HYDRIDE TRANSFER AND DIHYDROGEN ELIMINATION FROM OSMIUM AND RUTHENIUM METALLOPORPHYRIN HYDRIDES - MODEL PROCESSES FOR HYDROGENASE ENZYMES AND THE HYDROGEN ELECTRODE-REACTION

A series of metalloporphyrin hydride complexes of the type K[M(Por)(L)(H)] (M = Ru, Os; Por = OEP, TMP; L = THF, *Im, PPh3, pyridine) has been synthesized by stoichiometric protonation of the corresponding K2[M(Por)], followed by addition of L. The addition of excess acids to these hydrides resulted in the elimination of dihydrogen. The kinetics showed no evidence for a bimolecular mechanism for this process and suggest simple protonation of the metal-hydride bond followed by dihydrogen loss. One-electron oxidation of the metal hydrides also resulted in dihydrogen formation. The kinetics of the oxidatively induced hydrogen evolution step from K[Ru(OEP)(THF)(H)] were examined and indicate a bimolecular mechanism in which two metal hydrides reductively eliminate one dihydrogen molecule. The rate constant was determined to be 88 +/- 14 M-1 s-1. These reaction mechanisms are discussed in the context of designing bimetallic proton reduction catalysts. The metal hydride K[Ru(OEP)(THF)(H)], was also synthesized by heterolytic activation of H-2. This hydride is a good one-electron reductant (-1.15 V vs FeCp2) and is capable of reducing, by hydride transfer, the NAD+ analogue, 1-benzyl-N,N-diethyl-nicotinamide. This nicotinamide reduction by a hydride formed from heterolytic dihydrogen activation is suggested as the mechanism for hydrogenase enzymes.