exo and endo Isomerism of subvalent tin and germanium complexes derived from 1,3-diethers of p-tert-butylcalix[4]arene

A series of germanium and tin complexes of calix[4]arene diethers, namely [Calixt-Bu(O)2(OR)2]M, has been prepared by the reaction of [Calixt-Bu(OH)2(OR)2] with M[N(SiMe3)2]2 (M = Ge, Sn; R = Me, CH2Ph, CH2C6H4But, SiPr3i). X-ray diffraction studies demonstrate that [Calixt-Bu(O)2(OR)2]M exist as exo and endo isomers in which the metal is, respectively, located either outside or inside the calixarene cavity. The exo isomer is considered to be the kinetic product, whereas the endo isomer is thermodynamically more stable. The ability to access endo-[Calixt-Bu(O)2(OR)2]M, however, depends critically on the nature of R and M.