Charge Delocalization in a Cyclometalated Bisruthenium Complex Bridged by a Noninnocent 1,2,4,5-Tetra(2-pyridyl)benzene Ligand

Abstract

Two ruthenium atoms are covalently connected to the para positions of a phenyl ring in 1,2,4,5-tetra(2-pyridyl)benzene (tpb) to form a linear Ru–tpb–Ru arrangement. This unique structure leads to appealing electronic properties for the biscyclometalated complex [(tpy)Ru(tpb)Ru(tpy)]2+, where tpy is 2,2′;6′,2″-terpyridine. It could be stepwise oxidized at substantially low potential (+0.12 and +0.55 V vs Ag/AgCl) and with a noticeably large comproportionation constant (1.94 × 107). In addition to the routinely observed metal-to-ligand charge-transfer transitions, [(tpy)Ru(tpb)Ru(tpy)]2+ displays a separate and distinct absorption band at 805 nm with appreciable absorptivity (ε = 9000 M–1 cm–1). This band is assigned to the charge transition from the Ru–tpb–Ru motif to the pyridine rings of tpb with the aide of density functional theory (DFT) and time-dependent DFT calculations. Complex [(tpy)Ru(tpb)Ru(tpy)]2+ was precisely titrated with 1 equiv of cerium ammonium nitrate to produce [(tpy)Ru(tpb)Ru(tpy)]3+, which shows intense multiple NIR transitions. The electronic coupling parameters Hab of individual NIR components are determined to be 5812, 4942, 4358, and 3560 cm–1. DFT and TDDFT calculation were performed on [(tpy)Ru(tpb)Ru(tpy)]3+ to elucidate its electronic structure and spin density population and the nature of the observed NIR transitions. Electron paramagnetic resonance studies of [(tpy)Ru(tpb)Ru(tpy)]3+ exhibit a discernible rhombic signal with the isotropic g factor of ⟨g⟩ = 2.144. These results point to the strong orbital interaction of tpb with metal centers and that tpb behaves as a redox noninnocent bridging ligand in [(tpy)Ru(tpb)Ru(tpy)]2+. Complex [(tpy)Ru(tpb)Ru(tpy)]3+ is determined to be a Robin–Day class III system with full charge delocalization across the Ru–tpb–Ru motif