Synthesis, crystal structure, and solid-state NMR spectroscopy of K-2[Ga-4(C2O4)(PO4)(4)]center dot 2H(2)O, a new gallium phosphatooxalate with an intersecting tunnel structure

A new gallium phosphatooxalate, K-2[Ga-4(C2O4)(PO4)(4)]. 2H(2)O, has been s ynthesized hydrothermally and characterized by single-crystal X-ray diffrac tion and solid-state NMR spectroscopy. It crystallizes in the triclinic spa ce group P-1 (No. 2) with a = 7.9863(4), b = 8.2458(5), and c 8.7631(5) Ang strom, alpha = 116.075(1)degrees, beta = 100.825(1)degrees, and gamma = 104 .033(1)degrees, V = 413.3(1) Angstrom(3), and Z = 1. The structure consists of GaO6 octahedra and GaO4 tetrahedra connected by coordinating C2O42- and PO43- anions to generate intersecting channels in the three-dimensional fr amework with guest water molecules and the charge compensating Kf cations w ithin the channels. The water molecules and Kf cations sites are half occup ied. IH MAS NMR confirms two different sites for water molecules and also s hows that one is more mobile than the other. Three P-31 resonances at -10.9 , -12.3, and -14.9 ppm are observed at room temperature; the resonance at - 14.9 ppm splits into two peaks at 375 K. These four P-31 resonances, which have nearly the same intensity, result from different local environments du e to the ordering of the water molecules and Ki cations. The information ob tained from H-1 -> P-31 CP/MAS experiments provides insight into the intera ction between the guest water molecules in the structural tunnels and the p hosphates in the framework. Ga-71 MAS NMR confirms the presence of four- an d six-coordinated Ga atoms.