Expanded Mercaptocalixarenes: A New Kind of Macrocyclic Ligands for Stabilization of Polynuclear Thiolate Clusters

The syntheses and properties of expanded 4-tert-butyl-mercaptocalix[4]arenes, in which the methylene linkers are replaced by −CH₂NRCH₂− or −CH₂NRCH₂− and −CH₂NRCH₂CH₂CH₂NRCH₂− units, are described. The new macrocycles were obtained in a step-wise manner, utilizing fully protected, i. e. S-alkylated, derivatives of the oxidation-sensitive thiophenols in the cyclisation steps. Reductive cleavage of the macrobicyclic or macrotricyclic intermediates ( 6 , 7 , 11 ) afforded the free thiophenols (H₄ 8 , H₄ 9 , and H₄ 12 ) in preparative yields as their hydrochloride salts. The protected proligands can exist in two conformations, resembling the “cone” and “1,3-alternate” conformations found for the parent calix[4]arenes. The free macrocycles do not show conformational isomerism, but are readily oxidized forming intramolecular disulfide linkages. Preliminary complexation experiments show that these expanded mercaptocalixarenes can serve as supporting ligands for tetranuclear thiolato clusters.