Mixed valence aspects of diruthenium complexes [{(L)ClRu}(2)(mu-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)(mu-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 10(4) < K-c < 10(5) 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 L-1 < L-2 < L-4 much less than L-3, L-1 = 2-(2-pyridyl)benzoxazole, L-2 = 2-(2-pyridyl)benzthiazole, L-3 = 2-(2-pyridyl)benzimidazolate, L-4 = 1-methyl-2-(2-pyridyl)-1H-benzimidazole. Whereas the mixed valent complexes with L-1 and L-2 remain EPR silent at 4 K, the analogues with L-1 and L-3 exhibit typical ruthenium(III) EPR signals, albeit with some noticeable ligand contribution in the case of the L-3-containing complex, Intervalence charge transfer (IVCT) bands were found in the visible spectrum for the complex with L-3 but in the near-infrared range (at ca. 1500 nm) for the other systems.