Theoretical studies on C-heteroatom bond formation via reductive elimination from group 10 M(PH_3)_2(CH_3)(X) species (X = CH_3, NH_2, OH, SH) and the determination of metal-X bond strengths using density functional theory

摘要

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(PH_3)_2(CH_3)(X) species (X = CH_3, NH_2, OH). Both direct reaction from the four-coordinate species and a three-coordinate mechanism involving initial PH_3 loss have been considered. For the four-coordinate pathway the ease of reductive elimination to give M(PH_3)_2 and CH_3-X follows the trend M = Pd < Pt < Ni. The reaction of the cis-M(PH_3)_2(CH_3)(X) 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 CH_3 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(PH_3)(CH_3)(X) species is promoted by a labilisation of the cis-PH_3 ligand in the four-coordinate reactants when X = NH_2 or OH. For the three-coordinate pathway the energy change for reductive elimination to give M(PH_3) and CH_3-X again follows the trend M = Pd < Pt < Ni and in all cases the initial product is an M(PH_3)(XCH_3) adduct. The three-coordinate transition states again involve migration of the CH_3 ligand onto the cis-X ligand and for X = NH_2 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 CH_3 < NH_2 < OH. These trends in activation energy can be rationalised in terms of the strength of M-CH_3/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(PH_3)_2(CH_3)(SH) suggest that the more common experimental observation of C(sp~3)-S compared to C(sp~3)-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(sp~3)-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(PH_3)_2(CH_3)(X) systems the M-CH_3 homolytic bond strength increases down the group while for M-NH_2 and M-OH bonds the trend is M = Ni ≈ Pd < Pt. M-NH_2 and M-OH bonds are considerably stronger than M-CH_3 bonds and the presence of a heteroatom ligand serves to weaken M-CH_3 bonds even further.