Electronic configuration of five-coordinate high-spin pyrazole-ligated iron(II) porphyrinates

Article abstract of DOI:10.1021/ic101469e

Pyrazole, a neutral nitrogen ligand and an isomer of imidazole, has been used as a fifth ligand to prepare two new species, [Fe(TPP)(Hdmpz)] and [Fe(Tp-OCH₃PP)(Hdmpz)] (Hdmpz = 3,5-dimethylpyrazole), the first structurally characterized example

Full text of DOI:10.1021/ic101469e

1
0984 Inorg. Chem. 2010, 49, 10984–10991
DOI: 10.1021/ic101469e
Electronic Configuration of Five-Coordinate High-Spin Pyrazole-Ligated Iron(II)
Porphyrinates
,
†,‡
‡
,§
,‡
Chuanjiang Hu,* Bruce C. Noll, Charles E. Schulz,* and W. Robert Scheidt*
†
Key Laboratory of Organic Synthesis of Jiangsu Province, College of Chemistry, Chemical Engineering and
‡
Materials Science, Soochow University, Suzhou 215123, P.R. China, The Department of Chemistry and
Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556, United States, and
§
Department of Physics, Knox College, Galesburg, Illinois 61401, United States
Received July 22, 2010
Pyrazole, a neutral nitrogen ligand and an isomer of imidazole, has been used as a fifth ligand to prepare two new
species, [Fe(TPP)(Hdmpz)] and [Fe(Tp-OCH PP)(Hdmpz)] (Hdmpz = 3,5-dimethylpyrazole), the first structurally
3
characterized examples of five-coordinate iron(II) porphyrinates with a nonimidazole neutral ligand. Both complexes
are characterized by X-ray crystallography, and structures show common features for five-coordinate iron(II) species,
such as an expanded porphyrinato core, large equatorial Fe-N bond distances, and a significant out-of-plane
p
displacement of the iron(II) atom. The Fe-N(pyrazole) and Fe-N bond distances are similar to those in imidazole-
p
ligated species. These suggest that the coordination abilities to iron(II) for imidazole and pyrazole are very similar even
though pyrazole is less basic than imidazole. M €o ssbauer studies reveal that [Fe(TPP)(Hdmpz)] has the same
behavior as those of imidazole-ligated species, such as negative quadrupole splitting values and relative large
asymmetry parameters. Both the structures and the M €o ssbauer spectra suggest pyrazole-ligated five-coordinate
iron(II) porphyrinates have the same electronic configuration as imidazole-ligated species.
1-9
Introduction
theoretical studies.
A number of recent density functional
theory (DFT) studies for five-coordinate imidazole-ligated
Five-coordinate high-spin iron(II) porphyrinates have
been of interest as models of deoxyhemoglobin and deoxy-
myoglobin. In addition, high-spin five-coordinate states are
also found in many other heme proteins, such as reduced
cytochrome P450, chloroperoxidase, and horseradish perox-
idase. Knowledge of both the electronic and the geometric
structure at iron appears to be the key to understanding the
diverse and complicated functions of these hemoproteins.
Indeed, the electronic ground states of models of these iron-
1-4
hemes have appeared.
However, the conclusions reached
about the electronic ground state are by no means uniform.
Three of the four DFT calculations found that a triplet state
was lower in energy than the experimentally observed quintet
state. In all of these cases, a quintet state is predicted to be
lowest in energy when the iron(II) atom is constrained to be
further out of the porphyrin plane than the value obtained for
the triplet state. In other words, DFT calculations suggest
that large values of the displacement of Fe from the porphy-
rin plane stabilize higher spin multiplicities. The fourth
(II) species have been intensively investigated both experi-
mentally and theoretically and yet are far from being known
with certainty.
The difficulty in understanding the electronic structure of the
five-coordinate high-spin hemes is reflected in both the
relatively large amount published on the issue and some
inconsistent interpretations for both experimental and
3
study does predict a quintet ground state.
As part of a more general program of characterization of
10-13
high-spin iron(II) porphyrinates, our previous studies
(
(
6) Kent, T. A.; Spartalian, K.; Lang, G. J. Chem. Phys. 1979, 71, 4899.
7) Nakano, N.; Otsuka, J.; Tasaki, A. Biochem. Biophys. Acta 1971, 236,
2
22.
(8) Nakano, N.; Otsuka, J.; Tasaki, A. Biochem. Biophys. Acta 1972, 278,
355.
(9) Alpert, A.; Banerjee, R. Biochem. Biophys. Acta 1975, 405, 114.
*
To whom correspondence should be addressed. E-mail: cjhu@suda.edu.cn
C.H); scheidt.1@nd.edu (W.R.S.).
1) Rovira, C.; Kunc, K.; Hutter, J.; Ballone, P.; Parrinello, M. J. Phys.
Chem. A 1997, 101, 8914.
2) Kozlowski, P. M.; Spiro, T. G.; Zgierski, M. Z. J. Phys. Chem. B 2000,
(
(
(10) Hu, C.; Roth, A.; Ellison, M. K.; An, J.; Ellis, C. M.; Schulz, C. E.;
Scheidt, W. R. J. Am. Chem. Soc. 2005, 127, 5675.
(
1
04, 10659.
(11) Hu, C.; Noll, B. C.; Schulz, C. E.; Scheidt, W. R. J. Am. Chem. Soc.
2005, 127, 15018.
(12) Hu, C.; An, J.; Noll, B. C.; Schulz, C. E.; Scheidt, W. R. Inorg. Chem.
2006, 45, 4177.
(13) Hu, C.; Sulok, C. D.; Paulat, F.; Lehnert, N.; Twigg, A. I.; Hendrich,
M. P.; Schulz, C. E.; Scheidt, W. R. J. Am. Chem. Soc. 2010, 132, 3737.
(
(
3) Liao, M.-S.; Scheiner, S. J. Chem. Phys. 2002, 116, 3635.
4) Ugalde, J. M.; Dunietz, B.; Dreuw, A.; Head-Gordon, M.; Boyd, R. J.
J. Phys. Chem. A 2004, 108, 4653.
5) Ellison, M. K.; Schulz, C. E.; Scheidt, W. R. Inorg. Chem. 2002, 41,
173.
(
2
pubs.acs.org/IC
Published on Web 11/03/2010
r 2010 American Chemical Society

Article
Inorganic Chemistry, Vol. 49, No. 23, 2010 10985
show that there are two kinds of ligands on five-coordinate
high-spin iron(II) porphyrinates: (i) neutral ligands, such as
imidazole and (ii) anionic ligands, such as imidazolate,
thiolate, halogenate, phenolate, alkoxide, acetate, and so
Table 1. Brief Crystallographic Data and Data Collection Parameters for
[
3
Fe(TPP)(Hdmpz)] and [Fe(Tp-OCH PP)(Hdmpz)]
[
Fe(Tp-OCH
3
PP)-
[
Fe(TPP)(Hdmpz)]
36FeN
(Hdmpz)]
10,14-17
forth.
Even though all have a high-spin state (S=2),
formula
C
49
H
6
60 6 4
C H52FeN O
there are significant differences in their molecular structures
and M o€ ssbauer data. The electronic structure implied by the
M o€ ssbauer results shows that all of these deoxymyoglobin
models are distinctly different from those of high-spin iron(II)
porphyrinates ligated by anionic ligands. For the anionic
ligand coordinated species, the ground state has dxy as the
doubly occupied orbital, whereas for the imidazole-ligated
speciesthedoublyoccupiedorbitals can best be describedas a
low-symmetry hybrid orbital composed of a linear combina-
tion of the two axial d orbitals, along with a significant d
FW, amu
˚
764.69
976.93
a, A
11.3032(2)
30.8656(6)
11.7462(2)
105.358(1)
3951.67(12)
11.9585(3)
35.4853(9)
11.9857(3)
106.949(1)
4865.2(2)
˚
b, A
˚
c, A
β, deg
˚
V, A
3
space group
P2
1
/c
1
P2 /c
Z
4
4
3
D
F(000)
μ, mm-
c
, g/cm
1.285
1592
0.424
1.334
2048
0.367
1
π
xy
crystal dimensions, mm
absorption correction
radiation, MoKR, hλ
temperature, K
0.44 ꢀ 0.44 ꢀ 0.22
0.52 ꢀ 0.46 ꢀ 0.20
contribution.
SADABS
Surprisingly, although there are a large variety of anionic
ligands that yield high-spin five-coordinate iron(II) porphyr-
inates, imidazoles are the only neutral ligand that has yielded
this state. We are interested in the influence of axial ligands
on the electronic configuration and structure of high-spin
iron(II) porphyrinates. What is the electronic configuration
for the five-coordinate iron(II) porphyrinates with other
exogenous neutral ligands besides imidazole? Herein, pyra-
zole, an isomer of imidazole, has been studied as the fifth
ligand. Two high-spin five-coordinate pyrazole-ligated iron-
˚
0.71073 A
299(2)
84805
100(2)
86053
total data collected
unique data
11989 (Rint = 0.024)
12075 (Rint = 0.048)
10028
unique observed
data [I > 2σ(I)]
refinement method
final R indices
8806
2
full-matrix least-squares on F
R
1
= 0.0421,
wR = 0.1121
= 0.0645,
wR = 0.1272
R
1
= 0.0428,
wR = 0.1047
= 0.0554,
wR = 0.1132
[I > 2σ (I)]
final R indices (all data)
2
2
R
1
R
1
2
2
(
II) porphyrinates, [Fe(TPP)(Hdmpz)] and [Fe(Tp-OCH PP)-
3
18
2
1
(Hdmpz)], have been synthesized. We report the molecular
Fleischer preparation. M o€ ssbauer measurements were per-
formed on a constant acceleration spectrometer from 4.2 to
300 K with optional small field and in a 9 T superconducting
magnet system (Knox College). Samples for M o€ ssbauer spec-
troscopy were prepared by immobilization of the crystalline
material in Apiezon M grease. Sample homogeneity was shown
with zero field M o€ ssbauer spectra. Supporting Information,
Figure S1 gives the spectrum of [Fe(TPP)(Hdmpz)].
structures and further examine the electronic configuration
of these complexes.
Experimental Section
General Information. All reactions and manipulations for the
preparation of the iron(II) porphyrin derivatives (see below)
were carried out under argon using a double-manifold vacuum
line, Schlenkware, and cannula techniques. Toluene and hex-
anes were distilled over sodium benzophenone ketyl. Etha-
nethiol and 3,5-dimethylpyrazole was used as received. The
Synthesis of [Fe(Por)(Hdmpz)]. (Por=TPP or Tp-OCH
3
PP).
[Fe(II)(Por)] waspreparedby reductionof [Fe(Por)] O (0.05 mmol)
2
with ethanethiol (1 mL) in toluene (7 mL). The toluene solution was
stirred 3 days, then transferred into a Schlenk flask containing 3,5-
dimethylpyrazole (0.6 mmol). The mixture was stirred for 30 min,
then filtered. X-ray quality crystals were obtained in 8 mm ꢀ250 mm
sealed glass tubes by liquid diffusion using hexanes as nonsolvent.
X-ray Structure Determinations. Single-crystal experiments
were carried out on a Bruker Apex system with graphite-
2 3
free-base porphyrin H Por (Por = TPP or Tp-OCH PP) was
1
9
prepared according to Adler et al. The metalation of the free-
base porphyrin to give [Fe(Por)Cl] was done as previously
described. [Fe(Por)] O was prepared according to a modified
20
2
(
14) Schappacher, M.; Ricard, L.; Weiss, R.; Montiel-Montoya, R.;
Gonser, U.; Bill, E.; Trautwein, A. X. Inorg. Chim. Acta 1983, 78, L9.
15) Nasri, H.; Fischer, J.; Weiss, R.; Bill, E.; Trautwein, A. X. J. Am.
Chem. Soc. 1987, 109, 2549.
16) Caron, C.; Mitschler, A.; Riviere, G.; Schappacher, M.; Weiss, R.
J. Am. Chem. Soc. 1979, 101, 7401.
17) Mandon, D.; Ott-Woelfel, F.; Fischer, J.; Weiss, R.; Bill, E.;
Trautwein, A. X. Inorg. Chem. 1990, 29, 2442.
18) The following abbreviations are used in this paper: Hdmpz, 3,5-
dimethylpyrazole; Por, dianion of general porphyrin; Tp-OCH PP, dianion
˚
monochromated Mo-KR radiation ( λh =0.71073 A). The struc-
ture was solved by direct methods and refined against F using
SHELXTL;
2
(
2
2,23
subsequent difference Fourier syntheses led to
(
the location of most of the remaining non-hydrogen atoms. For
the structure refinement all data were used including negative
intensities. All non-hydrogen atoms were refined anisotropically
if not remarked otherwise below. Hydrogen atoms were added
with the standard SHELXL-97 idealization methods. The pro-
(
(
3
2
4
of meso-tetra- p-methoxyphenylporphyrin; TPP, dianion of meso-tetraphe-
nylporphyrin; TTP, dianion of meso-tetratolylporphyrin; OEP, dianion of
gram SADABS was applied for the absorption correction.
Brief crystal data for both structures are listed in Table 1. Complete
crystallographic details, atomic coordinates, anisotropic thermal
octaethylporphyrin; TpivPP, dianion of R,R,R,R-tetrakis(o-pivalamidophe-
0
nyl)porphyrin; Piv
2
C
8
P, dianion of R,R,5,15-[2,2 -(octanediamido)diphenyl]-R,
R,10-20-bis(o-pivalamidophenyl)porphyrin; Im, generalized imidazole; RIm,
generalized hindered imidazole; HIm, imidazole; 1-MeIm, 1-methylimidazole;
(21) (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.
-
2-MeHIm, 2-methylimidazole; 1,2-Me
2
Im, 1,2-dimethylimidazole; 2-MeIm ,
p
2-methylimidazolate; 222, Kryptofix 222; N , porphyrinato nitrogen; Nax,
nitrogen atom of axial ligand; Ct, the center of four porphyrinato nitrogen
atoms.
(22) Sheldrick, G. M. Acta Crystallogr., Sect. A 2008, A64, 112.
P
P
P
P
2
2 2
2 2 1/2
) }
(23) R
1
=
||F
o
| - |F
c
||/ |F
o
| and wR
2
={ w(F
o
- F
c
) / w(F
o
.
(
19) Adler, A. D.; Longo, F. R.; Finarelli, J. D.; Goldmacher, J.; Assour,
J.; Korsakoff, L. J. Org. Chem. 1967, 32, 476.
20) (a) Adler, A. D.; Longo, F. R.; Kampus, F.; Kim, J. J. Inorg. Nucl.
The conventional R-factors R
1
are based on F, with F set to zero for negative
2
2
2
F . The criterion of F > 2σ(F ) was used only for calculating R
1
. R-factors
2
(
based on F (wR ) are statistically about twice as large as those based on F,
2
Chem. 1970, 32, 2443. (b) Buchler, J. W. In Porphyrins and Metalloporphyrins;
Smith, K. M., Ed.; Elsevier Scientific Publishing: Amsterdam, The Netherlands,
and R-factors based on ALL data will be even larger.
(24) Sheldrick, G. M. SADABS; Universit €a t G €o ttingen: G €o ttingen,
1
975; Chapter 5.
Germany, 2006.

1
0986 Inorganic Chemistry, Vol. 49, No. 23, 2010
Hu et al.
Figure 1. ORTEP diagram of two pyrazole-ligated species. (a) [Fe(TPP)(Hdmpz)]. The major orientation of the pyrazole (92%) and phenyl ring atoms
C(42a), C(43a), C(45a), C(46a) with 53% occupancy) are shown. (b) [Fe(Tp-OCH PP)(Hdmpz)]. The major orientation of the pyrazole (92%) is shown.
The hydrogen atoms of the porphyrin ligand have been omitted for clarity. 50% probability ellipsoids are depicted.
(
3
parameters, and fixed hydrogen atom coordinates are given in
the Supporting Information.
Crystal data were collected at 100 K. The asymmetric unit
contains one porphyrin molecule, one 3,5-dimethylpyrazole
ligand, and one fully occupied toluene solvate. The 3,5-di-
methylpyrazole was found to be disordered over two positions,
a major and a minor position. The minor 3,5-dimethylpyrazole
was refined as a rigid group. ISOR was applied, together with
SIMU, to restrain the thermal displacement parameters of
atoms of the minor 3,5-dimethylpyrazole. After the final refine-
ment the occupancy of the major pyrazole orientation was
found to be 92%.
The pyrazole plane of the minor orientation is close to being
coplanar to the major orientation in both complexes studied; the
major and minor orientations are both shown in the Supporting
Information, Figure S2.
[
Fe(TPP)(Hdmpz)]. Three crystals from three distinct prep-
arations were measured by X-ray diffraction. The first two
crystals were measured at 100 K; attempts to refine the structure
gave unsatisfactorily high R /wR values. A detailed examina-
1
2
tion of the diffraction data showed that the rocking curves of
most diffraction maxima were broad with shoulders. Attempts
to integrate the data as twins failed. The third crystal was
measured first at room temperature and then at 100 K. The
data at room temperature gave satisfactory results, while the
data at 100 K gave the same unsatisfactory result as the previous
two measurements. In this paper, only the room temperature
result will be presented.
3
A red crystal with the dimensions 0.44 ꢀ 0.44 ꢀ 0.22 mm was
used for the structure determination. The asymmetric unit
contains one porphyrin molecule and one 3,5-dimethylpyrazole
ligand. The 3,5-dimethylpyrazole was found to be disordered
over two positions, a major and a minor position. The minor 3,
Results
The two new five-coordinate pyrazole-ligated iron(II)
porphyrinates were synthesized starting from the corre-
5
-dimethylpyrazole was refined as a rigid group, and ISOR was
sponding iron(III) μ-oxo derivatives ([Fe(por)] O). Etha-
2
applied, together with SIMU, to restrain their thermal displace-
ment parameters. After the final refinement the occupancy of
the major pyrazole orientation was found to be 92%. Four
carbon atoms of one phenyl group of the porphyrin were found
to be disordered over two positions; they were refined as C(42a),
C(43a), C(45a), C(46a) and C(42b), C(43b), C(45b), C(46b) with
nethiol reduction to give the iron(II) species [Fe(por)] was
followed by reaction with a hindered pyrazole ligand. The
structures of two derivatives have been obtained: [Fe(TPP)-
(
Hdmpz)] and [Fe(Tp-OCH PP)(Hdmpz)]. The data for
3
[Fe(TPP)(Hdmpz)] were collected at 100 K three times and
at room temperature once. Data collections at 100 K had
broadened peaks that were not adequately integrated, and
larger than acceptable values of R and wR were obtained.
5
3% and 47% occupancy, respectively.
Fe(Tp-OCH PP)(Hdmpz)]. A red crystal with the dimen-
[
3
3
sions 0.52 ꢀ 0.46 ꢀ 0.20 mm was used for the structure
determination. It was placed in inert oil, mounted on a glass
pin, and transferred to the cold gas stream of the diffractometer.
1
2
Nonetheless, the structures from the three low temperature
data sets were very similar. A satisfactory data set was only

Article
Inorganic Chemistry, Vol. 49, No. 23, 2010 10987
Scheme 1
The isomer shifts at these respective temperatures are 0.91
and 0.78 mm/s. The values of M o€ ssbauer quadrupole doub-
lets and isomer shifts at various temperatures are given in the
Supporting Information.
Discussion
The goal of this study was to further examine the electronic
configuration and structural differences caused by axial
ligands in high-spin five-coordinate iron(II) porphyrinates.
Pyrazole was used as fifth ligand in this paper; the difference
between the imidazole and the pyrazole rings is that a carbon
and a nitrogen atom exchange positions (see Scheme 1). A
sterically hindered pyrazole must be used to obtain five-
25
coordinate species rather than a six-coordinate species.
Because of the tautomeric forms possible for pyrazole,
both the 3- and 5- positions must be methylated. Thus, 3,
5
-dimethylpyrazole was chosen as the ligand, and two new
Figure 2. Formal diagrams of the porphyrinato cores. [Fe(TPP)-
five-coordinate iron(II) pyrazole-ligated porphyrinates were
obtained. To the best of our knowledge, these derivatives are
the first structurally characterized examples of five-coordi-
nate iron(II) porphyrinates with a nonimidazole neutral
ligand.
In addition to the atom position differences in the five-
membered ring, pyrazole is less basic than imidazole; the pKa
of protonated pyrazole (2.52) is much smaller that of proto-
3
(Hdmpz)] (top) and [Fe(Tp-OCH PP)(Hdmpz)] (bottom). Illustrated
are the displacements of each atom from the mean plane of the four
˚
pyrrole nitrogen in units of 0.01 A. Positive values of displacement are
toward the imidazole ligand. The diagrams also show the orientation of
the pyrazole ligand with respect to the atoms of the porphyrin core. The
location of the methyl group at the R-position of the coordinated
nitrogen is represented by the circle.
obtained at room temperature, but all data gave the same
structure.
26
nated imidazole (6.95). Although the Lewis basicity of
27
pyrazoles is also weaker than that of imidazoles, Sorrell
and Jameson have shown that two-coordinate pyrazole-
ligated Cu(I) will react with CO to form an adduct whereas
the two coordinate imidazole derivatives do not. How will
this influence the coordination behavior in their five-coordi-
nate iron(II) porphyrinate derivatives?
ORTEP diagrams for both new species are shown in
Figure 1. Only the major 3,5-dimethylpyrazole orientation
is given in this figure. The dihedral angles φ between the
pyrazole plane and the plane defined by N(1), Fe(1), N(5a)
are 36.8 and 45.4°, respectively. In each structure, the
pyrazole plane is almost perpendicular to the porphyrin
plane with a dihedral angle of 87.7 or 82.3°, respectively.
ORTEP diagrams showing both orientations of pyrazole are
provided in the Supporting Information, Figure S2.
Figure 2 gives formal diagrams of the porphyrin cores and
showsthe displacements of eachatom from themean plane of
28
Structural Studies. The structures of [Fe(TPP)(Hdmpz)]
and [Fe(Tp-OCH PP)(Hdmpz)] both show that the
3
overall features are as expected for a high-spin iron(II)
such as large equatorial Fe-N bond dis-
10,29
complex,
p
tances, a significant out-of-plane displacement of the iron-
(II) atom, and an expanded porphyrinato core.
˚
the four pyrrole nitrogen atoms in units of 0.01 A. The
˚
The average Fe-N bond length is 2.074(13) A for
p
orientation of the pyrazole ligand with respect to the core
atoms are shown by the line with the circle representing the
methyl group bound at the R-position of the coordinated
nitrogen. Also included on the diagrams of Figure 2 are the
individual Fe-Np bond lengths. An analogous diagram
showing atomic displacements from the mean plane of 24-
atom porphyrin core is given in the Supporting Information,
Figure S3. A complete listing of bond distances and angles is
given in the Supporting Information.
Crystalline [Fe(TPP)(Hdmpz)] was studied by variable-
temperature M o€ ssbauer spectroscopy at zero field and ap-
plied magnetic fields. Both the quadrupole splitting and
isomer shift values show a strong temperature dependence.
The quadrupole splitting for the crystalline species [Fe(TPP)-
˚
Fe(TPP)(Hdmpz)] and 2.068(9) A for [Fe(Tp-OCH PP)-
[
3
˚
(
Hdmpz)], and the Fe-N bond length is 2.1519(18) A or
ax
˚
.1414(18) A, respectively. These values are similar to
2
those of the imidazole-ligated porphyrinates as shown in
(
25) Collman, J. P.; Reed, C. A. J. Am. Chem. Soc. 1973, 95, 2048.
(26) (a) Elguero, E.; Gonzalez, E.; Jacquier, R. Bull. Soc. Chim. Fr. 1968,
an, J.; Elguero, J. J. Chem. Soc., Perkin Trans. 2 1983, 12, 1869.
5009. (b) Catal
ꢀ
(27) El Ghomari., M. J.; Mokhlisse, R.; Laurence, C.; Le Questel, J.-Y.;
Berthelot, M. J. Phys. Org. Chem. 1997, 10, 669.
(28) Sorrell, T. N.; Jameson, D. L. J. Am. Chem. Soc. 1983, 105, 6013.
(29) Scheidt, W. R.; Reed, C. A. Chem. Rev. 1981, 81, 543.
(30) (a) Collman, J. P.; Kim, N.; Hoard, J. L.; Lang, G.; Radonovich,
L. J.; Reed, C. A. Abstracts of Papers; 167th National Meeting of the
American Chemical Society; Los Angeles, CA, April 1974; American
Chemical Society: Washington, DC, 1974; INOR 29.
(Hdmpz)] at 4.2 K is 2.54 mm/s and at 298 K is 1.86 mm/s.

1
0988 Inorganic Chemistry, Vol. 49, No. 23, 2010
Hu et al.
a
˚
Table 2. Selected Bond Distances (A) and Angles (deg) for [Fe(TPP)(Hdmpz)], [Fe(T p-OCH
3
PP)(Hdmpz)], and Related Species
b
c,d
p
d
d,e
d,f
d
g,h
g,i
g,j
g,k
φ
complex
Fe-N
Fe-NIm
ΔN
4
Δ
Ct
3 3 N
Fe-N-C
Fe-N-X
θ
ref.
3
[
[
[
[
[
[
[
[
[
[
Fe(TPP)(Hdmpz)]
2.074(13)
2.068(9)
2.073(9)
2.087(7)
2.077(6)
2.079(8)
2.076(3)
2.080(6)
2.077(7))
2.086(8)
2.080(5)
2.072(6)
2.075(20)
2.113(4)
2.118(13)
2.11(2)
2.1519(18) 0.32
2.1414(18) 0.31
0.38
0.34
0.38
0.51
0.38
0.42
0.39
0.45
0.46
0.55
2.049
2.045
2.049
2.049
2.046
2.048
2.050
2.047
2.049
2.044
2.048(2)
2.033
2.051
2.036
2.044
2.045
2.040
2.033
2.037
2.033
2.030
139.38(16)
136.21(16)
131.1(2)
130.4(2)
131.9(3)
129.3(2)
132.8(1)
132.7(3)
131.3(3)
131.4(4)
131.4(12)
132.1(8)
126.5
116.14(14)
118.99(13)
122.9(2)
123.4(2)
122.7(3)
124.9(2)
121.4(1)
121.4(2)
122.4(3)
122.6(4)
122.7(11)
126.3(7)
120.4
7.1
9.8
8.3
8.6
6.1
11.4
6.6
3.8
6.9
10.3
7.8(24)
9.6
5.0
3.6
36.8
45.4
24.0
44.5 10
20.7 10
20.9 10
35.8 10
10.5 12
19.5 12
tw
tw
5
Fe(Tp-OCH
3
PP)(Hdmpz)]
Fe(TPP)(2-MeHIm)] 1.5C
l
3
6
H
5
Cl
2.127(3)l
2.155(2)
2.137(4)
0.32
0.39
0.35
0.36
0.32
0.37
0.34
0.42
Fe(Tp-OCH
3
PP)(2-MeHIm)]
PP)(1,2-Me Im)]
Im)]
Fe(TTP)(2-MeHIm)]
Fe(OEP)(1,2-Me Im)]
Fe(OEP)(2-MeHIm)]
Fe(TPP)(2-MeHIm)](2-fold)
Fe(Tp-OCH
3
2
l
Fe(TPP)(1,2-Me
2
2.158(2)
2.144(1)
2.171(3)
2.135(3)
2.161(5)
2.147(16)
2.095(6)
2.13(2)
2
6.5
30
average of the eight
0.36(3) 0.44(6)
[
[
[
[
[
[
[
[
[
[
Fe(TpivPP)(2-MeHIm)]
Fe(Piv P)(1-MeIm)]
K(222)][Fe(OEP)(2-MeIm )]
K(222)][Fe(TPP)(2-MeIm )]
Fe(TpivPP)(2-MeIm )]
Fe(TpivPP)Cl]
Fe(TpivPP)(O
Fe(TpivPP)(OC
Fe(TpivPP)(SC
0.40
0.31
0.56
0.56
0.52
0.53
0.55
0.56
0.42
0.52
0.43
0.34
0.65
0.66
0.65
0.59
0.64
0.62
NR
22.8 31
34.1 32
37.4 11
23.4 11
14.7 17
14
2
C
8
-
2.060(2)
1.999(5)
2.002(15)
136.6(2)
129.6(3)
NR
120.0(2)
126.7(3)
NR
-
9.8
5.1
-
-
m
-
n
2.108(15)
2.107(2)
2.114(2)
2.076(20)
2.096(4)
2.301(2)
2.034(3)
-
o
2
CCH
3
)]
15
15
14
16
-
o
6
H
5
)]
1.937(4)
2.370(3)
2.360(2)
-
p
6
HF
)]
4
)]
-
p
Fe(TPP)(SC
2
H
5
0.62
a
b
c
d
Estimated standard deviations are given in parentheses. All complexes are high spin. Averaged value. In A. Displacement of iron from the mean
e
˚
f
g
h
plane of the four pyrrole nitrogen atoms. Displacement of iron from the 24-atom mean plane of the porphyrin core. Value in degrees. The R-carbon of
i
j
the coordinated nitrogen. Imidazole 4-carbon or pyrazole 1-nitrogen. Off-axis tilt (deg) of the Fe-NIm bond from the normal to the porphyrin plane.
k
n
l
m
Dihedral angle between the plane defined by the closest N -Fe-NIm and the imidazole plane in deg. Major imidazole orientation. Not reported.
p
o
Chloride. Anionic oxygen donor. Thiolate.
p
Table 2. The comparison indicates that the coordination
abilities to iron(II) for imidazole and pyrazole are very
similar in the five-coordinate iron(II)porphyrinates even
major conformations are doming; other conformation
contributions, such as ruffling, saddling, are very small.
The large iron(II) ion is accommodated in the central
hole of the porphyrin ring not only by the iron atom
2
6
though pyrazole is less basic than imidazole.
The structural similarities between pyrazole- and imi-
dazole-ligated species are also shown in other aspects,
such as iron displacement and core expansion. As shown
in Table 2, the out-of-plane displacement of the iron atom
from the mean 24-atom porphyrin core (Δ) and the plane
displacements and long Fe-N bond distances but also
p
3
6
by radial expansion of the core. The radii of the central
hole (Ct N in Table 2) for the two new species are 2.049
3
3 3
˚
and 2.045 A, which are similar to those for imidazole-
ligated species.
The steric bulk of the methyl group at the R-position of
the coordinated nitrogen leads to, in all hindered imid-
azole and pyrazole derivatives examined to date, an off-
axis tilt of the axial Fe-N bond and a rotation of the axial
defined by the four pyrrole atoms (ΔN ) are similar to
4
those for imidazole-ligated porphyrinates. The iron dis-
˚
placements in the two species are 0.38 and 0.34 A out of
˚
the 24-atom plane and 0.32 and 0.31 A out of the four
ligand that leads to unequal Fe-N -X (X = C or N)
ax
pyrrole nitrogen atom plane. The difference in the two
values (note that Δ is always larger than ΔN ) is a measure
angles. The pyrazole ligand Fe-N bond vector is tilted off
of the normal to the 24-atom porphyrin mean plane. The
values of the tilt angle, given by θ in Table 2, are 7.1° for
4
˚
of porphyrin core doming and is 0.06 A for [Fe(TPP)-
˚
Hdmpz)] and 0.03 A for [Fe(Tp-OCH PP)(Hdmpz)].
(
3
[
Fe(TPP)(Hdmpz)] and 9.8° for [Fe(Tp-OCH PP)(Hdmpz)].
3
The porphyrin plane in [Fe(TPP)(Hdmpz)] shows a con-
formation similar to doming, but with three rings down
and one pyrrole ring slightly up, the effect may be better
This tilting is the partial result of minimizing the interac-
tion between the bulky imidazole methyl group and the
porphyrin core.
3
3
described as that of a tenting conformation. The por-
phyrin plane in [Fe(Tp-OCH PP)(Hdmpz)] is folded
One difference between pyrazole- and imidazole-li-
3
gated species is the Fe-N -X angles (Table 2). For
ax
along a pair of opposite methine carbons. Both of these
conformations have been observed in imidazole-ligated
species. The core conformations have also been analyzed
by the normal structural decomposition (NSD) method
the pyrazole-ligated species they are 139.38(16), 116.14(14)
and 136.2, 119.0°, much different from the angles for
imidazole-ligated species which have an average value of
131.4(12)° on the methyl side and 122.7(11)° on the other
3
4,35
provided by Shelnutt et al. and are given in Support-
ing Information, Table S15. The results show that the
side. Considering the difference between imidazole and
pyrazole, the unsubstituted R position of the coordinated
nitrogen in pyrazole is a nitrogen as opposed to the
carbon atom in imidazole. The corresponding N-H is
(
31) Jameson, G. B.; Molinaro, F. S.; Ibers, J. A.; Collman, J. P.; Brauman,
J. I.; Rose, E.; Suslick, K. S. J. Am. Chem. Soc. 1980, 102, 3224.
32) Momenteau, M.; Scheidt, W. R.; Eigenbrot, C. W.; Reed, C. A.
J. Am. Chem. Soc. 1988, 110, 1207.
33) Kirner, J. F.; Reed, C. A.; Scheidt, W. R. J. Am. Chem. Soc. 1977, 99,
557.
34) Sun, L.; Shelnutt, J. A. Program is available via the internet at http://
jasheln.unm.edu.
35) Jentzen, W.; Ma, J.-G.; Shelnutt, J. A. Biophys. J. 1998, 74, 753.
˚
about 0.1 A shorter than C-H, which causes less inter-
(
action between that hydrogen and porphyrin core and
allows the axial ligand to rotate more toward that direc-
tion. Figure 3 compares the differences.
(
2
(
(36) Scheidt, W. R.; Gouterman, M. In Iron Porphyrins, Part One; Lever,
A. B. P., Gray, H. B., Eds.; Addison-Wesley: Reading, MA, 1983; p 89.
(

Article
Inorganic Chemistry, Vol. 49, No. 23, 2010 10989
Figure 3. Diagram illustrating the differing average internal angles in imidazole-ligated (left) and pyrazole-ligated (right) high-spin iron(II) derivatives.
Besides the difference of the Fe-N -X angles, an-
Table 3. M o€ ssbauer Parameters for [Fe(TPP)(Hdmpz)] and Related Five-
Coordinate Imidazole-Ligated Iron(II) Porphyrinates
ax
other difference between pyrazole and imidazole isomers
is the possibility of forming hydrogen bonds involving the
N-H group. The N-H group faces toward the porphyrin
plane when pyrazole is coordinated to iron; this orienta-
tion rules out the possibility of hydrogen bonding. How-
ever, the N-H group is away from the porphyrin plane
when imidazole is coordinated to iron, and this orienta-
tion allows the formation of hydrogen bonds. Indeed, the
latter is common in biological systems.
a
a
b
complex
Fe(TPP)(Hdmpz)]
Fe(TPP)(2-MeHIm)]
ΔE
Q
δ
Fe
Γ
T, K ref.
[
[
[
-2.54 0.91 0.43 4.2 tw
-2.40 0.92 0.50 4.2
5
Fe(Tp-OCH
3
PP)(1,2-Me
PP)(2-MeHIm)]
Im)]
2
Im)]
-2.44 0.95 0.46 4.2 10
-2.18 0.94 0.58 4.2 10
-1.93 0.92 0.44 4.2 10
-1.96 0.86 0.55 4.2 10
-1.95 0.85 0.42 4.2 10
-2.06 0.86 0.43 4.2 10
-2.23 0.92 0.37 4.2 12
-1.96 0.90 0.41 4.2 12
-2.28 0.93 0.31 4.2 37
[Fe(Tp-OCH
3
[
[
[
[
Fe(TPP)(1,2-Me
Fe(TPP)(2-MeHIm)]
Fe(TTP)(2-MeHIm)]
2
2
Fe(TTP)(1,2-Me Im)]
Both pyrazole- and imidazole-ligated species have dis-
tinctly different structural features than porphyrins hav-
ing coordinated anionic ligand species (Table 2). In the
2
[Fe(OEP)(1,2-Me Im)]
[
[
[
[
Fe(OEP)(2-MeHIm)]
Fe(TPP)(2-MeHIm)(2-fold)
Fe(TPP)(1,2-Me
C
2
Im)]
Fe((Piv P)(1-MeIm)]
-2.16 0.92 0.25 4.2 37
latter, the Fe-N bond distances are much longer and the
c
p
2
8
-2.3
0.88 0.40 4.2 32
Fe-Nax distance is much shorter; further, the displace-
Hb
Mb
-2.40 0.92 0.30 4.2 37
-2.22 0.92 0.34 4.2 37
þ3.71 1.00 0.31 4.2 11
˚
ments of iron(II) are much larger (0.10-0.20 A) than
-
[
[
[
[
K(222)][Fe(OEP)(2-MeIm )]
K(222)][Fe(TPP)(2-MeIm )]
Fe(TPpivP)(2-MeIm )]
those of the imidazole- or pyrazole-ligated species. Thus
the structural data show that the pyrazole-ligated species
are much like the imidazole-ligated species and not the
anionic ligated species. Does this hold true for the spe-
cies’s electronic structures?
-
þ3.60 1.00 0.32 4.2 11
-
-
d
þ3.51 0.97
77
17
-
Fe(TpivPP)(SC
)(TPP)]
CCH
Fe(OCH )(TpivPP)]
Fe(OC
Fe(TpivPP)(SC
2
H
5
)]
þ2.18 0.83 0.30 4.2 38
-
[Fe(OC
6
H
5
þ4.01 1.03
4.2 39
þ4.25 1.05 0.30 4.2 40
-
[
[
[
[
Fe(O
2
3
)(TpivPP)]
-
d
3
þ3.67 1.03 0.40 4.2 15
Electronic Structure. Previous studies on imidazole-
and imidazolate-ligated five-coordinate iron(II) porphy-
rinates have shown that these represent two distinct groups
-
d
6
H
5
)(TpivPP)]
HF
þ3.90 1.06 0.38 4.2 15
d
6
4
)][NaC12
H
24
O ]
6
þ2.38 0.84 0.28 4.2 38
d
[Fe(TpivPP)(SC HF )][Na(222)]
4
6
þ2.38 0.83 0.32 4.2 38
10,11,13
-
d
M o€ ssbauer
[Fe(TpivPP)Cl]
þ4.36 1.01 0.31 77
14
with different electronic configurations.
spectra of the imidazolate-ligated and other anionic-ligated
species show a large positive value of the quadrupole
splitting with a small asymmetry parameter (η); they
a
b
mm/s. Line width, fwhm. Sign not determined experimentally,
c
d
presumed negative. Sign not determined experimentally, presumed
positive.
have the normal doubly occupied d ground state,
(
xy
The M
o€ ssbauer data of [Fe(TPP)(Hdmpz)] shows fea-
tures similar to those of imidazole-ligated species. The
2
1
1
1
2 1
d ) (d ) (d ) (d ) (d -y ) . On the other hand,
z2
x2
xy
xz
yz
M o€ ssbauer spectra of the imidazole-ligated species, in-
cluding deoxymyoglobin and deoxyhemoglobin, show a
negative value of the quadrupole splitting with large
asymmetry parameters; the doubly occupied d orbital
can be best described as a hybrid orbital composed of a
value of quadrupole splitting (ΔE ) observed at 4.2 K is
Q
2.54 mm/s, similar to those previously reported for high-
spin iron(II) species as shown in Table 3.
10,37
The large
value of the isomer shift (δ∼0.91 mm/s) is also consistent
41
with high-spin iron(II). There is substantial tempera-
ture variation of the quadrupole splitting values; com-
linear combination of d , dyz orbitals, and a modest
xz
contribution from the d orbital that is consistent with
xy
1
3
plete values of δ and ΔE
Supporting Information, Table S13. A plot of the data for
Fe(TPP)(Hdmpz)] and related species is given in Figure 4.
Q
versus T are tabulated in the
the low symmetry.
[
(
37) Kent, T. A.; Spartalian, K.; Lang, G.; Yonetani, T.; Reed, C. A.;
Collman, J. P. Biochem. Biophys. Acta 1979, 580, 245.
38) Schappacher, M.; Ricard, L.; Fisher, J.; Weiss, R.; Montiel-Montoya,
The value of ΔE decreases significantly as the tempera-
ture is increased. The observed temperature dependence
Q
(
R.; Bill, E. Inorg. Chem. 1989, 28, 4639.
(
39) Shaevitz, B. A.; Lang, G.; Reed, C. A. Inorg. Chem. 1988, 27, 4607.
(41) Debrunner, P. G. In Iron Porphyrins Part 3; Lever, A. B. P., Gray,
H. B., Eds.; VCH Publishers Inc.: New York, 1983; Chapter 2.
(42) Kent, T. A.; Spartalian, K.; Lang, G.; Yonetani, T. Biochem.
Biophys. Acta 1977, 490, 331.
(
40) Bominaar, E. L.; Ding, X.; Gismelseed, A.; Bill, E.; Winkler, H.;
Trautwein, A. X.; Nasri, H.; Fisher, J.; Weiss, R. Inorg. Chem. 1992, 31,
845.
1

1
0990 Inorganic Chemistry, Vol. 49, No. 23, 2010
Hu et al.
Figure 4. Plot illustrating the temperature-dependent quadrupole splitting values in the M o€ ssbauer data.
of ΔE is similar to the variation seen for imidazole-ligated
Q
6
,10,11,37,42-47
10
As discussed previously, the ex-
samples.
planation for this temperature variation is that there are
close-lying excited states. The excited states could have
the same or differing spin multiplicity relative to the
ground state. These data clearly suggest that the pyrazole-
ligated species have the same electronic structure as those
imidazole-ligated species, which is further confirmed by
the M o€ ssbauer data under applied magnetic fields.
The application of applied magnetic field M o€ ssbauer
spectroscopy provides more detailed information con-
cerning the electronic ground states. The M o€ ssbauer data
in strong magnetic fields shown in Figure 5 were fit with
37
the spin Hamiltonian model used by Kent et al.
2
z
2
x
2
y
Q
H ¼ D½S - 1=3SðS þ 1Þꢁ þ EðS - S Þ þ HB g~ SB þ H
3 3
ꢂ
ꢂ
~
-
g β HB BI þ SB A BI
3 3
N
N
3
where D and E are the axial and rhombic zero-field
splitting parameters that describe the fine structure of
Figure 5. M o€ ssbauer data and fits obtained for [Fe(TPP)(Hdmpz)] at
~
*
the S = 2 multiplet, A is the magnetic hyperfine tensor,
and H gives the nuclear quadrupole interaction:
3
9
T, 6 T, 9 T applied magnetic fields. Fitting parameters were D=
.4 cm , E/D=0.29 A/(g β )=(-6.8, -6.8, -2.2) T. Euler angles
n n
Q
-1
(
D relative to Q) (R,β,γ)=(-58, 36, 31)°. Lorentzian fwhm=0.43 mm/
s. Simulations of 3, 6, and 9 T spectra at 4.2 K assume slow spin
relaxation.
eQVzz
Q
2
z
2
x
2
y
H
¼
½3I - IðI þ 1Þ þ ηðI - I Þꢁ
1
2
5
7
An analysis of the spectra shows the largest component
of the electric field gradient, V , has a negative value, and
hence the sign of the quadrupole splitting value is also
negative. The asymmetry parameter for [Fe(TPP)-
Q is the quadrupole moment of the Fe nucleus and η=
Vxx - V )/V , where V are components of the electric
zz
(
yy
zz
ii
field gradient. The quadrupole splitting and isomer shift
were constrained to the values determined from the zero-
field data.
(Hdmpz)] (0.74) is relatively large, which is consistent
with those for imidazole-ligated species.
1
0,37,42-47
The
large asymmetry parameter indicates the low symmetry
of the electric field gradient (EFG), which is also reflected
in the solid-state structure. Our earlier study suggested
that imidazole-ligated species are unique and have an
unusual electronic configuration. M o€ ssbauer study of
(
(
(
43) Bade, D.; Parak, F. Biophys. Struct. Mechanism 1976, 2, 219.
44) Eicher, H.; Bade, D.; Parak, F. J. Chem. Phys. 1976, 64, 1446.
45) Huynh, B. H.; Paraefthymiou, G. C.; Yen, C. S.; Wu, C. S. J. Chem.
Phys. 1974, 61, 3750.
(
(
46) Eicher, H.; Trautwein, A. J. Chem. Phys. 1970, 52, 932.
47) Eicher, H.; Trautwein, A. J. Chem. Phys. 1969, 50, 2540.

Article
Fe(TPP)(Hdmpz)] indicates that pyrazole, another neu-
tral ligand, functions just as imidazole in the five-coordi-
nate iron(II) porphyrinates.
Summary. We have synthesized two pyrazole-ligated
five-coordinate high-spin iron(II) porphyrinates, [Fe(TPP)-
Inorganic Chemistry, Vol. 49, No. 23, 2010 10991
[
the National Institutes of Health for support of this
research under Grant GM-38401 to W.R.S. and the
National Natural Science Foundation of China (No.
20971093) to C.H. We thank the NSF for X-ray instru-
mentation support through Grant CHE-0443233.
(
Hdmpz)] and [Fe(Tp-OCH PP)(Hdmpz)]. Despite the
3
structural and ligand basicity difference between pyrazole
and imidazole, the X-ray crystallography suggests that
these new pyrazole species have similar structural features
as the imidazole-ligated species, which indicates that the
new neutral ligand, pyrazole, has similar coordination
ability as imidazole to the iron(II) porphyrinate. Consis-
tent with the structural studies, M o€ ssbauer spectra at
variable temperature and applied magnetic fields for
Supporting Information Available: Figure S1 displays the zero
field M o€ ssbauer spectrum of [Fe(TPP)(Hdmpz)]. Figure S2
shows ORTEP diagrams of [Fe(TPP)(Hdmpz)] and [Fe(Tp-
OCH PP)(Hdmpz)] including all the disordered atoms. Figure S3
3
displays displacements from the 24-atom plane. Tables S1-S12
give complete crystallographic details, atomic coordinates,
bond distances and angles, anisotropic temperature factors,
and fixed hydrogen atom positions for [Fe(TPP)(Hdmpz)] and
3
[Fe(Tp-OCH PP)(Hdmpz)]. Tables S13 and S14 give full details
[
Fe(TPP)(Hdmpz)] further confirm that the new species
on the observed M o€ ssbauer data and the fits. Table S15 shows
analysis results of the core conformations by the normal struc-
tural decomposition method. This information is available as a
PDF file. The crystallographic information files (CIF) are also
available. This material is available free of charge via the
Internet at http://pubs.acs.org.
have the same electronic structure as the imidazole-
ligated species.
Acknowledgment. We thank Professor N. Lehnert
Univ. of Michigan) for helpful discussions. We thank
(