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–(N_<i>x</i>H_<i>y</i>) 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 (<i>S</i> = 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