Synthesis, characterization and reactivity of heterobimetallic complexes (η5-C5R5)Ru(CO)(μ-dppm)M(CO)2(η5-C5H5) (R = H, CH3; M = Mo, W). Interconversion of hydrogen/carbon dioxide and formic acid by these complexes

Abstract

The heterobimetallic complexes [(η5-C5R5)Ru(CO)(μ-dppm)M(CO)2(η5-C5H5)] (M = Mo, W; R = H, CH3) (1–4) are prepared by metathetical reactions between (η5-C5R5)Ru(dppm)Cl and Na+[(η5-C5H5)M(CO)3]−. IR spectroscopic and X-ray structural studies show that each of these complexes contains a semi-bridging carbonyl ligand. The low activity of these complexes in catalytic CO2 hydrogenation to formic acid might be attributed to their non-facile reactions with H2 to yield the active dihydride species. The metal–metal bonds can be protonated to form the cationic complexes, which contain strong Ru–H–M bridges. The bridging hydrogen atom is weakly acidic since it can be removed by a strong base, and it protonates BPh4− to give BPh3 and benzene. It also reacts with the hydridic hydrogen of Et3SiH to yield H2. The bridging hydrogen, however, cannot be removed by hydride scavengers such as Ph3C+OTf− and Me3Si+OTf−. The sluggishness of the catalytic formic acid decomposition by 1–4 is attributable to the stability of the protonated bimetallic intermediate [(η5-C5R5)Ru(CO)(μ-dppm)(μ-H)M(CO)2(η5-C5H5)]+HCOO− formed during the catalysis.