Neutral Hexacoordinate Silicon Tris-Chelates: Structure and Stereodynamics

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Abstract

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)CNNHSiMe3 (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).

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