For the selective binding of the uranyl ion (UO22+) a new class of uranophiles has been designed from calixarenes: they are para-sulfonated calix[n]arenes (n = 4, 5, 6: 24H, 25H, and 26H, respectively) and their carboxylated derivatives (24CH2COOH, 25CH2COOH, and 26CH2COOH, respectively). We have found that the cyclic pentamers (25H and 25CH2COOH) and the cyclic hexamers (26H and 26CH2COOH) have remarkably large stability constants (Kuranyl= l018.4-19.2M-1), whereas the cyclic tetramers (24H and 24CH2COOH) have very small stability constants (Kuranyl= l03.1-3.2M-1). This trend is very compatible with the X-ray data which show that U022+complexes invariably adopt the coplanar penta- or hexacoordination geometry. Hence, the high stability is better explained by “coordination-geometry selectivity” than by “hole-size selectivity”. The selectivity factors (Kuranyl/KMn+) for 26H and 26CH2COOH were evaluated by comparing the Kuranylwith the stability constants for competing metal cations (KMn+). It was found that the selectivity factors for these calixarenes are surprisingly large, 1012-17as compared with competing Ni2+, Zn2+, and Cu2+ions! The remarkably high selectivity is attributed to the moderately rigid skeleton of calixarene which can provide the preorganized hexacoordination geometry for the binding of UO22+but cannot accommodate to the square-planar or tetrahedral coordination geometry for other metal cations in an “induced-Fit” manner. Thus, calixarene and calixarene, which are easily synthesized from cheap starting materials, serve as excellent basic skeletons for the design of superior uranophiles.
All Science Journal Classification (ASJC) codes
- Colloid and Surface Chemistry