Calixarene and Resorcarene Based Receptors: From Structural and Thermodynamic Studies to the Synthesis of a New Mercury(II) Selective Material

Abstract

Materials used in current technological approaches for the removal of mercury lack selectivity. Given that this is one of the main features of Supramolecular Chemistry, receptors based on calix[4]arene and calix[4]resorcarene containing functional groups able to interact selectively with polluting ions while discriminating against biologically essential ones were designed. Thus two receptors, a partially functionalised calix[4]arene derivative, namely, 5,11,17,23 tetra tert- butyl [25-27 bis(diethyl thiophosphate amino) dihydroxy] calix[4]arene (1) and a fully functionalised calix[4]resorcarene, 4,6,10,12,16,18,22,24-diethyl thiophosphate calix[4]resorcarene (2) are introduced,. Mercury (II) was the identified target due to the environmental and health problems associated with its presence in water Thus, following the synthesis and characterisation of 1 and 2 in solution ( 1HNMR) and in the solid state ( X-ray crystallography) the sequence of experimental events leading to cation complexation studies in acetonitrile and methanol (1HNMR, conductance, potentiometric and calorimetric measurements) with the aim of assessing their behaviour as mercury selective receptors are described .The cation selectivity pattern observed in acetonitrile follows the sequence Hg(II) > Cu(II) > Ag(I). In methanol 1 is also selective for Hg(II) relative to Ag(I) but no interaction takes place between this receptor and Cu(II). Based on previous results and experimental facts shown in this paper, it is concluded that the complexation observed with Cu(II) in acetonitrile occurs through the acetonitrile-receptor adduct rather than through the free ligand. Receptor 2 has an enhanced capacity for Hg(II) binding but forms metallate complexes with Cu(II). These studies in solution guided the anchoring of receptor 1 into a silica support to produce a new and recyclable material for the removal of Hg(II) from water. An assessment on Its capacity to extract this cation from water relative to Cu(II) and Ag (I) shows that the cation selectivity pattern of the Immobilised receptor is the same as that observed for the free receptor in methanol. These findings demonstrate that fundamental studies play a critical role in the selection of the receptor to be attached to silicate as well as in the reaction medium used for the synthesis of the new decontaminating agent.