Nickel(II) and copper(II) complexes of Schiff base ligands containing N4O2 and N4S2 donors with pyrrole terminal binding groups: Synthesis, characterization, X-ray structures, DFT and electrochemical studies

Article abstract of DOI:10.1016/j.ica.2010.08.019

A series of hexadentate ligands, H₂L^m (m = 1-4), [1H-pyrrol-2-ylmethylene]{2-[2-(2-{[1H-pyrrol-2-ylmethylene]amino}phenoxy) ethoxy]phenyl}amine (H₂L¹), [1H-pyrrol-2-ylmethylene]{2- [4-(2-{[1H-pyrrol-2-ylmethylene]amino}phenoxy)butoxy]phenyl}amine (H ₂L²), [1H-pyrrol-2-ylmethylene][2-({2-[(2-{[1H-pyrrol-2- ylmethylene]amino}phenyl)thio]ethyl}thio)phenyl]amine (H₂L ³) and [1H-pyrrol-2-ylmethylene][2-({4-[(2-{[1H-pyrrol-2-lmethylene] amino}phenyl)thio]butyl}thio) phenyl]amine (H₂L⁴) were prepared by condensation reaction of pyrrol-2-carboxaldehyde with {2-[2-(2-aminophenoxy)ethoxy]phenyl}amine, {2-[4-(2-aminophenoxy)butoxy]phenyl} amine, [2-({2-[(2-aminophenyl)thio]ethyl}thio)phenyl]amine and [2-({4-[(2-aminophenyl)thio]butyl}thio)phenyl]amine respectively. Reaction of these ligands with nickel(II) and copper(II) acetate gave complexes of the form ML^m (m = 1-4), and the synthesized ligands and their complexes have been characterized by a variety of physico-chemical techniques. The solid and solution states investigations show that the complexes are neutral. The molecular structures of NiL³ and CuL², which have been determined by single crystal X-ray diffraction, indicate that the NiL ³ complex has a distorted octahedral coordination environment around the metal while the CuL² complex has a seesaw coordination geometry. DFT calculations were used to analyse the electronic structure and simulation of the electronic absorption spectrum of the CuL² complex using TDDFT gives results that are consistent with the measured spectroscopic behavior of the complex. Cyclic voltammetry indicates that all copper complexes are electrochemically inactive but the nickel complexes with softer thioethers are more easily oxidized than their oxygen analogs.

Full text of DOI:10.1016/j.ica.2010.08.019

Inorganica Chimica Acta 363 (2010) 4080–4087
Contents lists available at ScienceDirect
Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Nickel(II) and copper(II) complexes of Schiff base ligands containing N4O2 and
N4S2 donors with pyrrole terminal binding groups: Synthesis, characterization,
X-ray structures, DFT and electrochemical studies
b
a
c
a
Ali Akbar Khandar a, , Christine Cardin , Seyed Abolfazl Hosseini-Yazdi , John McGrady , Marjan Abedi ,
⇑
Seyed Amir Zarei a, Yu Gan b
a Department of Inorganic Chemistry, Faculty of Chemistry, University of Tabriz, Tabriz 51666-14766, Iran
b Department of Chemistry, University of Reading, Whiteknights, Reading RG6 6AD, UK
c Department of Chemistry, University of Oxford, OX1 3QR, UK
a r t i c l e i n f o
a b s t r a c t
A series of hexadentate ligands, H2Lm (m = 1ꢀ4), [1H-pyrrol-2-ylmethylene]{2-[2-(2-{[1H-pyrrol-2-yl
methylene]amino}phenoxy)ethoxy]phenyl}amine (H2L1), [1H-pyrrol-2-ylmethylene]{2-[4-(2-{[1H-pyr-
rol-2-ylmethylene]amino}phenoxy)butoxy]phenyl} amine (H2L2), [1H-pyrrol-2-ylmethylene][2-({2-[(2-
{[1H-pyrrol-2-ylmethylene]amino}phenyl)thio]ethyl}thio)phenyl]amine (H2L3) and [1H-pyrrol-2-yl
methylene][2-({4-[(2-{[1H-pyrrol-2-lmethylene]amino}phenyl)thio]butyl}thio) phenyl]amine (H2L4)
were prepared by condensation reaction of pyrrol-2-carboxaldehyde with {2-[2-(2-aminophenoxy)eth-
oxy]phenyl}amine, {2-[4-(2-aminophenoxy)butoxy] phenyl}amine, [2-({2-[(2-aminophenyl)thio]ethyl}
thio)phenyl]amine and [2-({4-[(2-aminophenyl)thio]butyl}thio)phenyl]amine respectively. Reaction of
these ligands with nickel(II) and copper(II) acetate gave complexes of the form MLm (m = 1ꢀ4), and the
synthesized ligands and their complexes have been characterized by a variety of physico-chemical tech-
niques. The solid and solution states investigations show that the complexes are neutral. The molecular
structures of NiL3 and CuL2, which have been determined by single crystal X-ray diffraction, indicate that
the NiL3 complex has a distorted octahedral coordination environment around the metal while the CuL2
complex has a seesaw coordination geometry. DFT calculations were used to analyse the electronic struc-
ture and simulation of the electronic absorption spectrum of the CuL2 complex using TDDFT gives results
that are consistent with the measured spectroscopic behavior of the complex. Cyclic voltammetry indi-
cates that all copper complexes are electrochemically inactive but the nickel complexes with softer thio-
ethers are more easily oxidized than their oxygen analogs.
Article history:
Received 7 July 2010
Received in revised form 4 August 2010
Accepted 11 August 2010
Available online 19 August 2010
Keywords:
Copper(II) complexes
Nickel(II) complexes
X-ray structure
Pyrrol-2-carboxaldehyde
Cyclic voltammetry
DFT calculations
Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction
been extensively studied in this context, relatively little is known
about the Schiff base ligands, especially those derived from pyr-
Schiff base ligands and their complexes have been studied
extensively with the aim of shedding light on various aspects of
catalytic activity [1–5], magnetic, spectroscopic and anticancer
properties [6–11] as well as the role of metal ions in biological sys-
tems [12–14]. The active sites of many metalloenzymes contain
nickel and copper, and the transition metals play important roles
in controlling the catalytic function [15–18]. The relationship be-
tween the structure and the chelate ring size of multidentate li-
gands in coordination compounds is a subject of considerable
importance because the coordination chemistry of the metals is af-
fected by the both the type of donor atom and steric requirements
[19]. Although salicylidimine ligands and their complexes have
role-2-carboxaldehyde that can potentially act as hexadentate do-
nors [20–33]. Previously, we have considered the strain effects in
the copper and nickel complexes of two such Schiff base ligands
(Scheme 1), each of which possesses an N2O4 donor set [34]. In
the case where two methylene groups separate the ether oxygen
atoms the coordination geometry is distorted square planar geom-
etry but the more flexible analog with four methylene groups be-
tween the ether oxygen atoms acts as a hexadentate donor
giving a distorted octahedral geometry. Herein, we report the syn-
thesis and characterization of copper(II) and nickel(II) complexes
of the new ligands H2L1, H2L2, H2L3 and H2L4 (Scheme 2) which
are structurally related to those in Scheme 1 [34] but with different
donor sets of atoms, N4O2 and N4S2, and with pyrrole terminal moi-
eties derived from pyrrole-2-carboxaldehyde and {2-[2-(2-amino-
⇑
Corresponding author. Tel.: +98 (411) 3346494; fax: +98 (411) 3340191.
E-mail addresses: khandar@tabrizu.ac.ir, akhandar@yahoo.com (A.A. Khandar).
phenoxy)ethoxy]phenyl}amine
(1),
{2-[4-(2-aminophenoxy)
0020-1693/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2010.08.019

A.A. Khandar et al. / Inorganica Chimica Acta 363 (2010) 4080–4087
4081
2.3. X-ray crystallography
(CH2)n
Relevant data about the collections and structure solutions are
summarized in Table 1. Crystals of NiL3 and CuL2 for X-ray crystal-
lography were grown by slow evaporation of acetonitrile/chloro-
form and ethanol/dichloromethane (1:1) solution, respectively.
The data sets were collected on a Bruker SMART CCD detector for
NiL3 and on an Oxford Diffraction Gemini-S-Ultra diffractometer
O
N
O
N
N2Ph
HO
Ph2N
OH
for CuL2 using
u and x
scans. For the NiL3 complex a fine-focus
sealed tube was used as the radiation source. Data reduction for
NiL3 was carried out using the program SAINT [35]. An absorption
correction for NiL3 was applied using SADABS [36]. The CuL2 complex
crystallized as needle-shaped crystals that were very thin and
therefore weakly diffracting. The data were therefore collected at
low temperature (150 K) at station 9.8, Daresbury SRS (UK) using
synchrotron radiation source at a wavelength of 0.68829 Å. The
structures were determined by direct methods and were refined
by full-matrix least-squares procedures using the SHELXTL [37]. Scat-
tering factors were taken from International Tables for crystallog-
raphy [38].
n = 2: H2L, n = 4: H2L'
Scheme 1. Structure representation of Schiff base ligands H2L and H2L0.
butoxy] phenyl}amine (2), [2-({2-[(2-aminophenyl)thio]ethyl}
thio)phenyl]amine (3) and [2-({4-[(2-aminophenyl)thio]butyl}
thio)phenyl]amine (4), respectively. We report the crystal struc-
tures of NiL3 and CuL2 and discuss the effects of different donor
atom type, chelate size and the nature of the terminal binding
groups on the coordination chemistry of the ligands.
2.4. Syntheses
2. Experimental
2.4.1. Diamines (1–4)
2.1. Materials
{2-[2-(2-Aminophenoxy)ethoxy]phenyl}amine (1) and {2-[4-
(2-aminophenoxy)butoxy] phenyl}amine (2) were synthesized
according to the published procedure [39].
The solvents and reagents used in these studies were obtained
from commercial sources and were used as received.
[2-({2-[(2-Aminophenyl)thio]ethyl}thio)phenyl]amine (3) and
[2-({4-[(2-aminophenyl)thio]butyl}thio)phenyl]amine (4) were
prepared by a modification of a literature method [40]. In each case
2-aminothiophenol (60 mmol) was dissolved in ethanol (15 cm3),
then 6 cm3 of NaOH aqueous solution (40%) was added. The yel-
low-brown mixture was stirred and heated at reflux for 20 min.
1,2-Dibromoethane (5.64 g, 30 mmol) and 1,4-dibromobutane
(6.48 g, 30 mmol) were added dropwise to the solution. The solu-
tion was refluxed for 2 h. In the case of (3) the resulting solution
was cooled in water and the precipitate was filtered and recrystal-
lized from water/ethanol to give the white product. Yield: 2.49 g,
30%. M.p. 77–78 °C. Selected FT-IR data (cmꢀ1, KBr): 3290s and
3355s (NH), 3056w (CHarom), 2925w (CHaliph), 1617s and 1476s
(C@Carom). In the case of (4) the resulting solution was extracted
with water/chloroform. This product is liquid in room temperature.
Yield: 2.74 g, 70%. Selected FT-IR data (cmꢀ1, KBr): 3453s and
3353s (NH), 3016w and 3056w (CHarom), 2923w (CHaliph), 1611s
and 1478s (C@Carom).
2.2. Physical measurements
Elemental analyses were carried out using an Elemental Vario
EL III instrument. 1H NMR and 13C NMR spectra were recorded
at 400.13 and 100 MHz on a Bruker Avance 400 spectrometer
in CDCl3 using Me4Si as the internal reference. Cyclic voltam-
metric measurements were performed using an Auto lab poten-
tiostat PGSTAT 302 ECO CHEMIE. In all electrochemical studies a
three-electrode system was used, consisting of a glassy carbon
working electrode, a platinum wire auxiliary electrode, and Ag/
AgCl as the reference electrode. All of the electrochemical exper-
iments were carried out under an argon atmosphere at room
temperature using dimethyl formamide solution of complexes
containing 0.1 M lithium perchlorate as the supporting electro-
lyte. IR spectra were recorded on a Bruker Tensor 27 FT-IR spec-
trometer as KBr pellets. Electron impact (70 eV) mass spectrum
was recorded on a Shimadzu, QP1100EX spectrometer. Electronic
spectra were recorded on a Shimadzu, UV-1650 PC spectropho-
tometer from solution in dichloromethane. Conductivity data
were measured in DMF on a Metrohem 712 conductometer
instrument.
2.4.2. Ligands
2.4.2.1. [(1Z)-1H-Pyrrol-2-ylmethylene]{2-[2-(2-{[(1Z)-1H-pyrrol-2-
ylmethylene]amino} phenoxy)ethoxy]phenyl}amine (H2L1). A solu-
tion of pyrrole-2-carboxaldehyde (0.9510 g, 10 mmol) in ethanol
(CH2)n
(CH2)n
X
N
X
N
O
H
X
X
N
2
+
H
NH2
H2N
HN
NH
X = O, S
n = 2, 4
X = O, n = 2, H2L1
X = O, n = 4, H2L2
X = S, n = 2, H2L3
X = S, n = 4, H2L4
Scheme 2. Synthesis and structure representation of Schiff base ligands H2L1, H2L2, H2L3 and H2L4.

4082
A.A. Khandar et al. / Inorganica Chimica Acta 363 (2010) 4080–4087
Table 1
68.25 (C, s, alkyl), 108.65, 113.29, 115.80, 118.74, 120.42, 123.24,
125.17, 129.58, 141.33, 150.01, 151.51 (s, 22C, arom, imine).
Crystallographic data for NiL3 and CuL2.
UV–Vis [kmax/nm (e
/Mꢀ1 cmꢀ1)]: 334 (28 900), 299 (31 900), 227
NiL3
CuL2
(20 240) in CH2Cl2.
Formula
Formula weight
T (K)
C24H20N4NiS2
487.27
293(2)
C104H96Cu4ꢁN16O8
1952.13
150(2)
2.4.2.3. [(1Z)-1H-pyrrol-2-ylmethylene][2-({2-[(2-{[(1Z)-1H-pyrrol-
2-ylmethylene] amino}phenyl)thio]ethyl}thio)phenyl]amine (H2L3). A
solution of pyrrole-2-carboxaldehyde (0.9510 g, 10 mmol) in etha-
nol (10 cm3) was added dropwise to a solution of (3) (1.3821 g,
5 mmol) in warm ethanol (15 cm3) in the dark. The solution
was stirred and heated under reflux for 24 h. The solution was
evaporated at room temperature. The resulting yellow oil was
dissolved in the minimum amount of CH2Cl2 and was precipitated
by adding of n-hexane. The product was collected by filtration and
recrystallized from CH2Cl2 to n-hexane. Yield: 0.6459 g, 30%. M.p.
96–97 °C. Selected FT-IR data (cmꢀ1, KBr): 3208m (NHpyrrole),
2975w (CHalyph), 1621s (C@N), 1569s (C@Carom). Anal. Calc. for
Crystal color
Crystal size (mm)
Crystal system
Space group
a (Å)
orange
brown
0.55 ꢂ 0.45 ꢂ 0.25
trigonal
P3(2)21
8.9429(3)
8.9429(3)
23.2577(19)
90
90
120
1610.85(15)
3
1.507
0.71073
1.118
18 631/3141
(Rint = 0.0280)
756
0.1 ꢂ 0.02 ꢂ 0.02
monoclinic
P2(1)/c
25.037(3)
13.8374(18)
28.095(4)
90
108.062(2)
90
9254(2)
b (Å)
c (Å)
a
(°)
b (°)
c
(°)
V (Å3)
Z
4
Dcalc (Mg/m3)
k (Å)
1.401
0.68829
0.975
109 336/30 107
(Rint = 0.0715)
4048
l
(mmꢀ1
)
C24H22N4S2: C, 66.94; H, 5.15; N, 13.01. Found: C, 67.12; H, 5.14;
N, 12.91%. 1H NMR data (400.13 MHz, CDCl3, Me4Si): d 3.1 (4H, s,
Reflections collected/
unique
F(0 0 0)
h range for data
collection (°)
Index ranges
alkyl), 6.27–7.32 (14H, m, arom), 8.15 (2H, s, imine), 10.22 (2H, s,
br, H-pyrrole). UV–Vis [kmax/nm (e
/Mꢀ1 cmꢀ1)]: 310 (44 820), 280
2.63–30.01
1.52–31.52
(36 300), 257 (27 460), 232 (27 740) in CH2Cl2. Mass spectral parent
ion: m/z 431.
ꢀ12 6 h 6 12,
ꢀ12 6 k 6 12,
ꢀ31 6 l 6 32
3141/0/142
ꢀ35 6 h 6 35,
ꢀ20 6 k 6 20,
ꢀ40 6 l 6 40
30 107/0/1304
2.4.2.4.
[(1Z)-1H-pyrrol-2-ylmethylene][2-({4-[(2-{[(1Z)-1H-pyrrol-2-
Data/restraints/
parameters
ylmethylene] amino}phenyl)thio]butyl}thio)phenyl]amine (H2L4). This
ligand was prepared by a similar method to that of H2L3 using
pyrrole-2-carboxaldehyde (0.9510 g, 10 mmol) and (4) (1.5224 g,
5 mmol). In this case, a brown oil was obtained that was triturated
with diethylether to give the product as a brown solid which was
separated by filtration. Yield: 0.9861 g, 43%. M.p. 157–158 °C. Se-
lected FT-IR data (cmꢀ1, KBr): 3272m (NHpyrrole), 3010w (CHarom),
2928w (CHaliph), 1615s (C@N), 1566s (C@Carom). Anal. Calc. for
Goodness-of-fit (GOF)
1.008
1.002
on F2
Final R indices (I > 2
R indices (all data)
r
(I)) R1 = 0.0299,
R1 = 0.0470,
wR2 = 0.0980
R1 = 0.0962,
wR2 = 0.1146
0.607 and ꢀ0.783
wR2 = 0.0773
R1 = 0.0368,
wR2 = 0.0827
0.846 and ꢀ0.161
Largest difference in
peak and hole (e Åꢀ3
)
R1 = [
R
||Fo| ꢀ |Fc||]/
R
|Fo| (based on F), wR2 = {[
R
w (|F2o ꢀ F2c |)2]/[
R
w(Fo2)2]}1/2 (based
C
26H26N4S2: C, 68.09; H, 5.71; N, 12.22. Found: C, 68.23; H, 5.68;
on F2).
N, 12.29%. 1H NMR data (400.13 MHz, CDCl3, Me4Si): d 1.8–1.84
(4H, q, alkyl), 2.91–2.94 (4H, t, alkyl), 6.24–7.30 (14H, m, arom),
8.16 (2H, s, imine), 10.10 (2H, s, br, H-pyrrole). UV–Vis [kmax/nm
(10 cm3) was added dropwise to a solution of (1) (1.2214 g,
5 mmol) in warm ethanol (30 cm3) in the dark. The solution was
refluxed for 24 h. Then the solution was cooled and evaporated
at room temperature, the product precipitated as a cream solid
which was recrystallized from ethanol. Yield: 1.0011 g, 50.25%.
M.p. 74–75 °C. Selected FT-IR data (cmꢀ1, KBr): 3266m (NHpyrrole),
2930w (CHaliph), 1617s (C@N), 1583s and 1495s (C@Carom). Anal.
Calc. for C24H22N4O2: C, 72.34; H, 5.57; N, 14.06. Found: C, 72.48;
H, 5.40; N, 14.20%. 1H NMR data (400.13 MHz, CDCl3, Me4Si): d
4.39 (4H, s, alkyl), 6.23–7.17 (14H, m, arom), 8.25 (2H, s, imine).
13C NMR data (100 MHz, CDCl3, Me4Si): d 65.68, 65.74, 108.97,
110.92, 111.006, 116.86, 117.96, 120.64, 123.32, 125.34, 129.85,
(e
/Mꢀ1 cmꢀ1)]: 305 (48 800), 277 (44 020), 254 (32 640), 236
(30 300) in CH2Cl2.
2.4.3. Copper(II) and nickel(II) complexes
All nickel(II) and copper(II) complexes of the ligands were pre-
pared by addition of a solution of nickel acetate tetrahydrate
(0.1245 g, 0.5 mmol) or copper acetate monohydrate (0.0998 g,
0.5 mmol) in ethanol (25 cm3) to a solution of H2L1 and H2L2 and
to a suspension of H2L3 and H2L4 (0.5 mmol) in absolute ethanol
(25 cm3). In each case the precipitate was filtered and recrystal-
lized from CH3CH2OH/CH2Cl2 for NiL1, NiL2, NiL3, CuL1, CuL2 and
CuL3 and from THF/(C2H5)2O for NiL4 and CuL4.
151.12, 151.12. UV–Vis [kmax/nm (
302 (28 280), 242 (23 820) in CH2Cl2.
e
/Mꢀ1 cm-1)]: 333 (36 120),
2.4.3.1. NiL1. Reaction time: 4 h. Color: brown. Yield: 0.1707 g, 75%.
M.p. > 290 °C (dec). Selected FT-IR data (cmꢀ1
,
KBr): 3068w
(CHarom), 2937w (CHaliph), 1558s (C@N). Anal. Calc. for C24H20
NiN4O2: C, 63.34; H, 4.43; N, 12.31. Found: C, 63.38; H, 4.30; N,
12.25%. UV–Vis [kmax/nm
/Mꢀ1 cmꢀ1)]: 410 (46 020), 278
(28 160), 264 (28 980), 227 (20 460), 1004 (18) in CH2Cl2.
Km = 1.6
ꢀ1 cm2 molꢀ1 in DMF.
2.4.2.2. [(1Z)-1H-Pyrrol-2-ylmethylene]{2-[4-(2-{[(1Z)-1H-pyrrol-2-
ylmethylene] amino}phenoxy)butoxy]phenyl}amine (H2L2). This
ligand was prepared by a similar method to H2L1 using pyrrole-
2-carboxaldehyde (0.9510 g, 10 mmol) and (2) (1.3617 g, 5 mmol),
but the solution was refluxed for 12 h. The resulting yellow oil was
triturated with diethyl ether to give a cream solid which was
collected by filtration. Yield: 0.4905 g, 23%. M.p. 148–149 °C.
-
(e
X
Selected FT-IR data (cmꢀ1
,
KBr): 3222m (NHpyrrole), 3077w
2.4.3.2. NiL2. Reaction time: 4 h. Color: red-brown. Yield: 0.2150 g,
89%. M.p. > 300 °C (dec). Selected FT-IR data (cmꢀ1, KBr): 3067w
(CHarom), 2932w (CHaliph), 1557s (C@N). Anal. Calc. for C26H24Ni-
N4O2: C, 64.63; H, 5.01; N, 11.60. Found: C, 64.71; H, 4.90; N,
(CHarom), 2936w (CHaliph), 1623s (C@N), 1584s and 1493s
(C@Carom). Anal. Calc. for C26H26N4O2: C, 73.22; H, 6.14; N, 13.14.
Found: C, 73.10; H, 6.31; N, 12.98%. 1H NMR data (400.13 MHz,
CDCl3, Me4Si): d 1.94 (4H, s, alkyl), 4.03 (4H, s, alkyl), 6.13–7.15
(14H, m, arom), 8.24 (2H, s, imine), 11.63 (2H, s, br, H-pyrrole).
13C NMR data (100 MHz, CDCl3, Me4Si): d 25.13 (C, s, alkyl),
11.67%. UV–Vis [kmax/nm
(12 900), 256 (16 140), 231 (19 000), 1030 (22) in CH2Cl2.
Km = 1.8
ꢀ1 cm2 molꢀ1 in DMF.
(e
/Mꢀ1 cmꢀ1)]: 406 (42 840), 331
X

A.A. Khandar et al. / Inorganica Chimica Acta 363 (2010) 4080–4087
4083
2.4.3.3. NiL3. Reaction time: 4 h. Color: red. Yield: 0.2046 g, 84%.
M.p. > 310 °C (dec). Selected FT-IR data (cmꢀ1
KBr): 3064w
(CHarom), 2925w (CHaliph), 1540s (C@N). Anal. Calc. for C24H20
NiN4S2: C, 59.16; H, 4.14; N, 11.50. Found: C, 59.22; H, 4.03; N,
11.42%. UV–Vis [kmax/nm
/Mꢀ1 cmꢀ1)]: 439 (51 220), 424
(52 940), 339 (9800), 270 (8500), 231 (27 720), 849 (23) in CH2Cl2.
Km = 1.5
ꢀ1 cm2 molꢀ1 in DMF.
3. Results and discussion
,
-
3.1. Synthesis and characterization
The hexadentate Schiff base ligands H2L1, H2L2, H2L3 and H2L4
have been synthesized according to Scheme 2 and characterized
by IR, elemental analysis, 1H NMR, 13C NMR, UV–Vis spectra and
mass spectroscopy only for the H2L3. The IR and NMR data are in
accordance with the proposed structures. Disappearance of the
C@O and NH2 stretching vibrations, related to aldehyde and dia-
mine functional groups, respectively along with the growth of a
strong band in the region of 1615–1623 cmꢀ1 due to the C@N
bonds, indicate the formation of the Schiff base ligands. The peaks
in the region 8.15–8.25 ppm in the 1H NMR spectra of the ligands
are assigned to the resonance of the azomethine protons (CH@N),
also confirming the formation of the Schiff base ligands.
(e
X
2.4.3.4. NiL4. Reaction time: 4 h. Color: green. Yield: 0.2293 g, 89%.
M.p. > 310 °C (dec). Selected FT-IR data (cmꢀ1
KBr): 3057w
(CHarom), 2925w (CHaliph), 1549s (C@N). Anal. Calc. for C26H24
NiN4S2: C, 60.60; H, 4.69; N, 10.87. Found: C, 60.48; H, 4.82; N,
10.78%. UV–Vis [kmax/nm
/Mꢀ1 cmꢀ1)]: 440 (24 080), 417
(31 040), 271 (8420), 230 (19 500), 873 (23) in CH2Cl2.
Km = 2.7
ꢀ1 cm2 molꢀ1 in DMF.
,
-
(e
X
The reaction of the ligands with copper(II) and nickel(II) acetate
in a 1:1 ratio in ethanol gives CuLm and NiLm complexes. The ele-
mental analyses are consistent with the proposed molecular for-
mula that show the ratio of metal/ligand is 1:1. All complexes in
ca. 10ꢀ3 M solutions in DMF at 25 °C have very low conductance,
indicating that the complexes are all neutral [46]. Thus the ligands
must act as doubly negatively charged anions in complexation to
Cu(II) and Ni(II) ions. This hypothesis is supported by the absence
of pyrrole N–H stretches in the FT-IR spectra of the complexes.
Therefore, two nitrogen atoms of pyrrole groups are deprotonated
prior to complexation. The metals are also bound to ligands
through the azomethine nitrogens, deduced from the observed de-
crease in C@N stretching frequency, 49–65 cmꢀ1 and 59–81 cmꢀ1
upon complexation of ligands to Cu(II) and Ni(II) ions respectively.
On going from CuLm complexes to the NiLm analogs the C@N
stretching frequency decreases by 10, 1, 28 and 10 cmꢀ1, respec-
tively. In Cu(II) complexes the C@N stretching frequencies for
CuL1 and CuL3 and for CuL2 and CuL4 are the same. On the other
hand the C@N frequency difference between CuL1 and CuL2 is the
same as that between CuL3 and CuL4. So it seems that the differ-
ence in each pair depends only on the length of the aliphatic link-
age and the nature of the donor atom (S or O) has relatively little
impact. In the Ni (II) complexes, NiL1 with NiL3 and NiL2 with
NiL4, in contrast, the C@N stretching frequencies are very different,
suggesting that in this case the O or S atoms of the ligands are in
the coordination sphere of the Ni (II) ion. This assumption is sup-
ported by single crystal X-ray structures of CuL2 and NiL3 com-
plexes (vide infra) which show six coordination for Ni but four
coordination for Cu. These results are also consistent with the elec-
tronic spectra of the complexes. UV–Vis spectra of the ligands and
2.4.3.5. CuL1. Reaction time: 2 h. Color: brown. Yield: 0.2070 g, 90%.
M.p. > 260 °C (dec). Selected FT-IR data (cmꢀ1
KBr): 3098w
(CHarom), 2927w (CHaliph), 1568s (C@N). Anal. Calc. for C24H20
CuN4O2: C, 62.67; H, 4.38; N, 12.18. Found: C, 62.54; H, 4.49; N,
12.15%. UV–Vis [kmax/nm
/Mꢀ1 cmꢀ1)]: 381 (38 300), 309
(15 260), 244 (22 380), 232 (22 220), 624 (146) in CH2Cl2.
Km = 3.1
ꢀ1 cm2 molꢀ1 in DMF.
,
-
(e
X
2.4.3.6. CuL2. Reaction time: 2 h. Color: brown. Yield: 0.2123 g, 87%.
M.p. > 270 °C (dec). Selected FT-IR data (cmꢀ1
KBr): 3057w
(CHarom), 2929w (CHaliph), 1558s (C@N). Anal. Calc. for C26H24
CuN4O2: C, 63.99; H, 4.96; N, 11.48. Found: C, 64.05; H, 4.87; N,
11.44%. UV–Vis [kmax/nm
/Mꢀ1 cmꢀ1)]: 378 (41 500), 307
(17 700), 244 (24 540), 232 (25 900), 602 (131) in CH2Cl2.
Km = 2.2
ꢀ1 cm2 molꢀ1 in DMF.
,
-
(e
X
2.4.3.7. CuL3. Reaction time: 2 h. Color: dark green. Yield: 0.1968 g,
80%. M.p. > 160 °C (dec). Selected FT-IR data (cmꢀ1, KBr): 3066w
(CHarom), 2923w (CHaliph), 1568s (C@N). Anal. Calc. for
C
24H20CuN4S2: C, 58.58; H, 4.10; N, 11.38. Found: C, 58.65; H,
3.97; N, 11.29%. UV–Vis [kmax/nm (
/Mꢀ1 cmꢀ1)]: 400 (27 900),
253 (19 840), 232 (23 820), 635 (157) in CH2Cl2. Km
8.8
ꢀ1 cm2 molꢀ1 in DMF.
e
=
X
2.4.3.8. CuL4. Reaction time: 2 h. Color: green. Yield: 0.2263 g, 87%.
M.p. > 150 °C (dec). Selected FT-IR data (cmꢀ1
KBr): 3057w
(CHarom), 2922w (CHaliph), 1559s (C@N). Anal. Calc. for C26H24
CuN4S2: C, 60.04; H, 4.65; N, 10.77. Found: C, 60.12; H, 4.53; N,
10.80%. UV–Vis [kmax/nm
/Mꢀ1 cmꢀ1)]: 381 (28 760), 291
(18 200), 251 (22 200), 231 (24 260), 618 (209) in CH2Cl2.
Km = 2.3
ꢀ1 cm2 molꢀ1 in DMF.
,
their complexes were studied in dichloromethane, at 5 ꢂ 10ꢀ5
M
-
concentration in the 190–500 nm window and 10ꢀ2 M in the
400–1100 nm window. All the ligands, H2Lm, have similar spectral
features in the short wavelength region: the maxima of the bands
(e
*
*
corresponding to the
p ? p and n ? p transition are observed at
X
333, 334, 310 and 305 nm and 302, 299, 280 and 277 nm for H2Lm
(m = 1–4), respectively. These bands show a distinct red shift in
complexation. The electronic spectra for CuLm (m = 1–4) complexes
also show a broad shoulder in the visible region (620–670 nm), as-
signed to the d–d transition (dxz,yz ? dx2ꢀy2 and d2 ? dx2 ) of
2.5. Computational method
The gas phase geometry of the CuL2 complex was optimized in
the doublet state using density functional theory (B3LYP func-
tional) as implemented in GAUSSIAN 98 [41–43]. The calculations
were performed using the 6-311G(d) basis set for the metal cen-
ter, 6-31G(d) for the donor atoms, and 3-21G for all remaining
atoms [44]. No symmetry constraints were applied in the calcula-
tions. The electronic spectrum of the CuL2 complex was calcu-
lated using TDDFT starting from the optimized ground-state
geometry in the gas phase using the same B3LYP functional and
basis sets [45].
ꢀy2
z
Cu(II) ion based on TDFT and DFT calculations. This spectral feature
is typical of the square planar coordination geometry [47,48] re-
vealed by X-ray crystallography for CuL2. The nickel complexes,
NiLm, exhibit a broad absorption band centered in 1004, 1030,
849 and 873 nm for m = 1–4, respectively. These bands can be as-
signed to 3A2g ? 3T2g transition in an octahedral ligand field
[22,32], which is constructed from two nitrogen atoms of the pyr-
role groups, two nitrogens of azomethines and two etheric oxygens
or two thioetheric sulfurs. These transitions correspond to

4084
A.A. Khandar et al. / Inorganica Chimica Acta 363 (2010) 4080–4087
DO = 9960, 9708, 11 779 and 11 455 cmꢀ1, respectively, indicating
ligands with N4O2 donor set have weaker ligand fields than their
analogs with an N4S2 set. The results also indicate that the ligand
field strength decreases when the aliphatic linkage in the ligands
is elongated. In our earlier paper [34] we assumed that the ligand
H2L in Scheme 1 would be unable to form six coordinated com-
plexes with an N2O4 donor set due to the short length of the dim-
ethylene bridge. However, the new results with Ni(II) suggest that
this is not the case. Moreover, ligands similar to those of Schemes 1
and 2 with phenol and naphthol terminal groups with dimethylene
linkage act as a hexadentate ligand type O2N2O2 or O2N2S2 toward
the Ni(II) ion [49], in good agreement with our results. Therefore, it
is obvious that the length of the aliphatic linkage in these ligands is
not always a defining factor in complexation geometry, and the
nature of the metal ion and terminal groups are also important.
3.2. Structural studies
The crystal structure of the NiL3 complex with atomic num-
bering is illustrated in Fig. 1(A). Selected bond distances and
bond angles are listed in Table 2. The molecular structure of
the complex has a distorted octahedral geometry around the
Ni(II) center, as can be judged from the spread in its observed
angles [81.54(6)ꢀ99.41(6)°] and the trans angles [178.60(9)ꢀ
164.41(4)°]. The Ni(II) ion is bound through the N4S2 atoms,
where both the imine nitrogens are disposed trans to each other
with a bond angle of 178.60(9)° and both thioether sulfur atoms
and also both two pyrrole nitrogens occupy the cis coordination
sites with bond angles of 87.36(2)° and 96.09(9)°, respectively.
The Ni–N (imine) (2.0315(13) Å) and Ni–N (pyrrole)
(2.0308(16) Å) bond lengths are comparable to corresponding
Fig. 1. ORTEP drawing with atom labeling (A) and packing structure diagram (B) for NiL3.

A.A. Khandar et al. / Inorganica Chimica Acta 363 (2010) 4080–4087
4085
Table 2
distances reported in the literature [34,50] and the Ni–S
(2.4336(5) Å) bond length is comparable to the mean value of
the corresponding distances in Ref. [30]. The metric parameters
of the NiL3 complex molecule show that a twofold axis bisects
the complex, passing through the C1ꢀC(1A) bond and the Ni
atom. Z = 3 in the P3(2)21 space group reveals that the molecu-
lar symmetry axis is also the symmetry axis of the crystal. Com-
plexation of the H2L3 ligand with Ni results in the formation of
five-membered chelate rings. The crystal packing of NiL3 illus-
trated in Fig. 1(B) shows ligand–ligand interactions between
C11ꢁ ꢁ ꢁH(1A) and C4ꢁ ꢁ ꢁC8 of adjacent molecules with distances
of 2.874 and 3.382 Å, respectively. The aromatic rings stack with
adjacent ones in an offset face-to-face fashion with an interpla-
nar angle of 18.56° and shortest atom–atom distance of 3.57 Å.
The mean centroid–centroid distance of 4.271 Å indicates weak
interactions between phenyl rings.
Selected bond lengths (Å) and bond angles (°) for NiL3.
Bond lengths
Ni(1)–N(1)
Ni(1)–N(1A)
Ni(1)–N(2)
Ni(1)–N(2A)
Ni(1)–S(1)
Ni(1)–S(1A)
N(1)–C(7)
2.0315(13)
2.0315(13)
2.0308(16)
2.0308(16)
2.4336(5)
2.4336(5)
1.399(2)
N(1)–C(8)
N(2)–C(9)
N(2)–C(12)
S(1)–C(1)
S(1)–C(2)
C(2)–C(7)
C(8)–C(9)
1.308(2)
1.376(2)
1.337(2)
1.818(2)
1.7856(18)
1.411(3)
1.403(3)
C(1)–C(1A)
1.504(5)
Bond angles
N(1)–Ni(1)–N(1A)
N(1)–Ni(1)–S(1A)
N(1)–Ni(1)–S(1)
N(2)–Ni(1)–N(1)
N(2)–Ni(1)–N(1A)
N(2)–Ni(1)–S(1A)
N(2)–Ni(1)–S(1)
S(1A)–Ni(1)–S(1)
178.60(9)
95.61(4)
83.37(5)
81.54(6)
99.41(6)
90.22(4)
164.41(4)
87.36(2)
N(1A)–Ni(1)–S(1A)
N(1A)–Ni(1)–S(1)
N(2A)–Ni(1)–N(2)
N(2A)–Ni(1)–N(1)
N(2A)–Ni(1)–N(1A)
N(2A)–Ni(1)–S(1A)
N(2A)–Ni(1)–S(1)
83.37(5)
95.61(4)
96.09(9)
99.41(6)
81.54(6)
164.41(4)
90.22(4)
The structure of CuL2 is given in Fig. 2. Relevant bond distances
and angles are given in Table 3. There are four different molecules
Fig. 2. ORTEP drawing (A) (hydrogen atoms are omitted for clarity) and packing structure diagram along the c crystallographic axis (B) for CuL2 complex.

4086
A.A. Khandar et al. / Inorganica Chimica Acta 363 (2010) 4080–4087
Table 3
Selected bond lengths (Å) and bond angles (°) for CuL2.
Bond lengths
Cu(1)–N(5)
Cu(1)–N(6)
Cu(1)–N(7)
Cu(1)–N(8)
Cu(2)–N(9)
Cu(2)–N(10)
Cu(2)–N(11)
Cu(2)–N(12)
1.9909(18)
2.0207(19)
1.9559(19)
1.9677(19)
1.9437(19)
2.0063(18)
1.9421(19)
1.9998(17)
Cu(3)–N(1)
1.9560(19)
1.9979(18)
1.9548(19)
2.0020(18)
1.9516(19)
1.9980(18)
1.9883(18)
1.9468(19)
Cu(3)–N(2)
Cu(3)–N(3)
Cu(3)–N(4)
Cu(4)–N(13)
Cu(4)–N(14)
Cu(4)–N(15)
Cu(4)–N(16)
Bond angles
N(7)–Cu(1)–N(8)
N(7)–Cu(1)–N(5)
N(8)–Cu(1)–N(5)
N(7)–Cu(1)–N(6)
N(8)–Cu(1)–N(6)
N(5)–Cu(1)–N(6)
N(11)–Cu(2)–N(9)
N(11)–Cu(2)–N(12)
N(9)–Cu(2)–N(12)
N(11)–Cu(2)–N(10)
N(9)–Cu(2)–N(10)
N(12)–Cu(2)–N(10)
152.12(8)
82.72(8)
99.19(8)
100.59(8)
82.40(8)
169.88(8)
151.66(8)
82.80(8)
99.02(8)
100.61(8)
82.89(8)
169.19(8)
N(3)–Cu(3)–N(1)
N(3)–Cu(3)–N(2)
N(1)–Cu(3)–N(2)
N(3)–Cu(3)–N(4)
N(1)–Cu(3)–N(4)
N(2)–Cu(3)–N(4)
N(16)–Cu(4)–N(13)
N(16)–Cu(4)–N(15)
N(13)–Cu(4)–N(15)
N(16)–Cu(4)–N(14)
N(13)–Cu(4)–N(14)
N(15)–Cu(4)–N(14)
148.43(8)
100.04(8)
83.24(7)
82.52(8)
100.58(8)
168.37(7)
152.24(8)
98.69(8)
82.98(8)
83.03(8)
101.60(8)
166.90(8)
Fig. 3. Cyclic voltammogram of NiL3 in DMF at a scan rate of 0.05 Vsꢀ1
.
two pyrrole nitrogens are displaced to the opposite side of these
mean planes by distances of (0.353, 0.355), (0.359, 362), (0.398,
0.403) and (0.366, 0.375) Å, respectively. The coordination geome-
try can be views as a tetrahedral distortion of the square planar
geometry, or better as a seesaw structure, with the nearly linear
N (imine)–Cu–N (imine) angle forming the plank and the N (pyr-
role)–Cu–N (pyrrole) angle the pivot. The values of the structural
index s4 (defined as
s
4 = 360 ꢀ (a + b)/141 where a and b are the
two largest coordination angles in the four-coordinate species
[50] lie in range 0.27–0.3. The planes of Nimine–Cu–Nimine make
an angle of 80.90°–81.53° with the planes of Npyrrole–Cu–Npyrrole.
The metal–ligand interactions lead to a twisting of the ligand com-
pared to the free ligand, which shows a mirror plane according its
NMR spectrum. In this structure Cu(2)L2 and Cu(3)L2 units stack in
an offset face-to-face fashion with displacement angle of 14.36°.
Phenyl rings are nearly parallel to each other with an interplanar
angle of 4.13° and the closest atom–atom distance is 3.79 Å. The
mean centroid–centroid distance of 5.581 Å indicates very weak
interactions between phenyl rings.
Table 4
The selected structural parameters of CuL2 calculated by DFT and their X-ray observed
values for comparison (distances in (Å), angles in (°)).
Parameter
X-ray
DFT
Cu–Npyrrole
Cu–Nimine
1.9421–1.9677
1.97
1.9880–2.0207
2.022
167.07
83.27
100.56
100.58
83.28
145.76
N
N
imine–Cu–Nimine
pyrrole–Cu–Nimine
166.90(8)–169.88(8)
82.40(8)–82.98(7)
99.19(8)–100.04(8)
100.59(8)–101.60(8)
82.72(8)–83.24(7)
148.43(8)–152.24(8)
Npyrrole–Cu–Npyrrole
3.3. Geometry optimization and electronic and spectra structure
in this structure that show no element of symmetry in the solid
state. Coordination around the copper(II) ions is defined by the
N4 donor set of the H2L2 ligand. In each copper the two imine
and the two pyrrole nitrogen atoms of the potentially hexadentate
ligand are arranged in a distorted square planar environment with
bond distances of 1.9421–1.9677 Å for Cu–Npy and 1.9880–
2.0207 Å for Cu–Nimine. The weak interactions between etheric
oxygens and copper ion are reflected in Cuꢁ ꢁ ꢁO distances of approx-
imately 3 Å, and the angles between adjacent bonds in the coordi-
nation sphere of copper are in the range 82.40–101.60°. The
copper(II) ions and two imine nitrogens lie out of the mean planes
defined by N(7)N(8)Cu(1)N(5)N(6), N(9)N(11)Cu(2)N(10)N(12),
N(1)N(3)Cu(3)N(2)N(4) and N(13)N(16)Cu(4)N(14)N(15) by dis-
tances of (0.118, 0.291, 0.298), (0.115, 0.300, 0.306), (0.132,
0.334, 0.335) and (0.096, 0.317, 0.328) Å, respectively, while the
Selected geometric parameters of the fully optimized structure
of CuL2 are gathered in Table 4. There is good agreement between
the observed and computed structural parameters. The imine and
pyrrole nitrogens and the etheric oxygens of the complex have
high negative atomic charges: ꢀ0.470, ꢀ0.50 and ꢀ0.54, respec-
tively. The rather similar negative charge on the imine nitrogens
and the formally anionic pyrrole nitrogens suggests that the latter
donate more electron density to the copper atom, leading to rather
strong Cu–Npyrrole bonding. This idea is supported by the short Cu–
Npyrrole bond lengths in comparison to those of Cu–Nimine
.
The TDDFT calculation shows that two long wavelength bands
at 640.79 and 635.63 nm arise from transitions from the b-spin
HOMO-19, HOMO-18 and HOMO, having mainly d2, dyz,xz and
z
pimine characters, respectively, to the b-spin LUMO with dominant
Cu dx2
character (see Supplementary material). The result is in
ꢀy2
Table 5
Redox potentials (vs. Ag/Ag+ electrode) in DMF.
Compound
NiL1
Scan rate (m Vsꢀ1
)
Epa (V)
Epc(V)
E1/2 (V)
D
E (V)
50
100
not observed
not observed
ꢀ0.71045
ꢀ0.80505
ꢀ0.788
ꢀ0.808
ꢀ0.926
ꢀ1.03
not observed
not observed
–
–
–
–
NiL2
50
100
not observed
not observed
–
–
–
–
NiL3
50
100
0.402
0.627
0.7
–
1.32
1.65
NiL4
50
100
0.86
not observed
–
–
–
–
Ferrocene
50
0.52
0.34
0.43
0.18

A.A. Khandar et al. / Inorganica Chimica Acta 363 (2010) 4080–4087
4087
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The three ligands H2L2, H2L3 and H2L4 are irreversibly oxidized
in DMF solvent. The respective anodic peak potentials at scan rate
0.05 Vsꢀ1 are approximately 1.21, 1.24 and 1.22 V, respectively.
The two ligands H2L1 and H2L2 also show irreversible reduction
waves at approximately ꢀ1.27 and ꢀ1.8 V, respectively. Increasing
scan rates give a positive peak potential shift for anodic peaks and
a negative peak potential shift for cathodic peaks as well as
increasing in current intensity.
The electrochemical properties of the nickel complexes were
investigated in DMF with 0.1 M LiClO4 as a supporting electrolyte:
cyclic voltammetry data are summarized in Table 5. The two NiL1
and NiL2 complexes exhibit irreversible reduction waves but no
anodic wave is observed. The NiL4 complex, in contrast, exhibits
only an irreversible anodic peak. The reduction of NiL1 and NiL2
(scan rate 0.05 Vsꢀ1
)
occurred at approximately ꢀ0.71 and
ꢀ0.78 V, respectively, while the oxidation of NiL4 occurs at
0.86 V. The NiL3 complex shows a quasi-reversible anodic peak in
0.4 V and the corresponding cathodic peak in ꢀ0.92 V (Fig. 3).
These results suggest that the complexes with softer thioethers
are more easily oxidized than their oxygen analogs and stabilize
higher oxidation states [51]. For low-oxidation states S ligands
are expected to destabilize this state, as it is observed.
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Acknowledgement
We are grateful to University of Tabriz Research Council for the
financial support of this research.
Appendix A. Supplementary material
CCDC 611930 and 678398 contain the supplementary crystallo-
graphic data for NiL3 and CuL2. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif. Supplementary data associ-
ated with this article can be found, in the online version, at
doi:10.1016/j.ica.2010.08.019.
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