Spectroscopic and electrochemical characterization of di-tert-butylated sterically hindered Schiff bases and their phenoxyl radicals

Article abstract of DOI:10.1016/j.saa.2004.02.021

A series of sterically hindered N-arylsalicylaldimines (SAs) previously prepared from substituted salicylaldehydes (X-Sal, where X = H, Cl, Br, NO ₂, OH, OCH₃) and 2,6-di-t-butyl-1-hydroxyaniline (L _xH) and 2,5-di-t-butylaniline (L_x′H) were characterized by ¹H and ¹³C NMR, UV-Vis and electrochemical methods. The electronic spectra (ES) (X = OH, OCH₃) in alcoholic solvents (MeOH, EtOH, PrOH, iso-PrOH) unlike other solvents exhibit a new absorption band in the region 630-675nm (ε=19-242M ^-1cm^-1), which are not characteristic for other SA known in literature. The ESR studies of primary phenoxyl radicals generated from L_xH by their oxidation with PbO₂ reveal that some of them with the time are converted to more stable secondary Coppinger's radical. The cyclic voltammograms of L_xH and L_x′H except NO ₂-substituted ones in CH₃CN are similar and along with two or three irreversible anodic waves at the potentials ranging from 0.0 to +1.9V versus Ag/AgCl, also display one or two irreversible reduction waves at potentials -0.6 to +0.5V. A series of a new SA prepared from 3,5-di-t-butylsalicylaldehyde and mono-substituted anilines (X = H, o-, p-F, Cl, Br, OCH₃, p-t-butyl, 5,6-benzo) were characterized by analytical, spectroscopic (IR, UV-Vis, ¹H and ¹³C NMR), and electrochemical techniques. The ES spectra of o-, p-Cl, p-Br, o-CH₃ and 5,6-benzo-substituted SA did not exhibit expected absorptions at 400-500nm in alcoholic solutions.

Full text of DOI:10.1016/j.saa.2004.02.021

Spectrochimica Acta Part A 60 (2004) 3037–3047
Spectroscopic and electrochemical characterization of di-tert-butylated
sterically hindered Schiff bases and their phenoxyl radicals
V.T. Kasumova,∗, A.A. Medjidovb, N. Yaylic, Y. Zerena
a
Chemistry Department, Faculty of Arts and Sciences, Harran University, Sanliurfa, Turkey
b
Institute of Physical and Inorganic Chemistry, Academy of Sciences, Azerbaijan
c
Chemistry Department, Faculty of Arts and Science, KTU, Turkey
Received 15 December 2003; received in revised form 13 February 2004; accepted 13 February 2004
Abstract
A series of sterically hindered N-arylsalicylaldimines (SAs) previously prepared from substituted salicylaldehydes (X-Sal, where X = H,
Cl, Br, NO2, OH, OCH3) and 2,6-di-t-butyl-1-hydroxyaniline (LxH) and 2,5-di-t-butylaniline (Lxꢀ H) were characterized by 1H and 13C NMR,
UV-Vis and electrochemical methods. The electronic spectra (ES) (X = OH, OCH3) in alcoholic solvents (MeOH, EtOH, PrOH, iso-PrOH)
unlike other solvents exhibit a new absorption band in the region 630–675 nm (ε = 19–242 M−1 cm−1), which are not characteristic for
other SA known in literature. The ESR studies of primary phenoxyl radicals generated from LxH by their oxidation with PbO2 reveal that
some of them with the time are converted to more stable secondary Coppinger’s radical. The cyclic voltammograms of LxH and Lxꢀ H except
NO2-substituted ones in CH3CN are similar and along with two or three irreversible anodic waves at the potentials ranging from 0.0 to
+1.9 V versus Ag/AgCl, also display one or two irreversible reduction waves at potentials −0.6 to +0.5 V. A series of a new SA prepared
from 3,5-di-t-butylsalicylaldehyde and mono-substituted anilines (X = H, o-, p-F, Cl, Br, OCH3, p-t-butyl, 5,6-benzo) were characterized
by analytical, spectroscopic (IR, UV-Vis, 1H and 13C NMR), and electrochemical techniques. The ES spectra of o-, p-Cl, p-Br, o-CH3 and
5,6-benzo-substituted SA did not exhibit expected absorptions at 400–500 nm in alcoholic solutions.
© 2004 Elsevier B.V. All rights reserved.
Keywords: Sterically hindered N-di-t-butylaryl-salicylaldimines; N-Aryl-3,5-di-t-butyl-salicylaldimines; Phenoxyl radicals; ESR; 1H NMR; 13C NMR; IR;
UV-Vis; Electrochemistry
1. Introduction
oxidizable phenolate pendant arms, has led us to the synthe-
sis of sterically hindered N-(2,6-di-t-butyl-1-OH-phenyl)SA
ligands [5]. These SA and their transition metal complexes
are readily oxidized with PbO2, producing relatively sta-
ble, free and coordinated phenoxyl radicals. In the course
of studies on the redox chemistry of di-t-butylated N-aryl-
SA, we have synthesized a new series of SA on the basis
of 2,5-di-t-butylaniline [N-(2,5-di-t-butylphenyl)SA] [6a],
3,5-di-t-butylsalicylaldehyde [N-aryl-3,5-di-t-butylsalicylal-
dimines] [6b] and 3,5-di-t-butylaniline [N-(3,5-di-t-
butylphenyl)SA] [6c]. However, 1H and 13C NMR char-
acterization of N-(2,6-di-t-butyl-1-OH-phenyl)SA, and
phenoxyl radicals generated from their oxidation with PbO2,
as well as any spectroscopic data for recently prepared N-
aryl-3,5-di-t-butylsalicyladimines and electrochemical be-
havior of all of the above SA compounds were not reported.
The present report deals with the synthesis, spectroscopic
and electrochemical characterization of a series of bulky
Salicylaldimines (SAs) continues to be a subject of cur-
rent interest, not only because they are widely used as a
precursor compounds for a design of various new metal
complexes and as suitable models for pyridoxal and B6 vita-
mins [1], but also due to their interesting physico-chemical
properties such as phenomenon of proton tautomerization
[2], thermochromism and photochromism [3] as well as
because of different theoretical aspects of photo- and elec-
trochemistry [4].
Our interest in the synthesis, structural characterization,
and reactivity characteristics of complexes involving both
redox-active metals and redox-active ligands bearing readily
∗
Corresponding author. Tel.: +90-414-312-84-2447;
fax: +90-414-314-69-84.
E-mail address: vkasumov@harran.edu.tr (V.T. Kasumov).
1386-1425/$ – see front matter © 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2004.02.021

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V.T. Kasumov et al. / Spectrochimica Acta Part A 60 (2004) 3037–3047
SA bases of formula N-(2,6-di-t-butyl-1-hydroxy-phenyl)SA
(LxH), N-(2,5-di-t-butylphenyl)SA (Lxꢀ H) and N-XPh-3,5-
di-t-butylsalicylal-dimines (LxꢀꢀH).
of this work, perchlorate salts are potentially explosive and
should therefore be handled with appropriate care.
2.2. Preparation of LꢀxꢀH
2. Experimental
All of the salicylaldimine LxꢀꢀH compounds were pre-
pared in good yields (68–96%) as yellow crystalline solids
using standard procedures involving the condensation of
3,5-di-t-Bu-2-hydroxybenzaldehyde with various o- and
p-substituted anilines in refluxing methanol in the presence
of a catalytic amount of formic acid (five drops).
2.1. Materials and physical measurements
All the solvents were reagent grade and used without
further purification. The reagent grade chemicals, mono-
substituted anilines, 2,4-di-t-butylphenol, 2,6-di-t-butyl-
phenol, 2,5-di-t-butylaniline, 2-hydroxynaphthaldehyde,
salicylaldehyde, CH3O- and HO-substituted salicylaldehy-
des and tetrabutylammonium perchlorate (n-Bu4NClO4)
were purchased from the Aldrich and Fluka Chemi-
cal Co. and used without further purification. The Br
and NO2-substituted salicylaldehydes [5a], 3,5-di-t-butyl-
2-hydroxybenzaldehyde and 2,6-di-t-butyl-1-hydroxyaniline
were prepared from commercially available salicylalde-
hyde, 2,4-di-t-butylphenol and 2,6-di-t-butylphenol, re-
spectively, according to the literature procedures [6]. The
LxH and Lxꢀ H Schiff bases were prepared using stan-
dard procedures involving the condensation of equimolar
amounts of appropriate salicylaldehyde derivatives with
4-amino-2,6-di-t-butylphenol and 2,5-di-t-butylaniline, re-
spectively, as described earlier [5a,6a]. The oxidation of
LxH were carried out by mixing of degassed solutions of
salicylaldimines and PbO2 under high vacuum (10−3 to
10−4 mm Hg) in a 25 ml vessel equipped with a 3–4 mm
quartz tubes at one end for taking ESR spectra.
Elemental analyses were performed by the microanalyses
services of the Marmara Research Center of TÜBITAK in
Gebze and by Department of Chemistry at the University
of Fırat. IR spectra were recorded in KBr pellets using a
Perkin-Elmer FT-IR spectrophotometer. Electronic spectra
(ES) were measured on a Shimadzu UV 1601 UV-Vis spec-
trophotometer at 200–800 nm region in various solvents.
1H and 13C NMR spectra were recorded on a BRUKER
AC 200 or Varian XL-200 spectrometers with TMS as an
internal standard in CDCl3 and C6D6 solutions. The ESR
spectra were recorded on a Varian E-109 C model X-band
spectrometer with 100 kHz modulations. The g-values
were determined by comparison with a DPPH sample of
g = 2.0036. Errors for g- and A-parameters of the radicals
are 0.0005 and 0.005 G, respectively. Electrochemi-
cal measurements were carried out with a PC-controlled
Eco Chemie-Autolab-12 potentiostat/galvanostat electro-
chemical analyzer in a nitrogen-purged solution of acetoni-
trile 0.05 M tetrabutylammonium perchlorate (TBAP) as
the supporting electrolyte, in the potential range −1.5 to
2.25 V. Cyclic voltammetry was performed using a standard
three-electrode configuration with a platinum working and
counter electrodes and a Ag/AgCl reference electrode. The
potentials of the redox waves were referenced to Ag/AgCl.
Caution: whilst no problems were encountered in the course
2.2.1. 3,5-But2-2-(OH)C6H2CH=N–C6H5 (Lꢀ1ꢀH)
Aniline (0.93 g, 10 mmol) was added to a stirred solution
of 3,5-di-t-butyl-2-hydroxy-benzaldehyde (2.34 g, 10 mmol)
in methanol (80 ml). Formic acid (five drops) was added
and the solution refluxed for 6 h. After reducing the volume
to 15 ml by evaporating at 45–50 ◦C and allowing to cool
to room temperature, the precipitate yellow–orange crystals
were filtered and recrysallized from chloroform–MeOH
(1:5, v/v). Yield 88%. mp 115 ◦C. UV-Vis (C2H5OH):
λmax(log ε) = 206(4.41), 226(4.43), 238(sh), 277(4.23),
307(4.19), 322(sh) 350(4.08), 450(sh) nm. IR (KBr pellet):
=
ν = 1614 (CH N), 2450–2650 (intramolecular H-bonding),
2866–2959 cm−1 [C–H of –C(Me)3]. 1H NMR (CDCl3,
200 MHz): δ 1.33 [s, 9H, C(CH3)3], 1.48 [s, 9H, C(CH3)3],
7.22–7.45 (m, 5H, NAr–H), 7.21 [d, 1H, J(HH) 2.2 Hz,
C6H2] (meta-coupled doublets at 6C–H), 7.45 [d, 1H, J(HH)
2.5 Hz, C6H2] (meta-coupled doublets at 4C–H), 8.63 (s,
1H, CH N), 13.65 (s, 1H, OH). 13C NMR: δ –C21H27NO
=
(309.4): calcd. C 81.52, H 8.79, N 4.52; C 82.13, H 8.67,
N 4.65.
The SA L2ꢀꢀH–L H were prepared similarly. Some of
ꢀꢀ
13
them were isolated as viscous yellow oils, which were
readily crystallized from hexane. Yields and some physico-
chemical characterization data of LxꢀꢀH are presented in
Table 1.
Table 1
Physico-chemical properties and elemental analysis for LxꢀꢀH
Compound T (◦C) Yield (%) Elemental analyses (%)
(found/calcd.)
C
H
N
L1ꢀꢀH
L2ꢀꢀH
L3ꢀꢀH
L4ꢀꢀH
L5ꢀꢀH
L6ꢀꢀH
L7ꢀꢀH
L8ꢀꢀH
110
138
123
118
139
95
158
89
112
118
110
76
96
76
81
88
91
68
79
92
94
86
79
77
92
82.45/81.57 8.43/8.79 4.63/4.52
77.21/76.79 8.14/7.97 4.76/4.26
76.54/76.79 7.43/7.97 4.08/4.26
73.68/73.34 7.31/7.62 3.78/4.07
72.97/73.34 7.12/7.62 3.67/4.07
65.23/64.95 6.12/6.47 3.76/3.61
65.26/64.95 6.67/6.47 3.76/3.61
82.06/81.69 8.77/9.04 4.21/4.33
82.05/81.69 9.12/9.04 4.43/4.33
78.17/77.84 8.32/8.54 3.78/4.12
78.18/77.84 8.23/8.54 4.54/4.12
81.76/82.14 9.37/9.65 3.32/3.83
82.78/83.52 8.56/8.13 4.11/3.89
L9ꢀꢀH
ꢀꢀ
L
L
L
H
10
ꢀꢀ
H
H
11
ꢀꢀ
12
L1ꢀꢀ3H
145

V.T. Kasumov et al. / Spectrochimica Acta Part A 60 (2004) 3037–3047
3039
3.1. IR and electronic spectra of LꢀxH and LxH
2.2.2. 3,5-But2-2-(OH)C6H2CH=N-4-MeC6H4 (Lꢀ9ꢀH)
Yield 94%. mp 112 ◦C. UV-Vis (C2H5OH): λmax(log ε)
= 227(4.61), 277(4.37), 312(4.39), 354(4.33), 450(sh). IR
The IR and electronic spectral properties of LxH and Lxꢀ H
SA have been reported in the previous papers [5c,5d]. The
strong band observed in the region 1614–1622 cm−1 and a
broad weak absorption at about 2500–2800 cm−1 in the IR
=
(KBr pellet): ν = 1620 (CH N), 2450–2750 (intramolecular
H-bonding), 2866–2959 cm−1 [C–H of –C(Me)3]. 1H NMR
(200 MHz, CDCl3): δ 1.29 [s, 9H, C(CH3)3], 1.44 (s, 9H,
C(CH3)3), 7.19 (2H, d, C6H4–H), 7.39 (2H, d, C6H4–H),
ꢀꢀ
=
spectra of LxH are attributed to C N and intramolecular
hydrogen-bonded –OH, respectively (Table 2). Very strong
band near 1280 cm−1 has been assigned to phenolic C–O
stretching frequency of SA [7]. Narrow stronger bands in the
2868–2957 cm−1 region are assigned to ν(C–H) stretching
modes of the t-Bu groups.
8.59 (s, 1H, CH N), 13.80 (s, 1H, OH). 13C NMR: δ 162.8
=
=
(CH N), 158.2 (2-C), 146.0 (7-C), 140.4 (3-C), 136.8 (5-C),
136.4 (4-C), 129.9 (6-C), 127.7 (9-C and 11-C), 126.6
(10-C), 120.9 (1-C), 118.3 (8-C and 12-C), 35.1 [C(CH3)3],
34.2 [C(CH3)3], 31.5 [C(CH3)3], 29.4 [C(CH3)3], 21.0
(NAr-CH3). The products were characterized by complete
The electronic absorption spectral data of LxꢀꢀH in ethanol
are given in Table 2. The bands observed below 290 nm
and in the 329–365 range are assigned to intraligand ␲ →
1
assignment of H and 13C NMR signals.
∗
∗
␲ and n → ␲ transitions involving molecular orbitals of
=
the C N chromophore and the benzene ring, respectively
3. Results and discussion
[8]. The bands observed as a maximum or shoulder in the
440–460 nm region in ethanol solution are very sensitive to
the polarity of the solvents (in hexane and toluene was not
observed or appeared as a ve∗ry low intensity shoulders) and
is associated to n(O) → ␲ transitions in keto-amine or
dipolar zwitterionic keto-amine tautomer strꢀuꢀctures of LxꢀꢀH.
Surprisingly, for L4ꢀꢀH, L5ꢀꢀH, L7ꢀꢀH, L8ꢀꢀH and L13H compounds
unlike other LxꢀꢀH, as well as LxH and Lxꢀ H SA, no absorp-
tion was observed around 400–480 nm even in case of their
higher concentrated alcoholic solutions (Table 2). This find-
ing suggests that for these SAs, the keto-amine and dipolar
zwitterionic keto-amine tautomer structures either did not
form or their concentrations are insufficient for detection of
this absorption. The reason of this effect is not apparent. The
While the synthesis of the sterically hindered LxH
and Lxꢀ H SA (Scheme 1) have been described previ-
ously [5c,5d], their electrochemical behaviors and phe-
noxyl radicals generated from L1H and L9H–L12H, as
well as their 1H and 13C NMR spectral characteristics
have not been previously reported. Our preliminary ex-
amination reveals that the complexation abilities of LxꢀꢀH,
prepared from 3,5-di-tert-butylsalicylaldehyde, were es-
sentially different from those for LxH and Lxꢀ H prepared
from non-di-t-butylated salicylaldehydes [6]. Satisfactory
elemental analyses were obtained for LxꢀꢀH Schiff bases
(Table 1) and spectroscopic data are consistent with the
formulations depicted in Scheme 1.
X
X = H(L1H), 3-OH(L2H), 3-OCH3(L3H), 4-OH(L4H),
4-OCH3(L5H), 5-OH(L6H), 5-OCH3(L7H), 5-Cl(L8H),
5-Br(L9H), 5-NO2(L10H), 3,5-di-Br(L11H),
3-NO2-5-Br (L12H), 5,6-benzo(L13H)
HO
N=CH
HO
L H
x
X = H(L'1H), 3-OH(L'2H), 3-OCH3(L'3), 4-OH(L'4H),
4-OCH3(L'5H), 5-OH(L'6), 5-OCH3(L'7H),
5-Br(L'8H), 5-NO2(L'9H), 3,5-di-Br (L'10H)
X
N=CH
HO
L' H
x
5
6
12
11
R = H(L''1H), 2-F(L''2H), 4-F(L''3H), 2-Cl(L''4H), 4-Cl(L''5H),
2-Br(L''6H), 4-Br(L''7H), 2-CH3(L''8H), 4-CH3(L''9H),
2-OCH3(L''10H), 4-OCH3(L''11H), 4-t-Bu(L''12H),
11,12-Benzo(L''13H)
R
10
1
CH=N 7
4
2
3
9
8
OH
L'' H
x
C20H3
C18H3
t-Bu: C5 C19
t-Bu: C3 C17
CH3
CH3
CH3
CH3
Scheme 1.

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V.T. Kasumov et al. / Spectrochimica Acta Part A 60 (2004) 3037–3047
Table 2
IR (cm−1) and electronic (EtOH) spectral data for the LxꢀꢀH
Compound
IR spectra ν(CH=N)
Electronic spectra λmax(log ε) (nm, M−1 cm−1
)
L1ꢀꢀH
L2ꢀꢀH
L3ꢀꢀH
L4ꢀꢀH
L5ꢀꢀH
L6ꢀꢀH
L7ꢀꢀH
L8ꢀꢀH
L9ꢀꢀH
1614
1622
1618
1617
1617
1615
1616
1616
1620
1616
1619
1619
1614
225(4.65), 278(4.42), 307(4.51), 353(4.25), 450(1.69)
212a, 233(4.29), 279(4.20), 311(4.14), 358(4.06), 460a
210a, 226(4.40), 277(4.26), 308(4.21), 353(4.11), 460a
212(4.49), 229(4.46), 283(4.25), 359(4.06)
207(4.65), 229(4.62), 281(4.45), 312(4.44), 357(4.31)
206(4.65), 227(4.56), 281(4.39), 312(4.38), 357(4.25), 460(1.76)
206(4.68), 225(4.68), 280(4.55), 330a, 355(4.42)
211(4.39), 230(4.43), 275(4.25), 329(4.14), 353(4.12)
227(4.61), 277(4.37), 312(4.39, 354(4.33), 450a
210(4.65), 234(4.57), 276(4.34), 344(4.31), 450a
206(4.51), 228(4.51), 277(4.21), 331(4.33), 358(4.32), 460a
206(4.77), 227(4.73), 278(4.34), 312(4.34), 355(4.3), 440(1.98)
212(4.78), 227(4.79), 274(4.46), 290a, 365(4.48)
ꢀꢀ
L
L
L
L
H
H
H
H
10
ꢀꢀ
11
ꢀꢀ
12
ꢀꢀ
13
a
Shoulder.
lutions (5 × 10−2 M) of these compounds exhibit a new
very weak intensity broad band at around 630–675 nm
with molar extinction coefficients of 19–242 M−1 cm−1
(Table 3). The absorption in this region is characteristic
for the free 2,6-di-t-butyl-R-phenoxy radicals and other
free phenoxyl radicals [11]. Note that the appearance of
absorptions in this range is not characteristic for aryl-
salicylaldimines or other types of SA. Our examination
shows that this absorption appears only in alcoholic so-
lutions (Table 3). For X = H, Cl, Br, NO2 substituted
LxH SA [5a], as well as for Lxꢀ H and LxꢀꢀH compounds,
under similar conditions no bands were observed over
480 nm.
observation of this feature was only for some o-substituted
(Cl, CH3) and p-substituted (Cl, Br) SA, but not for other
substituted ones in this series; its interpretation on the ba-
sis of the electronic and steric effects of the substituents is
complicate.
Another interesting feature has been observed in
the electronic spectra of OH and OCH3-substituted
N-(2,6-di-t-butyl-1-hydroxyphenyl)SA (L2H–L7H) (Table 3).
The bands observed in the electronic spectra of these SA
in alcohol solutions below 500 nm∗(Table 2) have been
attributed to intramolecular ␲ → ␲ (207–280 nm), n →
∗
∗
␲
(340–370) and n(O) → ␲ (420–512 nm) transitions
[9,10]. Surprisingly, the highly concentrated alcohol so-
Table 3
Electronic absorption spectral data for the N-(1-OH-2,6-di-t-Bu-Ph)-X-Sal
Compound X
3-OH
Solvent
Electronic spectra λmax(log ꢀ) (nm, M−1 cm−1
)
EtOH
iso-PrOH
MeOH
222(5.1), 276(4.8), 341(5.03), 453(3.9)
218, 274, 341, 451, 630(1.43)
208(4.04), 277(3.49), 348(3.66), 457(3.79)
3-OCH3
4-OH
EtOH
MeOH
iso-PrOH
220(4.83), 275(4.44), 342(4.63), 447(3.91), 676(1.39)
218, 274, 341, 451, ∼620(1.36)
207(3.89), 274(3.45), 343(3.65), 451(2.84)
EtOH
MeOH
iso-PrOH
220(4.78), 285(4.39), 348(4.47), 416(4.38), 500a, 664(1.51)
209, 238, 284, 348, 418a, 500a 640(1.65)
209(4.89), 240(4.78), 285(4.26), 348(4.59), 421(3.87), 500a, 625(1.51)
4-OCH3
5-OH
EtOH
MeOH
iso-PrOH
218(4.55), 241(4.46), 284(4.26), 348(4.56), 420(4.15)
212, 240, 283, 348, 419
214, 243, 283, 348, 422
EtOH
MeOH
iso-PrOH
220(5.01), 237(5.05), 272(4.61), 370(4.84), 465(2.72), 677(1.46)
239, 272, 371, 469(2.63)
207(5.12), 239(4.91), 344(4.66), 373(4.69), 500a(2.47), 620(1.43)
5-OCH3
4,6-OH
EtOH
MeOH
iso-PrOH
227, 370(4.88), 467(2.74), 670(1.13),
210, 237, 272, 369, 470, 630a(1.17),
209(4.13), 237a(4.91), 371(2.49), 471(2.49), 630(1.47)
EtOH
MeOH
iso-PrOH
218(5.09), 392(4.98), 512(9.78), 675(2.38)
206, 261, 399, 500a
209, 399, 500a, 630a
a
Shoulder.

V.T. Kasumov et al. / Spectrochimica Acta Part A 60 (2004) 3037–3047
3041
Table 4
1H NMR chemical shifts δ (ppm) of N-(2,6-DTB-1-OH-Ph)-X-Sal (LxH) and N-(2,5-DTB-Ph-X-Sal (Lxꢀ H)
=
Compound
LxH
δ(OH/NH) δ(CH N)
δ(PhOH)
(3)
δ(CH3O)
(4)
δ(SalH) (5)
δ(PhH) (6)
δ(t-C(H3)3) (7)
S
(1)
(2)
a
H
13.85
13.65
7.97, 8.31 4.95 d
8.53, 8.59 5.31 d
–
–
6.91–7.15 m, 6.76 d,
6.88–7.43 m, 7.52 d
7.07 (s, 2H)
7.14 (s, 2H)
1.32 s
1.47, 1.60 d
b
a
5-Br
5-NO2
3,5-Br
13.86
13.65
7.97
8.53
4.99
5.33
–
–
6.76 s, 6.81 s, 6.95 d, 7.1 d, 7.11 d 7.09 (s, 2H)
1.32 s
1.48
b
6.91 d, 7.41 d, 7.51 d
7.14 (s, 2H)
a
14.90
14.99
7.85
8.66
5.01
5.41
–
–
6.67 d, 7.75 d
7.22 d, 8.21–8.39 m
7.08 (s, 2H)
7.14 (s, 2H)
1.32
1.48, 1.57
b
a
14.81
15.13
8.57
9.81
5.02
5.39
–
–
6.69, 6.71 d, 7.53 d
7.71 d, 7.48 d
7.06 (s, 2H
7.18 (s, 2H)
1.32
1.47
b
a
3-NO2-5-Br 16.2
16.65
8.31, 8.32 5.07
–
–
7.53 d, 7.59 s
8.54 d, 8.35 d
7.03 (s, 2H)
7.25 (s, 2H)
1.28, 1.31
1.48
8.59
5.54
a
3-OH
–
8.23
8.55
4.98
5.32
–
–
6.55–7.15 m
6.78–7.15 m
6.58 (s, 2H)
6.82 (s, 2H)
1.32
1.49
b
14.23
a
3-CH3O
4-OH
14.13
14.18
8.39
8.60
4.96
5.29
3.51
3.93
7.13 m
6.55–7.15 m
6.69 (s, 2H)
7.16 (s, 2H)
1.31
1.47
b
a
11.95
11.45
8.17
8.41
4.92
5.26
–
–
6.42 d, 6.50 s, 6.78 d
6.45 m, 7.09 d
7.15 (s, 2H)
7.26 (s, 2H)
1.06–1.43
1.26, 1.47 m
b
a
4-CH3O
5-OH
14.86
14.15
8.33
8.48
4.91
5.23
3.41
3.84
6.67 d, 6.81 d, 7.15 dd
6.49 dd, 7.11 s
7.32 (s, 2H)
7.23 (s, 2H)
1.32
1.47, 1.56
b
a
13.35
13.15
8.23
8.51
4.95
5.29
–
–
6.35 s, 6.48 d, 7.0 d
6.89 (s, 3 H)
7.13(d, J = 6.1 Hz, 2H) 1.41 s
b
7.13 (s, 2H)
1.47 s
a
5-OCH3
13.42
13.11
8.31
8.55
4.96
5.27
3.31
3.81
6.61 s, 6.80 d 6.95 d
6.95 m (3 H)
7.14 (s, 2H)
7.15 (s, 2H)
1.32
1.48
b
a
Lxꢀ H
H
13.30
13.75
8.13
8.31
–
–
–
–
6.90–7.32 m
7.24–7.37 m
6.65 dt, 6.80 d
6.86 d, 6.85–7.07 m
1.26, 1.43
1.34, 1.41
b
a
3-OH
13.72
13.60
8.06
8.41
5.74
5.77
–
–
7.03 dd, 7.18 m, 7.35 d
7.23–7.39 m
6.54–6.58 m, 6.78 d
6.85 t, 6.98–7.03 m
1.29, 1.40
1.34, 1.42
b
a
3-OCH3
13.52
13.43
8.25
8.44
–
–
3.51
3.95
7.16–7.21 m, 7.33 d
7.21–7.30 dd, 7.34 d
6.63–6.72 m, 6.85 d
6.85–7.04 m
1.24, 1.44
1.34, 1.41
b
b
b
4-OH
14.0
8.24
-
-
-
7.19–7.26 m,
7.23–7.38 m
6.44 dd, 6.52 d, 6.95 d 1.34, 1.41
6.45–6.55 m, 6.88 1.34, 141
6.37 d, 6.51 dd, 6.79 d 1.31, 1.38
4-OCH3
5-OH
13.64
8.35, 8.37
3.83
a
12.80
12.62
8.05
8.31
4.17
4.85
–
–
6.95 d, 7.17 m, 7.29 d
7.21 dd, 7.29, 7.34
b
6.78 d, 6.91 s
1.29, 1.33
a
5-OCH3
5-Br
12.85
12.63
8.09
8.39
–
–
3.31
3.82
7.17 m, 7.29 s, 7.34 s
7.25 dd, 7.34 s, 7.38 s
6.57 d, 7.81 m, 7.03 d
6.84 d, 6.95–6.99 m
1.29, 1.45
1.35, 1.41
b
a
13.25
13.10
7.78
8.36
–
–
–
–
7.13–7.17 m, 7.26 s, 7.31 s
7.27 t, 7.36 d, 7.44, 7.73, 7.55 d
6.72 q, 6.95 d
6.82 d, 6.94 d, 2.25 d
1.28, 1.39
1.34, 1.39
b
b
5-NO2
14.35
8.56
-
-
7.8 d, 8.10 d, 8.27 d, 8.43 t
7.21 t, 7.25 t, 7.33 d, 7.42 d
7.22 t, 7.38 t, 7.45 t, 7.78 q, 8.12 d 7.01 d, 7.16 d, 7.25 d
7.39 t, 7.57 t, 7.95 dd, 7.39 d 7.18 d, 7.30 m
6.91 d, 7.12 q, 7.35 m
6.92 d, 7.16 t
1.27, 136
a
b
c
5,6-Benzo
15.43
15.33 d
15.30
9.18,
9.20 d
9.48
–
–
–
–
–
–
1.26, 1.48
1.37, 1.46
1.37, 1.45
a
C6D6.
CDCl3.
CD3COCD3.
b
c
3.2. 1H and 13C NMR studies of LxH, Lxꢀ H and LꢀxꢀH
ble assignments are reported in Table 4. In general, a sig-
nificant downfield shift of the all-proton signal resonances
in CDCl3 was observed compared to those in C6D6. A de-
crease in shielding of the protons is associated with the ring
current effect of the benzene ring. A decrease in shielding of
the azomethine CH and phenolic OH protons on going from
1
The chemical shifts (δ) of the H NMR spectral data of
the N-(1-OH-2,6-di-t-butyl-phenyl)SA (LxH) with X = H,
Br, NO2 substituents on the salicylaldehyde moiety (L1H,
L9H–L12H), obtained in CDCl3 and C6D6, and their possi-

3042
V.T. Kasumov et al. / Spectrochimica Acta Part A 60 (2004) 3037–3047
Table 5
1H NMR spectral data of N-XPh-3,5-DTB-Sal (LxꢀꢀH) (δ, ppm)
=
Compounds
OH
CH N
CH3
OCH3
SalH
PhH
C(CH3)3
H
2-F
4-F
2-Cl
4-Cl
2-Br
4-Br
2-CH3
4-CH3
2-OCH3
4-OCH3
4-t-Bu
5,6-Benzo
13.65
13.54
13.58
13.51
13.50
13.42
13.49
13.77
13.80
14.05
13.87
13.86
13.72
8.63
8.71
8.59
8.62
8.61
8.59
8.60
8.57
8.59
8.67
8.62
8.64
8.72
–
–
–
–
–
–
–
2.41
2.33
–
–
–
–
–
–
–
–
–
–
–
7.21, 7.45
7.22, 7.47
7.22, 7.45
7.23, 7.48
7.23, 7.46
7.23, 7.48
7.22, 7.47
7.21, 7.46
7.18, 7.41
7.20, 7.44
7.21, 7.43
7.24, 7.44
7.27, 7.48
7.22–7.46 m
7.14–7.19 m, 7.26 d
7.03–7.21 m, 7.25 t
7.20 d, 7.24–7.44 dd
7.18–7.21 m, 7.36 d
7.07–7.18 m, 7.34 m, 7.65 d
7.52 d, 7.15 d
7.09–7.21 m, 7.23 d
1.33, 1.48
1.33, 1.48
1.33, 1.48
1.33, 1.49
1.33, 1.48
1.33, 1.49
1.33, 1.48
1.34, 1.48
1.29, 1.44
1.33, 1.49
1.33, 1.48
1.33, 1.49
1.35, 1.54
–
7.19 d, 7.39 d
3.85
3.81
–
6.91–6.98 m, 7.14–7.19 t,
6.90–6.95 t, 7.24–7.20 m
7.21 d, 7.24, 7.39 dd, 7.45 d
7.17 d, 7.53 m, 7.75 d, 7.95 m, 8.28 t
–
–
and 6. All LxꢀꢀH Schiff bases show a narrow intense singlet
signal at δ ca. 13.49–14.05, which is due to the exchange-
able H-bonded OH/NH proton. This peak, compared to
that in the 1H NMR spectra of Lxꢀ H and LxH, is very
slightly broadened, probably suggesting that intramolecular
H-bonding in the later is stronger than in LxꢀꢀH. The protons
=
C6D6 (δ 7.97–8.57 for CH N and δ 4.95–5.07 for phenolic
=
OH) to CDCl3 (δ 8.53–9.81 for CH N and δ 5.31–5.54 for
phenolic OH) is associated with the ring current effect of
benzene solvent. A similar trend in resonances, due to only
by the electron-withdrawing effect of substituents, were ob-
served for the intramolecular H-bonded salicylic OH proton
(δ 13.85–16.20 in C6D6 and 13.65–16.65 in CDCl3) (see
ꢀꢀ
=
of CH N, CH3 and OCH3 groups in LxH exhibit a singlet
resonance in the region δ 8.59–8.72, at δ 2.41 and 2.33 (for
2- and 4-CH3) and at δ 3.85 and 3.81 (for 2- and 4-OCH3),
respectively.
=
Table 4). The CH N proton resonances of L1H and L12H
appeared as doublets centered at δ 7.97, 8.31 in C6D6; at δ
8.53, 8.59 in CDCl3 (for L1H) and δ 8.31, 8.32 ppm in C6D6
(for L12H), respectively (Table 4), suggesting the existence
of the non-equivalency of azomethine proton in these lig-
ands. The observed splitting of the methine proton likely
originated from the keto-amine tautomer form [12]. Pheno-
lic meta-protons appear as a singlet but the salicylic ring
proton resonances as a singlet and doublet peaks in the re-
gion δ 6.67–8.54 (Table 4). Similar trends in chemical shifts
were also observed in the 1H NMR spectra of Lxꢀ H obtained
in CDCl3 and C6D6 (Table 4).
3.3. Oxidation of 1-OH-2,6-DTB-X-SA with PbO2
The one-electron oxidation of LxH with PbO2 in deoxy-
genated benzene, toluene or toluene/CHCl3 solutions un-
der vacuum at 300 K •leads to the formation of the stable
phenoxyl •radicals (Lx H). Some ESR spectra of the gen-
erated Lx H radicals are shown in Fig. 1. The values of
hyperfine splitting constants (hfsc) and g-factors extracted
from the ESR spectra of the phenoxyl radicals are listed in
Table 7.
The 1H and 13C NMR spectral characteristics of the
LxꢀꢀH compounds obtained in CDCl3 are given in Tables 5
Table 6
13C NMR chemical shifts δ (ppm) for N-XPh-3,5-DTB-Sal (LxꢀꢀH) in CDCl3
=
Compound X
2-F
CH N
Ar–C
C(CH3)3
C(CH3)3
165.75
163.83
165.7, 158.4, 140.5, 137.1, 128.4, 127.5, 127.3, 126.9, 124.6, 124.5,
121.5, 118.2, 116.7, 116.3
163.5, 158.9, 158.1, 144.7, 140.6, 136.9, 128.1, 126.8, 122.5, 122.4,
118.2, 116.3, 115.6
35.1, 34.2
31.4, 29.4
4-F
35.1, 34.2
31.6, 29.6
2-Cl
4-Cl
2-Br
4-Br
2-CH3
2-OCH3
164.61
164.06
164.5
164.1
163.47
163.81
158.5, 145.8, 140.5, 137.2, 130.1, 128.8, 127.7, 127.2, 126.9, 119.4, 118.1
158.2, 147.2, 140.6, 136.9, 132.0, 129.4, 128.3, 126.8, 122.4, 118.1
158.5, 147.3, 140.6, 137.2, 133.1, 128.6, 128.4 127.5, 127.0, 119.4, 118.1
158.2, 147.6, 140.7, 136.9, 132.4, 128.3, 126.9, 122.8, 119.9, 118.1
158.3, 147.8, 140.5, 136.9, 132.2, 130.5, 127.9, 126.7, 126.4, 118.4, 117.6
158.5, 152.5, 140.1, 137.6, 136.8, 127.7, 127.3, 126.5, 129.9, 120.0,
118.1, 111.7, 55.75b
35.1, 34.2
35.5, 34.2
35.2, 34.2
35.1, 34.2
35.1, 34.2
35.1, 34.1
31.6, 29.7
31.6, 29.7
31.43, 29.38
31.62, 29.65
31.45, 29.38, 18.3a
31.46, 29.42
4-OCH3
4-t-Bu
11,12-benzo
161.65
162.97
165.3
158.5, 157.9, 141.6, 141.4, 136.8, 127.5, 126.5, 118.4, 114.5, 55.4b
158.2, 149.7, 145.9, 140.4, 136.9, 127.7, 126.6, 126.2, 120.7, 118.3
159.1, 146.8, 140.8, 137.6, 134.5, 128.9, 128.24, 128.17, 128.0, 127.8,
126.8, 126.7, 126.0, 123.7, 119.4, 114.5
35.1, 34.2
35.8, 34.5, 34.2
35.5, 34.3
31.46, 29.39
31.5, 29.8, 29.44
31.7, 29.8
a
CH3.
OCH3.
b

V.T. Kasumov et al. / Spectrochimica Acta Part A 60 (2004) 3037–3047
3043
Fig. 1. ESR spectra for L1•H phenoxyl radicals generated by the oxidation of LxH with PbO2 at 300 K in benzene or toluene/CHCl3 solutions: (a) L1•H;
(b) L8•H; (c) L9•H; (d) Coppinger’s type radical formed from L8•H after 6 h; (e) L13•H; (f) L5•H; (g) L7•H; and (h) L9•H.
The ESR spectrum of oxidized L1H samples consists of
a less resolved eleven equally spaced lines, separated by
1.162 G (g = 2.0053) (Fig. 1a). Because of non-binomial
resolution of this spectrum, its detailed interpretation is dif-
ficult. However, the 10-line pattern qualitatively can be an-
alyzed in terms of an interaction of the unpaired electron
with one nitrogen (AN = 2.324 G) and two sets three pro-
tons (AHCH = 4.648 G) and AHm = 1.162 G (2H).
structure (hfs) (Fig. 1b–e). The ESR spectral pattern of
•
L8 H–L13•H radicals may be interpreted if it is assumed
that the splitting arises from the interaction of the un-
paired electron with two equivalent meta-protons (AHm), one
azomethine proton (ACHH) and one nitrogen nucleus (AN),
which hfsc related as AHm = AN/2 = ACHH/4. The values
of AHm, AN and ACHH parameters extracted from spectra us-
ing this relation are presented in Table 7. It is interesting
•
The ESR spectra of the phenoxyl radicals generated
from L8H–L13H ligands bearing electron-withdrawing sub-
stituents on salicylic moiety consists of various asymmetric
patterns some of which exhibit a less resolved hyperfine
that the spectra of the initially generated primary L1 H
•
and L8 H phenoxyl radicals after 5–6 h are converted to
a well-resolved nine-line hyperfine structure (g = 2.0045)
spacing of 1.063 G with an intensity distribution ratio of ca.

3044
V.T. Kasumov et al. / Spectrochimica Acta Part A 60 (2004) 3037–3047
Table 7
ESR parameters of the generated Lx•H (L1•H, L8•H–L12H)
Compound
AN
AmH
AHCH
g
Number of lines
H
3-OH
3-OCH3
4-OH
4-OCH3
5-OH
5-OCH3
4,6-di-OH
5-Cl
2.324
3.25
1.013
2.10
2.42
3.13
2.02
2.12
2.31
2.442
2.388
3.57
2.51
3.37
1.163
1.625
1.013
1.05
1.06
1.38
4.648
6.5
2.026
–
2.0012
2.0052
2.0045
2.0042
2.0042
2.0049
2.0048
2.0044
2.0047
2.0046
2.0049
2.0044
2.0054
2.0042
10
11
9
9
9
12
11
9
11
11
13
15
9
2.12
3.13
4.04
2.12
4.62
4.884
7.164
2.678
2.508
–
1.01
1.06
1.155
1.221
1.194
0.893
1.254
1.11
5-Br
5-NO2
3,5-di-Br
3-NO2-5-Br
5,6-Benzo
8
g = 2.0045 0.0003 and hfsc of AHm = 1.0625 G (Table 7).
The hfs of these spectra can be interpreted in terms of the
interaction of the unpaired electron spin density with two
equivalent meta-protons of the phenoxyl fragment (AHm),
1:4:7:8:8:8:7:4:1 (Fig. 1d). The stability of these secondary
radical species and their ESR parameters are very similar to
those observed for Coppinger’s radical [13]. The essentially
similar spectral pattern can be generated by the oxidation
of N-(3,5-di-t-butyl-4-hydroxyphenyl)-3,5-di-tert-butyl-p-
quinonemonoimine with PbO2 and some transition metal
complexes [5a,14].
=
with the hydrogen atom of the CH N azomethine group
(AHCH) and with the nitrogen nucleus (AN). Surprisingly,
the central components of the poorly resolved ESR spec-
•
•
•
While ESR spectra of L2 H–L7 H radicals bearing
electron-donated OH and OCH3 g•roups on the salicylic
trum of L2 H radical (g = 2.0052) exhibit a small hfsc
(0.875 G), whereas the large splitting (∼2.125 G) occurs on
the low-field and high-field sides of the spectrum (Fig. 1h).
It is thought that this spectrum originates from two different
•
ring, except spectra L2 H and L6 H which show more
complicated pattern, are characterized with different reso-
lution and line width (Fig. 1g and h) they have very similar
•
conformational structures of L2 H radical.
Fig. 2. Cyclic voltammograms (0.3 V s−1) of some LxH and Lxꢀ H in CH3CN (5 × 10−4 M) containing 0.05 M n-Bu4NClO4 at 25 ◦C: L1H in the potential
range −1 to +2 V (a) and 0 to +2 V (b); L2ꢀ H (c) and L7ꢀ H (d) in the potential range −1 to +2 V.

V.T. Kasumov et al. / Spectrochimica Acta Part A 60 (2004) 3037–3047
3045
X
X
PbO2
.
.
2
N=CH
HO
HO
O
=
N=CH
HO
=
N
O
O
.
L H
x
L H
Coppinger's Radical
x
Scheme 2.
Table 8
Cyclic voltammetric results for LxH and Lxꢀ H in acetonitrile
Compound LxH
Ep1a (V)
Ep2a (V)
Ep3a (V)
Ep4a (V)
Ep3c (V)
Ep2c (V)
Ep3c (V)
H
5-Br
5-NO2
3,5-di-Br
3-NO2-5-Br
3-OH
–
–
1.247
1.282
1.345
1.294
1.424
1.509
1.548
1.023
1.177
1.602
1.685
1.734
1.748
1758
–
1.749
1.885a
1.785
1.534
1.841
−0.271
−0.089
−1.041
−0.819
–
0.137
0.051
0.025
−0.309
0.052
–
–
–
–
–
0.117
0.187
0.629
1.152a
0.2055
–
0.892
0.841
−0.1314
–
−0.658
−0.092
–
0.301
0.482
0.242
0.647
–
−0.548
1.192a
1.311
0.528
–
–
–
–
–
5-OH
3-OCH3
4-OCH3
5-OCH3
Lxꢀ H
H
3-OH
0.545
−0.007
–
0.987
–
–
–
–
–
1.454
–
–
0.885
−0.767
0.789
–
1.837
–
−0.249
−1.27
0.081
−0.263
−0.643
−0.187
–
0.172
4-OH
5-OH
1.313
1.536
1.087
1.039
1.089
1.395
1.448
0.727
–
–
–
–
–
–
1.873
1.829
1.415
1.687
1.696a
–
1.769a
1.777
–
0.535
3-OCH3
4-OCH3
5-OCH3
5-Br
3,5-di-Br
5-NO2
3-NO2-5-Br
5,6-Benzo
–
–
–
–
–
–
–
–
−0.131
–
0.4332
0.835
1.102
−0.302
−0.249
0.797
–
0.994
−0.602
0.335
–
–
−1.124
−0.901
–
0.250
1.244
–
a
Weak shoulder.
Table 9
Cyclic voltammetric data for LxꢀꢀH
Compound
Ep1a (V)
Ep2a (V)
Ep1c (V)
Ep2c (V)
ꢁEp (mV)
E1/2 (V)
H
2-F
4-F
2-Cl
4-Cl
2-Br
4-Br
2-CH3
4-CH3
2-OCH3
4-OCH3
4-t-Bu
5,6-Benzo
1.258
1.303
1.463
1.492
1.371
1.335
1.051
1.205
1.173
1.149
1.173
1.195
0.182
–
–
–
–
–
–
1.119
1.132
1.014
–
–
–
–
–
–
–
–
139
171
449
–
1.188
1.217
1.238
–
–
−0.294
−0.359
–
1.088
1.078
−0.218
−0.362
−0.3404
0.512
0.538
–
–
117
95
–
–
–
–
1.281
–
1.146
1.125
1.111
1.085
–
–
1.703
1.625
1.735
1.317
0.518
0.546
0.915
The ESR spectra of phenoxyl radicals generated from
L3H and L7H consist of nine equally spaced lines with an
intensity ratios of 1:4:8:8:8:8:8:4:1 (Fig. 2). The observed
hfs pattern of these signals may be easily interpreted in
terms of the interaction of an unpaired electron spin density
with one nitrogen and four equivalent protons with the hy-
perfine splitting constants (G): AHm = 1.0625, AN = 2.125,
spectively. Note that these spectra according to their line
width, the intensity ratios of hfs lines as well as the val-
ues of g- and A-factors were nearly identical with those
that observed for Coppinger’s radical [13]. On the basis
of these experimental results, we suppose that the Cop-
pinger type radical has been generated in the one-electron
oxidation of L3H and L7H, by the following pathways
(Scheme 2):
•
•
A = 1.0625 and AN = 2.126 for L3 H and L7 H, re-

3046
V.T. Kasumov et al. / Spectrochimica Acta Part A 60 (2004) 3037–3047
3.4. Electrochemistry
+1.247 to 1.509 V are shifted anodically indicating their
difficulty in oxidation. But the expected opposite trend for
SA with electron-donating OH unlike their OCH3 analogues
were not observed (Table 8). Note that the unsubstituted
N-phenylsalicylaldimine under the same conditions exhibits
only a single irreversibly anodic wave at +1.529 V ver-
sus Ag/AgCl. Analysis of the results presented in Table 8
shows that the potentials of the highest irreversible anodic
waves in the range +1.68 to +1.9 V do not correlate with
the electron donor/acceptor ability of the substituents. The
observed anodic waves with Ep2a and Ep3a probably orig-
inate from the phenolate/phenoxyl and phenoxyl/quinone
redox processes of phenolic and salicylic OH group. The
highest oxidation waves with Ep4a can be associated with
the disproportionation products of the phenoxyl radicals or
Pt–phenoxyl species, which can be formed on a Pt electrode.
It is thought that the deviation from co-planarity of pheno-
lic and salicylic planes may be the main possible reason for
the less sensitivity of the oxidation potentials of phenolate
arms to electronic factors on the salicylic moiety of Lxꢀ H.
The voltammetric data of LxꢀꢀH are summarized in Table 9.
The CV of ligands exhibits a quasi reversible and irre-
versible redox waves (Fig. 3) (peak-to-peak separation,
The electrochemical properties of the LxH, Lxꢀ H and
LxꢀꢀH have been studied by cyclic voltammetry with a
platinum-working electrode in the potential range +2.25
to −1.5 V versus Ag/AgCl in CH3CN solutions and se-
lected data are summarized in Tables 8 and 9. The cyclic
voltammograms of LxH and Lxꢀ H, except Lꢀ H which ex-
hibits only a single irreversible anodic peak 1at 1.464 V, in
which X = H, 5-Br, 3-OCH3, 5-OCH3, 5-NO2 are similar
but not identical. They display two or three irreversible
anodic waves at the potentials ranging from 0 to +1.9 V
versus Ag/AgCl. No cathodic waves were observed in re-
verse scans from + 1.9 to + 0.6 V. For some SA along with
the main irreversible reduction waves over the range –0.5
to + 0.6 V versus Ag/AgCl, the minor ill-defined cathodic
peaks also were observed. A typical voltammograms of
some LxH and Lxꢀ H are presented in Fig. 2. The CV of some
SA shows the sweep-rate dependence behavior and multi-
ple scanning in the potential ranges +2.0 to 0 and +2.0 to
−1.5 V shows that some waves are not repeatable (Fig. 2a
and b). With an increase in the electron-withdrawing power
of the substituent groups, the Epa values in the range
Fig. 3. Cyclic voltammograms in CH3CN containing 0.05 M n-Bu4NClO4 at 25 ◦C with a Pt-electrode: (a) N-o-OCH3Ph-3,5-DTB-salicyaldimine (0.3 V s−1);
(b) N-o-FPh-3,5-DTB-salicylaldimine (0.2 V s−1); (c) N-phenyl-salicylaldimine (0.1 V s−1); (d) N-o-CH3Ph-3,5-DTB-salicylaldimine (0.1 V s−1).

V.T. Kasumov et al. / Spectrochimica Acta Part A 60 (2004) 3037–3047
3047
(b) G.O. Dudek, E.P. Dudek, J. Am. Chem. Soc. 88 (1963) 2407;
ꢁEp: 95–579 mV, E1/2: 1.125–1.235 V) (Table 9), which
are assigned to phenolate/phenoxyl couples. The E1/2 re-
dox potentials of L2ꢀꢀH (X = o-F) are more positive com-
pared to the unsubstituted L1ꢀꢀH or L8ꢀꢀH and L9ꢀꢀH bearing
electron-donating o-CH3 and o-OCH3 groups, respectively,
suggesting that the former are harder to oxidize than lat-
ter, due to electron-withdrawing ability of F atom. In this
connection, the unsubstituted N-phenylsalicylaldimine and
N-4-tolylsalicylaldimine under the same conditions exhibits
only one irreversible oxidation wave at Epa = 1.519 and
1.436 V (Fig. 3c), respectively. This fact clearly indicates
that the 3,5-di-t-Bu groups in the salicylaldehyde moiety
could efficiently sta+bilize the electrochemically generated
cation radicals, L2ꢀꢀ• H [15]. The CV curves of L5ꢀꢀH and
L6ꢀꢀH are very similar to each other and exhibit along
with a quasi-reversible couples around 0 V and, also an
irreversible oxidation wave at ca. Epa = 1.38 V versus
Ag/AgCl. Surprisingly, the CVs of 4-Br, 4-t-Bu, OCH3 and
5,6-benzo-substituted salicylaldimines are characterized by
two irreversible anodic waves in the potential ranges of
1.193–1.303 and 1.643–1.798 V versus Ag/AgCl under the
same conditions. Although the first irreversible response
can be attributed to phenolate/phenoxyl radical couple, an
unambiguous interpretation of the second irreversible oxi-
dation peaks is not easy. We assume that the second chemi-
cally irreversible responses at 1.643–1.798 V are associated
with phenoxyl/quinone couples or Pt-phenoxyl species.
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Acknowledgements
This work was partly supported by Science and Technol-
ogy Research Council Grant TBAG-1424. V.T.K. acknowl-
edges the Research Fund of Harran University for support
through project no. 233.
[10] R. Bhafta, M. Hellrwell, C.D. Garner, Inorg. Chem. 36 (1997)
2944.
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[13] M. Coppinger, Tetrahedron 18 (1962) 61.
[14] (a) V.T. Kasumov, B. Karabulut, I. Kartal, F. Köksal, Trans. Met.
Chem. 26 (2001) 64;
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