Models of the cytochromes. Axial ligand orientation and complex stability in iron(II) porphyrinates: The case of the noninteracting dπ orbitals

Article abstract of DOI:10.1021/ja9715657

The synthesis and characterization of seven bis-pyridine and bis-imidazole complexes of iron(II)) tetramesitylporphyrinate are reported. X-ray crystal structures of three of the complexes, [Fe(TMP)(4-CNPy)₂], [Fe(TMP)(3-CNPy)₂], and

Full text of DOI:10.1021/ja9715657

9438
J. Am. Chem. Soc. 1997, 119, 9438-9448
Models of the Cytochromes. Axial Ligand Orientation and
Complex Stability in Iron(II) Porphyrinates: The Case of the
Noninteracting dπ Orbitals
Martin K. Safo,1a,b Marlys J. M. Nesset,1c F. Ann Walker,*,1c
Peter G. Debrunner,*,1d and W. Robert Scheidt*,1a
Contribution from the Department of Chemistry and Biochemistry, UniVersity of Notre Dame,
Notre Dame, Indiana 46556, Department of Chemistry, UniVersity of Arizona,
Tucson, Arizona 85721, and Department of Physics, UniVersity of Illinois, Urbana, Illinois 61801
ReceiVed May 14, 1997X
Abstract: The synthesis and characterization of seven bis-pyridine and bis-imidazole complexes of iron(II)
tetramesitylporphyrinate are reported. X-ray crystal structures of three of the complexes, [Fe(TMP)(4-CNPy)2],
[Fe(TMP)(3-CNPy)2], and [Fe(TMP)(4-MePy)2], have been solved and all show parallel axial ligand orientations
with nearly planar porphyrinato cores. The Mo¨ssbauer spectra of six of the complexes, having pyridine ligands with
pKa(PyH+) ranging from ∼1.1 (4-CNPy) to 9.7 (4-NMe2Py), have been determined. The Mo¨ssbauer isomer shifts
at 120 K are in the range of 0.36-0.45 mm/s, and the quadrupole splittings (∆EQ) are in the range of 1.11-1.27
mm/s. Thus, unlike the corresponding Fe(III) complexes, the X-ray structures and Mo¨ssbauer spectroscopic parameters
of these (tetramesitylporphyrinato)iron(II)-bis(pyridine) complexes are shown to be essentially independent of the
basicity and π donor/acceptor properties of the axial pyridine ligands. These solid-state structural and spectroscopic
properties are compared to the thermodynamic properties of the same series of complexes in solution (Nesset, M. J.
M.; Shokhirev, N. V.; Enemark, P. D.; Jacobson, S. E.; Walker, F. A. Inorg. Chem. 1996, 35, 5188): The equilibrium
II
constants, â2 , for binding two ligands to [FeII(TMP)(DMF)] are also nearly independent of the basicity of the axial
pyridine ligand, although the FeIII/FeII reduction potentials vary strongly with ligand basicity due to the large variation
in â2III, the equilibrium constant for binding two ligands to the Fe(III) complex. Hence, it appears that low-spin d6
metalloporphyrins have a marked preference for parallel orientation of planar axial ligands, and that the charge
asymmetry at the iron nucleus (deduced from Mo¨ssbauer quadrupole splittings) and the thermodynamics of ligand
binding are unaffected by the electronic properties of the axial ligand. The major reason for the marked preference
for parallel ligand orientation for iron(II) porphyrinates appears to be lack of a means of energy stabilization of the
ruffled core of the perpendicular orientation.
Introduction
ring to either amide carbonyl groups of the protein backbone
or, possibly, hydrogen bond acceptors provided as amino acid
side chains. Physiologically relevant ligands provided by the
protein include the imidazole side chain of histidine,9,10 the
methyl thioether side chain of methionine,10 the thiolate of
cysteine,11 the phenolate of tyrosine,11 and in the case of
cytochrome f,12 the N-terminal amino group of the polypeptide.
For the case of heme centers coordinated to two planar
imidazole ligands of histidine residues, two limiting orientations
of the axial ligand planes have been implicated in the structures
of the cytochromes: imidazole planes oriented parallel to each
other (cytochromes b5,9 three of the heme centers of cyto-
chromes c3,13 the b hemes of sulfite oxidase14 and flavocyto-
chrome b2,15 and the heme a of cytochrome oxidase16 ), while
other bis-histidine-coordinated heme proteins are believed to
have their axial imidazole planes orientated perpendicular to
Model hemes based on iron(II) and iron(III) tetraphenylpor-
phyrinates have found considerable utility in elucidating and
understanding the properties of the heme proteins.2 However,
these synthetic hemes often introduce new or different properties
that the investigator may not have considered beforehand, for
example, rapid rotation of axial ligands in homogeneous solu-
tion.3-8 In comparison to the axial ligands in model hemes,
rotation of the axial ligands of heme proteins is prevented be-
cause they are linked by side chains that are covalently attached
to the protein backbone. The orientations of planar ligands
provided by the protein are also tightly controlled by protein
structural constraints that include steric crowding of other protein
side chains very near the heme and, in the case of histidine
ligands, hydrogen bonding of the NH group of the imidazole
X Abstract published in AdVance ACS Abstracts, September 15, 1997.
(1) (a) University of Notre Dame. (b) Present address: Department of
Medicinal Chemistry, Medical College of Virginia (VCU), P.O. Box 540,
Richmond, VA 23298. (c) University of Arizona. (d) University of Illinois.
(2) Walker, F. A.; Simonis, U. Iron-Porphyrin Chemistry. In Encyclo-
pedia of Inorganic Chemistry; Berliner, L. J., Reuben, J., Eds.; Plenum:
New York, 1993; pp 133-274.
(3) Nakamura, M.; Groves, J. T. Tetrahedron 1984, 44, 3225.
(4) Walker, F. A.; Simonis, U. J. Am. Chem. Soc. 1991, 113, 8652.
(5) Shokhirev, N. V.; Shokhireva, T. Kh.; Polam, J. R.; Watson, C. T.;
Raffii, K.; Simonis, U.; Walker, F. A. J. Phys. Chem. A 1997, 101, 2778.
(6) Momot, K. I.; Walker, F. A. J. Phys. Chem. A 1997, 101, 2787.
(7) Nakamura, M.; Tajima, K.; Tada, K.; Ishizu, K.; Nakamura, N. Inorg.
Chim. Acta 1994, 224, 113.
(9) Mathews, F. A.; Czerwinski, E. W.; Argos, P. In The Porphyrins;
Dolphin, D., Ed.; Academic Press: New York, 1979; Vol. VII, pp 107-
147.
(10) Dickerson, R. E.; Takano, T.; Eisenberg, D.; Kallai, O. B.; Samson,
L.; Cooper, A.; Margoliash, E. J. Biol. Chem. 1971, 246, 1971.
(11) Poulos, T. L. In Cytochrome P-450: Structure, Mechanism, and
Biochemistry; Ortiz de Montellano, P. R., Ed.; Plenum: New York, 1986;
pp 505-523.
(12) Martinez, S. E.; Huang, D.; Szczepaniak, A.; Cramer, W. A.; Smith,
J. L. Structure 1994, 2, 95.
(13) (a) Pierrot, M.; Haser, R.; Frey, M.; Payan, F.; Astier, J.-P. J. Biol.
Chem. 1982, 257, 14341. (b) Higuchi, Y.; Kusunoki, M.; Matsuura, Y.;
Yasuoka, N.; Kakudo, M. J. Mol. Biol. 1984, 172, 109.
(14) Kipke, C. A.; Cusanovich, M. A.; Tollin, G.; Sunde, R. A.; Enemark,
J. H. Biochemistry 1988, 27, 2918.
(8) Polam, J. R.; Shokhireva, T. Kh.; Raffii, K.; Simonis, U.; Walker,
F. A. Inorg. Chim. Acta In press.
S0002-7863(97)01565-5 CCC: $14.00 © 1997 American Chemical Society

Models of the Cytochromes
J. Am. Chem. Soc., Vol. 119, No. 40, 1997 9439
each other. Proteins of the latter group have been identified
largely on the basis of spectroscopic data for the oxidized
(Fe(III)) forms, and include the b hemes of mitochondrial
complex III, also known as cytochrome bc1,17 the similar b
hemes of cytochrome b6f of chloroplasts, one of the c-type
hemes of cytochrome c3,13 and the c-type heme of cytochrome
c′′ of Methylophilus methylotrophus.18
During the time over which our studies of bis(hindered
imidazole) and -(pyridine) complexes of Fe(III) porphyrinates
were being carried out and the above-described hypothesis was
developed, we had assumed that for the closed subshell
configuration of low-spin d6 Fe(II) porphyrinates, planar axial
ligands would prefer to align themselves in mutually perpen-
dicular planes, to maximize the π-bonding interactions between
the filled dπ orbitals of Fe(II) and the π* orbitals of the ligands.
However, the investigations reported herein, which involve
structural, spectroscopic, and thermodynamic measurements of
bis(pyridine) complexes of (tetramesitylporphyrinato)iron(II),
clearly show that low-spin Fe(II) porphyrinates strongly pre-
fer to have planar axial ligands oriented parallel to each other,
and that π-bonding interactions between Fe(II) and the axial
ligands are minimal. Herein we report the structures of three
complexes, [Fe(TMP)(4-CNPy)2], [Fe(TMP)(3-CNPy)2], and
[Fe(TMP)(4-MePy)2], and the Mo¨ssbauer parameters of a series
of FeII(TMP) complexes with pyridines of widely varying
basicities, where, in all cases, the analogous FeIII(TMP) deriva-
tive is known to have mutually perpendicular axial ligands.21,22
The solid state properties of these ferrocytochrome models will
then be compared to the thermodynamic properties of the same
complexes in homogeneous solution (reduction potentials of the
FeIII/FeII couples and the equilibrium constants for binding the
axial ligands to Fe(II) TMP at ambient temperatures).26 Possible
reasons for and consequences of the lack of sensitivity of low-
spin Fe(II) porphyrinate bis-ligand complexes to the σ-donor
and π-donor/acceptor properties of the axial ligands will be
evaluated.
Based on structural and spectroscopic investigations of the
bis(2-methylimidazole) complex of (tetraphenylporphyrinato)-
iron(III), [TPPFe(2-MeHIm)2]+,19,20 we concluded some time
ago that for low-spin d5 ferriheme centers, parallel orientation
of axial ligands is energetically favored, and that either bulky
axial ligands, such as 2-methylimidazole (or, as we later found,
the combination of tetrakis(2,6-disubstituted phenyl)porphyri-
nates together with pyridines21,22 or bulky imidazoles23 ), are
required to force the perpendicular relative orientation of planar
axial ligands in Fe(III) porphyrinates. We also proposed that
the reduction potentials of bis-histidine-ligated cytochromes
might in part be determined by the orientation of the axial ligand
planes, with perpendicular orientation creating the more positive
reduction potential.16 This appeared to be consistent with the
observation that the membrane-bound bis-histidine-coordinated
b cytochromes of the bc1 and b6f complexes, whose single-
feature EPR spectra suggest that the imidazole planes are
perpendicularly oriented,17 tend to have more positive reduction
potentials17b,24 than those for which the EPR signals are rhom-
bic25 and the structures are known or believed to have parallel-
oriented imidazole planes.9 Inherent in this hypothesis is the
belief that no change would take place in histidine imidazole
plane orientation when electron transfer occurred, and hence
both Fe(III) and Fe(II) forms of each limiting type of cytochrome
would have the same ligand orientations. This is known to be
the case for some of the small, water-soluble cytochromes such
Experimental Section
General Information. Reactions were performed with solvents
distilled under argon prior to use. THF and benzene were distilled
from sodium benzophenone ketyl. Dichloromethane, chloroform, and
hexane were distilled from CaH2. 4-Cyanopyridine was recrystallized
from CH2Cl2. Other imidazole and pyridine ligands were obtained from
Aldrich and used without further purification. Tetramesitylporphyrin
was prepared by slight modification of the procedures by Lindsey et
al.,27 and as described previously.21 The [Fe(TMP)(OH)] was prepared
by shaking a CH2Cl2 solution of [Fe(TMP)Cl] with a 2 M solution of
KOH. The reduced iron complex, [Fe(TMP)], was prepared by
reducing a benzene solution of [Fe(TMP)(OH)] with ethanethiol or a
chloroform solution with Zn(Hg). Solid [Fe(TMP)] precipitated from
the benzene solution. UV-vis (benzene): Split soret 419.0, 430.2; R
and â bands 527.5, 562.9 nm. This [Fe(TMP)] was immediately used
for synthesis of the bis-pyridine complexes.
9
as cytochrome b5 and c,10 but has not yet been elucidated for
the membrane-bound b cytochromes.
(15) (a) Xia, Z.-X.; Shamala, N.; Bethge, P. H.; Lim, L. W.; Bellamy,
H. D.; Xuong, N. H.; Lederer, F.; Mathews, F. S. Proc. Natl. Acad. Sci.
U.S.A. 1987, 84, 2629. (b) Dubois, J.; Chapman, S. K.; Mathews, F. S.;
Reid, G. A.; Lederer, F. Biochemistry 1990, 29, 6393.
(16) Iwata, S.; Ostermeier, C.; Ludwig, B.; Michel, H. Nature 1995, 376,
660.
(17) (a) Salerno, J. C. J. Biol. Chem. 1984, 259, 2331. (b) Tsai, A.;
Palmer, G. Biochim. Biophys. Acta 1982, 681, 484. (c) Tsai, A.-H.; Palmer,
G. Biochim. Biophys. Acta 1983, 722, 349.
(18) (a) Berry, M. J.; George, S. J.; Thomson, A. J.; Santos, H.; Turner,
D. L. Biochem. J. 1990, 270, 413. (b) Costa, H. S.; Santos, H.; Turner, D.
L.; Xavier, A. V. Eur. J. Biochem. 1992, 208, 427. (c) Costa, H. S.; Santos,
H.; Turner, D. L. Eur. J. Biochem. 1993, 215, 817.
Mo¨ssbauer samples were prepared as mulls in Apiezon L grease.
Mo¨ssbauer measurements were made at 4.2 and/or 120 K on a constant-
acceleration spectrometer. The spectra were fitted with two Lorentzians
of equal area. Isomer shifts are quoted relative to metallic iron at room
temperature. UV-visible spectra were recorded on a Perkin-Elmer
Lambda 4C spectrophotometer.
Synthesis of [Fe(TMP)(4-CNPy)2]. A chloroform solution (15 mL)
of [Fe(TMP)] prepared by reduction of [Fe(TMP)(OH)] (130 mg, 0.150
mmol) with Zn(Hg) was transferred by cannula filtration into a Schlenk
flask containing the 4-cyanopyridine ligand (300 mg, 2.88 mmol). The
reaction mixture was shaken for a minute, and then layered with hexane
for crystallization. X-ray quality crystals formed after 5 days. UV-
vis (CHCl3) λmax (log ꢀ): 421.0 (5.09), 527.2 (4.16), 558.0 (4.03) nm.
Synthesis of [Fe(TMP)(3-CNPy)2]. A chloroform solution (15 mL)
of [Fe(TMP)] prepared by reduction of [Fe(TMP)(OH)] (80 mg, 0.080
mmol) with Zn(Hg) was cannula-filtered into a Schlenk flask containing
the 3-cyanopyridine ligand (250 mg, 2.40 mmol). The reaction mixture
(19) Walker, F. A.; Huynh, B. H.; Scheidt, W. R.; Osvath, S. R. J. Am.
Chem. Soc. 1986, 108, 5288.
(20) Abbreviations used: Axial ligands: 2-MeHIm, 2-methylimidazole;
1,2-Me2Im, 1,2-dimethylimidazole; 1-MeIm, 1-methylimidazole; 1-VinIm,
1-vinylimidazole; 1-BzylIm, 1-benzylimidazole; 4-CNPy, 4-cyanopyridine;
3-CNPy, 3-cyanopyridine; 3-ClPy, 3-chloropyridine; 4-MePy, 4-methylpy-
ridine; Py, pyridine; 4-NMe2Py, 4-(dimethylamino)pyridine; PMe3, tri-
methylphosphine; DMF, dimethylformamide. Porphyrins: TMP, tetramesi-
tylporphyrin; TPP, tetraphenylporphyrin; (2,6-Cl2)4TPP, tetrakis(2,6-dichlo-
rophenyl)porphyrin; (2,6-Br2)4TPP, tetrakis(2,6-dibromophenyl)porphy-
rin; (2,6-F2)4TPP, tetrakis(2,6-difluorophenyl)porphyrin; (2,6-(OMe)2)4TPP,
tetrakis(2,6-dimethoxyphenyl)porphyrin; OEP, octaethylporphyrin. Other:
Np, porphinato nitrogen; MM2, molecular mechanics program; â2II, equi-
librium constant for binding two axial ligands to iron(II) porphyrinates;
â2III, equilibrium constant for bidning two axial ligands to iron(III)
porphyrinates.
(21) Safo, M. K.; Gupta, G. P.; Walker, F. A.; Scheidt, W. R. J. Am.
Chem. Soc. 1991, 113, 5497.
(22) Safo, M. K.; Gupta, G. P.; Watson, C. T.; Simonis, U.; Walker, F.
A.; Scheidt, W. R. J. Am. Chem. Soc. 1992, 114, 7066.
(23) Munro, O. Q.; Marques, H. M.; Debrunner, P. G.; Mohanrao, K.;
Scheidt, W. R. J. Am. Chem. Soc. 1995, 117, 935.
(24) von Jagow, G.; Engel, W. D. Angew. Chem., Int. Ed. Engl. 1980,
19, 659.
(25) Rivera, Barillas-Mury, C.; Christensen, K. A.; Little, J. W.; Wells,
M. A.; Walker, F. A. Biochemistry 1992, 31, 12233 and references therein.
(26) Nesset, M. J. M.; Shokhirev, N. V.; Enemark, P. D.; Jacobson, S.
E.; Walker, F. A. Inorg. Chem. 1996, 35, 5188.
(27) Watner, R. W.; Lawrence, D. S.; Lindsey, J. S. Tetrahedron Lett.
1987, 28, 3069. Lindsey, J. S.; Wagner, R. W. J. Org. Chem. 1988, 54,
828.

9440 J. Am. Chem. Soc., Vol. 119, No. 40, 1997
Safo et al.
Table 1. Summary of Crystal Data and Intensity Collection Parameters
complex
formula
fw, amu
space group
T, K
[Fe(TMP)(4-CNPy)2]‚2CHCl3
[Fe(TMP)(4-MePy)2]‚2C6H6
[Fe(TMP)(3-CNPy)2]‚2CHCl3
FeCl6N8C70H62
1283.88
P1h
FeN8C80H78
1179.39
P1h
FeCl6N8C70H62
1283.88
P1h
124
124
124
a, Å
b, Å
c, Å
R, deg
â, deg
γ, deg
Z
10.632(4)
12.742(4)
13.722(5)
97.38(3)
105.82(3)
107.92(3)
1
10.556(2)
11.188(2)
13.972(2)
90.93(1)
102.66(2)
94.64(2)
1
10.135(2)
12.326(2)
13.562(2)
106.46(1)
96.54(1)
102.78(2)
1
R1
R2
0.070
0.090
0.059
0.078
0.062
0.073
goodness of fit
2.321
2.350
3.084
was shaken for a minute, and then layered with hexane for crystal-
lization. X-ray quality crystals formed after 4 days.
hydrogen atom positions were idealized and included in subsequent
cycles of least-squares refinement as fixed contributors (C-H ) 0.95
Å and B(H) ) 1.3 × B(C), with additional reidealization as required.
All atoms were refined anisotropically except hydrogens and some
disordered (Vide infra) non-hydrogen atoms.
Synthesis of [Fe(TMP)(4-MePy)2]. A benzene filtrate (30 mL) of
[Fe(TMP)] prepared by reduction of [Fe(TMP)(OH)] with ethanethiol
(after [Fe(TMP)] solid crystals had been harvested) was reacted with
4-methylpyridine (200 mg, 2.15 mmol). The reaction mixture was
shaken for a minute, and then layered with hexane for crystallization.
In all three species, the six-coordinate complexes have required
inversion symmetry with the iron at the inversion center. All complexes
have a single disordered solvent molecule region. In the 4-CNPy
complex, two separate chloroform molecule positions were included
in the full-matrix least-squares refinements with atomic occupancies
of 0.5 for all atoms in one chloroform group and 0.25 for the other. A
third group was related to another chloroform molecule by an inversion
center and was refined by rigid-group methods: A C-Cl distance of
1.74 Å with atomic occupancies of 0.1 was used. Final atomic
coordinates can be found in Table S2, Supporting Information. In the
3-CNPy complex, the three chlorine atoms of the chloroform solvent
were disordered over two orientations. All positions were included in
the full-matrix least-squares refinements with atomic occupancies of
0.85 for all atoms in one group and 0.15 for the other. Final atomic
coordinates can be found in Table S3, Supporting Information. In the
4-MePy complex, the benzene solvent molecule was disordered over
two positions. These were included in the full-matrix least-squares
refinement with atomic occupancies of 0.6 and 0.4 for atoms in the
two respective groups. Final atomic coordinates can be found in Table
S4, Supporting Information. Anisotropic temperature factors, fixed
hydrogen atom positions, and group parameters for all three complexes
are also given in the Supporting Information.
X-ray quality crystals formed after 6 days. UV-vis (CHCl3) λmax
423.0, 530.0, 561.6 nm.
:
Synthesis of [Fe(TMP)(Py)2]. [Fe(TMP)] (50 mg, 0.060 mmol,
prepared by reduction of [Fe(TMP)(OH)] with ethanethiol) was stirred
in chloroform solution with Zn(Hg) for 10 min. The solution was then
transferred by cannula into a Schlenk flask containing the pyridine
ligand (200 mg, 2.24 mmol). The reaction mixture was shaken for a
minute, and then layered with hexane for crystallization. Crystals
formed after 6 days. UV-vis (CHCl3) λmax: 422.0, 528.7, 559.8 nm.
[Fe(TMP)(4-NMe2Py)2], [Fe(TMP)(3-ClPy)2], and [Fe(TMP)(1-
MeIm)2] were prepared as described for [Fe(TMP)(Py)2]. Only
microcrystalline material was obtained in each case. UV-vis data for
the first and third complexes are λmax 425.5, 536.5, 571.0 nm and 426.5,
536.9, 565.0 nm, respectively.
Structure Determinations. The three complexes [Fe(TMP)(4-
CNPy)2], [Fe(TMP)(3-CNPy)2], and [Fe(TMP)(4-MePy)2] were ex-
amined on an Enraf-Nonius CAD4 diffractometer equipped with a
locally modified Syntex LT-1 low-temperature attachment on the
diffractometer. Preliminary examination of crystals of the complexes
at a temperature of 124 K led to a one-molecule triclinic cell in each
case. Final cell constants and complete details of the intensity collection
and least-squares refinement parameters for the complexes are sum-
marized in Tables 1 and S1, Supporting Information. Precise cell
constants were determined from least-squares refinement of 25
automatically centered reflections. Four standard reflections were
monitored during each data collection for crystal movements and
possible deterioration of the crystal. No significant decay was observed
for any of the complexes.
Results and Discussion
General. All the FeIITMP complexes characterized and
reported herein are low-spin six-coordinate complexes as
indicated by UV-vis and/or Mo¨ssbauer spectroscopy, and/or
single-crystal structure determination. The UV-vis spectral
bands of all complexes are between 421-427 and 527-537
nm, with a shoulder at 558-571 nm, and are typical of other
low-spin iron(II) tetraphenylporphyrinates,30,31 a confirmation
of the iron(II) oxidation state. The spectral bands of the low-
basicity pyridine complexes are slightly blue shifted compared
to the high-basicity pyridine and imidazole complexes.
Intensity data were reduced by using the data reduction program
suite of R. H. Blessing.28 All data with Fo g 3.0σ(Fo) were retained
as observed and used in all subsequent refinements. The centrosym-
metric space group P1h was assumed for all complexes; this choice was
consistent with all subsequent developments of structure solution and
refinement. All structures of the complexes were solved with the direct
methods program MULTAN29 and difference Fourier syntheses. The
full-matrix least-squares programs ALLS or ORFLS were used for
structure refinement. After several cycles of least-squares refinement,
the possible hydrogen atoms were located in the complexes. The
Mo1ssbauer Spectra. The Mo¨ssbauer parameters of the
complexes of this study are given in Table 2 along with values
for other low-spin iron(II) porphyrinates.32-40 They are typical
(30) Collman, J. P.; Brauman, J. I.; Doxsee, K. M.; Halbert, T. R.;
Bunnenberg, E.; Linder, R. E.; La Mar, G. N.; Del Gaudio, J. D.; Lang,
G.; Spartalian, K. J. Am. Chem. Soc. 1980, 102, 4182.
(31) Safo, M. K.; Scheidt, W. R.; Gupta, G. P. Inorg. Chem. 1990, 29,
626.
(32) Dolphin, D.; Sams, J. R.; Tsin, T. B.; Wong, K. L. J. Am. Chem.
Soc. 1976, 98, 6970.
(33) Epstein, L. M.; Straub, D. K.; Maricondi, C. Inorg. Chem. 1967, 6,
1720.
(34) Medhi, O. K.; Silver, J. Inorg. Chim. Acta 1989, 166, 129.
(35) Mu¨nck, E. Methods Enzymol. 1978, 54, 346.
(36) Straub, D. K.; Connor, W. M. Ann. N.Y. Acad. Sci. 1973, 206, 383.
(28) Blessing, R. H. Crystallogr. ReV. 1987, 1, 3.
(29) Programs used in this study included local modifications of Main,
Hull, Lessinger, Germain, Declerq, and Woolfson’s MULTAN, Jacobson’s
ALLS, Zalkin’s FORDAP, Busing and Levy’s ORFFE, and Johnson’s
ORTEP2. Atomic form factors were from: Cromer, D. T.; Mann, J. B.
Acta Crystallogr., Sect. A 1968, A24, 321. Real and imaginary corrections
for anomalous dispersion in the form factor of the iron atom were from:
Cromer, D. T.; Liberman, D. J. J. Chem. Phys. 1970, 53, 1891. Scattering
factors for hydrogen were from: Stewart, R. F.; Davidson, E. R.; Simpson,
W. T. J. Chem. Phys. 1965, 42, 3175. All calculations were performed on
VAX 11/730 or 3200 computers.

Models of the Cytochromes
J. Am. Chem. Soc., Vol. 119, No. 40, 1997 9441
Table 2. Mo¨ssbauer Data for Bis(imidazole and pyridine) Iron(II) Porphyrintates
complex
mediuma
T, K (appl field)
δ,b mm s-1
∆EQ, mm s-1
lw, mm s-1
ref
[Fe(TMP)(1-MeIm)2]
crystalline
120
4.2
120
4.2
120
4.2
120
4.2
120
4.2
120
4.2
77
4.2
0.43(1)
0.45(1)
0.36(1)
0.39(1)
0.41(1)
0.42(1)
0.42(1)
0.43(1)
0.45(5)
0.46(2)
0.43(2)
0.45(1)
0.45
1.11(1)
1.09(1)
1.27(1)
1.20(1)
1.13(1)
1.11(1)
1.12(1)
1.09(1)
1.24(5)
1.24(2)
1.23(2)
1.24(1)
1.01
0.33(1)
0.30(1)
0.38(1)
0.27(1)
0.27(2)
0.24(2)
0.27(1)
0.26(1)
0.41(7)
0.32(3)
0.31(3)
0.29(1)
0.23
this work
[Fe(TMP)(4-NMe2Py)2]
[Fe(TMP)(4-CNPy)2]
[Fe(TMP)(4-MePy)2]
[Fe(TMP)(Py)2]
crystalline
crystalline
crystalline
crystalline
crystalline
crystalline
crystalline
this work
this work
this work
this work
this work
31
[Fe(TMP)(3-ClPy)2]
[Fe(TMP)(1-VinIm)2]
[Fe(TPP)(1-VinIm)2]
0.43
0.45
1.00
1.02
0.26
0.23
77
31
4.2
4.2 (6 T)
77
0.43
0.46
0.46
1.00
1.02
1.04
0.26
0.23
0.23
[Fe(TPP)(1-SiMe3Im)2]
crystalline
31
4.2
0.46
1.03
0.23
4.2 (6 T)
77
77
77
78
0.46
0.45
0.45
0.47
0.47
0.45
0.43
0.36
1.03
1.02
0.97
1.07
1.03
0.97
1.04
1.27
0.23
0.23
0.22
0.25
0.15
0.14
[Fe(TPP)(1-BzlIm)2]
[Fe(TPP)(1-AcIm)2]
[Fe(TPP)(1-MeIm)2]
[Fe(Proto IX)(1-MeIm)2]
[Fe(Proto IX)(HIm)2]
cytochrome b5, reduced
[Fe(PClPP)(Py)2]
crystalline
crystalline
crystalline
frozen EtOH:H2O
frozen EtOH:H2O
frozen H2O
crystalline
31
31
31
34
34
34
33
32
150
298
295
4.2
[Fe(OEP)(Py)2]
crystalline
0.38
0.46
1.21
1.13
[Fe(Proto IX)(Py)2]
[Fe(TPP)(Py)2]
crystalline
crystalline
77
300
77
0.45
0.35
0.40
0.39
0.39
0.39
0.34
1.21
1.22
1.15
1.64
1.61
1.73
1.67
32
38
[Fe(TMP)(2-MeHIm)2]
[Fe(TMP)(2-MeHIm)2]
[Fe(TMP)(1,2-Me2Im)2]
[Fe(OEP)(2-MeHIm)2]
frozen DMAc
77
39
40
39
39
4.2 (6 T)
77
77
0.36
frozen DMAc
frozen DMAc
a Crystalline samples contain natural abundance 57Fe; frozen solution samples contain enriched 57Fe. b All values relative to metallic iron. c DMA
) dimethylacetamide.
of low-spin Fe(II) porphyrinates having electronegative donor
atoms,32-41 and the isomer shifts and quadrupole splittings lie
within very narrow ranges (0.36-0.47 and 1.00-1.27 mm/s,
respectively). Since only small sample quantities were available
and spectral absorptions were correspondingly small, magnetic
Mo¨ssbauer measurements were not feasible. The sign and
symmetry of the EFG at the iron therefore remains unknown
for the complexes of this study, but we recall that in all bis-
ligated low-spin Fe(II) porphyrinates studied so far an excess
of negative charge has been found in the porphyrin plane relative
to the normal, suggesting a larger population in the dxy orbital
than in the dπ orbitals, perhaps as a result of charge transfer to
the ligands.42 In support of this expectation, the magnetic
Mo¨ssbauer spectra of [Fe(TMP)(1-MeIm)2], [Fe(TMP)(2-Me-
HIm)2], and [Fe(OEP)(PMe3)2] reported elsewhere40 show that
Vzz is positive in all three of these complexes, and that there is
a slight excess population in the dxy orbital as compared to the
Figure 1. ORTEP diagram of [Fe(TMP)(4-CNPy)2]. Labels assigned
to the crystallographically unique molecule are displayed and 50%
probability surfaces are shown.
dπ orbitals. However, on balance, the work summarized below
indicates that there is little π back-bonding from the dπ orbitals
to the axial ligands.
Structures of the Complexes. The molecular structures of
three of the seven complexes, [Fe(TMP)(4-CNPy)2], [Fe(TMP)-
(3-CNPy)2], and [Fe(TMP)(4-MePy)2], have been determined.
Shown in ORTEP diagrams in Figures 1-3 are the molecular
structures of these three complexes, respectively. The number-
ing scheme for the crystallographically unique atoms and bond
distances in each coordination group are displayed in these
(37) Bearden, A. J.; Moss, T. H.; Caughey, W. S.; Beaudreau, C. A.
Proc. Natl. Acad. Sci. U.S.A. 1965, 53, 1246. Bearden, A. J.; Moss, T. H.;
Caughey, W. S.; Beaudreau, C. A. Inorg. Chem. 1966, 5, 1255.
(38) Kobayashi, H.; Maeda, Y.; Yanagawa, Y. Bull. Chem. Soc. Jpn.
1970, 43, 2342.
(39) Polam, J. R.; Wright, J. L.; Christensen, K. A.; Walker, F. A.; Flint,
H.; Winkler, H.; Grodzicki, M.; Trautwein, A. X. J. Am. Chem. Soc. 1966,
118, 5272.
(40) Grodzicki, M.; Flint, H.; Winkler, H.; Walker, F. A.; Trautwein,
A. X. J. Phys. Chem. A 1997, 101, 4202.
(42) Debrunner, P. G. In Iron Porphyrins; Lever, A. B. P., Gray, H. B.,
(41) Abu-Soud, H. M.; Silver, J. Inorg. Chim. Acta 1989, 161, 139.
Eds.; VCH Publishers: New York, 1989; Vol. 3, p 170.

9442 J. Am. Chem. Soc., Vol. 119, No. 40, 1997
Safo et al.
Figure 4. Formal diagram of the porphinato core in [Fe(TMP)(4-
CNPy)2]. Deviations of each unique atom from the mean plane of the
core (in units of 0.01 Å) are shown. Averaged values for the chemically
unique bond distances and angles in the core are shown. The orientation
of the axial ligands with the closest Fe-NP vector (angle φ) are shown.
Individual values of the Fe-NP bond distances are shown.
Figure 2. ORTEP diagram of [Fe(TMP)(3-CNPy)2]. Labels assigned
to the crystallographically unique molecule are displayed and 50%
probability surfaces are shown.
Figure 3. ORTEP diagram of [Fe(TMP)(4-MePy)2]. Labels assigned
to the crystallographically unique molecule are displayed and 50%
probability surfaces are shown.
Figure 5. Formal diagram of the porphinato core in [Fe(TMP)(3-
CNPy)2]. Deviations of each unique atom from the mean plane of the
core (in units of 0.01 Å) are shown. The same information displayed
in Figure 4 is given.
figures. All three complexes have crystallographically required
inversion centers at the iron(II) atoms. This symmetry element
leads to several requirements for the molecular structure. First,
the iron(II) atom must be centered in the mean plane of the
24-atom core. Second, the axial ligand planes must be parallel
to each other, and third, the porphinato cores cannot be
significantly nonplanar. Previously reported molecular struc-
tures of three bis-imidazole iron(II) porphyrinates, [Fe(TPP)-
(1-VinIm)2],31 [Fe(TPP)(1-BzylIm)2],31 and [Fe(TPP)(1-MeIm)2],43
and two bis-pyridine complexes, [Fe(TPP)(Py)2]‚2Py44 and [Fe-
(TPP)(Py)2],45 also have crystallographically required inversion
symmetry and hence all conditions also apply to these molecules.
Averaged values for the chemically equivalent bond distances
and angles for the three complexes of this study are shown in
Figures 4-6. The number in parentheses following each
averaged value is the estimated standard deviation calculated
on the assumption that the individual values are all drawn from
the same population. Individual values of the Fe-NP bond
distances and their relationship to the axial ligand orientations
are also shown in the diagrams. Fixed atomic coordinates for
[Fe(TMP)(4-CNPy)2], [Fe(TMP)(3-CNPy)2], and [Fe(TMP)(4-
Figure 6. Formal diagram of the porphinato core in [Fe(TMP)(4-
MePy)2]. Deviations of each unique atom from the mean plane of the
core (in units of 0.01 Å) are shown. The same information displayed
in Figure 4 is given.
(43) (a) Steffen, W. L.; Chun, H. K.; Hoard, J. L.; Reed, C. A. Abstracts
of Papers; 175th National Meeting of the American Chemical Society,
Anaheim, CA; March 13, 1978; American Chemical Society: Washington,
D.C., 1978; INOR 15. (b) Hoard, J. L., personal communication to W.R.S.
(44) Li, N.; Petricek, V.; Coppens, P.; Landrum, J. Acta Crystallogr.,
Sect. C 1985, C41, 902.
MePy)2] are given in Tables S2-S4, respectively, in the
Supporting Information. Complete individual values of bond
distances and angles for [Fe(TMP)(4-CNPy)2], [Fe(TMP)(3-
CNPy)2], and [Fe(TMP)(4-MePy)2] are given in Tables 3-8,
respectively. Equatorial bond distances (Fe-NP) average to
(45) Li, N.; Coppens, P.; Landrum, J. Inorg. Chem. 1988, 27, 482.

Models of the Cytochromes
J. Am. Chem. Soc., Vol. 119, No. 40, 1997 9443
a
a
Table 3. Bond Lengths in [Fe(TMP)(4-CNPy)2]‚2CHCl3
Table 4. Bond Angles in [Fe(TMP)(4-CNPy)2]‚2CHCl3
type
length, Å
type
length, Å
type
value, deg
type
value, deg
Fe-N(1)
1.993(2)
1.991(2)
1.996(2)
1.380(3)
1.370(4)
1.382(4)
1.379(3)
1.354(4)
1.350(4)
1.443(4)
1.387(4)
1.441(4)
1.400(4)
1.444(4)
1.391(4)
1.431(4)
1.390(4)
1.354(4)
1.350(4)
1.496(4)
1.491(4)
1.394(4)
1.393(4)
1.515(5)
1.379(5)
1.42(3)
C(14)-C(15)
C(14)-C(18)
C(15)-C(16)
C(16)-C(11)
C(16)-C(19)
C(21)-C(22)
C(22)-C(23)
C(22)-C(27)
C(23)-C(24)
C(24)-C(25)
C(24)-C(28)
C(25)-C(26)
C(26)-C(21)
C(26)-C(29)
C(1)-C(2)
1.371(5)
1.508(5)
1.397(4)
1.399(4)
1.502(5)
1.401(4)
1.392(5)
1.508(5)
1.383(5)
1.392(5)
1.504(5)
1.388(4)
1.405(4)
1.511(4)
1.366(4)
1.392(5)
1.385(4)
1.368(4)
1.445(4)
1.143(4)
1.675(14)
1.350(22)
1.437(19)
1.90(5)
N(1)FeN(2)
N(1)FeN(2)′
N(1)FeN(3)
N(2)FeN(3)
FeN(1)C(a1)
FeN(1)C(a2)
C(a1)N(1)C(a2)
FeN(2)C(a3)
FeN(2)C(a4)
89.16(9)
90.84(9)
89.44(9)
C(11)C(12)C(13)
C(11)C(12)C(17)
C(13)C(12)C(17)
119.3(3)
120.9(3)
119.8(3)
121.7(3)
118.4(3)
121.3(4)
120.3(4)
122.2(3)
118.7(3)
121.4(3)
120.0(3)
Fe-N(2)
Fe-N(3)
N(1)-C(a1)
N(1)-C(a2)
N(2)-C(a3)
N(2)-C(a4)
N(3)-C(1)
91.04(10) C(12)C(13)C(14)
126.56(20) C(13)C(14)C(15)
128.27(18) C(13)C(14)C(18)
105.15(23) C(15)C(14)C(18)
128.07(18) C(14)C(15)C(16)
126.62(20) C(11)C(16)C(15)
105.25(23) C(11)C(16)C(19)
121.00(21) C(15)C(16)C(19)
N(3)-C(5)
C(a1)-C(b1)
C(a1)-C(m1)
C(a2)-C(b2)
C(a2)-C(m2)
C(a3)-C(b3)
C(a3)-C(m2)
C(a4)-C(b4)
C(a4)-C(m1)
C(b1)-C(b2)
C(b3)-C(b4)
C(m1)-C(11)
C(m2)-C(21)
C(11)-C(12)
C(12)-C(13)
C(12)-C(17)
C(13)-C(14)
C(8)-Cl(6)
C(a3)N(2)C(a4)
FeN(3)C(1)
FeN(3)C(5)
122.21(20) C(m2)C(21)C(22) 120.2(3)
C(1)N(3)C(5)
116.7(3)
110.4(3)
123.8(3)
125.8(3)
C(m2)C(21)C(26) 120.2(3)
N(1)C(a1)C(b1)
C(b1)C(a1)C(m1)
N(1)C(a1)C(m1)
N(1)C(a2)C(b2)
C(b2)C(a2)C(m2)
N(1)C(a2)C(m2)
N(2)C(a3)C(b3)
C(b3)C(a3)C(m2)
N(2)C(a3)C(m2)
N(2)C(a4)C(b4)
C(b4)C(a4)C(m1)′
N(2)C(a4)C(m1)
C(a1)C(b1)C(b2)
C(a2)C(b2)C(b1)
C(a3)C(b3)C(b4)
C(a4)C(b4)C(b3)
C(a1)C(m1)C(a4)′
C(a1)C(m1)C(11)
C(a4)'C(m1)C(11)
C(a2)C(m2)C(a3)
C(a2)C(m2)C(21)
C(a3)C(m2)C(21)
C(m1)C(11)C(12)
C(m1)C(11)C(16)
C(12)C(11)C(16)
C(22)C(21)C(26)
C(21)C(22)C(23)
C(21)C(22)C(27)
119.6(3)
119.0(3)
120.6(3)
120.4(3)
122.2(3)
118.1(3)
121.5(3)
120.3(3)
121.5(3)
119.6(3)
120.7(3)
119.9(3)
123.6(3)
118.5(3)
118.8(3)
119.0(3)
123.3(3)
120.2(3)
120.9(3)
178.2(4)
114.0(13)
107.3(10)
114.6(18)
129(3)
C(2)-C(3)
C(3)-C(4)
111.02(24) C(23)C(22)C(27)
C(4)-C(5)
123.1(3)
125.9(3)
110.2(2)
124.0(3)
125.8(3)
110.4(3)
123.8(3)
125.8(3)
107.0(3)
106.5(3)
106.6(3)
107.5(3)
124.3(3)
118.0(3)
117.6(3)
122.7(3)
118.9(3)
118.4(3)
120.4(3)
119.8(3)
119.8(3)
C(22)C(23)C(24)
C(23)C(24)C(25)
C(23)C(24)C(28)
C(25)C(24)C(28)
C(24)C(25)C(26)
C(21)C(26)C(25)
C(21)C(26)C(29)
C(25)C(26)C(29)
N(3)C(1)C(2)
C(1)C(2)C(3)
C(2)C(3)C(4)
C(3)C(4)C(5)
C(4)C(5)N(3)
C(2)C(3)C(6)
C(4)C(3)C(6)
C(3)C(6)N(4)
Cl(1)C(7)Cl(2)
Cl(1)C(7)Cl(3)
Cl(2)C(7)Cl(3)
Cl(4)C(8)Cl(5)
Cl(4)C(8)Cl(6)
Cl(5)C(8)Cl(6)
C(3)-C(6)
C(6)-N(4)
C(7)-Cl(1)
C(7)-Cl(2)
C(7)-Cl(3)
C(8)-Cl(4)
C(8)-Cl(5)
1.32(3)
a The estimated standard deviations of the least significant digits
are given in parentheses. Primed and unprimed symbols denote a pair
of atoms related by an inversion center at iron.
1.992(1) Å in [Fe(TMP)(4-CNPy)2], 1.996(0) Å in [Fe(TMP)-
(3-CNPy)2], and 1.988(0) Å in [Fe(TMP)(4-MePy)2]. These
compare with those of other iron(II) porphyrinates: 1.997(6)
Å in [Fe(TPP)(1-VinIm)2],31 1.993(9) Å in [Fe(TPP)(1-
BzylIm)2],31 and 1.997(6) Å in [Fe(TPP)(1-MeIm)2].43 The
distances in the two bis-pyridine complexes, [Fe(TPP)(Py)2]‚
2Py44 and [Fe(TPP)(Py)2],45 are 1.993(6) and 2.002(1) Å,
respectively. A pyrazine complex in which the ligands have a
dihedral angle to each other of 40°,46 has Fe-NP ) 2.004(3)
Å, a bis-piperidine complex47 has Fe-NP ) 2.004(3) Å, and a
bis-tetrahydrothiophene complex48 has Fe-NP ) 1.996(6) Å.
The structure of a highly distorted halogenated iron(II) porphy-
rinate with axial pyridine ligands in nearly perpendicular planes,
has recently been reported.49
Of the three complexes whose structures we report herein,
the axial bond lengths exhibit no clear trend that would indicate
strong dπ-pπ back-bonding for low-basicity pyridines: The Fe-
Nax bond distance is 1.996(2) Å in [Fe(TMP)(4-CNPy)2], 2.026
Å in [Fe(TMP)(3-CNPy)2], and 2.010 Å in [Fe(TMP)(4-
MePy)2], where the order given is that expected for increasing
σ-donor, decreasing π-acceptor characteristics of the axial
ligands. These distances are comparable to those of the bis-
imidazole FeIITPP complexes (see Table 9 for values), but
slightly shorter than the values found in the two bis-pyridine
FeIITPP complexes reported by Coppens and co-workers.44,45
An informative comparison can be made with the series of
low-spin bis-pyridine iron(III) TMP complexes, where the axial
Fe-Nax bond distances are also considered. Values for these
iron(III) complexes and the iron(II) complexes are given in Table
9. The axial bond distances of [Fe(TMP)(4-CNPy)2], [Fe-
95.0(18)
125.8(3)
a The estimated standard deviations of the least significant digits
are given in parentheses. Primed and unprimed symbols denote a pair
of atoms related by an inversion center at iron.
(TMP)(3-CNPy)2], and [Fe(TMP)(4-MePy)2] are longer than that
found for the iron(III) TMP complex, [Fe(TMP)(4-NMe2Py)2]-
ClO4 (1.984(8) Å).21 However, the distances are comparable
to those of the other three Fe(III) complexes, [Fe(TMP)(3-
EtPy)2]ClO4 (1.996(9) Å),22 [Fe(TMP)(4-CNPy)2]ClO4 (2.011-
(14) Å),22 and [Fe(TMP)(3-ClPy)2]ClO4 (2.012(8) Å).22 Slightly
lengthened Fe-Nax bonds are found among imidazole com-
plexes of Fe(II)31 as compared to Fe(III),50-53 Table 9.
For the three reported bis-pyridine FeIITMP complexes, the
dihedral angles between the pyridine planes and the porphyrin
cores show some modest deviations; the angles are 84.4°, 86.5°,
and 84.0° in [Fe(TMP)(4-CNPy)2], [Fe(TMP)(3-CNPy)2], and
[Fe(TMP)(4-MePy)2], respectively. The dihedral angles be-
tween the porphyrin core and the mesityl groups are 88.6° and
85.7° in [Fe(TMP)(4-CNPy)2], 80.7° and 88.5° in [Fe(TMP)-
(3-CNPy)2], and 86.9° and 87.8° in [Fe(TMP)(4-MePy)2].
A sterically bulky porphyrin and pyridine ligands were
successfully used to control axial ligand orientation in the low-
spin FeIIITMP systems.21,22 We naively expected that the
(46) Hiller, W.; Hanack, M.; Mezger, M. G. Acta Crystallogr., Sect. C
1987, C43, 1264.
(47) Radonovich, L. J.; Bloom, A.; Hoard, J. L. J. Am. Chem. Soc. 1972,
94, 2074.
(50) Quinn, R.; Valentine, J. S.; Byrn, M. P.; Strouse, C. E. J. Am. Chem.
Soc. 1987, 109, 3301.
(51) Scheidt, W. R.; Osvath, S. R.; Lee, Y. J. J. Am. Chem. Soc. 1987,
109, 1958.
(48) Mashiko, T.; Reed, C. A.; Haller, K. J.; Kastner, M. E.; Scheidt,
W. R. J. Am. Chem. Soc. 1981, 103, 5758.
(49) Grinstaff, M. W.; Hill, M. G.; Birnbaum, R.; Schaefer, W. P.;
Labinger, J. A.; Gray, H. B. Inorg. Chem. 1995, 34, 4896.
(52) Collins, D. M.; Countryman, R.; Hoard, J. L. J. Am. Chem. Soc.
1972, 94, 2066.
(53) Little, R. G.; Dymock, K. R.; Ibers, J. A. J. Am. Chem. Soc. 1975,
97, 4532.

9444 J. Am. Chem. Soc., Vol. 119, No. 40, 1997
Table 5. Bond Lengths in [Fe(TMP)(3-CNPy)2]‚2CHCl3
Safo et al.
a
a
Table 6. Bond Angles in [Fe(TMP)(3-CNPy)2]‚2CHCl3
type
length, Å
type
length, Å
type
value, deg
type
value, deg
Fe-N(1)
1.996(2)
1.996(2)
2.026(2)
1.386(3)
1.377(3)
1.374(3)
1.383(3)
1.355(3)
1.357(4)
1.433(4)
1.391(4)
1.437(4)
1.394(4)
1.445(4)
1.384(4)
1.442(4)
1.385(4)
1.343(4)
1.341(4)
1.505(4)
1.498(4)
1.398(4)
1.399(4)
1.493(4)
1.370(5)
C(14)-C(15)
C(14)-C(18)
C(15)-C(16)
C(16)-C(11)
C(16)-C(19)
C(21)-C(22)
C(22)-C(23)
C(22)-C(27)
C(23)-C(24)
C(24)-C(25)
C(24)-C(28)
C(25)-C(26)
C(26)-C(21)
C(26)-C(29)
C(1)-C(2)
1.395(5)
1.506(4)
1.394(4)
1.392(4)
1.505(4)
1.398(4)
1.388(4)
1.506(5)
1.379(5)
1.380(5)
1.510(4)
1.394(4)
1.402(4)
1.498(4)
1.384(4)
1.388(4)
1.384(4)
1.369(4)
1.441(4)
1.141(4)
1.735(4)
1.734(5)
1.758(5)
2.01(4)
N(1)FeN(2)
N(1)FeN(2)′
N(1)FeN(3)
N(2)FeN(3)
FeN(1)C(a1)
FeN(1)C(a2)
C(a1)N(1)C(a2)
FeN(2)C(a3)
FeN(2)C(a4)
90.31(9)
89.68(9)
89.43(9)
89.63(9)
127.83(18) C(13)C(14)C(15)
127.08(17) C(13)C(14)C(18)
104.95(21) C(15)C(14)C(18)
126.74(17) C(14)C(15)C(16)
127.41(18) C(11)C(16)C(15)
105.84(21) C(11)C(16)C(19)
122.04(19) C(15)C(16)C(19)
C(11)C(12)C(13)
C(11)C(12)C(17)
C(13)C(12)C(17)
C(12)C(13)C(14)
118.3(3)
121.4(3)
120.3(3)
122.9(3)
117.8(3)
121.3(3)
120.9(3)
121.4(3)
119.5(3)
120.7(3)
119.8(3)
Fe-N(2)
Fe-N(3)
N(1)-C(a1)
N(1)-C(a2)
N(2)-C(a3)
N(2)-C(a4)
N(3)-C(1)
N(3)-C(5)
C(a1)-C(b1)
C(a1)-C(m1)
C(a2)-C(b2)
C(a2)-C(m2)
C(a3)-C(b3)
C(a3)-C(m2)
C(a4)-C(b4)
C(a4)-C(m1)′
C(b1)-C(b2)
C(b3)-C(b4)
C(m1)-C(11)
C(m2)-C(21)
C(11)-C(12)
C(12)-C(13)
C(12)-C(17)
C(13)-C(14)
C(a3)N(2)C(a4)
FeN(3)C(1)
FeN(3)C(5)
121.09(18) C(m2)C(21)C(22) 120.85(24)
116.86(24) C(m2)C(21)C(26) 119.49(25)
C(1)N(3)C(5)
N(1)C(a1)C(b1)
C(b1)C(a1)C(m1)
N(1)C(a1)C(m1)
N(1)C(a2)C(b2)
C(b2)C(a2)C(m2)
N(1)C(a2)C(m2)
N(2)C(a3)C(b3)
C(b3)C(a3)C(m2)
N(2)C(a3)C(m2)
N(2)C(a4)C(b4)
C(b4)C(a4)C(m1)′
N(2)C(a4)C(m1)
C(a1)C(b1)C(b2)
C(a2)C(b2)C(b1)
C(a3)C(b3)C(b4)
C(a4)C(b4)C(b3)
C(a1)C(m1)C(a4)′
C(a1)C(m1)C(11)
C(a4)′C(m1)C(11)
C(a2)C(m2)C(a3)
C(a2)C(m2)C(21)
C(a3)C(m2)C(21)
C(m1)C(11)C(12)
C(m1)C(11)C(16)
C(12)C(11)C(16)
110.27(22) C(22)C(21)C(26)
124.73(24) C(21)C(22)C(23)
125.00(24) C(21)C(22)C(27)
110.37(22) C(23)C(22)C(27)
124.09(24) C(22)C(23)C(24)
125.53(24) C(23)C(24)C(25)
109.79(23) C(23)C(24)C(28)
123.86(25) C(25)C(24)C(28)
126.33(24) C(24)C(25)C(26)
109.78(23) C(21)C(26)C(25)
124.39(24) C(21)C(26)C(29)
125.83(23) C(25)C(26)C(29)
107.18(23) N(3)C(1)C(2)
107.22(24) C(1)C(2)C(3)
107.40(24) C(2)C(3)C(4)
107.18(23) C(3)C(4)C(5)
124.13(24) C(4)C(5)N(3)
117.69(23) C(1)C(2)C(6)
118.17(22) C(3)C(2)C(6)
123.94(25) C(2)C(6)N(4)
118.03(22) Cl(1)C(7)Cl(2)
117.99(23) Cl(1)C(7)Cl(3)
119.62(25) Cl(2)C(7)Cl(3)
120.23(24) Cl(4)C(7)Cl(5)
120.15(25) Cl(4)C(7)Cl(6)
Cl(5)C(7)Cl(6)
119.66(25)
119.5(3)
120.8(3)
119.6(3)
121.7(3)
118.4(3)
120.7(3)
120.9(3)
122.1(3)
118.7(3)
121.0(3)
120.3(3)
121.9(3)
120.8(3)
116.9(3)
120.2(3)
123.4(3)
119.4(3)
119.8(3)
178.7(4)
110.5(3)
110.06(23)
111.44(25)
93.9(14)
141.9(15)
85.9(13)
C(2)-C(3)
C(3)-C(4)
C(4)-C(5)
C(2)-C(6)
C(6)-N(4)
C(7)-Cl(1)
C(7)-Cl(2)
C(7)-Cl(3)
C(7)-Cl(4)
C(7)-Cl(5)
C(7)-Cl(6)
1.82(3)
1.77(3)
a The estimated standard deviations of the least significant digits
are given in parentheses. Primed and unprimed symbols denote a pair
of atoms related by an inversion center at iron.
corresponding low-spin iron(II) complexes would show the same
preferences for relative parallel or perpendicular orientations
of the ligand planes. However, the crystal structures of the three
bis-pyridine TMP complexes reported herein show the two axial
ligands to be parallel to each other. The ligand planes make
dihedral angles φ of 40°, 42°, and 41° to the closest Fe-NP
axis in [Fe(TMP)(4-CNPy)2], [Fe(TMP)(3-CNPy)2], and
[Fe(TMP)(4-MePy)2], respectively. The dihedral angles of the
two bis-pyridine iron(II) TPP complexes reported previously
are also large, 34° and 43°.44,45 Such large values are com-
mon for bis-pyridine complexes of both low-spin iron(II) and
-(III).21,31,44,45,54-56 In iron porphyrinate derivatives with planar
cores, placing the pyridine rings above the Fe-NP bonds is
always associated with longer axial bond lengths and hence the
intermediate-spin state.56-58 Because small dihedral angles are
observed for many bis-imidazole complexes of iron(II) and -(III)
porphyrinates,21,31,43,50-53 and all such complexes are strictly
low-spin, we conclude that the difference in “cone angle” as
measured by interference between the “ortho”-H of the imida-
zole or pyridine rings and the electron density of the Fe-NP
bonds is significant enough to allow 5-membered-ring hetero-
cycles to have small dihedral angles while precluding such
angles for 6-membered-ring heterocycles. Molecular mechanics
(MM2) calculations confirm that the lowest-energy position for
pyridine ligands is that with dihedral angles of 45°.55
a The estimated standard deviations of the least significant digits
are given in parentheses. Primed and unprimed symbols denote a pair
of atoms related by an inversion center at iron.
in [Fe(TMP)(4-CNPy)2], and 3.72 Å in [Fe(TMP)(3-CNPy)2].
It appears that none of these distances are short enough to cause
serious nonbonded contacts. In the Fe(III) TMP analogs,21,22
the porphinato core is S4 ruffled with the mesityl rings
concomitantly tipped away from the pyridines. This leads to
much larger mesityl carbon-pyridine carbon distances which
range from 3.8 Å upwards, with almost all greater than 4.2 Å.
The depths of the ligand binding pockets (as measured by the
o-methyl substituents of opposite mesityl rings) from the mean
porphinato plane in each complex of the present study are
similar, 2.52 and 2.52 Å in [Fe(TMP)(4-CNPy)2], 2.48 and 2.50
Å in [Fe(TMP)(3-CNPy)2], and 2.62 and 2.41 Å in [Fe(TMP)-
(4-MePy)2]. The square shape and symmetric depth contrast
with the iron(III) TMP complexes, which have oblong pockets
of unequal depth.21,22
In the three FeIITMP complexes the closest nonbonded
distance between pyridine ligand atoms and the o-methyl carbon
of the mesityl group is 3.80 Å in [Fe(TMP)(4-MePy)2], 3.76 Å
The phenomenon of ruffling among all iron(III) complexes
with perpendicular axial ligand orientations21-23,55,59,60 has been
explained in terms of steric interactions between the porphyrinate
ring and its substituents and the axial ligands for the five FeIII-
TMP complexes reported by us21-23 and also for [Fe(TPP)(2-
MeHIm)2]ClO4.60 However, these steric interactions cannot
explain the perpendicular axial ligand orientations and ruffled
(54) Inniss, D.; Soltis, S. M.; Strouse, C. E. J. Am. Chem. Soc. 1988,
110, 5644.
(55) Safo, M. K.; Walker, F. A.; Raitsimring, A. M.; Walters, W. P.;
Dolata, D. P.; Debrunner, P. G.; Scheidt, W. R. J. Am. Chem. Soc. 1994,
116, 7760.
(56) Scheidt, W. R.; Geiger, D. K.; Haller, K. J. J. Am. Chem. Soc. 1982,
104, 495.
(57) Safo, M. K.; Scheidt, W. R.; Gupta, G. P.; Orosz, R. D.; Reed, C.
A. Inorg. Chim. Acta 1991, 184, 251.
(58) Scheidt, W. R.; Geiger, D. K.; Hayes, R. G.; Lang, G. J. Am. Chem.
Soc. 1983, 105, 2625.
(59) Hatano, K.; Safo, M. K.; Walker, F. A.; Scheidt, W. R. Inorg. Chem.
1991, 30, 1643.
(60) Scheidt, W. R.; Kirner, J. L.; Hoard, J. L.; Reed, C. A. J. Am. Chem.
Soc. 1987, 109, 1963.

Models of the Cytochromes
J. Am. Chem. Soc., Vol. 119, No. 40, 1997 9445
a
a
Table 7. Bond Lengths in [Fe(TMP)(4-MePy)2]‚2C6H6
Table 8. Bond Angles in [Fe(TMP)(4-MePy)2]‚C6H6
type
length, Å
type
length, Å
type
value, deg
type
value, deg
Fe-N(1)
1.988(2)
1.988(2)
2.010(2)
1.379(3)
1.378(3)
1.372(3)
1.374(3)
1.341(3)
1.338(3)
1.440(3)
1.392(3)
1.439(3)
1.377(3)
1.434(3)
1.394(3)
1.446(3)
1.395(3)
1.345(4)
1.349(4)
1.502(3)
1.502(3)
1.396(4)
1.386(4)
1.504(4)
1.399(4)
1.372(5)
1.511(5)
1.389(4)
C(16)-C(11)
C(16)-C(19)
C(21)-C(22)
C(22)-C(23)
C(22)-C(27)
C(23)-C(24)
C(24)-C(25)
C(24)-C(28)
C(25)-C(26)
C(26)-C(21)
C(26)-C(29)
C(1)-C(2)
1.398(4)
1.502(4)
1.394(4)
1.392(5)
1.495(4)
1.367(4)
1.385(4)
1.510(4)
1.384(4)
1.393(4)
1.507(4)
1.376(3)
1.384(4)
1.384(4)
1.379(3)
1.499(3)
1.344(18)
1.302(20)
1.334(21)
1.487(20)
1.71(14)
1.306(14)
1.303(22)
1.20(4)
N(1)FeN(2)
N(1)FeN(2)′
N(1)FeN(3)
N(2)FeN(3)
FeN(1)C(a1)
FeN(1)C(a2)
C(a1)N(1)C(a2)
FeN(2)C(a3)
FeN(2)C(a4)
90.47(8)
89.53(7)
90.80(7)
91.55(7)
127.74(15) C(14)C(15)C(16)
126.85(16) C(11)C(16)C(15)
105.40(19) C(11)C(16)C(19)
126.86(16) C(15)C(16)C(19)
C(12)C(13)C(14)
C(13)C(14)C(15)
C(13)C(14)C(18)
C(15)C(14)C(18)
121.3(3)
118.1(3)
120.5(4)
121.4(4)
122.4(3)
118.2(3)
121.1(3)
120.1(3)
Fe-N(2)
Fe-N(3)
N(1)-C(a1)
N(1)-C(a2)
N(2)-C(a3)
N(2)-C(a4)
N(3)-C(1)
N(3)-C(5)
128.05(15) C(m2)C(21)C(22) 120.17(21)
104.93(19) C(m2)C(21)C(26) 120.14(22)
C(a1)-C(b1)
C(a1)-C(m1)
C(a2)-C(b2)
C(a2)-C(m2)
C(a3)-C(b3)
C(a3)-C(m2)
C(a4)-C(b4)
C(a4)-C(m1)′
C(b1)-C(b2)
C(b3)-C(b4)
C(m1)-C(11)
C(m2)-C(21)
C(11)-C(12)
C(12)-C(13)
C(12)-C(17)
C(13)-C(14)
C(14)-C(15)
C(14)-C(18)
C(15)-C(16)
C(a3)N(2)C(a4)
FeN(3)C(1)
FeN(3)C(5)
121.61(16) C(22)C(21)C(26)
122.35(16) C(21)C(22)C(23)
115.98(20) C(21)C(22)C(27)
110.18(20) C(23)C(22)C(27)
123.85(22) C(22)C(23)C(24)
125.96(21) C(23)C(24)C(25)
111.13(21) C(23)C(24)C(28)
124.02(22) C(25)C(24)C(28)
125.82(22) C(24)C(25)C(26)
110.72(22) C(21)C(26)C(25)
123.67(23) C(21)C(26)C(29)
125.60(22) C(25)C(26)C(29)
110.88(20) N(3)C(1)C(2)
123.20(22) C(1)C(2)C(3)
125.82(21) C(2)C(3)C(4)
107.03(21) C(3)C(4)C(5)
107.24(21) C(4)C(5)N(3)
107.37(22) C(2)C(3)C(6)
106.06(22) C(4)C(3)C(6)
122.87(22) C(31)C(30)C(35)
118.53(21) C(30)C(31)C(32)
118.59(21) C(31)C(32)C(33)
124.13(22) C(32)C(33)C(34)
118.35(21) C(33)C(34)C(35)
117.52(22) C(34)C(35)C(36)
120.18(22) C(37)C(36)C(41)
119.82(23) C(36)C(37)C(38)
120.00(24) C(37)C(38)C(39)
119.39(25) C(38)C(39)C(40)
120.91(23) C(39)C(40)C(41)
119.69(24) C(40)C(41)C(36)
119.69(23)
119.36(23)
120.65(23)
119.98(24)
121.87(24)
117.8(3)
121.9(3)
120.3(3)
122.5(3)
118.74(24)
121.17(24)
120.09(24)
123.81(22)
120.08(24)
116.29(22)
120.30(24)
123.52(23)
120.79(24)
122.89(25)
116.3(13)
120.5(15)
127.0(14)
113.5(11)
114.6(10)
127.0(11)
118.1(15)
133.4(15)
119.3(24)
113.4(20)
117.5(14)
118.2(13)
C(2)-C(3)
C(1)N(3)C(5)
C(3)-C(4)
N(1)C(a1)C(b1)
C(b1)C(a1)C(m1)
N(1)C(a1)C(m1)
N(1)C(a2)C(b2)
C(b2)C(a2)C(m2)
N(1)C(a2)C(m2)
N(2)C(a3)C(b3)
C(b3)C(a3)C(m2)
N(2)C(a3)C(m2)
N(2)C(a4)C(b4)
C(b4)C(a4)C(m1)′
N(2)C(a4)C(m1)
C(a1)C(b1)C(b2)
C(a2)C(b2)C(b1)
C(a3)C(b3)C(b4)
C(a4)C(b4)C(b3)
C(a1)C(m1)C(a4)′
C(a1)C(m1)C(11)
C(a4)′C(m1)C(11)
C(a2)C(m2)C(a3)
C(a2)C(m2)C(21)
C(a3)C(m2)C(21)
C(m1)C(11)C(12)
C(m1)C(11)C(16)
C(12)C(11)C(16)
C(11)C(12)C(13)
C(11)C(12)C(17)
C(13)C(12)C(17)
C(4)-C(5)
C(3)-C(6)
C(30)-C(31)
C(31)-C(32)
C(32)-C(33)
C(33)-C(34)
C(34)-C(35)
C(35)-C(31)
C(36)-C(37)
C(37)-C(38)
C(38)-C(39)
C(36)-C(37)
C(36)-C(37)
C(36)-C(37)
1.51(3)
1.53(3)
1.48(3)
1.415(20)
a The estimated standard deviations of the least significant digits
are given in parentheses. Primed and unprimed symbols denote a pair
of atoms related by an inversion center at iron.
porphinato cores of [Fe(TPP)(Py)2]ClO4,54 [Fe(TPP)(4-CNPy)2]-
ClO4,55 and [Fe((2,6-Cl2)4TPP)(1-VinIm)2]ClO4.59 For [Fe-
(TPP)(4-CNPy)2]ClO4 we have argued that there is an electronic
reason for the ruffling;55 the relatively strong π-acceptor (and
weak σ-donor) properties of the 4-CNPy ligands stabilize the
dxz,dyz orbitals of low-spin Fe(III) to the point where their energy
drops below that of the dxy orbital, causing the latter to be the
half-filled orbital. The filled dxz,dyz set could thus be involved
in π donation to the empty π* orbitals of the pyridine ligands
that have large wavefunction amplitude at the bonding nitrogen.
Meanwhile, the half-filled dxy orbital can then participate in
strong porphyrinfFe π donation from the filled 3a2u(π)
porphyrin orbital if and only if the porphyrinate ring ruffles
significantly so as to twist the porphyrin nitrogen pz orbitals by
15° or more from the normal to the mean plane of the porphyrin
so that there is a significant pz component in the xy plane,55
and it is thus the porphyrinfFe(III) π donation that stabilizes
this ruffled complex. We have shown, based on MM2 calcula-
tions, that the energy involved in the core ruffling is relatively
small in the iron(III) systems (4-8 kJ/mol).55
It is reasonable to expect that the porphyrin cores in the iron-
(II) TMP complexes will be ruffled if the axial ligands assume
perpendicular orientation with the ligand planes aligned along
the meso positions of the porphyrin because of a tendency of
the porphyrinate ring to bend or fold away from the unopposed
axial ligand (especially for pyridines or hindered imidazoles)
along a line perpendicular to its plane. Such a tendency has
been noted not only in the model ferriheme complexes discussed
above but also in the structures of several cytochromes c,61
where the histidine imidazole plane is aligned close to the R,γ
meso positions, as well as for cytochrome f12,62 and the siroheme
a The estimated standard deviations of the least significant digits
are given in parentheses. Primed and unprimed symbols denote a pair
of atoms related by an inversion center at iron.
of sulfite reductase.63 A different and more obvious cause of
ruffling arises if there are bulky substituents on both the meso
and â-pyrrole positions of the porphyrin, as in the highly
halogenated Fe(II) porphyrinate [Fe(TFPPBr8)(Py)2],49 where
the porphyrinate ring cannot be planar and the axial ligands
must follow the constraints of the ruffling pattern imposed by
the bulky porphyrin. For the low-spin Fe(III) porphyrinates
bound to ligands of moderate to high π-acceptor capability, we
have pointed out, as mentioned in the previous paragraph, a
third cause of ruffling that arises when the dxy orbital contains
the unpaired electron and can only receive electron density from
the porphyrinate ring by π donation if the porphyrinate ring
ruffles.55,64 Because the dxy orbital is filled in the case of low-
spin d6, this avenue for stabilizing perpendicular axial ligands
is not available for the Fe(II) porphyrinates. The parallel ligand
orientation, with the above-mentioned minimal contacts between
the pyridine and mesityl carbons, thus becomes the thermody-
namically stable form. Apparently, only with extremely bulky
porphyrinate and ligand substituents, as in the bis(1,2-dimeth-
ylimidazole) complex of FeIITMP8,26,39 discussed below, can
perpendicular orientation of axial ligands be stabilized for an
(61) Hobbs, J. D.; Shelnutt, J. A. J. Protein Chem. 1995, 14, 19.
(62) Martinez, S. E.; Smith, J. L.; Huang, D.; Szczepaniak, A.; Cramer,
W. A. In Research in Photosynthesis; Murata, N., Ed.; Proceedings of the
IXth International Congress on Photosynthesis; Kluwer Academic: Dor-
drecht, The Netherlands, 1992; Vol. 2, p 495.
(63) Crane, B. R.; Sigel, L. M.; Getzoff, E. D. Science 1995, 270, 29.
(64) Walker, F. A.; Nasri, H.; Turowska-Tyrk, I.; Mohanrao, K.; Watson,
C. T.; Shokhirev, N. V.; Debrunner, P. G.; Scheidt, W. R. J. Am. Chem.
Soc. 1996, 118, 12109.

9446 J. Am. Chem. Soc., Vol. 119, No. 40, 1997
Safo et al.
experimental error of each other,26 even though the basicities
of their conjugate acids, pKa(PyH+), range from ∼1.1 (4-CNPy)
to 9.7 (4-NMe2Py).65 For each of the porphyrinates studied the
Table 9. Summary of Fe-N Bond Distances in Low-Spin
Bis(imidazole and pyridine) Iron(II) and -(III) Complexesa
b
b
complex
Fe-Np
Fe-Nax
ref
II
same situation was found: log(â2 ) is “leveled” to a particular
A. Iron(II) Complexes
2.001(2)
value for each porphyrinate for the series of pyridines, indicating
no sensitivity on the part of Fe(II) to the σ-donor or π-donor/
acceptor characteristics of the axial ligands.26 In contrast, the
bis(1-MeIm) and bis(2-MeHIm) complexes of porphyrinates
[Fe(TPP)(1-VinIm)2]
[Fe(TPP)(1-BzlIm)2]
[Fe(TPP)(1-MeIm)2]
[Fe(TMP)(4-CNPy)2]
[Fe(TMP)(4-MePy)2]
[Fe(TMP)(3-CNPy)2]
[Fe(TPP)(Py)2]
2.004(2)
2.017(4)
2.014(5)
1.996(2)
2.010(2)
2.026(2)
2.037(1)
2.039(1)
31
31
43
this work
this work
this work
45
1.992(9)
1.997(6)
1.992(1)
1.988(0)
1.996(0)
2.001(2)
1.993(6)
II
having large ortho substituents have similar values of log(â2 )
(∼7.4 and ∼5.4, respectively), and do not appear to be sensitive
to the nature of the substituents, while less “hindered” porphy-
rinate complexes do not form bis(2-MeHIm) complexes at
ambient temperatures at ligand concentrations less than 1 M.26
[Fe(TPP)(Py)2]‚2Py
44
B. Iron(III) Complexes
d
[Fe(TMP)(1-MeIm)2]C1O4
1.988(20) 1.975(3)
21
II
While the values of log(â2 ) are quite similar, the FeIII/FeII
1.987(1)
1.989(8)
1.994(12) 1.977(3)
1.993(4) 1.964(3)
1.991(16) 1.977(16) 53
1.971(4)
1.992(5)
1.997(1)
1.965(3)
1.974(24) 52
51
[Fe(TPP)(HIm)2]Cl‚MeOH
[Fe(TPP)(HIm)2]Cl‚H2Od
reduction potentials of the [Fe(TMP)(L)2] complexes listed in
Table 10 vary strongly with the nature of the axial ligand. This
is due to the fact that the equilibrium constants for binding the
ligands to the FeIIITMP complexes vary significantly with the
basicity of the pyridine ligands and the steric factors of the
imidazole ligands.26 It should be noted that the FeIII/FeII
reduction potential is determined by the ratio of the equilibrium
[Fe(Proto IX)(1-MeIm)2]
[Fe(TPP)(2-MeHIm)2]ClO4
[Fe(TPP)(t-MU)2]SbF6
2.013(4)
1.983(4)
1.967(7)
60
50
50
d
[Fe(TPP)(c-MU)2]SbF6
1.995(17) 1.979(7)
[Fe(TMP)(4-NMe2Py)2]C1O4 1.964(10) 1.984(8)
21
22
22
II
constants, log(â2III/â2 ), measured under high enough ligand
[Fe(TMP)(3-EtPy)2]C1O4
[Fe(TMP)(3-ClPy)2]C1O4
[Fe(TMP)(4-CNPy)2]C1O4
[Fe(TPP)(4-CNPy)2]C1O4
[Fe(OEP)(4-NMe2Py)2]C1O4
1.964(4)
1.968(3)
1.961(7)
1.952(7)
2.002(4)
1.996(9)
2.012(8)
2.011(14) 22
2.002(8)
1.995(3)
concentration to ensure that the ratios of the concentrations of
the bis complexes of both oxidation states are independent of
ligand concentration.26,66 Thus, the differences in reduction
55
21
II
potentials, given the “leveled” values of â2 , are a measure of
the values of â2III, and do not, a priori, give any information
about bonding interactions in the Fe(II) (or Fe(III)) complexes.
Thus, the above-summarized work shows that only in the
presence of very bulky substituents on the ortho positions of
the phenyl rings of a (tetraphenylporphyrinato)iron(II) complex
are the bis-ligand complexes of 2- or 1,2-alkyl-substituted
imidazoles stable at room temperature at ligand concentrations
less than 1 M,26 and even for porphyrinates for which this
complex can be formed (TMP, for example), the stabilities of
the bis(2-MeHIm) complexes are approximately two orders of
magnitude less than those of the corresponding bis(1-MeIm)
and all bis(pyridine) complexes.26 Although no structure of a
bis(2-methylimidazole) or related complex of an Fe(II) porphy-
rinate has yet appeared, we have recently characterized the Fe-
(II) analog of this complex by 2D NMR techniques at very low
temperatures (-70 to -90 °C), where axial ligand rotation is
slow on the NMR time scale.8 Our studies confirm that the
axial 1,2-Me2Im ligands are aligned in perpendicular planes,
and the large difference in chemical shift of the two pairs of
o-CH3 mesityl resonances8 is very strong evidence that the
porphyrinate ring is strongly ruffled, as is observed in the
Fe(III) analog, [Fe(TMP)(1,2-Me2Im)2]+.23 The Mo¨ssbauer
isomer shifts and quadrupole splittings of [Fe(TMP)(2-Me-
HIm)2], [Fe(OEP)(2-MeHIm)2], and [Fe(TMP)(1,2-Me2Im)2]
have recently been reported39,40 (Table 2). The unusually large
quadrupole splittings, ∆EQ > 1.6 mm/s,39,40 are unique for low-
spin Fe(II) porphyrinates. Recent SCC-XR calculations in the
Local Density Approximation,40 discussed below, indicate that
the ruffling of the porphyrinate ring, which results in significant
shortening of the Fe-NP bonds, is the primary cause of the
exceptionally large quadrupole splittings observed for the bis-
(2-methyl- and -1,2-dimethylimidazole) complexes of Fe(II)
porphyrinates (Table 2).
a The numbers in parentheses are the estimated standard deviations
in the least significant digit(s). b Values in angstroms. c Two indepen-
dent half-molecules with required inversion symmetry.
Table 10. FeIII/FeII Reduction Potentials and Equilibrium
Constants for Binding Axial Ligands to FeIITMP in
Dimethylformamide To Form [Fe(TMP)(L)2]26
II
III
ligand
pKa(BH+)a FeIII/FeII E1/2, Vb log(â2
)
log(â2 )
4-CNPy
Py
3,4-Me2Py
4-NMe2Py
N-MeIm
2-MeHIm
∼1.1
+0.331
+0.140
+0.077
-0.121
-0.130
-0.212
7.5
7.4
7.3
7.9
7.3
5.5
0.2
3.3
4.4
8.3
7.9
7.4
5.22
6.46
9.7
7.33
7.56
a pKa(BH+) values taken from ref 65 except for that of 4-CNPy.
See ref 22 for discussion of the basicity of the latter pyridine.
b Potentials listed Vs SCE. Measured in dimethylformamide at 25 °C;
electrolyte ) 0.03 M TBAP.
Fe(II) porphyrinate that does not have bulky substituents at both
meso and â-pyrrole positions.
Equilibrium Constants for Formation of [Fe(TMP)L2]
Complexes. We have recently reported the complex stabilities
and reduction potentials for a series of tetrakis(2,6-disubstituted
phenyl)porphyrinatoiron(II)-bis-L complexes,26 where L in-
cludes some of the same series of substituted pyridine and
imidazole ligands as studied in this work. Among the so-called
“hindered” porphyrinates studied were the tetramesitylporphy-
rinate complexes. Table 10 summarizes the FeIII/FeII reduction
potentials of the bis-ligated complexes and the equilibrium
constants, log(â2II), for binding the ligands to FeIITMP.
Because the study was carried out in dimethylformamide
solution, the complex formation reaction in each case is:
[Fe(TMP)(DMF)] + 2L a [Fe(TMP)(L)2] + DMF (1)
The necessity of utilizing more highly “hindered” axial
ligands than the pyridines of this study (or highly congested
porphyrins substituted at both meso and â-pyrrole positions49)
â2II ) [Fe(TMP)(L)2]/[Fe(TMP)(DMF)][L]2
(2)
(65) Albert, A. In Physical Methods in Heterocyclic Chemistry; Katritzky,
A. R., Ed.; Academic Press: New York, 1971; Vol. I, pp 1-108, and Vol.
III, pp 1-26. See also ref 22 for discussion of the pKa(BH+) of 4-CNPy.
(66) Kolthoff, I. M.; Lingane, J. J. Polarography 1952, 1, 221.
The remarkable finding for the TMP complexes (as well as for
each of the other porphyrinates studied) is that the values of
log(â2 ) for the substituted pyridines studied are all within
II

Models of the Cytochromes
J. Am. Chem. Soc., Vol. 119, No. 40, 1997 9447
to force perpendicular orientation of unconstrained axial ligands
in Fe(II) porphyrinate complexes raises the intriguing question
of whether or not there may be an energetically unfavorable
interaction, heretofore unrecognized, that makes perpendicular
orientation of axial ligand planes unlikely in such unconstrained
systems. Such an unfavorable energy term could result from
the necessity for the porphyrinate ring to ruffle, and we have
pointed out above and previously23 that perpendicular orientation
of planar axial ligands “encourages” the porphyrinate ring to
ruffle. (We will explore the question of the energetics of ruffling
in the final section of this paper.) In support of the hypothesis
of an unfavorable energy term due to ruffling is the fact that
one of the two histidine ligands of Methylophilus methylotrophus
cytochrome c′′ is lost in a pH-dependent manner upon reduction
of this protein from Fe(III) to Fe(II).18b Since the spectroscopic
properties of ferric cytochrome c′′ strongly suggest that the axial
histidine ligands are in perpendicular planes,18 the loss of one
histidine upon reduction is consistent with there being an
unfavorable energy term involved in perpendicular orientation
of planar axial ligands. (However, other effects, as yet
unknown, imposed by the protein, may also contribute to this
loss of histidine ligand upon reduction.)
shown for the corresponding Fe(III) complexes,22 and were
expected to interact with the filled dπ orbitals of the low-spin
d6 metal to help stabilize the axial ligand bonds through dπ-pπ
back-bonding. For the bis-pyridine complexes of the present
study, we must consider the question of why the symmetrical
low-spin d6 electron configuration of [FeIITMP(L)2] complexes
prefer parallel alignment of identical planar axial ligands rather
than perpendicular (independent of the question of ruffling),
when the latter would allow, especially for low-basicity py-
ridines, π-back-bonding from one dπ orbital to the π* orbital
of one pyridine and a similar interaction from the other dπ orbital
to the other pyridine. This expectation assumes that such
π-back-bonding interactions provide large energies of stabiliza-
tion in low-spin Fe(II) porphyrinates having two identical axial
ligands. In fact, this may not be true, as suggested by the CO
complexes of hemoglobin and myoglobin, and their models:
There are, to our knowledge, no reports of stable bis-CO
complexes of Fe(II) porphyrinates, but for the unsymmetrical
axial ligand complexes where one ligand is a strong σ and π
donor while the other is a weak σ donor and a strong π acceptor,
the equilibrium constants for CO addition have been measured
by several research groups, and are in the 107-109 M-1 range
for simple iron(II) porphyrinates bound to imidazoles or
pyridines,69-71 many orders of magnitude larger than the
equilibrium constants for adding a second identical nitrogenous
base, K2 (∼102-104 M-1).26 In the case of CO addition it could
be argued that both σ donation from the lone pair and the filled-
filled π interaction of the nitrogen donor ligand with the dxz
and dyz orbitals of Fe(II) “fattens” these filled orbitals for π
back-donation to the CO. Without the “push” of a strong σ
and π donor ligand, Fe(II) shows little tendency to bind to strong
π acceptor ligands (and it should be noted that Fe(III) should
show even less, a conclusion that is consistent with the absence
of any report of a CO complex of a Fe(III) porphyrinate). For
two identical axial ligands, both have equal “push” and “pull”,
and hence the dπ orbitals of the d6 low-spin Fe(II) center may
be fairly passive in this case. In fact, for low-spin Fe(III)
porphyrinates, what we had previously attributed to π back-
donation from the dπ orbitals of Fe to the π* orbitals of low-
basicity pyridines or isonitriles may instead be a case of the
smaller crystal field strength of these ligands (a σ bonding effect)
as compared to high-basicity pyridines and imidazoles, that
places the dxz,dyz orbitals at lower energy than dxy for weak
σ-donor ligands.
As pointed out above, the FeIII/FeII reduction potentials
observed in these unconstrained model heme complexes are
simply a measure of the relative stability of the Fe(III) and
II
Fe(II) bis-ligand complexes, as measured by log(â2III/â2 ).26,66
As we have seen (Table 10), forcing the ligands to be in
perpendicular planes by choosing a “hindered” ligand such as
II
2-MeHIm reduces the value of â2 by a factor of nearly 100
over that for 1-MeIm, and this is the major contributor to the
negative shift in the reduction potential of 82 mV (Table 10),
for the values of â2III are very similar for the two ligands. Note
that for unconstrained models, the perpendicularly-aligned axial
ligand complex has the more negatiVe (because of its lower
stability) rather than the more positive reduction potential. We
had predicted the potential of the perpendicularly-aligned ligand
complex to be more positive previously,19 and certainly the
mitochondrial cytochromes b17b,24 (presumed perpendicular
imidazole planes) have more positive reduction potentials than
25
do the cytochromes b5 (known nearly parallel imidazole
planes). Thus, it appears that while these unconstrained com-
plexes are excellent spectroscopic and structural models of the
bis-histidine cytochromes, they are not the best models for the
effect of ligand orientation on reduction potential of the protein
centers. They do not benefit from pre-organization of the heme
binding site by hydrogen bonding of the histidine ligands in
particular orientations. Also, hydrogen bonding of the histidine
imidazole N-H itself can have profound effects upon complex
stabilities67 and thus reduction potentials. In this regard, it is
interesting to note that in the presence of strong hydrogen-
The hypothesis of passivity is supported by recent Self-
Consistent-Charge-XR calculations in the Local Density Ap-
proximation (LDA) for two bis-imidazole complexes of FeII-
TMP.40 The complexes studied were [Fe(TMP)(1-MeIm)2]72a
and [Fe(TMP)(2-MeHIm)2].72b The results show that in both
cases the dxy orbital is higher in energy than the dπ orbitals,
and that, as expected, the energy difference between dxz and dyz
is very small for the hindered imidazole complex (perpendicular
planes, 0.006 eV). But it is only slightly larger for the
nonhindered imidazole complex (parallel planes, 0.018 eV). The
population of the dxy orbital is very similar for the two complexes
(1.982 and 1.988, respectively), and is slightly larger than the
population of dxz (1.740 and 1.859, respectively) and dyz (1.756
and 1.805, respectively), as predicted by the observed positive
sign of the EFG found in the magnetic Mo¨ssbauer spectra.40
bond acceptors (triethylamine, for example), the “large gmax
”
signal of [Fe(TPP)(2-MeHIm)2]+ is replaced by a “normal
rhombic” signal,68 suggesting that in the presence of stronger
σ- and π-donation capabilities of the axial ligands the complex
finds a way to place the bulky axial ligands in relative orien-
tations that are closer to being parallel than perpendicular. Fur-
ther spectroscopic and structural characterization of this complex
is in progress.
π Donor/Acceptor Interactions in Low-Spin Fe(II) Por-
phyrinates. The imidazole and high-basicity pyridine ligands
of the FeIITMP derivatives of this study are all expected to form
strong σ-bonds.22,23 The low-basicity pyridines, on the other
hand, are weak σ-donors but reasonable π-acceptors, as we have
(69) El-Kasmi, D.; Tetreau, C.; Lavalette, D.; Momenteau, M. J. Am.
Chem. Soc. 1995, 117, 6041 and references therein.
(70) Hashimoto, T.; Dyer, R. L.; Crossley, M.; Baldwin, J. E.; Basolo,
F. J. Am. Chem. Soc. 1982, 104, 2101.
(71) Collman, J. P.; Brauman, J. I.; Iverson, B. L.; Sessler, J. L.; Morris,
R. M.; Gibson, Q. H. J. Am. Chem. Soc. 1983, 105, 3052.
(72) (a) Structural parameters taken from [Fe(TPP)(1-BzylIm)2].31 (b)
Structural parameters taken from [Fe(TMP)(1,2-Me2Im)2]ClO4.23
(67) Quinn, R.; Nappa, M.; Valetine, J. S. J. Am. Chem. Soc. 1992, 104,
2588.
(68) Walker, F. A. Unpublished results.

9448 J. Am. Chem. Soc., Vol. 119, No. 40, 1997
Safo et al.
The latter two orbital populations are, in fact, smaller for the
case of perpendicular as compared to parallel ligands, as pre-
dicted in the Mo¨ssbauer section above, but the difference is
very slightly saddled. What is quite apparant from the present
study is that a major difference between the low-spin d5 and d6
electron configurations is that complete filling of the three dπ
orbitals of octahedral symmetry removes the possibility of
porphyrinfFe π donation, and thereby removes any electronic
reason for ruffling. This is important for the (dxz,dyz)4(dxy)2
configuration of complexes of this study,40 in comparison to
the Fe(III) cases mentioned above, where only one electron is
in the dxy orbital, and PorfFe(III) π donation from the a2u(π)
orbital requires ruffling.55,64
2
2
small. The population of dx -y is also greater for the case of
perpendicular ligands than for parallel (0.584 and 0.500,
2
respectively), and greater than the population of dz in both cases
(0.479 and 0.438, respectively), and this population difference
also contributes to the positive electric field gradient found in
the magnetic Mo¨ssbauer spectra.40 Thus, based upon orbital
populations, both porphyrin and axial ligand σ donor and metal
π back-bonding interactions are calculated to be slightly greater
in the case of perpendicularly-aligned axial ligands, but these
effects are believed to be due mainly to the shorter NP-Fe bonds
that result from ruffling of the porphyrinate ring.40
It thus appears that the low-spin d6 electronic configuration
finds no stabilization due to ruffling of the porphyrinate ring
and, in fact, is destabilized by ruffling, if the nearly two orders
of magnitude lower stability of [Fe(TMP)(2-MeHIm)2] than of
[Fe(TMP)(1-MeIm)2] (Table 10) is considered. This difference
in complex stability alone amounts to a free energy difference
of 10.3 kJ/mol at 298 K. In comparison, for the Fe(III) analogs
of Table 10, the free energy difference is only 2.9 kJ/mol. The
difference between these two free energy differences, 7.4 kJ/
mol, may be thought of as a measure of the effect of the
difference in electron configuration of the two oxidation states,
assuming that the Fe(III) and Fe(II) derivatives have very similar
geometries.8,40 To this 7.4 kJ/mol we must now add the energy
required to ruffle the porphyrinate ring for the Fe(II) complex
(with its axial ligands in perpendicular planes), which has been
cancelled out in this ∆∆G calculation. Although we do not
have a direct measure of the ∆G of ruffling, we have calculated
the ∆H of ruffling of the TMP ring by MM2 methods.55
Assuming that the value obtained, 4 kJ/mol for [FeIII(TMP)-
(Py)2]+,55 is similar for the Fe(II) analog, and that the entropy
of ruffling is small, we can expect the barrier to ruffling of the
porphyrinate ring (and thus placing planar ligands in perpen-
dicular planes) will be at least another 4 kJ/mol higher. Thus,
it appears that the stabilization free energy of the parallel (planar
ring) over perpendicular (ruffled ring) structures of low-spin
Fe(II) porphyrinates may be of the order of at least -11.4 kJ/
mol, a sizable stabilization for the parallel alignment of axial
ligands.
The energy difference between the filled “eg(π)” orbitals is
calculated to be smaller for the hindered (0.039 eV) than for
the nonhindered imidazole complex (0.074 eV), although these
orbitals contain only 2% metal 3d character.40 However, more
interesting than the filled orbital energies is the difference in
energy of the LUMO “eg(π*)” orbitals, which is small (0.006
eV) for the case of ligands in perpendicular planes and quite
large (0.168 eV) for the case of ligands in parallel planes, even
though the percent Fe(3d) character is small in both cases (6-
10%) (although slightly larger for the case of perpendicular
ligands (9, 10%) than for parallel (6, 7%)). It is also interesting
to note that the percent Fe(3d) character is similar for both “eg-
(π*)” orbitals in each case, in spite of the fact that for the case
of parallel planes, the ligand π and π* orbitals can mix with
one metal dπ orbital and not the other. Thus, the effect upon
the energies of the porphyrin π* LUMO orbitals for the case
of parallel ligands is much greater than that upon the dπ orbitals
of the metal, and the metal d character of the π* LUMO orbitals
is fairly small, suggesting that the metal orbitals of low-spin
Fe(II) are, for the most part, noninteractingsthey may well
funnel information from the axial ligands to the porphyrinate π
system but are of themselves not very much affected by axial
ligand plane orientation.
Summary and Conclusions. From the above discussion of
bonding interactions it has become apparent that they do not
explain the observed stabilization of parallel ligand orientation
in Fe(II) porphyrinates, and to understand the reasons for this
stabilization we must focus on the porphyrinate ring, in particular
on the energetics of ruffling for low-spin d6 as compared to
low-spin d5 systems. In the case of low-spin Fe(III) porphy-
rinates the dπ orbitals appear to interact with the axial ligands;
the strongest evidence for this is the extreme ruffling of [Fe-
(TPP)(t-BuNC)2]ClO4 and [Fe(OEP)(t-BuNC)2]ClO4,64 where
there is no structural reason for the ruffling, but rather an
electronic reason, to stabilize the (dxz,dyz)4(dxy)1 electronic
configuration by porphyrinfFe(III) π donation, as discussed
above and elsewhere55 for [Fe(TPP)(4-CNPy)2]ClO4. However,
as suggested above, rather than π donation from the dπ orbitals
to the axial ligands, it may be that the weak σ-donor strength
of isonitriles and low-basicity pyridines is actually responsible
for placing dxy at higher energy than the dπ orbitals, and this
electron configuration is then stabilized in the low-spin Fe(III)
case by ruffling. It is interesting to note that in the iron(II)
derivative, [Fe(TPP)(t-BuNC)2],73 the porphinato core is only
Acknowledgment. We thank the reviewers of this paper for
helpful comments and the National Institutes of Health for
support of this research under Grant Nos. GM-38401 (W.R.S.),
DK-31038 (F.A.W.), and GM-16406 (P.G.D.).
Supporting Information Available: Table S1, complete
crystal data and intensity collection parameters; Tables S2-
S4, final atomic coordinates for [Fe(TMP)(4-CNPy)2], [Fe-
(TMP)(3-CNPy)2], and [Fe(TMP)(4-MePy)2], respectively; Tables
S5-S8, hydrogen atom fractional coordinates and isotropic
thermal parameters, anisotropic thermal parameters, group
parameters for the chloroform solvent, and fractional coordinates
for the fixed chloroform solvent for [Fe-(TMP)(4-CNPy)2];
Tables S9 and S10, hydrogen atom fractional coordinates and
isotropic thermal parameters, and anisotropic thermal parameters
for [Fe(TMP)(3-CNPy)2]; and Tables S11 and S12, hydrogen
atom fractional coordinates and isotropic thermal parameters,
and anisotropic thermal parameters for [Fe(TMP)(4-MePy)2] (17
pages). See any current masthead page for ordering and Internet
access instructions.
(73) Jameson, G. B.; Ibers, J. A. Inorg. Chem. 1979, 18, 1200.
JA9715657