A supramolecular approach towards the construction of molecular salts using phosphonic acid and pyrazole

Abstract

Systematic co-crystallization of a pyrazole analogue with various phosphonic acids resulted in robust supramolecular synthons between phosphonic acid and pyrazole, responsible for solid-state design. A series of phosphonic acids and methylenebis(3,5-dimethylpyrazole) (MBPz) has produced six molecular salts viz., [HPPA⁻·H₂PPA·HMBPz⁺·MBPz] (1), [(HtBPA⁻)₂·H₂MBPz²⁺·EtOH] (2), [HPAA⁻·HMBPz⁺·MBPz·MeOH] (3), [HPPRA⁻·HMBPz⁺] (4), [H₂EA²⁻·H₂MBPz²⁺·S] (5) and [H₃DPA⁻·(H₂DPA²⁻)_0.5·H₂MBPz²⁺·(H₂O)_0.4] (6) where H₂PPA = phenylphosphonic acid, H₂PAA = phosphonoacetic acid, H₂PPRA = 3-phosphonopropionic acid, H₂tBPA = tert-butylphosphonic acid, H₄EA = etidronic acid, H₄DPA = [1,4-phenylenebis(methylene)]diphosphonic acid and S = squeezed solvent molecule. Different substituent(s) on the phosphonic acid influenced the conformation of the MBPz molecule, which in turn led to the formation of distinct supramolecular organic frameworks (SOFs) with an array of reliable synthons. Structural analyses revealed that both compounds 1 and 2 exhibited a layered structure but the parallel stacked sheets are displaced in the former salt as compared to the latter. Salts 4 and 5 exhibited a three-dimensional (3D) hydrogen bonded supramolecular network and 2D porous supramolecular network, respectively. On the other hand, salts 3 and 6 display an interpenetrated supramolecular network and host–guest assembly, respectively. Furthermore, the interaction energy was calculated theoretically by using the Density Functional Theory (DFT) level energy to evaluate the stability of the corresponding salt, which was further corroborated by differential scanning calorimetry-thermogravimetric (DSC-TG) analysis.