Synthesis, spectroscopic and redox properties of the copper(II) complexes of N-(di-methylphenyl)-3,5-Bu2t-salicylaldimines

Article abstract of DOI:10.1016/j.saa.2005.05.018

A series of copper(II) complexes (CuL2x) with new N-di-methylphenyl-3,5-Bu2t-salicylaldimines (L^xH) were prepared and characterized by IR, UV/vis, ¹H NMR, ESR, cyclic voltammetry techniques and chemical oxidation. L^xH ligands have been found selectively bind to a Cu^II, rather than to Ni^II, Co ^II, Mn^II, VO^IV, Zn^II and Cd ^II. ESR examinations of the CuL2x complexes demonstrate that they exist in magnetically diluted mononuclear or coupled triplet-state structures in the solid. The temperature dependent (113-283 K) intensity of the powder ESR spectra for some CuL2x is characteristic of ferromagnetic coupling (J > 0). The reduction potentials of CuL2x in DMSO are sensitive to aniline moieties. Chemical oxidation of CuL2x with (NH₄)₂[Ce(NO ₃)₆] in CHCl₃ and MeCN solutions at 300 K affords gradually disappearance of their ESR signals and dramatic changes in the electronic spectra as well as the appearance of new maximum bands at 530-672 (CHCl₃) and 670-700 nm (MeCN), suggesting generation of Cu^II-phenoxyl radical species.

Full text of DOI:10.1016/j.saa.2005.05.018

Spectrochimica Acta Part A 63 (2006) 330–336
Synthesis, spectroscopic and redox properties of the copper(II) complexes
of N-(di-methylphenyl)-3,5-But2-salicylaldimines
Veli T. Kasumova,∗, Fevzi Koksal , Yasin Zeren
b
a
¨
a
Department of Chemistry, Faculty Arts and Sciences, Harran University, Sanliurfa, Turkey
b
Department of Physics, Ondokuz Mayis University, Samsun, Turkey
Received 24 March 2005; accepted 14 May 2005
Abstract
A series of copper(II) complexes (CuLx2) with new N-di-methylphenyl-3,5-But2-salicylaldimines (LxH) were prepared and character-
ized by IR, UV/vis, 1H NMR, ESR, cyclic voltammetry techniques and chemical oxidation. LxH ligands have been found selectively
bind to a CuII, rather than to NiII, CoII, MnII, VOIV, ZnII and CdII. ESR examinations of the CuLx2 complexes demonstrate that
they exist in magnetically diluted mononuclear or coupled triplet-state structures in the solid. The temperature dependent (113–283 K)
intensity of the powder ESR spectra for some CuLx2 is characteristic of ferromagnetic coupling (J > 0). The reduction potentials of
CuLx2 in DMSO are sensitive to aniline moieties. Chemical oxidation of CuLx2 with (NH4)2[Ce(NO3)6] in CHCl3 and MeCN solu-
tions at 300 K affords gradually disappearance of their ESR signals and dramatic changes in the electronic spectra as well as the
appearance of new maximum bands at 530–672 (CHCl3) and 670–700 nm (MeCN), suggesting generation of CuII–phenoxyl radical
species.
© 2005 Elsevier B.V. All rights reserved.
Keywords: N-(Di-methylphenyl)-3,5-But2-salicylaldimines; Cu(II) complexes; Spectroscopy; Electrochemistry; Chemical oxidation
the CuII and PdII complexes of above ligands, as well as
their Schiff base complexes derived from 2,5- or 3,5-But2-
anilines, unlike their non-But analogs, upon treatment with
PPh3 one-electron reduction via radical intermediates were
observed [2,3]. In this connection, we were interested in the
examining the effects of bulky But-groups introduced on the
salicylaldehyde ring on the redox chemistry of the related
complexes [4a,b]. Our recent studies have revealed that
bidentate N-(X-Ph)-3,5-But2-salicylaldimines prepared from
3,5-But2-salicylaldehyde (3,5-DTBS) exhibit very lower
complexing ability respect to CoII, NiII, VOII, MnII, ZnII
and CdII metal ions [5].
1. Introduction
There is a continues interest in investigating the relation-
ship between the redox reactivity and geometry of bidentate
salicylaldimine metal complexes in solution and solid states,
which could be the result of steric effects and electronic
effects or crystal packing forces [1].
Our early studies on the chemistry of transition
metal(II) chelates with redox-active sterically hindered
N-arylsalicylaldimines and other ligands bearing peripheral
bulky 2,6-But2-phenol group revealed that these family
complexes, unlike their non-But2 analogs, upon oxidation
with PbO2 besides easily generation of peripherally bonded
phenoxyl radical complexes, also displayed the possibility
proceeding intramolecular oxidative C–C coupling of
bis(salicylaldiminato)copper(II) complexes [2]. In addition,
In order to examine further the steric factors of
But2 groups that contribute towards complexation abil-
ity of ligands and electron transfer reactivity of their
complexes a new series of bis[N-dimethylphenyl-3,5-But2-
salicylideneiminato]copper(II) complexes, CuLx2 derived
,
from 3,5-DTBS and dimethyl substituted anilines are
reported (Scheme 1).
∗
Corresponding author. Tel.: +90 4143128456; fax: +90 4143121998.
E-mail address: vkasumov@harran.edu.tr (V.T. Kasumov).
1386-1425/$ – see front matter © 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2005.05.018

V.T. Kasumov et al. / Spectrochimica Acta Part A 63 (2006) 330–336
331
−1.5 to +2.0 V. The measured potentials were recorded with
respect to the Ag/AgCl reference electrode.
2.2. Materials
All chemicals and solvents were reagent grade and
were used without further purification. Cu(AcO)2·H2O,
(NH4)2Ce(NO3)6 (ACN), 2,4-di-tert-butylphenol, all
dimethyl substituted anilines (2,3-dimethylaniline, 2,4-
dimethylaniline, 2,5-dimethylaniline, 3,4-dimethylaniline
3,5-dimethylaniline, 2,6-dimethylaniline), were obtained
from Aldrich Chemical Co. 3,5-di-t-butyl-salicylaldehyde
was prepared by the literature method [6].
Scheme 1.
2. Experimental section
2.1. Measurements
C, H and N analyses were performed with a LECO CHNS
932 elemental analyzer. The electronic absorption spectra
were measured on a Shimadzu 1601 UV–vis spectropho-
tometer in the range 200–1100 nm. IR spectra were recorded
on a Perkin-Elmer FTIR spectrometer in the 450–4000 cm−1
range, in KBr pellets. 1H and 13C NMR spectra were recorded
on a Bruker Spectro spin Avance DPX-400 Ultra Shield
Model NMR or on a BRUKER AC 200 using TMS as an inter-
nal standard in CDCl3. The room temperature (RT) magnetic
susceptibility was obtained by using a Sherwood Scientific
magnetic balance and the diamagnetic corrections were eval-
uated using Pascal’s constants. ESR spectra were recorded at
300 K in the powder state and in CHCl3 solution and 150 K
in the glass state with a E109 C X-band Varian spectrometer,
using DPPH as a field marker. Errors for g- and A-parameters
of the complexes are 0.001 and 0.05 G, respectively.
Cyclic voltammetry experiments were performed
with a PC-controlled Eco Chemie-Autolab-12 potentio-
stat/galvanostat analyzer in acetonitrile (MeCN) or DMSO
under dry N2 at RT with 0.05 M Et4NBF4 as the supporting
electrolyte. Cyclic volammograms (CV) were obtained by
using a standard three-component system consisting of a
platinum working and counter electrodes and an Ag/AgCl
reference electrode. Cyclic voltammetry measurements were
obtained in degassed MeCN solutions of ca. 10−3 to 10−4
LxH and CuLx2 in degassed DMSO in the potential range
2.3. Preparation of ligands
All salicylaldimines, except L4H were prepared in
high yields (70–94%) via condensation of the 3,5-But2-
salicylaldehyde with the corresponding aniline (1:1 molar
ratio) in methanol at reflux for 5–6 h in the presence of
3–5 drops of formic acid and recrystallized from methanol.
All Schiff bases were obtained as yellow products. Ana-
lytical, and spectroscopic data for LxH are given in the
Tables 1 and 2. L4H: Yield: 48%, Mp. 80 ◦C. 1H NMR
(CDCl3): δ = 1.30 (s, 5-C(CH3)3, 9H), 1.54 (s, 3-C(CH3)3,
9H), 2.26 (s, CH3, 6H), 7.04 (t, JHH = 7.43 Hz, C4H of
Ph, 1H), 7.14 (d, JHH = 7.51 Hz, meta-coupled doublet from
anilinic CH-3 and CH-5 protons, 2H), 7.53 (d, JHH = 2.45 Hz,
meta-coupled doublet from salicylic CH-4 proton, 1H), 7.19
(d, 1H, JHH = 2.45 Hz, meta-coupled doublet from salicylic
CH-6 proton, 1H), 8.38 (s, 1H, CH N), 13.07 (a broad-
ened singlet, H-bonded salicylic OH, 1H). L6H: Yield: 91%,
Mp. 82 ◦C. 1H NMR (CDCl3), δ = 1.29, 1.44, 8.60 and
13.78 ppm, for 3,5-But2, CH N, OH groups as a singlet sig-
nals, respectively. Salicylic protons appear as a meta-coupled
doublets at δ = 7.49 (CH-4, JHH = 2.45 Hz, 1H) and 7.27 (CH-
6, JHH = 2.59 Hz, 1H). The methyl protons of L6H resonate at
δ = 2.39 (s, 6H). The signals arising from the aniline protons
onthe2, 6and4positionsappearat δ = 7.17(d, JHH = 7.49 Hz,
Table 1
Physico-chemical, IR and analytical data of LxH and CuL2x compounds
Compound
M.p. (◦C)
Yield (%)
IR spectra (cm−1
)
µeff (MB)
Found (Calcd.) (%)
C
νC
νC
H
N
N
O
L1H
111
103
96
82
114
80
260
177
230
260
230
235
70
78
84
48
94
92
61
59
83
90
84
89
1615
1618
1617
1622
1617
1615
1614
1617
1615
1614
1615
1611
–
–
–
–
–
–
–
1.97
1.57
1.77
2.07
1.83
2.06
72.14 (71.85)
71.86 (71.85)
72.34 (71.85)
82.07 (71.85)
81.96 (71.85)
82.42 (71.85)
75.95 (75.02)
75.91 (75.02)
75.41 (75.02)
76.08 (75.02)
76.45 (75.02)
73.82 (75.02)
9.35(9.26)
9.51(9.26)
9.18 (9.26)
8.45 (9.26)
9.32 (9.26)
9.09 (9.26)
8.36 (8.21)
8.33 (8.21)
7.61 (8.21)
7.57 (8.21)
7.72 (8.21)
6.58 (8.21)
4.21(4.15)
4.32(4.15)
4.28 (4.15)
4.26 (4.15)
4.31 (4.15)
3.96 (4.15)
3.85 (3.80)
3.76 (3.80)
3.97 (3.80)
3.88 (3.80)
3.90 (3.80)
3.68 (3.80)
L2H
L3H
–
–
–
–
1528
1530
1527
1529
1528
1526
L4H
L5H
L6H
CuL12
CuL22
CuL32
CuL42
CuL52
CuL62

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V.T. Kasumov et al. / Spectrochimica Acta Part A 63 (2006) 330–336
1H), 7.30 (d, JHH = 7.59 Hz, H) and 6.86 (s, 1H), respectively.
Table 2
For L1H: H NMR at δ = 1.37 (s, 9H, t-Bu), 1.52 (s, 9H, t-
1
Electronic spectral data for LxH ligands and CuL2x complexes
Bu), 2.30 (s, 3H, CH3), 2.37 (s, 3H, CH3), 6.92–7.24 (m,
4H, Ph), 7.26 (d, JHH = 2.4 Hz, 1H), 7.43 (d, JHH = 2.4 Hz,
1H), 8.56 (s, 1H, CH N), 13.79 (s, 1H, OH); L2H: δ = 1.36
(s, 9H, t-Bu), 1.51 (s, 9H, t-Bu), 2.37(s, 3H, CH3), 2.41 (s,
3H, CH3), 7.047–7.102 (m, 3H, Ph), 7.25 (d, JHH = 1.8 Hz,
1H), 7.47 (d, JHH = 1.6 Hz, 1H), 8.59 (s, 1H, CH N), 13.88
(s, 1H, OH); L3H: δ = 1.34 (s, 9H, t-Bu), 1.49 (s, 9H, t-Bu),
2.36 (s, 6H, CH3), 6.89 (s, 1H. Ph), 6.98 (s, 1H, Ph), 7.13 (d,
JHH = 7.4 Hz, 1H, Ph), 7.24 (d, JHH = 2.2 Hz, 1H), 7.46 (d,
JHH = 2.4 Hz, 1H), 8.57 (s, 1H, CH N), 13.82 (s, 1H, OH);
L5H: δ = 1.37 (s, 9H, t-Bu), 1.51 (s, 9H, t-Bu), 2.31 (s, 6H,
CH3), 2.33 (s, 6H, CH3), 7.06–7.25 (m, JHH = 6.8–8.0 Hz,
3H, Ph), 7.25 (d, JHH = 2.4 Hz, 1H), 7.47 (d, JHH = 2.4 Hz,
1H), 8.67 (s, 1H, CH N), 13.94 (s, 1H, OH).
Compound
L1H
Solvent
EtOH
Electronic spectra (λ/nm) (log ε, M−1 cm−1
)
214(4.6), 230a(1.4), 274(4.3), 320(4.2),
351(4.2)
272, 331, 353
330, 450a
Hexane
Acetone
EtOH
CuL12
214(2.8), 237(2.78), 387(2.15), 409(0.80),
500a, 661(2.54)
CHCI3
CHCI3
EtOH
290, 411, 320a, 490a, 660a
274, 314a, 348a, 635
CuL12 + Ox
L2H
214(4.5), 230(4.5), 275(4.2), 315a, 332(4.3),
355(4.4), 450a(1.08)
Hexane
Acetone
EtOH
275, 333, 356
334, 450a
CuL22
212(4.8), 237(4.7), 290(4.6), 360a, 400a,
500a(2.9), 670(2.42)
CHCI3
CHCI3
EtOH
287, 310a, 500a, 645a(2.48)
333, 355, 370, 530a
CuL22 + Ox
L3H
215(4.6), 230a(4.5), 275(4.3), 335(4.2),
352(4.2), 450a(1.08)
2.4. Synthesis of the complexes
Hexane
Acetone
EtOH
275, 335, 354
Complexes CuLx2 were prepared using following general
procedure. To a stirring solution of the ligand (1 mmol) in
MeOH (50–60 ml) at 60 ◦C was added a solution of copper
acetate monohydrate (0.5 mmol, 0.1 g) in MeOH (10 ml).
The resulting dark brown solution was refluxed under
vigorous stirring for 1.5–2 h. The mixture was concentrated
to ca. 25 ml and cooled to room temperature until green or
brown microcrystalline solid precipitated. It was filtered off,
washed with cold MeOH, dried in air and recrystallized from
CHCl3/n-hexane mixture. Analytical, and spectroscopic
data of CuLx2 are given in Tables 1–3.
330, 450a
CuL32
207(4.1), 236(4.1), 290(4.0), 413(3.6), 500a,
679(2.17)
287, 320a, 408, 500a, 673(2.35)
280, 330a, 350a, 410a, 635
206(4.7), 231(4.7), 272(4.5), 338(4.2)
273, 343
CHCI3
CHCI3
EtOH
Hexane
Acetone
EtOH
CuL32 + Ox
L4H
339
CuL42
232(5.0), 250a, 320a, 383(4.8),
404(4.3),490a(3.3), 660a(2.52)
285, 331, 406, 500a, 670a
275, 400a, 500a, 635
CHCI3
CHCI3
EtOH
CuL42 + Ox
L5H
215(4.7), 229(4.6), 276(4.4), 313(4.4),
328(4.4), 354(4.4), 450a(2.1)
276, 313, 328, 356
353, 450a
298, 415, 500a, 650a(2.52)
276, 325a, 400a, 500a, 670
215(4.6), 230a, 277(4.3), 310(4.3), 326(4.3),
353(4.3), 450a(2.06)
276, 310, 326, 355
321, 353, 450
207(4.4), 239(4.3), 294(4.2), 416(3.8),
500a(2.9), 660a(2.63)
293, 413, 320a, 500a, 660a(2.36)
277, 310, 325a, 354, 500a
Hexane
Acetone
CHCI3
CHCI3
EtOH
3. Results and discussion
CuL52
CuL52 + Ox
The analytical data of the CuLx2 complexes indicate 2:1
ligand to metal stochiometry. The synthesized complexes
are found to be sufficiently soluble in chloroform, diox-
ane, acetone, N;N-dimethylformamide (DMF) for spectral
measurements. Surprisingly that as it was observed for
N-aryl-3,5-But2-salicylaldimines [5] and N-alkyl-3,5-But2-
salicylaldimines [7], the presented N-dimethylphenyl-3,5-
But2-salicylaldimines also possess a very lower complexing
ability with respect to the metal ions such as NiII, CoII, VOIV,
MnII, CdII and ZnII. Our repeated attempts to prepare above
L6H
Hexane
Acetone
EtOH
CuL62
CHCI3
CHCI3
CuL62 + Ox
a
Shoulder.
Table 3
ESR parameters of bis(N-dimethylphenyl-3,5-But2-salisilaldiminato)Cu(II) complexes
Complex
Polycrystalline
Solution (CHCl3:toluene = 1:1)a
Cu
II
gII
g
A
giso
gII
g
Aiso
AII
A
⊥
gII/AII (cm)
⊥
⊥
CuL12
CuL22
CuL32
CuL42
CuL52
CuL62
a
2.216
2.229
2.229
2.225
gav = 2.072
2.262
2.068
2.049
2.045
2.045
150
150
149
152
–
2.121
2.124
2.128
2.128
2.118
2.120
2.238
2.238
2.238
2.228
2.227
2.226
2.063
2.067
2.073
2.078
2.064
2.067
63
68
64
68
64
65
160
160
160
160
164
164
14.5
22.0
16.0
22.0
14.6
15.9
151
151
151
150
147
147
2.055
85
Aiso, AII and A values in G (Gauss).
⊥

V.T. Kasumov et al. / Spectrochimica Acta Part A 63 (2006) 330–336
333
metal complexes starting from their acetates and LxH were
3.4. ESR spectra
unsuccessful. At the same time, there is no any problem in
the preparation of above metal chelates with Schiff bases
derived from non tert-butyl substituted salicylaldehydes and
2,6-di-tert-butyl or 2,6-di-phenyl substituted aminophenols
[2,4c,d].
3.4.1. Solid-state ESR spectra
The spin Hamiltonian parameters of CuLx2 complexes
are listed in Table 3. The ESR spectra of polycrystalline
samples of all compounds were obtained at 300 and 113 K.
No forbidden ꢀMs = 2 transition at half-field of ca.
1600 G was detected for these complexes. The powder
ESR spectra of the present complexes at room temperature
have a wide variety of line shapes, as shown in Fig. 1. The
polycrystalline powder ESR spectra of all CuLx2 at 300 K
3.1. IR and 1H NMR spectra
The assignment of the 1H NMR spectral data for LxH lig-
ands is presented in Section 2. The IR spectra of LxH and their
complexes exhibit strong sharp peaks in the 2860–2960 cm−1
region are assigned to ν(C H) stretching mode of t-Bu
groups. The ν(C N) stretches of CuLx2 are slightly blue
shifted (1611–1617 cm−1) relative to 1615–1622 cm−1, in
the free LxH, indicating coordination of the imine nitrogen
atom to copper (Table 1). A weak broad features centered
at 2500–2800 cm−1, due to ν(OH) of the intramolecularly
bonded OH· · ·N in LxH, are disappeared in the spectra of
CuLx2. A strong band at 1526–1530 cm−1 which appeared
only in the complexes, is attributed to coordinated ν(C O)
stretching mode [8].
are essential those for two-line axial gII > g > ge pattern
⊥
typical of tetragonal copper centers (Table 3) and consistent
with a dx2−y2 ground state [12]. Except CuL52 solid-state
ESR spectra all of other complexes exhibit well resolved gII
(2.216–2.229, AII = 149–152 G) components and unresolved
g (2.025–2.068), indicating that these complexes have very
⊥
weak intermolecular spin-exchange interactions in crystals
with |J| ≤ 0.02 cm−1 [13a]. The spectrum of CuL52 exhibits
only one an exchange-averaged broad signal (Fig. 1c), which
is attributable to dipolar interaction between crystallogra-
phycally non-equivalent CuII centers in the solid [13]. The
1
2
solid-state spectrum of CuL62 at 300 K is unusual for S =
3.2. Electronic spectra
ground-state systems and exhibits five equally spaced com-
ponents of a probably seven-line copper hyperfine multiplet
between 2700 and 3140 G with a separation of 85 G as well
as a strong x, y resonance at ca. 3200 G (Fig. 1d and e). The
spectrum is most likely due to weakly coupled di-copper(II)
Numerical data of absorbances and extinction coefficients
of the ligands recorded in ethanol, hexane and acetone are
presented in Table 2. Absorption spectra of the ligands except
L1H and L4H, along with ␲ → ␲ and n → ␲ transition
bands in ethanol and acetone solutions at 210–355 nm, also
exhibit a low intensity band at ca. 450 nm which is disap-
peared in hexane. This band is attributed to n → ␲* transition
in dipolar zwitterionic ketoamine tautomeric structures of
LxH (Table 2) [9]. The absence of a bands >380 nm in the
spectra of L1H and L4H suggests that for these ligands tau-
tomerization did not takes place even in the EtOH solutions.
*
*
species (gII = 2.262, g = 2.049) [14]. The signal intensities
⊥
of CuL22, CuL23 and CuL62 complexes increased on lowering
temperature from 300 to 113 K, following Curie’s law
(I = C/T) suggesting the existence ferromagnetic coupling
between copper centers [15a]. The spectral shapes of CuL26
are invariant with temperature changes, suggesting the lack
of conformational rotamers [15b]. At the same time, upon
cooling of CuL22 the increasing in gII (2.234 at 283 K and
*
All of the complexes along with intraligand ␲ → ␲ and
n → ␲* bands also exhibit a group of identical a broad asym-
metricvisiblebandsatca. 490–500and660–680 nm(Table2)
which are attributed to dxz/yz → dx2−y2 and dxy → dx2−y2
transitions in distorted tetragonal ligand field, respectively
[10a]. Note that a square planar CuN2O2 array is expected
to produce a d–d transition in the 500–570 nm region [10b].
An intense band in the 380–420 nm range having higher
molar coefficient extinction (104 M−1 cm−1) assignable to
phenolate (p )-to-Cu(II)(d ) LMCT transition [10,11].
␲
␲
3.3. Magnetic moments
The room temperature magnetic moments of CuLx2 except
CuL22 fall in the range of 1.77–2.06 BM which are typical for
1
2
mononuclear of Cu(II) compounds with a S = spin state
and did not indicate any antiferromagnetic coupling of spins
at this temperature. The lower magnetic moment (1.57µB)
obtained for CuL22 probably indicates the existence of the
antiferromagnetic interactions between copper(II) ions.
Fig. 1. Powder ESR spectra of CuLx2 complexes at 300 K: (a) CuL24, (b)
CuL32, (c) CuL25, (d, e) CuL62.

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V.T. Kasumov et al. / Spectrochimica Acta Part A 63 (2006) 330–336
Fig. 2. Solution ESR spectra for some CuLx2 and CuLx2 + (NH4)2Ce(NO3)6
systems in CHCl3/toluene solution: (a) CuL22, (b) CuL24, (c) CuL23, (d)
CuL32 + (NH4)2Ce(NO3)6, (e) CuL22, (f) CuL22 + (NH4)2Ce(NO3)6.
Fig. 3. Frozen-glass CHCl3 solution ESR spectra for some CuLx2 complexes
at 153 K: (a) CuL12, (b) CuL22, (c) CuL26, (d) CuL52.
2.269 at 113 K) and AII (152 G at 283 K and 164 G at 113 K)
were detected. The values of the exchange interaction
CHCl3 was accompanied by decreasing of the intensity of
Cu(II) ESR signals and appearance of the radical signals [5].
When the similar experiments were carried out by treatment
of CHCl3 solution of CuLx2 with three- to five-fold excess
amount of ACN along with color change from dark brown
to light brown or light green, the decrease in intensity ESR
spectra and appearance new broad line around lowest field
component were detected (Fig. 2d and f). The decreasing
of the signal intensity suggests that one-electron oxidation
results in the generation of a CuII–phenoxyl radical complex
parameter, G = (g − 2.0023)/(g − 2.0023), were found
ꢀ
⊥
to be in the range 4.23–5.31 for CuL22–CuL24 complexes,
suggesting that the local tetragonal axes are aligned parallel
or slightly misaligned and consistent with a dx2−y2 ground
state [16].
3.4.2. The solution ESR spectra
The room temperature ESR spectra of CuLx2 in
toluene/CHCl3 solution exhibit a typical four-line pattern
without 14N-shfs resolutions on the high-field compo-
nents (Fig. 2). The cryogenic solution spectra recorded in
CHCl3–toluene (1:3) glasses at 153 (Fig. 3) of all complexes
1
and there is antiferromagnetic coupling between an S =
2
1
CuII and an S = phenoxyl radical. Finally after 1–2 h the
2
ESR spectra are completely disappeared. It is necessary to
note that the generation of coordinated CuII–phenoxyl rad-
ical species recently reported for synthetic model systems
of the active site of galactose oxidase [17]. These param-
agnetic species give rise to electronic ground states ranging
from St = 1 to St = 0 depending on the number of coordinated
radicals [17d].
exhibit usual g > g > 2.03 and A ꢁ A pattern consistent
ꢀ
⊥
ꢀ
⊥
with a dx2−y2 ground state in a tetrahedrally distorted
copper(II) centers (Table 3) [12,14]. The gII and AII values of
these complexes are very similar to each other suggesting that
their molecular geometry is governed by the steric effect of
salicylaldehyde But2 groups. The G values (2.96–3.88) being
<4 are consistent with a dx2−y2 having and a small exchange
The spectral changes for CuLx2 + ACN systems were also
monitored by UV–vis spectroscopy during the progress of
the oxidation of CuLx2 complexes with ACN in CHCl3. The
oxidant is insoluble in CHCl3 so the reaction system was
heterogeneous and prior to each scanning, the solution of
sample was mixed with oxidant by using spatula during
5–6 s. Then solution was transferred into a 1 cm quartz
cuvette to measure spectrum. When three- to four-fold
excess of ACN was added to the solution of CuLx2 in CHCl3
at room temperature the dark brown solution turns to green
or light green with the disappearance of the original bands of
coupling between CuII centers [12a,16]. The g /A quotient
ꢀ
ꢀ
for all CuLx2 (147–151 cm) falls within the range (140–250)
expected for tetrahedrally distorted geometry [11].
3.5. Chemical oxidation
Our recent studies demonstrated that the chemical
oxidation of bis[N-alkyl(aryl)-3,5-But2-salicyilaldiminato]
copper(II) complexes by (NH4)2[Ce(NO3)6] (ACN) in

V.T. Kasumov et al. / Spectrochimica Acta Part A 63 (2006) 330–336
335
CuLx2 and the appearance of new absorptions at 530–672 nm
were observed (Table 2). After several scanning the low-
est energy d–d bands in the range of 600–675 nm were
completely disappeared and in some cases new maximum
bands begin appeared whose intensities gradually increased
by mixing the reaction mixture. Upon repeated scanning
without mixing of CuLx2 + ACN systems, the gradually
decreasing of new bands intensities were detected. It is
interesting that oxidation behavior of CuLx2 + ACN systems
in MeCN were different than that in CHCl3. As soon as the
oxidant and complexes were soluble in MeCN the oxidation
was carried out with eqvimolar molar amount CAN. Upon
adding the solid sample of oxidant to MeCN solution of
complexes (2.12–3.16 × 10−3 M) their dark brown color
was simultaneously changed to greenish-brown and the
ligand field bands of CuL12 (657 nm, ε = 374 M−1), CuL22
(666 nm, ε = 323 M−1), CuL23 (671 nm, ε = 364 M−1 cm1),
CuL42 (659 nm, ␧ = 383 M−1 cm−1) and CuL52 (655 nm (sh),
ε = 349 M−1 cm−1) were converted to maximum bands
at 680 (ε = 175 M−1 cm−1), 682 (ε = 118 M−1 cm−1), 681
(ε = 132 M−1 cm−1), 670 (ε = 136 M−1 cm−1) and 700 nm
(ε = 112 M−1 cm−1), respectively. At the same time the
absorption band of CuL62 (659 nm (sh), ε = 469 M−1 cm−1
was completely disappeared upon oxidation under simi-
lar conditions. The strong maximum bands in the range
282–290 and 398–410 nm observed in the spectra of CuL2x
were disappeared during oxidation.
)
3.6. Electrochemistry
The electrochemical properties of LxH were studied in
MeCN solution under anaerobic conditions at a scan rate of
0.2 V s−1 over a potential range from −1.5 to 2.0 V versus
Ag/AgCl by cyclic voltammetry (CV) using a platinum work-
ing electrode. The ligands exhibit a quasi-reversible redox
waves with Epa/Epc (V) potentials of 1.266/1.073 (L1H),
1.205/1.071 (L2H), 1.198/1.064 (L3H), 1.237/1.034 (L4H),
1.186/1.022 (L5H) and 1.208/1.017 (L6H) versus Ag/AgCl,
respectively, which are assigned to phenolate/phenoxyl
radical couples and an irreversible oxidation peak is present
at ca. +1.8 V for all ligands (Fig. 4a and b). While the
potentials of the first couples exhibit a slight dependence of
Fig. 4. Cyclic voltammograms for some ligands (in MeCN) and complexes (in DMSO): for L4H (a) and L2H (b); for CuL22 (c); initial recorded CV of CuL62
(d) (scan rate 0.1 V s−1) and repeated recorded wave of the same sample with scan rate 0.05 V s−1 (e).

336
V.T. Kasumov et al. / Spectrochimica Acta Part A 63 (2006) 330–336
substituents positions, a potential of second anodic oxidation
process is practically independent on the positions of the
methyl groups. In addition, the CV’s of L2H, L4H and L6H
unlike other ligands also exhibit redox couples at about 0 V
with Epa/Epc (V) potentials of 0.17/−0.13, 0.19/−0.12 and
0.14/−0.15 (Fig. 4a and b), respectively, which according
to [18] can be assigned to cathodic reduction of H+ evolved
in the anodic process and oxidation of adsorbed hydrogen.
The similar couples have been observed for Schiff bases
prepared from 2,6-di-tert-butylated phenols [19].
(b) V.T. Kasumov, A.A. Medjidov, R.Z. Rzaev, R.D. Kasumov, Rus.
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(2004) 591;
¨
(b) V.T. Kasumov, E. Tas¸, F. Ko¨ksal, S¸. Ozalp-Yaman, Polyhedron
24 (2005) 319.
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CV of some CuLx2 was examined in DMSO over the
potential range from −1.5 to 1.5 V versus Ag/AgCl at a scan
rate of 0.1 V s−1. The CV of CuL22, CuL24 and CuL62 com-
plexes together with quasi-reversible CuII/CuIII oxidation
waves with Epa/Epc at 0.35/0.12, 0.29/0.13 and 0.53/0.12 V,
respectively, also exhibit quasi-reversible CuII/CuI reductive
waves at E1/2 = −1.06 (CuL22), −0.98 (CuL24) and −0.79 V
(CuL62). Since all complexes have almost the similar coor-
dination geometry, the observed trend can be interpreted by
the electron-donating effect of the Me groups. It is of interest
that upon repeated running at scan rate of 0.05 V s−1 of CuL26
sample, the oxidation wave at −0.58 V decreases in size
while that for corresponding cathodic wave is significantly
increased in intensity (Fig. 4e) probably indicating that the
process becomes irreversible under these conditions.
[12] (a) B.J. Hathaway, D.E. Billing, Coord. Chem. Rev. 5 (1970) 1;
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1678.
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Acknowledgement
We are gratefully to the Harran University Research Fund
(HUBAK, Grant No.: 275) for financial support of this
research.
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(c) T.R. Felthouse, E.J. Laskowski, D.N. Hendrickson, Inorg. Chem.
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