Metal-Ligand Cooperativity in Iron Dinitrogen Complexes: Proton-Coupled Electron Transfer Disproportionation and an AnionicFe(0)N2Hydride

Metal-ligand cooperativity and redox-active ligands enablethe use of open-shellfirst-row transition metals in catalysis. However, thefleeting nature of the reactive intermediates prevents direct inspection ofthe relevant catalytic species. By employing phosphine alpha-iminopyridine(PNN)-based complexes, we show that chemical and redox metal-ligandcooperativity can be combined in the coordination sphere of irondinitrogen complexes. These systems show dual activation modes eitherthrough deprotonation, which triggers reversible core dearomatization, orthrough reversibly accepting one electron by reducing the iminefunctionality. (PNN)Fe(N2) fragments can be obtained under mildlyreducing conditions. Deprotonation of such complexes induces dearoma-tization of the pyridine core while retaining a terminally coordinated N2ligand. This species is nevertheless stable in solution only below-30 degrees Cand undergoes unusual ligand-assisted redox disproportionation through proton-coupled electron transfer at room temperature. Theorigin of this phenomenon is the significant lability of the alpha-imine C-H bonds in the dearomatized species, where the calculatedbond dissociation free energy is 48.7 kcal mol-1. The dispropotionation reaction yields an overreduced iron compound,demonstrating that the formation of such species can be triggered by mild bases, and does not require harsh reducing agents.Reaction of the dearomatized species with dihydrogen yields a rare anionic Fe hydride that binds dinitrogen and features arearomatized core.