Organotin(IV) compounds 2-(Me₂NCHR)C₆H₄(SnR′R″Br) with a fixed s-cis arrangement of the chiral carbon and tin centers. Crystal structure of 2-(Me₂NCHBu^t)C₆H₄SnMePhBr

A series of novel pentacoordinate triorganotin bromides SnR′R″Br[C₆H₄CH(R)NMe₂-2] have been synthesized and characterized in which R is H, Me, Et, i-Pr or t-Bu. The crystal structure of one of these compounds (R = t-Bu, R′ = Me and R″ = Ph) has been determined. The tin center has a distored trigonal-bipyramidal coordination geometry. The organic ligands occupy the equatorial sites, while the more electronegative Br and N ligands are in the axial positions. As a requirement of the space group symmetry the unit cell contains 8 enantiomeric pairs of diastereoisomers (R)_C(S)_Sn and (S)_C(R)_Sn. The given configuration at the chiral benzylic carbon atom is combined with a configuration at the chiral tin atom in such a way that the sterically most favorable diastereoisomer is formed. The influence of the steric requirements of the ortho-substituent Z = HRNMe₂ in 2-ZC₆H₄ (SnRR′Br) on the configuration of the Sn center has been studied. These compounds contain a fixed s-cis Z-C-C-Sn orientation. ¹H and ¹¹⁹Sn NMR spectroscopic studies show that in solution at low temperatures (-25°C) all these compounds are pentacoordinate as a result of intramolecular coordination. At higher temperatures a fluxional process becomes operative involving Sn-N bond dissociation/association. However, even at +110°C the Sn center is configurationally stable on the NMR time scale. For the compounds in which the Sn atom is also a chiral center (R′ = Me and R″ = Ph) in solution there is an equilibrium between two diastereoisomers, the ratio being dependent on the bulk of the benzylic substituent R. For R = t-Bu the equilibrium lies completely to the side of the sterically most favorable one.