Light Alters the NH₃ vs N₂H₄ Product Profile in Iron‐catalyzed Nitrogen Reduction via Dual Reactivity from an Iron Hydrazido (Fe=NNH₂) Intermediate

Abstract

Whereas synthetically catalyzed nitrogen reduction (N 2 R) to produce ammonia is widely studied, catalysis to instead produce hydrazine (N 2 H 4 ) has received less attention despite its considerable mechanistic interest. Herein, we disclose that irradiation of a tris(phosphine)borane (P 3 B ) Fe catalyst, P 3 B Fe + , significantly alters its product profile to increase N 2 H 4 versus NH 3 ; P 3

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

“…16 This manifests in the partial bending of the hydrazido ligand and (we propose) higher corresponding reactivity of N α via H + or H • transfer, leading toward N 2 H 4 generation; by contrast, the ground state of FeNNH 2 is protonated by relatively strong acids at the distal nitrogen (N β ) to release NH 3 . 16,30 We wondered whether a comparatively low-overpotential N 2 -to-N 2 H 4 catalytic process could be made highly selective using the same Fe catalyst via ground-rather than excited-state reactivity. Guiding our thinking, the singly reduced methyl analogue of FeNNH 2 , FeNNMe 2 − , exhibits a similar electronic structure to a low-lying excited state of FeNNH 2 .…”

“…Alternatively, reduction (favored by very strong reductants such as the Sm II species employed in this study) generates Fe NNH 2 – , which reacts with H + (or possibly H • ) at N α to ultimately evolve N 2 H 4 . As a third possibility, a low-lying excited state of Fe NNH 2 , accessed either thermally or by photoexcitation, reacts at N α in a fashion similar to Fe NNH 2 – . This multifaceted reactivity presents several methods to manipulate N 2 R selectivity by varying temperature, irradiation, acidity, and reductant strength.…”

“…Our laboratory recently discovered that blue light irradiation of N 2 R catalyzed by a tris­(phosphino)­borane iron complex ( Fe + , see Figure b for structure), with decamethylcobaltocene (Cp* 2 Co) as the reductant ( E °(Co III/II ) = −1.9 V vs Fc +/0 in MeCN) and anilinium acids (p K a ≤ 10.6 in MeCN), results in some selectivity for N 2 H 4 (1:2.3 N 2 H 4 :NH 3 ) . In the dark, this catalysis almost exclusively produces NH 3 (Figure b) .…”

“…Our laboratory recently discovered that blue light irradiation of N 2 R catalyzed by a tris­(phosphino)­borane iron complex ( Fe + , see Figure b for structure), with decamethylcobaltocene (Cp* 2 Co) as the reductant ( E °(Co III/II ) = −1.9 V vs Fc +/0 in MeCN) and anilinium acids (p K a ≤ 10.6 in MeCN), results in some selectivity for N 2 H 4 (1:2.3 N 2 H 4 :NH 3 ) . In the dark, this catalysis almost exclusively produces NH 3 (Figure b) . To rationalize this observation, we proposed that the selectivity change arises from a photochemically generated excited state of a hydrazido intermediate Fe NNH 2 * that features increased spin density at the proximal nitrogen (N α ) relative to its ground state, arising from the increased population of an Fe–N π antibonding orbital and poorer overlap in Fe–N π bonding orbitals .…”

“…Relatedly, while the vanadium-dependent nitrogenase enzyme produces some N 2 H 4 , the origin of this selectivity is unknown . Even among well-defined N 2 R catalysts that show moderate to high N 2 H 4 selectivity, − present understanding of factors that favor N 2 H 4 over NH 3 is limited . With NH 3 and N 2 H 4 emerging as prospective zero-carbon fuel alternatives in light of the energetic and infrastructural challenges associated with compression, storage, and transfer of hydrogen fuel, − delineating factors that dictate N 2 H 4 versus NH 3 catalytic selectivity is of considerable interest.…”