CRYSTAL-STRUCTURES AND STEREODYNAMICS OF NEUTRAL HEXACOORDINATE SILICON CHELATES - USE OF AN OPTICALLY-ACTIVE LIGAND FOR ASSIGNMENT OF AN INTRAMOLECULAR LIGAND-EXCHANGE PROCESS
D. Kost et al., CRYSTAL-STRUCTURES AND STEREODYNAMICS OF NEUTRAL HEXACOORDINATE SILICON CHELATES - USE OF AN OPTICALLY-ACTIVE LIGAND FOR ASSIGNMENT OF AN INTRAMOLECULAR LIGAND-EXCHANGE PROCESS, Journal of the American Chemical Society, 120(17), 1998, pp. 4209-4214
Crystal structures determined for several neutral hexacoordinate bis(N
-->Si) chelates revealed in all cases near octahedral geometries with
the nitrogen ligands in relative trans and the monodentate Ligands in
cis positions. To assign the two consecutive intramolecular ligand-sit
e exchange processes (reported earlier), a bis-chelate was prepared co
ntaining a chiral carbon center in each of the chelate rings (14). By
means of a phase sensitive NOESY NMR spectrum it was possible to concl
ude that the lower-barrier process involved exchange of diastereotopic
groups only within each diastereomer, and not between them, resulting
in the assignment of this process to the direct nondissociative inter
change of the monodentate ligands (X, Y). The Si-Cl bond lengths were
found to inversely correlate with the lower of the activation barriers
. Dissociation-recombination of-the N-->Si dative bond was also observ
ed by the high-temperature NMR spectra. Lack of correlation between Si
-N distances in the crystals and activation barriers led to the conclu
sion that Si-N dissociation was not involved in the measured rate proc
esses, but followed, at slightly higher temperature, the epimerization
at the silicon center by exchange of the two oxygen sites.