Alkyl substituent effects on reductive elimination reactions in zirconocene alkyl hydride complexes. Manipulation of the alkyl steric environment allows the synthesis of a zirconocene dinitrogen complex

The rate of reductive elimination as a function of alkyl ligand has been measured for a series of zirconocene alkyl hydride complexes, Cp*Cp″Zr(R)H (Cp* = η⁵-C₅Me₅, Cp″ = η⁵-C₅H₃-1,3-(SiMe₃)₂, R = CH₃, CH₂(CH₂)₂CH₃, CH₂(CH₂)₆CH₃, CH₂^cC₆H₁₁, CH₂CHMe₂, CH₂-CMe₃), where the steric disposition of the alkyl ligand has been systematically varied. The rate of reductive elimination increases modestly as the steric bulk of the alkyl ligand is increased. This trend is attributed to ground state destabilization arising from unfavorable steric interactions between the alkyl ligand and the cyclopentadienyl substituents. The effect is magnified when more voluminous cyclopentadienyl ligands are incorporated into the metallocene framework. Thus, the Cp*Cp‴Zr(R)H (Cp‴ = η⁵-C₅H₂-1,2,4- (SiMe₃)₃, R = CH₃, CH₂ (CH₂)₂CH₃, CH₂(CH₂)₆CH₃) series of alkyl hydride complexes lose alkane more readily than the corresponding Cp*Cp″Zr(R)H complexes. In addition, the rate of reductive elimination has also been examined for Cp*Cp″Zr(Ph)(H) and Cp*Cp″Zr(CH₂Ph)H (Ph = C₆H₅) and is slower than the alkyl hydride series. The sluggish rates of reductive elimination are a result of ground state stabilization imparted by a strong zirconium-phenyl bond and by η² coordination of the benzyl ligand, respectively. This interaction, along with the solid state structure of Cp*Cp″Zr(CH₃)H, has been characterized by X-ray diffraction. The kinetic data led to the synthesis of Cp*₂Zr(CH₂CMe₃)H, which undergoes reductive elimination of alkane and coordination of dinitrogen.