Synthesis, spectroscopic characterization and EPR studies on electron transfer reactions of bis[N-(2,5-di-tert-butylphenyl)salicylaldiminato]copper complexes with PPh3

Article abstract of DOI:10.1016/S1386-1425(01)00689-8

New bidentate N-(2,5-di-tert-butylphenyl)salicylaldimines bearing X = H, HO, CH₃O, Br, NO₂, 3,5-di-Br, 3-NO₂-5-Br and 5,6-benzo substituents on the salicylaldehyde moiety, L^xH, and their mononuclear bis[N-(2,5-d

Full text of DOI:10.1016/S1386-1425(01)00689-8

Spectrochimica Acta Part A 58 (2002) 2199–2211
www.elsevier.com/locate/saa
Synthesis, spectroscopic characterization and EPR studies
on electron transfer reactions of
bis[N-(2,5-di-tert-butylphenyl)salicylaldiminato]copper
complexes with PPh3
Veli T. Kasumov a,*, Fevzi Ko¨ksal b
a Department of Chemistry, Faculty of Arts and Sciences, Harran Uni6ersity, Sanliurfa, Turkey
b Physics Department, Faculty of Arts and Sciences, Ondokuz Mayis Uni6ersity, Samsun, Turkey
Received 7 August 2001; received in revised form 24 October 2001; accepted 19 November 2001
Abstract
New bidentate N-(2,5-di-tert-butylphenyl)salicylaldimines bearing X=H, HO, CH3O, Br, NO2, 3,5-di-Br, 3-NO2-
5-Br and 5,6-benzo substituents on the salicylaldehyde moiety, LxH, and their mononuclear bis[N-(2,5-di-tert-
butylphenyl)salicylaldiminato]copper(II) complexes, Cu(Lx)2, have been prepared and investigated by IR,
1
UV–Visible, H NMR, ESR spectroscopy, magnetic measurements, as well as reactions of Cu(Lx)2 with PPh3 were
studied. It has been found that some complexes with X=HO and CH3O unlike their electron-withdrawing and
unsubstituted analogues are readily reduced by PPh3 via intramolecular electron transfer from ligand to copper(II) to
give Cu-stabilized radical intermediates. The spectra of the primary radicals interpreted in terms of couplings of
unpaired electron with 63,65Cu, 31P, 14N nuclei and aromatic protons. © 2002 Elsevier Science B.V. All rights reserved.
Keywords: Redox-active bis(salicylaldiminato)copper(II) complexes; ESR; Optical spectra; Reactivities of copper(II) complexes
toward PPh3
complexes have been shown to react faster in
catalytic reactions compared to less bulky ones
1. Introduction
[3].
Previously we reported [4–16] a number of
The introduction of bulky groups in ligands
influences the overall catalytic and redox proper-
ties of the complexes [1,2]. Increased steric hin-
drance about the metal center of the complex
affects its reactivity, but the effects are not always
straightforward. In some cases sterically hindered
copper(II) complexes with salicylaldimine, 2-hy-
droxybenzylamine b-ketoamine and 2-hydroxy-
arylazo ligands containing sterically hindered
phenols in which unlike their unhindered
analogous in the treatment with (XPh)3P or oxi-
dation with PbO2, the formation of various radi-
cal intermediates (or oxidative CꢀC coupled
* Corresponding author. Fax: +90-414-314-6984
E-mail address: vkasumov@harran.edu.tr (V.T. Kasumov).
1386-1425/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved.
PII: S1386-1425(01)00689-8

2200
V.T. Kasumo6, F. Ko¨ksal / Spectrochimica Acta Part A 58 (2002) 2199–2211
copper(II) complexes) were observed. Although the
reduction of some copper(II) bischelates by
triphenylphosphine are well documented [17–28],
the copper(II)complexes in which reduction of
Cu(II) complexes with PPh3 proceeds via radical
intermediates are very limited [28]. At the same
time, the introduction of two tert-butyl groups in
the above-mentioned ligands promote the proceed-
ing of the intramolecular electron transfer in cop-
per(II), palladium(II) and cobalt(II) complexes, as
well as the catalytic activity of the palladium(II)
salicylaldiminates in the hydrogenation of nitro-
compounds and alkenes [7–16,29–31].
ities were corrected for ligand diamagnetism using
Pascal’s constants. The effective magnetic moments
were calculated using the expression veff
=
2.828(m×T)1/2. EPR spectra of the complexes
were measured in a Varian E-109 C model X-band
spectrometer with 100 kHz frequency modulation
in solution and polycrystalline state at 300 K and
in frozen solution at 113 K. The spectra were
calibrated with the diphenylpicrylhydrazyl (DPPH)
free radical of g=2.0036. The EPR parameters of
the complexes and radicals were determined di-
rectly from the spectra. Reported values involve
errors within 90.0005 for g-values and 90.005 G
for hyperfine coupling constants (hfcc) of Cu(Lx)2
and radicals, respectively. Reductions of Cu(Lx)2
with 2–4-fold excess of PPh3, were carried out by
mixing their deoxygenated toluene or chloroform
solutions in the EPR tubes at room temperature
under vacuum. The EPR spectra of the reaction
mixtures were immediately recorded under vacuum
at room temperature.
As a continuation of our earlier work on cop-
per(II) complexes with redox-active ligands, we
report the synthesis, spectral properties and redox
behavior toward PPh3 of a series of new Cu(Lx)2
salicylaldimine complexes, where Lx represents the
monoanions
of
bidentate
N-(2,5-di-tert-
butylphenyl)salicylaldimine ligands (Scheme 1).
2. Experimental
2.1. Preparation of ligands
The elemental analyses were carried out in the
Turkish Center of the Science and Technology
Salicylaldehyde, its hydroxy- and methoxy-sub-
stituted derivatives, 2,5-di-tert-butylaniline, cop-
per(II) acetate monohydrate and all solvents were
obtained commercially and used as received. Sol-
vents used for analytical purposes were spectro-
8
Research Council (TUBITAK) in Gebze and in the
Analytical Center of Chemistry Department, Fırat
University. Infrared spectra were recorded in a
Mattson 1000 FTIR spectrophotometer in the
region 4000–400 cm−1 using KBr discs. Electronic
spectra were measured in a Shimadzu UV 1601 and
Cary 1E spectrophotometers in various solvents.
Magnetic susceptibilities of powdered samples were
determined in a Sherwood Scientific Magnetic
Susceptibility Balance (Model MK 1) magnetome-
ter at room temperature and the molar susceptibil-
scopic
grade.
Br-
and
NO2-substituted
salicylaldehyde derivatives were prepared accord-
ing to the reported method [32,33]. All ligands
LxH were prepared using standard procedures
involving the condensation of 2,5-di-tert-butylani-
line with the appropriate salicylaldehyde in
methanol (MeOH). The yellow imines were
purified by recrystallization from MeOH (yields
85–92%). A solution of 2,5-di-tert-butyl-aniline
(10.5 mmol) in MeOH (10 ml) was added to a
solution of 3-methoxy salicylaldehyde (10 mmol)
in MeOH (50 ml). The resulting mixture was
stirred for 50 min at 45–50 °C. After reducing
the volume to 10 ml and cooling to room temper-
ature, the precipitate yellow–orange crystals were
filtered off and crystallized from chloroform–
MeOH (1:3 v/v) mixture, yield 92%. Elemental
analyses, melting points, yields of LxH are sum-
marized in Table 1.
Scheme 1.

V.T. Kasumo6, F. Ko¨ksal / Spectrochimica Acta Part A 58 (2002) 2199–2211
2201
Table 1
Analytical data for the LxH and Cu(Lx)2 complexes
Compound
Yield (%)
M.p. (°C)
Stoichiometry
Elemental analyses (found/calculated) (%)
C
H
N
L1H
L2H
L3H
L4H
L5H
L6H
L7H
L8H
L9H
85
88
92
86
89
88
93
87
93
96
95
87
91
86
85
76
85
79
82
86
88
86
90
102
126
129
189
119
215
132
165
163
152
212
143
241
\300
242
167
226
245
C21H27NO
C21H27NO2
C22H29NO2
C21H27NO2
C22H29NO2
C21H29NO2
C22H29NO2
C21H26NOBr
81.38/81.51
77.32/77.54
77.43I77.88
77.34/77.54
77.63/77.88
77.31/77.54
77.65/77.88
65.12/64.76
70.87/71.16
53.76/53.73
57.96/57.92
83.78/83.52
74.27/74.14
71.08/70.81
70.17/70.41
71.12/70.81
69.94/70.41
71.21/70.81
58.79/60.01
65.45/65.48
54.04/53.73
54.26/54.11
75.82/76.94
8.62/8.179
7.56/7.75
7.62/7.78
7.43/7.75
7.98/7.78
7.87/7.70
7.56/7.78
6.46/6.73
7.73/7.39
5.57/5.36
5.58/5.78
8.45/8.13
7.59/7.70
7.56/7.35
7.56/7.62
7.13/7.35
7.32/7.62
7.43/7.62
5.78/6.02
6.23/6.54
5.12/5.36
5.32/5.19
7.48/7.23
4.48/4.52
4.67/4.31
4.32/4.13
4.45/4.31
4.23/4.13
4.12/4.31
4.42/4.13
3.34/3.59
7.78/7.89
2.86/2.98
6.21/6.43
4.05/3.89
3.84/4.11
4.18/3.93
3.45/3.78
4.15/3.93
3.46/3.78
3.56/3.78
3.12/3.33
7.32/7.27
2.23/2.79
5.86/6.01
3.47/3.58
C
21H26N2O3
C21H25NOBr2
C21H25N2O3Br
L10
L11
L12
H
H
H
C25H29NO
C42H52N2O2Cu
C42H52N2O4Cu
Cu(L1)2
Cu(L2)2
Cu(L3)2
Cu(L4)2
Cu(L5)2
Cu(L7)2
Cu(L8)2
Cu(L9)2
C
44H56N2O4Cu
C42H52N2O4Cu
44H56N2O4Cu
C44H56N2O4Cu
C
\300
200
\300
233
C
C
42H50N2O2Br2
42H50N4O6Cu
Cu(L10
Cu(L11
Cu(L12
)
)
)
C42H48N2O2Br4Cu
C42H48N4O6Br2Cu
C50H56N2O2Cu
2
2
2
\300
2.2. Cu(Lx)2 complexes
3. Results and discussion
These complexes were prepared by following
general procedure. A solution of 200 mg (1
mmol) of copper(II) acetate monohydrate in 15
ml of MeOH was added to a magnetically
stirred and heated solution of 2 mmol of the
appropriate ligand in 80–90 ml of MeOH. The
reaction mixture was stirred at about 45–50 °C
for 1–1.5 h. The solution volume was decreased
to ca. 15 ml. After standing for 1–1.5 h at room
temperature, the precipitated product was
filtered off and washed with a small amounts
cold MeOH, n-hexane, dried in air and recrys-
tallized from acetone/n-hexane mixture, yield
76–91%. Analytical, spectroscopic and magnetic
moments data for the Cu(Lx)2 are given in Ta-
bles 1, 4 and 5.
The analytical data of the Cu(Lx)2 complexes
indicate 2:1 ligand to metal stoichiometry. The
synthesized complexes are found to be suffi-
ciently soluble in chloroform, toluene, dioxane,
acetone, N,N-dimethylformamide (DMF) and
pyridine for spectral measurements. The com-
plexes were obtained in good yields and high
purity by slow evaporation of the solvent mix-
ture at atmospheric pressure. None of the EPR
spectra of polycrystalline samples of Cu(Lx)2 ob-
tained at 298 and 113 K show any detectable
half-field absorption associated with a DMs= 9
2 transition. The room temperature EPR spectra
of Cu(L6)2 complex, along with copper hyperfine
patterns also exhibit low intensity radical signal
at g=2.0038.

2202
V.T. Kasumo6, F. Ko¨ksal / Spectrochimica Acta Part A 58 (2002) 2199–2211
3.1. Magnetic moments
L6H and 3438, 3259 cm−1 for Cu(L2)2, Cu(L4)2,
respectively. Thus in the complexation of L2H,
L4H, L6H, no coordination of 3- and 4-hydroxy
groups took place. New bands observed in the IR
spectra of Cu(Lx)2 at ca. 420–470 and 490–580
cm−1 regions, were assigned to the w(CuꢀN) and
w(CuꢀO) modes, respectively, according to Ref.
[34,35].
The room temperature magnetic moments of
Cu(Lx)2 fall in the range 1.89–2.19 BM which are
typical for tetrahedrally distorted mononuclear of
Cu(II) compounds with a S=1/2 spin state and
did not indicate any antiferromagnetic coupling of
spins at this temperature.
3.2. IR spectra
3.3. Electronic spectra
The w(CꢁN) stretching frequencies of the lig-
ands (Table 2) are located in the region 1615–
1635 cm−1 and are shifted to lower frequencies
(1604–1625 cm−1) upon chelation, which is con-
sistent with coordination through the azomethine
nitrogen atoms of the ligands (Table 4). A
medium intensity broad band at 2600–2800 cm−1
and a strong absorption in the 1280–1320 cm−1
range caused by the intramolecular OH···N in the
IR spectra of LxH are absent in the spectra of
Cu(Lx)2 and suggests the coordination of the lig-
and to the metal ion through the deprotonated
hydroxyl group in these complexes. These data
illustrate the monoanionic bidentate behavior of
the LxH ligands in the Cu(Lx)2 complexes. The IR
spectra of ligands and their complexes containing
OH in the 3-, 4- and 5-positions show a strong
bands at 3433, 3242, 3310 cm−1 for L2H, L4H,
Electronic absorption spectral data of LxH in
ethanol are given in Table 2. All LxH in ethyl
alcohol solutions show three group bands at
about 217–285, 324–366, and 410–480 nm re-
gions as maximum or a shoulder bands (Table 2).
The bands at the first region are assigned to
intraligand p“p* transitions. The band at 324–
366 nm are attributed to n“p* transitions. The
bands observed as a maxima or a shoulder in the
410–480 nm region in ethanol solutions, are more
sensitive to the polarity of the solvents (this band
was absent in hexane and toluene) and is at-
tributed to dipolar ketoamine tautomer forms of
LxH [36–38]. The electronic spectra confirm that
the LxH compounds exist in the phenolimine form
in non-polar solvents and as equilibrium mixtures
of phenolimine and ketoamine forms in ethanol.
All Cu(Lx)2 complexes show a group of identical a
Table 2
IR and electronic spectral data for LxH ligands
Compound
IR spectra (cm−1
)
Electronic spectra umax (log m, M−1 l−1
)
wCHꢁN
wOH
L1H
L2H
L3H
L4H
L5H
L6H
L7H
L8H
L9H
1620
1625
1617
1617
1615
1625
1628
1625
1625
1612
1604
1622
–
3438
–
3247
–
3310
–
–
–
–
–
–
268 (4.86), 337 (4.67), 430 (2.14)
224 (5.0), 277 (4.76), 324 (4.51), 447 (2.09)
218 (5.01), 274 (4.81), 330 (5.54), 444 (2.23)
239 (4.68), 285 (4.59), 333 (4.74), 410*(2.54)
210 (4.36), 218*, 236 (4.24), 245*, 282 (4.16), 330 (4.11), 420 (3.67)
240 (6.3), 272 (3.6), 366 (2.3), 480* (0.008)
237 (9.1), 271 (5.1), 362 (3.5), 470* (0.015)
217 (12), 230 (12.5), 268 (5.9), 349 (4.4)33, 440* (0.012)
210(4.1), 215*, 237 (2.5), 285 (2.1), 318*, 334 (1.6), 445*
206 (3.0), 219 (2.9), 230 (2.9), 271 (1.5), 350 (1.1), 450*
212 (8.9), 246 (5.6), 268 (4.2), 331 (4.1), 423 (2.4)
232 (11.8), 260*, 316 (3.2), 364 (2.6), 434 (2.2)
L10
L11
L12
H
H
H
* Shoulder.

V.T. Kasumo6, F. Ko¨ksal / Spectrochimica Acta Part A 58 (2002) 2199–2211
2203
Table 3
1H NMR chemical shifs (l in ppm) of some L6H in CDCl3
Compound OH/NH CHꢁN OH
OCH3 Sal.(H)
Phen (H)
t-Butyl (H)
1.29, 1.41
1.34, 1.41
L1H
L2H
13.09
13.60
8.44
8.41
–
–
–
7.33–7.45, 6.05–7.02
7.25 d, 7.23 d,
6.84 d
7.27 d, 7.23 d,
7.02 d
6.85–6.99 m
7.22, 7.18, 6.78
7.26, 7.23, 6.85
7.27 d, 7.24 d,
6.81 d
5.78
7.37 d, 7.08–7.04 td, 6.85 m
L3H
L6H
L7H
L8H
13.42
12.95
12.63
13.10
8.44
8.31
8.39
8.36
–
4.85
–
3.95
–
7.36 d, 7.04 m
7.34, 7.26, 6.88
1.31, 1.37
1.25, 1.34
134, 1.41
134, 1.39
3.82
–
7.37, 7.34, 9.55 m
7.55 d, 7.54 d, 7.48 d 7.38, 7.34, 7.26,
6.94 d
–
L12
H
15.33
15.35
9.21
9.19
–
–
8.14d, 7.77q
7.5 t, 7.14–7.72 m, 1.37, 1.46
7.01 d
broad asymmetric visible bands at ca. 470–500
and 645–730 nm (Table 4) which are attributed to
iminic nitrogen atom. Note that the linewidth at
half-peak height of OH/NH proton signal in the
spectra of LxH is very narrow (DH=1–2 Hz)
than that in the analogues salicylaldimines derived
from 2,6-di-tert-butyl-1-hydroxy-aniline (14–25
Hz) [44]. This effect may be due primarily to 14N
quadruple relaxation and broadening by interme-
diate proton exchange between oxygen and nitro-
gen in tautomeric structures in the LxH. It is
interesting that the resonances of hydrogen-
bonded OH and CHꢁN group of N-(2,5-di-tert-
dx
“d and dx
“d
transitions in
2
2
2
2
xy
xz, yz
tetr−agyonal ligand field−, yrespectively [39,40]. For
square planar complexes, the lower energy d–d
band is usually observed in the region 550–630
nm. The complexes reported in the present work
show the lower energy d–d band in the region
645–730 nm, and this red shift may be due to the
tetrahedral distortion from square planar environ-
ment of the CuII ion [41]. This behavior probably
due to the steric effects of the bulky tert-butyl
group in the 2-position of the ligands. The bands
in the 340–385 nm range and a shoulders at ca.
butylphenyl-2-hydroxy-naphthaldimine
(L12H)
were appeared as a doublet at l 15.33–15.35 ppm
(J=4 Hz) and 9.19–9.21 ppm (J=6 Hz) in
CDCl3, respectively, suggesting the existence of
two isomers for this compound in CHCl3. But, the
resonances of these groups appeared as a singlet
at l 15.43 and 9.18 ppm in C6D6. All the 3-OH,
4-OH and 5-OH-substituted salicylaldimines show
a broad and low intensity singlet in the range l
4.85–5.78 ppm assigned to salicylic OH protons.
The CHꢁN and OCH3 protons resonances ap-
peared as a sharp singlet in the ranges l 8.31–
8.44 and 3.82–3.95 ppm, correspondingly. The
aromatic protons appeared as a multiplet in the
6.66–7.54 ppm range. The protons of the tert-bu-
tyl groups of all LxH exhibit two sharp singlet
peaks in the ranges l 1.25–1.34 and 1.34–1.46
ppm indicating that tert-butyl protons in the 2-
and 5-positions of the aniline moieties of LxH are
magnetically non-equivalent.
400 nm are attributed to eg“p*
metal-to-
azomethine
ligand transitions and ligand-to-metal charge-
transfer transitions from the phenolate groups to
the copper center, respectively [42,43].
1
3.4. H NMR
1
The ligands structure is supported by H NMR
spectra, recorded in CDCl3 and C6D6. The 1H
NMR spectral results obtained for LxH in CDCl3,
together with the assignments are given in Table
3. All LxH salicylaldimines show a sharp intense
singlet in the low field l 12.63–13.60 ppm region,
which is due to the exchangeable hydrogen-
bonded hydroxyl/aminyl (OH/NH) proton. It dis-
appears upon exchange with D2O and can be
associated with the ꢀOH protons doubtlessly in-
volved in intramolecular hydrogen bonding with

2204
V.T. Kasumo6, F. Ko¨ksal / Spectrochimica Acta Part A 58 (2002) 2199–2211
coupling with two 14N (I=1) nuclei. But spectra
3.5. EPR spectra
other complexes neither at 300 nor 113 K did not
observe any additional 14N hyperfine splitting on
the high-field gꢀ component or at gÞ. A probable
mechanism for the absence of 14N hfsc may be a
relatively higher tetrahedral distortion from
square planar configuration around the copper
ion for these complexes. All complexes exhibit
EPR spectra corresponding to axial symmetry
with gꢀ\gÞ\2.03 and AꢀꢀAÞ, which indicates
The spin-Hamiltonian parameters of Cu(Lx)2
complexes are listed in Table 5. The EPR spectra
of polycrystalline samples of all compounds were
obtained at 300 and 113 K. None of the spectra
show resolvable metal hyperfine coupling or any
detectable half-field absorption associated with
DM= 92 forbidden transition indicating a small
zero field splitting effect. All the complexes show
a strong broad asymmetric signal centered at ca.
g=2.1. The spin-Hamiltonian parameters, elec-
tronic absorption spectra and magnetic moment
values for the Cu(Lx)2 complexes suggest a tetra-
hedral distorted square planar geometry around
Cu(II) in these complexes. The X=3-OH, and
3-CH3O substituted complexes displays a five-line
superhyperfine structure on high field copper hy-
perfine component with Aiso(N)=12–14 G due to
that the unpaired electron most likely resides in
2
the dx
orbital having B1g as a ground state
2
2
−y
term [45–49] in a distorted tetragonal copper site.
The value of the in-plane s-bonding parameter,
h2, evaluated using the expression h2=ꢁAꢁ/
0.036+(gꢀ−2.0023)+3/7(gÞ−2.0023)+0.04
[45,46], fall in the 0.48–0.52 range and indicate
the presence of significant in-plane s-bonding in
these complexes [50]. For Cu(L2)2, Cu(L8)2,
Table 4
IR and electronic spectra for Cu(Lx)2 complexes
Compound
veff (BM)
IR spectra (cm−1
wCHꢁN
1605
)
Solvent
Electronic spectra
wOH
Cu(L1)2
Cu(L2)2
Cu(L3)2
Cu(L4)2
Cu(L5)2
Cu(L7)2
Cu(L8)2
Cu(L9)2
2.03
2.18
1.99
1.92
1.94
1.89
1.93
2.19
2.18
2.08
2.14
–
CHCl3
Dioxane
CHCl3
Dioxane
CHCl3
Dioxane
CHCl3
Dioxane
CHCl3
Dioxane
EtOH
Dioxane
CHCl3
Dioxane
CHCl3
Dioxane
CHCl3
Dioxane
CHCl3
Dioxane
CHCl3
Dioxane
246, 270, 307, 362, 600a, 700a
246, 270, 306, 364, 610a, 710a
289, 375, 400a, 510a, 740
249, 265a, 292, 374, 747a
278, 325, 350a, 410a, 500a
248, 287, 400a, 600a, 700a
290, 345, 650
1618
3433b
1604
–
1615
3259c
248, 260a, 287, 350, 374, 645
260, 290, 310a, 376, 410a, 730a
256, 288, 354, 600a, 700a
240, 265, 360, 730
1610
–
–
–
–
–
–
–
1620
248, 378, 402, 727
1612
370a, 305, 365, 390a, 700a
250, 265a, 309, 361, 400a, 700a
370, 365, 600a
1617
258, 362, 400a, 609
Cu(L10
Cu(L11
Cu(L12
)
)
)
1614
254, 310a, 381, 400a, 610a, 730a
252, 270a, 330a, 379, 500a, 600a
265, 360, 450a, 620a
2
2
2
1615
245a, 265, 358, 618
1618
265a, 317, 384, 400, 450a, 700a
265, 290, 315, 390, 420a, 700a
a Shoulder.
b Broad.
c Narrow.

V.T. Kasumo6, F. Ko¨ksal / Spectrochimica Acta Part A 58 (2002) 2199–2211
2205
Table 5
ESR parametersa for the Cu(Lx)2 complexes
Compound
giso
gꢀ
gÞ
Aiso
Aꢀ
AÞ
h2
G
gꢀ/Aꢀ
Cu(L1)2
Cu(L2)2
Cu(L3)2
Cu(L4)2
Cu(L5)2
Cu(L6)2
Cu(L7)2
Cu(L8)2
Cu(L9)2
2.107
2.114
2.109
2.090
2.106
2.083
2.109
2.101
2.109
2.113
2.116
2.103
2.208
2.229
2.216
2.189
2.214
2.242
2.217
2.211
2.231
2.216
2.251
2.226
2.057
2.057
2.056
2.041
2.052
2.004
2.055
2.046
2.048
2.061
2.048
2.042
78.8
75
77.5
66.3
80
–
75
83.8
82.5
72.5
70
178
174
180
172
184
172
176
184
166
178
156
166
29.2
45.5
26.3
13.5
28
0.49
0.50
3.75
4.02
3.85
4.61
4.12
133
137
132
136
129
140
135
127
144
133
156
143
0.49
–
24.5
33.7
40.8
19.8
27
0.48
0.50
0.52
0.48
0.50
0.52
3.95
4.58
4.37
3.54
5.23
5.38
Cu(L10
Cu(L11
Cu(L12
)
)
)
2
2
85
44.5
a Copper hyperfine constants are given in Gauss.
Cu(L9)2, Cu(L11)2 and Cu(L12)2 complexes the ax-
ial symmetry parameter, G=(gꢀ−2.0023/gÞ−
2.0023), lie in the 3.54–5.38 range and indicates
that there is no exchange interaction between the
copper centers (G\4) [45,46,50]. According to
Sakaguchi and Addison, for N-, O- and S-donor
ligand Cu(II) complexes, the quotient gꢀ/Aꢀ may
be considered as a convenient empirical index of
tetrahedral distortion [51]. This quotient ranges
from ca. 105 to 132 cm for square-planar geome-
try with no appreciable tetrahedral distortion. The
gꢀ/Aꢀ quotient increases markedly on the intro-
duction of tetrahedral distortion to the chro-
2,4-di-tert-butylphenyl units unlike their unhin-
dered analogous, undergo one- or two-electron
transfer reactions in the interactions with tri-
arylphosphines and lead dioxide giving corre-
sponding metal-stabilized radical complexes
[7–16,29–31]. Attempts to generate radical spe-
cies by treatments of the free LxH ligands and
their Cu(Lx)2 complexes with PbO2 were unsuc-
cessful. When the HO- and CH3O-substituted
Cu(Lx)2 was treated with a 2–4-fold excess of
PPh3 in various solvents (dioxane, CHCl3, THF,
toluene) at room temperature under vacuum or in
air the decreasing of the intensities of copper
hyperfine lines and immediate appearance of radi-
cal signals centered at g=2.0056–2.0082 were
observed (Figs. 1–3). Surprisingly in the EPR
spectra of these systems any sign of adduct forma-
tion with PPh3 and noticeable changes in g-fac-
tors and copper hyperfine constants were not
observed. Upon treatment of Cu(L2)2 with a 2.5-
fold excess of PPh3 in toluene at 300 K in air
along with spectrum of the complex, a well-re-
solved seven-line pattern at g=2.0095 spacing of
mophore,
for
example,
as
in
bis(N-tert-butylsalicylaldiminato)copper(II) (157
cm) [52] and bis(N%-tert-butyl-pyrrol-2-carbal-
diminato)copper(II) (210 cm) [53]. As seen from
Table 5, the gꢀ/Aꢀ quotients except Cu(L5)2 and
Cu(L8)2, for all other complexes are slightly
higher than those for a square-planar geometry
(Table 5) suggesting that the steric hindrance of
the o-tert-butyl group induces a relatively higher
tetrahedral distortion.
11
G with an intensity distribution of ca.
3.6. Reacti6ities of Cu(Lx)2 toward PPh3
1:1:1:2:1:1:1 was observed (Fig. 1a). The observed
7-fold pattern with intensities of 1:1:1:2:1:1:1 may
be interpreted in terms of coupling of the un-
paired electron to one 63,65Cu (I=3/2) and 31P
(I=1/2) nuclei with hfcc ŽAP=3ŽACu. It can
be easily simulated with an isotropic coupling
constants Aiso(P)=33 G and Aiso(Cu)=11 G.
In our early works it has been demonstrated
that the copper(II) complexes with salicyl-
aldimine, 2-hydroxy-benzylamine, 2-hydroxy-aryl-
azo and b-ketoaminate ligands containing steri-
cally
hindered
2,6-di-tert-butylphenyl
and

2206
V.T. Kasumo6, F. Ko¨ksal / Spectrochimica Acta Part A 58 (2002) 2199–2211
ered to reflect the superimposed triplet and dou-
blet due to 14N and 31P nuclei having hfcc
Aiso(N)=32 and Aiso(P)=35 G, and centered at
g=2.0089 and 2.0096, respectively. The similar
septet pattern radical spectrum with an intensity
ratios of 1:1:1:2:1:1:1 spacing of 11.5 G is also
observed in the reduction of Cu(L4)2 with an
excess amount of PPh3 (Fig. 2a and b). As seen
from Fig. 2c, the deoxygenated samples of this
When this reaction is carried out by the mixing
deoxygenated solutions of Cu(L2)2 and PPh3 un-
der vacuum, each component of this septet ex-
hibits additional triplet superhyperfine splitting
(shfs) with a coupling constant of 2.5 G to the 14
N
nucleus (I=1) (Fig. 1b). This sample upon stand-
ing under vacuum or in air about 12 h showed the
copper hyperfine components disappearance and
a five-line pattern (Fig. 1c) which can be consid-
Fig. 1. EPR spectra of Cu(L2)2+PPh3 system: the initial stage spectrum of the reaction mixture Cu(L2)2+PPh3 in toluene at 300
K (a); radical part of (a) under vacuum (b); spectrum detected after 12 h under vacuum (c).

V.T. Kasumo6, F. Ko¨ksal / Spectrochimica Acta Part A 58 (2002) 2199–2211
2207
Fig. 2. Changes of the EPR spectra of Cu(L3)2+PPh3 system: the initial stage spectrum immediately recorded after mixing of
toluene solutions of Cu(L3)2 and PPh3 at 300 K (a); radical part of (a) in air (b); spectrum of (b) under vacuum (c); spectrum of
(c) after 16 h under vacuum in toluene at 300 K at modulation 1 G and scan 100 G (d).

2208
V.T. Kasumo6, F. Ko¨ksal / Spectrochimica Acta Part A 58 (2002) 2199–2211
Fig. 3. The initial stage EPR spectrum Cu(L6)2+PPh3 system recorded immediately after mixing of deoxygenated dioxane solutions
of Cu(L6)2 and PPh3 under vacuum at modulation 1.6 G and scan 200 G (a); spectrum of (a) under vacuum at modulation 1.6 G
and scan 100 G (b).

V.T. Kasumo6, F. Ko¨ksal / Spectrochimica Acta Part A 58 (2002) 2199–2211
2209
mixture shows additional superhyperfine structure
on each component arising from the 14N nucleus
of the ligand. In addition, weak couplings arising
probably from the azomethine proton were also
observed on some components. The spin-Hamil-
tonian parameters for this radical are: g=2.0082,
Aiso(Cu)=11.5 G, Aiso(P)=34.5 G, Aiso(N)=3.5
G, Aiso(H)=0.85 G. After 16 h in air a septet line
pattern disappeared and only very stable quartet
signal centered at g=2.0035 remained, with in-
tensity ratios 1:2:2:1 spacing of 1.875 G (Fig. 2d).
It is interesting to note that the similar quartet
pattern with an intensity ratios of 1:2:2:1 and
unreactivity towards atmospheric O2 (this signal
remains unchanged more than 3 weeks in air)
previously also were observed in the reduction of
bis(b-ketoaminato)copper(II) [9], bis(salicylaldimi-
nato)palladium(II) [29–31,54], bis(salicylaldimi-
nato)copper(II) [10,11,15], and 2-hydroxy-
aryl-azo ligands chelates of Cu(II) [7,8] containing
sterically hindered phenols fragments. The detec-
tion of stable secondary radical species exhibiting
identical quartet pattern and g- and A-factors in
the reduction of various complexes by PPh3, sug-
gests that these radical species probably contain
one 31P and 14N nuclei with hfcc AP=AN. Note
signals were observed. Although the solid state
EPR spectrum of Cu(L6)2 did not exhibit any
radical signal, the dioxane solution of this com-
plex reveals the presence a less intensity radical
singlet centered at g=2.0044 (DH=10 G). Upon
addition of a 3-fold excess of PPh3 to dioxane
solution of this sample in air, the appearance of a
seven-line pattern with a hyperfine splitting of
about 9.75 G at g=2.0063 was observed. The
presence a strong singlet centered at the center of
the spectrum complicated the accurate determina-
tion of the relative intensity ratios of hyperfine
lines. The complicated ESR spectra of this sample
in toluene under vacuum at 300 K are presented
in Fig. 3. It is assumed that these spectra result
from the overlapping signals due to the presence
at least two type of radicals. The central more
intense part of the spectra probably reflects con-
tributions arising from the phenoxyl type radical
presented in the starting complex (g=2.0042,
DH=10 G). It is thought that a less intense
patterns at high field and low field parts of the
spectra (g=2.0063) can be assigned to radical
species generated by the reduction with PPh3.
Some lines on wings of this pattern (Fig. 3b)
exhibit additional a triplet (3.5 G) of doublets
(1.125 G) splittings. We suppose that this spec-
that the generation of Ph3P+ cation radicals in
’
the process of reduction which can readily bond
trum is similar to above spectral features and may
Cu
to PPh3 to give corresponding s*-radicals
be readily simulated assuming ŽAPiso=3ŽA ,
iso
Ph3P ꢀPPh3 with two equivalent 31P nuclei have
with an additional shfs to one 14N and azome-
thine proton. From the spectra spin-Hamiltonian
parameters: Aiso(P)=33.6 G, Aiso(Cu)=11.175
G, Aiso(N)=3.5 G and Aiso(H)=1.125 G can be
calculated. Upon treatment of Cu(L7)2 with PPh3
in toluene, although the decreasing of the intensi-
ties of copper hyperfine lines takes place, but the
formation of any radical signal was not observed.
It is interesting that under the above-mentioned
conditions the reduction of Cu(L1)2 (X=H) and
Cu(Lx)2 (X=Br, NO2, 5,6-benzo) complexes with
a 5–10-fold excess of PPh3, even on heating, did
not take place.
+
’
been reported by Symons [55]. Similarly, in our
system the generated Ph3P+ cation radicals can
’
+
’
bond to ligand molecules giving Ph3P ꢀNR2 with
close hfcc of 31P and 14N nuclei can be considered
as one possible alternative. The EPR spectral data
suggest that the salicylate moiety is oxidized yield-
ing a phenoxyl coordinated radical ligand com-
x
’
plex, [(PPh3)2Cu(I)L ], as primary radical
intermediates in the reduction. It is thought that
the attack of PPh3 to Cu(Lx)2 molecule one ligand
removed as an anion radical and the afforded
(PPh3)2CuIILx intermediate, via one-electron
transfer from coordinated Lx to Cu(II) ion, con-
verts to oxidized forms.
In the reduction of Cu(L4)2 and Cu(L5)2 in
THF, which was used from their solubility, with
an excess of PPh3, without appearance any radical
species simultaneous disappearance of copper(II)
Thus EPR examination of the reaction mixture
of 2,5-di-butylated Cu(II)bis(salicylalimine) com-
plexes—PPh3 systems shows that some Cu(II)
chelates were readily reduced giving stable radical
ligand Cu(I) complexes and other stable sec-
ondary radical species. It has been found that the

2210
V.T. Kasumo6, F. Ko¨ksal / Spectrochimica Acta Part A 58 (2002) 2199–2211
[18] F. Len, K.M. Chan, Bull. Chem. Soc. Jpn. 45 (1972)
2709.
[19] G.A. Razuvaev, U.K. Cherkasov, G.A. Abakumov, J.
Organomet. Chem. 160 (1978) 361.
[20] B.B. Wayland, H.D. Wisinski, J. Chem. Soc., Chem.
Commun. (1971) 1025.
[21] F. Len, K.M. Chan, Bull. Chem. Soc. Jpn. 45 (1972)
2709.
reduction of some 2,5-di-tert-butylated bis(salicy-
lalimine)copper(II) complexes with PPh3 proceeds
via Cu-stabilized phosphine containing radical in-
termediates. The electron transfer ability of these
complexes is dependent upon both the position
and the nature of the substituent on the salicylic
fragment.
[22] G.A. Razuvaev, U.K. Cherkasov, G.A. Abakumov, J.
Organomet. Chem. 160 (1978) 361.
[23] H.S. Maslen, N.T. Waters, Coord. Chem. Rev. 17 (1975)
137.
Acknowledgements
[24] B.F. Anufrienko, A.A. Shklyaev, J. Struct. Chem. 17
(1976) 530.
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The authors would like to thank the Council
for Science and Technical Research of Turkey
8
(TUBITAK) for the financial support under the
Grant TBAG-1424 and to Dr I. Kartal for the H
1
NMR, Dr B. Karabulut for the EPR spectroscopy
measurements.
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