Highly Selective Fe-Catalyzed Nitrogen Fixation to Hydrazine Enabled by Sm(II) Reagents with Tailored Redox Potential...

Controlling product selectivity in multiproton, multielectron reductions of unsaturated small molecules is of fundamental interest in catalysis. For the N₂ reduction reaction (N₂RR) in particular, parameters that dictate selectivity for either the 6H⁺/6e^- product ammonia (NH₃) or the 4H⁺/4e^- product hydrazine (N₂H₄) are poorly understood. To probe this issue, we have developed conditions to invert the selectivity of a tris(phosphino)borane iron catalyst (Fe), with which NH₃ is typically the major product of N₂R, to instead favor N₂H₄ as the sole observed fixed-N product (>99:1). This dramatic shift is achieved by replacing moderate reductants and strong acids with a very strongly reducing but weakly acidic Sm^II-(2-pyrrolidone) core supported by a hexadentate dianionic macrocyclic ligand (Sm-PH) as the net hydrogen-atom donor. The activity and efficiency of the catalyst with this reagent remain high (up to 69 equiv of N₂H₄ per Fe and 67% fixed-N yield per H⁺). However, by generating N₂H₄ as the kinetic product, the overpotential of this Sm-driven reaction is 700 mV lower than that of the mildest reported set of NH₃-selective conditions with Fe. Mechanistic data support assignment of iron hydrazido(2-) species FeNNH₂ as selectivity-determining: we infer that protonation of FeNNH₂ at N_β, favored by strong acids, releases NH₃, whereas one-electron reduction to FeNNH₂^-, favored by strong reductants such as Sm-PH, produces N₂H₄ via reactivity initiated at N_α. Spectroscopic data also implicate a role for Sm^III-binding to anionic FeN₂^- (via an Fe-N₂- -Sm species) with respect to catalytic efficacy.