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

AbstractWe report on a new heterobimetallic manganese–ruthenium complex, in which a MnII bis(terpyridine) centre is covalently linked to two RuII 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)MnIII(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)2MnIV(terpy)(μ‐O)(terpy)MnIV(μ‐O)2MnIV(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 CF3CO2– anions are present in solution, as oxidation of the initially generated species, [(CF3CO2)(terpy)MnIII(μ‐O)2MnIV(terpy)(CF3CO2)]+, produces the stable dinuclear [(CF3CO2)(terpy)MnIV(μ‐O)2MnIV(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.