Visible-light-driven generation of high-valent oxo-bridged dinuclear and tetranuclear manganese terpyridine entities linked to photoactive ruthenium units of relevance to photosystem II

Abstract

We report on a new heterobimetallic manganese-ruthenium complex, in which a MnII bis(terpyridine) centre is covalently linked to two Ru II tris(bipyridine) units. By electro- or photoinduced oxidation of this Mn centre, high-valent multinuclear oxo-bridged manganese species can be generated. These were unambiguously identified by electrochemistry and UV/Vis and electron paramagnetic resonance (EPR) spectroscopy. In a CH3CN solution of water (1 M), the dinuclear core, [(H2O)(terpy)Mn III(μ-O)2MnIV(terpy)(H2O)] 3+ (terpy = 2,2':6',2″-terpyridine) is first obtained by dimerisation, and can be further oxidised to give rise to the tetranuclear core, [(H2O)(terpy)MnIV(μ-O)2Mn IV(terpy)(μ-O)(terpy)MnIV(μ-O)2Mn IV(terpy)(H2O)]6+. These cores are connected to two and four [Ru(bpy)3]2+ units, respectively. The second dimerisation process is prevented when coordinating CF3CO 2- anions are present in solution, as oxidation of the initially generated species, [(CF3CO2)(terpy)Mn III(μ-O)2MnIV(terpy)(CF3CO 2)]+, produces the stable dinuclear [(CF 3CO2)(terpy)MnIV(μ-O)2Mn IV(terpy)(CF3CO2)]2+. All these species are generated in high yields either by exhaustive electrolysis or continuous visible-light irradiation. A diazonium salt is required as the irreversible electron acceptor for photochemical experiments. Molecular mechanics calculations confirm that the formation of the higher nuclearity complex with a tetranuclear Mn central core linked to four Ru unit is feasible. The Ru-Ru distances are long enough to preclude possible steric hindrance of the Ru units. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.