Cyclopentadienyl substituent effects on reductive elimination reactions in group 4 metallocenes: Kinetics, mechanism, and application to dinitrogen activation

The rate of reductive elimination for a family of zirconocene isobutyl hydride complexes, Cp*(CpR_n)Zr(CH₂CHMe₂)H (Cp* = η⁵-C₅Me₅, CpR_n = substituted cyclopentadienyl), has been measured as a function of cyclopentadienyl substituent. In general, the rate of reductive elimination increases modestly with the incorporation of sterically demanding substituents such as [CMe₃] or [SiMe₃]. A series of isotopic labeling experiments was used to elucidate the mechanism and rate-determining step for the reductive elimination process. From these studies, a new zirconocene isobutyl hydride complex, Cp″₂Zr(CH₂CHMe₂)-(H) (Cp″ = η⁵-C₅H₃-1,3- (SiMe₃)₂), was designed and synthesized such that facile reductive elimination of isobutane and activation of dinitrogen was observed. The resulting dinitrogen complex, [Cp″₂Zr]₂(μ₂, η²,η²-N₂), has been characterized by X-ray diffraction and displays a bond length of 1.47 Å for the N₂ ligand, the longest observed in any metallocene dinitrogen complex. Solution magnetic susceptibility demonstrates that [Cp″₂Zr]₂(μ₂, η², η²-N₂) is a ground-state triplet, consistent with two Zr(III), d¹ centers. Mechanistic studies reveal that the dinitrogen complex is derived from the reaction of N₂ with the resulting cyclometalated zirconocene hydride rather than directly from reductive elimination of alkane.