Tetradentate Schiff base ligands and their complexes: Synthesis, structural characterization, thermal, electrochemical and alkane oxidation

Article abstract of DOI:10.1016/j.saa.2012.04.001

Three Schiff base ligands (H₂L¹-H₂L ³) with N₂O₂ donor sites were synthesized by condensation of 1,5-diaminonapthalene with benzaldehyde derivatives. A series of Cu(II), Co(II), Ni(II), Mn(II) and Cr(III) complexes were prepared and characterized by spectroscopic and analytical methods. Thermal, electrochemical and alkane oxidation reactions of the ligands and their metal complexes were investigated. Extensive application of 1D (¹H, ¹³C NMR) and 2D (COSY, HETCOR, HMBC and TOSCY) NMR techniques were used to characterize the structures of the ligands and establish the ¹H and ¹³C resonance assignments of the three ligands. Ligands H₂L¹ and H₂L³ were obtained as single crystals from THF solution and characterized by X-ray diffraction. Both molecules are centrosymmetric and asymmetric unit contains one half of the molecule. Catalytic alkane oxidation reactions with the transition metal complexes investigated using cyclohexane and cyclooctane as substrates. The Cu(II) and Cr(III) complexes showed good catalytic activity in the oxidation of cyclohexane and cyclooctane to desired oxidized products. Electrochemical and thermal properties of the compounds were also investigated.

Full text of DOI:10.1016/j.saa.2012.04.001

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 95 (2012) 382–398
Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Tetradentate Schiff base ligands and their complexes: Synthesis, structural
characterization, thermal, electrochemical and alkane oxidation
Gökhan Ceyhan a,c, Muhammet Köse b, Vickie McKee b, Serhan Urusß a,c, Aysßegül Gölcü a, Mehmet Tümer a,
⇑
a Kahramanmaraßs Sütçü Imam University, Chemistry Department, 46100 K. Maras, Turkey
b Chemistry Department, Loughborough University, LE11 3TU Leics, UK
c Kahramanmaraßs Sütçü Imam University, Research and Development Center for University-Industry-Public Relations, 46100 K. Maras, Turkey
a r t i c l e i n f o
a b s t r a c t
Three Schiff base ligands (H2L1–H2L3) with N2O2 donor sites were synthesized by condensation of 1,5-
diaminonapthalene with benzaldehyde derivatives. A series of Cu(II), Co(II), Ni(II), Mn(II) and Cr(III) com-
plexes were prepared and characterized by spectroscopic and analytical methods. Thermal, electrochem-
ical and alkane oxidation reactions of the ligands and their metal complexes were investigated. Extensive
application of 1D (1H, 13C NMR) and 2D (COSY, HETCOR, HMBC and TOSCY) NMR techniques were used to
characterize the structures of the ligands and establish the 1H and 13C resonance assignments of the three
ligands. Ligands H2L1 and H2L3 were obtained as single crystals from THF solution and characterized by X-
ray diffraction. Both molecules are centrosymmetric and asymmetric unit contains one half of the mol-
ecule. Catalytic alkane oxidation reactions with the transition metal complexes investigated using cyclo-
hexane and cyclooctane as substrates. The Cu(II) and Cr(III) complexes showed good catalytic activity in
the oxidation of cyclohexane and cyclooctane to desired oxidized products. Electrochemical and thermal
properties of the compounds were also investigated.
Article history:
Received 5 December 2011
Received in revised form 12 March 2012
Accepted 3 April 2012
Available online 21 April 2012
Keywords:
Schiff base
X-ray
Alkane oxidation
Electrochemical
Thermal
Oxidized products
Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction
synthesis. Cyclohexane (CyH) oxidation into cyclohexanol
(CyAOH) and cyclohexanone (Cy@O) has a great importance in
The ortho-hydroxy Schiff bases belong to the group of com-
pounds forming intramolecular hydrogen bonds generating ever
growing interest due to p-electron coupling between the acid and
base centers [1–4]. Such systems are interesting both from the the-
oretical and experimental point of view. The intramolecular proton
transfer reaction proceeds comparatively easily in these com-
industry. Over a billion tones of Cy@O and CyAOH are produced
each year and are generally used for the synthesis of Nylon-6 and
Nylon-6,6 [11]. A Co-naphthenate complex is used as a catalyst to
initiate the radical oxidation of cyclohexane using molecular oxygen
from the air at 160 °C and 15 bar pressure. On completion of the
reaction, only 4% of CyH is converted to oxidized products and with
selectivity towards Cy@O and CyAOH [11]. Tanase et al. have syn-
thesized polydentate pyridine based ligand and its iron complexes
[12]. These complexes were active in the catalytic oxidation of some
hydrocarbons, such as cyclohexane and cyclooctane [12,13].
pounds. Intramolecular p-electron coupling leads to the strength-
ening of the hydrogen bonds in these systems. These compounds
are potential materials for molecular memory and optical switch
devices [5] or fluorescent probes of biological systems [6].
In Schiff base metal complexes, the environment at the coordi-
nation center can be modified by attaching different substituents
to the ligand, which provides a useful range of steric and electronic
properties essential for the fine-tuning of structure and reactivity.
Therefore, Schiff base ligands are among the most fundamental
chelating systems in coordination chemistry [7,8] and complexes
of both transition and p-block metals based on this type ligands
were shown to catalyze a wide variety of reactions [9,10].
Microwave technology using the green oxidant hydrogen per-
oxide can reduce the reaction times and energy consumption with
an increase in cyclohexane conversion and CyAOH and Cy@O
selectivity [14–16]. We obtained high yields with high conversions
in short times, in our previous catalytic oxidation studies with
microwave power [15,16]. Carvalho et al. have studied the cyclo-
hexane oxidation under microwave conditions by using Fe(III)
complex catalysts, hydrogen peroxide as oxidant and acetonitrile
as solvent [17]. CyAOH, Cy@O and adipic acid were obtained as
major products in this study with a good CyH conversion and
CyAOH, Cy@O selectivity. According to the study of Clark et al.,
the Co-salen complex has shown a good catalytic activity for the
oxidation of cyclohexene to 2-cyclohexen-1-ol, cyclohexene oxide
and cyclohanediol with H2O2 under 300 W microwave power [14].
The transformation of hydrocarbons into oxygenated com-
pounds has been extensively investigated over the last decades,
because the products are valuable intermediates for organic
⇑
Corresponding author. Tel.: +90 344 2191093; fax: +90 344 2191042.
E-mail address: mtumer@ksu.edu.tr (M. Tümer).
1386-1425/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.saa.2012.04.001

G. Ceyhan et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 95 (2012) 382–398
383
In this paper, we report the synthesis and characterization
of three Schiff base ligands (H2L1 = N,N’-bis[3-hydroxy salicylid-
ene]-1,5-diamino naphthalene, H2L2 = N,N0-bis[4-hydroxy salicy-
lidene]-1,5-diamino naphthalene and H2L3 = N,N0-bis[3-methoxy
salicylidene]-1,5-diamino naphthalene) and their transition metal
complexes. The catalytic activity of the Cu(II), Co(II), Cr(III), Ni(II)
and Mn(II) complexes were investigated and the Cu(II) and Cr(III)
complexes showed enhanced catalytic effects for oxidation reac-
tions under microwave conditions using H2O2 as oxidant. The
structural characterization of the ligands H2L1 and H2L1 were done
by single crystal X-ray diffraction. The 1D (1H, 13C NMR) and 2D
(COSY, HETCOR, HMBC and TOCSY) NMR techniques were used
for characterization of the structures of the ligands.
12.6 mmol (1.74 g) 2,3- and 2,4-dihydroxy benzaldehyde for H2L1
and H2L2, respectively, and with 12.6 mmol (1.92 g) o-vanillin for
H2L3, each in 15 mL absolute methanol. The product was filtered,
washed with cold solvent and recrystallized from a methanol/hex-
ane mixture (1:1) to give colored crystals. Single crystals suitable
for X-ray diffraction studies for H2L1 and H2L3 were obtained from
slow evaporation of THF solution ligands. Physical properties and
other spectroscopic data are given in the experimental section.
H2L1: (C24H18N2O4). Color: dark orange, yield: 85%, m.p.: 265 °C,
elemental analyses: Found (calcd.), C, 72.41 (72.35); H, 4.49 (4.55);
N, 6.98 (7.03). 1H NMR (CDCl3,
D (ppm)): 6.84–6.88 (m, 2H, aro-
matic H), 7.02–7.04 (m, 2H, aromatic H), 7.22–7.24 (m, 2H, aro-
matic H), 7.52–7.54 (m, 2H, aromatic H), 7.68–7.72 (m, 2H,
aromatic H), 8.13–8.15 (m, 2H, aromatic H), 9.03 (s, 2H, CH@N),
9.38 (s, 2H, OH), 13.17 (s, 2H, OH). 13C NMR (CDCl3,
D (ppm)):
2. Experimental
115.69 (aromatic C), 119.53–119.83 (aromatic C), 120.32 (aromatic
C), 121.61 (aromatic C), 123.25 (aromatic C), 127.43 (aromatic C),
128.89 (aromatic C), 146.04–146.19 (CqAOH), 149.76 (CqAOH),
2.1. Materials and measurements
165.27 (C@N). FTIR (KBr, cmꢀ1): 3197
m(OH), 1615 m(C@N), 1350
All reagents and solvents were of reagent-grade quality and ob-
tained from commercial suppliers (Aldrich or Merck). Elemental
analyses (C,H,N) were performed using a LECO CHNS 932. Infrared
spectra were obtained using KBr discs (4000–400 cmꢀ1) on a Perkin
Elmer Spectrum 100 FT-IR. Far IR spectra of the complexes were re-
corded on a Perkin Elmer spectrum 400 FT-IR/FT-FAR. Electronic
spectra in the 200–900 nm range were obtained on a Perkin Elmer
Lambda 45 spectrophotometer. Molar conductances of the Schiff
base ligands and their transition metal complexes were determined
in DMF (ꢁ10ꢀ3 M) at room temperature using a Jenway Model 4070
conductivity meter. Magnetic measurements were carried out by
the Gouy method using Hg[Co(SCN)4] as a calibrant. Mass spectra
of the ligands were recorded on a LC/MS APCI AGILENT 1100 MSD
spectrometer. 1H and 13C NMR spectra were recorded on a Bruker
400 MHz instrument. TMS was used as internal standard and CDCl3
as solvent. The thermal analyses studies of the complexes were per-
formed on a Perkin Elmer Pyris Diamond DTA/TG Thermal System
under nitrogen atmosphere at a heating rate of 10 °C/min.
The microwave experiments were carried out in a Bergof
MWS3+ (Germany) oven equipped with pressure and temperature
control. Microwave experiments were done in closed DAP60 ves-
sels. The reaction products were characterized and analyzed by
using Perkin Elmer Clarus 600 GC (USA) equipped with MS detector
fitted with Elite-5 MS and FID detector fitted with BPX5 capillary
columns. A stock solution of concentration of 1 ꢂ 10–4 M of Schiff
base ligands was prepared in ethanol for electrochemical studies.
All voltammetric measurements were performed using a BAS
100 W (Bioanalytical System, USA) electrochemical analyzer. A
m
(phenolic CAOH), 867 m(aromatic ring CAH). Mass spectrum
(LC/MS APCI): m/z 400 ([M+2]+, 35%), 399 ([M+1]+, 100%), 398
([M]+, 18%).
H2L2: (C24H18N2O4). Color: dark yellow, yield: 88%, m.p.: 276 °C.
Elemental analyses: Found (calcd.), C, 72.30 (72.35); H, 4.59
(4.55); N, 7.08 (7.03). 1H NMR (CDCl3,
D (ppm)): 10.37 (s, 2H, OH),
8.88 (s, 2H, OH), 8.81 (s, 2H, CH@N), 8.06–8.08 (m, 2H, aromatic
H), 7.62–7.66 (m, 2H, aromatic H), 7.54–7.56 (m, 2H, aromatic H),
7.44–7.45 (m, 2H, aromatic H), 6.45–6.49 (m, 2H, aromatic H),
6.37–6.41 (m, 2H, aromatic H). 13C NMR (CDCl3,
D (ppm)): 115.69
(aromatic C), 119.53–119.83 (aromatic C), 120.32 (aromatic C),
121.61 (aromatic C), 123.25 (aromatic C), 127.43 (aromatic C),
128.89 (aromatic C), 146.04–146-19 (CqAOH), 149.76 (CqAOH),
165.27 (C@N). FTIR (KBr, cmꢀ1): 3268
m
(OH), 2865
m(CAH)alph,
1619
m(C@N), 1300
m
(phenolic CAOH), 834 (aromatic ring, CAH).
m
Mass spectrum (LC/MS APCI): m/z 399 ([M+1]+, 70%), 397
([Mꢀ1]+, 20%), 325 ([M-C3H5O2]+, 100%).
H2L3: (C25H22N2O4). Color: orange, yield: 83%, m.p.: 200 °C.
Elemental analyses: Found (calcd.), C, 73.28 (73.23); H, 5.16
(5.20); N, 6.52 (6.57). 1H NMR (CDCl3,
D (ppm)): 3.87 (singlet,
3H, OCH3), 6.95–6.99 (m, 3H, aromatic H), 7.19–7.21 (m, 2H,
aromatic H), 7.36–7.38 (m, 2H, aromatic H), 7.52–7.54 (m, 2H,
aromatic H), 7.76–7.70 (m, 3H, aromatic H), 8.12–8.14 (m, 2H, aro-
matic H), 9.05 (s, 2H, CH@N). 13C NMR (CDCl3,
D (ppm)): 56.45
(OCH3), 115.69 (aromatic C), 116.37 (aromatic C), 119.36 (aromatic
C), 120.21 (aromatic C), 121.74 (aromatic C), 124.23 (aromatic C),
127.42 (aromatic C), 128.88 (aromatic C), 146.13 (CqAOH),
148.48 (CqAOH), 150.93 (OCH3), 165.27 (C@N). FTIR (KBr, cmꢀ1):
glassy carbon working electrode (BAS;
U: 3 mm diameter), an Ag/
3201 m(OH), 1605 m(C@N), 1321 m(phenolic CAOH), 838 m(aromatic
AgCl reference electrode (BAS; 3 M KCl) and platinum wire counter
electrode and a standard one-compartment three electrode cell of
10 mL capacity were used in all experiments. The glassy carbon
electrode was polished manually with aqueous slurry of alumina
ring, CAH). Mass spectrum (LC/MS APCI): m/z 427 ([M+1]+, 100%),
428 ([M+2]+, 30%).
2.3. Preparation of the Schiff base complexes
powder (U: 0.01 lm) on a damp smooth polishing cloth (BAS velvet
polishing pad), before each measurement. All measurements were
performed at room temperature. A Mettler Toledo MP 220 pH meter
was used for the pH measurements using a combined electrode
(glass electrode reference electrode) with an accuracy of 0.05 pH.
Data collection for X-ray crystallography was completed using a
Bruker APEX2 CCD diffractometer and data reduction was per-
formed using Bruker SAINT [18]. SHELXTL was used to solve and
refine the structures [19].
The appropriate quantity of Schiff base ligands (H2L1, H2L2 and
H2L3) (1 mmol) was dissolved in EtOH (20 mL). To this solution, a
solution of the metal salts (1 mmol) in EtOH (20 mL) [1:2 M ratio
(L:M)] was added. The mixture was stirred for 20 h at 80 °C. The
precipitated complex was then filtered off, washed with cold eth-
anol and dried in a vacuum desiccator.
2.4. Cyclohexane and cyclooctane oxidation under microwave
irradiation
2.2. Synthesis of the Schiff base ligands
The catalytic oxidation of cycloclohexane and cyclooctane un-
der microwave irradiation was performed as follows: 0.02 mmol
catalyst, 2 mmol cyclohexane (Carlo Erba, 99.8%), 4 mmol H2O2
The ligands were synthesized by the reaction of 6.3 mmol
(1.0 g) naphthalene-1,5-diamine in 30 mL absolute methanol with

384
G. Ceyhan et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 95 (2012) 382–398
Table 1
Table 3
Crystallographic data of the Schiff base ligands H2L1 and H2L3.
Bond lengths [Å] and angles [°] for H2L1.
Identification code
H2L1
H2L3
C(1)AO(1)
C(1)AC(6)
C(2)AC(3)
C(4)AC(5)
C(6)AC(7)
N(1)AC(8)
C(8)AC(10)
C(10)AC(11)
C(11)AC(12)
O(1)AC(1)AC(2)
C(2)AC(1)AC(6)
O(2)AC(2)AC(1)
C(2)AC(3)AC(4)
C(4)AC(5)AC(6)
C(5)AC(6)AC(7)
N(1)AC(7)AC(6)
C(9)AC(8)AN(1)
N(1)AC(8)AC(10)
1.3598(14) C(1)AC(2)
1.3965(16)
1.3675(14)
1.3925(18)
1.4025(17)
1.2824(16)
1.3742(17)
1.4079(17)
1.424(2)
1.4079(17)
122.38(10)
119.20(10)
120.02(11)
120.01(11)
119.30(11)
120.29(11)
119.64(10)
120.91(11)
120.28(11)
122.36(11)
1.4065(17) C(2)AO(2)
1.3828(17) C(3)AC(4)
1.3796(18) C(5)AC(6)
1.4514(16) C(7)AN(1)
1.4190(15) C(8)AC(9)
1.4263(17) C(9)AC(12A)
1.4169(17) C(10)AC(10A)
1.3669(18) C(12)AAC(9A)
Empirical formula
Formula weight
Crystal system
Space group
Unit cell
a (Å)
b (Å)
c (Å)
C24H18N2O4
398.40
C26H22N2O4
426.46
Monoclinic
P21/n
Triclinic
ꢀ
P1
6.2762 (7)
8.6491 (10)
9.3158 (11)
86.398 (2)
82.908 (2)
69.683 (1)
470.50 (9)
1
0.097
5545
1914 [0.0182]
0.0337, 0.0873
0.0398, 0.0912
847259
6.7449(5)
24.2256(17)
6.8765(5)
90
112.2940(10)
90
1039.62(13)
2
0.093
117.92(10)
119.71(11)
120.78(10)
120.54(11)
120.39(12)
120.40(11)
121.84(11)
121.25(11)
117.78(10)
O(1)AC(1)AC(6)
O(2)AC(2)AC(3)
C(3)AC(2)AC(1)
C(5)AC(4)AC(3)
C(5)AC(6)AC(1)
C(1)AC(6)AC(7)
C(7)AN(1)AC(8)
C(9)AC(8)AC(10)
C(8)AC(9)AC(12A)
C(11)AC(10)AC(8)
a
(°)
b (°)
(°)
c
Volume (Å3)
Z
Abs. coeff. (mmꢀ1
Refl. collected
)
10504
Ind. Refl. [Rint
]
2575 [0.0166]
0.0374, 0.1018
0.0418, 0.1056
856306
R1, wR2 [I > 2
r
(I)]
C(11)AC(10)AC(10A) 119.54(14)
C(10A)AC(10)AC(8)
C(11)AC(12)AC(9A)
R1, wR2 (all data)
CCDC number
118.10(13)
120.50(11)
C(12)AC(11)AC(10) 120.67(11)
Symmetry operations for equivalent atoms A ꢀx + 1, ꢀy, ꢀz.
Table 2
Hyrogen bond geometry.
Table 4
Bond lengths [Å] and angles [°] for H2L3.
DAHꢃ ꢃ ꢃA
DAH Hꢃ ꢃ ꢃA Dꢃ ꢃ ꢃA
<(DHA)
H2L1
O1AH1ꢃ ꢃ ꢃN1
O1AH1ꢃ ꢃ ꢃN1
0.95
0.87
1.73
1.83
2.02
2.5860(12) 149.0
2.5987(12) 145.9
2.8333(12) 145.4
O(1)AC(1)
C(1)AC(2)
C(2)AC(4)
C(4)AC(5)
C(6)AC(7)
C(8)AN(1)
C(9)AC(10)
C(10)AC(13A)
C(11)AC(11A)
C(13)AC(10A)
1.3507(12) C(1)AC(7)
1.4088(14) C(2)AO(2)
1.3897(14) O(2)AC(3)
1.4006(15) C(5)AC(6)
1.4093(14) C(7)AC(8)
1.2860(13) N(1)AC(9)
1.3750(14) C(9)AC(11)
1.4083(15) C(11)AC(12)
1.4227(19) C(12)AC(13)
1.4084(15)
1.4068(14)
1.3612(13)
1.4296(13)
1.3763(15)
1.4535(14)
1.4145(12)
1.4298(14)
1.4174(14)
1.3692(15)
H2L3
H2L1(inter-
molecular)
O2AH2ꢃ ꢃ ꢃO1# 0.93
Symmetry operation # ꢀx + 1, ꢀy + 1, ꢀz + 1.
(Merck, 35%) were microwaved for 60 min at 400 W (40% of max-
imum output power). The catalyst:substrate:oxidant ratio was
1:100:200. The complexes were dissolved in 5 mL acetonitrile
and cyclohexane and H2O2 were added to the microwave vessels,
for each oxidation experiment. After it was immediately closed
the vessels, they were placed inside the Berghof MWS3+ micro-
wave oven and irradiated at 400 W for 60 min. The temperature
was controlled automatically by the microwave instrument at
about 140 °C. However, for some short times, it increased to
150–160 °C during the reaction and consequently, the pressure
also increased to 30–35 bar due to the evaporation of solvent and
substrate. In order to stop the oxidation before analysis, 1 mL
H2O was added in the vessels and the oxidized organic products,
except organic acids, were extracted with 10 mL CH2Cl2 and in-
jected to GC and GC–MS for analysis and characterization. The
amounts of CyH, CyAOH, Cy@O and CyON, CyONAOH, CyON@O
present were calculated from external calibration curves that were
prepared before analyses.
O(1)AC(1)AC(7)
C(7)AC(1)AC(2)
O(2)AC(2)AC(1)
C(2)AO(2)AC(3)
C(6)AC(5)AC(4)
C(1)AC(7)AC(6)
C(6)AC(7)AC(8)
C(8)AN(1)AC(9)
C(10)AC(9)AC(11)
122.69(9)
O(1)AC(1)AC(2)
O(2)AC(2)AC(4)
C(4)AC(2)AC(1)
C(2)AC(4)AC(5)
C(5)AC(6)AC(7)
C(1)AC(7)AC(8)
N(1)AC(8)AC(7)
C(10)AC(9)AN(1)
N(1)AC(9)AC(11)
117.71(9)
119.58(9)
114.50(9)
117.39(8)
120.53(10)
119.69(9)
119.70(9)
120.93(9)
120.58(9)
125.65(9)
119.83(9)
120.26(9)
120.10(10)
120.56(9)
121.60(9)
122.17(9)
117.18(9)
C(9)AC(10)AC(13A) 119.99(10)
C(12)AC(11)AC(9) 122.03(9)
C(13)AC(12)AC(11) 120.40(9)
C(12)AC(11)AC(11A) 119.27(11)
C(11A)AC(11)AC(9) 118.68(11)
C(12)AC(13)AC(10A) 121.06(10)
Symmetry operations for equivalent atoms A ꢀx + 2, ꢀy, ꢀz + 2.
3. Results and discussion
Analytical data for all of the complexes are given in Table 5 and
are in good agreement with the expected values. To prepare the li-
gands H2L1, H2L2 and H2L3, we used the carbonyl and diamine com-
pounds in ethanol media and the high yields suggest the reaction
conditions were appropriate. The ligands have polar groups, such
as AOH and ACH@N and are soluble in polar organic solvents, such
as EtOH, MeOH and CHCl3. The ligands are also stable for a long
time at room temperature without decomposition to oxidized
products. One difficulty in working with Schiff bases of salicylalde-
hyde derivatives arises from the fact that, even in neutral solutions,
a number of species may be present. In addition to enolimines,
hemiacetals and their hydrolysis products, tautomeric species,
such as ketoamines or cyclicdiamines formed from mono-Schiff
bases of diamines, may also occur. The metal complexes synthe-
sized are also stable at room temperature and are soluble in polar
organic solvents, such as EtOH, MeOH, DMSO and DMF.
2.5. X-ray structure solution and refinement for the compounds
Data were collected at 150(2)K° on a Bruker ApexII CCD diffrac-
tometer using Mo-Ka radiation (k = 0.71073 Å). The structure was
solved by direct methods and refined on F2 using all the reflections
[19]. All the non-hydrogen atoms were refined using anisotropic
atomic displacement parameters and hydrogen atoms bonded to
carbon atoms were inserted at calculated positions using a riding
model. Hydrogen atoms bonded to phenolic oxygen atoms (O1
and O2) were located from difference maps and allowed to ride
on their carrier atoms. Details of the crystal data and refinement
are given in Table 1. Hydrogen bond parameters are listed in Table
2 and bond lengths and angles are given in Tables 3 and 4.

G. Ceyhan et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 95 (2012) 382–398
385
Table 5
Analytical and physical data of the Schiff base ligands and their metal complexes.
% Found (Calcd.)
C
Compound
Color
Yield (%)
m.p. (°C)
H
N
Co2(L1)(H2O)2(Cl)2
Cu2(L1)(H2O)2(Cl)2
Cr2(L1)(H2O)4(NO3)4
Mn2(L1)(H2O)2(Cl)2
Ru2(L1)(H2O)4(Cl)4
Co2(L2)(H2O)2(Cl)2
Cu2(L2)(H2O)2(Cl)2
Cr2(L2)(H2O)4(NO3)4
Mn2(L2)(H2O)2(Cl)2
Ru2(L2)(H2O)4(Cl)4
Co2(L3)(H2O)2(Cl)2
Cu2(L3)(H2O)2(Cl)2
Cr2(L3)(H2O)4(NO3)4
Mn2(L3)(H2O)2(Cl)2
Ru2(L3)(H2O)4(Cl)4
Brown
Brown
Dark brown
Brown
Black
66
72
65
71
68
70
65
60
68
67
70
63
65
69
65
>250
>250
>250
>250
>250
>250
>250
>250
>250
>250
>250
>250
>250
>250
>250
46.45 (46.40)
46.97 (46.91)
35.19 (35.13)
47.06 (47.01)
35.54 (35.48)
46.45 (46.40)
45.77 (45.72)
35.18 (35.13)
47.06 (47.01)
35.54 (35.48)
48.15 (48.10)
47.46 (47.42)
36.86 (36.80)
48.75 (48.70)
37.21 (37.16)
3.30 (3.25)
3.34 (3.28)
3.02 (2.95)
3.35 (3.29)
3.04 (2.98)
3.31 (3.25)
3.25 (3.20)
3.01 (2.95)
3.35 (3.29)
3.03 (2.98)
3.78 (3.73)
3.72 (3.67)
3.38 (3.33)
3.82 (3.77)
3.40 (3.36)
4.54 (4.51)
4.61 (4.56)
10.31 (10.24)
4.63 (4.57)
3.50 (3.45)
4.55 (4.51)
4.50 (4.44)
10.30 (10.24)
4.62(4.57)
3.51 (3.45)
4.36 (4.31)
4.31 (4.25)
9.95 (9.90)
4.43 (4.37)
3.38 (3.33)
Green
Black
Brown
Brown
Black
Light brown
Black
Brown
Brown
Black
11
10
9
12
C
8 : 149.76
H3: 7.54
C1 : 164.04
2 : 128.94
C3 : 115.68
4 : 127.43
6
HO
7
C
C9 : 146.19
C10 : 119.83
H4: 7.68
H5: 8.14
H10: 6.86
8
3
2
OH
N
4
5
1
C
C
11 : 119.53
C5 : 121.61
C6 : 165.27
C
12 : 123.25
H
11: 7.02
N
OH
H12: 7.22
OH
C
7 : 120.32
H2L1
11
HO
10
9
12
C
8 : 146.30
C9 : 102.88
10 : 163.99
11 : 108.57
C12 : 135.01
H3: 7.44
C1 : 163.26
2 : 128.98
3 : 115.30
C4 : 127.23
5 : 121.05
6 : 163.42
C7 : 115.30
6
1
7
C
8
H4: 7.63
H5: 8.65
H9: 6.39
3
OH
N
2
C
C
4
5
C
C
H
11: 6.47
12: 7.54
N
OH
C
H
OH
H2L2
11
10
9
12
C
1 : 146.13
C2 : 128.88
3 : 115.69
H
3 : 7.44
C8 : 150.93
9 : 148.48
10 : 119.36
6
7
8
O
3
C
H
4 : 7.68
OH
N
2
4
13
C
H5 : 8.13
C
1
5
H10 : 6.97
C4 : 127.42
C5 : 121.74
C11 : 116.37
C12 : 124.23
N
OH
H
11 : 7.20
O
H12 : 7.37
C13 : 56.45
C
6 : 164.86
H2L3
H
13 : 3.87
C7 : 120.21
Fig. 1. H1(13C) NMR data of the Schiff base ligands.

386
G. Ceyhan et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 95 (2012) 382–398
Fig. 2. The HMBC (a: H2L1), TOCSY (b: H2L2) and COSY (c: H2L2) spectra of the Schiff base ligands H2L1 and H2L2.

G. Ceyhan et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 95 (2012) 382–398
387
Fig. 3. The 13C NMR spectrum of the Schiff base ligand H2L3.
Fig. 4. The absorption spectra of the ligand H2L1 (1 ꢂ 10–4 M) in the different pH values.
The 1H and 13C NMR spectra of the ligands were recorded in
CDCl3 and data are given in the experimental section. All 1H and
13C NMR signals were completely assigned using 1H–1H COSY,
HETCOR, TOSCY and HMBC spectra. The ring carbon atoms of the
ligands were elucidated by the interpretation of 13C 1H COSY,
1H–1H COSY, HETCOR and HMBC experiments and assignments
are given in Fig. 1. The HMBC spectrum of the ligand H2L1 and TOS-
CY and COSY spectra of the ligand H2L2 are shown in Fig. 2. It is
noteworthy that the influence of substitution on the chemical
shifts of the naphthalene moiety is weak. The OH groups on the
salicylidene moiety increases the electron density of the aromatic
rings due to the resonance or mesomeric effect. The Schiff base li-
gands have the proton donor OH groups in the ortho position of
their aromatic rings. Due to the presence of OH groups in the ortho
position to imine group, formation of intramolecular hydrogen
bonds is possible. The 1H resonance of the OAH groups of the li-
gands was shown in the 10.37–13.50 ppm range. The signal due
to the OH proton disappears on D2O shake. The singlets in the
8.87–9.38 ppm range can be attributed to the protons of the azo-
methine groups.
The COSY spectrum of the ligand H2L1 shows the protons H3, H4
and H5 are vicinal. The proton H4 interacts with both H3 and H5.
The proton H4 is shown at 7.68 ppm as a triplet. However, the pro-
tons H3 and H5 are doublets at 7.54 and 8.14 ppm, respectively. The
proton H11 interacts with H10 and H12. Therefore, the proton H11
appears as a triplet at 6.86 ppm, while H10 and H12 appear as dou-
blets at 7.02 and 7.23 ppm. Using the HETCOR spectrum of H2L1,
the carbon atoms C3 (D: 115.68), C4 (D: 127.43), C5 (D: 121.61),

388
G. Ceyhan et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 95 (2012) 382–398
bathochromic shift. Similar data were also obtained for the ligands
H2L2 and H2L3 in ethanol solution.
C10 (D: 119.83), C11 (D: 119.53) and C12 (D: 123.25) were assigned.
In the spectra of the other ligands H2L2 and H2L3, similar correla-
tions and interactions were determined. From the HMBC spectrum
of the ligand H2L1, the azomethine carbon atoms were investigated
at 165.27 ppm as C6. In the other ligands, this carbon atom was
seen at 163.42 and 164.86 ppm. In the spectrum of the ligand
H2L3, the methoxy carbon atom was identified at 56.45 ppm as
Electronic spectra of the metal complexes were measured in
EtOH and the numerical data are given in Table 6. All the com-
plexes show an intense band in the 330–300 nm range which is as-
signed to a p–
p⁄ transition associated with the azomethine linkage
[20]. The spectra of the complexes show intense bands in the high-
energy region in the 480–340 nm range which can be assigned to
charge transfer L ? M bands [21]. The bands observed in the
740–510 nm region can be attributed to d–d transitions of the me-
tal ions. In the spectra of the Cu(II) complexes, the d–d transitions
are observed in the 730–510 nm range. These values are of partic-
ular importance since they are highly dependent on the geometry
of the molecule. It is known that the transitions from a square-pla-
nar structure to a deformed tetrahedral structure lead to a red shift
of absorption in the electronic spectra [22]. Thus, the smaller value
of the wavelength of the band corresponding to the transitions is
resemblance between the geometry of the complex and that of
square–planar complex.
C
13. The 13C NMR spectrum of the Schiff base ligand H2L3 is given
in Fig. 3. The mass spectral data of the ligands are given in the
experimental section. The formulation of the ligands are deduced
from analytical data, 1H(13C) NMR and further supported by mass
spectroscopy. The relatively low intensities of the molecular ion
peaks, [M]+, are indicative of the ease of fragmentation of the com-
pounds, and this may reflect the number of heteroatoms present in
each structure. The spectra of the ligands H2L1 and H2L2 show
[M+1]+, peaks at m/e 399 while for H2L3 the signal is at
m/e = 427. The mass spectra of the investigated compounds are rel-
atively complicated and exhibit a large number of peaks. Neverthe-
less, all of the spectra are very similar, indicating the close
structural resemblance of the compounds. The fragmentation
mode, considered most likely to be the same for all of the com-
plexes, follow common steps. The mass spectra of all the com-
plexes exhibit the presence of molecular ion peaks of weak
IR data of the complexes are given in Table 6. In the spectra (in
the experimental section) of the Schiff base ligands H2L1, H2L2 and
H2L3, the broad bands in the 3268–3197 cmꢀ1 range can be attrib-
uted to the m
(OH) cmꢀ1 vibration. In the transition metal com-
intensity, dication [M2(Ln)](H2O)2(Cl)2]2+
.
plexes, the bands due to the OH modes are no longer observed,
suggesting that all the hydroxyl protons are displaced by MII and
Electronic spectra of the ligands (H2L1, H2L2 and H2L3) were
investigated at 1 ꢂ 10ꢀ3 M in ethanol solution and at different
pH values (pH 2–12) in phosphate buffer. The spectra are pH
dependent because of the weak acid character of the ligands. If
all ligands having H2Ln (n: 1, 2 or 3) are shown as H2A, in solution;
the equilibrium is:
MIII ions leading to covalent
m(MAO) bonding with the ligands. In
the ligands, the broad bands at 2750 and 2585 cmꢀ1 come from
intramolecular hydrogen bonding along the phenolic –OH and azo-
methine ACH@NA groups, these bands disappear on complexation
between the oxygen and nitrogen atoms with metal ions. The
m
(CH@N) band (1619–1605 cmꢀ1) is shifted to lower wavenumbers
H2A $ 2Hþ þ A2ꢀ
denoting that the nitrogen atom of the azomethine group is coor-
dinated to the metal(II) ion. The bonding of the metal ions to the
ligands through the nitrogen and oxygen atoms is further sup-
ported by the presence of new bands in the 565–470 and 475–
If we take the negative logarithm of the equilibrium constant, we
get pKa = pH + log [H2A]/[A2ꢀ]. When [H2A] = [A2ꢀ], then pKa has
to equal pH and this point is also named the half titration pH point,
because at this point, the half of the [H2A] concentration is titrated.
Using these equations, Ka values of all three species can be obtained.
To calculate the pKa values, the variation of absorption with pH was
investigated. The absorption spectra of the ligand H2L1 in the
1 ꢂ 10–4 M concentration are shown in Fig. 4. The ligand H2L1 has
two absorption maxima in the 270–400 nm range. It shows
430 cmꢀ1 range due to the
respectively.
m(MAO) and m(MAN) vibrations,
In order to investigate the metal–halogen, metal–nitrogen and
metal–oxygen vibrations, the far infrared spectra of the complexes
were studied and data for metal complexes of ligand H2L1 are given
in Fig. 5. The far infrared spectra of the chloro complexes revealed
some new bands in the 318–307, 294–293, 280–276 and 272–
270 cmꢀ1 range, which are assigned to
(MnACl) and
the 475–430 cmꢀ1 range in the complexes originates from the
(MAN) (azomethine) vibration and substantiates this coordina-
tion. In the Cr(III) complexes, the coordination of nitrate groups
m
(CoACl),
m(RuACl),
p
?
p⁄ transition at 276 and n ? p⁄ at 381 nm. The absorption
m
m(CuACl), respectively. The presence of bands in
maxima at 381 nm was shifted to shorter absorption values in the
pH 2–12 range. In the pH 2–12 range, the ligand shows a hypso-
chromic shift in this solution. As the pH values increase, the wave-
length shifted to the longer regions. In this pH range, there is also a
m
Table 6
Magnetic, infrareda and electronic spectral data of the metal complexes.
Compound
m
(OH)
m
(CH@N)
m
(CAOH)
m
(MAN)
m
(MAO)
m
(MACl)
m
(CrAONO2)
kmax (nm, MeOH)
Meff (B.M.)
Co2(L1)(H2O)2(Cl)2
Cu2(L1)(H2O)2(Cl)2
Cr2(L1)(H2O)4(NO3)4
Mn2(L1)(H2O)2(Cl)2
Ru2(L1)(H2O)4(Cl)4
Co2(L2)(H2O)2(Cl)2
Cu2(L2)(H2O)2(Cl)2
Cr2(L2)(H2O)4(NO3)4
Mn2(L2)(H2O)2(Cl)2
Ru2(L2)(H2O)4(Cl)4
Co2(L3)(H2O)2(Cl)2
Cu2(L3)(H2O)2(Cl)2
Cr2(L3)(H2O)4(NO3)4
Mn2(L3)(H2O)2(Cl)2
Ru2(L3)(H2O)4(Cl)4
3218
3250
3212
3285
3224
3268
3275
3206
3308
3288
3305
3280
3278
3374
3265
1604
1621
1605
1603
1612
1607
1617
1603
1606
1609
1613
1603
1600
1603
1605
1315
1322
1321
1345
1305
1318
1322
1315
1349
1350
1310
1305
1318
1325
1349
494
500
482
470
490
489
470
490
474
480
492
464
480
488
465
462
475
460
474
467
462
484
460
470
475
465
472
470
467
475
312
316
675, 610, 375, 360, 310, 290, 285
730, 405, 350, 320, 285
440, 390, 325, 280
4.22
1.82
3.6
5.88
1.81
4.26
1.90
3.8
5.90
1.83
4.24
1.88
3.8
243
240
235
278
294
310
272
735, 380, 305,290, 265
590, 355, 320, 295, 270
680, 600, 390, 380, 320, 278
620, 380, 320, 295, 280
480, 410, 375, 320, 280
738, 390, 325, 280
280
293
309
271
580, 400, 375, 330, 290, 280, 275
670, 610, 460, 380, 330, 280
510, 475, 340, 307, 280
400, 390, 300, 280
284
294
740, 380, 325, 275
600, 450, 340, 315, 290
5.90
1.82
a
cmꢀ1
.

G. Ceyhan et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 95 (2012) 382–398
389
in Cr(III), however, the unpaired spins are in t2g orbitals (for octa-
hedral coordination). Schiff base Cr(III) complexes exhibited mag-
netic moments in the range 3.6–3.8 B.M., consistent with S = 3/2
spin states of Cr(III) d3 centers. The Mn(II) Schiff base complexes
show the absorption bands in the 740–735 nm range and these
data are consistent with a four-coordinate, tetrahedral geometry
[25]. At room temperature, the magnetic moment data of the com-
plexes are in the 5.88–5.90 B.M range and these data are usually
observed for the Mn(II) compounds, regardless of stereochemistry
because the ground term is an 6A1, and thus, orbital contribution is
nil. The Mn(II) complexes have the tetrahedral geometry [26]. The
effective magnetic moments of the Ru(III) complexes are in the
1.81–183 B.M. at room temperature, a little higher than expected
for the spin-only value of a single-unpaired electron (1.73 B.M.),
which is consistent with a paramagnetic low-spin Ru(III) [27].
Fig. 5. The Far-IR spectra of the transition metal complexes of the Schiff base ligand
3.1 Crystal structures of the Schiff base ligand H2L1and H2L3
H2L1.
Perspective views of H2L1 and H2L3 are shown in Fig. 6. All bond
lengths and angles are within the normal ranges. All bond lengths
and angles in the phenyl rings and naphthalene ring have normal
Csp2–Csp2 values. The azomethine linkage distances in H2L1 and
H2L3 are 1.2824(16) and 1.2860(13) Å, respectively, which are
within the range of normal C@N values. Both H2L1 and H2L3 are
centrosymmetric. The molecular structures are broadly similar.
The dihedral angle between the two identical aromatic rings and
naphthalene ring in H2L1 and H2L3 are 56.40(6) and 47.03(7)°,
respectively; this difference is possibly due to an additional inter-
molecular hydrogen bond in H2L1.
There are intramolecular phenol-imine hydrogen bonds in both
H2L1 and H2L3 between O1AN1 (Table 2). Compound H2L1 contains
a second phenol group and this makes an intermolecular hydrogen
bond with the phenolic oxygen atom of a neighboring molecule
O2AO1 (under symmetry operation ꢀx + 1, ꢀy + 1, ꢀz + 1), linking
the molecules into H-bonded chains (Figs. 8 and 9).
is further supported by the appearance of new bands in the
245–230 cmꢀ1 range.
The molar conductivities (see Table 6) in DMF (ꢁ10ꢀ3 M solu-
tions) are too low to account for any dissociation of the Schiff base
ligands and complexes in MeOH. Therefore, the all compounds can
be regarded as non-electrolytes [23].
The magnetic moments (as B.M.) of the complexes were mea-
sured at room temperature and data are given in Table 6. All cobal-
t(II) complexes of the ligands H2L1–H3L2 have magnetic moments
values in the 4.23–4.26 B.M. range, and these values are consistent
with tetrahedral geometry [24]. The proposed structures of the
binuclear Cu(II) complexes are supported by magnetic moment
data. The observed magnetic moment values of the binuclear Cu(II)
complexes of the ligands H2L1–H2L3 are in the 1.82–1.90 B.M. We
can suggest that the binuclear Cu(II) complexes have tetrahedral
structure around the central metal ions. We may anticipate that
Fig. 6. Perspective view of H2L1 (a) and H2L3 (b) with atom labeling, thermal ellipsoid 50% probability, intramolecular hydrogen bonds are shown as dash lines, symmetry
operations: H2L1 A; ꢀx + 1, ꢀy, ꢀz H2L3 A; ꢀx + 2, ꢀy, ꢀz + 2.

390
G. Ceyhan et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 95 (2012) 382–398
Fig. 7. Molecular interactions within the structures (H2L1 left, H2L3 right).
In H2L1, intermolecular hydrogen linked chains link with neigh-
boring chain via edge to edge interactions; there are two sets
p–p
of interactions with neighboring molecules. First, one edge of the
naphthalene ring (C9 and C12) is stacked with the same section
of an adjacent molecule (under symmetry operation 1 + x, y, z) with
a interplanar separation of 3.383 Å. Second, phenol-imine section
(N1AC2) is stacked with the same section of an adjacent molecule
(under symmetry operation ꢀx, 1 ꢀ y, 1 ꢀ z); C1 and C1 are sepa-
rated by 3.393 Å (Fig. 7).
Similar
p–p
edge to edge interactions were observed in H2L3.
The C10AC13 edge of the naphthalene ring is stacked with the
same section of an adjacent molecule (under symmetry operation
1 + x, y, z) with a interplanar separation of 3.320 Å. Additionally,
there are also p–p edge to face (naphthalene–phenol) interactions
in H2L3; C10 is stacked with C1 of the neighboring molecule (under
symmetry operation x, y, ꢀ1 + z) with a separation of 3.388 Å
(Fig. 10). Unit cell packing is determined by intermolecular hydro-
gen bonds and
p–p p–p interactions in
interactions in H2L1 and
H2L3 (Figs. 8 and 9).
3.2. Electrochemical studies
Cyclic voltammograms of the complexes were run in DMF-
0.1 M Bu4NBF4 as supporting electrolyte at 293 K. All potentials
quoted refer to measurements run at a scan rates in the 100–
500 mVsꢀ1 range and against an internal ferrocene–ferrocenium
standard, unless otherwise stated. The electrochemical studies
were done in the 1 ꢂ 10ꢀ3 and 1 ꢂ 10ꢀ4 M solutions and obtained
data are given in Table 7. The voltammograms were recorded in the
range from ꢀ2.0 to 2.0 V vs Ag/AgCl. The selective voltammograms
of the complexes Co2(L1)(H2O)2(Cl)2 (a), Cu2(L1)(H2O)2(Cl)2 (b),
Mn2(L2)(H2O)2(Cl)2 (c) and Cu2(L3)(H2O)2(Cl)2 (d) are shown in
Fig. 6a–d. The complex Co2(L1)(H2O)2(Cl)2 (Fig. 11a) shows irre-
versible two anodic potentials at about 1.08 and 1.54 V. The sharp
oxidation peak at about 1.54 V is assigned to the oxidation of the
Co(II) complex. The sharpness of oxidation peak of cobalt is due
to the stability of Cu(II) in the solution. During the reverse scan,
a poorly defined oxidation wave appears at ꢀ0.68 V, which is asso-
ciated with the cathodic peak. The sharp oxidation peak assigned
to cobalt is also seen in a separate CV scan ranging from 1.20 to
0 V but not in a scan from 0 to 1.20 V (not shown). Thus, it is con-
cluded that the complex is decomposed. Other Co(II) complexes
also display similar properties.
Fig. 8. Packing plot of H2L1 viewing down b axis, hydrogen atoms are omitted for
clarity, intermolecular, and hydrogen bonds are shown as dash lines.
Ag+/AgCl, and one irreversible oxidation wave (
DE = 90 mV) on
the positive side. The redox process is assigned to Cu2+/Cu3+ oxida-
tion. In the reverse scan, Cu2+/Cu1+ was observed in the cathode
sweep. On the other hand, the Cu2(L2)(H2O)2(Cl)2 and Cu2(L3)
(H2O)2(Cl)2 complexes have two irreversible oxidation and
The complex Cu2(L1)(H2O)2(Cl)2 (Fig. 11b) shows one irrevers-
ible reduction wave (DE = 40 mV) on the negative side against

G. Ceyhan et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 95 (2012) 382–398
391
Fig. 9. Packing plot of H2L3 viewing down c axis, hydrogen atoms are omitted for clarity, intramolecular, and hydrogen bonds are shown as dash lines.
Fig. 10. Face to edge interactions within the structure of the ligand H2L3.
reduction waves in the 1.55–0.70 V range and ꢀ0.35 to 1.1 V range.
The irreversibility of the redox processes in the complexes is attrib-
uted to the changes in the coordination geometry or coordination
number upon change of the oxidation state or even to the expul-
sion of metal ions from the coordination sphere.
about the structure of these compounds. The complexes exhibit
two/three reduction–oxidation waves (see Table 7) in the range
ꢀ1.42 to 1.59 V (Epa) and ꢀ0.89 to 1.37 V (Epc), which suffer an irre-
versible one-electron oxidation process [MnII] ? [MnIII]+ eꢀ. The
Mn2(L2)(H2O)2(Cl)2 complex has reversible reduction–oxidation
process at about 1.40 V. This complex exhibits higher current
intensities in its redox waves than other Mn(II) complexes. These
Fig. 11c shows the cyclic voltammogram of Mn2(L2)(H2O)2(Cl)2.
Cyclic voltammetry studies provide additional useful information
Table 7
The electrochemical data of the Schiff base transition metal complexes.
Compound
Epa(V) (i)/(ii)
Epc(V) (i)/(ii)
E1/2(V) (i)/(ii)
DEp(V) (i)/(ii)
Co2(L1)(H2O)2(Cl)2
Cu2(L1)(H2O)2(Cl)2
Cr2(L1)(H2O)4(NO3)4
Mn2(L1)(H2O)2(Cl)2
Ru2(L1)(H2O)4(Cl)4
Co2(L2)(H2O)2(Cl)2
Cu2(L2)(H2O)2(Cl)2
Cr2(L2)(H2O)4(NO3)4
Mn2(L2)(H2O)2(Cl)2
Ru2(L2)(H2O)4(Cl)4
Co2(L3)(H2O)2(Cl)2
Cu2(L3)(H2O)2(Cl)2
Cr2(L3)(H2O)4(NO3)4
Mn2(L3)(H2O)2(Cl)2
Ru2(L3)(H2O)4(Cl)4
1.08, 1.54 (1.11, 1.58)
0.87 (0.91)
0.69, ꢀ0.68 (0.65, ꢀ0.71)
0.69 (0.78))
–
–
0.39 (0.46)
0.18 (0.22)
0.08 (0.64)
0.19 (0.03)
0.26 (0.08)
0.44 (0.65)
0.23 (0.54)
0.41 (0.51)
0.43 (0.55)
0.16 (0.11)
0.61 (0.51)
0.34 (0.45)
0.78 (0.14)
0.45 (0.55)
0.25 (0.69)
ꢀ1.61, 0.97 (ꢀ1.58, ꢀ1.08, 0.24)
0.88, 1.31 (0.83, 1.40)
1.07, 1.58 (0.36, 1.13)
ꢀ0.33, 1.59 (ꢀ0.27, 1.02)
0.20, 0.95 (0.18, 0.93)
ꢀ0.35, 1.01 (ꢀ0.32, 1.11)
0.98, 1.53 (ꢀ0.32, 1.04, 1.59)
0.71, 1.54 (0.22, 1.65)
ꢀ1.21, 1.67 (ꢀ1.45, 1.51)
0.21, 1.05 (0.24, 1.14)
0.89 (0.92)
0.91, 1.11 (1.12, ꢀ0.88, ꢀ1.72)
0.69, 0.37 (1.37, 0.60, 0.33)
0.81, 0.32, (0.85, 0.28)
1.15, ꢀ0.89 (1.09, ꢀ0.92)
0.70 (0.68)
1.20, ꢀ0.76 (1.22, ꢀ0.83)
1.16, ꢀ0.79 (1.04, ꢀ0.86)
0.95, ꢀ0.88 (0.97, ꢀ0.93)
1.06, ꢀ1.06 (1.00, ꢀ0.82)
1.26, 0.53 (1.24, 0.50)
1.14, 0.71 (1.19, 0.68)
1.06, ꢀ0.82 (1.03, ꢀ0.89)
0.97, ꢀ1.46 (0.96, ꢀ1.00)
0.94 (ꢀ)
– (1.38)
–
– (1.06)
– (1.12)
– (1.16)
– (1.04)
–
–
–
–
ꢀ1.42, ꢀ0.37, 1.46 (ꢀ1.31, 1.58)
–
ꢀ1.50, 0.72 (ꢀ1.44, 0.35)
ꢀ1.48 (–)
Supporting electrolyte: [NBu4](BF4) (0.1 M); concentrations of the compounds: 1 ꢂ 10ꢀ3 M. All the potentials are referenced to Ag+/AgCl; where Epa and Epc are anodic and
cathodic potentials, respectively. E1/2 = 0.5 ꢂ (Epa + Epc),
D
Ep = Epa ꢀ Epc. (i): these data have been obtained from scan rate 250 mVsꢀ1. Other data (ii) have been obtained by
scan rate 500 mVsꢀ1
.

392
G. Ceyhan et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 95 (2012) 382–398
complexes, that incorporate Schiff bases with electron donor sub-
stituents in the phenyl rings, show a greater tendency to stabilize
manganese (III).
The cyclic voltammograms of the Schiff base Cr(III) complexes
show two irreversible one-electron oxidation waves in the ꢀ1.61
to 1.11 V range. There are no redox waves observed for the free
Schiff base ligands, strongly suggesting that these potentials are
due to oxidation from Cr(III) to Cr(IV). The high oxidation potential
indicates that the Cr(III) oxidation state in Mn2(L3)(H2O)4(NO3)4 is
highly stabilized and it is difficult to oxidize the Cr(III) to Cr(IV) in
the other Cr(III) complexes. The Mn2(L2)(H2O)4(NO3)4 complex has
a reversible process at about 1.11 V. Comparison of cyclic voltam-
mograms of the Cr(III) complexes and the ligands indicate that
both redox waves involve only the metal center. We also examined
Fig. 11. (a–d) Cyclic voltammograms of the Co2(L1)(H2O)2(Cl)2 (a), Cu2(L1)(H2O)2(Cl)2 (b), Mn2(L2)(H2O)2(Cl)2 (c) and Cu2(L3)(H2O)2(Cl)2 (d) complexes at the different scan
rates.

G. Ceyhan et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 95 (2012) 382–398
393
Fig. 11 (continued)
the electrochemical response to the scan rate. In all cases, the ano-
dic and cathodic peak potentials (Epa and Epc) are relatively un-
changed for scan rates of 100, 150, 200, 250 and 500 mV/s.
However, the anodic and cathodic potential difference.
on the positive side. The former is assigned to Ru(III)/Ru(II) reduc-
tion and the latter is assigned to Ru(III)/Ru(IV) oxidation. The one-
electron nature of these waves can be established by comparing
the peak heights for each wave with that of the ferrocene–ferroce-
Schiff base ruthenium (III) complexes show two irreversible
nium couple
(
D
E = 40 mV,
D
E = ꢀ610 mV and Ipa/Ipc = 0.98)
reduction wave (
D
E = 40–780 mV) on the negative side against
E = 80–590 mV)
under the same conditions. Similar results were reported for
other octahedral ruthenium (III) complexes [27]. The effect of the
Ag+/AgCl, and two irreversible oxidation wave (
D

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G. Ceyhan et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 95 (2012) 382–398
Table 8
Catalytic oxidation of cyclohexane (CyH) and cyclooctane with H2O2 under microwave irradiationb.
a
Catalyst
Substrate
CyH conversion (mol%)
Desired products (mol%)
ol:one
By products (mol%)
Cu2(L1)(H2O)2(Cl)2
CyH
CyON
CyH
CyON
CyH
CyON
CyH
CyON
CyH
CyON
CyH
CyON
CyH
CyON
CyH
CyON
CyH
CyON
CyH
CyON
CyH
CyON
CyH
CyON
CyH
CyON
CyH
CyON
CyH
CyON
98.62
63.44
98.20
68.99
98.39
70.12
99.14
72.07
99.55
72.27
99.56
71.08
99.70
59.92
98.03
77.94
98.96
64.18
99.52
67.57
98.72
79.37
99.86
75.22
98.47
74.86
98.94
60.97
99.81
64.28
CyAOH (41.33); Cy@O (23.41)
CyONAOH (2.84); CyON@O (14.30)
CyAOH (13.92); Cy@O (6.57)
CyONAOH (1.87); CyON@O (6.45)
CyAOH (27.79); Cy@O (33.62)
CyONAOH (6.99); CyON@O (32.35)
CyAOH (10.81); Cy@O (13.20)
CyONAOH (3.67); CyON@O (11.40)
CyAOH (6.67); Cy@O (7.70)
CyONAOH (2.00); CyON@O (11.20)
CyAOH (56.45); Cy@O (18.96)
CyONAOH (2.64); CyON@O (12.75)
CyAOH (4.29); Cy@O (1.16)
CyONAOH (1.06); CyON@O (0.85)
CyAOH (25.00); Cy@O (25.19)
CyONAOH (7.77); CyON@O (24.95)
CyAOH (9.03); Cy@O (12.24)
CyONAOH (2.66); CyON@O (9.00)
CyAOH (5.08); Cy@O (6.00)
CyONAOH (0.72); CyON@O (3.90)
CyAOH (46.02); Cy@O (19.49)
CyONAOH (2.65); CyON@O (11.40)
CyAOH (2.88); Cy@O (0.72)
CyONAOH (0.61); CyON@O (0.65)
CyAOH (27.71); Cy@O (29.68)
CyONAOH (6.05); CyON@O (30.65)
CyAOH (9.82); Cy@O (12.47)
CyONAOH (3.44); CyON@O (11.60)
CyAOH (2.70); Cy@O (4.54)
1.77
0.20
2.12
0.29
0.83
0.22
0.82
0.32
0.87
0.18
2.98
0.21
3.70
1.25
0.99
0.31
0.74
0.30
0.85
0.18
2.36
0.23
4.00
0.94
0.93
0.20
0.79
0.30
0.60
0.30
33.88
46.30
77.71
60.67
36.98
30.78
75.13
57.00
85.18
59.07
24.15
55.69
94.25
58.01
47.84
45.22
77.69
52.52
88.44
62.95
33.21
65.32
96.26
73.96
41.08
38.16
76.65
45.93
92.57
63.11
Co2(L1)(H2O)2(Cl)2
Cr2(L1)(H2O)4(NO3)4
Ru2(L1)(H2O)4(Cl)4
Mn2(L1)(H2O)2(Cl)2
Cu2(L2)(H2O)2(Cl)2
Co2(L2)(H2O)2(Cl)2
Cr2(L2)(H2O)4(NO3)4
Ru2(L2)(H2O)4(Cl)4
Mn2(L2)(H2O)2(Cl)2
Cu2(L3)(H2O)2(Cl)2
Co2(L3)(H2O)2(Cl)2
Cr2(L3)(H2O)4(NO3)4
Ru2(L3)(H2O)4(Cl)4
Mn2(L3)(H2O)2(Cl)2
CyONAOH (0.27); CyON@O (0.90)
a
0.02 mmol catalyst:2 mmol cyclohexane:3 mmol hydrogene peroxide (1:100:150) and 5 mL acetonitrile were used for each reaction.
400 W power were applied for 60 min. The reaction temperature and pressure were held at around 140 °C and 30 bar in closed DAP60 vessels.
b
para-substituents (X) present in the Schiff base ligands is much
more pronounced in the case of ruthenium (III)–ruthenium (IV)
irreversible oxidation.
3.3. Cyclohexane and cyclooctane oxidation under microwave
irradiation
In order to determine the optimum reaction conditions, the ef-
fects of the temperature, pressure, co-catalyst, reaction time and
solvent on the cyclohexane and cyclooctane were investigated.
The effect of the catalyst amount on oxidation at a temperature
of 140 °C was studied. Although the conversion of cyclohexane
and cyclooctane are nearly unchangeable with the increasing of
the amount of catalyst, it is beneficial for the transformation of
the cyclohexane and cyclooctane to the cyclic alcohols and
ketones.
Fig. 12. Catalytic oxidation of cyclohexane and cyclooctane under microwave
irradiation.
The decrease in the activity could be mainly attributed to the
loss of metal atoms and catalyst during the reaction and filtration.
From the average of the results, the activities of the Schiff base
transition metal catalysts can be arranged in the following man-
ner: Ru(III) < Mn(II) < Co(II) < Cr(III) < Cu(II). Metal complexes of
the ligand H2L2 show the highest activity on the alkane oxidation
compared to the metal complexes of other ligands. In the ligand
H2L2, there is a hydroxyl group on the p-position in the salicylidene
moiety. This group increases the p-electron density of the ring by
the mesomeric effect. The optimized reaction conditions could be
applied to the oxidation reaction of cyclohexane and cyclooctane
by the Schiff base metal complexes as a catalyst and the obtained
data are given in Table 8. The catalytic oxidation of alkanes with
30% H2O2 under mild reaction conditions is especially an interest-
ing topic, because direct functionalization of inactivated CAH
bonds in saturated hydrocarbons usually requires drastic reaction
conditions, such as high pressure and temperature. The optimum
Fig. 13. Possible catalytic oxidation mechanism.

G. Ceyhan et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 95 (2012) 382–398
395
oxidation conditions were obtained as catalyst:substrate:oxidant
ratio of 1:100:200 in acetonitrile under 400 W microwave power
for 60 min. The temperature and pressure were controlled at about
140 °C and 30 bar by the instrument. A blank has been run under
the same conditions without any catalyst. According to a possible
oxidation mechanism, first and slow step is the oxidation of CyH
and CyON to cyclic-alcohols and cyclic-ketones and other further
oxidized products arise. If the first step is controlled, the selectivity
of the desired products, CyAOH, Cy@O from the substrate CyH and
CyONAOH, CyON@O from the substrate CyON, can be increased.
The obtained reaction products using H2O2 and catalyst under
microwave irradiation are shown in Fig. 12.
A possible catalytic oxidation mechanism is given in Fig. 13. In
this process, H2O2 is the oxidant, the metal complexes are the cat-
alyst and the cyclohexanol forms as an intermediate product. The
oxidation reactions come to pass around the metal of the com-
plexes. It has been proposed that the microwave power and the
novel catalysts could affect the selective oxidation of CyH to
Fig. 14. Influence of the complexes in cyclohexane and cyclooctane oxidation under microwave irradiation. 0.02 mmol catalyst: 2 mmol cyclohexane (cyclooctane): 4 mmol
hydrogene peroxide (1:100:200) and 5 mL acetonitrile were used for each reaction. 300 W power were applied for 30 min. The reaction temperature and pressure were held
at around 100 °C and 30 bar in closed DAP60 vessels.

396
G. Ceyhan et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 95 (2012) 382–398
Fig. 14 (continued)
CyAOH, Cy@O and CyON to CyONAOH, CyON@O. The synthesized
Cu(II) and Cr(III)complexes showed good catalytic activity in the
oxidation of cyclohexane and cyclooctane, yielding the desired oxi-
dized products. On the other hand, the Mn(II), Co(II) and Ru(III)
complexes did not show good selective catalytic effect when com-
pared to the literature data [11–17]. In all catalysis reactions
examined, the Cu(II) and Cr(III) complexes have the highest cata-
lytic activity compared to the other metal complexes for the selec-
tivities of the desired products CyAOH (cyclohexanol), Cy@O
(cyclohexanone), CyONAOH (cyclooctanol), CyON@O (Table 8,
Fig. 14).
In our study, the Co(II), Mn(II) and Ru(III) complexes showed
low catalytic activity on the cyclohexane and cyclooctane sub-
strates. The catalytic performance of the Cu(II) complexes may
comparable to the activity exhibited by the earlier reported com-
plexes [11,14]. The key point in the conversion of cyclohexane to
the oxidized products is the reduction of M(II)–L to M(I)–L (L: li-
gand). This reduction to M(I)–L is facilitated the ligands available
around the metal cation. The formation of the oxidation products
CyAOH and Cy O show the preferential attack of the activated
bonds. Tetrahedral geometry of Cu(II) complex may be the reason
of the having higher activity rather than the other complexes.

G. Ceyhan et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 95 (2012) 382–398
397
Fig. 14 (continued)
In order to give more insight into the structure of the com-
plexes, the thermal studies of the complexes have been carried
out using thermogravimetry (TG-DTA) techniques. The thermo-
gravimetric analyses for the ligands and metal complexes were
carried out within the temperature range from ambient tempera-
ture up to 900 °C. The thermal curve of the Cu2(L1)(H2O)2(Cl)2 com-
plex is given in Fig. 15. The thermal behavior of all the complexes
was almost the same. It was found from TG analysis that the binu-
clear metal complexes start losing mass in the 105–120 °C and end
in the 480–670 °C temperature range after losing big part of total
mass, but the decomposition did not finish completely at this tem-
perature range. The examination of TG curves showed that the
complexes decompose in least four or more stages. These decom-
position steps correspond to loss of the coordinated H2O mole-
cules, two/four Cl atoms, four nitrate groups and Schiff base
ligands. Fourth step did not finish completely within the tempera-
ture range 670–880 °C.
4. Conclusion
In this paper, we obtained three Schiff base ligands ligands
(H2L1–H2L3) and their Cu(II), Co(II), Ni(II), Mn(II) and Cr(III) com-
plexes and investigated as analytical, spectroscopic, thermal and
electrochemical. We obtained the ligand H2L1 and H2L3 as single

398
G. Ceyhan et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 95 (2012) 382–398
Fig. 15. The thermal curve of the Cu2(L1)(H2O)2(Cl)2 complex.
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crystals and investigated their structures by X-ray technique. We
studied alkane oxidation reactions of the transition metal com-
plexes using cyclohexane and cyclooctane as substrates. The Cu(II)
and Cr(III) complexes showed good catalytic activity in the oxida-
tion of cyclohexane and cyclooctane to the desired oxidized
products.
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We are grateful to the Scientific & Technological Research Coun-
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Appendix A. Supplementary data
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Full crystallographic data for the reported ligands (H2L1 and
H2L3) have been deposited with the Cambridge Crystallographic
Data Center, CCDC Nos. 847259 and 856306, respectively. Copies
of this information may be obtained by writing your request to:
The Director, CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax:
+44 1223 336 033; e-mail: deposit@ccdc.cam.ac.uk or http://
www.ccdc.cam.ac.uk).
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