1D polymeric and 2D layered zinc monoalkyl phosphates bearing long alkyl chains: synthesis, structural transformations and thermal behaviour

Abstract

The reactivity of two different alkyl phosphate esters with zinc metal ions has been explored. The reaction of zinc nitrate with n-octyl phosphate leads to a mixture of [(ⁿOctO)PO₃Zn(DMF)]_n (1) and [(ⁿOctO)PO₃Zn(H₂O)]_n (2a); similarly, the reaction of zinc nitrate with n-hexyl phosphate results in a mixture of [(ⁿHexO)PO₃Zn(DMF)]_n (3) and [(ⁿHexO)PO₃Zn(H₂O)]_n (4). While the DMF-coordinated compounds 1 and 3 are isolated as single crystals, the water-coordinated compounds 2a and 4 were obtained as polycrystalline samples. Exposure of these mixtures (1 and 2a or 3 and 4) to ambient humid conditions results in the exchange of coordinated DMF molecules by water molecules and the whole mixture transforming into either 2a or 4, respectively. The molecular structures of 1 and 3 have been determined by single-crystal X-ray diffraction studies, revealing them to be one-dimensional coordination polymers. The presence of flexible alkyl chains in 2a and 4 results in structural transformation, where the interlayer distance in 2a decreases over time under ambient conditions to form 2b, and it returns to its initial state on heating at 50 °C for 20 hours in a vacuum. DSC studies for 2a and 4 show two low-enthalpy reversible transitions below room temperature, which could be associated with the 3D organization of alkyl chains in the structure. Thermal decomposition studies reveal that compounds 2a and 4 can be utilized as a single-source precursor for ceramic materials. The water contact angles for 2a and 4 are 100.5° and 98.8°, respectively, which fall within the range of hydrophobic materials, indicating the importance of alkyl chains in imparting the hydrophobicity.