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Proton-Coupled Electron Transfer at the Pu^5+/4+ Couple.

ABSTRACT: The synthesis and solution and solid-state characterization of [Pu⁴⁺(NPC)₄], 1-Pu, (NPC = [NP^tBu(pyrr)₂]^-; ^tBu = C(CH₃)₃; pyrr = pyrrolidinyl) and [Pu³⁺(NPC)₄][K(2.2.2.-cryptand)], 2-Pu, is described. Cyclic voltammetry studies of 1-Pu reveal a quasi-reversible Pu^4+/3+ couple, an irreversible Pu^5+/4+ couple, and a third couple evincing a rapid proton-coupled electron transfer (PCET) reaction occurring after the electrochemical formation of Pu⁵⁺. The chemical identity of the product of the PCET reaction was confirmed by independent chemical synthesis to be [Pu⁴⁺(NPC)₃(HNPC)][B(ArF₅)₄], 3-Pu, (B(ArF₅)₄ = tetrakis(2,3,4,5,6-pentafluourophenyl)borate) via two mechanistically distinct transformations of 1-Pu: protonation and oxidation. The kinetics and thermodynamics of this PCET reaction are determined via electrochemical analysis, simulation, and density functional theory. The computational studies demonstrate a direct correlation between the changing nature of 5f and 6d orbital participation in metal-ligand bonding and the electron density on the N_im atom with the thermodynamics of the PCET reaction from Np to Pu, and an indirect correlation with the roughly 5-orders of magnitude faster Pu PCET compared to Np for the An⁵⁺ species.

SUBMITTER: Otte KS

PROVIDER: S-EPMC11311234 | biostudies-literature | 2024 Aug

REPOSITORIES: biostudies-literature

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Proton-Coupled Electron Transfer at the Pu5+/4+ Couple.

Otte Kaitlyn S KS Niklas Julie E JE Studvick Chad M CM Montgomery Charlotte L CL Bredar Alexandria R C ARC Popov Ivan A IA La Pierre Henry S HS

Journal of the American Chemical Society 20240725 31

The synthesis and solution and solid-state characterization of [Pu4+(NPC)4], 1-Pu, (NPC = [NPtBu(pyrr)2]-; tBu = C(CH3)3; pyrr = pyrrolidinyl) and [Pu3+(NPC)4][K(2.2.2.-cryptand)], 2-Pu, is described. Cyclic voltammetry studies of 1-Pu reveal a quasi-reversible Pu4+/3+ couple, an irreversible Pu5+/4+ couple, and a third couple evin ...[more]

PMID: 39051969

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Project description:Designing molecular platforms for controlling proton and electron movement in artificial photosynthetic systems is crucial to efficient catalysis and solar energy conversion. The transfer of both protons and electrons during a reaction is known as proton-coupled electron transfer (PCET) and is used by nature in myriad ways to provide low overpotential pathways for redox reactions and redox leveling, as well as to generate bioenergetic proton currents. Herein, we describe theoretical and electrochemical studies of a series of bioinspired benzimidazole-phenol (BIP) derivatives and a series of dibenzimidazole-phenol (BI2P) analogs with each series bearing the same set of terminal proton-accepting (TPA) groups. The set of TPAs spans more than 6 pK a units. These compounds have been designed to explore the role of the bridging benzimidazole(s) in a one-electron oxidation process coupled to intramolecular proton translocation across either two (the BIP series) or three (the BI2P series) acid/base sites. These molecular constructs feature an electrochemically active phenol connected to the TPA group through a benzimidazole-based bridge, which together with the phenol and TPA group form a covalent framework supporting a Grotthuss-type hydrogen-bonded network. Infrared spectroelectrochemistry demonstrates that upon oxidation of the phenol, protons translocate across this well-defined hydrogen-bonded network to a TPA group. The experimental data show the benzimidazole bridges are non-innocent participants in the PCET process in that the addition of each benzimidazole unit lowers the redox potential of the phenoxyl radical/phenol couple by 60 mV, regardless of the nature of the TPA group. Using a series of hypothetical thermodynamic steps, density functional theory calculations correctly predicted the dependence of the redox potential of the phenoxyl radical/phenol couple on the nature of the final protonated species and provided insight into the thermodynamic role of dibenzimidazole units in the PCET process. This information is crucial for developing molecular "dry proton wires" with these moieties, which can transfer protons via a Grotthuss-type mechanism over long distances without the intervention of water molecules.

Dataset Information

Proton-Coupled Electron Transfer at the Pu^5+/4+ Couple.

Publications

Proton-Coupled Electron Transfer at the Pu<sup>5+/4+</sup> Couple.

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