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

The complexes (OC)(3)ClRe(abpy) (1) and [(OC)(3)ClRe](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 Angstrom 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(mu-abpy) plane, the Re-Re distance is 5.033(7) Angstrom. The azo bond lengths at 1.272(9) (1) and 1.304(10) Angstrom (2) indicate substantial pi back donation from the rhenium(I) centers into the pi*(abpy) orbital. Spectroelectrochemistry reveals the successive occupation of the pi*(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 Re-185,Re-187 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. (C) 2000 Elsevier Science S.A. All rights reserved.