Department of Chemistry , University of Warwick , Coventry , CV4 7AL , U.K.
School of Chemistry, EaStCHEM and Centre of Magnetic Resonance , University of St Andrews , North Haugh , St Andrews , KY16 9ST , U.K.
Inorg Chem. 2018 Sep 4;57(17):11217-11224. doi: 10.1021/acs.inorgchem.8b01797. Epub 2018 Aug 14.
The rhodium(III) hydrogarnets CaRh(OH) and SrRh(OH) crystallize as polycrystalline powders under hydrothermal conditions at 200 °C from RhCl·3HO and either Ca(OH) or Sr(OH) 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 BaO 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), 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. 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. CaRh(OH) yields CaRhO and CaO above 650 °C, from which phase-pure CaRhO is isolated by washing with dilute nitric acid, a material previously only reported by high-pressure or high-temperature synthesis. SrRh(OH) decomposes to give a less crystalline material with a powder XRD pattern that is matched to the 2H-layered hexagonal perovskite SrRhO, which contains mixed-valent Rh, confirmed by Rh K-edge XANES spectroscopy. On heating BaNaRh(OH), a complex set of decomposition events takes place via transient phases.
三价铑氢石榴石 CaRh(OH) 和 SrRh(OH) 在 200°C 的水热条件下，由 RhCl·3HO 和 Ca(OH) 或 Sr(OH) 在 12 M NaOH 或 KOH 中反应生成多晶粉末。利用同步辐射粉末 X 射线衍射（XRD）数据的 Rietveld 精修，可以首次确定这两种材料的晶体结构。如果使用 BaO 作为试剂，并将氢氧化物的浓度增加到水通量条件（过量 NaOH），则在纯相样品中形成一种新的复杂铑氢氧化物 BaNaRh(OH)的单晶，其中钠离子来自通量。从单晶 XRD 数据中得到的结构解析揭示了孤立的八面体 Rh 中心，这些中心与 10 配位的 Ba 和两个独立的 8 配位 Na 位点共享氢氧化物。Na 魔角旋转 NMR 证实了两个结晶学上不同的 Na 位点的存在，并证实了样品的反磁性，这是 Rh(III)预期的性质。通过 X 射线热差分析研究了氢氧化物在空气中加热时的热行为，结果表明每种材料都有不同的分解途径。CaRh(OH)在 650°C 以上分解生成 CaRhO 和 CaO，从中用稀硝酸洗涤可分离出纯相的 CaRhO，这种材料以前仅通过高压或高温合成报道过。SrRh(OH)分解生成结晶性较差的物质，其粉末 XRD 图谱与 2H 层状六方钙钛矿 SrRhO 相匹配，其中包含混合价态的 Rh，这通过 Rh K 边 XANES 光谱证实。在加热 BaNaRh(OH)时，通过瞬态相发生了一系列复杂的分解事件。
