A Terminal N₂ Complex of High-Spin Iron(I) in a Weak, Trigonal Ligand Field

The role of Fe in biological and industrial N2 fixation has inspired the intense study of small molecule analogues of Fe–(NxHy) intermediates of potential relevance to these processes. Although a number of low-coordinate Fe–(N2) featuring varying degrees of fidelity to the nitrogenase active site are now known, these complexes frequently feature strongly donating ligands that either enforce low- or intermediate-spin states or result in linear Fe–(N2)–Fe bridging motifs. Given that the nitrogenase active site uses weak-field sulfide ligands to stabilize its reactive Fe center(s), N2 binding to high-spin Fe is of great interest. Herein, we report the synthesis and characterization of the first terminal N2 complex of high-spin (S = 3/2) Fe­(I) as well as a bridging Fe–(N2)–Fe analogue. Electron paramagnetic resonance and solution magnetic moment determination confirm the high-spin state, and vibrational experiments indicate a substantial degree of activation of the NN bond in these complexes. Density functional theory calculations reveal an electronic structure for the terminal adduct featuring substantial delocalization of unpaired spin onto the N2 ligand.