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

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.