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Article

Kinetics and Mechanism of N₂ Hydrogenation in Bis(cyclopentadienyl) Zirconium Complexes and Dinitrogen Functionalization by 1,2-Addition of a Saturated C−H Bond

Supporting Info

Wesley H. Bernskoetter, Emil Lobkovsky, and Paul J. Chirik*

Contribution from the Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell University, Ithaca, New York 14853

J. Am. Chem. Soc., 2005, 127 (40), pp 14051–14061

DOI: 10.1021/ja0538841

Publication Date (Web): September 17, 2005

In papers with more than one author, the asterisk indicates the name of the author to whom inquiries about the paper should be addressed.

, pc92@cornell.edu

Abstract

The rates of hydrogenation of the N₂ ligand in the side-on bound dinitrogen compounds, [(η⁵-C₅Me₄H)₂Zr]₂(μ₂,η²,η²-N₂) and [(η⁵-C₅Me₅)(η⁵-C₅H₂-1,2-Me₂-4-R)Zr]₂(μ₂,η²,η²-N₂) (R = Me, Ph), to afford the corresponding hydrido zirconocene diazenido complexes have been measured by electronic spectroscopy. Determination of the rate law for the hydrogenation of [(η⁵-C₅Me₅)(η⁵-C₅H₂-1,2,4-Me₃)Zr]₂(μ₂,η²,η²-N₂) establishes an overall second-order reaction, first order with respect to each reagent. These data, in combination with a normal, primary kinetic isotope effect of 2.2(1) for H₂ versus D₂ addition, establish the first H₂ addition as the rate-determining step in N₂ hydrogenation. Kinetic isotope effects of similar direction and magnitude have also been measured for hydrogenation (deuteration) of the two other zirconocene dinitrogen complexes. Measuring the rate constants for the hydrogenation of [(η⁵-C₅Me₅)(η⁵-C₅H₂-1,2,4-Me₃)Zr]₂(μ₂,η²,η²-N₂) over a 40 °C temperature range provided activation parameters of ΔH = 8.4(8) kcal/mol and ΔS = −33(4) eu. The entropy of activation is consistent with an ordered four-centered transition structure, where H₂ undergoes formal 1,2-addition to a zirconium−nitrogen bond with considerable multiple bond character. Support for this hypothesis stems from the observation of N₂ functionalization by C−H activation of a cyclopentadienyl methyl substituent in the mixed ring dinitrogen complexes, [(η⁵-C₅Me₅)(η⁵-C₅H₂-1,2-Me₂-4-R)Zr]₂(μ₂,η²,η²-N₂) (R = Me, Ph), to afford cyclometalated zirconocene diazenido derivatives.