MODELS OF HEME-D1 - MOLECULAR-STRUCTURE AND NMR CHARACTERIZATION OF AN IRON(III) DIOXOISOBACTERIOCHLORIN (PORPHYRINDIONE)

Crystallographic and NMR results are reported for 4, the ClFeIII complex of 2,7-dioxo-3,3',8,8',12,13,17,18-octaethylporphyrin (3). 4 serves as a model for heme d1, the unusual dioxoisobacteriochlorin (iBC = isobacteriochlorin) prosthetic group of bacterial dissimilatory nitrite reductases. The coordination and displacement of the Fe in 4 are similar to those found in pentacoordinate, high-spin iron(III) porphyrins, as is the domed conformation of its macrocycle. The major differences are the distinct elongation of the Fe-N distances to the saturated pyrroline rings, 2.118 (6) and 2.113 (6) angstrom, vs those to the pyrrole rings, 2.047 (6) and 2.033 (6) angstrom. The shorter C-alpha-C-beta pyrroline bonds bearing the keto groups vs those without the oxygens suggest that the keto groups are in conjugation with the macrocycle pi-system. These structural data thus provide a rationale for the most distinctive feature between iBCs and dioxo-iBCs: the large differences in Fe and macrocycle redox potentials. In contrast to the influence of the keto groups on the redox and pyrroline structural properties of 4, NMR data, particularly at the meso positions, are comparable to those recently reported for ClFeIIIOEiBC (Sullivan et al. J. Am. Chem. Soc. 1991, 113, 5264). Iterative extended Huckel calculations for the HOMOs and LUMOs of 4 correctly predict the effects of the keto groups on the redox properties of the macrocycle. Crystallographic results for the free base 3, although at poorer precision than for 4, indicate that the NH protons are localized on adjacent pyrrole rings and that the macrocycle is not inherently ruffled to any significant extent. Crystallographic data for 4: space group P1BAR, a = 11.635 (3) angstrom, b = 16.189 (5) angstrom, c = 9.003 (5) angstrom, alpha = 96.93 (3)-degrees, beta = 95.92 (3)-degrees, gamma = 93.75 (2)-degrees, V = 1669.2 angstrom 3, Z = 2. The structure was refined against 2421 data points with F(o) > 2-sigma-F(o) to R(F) = 0.065 and R(wF) = 0.064.