Structural Trends and Vibrational Analysis of N,N,N′,N′‐Tetramethylmalonamide Complexes Across the Lanthanide Series

Fundamental understanding of coordination chemistry across the lanthanide series is essential for explaining chemical behavior of rare‐earth metals in complex liquid‐liquid extraction processes, which in turn affects the distribution ratios and efficacy of separations as a whole. In this work, we explore the structural trends between the lanthanides and a neutral N,N,N′,N′‐tetramethylmalonamide (TMMA) ligand within four isolated families of solid‐state compounds: Ln(trans‐TMMA)2(NO3)3 Ln = La‐Nd, Sm; Ln(cis‐TMMA)2(NO3)3 Ln = Eu‐Tb, Er; [Ln(TMMA)3(NO3)2][Ln(TMMA)(NO3)4] Ln = Dy‐Tm; Ln(Κ2‐TMMA)(iPrOH)(NO3)3 and Ln(Κ1‐TMMA)(Κ2‐TMMA)(NO3)3 Ln = Yb, Lu. Moving across the lanthanide series, we note the formation of both discrete charge‐neutral complexes, as well as charged molecular anion‐cation pairs, with variations in spatial ligand arrangement, coordination numbers, and ligand denticities. IR and Raman spectroscopy paired with DFT frequency calculations were used for an in‐depth investigation of vibrational modes unique to each structural family. The collection of isolated model compounds was also discussed in the context of liquid‐liquid separations based on reported distribution ratios from malonamide extraction.