An unprecedented iron(III) complex supported by an asymmetric N-capped tripodal ligand incorporating the NO2S donor set

Article abstract of DOI:10.1016/j.ica.2009.09.010

The synthesis of an iron(III) complex [Fe(L)(1-Meim)] (1-Meim = 1-methylimidazole) coordinated by an asymmetric N-capped tripodal tetradentate ligand (L) equipped with three aromatic arms is described. X-ray crystallographic analysis shows that the comple

Full text of DOI:10.1016/j.ica.2009.09.010

Inorganica Chimica Acta 363 (2010) 1297–1300
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Note
An unprecedented iron(III) complex supported by an asymmetric N-capped
tripodal ligand incorporating the NO2S donor set
a,
c
a
Lok H. Tong a,b, , Yee-Lok Wong , Sofia I. Pascu , Jonathan R. Dilworth
*
*
a Chemistry Research Laboratory, Department of Chemistry, University of Oxford, 12 Mansfield Road, Oxford OX1 3TA, UK
b Section of Chemistry and Biochemistry, University of Geneva, 30 Quai Ernest-Ansermet, 1211 Geneva, Switzerland
c Department of Chemistry, University of Bath, Claverton Down, Bath, BA2 7AY, UK
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 15 May 2009
Received in revised form 21 August 2009
Accepted 2 September 2009
Available online 6 September 2009
The synthesis of an iron(III) complex [Fe(L)(1-Meim)] (1-Meim = 1-methylimidazole) coordinated by an
asymmetric N-capped tripodal tetradentate ligand (L) equipped with three aromatic arms is described.
X-ray crystallographic analysis shows that the complex adopts a distorted bipyramidal geometry with
a new type of N2O2S coordination environment around the iron centre.
Ó 2009 Elsevier B.V. All rights reserved.
Dedicated to Prof. Jonathan R. Dilworth
Keywords:
Asymmetric tripodal ligand
Iron(III) complex
Crystal structure
1. Introduction
We wished to develop a general synthetic strategy for N-capped
asymmetric ligands equipped with three aromatic arms in view of
Symmetric N-capped tripodal ligands of various types with
identical pendant arms have been extensively investigated as en-
zyme mimics [1–3], catalysts for organic transformations [4,5]
and molecular receptors [6]. Considerably less attention has been
given to asymmetric N-capped tripodal ligands bearing mixed N,
O and S donor groups despite their potential role in mimicking
the active sites of metalloenzymes with ternary coordination
environment, such as nitrile hydratase [7] and horse liver alcohol
dehydrogenase (HLADH) [8]. The coordination chemistry of Ni(II)
[9–12], Zn(II) [12], Mo(VI) [9,13], W(VI) [13] and V(V) [14] with
the asymmetric N-capped tripodal ligands has been reported and
in all cases, the tetradentate ligands contain one or more aliphatic
arms (Fig. 1). The chemistry of N-capped tripodal ligands with iron
focuses primarily on the preparation of synthetic and functional
models for a number of monomolecular non-heme enzymes such
as catechol dioxygenase [15,16], lipooxygenase [17] and methane
monooxygenase (MMO) [18] using symmetric NN3 type and par-
tially symmetric NNO2 type ligands with a combination of nitrogen
and oxygen in the donor set. To our knowledge, no iron complexes
supported by N-capped tripods bearing a mixed N/O/S donor set
have been reported.
the possibility of modifying the electronic properties of the ligated
metal centre by incorporation of substituents with different
electronic demands on each arm, and to extend the coordination
chemistry to other metal centres. Here we present the synthesis
of a new asymmetric N-capped tripodal ligand and the synthesis
and structure of an iron(III) complex. The tetradentate ligand fea-
tures substituted aryl alkoxide and aryl thiolate donors and more
importantly, it represents the first example of a ligand with an
NO2S (3° amine/phenolates/thiophenolate) donor set.
2. Experimental
2.1. General information
All reactions were carried out using standard Schlenk-line
techniques under an atmosphere of dinitrogen; workups were
performed in air. Tetrahydrofuran (THF) was distilled from so-
dium-benzophenone. Methanol (MeOH) and triethylamine (Et3N)
were distilled from magnesium methoxide and sodium respec-
tively. Silica gel (70–230 mesh) for flash column chromatography
was purchased from Fluka. All other reagents were purchased
from Aldrich and used as received. All 1H and 13C{1H} NMR spec-
tra were recorded on a Varian Mercury VX300 spectrometer (1H,
300 MHz and 13C, 75.4 MHz) using CDCl3 as solvent. Chemical
shifts were relative to internal SiMe4 (d = 0). Atmospheric pressure
* Corresponding authors. Address: Section of Chemistry and Biochemistry,
University of Geneva, 30 Quai Ernest-Ansermet, 1211 Geneva, Switzerland. Tel:
+41 22 379 60 36 (L.H. Tong).
E-mail address: lok.tong@unige.ch (L.H. Tong).
0020-1693/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2009.09.010

1298
L.H. Tong et al. / Inorganica Chimica Acta 363 (2010) 1297–1300
Chemistry Laboratory, University of Oxford, UK. 3,5-di-tert-bu-
tyl-2-hydroxybenzylbromide [19] and bis-aminothiophenol disul-
fide [20] was prepared according to literature procedures.
Diffraction data were collected on an Enraf–Nonius KappaCCD
diffractometer
with
graphite-monochromated
Mo
Ka
(k = 0.71073 Å). Crystals of [Fe(L)(1-Meim)] were mounted on a
glass fibre using perfluoropolyether oil and cooled rapidly to
150 K in a stream of cold nitrogen using an Oxford Cryosystems
CRYOSTREAM unit. The
h range for all data collection was
5.0 6 h 6 27.5. Intensity data were processed using the DENZO-SMN
package [21]. The structure was solved using the direct-methods
program SIR92 [22], which located all non-hydrogen atoms. Subse-
quent full-matrix least squares refinement was carried out using
the CRYSTALS program suite [23].
Fig. 1. Examples of previously reported asymmetric N-capped tripodal ligands
bearing a mixed N/O/S donor set. (R and R0 represent substituents on the phenols.)
2.2. Synthesis of the ligands
Table 1
2.2.1. {2-[N,N,N0,N0-Tetrakis(3,5-di-tert-butyl-2-
hydroxybenzyl)]aminothiophenolato} disulfide [H4(L)2]
Crystallographic data for complex [Fe(L)(1-Meim)].
Compound
[Fe(L)(1-Meim)]
To
a suspension of bis-aminothiophenol disulfide (0.25 g,
1.0 mmol) in THF (20 cm3) was added a solution of 3,5-di-tert-bu-
tyl-2-hydroxybenzylbromide (1.20 g, 4.0 mmol) in THF (30 cm3)
followed by triethylamine (0.56 cm3, 4.0 mmol) at room tempera-
ture. After heating under reflux overnight, the mixture was cooled
and filtered. The yellowish brown filtrate was concentrated and the
residue was chromatographed on a silica gel column using hexane
followed by CHCl3 as eluent. The second band was collected and
concentrated to give a pale yellow viscous liquid. Yield: 0.58 g
(52%). 1H NMR (CDCl3): d 7.36 (d, J = 8.1 Hz, 2H, ArH), 7.17–7.25
(m, 6H, ArH), 6.96 (dt, J = 7.6, 0.9 Hz, 2H, ArH), 6.70 (d, J = 2.4 Hz,
4H, ArH), 6.64 (dd, J = 8.0, 0.9 Hz, 2H, ArH), 4.17 (s, 8H, ArCH2),
Empirical formula
Formula weight
Temperature (K)
Colour
C40H54N3O2SFe
696.80
150
Black prism
0.06 ꢀ 0.08 ꢀ 0.24
Monoclinic
P21/n
10.5146(1)
29.2875(3)
12.7724(2)
90
102.5621
90
3839.06(8)
4
1.205
0.483
1493.834
0.89, 0.97
35,797
Crystal size (mm3)
Crystal system
Space group
a (Å)
b (Å)
c (Å)
a
(°)
b (°)
c
(°)
V (Å3)
t
t
1.39 (s, 36H, Bu), 1.21 (s, 36H, Bu). 13C{1H} NMR: d 152.6, 146.8,
141.8, 136.6, 135.5, 127.9, 127.0, 126.6, 125.8, 124.0, 123.8,
121.0, 57.2, 35.5, 34.3, 31.5, 30.1. MS (APCI): m/z 1122 (3%)
[M+H]+. Anal. calc. for C72H100N2O4S2: C, 77.1; H, 9.0; N, 2.5. Found:
C, 77.3; H, 9.3; N, 2.7%.
Z
Dcalcd (Mg/m3)
l
(mmꢁ1
)
F(000)
Transmission coefficients (min, max)
Number of data measured
Number of unique data (Rint
)
8907
Number of observed data (I > 3
Number of parameters, p
r(I))
6476
424
2.3. Synthesis of the complex
R
0.0369
2.3.1. [Fe(L)(1-Meim)] (1)
Rw
S(GOF)
0.0412
1.0925
+0.28 and ꢁ0.33
To a methanol/THF (1/1) solution (40 cm3) of [H4(L)2] (0.90 g,
0.8 mmol) was added NaBH4 (0.12 g, 3.2 mmol). The mixture was
stirred at room temperature for 1 h and then heated under reflux
for another 1 h. FeCl3 (0.26 g, 1.6 mmol) and three equivalents of
1-methylimidazole (1-Meim) (0.38 cm3, 4.8 mmol) were then
added. After heating under reflux overnight, the mixture was
cooled and left at room temperature for slow evaporation. The
black crystalline solids grown in the dark solution were collected,
washed with cold Et2O and dried in vacuo. Yield: 0.50 g (45%). Anal.
calc. for C40H54FeN3O2S: C, 69.0; H, 7.8; N, 6.0. Found: C, 69.1; H,
7.6; N, 5.9%. X-ray quality crystals of the complex were grown by
slow evaporation of methanol at room temperature.
Largest difference peak and hole (e Åꢁ3
)
Table 2
Selected bond distances (Å) and angles (°) for complex [Fe(L)(1-Meim)].
[Fe(L)(1-Meim)]
Fe(1)–S(1)
Fe(1)–O(1)
Fe(1)–O(2)
Fe(1)–N(1)
Fe(1)–N(2)
2.3175(5)
1.8615(12)
1.8604(12)
2.2468(13)
2.1148(14)
S(1)–Fe(1)–O(1)
S(1)–Fe(1)–O(2)
O(1)– Fe(1)–O(2)
S(1)–Fe(1)–N(1)
O(1)–Fe(1)–N(1)
O(2)–Fe(1)–N(1)
S(1)–Fe(1)–N(2)
O(1)–Fe(1)–N(2)
O(2)–Fe(1)–N(2)
N(1)–Fe(1)–N(2)
126.54(4)
123.69(4)
109.19(6)
82.72(4)
90.21(5)
90.29(5)
90.19(4)
93.87(5)
93.83(6)
172.90(6)
3. Results and discussion
The iron(III) complex [Fe(L)(1-Meim)] (1) was obtained in three
steps as shown in Scheme 1. The thiol functionality of the 2-amino-
thiophenol was first protected by aerial oxidation giving the
corresponding disulfide species in 93% yield. Subsequent reaction
with the 3,5-di-tert-butyl-2-hydroxybenzylbromide in the pres-
ence of triethylamine afforded the proligand (H4(L)2) in 52% yield.
Treatment of H4(L)2 with sodium borohydride cleaved the disulfide
linkage and deprotonated the phenol groups to give trisodium salt
of the tetradentate ligand (L) in situ, which was then followed by
chemical ionization (APCI) mass spectra were recorded on a Hew-
lett–Packard 1050 Series Mass Spectrometer. Elemental analyses
were performed by the microanalysis laboratory of the Inorganic

L.H. Tong et al. / Inorganica Chimica Acta 363 (2010) 1297–1300
1299
Scheme 1.
N(1) (90.21(5)° and 90.29(5)°, respectively), and (ii) the Fe(1)–
S(1) bond distance (2.3175(5)) is longer than Fe(1)–O(1) and Fe–
O(2) bonds (1.8615(12) and 1.8604(12) Å, respectively). The smal-
ler angle of N(2)–Fe–S(1) compared with that of N(2)–Fe–O(1) and
N(2)–Fe–O(2) indicates bending of the 1-methylimidazole towards
the five-membered chelate ring. The overall geometry and confor-
mation of 1 are similar to those in related iron(III) complexes sup-
ported by asymmetric N-capped tripodal NO3 ligands we reported
earlier [19], except that the thiophenolate arm in 1 is replaced by a
phenolate donor. The new type of N2O2S coordination environment
observed in 1 shows resemblance to an iron(III) complex reported
by Bashkin et al. which possesses a NO2S2 coordination geometry
around the iron centre [24].
In summary, we have described the synthesis and crystal struc-
ture of an iron(III) complex supported by an N-capped tetradentate
ligand. The tripod provides a new type of NO2S coordination sphere
for iron. Incorporation of substituents onto the aryl rings would be
possible in our facile synthetic approach by using different substi-
tuted 2-aminothiophenols and 2-hydroxybenzylbromides to give
rise to ligands possessing varying electronic demands and there-
fore offering new opportunities for a tunable Lewis basicity to
the bound metal centre of interest.
Fig. 2. ORTEP representation (40% probability ellipsoids) of the molecular structure
and atom labeling scheme for [Fe(L)(1-Meim)]. Hydrogen atoms are omitted for
clarity.
Supplementary material
CCDC 730372 contains the supplementary crystallographic data
for [Fe(L)(1-Meim)]. These data can be obtained free of charge from
The Cambridge Crystallographic Data Centre via www.ccdc.cam.
ac.uk/data_request/cif.
reaction with iron(III) chloride in the presence of 1-methylimidaz-
ole (1-Meim) to yield the desired complex 1 in 45% isolated yield.
The molecular structure of 1 is shown in Fig. 2. The crystallo-
graphic data for the complex is given in Table 1. Selected bond dis-
tances and angles are in Table 2. The compound crystallises in the
monoclinic space group P21/n. The iron centre is situated in a dis-
torted trigonal bipyramid geometry, with the equatorial plane de-
fined by two phenolate oxygens (O(1) and O(2)) and a sulfur atom
(S(1)). The axial sites are occupied by two nitrogen atoms (bridge-
head N(1) and imidazole N(2)), giving the N(1)–Fe–N(2) angle of
172.90(6)°. The iron atom is out of the plane (0.09 Å) of the three
equatorial donor atoms toward the imidazole nitrogen. The two
benzyl phenolate arms form six-membered chelate rings with
the metal centre, and a five-membered ring is observed with the
thiophenolate arm. The geometric features of the five-membered
ring displays differences with respect to those in the six-mem-
bered systems: (i) the angle S(1)–Fe(1)–N(1) (82.72(4)°) is consid-
erably smaller than that of the O(1)–Fe(1)–N(1) and O(2)–Fe(1)–
Acknowledgements
We thank the Croucher Foundation (Dr. Yee-Lok Wong) and
Royal Society (Dr. Sofia I. Pascu) for financial support. We are grate-
ful to Dr. Andrew Cowley for crystallographic assistance.
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