Iron(II) Carboxylate Complexes Based on a Tetraimidazole Ligand as Models of the Photosynthetic Non-Heme Ferrous Sites: Synthesis, Crystal Structure, and M?ssbauer and Magnetic Studies

Article abstract of DOI:10.1021/ic034907k

The preparations, X-ray structures, and detailed physical characterization are presented for new complexes involving an iron(II) center, a tetraimidazole ligand (TIM), and different carboxylates. [Fe(TIM)(C₆H ₅CH₂CO₂)](CIO₄) (1) crystallizes in the Pbca space group with a = 10.8947(13), b = 20.343(2), and c = 22.833(3) ?, Z = 8, and V = 5060.6(11) ?³. [Fe(TIM)(CH ₃CO₂)](CIO₄) (2) crystallizes in the la space group with a = 17.117(2), b = 10.3358(12), and c = 25.658(3) ?, β = 90.301(13)°, Z = 8, and V = 4539.5(9) ?³. In both structures, the iron(II) is hexacoordinated to the four N_imidazole donors of the TIM ligand and the two O donors of a bidentate carboxylate. The flexibility of the carboxylate bidentate coordination, symmetrical or more or less asymmetrical, associated with the steric demand of the TIM ligand results in a remarkable versatility of the Fe¹¹N₄O₂ coordination geometry. The diversity in carboxylate bidentate coordination modes has allowed us to clearly show the importance of the structural and electronic effects, through IR and M?ssbauer spectroscopy, of this apparently tenuous carboxylate shift. Comparison of the structural and M?ssbauer properties of these complexes with the non-heme ferrous site of photosynthetic systems (i) shows that the metric parameters of site 2b, including the symmetrically chelated bidentate carboxylate, are closer to those of the non-heme ferrous site in the bacterial reaction centers of Rhodopseudomonas viridis and R. sphaeroides and (ii) suggests that the ligand environment of the non-heme ferrous center of PS 2 is close to the axially distorted octahedral symmetry resulting from an asymmetrical bidentate coordination of the -CO₂ motif, as in complex 1.

Full text of DOI:10.1021/ic034907k

Inorg. Chem. 2004, 43, 2105−2113
Iron(II) Carboxylate Complexes Based on a Tetraimidazole Ligand as
Models of the Photosynthetic Non-Heme Ferrous Sites: Synthesis,
Crystal Structure, and Mo1ssbauer and Magnetic Studies
Gilles Lemercier,†,‡ Etienne Mulliez,§ Chantal Brouca-Cabarrecq,†,| Franc¸oise Dahan,† and
,†
Jean-Pierre Tuchagues*
Laboratoire de Chimie de Coordination, UPR CNRS 8241, 205 route de Narbonne,
31077 Toulouse Ce´dex 4, France, and Laboratoire de Chimie et Biochimie des Centres Re´dox
Biologiques, DBMS, CEA/CNRS/UniVersite´ Joseph Fourier, 17 AVenue des Martyrs,
38054 Grenoble Cedex 09, France
Received July 31, 2003
The preparations, X-ray structures, and detailed physical characterization are presented for new complexes involving
an iron(II) center, a tetraimidazole ligand (TIM), and different carboxylates. [Fe(TIM)(C H CH CO )](ClO ) (1) crystallizes
6
5
2
2
4
in the Pbca space group with a ) 10.8947(13), b ) 20.343(2), and c ) 22.833(3) Å, Z ) 8, and V ) 5060.6(11)
3
Å . [Fe(TIM)(CH CO )](ClO ) (2) crystallizes in the Ia space group with a ) 17.117(2), b ) 10.3358(12), and c )
3
2
4
3
25.658(3) Å, â ) 90.301(13)°, Z ) 8, and V ) 4539.5(9) Å . In both structures, the iron(II) is hexacoordinated
to the four Nimidazole donors of the TIM ligand and the two O donors of a bidentate carboxylate. The flexibility of the
carboxylate bidentate coordination, symmetrical or more or less asymmetrical, associated with the steric demand
II
of the TIM ligand results in a remarkable versatility of the Fe N O coordination geometry. The diversity in carboxylate
4
2
bidentate coordination modes has allowed us to clearly show the importance of the structural and electronic effects,
through IR and Mo¨ssbauer spectroscopy, of this apparently tenuous carboxylate shift. Comparison of the structural
and Mo¨ssbauer properties of these complexes with the non-heme ferrous site of photosynthetic systems (i) shows
that the metric parameters of site 2b, including the symmetrically chelated bidentate carboxylate, are closer to
those of the non-heme ferrous site in the bacterial reaction centers of Rhodopseudomonas viridis and R. sphaeroides
and (ii) suggests that the ligand environment of the non-heme ferrous center of PS 2 is close to the axially distorted
octahedral symmetry resulting from an asymmetrical bidentate coordination of the −CO motif, as in complex 1.
2
Introduction
loproteins, including lipoxygenases,2 Fe superoxide dismutase
(SOD),3 hemerythrin,4 isopenicilin N-synthase (IPSN),5 intra-
and extradiol dioxygenases,6 etc. A pyramidal coordination
Non-heme iron metalloproteins with mono active sites are
involved in various biological processes.1 The X-ray crystal
structure has been established for a number of such metal-
(2) (a) Minor, W.; Steckzko, J.; Stec, B.; Otwinowski, Z.; Bolin, J. T.;
Walter, R.; Axelrod, B. Biochemistry 1996, 35, 10687. (b) Skrzypczak-
Jankun, E.; Amzel, L. M.; Kroa, B. A.; Funk, M. O. J. Proteins:
Struct., Funct., Genet. 1997, 29, 15.
(3) (a) Cooper, J. B.; Mclntyre, K.; Badasso, M. O.; Wood, S. P.; Zhang,
Y.; Garbe, T. R.; Young, D. J. Mol. Biol. 1995, 246, 531. (b) Lah, M.
S.; Dixon, M. M.; Pattridge, K. A.; Stallings, W. C.; Fee, J. A.; Ludwig,
M. L. Biochemistry 1995, 34, 1646. (c) Ringe, D.; Petsko, G. A.;
Yamakura, K.; Suzuki, D.; Ohmori, D. Proc. Natl. Acad. Sci. U.S.A.
1983, 80, 3879. (d) Stallings, W. C.; Powers, T. B.; Pattridge, K. A.;
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3884. (e) Sttoddard, B. L.; Howell, P. L.; Ringe, D.; Petsko, G. A.
Biochemistry 1995, 29, 8885.
* To whom correspondence should be addressed. E-mail: tuchague@
lcc-toulouse.fr.
† Laboratoire de Chimie de Coordination.
‡ Present address: EÄ cole Normale Supe´rieure de Lyon, Laboratoire de
Chimie UMR CNRS and ENS-Lyon Nï. 5532, 46 alle´e d’Italie, 69364
Lyon Ce´dex 07, France. E-mail: gilles.lemercier@ens-lyon.fr.
§ Laboratoire de Chimie et Biochimie des Centres Re´dox Biologiques.
| Present address: Centre d’Elaboration de Mate´riaux et d’Etudes
Structurales, UPR CNRS 8011, 29 rue Jeanne Marvig, 31055 Toulouse
Ce´dex, France.
(1) (a) Solomon, E. I.; Brunold, T. C.; Davis, M. I.; Kemsley, J. N.; Lee,
S. K.; Lehnert, N.; Neese, F.; Skulan, A. J.; Yang, Y. S.; Zhou, J.
Chem. ReV. 2000, 100, 235. (b) Holm, R. H.; Kennepohl, P.; Solomon,
E. I. Chem. ReV. 1996, 96, 2239. (c) Lippard, S. J.; Berg, J. M.
Principals of Bioinorganic Chemistry; University Science Books: Mill
Valley, CA 94941, 1994.
(4) (a) Pulver, S.; Froland, W. A.; Fox, B. G.; Lipscomb, J. D.; Solomon,
E. I. J. Am. Chem. Soc. 1993, 115, 12049. (b) Reem, R. C.;
McCormick, J. C.; Richardson, D. E.; Devin, F. J.; Stephens, P. J.;
Musselman, R. L.; Solomon, E. I. J. Am. Chem. Soc. 1989, 111, 8.
(c) Stenkamp, R. E. Chem. ReV. 1994, 94, 715.
10.1021/ic034907k CCC: $27.50 © 2004 American Chemical Society
Published on Web 02/18/2004
Inorganic Chemistry, Vol. 43, No. 6, 2004 2105

Lemercier et al.
arrangement of three histidyl nitrogen donors is often found
at the metal sites of such proteins. A quite different
coordination structure characterizes the non-heme iron
located between the primary and secondary quinone electron
acceptors of the reaction centers in photosynthetic bacteria7
and photosystem 2 of oxygenic photosynthetic organisms.8
The X-ray structure determination of the reaction centers of
two different photosynthetic bacteria, Rhodopseudomonas
Viridis9 and R. sphaeroides,10 indicates that the ferrous ion
is in a distorted-octahedral ligand environment including four
nitrogen atoms pertaining to the imidazole moiety of histidine
residues and two oxygen atoms from a glutamic acid residue
of the surrounding protein.
Figure 1. Schematic representation of bis[(imidazol-4-methyl)-4′-imidazol-
2′-yl]methane (TIM).
Experimental Section
Materials. All reagents were of analytical grade and used without
further purification. Solvents were degassed under vacuum prior
to use. Ferrous acetate and ferrous formate were prepared according
to the method of Schreurer-Kestner as modified by Rhoda et al.13
Due to the high reactivity of iron(II) salts and complexes with
dioxygen, all complexes were prepared under a purified nitrogen
atmosphere in Schlenk-type vessels or in an inert-atmosphere box
(Vacuum Atmosphere HE 43-2) equipped with a Dry-Train (Jahan
EVAC 7).
A few iron(II) complexes including imidazole ligands have
been previously reported.11 However, none of them possess
the unique structural characteristics of the non-heme ferrous
iron of photosynthetic systems. Owing to the interest of
ferrous complexes as models of the ferrous center of
mononuclear non-heme iron-containing proteins, we have
synthesized and studied a novel series of chelates resulting
from the reaction of a tetraimidazole ligand, bis[(imidazol-
4-methyl)-4′-imidazol-2′-yl]methane,12 abbreviated TIM, with
ferrous salts. In this report, we describe the synthesis and
IR, Mo¨ssbauer, and variable-temperatue magnetic suscepti-
bility results for [Fe(TIM)(C6H5CH2CO2)](ClO4) (1), [Fe-
(TIM)(CH3CO2)](ClO4) (2), Fe2(TIM)2(C2O4)(ClO4)2 (3),
Fe(TIM)(HCO2)2 (4), and Fe(TIM)(CH3CO2)2 (5). X-ray
crystal structure determinations of 1 and 2 have also been
performed.
Ligand. The tetraimidazole ligand TIM (Figure 1) was prepared
as a faint-yellow microcrystalline powder, according to the
described procedure.12
Complexes. In a typical reaction for the synthesis of complexes
[Fe(TIM)(C6H5CH2CO2)](ClO4) (1), [Fe(TIM)(CH3CO2)](ClO4) (2),
and Fe2(TIM)2(C2O4)(ClO4)2 (3), TIM‚2H2O (0.6 mmol) was slowly
added as a solid to a yellow-green solution of Fe(ClO4)2‚6H2O (0.6
mmol) in methanol (7 mL) under stirring. The appropriate sodium
carboxylate was then added to the purple-red reaction mixture (2.1
equiv of solid sodium phenylacetate for 1, 1.3 equiv of a methanolic
solution of sodium acetate (105 mg in 1 mL) for 2, 0.55 equiv of
an aqueous solution of sodium oxalate (33.5 mg in 5 mL) for 3).
Complexes 1-3 slowly precipitated out of the reaction mixture and
were collected by filtration after 24 h (1, white-pale pink; 2, white-
light green; 3, brown-orange). In the case of complex 3, a strict
control of the sodium oxalate ratio was needed to minimize
simultaneous formation of the mononuclear species, Fe(TIM)(C2O4-
Na)(ClO4); on the other hand, attempts at preparing this mono-
nuclear species as a pure compound by increasing the sodium
oxalate ratio failed. Fe(TIM)(HCO2)2 (4) and Fe(TIM)(CH3CO2)2
(5) were obtained by direct reaction of TIM‚2H2O (1.37 mmol, as
a solid) with a methanolic solution (7 mL) of Fe(HCO2)2‚2H2O
and Fe(CH3CO2)2‚2H2O (1.37 mmol), respectively. After 14 h of
reaction, a small amount of precipitate was eliminated by filtration
and 6-7 equiv (vol) of CH3CN was added to the reaction mixture
yielding a precipitate which was collected by filtration (4, pale
yellow; 5, white-green). Complexes 1-5 were finally dried under
vacuum.
[Fe(TIM)(C6H5CH2CO2)](ClO4) (1): yield 160 mg (45%). Anal.
Calcd for FeC23H23N8O6Cl: C, 46.1; H, 3.9; N, 18.7; Cl, 5.9; Fe,
9.3. Found: C, 46.1; H, 3.8; N, 19.0; Cl, 5.8; Fe, 9.2. [Fe(TIM)-
CH3CO2](ClO4) (2): yield 190 mg (30%). Anal. Calcd for
FeC18H23N8O7Cl: C, 39.0; H, 4.2; N, 20.2; Cl, 6.4; Fe, 10.0.
Found: C, 39.2; H, 3.8; N, 20.5; Cl, 7.1; Fe, 10.0. Fe2(TIM)2(C2O4)-
(ClO4)2 (3): yield 160 mg (85%). Anal. Calcd for Fe2C32H32N16O12-
Cl2: C, 37.8; H, 3.8; N, 21.9; Cl, 7.0; Fe, 11.0. Found: C, 37.9;
H, 3.2; N, 21.8; Cl, 6.7; Fe, 11.3. Fe(TIM)(HCO2)2 (4): yield 300
mg (46%). Anal. Calcd for FeC17H18N8O4: C, 45.0; H, 4.0; N, 24.7;
Fe, 12.3. Found: C, 45.3; H, 4.1; N, 25.2; Fe, 11.2. Fe(TIM)(CH3-
(5) Roach, P. L.; Cliflon, I. J.; Hensgens, C. M. H.; Shibata, N.; Schofield,
C. J.; Hajdu, J.; Baldwin, J. E. Nature 1997, 387, 827.
(6) (a) Ohlendorf, D. H.; Lipscomb, J. D.; Weber, P. C. Nature (London)
1988, 336, 403. (b) Ohlendorf, D. H.; Orville, A. M.; Lipscomb, J. D.
J. Mol. Biol. 1994, 244, 586. (c) Kita, A.; Kita, S.; Fujisawa, I.; Inaka,
K.; Ishida, T.; Horiike, K.; Nozaki, M.; Miki, K. Structure 1999, 7,
25. (d) Sugimoto, K.; Senda, T.; Aoshima, H.; Masai, E.; Fukuda,
M.; Mitsui, Y. Structure 1999, 7, 953.
(7) Clayton, R. K., Sistrom, W. R., Eds. The Photosynthetic Bacteria;
Plenum: New York, 1978.
(8) (a) Nugent, J. H. A.; Diner, B. A.; Evans, M. C. W. FEBS Lett. 1981,
124, 241. (b) Petrouleas, V.; Diner, B. A. FEBS Lett. 1982, 147,
111.(c) Petrouleas, V.; Diner, B. A. AdV. Photosynth. Res., Proc. Int.
Congr. Photosynth., 6th 1984, 1, part 2, 195. (d) Rutherford, A.
W.; Zimmerman, J. L. Biochim. Biophys. Acta 1984, 767, 168. (e)
Diner, B. A.; Petrouleas, V. Biochim. Biophys. Acta 1986, 849,
264.
(9) (a) Deisenhofer, J.; Epp, O.; Miki, K.; Huber, R.; Michel, H. J. Mol.
Biol. 1984, 180, 385. (b) Deisenhofer, J.; Epp, O.; Miki, K.; Huber,
R.; Michel, H. Nature 1985, 318, 19. (c) Michel, H.; Epp, O.;
Deisenhofer, J. EMBO J. 1986, 5, 2445.
(10) (a) Allen, J. P.; Feher, G.; Yeates, T. O.; Komiya, H.; Rees, D. C.
Proc. Natl. Sci. U.S.A. 1988, 85, 8487. (b) Allen, J. P.; Feher, G.;
Yeates, T. O.; Komiya, H.; Rees, D. C. Proc. Natl. Sci. U.S.A. 1987,
84, 5730.
(11) (a) Holmes, F.; Jones, K. M.; Torrible, E. G. J. Chem. Soc. 1961,
4790. (b) Abushamleh, A. S.; Goodwin, H. A. Aust. J. Chem. 1979,
32, 513. (c) Guillot, G.; Mulliez, E.; Leduc, P.; Chottard, J. C. Inorg.
Chem. 1990, 29, 577. (d) Boinnard, D.; Cassoux, P.; Petrouleas, V.;
Savariault, J.-M.; Tuchagues, J.-P. Inorg. Chem. 1990, 29, 4114. (e)
Martinez Lorente, M. A.; Petrouleas, V.; Savariault, J.-M.; Poinsot,
R.; Drillon, M.; Tuchagues, J.-P. Inorg. Chem. 1991, 30, 3587. (f)
Bois, C.; Bousseksou, A.; Nitsche, W.; Mulliez, E.; Guillot-edeheit,
G.; Leduc, P.; Chottard, J.-C. New J. Chem. 1992, 16, 435. (g) Martinez
Lorente, M. A.; Dahan, F.; Petrouleas, V.; Bousseksou, A.; Tuchagues,
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Constant-Machado, H.; Lemercier, G.; Tuchagues, J.-P.; Varret, F.
Inorg. Chem. 1996, 35, 110.
(13) (a) Schreurer-Kestner, M. A. Bull. Soc. Chim. Fr. 1863, 5, 345. (b)
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(12) Mulliez, E. Tetrahedron Lett. 1989, 30, 6169.
2106 Inorganic Chemistry, Vol. 43, No. 6, 2004

Iron(II) Carboxylate Complexes
Table 1. Crystallographic Data for [Fe(TIM)(C6H5CH2CO2)](ClO4) (1)
and [Fe(TIM)(CH3CO2)](ClO4) (2)
CO2)2 (5): yield 286 mg (84%). Anal. Calcd for FeC20H26N8O4:
C, 46.7; H, 5.1; N, 21.8; Fe, 10.8. Found: C, 46.2; H, 4.7; N, 22.1;
Fe, 10.4.
1
2
Single crystals of 1 and 2 suitable for X-ray diffraction studies
were obtained by slow evaporation of the filtrate in the glovebox.
Caution! Perchlorate salts of compounds containing organic
ligands are potentially explosiVe. Although we haVe encountered
no such problems with these complexes, only small quantities should
be prepared and handled with care.
chem formula
fw
space group
a, Å
b, Å
C23H23ClFeN8O6
598.79
Pbca (No. 61)
10.8947(13)
20.343(2)
C17H19ClFeN8O6
522.7
Ia (No. 9)
17.117(2)
10.3358(12)
25.658(3)
90.301(13)
4539.5(9)
8
1.530
0.710 73
293(2)
c, Å
22.833(3)
â, deg
V, Å3
Z
5060.6(11)
8
1.572
0.710 73
293(2)
7.59
Physical Measurements. Elemental analyses were carried out
at the Laboratoire de Chimie de Coordination Microanalytical
Laboratory in Toulouse, France, for C, H, and N and at the Service
Central de Microanalyze du CNRS in Vernaison, France, for Cl
and Fe.
IR spectra were recorded in the 200-4000 cm-1 range on a
Perkin-Elmer 983 spectrophotometer coupled with a Perkin-Elmer
infrared data station. Samples were run as CsBr pellets prepared
under nitrogen in the glovebox.
F
calcd, g‚cm-3
γ, Å
T, K
µ(Mo KR), cm-1
Ra (obsd, all)
Rwb (obsd, all)
5.80
0.0259, 0.0404
0.0585, 0.0632
0.0383, 0.0431
0.0927, 0.0931
2
2
2 2
a R ) ∑||Fo| - |Fc||/∑|Fo|. b Rw ) [∑w(|Fo| - |Fc| )2/∑w|Fo | ]1/2
.
Table 2. Selected Interatomic Distances (Å) and Angles (deg) for
[Fe(TIM)(C6H5CH2CO2)](ClO4) (1)
Variable-temperature magnetic susceptibility measurements were
obtained with a Quantum Design MPMS SQUID susceptometer
as previously described.14 All samples were 3 mm diameter pellets
molded in the glovebox from microcrystalline samples. Magnetic
susceptibility measurements were performed in the 2-300 K
temperature range, and diamagnetic corrections were applied by
using Pascal’s constants. Least-squares computer fittings of the
experimental data were accomplished with an adapted version of
the function-minimization program STEPT.15
Fe-N(1)
2.1762(18)
2.1023(17)
2.1634(16)
86.21(7)
168.63(7)
94.28(7)
91.58(7)
89.26(6)
84.29(7)
116.64(7)
57.91(6)
Fe-N(3)
2.1367(18)
2.1064(19)
2.3259(16)
Fe-N(2)
Fe-N(4)
Fe-O(1)
Fe-O(2)
N(1)-Fe-N(2)
N(1)-Fe-N(3)
N(1)-Fe-N(4)
N(1)-Fe-O(1)
N(1)-Fe-O(2)
N(2)-Fe-N(3)
N(2)-Fe-N(4)
O(1)-Fe-O(2)
N(2)-Fe-O(1)
N(2)-Fe-O(2)
N(3)-Fe-N(4)
N(3)-Fe-O(1)
N(3)-Fe-O(2)
N(4)-Fe-O(1)
N(4)-Fe-O(2)
145.55(6)
87.66(6)
84.44(7)
99.79(7)
96.58(6)
97.81(7)
155.59(6)
Mo¨ssbauer measurements were obtained on a constant-accelera-
tion spectrometer with a 50 mCi source of 57Co (Rh matrix). Isomer
shift values (δ) are given with respect to metallic iron at room
temperature. The absorber was a sample of 100 mg of microcrys-
talline powder enclosed in a 20 mm diameter cylindrical plastic
sample holder, the size of which had been determined to optimize
the absorption. Variable-temperature spectra were obtained in the
80-305 K range, by using a MD306 Oxford cryostat, the thermal
scanning being monitored by an Oxford ITC4 servocontrol device
((0.1 K accuracy). A least-squares computer program16 was used
to fit the Mo¨ssbauer parameters and determine their standard
deviations of statistical origin (given in parentheses).
SHELXS-9719 and refined on F2 by full-matrix least-squares using
SHELXL-9720 with anisotropic displacement parameters for all non-
hydrogen atoms. H atoms were introduced in calculations using
the riding model with isotropic UH equal to 1.1 times that of the
riding atom. In 1, three oxygen atoms of the perchlorate anion were
found disordered and refined in the 60/40 ratio. In 2, four oxygen
atoms of one ot the two perchlorate anions were found disordered
and refined in the 60/40 ratio. The atomic scattering factors and
anomalous dispersion terms were taken from the standard compila-
tion.21 The maximum and minimum peaks on the final difference
Fourier map were 0.438 and -0.376 e A-3 for 1 and 0.210 and
-0.208 e Å-3 for 2. Drawings of the molecules were performed
with the program ZORTEP.22 Crystal data collection and refinement
parameters are given in Table 1, and selected bond distances and
angles are gathered in Tables 2 and 3 for 1 and 2, respectively.
Crystallographic Data Collection and Structure Determina-
tion. The selected crystals of 1 (translucid plate, 0.50 × 0.50 ×
0.10 mm3) and 2 (colorless parallelepiped, 0.40 × 0.35 × 0.30
mm3) were coated with vaselin under an inert atmosphere and sealed
in Lindemann glass capillaries and then mounted on an Enraf-
Nonius CAD4 diffractometer using a graphite-monochromated Mo
KR radiation (λ ) 0.710 73 Å). Data were collected at 293 K up
to 27° for 1 and to 26° for 2 in the ω-2θ scan mode and reduced
with the MolEN package.17 A total of 5493 reflections, all
independent, were collected for 1, and 6737 reflections, all
independent, were collected for 2. Absorption corrections18 from
ψ scans were applied (Tmin-max ) 0.9210-0.9999 for 1, Tmin-max
) 0.9800-0.9971 for 2). The structures were solved using
Results and Discussion
Syntheses. Reaction of the acyclic neutral tetraimidazole
ligand TIM with ferrous acetate and ferrous formate yielded
complexes analyzing for one TIM ligand and two carboxy-
lates per iron. These complexes are soluble in alcohols, and
addition of acetonitrile was required to separate them from
the reaction medium as faintly colored microcrystalline
(14) Clemente-Juan, J. M.; Mackiewicz, C.; Verelst, M.; Dahan, F.;
Bousseksou, A.; Sanakis, Y.; Tuchagues, J.-P. Inorg. Chem. 2002,
41, 1478.
(15) Chandler, J. P. Quantum Chemistry Program Exchange; Indiana
University: Bloomington, IN, 1973; Program 66.
(16) Varret, F. Proceedings of the International Conference on Mo¨ssbauer
Effect Applications, Jaipur, India, 1981; Indian National Science
Academy: New Delhi, 1982.
(17) Fair, C. K. MolEN. Structure Solution Procedures; Enraf-Nonius:
Delft, Holland, 1990.
(18) North, A. C. T.; Phillips, D. C.; Mathews, F. S. Acta Crystallogr.,
Sect. A 1968, A24, 351.
(19) Sheldrick, G. M. SHELXS-97. Program for Crystal Structure Solution;
University of Go¨ttingen: Go¨ttingen, Germany, 1990.
(20) Sheldrick, G. M. SHELXL-97. Program for the refinement of crystal
structures from diffraction data; University of Go¨ttingen: Go¨ttingen,
Germany, 1997.
(21) International Tables for Crystallography; Kluwer Academic Publish-
ers: Dordrecht, The Netherlands, 1992; Vol. C, Tables 4.2.6.8 and
6.1.1.4.
(22) Zsolnai, L.; Pritzkow, H.; Huttner, G. ZORTEP. Ortep for PC, Program
for Molecular Graphics; University of Heidelberg: Heidelberg,
Germany, 1996.
Inorganic Chemistry, Vol. 43, No. 6, 2004 2107

Lemercier et al.
Table 3. Selected Interatomic Distances (Å) and Angles (deg) for
[Fe(TIM)(CH3CO2)](ClO4) (2)
Fe(1)-N(1a)
Fe(1)-N(3a)
Fe(1)-N(5a)
Fe(1)-N(7a)
Fe(1)-O(1a)
Fe(1)-O(2a)
N(1a)-Fe(1)-N(3a)
2.166(3) Fe(2)-N(1b)
2.112(2) Fe(2)-N(3b)
2.148(3) Fe(2)-N(5b)
2.119(3) Fe(2)-N(7b)
2.108(2) Fe(2)-O(1b)
2.435(3) Fe(2)-O(2b)
86.59(10) N(1b)-Fe(2)-N(3b)
2.112(3)
2.192(3)
2.126(2)
2.217(3)
2.227(2)
2.263(2)
85.20(10)
N(1a)-Fe(1)-N(5a) 170.16(11) N(1b)-Fe(2)-N(5b) 114.00(11)
N(1a)-Fe(1)-N(7a)
N(1a)-Fe(1)-O(1a)
N(1a)-Fe(1)-O(2a)
N(3a)-Fe(1)-N(5a)
98.67(10) N(1b)-Fe(2)-N(7b)
91.56(10)
92.43(10) N(1b)-Fe(2)-O(1b) 157.51(9)
91.51(9)
N(1b)-Fe(2)-O(2b)
99.65(10)
83.45(10)
83.80(10) N(3b)-Fe(2)-N(5b)
N(3a)-Fe(1)-N(7a) 116.33(10) N(3b)-Fe(2)-N(7b) 166.49(10)
N(3a)-Fe(1)-O(1a)
97.14(9)
N(3b)-Fe(2)-O(1b) 100.48(9)
N(3b)-Fe(2)-O(2b) 104.12(10)
N(3a)-Fe(1)-O(2a) 153.05(9)
N(5a)-Fe(1)-N(7a)
83.79(10) N(5b)-Fe(2)-N(7b)
85.91(10)
88.37(9)
N(5a)-Fe(1)-O(1a)
N(5a)-Fe(1)-O(2a)
90.84(10) N(5b)-Fe(2)-O(1b)
98.00(9)
N(5b)-Fe(2)-O(2b) 146.11(10)
N(7a)-Fe(1)-O(1a) 145.15(9)
N(7b)-Fe(2)-O(1b)
N(7b)-Fe(2)-O(2b)
O(1b)-Fe(2)-O(2b)
87.50(9)
89.35(10)
57.88(8)
N(7a)-Fe(1)-O(2a)
O(1a)-Fe(1)-O(2a)
90.54(9)
56.05(9)
Figure 2. ORTEP view of the [Fe(TIM)(C6H5CH2CO2)](ClO4) (1)
complex with atom numbering.
solids, pale yellow for Fe(TIM)(HCO2)2 (4) and light green
for Fe(TIM)(CH3CO2)2 (5). However, we were unsuccessful
in growing crystals large enough to carry out an X-ray
structural study of 4 and 5. On the other hand, reaction of
TIM with ferrous perchlorate resulted in the immediate
appearance of an intense purple-red coloration of the reaction
mixture. The intensely colored compound formed in the
absence of coordinating anion, probably a low-spin FeIIN6
species, was very soluble and could not be precipitated out
of the solution. Subsequent methatheses reactions carried out
with different alkaline carboxylates yielded new species
which slowly precipitated out of the reaction medium as
faintly colored microcrystalline solids, white-pale pink for
1, white-light green for 2, and brown-orange for 3. Elemental
anlyses indicated the simultaneous presence of one carboxy-
late and one perchlorate anions for 1 and 2 and half an
oxalate and one perchlorate anions for 3, allowing in
conjunction with IR spectroscopy (see below) to formulate
these new species as Fe(TIM)(C6H5CH2CO2)](ClO4) (1), [Fe-
(TIM)(CH3CO2)](ClO4) (2), and Fe2(TIM)2(C2O4)(ClO4)2 (3).
While we succeeded to subsequently grow crystals of 1 and
2, we were unsuccessful in growing large enough crystals
to carry out an X-ray structural study of 3.
Figure 3. ORTEP view of the [Fe(TIM)(CH3CO2)](ClO4) (2) complex
with atom numbering.
to FeII is severely distorted from the octahedral one (see bond
lengths and angles in Table 2 and see Figure 2).
Molecular Structure of 1 and 2. The molecular structure
of [Fe(TIM)(C6H5CH2CO2)](ClO4) (1) consists of a [Fe-
(TIM)(C6H5CH2CO2)]+ complex cation, shown in Figure 2,
and a perchlorate counteranion.
Adjacent complex cations are hydrogen-bonded together
through NH(5a)‚‚‚O1 (-1/2 + x, 1 - y, z) and NH(7a)‚‚‚
O2 (-1/2 + x, 1/2 + y, 1/2 + z) imidazole-carboxylate
contacts and also to perchlorate counteranions through NH-
(6a)‚‚‚O3 (1/2 + x, 1 - y, z), NH(8a)‚‚‚O42 (1/2 + x, 1/2
+ y, -1/2 + z) and NH(8)‚‚‚O51 (1/2 + x, 1/2 + y, -1/2
+ z) imidazole-perchlorate contacts (2.79 < N‚‚‚O < 3.20
Å, CIF file) yielding a dense 3D network responsible for
the crystal packing.
The molecular structure of [Fe(TIM)(CH3CO2)](ClO4) (2)
consists of two slightly different [Fe(TIM)(CH3CO2)]+
complex cations, shown in Figure 3, and perchlorate coun-
teranions, both termed a and b.
The iron(II) is hexacoordinated to N1, N2, N3, and N4
from the TIM ligand (2.102-2.176 Å) and O1 and O2 from
the phenylacetate anion. The bidentate coordination mode
of the phenylacetate anion (Fe-O1 ) 2.164(2) Å, Fe-O2
) 2.328(2) Å and O1-Fe-O2 ) 57.9°) is significantly
asymmetric and results from the steric constraint imposed
by the three methylene bridges between the imidazole rings
of TIM which does not allow an equatorial tetradentate
coordination mode; i.e., tetracoordination through N1, N2,
N3, and N4 is not compatible with a planar conformation of
TIM. As a result, Fe is 0.6637(2) Å above the average N1,
N2, N3, N4 plane and the symmetry of the N4O2 donor set
The overall structure of the a and b complex cations of 2,
[Fe(TIM)(CH3CO2)]2+, is virtually the same as in complex
2108 Inorganic Chemistry, Vol. 43, No. 6, 2004

Iron(II) Carboxylate Complexes
Table 4. Carboxylate Anions: IR Data (cm-1) for Complexes 1-5 (∆
) νas(CO2-), νs(CO2-)) and for the Corresponding Sodium Carboxylate
Table 5. Effective Magnetic Moment (µEff/Fe) at Selected
Temperatures for Complexes 1-5
(∆ionic
)
temp, K
-
no.
complex
νas(CO2-), νs(CO2
) D ∆
ionic
complex (no.)
300 100
30
11
2
1
2
[FeII(TIM)(C6H5CH2CO2)](ClO4)
1557, 1405
1575, 1475
1575, 1425
1639, 1315
1595, 1341
1574, 1420
1574, 1400
152 190
100 165
150
324 320
254 240
154 165
174
[Fe(TIM)(C6H5CH2CO2)](ClO4) (1)
5.15 5.20 5.15 4.83 4.40a
[FeII(TIM)(CH3CO2)](ClO4)‚MeOH
[Fe(TIM)CH3CO2](ClO4)‚MeOH (2) 5.41 5.38 5.27 4.95 3.43
Fe2(TIM)2(C2O4)(ClO4) (3)
Fe(TIM)(HCO2)2 (4)
Fe(TIM)(CH3CO2)2‚MeOH (5)
4.68 4.52 3.87 2.38 1.29
5.30 5.18 4.87 4.13 2.24
5.10 5.12 5.05 4.78 3.58
3
4
5
(FeII)2(C2O4)(TIM)2(ClO4)2
FeII(TIM)(HCO2)2
FeII(TIM)(CH3CO2)2‚MeOH
a T ) 5 K.
1, with FeIIa and FeIIb ions hexacoordinated to N1, N3, N5,
and N7 from the TIM ligands (2.112-2.217 Å; see Table 3
for details) and O1 and O2 from bidentate acetate anions.
The most striking difference between the a and b complex
cations of 2 results from the variation in bidentate carboxylate
coordination mode: while the bidentate coordination of the
acetate is almost symmetrical in site b (Fe2-O1b ) 2.227-
(2) Å, Fe2-O2b ) 2.263(2) Å, and O1b-Fe2-O2b )
57.9°), it is even more asymmetrical in site a (Fe1-O1a )
2.108(2) Å, Fe1-O2a ) 2.435(3) Å, and O1a-Fe1-O2a
) 56.1°) than in complex 1. As in complex 1, the steric
constraint imposed by the three methylene bridges between
the imidazole rings does not allow an equatorial tetradentate
coordination mode of the TIM ligand, and consequently, the
symmetry of the N4O2 donor set to FeII is severely distorted
from the octahedral one (see bond lengths and angles in Table
3 and see Figure 3).
Adjacent complex cations are unevenly hydrogen-bonded
together through two NHa‚‚‚Ob, one NHa‚‚‚Oa, and one
NHb‚‚‚Oa imidazole-carboxylate contacts and also to per-
chlorate counteranions through two NHa‚‚‚Ob, one NHa‚‚‚
Oa, three NHb‚‚‚Ob, and one NHb‚‚‚Oa imidazole-
perchlorate contacts (2.70 < N‚‚‚O < 3.20 Å, CIF file)
yielding an intricate and dense 3D network between a and b
sites, responsible for the crystal packing.
The coordination modes of the phenylacetate in 1, oxalate
in 3, and formate in 4, may be tentatively assessed by
comparison with the ∆ value of the corresponding ionic
carboxylate in a way similar to that suggested by Deacon
and Phillips.23 Compared to the ∆ ) 190 cm-1 value for the
ionic form, the ∆ ) 150 cm-1 value obtained for the
phenylacetate in complex 1 suggests a bidentate coordination
mode, in agreement with the molecular structure of 1. The
∆ ) 325 cm-1 value obtained for the oxalate in complex 3
is close to that reported for the side-on η2 bridging bis-
bidentate coordination mode of the oxalate dianion.25 The
unique ∆ ) 255 cm-1 value obtained for the formate in
complex 4 is higher than the ∆ ) 240 cm-1 value reported
for the ionic formate, suggesting that both formate anions
of 4 may be monodentate.
The 660, 750, and 1155 cm-1 absorptions may be
attributed to the Γ and ∆ ring deformations and ∆ imidazole
ring breathing vibrations of TIM, respectively.
Magnetic Susceptibility. The magnetic susceptibility data
for complexes 1-5 were collected in the 2-300 K temper-
ature range, and µeff/Fe values for selected temperatures are
reported in Table 5.
The effective magnetic moment/iron at 300 K (5.10-5.41
µB) is larger than the S ) 2 spin-only value of 4.9 µB for 1,
2, 4, and 5, as usually observed for high-spin iron(II)
complexes.11g,26 Although values in excess of 5.2 µB at room
temperature may indicate the presence of impurities, the
Mo¨ssbauer spectra (see below) did not show the presence
of ferric contaminant, as also confirmed by EPR spectros-
copy. The temperature dependence of the magnetic moments
below 30 K indicates operation of zero-field splitting of the
high-spin iron(II) ground state.The µeff/Fe values for com-
plexes 1, 2, and 5 are practically constant, in the 300-30 K
range, in agreement with the mononuclear high-spin iron-
(II) nature of these complexes, as confirmed by the single-
crystal X-ray study of 1 and 2.
-
IR Spectroscopy. The asymmetric and symmetric CO2
-
-
absorptions (νas(CO2 ) and νs(CO2 ), respectively) observed
for the carboxylates in complexes 1-5 are summarized in
Table 4, together with their difference ∆ ()(νas(CO2 ) -
-
-
-
νs(CO2 )) and the usually reported ∆ionic ()νas(CO2 ) -
-
νs(CO2 ) for the corresponding free carboxylate).
In agreement with the structural determination, the
-
-
νas(CO2 ), νs(CO2 ), and ∆ values obtained for complex 2,
[Fe(TIM)(CH3CO2)](ClO4), clearly indicate the presence of
two different bidentate acetate anions, the coordination mode
of one of them being less symmetrical (∆ ) 150 cm-1), and
thus closer to a monodentate mode, than that of the other
one (∆ ) 100 cm-1).23,24 It is then very likely that the ∆ )
150 cm-1 value corresponds to the acetate anion of site a,
while the ∆ ) 100 cm-1 value is attributable to the acetate
anion of site b. The ∆ values obtained for the acetate anions
of complex 5, Fe(TIM)(CH3CO2)2, suggest that while one
acetate would not be coordinated to the iron (∆ ) 175 cm-1)
the other one would be coordinated in a bidentate mode (∆
) 155 cm-1), probably with significant asymmetry as in site
a of complex 2.
We have analyzed the variation of the magnetic suscep-
tibility of Fe(TIM)(HCO2)2 (4) by employing the expression
derived from the isotropic spin-exchange Hamiltonian H )
-2JS1S2 (S1 ) S2 ) 2) and the van Vleck equation27 modified
(25) (a) Lloret, F.; Julve, M.; Mollar, M.; Castro, I.; LaTorre, J.; Faus, J.;
Solans, X.; Morgenstern-Badarau, I. J. Chem. Soc., Dalton Trans.
1989, 729. (b) Lloret, F.; Julve, M.; Mollar, J.; Faus, J.; Solans, X.;
Journaux, Y.; Morgenstern-Badarau, I. Inorg. Chem. 1990, 29, 2232.
(c) Armentano, D.; De Munno, G.; Lloret, F.; Palii, A. V.; Julve, M.
Inorg. Chem. 2002, 41, 2007.
(26) (a) Kennnedy, B. J.; Murray, K. S. Inorg. Chem. 1985, 24, 1552. (b)
Mabad, B.; Cassoux, P.; Tuchagues, J.-P.; Hendrickson, D. N. Inorg.
Chem. 1986, 25, 1420.
(23) Deacon, G. B.; Phillips, R. J. Coord. Chem. ReV. 1980, 33, 227.
(24) Costes, J.-P.; Dahan, F.; Laurent, J.-P. Inorg. Chem. 1985, 24, 1018.
(27) O’Connor, C. J. Prog. Inorg. Chem. 1982, 29, 203.
Inorganic Chemistry, Vol. 43, No. 6, 2004 2109

Lemercier et al.
Figure 4. Variable-temperature magnetic susceptibility data for Fe(TIM)-
(HCO2)2 (4). The solid lines result from a least-squares fit of the data with
the model described in the text.
Figure 5. Variable-temperature magnetic susceptibility data for Fe2(TIM)2-
(C2O4)(ClO4) (3). The solid lines result from a least-squares fit of the data
with the model described in the text.
to take into account intermolecular interactions in the
molecular field approximation (zj′ with z ) number of nearest
neighbors and j′ ) coupling constant between nearest
neighbors).28 The least-squares refinement calculated from
this model yielded a reasonable fit (Figure 4) for the
parameters J ) - 0.5 cm-1, zj′ ) - 1 cm-1, g ) 2.17, and
Par ) 2.5% (Par accounts for a S ) 2 paramagnetic
impurity). Attempts to fit the experimental data with the
theoretical magnetic susceptibility calculated by exact di-
agonalization of the effective spin Hamiltonian taking into
account single-ion zero-field splitting (ZFS)29 did not
improve the quality of the fit. The minute value of the
isotropic interaction J and the larger value of the zj′
intermolecular interactions parameter, together with the
likeliness of a nonbridging monodentate coordination mode
of the formate anions indicated by IR spectroscopy, suggest
that intermolecular interactions are predominant, if not
exclusive, in complex 4.
The overall magnetic behavior of complex 3 clearly
indicates weak antiferromagnetic interactions between the
iron(II) centers, and the iron/TIM/oxalate/ClO4 ratios and
IR spectroscopy suggests the side-on η2 bis-bridging biden-
tate coordination mode of the oxalate dianion, i.e., oxalate-
bridged dinuclear iron(II) species. Analysis of the thermal
variation of the magnetic susceptibility of 3 by employing
the expression derived from the isotropic spin-exchange
Hamiltonian H ) -2JS1S2 (S1 ) S2 ) 2) and the van Vleck
equation modified to take into account intermolecular
interactions in the molecular field approximation did not yield
an acceptable fit. On the other hand, fitting the experimental
data with the theoretical magnetic susceptibility calculated
by exact diagonalization of the effective spin Hamiltonian
taking into account single-ion zero-field splitting (ZFS)29
dramatically improved the quality of the fit for the parameters
J ) - 2.1 cm-1, D ) - 3.35 cm-1, g ) 1.92, and Par )
7.2% (Figure 5). The relatively high percentage of S ) 2
paramagnetic impurity, resulting from the presence of a small
amount of mononuclear Fe(TIM)(C2O4Na)(ClO4) species (see
Experimental Section), may hinder an accurate simultaneous
evaluation of J, D, and g and lead to the low calculated g
value (1.92).
Mo1ssbauer Spectroscopy. The Mo¨ssbauer spectra of
complexes 1 and 3-5 recorded in the 4-293 K temperature
range consist of a single quadrupole-split doublet. The spectra
of 2 consist of two quadrupole-split doublets. All spectra
were least-squares-fitted with Lorentzian lines, and the
resulting isomer shift (δ) and quadrupole splitting (∆EQ)
parameters for selected temperatures are listed in Table 6.
The δ and ∆EQ values clearly indicate the presence of high-
spin FeII in all complexes. The δ values are weakly
temperature dependent due to second-order Doppler shift.30
The 1.11-1.16 mm‚s-1 isomer shift values obtained for
complexes 1-5 at 80 K are in the lower part of the δ range
observed for a [FeN4O2] core,11d-g likely as a result of the
charge delocalization at the bidentate carboxylate and π
interaction with the metal center.
The quadrupole splitting values may be divided into three
groups: the formate complex (4) is characterized by the
larger ∆EQ values, one of the sites of complex 2 is
characterized by the smaller ∆EQ values, and complexes 1,
3, and 5 and the remaining site of complex 2 are character-
ized by intermediate ∆EQ values. The large ∆EQ value
observed for complex 4 at 4 K (4.02 mm‚s-1) is characteristic
of an orbital singlet ground state, and its weak temperature
dependence (3.64 mm‚s-1 at 293 K) indicates that the energy
separation between the ground and higher orbital states is
quite large, i.e., that the crystal field distortion of the
octahedral symmetry has a strong axial term.31 This result
agrees with a monodentate coordination of both formate
anions as suggested from IR spectroscopy: indeed this
coordination mode may easily allow an axially distorted N4O2
octahedral ligand environment. The 3.06 mm‚s-1 ∆EQ value
observed for complex 1 at 4 K is large enough to indicate
an orbital singlet ground state, and its weak temperature
dependence (2.54 mm‚s-1 at 293 K) indicates that the crystal
field distortion of the octahedral symmetry is dominated by
an axial term.31 This result agrees with the structural
parameters indicating a distorted octahedral coordination
(28) Ginsberg, A. P.; Lines, E. P. Inorg. Chem. 1972, 11, 2289.
(29) Garge, P.; Chikate, R.; Padhye, S.; Savariault, J.-M.; de Loth, P.;
Tuchagues, J.-P. Inorg. Chem. 1990, 29, 3315.
(30) Greenwood, N. N.; Gibbs, T. C. Mo¨ssbauer Spectroscopy; Chapman
and Hall: New York, 1971.
(31) Sams, J. R.; Tsin, T. B. Inorg. Chem. 1975, 14, 1573.
2110 Inorganic Chemistry, Vol. 43, No. 6, 2004

Iron(II) Carboxylate Complexes
Table 6. Representative Least-Squares-Fitted Mo¨ssbauer Data (Isomer Shift, δ, Referenced to Metallic Iron at Room Temperature, Quadrupole
Splitting, ∆EQ, and Width at Half-Height, Γ/2) for Complexes 1-5
complex (no.)
T, K
δ, mm‚s-1
∆EQ, mm‚s-1
Γ/2, mm‚s-1
area ratio, %
[Fe(TIM)(C6H5CH2CO2)](ClO4) (1)
293
80
4
293
80
4
293
80
4
293
80
4
293
80
4
293
80
4
1.006(2)
1.111(1)
1.128(1)
1.006(4)
1.119(3)
1.134(2)
1.015(4)
1.139(2)
1.158(1)
1.039(1)
1.147(2)
1.195(2)
1.042(1)
1.165(1)
1.177(1)
0.987(1)
1.104(2)
1.112(3)
1.126(2)
2.536(2)
3.080(2)
3.059(1)
2.816(9)
3.261(5)
3.291(3)
2.130(8)
2.124(5)
2.232(5)
2.713(7)
3.265(4)
3.131(3)
3.636(2)
4.110(1)
4.023(2)
2.825(2)
3.252(3)
3.55(2)
0.115(2)
0.131(1)
0.135(1)
0.123(7)
0.123(4)
0.138(3)
0.133(7)
0.126(4)
0.179(4)
0.131(6)
0.141(3)
0.167(2)
0.127(2)
0.135(1)
0.147(2)
0.154(2)
0.184(3)
0.16(1)
100
100
100
46(3)
51(1)
49(1)
54(3)
49(1)
51(1)
100
100
100
100
100
100
100
100
36(6)
64(6)
[Fe(TIM)CH3CO2](ClO4)‚MeOH (2a)
[Fe(TIM)CH3CO2](ClO4)‚MeOH (2b)
Fe2(TIM)2(C2O4)(ClO4) (3)
Fe(TIM)(HCO2)2 (4)
Fe(TIM)(CH3CO2)2‚MeOH (5)
4
3.18(2)
0.186(6)
sphere as a result of the asymmetric bidentate coordination
mode of the phenylacetate: while five M-L bonds are in
the 2.102-2.176 Å range, the Fe-O2 bond is much longer
(2.326 Å). The structure of complex 2, site a, evidences an
even more asymmetric bidentate coordination mode of the
acetate with five M-L bonds in the 2.108-2.166 Å range
while the Fe1-O2a bond length equals 2.435 Å. Among the
two iron(II) sites distinguished by Mo¨ssbauer spectroscopy
the one characterized by the larger ∆EQ value at 4 K (3.29
mm‚s-1) and a weak temperature dependence (2.82 mm‚s-1
at 293 K) may be unambiguously assigned to site a by
comparison of the ∆EQ values of 1, and 2a,b: indeed, while
the ∆EQ values of 1 and 2a compare favorably, the ∆EQ
values of 2b (2.23 and 2.13 mm‚s-1 at 4 and 293 K,
respectively) clearly indicate a very different site symmetry
at the iron center. It is worth noting that IR spectroscopy
indicates very similar ∆ ) νas(CO2 ) - νs(CO2 ) values (150
cm-1) for the asymmetric bidentate coordination modes of
the phenylacetate in 1 and of the acetate in site a of complex
2, compared to the much lower ∆ ) 100 cm-1 value for the
symmetrical bidentate coordination mode of the acetate in
site b. The fact that both spectroscopies strongly sense the
difference between these symmetrical and asymmetrical
bidentate coordination modes of carboxylates clearly indi-
cates the importance of both the structural and electronic
effects of this apparently tenuous carboxylate shift.32 The
large ∆EQ values of complexes 3 and 5, 3.13 and 3.55
mm‚s-1 at 4 K, respectively, also indicate an orbital singlet
ground state, and their weak temperature dependence (Table
6) indicates that the crystal field distortion of the octahedral
symmetry is also dominated by an axial term.31 Considering
the closeness of the ∆EQ parameters of complexes 3 and 5
to those obtained for 1 and 2a, an asymmetric bidentate
coordination mode of the oxalate in 3 and of one of the
acetate anions in 5 (see IR section) may be inferred.
Discussion
Versatility of the FeIIN4O2 Coordination Geometry as
a Result of the Steric Demand of the Tetraimidazole
Ligand, TIM, and the Flexibility of the Carboxylate
Coordination. The TIM ligand has been selected for
preparing the present series of complexes because the steric
demand imposed by the three methylene bridges requires
coordination through the four imidazole nitrogen donors of
TIM with a geometry intermediate between planar equatorial
and folded conformations. The FeN4 set of atoms forms an
umbrella where the iron is 0.6637(2) Å above the four
nitrogen donors: coordination at the apical position below
the umbrella is thus at serious disadvantage. On the other
hand, the space needed for bidentate coordination of a
carboxylate, although larger than for the monodentate one,
is smaller than the space available around two cis positions
of a regular octahedron. It was thus expected that involve-
ment of TIM as a tetradentate ligand in the coordination
sphere of iron would favor the bidentate coordination of a
carboxylate, above the FeN4 umbrella, as actually shown by
the molecular structure of complexes 1 and 2. Indeed, both
structure clearly show that while there is not enough room
to accommodate monodentate coordination of acetate or
phenylacetate anions below the FeN4 umbrella, the space
available above the umbrella is large enough to accommodate
bidentate coordination of these carboxylates. Although the
crystal structures of complexes 3-5 could not be obtained,
the composition, IR, and Mo¨ssbauer data converge on
structural hypotheses consistent with the rationale above. In
complex 3, FeN4 umbrellas are most probably twinned
through bis-bidentate oxalate bridges. In complex 5, the
acetate anion is too bulky to coordinate in a monodentate
mode at the apical position below the FeN4 umbrella. Then,
while one acetate anion is coordinated in a bidentate mode
above the FeN4 umbrella, the second one is not coordinated.
In complex 4, the less bulky formate anions coordinate in a
monodentate mode at the apical positions below and above
the FeN4 umbrella.
-
-
(32) Rardin, R. L.; Tolman, W. B.; Lippard, S. J. New J. Chem. 1991, 15,
417.
Inorganic Chemistry, Vol. 43, No. 6, 2004 2111

Lemercier et al.
Another point of interest is the diversity in bidentate
coordination modes of carboxylates evidenced by the mo-
lecular structure of complexes 1 and 2. While the bidentate
coordination mode of the acetate anion is strongly asym-
metrical in site a and symmetrical in site b of complex 2,
the bidentate coordination mode of the phenylacetate anion
is asymmetrical in complex 1, although less than in site a of
complex 2. The origin of this diversity in carboxylate
bidentate coordination modes probably lies in the quite strong
and numerous but different hydrogen bonds in which these
carboxylates are involved with the NHimidazole functions of
adjacent molecules, to yield the 3D crystal packings of 1
and 2. As mentioned earlier, this diversity in carboxylate
bidentate coordination modes has allowed us to clearly show
the importance of both the structural and electronic effects
of this apparently tenuous carboxylate shift through both IR
and Mo¨ssbauer spectroscopy. This result further illustrates
possible intermediate states in the carboxylate shift and
supports the conclusions drawn by Lippard et al. about the
likeliness of a prominent role of the carboxylate shift in
biological systems.32
As previously underlined by Nordlund et al.,33 for the Fe1
site of the iron center in the E. coli ribonucleotide reductase
B2 protein, the bidentate coordination mode of the carboxy-
late endows the metal center with both trigonal bipyramidal
and octahedral features. The differences in bidentate coor-
dination modes of carboxylates evidenced by the molecular
structure of complexes 1 and 2 deserve an additional
comment with regard to the resulting versatility in symmetry
of the iron coordination sphere. Due to the small O1-Fe-
O2 angle (56-58°) and [O‚‚‚C‚‚‚O]- delocalization, the
bidentate coordination mode of the carboxylate may be
regarded as intermediary between one and two Fe-O bonds,
resulting in a coordination formally intermediary between
[FeN4O] and [FeN4O2]. While this picture is satisfactory for
a symmetrical bidentate coordination, it is as less accurate
as the bidentate coordination mode is more asymmetrical:
the picture is then closer to distorted octahedral than
intermediary between penta- and hexacoordinated. In the case
of complexes 1 and 2, we then expect site 2a (Fe1-O1a )
2.108, Fe1-O2a ) 2.435 Å) to be better described as
distorted octahedral, site 2b (Fe2-O1b ) 2.227, Fe2-O2b
) 2.263 Å) to be better described as intermediary between
penta- and hexacoordinated, and complex 1 (Fe-O1 ) 2.164,
Fe-O2 ) 2.328 Å) to be intermediary between sites 2a and
2b. As previously noted (Mo¨ssbauer section), the differences
in symmetry of the iron coordination sphere associated with
the differences in symmetry of the bidentate coordination
mode of the carboxylate (to the exclusion of TIM, the
tetradentate coordination of which is formally identical in 1
and 2a,b) is dramatically reflected in the values of the
quadrupole splitting parameter (Table 6).
Figure 6. Superimposed ORTEP plots of the iron coordination environ-
ment in site b of [Fe(TIM)(CH3CO2)](ClO4) (2) and in the non-heme ferrous
site of Rhodopseudomonas Viridis.
Figure 7. Compared temperature-dependence of the quadrupole splitting
for complexes 1-5, PS 2 particles, and bacterial reaction centers. The lines
drawn between the experimental values are intended to connect the values
corresponding to a given iron site and do not represent calculated values.
site of photosynthetic systems may be summarized as
follows: (i) In both types of systems, synthetic and biologi-
cal, the ligand environment to the ferrous iron includes four
Nimidazole and the two O donor atoms of a bidentate carboxy-
late. (ii) Among complex 1 and sites 2a,b, the metric
parameters of site 2b including the symmetrically chelated
bidentate carboxylate are closer to those in the non-heme
ferrous site of Rhodopseudomonas Viridis9 and R. sphaeroi-
des.10 The superimposed drawings of the iron coordination
environment in site 2b and in the non-heme ferrous site of
R. Viridis shown in Figure 6 illustrate the quality of the
structural modeling attained.
In Figure 7, the temperature dependence of the quadrupole
splitting of complexes 1-5 is compared to that of the non-
heme ferrous site in R. sphaeroides reaction centers34 and
of photosystem 2 (PS 2) particles from Chlamydomonas
reinhardtii.35 In the bacterial reaction centers, the iron is in
Comparison with the Non-Heme Ferrous Site of
Photosynthetic Systems. The noteworthy observations re-
sulting from a structural comparison of the iron ligand
environment in complexes 1 and 2 to the non-heme ferrous
(33) Nordlund, P.; Sjo¨berg, B. M.; Eklund, H. Nature 1990, 345, 593.
2112 Inorganic Chemistry, Vol. 43, No. 6, 2004

Iron(II) Carboxylate Complexes
a highly distorted octahedral ligand environment, as depicted
in Figure 6.
between penta- and hexacoordinated) has the quadrupole
splitting values closest to those of the non-heme ferrous site
of R. sphaeroides.34 Site 2a, which includes the more
asymmetrically chelated bidentate carboxylate, resulting in
an axially distorted octahedral ligand environment, is char-
acterized by quadrupole splitting values higher than those
of the non-heme ferrous site of photosynthetic systems.
Complex 1, which includes a bidentate carboxylate less
asymmetrically chelated than site 2a, is characterized by
quadrupole splitting values closer to those of the non-heme
ferrous center of PS 2 (although slightly higher). From this
comparison of the quadrupole splittings, it may be suggested
that the ligand environment of the non-heme ferrous center
of PS 2 is close to the axially distorted octahedral symmetry
resulting from an asymmetrical bidentate coordination of the
-CO2 motif.
On the basis of spectroscopic evidence and sequence
homologies with the structurally characterized photosynthetic
bacteria,9,10 the non-heme ferrous center of PS 2 is also
believed to have the four imidazole ligands, but the identity
of the remaining ligand(s) differ for bacteria and PS 2: in
PS 2, at least one of the remaining coordination sites is
occupied by bicarbonate, most likely in the bidentate
mode.35,36 Interestingly enough, the quadrupole splittings of
complexes 1-3 and 5, which include four Nimidazole and the
two O donor atoms of a bidentate carboxylate, are close
enough to those of the non-heme ferrous site of photosyn-
thetic systems but significantly different from those of
complex 4 in which the monodentate coordination of the
caxboxylates yields an axially distorted octahedral symmetry.
In complete agreement with the result of the structural
comparison reported in the previous section, site 2b including
the symmetrically chelated bidentate carboxylate (strongly
distorted ligand environment better described as intermediary
Acknowledgment. We are grateful to CNRS and Euro-
pean Community (TMR Contract FMRX-CT980174) for
financial support and to J.-C. Chottard for stimulating
discussions.
(34) Boso, B.; Debrunner, P.; Okamura, M. Y.; Feher, G. Biochim. Biophys.
Acta 1981, 638, 173.
(35) (a) Petrouleas, V.; Diner, B. A. Biochim. Biophys. Acta 1990, 1015,
131. (b) Diner, B. A.; Petrouleas, V. Biochim. Biophys. Acta 1990,
1015, 141.
Supporting Information Available: X-ray crystallographic data
in CIF format for the structures of complexes 1 and 2. This material
is available free of charge via the Internet at http://pubs.acs.org.
(36) Hienerwadel, R.; Berthomieu, C. Biochemistry 1995, 34, 16288.
IC034907K
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