Preparation, spectral behaviour and oxidation of some new zinc(II) salicylaldimines containing sterically hindered phenol

Article abstract of DOI:10.1016/S1386-1425(01)00430-9

A series of new bis[N-(2,6-di-tert-butyl-1-hyxyphenyl)salicylideneminato]zinc(II) complexes bearing one or two HO- and CH₃O-substituents on the salicylaldehyde moiety were prepared, and their spectroscopic properties (IR, electronic, ¹H NMR) as well as the ESR spectra of the one-electron oxidation products were examined. The ¹H NMR examination of Zn(L_x)₂ indicates that, contrary to expectations, the proton resonances of the complexes are shifted to the high-field side compared with those of the free ligands. Oxidation of these compounds with PbO₂ results in the formation of mono- and biradical types Zn(II)-stabilized phenoxyls in which there are both anisotropic and alternating line-width trends. Some spectral patterns are typical for weak interacting (J?A) or the case of J～A biradicals, in which ESR signal appeared as superimposition of two non-interacting monoradical centers with S = 1/2. ESR studies on biradicals type of Zn(L·_x)₂ at 300 and 77 K did not show any evidence of strong exchange coupling in these systems. All radical species were characterized by the ESR parameters.

Full text of DOI:10.1016/S1386-1425(01)00430-9

Spectrochimica Acta Part A 57 (2001) 2337–2344
www.elsevier.com/locate/saa
Preparation, spectral behaviour and oxidation of some new
zinc(II) salicylaldimines containing sterically hindered phenol
Veli T. Kasumov *
Chemistry Department, Faculty of Arts and Sciences, Harran Uni6ersity, Sanliurfa, Turkey
Received 17 November 2000; accepted 5 December 2000
Abstract
A series of new bis[N-(2,6-di-tert-butyl-1-hyxyphenyl)salicylideneminato]zinc(II) complexes bearing one or two
HO- and CH3O-substituents on the salicylaldehyde moiety were prepared, and their spectroscopic properties (IR,
1
electronic, 1H NMR) as well as the ESR spectra of the one-electron oxidation products were examined. The H NMR
examination of Zn(Lx)2 indicates that, contrary to expectations, the proton resonances of the complexes are shifted
to the high-field side compared with those of the free ligands. Oxidation of these compounds with PbO2 results in the
formation of mono- and biradical types Zn(II)-stabilized phenoxyls in which there are both anisotropic and
alternating line-width trends. Some spectral patterns are typical for weak interacting (JꢀA) or the case of JꢁA
biradicals, in which ESR signal appeared as superimposition of two non-interacting monoradical centers with S=1/2.
’
ESR studies on biradicals type of Zn(L x)2 at 300 and 77 K did not show any evidence of strong exchange coupling
in these systems. All radical species were characterized by the ESR parameters. © 2001 Elsevier Science B.V. All
rights reserved.
Keywords: Bis(salicylaldiminato)Zn(II) complexes; Coordinated phenoxyl radicals; ESR; 1H NMR; Optical spectra
process [1]. The fact that sterically hindered phe-
nol and its derivatives can readily undergo one or
1. Introduction
two-electron oxidation to the phenoxyl or p-qui-
none [5,6], respectively, offers the possibility of
The oxidation of phenol compounds catalyzed
by transition-metal complexes often involves elec-
preparing chelates with unusual oxidation states
tron transfer reactions from the substrates to the
of ligands. In particular, it is interesting to know
metal centers [1–4]. Specific questions concern the
in what ways the redox reactivities of the sterically
nature of the organic radical species, which is
hindered phenol systems change upon coordina-
often the initial product of an electron transfer
tion. The character of the coupling between the
two radical centers in the oxidized bis(ligand)
metal complexes is also considered another inter-
* Fax: +90-414-3146984.
esting aspect of these radical systems.
E-mail address: vkasumov@harran.edu.tr (V.T. Kasumov).
1386-1425/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.
PII: S1386-1425(01)00430-9

2338
V.T. Kasumo6 / Spectrochimica Acta Part A 57 (2001) 2337–2344
2. Experimental
Previously we reported the results of synthesis,
X-ray, spectroscopic (UV/Vis, IR, 1H NMR ESR)
investigation and redox reactivity of Cu(II),
VO(II), Co(II), Ni(II) and Pd(II) complexes with
bi-, tri- and tetradentate salicylideneiminate, b-ke-
toiminate and azo-ligands containing sterically
hindered phenol fragment [7–13]. All these com-
plexes, except Cu(II) and VO(II), readily undergo
one- or two-electron transfer oxidation with PbO2
resulting in the formation of the stable metal-con-
taining phenoxyl or secondary phenoxyl radical
species.
Salicylaldehyde, its hydroxy- and methoxy-sub-
stituted derivatives, zinc(II) acetate dihydrate and
all solvents were commercial (Merck, Fluka,
Sigma) products and were used without further
purification. 1-Hydroxy-2,6-di-tert-butyl-aniline
and ligands (LxH) were prepared by a previously
described procedure [7–9,14,15]. The analytical
and spectroscopic data of LxH and their oxida-
tion with PbO2 were reported [14,15].
2.1. Preparation of Zn(Lx)2 complexes
Although a series of radical containing com-
plexes with sterically hindered bidentate salicy-
laldimines bearing some electron-withdrawing
substituents (Cl, Br, NO2) on salicylic moiety have
been prepared and studied [7–15], the complexa-
tion capability of these ligands and the redox
behavior of their complexes are not completely
clear.
The present paper is devoted to the synthesis
and spectroscopic characterization of new salicyli-
deneiminate Zn(II) complexes containing periph-
eral sterically hindered phenol fragment and
electron-donating OH and CH3O substituents, at-
tached to the salicylaldehyde moiety. The radical
intermediates generated by the oxidation of the
synthesized complexes with PbO2 also were
examined.
A hot methanol solution (10 cm3) of zinc(II)
acetate dihydrate (0.2 g, 1 mmol) was added to a
stirred and heated methanol solution (70–80 cm3)
of the appropriate ligand (2.1 mmol). The stirred
mixture was heated at reflux (60–65°C) for 1–1.5
h and then allowed to cool to room temperature.
After standing for 2–3 h, the solution was evapo-
rated to c. 10–15 ml, and the precipitated yellow
product was filtered off, washed with n-hexane,
recrystallized from CHCl3-n-hexane and dried in a
vacuum at 50°C. The analytical and spectral data
of the complexes are presented in Tables 1 and 2.
3. Spectra
We report the synthesis, spectroscopic be-
haviours and oxidation with PbO2 of the zinc (II)
complexes [Zn(Lx)2] prepared from Schiff bases
(LxH) formed from 2,6-di-tert-butyl-1-hydrox-
yaniline and HO-, CH3O-substituted salicylalde-
hyde derivatives as shown in Scheme 1.
Elemental analyses were performed by the Sci-
ence and Technology Research Council of Turkey
8
(TUBITAK) in Gebze. The infrared spectra were
taken in KBr discs with a MATTSON 1000 FT-
IR spectrophotometer in the 400–4000 cm−1 re-
gion. The electronic absorption spectra were
Scheme 1. The structure of LxH, Zn(Lx)2 and generation of the radical ligand Zn(Lx)2 complexes.

V.T. Kasumo6 / Spectrochimica Acta Part A 57 (2001) 2337–2344
2339
Table 1
Melting point and analytical data for Zn(Lx)2 complexes
Compound
M.p. (°C) (dec.)
Yields (%)
Empirical formula
Elemental analysis, found/calculated (%)
C
H
N
Zn(L1)2
Zn(L2)2
Zn(L3)2
Zn(L4)2
Zn(L5)2
Zn(L6)2
Zn(L7)2
270
280
275
290
285
86.8
90.5
84.8
83.6
86.3
90.4
79.8
C
42H52N2O6Zn
67.63/68.25
67.16/67.60
68.80/68.25
66.72/67.60
69.26/68.25
63.30/64.82
67.87/68.93
7.04/7.28
6.55/7.02
7.15/7.28
6.66/7.02
6.84/7.28
7.02/6.73
6.96/7.16
2.98/3.61
3.10/3.75
3.28/3.61
2.97/3.75
3.67/3.61
3.49/3.59
3.57/3.82
C44H58N2O6Zn
C42H52N2O6Zn
C44H58N2O6Zn
C42H52N2O6Zn
C44H58N2O6Zn
(310)
295
C
42H52N2O10Zn
measured on a Shimadzu UV 160 A spectropho-
the formation of radical species during
complexation.
tometer in the 200–1100 nm region in various
1
solvents. H NMR was recorded on a BRUKER
AC 200 spectrometer using TMS as an internal
reference in CDCl3 solutions. The ESR spectra
were recorded on a Varian E-109 C model X-
band spectrometer with 100 kHz frequency modu-
lation. The g-values were determined by
comparison with a g=2.0036 of DPPH sample.
The errors for the g- and A-parameters of radi-
cals are 90.0005 and 90.005 G, respectively.
Oxidation of Zn(Lx)2 compounds was carried out
with lead dioxide in a sealed, degassed system at
room temperature under vacuum. The compound
dissolved in 4–5 ml toluene and PbO2 were trans-
ferred into separate glass tubes on a vacuum line.
Solution was deoxygenated under vacuum (10−3
mmHg) by repeated freeze–pump–thaw cycles.
Then, 90–110 mg of PbO2 were suspended in the
solution of sample and shaken for 30 s at room
temperature under vacuum. After the sedimenta-
tion of the heterogeneous phase, 1 ml of the
solution was taken for ESR measurements. All
ESR spectra of the reaction mixtures were imme-
diately recorded at room temperature under a
vacuum as oxidation proceeded.
4.1. IR and electronic spectra
The characteristic infrared group frequencies
and electronic spectral absorption data are given
in Table 2. The observed low-frequency shift of
nCꢀN stretching in the IR spectra of Zn(Lx)2
complexes (1594–1616 cm−1) relative to free LxH
ligands (1608–1649 cm−1) indicates the participa-
tion of the azomethine nitrogen atom of the lig-
ands in coordination with the zinc(II) ion.
Although the nOH stretching frequency of the
sterically hindered phenol usually appeared at ca.
3600 cm−1, in the spectra of Zn(L1)2, Zn(L2)2,
Pd(L5)2 and Pd(L6)2 complexes showed two bands
in the 3441–3570 cm−1 range. The broad medium
bands within the range 3441–3485 cm−1 can be
attributed to stretching vibrations of the associ-
ated sterically hindered phenol nOH or nNꢁH
(ketoamine tautomer forms of LxH in the solid
state). This assignment was made on the basis
that the low-energy stretching bands (ꢁ3400
cm−1) also appeared for methoxy-bearing Zn(L2)2
and Zn(L6)2 complexes. In addition, these com-
plexes, like others, were readily oxidized by PbO2
with the formation of phenoxyl radicals. Note
that the similar low-frequency shifts in the nOH
were also observed for some free LxH ligands
bearing OH and OCH3 [14,15]. It is interesting to
note that the IR spectra of the sterically hindered
phenol containing salicylaldimines with XꢀH, Cl,
Br and NO2 substituents on the salicylaldehyde
4. Results and discussion
The data of the elemental analyses are consis-
tent with the bis(chelate) structure of Zn(Lx)2
complexes. As supported by the ESR examina-
tion, the products did not show any evidence of

2340
V.T. Kasumo6 / Spectrochimica Acta Part A 57 (2001) 2337–2344
tral behaviors of the free and coordinated salicy-
laldimines are significantly different.
moiety and their metal complexes did not exhibit
any lower-frequency absorption bands in the
range 3100–3600 cm−1 [8].
On the basis of the recorded electronic spectra
of Zn(Lx)2 and LxH in dioxane as shown in Table
2, it is clear that the absorption bands except the
bands within the range 242–254 nm of all com-
plexes investigated are substantially different from
those for the corresponding free ligands. A com-
parison of the electronic spectra of the LxH and
their Zn(Lx)2 complexes (Table 2) measured in
dioxane solution indicates that some bands in the
ultraviolet region and all bands in the 400–460
nm range of the LxH disappeared in the spectra
of complexes. The bands observed in the 242–270
nm range can be assigned to the p“p* transitions
of the aromatic rings. The absorptions appeared
in the range 400–590 nm, presumably due to
intraligand or metal-to-ligand charge-transfer
transitions in the Zn(Lx)2.
The absorption bands centered at 480–590
more likely arise from the intraligand p“p* tran-
sitions in the quinonoid type structure in solu-
tions. It is known that the appearance of the
absorption bands in this range is not characteris-
tic for bis(salicylaldiminato)Zn(II) chelates [16].
The examination revealed that the electronic spec-
1
4.2. H NMR spectra
The 1H NMR spectral results obtained for
some LxH and their Zn(Lx)2 complexes in CDCl3,
together with the assignments, are given in Table
3. The proton resonance appears as a broad,
low-intensity singlet at d 11.45–14.86 ppm in the
spectra LxH attributable to the exchangeable sali-
cylic OH and NH ketoamine tautomer form of
LxH protons (hydrogen-bonded OH/NH) and is
absent in the spectra of Zn(Lx)2 (Table 3). The
absence of this signal in the spectra of Zn(Lx)2
suggests the coordination of the salicylaldehyde
through deprotonated oxygen atom. Another dif-
ference in the spectral feature is high-field shifts of
all proton signals of Zn(Lx)2 relative to the reso-
nances found for uncoordinated free ligands due
to the shelding upon complexation (Table 3). The
CHꢀN, OH and OCH3 signals appear as a singlet
in the range 7.75–8.37, 4.45–5.23 and 3.78–3.83
ppm, correspondingly, which was slightly shifted
to higher fields compared with those for the free
ligand. Interestingly, the similar high-field shift in
the protons resonances was also observed in
Table 2
IR and electronic spectral data for Zn(Lx)2 and LxH in dioxane
Compound
IR spectra cm−1
Electronic spectra, umax (log m), nm
wCꢀN
wOH
Zn(L1)2
L1H
Zn(L2)2
L2H
Zn(L3)2
L3H
Zn(L4)2
L4H
Zn(L5)2
L5H
Zn(L6)2
L6H
Zn(L7)2
L7H
1614
1649
1610
1614
1616
1620
1610
1620
1606
1618
1594
1608
1609
1618
3570, 3441
3635, 3437
3456, 3444
3440
3620
3631, 3304
3610
245, 290, 338, 410
244(4.26), 330a, 358(4.21), 375a, 480a
246, 290, 336, 353a, 376a, 405a
240 (4.2), 254(4.1), 276(4.2), 338(4.4), 380(3.3), 420(2.4)
254, 290a, 324, 387, 590a
242(4.1), 254(4.0), 275(4.0), 340(4.3), 380(3.6), 460 (.9)
254, 294a, 326, 385
3580
243 (4.3), 279 (4.2), 349 (4.3), 430 (3.5)
244, 270a, 335a, 342, 372, 435
3629, 3562
3618, 3443
3531, 3485
4490
3643
3632
245 (4.37), 282 (4.22), 351 (4.48), 420a
245, 270a, 336a, 340, 371, 431
244 (4.5), 271 (4.2), 344 (4.3), 368 (4.3)
242, 258a, 288a, 381, 400a, 450a, 480a
243(4.5), 272(4.1), 320(4.2), 343(4.3), 368(4.3), 395(3.5), 480a
a Shoulder.

V.T. Kasumo6 / Spectrochimica Acta Part A 57 (2001) 2337–2344
2341
Table 3
1H NMR chemical shifts (d in ppm) of LxH and Zn(Lx)2 complexes in CDCl3
a
Compound
OH/NH
CH(N
OH
OCH3
Sal. (H)
Phen (H)
t-Butyl (H)
1.49s
0.91, 1.25, 1.47
1.47s
1.27, 1.47
1.26, 1.47m
1.28, 1.46
1.47–1.56
1.30
L1H
Zn(L1)2
L2H
Zn(L2)2
L3H
Zn(L3)2
L4H
Zn(L4)2
L5H
L6H
14.25
–
14.18
–
11.45
–
14.15
–
8.55
7.75, 8.14
8.60
8.32
8.41
8.23, 8.37
8.48
8.29
8.50
8.55
5.32
4.75, 3.32
5.29
5.18
5.26
5.1, 5.23
5.29
5.12
5.28
5.27
–
–
3.93
3.83
–
6.55–6.60
6.19–6.29
6.88–6.99m
6.65–7.00
6.46m
6.15–6.35
6.49d
6.37–6.38
6.89s
7.15s
6.50–6.59
7.03–7.15s
7.15, 7.25
7.05m
6.96, 7.12
7.10s, 7.25d
7.03–7.11
7.13s, 7.26s
7.14m
–
3.84
3.78
–
3.81
3.78
13.15
13.11
–
1.42s, 1.59s
1.32
1.29, 1.48
6.95m
6.64, 6.86
Zn(L6)2
8.29
5.15
7.05, 7.14
a s, singlet; d, doublet; dd, doublet of doublets; t, triplet; m, multiplet.
NMR spectra of Pd(II) complexes with the same
LxH salicylaldimines [17]. Note that the high-field
shift of the proton resonances on the complexa-
tion has been reported previously by G. Schetty
and E. Steiner for some diamagnetic Co(III) com-
plexes with tridentate salicylaldimine and o,o%-di-
hydroxy-arylazo ligands [18]. The possible reason
for the observed high-field shift may be due to the
structural changes of the coordinated ligands,
which can form o-quinonoid type structures in the
solution. It is well known that, usually, the proton
signals must be shifted down-fields side upon
complexation ligand molecule as a result of the
deshielding effect of the diamagnetic metal ions,
which is in accordance with the decreased electron
density on the ligand. The protons of the tert-bu-
tyl groups of all complexes except Zn(L4)2 exhibit
two or three sharp singlet peaks at d 0.91–1.48
ppm indicating that the tert-butyl protons of
these compounds are magnetically non-equiva-
lent. The ring protons of salicylaldehyde and phe-
nol moieties of LxH and Zn(Lx)2 display
multiplets within 6.46–7.26 and 6.15–7.25 ppm
ranges, respectively, and many resonance of com-
plexes compared with those of the free ligands are
shifted to a high field c. 0.02–0.65 ppm. In some
cases, the singlet lines due to the aromatic protons
of LxH appeared as multiplets in the correspond-
ing Zn(Lx)2 1H NMR spectra (Table 3). Thus, the
1H NMR examination of Zn(Lx)2 indicates that in
spite of the expected low-field shifts in the proton
resonances of the complexes, the high-field shifts
were observed. The reason for the observed
shielding effect is not apparent. However, it is
thought that one alternative reason for this may
be the deviation of the planarity of free ligand
because of the unfavorable steric hindrance in the
coordinated salicylaldimines.
4.3. ESR spectra
The oxidation of Zn(Lx)2 was carried out with
PbO2 in a toluene/CHCl3 mixture at room tem-
perature under a vacuum. The ESR spectrum of
the reaction mixture was simultaneously recorded
as oxidation of the complex progressed. All of
these radicals are more stable than the phenoxyls
generated from uncomplexed free ligands. No ad-
ditional splittings were observed from the tert-bu-
tyl protons in any of these radical spectra. The
radicals generated from complexes were stable at
room temperature for at least 2–3 days, as shown
by the ESR spectra, but the intensity of radical
signals formed from the ligands decreased slowly
even under a vacuum. Although, in principle, in
the examined Zn(Lx)2-PbO2 systems, the forma-
tion of biradicals is expected, the oxidized com-
plexes did not show any evidence of ESR,
indicating the existence of strongly interacting
radical centers (JꢂA) attributable to the typical
half-field line (Dm= 92, g=4) of triplet state
biradical species. Some of the observed spectra
are similar to those found for weak interacting
(JꢀA) or the case of JꢁA biradicals, in which

2342
V.T. Kasumo6 / Spectrochimica Acta Part A 57 (2001) 2337–2344
close to 1:2:3:4:4:4:3:2:1 and ESR parameters such
ESR spectra appeared as a superimposition of
two non-interacting monoradical centers with S=
1/2 [19–21]. For some biradicals, anisotropic
spectra with an alternating line width were
observed.
H
H
’
as Am=1.063 G, g=2.0038 [Zn(L1)2], Am=
H
’
1.033, g=2.0039 [Zn(L3)2] and Am=1.325 G,
’
g=2.0045 [Zn(L4)2]. The number of hyperfine
lines of the ESR spectra of these radicals suggests
that the unpaired electron interacts with one ni-
trogen 14N(I=1) nucleus and two sets of three
hydrogens (one hydrogen atom of the CH group
and two equivalent protons of the phenoxyl frag-
X-band ESR spectra of the lead dioxide gener-
ated radical complexes are shown in Fig. 1. The
one-electron oxidation of Zn(L1)2, Zn(L3)2 and
Zn(L4)2 with PbO2 in CHCl3/toluene solutions
leads to the formation of the Zn(II)-containing
phenoxyl radicals, which are rather more stable
than those obtained from corresponding free un-
complexed ligands. The relatively broadening
(presumably due to intramolecular dipolar ex-
change interactions) spectral features observed for
these radicals are similar to each other and consist
of less-resolved (in some case very poor resolved)
nine lines with a relative intensity distribution
ment) with the values hfsc related as AN=AHCH
=
2AHm. Note that a more accurate determination of
the splitting constants is difficult, because of over-
lapping and broadening some lines of these spec-
tra. It is interesting that the spectral patterns of
the radicals generated from Zn(L2)2, Zn(L5)2 and
Zn(L6)2 are quite different from the abovemen-
tioned radical species (Fig. 1b, c, d). That ESR
’
spectrum of the radical Zn(L2)2 consists of 14
equidistant well-resolved lines (Fig. 1b). The ESR
spectrum of this radical (AH=1 G, g=2.0043)
was analyzed in terms of the interaction of the
unpaired electron spin density with two magneti-
cally equivalent nitrogen (AN=4 G) and two sets
of four protons [AC1H=2 G (1H), AC2H=2 G
(1H) and AHm=1G (2H)]. At the same time, the
’
ESR spectrum of Zn(L5)2 is considerably different
’
from those of all other Zn(Lx)2 in toluene solution
at room temperature (Fig. 1c). For this radical, a
quartet signal centered at g=2.0047 and spacing
of 1.75 G with relative intensity ratios of 1:2:2:1
were observed, which shows no evidence of any
additional hfsc from the m-protons of the phe-
noxyl rings. This four-line pattern probably arises
from splitting one nitrogen (AN=1.75 G) and
one proton of CH (ACHH=3.8 G). The absence of
an additional hyperfine structure from the meta-
protons of phenoxyl rings may be caused by a
broadening from anisotropic dipolar interactions
between radical centers and a broadening from
the modulation of isotropic hfsc due to the rota-
tion of the phenoxyl group [22]. The spectrum of
oxidized Zn(L6)2 also differs from all of other
’
Zn(Lx)2. This signal centered at g=2.0045 con-
sists of 11 well-resolved hyperfine lines with a
spacing of 1.1 G between central lines and an
intensity distribution of 1:2:4:6:7:8:7:6:4:2:1 (Fig.
1d). This spectrum was analyzed in terms of an
interaction of the unpaired with one nitrogen
’
Fig. 1. ESR spectra for Zn(L
) radicals generated by the
2
x
oxidation of Zn(Lx)2with PbO2 at 300 K in CHCl3/toluene
solutions: (a) Zn(L ) , (b) Zn(L ) , (c) Zn(L ) , (d) Zn(L ) .
’
’
’
’
1 2
2 2
5 2
6 2

V.T. Kasumo6 / Spectrochimica Acta Part A 57 (2001) 2337–2344
2343
(AN=2.22 G) and two sets of two hydrogens
[AHCH=2.22 G(2H) and AHm=1.11 G] assuming
hfsc related as AN=ACHH=2AHm. Additional,
equally spaced very-low-intensity four lines, are
also observed at both ends of the central multi-
plet. We assigned these peaks to the satellite lines
arising from 13C isotopes in natural abundance. It
should be noted that similar splitting patterns
were also observed in the ESR spectra of some
oxidized free ligand and complexes [14,15,23,24].
’
Thus, all of the observed spectra except Zn(L5)2
’
and Zn(L6)2 are typical for weak interacting (Jꢀ
A) or the case of JꢁA biradicals, in which the
ESR signal appeared as a superimposition of two
non-interacting monoradical centers with S=1/2.
It is interesting that in the oxidation of
bis(N-2,6-di-tert-butyl-1-hydroxyphenyl-5-chloro-
salicylaldiminato)zinc(II) [Zn(L8)2] and bis(N-2,6-
di-tert-butyl-1-hydroxyphenyl-3-nitro-5-bromo-
salicylaldiminato)zinc(II) [Zn(L9)2] complexes,
slightly different anisotropic spectral patterns are
observed (Fig. 2). These spectral features can be
considered as two superimposed signals. The cen-
tral lines of the spectrum (g=2.0034) generated
from [Zn(L8)2] in benzene under vacuum are nar-
rowed and split into doublets (c. 0.75 G), and
their intensity is decreased with respect to the low-
and high-field doublets (Fig. 2b). The unusual
asymmetric distribution of the intensity and alter-
nating line-width trends of these spectra are typi-
cal for weakly interacting biradicals [19–23].
However, the ESR spectrum of the oxidized sam-
ple of [Zn(L9)2] shows a relatively intense and
broadened central line (g=2.0029) (Fig. 2b). The
temperature dependence of this spectrum exhib-
ited a change in the intensity and resolution of the
ESR signal (Fig. 3). When the temperature of the
benzene solution is decreased from 283 to 253 K,
Fig. 2. ESR spectra of the radicals obtained by the oxidation
of Zn(L9)2 (a) and (b) Zn(L8)2 at 300 K in benzene.
significantly changed (Fig. 3b, c, d). Similar be-
havior was observed reversibly by either warming
or cooling the solution in this range. The intensity
of the spectrum dramatically decreased again and
the spectral features were changed over T\340
K. The temperature-dependence studies of this
spectrum indicate that a change of intensity of the
central lines in Fig. 3 did not follow Curie law
behavior (I81/T), so this spectrum cannot be
assigned to uncoupled monoradical species.
’
the intensity of the spectrum of Zn(L ) is de-
9 2
creased without a significant change in the values
of hfsc and remained low intensity slightly broad
9 lined equidistantly separated (AH=1.094 G)
spectral pattern. Upon warming the solution over
the temperature range of 283–303 K, the original
17 lined spectrum returned, but in the range of
303–333 K, the resolution of the spectrum im-
proved, and the intensity distributions as well as
the linewidth of some lines at its wings were
Summarizing, it can be said that the IR and
electronic spectral behaviors of Zn(Lx)2 com-
plexes are significantly different from those ob-
served
for
similar
complexes
bearing
electron-withdrawing substituents on salicylade-
hyde moiety. The ESR examinations per-
formed indicate that the spectral features of the
radicals generated from bis[N(1-OH-2,6-di-tert-

2344
V.T. Kasumo6 / Spectrochimica Acta Part A 57 (2001) 2337–2344
’
of Zn(L ) did not show any evidence of strong
x 2
exchange coupling in these systems.
Acknowledgements
The author would like to thank the Science and
Technology Research Council of Turkey
8
(TUBITAK) for financial support of this research
(Grant TBAG-1424) and Dr I. Kartal for record-
1
ing the H NMR spectra.
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