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.