Synthesis, spectroscopic (electronic, IR, NMR and ESR) and theoretical studies of transition metal complexes with some unsymmetrical Schiff bases

Article abstract of DOI:10.1016/j.molstruc.2013.10.046

Two unsymmetrical Schiff bases, glyoxal salicylaldehyde oxalic acid dihydrazone (gsodh) and glyoxal salicylaldehyde malonic acid dihydrazone (gsmdh) and their Co(II), Ni(II), Cu(II) and Zn(II) complexes have been synthesized. The structures of metal complexes are elucidated on the basis of elemental analyses, molar conductance, magnetic susceptibility measurements, electronic, ESR, IR and NMR (¹H and ¹³C) spectral studies. Both ligands show monobasic tetra-dentate behaviour, bonding through CO, two CN and a phenolate group. The electronic spectral studies in solid state indicate a square planar geometry for Ni(II) and Cu(II) complexes and a tetrahedral geometry for Co(II) complexes. However, Co(II) and Cu(II) complexes adopt octahedral geometry in DMSO solution. The ESR spectra of Cu(II) complexes in DMSO solution at 77 K predict an elongated tetragonal distorted octahedral geometry around metal ion and presence of unpaired electron in dx ²-y² orbital. Further, the structures of ligands and their Ni(II) complexes have been satisfactorily modelled by calculations based on density functional theory (DFT). The electronic spectra of Ni(II) complexes are also analyzed in depth with the help of time dependent-DFT (TD-DFT). The theoretical analyses of electronic structure and molecular orbitals have demonstrated that the high-energy absorption bands are M → L charge transfer and low energy transitions are d-d transitions.

Full text of DOI:10.1016/j.molstruc.2013.10.046

Journal of Molecular Structure 1058 (2014) 71–78
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Synthesis, spectroscopic (electronic, IR, NMR and ESR) and theoretical
studies of transition metal complexes with some unsymmetrical
Schiff bases
⇑
Vinod P. Singh , Shweta Singh, Divya P. Singh, K. Tiwari, Monika Mishra
Department of Chemistry, Banaras Hindu University, Varanasi 221005, India
g r a p h i c a l a b s t r a c t
h i g h l i g h t s
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
Metal complexes of unsymmetrical
Schiff bases have been prepared and
characterized.
Spectral studies of complexes show
variations in bonding mode and
stereochemistry.
ESR shows axial interaction of solvent
molecules in a square planar Cu(II)
complex.
DFT is used to correlate experimental
and calculated data of the Ni(II)
complexes.
TDDFT interprets M ? L charge
transfer and d–d transitions high and
low energy bands.
a r t i c l e i n f o
a b s t r a c t
Article history:
Two unsymmetrical Schiff bases, glyoxal salicylaldehyde oxalic acid dihydrazone (gsodh) and glyoxal sal-
icylaldehyde malonic acid dihydrazone (gsmdh) and their Co(II), Ni(II), Cu(II) and Zn(II) complexes have
been synthesized. The structures of metal complexes are elucidated on the basis of elemental analyses,
molar conductance, magnetic susceptibility measurements, electronic, ESR, IR and NMR (1H and 13C)
spectral studies. Both ligands show monobasic tetra-dentate behaviour, bonding through ˜C@O, two
˜C@NA and a phenolate group. The electronic spectral studies in solid state indicate a square planar
geometry for Ni(II) and Cu(II) complexes and a tetrahedral geometry for Co(II) complexes. However, Co(II)
and Cu(II) complexes adopt octahedral geometry in DMSO solution. The ESR spectra of Cu(II) complexes
in DMSO solution at 77 K predict an elongated tetragonal distorted octahedral geometry around metal ion
Received 20 September 2013
Received in revised form 17 October 2013
Accepted 17 October 2013
Available online 25 October 2013
Keywords:
Unsymmetrical Schiff bases
Transition metal complexes
NMR spectra
and presence of unpaired electron in dx2
orbital. Further, the structures of ligands and their Ni(II) com-
ꢁy2
ESR spectra
plexes have been satisfactorily modelled by calculations based on density functional theory (DFT). The
electronic spectra of Ni(II) complexes are also analyzed in depth with the help of time dependent-DFT
(TD-DFT). The theoretical analyses of electronic structure and molecular orbitals have demonstrated that
the high-energy absorption bands are M ? L charge transfer and low energy transitions are d–d
transitions.
TDDFT calculations
Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction
that coordinated ligands around central metal ions in natural sys-
tems are unsymmetrical [1,2]. Unsymmetrical Schiff base ligands
The interest in the field of transition metal complexes of
unsymmetrical Schiff base ligands has come from the realization
have offered many advantages over their symmetrical counterparts
in the elucidation of the composition and geometry of the metal
ion binding sites in metallo-proteins and metallo-enzymes and
selectivity of natural systems with synthetic materials [3–5]. The
metal complexes with similar ligands have also been used as
⇑
Corresponding author. Tel.: +91 9450145060.
E-mail address: singvp@yahoo.co.in (V.P. Singh).
0022-2860/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molstruc.2013.10.046

72
V.P. Singh et al. / Journal of Molecular Structure 1058 (2014) 71–78
catalysts in various organic reactions [6,7]. Recently, this class of
compounds has attracted much attention in the field of optoelec-
tronic technologies for their large nonlinear responses [8,9].
Understanding the coordination chemistry of these complexes,
which is affected by the strain of the ligands, has an important role
on the above properties [10].
was dried in a desiccator over anhydrous calcium chloride. Light
yellow, yield (78%). M.p. >300 °C. Anal. Calc. for C11H10N4O4
(262): C, 50.38; H, 3.82; N, 21.37. Found: C, 50.55; H, 3.86; N,
21.48%. IR (
m
cmꢁ1, KBr):
(NAN) 970w.
m(OH) 3449b; m(NH) 3207b; m(C@O)
1665b; (C@N) 1628s;
m
m
A number of biologically active transition metal complexes with
different asymmetric Schiff bases have been synthesized in our
laboratory and by others [11–15]. However, this is for the first time
that unsymmetrical Schiff base ligands have been synthesized by
the condensation of two different keto group containing com-
pounds (glyoxal and salicylaldehyde) with a dihydrazide (oxalic
and malonic acid dihydrazide), and used to prepare their metal
complexes. Since, many transition metal complexes of hydrazine
derivatives have been reported to show antialgal, antifungal, anti-
bacterial and antitumor activity [14–16], the present compounds
are also expected to be biologically active. Because of the various
applications and interesting structural diversities of such metal
chelates, we motivated to synthesize Co(II), Ni(II), Cu(II) and Zn(II)
complexes with two new unsymmetrical tetra-dentate Schiff bases
with N 2O2 donor sites, glyoxal salicylaldehyde oxalic acid dihydra-
zone (gsodh) and glyoxal salicylaldehyde malonic acid dihydra-
zone (gsmdh) and characterized them by various spectroscopic
techniques.
Currently, density functional theory (DFT) is commonly being
used to examine the electronic structure of transition metal com-
plexes owing to its accuracy, user friendliness and relative fastness
in studying the large molecules of transition metal complexes. To
understand and analyze the spectral properties, we have also car-
ried out extensive DFT calculations to optimize structure of the li-
gands and their Ni(II) complexes. DFT coupled with time
dependent density functional theory (TD-DFT) calculations have
been performed to establish the nature of the orbitals involved in
transition processes and to correlate structural parameters with
the spectroscopic properties of complexes.
2.2.2. Synthesis of glyoxal salicylaldehyde malonic acid dihydrazone
Glyoxal salicylaldehyde malonic acid dihydrazone (gsmdh) was
synthesized by reacting 100 ml aqueous solution of malonic acid
dihydrazide (10 mmol, 1.32 g) with 50 ml aqueous ethanolic solu-
tion of a mixture containing glyoxal (10 mmol, 0.58 ml) and sali-
cylaldehyde (10 mmol, 1.22 ml) in a round bottom flask. The
reactants were stirred on a magnetic stirrer continuously for 3
hours at room temperature. A cream coloured precipitate of the
product was obtained, which was filtered by suction and purified
by washing several times with aqueous ethanol (v/v, 1:1). The pure
ligand was dried in a desiccator over anhydrous calcium chloride at
room temperature. Cream yellow, yield (72%). M.p. 256d °C. Anal.
Calc. for C12H12N4O4 (276): C, 52.17; H, 4.35; N, 20.29. Found: C,
52.03; H, 4.31; N, 20.16%. IR (
m
cmꢁ1, KBr):
(NAN) 966w.
m(OH) 3448b; m(NH)
3209b; (C@O) 1668b; (C@N) 1630s; m
m
m
The ligands were characterized by elemental analyses (C, H, N),
melting points, NMR (1H and 13C) and IR spectra. The ligands could
not be crystallized because of their highly insoluble nature in or-
ganic solvents.
2.3. Synthesis of the metal complexes
The metal complexes were prepared either by the preformed li-
gands or by template synthesis.
2.3.1. By the preformed ligands
The Co(II), Ni(II), Cu(II) and Zn(II) complexes of gsodh and
gsmdh ligands were synthesized by reacting 50 ml ethanolic solu-
tions of each metal(II) chloride with 50 ml ethanolic slurry solution
of the powdered ligands gsodh (10 mmol, 2.62 g) and gsmdh
(10 mmol, 2.76 g), separately in 1:1 (M:L) molar ratio in a round
bottom flask keeping the amount of metal salts in slight excess
than the required amount (10 mmol). The reaction mixture was re-
fluxed for 3 hours for Cu(II) and Zn(II) complexes and 5 hours for
Co(II) and Ni(II) complexes of both the ligands with shaking at reg-
ular intervals. The solid products obtained were filtered, washed
thoroughly with ethanol to remove the un-reacted metal chloride
and dried in a desiccator at room temperature.
2. Experimental
2.1. Materials
Commercial reagents have been used without further purifica-
tion and all experiments have been carried out in the open atmo-
sphere. The metal salts, glyoxal, salicylaldehyde and solvents
were purchased from SD Fine Chemicals, India and were used as
such. Diethyl oxalate, diethylmalonate and hydrazine hydrate
(Merck Chemicals, India) were used in synthesis.
2.3.2. By template synthesis
The precursors oxalic acid dihydrazide, (CONHNH2)2 and
malonic acid dihydrazide, CH2(CONHNH2)2 were prepared by the
reported procedure [16] by reacting diethyl oxalate and diethyl-
malonate with hydrazine hydrate, respectively in 1:2 molar ratio
in a beaker containing 10 ml ethanol.
A mixture of 25 ml ethanolic solution of glyoxal (10 mmol,
0.58 ml) and salicylaldehyde (10 mmol, 1.22 ml) was added into
50 ml ethanolic solutions of each metal(II) chloride (10 mmol) in
a round bottom flask, separately. The above solution was stirred
on a magnetic stirrer for 30 minutes at room temperature. To this
reaction solution, 50 ml aqueous solution of oxalic acid dihydra-
zide (10 mmol, 1.18 g) and malonic acid dihydrazide (10 mmol,
1.32 g) was added, separately, drop wise with stirring. The metal
complexes were obtained as insoluble precipitates on stirring the
reaction solutions continuously for 2 hours at room temperature.
The complexes were filtered in a glass crucible by suction and
washed several times with ethanol and finally with diethyl ether.
2.2. Synthesis of ligands
2.2.1. Synthesis of glyoxal salicylaldehyde oxalic acid dihydrazone
The ligand glyoxal salicylaldehyde oxalic acid dihydrazone
(gsodh) was synthesized by reacting 100 ml hot aqueous solution
of oxalic acid dihydrazide (10 mmol, 1.18 g) with 50 ml aqueous
ethanolic solution of a mixture containing glyoxal (10 mmol,
0.58 ml) and salicylaldehyde (10 mmol, 1.22 ml) in a round bottom
flask. A light yellow coloured precipitate of the product was ob-
tained on stirring the above solution on a magnetic stirrer contin-
uously for 2 hours at 60 °C. The precipitate was filtered on a
Büchner funnel and washed several times with hot water followed
by ethanol to remove any un-reacted reactants. The pure product
2.3.2.1. [Co(gsodh-H)]Cl. Brown, yield (75%). M.p. >300 °C.
l
eff = 4.29 BM. Anal. Calc. for C11H9N4O4CoCl (355.5): Co, 16.60;
Cl, 9.98; C, 37.13; H, 2.53; N, 15.75. Found: Co, 16.50; Cl, 10.00;
C, 37.27; H, 2.56; N, 15.66%. IR ( (NH) 3208b;
cmꢁ1, KBr):
(C@N) 1614m;
(CAOꢁ) 1266m;
(MAO) 532m; (MAN) 470m.
m
m
m(C@O) 1664b, 1650m;
(NAN) 1006w,
m
m
m
m
m

V.P. Singh et al. / Journal of Molecular Structure 1058 (2014) 71–78
73
2.3.2.2. [Ni(gsodh-H)]Cl. Yellow, yield (76%). M.p. >300 °C. Anal.
Calc. for C11H9N4O4NiCl (355.2): Ni, 16.52; Cl, 9.99; C, 37.16; H,
2.53; N, 15.76. Found: Ni, 16.45; Cl, 10.00; C, 37.10; H, 2.51; N,
temperature on a Cahn-Faraday balance using Hg[Co(SCN)4] as
the calibrant. The X-band ESR spectra of Cu(II) complexes were re-
corded on a EMX 1444 EPR spectrometer at liquid nitrogen tem-
perature (LNT) in DMSO solution and at room temperature
(300 K) in solid state using 100 KHz field modulation. The g factors
were quoted relative to the standard g marker TCNE (g = 2.00277).
15.70%. IR (
m
cmꢁ1, KBr):
(C@N) 1615s;
(CAOꢁ) 1266m;
(MAN) 471w.
m(NH) 3208b;
m(C@O) 1664b, 1647m;
m
m
m
m(NAN) 1003m, m
(MAO) 532m;
2.3.2.3. [Cu(gsodh-H)]Cl. Dark brown, yield (72%). M.p. >300 °C.
eff = 1.80 BM. Anal. Calc. for C11H9N4O4CuCl (360): Cu, 17.64; Cl,
9.86; C, 36.67; H, 2.50; N, 15.55. Found: Cu, 17.51; Cl, 9.80; C,
36.56; H, 2.51; N, 15.62%. IR ( (NH) 3210b; (C@O)
cmꢁ1, KBr):
(C@N) 1610m; (NAN) 1006m,
(CAOꢁ) 1267s;
(MAN) 480w.
2.5. Theoretical calculations
l
The geometry optimization of gsodh and gsmdh ligands and
their Ni(II) complexes were performed on a computer using atomic
coordinates from ChemDraw structure as input, employing the
G03w suite of programs [18]. Both the ligands and the complexes
were treated as an open-shell system using spin restricted DFT
wave functions (B3LYP) [19,20], i.e. the Becke three-parameter ex-
change functional in combination with the LYP correlation func-
tional of Lee, Yang and Parr with 6-31Gꢂꢂ basis set for C, H, N and
O atoms, and for the Ni atom in the complexes effective core poten-
tials basis set LanL2DZ (Los Alamos National Laboratory 2 double
zeta) is used. The B3LYP method is commonly used for DFT calcu-
lation of transition metal complexes because of close relation of
calculated geometrical and spectral parameters with experimen-
tally observed results [21,22]. DFT optimized calculations were
carried out in spin states with S = 0, and the optimized structures
were confirmed to be local minima by performing harmonic vibra-
tion frequency analyses (no imaginary frequency found). No sym-
metry constraints were applied and only the default convergence
criteria were used during the geometric optimizations. Based on
the optimized geometries, TDDFT calculations were performed at
the same B3LYP level to calculate the vertical electron transition
energies. LanL2DZ was again used for Ni, while for C, H, N and O
atoms the 6-31Gꢂꢂ basis set was used. The electron density dia-
grams of molecular orbitals were obtained with the Gaussian
03W program at isosurface of 0.03.
m
m
m
1662b, 1645m;
(MAO) 526m;
m
m
m
m
m
2.3.2.4. [Zn(gsodh-H)]Cl. Yellow, yield (70%). M.p. >300 °C. Anal.
Calc. for C11H9N4O4ZnCl (361.9): Zn, 18.07; Cl, 9.81; C, 36.47; H,
2.49; N, 15.47. Found: Zn, 17.99; Cl, 9.76; C, 36.35; H, 2.44; N,
15.54%. IR (
m
m
cmꢁ1, KBr):
(C@N) 1611m;
(CAOꢁ) 1268m;
(MAN) 468m.
m
(NH) 3209b;
m
(C@O) 1666b, 1645s;
m
m(NAN) 1005w, m
(MAO)
530w;
m
2.3.2.5. [Co(gsmdh-H)]Cl. Green, yield (75%). M.p. >300 °C.
eff = 4.32 BM. Anal. Calc. for C12H11N4O4CoCl (369.5): Co, 15.97;
Cl, 9.60; C, 38.97; H, 2.98; N, 15.15. Found: Co, 16.00; Cl, 9.53; C,
38.89; H, 2.97; N, 15.22%. IR ( (NH) 3213b; (C@O)
cmꢁ1, KBr):
1669b, 1650m; (C@N) 1615s; (NAN) 1002m,
(CAOꢁ) 1271s;
(MAO) 533m; (MAN) 474m.
l
m
m
m
m
m
m
m
m
2.3.2.6. [Ni(gsmdh-H)]Cl. Orange, yield (77%). M.p. >300 °C. Anal.
Calc. for C12H11N4O4NiCl (369.2): Ni, 15.90; Cl, 9.61; C, 39.00; H,
2.98; N, 15.17. Found: Ni, 15.81; Cl, 9.56; C, 38.89; H, 2.96; N,
15.10%. IR (
m
m
m
cmꢁ1, KBr):
(C@N) 1617s;
(CAOꢁ) 1271s;
(MAN) 474m.
m
(NH) 3208b;
m
(C@O) 1667b, 1647m;
m
m
(NAN) 1003w, m
(MAO) 534w;
2.3.2.7. [Cu(gsmdh-H)]Cl. Dark brown, yield (68%). M.p. >300 °C.
eff = 1.78 BM. Anal. Calc. for C12H11N4O4CuCl (374): Cu, 16.98;
Cl, 9.49; C, 38.50; H, 2.94; N, 14.97. Found: Cu, 16.90; Cl, 9.43; C,
38.40; H, 2.96; N, 15.05%. IR ( (NH) 3213b; (C@O)
cmꢁ1, KBr):
1669b, 1651m; (C@N) 1613w; (NAN) 1003w,
(CAOꢁ) 1273s;
(MAO) 522w; (MAN) 472w.
3. Results and discussion
l
It appears from the analytical data that the ligands, gsodh and
gsmdh are formed by unsymmetrical condensation of glyoxal and
salicylaldehyde with oxalic acid dihydrazide and malonic acid
dihydrazide, respectively. These ligands deprotonate their phenolic
protons during complexation with metal(II) chloride and form 1:1
(M:L) complexes of general composition [M(gsodh-H)]Cl and
[M(gsodh-H)]Cl. The reactions may be written as:
m
m
m
m
m
m
m
m
2.3.2.8. [Zn(gsmdh-H)]Cl. Yellow, yield (66%). M.p. >300 °C. Anal.
Calc. for C12H11N4O4ZnCl (375.9): Zn, 17.40; Cl, 9.44; C, 38.31; H,
2.93; N, 14.90. Found: Zn, 17.32; Cl, 9.37; C, 38.20; H, 2.89; N,
14.86%. IR (
m
m
cmꢁ1, KBr):
(C@N) 1616m;
(CAOꢁ) 1270m;
(MAN) 479w.
m
(NH) 3210b;
m
(C@O) 1668b, 1649s;
MCl2.xH2O + gsodh
or gsmdh
[M(gsodh-H)]Cl + HCl + xH2O
or [M(gsmdh-H)]Cl
m
m(NAN) 1001m, m
(MAO)
533w;
m
where, M = Co(II), Ni(II), Cu(II) and Zn(II)
2.4. Physico-chemical measurements
The metal complexes have also been formed by template reac-
tion of metal salts, glyoxal, salicylaldehyde, oxalic acid dihydrazide
or malonic acid dihydrazide together in 1:1:1 molar ratio. They ex-
hibit similar compositions, magnetic moments and spectral data as
the complexes synthesized by preformed ligands.
Most of the metal complexes are coloured powdery solids. The
colour of both Co(II) complexes is green but changes to red on
addition of water or on dissolving in DMSO. Similarly, Cu(II) com-
plexes are dark brown and turn green in water or DMSO solution.
This behaviour of colour change is probably because Co(II) and
Cu(II) complexes increase their coordination number from four to
six in presence of water and DMSO solution. However, the Ni(II)
complexes of gsodh and gsmdh do not show colour variation under
similar conditions, and are yellow and orange in colour, respec-
tively. The ligands and their metal complexes are insoluble in
water and common organic solvents such as ethanol, methanol,
chloroform, benzene, DMF and diethyl ether. However, they are
The metal and chloride contents were analyzed gravimetrically
using standard literature procedures [17]. C, H and N contents
were determined on an Exeter Analytical Inc. CHN Analyser (Model
CE-440). The molar conductance of 10ꢁ3 M solutions of the com-
plexes in DMSO was measured at room temperature on a Eutech
Con 510 Conductivity meter. 1H and 13C NMR spectra of the ligands
were recorded in DMSO-d6 on a JEOL AL-300 FT-NMR multinuclear
spectrometer. Chemical shifts were reported in parts per million
(ppm) using tetramethylsilane (TMS) as an internal standard. All
exchangeable protons were confirmed by addition of D2O. Infrared
spectra were recorded in KBr on a Varian 3100 FT-IR spectropho-
tometer in 4000–400 cmꢁ1 region. Electronic spectra of the com-
plexes were recorded on a Shimadzu spectrophotometer, model,
Pharmaspec UV-1700 in nujol as well as in DMSO solution. Mag-
netic susceptibility measurements were performed at room

74
V.P. Singh et al. / Journal of Molecular Structure 1058 (2014) 71–78
soluble in DMSO. All the complexes are highly stable and do not
melt up to 300 °C. The molar conductance values of 10ꢁ3 M solu-
tions of the complexes in DMSO at room temperature have been
observed in the range 51.60–66.00
they are 1:1 electrolytes [23].
X
ꢁ1 molꢁ1 cm2 suggesting that
3.1. Electronic spectra and magnetic moments
Copper(II) complexes, in the present study, show leff values
1.80 and 1.78 B.M. corresponding to one unpaired electron. In solid
state (nujol medium), the copper(II) complexes of gsodh and
gsmdh show three bands at 640, 438, 352 nm and at 630, 464,
387 nm, respectively. These bands correspond to 2B1g ? 2A1g
,
? 2B2g and ? 2Eg transitions, indicating a square planar geometry
for the complexes [24]. In DMSO solution, however, a broad band
has been observed at 720 and 690 nm, respectively for gsodh and
gsmdh complexes, suggesting a distorted octahedral geometry.
The electronic spectral bands observed at 839, 480 and 369 nm
for [Ni(gsodh-H)]Cl complex and at 820, 480 and 358 nm for
[Ni(gsmdh-H)]Cl complex indicate
a square planar geometry
around the metal ion in solid state. These bands correspond to
the transitions 1A1g ? 1B2g, ? 1B1g and a charge transfer transition
similar to that reported for many Ni(II) complexes with a square
planar geometry [25]. It is further confirmed by their diamagnetic
character exhibited by both the nickel(II) complexes. The positions
of these bands only alter slightly in DMSO solution suggesting the
same stereochemistry in both media.
The leff values 4.29 and 4.32 B.M. observed for cobalt(II) com-
plexes of gsodh and gsmdh, respectively, correspond to three un-
paired electrons in a tetrahedral environment [26]. The bands
observed for [Co(gsodh-H)]Cl complex at 663, 398 nm and for
[Co(gsmdh-H)]Cl at 685, 373 nm, are in good agreement with the
bands reported for cobalt(II) tetrahedral complexes showing tran-
sitions as 4A2(F) ? 4T1(P) and a charge transfer transition, respec-
tively, in solid state. The DMSO solution spectra of cobalt(II)
complexes show a significant change in geometry due to solvent
interaction on axial positions. The bands observed at 1084, 520
and 440 nm for gsodh and at 1066, 524, and 430 nm for gsmdh
complexes indicate an octahedral geometry around Co(II) showing
Fig. 1. ESR spectra of Cu(II) complexes in DMSO solution at 77 K.
2.0405, 147 G, 110 G, 2.1440, 122 G, respectively. These values sug-
gest an elongated tetragonal distorted octahedral geometry for
both the complexes. Moreover, the trend g|| > g\ > ge (2.0023), sug-
gests that the ground state of copper(II) is predominantly dx2
ꢁy2
[29,30]. No
DMs = 2 transition at half field was observed for any
of these complexes, ruling out the possibility of dimeric forms.
4
transitions T1g(F) ? 4T2g(F), ? 4A2g(F), and ? 4T1g(P) [27].
It is interesting to observe that the g|| value obtained in DMSO
solution is markedly higher than those observed in solid state in
both the complexes. This variation may be considered to indicate
the possibility of axial bonding of the solvent molecules altering
the geometry of the complexes from square planar to distorted
octahedral. This type of axial bonding in a four coordinate complex
(with small g values) will then tend to pull the copper(II) ions out
of the equatorial plane, thus weakening the equatorial bonds. This
will also increase the g|| and g\ values, the change in g\ being very
small [31].
3.2. ESR spectra
ESR spectra of both the copper(II) complexes in solid state at
298 K are similar and exhibit axial signals with two g values,
g|| = 2.1827 and g\ = 2.0416 for [Cu(gsodh-H)]Cl and g|| = 2.2084
and g\ = 2.0432 for [Cu(gsmdh-H)]Cl. In these complexes, the g||
and g\ values are greater than 2.04 indicating that the copper(II)
ions have axial symmetry with all the principal axes aligned paral-
lel. The observed g|| values of both the complexes are consistent
with a square planar stereochemistry. The G factors [defined as
G = (g|| ꢁ 2)/(g\ ꢁ 2)] for gsodh and gsmdh complexes are 4.3918
and 4.8241, respectively, suggest that the local tetragonal axes
are only slightly misaligned and the exchange interactions be-
tween copper(II) centres in the solid state are negligible [28].
Improved resolutions of g and hyperfine anisotropy of the cop-
per(II) ion are observed in the frozen DMSO solution spectra of the
complexes at 77 K. Both the complexes show a well resolved
hyperfine structure due to copper(II) nuclei (I = 3/2) on the lower
field region (Fig. 1) to allow accurate calculation of g|| and g\ val-
ues. It is seen that the fourth hyperfine line of the parallel compo-
nent in the spectra is superimposed on the perpendicular
component. The gav and Aav values are calculated using the equa-
tions gav = (g|| + 2g\)/3; Aav = (A|| + 2A\)/3. The g||, g\, A||, A\, gav
and Aav values for the gsodh complex are 2.3468, 2.0600, 150 G,
93 G, 2.1556, 112 G, and for the gsmdh complex are 2.3511,
3.2.1. 1H and 13C NMR
The 1H NMR spectra of gsodh exhibits signals due to aromatic
protons as multiplet at d 6.80–7.40 (4H), two ANH protons as sin-
glet at 7.48 (1H) and 7.52 (1H), two ACH@N protons as singlet at
8.22 (1H) and doublet at 8.38 (1H, J = 7.3 Hz), ArAOH as singlet
at 9.95 (1H) and HC@O proton as singlet at 11.00 (1H) ppm. [32].
In the spectra of [Zn(gsodh-H)]Cl, both ANH protons show a
slightly downfield shift as compare to gsodh, which is indicative
of deshielding of ANH proton and in turn shows interaction of
the two ACH@N groups with the metal ion. The signals for aro-
matic protons show a minor shift in their position in the complex.
A significant downfield shift in HC@O signal is characteristic of the
involvement of the ˜C@O group of glyoxal in bonding with the me-
tal ion (Table 1). The gsmdh exhibits signals for ACH2 as singlet at
d 3.85 (2H), aromatic protons as multiplet at 6.70–6.92 (4H), two

V.P. Singh et al. / Journal of Molecular Structure 1058 (2014) 71–78
75
Table 1
1H and 13C NMR spectral data of ligands, and their complexes.
Compounds
1H NMR signals (ppm)
CH2
Aromatic protons
NH
ACH@N
ArAOH
HC@O
gsodh
[Zn(gsodh-H)]Cl
gsmdh
–
–
6.80–7.40m
6.89–7.42m
6.70–6.92m
6.72–7.31m
7.48s, 7.52s
7.53s, 7.61s
7.57s, 7.61s
7.63s, 7.65s
8.22s, 8.38d
8.27s, 8.41d
8.27s, 8.37d
8.38s, 8.41d
9.95s
9.98s
9.99s
9.98s
11.00s
11.15s
11.05s
11.18s
3.85s
3.90s
[Zn(gsmdh-H)]Cl
13C NMR signals (ppm)
Aromatic C
CAOH
ACH@N
HC@O
˜CH2
˜C@O
gsodh
[Zn(gsodh-H)]Cl
gsmdh
–
–
118.61–129.39
118.67–130.47
116.45–129.42
118.64–129.44
147.11
147.27
146.71
147.25
155.26, 156.32
156.42, 157.27
156.30, 157.31
157.41, 158.35
162.67, 162.88
162.78, 163.00
162.51, 162.92
162.67, 163.03
168.72
168.89
168.92
169.06
41.03
41.05
[Zn(gsmdh-H)]Cl
s = Singlet, d = doublet, m = multiplet.
Fig. 2. Representative structures of the metal complexes.
ANH protons as singlet at 7.57 (1H) and 7.61 (1H), two ACH@N as
singlet at 8.27 (1H) and doublet at 8.37 (1H, 7.2 Hz), ArAOH as sin-
glet at 9.99 (1H) and CH@O proton as singlet at 11.05 (1H) ppm.
The downfield shift of two ANH protons from 7.57, 7.61 to 7.63,
7.65 ppm in its Zn(II) complex suggests the coordination of the
two ACH@N nitrogen to the metal ion. There is minor shift in
the position of signals of ACH2 and aromatic protons. The absence
of the resonance signal due to ArAOH proton in Zn(II) complexes of
both the ligands provides an evidence for the deprotonation of the
phenolic OH group of ligand during complexation as inferred from
IR spectral data also. The disappearance of ANH, ACH@N and
ArAOH protons in both the ligands in their D2O exchanged 1H
NMR spectra confirm their assignments.
In Zn(II) complexes of gsodh and gsmdh ligands, the bonding
through phenolic oxygen has been inferred from the deshielding
observed in ˜CAOꢁ carbon as compared with the ligands. The
bonding of the ligands in their Zn(II) complexes through two azo-
methine nitrogen and HC@O group of glyoxal is indicated by the
deshielding observed in ACH@N and HC@O carbon (Table 1).
Fig. 3. DFT optimized structures of ligands.
appearance of a new band between 1266 and 1273 cmꢁ1, assigned
as
(CAO)ꢁ.
A broad band,
3.3. IR spectra
m
m
(C@O) is observed at 1665 cmꢁ1 in gsodh and at
The bonding of the ligands to the metal ions has been judged by
a careful comparison of the infrared spectra of the metal com-
plexes with those of the free ligand. A few significant bands have
been selected to observe the effect on group vibrations of ligands
in the complexes. The ligands show broad bands centred at 3449
and 3207 cmꢁ1 for gsodh and at 3448 and 3209 cmꢁ1 for gsmdh
1668 cmꢁ1 in gsmdh due to presence of three ˜C@O groups at the
same position in the ligands. The metal complexes show two
m
(C@O) ꢁb1ands in the ranges 1662–1666 cmꢁ1 and 1645–
1650 cm in gsodh complexes, and 1667–1669 cmꢁ1 and 1647–
1651 cmꢁ1 in gsmdh complexes. The first band is broad and occur
nearly at the same wave number as in parent ligands, suggesting
non-involvement of the two amide ˜C@O groups in bonding.
due to
(NH) either occur at the same frequency as in parent ligands or
shifted very slightly from their position suggesting no participation
of the ˜NH group in bonding [33]. The absence of (OH) bands in
m(OH) and m(NH), respectively. In the metal complexes,
m
However, the other
m(C@O) band due to ˜C@O group of glyoxal
is considerably shifted to lower frequency by 15–20 cmꢁ1 and
17–21 cmꢁ1 in gsodh and gsmdh complexes, respectively, indicat-
ing the participation of glyoxal ˜C@O in bonding with metal [15].
m
all the metal complexes indicates deprotonation of the phenolic
proton during complexation. This is further confirmed by the

76
V.P. Singh et al. / Journal of Molecular Structure 1058 (2014) 71–78
Fig. 4. DFT optimized structures of Ni(II) complexes.
Table 2
TDDFT calculations of electronic spectral transitions for Ni(II) complexes.
S.N.
Experimental band maxima (nm)
Calculated band maxima (Oscillator strength)
Transition
Assignments
[Ni(gsodh-H)Cl]
d2z (1A1g) ? dxy (1B2g
d–d transition
)
1
2
3
839
866
HOMO ? LUMO
(f = 0.0081)
456
d2z ð A1gÞ ! dx2ꢁy2 (1B1g
)
)
1
480
369
HOMO ? LUMO+1
HOMO ? LUMO+2
(f = 0.0421)
354
d–d transition
dz2
!
pꢂ (ligand)
M ? L charge transfer
(f = 1.0737)
[Ni(gsmdh-H)Cl]
d2z (1A1g) ? dxy (1B2g
d–d transition
)
1
2
3
820
798
HOMO ? LUMO
(f = 0.0169)
507
d2z ð A1gÞ ! dx2ꢁy2 (1B1g
1
480
358
HOMO ? LUMO+1
HOMO ? LUMO+2
(f = 0.0476)
391
d–d transition
dz2
!
pꢂ (ligand)
M ? L charge transfer
(f = 0.3690)
The
m
(C@N) bands observed at 1628 cmꢁ1 and 1630 cmꢁ1 in the
NiAN(3) = 2.035 NiAO(1) = 1.793, NiAO(4) = 1.952, and for
[Ni(gsmdh-H)]Cl, NiAN(1) = 1.958, NiAN(3) = 2.008 and
spectra of gsodh and gsmdh, respectively, shifted to lower fre-
quency by 12–18 cmꢁ1 and 13–17 cmꢁ1 in their metal complexes
suggesting coordination through the two azomethine groups
NiAO(1) = 2.007, NiAO(4) = 2.086. The bond angles for [Ni(gsodh-
H)]Cl are O(4)ANiAO(1) (87.7)°, N(3)ANiAN(1) (99.7)°,
O(4)ANiAN(3) (80.9)° and O(1)ANiAN(1) (93.1)°, and for
[Ni(gsmdh-H)]Cl, O(4)ANiAO(1) (85.7)°, N(3)ANiAN(1) (109.7)°,
O(4)ANiAN(3) (81.6)° and O(1)ANiAN(1) (98.9)°. The optimized
structure of the complexes suggests slightly distorted square pla-
nar geometry around the metal centre and the calculated bond
length and bond angles are in accordance with the other reported
square-planner nickel(II) complexes of N, O donar ligands [35–37].
The minor discrepancies between the calculated and experimental
geometries of Ni(II) complexes are reasonable, because the calculated
geometry refers to an isolated molecule while the experimental
geometry is for the molecule in a lattice in which intermolecular
interactions play a significant role [38,39]. In addition, the ˜C@NA
bond lengths, C(7)AN(1) = 1.324 and C(10)AN(3) = 1.311, and
˜C@O bond lengths, C(11)AO(4) = 1.243 and C(1)AO(1) = 1.912
in [Ni(gsodh-H)]Cl complex are over estimated by 0.019, 0.006
and 0.013, 0.002 Å, respectively, compared to the free gsodh ligand.
Similarly for the [Ni(gsmdh-H)]Cl complex, ˜C@NA bond lengths,
C(7)AN(1) = 1.323 and C(11)AN(3) = 1.321, and ˜C@O bond
[34]. A weak band due to m
(NAN) observed at 970 cmꢁ1 in gsodh
and 966 cmꢁ1 in gsmdh shifts to higher frequency by 33–
36 cmꢁ1 and 35–37 cmꢁ1, respectively in their complexes indicat-
ing the coordination of one of the nitrogen atom of the NAN group
with metal. All the metal complexes also show weak bands in the
ranges 522–532 cmꢁ1 and 468–480 cmꢁ1, which may be tenta-
tively assigned to m(MAO) and m(MAN), respectively. On the basis
of above discussion, general structures for the metal complexes are
proposed (Fig. 2).
3.4. Theoretical calculations (DFT)
3.4.1. Geometry optimization of ligands and their Ni(II) complexes
DFT calculation was carried out using three-dimensional
ChemDraw structural coordinates of ligands and their Ni(II) com-
plexes to fully optimize the ground state structure in the gaseous
phase. The optimized structures are shown in Fig. 3 and 4. The
optimized bond lengths for [Ni(gsodh-H)]Cl are NiAN(1) = 1.867,

V.P. Singh et al. / Journal of Molecular Structure 1058 (2014) 71–78
77
3.4.2. Theoretical studies (TD-DFT) on electronic spectra
In order to get a deeper understanding of the electronic transi-
tions occurring in the two representative Ni(II) complexes, TD-DFT
calculations in the gaseous phase at the B3LYP level have been per-
formed. A comparison of the experimental and calculated transi-
tions of Ni(II) complexes in the ground state, their oscillator
strengths and band assignments are listed in Table 2. The theoret-
ical assignments of the transitions (from TDDFT calculation) to the
experimentally observed bands are based on the criteria of energy
and oscillator strength of the calculated transitions. In the descrip-
tion of the electronic transitions, only the main components of the
molecular orbitals are taken into consideration (Figs. 5 and 6). Low
energy transitions theoretically found at 866 nm (f = 0.0081) for
[Ni(gsodh-H)]Cl and 798 nm (f = 0.0169) for [Ni(gsmdh-H)]Cl are
contributed mainly by transition from HOMO (dz2 (1A1g)) to LUMO
(dxy (1B2g)), and at 456 nm (f = 0.0421) for [Ni(gsodh] and 507 nm
(f = 0.0476) for [Ni(gsmdh-H)]Cl are contributed mainly by transi-
tion from HOMO (dz2 (1A1g)) to LUMO+1 (dx2ꢁy2 (1B1g)). The above
results suggest that both the low energy transitions are d–d tran-
sitions. However, the high energy transition theoretically calcu-
lated at 354 nm (f = 1.0737) for [Ni(gsodh] and 391 nm
(f = 0.3690) for [Ni(gsmdh] are metal to ligand charge-transfer
(MLCT) due to transition from HOMO (dz2 (Ni)) to LUMO+2 (pꢂ
(ligand)).
Fig. 5. Diagram showing the electronic transitions in [Ni(gsodh-H)]Cl complex
(orbital contour value = 0.03).
3.4.3. Theoretical studies on IR spectra
The IR spectra of gsodh and gsmdh ligands, and their nickel(II)
complexes were recorded in the range 4000–400 cmꢁ1 along with
the calculated frequencies obtained using the 6-31Gꢂꢂ basis set at
the B3LYP level (Table 3). The experimental results were inter-
preted and rationalized in light of the theoretical investigations
based upon the DFT calculations. The correlations between calcu-
lated and experimental spectral data are based on peak intensities
and peak frequencies (cmꢁ1) for IR spectra. Apart from some minor
deviations in theoretical group frequencies from the experimental
(1–20 cmꢁ1), the theoretical–experimental agreement is satisfac-
tory. Little deviations are due to the negligence of anharmonicity
in B3LYP method [40] and average error in frequencies calculated
with B3LYP method was reported to be of the order of 40–
50 cmꢁ1 [41].
4. Conclusions
The present paper includes the synthesis of two unsymmetrical
tetra-dentate Schiff bases, glyoxal-salicylaldehyde oxalic acid dihy-
drazone and malonic acid dihydrazone. The Co(II), Ni(II), Cu(II) and
Zn(II) complexes of above ligands have been synthesized by two
methods, first by complexation with preformed ligands and second
by template reaction. The IR and NMR spectra show that both the
ligands bond with the metal ion through ˜C@O, a phenolate group
and two ˜C@N groups. Magnetic moment, electronic and ESR spec-
tral studies indicate square planar environment in solid state for
Fig. 6. Diagram showing the electronic transitions in [Ni(gsmdh-H)]Cl complex
(orbital contour value = 0.03).
lengths, C(12)AO(4) = 1.903 and C(1)AO(1) = 1.980 are over esti-
mated by 0.011, 0.009, and 0.0116, 0.036 Å, respectively, compared
to free gsmdh ligand.
Table 3
Comparison of experimental and theoretical vibrational frequencies of the ligands and their Ni(II) complexes.
Band assign.
gsodh
Exptl.
gsmdh
Exptl.
[Ni(gsodh-H)]Cl
[Ni(gsmdh-H)]Cl
Calcd.
Intensity
Calcd.
Intensity
Exptl.
Calcd.
Intensity
Exptl.
Calcd.
Intensity
m
m
m
m
m
m
m
m
(OH)
(NH)
3449
3207
1665
1628
–
970
–
–
3450
3199
1673
1623
–
977
–
–
46.19
50.13
31.66
35.55
–
21.50
–
–
3448
3209
1668
1630
–
966
–
–
3440
3200
1674
1629
–
977
–
–
59.61
60.23
40.59
48.39
–
18.44
–
–
–
–
–
–
–
–
3208
1664, 1647
1615
1266
1003
532
3209
1676, 1655
1595
1235
1004
541
35.76
416.91, 95.67
31.76
30.98
67.98
13.67
10.98
3208
1667, 1647
1617
1271
1003
534
3212
1660, 1630
1608
1270
1009
555
65.76
109.89, 40.00
32.88
30.98
60.67
28.60
12.56
(C@O)
(C@N)
(CAO)ꢁ
(NAN)
(MAO)
(MAN)
471
494
474
490

78
V.P. Singh et al. / Journal of Molecular Structure 1058 (2014) 71–78
J. Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega, G.A.
Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa,
M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene, X. Li, J.E. Knox,
H.P. Hratchian, J.B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R.E.
Stratmann, O. Yazyev, A.J. Austin, R. Cammi, C. Pomelli, J.W. Ochterski, P.Y.
Ayala, K. Morokuma, G.A. Voth, P. Salvador, J.J. Dannenberg, V.G. Zakrzewski, S.
Dapprich, A.D. Daniels, M.C. Strain, O. Farkas, D.K. Malick, A.D. Rabuck, K.
Raghavachari, J.B. Foresman, J.V. Ortiz, Q. Cui, A.G. Baboul, S. Clifford, J.
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[19] A.D. Becke, J. Chem. Phys. 98 (1993) 5648–5652.
Ni(II), Cu(II) complexes and tetrahedral for Co(II) complex, which
changes to octahedral in DMSO solution. Theoretical calculations
on ligands and their nickel(II) complexes suggest agreement be-
tween theoretical analyses of electronic structure with the experi-
mental one. TD-DFT calculations indicate that the high-energy
absorption bands in UV–Vis spectra are charge transfer transitions
(MLCT) and low energy transitions are d–d transitions.
Acknowledgements
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ogy, Mumbai, for recording ESR spectra. The authors V.P.S and
D.P.S. are grateful to UGC, New Delhi and CSIR, New Delhi, respec-
tively, for financial assistance.
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