Alkaline-Earth Rhodium Hydroxides: Synthesis, Structures, and Thermal Decomposition to Complex Oxides.

The rhodium(III) hydrogarnets Ca3 Rh2 (OH)12 and Sr3 Rh2 (OH)12 crystallize as polycrystalline powders under hydrothermal conditions at 200 °C from RhCl3 ·3H2 O and either Ca(OH)2 or Sr(OH)2 in either 12 M NaOH or KOH. Rietveld refinements against synchrotron powder X-ray diffraction (XRD) data allow the first crystal structures of the two materials to be determined. If BaO2 is used as a reagent and the concentration of hydroxide increased to hydroflux conditions (excess NaOH), then single crystals of a new complex rhodium hydroxide, BaNaRh(OH)6 , are formed in a phase-pure sample, with sodium included from the flux. Structure solution from single-crystal XRD data reveals isolated octahedral Rh centers that share hydroxides with 10-coordinate Ba and two independent 8-coordinate Na sites. 23 Na magic-angle spinning NMR confirms the presence of the two crystallographically distinct Na sites and also verifies the diamagnetic nature of the sample, expected for Rh(III). The thermal behavior of the hydroxides on heating in air was investigated using X-ray thermodiffractometry, showing different decomposition pathways for each material. Ca3 Rh2 (OH)12 yields CaRh2 O4 and CaO above 650 °C, from which phase-pure CaRh2 O4 is isolated by washing with dilute nitric acid, a material previously only reported by high-pressure or high-temperature synthesis. Sr3 Rh2 (OH)12 decomposes to give a less crystalline material with a powder XRD pattern that is matched to the 2H-layered hexagonal perovskite Sr6 Rh5 O15 , which contains mixed-valent Rh3+/4+ , confirmed by Rh K-edge XANES spectroscopy. On heating BaNaRh(OH)6 , a complex set of decomposition events takes place via transient phases.