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.
All Science Journal Classification (ASJC) codes
- Colloid and Surface Chemistry