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.