A series of mononuclear nickel(II) complexes of Schiff-base ligands having N,N,O- and N,N,N-donor sites: Syntheses, crystal structures, solid state thermal property and catecholase-like activity

Article abstract of DOI:10.1016/j.poly.2012.07.088

Four new mononuclear nickel(II) complexes, namely [NiL¹(H ₂O)₃](NO₃)₂ (1), [NiL ²(H₂O)₃](NO₃)₂ (2), [NiL³(H₂O)₃]

Full text of DOI:10.1016/j.poly.2012.07.088

Polyhedron xxx (2012) xxx–xxx
Contents lists available at SciVerse ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
A series of mononuclear nickel(II) complexes of Schiff-base ligands having
N,N,O- and N,N,N-donor sites: Syntheses, crystal structures, solid state thermal
property and catecholase-like activity
Averi Guha a, Kazi Sabnam Banu a, Sudhanshu Das a, Tanmay Chattopadhyay b, Ria Sanyal a,
Ennio Zangrando c, , Debasis Das a,
⇑
⇑
a Department of Chemistry, University of Calcutta, 92, A.P.C. Road, Kolkata 700 009, India
b Department of Chemistry, Panchakot Mahavidyalya, Sarbari, Neturia, Purulia 723 121, India
c Dipartimento di Scienze Chimiche e Farmaceutiche, University of Trieste, Via L. Giorgieri 1, 34127 Trieste, Italy
a r t i c l e i n f o
a b s t r a c t
Four new mononuclear nickel(II) complexes, namely [NiL1(H2O)3](NO3)2 (1), [NiL2(H2O)3](NO3)2 (2),
[NiL3(H2O)3](NO3)2 (3) and [NiL4(ClBz)(H2O)]ꢀ1.25(H2O) (4) have been synthesized via Schiff-base forma-
tion by condensation between 2-benzoylpyridine and N-(2-aminoethyl)pyrrolidine for L1, salicylaldehyde
and N-(2-aminoethyl)piperazine (L2), 5-chlorosalicylaldehyde and N-(2-aminoethyl)piperazine (L3), and
5-chlorosalicylaldehyde and N-(2-aminoethyl)morpholine (L4). These complexes are comprehensively
characterized via routine physicochemical techniques as well as by single crystal X-ray structural analy-
ses. Despite all the nickel complexes are mononuclear, the catecholase activity shows prominent varia-
tion depending on the coordination environment around the metal center. Complexes 2 and 3 derived
from same amine bear an extra positive charge on the ligand system facilitating the substrate–catalyst
interaction to promote the oxidation of 3,5-DTBC to 3,5-DTBQ. On the contrary complexes 1 and 4 remain
inert in nature, although 1 shows structural similarities in terms of coordination environment with nickel
substituted catechol oxidase.
Article history:
Available online xxxx
Dedicated to Alfred Werner on the 100th
Anniversary of his Nobel prize in Chemistry
in 1913.
Keywords:
Mononuclear nickel(II) complex
X-ray crystallography
Thermogravimetry
Catecholase activity
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
metallobiosites like catechol oxidase, phosphatase, cytP-450 and
as possible mimics of nucleases [12]. Our work on modeling of cat-
For a long period of time considerable attention has been paid
to the chemistry of the metal complexes of Schiff-bases containing
nitrogen and other donors [1] and it has become an emerging area
of research. Modern chemists still synthesize Schiff-bases that, if
well-designed, are considered ‘‘privileged ligands’’. The corre-
sponding metal complexes have attracted increasing interest that
may be attributed to their stability, biological activity [2] and po-
tential applications in many fields such as in catalysis [3], material
sciences, hydrometallurgy and also to get insight into molecular
processes occurring in biochemistry [4–11]. Nickel(II) has very fas-
cinating coordination chemistry owing to its inherent ability to
adopt various geometries, which often interconvert and such con-
figurational switch is generally associated with color changes. In
recent years the development of nickel bio-chemistry and the
use of nickel compounds in the construction of new magnetic
materials give a thrust in this area. Our group is continuously
engaged in studying Schiff-base complexes of transition and
post-transition metal ions, mainly as small synthetic analogs of
echol oxidase [13], (a type-III copper protein catalyzing exclusively
the oxidation of catechols to quinones), demonstrates an extraordi-
nary efficiency of metal complexes towards this reaction. In partic-
ular this activity takes place when extra positive charges either on
imine nitrogen or other (usually nitrogen) atom are present on the
ligand system. It has already been verified that mononuclear cop-
per complexes are less active compared to their dinuclear counter-
parts. However, with manganese system the reverse trend is
observed to be reality [12c,12e]. Till date rare reports were found
in the literature where dinuclear or polynuclear nickel(II) com-
plexes of Schiff-base ligands have been used as catalysts in the oxi-
dation of catechols to quinones [12b,12f,12g]. Amongst these, one
of our dinuclear nickel(II) complexes having a positively charged li-
gand system is observed to display a high activity [12b]. However,
to the best of our knowledge no systematic study of mononuclear
nickel(II) complexes of Schiff-base ligands is reported in the litera-
ture on this subject. For the present study we have designed four
Schiff-base ligands L1–L4 (Scheme 1), and we are reporting herein
the synthesis and the comprehensive characterization of four
mononuclear nickel(II) complexes along with their spectroscopic,
solid state thermal property and catecholase-like activity using
⇑
Corresponding authors. Tel.: +91 33 24837031; fax: +91 33 23519755.
E-mail address: dasdebasis2001@yahoo.com (D. Das).
0277-5387/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2012.07.088
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2
A. Guha et al. / Polyhedron xxx (2012) xxx–xxx
Scheme 1. Chemical drawing of the ligand systems.
3,5-di-tert-butylcatechol as model substrate. Of these, L2 and L3
should get protonated upon coordination in order to confirm our
preliminary observations on the effect of this feature in the
catalysis.
(0.727 g, 2.5 mmol) was added and the resulting solution was stir-
red for 3–4 h. The green solution was filtered, kept in a CaCl2 des-
iccator in dark and after a few days crystals suitable for X-ray data
analysis were obtained. (Yield 70%). Anal. Calc. for C18H27N5NiO9
(1): C, 41.88; H, 5.27; N, 13.57. Found: C, 41.85; H, 5.26; N,
13.55%. UV–Vis-NIR (in methanol, nm) kmax = 590, 770, 895.
2. Experimental
2.3.2. [NiL2(H2O)3](NO3)2 (2)
2.1. Materials
A methanolic solution (5 mL) of N-(2-aminoethyl)pipeazine
(0.258 g, 2 mmol) was added dropwise to a hot methanolic solu-
tion (10 mL) of salicylaldehyde (0.244 g, 2 mmol) and the resulting
solution was refluxed for half an hour. Then, a methanolic solution
(10 mL) of Ni(NO3)2ꢀ6H2O (0.727 g, 2 mmol) was added stirring the
resulting solution for 3–4 h. The green solution was filtered, kept in
a CaCl2 desiccator in dark and after a few days crystals suitable for
X-ray data analysis were obtained. (Yield 73%). Anal. Calc. for
All the chemicals salicylaldehyde, 5-chlorosalicylaldehyde, 2-
benzoylpyridine,
N-(2-aminoethyl)piperazine,
N-(2-amino-
ethyl)pyrrolidine, N-(2-aminoethyl)morpholine and nickel nitrate
hexahydrate were obtained from commercial sources and used as
received. All other chemicals are of AR grade. Solvents were dried
according to standard procedure and distilled prior to use.
C
13H25N5NiO10 (1): C, 33.21; H, 5.36; N, 14.90. Found: C, 33.20;
2.2. Physical measurements
H, 5.33; N, 14.87%. UV–Vis-NIR (in methanol, nm) kmax = 378,
614, 750, 900.
Elemental analyses (carbon, hydrogen and nitrogen) were per-
formed using a Perkin Elmer 24 °C elemental analyzer. Infrared
spectra were recorded on KBr disks (400–4000 cmꢁ1) with a Shi-
madzu FTIR-8400S. Electronic spectra (800–200 nm) were re-
corded at 27 °C using a Shimadzu UV-3101PC in dry methanol.
Thermal analyses (TG–DTA) were carried out on a Mettler Toledo
(TGA/SDTA 851) thermal analyzer in flowing nitrogen at a rate of
30 cm3 minꢁ1. Ambient temperature magnetic susceptibility mea-
surements were performed with Magway MSB Mk1 magnetic sus-
ceptibility balance. Cyclic voltammetry was performed using a BAS
electrochemical workstation (Model Epsilon) under nitrogen atmo-
sphere in conventional three electrode configurations with tetrae-
thylammonium perchlorate as the supporting electrolyte in DMF. A
planar platinum milli-electrode and a platinum wire were used in
cyclic voltammetry as the working electrode and counter elec-
trode, respectively. The potentials are reported with respect to
Ag/AgCl as reference and are uncorrected for liquid junction
potential.
2.3.3. [NiL3(H2O)3](NO3)2 (3)
A methanolic solution (5 mL) of N-(2-aminoethyl)piperazine
(0.258 g, 2 mmol) was added dropwise to a hot methanolic solu-
tion (10 mL) of 5-chloro-salicylaldehyde (0.312 g, 2 mmol) and
the resulting solution was refluxed for an hour. Then, a methanolic
solution (10 mL) of Ni(NO3)2ꢀ6H2O (0.727 g, 2 mmol) was added
and the resulting solution was stirred for 3–4 h. The green solution
was filtered, kept in a CaCl2 desiccator in dark and after a few days
crystals suitable for X-ray data analysis were obtained. (Yield 71%).
Anal. Calc. for C13H24ClN5NiO10 (1): C, 30.95; H, 4.79; N, 13.88.
Found: C, 30.92; H, 4.75; N, 13.86%. UV–Vis-NIR (in methanol,
nm) kmax = 387, 614, 754, 915.
2.3.4. [NiL4(ClBz)(H2O)].1.25(H2O) (4)
A methanolic solution (5 mL) of N-(2-aminoethyl)morpholine
(0.2603 g, 2 mmol) was added dropwise to a hot methanolic solu-
tion (10 mL) of 5-chloro-salicylaldehyde (0.312 g, 2 mmol) and the
resulting solution was refluxed for an hour. Then, a methanolic
solution (10 mL) of Ni(NO3)2ꢀ6H2O (0.727 g, 2 mmol) was added
and the resulting solution was stirred for 3–4 h. The green solution
was filtered and kept in a CaCl2 desiccator in dark. Crystals suitable
for X-ray data analysis were obtained after a few days. (Yield 68%).
Anal. Calc. for C20H24.5Cl2N2NiO6.25 (1): C, 45.97; H, 4.73; N, 5.36.
Found: C, 45.95; H, 4.70; N, 5.33%. UV–Vis-NIR (in methanol, nm)
kmax = 385, 607, 755, 914.
2.3. Syntheses of the complexes
2.3.1. [NiL1(H2O)3](NO3)2 (1)
A methanolic solution (5 mL) of N-(2-aminoethyl)pyrrolidine
(0.2284 g, 2 mmol) was added dropwise to a hot methanolic solu-
tion (10 mL) of 2-benzoylpyridine (0.3664 g, 2 mmol) and the
resulting solution was refluxed for 12 h till a light yellow color ap-
peared. Then, a methanolic solution (10 mL) of Ni(NO3)2ꢀ6H2O
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A. Guha et al. / Polyhedron xxx (2012) xxx–xxx
3
2.4. X-ray structure determination
Diffraction data for compounds 1–4 were collected on a Bruker
Smart Apex diffractometer equipped with CCD. All the experiments
were performed at room temperature (except for 1 carried out at
273 K) with MoK
a radiation (k = 0.71073 Å). Cell refinement,
indexing and scaling of the data set were carried out using Bruker
Smart Apex and Bruker Saint packages [14]. The structures were
solved by direct methods and subsequent Fourier analyses and re-
fined by the full-matrix least-squares method based on F2 with all
observed reflections [15]. The contribution of H atoms (at calcu-
lated position generated by program SHELXL [15] or located from
the Fourier map for water molecules) was introduced in the final
cycles of refinement. The
DFourier map of 4 evidences an area of
disordered electron density interpreted as three water oxygen
atoms (one at half occupancy based on peak height). Crystallo-
graphic data and details of refinements are reported in Table 1.
All the calculations were performed using the WinGX System,
Ver 1.80.05 [16].
Fig. 1. ORTEP drawing (30% ellipsoid probability) and atom numbering scheme of
the complex cation of 1.
Table 2
Coordination bond lengths (Å) and angles (°) for 1.
Ni–N(1)
Ni–N(2)
Ni–N(3)
2.004(3)
2.168(3)
2.110(3)
Ni–O(1w)
Ni–O(2w)
Ni–O(3w)
2.077(3)
2.034(3)
2.078(3)
3. Results and discussion
3.1. Syntheses and characterization
N(1)–Ni–N(2)
N(1)–Ni–N(3)
N(1)–Ni–O(1w)
N(1)–Ni–O(2w)
N(1)–Ni–O(3w)
N(2)–Ni–N(3)
N(2)–Ni–O(1w)
N(2)–Ni–O(2w)
81.71(13)
77.56(13)
95.53(13)
174.40(14)
93.32(13)
159.20(14)
95.05(14)
103.74(14)
N(2)–Ni–O(3w)
N(3)–Ni–O(1w)
N(3)–Ni–O(2w)
N(3)–Ni–O(3w)
O(1w)–Ni–O(2w)
O(1w)–Ni–O(3w)
O(2w)–Ni–O(3w)
88.39(15)
88.65(13)
96.95(14)
91.12(14)
85.34(12)
170.88(13)
85.64(13)
All four complexes are prepared by applying template synthesis
technique by treating a methanolic solution of nickel(II) nitrate
hexahydrate with the Schiff-base formed in situ between alde-
hyde/ketone (salicylaldehyde, 5-chloro-salicylaldehyde and 2-ben-
zoylpyridine) and amines [N-(2-aminoethyl)piperazine, N-(2-
aminoethyl)pyrrolidine, N-(2-aminoethyl)morpholine]. Elemental
analyses suggest that the molecular composition of the complexes
should be NiL1–3(NO3)2ꢀ3H2O for complexes 1–3 and NiL4(-
NO3)2ꢀ2H2O for complex 4, which was further verified by thermo-
gravimetric analyses. It is evident from the thermograms (please
see SI) that all four complexes upon heating finally converted to
NiO. For complex 1, Tf (final temperature of decomposition) was
480 °C, to which the experimental weight loss corresponds to
86.83% (calculated 85.50%). The correspondent figures for 2 were
Tf = 430 °C, exp. weight loss = 86.47% (calc. 84.10%); for 3,
Tf = 590 °C, exp. weight loss = 85.65% (calc. 87.01%) and for 4,
Tf = 550 °C, exp. weight loss = 85.60% (calc. 85.90%). The metal con-
tent in the final residual species in each case has been estimated by
the usual gravimetric analysis [17] and the results corroborate well
with the calculated metal content based on the compound formu-
lation. Magnetic susceptibility measurements (
l
eff = ꢂ3.2 B.M.)
Table 1
Crystallographic data for compounds 1–4.
1
2
3
4.1.25H2O
Empirical formula
fw
C
18H27N5NiO9
C13H25N5NiO10
470.09
C13H24ClN5NiO10
504.53
C20H24.5Cl2N2NiO6.25
522.53
516.16
Crystal system
Space group
a (Å)
b (Å)
c (Å)
orthorhombic
P bca
11.9136(18)
17.157(3)
23.278(3)
triclinic
P1
triclinic
P1
monoclinic
P 21/c
18.049(4)
13.117(3)
20.279(5)
ꢀ
ꢀ
9.337(2)
9.829(3)
12.672(3)
94.981(4)
100.287(4)
118.349(4)
987.1(4)
2
9.035(5)
9.804(5)
13.264(7)
71.918(6)
79.389(6)
64.748(6)
1008.4(10)
2
a
(°)
b (°)
(°)
99.419(3)
c
V (Å)
4758.2(12)
8
1.441
4736.2(18)
8
1.466
Z
Dcalc, g cmꢁ3
1.582
1.045
1.662
1.157
l
(MoKa
), mmꢁ1
0.872
1.083
F(000)
2160
492
524
2164
h range (°)
1.75–28.34
26745
5625
0.0691
3487
2.40–28.30
8145
4359
0.0204
3728
2.38–26.28
6855
3547
0.0408
2608
1.86–26.58
33373
9097
0.0345
6735
No. of reflections collected
No. of independent reflections
Rint
No. of reflections [I > 2
r
(I)]
No. of refined parameters
310
1.113
0.0788,0.1509
0.880, ꢁ0.344
262
0.999
0.0437, 0.1289
0.564, ꢁ0.348
289
1.012
0.0699, 0.2038
1.571, ꢁ1.487
580
1.042
0.0651, 0.2082
1.647, ꢁ0.452
Goodness-of-fit (F2)
R1, wR2 (I > 2
r
(I))a
Residuals (eÅ3)
a
2
½
R1
=
R
||Fo| ꢁ |Fc||/
R
|Fo|, wR2 = [
R
w(Fo ꢁ Fc2)2/
R
w (Fo2)2]
.
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4
A. Guha et al. / Polyhedron xxx (2012) xxx–xxx
reveal that nickel(II) attains an octahedral configuration in all the
complexes and the appearance of three bands in UV–Vis-NIR
region corroborate this coordination geometry [18]. All the com-
plexes exhibit IR bands due to C@N stretch in the range 1638–
175, 185, 180 and 7 X
ꢁ1 cm2 Mꢁ1, respectively, indicating that 1–
3 behave as 2:1 electrolytes, while complex 4 is non-electrolytic
in nature.
1648 cmꢁ1 and skeletal vibration in the range 1549–1560 cmꢁ1
.
3.2. Description of crystal structure
Broad bands centered in the range 1360–1385 cmꢁ1 are exhibited
by the complexes 1–3 due to the presence of nitrate ion. The very
strong and broad bands observed at ꢂ3383, ꢂ3370, ꢂ 3407, and
ꢂ3361 cmꢁ1 in 1–4, respectively, are attributed to hydrogen
bonded O–H stretches of water molecules present in the com-
plexes. In order to understand the composition in solution, molar
conductance of the 10ꢁ3 M methanol solution of complexes 1–4
were determined. The conductance values at 298 K for 1–4 are
The ORTEP view of the complex cation of 1, [NiL1(H2O)3](NO3)2,
with the atom numbering scheme is illustrated in Fig. 1 and a
selection of bond distances and angles is reported in Table 2. The
independent unit contains two nitrate anions beside the metal
complex. The nickel ion shows a severely distorted octahedral
geometry where three co-ordination sites are occupied by N donor
atoms from the pyridine, the imino and the pyrrolidine, with
Fig. 2. (a) View along axis b of the 2D network in compound 1 formed by H-bonds realized among the aqua ligands and nitrate anions. (b) Side view of the 2D layer down axis
a.
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A. Guha et al. / Polyhedron xxx (2012) xxx–xxx
5
Table 5
Hydrogen bond parameters in complexes 1–4.
D–H
d(D–H)
d(H. . .A)
<DHA
d(D. . .A)
A
Symmetry code of A
Complex 1
O1w–H12
O1w–H11
O2w–H21
O2w–H22
O3w–H31
O3w–H32
0.760
0.844
0.776
0.704
0.803
0.702
2.014
1.933
1.942
2.001
1.897
2.200
170.36
173.15
168.52
178.13
167.64
178.89
2.766
2.773
2.707
2.705
2.687
2.902
O1
O3
O4
O1
O5
O6
[xꢁ1/2, y, ꢁz+1/2]
[xꢁ1/2, y, ꢁz+1/2]
[x+1/2, ꢁy+1/2, ꢁz+1]
Complex 2
N3–H3b
N3–H3b
N3–H3a
N3–H3a
O1w–H2c
O1w–H2d
O2w–H4c
O2w–H4d
O3w–H3c
O3w–H3d
0.901
0.901
0.901
0.901
1.022
0.754
0.792
0.960
0.812
0.990
2.224
2.052
2.028
2.302
2.039
2.510
1.849
1.911
2.060
1.763
145.25
152.30
153.22
144.35
157.80
123.35
169.25
162.19
161.70
166.25
3.009
2.880
2.861
3.079
3.010
2.992
2.631
2.840
2.842
2.734
O5
O7
O9
O10
O7
O10
O1
O9
O5
O8
[x, y+1, z]
[x, y+1, z]
[ꢁx+1, ꢁy+1, ꢁz]
[ꢁx+1, ꢁy+1, ꢁz]
[ꢁx, ꢁy, ꢁz]
[xꢁ1, y, z]
[xꢁ1, y, z]
[xꢁ1, y, z]
Complex 3
N3–H3a
N3–H3a
N3–H3b
N3–H3b
O1w–H11
O1w–H12
O2w–H21
O2w–H22
O3w–H31
O3w–H32
0.900
0.900
0.900
0.900
0.846
0.844
0.847
0.931
0.860
0.844
2.388
1.976
2.151
2.200
2.217
2.289
1.992
1.693
2.061
1.897
144.56
155.46
142.14
150.79
170.40
129.86
157.65
174.40
178.34
168.68
3.164
2.820
2.914
3.018
3.055
2.903
2.794
2.621
2.921
2.730
O5
O7
O9
O10
O7
O8
O9
O1
O5
O8
[x, yꢁ1, z]
[x, yꢁ1, z]
[ꢁx+1, ꢁy+1, ꢁz+1]
[ꢁx+1, ꢁy+1, ꢁz+1]
[xꢁ1, y, z]
[xꢁ1, y, z]
[ꢁx, ꢁy+2, ꢁz+1]
Complex 4
O1w–H11w
O2w–H21w
O1w–H12w
O2w–H22w
0.850
0.864
0.828
0.871
1.930
2.010
2.111
2.000
172.59
137.53
168.26
142.80
2.775
2.710
2.927
2.745
O6
O2
O3wa
O4wa
a
O3w and O4w are lattice water molecules of which no H atom was positioned.
Fig. 3. ORTEP drawing of the complex cation of 2.
Fig. 4. ORTEP drawing of the complex cation of 3.
formation of two 5-membered chelate rings. The metal completes
the coordination environment with three water molecules (Fig. 1).
Among the Ni–N bond distances, that involving the imino N do-
nor is the shorter (2.004(3) Å), while the Ni–N(py) is of 2.110(3) Å,
and the Ni–N(pyrrolidine) is the longest for the sp3 hybridization
of the nitrogen atom (2.168(3) Å). Correspondingly the Ni–O2w
bond length, trans to the imino nitrogen, is significantly shorter
(2.034(3) Å) with regard to the other Ni–Ow values of 2.077(3) and
2.078(3) Å. The distortion can be further evidenced from the devia-
tion of the cisoid and transoid angles from the ideal values (Table 2).
The nitrate anions behave as H-bond acceptor towards the aqua
ligands resulting in the formation of a 2D undulated layered net-
work, and a view of the packing down axis-b is shown in Fig. 2,
while the geometrical parameters of H-bonds are collected in Table
5. The compounds 2 and 3 crystallize in same space group (triclinic
ꢀ
system, P1) and are isostructural, being the unit cell parameters
close comparable, differing by ca. 20 Å3 in their volume, being lar-
ger in 3 for the presence of the chlorine atom. A thermal ellipsoid
diagram of 2 and 3 along with the atom numbering scheme is given
Figs. 3 and 4, respectively. In both the complexes the metal ions
have a distorted octahedral environment with the coordination
sites occupied by the tridentate Schiff base through the phenoxo
oxygen, the imino nitrogen and the N donor of the piperazine moi-
ety and by three water molecules. The structural similarity that ap-
pears from the figures of the two complexes is also confirmed by
the Ni–N and Ni–O coordination distances reported in Table 3
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6
A. Guha et al. / Polyhedron xxx (2012) xxx–xxx
Table 3
Coordination bond lengths (Å) and angles (°) for 2 and 3.
Complex 2
Complex 3
Ni–N(1)
Ni–N(2)
Ni–O(1)
Ni–O(1w)
Ni–O(2w)
Ni–O(3w)
1.994(2)
2.241(2)
2.003(2)
2.122(2)
2.025(2)
2.113(2)
Ni–N(1)
Ni–N(2)
Ni–O(1)
Ni–O(1w)
Ni–O(2w)
Ni–O(3w)
1.995(5)
2.216(5)
1.988(4)
2.127(5)
2.023(4)
2.110(4)
N(1)–Ni–N(2)
N(1)–Ni–O(1)
82.76(9)
91.47(9)
90.66(10)
175.00(9)
90.42(9)
174.04(8)
92.32(9)
92.24(9)
87.32(9)
89.21(10)
93.52(9)
91.26(9)
89.85(10)
178.81(9)
89.03(9)
N(1)–Ni–N(2)
N(1)–Ni–O(1)
83.48(17)
91.63(17)
89.2(2)
N(1)–Ni–O(1w)
N(1)–Ni–O(2w)
N(1)–Ni–O(3w)
N(2)–Ni–O(1)
N(2)–Ni–O(1w)
N(2)–Ni–O(2w)
N(2–)–Ni–O(3w)
O(1)–Ni–O(1w)
O(1)–Ni–O(2w)
O(1)–Ni–O(3w)
O(1w)–Ni–O(2w)
O(1w)–Ni–O(3w)
O(2w)–Ni–O(3w)
N(1)–Ni–O(1w)
N(1)–Ni–O(2w)
N(1)–Ni–O(3w)
N(2)–Ni–O(1)
N(2)–Ni–O(1w)
N(2)–Ni–O(2w)
N(2)–Ni–O(3w)
O(1)–Ni–O(1w)
O(1)–Ni–O(2w)
O(1)–Ni–O(3w)
O(1w)–Ni–O(2w)
O(1w)–Ni–O(3w)
O(2w)–Ni–O(3w)
176.26(17)
89.18(18)
175.05(15)
91.91(19)
92.79(17)
88.44(17)
88.75(19)
92.10(17)
90.76(18)
Fig. 7. ORTEP drawing (30% probability ellipsoid) of the molecular complex A of 4.
91.0(2)
178.31(19)
90.67(19)
Fig. 8. ORTEP drawing (30% probability ellipsoid) of the molecular complex B of 4.
Fig. 5. H-bonding scheme in compound 3. A similar arrangement is also observed in
compound 2.
Table 4
Coordination bond lengths (Å) and angles (°) for the two complexes of 4.
Complex A
Complex B
Ni(1)–N(1)
Ni(1)–N(2)
Ni(1)–O(1)
Ni(1)–O(2)
Ni(1)–O(3)
Ni(1)–O(1w)
2.006(3)
2.242(3)
2.082(3)
2.008(3)
2.072(3)
2.094(3)
Ni(2)–N(3)
Ni(2)–N(4)
Ni(2)–O(5)
Ni(2)–O(6)
Ni(2)–O(7)
Ni(2)–O(2w)
2.018(4)
2.224(4)
2.101(3)
1.992(3)
2.061(3)
2.084(3)
N(1)–Ni(1)–N(2)
N(1)–Ni(1)–O(1)
N(1)–Ni(1)–O(2)
N(1)–Ni(1)–O(3)
N(1)–Ni(1)–O(1w)
N(2)–Ni(1)–O(1)
N(2)–Ni(1)–O(2)
N(2)–Ni(1)–O(3)
N(2)–Ni(1)–O(1w)
O(1)–Ni(1)–O(2)
O(1)–Ni(1)–O(3)
O(1)–Ni(1)–O(1w)
O(2)–Ni(1)–O(3)
O(2)–Ni(1)–O(1w)
O(3)–Ni(1)–O(1w)
82.53(13)
89.62(12)
173.89(13)
86.16(12)
94.06(13)
170.90(12)
100.57(12)
93.38(12)
88.40(12)
87.68(11)
90.68(12)
87.57(11)
88.39(11)
91.31(12)
178.23(12)
N(3)–Ni(2)–N(4)
N(3)–Ni(2)–O(5)
N(3)–Ni(2)–O(6)
N(3)–Ni(2)–O(7)
N(3)–Ni(2)–O(2w)
N(4)–Ni(2)–O(5)
N(4)–Ni(2)–O(6)
N(4)–Ni(2)–O(7)
N(4)–Ni(2)–O(2w)
O(5)–Ni(2)–O(6)
O(5)–Ni(2)–O(7)
O(5)–Ni(2)–O(2w)
O(6)–Ni(2)–O(7)
O(6)–Ni(2)–O(2w)
O(7)–Ni(2)–O(2w)
84.22(17)
87.22(15)
174.82(15)
88.28(15)
92.11(15)
171.44(14)
100.27(15)
88.23(15)
94.79(15)
88.27(12)
91.30(12)
85.72(12)
89.27(13)
90.10(13)
176.97(13)
Fig. 6. The centrosymmetric pairing of complexes in 3 engaged in a double H-bond.
A similar arrangement is also detected in compound 2.
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A. Guha et al. / Polyhedron xxx (2012) xxx–xxx
7
where the corresponding values are well comparable. In fact for
each Ni center, the bond length involving the imine nitrogen is
the shortest (1.994(2) and 1.995(5) Å in 2 and 3, respectively)
and comparable in length with the metal-phenoxo of 2.003(2)
1.988(4) Å. On the other hand the Ni–N(piperidine) is the longest
(2.241(2), 2.216(5) Å). The Ni–O(water) bond distances show inter-
mediate values and lie in the range 2.023(4)–2.127(5) Å. Thus the
presence of the chorine atom in the tridentate ligand of 3 does
not seem to affect the coordination distances with respect to 2.
The piperazine ring assume the expected chair conformation
and the presence of two nitrate anions in each compound indicates
the protonation of piperazine nitrogen atom N3, as confirmed by
the bifurcated H-bonds formed by the NH2+ group with nitrate an-
ions. The H-bonding scheme is comparable in the two structures
leading to a 3D network. A detail of the H-bond interactions involv-
ing the water molecules, the amino group N3 and nitrate anion is
shown in Fig. 5 for complex 3. Moreover the complexes are paired
in the crystal about a center of symmetry giving rise to a double H-
bond (Fig. 6) for compound
3
(O2w. . .O10 = 2.621 Å, O2w–
H. . .O10 = 174.40°). The corresponding figures in 2 are 2.631 Å,
169.25°. The ligand L2, as well as the correspondent having a nitro
group in para position of the phenolato ring, has already been re-
ported in similar triaquo Ni(II) derivatives having sulfate as coun-
terion [19]. In those complexes the reported Ni–O and Ni–N
distances follow a trend close comparable to those of 2 and 3, as
expected for high spin Ni(II) complexes reported so far.
The X-ray structural analysis of compound 4 evidenced two
independent neutral complexes, depicted in Figs. 7 and 8, respec-
tively, beside disordered lattice water molecules. In both the com-
plexes (hereinafter indicated A and B) the nickel ion adopts a
distorted octahedral geometry being chelated by the tridentate
Schiff base meridionally bound through the phenolic oxygen, the
imino nitrogen and the N donor of the morpholine moiety, and
in addition by an unreacted 5-chloro-salicylaldehyde anion, com-
pleting the coordination geometry by an aqua ligand. The Ni–N
and N–O bond lengths and angles are comparable in the two com-
plexes within their esd’s (Table 4). The Ni–N(imine) bond distances
are significantly shorter than the Ni–N(morpholine) ones, as ex-
pected for the different hybridization of the N atoms, but also to
accomplish the five-membered Ni(NH2–CH2–CH2–NH2) ring. The
Ni1–O2 and Ni2–O6 bond distances of the salicylaldehyde phenoxo
oxygen, of 2.009(3) and 1.992(3) Å, are shorter with respect the
other Ni–O ones, which range from 2.061(3) to 2.101(3) Å. The
coordination bond distances of the tridentate ligand agree with
those measured in 2 and 3 with the exception of the Ni–O(phen-
oxo) that are sensibly longer here (mean 2.09 versus 2.00 Å).
A distinguished feature is the different conformation (d and k)
assumed by the ethylenediamine (en) in the two independent
complexes A and B (see Fig. 9). Incidentally, due to the centrosym-
metric space group the correspondent enantiomer of each is pres-
ent in the unit cell. The Fig. 9 evidences also the slight different
arrangement of the ligands about the metal ion, may be due to
packing requirements or induced by the interactions occurring in
the crystal between A and B (see below). In particular the imino-
phenolato moiety in A is almost coplanar with the equatorial plane
N1/N2/O1/O2 (dihedral angle of 6.6(1)°), while the correspondent
figure in B (with plane N3/N4/O5/O6) is of 15.5(5)°.
Fig. 9. Perspective view of the two conformational isomers of the five-membered
Ni(NH2–CH2–CH2–NH2) ring in molecule A and B.
Fig. 10. H-bonds and
p–p aromatic ring interactions occurring between the two
independent complexes of 4.
filled by solvent molecules, rather difficult to model and inter-
preted as two lattice water molecules plus another at half occu-
pancy based on the peak height. These molecules, O3w, O4w and
O5w, are H-bond connected with the aqua ligands and also among
them, connecting two A–B pair of complexes.
The two molecules A and B are firmly connected by two H-
bonds occurring between the aquo ligand and the hydroxy-benzal-
dehyde anion of the other complex (O1w. . .O6 = 2.775(4),
O2w. . .O2 = 2.710(4) Å) and through
p
–
p
interactions between
The ligand L4, but lacking of the chlorine atom, was used in the
construction of an unprecedented tetranuclear cyclic Ni cluster
where the metals are bridged by EE azide anions [20]. The Ni–N
and N–O distances involving the tridentate ligand and the water
molecule of the four metal centers compare well with those here
reported.
phenolato rings C1 and C14 of molecule A with aromatic rings
C34 and C21 of B (centroid to centroid distance of 3.539(3) and
3.571(3) Å, respectively). The pair of complexes along with the
interactions between them are shown in Fig. 10. As mentioned in
the experimental part, the electron density map evidences an area
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8
A. Guha et al. / Polyhedron xxx (2012) xxx–xxx
1.0
0.8
0.6
0.4
0.2
0.0
1.0
(a)
(b)
0.8
0.6
0.4
0.2
0.0
Complex 4 + 3,5-DTBC
3,5-DTBC
Complex 1
Complex 1 + 3,5-DTBC
3,5-DTBC
300
350
400
450
500
550
600
300
350
400
450
500
Wavelength / nm
Wavelength / nm
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
(c)
(d)
Complex 3 + 3,5-DTBC
Complex 2 + 3,5-DTBC
3,5-DTBC
3,5-DTBC
300
400
500
600
300
350
400
450
500
550
600
Wavelength / nm
Wavelength / nm
Fig. 11. The changes of UV–Vis spectral behavior at regular interval of time for (a) complex 1, (b) complex 4, (c) complex 2, (d) complex 3 in methanol medium upon addition
of 3,5-DTBC.
250000
200000
Complex 2
150000
Complex 3
Linear (Complex 3)
Linear (Complex 2)
100000
50000
0
0
200
400
600
800
1000
1200
1/S (M-1)
Fig. 12. Lineweaver–Burk plots of complexes 2 and 3.
3.3. Catechol oxidase activity
Table 6
Kinetic parameters for catecholase-like activity of 2 and 3 in MeOH medium.
In order to study the catecholase-like activity of present mono-
nuclear nickel(II) complexes, 3,5-di-tert-butylcatechol (3,5-DTBC)
has been chosen as the substrate. This molecule with two bulky
t-butyl substituents on the ring has a low quinone–catechol
Complex
Wavelength (nm)
Vmax (Msꢁ1
)
KM (M)
kcat (hꢁ1
)
2
3
430
395
1.46 ꢃ 10ꢁ5
1.92 ꢃ 10ꢁ3
5.26 ꢃ 101
3.57 ꢃ 10ꢁ5
4.24 ꢃ 10ꢁ5
1.29 ꢃ 102
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A. Guha et al. / Polyhedron xxx (2012) xxx–xxx
9
reduction potential. This makes it easily oxidized to the corre-
sponding o-quinone, 3,5-di-tert-butylbenzoquinone (3,5-DTBQ),
which is highly stable and shows a maximum absorption at about
400 nm in methanol. To check the ability of the Ni(II) complexes to
act as catalysts for catecholase-like activity, a 1 ꢃ 10ꢁ4 mol dmꢁ3
solution of a complex is treated with 1 ꢃ 10ꢁ2 mol dmꢁ3 (100
equiv) of 3,5-DTBC under aerobic condition. The course of the reac-
tion was followed by UV–Vis spectroscopy, and the time depen-
dent spectral scans of the four complexes are depicted in Fig. 11.
From the figure it is evident that a band ꢂ390 nm is observed to
increase with time after addition of 3,5-DTBC due to the gradual
increment of concentration of 3,5-DTBQ in case of complexes 2
and 3 catalyzed reactions, whereas nearly no change is noticed in
the spectral pattern of complexes 1 and 4. This data unambigu-
ously demonstrate that 2 and 3 are active catalysts for the aerial
oxidation of 3,5-DTBC to 3,5-DTBQ, whereas 1 and 4 reveal to be
inactive. The crystal structure analyses of 2 and 3 pointed out that
a positive charge center is created on the piperazine nitrogen atom,
as we previously observed in dinuclear Cu(II) and Ni(II) complexes
with 2,6-bis(N-ethylpiperazine-iminomethyl)-4-methyl-phenol as
ligand. This observation gives once again support to our earlier
suggestion [12b] that the positively charged piperazine moiety,
present in the tridentate ligand of complexes 2 and 3, creates a
channel to facilitate the catalyst–substrate interaction, a prerequi-
site for exhibiting better catalytic activity. A similar proposal has
now well accepted, especially to explain the activity of the Cu/
Zn-superoxide dismutase (SOD), where the Cu(II) lies at the bottom
of a narrow channel, and the positively charged arginine and lysine
residues are supposed to play a role in attracting the anions guid-
ing them through the channel towards the active site [21–23]. Here
it is noteworthy that in enzymes, the protein super-structure pro-
vides inherent spatial constraints that channels can be formed that
limit substrate access to the metal center within a buried active
site. It is difficult to imagine that similar scenarios can be operating
for our mononuclear complexes in which there is limited steric
protection on along the z-axis of the nickel centers. However, it
may be assumed that the extra positive charge on the backbone
of the ligand system is of immense help to facilitate the sub-
strate-metal center interaction via electrostatic as well as H-bond-
ing interaction. The conversion of catechol to quinone is a two-
electron oxidation process, and most of the investigators working
with copper catechol oxidase model systems believe that the metal
center redox participation is responsible for the catechol to qui-
none conversion [24]. The electrochemical analyses of complexes
2 and 3 in methanol are featureless, suggesting no preference for
the nickel(II) ions to undergo reduction to nickel(I) or oxidation
to nickel(III). However, when the same experiment is done in pres-
ence of 3,5-DTBC we observed reduction peaks at +0.28 and
ꢁ1.10 V which may be attributed to reduction of 3,5-DTBQ and
that of Ni(II) to Ni(I), respectively and only one oxidation peak at
+1.15 V corresponds to the oxidation of 3,5-DTBC to the quinone.
Thus, it may be stated that in presence of 3,5-DTBC our nickel(II)
complexes undergo reduction with concomitant oxidation of the
catechol to quinone. Therefore it may be assumed that the metal
center redox participation is responsible for the catecholase-like
activity of the complexes. It is to note that in spite of our several
attempts we failed to find out the oxidation peak corresponds to
the oxidation of Ni(I) to Ni(II).
in a 1 cm spectrophotometer quartz cell thermostated at 25 °C.
Then 0.04 mL of 0.005 mol dm–3 complex solution was quickly
added to it so that the ultimate concentration of the complex be-
comes 1 ꢃ 10–4 mol dm–3. The dependence of the initial rate on
the concentration of substrate, spectrophotometrically monitored
at respective wavelength, is given as SI file. Moreover the initial
rates method shows a first-order dependence on complex concen-
tration and exhibited saturation kinetics at higher substrate
concentrations. For this reason, a treatment based on Michaelis–
Menten model was seemed to be appropriate. The values of
Michaelis binding constant (Km), maximum velocity (Vmax) and rate
constant for the dissociation of substrates (i.e., turnover number,
kcat) were calculated for each complex from the Lineweaver–Burk
graph of 1/V versus 1/[S] (Fig. 12) by using the equation 1/
V = {Km/Vmax}{1/[S]} + 1/Vmax. The enzyme kinetics data are listed
in Table 6. From this study it is evident that, although mononuclear
nickel(II) complexes with appropriate Schiff-base ligands are
exhibiting catecholase-like activity with 3,5-DTBC as model sub-
strate, their efficiency is lower in comparison to the analogous
dinuclear nickel(II) species for which a kcat = 1.44 ꢃ 104 hꢁ1 was
derived [12b].
4. Conclusion
Four mononuclear nickel(II) complexes with designed Schiff-
base ligands have been synthesized and comprehensively structur-
ally characterized with the aim to investigate their catecholase-like
activity. Crystal structure analysis revealed that complex 1 exhibits
coordination environment around metal center very much similar
as in the native catechol oxidase in met state, but it is inactive in
catalyzing the oxidation of catechol. Complex 4, containing an
unreacted phenolate ligand, appears not to have further affinity
to interact with catechol substrate. On the other hand, complexes
2 and 3 are highly active in catalyzing the oxidation of 3,5-DTBC
to 3,5-DTBQ, and the positive charged ligand system of these is
supposed to be instrumental for their catecholase-like activity.
Acknowledgement
The authors wish to thank the CSIR for Research Project,
[01(2464)/11/EMR-II dt 16-05-11 to DD] and Shailabala Biswas Re-
search Foundation, University of Calcutta (AG) for financial
support.
Appendix A. Supplementary data
CCDC 878600–878603 contains the supplementary crystallo-
graphic data for complexes 1 to 4. These data can be obtained free
of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or
from the Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail: de-
posit@ccdc.cam.ac.uk. Supplementary data associated with this
article can be found, in the online version, at http://dx.doi.org/
10.1016/j.poly.2012.07.088.
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The kinetics for the oxidation of the substrate 3,5-DTBC were
determined by the initial rates method at 25 °C. The concentration
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than that of the complex and the increase of respective quinone
concentration were determined at a particular wavelength for each
complex. Solutions of substrates of concentration ranging from
0.001 to 0.05 mol dm–3 were prepared from a concentrated stock
solution in methanol. 2 mL of the substrate solution were poured
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