COFACIAL BISORGANOMETALLIC DIPORPHYRINS - SYNTHETIC CONTROL IN PROTON REDUCTION CATALYSIS

A series of cofacial bisorganometallic diporphyrin complexes, M2III/IIIR2DPB (M = Ru, Os; R = methyl, p-tolyl, 3,5-bis(trifluoromethyl)phenyl; DPB = diporphyrinatobiphenylene), has been synthesized by addition of the corresponding Grignard reagents to the dication of the dimetallodiporphyrins. These paramagnetic compounds have been characterized by H-1 NMR, cyclic voltammetry, UV-vis, and mass spectrometry. Quantitative two-electron chemical reduction of paramagnetic M2III/IIIR2DPB complexes produces diamagnetic species, [M2II/IIR2DPB]2-; subsequent protonation with suitable acids results in dihydrogen evolution. The overall processes were investigated with H-1 NMR. These complexes were examined as possible electrocatalysts for proton reduction by employing mercury-pool electrodes. Acids with a wide range of pK(a) values were employed to measure the basicity of these catalysts. Relative overpotentials have been estimated from these pK(a) values and the reduction potentials of these cofacial dimetallodiporphyrin complexes. Plausible catalytic proton reduction pathways involving either dihydrogen complexes or dihydrides are discussed.