Hydrogen Bonding of 1,10-Phen on Fe(II) Porphyrinates
heme center and also forms a hydrogen bond with a highly
conserved aspartate group Asp247, a stronger hydrogen bond
acceptor than that of backbone carbonyl oxygen. But they
have much different chemistry. The biological function of
globins is the reversible transport and storage of dioxygen,
whereas peroxidases can catalyze the conversion of hydrogen
peroxide to water and/or the oxidation of substrates. It has
been postulated that the hydrogen bond stabilizes higher
oxidation states of iron and distinctly alters the chemical
behavior of the peroxidases relative to the globins. The
importance of such hydrogen bonds has been commented
on and calculated on but, to our knowledge, has not been
experimentally investigated in a systematic manner.
We have been studying the nature of various five-
coordinate iron(II) porphyrinates.12-17 Much of this work
has focused on the nature of high-spin imidazole-ligated
species.13,14,16 As part of that work, we reported15 the effects
of deprotonating the imidazole ligand in five-coordinate
iron(II) porphyrinate derivatives of the type [Fe(Por)(2-
MeHIm)].18 Both imidazole- and imidazolate-ligated iron(II)
porphyrinates exhibit an S ) 2 (quintet) state, but the
structural parameters of the coordination groups are distinct
with both axial and equatorial bond distance differences and
large differences in the displacement of iron from the
porphyrin plane. Distinctive features in the Mo¨ssbauer spectra
obtained in applied magnetic fields show that the doubly
occupied d orbital is different in imidazole- vs imidazolate-
ligated iron(II) porphyrinates. The positive sign of the
quadrupole splitting in the imidazolate derivative shows that
the doubly occupied orbital must be the dxy orbital whereas
the negative sign in the imidazole derivative is consistent
only with a low-symmetry orbital comprised of a hybrid of
dxz, dyz, and dxy. This change in the d-electron configuration
is clearly consonant with all observed differing features of
the two classes.
deoxymyoglobin.19,20 These Mo¨ssbauer studies, in a strong
magnetic field, showed remarkable differences between
reduced HRP and deoxymyoglobin (deoxyMb) even though
both are five-coordinate hemes with histidine as the axial
ligand. Reduced HRP has a positive quadrupole splitting
constant (Vzz > 0) and a rather small asymmetry constant,20
whereas deoxyMb has a large asymmetry constant and a
negative value of the quadrupole splitting constant (Vzz
<
0). This strongly argues for different properties of the axial
ligand in the five-coordinate iron(II) states of reduced HRP
and deoxyMb.
On the basis of these studies, we have begun further
investigations with iron(II) porphyrinates of hydrogen-bonded
species to fill the gap between imidazole- and imidazolate-
ligated porphyrinates. They will also help us understand how
hydrogen bonds influence the molecular and electronic
structure of the high-spin species. Recently, we presented
the first of our investigations of hydrogen bond formation
with a coordinated imidazole in a high-spin iron(II) porphy-
rinate system, [Fe(TPP)(2-MeHIm)]2 ·2-MeHIm.21 This spe-
cies has two independent five-coordinate, high-spin iron(II)
porphyrinate sites: one with a strong hydrogen bond to the
imidazole and the second with a “neutral” imidazole ligand.
This complex has been studied by X-ray and neutron
diffraction as well as Mo¨ssbauer spectroscopy to assess the
effects of hydrogen-bonded imidazole as a ligand. These
studies show that there is a clear difference in the iron(II)
electronic structure between the two iron sites, distinctions
that can be attributed to the presence of a hydrogen bond to
the coordinated imidazole in the one site. But in this case,
the two different porphyrin molecules in the asymmetric unit
make it difficult to unambiguously assign their electronic
configuration. We have attempted to obtain additional
hydrogen-bonded imidazole-ligated iron(II) porphyrinates for
characterization. Success requires finding suitable hydrogen-
bond acceptors that are either weak ligands or nonligands
to avoid coordination competition with the imidazole (2-
methylimidazole).
These differences are similar to those of the globins and
the peroxidases. Reduced horse radish peroxidase (HRP) has
been studied by Mo¨ssbauer spectroscopy and compared with
Balch and co-workers22,23 have reported that 1,10-phenan-
throline (1,10-phen) forms hydrogen bonds with coordinated
imidazole in six-coordinate iron(III) porphyrinates.
(11) Gajhede, M.; Schuller, D. J.; Henriksen, A.; Smith, A. T.; Poulos,
T. L. Nat. Struct. Biol. 1997, 4, 1032.
(12) Nasri, H.; Ellison, M. K.; Krebs, C.; Huynh, B. H.; Scheidt, W. R.
J. Am. Chem. Soc. 2000, 122, 10795.
(13) Ellison, M. K.; Schulz, C. E.; Scheidt, W. R. Inorg. Chem. 2002, 41,
2173.
[Fe(TPP)(Cl)] + 2HIm h [Fe(TPP)(HIm)2]++Cl-
They showed that the equilibrium in the reaction above was
significantly shifted to the right by the addition of 1,10-
phenanthroline. Balch et al. investigated this system by
visible spectroscopy and NMR.22 In the NMR experiments,
diamagnetic cobalt(III) derivatives were substituted for the
(14) Hu, C.; Roth, A.; Ellison, M. K.; An, J.; Ellis, C. M.; Schulz, C. E.;
Scheidt, W. R. J. Am. Chem. Soc. 2005, 127, 5675.
(15) Hu, C.; Noll, B. C.; Schulz, C. E.; Scheidt, W. R. J. Am. Chem. Soc.
2005, 127, 5018.
(16) Hu, C.; An, J.; Noll, B. C.; Schulz, C. E.; Scheidt, W. R. Inorg. Chem.
2006, 45, 4177.
(17) Nasri, H; Ellison, M. K.; Shaevitz, B.; Gupta, G. P.; Scheidt, W. R.
Inorg. Chem. 2006, 45, 5284.
(18) The following abbreviations are used in this paper: Por, a generalized
porphyrin dianion; OEP, dianion of octaethylporphyrin; TPP, dianion
of meso-tetraphenylporphyrin; Tp-OCH3PP, dianion of meso-tetra-p-
methoxyphenylporphyrin; TTP, dianion of meso-tetratolylporphyrin,
(19) Debrunner, P. In Iron Porphyrins Part Three; Lever, A. B. P., Gray,
H. B., Eds.; VCH Publishers Inc.: New York, 1983; Chapter 2.
(20) Champion, P. M.; Chiang, R.; Mu¨nck, E.; Debrunner, P.; Hager, L. P.
Biochemistry 1975, 14, 4159.
(21) Hu, C.; Noll, B. C.; Piccoli, P. M. B.; Schultz, A. J.; Schulz, C. E.;
Scheidt, W. R J. Am. Chem. Soc. 2008, 130, 3127.
(22) Balch, A. L.; Watkins, J. J.; Doonan, D. J. Inorg. Chem. 1979, 18,
1228.
(23) The increase in imidazole binding constants upon the addition of 1,10-
phenanthroline had been noted previously by Abbott and Rafson, but
the origin of the effect was not established (a) Abbott, E. H.; Rafson,
P. A. J. Am. Chem. Soc. 1974, 96, 7378.
TpivPP, dianion of R,R,R,R-tetrakis(o-pivalamidophenyl)porphyrin;
Piv2C8P, dianion of R,R,5,15-[2,2′-(octanediamido)diphenyl]-R,R,10-
20-bis(o-pivalamidophenyl)porphyrin; Im, generalized imidazole; RIm,
generalized hindered imidazole; HIm, imidazole; 1-MeIm, 1-meth-
ylimidazole; 2-MeHIm, 2-methylimidazole; 1,2-Me2Im, 1,2-dimeth-
ylimidazole; phen, phenanthroline; DBU, 1,8-diazabicyclo[5.4.0]undec-
7-ene; Np, porphyrinato nitrogen; Ct, the center of four porphyrinato
nitrogen atoms.
Inorganic Chemistry, Vol. 47, No. 19, 2008 8885