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

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 (H 2 O) 2 ] ( Nd‐1 ), and an acac‐bridged Nd‐dimer, [Nd 2 (acac) 6 (H 2 O) 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‐ x Dy x (acac) 6 (H 2 O) 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.