ESR study of weak exchange interactions in binuclear copper(II) complexes with 2-hydroxyacetophenone acyldihydrazones

Article abstract of DOI:10.1023/A:1022195621106

The binuclear copper(II) complexes with 2-hydroxyacetophenone acyldihydrazones (H₄L) Cu₂L·2Py were studied by ESR spectroscopy. In these complexes, the coordination polyhedra are linked via the polymethylene chain containing from one to five units. In the complexes based on acyldihydrazones of malonic, succinic, glutaric, and adipic acids, weak spin-spin exchange interactions occur between the paramagnetic centers. An increase in the length of the polymethylene chain to five units hinders exchange interactions, and the ESR spectrum of the corresponding complex has a signal typical of monomeric copper(II) complexes.

Full text of DOI:10.1023/A:1022195621106

2268
Russian Chemical Bulletin, International Edition, Vol. 51, No. 12, pp. 2268—2272, December, 2002
ESR study of weak exchange interactions in binuclear copper(II) complexes
with 2ꢀhydroxyacetophenone acyldihydrazones
V. F. Shul´gin,a A. N. Gusev,a V. Ya. Zub,b and G. M. Larincꢀ
aV. I. Vernadskii Taurida National University,
4 ul. Yaltinskaya, 95007 Simferopol, Ukraine.
Fax: (065 2) 23 2310. Eꢀmail: vshul@ccssu.crimea.ua
bTaras Shevchenko Kiev State University,
60 ul. Vladimirskaya, 01033 Kiev, Ukraine.
Eꢀmail: vera@anorgchemie.univ.kiev.ua
cN. S. Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences,
31 Leninsky prosp., 119991 Moscow, Russian Federation.
Fax: +7 (095) 954 1279. Eꢀmail: lagema@igic.ras.ru
The binuclear copper(II) complexes with 2ꢀhydroxyacetophenone acyldihydrazones (H4L)
Cu2L•2Py were studied by ESR spectroscopy. In these complexes, the coordination polyhedra
are linked via the polymethylene chain containing from one to five units. In the complexes
based on acyldihydrazones of malonic, succinic, glutaric, and adipic acids, weak spinꢀspin
exchange interactions occur between the paramagnetic centers. An increase in the length of the
polymethylene chain to five units hinders exchange interactions, and the ESR spectrum of the
corresponding complex has a signal typical of monomeric copper(II) complexes.
Key words: ESR spectroscopy, hyperfine structure, spinꢀspin exchange interactions, biꢀ
nuclear copper(II) complexes, acyldihydrazones of 2ꢀhydroxyacetophenone and dicarboxyꢀ
lic acids.
catalyst were added to a suspension containing dihydrazide of
the corresponding dicarboxylic acid (from malonic to pimelic)
(5 mmol) in MeOH (50 mL). The reaction mixture was refluxed
with magnetical stirring until the dihydrazide was completely
dissolved (1—3 h). The resulting solution was cooled to ∼20 °C,
copper(II) acetate monohydrate (10 mmol) was added, and the
reaction mixture was stirred until crystals of copper acetate disꢀ
appeared (3—5 h). The precipitate that formed was filtered off,
washed with water and EtOH, and dried in air. The resulting
product was dissolved in pyridine (5—10 mL), kept for one
day, filtered off, and precipitated with water. The precipiꢀ
tate that formed was filtered off, washed with water and
EtOH, and dried in air. Grayꢀgreen or grayꢀsandꢀcolored finely
crystalline powders were obtained in yields of 1.6—2.5 g
(50—80%).
Three types of binuclear copper(II) complexes were
described in the literature, in which weak exchange interꢀ
actions between the paramagnetic centers occur through
the polymethylene chain.1—7 The majority of these comꢀ
plexes (molecular paramagnetics exhibiting weak antiferꢀ
romagnetism) were synthesized based on acyldihydrazones
of acetylacetone2—4 or salicylaldehyde.5—7 In the present
study, we synthesized binuclear copper(II) complexes with
2ꢀhydroxyacetophenone acyldihydrazones and investiꢀ
gated weak exchange interactions between the copper catꢀ
ions in these complexes by ESR spectroscopy.
According to the results of elemental and thermogravimetric
analyses, the compositions of the complexes correspond to the
general formula Cu2L•2Py.
[Cu2L1•2Py]•H2O (1). Found (%): Cu, 19.37; N, 13.11.
C29H28Cu2N6O5. Calculated (%): Cu, 19.03; N, 12.58.
IR, ν/cm–1: 1575 (ν(C=N)); 1515 (ν(N=C—O—)); 1350
(ν(COphen)).
H4L1 (n = 1), H4L2 (n = 2), H4L3 (n = 3),
H4L4 (n = 4), H4L5 (n = 5)
[Cu2L2•2Py]•MeOH•H2O (2). Found (%): Cu, 18.63;
N, 12.91. C31H34Cu2N6O6. Calculated (%): Cu, 17.80; N, 11.78.
IR, ν/cm–1: 1575 (ν(C=N)); 1510 (ν(N=C—O—)); 1360
(ν(COphen)).
Experimental
The coordination compounds were synthesized as follows.
2ꢀHydroxyacetophenone (10 mmol) and AcOH (0.5 mL) as the
[Cu2L3•2Py] (3). Found (%): Cu, 18.04; N, 12.88.
C
31H30Cu2N6O4. Calculated (%): Cu, 18.75; N, 12.40.
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 2107—2111, December, 2002.
1066ꢀ5285/02/5112ꢀ2268 $27.00 © 2002 Plenum Publishing Corporation

ESR study of copper(II) complexes
Russ.Chem.Bull., Int.Ed., Vol. 51, No. 12, December, 2002 2269
IR, ν/cm–1: 1570 (ν(C=N)); 1500 (ν(N=C—O—)); 1360
(ν(COphen)).
plexes contain the dimeric Cu2O2 fragments linked via
the polymethylene chain. Treatment of these complexes
with an excess of pyridine led to the cleavage of the pheꢀ
noxide bridges to form binuclear complexes. We demonꢀ
strated that the reactions of copper(II) acetate with
2ꢀhydroxyacetophenone acyldihydrazones proceeded acꢀ
cording to an analogous scheme to give binuclear comꢀ
plexes 1—5 in which the coordination polyhedra are sepaꢀ
rated by polymethylene chains of different lengths.
[Cu2L4•2Py] (4). Found (%): Cu, 18.16; N, 12.87.
C32H32Cu2N6O4. Calculated (%): Cu, 18.37; N, 12.15.
IR, ν/cm–1: 1585 (ν(C=N)); 1515 (ν(N=C—O—)); 1365
(ν(COphen)).
[Cu2L5•2Py] (5). Found (%): Cu, 18.38; N, 12.87.
C33H34Cu2N6O4. Calculated (%): Cu, 18.01; N, 11.91. IR,
ν/cm–1: 1580 (ν(C=N)); 1510 ((N=C—O—)); 1375 (ν(COphen)).
The copper percentage was calculated from the results of
trilonometric titration after thermal decomposition of a weighed
sample. The nitrogen percentage was determined by the Duma
micromethod. The thermogravimetric curves were obtained on
a Paulik—Paulik—Erdey Q derivatograph in a static air atmoꢀ
sphere; the rate of heating was 10 °C min–1; samples were placed
in a ceramic crucible without a lid; calcined aluminum oxide
was used as the standard.
The IR spectra of samples (KBr pellets) were measured in
the range of 4000—400 cm–1 on a Specord IRꢀ75 spectrophoꢀ
tometer. The ESR spectra were recorded on an ADANI PS 100Х
(9.45 Hz) radiospectrometer in a CHCl3—pyridine mixture
(10 : 1, v/v). Because of the poor solubility of the complexes
(the concentrations of the complexes were 5•10–4 mol L–1), the
spectra with a lower noise level could not be recorded. The
magnetic field was monitored on a nuclear magnetometer using
the diphenylpicrylhydrazyl (DPPH) radical as the standard. The
experimental spectra were processed on a computer by the
method of the best approximation of the experimental spectra
by the theoretical spectra with minimization of the error funcꢀ
tional
n = 1 (1), 2 (2), 3 (3), 4 (4), 5 (5)
Analysis of the IR spectra of compounds 1—5 conꢀ
firmed the assumption that the ligand is transformed into
the tetraprotonated imidol form because the spectra of
the complexes have no amide I band, which is observed in
the IR spectra of free 2ꢀhydroxyacetophenone acylꢀ
dihydrazones at ∼1600 cm–1, whereas the spectra
show two new absorption bands with maxima in the reꢀ
gions of 1585—1570 cm–1 (stretching vibrations of the
R = Σ (Iiexp – Iicalc )2/N,
i
where Iiobs are the experimental intensities of the ESR signal
processed on a computer as a file of points with a constant step
along the magnetic field H; Iit are the theoretical intensities at
the same H values calculated as the sum of derivatives of the
Lorentz and Gaussian functions centered at the resonance valꢀ
ues of H; and N is the number of points. The g factors, the
constants of hyperfine structure from the nuclei of the copper
isotopes, and the widths of the resonance lines were varied in the
course of minimization. The line width was specified by the
equation
Table 1. Results of thermogravimetric analysis of the copper(II)
complexes with 2ꢀhydroxyacetophenone acyldihydrazones
Comꢀ
pound
∆T a
T b
∆m c
(%)
Process
°С
2
1
2
40—90
90—260
260—570
40—130
170—240
300—520
130—200
300—640
110—190
280—420
100—190
300—730
—
200 (–)
340, 540 (+)
110 (–)
230 (–)
340, 410 (+)
150, 170 (–)
400 (+)
3
26
80
6
33
78
26
80
25
78
25
82
–Н2О
–2 Ру
Decomposition
–Н2О, MeОH
–2 Ру
Decomposition
–2 Ру
Decomposition
–2 Ру
∆H = α + βmI + γmI
,
where mI is the projection of the nuclear spin onto the direction
of the external magnetic field; and α, β, and γ are the parameters
of the line width. Since complexes containing polymethylene
bridges can be geometrically flexible, we also took into account
the contribution ∆Hp(1,2) = δ(m1 – m2)2 associated with the
influence of stereochemical nonrigidity on the isotropic exchange
through intramolecular motions about the polymethylene
bridge.3
3
4
5
170 (–)
330, 370 (+)
150, 170 (–)
390, 540 (+)
Decomposition
–2 Ру
Decomposition
Results and Discussion
a The temperature range according to the thermogravimetꢀ
ric data.
b The extremum in the DTA curve, the endothermic (–) and
exothermic (+) effects are indicated.
c The weight loss.
Previously,5 it has been demonstrated that the reacꢀ
tions of copper(II) acetate with salicylaldehyde acylꢀ
dihydrazones afforded polymeric complexes. These comꢀ

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Shul´gin et al.
>C=N—N=C< group) and 1515—1500 cm–1 (C—O
stretching vibrations of the —N=C—O— fragment).
It should be noted that the C—O stretching band of
the phenoxide group in the spectra of compounds 1—5 is
shifted (to 1375—1350 cm–1) as compared to that in the
IR spectra of the polymeric complexes (1320—1290 cm–1).
This is indirect evidence for the cleavage of the oxo
bridges9 and the formation of monomeric complexes in
which each copper cation is coordinated by two O atoms
of the deprotonated phenoxide and imidol groups and
also by the imine N atom. The fourth site in the coordinaꢀ
tion sphere is occupied by the N atom of the strongly
coordinated pyridine molecule. It is most probable that
the coordination polyhedron is a strongly distorted square.
In complexes 1 and 2, the water or MeOH molecule is,
apparently, involved in a weak additional coordination
bond to form a [4+1]ꢀtype polyhedron, viz., a tetragonal
pyramid.
The binuclear structures of the copper(II) complexes
with acyldihydrazones of oxalic acid and 2ꢀhydroxyacetoꢀ
phenone were confirmed10 by Xꢀray diffraction analysis.
According to the Xꢀray diffraction data, the copper(II)
complex prepared based on bis(salicylidene) hydrazone
of glutaric acid has an analogous structure.6,7
According to the results of thermogravimetric analysis
(Table 1), the elimination of the solvent molecules inꢀ
volved in complexes 1 and 2 occurred at 40—130 °C and
was not accompanied by noticeable thermal effects. The
pyridine molecule that is ivolved in a strong coordination
bond was eliminated at higher temperature (90—260 °C).
The process was accompanied by the endo effect
with a minimum in the DTA curve at 150—230 °C. At
260—300 °C, the acyldihydrazone molecule underwent
thermooxidative destruction grading into burningꢀout of
the organic residue. The latter process was characterized
by a strong exothermic effect.
a
b
1
1
2
2
2900
3100
3300
H/G
2900
3100
3300
H/G
c
d
1
1
2
2
2900
3100
3300
H/G
2900
3100
3300
H/G
e
1
2
2900
3100
3300
H/G
Fig. 1. Experimental (1) and calculated (2) ESR spectra of complexes 1 (a), 2 (b), 3 (c), 4 (d), and 5 (e) at 293 K.

ESR study of copper(II) complexes
Russ.Chem.Bull., Int.Ed., Vol. 51, No. 12, December, 2002 2271
Table 2. Parameters of the ESR spectra of the copper(II) complexes with 2ꢀhydroxyacetophenone
acyldihydrazones
Comꢀ
pound
g
aCu•104
/cm–1
Parameters of the line width/G
σ
(%)
g⊥
g||
А||
/G
α
β
γ
δ
1
2
3
4
5
2.110
2.112
2.117
2.112
2.110
38.2
38.1
39.8
40.3
72.1
29.5
32.2
32.0
32.6
50.6
3.0
9.5
7.6
8.9
10.0
–0.5
1.0
–1.1
–0.3
1.1
2.1
3.4
5.3
7.5
—
2.0
5.3
3.1
2.9
2.5
2.05
2.04
2.03
2.02
2.01
2.14
2.15
2.24
2.34
2.23
68
61
144
155
185
The ESR spectra of solutions of complexes 1—4
(Fig. 1, a—d) have an isotropic signals with a wellꢀreꢀ
solved sevenꢀline hyperfine structure with the coupling
constant of ∼40 G (Table 2), which is caused2—8 by the
exchange coupling between the unpaired electron and
two equivalent copper nuclei (I = 3/2). Previously,11,12 a
sevenꢀline hyperfine structure was observed for forbidꢀ
den transitions in the ESR spectra of polycrystalline
samples of some binuclear copper(II) complexes with short
metal—metal distances (∼3 Å) favorable for a strong antiꢀ
ferromagnetic exchange interaction (–2J is about several
hundreds of reciprocal centimeters). In the binuclear comꢀ
plexes under study, the direct overlapping of the magnetic
orbitals is impossible because of a large distance between
the paramagnetic centers (up to 10 Å), and the most
probable mechanism of spinꢀspin coupling involves the
indirect exchange through the chain of σ bonds of the
polymethylene bridge.
parallel orientation (Fig. 2, a). When the length of the
polymethylene chain is increased to three—four units, a
sevenꢀline hyperfine structure disappears and a poorly
a
2600
3000
3400
H/G
Theoretical analysis1 demonstrated that the sufficient
condition for a hyperfine structure from two copper nuꢀ
clei to occur is ACu << |–2J | or |–2J | >> 0.02 cm–1. In this
case, each unpaired electron interacts with two nuclear
spins and the ESR spectrum shows a sevenꢀline hyperfine
structure from two copper nuclei with oneꢀhalf the conꢀ
stant. A more rigorous treatment showed3 that in comꢀ
plexes characterized by a weak exchange interaction
through the polymethylene chain with n = 1—4, the exꢀ
b
change integral is 1—3 cm–1
.
2600
3000
3400
H/G
An increase in the length of the polymethylene chain,
which separates the coordination polyhedra, to five units
leads to a substantial decrease in –2J. As a result, the ESR
spectrum of complex 5 has an isotropic signal with a fourꢀ
line hyperfine structure from one nucleus having the "norꢀ
mal" value of the coupling constant, which is typical of
monomeric copper(II) complexes (Fig. 1, e).
c
An increase in the length of the polymethylene bridge
leads to interesting changes in the ESR spectra of soluꢀ
tions frozen at 77 K. Thus, the spectra of complexes 1
and 2 show an anisotropic signal typical of systems
with the axially symmetrical g factor (g⊥ = 2.04—2.05,
g|| = 2.14—2.15). In these spectra, a wellꢀresolved sevenꢀ
line hyperfine structure from two equivalent copper nuꢀ
clei with the constant of 61—68 G is observed in the
2600
3000
3400
H/G
Fig. 2. ESR spectra of solutions of complexes 1 (a), 4 (b), and
5 (c) at 77 K.

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Russ.Chem.Bull., Int.Ed., Vol. 51, No. 12, December, 2002
Shul´gin et al.
5. V. F. Shul´gin, E. A. Sarnit, and G. M. Larin, Koord. Khim.,
1998, 24, 222 [Russ. J. Coord. Chem., 1998, 24 (Engl.
Transl.)].
6. G. M. Larin, V. F. Shul´gin, and E. A. Sarnit, Mendeleev
Commun., 1999, 129.
7. G. M. Larin, V. F. Shul´gin, and E. A. Sarnit, Zh. Neorg.
Khim., 2000, 45, 1010 [Russ. J. Inorg. Chem., 2000, 4 (Engl.
Transl.)].
8. Yu. V. Rakitin, G. M. Larin, and V. V. Minin, Interpretatsiya
spektrov EPR koordinatsionnykh soedinenii [Interpretation of
EPR Spectra of Coordination Compounds], Nauka, Moscow,
1993, 399 pp. (in Russian).
resolved hyperfine structure from one copper nuclei with
the constant of 144—145 G is observed (Fig. 2, b). The
ESR spectrum of a frozen solution of the complex based
on acyldihydrazone of pimelic acid (n = 5) has a proꢀ
nounced hyperfine structure from one copper nucleus in
the parallel orientation (Fig. 2, c), and the constant of the
hyperfine structure increases to 185 G. This indicates that
weak exchange interactions between the copper cations
through the polymethylene chain are retained in frozen
solutions, but these interactions act at substantially smaller
distances.
9. V. A. Kogan, V. V. Zelentsov, G. M. Larin, and V. V. Lukov,
Kompleksy perekhodnykh metallov s gidrazonami. Fizikoꢀ
khimicheskie svoistva i stroenie [Transition Metal Complexes
with Hydrazones. Physicochemical Properties and Structure],
Nauka, Moscow, 1990, 112 pp. (in Russian).
10. M. T. Toshev, V. G. Yusupov, Z. T. Karimov, Kh. B. Dustov,
B. B. Umarov, N. A. Parpiev, and G. G. Aleksandrov, Koord.
Khim., 1990, 16, 1092 [Sov. J. Coord. Chem., 1990, 16 (Engl.
Transl.)].
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Received January 22, 2002;
in revised form May 23, 2002