Reactivity of the Verdoheme Analogues, 5-Oxaporphyrin Complexes of Cobalt(II) and Zinc(II), with Nucleophiles: Opening of the Planar Macrocycle by Alkoxide Addition To Form Helical Complexes.

Abstract

The 5-oxaporphyrin macrocycle is a modified porphyrin (with an oxygen atom replacing a meso-methine unit) that is produced during heme degradation in biological and chemical systems. We have undertaken an investigation into the chemical reactivity of metal complexes of this macrocycle. Here we report that the diamagnetic zinc(II) and paramagnetic cobalt(II) verdoheme analogues, [Zn(II)(OEOP)](O(2)CCH(3)) and [Co(II)(OEOP)](PF(6)) (OEOP is the monoanion of octaethyl-5-oxaporphyrin), undergo ring opening when treated with alkoxide ions. The zinc(II) complex Zn(II)(OEBOMe), where OEBOMe is the dianion of octaethylmethoxybiliverdin, is sufficiently stable to be isolated in crystalline form, but the cobalt(II) analogue, Co(II)(OEBOMe), is less stable and has been characterized primarily by (1)H NMR spectroscopy in solution. The reactions are accompanied by a color change from blue-green for the verdoheme complexes to yellow-green for the ring-opened compounds. The ring-opened product Zn(II)(OEBOMe) has been subjected to a crystallographic study which reveals that the complex contains a four-coordinate zinc(II) ion whose geometry is significantly controlled by the helical tetrapyrrole ligand so that tetrahedral coordination is not achieved. The structure of this complex is compared to that of a closely related free ligand and those of copper and cobalt complexes of tetrapyrrole ligands that also form helical complexes.