Two crystalline forms of low-spin [Fe(TMP)(5-MeHIm)2]ClO4. Relative parallel and perpendicular axial ligand orientations

Article abstract of DOI:10.1021/ja991551w

The preparation and characterization of two crystalline forms of [Fe(TMP)(5-MeHIm)₂]ClO₄ with distinctly different molecular structures are reported. Crystal structure analysis shows that paral-[Fe(TMP)(5- MeHIm)₂]ClO₄ has the axial imidazole ligands arranged in a relative parallel orientation (over a slightly S₄-ruffled porphyrin core) and perp- [Fe(TMP)(5-MeHIm)₂]ClO₄ has the axial imidazole ligands arranged in a relative perpendicular orientation (over a considerably S₄-ruffled porphyrin core). The two species have different Mossbauer and solid-state EPR spectra. The small quadrupole splitting (ΔE(q) = 1.78(1) mm/s, 120 K) and a single observable EPR g(max) value (3.43 at 4.2 K) for perp-[Fe(TMP)(5- MeHIm)₂]ClO₄ are indicative of the relative perpendicular arrangement of the axial ligands. The larger quadrupole splitting (ΔE(q) = 2.557(3) mm/s, 120 K) and rhombic g-tensor (g₁ = 2.69, g₂ = 2.34-2.43, and g₃ = 1.75) in the solid state and in frozen DMF-acetonitrile 3:1 (g₁ = 2.64, g₂ = 2.30, and g₃ =1.80) at 4.2 K for paral-[Fe(TMP)(5-MeHIm)₂]ClO₄ are indicative of a relative parallel axial ligand orientation. The actual axial ligand dihedral angles are Δφ = 76°and Δφ = 26 or 30°for perp- and paral- [Fe(TMP)(5-MeHIm)₂]ClO₄, respectively, and thus the dihedral angle at which the EPR spectral type changes from large g(max) to rhombic must be 30 < Δφ < 76°. Because the porphyrin and axial ligands are similar for both crystalline forms of [Fe(TMP)(5-MeHIm)₂]ClO₄, a more direct correlation between molecular and electronic structure has been established. Molecular mechanics calculations indicate that nonbonded interactions between the axial ligands and meso-mesityl groups of [Fe(TMP)(5-MeHIm)₂]⁺ destabilize a relative parallel orientation for the axial ligands, yet the parallel orientation is observed in all frozen solution samples as confirmed by EPR investigations. This is believed to be due to the competing stabilization of the electronic state of the rhombically distorted parallel complex with an energy stabilization of 2.8-3.7 kcal/mol, as compared to the energy destabilization of 2.6 kcal/mol obtained from MM calculations.

Full text of DOI:10.1021/ja991551w

11144
J. Am. Chem. Soc. 1999, 121, 11144-11155
Two Crystalline Forms of Low-Spin [Fe(TMP)(5-MeHIm)2]ClO4.
Relative Parallel and Perpendicular Axial Ligand Orientations
Orde Q. Munro,*,†,‡ Judith A. Serth-Guzzo,† Ilona Turowska-Tyrk,† K. Mohanrao,†
Tatjana Kh. Shokhireva,§ F. Ann Walker,*,§ Peter G. Debrunner,*, and
W. Robert Scheidt*,†
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,
Department of Physics, UniVersity of Illinois, Urbana, Illinois 61801, and Department of Chemistry,
UniVersity of Natal, PriVate Bag X01, ScottsVille 3209, South Africa
ReceiVed May 10, 1999
Abstract: The preparation and characterization of two crystalline forms of [Fe(TMP)(5-MeHIm)2]ClO4 with
distinctly different molecular structures are reported. Crystal structure analysis shows that paral-[Fe(TMP)-
(5-MeHIm)2]ClO4 has the axial imidazole ligands arranged in a relative parallel orientation (over a slightly
S4-ruffled porphyrin core) and perp-[Fe(TMP)(5-MeHIm)2]ClO4 has the axial imidazole ligands arranged in a
relative perpendicular orientation (over a considerably S4-ruffled porphyrin core). The two species have different
Mo¨ssbauer and solid-state EPR spectra. The small quadrupole splitting (∆Eq ) 1.78(1) mm/s, 120 K) and a
single observable EPR gmax value (3.43 at 4.2 K) for perp-[Fe(TMP)(5-MeHIm)2]ClO4 are indicative of the
relative perpendicular arrangement of the axial ligands. The larger quadrupole splitting (∆Eq ) 2.557(3) mm/
s, 120 K) and rhombic g-tensor (g1 ) 2.69, g2 ) 2.34-2.43, and g3 ) 1.75) in the solid state and in frozen
DMF-acetonitrile 3:1 (g1 ) 2.64, g2 ) 2.30, and g3 )1.80) at 4.2 K for paral-[Fe(TMP)(5-MeHIm)2]ClO4
are indicative of a relative parallel axial ligand orientation. The actual axial ligand dihedral angles are ∆φ )
76° and ∆φ ) 26 or 30° for perp- and paral-[Fe(TMP)(5-MeHIm)2]ClO4, respectively, and thus the dihedral
angle at which the EPR spectral type changes from large gmax to rhombic must be 30 < ∆φ < 76°. Because
the porphyrin and axial ligands are similar for both crystalline forms of [Fe(TMP)(5-MeHIm)2]ClO4, a more
direct correlation between molecular and electronic structure has been established. Molecular mechanics
calculations indicate that nonbonded interactions between the axial ligands and meso-mesityl groups of [Fe-
(TMP)(5-MeHIm)2]+ destabilize a relative parallel orientation for the axial ligands, yet the parallel orientation
is observed in all frozen solution samples as confirmed by EPR investigations. This is believed to be due to
the competing stabilization of the electronic state of the rhombically distorted parallel complex with an energy
stabilization of 2.8-3.7 kcal/mol, as compared to the energy destabilization of 2.6 kcal/mol obtained from
MM calculations.
Introduction
The pair of axial histidines of bovine cytochrome b5, a
monoheme protein, also show a relative near-parallel orienta-
tion.4 In both the cytochromes c3 and cytochrome b5, the exact
relative orientation(s) of the histidine ligands (and indeed the
orientations of the ligand planes relative to the Fe-Np bonds5
within the heme group) appear to be important in fine-tuning
the heme redox potential. For example, EPR and redox
measurements on cytochrome c3 isolated from several species
of sulfate reducing bacteria indicate that each heme iron has a
unique electronic structure characterized by well-resolved g-
Currently there are three X-ray structures of tetraheme
cytochromes with bis-histidine coordination of the heme iron
(the cytochromes c3 from the sulfate reducing bacteria Des-
ulfoVibrio Vulgaris,1 DesulfoVibrio desulfuricans,2 and Desulfo-
microbium baculatum3) which clearly show unique relative
orientations for the pair of axial histidines coordinated to each
heme group. In cytochrome c3 from D. Vulgaris, for example,
three of the four heme groups have relative parallel histidine
orientations, while the fourth has a more staggered conformation
in which the dihedral angle between the histidine planes is 64°
and therefore closer to a relative perpendicular arrangement.1
(4) Mathews, F. S.; Czerwinski, E. W.; Argos, P. In The Porphyrins;
Dolphin, D., Ed.; Academic: New York, 1979; Vol. 7, pp 108-148.
(5) Abbreviations used: BzHIm, benzimidazole; 4-CNPy, 4-cyanopyri-
dine; |Cm|, mean absolute displacement of the meso carbons from the
porphyrin mean plane; 1,2-Me2Im, 1,2-dimethylimidazole; HIm; imidazole;
L, ligand in general; K222, potassium 4,7,13,16,21,24-hexaoxa-1,10-
diazabicyclo[8.8.8]hexacosane; 2-MeHIm, 2-methylimidazole; 4(5)-MeHIm,
4(5)-methylimidazole; 1-MeIm, 1-methylimidazole; MM, molecular me-
chanics; c-MU, cis-methyl urocanate; Nax, axial nitrogen donor; Np,
porphinato nitrogen; 4-NMe2Py, 4-N,N-dimethylpyridine; OEP, 2,3,7,8,-
12,13,17,18-octaethylporphyrin dianion; Proto IX, trianion protoporphyrin
IX.; T-2,6-Cl2PP, 5,10,15,20-tetra(2,6-dichlorophenyl)porphyrin dianion;
TMP, 5,10,15,20-tetramesitylporphyrin dianion; TPP, 5,10,15,20-tetraphe-
nylporphyrin dianion; t-MU, trans-methyl urocanate; 1-VinIm, 1-vinylimi-
dazole.
* Authors to whom correspondence should be sent.
† University of Notre Dame.
‡ University of Natal.
§ University of Arizona.
University of Illinois.
(1) Higuchi, Y.; Kusunoki, M.; Matsuura, Y.; Yasuoka, N.; Kakudo, M.
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10.1021/ja991551w CCC: $18.00 © 1999 American Chemical Society
Published on Web 11/17/1999

Low-Spin [Fe(TMP)(5-MeHIm)2]ClO4
J. Am. Chem. Soc., Vol. 121, No. 48, 1999 11145
values and a discrete redox potential.6-9 We (and others10,11
)
residues within the heme binding pocket.32 In simple low-spin
bis-imidazole and bis-pyridine iron(III) porphyrins, axial ligand
orientation also appears to hinge on several factors. For meso-
tetraaryl porphyrins with sterically bulky imidazoles and py-
ridines, complexes of the type [Fe(TPP)L2]+ and [Fe(TMP)L2]+
show strong S4-ruffling of the porphyrin core and a relative
perpendicular orientation for the axial ligands. From the crystal
structures of several of these complexes,15,18,19,33 and MM-
calculated potential surfaces for [Fe(TMP)(1,2-MeIm)2]+ and
related species,34 we have shown that this type of conformation
best minimizes unfavorable nonbonded interactions between the
axial ligands and the meso-aryl substituents. We have also found
that strong π-acceptor, weak σ-donor ligands such as 4-cyan-
opyridine, for example in [Fe(TPP)(4-CNPy)2]ClO4,35 favor an
unusual iron(III) electronic ground state (dyz,dxz)4(dxy)1 which,
through enhanced dπ-porphyrin π-backbonding, leads to S4-
ruffling of the porphyrin core and a relative perpendicular
orientation for the axial ligands.36 Finally, in low-spin iron(III)
complexes where the steric bulk of the axial and porphyrin
ligands is not overly large, relative parallel imidazole and high-
basicity pyridine orientations are observed along with near-
planar porphyrin core conformations. A relative parallel axial
ligand orientation appears favored in these systems as a result
of the geometry of the imidazole pπ-metal pπ interaction,12
as well as the Jahn-Teller stabilization derived from parallel
orientations. Structurally characterized examples of bis-imida-
zole and bis-pyridine complexes with relative parallel axial
ligand orientations include [Fe(TMP)(1-MeIm)2]ClO4,15 [Fe-
(TPP)(1-MeIm)2]ClO4,17 [Fe(TPP)(HIm)2]Cl,16 [Fe(TPP)(c-
MU)2]SbF6,37 [Fe(TPP)(t-MU)2]SbF6,37 and [Fe(OEP)(4-NMe2-
Py)2]ClO4.15
Although the relative orientations of axial imidazoles and
pyridines in low-spin iron(III) porphyrins clearly affect the
electronic structure of the metal and thus the type of EPR,
Mo¨ssbauer, and NMR19,38,39 spectra of these systems, the
evidence to date has largely been gathered from studies on
complexes with a variety of axial and porphyrin ligands. In this
paper we describe the synthesis and characterization of two
novel crystalline forms of [Fe(TMP)(5-MeHIm)2]ClO4 in which
the axial ligands adopt relative parallel and perpendicular
orientations, respectively. We have labeled these two crystalline
forms paral-[Fe(TMP)(5-MeHIm)2]ClO4 and perp-[Fe(TMP)-
(5-MeHIm)2]ClO4 to distinguish their near-parallel and near-
perpendicular axial ligand arrangements, respectively. Interest-
ingly, these two forms reduce the dihedral angle necessary to
have shown that the relative orientations of the axial ligands in
synthetic low-spin bis-imidazole and bis-pyridine iron(III)
porphyrins directly affect the relative energies of the dπ orbitals,
particularly the half-filled dyz orbital,12 and thus the type of low-
spin EPR spectrum observed.13,14 Specifically, a relative parallel
axial ligand orientation leads to a normal rhombic low-spin g
tensor with a low-field gmax value e3.0,15-17 while a relative
perpendicular axial ligand orientation leads to near-degeneracy
of the dxz and dyz orbitals10,13 and an EPR spectrum in which
the sole observable signal below ∼20 K has a gmax value
>3.2.15,18,19 This so-called “large gmax” value, furthermore, varies
with the basicity of the perpendicularly aligned axial ligands.19
The crystal structures of the cytochromes c3,1-3 cytochrome
b5,4 and other heme proteins with a single histidine residue
coordinated to the heme iron20-30 reveal that the orientations
of the axial ligands are probably controlled by a combination
of three factors: (i) covalent attachment of the imidazole group
to the protein backbone, (ii) hydrogen bonding between the
imidazole N-H proton and H-bond acceptors, most commonly
carbonyl groups of the protein backbone,31 and (iii) nonbonded
interactions both with the porphyrin ring and with amino acid
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2829.
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1163-1169.
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Chem. Soc. 1986, 108, 5288-5297.
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1991, 30, 1643-1650.
(34) Munro, O. Q.; Marques, H. M.; Debrunner, P. G.; Mohanrao, K.;
Scheidt, W. R. J. Am. Chem. Soc. 1995, 117, 935-954.
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Dolata, D. P.; Debrunner, P. G.; Scheidt, W. R. J. Am. Chem. Soc. 1994,
116, 7760-7770.
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1984, 175, 159-174.
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Mol. Biol. 1985, 186, 627-643.
(36) We have confirmed this mechanism of S4-ruffling in low-spin ferric
porphyrins with strong π-accepting isocyanide ligands in the ruffled
complexes [Fe(TPP)(t-BuNC)2]ClO4 and [Fe(OEP)(t-BuNC)2]ClO4: 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, 12109-12118.
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214, 585-595.
(31) Histidine N-H groups are H-bonded to carbonyl oxygens in the
following hemoproteins: human deoxyHb, R-chain (His-87‚‚‚Leu-83);
human deoxyHb, â-chain (His-92‚‚‚Leu-88);22 sperm whale myoglobin (His-
92‚‚‚Leu-89);20 Aplysia Mb (His-95‚‚‚Phe-93);21 and horse, yeast, rice, and
tuna cytochrome c (His-18‚‚‚Pro-30).27-30
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11146 J. Am. Chem. Soc., Vol. 121, No. 48, 1999
Munro et al.
Table 1. Crystallographic Details for paral- and
perp-[Fe(TMP)(5-MeHIm)2]ClO4
produce the large gmax signal and increase that necessary to retain
the normal rhombic EPR signal to a difference of 46°, rather
than the 90° implied by the terms “parallel” and “perpendicular”.
The two crystalline forms of [Fe(TMP)(5-MeHIm)2]ClO4 have
been prepared by two different synthetic routes; that for perp-
[Fe(TMP)(5-MeHIm)2]ClO4 is somewhat unusual, but was
discovered in our efforts to synthesize the mixed-ligand deriva-
tive [Fe(TMP)(4-CNPy)(5-MeHIm)]ClO4. Importantly, this
unusual preparation has been reproduced by three of us
independently (more than once) and leads to the same crystal
polymorph. To our knowledge, this appears to be the first
example of two different conformations of the same low-spin
iron(III) porphyrin with identical axial ligands.
molecule
paral-[Fe(TMP)(5-
MeHIm)2]ClO4
FeC70H73ClN11O4
1223.72
3[4(5)-MeHIm]
16.536(6)
19.462(7)
20.127(2)
100.952(15)
6359(6)
P21
4
1.267
0.330
perp-[Fe(TMP)(5-
MeHIm)2]ClO4
FeC65.2H66Cl4.6N8O4.4
1249.75
formula
formula wt
solvent/asym unit
a, Å
b, Å
c, Å
1.2CHCl3‚0.4H2O
15.63(2)
20.96(3)
19.78(3)
99.88(3)
6434(20)
P21/n
â, deg
V, Å3
space group
Z
4
Dc, g/cm3
µ, mm-1
λ, Å
1.382
0.540
0.71073
127
R1 ) 0.082;
R2 ) 0.0890
Experimental Section
T, K
final R indicesa
[I > 1.5σ(I)]
R1 ) 0.072;
R2 ) 0.083
General Information. All reactions were performed under an argon
atmosphere with Schlenk-ware and cannula techniques. All solvents
were distilled under argon prior to use. Chloroform and hexane were
distilled from CaH2 and sodium/benzophenone, respectively. 4(5)-
Methylimidazole and 4-cyanopyridine were recrystallized from diethyl
ether. meso-Tetramesitylporphyrin was prepared by a modified version
of the procedure published by Lindsey et al.40 and iron was inserted
into H2TMP by standard techniques.41 [Fe(TMP)OClO3] was prepared
as previously described.15 Caution! These perchlorate salts can detonate
spontaneously and should be handled only in small quantities; other
safety precautions are also warranted. UV-vis spectra were recorded
on a Perkin-Elmer Lambda 6 spectrophotometer. Mo¨ssbauer measure-
ments were performed at 120 K on a constant acceleration Mo¨ssbauer
spectrometer on ground single-crystal samples prepared as Apiezon L
grease mulls, as previously described.34 The spectra were least-squares
fitted with Lorentzian line shapes. Isomer shifts are quoted relative to
iron metal at 300 K. The EPR spectra were obtained on polycrystalline
samples at 4.2 K with a Bruker ESP-300E EPR spectrometer operating
at X-band and equipped with an Oxford helium cryostat. Glassy EPR
samples with varying ratios of 5-MeHIm:Fe were prepared in DMF-
acetonitrile 3:1.
Synthesis of paral-[Fe(TMP)(5-MeHIm)2]ClO4. [Fe(TMP)OClO3]
(210 mg, 0.224 mmol) and 4(5)-MeHIm (210 mg, 2.56 mmol) were
placed in a 25-mL Schlenk flask. Chloroform (∼14 mL) was added
and the solution was stirred for 10 min. The reaction mixture was
transferred to eight 15 × 1.5 cm test tubes and layered with 15 mL of
hexane. X-ray quality crystals formed after 4 days. Bulk samples were
obtained by selecting the large octagonal plate crystals obtained in the
procedure. UV-vis (CHCl3) λmax: 417 (Soret), 552, 580 nm.
Synthesis of perp-[Fe(TMP)(5-MeHIm)2]ClO4. Two slightly dif-
ferent procedures were used for this compound. Method A: In each of
four 15 × 1.5 cm test tubes were placed [Fe(TMP)OClO3] (15 mg,
0.016 mmol), 4-CNPy (17 mg, 0.163 mmol), and 4(5)-MeHIm (1.5
mg, 0.018 mmol). Each test tube was evacuated and backfilled with
argon, and 2 mL of chloroform was added. The solution was then stirred
for a few minutes and layered with hexane. After 3 days, X-ray quality
crystals were harvested, washed three times with hexane, and dried.
These crystals were square plates which were separated from other solid
material by hand selection for both bulk samples and X-ray analysis.
Method B: [Fe(TMP)OClO3] (75 mg, 0.080 mmol) and 4-CNPy (84
mg, 0.806 mmol) were placed in a 25-mL Schlenk flask. Chloroform
(∼5.4 mL) was added and the solution was stirred for 20 min. The
UV-vis (CHCl3) spectrum (412, 535, 572 nm) is that of [Fe(TMP)-
(4-CNPy)2]ClO4.15 A chloroform solution of 4(5)-MeHIm (0.080 mmol)
was added to the solution with a syringe and stirred for an additional
20 min. A new UV-vis spectrum results (CHCl3): λmax 418 (Soret),
547, 577 nm. The reaction mixture was transferred to four 15 × 1.5
cm test tubes and layered with 15 mL of hexane. X-ray quality crystals
formed after 4 days.
a R1 ) ∑||Fo| - |Fc||/∑|Fo| and R2 ) [∑w(|Fo| - |Fc|)2/∑w(Fo)2]1/2
.
X-ray Structure Determinations. The two different black, crystal-
line forms of [Fe(TMP)(5-MeHIm)2]ClO4 were examined with graphite-
monochromated Mo KR radiation on an Enraf-Nonius FAST area
detector diffractometer at 127 K. Unit cell determination and data
collection procedures with the area detector have been described
previously.42 A summary of cell constants and refinement results is
given in Table 1; complete details are given in Table S1. Slight
variations in data collection instrument settings were used owing to
differing crystal quality. Data sets were corrected for Lorentz-
polarization and absorption effects. The structures were solved by
Patterson methods with the SHELXS-86 program.43 In the perp-[Fe-
(TMP)(5-MeHIm)2]ClO4 structure there is one independent [Fe(TMP)-
(5-MeHIm)2]ClO4 per asymmetric unit, but in the paral-[Fe(TMP)(5-
MeHIm)2]ClO4 structure there are two such independent moieties in
the asymmetric unit. The solvent content of the two crystalline forms
was determined during the course of structure solution and refinement.
For perp-[Fe(TMP)(5-MeHIm)2]ClO4, there is one ordered chloroform
molecule and a partially occupied chloroform molecule that is
disordered around the inversion center at 1/2, 0, 1/2. There were, in
addition, two extra peaks that were refined as partial water molecules.
The final model thus had a solvent region divided between two
asymmetric units that was occupied either by a CHCl3 (40%) or two
water molecules (40%). In paral-[Fe(TMP)(5-MeHIm)2]ClO4, there are
three imidazole molecules in the asymmetric unit that form a hydrogen
bond network with the perchlorate ions and the coordinated imidazoles.
Least-squares refinement of the structural model for both crystals
was carried to convergence with anisotropic temperature factors for
all nonhydrogen atoms. Hydrogen atoms were included as fixed,
idealized contributors. For paral-[Fe(TMP)(5-MeHIm)2]ClO4, which
crystallizes in the polar space group P21, the assignment of the
enantiomer was made. The opposite hand yielded both weighted and
unweighted R’s that were 0.1% higher and the coordinates of the first
hand are reported herein. Final discrepancy indices are listed in Table
1. Because of the unusual nature of perp-[Fe(TMP)(5-MeHIm)2]ClO4,
a second structure determination of this form was undertaken using a
new, independent crystal preparation. The structure was independently
solved from this data set and carried through the refinement process
until it was clear that the same structure had been obtained. Final atomic
coordinates are listed in Tables S2 and S7. Fixed hydrogen atom
coordinates and anisotropic temperature factors are given in the
Supporting Information.
(42) Scheidt, W. R.; Turowska-Tyrk, I. Inorg. Chem. 1994, 33, 1314-
1318.
(43) Programs used in this study included SHELXS-86: Sheldrick, G.
M. Acta Crystallogr., Sect. A 1990, A46, 467. Local modifications of Busing
and Levy’s ORFFE and ORFLS, Jacobson’s ALLS, Zalkin’s FORDAP,
and Johnson’s ORTEP2. Scattering factors were taken from: International
Tables for Crystallography; Wilson, A. J. C., Ed.; Kluwer Academic
Publishers: Dordrecht, 1992; Vol. C.
(40) Wagner, R. W.; Lawrence, D. S.; Lindsey, J. S. Tetrahedron Lett.
1987, 28, 3069-3070. Lindsey, J. S.; Wagner, R. W. J. Org. Chem. 1988,
54, 828-836.
(41) Adler, A. D.; Longo, F. R.; Kampas, F.; Kim, J. J. Inorg. Nucl.
Chem. 1970, 32, 2443.

Low-Spin [Fe(TMP)(5-MeHIm)2]ClO4
J. Am. Chem. Soc., Vol. 121, No. 48, 1999 11147
Molecular Mechanics Calculations. These were performed on an
IBM-compatible computer with our modified version44 of Allinger’s
MM program MM2(87).45 Input structures were either the X-ray
structures of [Fe(TMP)(5-MeHIm)2]ClO4 (orthogonalized coordinates)
or idealized structures with planar core conformations.46 An energy
cut-off for minimization of ∆UT e 6.0 × 10-5 kcal/mol between
successive iterations was used with our previously described force
field.34,47,48 A dielectric constant of 10 D was used to take into account
a fairly polar crystal environment. Partial atomic charges were not
included in the calculations.34,49,50
The three X-ray structures of [Fe(TMP)(5-MeHIm)2]ClO4 were
modeled by fixing the orientations of the axial ligands at their observed
values using MM2’s dihedral angle locking routine prior to refining
all other structural parameters. Comparison of the energy-minimized
and X-ray structures of [Fe(TMP)(5-MeHIm)2]ClO4 gave acceptable
rmsd’s (bond lengths, bond angles, torsion angles, 24-atom mean
planes).
The orientation of an axial imidazole ligand relative to the porphyrin
core may be defined by the torsion angle that involves a porphyrin
nitrogen atom, the central metal ion, a coordinated axial imidazole
nitrogen, and an imidazole R-carbon (Np-Fe-Nax-CIm, φ). The
orientations of a pair of trans imidazole ligands are then defined by
the torsion angles φ1 and φ2, which are measured relative to the same
porphyrin nitrogen.51 Conformational surfaces for [Fe(TMP)(5-Me-
HIm)2]+, [Fe(TPP)(5-MeHIm)2]+, [Fe(porphine)(5-MeHIm)2]+, and [Fe-
(TMP)(HIm)2]+ were calculated by counter-rotating the axial ligands
from 0 to 90° (φ1) and 0 to 180° (φ2) in 10° increments, producing a
total of 10 × 19 starting conformations for refinement.52 (This dihedral
angle range maps the symmetry-unique portion of conformational
space.) The method used by MM2 to fix the driven torsion angles during
geometry optimization has been described elsewhere.34 The strain
energy components and coordination sphere structural parameters of
the energy-optimized conformations were extracted from the MM2
output files and analyzed as a function of the driven torsion angles.
Figure 1. ORTEP diagram of perp-[Fe(TMP)(5-MeHIm)2]ClO4.
Thermal ellipsoids are drawn at the 50% probability level. Hydrogen
atoms are omitted for clarity.
and concomitant structural features. One form, which we denote
as perp-[Fe(TMP)(5-MeHIm)2]ClO4, has the axial ligands in a
relative perpendicular orientation; the second form (paral-[Fe-
(TMP)(5-MeHIm)2]ClO4) has the axial ligands in a relative
parallel orientation. Both complexes have been characterized
by single-crystal X-ray structure determinations and by Mo¨ss-
bauer and EPR spectroscopies. The structural results for perp-
[Fe(TMP)(5-MeHIm)2]ClO4 are reported first, followed by those
for paral-[Fe(TMP)(5-MeHIm)2]ClO4.
The molecular structure and numbering scheme for the
crystallographically unique atoms of perp-[Fe(TMP)(5-MeHIm)2]-
ClO4 are shown in the ORTEP diagram of Figure 1. The ORTEP
diagram shows that perp-[Fe(TMP)(5-MeHIm)2]ClO4 has a
near-perpendicular relative orientation of the axial imidazole
ligands. The actual dihedral angle, ∆φ, between the two axial
ligands is 76°. The projection of the two imidazole ligand planes
onto the 24-atom porphyrin mean plane makes angles of 46°
(φ1) and 30° (φ2) to the same Fe-Np vector. (φ1 denotes the
orientation of the top ligand in all ORTEP diagrams.) The
dihedral angles between the two ligand planes and the porphyrin
plane are 82.4° and 77.5°. The dihedral angles between the four
mesityl rings and the mean porphinato core are 82.0, 80.9, 84.3,
and 84.2°.
Averaged values for the chemically equivalent bond distances
and angles in the core are shown in the formal diagram of Figure
2. Also displayed are the individual displacement values of the
crystallographically unique atoms from the mean plane of the
24-atom core (in units of 0.01 Å) and the orientation of the
axial ligands. The porphinato core of perp-[Fe(TMP)(5-MeHIm)2]-
ClO4 exhibits a modest degree of S4-ruffling, in accord with
observations for other [FeIII(TMP)L2]+ derivatives having axial
ligands with relative perpendicular orientations. Also consistent
with the ruffled core are the relatively short equatorial Fe-Np
bond distances which average to 1.981(7) Å and bending of
the methine carbon atoms out of the plane of the individual
pyrrole rings by an average of (0.32 Å. The two independent
axial bond distances (FeIII-Nax) are 1.973(6) and 1.957(6) Å.
The axial Nax-Fe-Nax angle is 176.6(2)°; the Nax-Fe-Np
angles range from 87.6(3) to 92.9(3)°. Selected values of the
bond distances and bond angles in perp-[Fe(TMP)(5-MeHIm)2]-
ClO4 are given in Table 2; complete listings of structural data
are given in the Supporting Information.
Results
[Fe(TMP)(5-MeHIm)2]ClO4 has been obtained in two crystal-
line forms with distinctly different molecular structures. The
structures differ in the relative orientation of the axial ligands
(44) Munro, O. Q.; Bradley, J. C.; Hancock, R. D.; Marques, H. M.;
Marsicano, F.; Wade, P. W. J. Am. Chem. Soc. 1992, 114, 7218-7230.
(45) (a) Allinger, N. L. J. Am. Chem. Soc. 1977, 99, 8127-8134. (b)
Allinger, N. L.; Yuh, Y. MM2(87). Distributed to academic users by QCPE,
under special agreement with Molecular Design Ltd., San Leandro, CA.
(c) Sprague, J. T.; Tai, J. C.; Young, Y.; Allinger, N. L. J. Comput. Chem.
1987, 8, 581.
(46) ALCHEMY III, 3D Molecular Modeling Software. Tripos Associ-
ates Inc., 1699 S. Hanley Rd., St. Louis, MO. Other programs used in this
study: (1) XANADU, program for manipulation of crystallographic data:
Roberts, P.; Sheldrick, G. M. 1976/7. This program was routinely used to
fit least-squares planes through the non-hydrogen atoms of the computed
porphyrin cores. (2) AXUM, Technical Graphics and Data Analysis, V.
3.0. TriMetrix Inc., 444 NE Ravenna Boulevard, Suite 210, Seattle, WA
98115.
(47) Marques, H. M.; Munro, O. Q.; Grimmer, N. E.; Levendis, D. C.;
Marsicano, F.; Pattrick, G.; Markoulides, T. J. Chem. Soc., Faraday Trans.
1995, 91, 1741-1749.
(48) The total steric energy of a given conformation, UT, corresponds to
the sum of the strain energy contributions from bond stretching/compression
(Ub), angle-bending (Uθ), stretch-bend deformations (USB), 1,4-nonbonded
interactions (U1,4-NB), through-space nonbonded interactions (UNB), torsion
angle deformations (Uφ), and dipole-dipole interactions (Uµ).
(49) Shelnutt, J. A.; Medforth, C. J.; Berber, M. D.; Barkigia, K. M.;
Smith, K. M. J. Am. Chem. Soc. 1991, 113, 4077-4087.
(50) The force field includes the standard MM2 bond dipoles for the
C-C and C-N bonds. All M-L bond dipoles have an assigned value of
zero.
(51) The orientations of imidazole and pyridine ligands in metallopor-
phyrin complexes are normally defined from crystallographic coordinates
by the angle φ between the projection of the plane of the axial ligand onto
the porphyrin core and the closest Fe-Np vector.53
(52) Due to counter-rotation of the two torsion angles φ1 and φ2, a φ2
value of +45°, for example, corresponds to an angle of - 45° in the frame
of reference for φ1.
The (uncoordinated) N-H groups of the two imidazole
ligands of perp-[Fe(TMP)(5-MeHIm)2]ClO4 are within hydrogen
bonding distance to oxygen atoms of the perchlorate anion.
Distances are N(7)‚‚‚O(2) ) 2.872 Å and N(8)‚‚‚O(4) ) 2.842

11148 J. Am. Chem. Soc., Vol. 121, No. 48, 1999
Munro et al.
Figure 2. Formal diagram of the porphinato core in perp-[Fe(TMP)-
(5-MeHIm)2]ClO4 displaying the perpendicular displacement of each
unique atom from the 24-atom mean plane. All displacements are given
in units of 0.01 Å. The orientation of the axial ligands is given with
the position of the methyl group of the imidazole indicated by the filled
circle. Also entered on the diagram are the averaged values of all bond
distances and angles of the core.
Table 2. Selected Bond Lengths and Bond Angles for
perp-[Fe(TMP)(5-MeHIm)2]ClO4 and
paral-[Fe(TMP)(5-MeHIm)2]ClO4
Figure 3. ORTEP diagram of paral-[Fe(TMP)(5-MeHIm)2]ClO4 for
Molecule A and Molecule B. Thermal ellipsoids are drawn at the 50%
probability level. Hydrogen atoms are omitted for clarity.
perp-[Fe(P)L2]- paral-[Fe(P)L2]- paral-[Fe(P)L2]-
b
ClO4
ClO4(A)c
ClO4(B)d
(A) bond lengths (Å)a
Fe-N(1)
Fe-N(2)
Fe-N(3)
Fe-N(4)
Fe-N(5)
Fe-N(6)/(7)e
1.985(6)
1.979(6)
1.988(6)
1.973(6)
1.957(6)
1.973(6)
1.986(5)
1.981(5)
1.974(5)
1.985(5)
1.984(5)
1.980(5)
1.985(5)
mean plane makes dihedral angles of 10° (φ1) and 20° (φ2) to
the closest Fe-Np vector for molecule A, and angles of 14°
(φ1) and 12° (φ2) for molecule B. The dihedral angles between
the ligand planes and the porphyrin plane are 86.0° and 88.3°
(molecule A), and 83.2° and 86.6° (molecule B). The dihedral
angles between the four mesityl rings and the 24-atom porphyrin
mean plane are 77.3, 85.8, 89.5, and 86.0° for molecule A; those
for molecule B are 86.1, 88.4, 87.2, and 78.9°.
1.978(5)
1.985(5)
1.984(5)
1.978(6)
1.961(5)
(B) bond angles(°)
89.43(19)
N(1)-Fe-N(2)
N(1)-Fe-N(3)
N(1)-Fe-N(4)
N(2)-Fe-N(3)
N(2)-Fe-N(4)
N(3)-Fe-N(4)
N(5)-Fe-N(1)
N(5)-Fe-N(2)
N(5)-Fe-N(3)
N(5)-Fe-N(4)
N(6)/(7)e-Fe-N(1)
N(6)/(7)e-Fe-N(2)
N(6)/(7)e-Fe-N(3)
N(6)/(7)e-Fe-N(4)
N(6)/(7)e-Fe-N(5)
91.16(24)
178.95(23)
89.07(24)
89.08(25)
179.52(25)
90.67(24)
88.38(23)
89.13(25)
90.60(24)
90.47(25)
91.08(24)
87.55(25)
89.95(24)
92.86(25)
176.62(24)
90.44(19)
179.54(23)
89.87(19)
89.66(19)
178.85(23)
90.03(18)
89.98(21)
88.88(20)
89.58(21)
90.02(21)
90.42(21)
92.22(21)
90.03(20)
88.88(21)
178.83(21)
179.19(21)
90.29(19)
90.30(19)
178.87(22)
90.00(19)
90.10(21)
90.79(22)
89.15(21)
90.31(21)
90.08(21)
89.14(21)
90.67(21)
89.77(21)
179.80(22)
Figure 4 shows the perpendicular displacements of the
crystallographically unique atoms from the mean planes of the
24-atom porphyrin cores (in units of 0.01 Å) of molecules A
and B of paral-[Fe(TMP)(5-MeHIm)2]ClO4 and the orientation
of the axial ligands. Averaged values for the chemically
equivalent bond distances and angles of the porphyrin core are
displayed in each formal diagram. paral-[Fe(TMP)(5-MeHIm)2]-
ClO4 has a relatively planar porphinato core; molecule B is more
planar than molecule A. The Fe-Nax distances for molecule A
are 1.978(6) and 1.961(5) Å; those for molecule B are 1.980(5)
and 1.985(5) Å. Equatorial bond distances (Fe-Np) average to
1.983(4) Å for molecule A and 1.981(5) Å for molecule B. The
Nax-Fe-Nax angle is 179.80(22)° (molecule A) and 178.83-
(21)° (molecule B); the Nax-Fe-Np angles range from 89.14-
(21)° to 90.79(22)° (molecule A) and 88.88(21)° to 92.22(21)°
(molecule B). Selected values of the bond distances and bond
angles for both molecules are given in Table 2; complete listings
of structural data are given in the Supporting Information.
The N-H groups of all imidazole ligands of both paral-[Fe-
(TMP)(5-MeHIm)2]+ ions are within hydrogen bonding distance
of acceptors in the solid-state structure. One imidazole N-H
of each cation is hydrogen bonded to an oxygen atom of the
perchlorate anions. Distances are N(6)A‚‚‚O(1) ) 2.929 Å and
N(6)B‚‚‚O(5) ) 2.866 Å. The N-H of the other imidazole of
each molecule is involved in a much stronger hydrogen bond
in which the acceptors are the unprotonated nitrogen atoms of
the solvate 4(5)MeHIm molecules. The N(8)A‚‚‚N(4)S and
a The estimated standard deviations of the least significant digits
are given in parentheses. b P ) TMP; L ) 5-MeHIm. c Independent
molecule A. d Independent molecule B. e Axial nitrogen trans to N(5)
for molecules A and B of paral-[Fe(TMP)(5-MeHIm)2]ClO4.
Å. Hydrogen bonds of this sort (between the uncoordinated
N-H group and the counteranion) are frequently observed in
porphinatoiron(III) imidazole complexes with N-H groups.
The crystal structure of paral-[Fe(TMP)(5-MeHIm)2]ClO4
consists of two independent molecules in the asymmetric unit
which we denote as molecule A and molecule B; the molecular
structures of both are shown in the ORTEP diagrams of Figure
3. The numbering schemes for the crystallographically unique
atoms are also displayed. The ORTEP diagrams show that the
imidazole ligands are arranged in nearly parallel orientation for
both molecules. The dihedral angles, ∆φ, between the two axial
ligands are 30° for molecule A and 26° for molecule B. The
projection of the two ligand planes onto the 24-atom porphyrin

Low-Spin [Fe(TMP)(5-MeHIm)2]ClO4
J. Am. Chem. Soc., Vol. 121, No. 48, 1999 11149
A
B
Figure 5. X-band EPR spectra of (a) perp-[Fe(TMP)(5-MeHIm)2]ClO4
and (b) paral-[Fe(TMP)(5-MeHIm)2]ClO4 at 4.2 K.
electronic symmetry) and a relative perpendicular axial ligand
orientation.15,19,54
The Mo¨ssbauer spectrum of paral-[Fe(TMP)(5-MeHIm)2]-
ClO4 taken at 120 K is shown in Figure S2b. The spectrum
reveals a quadrupole doublet with a splitting (∆Eq) of 2.56(1)
mm/s and an isomer shift (δ) of 0.22(3) mm/s. The values for
the quadrupole splitting are representative of other low-spin iron-
(III) porphyrinates with axial imidazoles and pyridines in a
relative parallel orientation.15,19 The EPR spectrum of a poly-
crystalline sample of paral-[Fe(TMP)(5-MeHIm)2]ClO4 taken
at 4.2 K (Figure 5b) displays a rhombic g tensor with g-values
of 2.69, 2.34-2.43, and 1.75, consistent with similar systems
that have relative parallel axial ligand orientations.13,15,16,37 There
is some apparent variation in the position of the central line of
the spectrum, but the two outer features remain at the same
field positions as the EPR tube is rotated.
MM-calculated and crystallographically observed bond dis-
tances, bond angles, and torsion angles for the three structures
of [Fe(TMP)(5-MeHIm)2]+ are compared in Table S13. The
mean differences between the calculated and observed structural
parameters are 0.016 Å (bond distances), 1.8° (bond angles),
and ∼5° (torsion angles) for the three structures. Figure 6
compares the calculated and observed conformations of the three
[Fe(TMP)(5-MeHIm)2]+ cations;55 the average absolute dis-
placements of the various classes of porphyrin core atoms from
the porphyrin mean plane are compared in Table 3. The S4-
ruffled structure of perp-[Fe(TMP)(5-MeHIm)2]+ is well mod-
eled. However, the calculated conformations of paral-[Fe(TMP)-
(5-MeHIm)2]+ are somewhat more ruffled than the X-ray
structures. The near-orthogonal dihedral angles between the
planes of the axial imidazole ligands and the porphyrin mean
plane are reproduced in the two calculated structures of paral-
Figure 4. Formal diagram of the porphinato core in paral-[Fe(TMP)-
(5-MeHIm)2]ClO4 for Molecule A and Molecule B displaying the
perpendicular displacement of each unique atom from the 24-atom mean
plane. All displacements are given in units of 0.01 Å. The orientation
of the axial ligands is given with the position of the methyl group of
the imidazole indicated by the filled circle. Also entered on the diagram
are the averaged values of all bond distances and angles of the core.
N(8)B‚‚‚N(2)S distances are 2.715 and 2.846 Å. These distances
are indicative of relatively strong hydrogen bonds. The solvate
imidazole molecules are further involved in a three-dimensional
hydrogen bonding network. A complete tabulation of the
hydrogen bonding distances is available in Table S12 of the
Supporting Information.
The Mo¨ssbauer spectrum of polycrystalline perp-[Fe(TMP)-
(5-MeHIm)2]ClO4 taken at 120 K is shown in Figure S2a. The
spectrum shows a quadrupole doublet with a small splitting
(∆Eq) of 1.78(1) mm/s and an isomer shift (δ) of 0.22(1) mm/
s. The values of the quadrupole splitting and isomer shift are
consistent with other low-spin iron(III) porphyrinates with the
axial ligands in a relative perpendicular orientation.15,19 The EPR
spectrum of a polycrystalline sample of perp-[Fe(TMP)(5-
MeHIm)2]ClO4 recorded at 4.2 K is shown in Figure 5a. A single
feature with gmax ) 3.43 is observed for the majority species;
very minor contamination of the sample by two rhombic low-
spin iron(III) species (g1 ) 2.90 and 2.77; g2 ) 2.21) and a
high-spin iron(III) species (g ) 6.03) is also evident. The “large
gmax” signal from perp-[Fe(TMP)(5-MeHIm)2]ClO4 is consistent
with near-degeneracy of the dxz and dyz orbitals (near-axial
(53) Scheidt, W. R.; Lee, Y. J. Struct. Bonding 1987, 64, 1-70.
(54) Palmer, G. Biochem. Soc. Trans. 1985, 13, 548-560.
(55) The rmsd’s for the fits are: perp-[Fe(TMP)(5-MeHIm)2]ClO4, 0.085
Å; paral-[Fe(TMP)(5-MeHIm)2]ClO4 (molecule A), 0.086 Å; and paral-
[Fe(TMP)(5-MeHIm)2]ClO4 (molecule B), 0.091 Å.

11150 J. Am. Chem. Soc., Vol. 121, No. 48, 1999
Munro et al.
Table 3. Comparison of Crystallographically Observed and
Calculated Core Conformations for [Fe(TMP)(5-MeHIm)2]ClO4
Figure 7 compares conformational surfaces for [Fe(TMP)-
(5-MeHIm)2]+ and [Fe(TPP)(5-MeHIm)2]+ as plots of the
change in total steric energy (relative to the global minimum)
as a function of the orientations (φ1 and φ2) of the axial
imidazole ligands relative to one of the four equivalent porphyrin
nitrogens. A three-dimensional surface and a map of the surface
are shown for each complex. The minimum energy conformation
of [Fe(TMP)(5-MeHIm)2]+ occurs at φ1, φ2 ) 45°; the axial
ligands have a relative perpendicular orientation and the
porphyrin core is S4-ruffled. A strain energy maximum occurs
at φ1, φ2 ) 45°, 135° when the axial ligands adopt a relative
parallel orientation over a planar porphyrin core and point
directly toward the meso-mesityl groups. In the case of [Fe-
(TPP)(5-MeHIm)2]+, a local minimum is observed at φ1, φ2 )
45°, 135° in addition to the global minimum at φ1, φ2 ) 45°.
The locations of two of the three X-ray structures of [Fe(TMP)-
(5-MeHIm)2]ClO4 are shown by the arrows on the map of the
conformational surface for [Fe(TMP)(5-MeHIm)2]+. The X-ray
structure of perp-[Fe(TMP)(5-MeHIm)2]ClO4 lies close to the
calculated lowest energy conformation (point 2). The X-ray
structure of paral-[Fe(TMP)(5-MeHIm)2]+ (molecule A) is
found in a region of higher steric strain at point 1.56
a
perp-[Fe(TMP)- paral-[Fe(TMP)- paral-[Fe(TMP)-
(5-MeHIm)2]+ (5-MeHIm)2]+ Ab (5-MeHIm)2]+ Bc
obsd
calcd
obse
calce
obsf
calcf
|Fe|
|N4|
|Ca|
|Cb|
|Cm|
|C20|
3
4(2)
18(3)
13(5)
32(3)
19(8)
1
3(2)
20(7)
15(14) 11(8)
38(1) 16(3)
21(13) 12(6)
5
4(2)
10(6)
1
3(1)
14(3)
9(4)
27(1)
15(7)
12(8)
3
0
1(1)
12(1)
8(2)
24(1)
13(6)
11(7)
2(1)
4(2)
4(3)
7(3)
5(3)
4(3)
|C20N4Fe| 16(10) 18(14) 10(6)
a
|Ca|, |Cb|, |Cm|, |C20|, |N4|, |C20N4Fe|, and |Fe| are the mean absolute
displacements of the R-, â-, meso-, 20-porphyrin carbons, pyrrole
nitrogens, all atoms, and iron from the mean plane of the porphyrin
(in units of 0.01 Å), respectively. b Molecule A. c Molecule B. d φ1 )
-42.8°, φ2 ) -35.3°. e φ1 ) -6.4°, φ2 ) 159.0°. f φ1 ) 8.4°, φ2 )
-167.4°.
Varying the relative orientations of the axial 5-MeHIm ligands
of [Fe(TMP)(5-MeHIm)2]+ leads to significant conformational
changes (Figure 7) as well as structural changes in the iron(III)
coordination sphere (Figure 8). In Figure 8a, the S4-ruffled
conformation of [Fe(TMP)(5-MeHIm)2]+ (φ1, φ2 ) 45°) is
calculated to have shorter Fe-Np bonds (∼1.967 Å) than the
planar conformation (φ1, φ2 ) 45°, 135°; average Fe-Np
distance ∼1.984 Å). From Figure 8b, the difference in the
average bond length, ∆xy, for the two pairs of trans Fe-Np bonds
is zero when the axial ligands have a relative perpendicular
orientation and are positioned over the Fe-Np bonds (φ1, φ2 )
0°, 90°), or over the methine carbons (φ1, φ2 ) 45°). A
maximum difference (0.011 Å) is observed when the axial
ligands are slightly staggered and nearly eclipse one pair of trans
Fe-Np bonds (φ1, φ2 ) 18°, 162°). The average Fe-Nax
distance also varies with axial ligand orientation (Figure 8c).
The axial bonds are calculated to be longest (∼1.981 Å) when
the 5-MeHIm ligands eclipse a trans pair of Fe-Np bonds and
have a relative parallel orientation (φ1, φ2 ) 0°); the shortest
average Fe-Nax distance (∼1.958 Å) is observed when the axial
ligands adopt a relative perpendicular orientation and point
toward the porphyrin meso carbons (S4-ruffled conformation).
Discussion
Molecular Structures. The structures of [Fe(TMP)(5-
MeHIm)2]ClO4 are remarkable in several respects. Most im-
portantly, they represent the first example of a low-spin iron(III)
porphyrin complex with two different relative orientations for
the same pair of axial ligands. The two crystalline forms of
[Fe(TMP)(5-MeHIm)2]ClO4 have distinctly different EPR and
Mo¨ssbauer spectroscopic parameters which allow us to defini-
tively correlate the electronic structure of the iron(III) ion with
the relative orientations of the axial imidazole ligands.
Figure 6. Comparison of calculated and crystallographically observed
structures of [Fe(TMP)(5-MeHIm)2]ClO4. The X-ray structures to which
the calculated structures have been fitted are, from top to bottom, perp-
[Fe(TMP)(5-MeHIm)2]ClO4, paral-[Fe(TMP)(5-MeHIm)2]ClO4 (mol-
ecule A), and paral-[Fe(TMP)(5-MeHIm)2]ClO4 (molecule B). Hydro-
gen atoms have been omitted for clarity.
Bis-imidazole complexes of low-spin iron(III) porphyrins with
relative perpendicular axial ligand orientations and S4-ruffled
core conformations are normally observed with bulky axial
ligands, for example, [Fe(TPP)(2-MeHIm)2]ClO4,18 [Fe(TMP)-
[Fe(TMP)(5-MeHIm)2]+ (Figure 6). However, the tilt of the
axial ligands observed in the X-ray structure of perp-[Fe(TMP)-
(5-MeHIm)2]+ is not well modeled.
(56) The location of the X-ray structure of paral-[Fe(TMP)(5-MeHIm)2]-
ClO4 (molecule B) on the conformational surface is related to the position
for molecule A (point 1) by an approximate two-fold rotation about an
axis orthogonal to the map in Figure 7 centered at φ1, φ2 ) 0°.

Low-Spin [Fe(TMP)(5-MeHIm)2]ClO4
J. Am. Chem. Soc., Vol. 121, No. 48, 1999 11151
Figure 7. Plot of the change in steric energy (∆UT) as a function of axial 5-MeHIm orientation for [Fe(TMP)(5-MeHIm)2]+ and [Fe(TPP)(5-
MeHIm)2]+. A contour map of the three-dimensional surface is shown in each case. φ1 and φ2 correspond to the torsion angles Np-Fe-Nax-CIm
for the top and bottom ligands, respectively. The locations of the X-ray structures of [Fe(TMP)(5-MeHIm)2]ClO4 on the conformational surface are
shown: point 1, paral-[Fe(TMP)(5-MeHIm)2]ClO4 (molecule A); point 2, perp-[Fe(TMP)(5-MeHIm)2]+.
(1,2-Me2Im)2]ClO4,34 and [Fe(TMP)(BzHIm)2]ClO4.57 The struc-
ture of perp-[Fe(TMP)(5-MeHIm)2]ClO4 is unusual since the
axial ligands are staggered and the porphyrin S4-ruffled, even
though the 5-MeHim ligands are not particularly bulky. The
conformation of perp-[Fe(TMP)(5-MeHIm)2]ClO4 is therefore
similar to Hoard’s structure of [Fe(TPP)(HIm)2]Cl.58 Both
derivatives display staggered imidazole orientations and quasi
S4-ruffled porphyrin cores. However, the dihedral angle between
the 5-MeHIm groups of perp-[Fe(TMP)(5-MeHIm)2]ClO4 (76°)
is closer to 90° than the dihedral angle (57°) between the two
imidazole ligands of [Fe(TPP)(HIm)2]Cl.58 The porphinato core
of perp-[Fe(TMP)(5-MeHIm)2]ClO4 (|Cm| ) 0.32 (3) Å)
exhibits a similar magnitude of S4-ruffling to that of [Fe(TPP)-
(HIm)2]Cl (|Cm| ) 0.31 (3) Å). The average Fe-Np bond
lengths for the two complexes (1.981(7) Å for perp-[Fe(TMP)-
(5-MeHIm)2]ClO4 and 1.989(5) Å for [Fe(TPP)(HIm)2]Cl58) are,
within the estimated uncertainties, equivalent. This reflects the
similar degree of S4-ruffling observed in the two complexes.
However, the mean Fe-Np bond length for perp-[Fe(TMP)(5-
MeHIm)2]ClO4 is notably shorter than that typical for planar
low-spin iron(III) porphyrins (∼1.990 Å),59 consistent with
moderate S4-ruffling of the porphyrin.
respectively) from the plane of the heme normal. Such tilting
is not without precedent. Structurally characterized examples
of off-axis imidazole coordination in low-spin iron(III) porphy-
rins include [Fe(TMP)(1-MeIm)2]ClO4 (6.3° and 1.2°),15 [Fe-
(TPP)(t-MU)2]SbF6 (4°),37 and [Fe(TPP)(HIm)2]Cl (3.6° and
3.0°).16 Obviously, the tilt angle of perp-[Fe(TMP)(5-MeHIm)2]-
ClO4 is larger than the listed cases. Crystal packing effects and/
or hydrogen bonding to the N-H proton of coordinated
imidazoles may influence whether the ligands bind in an off-
axis manner since not all mono- and bis-imidazole complexes
show this type of distortion. Off-axis binding of imidazole-
(histidine) ligands has also been seen in a number of heme
protein derivatives.
The Fe-Nax bond lengths of perp-[Fe(TMP)(5-MeHIm)2]-
ClO4 are significantly different; the bond to the 5-MeHIm ligand
with φ1 ) 46° (1.957(6) Å) is shorter than the bond (1.973(6)
Å) to the trans ligand with φ2 ) 30°. This trend of longer Fe-
Nax bonds for smaller values of φ has been observed in other
bis-imidazole iron(III) porphyrinates,16,37,58,60 including the
present structure of paral-[Fe(TMP)(5-MeHIm)2]ClO4 (molecule
A). As pointed out by Hoard,58 lengthening of the Fe-Nax bonds
as φ f 0° reflects increased nonbonded repulsion between the
R-hydrogens of the axial ligands and the porphinato nitrogens.
However, it is noteworthy that the value of φ is not a quantitative
indicator of axial bond length in these complexes.53
In contrast to [Fe(TPP)(HIm)2]Cl,58 the axial imidazoles of
perp-[Fe(TMP)(5-MeHIm)2]ClO4 are tilted (by 12.5° and 7.6°,
(57) The structure of [Fe(TMP)(BzHIm)2]ClO4 has an S4-ruffled por-
phyrin core and a relative perpendicular arrangement of the axial ligands
(∆φ ) 89°). Serth-Guzzo, J. A.; Turowska-Tyrk, I.; Scheidt, W. R.
Manuscript in preparation.
(58) Collins, D. M.; Countryman, R.; Hoard, J. L. J. Am. Chem. Soc.
1972, 94, 2066-2072.
(59) Scheidt, W. R.; Reed, C. A. Chem. ReV. 1981, 81, 543-555.
The axial ligands of paral-[Fe(TMP)(5-MeHIm)2]ClO4 adopt
a relative parallel orientation. However, the 5-MeHIm groups
are somewhat staggered rather than exactly eclipsed; ∆φ ) 30°
(60) Little, R. G.; Dymock, K. R.; Ibers, J. A. J. Am. Chem. Soc. 1975,
97, 4532-4539.

11152 J. Am. Chem. Soc., Vol. 121, No. 48, 1999
Munro et al.
relative to the closest Fe-Np bond differ for the two molecules
(10 and 20° for molecule A, 12 and 14° for molecule B).
Interestingly, the porphyrin cores of both molecules show a
modest degree of S4-ruffling, with molecule A (|Cm| ) 0.16(3)
Å) somewhat more distorted than molecule B (|Cm| ) 0.07(3)
Å). The nonplanar porphyrin cores of the two structures of
paral-[Fe(TMP)(5-MeHIm)2]ClO4 (Figures 2 and 4) clearly
reflect the different axial ligand orientations in each case,
particularly since the dihedral angles between the porphyrin
mean plane and mesityl groups are similar for both molecules.
A small staggering of near-parallel axial imidazoles has been
observed in other low-spin iron(III) porphyrinates, indeed all
those for which inversion symmetry at the metal is not
crystallographically required. Thus, ∆φ is 11° in [Fe(TPP)(1-
MeIm)2]ClO4, 6° in [Fe(T-2,6-Cl2PP)(1-VinIm)2]ClO4, and 13°
in [Fe(Proto IX)(1-MeIm)2]. Interestingly, the porphyrin cores
of each exhibit modest distortions which appear to reflect (i)
the orientations of the axial ligands relative to the Fe-Np bonds
and (ii) the orientations of the meso-aryl groups relative to the
porphyrin mean plane (TPP and T-2,6-Cl2PP derivatives).
However, the contribution made by nonbonded interactions
between the meso-mesityl groups and pyrrole rings to possible
nonplanar distortions of the porphyrin core in both molecules
of paral-[Fe(TMP)(5-MeHIm)2]ClO4 is small since the mesityl
rings are not tilted to angles <77° relative to the heme plane.
Finally, the dispersion in the Fe-Np bond distances for
molecules A and B of paral-[Fe(TMP)(5-MeHIm)2]ClO4 is
small. There is no experimental evidence for an in-plane rhombic
distortion (shorter Fe-Np bonds perpendicular to the mean plane
of the axial imidazoles; longer Fe-Np bonds parallel to the mean
plane of the axial imidazoles) for either molecule. This might
have been expected given the structural data for analogous
complexes with relative parallel axial ligand orientations, for
example, [Fe(TMP)(1-MeIm)2]ClO4 (molecule 1),15 [Fe(TPP)-
(1-MeIm)2]ClO4,17 [Fe(TPP)HIm2]Cl (molecule A),16 and [Fe-
(TPP)(c-MU)2]SbF6 (molecule B),37 all of which show this type
of distortion. However, all have more planar porphyrin cores
than molecules A and B of paral-[Fe(TMP)(5-MeHIm)2]ClO4
and, with the exception of [Fe(TPP)(1-MeIm)2]ClO4,17 each
displays a value of ∆φ ) 0°. Clearly, the conformation of the
porphyrin and the magnitude of ∆φ may determine whether an
in-plane rhombic distortion in the Fe-Np bonds is observed,
especially since such distortions are probably related, in part,
to imidazole pπ-iron(III) dyz π-bonding17 which is at a
maximum when ∆φ ) 0°. Interestingly, the average Fe-Np
bond distances of 1.983(4) Å for molecule A and 1.981(5) Å
for molecule B of paral-[Fe(TMP)(5-MeHIm)2]ClO4 are shorter
than the nominal distance of 1.990 Å for planar low-spin iron-
(III) porphyrins.59 This undoubtedly reflects the small S4-ruffling
displayed by each molecule. It is nonetheless surprising that
perp-[Fe(TMP)(5-MeHIm)2]ClO4, with the most ruffled core
conformation, has a mean Fe-Np distance equivalent to that of
molecule B of paral-[Fe(TMP)(5-MeHIm)2]ClO4.
Why have we been able to isolate two crystalline forms of
[Fe(TMP)(5-MeHIm)2]ClO4 with differing relative axial ligand
orientations? Previous studies strongly suggest that the preferred
relative ligand orientation for bis(imidazole)iron(III) porphyri-
nates, in the absence of introduced steric effects, is the parallel
orientation. The solid-state structure of paral-[Fe(TMP)(5-
MeHIm)2]ClO4 is observed to be in a hydrogen-bonding network
and this feature might be considered responsible for the parallel
orientation of the axial ligands. However, the observation of
solution-state, rhombic EPR spectra under conditions where
hydrogen bonding can not be present as well as when it is
Figure 8. Contour maps of (a) the average Fe-Np bond distance, (b)
the difference between the mean Fe-Np bond distance along the x-
and y-directions of the heme group, ∆xy, and (c) the average Fe-Nax
bond distance with the orientation of the axial 5-MeHIm ligands for
[Fe(TMP)(5-MeHIm)2]+.
for molecule A and 26° for molecule B. Moreover, the
projections of the axial ligand planes onto the porphyrin core

Low-Spin [Fe(TMP)(5-MeHIm)2]ClO4
J. Am. Chem. Soc., Vol. 121, No. 48, 1999 11153
present (vide infra) suggests that the observed solid-state
H-bonding is not required for the parallel ligand orientation for
this ligand combination. Thus an explanation of why the relative
perpendicular orientation can be observed must be sought. The
preparation of crystalline perp-[Fe(TMP)(5-MeHIm)2]ClO4
requires the presence of an “impurity” ligand, the weakly basic,
strong π-accepting ligand 4-cyanopyridine as well as 4(5)-
MeHIm. Similar ligand combinations can give rise to a mixed
ligand complex in which pyridine and imidazole have a relative
perpendicular orientation.61 In the current circumstances, we
believe that a small amount of a mixed ligand complex with
relative perpendicular orientations provides a template for the
crystallization of perp-[Fe(TMP)(5-MeHIm)2]ClO4. It is to be
emphasized that the procedure described is quite reproducible,
even if empirical.
EPR and Mo1ssbauer Spectroscopy. The EPR spectrum of
polycrystalline perp-[Fe(TMP)(5-MeHIm)2]ClO4 is of the “large
g
max” spectral type,14 consistent with a near perpendicular
arrangement for the axial 5-MeHIm groups15,19 and near-
degeneracy of the dxz and dyz orbitals in a (dxy)2(dxz,dyz)3 ion.
The value of gmax (3.43) is bracketed by the g-values reported
for polycrystalline samples of [Fe(TPP)(2-MeHIm)2]ClO4 (gmax
) 3.56)18 and [Fe(TMP)(2-MeHIm)2]ClO4 (gmax ) 3.17),19 both
of which have relative perpendicular axial ligand orientations.62
Moreover, several structurally characterized bis-pyridine deriva-
tives with relative perpendicular axial ligand orientations,
including [Fe(TMP)(4-NMe2Py)2]ClO4 (gmax ) 3.48)15 and [Fe-
(TMP)(4-NH2Py)2]ClO4 (gmax ) 3.40),19 have gmax-values
bracketing those of the gmax of perp-[Fe(TMP)(5-MeHIm)2]-
ClO4. It is interesting that the large gmax spectral type is observed
and is not significantly different in g-value from the bis-ligand
complexes having dihedral angles of very close to 90°, when
the dihedral angle of perp-[Fe(TMP)(5-MeHIm)2]ClO4 is only
76°.
Figure 9. X-band EPR spectra of [Fe(TMP)(5-MeHIm)2]ClO4 in 3:1
DMF-acetonitrile glasses at 4.2 K. The top spectrum has an imida-
zole-Fe ratio of 2:1; the bottom panel has an imidazole-Fe ratio of
60:1.
The EPR g-values for paral-[Fe(TMP)(5-MeHIm)2]ClO4 (g1
) 2.69, g2 ) 2.34-2.43, and g3 ) 1.75) show a smaller spread
than the range found for other structurally characterized low-
spin bis-imidazole iron(III) porphyrinates with relative parallel
axial ligand orientations, including various cytochromes b5 (g1
) 3.03-3.07, g2 ) 2.22-2.24, g3 ) 1.35-1.46, V/λ ) 1.55-
1.71, ∆ /λ ) 3.00-3.32, V/∆ ) 0.51-0.52),65 [Fe(TMP)(1-
MeIm)2]ClO4 (g1 ) 2.886, g2 ) 2.325, g3 ) 1.571; V/λ ) 2.07,
∆ /λ ) 3.09, V/∆ ) 0.67),15 [Fe(TPP)(HIm)2]Cl (g1 ) 2.84, g2
) 2.32, g3 ) 1.59; V/λ ) 2.16, ∆/λ ) 3.12, V/∆ ) 0.69),16 and
[Fe(TPP)(1-MeIm)2]ClO4 (g1 ) 2.866, g2 ) 2.276, g3 ) 1.535,
V/λ ) 2.01, ∆/λ ) 3.16, V/∆ ) 0.64),17 and are close to those
shown by bis-imidazolate complexes. An investigation of the
EPR spectrum of [Fe(TMP)(5-MeHIm)2]ClO4 in 3:1 DMF-
acetonitrile glasses as a function of 5-MeHIm concentration
showed that at a ligand-Fe ratio of ∼2, the observed g-values
were 2.89, 2.31, and 1.58, yielding V/λ ) 2.07, V/λ ) 3.22,
and V/∆ ) 0.64. As the ligand-Fe ratio was increased, a new
rhombic signal began to grow until at a ratio of ∼60:1 only the
new signal was observed. Its g-values were 2.64, 2.30, and 1.80,
yielding V/λ ) 3.10, ∆/λ ) 4.09, and V/∆ ) 0.76. Figure 9
illustrates these limiting EPR spectra. These values are much
more reasonable for a bis-imidazolate complex and we thus
conclude that the middle g-value observed for the crystalline
sample is skewed by a non-random orientation of the molecules
in the EPR sample. The value of ∆/λ observed for the crystalline
sample (2.83) is much too small for imidazolate, which is a
stronger field ligand than neutral imidazole, for which values
Effective axial electronic symmetry at the metal occurs when
the rhombic splitting, V, is much smaller than the tetragonality,
∆.63 An important consequence is that the electric field gradient
(EFG) at the iron nucleus is reduced below the single-electron
value and smaller quadrupole splittings (∆Eq < 2.0 mm/s)15,19,64
are observed in the 57Fe Mo¨ssbauer spectra of such species. In
contrast, ∆Eq values >2.0 are consistent with a relative parallel
axial ligand orientation. The relative orientations of the axial
ligands in bis-imidazole iron(III) porphyrinates may therefore
be assigned from a Mo¨ssbauer spectrum taken under conditions
of small applied magnetic field. Together with EPR and
structural data, a definitive correlation between the relative
orientations of the axial ligands and the electronic structure of
the metal may be established. In the case of perp-[Fe(TMP)-
(5-MeHIm)2]ClO4, the small Mo¨ssbauer quadrupole splitting
(∆Eq ) 1.78(1) mm/s) and “large gmax” EPR spectral type
provide unambiguous evidence that a relative perpendicular axial
ligand orientation leads to near degeneracy of the dxz and dyz
orbitals.
(61) Scheidt, W. R.; Serth-Guzzo, J.; Turowska-Tyrk, I.; Safo, M. K.;
Walker, F. A.; Debrunner, P. G. Abstracts of Papers; 208th National Meeting
of the American Chemical Society, Washington, D. C., Aug 21-25, 1994;
American Chemical Society: Washington, D.C., 1994; INORG 376.
(62) The X-ray structure of [Fe(TMP)(2-MeHIm)2]ClO4 has not been
determined. However, the 1H NMR spectrum of this species suggests an
S4-ruffled porphyrin conformation in solution at -74 °C.38
(63) Values of V and ∆ were calculated following Taylor. Taylor, C. P.
S. Biochim. Biophys. Acta 1977, 491, 137-148. We have assumed that “z”
is along the heme normal to calculate the ligand field parameters.
(64) (a) Medhi, O. K.; Silver, J. J. Chem. Soc., Dalton Trans. 1990, 263-
270. (b) Medhi, O. K.; Silver, J. J. Chem. Soc., Dalton Trans. 1990, 555-
559.
(65) (a) Bois-Poltoratsky, R.; Ehrenberg, A. Eur. J. Biochem. 1967, 2,
361-365. (b) Passon, P. G.; Reed, D. W.; Hultquist, D. E. Biochim. Biophys.
Acta 1972, 275, 51-61. (c) von Bodman, S. B.; Schuler, M. A.; Jollie, D.
R.; Sligar, S. G. Proc. Natl. Acad. Sci. U.S.A. 1986, 83, 9443-9447. (d)
Rivera, M.; Barillas-Mury, C.; Christensen, K. A.; Little, J. W.; Wells, M.
A.; Walker, F. A. Biochemistry 1992, 31, 12233-12240. (e) Guzov, V.
M.; Houston, H. L.; Murataliev, M. B.; Walker, F. A.; Feyereisen, R. J.
Biol. Chem. 1996, 271, 26637-26645.

11154 J. Am. Chem. Soc., Vol. 121, No. 48, 1999
Munro et al.
of 3.00-3.32 are common (vide supra). Thus the frozen solution
value of ∆/λ ) 4.09 is much more consistent with the
imidazolate formulation. This is an example of the distortion
of the EPR spectra of powdered solid samples by non-random
orientation of crystallites, and points out the importance of
parallel solution measurements.
(4-NMe2Py)2]+ (0.7-0.9)15 and [Fe(TPP)(2-MeHIm)2]+ (0.9).13
A lower limit of ∆/λ for perp-[Fe(TMP)(5-MeHIm)2]ClO4
would be ∼0.4, given the magnitude of the spitting observed
in [Fe(TPP)(CN)2]- (0.44).10
The Jahn-Teller stabilization energy thus depends on the
magnitude of the spin-orbit coupling constant, λ. For Fe3+ (g),73
λ is 460 cm-1 and is expected to be reduced to between ∼300
and 400 cm-1 for ferrihemes.13,74 For perp-[Fe(TMP)(5-MeHIm)2]-
ClO4, the crystal field stabilization energy can then be estimated
at e0.9-1.2 kcal/mol, while for paral-[Fe(TMP)(5-MeHIm)2]-
ClO4 the estimated value will be 1.8-3.9 kcal/mol. The
electronic stabilization for a relative parallel orientation of the
axial ligands is thus less than ∼3.0 kcal/mol.
Using the solution g-values, it is interesting to note that ∆/λ
(4.09) for [Fe(TMP)(5-MeHIm)2]ClO4 is close to that typical
for iron(III) porphyrinates with the mixed axial ligand combina-
tion imidazole/imidazolate (∆/λ ) 4.44)66 and smaller than that
with two axial imidazolate ligands, for example, [(K222)][Fe-
(TPP)(5-MeIm)2] (∆/λ ) 4.94).67 The value of ∆/λ for [Fe-
(TMP)(5-MeHIm)2]ClO4 is similar to the values reported for
several heme proteins at high pH, for example, cytochrome b5
at pH 11.5 (∆/λ ) 3.87, histidine/histidinate form)68 and the
5-MeIm- complex of metmyoglobin at pD 10.8 (∆/λ ) 4.29
histidine/imidazolate).69 This suggests that H-bonding between
the coordinated 5-MeHIm ligands of paral-[Fe(TMP)(5-MeHIm)2]-
ClO4 and the “solvent” 4(5)-MeHIm molecules within the
asymmetric unit indeed imparts partial imidazolate character
to at least one axial ligand of each cation.
There are two final considerations that should be drawn from
the EPR data. As has been noted, the axial ligand dihedral angles
in the two crystalline forms deviate significantly from the ideal
values for perpendicular and parallel orientations. Nonetheless,
the complexes show limiting type EPR spectra, with the solid-
state EPR spectrum of perp-[Fe(TMP)(5-MeHIm)2]ClO4 (di-
hedral angle ) 76°) that of a large gmax signal while that for
paral-[Fe(TMP)(5-MeHIm)2]ClO4 (dihedral angle ) 30°, two
molecules) is rhombic. This leads to the question of where the
dividing line in EPR spectral type as a function of the ligand
orientation is to be found, and thus what the histidine dihedral
angles may be for the membrane-bound cytochromes b of the
mitochondrial electron-transport chain and the similar cyto-
chrome b6 heme centers found in chloroplasts.70 Structural data
for the former are approaching a resolution where it may soon
be possible to determine these angles.71,72 It is unfortunate that
the EPR spectral type of the first reported imidazole derivative,
[Fe(TPP)(HIm)2]Cl,58 with its 57° imidazole dihedral angle, was
not reported. (To this day it has not been possible to regrow
crystals of this particular form.)
Molecular Mechanics Calculations. Two objectives of this
study were to determine how steric interactions affect the relative
stabilities of isomers of [Fe(TMP)(5-MeHIm)2]+ (and related
species such as [Fe(TPP)(5-MeHIm)2]+) and to understand how
conformational changes might affect the coordination geometry
of the iron(III) ion in [Fe(TMP)(5-MeHIm)2]+.
The conformational surfaces for [Fe(TMP)(5-MeHIm)2]+ and
[Fe(TPP)(5-MeHIm)2]+ shown in Figure 7 differ mainly in the
number of maxima and minima. Although both have global
minima at φ1, φ2 ) 45° (S4-ruffled core conformation), the steric
energy for [Fe(TMP)(5-MeHIm)2]+ reaches a maximum when
the axial ligands have a relative parallel orientation and point
at the meso-mesityl groups (planar core conformation; φ1, φ2
) 45°, 135°). For [Fe(TPP)(5-MeHIm)2]+, this region clearly
corresponds to a local minimum. Since a plot of the change in
nonbonded energy with axial ligand orientation (Figure S1) for
[Fe(TMP)(5-MeHIm)2]+ also reaches a maximum at this point
(φ1, φ2 ) 45°, 135°), nonbonded interactions between the axial
5-MeHIm ligands and the o-methyl groups of TMP destabilize
a relative parallel orientation for the axial ligands.75 A relative
parallel axial ligand orientation is also destabilized when the
ligands eclipse a pair of trans Fe-Np bonds (e.g., when φ1, φ2
) 0°, 180°).
Interestingly, the conformation observed in the X-ray structure
of perp-[Fe(TMP)(5-MeHIm)2]+ lies ∼0.4 kcal/mol from the
global minimum (point 2 in Figure 7). The marked stability of
this conformation relative to all other conformational isomers
is brought about by S4-ruffling of the porphyrin core (Table
3).34 Point 1 in Figure 7, in contrast, shows the location of
molecule A of the X-ray structure of paral-[Fe(TMP)(5-
MeHIm)2]ClO4 on the potential surface.56 This conformation
is ∼2.6 kcal/mol higher in energy than the global minimum
and is located close to the local maximum that has the imidazole
planes eclipsed and positioned over a pair of trans Fe-Np
vectors (φ1, φ2 ) 0°, 180°). Since the axial 5-MeHIm ligands
are close to eclipsing a pair of trans Fe-Np bonds at point 1,
nonbonded interactions between the imidazole R-hydrogens and
the porphyrin nitrogens (Figure S1) account for the higher steric
energy of this conformer relative to that of perp-[Fe(TMP)(5-
MeHIm)2]ClO4.
The major change in electronic structure that accompanies
changes in the axial ligand dihedral angles is the energy
difference between the dyz and dxz orbitals, e.g., the Jahn-Teller
stabilization energy. A convenient measure of the relative Jahn-
Teller stabilization energy, ∆/λ, can be obtained from EPR
spectra. ∆/λ for complexes with relative parallel orientations is
in the range 2-3.3. For the particular case of paral-[Fe(TMP)-
(5-MeHIm)2]ClO4 in solution, ∆/λ is in the range of 2.1 to 3.1.
Although ∆/λ cannot be determined from the powder EPR
spectrum of perp-[Fe(TMP)(5-MeHIm)2]ClO4, we estimate that
∆/λ e 1 based on the values found previously for [Fe(TMP)-
(66) Quinn, R.; Nappa, M.; Valentine, J. S. J. Am. Chem. Soc. 1982,
104, 2588-2595.
Varying the axial ligand orientations of [Fe(TMP)(5-Me-
HIm)2]+, and thus the porphyrin conformation, perturbs the
coordination geometry of the metal ion.76 Figure 8a shows that
(67) Quinn, R.; Strouse, C. E.; Valentine, J. S. Inorg. Chem. 1983, 22,
3934-3940.
(68) Bois-Poltoratsky, R.; Ehrenberg, A. Eur. J. Biochem. 1967, 2, 361-
365.
(73) Figgis, B. N.; Lewis, J. In Techniques of Inorganic Chemistry;
Jonassen, H. B., Weissberger, A., Eds.; Wiley Interscience: New York,
1965; Vol. IV, p 159.
(74) Maltempo, M. M. J. Chem. Phys. 1974, 61, 2540-2547. Levin, P.
D.; Brill, A. S. J. Phys. Chem. 1988, 92, 5103-5110.
(75) Previously we found that nonbonded interactions between the
porphyrin mesityl substituents and pyridine ligands in low-spin bis-pyridine
derivatives of [Fe(TMP)]+ favor a relative perpendicular axial ligand
orientation,19,35 consistent with the conformational surface for [Fe(TMP)-
(5-MeHIm)2]+ shown in Figure 7.
(69) Gadsby, P. M. A.; Thomson, A, J. J. Am. Chem. Soc. 1990, 112,
5003-5011.
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Low-Spin [Fe(TMP)(5-MeHIm)2]ClO4
J. Am. Chem. Soc., Vol. 121, No. 48, 1999 11155
the calculated average Fe-Np bond length increases from
∼1.965 Å for an S4-ruffled conformation (φ1, φ2 ) 45°) to
∼1.985 Å for a planar conformation (φ1, φ2 ) 45°, 135°). This
trend is in fact observed experimentally. For example, the mean
Fe-Np bond distance varies from 1.937(12) Å in the strongly
only in special circumstancessprimarily that of introduced steric
effects. However, [Fe(TMP)(5-MeHIm)2]ClO4 has been obtained
in two distinct crystalline forms with different relative ligand
orientations. One form, perp-[Fe(TMP)(5-MeHIm)2]ClO4, has
a relative perpendicular axial ligand arrangement, a small
Mo¨ssbauer quadrupole splitting, and a large gmax EPR spectrum
with a single observable g-value. The second form, paral-[Fe-
(TMP)(5-MeHIm)2]ClO4, has a near-parallel axial ligand ori-
entation, a large Mo¨ssbauer quadrupole splitting, and a EPR
spectrum with a rhombic g tensor. The appearance, in the solid
state, of ligand orientations approaching the two limiting forms
speaks to a near energetic equivalence of conformational
isomers. The energy balance between the two forms is the result
of crystal field stabilization effects favoring the parallel form
and steric strain effects that favor the perpendicular form.
Estimates of the two opposing energetic effects are both less
than 3.0 kcal/mol. Indeed, stabilization energy estimates are so
close that it is easily seen that the energy balance may shift in
favor of either conformation depending on factors such as the
degree of ruffling of the porphyrin ligand, the nature of the
peripheral substituents on the porphyrin ligand including the
size of the ortho substituents of the meso-aryl groups, and
variations in the actual dihedral angle, ∆φ, between the axial
ligand planes.
34
S4-ruffled complex [Fe(TMP)(1,2-Me2Im)2]ClO4 to 1.987(1)
Å in [Fe(TMP)(1-MeIm)2]ClO4 (planar core conformation).15
The difference in the mean calculated bond length for
orthogonal pairs of trans Fe-Np bonds (i.e., those along the x-
and y-directions) also depends on ligand orientation. Figure 8b
indicates that this difference, ∆xy, becomes largest when the
axial ligands have a relative parallel orientation and nearly
eclipse a pair of trans Fe-Np bonds (e.g., when φ1, φ2 ) 18°,
162°). Unfavorable nonbonded interactions between the axial
ligands and eclipsed atoms of the porphyrin core are apparently
minimized by slight adjustment of the Fe-Np bonds. Thus, some
asymmetry in metal-porphyrin bonding may be introduced by
changes in porphyrin-ligand nonbonded interactions. However,
although the X-ray structures of [Fe(TPP)(HIm)2]+ 16 and [Fe-
(TPP)(t-MU)2]+ 37 show such a distortion, the Fe-Np bonds of
the X-ray structures of paral-[Fe(TMP)(5-MeHIm)2]+ do not
exhibit an experimentally significant degree of asymmetry.
Finally, Figure 8c shows that the average calculated Fe-Nax
bond distance increases from 1.958 Å in the S4-ruffled confor-
mation of [Fe(TMP)(5-MeHIm)2]+ (φ1, φ2 ) 45°) to 1.981 Å
when the planes of the axial ligands are parallel and eclipse a
pair of trans Fe-Np bonds (φ1, φ2 ) 0°, 180°). These structural
changes largely reflect the increase in ligand-porphyrin non-
bonded interactions as the ligand planes switch from a staggered
arrangement over an S4-ruffled core to an eclipsed arrangement
over a pair of trans Fe-Np vectors. Although φ is not a
quantitative predictor of Fe-Nax bond length,53 experimental
evidence for variation of the mean Fe-Nax distance with ligand
orientation exists. For example, the X-ray structures of [Fe-
(TPP)(HIm)2]Cl16 have mean Fe-Nax bond distances of 1.964-
(3) and 1.977(3) Å for axial imidazole orientations of 41° and
6°, respectively.
Acknowledgment. We thank the National Institutes of
Health for support of this research under Grants GM-38401
(W.R.S.), DK-31038 (F.A.W.), and GM-16406 (P.G.D.). Funds
for the purchase of the FAST area detector diffractometer were
provided through NIH Grant RR-06709.
Supporting Information Available: Complete crystal-
lographic details, final atomic coordinates, anisotropic thermal
parameters, fixed hydrogen atom positions, and fractional
coordinates for all non-hydrogen atoms of paral-[Fe(TMP)(5-
MeHIm)2]ClO4 and perp-[Fe(TMP)(5-MeHIm)2]ClO4 are given
in Tables S1-S11; Table S12 gives a table of hydrogen bond
distances observed in the solid-state structure of paral-[Fe-
(TMP)(5-MeHIm)2]ClO4; Figure S1 is a plot of the change in
van der Waals energy (kcal/mol) for [Fe(TMP)(5-MeHIm)2]+
with axial ligand orientation (PDF). This material is available
free of charge via the Internet at http://pubs.acs.org.
Ligand Orientation Preferences: Final Comments. Rela-
tive parallel orientations are the apparently preferred orientation
for most characterized iron(III) porphyrinates having two planar
axial ligands, with relative perpendicular orientations observed
(76) A noteworthy caveat here is that the calculated changes in the Fe-
Np and Fe-Nax bond lengths do not include contributions from imidazole-
and porphyrin-iron(III) π-bonding.12
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