Cyclopentadienyl Substituent Effects on Reductive Elimination Reactions in Group 4 Metallocenes: Kinetics, Mechanism, and Application to Dinitrogen Activation

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Abstract
The rate of reductive elimination for a family of zirconocene isobutyl hydride complexes, Cp*(CpRn)Zr(CH2CHMe2)H (Cp* = η5-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‘ ‘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.

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