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)CNN(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.
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