Bis-amino Acid Derivatives of 1,1′-Ferrocenedicarboxylic Acid: Structural, Electrochemical, and Metal Ion Binding Studies

Abstract

We report on the structural and electrochemical investigation of 1,1′-ferrocenedicarboxylic acid derivatives of tryptophan (Fc[CO-Trp-OMe]2 - Fc-conjugate 1), threonine (Fc[CO-Thr-OMe]2 – Fc-conjugate 2), aspartic acid (Fc[CO-Asp-OMe]2 - Fc-conjugate 3) and glutamic acid (Fc[CO-GluOMe]2 - Fc-conjugate 4) and their hydrolyzed analogues 1a–4a respectively (Scheme 1). CD and NMR spectroscopy established 1,2′-“Herrick conformation” in solution, having intramolecular interstrand hydrogen bonds for all Fc-conjugates. However, in solid state, Fc[CO-Trp-OMe]2 exists in “Herrick conformation” whereas Fc[CO-Thr-OMe]2 is present in anti conformation. In solution, the involvement of indole NH of Trp and alcoholic proton of Thr in intermolecular hydrogen bonding has been explored by temperature- and concentration-dependent NMR studies. The half-wave potentials (E1/2) of ferrocene-conjugates follow the sequence 1 < 2 < 4 < 3 which is explained by the contribution of amino acid side chain functionalities toward the stability of ferrocenium ion. The CV of the Fc-conjugate 1/1a (having Trp moiety) displays two redox processes, one of which is assigned to the Fc group, and the other being related to the indole group. The oxidation peak potential of indole was found to depend strongly on the pH of the medium. The values of diffusion coefficient (D) and electron transfer rate constant (ksh) for all Fc-conjugates were determined from their corresponding cyclic voltammograms. In addition, metal ion interactions were studied with hydrolyzed Fc-conjugates 2a–4a using CV and DPV. Upon binding to metal ions, the electrochemical changes associated with the hydrolyzed Fc-conjugates correlated to the charge density of the binding metal ion.