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)2RuII(L)RuII(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 (Kc = 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.