ELECTROPHILIC METHYLPLATINUM COMPLEXES - A THEORETICAL-STUDY OF THE MECHANISM OF C-C AND C-H BOND FORMATION AND ACTIVATION

The reductive elimination of methane or ethane from the five-coordinat e intermediate model complexes [PtHMe2L2](+), or [PtMe3L2](+) respecti vely, and the corresponding C-H or C-C bond activation from the alkane complexes [PtMe(CH4)L-2](+) or [PtMe(C2H6)L-2](+), 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-[PtL2-Me-3](+), L = PH3, show that the regular trigonal-bipyramidal (TBP) structure has an orbitally degener ate ground state and should undergo distortion to either the square-py ramidal (SP) or pinched trigonal-bipyramidal (PTBP) structure. In the PTBP structure, two methyl groups are in close proximity (C-Pt-C ca. 7 0 degrees) 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 eliminat ion and oxidative addition are much easier than C-C reductive eliminat ion and oxidative addition, but there is no major difference between t he activation energies when L = NH3 or PH3. However, the platinum(IV) complexes are relatively more stable when L = NH3 than when L = PH3 co mpared to the platinum(II) alkane complexes, and so the activation ene rgies 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, respe ctively. In the platinum(II) alkane complexes, the stereochemistry wit h ligands L mutually trans is preferred. The oxidative-addition/reduct ive-elimination reactions occur by a concerted mechanism, probably wit h a PTBP complex on the reaction coordinate. For C-H reductive elimina tion, the methane remains coordinated to platinum through the C-H a co mplex. For C-C reductive elimination, the transition state is a C-C si gma complex but in the final ethane complex the binding is as a C-H a complex. For methane complexes, the binding is eta(3) but one platinum C-H contact is shorter than the other, while for ethane complexes, th e binding is usually eta(4) through two eclipsed platinum C-H sigma-co mplex interactions, but one appears much stronger than the other. The weaker of these sigma-complex interactions is just strong enough to ov ercome the tendency of ethane to adopt the staggered conformation (ca. 3 kcal mol(-1)). Activation of the C-C bond of ethane is likely only in systems where the much easier C-H activation is rapid and reversibl e.