Synthesis, crystal structure, and solid-state NMR spectroscopy of K₂[Ga₄(C₂O₄)(PO₄)₄]·2H₂O, a new gallium phosphatooxalate with an intersecting tunnel structure

A new gallium phosphatooxalate, K₂[Ga₄(C₂O₄)(PO₄)₄]·H₂O, has been synthesized hydrothermally and characterized by single-crystal X-ray diffraction and solid-state NMR spectroscopy. It crystallizes in the triclinic space group P-1 (No. 2) with a = 7.9863(4), b = 8.2458(5), and c = 8.7631(5) Å, α = 116.075(1)°, β = 100.825(1)°, and γ = 104.033(1)°, V = 473.3(1) Å ³, and Z = 1. The structure consists of GaO₆ octahedra and GaO₄ tetrahedra connected by coordinating C₂O₄^2- and PO₄^3- anions to generate intersecting channels in the three-dimensional framework with guest water molecules and the charge compensating K⁺ cations within the channels. The water molecules and K⁺ cations sites are half occupied. ¹H MAS NMR confirms two different sites for water molecules and also shows that one is more mobile than the other. Three ³¹P 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 ³¹P resonances, which have nearly the same intensity, result from different local environments due to the ordering of the water molecules and K⁺ cations. The information obtained from ¹H → ³¹P CP/MAS experiments provides insight into the interaction between the guest water molecules in the structural tunnels and the phosphates in the framework. ⁷¹Ga MAS NMR confirms the presence of four- and six-coordinated Ga atoms.