Fe-Mediated HER vs N2RR: Exploring Factors That Contribute to Selectivity in P3EFe(N2) (E = B, Si, C) Catalyst Model Systems

Mitigation of the hydrogen evolution reaction (HER) is a key challenge in selective small-molecule reduction catalysis. This is especially true of catalytic nitrogen (N2) and carbon dioxide (CO2) reduction reactions (N2RR and CO2RR, respectively) using H+/e– currency. Here we explore, via DFT calculations, three iron model systems, P3EFe (E = B, Si, C), known to mediate both N2RR and HER, but with different selectivity depending on the identity of the auxiliary ligand. It is suggested that the respective efficiencies of these systems for N2RR trend with the predicted N–H bond strengths of two putative hydrazido intermediates of the proposed catalytic cycle, P3EFe­(NNH2)+ and P3EFe­(NNH2). Further, a mechanism is presented for undesired HER consistent with DFT studies, and previously reported experimental data, for these systems; bimolecular proton-coupled electron transfer (PCET) from intermediates with weak N–H bonds is posited as an important source of H2, instead of more traditional scenarios that proceed via metal hydride intermediates and proton transfer/electron transfer (PT/ET) pathways. Wiberg bond indices provide additional insight into key factors related to the degree of stabilization of P3EFe­(NNH2) species, factors that trend with overall product selectivity.

析氢反应（hydrogen evolution reaction, HER）的抑制是选择性小分子还原催化中的关键挑战。对于以质子-电子对（H+/e–）为电荷载体的催化氮（nitrogen, N₂）还原反应（nitrogen reduction reaction, N2RR）与二氧化碳（carbon dioxide, CO₂）还原反应（carbon dioxide reduction reaction, CO2RR）而言，这一难题尤为突出。本研究通过密度泛函理论（density functional theory, DFT）计算，探究了三类铁基模型体系P3EFe（其中E=B、Si、C）：这类体系已被证实可同时介导N2RR与HER，但根据辅助配体的差异，其催化选择性各不相同。研究表明，该三类体系的N2RR催化效率，与所提出的催化循环中两种推定的肼基中间体P3EFe(NNH2)+与P3EFe(NNH2)的预测N-H键键强呈相关趋势。此外，本研究针对上述体系提出了与DFT计算结果及已有实验数据相符的副反应HER发生机制：相较于传统的经由金属氢化物中间体与质子转移/电子转移（proton transfer/electron transfer, PT/ET）路径的反应模型，我们认为N-H键较弱的中间体发生双分子质子耦合电子转移（proton-coupled electron transfer, PCET）是生成H₂的重要途径。威伯格键指数（Wiberg bond indices）为理解与P3EFe(NNH2)物种稳定化程度相关的关键因素提供了额外视角，而这些因素与体系的整体产物选择性呈相关趋势。