views of molecule 1 of both compounds are shown in Figure 2, and the relevant

Figure 2. Structural diagrams for molecules 1 in the crystal structures of 1c and 5c (note: displacement ellipsoid plots and overlaps of the two symmetrically independent molecules are available in the Supporting Information).

structural data are presented in Table 2. Notably, the pairs of independent molecules in both structures differ in the orientation of the amide plane with respect to the phosphinoferrocene moiety (the O and NHMe moieties appear transposed; see Figure S5 in the Supporting Information) and, further, in the conformation of the ferrocene moiety. In one molecule, the substituents assume almost ideal anticlinal eclipsed positions (expected: τ = 144°); in contrast, in the other molecule, the ferrocene unit is more opened up, with τ ≈ 155−160°. The multiplication of the moieties in the crystallographic asymmetric unit is very likely associated with intermolecular interactions, because both amides form infinite

Scheme 2. Synthesis of Phosphine Selenides 6a

a

C

Legend: R = i-Pr (a), t-Bu (b), cyclohexyl (c), Ph (d). DOI: 10.1021/acs.organomet.7b00181 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

6a−c) or between the independent molecules (N1 ···O2/N2··· O1; 6c). The ferrocene units in 6a−d are tilted by less than 5°, except for the bulky tert-butyl derivative, in which the cyclopentadienyl planes subtend an angle of 7.2(1)°. The PSe bond lengths vary by less than approximately 0.02 Å in the entire series and compare well with the values reported for dppfSe229 and analogous compounds.30 Likewise, the parameters describing the amide moieties do not substantially differ from those determined for 1d,16 1c, and 5c (see above). Complexes 7a−d were prepared from the respective ligands and the dimeric complex [Rh(μ-Cl)(CO)2]2 (Scheme 3). The

The complete series of the P-selenides, compounds 6a−d, were structurally characterized by X-ray diffraction analysis. The structures are presented in Figure 3, and the pertinent

Scheme 3. Synthesis of Rh(I) Complexes 7a

a

R = i-Pr (a), t-Bu (b), cyclohexyl (c), and Ph (d).

coordination of two phosphine moieties to the RhCl(CO) fragment was manifested through a shift of the 31P NMR signals to a lower field and their splitting into Rh-coupled doublets. In addition, the resonances of the 31P-coupled carbon atoms in the 13 C NMR spectra were observed as characteristic apparent triplets resulting from virtual coupling in the ABX spin systems of the type 12C−31P(A)−Rh−31P(B)−13C(X).31 The presence of the Rh-bound CO ligand was corroborated by the presence of doublets of triplets due to interactions with the central atom (1JRhC ≈ 75 Hz) and two equivalent phosphorus atoms (1JPC ≈ 16 Hz) at δC 187−190 in the 13C NMR spectra and by strong νCO bands in the IR spectra at 1935−1960 cm−1. The crystal structure of complex 7c (in solvated form, 7c· AcOEt) is shown in Figure 4, and the structural parameters are given in Table 4. Complex 7c crystallizes with its Cl−Rh−CO moiety lying on a crystallographic 2-fold axis. As a result, the coordination environment of the Rh(I) ion is exactly planar but slightly

Figure 3. Views of molecule 1 in the structures of 6a,c,d. Displacement ellipsoid plots are presented in the Supporting Information.

structural parameters are given in Table 3. Whereas compounds 6a,c,d crystallize with two structurally independent molecules, the asymmetric unit of 6b contains only one molecule of this amide. The independent molecules of 6a differ predominantly in the orientation of the amide moieties, which appear inverted. A similar situation is observed in the case of 6c, except that the independent molecules also differ in the mutual position of the ferrocene substituent (parameter τ). In contrast, the amide units are similarly oriented in the independent molecules of 6d, which then differ mainly in the conformation of the ferrocene unit (see τ in Table 3). These features again appear to be connected with crystal packing. All compounds form intermolecular N−H···OC hydrogen bonds in the solid stateeither between the same molecules (N1···O1/N2···O2;

Table 3. Pertinent Geometric Parameters for Phosphine Selenides 6a 6cc

6a

6d

param

mol 1

mol 2

6b

mol 1

mol 2

mol 1

mol 2

Fe−Cg1 Fe−Cg2 ∠Cp1,Cp2 τ CO C−NH OC−N φ PSe N···Ob

1.649(2) 1.645(2) 3.2(2) 148.8(3) 1.247(5) 1.343(5) 121.5(3) 9.8(4) 2.110(1) 2.794(4)

1.645(2) 1.643(2) 3.6(2) −148.0(3) 1.239(5) 1.336(5) 121.3(4) 12.5(4) 2.112(1) 2.867(5)

1.650(1) 1.641(1) 7.2(1) 157.1(2) 1.234(3) 1.332(3) 121.6(2) 8.3(2) 2.1254(7) 2.762(2)

1.654(1) 1.646(1) 3.8(1) 158.4(2) 1.245(2) 1.333(3) 122.2(2) 6.0(2) 2.1169(5) 2.902(2)

1.653(1) 1.647(1) 4.4(1) 142.9(1) 1.231(2) 1.341(3) 121.6(2) 9.9(2) 2.1150(5) 2.953(2)

1.652(2) 1.644(2) 2.3(2) 101.3(3) 1.231(4) 1.338(4) 121.6(3) 10.0(4) 2.1071(9) 2.745(3)

1.646(2) 1.640(1) 1.7(2) 88.0(2) 1.241(4) 1.332(5) 122.0(3) 7.3(3) 2.1080(8) 2.761(3)

a

All parameters are defined as for amides 1c and 5c. bThe N···O distances for the intermolecular hydrogen bonds N1−H1N···O1 (molecule 1)/ N2−H2N···O2 (molecule 2) in 6a and 6d, the N−H1N···O hydrogen bonds in 6b, and the N1−H1N···O2 (molecule 1)/N2−H2N···O1 (molecule 2) hydrogen bonds in 6c. cAdditional data: P1−C6 1.793(2) Å, P1−C12 1.839(2) Å, P1−C18 1.833(2) Å, C−P−C angles in molecule 1 105−108°; P2−C56 1.797(2) Å, P2−C62 1.836(2) Å, P2−C68 1.832(2) Å, C−P−C angles in molecule 2: approximately 106° all. D


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Organometallics

Figure 5. Simplified packing diagram for 7c·AcOEt as viewed in the direction of the crystallographic c axis. The solvent, all CHn hydrogens, and the carbons of the cyclohexyl rings (except for the pivotal ones) are omitted for clarity.

Table 5. Parameters Reflecting the Steric and Electronic Properties of 1a−d

Figure 4. View of the complex molecule in the structure of 7c·AcOEt with the hydrogen atoms omitted for clarity (a displacement ellipsoid plot is available in the Supporting Information). Primed atoms are generated by a crystallographic 2-fold axis.

R

Table 4. Selected Geometric Data for 7c·AcOEta distance Rh−P Rh−Cl Rh−C25 C25−O2 Fe−Cg1 Fe−Cg2 C11O1 C11−N1 a

angle 2.3556(7) 2.3869(9) 1.790(4) 1.158(5) 1.647(1) 1.644(1) 1.239(3) 1.332(3)

P−Rh−Cl P−Rh−C25 P−Rh−P′ Rh−C25−O2 ∠Cp1,Cp2 τ O1C11−N1 φ

88.25(2) 91.75(2) 176.50(2) 179.98(3) 2.1(2) 145.2(2) 121.6(2) 8.5(4)

param

i-Pr (a)

t-Bu (b)

Cy (c)

Ph (d)

νCO for 7 (cm−1)a 1 JSeP for 6 (Hz)b Epa for 1 (V)c E°′ for 5 (V)c Epa for 6 (V)c ΩSe (deg)d,e θSe (deg)d

1961 693 0.20 0.36 0.40 155 184

1950 688 0.21 0.35 0.37 165 195

1956 686 0.18 0.35 0.38 159 187

1978 718 0.27 0.40 0.44 152 200

a In chloroform. bIn CDCl3. Chemical shifts are given relative to 85% H3PO4. cIn CH2Cl2. Potentials are given vs a ferrocene/ferrocenium reference. Epa is the anodic peak potential for irreversible oxidation (at a scan rate of 100 mV s−1); E°′ is the formal potential obtained as an average of the peak potentials in cyclic voltammetry. dLigand solid (Ω) and cone (θ) angles calculated from the crystal structure data of selenides 6. eFor an easier comparison, the solid angles are also expressed in degrees: Ωdeg = 2 arccos[1 − Ωsr/2π].

Parameters are defined as for 1c (see Table 2).

angularly distorted (the adjacent interligand angles deviate by ±1.75° from 90°).32 Whereas the Rh−Cl bond length in 7c is practically halfway between those determined for trans[RhCl(CO) (FcPPh2)2]33 (Fc = ferrocenyl) and trans-[RhCl(CO) (PCy3)2],34 the Rh−P and Rh−C distances are closer to only one of these reference compounds (i.e., the FcPPh2 complex for the rather long Rh−P distance and the PCy3 complex for the Rh−C distance).35 The cyclopentadienyl rings in 7c adopt an anticlinal eclipsed conformation (ideal τ = 144°) and are tilted by only ca. 2°. The amide units are rotated by 8.5(4)° with respect to their parent cyclopentadienyl rings and participate in intermolecular N−H···OC interactions (N···O  2.832(3) Å) that result in the formation of layers oriented parallel to the crystallographic ab plane (Figure 5). Stereoelectronic Properties of 1a−d. The electronic properties of donors 1a−d were probed through trends in the carbonyl stretching (νCO) frequencies of trans-[RhCl(CO)(1κP)2] (7a−d) and the 77Se−31P coupling constants (1JSeP) determined for phosphine selenides 6a−d. The former parameter serves as a practical alternative36,37 to νCO frequencies of Ni carbonyl complexes.38 In contrast, the 1JSeP coupling constants38,39 are believed to reflect both the steric and electronic influence because the s character of the phosphorus lone pair is affected not only by the electronic properties of the phosphine substituents but also by their steric demands (through changes in the C−P−C angles). These parameters (see Table 5; an extended version of this table is

available in the Supporting Information) were complemented by electrochemical data and DFT computations. In the series 7a−d, the wavenumber of the νCO band increased in the sequence 7d (Ph) < 7a (i-Pr) < 7c (Cy) < 7b (t-Bu), suggesting an increasing donor ability of the phosphine ligands (as well as Rh→CO π back-bonding) in the same order. Notably, the wavelengths of the νCO bands were well reproduced by DFT computations (Figure 6) and also showed a good correlation with the Hammett σp constants (Figure 6).40 The |1JSeP| values determined for phosphine selenides 6a−d increased as follows: 6c (Cy) ≤ 6b (t-Bu) < 6a (i-Pr) < 6d (Ph). This trend, where a lower |1JSeP| value suggests a higher electron-donating ability of the phosphorus atom, is consistent with that implicated by the νCO frequencies, except that 6b,c appeared inverted. Again, the 1JSeP coupling constants corresponded well to the calculated shielding constants (see the Supporting Information). Electrochemical measurements provided little diagnostic information regarding possible variations in the overall electron density because the changes in the potentials of the ferrocene oxidation determined by cyclic voltammetry (Table 5) were rather minor. In the anodic region, phosphines 1a−d first underwent irreversible oxidations attributable to the ferrocene unit (Figure 7). The anodic peak potentials (Epa) were approximately 0.2 V vs a ferrocene/ferrocenium reference for E


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Organometallics

with those determined analogously from the DFT-optimized structures of 1a−d (see Table S2 in the Supporting Information). Moreover, the Ω angles for 1c calculated similarly from the crystal structure data of free 1c and for compounds 5c−7c varied surprisingly little (cf. 1c, Ω = 157°; 5c, Ω = 159°; 6c, ΩSe = 159°; 7c, Ω = 152°), indicating the robustness of this approach. The standard ligand cone angles (θSe),38 which were calculated in a similar manner,48 showed a different trend, 1a (i-Pr) < 1c (Cy) ≪ 1b (t-Bu) < 1d (Ph) (i.e., with 1d constituting the opposite extreme in the series), which, however, followed the Taft−Dubois steric parameter −Es (see Figure S14 in the Supporting Information).49 The apparent discrepancy between the two scales (θ vs Ω) is explained by the different approaches behind the two measures of a ligand’s steric properties. Whereas cone angles represent the maximum steric demands, solid angles signify the minimum50 (Ω includes meshing of the molecular parts, which can especially affect the sterically anisotropic phenyl substituents). Catalytic Tests. Characterization of 1a−d would not be validated without establishing its relationship with real catalytic results. Hence, the amidophosphines were evaluated as ligands for Pd-catalyzed Suzuki−Miyaura cross-coupling (SM), which is one of the most widely utilized metal-mediated C−C bondforming reaction.51 To simplify the reaction system, we utilized the coupling of 4-bromoanisole with triolborate 852 to give 4methyl-4′-methoxybiphenyl (10 in Scheme 4); this reaction

Figure 6. Relationship between the experimental CO stretching frequencies in 7a−d and the calculated values (left) or Hammett’s σp constants (right). Regression results: ν(CO)exp = [140(2)]σp + 1980(3) cm−1, r2 = 0.9686.

Scheme 4. Suzuki−Miyaura Cross-Coupling of Triolborate 8 with Anisyl Bromide 9 Figure 7. (left) Representative cyclic voltammograms of 1c (black), 5c (blue), and 6c (red), as recorded on a Pt-disk electrode at a scan rate of 100 mV s−1. (right) Cyclic voltammograms of 1c recorded over different potential ranges at a constant scan rate (100 mV s−1).

dialkylphosphino derivatives 1a−c and 0.27 V for 1d, which features the least electron donating phosphine substituent. In all cases, the primary oxidation was followed by several consecutive redox steps involving electrogenerated species and/or their decomposition products (Figure 7).41 The formation of borane adducts decreased the reactivity of the phosphine moiety, which was reflected in the cyclic voltammetric response. Thus, adducts 5a−d were oxidized in a single reversible step at potentials more positive than those for the parent phosphines because of the reduction of the electron density associated with P→B donation (Figure 7).42 The oxidation of the phosphines to selenides 6 resulted in a similar anodic shift, and the oxidation was observed to be fully irreversible (Figure 7).43 Similar to the case for the parent phosphines, the oxidation potentials determined for the representative bearing phenyl substituents were higher than those for its dialkylphosphino analogues. The steric properties44 of 1a−d were mapped through their ligand solid angles (Ω)45 and Tolman cone angles (θ).38 These parameters were calculated from the solid-state structures of selenides 6 (the complete series was structurally characterized) in which the selenium atom was replaced with palladium at a Pd−P distance of 2.30 Å.46 The solid angles ΩSe estimated in such a manner using an algorithm developed by Allen et al.47 increased in the following sequence: 1d (Ph) < 1a (i-Pr) < 1c (Cy) < 1b (t-Bu) (Table 5). These ΩSe values compared well

avoids the use of external bases, thereby presumably streamlining the overall reaction mechanism by eliminating complicated equilibria between various Pd (e.g., Pd−OH) and boron (e.g., boronic acid vs borate anion) species.53 A further simplification of the testing reaction was achieved through the use of a defined and reasonably stable Pd(0) precursor with labile ligands, [Pd(η2:η2-cod)(η2-ma)] (11; cod = cycloocta-1,5-diene, ma = maleic anhydride),54 which does not require activation by reduction (Pd(II) → Pd(0)). Catalytic tests were conducted in an HCON(CD3)2/D2O mixture (3/1) at 40 °C and monitored by 1H NMR spectroscopy. The reaction performed without any supporting ligand revealed that even complex 11 mediates the coupling reaction, achieving an approximately 50% NMR yield of 10 within 3 h; however, it is deactivated relatively quickly (Figure 8). Catalytic systems comprising ligands 1a−d all provided better yields in comparison to unsupported 11 at both tested Pd to ligand ratios (i.e., 1:1 and 1:2). However, the observed kinetic profiles differed substantially, suggesting dissimilar structures of the operative Pd complexes as well as changes in their catalytic activation. As expected, the catalyst containing ligands bearing electron-rich dialkylphosphino substituents (1a−c) afforded yields generally better (>90% in 3 h) than those for catalysts F


Article

Organometallics

c)2(ma)], which exhibited very similar catalytic activities (Figure 8, top). Notably, catalytic systems resulting from 11 and 1 equiv of the ligands 1a,b,d reacted more quickly because of the additive effect of the two species. Nonetheless, because of the rapid deactivation of their components, their yield of 10 was limited to that afforded by the more slowly reacting species [Pd(1d)2(ma)]. Ligand 1b (1 equiv) reacted with 11 to afford the monophosphine complex [Pd(1b)(ma)] (reactions with 2 equiv of 1b were complicated by rapid decomposition). This species catalyzed the coupling reaction even at early stages (without an observable induction period) but achieved slightly lower conversions (after 3 h) in comparison to bis-phosphine complexes featuring 1a,c as the donors (compare Figures 8 and 9). A catalyst resulting from the addition of 2 equiv of ligand 1b

Figure 8. (top) Kinetic profiles for catalysts resulting from 11 and ligands 1a (circles) and 1c (triangles) at Pd to ligand ratios of 1:1 (blue symbols) and 1:2 (red symbols). (bottom) Kinetic profiles for the model SM reaction mediated by 11 itself (black) and catalysts resulting from 11 and 1 (blue) or 2 equiv (red) of ligand 1d.

Figure 9. Kinetic profiles for the model SM reaction mediated by 11 itself (black; reproduced from Figure 8) and catalysts resulting from 11 and 1 (blue) or 2 equiv (red) of ligand 1b.

attained the best yields of the coupling product (similar to the cases of 1a,c) but deteriorated relatively rapidly (within approximately 70−90 min). Even in this case, complex 11 showed higher catalytic activity during the first ca. 30 min of the reaction in comparison to any 11/1b catalysts. This can be ascribed to the faster activation of the former complex simply by the dissociation of the labile cod ligand, whereas activation of the phosphine complexes requires the dissociation of the more tightly bound ma ligand, which is influenced by the donor properties of 1a−d (see above). NMR Characterization of the Pd-1 Complexes. NMR spectra recorded for mixtures resulting from the addition of 1 or 2 equiv of ligands 1a−d to 11 in CDCl3 confirmed the predictions made on the basis of the kinetic data (for the NMR spectra, see the Supporting Information). At Pd to phosphine ratios of 1:2, ligands 1a,c,d reacted with 11 to afford trigonal complexes of the type [Pd(1)2(ma)],56,57 whereas a similar reaction with 1b was accompanied by extensive decomposition (the reaction mixture immediately darkened), which precluded acquisition of any NMR spectra. The replacement of the cod ligand with phosphines 1 was indicated by a shift of the signals corresponding to CHCHcod protons to higher field in the 1H NMR spectra, and the formation of bis-phosphine complexes was suggested by the splitting of the proton signal of the ma ligand into a multiplet corresponding to an AA′XX′ spin system (A = 1H, X = 31P).56 Ferrocene protons were diastereotopic in these complexes, giving rise to eight 1H NMR signals. The most

obtained from 1d. However, the kinetic data indicated that the latter catalysts were activated more quickly (Figure 8). Assuming that the active catalyst is formed through the replacement of the diene ligand by phosphine donors and dissociation of π-bound ma, this observation can be rationalized in terms of the Dewar−Chatt−Duncanson bonding model55 as follows. A weaker coordination of ma is expected when the other ligands at palladium are weaker donors (less π backdonation), which is the case for complexes containing 1d (for instance, the reaction in the presence of 11 and 1 equiv of 1d showed practically no “activation period”). More importantly, the kinetic data indicated different reaction behaviors of the phosphine ligands. Subsequent NMR analyses (see below) confirmed that ligand 1d gave rise to [Pd(1d)2(ma)], irrespective of the reaction stoichiometry (Pd:1d = 1:1 and 1:2). Thus, while the kinetic profile of the reaction performed at a Pd:1d ratio of 1:2 represents the activity of [Pd(1d)2(ma)], the profile at a Pd:1d ratio of 1:1 is the sum of the catalytic performances of [Pd(1d)2(ma)] and 11 (Figure 8, bottom). In contrast, ligands 1a,c reacted with 1 equiv of 11 to provide a mixture containing the respective mono- and bis-phosphine complexes and unreacted 11; hence, the kinetic profiles were again a convolution representing three species. When the stoichiometric amount of ligands was increased to 2 equiv, both 1a and 1c provided the bis-phosphine complexes [Pd(1a/ G


Article

Organometallics notable feature in the 13C NMR spectra was the shift of signals corresponding to the CH groups in the ma ligand to higher fields as the donating ability of the amidophosphine ligands increased (δC 53.63 (1d), 47.00 (1a), and 47.23 (1c)). A similar trend was noted in the 1H NMR spectra (δH 4.39 (1d), 4.24 (1a), and 4.19 (1c)).56 The 13C NMR resonances of the amide CO groups appeared shifted by ca. 1.2 ppm to lower field with respect to the free ligands. Finally, coordination of the phosphine moieties was indicated by a shift of the 31P NMR resonances to lower fields (ΔδP = 33.7−35.2 ppm). As previously stated, the reactions of 11 with 1 at a 1:1 molar ratio were less predictable. The most bulky ligand (1b) cleanly afforded [Pd(1b)(ma)], 1d furnished a mixture of [Pd(1d)2(ma)] and unreacted 11, and the remaining ligands 1a,c provided mixtures of mono- and (predominantly) bisphosphine complexes and unreacted 11. The NMR data for [Pd(1b)(ma)] suggested that 1b coordinates as an O,Pchelating ligand. The cod ligand was completely displaced, and the resonances of the amide units were shifted to lower field (ΔδH(NH) = 0.5 ppm, ΔδC(CO) = 3.5 ppm). The 31P NMR signal was observed at δP 64.3 (ΔδP = 36.3 ppm).

donors 1a,c. The greater stability of the catalysts [Pd(ma)(1a/ c)2] even enabled the achievement of results similar to those obtained when faster reacting but less stable analogous catalysts prepared with only 1 equiv of ligand were used. Although these observations may appear natural in view of the parameters describing the stereoelectronic properties of amides 1, the conspicuous change in the ligating properties observed as a result of a rather minor structural variation of the ligands demonstrates the need for careful investigations and optimizations of catalytic reactions because the nature of the products arising from a metal precursor and the respective donors (precatalysts!) can strongly affect their course. Although detailed studies into the nature of species involved in a catalytic process may appear tedious and superfluous for a synthetic chemist, they provide valuable information regarding the factors controlling the course of a catalytic process and may aid in facilitating its optimization.

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EXPERIMENTAL SECTION

Materials and Methods. All syntheses were performed under an argon atmosphere using standard Schlenk techniques with protection from direct sunlight. Compounds 1d16 and 1154 were prepared using literature procedures. Anhydrous dichloromethane and tetrahydrofuran were obtained from a PureSolv MD5 solvent purification system (Innovative Technology, Amesbury, MA, USA). Other chemicals and the solvents used for crystallizations and chromatography were of reagent grade (Sigma-Aldrich, Alfa-Aesar; solvents from Lach-Ner, Czech Republic) and were used without purification. Analytical measurements aimed at establishing the purity of the prepared compounds were performed for samples isolated from the syntheses described below. The NMR spectra were collected at 25 °C using a Varian UNITY Inova 400 or a Bruker AVANCE III 600 spectrometer. Chemical shifts (δ/ppm) are expressed relative to an internal tetramethylsilane (1H and 13C) or an external 85% H3PO4 standard (31P). In addition to the standard notation of signal multiplicity, vt and vq denote virtual multiplets arising from protons in the amide- and PPh2-substituted cyclopentadienyl rings, respectively (AA′BB′ and AA′BB′X spin systems, respectively; fc = ferrocene-1,1′diyl). FTIR spectra were recorded on a Nicolet 6700 spectrometer in the range 400−4000 cm−1. Low-resolution electrospray ionization (ESI) mass spectra were obtained on an Esquire 3000 spectrometer (Bruker) using samples dissolved in HPLC-grade methanol. Highresolution (HR) measurements were performed using an LTQ Orbitrap XL spectrometer (Thermo Fisher Scientific). The assignment of fragment ions was based on a comparison of the theoretical and experimental isotopic patterns. Elemental analyses were determined with a PerkinElmer PE 2400 CHN analyzer. Cyclic voltammograms were recorded at various scan rates using a μAUTOLAB III instrument (Eco Chemie, The Netherlands) at room temperature with dry dichloromethane as the solvent (sample concentration: 0.5 mM) and Bu4N[PF6] as the supporting electrolyte (0.1 M). A glassy-carbon disk (2 mm diameter) was used as a working electrode, Ag/AgCl (3 M LiCl/EtOH) as a reference electrode, and a platinum sheet as a counter electrode. Ferrocene was added as an internal reference for the final scans. Synthesis of 3a. 1,1′-Dibromoferrocene (2; 8.60 g, 25 mmol) was dissolved in dry THF (100 mL) in an oven-dried two-necked reaction flask under argon. The solution was cooled in a dry ice/ethanol cooling bath to ca. − 75 °C, and LiBu (10 mL of a 2.5 M solution in hexanes, 25 mmol) was added. After the mixture was stirred for 30 min (an orange precipitate formed), neat chlorodiisopropylphosphine (4.40 mL, 27.5 mmol) was added dropwise, and the stirring was continued at −75 °C for 30 min and then at room temperature for another 90 min. Next, borane−dimethyl sulfide (16.25 mL of a 2 M solution in THF, 32.5 mmol) was introduced, and the reaction mixture was stirred for an additional 30 min. The reaction was terminated by the addition of saturated aqueous NaHCO3 (20 mL), and the reaction mixture was partitioned between diethyl ether (100 mL) and brine (80

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CONCLUSION First, the results presented in this paper confirm a good agreement between the trends in the donor ability of phosphine ligands 1, as inferred from the 31P−77Se coupling constants determined for their respective P-selenides and from the νCO frequencies of the Rh carbonyl complexes trans-[RhCl(CO)(1κP)2]. Both of these parameters indicate an increase in the donor properties upon replacement of PPh2-substituted amidophosphine ligand 1d with compounds containing isopropyl and cyclohexyl substituents (1a,c), which are quite similar, and, finally, with tert-butyl derivative 1b. This sequence is in line with the trend in, for example, Hammett’s σp constants. Evaluation of the steric parameters through Tolman’s cone angles (θ) and solid angles (Ω) led to more ambiguous results. The former values representing the maximum steric demands suggested 1d to be the most bulky in the series, and the trend correlated with the Taft−Dubois steric parameters (1a ≈ 1c < 1b < 1d). In contrast, the solid angles, which seem to reflect possible ligand anisotropy, varied less in the series and increased in the series 1d < 1a < 1c < 1b. Notably, both the spectroscopic (1JSe,P and νCO) and steric parameters (θ and Ω) were very well reproduced by DFT computations, which in turn validates theoretical approaches to evaluating ligand properties. Second, catalytic tests and in situ NMR studies revealed differences in the coordination behavior of individual amidophosphine ligands 1. While the interaction of 11 with the bulky and electron-rich donor 1b afforded the monophosphine chelate complex [Pd(1b)(ma)], those involving the other ligands led to bis-phosphine complexes [Pd(1)2(ma)] either irrespective of the metal to ligand ratio (1d) or in a mixture when the ideal stoichiometry was not met (1a,c). The best results from the model SM reaction were obtained using a catalyst formed from 11 and 2 equiv of 1b and catalysts formed from 11 and ligands 1a,c (1 equiv), presumably because of the synergistic activity of 11 itself and the respective Pd-1 complexes in the latter case. Among the catalysts resulting from systems with just one Pd-1 complex present (i.e., at Pd:P = 1:2 for 1a,c,d and at Pd:P = 1:1 for 1b), the best results in terms of reaction yields and catalyst lifetime were yielded from H


Article

Organometallics

74.21 (d, JPC = 7 Hz, 2 C, CH of fc), 78.14 (s, Cipso-Br of fc). 31P{1H} NMR (161.90 MHz, CDCl3): δ 24.2 (br d). IR (Nujol): νmax 2378 s, 2359 s, 2332 s, 2252 w, 1671 w, 1456 s, 1410 w, 1341 w, 1310 w, 1275 w, 1204 m, 1170 m, 1154 w, 1113 w, 1062 m, 1034 m, 1004 w, 917 w, 872 s, 861 w, 855 w, 833 m, 818 m, 766 m, 755 m, 635 m, 606 m, 504 m, 497 m, 486 m, 463 m, 448 w, 439 w cm−1. MS (ESI+): m/z 497 ([M + Na]+). Anal. Calcd for C22H33BBrFeP (475.03): C, 55.62; H, 7.00. Found: C, 55.65; H, 6.99. General Procedure for the Synthesis of Acids 4. The respective bromide 3 (8.0 mmol) was dissolved in anhydrous THF (40 mL) in a three-necked flask equipped with a gas inlet and a rubber septum. The solution was cooled in a dry ice/ethanol bath before LiBu (3.6 mL of a 2.5 M solution in hexanes, 9.0 mmol) was added dropwise. The resulting mixture was stirred at ca. −75 °C for 1 h and then poured onto crushed dry ice (ca. 100 g) and warmed to room temperature. The obtained solution was acidified with 1.25 M HCl in methanol (15 mL, approximately 19 mmol) and concentrated under reduced pressure. The residue was transferred into a separatory funnel, diluted with dichloromethane (50 mL), and washed with brine (3 × 50 mL). The organic layer was separated, dried over MgSO4, and evaporated under vacuum to afford the crude product, which in turn was purified by flash chromatography over silica gel. The first band containing FcPR2·BH3 was eluted with dichloromethane/methanol (75/1) and discarded. The target product was collected from a second band eluted using dichloromethane/methanol (10/1) and recrystallized from boiling ethyl acetate. Single crystals of 4a−c used for structure determination were grown from chloroform/hexane. Preparation of 4a. Compound 4a was prepared according to the general procedure and was obtained as burgundy red needles. Yield: 2.51 g (87%). 1 H NMR (399.95 MHz, CDCl3): δ 0.15−1.08 (br m, 3 H, BH3), 1.15 (dd, 3JHH = 7.1 Hz, 3JPH = 3.5 Hz, 6 H, CHMe2), 1.19 (dd, 3JHH = 7.1 Hz, 3JPH = 4.3 Hz, 6 H, CHMe2), 2.16 (d of sept, 2JPH = 10.2 Hz, 3 JHH = 7.1 Hz, 2 H, CHMe2), 4.47 (vq, J′ = 1.7 Hz, 2 H, fc), 4.52 (d of vt, J ≈ 0.9, 1.8 Hz, 2 H, fc), 4.68 (vt, J′ = 2.0 Hz, 2 H, fc), 4.94 (vt, J′ = 2.0 Hz, 2 H, fc). Note: the signal of the carboxyl proton was not identified because of extensive broadening. 13C{1H} NMR (100.58 MHz, CDCl3): δ 17.20 (s, 2 C, CHMe2), 17.51 (d, 2JPC = 1 Hz, 2 C, CHMe2), 22.61 (d, 1JPC = 35 Hz, 2 C, CHMe2), 69.97 (d, 1JPC = 54 Hz, Cipso-P of fc), 70.75 (s, Cipso-CO2H of fc), 71.97 (d, JPC = 7 Hz, 2 C, CH of fc), 73.25 (d, JPC = 7 Hz, 2 C, CH of fc), 73.44 (d, JPC = 6 Hz, 2 C, CH of fc), 74.64 (s, 2 C, CH of fc), 177.17 (s, C = O). 31P{1H} NMR (161.90 MHz, CDCl3): δ 31.7 (br d). IR (Nujol): νmax 2370 s, 2330 s, 2251 m, 1676 vs, 1299 s, 1250 w, 1203 w, 1167 s, 1103 w, 1067 m, 1038 m, 1028 m, 920 m, 889 w, 848 w, 836 m, 823 m, 780 m, 747 m, 694 m, 633 m, 592 w, 561 m, 496 m, 485 m, 463 w, 442 w, 428 w cm−1. MS (ESI+): m/z 383 ([M + Na]+), 399 ([M + K]+); MS (ESI−): m/z 359 ([M − H]−). Anal. Calcd for C17H26BFeO2P (360.01): C, 56.71; H, 7.28. Found: C, 56.57; H, 7.09. Preparation of 4b. Acid 4b was synthesized according to the previously described general procedure and was isolated as burgundy red needles. Yield: 2.68 g (86%). 1 H NMR (399.95 MHz, CDCl3): δ 0.15−1.10 (br m, 3 H, BH3), 1.29 (d, 3JPH = 12.9 Hz, 18 H, CMe3), 4.54 (d of vt, J ≈ 0.9, 1.9 Hz, 2 H, fc), 4.58 (vq, J′ = 1.7 Hz, 2 H, fc), 4.67 (vt, J′ = 2.0 Hz, 2 H, fc), 4.92 (vt, J′ = 2.0 Hz, 2 H, fc). Note: the signal due to carboxyl proton was not found due to broadening. 13C{1H} NMR (100.58 MHz, CDCl3): δ 28.64 (d, 2JPC = 2 Hz, 6 C, CMe3), 33.37 (d, 1JPC = 28 Hz, 2 C, CMe3), 70.72 (s, Cipso-CO2H of fc), 71.95 (d, 1JPC = 49 Hz, Cipso-P of fc), 72.10 (s, 2 C, CH of fc), 73.42 (d, JPC = 6 Hz, 2 C, CH of fc), 74.70 (d, JPC = 6 Hz, 2 C, CH of fc), 75.14 (s, 2 C, CH of fc), 177.29 (s, CO). 31P{1H} NMR (161.90 MHz, CDCl3): δ 45.4 (br d). IR (Nujol): 2378 s, 2366 s, 2342 s, 2258 m, 1667 vs, 1403 m, 1348 m, 1292 s, 1214 w, 1204 w, 1181 m, 1161 s, 1139 m, 1062 m, 1034 s, 1025 s, 939 m, 915 m, 892 w, 853 w, 838 s, 813 m, 783 m, 744 m, 645 m, 626 w, 599 w, 565 m, 519 m, 497 m, 490 m, 478 m, 444 w. MS (ESI+): m/z 411 ([M + Na]+); MS (ESI−): m/z 387 ([M − H]−). Anal. Calcd for C19H30BFeO2P (388.06): C, 58.80; H, 7.79. Found: C, 58.75; H, 7.60.

mL). The organic layer was separated, dried over anhydrous MgSO4, and evaporated under reduced pressure. The resulting residue was dissolved in dichloromethane (100 mL) and evaporated with chromatography-grade silica gel (ca. 150 mL). The preadsorbed product was transferred onto a silica gel column packed in hexane. Elution with hexane removed unreacted 2 and bromoferrocene. Subsequent elution with toluene/hexane (3/1) led to the development of an orange band, which afforded analytically pure 3a after evaporation. Yield: 8.00 g (81%), orange-brown microcrystalline solid. The compound was recrystallized from hot heptane. 1 H NMR (399.95 MHz, CDCl3): δ 0.20−1.05 (br m, 3 H, BH3), 1.12−1.21 (m, 12 H, CHMe2), 2.15 (d sept, 2JPH = 10.2, 3JHH = 7.1 Hz, 2 H, CHMe2), 4.30 (vt, J′ = 1.9 Hz, 2 H, fc), 4.41 (vq, J′ = 1.8 Hz, fc), 4.46 (d of vt, J = 0.9, 1.8 Hz, 2 H, CH of fc), 4.52 (vt, J′ = 1.9 Hz, 2 H, CH of fc). 13C{1H} NMR (100.58 MHz, CDCl3): δ 17.20 (s, 2 C, CHMe2), 17.50 (d, 2JPC = 2 Hz, 2 C, CHMe2), 22.60 (d, 1JPC = 35 Hz, 2 C, CHMe2), 69.16 (d, 1JPC = 55 Hz, Cipso-P of fc), 69.72 (s, 2 C, CH of fc), 71.63 (s, 2 C, CH of fc), 73.68 (d, JPC = 7 Hz, 2 C, CH of fc), 74.66 (d, JPC = 6 Hz, 2 C, CH of fc), 78.07 (s, Cipso-Br of fc). 31P{1H} NMR (161.90 MHz, CDCl3): δ 31.8 (br m). IR (Nujol): νmax 2373 s, 2362 s, 2326 s, 2248 w, 1665 w, 1410 m, 1349 m, 1305 w, 1262 w, 1250 m, 1204 w, 1168 m, 1151 m, 1105 w, 1069 s, 1037 s, 1026 m, 1009 w, 970 w, 937 w, 888 m, 869 s, 841 m, 825 s, 752 m, 692 s, 633 m, 588 m, 510 m, 496 s, 487 m, 465 m, 446 m cm−1. MS (ESI+): m/z 417 ([M + Na]+), 433 ([M + K]+). Anal. Calcd for C16H25BBrFeP (394.90): C, 48.66; H, 6.38. Found: C, 48.85; H, 6.16. Synthesis of 3b. Compound 3b was prepared similarly to 3a, starting from 2 (8.60 g, 25 mmol) and using LiBu (10 mL of a 2.5 M solution in hexanes, 25 mmol), chlorodi-tert-butylphosphine (5.44 mL, 27.5 mmol), and BH3·SMe2 (16.25 mL of a 2 M solution in THF, 32.5 mmol). Workup and chromatographic purification was performed as previously described (note: the initial elution with hexane removed some 2 and bromoferrocene, whereas the subsequent elution with toluene/hexane (3/1) provided a minor band due to side products and a third, major band containing 3b). Compound 3b was isolated as an orange-brown oil that solidified to a microcrystalline solid. Yield: 6.95 g (66%). The compound was recrystallized from hot heptane. 1 H NMR (399.95 MHz, CDCl3): δ 0.20−1.10 (br m, 3 H, BH3), 1.29 (d, 3JPH = 12.8 Hz, 18 H, CMe3), 4.28 (vt, J′ = 1.9 Hz, 2 H, fc), 4.48 (d of vt, J = 0.9, 1.8 Hz, 2 H, fc), 4.49 (vt, J′ = 2.0 Hz, 2 H, fc), 4.54 (vq, J′ = 1.8 Hz, 2 H, fc). 13C{1H} NMR (100.58 MHz, CDCl3): δ 28.67 (d, 2JPC = 2 Hz, 6 C, CMe3), 33.36 (d, 1JPC = 28 Hz, 2 C, CMe3), 70.26 (s, 2 C, CH of fc), 71.35 (d, 1JPC = 50 Hz, Cipso-P of fc), 71.76 (s, 2 C, CH of fc), 74.67 (d, JPC = 6 Hz, 2 C, CH of fc), 75.18 (d, JPC = 6 Hz, 2 C, CH of fc), 78.06 (s, Cipso-Br of fc). 31P{1H} NMR (161.90 MHz, CDCl3): δ 45.4 (br m). IR (Nujol): νmax 2386 s, 2375 m, 2363 s, 2344 m, 2277 w, 2264 w, 1685 w, 1637 w, 1301 w, 1191 m, 1182 m, 1157 m, 1067 m, 1059 m, 1031 m, 1023 m, 936 w, 896 w, 875 m, 855 w, 824 s, 759 m, 749 m, 675 w, 647 m, 626 w, 572 w, 515 m, 505 m, 478 s, 439 w cm−1. MS (ESI+): m/z 445 ([M + Na]+), 461 ([M + K]+). Anal. Calcd for C18H29BBrFeP (422.96): C, 51.11; H, 6.91. Found: C, 51.27; H, 6.68. Synthesis of 3c. Compound 3c was prepared analogously to 3a using 2 (8.60 g, 25 mmol), LiBu (10 mL of a 2.5 M solution in hexanes, 25 mmol), chlorodicyclohexylphosphine (6.25 mL, 27.5 mmol), and BH3·SMe2 (16.25 mL of a 2 M solution in THF, 32.5 mmol). The workup was performed as previously described, except that chloroform was used for the partitioning of the reaction mixture (instead of diethyl ether) for solubility reasons. After chromatographic purification, compound 3c was isolated as a yellow-orange microcrystalline solid. Yield: 10.60 g (89%). 1 H NMR (399.95 MHz, CDCl3): δ 0.10−1.05 (br m, 3 H, BH3), 1.10−1.40 (m, 10 H, Cy), 1.60−2.00 (m, 12 H, Cy), 4.27 (vt, J′ = 2.0 Hz, 2 H, fc), 4.37 (vq, J′ = 1.8 Hz, 2 H, fc), 4.45 (d of vt, J′ = 0.9, 1.9 Hz, 2 H, fc), 4.50 (vt, J′ = 1.9 Hz, 2 H, fc). 13C{1H} NMR (100.58 MHz, CDCl3): δ 25.93 (d, JPC = 1 Hz, 2 C, CH2 of Cy), 26.83 (s, 2 C, CH2 of Cy), 26.93 (d, JPC = 2 Hz, 2 C, CH2 of Cy), 26.99 (s, 2 C, CH2 of Cy), 27.25 (d, JPC = 2 Hz, 2 C, CH2 of Cy), 32.42 (d, 1JPC = 34 Hz, 2 C, CH of Cy), 69.57 (s, 2 C, CH of fc), 70.04 (d, 1JPC = 56 Hz, CipsoP of fc), 71.68 (s, 2 C, CH of fc), 73.97 (d, JPC = 7 Hz, 2 C, CH of fc), I


Article

Organometallics Synthesis of 4c. Compound 4c was prepared according to the previously described general procedure, resulting in the formation of burgundy red needlelike crystals. Yield: 2.62 g (74%). 1 H NMR (399.95 MHz, CDCl3): δ 0.15−1.00 (br m, 3 H, BH3), 1.10−1.45 (m, 10 H, Cy), 1.61−2.03 (m, 12 H, Cy), 4.43 (vq, J′ = 1.8 Hz, 2 H, fc), 4.51 (d of vt, J ≈ 0.9, 1.8 Hz, 2 H, fc), 4.66 (vt, J′ = 2.0 Hz, 2 H, fc), 4.92 (vt, J′ = 2.0 Hz, 2 H, fc). Note: the signal due to carboxyl proton is broadened. 13C{1H} NMR (100.58 MHz, CDCl3): δ 25.91 (d, JPC = 2 Hz, 2 C, CH2 of Cy), 26.80 (d, JPC = 1 Hz, 2 C, CH2 of Cy), 26.91 (s, 2 C, CH2 of Cy), 27.04 (s, 2 C, CH2 of Cy), 27.28 (d, JPC = 2 Hz, 2 C, CH2 of Cy), 32.39 (d, 1JPC = 34 Hz, 2 C, CH of Cy), 70.72 (d, 1JPC = 54 Hz, Cipso-P of fc), 70.76 (s, Cipso-CO2H of fc), 72.01 (s, 2 C, CH of fc), 73.21 (d, JPC = 6 Hz, 2 C, CH of fc), 73.38 (d, JPC = 7 Hz, 2 C, CH of fc), 74.58 (s, 2 C, CH of fc), 177.45 (s, CO). 31P{1H} NMR (161.90 MHz, CDCl3): δ 24.2 (br s). IR (Nujol): νmax 2380 s, 2366 s, 2331 s, 2254 m, 1672 vs, 1404 m, 1357 w, 1347 w, 1300 s, 1272 w, 1204 m, 1165 s, 1141 w, 1133 w, 1115 w, 1069 s, 1063 s, 1050 m, 1042 w, 1037 w, 1027 m, 1006 m, 937 m, 913 m, 897 w, 853 m, 840 s, 822 m, 780 w, 767 w, 757 w, 748 m, 669 w, 634 m, 605 m, 563 m, 519 w, 495 m, 461 m, 444 w, 428 m cm−1. MS (ESI+): m/z 463 ([M + Na]+), 479 ([M + K]+); MS (ESI−): m/z 439 ([M − H]−). Anal. Calcd for C23H34BFeO2P (440.14): C, 62.76; H, 7.79. Found: C, 62.50; H, 7.65. General Procedure for the Synthesis of Protected Amides 5a−c. Under argon, the corresponding carboxylic acid 4 (3.0 mmol) and 1-hydroxybenzotriazole (486 mg, 3.6 mmol) were suspended in anhydrous dichloromethane (20 mL). The mixture was cooled in an ice bath and treated with neat 1-ethyl-3-[3-(dimethylamino)propyl]carbodiimide (0.62 mL, 3.6 mmol), at which point all solids (i.e., the triazole) dissolved. The reaction mixture was stirred at 4 °C for another 30 min before methylamine was added (12 mL of a 2.0 M solution in THF, 24.0 mmol), and stirring was continued for 48 h at room temperature. The reaction was terminated by the addition of 3 M HCl (30 mL), and the reaction mixture was transferred to a separatory funnel and diluted with ethyl acetate (30 mL). The organic layer was separated and washed with saturated aqueous NaHCO3 and brine (30 mL each). The organic layer was dried over MgSO4 and evaporated, affording crude product that was purified by chromatography over neutral alumina and recrystallized as described below. Preparation of 5a. Amide 5a was synthesized according to the previously described general procedure from 4a (1.08 g, 3.0 mmol). Dichloromethane/methanol (20/1) was used during the chromatography, and the compound was crystallized from boiling heptane. Yield: 0.87 g (78%), orange prismatic crystals. 1 H NMR (399.95 MHz, CDCl3): δ 0.22−1.08 (br m, 3 H, BH3), 1.14 (dd, 3JHH = 7.0 Hz, 3JPH = 6.4 Hz, 6 H, CHMe2), 1.18 (dd, 3JHH = 7.1 Hz, 3JPH = 5.1 Hz, 6 H, CHMe2), 2.15 (d of sept, 2JPH = 10.0 Hz, 3 JHH = 7.1 Hz, 2 H, CHMe2), 2.94 (d, 3JHH = 4.9 Hz, 3 H, NHMe), 4.22 (vq, J′ = 1.7 Hz, 2 H, fc), 4.36 (vt, J′ = 2.0 Hz, 2 H, fc), 4.55 (d of vt, J′ ≈ 0.9, 1.8 Hz, 2 H, fc), 4.82 (vt, J′ = 1.9 Hz, 2 H, fc), 6.71 (br q, 3 JHH = 4.4 Hz, 1 H, NH). 13C{1H} NMR (150.93 MHz, CDCl3): δ 17.01 (s, 2 C, CHMe2), 17.24 (d, 2JPC = 2 Hz, 2 C, CHMe2), 22.22 (d, 1 JPC = 36 Hz, 2 C, CHMe2), 26.16 (s, NHMe), 68.14 (d, 1JPC = 55 Hz, Cipso-P of fc), 70.17 (s, 2 C, CH of fc), 71.65 (s, 2 C, CH of fc), 71.97 (d, JPC = 6 Hz, 2 C, CH of fc), 73.41 (d, JPC = 7 Hz, 2 C, CH of fc), 78.37 (s, Cipso-CO of fc), 169.84 (s, CO). 31P{1H} NMR (161.90 MHz, CDCl3): δ 31.7 (br d). IR (Nujol): νmax 3307 s, 2376 s, 2327 s, 2250 m, 1632 s, 1559 s, 1408 m, 1348 m, 1307 s, 1250 w, 1195 m, 1168 m, 1149 m, 1069 m, 1036 m, 1006 w, 960 w, 888 m, 829 m, 788 w, 774 w, 693 m, 669 w, 633 w, 593 w, 531 m, 517 m, 490 m, 463 w cm−1. MS (ESI+): m/z 396 ([M + Na]+), 412 ([M + K]+); MS (ESI−): m/z 372 ([M − H]−). Anal. Calcd for C18H29BFeNOP (373.06): C, 57.95; H, 7.84; N, 3.75. Found: C, 57.83; H, 7.62; N, 4.05. Preparation of 5b. Compound 5b was synthesized according to the previously described general procedure from acid 4b (1.16 g, 3.0 mmol). Chromatographic purification was performed with dichloromethane/methanol (20/1), and the product was crystallized from hot ethyl acetate/heptane (approximately 1/1). Yield: 697 mg (58%), orange crystalline solid.

H NMR (399.95 MHz, CDCl3): δ 0.28−1.20 (br m, 3 H, BH3), 1.31 (d, 3JHH = 12.9 Hz, 18 H, CMe3), 2.94 (d, 3JHH = 4.9 Hz, 3 H, NHMe), 4.32−4.35 (m, 4 H, 2× CH of fc), 4.58 (d of vt, J′ ≈ 0.9, 1.9 Hz, 2 H, fc), 4.78 (vt, J′ = 2.0 Hz, 2 H, fc), 6.74 (broad q, 3JHH ≈ 3.4 Hz, 1 H, NH). 13C{1H} NMR (100.58 MHz, CDCl3): δ 26.14 (s, NHMe), 28.75 (d, 2JPC = 2 Hz, 6 C, CMe3), 33.38 (d, 1JPC = 28 Hz, 2 C, CMe3), 70.41 (s, 2 C, CH of fc), 70.93 (d, 1JPC = 50 Hz, Cipso-P of fc), 71.71 (d, JPC = 6 Hz 2 C, fc), 72.07 (s, 2 C, CH of fc), 74.86 (d, JPC = 6 Hz, 2 C, CH of fc), 78.60 (s, Cipso-CO of fc), 169.90 (s, C O). 31P{1H} NMR (161.90 MHz, CDCl3): δ 44.8 (s). IR (Nujol): νmax 3269 s, 2379 s, 2352 s, 2258 m, 1628 s, 1559 s, 1408 m, 1344 m, 1310 s, 1216 w, 1194 m, 1156 m, 1069 m, 1057 w, 1032 m, 1024 m, 984 w, 958 w, 941 w, 915 w, 890 w, 849 w, 838 s, 813 m, 794 w, 779 w, 749 m, 645 m, 625 w, 596 w, 569 w, 525 m, 515 m, 485 m, 465 w cm−1. MS (ESI+): m/z 424 ([M + Na]+), 440 ([M + K]+); MS (ESI−): m/z 400 ([M − H]−). Anal. Calcd for C20H33BFeNOP (401.11): C, 59.89; H, 8.29; N, 3.49. Found: C, 59.81; H, 8.28; N, 3.31. Preparation of 5c. Amide 5c was obtained from 4c (1.32 g, 3.0 mmol) according to the general procedure described. Dichloromethane/methanol (20/1) was used to elute the product, which was subsequently recrystallized from hot ethyl acetate. Yield: 1.15 g (85%), orange crystalline solid. The crystals used for structure determination were selected from the reaction batch. 1 H NMR (399.95 MHz, CDCl3): δ 0.20−1.06 (br m, 3 H, BH3), 1.10−1.38 (m, 10 H, Cy), 1.65−2.06 (m, 12 H, Cy), 2.93 (d, 3JHH = 4.9 Hz, 3 H, NHMe), 4.19 (vq, J′ = 1.8 Hz, 2 H, fc), 4.33 (vt, J′ = 2.0 Hz, 2 H, fc), 4.54 (d of vt, J′ ≈ 0.9, 1.8 Hz, 2 H, fc), 4.80 (vt, J′ = 2.0 Hz, 2 H, fc), 6.71 (broad q, 3JHH = 4.6 Hz, 1 H, NH). 13C{1H} NMR (150.93 MHz, CDCl3): δ 25.86 (d, JPC = 1 Hz, 2 C, Cy), 26.15 (s, NHMe), 26.63 (s, 2 C, Cy), 26.76 (d, JPC = 7 Hz, 2 C, Cy), 26.83 (d, JPC = 9 Hz, 2 C, Cy), 26.98 (d, JPC = 3 Hz, 2 C, Cy), 31.84 (d, 1JPC = 35 Hz, 2 C, CH of Cy), 68.75 (d, 1JPC = 55 Hz, Cipso-P of fc), 70.16 (s, 2 C, CH of fc), 71.68 (s, 2 C, CH of fc), 71.89 (d, JPC = 6 Hz, 2 C, CH of fc), 73.49 (d, JPC = 7 Hz, 2 C, CH of fc), 78.41 (s, Cipso-CO of fc), 169.89 (s, CO). 31P{1H} NMR (161.90 MHz, CDCl3): δ 24.0 (br d). IR (Nujol): νmax 3319 s, 3091 m, 2390 s, 2371 s, 2331 s, 2253 m, 1645 m, 1628 s, 1559 s, 1547 s, 1407 m, 1347 m, 1307 s, 1224 w, 1203 w, 1190 w, 1166 m, 1150 w, 1071 m, 1057 w, 1003 w, 917 w, 853 m, 831 s, 770 w, 757 m, 669 m, 635 m, 606 w, 523 m, 507 m, 488 m, 466 w cm−1. MS (ESI+): m/z 476 ([M + Na]+), 492 ([M + K]+); MS (ESI−): m/z 439 ([M − H − BH3]−), 452 ([M − H]−). Anal. Calcd for C24H37BFeNOP (453.19): C, 63.61; H, 8.23; N, 3.09. Found: C, 63.41; H, 8.11; N, 3.00. Preparation of 5d. A solution of 1d (214 mg, 0.5 mmol) in anhydrous THF (10 mL) was treated with BH3·SMe2 (0.4 mL of a 2.0 M solution in THF, 0.8 mmol). After the mixture was stirred for 30 min, it was diluted with methanol (1 mL; caution! gas evolution) and the volatiles were removed under vacuum. The crude product was purified by chromatography over silica gel using dichloromethane/ methanol (50/1) as the eluent. The solitary band was collected and evaporated to afford 5d as an orange solid. Yield: 202 mg (91%). If necessary, the compound can be recrystallized from hot ethyl acetate. 1 H NMR (399.95 MHz, CDCl3): δ 0.80−1.80 (br m, 3 H, BH3), 2.86 (d, 3JHH = 4.8 Hz, 3 H, NHMe), 4.12 (vt, J′ = 2.0 Hz, 2 H, fc), 4.25 (vq, J′ = 1.9 Hz, 2 H, fc), 4.62 (d of vt, J′ ≈ 1.0, 1.9 Hz, 2 H, fc), 4.77 (vt, J′ = 2.0 Hz, 2 H, fc), 6.21 (broad q, 3JHH = 4.8 Hz, 1 H, NH), 7.41−7.46 (m, 4 H, Ph), 7.47−7.53 (m, 2 H, Ph), 7.54−7.61 (m, 4 H, Ph). 13C{1H} NMR (100.58 MHz, CDCl3): δ 26.35 (s, NHMe), 70.15 (s, 2 C, CH of fc), 70.34 (d, 1JPC = 68 Hz, Cipso-P of fc), 71.60 (s, 2 C, CH of fc), 73.54 (d, JPC = 8 Hz, 2 C, CH of fc), 74.37 (d, JPC = 10 Hz, 2 C, CH of fc), 78.08 (s, Cipso-CO of fc), 128.61 (d, JPC = 10 Hz, 4 C, CH of Ph), 130.36 (d, 1JPC = 60 Hz, 2 C, Cipso of Ph), 131.24 (d, JPC = 2 Hz, 2 C, CH of Ph), 132.64 (d, JPC = 9 Hz, 4 C, CH of Ph), 169.68 (s, CO). 31P{1H} NMR (161.90 MHz, CDCl3): δ 16.2 (br d). IR (Nujol): νmax 3295 s, 2373 s, 2339 s, 2246 w, 1635 s, 1541 s, 1403 w, 1303 s, 1217 w, 1171 m, 1153 w, 1131 w, 1107 m, 1058 s, 1025 s, 999 w, 837 m, 819 m, 797 w, 759 w, 740 s, 699 s, 637 m, 624 w, 610 w, 517 s, 504 w, 480 m, 455 w cm−1. MS (ESI+): m/z 464 ([M + Na]+), 480 ([M + K]+); MS (ESI−): m/z 426 ([M − H − BH3]−), 440 ([M − 1

J


Article

Organometallics H]−). Anal. Calcd for C24H25BFeNOP (441.09): C, 65.35; H, 5.71; N, 3.18. Found: C, 65.24; H, 5.75; N, 3.05. General Procedure for the Deprotection of Borane Adducts 5a−c. A Schlenk flask was charged with the respective borane adduct 5 (0.5 mmol) and 1,4-diazabicyclo[2.2.2]octane (224 mg, 2.0 mmol). The reaction vessel was flushed with argon by three vacuum−argon cycles and sealed with a rubber septum. Anhydrous THF (10 mL) was introduced, and the resulting mixture was heated at 65 °C overnight. The solvent was evaporated under vacuum, and the crude product was purified by flash column chromatography to provide analytically pure amide 1. Preparation of 1a. Amide 1a was isolated by chromatography over silica gel using dichloromethane/methanol (50/1) as the eluent and was isolated as an orange solid. Yield: 170 mg (95%). 1 H NMR (399.95 MHz, CDCl3): δ 1.09 (dd, 3JHH = 7.1 Hz, 3JPH = 2.4 Hz, 6 H, CHMe2), 1.12 (dd, 3JHH = 7.1 Hz, 3JPH = 4.8 Hz, 6 H, CHMe2), 1.94 (sept of d, 3JHH = 7.1 Hz, 2JPH = 2.6 Hz, 2 H, CHMe2), 2.95 (d, 3JHH = 4.9 Hz, 3 H, NHMe), 4.12 (vq, J′ = 1.6 Hz, 2 H, fc), 4.30 (vt, J′ = 1.9 Hz, 2 H, fc), 4.35 (vt, J′ = 1.8 Hz, 2 H, fc), 4.56 (vt, J′ = 1.9 Hz, 2 H, fc), 6.34 (br m, 1 H, NH). 13C{1H} NMR (100.58 MHz, CDCl3): δ 19.72 (d, 2JPC = 10 Hz, 2 C, CHMe2), 20.08 (d, 2JPC = 15 Hz, 2 C, CHMe2), 23.33 (d, 1JPC = 11 Hz, 2 C, CHMe2), 26.27 (s, NHMe), 70.08 (d, JPC = 1 Hz, 2 C, CH of fc), 71.22 (d, JPC = 2 Hz, 2 C, CH of fc), 71.35 (s, 2 C, CH of fc), 72.93 (d, JPC = 9 Hz, 2 C, CH of fc), 76.31 (d, 1JPC = 17 Hz, Cipso-P of fc), 77.16 (s, Cipso-CO of fc), 170.96 (s, CO). 31P{1H} NMR (161.90 MHz, CDCl3): δ 0.8 (s). IR (Nujol): νmax 3341 s, 1634 m, 1537 s, 1405 m, 1296 s, 1240 w, 1215 w, 1203 w, 1188 m, 1152 m, 1077 w, 1037 w, 1029 m, 957 w, 832 s, 776 w, 635 w, 610 w, 590 w, 531 m, 513 m, 501 m, 492 m, 465 w cm−1. MS (ESI+): m/z 360 ([M + H]+), 382 ([M + Na]+). HRMS: calcd for C18H27FeNOP ([M + H]+) 360.1174, found 360.1177. Preparation of 1b. Compound 1b was prepared according to the general procedure. Silica gel and dichloromethane/methanol (50/1) were used during the chromatography. Yield: 165 mg (85%), orange solid. 1 H NMR (399.95 MHz, CDCl3): δ 1.22 (d, 3JPH = 11.3 Hz, 18 H, CMe3), 2.95 (d, 3JHH = 4.9 Hz, 3 H, NHMe), 4.23 (vq, J′ = 1.7 Hz, 2 H, fc), 4.30 (vt, J′ = 1.9 Hz, 2 H, fc), 4.39 (vt, J′ = 1.8 Hz, 2 H, fc), 4.59 (vt, J′ = 1.9 Hz, 2 H, fc), 6.15 (br s, 1 H, NH). 13C{1H} NMR (100.58 MHz, CDCl3): δ 26.40 (s, NHMe), 30.79 (d, 2JPC = 13 Hz, 6 C, CMe3), 32.92 (d, 1JPC = 20 Hz, 2 C, CMe3), 69.97 (s, 2 C, CH of fc), 71.14 (d, JPC = 2 Hz, 2 C, CH of fc), 71.98 (s, 2 C, CH of fc), 74.14 (d, JPC = 11 Hz, 2 C, CH of fc), 78.95 (d, 1JPC = 27 Hz, Cipso-P of fc), 170.89 (s, CO). Signal due to the other Cipso carbon of the ferrocene unit was not observed. 31P{1H} NMR (161.90 MHz, CDCl3): δ 28.0 (s). IR (Nujol): νmax 3335 s, 3302 s, 3092 m, 1629 s, 1560 s, 1407 m, 1363 m, 1307 s, 1225 w, 1194 m, 1175 m, 1148 m, 1054 w, 1037 w, 1028 m, 935 w, 911 w, 890 w, 867 w, 847 w, 825 s, 811 m, 788 w, 775 w, 691 m, 633 w, 586 w, 571 w, 527 m, 513 m, 491 m, 483 m, 460 w, 442 w cm−1. MS (ESI+): m/z 388 ([M + H]+). HRMS: calcd for C20H31FeNOP ([M + H]+) 388.1487, found 388.1490. Preparation of 1c. Amide 1c was obtained from 5c according to the general procedure and using dichloromethane/methanol (75/1) and silica gel for chromatographic purification. Yield: 181 mg (82%), orange solid. The crystals for X-ray diffraction analysis were grown from hot ethyl acetate. 1 H NMR (399.95 MHz, CDCl3): δ 1.02−1.38 (m, 10 H, Cy), 1.63−1.84 (m, 10 H, Cy), 1.88−2.20 (m, 2 H, Cy), 2.94 (d, 3JHH = 4.8 Hz, 3 H, NHMe), 4.09 (vq, J′ = 1.6 Hz, 2 H, fc), 4.28 (vt, J′ = 1.9 Hz, 2 H, fc), 4.35 (vt, J′ = 1.8 Hz, 2 H, fc), 4.53 (vt, J′ = 1.9 Hz, 2 H, fc), 6.34 (br m, 1 H, NH). 13C{1H} NMR (100.58 MHz, CDCl3): δ 26.28 (s, NHMe), 26.37 (d, JPC = 1 Hz, 2 C, CH2 of Cy), 27.22 (d, JPC = 8 Hz, 2 C, CH2 of Cy), 27.38 (d, JPC = 11 Hz, 2 C, CH2 of Cy), 30.07 (d, JPC = 2 Hz, 2 C, CH2 of Cy), 30.18 (d, JPC = 2 Hz, 2 C, CH2 of Cy), 33.29 (d, 1JPC = 11 Hz, 2 C, CH of Cy), 70.09 (d, JPC = 2 Hz, 2 C, CH of fc), 71.15 (d, JPC = 2 Hz, 2 C, CH of fc), 71.32 (s, 2 C, CH of fc), 73.08 (d, JPC = 10 Hz, 2 C, CH of fc), 77.22 (s, Cipso-CO of fc), 171.00 (s, CO). The signal due to one Cipso of the ferrocene unit overlaps with the solvent resonance. 31P{1H} NMR (161.90 MHz,

CDCl3): δ − 7.6 (s). IR (Nujol): νmax 3315 s, 1630 s, 1559 s, 1407 m, 1344 m, 1304 s, 1263 w, 1191 m, 1166 w, 1147 m, 1053 w, 1029 m, 1020 w, 1000 w, 890 w, 884 w, 845 w, 824 s, 788 w, 775 w, 680 m, 650 m, 629 w, 618 w, 583 w, 528 m, 516 m, 494 m, 472 m, 452 w, 441 w cm−1. MS (ESI+): m/z 440.2 ([M + H]+). HRMS: calcd for C24H35FeNOP ([M + H]+) 440.1801, found 440.1800. General Procedure for the Synthesis of Phosphine Selenides 6. Amide 1 (0.5 mmol) and KSeCN (79 mg, 0.55 mmol) were dissolved in anhydrous methanol (5 mL), and the resulting solution was stirred at room temperature overnight. The reaction mixture was filtered through a PTFE syringe filter (pore size 0.45 μm) to remove separated KCN, and the filtrate was evaporated under vacuum, affording crude 6, which was purified by flash chromatography as described below. Preparation of 6a. The synthesis of 6a was performed as previously described, and the crude product was chromatographed over silica gel with dichloromethane/methanol (75/1). After evaporation of the solvent, pure 6a was obtained as an orange solid. Yield: 178 mg (81%). Crystals suitable for structure determination were obtained by recrystallization from hot heptane. 1 H NMR (399.95 MHz, CDCl3): δ 1.18 (dd, 3JHH = 6.9 Hz, 3JPH = 5.5 Hz, 6 H, CHMe2), 1.22 (dd, 3JHH = 7.0 Hz, 3JPH = 5.1 Hz, 6 H, CHMe2), 2.30 (d of sept, 2JPH = 8.2 Hz, 3JHH = 7.0 Hz, 2 H, CHMe2), 2.93 (d, 3JHH = 4.8 Hz, 3 H, NHMe), 4.25 (vq, J′ = 1.7 Hz, 2 H, fc), 4.35 (vt, J′ = 1.8 Hz, 2 H, fc), 4.59 (vq, J′ = 1.6 Hz, 2 H, fc), 4.88 (vt, J′ = 1.8 Hz, 2 H, fc), 7.64 (broad q, 3JHH = 3.2 Hz, 1 H, NH). 13C{1H} NMR (100.58 MHz, CDCl3): δ 16.64 (d, 2JPC = 1 Hz, 2 C, CHMe2), 17.33 (d, 2JPC = 2 Hz, 2 C, CHMe2), 25.97 (s, NHMe), 27.54 (d, 1JPC = 45 Hz, 2 C, CHMe2), 70.97 s, (2 C, CH of fc), 71.59 (s, 2 C, CH of fc), 71.67 (d, JPC = 9 Hz, 2 C, CH of fc), 72.41 (d, 1JPC = 69 Hz, CipsoP of fc), 73.69 (d, JPC = 10 Hz, 2 C, CH of fc), 79.25 (s, Cipso-CO of fc), 169.77 (s, CO). 31P{1H} NMR (161.90 MHz, CDCl3): δ 59.3 (s with 77Se satellites, 1JSeP = 693 Hz). IR (Nujol): νmax 3306 s, 1631 s, 1556 s, 1407 m, 1305 s, 1222 w, 1244 w, 1195 m, 1166 m, 1149 m, 1054 w, 1030 m, 935 w, 889 w, 827 s, 775 m, 674 s, 651 m, 592 w, 560 m, 531 m, 511 m, 488 m, 471 w, 441 w cm−1. MS (ESI+): m/z 440 ([M + H]+), 462 ([M + Na]+), 478 ([M + K]+); MS (ESI−): m/z 438 ([M − H]−). Anal. Calcd for C18H26FeNOPSe (438.19): C, 49.34; H, 5.98; N, 3.20. Found: C, 49.22; H, 6.03; N, 3.05. Preparation of 6b. Selenide 6b was synthesized from phosphine 1b according to the aforementioned general procedure and was purified by chromatography over silica gel using dichloromethane/methanol (20/1) as the eluent. Yield: 169 mg (72%). The crystals used for structure determination were grown from hot ethyl acetate/heptane. 1 H NMR (399.95 MHz, CDCl3): δ 1.40 (d, 3JPH = 15.7 Hz, 18 H, CMe3), 2.93 (d, 3JHH = 4.9 Hz, 3 H, NHMe), 4.33 (vt, J′ = 1.9 Hz, 2 H, fc), 4.39 (vq, J′ = 1.7 Hz, 2 H, fc), 4.61 (vq, J′ = 1.7 Hz, 2 H, fc), 4.85 (vt, J′ = 1.9 Hz, 2 H, fc), 7.76 (broad q, 3JHH = 3.8 Hz, 1 H, NH). 13 C{1H} NMR (100.58 MHz, CDCl3): δ 25.95 (s, NHMe), 28.68 (d, 2 JPC = 2 Hz, 6 C, CMe3), 38.32 (d, 1JPC = 36 Hz, 2 C, CMe3), 71.23 (s, 2 C, CH of fc), 71.33 (d, JPC = 8 Hz, 2 C, CH of fc), 71.90 (s, 2 C, CH of fc), 75.28 (d, JPC = 9 Hz, 2 C, CH of fc), 75.42 (d, 1JPC = 62 Hz, Cipso-P of fc), 79.43 (s, Cipso-CO of fc), 169.85 (s, CO). 31P{1H} NMR (161.90 MHz, CDCl3): δ 75.5 (s with 77Se satellites, 1JSeP = 688 Hz). IR (Nujol): νmax 3264 s, 1626 s, 1559 s, 1409 m, 1341 m, 1309 s, 1215 m, 1196 m, 1174 m, 1154 m, 1052 w, 1029 m, 941 w, 838 s, 805 m, 778 w, 670 s, 650 m, 592 w, 568 w, 509 m, 492 m, 454 w cm−1. MS (ESI+): m/z 468 ([M + H]+), 490 ([M + Na]+), 506 ([M + K]+); MS (ESI−): m/z 466 ([M − H]−). Anal. Calcd for C20H30FeNOPSe (466.24): C, 51.52; H, 6.49; N, 3.00. Found: C, 51.63; H, 6.47; N, 2.99. Preparation of 6c. Compound 6c was prepared as previously described from 1c and was purified by column chromatography over silica gel with dichloromethane/methanol (10/1). Yield: 145 mg (56%), orange solid. The crystals used for structure determination were obtained from hot ethyl acetate. 1 H NMR (399.95 MHz, CDCl3): δ 1.10−1.41 (m, 10 H, Cy), 1.66−1.74 (m, 2 H, Cy), 1.78−1.92 (m, 4 H, Cy), 1.96−2.10 (m, 6 H, Cy), 2.92 (d, 3JHH = 4.9 Hz, 3 H, NHMe), 4.23 (vq, J′ = 1.8 Hz, 2 H, fc), 4.32 (vt, J′ = 2.0 Hz, 2 H, fc), 4.58 (vq, J′ = 1.7 Hz, 2 H, fc), 4.87 K


Article

Organometallics

Anal.58 Calcd for C37H52ClFe2N2O3P2Rh (884.82): C, 50.22; H, 5.92; N, 3.17. Found: C, 49.70; H, 6.01; N, 2.89. Synthesis of 7b. Complex 7b was synthesized as previously described and isolated by chromatography on silica gel using dichloromethane/methanol (20/1). Yield: 77 mg (82%), orange solid. 1 H NMR (399.95 MHz, CDCl3): δ 1.48 (apparent t, J′ = 6.8 Hz, 36 H, CMe3), 2.92 (d, 3JHH = 4.7 Hz, 6 H, NHMe), 4.44 (br m, 4 H, fc), 4.50 (br m, 4 H, fc), 4.67 (br m, 4 H, fc), 4.83 (br m, 4 H, fc), 6.48 (br q, 3JHH = 4.3 Hz, 2 H, NH). 13C{1H} NMR (150.93 MHz, CDCl3): δ 26.49 (s, 2 C, NHMe), 31.62 (s, 12 C, CMe3), 38.76 (apparent t, J′ = 7 Hz, 4 C, CMe3), 70.38 (s, 4 C, CH of fc), 71.87 (br m, 4 C, CH of fc), 74.42 (s, 4 C, CH of fc), 77.32 (br m, 4 C, CH of fc), 78.13 (apparent t, J′ = 13 Hz, 2 C, Cipso-P of fc), 170.53 (s, 2 C, CO), 189.95 (dt, 1 JRhC = 74 Hz, 2JPC = 15 Hz, CO). The signal due to ferrocene CCO overlaps with the solvent resonance. 31P{1H} NMR (161.90 MHz, CDCl3): δ 55.7 (d, 1JRhP = 126 Hz). IR (Nujol): νmax 3292 s, 1943 s, 1634 s, 1541 s, 1411 w, 1302 s, 1171 m, 1147 m, 1030 m, 935 w, 835 m, 773 w, 656 w, 581 w, 480 m, 458 w cm−1. MS (ESI+): m/z 905 ([M − Cl]+). Anal. Calcd for C41H60ClFe2N2O3P2Rh (940.92): C, 52.34; H, 6.43; N, 2.98. Found: C, 51.85; H, 6.51; N, 2.82. Synthesis of 7c. Compound 7c was prepared by the previously outlined general procedure and was purified by chromatography over silica gel using dichloromethane/methanol (20/1) as the eluent. Yield: 100 mg (96%), yellow solid. The crystals used for structure determination were obtained from ethyl acetate. 1 H NMR (399.95 MHz, CDCl3): δ 1.14−1.38 (m, 12 H, Cy), 1.44−1.61 (m, 8 H, Cy), 1.66−1.75 (m, 4 H, Cy), 1.76−1.88 (m, 8 H, Cy), 2.00−2.09 (m, 4 H, Cy), 2.19−2.28 (m, 4 H, Cy), 2.41−2.52 (m, 4 H, Cy), 2.92 (d, 3JHH = 4.9 Hz, 6 H, NHMe), 4.45 (vt, J′ = 1.8 Hz, 4 H, fc), 4.49 (vt, J′ = 2.0 Hz, 4 H, fc), 4.75 (br m, 4 H, fc), 4.84 (vt, J′ = 2.0 Hz, 4 H, fc), 6.37 (q, 3JHH = 4.7 Hz, 2 H, NH). 13C{1H} NMR (150.93 MHz, CDCl3): δ 26.24 (s, 4 C, Cy), 26.51 (s, 2 C, NHMe), 27.28 (apparent t, J′ = 6 Hz, 4 C, Cy), 27.41 (apparent t, J′ = 6 Hz, 4 C, Cy), 28.91 (s, 4 C, Cy), 29.98 (s, 4 C, Cy), 37.18 (apparent t, J′ = 12 Hz, 4 C, CH of Cy), 69.97 (s, 4 C, CH of fc), 72.19 (apparent t, J′ = 3 Hz, 4 C, CH of fc), 72.73 (s, 4 C, CH of fc), 75.08 (apparent t, J′ = 3 Hz, 4 C, CH of fc), 76.33 (apparent t, J′ = 18 Hz, 2 C, Cipso-P of fc), 77.65 (s, 2 C, Cipso-CO of fc), 170.43 (s, 2 C, CO), 189.13 (dt, 1JRhC = 75 Hz, 2JPC = 16 Hz, CO). 31P{1H} NMR (161.90 MHz, CDCl3): δ 28.9 (d, 1JRhP = 121 Hz). IR (Nujol): νmax 3300 s, 1947 s, 1635 s, 1544 s, 1411 w, 1301 s, 1193 m, 1159 m, 1054 w, 1030 m, 1003 w, 912 w, 889 w, 833 m, 773 w, 575 w, 487 m cm−1. MS (ESI+): m/z 1009 ([M − Cl]+). Anal. Calcd for C49H68ClFe2N2O3P2Rh (1045.07): C, 56.31; H, 6.56; N, 2.68. Found: C, 55.93; H, 6.66; N, 2.30. Preparation of 7d. Complex 7d was prepared according to the previously described general procedure and was purified by column chromatography over silica gel with dichloromethane/methanol (50/ 1) as the eluent. Yield: 95 mg (93%), yellow solid. 1 H NMR (399.95 MHz, CDCl3): δ 2.67 (d, 3JHH = 4.8 Hz, 6 H, NHMe), 4.44 (vt, J′ = 2.0 Hz, 4 H, fc), 4.46−4.50 (m, 8 H, fc), 4.85 (vt, J′ = 2.0 Hz, 4 H, fc), 6.16 (q, 3JHH ≈ 5.0 Hz, 2 H, NH), 7.36−7.45 (m, 12 H, Ph), 7.62−7.69 (m, 8 H, Ph). 13C{1H} NMR (150.93 MHz, CDCl3): δ 26.28 (s, 2 C, NHMe), 70.04 (s, 4 C, CH of fc), 72.57 (s, 4 C, CH of fc), 73.24 (m, 4 C, CH of fc), 75.89 (apparent t, J′ = 25 Hz, 2 C, Cipso-P of fc), 76.08 (m, 4 C, CH of fc), 78.37 (s, 2 C, Cipso-CO of fc), 128.09 (apparent t, J′ = 5 Hz, 8 C, CH of Ph), 130.26 (s, 4 C, CH of Ph), 133.78 (apparent t, J′ = 6 Hz, 8 C, CH of Ph), 134.40 (apparent t, J′ = 23 Hz, 4 C, CH of Ph), 169.87 (s, 2 C, CO), 187.22 (dt, 1JRhC = 75 Hz, 2JPC = 16 Hz, CO). 31P{1H} NMR (161.90 MHz, CDCl3): δ 21.0 (d, 1JRhP = 126 Hz). IR (Nujol): νmax 3290 s, 1961 s, 1951 s, 1625 s, 1553 s, 1435 m, 1409 m, 1366 m, 1310 m, 1301 m, 1259 w, 1196 m, 1163 m, 1098 m, 1056 m, 1037 m, 998 w, 836 s, 746 s, 726 s, 697 s, 577 m, 531 m, 510 s, 503 s, 479 m, 467 w, 456 m cm−1. MS (ESI+): m/z 957 ([M − Cl − CO]+), 985 ([M − Cl]+). Anal. Calcd for C49H44ClFe2N2O3P2Rh·0.2CH2Cl2 (1037.85): C, 56.93; H, 4.31; N, 2.70. Found: C, 56.87; H, 4.59; N, 2.69. Catalytic Experiments. Borate 8 (28.3 mg, 0.11 mmol), 4bromoanisole (9; 18.7 mg, 0.10 mmol), and mesitylene (internal standard; 12.0 mg, 0.10 mmol) were dissolved in a mixture of N,Ndimethylformamide-d7 (0.4 mL) and D2O (0.15 mL). The resulting

(vt, J′ = 2.0 Hz, 2 H, fc), 7.65 (broad q, 3JHH = 4.6 Hz, 1 H, NH). 13 C{1H} NMR (100.58 MHz, CDCl3): δ 25.72 (d, JPC = 2 Hz, 2 C, CH2 of Cy), 25.97 (s, NHMe), 26.09 (d, JPC = 2 Hz, 2 C, CH2 of Cy), 26.21 (d, JPC = 2 Hz, 2 C, CH2 of Cy), 26.46 (d, JPC = 1 Hz, 2 C, CH2 of Cy), 27.12 (d, JPC = 3 Hz, 2 C, CH2 of Cy), 36.94 (d, JPC = 45 Hz, 2 C, CH of Cy), 70.94 (s, 2 C, CH of fc), 71.59 (d, JPC = 6 Hz, 2 C, CH of fc), 71.61 (s, 2 C, CH of fc), 73.05 (d, 1JPC = 69 Hz, Cipso-P of fc), 73.76 (d, JPC = 10 Hz, 2 C, CH of fc), 79.33 (s, Cipso-CO of fc), 169.79 (s, CO). 31P{1H} NMR (161.90 MHz, CDCl3): δ 50.9 (s with 77Se satellites, 1JSeP = 686 Hz). IR (Nujol): 3322 s, 3092 m, 3074 m, 1645 m, 1628 s, 1548 s, 1407 m, 1344 m, 1306 s, 1223 w, 1199 m, 1165 m, 1152 w, 1114 w, 1081 w, 1056 m, 1042 w, 1029 m, 1022 m, 1004 m, 917 w, 894 m, 887 m, 851 m, 830 s, 776 w, 748 m, 738 m, 674 s, 626 m, 590 m, 550 m, 527 m, 504 m, 484 m, 475 w, 459 w, 452 m, 439 w cm−1. MS (ESI+): m/z 542 ([M + Na]+), 558 ([M + K]+); MS (ESI−): m/z 518 ([M − H]−). Anal. Calcd for C24H34FeNOPSe (518.31): C, 55.61; H, 6.61; N, 2.70. Found: C, 55.41; H, 6.52; N, 2.71%. Preparation of 6d. Compound 6d was prepared from 1d according to the previously outlined general procedure and was purified by chromatography over silica gel using dichloromethane/methanol (10/ 1) as the eluent. Yield: 226 mg (89%), orange solid. The crystals used for structure determination were obtained from hot ethyl acetate. 1 H NMR (399.95 MHz, CDCl3): δ 2.92 (d, 3JHH = 4.8 Hz, 3 H, NHMe), 3.96 (vt, J′ = 1.9 Hz, 2 H, fc), 4.25 (vq, J′ = 2.0 Hz, 2 H, fc), 4.65 (vq, J′ = 1.8 Hz, 2 H, fc), 4.91 (vt, J′ = 2.0 Hz, 2 H, fc), 7.36 (broad q, 3JHH = 4.0 Hz, 1 H, NH), 7.42−7.54 (m, 6 H, Ph), 7.67− 7.76 (m, 4 H, Ph). 13C{1H} NMR (100.58 MHz, CDCl3): δ 26.11 (s, NHMe), 71.05−71.20 (m, 4 C, CH of fc), 73.29 (d, JPC = 10 Hz, 2 C, CH of fc), 75.08 (d, 1JPC = 88 Hz, Cipso-P of fc), 75.09 (d, JPC = 12 Hz, 2 C, CH of fc), 78.96 (s, Cipso-CO of fc), 128.43 (d, JPC = 13 Hz, 4 C, CH of Ph), 131.70 (d, JPC = 3 Hz, 2 C, CH of Ph), 132.04 (d, JPC = 11 Hz, 4 C, CH of Ph), 132.55 (d, 1JPC = 79 Hz, 2 C, Cipso of Ph), 169.77 (s, CO). 31P{1H} NMR (161.90 MHz, CDCl3): δ 32.6 (s with 77Se satellites, 1JSeP = 718 Hz). IR (Nujol): νmax 3296 s, 1635 s, 1538 s, 1400 m, 1301 s, 1218 w, 1192 m, 1169 m, 1152 m, 1122 w, 1095 s, 1071 w, 1054 w, 1037 w, 1024 m, 1018 m, 997 w, 909 w, 890 w, 870 w, 838 m, 819 s, 797 m, 757 w, 750 m, 711 m, 695 m, 690 m, 630 w, 619 w, 618 w, 574 m, 534 w, 520 m, 509 w, 492 w, 477 m, 452 w, 444 w cm−1. MS (ESI+): m/z 508 ([M + H]+), 530 ([M + Na]+), 546 ([M + K]+); MS (ESI−): m/z 506 ([M − H]−). Anal. Calcd for C24H22FeNOPSe (506.22): C, 56.94; H, 4.38; N, 2.77. Found: C, 56.70; H, 4.24; N, 2.77. General Procedure for the Synthesis of [RhCl(CO)(L-κP)2] (7; L = 1a−d). Solid [Rh(μ-Cl) (CO)2]2 (19.4 mg, 0.05 mmol) was added to a solution of the respective amide 1 (0.22 mmol) in dichloromethane (5 mL). The resulting orange solution was stirred for 2 h and then filtered through a PTFE syringe filter (pore size 0.45 μm) and evaporated to dryness. The crude product was purified by column chromatography, as specified below. Synthesis of 7a. Complex 7a was prepared as previously described and was purified by chromatography over silica gel using dichloromethane/methanol (50/1) as the eluent. Yield: 78 mg (88%), yellow solid. 1 H NMR (399.951 MHz, CDCl3): δ 1.31 (d of apparent t, J1 ≈ J2 ≈ 7.0 Hz, 12 H, CHMeA), 1.39 (d of apparent t, J1 ≈ J2 ≈ 8.0 Hz, 12 H, CHMeB), 2.69−2.83 (m, 4 H, CHMe2), 2.91 (d, 3JHH = 4.8 Hz, 6 H, NHMe), 4.46 (vt, J′ = 1.8 Hz, 4 H, fc), 4.51 (vt, J′ = 2.0 Hz, 4 H, fc), 4.60 (m, 4 H, fc), 4.86 (vt, J′ = 2.0 Hz, 4 H, fc), 6.36 (q, 3JHH = 4.8 Hz, 2 H, NH). 13C{1H} NMR (150.93 MHz, CDCl3): δ 18.88 (s, 4 C, CHMeA), 20.31 (vt, J′ = 2 Hz, 4 C, CHMeB), 26.44 (s, 2 C, NHMe), 26.60 (apparent t, J′ = 13 Hz, 4 C, CHMe2), 69.95 (s, 4 C, CH of fc), 72.34 (apparent t, J′ = 3 Hz, 4 C, CH of fc), 72.73 (s, 4 C, CH of fc), 74.95 (apparent t, J′ = 5 Hz, 4 C, CH of fc), 77.68 (s, 2 C, Cipso-CO of fc), 170.31 (s, 2 C, CO), 188.55 (dt, 1JRhC = 75 Hz, 2JPC = 16 Hz, CO). The signal due to ferrocene C−P is obscured by the solvent resonance. 31P{1H} NMR (161.90 Hz, CDCl3): δ 38.0 (d, 1JRhP = 120.9 Hz). IR (Nujol): νmax 3296 s, 1950 s, 1633 s, 1544 s, 1410 m, 1301 m, 1246 w, 1193 w, 1158 m, 1030 m, 930 w, 883 w, 834 m, 624 m, 631 m, 572 m, 498 m cm−1. MS (ESI+): m/z 849 ([M − Cl]+). L


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Organometallics ORCID

solution was transferred to an NMR tube, and the tube was heated to 40 °C in the NMR spectrometer for several minutes. The NMR tube was then briefly removed from the spectrometer, a stock solution of the respective catalyst was added (50 μL of a solution freshly prepared by mixing 11 (1.6 mg, 0.005 mmol) and ligand 1 (0.010 or 0.0050 mmol) in 0.5 mL of N,N-dimethylformamide-d7), and the progress of the reaction was monitored by 1H NMR spectroscopy. X-ray Crystallography. Diffraction data were collected with a Nonius KappaCCD diffractometer with an Apex2 detector (Bruker) or a Bruker D8 VENTURE Kappa Duo diffractometer with a PHOTON 100 CMOS detector at 150(2) K. Graphite-monochromated Mo Kα radiation was used, with the exception of amide 1c, for which Cu Kα radiation was utilized. The structures were solved by direct methods (SHELXT)59 and refined by full-matrix least-squares methods based on F2 (SHELXL-97 or SHELXL-2014).60 Non-hydrogen atoms were assigned anisotropic displacement parameters. The BH3, carboxyl, and amide (NH) hydrogens were identified on the difference electron density maps and refined as riding atoms with Uiso(H) set to 1.2Ueq of their bonding atom. Hydrogen atoms bonded to carbons were included in their theoretical positions using the HFIX instructions in SHELXL and were refined similarly. Compound 6d crystallizes with the symmetry of the noncentrosymmetric space group Pca21. The Flack enantiomorph parameter was −0.008(2). Compound 1c also crystallizes noncentrosymmetrically (space group Pn) but as a racemic twin. The refined ratio of the enantiomeric domains was ca. 96:4. Relevant crystallographic data and structure refinement parameters are available in the Table S1 in the Supporting Information. The geometric parameters and structural drawings were obtained with a recent version of the PLATON software.61 DFT Computations. All calculations were performed using DFTspecifically, the Becke three-parameter hybrid functional62 with the nonlocal Lee−Yang−Parr exchange-correlation functional63 (B3LYP) and the Gaussian program package.64 The standard splitvalence basis set65 was used, except for selected atoms (P, Fe, and Se), for which a quadruple-ζ basis set65 was employed. For Fe atoms and heavier elements, relativistic effective core potentials (ECP) were used.66 Geometry optimizations started from the experimental X-ray structures where available. After the stationary points were located on the potential energy surface (PES), harmonic vibrational analysis was carried out using the analytically calculated force-constant matrix to ensure that the points were energy minima. The optimized geometries are available in the Supporting Information.

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Petr Štěpnička: 0000-0002-5966-0578 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Thanks are due to Dr. Simona Hybelbauerová for her assistance during the kinetic measurements. The research leading to these results has received funding from the Norwegian Financial Mechanism 2009−2014 and the Ministry of Education, Youth and Sports under Project Contract no. MSMT-23681/2015-2.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.7b00181. Summary of crystallographic data, structural drawings, an extended version of Table 5, additional correlation diagrams, and NMR spectra (PDF) Cartesian coordinates for the DFT-optimized structures (XYZ) Accession Codes

CCDC 1536168−1536177 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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REFERENCES

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