Structures and spectroelectrochemistry (UV–vis, IR, EPR) of complexes [(OC) 3ClRe] n(abpy), n=1, 2; abpy=2,2′-azobispyridine

Abstract

The complexes (OC) 3ClRe(abpy) ( 1) and [(OC) 3ClRe] 2(abpy) ( 2) with abpy=2,2′-azobispyridine were structurally characterized and studied by spectroelectrochemistry in the UV–vis and in the IR carbonyl stretching region. Compound 1 exhibits a s-cis/ E/ s-trans configurated abpy ligand with metal coordination to one pyridyl and one azo function and one non-bonding ReN interaction at 3.293 Å to the second pyridyl nitrogen atom. The dinuclear complex 2 with two azoimine-coordinated metal centers is distinguished by the trans arrangement of the chloride ligands with respect to the Re 2(μ-abpy) plane, the ReRe distance is 5.033(7) Å. The azo bond lengths at 1.272(9) ( 1) and 1.304(10) Å ( 2) indicate substantial π back donation from the rhenium(I) centers into the π*(abpy) orbital. Spectroelectrochemistry reveals the successive occupation of the π*(abpy) orbital by electrons; metal-to-ligand charge transfer features are shifted to higher energies and carbonyl stretching bands to lower wavenumbers. The EPR spectra of both radical anion intermediates 1 − and 2 − are dominated by very similar 185,187Re hyperfine splitting, ruling out a mixed-valent formulation. The second electron uptake is followed by the rapid loss of one chloride, leading to an unsymmetrical species in the dinuclear system.