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Reduction of Cp*( Tripp TerN)UI with KC 8 generates (KCp*( Tripp TerN)UI) 2 , the first example of a trivalent uranium imido, a previously elusive species, which are commonly unstable. Experimental and computational results indicate that the K + coordination is responsible for this isolable U(III) monoimido complex., (© 2024 The Author(s). Chemistry - A European Journal published by Wiley-VCH GmbH.)

Neptunium is an actinide element sourced from anthropogenic production, and, unlike naturally abundant uranium, its coordination chemistry is not well developed in all accessible oxidation states. High-valent neptunium generally requires stabilization from at least one metal-ligand multiple bond, and departing from this structural motif poses a considerable challenge. Here we report a tetrahedral molecular neptunium(V) complex ([Np 5+ (NPC) 4 ][B(ArF 5 ) 4 ], 1-Np) (NPC = [NP t Bu(pyrr) 2 ] - ; t Bu = C(CH 3 ) 3 ; pyrr = pyrrolidinyl (N(C 2 H 4 ) 2 ); B(ArF 5 ) 4  = tetrakis(2,3,4,5,6-pentafluourophenyl)borate). Single-crystal X-ray diffraction, solution-state spectroscopy and density functional theory studies of 1-Np and the product of its proton-coupled electron transfer (PCET) reaction, 2-Np, demonstrate the unique bonding that stabilizes this reactive ion and establishes the thermochemical and kinetic parameters of PCET in a condensed-phase transuranic complex. The isolation of this four-coordinate, neptunium(V) complex reveals a fundamental reaction pathway in transuranic chemistry., (© 2024. The Author(s), under exclusive licence to Springer Nature Limited.)

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.

A series of Ce 3+ complexes with counter cations ranging from Li to Cs are presented. Cyclic voltammetry data indicate a significant dependence of the oxidation potential on the alkali metal identity. Analysis of the single-crystal X-ray diffraction data indicates that the degree of structural distortion of the secondary coordination sphere is linearly correlated with the measured oxidation potential. Solution electronic absorption spectroscopy confirms that the structural distortion is reflected in the solution structure. Computational studies further validate this analysis, deciphering the impact of alkali metal cations on the Ce atomic orbital contributions, differences in energies of Ce-dominant molecular orbitals, energy shift of the 4f-5d electronic transitions, and degree of structural distortions. In sum, the structural impact of the alkali metal cation is demonstrated to modulate the redox and electronic properties of the Ce 3+ complexes, and provides insight into the rational tuning of the Ce 3+ imidophosphorane complex oxidation potential through alkali metal identity., Competing Interests: The authors declare no conflict of interest., (This journal is © The Royal Society of Chemistry.)

The study of the redox chemistry of mid-actinides (U-Pu) has historically relied on cerium as a model, due to the accessibility of trivalent and tetravalent oxidation states for these ions. Recently, dramatic shifts of lanthanide 4+/3+ non-aqueous redox couples have been established within a homoleptic imidophosphorane ligand framework. Herein we extend the chemistry of the imidophosphorane ligand (NPC=[N=P t Bu(pyrr) 2 ] - ; pyrr=pyrrolidinyl) to tetrahomoleptic NPC complexes of neptunium and cerium (1-M, 2-M, M=Np, Ce) and present comparative structural, electrochemical, and theoretical studies of these complexes. Large cathodic shifts in the M 4+/3+ (M=Ce, U, Np) couples underpin the stabilization of higher metal oxidation states owing to the strongly donating nature of the NPC ligands, providing access to the U 5+/4+ , U 6+/5+ , and to an unprecedented, well-behaved Np 5+/4+ redox couple. The differences in the chemical redox properties of the U vs. Ce and Np complexes are rationalized based on their redox potentials, degree of structural rearrangement upon reduction/oxidation, relative molecular orbital energies, and orbital composition analyses employing density functional theory., (© 2023 Wiley-VCH Verlag GmbH.)

The development of high-valent transuranic chemistry requires robust methodologies to access and fully characterize reactive species. We have recently demonstrated that the reducing nature of imidophosphorane ligands supports the two-electron oxidation of U 4+ to U 6+ and established the use of this ligand to evaluate the inverse-trans-influence (ITI) in actinide metal-ligand multiple bond (MLMB) complexes. To extend this methodology and analysis to transuranic complexes, new small-scale synthetic strategies and lower-symmetry ligand derivatives are necessary to improve crystallinity and reduce crystallographic disorder. To this end, the synthesis of two new imidophosphorane ligands, [N═P t Bu(pip) 2 ] - ( NPC 1 ) and [N═P t Bu(pyrr) 2 ] - ( NPC 2 ) (pip = piperidinyl; pyrr = pyrrolidinyl), is presented, which break pseudo- C 3 axes in the tetravalent complexes, U[NPC 1 ] 4 and U[NPC 2 ] 4 . The reaction of these complexes with two-electron oxygen-atom-transfer reagents (N 2 O, trimethylamine N -oxide (TMAO) and 2,3:5,6-dibenzo-7-azabicyclo[2.2.1]hepta-2,5-diene (dbabhNO)) yields the U 6+ mono-oxo complexes U(O)[NPC 1 ] 4 and U(O)[NPC 2 ] 4 . This methodology is optimized for direct translation to transuranic elements. Of the two ligands, the NPC 2 framework is most suitable for facilitating detailed bonding analysis and assessment of the ITI. Theoretical evaluation of the U-(NPC) bonding confirms a substantial difference between axially and equatorially bonded N atoms, revealing markedly more covalent U-N ax interactions. The U 6d + 5f combined contribution for U-N ax is nearly double that of U-N eq , accounting for ITI shortening and increased bond order of the axial bond. Two distinct N-atom hybridizations in the pyrrolidine/piperidine rings are noted across the complexes, with approximate sp 2 and sp 3 configurations describing the slightly shorter P-N "planar" and slightly longer P-N "pyramidal" bonds, respectively. In all complexes, the NPC 2 ligands feature more planar N atoms than NPC 1 , in accordance with a higher electron-donating capacity of the former.