Bernskoetter Wesley H, Lobkovsky Emil, Chirik Paul J
Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell University, Ithaca, New York 14853, USA.
J Am Chem Soc. 2005 Oct 12;127(40):14051-61. doi: 10.1021/ja0538841.
The rates of hydrogenation of the N2 ligand in the side-on bound dinitrogen compounds, [(eta(5)-C5Me4H)2Zr]2(mu2,eta(2),eta(2)-N2) and [(eta(5)-C5Me5)(eta(5)-C5H2-1,2-Me2-4-R)Zr]2(mu2,eta(2),eta(2)-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 [(eta(5)-C5Me5)(eta(5)-C5H2-1,2,4-Me3)Zr]2(mu2,eta(2),eta(2)-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 [(eta(5)-C5Me5)(eta(5)-C5H2-1,2,4-Me3)Zr]2(mu2,eta(2),eta(2)-N2) over a 40 degrees C temperature range provided activation parameters of deltaH(double dagger) = 8.4(8) kcal/mol and deltaS(double dagger) = -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, [(eta(5)-C5Me5)(eta(5)-C5H2-1,2-Me2-4-R)Zr]2(mu2,eta(2),eta(2)-N2) (R = Me, Ph), to afford cyclometalated zirconocene diazenido derivatives.
通过电子光谱法测定了侧基配位的二氮化合物[(η⁵-C₅Me₄H)₂Zr]₂(μ₂,η²,η²-N₂)和[(η⁵-C₅Me₅)(η⁵-C₅H₂-1,2-Me₂-4-R)Zr]₂(μ₂,η²,η²-N₂)(R = Me, Ph)中N₂配体氢化生成相应的氢化锆茂二氮烯基配合物的速率。对[(η⁵-C₅Me₅)(η⁵-C₅H₂-1,2,4-Me₃)Zr]₂(μ₂,η²,η²-N₂)氢化反应速率定律的测定确定了该反应总体为二级反应，对每种试剂均为一级反应。这些数据，结合H₂与D₂加成时2.2(1)的正常一级动力学同位素效应，确定了第一次H₂加成是N₂氢化反应的速率决定步骤。对于另外两种锆茂二氮配合物的氢化（氘化）反应，也测定了类似方向和大小的动力学同位素效应。在40℃温度范围内测量[(η⁵-C₅Me₅)(η⁵-C₅H₂-1,2,4-Me₃)Zr]₂(μ₂,η²,η²-N₂)氢化反应的速率常数，得到活化参数ΔH‡ = 8.4(8) kcal/mol和ΔS‡ = -33(4) eu。活化熵与有序的四中心过渡结构一致，其中H₂以形式上的1,2-加成方式加成到具有相当多重键特征的锆-氮键上。这一假设的证据来自于在混合环二氮配合物[(η⁵-C₅Me₅)(η⁵-C₅H₂-1,2-Me₂-4-R)Zr]₂(μ₂,η²,η²-N₂)(R = Me, Ph)中，通过环戊二烯基甲基取代基的C-H活化实现N₂官能化，生成环金属化锆茂二氮烯基衍生物。
