Proton-Coupled Electron Transfer at the Pu<sup>5+/4+</sup> Couple
K. Otte, Julie E. Niklas, Chad M. Studvick, Charlotte L. Montgomery, Alexandria R. C. Bredar, Ivan A. Popov, Henry S. La Pierre
Abstract
High Resolution Image Download MS PowerPoint Slide The synthesis and solution and solid-state characterization of [Pu 4+ (NPC) 4 ], 1-Pu, (NPC = [NP t Bu(pyrr) 2 ] −; t Bu = C(CH 3 ) 3; pyrr = pyrrolidinyl) and [Pu 3+ (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 Pu 5+ . The chemical identity of the product of the PCET reaction was confirmed by independent chemical synthesis to be [Pu 4+ (NPC) 3 (HNPC)][B(ArF 5 ) 4 ], 3-Pu, (B(ArF 5 ) 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 5 f and 6 d 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 An 5+ species.