On the prerequisites for the formation of solution complexes from [60]fullerene and calix[n]arenes: A novel allosteric effect between [60]fullerene and metal cations in calix[n]aryl ester complexes

In order to find calix[n]arenes which can interact with [60]fullerene in solution we have screened a number of different calix[n]arenes by spectroscopic methods. We eventually discovered such three calix[n)arenes (1₅·Bu(t)·H, 1₆·Bu(t)·H, and 2₃·Bu(t)·H) which could accept [60]fullerene even in solution. They commonly possess a cone conformation and a benzene ring inclination suitable to [60]fullerene inclusion. Furthermore, it was found that calix[n]aryl ester derivatives, which cannot interact with [60]fullerene, become excellent [60]fullerene-acceptors in the presence of certain specific metal cations. This was attributed to a metal-induced conformational change to preorganize cone calixi[n]aryl esters. This phenomenon is a sort of positive allosterism, which is the first example in fullerene chemistry. We believe that these findings will open a door to new fullerene-calix[n]arene conjugate chemistry.