Enhancement of Alkali Metal Ion Recognition by Metalation of a Tris(saloph) Cryptand Having Benzene Rings at the Bridgeheads.

Abstract

A cryptand derivative, H6L, which has three H2saloph arms connected by two benzene ring bridgeheads, was synthesized and converted into the trinuclear metallocryptand, LNi3. The nonmetalated host, H6L, was found to bind to alkali metal ions (Na+, K+, Rb+, Cs+; logKa = 3.37-6.67) in its well-defined cavity in DMSO/chloroform (1:9). The binding affinity was enhanced by 1-2 orders of magnitude upon the conversion into the metallocryptand, LNi3, which can be explained by the more polarized phenoxo groups in the [Ni(saloph)] arms. The guest binding affinity of Na+ < K+ < Rb+ ≈ Cs+ was clearly demonstrated by the 1H NMR competition experiments. The DFT calculations suggested that the Rb+ ion most suitably fit into the benzene-benzene spacing with a cation-π interaction and that only the largest Cs+ ion can almost equally interact with all six phenoxo oxygen donor atoms. The metallocryptand, LNi3, also showed a strong binding affinity to Ag+ by taking advantage of cation-π interactions, which was confirmed by spectroscopic titrations and crystallographic analysis as well as DFT calculations. Thus, the well-defined three-dimensional cavity of LNi3 was found to be suitable for strong binding with alkali metal ions as well as Ag+.