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) << 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(4) 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.