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

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(C10N4H24)], 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.