Evidence for catalytic water oxidation by a dimanganese tetrakis-Schiff base macrocycle

摘要

A dimanganese tetrakis-Schiff base complex [(Mn2L)-L-II](2+), where L provides a N6O2 coordination environment, has been synthesized and characterized via electrochemical and spectroscopic studies. Cyclic voltammograms (CV) of [ (Mn2L)-L-II](2+) in propylene carbonate, to which water has been added as a limiting reagent, show an increase in current starting at +1.3 V versus Fc(+/o). The current shows a first order dependence on H2O and [(Mn2L)-L-II](2+), indicative of electrocatalytic water oxidation. Controlled potential electrolysis produced O-2 in 39-45% Faradaic efficiency. Blocking the axial coordination site to the manganese centers by a mu-carboxylato axial ligand in an analogous Schiff base complex [(Mn2LAc)-L-II](+) lowers the catalytic activity, suggesting the importance of an open coordination site to the central manganese atoms for aqua ligands. Photosensitization of [(Mn2L)-L-II](2+) with [Ru(bpy)(3)](2+) and K2S2O8 in acetonitrile-water mixtures produced O-2 as measured by a Clark electrode, with the rate of O-2 evolution proportional to [(Mn2L)-L-II](2+). (C) 2014 Elsevier B.V. All rights reserved.