Water photolysis. Part 1. The photolysis of co-ordinated water in [{MnL(H2O)}2][ClO4]2(L = dianion of tetradentate O2N2-donor Schiff bases). A model for the manganese site in photosystem II of green plant photosynthesis

Abstract

A number of manganese(III) complexes of the type [{MnL(H₂O)}₂]²⁺(L = dianion of O₂N₂ tetradentate Schiff base), in aqueous solution, have been shown to liberate dioxygen and reduce p-benzoquinone to hydroquinone when irradiated with visible light. The photoactivity is critically dependent on the structure of the ligand, the complex [{Mn(salpd)(H₂O)}₂][ClO₄]₂[salpd = propane-1,3-diylbis(salicylideneiminate)] being the most active. All the active complexes exhibit a band at 590 nm in the electronic spectrum, which is absent for the inactive complexes. Amongst the parameters of the photolysis which have been studied are: wavelength of light, temperature, complex and quinone concentrations, ligand structure, pH, and solvent nature. The rate of dioxygen evolution is dependent on the manganese(III) complex (first order) and quinone concentrations (order, 0.5) and the pH of the reaction medium, but is independent of solvent. The water which is photolysed is that bound to the manganese (suggesting that a model for the manganese site of photosystem II of the green plant is represented by these complexes). The energy of activation for O₂ evolution in [{Mn(salpd)(H₂O)}₂][ClO₄]₂ is ca. 80 kJ mol^–1, and the evidence points to homolytic, rather than heterolytic, fission of water. The manganese complex is converted to [{Mn(salpd)}₂O] in the photolysis, i.e. no oxidation state change of manganese is involved. In addition to p-benzoquinone, methylene blue has been shown to be a hydrogen acceptor in this system.