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Influence of Countercation Hydration Enthalpies on the Formation of Molecular Complexes: A Thorium–Nitrate Example

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posted on 17.11.2017, 00:00 by Geng Bang Jin, Jian Lin, Shanna L. Estes, S. Skanthakumar, L. Soderholm
The influence of countercations (An+) in directing the composition of monomeric metal–ligand (ML) complexes that precipitate from solution are often overlooked despite the wide usage of An+ in materials synthesis. Herein, we describe a correlation between the composition of ML complexes and A+ hydration enthalpies found for two related series of thorium (Th)–nitrate molecular compounds obtained by evaporating acidic aqueous Th–nitrate solutions in the presence of A+ counterions. Analyses of their chemical composition and solid-state structures demonstrate that A+ not only affects the overall solid-state packing of the Th–nitrato complexes but also influences the composition of the Th–nitrato monomeric anions themselves. Trends in composition and structure are found to correlate with A+ hydration enthalpies, such that the A+ with smaller hydration enthalpies associate with less hydrated and more anionic Th–nitrato complexes. This perspective, broader than the general assumption of size and charge as the dominant influence of An+, opens a new avenue for the design and synthesis of targeted metal–ligand complexes.