Synthesis and characterization of two binuclear iron(III) complexes of aminoethanol derivatives of aminophenol as models for non-heme iron enzymes active sites

Article abstract of DOI:10.1016/j.poly.2011.01.019

Two aminoethanol derivatives of aminophenol ligands were synthesized and characterized by IR and ¹H NMR spectroscopies. The binuclear iron(III) complexes of these ligands have been prepared and characterized by IR, ¹H NMR and UV-Vis

Full text of DOI:10.1016/j.poly.2011.01.019

Polyhedron 30 (2011) 1143–1148
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Synthesis and characterization of two binuclear iron(III) complexes of
aminoethanol derivatives of aminophenol as models for non-heme iron enzymes
active sites
a
b
b
ˇ ´ c
Elham Safaei a, , Iraj Saberikia , Andrzej Wojtczak , Zvonko Jaglicic , Anna Kozakiewicz
⇑
a Institute for Advanced Studies in Basic Sciences (IASBS), 45195 Zanjan, Iran
b Nicolaus Copernicus University, Department of Chemistry, 87-100 Torun, Poland
c Institute of Mathematics, Physics and Mechanics and Faculty of Civil and Geodetic Engineering, University of Ljubljana, Jadranska 19, SI-1000 Ljubljana, Slovenia
a r t i c l e i n f o
a b s t r a c t
Two aminoethanol derivatives of aminophenol ligands were synthesized and characterized by IR and 1
H
Article history:
Received 22 October 2010
Accepted 13 January 2011
Available online 31 January 2011
NMR spectroscopies. The binuclear iron(III) complexes of these ligands have been prepared and charac-
terized by IR, 1H NMR and UV–Vis spectroscopic techniques, cyclic voltammetry, single crystal X-ray
diffraction and magnetic susceptibility studies. X-ray analysis revealed binuclear complexes, Fe2(L2), in
which Fe(III) centers are surrounded by two phenolate and hydroxyl oxygen atoms, and amine nitrogens
of the ligands. The metal active sites of both complexes are held together by the two above mentioned
hydroxyl bridges. Variable temperature magnetic susceptibility indicates antiferromagnetic coupling
between the iron centers of both complexes. This exchange coupling is stronger for Fe2(Lae)2, such that
it shows a room temperature strong coupling between the two iron centers. The investigated complexes
undergo irreversible electrochemical oxidation and reduction.
Keywords:
Aminophenol
Non-heme
Model complex
Iron complexes
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
and reported as a result of their close relationship with phenol-
containing ligands found in non-heme iron-containing metalloen-
Metalloenzymes are large macromolecules that control a wide
range of biological and physiological functions essential for life.
These molecules can be active in a wide variety of reactions,
including oxidations, reductions and other metal-driven electron
transfers in enzymatic reactions [1]. Binuclear non-heme iron cen-
ters are present in some of these enzymes that perform important
biological functions, such as binding and activation of molecular
oxygen for biochemical oxidations. Methanemonooxygenase
(MMO), ribonucleotide reductase (RNR) and D9-desaturase (D9D)
are some of the important non-heme iron enzymes [1–5].
zymes [8–19].
In the present work, we have described the coordination,
magnetic and redox properties of the iron(III) complexes of
two aminophenol ligands, H2Lae [2-(((2-(3,5-di-tert-butyl-2-
hydroxybenzylamino)ethyl)(2-hydroxyethyl)amino)methyl)-4,6-
di-tert-butylphenol] and H2Lmae [2,4-di-tert-butyl-6-(((2-hydroxy-
ethyl)(methyl)amino)methyl)phenol], with nitrogen and oxygen
donor atoms possessing pendent arms containing 2-((2-amino-
ethyl)amino)ethanol
(Scheme 1).
and
2-(methylamino)ethanol
groups
Dinuclear iron complexes have drawn much attention for the
investigation of metalloenzymes and redox catalysts, as well as
the exchange coupling between metal centers [1,6–8]. Extensive
efforts have been made to provide valuable insight into the struc-
ture and reactivity of non-heme iron enzymes. Several research
groups have synthesized biomimetic model complexes with the
O, N, O coordination sphere [9–14]. The use of chelating tri-, tetra-
and pentadentate amine-bis(phenolate) ligands plays an increas-
ingly important role in providing a coordination sphere similar to
that of the enzymes in designing transition metal catalysts. A num-
ber of Fe(III) complexes of this class of ligands have been studied
2. Materials and methods
Reagents and analytical grade materials were purchased from
commercial suppliers and were used without further purification,
except those for electrochemical measurements. Elemental analy-
ses (C, H, and N) were performed by the Research Institute of Petro-
leum Industry (RIPI). Fourier transform infrared spectroscopy on
KBr pellets was performed on a FT IR Bruker Vector 22 instrument.
NMR measurements were performed on a Bruker 400 instrument.
UV–Vis absorbance digitized spectra were collected using a CARY
100 spectrophotometer. Magnetic susceptibility was measured
using powder samples of solid material in the temperature range
⇑
Corresponding author.
E-mail address: safaei@iasbs.ac.ir (E. Safaei).
0277-5387/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2011.01.019

1144
E. Safaei et al. / Polyhedron 30 (2011) 1143–1148
H3C
NH
OH
N
OH
N
OH
tBu
HO
tBu
tBu
OH
tBu
tBu
Scheme 1. (a) H2Lae and (b) H2Lmae
tBu
.
2–300 K by means of a SQUID susceptometer (Quantum Design
MPMS-XL-5) in a magnetic field of 1000 Oe.
1H); 7.257 (s, 1H); IR (cmꢀ1): 3218 (OH); 2955 (CAH); 1612
(C@C, phenyl ring). M.p. 103–104 °C.
Voltammetric measurements were made with a computer con-
trolled electrochemical system (ECO Chemie, Ultrecht, The Nether-
lands) equipped with a PGSTA 30 model and driven by GPES (ECO
Chemie). A glassy carbon electrode with a surface area of 0.035 cm2
was used as the working electrode and a platinum wire served as
the counter electrode. The reference electrode was an Ag wire as
the quasi reference electrode. Ferrocene was added as an internal
standard after completion of a set of experiments, and potentials
were referenced versus the ferrocenium/ferrocene couple (Fc+/Fc).
3.1.2. Synthesis of the complexes
3.1.2.1. Synthesis of Fe2(Lae)2. Triethylamine (0.2 g, 2.00 mmol) was
added to a solution of H2Lae (0.540 g, 1.00 mmol) in ethanol. Iro-
n(III) acetate was added to this solution and the resulting mixture
was refluxed for 1 h, resulting in an intense purple solution. The
solvent was removed and the solid material was crystallized in a
1:1 dichloromethane/methanol mixture. Yield: 0.95 g (80%). Anal.
Calc. for C68H104Fe2N4O6 (considering the EtOH and MeOH mole-
cules) (1184.67 g/mol): C, 66.4; H, 8.7; N, 4.5. Found: C, 67.5; H,
9.1; N, 4.4%. Improved elemental analytical data could not be
obtained even from crystalline samples.
The X-ray data for the reported complexes Fe2(Lae
)
2
and
Fe2(Lmae)2 were collected with an Oxford Sapphire CCD diffractom-
eter using Mo K radiation, k = 0.71073 Å, at 293(2) K by the
a
IR (KBr, cmꢀ1): 3445, 3218, 2957, 2863, 1604, 1468, 1357, 1263,
x
ꢀ 2h method. The structures were solved by direct methods
and refined with the full-matrix least-squares method on F2 by
means of the SHELX97 [20] program package. A numerical absorp-
tion correction was applied (RED171 package of programs [21] Ox-
ford Diffraction, 2000), the maximum and minimum transmissions
1080, 1025, 878, 805, 745, 643, 536, 469. UV–Vis in CH2Cl2, kmax
,
nm (
ꢀ
, Mꢀ1 cmꢀ1): 280 (63433), 510 (16418).
3.1.2.2. Synthesis of Fe2(Lmae)2. Triethylamine (0.2 g, 2.00 mmol)
was added to a solution of H2Lmae (0.293 g, 1.00 mmol) in ethanol.
Colored iron(III) chloride hexahydrate (0.270 g, 1.00 mmol) was
added to this solution and the resulting mixture was refluxed for
1 h, giving an intense purple solution. The solvent was removed
and the solid material crystallized in a 1:1 dichloromethane/aceto-
nitrile mixture. Yield: 0.57 g (75%). Anal. Calc. for C34H52Cl2Fe2N2O4
(764.2 g/mol): C, 54.9; H, 7.1; N, 3.8. Found: C, 56.5; H, 7.6; N, 3.7%.
Improved elemental analytical data could not be obtained, even
from crystalline samples.
being 0.9737/0.7200 and 0.8898/0.6031 for Fe2(Lae
) and Fe2(L-
2
mae)2, respectively. No extinction correction was applied. Hydrogen
atoms were located from the electron density maps and con-
strained during the refinement.
3. Results
3.1. Preparations
IR (KBr, cmꢀ1): 3447, 2956, 2912, 2872, 1636, 1467, 1360, 1303,
1253, 1171, 1063, 907, 841, 807, 755, 642, 567, 518, 444. UV–Vis in
3.1.1. Synthesis of the ligands
Both ligands were synthesized according to a modified litera-
ture procedure [22].
CH2Cl2, kmax, nm (ꢀ
, Mꢀ1 cmꢀ1): 277 (51000), 520 (13000).
3.1.1.1. Synthesis of H2Lae. A solution of 2,4-di-tert-butylphenol
(11.25 g, 55.00 mmol), 2-(2-aminoethylamino)ethanol (3 mL,
27.00 mmol) and 37% aqueous formaldehyde (4.58 mL,
55.00 mmol) was stirred and refluxed for 48 h. Upon cooling, a
large quantity of beige solid was formed. The solvent was dec-
anted, and the remaining solid residue was washed with cold
methanol to give a pure white powder (10 g, 67% yield).
4. Discussion
The substituted aminophenol compounds [2-((2-((3-5-di-
tert-butyl-2-hydroxybenzyl)(2-hydroxyethyl)amino)ethyl amino)-
methyl)-4,6-di-tert-butylphenol], H2Lae, and [2,4-di-tert-butyl-6-
(((2-hydroxyethyl)(methyl)amino)methyl)phenol], H2Lmae
, were
prepared by the methanol free Mannich condensation of the corre-
sponding phenol, amine and formaldehyde. These ligands in meth-
anol were treated with iron(III) acetate or iron(III) chloride
hexahydrate and triethylamine in a suitable ratio, and the solution
1H NMR (400 MHz, CDCl3, 298 K) d: 1.311 (s, 18H); 1.389 (s,
18H); 2.597 (t, 2H); 2.675 (t, 2H); 2.773 (t, 2H); 3.523(t, 2H);
3.773 (s, 2H); 3.817 (s, 2H); 6.793 (s, 1H); 6.901 (s, 1H); 7.191 (s,
1H); 7.283 (s, 1H); 7.290 (s, 1H). IR (cmꢀ1): 3229 (OH); 2956
(CAH); 1604 (C@C, phenyl ring). M.p. 135–136 °C.
was refluxed to give the iron complexes Fe2(Lae
high yields.
)
2 and Fe2(Lmae)2 in
In the IR spectra of both ligands, the OH stretch is observed in
the range 3220–3230 cmꢀ1. These strong and sharp bands were re-
placed by a broad band in IR spectra of the complexes, proving the
coordination of the phenol groups to the metal ion.
3.1.1.2. Synthesis of H2Lmae. A solution of 2,4-di-tert-butylphenol
(6.5 g, 32.00 mmol), 2-(methylamino)ethanol (2.5 mL, 32.00
mmol) and 37% aqueous formaldehyde (2.7 mL, 32.00 mmol) was
stirred and refluxed for 48 h. Upon cooling, a large quantity of
beige solid was formed. The solvent was decanted, and the remain-
ing solid residue was washed with cold methanol to give a pure
white powder (3.3 g, 35% yield).
The electronic spectra of the complexes recorded in CH2Cl2 are
presented in Sections 3.1.2.1 and 3.1.2.2. The electronic absorption
spectra of the two reported complexes show multiple intense
bands in the UV and visible regions. In both complexes, the absorp-
tion maxima observed in the near-UV regions (below 300 nm) are
1H NMR (400 MHz, CDCl3, 298 K) d: 1.316 (s, 9H); 1.446 (s, 9H);
2.420 (s, 3H); 2.700 (t, 2H); 3.785 (s, 2H); 3.800 (t, 2H); 6.867 (s,
caused by
p ?
p⁄ transitions involving the phenolate units. The

E. Safaei et al. / Polyhedron 30 (2011) 1143–1148
1145
Table 1
Crystal data and structure refinement for Fe2(Lae
)
and Fe2(Lmae)2.
2
Identification code
Fe2(Lae
)
2
Fe2(Lmae
)
2
Empirical formula
Formula weight
T (K)
Wavelength (Å)
Crystal system
Space group
Unit cell dimensions
a (Å)
C34.50H55FeN2O4.25 C36H58Cl2Fe2N2O4
621.65
292(2)
0.71073
triclinic
765.44
293(2)
0.71073
monoclinic
P2(1)/c
ꢀ
P1
14.1810(5)
15.3205(6)
17.2593(6)
101.317(3)
3623.8(2)
4
11.9075(4)
10.0555(3)
34.1664(16)
98.422(4)
4046.8(3)
4
b (Å)
c (Å)
b (°)
V (Å3)
Z
Dcalc (Mg/m3)
Absorption coefficient
1.139
1.256
0.452
0.885
(mmꢀ1
F(0 0 0)
Crystal size (mm)
)
1344
1624
0.78 ꢂ 0.30 ꢂ 0.06
0.64 ꢂ 0.62 ꢂ 0.13
h Range for data collection (°) 2.09–26.00
2.11–27.00
Limiting indices
ꢀ17 6 h 6 14,
ꢀ18 6 k 6 16,
ꢀ16 6 l 6 21
ꢀ15 6 h 6 14,
ꢀ9 6 k 6 12,
ꢀ43 6 l 6 42
27023/8720
[Rint = 0.0377]
98.8% (h = 27.00)
numerical
Fig. 1. ORTEP diagram and atom labeling scheme for complex Fe2(Lae)2. Ellipsoids
are plotted at 30% probability level.
Reflections collected/unique
23564/14220
[Rint = 0.0447]
99.7% (h = 26.00)
numerical
Completeness
Absorption correction
Maximum and minimum
transmission
0.9737 and 0.7200
0.8898 and 0.6031
Table 2
Selected bond lengths [Å] and angles [°] for Fe2(Lae)2.
Refinement method
full-matrix least-
squares on F2
14220/564/829
0.947
R1 = 0.0608,
wR2 = 0.1687
R1 = 0.1138,
full-matrix least-
squares on F2
8720/0/443
Coordination sphere of Fe1
Coordination sphere of Fe2
Data/restraints/parameters
Fe1–O1
Fe1–O2
Fe1–O3
Fe1–O23
Fe1–N2
Fe1–N1
O1–C1
C16–O2
C7–N1
N1–C8
1.891(3)
Fe2–O22
Fe2–O21
Fe2–O23
O3–Fe2
Fe2–N3
Fe2–N4
O21–C41
C56–O22
C47–N3
N3–C48
C48–C49
C49–N4
N4–C73
C73–C74
C74–O23
N4–C50
1.909(3)
Goodness-of-fit (GOF) on F2
1.074
1.932(2)
2.098(3)
1.972(3)
2.232(3)
2.235(3)
1.346(4)
1.357(4)
1.500(5)
1.475(5)
1.512(6)
1.495(5)
1.486(5)
1.512(6)
1.424(4)
1.504(5)
1.417(10)
1.906(3)
2.049(3)
2.018(3)
2.169(3)
2.210(3)
1.346(4)
1.334(5)
1.488(5)
1.472(5)
1.511(5)
1.478(5)
1.487(5)
1.513(6)
1.405(4)
1.478(5)
Final R indices [I > 2
r
(I)]
R1 = 0.0564,
wR2 = 0.1120
R1 = 0.0774,
wR2 = 0.1184
0.663 and ꢀ0.556
R indices (all data)
wR2 = 0.1938
1.865 and ꢀ0.389
Largest difference in peak
and hole (e Aꢀ3
)
lowest energy bands (between 450 and 700 nm) are proposed to
C8–C9
C9–N2
arise from charge-transfer transitions from the phenolate(
p
) to
Fe(III)(dp⁄)[11].
N1–C33
C33–C34
O3–C34
N2–C10
N2–C35B
4.1. X-ray data collection and crystal structure determination of
Fe2(Lae and Fe2(Lmae
)
2
)
2
O1–Fe1–O2
O1–Fe1–O23
O2–Fe1–O23
O1–Fe1–O3
O2–Fe1–O3
O23–Fe1–O3
O1–Fe1–N2
O2–Fe1–N2
O23–Fe1–N2
O3–Fe1–N2
O1–Fe1–N1
O2–Fe1–N1
O23–Fe1–N1
O3–Fe1–N1
N2–Fe1–N1
100.49(11)
97.56(11)
94.90(11)
160.60(11)
97.80(11)
74.41(10)
97.12(12)
84.51(11)
165.17(12)
90.96(12)
86.64(11)
162.00(11)
100.56(11)
77.77(11)
78.18(12)
O21–Fe2–O22
O21–Fe2–O3
O22–Fe2–O3
O21–Fe2–O23
O22–Fe2–O23
O3–Fe2–O23
O21–Fe2–N3
O22–Fe2–N3
O3–Fe2–N3
O23–Fe2–N3
O21–Fe2–N4
O22–Fe2–N4
O3–Fe2–N4
92.35(11)
99.84(11)
110.61(11)
172.84(11)
93.78(11)
74.52(10)
84.55(11)
161.35(12)
88.04(11)
90.78(11)
106.75(12)
84.87(11)
148.68(12)
77.50(11)
78.47(11)
4.1.1. Crystal structure of Fe2(Lae
)
2
The crystal data for the complex Fe2(Lae
)
2
are given in Table 1.
The asymmetric part of the structure is formed by the binuclear
complex of the formula Fe2(Lae)2 (Fig. 1) and two water molecules.
The oxygen atoms of the NACH2CH2Oꢀ groups from two ligands
(O3, O23) act as bridges between two Fe(III) ions, which results
in the formation of the central Fe2O2 ring in the complex molecule.
The angles within that ring are Fe2–O3–Fe1 102.80(11)°, Fe1–O23–
Fe2 106.22(10)°, O23–Fe1–O3 74.41(10)° and O3–Fe2–O23
74.51(10)°. The FeAO bonds in this central ring range from Fe1–
O23 1.971(3) to Fe1–O3 2.098(3) Å, and are longer by up to 0.2 Å
than the FeAO bonds involving the phenolate O atoms O1, O2,
O21 and O22 (Table 2).
O23–Fe2–N4
N3–Fe2–N4
Analysis of the electron density maps revealed the partial sub-
stitution of N2 in the equatorial position, and the C35B–C36B–
O3B occupancy of 50%, assigned based on the electron density le-
vel. Additionally, the hydroxyl group is disordered, and the two
discrete positions O3B and O3BB are equally populated (25%). That
group is not involved in coordination to Fe, but forms an intermo-
lecular H-bond O3Bꢁ ꢁ ꢁO3BB [ꢀx, ꢀy, ꢀz], of length 3.062 Å. How-
ever, the hydrogen atoms could not be located in the electron
density maps due to the 25% occupancy of the hydroxyl groups.
The A pair of these interactions forms a centrosymmetric dimer
of complex molecules.
The overall architecture of the complex reveals different posi-
tioning of the Lae ligands around the Fe centers. In the coordination
sphere of Fe1, the axial positions relative to the Fe2O2 ring plane
are occupied by phenolate O2 atom and N1 carrying the CH2CH2Oꢀ
group, which in turn forms an O3 bridge between the metal cen-
ters. Conversely, in the Fe2 coordination sphere the N4 atom with
the bridging CH2CH2O23 group is positioned equatorially, while

1146
E. Safaei et al. / Polyhedron 30 (2011) 1143–1148
Table 4
the axial positions are occupied by unsubstituted N3 and pheno-
late O22. Considering only the orientation of the phenolate rings
of both ligands and the position of the donor atoms in the coordi-
nation sphere, the non-crystallographic symmetry of the complex
molecule might be described as a center of symmetry positioned
at the center of the Fe2O2 ring. However, the double substitution
of both nitrogen atoms (N1 and N2) of one ligand breaks that sym-
metry. The spatial orientation of the coordinating C33–C34–O3
group near Fe1 is similar to the axial orientation of the C1–C6 phe-
nolate, while around Fe2 the equatorial position of the coordinat-
ing group C73–C74–O23 is close to the plane of the central Fe2O2
ring.
Selected bond lengths [Å] and angles [°] for Fe2(Lmae)2.
Coordination sphere of Fe1
Coordination sphere of Fe2
Fe1–O1
Fe1–O2
Fe1–O2#1
Fe1–N1
Fe1–Cl1
O2–Fe1#1
O1–C1
1.821(2)
Fe2–O21
Fe2–O22
Fe2–O22#2
Fe2–N21
Fe2–Cl2
O22–Fe2#2
O21–C21
C27–N21
N21–C36
N21–C38
C37–O22
1.828(2)
1.954(2)
1.984(2)
2.186(3)
2.2193(13)
1.984(2)
1.349(3)
1.482(4)
1.486(4)
1.472(4)
1.419(4)
1.960(2)
1.9971(19)
2.196(2)
2.2160(9)
1.9970(19)
1.354(3)
1.489(3)
1.478(4)
1.483(4)
1.420(3)
C7–N1
N1–C16
N1–C18
C17–O2
Each Fe center has a deformed octahedral N2O4 coordination
sphere. For the Fe1 center, the Fe1–N1 and Fe1–N2 distances are
2.237(3) and 2.233(3) Å, respectively, while the Fe1–O distances
range from Fe1–O1 1.890(3) to Fe1-O3 2.098(3) Å. For Fe2, the
Fe2–N3 and Fe2–N4 distances are 2.169(3) and 2.208(3) Å, respec-
tively. The Fe2–O distances vary between Fe2–O21 1.906(3) and
Fe2–O23 2.050(3) Å. The pentadentate coordination of the Lae li-
gands results in deformation of the angles within the coordination
sphere. The angles at Fe1 range from O23–Fe1–O3 74.41(10) to
O23–Fe1–N1 100.56(11)°, and from O1–Fe1–O3 160.59(11) to
O23–Fe1–N2 165.17(12)°. For Fe2 the angles vary between O3–
Fe2–O23 74.51(10) and O22–Fe2–O3 110.63(11)°, and O3–Fe2–
N4 148.66(12) and O21–Fe2–O23 172.84(11)°. For both Fe centers,
the smallest values correspond to the O3–Fe–O23 angle involving
the bridging oxygen atoms.
O1–Fe1–O2
121.55(10)
101.06(9)
74.93(9)
89.39(9)
78.59(8)
153.23(9)
113.37(8)
125.04(7)
97.41(6)
100.94(7)
115.6(2)
108.7(2)
110.1(2)
112.4(2)
112.65(18)
110.15(17)
102.85(16)
120.46(17)
132.80(18)
105.07(9)
O21–Fe2–O22
O21–Fe2–O22#2
O22–Fe2–O22#2
O21–Fe2–N21
O22–Fe2–N21
O22#2–Fe2–N21
O21–Fe2–Cl2
O22–Fe2–Cl2
O22#2–Fe2–Cl2
N21–Fe2–Cl2
N21–C27–C26
C38–N21–C27
C38–N21–C36
C27–N21–C36
C38–N21–Fe2
C27–N21–Fe2
C36–N21–Fe2
C37–O22–Fe2
C37–O22–Fe2#2
Fe2–O22–Fe2#2
121.86(12)
96.27(10)
75.48(10)
91.09(9)
O1–Fe1–O2#1
O2–Fe1–O2#1
O1–Fe1–N1
O2–Fe1–N1
O2#1–Fe1–N1
O1–Fe1–Cl1
O2–Fe1–Cl1
O2#1–Fe1–Cl1
N1–Fe1–Cl1
N1–C7–C6
C18–N1–C7
C16–N1–C18
C16–N1–C7
C18–N1–Fe1
C7–N1–Fe1
79.48(9)
154.03(10)
119.75(9)
118.24(10)
102.08(9)
95.68(8)
114.2(2)
109.2(3)
110.3(3)
109.8(2)
116.5(2)
106.28(17)
104.5(2)
120.7(2)
134.7(2)
C16–N1–Fe1
C17–O2–Fe1
C17–O2–Fe1#1
Fe1–O2–Fe1#1
Each H2Lae ligand contains a central ethylenediamino bridge
linking the two 2,4-di-tert-butyl-6-benzoate moieties. In the two
ligands, the conformation of these bridges is opposite, as described
with torsion angles N1–C8–C9–N2 52.3(5) and N3–C48–C49–N4
ꢀ52.0(5)°. Conversely, the conformation of the N-bound EtO
groups bridging the Fe centers is similar, with the torsion angles
being N1–C33–C34–O3 ꢀ43.9(5) and N4–C73–C74–O23
ꢀ44.0(4)°. For the non-coordinating group, the alternative-anticli-
nal and trans conformations are found with torsion angles N2–
C35B–C36B–O3B ꢀ105(2) and N2–C35B–C36B–O3BB 168.0(9)°.
An intramolecular H-bond is formed between N3–H and O2, the
Nꢁ ꢁ ꢁO distance being 2.994 Å. Analysis of the crystal packing
reveals that both water molecules participate in the H-bonds (Ta-
ble 3), with the distances O1W–H1O1ꢁ ꢁ ꢁO2W [ꢀx, ꢀy, ꢀz ꢀ 1]
2.807 and O2W–H2O2ꢁ ꢁ ꢁO1W [ꢀx, ꢀy, ꢀz ꢀ 1] 2.807 Å.
104.52(10)
Fig. 2. ORTEP diagram and atom labeling scheme for molecule 1 of complex
Fe2(Lmae)2. Ellipsoids are plotted at 30% probability level. For the half of the
molecule related by the center of symmetry [ꢀx, ꢀy, ꢀz], heteroatoms are also
labeled.
4.1.2. Crystal structure of Fe2(Lmae
)
2
The crystal data for the complex Fe2(Lmae)2 are given in Table 1,
selected bond lengths and angles are presented in Table 4, and the
labeling scheme is shown in Fig. 2.
tral part of each molecule is a four-membered Fe2O2 ring. Almost
identical distances between two Fe ions are found in the two mol-
ecules, those being Fe1ꢁ ꢁ ꢁFe1 [ꢀx, ꢀy, ꢀz] 3.1411 Å in molecule 1
and Fe2ꢁ ꢁ ꢁFe2 [1 ꢀ x, 1 ꢀ y, ꢀz] 3.1143 Å.
The asymmetric part of the Fe2(Lmae
) structure contains the
2
halves of two binuclear complex molecules Fe2(Lmae)2 (Fig. 2). Each
Lmae ligand forms three coordination bonds to one Fe ion via two O
and one N atom. Both complex molecules are centrosymmetric,
and the other halves are generated from those in the asymmetric
unit by the transformations [ꢀx, ꢀy, ꢀz] for molecule 1 and
[1 ꢀ x, 1 ꢀ y, ꢀz] for molecule 2. In each complex molecule, the pair
of alkoxo atoms (O2 or O22), related by the center of symmetry,
form bridges between the two Fe centers. Consequently, the cen-
The coordination sphere around each Fe(III) is ClNO3 and this
has the geometry of a deformed trigonal bipyramid (TB). In both
molecules the coordination sphere consists of equatorially bound
phenolate O1, alkoxo O2 and Cl1, and axially positioned N and al-
koxo O atoms from the ligand related to the center of symmetry.
The FeAO bond formed by the phenolate oxygen atoms are shorter
than those formed by the alkoxo O by 0.12–0.14 Å, an effect similar
to that found in the [Fe2(Lae)2] complex is reported here.
Table 3
Hydrogen bonds for Fe2(Lae
)
[Å and °].
2
Small but statistically significant differences are found in the
geometries of the coordination spheres of the two complex mole-
cules. In molecule 1 the equatorial distances Fe1–O1, Fe1–O2 and
Fe1–Cl1 are 1.821(2), 1.960(2) and 2.2160(9) Å, respectively. The
axial bonds Fe1–N1 2.196(2), and Fe1–O2 [ꢀx, ꢀy, ꢀz]
1.9971(19) Å, complete the coordination. The deformation of the
sphere from an idealized TB geometry is significant, with the an-
gles between the equatorial bonds ranging from O1–Fe1–Cl1
D–Hꢁ ꢁ ꢁA
d(D–H)
d(Hꢁ ꢁ ꢁA)
d(Dꢁ ꢁ ꢁA)
<(DHA)
N3–H3Nꢁ ꢁ ꢁO2
0.910
0.910
0.884
2.189
2.189
2.268
2.994
2.994
2.807
146.94
146.94
119.12
N3–H3Nꢁ ꢁ ꢁO2
O1W–H1O1ꢁ ꢁ ꢁO2W
[ꢀx, ꢀy, ꢀz ꢀ 1]
O2W–H2O2ꢁ ꢁ ꢁO1W
[ꢀx, ꢀy, ꢀz ꢀ 1]
0.723
2.157
2.807
150.09

E. Safaei et al. / Polyhedron 30 (2011) 1143–1148
1147
113.37(8) to O2–Fe1–Cl1 125.04(7)°. The angles between the axial
and equatorial bonds range from O2–Fe1–O2 [ꢀx, ꢀy, ꢀz] 74.93(9)
to O1–Fe1–O2 [ꢀx, ꢀy, ꢀz] 101.06(9)°, and the O2 [ꢀx, ꢀy, ꢀz]–
Fe1–N1 angle is 153.23(9)°. In molecule 2, the corresponding dis-
tances of the equatorial Fe2–O21, Fe2–O22 and Fe2–Cl2 bonds
are 1.828(2), 1.954(2) and 2.2193(13) Å, while the axial distances
are Fe2–N21 2.186(3) and O22–Fe2 [1ꢀx, 1ꢀy, ꢀz] 1.984(2) Å.
The deformation of the FeClNO3 trigonal bipyramid is smaller than
that in molecule 1. The angles between the equatorial bonds vary
from O22–Fe2–Cl2 118.24(10) to O21–Fe2–O22 121.86(12)°, the
angles between axial and equatorial bonds range from O22–Fe2–
O22 [1ꢀx, 1ꢀy, ꢀz] 75.48(10) to O22 [1ꢀx, 1ꢀy, ꢀz]–Fe2–Cl2
102.08(9)°, while the axial O22 [1ꢀx, 1ꢀy, ꢀz]–Fe2–N21 angle is
154.03(10)°.
The conformational analysis [23] revealed that in molecule 1
the conformation of the six-membered chelate ring Fe1–O1–C1–
C6–C7–N1 is screw-boat, and that for the five-membered Fe1–
N1–C16–C17–O2 ring is twisted around C16–N1. In molecule 2
the conformations of the corresponding Fe2–O21–C21–C26–C27–
N21 and Fe2–N21–C36–C37–O22 rings are also screw-boat and
twisted on C36–N21, respectively. However, the differences in
the values of the corresponding torsion angles between two com-
plex molecules vary from 1° to 8° for the six-membered rings
and 2° to 5° for the five-membered rings. The largest differences
are observed for the conformation of the C6–C7–N1–C16–C17–
O2 fragment of two molecules, with the O1–C1–C6–C7 and C1–
C6–C7–N1 torsion angles and their equivalents in molecule 2 being
49.81°, ꢀ59.09° and 57.11°, ꢀ65.59°, respectively.
Fig. 3. Susceptibilities as a function of temperature for both complexes. Full lines
are fits with the Eq. (1) with parameters as described in the text.
where x = J/kT and the last term accounts for small ratio of paramag-
netic (non-coupled) ions that are responsible for a Curie-tail at low
temperatures. The results of the fitting procedure are given as solid
lines in Fig. 3, with parameters g = 1.90, J = ꢀ15.4 cmꢀ1 and 0.2% of
paramagnetic ions for the [Fe2(Lae)2] sample, while for the [Fe2(L-
Analysis of the electron density maps revealed rotational disor-
der of the C13–C15 tert-butyl group in molecule 1, with the occu-
pancies of 58.3% and 41.7% assigned based on the electron density
levels.
Analysis of the crystal packing revealed interactions between
the C–H groups and the aromatic ring. The Hꢁ ꢁ ꢁ
p distance between
the hydrogen atom and the center of gravity (Cg) of the ring, C7–
H7Bꢁ ꢁ ꢁCg(C21–C26)[1 + x, y, z], is 2.87 Å. For C33–H33Cꢁ ꢁ ꢁCg(C1–
C6) [ꢀ1 + x, y, z] the distance is 2.94 Å.
4.2. Magnetic susceptibility measurements
The magnetic susceptibility
v for powdered samples of [Fe2(-
Lae)2] and [Fe2(Lmae)2] was measured in a magnetic field of
1000 Oe as a function of temperature, over the temperature range
2–300 K. The measured data were corrected for the temperature
independent Larmor diamagnetic susceptibility obtained from Pas-
cal’s tables [24] and for the sample holder contribution. A typical
magnetic diagram is presented in Fig. 3 in the form of
v versus T.
For both complexes the susceptibility increases with decreasing
temperature from room temperature and shows a maximum at
approximately Tmax = 66 K for [Fe2(Lae)2] and 115 K for [Fe2(Lmae)2].
Below Tmax the susceptibility decreases down to a minimal value
which is close to zero for [Fe2(Lmae)2] and below 20% of the maxi-
mal value for the [Fe2(Lae)2] sample. Only below 5 K can a weak
Curie upturn due to paramagnetic ions be observed in both sam-
ples. The measured maximum in
v(T) is a clear indication of an
antiferromagnetic interaction between the Fe(III) ions in the di-
meric units. The interaction is effective even at room temperature
in comparison to thermal fluctuations in both samples, as the
effective magnetic moments are well below the expected value
of 5.9 lB for Fe(III) paramagnetic units [25].
The molar magnetic susceptibility for a dinuclear Fe(III) system
based on the isotropic Heisenberg model (H = ꢀJ SA SB) can be de-
scribed by the function [26].
2
ex þ 5e3x þ 14e6x þ 30e10x þ 55e15x
C
T
v
¼
þ
a
ð1Þ
kT 1 þ 3ex þ 5e3x þ 7e6x þ 9e10x þ 11e15x
Fig. 4. Cyclic voltammograms of Fe2(Lae
)
2
and Fe2(Lmae
)
2
in CH2Cl2 at ꢀ40 °C.

1148
E. Safaei et al. / Polyhedron 30 (2011) 1143–1148
mae)2] sample we obtained g = 2.10, J = ꢀ27.2 cmꢀ1 and 0.04% of
sity of Ljubljana for their valuable help. E. Safaei gratefully
acknowledges support from the Institute for Advanced Studies in
Basic Sciences (IASBS) Research Council under Grant No.
G2011IASBS127.
paramagnetic ions.
4.3. Electrochemistry
The cyclic voltammograms (CV) of the complexes Fe2(Lae
) and
2
Fe2(Lmae)2 have been recorded in CH2Cl2 solutions containing 0.1 M
[(nBu)4N]ClO4 as a supporting electrolyte. Prior to the measure-
ment, the GC electrode was polished with 0.1 mm alumina powder
and washed with distilled water. The voltage scan rate was set at
50 mV secꢀ1. The solutions were deoxygenated by bubbling nitro-
gen gas through them. Ferrocene was added as an internal stan-
dard after completion of a set of experiments, and the potentials
were referenced versus the ferrocenium/ferrocene couple (Fc+/
Fc). The cyclic voltammograms (CV) observed for both complexes
revealed oxidation and reduction peaks (Fig. 4).
Appendix A. Supplementary data
CCDC 796123 and 796124 contain the supplementary crystallo-
graphic data for Fe2(Lae
)
and Fe2(Lmae)2. These data can be ob-
2
tained free of charge via http://www.ccdc.cam.ac.uk/conts/
retrieving.html, or from the Cambridge Crystallographic Data Cen-
tre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-
033; or e-mail: deposit@ccdc.cam.ac.uk.
The oxidation processes were assigned to a ligand-centered oxi-
dation yielding the phenoxyl radical in the complex. These ligand-
centered voltammograms are electrochemically irreversible, as is
seen from the high separation of the peaks of reduction and re-oxi-
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D
Ep = Epa ꢀ Epc). In addition, the current relationship (ipa/
ipc) is less than unity in each case. Metal-centered voltammograms
have been observed in the negative potential range, which corre-
sponds to the FeIII/FeII reduction of Fe2(Lae
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and Fe2(Lmae)2. They
2
are chemically irreversible, therefore the reduced species are
unstable.
5. Conclusions
Two iron(III) complexes of the type Fe2(Lae
)
and Fe2(Lmae)2,
2
where L is a newly synthesized [N–O]-donor tripodal ligand con-
taining aminoethanol derivatives of aminophenol, have been iso-
lated and studied as the structural models for non-heme iron
enzymes active sites. X-ray crystal structures of the two complexes
reveal that both complexes have dimeric structures in which Fe(III)
centers are surrounded by two phenolate and hydroxyl oxygen
atoms, and amine nitrogen atoms of the ligands.
Magnetostructural studies of both complexes displayed an anti-
ferromagnetic coupling between the Fe3+ ions through hydroxyl
bridges. A room temperature strong coupling between the two iron
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) complex. Oxidation and
2
reduction of the investigated complexes yielded the corresponding
oxidized or reduced species due to ligand or metal centered elec-
trochemical processes.
}
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The authors are grateful to the Institute for Advanced Studies in
Basic Sciences (IASBS), Nicolaus Copernicus University and Univer-