Tripodal ligand frameworks based on 1,3,5-trisubstituted-2,4,6 triethylbenzene provide a highly preorganized cavity for binding of cations, such as NH4+, Ln3+ and An3+. Previous work by Chin et al. (Angew. Chem. Int. Ed. 1999, 38, 2756) and Ahn et al. (Chem. Lett. 2000, 170) has shown the potential of such frameworks for binding NH4+. We have also shown remarkable selectivity of these frameworks for binding NH4+ over K+ both by fluorescence and NMR titrations. Herein we undertake a systematic study of the effects of pyrazole substitution on their cation-binding properties, by X-ray crystallography, NMR spectroscopy, fluorescence spectroscopy, and density functional theory (DFT) calculations. Specifically, it was found that substituting two methyl groups on each pyrazole with phenyl groups results in a drop of binding constant (as measured by 1H-NMR titrations) from 74000 M-1 to less than 20 M-1. Furthermore, Ln(III) extraction by those ligands showed similar effects by these substitution patterns. Comparison of X-ray structures for the NH4PF6 complexes with the diphenyl- vs the dimethyl- analog showed preservation of ion pairing via hydrogen bonding for the dimethyl analog, but not for the diphenyl analog. This finding, which was also corroborated by DFT calculations, indicates the critical role of ion pairing in cation complexation by these ligands. Such effects can potentially be utilized for facilitating selective metal ion separations via solvent extraction.

Structural, spectroscopic, and theoretical studies on the effects of pyrazole substitution and ion-pairing in binding and sensing of NH4+ and Lanthanides(III) by tripodal tris-pyrazole receptors

254th National Meeting (2017)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.