Zirconium Oxalates: Zr(OH)(CO), (HO)[Zr(CO)(HO)], and MM'[Zr(CO)]·HO with M and M' = Ammonium, Alkali Metal, and Hydroxonium Ion HO ( = 2, 3, 4).

Crystallized powder of dihydroxide zirconium oxalate Zr(OH)(CO) () was obtained by precipitation and the structure determined from powder X-ray data. The three-dimensional (3D) framework observed in () results from the interconnection of zirconium hydroxide chains [Zr(OH)] and zirconium oxalate chains [{Zr(CO)}]. Single crystals of (HO)[Zr(CO)(HO)] () were obtained by evaporation. The structure contains dimeric anions [Zr(CO)(HO)] connected through hydrogen bonds to hydroxonium ions (HO) to create a 3D supramolecular framework. The addition of ammonium or alkali nitrate led to the formation of single crystals of Na[Zr(CO)]·2HO (), M(HO)[Zr(CO)]·HO, M = K (), M = NH (), M(HO)(HO)[Zr(CO)], M = Rb (), and M = Cs (). For the five compounds, the structure contains ribbons [{ZrOx}] formed by entities Zr(CO) sharing two oxalates. In (), the shared oxalates are in cis positions and the chain [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 [Zr-Ox] are corrugated with Zr-Zr-Zr angles in the range 140.34(1)-141.07(1)°. In the compounds (), the cohesion between the ribbons is ensured by the alkaline or ammonium cations and the hydroxonium ions (HO) for M = K, NH, (HO), and (HO) for M = Rb and Cs. During the thermal decomposition of the alkaline-free zirconium oxalates (), (), and (), 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.