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 P2PtMe3R [P-2 = dppe (PPh2(CH2)(2)PPh2), dppbz (o-PPh2(C6H4)PPh2); 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-P2PtMe3R (P-2 = dppe, R = Me, H; P-2 = dppbz, R = Me) have been characterized crystallographically. The Pt(IV) hydrides, fac-P2PtMe3H (P-2 = dppe, dppbz), are rare examples of stable phosphine ligated Pt(IV) alkyl hydride complexes.