Synthesis of a Single Four-Ring (S4R) Molecular Zinc Phosphate and Its Assembly to an Extended Polymeric Structure: A Single-Crystal and in-Situ MAS NMR Investigation

A reaction of ZnO, HCl, H3PO4, and 2-pyridylpiperazine in THF/H2O mixture at 75 °C for 72 h produces a new zinc phosphate, [(C5NH5)(C4N2H10)][Zn (H2PO4)2(HPO4)], I. Zinc phosphate I consists of single four-ring (S4R) units with terminal phosphoryl groups hanging from the Zn center. On reaction with zinc acetate dihydrate in the presence of water at 100 °C, I gave another new zinc phosphate, [(C5NH5)(C4N2H10)][Zn2(H2PO4)(HPO4)(PO4)]·2H2O, II. II has a layer structure with apertures formed by 4- and 8-T atoms (T = Zn, P). An examination of the two structures reveals that I and II are related, II being formed by the direct addition of Zn^2+ ions to I. Room-temperature 31P MAS NMR studies show the presence of different phosphorus species in both compounds. An in-situ 31P MAS NMR investigation on the formation of II from I in the presence of Zn^2+ ions and water reveals the transformation to be facile. What is noteworthy in this study is that the structural integrity of the S4Rs has been maintained during the formation of II. Donor-acceptor hydrogen bond interactions and (pi)-(pi) interactions involving the pyridyl groups also appear to play subtle roles in both phosphates. This study, the first attempt of its kind, combines the principles of supramolecular organic chemistry with inorganic building units and contributes to our understanding of the formation of framework solids.