Photosensitized Water Oxidation by Use of a Bioinspired Manganese Catalyst

Abstract

In an artificial version of photosynthesis, sunlight and water are used to produce fuels. Our research focuses on the bottleneck in this process, the photooxidation of water. In the course of developing a water oxidation catalyst, a number of metal complexes have been synthesised, characterised, and studied for catalytic activity. Three of them are dinuclear complexes (Ru, Co and Cu) of 2,6-bis[(2-hydroxybenzyl)-(2-pyridylmethyl)aminomethyl]-4-methylphenol (H3bbpmp). The fourth is a dimeric Ru complex with a ligand containing imidazole and phenol motifs. Additionally, a dinuclear Mn complex with a ligand that contains benzimidazoles and carboxylates coordinating to the metal atoms was also developed. This Mn complex was then covalently linked to [Ru(bpy)3]2+-type photosensitisers, resulting in three different bimetallic dyads. Finally, a dinuclear Fe complex containing the same ligand as the dinuclear Mn complex was synthesised.The potential of the three H3bbpmp complexes as catalysts for oxidation of organic compounds was investigated and it was found that the Ru complex catalyses the oxidation of alcohols to the corresponding ketone or aldehyde using (diacetoxyiodo)benzene as oxidant. The Co complex functions as an electron transfer mediator in a coupled catalytic system for allylic oxidation using oxygen gas. The oxidation of 3,5-di-tert-butylcatechol to the corresponding ortho-quinone with oxygen gas using the copper complex proved that it can be considered as a model of catecholase. The dimeric Ru complex and the dinuclear Mn and Fe complexes proved to catalyse water oxidation when employing stoichiometric amounts of the oxidant [Ru(bpy)3](PF6)3. Furthermore, using [Ru(bpy)2(deeb)](PF6)2 as photosensitiser together with Na2S2O8 as sacrificial electron acceptor in aqueous phosphate buffer at pH = 7.2, photochemical water oxidation was demonstrated. The bimetallic dyads however, did not show catalytic activity for the oxidation of water.