Binding properties of p-tert-butyldihomooxacalix[4]arene tetra(2-pyridylmethoxy) derivative towards alkali, alkaline earth, transition and heavy metal cations

The binding properties of tetra(2-pyridylmethoxy) (2) derived from p-tert-butyldihomooxacalix[4]arene, in the cone conformation, towards alkali, alkaline earth, transition (Mn2+, Fe2+, Co2+, Ni2+, Cu2+ and Zn2+) and heavy (Ag+, Cd2+, Hg2+ and Pb2+) metal cations have been assessed by extraction studies of metal picrates from water into dichloromethane and stability constant measurements in methanol and acetonitrile. Results concerning the calorimetric study of K+, Ba2+, Cu2+ and Ag+ complexes in acetonitrile are presented. The affinity of 2 for some cations (K+, Ag+, Zn2+, Hg2+ and Pb2+) has also been investigated by proton NMR spectrometry. The analogous derivative (3) of p-tert-butylcalix[ 4] arene was also studied, and the results of both compounds are compared. Towards the hard alkali and alkaline earth metal cations ligands 2 and 3 display a low efficiency of extraction. The complexation levels are also low in methanol becoming, however, much higher in acetonitrile. The highest stability is found for Ca2+ with ligand 2 in this solvent. Both ligands show a very high affinity for the soft heavy metal ions with preference for Ag+, but other ions such as Hg2+, Pb2+ and Cu2+ are also well bound. Proton NMR titrations confirm the formation of 1 : 1 complexes between 2 and all the cations studied, also indicating that they should be located inside the cavity defined by the phenoxy oxygen and the pyridyl nitrogen atoms