Design and synthesis of a C₄-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 C₄ symmetry in both the tetraphenylporphyrin and the calix[4]arene this molecule still retains C₄ 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 Kass = 5.23 (CHCI3:MeCN = 4:1 v/v; 25 °C) whereas KC104 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.