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-TERT)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 distorted 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 = CHRNMe2 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. H-1 and Sn-119 NMR spectroscopic studies show that in solution at low temperatures (-25-degrees-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-degrees-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.