Abstract
The free-base and metal complexes of mono- and di-/3-oxoporphyrins obtained by hydrogen peroxide oxidation of octaethylporphyrin have been prepared and certain chemical and physicochemical properties investigated. One of these complexes, which are formally analogous to hydroporphyrins, has been proposed to be the essential chromophoric unit of the macrocycle in heme-ifv The structure of [3,3,7,8,12,13,17,18-octaethyl-2(3Ff)-porphinonato(2-)]nickel, C36H44N4ONi, has been determined by X-ray diffraction. The complex crystallizes in the monoclinic space group C2/c with a = 38.565 (8) Á, b = 14.767 (4) Á, c = 17.327 (5) Á, and ß = 102.19 (3)°. There are 12 molecules of the oxoporphyrin per unit cell including four disordered molecules on a special position. The structure has been refined by full-matrix least squares to R = 0.106. The results confirm the structure of the oxoporphyrin macrocycle previously deduced by spectroscopic evidence. The conformations of the two independent molecules are different. The general position molecule is nearly planar, whereas the special position molecule is S4 ruffled. Both deviate from planarity to a lesser extent than nickel hydroporphyrin complexes. The /3-carbonyl group of oxoporphyrins has been found to be remarkably inert. The electrochemical properties of oxoporphyrins are unlike those of hydroporphyrins. The first oxidations of mono- and dioxoporphyrins occur at potentials quite similar to that of octaethylporphyrin. In contrast, the potentials of the first reductions become successively less negative as more carbonyl groups are introduced.