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Inspired by the fact that the active centers of natural nitrogenases are polynuclear iron-sulfur clusters, the reactivity of isolated iron-sulfur clusters toward N 2 has received considerable attention to gain fundamental insights into the activation of the N≡N triple bond. Herein, a series of gas-phase iron-sulfur cluster anions Fe x S y - ( x = 1-8, y = 0- x ) were prepared and their reactivities toward N 2 were investigated systematically by mass spectrometry. Among the 44 investigated clusters, only Fe 5 S 2 - and Fe 5 S 3 - showed superior reactivity toward N 2 . Theoretical results revealed that N 2 binds molecularly to the iron sites of Fe 5 S 2,3 - in a common end-on coordination mode with an unprecedented back-donation interaction from the localized d - d bonding orbitals of Fe-Fe sites to the π* antibonding orbitals of N 2 . This is the first example to disclose the significant contribution of the dual metal sites rather than the single metal atom to N 2 adsorption in the prevalent end-on binding mode.

It is of great importance to study the mechanisms to activate dinitrogen (N 2 ), the very inert molecule, under mild conditions. Gas-phase metal clusters are being actively generated to react with N 2 to identify new reaction types and mechanisms. Herein, an unprecedented, mechanistically unique metal atom (Fe or V) ejection in the thermal reaction of FeV 2 C 2 - with N 2 has been identified using mass spectrometry, photoelectron imaging spectroscopy, and quantum chemistry calculations. Strong evidence suggests that the complete cleavage of the N≡N triple bond and subsequent functionalization of two N atoms via C-N coupling were achieved in this reaction. The complementary cooperation between V atoms with strong electron-donating ability and an Fe atom with large electron-withdrawing ability as well as the geometric flexibility of the Fe-V-V ring drives the whole reaction. The important role of C ligands in N≡N cleavage was also revealed. This study emphasizes the importance of heteronuclear metal systems for N 2 fixation.