Search NASA⌕ Search

SEARCH · Search NASA

Results for “Organometallic”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 91 records · Page 5
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Entangled Electrons Drive a Non-superexchange Mechanism in a Cobalt Quinoid Dimer Complex

A central theme in chemistry is the understanding of the mechanisms that drive chemical transformations. A well-known, highly cited mechanism in organometallic chemistry is the superexchange mechanism in which unpaired electrons on two or more metal centers interact through an electron pair of the bridging ligand. We use a combination of novel synthesis and computation to show that such interactions may in fact occur by a more direct mechanism than superexchange that is based on direct quantum entanglement of the two metal centers. Specifically, we synthesize and experimentally characterize a novel cobalt dimer complex with benzoquinoid bridging ligands and investigate its electronic structure with the variational two-electron reduced density matrix method using large active spaces. The result draws novel connections between inorganic mechanisms and quantum entanglement, thereby opening new possibilities for the design of strongly correlated organometallic compounds whose magnetic and spin properties have applications in superconductors, energy storage, thermoelectrics, and spintronics.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Supported Organoiridium-Pincer Catalysts for the Nonoxidative Dehydrogenation of High-Density Polyethylene

The iridium-pincer complex {p-OP( t Bu) 2 -C 6 H 2 -2,6-OP( t Bu) 2 ] 2 }Ir(C 2 H 4 ) ( P [Ir]) has been reported as a stable and active catalyst toward alkane dehydrogenation in homogeneous and supported heterogeneous systems. Dehydrogenation has been shown as a practical method toward functional polyolefins, with dehydrogenated high-density polyethylene (deHDPE) demonstrated as a valuable synthon for upcycling, as orthogonal C-H strategies are key to end-of-life upcycling. The heterogenization of P [Ir] on oxides (SiO 2 , Al 2 O 3 , and TiO 2 ; P [Ir]/E y O x ) yields a mixture of organometallic Ir-fragments whose catalytic nonoxidative dehydrogenation activity is modulated by the binding modes of the active metal on the surface. The binding mode was elucidated by a combination of solid-state NMR and XAFS analyses and supported by DFT calculations. Surface binding through the ligand enables active organoiridium that catalyzes internal olefination of deHDPE up to 1.23 mol % at 200 °C under dynamic vacuum. Alternatively, when the organoiridium is bonded though the metal center (Ir-O SiO ), catalyst activity is negligible. Furthermore, the catalytic activity of P [Ir]/SiO 2 showed comparable reactivity with the homogeneous analogue under the same catalytic conditions, and the heterogenized catalyst can be reused up to three cycles. In conclusion, this work highlights the importance of understanding how organometallic precursors react with hydroxylated metal oxide surfaces to establish structure-property relationships.

Hunt, Samuel B. [Argonne National Laboratory (ANL)↗
DOE OSTI2022

Surface Immobilization of a Re(I) Tricarbonyl Phenanthroline Complex to Si(111) through Sonochemical Hydrosilylation

A sonochemical-based hydrosilylation method was employed to covalently attach a rhenium tricarbonyl phenanthroline complex to silicon(111). fac-Re(5-(p-Styrene)-phen)(CO) 3 Cl (5-(p-styrene)-phen = 5-(4-vinylphenyl)-1,10-phenanthroline) was reacted with hydrogen-terminated silicon(111) in an ultrasonic bath to generate a hybrid photoelectrode. Subsequent reaction with 1-hexene enabled functionalization of remaining atop Si sites. Attenuated total reflectance–Fourier transform infrared spectroscopy confirms attachment of the organometallic complex to silicon without degradation of the organometallic core, supporting hydrosilylation as a strategy for installing coordination complexes that retain their molecular integrity. Detection of Re(I) and nitrogen by X-ray photoelectron spectroscopy (XPS) further support immobilization of fac-Re(5-(p-styrene)-phen)(CO) 3 Cl. Cyclic voltammetry and electrochemical impedance spectroscopy under white light illumination indicate that fac-Re(5-(p-styrene)-phen)(CO) 3 Cl undergoes two electron reductions. Mott–Schottky analysis indicates that the flat band potential is 239 mV more positive for p-Si(111) co-functionalized with both fac-Re(5-(p-styrene)-phen)(CO) 3 Cl and 1-hexene than when functionalized with 1-hexene alone. XPS, ultraviolet photoelectron spectroscopy, and Mott–Schottky analysis show that functionalization with fac-Re(5-(p-styrene)-phen)(CO) 3 Cl and 1-hexene introduces a negative interfacial dipole, facilitating reductive photoelectrochemistry.

X-ray photoelectron spectroscopy↗
DOE OSTI2025

Dimolybdenum Paddlewheel Complexes with Cation Binding Sites as Electrolyte Additives to Manipulate the Solid-Electrolyte Interphase at Lithium Metal Anodes

The use of electrolyte additives at millimolar loadings to control the surface chemistry of lithium metal anodes (LMAs) is a leading strategy to improve lithium metal batteries and promote electrosynthetic reactions. Whereas previous studies employed either inorganic or organic additives, in this study, we report the first organometallic additive, Mo 2 (mea) 4 [1, mea = 2-(2-methoxyethoxy)acetate], a dimolybdenum paddlewheel complex that is stable under Li plating conditions and features cation binding sites in the second coordination sphere that promote reversible Li + coordination. Binding of Li + ions to 1 induces immobilization of cationically charged aggregates (or products thereof) into the solid electrolyte interphase (SEI), imparting multiple beneficial functions. The modified SEI was found to protect the LMA against parasitic side reactions, produce modest but measurable improvements to Li plating properties (e.g., overpotential, surface structure, and Coulombic efficiency), and improve interfacial charge transport properties. Furthermore, the most notable benefit to battery cycling performance appears in calendar aging tests, which show that the presence of the additive protects the LMA from parasitic side reactions that would otherwise decrease overall cell cycling efficiency over time. Collectively, these data disclose a tactic for designing electrolyte additives using principles of organometallic synthesis.

Additives↗
DOE OSTI2025

Supported-Single Nickel Atom Catalysts for the Methanation of Carbon Dioxide

Synthesis of twenty-seven bimetallic catalysts consisting of nickel and one of nine different dopants (B, Co, Cu, Fe, Mg, Mn, Sn, V, and Zn) supported on three different metal oxides (Al 2 O 3 , CeO 2 , and SiO 2 ) is carried out via organometallic grafting. The catalysts are evaluated for their activity and selectivity for the CO 2 methanation reaction at a feed ratio of H 2 /CO 2 of 4 at 300 °C in a high-throughput flow reactor system. After in situ pre-activation (500 °C in H 2 ), Ni/Co/CeO 2 exhibited high conversion (84.3%) and selectivity for methane (99.6%). Ni/Co/CeO 2 was characterized by high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction, H 2 -temperature-programmed reduction (H 2 -TPR), and CO 2 -temperature-programmed desorption (CO 2 -TPD). HRTEM showed the presence of single Ni and Co atoms on ceria after pre-reduction at 500 °C and after the methanation reaction at 300 °C for 15 h. XPS determined that the strong interaction between Ni, Co, and ceria increased after the reduction, leading to a charge transfer between Ni and Ce that created oxygen vacancies in ceria. Nickel was found to be Ni 2+ in the as-prepared material and was partially reduced in the presence of cobalt and after the activation in H 2 at 500 °C. The DFT results show that both nickel and cerium exhibit lower Bader charges in the Ni/Co/CeO 2 system, confirming that the presence of cobalt enhances the reduction of both Ni and Ce through electronic interactions. This indicates that single cationic Ni atoms are highly effective for the methanation reaction. In conclusion, the organometallic grafting technique is found to be efficient for synthesizing catalysts with highly homogeneous dispersed species at low metal loadings (0.16 wt % Ni–0.15 wt % Co), which leads to high turnover frequency (up to 248.7 h –1 ) and durability for methanation.

CO2 conversion↗
DOE OSTI2022

Cobalt-Catalyzed Hydrogenation Reactions Enabled by Ligand-Based Storage of Dihydrogen

The use of supporting ligands that can store either protons or electrons has emerged as a powerful strategy in catalysis. While these strategies are potent individually, natural systems mediate remarkable transformations by combining the storage of both protons and electrons in the secondary coordination sphere. As such, there has been recent interest in using this strategy to enable fundamentally different transformations. Furthermore, outsourcing H-atom or hydrogen storage to ancillary ligands can also enable alternative mechanistic pathways and thereby selectivity. Here, we describe the application of this strategy to facilitate radical reactivity in Co-based hydrogenation catalysis. Metalation of previously reported dihydrazonopyrrole ligands with Co results in paramagnetic complexes, which are best described as having Co(II) oxidation states. These complexes catalytically hydrogenate olefins with low catalyst loadings under mild conditions (1 atm H 2 , 23 °C). Mechanistic, spectroscopic, and computational investigations indicate that this system goes through a radical hydrogen-atom transfer (HAT) type pathway that is distinct from classic organometallic mechanisms and is supported by the ability of the ligand to store H 2 . These results show how ancillary ligands can facilitate efficient catalysis, and furthermore how classic organometallic mechanisms for catalysis can be altered by the secondary coordination sphere.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

Formal Oxidation States and Coordination Environments in the Catalytic Reduction of CO to Methanol

Fundamental insight into multi-electron, multi-proton redox reactions with organometallic catalysts is greatly facilitated by knowledge of the formal oxidation state of the metal center in each of the elementary reaction steps that comprise the catalytic cycle. X-ray absorption near edge structure (XANES) is utilized herein to quantify the oxidation states and coordination environment of the organometallic resting state and intermediates in a newly proposed catalytic reduction of carbon monoxide to methanol.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Mechanistic Analysis of Metallaphotoredox C–N Coupling: Photocatalysis Initiates and Perpetuates Ni(I)/Ni(III) Coupling Activity

In this work, the combined use of reaction kinetic analysis, ultrafast spectroscopy, and stoichiometric organometallic studies has enabled the elucidation of the mechanistic underpinnings to a photocatalytic C–N cross-coupling reaction. Steady-state and ultrafast spectroscopic techniques were used to track the excited-state evolution of the employed iridium photocatalyst, determine the resting states of both iridium and nickel catalysts, and uncover the photochemical mechanism for reductive activation of the nickel cocatalyst. Stoichiometric organometallic studies along with a comprehensive kinetic study of the reaction, including rate–driving force analysis, unveiled the crucial role of photocatalysis in both initiating and sustaining a Ni(I)/Ni(III) cross-coupling mechanism. The insights gleaned from this study further enabled the discovery of a new photocatalyst providing a >30-fold rate increase.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Generating Potent C–H PCET Donors: Ligand-Induced Fe-to-Ring Proton Migration from a Cp*Fe III –H Complex Demonstrates a Promising Strategy

Highly reactive organometallic species that mediate reductive proton-coupled electron transfer (PCET) reactions are an exciting area for development in catalysis, where a key objective focuses on tuning the reactivity of such species. Here, this work pursues ligand-induced activation of a stable organometallic complex toward PCET reactivity. This is studied via the conversion of a prototypical Cp*Fe III –H species, [Fe III (η 5 -Cp*)(dppe)H] + (Cp* = C 5 Me 5 –, dppe = 1,2-bis(diphenylphosphino)ethane), to a highly reactive, S = 1/2 ring-protonated endo-Cp*H–Fe relative, triggered by the addition of CO. Our assignment of the latter ring-protonated species contrasts with its previous reported formulation, which instead assigned it as a hypervalent 19-electron hydride, [Fe III (η 5 -Cp*)(dppe)(CO)H] + . Herein, pulse EPR spectroscopy ( 1,2 H HYSCORE, ENDOR) and X-ray crystallography, with corresponding DFT studies, cement its assignment as the ring-protonated isomer, [Fe I (endo-η 4 -Cp*H)(dppe)(CO)]+. A less sterically shielded and hence more reactive exo-isomer can be generated through oxidation of a stable Fe 0 (exo-η 4 -Cp*H)(dppe)(CO) precursor. Both endo- and exo-ring-protonated isomers are calculated to have an exceptionally low bond dissociation free energy (BDFE C–H ≈ 29 kcal mol –1 and 25 kcal mol –1 , respectively) cf. BDFE Fe–H of 56 kcal mol –1 for [Fe III (η 5 -Cp*)(dppe)H] + . These weak C–H bonds are shown to undergo proton-coupled electron transfer (PCET) to azobenzene to generate diphenylhydrazine and the corresponding closed-shell [Fe II (η 5 -Cp*)(dppe)CO] + byproduct.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Architector for high-throughput cross-periodic table 3D complex building

Abstract Rare-earth and actinide complexes are critical for a wealth of clean-energy applications. Three-dimensional (3D) structural generation and prediction for these organometallic systems remains a challenge, limiting opportunities for computational chemical discovery. Here, we introduce Architector , a high-throughput in-silico synthesis code for s-, p-, d-, and f-block mononuclear organometallic complexes capable of capturing nearly the full diversity of the known experimental chemical space. Beyond known chemical space, Architector performs in-silico design of new complexes including any chemically accessible metal-ligand combinations. Architector leverages metal-center symmetry, interatomic force fields, and tight binding methods to build many possible 3D conformers from minimal 2D inputs including metal oxidation and spin state. Over a set of more than 6,000 x-ray diffraction (XRD)-determined complexes spanning the periodic table, we demonstrate quantitative agreement between Architector-predicted and experimentally observed structures. Further, we demonstrate out-of-the box conformer generation and energetic rankings of non-minimum energy conformers produced from Architector , which are critical for exploring potential energy surfaces and training force fields. Overall, Architector represents a transformative step towards cross-periodic table computational design of metal complex chemistry.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Mechanistic roles of metal- and ligand-protonated species in hydrogen evolution with [Cp*Rh] complexes

Protonation reactions involving organometallic complexes are ubiquitous in redox chemistry and often result in the generation of reactive metal hydrides. However, some organometallic species supported by η 5 -pentamethylcyclopentadienyl (Cp*) ligands have recently been shown to undergo ligand-centered protonation by direct proton transfer from acids or tautomerization of metal hydrides, resulting in the generation of complexes bearing the uncommon η 4 -pentamethylcyclopentadiene (Cp*H) ligand. Here, time-resolved pulse radiolysis (PR) and stopped-flow spectroscopic studies have been applied to examine the kinetics and atomistic details involved in the elementary electron- and proton-transfer steps leading to complexes ligated by Cp*H, using Cp*Rh(bpy) as a molecular model (where bpy is 2,2′-bipyridyl). Stopped-flow measurements coupled with infrared and UV-visible detection reveal that the sole product of initial protonation of Cp*Rh(bpy) is [Cp*Rh(H)(bpy)] + , an elusive hydride complex that has been spectroscopically and kinetically characterized here. Tautomerization of the hydride leads to the clean formation of [(Cp*H)Rh(bpy)] + . Variable-temperature and isotopic labeling experiments further confirm this assignment, providing experimental activation parameters and mechanistic insight into metal-mediated hydride-to-proton tautomerism. Spectroscopic monitoring of the second proton transfer event reveals that both the hydride and related Cp*H complex can be involved in further reactivity, showing that [(Cp*H)Rh] is not necessarily an off-cycle intermediate, but, instead, depending on the strength of the acid used to drive catalysis, an active participant in hydrogen evolution. Identification of the mechanistic roles of the protonated intermediates in the catalysis studied here could inform design of optimized catalytic systems supported by noninnocent cyclopentadienyl-type ligands.

08 HYDROGEN↗
DOE OSTI2022

Architector

Architector is a 3D chemical structure generation software package designed to take minimal 2D information about ligands and metal centers and generates chemically sensible 3D conformers and stereochemistry of the organometallic compounds. Architector is capable of high-throughput in-silico construction of s-, p-, d-, and f-block organometallic complexes. Architector represents a transformative step towards cross-periodic table computational design of metal complex chemistry.

Yang, Ping↗
DOE OSTI2024

Modulating Transition Metal Reactivity with Force

The reactivity and selectivity of a transition metal catalyst is intimately related to its ligand-sphere geometry, and, in many cases, the ideal ligand geometry for one step of a catalytic cycle is poorly matched to the ideal ligand geometry for another. For this reason, methods for reversibly modulating ligand geometry on the time scale of catalytic turnover or monomer enchainment are highly desirable. Mechanical force represents a heretofore untapped approach to modulate catalyst geometry and/or reactivity, with the potential to do so on the timescale of catalytic turnover or monomer enchainment. Macroscopic mechanical forces are large, directional and localized to an extent that differentiates them from other forms of energy input such as heat or light. Here, in this Concept, we describe our efforts to address the fundamental challenges associated with force-modulated transition metal catalysis by employing molecular force probe ligands comprising a stiff stilbene photoswitch tethered to rotationally flexible biaryl bisphosphine ligand. Our efforts to date include the modulation of catalytic activity through force-mediated ligand perturbations, quantification of the force-coupled ligand effects on the energetics of elementary organometallic transformations, and evaluation of the mechanisms of force transduction in these systems.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Surface vs Homogeneous Organo-Hafnium Catalyst Ion-Pairing and Ligand Effects on Ethylene Homo- and Copolymerizations

Heterogeneous catalysts have long dominated polyethylene and polypropylene production, but understanding their catalysis is challenged by uncertainties in active site structures and percentages. Surface-bound organometallic catalysts are an emerging strategy to combine successful homogeneous catalysts having well-understood structures, relatively high percentages of active sites, and exceptional control of selectivity, with the attractions of surface catalysts, to transition promising homogeneous systems to large-scale heterogeneous ones. Nevertheless, surface-bound olefin polymerization catalysts typically produce ultrahigh M w ’s but with low activity and comonomer selectivity. Here, we report the systematic synthesis and characterization of a series of pyridylamido–Hf complexes and their corresponding surface catalysts chemisorbed on sulfated alumina (AlS) and zirconia (ZrS). Comparative ethylene homo- and 1-octene copolymerizations reveal similar activity and 1-octene selectivity trends in the homogeneous and heterogeneous systems. For the surface pyridylamido–Hf catalyst series, large variations in activity (up to 10×) and 1-octene incorporation (up to 28×) are achieved by ligand and support manipulation. Interestingly, while the homogeneous catalysts exhibit positive comonomer effects in ethylene/1-octene copolymerization, the surface catalysts behave oppositely. Extended X-ray absorption fine structure (EXAFS) reveals significantly elongated Hf···O bond distances vs typical Hf–O covalent bonds (2.06 vs 1.97 Å). Furthermore, density functional theory (DFT) analysis of the heterolytic ion pair separation enthalpies, olefin insertion energetics, and NBO/Bader charges also suggest electrostatic Hf cation–anionic support binding and catalytic patterns, which are modulated by the ion-pairing energetics and ligand architecture.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Computational Investigation of the Role of Active Site Heterogeneity for aSupported Organovanadium(III) Hydrogenation Catalyst

A crucial consideration for supported heterogeneous catalysts is the nonuniformity of the active sites, particularly for supported organometallic catalysts. Standard spectroscopic techniques, such as X-ray absorption spectroscopy, reflect the nature of the most populated sites, which are often intrinsically structurally distinct from the most active catalytic sites. Additionally, with computational models, often, only a few representative structures are used to depict catalytic active sites on a surface, even though there are numerous observable factors of surface heterogeneity that contribute to the kinetically favorable active species. A previously reported study on the mechanism of a surface organovanadium-(III) catalyst [(SiO 2 )V III (Mes)(THF)] for styrene hydrogenation yielded two possible mechanisms: heterolytic cleavage and redox cycling. These two mechanistic scenarios are challenging to differentiate experimentally since the kinetic readouts of the catalyst are identical. To showcase the importance of modeling surface heterogeneity and its effect on catalytic activity, density functional theory (DFT) computational models of a series of potential active sites of [(SiO 2 )V III (Mes)(THF)] for the reaction pathways are applied in combination with kinetic Monte Carlo (kMC) simulations. Computed results were then compared to the previously reported experimental kinetic study: (1) DFT free-energy reaction pathways indicated the likely active site and pathway for styrene hydrogenation, a heterolytic cleavage pathway requiring a bare tripodal vanadium site. (2) From the kMC simulations, a mixture of different bond lengths from the support oxygen to the metal center was required to qualitatively describe the experimentally observed kinetic aspects of a supported organovanadium(III) catalyst for olefin hydrogenation. This work underscores the importance of modeling surface heterogeneity in computational catalysis.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Supported Electrophilic Organoruthenium Catalyst for the Hydrosilylation of Olefins

A series of supported electrophilic organoruthenium complexes has been synthesized via surface organometallic chemistry (SOMC) techniques and applied to the selective hydrosilylation of olefins. The air-sensitive 16e - complex Cp*RuMes(PCy 3 ) (1) (Cp* = pentamethylcyclopentadienyl, Mes = mesityl) was synthesized by the treatment of Cp*RuCl(PCy 3 ) with mesityl Grignard MesMgBr. This species was chemisorbed onto sulfated zirconia SO 4 /ZrO 2 , but the resulting material was inactive toward cyclohexene hydrosilylation with phenylsilane. Instead, Cp*RuMes(PCy 3 ) was treated with phenylsilane (PhSiH 3 ) to provide a ruthenium disilyl hydride complex Cp*RuH(SiH 2 Ph) 2 (PCy 3 ) (3), which was fully characterized by NMR spectroscopy and single-crystal X-ray diffraction. Grafting this species onto SO 4 /ZrO 2 resulted in the formation of phenylsilane along with the surface electrophilic species [Cp*RuH(R)(X-SiHPh)(PCy 3 )] (R = H, O 3 S-O or O 3 Zr-O; 4a, 4b, X = O 3 S-O, and O 3 Zr-O, respectively) as the major species. Material 4 was characterized via a combination of spectroscopic techniques including dynamic nuclear polarization (DNP)-enhanced solid-state NMR spectroscopy, diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), X-ray absorption spectroscopy (XAS), and density function theory (DFT) calculations. Further, capping the remaining acid sites on 4 with Me 3 Si-SiMe 3 provides 5, which significantly reduces side reactions, such as olefin isomerization and silane redistribution. Catalyst 5 is a highly robust and selective hydrosilylation catalyst and can be recycled up to 5 times without significant diminishment of activity. Exclusive anti-Markovnikov regiochemistry, cis-addition selectivity, and the inactivity of secondary and tertiary silanes provide support for the proposed Glaser-Tilley mechanism involving cationic ruthenium silylene species analogous to homogeneous systems.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

Supported Organochromium Ethylene Oligomerization Enabled by Surface Lithiation

Here, in this work, supported organochromium ethylene polymerization catalysts have been tuned to mediate ethylene oligomerization via surface lithiation, which provides a generalizable protocol to control stereoelectronics and redox states of surface organometallic active sites. The homoleptic chromium(IV) alkyl complex Cr(CH 2 SiMe 3 ) 4 was grafted on high-surface-area anatase titania (TiO 2 ) nanoparticles as well as on silica to produce Cr/TiO 2 and Cr/SiO 2 , respectively. Treatment of these materials with excess n-butyllithium led to the reduced chromium complexes Cr/Li x TiO 2 and Cr/Li/SiO 2 , each of which still retains one hydrocarbyl ligand on chromium. A set of heterogeneous complexes were studied by electron paramagnetic resonance and X-ray absorption spectroscopy, which indicate a reduction in the oxidation state of the major chromium species to Cr II upon lithiation. Cr/Li x TiO 2 converts ethylene to hexenes with a high selectivity (>80%), which was persistent over 10 days at 80 °C, achieving >950 turnovers. The exclusive formation of C 4 and C 6 olefins, preferring the trimerization product, without a statistical (Flory–Schulz) distribution is characteristic of the oxidative cyclization oligomerization mechanism rather than the traditional Cossee–Arlman mechanism, whereas Cr/Li/SiO 2 produced a mixture of trimerization and polymerization products, suggesting site heterogeneity in the silica-based material. On the other hand, the unreduced chromium(IV) materials as well as low lithium-containing Cr/Li x TiO 2 (x < 0.16) exclusively produced ultrahigh molecular weight polyethylene, determined by differential scanning calorimetry and gel permeation chromatography analysis, likely formed via a linear-insertion mechanism, with a crossover from the polymerization to oligomerization regime observed at ~16% Li intercalation.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Effect of hydride vapor phase epitaxy growth conditions on the degree of atomic ordering in GaInP

We report a systematic study of CuPt-type ordering in hydride vapor phase epitaxy (HVPE)-grown Ga 0.5 In 0.5 P. Selected-area electron diffraction reveals ordering in samples grown on (001) GaAs substrates offcut toward (1$\overline{1}$1)B. The ordering is single-variant, occurring only on (1$\overline{1}$1)B planes and not on ($\overline{1}$11)B. Quantitative analysis of the order parameter by high-resolution x-ray diffraction (HRXRD) indicates that ordering increases with deposition temperature in samples grown at 600–700 °C with a constant gas-phase V/III ratio ~3. Ordering increases with V/III ratio in the range of 1.3–6.7 at a constant deposition temperature of 650°C. Photoluminescence measurements correlate the order parameter with Ga0.5In0.5P bandgap contraction, though the contraction is larger than expected based on the magnitude of order parameters measured by HRXRD. A possible reason for this discrepancy is that the photoluminescence emission occurs in the lower bandgap ordered domains, which are small and evenly dispersed throughout the material. We also show that the degree of ordering decreases with growth rate, disappearing at ~60 µm/h. The HVPE-grown material exhibits a generally weaker ordering than organometallic vapor phase epitaxy (OMVPE)-grown material, likely due to the moderate V/III ratios employed, in contrast to the V/III ratios in the 100s typical of OMVPE. However, the tendency for ordering to increase with V/III ratio suggests that the same dimer-induced stress mechanism used to explain the occurrence of ordering in OMVPE-grown material also applies to HVPE. The tendencies for ordering to increase with deposition temperature and decrease with growth rate show that kinetics limit the degree of ordering, also in agreement with OMVPE trends.

74 ATOMIC AND MOLECULAR PHYSICS↗