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

Dinuclear compounds [L2Ru(μ-E)RuL2]n where L is acetylacetonate (acac–, 2,4-pentanedionate), 2,2′-bipyridine (bpy), or 2-phenylazopyridine (pap) and EH4 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)2Ru(μ-E)Ru(acac)2] ((NBu4)2[1]), [(bpy)2Ru(μ-E)Ru(bpy)2]ClO4 ([2]ClO4), and [(pap)2Ru(μ-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(μ-E)Ru center. Several additional charge states of the compounds were electrochemically accessible and were identified mostly as complexes with noninnocently behaving pap0/•– 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-depen...