Division of Chemistry and Chemical Engineering , California Institute of Technology (Caltech) , Pasadena , California 91125 , United States.
J Am Chem Soc. 2018 May 16;140(19):6122-6129. doi: 10.1021/jacs.8b02335. Epub 2018 May 2.
Substrate selectivity in reductive multielectron/proton catalysis with small molecules such as N, CO, and O is a major challenge for catalyst design, especially where the competing hydrogen evolution reaction (HER) is thermodynamically and kinetically competent. In this study, we investigate how the selectivity of a tris(phosphine)borane iron(I) catalyst, PFe, for catalyzing the nitrogen reduction reaction (NRR, N-to-NH conversion) versus HER changes as a function of acid p K. We find that there is a strong correlation between p K and NRR efficiency. Stoichiometric studies indicate that the anilinium triflate acids employed are only compatible with the formation of early stage intermediates of N reduction (e.g., Fe(NNH) or Fe(NNH)) in the presence of the metallocene reductant CpCo. This suggests that the interaction of acid and reductant is playing a critical role in N-H bond-forming reactions. DFT studies identify a protonated metallocene species as a strong PCET donor and suggest that it should be capable of forming the early stage N-H bonds critical for NRR. Furthermore, DFT studies also suggest that the observed p K effect on NRR efficiency is attributable to the rate and thermodynamics of CpCo protonation by the different anilinium acids. Inclusion of Cp*Co as a cocatalyst in controlled potential electrolysis experiments leads to improved yields of NH. The data presented provide what is to our knowledge the first unambiguous demonstration of electrocatalytic nitrogen fixation by a molecular catalyst (up to 6.7 equiv of NH per Fe at -2.1 V vs Fc).
小分子如 N、CO 和 O 的还原多电子/质子催化中的底物选择性是催化剂设计的主要挑战，尤其是在竞争的析氢反应 (HER) 在热力学和动力学上都具有竞争力的情况下。在这项研究中，我们研究了三(膦)硼烷铁(I)催化剂 PFe 催化氮还原反应 (NRR，N 到 NH 的转化) 与 HER 的选择性如何随酸 pK 值而变化。我们发现 pK 值与 NRR 效率之间存在很强的相关性。计量研究表明，所使用的三氟甲磺酸苯胺仅与金属配合物还原剂 CpCo 存在时氮还原（例如 Fe(NNH)或 Fe(NNH))早期中间体的形成相容。这表明酸和还原剂的相互作用在 N-H 键形成反应中起着关键作用。DFT 研究确定了质子化的金属配合物物种作为强 PCET 供体，并表明它应该能够形成对于 NRR 至关重要的早期 N-H 键。此外，DFT 研究还表明，观察到的 pK 值对 NRR 效率的影响归因于不同的苯胺酸对 CpCo 的质子化的速率和热力学。在控制电位电解实验中包含 Cp*Co 作为共催化剂会导致 NH 的产率提高。所提出的数据提供了据我们所知的第一个明确的分子催化剂电催化固氮的例证（在-2.1 V 相对于 Fc 下，每个 Fe 可达 6.7 当量的 NH）。
