Synthesis, structure, and polymorphism studies in amine-templated open-framework zinc phosphites

Abstract

Five new zinc phosphites, [C₁₀N₄H₂₆][Zn₂(HPO₃)₄]·2H₂O, 1, [C₁₀N₄H₂₆] [Zn₅(H₂O)₄ (HPO₃)₆]·4H₂O, 2, [C₁₀N⁴H₂₆][Zn₄(HPO₃) ₆]·2H₂O, 3, [C₁₀N₄H₂₆] [Zn₄(HPO₃)₆]·2H₂O, 4, and [Zn₂(HPO₃)₂(C₁₀N₄H ₂₄)], 5, were synthesized employing solvo/hydrothermal reactions in the presence of 1,4-bis (3-aminopropyl) piperazine (APPIP). Single crystal X-ray diffraction studies indicate that all the compounds form a hierarchy of structures. While the structures 1 and 2 are low-dimensional, 3-5 have three-dimensional connectivity. ZnO₄ and HPO₃ units form a 4-membered ring and are connected through their corners forming a one-dimensional chain structure in 1. 2 has one-dimensional ladders connected by ZnO₂(H₂O)₄ octahedral units forming a layer with 4- and 8-membered apertures. Compounds 3 and 4 have similar molecular formulae and connectivity, which makes them polymorphic in nature. The amine molecules exist in gauche and all-trans form in 3 and 4, respectively. The amine molecule binds with the zinc center in 5 and acts as a pillar between the two zinc phosphite layers. The present study outlines the possible role of synthesis parameters for the isolation of a number of different structures by employing a single amine molecule, APPIP. The observation of polymorphic structures along with the interconvertibilities of one of the phases is important and noteworthy. The ³¹P chemical shifts observed in NMR studies, consistent with the single crystal data, have been correlated with the valence sum values of the oxygen that are bound with the distinct phosphorus.