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 |
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Authors: | Roberta Cacciapaglia Luigi Mandolini David N. Reinhoudt Riccardo Salvio Rocco Ungaro |
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Affiliation: | a Dipartimento di Chimica and IMC - CNR Sezione Meccanismi di Reazione, Università di Roma La Sapienza, Box 34 - Roma 62, 00185 Rome, Italy b Dipartimento di Chimica Organica e Industriale, Università degli Studi di Parma, V.le G. P. Usberti 17/A, 43100 Parma, Italy c Laboratory of Supramolecular Chemistry and Technology, MESA+ - Research Institute, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands |
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Abstract: | 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 para-positions 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. |
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Keywords: | Calixarenes Zinc complexes Metallocatalysts Ester methanolysis Supramolecular catalysis Nitrogen ligands |
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