DIRECT EVIDENCE OF A HAFNIUM ETA(2)-IMINOACYL COMPLEX AS THE COMMON INTERMEDIATE IN THE COMPETITIVE REDUCTIVE COUPLING AND 1,2-SILYL SHIFT PATHWAYS ASSOCIATED WITH THE ISOCYANIDE INSERTION CHEMISTRY OF CP2HF(CH2SIME2CH2)

The reaction of 1 equiv of tert-butyl isocyanide with Cp2Hf(CH2SiMe2CH2), 1, affords a 1:1 equilibrium mixture of the eta2-iminoacyl complex Cp2Hf(N(CMe3)CCH2SiMe2CH2), 2a, and the cyclic enamido complex Cp2Hf(N(CMe3)C(=CH2)SiMe2CH2), 3a. The subsequent addition of a second equivalent of tert-butyl isocyanide to this equilibrium mixture proceeds exclusively with the formation of the eta2-iminoacyl imine compound Cp2Hf(N(CMe3)C-C(=NCMe3)CH2SiMe2CH2), 4, which when heated rearranges to the bicyclic enediamido complex Cp2Hf(N(CMe3)C(CH2SiMe2CH2)=CN(CMe3)), 5. The reversibility associated with 2a <-> 3a enables the overall reductive coupling reaction to proceed with complete consumption of both 2a and 3a and provides direct evidence of 2a as the common intermediate in both the 1,2-silyl shift and the reductive coupling pathways. The enthalpy of activation for the conversion of 4 to 5 is 28.8(11) kcal/mol, which is ca. 3 kcal/mol larger than that for the analogous zirconium system. This difference is attributed to a stronger Hf-C(methylene) bond, which was determined by an X-ray crystallographic analysis of 4 to be 0.03 angstrom shorter than the Zr-C(methylene) bond of Cp2Zr(N(CMe3)C-C(=NCMe3)CH2SiMe2CH2). Crystal data for 4 at 25-degrees-C: monoclinic space group P2(1)/c with a = 8.408(2) angstrom, b = 18.534(4) angstrom, c = 16.326(4) angstrom, beta = 101.71(2)degrees, and Z = 4. Full-matrix least-squares refinement (based on F(o)2) of 4384 diffractometry data converged with R(F(o)) = 0.039.