Hydride transfer and dihydrogen elimination from osmium and ruthenium metalloporphyrin hydrides: model processes for hydrogenase enzymes and the hydrogen electrode reaction

Abstract

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 H₂. 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-diethylnicotinamide. This nicotinamide reduction by a hydride formed from heterolytic dihydrogen activation is suggested as the mechanism for hydrogenase enzymes.

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