Syntheses, crystal structures, thermal stabilities and luminescence of two new zinc phosphonates

Two new zinc phosphonates, [(C5H14N2)0.5Zn5(HO3PCH(OH)CO2)(O3PCH(OH)CO2)3 x 2 H2O] x 2 H2O (1) and [(C4H12N2)Zn2(O3PCH(OH)CO2)2] (2), have been synthesized under hydrothermal conditions and characterized by single-crystal X-ray diffraction along with powder XRD, elemental analysis (EA), IR, and thermogravimetric analysis (TGA). In compound 1, [ZnO6] octahedra and [ZnO5] square-pyramids are bridged by 2-hydroxyl(phosphono)acetate with tri-, tetra-, and pentadentate coordination modes, resulting in a 3D framework possessing 1D 24-atom channels. In compound 2, 2-hydroxyl(phosphono)acetate exhibits a new tridentate coordination mode to link [ZnO4] tetrahedrons into a 2D hybrid layer. Both 2-methylpiperazine and piperazine serve as structure-directing agents, which are encapsulated into frameworks through electrostatic interactions and hydrogen bonding. Solids 1 and 2 display purple and yellow-green emission, respectively. It is interesting that solid 1 can give off bright white, yellow and orange-red emission under different heat-treatment temperatures. Furthermore, the luminescent color of solid 1-250 can be reversibly tuned among yellow-green, purple-blue, bright white and blue-green by shifting the wavelength of excited light.