Structural and dynamic behaviour of cadmium(II) and mercury(II) complexes of 1,4,8,11-tetra-azacyclotetradecane and 1,4,8,11-tetramethyl-1,4,8,11-tetra-azacyclotetradecane

Abstract

The macrocyclic ligand 1,4,8,11-tetra-azacyclotetradecane (L_a) gives a complex with cadmium(II) which is a mixture of two isomers in solution. The isomers are observed in the ¹³C n.m.r. spectra at low temperatures, and are postulated to involve square and dynamically folded trigonal-bipyramidal (t.b.p.) co-ordination of the macrocycle, with different configurations at the co-ordinated nitrogen donors (square, R, S, S, R; t.b.p., R, S, R, S). These two isomers are found to interconvert on the n.m.r. timescale at elevated temperatures, a process which requires double nitrogen-atom inversions. The complex of L_a with mercury(II) has a ¹³C n.m.r. spectrum consistent with a stable ‘trans’ co-ordination of the macrocycle which may only be achieved by the adoption of a geometry, unique for L_a with R, S, R, S donor-atom configurations. This geometry is confirmed by an X-ray analysis of the complex [Hg(L_a)Cl]₂[HgCl₄] where the conformation of the macrocycle forces distorted square-pyramidal five-co-ordination upon the mercury(II) ion. The complex crystallises in the monoclinic space group C2/c, Z= 4, with a= 15.686(9), b= 15.058(8), c= 14.979(6)Å, and β= 103.03(4)°(T= 153 K). 2 226 Reflections with I/σ(I) 3.0 have been recorded by a four-circle diffractometer and least-squares refinement has produced a final R of 0.037. Both cadmium(II) and mercury(II) ions form complexes with 1,4,8,11-tetramethyl-1,4,8,11-tetra-azacyclotetradecane (L_b) which are best envisaged as five-co-ordinate with macrocyclic configurations R, S, R, S. These complexes are found to undergo Berry-type dynamic rearrangements between equivalent trigonal bipyramids as found previously for the analogous complexes of zinc(II) ion. The behaviours of the complexes are compared and discussed in relation to metal-ion size and macrocycle configurations.