Steric influences of phosphorus-donor ligands on the structure and ligand-exchange rates of cobaloximes

Abstract

The crystal structures of two cobaloximes, [CoMe(Hdmg)₂(PMe₃)](1) and [CoMe(Hdmg)₂{P(C₆H₁₁)₃}](2)(H₂dmg = dimethylglyoxime), are reported and discussed. Compound (1) crystallizes in the space group P with cell parameters a= 15.830(8), b= 12.279(7), c= 12.257 Å, α= 94.85(7), β= 84.49(8), γ= 130.07(9)°, and Z= 4; (2) crystallizes in the space group P2₁2₁2₁ with a= 18.50(1), b= 16.83(1), c= 9.943(8)Å, and Z= 4. Both structures have been solved by Patterson and Fourier methods and refined by block-diagonal anisotropic least-squares methods to final R values of 0.037 (1) and 0.038 (2), using 4 812(1) and 2 710(2) independent reflections. The Me–Co–P fragment of (1) is characterized by a C–Co–P angle of 178.9(2)°(mean) and Co–P and Co–C bond lengths of 2.293(1)(mean) and 2.015(3)Å(mean); the corresponding figures for (2) are 179.1(2)°, 2.463(1), and 2.016(5)Å respectively. The value of 2.463(1)Å is the largest so far reported for a Co^III–P bond length. The two nearly planar Hdmg units make a mean interplanar angle of 4° in (1) and are bent towards the axial methyl group. A similar but more pronounced bending is observed in (2). The trend in Co–P distances in these and other similar complexes [CoX(Hdmg)₂(PR₃)](X = Cl or Me; R = OMe, Me, Buⁿ, Ph, or C₆H₁₁) is discussed and compared with ³¹P and ¹H chemical shifts. Deformations observed in the geometry of these octahedral cobalt(III) complexes are interpreted in terms of ‘steric’ and ‘electronic’ influences. Finally, kinetic measurements of the conversion of [CoMe(Hdmg)₂{P(C₆H₁₁)₃}] to [CoMe(Hdmg)₂(py)](py = pyridine) give no indication of a significant contribution from steric effects on the rate of the five- to six-co-ordinate step. However, the relatively high rate of displacement of the P(C₆H₁₁)₃ ligand compared to other phosphines of smaller cone angle almost certainly arises from the steric interaction of the C₆H₁₁ rings with the Co(Hdmg)₂ moiety.