Preparation, characterisation and crystal structure of two zinc(II) benzoate complexes with pyridine-based ligands nicotinamide and methyl-3-pyridylcarbamate

Article abstract of DOI:10.1016/j.ica.2004.01.016

Two benzoate complexes namely tetrakis(μ₂-benzoato-O,O ^′)-bis(μ₂-benzoato-O,O)-bis(nicotinamide-N) -tri-zinc(II), [Zn₃(benz)₆(nia)₂] (I) and bis(benzoato-O)-bis(methyl-3-pyridylcarbamate-N)-zinc(II), [Zn(benz) ₂(mpcm)₂] (II) (benz=benzoate anion, nia=nicotinamide, mpcm=methyl-3-pyridylcarbamate) were prepared and characterised by elemental analysis, IR spectroscopy, thermal analysis and X-ray structure determination. The structure of the complex I is centrosymmetric, formed by a linear array of three zinc atoms. The central zinc atom shows octahedral coordination and is bridged to each of the terminal zinc atoms by three benzoate anions. Two of them act as bidentate, one as monodentate ligand. By additional coordination of the nia ligand, the terminal Zn atoms adopt tetrahedral surrounding. The structure of complex II contains two crystallographically independent [Zn(benz) ₂(mpcm)₂] molecules. In each molecule, the zinc atom is tetrahedrally coordinated by two monodentate benzoate and two methyl-3-pyridylcarbamate ligands. Intermolecular hydrogen bonds of the N-H?O type connect molecules in the structures of complexes I and II to form a two-dimensional network. The three different types of carboxylate binding found in the complexes were distinguished also by values of carboxylate stretching vibrations in FT-IR spectra as well as by thermal decomposition of the complexes in nitrogen.

Full text of DOI:10.1016/j.ica.2004.01.016

Inorganica Chimica Acta 357 (2004) 2049–2059
www.elsevier.com/locate/ica
Preparation, characterisation and crystal structure of two
zinc(II) benzoate complexes with pyridine-based ligands
nicotinamide and methyl-3-pyridylcarbamate
a,
a
b
c
V. Zelenak *, M. Sabo , W. Massa , P. Llewellyn
ꢀꢁ
a
ꢀ
Department of Inorganic Chemistry, Faculty of Natural Science, Institute of Chemical Sciences, P.J. Safꢁarik University,
Moyzesova 11, Koꢀsice SK-041 54, Slovakia
Department of Chemistry, Philipps University, Marburg D-35032, Germany
b
c
Madirel Laboratory (UMR 6121), Provence University and CNRS, Centre de St-Jꢁer^ome, Marseille F-13331, France
Received 17 June 2003; accepted 18 January 2004
Abstract
Two benzoate complexes namely tetrakis(l2-benzoato-O,O0)-bis(l2-benzoato-O,O)-bis(nicotinamide-N)-tri-zinc(II), [Zn3(benz)6-
(nia)2] (I) and bis(benzoato-O)-bis(methyl-3-pyridylcarbamate-N)-zinc(II), [Zn(benz)2(mpcm)2] (II) (benz ¼ benzoate anion,
nia ¼ nicotinamide, mpcm ¼ methyl-3-pyridylcarbamate) were prepared and characterised by elemental analysis, IR spectroscopy,
thermal analysis and X-ray structure determination. The structure of the complex I is centrosymmetric, formed by a linear array of
three zinc atoms. The central zinc atom shows octahedral coordination and is bridged to each of the terminal zinc atoms by three
benzoate anions. Two of them act as bidentate, one as monodentate ligand. By additional coordination of the nia ligand, the
terminal Zn atoms adopt tetrahedral surrounding. The structure of complex II contains two crystallographically independent
[Zn(benz)2(mpcm)2] molecules. In each molecule, the zinc atom is tetrahedrally coordinated by two monodentate benzoate and two
methyl-3-pyridylcarbamate ligands. Intermolecular hydrogen bonds of the N–Hꢀ ꢀ ꢀO type connect molecules in the structures of
complexes I and II to form a two-dimensional network. The three different types of carboxylate binding found in the complexes were
distinguished also by values of carboxylate stretching vibrations in FT-IR spectra as well as by thermal decomposition of the
complexes in nitrogen.
Ó 2004 Elsevier B.V. All rights reserved.
Keywords: Zinc benzoate; Nicotinamide; Methyl-3-pyridylcarbamate; Crystal structure; FT-IR spectra; Thermogravimetry
1. Introduction
to preserve food from growth of microorganisms and it
can be found in beverages, fruit products, chemically
leavened baked goods, condiments as well as in the
drink industry (E-codes 210–213) [8–12].
Aromatic carboxylic acids are widely used in medi-
cine as non-steroidal anti-inflammatory drugs (e.g.,
ibuprofen, naproxen, diclophenac and fenclofenac) [1–
4]. Moreover, some of the simple aromatic carboxylic
acids (benzoic acid and cinnamic acid) are known to
have anti-bacterial and anti-fungal properties. Benzoic
acid is used in combination with salicylic acid in der-
matology as a fungicidal treatment (WhitfieldÕs oint-
ment) for fungal skin diseases (ringworm) [5]. It can be
found in cosmetics, deodorants and toothpastes [6,7].
More widespread is the use of benzoic acid and its salts
The carboxylic group of the carboxylic acids is
available for interactions with metals which can have
synergetic or antagonistic effect to the biological activ-
ity. It was found that anti-inflammatory and anti-
bacterial activity of metal complexes was higher than
in the parent carboxylic acids [3,13]. Moreover, the
anti-bacterial effect of some drugs could be enhanced
when they are chelated to a metal [14]. Therefore, for
the preparation of effective anti-microbial species it
is very important to gain knowledge about the struc-
ture and bonding relations of the complexes. On the
contrary, undissociated benzoic acid is a more effective
* Corresponding author. Tel.: +6228114; fax: +6222124.
ꢀꢁ
E-mail address: zelenak@kosice.upjs.sk (V. Zelenak).
0020-1693/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2004.01.016

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V. Zeleꢀnꢁak et al. / Inorganica Chimica Acta 357 (2004) 2049–2059
anti-microbial agent than its sodium salt, however, so-
dium benzoate is still used preferably due to its higher
solubility in water [12,15].
71%. Anal. Calc. for C54H42N4O14Zn3: C, 55.57; H,
3.63; N, 4.80; Zn, 16.81. Found: C, 55.49; H, 3.45; N,
4.93; Zn, 16.1%. IR(KBr disk): 3376 s, 3200 s, 3066 w,
3027 vw, 1670 vs, 1630 vs, 1600 vs, 1568 vs, 1541 vs,
1492 w, 1448 m, 1401 vs, 1343 vs, 1308 m, 1204 m, 1176
w, 1157 w, 1133 m, 1097 w, 1071 ww, 1062 m, 1024 m,
1002 w, 942 m, 843 w, 822 w, 802 w, 723 vs, 695 s, 680 m,
660 vw, 639 w, 548 vw, 523 vw, 477 w, 462 w, 422 w,
It is of our interest to study the interaction of the
carboxylic acids with zinc, which is an essential and
beneficial element in human growth. This element is
known to regulate activity over 300 metalloenzymes and
it is a component of special proteins, called ‘‘zinc fin-
gers’’ which participate in the reliable transfer of genetic
information [16–18]. Zinc and its compounds have anti-
bacterial and anti-viral activity and the wound-healing
effect of zinc-containing ointments has been known for
several centuries [3,19–23]. Zinc may be used as a ther-
apeutic agent, it may act as an anti-sickling agent and
play a role in the prevention of pain crisis in sickle-cell
disease. Zinc was successfully used in the treatment of
acrodermatitis enteropathica, WilsonÕs disease, gastro-
intestinal disorders, infertility and other diseases [24].
Complex of zinc(II) acetate with erythromycin is used in
clinical medicine for acne therapy [25,26].
In our previous works, we have described physico-
chemical properties, crystal structures and biological
activity of aliphatic carboxylic acid zinc(II) complexes
containing additional N-donor ligands [27]. As a part of
our ongoing research with aromatic carboxylic acids, we
report in the present study synthesis, crystal structure,
thermal and spectral properties of two benzoate com-
plexes, namely tetrakis(l2-benzoato-O,O0)-bis(l2-ben-
zoato-O,O)-bis(nicotinamide-N)-tri-zinc(II), [Zn3(benz)6
(nia)2] (I) and bis(benzoato-O)-bis(methyl-3-pyridylcar-
bamate-N)-zinc(II) [Zn(benz)2(mpcm)2] (II) (benz ¼
benzoate anion, nia ¼ nicotinamide, mpcm ¼ methyl-3-
pyridylcarbamate). The biological activity of these
complexes against common bacterial strains is under
investigation.
cmꢁ1
.
2.1.2. [Zn(Benz)2(MPCM)2] (II)
A solution of 0.280 g zinc(II) sulfate (1.73 mmol) in
40 cm3 of water was added to a solution of 0.500 g (3.47
mmol) of sodium benzoate in 40 cm3 of water. The pH
of the solution was adjusted by free benzoic acid to 6.
The reaction mixture was stirred for 30 min and filtered.
Then, 25 cm3 of an aqueous solution of methyl-3-pyr-
idylcarbamate (0.528 g, 3.47 mmol) was added to this
mixture. After stirring for 2 h, the solution was filtered
and left to crystallise at room temperature. After two
days, needle-like crystals appeared, which were filtered
off, washed with water and dried in air. Yield 80%. Anal.
Calc. for C28H26N4O8Zn: C, 54.96; H, 4.28; N, 9.16; Zn,
10.68. Found: C, 54.72; H, 4.47; N, 9.35; Zn, 10.15%.
IR(KBr disk): 3249 m, 3134 w, 3067 m, 2952 w, 1736 vs,
1610 vs, 1556 vs, 1491 s, 1439 s, 1427 s, 1389 s, 1335 m,
1300 w, 1259 s, 1236 vs, 1193 w, 1134 w, 1110 vw, 1076
m, 1056 m, 1025 w, 955 vw, 939 vw, 910 vw, 876 w, 845
w, 808 m, 770 w, 721 s, 698 m, 650 vw, 639 vw, 566 w,
450 w, 417 w, 389 w, cmꢁ1
.
2.2. Physical measurements
The infrared spectra of the complexes were recorded
with a EQUINOX 55 (Bruker) FT-IR spectrophotom-
eter as KBr discs (complex/KBr mass ratio 1:100).
Thermogravimetric (TG) measurements were carried
out in dynamic nitrogen atmosphere (sample flow rate
60 ml/min) using Hi-Res mode (G) and an initial heating
rate of 5 °C/min by means of TGA Q500 apparatus (TA
Instruments). Elemental analyses were performed with a
Perkin–Elmer 2400 CHN Elemental Analyser. Zinc
content was determined spectrophotometrically by the
dithiazone method with a Varian 635 spectrophotome-
ter. The final solid products of the thermal decomposi-
2. Experimental
2.1. Syntheses
All reagents were purchased from Aldrich and used
as received.
2.1.1. [Zn3(benz)6(nia)2] (I)
About 25 cm3 of aqueous solution of sodium ben-
zoate (0.500 g; 3.47 mmol) was mixed with 25 cm3 of an
aqueous solution of zinc sulfate (0.280 g; 1.73 mmol).
The pH of the solution was adjusted to 6 using free
benzoic acid. After 30 min stirring, the solution was
filtered and 20 cm3 of an aqueous solution of nicotin-
amide (0.141 g; 1.16 mmol) was added to the filtrate.
The reaction mixture was stirred for 3 h, then filtered
and left to stand at room temperature. After two days,
colourless prisms of the complex crystallised. The crys-
tals were filtered off, washed with water and dried. Yield
tion were analysed by Siemens a D5005 XRD
diffractometer using Cu Ka radiation.
2.3. Structure determination and refinement
Complex I crystallises as air-stable colourless prisms,
whereas complex II forms needle-like crystals. The
crystallographic data for both complexes are given in
Table 1. Diffraction experiments were performed on
IPDS area detector systems (Mo Ka radiation,
ꢂ
k ¼ 0:71070 A) equipped with graphite monochroma-

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2051
Table 1
Crystal data and structure refinement
Formula
Empirical formula
Formula weight
[Zn3(benz)6(nia)2]
C54H42N4O14Zn3
1167.02
[Zn(benz)2(mpcm)2]
C28H26N4O8Zn
611.90
Crystal parameters
Crystal shape
Crystal colour
prism
colourless
flat needle
colourless
Crystal size (mm)
Crystal system
Space group
0.30 ꢂ 0.24 ꢂ 0.20
orthorhombic
Pbca (No. 61)
0.60 ꢂ 0.30 ꢂ 0.13
orthorhombic
P212121 (No. 19)
Unit-cell dimensions
ꢂ
a (A)
ꢂ
10.1343(4)
20.3514(11)
24.3803(12)
5028.4(4)
4
10.2866(4)
22.3253(8)
24.4093(15)
5605.6(5)
8
b (A)
ꢂ
c (A)
ꢁ3
ꢂ
Vc (A
Molecules per cell, Z
Dx (g cmꢁ3
)
)
1.542
1.450
Measurement of intensity data
Instrument
Temperature (K)
IPDS (Stoe)
193(1)
IPDS-II (Stoe)
153(1)
ꢂ
Radiation k (A)
Monochromator
Scan mode
0.71073
graphite
0.71073
graphite
x
2.17–28.0
x
1.82–29.44
h range (°)
Reflections measured
34 593
5963
15 376
15 376
Independent reflections
Reflections observed
Linear absorption coefficient l (mmꢁ1
4512 (I P 2rðIÞ)
10 592
0.933
)
1.492
2384
F ð000Þ
2528
37 641 with 1:73 6 h 6 29:41
Reflections for cell measurement
Indices limits
8000 with 2:25 6 h 6 27:95
h
k
l
)12 to 12
)26 to 26
)28 to 32
)13 to 14
)30 to 30
)22 to 33
Refinement
Refinement method
full-matrix least-squares on F 2
full-matrix least-squares on F 2 as racemic twin
(twin ratio 0.483(5))
741
Parameters
Goodness-of-fit on F 2
346
0.922
0.803
R1
0.0263 (observed); 0.0421 (all)
0.0584 (observed); 0.0617 (all)
0.405 and )0.237
0.0275 (observed); 0.0463 (all)
0.0527 (observed); 0.0556 (all)
0.255 and )0.394
wR2
Maximum and minimum height in final
ꢂ
difference synthesis (e A
ꢁ3
)
tors. The details about determination of unit cell pa-
rameters and data collection are in Table 1. Data were
corrected for Lorentz and polarisation factors. A nu-
merical absorption correction based on indexed faces
was applied for both crystals [28]. In both structures the
zinc atoms were located from Patterson synthesis using
the SHELXS 97 program [29]. Fourier maps, calculated
using SHELXL 97 [30], enabled the location of all non-
hydrogen atoms. Full-matrix least-squares refinements
on F 2 were carried out with anisotropic thermal pa-
rameters. Hydrogen atoms were located from difference
Fourier syntheses but kept riding on idealised positions
[30]. The H atoms of the NH2 group of complex I only
were refined without any constraints. The isotropic
thermal parameters were fixed to 1:2Ueq of the parent
atom for all H atoms. All structure drawings were per-
formed using DIAMOND [31]. Selected bond distances
and angles are listed in Tables 2 and 3.
3. Results and discussion
3.1. Synthesis
Generally in the preparation of zinc(II) carboxylates,
the reaction between carboxylic acid and zinc(II) car-
bonate or hydroxide is often used and this procedure is
also applicable for the preparation of aromatic zinc(II)
carboxylates [32–34]. However, due to the lower solu-
bility of aromatic carboxylic acids in water, the synthetic
procedure involving the sodium salt of the acid is
often used [35,36]. For the preparation of copper(II)

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V. Zeleꢀnꢁak et al. / Inorganica Chimica Acta 357 (2004) 2049–2059
Table 2
ꢂ
Selected bond distances (A) and angles (°) in the complex [Zn3(benz)6(nia)2]
Bond distances
Zn1–O12
Zn1–O21
Zn1–O32
Zn2–O11
Zn2–O21
Zn2–O31
Zn2–N1
2.061(1)
2.201(1)
2.050(1)
1.962(1)
1.996(1)
1.933(1)
2.023(1)
1.270(2)
1.257(2)
1.499(2)
1.306(2)
O22–C20
C20–C21
O31–C30
O32–C30
C30–C31
N1–C1
1.228(2)
1.500(3)
1.275(2)
1.247(2)
1.501(2)
1.342(2)
1.344(2)
1.504(2)
1.232(2)
1.326(2)
N1–C5
C2–C6
O11–C10
O12–C10
C10–C11
O21–C20
O1–C6
N12–C6
Bond angles
O31–Zn2–O11
O31–Zn2–O21
O11–Zn2–O21
O31–Zn2–N1
O11–Zn2–N1
O21–Zn2–N1
O32i–Zn1–O12
O32–Zn1–O12
O32–Zn1–O12i
O32i–Zn1–O21i
O32–Zn1–O21i
O12–Zn1–O21i
O12i–Zn1–O21i
O32i–Zn1–O21
O32–Zn1–O21
O12–Zn1–O21
O12i–Zn1–O21
117.24(6)
116.47(5)
95.73(5)
105.09(5)
103.13(5)
118.80(5)
84.72(5)
95.28(5)
84.72(5)
91.95(5)
88.05(5)
92.95(4)
87.05(4)
88.05(5)
91.95(5)
87.05(4)
92.95(4)
Zn2–O21–Zn1
C1–N1–C5
N1–C1–C2
98.57(5)
118.8(1)
122.7(1)
118.2(1)
122.3(1)
120.2(1)
117.4(1)
124.1(1)
119.2(1)
116.6(1)
121.1(2)
121.7(2)
117.1(2)
125.5(2)
118.1(2)
116.5(2)
C1–C2–C6
O1–C6–N12
O1–C6–C2
N12–C6–C2
O12–C10–O11
O12–C10–C11
O11–C10–C11
O22–C20–O21
O22–C20–C21
O21–C20–C21
O32–C30–O31
O32–C30–C31
O31–C30–C31
i, 1 ꢁ x; ꢁy; ꢁz.
complexes of aromatic carboxylic acids containing ad-
ditional N-donor ligands, Melnik et al. used a two-step
synthesis. They prepared crystalline copper carboxylates
in a first step, which were consequently used in the
second step in the reaction with the N-donor ligand
[37,38]. The complexes in this study were prepared by a
modified procedure – direct one-step reaction of the
sodium benzoate with zinc sulfate and nicotinamide or
methyl-3-pyridylcarbamate.
groups and by the nitrogen atom of the pyridine ring of
the nicotinamide molecule.
Two different types of carboxylate coordination were
found in this complex. Four benzoate ligands are co-
ordinated bidentately by both oxygen atoms of car-
boxylate group, forming syn–syn bridges between
central and terminal zinc ions. The other two benzoate
ligands are coordinated monodentately and function as
monatomic Zn–O–Zn bridges with one oxygen atom
which remains uncoordinated and involved only in a
weak interaction with the zinc atom (Zn2ꢀ ꢀ ꢀO22 ¼
3.2. Description of the structure
ꢂ
2.780(1) A).
3.2.1. Complex [Zn3(benz)6(nia)2] (I)
Similar structures of a linear array of three metal ions
bridged by carboxylate ligands were reported by Clegg
et al. [39,40]. They reported the crystal structures of
complexes of general formula [MZn2(crot)6(base)2],
where M ¼ Zn, Mn, Co, Ni, Cd, Mg, Ca, Sr;
crot ¼ crotonate ¼ CH3CHCHCOOꢁ and base ¼ quino-
line ¼ C9H7N or 6-methylquinoline ¼ C10H9N. In this
series, the structures of complexes with M ¼ Zn, Mn,
Co, Ni, Cd contained, as with complex I, four syn–syn
and two monodentate monatomic crotonate bridges.
Moreover, to monatomic crotonate, monatomic ace-
tate ligands were also found in zinc complexes, namely
[Zn3(CH3COO)6(py)2], [Zn3(CH3COO)6(bppz)2], where
A view of the structure is shown in Fig. 1, principal
bond lengths and angles are summarised in Table 2. The
complex crystallises in the orthorhombic space group
Pbca with four formula units in the unit cell. The
structure is formed by a centrosymmetric linear array of
three zinc ions, which are coordinated by six benzoate
anions and two neutral nicotinamide ligands. The mid-
dle zinc atom occupies a special position and lies on a
crystallographic inversion centre. This atom is octahe-
drally coordinated by six oxygen atoms of the benzoate
groups. Each of the two terminal zinc ions is coordi-
nated tetrahedrally by three oxygen atoms of benzoate

V. Zeleꢀnꢁak et al. / Inorganica Chimica Acta 357 (2004) 2049–2059
2053
Table 3
ꢂ
Selected bond distances (A) and angles (°) in the crystallographically independent molecules of the complex [Zn(benz)2(mpcm)2]
Bond distances
Zn1–O41
Zn1–O31
Zn1–N21
Zn1–N11
O31–C30
O41–C40
N22–C26
N22–C22
C30–O32
C30–C31
O21–C26
O42–C40
C12–N12
N12–C16
C40–C41
C26–O22
O12–C16
O11–C16
O12–C17
O22–C27
1.935(1)
1.960(1)
2.017(1)
2.052(1)
1.282(2)
1.279(2)
1.354(3)
1.398(2)
1.243(2)
1.499(3)
1.212(3)
1.236(2)
1.397(2)
1.370(3)
1.498(3)
1.341(2)
1.346(2)
1.197(2)
1.441(3)
1.442(3)
Zn2–O71
Zn2–O81
Zn2–N51
Zn2–N61
O82–C80
C70–O72
C70–O71
C70–C71
O81–C80
C80–C81
N62–C66
N62–C62
O61–C66
C66–O62
O52–C56
C52–N52
N52–C56
O51–C56
O52–C57
O62–C67
1.936(1)
1.954(1)
2.008(1)
2.052(1)
1.238(2)
1.242(2)
1.285(2)
1.497(3)
1.285(2)
1.500(3)
1.374(2)
1.383(2)
1.202(2)
1.337(2)
1.352(3)
1.402(2)
1.361(3)
1.196(3)
1.438(3)
1.443(2)
Bond angles
O41–Zn1–O31
O41–Zn1–N21
O31–Zn1–N21
O41–Zn1–N11
O31–Zn1–N11
N21–Zn1–N11
C26–N22–C22
O32–C30–O31
O32–C30–C31
O31–C30–C31
C16–N12–C12
O42–C40–O41
O42–C40–C41
O41–C40–C41
O21–C26–O22
O21–C26–N22
O22–C26–N22
C26–O22–C27
C16–O12–C17
O11–C16–O12
O11–C16–N12
O12–C16–N12
108.91(6)
121.16(7)
112.74(6)
109.94(6)
95.56(6)
105.38(6)
127.2(2)
123.2(2)
120.5(2)
116.2(2)
125.9(2)
123.7(2)
120.5(2)
115.9(2)
124.7(2)
126.7(2)
108.6(2)
115.7(2)
114.5(2)
125.7(2)
126.4(2)
107.8(2)
O71–Zn2–O81
O71–Zn2–N51
O81–Zn2–N51
O71–Zn2–N61
O81–Zn2–N61
N51–Zn2–N61
O72–C70–O71
O72–C70–C71
O71–C70–C71
O82–C80–O81
O82–C80–C81
O81–C80–C81
C66–N62–C62
O61–C66–O62
O61–C66–N62
O62–C66–N62
C66–O62–C67
C56–O52–C57
C56–N52–C52
O51–C56–O52
O51–C56–N52
O52–C56–N52
109.09(6)
118.51(7)
111.96(6)
111.62(6)
94.92(6)
108.24(6)
123.7(2)
120.1(2)
116.2(2)
124.1(2)
120.6(2)
115.2(2)
127.1(2)
126.0(2)
126.0(2)
108.0(2)
114.9(2)
114.3(2)
126.3(2)
125.2(2)
127.4(2)
107.4(2)
py ¼ pyridine, bppz ¼ 2,5-bis(2-pyridyl)pyrazine) [41,
42]. However, to our knowledge, the structure presented
is the first where a trinuclear array of zinc ions is formed
by aromatic carboxylate species. Such structures formed
by benzoate anions were reported with cobalt [43], but
not with zinc. Although Clegg et al. [44] successfully
synthesised the complex [Zn2Co(benz)6(quinoline)2],
which on the basis of spectral data has probably a
similar structure, however, the structure of this complex
was not solved.
The Zn–O bonds in carboxylate ligands of complex I
ꢂ
range from 1.933(1) to 2.201(1) A and, on an average,
they are somewhat longer than values observed for a
series of structurally characterised zinc benzoate com-
plexes [32–36,45,46]. Four oxygen atoms of the biden-
tate carboxylate groups form an equatorial plane of the
octahedron around Zn1, with distances 2 ꢂ 2.050(1) and
ꢂ
2 ꢂ 2.061(1) A, respectively. Apical positions of the oc-
tahedron are occupied by oxygen atoms from mon-
ꢂ
atomic benzoates (Zn1–O21 distances 2 ꢂ 2.201(1) A).
The angles in this octahedron differ only slightly from
the ideal 90° value (see Table 2). The tetrahedron
around the terminal zinc atoms is formed by two
oxygen atoms of two bidentate benzoate groups
ꢂ
(Zn2–O31 ¼ 1.933(1) and Zn2–O11 ¼ 1.962(1) A, re-
spectively), one oxygen atom of monatomic benzoate
ꢂ
(Zn2–O21 ¼ 1.996(1) A) and the nitrogen atom of
the pyridine ring of nicotinamide (Zn2–N1 ¼ 2.023(1)).
The tetrahedron is slightly deformed, due to the mon-
atomic binding mode of the carboxylate group and weak

2054
V. Zeleꢀnꢁak et al. / Inorganica Chimica Acta 357 (2004) 2049–2059
Fig. 1. The structure of the complex [Zn3(benz)6(nia)2].
interaction of the non-bonded carboxylate oxygen with
ꢂ
dependent [Zn(benz)2(mpcm)2] molecules differing
strongly in the arrangement of the ligand molecules. A
view of the structure is given in Fig. 3. The bond lengths
and angles in both molecules differ only slightly and they
are summarised in Table 3.
the zinc atom (Zn2ꢀ ꢀ ꢀO22 ¼ 2.780(1) A). The inter-
ꢂ
atomic Znꢀ ꢀ ꢀZn distance (3.1845(2) A) is somewhat
shorter than Znꢀ ꢀ ꢀZn distance in trinuclear complexes
formed by aliphatic carboxylates [40–42].
The trinuclear molecules are linked by two sets of
intermolecular N–Hꢀ ꢀ ꢀO hydrogen bonds. In the first of
these sets, hydrogen bonds are formed between the hy-
drogen atom of the amino group of nicotinamide and
uncoordinated carboxylate oxygen O22 of monatomic
carboxylate bridge. This set of hydrogen bonds connects
the trinuclear molecules into chains in the c-axis direc-
tion. The second set of hydrogen bonds is formed be-
tween the second hydrogen of the amino group of
nicotinamide and an oxygen atom O1 of an adjacent
nicotinamide molecule. This set of hydrogen bonds is
running along the a-axis. Considering both sets of hy-
drogen bonds, trinuclear molecules are interconnected
forming a plane parallel with the ac-plane (Fig. 2(a)).
However, this plane is not ‘‘flat’’ but possesses sinusoi-
dal-like shape (Fig. 2(b)). No hydrogen bonds were
found between such planes in the b-axis direction. The
important parameters of the hydrogen bonding geome-
try are listed in Table 4.
The structure of the complex is molecular. The zinc
ion is tetrahedrally coordinated by two monodentate
benzoate anions and two neutral methyl-3-pyridylcar-
bamate molecules. The tetrahedron around the zinc
atom is distorted due to the presence of a weak inter-
action between the uncoordinated carboxylate oxygens
and the zinc atoms. The maximum deviations from the
ideal value of 109°280 have the angles O31–Zn1–N11
and O81–Zn2–N51 of 95.56(6)° and 94.92(6)°, respec-
tively. The distances of the uncoordinated oxygens from
ꢂ
the zinc atoms vary from 2.847(1) to 2.946(1) A. These
distances are longer than the distances of non-bonded
carboxylate oxygen in complex I and are comparable to
the other zinc(II) benzoate complexes with monodentate
coordination of the benzoate group and containing an
additional organic ligand [46–48]. In each of these two
crystallographically independent molecules, the Zn–O
distances are not the same (see Table 3). A similar be-
haviour can be observed in the Zn–N bond lengths. This
is a consequence of the hydrogen bonding geometry
discussed below. The bond lengths in the methyl-3-
pyridylcarbamate molecule are within the distances ob-
served for similar complexes containing this ligand
[49,50].
3.2.2. Complex [Zn(benz)2(mpcm)2] (II)
The complex crystallises in an orthorhombic space
group P212121 with eight formula units in the unit cell.
The formula unit contains two crystallographically in-

V. Zeleꢀnꢁak et al. / Inorganica Chimica Acta 357 (2004) 2049–2059
2055
Fig. 2. Hydrogen bonding geometry in the complex [Zn3(benz)6(nia)2]. Zinc atoms are displayed as black spheres, oxygen atom as dashed and
nitrogen atoms as grey spheres. (a) View of two-dimensional network parallel with ac-plane. Benzene rings are omitted for clarity purposes. (b) View
along a-axis displaying sinusoidal-like shape of the formed planes.
The molecules are held together by hydrogen bonds
formed by the hydrogen atoms of the imino groups in
the methyl-3-pyridylcarbamate ligands and non-coordi-
nated oxygens of the benzoate anions. These bonds run
along the a and b axes forming a two-dimensional
network parallel to the ab-plane and connect as well
crystallographically same and crystallographically in-
dependent molecules. No hydrogen bonds were found
between these planes, in the c-direction. These hydrogen
bonds significantly influence the Zn–O and Zn–N bond
lengths in both molecules. If the hydrogen atom and
oxygen atom, which form the intermolecular hydrogen
bond, are in the crystallographically same molecules (see
bonds N12–H12ꢀ ꢀ ꢀO42i and N62–H62ꢀ ꢀ ꢀO72ii in Table
4), the Zn–N distances have longer values (Zn1–
ꢂ
N11 ¼ 2.052(1) and Zn2–N61 ¼ 2.052(1) A, respectively)

2056
V. Zeleꢀnꢁak et al. / Inorganica Chimica Acta 357 (2004) 2049–2059
Table 4
Intermolecular hydrogen bonds in the complexes (I) and (II)
ꢂ
D–H (A)
ꢂ
ꢂ
D–H
Hꢀ ꢀ ꢀA (A)
DHA (°)
Dꢀ ꢀ ꢀA (A)
A
Complex I
N12–H12B
N12–H12A
0.85(2)
0.88(2)
2.07(2)
1.97(2)
159(2)
173(2)
2.880(2)
2.847(2)
O1 [x ꢁ 1=2; y; ꢁz þ 1=2]
O22 [x ꢁ 1=2; y; ꢁz þ 1=2]
Complex II
N12–H12
N22–H22
N52–H52
N62–H62
0.880
0.879
0.879
0.880
2.04
1.99
1.96
2.04
160
165
164
168
2.887(2)
2.845(2)
2.820(2)
2.905(2)
O42 [1=2 þ x; 5=2 ꢁ y; ꢁz]
O82
O32
O72 [ꢁ1=2 þ x; 3=2 ꢁ y; ꢁz]
and the Zn–O distances have shorter values (Zn1–
ꢂ
For the IR spectra of carboxylates, the stretching
vibrations of the carboxylate group mas(COO) and
ms(COO) are characteristic. In the studied complexes,
their assignment has to be made carefully due to the
presence of aromatic rings in both benzoate and N-do-
nor organic ligands, whose stretching vibrations fall in
the same region of the spectrum as the stretching vi-
brations of the carboxylate group. Therefore, we have
compared the spectra of these complexes with the data
presented in the literature for related complexes [34,44]
as well as with the spectrum of zinc benzoate, nicotin-
amide and methyl-3-pyridylcarbamate. The anti-sym-
metric carboxylate stretches mas(COO) were found at
1630, 1568 cmꢁ1 in complex I and at 1610 cmꢁ1 in
complex II. The symmetric ms(COO) stretches were
found at 1448 and 1401 cmꢁ1 in complex I and 1427
cmꢁ1 in complex II. In accordance with the literature
data, we suppose that the bands at 1630 and 1448 cmꢁ1
in complex I are due to anti-symmetric and symmetric
stretching vibrations of the two monodentate benzoate
O41 ¼ 1.935(1) and Zn2–O71 ¼ 1.936(1) A, respectively)
than in the case when the intermolecular hydrogen
bonds (N22–H22ꢀ ꢀ ꢀO82 and N52–H52ꢀ ꢀ ꢀO32 in Table
4) are formed between crystallographically independent
molecules. The details of this hydrogen bonding geom-
etry are shown in Fig. 4 and are summarised in Table 4.
3.3. Spectroscopic studies
The IR data of the complexes are listed in Section 2.
In the spectra of both complexes, the stretching vibra-
tions of N–H and C–H groups are observed in the re-
gion above 3000 cmꢁ1. The stretching vibrations of
aliphatic C–H groups of methyl-3-pyridylcarbamate are
at 2952 cmꢁ1. Moreover, the presence of N-donor or-
ganic ligands of nicotinamide and methyl-3-pyridylcar-
bamate is evident from the presence of strong
absorption bands of the carbonyl m(C@O) vibration at
1670 and 1732 cmꢁ1, respectively.
Fig. 3. The two crystallographically independent molecules of [Zn(benz)2(mpcm)2].

V. Zeleꢀnꢁak et al. / Inorganica Chimica Acta 357 (2004) 2049–2059
2057
Fig. 4. Two-dimensional network formed by hydrogen bonds (dashed lines) in the complex [Zn(benz)2(mpcm)2]. View along the c-crystallographic
axis. The zinc atoms in crystallographically independent molecules are displayed as black and hatched spheres, respectively. The other atoms in the
crystallographically independent molecules are displayed as grey and white spheres, respectively. The benzene rings as well as hydrogen atoms not
involved in the intermolecular hydrogen bonds are omitted for clarity.
groups and the bands at 1568 and 1401 cmꢁ1 are due to
stretches of the four syn–syn bidentate benzoate ligands
[34,44]. It is important to notice the trend of the change
of the values of the carboxylate stretches in both com-
plexes. The lowest values of mas(COO) and ms(COO)
(1568 and 1401 cmꢁ1, respectively) are observed for syn–
syn benzoate groups in complex I, in which the similar
distribution of electron density is expected for both C–O
bonds. These values are higher in complex II (1610 and
1427 cmꢁ1), where only monodentate carboxylate co-
ordination occurs and the electron density distribution
in both C–O bonds is different. As a consequence, the
frequencies of the carboxylate stretches are shifted to
higher values, closer to the region of m(C@O) vibration
in free carboxylic acids. Finally, the vibrations of
monodentate benzoate groups occur at the highest val-
ues (1630 and 1448 cmꢁ1, respectively). The coordinated
oxygen atom is simultaneously bonded to two zinc ions,
which probably leads to the higher electron withdraw-
ing from unbounded oxygen and to the shift in
Fig. 5. TG/DTG curves of the complexes [Zn3(benz)6(nia)2] (curve 1)
and [Zn(benz)2(mpcm)2] (curve 2) during heating in N2.

2058
V. Zeleꢀnꢁak et al. / Inorganica Chimica Acta 357 (2004) 2049–2059
anti-symmetric as well symmetric vibrations of the car-
boxylate group.
In the region below 1300 cmꢁ1, a number of bands
due to deformation vibrations of C–H and N–H groups
as well as vibrations of aromatic ring were observed and
respective values are listed in Section 2.
The thermal decomposition of the sample
[Zn(benz)2(mpcm)2] significantly differs from complex I
described above. The first striking difference can be
found already by visual comparison of the solid resi-
dues. Instead of a white zinc oxide observed after the
thermal analysis of the complex [Zn3(benz)6(nia)2], a
black powder was found after the thermal analysis of
[Zn(benz)2(mpcm)2]. The powder XRD showed the
3.4. Thermal studies
2
presence of zinc as well as an amorphous component,
The results of the thermogravimetric analysis are
presented in Fig. 5. The thermal decomposition of the
complex [Zn3(benz)6(nia)2] well reflects its crystal
structure. The sample is thermally stable up to 165 °C.
Above this temperature, in the range 165–210 °C, two
molecules of nicotinamide are evolved (calc. mass loss
20.9%, observed 20%). This decomposition takes place
in one step, which is in accordance with the symmetry of
the complex and the equivalence of the nicotinamide
molecules through the inversion centre. Further heating
leads to the decomposition of the zinc(II) benzoate,
which takes place in two different steps. In the first step
in the range 210–305 °C, decomposition of two benzoate
anions takes place and we suppose that the two mono-
dentately bonded bridging benzoate ligands are evolved
in this step. Heating above 320 °C leads to very rapid
and complete pyrolysis of the rest of the organic part of
the molecule, i.e., decomposition of four syn–syn ben-
zoate bridges. The white colour of the final solid residue
was in accordance with the results of XRD, which
which we suppose to be carbon. The thermogravimetric
curves of the thermal decomposition of this complex are
shown in Fig. 5. The complex is thermally stable up to
130 °C. We suppose that in the first step the benzoate
ligands are broken in the temperature range 130–200 °C.
The temperature 130 °C is approximately 80 °C lower
than the decomposition temperature of the benzoate li-
gands in complex I. We suppose that this is due to the
fact that in complex I each of the benzoate ligands is
bonded to zinc atoms by two bonds, instead of one bond
formed by the monodentate coordination in complex II.
Moreover, considering the composition of the solid
residue, we suppose that no fracture of C–O bonds took
place. The calculated weight loss, considering the release
of two benzoate ligands, is 39.6% and observed weight
loss in the range 130–200 °C is 37%. The release and
pyrolysis of methyl-3-pyridylcarbamate ligands starts at
200 °C. This temperature is similar to that observed by
Mojumdar et al. [52] for the release of methyl-3-pyr-
idylcarbamate from magnesium complexes. The pyro-
lysis of mpcm ligands took place in two steps in the
temperature ranges 200–310 and 310–450 °C. However,
the release of the ligands was not complete and some
carbonisation occurred during the pyrolysis. Due to this
carbonisation, the amount of solid residue (22 wt%) is
higher than the value calculated for zinc or zinc oxide
(10.7% and 13.3%, respectively).
1
confirmed a hexagonal wurtzite structure. The careful
inspection of the weight losses during the last two
thermal decomposition steps enabled us to make some
further conclusions about the decomposition of benzo-
ate ligands and formation of final solid residue. From
the observed weight loss in the range 210–305 °C, we
suppose that in the monatomic benzoate bridges fracture
of C–O bonds between carbon atoms of carboxylate
group and bridging oxygen atoms took place to form
two free radicals C6H5C(O)ꢀ, which were liberated and
probably further reacted to form benzil C6H5CO-
COC6H5 (calc. mass loss 18.1%, observed 18%). Such
formation of radicals was observed also by Sun et al.
[51] in zinc phthalate. The two oxide atoms formed by
this fracture remained bonded to zinc atoms and used in
zinc oxide formation found in the final solid residue.
Similar fracture of one of the four syn–syn carboxylate
bridges also probably took place in the final decompo-
sition step above 320 °C, while the other three syn–syn
benzoate ligands decompose by another mechanism and
we suppose that they are liberated entirely. The calcu-
lated weight loss considering this proposal is 40.1% and
observed weight loss was 42%. The amount of the final
solid residue (20%) is in accordance with the value
20.9% calculated for ZnO.
4. Supplementary material
Complete crystallographic data have been deposited
with the Cambridge Crystallographic Data Centre as
supplementary publication Nos. CCDC 211577 (com-
plex I) and CCDC 211576 (complex II), respectively.
These data can be obtained free of charge via
www.ccdc.cam.ac.uk/conts/retrieving.html (or from the
CCDC, 12 Union Road, Cambridge CB2 1EZ, UK; fax:
+44 1223 336033; e-mail: deposit@ccdc.cam.ac.uk).
Acknowledgements
This work was supported by the Grant Agency
VEGA of the Slovak Ministry of Education (Grant No.
1
2
ICDD Card No. 36-1451.
ICDD Card No. 4-0831.

V. Zeleꢀnꢁak et al. / Inorganica Chimica Acta 357 (2004) 2049–2059
2059
[26] E.J. van Hoogdalem, J. Eur. Acad. Dermatol. Venereol. 11 (1998)
S13.
ꢃ
1/9247/02). One of the authors (V.Z.) thanks ‘‘Ministere
delegue a la Recherche et aux Nouvelles Technologies’’
of the French republic for the financial support during
his stay in laboratory Madirel in the frame of the pro-
ꢁ ꢁ ꢁ ꢃ
ꢀꢁ €
[27] (a) For example: V. Zelenak, K. Gyoryova, J. Simon, J. Therm.
ꢁ
Anal. 46 (1996) 573;
ꢀꢁ ꢁ ꢁ ꢀ ꢁ
(b) V. Zelenak, K. Gyoryova, I. Cısarova, J. Loub, Acta
€
Crystallogr., Sect. C 52 (1996) 1917;
ꢁ
gramme ‘‘Accueil de jeunes chercheurs etrangers en
ꢀ
(c) J. Skorsepa, K. Gyoryova, M. Melnık, K. Smolander, M.
Ahlgren, Acta Crystallogr., Sect. C 51 (1995) 1069;
€
ꢁ
ꢁ
ꢁ
sejour de recherche post-doctorale’’.
ꢀꢁ
(d) V. Zelenak, K. Gyoryova, D. Mlynarcık, Metal Based Drugs 8
(2002) 269.
€
ꢁ
ꢀꢁ
References
[28] G.M. Sheldrick, SHELXTL 5.03, Siemens Analytical X-ray Instru-
ments, Inc., Madison, WI, 1996.
[29] G.M. Sheldrick, SHELXS 97, Program for the Solution of Crystal
[1] P. Emery, S.X. Kong, E.W. Ehrich, D.J. Watson, T.E. Towheed,
Clin. Ther. 24 (2002) 1225.
€
Structures, University of Gottingen, Gottingen, Germany, 1997.
[30] G.M. Sheldrick, SHELXL 97, Program for the Refinement of
€
[2] J.R.J. Sorenson, in: H. Sigel (Ed.), Metal Ions in Biological
Systems, vol. 14, Marcel Dekker, New York, 1982, p. 77.
[3] C. Dendrinou-Samara, G. Tsotsou, L.V. Ekateriniadou, A.H.
Kortsaris, C.P. Raptopoulou, A. Terzis, D.A. Kyriakidis, D.P.
Kessissoglou, J. Inorg. Biochem. 71 (1998) 171.
€ €
Crystal Structures, University of Gottingen, Gottingen, Germany,
1997.
[31] K. Brandenburg, DIAMOND 2.1d, Crystal Impact GbR, Bonn,
2000.
[4] K.M. Stuhlmeier, H. Li, J.J. Kao, Biochem. Pharmacol. 57 (1999)
313.
[32] W. Clegg, D.R. Harbron, P.A. Hunt, I.R. Little, B.P. Straughan,
Acta Crystallogr., Sect. C 46 (1990) 750.
[5] (a) K.B. Diehl, Am. Fam. Physician 54 (1996) 1687;
(b) D. Russell, A.D. Russell, J. Infect. 24 (1992) 333.
[6] S.C. Rastogi, J.P. Lepoittevin, J.D. Johansen, P.J. Frosch, T.
[33] W. Clegg, D.R. Harbron, B.P. Straughan, J. Crystallogr. Spectr.
Res. 20 (1990) 17.
ꢁ
Menne, M. Bruze, B. Dreier, K.E. Andersen, I.R. White, Contact
Dermatitis 39 (1998) 293.
[34] W. Clegg, D.R. Harbron, Ch.D. Homan, P.A. Hunt, I.R. Little,
B.P. Straughan, Inorg. Chim. Acta 186 (1991) 51.
[35] Y. Nakacho, T. Misawa, T. Fujiwara, A. Wakahara, K. Tomita,
Bull. Chem. Soc. Jpn. 49 (1976) 58.
[7] E. Sainio, L. Kanerva, Contact Dermatitis 33 (1995) 100.
[8] M. Hanssen, E for additives, Thorsons, HarperCollins Publishers,
UK, 1987.
[36] G.A. Gusejnov, F.N. Musaev, B.T. Usubaliev, I.R. Amiraslanov,
Ch.S. Mamedov, Koord. Khim. 10 (1984) 117.
[9] L.L. Dock, J.D. Floros, R.H. Linton, J. Food Prot. 63 (2000)
1026.
ꢁ
ꢁꢀ
ꢁ
[37] M. Melnık, L. Macaskova, C.E. Holloway, J. Mrozinski, B.
Kalinska, Inorg. Chim. Acta 299 (2000) 284.
[10] M. Islam, J. Chen, M.P. Doyle, M. Chinnan, J. Food Prot. 65
(2002) 1411.
ꢁ
ꢁ
ꢁ
[38] M. Melnık, M. Koman, D. Hudecova, J. MoncolÕ, B. Hudova, T.
Glowiak, J. Mrozinky, C.E. Holloway, Inorg. Chim. Acta 308
(2000) 1.
[11] European Union Directive 95/2/CE from 20.02.1995 on food
additives, colourants and sweeteners. European Commission.
[12] World Health Organisation. International Programme on Chem-
ical Safety. Benzoic acid and sodium benzoate. Concise Interna-
tional Chemical Assessment Document No. 26. Geneva, 2000.
[13] J.R.J. Sorenson, V. Kishore, A. Pezeshk, L.W. Oberley, S.W.C.
Leuthauser, T.D. Oberley, Inorg. Chim. Acta 91 (1984) 285.
[14] Z.H. Chohan, S.K.A. Sherazi, Metal-Based Drugs 4 (1997) 327.
[15] J.R. Chipley, Sodium benzoate and benzoic acid, in: A.L. Branen,
P.M. Davidson (Eds.), Antimicrobials in Foods, Marcel Decker,
New York, NY, 1983, pp. 11–35.
[39] W. Clegg, I.R. Little, B.P. Straughan, Inorg. Chem. 27 (1988)
1916.
[40] W. Clegg, I.R. Little, B.P. Straughan, J. Chem. Soc., Dalton
Trans. (1986) 1283.
[41] B. Singh, J.R. Long, F.F. de Biani, D. Gatteschi, P. Stavropoulos,
J. Am. Chem. Soc. 119 (1997) 7030.
[42] A. Neels, H. Stoeckli-Evans, Inorg. Chem. 38 (1999) 6164.
[43] J. Catterick, P. Thornton, J. Chem. Soc., Dalton Trans. (1976)
1634.
[44] W. Clegg, P.A. Hunt, B.P. Straughan, M.A. Mendiola, J. Chem.
Soc., Dalton Trans. (1989) 1127.
[16] B.L. Vallee, D.S. Auld, Biochemistry 24 (1990) 5647.
[17] E. Kimura, T. Koike, Adv. Inorg. Chem. 44 (1997) 229.
[18] W. Kaim, B. Schwederski, Bioinorganic Chemistry: Inorganic
Elements in the Chemistry of Life, Wiley, Chichester, England,
1994.
ꢀ
[45] I. Potocnak, M. Dunaj-Jurco, J. Cernak, Acta Crystallogr., Sect.
ꢀꢀꢁ
C 49 (1993) 1496.
ꢀ
ꢁ
ꢀ
[46] J. Cernak, I. Adzimova, F. Gerard, A.M. Hardy, Acta Crystal-
ꢁ
ꢁ
logr., Sect. C 51 (1995) 392.
[19] B.D. Korant, J.C. Kauer, B.F. Butterworth, Nature 248 (1974)
588.
[20] E. De Clercq, Metal-Based Drugs 4 (1997) 173.
[47] S.S.S. Raj, H.K. Fun, P.S. Zhao, F.F. Jian, D. Lu, X.J. Yang, X.
Wang, Acta Crystallogr., Sect. C 56 (2000) 742.
[48] M.R. Sundberg, Acta Chem. Scand. 48 (1994) 195.
ꢁ
ꢁ
~
[21] B. Simo, L. Perello, R. Ortiz, A. Castineiras, J. Latorre, E.
ꢀ
ꢁ
ꢁ
ꢁ
ꢁ
[49] J. MoncolÕ, M. Koman, M. Melnık, M. Cernakova, T. Glowiak,
Polyhedron 19 (2000) 2573.
Canton, J. Inorg. Biochem. 81 (2000) 275.
[22] O. Yamamoto, Int. J. Inorg. Mater. 3 (2001) 643.
[23] Y.H. Cho, S.J. Lee, J.Y. Lee, S.W. Kim, Ch.B. Lee, W.Y. Lee,
M.S. Yoon, Int. J. Antimicrob. Agents 19 (2002) 576.
[24] S.C. Cunnane, Zinc: Clinical and Biochemical Significance, CRC
Press, Boca Raton, FL, 1988.
ꢁ
[50] M. Koman, M. Melnık, T. Glowiak, Acta Crystallogr., Sect. C 54
(1998) 1604.
[51] J. Sun, L. Yuan, K. Zhang, D. Wang, Thermochim. Acta 343
(2000) 105.
ꢁ
[52] S.C. Mojumdar, M. Melnık, E. Jona, Thermochim. Acta 352–353
ꢁ
[25] C.L. Feucht, B.S. Allen, D.K. Chalker, J. Am. Acad. Dermatol. 3
(1980) 483.
(2000) 127.