Realization of Organocerium-Based Fullerene Molecular Materials Showing Mott Insulator-Type Behavior

Electron-rich organocerium complexes (C5Me4H)3Ce and [(C5Me5)2Ce(ortho-oxa)], with redox potentials E1/2 = -0.82 V and E1/2 = -0.86 V versus Fc/Fc+, respectively, were reacted with fullerene (C60) in different stoichiometries to obtain molecular materials. Structurally characterized cocrystals: [(C5Me4H)3Ce]2·C60 (1) and [(C5Me5)2Ce(ortho-oxa)]3·C60 (2) of C60 with cerium-based, molecular rare earth precursors are reported for the first time. The extent of charge transfer in 1 and 2 was evaluated using a series of physical measurements: FT-IR, Raman, solid-state UV-vis-NIR spectroscopy, X-ray absorption near-edge structure (XANES) spectroscopy, and magnetic susceptibility measurements. The physical measurements indicate that 1 and 2 comprise the cerium(III) oxidation state, with formally neutral C60 as a cocrystal in both cases. Pressure-dependent periodic density functional theory calculations were performed to study the electronic structure of 1. Inclusion of a Hubbard-U parameter removes Ce f states from the Fermi level, opens up a band gap, and stabilizes FM/AFM magnetic solutions that are isoenergetic because of the large distances between the Ce(III) cations. The electronic structure of this strongly correlated Mott insulator-type system is reminiscent of the well-studied Ce2O3.

Electronic Structure Studies and Photophysics of Luminescent Th(IV) Anilido and Imido Complexes

A series of thorium anilide compounds [ThNHAr^R(TriNOx)] (R = para-OCH₃ (1-Ar^OMe), para-H (1-Ar^H), para-Cl (1-Ar^Cl), para-CF₃ (1-Ar^4-CF3), TriNOx^3- = tris(2-tert-butylhydroxylaminato)benzylamine), and their corresponding imido compounds [Li(DME)][Th═NAr^R(TriNOx)] (2-Ar^R) as well as the alkyl congeners [ThNHAd(TriNOx)] (1-Ad) and [Li(DME)][Th═NAd(TriNOx)] (2-Ad), have been prepared. The para-substituents on the arylimido moiety were introduced for systematic variation of their electron-donating and withdrawing abilities, changes that were evident in measurements of the ¹³C{¹H} NMR chemical shifts of the ipso-C atom of the Ar^R moiety. Room temperature, solution-state luminescence of the four new thorium imido compounds, along with the previously reported [Li(THF)₂][Th═NAr^3,5-CF3(TriNOx)] (2-Ar^3,5-CF3) and [Li(THF)(Et₂O)][Ce═NAr^3,5-CF3(TriNOx)] (3-Ar^3,5-CF3) have been described. Among these complexes, 2-Ar^3,5-CF3 demonstrated the most intense luminescence feature with excitation at 398 nm and emission at 453 nm. The luminescence measurements, together with a time-dependent density functional theory (TD-DFT) study, helped uncover an intra-ligand n → π* transition that was assigned as the origin of the bright blue luminescence; 3-Ar^3,5-CF3 has an 1.2 eV redshift in excitation energy compared with its proligand. The weak luminescence of other derivatives (2-Ar^R and 3-Ar^3,5-CF3) was attributed to non-radiative decay from low-lying excited states originating from inter-ligand transitions (2-Ar^R) or ligand-to-metal charge transfer bands (3-Ar^3,5-CF3). Overall, the results expand the range of the thorium imido organometallic compounds and demonstrate that thorium(IV) complexes can support strong ligand luminescence. The results also demonstrate the utility of applying a Th(IV) center for tuning the n → π* luminescence energy and intensity of an associated imido moiety.

Electronic structure studies reveal 4f/5d mixing and its effect on bonding characteristics in Ce-imido and -oxo complexes

This study presents the role of 5d orbitals in the bonding, and electronic and magnetic structure of Ce imido and oxo complexes synthesized with a tris(hydroxylaminato) [((2-^tBuNO)C₆H₄CH₂)₃N]³⁻ (TriNO_x³⁻) ligand framework, including the reported synthesis and characterization of two new alkali metal-capped Ce oxo species. X-ray spectroscopy measurements reveal that the imido and oxo materials exhibit an intermediate valent ground state of the Ce, displaying hallmark features in the Ce L_III absorption of partial f-orbital occupancy that are relatively constant for all measured compounds. These spectra feature a double peak consistent with other formal Ce(iv) compounds. Magnetic susceptibility measurements reveal enhanced levels of temperature-independent paramagnetism (TIP). In contrast to systems with direct bonding to an aromatic ligand, no clear correlation between the level of TIP and f-orbital occupancy is observed. CASSCF calculations defy a conventional van Vleck explanation of the TIP, indicating a single-reference ground state with no low-lying triplet excited state, despite accurately predicting the measured values of f-orbital occupancy. The calculations do, however, predict strong 4f/5d hybridization. In fact, within these complexes, despite having similar f-orbital occupancies and therefore levels of 4f/5d hybridization, the d-state distributions vary depending on the bonding motif (Ce Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> O vs. CeN) of the complex, and can also be fine-tuned based on varying alkali metal cation capping species. This system therefore provides a platform for understanding the characteristic nature of Ce multiple bonds and potential impact that the associated d-state distribution may have on resulting reactivity.

Ce(iv) complexes with multiple bonds display similar f⁰ fractions, but different f/d hybridization, 5d-orbital energies, and TIP levels.

Tantalum, easy as Pi: understanding differences in metal–imido bonding towards improving Ta/Nb separations

The separation and purification of niobium and tantalum, which co-occur in natural sources, is difficult due to their similar physical and chemical properties. The current industrial method for separating Ta/Nb mixtures uses an energy-intensive process with caustic and toxic conditions. It is of interest to develop alternative, fundamental methodologies for the purification of these technologically important metals that improve upon their environmental impact. Herein, we introduce new Ta/Nb imido compounds: M(^tBuN)(TriNOx) (1-M) bound by the TriNOx³⁻ ligand and demonstrate a fundamental, proof-of-concept Ta/Nb separation based on differences in the imido reactivities. Despite the nearly identical structures of 1-M, density functional theory (DFT)-computed electronic structures of 1-M indicate enhanced basic character of the imido group in 1-Ta as compared to 1-Nb. Accordingly, the rate of CO₂ insertion into the M Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> N_imido bond of 1-Ta to form a carbamate complex (2-Ta) was selective compared to the analogous, unobserved reaction with 1-Nb. Differences in solubility between the imido and carbamate complexes allowed for separation of the carbamate complex, and led to an efficient Ta/Nb separation (S_Ta/Nb = 404 ± 150) dependent on the kinetic differences in nucleophilicities between the imido moieties in 1-Ta and 1-Nb.

A selective separation of the critical metals tantalum and niobium was accomplished from π-bonding-based reactivity differences of imido complexes. New insights into Ta/Nb separations were gained through detailed kinetic and computational studies.

A hydrolytically stable Ce(iv) complex of glutarimide-dioxime

Glutarimide-dioxime, a proligand known for its strongly electron-donating properties that has been studied for applications in uranium-sequestration from seawater, is considered here for stabilization of the cerium(iv) cation.

Uranyl Functionalization Mediated by Redox-Active Ligands: Generation of O-C Bonds via Acylation

A series of uranyl compounds with the redox-active iminoquinone ligand have been synthesized, and their electronic structures elucidated using multinuclear NMR, EPR, electronic absorption spectroscopies, SQUID magnetometry, and X-ray crystallography. Characterization and analysis of the iminoquinone (iq⁰) complex, (^dippiq)UO₂(OTf)₂THF (1-iq), the iminosemiquinone (isq^1-) complex, (^dippisq)₂UO₂THF (2-isq), and the amidophenolate (ap^2-) complex, [(^dippap)₂UO₂THF][K(18-crown-6)(THF)₂]₂(3-ap crown) show that reduction events are ligand-based, with the uranium center remaining in the hexavalent state. Reactivity of 2-isq with B-chlorocatecholborane or pivaloyl chloride leads to U-O_uranyl bond scission and reduction of U(VI) to U(IV) concomitant with ligand oxidation along with organic byproducts. ¹⁸O isotopic labeling experiments along with IR spectroscopy, mass spectrometry, and multinuclear NMR spectroscopy confirm that the organic byproducts contain oxygen atoms which originate from U-O_uranyl bond activation.