New ionophoric calix[4]diquinones: Coordination chemistry, electrochemistry, and X-ray crystal structures

A new library of ionophoric p-tert-butylcalix[4]diquinones containing ester (1), primary, secondary, and tertiary amide (2-4), and crown ether (5) substituents has been synthesized by treatment of the respective 1, 3-bis-substituted p-tert-butylcalix[4]arene with Ti(OCOCF3)(3) in trifluoroacetic acid. The structures of the free ligands 1 and 2, a p-tert-butylcalix[4]diquinone bridged at the lower rim by two linked veratrole groups (7), and a previously synthesized p-tert-butylcalix[4]diquinone bis(methyl ether) species (8) have been elucidated by X-ray crystallography. The X-ray crystal structures of 1-Sr(ClO4)(2), 1-KClO4, 2-NaClO4, 4-n-BuNH3BF4, 5-NaOCOCF3, and 5-KPF6 demonstrate that these complexes adopt the cone conformation in the solid state. Interestingly, cation-quinone oxygen atom coordination occurs at both the upper and lower rim of 2-NaClO4 and 4-n-BuNH3BF4. Solution coordination properties have been studied by both H-1 NMR and UV/vis techniques. The electrochemical properties of the ''acyclic'' p-tert-butylcalixdiquinones 1, 4, and 8 and their complexes have been studied using eyelid and square wave voltammetric techniques. The reduction potentials of the group 1 or 2 metal, ammonium, and alkylammonium complexes are significantly anodically shifted with respect to that of the free ligand. Addition of cations to electrochemical solutions of a p-tert-butylcalixdiquinone-crown-5 compound (5) caused large anodic shifts (by up to 555 mV in the presence of Ba2+) in a manner similar to that of the acyclic species.