Mixed valence aspects of diruthenium complexes [{(L)ClRu}₂(μ -tppz)]ⁿ⁺ incorporating 2-(2-pyridyl)azoles (L) as ancillary functions and 2,3,5,6-tetrakis(2-pyridyl)pyrazine (tppz) as bis-tridentate bridging ligand

Abstract

Tppz [2,3,5,6-tetrakis(2-pyridyl)pyrazine]-bridged complexes [{(L)ClRu}₂(μ -tppz)]ⁿ⁺ with structurally similar but electronically different ancillary ligands, 2-(2-pyridyl)azoles (L), were synthesized as diruthenium(II) species. Cyclic voltammetry, EPR of paramagnetic states, and UV-vis-NIR spectroelectrochemistry show that the first two reduction processes occur at the tppz bridge and that oxidation involves mainly the metal centers. The mixed valent intermediates from one-electron oxidation exhibit moderate comproportionation constants 10⁴ < K_c < 10⁵ but appear to be valence-averaged according to the Hush criterion. Redox potentials, EPR, and UV-vis-NIR results show the effect of increasing donor strength of the ancillary ligands along the sequence L¹ < L² < L⁴≪ L³, L¹ = 2-(2-pyridyl)benzoxazole, L² = 2-(2-pyridyl)benzthiazole, L³ = 2-(2-pyridyl)benzimidazolate, L⁴ = 1-methyl-2-(2-pyridyl)-1H-benzimidazole. Whereas the mixed valent complexes with L1 and L2 remain EPR silent at 4 κ, the analogues with L⁴ and L³ exhibit typical ruthenium(III) EPR signals, albeit with some noticeable ligand contribution in the case of the L³-containing complex. Intervalence charge transfer (IVCT) bands were found in the visible spectrum for the complex with L³ but in the near-infrared range (at ca. 1500 nm) for the other systems.