Synthesis, structure and spectroelectrochemical properties of a dinuclear ruthenium complex exhibiting a strong electronic interaction across a 1,2,4,5-tetrazine bridging ligand

Abstract

The reaction of [Ru(bpy)₂(EtOH)₂]²⁺ (bpy = 2,2'-bipyridine) with the potentially bridging ligand 3,6-bis(3,5-dimethylpyrazol-1-yl)-1,4-dihydro-1,2,4,5-tetrazine) (H₂L) results in formation of the diruthenium complex [(bpy)₂Ru^II(L)Ru^II(bpy)₂]⁴⁺ ([1]⁴⁺) in which the dihydrotetrazine unit of H₂L has undergone oxidative dehydrogenation to give an aromatic tetrazine unit connecting the two metal centres [L = 3,6-bis(3,5-dimethylpyrazol-1-yl)-1,2,4,5-tetrazine]. The crystal structures of H₂L and [1(ClO₄)₄].2H₂O have been determined. In MeCN solution, complex [1]⁴⁺ undergoes two successive one-electron oxidation processes at 1.25 and 1.70 V versus SCE, corresponding to successive Ru(II)/Ru(III) couples, affording a stable mixed-valence Ru(II)/Ru(III) state (K_c = 4 × 10⁷) at intermediate potentials. Four successive ligand-based reduction processes at -0.13, -0.99, -1.58 and -1.94 V versus SCE are also observed, of which the first two are associated with the bridging tetrazine unit and the other two are reductions of the coordinated bipyridine molecules. A UV/VIS/NIR spectroelectrochemical study was carried out on [1]ⁿ⁺ for n = 3-6; the mixed-valence species [1]⁵⁺ displays a narrow π-π transition at 1534 nm, indicative of a class III mixed-valence state. The chemically generated one-electron-reduced species [1]³⁺ shows an EPR signal (g = 2.0085) characteristic of a ligand-centred radical. [1]⁴⁺ is weakly luminescent at 77 K.