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 [Tb_6-xEu_x(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.