Voltammetric Characterization of Redox-Inactive Guest Binding to Ln^III[15-Metallacrown-5] Hosts Based on Competition with a Redox Probe

Abstract

A novel competitive binding assay was implemented to monitor the binding of a redox inactive substrate to a redox inactive metallacrown host based on its competition with ferrocene carboxylate (FcC⁻) using cyclic voltammetry (CV). First, the binding of FcC⁻ to Ln^III[15-MC-5] (LnMC) hosts was characterized by cyclic voltammetry. It was shown that the voltammetric half wave potentials, E_1/2, shifted to more positive potentials upon the addition of LnMC. The explicit dependence of E_1/2 with the concentration of LnMC was used to determine the association constants for the complex. The FcC⁻ binding strength decreased with larger central lanthanide metals in the LnMC hosts, and substantially weaker binding was observed with La^III. X-ray crystallography revealed that the hydrophobic host cavity incompletely encapsulated FcC⁻ when the guest was bound to the nine-coordinate La^III, suggesting the LnMC’s ligand side chains play a substantial role in guest recognition. With knowledge of the MC-FcC⁻ solution thermodynamics, the binding affinity of a redox inactive guest was then assessed. Addition of sodium benzoate to a LnMC and FcC⁻ mixture resulted in E_1/2 shifting back to the value observed for FcC⁻ in the absence of LnMC. The association constants between benzoate and LnMC’s were calculated via the competitive binding approach. Comparison with literature values suggests this novel assay is a viable method for determining association constants for host–guest systems that exhibit the proper electrochemical behavior. Notably, this CV competitive binding approach does not require the preparation of a modified electrode or a tethered guest, and thus can be generalized to a number of host–guest systems.