X-RAY STRUCTURAL AND IMIDAZOLE-BINDING STUDIES OF NICKEL BETA-OXOPORPHYRINS

X-ray structures for [3,3,8,8,12,13,17,18-octaethyl-3H,8H-porphine-2,7-dionato(2-)]nickel (nickel di-beta-oxoporphyrin-II, NiDP-II) (2) and [3,3,7,8,12,12,18,18-octaethyl-3H,12H,18H-porphine-2,13,17-trionato(2-)] nickel (nickel tri-beta-oxoporphyrin-A, NiTP-A) (3) are reported for the first time along with a structure of higher precision for the previously studied complex [3,3,7,8,12,13,17,18-octaethyl-3H-porphin-2-onato(2-)]nickel (nickel mono-beta-oxoporphyrin, NiMP) (1) (Stolzenberg, A. M.; Glazer, P. A.; Foxman, B. M. Inorg. Chem. 1986,25,983-991). Average methine carbon displacements from the four-nitrogen plane increase with increasing number of beta-oxo groups: < 0.10 angstrom (NiMP, general), 0.35 angstrom (NiMP, special), 0.45 angstrom (NiDP-II), and 0.52 angstrom (NiTP-A). These values correlate with the decreased aromaticity and concomitant greater ring flexibility which result from the presence of one, two, or three beta-oxo substituents. NiMP shows essentially no tendency to bind imidazole, whereas K1 and K2 values for imidazole binding are 10 +/- 4 and 7 +/- 2 M-1 for NiDP-II and 1700 +/- 300 and 500 +/- 150 M-1 for NiTP-A. The structural mechanism for the parallel increases in ring ruffling and strength of imidazole binding is the significant flexibility of the beta-oxoporphyrin rings, which facilitates the core expansion and ring flattening accompanying formation of 6-coordinate high-spin Ni(II). NiTP-A exhibits a higher affinity for imidazole than positively charged porphyrins and the Ni-containing tetrahydrocorphin Factor 430. Ni3+/2+ potentials of NiDP-II and NiTP-A are sensitive to imidazole concentration, shifting in the negative direction as imidazole is added, which indicates imidazole ligation preferentially stabilizes Ni(III). Ni(II)TP-A is oxidized to the Ni(III) complex at 0.27 V vs SCE in the presence of imidazole, 0.36 V lower than the value for Ni3+/2 oxidation in methylene chloride solution. This result illustrates the powerful modulation of Ni3+/2+ potentials which can be accomplished by a biologically relevant porphyrinic ligand. Crystallographic data are as follows. NiMP: monoclinic space group C2/c, a = 38.549(9) angstrom, b = 14.744(5) angstrom, c = 17.329(4) angstrom, beta = 102.47(2)-degrees, V = 9617(8) angstrom3, Z = 12, R(F) = 0.079 and R(wF) = 0.102, based on 6914 unique data points with F(o) > 3sigma(F(o)), T = 294 +/- 1 K. NiDP-II: monoclinic space group P2(1)/c, a = 12.370(1) angstrom, b = 17.199(3) angstrom, c = 15.172(1) angstrom, beta = 91.09(1)-degrees, V = 3227.4(6) angstrom3, Z = 4, R(F) = 0.044 and R(wF) = 0.057, based on 6012 unique data points with F(o) > 3sigma(F(o)), T = 294 +/- 1 K. NiTP-A: monoclinic space group P2(1)/c, a = 11.880(1) angstrom, b = 23.301(3) angstrom, c = 12.585(1) angstrom, beta = 106.03(2)-degrees, V = 3348.4(9) angstrom3, Z = 4, R(F) = 0.088 and R(wF) = 0.076, based on 3576 unique data points with F(o) > 3sigma(F(o)), T = 294 +/- 1 K.