Reactions of Grubbs Catalysts with Excess Methoxide: Formation of Novel Methoxyhydride Complexes

On exposure to NaOMe (>= 3 equiv) in CH2Cl2-MeOH at 23 degrees C, the first-generation Grubbs catalyst RuCl2(PCy3)(2)(=CHPh) (1a) is immediately transformed into the six-coordinate methoxyhydride complexes RuH(OMe)(CO)(2)(PCy3)(2) (4a) and RuH(OMe)(CO)(H-2)(PCy3)(2) (5a). Complex 5a can be recycled into 4a under conditions conducive to removal of H-2. The second-generation catalyst RuCl2(IMes)(PCy3)(=CHPh) (lb; IMes = N,N'-bis(mesityl)imidazol-2-ylidene) reacts more slowly, requiring several hours even at 20 equiv of NaOMe, and terminates at five-coordinate RuH(OMe)(CO)(IMes)(PCy3) (3b). Experiments in the presence of added PCy3 reveal that consumption of la, but not lb, proceeds via the four-coordinate intermediate formed by equilibrium loss of phosphine, a function of the lability of the PCy3 ligand at ambient temperatures. The poor accessibility of such an intermediate for lb at 23 degrees C retards salt metathesis and inhibits further reaction of 3b. For the bis(PCy3) analogue 3a, fast transformation into 4a is proposed to involve reversible loss of PCy3, coordination of methanol, sigma-metathesis of methanol at the hydride site to liberate H-2, and beta-elimination/decarbonylation of bound methoxide. Competitive uptake of H-2 by 3a yields six-coordinate 5a (the dihydrogen adduct of 3a). Independent routes to RuH(OMe)(CO)(2)(L)(PCy3) (4a/b; a, L = PCy3; b, L = IMes) were developed: these involved sequential transformation of RuHCl(CO)(L)(PCy3) (2a/b) into the bis-carbonyl adducts RuHCl(CO)(2)(L)(PCy3) (7a/b) under CO, conversion of 7a/b into the more reactive triflates RuH(OTf(CO)(2)(L)(PCy3) (8a/b), and reaction of 8a/b with equimolar NaOMe. Dihydride 6b was also prepared, by reaction of 8b with NaH.