Synthesis, characterization, and thermal behavior of Cu(II) and Zn(II) complexes with (E)-2-[ N′ -(1-pyridin-2-yl-ethylidene)hydrazino]acetic acid (aphaOH) Crystal structure of [Zn2(aphaO)2Cl 2]

Article abstract of DOI:10.1080/00958972.2013.786052

Copper(II) and zinc(II) complexes with the condensation derivative of 2-acetylpyridine and hydrolyzed ethyl hydrazinoacetate were synthesized in a one pot reaction Crystal structure of the zinc(II) complex was determined by single-crystal X-ray diffraction In this binuclear complex, the ligand is coordinated via pyridine and imine nitrogen, while carboxylate oxygen is a bridge between metal centers The fifth coordination site is occupied by a chloride, giving a distorted square pyramidal geometry around each zinc(II) The molecular structure of the copper(II) complex was determined from elemental analysis, infrared and electron paramagnetic resonance spectroscopy, molar conductivity, mass spectrometry, and measurements of the magnetic moment Thermal behavior of both complexes was studied by thermogravimetric analysis coupled with mass spectrometer

Full text of DOI:10.1080/00958972.2013.786052

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Synthesis, characterization, and
thermal behavior of Cu(II) and Zn(II)
complexes with (E)-2-[N′-(1-pyridin-2-
yl-ethylidene)hydrazino]acetic
acid (aphaOH). Crystal structure of
[
Zn (aphaO) Cl ]
2 2 2
a
b
c
Nenad Filipović , Marija Borna , Olivera Klisurić , Matej
d
e f
g
Pregelj , Marko Jagodič
, Katarina Anđelković & Tamara
g
Todorović
a
Faculty of Agriculture , University of Belgrade , Belgrade ,
Republic of Serbia
b
Max Planck Institute for Chemical Physics of Solids , Dresden ,
Germany; Computer-Chemistry-Center, Friedrich-Alexander-
Universität Erlangen-Nürnberg, Erlangen, Germany
c
Faculty of Sciences , University of Novi Sad , Novi Sad , Republic
of Serbia
d
Jožef Stefan Institute , Ljubljana , Slovenia
e
Institute of Mathematics, Physics and Mechanics , Ljubljana ,
Slovenia
f
EN-FIST Centre of Excellence , Ljubljana , Slovenia
g
Faculty of Chemistry , University of Belgrade , Belgrade ,
Republic of Serbia
Accepted author version posted online: 18 Mar 2013.Published
online: 19 Apr 2013.
To cite this article: Nenad Filipovi , Marija Borna , Olivera Klisuri , Matej Pregelj , Marko Jagodi ,
Katarina Anelkovi & Tamara Todorovi (2013) Synthesis, characterization, and thermal behavior
of Cu(II) and Zn(II) complexes with (E)-2-[N′-(1-pyridin-2-yl-ethylidene)hydrazino]acetic acid
(
aphaOH). Crystal structure of [Zn (aphaO) Cl ], Journal of Coordination Chemistry, 66:9,
2 2 2
1549-1560, DOI: 10.1080/00958972.2013.786052
To link to this article: http://dx.doi.org/10.1080/00958972.2013.786052

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Journal of Coordination Chemistry, 2013
http://dx.doi.org/10.1080/00958972.2013.786052
Synthesis, characterization, and thermal behavior of Cu(II)
and Zn(II) complexes with (E)-2-[N′-(1-pyridin-2-yl-
ethylidene)hydrazino]acetic acid (aphaOH). Crystal structure
of [Zn (aphaO) Cl ]
2
2
2
NENAD FILIPOVIƆ, MARIJA BORNA‡, OLIVERA KLISURIĆx, MATEJ PREGELJ{,
MARKO JAGODIČk,††, KATARINA ANĐELKOVIƇ‡ and TAMARA
TODOROVIĆ*‡‡
†
Faculty of Agriculture, University of Belgrade, Belgrade, Republic of Serbia
‡
Max Planck Institute for Chemical Physics of Solids, Dresden, Germany; Computer-
Chemistry-Center, Friedrich-Alexander-Universität Erlangen-Nürnberg, Erlangen, Germany
xFaculty of Sciences, University of Novi Sad, Novi Sad, Republic of Serbia
{
Jožef Stefan Institute, Ljubljana, Slovenia
kInstitute of Mathematics, Physics and Mechanics, Ljubljana, Slovenia
†EN-FIST Centre of Excellence, Ljubljana, Slovenia
‡Faculty of Chemistry, University of Belgrade, Belgrade, Republic of Serbia
†
‡
(Received 29 June 2012; in final form 10 January 2013)
Copper(II) and zinc(II) complexes with the condensation derivative of 2-acetylpyridine and
hydrolyzed ethyl hydrazinoacetate were synthesized in a one pot reaction. Crystal structure of the
zinc(II) complex was determined by single-crystal X-ray diffraction. In this binuclear complex, the
ligand is coordinated via pyridine and imine nitrogen, while carboxylate oxygen is a bridge between
metal centers. The fifth coordination site is occupied by a chloride, giving a distorted square pyrami-
dal geometry around each zinc(II). The molecular structure of the copper(II) complex was deter-
mined from elemental analysis, infrared and electron paramagnetic resonance spectroscopy, molar
conductivity, mass spectrometry, and measurements of the magnetic moment. Thermal behavior of
both complexes was studied by thermogravimetric analysis coupled with mass spectrometer.
Keywords: Copper(II); Zinc(II); X-ray analysis; EPR spectroscopy; Thermogravimetric analysis
1. Introduction
Hydrazone ligands are condensation products of carbonyl compounds with hydrazine or its
alkyl and acyl derivatives. They are known for their versatile biological activities which,
in some cases, may be enhanced by coordination with metal ions [1]. Some complexes
with hydrazone type ligands show catalytic activity [2], while some polynuclear hydrazone
complexes can serve as molecular-based information storage devices [3].
*
Corresponding author. Email: tamarat@chem.bg.ac.rs
Ó 2013 Taylor & Francis

1
550
N. Filipović et al.
Hydrazones that contain a carbonyl oxygen (Ohyd) can be obtained from various
hydrazides and carbonyl compounds. If a carbonyl group is directly bound to a hydrazine
moiety, then Ohyd has a strong coordination tendency. This was previously shown by a
Cambridge Structural Database (CSD) [4] survey for hydrazone ligands derived from
2
-acetylpyridine (ap) and 2,6-diacetylpyridine (dap) [5], where coordination to Ohyd was
observed in more than 90% of all the complexes with this ligand type. In N-heteroaromatic
hydrazones derived from ethyl hydrazinoacetate hydrochloride (haOEt·HCl) as hydrazine
derivatives, which were introduced earlier [5–9], the carbonyl group is not part of a hydra-
zide, but an ester (scheme 1). Consequently, the hydrazinic NH proton becomes less
acidic, thus hydrazones derived from haOEt·HCl can coordinate in a neutral form [5–9],
which is the case for Cu(II) complexes with in situ obtained ligands fphaOEt, aphaOEt,
qahaOEt, and dap(haOMe) (see scheme 1) [6, 7]. The ester oxygens do not coordinate to
2
the metal in any of these complexes, despite the fact that a stable 6-membered chelate ring
may be formed. The same is observed for Cd(II) complexes with aphaOEt and dapha
(
OEt) [5], and Pd(II) and Pt(II) complexes with monohydrazone ligands [8, 9]. Coordina-
2
tion of the ligands in mono-ionic form is observed only when hydrolysis of an ester part
of the ligands takes place [8], like in Co(III) and Zn(II) complexes with tridentate aphaOH
ligand (see scheme 1). A deprotonated carboxyl coordinates to the metal center via an
oxygen (Ocarbox), in addition to coordination of pyridine and imine nitrogen (Npy and Nim,
respectively).
The fact that complexes of fphaOEt, aphaOEt, qahaOEt, dap(haOMe) , and dap(haOEt)
2
2
ligands were easily obtained by in situ reactions of haOEt·HCl with metal salts and corre-
sponding carbonyl compounds, while complexes with the hydrolyzed form of the ligands
were obtained as unexpected products, opened a possibility for investigation of specific
reaction conditions towards formation of compounds with the hydrolyzed form of the
ligands. Ester hydrolysis occurs during formation of metal organic frameworks by hydro-
thermal synthesis [10]. Also, syntheses of desired complexes can be done by direct reac-
tion of the hydrolyzed form of the ligands and corresponding metal salts in an appropriate
solvent. This conventional direct approach implies three steps: synthesis of the ligands,
their hydrolysis and afterwards coordination to metal ions. Template synthesis of
complexes via in situ hydrolysis has two benefits: (1) higher yield of desired complexes;
(2) it is not time and reagent consuming. In this work, one pot reaction conditions towards
complex compounds with aphaOH were investigated. Structural characterization and
Scheme 1. N-heteroaromatic hydrazones derived from (haOEt·HCl).

Cu(II) and Zn(II) hydrazone complexes
1551
investigation of thermal stabilities of two compounds with aphaOH, [Cu(aphaO)Cl] (1)
and [Zn (aphaO) Cl ] (2) are discussed.
2
2
2
2
. Experimental
2
.1. General remarks
2
-Acetylpyridine (99+%) and CuCl ·2H O (p.a.) were obtained from Acros Organics,
2
2
while (haOEt·HCl) (97%) and Zn(AcO) ·2H O (p.a.) were obtained from Fluka. Solvents
2
2
(ethanol 96%, diethyl ether, and acetonitrile 99.5%) were used without purification, except
for absolute ethanol, which was obtained by standard methods. Elemental analyses (C, H,
N) were performed by standard micro-methods using the ELEMENTAR Vario ELIII C.H.
N.S=O analyzer. Infrared (IR) spectra were recorded on a Thermo Scientific Nicolet 6700
FT-IR spectrophotometer by the Attenuated Total Reflection (ATR) in the region
ꢀ
1
4
000ꢀ400 cm . Molar conductivity measurements were performed at ambient temperature
on the Crison Multimeter MM41. The 1-D NMR spectra of DMSO-d solutions of the Zn
6
(
II) complex (for labeling see figure 1) were obtained using a Bruker Avance 500 instru-
1
13
ment equipped with a broad-band direct probe ( H at 500 MHz, C at 125.8 MHz). The 2-
D NOESY spectrum of the complex was performed on the same instrument, with 8 scans
per t1-increment and mixing time of 0.8 s. All spectra were measured at 296 K. Chemical
1
shifts are given on δ scale relative to tetramethylsilane as an internal standard for H and
1
3
C. The magnetic measurement of the Cu(II) complex was performed at 304 K by the
Evans’ method using a MSB-MK1 balance (Sherwood Scientific Ltd.) with Hg[Co(SCN)4]
as a calibrant. Diamagnetic corrections were calculated from Pascal’s constants.
Continuous wave EPR experiments were performed at ambient temperature on a home-
made spectrometer (Laboratory for EPR spectroscopy, Jožef Stefan Institute, Ljubljana,
Slovenia). A Larmor frequency of 9.38 GHz (X-band) was used. Electron spray ionization
Figure 1. Molecular structure of 2. The ellipsoids are drawn at 50% probability level, hydrogens at arbitrary
size. Equivalent atoms are generated by transformation ꢀx + 1, ꢀy, ꢀz + 1.

1
552
N. Filipović et al.
mass spectrum (ESI-MS) of the copper(II) complex was recorded on a Agilent Technolo-
gies 6210-1210 TOF-LC-ESI-MS instrument using a methanol/DMSO (7 : 3) solvent
mixture. The mass spectrometer was operated in the positive ion mode.
2.2. Preparation of [Cu(aphaO)Cl] (1)
The solution of haOEt·HCl (0.15 g, 1.0 mM) and ap (0.12 g, 1.0 mM) in a MeCN/H O
2
mixture (20 mL/5 mL) was heated with stirring until the temperature reached 313 K, fol-
lowed by addition of CuCl ·2H O (0.16 g, 0.94 mM). The reaction mixture was refluxed
2
2
for 30 min and cooled to ambient temperature. Green crystals of 1 were obtained from the
mother liquor, filtered off and washed with ether. Yield: 0.19 g (58.8%). Anal. Calcd for
C H ClCuN O (%): C, 37.12; H, 3.46; N, 14.43. Found: C, 36.91; H, 3.67; N, 14.47.
9
10
3 2
ꢀ
1
IR (ATR, cm ): 3243 (m), ν(N–H); 1610 (vs. br), ν (COO) and ν(C=N); 1374 (s),
as
ν (COO); 1023 (m), ν(N–N); 650 (w), (pyridine, in plane); 478 (w), (pyridine, out of
s
ꢀ
3
ꢀ1
2
ꢀ1
plane). ΛM (1 ꢁ10 M, DMF): 19.5 Ω cm M . μ (304 K) = 1.91 μB. ESI-MS: [M–Cl
+
+
DMSO] (C H N O SCu) Calcd m/z 333.0208, found 333.0216.
11 16 3 3
2
.3. Preparation of [Zn (aphaO) Cl ] (2)
2 2 2
A warm solution of ap (0.12 g, 1.0 mM) and haOEt·HCl (0.15 g, 1.0 mM) in absolute etha-
nol (10 mL) was added to a warm solution of Zn(AcO) ·2H O (0.22 g, 1.0 mM) in abso-
2
2
lute ethanol (10 mL). The reaction mixture was refluxed for 1 h and cooled to ambient
temperature. After two days white crystals of 2 formed were filtered from the mother
liquor and washed with ether. Yield 0.20 g (68.2%). Anal. Calcd for C H Cl N O Zn
2
1
8
20
2
6
4
ꢀ1
(
3
%): C, 36.89; H, 3.44; N, 14.34. Found: C, 36.81; H, 3.41; N, 14.35. IR (ATR, cm ):
235 (m), ν(N–H); 1646 (vs), ν (COO) and ν(C=N); 1372 (s), ν (COO); 670 (w), (pyri-
dine, in plane); 490 (w), (pyridine, out of plane). H NMR (500 MHz, DMSO-d ): δ = 2.23
as
s
1
6
(
(
s, 3H, H–C9), 3.73 (d, 2H, H–C7), 7.55 (br t, 1H, H–N3), 7.84 (d, 1H, H–C3), 8.10
ovlp, m, 2H, H–C4, and H–C5), 8.49 (br, m, 1H, H–C6). C NMR (125.8 MHz, DMSO-
1
3
d ): δ = 11.3 (C9), 53.0 (C7), 121.1 (C3), 124.6 (C5), 137.0 (C4), 141.1 (C6), 148.8 (C2),
6
ꢀ
3
ꢀ1
2
ꢀ1
1
50.0 (C1), 173.1 (C8). ΛM (1 ꢁ10 M, H O): 55.2 Ω cm M . IR and NMR spectro-
2
scopic data, as well as molar conductivity measurements are in agreement with the data
previously published [8].
2
.4. X-ray crystallography
Although 1 was obtained as crystalline material, no crystals suitable for single-crystal
X-ray diffraction analysis (XRD) could be found. The unit cell parameters of 1 were deter-
mined at ambient temperature from powder X-ray diffraction data obtained by employing
a HUBER G670 imaging-plate Guinier camera equipped with a Ge monochromator and
Co Kα radiation (λ = 1.788965 Å). LaB (a = 4.15692(1) Å) was used as internal standard.
1
6
A single crystal of 2 was selected and mounted on a glass fiber. Diffraction data were
collected using an Oxford Diffraction KM4 four-circle goniometer equipped with a
Sapphire CCD detector. The crystal to detector distance was 45.0 mm and graphite
monochromated Mo Kα (λ = 0.71073 Å) radiation was used for the experiments. The data
were reduced using CrysAlisPRO [11]. A semi-empirical absorption-correction based upon
the intensities of equivalent reflections was applied, and the data were corrected for

Cu(II) and Zn(II) hydrazone complexes
1553
Lorentz, polarization, and background effects. Scattering curves for neutral atoms, together
with anomalous dispersion corrections, were taken from the International Tables for Crys-
tallography [12]. The structure was solved by direct methods using the Sir97 program [13]
2
and refined by full-matrix least-squares procedures on F using SHELXS-97 and SHEL-
XL-97 [14] as implemented in the WinGX program suite [15]. The non-H atoms were
refined anisotropically. Positions of hydrogens were found from inspection of the differ-
ence Fourier maps. At the final stage of refinement, hydrogens from methyl were posi-
tioned geometrically (CꢀH = 0.96 Å) and refined using a riding model with fixed isotropic
displacement parameters. Analysis of the crystal geometry was performed by use of the
Cambridge Crystallographic Data Center packages [16]. Crystallographic data and refine-
ment parameters are listed in table 1. Figures representing the molecular structure were
made using ORTEP-3 [17] and PLATON [18].
2
.5. Thermal analysis
A coupled TG-MS system was used to study gaseous species evolved during dynamic
thermal decomposition (or fragmentation) processes. Thermogravimetric measurements
were carried out on a NETZSCH STA409 CD combined with a quadruple mass spectrom-
eter system, QMG422, incorporated in an Argon filled glove-box. The samples were
heated to 1073 K under argon at 10 K/min followed by immediate cooling to ambient
temperature. Mass spectra were recorded over the whole temperature range. The resulting
products were analyzed by powder X-ray diffraction. Long-term annealing (120 h) at
1
073 K was performed for both compounds to determine the final products of thermal
Table 1. Crystallographic data and crystal structure refinement details for 2.
Empirical formula
Formula weight (g/M)
Wavelength (Å)
Temperature (K)
Crystal system
Space group
a (Å)
b (Å)
c (Å)
α (°)
β (°)
9 3 2
C H10ClN O Zn
293.02
0.71073
293(2)
Triclinic
ꢀ
P1 (No. 2)
7.9449(7)
8.8623(8)
9.4347(7)
69.711(7)
71.303(7)
64.926(9)
552.45(8)
2
γ (°)
V (Å )
3
Z
ꢀ3
ρcalc (g cm
)
1.761
2.451
ꢀ
1
μ (mm
)
F(0 0 0)
296
hmin–hmax (°)
3.1–28.9
Refl. collected/unique [Rint
]
3618/1937(0.030)
1937/0/174
1.075
Data/restraints/parameters
2
Goodness-of-fit on F
R indices (all data)
R
1
= 0.0324
wR = 0.0664
= 0.0283
wR = 0.0639
0.28/ꢀ0.26
2
Final R indices (I > 2σ(I))
R
1
2
ꢀ3
Largest diff. peak/hole (e·Å
)

1
554
N. Filipović et al.
decomposition. For the thermal annealing experiments, samples were placed in glassy car-
bon crucibles thereafter enclosed in quartz glass reactors, with connections to an argon
supply and relief pressure valve, under flowing argon. The complete reaction setup was
thereafter annealed under constant argon flow using one-zone vertical tube furnace. The
following heating and cooling procedures were applied: 298 K/8 h/ → 1073 K (const.
temperature for 120 h) → 298 K/8 h/. The resulting products were analyzed by means of
powder X-ray diffraction.
3
3
. Results and discussion
.1. Synthetic aspects
To obtain a Cu(II) complex with aphaOEt, a template reaction between ap and haOEt·HCl
with CuCl ·2H O in ethanol was previously performed [6]. [CuCl (aphaOEt)] was
2
2
2
obtained as a green crystalline material, which was structurally characterized by a single
crystal XRD. In the present work a way to obtain the Cu(II) complex with aphaOH, which
is the hydrolyzed form of aphaOEt, by in situ reaction was investigated. In order to obtain
the desired Cu(II) complex, starting from the same reactants as in the synthesis of
[
CuCl (aphaOEt)], several mixtures of solvents were used. A higher content of water in
2
the reaction mixture increases the crystallization time and, in some cases, even disabled
the condensation reaction. The solvent mixture MeCN : H O (4 : 1 v/v) was found to be
2
most suitable, and its use yielded a green polycrystalline compound 24 h after the reaction.
Results of the elemental analysis suggested that the Cu : Cl : aphaO molar ratio was 1 : 1 : 1.
The ESI-MS spectrum of the complex was recorded in DMSO/methanol (3 : 7). The high-
+
est (m/z = 333) peak corresponds to [M–Cl+DMSO] (exhibiting the appropriate isotope
pattern), as the result of DMSO for chloride substitution in the parent complex. The value
of the magnetic moment at ambient temperature corresponds to the spin only case, indicat-
ing that a neutral, mononuclear compound was obtained (scheme 2). Its composition is in
Scheme 2. Proposed model of the molecular structure of [Cu(aphaO)Cl] (1).

Cu(II) and Zn(II) hydrazone complexes
1555
accord with the conductivity measurement, IR spectroscopy and observed green color of
the complex. Mononuclear compounds with similar ligands are green, such as
[
CuCl (aphaOEt)] and [CuCl (fphaOEt)], while the binuclear complex [Cu Cl (qahaOEt) ]
2 2 2 4 2
is brown [6]. Under the same reaction conditions, replacement of ap with aldehydes fp and
qa did not result in hydrolysis reactions, thus Cu(II) complexes with the hydrolyzed forms
of fphaOEt and qahaOEt were not obtained.
For Zn(II), a polycrystalline complex with the hydrolyzed aphaOEt was previously
1
13
obtained [8]. The complex was characterized by NMR ( H and C) and IR spectroscopy,
elemental analysis and molar conductivity. To obtain the Zn(II) complex with aphaOEt and
prevent hydrolysis, 96% ethanol was replaced with absolute ethanol, while the molar ratio
of all reactants was 1 : 1 : 1. In this reaction, a clear solution was obtained, and after several
days, a crystalline product suitable for single-crystal XRD was isolated. Spectral data, ele-
mental analysis, and molar conductivity measurements were consistent with data previously
obtained for the polycrystalline Zn(II) compound with the mono-ionic form of aphaOH [8].
A very small amount of water, present in the starting salt Zn(AcO) ·2H O, was enough to
2
2
cause hydrolysis. In another attempt to prepare the desired Zn(II) complex with aphaOEt
ligand, the starting acetate was replaced by anhydrous ZnCl . The presence of only
2
haOEt·HCl and ap in the reaction mixture, after reflux with ZnCl in absolute ethanol or
2
acetonitrile, was proven by thin layer chromatography [silica gel, ethanol-CH Cl
2
2
(
7 : 3, v/v)]. The presence of acetate in the reaction mixture is needed for the condensation
reaction to take place, since acetate is a stronger base than chloride and deprotonates
haOEt·HCl.
3.2. EPR spectroscopy
To obtain more information about the coordination environment of Cu(II) in 1, continuous
wave EPR experiments were performed at ambient temperature. Line shape simulations for
Cu(II) (S = 1/2) yielded g = 2.041(5), g = 2.061(5), and g = 2.254(5). The shape of the
x
y
z
obtained spectrum was characteristic for axial symmetry and consistent with a tetragonal
geometry around Cu(II). The spectrum showed no obvious hyperfine splitting, probably
due to weak spin-spin interactions. The effective magnetic moment per Cu(II) ion obtained
from EPR measurements is 1.9 μ , which is in agreement with the value obtained from
B
magnetic measurements and the expected value from the literature [19].
3
.3. Structure determination
Due to the poor quality of the crystals, the crystal structure of 1 remains unknown. However,
the unit cell parameters and the space group were determined from the results of powder X-
ꢀ
ray diffraction. Compound 1 crystallizes in triclinic space group P1 with the following lattice
parameters: a = 8.234(9) Å, b = 9.123(10) Å, c = 10.057(10) Å, α = 64.11(8)°, β = 76.13(9)°,
γ = 80.62(16)°. The solution of the crystal structure of 1 from powder X-ray diffraction data,
somewhat complex due to the presence of light atoms in the structure, is in progress.
An ORTEP drawing of binuclear [Zn (aphaO) Cl ] (2) is depicted in figure 1, while
2
2
2
selected bond distances and angles within this complex are given in table 2. The complex
lies around an inversion center giving rise to a centrosymmetric dimer. In the coordination
sphere of 2, the mono-deprotonated ligand is coordinated via Npy and N , while carboxyl-
im
ate oxygens bridge metal centers. The fifth coordination site is occupied by a chloride,

1
556
N. Filipović et al.
Table 2. Selected bond distances (Å) and angles (°) for 2.
Zn(1)–N(1)
Zn(1)–N(2)
Zn(1)–O(1)
Zn(1)–O(1)
2.130(2)
2.131(2)
2.2924(18)
1.9963(18)
2.2245(8)
C(1)–N(1)
N(1)–N(3)
N(3)–C(7)
C(7)–C(8)
C(8)–O(2)
1.292(3)
1.356(3)
1.454(4)
1.513(4)
1.220(3)
i
Zn(1)–Cl(1)
i
i
O(1) –Zn(1)–N(1)
130.06(8)
O(1) –Zn(1)–O(1)
72.49(7)
84.23(7)
151.60(8)
99.66(5)
107.51(7)
N(1)–Zn(1)–N(2)
75.95(8)
113.86(6)
113.22(6)
106.72(6)
O(1)–Zn(1)–N(1)
O(1)–Zn(1)–N(2)
O(1)–Zn(1)–Cl(1)
i
O(1) –Zn(1)–Cl(1)
N(1)–Zn(1)–Cl(1)
N(2)–Zn(1)–Cl(1)
i
Zn(1) –O(1)–Zn(1)
i
=
ꢀx + 1, ꢀy, ꢀz + 1.
thus the complex is neutral and the coordination number of each Zn(II) is five. The overall
geometry of 2 may be described as distorted square pyramidal, since the value for the
respective angular structural parameter τ is 0.36 [20]. Heavy distortion of the geometry is
further supported by significant displacement of Zn(1) from the mean basal plane [O(1)N
i
(
1)N(2)O(1) ] at 0.6701(3) Å. Due to tridentate coordination of the ligand and formation of
binuclear species via carboxylate oxygen bridges, one five-, one six- and one four-mem-
bered chelate rings are formed. The pyridine ring and the five-membered chelate ring are
almost planar (maximum deviation from the average ring skeleton excluding the metal is
0
.1 Å and the angle between the mean planes defined by the fused pyridine ring and the
five-membered chelate ring including the metal is 5.6°). Despite the fact that the bond dis-
tance range/average is higher than 25%, the results of Cremer and Pople’s ring puckering
analysis [21] showed that the six-membered chelate ring could be described in a boat con-
formation [Zn(1)–O(1)–C(8)–C(7)–N(3)–N(1): Q = 0.774 Å, u = 183.0°, h = 82.5°]. The
T
2
2
i
Zn–(μ-O
) –Zn skeleton exhibits rhombohedral geometry [Zn(1)···Zn(1) = 3.463 Å and
carbox 2
i
i
O(1)···O(1) = 2.547 Å]. The Zn(1)–Zn(1) distance is similar to metal–metal separations
observed in zinc complexes with (μ-Ocarbox)2 bridges (3.11–3.50 Å) [22]. The Zn–O bond
lengths in 2 are comparable to those found in similar zinc(II) complexes with Zn–(μ-Ocar-
box)2–Zn bridges [22].
The crystal packing is dominated by intermolecular hydrogen bonds between NH and
ꢀ
noncoordinated carbonyl oxygens that give rise to infinite 1-D chains along [1 1 0] direc-
tion [N3–H···O2 (2–x, –1–y, 1–z) = 2.967(4) Å, N–H···O = 161(3)°]. Intermolecular π–π
interactions are also present in this crystal structure. The centroid-centroid distance of
stacked pyridine rings is 3.794 Å, while the corresponding displacement angle is 23.79° in
accord with a shift of 1.53 Å, thus indicating almost perfect face-to-face alignment [23].
The stacking interactions between neighboring pyridine fragments expand 1-D chains
formed by hydrogen bonding into 2-D supramolecular layers parallel to (1 1 0). Finally,
these 2-D layers are further interlinked into a 3-D framework (figure 2) by intermolecular
C–H···Cl [C5–H5···Cl1 (1–x, 1–y, –z) = 3.754 Å, C–H···Cl = 175.02°] hydrogen bonds.
3
.4. Thermal stability
In the thermograms of 1 and 2 (Supplementary material), no mass loss is observed up to
organic decomposition above 433 K (for 1) and 513 K (for 2), which means that the title
compounds do not contain coordinated or crystalline water.

Cu(II) and Zn(II) hydrazone complexes
1557
Figure 2. 3-D framework of 2 formed by NH···O and CH···Cl hydrogen bonds (denoted by dashed lines), as
well as π-π interactions (denoted by dash-dot line). Hydrogens, except those involved in hydrogen bonding, are
omitted for clarity. View direction is parallel to [0 1 0].
Thermal decomposition of both compounds proceeds in four stages and for 2 is not
complete in the temperature range from 298 to 1073 K. The observed mass losses in these
four stages are presented in table 3. However, a decomposition mechanism for both
complexes cannot be proposed by a simple analysis of the mass losses. This indicates that
simultaneous decomposition processes take place, which is further supported by analysis
of the mass spectra recorded during all four stages.
Data obtained from the MS suggest that thermal degradation of both complexes starts
with a decarboxylation process. The most abundant fragment in the mass spectra recorded
during the first decomposition stage of both compounds was at a m/z value of 44 (with sat-
ellites at 45 and 46) originating from CO . Also, at the end of the first stage, less intense
2
peaks at 106 (py-CH CH ) and 117–123 (py-CNCH , n 6 6) are observed with their
2
3
n
decomposition fragments with lower m/z values. The same fragments were observed in
mass spectra at the end of the second decomposition stage. At the end of the third stage,
the intensities of the signals below m/z = 50 are much larger than those in the higher m/z
region, indicating further decomposition of larger organic fragments. During the last stage
of decomposition, the fragments in the higher m/z region slowly vanish, while the intensity
Table 3. Thermal decomposition stages for 1 and 2. Temperature ranges and corresponding mass losses.
Compound 1
Compound 2
Decomposition stage I (K)
Observed mass loss (wt.%)
Decomposition stage II (K)
Observed mass loss (wt.%)
Decomposition stage III (K)
Observed mass loss (wt.%)
Decomposition stage IV (K)
Observed mass loss (wt.%)
433–483
22.02
483–533
8.38
533–683
20.33
683–1073
9.41
513–553
19.40
553–643
7.10
643–773
5.15
673–1073
31.77

1
558
N. Filipović et al.
of those in the lower m/z region increased. The total mass loss during thermal decomposi-
tion in the studied temperature region was ꢂ60 wt.% for 1 and ꢂ63 wt.% for 2.
Powder X-ray diagram of the resulting product for 1 (Supplementary material) indicated
a mixture of amorphous and crystalline materials. Several sharp intense lines were
observed and identified as carbon, earlier named n-diamond [24]. In this crystal structure,
six formula units are present in the rhombohedral unit cell with lattice parameters
a = b = 2.5321 Å and c = 6.2023 Å, space group R3m (No. 160). This observation was later
confirmed by powder X-ray diffraction data of the resulting product from long term
annealing experiments of the same starting material.
The powder X-ray diagram of the resulting solid after decomposition of 2 indicated only
amorphous material. In the powder X-ray diagram of the resulting product from long-term
annealing experiments of the same starting material, a new modification of zinc oxide,
ZnO, may be assigned, next to another phase with unknown crystal structure (Supplemen-
tary material). Two sets of peaks can be distinguished, a set of sharp and narrow peaks
and a set of broad peaks. All sharp peaks can be indexed on the basis of the Nb0.92S [25]
structure type [P6 mc space group (No. 186)], with lattice parameters a = b = 5.8021(20) Å
3
and c = 5.6651(21) Å. ZnO is the first oxide compound adopting this structure type, next to
a few sulfide and nitride compounds, that is, δ-MoN [26]. Due to the presence of a second
phase with an unknown crystal structure in the resulting product, the structure of the new
zinc oxide could not be precisely refined.
In the thermal decomposition of 1 and 2, only the first two decomposition stages can be
compared. According to the mass spectra, the most intense process in the first stage is
decarboxylation of the ligand. In the second stage, besides decarboxylation, loss of 2-ap
imine fragment dominates. Although in the later stages further decomposition of organic
fragments takes place, one big difference has to be noted. In the product after thermal
decomposition of 1, no compound containing copper could be identified.
4
. Conclusions
The copper(II) complex with the aphaOEt ligand was previously synthesized starting from
ap, haOEt·HCl and CuCl in an ethanolic solution. In the present study, a variety of reac-
2
tion conditions towards formation of the Cu(II) complex with the hydrolyzed form of the
aphaOEt ligand was investigated. It was found that mixture of MeCN and H O (4:1 v/v)
2
is most suitable for one pot formation of 1. Tridentate coordination (N N ,Ocarbox) of the
py, im
mono-anionic form of aphaOH is postulated, while the fourth coordination site in the inner
sphere of 1 is occupied by a chloride. Attempts to synthesize Cu(II) complexes with the
hydrolyzed form of the corresponding ligands by in situ reactions of haOEt·HCl with
N-heteroaromatic aldehydes (fp and qa) remained unsuccessful. This may indicate that the
type of carbonyl compound plays a significant role in the synthesis, next to impact of the
solvent.
Regardless of experimental conditions such as choice of solvent, molar ratio of reac-
tants, presence of water in the reaction mixture, the Zn(II) complex 2 with the hydrolyzed
form of aphaOEt is exclusively formed. It was previously postulated that a mononuclear
tetrahedral complex was obtained, but in the present study, a single crystal XRD revealed
the binuclear nature of this compound in the solid state. In the inner sphere of 2, chloride
is coordinated to Zn(II). The ligand is coordinated in the mono-anionic form via Npy and

Cu(II) and Zn(II) hydrazone complexes
1559
N , while the carboxylate oxygen is a double donor, bridging two metal centers. This
im
coordination of carboxylate oxygen to Zn(II) is rare, as shown from a survey of
Cambridge Structural Database (CSD). Overall geometry of both metal centers in 2 can be
described as a distorted square pyramid. The crystal packing of 2 is dominated by intermo-
lecular hydrogen bonds and π–π interactions. In the 2-D NOESY spectrum of the Zn(II)
complex, there is no correlation signal between protons that are spatially close in the
molecular structure of the complex in the solid state (CH from one ligand and aromatic
2
C6 proton from the second one). Since a signal can be observed only if the distance is
smaller than 5 Å, and the distance between the above mentioned protons is close to the
limit, it remains unknown if the geometry and nuclearity of the Zn(II) complex is
preserved in solution.
Thermal decompositions of 1 and 2 occur via four stages. Both complexes show moder-
ate thermal stability, with the Zn(II) complex showing slightly higher thermal stability.
Supplementary material
CCDC 875363 contains the supplementary crystallographic data for 2. These data can be
obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the
Cambridge Crystallographic Data Center, 12 Union Road, Cambridge CB2 1EZ, UK; Fax:
(+44) 1223-336-033; or E-mail: deposit@ccdc.cam.ac.uk. Supplementary data associated
with this article can be found in the online version at doi:10.1080/00958972.2013.786052.
Acknowledgments
The authors would like to acknowledge the help of Dr S. Hoffmann and S. Scharsach
from MPI CPfS in Dresden with the TG-MS measurements and data analyses, and Dr H.
Borrmann for his help, guidance and support. N.F., M.J. and T.T. gratefully acknowledge
the financial support by the Max Planck Society and the opportunity to work at MPI in
Dresden, where a part of this work was done. This work was supported by the Ministry of
Education and Science of the Republic of Serbia (Grant 172055).
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