Theoretical studies on C–heteroatom bond formation via reductive elimination from group 10 M(PH3)2(CH3)(X) species (X = CH3, NH2, OH, SH) and the determination of metal–X bond strengths using density functional theory

Abstract

Density functional calculations have been used to investigate C–C, C–N and C–O bond forming reactions via reductive elimination from Group 10 cis-M(PH₃)₂(CH₃)(X) species (X = CH₃, NH₂, OH). Both direct reaction from the four-coordinate species and a three-coordinate mechanism involving initial PH₃ loss have been considered. For the four-coordinate pathway the ease of reductive elimination to give M(PH₃)₂ and CH₃–X follows the trend M = Pd < Pt < Ni. The reaction of the cis-M(PH₃)₂(CH₃)(NH₂) species is promoted by the formation of methylamine adducts. Non-planar transition states are located and the C–heteroatom bond forming processes are characterised by migration of CH₃ onto the cis-heteroatom ligand. For a given ligand, X, activation energies follow the trend M = Ni < Pd < Pt. Formation of the three-coordinate M(PH₃)(CH₃)(X) species is promoted by a labilisation of the cis-PH₃ ligand in the four-coordinate reactants when X = NH₂ or OH. For the three-coordinate pathway the energy change for reductive elimination to give M(PH₃) and CH₃–X again follows the trend M = Pd < Pt < Ni and in all cases the initial product is an M(PH₃)(XCH₃) adduct. The three-coordinate transition states again involve migration of the CH₃ ligand onto the cis-X ligand and for X = NH₂ or OH activation energies follow the trend Ni > Pd < Pt. For a given metal activation energies in both the four- and three-coordinate pathways increase along the series CH₃ < NH₂ < OH. These trends in activation energy can be rationalised in terms of the strength of M–CH₃/M–X bonding as long as the extent of geometrical distortion required to obtain the transition state geometry is taken into account. Further calculations on cis-Pd(PH₃)₂(CH₃)(SH) suggest that the more common experimental observation of C(sp³)–S compared to C(sp³)–O reductive elimination arises from the greater kinetic accessibility of the former process rather than an intrinsic thermodynamic preference for C–S bond formation. By comparison, the calculations indicate that C(sp³)–N reductive elimination should be feasible from Ni and Pd systems. DF calculations are shown to reproduce the relative homolytic bond strengths determined experimentally for Pt–X bonds. In the cis-M(PH₃)₂(CH₃)(X) systems the M–CH₃ homolytic bond strength increases down the group while for M–NH₂ and M–OH bonds the trend is M = Ni ≈ Pd < Pt. M–NH₂ and M–OH bonds are considerably stronger than M–CH₃ bonds and the presence of a heteroatom ligand serves to weaken M–CH₃ bonds even further.