Iron(III) complexes of pyridine-based tetradentate aminophenol ligands as structural model complexes for the catechol-bound intermediate of catechol dioxygenases

Article abstract of DOI:10.1016/j.poly.2013.02.067

Some pyridine-based tetradentate aminophenol ligands and their acetylacetonato iron(III) LFe(acac) complexes have been synthesized and characterized by spectroscopic techniques, cyclic voltammetry, single crystal X-ray diffraction and magnetic susceptibility studies. X-ray analysis revealed monomer complexes in which the iron centers have been surrounded by pairs of acetyl acetonate and phenolate oxygens and two nitrogen atoms of the ligands. It has been shown that the two Fe-O_acac bonds in these complexes are not equal, as seen for Fe-O_cat bonds in the catechol-bounded intermediate of catechol dioxygenases. Variable temperature magnetic susceptibility indicates a paramagnetic behavior for all the mononuclear high spin iron complexes. The investigated complexes undergo electrochemical oxidation and reduction.

Full text of DOI:10.1016/j.poly.2013.02.067

Polyhedron 55 (2013) 109–116
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Polyhedron
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Iron(III) complexes of pyridine-based tetradentate aminophenol ligands
as structural model complexes for the catechol-bound intermediate
of catechol dioxygenases
b
b
ˇ c
Sima Heidari a, Elham Safaei a, , Andrzej Wojtczak , Patricia Cotic , 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, 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
Article history:
Some pyridine-based tetradentate aminophenol ligands and their acetylacetonato iron(III) LFe(acac)
complexes have been synthesized and characterized by spectroscopic techniques, cyclic voltammetry,
single crystal X-ray diffraction and magnetic susceptibility studies. X-ray analysis revealed monomer
complexes in which the iron centers have been surrounded by pairs of acetyl acetonate and phenolate
oxygens and two nitrogen atoms of the ligands. It has been shown that the two Fe–Oacac bonds in these
complexes are not equal, as seen for Fe–Ocat bonds in the catechol-bounded intermediate of catechol
dioxygenases. Variable temperature magnetic susceptibility indicates a paramagnetic behavior for all
the mononuclear high spin iron complexes. The investigated complexes undergo electrochemical oxida-
tion and reduction.
Received 1 November 2012
Accepted 15 February 2013
Available online 5 March 2013
Keywords:
Aminophenol
Substrate adducts models
Catechol dioxygenases
Iron complexes
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
for these enzymes [9–22]. Among these ligands, tetradentateamine-
bis(phenolato) ligands have an important role in a designing model
Aromatic hydrocarbons are environmental pollutants which
pose a major hazard to the environment, being mutagenic, carcin-
ogenic and recalcitrant [1]. Aromatic ring-cleaving enzymes, such
as catechol dioxygenases, play an important role in the degrada-
tion of aromatic compounds. This process is common in soil
micro-organisms which participate in the recycling of aromatic
compounds as sole the carbon and energy sources through aerobic
catabolism pathways [2,3]. Catechol dioxygenases catalyze the oxi-
dative cleavage of catechol substrates. These types of enzymes are
divided into two subclasses, namely intradiol and extradiol, which
differ in their mode of ring cleavage [4–6]. The Fe(III)-containing
intradiol dioxygenases cleave the vicinal hydroxyl groups (meta-
cleavage) to yield muconic acid as a product, whereas extradiol
dioxygenases catalyze the extradiol ring cleavage at the C–C bond
for phenol-containing non-heme iron-containing metalloenzymes
[23–28]. Many of the reported compounds of amine-bis(phenolato)
ligands have focused on the iron acetylacetonato complexes of these
ligands [29–32].
In this paper, we present a series of pyridine-based tetradentate
aminophenol ligands, H2(LAMP) (Scheme 1) and their acetylaceto-
nato iron(III) complexes, LFe(acac), as enzyme-substrate adduct
models for catechol dioxygenases. The phenol moieties and acetyl-
acetone ligand mimic the coordinated tyrosine ligand and Fe–Ocat
bond in the enzyme-catechol adduct, respectively. The coordina-
tion, magnetic and redox properties of related iron complexes of
the above mentioned ligands H2L are described.
2. Materials and methods
inside the vicinal hydroxyl groups to form
a-hydroxymuconic
semi-aldehyde [7]. Although intradiol and extradiol dioxygenases
oxidize an overlapping set of substrates, they function by two basi-
cally different mechanisms due to their different structures [8].
Many attempts to model the catechol dioxygenases have been re-
ported. In this way, iron complexes of tri- and tetradentate ligands
containing pyridine, imidazole, pyrazol, carboxylate and phenolate
moieties have been synthesized as structural and functional models
Reagents and analytical grade materials were purchased from
commercial suppliers and used without further purification, except
those for electrochemical measurements. Elemental analyses (C, H,
N) were performed by the Isfahan University of Technology. Fou-
rier 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 spectrophotom-
eter. Magnetic susceptibility was measured using powder samples
of solid materials in the temperature range 2–300 K by means of a
⇑
Corresponding author. Tel.: +98 241 415 3200; fax: +98 241 415 3232.
E-mail address: safaei@iasbs.ac.ir (E. Safaei).
0277-5387/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2013.02.067

110
S. Heidari et al. / Polyhedron 55 (2013) 109–116
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).
The X-ray data for the reported complexes were collected with
an Oxford Sapphire CCD diffractometer using Mo K
a radiation,
k = 0.71073 Å, at 293(2) K, by the -2h method. The structures
x
were solved by direct methods and refined with the full-matrix
least-squares method on F2 by means of the SHELX97 [33] program
package. An analytical absorption correction was applied
(RED171 package of programs [34] Oxford Diffraction, 2000), the
maximum and minimum transmissions are given in Table 1. The
absolute structure for FeLAMPOMe was determined by the Flack
method [35], with the Flack x = 0.018(11). Hydrogen atoms were
located from the electron density maps and constrained during
the refinement.
Ligand
H2(LAMPB
H2(LAMPC
R1
-Br
R2
-Br
)
)
-Cl
-Cl
H2(LAMPBM
)
3. Results
-C(CH3)3
-C(CH3)3
-CH3
-OCH3
3.1. Preparations
H2(LAMPOMe
)
3.1.1. Synthesis of ligands
The syntheses of H2LAMPC [36], H2LAMPOMe [37] and H2LAMPBM
Scheme 1. Schematic drawing of tetradentate bisphenolate ligands.
[38,39] were conducted by
a modified literature procedure,
employing the Mannich condensation of the corresponding phenol,
amine and formaldehyde without using any more water or meth-
SQUID susceptometer (Quantum Design MPMS-XL-5) in a magnetic
field of 1000 Oe.
anol, as described below for the synthesis of H2LAMPB
.
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 a working electrode and a platinum wire served as the
counter electrode. The reference electrode was an Ag wire as the
3.1.1.1. Synthesis of H2LAMPB. A mixture of 2,4-di-bromophenol
(30.00 mmol), 2-aminomethylpyridine (1.56 mL, 15.00 mmol) and
37% aqueous formaldehyde (2.5 mL, 30.00 mmol) was stirred and
refluxed for 48 h. The solvent was decanted, and the remaining
yellowish oil was triturated with cold n-hexane and cold methanol
Table 1
Crystal data and structure refinement for FeLAMPB, FeLAMPC, FeLAMPBM and FeLAMPOMe
.
Identification code
FeLAMPB
FeLAMPC
FeLAMPBM
FeLAMPOMe
Empirical formula
Formula weight
C25H21Br4FeN2O4
788.93
C25H21Cl4FeN2O4
611.09
C35H45FeN2O4
613.58
C35H45FeN2O6
645.58
T (K)
293(2)
293(2)
293(2)
293(2)
Wavelength (Å)
Crystal system
Space group
0.71073
trigonal
R-3
0.71073
triclinic
0.71073
monoclinic
P21/n
0.71073
orthorhombic
P212121
ꢀ
P1
Unit cell dimensions
a (Å)
b (Å)
c (Å)
34.7954(13)
34.7954(13)
11.8617(4)
12.6710(8)
13.6811(8)
17.2776(11)
109.397(6)
2718.1(3)
4
9.4159(7)
29.6968(18)
12.2485(10)
104.977(8)
3308.6(4)
4
10.4561(3)
23.3893(6)
27.7992(8)
b (°)
V (Å3)
12437.1(8)
18
1.896
6798.6(3)
8
1.261
0.489
Z
Calculated density (Mg/m3)
Absorption coefficient
1.493
0.982
1.232
0.494
6.362
(mmÀ1
F(000)
)
6894
1244
1308
2744
Crystal size (mm)
h (°)
0.60 Â 0.16 Â 0.12
0.38 Â 0.13 Â 0.09
0.43 Â 0.38 Â 0.23
0.50 Â 0.16 Â 0.12
2.19–28.42
2.01–28.37
2.20–28.28
2.13–28.23
Limiting indices
À44 6 h 6 46, À46 6 k 6 45,
À14 6 l 6 15
À16 6 h 6 15, À17 6 k 6 17,
À20 6 l 6 21
À11 6 h 6 10, À33 6 k 6 36,
À16 6 l 6 16
À13 6 h 6 13, À30 6 k 6 30,
À35 6 l 6 36
Reflections collected/unique
27178/6434 (0.0613)
19064/11850 (0.0756)
21125/7331 (0.0826)
46151/14879 (0.0791)
(Rint
)
2h, completeness (%)
Maximum and minimum
transmission
26.00 99.9
0.5047 and 0.1145
25.00 99.0
0.9208 and 0.7037
25.00 99.9
0.8941 and 0.8164
25.00 99.9
0.9451 and 0.7939
Data/restraints/parameters
6434/0/325
0.955
11850/0/649
0.889
7331/0/379
1.064
14879/0/793
0.813
Goodness-of-fit (GOF) on F2
Final R indices [I > 2
R indices (all data)
r
(I)]
R1 = 0.0417, wR2 = 0.0888
R1 = 0.1091, wR2 = 0.1058
R1 = 0.0616, wR2 = 0.1265
R1 = 0.1667, wR2 = 0.1700
R1 = 0.0792, wR2 = 0.1843
R1 = 0.1324, wR2 = 0.2086
R1 = 0.0467, wR2 = 0.0704
R1 = 0.1131, wR2 = 0.0807
0.018(11)
0.378 and À0.302
Absolute structure parameter
Largest difference peak and
1.408 and À1.073
0.685 and À0.459
0.433 and À0.486
hole (e AÀ3
)

S. Heidari et al. / Polyhedron 55 (2013) 109–116
111
to give a creamy yellow powder. The remaining solid residue was
4. Discussion
washed with cold methanol and boiling water for further purifica-
tion and then dried in air (4.30 g, 45% yield). 1H NMR (400 MHz,
CDCl3, 298 K) d: 3.839 (s, 4H); 3.910 (s, 2H); 7.168 (d, 2H); 7.239
(d, 1H); 7.412 (t, 1H); 7.617 (d, 2H); 7.848 (t, 1H); 8.743 (d, 1H);
11.539 (s, 1H). IR (cmÀ1): 3446 (OH); 2912 (C–H); 1447 (C@C, phe-
nyl ring). M.p.: 182–183 °C.
Some pyridine-based tetradentate aminophenol ligands were
prepared by a methanol free Mannich condensation of the corre-
sponding phenol, amine and formaldehyde. H2LAMPC, H2LAMPOMe
and H2LAMPBM were compared characteristically with those re-
ported in the literature [36–40]. These ligands were treated with
an ethanol or acetonitrile solution of tris(acetylacetonato)iron(III)
and triethylamine in a suitable ratio, and the solution was refluxed
to give the iron complexes FeLAMPX (X: B, C, OMe, BM) in high
yields.
In the IR spectra of all the ligands, the OH stretch is observed in
the range 3300–3500 cmÀ1. These strong and sharp bands were re-
placed by a broad band in the IR spectra of the complexes, proving
the coordination of the phenol groups to the metal ion. A strong
band was observed in the range 1600–1500 cmÀ1 related to the
carbonyl stretching. The frequency of the CO stretching vibration
changes with the variation of the substituents on the phenol
groups of the ligands. The electron-donating or accepting proper-
ties of the substituents, transmitted through the phenolate and
iron atoms, changes the positive charge on the carbonyl carbon,
varying the force constant and hence the frequency of the CO
stretching vibration.
3.1.1.2. Synthesis of H2LAMPC. (4.47 g, 65% yield). 1H NMR (400 MHz,
CDCl3, 298 K) d: 3.837 (s, 4H); 3.907 (s, 2H); 6.995 (d, 2H); 7.225 (d,
1H); 7.321 (d, 2H); 7.395 (m, 1H); 7.820 (m, 1H); 8.731 (d, 1H);
11.433 (s, 2H). IR (cmÀ1): 3432 (OH); 2845 (C–H); 1465 (C@C, phe-
nyl ring). M.p.: 181.2–181.9 °C.
3.1.1.3. Synthesis of H2LAMPOMe. (5.76 g, 78% yield). 1H NMR
(400 MHz, CDCl3, 298 K) d: 1.380 (s, 18H); 3.782 (s, 6H); 3.815 (s,
4H); 3.831 (s, 2H); 6.540 (d, 2H); 6.861 (d, 2H); 7.165 (d, 1H);
7.336 (s, 1H); 7.765 (t, 1H); 8.729(d, 1H); 10.342(s, 2H). IR
(cmÀ1): 3062.87 (OH); 2952.60 (C–H); 1477.3 (C@C, phenyl ring).
M.p.: 179.1–180.6 °C.
3.1.1.4. Synthesis of H2LAMPBM. (4.97 g, 73% yield). 1H NMR
(400 MHz, CDCl3, 298 K) d: 1.390 (s, 18H); 2.284 (s, 6H); 3.795
(s, 4H); 3.862 (s, 2H); 6.540 (d, 2H); 6.861 (d, 2H); 7.157 (d, 1H);
7.321 (q, 1H); 7.736 (t of d, 1H); 8.735(d, 1H); 10.546 (s, 2H).
M.p.: 188.8–190.1 °C.
The electronic spectra of all the complexes were recorded in
CH2Cl2. The electronic absorption spectra of all the reported com-
plexes show multiple intense bands in the UV and visible regions.
In all the complexes, the absorption maxima observed in the near-
UV regions (below 300 nm) are caused by
p ?
p⁄ transitions
3.1.2. Synthesis of the complexes
involving the phenolate units. The lowest energy bands (between
Triethylamine (0.28 ml, 2.00 mmol) was added to a solution of
H2L (1.00 mmol) in ethanol, except in the case of FeAMPC where
the solvent was acetonitrile. Tris(acetylacetonato)iron(III) (0.35 g,
1.00 mmol) was added to this solution and the mixture was
refluxed for 2 h, resulting in an intense dark red solution.
400 and 700 nm) are proposed to arise from charge-transfer tran-
sitions from phenolate(p
) to Fe(III)(dp⁄). The energies of the low-
energy LMCT band follow the trend FeLAMPB > FeLAMPC > FeLAMP-
BM > FeLAMPOMe. This criteria can be attributed to the transmission
of the electron-accepting properties of the halo groups through
the phenolate to the iron atoms, which reduces the Fe(III)(dp⁄) en-
3.1.2.1. Synthesis of FeLAMPB. FeLAMPB was crystallized in a 1:1
dichloromethane/ethanol mixture. (0.54 g, 69% yield). Anal. Calc.
ergy and hence increases the energy of the phenolate(p) to
Fe(III)(dp⁄) transition (Fig. 1).
for
C25H21Br4FeO4N2 (788.758 g/mol): C, 54.18; H, 3.629; N,
5.070. Found: C, 52.47; H, 3.44; N, 4.69%. IR (KBr, cmÀ1): 3446,
2901, 1570, 1523, 1453, 1359, 1314, 1271, 1155, 1102, 1016,
4.1. Crystal structure determinations of the FeLAMPB, FeLAMPC, FeLAMPBM
and FeLAMPOMe complexes
936, 849, 795, 708, 545, 491, 451. UV–Vis in CH2Cl2, kmax, nm (e,
MÀ1 cmÀ1): 291 (42733), 476 (11351).
3.1.2.2. Synthesis of FeLAMPC. Fe LAMPC was crystallized in a 1:1 ace-
tone and water mixture. (0.45 g, 73% yield). Anal. Calc. for C25H21-
Cl4FeO4N2 (611.102 g/mol): C, 40.65; H, 2.667; N, 3.632. Found:
C, 44.3; H, 3.07; N, 2.18%. IR (KBr, cmÀ1): 3446, 2910, 1585,
1521, 1456, 1371, 1305, 1269, 1216, 1172, 1091, 1018, 968, 931,
The X-ray experimental data and structure refinement for the
crystal structures are summarized in Table 1. Selected bond
lengths and angles within the coordination sphere are presented
in Table 2.
863, 799, 759, 666, 571, 467. UV–Vis in CH2Cl2, kmax, nm (e -
, MÀ1
cmÀ1): 289 (49657), 480 (12784).
3.1.2.3. Synthesis of FeLAMPOMe. FeLAMPOMe was crystallized in a 1:1:1
dichloromethane/ethanol/acetonitrile mixture. (0.52 g, 80% yield).
Anal. Calc. for C35H45FeO6N2 (645.263 g/mol): C, 65.29; H, 6.814;
N, 4.239. Found: C, 65.12; H, 7.03; N, 4.34%. IR (KBr, cmÀ1): 3446,
2954, 2905, 2868, 1593, 1520, 1470, 1383, 1293, 1200, 1165,
1090, 1017, 922, 870, 837, 759, 543, 476, 437. UV–Vis in CH2Cl2,
kmax, nm (e
, MÀ1 cmÀ1): 232 (39084), 528 (7309).
3.1.2.4. Synthesis of FeLAMPBM. FeLAMPBM was crystallized in a 1:1
dichloromethane/ethanol mixture. (0.46 g, 75% yield). Anal. Calc.
for C35H45FeO4N2 (613.588 g/mol): C, 68.15; H, 7.196; N, 4.472.
Found: C, 68.51; H, 7.39; N, 4.57%. IR (KBr, cmÀ1): 3413, 2951,
2908, 2355, 1596, 1519, 1458, 1434, 1383, 1261, 1206, 1149,
Fig. 1. Electronic absorption spectra of the synthesized complexes (4.68 Â 10À5) in
1090, 1020, 927, 859, 811, 662, 597, 544. UV–Vis in CH2Cl2, kmax
,
nm (
e
, MÀ1 cmÀ1): 231 (34836), 510 (6552).
CH2Cl2 solution.

112
S. Heidari et al. / Polyhedron 55 (2013) 109–116
Table 2
Selected bond lengths (Å) and angles (°) for the coordination sphere of the central Fe ion for FeLAMPB, FeLAMPC, FeLAMPBM and FeLAMPOMe
.
FeLAMPB
FeLAMPB
FeLAMPC
Mol. 1
FeLAMPC
Mol. 2
FeLAMPBM
FeLAMPBM
FeLAMPOMe
Mol. 1
FeLAMPOMe
Mol. 2
Fe1–O1
Fe1–O2
Fe1–O4
Fe1–O3
Fe1–N2
Fe1–N1
1.920(3)
1.943(3)
1.963(3)
1.987(3)
2.174(3)
2.234(3)
Fe1–O1
Fe1–O2
Fe1–O4
Fe1–O3
Fe1–N2
Fe1–N1
1.925(4)
1.926(3)
1.976(4)
2.044(4)
2.187(4)
2.187(4)
Fe2–O5
Fe2–O6
Fe2–O7
Fe2–O8
Fe2–N4
Fe2–N3
1.911(4)
1.922(4)
1.970(4)
2.044(4)
2.199(4)
2.188(4)
Fe1–O1
Fe1–O2
Fe1–O3
Fe1–O4
Fe1–N2
Fe1–N1
1.890(3)
1.894(3)
2.071(3)
1.975(3)
2.199(4)
2.201(4)
Fe1–O1
Fe1–O2
Fe1–O5
Fe1–O6
Fe1–N2
Fe1–N1
1.925(2)
1.873(2)
2.089(2)
1.974(2)
2.194(3)
2.215(3)
Fe2–O21
Fe2–O22
Fe2–O25
Fe2–O26
Fe2–N22
Fe2–N21
1.882(2)
1.937(2)
1.962(2)
2.092(2)
2.201(3)
2.203(2)
O1–Fe1–O2 172.96(13) O1–Fe1–O2 99.97(15) O5–Fe2–O6 99.39(16) O1–Fe1–O2 101.57(13) O2–Fe1–O1 101.51(9) O21–Fe2–O22 101.19(9)
O1–Fe1–O4 96.66(13)
O1–Fe1–N2 94.61(13)
O1–Fe1–O4 90.62(16) O5–Fe2–O7 90.91(15) O1–Fe1–O4 99.27(14)
O1–Fe1–N2 85.29(16) O5–Fe2–N3 92.23(15) O1–Fe1–N2 162.93(14) O1–Fe1–N2 89.43(9)
O1–Fe1–O6 96.18(9)
O21–Fe2–O26 87.86(10)
O21–Fe2–N22 163.01(10)
O3–Fe1–N2 173.60(13) O3–Fe1–N2 84.02(16) O8–Fe2–N3 88.49(16) O3–Fe1–N2 80.48(14)
O1–Fe1–N1 85.25(12) O1–Fe1–N1 92.01(15) O5–Fe2–N4 85.38(16) O1–Fe1–N1 89.26(13)
O3–Fe1–N1 100.24(12) O3–Fe1–N1 88.37(15) O8–Fe2–N4 84.45(16) O3–Fe1–N1 85.38(12)
O5–Fe1–N2 80.80(10) O25–Fe2–N22 91.94(10)
O1–Fe1–N1 88.21(9)
O5–Fe1–N1 86.05(9)
O21–Fe2–N21 90.51(9)
O25–Fe2–N21 167.02(10)
N2–Fe1–N1 77.92(13)
N2–Fe1–N1 76.76(15) N3–Fe2–N4 77.01(16) N2–Fe1–N1 76.63(14)
N2–Fe1–N1 76.36(10) N22–Fe2–N21 76.69(10)
of the acac bidentate ligand resulted in an octahedral geometry
of the coordination sphere of Fe(III) or Mn(III) complexes [31]. In
all the complexes reported here, the coordination sphere is octahe-
dral (Table 2, Figs. 2–5). In the catechol 1,2-dioxygenases, the coor-
dination sphere of Fe is formed by two tyrosine residues (Y408 and
Y477) occupying a cis orientation, two histidine residues and a
water or OH group. It was suggested, that the equatorial Y408 is
a Lewis base affecting the Lewis acidity of the central Fe ion and
facilitating the rapid exchange of ligands in the catalytic cycle
[41]. Tyrosine occupies the trans position relative to one of the
catechol oxygens. One of the histidine residues is positioned trans
relative to the other catechol oxygen.
In the FeLAMPC, FeLAMPBM and FeLAMPOMe complexes reported
here, the coordination sphere is octahedral with the tetradentate
amino-pyridyl-bis(phenolate) and bidentate acac ligands. The rela-
tive position of the two phenolate moieties in the coordination
sphere is cis (Figs. 3–5). One phenolate O and the ternary N1 are
positioned trans relative to the acac oxygen atoms. Such an archi-
tecture resembles that suggested for catechol 1,2-dioxygenases. On
the contrary, in FeLAMPB, the acac oxygen atoms are positioned
trans to the tertiary N1 and pyridil N2 atoms, while the phenolate
O atoms are trans to each other (Fig. 2). In all the structures
reported in this paper, the bond distances for the analogous bonds
within the Fe coordination sphere are similar to each other, with
the Fe–O bonds formed by the phenolate oxygens being the short-
est, while the Fe–N bonds are the longest within the sphere (Ta-
ble 2). It has to be noted that in FeLAMPC and FeLAMPBM there is no
statistically significant difference in the length of the Fe–O coordi-
nation bonds formed by both phenolates. However, in the FeLAMPB
Fig. 2. ORTEP diagram and atom labeling scheme for the complex FeLAMPB. The
atomic ellipsoids are plotted at the 30% probability level.
Complexes similar to these reported here were designed as
small-molecular models of enzymes, in particular catechol 1,2-
dioxygenases [24,31,38]. In that enzyme, the research revealed a
multi-step binding of the substrate and inhibitors, with the conver-
sion of the Fe coordination sphere from trigonal bypyramidal into
octahedral [41,42]. The trigonal bypyramidal geometry is known
for Fe(III) complexes with diamine-bis(phenolate) ligands and
monodentate Cl or Br counterions [38]. On the contrary, the use
Fig. 3. ORTEP diagram and atom labeling scheme for two molecules of the complex FeLAMPC, constituting the asymmetric part of the structure. The atomic ellipsoids are
plotted at the 30% probability level. The hydrogen atoms are omitted for clarity.

S. Heidari et al. / Polyhedron 55 (2013) 109–116
113
Table 3
Electrode potentials (in V) for the oxidation and reduction of the complexes FeLAMPB
,
FeLAMPC, FeLAMPOMe and FeLAMPBM measured at T = À70 °C in CH2Cl2 solutions and
referenced vs. the Fc+/Fc couple. The scan rate is 25 mV/s.
red
red
ox
ox
Complexes
E1 (V)
E2 (V)
E1 (V)
E2 (V)
FeLAMPB
À0.74a
À
À1.25a
À0.96a
À1.67b
À1.75a
0.74a
0.91b
0.12b
0.16b
1.11a
1.20a
0.47b
0.65b
FeLAMPC
FeLAMPOMe
FeLAMPBM
À1.36a
À1.49a
a
Irreversible reaction, peak potential is given.
Electrochemical quasi-reversible reaction.
b
Fe–N distance of 2.2706(15) Å in the trigonal bipyramidal complex
employing the same ligand, in which the central nitrogen atom of
the ligand and the chloride ion occupy the apical sites [38].
X-ray crystal structure analysis revealed that the dihedral angle
between the two phenolate rings are 61.2° and 58.3°, 72.8° and
73.3° and 71.1° for the FeLAMPC, FeLAMPBM and FeLAMPOMe complexes
respectively. In the FeLAMPB complex, with a flat arrangement of the
phenolic rings of the ligand, the angle is 45.3°.
The packing analysis of the reported structures revealed some
intermolecular interactions that are listed in Table 3.
Fig. 4. ORTEP diagram and atom labeling scheme for the complex FeLAMPBM. The
atomic ellipsoids are plotted at the 30% probability level. The hydrogen atoms are
omitted for clarity.
4.1.1. Crystal structure of FeLAMPB
The structure of FeLAMPB consists of a single molecule of the
complex (Fig. 2). The bond distances and bond angles relevant to
the octahedral coordination sphere of the complex are listed in Ta-
ble 2. The coordination arrangement around the iron atom of this
complex is different from the trigonal bypyramidal arrangement
found for the active site of the PCA-bound intermediate of 3,4-
PCD [3]. Both acac oxygen atoms of the complex have a trans posi-
tion relative to the nitrogen atoms of the ligand, but the catechol
oxygen atoms in 3,4-PCD are trans to the nitrogen atom of
His460 and the oxygen atom of Tyr408. The Fe–Oacac bond lengths
(1.963(3) and 1.987(3) Å) of this complex are longer than the Fe–
Ocat bond lengths in 3,4-PCD [3], the distance of Fe–O3 trans to
the pyridine group is longer than that of Fe–O4 trans to the tertiary
amine ligand. This indicates that the pyridine ligand has a greater
trans influence than the tertiary amine ligand in the FeLAMPB com-
plex. The Fe–O1 and Fe–O2 (phenolato) bond lengths (1.920(3) and
1.943(3) Å, respectively) agree with the Fe–O (Y408) bond length
(2.199(4) Å) of the PCA-bound intermediate of 3,4-PCD [3]
(Table 2).
and FeLAMPOMe complexes, the difference between these bonds is
significant (0.02 and 0.05 Å, respectively). This seems to be consis-
tent with the different role of both tyrosine residues mentioned
above, as concluded for catechol 1,2-dioxygenase [41]. Also, it
has to be noted, that the Fe–O(phenolate) distances for the chlori-
nated or brominated ligands are significantly longer than those
found for the FeLAMPBM and FeLAMPOMe complexes (Table 2). The
two different distances found for FeLAMPOMe can be classified as
belonging to one of the groups mentioned above. These bond
lengths are similar to the Fe–O(phenolato) bond lengths observed
in related octahedral iron(III) complexes possessing phenolato
ligands [29–32]. They are, however, longer than the average Fe–O
bond length in trigonal bipyramidal or square pyramidal com-
plexes, as expected because of the higher coordination number
[12,38].
In FeLAMPC, the distance between the iron atom and the central
nitrogen of the ligand is shorter than the previously reported Fe–N
distance of (amine)bis(phenolato)Fe(acac) complexes. However, in
the other three structures this bond length is longer than the re-
lated Fe–N distance of the related octahedral complexes [29–32].
In FeLAMPBM, the Fe1–N1 distance is shorter than the corresponding
The extended conformation unique for that complex is reflected
in the torsion angles C6–C7–N1–C8 and C7–N1–C8–C9 of 176.1(4)°
and 179.3(4)°. Other torsion angles describing the molecular
Fig. 5. ORTEP diagram and atom labeling scheme for the asymmetric part of the complex FeLAMPOMe. The atomic ellipsoids are plotted at the 30% probability level. The
hydrogen atoms are omitted for clarity.

114
S. Heidari et al. / Polyhedron 55 (2013) 109–116
conformation are N1–C15–C16–N2 of À32.9(5)°, and the bridge
C1–C6–C7–N1 and N1–C8–C9–C1 angles of À46.3(6)° Àand
60.4(5)°, respectively.
4.1.4. Crystal structure of FeLAMPOMe
The absolute structure of the complex was determined accord-
ing to the Flack method [19]. The asymmetric part of the structure
consists of two molecules of FeLAMPOMe, with an octahedral geom-
etry of the coordination sphere (Fig. 5). In both molecules the phe-
nolate rings are positioned cis and are almost perpendicular in the
coordination sphere, the dihedral angles between their best planes
are 73.28(9)° and 71.1(1)° for Molecules 1 and 2, respectively.
The LAMPB ligand is bound with two phenolate O atoms in trans
positions. For the observed flat arrangement of the ligand, the
dihedral angle between the best planes of the two phenolate rings
is 45.3°. The acac ligand is positioned almost perpendicular to the
LAMPB ligand, with the dihedral angle between its plane and the
phenolate ring planes being 67.0(1)° and 67.8(1)° for the C1–C6
and C9–C14 rings, respectively. The dihedral angles between the
best planes of the pyridine ring and C1–C6, C9–C14 and acac are
77.6(2)°, 33.4(2)° and 37.4(1)°, respectively.
These values are similar to those found in FeLAMPBM and FeLAMPC
.
The conformation of both molecules is similar to that observed
for FeLAMPBM. In Molecule 1, the N1–C25–C26–N2 torsion angle is
36.5(4)° and the C1–C6–C7–N1, C6–C7–N1–C8, C7–N1–C8–C9
and N1–C8–C9–C14 bridge torsion angles are À65.1(5), 170.3(3),
À59.5(3) and À59.2(4)°, respectively. For Molecule 2, the corre-
sponding values are À36.2(4)°, 60.9(4)°, 58.9(3)°, À170.5(3)° and
65.2(4)°.
The dihedral angles between the acac plane and the phenolate
C1–C6 and C9–C14 rings are 59.37(7)° and 68.61(7)° for Molecule
1 and 68.19(9)° and 45.7(1)° for Molecule 2. The dihedral angles
between the best planes of the pyridine ring and C1–C6, C9–C14
and acac or their equivalents are 40.7(1), 32.6(1)° and 57.61(6)°
Five-membered and six-membered chelate rings are formed by
the ligands. Among those, Fe–N1–C15–C16–N2 has a conformation
twisted on N1–C15, Fe1–O1–C1–C6–C7–N1 is half-chair, Fe1–O2–
C14–C9–C8–N1 has a boat conformation and Fe1–O3–C22–C23–
C24–O4 is an envelope.
4.1.2. Crystal structure of FeLAMPC
The asymmetric part of the structure consists of two molecules
of FeLAMPC with the geometry corresponding to the octahedral
mode for catechol 1,2-dioxygenase. Two phenolate rings are posi-
tioned trans (Fig. 3) and the different dihedral angles between their
best planes are 61.2° and 58.3° for Molecules 1 and 2, respectively.
These values are smaller by 10° than those found in FeLAMPBM and
FeLAMPOMe reported later in this paper.
and 33.0(1)°, 38.1(1)° and 81.1(1)° for Molecules
respectively.
1 and 2,
For the chelate rings formed by the ligands in Molecule 1, the
Fe–N1–C25–C26–N2 is twisted on N1–C25, Fe1–O1–C1–C6–C7–
N1 and Fe1–O2–C14–C9–C8–N1 have a boat conformation, while
Fe1–O3–C22–C23–C24–O4 is planar. In Molecule 2, the ring Fe2–
N21–C65–C66–N22 is twisted on N21–C65, Fe2–O21–C41–C46–
C47–N21 and Fe2–O22–C54–C49–C48–N21 are in a boat confor-
mation and Fe2–O25–C72–C73–C74–O26 is a screw-boat.
The molecular conformation is slightly different from that
detected for FeLAMPB, which is reflected by the N1–C15–C16–N2
torsion angle of À16.7(7)°, and the C1–C6–C7–N1, C6–C7–N1–C8,
C7–N1–C8–C9 and N1–C8–C9–C14 bridge torsion angles of
56.6(6)°, À179.7(4)°, 53.3(5)° and 46.1(7)°, respectively. A similar
conformation is found for Molecule 2, with the corresponding val-
ues being À16.6(7)°, 57.7(6)°, 177.8(4)°, 56.6(6)° and 38.9(7)°. The
dihedral angles between the acac plane and the phenolate C1–C6
and C9–C14 rings are 37.6(2)° and 82.7(2)° for Molecule 1 and
37.2(2) and 86.8(2)° for Molecule 2. The dihedral angles between
the best planes of the pyridine ring and C1–C6, C9–C14 and acac
or their equivalents are 55.5(2)°, 10.6(4)° and 87.2(2)° and
57.2(2)°, 15.5(3)° and 85.7(2)° for Molecules 1 and 2, respectively.
Among the chelate rings formed by the ligands in Molecule 1,
Fe–N1–C15–C16–N2 is an envelope on N1, Fe1–O1–C1–C6–C7–
N1 is a screw-boat, Fe1–O2–C14–C9–C8–N1 has an envelope con-
formation, while Fe1–O3–C22–C23–C24–O4 is planar. In Molecule
2, the ring Fe2–N3–C45–C46–N4 is an envelope on N3, Fe2–O5–
C31–C36–C37–N3 is a screw-boat, Fe2–O6–C44–C39–C38–N3 is
an envelope and Fe1–O7–C52–C53–C54–O8 is planar.
4.2. Magnetic susceptibility measurements
Magnetic susceptibilities for powdered samples of FeLAMPB
,
FeLAMPC, FeLAMPOMe and FeLAMPBM were measured in a magnetic
field of 1000 Oe as a function of temperature in the range 2–
300 K. The measured data were corrected for the temperature-
independent Larmor diamagnetic susceptibility obtained from Pas-
cal’s tables [43] and for the sample holder contribution. The tem-
perature variation of the susceptibility
v(T) and the effective
magnetic moment leff in Bohr magnetons (BM) are shown in
Fig. 6. The susceptibilities of all four samples are practically the
same and follow a Curie 1/T law, indicating paramagnetic behavior,
4.1.3. Crystal structure of FeLAMPBM
The asymmetric part of the structure consists of a molecule of
FeLAMPBM with an octahedral geometry of the coordination sphere
(Fig. 4), analogous to FeLAMPC. The two phenolate rings are posi-
tioned cis and the dihedral angles between their best planes, being
72.8°, is significantly larger than those found in the two molecules
of FeLAMPC, as described above. The molecular conformation is re-
flected by the N1–C25–C26–N2 torsion angle of 34.8(6)° and the
C1–C6–C7–N1, C6–C7–N1–C8, C7–N1–C8–C9 and N1–C8–C9–C14
bridge torsion angles of À58.2(5)°, À55.9(5)°, 171.7(4)° and
À62.3(5)°, respectively. The dihedral angles between the acac
plane and the phenolate C1–C6 and C9–C14 rings are 42.2(2)°
and 30.6(2)°. The dihedral angles between the best planes of the
pyridine ring and C1–C6, C9–C14 and acac are 30.6(2)°, 42.2(4)°
and 82.7(2)°, respectively.
Among the chelate rings formed by the ligands in Molecule 1,
the Fe–N1–C25–C26–N2 is twisted on N1–C25, Fe1–O1–C1–C6–
C7–N1 and Fe1–O2–C14–C9–C8–N1 are in a boat conformation,
while Fe1–O3–C22–C23–C24–O4 has an envelope conformation.
Fig. 6. Temperature dependent susceptibility
eff (inset) of FeLAMPX (X = B, C, OMe, BM) measured in a magnetic field of H = 1000
Oe.
v(T) and effective magnetic moment
l

S. Heidari et al. / Polyhedron 55 (2013) 109–116
115
which has also been reported for LFe(acac) down to 20 K in [29].
Paramagnetic behavior can also be observed from the temperature
independent effective magnetic moment of the complexes. The
effective magnetic moments for all the complexes lie between
5.7 and 5.8 BM, which agrees with the expected effective magnetic
moment of a high spin Fe3+ ion of 5.9 BM [44]. Similar values of the
effective magnetic moment at room temperatures have been
reported for high-spin octahedral FeIII complexes in [30]. A small
downturn of the measured effective moment at very low temper-
atures can be attributed to a zero field splitting [45] rather than
a weak antiferromagnetic interaction.
observed to shift to more negative values on varying the substitu-
ents on the phenolato group from electron withdrawing
(R1 = R2 = –Cl, –Br) to electron donating (R1 = –C(CH3)3, R2 = –
CH3) due to the lower Lewis acidity of the iron centers.
5. Conclusions
A few iron(III) complexes of the type FeLAMPX, where L is a syn-
thesized [N-O]-donor tripodal ligand containing a 2-aminomethyl-
pyridine derivative of aminophenol, have been isolated and
studied as structural models for substrate adducts of catechol diox-
ygenases. The X-ray crystal structures of the complexes reveal that
these complexes have monomer crystal structures, in which iron
centers have been surrounded by two acetylacetonate and pheno-
late oxygens and two nitrogens of the ligands. The two Fe–Oacac
bonds in the complexes are different in length, as is seen for the
iron-catechol oxygen bonds (Fe–Ocat) in the catechol-bounded
intermediate of catechol dioxygenases.
Magnetostructural studies of all the complexes displayed a
paramagnetic high spin iron(III) complex in almost the whole
investigated temperature range. Oxidation and reduction of the
investigated complexes yielded the corresponding oxidized or re-
duced species due to ligand or metal centered electrochemical
process.
4.3. Electrochemistry
The cyclic voltammograms (CVs) of all the complexes have been
recorded in CH2Cl2 solutions containing 0.1 M [(nBu)4N]ClO4 as a
supporting electrolyte. Prior to the measurements, the GC elec-
trode was polished with 0.1 mm alumina powder and washed with
distilled water. The voltage scan rate was set at 25 mV secÀ1. The
solutions were deoxygenated by bubbling nitrogen gas through
them. The cyclic voltammograms (CVs) show electrochemical oxi-
dation and reduction peaks for the iron complexes (Fig.7 and
Table 3).
The oxidation processes were assigned to a ligand-centered
oxidation, yielding the phenoxyl radical in the complex. This
ligand-centered voltammogram is electrochemically irreversible
Acknowledgments
for the E2 FeLAMPB complex, as is seen from the separation of
OX
the oxidation and reduction peaks (DEp = Epa – Epc). In addition,
The authors are grateful to the Institute for Advanced Studies in
Basic Sciences (IASBS), Nicolaus Copernicus University and the Uni-
versity of Ljubljana for their valuable help. E. Safaei gratefully
acknowledges support by the Institute for Advanced Studies in Ba-
sic Sciences (IASBS) Research Council under Grant No.
G2012IASBS127.
the current relationship (ipa/ipc) is less than unity in each case.
For the FeLAMPC, FeLAMPOMe and FeLAMPBM complexes, ligand-cen-
tered voltammograms are electrochemically quasi-reversible,
based on the deviation of
DEp from 1. The electrochemical behav-
ior of the FeLAMPOMe and FeLAMPBM complexes is consistent with
electrochemical studies on related Fe(acac) complexes bearing an
(amine)-bis(phenolato) ligand with an N,N-dimethyl-1,2- or 2-
methoxyethylamin arm [32]. However the FeLAMPBM complex, de-
spite it being a similar complex with a 2-methoxyethylamin arm,
exhibits irreversible oxidation responses [30]. This may be attrib-
uted to the more electron accepting pyridyl arm than the 2-meth-
oxyethylamin arm, which is likely to destabilize the phenoxyl
radical. Metal-centered voltammograms have been observed in
the negative potential range, which correspond to the Fe(III)/Fe(II)
reduction of the complexes. The Fe(III)/Fe(II) redox potentials are
Appendix A. Supplementary data
The structural data have been deposited with the Cambridge
Crystallographic Data Centre, CCDC Nos. 882058, 882059, 882060
and 882061 for FeLAMPB, FeLAMPC, FeLAMPBM and FeLAMPOMe respec-
,
tively. These data can be obtained free of charge via http://
www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge, CB2 1EZ,
UK; fax: +44 1223 336 033; or e-mail: deposit@ccdc.cam.ac.uk.
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