Tetrazine derived mononuclear Ru^II(acac)₂(L) (1), [Ru^II(bpy)₂(L)](ClO₄)₂ (2) and [Ru^II(bpy)(L)₂](ClO₄)₂ (3) (L = 3-amino-6-(3,5-dimethylpyrazol-1-yl)-1,2,4,5-tetrazine, acac = acetylacetonate, bpy = 2,2'-bipyridine): syntheses, structures, spectra and redox properties

Abstract

Mononuclear ruthenium complexes of tetrazine derived L, Ru^II(acac)₂(L) (1), [Ru^II(bpy)₂(L)](ClO₄)₂ (2) and [Ru^II(bpy)(L)₂](ClO₄)₂ (3) (L = 3-amino-6-(3,5-dimethylpyrazol-1-yl)-1,2,4,5-tetrazine, acac = acetylacetonate and bpy = 2,2'-bipyridine) were prepared. The free L exists as a dimeric entity in the solid state via hydrogen bonding interactions involving L and water molecules present in the crystal lattice. 1 exhibits unusually strong bonds from Ru^II to coordinating pyrazolyl-N (2.040(2) Å) and especially to tetrazine-N (1.913(2) Å). The Ru^III/Ru^II couples of 1–3 appeared at 0.28, 1.34 and 1.50 V versus SCE, respectively. The tetrazine and bpy-based reductions were observed at -1.33 (1); -0.55 and -1.55/-1.75/-1.98 (2); -0.47/-0.78 and -1.80/-2.02 V (3), respectively. 1, 2 and 3 displayed two MLCT bands each, corresponding to dp(Ru^II) → Π (L, tetrazine) and dπ(Ru^II) →π^∗ (acac or bpy or L) transitions. 1⁺ and 2⁺ showed rhombic EPR spectra at 110 and 4 K, respectively and 1^-, 2^- and 3^- exhibited multiple line EPR spectra at 300 K. 1–3 exhibited moderately strong emission spectra in EtOH–MeOH glass at 77 K.