Mechanisms of C−C and C−H Alkane Reductive Eliminations from Octahedral Pt(IV):  Reaction via Five-Coordinate Intermediates or Direct Elimination?

The Pt(IV) complexes P(2)PtMe(3)R [P(2) = dppe (PPh(2)(CH(2))(2)PPh(2)), dppbz (o-PPh(2)(C(6)H(4))PPh(2)); R = Me, H] undergo reductive elimination reactions to form carbon-carbon or carbon-hydrogen bonds. Mechanistic studies have been carried out for both C-C and C-H coupling reactions and the reductive elimination reactions to form ethane and methane are directly compared. For C-C reductive elimination, the evidence supports a mechanism of initial phosphine chelate opening followed by C-C coupling from the resulting five-coordinate intermediate. In contrast, mechanistic studies on C-H reductive elimination support an unusual pathway at Pt(IV) of direct coupling without preliminary ligand loss. The complexes fac- P(2)PtMe(3)R (P(2) = dppe, R = Me, H; P(2) = dppbz, R = Me) have been characterized crystallographically. The Pt(IV) hydrides, fac-P(2)PtMe(3)H (P(2) = dppe, dppbz), are rare examples of stable phosphine ligated Pt(IV) alkyl hydride complexes.