Benzyl phenol derivatives: Extraction properties of calixarene fragments

The remarkable self-assembly of benzyl phenol in binding Cs⁺ has been shown in this work to exhibit sensitivity to alkyl substitution effects in liquid-liquid extraction experiments. Substituted benzyl phenols, particularly 4-sec-butyl-2-(α-methylbenzyl)phenol (BAMBP), have been known for over 30 years for their high cesium extraction efficiency and selectivity. The selectivity demonstrated by BAMBP has only been exceeded by calix[4]arenes functionalized with crown ethers. Some structural analogy exists between these two classes of molecules, as substituted benzyl phenols can be considered as calix[4]arene fragments. Additionally, both classes bind Cs⁺ through oxygen donor and π-arene interactions. Based on computational results and crystal structures, a series of benzyl phenol derivatives were synthesized to allow insightful comparisons of extraction efficiency and selectivity as influenced by substituents on key sites. Solvent extraction data are presented regarding cesium extraction efficiencies of various benzyl phenol derivatives from alkaline media along with their separation factors for Cs⁺ vs. Na⁺. Substitution of alkyl groups at para and meta positions of the phenol ring of benzyl phenol and substitution of a second benzyl group at the ortho position all lower cesium extraction strength. Methyl substitution at the methylene carbon at best slightly increases extraction strength. Selectivity for for Cs⁺ vs. Na⁺ varies from 25 to 3800 with no apparent trend. Since the results do not reflect the expected effects on conformational strain predicted from molecular mechanics, it is concluded that effects of substitution on the self-assembly properties of the free ligands and their complexes are more important than effects on conformational strain.