Neutral hexacoordinate silicon tris-chelates: Structure and stereodynamics

Neutral hexacoordinate silicon chelates with two chloro ligands (1) have be en converted to neutral tris-chelates (6 and 7) by reaction with bis-(OO-tr imethylsilyl)catechol (4) and (Z)-Me3SiO(Ph)C=NNHSiMe3 (5), respectively. L ikewise, zwitterionic dichloro-hexacoordinate chelate (2a) was converted to the zwitterionic tris-chelate 8 with three different chelate rings, by rea ction with 4. 2D-NOESY NMR spectra at various temperatures for 6a and 7a an d for the chirally labeled 6b and 7c showed two consecutive ligand-site exc hange processes, assigned to interchange of diraethylhydrazido-chelate ring s via (O,O)-1,2-shift (6a, DeltaG* 20.8 kcal mol(-1), 7a, 18.5 kcal mol(-1) ) and dissociation-recombination of the N-Si bond. In 8, the NMR evidence s hows that the lower process is N-Si dissociation-recombination, followed by enantiomerization at silicon via (O,O)-exchange (DeltaG* = 16.1 and 20.9 k cal mol(-1), respectively).