Zirconium Oxalates: Zr(OH)(2)(C(2)O(4)), (H(11)O(5))(2)[Zr(2)(C(2)O(4))(5)(H(2)O)(4)], and MM′[Zr(C(2)O(4))(3)]·xH(2)O with M and M′ = Ammonium, Alkali Metal, and Hydroxonium Ion H(2n+1)O(n)(+) (n = 2, 3, 4)

[Image: see text] Crystallized powder of dihydroxide zirconium oxalate Zr(OH)(2)(C(2)O(4)) (ZrOx) was obtained by precipitation and the structure determined from powder X-ray data. The three-dimensional (3D) framework observed in (ZrOx) results from the interconnection of zirconium hydroxide chains (1)(∞)[Zr(OH)(2)](2+) and zirconium oxalate chains (1)(∞)[{Zr(C(2)O(4))}(2+)]. Single crystals of (H(11)O(5))(2)[Zr(2)(C(2)O(4))(5)(H(2)O)(4)] (H2Zr2O5) were obtained by evaporation. The structure contains dimeric anions [Zr(2)(C(2)O(4))(5)(H(2)O)(4)](2–) connected through hydrogen bonds to hydroxonium ions (H(11)O(5))(+) to create a 3D supramolecular framework. The addition of ammonium or alkali nitrate led to the formation of single crystals of Na(2)[Zr(C(2)O(4))(3)]·2H(2)O (Na2ZrOx3), M(H(7)O(3))[Zr(C(2)O(4))(3)]·H(2)O, M = K (KHZrOx3), M = NH(4) (NH4HZrOx3), M(H(5)O(2))(0.5)(H(9)O(4))(0.5)[Zr(C(2)O(4))(3)], M = Rb (RbHZrOx3), and M = Cs (CsHZrOx3). For the five compounds, the structure contains ribbons (1)(∞)[{ZrOx(3)}(2–)] formed by entities Zr(C(2)O(4))(4) sharing two oxalates. In (Na2ZrOx3), the shared oxalates are in cis positions and the chain (1)(∞)[Zr–Ox] is stepped with a Zr–Zr–Zr angle of 98.27(1)°. In the other compounds, the shared oxalates are in trans positions and the chains (1)(∞)[Zr–Ox] are corrugated with Zr–Zr–Zr angles in the range 140.34(1)–141.07(1)°. In the compounds (MHZrOx3), the cohesion between the ribbons is ensured by the alkaline or ammonium cations and the hydroxonium ions (H(7)O(3))(+) for M = K, NH(4), (H(5)O(2))(+), and (H(9)O(4))(+) for M = Rb and Cs. During the thermal decomposition of the alkaline-free zirconium oxalates (ZrOx), (H2Zr2Ox5), and (NH4HZrOx3), the formed amorphous zirconia is accompanied by carbon; the oxidation of carbon at about 540 °C to carbon dioxide is concomitant with the crystallization of the stabilized tetragonal zirconia.