研究室工作進展 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.
氮氣是空氣的主要成分，占空氣體積比的78%以上，是最豐富、最“廉價”的氮源,可謂取之不盡用之不竭。但是氮氣分子是最穩定的分子之一，實現溫和條件下氮氣的高效活化與轉化，從而不經過氨直接合成含氮有機化合物，是自然預設給科學家的“聖杯”（ 李嘉鵬 殷劍昊 俞超 張文雄 席振峰，從氮氣直接合成含氮有機化合物，化學學報 2017, 75, 733-743.）。
利用過渡金屬與配體的協同效應是實現溫和條件下氮氣的高效活化與轉化的途徑之一。雖然該研究領域已經取得了一些重要進展，但是效率依然很低，尤其是反應機理還很不清楚。例如，已經有文獻報道鉻與一些配體結合可以催化N2與Me3SiCl反應生成N(SiMe3)3，但是其反應的中間體未知，催化機理不詳，限制了該催化反應的進一步優化和應用。本文利用我們自己發展的一類膦-環戊二烯配體（Inorg. Chem. Front. 2019, 6, 428.），與Cr一起形成了結構新穎的多核Cr-N2配合物，并首次獲得了其催化生成N(SiMe3)3的活性反應中間體。該工作為固氮化學中配體的設計，以及多功能配體與多核過渡金屬協同作用于氮氣分子的活化模式提供了一個新途徑。