E0015

X-ray Crystallographic Studies of Cobalt-Dioxygen Complexes which Model the Active Sites in Oxyhemoproteins. Charles J. Simmons and Mai Toyofuku, Dept. of Chemistry, Univ. of Hawaii at Hilo, Hilo, HI 96720-4091 U.S.A.

A series of monomeric dioxygen cobalt-Schiff base complexes with identical equatorial ligands and different N-axial bases have been synthesized and characterized by X-ray diffraction at 120 K. Their molecular architectures are relatively simple compared to those of the myriad varieties of exotic synthetic iron porphyrins and other cobalt complexes reported in the literature, which are very intractable to crystallize. Incorporated into the ligand framework are tert-butyl and tert-amyl groups, instead of ‘straps,’ ‘picket fences,’ ‘caps,’ etc. Low temperatures and an appropriate polar solvent (acetonitrile) are used to minimize dimerization and irreversible oxidation, facilitating oxygenation and crystallization of complexes. The dioxygen molecule binds to Co in a bent, Pauling configuration with Co-O-O bond angles of ca. 120% and O-O bond lengths between 1.29-1.32 Å, suggesting considerable superoxide character. The Co-N_ax bond lengths show considerable variation (1.98-2.10 Å), resulting mainly from differences in steric interactions between the N-axial bases and equatorial ligands. This causes changes in the electronic structures of the CoO₂ groups, which, in turn, influence the O-O bond lengths, the rotational order/disorder of the bound O₂ groups, and the relative strengths of the C-HÃÃÃO₂ hydrogen bonds.

X-ray structure of one member of a series of dioxygen cobalt-Schiff base complexes, viz., Co(3-t-BuSalchxn)(Py)(O₂).