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Volume 124, 2003
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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

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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.

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Publication details

The article was received on 11 Dec 2002, accepted on 03 Jan 2003 and first published on 01 May 2003


Article type: Paper
DOI: 10.1039/B212309F
Citation: Faraday Discuss., 2003,124, 111-127
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    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

    S. A. Macgregor, G. W. Neave and C. Smith, Faraday Discuss., 2003, 124, 111
    DOI: 10.1039/B212309F

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