Synthesis and characterization of an iron(III) complex of glycine derivative of bis(phenol)amine ligand in relevance to catechol dioxygenase active site

Article abstract of DOI:10.1016/j.poly.2011.01.036

A glycine derivative of bis(phenol)amine ligand (HL^Gly) was synthesized and characterized by ¹H NMR and IR spectroscopies. The iron(III) complex (L^GlyFe) of this ligand was synthesized and characterized by IR, UV-Vis, X-ray and magnetic susceptibility studies. X-ray analysis reveals that in L^GlyFe the iron(III) center has a distorted trigonal bipyramidal coordination sphere and is surrounded by an amine nitrogen, a carboxylate and two phenolate oxygen atoms. The mentioned carboxylate group acts as μ-bridging ligand for iron centers of neighbor complexes. The variable-temperature magnetic susceptibility indicates that L^GlyFe is the paramagnetic high spin iron(III) complex. It has been shown that electrochemical oxidation of this complex is ligand-centered due to the oxidation of phenolate to the phenoxyl radicals. The L^GlyFe complex also undergoes an electrochemical metal-centered reduction of ferric to ferrous ion. The oxygenation of 3,5-di-tert-butyl-catechol, with L^GlyFe in the presence of dioxygen was investigated.

Full text of DOI:10.1016/j.poly.2011.01.036

Polyhedron 30 (2011) 1219–1224
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Polyhedron
journal homepage: www.elsevier.com/locate/poly
Synthesis and characterization of an iron(III) complex of glycine derivative
of bis(phenol)amine ligand in relevance to catechol dioxygenase active site
a
b
b
ˇ ´ c
Elham Safaei a, , Hamid Sheykhi , 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
A glycine derivative of bis(phenol)amine ligand (HLGly) was synthesized and characterized by 1H NMR
and IR spectroscopies. The iron(III) complex (LGlyFe) of this ligand was synthesized and characterized
by IR, UV–Vis, X-ray and magnetic susceptibility studies. X-ray analysis reveals that in LGlyFe the iron(III)
center has a distorted trigonal bipyramidal coordination sphere and is surrounded by an amine nitrogen,
Article history:
Received 11 November 2010
Accepted 28 January 2011
Available online 17 February 2011
a carboxylate and two phenolate oxygen atoms. The mentioned carboxylate group acts as
l-bridging
Keywords:
Catechol dioxygenases
Non-heme
ligand for iron centers of neighbor complexes. The variable-temperature magnetic susceptibility indicates
that LGlyFe is the paramagnetic high spin iron(III) complex. It has been shown that electrochemical oxi-
dation of this complex is ligand-centered due to the oxidation of phenolate to the phenoxyl radicals. The
Model complex
L
GlyFe complex also undergoes an electrochemical metal-centered reduction of ferric to ferrous ion. The
oxygenation of 3,5-di-tert-butyl-catechol, with LGlyFe in the presence of dioxygen was investigated.
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
iron complexes with phenolate moieties. In the present work, the
coordination, magnetic and redox properties of the iron(III) com-
Mono- and binuclear non-heme iron centers are present in
some metalloproteins that perform important biological functions
involving dioxygen [1]. The catechol dioxygenases are non-heme
iron enzymes that catalyze the oxidative degradation of catechols
which are aromatic pollutants. The active site of these enzymes
contains iron(III) and amino acid residues such as histidine, tyro-
sine and glutamic acid [2–10]. These enzymes are divided into
two subclasses: the intradiol dioxygenases, catalyzing the cleavage
of the carbon–carbon bond between the two catechol oxygen
atoms to give muconic anhydride; and the extradiol dioxygenases,
which catalyze the cleavage of the carbon–carbon bond adjacent to
the catechol oxygen atoms (Scheme 1) giving 2-hydroxymuconic
semialdehyde as the product [11].
Tripodal ligands have been used to provide a coordination
sphere for synthetic models of enzyme active sites. Iron(III) com-
plexes with N-centered quadridentate tripodal ligands comprising
pyridyl [12], carboxylic [13], imidazole [14] moieties, have at-
tracted much attention due to their similarities to the structure
and functional of some metalloenzymes including the catechol
dioxygenases [3,15–18] purple acid phosphatases [19–21], soy-
bean lipoxygenases [22,23] or galactose oxidase [24]. Several re-
search groups became interested in designing ligands and their
plex LGlyFe obtained from a glycine derivative of amine bis(phenol)
ligand HLGly are described (Scheme 2).
2. Experimental
2.1. Materials and physical measurements
Reagents or analytical grade materials were obtained from com-
mercial suppliers and used without further purification, except
those for electrochemical measurements.
Elemental analyses (C, H, N) were performed by the Research
Institute of Petroleum Industry (RIPI). Fourier transform infrared
spectroscopy on KBr pellets was performed on a FT-IR Bruker Vec-
tor 22 instrument. NMR measurements were performed on a Bru-
ker 250 instrument. UV–Vis absorbance digitized spectra were
collected using a Pharmacia Biotech spectrophotometer. The elec-
tronic spectra of all complexes were recorded in CH3OH.
Magnetic susceptibility was measured from powder samples of
solid material at the temperature range 2–300 K with a SQUID sus-
ceptometer (Quantum Design MPMS-XL-5) in a magnetic field of
1000 Oe.
Voltammetric measurements were performed with a computer
controlled Auto Lab electrochemical system (ECO Chemie, Ultrecht,
The Netherlands) equipped with a PGSTA 30 model and driven by
GPES (ECO Chemie). A glassy carbon electrode with a surface area
⇑
Corresponding author. Tel.: +98 241 4153200; fax: +98 241 4153232.
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.2011.01.036

1220
E. Safaei et al. / Polyhedron 30 (2011) 1219–1224
2.2.2. Synthesis of LGlyFe
HLGly (0.511 g, 1 mmol) was added to a solution of triethyl-
amine (0.42 ml, 3 mmol) in methanol (50 ml). The solution was
stirred for 10 min at room temperature. Then Fe(NO3)2Á4H2O
(0.404 g, 1 mmol) was added to the solution, the color changed
to violet. Subsequently the solution was further stirred at room
temperature for 60 min. It was filtered and the solvent was evapo-
rated to give a violet solid, which was subsequently washed with
warm water and n-hexane. Violet crystals were isolated from a
1:3 mixture of water/acetone. Yield = 0.36 g (63.7%): Anal. Calc.
for LGlyFe (C32H46NO4Fe): C, 68; H, 8.2; N, 2.4; Fe, 9.8. Found: C,
64.8; H, 8.5; N, 1.8; Fe, 9.3%. IR (KBr, cmÀ1): 3420w, 2954s,
2491w, 1627sh, 1576s, 1459s, 1386s, 1284s, 1104m, 1023w,
971m, 916m, 839m, 738m, 611w, 558m, 490m. UV–Vis in CH2Cl2:
Scheme 1. Scheme of cleavage by intradiol and extradiol dioxygenases [7].
kmax, nm (e
, MÀ1 cmÀ1): 330 (4980), 474 (3496).
HOOC
2.2.3. Spectrophotometric investigation of HLGly and Fe complexation
In an experiment, 2 ml of FeNO3Á4H2O solution in methanol
(2.5 Â 10À4 M) was transferred into a cuvette. UV–Vis spectra were
recorded in the range of 300–800 nm about 5 min after each addi-
t-Bu
t-Bu
N
OH
HO
tion of 10
l
L of HLGly (5 Â 10À3 M) solution. Changes in the absor-
bance of iron nitrate complex upon addition of HLGly solution were
monitored at the maximum of the 540 nm wavelength.
t-Bu
t-Bu
Scheme 2. The structure of bis(phenol)amine HLGly
.
2.2.4. Catalytic oxidation of 3,5-di-tert-butyl-catechol by LGlyFe
LGlyFe (5% mmol) was added to a solution of triethylamine
(2 mmol) in methanol (5 ml) and (1 mmol) 3,5-di-tert-butyl-cate-
chol (3,5-DTBC). The solution exposed to dioxygen and stirred for
48 h, the violet color slowly changed to dark green. The progress
of the reaction was followed by TLC and 1H NMR spectroscopy.
Meanwhile both techniques showed the disappearance of 3,5-
DTBC, the products were extracted from the aqueous solution with
diethyl ether (3 Â 30 ml). The organic layer was separated, washed
with 2 M HCl (2 Â 20 ml) and then dried over anhydrous Na2SO4 at
room temperature and then filtered off and the filtrate was evapo-
rated to give 3,5-di-tert-butyl-o-benzoquinone (3,5-DTBQ) as the
major product, and also cleavage products in small amounts. The
major product was quantified by comparing the TLC retention fac-
tor (Rf) values and 1H NMR signals with the related values of the
same sample reported previously [27].
of 0.035 cm2 was used as a working electrode and a platinum elec-
trode served as the counter electrode. The reference electrode was
an Ag wire. Ferrocene was added as an internal standard after com-
pletion of a set of experiments, and potentials are referenced vs.
the ferrocenium/ferrocene couple (Fc+/Fc).
The X-ray data for the reported complex FeLGly were collected
with an Oxford Sapphire CCD diffractometer using Mo K
a radiation
k = 0.71073 Å, at 292(2) K, by
x
À 2h method. Structure has been
solved by direct methods and refined with the full-matrix least-
squares method on F2 with the use of SHELX97 [25] program pack-
age. The numerical absorption correction was applied using
RED171 package of programs (Oxford Diffraction, 2000) [26], the
maximum and minimum transmission of 0.9492 and 0.8091. Posi-
tions of hydrogen atoms have been found from the electron density
maps, and hydrogen atoms were constrained during the refine-
ment. The crystallographic data have been deposited with CCDC,
the deposition number CCDC 798748.
The same experiment was repeated in the absence of any cata-
lyst, no oxidation products were obtained.
2.2. Preparations
3. Results and discussion
2.2.1. Synthesis of HLGly
Bis-(3,5-di-tert-butyl-2-hydroxy-benzyl)-amino-acetic
acid
This ligand was synthesized by the modified literature proce-
dure [27]. A mixture of 2,4-di-tert-butylphenol (4.9 g, 24.2 mmol),
glycine (0.907 g, 12.1 mmol), and 36% aqueous formaldehyde
(4 ml, 48 mmol) was stirred and refluxed for 2 days. The mixture
was cooled and filtered and the residue was washed with cold
methanol to give the product as a white powder. Further purifica-
tion was then achieved by washing the precipitate with boiling
water and drying the solid in air. The solution remaining after
the removal of the solid was left to give more product (5.6 g, 91%
yield). Anal. Calc. for C32H49NO4 (511.7 g/mol): C, 75.1; H, 8.6; N,
2.7. Found: C, 73.7; H, 9.9; N, 2.6%. 1H NMR (CDCl3-250 MHz): d
7.27 (s, 2H), 6.93 (s, 2H), 3.85 (s, 4H), 3.49 (s, 2H), 1.53 (s, 18H),
1.14 (s, 18H). 13C NMR(CDCl3-250 MHz): 173.97, 152.3, 141.6,
136.5, 125.4, 124.5, 119.56, (77.54, 77.03, 76.53, CDCl3), 57.25,
54.04, 34.86, 34.15, 31.61, 29.70. IR (KBr, cmÀ1): 3387w, 2958s,
2871sh, 1717s, 1613w, 1476s, 1426sh, 1365s, 1296m, 1227s,
1125s, 1021m, 977m, 874m, 810m, 693m, 654m, 591m. m.p.
122 °C.
HLGly was synthesized from glycine, formaldehyde, and 2,4-di-
tert-butyl phenol, in a simple Mannich condensation. Since the
formaldehyde used in the reaction contained 63% water, the pro-
cess was carried in water instead of methanol as reported for sim-
ilar aminophenol ligands [28].
The ligand HLGly was treated with iron nitrate, triethylamine in
suitable ratio and the solution was refluxed to yield the iron com-
plex LGlyFe with high yield.
In IR spectra of this complex, the strong and sharp band at
3387 cmÀ1 corresponding to the mOH stretch of ligand (HLGly) was
replaced by a broad band, proving the coordination of phenol
groups to the metal.
Electronic absorption spectra of complexes presented in Section
2.2.2 exhibit intense bands in the near-UV regions (below 300 nm)
which are assigned to
p ?
p⁄ transitions involving the phenolate
units. The lowest energy bands (between 450 and 700 nm) are pro-
posed to arise from charge-transfer transitions from the pheno-
late(p
) to Fe(III)(dp⁄)[29,30].

E. Safaei et al. / Polyhedron 30 (2011) 1219–1224
1221
3.1. Spectral data analysis of HLGly–Fe complexation
Table 1
Crystal data and structure refinement for LGlyFe.
The titration of ligand (HLGly) solution at fixed concentration of
iron and varying concentration of HLGly have been conducted. The
titration spectra of iron nitrate upon increasing addition of HLGly
are shown in Fig. 1. During the titration, the hypochromicity was
observed without any shift in kmax band of 540 nm, which repre-
sents the existence of interaction between HLGly and the iron ion.
The appearance of two isosbestic points in iron nitrate spectra
clearly indicates the existence of simple equilibrium between HLGly
and HLGly–Fe. The complex composition of 1:1 was determined by
plotting the absorption changes vs. ligand to metal mole ratio
(nHLGly/nFe). Fig. 2 shows a mole ratio plot of iron nitrate upon
Identification code
Empirical formula
Formula weight
Temperature (K)
Wavelength (Å)
Crystal system, space group
Unit cell dimensions
a (Å)
LGlyFe
32H46FeNO4
564.55
292(2)
0.71073
C
orthorhombic, Iba2
33.9445(17)
20.0897(8)
9.2855(4)
6332.1(5)
8, 1.184
b (Å)
c (Å)
Volume (Å3)
Z, Dcalc (mg/m3)
Absorption coefficient (mmÀ1
F(0 0 0)
)
0.510
2424
increasing addition of HLGly
.
Crystal size (mm)
h Range for data collection (°)
Limiting indices
0.43 Â 0.14 Â 0.10
2.36–26.00
À41 ꢀ h ꢀ 41
À24 ꢀ k ꢀ 23
À11 ꢀ l ꢀ 8
21621/5203 [Rint = 0.1413]
99.8%
3.2. X-ray crystal structure of LGlyFe
Reflections collected/unique
Completeness to h = 26.00
Absorption correction
Maximum and minimum transmission
Refinement method
The diffraction experiment and the crystal data are summarized
in Table 1. The selected bond lengths and angles are given in Table
2.
Numerical
0.9492 and 0.8091
Full-matrix least-squares on F2
5203/7/343
The asymmetric part of the reported structure consists of the
LGly ligand forming four bonds to the central Fe(III) ion (Fig. 3).
The Fe(LGly) units form the infinite chain along the crystallographic
z-axis, in which the adjacent Fe centers are bridged by the carbox-
ylic group of the central glycyl moiety of the ligand (Fig. 4). There-
fore, each Fe ion has a FeNO4 coordination sphere which has a
geometry of a square pyramid deformed towards the trigonal
bipyramid (Table 2). The phenolate O1 and O2 form short bonds
Data/restraints/parameters
Goodness-of-fit on F2
1.021
Final R indices [I > 2
r
(I)]
R1 = 0.0712, wR2 = 0.1541
R1 = 0.1319, wR2 = 0.1773
0.05(4)
0.523 and À0.828
R indices (all data)
Absolute structure parameter
Largest differences in peak and hole (e ÅÀ3
)
to the Fe(III), the distances being 1.859(5) and 1.824(5) Å, respec-
tively. The carboxylate O3 participates in the Fe1–O3 bond of
2.020(5) Å, while the O4 atom forms the 1.985(5) Å bond to the
adjacent Fe1 [x, Ày À 1, z À 1/2]. A series of such bonds formed
by the carboxylate groups leads to the formation of the one-dimen-
sional chain of Fe(LGly) units. The longest bond within the coordi-
nation sphere is formed by N1 atom, with the Fe–N distance of
2.207(5) Å. The steric demands for the pentadentate coordination
of LGly result in the elongation of the Fe–N1 bond and the wide
range of the bond angles found in the coordination sphere. The an-
gles between the bonds formed by O1, O2 and O3 are in a range
110.4(2)–134.1(2)°. The O–Fe–N1 angles involving these O atoms
vary from O3–Fe1–N1 76.80(19)° to O2–Fe1–N1 92.4(2)°. The an-
gles between bonds involving O1, O2 or O3 and the Fe1–O4 [x,
Ày À 1, z + 1/2] are also close to 90° (from 81.06(19)° to
108.7(2)°). The O4 [x, Ày À 1, z + 1/2] and N1 atoms are positioned
axially, with the N–Fe–O angle being 153.7(2)°. The Fe(III) com-
plexes containing LGly or its analogs with different substituents
1.6
1.2
0.8
0.4
0
300
400
500
600
λ/nm
700
800
Fig. 1. The titration absorption spectra of Fe(NO3)3 (2.5 Â 10À4 M) by HLGly
(2.5 Â 10À5, 5 Â 10À5, 7.5 Â 10À5, 1 Â 10À4, 1.25 Â 10À4, 1.5 Â 10À4, 1.75 Â 10À4
2 Â 10À4, 2.25 Â 10À4, 2.5 Â 10À4, 2.75 Â 10À4, 3 Â 10À4, 3.25 Â 10À4, 3.5 Â 10À4
3.75 Â 10À4, 4 Â 10À4, 4.25 Â 10À4, 4.5 Â 10À4 M).
,
,
t
replacing Bu groups and additional N2-type ligand have been re-
ported before [31,32]. For these complexes, the similar bond
lengths were reported within the coordination sphere. However,
only in the Fe(LGly) complex reported here the coordination sphere
is so deformed and the carboxylate group participates in the bridg-
ing interactions between adjacent Fe centers.
0.8
0.6
0.4
0.2
0
The valence geometry of the LGly ligand is typical for such mol-
ecules. Conformation of the central C1–C6–C7–N1–C8–C9–C14
fragment is described with the consecutive torsion angles of
61.7(8)°, À171.5(6)°, 61.4(7)° and 62.2(8)°. The dihedral angle be-
tween the phenolate rings of the same ligand is 61.5(3)°, while
the angles between the C1–C6 and C9–C14 rings and the five-
membered Fe1–O3–C32–C31–N1 chelate ring are 84.6° and 33.9°,
respectively. Such a geometry results in the quasi-helical arrange-
ment of the central C3–N–C3 chain and phenolate rings along the
infinite Fe(LGly) chain (Fig. 5). That arrangement is the reason for
the non-centrosymmetric space group determined for the investi-
gated compound.
0
0.5
1
1.5
2
2.5
nHLGly/ nFe
Fig. 2. Determination of complex composition by the mole ratio plot.

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E. Safaei et al. / Polyhedron 30 (2011) 1219–1224
Table 2
The six-membered chelate rings Fe1–N1–C7–C6–C1–O1 and
Selected bond lengths [Å] and angles [°] for LGlyFe.
Fe1–N1–C8–C9–C14–O2 have a boat conformation, while the
five-membered Fe1–O3–C32–C31–N1 ring is an envelope on N1.
Analysis of the non-bonding interactions in the reported struc-
ture revealed four intramolecular interactions involving C17, C18,
Fe1–O2
Fe1–O1
Fe1–O4#1
Fe1–O3
Fe1–N1
O1–C1
C6–C7
C7–N1
N1–C31
N1–C8
C8–C9
C14–O2
C31–C32
C32–O4
C32–O3
O4–Fe1#2
O2–Fe1–O1
O2–Fe1–O4#1
O1–Fe1–O4#1
O2–Fe1–O3
O1–Fe1–O3
O4#1–Fe1–O3
O2–Fe1–N1
O1–Fe1–N1
O4#1–Fe1–N1
O3–Fe1–N1
C1–O1–Fe1
O1–C1–C6
O1–C1–C2
C5–C6–C7
C1–C6–C7
N1–C7–C6
C31–N1–C8
C31–N1–C7
C8–N1–C7
C31–N1–Fe1
C8–N1–Fe1
C7–N1–Fe1
C9–C8–N1
O2–C14–C9
O2–C14–C13
C9–C14–C13
C14–O2–Fe1
N1–C31–C32
O4–C32–O3
O4–C32–C31
O3–C32–C31
C32–O3–Fe1
C32–O4–Fe1#2
1.824(5)
1.859(5)
1.985(5)
2.020(5)
2.207(5)
1.343(8)
1.489(10)
1.488(8)
1.461(9)
1.484(8)
1.476(9)
1.353(8)
1.477(9)
1.258(8)
1.259(8)
1.986(5)
113.6(2)
108.7(2)
96.6(2)
t
C28 and C29 C–H groups of Bu and phenolate O1 or O2, with the
CÁ Á ÁO distances varying from 3.0323 to 3.141 Å. Also the C–HÁ Á Á
p
interaction between C17–H17B and the ring C1–C6 is found, the
distance between H17B and the center of gravity (Cg) of the ring
being HÁ Á ÁCg [x, 1 À y, 1/2 + z] 2.96 Å.
3.3. Magnetic susceptibility measurements
Magnetic susceptibility for powdered samples of LGlyFe was
measured in a magnetic field of 1000 Oe as a function of tempera-
ture in the range 2–300 K. The measured data were corrected for
the temperature-independent Larmor diamagnetic susceptibility
obtained from the Pascal’s tables [33] and for the sample holder
contribution. The magnetic diagram is presented in Fig. 6 in the
form of leff vs. T.
110.4(2)
134.1(2)
81.06(19)
92.4(2)
Above T ꢁ 15 K, the effective magnetic moment (
plex LGlyFe is essentially temperature-independent and has
lue of 5.45 B, what is in a good agreement with the expected value
(5.92
l
eff) of com-
88.8(2)
l
eff va-
153.7(2)
76.80(19)
129.2(4)
118.7(7)
121.0(6)
121.1(7)
119.5(7)
113.5(5)
109.4(5)
111.9(5)
110.5(5)
106.8(4)
105.9(4)
112.1(4)
116.1(6)
118.6(6)
121.4(6)
119.9(7)
128.8(4)
110.4(6)
124.2(6)
117.8(6)
118.0(6)
119.4(4)
137.7(4)
l
l
B) for the isolated S = 5/2 Fe(III) ion. Thus the magnetic mea-
surement of this complex shows unambiguously that LGlyFe con-
tains the magnetically diluted high-spin d5 iron(III) ion.
Below 15 K, the effective magnetic moment (leff) slightly de-
creases and the Curie–Weiss fit of the high temperature data
(above 100 K) gave h = À0.8 K. Decrease of the effective magnetic
moment and a small negative value of Curie–Weiss temperature
h might be due to a zero field splitting or(and) a weak antiferro-
magnetic interaction between adjacent Fe(III). In the latter case
from the mean field theory [34], we have estimated the exchange
integral J/kB = 3h/(2zS(S + 1)) = À0.07 K (or À0.05 cmÀ1), where we
used z = 2 nearest neighbours and S = 5/2.
3.4. Electrochemistry
Cyclic voltammograms (CV) of LGlyFe (3 Â 10À3 M) have been
recorded in CH3CN solutions containing 0.1 M LiClO4 as a support-
ing electrolyte at À40 °C (Fig. 7). Prior to the measurement, 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 sÀ1
.
Symmetry transformations used to generate equivalent atoms: #1 x, Ày À 1, z + 1/2;
#2 x, Ày À 1, z À 1/2.
The solutions were deoxygenated by bubbling the nitrogen gas
through them. Ferrocene was added as an internal standard after
Fig. 3. ORTEP diagram and atom labeling scheme for the asymmetric unit of complex LGlyFe. Ellipsoids are plotted at 30% probability level. Hydrogen atoms omitted for clarity.

E. Safaei et al. / Polyhedron 30 (2011) 1219–1224
1223
Fig. 4. The details of the interactions between the adjacent LGlyFe complex molecules leading to the formation of the chain supramolecular structure.
80
40
0
-40
-80
-2.5
-1.5
-0.5
E/V vs. Fc
0.5
1.5
Fig. 5. The chain structure of LGlyFe. For clarity, the hydrogen atoms are omitted.
Fig. 7. Cyclic voltammetry of LGlyFe (3 Â 10À3 M) in CH3CN with M LiClO4 as
supporting electrolyte. Potentials are referenced vs. Fc, Scan rate is 50 mV/s and
T = À40 °C.
6.00
5.00
4.00
3.00
2.00
1.00
0.00
negative potential range, reveal the FeIII/FeII reduction of LGlyFe.
They are chemically irreversible, what suggests the instability of
the oxidized and reduced species.
3.5. LGlyFe-catalyzed oxidation of 3,5-di-tert-butyl-catechol
0
50
100
150
200
250
300
T/K
The focus of this experimental part was to catalyze the oxygen-
ation of catechols, with dioxygen as an oxidant. In the presence of
5% of the catalyst, 100% of 3,5-di-tert-butyl-catechol (3,5-DTBC)
was converted to 3,5-di-tert-butyl-o-benzoquinone (3,5-DTBQ) as
the major product, and also cleavage products in small amounts
(Scheme 3). Progress of the reaction was followed by 1H NMR spec-
troscopy. The signals of the free substrate (d 6.90, 1.37, 1.23) disap-
peared along with the appearance of signals of the 3,5-DTBQ (d
6.93, 6.22, 1.28, 1.23) [35,36]. The results show that the LGlyFe com-
plex can act as a good catalyst of the 3,5-DTBC oxidation to 3,5-
DTBQ in a presence of oxygen. The 1H NMR shows the 100% con-
version of 3,5-DTBC producing 69.9% benzoquinone, 17.9% of a
Fig. 6. Magnetic measurements for complex LGlyFe.
completion of a set of experiments, and potentials are referenced
vs. the ferrocenium/ferrocene couple (Fc+/Fc).
The CV voltammograms observed with LGlyFe exhibit the irre-
versible oxidation (Eo1x) and reduction peaks at 0.18 V and À1.1 V,
respectively. The oxidation process was probably attributed to
the ligand-centered oxidation yielding the phenoxyl radical in
the complex, but there is no direct evidence available yet. The me-
tal-centered voltammograms, which have been observed at the

1224
E. Safaei et al. / Polyhedron 30 (2011) 1219–1224
t-Bu
O
O
O
t-Bu
OH
OH
t-Bu
O
O
LGlyFe (5% mmol), Et3N, O2
t-Bu
+
24 h, 25 ºC, Methanol
O
t-Bu
t-Bu
t-Bu
t-Bu
+
Other
Products
Scheme 3. Oxidation of 3,5-DTBC by LGlyFe.
cleavage product (Scheme 3) and 12.2% of a mixture of unknown
products (Figs. 1S–3S).
Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ,
UK; fax: (+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.
4. Conclusion
References
The ligand bis-(3,5-di-tert-butyl-2-hydroxy-benzyl)-amino-
acetic acid, HLGly was prepared by a simple green synthesis. The
two phenolate moieties, one carboxylate group and the central
tertiary amine in HLGly provide a NO3 donor set. The iron(III) com-
plex of this tripodal tetradentate ligand. Was synthesized and
characterized.
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LGlyFe units.
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whole investigated temperature range. Only below 15 K the effec-
tive magnetic moment slightly decreases which may be due to a
zero field splitting or a very week antiferromagnetic interaction
between iron(III) ions effective only at low temperature.
Redox process of LGlyFeIII yielded the corresponding Fe(III)-phe-
noxyl radical and LGlyFeII species during the cyclic voltammetry
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Authors are grateful to the Institute for Advanced Studies in
Basic Sciences (IASBS), Nicolaus Copernicus University and Univer-
sity of Ljubljana, for their valuable help. E. Safaei gratefully
acknowledges the support by the Institute for Advanced Studies
in Basic Sciences (IASBS) Research Council under Grant No.
G2011IASBS127.
Appendix A. Supplementary data
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CCDC 798748 contains the supplementary crystallographic data
for LGlyFe. These data can be obtained free of charge via http://
www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge