The room temperature structures of anhydrous zinc(II) hexanoate and pentadecanoate Richard A Taylor, Henry A. Ellis * , Paul T. Maragh, Nicole A.S. White Department of Chemistry, University of the West Indies, Mona, Kingston 7, Jamaica Received 13 September 2005; received in revised form 1 November 2005; accepted 1 November 2005 Available online 20 December 2005 Abstract The room temperature structures of anhydrous zinc (II) hexanoate and pentadecanoate have been studied using infrared spectroscopy, X-ray diffraction and polarizing light microscopy. Lattice parameters from single crystal X-ray and powder diffraction data, for short chain length hexanoate, are compared to validate the powder method which is then used, in conjunction with density and other molecular calculations to determine the structure of the longer chain length pentadecanoate. The compounds are isostructural; in that, each zinc atom is tetrahedrally coordinated to oxygen atoms of four different carboxylate groups and each ligand forms a bidentate bridge with two tetrahedral zinc atoms in a syn–anti arrangement. Bonding is unsymmetrical around the zinc atom. For both compounds, hydrocarbon chains, in the fully extended all-trans configuration, are tilted at an average angle between 58 and 618 to the planes containing zinc ions, though, in the case of the hexanoate, a bilayer arrangement of hydrocarbon chains within a lamella is indicated. The arrangement of the chains within the bilayer is regular with a similar side chain interaction for all the hydrocarbon moieties. For the longer chain length compound, an interdigitated bilayer arrangement of chains within a lamella is proposed. For this, an alternating head-to-tail arrangement within the layers between zinc atoms is indicated from the X-ray data. Nevertheless, both compounds crystallize within a monoclinic unit cell with P1c1 symmetry, at least for the hexanoate, with the chains arranged in a two dimensional network along the ac plane within the unit cell. q 2005 Elsevier B.V. All rights reserved. Keywords: Zinc(II) hexanoate; Zinc(II) pentadecanoate; X-ray diffraction; Polarising light microscopy; Lattice parameters; Lamellar; Infrared spectroscopy; Unit cell 1. Introduction Long chain carboxylates of polyvalent metal ions are substances of considerable importance and are used commer- cially in many applications [1,2]. However, they do not readily form crystals suitable for single-crystal X-ray analysis. Usually, the crystals are thin needles that are very fragile. Consequently, the few single crystal structures that have been reported have been those of the short chain compounds [3–11]. For example, anhydrous zinc acetate was reported to exist in a monoclinic or orthorhombic form, depending on the method of preparation [9] and coordination was reported to be by chelating bidentate bonding of carboxylate groups to zinc [10]. However, more recent studies have indicated that the structure was monoclinic, in which a zinc atom was tetrahedrally coordinated, in a bridging bidentate mode, to four oxygen atoms from four carboxylate groups in a syn–anti arrangement [12]. Indeed, zinc(II) hexanoate and propionate are reported to have similar syn–anti structures [8,11]. Additionally, infrared and 13 C solid state NMR studies [13] have indicated that the hexanoate and heptanoate exist in different polymorphic forms, differing only in interaction between hydrocarbon chains or in their orientations [13]. Whilst a crystal structure has been reported for anhydrous zinc(II) hexanoate [13], (CH 3 (CH 2 ) 4 COOZn; ZnC 6 ) in which each ligand is tetrahedrally coordinated to oxygen atoms from four different carboxylate groups, the coordination structure of the carboxylate groups around zinc for long chain compounds is not known, but for a relatively few compounds [14]. These are of such low symmetry that it is difficult to grow single crystals from them for X-ray diffraction studies. In this study, X-ray single crystal data are collected and analyzed to confirm or repudiate the reported structure of anhydrous zinc(II) hexanoate and to validate the X-ray powder method which is then used, in conjunction, with infrared, microscopy and density data, to determine the molecular Journal of Molecular Structure 787 (2006) 113–120 www.elsevier.com/locate/molstruc 0022-2860/$ - see front matter q 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2005.11.004 * Corresponding author. Tel.: C876 9358460; fax: C876 9771835. E-mail address: henry.ellis@uwimona.edu.jm (H.A. Ellis).