Synthesis and reactivity of a diruthenium diazoalkane complex

Reaction of [RU2(mu -CO)(CO)(4) (mu -dppm)(2)], 1, dppm = Ph2PCH2PPh2, with N2CHCO2Et gives the complex [RU2(mu -NNCHCO2Et)(CO)(4)(mu -dppm)(2)], 2, in which the diazoalkane acts as a novel 2-electron ligand bridging a metal-metal bond. Complex 2 reacts with phenylacetylene, With displacement of a carbonyl ligand, to give [RU2(mu -CO)(CO)(2){mu -NN(CHCO2Et)(CPh=CH)}(mu -dPPM)(2)], 3a, which contains a new type of metallacycle formed by coupling of the diazoalkane and alkyne ligands at the diruthenium center. Protons attack complex 2 first at the NCH center to give the alkyldiazonium complex [RU2(CO)(4)(mu -NNCH2CO2Et)(mu -dppm)(2)](+), 4, and then, with the strong acid H[BF4], at the Ru-Ru bond to give the dicationic complex [Ru-2-(mu -H)(CO)(4)(mu -NNCH2CO2Et)(mu -dppm)(2)](2+), 5. Complex 2 is a precatalyst for the decomposition of formic acid to give CO2 and H-2, and the chemistry leading to the active catalyst is described.