PC(sp³)P pincer carbonyl complexes of iridium(I), and iridium(III)

Abstract

The previously reported complex trans-[IrHCl{cis-1,3-bis-(di-tert-butylphosphino)methyl}cyclohexane] (2) forms the 18-electron carbonyl compound anti-[Ir(CO)HCl{cis-1,3-bis-((di-tert-butylphosphino)methyl)}cyclohexane] (5a) upon reaction with 1 atm CO. The structural isomer syn-[IrH(CO)Cl{cis-1,3-bis-((di-tert-butylphosphino)methyl)}cyclohexane] (5b) is obtained directly upon complexation of the ligand (1) with IrCl₃·H₂O in refluxing DMF. syn-5b is the first iridium aliphatic pincer complex with this orientation of the hydrogens and is the thermodynamically more stable isomer. Both compounds 5a and 5b afford the Ir(I) complex trans-[Ir(CO){cis-1,3-bis-((di-tert-butylphosphino)methyl)}cyclohexane] (4) upon treatment with KO^tBu. Complex 4 was also synthesised in a more straightforward fashion from the previously known terminal nitrogen complex trans-[Ir(N₂){cis-1,3-bis-((di-tert-butylphosphino)-methyl)}cyclohexane] (3) under atmospheric CO. The complexes 4, 5a and 5b were characterised spectroscopically and in the solid state. IR data point to a more electron rich metal centre as compared to the corresponding aromatic complexes.

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Introduction

The chemistry of iridium PCP pincer-type complexes has been continuously developed over the last decades, mainly owing to their applications as active homogeneous catalysts in the dehydrogenation¹ of alkanes,^2–6 alcohols^7,8 and amine-boranes.^9,10 Oxidative additions and reductive eliminations are fundamental processes in these and many other catalytic transformations and stoichiometric reactions, and are highly influenced by the electronic properties of the metal centre.¹¹ In this aspect, the application of all-aliphatic pincer backbones is a relevant task, since the properties of a C(sp³)-compared to the more common C(sp²)-based PCP complexes might differ significantly due to electronic factors such as stronger trans influence by the metallated carbon and a metal centre with higher nucleophilicity.¹² Also, the hybridization is expected to influence the rate of any concerted reaction.^13,14 Carbon monoxide has been long known to coordinatively add to both PC(sp²)P-¹⁵ and PC(sp³)P-supported¹⁶ iridium(III) complexes, and such iridium carbonyl complexes have later been found to be involved in catalytic transformations such as transfer hydrogenations of ketones⁸ and olefin hydroformylation.¹⁷ PCP iridium(I) carbonyl complexes are well known for benzene based pincer structures,^7,18–22 and have been reported to catalyse the decarbonylation of 2-naphtaldehyde²³ and the partial deoxygenation of diols²⁴ and glycerol,²⁵ but there are no PC(sp³)P-supported iridium(I) carbonyl complexes reported to this date.

Here we report on the synthesis and interconversion of PC(sp³)P pincer carbonyl complexes with iridium(I) and iridium(III). The electronic properties of the PC(sp³)P pincer ligand is also probed using carbonyl stretching frequencies.

Experimental section

General comments

All manipulations were performed under a nitrogen or argon atmosphere using standard Schlenk or glovebox techniques, except where noted. Solvents were purified by vacuum distillation from sodium/benzophenone ketyl radical. The ligand cis-1,3-bis-((di-tert-butylphosphino)methyl)}cyclohexane, 1, and the complexes 2 and 3 were prepared according to previously reported procedures,^26,27cf.Scheme 1 for numbering. All other chemicals were purchased from commercial suppliers and used as received. ¹H-, ¹³C and ³¹P-NMR experiments were recorded on a Varian Unity INOVA 500 spectrometer, operating at 499.76 (¹H), 125.68 (¹³C) and 202.31 (³¹P) MHz. For ¹H- and ¹³C-NMR spectra, the residual solvent peak was used as an internal reference. ³¹P-NMR spectra were referenced externally using 85% H₃PO₄ at δ = 0 ppm. Multiplicities are abbreviated as follows: (s) singlet, (d) doublet, (t) triplet, (q) quartet, (m) multiplet, (br) broad, (v) virtual. IR spectra were obtained on a Bruker ALPHA FT-IR spectrometer. Elemental analyses were performed by H. Kolbe Microanalytisches Laboratorium, Mülheim an der Ruhr, Germany.

Scheme 1

Crystallography

XRD-quality crystals of compounds 4, 5a and 5b were obtained through recrystallization from toluene or hexane. Intensity data were collected with an Oxford Diffraction Excalibur 3 system, using ω-scans and MoKα (λ = 0.71073 Å) radiation.²⁸ The data were extracted and integrated using Crysalis RED.²⁸ The structure was solved by direct methods and refined by full-matrix least-squares calculations on F² using SHELXTL5.1.²⁹ Compound 4 formed small, weakly diffracting crystals, giving rise to a high R_int.³⁰ Non-H atoms were refined with anisotropic displacement parameters. Hydrogen atoms were constrained to parent sites, using a riding model. For 5a and 5b attempts were made to locate the hydride atoms. Although residual electron density could be located in the expected area trans to CO and Cl, respectively, all attempts to model this as a hydride failed, giving unreasonable distances and angles and negative isotropic thermal parameters. Molecular graphics were generated using CrystalMaker® 8.3.5.³¹

Preparation of trans-[Ir(CO){cis-1,3-bis-((di-tert-phosphino)methyl)}-cyclohexane] (4). Compound 3 (10.0 mg, 0.016 mmol) was dissolved in toluene (3 mL), and the solution was freeze–pump–thawed prior to addition of CO (1 atm). After stirring at room temperature for 1.5 h, the solvent was removed in vacuo, and the yellow solid residue was recrystallized from hexane. Yield: 7.8 mg (78%). ¹H-NMR (C₆D₆): δ 2.32–2.27 (m, PCH₂CH, 2H), 2.22–2.19 (m, Cy, 2H), 2.02–1.98 (br m, Cy, 1H), 1.73–1.64 (m, Cy, 2H), 1.52 (tt, J = 4.0 Hz, J = 13.5 Hz, PCH₂CH, 2H), 1.48–1.42 (m, Cy, 1H), 1.32 (vt, J_PH = 13.0 Hz, ^tBu, 18H), 1.26 (vt, J_PH = 13.0 Hz, ^tBu, 18H), 1.19 (t, J = 11.0, HC–Ir, 1H), 0.95 (dq, J = 3.5 Hz, J = 12.5 Hz, Cy, 2H). ¹³C{¹H}-NMR (C₆D₆): δ 194.8 (vt, J_PC = 15 Hz, Ir–CO, 1C), 71.6 (vt, J_PC = 7.2 Hz, HC–Ir, 1C), 50.3 (vt, J_PC = 19 Hz, PCH₂, 2C), 36.9 (vt, J_PC = 25 Hz, Cy, 2C), 36.4 (vt, J_PC = 21 Hz, C(CH₃)₃, 2C), 35.8 (vt, J_PC = 22 Hz, C(CH₃)₃, 2C), 34.9 (vt, J_PC = 19 Hz, Cy, 2C), 29.8 (vt, J_PC = 5.6 Hz, C(CH₃)₃, 6C), 29.7 (vt, J_PC = 5.2 Hz, C(CH₃)₃, 6C), 27.7 (vt, J_PC = 2.8 Hz, CH₂CH₂CH₂, 1C). ³¹P{¹H}-NMR (C₆D₆): δ 81.8 (s). IR (NaCl/nujol) ν_CO = 1917 cm⁻¹, (hexane) ν_CO = 1920 cm⁻¹, (CH₂Cl₂) ν_CO = 1896 cm⁻¹.

Preparation of anti-[Ir(CO)HCl{cis-1,3-bis-((di-tert-butyl-phosphino)methyl)}-cyclohexane] (5a). Compound 2 (25.0 mg, 0.040 mmol) was dissolved in THF (3 mL), and the solution was freeze–pump–thawed prior to addition of CO (1 atm). A colour change from deep red to colourless was observed within seconds. After stirring at room temperature for 2 h, the solvent was removed in vacuo, and the white solid residue was recrystallized from hexane. Yield: 22.2 mg (85%). ¹H-NMR (C₆D₆): δ 1.89–1.84 (m, PCH₂CH, 2H), 1.82–1.77 (br m, Cy, 2H + 1H), 1.61 (t, J = 10.5 Hz, HC–Ir, 1H), 1.51–1.45 (br m, PCH₂CH, 2H + Cy, 1H), 1.41 (vt, J_PH = 13.0 Hz, ^tBu, 18H), 1.37 (vt, J_PH = 13.0 Hz, ^tBu, 18H), 1.12 (tt, J = 3.5 Hz, J = 14.0 Hz, Cy, 2H), 0.90 (dq, J = 4.0 Hz, J = 13.0 Hz, Cy, 2H), −8.59 (dt, J_HH = 1.5 Hz, J_PH = 17.0 Hz, Ir–H, 1H). ¹³C{¹H}-NMR (C₆D₆): δ 226.7 (s, Ir–CO, 1C), 51.5 (vt, J_PC = 11 Hz, CH–Ir, 1C), 37.8 (vt, J_PC = 27 Hz, PCH₂, 2C), 36.8 (vt, J_PC = 21 Hz, Cy, 2C), 36.2 (vt, J_PC = 23 Hz, C(CH₃)₃, 2C), 33.9 (vt, J_PC = 17 Hz, C(CH₃)₃, 2C), 32.7 (s, Cy, 2C), 30.8, (vt, J_PC = 3.2 Hz, C(CH₃)₃, 6C), 30.6 (vt, J_PC = 3.0 Hz, C(CH₃)₃, 6C), 27.8, (s, CH₂CH₂CH₂, 1C). ³¹P{¹H}-NMR (C₆D₆): δ 50.2 (s). IR (ATR) ν_CO = 1977 cm⁻¹. Anal. calcd for C₂₅H₅₁ClIrOP₂ (657.29): C, 45.68; H, 7.82. Found: C, 45.60; H, 7.65.

Preparation of syn-[IrH(CO)Cl{cis-1,3-bis-((di-tert-butyl-phosphino)methyl)}-cyclohexane] (5b). cis-1,3-Bis-[(di-tert-butylphosphino)methyl]cyclohexane (1) (24.8 mg, 0.062 mmol) and IrCl₃·H₂O (18.6 mg, 0.062 mmol) was mixed with dry degassed DMF (4 mL) under a stream of N₂. The mixture was heated to 150 °C for 24 h. Upon cooling to RT a yellow precipitate came out of solution. The solvent was removed in vacuo, followed by repeated crystallisation from THF to afford 5b as a pale yellow crystalline powder. Yield: 23.2 mg (54%). ¹H-NMR (C₆D₆): δ 2.61–2.52 (m, PCH₂CH, 2H), 2.09–2.01 (m, PCH₂CH, 2H + Cy, 2H), 1.86–1.81 (m, Cy, 1H), 1.55 (vt, J_PH = 13.5 Hz, ^tBu, 18H), 1.53–1.51 (m, Cy, 1H) 1.42 (t, J = 11.0 Hz, HC–Ir, 1H), 1.15 (tt, J = 4.0 Hz, J = 14.0 Hz, Cy, 2H), 1.06 (vt, J_PH = 12.5 Hz, ^tBu, 18H), 0.92 (dq, J = 3.5 Hz, J = 13.0 Hz, Cy, 2H), −18.7 (t, J_PH = 13.0 Hz, Ir–H, 1H). ³¹P{¹H}-NMR (C₆D₆): δ 56.4 (d, J_PH = 13.0 Hz). IR (ATR) ν_CO = 1989 cm⁻¹. Anal. calcd for C₂₅H₅₁ClIrOP₂ (657.29): C, 45.68; H, 7.82. Found: C, 45.59; H, 7.79.

Results and discussion

We have earlier reported on the cyclometallation of ligand 1 with [Ir(COD)Cl]₂ to give trans-[IrHCl{cis-1,3-bis-(di-tert-butylphosphino)methyl}cyclohexane] (2), and also on the reduction of this compound with metallic potassium under an N₂ atmosphere at elevated temperatures, affording the Ir(I) terminal nitrogen complex 3.²⁶ We here report an alternative synthesis of 3 from 2 under slightly milder conditions and in comparable yields, using KO^tBu (Scheme 1) as was previously reported by Milstein and Frech for the preparation of a naphthyl based PCP Rh(I) η¹-N₂ complex.³² Upon addition of CO to a degassed toluene solution of 3, a colour change from orange to yellow was observed within minutes, consistent with what is expected upon substitution to a stronger π–acceptor ligand. Following this route, the Ir(I) carbonyl complex 4 was isolated and characterised by IR and NMR spectroscopy and the structure was confirmed by means of X-ray crystallography. It shows a characteristic carbonyl shift at 194.8 ppm in the ¹³C-NMR spectrum. Complex 4 failed to give satisfactory elemental analysis, possibly due to a limited stability at room temperature similarly to what was found for complex 3. However, based on NMR spectra (see ESI†) it is essentially pure. The molecular structure of compound 4 is shown in Fig. 1, and the crystallographic data for the compounds 4–5 are given in Table 1. The structure adopts a distorted square planar geometry around iridium. While the angle between the PCP coordinated carbon and the carbonyl ligand is close to ideal (177.1°), the P–Ir–P angle is much more distorted (164.46°) due to the usual geometric constraints imposed by the chelating pincer arms. With respect to bond lengths and angles around iridium, complex 4 resembles its aromatic analogue very closely,⁷ and, surprisingly, there is no substantial change of the Ir–CO or C–O distances (PC_ArP mean distances: Ir–CO = 1.863 Å; C–O = 1.147 Å, 4: Ir–CO = 1.860(7) Å; C–O = 1.143(7)); a similar observation was made regarding the Ir(I)–N₂ complexes where the aliphatic ligand was also observed to induce a small decrease in the N–N bond distance.²⁶ However, both the N₂- and current CO-ligands are subject to substantial libration,³³ an explanation that is unambiguous in the N₂-case since the complex actually showed a shorter distance than in free N₂. Therefore, a better measure of the electron density is the ν_CO stretching frequency. In hydrocarbons this is 1920 cm⁻¹ for 4 compared with 1928 cm⁻¹ for the corresponding aromatic compound.³⁴ In dichloromethane the corresponding values are 1896 and 1913 cm⁻¹, respectively,³⁵ and overall this points to a more electron rich metal centre in 4 compared to the aromatic analogue, a trend that agrees with the observations of ν_NN stretching frequencies for Ir(I)–N₂ complexes and CV-measurements for Ni(II) complexes.^12c,26

Fig. 1 Molecular structure of 4 at the 30% probability level. For clarity, hydrogen atoms are only depicted in the cyclohexyl ring. Selected bond lengths (Å) and bond angles (°) with estimated standard deviations: Ir1–C1 = 2.143(6), Ir1–C9 = 1.860(7), C9–O1 = 1.143(7), Ir1–P1 = 2.3073(16), Ir1–P2 = 2.3060(15), P1–Ir1–P2 = 164.46(6), C1–Ir–C9 = 177.1(3), Ir1–C9–O1 = 179.1(7), P1–Ir1–C1 = 82.38(16), P2–Ir1–C1 = 82.14(16).

Table 1 Crystallographic data for compounds 4–5

	4	5a	5b
a R = ∑(|Fo| − |Fc|)/∑|Fo|. b wR2 = [∑w(|Fo| − |Fc|)^2/∑(|Fo|)^2]^1/2. c S = [∑w(|Fo| − |Fc|)^2/∑(|Fo|)^2]^1/2.
Formula	C25H49IrOP2	C25H50ClIrOP2	C25H50ClIrOP2
F w	619.78	656.24	656.24
Space group	Pbca	P21/n	Pbca
a/Å	12.4581(9)	12.5453(2)	12.3770(2)
b/Å	15.3030(9)	15.2101(3)	15.3452(2)
c/Å	29.2263(16)	15.5649(3)	28.8663(4)
β/deg	90	93.996(2)	90
V/Å^3	5571.9	2962.80	5482.51
Z	8	4	8
D calcd/g cm^−3	1.478	1.469	1.590
μ/mm^−1	4.920	4.718	5.100
θ/range/deg	2.47–28.12	2.42–28.96	2.23–33.14
Reflns collected	90 480	70 507	39 527
Unique reflns	6596	7422	9768
R(F) (I > 2σ(I))^a	0.0520	0.0332	0.0389
wR2(F^2) (all)^b	0.1167	0.1030	0.1221
S ^c	1.224	1.425	1.124
R int	0.126	0.0594	0.0317
CCDC	1029323	1029333	1029332

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Subjecting the deep red solution of complex 2 to 1 atm CO resulted in a colourless solution of the 18 electron complex 5a within seconds. Treating a C₆D₆ solution of 5a with an excess of KO^tBu afforded reduction to the iridium(I) complex 4, as confirmed by comparison with the NMR-spectrum of the isolated compound. The lower route is, however, slower and slightly less clean than the synthesis starting from compound 3 (Scheme 1).

Refluxing ligand 1 and IrCl₃·H₂O in DMF gave a yellow solid material that was shown to be complex 5b, an isomer of 5a (Scheme 2). This type of cyclometallation where the solvent is the carbonyl source, was previously observed by Azerraf and Gelman in the formation of a dibenzobarrelene based PC(sp³)P iridium complex.^8a,8c

The structural isomers 5a and 5b are clearly distinguishable by means of NMR-spectroscopy, most notably in the ³¹P-NMR shifts (δ = 50.2 ppm and 56.4 ppm respectively in C₆D₆) and the ¹H-NMR hydride shifts (δ = −8.59 ppm and −18.7 ppm respectively in C₆D₆), and both compounds are seemingly resistant towards isomerisation upon standing in solution at room temperature for several days. A significantly lower solubility of compound 5b made attempts to obtain a satisfactory ¹³C-NMR spectrum of this compound unsuccessful. However, crystallographic and IR spectroscopic data clearly confirm the presence of a carbonyl ligand. The ν_CO stretching frequencies for 5a and 5b are found at 1977 cm⁻¹ and 1989 cm⁻¹ respectively, which can be compared to the value reported for the aromatic analogue of 5a (ν_CO = 1985 cm⁻¹, KBr).¹⁵ Thus, it is again clear that the electron density at iridium bonded to a C(sp³)-carbon is higher than in an analogous aromatic complex. Also, the π-back donation is weaker trans to a σ-bonded carbon than trans to the hydride ligand. As expected the ν_CO values in the Ir(III) complexes 5a and 5b are substantially higher than the value in the Ir(I) complex 4.

The molecular structures of compound 5a and 5b are given in Fig. 2. Notably, the two isomers 5a and 5b have different orientations of their respective hydride ligands relative to the α-hydrogen, as illustrated in Schemes 1 and 2. In case of 5a, the hydride and α-hydrogen are located anti to each other, while in 5b they are syn. All previously reported PC(sp³)P complexes with iridium^26,36 show an anti configuration and this seems to be the preferred outcome of a metallation involving a concerted oxidative addition process via a C–H σ-complex. This is therefore what is observed in the fast CO addition to 2 which has an anti configuration. Gelman observed that the quality of the DMF influenced the outcome of the cyclometallation reaction, affording a PC(sp³)PIrH(CO)Cl complex in the presence of water and a PC(sp³)PIr(CO)(Cl)₂ complex in dry solvent,^8c but 5b is analogous to the complex reported in wet DMF, featuring the carbonyl ligand located in a trans position and the hydride and chloride both in cis position to the metallated PCP carbon, although DMF freshly distilled from CaH₂ was used.

Fig. 2 Molecular structure of 5a and 5b at the 30% probability level. For clarity, hydrogen atoms are only depicted in the cyclohexyl ring. Selected bond lengths (Å) and bond angles (°) with estimated standard deviations: 5a: Ir1–C1 = 2.137(4), Ir1–C9 = 1.943(4), C9–O1 = 1.101(5), Ir1–Cl1 = 2.5374(10), Ir1–P1 = 2.3591(10), Ir1–P2 = 2.3643(10), P1–Ir1–P2 = 158.44(4), C1–Ir–C9 = 87.16(16), Ir1–C9–O1 = 173.7(4), C1–Ir1–Cl1 = 179.40(11), P1–Ir1–Cl1 = 95.53(4), P2–Ir1–Cl1 = 96.39(4). 5b: Ir1–C1 = 2.159(4), Ir1–C9 = 1.909(5), C9–O1 = 1.111(6), Ir1–Cl1 = 2.5340(12), Ir1–P1 = 2.3578(11), Ir1–P2 = 2.3555(11), P1–Ir1–P2 = 161.82(4), C1–Ir–C9 = 174.72(18), Ir1–C9–O1 = 174.1(4), C1–Ir1–Cl1 = 90.49(12), P1–Ir1–Cl1 = 94.15(4), P2–Ir1–Cl1 = 94.44(4).

Scheme 2

The observations by Gelman and the syn configuration of the hydride ligand and α-hydrogen in 5b probably means that the mechanism for formation of the cyclometallated species in DMF is not a simple C–H oxidative addition but involves several deprotonation/protonation steps. There was no tendency for isomerization of 5b. Overall, this indicates that the syn configuration is thermodynamically more stable than the anti one and this is also in line with the higher density for 5b. To test this hypothesis, we attempted isomerisation of 5a to 5b (Scheme 3). Indeed, when a solution of 5a in C₆D₆ was heated at 90 °C, signals of 5b appeared, together with very small amounts of 4 and another compound, which is characterized by a doublet at 57.6 ppm in the ³¹P{¹H} NMR spectrum and a triplet at −18.78 (J_PH = 11.8 Hz) in the ¹H NMR spectrum. Based on the similarity of the NMR signals of this new compound and those of 5b, we tentatively ascribe it to the structure 5c, i.e. the anti isomer with CO trans to the σ-C bond. After 36 h the reaction was complete and only 5b together with traces of 4 was observed. These observations suggest that most likely the isomerisation of 5a to 5b proceeds via a reversible dehydrochlorination to give 4, followed by protonation syn (to give 5b) or anti (to give 5c) with respect to the α-CH of 4.

Scheme 3

DFT calculations confirm the relative thermodynamic stability of 5a, 5b and 5c. Thus, energies of 5a and 5c are almost equal, while complex 5b is 7.4 kcal lower than 5a and 5c in agreement with experimental observations (see ESI† for details).

Conclusions

In summary, we have described the synthesis of new cyclohexyl-based PCP carbonyl complexes with iridium(I) and iridium(III). As noted earlier, the C(sp³) ligand gives a more electron rich metal complex than observed for the corresponding aromatic systems. Furthermore, we have, for the first time, isolated the thermodynamically more stable syn isomer of a PC(sp³)P complex with iridium.

Acknowledgements

Financial support from the Swedish Research Council, the Knut and Alice Wallenberg Foundation and the Royal Physiographic Society in Lund is gratefully acknowledged.

References

1. For an excellent review see: J. Choi, A. H. R. MacArthur, M. Brookhart and A. S. Goldman, Chem. Rev., 2011, 111, 1761.
2. M. Gupta, C. Hagen, R. J. Flesher, W. C. Kaska and C. M. Jensen, Chem. Commun., 1996, 2083.
3. K. B. Renkema, Y. V. Kissin and A. S. Goldman, J. Am. Chem. Soc., 2003, 125, 7770.
4. (a) I. Gottker-Schnetmann, P. White and M. Brookhart, J. Am. Chem. Soc., 2004, 126, 1804; (b) I. Gottker-Schnetmann and M. Brookhart, J. Am. Chem. Soc., 2004, 126, 9330.
5. D. Morales-Morales, R. Redon, C. Yung and C. M. Jensen, Inorg. Chim. Acta, 2004, 357, 2953.
6. R. Ahuja, B. Punji, M. Findlater, C. Supplee, W. Schinski, M. Brookhart and A. S. Goldman, Nat. Chem., 2011, 3, 167.
7. D. Morales-Morales, R. Redon, Z. H. Wang, D. W. Lee, C. Yung, K. Magnuson and C. M. Jensen, Can. J. Chem., 2001, 79, 823.
8. (a) C. Azerraf and D. Gelman, Chem.–Eur. J., 2008, 14, 10364; (b) R. Levy, C. Azerraf, D. Gelman, K. Rueck-Braun and P. N. Kapoor, Catal. Commun., 2009, 11, 298; (c) C. Azerraf and D. Gelman, Organometallics, 2009, 28, 6578.
9. (a) M. C. Denney, V. Pons, T. J. Hebden, D. M. Heinekey and K. I. Goldberg, J. Am. Chem. Soc., 2006, 128, 12048; (b) T. J. Hebden, M. C. Denney, V. Pons, P. M. B. Piccoli, T. F. Koetzle, A. J. Schultz, W. Kaminsky, K. I. Goldberg and D. M. Heinekey, J. Am. Chem. Soc., 2008, 130, 10812.
10. A. Staubitz, M. E. Sloan, A. P. M. Robertson, A. Friedrich, S. Schneider, P. J. Gates, J. S. A. D. Gunne and I. Manners, J. Am. Chem. Soc., 2010, 132, 13332.
11. P. W. N. M. van Leeuwen, in Homogeneous Catalysis: Understanding the Art, Kluwer Academic Publishers, Dordrecht, the Netherlands, 1st edn, 2004.
12. (a) J. Choi, D. Y. Wang, S. Kundu, Y. Choliy, T. J. Emge, K. Krogh-Jespersen and A. S. Goldman, Science, 2011, 332, 1545; (b) A. V. Polukeev, R. Gritcenko, K. J. Jonasson and O. F. Wendt, Polyhedron, 2014, 84, 63; (c) K. J. Jonasson and O. F. Wendt, Chem.–Eur. J., 2014, 20, 11894.
13. (a) M. Brookhart, A. F. Volpe, D. M. Lincoln, I. T. Horvath and J. M. Millar, J. Am. Chem. Soc., 1990, 112, 5634; (b) M. Brookhart, E. Hauptman and D. M. Lincoln, J. Am. Chem. Soc., 1992, 114, 10394.
14. B. J. Burger, M. E. Thompson, W. D. Cotter and J. E. Bercaw, J. Am. Chem. Soc., 1990, 112, 1566.
15. C. J. Moulton and B. L. Shaw, J. Chem. Soc., Dalton Trans., 1976, 1020.
16. C. Crocker, H. D. Empsall, R. J. Errington, E. M. Hyde, W. S. Mcdonald, R. Markham, M. C. Norton, B. L. Shaw and B. Weeks, J. Chem. Soc., Dalton Trans., 1982, 1217.
17. S. Musa, O. A. Filippov, N. V. Belkova, E. S. Shubina, G. A. Silantyev, L. Ackermann and D. Gelman, Chem.–Eur. J., 2013, 19, 16906.
18. B. Rybtchinski, Y. BenDavid and D. Milstein, Organometallics, 1997, 16, 3786.
19. D. W. Lee, C. M. Jensen and D. Morales-Morales, Organometallics, 2003, 22, 4744.
20. I. Gottker-Schnetmann, P. S. White and M. Brookhart, Organometallics, 2004, 23, 1766.
21. S. A. Kuklin, A. M. Sheloumov, F. M. Dolgushin, M. G. Ezernitskaya, A. S. Peregudov, P. V. Petrovskii and A. A. Koridze, Organometallics, 2006, 25, 5466.
22. B. Punji, T. J. Emge and A. S. Goldman, Organometallics, 2010, 29, 2702.
23. J. J. Adams, N. Arulsamy and D. M. Roddick, Dalton Trans., 2011, 40, 10014.
24. T. J. A. Foskey, D. M. Heinekey and K. I. Goldberg, ACS Catal., 2012, 2, 1285.
25. D. B. Lao, A. C. E. Owens, D. M. Heinekey and K. I. Goldberg, ACS Catal., 2013, 3, 2391.
26. A. Arunachalampillai, D. Olsson and O. F. Wendt, Dalton Trans., 2009, 8626.
27. S. Sjövall, O. F. Wendt and C. Andersson, J. Chem. Soc., Dalton Trans., 2002, 1396.
28. Crysalis CCD and Crysalis RED, Oxford Diffraction Ltd., Abingdon, Oxfordshire, UK, 2005.
29. G. M. Sheldrick, Acta Crystallogr., Sect. A: Found. Crystallogr., 2008, 64, 112.
30. M. S. Weiss and R. Hilgenfeld, J. Appl. Crystallogr., 1997, 30, 203.
31. CrystalMaker® Software, Begbroke Science Park, Sandy Lane, Yarnton, Oxfordshire, OX5 1PF, United Kingdom, 2010.
32. C. M. Frech and D. Milstein, J. Am. Chem. Soc., 2006, 128, 12434.
33. J. P. Clusker, M. Lewis and M. Rossi, Crystal Structure Analysis for Chemists and Biologists, Wiley-VCH, 1994.
34. F. Liu and A. S. Goldman, Chem. Commun., 1999, 655.
35. A. V. Polukeev, S. A. Kuklin, P. V. Petrovskii, S. M. Peregudova, A. F. Smol'yakov, F. M. Dolgushina and A. A. Koridze, Dalton Trans., 2011, 40, 7201.
36. (a) K. J. Jonasson, N. Ahlsten and O. F. Wendt, Inorg. Chim. Acta, 2011, 379, 76; (b) H. A. Mayer, R. Fawzi and M. Steimann, Chem. Ber., 1993, 126, 1341.

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Footnote

† Electronic supplementary information (ESI) available: NMR spectra for 4. Computational details. Crystal data for 4, 5a and 5b. CCDC 1029323, 1029332 and 1029333. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4ra15562a

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