Zinc(II)di(o-hydroxybenzoate) complexes with imidazole and its derivatives. Synthesis, thermal characterization and molecular structures

Article abstract of DOI:10.1023/B:JTAN.0000011021.41412.4a

Mixed complexes of the type: Zn(Hsal)₂(Him)₂, Zn(Hsal)₂(Him)₅, Zn(Hsal)₂(4-MeHim) ₂ and Zn(Hsal)₂(l,2-diMeim)₂ (where Hsal=OHC₆H₄COO^-,

Full text of DOI:10.1023/B:JTAN.0000011021.41412.4a

Journal of Thermal Analysis and Calorimetry, Vol. 74 (2003) 895–904
ZINC(II)DI(o-HYDROXYBENZOATE) COMPLEXES
WITH IMIDAZOLE AND ITS DERIVATIVES
Synthesis, thermal characterization and molecular structures
M. Olczak-Kobza*, R. Czylkowski and J. Karolak-Wojciechowska
Institute of General and Ecological Chemistry, Technical University, 90–924 ˜ódü, Poland
(Received November 29, 2002; in revised form July 22, 2003)
Abstract
Mixed complexes of the type: Zn(Hsal)2(Him)2, Zn(Hsal)2(Him)5, Zn(Hsal)2(4–MeHim)2 and
Zn(Hsal)2(1,2-diMeim)2 (where Hsal=OHC6H4COO–, Him=imidazole, 4-MeHim=4-methylimidazole,
1,2-diMeim=1,2-dimethylimidazole) have been synthesized. The application of chemical, thermal and
X-ray methods has enabled us to analyze the complexes and their sinters. The gaseous products of py-
rolysis of Zn(Hsal)2(Him)2 and Zn(Hsal)2(4–MeHim)2 have been investigated. Thermal decomposi-
tion pathways have been postulated for synthesized complexes. The molecular structures of the
Zn(Hsal)2(Him)2 and Zn(Hsal)2(1,2-diMeim)2 have been solved.
Keywords: heteroligand complexes, o-hydroxybenzoic acid, thermal analysis, zinc(II), X-ray
structures
Introduction
The present work is a continuation of our studies on the synthesis and properties of
zinc(II) and cadmium(II) heteroligand complexes. Organic ligands of the H2L type (such
as: o-hydroxybenzoic acid, o-aminobenzoic acid, o-hydroxybenzaldoxime) and bivalent
metals form either di-complexes M(HL)2 or mono-complexes ML (where M=metal ion,
HL=OHC6H4COO–, NH2C6H4COO–, OC6H4CHNOH–, L=OC6H4COO2–, NHC6H4COO2–,
OC6H4CHNO2–). The mono-compounds may bind to the other monodentate lig-
ands [1, 2] as well as to bidentate [3–5], whereas the di-compounds – only to mono-
dentate ones [4].
The complexes of zinc(II) with imidazole and its derivatives have been investi-
gated in the solid-state as well as in the solution for a long time now [6–8]. In the mol-
ecule of imidazole, nitrogen N3 occupies a preference place of the coordination and
the property of coordination is determined by the type and position of the substituent
in the ring. The methyl group decreases the complexing power in the following order:
imidazole>4-methylimidazole>1,2-dimethylimidazole [7].
*
Author for correspondence: E-mail: wkobza@p.lodz.pl
1388–6150/2003/ $ 20.00
Akadémiai Kiadó, Budapest
© 2003 Akadémiai Kiadó, Budapest
Kluwer Academic Publishers, Dordrecht

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Zinc(II) forms simple as well as mixed complexes with imidazole (or its deriva-
tives) and e.g. with aliphatic and amino acid [9–12]. In analogous surroundings, zinc
occurs in important biological molecules (metalloproteins and metalloenzymes) and,
therefore, complexes containing such ligands are frequently researched by means of
the spectroscopic, structural, and thermal investigation [11–13].
The investigations of these complexes in solution provide the important informa-
tion about properties of biologically active molecules and the investigations in the
solid-state are complemented. Recently, we have prepared new zinc complexes with
aromatic acids and imidazoles of general formulas: Zn(Hsal)2(Him)2, Zn(Hsal)2(Him)5,
Zn(Hsal)2(4-MeHim)2 and Zn(Hsal)2(1,2-diMeim)2. Structure and thermal characteris-
tic of these complexes have been determined.
Experimental
Materials
P.a.: ZnCO3, o-hydroxybenzoic acid, ethanol (POCh – Gliwice), imidazole (Fluka
Chemie AG), 4-methylimidazole, 1,2-dimethylimidazole (Sigma) were used without
additional purification.
Preparation
Zinc di(o-hydroxybenzoate) was obtained according to the method described in liter-
ature [14].
Table 1 Results of chemical analysis
Zn/%
C/%
found
H/%
N/%
found
Compound
calc. found calc.
calc. found calc.
Zn(Hsal)2(Him)2
13.81
9.62
13.8 50.72
9.6 51.21
12.5 52.68
12.3 54.22
49.7
51.1
51.1
53.7
–
3.38
4.41
3.98
4.89
–
3.8
4.4
4.7
4.9
–
11.82
20.60
11.12
10.54
28.06
24.62
9.41
11.5
20.0
10.9
10.1
24.1
19.3
7.7
Zn(Hsal)2(Him)5
Zn(Hsal)2(4-MeHim)2
12.98
Zn(Hsal)2(1,2-diMeim)2 12.30
a
Zn(Him)2
32.76
27.70
22.10
32.0
–
a
Zn(4-MeHim)2
Zn(sal)(1,2-diMeim)a
28.4 42.21
22.5 48.42
44.5
47.6
4.40
4.03
4.5
4.3
Zn(Hsal)2(Him)2 (a); Zn(Hsal)2(Him)5 (b); Zn(Hsal)2(4-MeHim)2 (c). A mixture
of ZnCO3, H2sal, and imidazole or 4-methylimidazole in absolute ethanol was heated to
333 K, using magnetic stirring. The mixture was allowed to reflux for two hours (com-
pound a and b) or for ten hours (compound c) and then the hot mixture was filtered. The
obtained solutions were left to crystallize. The white precipitates were washed with eth-
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anol (a, b). When the synthesis of (c) was carried out, oil was obtained. The oil was
treated 5-fold with ether and the brownish precipitate was obtained. The reactions were
carried out at different molar ratios: 1:2:3 (a), 1:2:5 (b) and 1:2:2 (c).
Zn(Hsal)2(1,2-diMeim)2: Zn(Hsal)2⋅2H2O was dissolved in a water solution of
1,2-dimethylimidazole (molar ratio 1:2). This procedure yielded a white precipitate.
It was washed with water and dried at room temperature.
Chemical analysis
The zinc(II) was detected by complexometric titration with EDTA [15], while carbon,
hydrogen, and nitrogen – by elemental analysis. The results are presented in Table 1.
X-ray analysis
Powder analysis
The X-ray analysis was carried out by means of a Siemens D 5000 powder diffrac-
tometer, using CuKa radiation, 2θ range 2–80°. The products of synthesis and the sinters,
formed as a result of thermal decomposition, were studied using X-ray method. Figure 1
presents powder diffraction patterns of Zn(Hsal)2(Him)2 and its sinters.
Fig. 1 X-ray analysis of Zn(Hsal)2(Him)2 complex: a – before sintering, b – after
sintering at 653 K and c – after sintering at 973 K
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X-ray structure analysis
Colorless crystals, suitable for X-ray experiment, were obtained only for
Zn(Hsal)2(Him)2 and Zn(Hsal)2(1,2-diMeim)2 by slow evaporation of ethanol solu-
tions. At first, both structures were solved based on X-ray experiment with CuKα radia-
tion (KM4 four-cycle diffractometer) and no absorption correction. MoKα radiation
was also used for Zn(Hsal)2(1,2-diMeim)2 analysis (KM4 four-cycle diffractometer
with CCD camera). The structures were solved by direct methods using
SHELXS97 [16] and refined with all data on F2 using SHELXL97 [17]. All details of
X-ray structure analysis are summarized in Table 2.
Table 2 Crystal data and summary of intensity data collection and structure refinement
Zn(Hsal)2(1,2-diMeim)2
Zn(Hsal)2(Him)2
1
ZnC24H26N4O6
531.86
2
ZnC24H26N4O6
531.86
Empirical formula
Formula mass
Temperature/K
Wavelength/
Crystal system
Space group
a/
ZnC20H18N4O6
475.75
293(2)
293(2)
298(2)
1.54178
tetragonal
P 41212
7.871(1)
7.871(1)
33.243(7)
2059.5(6)
4
1.54178
orthorhombic
P 212121
8.030(2)
14.943(39
20.863(4)
2503.4(9)
4
0.71073
orthorhombic
P 212121
8.0110(16)
14.919(3)
20.837(4)
2490.4(9)
4
b/
c/
3
Volume/
Z
Calculated density/mg m–3
Absorption coefficient
F(000)
1.534
1.411
1.419
2.062
1.755
1.032
976
1104
1104
θ range/°
5.32 to 80.26
to 80.26
3.24 to 22.99
–10<h<10
–9<k<0
0<l<42
0<h<9
0<k<19
0<l<26
–8<h<7
–16<k<16
–22<l<22
Index ranges
Reflections collected/unique
Refinement method
4517/2254
F2
2906/2881
F2
13755/2789
F2
Data/restraints/parameters
Goodness-of-fit on F2
R1 indices [I>2σ(I)]
2254/0/143
1.156
2881/0/319
1.535
2789/0/317
1.130
0.0549
0.0415
0.0711
wR2
0.1209
0.4489
0.1181
–3
Largest difference peak and hole/e 
0.973 and –1.179
3.36 and –0.69
0.351 and –0.496
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Fig. 2 Thermal curves of complexes
Thermal studies
The thermoanalytical measurements were carried out using
a
Balzers
TG/DTA–SETSYS–16/18 thermoanalyzer with a Balzers mass spectrometer. The
temperature range 293–1273 K; the heating rate 10 K min–1, in air atmosphere; the
sample mass <10 mg. The sinters were obtained on the basis of the TG curves. XRD
and elemental analyses of the sinters were carried out (Table 3). The gaseous decom-
position products of Zn(Hsal)2(Him)2 and Zn(Hsal)2(4-MeHim)2 complexes were an-
alyzed. Figure
Zn(Hsal)2(4-MeHim)2 as an example.
2
presents thermal curves of Zn(Hsal)2(Him)2 and
Results and discussion
The heteroligand complexes: Zn(Hsal)2(Him)2, Zn(Hsal)2(Him)5, Zn(Hsal)2(4-MeHim)2,
and Zn(Hsal)2(1,2-diMeim)2 were obtained in the reaction of zinc salts with imidazole or
its derivatives. Depending on the molar ratio of the reagents, two different mixed com-
plexes were obtained for imidazole. The analysis of powder diffraction patterns showed
that the three complexes have the crystalline structures, while 4-methylimidazole com-
plex is amorphous.
X-ray structure analyses of Zn(Hsal)2(Him)2 and Zn(Hsal)2(1,2-diMeim)2 were
conducted. Figure 3 shows the molecular units of the compounds Zn(Hsal)2(Him)2 and
Zn(Hsal)2(1,2-diMeim)2. All selected geometrical details are gathered in Table 4. It
should be mentioned that geometrical parameters for the structures of Zn(Hsal)2(1,2-
diMeim)2, obtained by two methods, are comparable. However, the structure deter-
mined based on experiment with MoKα radiation has better validation (Table 2). There-
fore, in Table 4 and in subsequent discussion only the values obtained based on these
data will be used. As it is clear from the Fig. 3, Zn(Hsal)2(Him)2 and Zn(Hsal)2(1,2-
diMeim)2 consist of a Zn polyhedra (here tetrahedron). In both solved structures, the Zn
ion is surrounded by two carboxylate groups, which are monodentate, and two
imidazole or 1,2-dimethylimidazole molecules (Fig. 3). Hence, for full tetrahedron de-
scription, the two Zn–O and two Zn–N distances are essential. Half of chemical unit of
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Fig. 3 Molecular structure of complexes: a – Zn(Hsal)2(Him)2 and b – Zn(Hsal)2(1,2-diMeim)2
Zn(Hsal)2(Him)2 is crystallographically independent. From the data collected in Ta-
ble 4, it is visible that Zn polyhedra coordination in two solved examples differs geo-
metrically. In the structure of Zn(Hsal)2(Him)2 the Zn–O distance is significantly lon-
ger than the Zn–N one. But in the structure of Zn(Hsal)2(1,2-diMeim)2 both Zn–N dis-
Table 4 Selected geometrical details for complexes. Bond length in  and all angles in (°)
Zn(Hsal)2(1,2-diMeim)2
Zn(Hsal)2(Him)2
1
2
Zn-N3
Zn-N3’
Zn-O2
Zn-O2’
Zn-O1
Zn-O1’
1.985(2)
1.985(2)
2.030(1)
2.030(1)
2.700
2.008(6)
1.978(6)
1.990(5)
1.958(5)
2.911
2.035(3)
1.993(3)
1.995(2)
1.959(2)
2.889
–
3.078
3.057
N3-Zn-N3’
N3-Zn-O2
N3-Zn-O2’
N3’-Zn-O2’
O2-Zn-O2’
O2-Zn-N3’
120.74(1)
104.90(7)
104.4(2)
109.4(2)
128.8(2)
106.3(2)
105.2(2)
98.7(2)
104.5(1)
109.3(1)
128.2(1)
106.5(1)
105.7(9)
98.9(1)
–
–
97.42(8)
–
Zn-O2-C1-C11
Zn-O2’-C1’-C11’
168.82(0.17)
–
179.0(5)
178.3(5)
–178.4(2)
–176.8(2)
O12-H12A...O1
O12’-H12B...O1’
1.703
–
1.841
1.785
1.757
1.783
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tances are longer than respective Zn–O ones. The ‘reversing’ of these distances lengths
reflects on the values of bond angles formed by Zn with respective O and/or N (Ta-
ble 4). The volume of substituted and unsubstituted imidazole differs significantly.
Therefore, space filled by two methyl substituents reflects on pulling up imidazole moi-
ety and enlarging the Zn–N distances in Zn(Hsal)2(1,2-diMeim)2. At the same time, ox-
ygen atoms are coming nearer to Zn to fill out added free space. It reflects on stretching
the torsion angles of Zn-O2-C1-C11 and on increasing the distances between Zn and
the second carboxy-oxygen in Zn(Hsal)2(1,2-diMeim)2 in comparison with the struc-
ture of Zn(Hsal)2(Him)2. Additionally, in Zn(Hsal)2(1,2-diMeim)2, the size of
1,2-dimethylimidazole ligand is responsible for the difference between two analogical
bonds Zn–N and two analogical bonds Zn–O. In both structures under discussion, crys-
tals include isolated chemical units. However, in such units two intramolecular hydro-
gen bonds O12-H12A....O1 and O12’-H12B....O1’ were identified. In the structure of
Zn(Hsal)2(Him)2, mentioned above H-bonds are stronger than those in
Zn(Hsal)2(1,2-diMeim)2 (Table 4).
The mass loss, the chemical and powder diffractometric analysis of the sinters,
endo- and exothermic effects indicate that the strength of bond between the metal and
ligand determines the course of thermal decompositions for all the complexes.
In the pyrolysis of the Zn(Hsal)2(Him)2 and Zn(Hsal)2(4-MeHim)2 two mole-
cules of o-hydroxybenzoic acid are lost at first. The chemical analysis indicated that
the sinters have formulae: Zn(im)2 and Zn(4-Meim)2. Their diffractograms are differ-
ent from diffractograms of initial compounds (e.g. Figs 1a and 1b). Inorganic com-
pound (ZnO) forms in the next stage (Fig. 1c). Thermal decomposition of the com-
pounds named above proceeds in accordance with reactions I and II:
Zn(Hsal)2(Him)2→Zn(im)2→ZnO
(I)
Zn(Hsal)2(4-MeHim)2→fusion process→Zn(4-Meim)2→ZnO
(II)
The analysis of the gaseous products of decomposition of Zn(Hsal)2(Him)2 con-
firmed the emission of the o-hydroxybenzoic acid molecules. The ionic current that
originates from emission of acid (molecular ion m/z 138), phenol, and benzene (frag-
ment ions m/z 92 and 78, 77, 52, 51, 50) increases at the first stage temperature range.
Since the amount of nitrogen in the 613 K sinters is equal to 24.1% (theor. in
Zn(im)2 – 28.06%), it is assumed that also some imidazole evaporates in the first
stage (fragment ion m/z 68). The complete decomposition of the Zn(im)2 takes place
in the second stage. Then, the gaseous products include mostly NO (m/z 30),
CO2 (m/z 44) and H2O (m/z 18). Analogous gaseous products are generated in the
case of decomposition of the 4-methylimidazole complex, and the ionic current from
emission of CH3 group does not increase till the second stage. This is confirmed by
the formation of Zn(4-Meim)2 in the stage after fusion process.
The decomposition of the Zn(Hsal)2(Him)5 complex proceeds in three stages.
The course of the thermal curves indicates that there is the endothermic peak at 353 K
and no mass loss in the first stage. In the second stage zinc complex decomposes to
Zn(im)2. The final product of the decomposition is zinc oxide (Table 3, reaction III).
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903
Zn(Hsal)2(Him)5→fusion process→Zn(im)2→ZnO
(III)
Also the pyrolysis of Zn(Hsal)2(1,2-diMeim)2 complex begins with the phase
transition (endothermic peak at 453 K). The next stages proceed with the gradual
mass loss. In the second stage, one molecule of the 1,2-dimethylimidazole and one
molecule of the o-hydroxybenzoic acid are lost. Zn(sal)(1,2-diMeim) is the product
of this reaction. The diffractometric analysis showed that zinc oxide forms in the third
stage of decomposition together with organic fragments of the ligands. The pure zinc
oxide is the product of the fourth stage. (Table 3, reaction IV).
organ.
→ZnO (IV)
fragm.
fusion
Zn(Hsal)2(1,2-diMeim)2→
→Zn(sal)(1,2-diMeim)→ZnO+
process
The course of the pyrolysis (in conditions determined in this paper) and molecu-
lar structures of Zn(Hsal)2(Him)2 and Zn(Hsal)2(1,2-diMeim)2 lead to the same con-
clusions about the strength of zinc-imidazole or 1,2-dimethylimidazole and zinc-o-
hydroxybenzoate bonds.
The thermal stability of the compounds decreases in the following order:
Zn(Hsal)2(Him)2>Zn(Hsal)2(4-MeHim)2>Zn(Hsal)2(1,2-diMeim)2>Zn(Hsal)2(Him)5
Ti= 478 K
468 K
453 K
418 K
where Ti – initial temperature of decomposition.
Conclusions
• The composition of di(o-hydroxybenzoate)zinc(II) complexes with imidazole de-
pends on molar ratio reagents, but that of complexes with derivatives of imidazole
does not. Compounds with imidazole and 1,2-dimethylimidazole have a crystalline
structure, while compound with 4-methylimidazole is amorphous.
• Thermal decomposition of the mixed complexes has a multi-stage course. The com-
plex containing imidazole is thermally most stable from among the complexes of the
same generalized formula Zn(A)2(B)2 (where A – Hsal–; B – Him, 4-MeHim,
1,2-diMeim).
• The course of pyrolysis (in conditions determined in this paper) and molecular
structure of the complexes lead to the same conclusions about the strength metal-
ligand bonds.
• Zinc oxide is the final pyrolysis product of all the investigated complexes.
• Zn(Hsal)2(Him)2 and Zn(Hsal)2(1,2-diMeim)2 have the tetrahedron structure.
Metal is surrounded by two monodentate carboxylate groups and two imidazole or
• 1,2-dimethylimidazole molecules. In Zn(Hsal)2(Him)2 complex, the Zn–N bonds
are stronger than the Zn–O bonds, in Zn(Hsal)2(1,2-diMeim)2 the inverse of bond
strength is observed.
• In Zn(Hsal)2(1,2-diMeim)2 every o-hydroxybenzoate ion and every 1,2-
dimethylimidazole molecule are differently bound by zinc. We encounter two dif-
ferent Zn–N bonds as well as two different Zn–O bonds.
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• Intramolecular hydrogen bonds are found in both investigated structures. These
bonds are stronger in Zn(Hsal)2(Him)2 than those in (Hsal)2(1,2-diMeim)2.
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