Doubly phenoxo-bridged M-Na (M = Cu(II), Ni(II)) complexes of tetradentate Schiff base: Structure, photoluminescence, EPR, electrochemical studies and DFT computation

Article abstract of DOI:10.1016/j.poly.2014.04.032

Phenoxo-bridged complexes [M(μ-L)Na(ClO₄)(CH₃OH)] (M = Cu(II) (1) and Ni(II) (2)) (H₂L = N,N′-bis(3- methoxysalicylidenimino)-1,3-diaminopropane, C₆H₃(OMe)(OH) -CH = N-(CH₂)₃-N =

Full text of DOI:10.1016/j.poly.2014.04.032

Polyhedron 78 (2014) 62–71
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Doubly phenoxo-bridged M–Na (M = Cu(II), Ni(II)) complexes
of tetradentate Schiff base: Structure, photoluminescence, EPR,
electrochemical studies and DFT computation
a
c
d
a,
Kuheli Das a, Amitabha Datta b, , Suman Roy , Jack K. Clegg , Eugenio Garribba , Chittaranjan Sinha
,
⇑
⇑
Hulya Kara b
a Department of Chemistry, Inorganic Chemistry Section, Jadavpur University, Kolkata 700032, India
b Fizik Bölümü, Fen-Edebiyat Fakültesi, Balikesir Üniversitesi, Balikesir 10100, Turkey
c School of Chemistry and Molecular Biosciences, The University of Queensland, Brisbane, St Lucia, QLD 4072, Australia
d Dipartimento di Chimica e Farmacia and Centro Interdisciplinare per lo Sviluppo della Ricerca Biotecnologica e per lo Studio della Biodiversità della Sardegna, Università di
Sassari, Via Vienna 2, I-07100 Sassari, Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
Phenoxo-bridged complexes [M(
l
-L)Na(ClO4)(CH3OH)] (M = Cu(II) (1) and Ni(II) (2)) (H2L = N,N0-bis(3-
Received 3 January 2014
Accepted 11 April 2014
Available online 23 April 2014
methoxysalicylidenimino)-1,3-diaminopropane, C6H3(OMe)(OH)-CH = N-(CH2)3-N = CH-C6H3(OMe)(OH))
are structurally characterised by single-crystal X-ray diffraction study. M(II) appears in the square-plane
geometry with MN2O2 coordination sphere, while the sodium ion exists with NaO6 distorted octahedral
environment. The molecules pack in the solid-state forming three-dimensional arrays through C–H---O,
Keywords:
Schiff base
Cu(II) and Ni(II) complexes
EPR
O–H---O, C–H---
p, and p---p interactions. The spectral and redox properties are theoretically explained
by DFT computation of optimised geometry of the complexes.
Ó 2014 Elsevier Ltd. All rights reserved.
Cyclic voltammetry
Photoluminescence
1. Introduction
useful molecules in small-molecule activation [16], electron storage
[17], and they carry polar organometallics [18].
Metal complexes incorporating different N,O donor ligands have
received considerable attention due to their ability to bind different
cations [1], anions [2,3], and neutral compounds [4–7]. The choice
of ligands encoding some degree of flexibility is an important factor
in the design of metal-ligand complexes of different nuclearity,
dimensionality, redox ability, chirality, etc. which illustrates a
range of applications in catalysis [8,9], selective host-guest recogni-
tion [10–13] and use as building blocks in the formation of
extended structures [14,15]. The chemistry of Schiff base ligands
has received continuous and intensive attention in many fields of
research because of their unique coordination and structural
properties. Metal-salicylaldimines with o-phenoxo group(s) are
fascinating ligands that can coordinate with not only p- and d-block
metal elements but also alkali-metal ions. Hetero-binuclear
complexes of alkali- and transition metal-salicylaldimines are
Ligands based on mixed donor Schiff bases have been used for
the formation of hetero-metallic complexes [19–21]. The phenoxy
bridging tetradentate N2O2 ligand is a compartmental system
[22–25], sometime forms coordinatively unsaturated metal centers
[26–29] allowing for their incorporation into coordination poly-
mers [30–32]. N,N0-Bis(3-methoxysalicylidenimino)-1,3-diamino-
propane (H2L) has been used by us to prepare heterometallic 3d/
4f [28,33–37], 3d/3d [28,38,39] and 3d/alkali metal complexes
[28,40,41]. It is reported that some copper(II)-sodium(I) and
nickel(II)-sodium(I) complexes with salen-type compartments
Schiff bases [42–44] where structural determination has revealed
that the transition metal ions are placed in the N2O2 compartment
of the ligand, and sodium(I) is placed in the O4 compartment of the
complexes. In the extension to these studies, herein we report
spectroscopic studies (IR, UV–Vis, Fluorescence), X-ray structures,
EPR, electrochemical measurements and DFT calculations, of two
heterometallic complexes, [Cu(
l-L)Na(ClO4)(CH3OH)] (1) and
⇑
Corresponding authors. Tel.: +91 9433621872; fax: +91 33 2413 7121
(C. Sinha).
[Ni( -L)Na(ClO4)(CH3OH)] (2). To the best of our knowledge no
l
measurements of their EPR spectra or DFT calculations of these
E-mail addresses: amitd_ju@yahoo.co.in (A. Datta), c_r_sinha@yahoo.com
complexes have emerged in the literature.
(C. Sinha).
http://dx.doi.org/10.1016/j.poly.2014.04.032
0277-5387/Ó 2014 Elsevier Ltd. All rights reserved.

K. Das et al. / Polyhedron 78 (2014) 62–71
63
2.3.2. Preparation of complex [Ni(-L)Na(ClO4)(CH3OH)] (2)
2. Experimental
Upon addition of H2L (0.34 g, 1 mmol) in methanol (20 mL) to
Ni(ClO4)2ꢄ6H2O (0.36 g, 1 mmol) in the same solvent produced
instantly a green solution. The mixture was stirred for half an hour
and then a solution of NaClO4 (0.12 g, 1 mmol) in the minimum
volume of water was added, and the reaction mixture was kept
undisturbed and allowed to evaporate slowly. After ten days,
dark-green, rectangular-shaped single crystals of 2 were obtained.
The crystals were filtered off, washed with water and dried in air.
Yield: 68% (0.38 g). Anal. Calc. for C20H24NaNiN2O9Cl: C, 43.40; H,
4.37; N, 5.06. Found: C, 43.58; H, 4.48; N, 4.92%.
2.1. Materials
o-Vanillin and 1,3-diaminopropane (Merck, India), nickel per-
chlorate hexahydrate, copper perchlorate hexahydrate and sodium
perchlorate (Sigma-Aldrich) were purchased and used as received
without further purification. All solvents used were of reagent
grade. The ligand (H2L) synthesis was carried out following the
published procedure [45].
2.2. Physical techniques
2.4. X-Ray crystallography
Elemental analyses were performed on a Heraeus CHN-OS
Rapid Elemental Analyzer at the Instrument Centre of the NCHU.
Infrared spectra were recorded on a Perkin-Elmer 883-Infrared
spectrophotometer in the range 4000–400 cmꢀ1 as KBr pellets.
Electronic spectra were measured on a Hitachi U 3400 (UV–Vis–
NIR) spectrophotometer in methanol. EPR spectra were recorded
from 0 to 10000 Gauss in the temperature range 77–298 K with
an X-band (9.15 GHz) Varian E-9 spectrometer. The EPR parame-
ters reported in the text were obtained by simulating the spectra
with the computer program Bruker WinEPR SimFonia [46]. In all
the simulations, second-order effects were taken into account,
the ratio Lorentzian/Gaussian, affecting the line shape, was set to
1 and the line width used for x, y, z axes was 22, 25 and 30 Gauss,
respectively. Emission spectra were examined by LS 55 Perkin–
Elmer spectrofluorimeter at room temperature (298 K) in CH3CN
solution under degassed condition. The fluorescence quantum
yield of the complexes was determined using carbazole as a refer-
ence with known /R of 0.42 in Benzene [47]. The complex and the
reference dye were excited at same wavelength, maintaining
nearly equal absorbance (ꢁ0.1), and the emission spectra were
recorded. The area of the emission spectrum was integrated using
the software available in the instrument and the quantum yield is
The crystals [Cu(
mm) and [Ni(
-L)Na(ClO4)(CH3OH)] (2) (0.30 ꢃ 0.30 ꢃ 0.20 mm)
were used for data collection. The Oxford Gemini Ultra employing
confocal mirror monochromated Cu K radiation generated from a
sealed tube (k 1.54184 Å) used for data collection of 1 and graphite
monochromated Mo K radiation generated from a sealed tube
(k 0.71073 Å) was used for 2 with and scans at 120(2) K. Data
integration and reduction were undertaken with CrysAlisPro [48]
and subsequent computations were carried out using the
WinGX-32 graphical user interface [49]. Gaussian and empirical
absorption corrections were applied using CryAlisPro [48] in the
l
-L)Na(ClO4)(CH3OH)] (1) (0.41 ꢃ 0.17 ꢃ 0.12
l
a
a
x
w
hkl range ꢀ17 6 h 6 20, ꢀ9 6 k 6 9; ꢀ23 6 l 6 21 for
1 and
ꢀ10 6 h 6 10, ꢀ20 6 k 6 19; ꢀ32 6 l 6 31 for 2 in the h range
3.24–72.02° for 1 and 2.98–30.82° for 2. Structures were solved
by direct methods using SHELXS-97 [50], then refined and extended
with SHELXL-97 [50]. Carbon-bound hydrogen atoms were included
in idealized positions and refined using a riding model. Oxygen-
bound hydrogen atoms were first located in the difference Fourier
map before refinement with bond length and angle restraints.
Compound 2 crystallizes with two molecules in the asymmetric
unit. One of the oxygen atoms from the Cl(1) containing perchlo-
rate anion shows disorder over two positions with occupancies of
0.6 and 0.4. These atoms were modeled with equal thermal param-
eters. The propylene section of 1 is disordered and was modeled
over two equal occupancy positions. Each individual pair of disor-
dered atoms was also included with equal thermal parameters. The
/S=/R ¼ ½AS=ARꢂ ꢃ ½ðAbsÞR=ðAbsÞSꢂ ꢃ ½gS2
=
gR2
ꢂ
Here, /S and /R are the fluorescence quantum yield of the sample
and reference, respectively. AS and AR are the area under the fluo-
rescence spectra of the sample and the reference, respectively,
(Abs)S and (Abs)R are the respective optical densities of the sample
and the reference solution at the wavelength of excitation, and gS
and gR are the values of refractive index for the respective solvent
used for the sample and reference. Electrochemical measurements
were performed using computer-controlled CH-Instruments,
Electrochemical workstation, Model No CHI 600D (SPL) with Pt-disk
electrodes. All measurements were carried out under nitrogen
environment at 298 K with reference to SCE electrode in acetonitrile
using [n-Bu4N]ClO4 as supporting electrolyte. The reported poten-
tials are uncorrected for junction potential.
Table 1
Crystallographic data of [Cu(
l
-L)Na(ClO4)(CH3OH)] (1) and [Ni(l-L)Na(ClO4)(CH3OH)]
(2).
1
2
Empirical formula
Formula weight
Crystal system
Space group
a (Å)
C
20H24NaCuN2O9Cl
C20H24NaNiN2O9Cl
553.56
triclinic
558.39
monoclinic
P21/n
16.4736(2)
7.5792(10)
18.9289(2)
90
102.0580(10)
90
ꢀ
P1
7.3822(7)
14.0384(17)
22.8738(18)
98.533(8)
95.345(7)
104.276(9)
2251.0(4)
4
b (Å)
c (Å)
a
(°)
b (°)
2.3. Synthesis of the complexes
c
(°)
V(Å)3
2311.26(5)
4
1.605
2.3.1. Preparation of complex [Cu(l-L)Na(ClO4)(CH3OH)] (1)
Z
Dcalc (mg mꢀ3
)
1.633
Upon addition of H2L (0.34 g, 1 mmol) in methanol (20 mL) to
Cu(ClO4)2ꢄ6H2O (0.37 g, 1 mmol) in the same solvent the mixture
was stirred for half an hour and then a solution of NaClO4
(0.12 g, 1 mmol) in the minimum volume of water was added,
and the reaction mixture was kept undisturbed and allowed to
evaporate slowly. After ten days, dark brown coloured, rectangu-
lar-shaped single crystals of 1 were obtained. The crystals were
filtered off, washed with water and dried in air. Yield: 71%
(0.39 g). Anal. Calc. for C20H24NaCuN2O9Cl: C, 43.02; H, 4.33; N,
5.02. Found: C, 42.83; H, 4.41; N, 5.18%.
l
(mmꢀ1
)
3.071 (Cu K
a
)
1.056 (Mo K
a)
F(000)
1148
1144
Parameters
331
628
a
R1 [I > 2
wR2
r(I)]
0.0689
0.2179
1.068
0.0484
0.1016
1.028
b
Goodness of fit
a
R1
wR2 = (
=
R
||Fo| ꢀ |Fc||/
R
|Fo| for Fo > 2
R
r
(Fo).
(wF2c)2)1/2 all reflections, w = 1/[
2(F2o)+(0.0414P)2 + 1.4241P] for 2, respectively, where
b
R
w(F2o ꢀ F2c)2/
r +
2(F2o)+(0.1511P)2
1.8729P] for 1 and w = 1/[
r
P = (F2o + 2F2c)/3.

64
K. Das et al. / Polyhedron 78 (2014) 62–71
Fig. 1. ORTEP representation of 1 shown with 50% probability ellipsoids. Regions of disorder omitted for clarity.
Fig. 2. ORTEP representation of one of the two chemically identical, but crystallographically independent molecules in 2 shown with 50% probability ellipsoids.
Table 2
Table 3
Selected bond lengths (Å) and angles (°) for [Cu(
l-L)Na(ClO4)(CH3OH)] (1).
Selected bond lengths (Å) and angles (°) for [Ni(l-L)Na(ClO4)(CH3OH)] (2).
Cu(1)–N(1B)
Cu(1)–O(2)
Cu(1)-N(2)
Cu(1)–O(3)
Na(1)–O(1)
Na(1)–O(3)
Na(1)–O(9)
Na(1)–O(5B)
Na(1)–O(2)
Na(1)–O(4)
1.905(19)
1.870(2)
1.905(3)
1.860(2)
2.395(3)
2.315(3)
2.378(4)
2.469(11)
2.333(3)
2.343(3)
N(2)–Cu(1)–N(1B)
O(2)–Cu(1)–O(3)
O(3)–Cu(1)–N(2)
N(1B)–Cu(1)–O(2)
Cu(1)–O(3)–Na(1)
Cu(1)–O(2)–Na(1)
O(2)–Na(1)–O(3)
O(3)–Na(1)–O(4)
O(2)-Na(1)–O(9)
O(4)–Na(1)-O(9)
95.0(6)
Ni(2)–N(3)
Ni(2)–O(11)
Ni(2)–N(4)
Ni(2)–O(12)
Na(2)–O(10)
Na(2)–O(12)
Na(2)–O(13)
Na(2)–O(11)
Na(2)–O(18)
Na(2)---Ni(2)
1.875(2)
1.847(2)
1.881(2)
1.860(2)
2.423(2)
2.328(2)
2.478(2)
2.318(2)
2.326(2)
3.3793(11)
N(4)–Ni(2)–N(3)
N(4)–Ni(2)–O(12)
O(12)–Ni(2)–O(11)
O(11)–Ni(2)–N(3)
Na(2)–O(14)
Ni(2)–O(11)–Na(2)
Ni(2)–O(12)–Na(2)
O(12)–Na(2)–O(11)
92.56(9)
92.68(9)
81.88(8)
93.78(9)
2.371(2)
107.92(8)
107.06(8)
63.04(7)
79.31(10)
92.66(13)
92.9(6)
110.11(11)
108.97(11)
61.60(9)
68.37(10)
114.34(13)
90.35(13)

K. Das et al. / Polyhedron 78 (2014) 62–71
65
Fig. 3. View of the 1 down the crystallographic ac-vector showing part of the two-dimensional layers formed through herringbone-like arrangements of one-dimensional
chains. Chains are colored alternately.
perchlorate anion was also disordered and modeled over two 50%
occupancy positions. Crystallographic data is summarized in
Table 1.
2.5. Theoretical calculations
Full geometry optimization of 1 and 2 were carried out using
density functional theory (DFT) at the B3LYP level [51]. All calcula-
tions were performed using the GAUSSIAN 03 program package [52]
with the aid of the Gauss View visualization program [53]. For C,
H, N, O, and Cl the 6-31G(d) basis set were assigned, while for Cu
and Ni the LanL2DZ basis set with effective core potential were
employed [54]. The vibrational frequency calculations were per-
formed to ensure that the optimized geometries represent the local
minima and there are only positive eigen values. Vertical electronic
excitations based on B3LYP optimized geometries were computed
using the time-dependent density functional theory (TD-DFT) for-
malism [55–57] in acetonitrile using conductor-like polarizable
continuum model (CPCM) [58]. Gauss Sum was used to calculate
the fractional contributions of various groups to each molecular
orbital [59].
Fig. 4. View of the 1 down the crystallographic ac-diagonal. Each layer interacts
through a series of hydrogen bonds (not shown).
3. Results and discussion
3.1. Syntheses and formulation
The ligand, H2L (N,N0-bis(3-methoxysalicylidenimino)-1,
3-diaminopropane), is prepared by the condensation of o-vanillin
and 1,3-diaminopropane (1:2 mole ratio) in methanol, following
the literature method [45]. H2L is then reacted with M(ClO4)2ꢄ6H2O
in methanol followed by the addition of NaClO4 with 1:1:1 ratio
and the complexes of composition, [M(l-L)Na(ClO4)(CH3OH)]
(M = Cu(II) (1) and Ni(II) (2)) are crystallized by slow evaporation
of the solvent.
The IR spectra of the complexes (1 and 2) show m(C–O) band at
1230 cmꢀ1 and 1244 cmꢀ1 for 1 and 2, respectively corresponding
to the deprotonated phenoxy group (the corresponding peak
appears at 1254 cmꢀ1 in H2L). Infrared stretching at 1620 and
1616 cmꢀ1 for 1 and 2, respectively are assigned to coordinated
m
(C@N) [60], whereas in free ligand, H2L, the band appears at
higher wave number (1632 cmꢀ1). The
(M–N) and (M–O) stretching
frequencies are observed at 562 and 434 cmꢀ1 for 1, 576 and
m
m
474 cmꢀ1 for 2. The
m(ClO4) shows band at 1074, 1103 and
1119 cmꢀ1 for 1 and 1085, 1107, 1122 cmꢀ1 for 2 corresponds to
the M–O–(ClO3) vibration [61] along with a weak shoulder at
Fig. 5. Schematic representation of part of the one-dimensional ribbon-like
polymer formed through
p
–p
interactions in 2. Arrows indicate
p–p
interactions.
625–630 cmꢀ1
.

66
K. Das et al. / Polyhedron 78 (2014) 62–71
The measurement has been performed in polar solvent, MeCN
while in nitrobenzene the complex is nonconducting, and is sup-
porting to no-dissociation.
and nonpolar solvent, nitrobenzene. The molar conductance (KM
)
data of 1 and 2 are 12 and 19 S m2 molꢀ1 in nitrobenzene and
132 and 163 S m2 molꢀ1 in acetonitrile respectively which indicate
that in nitromethane both complexes appear nonionic where as in
acetonitrile they appear 1:1 electrolyte. In both the complexes
3.2. The molecular structures of 1 and 2
The molecular structures of [Cu(
l-L)Na(ClO4)(CH3OH)] (1)
[Cu(l-L)Na(ClO4)(CH3OH)] (1) and [Ni(l-L)Na(ClO4)(CH3OH)] (2),
(Fig. 1) and [Ni( -L)Na(ClO4)(CH3OH)] (2) (Fig. 2) show that the
l
ClO4-is weakly bonded to sodium center which exhibits solvent
polarity dependent coordination. In coordinating polar solvent like,
MeCN, the perchlorate group dissociates and shows conductivity
M(II) centre adopts a square-planar geometry with MN2O2 coordi-
nation (M = Cu(II) (1) and Ni(II) (2)). The bond lengths are given in
Tables 2 and 3. In the complexes, the sodium ion is coordinated to
Fig. 6. Schematic representation of parts of the formal (a) and informal (b) hydrogen bond interactions in 2. Dashed lines indicate hydrogen bonds.
HOMO-1, E = -5.87 HOMO, E = -5.66
eV; Cu, 2%; L, 98% eV; L, 99%
[Cu(L)Na(MeOH)(ClO4)] (1)
LUMO, E = -3.32
eV; ClO4, 98%
LUMO+1, E = -2.27
eV; Cu, 2%, L, 98%
HOMO-1, E = -5.83 HOMO: E = -5.56
LUMO,E = -3.35
eV; ClO4, 97%
LUMO+1, E = -2.22
eV; Ni, 9%; L, 91%
eV; Ni, 4%; L, 96%
eV; Ni, 7%; L, 93%
[Ni(L)Na(MeOH)(ClO4)] (2)
Fig. 7. Surface plots of some MOs (HOMOꢀ1, HOMO, LUMO and LUMO+1) of [Cu(L)Na(MeOH)(ClO4)] (1) and [Ni(L)Na(MeOH)(ClO4)] (2).

K. Das et al. / Polyhedron 78 (2014) 62–71
67
two O-methoxy groups, two bridging O-phenolato groups in a
hydrogen bonding interaction results in
a
three-dimensional
pseudo-macrocylic array along with methanol and a perchlorate
anion resulting in a six-coordinate O6-coordination sphere. The
Cu(II)---Na(I) separation is 3.4318(14) Å in 1 and the Na(I) and
Ni(II) centers in 2 are separated by 3.3793(11) (molecule 1) and
3.4289(11) Å (molecule 2). Interestingly, the crystal structure 2
consists of two conformationally related molecules (Supplemen-
tary Material, Fig. S1) with slightly different orientation among
bond parameters (Fig. 2 and Table 3 record the molecule 2;
molecule 1 is given in Supplementary Material, Table S1). The
corresponding Ni–N or Ni–O bond distances (molecule 1),
[(Ni(1)–N(1), 1.917(2) Å; (Ni(1)–N(2), 1.893(2) Å; Ni(1)–O(3);
1.857(2) Å); Ni(1)–O(2); 1.874(2) Å data are given in Supplemen-
tary Material, Table S1] among square planar environment, show
higher value in comparing with other molecular unit, [(Ni(2)–
N(4), 1.881(2) Å; Ni(2)–N(3), 1.875(2) Å; Ni(2)–O(11), 1.847(2) Å;
Ni(2)–O(12), 1.860(2) Å)] (molecule 2). Similarly, the correspond-
ing Na-O bond distance in six-coordinated arrangement (molecule
1) belongs in the range, 2.313–2.403 Å; slightly shorter than in
molecule 2, ranging 2.318–2.478 Å. While Na–O bond distances
differ inversely i.e., the distances are shorter in molecule 1 than
molecule 2.
network.
3.3. Absorption and emission spectra
The UV–Vis spectra of the complexes 1 and 2 are recorded in
methanol solution in 200–800 nm and have been compared with
free ligand data [45]. The free ligand shows high intense bands
(e
, 104 molꢀ1 cmꢀ1) at 220, 260, 293 nm which are assigned to
p
? e )
p⁄ transition and less intense absorptions ( , 103 molꢀ1 cmꢀ1
at 332 and 419 nm are due to n ? p⁄ transition. On complexation,
the n ? p⁄ band in both the complexes are shifted from 332 to 361
and 372 nm for 1 and 2, respectively. A characteristic feature to
copper(II) complex (1) is the appearance of broad d–d weak
absorption band at 595 nm and [Ni(l-L)Na(ClO4)(CH3OH)] (2)
shows weak d–d bands at 475 and 590 nm [65,66].
The spectral characteristics are explained with the help of DFT
computation of optimized structures of 1 and 2. The orbital ener-
gies along with contributions from the ligands and metal for few
MOs are given in Fig. 7 and details are given in the Supplementary
The complex 1 shows one-dimensional polymers through
p–p
150
interactions (Cg---Cg separation 3.57–3.65 Å) along the crystallo-
graphic b-axis. Neighboring ribbons are arranged in a herring-
bone-like motif with a series of edge-to-face (C–H – centroid
445 nm
Ligand
Complex2
distances of 2.74–3.13 Å)
p-interactions to form two-dimensional
Complex1
sheets perpendicular to the crystallographic ac-vector (Fig. 3). Each
sheet further interacts through (Me)O–H---O(ClO3), 2.02(3) Å and
((N=)CH---O(ClO3), 2.70 Å; and (Phenyl)C–H---O(ClO3), 2.40–
2.70 Å to form a three-dimensional network (Fig. 4). In 2, the adja-
cent molecules pack closely together forming a one-dimensional
ribbon-like polymer through a combination of offset face-to-face
100
λexc= 327 nm
50
p–p (phenyl ring---phenyl ring) interactions and phenyl---chelate
419 nm
ring stacking, which propagates along the crystallographic a-axis
(Fig. 5). The presence of phenyl-phenyl interactions is indicated
by centroid–centroid separations of 3.63 Å, while the phenyl---
chelate ring separation is slightly shorter (3.5 Å) [62–64]. The
one-dimensional ribbons interact through coordinated methanol
– perchlorate, (Me)O–H---O(ClO3) = 1.979(15) Å (x ꢀ 1, y, z) and
2.009(14) Å (x + 1, y, z)) (Fig. 6a), whereas the C–Hphenyl – perchlo-
rate (2.60–2.70 Å) and CHimine – perchlorate (2.60 Å) (Fig. 6b)
λexc= 370 nm
0
400
450
500
550
600
650
Wavelenghth (nm)
Fig. 8. Emission spectra of (a) H2L (in DMF), (b) complex 2 (in MeOH) and (c)
complex 1 (in DCM).
Table 4
Assignment of spectral transitions of the complexes following TD-DFT computation.
Excitation energy (eV)
[Cu( -L)Na(MeOH)(ClO4)] (1)
k (nm)
f
Key transitions
Character
l
1.8807
2.1514
2.2676
2.5702
3.0424
3.3248
3.3823
3.5051
3.7816
659.24
576.31
546.75
482.39
407.51
372.91
366.57
353.72
327.86
0.0012
0.0019
0.0053
0.0145
0.0660
0.0118
0.0592
0.0384
0.0050
(51%) HOMO ? LUMO + 1
(98%) HOMOꢀ1 ? LUMO + 1
(40%) HOMOꢀ7 ? LUMO
(49%) HOMOꢀ7 ? LUMO
(52%) HOMO ? LUMO + 5
(52%) HOMOꢀ1 ? LUMO + 4
(66%) HOMOꢀ1 ? LUMO + 5
(66%) HOMOꢀ1 ? LUMO + 6
(65%) HOMOꢀ7 ? LUMO + 1
LLCT
LLCT
IPCT
IPCT
LPCT
LPCT
LPCT
LPCT
PLCT
[Ni(l-L)Na(MeOH)(ClO4)] (2)
2.3322
2.6854
3.2601
3.3847
3.4980
3.8413
4.4824
5.0094
5.2612
531.62
461.70
380.31
366.31
354.45
322.77
276.60
247.50
235.66
0.0013
0.0018
0.0778
0.0217
0.0633
0.0042
0.1150
0.0030
0.0115
(72%) HOMOꢀ1 ? LUMO + 1
(95%) HOMOꢀ1 ? LUMO + 2
(37%) HOMO ? LUMO + 5
(39%) HOMOꢀ1 ? LUMO + 4
(37%) HOMOꢀ1 ? LUMO + 5
(20%) HOMO ? LUMO + 6
(32%) HOMOꢀ4 ? LUMO + 5
(24%) HOMOꢀ9 ? LUMO + 1
(37%) HOMO ? LUMO + 8
LLCT
LLCT
LPCT
LPCT
LPCT
LMCT
LPCT
PLCT
ILCT
LLCT (L(
ILCT (intra ligand charge transfer).
p p) ? Cu(dp)); PLCT (perchlorate to ligand charge transfer);
) ? L(p⁄)); IPCT (intra perchlorate charge transfer); LPCT (ligand toperchlorate charge transfer); LMCT (L(

68
K. Das et al. / Polyhedron 78 (2014) 62–71
1000
100
10
1000
100
10
1000
100
10
Complex 2
Complex 1
)
Ligand (LH2
1
1
1
0
10
20
30
40
50
60
0
10
20
30
40
50
60
0
10
20
30
40
50
60
Time (ns)
Time (ns)
Time (ns)
(a)
(b)
(c)
Fig. 9. Exponential decay profile (d) and fitting curve (ꢀ) of (a) H2L (b) complex 1(in DCM) and (c) complex 2 (in MeOH). Excitation is carried out at 370 nm.
Fig. 11. X-band EPR spectra of the polycrystalline complex 1 at (a) RT and (b) 77 K.
Diphenylpicrylhydrazyl (dpph) is the standard field marker (g = 2.0036).
Fig. 10. Anisotropic X-band EPR spectra of: (a) 2 in DMF, (b) 1 in DMF, (c) 1 in
DMSO and (d) simulated spectrum of 1 in DMSO. Diphenylpicrylhydrazyl (dpph) is
the standard field marker (g = 2.0036). The spectra in the traces (a)–(c) were
recorded with the same instrumental gain. Inset figure shows spin density of
is recorded to [Ni(
to HOMOꢀ8, LUMO, LUMO+4 to LUMO+6 have >45% share of
ClO4. In [Cu( -L)Na(ClO4)(CH3OH)] (1), Cu contributes 12% to
l
-L)Na(ClO4)(CH3OH)] (2) in which HOMOꢀ5
l
HOMOꢀ3, 6% to HOMOꢀ4, 9% to LUMO+7, 59% to LUMO+9. In
the complex 2, Ni contributes irregularly to MOs such as 4% to
HOMOꢀ1, 12% to HOMOꢀ2, 90% to HOMOꢀ4, 9% to LUMO+1,
10% to LUMO+2, 32% to LUMO+7, etc. Thus the ligand L and ClO4
group control over the molecular function and hence the electronic
properties of the complexes. Thus, HOMO ? LUMO is considered
[Cu(l-L)Na(MeOH)(ClO4)] considering optimized structure.
material (Figs. S2, S3, S4 and Tables S2, S4 for 1 and Fig. S5 and
Table S6 for 2). Both filled and vacant MOs around HOMO and
LUMO are constituted mainly by ligand orbitals (L and ClO4). It is
observed that HOMOꢀ5 and other lower energetic occupied MOs
carry >80% ClO4 function in the complex 1 and in LUMO, LUMO+3
to LUMO+5 also have >80% ClO4 contribution. Similar observation
as L(p
) ? ClO4(p⁄); HOMOꢀ1 ? LUMO+1 is LLCT (involving ligand
L
functions) and HOMO ? LUMO+6 is designated LMCT
(L(p) ? Cu(dp)) transitions in [Cu(l-L)Na(ClO4)(CH3OH)] (1). Same
Table 5
EPR parameters of 1 in different solvents.
Species
Solvent
gx
Ax (10ꢀ4 cmꢀ1
)
gy
Ay (10ꢀ4 cmꢀ1
)
gz
Az (10ꢀ4 cmꢀ1
)
2
2
2
DMSO
DMF
CH3OH
2.037
2.037
2.036
11
11
12
2.069
2.070
2.069
13
12
13
2.243
2.243
2.243
187
187
188

K. Das et al. / Polyhedron 78 (2014) 62–71
69
argument for [Ni(
l
-L)Na(ClO4)(CH3OH)] (2) along with an addi-
Anisotropic solution EPR spectra of 1 are typical of mononuclear
Cu(II) centres and show well-resolved hyperfine structure
tional HOMOꢀ4 ? LUMO+2 transition which is (Cu(d
p
) ? L(p⁄))
a
MLCT. The calculated transitions are grouped in Table 4. The inten-
sity of these transitions has been assessed from oscillator strength
(f). In MeCN the longest wavelength band calculated at >659 nm (f,
(Fig. 10, traces b and c). The spectra measured in DMSO, DMF
and CH3OH are comparable and the parameters, very similar in
the three solvents, are reported in Table 5. They were simulated
with software WinEPR [46]. The EPR signals show a slight rhombic
distortion; for example, in DMSO the following parameters
0.0012) for 1 is assigned to L(
intense transitions at 482 (intra perchlorate CT, IPCT), 407, 372,
353 nm (L(
) ? ClO4(p⁄)) (detail are given in Supplementary
p
) ? L(p⁄) transition followed by high
p
were simulated: gz = 2.243, Az = 187 ꢃ 10–4 cm–1
,
gy = 2.069,
material Tables S3, S5 for 1). In complex 2, the transitions at
Ay = 13 ꢃ 10–4 cm–1, gx = 2.043, Ax = 11 ꢃ 10–4 cm–1 (Fig. 10, trace
d). The results are consistent with dx2-y2 ground state and a geom-
etry close to the limit of square planar in solution; the rhombicity
may be due to the slight anisotropy along the x and y axes (for
example, the value of the two trans angles are slightly different).
The spectra of 1 in DMSO, DMF and CH3OHare typical of a Cu(II)
species with an equatorial donor set (Ophenolic, Nimine, Nimine, Opheno-
lic), even if the characteristic super-hyperfine structure is not
detected (Fig. 10, traces b and c). The spectral parameters are in
agreement with those reported for square-planar Cu(II) complexes
formed by salpn derivatives ((6,6,6)-chelate rings) [28].
531 nm (f, 0.0013) and 461 nm (f, 0.0018) are referred to
L(
are referred to L(
p
) ? L(p⁄) transitions; the transitions at 380, 366, 354 and 276
p
) ? ClO4(p⁄) transitions (detail are given in Sup-
plementary material Table S7 for 2).
Emission spectroscopy shows that H2L emits strongly while the
complexes, 1 and 2, are weak emitter. (Fig. 8). The emission band of
H2L appears at 445 nm upon excitation at 327 nm which is a
p
?
p⁄ emission. The complexes, 1 and 2, fluoresce weakly at
419 nm when they are excited at 370 nm. Longer wavelength of
excitation (MLCT/d–d bands) does not show any emission. The
fluorescence quantum yield of the ligand (/L, 0.02) and complexes
(/Ni, 0.014; /Cu, 0.01) were determined using carbazole as refer-
ence with known quantum yield value in benzene (/R = 0.42).
The fluorescence spectra revealed that fluorescence emission
intensity of Schiff bases decreased dramatically on complex forma-
tion with transition metal ions which refers to paramagnetic
quenching and heavy atom affect. Upon excitation on UV light irra-
diation (370 nm) ligand dissociation from the complexes (1 and 2)
may cause weak emission and it is observed indeed (Fig. 8).
The decay profiles of the ligand and the complexes have been
generated and shown in Fig. 9. The fluorescence life time has been
measured for free ligand and the complexes upon excitation at
370 nm. The decay profile of ligand fits with bi-exponential decay
whereas in case of complexes, he nature of fitting is tri-exponential
3.5. Electrochemistry
The cyclic voltammograms of the complexes in acetonitrile dis-
play both oxidative and reductive responses at positive and nega-
tive to Ag/AgCl reference electrode (Fig. 12). At positive potential,
one quasi-reversible (E1/2,0.69 V(220 mV)) and an irreversible
(Epa, 1.05 V) oxidative responses are recorded for complex 2 along
with ligand reduction at ꢀ1.12 V (
DE, 140 mV). First couple may be
referred to nickel(II) oxidation, Ni(III)/Ni(II) (2) [71]. The complex 1
(Fig. 9). The mean life time (sf = a1s1 + a2s2 + a3s3 where a1, a2 and
a3 are relative amplitudes of decay process) have been calculated
to compare excited state stability of the complexes. The life times
of both complexes (0.16–0.17 ns) are much lower than the ligand
itself (3.16 ns). This indicates that excited states of the complexes
are unstable than those of the free ligand. The radioactive and
non-radioactive rate constants values are also calculated and they
show usual higher knr (3.08 ꢃ 108(H2L), 55.05 ꢃ 108(1) and
55.05 ꢃ 108(2)) value than the kr (6.94 ꢃ 106, 55.61 ꢃ 106(1) and
78.9 ꢃ 106(2)).
3.4. The EPR spectra
Room temperature (298 K) bulk magnetic measurement
accounts diamagnetic property of Ni(II) (d8) in 2 (
l
, 0.14 BM) while
one electron subnormal paramagnetism is calculated for Cu(II) (d9)
in 1 ( , 1.58 BM). The polycrystalline powder of 2 is EPR-silent both
l
at room temperature and 77 K. The behaviour does not change
when this is dissolved in organic solvent, such as DMSO, DMF
and CH3OH; the spectrum recorded in DMSO is shown in
Fig. 10a. The explanation of the lack of spectral signals is due to
the fact that a square planar Ni(II) is diamagnetic with all the elec-
tron coupled (S = 0) [67].
EPR spectra of polycrystalline powder of 1 were recorded at
room temperature and 77 K. They do not change significantly with
lowering the temperature. No hyperfine coupling constant
between the unpaired electron and 63,65Cu nucleus was detected.
A half-field signal around 1700 Gauss, indicative a weak coupling
between the copper ions, is observed (Fig. 11). The spectra appear
to be tetragonal with g = 2.182 and g\ = 2.053 at RT and g = 2.187
k
k
and g\ = 2.054 at 77 K. The order g > g\ > ge is characteristic of a
k
Fig. 12. Cyclic voltammogram of complexes 1 and 2 in MeCN using Pt-disk working
electrode, Pt-wire auxiliary and SCE reference electrode in presence of [n-Bu4N]
(ClO4) supporting electrolyte at 50 mV Sꢀ1 scan rate at 300 K.
d
x2ꢀy2 ground state, [68–70] and is consistent with the square pla-
nar geometry of 1.

70
K. Das et al. / Polyhedron 78 (2014) 62–71
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E, 160 mV) and 1.18 V (Epa) fol-
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D
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Cu(III)/Cu(II) oxidation [72] and irreversible oxidation may be
assigned to phenolato ? quinonoid oxidation [73]. Reductive
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4. Conclusion
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Doubly phenoxo-bridged heterodinuclear [Cu(l-L)Na(ClO4)
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Mitra, Inorg. Chem. 47 (2008) 6227.
(CH3OH)] (1) and [Ni( -L)Na(ClO4)(CH3OH)] (2) complexes, with
l
L being a Schiff base ligand, have been prepared and characterised
in solution and the solid state. The crystal structures show that the
complexes pack closely into intricate three-dimensional arrays
through a variety of intermolecular interactions. The EPR discus-
sion of 1 and systematic discussion of emission spectra with DFT
computation of the complexes have been carried out.
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India for the grant to carry out the present study. A.D. would like to
thank to The Scientific & Technological Research Council of Turkey
(TÜBITAK) for the grant (2221 – Fellowship for Visiting Scientist).
J.K.C. acknowledges the support of the Australian Research Council.
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Appendix A. Supplementary material
CCDC 870813 and 894362 contains the supplementary crystal-
lographic data for 1 and 2. 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:
deposit@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.2014.04.032.
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