An unusual dinuclear Ruthenium(III) complex with a conjugated bridging ligand derived from cleavage of a 1,4-dihydro-1,2,4,5-tetrazine ring. Synthesis, structure, and UV-Vis-NIR spectroelectrochemical characterization of a five-membered redox chain incorporating two mixed-valence states

Abstract

Reaction of [Ru(acac)₂(CH₃CN)₂] with 3,6-bis(3,5-dimethylpyrazol-1-yl)-1,4-dihydro-1,2,4,5-tetrazine (H₂L) results in formation of an unexpected dinuclear complex [(acac)₂Ru^III(L¹)Ru^III(acac)₂] (1) in which the bridging ligand [L¹]^2- contains an -HN—C=N—N=C—NH^- unit arising from two-electron reduction of the 1,4-dihydro-1,2,4,5-tetrazine component of H₂L. The crystal structure of complex 1 confirms the oxidation assignment of the metal ions as Ru(III) and clearly shows the consequent arrangement of double and single bonds in the bridging ligand, which acts as a bis-bidentate chelate having two pyrazolyl/amido chelating sites. Cyclic voltammetry of the complex shows the presence of four reversible one-electron redox couples, assigned as two Ru(III)/Ru(IV) couples (oxidations with respect to the starting material) and two Ru(II)/Ru(III) couples (reductions with respect to the starting material). The separation between the two Ru(III)/Ru(IV) couples (ΔE_½ = 700 mV) is much larger than that between the two Ru(II)/Ru(III) couples (ΔE_½ = 350 mV) across the same bridging pathway, because of the better ability of the dianionic bridging ligand to delocalize an added hole (in the oxidized mixed-valence state) than an added electron (in the reduced mixed-valence state), implying some ligand-centered character for the oxidations. UV-vis-NIR spectroelectrochemical measurements were performed in all five oxidation states; the Ru(II)-Ru(III) mixed-valence state of [1]- has a strong IVCT transition at 2360 nm whose parameters give an electronic coupling constant of V_ab ≈ 1100 cm^-1, characteristic of a strongly interacting but localized (class II) mixed-valence state. In the Ru(III)-Ru(IV) mixed-valence state [1]⁺, no low-energy IVCT could be detected despite the strong electronic interaction, possibly because it is in the visible region and obscured by LMCT bands.