Differential Impacts of Pyrophosphate on Ferrates(VI, V, and IV): Through Its Unique Inhibition to Identify Fe(V) Species

High-valent iron species [Fe­(V) and Fe­(IV)] exhibit remarkable oxidative activity in environmental chemistry. However, the distinctions between the properties of Fe­(V) and Fe­(IV) remain poorly understood due to the challenges of distinguishing them. Herein, using pyrophosphate as a model ligand, we comprehensively investigated the influence of oxo-ligands on the reactivity of high-valent iron­(VI, V, IV) species. An innovative strategy to selectively generate Fe­(IV) using the Fe­(VI)-initiated system was proposed, enabling an in-depth investigation of the interaction between Fe­(IV) and pyrophosphate. The results reveal that pyrophosphate strongly inhibits Fe­(V) oxidation, while it has minimal impact on the reactivity of Fe­(VI) and Fe­(IV). Based on ligand field theory, pyrophosphate complexation can induce iron 3d orbital resplitting, leading to spin electron rearrangement. Specifically, the hexa-coordinated Fe­(V)-oxo complex ligated by pyrophosphate exhibits higher orbital energy, reducing its stability and effective collisions with contaminants, whereas, the potential Jahn–Teller distortion of the Fe­(IV)-oxo complex could enhance its stability and preserve its significant reactivity. Given its selective inhibition of Fe­(V) oxidation, pyrophosphate can emerge as a promising targeted quenching agent for Fe­(V) species. This study provides valuable theoretical insights to guide the identification and characterization of intermediate iron species in iron-based oxidation processes.高价铁物种[Fe(V)和Fe(IV)]在环境化学领域展现出优异的氧化活性。然而，由于难以区分Fe(V)与Fe(IV)，二者的性质差异至今仍未得到充分阐释。本研究以焦磷酸根（pyrophosphate）为模型配体，系统探究了氧配体（oxo-ligands）对高价铁(VI、V、IV)物种反应活性的影响。我们提出了一种利用Fe(VI)引发体系选择性生成Fe(IV)的创新策略，从而能够深入研究Fe(IV)与焦磷酸根之间的相互作用。研究结果表明，焦磷酸根可显著抑制Fe(V)的氧化过程，但对Fe(VI)与Fe(IV)的反应活性仅产生微弱影响。基于配体场理论（ligand field theory），焦磷酸根配位可诱导铁的3d轨道发生重裂，进而引发自旋电子重排。具体而言，经焦磷酸根配位的六配位Fe(V)-氧代复合物具有更高的轨道能级，降低了其稳定性以及与污染物发生有效碰撞的概率；而Fe(IV)-氧代复合物潜在的姜-泰勒畸变（Jahn–Teller distortion）则可增强其稳定性，并保留其显著的反应活性。鉴于焦磷酸根能够选择性抑制Fe(V)的氧化过程，其有望成为一种极具应用前景的Fe(V)物种靶向淬灭剂（targeted quenching agent）。本研究可为铁基氧化过程中中间铁物种的鉴定与表征提供重要的理论指导依据。