Characterization of an FeN–NH2 Intermediate Relevant to Catalytic N2 Reduction to NH3

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.

特定过渡金属催化氮气还原为氨的能力，在生物无机化学领域引起了广泛关注。早期过渡金属如钼（Mo）和钨（W）能够轻易地与氮气结合，并在一个或多个氮原子上进行质子化，从而形成M(NxHy)物种，这些物种可被表征并进而生成氨。相比之下，直接将Fe-N2物种质子化形成可表征的Fe(NxHy)产物一直难以实现。本研究中，我们展示在低温下向[(TPB)Fe(N2)]-[Na(12-冠醚-4)]中添加酸，会导致一种新的S = 1/2铁物种的生成。电子顺磁共振（EPR）、电子核双共振（ENDOR）、莫塞巴效应（Mössbauer）和扩展X射线吸收精细结构（EXAFS）分析，结合密度泛函理论（DFT）研究，明确将该新物种指认为[(TPB)Fe=N-NH2]+，这是一种具有Fe到N三键的双质子化叠氮化物(2-)络合物。这一不稳定的物种为[(TPB)Fe(N2)]-[Na(12-冠醚-4)]介导的Fe催化氮还原的第一步可能沿远程或“Chatt型”途径进行提供了强有力的证据。此外，还简要讨论了后续催化步骤是否可能涉及早期或晚期N-N键的断裂，正如在限制性远程或交替机制中观察到的，分别对应于早期或晚期阶段。