Layered microporous tin(iv) bisphosphonates

Abstract

This article reports the hydrothermal synthesis and characterization of two new series of porous tin(IV) phosphonophenoxyphenylphosphonates with controlled pore size distributions, using as precursor the 4-(4′-phosphonophenoxy)phenyl phosphonic acid, [H₂O₃P–C₆H₄]₂–O. Supermicroporous solids (S_BET, 300–400 m² g⁻¹) were obtained employing n-alcohol (C1-C6)–water mixtures (solvents ratio 1 : 1), in the presence of hydrofluoric acid. X-Ray powder diffraction shows that these compounds are semi-crystalline and the local environments around the phosphorus and tin elements have been studied by ³¹P and ¹¹⁹Sn MAS-NMR spectroscopy, respectively. The microstructure (particle sizes and shapes) of these phosphonates has been analyzed by scanning and transmission electron microscopy. This study shows that the microstructures of single-ligand (for instance tin(IV) phenylphosphonate) and cross-linked tin(IV) bisphosphonates are different. Tin(IV) phenylphosphonate crystallizes as micron-sized spheres, ∅ ∼ 1–2 µm, formed by the aggregation of nanospheres, whereas tin(IV) bisphosphonates crystallize as microparticles larger than 20 µm. The textural properties of these porous solids were characterized by N₂ and CO₂ sorption isotherms. The key result of this work is that maxima of pore size distributions smoothly shift from 12 to 16 Å upon increasing the chain length of the alcohol. The microporosity of tin(IV) bisphosphonates is compatible with a double role played by the phosphonate groups acting as a pillar between adjacent layers and as a component of the hybrid organic–inorganic layers.