Synthesis, characterization and reactivity of heterobimetallic complexes ((eta)^5-C5R5)Ru(CO)((mu)-dppm)M(CO)2((eta)5-C5H5)(R = H, CH3; M = Mo, W). Interconversion of hydrogen/carbon dioxide and formic acid by these complexes

The heterobimetallic complexes [((eta)5-C5R5)Ru(CO)((mu)-dppm)M(CO)2((eta)5-C5H5)](M = Mo, W; R = H, CH3)(1-4) are prepared by metathetical reactions between ((eta)^5-C5R5)Ru(dppm)Cl and Na+[((eta)^5-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 [((eta)^5-C5R5)Ru(CO)((mu)-dppm)((mu)-H)M(CO)2((eta)^5-C5H5)]+HCOO- formed during the catalysis.