Synthesis and Structure Evolution in Metal Carbazole Diphosphonates Followed by Electron Diffraction

To access porous metal phosphonates, a new V-shaped, rigid, and sterically demanding diphosphonic acid, namely 3,6-diphosphono-9H-carbazole (H₄L), was designed and employed in a high-throughput investigation. Screening of different metal salts and subsequent optimization studies resulted in the isolation of two porous metal phosphonates [Cu₂(H₂O)₂(L)]·2H₂O (CAU-37) and [Zn_6.75(H₂O)_1.5(HL)_2.5(L)_1.5]·8H₂O (CAU-57). Structure determination was accomplished by electron diffraction and the dehydration behavior of CAU-37 was followed in situ. A rare case of intralayer water de-/adsorption in CAU-37 was found which leads to a cell volume change of 11.9%. Rod-shaped inorganic building units (IBUs) are connected to layers and structural flexibility is due to "accordion-like" structural changes within the layers. In contrast, in CAU-57 a layered IBU is found, which usually results in the formation of dense structures. Due to the shape and rigidity of the linker, the interconnection of the IBUs results in the formation of pores. Water sorption measurements in combination with powder X-ray diffraction data confirmed the reversibility under structural retention.