173 Acta Chim. Slov. 1999, 46(2), pp. 173-184 SYNTHESIS AND CHARACTERIZATION OF BIS(CARBOXYLATO)ZINC(II) (C6 - C8) - CRYSTAL STRUCTURE OF BIS(HEXANOATO)ZINC(II), Zn(O2CC5H11)2 - FORM A# Primož Šegedin, Nina Lah, Mojca Zefran, Ivan Leban and Ljubo Golič Faculty of Chemistry and Chemical Technology, University of Ljubljana, Aškerčeva 5, P.O.B. 537, SI-1001 Ljubljana, Slovenia, e-mail: Primoz.Segedin@uni-lj.si (received 29.3.1999) Abstract Different synthetic routes were used for the preparation of zinc(II) alkyl carboxylates with 6 - 8 carbon atoms. Isolated products were characterised by standard physicochemical methods. Powder diffraction patterns, IR spectra, DTA curves and 13C solid state NMR spectra indicate presence of two different polymorphic forms in the case of hexanoate and heptanoate. No fungicidal activity of isolated compounds was observed. Crystal structure of Zn(O2CC5H11)2 - form A was determined by X-ray single-crystal method. Each Zn atom is coordinated tetrahedrally by oxygen atoms of four different hexanoate ligands while each hexanoate ligand is bridging two tetrahedral zincs in syn-anti mode thus forming a typical layered structure. Introduction Carboxylate complexes of transition metals have been subject of many studies. Besides their practical importance in industry, these compounds exhibit very interesting physicochemical properties which also make them a challenge to fundamental investigation [1]. Recent studies on the formation of zinc carboxylates revealed that the structures of the # Dedicated to the memory of Prof. Dr. Jože Šiftar 174 compounds formed depend mainly on the R group of the carboxylic acid and only partly on reaction conditions [2] and suggested that the alkyl chain has a much more important role in the formation and stability of the structure through the interactions between neighbouring chains than previously believed [3]. Our interest in metal carboxylates evolves from their biological implications with emphasis on possible correlation between chemical and biological properties of synthesised compounds. Experimental Commercially available substances as ZnO, ZnSO4-7H2O, Zn(O2CCH3)2-2H2O, hexanoic, heptanoic and octanoic acid and all solvents were used without further purification. The zinc contents of weighed samples were determined by EDT A titration after fuming the samples twice with a 1 : 2 mixture of concentrated sulphuric and nitric acids. Synthesis Bis(hexanoato)zinc(II), Zn(O2CC5H11)2 - form A 0.814 g ZnO and 5.0 mL hexanoic acid (100 % excess) were added to 25 mL of ethanol and the obtained suspension was refluxed. After one hour the solution became transparent, but reflux was continued for one hour. The solution was left at room temperature for 20 h. The colourless plate-like crystals were filtered off, washed with ethanol and dried in evacuated desiccator over KOH. Average yield 96 %. Anal. Calcd. for Zn(O2CC5H11)2 (Mr=295.7): Zn 22.11 %; found 22.4 %. From X-ray powder data the following d spacings (A) were observed (relative intensities in parentheses): 8.1 (4), 5.4 (4), 4.72 (1), 4.58 (3), 4.38 (10), 4.16 (5), 4.00 (2), 3.88 (8), 3.83 (7). The same form of bis(hexanoato)zinc(II) was obtained by alternative methods: from ZnSO4-7H2O with hexanoic acid and KOH in ethanol by already described procedure [4], from ZnSO4-7H2O with sodium hexanoate in water and from Zn(O2CCH3)2-2H2O with sodium hexanoate in water. In all cases the obtained yields are lower therefore the details of the procedures are not given. 175 Monocrystals of Zn(O2CC5H11)2 - form A were obtained by following procedure: 0.5 g of crude product was added to 100 mL of water, warmed and left at boiling temperature for one minute. The undissolved product was filtered off and the solution was cooled to room temperature and left in refrigerator at 0 oC for one week. 100 mL of ethanol were added and the solution was left at 0 oC. Monocrystals suitable for structure determination were collected from solution after another week. Bis(hexanoato)zinc(II), Zn(O2CC5H11)2 - form B 0.90 g of form A, obtained from ZnO by reaction with hexanoic acid in ethanol, were dissolved in 125 mL of benzene at boiling temperature. The undissolved starting material was filtered off and the solution was cooled slowly to room temperature and left in the refrigerator over night. Transparent colourless crystalline product was filtered, dried on air without washing and then over KOH for 24 hours. Average yield 33.5 %. Anal. Calcd. for Zn(O2CC5H11)2 (Mr= 295.7 ): Zn 22.11 %; found 22.2 %. The following d spacings (A) were observed (relative intensities in parentheses): 8.1 (4), 5.4 (3), 4.59 (7), 4.38 (8), 4.13 (10), 3.98 (3), 3.90 (8), 3.84 (2), 3.63 (6). Bis(heptanoato)zinc(II), Zn(O2CC6H13)2 - A and B, and bis(octanoato)zinc(II), Zn(O2CC7H15)2 Similar synthetic procedures were used for reactions with heptanoic and octanoic acid. While in the case of bis(heptanoato)zinc(II) the formation of two polymorphic forms was observed - form A after different synthetic routes described above, and form B with recristallization from benzene, only one form of bis(octanoato)zinc(II) was obtained. Measured d-values with relative intensities for three isolated products are given in Table 1. Results of Zn analysis were within expected experimental errors. 176 Table 1. Measured d-values in A (with relative intensities in parentheses) for Zn(O2CC6H13)2 - form A (2A), Zn(O2CC6H13)2 - form B (2B), and Zn(O2CC7H15)2 (3). 2A 9.3(4) 6.15(3) 4.61(2) 4.52(2) 4.45(7) 4.37(8) 4.19(1) 4.07(1) 4.00(8) 3.9(1) 3.77(6) 3.72(9) 2B 9.3(3) 6.2(3) 4.62(5) 4.53(6) 4.25(8) 4.2(1) 3.87(2) 3.78(6) 3.72(8) 3 10.5(3) 7.0(3) 4.63(2) 4.59(2) 4.35(6) 4.28(5) 4.07(1) 3.93(8) 3.9(1) 3.79(1) 3.52(3) 3.47(3) Physical Measurements Interplanar spacings were obtained with HUBER GUINIER CAMERA 620 using CuKa radiation (Johansson monochromator). Infrared spectra were measured on Nujol or Fluorolube mulls between CsI plates using Perkin-Elmer FT-IR 1720X spectrometer in the range 4000-400 cm-1. DTA curves were obtained using a Mettler 2000 C thermoanalyzer. Experimental conditions: platinum crucibles 7.0 mm in diameter, sample weights 20 mg, heating rate 2 oC min-1, atmosphere of dry argon., a-Al2O3 as a reference substance for DTA measurements. 13C solid state NMR spectra were recorded with Varian Unity-plus 300 at the National NMR Center of Slovenia. Experiments were carried out at 75.41 MHz using cross-polarization (CP) and magic-angle spinning (MAS) at 3.5-4.0 kHz with high-power 1H decoupling, 1.9 ms CP contact time and a 5 s recycle time, using 7 mm probe from Doty. Typically 1000-5000 accumulations were acquired. Spinning sidebands were eliminated using the TOSS technique. Single crystal diffraction measurements were carried out on an Enraf-Nonius CAD-4 diffractometer with MoKa radiation (graphite monochromator). Cell parameters were obtained from refinement of 75 reflections in 9 range 8.53 - 15.56 o using PARAM [5]. Common data corrections for variations in reference reflections and Lorentz-polarization effects were applied (DATRD2 in NRCVAX) [6]. Absorption correction was preformed by 177 Gaussian integration method (ABSORP in NRCVAX) [6]. SHELXS [7] was used to solve and SHELXL [8] to refine the structure. Details of crystal data and data collection are presented in Table 2. All H atoms were placed at calculated positions with isotropic thermal displacement parameters taken from the parent non-H atoms and multiplied by 1.2. Scattering factors of all atoms were taken from the reference [9]. Final values of R1 (wR2) factors are 0.0302 (0.0719) for 2670 observed reflections and 0.0512 (0.0785) for all 3466 data. Table 2. Crystal and data collection summary for Zn(O2CC5H11)2 - form A Asymmetric Unit Formula Crystal System Space Group a (A) b (A) c (A) ßo) Volume (A3) Z Dx (Mg/m3) [i (m-1) Crystal Colour Crystal Shape Crystal Size (mm) Intensity Decay (%) T /T x min' x max Radiation Diffractometer Scan Type Measured Reflections Observed Reflections [I>2(g)I] Rint Zn(O2CC5H11)2 monoclinic P21/c No.14 32.395(3) 4.7914(2) 9.3450(8) 93.661(1) 1447.5(2) 4 1.357 0.1697 colourless plate 0.15x0.61x0.69 1.1 0.3774/0.8171 MoKa, X = 0.710930 A Enraf-Nonius CAD-4 00/20 3466 2670 0.0441 Results and Discussion Different synthetic routes were used for the preparation of bis(carboxylato)zinc(II) complexes with 6-8 carbon atoms. Reproducible, although small differences in powder 178 patterns were the first indication for the presence of two modifications in the case of hexanoato and heptanoato ligands. The most prominent features of the IR spectra are the carboxylate stretching frequencies characterised by strong bands between 1650 and 1400 cm-1. Very similar patterns for antisymmetric (vasCO2) and symmetric (vsCO2) stretching modes of carboxylate groups and almost identical values of differences A between them (Table 3) are the consequence of the bidentate bridging positions of carboxylate ligands in all isolated compounds [10]. Small differences in the structure of strong band centered at 1460 cm1 corresponding to the antisymmetric deformations of CH3 and scissoring modes of CH2 groups (Figure 1) indicate different interactions of alkyl chains in different polymorphic forms. This proposal is confirmed with the observed values of 13C chemical shifts in the solid state NMR spectra (Table 4). Table 3. IR absorption bands in carboxylate stretching region for Zn(O2CC5H11)2 - form A (1A), Zn(O2CC5H11)2 - form B (IB), Zn(O2CC6H13)2 - form A (2A), Zn(O2CC6H13)2 - form B (2B), and Zn(O2CC7H15)2 (3). Compound vas(CO2), cm1 vs(CO2), cm1 Av, cm 1A 1548, 1532 1409, 1398 136 IB 1548, 1537, 1532 1408, 1398 136 2A 1548, 1531 1409, 1399 136 2B 1546, 1532 1409, 1399 135 3 1549, 1532, 1527 1408, 1398 133 Table 4. 13C NMR chemical shift data (ppm relative to TMS) Compound Ö(CH3) Ö(CH2) 6(CO) Zn(O2CC5H11)2 ¦ A 15.3 27.4, 30.2, 36.3, 38.5 186.3 Zn(O2CC5H11)2 ¦ B 15.3 26.1, 28.5, 37.3a 185.2 Zn(O2CC6H13)2 ¦ A 16.9 27.4, 30.2, 34.9, 37.7 186.4 Zn(O2CC6H13)2 ¦ B 16.7 27.4, 30.0, 34.9, 37.5 186.0 Zn(O2CC7H15)2 16.8 27.2, 30.2, 37.2a 186.3 Broad 179 Figure 1. IR spectra of Zn(O2CC6H13)2 - form A, (A) and Zn(O2CC6H13)2 - form B, (B) in carboxylate stretching region. There are three types of carbon for each carboxylate ligand. A single resonance at 185.8 " 0.6 ppm in all spectra corresponds to carboxyl carbon and another at 16.1 " 0.8 ppm to methyl carbon. Significant differences in the positions and shapes of resonances corresponding to methylen carbons in the range 26.1 - 38.5 ppm in the case of two polymorphic forms of hexanoato and heptanoato complexes also indicate possible differences in interactions between carboxylate chains or their different orientations. All observed values of chemical shifts are in agreement with published results for other metal carboxylate complexes [11]. Alt hough t hermal deco mpo sit io n o f isolat ed co mpo unds has no t been st udied in det ails, two stage decomposition leading to ZnO as the only obtained solid residue was proved by powder pattern. Symmetrical ketones and carbon dioxide are the most probable gaseous 180 products in all cases in agreement with observed thermal decomposition for salts of many carboxylic acids [12]. The only significant difference is observed in DTA curves of two polymorphic forms of hexanoato and heptanoato complexes in low temperature range, where no mass loss was observed (Figure 2). Polymorphism was observed in some other metal carboxylates, with different crystalline modifications being formed on crystallisation from solution and on cooling from melt [12, p. 4]. In the case of zinc carboxylates the differences in DTA curves, indicating different phase changes on heating premelted samples or samples obtained by recrystallisation from benzene was already described [13]. The irreversible transformation of Zn(O2CC5H11)2 - form B and Zn(O2CC6H13)2 - form B to form A was observed in both cases, when the samples were repeatedly heated to 155 oC (slightly over melting point) and cooled to room temperature. The observed changes in DTA curves were confirmed by powder patterns of solid residues after the third cycle. The presence of two polymorphic forms was confirmed in the case of bis(acetato)zinc(II) [14a,b] and bis(propionato)zinc(II) [14c,d] by X-ray structure determination. All isolated compounds were screened for fungicidal activity by preliminary tests described previously [15]. All of them showed no significant fungicidal activity against Trametes versicolor. 300 330 360 390 420 T/K Figure 2. DTA curves of two polymorphic forms of Zn(O2CC5H11)2 in temperature range, where no mass loss was observed: form A - (A), and form B - (B). 181 Crystal structure of Zn(O2CC5H11)2 - form A The title compound belongs to the family of polymeric anhydrous zinc(II) carboxylates and crystallizes in monoclinic space group P21/c. It is isostructural with the previously published Zn(II) propionate [14d]. Each Zn atom is tetrahedrally coordinated by carboxylate O atoms from four different monodentate hexanoato ligands as illustrated in Figure 3. Selected bond lengths and angles are presented in Table 5. Figure 3. ORTEP-III view of the coordination around zinc. Thermal ellipsoids are drawn with 30% probability level. Hydrogen atoms are omitted for clarity. It is noticeable that the Zn-O distances are significantly different in two crystallographicaly different hexanoate groups. Zn-O21 and Zn-O22 are lengthened slightly 182 compared to Zn-O11 and Zn-O12. Similar discrepancy was observed in Zn(II) propionate [14d]. Alkyl chains are in fully extended, all-trans configuration. Mean value of C-C bond lengths is 1.516(7) A which is common for similar systems. Table 5. Interatomic Distances (A) and Angles (o) with Estimated Standard Deviations Bond lengths and ang les Zn-O11 1.954(1) Zn-O21ii 1.964(1) Zn-O12i 1.952(1) Zn-O22 1.964(1) O11-C11 1.261(2) O21-C21 1.270(2) O12-C11 1.262(2) O22-C21 1.263(2) C11-C12 1.502(2) C21-C22 1.507(3) C12-C13 1.519(3) C22-C23 1.511(3) C13-C14 1.525(3) C23-C24 1.526(3) C14-C15 1.520(3) C24-C25 1.518(3) C15-C16 1.521(3) C25-C26 1.516(4) O11-C11-O12 120.9(2) O21-C21-O22 120.5(2) Symmetry code: i) x, y-1, z and ii) x, 0.5-y, 0.5+z Zn atoms are linked with syn-anti hexanoate bridges forming two-dimensional polymer sheets paralel to bc plane. A similar sheet structure is observed not only for isostructural propionate [14d], but also for monoclinic form of Zn(II) acetate [14b] . There is no covalent bonding between sheets (Figure 4). The closest contacts between alkyl chains of hexanoate groups are: 4.105(3) A for C12-C13 (x,1+y,z), 4.139(3) A for C14-C15(x,1+y,z), 4.072(3) A for C22-C24 (x, y-1,z) and 4.115(4) A for C23-C25(x, y-1,z) which might be characterized as weak hydrophobic interactions. 183 Figure 4. The PLUTON [16] stereoview of the packing in the unit cell. Acknowledgements The work was supported by the Ministry of Science and Technology, Republic of Slovenia, through grants J1-7313-103 and J1-0442-103. 13C solid state NMR spectra were recorded at the National NMR Center of Slovenia. We thank Mr. Aleksander Gačeša for technical assistance. Screening tests on fungicidal activity were carried out at Department of Wood Science and Technology, Biotechnical Faculty, University of Ljubljana, Ljubljana, Slovenia. 184 References [I] M. F. R. Moita, M. L. T. S. Duarte, R. Fausto, J. Chem. Soc. Faraday Trans., 1994, 90, 2953-2960, and references therein. [2] O. Berkesi, T. Katona, I. Dreveni, J. A. Andor, J. 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Šegedin,^cta Chim. Slovenica, 1994, 41, 393- 404. [16] A.L. Spek, PLUTON. Molecular graphics Program. University of Utrecht, The Netherlands, 1991. Povzetek Po različnih sintetskih poteh smo pripravili cinkove soli karboksilnih kislin s 6 - 8 C atomi. Vse izolirane spojine so bile okarakterizirane s standardnimi fizikalno kemijskimi metodami. Praškovni posnetki, IR spektri, DTA krivulje in 13C spektri v trdnem kažejo na prisotnost dveh polimorfnih oblik v primeru koordinacijski spojin s heksanoato in heptanoato ligandom. Nobena od sintetiziranih spojin ne kaže fungicidne aktivnosti. Kristalna struktura Zn(O2CC5H11)2 - oblika A je bila določena z rentgensko strukturno analizo. Cinkovi atomi so tetraedrično koordinirani s kisikovimi atomi štirih različnih heksanoato ligandov. Heksanoato skupine kot mostovni ligandi povezujejo po dva cinkova atoma na syn-anti način, pri čemer nastane tipična plastovita struktura.