Neutral hexacoordinate silicon tris-chelates: Structure and stereodynamics

Neutral hexacoordinate silicon chelates with two chloro ligands (1) have been converted to neutral tris-chelates (6 and 7) by reaction with bis-(O,O-trimethylsilyl)catechol (4) and (Z)-Me₃SiO(Ph)C=NNHSiMe₃ (5), respectively. Likewise, zwitterionic dichloro-hexacoordinate chelate (2a) was converted to the zwitterionic tris-chelate 8 with three different chelate rings, by reaction with 4. 2D-NOESY NMR spectra at various temperatures for 6a and 7a and for the chirally labeled 6b and 7c showed two consecutive ligand-site exchange processes, assigned to interchange of dimethylhydrazido-chelate rings via (O,O)-1,2-shift (6a, ΔG* = 20.8 kcal mol^-1, 7a, 18.5 kcal mol^-1) and dissociation-recombination of the N-Si bond. In 8, the NMR evidence shows that the lower process is N-Si dissociation-recombination, followed by enantiomerization at silicon via (O,O)-exchange (ΔG* = 16.1 and 20.9 kcal mol^-1, respectively).