Intramolecular Halogen Stabilization of Silylium Ions Directs Gearing Dynamics

2,6-Bis(2,6-difluorophenyl)phenyldimethylsilanium ion (1a) adopts a ground-state C 2 trigonal-bipyramidal geometry in which fluorines from opposing 2,6-difluorophenyl groups coordinate to the apical positions of the pentavalent silyl cation. Exchange of fluorine at silicon occurs by a disrotatory gearing mechanism wherein one fluorine remains coordinated to silicon throughout the circuit. The “cogwheel” transition state is C s -symmetric, with one ring having a dihedral angle of 0° and the other a dihedral angle of 90°. This correlated dynamic process is a function of the coordinating halogen. The chloro derivative (1b) adopts a similar ground-state geometry, but exchange of chlorine at silicon follows a conrotatory process.