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.