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-star(CpRn)Zr(CH2CHMe2)H (Cp-star = eta(5)-C5Me5, CpRn = 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"Zr-2(CH2CHMe2)-(H) (Cp" = eta(5)-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"Zr-2](2)(mu(2), eta(2), eta(2-)N(2)), has been characterized by X-ray diffraction and displays a bond length of 1.47 Angstrom for the N-2 ligand, the longest observed in any metallocene dinitrogen complex. Solution magnetic susceptibility demonstrates that [Cp"Zr-2](2)(mu(2), eta(2), eta(2)-N-2) is a ground-state triplet, consistent with two Zr(III), d(1) centers. Mechanistic studies reveal that the dinitrogen complex is derived from the reaction of N-2 with the resulting cyclometalated zirconocene hydride rather than directly from reductive elimination of alkane.