Dual Iron Sites in Activation of N2 by Iron-Sulfur Cluster Anions Fe5S2- and Fe5S3-

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 equivalent to N triple bond. Herein, a series of gas-phase iron-sulfur cluster anions FexSy- (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 Fe5S2- and Fe5S3- showed superior reactivity toward N-2. Theoretical results revealed that N-2 binds molecularly to the iron sites of Fe5S2,3- 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 pi* 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.