Structural Variety within Gallium Diphosphonates Affected by the Organic Linker Length

Three new gallium diphosphonates: Ga(3)(OH)(O(3)PC(3)H(6)PO(3))(2) (1), Ga(4)(O(3)PC(5)H(10)PO(3))3(C(5)H(5)N)(2) (2), and Ga(HO(3)PC(10)H(20)PO(3)) (3), in which the diphosphonate bridging ligands have 3, 5, and 10 methylene units, respectively, have been synthesized using solvothermal methods and their structures determined using single-crystal laboratory and synchrotron X-ray diffraction data. All three materials contain Ga-centered tetrahedra and octahedra linked together through the -PO(3) groups of the diphosphonate ligands to form two-dimensional pillared slab (1) and three-dimensional pillared (2 and 3) materials. Compound 1 contains bridging hydroxide anions that connect Ga-centered octahedra and tetrahedra, and contains pillared slabs in which one side of the Ga-P-O/OH/CH hybrid layers are connected by the propylenediphosphonate groups only. This slab also contains propylenediphosphonate groups arranged orthogonally to the pillaring direction in the outermost layer of the Ga-P-O/OH/CH hybrid layers. Compound 2 is a framework structure that contains framework pyridine molecules between alternate layers of diphosphonate-pillared Ga-P-O layers and is structurally stable to loss of 1 equiv of pyridine molecules from the structure. Compound 3 is a partially condensed pillared framework structure with one P-O-H bond per diphosphonate group remaining in the resulting material. The structural changes observed as the alkylene chain in the diphosphonate ligand is increased in these compounds is compared to other members of the gallium diphosphonate family synthesized in a similar manner, and other metal diphosphonate series, to gain some general oversight of the structural trends observed in series of metal diphosphonate materials in which the alkylene chain length is varied systematically.