Inorganica Chimica Acta 372 (2011) 237–242
Contents lists available at ScienceDirect
Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Interaction of copper(II) complexes with bis(p-nitrophenyl)phosphate:
Structural and spectral studies
Thirumanasekaran Dhanalakshmi a, Rangasamy Loganathan a, Eringathodi Suresh b, Helen Stoeckli-Evans c,
Mallayan Palaniandavar a,⇑
a
b
c
Centre for Bioinorganic Chemistry, School of Chemistry, Bharathidasan University, Tiruchirappalli 620 024, India
Analytical Science Discipline, Central Salt and Marine Chemicals Research Institute, Bhavnagar 364 002, India
Department of Chemistry, University of Neuchatel, Neuchatel, Switzerland
a r t i c l e
i n f o
Article history:
Available online 25 February 2011
Dedicated to S.S. Krishnamurthy
Keywords:
Phosphate ester hydrolysis
Copper(II) complexes
Tridentate 3N ligands
Bis(p-nitrophenyl)phosphate
Cu(II)/Cu(I) redox potential
a b s t r a c t
When the complexes [Cu(L1)(H2O)](ClO4)2 1, where L1 = 4-methyl-1-(pyrid-2-ylmethyl)-1,4-diazacycloheptane, and [Cu(L2)Cl2] 2, where L2 = 4-methyl-1-(quinol-2-ylmethyl)-1,4-diazacycloheptane are interacted with one/two equivalents of bis(p-nitrophenylphosphate, (p-NO2Ph)2PO2, BNP), no hydrolysis of
BNP is observed. From the solution the adducts of copper(II) complexes [Cu2(L1)2((p-NO2Ph)2PO2)2](ClO4)2 3 and [Cu(L2)((p-NO2Ph)2PO2)2]H2O 4 have been isolated and structurally characterised. The
X-ray crystal structure of 3 contains two Cu(L1) units bridged by two BNP molecules. The CuCu distance
(5.1 Å) reveals no Cu–Cu interaction. On the other hand, the complex 4 is mononuclear with Cu(II) coordinated to the 3N ligand as well as BNP molecules through phosphate oxygen. The trigonality index
(s, 0.37) observed for 4 is high suggesting the presence of significant trigonal distortion in the
coordination geometry around copper(II). The complexes are further characterized by spectral and
electrochemical studies.
Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
Hydrolysis of phosphate esters by metal ions remains an active
area of research due to the fact that the backbones of DNA and RNA
contain phosphodiester bonds and phosphate esters are involved
in important biological reactions [1–12]. Nature has developed
metalloenzymes [13–15], often with two or more metal ions, especially zinc, in their active sites, to hydrolyze phosphate diester
bonds. The design and synthesis of model complexes that mimic
the function, structure, and reactivity of the active sites of the enzymes will provide valuable insight into the structure and function
of enzymes. Complexes, either mononuclear or polynuclear,
involving a wide variety of metals ranging from d-block transition
metals [1–4,12,16–18] to lanthanides [19,20] have been used for
studying the cleavage of phosphate diester bonds. Although zinc
is the most commonly found metal in these enzymes, an extensive
range of metal ions has been found to promote the cleavage of
phosphate esters [2,18–20]. Metal complexes of different phosphate moieties in which the phosphate moieties adopt monodentate, [21,22] chelating, [23–33] and bridging [34–39] modes have
been structurally characterized to explore the metal binding
⇑ Corresponding author.
E-mail addresses: [email protected], [email protected], palaniandavarm@
gmail.com (M. Palaniandavar).
0020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2011.02.030
properties of such ligands and to elucidate the importance of these
interactions in biological systems. Also, several metal complexes
containing nucleic acid fragments, ATP, polyphosphates and polyphosphate esters have been structurally characterized [40–45].
Many copper(II) complexes, either mononuclear [1,3–6,46,47] or
dinuclear [48], have been reported to effect the phosphate ester
cleavage very efficiently and only a few copper(II) complexes with
the coordinated phosphates and phosphate esters have been structurally characterized [5,6,22,45].
From our laboratory, we have reported a few mononuclear copper(II) complexes of tridentate 3N ligands with varying steric
crowding around copper(II) as catalysts for olefin aziridination
[49,50]. We found that effective tuning of the ligand environment
around copper(II) led to interesting trends in reactivity. Very recently, a cis-diaqua copper(II) complex of the ligand bis(benzimidazol-2-yl-methyl)amine has been shown to promote the
transesterification of a phosphate diester [5,6]. This prompted us
to use the copper(II) complexes [Cu(L1)(H2O)](ClO4)2 1 and
[Cu(L2)Cl2] 2 (Scheme 1) as catalysts for phosphate ester hydrolysis. The primary criterion for the synthesis of mononuclear copper(II) complexes as catalysts is that the copper(II) center has
sites available [49,50] for coordination of the phosphate ester.
The interaction of the complexes with one mole of bis(p-nitrophenyl)phosphate [(p-NO2Ph)2PO2], which is a widely used phosphate diester substrate, has been probed by employing spectral
238
T. Dhanalakshmi et al. / Inorganica Chimica Acta 372 (2011) 237–242
H3C
N
N
H3C
N
N
N
L1
N
L2
Scheme 1. Ligands employed for the present study.
and electrochemical methods. The complexes 1 and 2 fail to display any cleavage of phosphate esters and the adducts of the complexes with the phosphate have been obtained from a solution of
equimolar amounts of the complex and bis(p-nitrophenyl)phosphate. Interestingly, while 1 forms a dinuclear adduct with two
phosphates involved in bridging the two copper(II) centers, 2 forms
a mononuclear adduct with two phosphate esters coordinated via
oxygen.
2. Experimental
2.1. Materials and methods
CuCl22H2O (Merck, India), tetra-butylammonium bromide (G.F.
Smith), N-methylhomopiperazine, Cu(ClO4)26H2O, 2-picolylchloride hydrochloride, 2-quinolyl chloride hydrochloride, and bis(pnitrophenyl)phosphate monohydrate (Aldrich) were used as received. Tetra-n-butylammonium perchlorate (TBAP) was prepared
by the addition of sodium perchlorate to a hot ethanol solution
of tetra-n-butylammonium bromide. The product was recrystallised from aqueous ethanol and was tested for the absence of bromide. Electronic absorption spectra were acquired using a Varian
300 spectrophotometer (200–1100 nm). Electron paramagnetic
resonance (EPR) spectra were recorded on a JEOL JES-TE 100 Xband spectrometer, the field being calibrated with diphenylpicrylhydrazyl (dpph). The g0 and A0 values were estimated
at ambient temperature and g|| and A|| at 77 K. The values of g ?
and A? were computed as ½(3g0 g||) and ½(3A0 A||), respectively. Electrochemical experiments were conducted using a EG &
G PAR 273 Potentiostat/Galvanostat with EG & G M270 software,
using a platinum sphere working electrode, a Ag/AgNO3 reference
electrode, and a platinum plate auxiliary electrode. Cyclic voltammograms were obtained in methanol using 0.1 M TBAP as supporting electrolyte. Elemental analyses were performed in Department
of Chemistry, Bharathiar University, Coimbatore.
Caution! Perchlorate salts of transition metal complexes containing organic ligands are potentially explosive and should be prepared in small quantities and handled with appropriate
precautions. While no difficulties were encountered with the complexes reported herein, due caution should be exercised.
obtained. Yield: 0.36 g (25%); Anal. Calc. for C48H54Cl2Cu2N10O24P2:
C, 40.74; H, 3.85; N, 9.90. Found: C, 40.72; H, 3.84; N, 9.93%.
2.1.3. [Cu(L2)((p-NO2Ph)2PO2)2]H2O 4
This was prepared by the addition of bis(p-nitrophenyl)phosphate (0.26 g, 1 mmol) in methanol (15 mL) to a solution of
[Cu(L2)Cl2] (0.40 g, 1 mmol) in methanol with stirring. After
15 min of stirring the blue solution was layered with diethyl ether
and left as such for crystallization. Blue blocks of crystals suitable
for X-ray diffraction were deposited after two days. Yield: 0.52 g
(65%); Anal. Calc. for C40H39CuN7O17P2: C, 47.32; H, 3.87; N, 9.66.
Found: C, 47.34; H, 3.89; N, 9.70%.
2.2. X-ray crystallography
The single-crystal X-ray diffraction data for the complex 3 were
collected on a Bruker SMART Apex diffractometer equipped with a
CCD area detector at 293 K with Mo–Ka radiation (k, 0.71073 Å). A
crystal of suitable size was immersed in paraffin oil and then
mounted on the tip of a glass fiber and cemented using epoxy resin.
The SMART [51–53] program was used for collecting frames of data,
indexing the reflections, and determination of lattice parameters;
SAINT [51–53] program for integration of the intensity of reflections
and scaling; SADABS [51–53] program for absorption correction, and
the SHELXTL [54,55] program for space group and structure determination, and least-squares refinements on F2. The structure was
solved by heavy atom method. Other non-hydrogen atoms were located in successive difference Fourier syntheses. The final refinement was performed by full-matrix least-squares analysis.
Hydrogen atoms attached to the ligand moiety were located from
the difference Fourier map and refined isotropically.
A blue crystal of compound 4 was mounted on a Stoe Imaging
Plate Diffractometer System (Stoe & Cie, 1995) equipped with a
one-circle u goniometer and a graphite-monochromator. Data collection was performed at 173(2) K using Mo–Ka radiation
(k = 0.71073 Å). A total of 200 exposures (3 min per exposure) were
obtained at an image plate distance of 90 mm with 0 < u < 200°
and with the crystal oscillating through 1° in u. The resolution
(Dmin Dmax) is 12.45–0.81 Å. The molecular formula of this compound is [Cu(PO2(C6H4NO3)2)2(C16H21N3)]H2O. The high Rint and
residual R-values are due too the poor quality of the crystal. There
are only 3201 observed reflections for 286 parameters. The structure was solved by direct methods using the program SHELXS-97
[54,55] and refined by full matrix least squares on F2 with SHELXL97 [54,55] the hydrogen atoms were included in calculated
positions and treated as riding atoms using SHELXL-97 default
parameters. Only the heaviest atoms such as Cu and P were refined
anisotropically. Relevant crystallographic informations for 3 and 4
are summarized in Table 1.
3. Results and discussion
3.1. Synthesis of complexes
2.1.1. Preparation of phosphate adduct complexes
The ligands L1 and L2 and the complexes [Cu(L1)(H2O)](ClO4)2 1
and [Cu(L2)Cl2] 2 were prepared by using the procedures reported
already [33].
The reaction of [Cu(L1)(H2O)](ClO4)2 1 with bis(p-nitrophenyl)phosphate (BNP) yielded bright blue crystals of the complex 3. Similarly, upon treating [Cu(L2)Cl2] 2 with BNP pale blue
crystals of the complex 4 were obtained.
2.1.2. [Cu2(L1)2((p-NO2Ph)2PO2)2](ClO4)2 3
A solution of bis(p-nitrophenyl)phosphate (0.26 g, 1 mmol) in
methanol (15 mL) was added to a methanolic solution of 1
(0.48 g, 1 mmol) with stirring. After 15 min of stirring, the deep
blue solution obtained was left as such for crystallization. Dark
blue blocks of crystals suitable for X-ray data collection were
3.2. Structural characterization
3.2.1. Structure of [Cu2(L1)2((p-NO2Ph)2PO2)2](ClO4)2 3
The ORTEP view of complex 3 is depicted in Fig. 1 along with
atom numbering scheme. The relevant bond lengths and bond angles are given in Table 2. The unit cell of 3 consists of the dication
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T. Dhanalakshmi et al. / Inorganica Chimica Acta 372 (2011) 237–242
Table 1
Crystal data and structure refinement details for 3 and 4.
Empirical formula
Formula weight
Crystal system
Crystal size (mm)
Space group
a (Å)
b (Å)
c (Å)
a (°)
b (°)
c (°)
V (Å3)
Z
k (Å)
Dcalc (g cm3)
Goodness-of-fit (GOF) on F2
Number of reflections
measured
Number of reflections used
Number of LS restraints
Number of refined parameters
Final R indices [I > 2r(I)]
R1a
wR2b
a
b
Table 2
Selected bond lengths (Å) and bond angles (°) for 3 and 4.
3
4
3
C48H54Cl2Cu2N10O24P2
1414.93
monoclinic
0.33 0.39 0.45
P21/c (No. 14)
11.162(2)
20.987(4)
14.132(3)
90.000
113.26
90
3041.3 (10)
2
Mo Ka, 0.71073
1.545
0.82
13 285
C40H39CuN7O17P2
1014.13
triclinic
0.40 0.20 0.10
P1
Cu(1)–N(1)
Cu(1)–N(2)
Cu(1)–N(3)
Cu(1)–O(1)
Cu(1)–O(3)#1
7.2087(9)
12.5304(14)
25.181(3)
86.243(13)
84.550(13)
74.450(13)
2179.5(4)
2
Mo Ka, 0.71073
1.478
0.860
17 552
N(1)–Cu(1)–N(2)
N(1)–Cu(1)–N(3)
N(2)–Cu(1)–N(3)
N(2)–Cu(1)–O(1)
N(1)–Cu(1)–O(1)
N(3)–Cu(1)–O(1)
O(3)#1–Cu(1)–N(2)
O(3)#1–Cu(1)–N(1)
O(3)#1–Cu(1)–N(3)
O(3)#1–Cu(1)–O(1)
5916
0
398
8056
2
606
0.0580
0.1581
0.0991
0.2463
R1 = [R(||Fo| |Fc||)/R|Fo|].
wR2 ¼ f½RðwðF 2o F 2c Þ2 Þ=RðwF 4o Þ1=2 g.
4
2.008(4)
2.006(4)
2.025(4)
2.163(3)
1.942(4)
82.48(17)
157.49(17)
79.99(17)
96.93(16)
98.28(16)
97.75(16)
162.29(16)
96.88(17)
95.55(17)
100.67(14)
N(5)–Cu(1)
N(6)–Cu(1)
N(7)–Cu(1)
O(1)–Cu(1)
O(9)–Cu(1)
N(6)–Cu(1)–N(5)
N(6)–Cu(1)–N(7)
N(5)–Cu(1)–N(7)
N(6)–Cu(1)–O(9)
N(5)–Cu(1)–O(9)
N(7)–Cu(1)–O(9)
N(6)–Cu(1)–O(1)
N(5)–Cu(1)–O(1)
N(7)–Cu(1)–O(1)
O(9)–Cu(1)–O(1)
2.016(7)
2.011(7)
2.021(6)
2.120(5)
2.023(5)
84.0(3)
79.4(2)
163.4(3)
140.7(3)
99.7(2)
92.1(2)
120.0(3)
91.1(2)
98.6(2)
99.1(2)
distances have been observed in the case of complexes with only
O–P–O bridges. The X-ray crystal structure of [Cu2(L3)(Ph2PO4)2]-,
where L3 is a dinucleating ligand and the CuCu distance
(4.812 Å) is related to that of 3 [46].
3.2.2. Structure of [Cu(L2)((p-NO2Ph)2PO2)2]H2O 4
The ORTEP representation of 4 is depicted in Fig. 2 together with
the atom numbering scheme. Selected bond lengths and bond angles are shown in Table 2. The unit cell of 4 contains copper(II)
coordinated by three nitrogen atoms, two (N2, N3) from the homopiperazine unit and one from the quinoline moiety (N1) of the
ligand L2, and two oxygen atoms (O1 and O9) of two bis(p-nitrophenyl)phosphate molecules. The value of the structural index s
(0.37) reveals that the coordination geometry around copper(II)
is best described as trigonal bipyramidal distorted square based
pyramidal (TBDSBP) with the corners of the square plane being
occupied by the three nitrogen atoms and one oxygen atom (O9)
of one of the two phosphate esters, and the apical position by
the other oxygen atom (O1) of the second phosphate ester.
Although both the oxygen atoms that are not involved in ester linkages are equivalent, only one is coordinated to the copper ion. The
axial oxygen atom is located at a distance (Cu–O1, 2.120(5) Å)
longer than the equatorial oxygen (Cu–O9, 2.023(5) Å), obviously
because of the presence of two electrons in the dz2 orbital of copper(II) in the square-based environment. The geometries and bond
Fig. 1. ORTEP drawing of 3 showing the atom numbering scheme and the thermal
motion ellipsoids (50% probability level).
[Cu2(L1)2((p-NO2Ph)2PO2)2]2+ and two perchlorate ions. The two
copper atoms in the complex cation are bridged in a l-1,3 mode
by two (p-NO2Ph)2PO2 molecules. Each copper is coordinated by
three nitrogen atoms, two (N2, N3) from the homopiperazine moiety and one from the pyridine moiety (N1) of the ligand L1, and
two phosphate oxygen atoms (O1, O3) of BNP. The CuN3O2 coordination polyhedron is best described as trigonal bipyramidal distorted square based pyramidal (TBDSBP) [56–58], as revealed by
the value of the structural index [59] s of 0.08. The basal plane is
constituted by N1, N2, N3 and O(3)#1 (Cu(1)–N(1), 2.008(4) ;
Cu(1)–N(2), 2.006(4) ; Cu(1)–N(3), 2.025(4) and Cu(1)–O(3)#1,
1.942 Å) with the axial oxygen atom O1 (Cu–O1, 2.163(3) Å) being
significantly longer due to the presence of two electrons in dZ2
orbital. The CuCu distance is 5.1 Å, which falls in the range
(2.9–5.5 Å) observed for metal–metal distances in polynuclear metal complexes dimerised [60] by the O–P–O bridges. Longer CuCu
Fig. 2. ORTEP drawing of 4 showing the atom numbering scheme and the thermal
motion ellipsoids (50% probability level).
T. Dhanalakshmi et al. / Inorganica Chimica Acta 372 (2011) 237–242
length around the phosphorous atoms are similar to those of the
phosphate diesters [61].
It is interesting that while complex 1 interacts with BNP to give
the dimeric copper(II)–phosphate adduct 3, the complex 2 gives
the mononuclear phosphate adduct 4. The value of trigonality index (s) decreases from 0.48 for the precursor complex 2 to 0.37
for the adduct 4 upon replacing the two chloride ions in 2 by
two (p-NO2Ph)2PO2 molecules and the structure is relaxed towards
one with higher square planarity. However, the copper(II) geometry in 4 is sterically less constrained than that in 3 due to the presence of the bulky quinolyl moiety of L2. This illustrates why the
mononuclear complex 2 forms only a mononuclear phosphate adduct 4. The presence of the bulky quinolyl moiety in L2 would hinder the formation of a dinuclear adduct similar to 3. The equatorial
oxygen atom in 3 is more tightly bound to copper than that in 4, as
evident from the difference in the Cu–O bond lengths (0.081 Å).
1
0.8
Absorbance
240
A
0.6
B
0.4
0.2
0
350
450
550
650
750
850
950
1050
Wavelength (nm)
Fig. 3. Electronic absorption spectra of complexes 3 (A) and 4 (B) in methanol
solution. Concentration of the complexes: 3 103 M.
3.3. Spectral and electrochemical properties
The solid state reflectance spectra of both 3 and 4 show a broad
ligand field feature (600–750 nm, Table 3) in the visible region,
which appears to contain more than one band and this is typical
of Cu(II) located in a square-based environment. In methanol solution only one ligand field feature is observed (3, 660 nm; 4,
720 nm, Fig. 3) for the complexes suggesting changes in coordination geometries upon dissolution.
The polycrystalline EPR spectra of 3 and 4 are axial (Table 3).
The frozen-solution spectra (Fig. 4) of the two complexes are also
axial [g|| > g\ > 2.0, G = (g|| 2)/(g\ 2) = 3.0–4.3] [62,63]. The g||
and A|| values of 1 increase slightly upon adduct formation followed by dimerisation to obtain 3 suggesting the incorporation
of phosphate oxygen donor in the coordination sphere. The g|| (3,
2.240; 4, 2.245) and A|| (3, 190; 4, 168 104 cm1) values suggest
the presence of a square-based [CuN3O]+/[CuN3O2] chromophore,
as the replacement of one or more nitrogen atoms from the CuN4
chromophore is expected to increase the g|| value and decrease
the A|| value (g||, 2.200; A||, 200 104 cm1 for CuN4 chromophore) [40]. In contrast, the A|| value of 3 decreases upon adduct
formation with two BNP molecules to give 4 suggesting that the
Table 3
Electronic absorption and EPR spectral data for the copper(II) complexes.
Complexes
Electronic spectra
a
EPR spectra
b
kmax/nm (emax/M1cm1)
Solid
Methanol
Solid
Frozenc solution
1
545–645
642 (150)
268 (34 310)d
g|| 2.223
g\ 2.109
2
700–850
758 (270)
275 (27 430)d
238 (36 005)d
g3 2.162
g2 2.136
g1 2.078
3
600–750
660 (260)
287 (63 380)d
g|| 2.197
g\ 2.095
g|| 2.227
A|| 186
g\ 2.066
g||/A|| 119
g|| 2.231
A|| 130
g\ 2.077
g||/A|| 171
g|| 2.240
A|| 190
g\ 2.060
g||/A|| 117
4
500–700
720 (200)
270 (64 170)d
g||
g\ 2.147
g|| 2.245
A|| 168
g\ 2.056
g||/A|| 133
a
Concentration, 3 103 M for ligand field and 2 105 M for ligand-based
transitions.
b
A|| in 104 cm1.
c
Methanol:acetone (4 :1 V/V) glass at 77 K.
d
p–p⁄ transitions within the ligand.
Fig. 4. X band EPR spectra of complexes 3 and 4 at 77 K in methanol/acetone (4:1 V/
V) glass.
coordination geometry of 4 is distorted from square planarity
much more than that of 3. This is consistent with the observed s
value of 4, which is higher than that of 3 (cf. above). In fact, the value of g||/A|| quotient for 4 (133 cm) is also higher than that for 3
(117 cm). It is interesting to note that the g||/A|| quotient for 4
(133 cm) is much lower than that for the parent complex 2
(171 cm) suggesting that with the replacement of the two chloride
ions in 2 by two phosphate oxygen atoms, the distortion of the
copper(II) coordination geometry from square planarity is lowered
and this is evident also from the lower s values of 4 (4, 0.37; 2,
0.48).
The electrochemical data obtained for the present complexes in
methanol solution using TBAP as supporting electrolyte are collected in Table 4. The cyclic (CV) and differential pulse voltammograms (DPV) have been obtained using a Pt sphere as working
electrode and Ag/AgNO3 as reference electrode. While 3 shows
irreversible Cu(II) to Cu(I) reduction, 4 exhibits reversible Cu(II)
to Cu(I) reduction with E1/2 of 0.198 V (Fig. 5). The value of the
limiting peak-to-peak separation (DEp, 138 mV) is higher than that
for Fc/Fc+ couple (DEp, 88 mV) under identical conditions. This suggests that the heterogeneous electron transfer process in the present complexes is far from reversible and that on electron transfer
considerable stereochemical reorganization of the coordination
sphere occurs.
241
T. Dhanalakshmi et al. / Inorganica Chimica Acta 372 (2011) 237–242
Table 4
Electrochemical dataa for copper(II) complexes at 25.0 ± 0.2 °C in methanol solution.
Complexes
Epc (V)
1
2
3
4
Epa (V)
0.476
0.326
0.484
0.278
0.318
–
–
0.140
E1/2 (V)
DEp (mV)
ipa/ipc
D (106 cm2 s1)
158
–
–
138
0.6
–
–
0.9
3.4
5.8
1.14
4.0
b
CV
DPV
0.397
0.164c
0.242c
0.209
0.378
0.252
0.378
0.198
a
Potential measured (±0.002 V) vs. non-aqueous Ag/AgNO3 reference electrode; add 0.544 V to convert to standard hydrogen electrode (SHE); Fc/Fc+ couple, E1/2, 0.038 V
(CV), DEp, 88 mV; scan rate 50 mV s1; supporting electrolyte, tetra-N-butylammonium perchlorate (0.1 M); complex concentration, 1 103 M.
b
Differential pulse voltammetry (DPV), scan rate 1 mV s1, pulse height 50 mV.
c
Potential at half-height, Ep1/2.
40
I (µA)
20
0
-20
100
0
-100
-200
-300
-400
-500
E (mV)
Fig. 5. Cyclic and differential pulse voltammograms of complex 4 in methanol
solution at 25 °C at 0.05 V s1 scan rate. Complex concentration: 0.001 M.
4. Conclusions
Two copper(II) complexes with bis(p-nitrophenyl)phosphate
bound to copper(II) in both monodentate and bidentate bridging
modes have been isolated and studied. This study focuses on the
choice of ligands that has to be incorporated for the synthesis of
exact models for metalloenzymes.
Acknowledgements
We sincerely thank the Council of Scientific and Industrial Research, New Delhi for a Senior Research Fellowship to T.D. Professor M. Palaniandavar is a recipient of DST Ramanna Fellowship
[Scheme No. SR/S1/RFIC-01/2010]. We also thank the Department
of Science and Technology, New Delhi for supporting this research
[Scheme No. SR/S5/BC-05/2006]. We thank Dr. P. Sambasiva Rao,
Pondicherry University, Puducherry for providing the EPR facility.
Appendix A. Supplementary material
CCDC 746356 and 746357 contain the supplementary crystallographic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
http://www.ccdc.cam.ac.uk/data_request/cif. Supplementary data
associated with this article can be found, in the online version, at
doi:10.1016/j.ica.2011.02.030.
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