Charge delocalization in a cyclometalated bisruthenium complex bridged by a noninnocent 1,2,4,5-Tetra(2-pyridyl)benzene ligand

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 H_ab 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.