AGOSTIC BETA-SI-H INTERACTIONS IN SILYLAMIDO COMPLEXES OF ZIRCONOCENE

A series of silylamido complexes of zirconium, Cp2Zr(X)(NtBuSiMe2H) (X = H, I, Br, Cl, F; 1-5), have been prepared. These complexes exhibit agostic beta-Si-H interactions with the metal center and have been characterized by spectroscopic and structural methods. Spectroscopic evidence for the interaction of the Si-H sigma-bond with zirconium include (1) abnormally large upfield chemical shifts for the silicon hydride and silicon nuclei in the H-1 and Si-29 NMR spectra, (2) unusually small values of the silicon-hydrogen coupling constants (1J(SiH)), and (3) low-energy Si-H stretching frequencies in the infrared spectra. All of the spectroscopic data also establish a clear trend for the strength of the nonclassical Zr-H-Si interaction in Cp2Zr(X)(NtBuSiMe2H): X = H > I > Br > Cl > F. This ordering directly reflects the relative electrophilicity of the zirconium center. The molecular structures of the hydride and chloride derivatives 1 and 4 as determined by single-crystal X-ray diffraction studies are also consistent with coordination of the Si-H bond to the metal center. In particular, short Zr-Si distances and acute Zr-N-Si angles point to a severe bending of the silyl group toward zirconium, and the location of the amido group near the center of the metallocene equatorial wedge is consistent with a Cp2ML3 Coordination environment, not the Cp2ML2 geometry implied by the formula CP2Zr(NR2)(X).