Pincher ferrocene-derived cation carboxylate ion pairs in aqueous DMSOElectronic supplementary information (ESI) available: Raw isotherm data and fitting parameters. New compound characterization data. Absolute energies and Cartesian coordinates for computed complexes. See DOI: 10.1039/c3ob40817e

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 ∼10 6 M −1 to acetate in 9 : 1 DMSO-H 2 O and a K a of 850 M −1 in neat D 2 O, 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. Pincher ferrocene-derived (bis)guanidinium (di)cations form complexes with monocarboxylates in competitive solvents (H 2 O-DMSO).