Kinetics and mechanism of N₂ hydrogenation in bis(cyclopentadienyl) zirconium complexes and dinitrogen functionalization by 1,2-addition of a saturated C-H bond

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-C₅Me ₅)(η⁵-C₅H₂-1,2-Me ₂-4-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-C ₅H₂-1,2,4-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 H ₂ 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-C ₅Me₅)(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-C₅H ₂-1,2-Me₂-4-R)Zr]₂(μ₂, η²,η²-N₂) (R = Me, Ph), to afford cyclometalated zirconocene diazenido derivatives.