Synthesis and Structure of Low-Dimensional Gallium Fluorodiphosphates Seen during the Crystallization of the Three-Dimensional Microporous Gallium Fluorophosphate ULM-3

Two one-dimensional gallium fluorodiphosphates Ga(P 2 O 7 )F·H 3 N(CH 2 ) 3 NH 3 ·3H 2 O, 1, and Ga(P 2 O 7 )F·H 3 N(CH 2 ) 3 NH 3 ·H 2 O, 2, have been synthesized under mild conditions (<100 °C) from aqueous mixtures of Ga(NO 3 ) 3 ·6H 2 O, P 2 O 5, 1,3-diaminopropane, and HF. Their structures consist of anionic chains of formula Ga(P 2 O 7 )F 2 - containing corner-sharing {GaO 4 F 2 } octahedra linked together via fluorine and P 2 O 7 units, which are charge-balanced by propyldiammonium dications. During the hydrothermal synthesis of the open-framework three-dimensional ULM-3, compound 2 has been identified as the crystalline intermediate phase when phosphorus pentoxide is used as a starting material. This process has been followed in situ with time-resolved energy-dispersive X-ray diffraction (EDXRD), which reveals that dissolution of the one-dimensional phase 2 occurs before the rapid crystallization of ULM-3. If compound 2 is quenched from hydrothermal conditions, rapid and reversible conversion to compound 1 takes place. The two chain phases are thus solid precursors of the microporous ULM-3, and tracking the transformation process using in situ EXDRD shows that dissolution of 1 takes place before crystallization of 2 and then further transformation into the 3D phase, with the 1 and 2 acting as reservoirs of reactive species for ULM-3 crystallization.