Dihydrogen complexes of metalloporphyrins: characterization and catalytic hydrogen oxidation activity

Abstract

A series of monometallic dihydrogen complexes of the type M(OEP)(L)(H₂) (M = Ru, Os; L = THF, *Im) was synthesized and characterized by ¹H NMR. The H-H bond length was found to increase when Os was replaced by Ru or when *Im was replaced by THF. The bond distances (as determined by T₁) range from 0.92 to 1.18 Á. The first example of a bimetallic bridging dihydrogen complex, Ru₂(DPB)(*Im)₂(H₂), was also prepared. The H₂ ligand is simultaneously bound to both Ru-metal centers. High-field ¹H NMR experiments (620 MHz) revealed a -7.37 Hz dipolar splitting of the H₂ ligand for this complex. Analysis of this splitting suggests that the H₂ ligand is bound with the H-H axis perpendicular to the Ru-Ru axis. These complexes were examined as possible catalysts for the oxidation of dihydrogen through prior heterolytic activation of H₂. Only Ru(OEP)(THF)(H₂) can be conveniently deprotonated. Ru(OEP)(THF)(H₂) is also implicated in the Ru- (OEP)(THF)₂ catalyzed isotopic exchange between H₂ and D₂O in THF solution. Each step for this mechanism has been elucidated. We have also achieved catalytic dihydrogen oxidation using [Ru(OEP)]₂ adsorbed onto graphite. Two mechanisms for this ruthenium porphyrin catalyzed dihydrogen oxidation are presented and compared.

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