DESIGN AND SYNTHESIS OF A C-4-SYMMETRICAL HARD-SOFT DITOPIC METAL RECEPTOR BY CALIXARENE-PORPHYRIN COUPLING

A tetraphenylporphyrin capped with a calix[4]aryl amide through L-alanine pillars (compound 1) has been synthesized. Owing to C-4 symmetry in both the tetraphenylporphyrin and the calix[4]arene this molecule still retains C-4 symmetry. As the calix[4]aryl amide moiety can bind alkali metal cations, and the porphyrin moiety can bind transition metal cations, compound 1 acts as a host molecule with a hard-soft ditopic metal-binding site. It was found that the Zn-II complex of 1 (complex 1 . Zn-II) can bind KI very strongly with log K-ass = 5.23 (CHCl3:MeCN = 4:1 v/v; 25 degrees C) whereas KClO4 is scarcely captured by 1 . Zn-II. Absorption and fluorescence spectroscopic studies showed that I- ion is bound to this cavity, interacting with both calix[4]arene-bound K+ through electrostatic attraction and porphyrin-bound Zn-II through coordination as an axial ligand. The results establish that complex 1 . Zn-II acts as a unique ditopic receptor for I- ion.