DIHYDROGEN COMPLEXES OF METALLOPORPHYRINS - CHARACTERIZATION AND CATALYTIC HYDROGEN OXIDATION ACTIVITY

A series of monometallic dihydrogen complexes of the type M(OEP)(L)(H-2) (M = Ru, Os; L = THF, *Im) was synthesized and characterized by H-1 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 angstrom. The first example of a bimetallic bridging dihydrogen complex, Ru2(DPB)(*Im)2(H-2), was also prepared. The H-2 ligand is simultaneously bound to both Ru-metal centers. High-field H-1 NMR experiments (620 MHz) revealed a -7.37 Hz dipolar splitting of the H-2 ligand for this complex. Analysis of this splitting suggests that the H-2 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 H-2. Only Ru(OEP)(THF)(H-2) can be conveniently deprotonated. Ru(OEP)(THF)(H-2) is also implicated in the Ru-(OEP)(THF)2 catalyzed isotopic exchange between H-2 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.