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-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–.