Di- and Trinuclear Arrangements of Zinc(II)- 1,5,9-Triazacyclododecane Units on the Calix[4]arene Scaffold: Efficiency and Substrate Selectivity in the Catalysis of Ester Cleavage

The catalytic activity of the zinc(II) complexes of calix[4]arenes decorated with 1,5,9-triazacyclododecane ligands at the 1,2-, 1,3-, and 1,2,3-positions of the upper rim was investigated in the basic methanolysis (pH 10.4) of aryl acetates functionalised at the meta- and parapositions with a carboxylate anchoring group. Michaelis–Menten kinetics and turnover catalysis were observed. High rate accelerations, up to more than 104-fold at 0.2 mM catalyst, were recorded in the most favourable catalyst–substrate combinations. The order of catalytic efficiency of regioisomeric bimetallic complexes is 1,2-vicinal 1,3-distal, resulting from a significant degree of synergism between metal ions in the former, and a complete lack in the latter. The moderately higher efficiency of the trimetallic compared with the 1,2- vicinal bimetallic catalyst provides an indication of a possible cooperation of three zinc(II) ions in the catalysis.