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