Electrocatalytic H2O Reduction with f‑Elements: Mechanistic Insight and Overpotential Tuning in a Series of Lanthanide Complexes

Electrocatalytic energy conversion with molecular f-element catalysts is still in an early phase of its development. We here report detailed electrochemical investigations on the recently reported trivalent lanthanide coordination complexes [((Ad,MeArO)3mes)­Ln] (1–Ln), with Ln = La, Ce, Pr, Nd, Sm, Gd, Dy, Er, and Yb, which were now found to perform as active electrocatalysts for the reduction of water to dihydrogen. Reactivity studies involving complexes 1–Ln and the Ln­(II) analogues [K­(2.2.2-crypt)]­[((Ad,MeArO)3mes)­Ln] (2–Ln) suggest a reaction mechanism that differs significantly from the reaction pathway found for the corresponding uranium catalyst [((Ad,MeArO)3mes)­U] (1–U). While complexes 1−Ln activate water via a radical pathway, only upon a 1 e− reduction to yield the reduced species 2−Ln, the 5f analogue 1−U directly reduces H2O via a 2 e− pathway. The electrocatalytic H2O reduction by complexes 1–Ln is initiated by the respective Ln­(III)/Ln­(II) redox couples, which gradually turn to more positive values across the Ln series. This correlation has been exploited to tune the catalytic overpotential of water reduction by choice of the lanthanide ion. Kinetic studies of the 1–Ln series were performed to elucidate correlations between overpotential and turnover frequencies of the 4f-based electrocatalysts.

以分子f区元素催化剂开展电催化能量转化的研究仍处于早期发展阶段。本文针对近期报道的三价镧系配位配合物[((Ad,MeArO)₃mes)­Ln]（1–Ln，其中Ln=La、Ce、Pr、Nd、Sm、Gd、Dy、Er、Yb）开展了详细的电化学研究，本研究发现该类配合物可作为活性电催化剂用于水还原制备氢气。针对配合物1–Ln及其二价镧系类似物[K(2.2.2-穴醚)]­[((Ad,MeArO)₃mes)­Ln]（2–Ln）的反应性研究表明，其反应机理与对应铀系催化剂[((Ad,MeArO)₃mes)­U]（1–U）所遵循的反应路径存在显著差异。1–Ln类配合物可通过自由基路径活化水，而仅当经单电子还原生成还原物种2–Ln后方可实现该过程；与之不同的是，5f区的对应物1–U可直接通过双电子路径还原H₂O。1–Ln类配合物催化的电催化水还原反应由相应的Ln(III)/Ln(II)氧化还原对引发，且该氧化还原对的电位随镧系元素序列逐渐向正电位偏移。利用这一相关性，可通过选择不同镧系离子来调控水还原反应的催化过电位。本研究针对1–Ln系列配合物开展了动力学研究，以阐明该类4f基电催化剂的过电位与周转频率之间的关联。