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

Citation
Pa. Connick et al., X-RAY STRUCTURAL AND IMIDAZOLE-BINDING STUDIES OF NICKEL BETA-OXOPORPHYRINS, Inorganic chemistry, 32(15), 1993, pp. 3256-3264
Citations number
36
Categorie Soggetti
Chemistry Inorganic & Nuclear
Journal title
ISSN journal
00201669
Volume
32
Issue
15
Year of publication
1993
Pages
3256 - 3264
Database
ISI
SICI code
0020-1669(1993)32:15<3256:XSAISO>2.0.ZU;2-9
Abstract
X-ray structures for 18-octaethyl-3H,8H-porphine-2,7-dionato(2-)]nicke l (nickel di-beta-oxoporphyrin-II, NiDP-II) (2) and ctaethyl-3H,12H,18 H-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 2,13,17,18-octa ethyl-3H-porphin-2-onato(2-)]nickel (nickel mono-beta-oxoporphyrin, Ni MP) (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 angs trom (NiDP-II), and 0.52 angstrom (NiTP-A). These values correlate wit h the decreased aromaticity and concomitant greater ring flexibility w hich result from the presence of one, two, or three beta-oxo substitue nts. 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 N iDP-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-oxoporph yrin rings, which facilitates the core expansion and ring flattening a ccompanying formation of 6-coordinate high-spin Ni(II). NiTP-A exhibit s a higher affinity for imidazole than positively charged porphyrins a nd the Ni-containing tetrahydrocorphin Factor 430. Ni3+/2+ potentials of NiDP-II and NiTP-A are sensitive to imidazole concentration, shifti ng in the negative direction as imidazole is added, which indicates im idazole ligation preferentially stabilizes Ni(III). Ni(II)TP-A is oxid ized to the Ni(III) complex at 0.27 V vs SCE in the presence of imidaz ole, 0.36 V lower than the value for Ni3+/2 oxidation in methylene chl oride solution. This result illustrates the powerful modulation of Ni3 +/2+ potentials which can be accomplished by a biologically relevant p orphyrinic ligand. Crystallographic data are as follows. NiMP: monocli nic 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) angstrom 3, Z = 12, R(F) = 0.079 and R(wF) = 0.102, based on 6914 unique data p oints with F(o) > 3sigma(F(o)), T = 294 +/- 1 K. NiDP-II: monoclinic s pace 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 poi nts with F(o) > 3sigma(F(o)), T = 294 +/- 1 K. NiTP-A: monoclinic spac e group P2(1)/c, a = 11.880(1) angstrom, b = 23.301(3) angstrom, c = 1 2.585(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 point s with F(o) > 3sigma(F(o)), T = 294 +/- 1 K.