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 (13)C-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.

Odd–even chain packing, molecular and thermal models for some long chain sodium(I) n-alkanoates

“…Indeed, the intensity ratios for these two bands, I s /I as , ( Table 2) confirm the foregoing, though they show no clear trend with increasing chain length. Additionally, the difference in frequencies between the asymmetric and symmetric stretching bands, Dm = (m as À m s ) cm À1 , has been used as an indicator for determining metal-carboxylate coordination type; though, it is not definitive [1][2][3][4]11,13,14,19,20]. Nevertheless, this approach has yielded excellent results for several coordination modes.…”

“…Representative spectra, shown in Fig. 1, are typical for metal carboxylates [1][2][3]13,14,18,19]. For example, the absence of the hydroxyl vibration, in the region of 3300-3500 cm À1 and replacement of the carbonyl (C@O) stretching band, at ca.…”

“…The presence of such bands indicate that the methylene groups adopt the all-trans conformation at room temperature; hence, the hydrocarbon chains are fully extended. According to several authors [1][2][3][4]13,14,[18][19][20][21][22], the type of crystal packing can be adduced from the shape of the dCH 2 absorption, at ca. 720 cm À1 , and bending vibration band at ca.…”

“…Diversity in the structural chemistry of metal carboxylates (soaps) and their physio-chemical properties arising therefrom, have been the subject of recent studies from our group [1][2][3]. This is mainly due to the possible application of these compounds as liquid crystals in the manufacturing of Liquid Crystal Display (LCD) devices, where they offer shorter switching times, a larger field of view and higher complexity relative to conventional twisted nematics.…”

“…Nevertheless, X-ray power diffraction has provided many useful insights into the molecular and lattice structures for these long-chain adducts [1,2,[13][14][15]. For example, Ellis and co-workers [2,16] have shown that the types and numbers of intermediate transitions are depended on chain length, changes in molecular symmetry, packing of the molecules in the crystal lattice and the relative strengths of metal-carboxyl (head groups) to van der Waals forces between hydrocarbon chains. Clearly, knowledge of their room temperature molecular structure and lattice packing is a prerequisite for understanding the phase behaviors of these compounds.…”