Iron(III) complexes using NNS reduced Schiff bases and NNOS coordinating tetradentate ligands: Synthesis, structure and catecholase activity

Article abstract of DOI:10.1016/j.ica.2010.05.027

The orange-red colored complexes of the type [Fe(L_SB)Cl ₃], 1, have been synthesized in excellent yields by reacting FeCl₃·6H₂O with L_SB in methanol. Here, L_SB is (2-(ethylthio)-N-(pyridin-

Full text of DOI:10.1016/j.ica.2010.05.027

Inorganica Chimica Acta 363 (2010) 3131–3138
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Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Iron(III) complexes using NNS reduced Schiff bases and NNOS coordinating
tetradentate ligands: Synthesis, structure and catecholase activity
*
Reena Singh, Atanu Banerjee, Kajal Krishna Rajak
Inorganic Chemistry Section, Department of Chemistry, Jadavpur University, Kolkata 700032, India
a r t i c l e i n f o
a b s t r a c t
Article history:
The orange–red colored complexes of the type [Fe(LSB)Cl3], 1, have been synthesized in excellent yields by
reacting FeCl3ꢀ6H2O with LSB in methanol. Here, LSB is (2-(ethylthio)-N-(pyridin-2-ylmethyl)ethanamine),
(LSB1) and (2-(benzylthio)-N-(pyridin-2-ylmethyl)ethanamine) (LSB2). Similarly, FeCl3ꢀ6H2O reacted with
2-(((2-(ethylthio) ethyl) (pyridin-2-ylmethyl)amino)methyl)phenol (HL1), 2-(((2-(ethylthio)ethyl)(pyri-
din-2 ylmethyl)amino)methyl)-4-nitrophenol (HL2), 4-chloro-2-(((2-(ethylthio)ethyl)(pyridin-2-ylmethyl)-
amino)methyl)phenol (HL3), 2-(((2-(benzylthio)ethyl)(pyridin-2-ylmethyl) amino)methyl)phenol (HL4),
2-(((2-(benzylthio)ethyl)(pyridin-2-ylmethyl)amino)methyl) -4-nitrophenol (HL5), and 4-chloro-2-(((2-
(benzylthio)ethyl)(pyridin-2-ylmethyl)amino) methyl)phenol (HL6) to give dichloro complexes of the
type [Fe(L)Cl2], 2. The solid and solution structure of the complexes, as well as their properties, were
probed using X-ray diffraction, spectroscopic and electrochemical methods. The Mössbauer spectral
study at 80 K for complexes reveals the existence of (III) oxidation state and high-spin state of the metal
center in the complex. Dioxygenase activity of the complexes has been studied and both 1 and 2 have
been found to display the intradiol-cleaving pathway. However, no extradiol cleavage products have been
isolated.
Received 27 January 2010
Received in revised form 12 May 2010
Accepted 14 May 2010
Available online 24 May 2010
Dedicated to Animesh Chakravorty
Keywords:
Iron(III)
NNS and NNOS ligands
Structure
Catecholase activity
Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction
contain, Fe(II) center in which Fe(II) center is bonded with histidine
residue, one glutamic acid residue and two water molecules form-
The chemistry of mononuclear iron(III) complexes has aroused
considerable interest as iron has been found in active sites of the
large number of metalloenzymes [1–11]. Among them, the cat-
echolase dioxygenases are non-heme iron containing enzymes,
mainly found in soil bacteria, and play a key role in biodegradation
of naturally occurring aromatic 1,2-dihydroxy compound to ali-
phatic product by oxidative ring cleavage [12,13]. The catechol
dioxygenases can be subdivided into two classes [11,14,15]. The
first one is the intradiol cleaving enzymes containing a non-heme
iron(III) co-factor that catalyze a C1–C2 bond between the two cat-
echol oxygen atoms giving muconic acid and the second one is the
extradiol dioxygenases which utilizes the non-heme iron(II) that
catalyze either C2–C3 or C1–C6 bond cleavage of catechol yielding
2-hydroxy muconic semialdehyde (Scheme 1).
The X-ray structure of intradiol cleaving enzyme protocatechu-
ate 3,4-dioxygenase from Pseudomonas putida reveals that, in the
active site of the enzyme the iron(III) center is coordinated by
two nitrogen atoms from histidine residue, two oxygen atoms from
tyrosinase moiety and a solvent derived hydroxide ligand in a dis-
torted trigonal geometry arrangement (Scheme 2(A)) [16,17]. On
the other hand, in the active sites of the extradiol cleaving enzyme
ing a distorted square pyramid geometry (Scheme 2B) [18,19]. A
considerable number of mononuclear Fe(III) complexes of nitrogen,
carboxylate and phenolate donor have been synthesized as func-
tional model to explore their dioxygenase activity towards the dif-
ferent catechol substrates as well as to investigate the mechanism
of catechol oxidation [20–31]. Despite the large number of Fe(III)
complexes with O,N coordinating ligands discovered so far, the
chemistry of mononuclear Fe(III) complexes with thioether ligands
is still restricted and much remains to be done. A very few Fe(III)
thioether complexes have been isolated and characterized [32–
37] and the hardness of iron(III) has been probably found to be
responsible for lesser known FeIII–S (thioether) species in pure
form. This has prompted us to undertake the task of synthesis of
Fe(III) complexes incorporating N,O,S coordinating ligands and to
understand how the small variation in the ligand, modulate the de-
tails of molecular structure as well as their catecholase activity.
Here, we describe the synthesis of two families of mononuclear
Fe(III) complexes of types [FeIII(LSB)Cl3], 1 and [FeIII(L)Cl2], 2 using
conformationally labile N,N,S-coordinatingtridentate reducedSchiff
base ligands and N,N,O,S-donating tetradentate ligands, respec-
tively. The X-ray structures of two representative complexes have
been determined. The spectral and electrochemical behaviors of
the complexes have been described. The catecholase activity of the
complexes has also been studied.
* Corresponding author.
E-mail address: kkrajak@chemistry.jdvu.ac.in (K.K. Rajak).
0020-1693/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2010.05.027

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R. Singh et al. / Inorganica Chimica Acta 363 (2010) 3131–3138
2. Results and discussion
2.1. Syntheses
1
2
The two tridentate (N,N,S) reduced Schiff bases LSB and LSB
(general abbreviation, LSB) and six tetradentate (N,N,O,S) HL1–HL6
Chart 2.
Scheme 1.
Fig. 1. Molecular view and labeling scheme for selected atoms of [FeIII(LSB1)Cl3]. All
the hydrogen atoms are omitted for clarity.
(generally written as HL) ligands are used (Chart 1) in the present
work and the types of complexes synthesized are listed in Chart 2.
The stoichiometric reaction of FeCl3ꢀ6H2O with LSB in refluxing
MeOH afforded the orange–red complexes of general formula
[FeIII(LSB)Cl3], 1 in good yields (Eq. (1)).
h
i
LSB
FeCl3 ꢀ 6H2O ꢁꢁꢁꢁꢁꢁꢁꢁꢁ! FeIIIðLSBÞCl3
ð1Þ
Scheme 2.
MeOH; reflux ð2 hÞ
Chart 1.

R. Singh et al. / Inorganica Chimica Acta 363 (2010) 3131–3138
3133
Table 3
Crystal data and structure refinement parameters for [FeIII(LSB1)Cl3], 1a and
[FeIII(L2)Cl2], 2b.
1a
2b
Formula
Formula weight
Crystal system
Space group
a (Å)
C
10H16Cl3FeN2S
C17H20Cl2FeN3O3S
473.17
monoclinic
P21/c
20.0756(4)
7.58790(10)
13.4287(2)
106.0070(10)
1966.30(6)
4
358.51
orthorhombic
F2dd
7.654(2)
25.597(7)
29.921(8)
90.00
5862(3)
16
1.929
b (Å)
c (Å)
b (°)
V (Å3)
Z
Dcalc (mg mꢁ3
)
1.598
1.168
l
(mmꢁ1
)
2.016
h (°)
2.09–29.42
293(2)
0.0503, 0.1172
0.956
1.06–27.73
293(2)
0.0255, 0. 0.0716
1.116
Fig. 2. Molecular view and labeling scheme for selected atoms of [FeIII(L2)Cl2]. All
the hydrogen atoms are omitted for clarity.
T (K)
R1a, wR2 [I > 2
r(I)]
b
GOF on F2
a
R1
wR2 = [
=
R
|Fo| ꢁ |Fc|/
R|Fo|.
R
w(F2o ꢁ F2c )2/
R .
w(F2o )2]1/2
b
Table 1
Selected bond lengths (Å) and angles (°) for [FeIII(LSB1)Cl3].
On the other hand, complexes of the type [FeIII(L)Cl2], 2 were
Distances
obtained as deep blue solid from a reaction of FeCl3ꢀ6H2O with
Fe(1)–N(1)
Fe(1)–N(2)
Fe(1)–S(1)
Fe(1)–Cl(1)
Fe(1)–Cl(2)
Fe(1)–Cl(3)
2.181(4)
2.212(4)
2.6280(14)
2.2836(14)
2.3149(13)
2.2930(14)
HL in a 1:1 ratio (Eq. (2)).
h
i
HL
FeCl3 ꢀ 6H2O ꢁꢁꢁꢁꢁꢁꢁꢁꢁ! FeIIIðLÞCl2
ð2Þ
MeOH; reflux ð2 hÞ
The IR and UV–Vis spectral data of the complexes are given in
the Experimental Section. The complexes display Fe–S and Fe–Cl
stretches near 340 cmꢁ1 and 390 cmꢁ1, respectively. The type 2
compounds display [38] Fe–O and C–O stretches in the region
760–770 cmꢁ1 and 1270–1300 cmꢁ1, respectively.
All the iron(III) complexes have magnetic moments in the range
5.2–5.8 BM at room temperature which is consistent with a high-
spin ferric center.
Angles
N(1)–Fe(1)–N(2)
N(1)–Fe(1)–Cl(1)
N(1)–Fe(1)–Cl(2)
N(1)–Fe(1)–Cl(3)
N(1)–Fe(1)–S(1)
N(2)–Fe(1)–Cl(1)
N(2)–Fe(1)–Cl(2)
N(2)–Fe(1)–Cl(3)
N(2)–Fe(1)–S(1)
Cl(1)–Fe(1)–Cl(2)
Cl(1)–Fe(1)–Cl(3)
Cl(1)–Fe(1)–S(1)
Cl(2)–Fe(1)–Cl(3)
Cl(2)–Fe(1)–S(1)
Cl(3)–Fe(1)–S(1)
76.48(15)
164.30(12)
94.17(11)
88.72(12)
77.94(12)
89.74(11)
164.90(11)
89.10(12)
79.59(12)
97.64(5)
98.69(5)
92.36(5)
102.68(5)
86.94(5)
164.16(4)
The complexes were found to be NMR silent in CDCl3 solution at
room temperature.
2.2. Crystal structure
The structures of 1a and 2b have been determined. Molecular
views excluding the hydrogen atoms are shown in Figs. 1 and 2,
respectively. The selected bond parameters are given in Tables 1
and 2, respectively. Significant crystal data are given in Table 3.
Table 2
Selected bond lengths (Å) and angles (°) for [FeIII(L2)Cl2].
Distances
2.2.1. [FeIII(LSB1)Cl3], 1a
Fe(1)–O(1)
Fe(1)–N(1)
Fe(1)–N(2)
Fe(1)–S(1)
Fe(1)–Cl(1)
Fe(1)–Cl(2)
1.9060(12)
2.1697(14)
2.2688(13)
2.6761(5)
2.2744(5)
2.3026(5)
The complex 1a is a neutral Fe(III) complex in which iron is hexa-
coordinated with a distorted octahedral geometry. The iron(III) cen-
ter is facially coordinated by two nitrogen atoms and one sulfur
donor atom (N1, N2, S1) of the ligand LSB1, and the three chloride
ions complete the remaining octahedral sites. The Fe–NPy bond dis-
tance (2.18(4) Å) falls in the range of Fe–NPy bond distances [39–47].
As the secondary amine and pyridine nitrogen atoms are sp3 and
sp2-hybridized, the distances between the iron(III) and secondary
amine nitrogen atom (2.210(4)) is expected to be longer than that
between the iron(III) and pyridine nitrogen atom. Further, the
Fe–S bond is significantly longer than the Fe–N bond due to inability
of sterically hindering S–(CH2CH3) group to properly orient [22,48,
49–53] itself towards iron(III). The deviation [48] from a perfect
octahedral geometry is best illustrated by the S–Fe–Cl3(164.16o),
N1–Fe–Cl1(164.30o) and N2–Fe–Cl2(164.90o) bond angles, which
markedly deviate from that (180o) of an ideal octahedron. The
Fe–Clequatorial distances (2.293(2) Å) and (2.283(2) Å) are, as
expected, shorter than the Fe–Claxial (2.315(2) Å).
Angles
O(1)–Fe(1)–N(1)
O(1)–Fe(1)–N(2)
O(1)–Fe(1)–S(1)
O(1)–Fe(1)–Cl(1)
O(1)–Fe(1)–Cl(2)
N(1)–Fe(1)–N(2)
N(1)–Fe(1)–S(1)
N(1)–Fe(1)–Cl(1)
N(1)–Fe(1)–Cl(2)
N(2)–Fe(1)–S(1)
N(2)–Fe(1)–Cl(1)
N(2)–Fe(1)–Cl(2)
S(1)–Fe(1)–Cl(1)
S(1)–Fe(1)–Cl(2)
Cl(1)–Fe(1)–Cl(2)
162.70(5)
87.87(5)
92.63(4)
98.21(4)
97.31(4)
75.88(5)
78.47(4)
95.91(4)
90.12(4)
78.45(4)
163.31(4)
95.00(4)
85.727(18)
167.907(18)
99.58(2)

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R. Singh et al. / Inorganica Chimica Acta 363 (2010) 3131–3138
2.2.2. [FeIII(L2)Cl2], 2b
and [4,5-dichloro-1,2-bis(pyridinecarboxamido)benzene]Fe–(NEt4)Cl2
[72] (g = 4.28).
The complex crystallizes in P21/c space group. The molecule has
FeN2OSCl2 coordination sphere with a distorted octahedral geome-
try, constituted by two nitrogen atoms, one sulfur, one phenolato
oxygen and two chloride ions. The ligand binds in a facial coordina-
tion mode in which the amine nitrogen (N2) and chloride (Cl1) ion
occupy the axial position whereas the equatorial plane is occupied
by S1, N1, Cl2 and O1. The Fe–Npy bond length (2.170(2) Å) is
shorter than the Fe–Namine bond distance (Fe–N2), (2.269(2) Å),
which is expected of the difference in hybridization of nitrogen
atoms.
The Mössbauer spectra of complexes 1a and 2b further confirm
both the oxidation state (III) and high-spin state of metal center in
the complex. The spectra were recorded at 80 K and show a sym-
metric quadruple doublet depicted in Fig. S2. This doublet accounts
for 100% and 99.84% of the iron in the Mössbauer sample of 1a and
2b, respectively. The isomer shift and the quadruple splitting (DEQ)
values are 0.47 and 0.40 mmsꢁ1 for 1a whereas 0.49 and 0.82
mmsꢁ1 for 2b and these values are in good agreement with those
reported for related high-spin iron(III) complexes [73–75].
The Fe–O(phenolate) bond is significantly different (Fe–O1,
1.9 Å) but are similar to those reported for six-coordinated iron(III)
complexes [54–62]. The phenolate oxygen in the six membered
ring has an Fe–O1–C13 bond angle 134.41°, which is higher than
the ideal value of 120° for an sp2 hybridized phenolate oxygen
2.4. UV–Vis spectra
The iron(III) complexes show an intense absorption band near
360 nm, which can be assigned to as the ligand to metal charge
transfer transition from Clꢁ to Fe(III) [39–42]. Bands around
atom indicating that the latter (in-plane p–
p orbital) interacts
[62] less strongly for a half-filled dꢂ orbital on iron(III). Also the an-
p
300–315 nm may be attributed to as S(r)–Fe(III) ligand to metal
gle is higher than the average Fe–O–C bond angle of ꢃ128.5° ob-
served in other octahedral iron(III) complexes [56–65].
The longer Fe–S (2.676(2) Å) is probably due to the presence of
sterically hindering S-(CH2CH3) group which leads to an improper
orientation [22,48,43] of the lone pair orbital on sulfur atom to-
wards the iron(III) orbital. The S–Fe–Cl2 bond angle being
167.91° also deviates significantly from 180°. The Fe–Cl distances
(ꢃ2.28 Å) are in the same range [52,56,62,64] as those in octahe-
dral iron(III) complexes suggesting that the chloride ions should
be labile in solution.
charge transfer transition [76] and the bands ꢃ250 nm is due to
2.3. EPR and Mössbauer spectra
An important requirement of an intradiol catechol dioxygenase
model is the high-spin nature of the iron(III) metal center. In this
context, the electron paramagnetic resonance (EPR) studies de-
serve mention. The X-band EPR of the complexes were recorded
in frozen solution and exhibits a well resolved EPR signal near
g = 4.1 typical of a monomeric high-spin Fe(III) center in a distorted
octahedral environment [66–71] (Fig. S1). For example, similar EPR
spectra were observed for related monomeric high-spin octahedral
complexes, namely, [bis(pyridin-2-ylmethyl)amine]FeCl3 [55], (g =
4.30), [bis(benzimidazol-2-ylmethyl)amine]FeCl3 [55] (g = 4.22),
Table 4
Absorption maxima of bands observed in the UV–Vis spectra of Fe(III) and Fe(III)–
catecholate complexes in CH3CN solution.
kmax (nm) (e )
, Mꢁ1 cmꢁ1
Ligand added
Complex None
H2DBC
1a
1b
2a
2b
2c
2d
2e
2f
360(4100),310(4475), 240 (10185)
355(2235), 315(2985), 255(9165)
870(2580),
500(1555)
910(2060),
580(1770)
765(1945),
480(1820)
790(1215),
500(1980)
775(2255),
485(2320)
800(2025),
485 (2180)
820 (2245),
500 (2060)
810(2130),
492(2200)
600(2915), 310(4475), 280(9920),
250(9640)
610(525), 365(15575), 300(11665),
270(33385), 245(32880)
605(3235), 315(7630. 290(10100),
245 (12430)
515(2615), 310(4140), 285(8415),
245(10625)
560(3160), 425(3790), 335(14675),
265(32370), 250(10340)
545(2320), 320(2680), 280(7615),
250(8110)
Fig. 3. Progress of the reaction of adduct [Fe(LSB1)(DBC)Cl] (a) and [Fe(LSB2)(DBC)Cl]
(b) with O2 in CH3CN solution. The disappearance of the catecholate to iron(III)
charge-transfer band was monitored.
H2DBC = 3,5-di-tert-butylcatechol.

R. Singh et al. / Inorganica Chimica Acta 363 (2010) 3131–3138
3135
intra-ligand transition [48]. An additional band appears as a high
energy band in the region 310–350 nm for the type 2 compounds
which can be assigned to the charge transfer transition from out-
of-plane p
p orbital (HOMO) of the phenolate oxygen to the half
2
2
filled dx ꢁ y2/dz orbital of iron(III). The lowest energy band
(500–600 nm) in the type 2 complexes would arise from the charge
transfer transition [51,77] from the in-plane pp orbital (POMO) of
*
the phenolate ion to the half filled d orbital of iron(III).
p
To throw light on the ability of the present complexes to inter-
act with catechol substrates, simple and sterically hindered cate-
chol was added to them in excess (1:1.2) to ensure complete
adduct formation and their spectra were measured. Two new visi-
ble bands (480–550 nm, 750–850 nm) (Table 4) which appear on
adding catechol dianions are assignable to a catecholate ? Fe(III)
Fig. 6. Plot of [1+log (abs)] versus time for the reaction of [Fe(L2)(DBC)] with O2 in
CH3CN solution.
LMCT transition, [48,24,78–80] involving two different catecholate
orbitals on the chelated catecholate.
2.5. Catechol dioxygenase activity of iron(III) complexes
The catechol cleaving dioxygenase activity of the complexes
was explored in acetonitrile solution. The catechol adducts
[Fe(LSB)(DBC)Cl] for type 1 and [Fe(L)(DBC)] adducts for type 2 com-
plexes were generated in situ by the addition of equimolar quanti-
ties of 3,5-di-tert-butylcatechol (H2DBC) and two equivalents of
piperidine and exposing the mixtures to molecular oxygen. The
decomposition of complex–substrate adducts were monitored by
detecting the decay of the low energy LMCT bands (Figs. 3 and 4)
at ambient temperature. The oxygenated products were identified
by 1H NMR and HRMS techniques. The catecholate adducts of all
the present complexes exclusively afford the intradiol cleavage
products. The first step involves the binding of the 3,5-di-tert-
butylcatecholate moiety to the iron complex as a mononuclear
center and this step is assumed to be followed by the formation
of substrate-alkylperoxo-Fe3+ intermediate [81–84] by O2 attack
which on acyl migration yields intradiol cleavage products.
The catecholase activity of the iron(III) complexes was also
measured in absence of O2 and the complexes were found to show
no catecholase activity in the absence of O2.
Fig. 4. Progress of the reaction of adduct [Fe(L2)(DBC)] with O2 in CH3CN solution.
The disappearance of the catecholate to iron(III) charge-transfer band was
monitored.
All the complexes were found to react with the reactants
(molecular oxygen and catechol) and the oxygenation reactions ex-
hibit pseudo-first order kinetics as judged by the linearity of the
plot of [1+log(abs)] versus time. Plot of [1+log(abs)] versus time
for complexes 1a and 1b are depicted in Fig. 5 while that for 2b
is given in Fig. 6.
Table 5
Kinetic dataa for catalytic reaction.
Complex
kO2 (Mꢁ1 Sꢁ1
)
[FeIII(LSB1)Cl3]
[FeIII(LSB2)Cl3]
[FeIII(L1)Cl2]
[FeIII(L2)Cl2]
[FeIII(L4)Cl2]
[FeIII(L5)Cl2]
9.1 ꢄ 10ꢁ3
10.2 ꢄ 10ꢁ3
1.29 ꢄ 10ꢁ3
6.01 ꢄ 10ꢁ3
1.44 ꢄ 10ꢁ3
7.8 ꢄ 10ꢁ3
a
kO2 = kobs/[O2]. The solubility of O2 in CH3CN at 25 °C is 8.1 ꢄ 10ꢁ3 M. The
Fig. 5. Plot of [1+log(abs)] versus time for the reaction of [Fe(LSB1)(DBC)Cl] (a) and
kinetic data were obtained by time dependent spectral measurements for a period
of 3 h.
[Fe(LSB2)(DBC)Cl] (b) with O2 in CH3CN solution.

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R. Singh et al. / Inorganica Chimica Acta 363 (2010) 3131–3138
The rate of the reaction was calculated using the equation
Instruments Variox cryostat or an Oxford Instruments Mössbauer-
Spectromag cryostat. The latter was used for measurement in
applied magnetic fields with the field oriented perpendicular to
(kO2 = kobs/[O2]) [85–87]. The value of kobs is deduced from the
slope of the linear graph. The rate of the reaction calculated for
1a, 1b, 2a, 2b, 2d and 2e are reported in Table 5. The rates are com-
parable with other reported complexes incorporating O,N-donor li-
gands [21,30,43,48,88,89].
Time dependent spectral measurements for 1a and 1b show
that, 1b reacts faster than 1a which is due to the fact that there
is an increase in positive charge on iron(III) facilitating a stronger
interaction of catechols with iron(III) center. On the other hand,
the rates of reaction decrease in the order 2e > 2b > 2d > 2a. Due
to the bulky benzyl group, the lone pair on sulfur in 2e is not ex-
actly oriented towards iron causing an increase in Lewis acidity
on Fe(III) center facilitating the Fe(III)–catecholate binding and
hence increasing the reaction rate slightly. Furthermore, replace-
ment of p–H on the phenolato ring in 2a and 2d by an electron
withdrawing group (–NO2) in 2b and 2e also increases the Lewis
acidity on Fe(III) center enhancing the rate of the reaction.
the c-beam. Isomer shifts are given relative to a-Fe at room temper-
ature. Electrospray ionization mass spectra (ESI-MS) (70 eV) were
recorded on a Qtof Micro YA263 spectrometer. 1H NMR data were
recorded in CDCl3 solvent at 298 K with the help of Bruker
300 MHz FT spectrometer using tetramethylsilane (TMS) as an internal
standard. The C, H, N content of the samples was determined with
the help of a Perkin–Elmer 2400 Series II elemental analyzer.
4.3. Crystallographic studies
Single crystals of suitable quality for X-ray diffraction study of
the complexes 1a and 2b were grown by slow evaporation of the
complexes in acetonitrile solution. The X-ray intensity data were
measured at 293 K on Bruker-Nonious SMART APEX CCD diffrac-
tometer (Mo Ka, k = 0.71073 Å). The detector was placed at a dis-
tance 6.0 cm from the crystal. Total 606 frames were collected
with a scan width of 0.3° in different settings of . The data were re-
duced in SAINTPLUS [90] and empirical absorption correction was ap-
plied using the SADABS package [90]. Metal atoms were located using
direct method and the rest of the non-hydrogen atoms emerged
from successive Fourier synthesis. The structures were refined by
full matrix least-square procedure on F2. All non-hydrogen atoms
were refined anisotropically. The hydrogen atoms were included
in calculated positions. Calculations were performed using the
SHELXTL v.6.14 program package. Molecular structure plots were
drawn using ORTEP [91].
3. Conclusion
The synthesis and structural characterization of new mononu-
clear iron(III) complexes incorporating N,N,S coordinating triden-
tate and a few monophenolato iron(III) complexes of new N,N,O,S
coordinating tetradentate ligands have been isolated and studied
as structural and functional models for catechol dioxygenases. As
revealed by the X-ray crystal structure, the ligand is bound to the
metal center in a facial coordination mode. Dioxygen cleaving cat-
echolase activity of the complexes was carried out and all the com-
plexes are found to be highly reactive towards intradiol cleavage of
H2DBC in the presence of dioxygen. However, in the absence of O2,
no catalytic activity has been observed. Besides this, the complexes
were also characterized by UV–Vis, EPR, Mössbauer and electro-
chemical measurements.
4.4. Reactivity studies
The catechol cleavage activity of all of the complexes toward
H2DBC was examined by exposing the adducts [Fe(LSB)(DBC)Cl]
and [Fe(L)(DBC)] generated in situ in CH3CN solution to dioxygen.
Kinetic analyses of the catechol cleavage reactions were carried out
by time-dependent measurement of the disappearance of the low-
er energy LMCT bands at ambient temperature. The complexes
[Fe(LSB)(DBC)Cl] or [Fe(L)(DBC)] (0.1 mmol) in CH3CN solvent
(5 ml) were exposed to dioxygen and stirred for 24 h. The oxygen-
ation reaction was quenched by the addition of 6(M) HCl (5 ml).
The products were extracted from the aqueous solution with
CH2Cl2 (3 ꢄ 20 ml). The clear yellow organic layer was separated
and dried over sodium sulfate. The products were subjected to col-
umn chromatography using silica gel and CH2Cl2/CH3OH (8:1)
mixture.
4. Experimantal
4.1. Materials
All the starting materials were analytically pure and were used
without further purification, and the solvents were purified by
standard procedure. The ligands were prepared as described
below.
4.2. Physical measurements
UV–Vis spectra were measured on a Perkin–Elmer LAMBDA 25
spectrophotometer and IR spectra were recorded with Perkin–
Elmer L-0100 spectrophotometer. Electrochemical measurements
were performed (acetonitrile solution) on a CHI 620A electrochem-
ical analyzer using a platinum electrode under dinitrogen atmo-
sphere. Tetraethylammonium perchlorate (TEAP) was used as a
supporting electrolyte and the potentials are referenced to the
standard calomel electrode (SCE) without junction correction.
The cyclic voltammogram were recorded with a scan rate of
50 mV/s with iR compensation in all cases. X-band (9.1 GHz) EPR
spectra were recorded on a Varian E-112 spectrometer at a micro-
wave power of 1 mW and modulation amplitude of 0.5 gauss using
quartz Dewar. The Dewar was filled with liquid nitrogen and the
quartz sample tube was immersed in it. During the measurements,
the microwave cavity was continuously purged with pure and dry
nitrogen. The spectra were calibrated with respect to diph-
enylpicrylhydrazyl (dpph, g = 2.0037). Mössbauer data were re-
corded on alternating constant-acceleration spectrometers. The
sample temperature was maintained constant in an Oxford
4.5. Determination of the oxidation products
The oxidation products of [Fe(LSB)(DBC)Cl] and [Fe(L)(DBC)] in
stoichiometric reaction were analyzed by high resolution mass
spectra (HRMS) and 1H NMR techniques. After the reaction ran
for 24 h, the major products were determined as intradiol cleavage
products (Scheme 3) (Table 6), composed of I, II and III.
4.5.1. 3,5-Di-tert-butyl-5-(N,N–dimethylamidomethyl)-2-furanone (I)
Mass spectral data found for C16H27NO3, [M+Na]+, 303.3701; 1H
NMR (300 MHz. CDCl3): 1.03(s, 9H), 1.21(s, 9H), 1.9 and 3.1(2H),
2.7(s, 6H), 6.98(s,1H) and the mass, 1H NMR data were in agree-
ment with literature data.
4.5.2. 3,5-Di-tert-butyl-5-(2-oxo-2-piperidinylethyl)-5H-furanone (II)
Mass spectral data found for C19H31NO3, [M+Na]+, 343.9416; 1H
NMR (300 MHz. CDCl3): 1.00(s, 9H), 1.21(s,9H), 1.47(m, 6H), 2.74
and 2.91 (2H), 3.47(m, 4H), 7.05(s, 1H).

R. Singh et al. / Inorganica Chimica Acta 363 (2010) 3131–3138
3137
Scheme 3.
4.7.4. [FeIII(L2)Cl2], 2b
Table 6
HL2 (207 mg, 0.60 mmol) was added to a solution of 100 mg
(0.60 mmol) of FeCl3ꢀ6H2O in 20 ml methanol. The reaction mix-
ture was refluxed for 2 h producing a deep blue crystalline precip-
itate. It was then filtered and the crystalline solid was washed
thoroughly with methanol. Deep blue colored crystals were ob-
tained by slow evaporation of the solid in acetonitrile solution.
Yield: 255 mg (89%). Anal. Calc. for C17H20Cl2FeN3O3S: C, 43.21;
Percentage yield of the cleavage products I–III.
Catechol adducts
Percentage yield
[Fe(LSB1)(DBC)Cl]
[Fe(LSB2)(DBC)Cl]
[Fe(L1)(DBC)]
[Fe(L2)(DBC)]
[Fe(L3)(DBC)]
[Fe(L4)(DBC)]
[Fe(L5)(DBC)]
[Fe(L6)(DBC)]
I (14.5), II (68.2), III (4.0)
I (16.4), II (65.5), III (14.0)
I (5.7), II (73.8), III (5.1)
I (5.2), II (66.9), III (4.0)
I (5.8), II (67.8), III (4.7)
I (5.5), II (75.1), III (4.2)
I (6.4), II (70.2), III (4.2)
I (6.0), II (72.7), III (3.4)
H, 4.22; N, 8.89. Found: C, 43.15; H, 4.26; N, 8.88%. UV–Vis (kmax
/
nm (
e
/Mꢁ1 cmꢁ1) CH3CN): 610(526); 365(15576); 300(11664);
270(33383); 245(32877). IR (KBr, cmꢁ1):
m(C–O) 1264, m(Fe–O)
770,
m
(Fe–S) 324,
m
(Fe–Cl) 456. Epc (FeIII/FeII): ꢁ0.181 V.
4.5.3. 3,5-Di-tert-butyl-1-oxacyclohepta-3,5-diene-2,7-dione (III)
Mass spectral data found for C14H20O3, [M+Na]+, 258.8596; 1H
NMR (300 MHz. CDCl3): 1.13(s, 18H), 5.49(s, 1H), 6.43(s, 1H).
4.7.5. [FeIII(L3)Cl2], 2c
Yield: 232 mg (83%). Anal. Calc. for C17H20Cl3FeN2OS: C, 44.18;
H, 4.31; N, 6.10. Found: C, 44.18; H, 4.35; N, 6.05%. UV–Vis (kmax
nm (
/Mꢁ1 cmꢁ1) CH3CN): 605(3235); 315(7631); 290(10100);
/
e
4.6. Synthesis of the ligands
274(12428). IR (KBr, cmꢁ1):
m(C–O) 1266, m(Fe–O) 768, m(Fe–S)
336,
m
(Fe–Cl) 481. Epc (FeIII/FeII): ꢁ0.186 V.
The reduced Schiff bases, LSB, were synthesized from the parent
Schiff bases by NaBH4 reduction [92] and the tripodal ligands HL
were synthesized by a reported procedure [93].
4.7.6. [FeIII(L4)Cl2], 2d
Yield: 245 mg (79%). Anal. Calc. for C24H28Cl2FeN2OS: C, 53.91;
H, 4.68; N, 5.76. Found: C, 53.99; H, 4.72; N, 5.71%. UV–Vis (kmax
nm CH3CN): 515(2617); 308(4142); 285(8414);
/Mꢁ1 cmꢁ1
/
4.7. Preparation of type 1 and 2 complexes
(
e
)
245(10626). IR (KBr, cmꢁ1):
334,
m(C–O) 1287, m(Fe–O) 772, m(Fe–S)
The [FeIII(LSB)Cl3] and [FeIII(L)Cl2] complexes were prepared by
using a general method with the FeCl3.6H2O as starting material.
Details are given here for two representative cases (1a and 2b).
m
(Fe–Cl) 459. Epc (FeIII/FeII): ꢁ0.192 V.
4.7.7. [FeIII(L5)Cl2], 2e
Yield: 310 mg (91%). Anal. Calc. for C24H27Cl2FeN3O3S: C, 49.33;
H, 4.20; N, 7.81. Found: C, 49.37; H, 4.14; N, 7.85%. UV–Vis (kmax
nm (
/Mꢁ1 cmꢁ1) CH3CN): 560(3159); 427(3792); 334(14673);
4.7.1. [FeIII(LSB1)Cl3], 1a
/
1
LSB (117 mg, 0.60 mmol) was added to a solution of 100 mg
e
265(32371); 252(10343). IR (KBr, cmꢁ1):
m(C–O) 1290, m(Fe–O)
(0.60 mmol) of FeCl3ꢀ6H2O in 20 ml methanol. The reaction mix-
ture was refluxed for 2 h producing an orange–red precipitate. It
was filtered and the crystalline solid was washed thoroughly with
methanol. Orange–red colored crystals were obtained by slow
evaporation of the solid in acetonitrile solution. Yield: 91 mg
(69%). Anal. Calc. for C10H16Cl3FeN2S: C, 33.35; H, 4.50; N, 7.76.
764,
m(Fe–S) 330,
m
(Fe–Cl) 472. Epc (FeIII/FeII): ꢁ0.180 V.
4.7.8. [FeIII(L6)Cl2], 2f
Yield: 296 mg (89%). Anal. Calc. for C27H27Cl3FeN2OS: C, 50.34;
H, 4.26; N, 5.30. Found: C, 50.38; H, 4.22; N, 5.34%. UV–Vis (kmax
nm CH3CN): 547(2320); 320(2681); 280(7613);
/Mꢁ1 cmꢁ1
/
Found: C, 33.50; H, 4.49; N, 7.81%. UV–Vis (kmax/nm (
e
/Mꢁ1 cmꢁ1
(N–
)
(
e
)
CH3CN): 358 (4106); 309 (4474); 241 (10183). IR (KBr, cmꢁ1):
m
249(8112). IR (KBr, cmꢁ1):
342,
m(C–O) 1296, m(Fe–O) 760, m(Fe–S)
(Fe–Cl) 380. Epc (FeIII/FeII): ꢁ0.524 V.
m
(Fe–Cl) 495. Epc (FeIII/FeII): ꢁ0.184 V.
H) 3430,
m(Fe–S) 342,
m
4.7.2. [FeIII(LSB2)Cl3], 1b
Acknowledgements
Yield: 210 mg (78%). Anal. Calc. for C17H22Cl3FeN2S: C, 42.78; H,
5.00; N, 6.61. Found: C, 42.83; H, 5.03; N, 6.66%. UV–Vis (kmax/nm
Financial support from Council of Scientific and Industrial Re-
search, New Delhi, India, Department of Science and Technology,
New Delhi, India and the University Grant Commission, New Delhi
are greatly acknowledged. We are also thankful to Indian Associa-
tion for cultivation of Science, Kolkata, India for the data collection
on the CCD facility setup.
(e
/Mꢁ1 cmꢁ1) CH3CN): 357 (2236); 313 (2985); 255 (9165). IR (KBr,
cmꢁ1):
m(N–H) 3424, m(Fe–S) 330, m
(Fe–Cl) 368. Epc (FeIII/FeII):
ꢁ0.681 V.
4.7.3. [FeIII(L1)Cl2], 2a
Yield: 205 mg (80%). Anal. Calc. for C17H21Cl2FeN2OS: C, 47.56;
H, 4.96; N, 6.58. Found: C, 47.68; H, 4.94; N, 6.54%. UV–Vis (kmax
nm CH3CN): 600(2915); 310(4474); 278(9919);
/Mꢁ1 cmꢁ1
/
Appendix A. Supplementary material
(e
)
250(9639). IR (KBr, cmꢁ1):
m
(C–O) 1256,
m(Fe–O) 762,
m
(Fe–S)
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.ica.2010.05.027.
346,
m
(Fe–Cl) 452. Epc (FeIII/FeII): ꢁ0.22 V.

3138
R. Singh et al. / Inorganica Chimica Acta 363 (2010) 3131–3138
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