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

On exposure to NaOMe (≥3 equiv) in CH 2 Cl 2 –MeOH at 23 °C, the first-generation Grubbs catalyst RuCl 2 (PCy 3 ) 2 (═CHPh) ( 1a ) is immediately transformed into the six-coordinate methoxyhydride complexes RuH(OMe)(CO) 2 (PCy 3 ) 2 ( 4a ) and RuH(OMe)(CO)(H 2 )(PCy 3 ) 2 ( 5a ). Complex 5a can be recycled into 4a under conditions conducive to removal of H 2 . The second-generation catalyst RuCl 2 (IMes)(PCy 3 )(═CHPh) ( 1b; 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)(PCy 3 ) ( 3b ). Experiments in the presence of added PCy 3 reveal that consumption of 1a, but not 1b, proceeds via the four-coordinate intermediate formed by equilibrium loss of phosphine, a function of the lability of the PCy 3 ligand at ambient temperatures. The poor accessibility of such an intermediate for 1b at 23 °C retards salt metathesis and inhibits further reaction of 3b . For the bis(PCy 3 ) analogue 3a, fast transformation into 4a is proposed to involve reversible loss of PCy 3, coordination of methanol, σ-metathesis of methanol at the hydride site to liberate H 2, and β-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)(PCy 3 ) ( 4a/b; a, L = PCy 3; b, L = IMes) were developed: these involved sequential transformation of RuHCl(CO)(L)(PCy 3 ) ( 2a/b ) into the bis-carbonyl adducts RuHCl(CO) 2 (L)(PCy 3 ) ( 7a/b ) under CO, conversion of 7a/b into the more reactive triflates RuH(OTf)(CO) 2 (L)(PCy 3 ) ( 8a/b ), and reaction of 8a/b with equimolar NaOMe. Dihydride 6b was also prepared, by reaction of 8b with NaH.