Introduction

Zinc is found in numerous essential enzymes which catalyze the metabolic conversion or degradation of proteins, nucleic acid, lipids and other important bioorganic compounds. Other functions are in structural stabilization of insulin, of hormone complexes or of transcription-regulating factors for the transfer of genetic information (‘zinc fingers’) [1]. Zinc is used in the prevention and therapy of many illnesses as a component of drugs and biopreparations [2]. It may be used in treatments of acrodermatitis enteropathica, gastrointestinal disorders, infertility, in the prevention of sickle-cell disease and other diseases [3]. Some aromatic carboxylic acids (e.g. benzoic acid, salicylic acid) are known to have antimicrobial properties. Benzoic acid is used in combination with salicylic acid in dermatology as a fungicidal treatment for fungal skin diseases [4]. The synthesis and investigation of physicochemical properties and biological activity of metal carboxylate complexes are of increasing interest [5–7]. Köse [8] studied the spectral and magnetic properties of mixed-ligand m-hydroxybenzoate complexes of Zn(II), Co(II), Ni(II) and Cu(II) with nicotinamide. The crystal structures and spectroscopic properties of copper(II) chloroacetates with isonicotinamide, N-methylnicotinamide and N,N-diethylnicotinamide were studied by Moncol et al. [9]. Mojumdar et al. [10] studied the thermal properties of Cu(II) and Mg(II) carboxylates with N-donor heterocyclic ligands and proposed their structure by means of spectral analyses. Several 2-bromobenzoatocopper(II) complexes were synthesised and their spectral, structural and magnetic properties were investigated [11, 12]. In our previous works we described the preparation, thermal, spectral and biological properties of aliphatic zinc(II) carboxylates [13–15], salicylates and halogenosalicylates [16, 17] and benzoates [18, 19]. It was found that the thermal decomposition of zinc(II) benzoate complexes with urea and caffeine starts with the release of organic ligand, which is followed by the release of carbon dioxide and diphenylketone. Carboxylato- and halogenocarboxylatozinc(II) complexes inhibited photosynthetic electron transport in spinach chloroplasts and in green alga Chlorella vulgaris [20]. The structural properties of zinc(II) 2-bromobenzoate and its complexes with N-methylnicotinamide, methyl-3-pyridylcarbamate, N,N-diethylnicotinamide and nicotinamide were published earlier [21–24]. In this paper the spectral, thermal and biological properties 2-bromobenzoatozinc(II) complexes with organic ligand are reported.

Experimental

Synthesis of the compounds

These A.R. grade chemicals were used for the preparation of the studied compounds: ZnCl2 (Fluka, Germany), Na2CO3 (Mikrochem a.s., Slovakia), 2-bromobenzoic acid 97% (Aldrich, Germany), urea, N-methylnicotinamide, nicotinamide, isonicotinamide, N,N-diethylnicotinamide and phenazone (Merck, Germany).

The following compounds were prepared: Zn(2-Brbenz)2 (I), Zn(2-Brbenz)2(u)2 (II), Zn2(2-Brbenz)4(mnad)2 (III), Zn(2-Brbenz)2(inad)2·H2O (IV), Zn(2-Brbenz)2(denad)2·2H2O (V), Zn(2-Brbenz)2(nad)2 (VI) and Zn2(2-Brbenz)4(phen)2 (VII)

The syntheses may be expressed by the following equations:

$$ {\text{ZnCl}}_{ 2} + {\text{ Na}}_{ 2} {\text{CO}}_{ 3} \to {\text{ZnCO}}_{ 3} \downarrow + {\text{ 2NaCl}} $$
(1)
$$ {\text{ZnCO}}_{ 3} + { 2}\left( { 2{\text{-BrC}}_{ 6} {\text{H}}_{ 4} {\text{COOH}}} \right) \to {\text{Zn}}\left( { 2{\text{-BrC}}_{ 6} {\text{H}}_{ 4} {\text{COO}}} \right)_{ 2} + {\text{ H}}_{ 2} {\text{O }} + {\text{ CO}}_{ 2} \uparrow $$
(2)
$$ {\text{Zn}}\left( { 2{\text{-BrC}}_{ 6} {\text{H}}_{ 4} {\text{COO}}} \right)_{ 2} + {\text{ 2L}} \to {\text{Zn}}\left( { 2{\text{-BrC}}_{ 6} {\text{H}}_{ 4} {\text{COO}}} \right)_{ 2} {\text{L}}_{ 2} $$
(3)

2-Bromobenzoic acid (2.58 g, 97%, 12 mmol) dissolved in methanol (40 cm3) was added to the excess of aqueous suspension of ZnCO3 freshly prepared by the reaction of aqueous solution of ZnCl2 and Na2CO3. The reaction mixture was stirred for 1.5 h and the excess of ZnCO3 was filtered off. Then, to the filtrate of zinc 2-bromobenzoate the solution of bioactive ligands (urea, nicotinamide, isonicotinamide, N-methylnicotinamide, N,N-diethylnicotinamide and phenazone) were added in stoichiometric ratio and stirred for 2 h. The reaction mixture was reduced to a half of its volume at 343 K and left to crystallize at room temperature. In a few days, crystalline (I, III–VII) and powdery (II) complex compounds were obtained in 78–86% yields.

Instrumentation

The carbon, hydrogen and nitrogen content in the newly synthesised compounds were determined by the CHN analyzer PERKIN ELMER 2400. The zinc content was determined using Complexone III as an agent and Eriochrome black T as an indicator.

The IR spectra of the prepared zinc complex compounds and the solid intermediates of thermal decomposition were recorded on an AVATAR 330 FT-IR Thermo Nicolet spectrometer using KBr pellets (2 mg/200 mg KBr), in the range 4000–400 cm−1.

Thermal decomposition was studied in nitrogen atmosphere using a Perkin-Elmer TGA7 with the heating rate of 10 K min−1 up to 1073 K in platinum crucibles.

Mass spectrometer GC/MS Agilent 7890A was used for determination of volatile products of the thermal decomposition.

Antimicrobial assay

The antibacterial activities of the studied Zn(II) complexes, organic ligands (urea, thiourea, methyl-3-pyridylcarbamate, phenazone, N-methylnicotinamide, isonicotinamide and N,N-diethylnicotinamide) and 2-bromobenzoic acid were evaluated by a micro-dilution method using G+ bacteria Staphylococcus aureus CCM 3953, G− bacteria Escherichia coli CCM 3988 [25]. The effects of these compounds on the yeasts Candida albicans (purchased from the Laboratory of Medical Mycology, Slovak Medical University, Bratislava, Slovakia) were determined by macro-dilution method in L-shapes tubes adapted for direct measurement of absorbance [26]. The cultures of bacteria (in Mueller–Hinton growth medium) and yeasts (Sabouraud′s growth medium) were incubated under vigorous shaking. The effect of tested compounds on the growth of filamentous fungi Rhizopus oryzae CCM F-8284, Alternaria alternata CCM F-128 and Microsporum gypseum CCM F-8342 was observed by macro-dilution technique on solidified broth medium during static culturing [27, 28] and the diameters of growing fungal colonies were measured at intervals. Strains designed “CCM” were originally obtained from the Czech Collection of Microorganisms, Masaryk University, Brno, Czech Republic.

Chromatographically pure compounds were dissolved in DMSO; its final concentration never exceeded 1.0 vol.% in either control or treated samples. Concentration of tested compounds was in the range of 0.01–2.0 mmol dm−3 (bacteria, yeasts) or of 0.1–3.0 mmol dm−3 (filamentous fungi) in all experiments. The antimicrobial activity was characterized by the IC50 values (concentration of a compound which in comparison to the control inhibits the growth of model microorganisms to 50%) and MIC values (minimal inhibitory concentration of a compound which inhibits microbial growth by 100%). The IC50 and MIC values were read from toxicity curves. MIC experiments on subculture dishes were used to assess the minimal microbicidal concentration (MMC). Subcultures were prepared separately in Petri dishes containing appropriate agar medium and incubated at 303 K for 48 h (bacteria, yeasts) and 298 K for 96 h (filamentous fungi). The MMC value was taken as the lowest concentration which showed no visible growth of microbial colonies on the subculture dishes.

Results and discussion

The prepared compounds (I–VII) are white in colour, stable in air and light. Elemental analyses (Table 1) are in good agreement with the calculated ones. The solubility of the studied compounds in various solvents is presented in Table 2.

Table 1 Elemental analysis of the prepared zinc(II) compounds
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Table 2 Solubility of the prepared compounds
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IR spectra

The characteristic IR bands for the compounds (I–VII) are reported in Table 3. The assignments were done according to the literature data [29, 30]. The magnitude of separation of asymmetric νas(COO−) and symmetric νs(COO−) stretching vibrations of carboxylate group, Δ(COO−), can be used as a criterion to assign the type of the carboxylate coordination in inorganic complexes. In general the following order is proposed for divalent metal carboxylates: Δ(monodentate) ≫ Δ(ionic) ≥ Δ(bridging) ≫ Δ(chelating) [29, 31]. The Δ value determined from the IR spectra of sodium 2-bromobenzoate is 168 cm−1. By comparing the values of Δ(COO−) of prepared compounds with that of sodium 2-bromobenzoate we can assume a monodentate coordination of 2-bromobenzoate group in compounds (II) (184 cm−1), (IV) (187 cm−1), (V) (200 cm−1), (VI) (211 cm−1) and (VII) (224 cm−1) and a bridging mode of binding in compounds (I) (168 cm−1), (III) (168 cm−1) and (VII) (156 cm−1).

Table 3 Characteristic absorption bands ν/cm−1 in IR spectra of compounds (I–VII)
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The strong absorption band of the carbonyl ν(C=O) vibration of compounds (III, IV, V, VI) at 1679, 1701, 1632 and 1682 cm−1, respectively, is shifted to a higher wavenumber as compared with the free ligands (νmnad(C=O) = 1644 cm−1, νinad(C=O) = 1666 cm−1, νdenad(C=O) = 1628 cm−1, νnad(C=O) = 1679 cm−1). It can be explained by the fact that the pyridine nitrogen of of these ligands is involved in coordination with zinc, therefore, the electron density is shifted towards the pyridine nitrogen, leading to the increase in the double bond character of the carbonyl group and shift the stretching vibration ν(C=O) to a higher value. On the other hand in the case of compounds (II) and (VII) the absorption band of the carbonyl ν(C=O) vibration appeared at 1633 and 1647 cm−1, respectively, exhibited a shift to lower wavenumber in comparison with free ligands (νu(C=O) = 1670 cm−1, νphen(C=O) = 1666 cm−1). This phenomenon can be explained by the coordination of the carbonyl oxygen to the central zinc atom, leading to a decrease of the double bond character of the carbonyl group and shifting the stretching vibration ν(C=O) to lower values. These assumptions were proved by the results of the X-ray structural analyses of compounds (III, V, VI) and (VII) [21–24].

Thermal behaviour

Thermal decomposition of the prepared compounds is given in Table 4.

Table 4 Thermal decomposition of the prepared compounds
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Compound Zn(2-BrC6H4COO)2

As it follows from Fig. 1, the compound is thermally stable up to 523 K. On heating above this temperature thermal decomposition takes place. The release of bis(2-bromophenyl)ketone and carbon dioxide (exp. mass loss 84.02%, calc. mass loss 82.51%) in temperature range 523–983 K are observed on TG/DTG curve. The final solid product of thermal decomposition is ZnO (exp. 15.98%, calc. 17.49%). The following mechanism is proposed for the thermal decomposition:

$$ {\text{Zn}}\left( { 2{\text{-BrC}}_{ 6} {\text{H}}_{ 4} {\text{COO}}} \right)_{ 2} \to \, \left( {{\text{C}}_{ 5} {\text{H}}_{ 4} {\text{Br}}} \right)_{ 2} {\text{CO }} + {\text{ CO}}_{ 2} + {\text{ ZnO}} $$
(4)
Fig. 1
figure 1

Thermal decomposition of Zn(C6H4COO)2

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Compound Zn(2-BrC6H4COO)2(u)2

The compound is stable up to 403 K. The thermal decomposition may be characterized as a two step reaction in temperature range from 403 to 1073 K. In the first step two moles of urea are released and than bis(2-bromophenyl)ketone and carbon dioxide (exp. mass loss 86.01%, calc. mass loss 86.11%) are evolved. The final solid product of thermal decomposition is ZnO (exp. 13.99, calc. 13.89) (Fig. 2). Mass spectrum measured at 438 K confirmed the release of urea (m/z: 60, 44, 16). Based on the mass spectrum we propose the following fragmentation scheme of urea:

Fig. 2
figure 2

Thermal decomposition of Zn(C6H4COO)2(u)2

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In the IR spectrum of solid intermediate at 573 K showed that the absorption bands of urea (ν(C=O) = 1633 cm−1 and ν(N–H) = 3454, 3350 cm−1) were missing. The following reaction is proposed for the decomposition process:

$$ {\text{Zn}}\left( { 2{\text{-BrC}}_{ 6} {\text{H}}_{ 4} {\text{COO}}} \right)_{ 2} \left( {\text{u}} \right)_{ 2} \to {\text{ 2u }} + \, \left( {{\text{C}}_{ 6} {\text{H}}_{ 4} {\text{Br}}} \right)_{ 2} {\text{CO }} + {\text{ CO}}_{ 2} + {\text{ ZnO}} $$
(5)

Compound Zn2(2-BrC6H4COO)4(mnad)2

The thermal decomposition starts at 453 K with the release of two moles of N-methylnicotinamide and in the next step the release of two moles of bis(2-bromophenyl)ketone and two moles of carbon dioxide (exp. mass loss 86.18%, calc. mass loss 86.46%) (Fig. 3) are evolved. The final solid product of thermal decomposition is ZnO (exp. 13.82%, calc. 13.54%).

Fig. 3
figure 3

Thermal decomposition of Zn2(C6H4COO)4(mnad)2

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The release of N-methylnicotinamide was confirmed by mass spectrometry (m/z: 136, 107, 79, 52) measured at 473 K. We propose the following fragmentation scheme of N-methylnicotinamide:

In the IR spectrum of the solid intermediate product at 633 K the absorption bands of N-methylnicotinamide (ν(C=O) = 1679 cm−1, ν(N–H) = 3360 cm−1, ν(C–H)aliph = 2936 cm−1, \( \delta_{\text{as}} \left( {\text{C}}{-}{\text{H}} \right)_{{{\text{CH}}_{3} {-} }} = 1439\,{\text{cm}}^{ - 1} \) and \( \delta_{\text{s}} \left( {\text{C}}{-}{\text{H}} \right)_{{{\text{CH}}_{3} {-} }} = 1296\,{\text{cm}}^{ - 1} \)) were missing.

The mechanism of thermal decomposition can be expressed as follows:

$$ {\text{Zn}}_{ 2} \left( { 2{\text{-BrC}}_{ 6} {\text{H}}_{ 4} {\text{COO}}} \right)_{ 4} \left( {\text{mnad}} \right)_{ 2} \to {\text{ 2mnad }} + { 2}\left( {{\text{C}}_{ 6} {\text{H}}_{ 4} {\text{Br}}} \right)_{ 2} {\text{CO }} + {\text{ 2CO}}_{ 2} + {\text{ 2ZnO}} $$
(6)

Compound Zn(2-BrC6H4COO)2(inad)2·H2O

The compound is thermally stable up to 363 K (Fig. 4). Release of water takes place above this temperature (exp. mass loss 2.30%, calc. mass loss 2.47%). The thermal decomposition of anhydrous product may be characterized as a two step reaction in temperature range from 423 to 1073 K. In the first step two moles of isonicotinamide are released. In the next step bis(2-bromophenyl)ketone and carbon dioxide are lost (exp. mass loss 85.35%, calc. mass loss 86.35%). In the IR spectrum of solid intermediate product at 583 K the absorption bands of isonicotinamide (ν(C=O) = 1701 cm−1 and ν(N–H) = 3379, 3250 cm−1) were missing. The final solid product of thermal decomposition is ZnO (exp. 12.35%, calc. 11.18%).

$$ {\text{Zn}}\left( { 2{\text{-BrC}}_{ 6} {\text{H}}_{ 4} {\text{COO}}} \right)_{ 2} \left( {\text{inad}} \right)_{ 2} {\text{ H}}_{ 2} {\text{O}} \to {\text{ H}}_{ 2} {\text{O }} + {\text{ 2inad }} + \, \left( {{\text{C}}_{ 6} {\text{H}}_{ 4} {\text{Br}}} \right)_{ 2} {\text{CO }} + {\text{ CO}}_{ 2} + {\text{ ZnO}} $$
(7)
Fig. 4
figure 4

Thermal decomposition of Zn(C6H4COO)2(inad)2.H2O

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Compound Zn(2-Brbenz)2(denad)2·2H2O

From Fig. 5 it followed that the thermal decomposition of Zn(2-Brbenz)2(denad)2(H2O)2 starts at 333 K with the dehydration process (exp. mass loss 4.21%, calc. mass loss 4.19%). In temperature range 403–823 K two moles of N,N-diethylnicotinamide, one mole of bis(2-bromophenyl)ketone and one mole of carbon dioxide were evolved (exp. mass loss 82.94%, calc. mass loss 82.12%). In IR spectra of the solid intermediate product at 603 K the characteristic absorption bands of N,N-diethylnicotinamide (ν(C=O) = 1632 cm−1, ν(C–H)aliph = 2988 cm−1, \( \delta_{\text{as}} \left( {\text{C}}{-}{\text{H}} \right)_{{{\text{CH}}_{3} {-} }} = 1444\,{\text{cm}}^{ - 1} \) and \( \delta_{\text{s}} \left( {\text{C}}{-}{\text{H}} \right)_{{{\text{CH}}_{3} {-} }} = 1366\,{\text{cm}}^{ - 1} \)) were missing. The final product of thermal decomposition is ZnO (exp. 12.85%, calc. 13.69%). The following reaction is proposed for the decomposition process:

$$ {\text{Zn}}\left( { 2{\text{-BrC}}_{ 6} {\text{H}}_{ 4} {\text{COO}}} \right)_{ 2} \left( {\text{denad}} \right)_{ 2} {\text{ 2H}}_{ 2} {\text{O}} \to {\text{ 2H}}_{ 2} {\text{O }} + {\text{ 2denad }} + \, \left( {{\text{C}}_{ 6} {\text{H}}_{ 4} {\text{Br}}} \right)_{ 2} {\text{CO }} + {\text{ CO}}_{ 2} + {\text{ ZnO}} $$
(8)
Fig. 5
figure 5

Thermal decomposition of Zn(C6H4COO)2(denad)2.2H2O

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Compound Zn(2-BrC6H4COO)2(nad)2

The compound is stable up to 473 K. The thermal decomposition may be characterized as a two step reaction in temperature range from 473 to 1073 K. In the first step two moles of nicotinamide release and than bis(2-bromophenyl)ketone and carbon dioxide (exp. mass loss 88.42%, calc. mass loss 88.53%) are evolved. The final solid product of thermal decomposition is ZnO (exp. 11.56%, calc. 11.47%) (Fig. 6). Mass spectrum measured at 473 K confirmed the release of nicotinamide (m/z: 122, 106, 78, 51).

Fig. 6
figure 6

Thermal decomposition of Zn(C6H4COO)2(nad)2

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The fragmentation scheme for nicotinamide is proposed as follows:

The absence of nicotinamide was confirmed by IR spectra in the solid intermediate product at 598 K where the absorption bands of characteristic groups of nicotinamide (ν(C=O) = 1689 cm−1 and ν(N–H) = 3383 cm−1) were missing. The following reaction is proposed for the decomposition process:

$$ {\text{Zn}}\left( { 2{\text{-BrC}}_{ 6} {\text{H}}_{ 4} {\text{COO}}} \right)_{ 2} \left( {\text{nad}} \right)_{ 2} \to {\text{ 2nad }} + \, \left( {{\text{C}}_{ 6} {\text{H}}_{ 4} {\text{Br}}} \right)_{ 2} {\text{CO }} + {\text{ CO}}_{ 2} + {\text{ ZnO}} $$
(9)

Compound Zn2(2-BrC6H4COO)4(phen)2

As it can be seen from Fig. 7, the compound is thermally stable up to 473 K. On heating above this temperature thermal decomposition takes place. The release of two moles of phenazone and than two moles of bis(2-bromophenyl)ketone and two moles of carbon dioxide (exp. mass loss 86.44%, calc. mass loss 87.55%) are observed in temperature range 473–1073 K on TG/DTG curves. The final solid product of thermal decomposition is ZnO (exp. 13.56%, calc. 12.45%). The release of phenazone was confirmed by mass spectrometry (m/z: 188, 173, 96) measured at 483 K. The fragmentation scheme of phenazone is as follows:

Fig. 7
figure 7

Thermal decomposition of Zn2(C6H4COO)4(phen)2

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The following mechanism is proposed for the thermal decomposition:

$$ {\text{Zn}}_{ 2} \left( { 2{\text{-BrC}}_{ 6} {\text{H}}_{ 4} {\text{COO}}} \right)_{ 4} \left( {\text{phen}} \right)_{ 2} \to {\text{ 2phen }} + { 2}\left( {{\text{C}}_{ 6} {\text{H}}_{ 4} {\text{Br}}} \right)_{ 2} {\text{CO }} + {\text{ 2CO}}_{ 2} + {\text{ 2ZnO}} $$
(10)

Biological properties

The results of determination of antimicrobial activity of tested compounds (characterized by the IC50 and MIC values [mmol dm−3]) are summarized in Table 5. In general, it could be concluded that the presence of zinc(II) ion in complexes led to the increase of the inhibitory activity on the growth of bacteria, yeasts and filamentous fungi in comparison with 2-bromobenzoic acid (XVII), except of A. alternata. Neither 2-bromobenzoic acid (XVII) nor any free ligand (X–XVI) affected the growth of selected microorganisms (IC50 > 3.0 mmol dm−3, MIC > 3.0 mmol dm−3). In comparison with zinc(II) 2-bromobenzoate (I) (IC50 = 0.13 mmol dm−3, MIC = 1 mmol dm−3) the increase of antibacterial activity against G+ pathogenic bacterium S. aureus was observed only in case of complex (VII) (IC50 = 0.07 mmol dm−3, MIC = 1 mmol dm−3); the efficiency of complexes (IV) and (V) is comparable with complex I. On the other hand, the inhibitory activity of compounds (II, III, VIII, IX) (IC50 = 0.20–0.21 mmol dm−3, MIC = 1.0–2.0 mmol dm−3) was lower than that of zinc(II) 2-bromobenzoate (I). In comparison with complex (I) (IC50 = 0.37 mmol dm−3, MIC = 1.0 mmol dm−3) only in case of complex (II) (IC50 = 0.10 mmol dm−3, MIC = 1.0 mmol dm−3) was observed a higher antibacterial activity against E. coli. The inhibitory activity of complex (VII) (IC50 = 0.34 mmol dm−3, MIC = 1.0 mmol dm−3) was comparable with that of zinc(II) 2-bromobenzoate (I) and the remaining complexes (III–V, VIII, IX) have had lower inhibition efficiency than complex (I) (IC50 = 0.40–0.90 mmol dm−3, MIC = 1.0–2.0 mmol dm−3) against this G− bacterium. Complex (VII) had the highest antimicrobial activity against yeast C. albicans ((IC50 = 0.50 mmol dm−3). Except of complex (III) (IC50 = 0.72 mmol dm−3), which efficiency was comparable with that of complex (I) (IC50 = 0.50 mmol dm−3), the remaining zinc(II) compounds (II, IV, V, VIII, IX) had a lower antimicrobial activity against this yeast (IC50 = 0.80–1.35 mmol dm−3). The growth of R. oryzae was the most strongly inhibited by complex (VII) (IC50 = 1.80 mmol dm−3), but the presence of N-methylnicotinamide, thiourea and methyl-3-pyridylcarbamate in complexes (III, VIII, IX) also increased their antifungal activity (IC50 = 2.0–2.2 mmol dm−3) in comparison with zinc(II) 2-bromobenzoate (I) (IC50 > 3.0 mmol dm−3). Neither the studied complexes (I–VII, IX), nor the free ligands (X–XVI) and 2-bromobenzoic acid (XVII) influenced the growth of filamentous fungi A. alternata (IC50 > 3.0 mmol dm−3). Only complex (VIII) (IC50 = 2.8 mmol dm−3) slightly inhibited the growth of this fungi. The highest antifungal activity against dermatophytic fungi M. gypseum was observed in the presence of complex (IX) (IC50 = 1.0 mmol dm−3, MIC > 3.0 mmol dm−3). In comparison with the inhibitory activity of compound I (IC50 = 2.0 mmol dm−3), the inhibitory activity of compounds (III, IV, VII, VIII) was positively influenced by the ligands N-methylnicotinamide, isonicotinamide phenazone and thiourea (IC50 = 1.3–1.6 mmol dm−3). The selected bacteria S. aureus and E. coli were more sensitive to the studied zinc(II) complex compounds than yeast C. albicans or filamentous fungi M. gypseum, R. oryzae and A. alternata, respectively.

Table 5 Antimicrobial activity of zinc(II) 2-bromobenzoate complexes characterized by IC50 and MIC values/mmol dm−3
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Conclusions

The thermal decomposition of hydrated compounds started from 333 K with dehydration process. During the thermal decomposition the organic ligand, carbon dioxide and bis(2-bromophenyl)ketone were evolved. The final solid product of the thermal decomposition heated up to 1073 K was zinc oxide. The solid intermediates and volatile products of thermal decomposition were confirmed by IR spectroscopy and mass spectrometry. It was found that zinc(II) 2-bromobenzoate starts to decompose at the highest temperature and the thermal stability of anhydrous compounds increases in the following order:

In the case of the compounds with solved crystal structure: Zn(2-BrC6H4COO)2 (I), Zn2(2-BrC6H4COO)4(mnad)2 (III), Zn(2-BrC6H4COO)2(denad)2·2H2O (V), Zn(2-BrC6H4COO)2(nad)2 (VI), Zn2(2-BrC6H4COO)4(phen)2 (VII) the values of Δ were in agreement with the results of structural analysis:

Δ(monodentate) = 200, 211 and 224 cm−1 (V–VII)

Δ(bridging) = 168, 168 and 156 cm−1 (I, III, VII) [21–24].

The compound with phenazone (VII) had the highest inhibition activity on the growth of S. aureus, C. albicans and R. oryzae. The compound with urea (II) had the highest antibacterial activity against E. coli. The growth of A. alternata was influenced only by compound with thiourea (VIII). The highest antifungal activity against M. gypseum was observed in the presence of compound with methyl-3-pyridylcarbamate (IX). The presence of free organic ligands and 2-bromobenzoic acid did not affect the growth of microorganisms.