The first dimeric trioganotin fluoride : stabilization by unsymmetrically oriented menthyl substituents

In the solid state, (-)-tris([1<i>R</i>,2<i>S</i>,5<i>R</i>]-menthyl)tin fluoride, Men₃SnF (<b>6</b>), exhibits a dimeric structure in which one tin atom is four-coordinate and the other five-coordinate. This novel dimeric association mode is attributed to the orientation of the unsymmetrical menthyl groups, which are arranged to allow a minimum Sn···Sn separation of 4.84 Å within the dimer. The exocyclic isopropyl groups of the menthyl groups point in opposite directions on each tin atom of the dimer, thus preventing further association (Sn···Sn separation outside the dimer 10.49 Å). ¹¹⁹Sn and ¹⁹F MAS NMR spectroscopy were utilized to probe the diverse coordination numbers found by X-ray crystallography for the geometries of the two tin and fluorine sites. In solution, <b>6</b> is a monomer at both room temperature and -100 °C. However, upon addition of Bu₄NF, <b>6</b> is in equilibrium with [(Men₃SnF)₂F]- (<b>6a</b>), [Men₃SnF₂]^- (<b>6b</b>), and noncoordinating fluoride anions.<br />