Breathing Effect in a Tetrapodal Lanthanide‐Phosphonate Organic Framework

Abstract The highly flexible organic linker hexamethylenediamine‐ N,N,N’,N’ ‐tetrakis(methylphosphonic acid) (H 8 htp) was used in the preparation of a new 3D MOF formulated as [La 2 (SO 4 ) 2 (H 6 htp)(H 2 O) 4 ] ⋅ 2H 2 O ( UAV‐15) by using hydrothermal synthesis. The flexibility of the organic linker posed a serious challenge in the preparation of crystalline materials: for this, the judicious selection of sulfuric acid to act as a mineralizing agent proved to be instrumental, acting in the retardation of the coordination of the phosphonic acid groups, as well as being an integral part the UAV‐15 by coordinating to the metal center. The material is a 3D network having channels filled with crystallization water molecules that can be easily removed by increasing of temperature (120 °C for 24 h), leading to a single‐crystal‐to‐single‐crystal transformation originating [La 2 (SO 4 ) 2 (H 6 htp)(H 2 O) 3 ] ( UAV‐15 d ). This transformation maintains the overall topology of UAV‐15 with only a small distortion of the framework. The reversibility of this transformation was studied, with the material reverting naturally to its original structure at ambient conditions for a period of 3 months or, in a faster way, when immersed in water at 120 °C for 24 h.