Neutral Hexacoordinate Silicon Complexes. Synthesis, Structure, and Stereodynamics: Evidence for Two 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 XSiCl₃(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 ¹H, ¹³C, and ²⁹Si 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 ²⁹Si–¹⁹F one-bond coupling, observed in the ²⁹Si 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.