Noninnocently Behaving Bridging Anions of the Widely Distributed Antioxidant Ellagic Acid in Diruthenium Complexes

Dinudear compounds [L2Ru(mu-E)RuL2](n) where L is acetylacetonate (acac(-), 2,4-pentanedionate), 2,2'-bipyridine (bpy), or 2-phenylazopyridine (pap) and Eat is ellagic acid, an antioxidative bis-catechol natural product, were studied by voltammetric and spectroelectrochemical techniques (UV-vis-NIR and electron paramagnetic resonance (EPR)). The electronic structures of the isolated forms (NBu4)(2)[(acac)(2)Ru(mu-E)Ru(acac)(2)] ( (NBu4)(2)[1]), (bpy)(2)Ru(mu-E)Ru(bpy)(2)]ClO4 ([2]ClO4), and [(pap)(2)Ru(mu-E)Ru(pap)(2)] ([3]) were characterized by density functional theory (DFT) in conjunction with EPR and UV vis NIR measurements. The crystal structure of (NBu4)(2)[1] revealed the meso form and a largely planar Ru(itE)Ru center. Several additional charge states of the compounds were electrochemically accessible and were identified mostly as complexes with noninnocently behaving pap(0/center dot-) or bridging ellagate (En-) anions (n = 2, 3, 4) but not as mixed-valence intermediates. The free anions En-, n = 1-4, were calculated by time-dependent DFT to reveal NIR transitions for the radical forms with n = 1 and 3 and a triplet ground state for the bis(o-semiquinone) dianion E2-.