Di(μ-oxo) binuclear manganese(III,IV) poly(bipyridyl) complexes bearing four ruthenium(II) photoactive units:: Synthesis, characterization, and photoinduced electron-transfer properties

In order to model the photoinduced electron-transfer reactions from the manganese cluster to the photoactive P-680 chlorophylls in photosystem II, three heterohexanuclear complexes, [(Mn2O2)-O-III,IV{Ru-II(bpy)(2)(L-n)}(4)](11+) [bpy = 2,2'-bipyridine, n = 2 (1a), 4 (1b), 6 (1c)], in which one Mn-III,Mn-IV(mu-O)(2) center is covalently linked to four Ru-II(bpy)(3)-like moieties by bridged bis(bipyridine) L-n ligands, have been synthesized and characterized. The electrochemical, photophysical, and photochemical properties of these complexes have been investigated in CH3CN. The cyclic voltammograms and rotating-disk electrode curves of the three complexes show the presence of two very close successive reversible oxidation processes corresponding to the Mn-2(III,IV)/Mn-2(IV,IV) and Ru-II/Ru-III redox couples (estimated E-1/2 approximate to 0.82 and 0.90 V, respectively). The lower potential of the Mn-2(III,IV) subunit compared to those of the Ru-II moieties indicates that the Ru-III species can act as an efficient oxidant toward the Mn-2(III,IV) core. The two oxidized forms of the complexes [(Mn2O2)-O-IV,IV{Ru-II(bpy)(2)(L-n)}(4)](12+) (2a-c) and [(Mn2O2)-O-IV,IV{Ru-III(bpy)(2)(L-n)}(4)](16+) (3a-c) obtained in good yields (> 90% for 2a<-c and > 85% for 3a-c) by sequential electrolyses are very stable. Photophysical studies show that the (MLCT)-M-3 excited state of the Ru(bpy)(3) centers is moderately quenched by the Mn-2(III,IV)(mu-O)(2) core (15-25% depending on the length of the bridging alkyl chain). Nevertheless, this energy transfer can be easily short-circuited in the presence of an external irreversible electron acceptor like the (4-bromophenyl)diazonium cation, by an electron transfer leading, in a stepwise fashion, to the stable one- and five-electron-oxidized species 2a-c and 3a-c, respectively, also in good yields, under continuous irradiation of the solutions. Electro- and photoinduced oxidation experiments have been followed by UV-visible and electron paramagnetic resonance spectroscopy.