Controlling the activity of components is an important factor in designing adaptive systems. Thus, the binding of anionic guests towards a catalytic host triggering its activity is a major step in controlling catalysis. Several 1,2,3-triazoles can be used to cooperatively bind anions and activate an enantioselective catalyst. However, to precisely tune these reactions, detailed knowledge of the binding process is necessary. Here, we study the cooperative binding of a range of different anions to a chiral tetrakis-triazole (1). 1H-NMR titrations of host (1) and guest reveal binding constants using the program BindFit. Thermodynamic parameters are examined by isothermal titration calorimetry (ITC) and give further insight into the binding process. Large differences between anions such as triflate, acetate, chloride, fumarate or benzoate are observed and mainly relate to the anions charge density. Effects of anion isomers and of fluorinated anions are investigated by comparing the anion species. Regarding binding constants, isomeric and fluorinated anions show significant differences towards their counterparts. Interestingly, comparing ammonium and tetraalkylammonium salts, even the cation shows a major effect on anion binding to the catalyst. Electrophoretic NMR experiments highlight the relevance of ion pairing in solution, which is reducing the effective amount of available anions. Ion pairing and the choice of a suitable cation are consequently having a dramatical impact on the binding equilibrium and should be considered in optimizing catalytic activity.

Cooperative anion binding by triazole containing catalysts: Variation of anion structure

ACS Spring Meeting 2021

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