Synthesis and Spectroscopic Studies of New Bis(N-1-hydroxy-2,6-di-tert-butylphenyl-salicylideneaminato)cobalt(II) Complexes and their Oxidation with PbO2

Article abstract of DOI:10.1515/znb-2001-0811

A series of new bis[N-(1-OH-2,6-di-tert-butylphenyl)salicylaldiminato]cobalt(II) complexes possessing one or two HO- and CH₃O- substituents on the salicylaldehyde moiety were prepared, and their spectroscopic properties as well as their oxidation with PbO₂ were examined. ESR data indicate that oxidation of the complexes produces stable phenoxyl radicals. All phenoxyl radicals have similar g-values and hyperfine coupling constants, which are influenced very little by the substituents and by coordination. The experimental observations indicate that the Co(L^·x)₂ radicals are ligand-localized and that there is no notable contribution from the metal d-orbitals to the values of the g-factors. Satellite peaks from the ¹³C nuclei in the aromatic ring were also observed. The ESR spectra of some of the complexes showed partial conversion of primary radicals to secondary paramagnetic species exhibiting interaction with ⁵⁹Co(I = 7/2) and ¹⁴N nuclei.

Full text of DOI:10.1515/znb-2001-0811

Synthesis and Spectroscopic Studies of New Bis(N-l-hydroxy-2,6-di-fe/tf-butyl-
phenyl-salicylideneaminato)cobalt(II) Complexes and their Oxidation with P b02
Veli T. Kasumov3, Biinyamin K arabulutb, Ibrahim K artalb, and Fevzi Köksalb
a Department of Chemistry, Faculty of Arts and Sciences, Harran University,
Sanliurfa, Turkey
h Physics Department, Faculty of Arts and Sciences, Ondokuz Mayis University,
Samsun, Turkey
Reprint requests to Prof. V. T. Kasumov. E-mail: vkasumov@harran.edu.tr
Z. Naturforsch. 56b, 778-786 (2001); received March 28, 2001
A series of new bis[7V-(l-OH-2,6-di-ferr-butylphenyl)salicylaldiminato]cobalt(II) com-
plexes possessing one or two HO- and CH30- substituents on the salicylaldehyde moiety
were prepared, and their spectroscopic properties as well as their oxidation with Pb02were
examined. ESR data indicate that oxidation of the complexes produces stable phenoxyl radi-
cals. All phenoxyl radicals have similar g-values and hyperfine coupling constants, which are
influenced very little by the substituents and by coordination. The experimental observations
indicate that the Co(L x)2 radicals are ligand-localized and that there is no notable contribu-
tion from the metal d-orbitals to the values of the g-factors. Satellite peaks from the 13C
nuclei in the aromatic ring were also observed. The ESR spectra of some of the complexes
showed partial conversion of primary radicals to secondary paramagnetic species exhibiting
interaction with 59Co(I = 7/2) and 14N nuclei.
Introduction
sponding expected bis-salicylaldiminates. Their ox-
idation with P b 0 2 leads to the formation of coor-
dinated phenoxyl radicals with different ESR
spectra. The ESR spectra of the radical interm edi-
ates generated by the oxidation of Co(II) com-
plexes were quite different from those obtained
for other metal complexes. The redox behaviour
of the complexes and their oxidized products are
not fully understood. In order to obtain further
information on the nature of these radical species
we decided to investigate similar metal chelates
bearing electron-releasing substituents on the sali-
cylaldehyde moiety.
In this paper we report the synthesis, spectro-
scopic characterization and oxidation behavior of
the cobalt(II) complexes obtained from Schiff
bases formed from 2,6-di-te/t-butyl-l-hydroxyani-
line and HO-, CH30-substituted salicylaldehyde
derivatives.
Oxidation processes catalyzed by transition-
m etal complexes capable to activate substrates
and/ or reactants by electron transfer reactions
from the substrates to the m etal centers are of cur-
rent interest from viewpoints of synthetic prob-
lems [1, 2]. The ability of m etal ions to control
the oxidation potentials of organic molecules by
complexation also plays a significant role in bio-
logical electron transfer systems. Examples are
transition metal complexes containing sterically
hindered phenol (SHP) fragments. The fact that
SH P’s can undergo one- or two-electron oxid-
ations, to the phenoxyl or quinone, respectively,
offers the possibility of preparing chelates with un-
usual oxidation states [5-8].
Previously [5], we reported the unusual reac-
tions of CI-, Br-, N 0 2-substituted bidentate salicy-
laldimines containing 2,6-di-te/t-butyl-l-hydroxy-
phenyl fragments with Cu(II), which instead of the
expected bis(salicylidenaiminato)copper(II) com-
plexes formed Cu(II) chelates with a tetradentate
salicylaldiminate ligand as a result of C-C oxida-
tive coupling of phenols moieties. Complexation
of these ligands with Co(II), VO(IV), Ni(II),
Experimental
The elemental analyses were carried out in the
Turkish Center of the Science and Technology R e-
search Council (TÜBITAK) in Gebze. Infrared
spectra were recorded on a MATTSON 1000 FTIR
Pd(II) and Zn(II), however, yielded the corre- spectrophotom eter in the region 4000-400 cm -1
0932-0776/2001/0800-0778 $06.00 © 2001 Verlag der Zeitschrift für Naturforschung, Tübingen • www.znaturforsch.com
D
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V. Kasumov • Bis(N-l-hydroxy-2,6-di-fert-butyl-phenyl-salicylideneaminato)cobalt(II) Complexes
H(j
Co— Cr
C a - c /
Co(Lx)2
Co(L‘x)2
X= H(L'H), 3-OH(L2H), 3-OCH3(L3H), 4-OH(L4H), 4-OCH3(L5H),
5-OH(L6H), 5-OCH3(L7H), 4,6-di-OH(L8H), 3,4-di-OH(L9H)
Scheme 1
Table 1. Electronic spectral data for the LXH salicylal-
dimines in dioxane.
using KBr discs. Electronic spectra were measured
on a Shimadzu UV 160 A spectrophotom eter in the
200-1100 nm region in various solvents. Magnetic
susceptibilities were measured by the Faraday tech-
nique at room tem perature. The apparatus was cal-
ibrated using Hg[Co(NCS)4]. Effective magnetic
m om ents were calculated by the equation, /uefi =
2.828[(^M)xT)]1/2, where xm is the molar magnetic
susceptibility corrected for diamagnetism by the
use of Pascal’s constants [9]. The ESR spectra were
recorded on a Varian E-109 C model X-band spec-
trom eter with 100 kHz frequency modulation. The
g-values were determ ined by comparison with g =
2.0036 of a D PPH sample. The errors for g- and
A -param eters of radicals are ± 0.0002 and ± 0.005
G, respectively. Oxidation of Co (Lx) 2 compounds
was carried out by lead dioxide in a sealed, de-
gassed system. The compound (dissolved in 4 ml
of toluene) and P b 0 2 were transferred into sepa-
rate glass tubes on a vacuum line. The solution was
deoxygenated under vacuum (10-3 mm Hg) by re-
peated freeze-pump-thaw cycles. Then 90-110 mg
of P b 0 2 was suspended in the solution of the sam-
ple and shaken for 30 s under vacuum. A fter the
sedim entation of a heterogeneous phase, 1 ml of
the solution was taken for ESR measurements. All
E SR spectra of the reaction mixtures were re-
corded at room tem perature under vacuum as the
oxidation proceeded.
Electronic spectra Amax (log £•), nm
LXH
L!H
L2H
244(4.26), 330*, 358(4.21), 375*, 480*
240(4.20), 254(4.14), 276(4.24), 338(4.39),
380(3.28), 420(2.44)
242(4.12), 254(4.01), 275(4.07), 340(4.26),
380(358), 460(1.95)
L3H
L4H
L5H
L6H
L7H
243(4.32), 279(4.18), 349(4.28), 430(3.52)
245(4.37), 282(4.22), 351(4.48), 420*
244(4.51), 271(4.18), 344(4.27), 368(4.29)
243(4.51), 272(4.17), 320(4.26), 343(4.31),
368(4.36), 395(3.54), 480*
L8H
253(3.89), 381(4.18)
* Shoulder.
Co(Lx)2 complexes were prepared by the
following procedure: A slow stream of nitrogen
was passed through a stirred solution of the corre-
sponding salicylaldimine (3 mmole) refluxed at
65-75 °C in ethanol (50-70 ml) and a deoxyge-
nated solution of cobalt(II) acetate tetrahydrate
(1.5 mmole) in m ethanol (10 ml) was added. The
mixture was heated at ca. 60-65 °C with stirring
for about 4 0-50 min under nitrogen and evapora-
tion to 20-25 ml left to cool to room tem perature.
The precipitated brown-red complex was filtered,
washed with a small am ount of methanol and fi-
nally with hexane. The crude product was crystal-
lized from a acetone-CH Cl3 mixture, yield 78-
95%. Analysis for C o(L1)2: M.p. 270 °C.-UV/vis
(dioxane): Amax = 208, 240, 333, 380, 508 nm. - IR-
(pellet): v = 1610 (CH=N), 3610 cm "1 (O-H).-
fieff= 4.47 B. M. - C42H52N 20 4Co: calcd. C 72.26,
H 7.42, N 3.96; found C 71.15, H 7.57, N 3.47.
In the alternative m ethod, the Co(II) complexes
Preparation o f the compounds
Salicylaldehyde, its hydroxy- and methoxy-sub-
stituted derivatives, Co(OAc)2-4H20 , and all
solvents were commercial (Merck, Fluka, Sigma)
products and used without further purification. 1-
Hydroxy-2,6-di-/m-butyl-aniline
and
ligands
(LXH ) were prepared as previously described [5, were prepared in a similar m anner under atm o-
10], The analytical and spectroscopic data of LXH
and their oxidation by P b 0 2 have been reported
[10] .The electronic spectral data of LXH in dioxane
are given in Table 1.
spheric air conditions. In this case, except for the
compound synthesized from L JH, all complexes
were identical with those of the corresponding
Co(Lx)2 compounds obtained under nitrogen. The
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Table 2. Physical and analytical data for the complexes.
V. Kasumov • Bis(N-l-hydroxy-2,6-di-fert-butyl-phenyl-salicylideneaminato)cobalt(II) Complexes
Compound
M.p.°C
(dec.)
Elemental analyses, found/calcd [%]
Color
Yield
(%)
^eff
(B.M.)
C
H
N
brown
reddish
brown
green
brown
orange
brown
reddish
brown
brown
reddish
brown
brown
270
(200)
80
90
71.15/72.26
66.92/68.19
7.57/7.42
6.35/7.08
4.49
5.25
3.47/3.96
2.80/3.78
Co(L1)2
Co(L2)2
Co(L3)2
Co(L4)2
Co(L5)2
275
270
289
88
85
95
4.30
5.18
4.95
67.20/68.82
67.93/68.19
69.23/68.82
6.16/7.35
6.71/7.08
7.67/7.35
2.65/3.64
3.12/3.78
3.71/3.64
Co(L6)2
(255)
78
5.12
69.25/68.19
7.31/7.08
3.66/3.78
Co(L7)2
Co(L«)2
(190)
275
90
70
4.87
4.25
66.35/6882
64.28/65.36
7.32/7.35
6.28/6.73
3.55/3.64
3.78/3.62
Co(L9)2
286
82
4.95
64.86/65.36
7.16/6.73
3.26/3.62
physical and analytical data of the complexes are
summarized in Table 2.
upon chelation, which is consistent with coordina-
tion through the azomethine nitrogen atoms of the
ligands. The stretching frequency of the sterically
hindered O H of LXH and Co(Lx)2 appears as a
sharp strong peak in the range 3440-3637 cm -1
and 3482-3648 cm -1, respectively (Table 3). The
appearance of two v(OH) frequencies, observed
for Co(L2)2 and Co(L5)2 - Co(L7)2, indicate the
existence of sterically hindered O H in different
Results and Discussion
The elemental analyses, magnetic susceptibility
measurements, IR and electronic absorption
spectral data indicate that the Co(Lx)2 complexes
exist in the high-spin 1:2 metal:ligand stoichiome-
try. ESR spectra of the starting complexes con- environments, or of isomers of these complexes.
firmed that no Co(II) species is present even at
liquid nitrogen tem perature. All complexes can be
obtained as orange-brown or red powders and are
sparingly soluble in common organic solvents but
dissolve appreciably in dioxane and toluene. The
comparison of analytical, magnetic and spectral
The IR spectra of complexes with hydroxy substit-
uents in the 4- and 5-positions show a broad m e-
dium band at ca.3428 and 3536 cm -1 which can be
attributed to vOH. In agreement with the data for
other N-aryl-salicylaldimine complexes of Co(II)
[11, 12], the bands at about 650-550 and 550-420
data indicates that the attem pted complexation of cm -1 for the present compounds are assigned to
L XH with Co(OAc)2 in air gives instead of the ex- vCo-O and vCo-N stretching frequencies, corre-
pected Co(L1)2 complex - a red organic com - spondingly.
pound identified as indophenol (I) [8] (Scheme 2).
In the oxidative conversion of L XH to I the Co(II)
The effective magnetic moments (4.25-5.25
B. M.) of the Co(Lx)2 complexes at room tem per-
ion probably plays as catalytic role. The C o(L1)2 ature per molecule fall in the range 4 -5 B. M.,
complex was finally prepared under nitrogen at- which suggest a spin quartet state S = 3/2. For
mosphere.
C o(II) (d7) a spin quartet state S = 3/2 arises either
in a weak field octahedral configuration or in a
tetrahedral configuration [13-15], The electronic
spectra of the complexes and the fact that LXH
The C=N stretching vibrations of the ligands are
located in the region 1614-1649 cm -1 and are
shifted to lower frequencies (1598-1618 cm -1)
Scheme 2
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V. Kasumov • Bis(N-l-hydroxy-2,6-di-fert-butyl-phenyl-salicylideneaminato)cobalt(II) Complexes
781
Table 3. IR and electronic spectral data for Co(Lx)2 complexes.
Co(Lx)2
Electronic spectra**, Amax (log e), nm
IR spectra (cm-1)
vC=N
vOH
Co(L1)2
Co(L2)2
Co(L3)2
Co(L4)2
Co(L5)2
Co(L6)2
Co(L7)2
Co(L8)2
Co(L9)2
3614
208, 240, 333, 380, 508
1610
1618
1619
1598
1612
1610
1602
1606
1612
3629, 3646
3626
249(4.66), 327(4.48), 400*, 480*, 750*, 900, A>1100
245(4.57), 317(4.29), 388(4.32), 600*, 690*,750*, A>1100
245(4.59), 317(4.42), 380(4.38), 600*, 690*, 890, A>1100
254(4.72), 301(4.42), 381(4.32), 640*, 735, A>1100
246(4.62), 318(4.6), 432 (4.27), 698, 870*, A>1100
247(4.69), 317(4.38), 426(4.25), 500*, A>1100
250, 280*, 354, 500*
3626
3619, 3648
3625, 3566
3621, 3482
3625, 3578
3635, 3470
257, 330, 368, 490*
* Shoulder; ** in dioxine.
ine ligand and the easy oxidation of the cobalt(II)
chelates. The spectra of some complexes show low
intensity shoulders at ca.500-670nm which are
probably due to d-d transitions. The spectra of the
Co(L2)2-Co(L7)2 complexes show another band in
the near-IR region Amax>1100 nm, along with max-
imum at 900 and 735 nm (Table 1). Although the
spectral region of the spectrophotom eter does not
allow us to determ ine the absorption bands higher
could be coordinated only as a bidentate ligand
suggests that the tetrahedral geometry is most fa-
vored for Co(Lx)2 .
The electronic spectra of the Co(Lx)2 complexes
in dioxane show two or three peaks in the ultravio-
let region. The bands beyond 400 nm are very in-
tense (logf = 4.27-4.72) and are observed only on
complex formation. The band at 380-432 nm
seems to be due to both the
transitions of
C=N and charge-transfer transition arising from n-
electron interactions between metal and ligand
which involves either a metal-to-ligand or ligand-
to-m etal electron transfer [14-16]. The observed
bathochrom ic shift in the higher-energy spectral
bands are probably caused by the electron transfer
from the metal to the ligand [16]. This is in
agreem ent with both the strong electron acceptor
properties of the imine group of the salicylaldim-
than 1100 nm (Fig. 1), the
ff values of Co(Lx)2
and the similarity of the absorption spectra with
those of known bis(Ar-aryl-salicylideneaminato)-
cobalt(II) complexes [15, 16], probably indicate
that all these complexes possess tetrahedral coor-
dination around the C o(II) ion. The shoulders in
the range 500-670 nm and absorption maximum
at about 690-900 nm may tentatively be assigned
to 4A 2-* 4T2 and 4A 2->4Ti(P) transitions, respec-
Fig. 1. ESR spectra observed in the oxidation of Co(L1)2 with Pb02 in benzene at room temperature: (a) initial
stage spectrum; (b) after 2h.
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782
V. Kasumov • Bis(N-l-hydroxy-2,6-di-rert-butyl-phenyl-salicylideneaminato)cobalt(II) Complexes
arises from the interaction of the unpaired elec-
tron with 59Co(I = 7/2), one 14N( = 1) and one
azomethine proton nucleus with A HCH= A N and
A Co = 3AN. In this case from the spectra the
tively [16]. Although the electronic spectra are in
good agreement with tetrahedral geometry, the
possibility of penta- and hexacoordinate adducts
of the Co(Lx)2 complexes with dioxane cannot be
ruled out completely. Comparison of the elec- A h c h = A n = 1.975 G, ACo = 5.93 G values can be
estimated. A t the same time, when the spectrum of
this sample was recorded after standing 3 h under
tronic spectra of the LXH ligands (Table 1) and
their Co(Lx)2 (Table 3) complexes m easured in di-
oxane solution indicate that some bands in the ul- vacuum, its phenoxyl part was disappeared com-
traviolet region and all bands in the 400-460 nm
range of the LXH have disappeared in the spectra
of complexes.
pletely and only the 27-line signal with the un-
changed ESR param eters remains (Fig. lb). A low
intensity asymmetric signal (g = 2.009) without of
hyperfine structure was observed at 77 K. No solid
m atter was found in ESR tubes excluding the pos-
sibility that the radical species might decay to a
Co(salicylaldehyde)2 type complex and another
radical product.
Oxidation o f Co (Lx)2
No ESR signals were observed for powders and
freshly prepared solutions of Co(Lx)2 at 300 and at
77 K because rapid spin-lattice relaxation of Co2+
broadens the lines at higher tem peratures [15].
The Co(Lx)2 compounds are readily oxidized with
lead dioxide in toluene/CHCl3 solution at room
tem perature and under vacuum with the form a-
tion of stable phenoxyl radicals. In general, while
oxidation proceeded the intensity of the signal
increased, and no decrease was observed during
5 -6 h. W hen the tem perature was lowered, the
intensity of the ESR spectra of Co(L x)2 decreased
markedly and finally a slight asymmetric unre-
solved signal remained at 77 K. As the sample was
heated to room tem perature the original spectra
was regenerated. The substantial decrease in the
intensity of the ESR signal at lower tem perature
is probably caused by dimerization of the radicals.
The ESR spectra of the reaction mixtures did not
show any triplet state biradical species, for which
the appearance of two sets of A m = 1 lines, about
Oxidation of the Co(L2)2, Co(L5)2, Co(L6)2
complexes in toluene/chloroform solutions imme-
diately leads to the formation of the radical
species having
a well-resolved nine-line ESR
signal (g = 2.0045-2.0052) of spacing 1.063 G
with an intensity distribution approximately
1:4:7:8:8:8:7:4:1 ratio. The ESR spectra of these
radicals are shown in Fig. 2. A t both sides of these
symmetric ESR spectra the low intensity five sat-
ellite lines arising from 13C isotopes in natural
abundance (Figs. 2 (a)-(c)) have been observed. It
is interesting that these spectra were identical with
spectrum II (Coppinger’s radical), which was
form ed by the oxidation of indophenol
I
(Scheme 2). The observed nine-line spectrum may
be easily analyzed by assuming that the unpaired
electron spin density interacts with one nitrogen
and four equivalent raeta-protons of II with the
values of hfsc related by A HW = 1/2AN. A t the
g = 2 and a half-field Am = 2 signal at ca. g = 4 same time, the simultaneous observation of a se-
signal, were expected.
condary radical without any evidence of primary
The oxidation of Co(L1)2 with P b 0 2 at room
tem perature in benzene solution immediately
leads to the formation of the stable param agnetic
species and its spectrum is given in Fig. la. This
spectrum consists of two superim posed signals
with similar intensity. One has a triplet pattern
spacing of 4.25 G in which each com ponent shows
additional less resolved hfsc constant of 0.98 G
and centered at g = 2.0049, probably, arising from
the phenoxyl radical. A nother signal centered at
g = 2.0094 consists of 27 equidistant lines spacing
of 1.975 G. The observed hyperfine structure may
be interpreted if it is assumed that the splitting
9
Fig. 2. ESR spectra of Co(L2)2 (a), Co(L 5)2 (b) and
Co(L 6)2 c ) in toluene/chloroform solutions at 300K.
(
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V. Kasumov • Bis(N-l-hydroxy-2,6-di-rert-butyl-phenyl-salicylideneaminato)cobalt(II) Complexes
783
radicals seems to be less likely. In addition, unlike
at g = 2.0048. The ESR spectrum consists of 10
II, when the m entioned radicals standing in air and equidistant lines with spacing of 1.125 G. We did
were shaken, the intensity of their spectra slightly
decreases. We suggest that another alternative ex-
planation for the observed spectral pattern may
be interaction of the unpaired electron with one
azomethine proton, one nitrogen nucleus and
three equivalent (two m eta-protons on the phe-
noxy ring and one proton in the 6-position of the
not observe additional lines when the m odulation
amplitude is varied over a range of 0 .1 -2 G. The
spectrum was analyzed by assuming that the un-
paired electron spin density interacts with one ni-
trogen and three sets of four hydrogens, which hfsc
related as A HCH=A N - 2 A Hm = 2A Hsal [AHCH -
2.05 G (1H), A Hm = 1.025 G (2H), A HSai = 1.025
salicylaldehyde ring) by assuming A wH=AsalH= G and A N = 1.025 G]. It is interesting that when
A N = 1/3ACHh . The simulation of these spectra, the reaction mixture was allowed to stand at room
which were obtained using a com puter program of tem perature and under vacuum overnight (—20 h),
McKelvey [14c], is shown in Fig. 3. A match of the
com puted plot with the experim ental spectra was
obtained with the hfsc A cHH = 3.075 G (1H),
A mH= 1.025 G (2H), A Sal.H - 1.025 G (1H), A N =
the spectrum of this sample along with the above
ten-line signal, also show a new low intensity octet
centered at g = 2.00 with a 59Co hyperfine coupling
constant of 11G. No further splitting of this eight-
1.025 G (1 N) and a line width of 0.2 G. The simu- line pattern from coupling of 14N and proton was
lated spectrum does not reflect the observed satel-
lite lines of 13C coupling of 1.025 G. In the ESR
observed.
The spectrum of oxidized Co(L4)2 in CHC13 dif-
spectra of these systems additional hyperfine cou- fers from all the other experimentally observed
pling with the 59Co(I = 7/2) nucleus from delocal-
ization of the unpaired electron is not observed.
No further change in the ESR spectra were noted
radical spectra and has an unresolved ESR signal
centered at g = 2.0049 with A H = 6 G. It is prob-
able that the observed broadening of radical signal
on keeping these reaction mixtures under vac- arises from intram olecular interaction of the radi-
uum overnight.
The one-electron oxidation of Co(L3)2 with
cal centers with the unpaired electron of Co(II),
and as a consequence of this the ESR spectrum of
P b 0 2 in toluene/CHCl3 solution leads to form a- radical species occurs as a singlet. A nother alter-
tion of the coordinated phenoxyl radical centered
native reason of this phenom enon may be inter-
molecular exchange between radical centers. If in-
terconversion rates of isomeric forms are much
lower or comparable to the ESR time scale, a rela-
tive sharpening or broadening of the spectrum
must be expected. In the oxidation of this complex
in toluene solution slightly asymmetric well-re-
solved nine-line spectrum centered at g = 2.0044
and hfsc A H = 1.1 G was observed.
Oxidation of the complex Co(L7)2 with P b 0 2 in
toluene solution leads to the form ation of radical
species with an ESR spectrum which consists of
nine well-resolved lines spacing of 1.125 G (g =
2.0047) with an intensity distribution close to
1:4:7:8:8:8:7:4:1 (Fig. 4a). Five additional low inten-
sity satellite lines are also observed at both ends
of this spectrum, probably, arising from 13C iso-
topes in natural abundance. The observation of a
nine splitting pattern seems to suggest a ligand-
based C o(L 7)2 radical. The observed intensity dis-
tribution was analyzed in terms of an interaction
of the unpaired electron with one nitrogen nucleus
(A N = 1.125 G) and three sets of four protons
Fig. 3. ESR spectrum of Co(L6)2; experimental (top) in
toluene/chloroform at 300 K and its simulation (below).
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784
V. Kasumov • Bis(N-l-hydroxy-2,6-di-rerr-butyl-phenyl-salicylideneaminato)cobalt(II) Complexes
tween the phenoxy ring and the para substituent
depends on the twist angle, between the plane of
the phenoxy ring and a plane projected through
the N=CH-Sal group. If the twist angle is large,
conjugation between these two moieties of the
molecule is reduced [19, 20], It should be noted
that the X-ray structural analysis of bis[7V-(2,6-di-
ter?-butyl-l-methoxyphenyl)salicylaldiminato]-
Cu(II), which has nearly the same ligand as in
Co(Lx)2 complexes, shows that the twist angle be-
tween N-phenyl ring and -N=CH-Sal. group is
82.7° [21]. The conjugation of the C=N bond to
the phenolic ring plays an im portant role in the
interaction of the radical centers in Co(L*)2. The
values of cobalt hyperfine coupling constants in
the present Co(III)-phenoxyl radicals is the same
range of the Co(III)-o-sem iquinone system [22-
28], and are much smaller than low-spin Co(II)
complexes [3].
The octet signal arises from the complex of dia-
magnetic Co(III) ions coordinated with phenoxyl
radical anion as a result of the Co(II) —►ligand
electron transfer. A n alternative structure for the
Co(III)- stabilized radical interm ediate may be
shown as III (Scheme 3).
Fig. 4. ESR spectra observed in the oxidation of Co(L7)2
with Pb02 in toluene solution at room temperature: ini-
tial stage spectrum (a); spectrum recorded after 16 h un-
der vacuum and at room temperature (b).
[Ah c h = 4.5 G (1 H), A Hm = 2.25 G (2H) and
A HSai = 1.125 G (H)]. W hen the reaction mixture
was allowed to stand at room tem perature under
vacuum overnight (—16 h), the spectrum of this
sample showed along with the nine-line signal a
new low intensity octet of triplets (1:1:1) centered
at g = 2.00 (Fig. 4 b). The hyperfine splitting
pattern of this signal is consistent with a coupling
of the unpaired electron with 59Co(I = 7/2) and
14N(I = 1) nuclei (ACo = 11 G, A N = 1.1 G). The
L- solvent
X= H, 3- OCHj, 5-OCH3
Scheme 3
The generated radicals III are stable more than
2 -3 days and did not disappear even after heating
appearance of cobalt nucleus splitting of 11 G in- the reaction mixture to 340 K. No changes in the
dicates that the highest occupied MO has approxi-
mately 20% m etal character. Note that similar cals are observed at these tem peratures. The nitro-
isotropic coupling constants for any of these radi-
gen splitting in these radicals is smaller than nit-
roxide radicals and the m ajor splitting is produced
by the azomethine proton. The small coupling con-
ESR spectra have also been observed in the oxida-
tion of bis[./V-(l-hydroxy-2.6-di-terr-butyl-phenyl)-
2-hydroxy-naphthaldim inato]Co(II) by P b 0 2 (g =
1.998, A Co = 6.92, A N= A H = 1.78) [18]. No split- stant indicates that the phenoxy ring contains the
ting was observed from the terr-butyl groups for
any of the radicals. A possible explanation for the
lower unpaired electron spin density. For all these
radicals no half-field forbidden signal (zlM = 2)
small coupling constant in C o (L x)2 radicals is a which is characteristic for triplet state biradicals
large twist angle. It is known that conjugation be- was observed. The absence of the characteristic
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V. Kasumov • Bis(N-l-hydroxy-2,6-di-fm-butyl-phenyl-salicylideneaminato)cobalt(II) Complexes
785
triplet state signal, indicates that there is a weak Conclusions
spin-exchange interaction betw een radical centers
The chemical oxidation of bis[/V-(l-hydroxy-2,6-
di-tert-butylphenyl)salicylideneaminato] cobalt(II)
complexes produces various stable free radical
species. The unpaired electron is localized pre-
dominantly on the ligand in these kinds of radicals.
The hyperfine coupling constants and g-factors of
these radicals are very little influenced by coordi-
nation or by the nature and positions of the sub-
stituents in salicylaldehyde moiety. ESR spectra of
some complexes showed the partial conversion of
primary radicals to secondary param agnetic spe-
cies exhibiting interaction radical center with
59Co(I = 7/2) and 14N (I = 1) nuclei and Coppinge’s
type phenoxyl radicals. There is not ESR evidence
indicating the existence of a strong exchange in-
teraction between two phenoxyl centers in
Co(L x)2 radical complexes. Satellite peaks from
the 13C nuclei in the aromatic ring were also ob-
served in the ESR spectra of some radicals.
in the present Co-stabilizing phenoxyl radicals.
The fact that the isotropic hyperfine coupling to
14N and *H are almost identical and independent
of the nature and position of the substituents on
the salicylaldehyde moiety for a series C o(L x)2
radicals suggest that the unpaired electron is local-
ized predom inantly on the phenoxyl ring of the
ligands. No splitting was observed from the tert-
butyl groups for any of these radicals. On the other
hand, the similarity in shape, num ber of lines and
values of hfsc of the spectra under consideration
with spectra obtained from the corresponding oxi-
dized free ligands also confirmed the localization
of unpaired electron on phenoxy ring. Although
great num bers of phenoxyl radicals have been
studied [4,19, 29], the appearance of 13C hyperfine
splitting in the ESR spectra of phenoxyl type radi-
cals is observed in very few cases [29]. According
to theory developed by Karpuls and Fraenkel [30],
the magnitude of the 13C splitting depends upon
the spin density both at the carbon atom in ques-
tion and at neighboring carbons in the jt system.
The small values of the isotropic 14N-hfsc con-
stants in a series C o(L x)2 is an indication that
there is a very little direct spin density in the nitro-
gen s orbital.
Acknowledgement
We thank the Turkish Center of the Science and
Technology Research Council (TÜBITAK) for fi-
nancial support (G rant TBAG-1424) of this re-
search.
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