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

Abstract

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 SmII-(2-pyrrolidone) core supported by a hexadentate dianionic macrocyclic ligand (SmII-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 Nβ, favored by strong acids, releases NH3, whereas one-electron reduction to FeNNH2-, favored by strong reductants such as SmII-PH, produces N2H4 via reactivity initiated at Nα. Spectroscopic data also implicate a role for SmIII-binding to anionic FeN2- (via an Fe-N2- -SmIII species) with respect to catalytic efficacy.