Voltammetric characterization of redox-inactive guest binding to Ln(III)[15-Metallacrown-5] hosts based on competition with a redox probe.

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(Cu(II),N,L-pheHA)-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.