Characterization of an Fe≡N–NH₂Intermediate Relevant to Catalytic N₂Reduction to NH₃

Abstract

The ability of certain transition metals to mediate the reduction of N2 to NH3 has attracted broad interest in the biological and inorganic chemistry communities. Early transition metals such as Mo and W readily bind N2 and mediate its protonation at one or more N atoms to furnish M(NxHy) species that can be characterized and, in turn, extrude NH3. By contrast, the direct protonation of Fe-N2 species to Fe(NxHy) products that can be characterized has been elusive. Herein we show that addition of acid at low temperature to [(TPB)Fe(N2)][Na(12-crown-4)] results in a new S = 1/2 Fe species. EPR, ENDOR, Mössbauer, and EXAFS analysis, coupled with a DFT study, unequivocally assign this new species as [(TPB)Fe≡N-NH2]+, a doubly protonated hydrazido(2-) complex featuring an Fe-to-N triple bond. This unstable species offers strong evidence that the first steps in Fe-mediated nitrogen reduction by [(TPB)Fe(N2)][Na(12-crown-4)] can proceed along a distal or `Chatt-type' pathway. A brief discussion of whether subsequent catalytic steps may involve early or late stage cleavage of the N-N bond, as would be found in limiting distal or alternating mechanisms, respectively, is also provided.

A Synthetic Single-Site Fe Nitrogenase: High Turnover, Freeze-Quench 57Fe Mössbauer Data, and a Hydride Resting State

“…(ii) The most reduced state, P 3 B Fe-N 2 − 1 , can be doubly protonated at low temperature to generate P 3 B Fe=N-NH 2 + , a distal pathway intermediate. 18 This S = 1/2 species features a short Fe-N multiple bond (~1.65 Å). Its diamagnetic relative, P 3 sSi Fe=N-NH 2 + , has very recently been structurally characterized.…”

“…We have shown in previous work that 1 reacts favorably with HBAr F 4 in Et 2 O at −78 °C along a productive N 2 fixation pathway. 18 In brief, 1 can be doubly protonated in Et 2 O at −78 °C to generate P 3 B Fe=NNH 2 + (Eqn. 2).…”

An Fe-N2 Complex That Generates Hydrazine and Ammonia via Fe═NNH2: Demonstrating a Hybrid Distal-to-Alternating Pathway for N2 Reduction

“…The related S = 1 / 2 {[TPB]Fe(NNH 2 )} + exhibited 1 H hyperfine coupling as large as 18 MHz between g 2 and g 3 . 7 …”

“…7 This species cannot be isolated and independently studied owing to its greater instability and the presence of additional iron components, and it remains unclear whether NH 3 production in this case derives from similar late-stage cleavage to first produce N 2 H 4 , akin to [SiP iPr 3 ]Fe═NNH 2 , or if an early-stage cleavage pathway instead generates a terminal iron-bound nitride, such as [TPB]Fe═N or (TPB)Fe═N + . The greater flexibility of the Fe–B bond relative to the Fe–Si bond may afford access to different intermediates.…”

“…5,7 Its detection in operando when both strong acid and reductant are present is not feasible; the species is far too reactive under such conditions. We therefore generated {[TPB]Fe═NNH 2 } + at low temperature by double protonation of {[TPB]Fe(N 2 )} − in the absence of exogenous reductant and characterized this species with a suite of spectroscopic techniques including EPR/ ENDOR, XAS, and Mössbauer spectroscopies.…”