M. Dolaz et al. / Spectrochimica Acta Part A 76 (2010) 174–181
175
complexes were characterized by the spectroscopic and analyti-
3. Results and discussions
cal methods. We obtained the suitable single crystals of the ligand
for the X-ray analysis. Ligand and its metal complexes show the
reversible or irreversible peak potentials in DMF solution. The cat-
alytic properties of the Cu(II), Co(II) and Ni(II) metal complexes
were investigated using the cyclohexane as a substrate. The results
showed that the ligand has enhanced the catalytic properties of the
complexes under microwave conditions using H2O2 as oxidant.
The ligand trans-N,Nꢀ-bis[(3,4-dichlorophenyl)methylidene]cy-
talline, non-hygroscopic, partially soluble in most common organic
solvents but entirely soluble in DMF and DMSO. The analytical data
were showed that the metal to ligand ratio is 1:1 stoichiometry (in
the Supporting information). The compounds are very stable solids
at room temperature without decomposition for a long time. The
molar conductance values are in the 6.5–8.2 ꢀ−1 mol−1 cm2 range,
diffraction analysis.
2. Physical measurement
The 1H(13C) NMR spectra of the ligand and its nickel(II) com-
plex were recorded using DMSO-d6 as a solvent, and the spectral
data are given in the Supporting information. The ligand has a sym-
metric nature. Therefore, the hydrogen atoms in the benzene and
cyclohexane rings are shown as one signal for each other. For exam-
ple, two hydrogen atoms of the azomethine groups have one signal
in the 1H NMR spectrum. The azomethine groups were shown at ı
8.27 ppm as a singlet. In the nickel(II) complex, these groups shifted
to the lower regions (ı 8.34 ppm) and this shift shows that the
nitrogen atom of the CH N group coordinated to the nickel ion.
Aromatic rings protons are shown in the range ı 7.26–8.08 ppm
range as a multiplet. In the 1H NMR spectrum of the complex, the
aromatic ring protons shifted to the lower regions. In the ligand, the
hydrogen atoms of the CH2/CH groups in the cyclohexane ring are
observed in the ı 3.40–1.21 ppm range, and, in the complex, these
signals slightly shifted to the lower regions.
In order to further information about the ligand and its com-
plexes, the 13C NMR spectra were investigated. In the ligand, the
azomethine carbon atom is shown at ı 160.17 ppm. In the nickel(II)
complex, this signal shifted to the lower region. This situation con-
firms that the nitrogen atom of the CH N group coordinated to
the nickel(II) ion. The aromatic ring C atoms were shown in the
ı 138.25–129.01 ppm range. The C atoms of the cyclohexane ring
atoms shifted to the lower regions.
The mass spectral studies for the ligand and its metal com-
plexes were done and obtained data are given in the Supporting
information. The mass spectra for the ligand and its metal com-
plexes were characterized by a peak corresponding to Schiff base
fragmentation. The mass spectrum of the ligand (L) shows molecu-
lar ion peak ([M]+, 100%) at m/z 428. Moreover, the fragmentation
peaks at m/z 427 (82%) and 430 (30%) can be attributed to the
[M−1]+ and [M+2]+2 ions, respectively. The mass spectra of the
can be attributed to the molecular ion peaks ([M]+) of the copper(II),
cobalt(II) and nickel(II) complexes, respectively.
The infrared spectral data of all compounds were given in the
Supporting information. In the spectrum of the ligand, the ꢁ(CH N)
vibration of the azomethine group is shown at the 1643 cm−1. In
the complexes, this vibration shifted to the lower regions and can
be attributed to the complexation of the metal ion and nitrogen
atoms of the azomethine groups. In the spectra of the complexes,
range are ꢁ(M–N) vibration.
2.1. General
The metal salts CuCl2·2H2O, CoCl2·6H2O and NiCl2·6H2O
and organic solvents were purchased from commercial
sources and used as received, unless noted otherwise. 3,4-
Dichlorobenzaldehyde,
( )trans-1,2-cyclohexanediamine and
([CH3(CH2)3]4NBF4) were obtained from Fluka. Elemental analyses
(C, H, N) were performed using a LECO CHNS 932 elemental anal-
yser. IR spectra were obtained using KBr discs (4000–400 cm−1
)
on a Shimadzu 8300 FT-IR spectrophotometer. The electronic
spectra in the 200–900 nm range were obtained on a PerkinElmer
Lambda 45 spectrophotometer. Magnetic measurements were
carried out by the Gouy method using Hg[Co(SCN)4] as calibrant.
Molar conductances of the Schiff base ligands and their transition
metal complexes were determined in DMF (∼10−3 M) at room
temperature using a Jenway Model 4070 conductivity meter. Mass
spectra of the ligands were recorded on a LC/MS APCI AGILENT
1100 MSD spectrophotometer. 1H and 13C NMR spectra were
recorded on a Varian XL-200 instrument. TMS was used as internal
standard and DMSO-d6 as solvent. The metal and chloride contents
of the complexes were determined as gravimetrically according
to the known procedure [15]. The thermal analysis studies of
the complexes were performed on a PerkinElmer Pyris Diamond
DTA/TG Thermal System under nitrogen atmosphere at a heating
rate of 10 ◦C/min.
Cyclic voltammograms were recorded on an Iviumstat Electro-
chemical workstation equipped with a low current module (BAS
PA–1) recorder. The electrochemical cell was equipped with a
BAS glassy carbon working electrode (area 4.6 mm2), a platinum
coil auxiliary electrode and an Ag/AgCl reference electrode filled
with tetrabutylammonium tetrafluoroborate (0.1 M) in DMSO and
DMF solvents and adjusted to 0.00 V vs SCE. Cyclic voltammetric
measurements were made at room temperature in an undivided
cell (BAS model C–3 cell stand) with a platinum counter elec-
trode and an Ag/AgCl reference electrode (BAS). All potentials are
reported with respect to Ag/AgCl. The solutions were deoxygenated
by passing dry nitrogen through the solution for 30 min prior to
the experiments, and during the experiments the flow was main-
tained over the solution. Digital simulations were performed using
DigiSim 3.0 for windows (BAS, Inc.). Experimental cyclic voltammo-
grams used for the fitting process had the background subtracted
and were corrected electronically for ohmic drop.
The microwave experiments were carried out using a Bergof
MWS3+ (Germany) equipped with pressure and temperature con-
trol. Microwave experiments were done in closed DAP60 vessels.
The reaction products were characterized and analyzed by using
PerkinElmer Clarus 600 GC (USA) equipped with MS detector fit-
columns.
Preparation of the ligand (L), preparation of the complexes,
cyclohexane oxidation under microwave irradiation and X-ray
structure solution and refinement for the Schiff base ligand (L) were
given in the Supporting information.
The electronic spectra of the ligand and their complexes were
recorded in DMF as a solvent. The spectral data are given in the
Supporting information. In the spectra of the ligand, the band at
the 342 nm may be assigned to the n–* transitions. The observed
bands in the 310–290 nm range can be attributed to the –* and
–␦* transitions. The complexes exhibit multiple absorptions in