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

The synthesis and solution and solid-state characterization of [Pu⁴⁺(NPC)₄], 1-Pu, (NPC = [NPBu(pyrr)₂]^-; Bu = 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.