E−H Bond Activation Reactions (E = H, C, Si, Ge) at Ruthenium: Terminal Phosphides, Silylenes, and Germylenes

The placement of a strongly trans-influencing ligand on a ruthenium center opposite an anchoring silyl group of the tetradentate tripodal tris(phosphino)silyl ligand, [SiPPh3]− ([SiPPh3]− = tris(2-(diphenylphosphino)phenyl)silyl), has been explored. Installation of alkyl or terminal phosphide ligands trans to the anchoring silyl group affords the complexes [SiPPh3]RuR (R = Me (2), CH2Ph (4), PPh2 (5), PiPr2 (6)). Complexes 2, 4, and 5 are thermally unstable. Complexes 2 and 4 decay to the cyclometalated complex [SiPPh2P′Ph]Ru (3), whereas complex 5 decays to the cyclometalated phosphine adduct [SiPPh2P′Ph]Ru(PHPh2) (7). Complex 3 is found to effect E−H (E = H, C, Si, Ge) bond activation of substrates such as secondary silanes and germanes to yield the structurally unusual silylene complexes [SiPPh3]Ru(H)(SiRR′) (R = R′ = Ph (10a), R = Ph R′ = Me (10b)) and the germylene complex [SiPPh3]Ru(H)(GeR2) (R = Ph) (11) via double E−H activation transformations. Both theory and experiments suggest electrophilic character at the silylene moiety. Reaction of 3 with catecholborane, in contrast to silanes and germanes, results in insertion of the B−H unit into the M−C bond of the cyclometalated species to yield the borate complex [SiPPh2PPh-B(cat)]Ru(μ-H) (14). Complex 3 also reacts with bis(catecholato)diboron to yield a similar complex, [SiPPh2PC6H3B(cat)-B(cat)]Ru(μ-H) (15), with selective borylation of an ortho C−H bond.