Structural, odd–even chain alternation and thermal investigation of a homologous series of anhydrous silver(i) n-alkanoates

Abstract

Molecular and lattice structures of a homologous series (n_c = 8–20, inclusive) of silver (I) n-alkanoates are determined from X-ray Powder Diffraction, Solid State spin decoupled ¹³C-NMR and variable temperature Fourier Transform Infrared Spectroscopies. The compounds crystallize in a monoclinic crystal system with hydrocarbon chains in the fully extended all-trans conformation. Moreover, the chains are tilted ca. 75° with respect to the metal basal plane and are arranged as methyl(tail)-to-methyl(tail) bilayers within a lamellar. The methyl chain ends, from different layers in the bilayer, do not overlap but are in such close proximity to cause methyl–methyl interactions. In a molecule, two carboxylate groups bind in a syn–syn type bridging bidentate mode to two silver atoms to form an eight-membered structure. Intramolecular silver–silver and intermolecular Ag–O–Ag interactions stabilize the head group and promote the formation of layer type polymeric sheets. Though the compounds are nearly isostructural, odd–even chain alternation is observed in density, anti-symmetric stretching vibrations of methyl and unusually, carboxylate (head) groups, as a result of packing differences of hydrocarbon chains within the crystal lattice. These arise from the relative vertical distances between polymeric sheets, which are not in the same plane. Thus, for odd chain length compounds, where those distances are less than for even chains, more ordered packing and hence higher densities are observed for these adducts. Also, the numbers and natures of the thermotropic phase transitions are chain length dependent and irreversible.