Donor-Acceptor Interaction-Driven Inclusion of Tetraimidazolium Cyclophanes to Carbanions That Induces pK a Shifting of Conjugated Carbonic Acids in Water*

Abstract

Two tetraimidazolium-incorporated macrocycles have been prepared and used to include carbanionic conjugate bases of phenylmalononitriles through donor-acceptor interaction. X-ray diffraction analysis revealed that the two substrates form stable 1:1 complexes, with the carbanion being inserted in the cavity of the macrocycle. Fluorescence titration assays in water were used to determine the binding constants of totally eighteen complexes, which range from 1.83 x 105 M-1 to 8.96 x 106 M-1. The high binding stability induced the formation of charge-transfer absorption band around 400 nm and the pKa of the carbon acids in water to shift by -0.79 to -2.52 units, which correspond to 6.2 to 332-fold acidity enhancement. The result shows that conjugate carbanions are new, useful electron-rich donors to form donor-acceptor complexes with electron-deficient substrates for the research of supramolecular chemistry.