Weakly Coordinated Zinc and Aluminum σ-Complexes of Copper(I

Communication pubs.acs.org/Organometallics

Weakly Coordinated Zinc and Aluminum σ‑Complexes of Copper(I) Adi E. Nako, Qian Wen Tan, Andrew J. P. White, and Mark R. Crimmin* Department of Chemistry, Imperial College London, South Kensington, London SW7 2AZ, U.K. S Supporting Information *

ABSTRACT: We report the synthesis and isolation of three new σcomplexes of Cu(I) in which E−H (E = Al, Zn) σ-bonds are coordinated to copper. The addition of the main group hydride to a toluene-solvated Cu(I) complex results in reversible ligand exchange, and the Cu(I) σ-complexes have been crystallized. Experimental and computational data provide a wealth of evidence for weak binding of the E−H bond to Cu(I), which can be ascribed to σ-donation from the E−H bond into the 4s orbital of copper and back-donation from copper into the E−H σ* orbital. Scheme 1. Synthesis of Intermolecular σ-Complexes of Copper(I)a

ince the discovery of the first dihydrogen complex, [W(CO)3(PiPr3)2(H2)], by Kubas and co-workers, there has been keen interest in σ-complexes of the transition metals.1,2 While these types of compounds are of fundamental interest, they are often invoked as intermediates in catalysis. For example, mechanistic studies on C−H borylation provide experimental support for the formation of σ-borane complexes as intermediates.3 Furthermore, σ-silane complexes are potential intermediates in alkene hydrosilylation via the Chalk−Harrod mechanism.4 Despite these studies and the importance of the group 11 metals in catalysis, intermolecular σ-complexes of copper are unknown, to the best of our knowledge.5 In this regard, Bourissou and co-workers have reported the isolation of a number of copper(I) complexes, in which chelating phosphine ligands direct Si−Si, Si−H, or Sn− Sn σ-bonds toward the metal.5 Similarly, Stack and co-workers have provided computational and experimental (EPR spectroscopy) support for the formation of an agostic complex by coordination of a C−H bond in a triazamacrocyclic ligand to copper(II).6 If a system could be found that would allow the isolation of σ-complexes of copper, it could open up new avenues in catalysis and bond activation with this abundant and inexpensive element. The reaction of the inverse sandwich complex 12·toluene with the series of main-group hydrides 2-Al, 3-Al, and 4-Zn in C6D6 solution was studied by 1H NMR spectroscopy.7−9 In all cases, new species were observed within the first point of analysis (15 min, 298 K) and following preparative-scale experiments the σ-complexes 2−4 were isolated as yellow crystalline solids in 66−92% yields. The compounds have been fully characterized by NMR spectroscopy, X-ray crystallography, and CHN analysis (Scheme 1; see the Supporting Information for experimental details). In the solid state, two distinct coordination modes are observed in 2−4 (Figure 1). Although in all cases the hydrides were located in the Fourier map and were freely refined, DFT calculations have been used to verify the experimentally determined distances and angles (Figure 2).

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© XXXX American Chemical Society

a

Ar1 = 2,4,6-Me3C6H2; Ar2 = 2,6-Cl2C6H3; Ar3 = 2,6-iPr2C6H3.

While the Cu(I) fragment resides within the wedge formed by the AlH2 group for 2, it sits outside this wedge for 3. The Cu−H−Al angle of 153.97(12)° and extremely long Cu···Al distance of 3.1231(5) Å of 3 contrast with the Cu···E bond lengths of 2.6143(7) and 2.4684(5) Å and Cu−H−E angles of 110.5(12) and 111(2)° of 2 and 4, respectively. These data suggest two coordination modes reminiscent of those found in σ-complexes.10−13 For example, Aldridge and co-workers have Received: April 10, 2014

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Figure 1. Crystal structures of (a) 2, (b) 3 and (c) 4. Selected bond angles (deg) and bond lengths (Å): 2, Al−Cu 2.6143(7), Al−H(1) 1.68(2), Al− H(2) 1.49(2), Cu−H(1) 1.50(2), Al−H(1)−Cu 110.5(12); 3, Al−Cu 3.1231(5), Al−H(1) 1.66(2), Al−H(2) 1.53(2), Cu−H(1) 1.54(2), Al− H(1)−Cu 153.9(17); 4, Zn−Cu 2.4684(5), Zn−H 1.51(3), Cu−H 1.48(3), Zn−H−Cu 111(2).

Scheme 2. Reversible Formation of σ-Complexes of Copper(I)a

a

Figure 2. Comparison of the calculated (m062x functional, 6-31G +d,p/Lanl2DZ basis set) geometries of 2−4. Distances are given in Å.

Ar2 = 2,6-Cl2C6H3; HERn = 2-Al, 3-Al, 4-Zn.

Although 12·toluene is dimeric in the solid state, in toluene solution (37 mM at 303 K) it is solvated to form 1·toluene.7c This statement is supported by the observation that 1·toluene possesses a diffusion coefficient within the range recorded for 2-Al, 3-Al, 4-Zn and the control sample 1·cyclohexene (D = 0.967−0.944 × 10−9 m2 s−1) and a solution hydrodynamic radius (rsolution = 4.1 Å) that is inconsistent with the solid-state value calculated for the analogue 12·benzene (rsolid = 5.3 Å).15 In contrast, 3 possess a smaller diffusion coefficient (D = 0.775 × 10−9 m2 s−1) and a solution hydrodynamic radius that matches the solid-state data (rsolution = 5.2 Å; rsolid = 5.3 Å).16 These experiments exclude the formation of higher nuclearity species in solution and imply the reaction stoichiometry presented in Scheme 2. A van’t Hoff analysis on a 37 mM toluene-d8 sample of 3 (213−333 K) gave ΔH° = −1.75(4) kcal mol−1 and ΔS° = −5.0(2) cal K−1 mol−1, resulting in ΔG298 K = −0.23(4) kcal mol−1. A similar analysis of 4 (193−243 K) gave ΔH° = −2.47(9) kcal mol−1 and ΔS° = −10.8(4) cal K−1 mol−1, resulting in ΔG°298 K = +0.65(8) kcal mol−1. These data are consistent with competitive binding of the arene and the E−H σ-bond to Cu(I). The reversibility of σ-complex formation was confirmed by a crossover experiment in which 2 was reacted with 3-Al in a 1:1 stoichiometry in toluene-d8 to form 2-Al and 3. DFT calculations were undertaken in order to gain a further understanding of these equilibria. The reaction of 1 with exogeneous ligands is calculated to be increasingly exergonic across the series C6F6 < HBpin < HSiH2Ph < C6H6 ≈ C7H8 ≈ 4-Zn < 2-Al ≈ 3-Al (see the Supporting Information, Figure S5). In line with these findings, we have been unable to experimentally verify the formation of σ-borane and σ-silane complexes from reaction of HBpin and H3SiPh with 12·toluene in toluene-d8 solution at 298 K. Furthermore, the measured equilibrium constants for the reaction 1·arene + 3-Al → 3 + arene are solvent dependent, taking relative values of 12.2, 1.6,

documented a series of σ-alane complexes of Mn(I), W(0), and Mo(0) that display a coordination geometry similar to that of 2 in the solid state.12,13 The reported M···Al distances range from 2.446(1) to 2.841(1) Å, and all are smaller than that observed in 3. Parallels may also be drawn with σ-borane complexes of the transition metals. Shimoi and Hartwig have characterized a series of σ-borane complexes of Mn(I) in which the coordination environment at manganese is dependent upon the geometry at boron.14 For example, the Mn···B distances in [(η5-C5H4Me)Mn(CO)2(HBpin)] and [CpMn(CO)2(H3B· NMe3)] are 2.149(2) and 2.682(3) Å, respectively.11 The Cu−H distances in 2−4, determined by X-ray diffraction, range from 1.48(2) to 1.54(2) Å. Low-temperature 1 H NMR spectroscopy confirmed the retention of a Cu−H bond in solution. While the fast-exchange equilibria operating in samples of 2 (vide infra) did not allow deconvolution of the hydride resonances, data collected on 3 and 4 at 193 K in toluene-d8 reveal the bridging hydrides as resonances at δ −0.12 ppm (fwhm = 47 Hz) and −0.55 ppm (fwhm = 8 Hz) shifted upfield by Δδ = 4.1 and 5.0 ppm from 3-Al and 4-Zn, respectively. For complex 3, the assignment was confirmed by synthesis of the deuterium analogue; this experiment also resulted in identification of the noncoordinated Al−H as a broad resonance centered at δ 3.38 ppm. Dissolving pure crystalline samples of 3 in toluene-d8 gave mixtures of 1·toluene, 3-Al, and 3, the ratio of which was temperature dependent. Similarly, variable temperature 1H NMR data on 2 and 4 are consistent with reversible σ-complex formation through dynamic equilibria (Scheme 2). While in the case of 2 the rate of exchange is fast on the NMR time scale, for 3 and 4 (below 243 K) the rate of exchange is slow enough for each component of the mixture to be resolved. In order to probe the nuclearity of each component in solution, a series of diffusion order spectroscopy experiments were conducted. B

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or interactions and suggests that σ-donation from the E−H bond to the 4s orbital of Cu(I) is augmented by back-donation to the E−H σ*-orbital. In solution, E−H binding to Cu(I) is weak and reversible and competes with solvation of 1 with arenes. Taken in combination, the theoretical and experimental evidence imply that the contribution of the resonance structure B to the ground-state structures of 2−4 is considerably less important than that of A1 (Figure 3). Regardless of the extent of back-bonding and the contribution of the resonance structure A2, these species can be considered as true σcomplexes of Cu(I).

and 1.0 in 1,2-difluorobenzene, toluene-d8 and C 6 D 6 , respectively (298 K, 55 mM in 3). In order to probe the nature of bonding in 2−4, gas-phase DFT calculations were undertaken using the m062x functional and a hybrid 6-31G+d,p (C, H, N, Cl, Cu)/Lanl2DZ (Al, Zn) basis set. Natural bond orbital (NBO) analysis reveals the importance of a large electrostatic component to the bonding and demonstrates that the charge on the bridging hydride (2, −0.48 e; 3, −0.53 e) is more negative than that on the terminal hydride (2, −0.42 e; 3, −0.44 e). The calculated Wiberg bond indices (WBIs) for the Cu−H bonds (2, 0.18; 3, 0.13; 4, 0.28) are consistently lower than those of the E−H bonds (2, 0.53; 3, 0.54; 4, 0.31). Comparison of these latter values with the WBIs of the noncoordinated Al−H bonds (2, 0.74; 3, 0.76) suggests a weakening of the E−H bond upon coordination to Cu(I). The Cu···E WBIs highlight the difference in the two coordination modes, and the value for 3 is considerably smaller than that calculated for the remaining members of the series (2, 0.30; 3, 0.14; 4, 0.26). An atoms in molecules (AIM) analysis on 2−4 returns, in all cases, bond critical points (BCPs) between Cu and H (electron density of the bond-critical points ρbcp = 0.079−0.092; Laplacian of the electron density at the bondcritical points ∇ρ2bcp = 0.133−0.169) which are similar to those observed between E and H (ρbcp = 0.052−0.068; ∇ρ2bcp = 0.125−0.193). No BCPs were found between E and Cu. Further calculations reveal that the d10 Cu(I) fragment 1 possesses a LUMO that is largely 4s in character. Second-order perturbation theory analysis allowed the quantification of donor−acceptor interactions in 2−4 and electron donation occurs from the E−H σ-bond to the 4s orbital of copper (2, 45.0 kcal mol−1; 3, 35.1 kcal mol−1; 4, 84.4 kcal mol−1), while back-donation from copper occurs primarily to the Al−H σ* orbital and is significantly weaker in the more electron-rich aluminum complex (2, 21.2 kcal mol−1; 3, 6.6 kcal mol−1; 4, 22.7 kcal mol−1). While the difference in the observed coordination modes is likely influenced by steric effects, it may also be explained by the increased coordination number of 3, resulting in the Al−H σ* orbital becoming energetically inaccessible. Similar observations have been used to explain the divergence in the solid state structures of σ-borane adducts. Hence, the coordination of catecholborane or pinacolborane to transition metals is believed to occur through not only σdonation from the B−H bond to a vacant d orbital but also back-donation from the metal to the p orbital of boron. In contrast, σ-complexes of BH3·L (L = NMe3, PMe3) are not stabilized by back-donation; here both the p orbital and σ*(B− H) of the borane are too high in energy.14 Caulton, Kubas, and others have documented homobimetallic17 and heterobimetallic18 hydride complexes of Cu(I). For example, Sadighi has reported neutral and cationic Nheterocyclic carbene stabilized hydrides of copper(I).19 The bonding in the cationic complex [{IPrCu}2(μ-H)]+ (IPr = 1,3bis(2,6-diisopropylphenyl)imidazol-2-ylidene) has been described as an open three-center interaction in which the only metal−metal interaction occurs through a symmetrically bound bridging hydride ligand.19b Although evidently a three-centered interaction, a balanced consideration of the data presented herein suggests that the σ complex is a more apt description than bridging hydride for the bonding in 2−4. AIM analysis allows the identification of BCPs between Cu/H and E/H with similar electron densities, while NBO analysis denotes a bonding picture with a large electrostatic component. Second-order perturbation theory considers the donor−accept-

Figure 3. Possible resonance contributions to the bonding in 2−4.

In summary, we have isolated a series of new σ-complexes containing E−H bonds (E = Al, Zn) coordinated to Cu(I). The complexes are formed by reversible ligand exchange with a Cu(I)−arene precursor. We are continuing to investigate the coordination of E−H bonds to 1 and the reaction chemistry of Cu(I) σ-complexes.

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ASSOCIATED CONTENT

S Supporting Information *

Details of the experimental procedures and calculations are provided in the supporting information. These include, X-ray crystallographic data for 2−4, diffusion-ordered spectroscopy data, details of the van’t Hoff analysis and DFT studies, Z matrices and cartesian coordinates for the calculated structures and multinuclear NMR data. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail for M.R.C.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for financial support from The Royal Society in the form of a University Research Fellowship (M.R.C.) and Imperial College London for funding a Ph.D. studentship (A.E.N.). We are also grateful to the EPSRC for funding (EP/ L011514/1). We are grateful to Dr. Neil J. Brown, Prof. Charlotte K. Williams, and Prof. Milo Shaffer for useful discussions.

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REFERENCES

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