Electron transfer behavior and solid state structures of the helical cobalt complexes of the open-chain tetrapyrrole ligand, octaethylbilindione

A study of the redox behavior of the cobalt complex of octaethylbilindione (H(3)OEB), a biliverdin analog, reveals that coordination of the ligand allows its complexes to undergo reversible, one-electron transfer processes, whereas the free ligand itself undergoes an irreversible two-electron oxidation. Thus, the four-membered electron transfer series involving [Co(OEB)](n) with n = +1, 0, -1, -2 has been observed electrochemically. The most highly oxidized member of this series has been isolated in the form of its triiodide salt, [Co-II(OEBOx)]I-3.0.5CH(2)Cl(2), and characterized by X-ray diffraction. The structure involves helical coordination of the linear tetrapyrrole ligand about the cobalt with all four nitrogen atoms coordinated with Co-N distances falling in the narrow range, 1.898(7)-1.926(7) Angstrom. The triiodide ion, which is disordered over three orientations, is ion paired to the complex. The Co ... I distance (2.818 Angstrom in the predominant orientation) is much longer than expected for a Co-I covalent bond. In the solid state, pairs of [Co-II(OEBOx)]I-3 crystallize about a center of symmetry so that two identical tab/slot arrangements, which involve CH ... O hydrogen bonds, occur. These supramolecular arrangements involve hydrogen bonding between the lactam oxygen of one complex and methine and two methylene protons of an adjacent complex. Similar hydrogen bonded motifs are found in other complexes derived from octaethylbilindione and may occur for biliverdin IX derivatives as well.