State Key Laboratory for Structural Chemistry of Unstable and Stable Species, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, People's Republic of China.
J Chem Phys. 2022 Feb 14;156(6):064303. doi: 10.1063/5.0077183.
Nitrogen (N) fixation is a challenging task for chemists. Adsorption of N on transition metal (TM) sites has been identified as a prerequisite for activating the very stable N≡N triple bond in both industrial and biological processes. The importance of π back-donation (filled orbitals of TM → π* orbitals of N) between metal sites and N has been well elucidated while the role of another classic orbital interaction, namely σ donation (σ orbitals of N → empty orbitals of TM), remains ambiguous. Herein, the size-dependent reactivity of trinuclear rhodium deuteride cluster anions RhD (n = 0-3) toward N adsorption in the gas phase was investigated experimentally and theoretically. A reverse relationship that higher electron-donating ability of clusters corresponds to lower N adsorption reactivity was experimentally observed, which is uncommon in N activation by gas-phase species. Theoretical analysis revealed that the σ donation rather than the π back-donation plays a predominant role in the adsorption complexes RhDN and the enhanced reactivity upon D addition is ascribed to the lowered energy levels of active orbitals in RhD as n increases. This study provides the first experimental evidence to declare the important role of σ donation and new clues for the design of reactive metal species in nitrogen fixation.
氮气（N）固定对于化学家来说是一项具有挑战性的任务。在工业和生物过程中，吸附 N 到过渡金属（TM）位点被认为是激活非常稳定的 N≡N 三键的先决条件。金属位点和 N 之间的π反馈（TM 的填充轨道→N 的π*轨道）的重要性已经得到了很好的阐明，而另一种经典轨道相互作用，即σ给予（N 的σ轨道→TM 的空轨道）的作用仍然存在争议。在此，通过实验和理论研究了三核铑氘化物簇阴离子 RhD（n = 0-3）在气相中对 N 吸附的尺寸依赖性反应性。实验观察到一个反相关关系，即簇的更高电子给体能力对应于更低的 N 吸附反应性，这在气相物种的 N 活化中并不常见。理论分析表明，σ给予而不是π反馈在吸附配合物 RhDN 中起主要作用，并且随着 n 的增加，D 添加增强了反应性，这归因于 RhD 中活性轨道能级的降低。这项研究提供了第一个实验证据，证明了σ给予的重要作用，并为氮气固定中反应性金属物种的设计提供了新的线索。
