Structure and thermal behavior of the layered zincophosphate [NH3–CH2–CH(NH3)–CH3](ZnPO4)2

A zinc phosphate with the composition [NH3–CH2–CH(NH3)–CH3](ZnPO4)2, containing the doubly protonated 1,2-diaminopropane (abbr. HDAP), was synthesized by hydrothermal crystallization of zinc nitrate, phosphoric acid, 1,2-diaminopropane and trimethylenedipiperidine. The single crystal analysis shows a layered inorganic–organic structure built up of a sandwich-like motif of alternating inorganic layers and HDAP cations (trimethylenedipiperidine not being a constituent of the structure). The HDAP species are found to be disordered and serve as bridges between two adjacent inorganic layers which are separated by 3.95 Å. The bridging interaction occurs via a hydrogen-bonding network. The inorganic layer features a pattern of four-membered rings involving two ZnO4 and two PO4 tetrahedra connected by sharing O atoms. Thermal analysis shows that the compound is stable up to 370 °C and that the thermal decomposition of HDAP occurs in two steps between 370 and 460 °C, causing the collapse of the structure. The first decomposition step corresponds to ammonia removal which proceeds with a high activation energy (Ea = 282 kJ mol−1). The high Ea value is mainly attributed to strong electrostatic interactions between organic cations and anionic inorganic layers, the disruption of the interactions being the main reason for structural collapse after the HDAP removal.