Zinc–salophen complexes as selective receptors for tertiary amines

Zinc–salophen compounds 1–3 incorporating in the given order 1,2-diaminobenzene, 2,3-diaminonaphthalene, and 9,10-diaminophenantrene moieties were synthesised. Their binding properties toward a series of tertiary amines were assessed by UV-Vis and fluorescence spectroscopy in chloroform solution. Unprecedented selectivities of quinuclidine vs. triethylamine higher than 105 were recorded, thereby revealing the dramatic influence of steric effects on axial coordination of tertiary amines. X-Ray diffraction analyses showed that in the solid state compound 2 is dimeric, but its 1 : 1 quinuclidine complex is monomeric. Strong indications were obtained that both free receptors and their amine adducts are monomeric in dilute chloroform solution.