Influence of co-ligands in synthesis, photoluminescence behavior and catalytic activities of zinc complexes of 2-((E)-((pyridin-2-yl)methylimino)methyl)phenol

Article abstract of DOI:10.1016/j.poly.2014.08.011

Four new zinc(II) complexes of a tridentate Schiff base ligand 2-((E)-((pyridin-2-yl)methylimino)methyl)phenol (HL), namely [Zn₂L₂Cl₂] (1); [Zn₂L₂Br₂] (2); [ZnL(SCN)(MeOH)] (3) and [Zn

Full text of DOI:10.1016/j.poly.2014.08.011

Accepted Manuscript
Influence of co-ligands in synthesis, photoluminescence behavior and catalytic
activities of zinc complexes of 2-((E)-((pyridin-2-yl)methylimino)methyl)phe-
nol
Priyanka Kundu, Prateeti Chakraborty, Jaydeep Adhikary, Tanmay
Chattopadhyay, Roland C. Fischer, Franz A. Mautner, Debasis Das
PII:
S0277-5387(14)00533-6
DOI:
http://dx.doi.org/10.1016/j.poly.2014.08.011
POLY 10912
Reference:
Polyhedron
To appear in:
Received Date:
Accepted Date:
6 June 2014
2 August 2014
Please cite this article as: P. Kundu, P. Chakraborty, J. Adhikary, T. Chattopadhyay, R.C. Fischer, F.A. Mautner,
D. Das, Influence of co-ligands in synthesis, photoluminescence behavior and catalytic activities of zinc complexes
of 2-((E)-((pyridin-2-yl)methylimino)methyl)phenol, Polyhedron (2014), doi: http://dx.doi.org/10.1016/j.poly.
2014.08.011
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“REVISED”
Influence of co-ligands in synthesis, photoluminescence behavior and catalytic
activities
of
zinc
complexes
of
2-((E)-((pyridin-2-
yl)methylimino)methyl)phenol
Priyanka Kundua, Prateeti Chakrabortya, Jaydeep Adhikarya, Tanmay Chattopadhyayb, Roland C.
Fischerc, Franz A. Mautnerc,*, Debasis Dasa,*
a Department of Chemistry, University of Calcutta, 92 A. P. C. Road, Kolkata 700009, India.
b Department of Chemistry, Panchakot Mahavidyalaya, Sarbari, Purulia, Pin 723121, India.
c Institut fuer Physikalische und Theoretische Chemie, Technische Universitaet Graz, 8010 Graz,
Austria.
Corresponding authors. Tel.: +91 33 24837031; fax: +91 33 23519755.
E-mail address: dasdebasis2001@yahoo.com(D.Das)
ABSTRACT
Four new zinc(II) complexes of a tridentate Schiff base ligand 2-((E)-((pyridin-2-
yl)methylimino)methyl)phenol
(HL),
namely
[Zn2L2Cl2]
(1);
[Zn2L2Br2]
(2);
[ZnL(SCN)(MeOH)] (3) and [Zn2L(OAc)3(MeOH)] (4) have been synthesized with the view to
investigate the role of co-ligands in controlling the structural diversity, photoluminescence
property and catalytic activities. The complexes have been characterized through common
physicochemical techniques as well as by X-ray single crystal structural analysis. All of them
display interesting co-ligands mediated photoluminescence property the origin of which may be
attributed due to ligand to metal charge transfer (LMCT) and the order of photoluminescence
efficiency is 3>4>1~2. Catecholase and phosphatase activities of the complexes have been
investigated in DMF medium using 3,5-di-tert-butylcatechol (3,5-DTBC) and 4-
nitrophenylphosphate as substrates respectively. Co-ligands have prominent role on phosphatase
activity of the complexes where the order of activity in terms of kcat value is 4>1>2>3.
1

Interestingly, only complex 4 is active in catalyzing the aerobic oxidation of 3,5-DTBC. The
presence of readily labile solvent molecule in one of the zinc centers of complex 4 perhaps takes
the key role for initiating substrate-Zn interaction and thereby the activity. Radical pathway is
the probable reason behind the catecholase activity as is evident from EPR study. Solid state
thermal analysis suggests that ZnO is the thermally stable end product for complexes 1, 2 and 4,
whereas it is ZnS in case of complex 3.
Keywords: Schiff-base, Structural diversity, Phosphatase activity, Catecholase activity,
Photoluminescence, Thermal analysis.
1. Introduction
Zinc complexes of tridentate Schiff-base ligands recently become an interdisciplinary
area of research due to their versatile application in various fields e.g. as models of biomolecules,
as catalysts in many reactions, as materials for optoelectronic application and as important
materials in industry [1-6]. Exploring the coordination unsaturation property of tridentate Schiff
base ligand a variety of complexes can be designed having structural diversity just by tuning the
metal ions and co-ligands [7-10]. Recently, we reported a series of Zn and Cd complexes of a
tridentate Schiff-base ligand 2-((E)-((pyridin-2-yl)ethylimino)methyl)phenol (HL1) where the co-
ligands were Cl, Br and I [11]. Although no structural diversity was obtained but interesting
anion dependent photo-fluorescence property was explored. However, that interesting
observation fueled our curiosity further to investigate the role of co-ligands on creating structural
diversity and thereby on catalytic activities as well as on photoluminescence property of zinc
complexes with homologous ligand of HL1. With those aims in mind we have synthesized a
series of zinc complexes selecting 2-((E)-((pyridin-2-yl)methylimino)methyl)phenol (HL) where
the co-ligands used are Cl, Br, SCN and OAc (Scheme 1). The comprehensively characterized
complexes have been employed as catalysts to catalyze (i) the aerobic oxidation of catechol,
popularly known as catecholase activity using 3,5-di-tert-butylcatechol (3,5-DTBC) as substrate
and (ii) hydrolytic cleavage of phosphoester bond, i.e. phosphatase activity employing p-
nitrophenylphosphate (PNPP) as substrate. Additionally, the solid state thermal property and
photoluminescence property of the complexes have been investigated. All those new and
exciting outcomes have been vividly reported in this presentation.
2

Scheme 1. Synthetic route of the complexes.
2. Experimental Section
2.1. Starting Materials
High purity salicylaldehyde, 2-aminomethylpyridine (ampy), zinc perchlorate
hexahydrate were obtained from Sigma-Aldrich company. Zinc(II) chloride, zinc(II) bromide,
zinc acetate dehydrate, sodium thiocyanate were purchased from Merck Chemical Company and
used as received. Solvents were dried according to literature procedure. All other chemicals were
of AR grade.
2.2. Synthesis of the Complexes
2.2.1. [Zn2(L)2Cl2] (1)
A methanolic solution (5 mL) of salicylaldehyde (0.122 g, 1 mmol) was added to a
methanolic solution(10 mL) of 2-(2-aminomethyl)pyridine (0.122 g, 1 mmol) and the resulting
mixture was refluxed for 2 h. Then a methanolic solution of zinc(II) chloride (0.204 g, 1.5 mmol)
was added to it and the solution was allowed to stir for an additional 2 h. After that the resulting
3

solution was filtered and the filtrate was kept in a CaCl2 desiccator. Yellow colored single
crystals suitable for X-ray data collection were obtained from the filtrate after two days. Yield
86%. Anal. Calc. for C26H22Cl2N4O2Zn2 : C,50.07 ; H,3.55 ; N,8.98 . Found: C,50.28 ; H,3.45 ;
N,8.92 %. IR: ν (C=N) 1638cm-1, ν (skeletal vibration) 1595 and 1551 cm-1.
2.2.2. [Zn2(L)2Br2] (2)
The complex was prepared following the same procedure like 1 just by replacing zinc
chloride with zinc (II) bromide(0.338 g, 1.5 mmol ). Light yellow colored single crystals suitable
for X-ray data collection were obtained from the filtrate after a few days on keeping it in room
temparature. Yield 85%. Anal. Calc. for C26H22Br2N4O2Zn2: : C,43.81 ; H,3.11 ; N,7.86 . Found:
C,43.51 ; H,3.01 ; N,7.46 %. IR: ν (C=N)1637 cm-1, ν (skeletal vibration) 1595 and 1551 cm-1.
2.2.3. [Zn(L)(SCN)(MeOH)] (3)
A methanolic solution (5 mL) of salicylaldehyde (0.122 g,1 mmol) was added to a
methanolic solution (10 mL) of 2-(2-aminomethyl)pyridine (0.122 g, 1 mmol) resulting mixture
was boiled for 2 h.Then a methanolic solution of zinc(II) acetate (0.329 g,1.5 mmol) was added
to it and the solution was allowed to stir for an additional 2h. After stirring the resulting mixture
for 2 h a methanol-water (1:1) solution of sodium thiocyanate (0.243 g, 3 mmol) was added to it
and the stirring was continued for an additional 1 h. Then the solution was filtered. Deep yellow
colored single crystals suitable for x-ray data collection were obtained on keeping the filtrate at
room temperature for few days. Yield 82%. Anal. Calc. for C14H11N3OZnS: : C,45.84 ; H,3.02 ;
N,11.45 . Found: C,45.79 ; H, 3.00; N,11.22 %. IR: ν (C=N) 1640 cm-1, ν (skeletal vibration)
1542 cm-1.
2.2.4. [Zn2(L)(OAc)3(MeOH)] (4)
A methanolic solution (5 mL) of salicylaldehyde (0.122 g,1 mmol) was added to a
methanolic solution (10 mL) of 2-(2-aminomethyl)pyridine (0.122 g, 1 mmol) and the resulting
mixture was refluxed for 2 h.Then a methanolic solution of zinc(II) acetate (0.329 g,1.5 mmol)
was added to it and the solution was further refluxed for an additional 2h. Deep yellow single
crystals suitable for X-ray data collection were obtained after a week on keeping the filtrate in a
4

CaCl2 dessicator Yield 82%. Anal. Calc. for C20H24N2O8Zn2: C,43.58 ; H,4.38 ; N,5.08 . Found:
C,43.55 ; H,4.11 ; N,5.01 %. IR: ν (C=N) 1642cm-1, ν (skeletal vibration) 1599 cm-1.
2.3. Methods and Instrumentation
Elemental analyses (C, H and N) were performed using a Perkin-Elmer 240C elemental
analyzer. Infrared spectra in KBr pellets (400-4500 cm-1) were recorded using a Perkin-Elmer
RXI FT-IR spectrophotometer. Electronic Spectra (UV-VIS) in dimethylformamide (900-250
nm) were recorded in a Hitachi U-3501 spectrophotometer. Thermal analysis (DT-TGA) was
carried out using Mettler Toledo (TGA/SDTA851) thermal analyzer in a dynamic atmosphere of
dinitrogen (flow rate: 30 cc/min-1). All the samples were heated in a platinum crucible at a rate of
10 degree per minute with inert alumina as a reference from 30 to 700°C. Fluorescence spectra
of the ligand and of the complexes were recorded in DMF solution as well as in solid state with a
Perkin-Elmer model LS 55 Luminescence Spectrometer. The electron paramagnetic resonance
(EPR) experiment was done at 77K in pure DMF using a Bruker BioSpin GmbH diffractometer.
2.4. X-ray crystal structure analysis
The X-ray single-crystal data of compounds 1 - 4 were collected on a Bruker-AXS
APEX-II CCD diffractometer. The crystallographic data, conditions retained for the intensity
data collection and some features of the structure refinements are listed in Table 1. The
intensities were collected with Mo-Kα radiation (λ= 0.71073 Å). Data processing, Lorentz-
polarization and absorption corrections were performed using APEX, SAINT and the SADABS
computer programs [12]. The structures were solved by direct methods and refined by full-matrix
least-squares methods on F2, using the SHELXTL [13] program package. All non-hydrogen
atoms were refined anisotropically. The hydrogen atoms were located from difference Fourier
maps, assigned with isotropic displacement factors and included in the final refinement cycles by
use of geometrical constraints. Molecular plots were performed with the Mercury [14] program.
CCDC-982626 (for 1), -982627 (for 2), -982628 (for 3), and -982629 (for 4) contain the
supplementary crystallographic data for this paper. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
5

Table 1
Crystallographic data and processing parameters.
Compound
1
2
3
4
Empirical
formula
Formula mass
System
Space group
a (Å)
C26H22Cl2N4O2Zn2 C26H22Br2N4O2Zn2 C15H15N3O2SZn
C20H24N2O8Zn2
624.16
Monoclinic
P21/n
713.06
Monoclinic
P21/n
366.76
Monoclinic
P21/n
551.19
Triclinic
P-1
9.2978(18)
8.9738(18)
15.281(3)
90
9.3630(9)
8.9795(8)
15.2246(17)
90
5.1136(4)
34.582(3)
8.9631(8)
90
9.4233(7)
11.4800(9)
11.6074(11)
105.540(3)
105.947(3)
103.670(3)
1096.19(16)
2
b (Å)
c (Å)
α (°)
103.397(2)
90
101.151(18)
90
97.17(3)
90
β (°)
γ (°)
V (Å3)
1240.3(4)
2
296(2)
1255.8(2)
2
100(2)
1572.6(3)
4
296(2)
Z
T (K)
100(2)
2.238
µ (mm-1)
Dcalc (Mg/m3)
Crystal size
(mm)
2.182
5.124
1.704
1.671
1.886
1.549
1.670
0.42 × 0.35 ×
0.31
0.34 × 0.32 ×
0.14
0.44 × 0.37 ×
0.30
0.21 × 0.16
×0.12
28.00
25.77
26.36
28.04
θ max (°)
Data collected
Unique refl. /
Rint
8417
2326 / 0.0246
9708
2557 / 0.0293
19173
3632 / 0.0521
30144
5261 / 0.0651
Parameters
163
163
1.050
203
1.062
296
0.983
Goodness-of-Fit 1.039
on F2
R1 / wR2 (all
data)
Residual
extrema (e/Å3)
0.0267 / 0.0724 0.0309 / 0.0665
0.0584 / 0.1574 0.0278 / 0.0679
0.86 / -0.76 0.58 / -0.49
0.34 / -0.23 0.60 / -0.29
3. Results and discussion
In one of our earlier studies [11] we observed that the treatment of ligand 2-[[[2-(2-
pyridyl)ethyl]imino]methyl]phenol formed in-situ via reaction of salicylaldehyde and N-(2-
aminoethyl)pyridine generates dinuclear Zinc(II) complexes where halide ions play an important
6

role in controlling the photoluminescence property. Thus the Schiff base ligand formed by
condensation of salicylaldehyde and N-(2-aminomethyl)pyridine, a lower homologous amine of
N-(2-aminoethyl)pyridine may also generates interesting anion controlled structural features,
catalytic activities and photophysical property. With those aims in mind for the present study we
have prepared four complexes following template synthesis technique by treatment of
methanolic solution of ZnX2 (X= Cl, Br, OAc) with the N,N,O-chelating ligand HL formed in
situ via condensation of salicylaldehyde and N-(2-aminomethyl)pyridine. Interestingly, here also
we got phenoxo bridged dinuclear zinc(II) complexes in presence of Cl- and Br- as a co-ligand, a
consequence observed earlier [11]. But in presence of SCN- and OAc- as co-ligand we got a
mononuclear and dinuclear complexes respectively. The molecular composition of the
complexes may generally be formulated as [M2L2X2] (X=Cl and Br); [Zn(L)(SCN)].(MeOH) and
[Zn2(L)(OAc)3].(MeOH) on the basis of elemental analyses. The complexes exhibit very low
molar conductance (7-10 Ω-1cm2M-1) in DMF medium suggesting that they are non-electrolyte in
solution. FT-IR spectral study reveals that all the complexes exhibit bands due to C=N stretching
mode in the range 1637-1643 cm-1 and skeletal vibration in the range 1550-1600 cm-1 (but no
bands for the free carbonyls or primary amines are observed indicating that a complete
condensation has occurred (Fig. S1-S4 in supporting information).
3.1. Description of the structures
.
3.1.1. [Zn(L)Cl]2 (1)
Compound 1 consists of centrosymmetric dinuclear [Zn(L)Cl]2 complexes. A perspective
view of the neutral dinuclear complex together with the atom numbering scheme is given in Fig.
1, and selected bond parameters are collected in Table 2. Zn(1) is penta-coordinated by N(1),
N(2), further by O(1) and O(1’) [(‘): 1-x,-y,-z], which form the common edge within the
dinuclear units, and Cl(1) atoms. The ZnN2O2Cl coordination polyhedron may be described as a
distorted trigonal bipyramid with τ value of 0.59 [15]. The axial sites are occupied by O(1) and
N(2) atoms of the blocking ligand(L) [Zn(1)-O(1) = 2.1464(16)]; Zn(1)-N(2) = 2.1545(19) Å;
O(1)-Zn(1)-N(2) = 160.90(7)°]. The equatorial bond lengths are: Zn(1)-N(1) = 2.056(2), Zn(1)-
O(1’) = 2.0068(16), Zn(1)-Cl(1) = 2.2511(8) Å, respectively. The O(1)-Zn(1)-O(1’) and Zn(1)-
7

O(1)-Zn(1’) angles are 80.65(7) and 99.35(7)°, respectively. The Zn(1)…Zn(1’) distance is
3.1674(8) Å, whereas the shortest inter-dimer metal-metal separation is 6.6571(14) Å (Fig. 2).
Fig. 1. Perspective view (30% probability ellipsoids) of 1 together with the atom numbering
scheme. Symmetry code: (‘) 1–x,-y,-z.
Fig. 2. Crystallographic Packing plot of (1).
Table 2
Selected bond lengths (Å) and angles (°) for compounds 1 - 4.
Compound 1
Zn(1)-N(1)
Zn(1)-N(2)
2.056(2)
2.1545(19)
Zn(1)-O(1)
Zn(1)-O(1’)
2.1464(16)
2.0068(16)
8

Zn(1)-Cl(1)
2.2511(8)
116.86(7)
80.65(7)
83.70(7)
102.61(7)
78.04(8)
99.35(7)
O(1’)-Zn(1)-N(1)
O(1’)-Zn(1)-O(1)
N(1)-Zn(1)-O(1)
O(1’)-Zn(1)-N(2)
N(1)-Zn(1)-N(2)
Zn(1)-O(1)-Zn(1’)
Compound 2
O(1)-Zn(1)-N(2)
O(1’)-Zn(1)-Cl(1)
N(1)-Zn(1)-Cl(1)
O(1)-Zn(1)-Cl(1)
N(2)-Zn(1)-Cl(1)
160.90(7)
117.07(5)
125.64(6)
98.63(5)
96.54(6)
Zn(1)-N(1)
Zn(1)-N(2)
Zn(1)-Br(1)
O(1’)-Zn(1)-N(1)
O(1’)-Zn(1)-O(1)
N(1)-Zn(1)-O(1)
O(1’)-Zn(1)-N(2)
N(1)-Zn(1)-N(2)
Zn(1)-O(1)-Zn(1’)
Compound 3
2.050(2)
2.160(2)
2.4037(5)
120.14(8)
79.75(8)
83.93(8)
104.95(8)
78.29(8)
100.25(8)
Zn(1)-O1)
Zn(1)-O(1’)
2.1340(18)
1.9959(18)
O(1)-Zn(1)-N(2)
O(1’)-Zn(1)-Br(1)
N(1)-Zn(1)-Br(1)
O(1)-Zn(1)-Br(1)
N(2)-Zn(1)-Br(1)
161.54(8)
115.29(5)
123.98(6)
98.64(5)
95.39(6)
Zn(1)-N(1)
Zn(1)-N(2)
Zn(1)-N(3)
2.065(3)
2.167(3)
1.998(4)
Zn(1)-O(1)
Zn(1)-O(2)
N(3)-C(14)
2.018(3)
2.021(3)
1.150(6)
C(14)-S(1)
1.618(4)
N(3)-Zn(1)-O(1)
N(3)-Zn(1)-O(2)
O(1)-Zn(1)-O(2)
N(3)-Zn(1)-N(1)
O(1)-Zn(1)-N(1)
N(3)-C(14)-S(1)
Compound 4
93.06(14)
100.45(16)
103.20(14)
159.71(16)
88.34(12)
179.6(5)
O(2)-Zn(1)-N(1)
N(3)-Zn(1)-N(2)
O(1)-Zn(1)-N(2)
O(2)-Zn(1)-N(2)
N(1)-Zn(1)-N(2)
Zn(1)-N(3)-C(14)
98.92(13)
93.79(15)
156.67(14)
97.44(14)
77.73(13)
174.9(4)
Zn(1)-N(1)
Zn(1)-N(2)
Zn(1)-O(1)
Zn(1)-O(2)
2.0772(19)
2.1034(19)
2.0728(15)
2.1348(16)
2.0349(16)
99.82(6)
166.08(7)
89.15(7)
91.53(7)
168.64(7)
79.60(7)
91.07(7)
176.45(6)
118.48(7)
105.55(6)
112.70(6)
Zn(2)-O(1)
Zn(2)-O(3)
Zn(2)-O(5)
Zn(2)-O(6)
1.9996(15)
1.9539(16)
1.9582(16)
1.9500(15)
2.2256(17)
93.18(7)
87.85(6)
97.82(7)
92.07(7)
85.10(7)
89.39(6)
84.36(7)
110.77(6)
101.95(6)
106.95(7)
102.56(7)
Zn(1)-O(4)
Zn(1)-O(8)
O(4)-Zn(1)-O(1)
O(4)-Zn(1)-N(1)
O(1)-Zn(1)-N(1)
O(4)-Zn(1)-N(2)
O(1)-Zn(1)-N(2)
N(1)-Zn(1)-N(2)
N(2)-Zn(1)-O(8)
O(2)-Zn(1)-O(8)
O(3)-Zn(2)-O(6)
O(6)-Zn(2)-O(5)
O(3)-Zn(2)-O(5)
O(4)-Zn(1)-O(2)
O(1)-Zn(1)-O(2)
O(2)-Zn(1)-N(1)
O(2)-Zn(1)-N(2)
O(4)-Zn(1)-O(8)
O(1)-Zn(1)-O(8)
N(1)-Zn(1)-O(8)
O(6)-Zn(2)-O(1)
O(3)-Zn(2)-O(1)
O(1)-Zn(2)-O(5)
Zn(1)-O(1)-Zn(2)
9

Symmetry code for 1: (‘) 1–x,-y,-z; symmetry code for 2: (‘) –x,1-y,1-z.
.
3.1.2. [Zn(L)Br]2 (2)
The crystal structure of compound 2, which is isomorphic to that of compound 1, consists
of centrosymmetric dinuclear [Zn(L)Br]2 complexes. A perspective view of the neutral dinuclear
complex together with the atom numbering scheme is given in Fig. 3, and selected bond
parameters are collected in Table 2. Zn(1) is penta-coordinated by N(1), N(2), further by O(1)
and O(1’) [(‘): -x,1-y,1-z], which form the common edge within the dinuclear units, and Br(1)
atoms. The ZnN2O2Br coordination polyhedron may be described as a distorted trigonal
bipyramid with τ value of 0.63 [15]. The axial sites are occupied by O(1) and N(2) atoms of the
blocking ligand (L) [Zn(1)-O(1) = 2.1340(18), Zn(1)-N(2) = 2.160(2) Å; O(1)-Zn(1)-N(2) =
161.54(8)°]. The equatorial bond lengths are: Zn(1)-N(1) = 2.050(2), Zn(1)-O(1’) = 1.9959(18),
Zn(1)-Br(1) = 2.4037(5) Å, respectively. The O(1)-Zn(1)-O(1’) and Zn(1)-O(1)-Zn(1’) angles
are 79.75(8) and 100.25(8)°, respectively. The Zn(1)…Zn(1’) distance is 3.1708(5) Å, whereas
the shortest inter-dimer metal-metal separation is 6.8085(9) Å (Fig. S5 in Supporting
Information).
Fig. 3. Perspective view (50% probability ellipsoids) of 2. together with the atom numbering
scheme. Symmetry code: (‘) -x,1-y,1-z.
10

3.1.3. [Zn(L)(NCS)(MeOH)] (3)
The crystal structure of compound 3 consists of mono-nuclear neutral complexes
[Zn(L)(NCS)(MeOH)]. A perspective view of 3 together with the atom numbering scheme is
presented in Fig. 4, and selected bond parameters are summarized in Table 2. The Zn(1) center is
penta-coordinated by N(1), N(2) and O(1) donor atoms of the blocking ligand (L), N(3) of
isothiocyanate anion, and O(2) of methanol ligand. The ZnN3O2 coordination polyhedron may be
described as a distorted square pyramid with τ value of 0.05 [15]. The apical site is occupied by
O(2) of the MeOH molecule [Zn(1)-O(2) = 2.021(3)] Å]. The basal Zn(1)-N/O bonds lengths
vary from 1.988(4) to 2.167(3) Å. The metal center deviates from its basal N3O-plane by 0.361
Å. The terminal isothiocyanato ligand has the following bond parameters: N(3)-C(14) =
1.150(6), C(14)-S(1) = 1.618(4) Å; Zn(1)-N(3)-C(14) = 174.9(4) and N(3)-C(14)-S(1) =
179.6(5)°. The shortest metal-metal separation is 5.1136(8) Å. O(2) of methanol ligand acts as
donor for a hydrogen bond of type O-H…O to acceptor O(1’) [(‘): 1+x,y,z] atom to generate a
supramolecular 1D system oriented along the a-axis of the monoclinic unit cell. [O(2)-H(1) =
0.84(4) Å, O(2)…O(1’) = 2.606(4) Å, O(2)-H(1)…O(1’) = 173(5)°] (Fig. S6 in Supporting
Information).
Fig. 4. Perspective view (30% probability ellipsoids) of 3 together with the atom numbering
scheme.
11

3.1.4. [Zn2(L)(OAc)3(MeOH)] (4):
The crystal structure of compound 4 consists dinuclear neutral complexes
[Zn2(L)(OAC)3(MeOH)]. A perspective view of 4 together with the atom numbering scheme is
presented in Fig. 5, and selected bond parameters are summarized in Table 2. The Zn(1) center is
hexa-coordinated by N(1), N(2) and O(1) donor atoms of the blocking ligand (L), O(2), O(4) of
two different acetate anions and O(8) of a methanol ligand. The ZnN2O4 coordination
polyhedron around Zn(1) may be described as elongated octahedron with O(2) and O(8) in the
axial sites [Zn(1)-O(2) = 2.1348(16), Zn(1)-O(8) = 2.2256(16) Å, O(2)-Zn(1)-O(8) =
176.45(6)°], and the other Zn(1)-N/O bond distances are in the range from 2.0349(16) to
2.1034(19) Å. The tetrahedron around Zn(2) is formed by O(1) of the blocking ligand L, and
O(3), O(5) and O(6) of acetate groups. The Zn(2)-O bond distances vary in the range from
1.9500(15) to 1.9996(15) Å, and the O-Zn(2)-O bond angles and are in the range from 105.55(6)
to 118.48(7)°. O(1) of the blocking ligand L is bridging the two zinc centers [Zn(1)-O(1)-Zn(2) =
102.56(7)°, Zn(1)…Zn(2) = 3.1776(5) Å]. The two zinc centers are further bridged by by two
acetate anions, both acting in the syn-syn mode, whereas the third acetate anion acts as a
monodentate only ligand via its oxygen atom O(6). The Zn-O-C bond angles of acetate groups
are in the range from 114.57(14) to 134.93(15)°. O(8) of methanol ligand acts as donor for a
hydrogen bond of type O-H…O to acceptor O(7’) [(‘): -1+x,y,z] atom to generate a
supramolecular 1D system oriented along the a-axis of the triclinic unit cell (Figure 6) [O(8)-
H(2) = 0.86(2) Å, O(8)…O(7’) = 2.678(3) Å, O(8)-H(2)…O(7’) = 177(2)°]. The shortest inter-
dimer metal-metal separation is 6.4302(8) Å.
Fig. 5. Perspective view (50% probability ellipsoids) of 4 together with the atom numbering
scheme.
12

Fig. 6. Crystallographic Packing plot of (4).
3.2. Phophatase Activity
To study the phosphatase activity of the zinc complexes, disodium salt of (4-nitrophenyl)
phosphate hexahydrate was chosen as the substrate. Their hydrolytic tendency was detected
spectrophotometrically by monitoring the increase in absorbance at λmax=427 nm through
wavelength scan from 200-800 nm in 97.5% DMF medium (DMF : water = 97.5 : 2.5) where the
substrate was taken 20 equivalents of the catalyst, till roughly 2% reaction conversion as we
reported earlier [16]. The change in spectral behavior of complex 4 is shown in Fig. 7 as a
representative one and the others are given in SI file (Fig. S7-S9 in Supporting Information).
Fig. 7. Wavelength scan for the hydrolysis of 4-NPP in the absence and presence of complex 4
(substrate : catalyst = 20:1) in 97.5% DMF medium at 250C. [4-NPP]=1 × 10-3(M), [Complex] =
0.05 ×10-3(M).
13

The kinetic study of complexes were done by the initial slope method following the rate of
increase in absorption of the band at ∼427nm corresponding to rise in 4-nitrophenolate
concentration. The initial first-order rate constants, V (min-1), for the cleavage of NPP were
obtained directly from the plot of log[Aα/(Aα – At)] values versus time (through origin) which
were linear with R2 ≥ 0.99. In order to determine the dependence of rate constants on substrate
concentration, complexes 1-4 were treated with five different concentrations of substrate. It can
be observed that it follows a first order kinetics at lower concentration and gradually differs from
unity at higher concentration. (Fig. S10-S13 in Supporting Information) The kinetic parameters
(Vmax : maximum initial rate attained for a particular concentration of the metal complex in
presence of a large excess of the 3,5-DTBC; kM : Michaelis–Menten constant for the metal
complex kcat, : turnover number) for the catalyzed reactions were determined from the linear plots
of 1/V versus 1/[NPP] values (Lineweaver-Burk plot) as per the Michaelis-Menten approach of
enzymatic kinetics. The results of calculation remained the same when the experiment was done
by varying the [complex] under investigation at constant [NPP]. Phosphatase activity of the
complexes shows the order of activity in terms of kcat value is 4>1>2>3. All the kinetic
parameters are given in Table 3.
To correct for the spontaneous cleavage of 4-NPP, each reaction was measured against a
reference cell that was identical to the sample cell in composition except for the absence of Zn
complex. The spontaneous hydrolysis of 4-NPP had a much lower reaction rate when compared
to the metal catalyzed reaction and hence the rate of spontaneous hydrolysis of 4-NPP was too
low to be taken into account in the kinetic measurements. Therefore, the general rate of
spontaneous hydrolysis was not separately determined [17]. Most of the recently reported Zn
complexes along with their rate constants and relevant references [16, 18-21] are present in
Table 4. On comparison of other zinc complexes it can be inferred that our synthesized Zn(II)
complexes have moderate catalytic efficiency to cleave P-O bond of 4-NPP.
14

Table 3
First order rate constants for phosphatase activity obtained by Michaelis−Menten treatment of
complex 1−4.
Complex
104 Vmax (M min-1)
104 kM (M)
4.45
kcat (sec−1)
0.292
8.77
4.33
3.463
13.6
1
2
3
4
5.45
0.144
4.19
0.115
4.80
0.453
Table 4
First-order rate constants for the hydrolysis of various monophosphate esters by some previously
reported Zinc(II )complexes.
Complex
Substrate Conditions
kcat (s-1)
Reference
(2.73-9.86) ×10-2
[Zn2L3]and [Zn2L4-8]
NPP
[18]
Acetonitrilewater,
pH 6.8,
25°C
4.10 ×10-3
3.44 ×10-4
[19]
[20]
[Zn2(L9O)]
pH 6-6.5, 25°C
HPNP
HPNP
[ZnII2(L10)-(µ-
O2CMe)2(MeCN)2][PF6]
Methanol-water
(33%, v/v),
30°C
[Zn2(L11)2X2]
[Zn(HL12)X2]
NPP
NPP
DMF-Water
(97.5:2.5 v/v)
25°C
(8.33-9.35)
(2.88-4.68)
[16]
[16]
DMF-Water
(97.5:2.5 v/v)
25°C
Methanol-water (7:3,
v/v). 25°C
[Zn3(L )2(OAc)4]
and NPP
(0.484-7.11) ×10-2 [21]
1
2
[Zn(L )(OAc)]
3.3. Catecholase activity
In order to determine the catecholase-like activity of our complexes 1 × 10-4 mol dm-3
solutions of 1-4 were treated with 1 × 10-2 mol dm-3 (100 equiv) of 3, 5-DTBC under aerobic
15

condition. The course of the reaction was followed by UV-vis spectroscopy. The time dependent
UV-vis spectral scan was performed in pure DMF. Our study reveals that only complex 4 is
active in catalyzing the aerobic oxidation of 3, 5-DTBC (Fig. 8) whereas all other complexes are
totally inactive (Fig. S14 in Supporting Information). From structural analyses it is evident that
only in case of complex 4 one zinc center contains readily labile solvent molecule and more
importantly accessible and that very feature is likely to be responsible to facilitate the catalyst-
substrate interaction and thereby the activity.
Fig. 8. UV−vis spectra of (i) complex 4 (ii) 3, 5-DTBC, and (iii) changes in UV−vis spectra of
complex 4 upon addition of 3, 5-DTBC.
The binding constant (Km), maximum velocity (Vmax), and rate constant for dissociation of the
substrate (i.e., turnover number, kcat) were calculated for the complex using the
Lineweaver−Burk graph of 1/[V]versus1/[S] with the equation 1/V = (Km/Vmax)(1/[S]) + 1/Vmax
.
The kinetic parameters are as follows: Vmax (M S−1), 2.8395×10-6 ; Km (M), 1.4872×10-3 and kcat
(h−1) 10.22×101. The kinetic plots are given as supporting information (Fig. S15-S16). Catalytic
oxidation of 3,5-DTBC to 3,5-DTBQ needs either metal centered redox participation or radical
(generally ligand based) involvement. It is unlikely that in zinc complexes with Schiff-base-type
ligands the metal center(s) are involved in the redox process with regard to the two-electron
16

oxidation of 3,5-DTBC, as proposed for the copper system [22,23]. The EPR study at 77 K,
performed immediately after mixing the ZnII complex with 3,5-DTBC in an inert atmosphere,
shows a broad, nearly isotropic EPR signal at g ≈ 2.00 (Fig. 9), a definite indication of any
ligand centered radical as we observed in our earlier studies also [24] and that radical generation
is most likely responsible for the oxidation of 3,5-DTBC.
Fig. 9. EPR spectrum of the mixture of complex 4 and 3,5-DTBC.
On the basis of our previous experience [24] and the present experimental observation we are
now in a position to put forward a probable mechanistic pathway, as depicted in Fig. 10 for the
oxidation of 3,5-DTBC to 3,5-DTBQ catalyzed by complex 4.
17

CH3
O
O
N
O
Zn
Zn
O
O
O
O
N
O
CH3
H
CH
CH3
3
CH3OH
t-Bu
OH
OH
t-Bu
CH3
O
O
N
O
O
Zn
Zn
O
O2
O
CH3
t-Bu
O
N
H+
CH3
O
CH3
O
CH3
O
O
t-Bu
O
O
N
O
O
N
O
Zn
Zn
Zn
O
Zn
O
Oxygen
O
O
O
O
O
N
N
t-Bu
CH3
CH3
OH
O
CH3
CH3
O
O
t-Bu
t-Bu
t-Bu
CH3
O
O
N
O
Zn
Zn
O
O
O
O
N
H+
CH3
CH3
O
O
t-Bu
t-Bu
Fig. 10. Probable mechanistic pathway for oxidation of 3,5DTBC catalyzed by complex 4.
18

3.4. Electronic property
Electronic spectra of the ligand as well as the four zinc(II) complexes were recorded in
DMF medium. The ligand shows band maxima at 268 and 317 nm, which may be attributed to
π→π* and n→π* transitions respectively. All four complexes display three bands near 234, 265
and 375 nm and only 1 and 2 show one additional band at 318 nm (Fig. S17 in Supporting
Information). The band at lower wavelength (~234 nm) corresponds to intraligand charge
transfer, whereas the other band originates from ligand to metal charge transfer. For complexes 1
and 2 the low energy band can be assigned to be either due to X→M and the other due to
PhO→M charge transfers [25].
3.5. Photoluminescence property
Fluorescence spectral study of the four complexes has been performed in DMF medium
at ambient temperature. Photo-excitations have been performed at λmax(ex) of 316 and 370 nm.
However, it is interesting to note that no appreciable change in emission band maxima or in
quantum yields for the complexes were observed on changing the excitation wavelength. Fig. 11
depict the emission spectra of the ZnII complexes along with that of the ligand with an excitation
λmax(ex) of 370 nm. The emission band maxima, λmax(ex) for all the complexes lay in the range
445-460 nm whereas λmax(ex) of the ligand is 492 nm and thus on complexation a blue shift in
λmax(ex) is observed. We can explain this blue shift by the fact that excited states resulting from
complexes of Zn2+ are typically ligand-centered (LC) in nature due to the inability of the d10
metal center to participate in low-energy charge transfer for metal-centered transitions. Quantum
yield calculations (Table 5) show that all the metal complexes are more fluorescent in compare
to the ligand and this is probably due to the greater rigidity of the ligand system attained upon
complexation [26-28]. From the figures, as well as the quantum yield calculations it is also clear
that the co-ligands have a significant role on the photoluminescence property of the complexes.
The fluorescence efficiency of the complexes shows the following trend: (3>4>1~2), thus the
highest efficiency is seen for the mononuclear thiocyanato complex. On examining the structures
of the complexes it may be inferred that maximum planarity is attained in thiocyanato complex
(complex 3), a feature which is supposed to facilitate the π →π* transition and thereby the
highest photoluminescence activity.
19

Fig. 11. Fluorescence spectra of complexes 1-4 in DMF medium. (λexc = 370 nm). Probe
concentrations are 10-5 M.
Table 5
Photophysical parameters of metal complexes 1-4 in DMF with excitation at λ= 370 nm.
Complex
Emission
(λmax, nm)
457
Quantum yield
[Zn2(L)2Cl2] (1)
0.317
0.316
0.684
0.466
0.221
[Zn2(L)2Br2] (2)
457
[Zn(L)(SCN)(MeOH)] (3)
[Zn2(L)(ac)3(MeOH)] (4)
Ligand HL
457
445
492
20

3.6. Thermal property
Solid state thermal analyses have been performed in order to (i) understand the thermal
decomposition patterns of the complexes, (ii) to verify the molecular composition of the
complexes, and (iii) to synthesize the thermally stable end products. The analyses performed in
the temperature range 30–700°C suggest that complexes are stable up to nearly 200°C. Three
dinuclear Zn complexes yield ZnO as the thermally stable end product, whereas the mononuclear
thiocyanato species yields ZnS as the final product. Table 6 reports the calculated and
experimental weight losses, and temperature range of decomposition for all the complexes
studied and the thermograms are provided in the supplementary information (Fig. S18-S21 in
Supporting Information). The initial temperature of decomposition (Ti) of the zinc complexes
follows the trend 4 < 3 < 1 < 2 which indicates that complex 2 is thermally most stable.
Table 6
Theoretical and experimental weight loss (from thermograms) of the complexes.
Complex
End product
(assumed)
Final
Theoretical
weight
(%)
Experimental
temperature
(°C)
loss weight loss (%)
1
2
3
4
ZnO
ZnO
ZnS
ZnO
650
670
640
510
73.9
78.72
87.33
76.78
69.36
77.16
73.4
70.46
4. Conclusion
The influence of anions, more particularly Cl, Br, SCN and OAc on the structural
diversity in ZnII complexes of a N,N,O donor Schiff-base ligand 2-((E)-((pyridin-2-
yl)methylimino)methyl)phenol and thereby on their catalytic activities and photoluminescence
behavior have been investigated via synthesizing four complexes. Cl and Br generate dinuclear
complexes with same structural feature where the phenoxo oxygen is acting as a bridging donor
21

to the metal ions and coordination environment around each Zn centre is distorted trigonal
bipyramid. On the other hand thiocyanato complex is a mononuclear species and geometry is
distorted square-pyramidal. Interestingly, complex 4 (acetato complex) although dinuclear but
two zinc centers have different coordination geometry, one is elongated octahedral and other is
tetrahedral. Therefore, very appealing anion mediated structural diversity is achieved from this
investigation. All the complexes are highly efficient catalysts to catalyze the hydrolysis of P-O
bond of a phosphoester, 4-NPP and the order of activity is 4>1>2>3. Interestingly, only complex
4 has shown catecholase activity towards 3,5-DTBC as substrate. The presence of readily labile
solvent molecule in one of the zinc centers of complex 4 perhaps takes the key role for initiating
substrate-Zn interaction and radical pathway, as is evident from EPR study, is supposed to be
responsible for the oxidation of the substrate. All four complexes are highly fluorescent and the
order of photoluminescence activity is 3>4>1~2. Solid state thermal analyses reveal that for 3 it
is ZnS and for the other three complexes ZnO are thermally stable end products.
Acknowledgments
The authors wish to thank CSIR, New Delhi [CSIR project No. 01(2464)/11/EMR-II dated 16-
05-2011] for financial support.
Supporting Information
X-ray crystallographic data in CIF format of complexes 1, 2, 3 and 4, FTIR spectra, packing
diagrams of 2 and 3, electronic spectra, UV-vis and kinetics data for phosphatase and catecholase
activities, thermograms.
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24

Graphical abstract (Pic)
25

Graphical abstract (synopsis)
The role of co-ligands on the structural diversity, catalytic activities and on photoluminescence
behavior of a tridentate Schiff-base ligand complexes of zinc(II) have been explored.
26