COBALT COMPLEXES OF OCTAETHYLOXOPHLORIN - METAL-CENTERED REDOX CHEMISTRY IN THE PRESENCE OF A REDOX-ACTIVE LIGAND

Two cobalt complexes of octaethyloxophlorin (H2OEPOH), which are related by one-electron oxidation and proton transfer, have been characterized by X-ray diffraction. Deep orange {(py)Co(II)(OEPOH..py)} crystallizes in the triclinic space group P1 with a = 9.267(3) angstrom, b = 14.007(4) angstrom, c = 15.816(6) angstrom, alpha = 101.70(2)-degrees, beta = 92.41(3)-degrees, and gamma = 101.51(2)-degrees at 130(2) K with Z = 2. Refinement of 4615 reflections with F > 4.0 sigma(F) and 490 parameters yielded R = 0.053, R(w) = 0.055. The structure involves a five-coordinate cobalt ion with the axial Co-N distance (2.204(3) angstrom) longer than the in-plane Co-N distances (average 1.988 angstrom). This elongation is consistent with the presence of a low-spin (S = 1/2) d7 ion with the d(z2) orbital filled. The macrocycle has an ordered structure with a second pyridine participating in a hydrogen bond to the meso hydroxyl group. Lime-green solutions of {py)2Co(III)-(OEPO)} produce blue crystals that form in the monoclinic space group P2(1) with a = 19.177(4) angstrom, b = 20.039(4) angstrom, c = 10.547(2) angstrom, and beta = 100.36-degrees at 130(2) K with Z = 4. Refinement of 4874 reflections with I > 2sigma(I) and 973 parameters yielded R = 0.054. The structure was refined as a racemic twin. There are two independent molecules in the unit cell. Each contains a six-coordinate cobalt with short in- and out-of-plane Co-N distances, which average 1.958 angstrom for the Co-N(pyrrole) and 1.961 angstrom for the Co-N(pyridine) distances, and a severely saddle-shaped macrocycle. In both molecules the meso keto groups show some disorder, and in the second molecule three of the ethyl groups are disordered. The electronic spectrum of {(py)Co(II)(OEPOH..py)} shoes a typical poryhyrin-like spectrum (lambda(max), nm: 404 (Soret), 530) while the spectrum of {(PY)2Co(III)(OEPO)} shows characteristic low-energy absorption (lambda(max), nm: 442 (Soret), 532, 622, 676) that distinguishes this trianionic macrocycle from oxophlorin radical complexes like {(py)Zn(II)(OEPO.)}.