THIOPHENE ANALOGUES OF CROWN ETHERS

Joan Halfpenny

Department of Chemistry and Physics, Nottingham Trent University, Clifton Lane, Nottingham NG11 8NS, UK e-mail joan.halfpenny@ntu.ac.uk

Keywords: thiophene, macrocycle, metal complexation

A series of bis-Mannich bases containing 14- to 22- membered rings which can be related to crown ethers and cryptands have been shown to form metal complexes with Li⁺, Na⁺, K⁺, Cs⁺, Ag⁺, Mg²⁺, Ca²⁺, and Sr²⁺ which are soluble in dichloromethane [1]. The compounds have greater extracting ability than 18-crown-6 and the metal complexes are stable in solution even though some of the metal ions are too large to fully enter the macrocycle cavity and the number of binding sites is therefore limited.

Results of theoretical calculations on the most stable conformations in solution show that the macrocycles will twist and adopt a cup-shaped geometry in order to coordinate in the most effective way to a given metal. The energy-minimised conformation of the free macrocyclic ligand is substantially different from that of the complex, showing that the macrocycles must be more flexible in solution than in the solid state. This prevents crystallisation of the metal complexes on cooling or on evaporation of solvent, except where the metal ionic radius is of a suitable size to exactly fit the coordination requirements of the macrocycle. Attempts at crystallisation when other metals are present result in separation (and preferential crystallisation) of the free macrocycle from the metal and its counterion. This is currently being investigated as a separation/recyling technique.

Single crystal structure determination of a typical free macrocycle will be compared with theoretical stability calculations for the conformational behaviour of the molecule in solution with and without the presence of metal ions. Some conclusions will be drawn regarding the relative stability and metal complexing power of various macrocyclic systems.

1. J.M. Barker, J.D.E. Chaffin, J. Halfpenny, P.R. Huddleston, P.F. Tseki: J. Chem. Soc. Chem. Comm. (1993) 1733-4