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 H-2 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 H-2. 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.