研究室工作进展 Mar. 6th, 2019
Dinitrogen fixation and functionalization mediated or catalyzed by transition-metal complexes have been a central research theme in chemistry. Although some successful examples have been reported in the literature, the search for novel bonding diversity of structurally well-defined transition metal-dinitrogen complexes and further transformation of coordinated N2 is still in great demand and of great challenge.
In this work, a series of trinuclear and dinuclear Cr(I)-N2 complexes bearing cyclopentadienyl-phosphine ligands were synthesized and characterized. Further reduction of the Cr(I)-N2 complexes generated anionic Cr(0)-N2 complexes, which could react with Me3SiCl to afford the first chromium hydrazido complex from N2 functionalization. These complexes were found to be effective catalysts for the transformation of N2 into N(SiMe3)3.
The isolation of the chromium hydrazido complex represented one of the key intermediates in the Cr-catalyzed N2 reduction cycle. This study should have a major scientific impact on the chemistry of transition-metal (Cr in particular) mediated or catalyzed dinitrogen fixation and transformation.
氮气是空气的主要成分，占空气体积比的78%以上，是最丰富、最“廉价”的氮源,可谓取之不尽用之不竭。但是氮气分子是最稳定的分子之一，实现温和条件下氮气的高效活化与转化，从而不经过氨直接合成含氮有机化合物，是自然预设给科学家的“圣杯”（ 李嘉鹏 殷剑昊 俞超 张文雄 席振峰，从氮气直接合成含氮有机化合物，化学学报 2017, 75, 733-743.）。
利用过渡金属与配体的协同效应是实现温和条件下氮气的高效活化与转化的途径之一。虽然该研究领域已经取得了一些重要进展，但是效率依然很低，尤其是反应机理还很不清楚。例如，已经有文献报道铬与一些配体结合可以催化N2与Me3SiCl反应生成N(SiMe3)3，但是其反应的中间体未知，催化机理不详，限制了该催化反应的进一步优化和应用。本文利用我们自己发展的一类膦-环戊二烯配体（Inorg. Chem. Front. 2019, 6, 428.），与Cr一起形成了结构新颖的多核Cr-N2配合物，并首次获得了其催化生成N(SiMe3)3的活性反应中间体。该工作为固氮化学中配体的设计，以及多功能配体与多核过渡金属协同作用于氮气分子的活化模式提供了一个新途径。