Low-spin bis(2-methylimidazole)(octaethylporphyrinato)iron(III) chloride (perp-[Fe(OEP)(2-MeHIm)2]Cl): A consequence of hydrogen bonding?

Article abstract of DOI:10.1021/ic061014u

The synthesis and characterization of low-spin bis(2-methylimidazole) (octaethylporphyrinato)iron(III) chloride (perp-[Fe(OEP)(2-MeHIm) ₂]Cl) is reported. The structure shows that the cation is a low-spin species with two imidazole ligands having a relative perpendicular orientation. The porphyrin core is very ruffled, which leads to shortened equatorial bonds of 1.974(4) A and slightly elongated axial Fe-N bond lengths of 2.005(10) A that are about 0.02 A shorter and 0.03 A longer, respectively, in comparison to bis-imidazole ligated iron(III) species with parallel oriented axial ligands. A one-dimensional hydrogen-bond chain is formed between chloride anions and uncoordinated imidazole nitrogen atoms. Compared with paral-[Fe(OEP)(2-MeHIm)₂]ClO₄, hydrogen bonding may play an important role in the differences in the two structures. Moessbauer spectra show broadened quadrupole doublets with quadrupole splittings of 1.81 mm/s at RT and 1.94 mm/s at 20 K. The isomer shift ranges from 0.26 to 0.36 mm/s. These confirm that the title complex is a low-spin iron(III) species with the ground state (d_xy)²(d_xz,d _yz)³. Crystal data: monoclinic, space group P2 ₁/c, a = 14.066(3) A, b, 20.883(4) A, c = 19.245(4) A, β = 109.67°, and Z = 4.

Full text of DOI:10.1021/ic061014u

Inorg. Chem. 2006, 45, 9721−9728
Low-Spin Bis(2-methylimidazole)(octaethylporphyrinato)iron(III) Chloride
(perp-[Fe(OEP)(2-MeHIm)2]Cl): A Consequence of Hydrogen Bonding?
Chuanjiang Hu,† Bruce C. Noll,† Charles E. Schulz,‡ and W. Robert Scheidt*,†
Department of Chemistry and Biochemistry, UniVersity of Notre Dame,
Notre Dame, Indiana 46556, and Department of Physics, Knox College, Galesburg, Illinois 61401
Received June 7, 2006
The synthesis and characterization of low-spin bis(2-methylimidazole)(octaethylporphyrinato)iron(III) chloride (perp-
[Fe(OEP)(2-MeHIm)2]Cl) is reported. The structure shows that the cation is a low-spin species with two imidazole
ligands having a relative perpendicular orientation. The porphyrin core is very ruffled, which leads to shortened
equatorial bonds of 1.974(4) Å and slightly elongated axial Fe−N bond lengths of 2.005(10) Å that are about 0.02
Å shorter and 0.03 Å longer, respectively, in comparison to bis-imidazole ligated iron(III) species with parallel
oriented axial ligands. A one-dimensional hydrogen-bond chain is formed between chloride anions and uncoordinated
imidazole nitrogen atoms. Compared with paral-[Fe(OEP)(2-MeHIm)2]ClO4, hydrogen bonding may play an important
role in the differences in the two structures. Mo¨ssbauer spectra show broadened quadrupole doublets with quadrupole
splittings of 1.81 mm/s at RT and 1.94 mm/s at 20 K. The isomer shift ranges from 0.26 to 0.36 mm/s. These
confirm that the title complex is a low-spin iron(III) species with the ground state (dxy)2(dxz,dyz)3. Crystal data:
monoclinic, space group P21/c, a
) 14.066(3) Å, b, 20.883(4) Å, c ) 19.245(4) Å, â ) 109.67°, and Z ) 4.
Introduction
conformation in which the dihedral angle between the
histidine planes is closer to a relative perpendicular arrange-
ment (64°). The relative orientation of the two imidazole
planes with respect to each other has been believed to play
an important role in modeling the spectroscopic properties
(such as EPR) and possibly also the redox properties of these
heme proteins.
Bis-histidine-coordinated hemes have been of interest due
to their importance in electron-transfer biological systems.
In current protein structures, two limiting orientations of the
axial ligand planes are found: imidazole planes oriented
parallel to each other (cytochromes b5,1 three of the heme
centers of cytochromes c3,2-4 the b hemes of sulfite oxidase5
and flavocytochrome b2,6 and the heme a of cytochrome
oxidase7) while one of the four heme groups in cytochrome
c3 from DesulfoVibrio Vulgaris has a more staggered
The orientations of planar axial ligands in bis-ligated
hemes of iron(III) have been intensively investigated. The
earliest studies showed that the relative and absolute orienta-
tions of the two axial ligands have significant effects on the
electronic structure. The first example was that of [Fe(OEP)-
(3-ClPy)2]ClO4,8 where two different crystalline polymorphs
were isolated. The polymorphs are those of a triclinic, spin-
* To whom correspondence should be addressed. E-mail: scheidt.1@
nd.edu.
† University of Notre Dame.
‡ Knox College.
(1) Mathews, F. S.; Czerwinski, E. W.; Argos, P. In The Porphyrins;
Dolphin, D., Ed.; Academic Press: New York, 1979; Vol. VII, p 108.
(2) Pierrot, M.; Haser, R.; Frey, M.; Payan, F.; Astier, J.-P. J. Biol. Chem.
1982, 257, 14341.
(3) Higuchi, Y.; Kusunoki, M.; Matsuura, Y.; Yasuoka, N.; Kakudo, M.
J. Mol. Biol. 1984, 172, 109.
(4) Czjzek, M.; Guerlesquin, F.; Bruschi, M.; Haser, R. Structure 1996,
4, 395.
(5) Kipke, C. A.; Cusanovich, M. A.; Tollin, G.; Sunde, R. A.; Enemark,
J. H. Biochemistry 1988, 27, 2918.
(6) (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.
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(7) Iwata, S.; Ostermeier, C.; Ludwig, B.; Michel, H. Nature (London)
1995, 376, 660.
(8) The following abbreviations are used in this paper: TMP, dianion of
meso-tetramesitylporphyrin; TPP, dianion of meso-tetraphenylporphy-
rin; Proto IX, dianion of protoporphyrin IX.; t-Mu, trans-methyluro-
canate; c-Mu, cis-methylurocanate; OMTPP, dianion of octameth-
yltetraphenylporphyrin;OETPP,dianionofoctaethyltetraphenylporphyrin;
RIm, a generalized hindered imidazole; HIm, imidazole; 1-MeIm,
1-methylimidazole; 2-MeHIm, 2-methylimidazole; 4-MeHIm, 4-me-
thylimidazole; 5-MeHIm, 5-methylimidazole; 1,2-Me2Im, 1,2-di-
methylimidazole; Py, pyridine; 3-ClPy, 3-chloropyridine; 4-CNPy,
4-cyanopyridine; 3-CNPy, 3-cyanopyridine; 4-MePy, 4-methylpyri-
dine; 3-EtPy, 3-ethylpyridine; 4-NMe2Py, 4-(dimethylamino)pyridine;
Np, porphyrinato nitrogen; Ct, the center of four porphyrinato nitrogen
atoms; NAx, nitrogen of axial ligands; Nim, nitrogen of imidazole
ligands; Cim, carbon of imidazole ligands.
10.1021/ic061014u CCC: $33.50
Published on Web 10/27/2006
© 2006 American Chemical Society
Inorganic Chemistry, Vol. 45, No. 24, 2006 9721

Hu et al.
equilibrium (S ) 1/2 h S ) 5/2)9,10 system and a monoclinic,
intermediate-spin system, S ) 3/2.11 In these complexes, the
two pyridines maintain a relative parallel orientation but the
absolute orientation of the two axial ligands with respect to
the porphinato core changed. Although this system showed
large changes in the electronic structure of iron(III), control-
ling axial ligand orientation more likely will affect the
relative energies of the three lowest d-orbitals of iron, namely
the dxy, dxz, and dyz orbitals.
identical axial ligands that have different structure and/or
physical properties have been studied.18,24,25 Both [Fe(TPP)-
(HIm)2]+ and [Fe(TMP)(5-MeIm)2]+ have been shown to
form two different low-spin crystalline species. They are
structurally distinguished by the relative orientation of the
two axial ligands. We have synthesized and structurally
characterized a new bis-ligated complex, perp-[Fe(OEP)(2-
MeHIm)2]Cl. Interestingly, this new complex is a low-spin
species with two perpendicular ligands that is distinct from
the related complex paral-[Fe(OEP)(2-MeHIm)2]ClO4 26 with
parallel axial ligands and a high-spin state. As expected from
analogous iron(III) species, the porphyrin core in this new
species is necessarily ruffled to accommodate the two
sterically hindered ligands. The solid-state structure shows
that the coordinated imidazole ligands form hydrogen bonds
with the chloride anions and provides a probable explanation
for the apparent stronger ligand field observed in this
complex.
In those model compounds, most bis-ligated d5 iron(III)
complexes have a characteristic rhombic EPR spectrum,12
with three observed g values, while some iron(III) species
have an unusual EPR spectrum: a single-feature low-spin
EPR signal with g g 3.2. This EPR spectral type has been
called large gmax13 or highly anisotropic low-spin (HALS).14
The origin of the large gmax EPR spectrum was first studied
in the complex [Fe(TPP)(2-MeHIm)2]+.15,16 This study
showed that the spectrum resulted from mutually perpen-
dicular axial ligands that lead to nearly degenerate iron dπ
orbitals. Subsequently, a number of additional iron(III)
species were shown to have the two planar ligands oriented
perpendicular to each other.17-19,22 Most of these species
display a large gmax EPR spectrum17,18,20,23 and an unusually
small value of the Mo¨ssbauer quadrupole splitting con-
stant.16,17,19,21,22 A final case is found for strong π-accepting
ligands, such as 3- and 4-cyanopyridine, where the interaction
with axial ligands lowers the energy of iron dπ orbitals below
dxy so that the ground state changes to (dxz,dyz)4(dxy)1. This
leads to a final type of EPR spectrum observed in low-spin
iron(III): an axial EPR spectrum. All of the complexes with
relative perpendicular ligands are found to have strongly
ruffled porphyrinato cores. For these systems, this conforma-
tion allows for the possibility of a π interaction between iron
and the porphyrin.
Experimental Section
General Information. All solvents were used as received. The
free-base porphyrin octaethylporphyrin (H2OEP)8 was purchased
from Mid-century. The metalation of the free-base porphyrin to
give [Fe(OEP)Cl] was done as previously described.27 [Fe(OEP)]2O
was prepared according to a modified Fleischer preparation.28
Synthesis of perp-[Fe(OEP)(2-MeHIm)2]Cl. To a solution of
[Fe(OEP)Cl] (30 mg, 0.05 mmol) in 10 mL of chloroform,
2-methylimidazole (20 mg, 0.24 mmol) was added. The mixture
was stirred for 4 h, then transferred into several 8 mm × 250 mm
glass tubes, and layered by hexanes as nonsolvent for crystallization.
After 10 days, block crystals were formed along with variable
amount of microcrystalline materials. Samples for Mo¨ssbauer
spectroscopy were made from single crystals enriched to 95% in
57Fe. In addition, Mo¨ssbauer spectroscopy verified that the micro-
crystalline material was identical to the block crystals.
X-ray Structure Determination. A dark red crystal with the
dimensions 0.46 × 0.20 × 0.12 mm3 was used for the structure
determination. The single-crystal experiment was carried out on a
Bruker Apex system with graphite-monochromated Mo KR radiation
(λ ) 0.71073 Å). The crystalline sample was placed in inert oil,
mounted on a glass pin, and transferred to the cold gas stream of
the diffractometer. Crystal data were collected at 100 K, Table 1.
The structure was solved by direct methods using SHELXS-
9729 and refined against F 2 using SHELXL-97;30,31 subsequent
difference Fourier syntheses led to the location of most of the
remaining nonhydrogen atoms. For the structure refinement all data
were used including negative intensities. The structure was refined
in space group P21/c. The program SADABS32 was applied for
In addition to the above-mentioned [Fe(OEP)(3-ClPy)2]-
ClO4,8 several other examples of iron(III) porphyrins with
(9) Scheidt, W. R.; Geiger, D. K. J. Chem. Soc., Chem. Commun. 1979,
1154.
(10) Scheidt, W. R.; Geiger, D. K.; Haller, K. J. J. Am. Chem. Soc. 1982,
104, 495.
(11) Scheidt, W. R.; Geiger, D. K.; Hayes, R. G.; Lang, G. J. Am. Chem.
Soc. 1983, 105, 2625.
(12) Blumberg, W. E.; Peisach, J. AdV. Chem. Ser. 1971, 100, 271.
(13) Walker, F. A.; Reis, D.; Balke, V. L. J. Am. Chem. Soc. 1984, 106,
6888.
(14) Magita, C. T.; Iwaizumi, M. J. Am. Chem. Soc. 1981, 103, 4378.
(15) Scheidt, W. R.; Kirner, J. L.; Hoard, J. L.; Reed, C. A. J. Am. Chem.
Soc. 1987, 109, 1963.
(16) Walker, F. A.; Huynh, B. H.; Scheidt, W. R.; Osvath, S. R. J. Am.
Chem. Soc. 1986, 108, 5288.
(17) Safo, M. K.; Gupta, G. P.; Walker, F. A.; Scheidt, W. R. J. Am. Chem.
Soc. 1991, 113, 5497.
(18) Munro, O. Q.; Serth-Guzzo, J. A.; Turowska-Tyrk, I.; Mohanrao, K.;
Shokhireva, T. Kh.; Walker, F. A.; Debrunner, P. G.; Scheidt, W. R.
J. Am. Chem. Soc. 1999, 121, 11144.
(24) Scheidt, W. R.; Osvath, S. R.; Lee, Y. J. J. Am. Chem. Soc. 1987,
109, 1958.
(25) Collins, D. M.; Countryman, R.; Hoard, J. L. J. Am. Chem. Soc. 1972,
94, 2066.
(26) Geiger, D. K.; Lee, Y. J.; Scheidt, W. R. J. Am. Chem. Soc. 1984,
106, 6339-6343.
(19) Munro, O. Q.; Marques, H. M.; Debrunner, P. G.; Mohanrao, K.;
Scheidt, W. R. J. Am. Chem. Soc. 1995, 117, 935.
(20) Inniss, D.; Soltis, S. M.; Strouse, C. E. J. Am. Chem. Soc. 1988, 110,
5644.
(21) Safo, M. K.; Gupta, G. P.; Watson, C. T.; Simonis, U.; Walker, F.
A.; Scheidt, W. R. J. Am. Chem. Soc. 1992, 114, 7066.
(22) 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.
(27) (a) Adler, A. D.; Longo, F. R.; Kampus, F.; Kim, J. J. Inorg. Nucl.
Chem. 1970, 32, 2443. (b) Buchler, J. W. In Porphyrins and
Metalloporphyrins; Smith, K. M., Ed.; Elsevier Scientific Publishing:
Amsterdam, The Netherlands, 1975; Chapter 5.
(28) (a) Fleischer, E. B.; Srivastava, T. S. J. Am. Chem. Soc. 1969, 91,
2403. (b) Hoffman, A. B.; Collins, D. M.; Day, V. W.; Fleischer, E.
B.; Srivastava, T. S.; Hoard, J. L. J. Am. Chem. Soc. 1972, 94, 3620.
(29) Sheldrick, G. M. Acta Crystallogr. 1990, A46, 467.
(30) Sheldrick, G. M. Program for the Refinement of Crystal Structures;
Universita¨t Go¨ttingen: Germany, 1997.
(23) Walker, F. A. Chem. ReV. 2004, 104, 589.
9722 Inorganic Chemistry, Vol. 45, No. 24, 2006

Low-Spin (perp-[Fe(OEP)(2-MeHIm)2]Cl)
Table 1. Complete Crystallographic Details for [Fe(OEP)(2-
MeHIm)2]Cl·3CHCl3
formula
C47H59Cl10FeN8
fw, amu
1146.37
a, Å
14.066(3)
b, Å
20.883(4)
c, Å
19.245(4)
â, deg
109.67(3)
5323.1(18)
V, Å3
space group
Z
P21/c
4
1.430
Dc, g/cm3
F(000)
2372
0.827
µ, mm-1
crystal dimensions, mm
radiation
temp, K
0.46 × 0.20 × 0.12
MoKR, λh ) 0.71073 Å
100(2)
total data collected
abs correction
unique data
unique obsd data [I > 2σ(I)]
refinement method
final R indices [I > 2σ(I)]
final R indices (all data)
86704
semiempirical from equivalents
13213 (Rint ) 0.035)
10576
Figure 1. ORTEP diagram of six-coordinate perp-[Fe(OEP)(2-MeHIm)2]+.
The hydrogen atoms of the porphyrin ligand have been omitted for clarity;
the hydrogen atoms of the imidazole ligand are shown. 50% probability
ellipsoids are depicted.
full-matrix least-squares on F2
R1 ) 0.0341, wR2 ) 0.0916
R1 ) 0.0461, wR2 ) 0.0962
the absorption correction. Complete crystallographic details, atomic
coordinates, bond distances and angles, anisotropic thermal param-
eters, and fixed hydrogen atom coordinates are given in the
Supporting Information.
The asymmetric unit was found to contain one iron(III) porphy-
rinate cation, one chloride ion, and three chloroform solvent
molecules. The iron porphyrinate is a bis-ligated species, perp-
[Fe(OEP)(2-MeHIm)2]+. The terminal carbon of one ethyl group
is disordered over two positions. Both disordered parts (C(42(a)
and C(42(b)) are refined with the same anisotropic displacement
parameters and the same bond distance to C(41). After the final
refinement, the occupancy of the major position was found to be
53%. All three chloroform solvent molecules are disordered over
three positions. All non-hydrogen atoms were refined anisotropi-
cally. Hydrogen atoms were added with the standard SHELXL-97
idealization methods.
Figure 2. Formal diagram of the porphyrinato cores of perp-[Fe(OEP)-
(2-MeHIm)2]+. Illustrated are the displacements of each atom from the 24-
atom core plane in units of 0.01 Å. The diagrams also show the orientation
of the imidazole ligands with respect to the atoms of the porphyrin core.
The position of the methyl group at the 2-carbon position is represented by
the circle.
Results
The reaction of [Fe(OEP)Cl] with excess 2-methylimida-
zole yielded block crystals and microcrystalline material. The
structure of the crystalline sample was determined by X-ray
crystallography. The ORTEP diagram of the perp-[Fe(OEP)-
(2-MeHIm)2]+ cation is shown in Figure 1. The cation is a
six-coordinate iron(III) porphyrinate with two relative per-
pendicular imidazole ligands. Selected bond distances and
angles are listed in Table 2. Equatorial bond distances (Fe-
Np) average 1.974(4) Å. The axial bond lengths are 2.0123-
(15) Å and 1.9984(15) Å. The Nim-Fe-Nim angle is
175.73(6)°.
Table 2. Selected Bond Lengths and Angles for
[Fe(OEP)(2-MeHIm)2]Cl·3CHCl3
bond
length (Å)
bond
length (Å)
Fe(1)-N(1)
Fe(1)-N(3)
Fe(1)-N(5)
1.9749(14)
1.9666(14)
2.0123(15)
Fe(1)-N(2)
Fe(1)-N(4)
Fe(1)-N(7)
1.9765(14)
1.9734(14)
1.9984(15)
angle
value(°)
angle
value(°)
C(a1)-N(1)-Fe(1)
C(a3)-N(2)-Fe(1)
C(a5)-N(3)-Fe(1)
C(a7)-N(4)-Fe(1)
C(1)-N(5)-Fe(1)
C(5)-N(7)-Fe(1)
N(7)-Fe(1)-N(5)
127.10(11)
127.03(11)
127.19(11)
126.49(11)
132.85(11)
132.54(11)
175.73(6)
C(a2)-N(1)-Fe(1)
C(a4)-N(2)-Fe(1)
C(a6)-N(3)-Fe(1)
C(a8)-N(4)-Fe(1)
C(3)-N(5)-Fe(1)
C(7)-N(7)-Fe(1)
127.08(11)
126.94(11)
126.79(11)
127.42(11)
120.88(11)
121.31(11)
The displacement of each atom of the porphyrin core from
the 24-atom mean plane is shown in Figure 2. The orienta-
tions of the 2-MeHIm ligands including the value of the
dihedral angle are also shown; the circle represents the
position of the methyl group. The six-coordinate cation has
a significantly S4-ruffled core. The absolute ligand orientation
is given by the dihedral angle between the axial ligand plane
and the closest NaxFeNp plane and is conventionally denoted
by φ. The ligand planes make dihedral angles of 40.8° and
43.4° to the closest NaxFeNp plane in perp-[Fe(OEP)(2-
MeHIm)2]+. The relative ligand orientation is simply the
2 2
4
(31) R1 ) ∑||Fo| - | Fc||/∑|Fo| and wR2 ) {∑[w(Fo2 - Fc ) ]/∑[wFo ]}1/2
.
The conventional R factors R1 are based on F, with F set to zero for
negative F2. The criterion of F2 > 2σ(F2) was used only for calculating
R1. R factors based on F2 (wR2) are statistically about twice as large
as those based on F, and R factors based on ALL data will be even
larger.
(32) Sheldrick, G. M. Program for Empirical Absorption Correction of
Area Detector Data; Universita¨t Go¨ttingen: Germany, 1996.
Inorganic Chemistry, Vol. 45, No. 24, 2006 9723

Hu et al.
Table 3. Selected Bond Distances (Å) and Angles (deg) for [Fe(OEP)(2-MeHIm)2]Cl and Related Speciesa
φe,f
relative orientatione,g
ref
b,c
c
c,d
complex
Fe-Np
Fe-NAx
Cm
[Fe(TMP)(1-MeIm)2]ClO4
1.988(20)
1.987(1)
1.994(12)
1.993(4)
2.000(11)
1.995(10)
1.992(5)
1.997(1)
1.995(17)
1.982(11)
2.002(4)
1.991(16)
1.983(4)
1.981(5)
1.990(2)
1.991(6)
1.975(3)
1.965(3)
1.977(3)
1.964(3)
1.975(2)
1.987(2)
1.983(4)
1.967(7)
1.979(7)
23
41
6
41
3.1
4.6
22
29
15
22/32
36
3/16
10/46
14/12
12.6/6.9
0h
0h
17
[Fe(TPP)(HIm)2]Cl‚CHCl3
0h
24
0h
[Fe(TPP)(4-MeHIm)2]Cl
0h
33
33
34
34
0h
[Fe(TPP)(t-Mu)2]SbF6
[Fe(TPP)(c-Mu)2]SbF6
0h
0h
0h
[Fe(TPP)(1-MeIm)2]ClO4
[Fe(OEP)(4-NMe2Py)2]ClO4
[Fe(Proto IX)(1-MeIm)2]
1.974(6)b
1.995(3)
11
35
17
36
18
0h
1.977(16)b
1.970(12)b
1.982(3)b
1.996(29)b
1.978(10)
13
30
26
19.5
paral-[Fe(TMP)(5-MeHIm)2]ClO4
paral-[Fe(OMTPP)(1-MeIm)2]Cl
average of the above 10
0.01
37
[Fe(TPP)(HIm)2]Cl‚MeOH
perp-[Fe(TMP)(5-MeHIm)2]ClO4
[Fe(TPP)(2-MeHIm)2]ClO4
[Fe(TMP)(1,2-Me2Im)2]ClO4
[Fe(TPP)(Py)2]ClO4
[Fe(TMP)(3-ClPy)2]ClO4
[Fe(TMP)(4-CNPy)2]ClO4
[Fe(TMP)(3-EtPy)2]ClO4
[Fe(TMP)(4-NMe2Py)2]ClO4
[Fe(TPP)(4-CNPy)2]ClO4
average of the above 10
1.989(8)
1.981(7)
1.971(4)
1.937(12)
1.982(7)
1.968(3)
1.961(7)
1.964(4)
1.964(10)
1.952(7)
1.967(14)
1.974(24)b
1.965(11)b
2.012(4)b
2.004(0)b
2.003(3)b
2.012(8)b
2.011(14)b
1.996(9)b
1.984(8)b
2.002(8)b
1.996(16)
0.31
0.32
0.40
0.72
0.25
0.36
0.41
0.43
0.51
0.55
18/39
30/12
32/32
44.8/45.4
34/38
48/29
43/44
43/43
37/42
35/36
57
76
89.3
89.4
86
77
90
90
79
25
18
15
19
20
21
22
21
17
22
89
perp-[Fe(OMTPP)(1- MeIm)2]Cl
[Fe(OETPP)(1-MeIm)2]Cl
[Fe(OETPP)(4-NMe2Py)2]Cl
[Fe(OMTPP)(4-NMe2Py)2]ClO4
[Fe(OMTPP)(Py)2]ClO4
[Fe(OETPP)(4-NMe2Py)2]ClO4
average of the above 6
average of the above 16
1.969(7)
1.970(7)
1.951(5)
1.979(3)
1.973(3)
1.977(2)
1.970(10)
1.968(13)
1.982(10)
1.977(1)b
2.000(22)b
2.009b
2.024(4)
2.030(3)b
2.004(22)
1.999(18)
0.10
0.03
0.28
0.01
0.18
0.08
29.3
90.0h
73
70
84.2
90.0h
53.2
37
37
40
38
38
38
9.6/82.7
9.0/29.0
1.3/2.4
23.4
10.6/20.5
perp-[Fe(OEP)(2- MeHIm)2]Cl
paral-[Fe(OEP)(2- MeHIm)2]ClO4
1.974(4)
2.041(9)
2.005(10)
2.275(1)
0.46
0.05
40.8/43.4
22.2
87.6
0h
this work
26
a Estimated standard deviations are given in parentheses. b Averaged value. c In Å. d Average absolute values of displacements of the methine carbons
from the 24-atom mean plane. e Value in degrees. f Dihedral angle between the plane defined by the closest Np-Fe- Nim and the imidazole plane. g Dihedral
angle between two axial ligands. h Exact value required by symmetry.
dihedral angle between the two axial ligand planes. This
results in a relative ligand orientation of 87.6° between the
two imidazole planes. Both ligands are almost perpendicular
to the porphyrin plane with dihedral angles 86.1° and 88.6°.
The Fe-Nim-Cim angles are also listed in Table 2.
The crystalline species was studied with variable temper-
ature Mo¨ssbauer spectroscopy. The quadrupole splitting and
isomer shift values show a weak temperature dependence.
The quadrupole splitting for the crystalline species perp-
[Fe(OEP)(2-MeHIm)2]Cl at 295 K is 1.81 mm/s. The isomer
shifts at the corresponding temperature is 0.26 mm/s. These
Mo¨ssbauer parameters at various other temperatures are given
in Table S7 of the Supporting Information.
clear that the complex [Fe(OEP)(2-MeHIm)2]+ is close to
the spin crossover point and that subtle phenomena might
shift the balance between the two spin states.
Structure of perp-[Fe(OEP)(2-MeHIm)2]Cl. As shown
in Figure 1, the cation is a six-coordinate species with relative
perpendicular imidazole ligands. It has a ruffled core
conformation, which is also shown in Figure 2, a formal
diagram giving atomic displacements of core atoms from
the 24-atom mean plane. The large absolute value of
displacements of meso-carbon atoms (Cm) is a feature of
ruffled conformations. Saddled structures usually have large
displacement of â-carbon atoms but not large Cm displace-
ments. For comparison, related structural data for low-spin
bis-ligated iron(III) porphyrinates are listed in Table 3. There
are three groups of complexes in this table. The top group
gives values for species having the two axial ligands with
relative parallel orientations, while the second and third
groups give values for species having the two axial ligands
with relative perpendicular orientations. The top group has
porphyrin core conformations that are effectively planar; the
second group are largely those with strongly ruffled por-
phyrin cores, while the third group are extremely saddled.
An estimate of the ruffling magnitude can be given by the
Discussion
The data given below clearly show that perp-[Fe(OEP)-
(2-MeHIm)2]Cl is a low-spin species in the solid state. This
is in distinct contrast with that observed for paral-[Fe(OEP)-
(2-MeHIm)2]ClO4,26 which has been shown to display a high-
spin state in the crystalline state. However, as shown
previously by Geiger et al.,26 solutions of the salt [Fe(OEP)-
(2-MeHIm)2]ClO4 display a thermal spin-equilibrium be-
tween a high-spin species and a low-spin species. It is thus
9724 Inorganic Chemistry, Vol. 45, No. 24, 2006

Low-Spin (perp-[Fe(OEP)(2-MeHIm)2]Cl)
average displacement of Cm from the 24-atom mean plane.
For the title complex, the average absolute value of Cm (0.46
Å) is in the middle of those values of ruffled iron(III)
porphyrinates shown in Table 3. In this complex, a strongly
ruffled porphyrin core is required in order to allow the close
approach with the sterically bulky 2-methylimidazole ligands,
with the formation of two oblong cavities at right angles on
opposite sides of the porphyrin ring and a relative perpen-
dicular orientation of the two axial imidazole ligands.
The average equatorial distance observed in perp-[Fe-
(OEP)(2-MeHIm)2]Cl is 1.973(4) Å which is similar to those
values for the ruffled and saddled structures in Table 3.26
The average value for the equatorial Fe-Np distances in those
six-coordinate iron(III) porphyrinates with two perpendicular
ligands is ∼1.968(13) Å, about ∼0.02 Å shorter than those
iron(III) porphyrinates with two parallel ligands (1.991(6)
Å). These former complexes have ruffled or saddled por-
phyrin core conformations rather than planar conformations.
This decrease (bond shortening) is the clear result of the core
distortion. The average value of the two axial Fe-Nim
distances in the title complex is 2.005(10) Å, which is similar
to the corresponding distances observed in other ruffled bis-
(imidazole-ligated) iron(III) porphyrinates (1.996(17) Å), but
longer than those in planar structures (1.978(10) Å) where
the two ligand planes have a relative parallel orientation.
The above structural data suggest similarity between this
OEP complex and those tetraaryl porphyrin complexes shown
in Table 3. When hindered methylimidazoles are used as
ligands, in the low-spin structures of OEP, TPP, and TMP,
they all have very ruffled porphyrin cores with two perpen-
dicular ligands. There could be some weak interaction
between imidazole atoms and methyl atoms of the mesityl
groups in the TMP complex but no such interaction in TPP
and OEP complexes. So the dominant factor of the ruffling
must be the interaction between imidazole atoms and
porphyrin core atoms. To clearly represent the interaction
between imidazole atoms and the porphyrin core, the
distances between imidazole hydrogen atoms and their close
five-membered atom plane in the porphyrin core have been
chosen as shown in Figure 3. In perp-[Fe(OEP)(2-Me-
HIm)2]+, H(3) and H(7) are hydrogen atoms bonded to the
4-position imidazole carbon; H(4) and H(8) are hydrogen
atoms bonded to the 2-methyl carbon. These distances are
listed in Table 4. The average distance is 2.34 Å between
4-position hydrogen atoms and the close plane and 2.52 Å
between 2-methyl hydrogen atoms and the close plane. The
corresponding distances are 2.30 and 2.48 Å for [Fe(TPP)-
(2-MeHIm)2]ClO4,15 2.34 and 2.60 Å for [Fe(TMP)(1,2-Me2-
Im)2]ClO4.19 If the porphyrin core were planar, the calculated
values were 2.22 and 2.34 Å. They are thus about 0.1 Å
shorter than those found in the ruffled structures, which
would cause much larger repulsion interaction between the
imidazole group and the porphyrin core. The paral-[Fe-
(OEP)(2-MeHIm)2]ClO4 complex has a planar core but the
H‚‚‚core distances are increased by both an appropriate
absolute orientation of the imidazole and an increased axial
Fe-Nim distance of 2.275 Å appropriate for the high-spin
state. Thus the steric interaction between the 2-methylimi-
Figure 3. Diagram showing the interaction between imidazole hydrogen
atoms and their close five-membered atom plane in the 24-atom core. Yellow
planes are mean planes 1 and 3 that have close interactions with the
hydrogen atoms of 2-methyl and 4-carbon atoms of the upper imidazole in
the figure. Green planes are mean planes 2 and 4 that have close interactions
with the hydrogen atoms of the lower imidazole in the figure. Dashed lines
indicate the distance between hydrogen atom and the closest mean plane.
dazole and the core in high-spin paral-[Fe(OEP)(2-MeHIm)2]-
ClO4 is similar to that of perp-[Fe(OEP)(2-MeHIm)2]Cl.
The steric interaction of the 2-methyl group adjacent to
the porphyrin core atoms leads to a small tilting of the Fe-
Nim bond from the normal to the porphyrin plane; values
are 3.7 and 3.2°. The direction of the tilts always serve to
minimize the imidazole methyl group‚‚‚porphyrin core
contacts. In addition, the methyl group leads to significantly
different pairs of Fe-Nim-Cim angles; the angle involving
the methyl side of imidazole is 132.85(11) and 132.54(11)°,
while the unhindered Cim has Fe-Nim-Cim angles of 120.88-
(11) and 121.31(11)°. These are similar to the values in [Fe-
(TPP)(2-MeHIm)2]+,15 where small tilt angles(∼4°) and large
difference between two different Fe-Nim-Cim angles are also
observed.
In comparing perp-[Fe(OEP)(2-MeHIm)2]Cl with paral-
[Fe(OEP)(2-MeHIm)2]ClO4,26 one obvious difference is the
counterion. For the title complex, the chloride counterion
plays an important role in hydrogen bonding in the solid
state as shown in Figure 4. The chloride ion (Cl(1)) is
hydrogen bonded to two uncoordinated imidazole nitrogen
atoms (N(6) and N(8)) from two different units and forms a
one-dimensional zigzag chain. The corresponding distances,
3.16 Å (Cl(1)‚‚‚N(6)) and 3.22 Å (Cl(1)‚‚‚N(8)), are a little
shorter than the standard hydrogen bond N‚‚‚Cl distance (3.3
Å).41 The moderately strong hydrogen bonding can make
(33) Silver, J.; Marsh, P. J.; Symons, M. C. R.; Svistunenko, D. A.;
Frampton, C. S.; Fern, G. R. Inorg. Chem. 2000, 39, 2874.
(34) Quinn, R.; Valentine, J. S.; Byrn, M. P.; Strouse, C. E. J. Am. Chem.
Soc. 1987, 109, 3301.
(35) Higgins, T.; Safo, M. K.; Scheidt, W. R. Inorg. Chim. Acta 1990,
178, 261.
Inorganic Chemistry, Vol. 45, No. 24, 2006 9725

Hu et al.
Table 4. Selected Distances of Imidazole Hydrogen Atoms to the Corresponding Plane for perp-[Fe(OEP)(2-MeHIm)2]Cl and Related Species
perp-[Fe(OEP)-
(2-MeHIm)2]Cl
[Fe(TPP)-
[Fe(TMP)-
paral-[Fe(OEP)-
(2-MeHIm)2]ClO4
26
(2-MeHIm)2]+ 15
(1,2-Me2Im)2]+ 19
H(3)‚‚‚plane 1a(Å)
H(7)‚‚‚plane 2c(Å)
average of the above 2
H(4a)‚‚‚plane 3d(Å)
H(4c)‚‚‚plane 3d(Å)
H(8a)‚‚‚plane 4e(Å)
H(8c)‚‚‚plane 4e(Å)
average of the above 4
2.31
2.37
2.34
2.46
2.56
2.47
2.58
2.52
2.31
2.29
2.30
2.36
2.55
2.41
2.61
2.48
2.37
2.31
2.34
2.57
2.63
2.57
2.64
2.60
2.42b
2.63b
2.65b
a Mean plane 1 is defined by N(3), C(a5), C(m2), C(a4), and N(2). b Two equal distances as required by inversion symmetry. c Mean plane 2 is defined
by N(1), C(a3), C(m1), C(a2), and N(2). d Mean plane 3 is defined by N(1), N(4), C(m4), C(a1), and C(a8). e Mean plane 4 is defined by N(3), N(4), C(a6),
C(m3), and C(a7).
Figure 4. Hydrogen bonding network in the crystal structure of perp-[Fe(OEP)(2-MeHIm)2]Cl.
the coordinated imidazole have some imidazolate character
and thus act as a stronger ligand field.42 For the [Fe(OEP)-
(2-MeHIm)2]+ system, which is known to be close to the
spin crossover point,26 the hydrogen-bonding effect on the
ligand field strength is apparently sufficient to favor the low-
spin state observed for perp-[Fe(OEP)(2-MeHIm)2]Cl. In-
terestingly, a hydrogen bond network was found for paral-
[Fe(OEP)(2-MeHIm)2]ClO4 as shown in Figure S1. The
26
hydrogen bonding appears weaker in this system; the
perchlorate is disordered so that the H-bonding to the
imidazole N-H distance is either 2.92 or 3.08 Å for N‚‚‚O
with the distance to the second imidazole is then 3.08 or
2.92 Å, respectively. Although other factors may well also
contribute, the weaker H-bonding then leads to the high-
spin state observed for paral-[Fe(OEP)(2-MeHIm)2]ClO4.26
The hydrogen bond distances and angles for both low-spin
perp-[Fe(OEP)(2-MeHIm)2]Cl and high-spin paral-[Fe(OEP)-
(2-MeHIm)2]ClO4 are listed in Table S8. Hydrogen bonding
to the uncoordinated N-H group of a ligated imidazole is
also known to substantially enhance the equilibrium binding
constant of the ligated imidazole in iron(II) and cobalt(III)
porphyrinates.43-45
(36) Little, R. G.; Dymock, K. R.; Ibers, J. A. J. Am. Chem. Soc. 1975,
97, 4532.
(37) Yatsunyk, L. A.; Carducci, M. D.; Walker, F. A. J. Am. Chem. Soc.
2003, 125, 15986.
(38) Ohgo, Y.; Ikeue, T.; Takahashi, M.; Takeda, M.; Nakamura, M. Eur.
J. Inorg. Chem. 2004, 798.
(39) Epstein, L. M.; Straub, D. K.; Maricondi, C. Inorg. Chem. 1967, 6,
1720.
(40) Ogura, H.; Yatsunyk, L.; Medforth, C. J.; Smith, K. M.; Barkigia, K.
M.; Renner, M. W.; Melamed, D.; Walker, F. A. J. Am. Chem. Soc.
2001, 123, 6564.
(41) Hamilton, W. C.; Ibers, J. A. In Hydrogen Bonding in Solids: Methods
of Molecular Structure Determination; Hamilton, W. C., Ed.; W. A.
Benjamin: New York, 1968.
(42) Reed, C. A.; Mashiko, T.; Bentley, S. P.; Kastner, M. E.; Scheidt, W.
R.; Spartalian, K.; Lang, G. J. Am. Chem. Soc. 1979, 101, 2948.
9726 Inorganic Chemistry, Vol. 45, No. 24, 2006

Low-Spin (perp-[Fe(OEP)(2-MeHIm)2]Cl)
Table 5. Solid-State Mo¨ssbauer Parameters for the Title Complex and Related Species
relative orientationb
confc
ref
a
a
complex
∆Eq
δFe
sample phase
T (K)
Type I
cryst solid
cryst solid
cryst solid
cryst solid
cryst solid
cryst solid
cryst solid
cryst solid
solid
cryst solid
cryst solid
cryst solid
cryst solid
cryst solid
cryst solid
perp-[Fe(OEP)(2-MeHIm)2]Cl
[Fe(TMP)(4-NMe2Py)2]ClO4
perp-[Fe(TMP)(5-MeHIm)2]ClO4
[Fe(TPP)(2-MeHIm)2]ClO4
[Fe(TMP)(1,2-Me2Im)2]ClO4
[Fe(TMP)(2-MeHIm)2]ClO4
[Fe(TMP)(3-EtPy)2]ClO4
1.85
1.74
1.78
1.77
1.26
1.48
1.25
1.36
1.25
1.70
1.94
2.13
1.89
2.18
2.31
0.20
0.20
0.22
0.22
0.17
0.20
0.18
0.20
0.16
0.27
0.26
0.26
0.23
0.25
0.26
87.6
79
76
89.3
89.4
e
90
77
e
90
73
70
84
90
53
ruf
ruf
ruf
ruf
ruf
e
ruf
ruf
e
sad
sad
sad
sad
sad
sad
this work
17
18
16
19
21
21
21
39
46
46
46
38
38
38
77
120
120
120
77
77
77
77
4.2
4.2
4.2
70
78
80
[Fe(TMP)(3-ClPy)2]ClO4
[Fe(TPP)(Py)2]Cl
perp-[Fe(OMTPP)(1-MeIm)2]Cl
[Fe(OETPP)(1-MeIm)2]Cl
[Fe(OETPP)(4-NMe2Py)2]Cl
[Fe(OMTPP)(4-NMe2Py)2]ClO4
[Fe(OMTPP)(Py)2]ClO4
[Fe(OETPP)(4-NMe2Py)2]ClO4
Type II
cryst solid
cryst solid
cryst solid
cryst solid
crystalline(?)
cryst solid
paral-[Fe(TMP)(5-MeHIm)2]ClO4
[Fe(TMP)(1-MeHIm)2]ClO4
[Fe(OEP)(4-NMe2Py)2]ClO4
[Fe(TPP)(4-MeHIm)2]Cl
[Fe(TPP)(HIm)2]Cl
paral-[Fe(OMTPP)(1-MeIm)2]Cl
2.56
2.28
2.14
2.26
2.23
2.78
0.22
0.28
0.26
0.34
0.23
0.28
120
77
77
77
77
26/30
0
0
0
d
pla
pla
pla
pla
d
18
17
17
33
39
46
4.2
19.5
sad
Type III
cryst solid
cryst solid
[Fe(TMP)(4-CNPy)2]ClO4
[Fe(TPP)(4-CNPy)2]ClO4
0.97
0.65
0.20
0.19
77
4.2
70
89
ruf
ruf
21
22
a mm/s. b Dihedral angle between two axial ligands. c Predominant core conformation contribution: pla, planar; ruf, ruffled; sad, saddled. d Not determined,
presumed parallel and planar. e Not determined, presumed perpendicular and ruffled.
Mo1ssbauer Spectra. Among the bis-ligated Fe(III) por-
phyrinates with imidazole or pyridine as ligands, three
structural/spectroscopic categories can be identified: (Type
I) perpendicular ligands, normally ruffled or saddled por-
phyrin core, with “large gmax” EPR, and quadrupole splittings
usually in the range from 1.2 to 1.8 mm/s; (Type II) parallel
ligands, usually planar porphyrin cores although a saddled
example exists, with classic rhombic EPR and quadrupole
splittings in the range from 2.1 to 2.8 mm/s; and (Type III)
axial ligands nearly perpendicular, ruffled porphyrin core,
axial EPR spectrum, very small quadrupole splitting. Type
I and Type II have the ground state (dxy)2(dxz,dyz)3; type III
has the ground state (dxz,dyz)4(dxy)1.
hyperfine interactions with intermediate fluctuation rates on
the Mo¨ssbauer time scale. The failure to see an EPR
spectrum, even at 4.2 K, could be related to this magnetic
broadening, either through a spin-state equilibrium or through
incipient intermolecular magnetic coupling. We note that
some compounds in Type I have ∆Eq larger than 1.8 mm/s.
These porphyrins all have strongly saddled cores, which
could be the reason for the larger ∆Eq values. A comparison
of ∆Eq of the planar species with the same porphyrin (for
example, 2.8 mm/s for paral-[Fe(OMTPP)(1-MeIm)2]Cl),
shows that the saddled species with relative perpendicular
axial ligands still have much smaller quadrupole splitting
values.
For the title complex, Mo¨ssbauer spectra were measured
from room temperature to 20 K. The isomer shift (0.26 mm/
s) indicates it is a low-spin species. Both quadrupole splitting
and isomer shift are typical for type I compounds, which
suggests that the electronic ground state is (dxy)2(dxz,dyz)3.
Table 5 shows the strong similarity between the Mo¨ssbauer
data observed for the current complex and related species.
The Mo¨ssbauer spectra clearly show that perp-[Fe(OEP)(2-
MeHIm)2]Cl undergoes significant magnetic broadening.
Line widths at RT are somewhat broadened to 0.40 and 0.54
mm/s but are broadened to 1.05 and 1.02 mm/s at 20 K.
This broadening is likely due to unresolved magnetic
Summary. The synthesis and isolation of a six-coordinate
bis(2-methylimidazole)(octaethylporphyrinato)iron(III) chlo-
ride, perp-[Fe(OEP)(2-MeHIm)2]Cl, has been accomplished.
This complex has been characterized by an X-ray diffraction
study. Distinct from the high-spin paral-[Fe(OEP)(2-MeHIm)2]-
ClO4 derivative,26 this complex is a low-spin iron(III)
porphyrinate with its axial ligands in a relative perpendicular
orientation. Consistent with the necessary use of a hindered
imidazole to achieve a relative perpendicular orientation, the
porphyrin core is strongly ruffled. This complex has been
found to have shorter equatorial Fe-Np bonds and longer
Fe-Nim bonds than those found in analogous planar com-
plexes that have the two axial imidazoles in a relative parallel
orientation. A one-dimension hydrogen-bond chain is formed
between chloride anions and uncoordinated imidazole ni-
trogen atoms. Mo¨ssbauer spectroscopy verified the low-spin
state of the species and confirmed that it is a type I low-
spin species with ground state (dxy)2(dxz,dyz)3.
(43) Coyle, C. L.; Rafson, P. A.; Abbott, E. H. Inorg. Chem. 1973, 12,
2007.
(44) Walker, F. A.; Lo, M.-W.; Ree, M. T. J. Am. Chem. Soc. 1976, 98,
5552.
(45) Balch, A. L.; Watkins, J. J.; Doonan, D. J. Inorg. Chem. 1979, 18,
1228.
(46) Teschner, T.; Yatsunyk, L.; Schu¨×b6nemann, V.; Paulsen, H.; Winkler,
H.; Hu, C.; Scheidt, W. R.; Walker, F. A.; Trautwein, A. X. J. Am.
Chem. Soc. 2006, 128, 1379.
Inorganic Chemistry, Vol. 45, No. 24, 2006 9727

Hu et al.
Mo¨ssbauer parameters; Table S8 showing hydrogen bond distances
and angles for perp-[Fe(OEP)(2-MeHIm)2]Cl and paral-[Fe(OEP)-
(2-MeHIm)2]ClO4; Figure S1 showing the hydrogen bonding
network in the structure of paral-[Fe(OEP)(2-MeHIm)2]ClO4; X-ray
crystallographic file. This material is available free of charge via
the Internet at http://pubs.acs.org.
Acknowledgment. We thank the National Institutes of
Health for support of this research under Grant GM-38401
and the NSF for X-ray instrumentation (CHE-0443233).
Supporting Information Available: Tables S1-S6, giving
complete crystallographic details, atomic coordinates, bond dis-
tances and angles, anisotropic temperature factors, and fixed
hydrogen atom positions; Table S7 showing variable-temperature
IC061014U
9728 Inorganic Chemistry, Vol. 45, No. 24, 2006