Journal of the American Chemical Society, Vol.117, No.46, 11512-11522, 1995
Neutral Hexacoordinate Silicon Complexes - Synthesis, Structure, and Stereodynamics - Evidence for 2 Nondissociative Ligand-Exchange Mechanisms
Novel hexacoordinate silicon compounds 1-4 with two identical bidentate ligands, respectively, are readily prepared in high yields from the reaction of XSiCl(3) (X = H, Me, Ph, Cl) with the O-trimethylsilyl derivatives of N,N-dimethyl carbohydrazides 6. The NMR spectra of all compounds indicate that a single diastereomer is present in solution, of the possible six, within NMR detection limits. The published X-ray crystallographic structure of one of the complexes, as well as an analysis of the H-1, C-13 and Si-29 NMR spectra and their temperature dependence, conforms to an octahedral geometry with the oxygen ligands and the pair of monodentate ligands cis to each other, respectively, and the nitrogens in a trans position. All of the complexes show temperature dependence of their NMR spectra, characteristic of fluxional behavior. Two rate processes (topomerizations) take place on the NMR time scale in compounds 1-3, at activation free energies ranging, respectively, between 10.6 and 16.4 kcal/mol, and 15.0 and 18.5 kcal/mol. A single process is observed for the C-2v symmetric 4 complexes. A remarkable solvent dependence of barriers is observed, suggesting that ligand site exchange is associated with dissociation or weakening of Si-ligand bonds. The simultaneous exchange of N-methyl groups and benzyl methylene protons in 3d is evidence that no Si-N bond cleavage and chelate ring opening take place during topomerization. The persistence of Si-29-F-19 one-bond coupling, observed in the Si-29 NMR spectrum at temperatures well above the fast exchange limit temperature, proves that no ionic dissociation of the Si-halogen bond takes place. It is concluded that topomerization occurs in a nondissociative, intramolecular Ligand site exchange process. A likely mechanism that accounts for all of these observations is a 1,2-shift of adjacent ligands, X and Cl, or the two oxygen ligands, via a "bicapped tetrahedron" intermediate or transition state.