X-ray diffraction, magnetochemical, and quantum chemical study of the structure and properties of binuclear copper(II) complexes

Article abstract of DOI:10.1134/S1070363212110047

The binuclear chelates of copper(II) based on the tridentate azomethine ligands were synthesized and characterized by elemental analysis, IR, ¹H NMR spectroscopy, X-ray absorption spectroscopy (XANES and EXAFS), and magnetic measurements. The quantum-chemical study of the structure and calculation of magnetic properties of the obtained metal-chelates was performed using the density functional theory with the broken symmetry technique. The performed magnetochemical studies in the temperature range 2-300 K suggest the existence of antiferromagnetic exchange interaction in all obtained complexes. The parameters of the exchange interaction -2J > 260 cm^-1 were determined experimentally for all compounds, the experimental data is in a good agreement with the results of quantum-chemical calculations.

Full text of DOI:10.1134/S1070363212110047

ISSN 1070-3632, Russian Journal of General Chemistry, 2012, Vol. 82, No. 11, pp. 1770–1776. © Pleiades Publishing, Ltd., 2012.
Original Russian Text © S.A. Nikolaevskii, A.S. Burlov, A.S. Bogomyakov, V.G. Vlasenko, D.A. Garnovskiy, A.G. Starikov, Ya.V. Zubavichus,
I.S. Vasilchenko, S.I. Levchenkov, A.I. Uraev, 2012, published in Zhurnal Obshchei Khimii, 2012, Vol. 82, No. 11, pp. 1778–1784.
X-Ray Diffraction, Magnetochemical,
and Quantum Chemical Study of the Structure
and Properties of Binuclear Copper(II) Complexes
S. A. Nikolaevskiia, A. S. Burlova, A. S. Bogomyakovb, V. G. Vlasenkoc, D. A. Garnovskiyd,
A. G. Starikovd, Ya. V. Zubavichuse, I. S. Vasilchenkoa, S. I. Levchenkovd, and A. I. Uraeva
a Research Institute of Physical and Organic Chemistry, Southern Federal University,
ul. Stachki 194/2, Rostov-on-Don, 344090 Russia
e-mail: sanikoil@inbox.ru
b Institute “International Tomography Center,” Siberian Branch, Russian Academy of Sciences, Novosibirsk, Russia
c Research Institute of Physics, Southern Federal University, Rostov-on-Don, Russia
d Southern Scientific Center, Russian Academy of Sciences, Rostov-on-Don, Russia
e National Research Centre “Kurchatov Institute,” Moscow, Russia
Received September 13, 2011
Abstract—The binuclear chelates of copper(II) based on the tridentate azomethine ligands were synthesized
1
and characterized by elemental analysis, IR, H NMR spectroscopy, X-ray absorption spectroscopy (XANES
and EXAFS), and magnetic measurements . The quantum-chemical study of the structure and calculation of
magnetic properties of the obtained metal-chelates was performed using the density functional theory with the
broken symmetry technique. The performed magnetochemical studies in the temperature range 2–300 K
suggest the existence of antiferromagnetic exchange interaction in all obtained complexes. The parameters of
the exchange interaction –2J > 260 cm–1 were determined experimentally for all compounds, the experimental
data is in a good agreement with the results of quantum-chemical calculations.
DOI: 10.1134/S1070363212110047
Systematic studies on the synthesis, structure, and
magnetic properties of bi- and polynuclear transition
metal coordination compounds with azomethine ligands
have contributed significantly to a rapidly developing
area of modern chemistry, the molecular design of mag-
netic materials [1–4]. We have previously described
the binuclear copper(II) complexes with tridentate
azomethine ligands exhibiting both ferromagnetic and
antiferromagnetic exchange interactions [5–11]. In the
extension of these studies, we synthesized and
characterized ligand systems I and binuclear chelate
complexes II with oxygen bridges.
R2
R1
OH
N
R1
O
O
O
N
CH
Cu
Cu
HC
OH
HC
N
O
R1
R2
R2
I
II
R1 = Br, R2 = H (a); R1 = Br, R2 = NO2 (b); R1 = –N=NC6H4-p-Ме, R2 = H (c).
1770

X-RAY DIFFRACTION, MAGNETOCHEMICAL, AND QUANTUM CHEMICAL STUDY
1771
Е, eV
Е, eV
Е, eV
Fig. 1. XANES CuK-edge X-ray absorption spectra of metal complexes of copper. (solid line) IIa, (points) IIb, (dashed line) IIc.
The inserts show the pre-edge area (A) and the first derivatives of the CuK-absorption edges.
Elemental analysis of the complexes corresponds to
the ratio M:L = 1:1, which is consistent with the
expected structure of the binuclear compounds II. In
the IR spectra of complexes II the stretching OH
vibrations in the region 3450 cm–1 disappear and the
frequency of the C=N stretching vibrations decreases
(near 1600 cm–1) by 10–15 cm–1compared with the
spectra of ligands I.
possibility of p–d mixing of the metal atomic orbitals.
In addition, all the CuK edges have a pronounced
shoulder B, which is well seen in the first derivatives
of the edges. The first derivatives of the CuK edges of
the complexes have two distinct peaks split by 7.5 eV.
These features are usually typical of the XANES for
the tetrahedral environment of the absorbing atom, in
this case copper, in the metal complex IIa, IIb, and
IIc.
The dimeric structure and type of the ligand
environment is revealed by the analysis of the CuK
edges of X-ray absorption of complexes II.
Quantitative characteristics of the atomic local
structures of the copper metal complexes IIa, IIb, and
IIc were obtained by the analysis of EXAFS (Extended
X-ray absorption fine structure) of the CuK edges in
the X-ray absorption spectra of these compounds.
Figure 2 shows the Fourier transforms modules (FTM)
of the EXAFS in the CuK-absorption edges of copper
complexes IIa, IIb, and IIc. As can be seen from
Figure 2, the EXAFS MTS consist of the main peak at
low r = 1.54 Å, which uniquely corresponds to the
scattering at the nearest light coordination spheres
consisting of the O and N atoms, and the peak at r =
2.46–2.50 Å, which can be candidate for the
Figure 1 shows the normalized XANES (X-ray
absorption near edge structure) at the CuK edges in the
X-ray absorption spectra of compounds IIa, IIb, and
IIc. The XANES of the metal complexes IIa, IIb, and
IIc are similar. The pre-edge peaks in the spectra of
these compounds have the same energy position in the
region of 8968 eV and similar amplitude. The
appearance of the pre-edge peaks is associated with the
electron transitions 1s→3d and is characteristic only of
the compounds with low-symmetry coordination site
(the absence of an inversion center), with the
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1772
NIKOLAEVSKII et al.
complexes IIa, IIb, and IIc corresponds to the
scattering of the photoelectron wave at the adjacent
copper atoms, that is, points to a dimeric structure of
the IIa, IIb, and IIc molecules.
Table 1 lists the results of calculation of the
parameters of local atomic environment in the
complexes at the choice of appropriate models of
atomic structure of the coordination site. Analysis of
these data shows that the first coordination shell of the
copper atom in compounds IIa, IIb, and IIc with
oxygen bridges consists of four nitrogen and oxygen
atoms with the same distance Cu···O/N, r = 1.92 Å.
Since the peak corresponding to the appearance of
Cu···Cu distances in these compounds have different
amplitudes, the parameters of this coordination sphere
were found using two models. In one case the same
coordination numbers CN = 1 were selected and values
of the Debye–Waller factor σ2 were varied, in the other
the σ2 value was fixed and the CN value was found.
The choice of these models is due to the strong
correlation of these parameters. The best model was
selected on the basis of the lowest values of the Q
factor. As can be seen from Table 1, in the case of
complexes IIb and IIc, the lowest Q values are
obtained for the model of dimeric molecular structure,
but with a lower coordination number (~0.7). Such a
lowering of the coordination numbers can be
associated with an admixture of monomeric copper
complexes. For the complex IIa we obtained the
single-phase state with dimeric structure of molecules,
the Cu···Cu distance was 2.98 Å.
IIc
IIb
IIа
r, Å
Fig. 2. Modules of the Fourier transformed EXAFS CuK-
edges of the X-ray absorption spectra of the metallo-
complexes IIa–IIc. (Solid line) experiment; (circles)
calculation.
manifestation of the Cu···Cu distances. Varying the
integration limits of the Fourier transformation of
EXAFS clearly showed that this peak in the case of
Our magnetochemical studies showed that in all the
described complexes the exchange interaction of
antiferromagnetic type was realized. The temperature
dependence of magnetic susceptibility of the copper(II)
complexes were satisfactorily interpreted in the
framework of the model of exchange-linked dimer (for
the Hamiltonian H = –2JS1S2) [12]. The 2J parameter
of the antiferromagnetic exchange interaction was
calculated using the modified Bleaney–Bowers
equation [13], taking into account the presence in the
complex of some fraction of paramagnetic impurities:
Table 1. Structural data on the local atomic environment of
the copper metallocomplexes obtained by fitting the EXAFS
dataa
Comp.
no.
N
r
σ2
Atom
Q
4
1
1.92
2.98
0.0042
0.0052
O/N
Cu
0.5
IIа
IIbb
IIcb
4
1
1.92
2.99
0.0054
0.0076
O/N
Cu
1.1
0.9
1.3
0.8
4
0.7
1.92
2.98
0.0054
0.0052
O/N
Cu
–1
NAg2β2
–2J
4
1
1.92
2.93
0.0050
0.0068
O/N
Cu
χ' = –——– (1 – p) 1 + 1/3exp —— + pS(S + 1) + Nα.
m
3kT
kT
4
0.7
1.92
2.92
0.0050
0.0052
O/N
Cu
Here χ'M is the molar magnetic susceptibility corrected
for the atomic diamagnetism within the Pascal additive
scheme, p is the mole fraction of paramagnetic
a
r is interatomic distance, N is coordination number, σ2 is Debye-
Waller factor, Q is quality fit function. b Fit in the range of k 1.1–
2.87 Å–1.
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X-RAY DIFFRACTION, MAGNETOCHEMICAL, AND QUANTUM CHEMICAL STUDY
1773
Т, K
Т, K
Fig. 3. The temperature dependence of the effective
magnetic moment of complex IIa.
Fig. 4. The temperature dependence of the effective
magnetic moment of complex IIb.
impurity, Nα is the temperature-independent para-
magnetism that for the Cu(II) ion is taken equal to
60×10–6.
compounds IIa, IIb, and IIc, we performed the
quantum-chemical study of the structures II and III
simulating their coordination sites.
The calculations showed that structure II has
almost planar metal-chelate rings. The geometry of the
isomeric structure III is characterized by a rotation of
two ligands relative to each other (at an angle of 19°)
due to their repulsive interactions. It follows from Fig. 6
that the Cu···Cu distances in the structures II and III
are similar: 3.06 and 3.04 Å, respectively. The
calculated values of the 2J constants for these
structures (–250 and –342 cm–1 for II and III
respectively) are also consistent with the experimental
data of the magnetochemical investigations of the
complexes IIa–IIc. Thus, the characteristics of
structures II and III calculated by DFT method do not
allow us to make a choice in favor of one or another
stereochemistry of the coordination site. However,
destabilization of structure III with respect to II by 4.5
kcal mol–1 indicates a disadvantage of this structure for
the complexes IIa, IIb, and IIc.
The experimental curves of μeff.(T) of the com-
plexes II are shown in Figs. 3–5, solid lines cor-
respond to theoretical curves. Table 2 lists the op-
timum parameters.
The values of the parameters of antiferromagnetic
exchange interaction 2J in the complexes IIa–IIc are –
322 cm–1 (g = 2.00, p = 0.005), –405 cm–1 (g = 2.00,
p = 0.005) and –263 cm–1 (g = 2.10, p = 0.018),
respectively.
The exchange interaction in complex IIc is
somewhat weaker than in IIa and IIb, which may be
caused by distortion of the exchange fragment due to
the presence of bulky substituents R1. This assumption
is consistent with the structural parameters of the
complexes obtained from the data of EXAFS (Table 1).
A decrease in the Cu···Cu distance in complex IIc
compared with IIa and IIb is connected, apparently,
with a bend of the binuclear molecules along the line
connecting the bridging atoms, which reduces the Cu–
Ophen–Cu bond angle. The value of the latter affects
considerably the strength of antiferromagnetic
exchange in binuclear copper complexes with
phenoxide bridges [4, 14–17].
Table 2. The magnetic properties of complexes II
Comp.
Т, K
μeff, BM
2J, cm–1
g
p
no.
300
2
1.556
0.153
–322
2.00
0.005
IIа
The binuclear coordination compounds based on
the ligands of type I are known [4] to be capable to
exist as two isomers differing by the inclusion into the
exchange fragment of the heteroatoms of either five-
membered or six-membered metallocycles (structures
II and III, respectively). To clarify the structure of
300
2
1.554
0.254
–405
–263
2.00
2.10
0.015
0.018
IIb
IIc
300
2
1.722
0.246
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 82 No. 11 2012

1774
NIKOLAEVSKII et al.
The spectra processing was performed using the
standard procedures of the background abstraction,
normalization according to the value of the K-edge
jump, and isolation of the atomic absorption μ0 [18],
followed by Fourier transformation of the obtained
EXAFS (χ)-spectra in the range of photoelectron wave
vector k from 2.6 to 13 Å–1 with the weight function k3.
The threshold ionization energy E0 was chosen
corresponding to the maximum of the first derivative
of the K-edge and further was varied for fitting.
The exact values of the structural parameters of the
nearest environment of copper ion were determined by
nonlinear fitting the parameters of the coordination
spheres comparing the calculated EXAFS signal and
the signal extracted from the full EXAFS spectrum by
Fourier filtering of the Fourier-transformed modules.
The non-linear fitting was performed using the
IFFEFIT-1.2.5 software package [19]. The phases and
amplitudes of the photoelectron wave scattering for the
model spectrum were calculated using the FEFF7
program [20]. The initial atomic coordinates to cal-
culate the phase and amplitude of the scattering and
further adjustment were taken from the X-ray data
found in the Cambridge structural database for the
complex with similar local environment of copper
ions.
Т, K
Fig. 5. The temperature dependence of the effective
magnetic moment of complex IIc.
EXPERIMENTAL
IR spectra were obtained on a Varian 3100-FTIR
Excalibur instrument from the powders by the method
of attenuated total internal reflection. The H NMR
spectra were recorded on a Varian Unity-300 device
(300 MHz) in the mode of internal stabilization of the
polar-resonance 2D line in CDCl3.
1
The X-ray CuK-edge absorptions were obtained in
the transmission mode on an EXAFS-spectrometer of
the Structural Material Research station at the
Kurchatov synchrotron center (Moscow). The energy
of the electron beam used as a source of X-ray
synchrotron radiation was 2.5 GeV at a current of 80–
100 mA. For monochromation of the X-ray radiation a
double-crystal Si(111) monochromator was used.
The fit quality function Q minimized for finding the
parameters of the nearest environment structure was
calculated by formula (1).
Fig. 6. The results of quantum-chemical calculations of structure of the intra-chelate complexes II and III.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 82 No. 11 2012

X-RAY DIFFRACTION, MAGNETOCHEMICAL, AND QUANTUM CHEMICAL STUDY
1775
acetate monohydrate in 20 ml of ethanol. The resulting
mixture was refluxed for 4 h. The precipitated fine
crystalline complexes were filtered off, washed with
boiling ethanol (3×5 ml) and hexane (2×5 ml), and
dried in a vacuum drying cabinet at 100°C.
Σ[kχexp(k) – kχth(k)]2
Q(%) = ————————×100%.
Σ[kχexp(k)]2
(1)
The magnetic properties of polycrystalline samples
were studied on Quantum Design SQUID
a
Bis{4-bromo-2-[2-(hydroxyphenylimino)methyl]-
phenoxido}dicopper(II) (IIa). Yield 65%. Green
powder, mp > 250°C. IR spectrum, ν, cm–1: 1613 v.s
(C=N), 1337 s (Ph–O). Found, %: C 44.19, H 2.35, N
3.97. C26H16Br2Cu2N2O4. Calculated, %: C 44.15, H
2.28, N 3.96.
magnetometer MPMS-XL in the temperature range 2–
300 K, in the magnetic field of 5 kOe. At calculating
the paramagnetic component of the molar magnetic
susceptibility (χ) corrections were introduced in
accordance with the diamagnetic Pascal additive
scheme. In the paramagnetic region, the effective
magnetic moment was determined from Eq. (2).
Bis{4-bromo-2-[5-nitro-2-(hydroxyphenylimino)-
methyl]-phenoxydo}dicopper(II) (IIb). Yield 67%.
Brown powder, mp > 250°C. IR spectrum (cm–1): 1611
v.s (C=N), 1337 s ( Ph–O). Found, %: C 40.01, H 1.58,
N 7.13. C26H14Br2Cu2N4O8. Calculated, %: C 39.17, H
1.77, N 7.03.
1/2
3k
μeff = ——– χТ ≈ (8χТ)1/2,
(2)
NAβ2
where k is Boltzmann constant, NA is Avogadro
number, β is the Bohr magneton.
Bis{[2-(hydroxyphenylimino)methyl]4-(p-
tolyldiazenyl)phenoxido}dicopper(II) (IIc). Yield
57%. Black powder, mp > 250°C. IR spectrum, (ν, cm–1):
1617 v.s (C=N), 1332 m (Ph–O). Found, %: C 61.08,
H 3.99, N 10.57. C40H30Cu2N6O4. Calculated, %:
C61.14, H 3.85, N 10.69.
All quantum-chemical calculations were performed
within the density functional theory DFT B3LYP/6-
311++G(d,p) [21] with the Gaussian 03 program [22].
To find the stationary points on potential energy
surface a full geometry optimization of molecular
structures was carried out. Calculation of exchange
coupling constant J was performed using the technique
ACKNOWLEDGMENTS
This work was supported by the program of Pre-
sidium of Russian Academy of Sciences “Molecular
Design of magnetically active substances and
materials,” Russian Foundation for Basic Research
(grants nos. 10-03-00434 and 11-03-00475), the
President of the Russian Federation (NSh-
3233.2010.3), and Ministry of Education and Science
(AVTsP RNP VSh-2.2.1.1/12630).
according to the formula proposed in [23]: ET – EBS
=
–2J, where ET is the triplet state energy, EBS is the
energy of the symmetry breakdown state.
Azomethines Ia, Ib were obtained by the known
method [24].
2-{[(2-Hydroxyphenylimino)methyl]-4-(p-tolyl-
diazenyl)}phenol (Ic). A suspension of 0.721 g (3 mmol)
of 2-hydroxy-5-[(4-methylphenyl)diazenyl]benzalde-
hyde and 0.327 g (3 mmol) of 2-aminophenol in 15 ml
of toluene was refluxed for 5 h. The red precipitate
was filtered off, washed with hot isopropyl alcohol
(3×5 ml), then with hexane (2×5 ml). Yield 98%, mp
239–240°C. Found, %: C 72.35; H 5.22; N 12.60.
C20H17N3O2. Calculated, %: C 72.49; H 5.17; N 12.68.
1H NMR spectrum (CDCl3), δ, ppm: 2.42 s (3H, CH3),
5.75 br.s (1H, OH-phenol), 6.98–8.10 m (11H, Ar),
8.83 s (1H, CH=N), 12.87 br.s (1H OH of salycilic
aldehyde). IR spectrum, (ν, cm–1): 3415 s (OH), 1608
v.s (C=N), 1288 w (Ph–O).
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