Formation of three photoluminescent zinc(II) complexes with Zn2O2 cores: Examples of bi-dentate bonding modes of potentially tri- and tetra-dentate Schiff bases

Article abstract of DOI:10.1016/j.poly.2014.12.018

The 1:1 condensation of N,N-dimethyl-1,3-diaminoethane with 5-bromosalicylaldehyde, 3-methoxysalicylaldehyde and 3-ethoxysalicyalaldede produces three Schiff bases, HL¹ [2-(-(3-(dimethylamino)propylimino)methyl)-4-bromophenol], HL² [

Full text of DOI:10.1016/j.poly.2014.12.018

Polyhedron 88 (2015) 156–163
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Formation of three photoluminescent zinc(II) complexes with Zn2O2
cores: Examples of bi-dentate bonding modes of potentially tri- and
tetra-dentate Schiff bases
Anik Bhattacharyya a, Suresh Sen a, Klaus Harms b, Shouvik Chattopadhyay a,
⇑
a Department of Chemistry, Inorganic Section, Jadavpur University, Kolkata 700032, India
b Fachbereich Chemie, Philipps-Universität Marburg, Hans-Meerwein-Strasse, D-35032 Marburg, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
The 1:1 condensation of N,N-dimethyl-1,3-diaminoethane with 5-bromosalicylaldehyde, 3-methoxysali-
cylaldehyde and 3-ethoxysalicyalaldede produces three Schiff bases, HL1 [2-(-(3-(dimethylamino)propyli-
mino)methyl)-4-bromophenol], HL2 [2-(-(3-(dimethylamino)propylimino)methyl)-6-methoxyphenol] and HL3
[2-(-(3-(dimethylamino)propylimino)methyl)-6-ethoxyphenol], respectively. Addition of zinc(II) perchlo-
rate to the methanol solution of HL1 produces a phenoxo bridged dinuclear zinc(II) complex, [Zn2(HL1)4]
(ClO4)4 (1), where the potential tridentate N2O donor Schiff base, HL1, is trapped in its zwitterionic
form and behaves like a bi-dentate ligand. On the other hand, phenoxo bridged dinuclear complexes,
[(HL2)Zn2(L2)(dca)2]ClO4 (2) and [(HL3)Zn2(L3)(NCS)2]ClO4ꢀCH3OH (3) are produced by the addition of
zinc(II) perchlotate into the methanol solutions of the potential tetradentate Schiff bases, HL2 and HL3,
respectively, followed by the addition of sodium dicyanamide or sodium thiocyanate. In both complexes,
Schiff bases are trapped in their deprotonated anionic forms and non-deprotonated zwitterionic forms as
well. The deprotonated ligands are coordinated in their usual chelating tetradentate manners whereas
zwitterionic forms of the non-deprotonated ligands are coordinated to zinc(II) only through the imine
nitrogen and phenoxo oxygen, i.e., they behave as bi-dentate ligands. The structures of the complexes
have been confirmed by single crystal X-ray diffraction studies. All three complexes are dinuclear and
contain Zn2O2 cores. All three complexes show fluorescence.
Received 3 September 2014
Accepted 17 December 2014
Available online 24 December 2014
Keywords:
Schiff base
Crystal structure
Zn2O2 core
Zwitterionic
Photoluminescence
Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction
interesting molecular and crystalline architectures and different
dimensionality [10,17,21–25]. A large number of phenoxo-bridged
Design and synthesis of di and polynuclear complexes of transi-
tion metals attract a lot of interests now-a-days because of their
potential uses in magnetism, catalysis, opto-electronic devices,
molecular architectures, material science, etc. [1–6]. Schiff bases
are very well-known chelating ligands to the synthetic inorganic
chemists to prepare such di and polynuclear complexes [7–10].
Among all the Schiff bases, H2salen type Schiff bases are definitely
receiving the most attention, as they can form poly-nuclear com-
plexes exploiting the bridging ability of phenoxo oxygen atoms
[11–16]. N2O donor tridentate Schiff bases could also be prepared
easily and used to synthesize several di and polynuclear transition
metal complexes [17–20]. The versatile ability of phenoxo oxygen
atoms to bind metal ions in different coordination modes, have led
to the isolation of a large number of coordination compounds with
transition metal complexes have been prepared in order to exam-
ine the extent of magnetic exchange interaction in these com-
plexes [26,27]. However, phenoxo-bridged complexes derived
from d10 metal ions did not receive such high attention compared
to the other mentioned metal ions, probably because of the lack of
magnetic exchange interaction in it. In this context, it is notewor-
thy that the importance of the d10 metal complexes lies in their
strong luminescence properties and potential applications in elec-
tronic and optoelectronic devices [28,29].
Focusing to zinc, it has a vital importance in metabolism, and is
necessary for proper functioning of the immune system [30]. Zinc
is the second most abundant transition metal in the human body
and is an essential cofactor in many biological processes [31,32].
Zinc(II) may adopt varieties of geometries as a consequence of
its d10 electronic configuration with no CFSE in any geometry.
Exploiting these varieties of coordination geometries around
zinc(II), various molecular architectures may be produced [33–38].
⇑
Corresponding author. Tel.: +91 3324572147.
E-mail address: shouvik.chem@gmail.com (S. Chattopadhyay).
http://dx.doi.org/10.1016/j.poly.2014.12.018
0277-5387/Ó 2015 Elsevier Ltd. All rights reserved.

A. Bhattacharyya et al. / Polyhedron 88 (2015) 156–163
157
The synthesis and characterization of new zinc(II) complexes
deserve further exploration following the discovery of zinc(II) con-
taining enzymes, e.g., carbonic anhydrase (the biological counter-
part to haemoglobin), which transports CO2 from the body. On the
other hand, supramolecular systems based on coordination com-
plexes have widely been used as photonic devices, sensors and cat-
alysts and in host–guest chemistry [39–47]. A significant number
of supramolecular zinc(II) complexes has been synthesized in the
last several years [33–38]. In the present work, we have used three
flexidentate Schiff bases as blocking ligands to prepare dinuclear
zinc(II) complexes with Zn2O2 cores. In each of the complexes, zwit-
terionic forms of the Schiff bases are trapped. Herein, we would like
to report the synthesis and characterization of two phenoxo bridged
dinuclear zinc(II) complexes with pseudo-halide co-ligands. All
complexes exhibit fluorescence in acetonitrile medium.
the methanol solution of the ligand HL2 and the resulting solution
was refluxed for ca. 30 min. A methanol–water (2:1) solution
(10 ml) of sodium dicyanamide (89 mg, 1 mmol) was then added
and the refluxing was continued for an additional 1 h. A light yel-
low crystalline precipitate of complex 2 was separated out after
2 days and collected by filtration. Single crystals, suitable for
X-ray diffraction, were obtained after few days on slow evaporation
of the of the methanol solution of the complex in open atmosphere.
Yield: 609 mg, 73%. Anal. Calc. for C30H39ClN10O8Zn2
(M = 833.9 g molꢁ1): C, 43.21; H, 4.71; N, 16.80. Found: C, 43.1;
H, 4.5; N, 16.9%. FT-IR (KBr, cmꢁ1): 1085 (Cl–O); 1629 (C@N);
2173, 2226, 2292 (dca). kmax (nm) [e
max(L molꢁ1 cmꢁ1)] (acetoni-
trile): 230 (1.85 ꢂ 104); 271 (9.64 ꢂ 103); 363 (3.01 ꢂ 103). M.P.:
>200 °C.
2.1.2.3. [(HL3)Zn2(L3)(NCS)2]ClO4ꢀCH3OH (3). A methanol solution
(10 ml) of zinc(II) perchlorate hexahydrate (370 mg, 1 mmol) was
added to the methanol solution of the ligand HL3 and the resulting
solution was refluxed for ca. 30 min. A methanol–water (2:1) solu-
tion (10 ml) of sodium thiocyanate (81 mg, 1 mmol) was then
added and the refluxing was continued for an additional 1 h. It
was cooled and kept for few days to get pale yellow crystals of
complex 3, which were collected by filtration. Single crystals, suit-
able for X-ray diffraction, were obtained after few days on slow
evaporation of the methanol solution in open atmosphere.
2. Experimental
All chemicals were of reagent grade and used as purchased from
Sigma–Aldrich without further purification.
Caution!!! Although no problems were encountered in this
work, organic ligands in the presence of perchlorates are poten-
tially explosive. Only a small amount of the material should be pre-
pared and it should be handled with care.
Yield: 676 mg, 77%. Anal. Calc. for C31H47ClN6O9S2Zn2
(M = 878.05 g molꢁ1): C, 42.40; H, 5.39; N, 9.57. Found: C, 42.2;
H, 5.1; N, 9.7%. FT-IR (KBr, cmꢁ1): 1085 (Cl–O); 1615 (C@N);
2.1. Preparations
2.1.1. Preparation of ligands
2091 (NCS). kmax (nm) [e
max(L molꢁ1 cmꢁ1)] (acetonitrile): 233
2.1.1.1. HL1 (2-(-(3-(dimethylamino)propylimino)methyl)-4-bromo-
phenol). The tridentate Schiff base ligand, HL1, was prepared by
refluxing N,N-dimethyl-1,3-diaminopropane (0.10 ml, 1 mmol)
with 5-bromosalicylaldehyde (200 mg, 1 mmol) in methanol solu-
tion (20 ml) for ca. 1 h. It was not purified and used directly for the
preparation of complex 1.
(2.01 ꢂ 104); 275 (9.29 ꢂ 103); 365 (3.71 ꢂ 103). M.P.: >200 °C.
2.2. Physical measurements
Elemental analysis (carbon, hydrogen and nitrogen) was per-
formed using a Perkin–Elmer 240C elemental analyzer. IR spectra
in KBr (4500–500 cmꢁ1) were recorded with a Perkin–Elmer Spec-
trum Two spectrophotometer. Electronic spectra in acetonitrile
were recorded on a Perkin–Elmer Lambda 35 UV–Vis spectropho-
tometer. Fluorescence spectra in acetonitrile were obtained in
Shimadzu RF-5301PC spectrofluorometer at room temperature.
Lifetime measurements were recorded using Hamamatsu MCP
photomultiplier (R3809) and were analyzed by using IBHDAS6
software. Intensity decay profiles were fitted to the sum-of
2.1.1.2. HL2 (2-(-(3-(dimethylamino)propylimino)methyl)-6-methoxy-
phenol). The tetradentate Schiff base ligand, HL2, was prepared by
refluxing N,N-dimethyl-1,3-diaminopropane (0.10 ml, 1 mmol) with
3-methoxysalicylaldehyde (152 mg, 1 mmol) in methanol solution
(20 ml) for ca. 1 h. It was not purified and used directly for the
preparation of complex 2.
2.1.1.3. HL3 (2-(-(3-(dimethylamino)propylimino)methyl)-6-ethoxy-
phenol). It was prepared in a method similar to that for the ligand,
HL2, except that 3-ethoxysalicylaldehyde (166 mg, 1 mmol) was
used instead of 3-methoxysalicylaldehyde. It was not purified
and used directly for the preparation of complex 3.
exponentials series
X
IðtÞ ꢁ
Aiexpðꢁt=siÞ
i
where Ai is a factor representing the fractional contribution to the
time-resolved decay of the component with a lifetime of i. The
av) are determined using the follow-
s
2.1.2. Preparation of complexes
intensity-averaged life times (
s
2.1.2.1. [Zn2(HL1)4](ClO4)4 (1). A methanol solution (10 ml) of
zinc(II) perchlorate hexahydrate (185 mg, 0.5 mmol) was added
to the methanol solution of the ligand HL1 and the resulting
solution was refluxed for ca. 1 h. A light yellow crystalline solid
was separated out after 2 days and collected by filtration. X-ray
quality single crystals of complex 1 were obtained after few days
on slow evaporation of the methanol solution of the complex in
open atmosphere.
ing equation:
X
X
<
s
>¼
Ais2i
=
Aisi
i
i
Powder X-ray diffractions were performed on a Bruker D8
instrument with Cu K radiation.
a
2.3. X-ray crystallography
Yield: 1235 mg, 74% (based on zinc). Anal. Calc. for C48H68Br4Cl4
N8O20Zn2 (M = 1669.28 g molꢁ1): C, 34.54; H, 4.11; N, 6.71. Found:
C, 34.1; H, 4.0; N, 6.9%. FT-IR (KBr, cmꢁ1): 1085 (Cl–O); 1630
Suitable crystals of the complexes were used for data collection
using a ‘Bruker D8 QUEST area detector’ diffractometer equipped
(C@N). kmax (nm)
[e
max(L molꢁ1 cmꢁ1)] (acetonitrile): 228
with graphite-monochromated Mo K
a radiation (k = 0.71073 Å).
(1.97 ꢂ 104); 267 (8.31 ꢂ 103); 371 (2.67 ꢂ 103). M.P.: >200 °C.
The molecular structures were solved by direct method and refined
by full-matrix least squares on F2 using the SHELX-97 and SHELX-2013
package [48,49]. Non-hydrogen atoms were refined with aniso-
tropic thermal parameters. The hydrogen atoms were placed in
2.1.2.2. [(HL2)Zn2(L2)(dca)2]ClO4 (2). A methanol solution (10 ml) of
zinc(II) perchlorate hexahydrate (370 mg, 1 mmol) was added to

158
A. Bhattacharyya et al. / Polyhedron 88 (2015) 156–163
Table 2
their geometrically idealized positions and constrained to ride on
their parent atoms. Multi-scan empirical absorption corrections
were applied to the data using the program SADABS [50]. In the
crystal structure of complex 3, the thiocyanate group has been
refined using two positions with occupation factor 0.84 and 0.16,
respectively. Also the methanol molecule has been refined with
the oxygen atom at two positions with occupation factors 0.8
and 0.2, respectively. The details of crystallographic data and
refinements are given in Table 1. Selected bond lengths and bond
angles are gathered in Tables 2 and S1 (Supplementary informa-
tion), respectively.
Selected bond lengths (Å) of 1, 2 and 3.
1
2
3
Zn(1)–O(2)
Zn(1)–O(2)⁄
Zn(1)–O(8)
Zn(1)–N(5)
Zn(1)–N(21)
Zn(1)–N(21)⁄
Zn(1)–N(1)
Zn(2)–O(2)
Zn(2)–O(8)
Zn(2)–O(15)
Zn(2)–N(33)
Zn(2)–N(38)
2.067(5)
2.088(5)
1.979(5)
2.038(6)
2.089(2)
–
2.077(3)
–
2.021(2)
2.094(2)
2.025(2)
–
2.036(3)
2.076(3)
2.031(4)
–
–
2.078(6)
–
–
–
–
–
–
2.102(2)
2.033(2)
1.999(2)
2.355(2)
1.966(2)
1.954(2)
2.112(4)
2.021(3)
1.993(3)
2.387(3)
1.925(4)
1.967(5)
3. Results and discussion
Symmetry transformation: * = 1 ꢁ x, ꢁy,1 ꢁ z.
3.1. Synthesis
Schiff bases is present in its zwitterionic form. The increase in pH
due to the additionof dicyanamide or thiocyanate may be responsible
for the deprotonation of one of the Schiff bases. The formation of all
three complexes is shown in Scheme 1.
The tridentate Schiff-base ligands, HL1, HL2 and HL3, were synthe-
sized by the condensation of N,N-dimethyl-1,3-diaminopropane,
respectively, with 5- bromosalicylaldehyde, 3-methoxysalicylalde-
hyde and 3-ethoxysalicyalaldede in 1:1 M ratio, following the litera-
ture method [7–10]. Addition of zinc(II) perchlorate hexahydrate to
the methanol solution of HL1 produced a deep yellow solution. Single
crystals of phenoxo-bridged dinuclear complex 1, suitable for X-ray
diffraction, were grown from it. On the other hand, complexes 2
and 3 were produced by the addition of zinc(II) perchlotate hexahy-
drate into the methanol solution of HL2 and HL3, respectively, fol-
lowed by the addition of sodium dicyanamide or sodium
thiocyanate. Both are phenoxo bridged dinuclear complexes. It is to
be noted here that the use of zinc(II) bromide or zinc(II) chloride
and HL1 produced mononuclear zinc(II) complexes, [Zn(HL1)Br2] and
[Zn(HL1)Cl2] respectively, the structures of which were reported by
different groups [51,52]. HL2 was also used to prepare similar mono-
nuclear zinc(II) complex, [Zn(HL2)(SCN)2]ꢀCHCl3, [53]. Use of zinc(II)
iodide with a very similar ligand produced similar mononuclear com-
It is to be noted here, HL1 is a potential tridentate ligand and
uses its two donor sites to bind zinc(II) in complex 1. On the other
hand, HL2 and HL3 are potential tetradentate ligands. Their bi-den-
tate and tetradentate bonding modes are observed in complexes 2
and 3. Thus all three Schiff bases are behaving as pendant ligands.
The term ‘pendant’ is applied to a potential polydentate ligand hav-
ing additional donor groups attached to its periphery. A large num-
ber of pendant ligands are used by different groups in a variety of
different chemical applications [55–59]. The importance of these
pendant ligands lies in their ability to perturb metal ions at addi-
tional coordination sites, in competition with external substrates,
as well as to provide connection points for other molecules.
3.2. Structure description
plex,
[Zn(HL)I2]ꢀCH3OH
{HL = 2-(-(3-(diethylamino)propylimi-
no)methyl)-4-bromophenol} [54]. It is to be noted here, in all
complexes, the Schiff bases are present in their zwitterionic forms.
The increased acidity of the medium due to the presence of perchlo-
rate or halide is responsible for the non-deprotonation of the Schiff
base. The lower coordinating ability of perchlorate compared to
halidesmaybe thedrivingforce fortheformationofphenoxo-bridged
dinuclear complex, 1. On the other hand, in both 2 and 3, one of the
3.2.1. Complex [Zn2(HL1)4](ClO4)4 (1)
ꢀ
Complex 1 crystallizes in the triclinic space group P1. A per-
spective view of complex 1 with selective atom numbering scheme
is shown in Fig. 1. In the asymmetric unit, each of the zinc(II) cen-
tres is coordinated by two phenoxo oxygen atoms, O(1) and O(2)
and two imine nitrogen atoms, N(1) and N(3) from two zwitter-
ionic Schiff bases (HL1). The fifth coordination site of each zinc(II)
Table 1
Crystal data and refinement details of complexes 1, 2 and 3.
Complex
1
2
3
Formula
T (K)
C
100
48H68Br4Cl4N8O20Zn2
C30H39ClN10O8Zn2
100
C31H47ClN6O9S2Zn2
100
Crystal system
Space group
a (Å)
b (Å)
c (Å)
triclinic
P1
triclinic
P1
monoclinic
P21/c
9.305(1)
23.758(2)
17.394(1)
–
98.345(2)
–
4
ꢀ
ꢀ
10.222(5)
13.091(5)
14.074(5)
74.665(5)
73.008(5)
67.572(5)
1
1.690
3.405
840
9.620(1)
11.749(1)
16.827(1)
84.948(2)
81.592(2)
73.888(2)
2
1.534
1.465
860
a
(°)
b (°)
c
(°)
Z
Dcalc (g cmꢁ3
)
1.533
1.499
1824
l
(mmꢁ1
F(000)
)
Total reflections
Unique reflections
Observed data [I > 2
No. of parameters
Rint
24840
6224
3961
364
32676
8307
6439
470
79287
7005
5560
505
r
(I)]
0.050
0.056
0.083
R1, wR2 (all data)
R1, wR2 [I > 2r(I)]
0.1020, 0.1968
0.0612, 0.1724
0.0573, 0.0767
0.0352, 0.0701
0.0675, 0.1190
0.0476, 0.1094

A. Bhattacharyya et al. / Polyhedron 88 (2015) 156–163
159
Scheme 1. Preparation of the complexes.
centre is occupied by a symmetry related phenoxo oxygen atom,
O(2)⁄ (symmetry transformation: * = 1 ꢁ x, ꢁy,1 ꢁ z). The
zinc(II)ꢀ ꢀ ꢀzinc(II) distance within the dinuclear unit is 3.277(2) Å.
The geometry of the penta-coordinated metal center may conve-
perchlorate anion. A perspective view of complex 2 is shown in
Fig. 3. Within the dinuclear unit, two penta-coordinated zinc(II) cen-
tres are bridged by phenoxo oxygen atoms, O(2) and O(8) of two
molecules of the Schiff base. The other coordination sites of Zn(1)
centre are occupied by an amine nitrogen atom, N(1) and an imine
nitrogen atom, N(5), from a deprotonated Schiff base (L2)ꢁ and an
imine nitrogen atom, N(21) from a zwitterionic Schiff base (HL2)
and that of Zn(2) are occupied by a methoxy oxygen atom, O(15)
of the deprotonated Schiff base (L2)ꢁ and two nitrogen atoms,
N(33) and N(38), from two terminal dicyanamides. The zin-
c(II)ꢀ ꢀ ꢀzinc(II) distance within the dinuclear unit is 3.1828(5) Å.
The saturated six membered chelate ring, Zn(1)–N(1)–C(2)–C(3)–
C(4)–N(5), exhibits an intermediate conformation between chair
and half-chair with puckering parameters q = 0.616(2) Å,
h = 2.64(19)°, / = 252(4)° [61–63]. The Addison parameters are
0.67 for Zn(1) and 0.41 for Zn(2), indicating the actual geometries
of zinc(II) are intermediate between trigonal bipyramid and square
pyramid.
niently be measured by the Addison parameter (
60, where and b are the two largest Ligand–Metal–Ligand angles
of the coordination sphere ( = 0 infers an ideal square pyramid
and = 1 infers a perfect trigonal bipyramid)] [60]. The Addison
s
) [s
= (a
ꢁ b)/
a
s
s
parameter is 0.76 indicating the actual geometry of the zinc(II)
centre is close to trigonal bipyramid.
The complex forms two intermolecular hydrogen bonds. The
hydrogen atom H(1) attached with the free amine nitrogen atom
N(1) ofthe zwitter-ionicSchiff base is hydrogenbonded withan oxy-
gen atom O(5) of a perchlorate anion. Similarly, the hydrogen atom
H(17) attached with N(17) of another zwitter-ionic Schiff base forms
a hydrogen bond with the oxygen atom O(10) as shown in Fig. 2. The
details of the hydrogen bonding interactions are given in Table 3.
3.2.2. Complex [(HL2)Zn2(L2)(dca)2]ClO4 (2)
The hydrogen atom H(1) attached with the free amine nitrogen
atom N(17) of the zwitter-ionic Schiff base is hydrogen bonded
ꢀ
Complex 2 crystallizes in the triclinic space group P1. The asym-
with
a
symmetry related nitrogen atom N(42)b (Symmetry
metric unit consists of dinuclear [(HL2)Zn2(L2)(dca)2]+ cation and

160
A. Bhattacharyya et al. / Polyhedron 88 (2015) 156–163
Fig. 1. Perspective view of the complex 1 with selective atom numbering scheme of
the metal and coordinating atoms. Hydrogen atoms are omitted for clarity.
Fig. 3. Perspective view of the complex 2 with selective atom numbering scheme of
the metal and coordinating atoms. Hydrogen atoms are omitted for clarity.
with the methyl carbon atom C(30) shows another inter-molecular
C–Hꢀ ꢀ ꢀ
ring R7 [C(23)–C(24)–C(25)–C(26)–C(27)–C(28)]. The combination
of these two inter-molecular C-Hꢀ ꢀ ꢀ interactions produced a 2D
sheet structure as shown in Fig. 5. The details of the geometric fea-
tures of the C–Hꢀ ꢀ ꢀ interactions are given in Table 4.
p
interaction with a symmetry related (x,ꢁ1 + y,z) phenyl
p
p
3.2.3. Complex [(HL3)Zn2(L3)(NCS)2]ClO4ꢀCH3OH (3)
The structure of complex 3 is very similar to that of complex 2.
It crystallizes in the monoclinic space group P21/c. The asymmetric
unit consists of a dinuclear [(HL3)Zn(L3)Zn(NCS)2]+ cation, one per-
chlorate anion and a methanol of crystallization. A perspective
view of complex 3 is shown in Fig. 6. The Zn(1) centre is sur-
rounded by an amine nitrogen atom, N(1), one imine nitrogen
atom, N(5), and a phenoxo oxygen atom, O(8), from a deprotonated
Schiff base ligand, (L3)ꢁ, and a phenoxo oxygen atom, O(2), and an
amine nitrogen atom, N(21), from a zwitterionic Schiff base (HL3).
On the other hand, Zn(2) centre is coordinated by a phenoxo oxy-
gen atom, O(8) and an ethoxy oxygen atom, O(15), of the deproto-
nated Schiff base, one phenoxo oxygen atom, O(2), of the
zwitterionic Schiff base and two nitrogen atoms, N(33) and
N(38), from two terminal thiocyanates. Thus two penta-coordi-
nated zinc(II) centres are bridged by phenoxo oxygen atoms, O(2)
and O(8). The zinc(II)ꢀ ꢀ ꢀzinc(II) distance within the dinuclear unit
is 3.1427(7) Å. The saturated six membered chelate ring, Zn(1)–
N(1)–C(2)–C(3)–C(4)–N(5), exhibits an intermediate conformation
between chair and half-chair with puckering parameters
q = 0.606(4) Å, h = 173.8(4)°, h = 73(3)° [61–63]. The Addison
parameters are 0.67 for Zn(1) and 0.35 for Zn(2), indicating the
actual geometries of zinc(II) are intermediate between trigonal
bipyramid and square pyramid.
Fig. 2. Inter-molecular hydrogen bonding interactions among the complex moiety
and the counter anions in complex 1. Only the interacting hydrogen atoms are
shown in figure. Methyl groups of amine nitrogen atoms are omitted for clarity.
Table 3
Hydrogen bonding distances (Å) and angles (°) for complex 1 and 2.
Complex D–Hꢀ ꢀ ꢀA
D–H (Å) Dꢀ ꢀ ꢀA (Å) Hꢀ ꢀ ꢀA (Å) \D–Hꢀ ꢀ ꢀA (°)
1
N(1)–H(1)ꢀ ꢀ ꢀO(5)
0.91
N(17)–H(17)ꢀ ꢀ ꢀO(10) 0.91
N(17)–H(1)ꢀ ꢀ ꢀN(42)a 0.89
2.790(14) 1.89
2.857(12) 1.98
2.909(3) 2.09
171
162
153
2
Symmetry transformations: a = 1 ꢁ x,1 ꢁ y,1 ꢁ z. D = donor; H = hydrogen;
A = acceptor.
transformation: b = 1 ꢁ x,1 ꢁ y,1 ꢁ z) of a dicyanamide anion from
a neighbouring molecule to form a dimeric structure (Fig. 4). The
details of the hydrogen bonding interactions are given in Table 3.
The complex shows significant intra and inter-molecular
The complex shows both intra and inter-molecular C–Hꢀ ꢀ ꢀ
p
C–Hꢀ ꢀ ꢀ
p interactions. The hydrogen atom, H(19B) attached with
interactions. The hydrogen atom, H(31B) attached with C(31) is
C(19) is involved in an intra-molecular C–Hꢀ ꢀ ꢀ
p interaction with
involved in an intra-molecular C–Hꢀ ꢀ ꢀ
p interaction with the phenyl
the chelate ring R4 [Zn(1)–O(8)–C(8)–C(7)–C(6)–N(5)]. On the
ring R6 [C(7)–C(8)–C(9)–C(10)–C(11)–C(12)] On the other hand,
other hand, the hydrogen atom, H(28) attached with C(28) is
the hydrogen atom, H(206) attached with C(200) is involved in
p
interaction with the phenyl ring R7
[C(24)–C(25)–C(26)–C(27)–C(28)–C(29)] of a symmetry related
involved in inter-molecular C–Hꢀ ꢀ ꢀ
p interaction with the phenyl
an inter-molecular C–Hꢀ ꢀ ꢀ
ring R6 [C(7)–C(8)–C(9)–C(10)–C(11)–C(12)] of a symmetry related
(1 + x,y,z) molecule. Similarly, the hydrogen atom, H(30B), attached
(x,1/2-y,1/2 + z) methanol molecule as shown in Fig. 7. The details

A. Bhattacharyya et al. / Polyhedron 88 (2015) 156–163
161
Table 4
Geometric features (distances in Å and angles in °) of the C–Hꢀ ꢀ ꢀ
p
interactions
obtained for complex 2 and 3.
Complex
C–Hꢀ ꢀ ꢀCg(Ring)
Hꢀ ꢀ ꢀCg (Å)
C–Hꢀ ꢀ ꢀCg (°)
Cꢀ ꢀ ꢀCg (Å)
2
C(19)–H(19B)–Cg(4)
C(28)–H(28)–Cg(6)b
C(30)–H(30B)–Cg(7)c
C(31)–H(31B)–Cg(6)
C(200)–H(206)–Cg(7)d
2.64
2.84
2.60
2.93
2.67
146
126
159
132
142
3.506(3)
3.481(3)
3.537(3)
3.657(9)
3.500(7)
3
Symmetry transformations: b = 1 + x,y,z; c = x, ꢁ1 + y,z; d = x,1/2 ꢁ y,1/2 + z.
Fig. 4. Hydrogen bonding interactions in complex 2 forming a dimeric structure.
The methyl groups of amine nitrogens and hydrogen atoms except the interacting
ones are omitted for clarity.
of the geometric features of the C–Hꢀ ꢀ ꢀ
p interactions are given in
Table 4.
3.3. IR, electronic and luminescence spectra
In IR spectra of all three complexes distinct bands due to the
azomethine (C@N) groups within the range of 1615–1630 cmꢁ1
are routinely noticed [64,65]. The strong band at 1085 cmꢁ1 give
evidence for the presence of ionic perchlorate in each of the com-
plex [66]. The dicyanamide anion in complex 2 shows three char-
acteristic IR absorptions located in the region 2170–2300 cmꢁ1
[67]. A strong band at 2091 cmꢁ1 in the IR spectrum of 3 indicates
the presence of thiocyanate, which is evident from the crystal
structure determination [68]. IR spectra of all three complexes
are given in Figs. S1–S3. The IR spectra of the complexes have also
been compared with those of the free ligands in order to determine
the coordination sites that may get involved in chelation. The
imine (C@N) stretching vibration is found in free ligands, HL1,
HL2 and HL3, at 1647, 1643 and 1633 cmꢁ1, respectively. The bands
are shifted to lower frequency in the complexes. These shifts are
due to the reduction of double bond characters of the C@N bonds,
caused by the coordination of nitrogen atoms to the zinc(II) centers
and are in agreement with results obtained from other similar
complexes described previously [69]. The intensity of strong bands
around 3300 cmꢁ1 due to O–H stretching vibrations in the IR spec-
tra of free ligands has been drastically reduced due to the partici-
pation of phenoxo oxygen atoms in coordination resulting with
concomitant deprotonation of the ligands. Positions of bands corre-
sponding to C–O stretching vibrations in free ligands also shift
towards lower frequency region in complexes due to participation
of the oxygen atoms in coordination. At the same time, new bands
in the region of 500 cmꢁ1 arise in the complexes indicating zin-
c(II)–O vibrations.
Fig. 6. Perspective view of the complex 3 with selective atom numbering scheme of
the metal and coordinating atoms. Hydrogen atoms are omitted for clarity.
The electronic spectra of complexes and ligands are recorded in
acetonitrile medium in the range 200–1000 nm. Absorption spec-
tra of ligands (HL1, HL2 and HL3) exhibit three bands, of which
bands around 230 nm may be attributed to
p–
p⁄ transitions and
the bands observed around 310 and 370 nm may be assigned to
n–p⁄ transitions [70]. Although the same trends have been main-
tained in the complexes, slight shifts in the positions of these
bands are observed as a result of complexation. There is no bands
corresponding to d–d electronic transitions as expected for zinc(II)
complexes with d10 electronic configurations [69]. The UV–Vis
absorption spectra of all the complexes are shown in Figs. S4–S6,
Supplementary information.
All the complexes and ligands exhibit luminescence in acetoni-
trile medium. These are assigned to intra-ligand (
p–
p⁄) fluores-
cence [21]. The ligands, HL1, HL2 and HL3, display emission
Fig. 5. The two-dimensional sheet structure formed in complex 2 via inter-molecular C–Hꢀ ꢀ ꢀ
p interactions.

162
A. Bhattacharyya et al. / Polyhedron 88 (2015) 156–163
Fig. 10. Steady-state photoluminescence decay profile of HL3 and complex 3.
Fig. 7. Inter-molecular C–Hꢀ ꢀ ꢀp interaction in complex 3.
Fig. 11. Time dependent photoluminescence decay profile of all three complexes.
Table 5
The details data of the photoluminescence and time-resolved photoluminescence
decays of complexes 1, 2 and 3.
Fig. 8. Steady-state photoluminescence decay profile of HL1 and complex 1.
Samples
kex (nm)
kem (nm)
s
(ns)
v2
1
2
3
371
363
365
420
414
417
2.39
9.14
8.78
1.087132
1.073078
1.023388
maxima at 415, 407 and 413 nm when excited at 364, 357 and
361 nm, respectively. The emission spectra of the zinc(II) com-
plexes closely resemble the emission spectra of the ligands (Figs. 8–
10). The emission intensities of zinc(II) complexes are stronger
than that of free ligands, as were also observed in similar com-
plexes [69]. These results suggest that these ligands may be used
as suitable agents to detect zinc(II) and may have potential uses
as zinc(II) sensors [71]. The strong fluorescence of zinc(II) com-
plexes compared to ligands may be facilitated by the difficulty to
oxidise or reduce zinc(II) with its stable d10 configuration [72].
Enhancement of fluorescence through complexation opens up the
opportunity for potential photochemical applications of these
complexes [73].
The fluorescence lifetimes of the complexes are investigated in
acetonitrile solution at room temperature. The lifetimes of 1, 2 and
3 are about 2.39, 9.14 and 8.78 ns respectively (Fig. 11). Details of
the luminescence data of the complexes are listed in Table 5.
Fig. 9. Steady-state photoluminescence decay profile of HL2 and complex 2.

A. Bhattacharyya et al. / Polyhedron 88 (2015) 156–163
163
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The experimental powder XRD patterns of the bulk product of
all the complexes are in good agreement with the simulated XRD
patterns from single crystal X-ray diffraction, confirming purity
of the bulk samples (Figs. S7–S9). The simulated patterns were cal-
culated from the single crystal structural data (cif file) using the
CCDC Mercury software.
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The whole work can be concluded in three statements. Firstly,
the synthesis and characterization of three new zinc(II) Schiff base
complexes containing Zn2O2 cores unambiguously show that the
potential tri and tetradentate Schiff bases are acting as bi-dentate
ligands, keeping the remaining donor sites pendant. Secondly, in
all three complexes, zwitterionic forms of the Schiff bases are
trapped. Lastly, the increase in emission intensities of zinc(II) com-
plexes compared to that of ligands suggests the potential uses of
the ligands as zinc(II) sensors. Keeping in mind the novel role of
zinc(II) complexes in the field of electronic and opto-electronic
research, the present findings provide insight and serve as a proto-
type for preparing other such complexes.
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