Spectroscopic studies of metal complexes with redox-active hydrogenated Schiff bases

Article abstract of DOI:10.1016/S1386-1425(00)00403-0

Synthesis and spectroscopic (IR, UV-visible, ESR) characterization of metal(II) complexes M(L′_x)₂, (where M = Co(II), NI(II), VO(II), Pd(II), L′_x = L′₁, L′₂, L′₃ are monoanion of unsubstitu

Full text of DOI:10.1016/S1386-1425(00)00403-0

Spectrochimica Acta Part A 57 (2001) 451–460
www.elsevier.nl/locate/saa
Spectroscopic studies of metal complexes with redox-active
hydrogenated Schiff bases
Veli T. Kasumov *
Department of Chemistry, Faculty of Arts and Sciences, Harran Uni6ersity, Sanhurfa, Turkey
Received 27 July 2000; accepted 21 August 2000
Abstract
Synthesis and spectroscopic (IR, UV-visible, ESR) characterization of metal(II) complexes M(L% )2, (where
x
M=Co(II), NI(II), VO(II), Pd(II), L% =L% , L% , L% are monoanion of unsubstituted, 5-Cl and 5-Br substituted-2-hy-
x
1
2
3
droxybenzylamine) with redox-active N-(3,5-ditert-butyl-4-hydroxyphenyl)-2-hydroxybenzylamine ligands as well as
radical species generated from these compounds by the oxidation with PbO2 are reported. ESR studies indicate that
the VO(L% )2 and Ni(L% )2 complexes, in opposite to their salicylaldimine precursors, are more readily oxidized with
x
x
lead dioxide and results in the formation of the indophenoxyl type stable radical. The formed radical species are very
similar to each other and quite different from those of the salicylaldimine analogous according to their g-factors and
hyperfine coupling constants. The nine line radical spectra observed in the oxidation of Co(L% )2, on standing under
x
Co
xy
vacuum, gradually converted to the signals characteristic of the low-spin Co(II) (gx,y=2.276, gz=1.998, A =122.7
G, ACz o=150 G) and radical containing Co(III) intermediate with gx,y=2.015, A =4.66 G, gz=1.989, ACz o=10 G
Co
xy
were also observed. © 2001 Elsevier Science B.V. All rights reserved.
Keywords: Redox-active benzylamine-complexes; Radical species; IR; UV-visible; ESR
provided by chelates of transition metal ions
which are capable of oxidizing a coordinated or-
ganic figand to the neutral radical. From this
viewpoint, one interesting redox-active system
consists of transition metal complexes containing
sterically hindered phenol fragments. It is well
known that sterically hindered phenol derivatives
are effective antioxidants against photo-destruc-
tion of materials such as polymers, petroleum
products, and in the rancidification of fats and
oils [2,3]. The fact that sterically hindered phenols
can readily undergo one or two-electron transfer
offers the possibility of preparing chelates with
unusual oxidation states [4].
1. Introduction
Intramolecular electron transfer is a fundamen-
tal chemical phenomenon and relates specifically
to biological and catalytic redox processes which
occur in natural and synthetic electron transfer
systems [1]. The ability of the metal ions to con-
trol the oxidation potentials of organic molecules
by complexing are significant in redox processes.
A particularly interesting class of redox system is
* Corresponding author.: Fax: +90-414-3146984.
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(00)00403-0

452
V.T. Kasumo6 / Spectrochimica Acta Part A 57 (2001) 451–460
Although there have been intensive studies on
tibilities were measured by the Faraday
technique at room temperature (2592°C). The
apparatus was calibrated by the use of Hg[-
Co(SCN)4]. The molar susceptibilities were cor-
rected for ligand diamagnetism using Pascal’s
constants [7]. The ESR spectra of M(Lx)2 were
recorded on a Radiopan model SE/X-2547 (X-
Band) in solution and in polycrystalline state at
300 and 77 K using DPPH (g=2.0036) as the
standard. The ESR parameters of the complexes
were determined directly from the spectra. The
magnetic field was calibrated using a standard
Mn2+ sample. Reported values involve errors
within90.005 for g-values and 90.05 G A-val-
ues of VO(Lx)2. The errors for radical g-factors
and hyperfine splitting constant (hfsc) parame-
ters are 90.0005 and 90.005 G, respectively.
coordination compounds of salicylaldimines with
transition metal ions, relatively little is known
about Schiff base chelates containing redox-ac-
tive fragments. The coordination chemistry of
some transition metals with potentially redox-ac-
tive chelating ligands such as bi-, tri- and tetra-
dentate salicylaldimines, naphthaldi mines,
azoligands, b-ketoiminates and other species
containing sterically hindered phenol fragments
has been studied [4–6]. The present investigation
was undertaken to explore the changes which
can occur in redox reactivity of LxH and
M(Lx)2 when the ꢀCH=Nꢀgroup was replaced
by saturated ꢀCH2ꢀNHꢀ linkage in the structure
of salicylaldimine. In particular, it is of interest
to know to what extent and how this structural
change affects the redox properties of free and
coordinated benzylamine ligands (L% H).
x
In this paper we describe the coordination and
redox chemistry of new complexes of M(L% )
[M=Ni(II), Pd(II), VO(II)] on the basis of the
redox-active N-(3,5-di-tert-butyl-4-hydroxyphenyl)-
x 2
3. Synthesis of ligands and complexes
Salicylaldimines LxH were synthesized by the
method described previously 4a. N-(3,5-di-tert-
butyl-4-hydroxyphenyl)-2-hydroxybenzylamines
2-hydroxybenzylamines (L% H) ligands, prepared
according to Scheme 1.
x
(L% H) were prepared by using the following
x
procedure: NaBH4 (0.05 mol) was added slowly
with small portions of about 10–15 mg to
0.01 mol stirring solutions of LxH in 40–50 ml
of isopropanol for 15–20 min. Stirring con-
tinued for another 30–40 min and the mixture,
cooled to room temperature, was then poured
into 150–200 ml water. The resulting solution
was vigorously stirred for 1–1.5 h and allowed
to stand for 3–4 h. The precipitated white cry-
stals were collected by filtration and then
washed 6–8 times with water, dried under vac-
2. Experimental
Elemental analyses were performed by the Sci-
ence and Technical Research Council of Turkey
(TUBITAK) in Gebze. IR spectra were obtained
using a Carl Zeiss Jena Specord M 80 spec-
trophotometer in the 4000–400 cm−1 region as
KBr discs. Electronic spectra were recorded on a
Carl Zeiss Jena Specord M 40 spectrophotome-
ter in the 200–900 nm region. Magnetic suscep-
Scheme 1.

V.T. Kasumo6 / Spectrochimica Acta Part A 57 (2001) 451–460
453
Table 1
Analytical data for L% H and M(L% )2 compounds
x
x
Compound
Yields (%)
Color
M.p. (°C)
Elemental analyses, found/calc’d (%)
C
H
N
L% H
91.5
92.8
85.5
56.4
62.4
48.6
65.8
63.4
71.6
48.7
52.9
56.4
44.67
56.5
59.4
White
White
White
146
149
156
74.44/74.71
70.43/67.78
61.19/60.54
71.56/70.85
64.35/64.59
58.45/57.97
70.98/70.87
65.38/64.61
49.56/58.05
67.12/66.42
60.68/60.88
54.78/54.98
69.85/70.06
63.67/63.91
58.02/57.45
8.27/8.66
7.42/7.86
6.98/7,02
7.64/7.93
6.96/7.23
5.85/6.26
7.68/7.93
6.76/6.98
6.12/6.27
7.16/7.44
6.49/6.57
6.19/5.94
7.47/7.56
6.67/6.91
6.52/6.21
3.78/8.30
7.44/7.53
6.57/6.73
3.45/3.93
3.32/3.22
4.15/3.96
4.15/3.94
3.89/3.59
3.32/3.22
3.34/3.68
3.56/3.38
3.56/3.05
4.18/3.89
3,66/3.55
3.34/3.19
1
L% H
2
L% H
3
Co(L% )
Co(L% )
Co(L% )
Ni(L% )
Ni(L% )
Ni(L% )
Pd(L% )
Pd(L% )
Pd(L% )
VO(L% )
VO(L% )
VO(L% )
Brown
Brown
Brown
Green
Green
Dark green
Brown
Brown
Brown
Green
Green
Green
\250
\250
\250
\250
\250
\250
\250
\250
\250
\250
\250
\250
1 2
2 2
3 2
1 2
2 2
3 2
1 2
2 2
3 2
1 2
2 2
3 2
uum and recrystallized from hexane-acetone,
yielded 85–93%.
The complexes of M(Lx)2 [M=Co(II), Ni(11),
VO(II)] were prepared by the following general
method. Metal(II) acetate (2 mmol) was dissolved
in the minimum volume of methanol (10–15 ml)
Solution was degassed under vacuum (10−3
–
10−4 mmHg) by four to five freeze–pump–thaw
cycles. Then degassed solution was mixed with
PbO2 (70–80 mg) and shaken for 30 s at room
temperature. After the sedimentation of heteroge-
neous phase, 1 ml of the solution was introduced
into an ESR sample tube for monitoring the
spectrum. The ESR spectra taken rapidly after the
sedimentation of reaction mixture. Reduction of
and added to an equivalent amount L% H (4
x
mmol) in warm methanol (40–50 ml). Soon green
or brown crystals were precipitated. After the
mixture was warmed at 50–60°C with stirring for
50–60 min and left to cool. The precipitated
crystals were by suction filtration, washed with
methanol and diethyl ether, and then dried in the
open air and recrystallized from methanol–ace-
Pd(L% ) complexes with an excess of PPh3 was
x 2
carried out by mixing their degassed CHCl3 solu-
tions at room temperature under vacuum.
tone mixture. Pd(L% ) complexes were prepared
according to previously reported procedure [6a]
x 2
4. Results and discussion
using Pd(II) acetate and L% H in glacial acetic acid
x
The analytical data of the complexes (Table 1)
indicate 2:1 (ligand/metal) stoichiometry and
hence the figands are behaving as bidentate
(20–25 ml). Analytical and spectroscopic data of
the compounds synthesized are given in the Ta-
bles 1–3.
monobasic anions in the M(L% ) complexes. The
x 2
examination show a higher complexation ability
3.1. Generation of radicals
of hydrogenated L% H ligands than that of corre-
x
sponding salicylaldimines precursors. This behav-
ior is undoubtedly caused by the higher basicity of
the nitrogen atom in hydrogenated benzylamine
ligands than in their salicylaldimine analogous.
Additionally, the Ni(L% ) and VO(L% ) com-
L% H and M(L% ) compounds were oxidized as
x
x 2
follows. The compound, dissolved in 5 ml toluene
or chloroform and PbO2 (70–90 mg), were trans-
ferred into separate glass tubes on vacuum line.
x 2
x 2

454
V.T. Kasumo6 / Spectrochimica Acta Part A 57 (2001) 451–460
plexes, unlike their salicylaidiminate precursors,
were readily oxidized by PbO2 giving very stable
secondary radical. Note that the nonhindered
bis(benzylaminato) complexes reported by West
[8], does not undergo any oxidative conversion in
the interaction with PbO2. The prepared ligands,
plane bending mode, and very weak broad ab-
sorption near 2700 cm−1 assignable to the
intramolecular hydrogen bonding of OꢀH stretch-
ing vibration were not observed in the complexes
as a result of the displacement of the acidic pro-
ton by the metal ions. The w(V=O) frequencies
of the VO(Lx)2 complexes occur in the range
910–960 cm−1 and this is in the usual range
(960950 cm−1) observed for the majority of
VO(11) complexes [9].
except L%H, are sensitive to air oxidation even in
1
the solid state. However, the bis(benzylaminato)
complexes (M(L% ) , are quite stable in air.
x 2
5. Infrared spectra
6. LxH benzylamines
The IR spectra of the complexes provide direct
information about the coordination of the ligand
via NH nitrogen atom and deprotonated oxygen
of the OH group (Table 2). The LxH spectra
exhibit broad medium bands at 3320–3345 cm−1
and a sharp strong band at 3610–3640 cm−1 due
to the w(NH) and sterically hindered phenolic
The electronic spectra of L% H (Table 2) exhibit
x
bands at about 230 and 250–289 nm attributable
to the y“y* transition of benzene rings. The
bands in the range 308–314 and 362–365 nm may
assigned to n“y* charge transfer transition.
These CT seems to originate from the OH and
NH groups to a phenyl rings of the ligand. The
low intensity broad band appeared at 446–454
nm which can be attributed to the bipolar ions
formed as a result of the intramolecular proton
transfer from OH to NH group, disappeared in
w(OH) stretching vibrations, respectively.
A
strong band in the salicylaldimines in the 1615–
1635 cm−1 region due to the w(C=N) stretch
disappeared in the spectra of hydrogenated L% H
x
and M(L% ) compounds. A strong band appeared
the spectra of M(L% ) complexes. In all complexes
the intraligand y“y* band at 230 nm is red
x 2
in the IR spectra of L% H at 1280–1310 cm−1
x 2
x
which was attributed to the hydrogen bond in-
shifted and appeared at 248–254 nm (Table 2).
Table 2
Spectroscopic and magnetic data of ligands and their metal complexes
Compound
mef M.B.
IR spectra (cm−1
)
Electronic spectra, u/nm (ml mol−1 cm−1
)
wNH
wOH
L% H
–
–
–
3325
3320
3345
3380
3398
3385
3395
3382
3379
3388
3390
3385
3395
3388
3395
3630
3620
3630
3628
3640
3615
3610
3615
3625
3620
3649
3620
3620
3645
3620
230,289,308,362,454
232,250,289,308,365,454
230,289,311,364,446
1
L% H
2
L% H
3
VO(L% )
VO(L% )
VO(L% )
Co(L% )
Co(L% )
Co(L% )
Ni(L% )
Ni(L% )
Ni(L% )
Pd(L% )
Pd(L% )
Pd(L% )
1.75
1.78
1.73
5.16
4.49
4,46
3.36
3.32
3.38
Dia.
–
255,260,275,288,338,365,485,666,890
1 2
256,267, 320, 336, 365, 382, 490, 625, 790
260,278,289,315,340,365,385,490,665,725
3 12, 328, 373, 431, 556 (102)
287, 318, 335, 364, 382, 406, 510, 602(124)
276,288,335,346,385,417, 500, 610(614)
273, 286, 370, 382, 458, 588, 790 (320)
256,270,322,335,365,381, 515,685(450)
251, 287, 335, 364, 386, 521, 685 (380)
253,285,317,334,350,362, 373,485,526
285,317,333,362,382,434, 524
2 2
3 2
1 2
2 2
3 2
1 2
2 2
3 2
1 2
2 2
–
253,285,317,334,350,362, 373,485,526
3 2

V.T. Kasumo6 / Spectrochimica Acta Part A 57 (2001) 451–460
455
Table 3
ESR parameters of VO(L% )2 complexes
x
Compound
giso
g//
gÞ
*Aiso
*A//
*AÞ
Oxidized complex
g
AHa
VO(L% )
1.996
1.993
1.992
1.963
1.950
1.950
2.013
2.003
2.013
96.3
94.5
94.5
188.2
182.3
182.8
50.4
52.3
50.4
2.0031
2.0041
2.0034
1.129
1.129
1.129
1 2
VO(L% )
2 2
VO(L% )
3 2
a In Causs.
In the oxidation of L% H with PbO2 at 300 K in
pling constant is positive and the orbital contribu-
tion is almost completely quenched in
oxovanadium(IV) complexes [10a]. The room
temperature effective magnetic moments of the
x
toluene, THF and CHCl3 solutions, the formation
of less resolved asymmetric ESR spectra with
similar magnetic resonance parameters have been
observed. For oxidized L%H and L%H in CHCl3
VO(L% ) (1.72–1.74 B.M.) are close to that ex-
1
2
x 2
or toluene, at room temperature, 7 line (g=
2.0044, AHNH=1.85 G, AN=3.8 G) and 5 line
(g=20046, AN=ANHH=3.82 G) asymmetric
spectral patterns, respectively, were observed.
These spectra are quite different from those of the
corresponding salicylaldimine ligands [4a,10].
Note that generation of the aminyl radicals in
these systems can not be ruled out and it is seems
that the observed less resolved unusual spectral
features are originated from the aminyl radicals
[11]. Aminyl radicals might be formed by remov-
ing of the ꢀNꢀH hydrogen atom by oxidation
with PbO2. On the other hand, relatively less
resolved 9-line isotropic spectrum (g=2.0049,
AHm=1.23 G, ANHH=AN=2.46 G) generated
pected [10b], spin-only values of 1.73 B.M. and
indicate the magnetically diluted nature and ab-
sence of antiferromagnetic exchange interaction in
these complexes at room temperature.
The electronic spectra of the VO(L% ) com-
x 2
plexes in chloroform (Table 2) exhibit three ab-
sorption bands in the range 485–490, 625–666
and 790–890 nm. These bands, according to Ball-
hausen and Gray molecular orbital scheme [12],
are assigned to the d “d , d “d
and
2
2
2
−y
xy
xy
z
x
dxy“dxz,yz transitions, respectively. This assign-
ment also is agreement with our ESR data. Some
charge-transfer bands are also observed at 365–
385 mm.
The ESR parameters of VO(L% ) determined
x 2
from L%H under similar conditions, can be con-
sidered as phenoxyl radicals.
direct from the spectra recorded at room tempera-
ture (77 K) are presented in Table 3. The room
3
temperature isotropic ESR spectra of VO(L% ) in
x 2
chloroform consist of the usual eight-line hy-
perfine pattern resulting from coupling of the
unpaired electron to the 51V(I=7/2) nucleus (Fig.
1(a)). There was no observed additional splitting
from 14N(I=1) nucleus. The isotropic vanadium
electron spin–nuclear spin hyperfine coupling
constants (Aiso), were obtained by averaging the
separation of the −mI to mI transitions. The
anisotropic spectra of these complexes at 77 K
exhibit axial rather than rhombic symmetry (Fig.
1(c)) with the values of Az\Ax=Ay and gzB
gx=gy. The following expressions can be used as
good approximations for the estimation of g
values
7. Bis chelates of VO(11)
On mixing hot methanolic solutions equivmolar
amount of L% H and VO(acet)2 in a 2:1 ratio, the
x
dark green complex precipitated simultaneously.
Upon complexation of L% H as evidenced by ESR,
x
unlike parent salicylaldimine analogous, the for-
mation of the radical species were not observed.
The oxovanadium(IV) complexes belong to
S=1/2 system and it is expected that the mag-
netic moments of magnetically dilute oxovanadiu-
m(IV) complexes would be very close to the
spin-only magnetic moment, as the spin-orbit cou-

456
V.T. Kasumo6 / Spectrochimica Acta Part A 57 (2001) 451–460
gx,y=ge−2h2k2u/Exy“Exz,yz
(1)
(2)
The one-electron oxidation of VO(L% ) with
x 2
PbO2 in CHCl3 at room temperature, unlike their
gz=ge−8h2i2u/Exy“Ex
salicylaldiminate VO(II), readily leads to the for-
mation of stable phenoxyl radicals and gradually
disappearance of VO(II) signals. These spectral
patterns are similar to each other and have quite
close g-factors and hfsc values (Table 3), but quite
different from those of the radicals generated
2
2
−y
where h, i and k are modified molecular orbital
coefficients of the dxy, dx
2, and dxz, dyz orbitals,
2
respectively, and u is the −spyin-orbit coupling con-
stant of the vanadium ion in the complex [13].
The absence of hyperfine splitting from nitrogen
nucleus in the ESR spectra, in agreement with
Ballhausen and Gray, indicate that the unpaired
electron occupies an essentially non-bonding
vanadium dxy orbital which is largely metal in
character and thus independent of the covalent
character of the bonding orbitals.
from L% H and parent salicylaldimine ligands. The
x
spectra of all of the radicals generated from
VO(L% ) exhibit well resolved equidistant nine-
x 2
line pattern spacing of 1.129 G with an intensity
distribution of 1:4:7:8:9:7:4:1 (Fig. 1(b)). These
spectra can be analyzed in terms of an interaction
of the unpaired electron spin density with one
Fig. 1. ESR spectra of VO(L% ) . (a) at 300 and (c) 77 K; (b) oxidized sample of VO(L% ) at 300 K, spectra detected for the oxidized
1 2
1 2
samples of Ni(L% )2 at 300 K: (d) Ni(L% ) , (e) NI(L% ) . All spectra recorded in CHCl3.
x
2 2
3 2

V.T. Kasumo6 / Spectrochimica Acta Part A 57 (2001) 451–460
457
nitrogen (AN=2.22 G) and two sets of three
protons [(AHm=1.11 G (2H) and ANHH=2.22 G
(1H)] assuming that AN=ANHH=2AHm. It is inter-
esting to note that in the oxidation of parent
bis(salicylaidiminato)VO(II) chelates, only for
X=H complex the formation of low intensity
radical was observed. At the same time, our at-
tempts to oxidize X=Cl, Br, NO2 bearing bis(sal-
icylaldiminato)VO(II) complexes in various
solvents were unsuccessful [4c]. Thus, the hydro-
this signal is similar to that of the oxidized
NI(L%) sample. Note that essentially similar ESR
2 2
spectra were also observed in the oxidation of
Cu(II) [15] complexes with the same ligands.
These experimental observations allow us to con-
clude that, the observed ESR spectra originated
from secondary radical species.
9. Bis chelates of Pd(II)
genated VO(L% ) complexes, unlike their salicy-
x 2
laldimine precursors, are readily oxidized with
PbO2 with the formation of the stable phenoxyl
radicals.
All the Pd(Lx)2 complexes are diamagnetic, sug-
gesting a square planar geometry for these com-
pounds. The electronic spectra of these
compounds are different from those of the parent
salicylaldimine Pd(II) chelates. In the electronic
8. Bis chelates of Ni(II)
spectra of the Pd(L% ) complexes together intrali-
x 2
gand absorptions in the UV region, the new
bands at 378–385, 434–485 and 515–526 nm
were also observed. The bands in the regions of
434–485 and 515–526 nm may be assigned to the
All Ni(L% ) complexes are paramagnetic in the
x 2
solid state with magnetic moments in the range
3.32–3.38 B.M. (Table 1), which are close to
those found for tetrahedral Ni(II) complexes [14].
1
1Ag“1A2g and Ag“1B1g transitions, respectively
Electronic spectra of NI(L% ) in CHCl3 in the
[16]. The other intense band observed in the UV
region at 378–385 nm, probably has metal to
ligand charge transfer character.
x 2
visible region show absorption bands in the range
458–590 and 685–790 nm and according to the
values of coefficient extinction (m=320–450) can
Upon treatment of Pd(L%) and Pd(L%) com-
1 2
3 2
3
3
be assigned to T1“3A2 and T1“3T(P) in tetra-
hedral symmetry [14]. The bands appeared at
364–385 nm, and can be attributed to the ligand-
to-metal charge-transfer transitions.
plexes with PbO2 in CHCl3 solution under the
above mentioned conditions, well resolved ESR
spectral patterns having quite close magnetic reso-
nance parameters were observed. In all cases dur-
ing oxidation the gradually precipitation of the
These complexes, unlike their salicylaldimine
analogous, rather readily oxidized with PbO2 in
CHCl3 at room temperature giving corresponding
metallic Pd occurred. Upon oxidation of Pd(L%)
1 2
and Pd(L%) complexes, relatively stable radical
3 2
radical species. In the oxidation of Ni(L% ) low
species exhibiting nine line hyperfine splitting
spectral features with an intensity distribution of
1:4:7:8:8:8:7:4:1 and ESR parameters such as g=
2.0062, AH=1.25 G and g=2.0045, AH=l.223
G, respectively, were observed (Fig. 2(e)). In the
x 2
intensity unresolved signal centered at g=2.0038
was observed. However, under the same condi-
tions, oxidation of Ni(L%) and Ni(L%) results in
2 2
3 2
an ESR spectra which consist of equidistant nine-
and ten-line patterns (Fig. 1(d) and (e)) with the
parameters such as g=2.0041, AH=1.013 G and
g=2.0038, AH=1.127 G, respectively. The nine-
line spectrum, according to line shapes, number of
lines, intensity ratios and the values of g-factors
and hfsc (within experimental error) are very sim-
ilar to those of the oxidized VO(Lx)2 and Pd(II)
samples spectra. The highest field component of
the spectrum in Fig. 1(e) most probably has an-
other origin and equal spacing nine-line part of
case of oxidized Pd(L%) sample in CHCl3, at 300
2 2
K, the unusual spectrum having unresolved low
field wing and a well resolved nine-line pattern on
the high field side (g=2.0053, AH=1.166 G) was
detected (Fig. 2(g)). This spectral pattern remains
unchanged on varying in concentration of com-
plex over range of 10−4–10−3 M as well as with
changes in gain and modulation amplitude from
0.05 to 2 G. The unusual broadness and the
asymmetry of the overall pattern was probably

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V.T. Kasumo6 / Spectrochimica Acta Part A 57 (2001) 451–460
Fig. 2. ESR spectra of the oxidation products of Co(L% )2 and Pd(L% )2 in CHCl3 at 300 K. (a, b): Co(L% ) , (c, d): Co(L% ) , (e):
x
x
1 2
2 2
Pd(L% ) and (g): Pd(L% ) . The central multiplets (b) and (d) were recorded at scan range of 50 G and modulation amplitude of 0.2
1 2
3 2
G.
immediate precipitation of Pd, a very weak inten-
sity hardly detectable spectral patterns centered at
g=2 were observed.
due to the anisotropy of the hyperfine coupling
tensors and that of the g-tensor [17]. At 77 K, for
this sample, unresolved symmetric singlet centered
at g=2.005 was detected.
Interaction of Pd(L% ) with an excess of PPh3
x 2
in CHCl3 solutions at 300 K under vacuum, by
analogy with those which were observed for corre-
sponding bis(salicylaldiminato) palladium(II) [6a],
results in reduction of the complexes accompanied
by the precipitation of the metallic Pd. Upon
10. Bis chelates of Co(II)
The values of the effective magnetic moments
of these complexes ranging between 4.46 and 5.16
B.M. suggest a spin quartet state S=3/2, and
reduction of Pd(L%) with PPh3 in CHCl3 the
1 2
electronic spectra in CHCl3 (m=102–614
l
complicated ESR spectrum g=2.0045, AH=l.42
mol−1cm−1) in the visible region (Table 2) are
indicative of tetrahedral coordinate Co(II) (d7)
complexes [18]. The absorption band in the region
G was detected. But in the cases of Pd(L%) and
2 2
Pd(L%) , under similar conditions, along with
3 2

V.T. Kasumo6 / Spectrochimica Acta Part A 57 (2001) 451–460
459
of 556–610 nm may be assigned to the 4A2“
g=2.009 (DH=16.5 G) and g=2.0012 (DH=
3.3 G), respectively, However, exact analysis of
this spectrum is not easy, the observed frozen
solution signal undoubtedly indicate that the
room temperature six-line pattern of the oxidized
4T1(P) transition in tetrahedral symmetry.
Oxidation of Co(L% ) complexes in the toluene
x 2
and CHC13 solutions under the above mentioned
conditions resulted in ESR spectra which were
Co(L%) sample, can be attributed to superim-
quite different from those of the oxidized Ni(L% ) ,
1 2
x 2
posed two signals.
Pd(L% ) and Co(L% ) (Fig. 2(a), (c)). For instance,
x 2
x 2
The performed studies indicate that except
Co(Lx)2 complexes practically all of the oxidized
complexes independently of the ligand and metal
nature display nearly identical ESR spectral fea-
ture. The ESR parameters and intensity distribu-
tion of 1:4:7:8:8:8:7:4:1 of the detected spectra are
similar of those reported by Coppinger [20] in-
dophenoxyl type radical 5 in Scheme 2.
According to Scheme 2, the indirectly generated
unstable radical 2 immediately distorted 3 and
this radical product may result from dispropor-
tion of the aminophenoxyl-aminophenoxyl ligand
can be to give quinine-phenoxyl type stable radi-
cals such as 5 or 6. It is necessary to note that the
nine line spectral pattern with similar intensity
ratios were also observed in the oxidation of some
salicylaldimines and their complexes [4a,10].
Finally, this work clearly demonstrates the re-
placement of the ꢀC=Nꢀ group by the
ꢀCH2ꢀNHꢀ linkage in bis[N-(2,6-di-tert-butyl-l-
hydroxyphenyl)salicylaldimine] metal(II) com-
plexes significantly increases their oxidative
reactivity towards lead dioxide. ESR studies indi-
cate that bis(benzylaminato) complexes of Ni(II)
and VO(II) containing peripheral sterically hin-
dered phenol fragment, in opposite to their salicy-
laldimine precursors are more readily oxidized
on treatment with PbO2, toluene solution of
Co(L%) gives a spectrum consisting of superimpo-
1 2
sition of less resolved very intense radical signal
(g=2.0035, AH=1.11 G) and low intensity octet
pattern spacing of 6.2 G (g=2.01) due to interac-
tion of the unpaired electron to the 59Co(I=7/2)
nucleus (Fig. 2(a)) was observed. A frozen solu-
tion (77 K) anisotropic spectrum of this sample
shows resolved gxy and gz components with the
parameters such as
gx,y=2.276, gz=1.998,
Ax,y(59Co)=122.7 G, and Az(59Co)=150
G
clearly indicate a (d ) ground configuration char-
2
z
acteristic of the low-spin Co(II) complexes [19]. In
the presence of atmospheric oxygen the radical
part of this spectrum was disappeared. ESR spec-
trum of oxidized of Co(L%) in toluene at room
2 2
temperature, also consists of overlapped radical
signal (g=2.0054, AH=1.18 G) and anisotropic
eight-line pattern typical of the paramagnetic
Co(III) with parameters as gx,y=2.015, Ax,y
=
4.66 G, gz=1.989, Az=10 G (Fig. 2(c)). On the
other hand, the spectrum generated from the com-
plex of Co(L%) under the similar conditions, ex-
3 2
hibit six line pattern centered at g=2.0037 and
has an hfsc of 4.09 G with an intensity distribu-
tion of 1:2:3:3:2:1. At 77 K this spectrum trans-
formed into two different signals centered at
Scheme 2.

460
V.T. Kasumo6 / Spectrochimica Acta Part A 57 (2001) 451–460
[4] (a) A.A. Medjidov, V.T. Kasumov, H.S. Mamedov, Ko-
ord. Khim. (1981) 66; (b) V.T. Kasumov, M.K.
with lead dioxide and results to the formation of
secondary phenoxyl radicals. On the other hand
in the cases of Pd(II) and Co(II) the radical
intermediates generated from salicylaldiminate
and benzylaminate are significantly different from
each other. In the oxidation of benzylaminate,
unlike their salicylaldiminate precursors, the de-
tection of primery radical species In generally,
ESR spectra of the generated radical species ac-
cording to their g-factors and hyperfine coupling
constants are very similar each other and essen-
tially different from those of the salicylaldimine
analogous and free benzylamine ligands. In addi-
tion, the complexation ability of the hydrogenated
benzylamine ligands was found rather higher than
that of the parent salicylaldinine precursors.
7
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Acknowledgements
The author thank the Council of Scientific and
Technical Research (TUBITAK) for financial
support of the work (Grant No. TBAG- 1424).
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