PENTACOORDINATE SILICON-COMPOUNDS - STEREOCHEMICAL NONRIGIDITY OF CHELATES FORMED BY INTRAMOLECULAR RING-CLOSURE - CRYSTAL-STRUCTURE OF 8-DIMETHYLAMINO-1-TRIFLUOROSILYLNAPHTHALENE

The application of dynamic NMR spectroscopy to the study of stereochem ical non-rigidity in pentacoordinate chelated organosilicon compounds is described. It is shown that in the compounds Me2NCH(Me)C6H4SiXYZ, n on-dissociative ligand permutation at silicon can be distinguished una mbiguously from processes associated with rupture of the chelate ring and nitrogen inversion. The crystal and molecular structure of 8-Me2NC 10H6SiF3 has been determined. Pentacoordination of the silicon atom is confirmed, with the donor nitrogen atom and a fluorine atom occupying axial sites in an overall trigonal bipyramidal geometry. The N --> Si separation is 2.3 angstrom (average of two distinct but closely relat ed molecular conformations), which is less than the C1-C-8 distance in the naphthalene nucleus, indicating a substantial bonding interaction . NMR studies of the dynamic behaviour of the Me2N group, and where po ssible (F-19, H-1) of the monodentate ligands in 8-dimethylamino-1-sil ylnaphthalene compounds, together with the results for the chelated be nzylaminosilicon compounds, confirm that inversion of the absolute con figuration at the silicon atom is not achieved by this process. The fr ee energies of activation for non-dissociative ligand permutation at a silicon range from less than 7 kcal mol-1 [SiH3, Si(OR)3], which is b elow the limit of direct measurement, to 13 kcal mol-1 for Me2NCH(Me)C 6H4SiF3; difunctional silicon chelate compounds (Cl, F, OR) display va lues from 9-12 kcal mol-1. These are comparable with those determined for fluxional processes in acyclic pentacoordinate silicon compounds.