Dinitrogen Coordination to a High‐Spin Diiron(I/II) Species

Dinitrogen coordination to iron centers underpins industrial and biological fixation in the Haber–Bosch process and by the FeM cofactors in the nitrogenase enzymes. The latter employ local high‐spin metal centers; however, iron–dinitrogen coordination chemistry remains dominated by low‐valent states, contrasting the enzyme systems. Here, we report a high‐spin mixed‐valent cis‐(μ‐1,2‐dinitrogen)diiron(I/II) complex [(FeBr)2(μ‐N2)Lbis]− (2), where [Lbis]− is a bis(β‐diketiminate) cyclophane. Field‐applied Mössbauer spectra, dc and ac magnetic susceptibility measurements, and computational methods support a delocalized S=7/2 Fe2N2 unit with D=−5.23 cm−1 and consequent slow magnetic relaxation. Dinitrogen coordination to multiiron complexes is instrumental in understanding the reactivity of nitrogenases cofactors. The synthesis of a high‐spin mixed‐valent cis‐(μ‐1,2‐dinitrogen)diiron(I/II) complex obtained by a one‐electron reduction of a diiron(II/II) precursor is reported. The electronic structure of the complex was studied using Mössbauer spectroscopy, SQUID magnetometry, and DFT calculations, which support a S=7/2 ground state.