Revisiting Dinuclear Ruthenium Water Oxidation Catalysts: Effect of Bridging Ligand Architecture on Catalytic...

An attractive catalytic pathway for the conversion of water to oxygen would involve two metal oxide centers combining in a constructive sense to make O═O. This prospect makes the study of certain dinuclear transition metal complexes particularly attractive. In this work, we describe the design and synthesis of two symmetrical bis-tridentate polypyridine ligands 6 and 12 that bind two Ru^II centers at a separation of 3.6 Å in 7 and 5.7 Å in 13. In the presence of Ce^IV at pH = 1, these systems oxidize water with the system having the more proximal metals being more reactive. In the case of the more proximal metal centers, the bridging ligand is a 3,6-disubstituted pyridazine which, under the influence of Ce^IV, cleaves into two [Ru(bpc)(pic)₂CH₃CN]⁺ fragments (14) which then function as the actual catalyst (bpc = 2,2'-bipyridine-6-carboxylate, pic = 4-methylpyridine). The second dinuclear catalyst contains a central pyrimidine ring which is less sensitive to oxidative decay and hence less reactive. Caution is advised in the use of Ce^IV as a sacrificial electron acceptor due to unexpected oxidative decay of the catalyst.