Alexander Altman

© 2016 American Chemical Society.A tetravalent cerium macrocyclic complex was prepared with an octadentate terephthalamide ligand comprised of hard catecholate donors and characterized in the solution state by spectrophotometric titrations and electrochemistry and in the crystal by X-ray diffraction. The solution-state studies showed that L exhibits a remarkably high affinity toward Ce4+, with log β110 = 61 and ΔG = -348 kJ/mol, compared with log β110 = 32.02 for the analogous Pr3+ complex. In addition, L exhibits an unusual preference for forming CeL4- relative to formation of the analogous actinide complex, ThL4-, which has β110 = 53.7. The extreme stabilization of tetravalent cerium relative to its trivalent state is also evidenced by the shift of 1.91 V in the redox potential of the Ce3+/Ce4+ couple of the complex. The unprecedented behavior prompted an electronic structure analysis using L3- and M5,4-edge X-ray absorption near-edge structure spectroscopies and configuration interaction calculations, which showed that 4f-orbital bonding in CeLK4 has partial covalent character due to ligand-to-metal charge transfer in the ground state. The experimental results are presented in the context of earlier measurements on tetravalent cerium compounds, indicating that the amount of LMCT for CeLK4 is similar to that observed for [Et4N]2[CeCl6] and CeO2 and significantly less than that for the organometallic sandwich compound cerocene, 2Ce. A simple model to rationalize changes in 4f orbital bonding for tri- and tetravalent lanthanide and actinide compounds is also provided.

© 2016 The Royal Society of Chemistry.The electronic structure in the complete series of stable lanthanide sesquioxides, Ln2O3, has been evaluated using oxygen K-edge X-ray absorption spectroscopy with a scanning transmission X-ray microscope. The experimental results agree with recent synthetic, spectroscopic and theoretical investigations that provided evidence for 5d orbital involvement in lanthanide bonding, while confirming the traditional viewpoint that there is little Ln 4f and O 2p orbital mixing. However, the results also showed that changes in the energy and occupancy of the 4f orbitals can impact Ln 5d and O 2p mixing, leading to several different bonding modes for seemingly identical Ln2O3 structures. On moving from left to right in the periodic table, abrupt changes were observed for the energy and intensity of transitions associated with Ln 5d and O 2p antibonding states. These changes in peak intensity, which were directly related to the amounts of O 2p and Ln 5d mixing, were closely correlated to the well-established trends in the chemical accessibility of the 4f orbitals towards oxidation or reduction. The unique insight provided by the O K-edge XAS is discussed in the context of several recent theoretical and physical studies on trivalent lanthanide compounds.

© 2015 American Chemical Society.Polarized aluminum K-edge X-ray absorption near edge structure spectroscopy and first-principles calculations were used to probe electronic structure in a series of Al, AlX 2, and AlR 2 coordination compounds. Spectral interpretations were guided by examination of the calculated transition energies and polarization-dependent oscillator strengths, which agreed well with the XANES spectroscopy measurements. Pre-edge features were assigned to transitions associated with the Al 3p orbitals involved in metal-ligand bonding. Qualitative trends in Al 1s core energy and valence orbital occupation were established through a systematic comparison of excited states derived from Al 3p orbitals with similar symmetries in a molecular orbital framework. These trends suggested that the higher transition energies observed for AlX 2 systems with more electronegative X 1- ligands could be ascribed to a decrease in electron density around the aluminum atom, which causes an increase in the attractive potential of the Al nucleus and concomitant increase in the binding energy of the Al 1s core orbitals. For Al and AlH 2 the experimental Al K-edge XANES spectra and spectra calculated using the eXcited electron and Core-Hole approach had nearly identical energies for transitions to final state orbitals of similar composition and symmetry. These results implied that the charge distributions about the aluminum atoms in Al and AlH 2 are similar relative to the AlX 2 and AlMe 2 compounds, despite having different formal oxidation states of +1 and +3, respectively. However, Al was unique in that it exhibited a low-energy feature that was attributed to transitions into a low-lying p-orbital of b 1 symmetry that is localized on Al and orthogonal to the Al plane. The presence of this low-energy unoccupied molecular orbital on electron-rich Al distinguishes its valence electronic structure from that of the formally trivalent compounds AlX 2 and AlR 2. The work shows that Al K-edge XANES spectroscopy can be used to provide valuable insight into electronic structure and reactivity relationships for main-group coordination compounds.