Pentacoordinate Siliconate Tetraanionic Molecular Squares. Synthesis, Structure, and Solution Conformation of a Novel Tetraanion−Tetracation Ion Pair

Abstract

A tetraanionic molecular square containing four pentacovalent siliconates is produced in high yield by the condensation of phenyltriethoxysilane and spirocatechol 1. A single isomer, meso 2, is produced from this diastereoselective condensation reaction. The steps leading up to the formation of meso-2 are not known, but the reaction appears to be driven by the insolubility of the tetrakistriethylammonium salt 2a. The tetraanionic macrocycle is a stable salt that undergoes ion-exchange reactions. An ion pair composed of the tetraanionic siliconate macrocycle and cyclobis(paraquat-p-phenylene) tetracation (2e) has been synthesized by ion exchange. Following complete assignment of all proton resonances in this 4 × 4 complex, the solution conformation was analyzed by an intermolecular double pulsed field gradient spin−echo (DPFGSE) NOE study. These results, aided by a molecular mechanics modeling study, have led to a proposed dynamic solution structure that features the cyclic bisparaquat tetracation selectively intercalating around a phenyl ring of the C2 face of the macrocyclic tetraanion which in turn is engulfed by the interior of the tetraanion (Figure 10). This novel host−host complexation calls attention to the conformational flexibility of the macrocyclic tetrasiliconates and, in all probability, all macrocycles be they charged or neutral.