Pincher ferrocene-derived cation carboxylate ion pairs in aqueous DMSO

Abstract

Pincher cationic ferrocene hosts for carboxylate ion guests were synthesized and the binding constants were determined by NMR or UV-Vis titrations. These (di)cationic hosts form tight complexes with benzoate or acetate even in competitive aqueous DMSO solvent. A bis(acylguanidinium) ferrocene dication achieves a remarkable K_a of ∼10⁶ M⁻¹ to acetate in 9 : 1 DMSO–H₂O and a K_a of 850 M⁻¹ in neat D₂O, one of the highest association constants known for a carboxylate complex exploiting only electrostatic interactions in neat water. DFT computations of the binding enthalpy are in good agreement with the experimentally determined association constants. The ferrocene backbone used in these pincher complexes may prove to be a useful semi-flexible scaffold for redox detectable/switchable self-assemblies in aqueous solutions.