Synthesis and alkali metal complexation studies of novel cage-functionalized cryptands

The synthesis of novel cage-functionalized cryptands 1–5 containing adamantane-, 2-oxaadamantane- or noradamantane-moiety [i.e., 1,3-diethyladamantano[2.2.0]cryptand (1), 1,3-diethoxyadamantano[2.2.2]cryptand (2), 1,3-di[(ethyloxy)methyl]adamantano[2.2.2]-cryptand (3), 1,3-di[(ethyloxy)methyl]-2-oxaadamantano[2.2.3]cryptand (4), and 1,2-diethyloxynoradamantano[2.2.2]cryptand (5)] and their alkali metal binding properties are reported. The results obtained by extraction experiments showed that all the cryptands displayed lower extraction capabilities than the parent [2.2.2]cryptand. However, cryptands 1 and 2 showed much higher selectivity toward K⁺ than the reference [2.2.2]cryptand. When the third bridge is enlarged by two additional CH₂-groups as well as by two oxygen atoms, as in cryptands 3 and 4, the complexational abilities for bigger cations (K⁺, Rb⁺ and Cs⁺) are enhanced. Cryptand 5 displayed very good extraction capabilities of all cations, but showed practically no selectivity towards any of the alkali metal cation. The experimental findings are corroborated by calculation studies consisting of force field based conformational search using Monte Carlo method followed by investigation of the stabilities of the complexes of cryptands with Na⁺ and K⁺ metal ions in chloroform by means of quantum chemical calculations at the density functional theory level.