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 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.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/77442/1/chem_200903015_sm_miscellaneous_information.pd