CRYSTAL-STRUCTURES AND STEREODYNAMICS OF NEUTRAL HEXACOORDINATE SILICON CHELATES - USE OF AN OPTICALLY-ACTIVE LIGAND FOR ASSIGNMENT OF AN INTRAMOLECULAR LIGAND-EXCHANGE PROCESS

Citation
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
Citations number
27
Language
INGLESE
art.tipo
Article
Categorie Soggetti
Chemistry
Volume
120
Issue
17
Year of publication
1998
Pages
4209 - 4214
Database
ISI
SICI code
Abstract
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.