Tetravalent Cerium Alkyl and Benzyl Complexes.

High-valent cerium complexes of alkyl and benzyl ligands are unprecedented due to the incompatibility of the typically highly oxidizing Ce4+ ion and the reducing alkyl or benzyl ligand. Herein we report the synthesis and isolation of the first tetravalent cerium alkyl and benzyl complexes supported by the tri-tert-butyl imidophosphorane ligand, [NP(tBu)3]1-. The Ce4+ monoiodide complex, [Ce4+I(NP(tert-butyl)3)3] (1-CeI), serves as a precursor to the alkyl and benzyl complexes, [Ce4+(Npt)(NP(tert-butyl)3)3] (2-CeNpt) (Npt = neopentyl, CH2C(CH3)3) and [Ce4+(Bn)(NP(tert-butyl)3)3] (2-CeBn) (Bn = benzyl, CH2Ph). The bonding and structure of these complexes are characterized by single-crystal XRD, NMR and UV-vis-NIR spectroscopy, cyclic voltammetry, and DFT studies.

Michael Addition-Elimination Ring-Opening Polymerization.

A cyclic thioenone system capable of controlled ring-opening polymerization (ROP) is presented that leverages a reversible Michael addition-elimination (MAE) mechanism. The cyclic thioenone monomers are easy to access and modify and for the first time incorporate the dynamic reversibility of MAE with chain-growth polymerization. This strategy features mild polymerization conditions, tunable functionalities, controlled molecular weights (Mn), and narrow dispersities. The obtained polythioenones exhibit excellent optical transparency and good mechanical properties and can be depolymerized to recover the original monomers. Density functional theory (DFT) calculations of model reactions offer insights into the role of monomer conformation in the polymerization process, as well as explaining divergent reactivity observed in seven-membered thiepane (TP) and eight-membered thiocane (TC) ring systems. Collectively, these findings demonstrate the feasibility of MAE mechanisms in ring-opening polymerization and provide important guidelines toward future monomer designs.

Divergent Stabilities of Tetravalent Cerium, Uranium, and Neptunium Imidophosphorane Complexes.

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=Pt 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 M4+/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 U5+/4+ , U6+/5+ , and to an unprecedented, well-behaved Np5+/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.

A tetrahedral neptunium(V) complex.

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 ([Np5+(NPC)4][B(ArF5)4], 1-Np) (NPC = [NPtBu(pyrr)2]-; tBu = C(CH3)3; pyrr = pyrrolidinyl (N(C2H4)2); B(ArF5)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.