Electrophilic Methylplatinum Complexes:  A Theoretical Study of the Mechanism of C−C and C−H Bond Formation and Activation

• Published in: Organometallics 1998-04, Vol.17 (8), p.1478-1486
• Main Authors: Hill, Geoffrey S, Puddephatt, Richard J
• Format: Article
• Language: English
• ISSN: 0276-7333; 1520-6041
• DOI: 10.1021/om9709291

The reductive elimination of methane or ethane from the five-coordinate intermediate model complexes [PtHMe2L2]+, or [PtMe3L2]+ respectively, and the corresponding C−H or C−C bond activation from the alkane complexes [PtMe(CH4)L2]+ or [PtMe(C2H6)L2]+, respectively, have been studied by carrying out extended Huckel molecular orbital (EHMO) calculations and density functional theory (DFT) calculations on both the ground-state and transition-state structures with L = NH3 or PH3. The EHMO calculations on trans-[PtL2Me3]+, L = PH3, show that the regular trigonal-bipyramidal (TBP) structure has an orbitally degenerate ground state and should undergo distortion to either the square-pyramidal (SP) or pinched trigonal-bipyramidal (PTBP) structure. In the PTBP structure, two methyl groups are in close proximity (C−Pt−C ca. 70°) and tilted away from each other. Although the tilting leads to a close Pt···HC contact, no attractive agostic Pt···H bonding is indicated. The DFT calculations predict that C−H reductive elimination and oxidative addition are much easier than C−C reductive elimination and oxidative addition, but there is no major difference between the activation energies when L = NH3 or PH3. However, the platinum(IV) complexes are relatively more stable when L = NH3 than when L = PH3 compared to the platinum(II) alkane complexes, and so the activation energies for C−H or C−C oxidative addition are calculated to be lower for the NH3 complexes. The platinum(IV) complexes with ligands L mutually cis or trans are most stable in the SP or PTBP stereochemistry, respectively. In the platinum(II) alkane complexes, the stereochemistry with ligands L mutually trans is preferred. The oxidative-addition/reductive-elimination reactions occur by a concerted mechanism, probably with a PTBP complex on the reaction coordinate. For C−H reductive elimination, the methane remains coordinated to platinum through the C−H σ complex. For C−C reductive elimination, the transition state is a C−C σ complex but in the final ethane complex the binding is as a C−H σ complex. For methane complexes, the binding is η3 but one platinum C−H contact is shorter than the other, while for ethane complexes, the binding is usually η4 through two eclipsed platinum C−H σ-complex interactions, but one appears much stronger than the other. The weaker of these σ-complex interactions is just strong enough to overcome the tendency of ethane to adopt the staggered conformation (ca. 3 kcal mol-1). Activatio