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.

Anion Encapsulation Drives the Formation of Dimeric GdIII[15-metallacrown-5]3+ Complexes in Aqueous Solution

“…Numerous studies have examined the formation, stability, and kinetics of these complexes, and detailed studies describing the structure of M(15-MC-5) with various metals and ligands have appeared. 7 Most interesting among these complexes are those prepared with chiral ligands, such as phenylalanine hydroxamic acid (pheHA), 8 which lead to the formation of face-differentiated MC, which places five ligand side chains on the same face (Scheme 1). …”

“…While crystallographic studies clearly demonstrate that hydrophobic compartments of different volume can be prepared by varying the hydroxamic acid ligand side chain, 10 there is limited information on whether such dimeric structures exist in solution and are competent to sequester guests into the generated molecular capsule. 8a,f A continuing issue that has plagued interpretation and implementation of MCs for guest recognition is the understanding of the composition of these molecules in solution. To this end, we have examined the binding features of the well-characterized Gd III [15-MC Cu II NpheHA -5] 3+ ( MC ; Scheme 1) 8e host with six different dicarboxylate guest molecules (Scheme 1) having variable lengths and different degrees of unsaturation and have explored to what extent MC is still capable of forming effective dimeric complexes in aqueous solution.…”

“…8a,f A continuing issue that has plagued interpretation and implementation of MCs for guest recognition is the understanding of the composition of these molecules in solution. To this end, we have examined the binding features of the well-characterized Gd III [15-MC Cu II NpheHA -5] 3+ ( MC ; Scheme 1) 8e host with six different dicarboxylate guest molecules (Scheme 1) having variable lengths and different degrees of unsaturation and have explored to what extent MC is still capable of forming effective dimeric complexes in aqueous solution. [Simplified MC nomenclature uses the formula M(X)[# ring n + , where M is the captured atoms-MC M′ (ox)NL -# ring oxygens] central ion, X represents ligands that may bridge between the central and ring metals, MC is the abbreviation for a metallacrown, M′ (ox) is the ring ion and oxidation state, N is the oxime nitrogen that forms part of the MC ring, and L is the ligand templating the MC.…”

“…Previous attempts to examine guest binding to MC quantitatively in solution using NMR failed because of the severe line broadening due to the presence of copper(II) in the MC backbone; likewise, the use of UV–vis was precluded by the essentially nonexistent change in the extinction coefficient upon going from host to host–guest complexes. Because the calorimetric technique has been proven to be a straightforward, convenient, and accurate method for assessing the stoichiometries and binding affinities of different guests with several hosts 8d,11,12 the interaction of MC with guests 1–6 in water was probed using isothermal titration calorimetry (ITC). 13,14 An example of typical experimental results is shown in Figure 1.…”

Crystal structure of bis(μ2-ferrocenecarboxylato-κ2 O:O′)-bis(1,10-phenanthroline-κ2 N,N′)-(μ2-methanolato-κ2 O,O)dicopper(II) tetrafluoroborate – acetonitrile (1/1), C49H40BCu2F4Fe2N5O5

“…As a versatile functionalized ligand, ferrocenecarboxylate is widely used for the construction of heterometallic complexes with other metals. Among them, most of heterodimetallic complexes, such as mononuclear complexes [14][15][16], polymers [17], clusters [18], and MOFs [19] are reported in the last few years.…”