Anhydrous polymeric zinc(II) pentanoate

The structure of the title compound, poly[di-μ-pentanoato-zinc(II)], [Zn{CH3(CH2)3COO}2]n, consists of a three-dimensional polymeric layered network with sheets parallel to the (100) plane, in which tetrahedrally coordinated zinc(II) ions are connected by pentanoate bridges in a syn–anti arrangement. The hydrocarbon chains are in the fully extended all-trans conformation and are arranged in a tail-to-tail double bilayer.

The structure of the title compound, poly[di--pentanoatozinc(II)], [Zn{CH 3 (CH 2 ) 3 COO} 2 ] n , consists of a three-dimensional polymeric layered network with sheets parallel to the (100) plane, in which tetrahedrally coordinated zinc(II) ions are connected by pentanoate bridges in a syn-anti arrangement. The hydrocarbon chains are in the fully extended alltrans conformation and are arranged in a tail-to-tail double bilayer.
The structure ( Fig. 1) is four-coordinate, where each zinc ion is tetrahedrally coordinated by oxygen atoms from four different pentanoate ligands. The four pentanoate ligands around zinc are of the Z,E-type bridging bidentate mode; that is, they are bonded in a syn-anti arrangement to two tetrahedral zinc ions. Geometric data indicate that the Zn-O bond lengths are not equivalent and clearly point to unsymmetrical bonding around the zinc ion.
The alkyl chains of the pentanoate groups are in the fully extended all-trans conformation. There is excellent agreement of the C-C bond lengths and C-C-C angles with published values for hydrocarbon chains in a fully extended all-trans conformation (Lomer & Perera, 1974). There are four formula units in the unit cell and two distinct basal planes, resulting in a double bilayer lamella arrangement forming a polymeric network ( Fig. 2) with an alternating packing of the hydrocarbon chains in neighbouring bilayers. When viewed down the b axis, the hydrocarbon chains, which are tilted with respect to the zinc basal planes, are in each bilayer aligned in different planes. The structure appears very different when viewed down the a axis (Fig. 3), where in one bilayer the chains appear to zigzag and cross at the bonds along the C-C axis. In the other bilayer the chains are tilted towards each other and appear to cross each other at carbon atom number 4.
The molecular packing (Fig. 4) highlights the distorted tetrahedra around the zinc ions. In one basal plane, the vertices of the tetrahedra alternate parallel and perpendicular to the vertical plane throughout and in the other basal plane the vertices alternate at the top and bottom throughout. This arrangement allows for alternating basal planes in the overall structure to be identical.
There is interaction between parallel sheets through bidentate bridging, resulting in a three-dimensional sheet-like/layered polymeric network where the chains are arranged tail-to-tail, arising from van der Waals interactions in sheets parallel to the ac plane.

Experimental
Single crystals of zinc(II) pentanoate were prepared from the reaction of zinc oxide (0.407 g) and n-pentanoic acid (5.0 cm 3 ; >100% excess) in approximately 100 cm 3 of ethanol. The white suspension was refluxed until the solution was transparent.
supplementary materials sup-2 The resulting hot, colorless solution was filtered by suction and the filtrate left to cool to room temperature. After about six days, long, thin, colourless, plate-like single crystals, some in clusters, crystallized from solution. The crystals were then removed, air-dried, and kept in sealed vials at ambient temperature.

Refinement
H atoms were positioned geometrically and refined as riding, with C-H = 0.97 Å and U iso (H) = 1.2U eq (C) for methylene, and C-H = 0.96 Å and U iso (H) = 1.5U eq (C) for methyl groups. The crystal was weakly diffracting at high angles. Fig. 1. : Asymmetric unit of zinc(II) n-pentanoate: Displacement ellipsoids are drawn at the 75% probability level.    Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating Rfactors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.