Lanthanide Differentiation Using Acetylacetonate: Insights into the Solution Speciation and Crystallization Behavior of Homo- and Heterometal Complexes

Abstract

The lanthanides form a chemically cohesive series, and identifying differences that can be leveraged to separate near neighbors remains an ongoing need. Here, the synthesis, structural chemistry, and spectroscopic properties of a neodymium-acetylacetonate monomer, [Nd(acac)₃(H₂O)₂] (Nd-1), and an acac-bridged Nd-dimer, [Nd₂(acac)₆(H₂O)₂] (Nd-2), are described. Interestingly, the Nd-dimer readily precipitates from solution; however, efforts to crystallize the Dy analog using the same synthetic approach are unsuccessful. Yet heterometal dimers, [Nd_2-xDy_x(acac)₆(H₂O)₂] (NdDy-2), can be prepared from mixed-metal solutions. To understand these differences in crystallization behavior of Nd- and Dy-acac complexes, solution speciation of Ln-acac systems is examined using nano-electrospray ionization-mass spectrometry (nESI-MS). Both homometal and heterometal solutions are prepared (Ln = Nd, Dy, Nd/Dy). nESI-MS of the solutions showed presence of homometallic dimeric species for Nd and Dy and peaks consistent with both homometallic and heterometallic complexes in mixed-metal solutions. Density functional theory is used to further understand the intrinsic differences in nucleation energetics in dimeric homo- and heterometallic species. Overall, this work provides fundamental insight into the correlation between solution- and solid-state structural units and differences in the crystallization behavior of Ln complexes; the latter is particularly relevant to separating metal ions with otherwise very similar chemical behavior.