Anomalous line broadening in the ESR spectra of copper(II) dithizonate

Article abstract of DOI:10.1023/A:1011344704024

The structure of the copper(II) complex with dithizone (H₂Dz) was studied by ESR spectroscopy. The lines of the ESR spectra of the Cu(HDz)₂ complex in solutions are so broad that the superhyperfine structure (SHFS) from the ligand at

Full text of DOI:10.1023/A:1011344704024

Russian Chemical Bulletin, International Edition, Vol. 50, No. 3, pp. 413—417, March, 2001
413
Anomalous line broadening in the ESR spectra of copper(II) dithizonate
G. M. Larin« and G. A. Zvereva
N. S. Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences,
31 Leninsky prosp., 117907 Moscow, Russian Federation.
Fax: +7 (095) 954 1279. E-mail: lagema@ionchran.rinet.ru
The structure of the copper(II) complex with dithizone (H2Dz) was studied by ESR
spectroscopy. The lines of the ESR spectra of the Cu(HDz)2 complex in solutions are so broad
that the superhyperfine structure (SHFS) from the ligand atoms is unobservable and even the
hyperfine structure (HFS) from the copper nuclei is poorly resolved. In polycrystalline
magnetically dilute powders with the ratio Ni(HDz)2 : Cu(HDz)2 = 60 : 1, the SHFS from the
N atoms is well resolved in both perpendicular and parallel orientations. The copper(II)
complex with dithizone has a square-planar structure in both liquid solutions and Ni(HDz)2
matrix. Reasons for the unusual linewidths in the ESR spectra of Cu(HDz)2 are discussed.
Key words: copper(II) complexes, dithizone, ESR spectroscopy, superhyperfine structure,
distortions, relaxation.
Structures of coordination compounds in the solid
phase are usually established by single crystal X-ray
diffraction analysis. However, the structures of coordi-
nation compounds in the solid phase and in solutions
are often different. Reliable information on their geo-
metric structure in solutions is of doubtless interest. This
is especially important for complexes with polyfunctional
and polydentate ligands, such as diphenylthiocarbazone*
(dithizone, H2Dz).
Reliable information on the structure of coordination
compounds can be obtained by the ESR study of mag-
netically dilute polycrystalline samples. Therefore, we
prepared monosubstituted copper(II), nickel(II), and
zinc(II) bis-dithizonates and magnetically dilute powders
of nickel and zinc dithizonates containing ∼2% copper(II)
dithizonate.
Experimental
Dithizone is widely used in analytical chemistry for
detecting metal traces.1 According to published data,1
dithizone can act as both singly (HDz) and doubly
protonated (Dz)2– bidentate ligand.
X-ray diffraction analysis2 of dithizone crystals shows
that the H atoms are localized at the utmost N atoms
bound to the phenyl rings.
Solid metal dithizonates were synthesized by the extraction
method. An aqueous solution of the metal salt with the corre-
sponding pH was shaken with a solution of dithizone in chloro-
form, and the organic phase was evaporated. The pH regions of
extraction of mono- and disubstituted dithizonates in the ex-
traction method are presented in the monograph.1 CuCl2•2H2O,
NiCl2•6H2O, ZnCl2•1.5H2O, and diphenylthiocarbazone (ana-
lytically pure grade) were used as starting compounds. The
composition of the synthesized compounds was established
from elemental analysis data.
Monosubstituted nickel dithizonate Ni(HDz)2. Ammonia
was added to a solution of NiCl2•6H2O (0.26 g) in water
(20 mL) to pH 8.5—9. The reaction mixture was shaken with a
solution of dithizone (0.5 g) in chloroform (250 mL) for
∼30 min until the color of the organic layer changed completely
from emerald-green to brown-violet. The aqueous layer was
separated, and chloroform was distilled off on a rotary evapora-
tor. The product was obtained in a 37% yield (0.23 g). Found
(%): C, 54.75; H, 3.90; N, 19.35; S, 11.02. NiC26H22N8S2.
Calculated (%): C, 54.83; H, 3.86; N, 19.68; S, 11.25.
–
S
C
H
H
N
N
N
N
Ph
Ph
According to the X-ray diffraction data,3—5 dithizone
forms bis-chelate complexes with NiII, CuII, and ZnII
with the general formula Ì(HDz)2, in which dithizone
manifests bidentate coordination through the S and N
atoms to form five-membered metallocycles.
Monosubstituted copper dithizonate Cu(HDz)2. A solution
of CuCl2•2H2O (0.18 g) in water (20 mL) was shaken with a
solution of dithizone (0.6 g) in chloroform (250 mL) at pH 2—4
for ∼1 h until the color of the organic layer changed completely
from emerald-green to red-violet. The aqueous layer was sepa-
rated, and chloroform was distilled off on a rotary evaporator.
The product was obtained in 83% yield (0.50 g). Found (%):
C, 55.30; H, 3.50; N, 19.01; S, 11.07. CuC26H22N8S2. Calcu-
lated (%): C, 54.35; H, 3.8; N, 19.51, S, 11.16.
This work is devoted to the ESR study of solutions of
the CuII complex with dithizone to determine its geo-
metric structure and reveale the influence of low-sym-
metry distortions in solution on the linewidths in the
ESR spectra.
* Diphenylthiocarbazone is shortly named dithizone (molecular
formula C13H12N4S; molecular weight 256.3).
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 395—399, March, 2001.
1066-5285/01/5003-413 $25.00 © 2001 Plenum Publishing Corporation

414
Russ.Chem.Bull., Int.Ed., Vol. 50, No. 3, March, 2001
Larin and Zvereva
Monosubstituted zinc dithizonate Zn(HDz)2. Ammonia was
added to a solution of ZnCl2•1.5H2O (0.1 g) in water (20 mL)
to pH 6.5—7.5. The reaction mixture was shaken with a solution
of dithizone (0.24 g) in chloroform (250 mL) for ∼15 min until
the color of the organic layer changed completely from emer-
ald-green to purple-red. The aqueous layer was separated, and
chloroform was distilled off on a rotary evaporator. The product
was obtained in 60% yield (0.21 g). Found (%): C, 55.10;
H, 3.40; N, 19.05; S, 11.01. ZnC26H22N8S2. Calculated (%):
C, 54.19; H, 3.89; N, 19.50, S, 11.14. The magnetically dilute
samples of Cu and Ni, Cu and Zn compounds were prepared as
follows. A mixture of Cu dithizonate (1 mg) and Ni dithizonate
(60 mg) was dissolved in chloroform (20 mL). Then both
complexes were precipitated in combination with a slow evapo-
ration of the solvent and maintaining of a constant temperature.
Magnetically dilute samples of the Cu and Zn compound were
obtained similarly.
a
2
1
3000
3100
3200
3300
H/G
ESR spectra were recorded on a Radiopan SE/X-2542
radiospectrometer with a working frequency of 9.45 GHz. The
magnetic field was calibrated by a nuclear magnetometer. The
diphenylpicrylhydrazine (DPPH) radical was used as a standard.
Experimental spectra were processed on a computer using the
procedure6 that provides the best correspondence between ex-
perimental and theoretical spectra by minimization of the error
functional R. Dioxane and chloroform were used as solvents.
The spectra were obtained at a concentration (in solutions) of
b
2
1
10 mol L–1. Dilution to 10 mol L showed that, within
these boundaries, the linewidth is independent of the concen-
tration of the copper(II) compound.
–3
–5
–1
The values of g factor, hyperfine structure (HFS) constants
from the copper nuclei, superhyperfine structure (SHFS) from
the ligand atoms, and resonance linewidths were varied during
minimization. The linewidth was specified by the expression
3000
3100
3200
3300
3400 H/G
Fig. 1. Experimental (1) and theoretical (2) ESR spectra of
solutions of Cu(HDz)2 in dioxane (a) and in chloroform (b).
∆H = α +βmI + γmI2,
Fig. 1, a) and is described by the following isotropic spin
where mI is the projection of a nuclear spin on the direction of
the external magnetic field, and α is the term taking into
account all broadening effects similar for all HFS lines. The β
coefficient is determined by the product of the g tensor and
hyperfine coupling (HFC) tensor. The γ coefficient corresponds
to HFC anisotropy and depends on the correlation time of the
rotation motion of a paramagnetic species in the liquid. The α,
β, and γ coefficients determine the contributions of the ESR
spectra of coordination compounds to the linewidths due to the
relaxation times of the spin of an unpaired electron and the spin
of the central metal nucleus and are measured in units of
magnetic field strength. Minimization was stopped at R for
which a good agreement of the theoretical and experimental
spectra was achieved and further iterations did not change R
(the amplitude of the spectrum was normed by 1). This usually
occurs at R < 0.003. The obtained degree of correspondence is
illustrated, in particular, in Fig. 1.
Hamiltonian (SH):
Í = <g>βSH + <aCu>SI,
where β is the Bohr magneton, S = 1/2 is the spin of an
electron, I = 3/2 is the nuclear spin of two copper 63Cu
and 65Cu isotopes (natural content 69.1 and 30.9%,
respectively), and Í is the external magnetic field
strength. Other SH parameters are presented in Table 1.
According to the X-ray diffraction data,4 the
Cu(HDz)2 molecule has a square-planar structure. The
Cu atom is linked to two S atoms and two N atoms with
trans-arrangement. However, the Cambridge Structural
Database does not cite this work because the X-ray
structural study4 was incorrect.
In the square-planar copper(II) complexes with the
d9 electronic configuration, an unpaired electron is on
the <dx –y2> orbital.9 The appearance of S atoms in the
2
Results and Discussion
coordination sphere of complex compounds of transition
elements usually results in a strong line narrowing in the
ESR spectra and favors a good resolution of SHFS from
the ligand atoms.10 For example, the lines in the ESR
spectra of the copper complexes with bis(α-thiopico-
line anilide) {Cu(α-tpa)2} and bis(8-thioquinoline)
{Cu(tox)2}, in which the coordination sphere consists of
The CuII complexes with dithizone have been stud-
ied7,8 by the ESR method. The ESR spectrum of the
Cu(HDz)2 complex at 293 Ê in dioxane consists of four
HFC lines from the interaction of the spin of an un-
paired electron with the spin of the Cu nucleus (see

ESR spectra of copper(II) dithizonate
Russ.Chem.Bull., Int.Ed., Vol. 50, No. 3, March, 2001
415
Table 1. Parameters of SH for the copper compounds in solutions
–1
CompoundSolvent
Cu(HDz)2
<
g>a
<aCu>•104/cm
<aN>/G
Ref.
Dioxane
Dioxane
Benzene
Benzene
Toluene
2.075
2.068
2.073
2.076
2.068
2.068
2.056
57
—
—
15.0
14.0
16.0
—
7,8
This work
11
b
55.5, 59.5
Cu(α-tpa)2
Cu(tox)2
Cu(tag)2
Cu(HDz)2
Cu(HDz,ddc)
58
61
12
15
b
74.9, 80.2
54.3, 58.2
74.2, 79.4
b
Chloroform
Chloroform
This work
This work
b
12.1
a
Some difference between the SH parameters is most likely related to different methods of their determination.
HFS constants for the 65Cu isotope.
b
two N atoms and two S atoms ([trans-N2S2]), are so
It is seen from comparison of the spectra of Cu(HDz)2
(see Fig. 1) that the HFS from the Cu atoms in a
chloroform solution is better resolved than in dioxane,
but the linewidth in the ESR spectra is still so large
(Table 2) that the SHFS from the N atoms is not
observed either in this case.
The isotropic SH parameters for several copper(II)
compounds in solutions (see Table 1) with the
same coordination atoms [trans-N2S2] coincide. It is
known11,12,15 that Cu(α-tpa)2, Cu(tox)2, and Cu(tag)2
(tag are thioacylhydrazones of monocarbonyl com-
pounds) have a square-planar structure in both the solid
phase and solutions. Since the SH parameters in solu-
tions for all compounds are the same, Cu(HDz)2 in a
solution also has a square-planar structure. However, the
ESR spectra of Cu(HDz)2 in solutions contain an anoma-
lously broad line (see Table 2).
narrow that the SHFS from two N atoms is resolved to
HFS several components from the Cu atoms.11,12 Based
on this, we could expect that the ESR spectrum of
Cu(HDz)2 should contain, along with four HFS lines
from the Cu nuclei, the SHFS from two N atoms, but it
is not observed. In the ESR spectrum of Cu(HDz)2,
even the HFS from the Cu nucleus is poorly resolved
(see Fig. 1, à), although it is known9 that the lines in the
ESR spectra are narrowed with an increase in the num-
ber of the S atoms in the coordination sphere.
This raises the question: why are the lines in the ESR
spectrum of Cu(HDz)2 with the [trans-N2S2] coordina-
tion sphere so broadened that the SHFS from two N
atoms is not resolved? Several reasons can be found for
this broadening of the lines in the spectra. One of them
is a low-symmetry distortion of the square-planar coor-
dination sphere (for example, the tetrahedral distortion
associated with the turn of two metallocycles about the
central metal atom, as well as the deviation of the
coordinated chelate cycles to the same or different
sides).13,14 Another possible reason is a change in the
shape and sizes of the copper(II) dithizonate molecule,
which affects the time of spin-lattice relaxation. These
possibilities cannot be examined without additional stud-
ies of the Cu(HDz)2 structure.
To decrease the linewidth in the ESR spectra, we
increased the number of S atoms in the coordination
sphere of CuII. With this purpose, we added copper
diethyl dithiocarbamate {Cu(ddc)2} to a solution of
Cu(HDz)2 in chloroform. The ESR spectrum shows
–
–
(Fig. 2) that ddc substitutes one HDz anion in the
internal coordination sphere to form a heteroligand CuII
complex. A new spectrum appears against the back-
ground of unreacted Cu(HDz)2, in which four HFS lines
The ESR study of Cu(HDz)2 was performed in solu-
tions and magnetically dilute polycrystalline powders at
room temperature. The ESR spectra of Cu(HDz)2 solu-
tions in dioxane and chloroform are presented in Fig. 1.
The SH parameters obtained by the computer simulation
of the experimental spectra are summarized in Table 1.
Table 2. Coefficients in the equation for determination of lines
in the ESR spectra of the copper(II) compounds
CompoundCi-oord
Solvent
nation
sphere
α
β
γ
G
Cu(HDz)2
[trans-N2S2]
Dioxane
Chloroform
Toluene
Chloroform
Toluene
45.2 13.7 3.4
35.0 7.4 1.8
12.6 6.4 1.1
9.7 5.0 1.7
7.5 1.7 0.46
3000
3100
3200
3300
3400 H/G
Cu(tag)2
Cu(HDz,ddc)
Cu(ddc)2
[trans-N2S2]
[NS3]
Fig. 2. ESR spectrum of the Cu(HDz,ddc) heteroligand com-
plex formed by the addition of Cu(ddc)2 to a solution of
Cu(HDz)2 in chloroform and Cu(HDz)2.
[S4]

416
Russ.Chem.Bull., Int.Ed., Vol. 50, No. 3, March, 2001
Larin and Zvereva
Table 3. Anisotropic SH parameters for the copper compounds
Com-
pound
g||
g⊥
A
B
<aN>|| <aN>⊥ Refs.
2
104 cm
G
–1
Cu(α-tpa)2 2.131
Cu(tox)2 2.147
Cu(HDz)2 2.145
2.037 154 12.0
2.036 172 25.0
2.033 176 26.0
15.0
—
17.2
15.0
15.2
16.5
11
12
*
1
* This work.
copper(II) compounds with biaxial anisotropy of the
parameters (Fig. 4) and described by the axial-symmetry
SH in the form
3000
3100
3200
3300
3400 H/G
Fig. 3. Experimental (1) and theoretical (2) ESR spectra of a
solution of the Cu(HDz,ddc) heteroligand complex in chlo-
roform.
H = [g||SzHz + g⊥(SxHx + SyHy)] + ASzIz + B(SxIx + SyIy),
where S = 1/2 is the spin of an electron, I = 3/2 is the
spin of the Cu nucleus, À is the HFS constant from the
Cu nucleus in parallel orientation, and  is the HFS
constant from the Cu nucleus in perpendicular orien-
tation.
A comparison of the anisotropic SH parameters for
Cu(HDz)2, Cu(α-tpa)2, and Cu(tox)2 (Table 3) shows
that they are close and can be attributed to the square-
planar copper(II) complexes with the [trans-N2S2] coor-
dination sphere.
from the Cu atom are additionally split into three com-
ponents. The ESR spectrum of the pure heteroligand
complex prepared by mixing of Cu(HDz)2 and Cu(ddc)2
in a ratio of 1 : 1 has the SHFS from one N atom
(Fig. 3). As we assumed, the appearance of the third S
atom in the coordination sphere of the CuII atom
narrows, indeed, the lines in the ESR spectrum (see
Table 2).
The formation of the Cu(HDz,ddc) heteroligand
complex at the 1 : 1 ratio of reactants suggests the
following: first, the starting copper (II) dithizonate has
the Cu(HDz)2 composition; second, when the coordina-
tion sphere of CuII contains only one dithizonate ligand,
the lines in the ESR spectra are not broadened, which is
indicated by the well-resolved SHFS from one N atoms
(see Fig. 3) and the linewidths in the ESR spectra of the
heteroligand complex (see Table 2).
In the ESR spectrum of the magnetically dilute
polycrystalline powder of Ni and Cu dithizonate, the
SHFS of five lines is well resolved in both perpendicular
and parallel orientations (see Fig. 4). The computer
simulation of the experimental spectrum showed that
five SHFS lines have an intensity ratio of 1 : 2 : 3 : 2 : 1.
Therefore, their appearance is due to the hyperfine
splitting from two equivalent N atoms.
The ESR study of the Zn and Cu dithizonate pow-
ders showed that copper dithizonate does not enter the
slightly distorted tetrahedral lattice of ZnII dithizonate.
Therefore, CuII dithizonate is such a structurally rigid
coordination compound14 that it retains its square-pla-
nar structure and does not undergo even insignificant
distortions, which is indicated by the absence of traces of
CuII dithizonate in the lattice of ZnII dithizonate.
Thus, the larger linewidth in the ESR spectra of the
Cu(HDz)2 complex in solutions is related to the low-
symmetry distortions of the coordination sphere14 be-
cause Cu(HDz)2 has a square-planar structure in the
solution and solid state.
The spectrum of the magnetically dilute polycrystal-
line powder of NiII and CuII dithizonate is typical of the
1
Consider the influence of the shape and sizes of the
Cu(HDz)2 molecule on the time of spin-lattice relax-
ation. The time of spin-lattice relaxation in coordination
copper(II) compounds is usually rather long, so that the
ESR spectra have narrow lines. However, the spectra of
the hexacoordinated copper(II) complexes with the
{[CuX6]n–} monodentate ligands in solutions exhibit16
broad lines with the poorly resolved HFS from the Cu
atoms. This is due to the dynamic Jahn—Teller effect.
When at least one Cu—ligand bond is strengthened, the
lines in the ESR spectra are sharply narrowed, which
2
2600
2800
3000
3200
3400
H/G
Fig. 4. Experimental (1) and theoretical (2) ESR spectra of the
magnetically dilute powder of a Ni(HDz)2 and Cu(HDz)2
(60 : 1) mixture.

ESR spectra of copper(II) dithizonate
Russ.Chem.Bull., Int.Ed., Vol. 50, No. 3, March, 2001
417
along with the HFS at the most narrow upfield line from
the 63Cu, 65Cu isotopes (Fig. 5).
The coordination compounds of copper with tag
ligands are close in structure to copper dithizonate in
which some dithizone molecules are replaced by the R3
group. It is found15 that, regardless of the nature, shape,
and sizes of the R3 substituents, the linewidths in the
ESR spectra of the copper complexes with tag remain
almost unchanged.
1
2
Based on the aforesaid, we may assert that the time
of spin-lattice relaxation does not define the linewidths
in the ESR spectra of the copper complexes with
dithizone, and thus, the question about reasons for the
line broadening in the spectra of Cu(HDz)2 in solutions
remains open.
3000
3100
3200
3300
H/G
Fig. 5. Experimental (1) and theoretical (2) ESR spectra of a
solution of Cu(tag)2 in toluene.
References
occurs during the coordination of only one bidentate
ligand by the Cu atom.
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Micro- und Spurenanalyse, Verlag Chemie, GMBH,
Weinheim, 1958.
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Khim., 1974, 704 [Bull. Acad. Sci. USSR, Div. Chem. Sci.,
1974, 23 (Engl. Transl.)].
At the same time, the lines in the ESR spectra of
viscous solutions or standard solutions for very high
molecular weight of the polymacrocyclic ligand also
have a large width and poorly resolved HFS from the
CuII nuclei due to the influence of unresolved anisot-
ropy of the HS parameters.16 In all other cases, the lines
in the ESR spectra of the copper(II) coordination com-
pounds, especially with the coordination sphere contain-
ing S atoms, are narrow, due to which the HFS from the
CuII atoms and SHFS from the ligand atoms are well
resolved.
Evidently, both factors of line broadening in the ESR
spectra of solutions are absent in the case of the CuII
complexes with dithizone.
We have previously15 studied the following copper(II)
coordination compounds with tag ligands:
Cu/2
R2
S
N
C
R1
C
N
R3
13. G. M. Larin, V. A. Kolosov, and Yu. N. Dubrov, Koord.
Khim., 1978, 4, 36 [J. Coord. Chem. USSR, 1978, 4 (Engl.
Transl.)].
14. G. M. Larin, Koord. Khim., 1994, 20, 883 [Russ. J. Coord.
Chem., 1994, 20 (Engl. Transl.)].
15. G. M. Larin, V. G. Yusupov, B. B. Umarov, V. V. Minin,
K. N. Zelenin, A. M. Abdullaev, L. A. Khorseeva, and
N. A. Parpiev, Zh. Obshch. Khim., 1993, 63, 183 [Russ. J.
Gen. Chem., 1993, 63 (Engl. Transl.)].
16. S. A. Al´tshuler and B. M. Kozyrev, Elektronnyi para-
magnitnyi rezonans soedinenii elementov promezhutochnykh
grupp [Electron Paramagnetic Resonance of Compounds of
Elements of Intermediate Groups], Nauka, Moscow, 1972,
672 pp. (in Russian).
The R1, R2, and R3 substituents were represented by
various groups from H to 4-C6H4OMe, and the molecu-
lar weight of the CuII coordination compounds consisted
of 500—700 units; the linewidth of the ESR spectra
remained almost unchanged. The molecular weight of
Cu(tag)2 with R1 = H, R2 = 4-C6H4OMe, and
R3 = CH2Ph is 597.8, whereas for Cu(HDz)2 it is 573.8.
Thus, the shape, size, and molecular weight of both
compounds are almost the same, but the linewidth in the
ESR spectra differ by nearly an order of magnitude (see
Table 2). The lines in the ESR spectra of the Cu(tag)2
compound with the [trans-N2S2] coordination sphere
are so narrow that the HFS from the Cu atoms and the
SHFS from two equivalent N atoms are well resolved
Received July 20, 2000;
in revised form October 30, 2000