Lithium-Assisted Dinitrogen Reduction Mediated by Nb₂LiNO_1-4^- Cluster Anions: Electron Donors or Structural Units

Alkali atoms are usually used as promoters to significantly increase the catalytic activity of transition-metal catalysts in a wide range of reactions such as dinitrogen conversion reactions. However, the role of alkali metal atoms remains controversial. Herein, a series of quaternary cluster anions containing lithium atoms Nb₂LiNO_1-4^- have been synthesized and reacted with N₂ at room temperature. The detailed experimental and theoretical investigations indicate that Nb₂LiNO^- is capable to cleave the N≡N bond and the Li atoms in Nb₂LiNO_1,2^- act as electron donors in the N₂ reduction reaction. With the increase in the number of oxygen atoms, the reactivity toward N₂ is reduced from adsorption via a side-on end-on mode in Nb₂LiNO₂^- to the inertness of Nb₂LiNO₄^-. In Nb₂LiNO_3,4^- anions, the Li atoms are bonded with oxygen atoms, acting as structural units to stabilize structures. Therefore, the roles of alkali atoms are able to change with different chemical environments of active sites. For the first time, we reveal how the number of ligands (oxygen atoms herein) can be used to finely regulate the reactivity toward N₂.