Highly selective and efficient extraction of two Pb2+ ions with a p-tert-butylcalix[6]arene hexacarboxylic acid ligand: an allosteric effect in extraction
作者:Birendra Babu Adhikari、Manju Gurung、Anup Basnet Chetry、Hidetaka Kawakita、Keisuke Ohto
DOI:10.1039/c3ra44289f
日期:——
Solvent extraction behavior of p-tert-butylcalix[6]arene hexacarboxylic acid towards Pb2+ and other divalent transition metal ions has been studied to investigate the selectivity, extraction mechanism, stoichiometry of complexation and composition of the complex in the organic phase. Extractability and selectivity of the cyclic ligand were analyzed in comparison with the acyclic analog. The macrocyclic effect was genuinely operative and the size-match effect was the governing factor for cation selectivity. Accordingly, the cyclic ligand selectively extracted the Pb2+ ion in a pH range fairly more acidic than the acyclic ligand. The cations were extracted by ion exchange mechanism, and the electroneutral complexes were resulted by the exchange of two protons for a divalent cation. A molecule of p-tert-butyl calix[6]arene hexacarboxylic acid ligand extracted two Pb2+ ions. One Pb2+ was extracted due to the size-fit effect inside the hydrophilic cavity of calix[6]arene composed by phenoxy and carbonyl oxygen atoms. The macrocyclic ring inversion was obstructed due to guest encapsulation, and the encapsulated Pb2+ ion acted as a template for aggregation of the carboxyl groups. This binding event caused a positive allosteric effect and led to the extraction of the second Pb2+ ion at the aggregated functional group site.
研究了p-叔丁基杯[6]芳烷六羧酸对Pb2+及其他二价过渡金属离子的溶剂萃取行为,以探究其选择性、萃取机理、配合物的化学计量学及有机相中配合物的组成。通过与非环状类似物的比较,分析了环状配体的萃取能力和选择性。大环效应确实发挥了作用,而离子与配体孔穴尺寸匹配效应对阳离子选择性起主导作用。因此,环状配体在比非环状配体更酸性的pH范围内选择性萃取Pb2+离子。阳离子的萃取是通过离子交换机制进行的,通过交换两个质子与一个二价阳离子形成电中性配合物。一个p-叔丁基杯[6]芳烷六羧酸配体萃取两个Pb2+离子。一个Pb2+的萃取是由于杯[6]芳烷的亲水性腔体内由苯氧基和羰基氧原子构成的尺寸匹配效应。由于客体分子的包封,大环的翻转受阻,包封的Pb2+离子作为模板促进了羧基的聚集。这种结合事件产生了正协同效应,并在聚集的功能团位点导致了第二个Pb2+离子的萃取。