Structural studies of donor-free and donor-solvated sodium carboxylates

Focusing mainly on sodium 2-ethylhexanoate, this study reveals that the carboxylate exists as a dimer in MeOD solution as evidenced by Diffusion Ordered NMR SpectroscopY (DOSY). Two crystalline varieties with distinct polymeric structures have been synthesised and crystallographically characterised. A mixed 1,10-phenanthroline–water solvate [{(C5H10)(C2H5)COONa.(H2O)[1,10-phen]}2]∞ contains dimeric [Na(OH)2]2 subunits, which propagate through hydrogen bonds between O atoms of the carboxylate and OH water bonds. Adjacent polymeric chains interdigitate with each other through π-π interactions between 1,10-phen rings. Solvent-free sodium 2-ethylhexanoate has five-coordinate cations comprising one bidentate chelating and three monodentate carboxylate oxygen atoms. Here, the packing arrangement is different with the central hydrophilic (NaO2)∞ core surrounded by a wrapping of disordered alkyl groups. A similar polymeric structure is observed for the crystalline DMSO-solvated sodium valproate [{(C3H7)(C4H8)COONa.(DMSO)}]∞. This adopts a layered arrangement comprising alternating sodium carboxylate hydrophilic layers and hydrophobic organic bilayers.