Ca₁₂InC_{13–<i>x</i>} and Ba₁₂InC₁₈H₄: Alkaline-Earth Indium Allenylides Synthesized in AE/Li Flux (AE = Ca, Ba)

Two new complex main-group metal carbides were synthesized from reactions of indium, carbon, and a metal hydride in metal flux mixtures of an alkaline earth (AE = Ca, Ba) and lithium. Ca₁₂InC_{13–<i>x</i>} and Ba₁₂InC₁₈H₄ both crystallize in cubic space group <i>Im</i>3̅ [<i>a</i> = 9.6055(8) and 11.1447(7) Å, respectively]. Their related structures are both built on a body-centered-cubic array of icosahedral clusters comprised of an indium atom and 12 surrounding alkaline-earth cations; these clusters are connected by bridging monatomic anions (either H^– or C^4–) and allenylide anions, C₃^4–. The allenylide anions were characterized by Raman spectroscopy and hydrolysis studies. Density of states and crystal orbital Hamilton population calculations confirm that both compounds are metallic

Luminescent zero-dimensional organic metal halide hybrids with near-unity quantum efficiency

Single crystalline zero-dimensional (0D) organic-inorganic hybrid materials with perfect host-guest structures have been developed as a new generation of highly efficient light emitters. Here we report a series of lead-free organic metal halide hybrids with a 0D structure, (C4N2H14X)4SnX6 (X = Br, I) and (C9NH20)2SbX5 (X = Cl), in which the individual metal halide octahedra (SnX64−) and quadrangular pyramids (SbX52−) are completely isolated from each other and surrounded by the organic ligands C4N2H14X+ and C9NH20+, respectively. The isolation of the photoactive metal halide species by the wide band gap organic ligands leads to no interaction or electronic band formation between the metal halide species, allowing the bulk materials to exhibit the intrinsic properties of the individual metal halide species. These 0D organic metal halide hybrids can also be considered as perfect host-guest systems, with the metal halide species periodically doped in the wide band gap matrix. Highly luminescent, strongly Stokes shifted broadband emissions with photoluminescence quantum efficiencies (PLQEs) of close to unity were realized, as a result of excited state structural reorganization of the individual metal halide species. Our discovery of highly luminescent single crystalline 0D organic-inorganic hybrid materials as perfect host-guest systems opens up a new paradigm in functional materials design

Mechanism of Initiation in the Phillips Ethylene Polymerization Catalyst: Redox Processes Leading to the Active Site

The detailed mechanism by which ethylene polymerization is initiated by the inorganic Phillips catalyst (Cr/SiO₂) without recourse to an alkylating cocatalyst remains one of the great unsolved mysteries of heterogeneous catalysis. Generation of the active catalyst starts with reduction of Cr^VI ions dispersed on silica. A lower oxidation state, generally accepted to be Cr^II, is required to activate ethylene to form an organoCr active site. In this work, a mesoporous, optically transparent monolith of Cr^VI/SiO₂ was prepared using sol–gel chemistry in order to monitor the reduction process spectroscopically. Using in situ UV–vis spectroscopy, we observed a very clean, stepwise reduction by CO of Cr^VI first to Cr^IV, then to Cr^II. Both the intermediate and final states show XANES consistent with these oxidation state assignments, and aspects of their coordination environments were deduced from Raman and UV–vis spectroscopies. The intermediate Cr^IV sites are inactive toward ethylene at 80 °C. The Cr^II sites, which have long been postulated as the end point of CO reduction, were observed directly by high-frequency/high-field EPR spectroscopy. They react quantitatively with ethylene to generate the organoCr^III active sites, characterized by X-ray absorption and UV–vis spectroscopy, which initiate polymerization

Mechanism of Initiation in the Phillips Ethylene Polymerization Catalyst: Redox Processes Leading to the Active Site

The detailed mechanism by which ethylene polymerization is initiated by the inorganic Phillips catalyst (Cr/SiO₂) without recourse to an alkylating cocatalyst remains one of the great unsolved mysteries of heterogeneous catalysis. Generation of the active catalyst starts with reduction of Cr^VI ions dispersed on silica. A lower oxidation state, generally accepted to be Cr^II, is required to activate ethylene to form an organoCr active site. In this work, a mesoporous, optically transparent monolith of Cr^VI/SiO₂ was prepared using sol–gel chemistry in order to monitor the reduction process spectroscopically. Using in situ UV–vis spectroscopy, we observed a very clean, stepwise reduction by CO of Cr^VI first to Cr^IV, then to Cr^II. Both the intermediate and final states show XANES consistent with these oxidation state assignments, and aspects of their coordination environments were deduced from Raman and UV–vis spectroscopies. The intermediate Cr^IV sites are inactive toward ethylene at 80 °C. The Cr^II sites, which have long been postulated as the end point of CO reduction, were observed directly by high-frequency/high-field EPR spectroscopy. They react quantitatively with ethylene to generate the organoCr^III active sites, characterized by X-ray absorption and UV–vis spectroscopy, which initiate polymerization

Electronic Structure and Properties of Berkelium Iodates

The reaction of ²⁴⁹Bk­(OH)₄ with iodate under hydrothermal conditions results in the formation of Bk­(IO₃)₃ as the major product with trace amounts of Bk­(IO₃)₄ also crystallizing from the reaction mixture. The structure of Bk­(IO₃)₃ consists of nine-coordinate Bk^III cations that are bridged by iodate anions to yield layers that are isomorphous with those found for Am^III, Cf^III, and with lanthanides that possess similar ionic radii. Bk­(IO₃)₄ was expected to adopt the same structure as M­(IO₃)₄ (M = Ce, Np, Pu), but instead parallels the structural chemistry of the smaller Zr^IV cation. Bk^III–O and Bk^IV–O bond lengths are shorter than anticipated and provide further support for a postcurium break in the actinide series. Photoluminescence and absorption spectra collected from single crystals of Bk­(IO₃)₄ show evidence for doping with Bk^III in these crystals. In addition to luminescence from Bk^III in the Bk­(IO₃)₄ crystals, a broad-band absorption feature is initially present that is similar to features observed in systems with intervalence charge transfer. However, the high-specific activity of ²⁴⁹Bk (<i>t</i>_1/2 = 320 d) causes oxidation of Bk^III and only Bk^IV is present after a few days with concomitant loss of both the Bk^III luminescence and the broadband feature. The electronic structure of Bk­(IO₃)₃ and Bk­(IO₃)₄ were examined using a range of computational methods that include density functional theory both on clusters and on periodic structures, relativistic <i>ab initio</i> wave function calculations that incorporate spin–orbit coupling (CASSCF), and by a full-model Hamiltonian with spin–orbit coupling and Slater–Condon parameters (CONDON). Some of these methods provide evidence for an asymmetric ground state present in Bk^IV that does not strictly adhere to Russel–Saunders coupling and Hund’s Rule even though it possesses a half-filled 5<i>f</i> ⁷ shell. Multiple factors contribute to the asymmetry that include 5<i>f</i> electrons being present in microstates that are not solely spin up, spin–orbit coupling induced mixing of low-lying excited states with the ground state, and covalency in the Bk^IV–O bonds that distributes the 5<i>f</i> electrons onto the ligands. These factors are absent or diminished in other <i>f</i>⁷ ions such as Gd^III or Cm^III