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

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 Re2(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 1radical dot− and 2radical dot− 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.