Dual Iron Sites in Activation of N by Iron-Sulfur Cluster Anions FeS and FeS.

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 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 FeS ( = 1-8, 0-) were prepared and their reactivities toward N were investigated systematically by mass spectrometry. Among the 44 investigated clusters, only FeS and FeS showed superior reactivity toward N. Theoretical results revealed that N binds molecularly to the iron sites of FeS in a common end-on coordination mode with an unprecedented back-donation interaction from the localized - bonding orbitals of Fe-Fe sites to the π* antibonding orbitals of N. This is the first example to disclose the significant contribution of the dual metal sites rather than the single metal atom to N adsorption in the prevalent end-on binding mode.