Developing Lanthanide-Nitrate Cluster Chemistry toward Rare Earth Separations.

Nitrate-decorated hexamers with a [Ln6(μ6-O)(μ3-OH)8]8+ core have been reported for nearly every lanthanide ion and are used as precursors for the assembly of functional metal-organic frameworks. Yet, few studies have examined the correlation between the solution and solid-state species, and the formation of mixed-metal clusters. Toward this end, a series of homo- and heterometal lanthanide nitrate hexamers was prepared via pH adjustment of aqueous lanthanide nitrate solutions. Examination of the homometallic europium solutions using Small Angle X-ray Scattering and nESI-MS showed that lower order complexes dominate lanthanide speciation in nitrate media. Yet, powder X-ray diffraction data of the precipitated phase confirmed the formation of [Ln6(μ6-O)(μ3-OH)8(NO3)6(H2O)12]·2(NO3)·n(H2O), Ln6, for Ln = Eu and Tb. For heterometal systems, analysis of the solid-state product by ICP-MS showed the selective incorporation of the heavier rare earths into Ln6. Selectivity was quantified by calculating an average separation factor, which is defined as the ratio of recovery factors of both metals. Further examination of the luminescence behavior of mixed metal [Tb6-xEux(μ6-O)(μ3-OH)8(NO3)6(H2O)12]·2(NO3)·n(H2O), with x = 1.1-3.6, showed that the relative intensities of the peaks at 489 nm (terbium, 5D4 → 7F6) and 690 nm (europium, 5D0 → 7F4) trend with the percent incorporation of europium and terbium into the cluster.