Pool Jaime A, Lobkovsky Emil, Chirik Paul J
Department of Chemistry and Chemical Biology, Baker Laboratory Cornell University, Ithaca, New York 14853, USA.
Nature. 2004 Feb 5;427(6974):527-30. doi: 10.1038/nature02274.
Molecular nitrogen is relatively inert owing to the strength of its triple bond, nonpolarity and high ionization potential. As a result, the fixation of atmospheric nitrogen to ammonia under mild conditions has remained a challenge to chemists for more than a century. Although the Haber-Bosch process produces over 100 million tons of ammonia annually for the chemical industry and agriculture, it requires high temperature and pressure, in addition to a catalyst, to induce the combination of hydrogen (H2) and nitrogen (N2). Coordination of molecular nitrogen to transition metal complexes can activate and even rupture the strong N-N bond under mild conditions, with protonation yielding ammonia in stoichiometric and even catalytic yields. But the assembly of N-H bonds directly from H2 and N2 remains challenging: adding H2 to a metal-N2 complex results in the formation of N2 and metal-hydrogen bonds or, in the case of one zirconium complex, in formation of one N-H bond and a bridging hydride. Here we extend our work on zirconium complexes containing cyclopentadienyl ligands and show that adjustment of the ligands allows direct observation of N-H bond formation from N2 and H2. Subsequent warming of the complex cleaves the N-N bond at 45 degrees C, and continued hydrogenation at 85 degrees C results in complete fixation to ammonia.
由于氮分子的三键强度、非极性和高电离势，分子氮相对惰性。因此，在温和条件下将大气中的氮固定为氨，一个多世纪以来一直是化学家面临的挑战。尽管哈伯-博施法每年为化学工业和农业生产超过1亿吨氨，但它除了需要催化剂外，还需要高温高压来促使氢气（H2）和氮气（N2）结合。分子氮与过渡金属配合物的配位可以在温和条件下激活甚至断裂强N-N键，质子化以化学计量甚至催化产率生成氨。但是直接由H2和N2组装N-H键仍然具有挑战性：向金属-N2配合物中加入H2会导致形成N2和金属-氢键，或者在一种锆配合物的情况下，形成一个N-H键和一个桥连氢化物。在这里，我们扩展了对含环戊二烯基配体的锆配合物的研究，并表明配体的调整允许直接观察到由N2和H2形成N-H键。随后将配合物升温至45℃时会断裂N-N键，在85℃继续氢化会完全固定为氨。
