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

Do Mono-oxo Sites Exist in Silica-Supported Cr(VI) Materials? Reassessment of the Resonance Raman Spectra

The monomeric, single-atom oxochromium species present on the surface of silica-supported Cr­(VI) catalysts was characterized in detail using resonance Raman (RR) spectroscopy over a range of excitation wavelengths corresponding to the primary electronic transitions of Cr­(VI)/SiO₂. The findings resolve a long-standing controversy regarding the possible contribution of mono-oxoCr­(VI) sites, (SiO)₄CrO, postulated to coexist with the well-established dioxoCr­(VI) sites, (SiO)₂Cr­(O)₂. Density functional theory (DFT) calculations and a normal coordinate analysis conducted using a chromasiloxane model cluster confirm prior assignments of bands in the nonresonant Raman spectrum at 986 and 1001 cm^–1 to the symmetric and antisymmetric stretching modes, respectively, of the dioxoCr­(VI) sites. For all excitation energies, the symmetric stretch shows apparent resonant enhancement. Since all of the electronic transitions are strongly allowed, this finding is consistent with A-term enhancement. UV excitation at 257 nm (into the high energy electronic transition centered at 271 nm) also results in modest resonant enhancement of the antisymmetric stretch, due to the low average symmetry of the surface sites. Excitation at 351 nm (into the electronic transition centered at 343 nm) results in a strong increase in the relative intensity of the antisymmetric stretch, which is likely caused by B-term enhancement. Previously reported evidence for a mono-oxoCr­(VI) site consists of a vibrational band observed at ca. 1011 cm^–1 and assigned to its CrO stretch. However, the band is observed only upon excitation into the lowest-energy electronic transition, at 439 nm. We show that excitation into this electronic transition causes photoinduced decomposition. The process depends on the laser power and duration of exposure, and it yields the band previously assigned to a mono-oxo species. The resonance Raman study reported here, in combination with our recent rigorous analysis of the corresponding electronic spectra, lead us to conclude that there is no credible spectroscopic evidence for the existence of mono-oxochromate species in highly dispersed Cr/silica materials

Reassessment of the Electronic Structure of Cr(VI) Sites Supported on Amorphous Silica and Implications for Cr Coordination Number

The electronic structure of isolated Cr­(VI) sites supported on silica was reinvestigated using multiple, complementary electronic spectroscopies applied to transparent xerogel monoliths. The absorption spectrum exhibits three previously reported peaks, at 22 800, 29 100, and 41 500 cm^–1, as well as a previously unresolved band at ca. 36 900 cm^–1. The emission is a long-lived red luminescence with λ_max = 13 600 cm^–1, emanating from the lowest excited state. Assignment of the excited states was facilitated using time-dependent density functional theory (TD-DFT) calculations performed on cluster models. All of the observed electronic transitions and their energies are accounted for by dioxoCr­(VI) sites. The lowest energy observed excitation at 22 800 cm^–1 populates a singlet excited state, while the emitting state is the corresponding triplet state, accessed by intersystem crossing from the singlet state. Spectroscopic bands observed at 29 100, 36 900, and 41 500 cm^–1 were assigned, based on the TD-DFT calculation, to spin-allowed transitions that are consistent with emission polarization anisotropy measurements. Small variations in site symmetry at Cr result principally in inhomogeneous broadening of the spectral bands, as well as a red-edge effect in the photoemission spectrum. There is no evidence for a significant contribution from five-coordinate mono-oxoCr­(VI) sites

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