Spectroscopic studies on bis(3,5-di-t-butyl-1,2-benzoquinone 1-oximato)copper(II) and its mixed-ligand complexes. Copper(II)-radical ferromagnetic coupling

Article abstract of DOI:10.1515/znb-2002-0504

Bis(3,5-di-t-butyl-1,2-benzoquinone 1-oximato)copper(II), Cu(ox)₂, and its mixed-ligand complexes, Cu(ox)L^x [L^x = 8-hydroxyquinolinato (L¹), N-Phenyl-salicylaldimine (L²), N-phenyl-3,5-di-t-butylsalicylaldiminato (L³) and N-(2-hydroxyphenyl)-3,5-di-t-butylsalicylaldiminato (L⁴)], were prepared and their spectral behavior as well as redox-reactivity towards PPh₃, (m-ClC₆H₄)₃P, (m-CH₃C₆H₄)₃P and PPh₂-(CH₂)₄-PPh₂ studied by analytical and spectroscopic (IR, UV-vis, ESR) techniques and magnetic measurements. Cu(ox)₂ and Cu(ox)L⁴ complexes prepared in air show μ_ef values of 2.84 and 3.33 μ_B, respectively, and are consistent with an S = 1 and S = 3/2 ground states. Both compounds are formulated as copper(II)-radical complex exhibiting intramolecular ferromagnetic coupling between the orthogonal d_x²-_y² magnetic orbital of the Cu^II ion and that of the π-radical ligand.

Full text of DOI:10.1515/znb-2002-0504

Spectroscopic Studies on Bis(3,5-di-t-butyl-1,2-benzoquinone
1-oximato)copper(II) and its Mixed-Ligand Complexes.
Copper(II)-Radical Ferromagnetic Coupling
Veli T. Kasumova, Es¸ref Tas¸a, Yasin Yakara, Fevzi Ko¨ksalb, and Rahmi Ko¨s¸eog˘lub
a
Chemistry Department, Faculty of Arts and Sciences, Harran University, Sanlıurfa, Turkey
b Physics Department Faculty of Arts and Sciences, Ondokuz Mayıs University Samsun, Turkey
Reprint requests to Prof. Dr. V. T. Kasumov. E-mail: vkasumov@harran.edu.tr
Z. Naturforsch. 57 b, 495–502 (2002); received November 19, 2001
Mixed-Ligand Complexes, Copper(II)-Radical Ferromagnetic Coupling
Bis(3,5-di-t-butyl-1,2-benzoquinone 1-oximato)copper(II), Cu(ox)2, and its mixed-ligand
complexes, Cu(ox)Lx [Lx = 8-hydroxyquinolinato (L1), N-Phenyl-salicylaldimine (L2), N-
phenyl-3,5-di-t-butylsalicylaldiminato (L3) and N-(2-hydroxyphenyl)-3,5-di-t-butylsalicyl-al-
diminato (L4)], were prepared and their spectral behavior as well as redox-reactivity towards
PPh3, (m-ClC6H4)3P, (m-CH3C6H4)3P and PPh2-(CH2)4-PPh2 studied by analytical and spec-
troscopic (IR, UV-vis, ESR) techniques and magnetic measurements. Cu(ox)2 and Cu(ox)L4
complexes prepared in air show
values of 2.84 and 3.33 B, respectively, and are consistent
with an S= 1 and S= 3/2 groundefstates. Both compounds are formulated as copper(II)-ra-
dical complex exhibiting intramolecular ferromagnetic coupling between the orthogonal dx
2
2
y
magnetic orbital of the CuII ion and that of the -radical ligand.
Introduction
In this report, we present studies on synthesis,
spectroscopy and redox behavior of 3,5-di-t-butyl-
1,2-benzoquinone 1-monoxime (Hox), bis(3,5-
di-t-butyl-1,2-benzoquinone 1-oximato)copper(II)
[1a 1b, referred to as Cu(ox)2] (Scheme 1), and
mixed ligand chelates, Cu(ox)Lx, containing the
anion of Hox as primary ligand and various N-,
O-donors Lx = 8-hydroxyquinolinato (L1), N-Phe-
nyl-salicylaldimine (L2), N-phenyl-3,5-di-t-butyl-
salicylaldiminato (L3) and N-(2-hydroxyphenyl)-
3,5-di-t-butylsalicylaldiminato (L4) as secondary
ligands. In order to explore the effects of secondary
-acceptor aromatic ligands on the redox reactivity
of Cu(ox)2 towards PPh3 and the nature of the radi-
cal species we also decided to investigate the redox-
reactivity of the mixed ligand Cu(ox)Lx complexes.
Quinone and quinoid cofactors appear in biolog-
ical systems and sometimes function in conjunc-
tion with a redox-active metal center [1]. Transition
metal o-quinone complexes have shown a unique
facility for internal electron transfer between the
metal and chelated quinone ligands via the cate-
cholate and semiquinone forms [2]. Although the
chemistry of transition metal and Main Group el-
ement complexes with di-tert-butylated o-benzo-
quinone (DTBQ) has been extensively studied by
C. G. Pierpont, D. N. Hendrickson, G. A. Abaku-
mov, D. G. Tuck and their co-workers as well as
other researches [3 - 9], these compounds constitute
an interesting class of bidentate ligands. In particu-
lar relatively little is known about the chemistry of
the transition metal complexes with DTBQ-oxime
ligands. Transition metal(II) complexes of o-quin-
one mono-oximes have been thoroughly studied es-
pecially in view of their extensive reactivity towards
organic substances [10]. Recently we have found
that the complexation of a 3,5-DTBQ-oxime lig-
and, which was used without isolation in pure state,
with some transition metal ions was accompanied
by the formation of various stable radical species
which was difficult to interpret [11].
Experimental Section
Physical property measurements
The experimental procedures and instruments used for
the physical property measurements were as previously
described [11]. The measurement of the ESR spectra of
Cu(ox)2 and Cu(ox)Lx, as well as their reduction with an
excess of triarylphosphines and 1,4-bis-(diphenyldiphos-
phino)butane (dppb) were carried out as described pre-
viously [11]. Errors for g- and A-parameters of radicals
c
¨
¨
0932–0776/02/0500–0495 $ 06.00
2002 Verlag der Zeitschrift fur Naturforschung, Tubingen www.znaturforsch.com
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V. T. Kasumov et al. · Bis(3,5-di-t-butyl-1,2-benzoquinone 1-oximato)copper(II)
Scheme 1.
Table 1. Physical and analytical
Compound
M.p.
( C)
Yield
Elemental analyses, found/calcd. [%]
eff
data for the Cu(ox)Lx complexes.
(%) (B.M.)
C
H
N
Cu(ox)L1
Cu(ox)L2
Cu(ox)L3
262
253
263
87
86
92
1.85 62.89/62.50 6.21/6.33 5.67/5.93
1.84 65.78/65.63 6.43/6.33 5.43/5.66
2.05 69.63/68.96 7.89/8.14 4.82/4.50
3.33 66.86/67.77 7.34/7.15 4.65/4.51
Cu(ox)L4 (280, dec.) 78
are 0.0002 and 0.0005 G, respectively. The g-values protons on phenol), 7,25 (d, 1 H, J = 2.41 Hz), 7.48 (d, 1
were determined by comparison with g = 2.0036 for H, J = 2.41 Hz) (meta-coupled doublets ring protons on
DPPH.
the salicylic moiety), 8.69 (s, 1H, CH=N), 12.69 (s, 1 H,
HO/NH). – C21H27NO2 (325.4): calcd. C 77.5, H 8.4, N
4.3; found. C 77.8, H 9.4, N 4.3.
Preparation of the compounds
Cu(ox)2 was prepared as follow: Nitrogen gas was
passed through a solution containing Hox (1 mmol,
0.335 g) in methanol (40 ml) for 10 min. When a solution
of Cu(OAc)2 H2O (0.5 mmol, 0.1 g) in deoxygenated
methanol (10 ml) was added to a dark red solution of
Hox a deep blue color immediately appeared. The re-
sulting mixture was stirred under N2 at 40 - 45 C for
ca. 1 h, during which solid crystalline products precipi-
tated. The volume of the solution was then reduced with
a flow of N2 to ca. 6 - 8 ml. The dark blue microcrys-
talline precipitate of Cu(ox)2 was isolated by filtration,
washed with minimal amounts of cold MeOH (2 ml) and
hexane, and dried in air. The product was crystallized
from methanol-acetone (2:1 v/v) mixture (0.285 g, 85 %).
Hox, hydroxylamine hydrochloride, 8-hydroxyquin-
oline (HL1), PPh3, dppb, Cu(OAc)2 H2O and sol-
vents were obtained commercially and used as received.
3,5-Di-t-butyl-1,2-benzoquinone 1-monoxime [12a] and
3,5-di-t-butyl-2-hydroxy-benzaldehyde [12b] were pre-
pared by described procedures. (m-ClC6H4)3P and
(m-CH3C6H4)3P were synthesized by using a previ-
ously reported method [13]. N-Phenyl-salicylaldimine
(HL2), N-Phenyl-3,5-di-t-butylsalicylaldimine (HL3), N-
(2-hydroxyphenyl)-3,5-di-t-butylsalicylaldimine (H2L4)
were obtained by the condensation of 3,5-di-t-butyl-2-
hydroxybenzaldehyde with aniline and o-aminophenol in
methanol, respectively, in equimolar ratio. HL3: Yield
88%. M. p. 115 C. – UV/vis (C2H5OH): max(lg ) =
206(4.41), 226(4.43), 238(sh), 277(4.23), 307(4.19),
322(sh) 350(4.08), 450 nm (sh). – IR (pellet): = 1614
(CH=N), 2450 - 2650 (intramolecular H-bonding), 2866
Cu(ox)2 black microcrystals, M. p. 245 (decomp). – IR:
1
= 1656 (C=O), 1608 cm (C=N). –
per molecule:
eff
2.01 B. M. at 288 K. – C28H40N2O4Cu (532.16): calcd.
C 63.32, H 7.57, N 5.26; found C 63.17, H 7.47, N 5.14.
The spectral behavior and redox reactivity of this com-
1
- 2959 cm [C-H of -C(Me)3]. – 1H- NMR (200 MHz,
CDCl3): = 1.33 (s, 9H), 1.48 (s, 9 H), 7.22 - 7.45 (m, 5 H,
Ph), 7.21 (d, 1 H, J = 2.22 Hz), 7.46 (d, 1 H, J = 2.56 Hz)
(meta-coupled doublets of ring protons on the salicylic
moiety), 8.63 (s 1H, CH=N), 13.65 (s 1 H, OH/NH). -
C21H27NO (309.4): calcd. C 81.52, H 8.79, N 4.52; C
82.13, H 8.67, N 4.65. H2L4: Yield 90%. M. p. 146 C
– UV/vis. (C2H5OH): max(lg ) = 203(4.58), 215(4.55),
240(4.45), 274(4.30), 356(4.31), 450 (3.0) nm (sh). – IR
(pellet): = 1614 (CH=N), 2500 - 2800 (intramolec-
ular H-bonding OH), 3450 (intermolecular H-bonding),
plex except
in air.
are similar to those of Cu(ox)2 prepared
eff
The mixed-ligand Cu(ox)Lx complexes were prepared
by mixing methanolic solutions of Cu(OAc)2 H2O, Hox
and HLx in a 1:1:1 molar ratio, and heating the reac-
tion mixture in the range 45 - 50 C for about 1.5 h in
air described above. The physical and analytical data of
the complexes are summarized in Table 1. Analysis for
Cu(ox)L1: M. p. 262 C. – UV/vis (CHCl3): max(lg )
265, 354(3.91), 403(3.77), 529(3.48), 600s nm. – IR (pel-
let): = 1632 (C=O), 1605 cm (C=N). –
B. M. – C23H26N2O3Cu (441.99): calcd. C 62.50, H 6.33
1
1
3554 cm (free OH). – H NMR (200 MHz, CDCl3):
= 1.33 (s, 9 H, C(Me)3), 1.47 (s, 9 H, C(Me)3), 5.84 (s,
1 H, HO on phenol), 6.95, 7.15 (t, 2 H, J = 7.23 Hz), 6.9
1
= 1.85
eff
(d, 1 H, J = 1.22 Hz), 7.24 (d, 1 H, J = 1.22 Hz) (ring N 5.93; found: C 62.89, H 6.21, N 5.67.
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V. T. Kasumov et al. · Bis(3,5-di-t-butyl-1,2-benzoquinone 1-oximato)copper(II)
497
Results and Discussion
The results of elemental analysis for C, H, N
are consistent with the stoichiometry for Cu(ox)2
and Cu(ox)Lx complexes. The spectroscopic and
magnetic data of Cu(ox)2 prepared in air and under
N2 and of some radical intermediates generated by
their reduction with PPh3 do not coincide with those
for bis(oximato)Cu(II) prepared without isolation of
the Hox in pure state [11]. Note that the versatility
of Hox as a ligand is enhanced by its ability to act
as suggested by two tautomeric valence isomeric
forms, and that it is air sensitive.
The IR spectrum of Hox exhibits a broad strong
1
band at 3215 - 3235 cm and narrow bands at
1
1680 and 1640 cm due to the oxime O-H, C=O
and C=N stretching frequencies, respectively. The
1
bands at 3450 and 3550 cm attributable to the
intermolecular H-bonding and free OH stretching
frequencies of the HL3 and H2L4 salicylaldimines
are absent in the IR spectra of the Cu(ox)Lx com-
plexes. The C=N and C=O stretching frequencies
appear in the 1605 - 1614 and 1625 - 1648 cm 1 re-
gions, respectively. The lack of strong absorptions
1
in the 1680 - 1740 cm range indicates the ab-
sence of coordinated o-nitroso-phenolato isomer in
the complexes.
The spectral data of Hox agree with previous re-
1
ports [11b, 12a]. The electronic, infrared and H
NMR characteristics of the new HL3 and H2L4 lig-
ands and their assignments are presented in the ex-
perimental section. In the electronic spectra of these
ligands along with bands in the 206 - 356 nm re-
gion which are due to intraligand
* and n
*
transitions involving molecular orbitals of the C=N
chromophore and benzene ring, a broad shoulder
around at 450 nm was observed. This absorption (in
EtOH solutions) is attributed to the n
* transi-
tion of the dipolar ketoamine tautomeric structure
of HL3 and H2L4 [14, 15]. After standing in air
for several month a conversion of the orange Hox
crystals into dark brown oil was observed. The elec-
Fig. 1. ESR spectra of radicals generated from Hox oxime
in toluene solutions at 300 K under vacuum: (a) air oxi-
dized samples; (b) oxidation with PbO2 in air; (c) oxida-
tion with PbO2 under vacuum.
tronic spectrum of this oil in toluene solution (
max
and A(14N) = 25.85 G) and the phenoxyl type rad-
ical which is characterized by a partially resolved
pattern of low intensity (Fig. 1a). A similar super-
imposed spectrum with a more complicated cen-
tral multiplet was detected in the oxidation of Hox
with PbO2 in air (Fig. 1b). The spectrum of radicals
generated by the oxidation of Hox with PbO2 in a
at 812, 511, 348 and a shoulder at 380 nm) is sig-
nificantly different from those observed for the par-
ent Hox in toluene [ max(lg ) 324(3.9), 431(3.6),
684(1.7)]. The ESR spectrum of a toluene solution
of this oil recorded at 300 K reveals a superposition
of the spectra of 3,5-di-t-butyl 2-quinone-nitroxide
(a well-resolved 1:1:1 triplet pattern with g = 2.0035
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498
V. T. Kasumov et al. · Bis(3,5-di-t-butyl-1,2-benzoquinone 1-oximato)copper(II)
Table 2. Electronic spec-
1
Compound Solvent
Electronic absorption spectra
(log
M
1cm
)
max
tral data for Cu(ox)2,
Cu(ox)Lx and complex
Cu(ox)2
EtOH
204(4.32), 236(4.3), 264(4.0), 305(4.1), 352(4.2),
410s, 528(2.8)
+
phosphine systems
(s shoulder).
MeOH
Acetone
CHCl3
208, 237, 264, 302, 351, 410, 525
349(4.22), 422(3.79), 544(3.79), 620s
270s, 300(4.14), 353(4.2), 415(3.87), 543(3.7), 625s
308, 340s, 490, 690
CHCl3 + PPh3
PhMe
295(4.49), 349(4.46), 423(4.2), 566(4.08), 630s
319, 413, 480, 510s, 545s
PhMe+PPh3
CHCl3 + m-ClPh3P 310s, 345s, 400, 500s, 690s
CHCl3 + m-CH3Ph3P 310s, 342s, 430, 490s, 515s, 700
CHCl3 + dppb
Cu(ox)L1 CHCl3
CHCl3 + PPh3
PhMe
Cu(ox)L2 EtOH
CHCl3
CHCl3 + PPh3
Cu(ox)L3 CHCl3
CHCl3 + PPh3
Cu(ox)L4 MeOH
MeOH + PPh3
310, 480, 505s, 550s, 703
265, 354(3.91), 403(3.77), 529(3.48), 600s
360, 370s, 375s, 410, 500s, 550s
340, 415, 537, 610s
232, 370, 295s, 346, 528
275, 300s, 352, 380s, 536, 600s
345, 390, 490s, 510s, 550s
283, 352, 400s, 511, 600s
308, 345, 415, 500s, 550s, 700
206, 230, 260, 296, 304s, 352, 427, 448s, 526
340, 445, 500s, 660
vacuum in toluene solution consists of a superposi- red is observed (Table 1). In addition, the maxi-
tion of a 1:1:1 triplet (g = 2.0066, AN = 25.88 G) mum in the 528 - 566 nm range and the shoul-
and a triplet of triplets (1:2:1) (g = 2.0066 with hfsc der at ca. 600 nm observed in the spectra of the
of AN = 10 G and AH = 2.5 G) (Fig. 1c) due to hy- unreduced parent complexes have disappeared and
perfine coupling with the 14N nucleus for nitroxide broad structured shoulders in the 490 - 550 nm re-
1
and with one 14N and two identical H nuclei for gion were observed in the spectra of the reduced
the phenoxyl type radicals, respectively.
samples (Table 1). The strong absorption bands (
103 - 104 M 1cm 1) in the 600 - 850 nm range
and their solvent dependence are characteristic for
coordinated semiquinone and phenoxyl type radi-
The spectra of the mixed-ligand complexes
Cu(ox)Lx show a similar pattern in the visible re-
gion consisting of maxima in the range 526 - 566 nm
and a weak shoulder at ca. 600 nm (Table 2). The
location of these maxima varies depending upon
the nature of the complex and solvents. Although
the appearance of the absorption bands in this re-
cals and can be assigned to ligand based n
* or
metal-to-ligand charge-transfer (d
[1, 4].
*) transitions
The room temperature effective magnetic mo-
ments of the complexes except Cu(ox)2 and
2
2
2
2
gion attributable to B1g A1g, B1g B2g and
Cu(ox)L4 prepared in air are in the 1.84 - 2.05
2B1g E1g transitions in D4h symmetry is typical
2
eff
for CuII in a square-planar environment [16], owing
to their intensities, none of the observed absorptions
(lg = 2.5 - 3.5) can be attributed to d-d transi-
tions but must be related to charge-transfer transi-
tions between the metal ion and the oximato ligand.
Bands below 500 nm have very high extinction co-
efficients and may be assigned to the intraligand
range (Table 2) which is typical for mononuclear
copper(II) complexes with a S= 1/2 spin state.
Complexes of Cu(ox)2 and Cu(ox)L4 prepared in
air unlike the same complexes prepared under N2,
show
values of 2.83 and 3.33 B.M., which are
eff
consistent with an S= 1 and S= 3/2 ground states,
respectively, indicating a strong ferromagnetic cou-
pling between the radical ligands and the copper(II)
*, n
* and metal-to-ligand charge-transfer
centers. The observation of very higher
values
eg
* transitions [17].
eff
Upon treatment of toluene, EtOH, or CHCl3 so- unambiguously indicates that the ligands in these
lutions of Cu(ox)2 and Cu(ox)Lx with a 2-3 fold complexes are coordinated in radical form. The al-
excess of triarylphosphines and dppb (under vac- ternative formalism, such as the formation of a cop-
uum or in air), a color change from green to deep per(III) complex is ruled out on the basis of spectral
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V. T. Kasumov et al. · Bis(3,5-di-t-butyl-1,2-benzoquinone 1-oximato)copper(II)
499
and elemental analysis data. The origin of a ferro-
magnetic coupling can be easily explained by an
orbital model: The magnetic orbital of copper(II) is
y2
dx2 , with a -bonding character in the basal plane
of the square planar (or pyramidal) complex, while
the * magnetic orbital of the semiquinone radi-
cal is orthogonal to the d electrons of the metal;
therefore, according to well-established rules [18], a
ferromagnetic coupling must be observed. Recently
it was reported that semiquinone radical ligands
chelated in the basal plane of a square pyramidal
complex of Cu(II) interact ferromagnetically with
the orthogonal d electrons of the metal and give a
S= 1 molecular spin state at 300 K [19].
The ESR spectra of Cu(ox)2 and Cu(ox)Lx
ESR spectra of polycrystalline samples of
Cu(ox)2 and Cu(ox)Lx were obtained at 300 and
113 K. No half-field signals associated with a
Ms = 2 forbidden transition and Ms = 1 typi-
cal for binuclear Cu(II) compounds were observed.
The room temperature solid state ESR spectra of
these complexes except Cu(ox)L2 are quite similar
and exhibit an axially symmetric g-tensor parame-
ters with g > g > 2.0023 [g = 2.203, g = 2.049
for Cu(ox)2; g = 2.206, g = 2.041 for Cu(ox)L1;
g = 2.295, g = 2.096 for Cu(ox)L3, and g =
2.204, g = 2.043 for Cu(ox)L4], indicating that the
Fig. 2. (a) Temperature dependence of the solid state ESR
spectrum of Cu(ox)2; (b) ESR spectrum of Cu(ox)2 in
toluene recorded at 300 K.
nuclei. A five to six lines splitting (as a result of the
superposition of two spectra) appeared on the high
field components of the spectrum due to 14N (I = 1)
superhyperfine (shf) coupling with two equivalent
nitrogen donor atoms [21]. It is interesting that the
toluene solution spectra of Cu(ox)2 and Cu(ox)L4
complexes synthesized in air exhibit an additional
three line splitting of 7.5 G centered at g = 2.0067
(Fig. 2b). This pattern probably originates from co-
ordinated phenoxyl or semiquinone type radicals.
On the basis of the magnetic moment data and the
ESR spectra, Cu(ox)2 and Cu(ox)L4 complexes are
considered as radical ligand Cu(II) complexes.
The toluene frozen-glass (113 K) ESR spectra of
these complexes are also of axial symmetry with
spin-Hamiltonian parameters g > g > 2.0023 (Ta-
ble 3), suggesting that the copper(II) atom has a
y2
copper site has a dx2
ground-state characteristic
of square planar, square base pyramidal, or octahe-
dral stereochemistry [19]. The exchange interaction
parameter, G = (g – 2.0023 / g – 2.0023), for
Cu(ox)2, Cu(ox)L1 and Cu(ox)L3 show G>4, sug-
gesting the absence of exchange coupling between
copper(II) centers in the solid state [20]. Complex
Cu(ox)2 prepared in air unlike Cu(ox)2 prepared
under N2, displays a single isotropic line of peak-
to-peak line width of 90 G at g = 2.076. The temper-
ature dependence of the solid state ESR spectrum
of Cu(ox)2 prepared in air is shown Fig. 2a. When
the sample was cooled from 293 to 113 K the signal
intensity increased following Curie’s law (I = C/T).
This behavior is quite characteristic of a strong in-
tramolecular ferromagnetic interaction [18].
y2
dx2
ground state [20]. The patterns for Cu(ox)2,
Cu
N
The isotropic giso, Aiso and Aiso values for Cu(ox)L2 and Cu(ox)L3 are similar and show a five
Cu(ox)2 and Cu(ox)Lx complexes are close to each nitrogen shf splitting on both the perpendicular and
other (Table 3). The room temperature ESR spec- parallel components (mI = +3/2). In the frozen-glass
trum of Cu(ox)2 in toluene shows the usual four line spectra of Cu(ox)L1 and Cu(ox)L3 the g com-
pattern due to hyperfine splitting of 63 65Cu (I = 3/2) ponents overlap with another broad line which is
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V. T. Kasumov et al. · Bis(3,5-di-t-butyl-1,2-benzoquinone 1-oximato)copper(II)
Table 3. ESR parameters for the Cu(ox)2 and Cu(ox)Lx complexes.
2
Compound
giso
g
g
Aiso
A
A
ANiso
g /A
G
Cu(ox)2
2.094
2.099
2.095
2.087
2.096
2.203
2.221
2.201
2.194
2.208
2.039
2.038
2.044
2.044
2.040
85
84
84
82
85
170
180
176
168
188
43
14
11
12
12
14
0.49
0.51
0.49
0.48
0.50
130
123
125
131
118
4.83
5.75
4.76
4.59
5.2
Cu(ox)L1
Cu(ox)L2
Cu(ox)L3
Cu(ox)L4
36
38
39
25
Fig. 3. ESR spectra detected for Cu(ox)2 + arylphosphine systems in toluene at 300 K under vacuum: (a) Cu(ox)2 +
PPh3; (b) Cu(ox)2 + dppb; (c) Cu(ox)2 + (m-CH3C6H4)3P.
spread over a range of 1100 G. The spectrum of change from dark green to deep red and a disap-
Cu(ox)L3 also shows a broad line at g = 1.887. pearance of the ESR signal of Cu(ox)2, as well as
The observed broadening features on either side the appearance of a new seven-line pattern at g =
of the central resonance may be due to dimeric or 2.0036 with hfcc of 2.25 G were observed (Fig. 3a).
polymeric species with weak exchange and dipole- The asymmetric feature of this spectrum, and the
dipole interactions between the paramagnetic cen- appearance of a low intensity middle line com-
ters [22].
plicate the accurate simulation of this signal. The
spectrum may be assigned to the radical species
which reflects interaction of the unpaired electron
spin density with one 31P, one 14N and one 1H nu-
cleus with AP = 7.75 G, AN = 2.25 G and AH =
2.25 G. Quite similarspectra were detected in the re-
duction of mixed-ligand Cu(ox)Lx complexes with
PPh3 in toluene. Surprisingly, a similar signal was
generated in the interaction of Mn(3,5-di-t -butyl-
1,2-benzoquinone 1-oximato)2 with PPh3. Note that
the observed radical species are stable in air, and no
changes in the spectrum on keeping the samples at
room temperature for 5 h were observed. A simi-
lar septet pattern was observed also on treatment of
Cu(ox)2 with an excess of dppb in toluene at 300 K
(Fig. 3b), but the g-factor (2.013) and the hyperfine
coupling constant (2.5 G) observed in the Cu(ox)2
+ dppb system is significantly different from those
For all complexes the g /A quotient ranges from
118 to 131 cm, evidence in support of the square-
planar geometry with no appreciable tetrahedral dis-
tortion [23]. The axial symmetry parameters, G, for
these complexes lie in the 4.59 - 5.45 range and
suggest that there is no exchange interaction be-
tween the copper centers (G>4) [20a,b]. The value
of the in-plane -bonding parameter, 2, evaluated
using the expression 2 = A /0.036 + (g – 2.0023)
+
37 (g – 2.0023) + 0.04 [19], fall in the 0.48 -
0.51 range and indicate the presence of significant
in-plane -bonding in these complexes [20c].
Reduction of Cu(ox)2 and Cu(ox)Lx with
triarylphosphines
Upon addition of a 2 - 3 fold excess of PPh3 to
toluene solutions of Cu(ox)2 under vacuum, a color
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V. T. Kasumov et al. · Bis(3,5-di-t-butyl-1,2-benzoquinone 1-oximato)copper(II)
501
found for the Cu(ox)2 + PPh3 system. The higher (g = 2.0065), in which central components show
value of the g-factor compared to that of free radi- a poorly resolved hyperfine structure, is observed.
cals (g = 2.003 - 2.008) undoubtedly indicates that The latter system exhibits a nine line spectral pattern
there is a noticeable contribution from the metal centered at g = 2.007 with hfcc of 3.75 G, but on the
d-orbitals to the molecular orbital containing the central components of this signal a relatively well
unpaired electron. The electronic spectral pattern of resolved additional shf structure with a coupling of
the Cu(ox)2 + dppb system (Table 1) is also differ- 0.5 G appeared (Fig. 3c). These spectra are practi-
ent from that observed for Cu(II) oxime complexes cally identical with those found for the reduction of
prepared without isolation of Hox in pure state [11]. the bis(oximato)CuII complex with (m-ClC6H4)3P
The ESR spectra of Cu(ox)2 + (m-ClC6H4)3P and or (m-CH3C6H4)3P [11b]. The similarity of these
Cu(ox)2 + (m-CH3C6H4)3P systems in toluene at spectra to that reported for the [Cu(PPh3)2 (3,5-
300 K under vacuum show similar patterns. For the DTBSQ)] [7] allow us to formulate these radicals as
former system a nine line signal spacing of 3.75 G [(m-XC6H4)3P]2CuI (oxsq) semiquinonate adducts.
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