Specificity of complex formation of benzoin (phthalazin-1-yl)hydrazone with copper(II) and nickel(II): Physicochemical study and quantum-chemical simulation

Article abstract of DOI:10.1134/S1070363209040240

Benzoin (phthalazin-1-yl)hydrazone and its complexes with copper(I), copper(II), and nickel(II) were synthesized. Acid-base properties of benzoin (phthalazin-1-yl)hydrazone were studied, and its ionization constants and energies of possible conformers wer

Full text of DOI:10.1134/S1070363209040240

ISSN 1070-3632, Russian Journal of General Chemistry, 2009, Vol. 79, No. 4, pp. 826–832. © Pleiades Publishing, Ltd., 2009.
Original Russian Text © I.N. Shcherbakov, L.D. Popov, S.I. Levchenkov, A.N. Morozov, V.A. Kogan, A.D. Vikrishchuk, 2009, published in Zhurnal Obshchei
Khimii, 2009, Vol. 79, No. 4, pp. 663–669.
Specificity of Complex Formation
of Benzoin (Phthalazin-1-yl)hydrazone with Copper(II)
and Nickel(II): Physicochemical Study
and Quantum-Chemical Simulation
I. N. Shcherbakova, L. D. Popova, S. I. Levchenkovb, A. N. Morozova,
V. A. Kogana, and A. D. Vikrishchukb
a Southern Federal University, ul. Zorge 7, Rostov-on-Don, 344090 Russia
e-mail: physchem@yandex.ru
b Southern Research Center, Russian Academy of Sciences, Rostov-on-Don, Russia
e-mail: s.levchenkov@gmail.com
Received May 30, 2008
Abstract—Benzoin (phthalazin-1-yl)hydrazone and its complexes with copper(I), copper(II), and nickel(II)
were synthesized. Acid–base properties of benzoin (phthalazin-1-yl)hydrazone were studied, and its ionization
constants and energies of possible conformers were calculated by quantum-chemical methods. The structure of
the isolated complexes was determined on the basis of their elemental compositions, IR spectra, and data of
thermogravimetric, conductometric, and magnetochemical measurements. Copper(II) acetate with benzoin
(phthalazin-1-yl)hydrazone forms dinuclear complex, nickel(II) acetate and chloride produce mononuclear
octahedral complexes, and copper(II) halides give rise to copper(I) complexes with the oxidized hydrazone.
DOI: 10.1134/S1070363209040240
Benzoin hydrazones belong to a fairly poorly
explored group of ligand systems. The hydroxy group
in benzoin and its condensation products may be
regarded as alcoholic rather than phenolic (as in
salicylaldehyde hydrazones); therefore, it is con-
siderably less acidic. However, unlike α-diketone
hydrazones, semicarbazones and acylhydrazones derived
from benzoin are capable of coordinating metal ions as
both singly and doubly deprotonated species [1–4].
Benzoin hetarylhydrazone complexes have not been
reported; on the other hand, the available published
data indicate that the presence of a heteroaromatic
fragment in a ligand molecule could strongly affect
both structure and magnetic properties of complexes
based thereon [5–8]. As subjectes for study we selected
benzoin (phthalazin-1-yl)hydrazone (I) and its copper(II)
and nickel(II) complexes.
and their complexes [11, 12]. The complexes were
synthesized according to the scheme shown below.
Cu(CH3COO)2
II
Ph
Ph
OH
CH
Ni(CH3COO)2
NiCl2
H
III
IV
Va
Vb
N
N N
N
CuCl2
CuBr2
I
(H2L)
Hydrazone I was identified by the data of elemental
1
analysis (Table 1) and IR and H NMR spectroscopy.
The following absorption bands were observed in the
IR spectrum of hydrazone I: slightly broadened band at
3269 cm–1 was assigned to stretching vibrations of the
O–H group, N–H bond gave a band at 3325 cm–1,
stretching vibrations of the azomethine C=N bond
appeared at 1677 cm–1, two bands at 1591 and 1570 cm–1
As shown in [9, 10], hydrazones derived from
phthalazin-1-ylhydrazine exist as the corresponding
1,2-dihydrophthalazin-1-ylidene tautomers; this was
also confirmed by X-ray analysis of some hydrazones
826

SPECIFICITY OF COMPLEX FORMATION OF BENZOIN (PHTHALAZIN-1-YL)HYDRAZONE
827
Table 1. Colors, melting points, elemental analyses, and molar conductivities of compounds I–V
Found/calculated, %
Comp.
λ,
Color
mp, °C
Formula
no.
Ω–1 cm3 mol–1
C
H
N
M
Yellow
218
>250
>250
>250
>250
>250
73.7/74.6
62.5/63.5
57.3/56.8
66.9/66.6
50.2/49.9
43.9/44.4
4.99/5.12
4.09/3.88
4.50/4.77
5.35/5.10
3.02/2.79
2.70/2.48
16.1/15.8
13.8/13.5
12.8/12.0
13.1/13.5
15.1/15.5
14.2/13.8
–
C22H18N4O
C22H16CuN4O
C22H22NiN4O4
C46H42NiN8O4
C15H10ClCuN4O
C15H10BrCuN4O
–
I
Brown
14.9/15.3
13.7/12.6
7.30/7.08
16.9/17.6
15.4/15.7
3.0
8.6
4.6
6.8
7.5
II
Brown–green
Yellow
III
IV
Va
Vb
Yellow
Yellow
corresponded to C=N bonds in the heterocyclic frag-
ment, and stretching vibrations of the C–O bond had a
frequency of 1254 cm–1 [13–16].
structures in which the OH group is deprotonated much
more readily than the NH group [1, 3, 4]. The same
follows from the ionization constants of hydrazone I,
calculated by quantum-chemical method according to
the procedure described in [17]: the pKa1 and pKa2
values were estimated at 10.89 and 20.67, respectively,
and the first ionization step corresponds to proton
abstraction from the NH group (Table 2). According to
the calculations, the most favorable conformer of
neutral hydrazone I has structure A stabilized by two
hydrogen bonds (Fig. 1).
The 1H NMR spectrum of hydrazone I in DMSO-d6
contained the following signals, δ, ppm: 7.24–7.69
(10H, Ph), 6.58 d (1H, OH, J = 6.3 Hz), 6.9 d (1H, CH,
J = 6.3 Hz), 11.78 s (1H, NH), 8.27 d (1H, 8-H, J =
7.8 Hz), 7.89 m (3H, 5-H, 6-H, 7-H), 8.00 br.s (1H, 4-
H). The chemical shift of the hydroxy proton suggests
a weak acidity of the hydroxy group; nevertheless,
both NH and OH signals disappeared upon addition of
D2O. The downfield position of the NH signal
confirms the assumed structure of hydrazone I as 1,2-
dihydrophthalazin-1-ylidene tautomer.
Coordination compounds based on hydrazone I
were synthesized according to the scheme given above.
Their structure was determined on the basis of their
elemental compositions (Table 1) and conductometric
(Table 1), IR spectral, and magnetochemical data
(Table 3).
The electronic absorption spectra of hydrazone I
displayed a distinct dependence of the spectral pattern
on the solvent nature and pH value. A 20-nm blue shift
of the long-wave absorption maximum (λ 350 nm) was
observed in aqueous ethanol at pH < 5; this means that
the hydrazone molecule is protonated [1–3]; no
deprotonation occurred in the pH range from 5 to 12.
Complex II obtained from hydrazone I and copper(II)
acetate had a composition of 1:1 (CuL, Table 1). Its
thermogravimetric analysis revealed no endothermic
effects in the temperature range from 120 to 180°C;
i.e., no inner-sphere solvent molecules were present in
complex II [13, 18]. Thermooxidative decomposition
of the ligand occurs at 240–400°C. Measurement of
the electric conductivity of a 0.001 M solution of com-
plex II in DMF showed that it is a nonelectrolyte [19].
Analysis of the IR and NMR spectra of hydrazone I
led us to presume that its deprotonation should involve
proton abstraction from the phthalazine rather than
benzoin fragment. This distinguishes compound I from
well studied salicylaldehyde hydrazones and related
Table 2. Total energies (E), energies of zero-point vibrations (E0), differences in the Gibbs energies at 0 and 298 K (ΔG0→298),
and energies of solvattion (ΔGs) of neutral (A), singly deprotonated (B) and doubly deprotonated (C) forms of hydrazone I
and calculated ΔGc0 and pKa values of structures A and B
Species
ΔGs, kcal mol–1
E, hartree
E0, hartree
0.356799
0.341025
0.323762
ΔG0→298, hartree ΔGc0, kcal mol–1
pKa
–15.95
–1143.642041
–1143.082478
–1142.383435
193.60
183.85
170.89
–14.9
–28.2
10.89
20.67
A
B
C
–63.68
–181.45
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 79 No. 4 2009

828
SHCHERBAKOV et al.
1.907
1.732
2.173
А
B
Fig.1. Preferential conformations of the neutral (A) and singly deprotonated forms (B) of hydrazone I.
The IR spectrum of complex II lacked absorption
bands typical of OH and NH stretching vibrations,
while the band belonging to the azomethine C=N bond
slightly displaced to lower frequencies (1655 cm–1);
the positions of absorption bands due to heterocyclic
C=N bonds and C–O remained almost unchanged.
Measurement of the magnetic susceptibility of
complex II in the solid phase by the Faraday method
showed that its magnetic moment decreases from 1.85
B.M. at room temperature to 1.28 B.M. at –196°C
(nitrogen boiling point). These data suggest the
existence of moderate antiferromagnetic exchange
interaction; therefore, complex II is a dimer. We
succeeded in satisfactorily interpreting the temperature
dependence of magnetic susceptibility of complex II in
terms of the Heisenberg–Dirac–Van Vleck isolated
exchange cluster model [20–23]. The antiferromag-
netic exchange coupling parameter 2J was calculated
using modified Bleaney–Bowers Eq. (1) which takes
into account the presence of some paramagnetic
impurity in the complex [20, 21].
N
Ph
Ph
Ph
Ph
N
O
N N
N N
O
N
CH
Cu
Cu
CH
N
2NAg2β2
3kT
1
(1 – f) 1 + exp
3
–2J
–1
χ'M =
II
kT
+ f S(S + 1) + Nα.
(1)
Table 3. Magnetic properties of complexes II–V
Here, χ'M is the molar magnetic susceptibility
corrected for diamagnetism of atoms according to the
Pascal model [20], f is the mole fraction of a para-
magnetic impurity, and Nα is the temperature-indepen-
dent paramagnetism which was assumed to be equal to
60 × 10–6 for Cu(II) ion.
Compound no.
Temperature, K
293, 77.4
293, 77.4
293, 77.4
295
μeff,b B.M.
1.88, 1.28
2.96, 2.90
2.90, 2.87
Diamagnetic
IIa
III
IV
Va
Vb
The presence of several donor centers in the ligand
molecule provides the possibility for several mode of
dimerization of the complexes, in particular through
nitrogen atoms in the phthalazine fragment with
295
Diamagnetic
Exchange parameter 2J = –93 cm–1; g = 2.20. Calculated per
copper ion.
a
b
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 79 No. 4 2009

SPECIFICITY OF COMPLEX FORMATION OF BENZOIN (PHTHALAZIN-1-YL)HYDRAZONE
829
formation of six-membered Cu2(NN)2 exchange entity
(D) or through oxygen atoms in the benzoin fragment
with formation of four-membered Cu2O2 exchange
entity (E). The energies of isomeric structures D and E
were calculated in terms of the density functional
theory (DFT). The results are shown in Fig. 2. It is
seen that dimerization through the phthalazine nitrogen
atoms (structure D) is more favorable (by 2.35 kcal mol–1).
Ni(HL)2(MeOH)3 (IV), respectively (here, L2– and HL–
are, respectively, doubly and singly deprotonated
hydrazone I species). Both these complexes are
nonelectrolytes (Table 1); their effective magnetic
moments are 2.96 and 2.90 B.M., respectively, and are
almost independent of temperature. Thermogravimetric
analysis of complexes III and IV revealed endothermic
effects in the temperature range from 120–160°C with
weight loss corresponding to elimination of inner-
sphere water or methanol molecules, res-pectively [13,
18]; thermooxidative decomposition of the ligand was
observed at 240–400°C.
The exchange parameter 2J in complex II is
–93 cm–1. On the other hand, some dimeric copper
complexes with bridging nitrogen atoms belonging to
phthalazine or pyridazine fragment showed strong
antiferromagnetic exchange characterized by a 2J
value of –300 to –450 cm–1 [24, 25]. The low exchange
parameter suggests that the exchange moiety in
dimeric complex II is nonplanar [7, 8]. In fact, DFT
calculations showed that the planes of the monomer
fragments are turned with respect to each other by an
angle of about 15° due to steric repulsion of hydrogen
atoms in position 4 of the phthalazine rings and donor
oxygen atoms.
According to the IR data, nickel(II) acetate and
chloride coordinate, respectively, doubly and singly
deprotonated ligand. The IR spectrum of IV contains a
broad absorption band at 3370 cm–1 due to coordinated
methanol molecules and a band at 3251 cm–1 due to
stretching vibrations of the OH group in the ligand. In
the IR spectrum of III we observed only a broad band
with its maximum at 3391 cm–1, which was assigned to
coordinated water molecules.
The reaction of hydrazone I with nickel(II) acetate
and chloride gave complexes NiL(H2O)3 (III) and
The assumed structures of nickel(II) complexes III
and IV are shown below.
D
E
E = –5593.158149 au,
ΔE= 0.0 kcal mol–1
E = –5593.154408 au,
ΔE = 2.35 kcal mol–1
Fig. 2. Probable structures (D, E) of dinuclear copper(II) complex with hydrazone I; total energies (E) and relative stabilities (ΔE)
are given.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 79 No. 4 2009

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SHCHERBAKOV et al.
Ph
aq
CH
HO
Ph
L'
Ph
Ph
O
N
aq
N
N
CH
Ni
aq
N
N
Ni
N N
N
N
N
N
L'
OH
N
Ph
IV
CH
Ph
III
L' = CH3OH.
Hydrazone I reacted with copper(II) chloride and
bromide in a quite different mode than with copper(II)
acetate or nickel(II) chloride. After addition of copper
(II) halide to a solution of hydrazone I, the mixture
turned colorless, and a weakly colored solid separated.
Its elemental composition, IR spectrum, and magnetic
properties sharply differed from those found for the
other examined complexes. The complexes obtained
from hydrazone I and copper(II) chloride and bromide
X
Cu
N N
O
N
H4
H5
H13
N
H8
H9
H12
H7
H6
had
the
compositions
C15H10ClCuN4O
and
H11
H10
C15H10BrCuN4O, respectively. According to the data of
thermogravimetric analysis, the complexes contained
no solvent molecules. Measurement of the conduc-
tivity of their 0.001 M solutions in DMF showed that
these complexes are nonelectrolytes; this means that
the halide ions therein are coordinated in non-
ionogenic mode [19].
V, X = Cl (a), Br (b).
the phthalazine fragment of Va resonated at δ, ppm:
9.05 s (1H, 4-H), 8.74 br.s (1H, 8-H), 8.18 d (1H, 5-H,
J = 6.7 Hz), 8.02 d.d (1H, 7-H, J = 7.6 Hz), 7.86 d
(1H, 6-H, J = 7.6 Hz); phenyl proton signals: 8.39 br.s
The IR spectra of the complexes contained an
absorption band at 1710 cm–1, which was absent in the
spectrum of initial hydrazone I; this band may be
attributed to stretching vibrations of carbonyl group.
By contrast, no band at 1250 cm–1 typical of C–O
stretching vibrations was observed. The complexes
were diamagnetic, i.e., copper(II) ion was reduced to
copper(I). Therefore, we were able to examine them by
1
(2H, 9-H, 13-H), 7.50 m (3H, 10-H, 11-H, 12-H). H
NMR spectrum of Vb, ppm: 9.18 s (1H, 4-H), 8.61 d
(1H, 8-H, J = 7.9 Hz), 8.26 d (1H, 5-H, J = 7.6 Hz),
8.09 d.d (1H, 7-H, J1 = J2 = 7.6 Hz), 7.96 d.d (1H, 6-H,
J1 = J2 = 7.5 Hz); 8.34 d (2H, 9-H, 13-H, J = 7.2 Hz),
7.60 m (3H, 10-H, 11-H, 12-H).
However, the obtained data did not allow us to
unambiguously determine which donor centers in the
hydrazone residue are involved in coordination to
copper(I) ion. The position of the carbonyl absorption
band (1710 cm–1) suggests that the carbonyl oxygen
atom is not coordinated to copper(I); therefore, the
donor center is either azomethine or heterocyclic
nitrogen atom or both these simultaneously. Pre-
sumably, the hydrazone residue in complexes Va and
Vb acts as a unidentate ligand, and the coordination
number of copper ion is equal to 2.
1
1H NMR spectroscopy. The H NMR spectra of both
complexes were very similar, indicating formation of
the same oxidation product from hydrazone I. The
spectra displayed signals belonging to 10 protons. The
above findings allowed us to presume that the
complexation products of hydrazone I with copper(II)
chloride and bromide have structure V.
In the 1H NMR spectrum of complex Va in DMSO-
d6 all signals were broadened due to the presence of a
small amount of a paramagnetic impurity. Protons in
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SPECIFICITY OF COMPLEX FORMATION OF BENZOIN (PHTHALAZIN-1-YL)HYDRAZONE
831
A probable reason for the observed differences in
the reactivities of hydrazone I toward copper(II) and
nickel(II) acetates, on the one hand, and copper(II)
halides, on the other, is as follows. Copper(II) and
nickel(II) acetates are involved in coordination with
doubly deprotonated hydrazone. It is well known that
the nature of metal salt determines the degree of ligand
deprotonation in complex formation with salicylal-
dehyde hydrazones: metal salts derived from strong
acids give rise to complexes containing singly
deprotonated hydrazone species as ligand, whereas
complexes obtained from metal salts with weak acids
contain doubly deprotonated ligand species [4, 26, 27].
Presumably, oxidation of the benzoin fragment with
copper(II) ion occurs upon coordination with the
ligand with non-ionized hydroxy group, which is
possible in the reaction of hydrazone I with copper(II)
chloride and bromide. Analogous process is likely to
occur in the reactions of hydrazone I with copper(II)
nitrate and perchlorate; the latter, being salts of strong
acids, should also stabilize singly deprotonated
hydrazone species. In these reactions we obtained
samples containing mixtures of copper(II) and copper(I)
complexes, but we failed to isolate pure diamagnetic
complexes suitable for NMR study.
strength of 9000 Oe; the setup was calibrated against
HgCo(CNS)4. The temperature dependences of the
magnetic susceptibilities of the dinuclear complexes
were interpreted in terms of the Heisenberg–Dirac–Van
Vleck isotropic exchange model [20–23]. The ex-
change coupling parameters for the dinuclear copper
complexes were calculated using Bleaney–Bowers
equation (1).
Quantum-chemical calculations of the ligands and
coordination compounds were performed in terms of
the density functional theory (DFT) with Perdew–
Burke–Ernzerhof potential [29] using PRIRODA
software [30]. A three-exponent sent of Gauss
functions was used as calculation basis set. The geo-
metric parameters of model structures were optimized
with respect to all natural coordinates without
symmetry constraints. No scaling factor was applied to
normal vibration frequencies.
The energies of solvation (ΔGs) of neutral and
ionized species were calculated in terms of the
polarized continuum model (PCM) [31, 32] with
account taken of dispersion contribution using
PCGAMESS program [33] (DFT, B3LYP/6-311G(d,p).
The energies of solvation were calculated after
preliminary optimization of geometric parameters of
molecules and ions in solution. The calculated energies
of solvation are given in Table 2. The equilibria
involving isomeric structures D and E of copper(II)
complex II were examined assuming triplet electronic
states.
EXPERIMENTAL
1
The H NMR spectra were recorded on a Varian
Unity 300 (300 MHz) spectrometer with Fourier trans-
form using DMSO-d6 as solvent and hexamethyl-
disiloxane as internal reference. The IR spectra were
measured on a Varian Scimitar instrument in the
frequency range from 400 to 4000 cm–1; samples were
prepared as suspensions in mineral oil and fluorinated
hydrocarbons. The electronic absorption spectra were
obtained on a Unicam Helios Gamma spectrophoto-
meter in the λ range from 195 to 1100 nm; acetone,
acetonitrile, dioxane, DMSO, DMF, methanol, ethanol,
carbon tetrachloride, and chloroform were used as
solvents (c = 10–5 M); cell path length 1 cm.
Hydrazone (I) was synthesized from commercial
benzoin and phthalazin-1-ylhydrazine hydrochloride. A
hot solution of 0.01 mol of benzoin was added to a hot
suspension of 0.01 mol of phthalazin-1-ylhydrazine
hydrochloride in 50 ml of methanol. After 5 min, an
equivalent amount of sodium acetate was added, and a
bright yellow solid immediately began to separate. The
mixture was heated for 3 h under reflux and left
overnight, and the precipitate was filtered off, washed
first with two portions of warm water and then with
hot methanol, and recrystallized from methanol–DMF
(2:1). Yield 65%, bright yellow amorphous powder,
mp 218–219°C. The product was poorly soluble in
alcohol, chloroform, and dioxane and insoluble in
water and benzene.
Thermal gravimetric analysis of the complexes was
performed on a Diamond TG/DTA instrument (Perkin–
Elmer, Japan). Samples were heated to 650°C at a rate
of 10 deg/min. The molar conductivities of 0.001 M
solutions of the complexes in DMF were measured
using an R-38 rheocord bridge.
Complexes of transition metals with hydrazone
I. A hot solution of 0.001 mol of the corresponding
copper(II) or nickel(II) salt in methanol was added to a
hot suspension of 0.001 mol of hydrazone I in 20 ml of
The specific magnetic susceptibilities were deter-
mined by the relative Faraday method in the
temperature range from 80 to 300 K at a magnetic field
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 79 No. 4 2009

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SHCHERBAKOV et al.
13. Samus’, N.M., Tsapkov, V.I., and Kravchenko, V.N.,
methanol. The mixture changed its color, and
hydrazone I dissolved. The mixture was heated for 3 h
under reflux, and the precipitate was filtered off,
washed with hot methanol, and recrystallized from
dioxane or methanol–DMF. The elemental composi-
tions, melting points, and colors of the complexes, as
well as molar conductivities of their 0.001 M solutions
in DMF, are collected in Table 1.
Russ. J. Gen. Chem., 1997, vol. 67, no.12, p. 1929.
14. Paolucci, G., Stelluto, S., Sitran, S., Ajo, D., Benetollo, F.,
Polo, A., and Bombieri, G., Inorg. Chim. Acta, 1992,
vol. 57, no. 1, p. 193.
15. Odashima, T., Yamada, M., Yonemori, N., and Ishii, H.,
Bull. Chem. Soc. Jpn., 1987, vol. 60, no. 9, p. 3225.
16. Moshon, M.C., Gallego, M.C., and Perez, A.G., Talanta,
1986, vol. 33, no. 7, p. 627.
17. Popov, L.D., Levchenkov, S.I., Shcherbakov, I.N., and
Kogan, V.A., Russ. J. Gen. Chem., 2007, vol. 77, no. 7,
p. 1284.
18. Kukushkin, Yu.N., Khodzhaev, O.F., Budanova, V.F.,
and Parpiev, N.A. Termoliz koordinatsionnykh
soedinenii (Thermolysis of Coordination Compounds),
Tashkent: Fan, 1986.
ACKNOWLEDGMENTS
This study was performed under financial support
by the Russian Foundation for Basic Research (project
no. 08-03-91316-IND_a) and by the Southern Federal
University (internal project).
19. Geary, W.J., Coord. Chem. Rev., 1971, vol. 7, no. 1,
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