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

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On exposure to NaOMe (≥3 equiv) in CH2Cl2–MeOH at 23 °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)(H2)(PCy3)2 (5a). Complex 5a can be recycled into 4a under conditions conducive to removal of H2. The second-generation catalyst RuCl2(IMes)(PCy3)(═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)(PCy3) (3b). Experiments in the presence of added PCy3 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 PCy3 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(PCy3) analogue 3a, fast transformation into 4a is proposed to inv...