Highly Selective Fe-Catalyzed Nitrogen Fixation to Hydrazine Enabled by Sm(II) Reagents with Tailored Redox Potential and p<i>K</i>_a

Abstract

Controlling product selectivity in multiproton, multielectron reductions of unsaturated small molecules is of fundamental interest in catalysis. For the N 2 reduction reaction (N 2 RR) in particular, parameters that dictate selectivity for either the 6H + /6e – product ammonia (NH 3 ) or the 4H + /4e – product hydrazine (N 2 H 4 ) 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 NH 3 is typically the major product of N 2 R, to instead favor N 2 H 4 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 II –PH) as the net hydrogen-atom donor. The activity and efficiency of the catalyst with this reagent remain high (up to 69 equiv of N 2 H 4 per Fe and 67% fixed-N yield per H + ). However, by generating N 2 H 4 as the kinetic product, the overpotential of this Sm-driven reaction is 700 mV lower than that of the mildest reported set of NH 3 -selective conditions with Fe . Mechanistic data support assignment of iron hydrazido(2−) species FeNNH 2 as selectivity-determining: we infer that protonation of FeNNH 2 at N β, favored by strong acids, releases NH 3, whereas one-electron reduction to FeNNH 2 –, favored by strong reductants such as Sm II –PH, produces N 2 H 4 via reactivity initiated at N α . Spectroscopic data also implicate a role for Sm III -binding to anionic Fe N 2 – (via an Fe –N 2 - - Sm III species) with respect to catalytic efficacy.

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