Novel gallium phosphatooxalate with pendant oxalate ligands: Preparation, crystal structure, NMR spectroscopy, and thermal stability

A new gallium phosphatooxalate hybrid material, (C10H28N4)[Ga-6(OH)(2)(PO4)(2)(HPO4)(4)-(C2O4)(3)].3.5H(2)O (1), has been synthesized under mild hydrothermal conditions (150 degreesC) in the presence of the amine 1,4-bis(3-aminopropyl)piperazine (APPIP) as a structure-directing agent. The compound was structurally characterized by single-crystal X-ray diffraction, thermogravimetric analysis, and H-1, H-2, C-13, P-31, and Ga-71 solid-state NMR spectroscopy. The structure of (1) consists of gallium phosphate double layers formed of GaO6 octahedra, GaO4 tetrahedra, and HPO4 and PO4 groups sharing corners. The double layers are crosslinked by oxalate ligands to form a three-dimensional framework with intersecting channels where the amine and water molecules are located. The oxalate units play a dual role as nonbridging mono-bidentate ligands with two terminal C-O groups pendant inside the channels and bis-bidentate ligands bridging the double layers. Crystal data are as follows: triclinic P (1) over bar (No. 2), alpha = 9.0220(7) Angstrom, b = 9.1864(9) Angstrom, b = 13.4912(9) Angstrom, alpha = 73.988(9)degrees, beta = 79.199(7)degrees, gamma = 78.775(7)degrees, V = 1043.6(2) Angstrom(3), and Z = 1. Ga-71 MAS NMR confirms the presence of four- and six-coordinated Ga atoms. Three P-31 resonances at -0.2, -4.15, and -7.7 ppm in a 1: 1: 1 ratio are observed, consistent with the structural model determined from the XRD structural analysis. Thermogravimetric analysis and X-ray diffraction show that the framework is stable up to about 300 degreesC after complete dehydration. The compound partially reabsorbs water, giving a cell volume intermediate between the completely hydrated and dehydrated phase.