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-site exchange processes (reported earlier), a bis-chelate was prepared containing a chiral carbon center in each of the chelate rings (14). By means of a phase sensitive NOESY NMR spectrum it was possible to conclude 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 interchange 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 observed by the high-temperature NMR spectra. Lack of correlation between Si−N distances in the crystals and activation barriers led to the conclusion that Si−N dissociation was not involved in the measured rate processes, but followed, at slightly higher temperature, the epimerization at the silicon center by exchange of the two oxygen sites.

Novel chelating agents derived from N-isopropylidene hydrazides, N-isopropylideneimino-O-(trimethylsilyl)acylimidates ( 1), have been utilized to prepare hexacoordinate dihalosilicon complexes (2) unique in their trans geometry and in their facile ionization to pentacoordinate siliconium halide salts (3).

The zwitterionic monocyclic λ5 Si-monofluorosilicates 3a−c were synthesized by reaction of the zwitterionic λ5 Si-trifluorosilicates 1a−c with 1 molar equiv of (Z)-Me3SiO(Ph)CNN(H)SiMe3 (2). The zwitterionic monocyclic λ5 Si-difluorosilicate 8 and the zwitterionic spirocyclic λ5 Si-silicate 9 (isolated as the isomer 9-I) were prepared in an analogous manner, starting from the zwitterionic λ5 Si-tetrafluorosilicate 7 and using 1 (→ 8) or 2 (→ 9) molar equiv of 2. Single-crystal X-ray diffraction studies showed that the Si-coordination polyhedra of 1a, 3a, 8, 9-I·CHCl3, and 9-I·CH3CN are distorted trigonal bipyramids. The axial positions are occupied by two fluorine atoms (1a), one fluorine atom and one oxygen atom (3a, 8), or two oxygen atoms (9-I·CHCl3, 9-I·CH3CN). These results are in agreement with the NMR data obtained for these compounds in solution. The chiral monocyclic compounds (3a−c and 8) undergo two distinct rate processes, inversion of absolute configuration of the chiral λ5 Si-silicate skeleton (enantiomerization) and a novel “ammonium-nitrogen inversion”, measured by variable-temperature and selective inversion recovery NMR experiments (activation barrier for “ammonium-nitrogen inversion”:  3a−c, 17.0 ± 0.3 kcal mol-1; 8, 20.8 ± 0.3 kcal mol-1; activation barrier for inversion of absolute configuration at the silicon atom:  3c, 20.7 ± 0.3 kcal mol-1; 8, 11.7 ± 0.3 kcal mol-1). In the spirocyclic λ5 Si-silicate 9-I only one of these processes, the “ammonium-nitrogen inversion” (ΔG ⧧ = 20.8 ± 0.3 kcal mol-1), could be observed, while inversion of absolute configuration at the silicon atom could not be detected by NMR spectroscopy. Furthermore, no interconversion of diastereomers 9-I ⇄ 9-II was observed.

Attempted preparation of neutral hexacoordinate silicon chelates with two OfSi dative bonds (4), in analogy to previously reported bis-NfSi isomers (2), resulted in unexpected formation of pentacoordinate siliconium complexes (5 and 6), characterized by a crystal structure. The barrier for inversion of configuration at silicon was determined by selective inversion recovery NMR measurements: ∆G* ) 20.5 ( 0.2 kcal mol -1 .

Two-, three-, and even four-bond coupling constants are reported for the first time across the N→Si coordination bond in neutral, hexacoordinate silicon chelates. Two-bond coupling constants depend critically on bond angles, and three-bond couplings follow a Karplus-type correlation with dihedral angles obtained from X-ray crystallographic analyses.