One-Electron-Redox Activation of the Reduced Phillips Polymerization Catalyst, via Alkylchromium(IV) Homolysis: A Computational Assessment

In ethylene polymerization by the Phillips catalyst, inorganic Cr­(II) sites are believed to be activated by reaction with ethylene to form (alkyl)­Cr^III sites, in a process that takes about 1 h at ca. 373 K. The detailed mechanism of this spontaneous self-initiation has long remained unknown. It must account both for the formation of the first Cr–C bond and for the one-electron oxidation of Cr­(II) to Cr­(III). In this study, we used density functional theory to investigate a two-step initiation mechanism by which ethylene oxidative addition leads first to various (organo)­Cr^IV sites, and subsequent Cr–C bond homolysis gives (organo)­Cr^III sites capable of polymerizing ethylene. Pathways involving spin crossing, C–H oxidative addition, H atom transfer, and Cr–C bond homolytic cleavage were explored using a chromasiloxane cluster model. In particular, we used classical variational transition theory to compute free energy barriers and estimate rates for bond homolysis. A viable route to a four-coordinate bis­(alkyl)­Cr^IV site was found via spin crossing in a bis­(ethylene)­Cr^II complex followed by intramolecular H atom transfer. However, the barrier for subsequent Cr–C bond homolysis is a formidable 209 kJ/mol. Increasing the Cr coordination number to 6 with additional siloxane ligands lowers the homolysis barrier to just 47 kJ/mol, similar to reported homolysis paths in molecular [CrR­(H₂O)₅³⁺] complexes. However, siloxane coordination also raises the barrier for the prior oxidative addition step to form the bis­(alkyl)­Cr^IV site. Thus, we suggest that hemilability in the silica “ligand” may facilitate the homolysis step while still allowing the oxidative addition of ethylene

Computational Support for Phillips Catalyst Initiation via Cr–C Bond Homolysis in a Chromacyclopentane Site

Using density functional theory, we examine a possible homolysis initiation mechanism for the Phillips catalyst, starting from Cr^II sites exposed to ethylene. Spin-crossing in an abundant quintet <i>bis</i>(ethylene) Cr^II site leads to cycloaddition to form a chromacyclopentane site. One Cr–C bond then homolyzes to generate a tethered <i>n</i>-butyl radical: [Cr­(CH₂)₃CH₂^•]. If the radical attaches to a nearby inorganic Cr site, it yields two alkylCr^III sites capable of Cossee–Arlman polymerization. The overall computed barrier for this initiation process is 132 kJ/mol, which is comparable to the 120 kJ/mol value that we estimated from reported initiation times in industrial reactors. Poisson statistics suggest that this mechanism could activate ∼35% of Cr sites on a commercial catalyst with a loading of 0.4 Cr/nm². Pairwise Cr grafting, amplification by complementary initiation reactions, or the creation of dangling bonds that form as the silica support fractures, might explain the apparent increase in per-site activity at lower Cr loadings

Spectroscopic Evidence of Extra-Framework Heterometallic Oxo-Clusters in Fe/Ga-ZSM-5 Catalysts

The effect of introducing extra-framework Ga on the local structure of the metal sites in Fe/ZSM-5, resulting in enhanced reactivity toward N₂O, was investigated using a combination of Raman and X-ray absorption spectroscopies. The Raman spectra indicate an increased abundance of oxo- and/or hydroxo-bridged diiron sites, whereas the Fe <i>K</i>-edge XANES reveals more extensive reduction of Fe(III) to Fe(II). Curvefits of the EXAFS at both the Ga and Fe <i>K</i>-edges are consistent with heterometallic oxo-clusters containing both Ga−Fe and Fe−Fe paths. The spectroscopic evidence suggests a tetranuclear [Fe₂Ga₂O₄²⁺] core, possessing an open dicubane structure

Water-Catalyzed Activation of H₂O₂ by Methyltrioxorhenium: A Combined Computational–Experimental Study

The formation of peroxorhenium complexes by activation of H₂O₂ is key in selective oxidation reactions catalyzed by CH₃ReO₃ (methyltrioxorhenium, MTO). Previous reports on the thermodynamics and kinetics of these reactions are inconsistent with each other and sometimes internally inconsistent. New experiments and calculations using density functional theory with the ωB97X-D and augmented def2-TZVP basis sets were conducted to better understand these reactions and to provide a strong experimental foundation for benchmarking computational studies involving MTO and its derivatives. Including solvation contributions to the free energies as well as tunneling corrections, we compute negative reaction enthalpies for each reaction and correctly predict the hydration state of all complexes in aqueous CH₃CN. New rate constants for each of the forward and reverse reactions were both measured and computed as a function of temperature, providing a complete set of consistent activation parameters. New, independent measurements of equilibrium constants do not indicate strong cooperativity in peroxide ligand binding, as was previously reported. The free energy barriers for formation of both CH₃ReO₂(η²-O₂) (<b>A</b>) and CH₃ReO­(η²-O₂)₂(H₂O) (<b>B</b>) are predominantly entropic, and the former is much smaller than a previously reported value. Computed rate constants for a direct ligand-exchange mechanism, and for a mechanism in which a water molecule facilitates ligand-exchange via proton transfer in the transition state, differ by at least 7 orders of magnitude. The latter, water-assisted mechanism is predicted to be much faster and is consequently in much closer agreement with the experimentally measured kinetics. Experiments confirm the predicted catalytic role of water: the kinetics of both steps are strongly dependent on the water concentration, and water appears directly in the rate law

Computational Kinetic Discrimination of Ethylene Polymerization Mechanisms for the Phillips (Cr/SiO₂) Catalyst

The mechanism of ethylene polymerization by the widely used Phillips catalyst remains controversial. In this work, we compare initiation, propagation, and termination pathways computationally using small chromasiloxane cluster models for several previously proposed and new mechanisms. Where possible, we consider complete catalytic cycles and compare predicted kinetics, active site abundances, and polymer molecular weights to known properties of the Phillips catalyst. Prohibitively high activation barriers for propagation rule out previously proposed chromacycle ring expansion and Green–Rooney (alternating alkylidene/chromacycle) mechanisms. A new oxachromacycle ring expansion mechanism has a plausible propagation barrier, but initiation is prohibitively slow. On sites with adjacent bridging hydroxyls, either Si­(OH)­Cr^II-alkyl or Si­(OH)­Cr^III-alkyl, initiated by proton transfer from ethylene, chain growth by a Cossee–Arlman-type mechanism is fast. However, the initiation step is uphill and extremely slow, so essentially all sites remain trapped in a dormant state. In addition, these sites make only oligomers because when all pathways are considered, termination is faster than propagation. A monoalkylchromium­(III) site without an adjacent proton, (SiO)₂Cr-alkyl, is viable as an active site for polymerization, although its precise origin remains unknown

An Organometallic Cu₂₀ Nanocluster: Synthesis, Characterization, Immobilization on Silica, and “Click” Chemistry

The development of atomically precise nanoclusters (APNCs) protected by organometallic ligands, such as acetylides and hydrides, is an emerging area of nanoscience. In principle, these organometallic APNCs should not require harsh pretreatment for activation toward catalysis, such as calcination, which can lead to sintering. Herein, we report the synthesis of the mixed-valent organometallic copper APNC, [Cu₂₀(CCPh)₁₂(OAc)₆)] (<b>1</b>), via reduction of Cu­(OAc) with Ph₂SiH₂ in the presence of phenylacetylene. This cluster is a rare example of a two-electron copper superatom, and the first to feature a tetrahedral [Cu₄]²⁺ core, which is a unique “kernel” for a Cu-only superatom. Complex <b>1</b> can be readily immobilized on dry, partially dehydroxylated silica, a process that cleanly results in release of 1 equiv of phenylacetylene per Cu₂₀ cluster. Cu K-edge EXAFS confirms that the immobilized cluster <b>2</b> is structurally similar to <b>1</b>. In addition, both <b>1</b> and <b>2</b> are effective catalysts for [3+2] cycloaddition reactions between alkynes and azides (i.e., “Click” reactions) at room temperature. Significantly, neither cluster requires any pretreatment for activation toward catalysis. Moreover, EXAFS analysis of <b>2</b> after catalysis demonstrates that the cluster undergoes no major structural or nuclearity changes during the reaction, consistent with our observation that supported cluster <b>2</b> is more stable than unsupported cluster <b>1</b> under “Click” reaction conditions

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

Ligand-Exchange-Induced Growth of an Atomically Precise Cu₂₉ Nanocluster from a Smaller Cluster

The copper hydride nanocluster (NC) [Cu₂₉Cl₄H₂₂­(Ph₂phen)₁₂]Cl (<b>2</b>; Ph₂phen = 4,7-diphenyl-1,10-phenanthroline) was isolated cleanly, and in good yields, by controlled growth from the smaller NC, [Cu₂₅H₂₂(PPh₃)₁₂]­Cl (<b>1</b>), in the presence of Ph₂phen and a chloride source at room temperature. Complex <b>2</b> was fully characterized by single-crystal X-ray diffraction, XANES, and XPS, and represents a rare example of an <i>N*</i> = 2 superatom. Its formation from <b>1</b> demonstrates that atomically precise copper clusters can be used as templates to generate larger NCs that retain the fundamental electronic and bonding properties of the original cluster. A time-resolved kinetic evaluation of the formation of <b>2</b> reveals that the mechanism of cluster growth is initiated by rapid ligand exchange. The slower extrusion of CuCl monomer, its transport, and subsequent capture by intact clusters resemble elementary steps in the reactant-assisted Ostwald ripening of metal nanoparticles

Ligand-Exchange-Induced Growth of an Atomically Precise Cu₂₉ Nanocluster from a Smaller Cluster

The copper hydride nanocluster (NC) [Cu₂₉Cl₄H₂₂­(Ph₂phen)₁₂]Cl (<b>2</b>; Ph₂phen = 4,7-diphenyl-1,10-phenanthroline) was isolated cleanly, and in good yields, by controlled growth from the smaller NC, [Cu₂₅H₂₂(PPh₃)₁₂]­Cl (<b>1</b>), in the presence of Ph₂phen and a chloride source at room temperature. Complex <b>2</b> was fully characterized by single-crystal X-ray diffraction, XANES, and XPS, and represents a rare example of an <i>N*</i> = 2 superatom. Its formation from <b>1</b> demonstrates that atomically precise copper clusters can be used as templates to generate larger NCs that retain the fundamental electronic and bonding properties of the original cluster. A time-resolved kinetic evaluation of the formation of <b>2</b> reveals that the mechanism of cluster growth is initiated by rapid ligand exchange. The slower extrusion of CuCl monomer, its transport, and subsequent capture by intact clusters resemble elementary steps in the reactant-assisted Ostwald ripening of metal nanoparticles

Sustainable Solvent Systems for Use in Tandem Carbohydrate Dehydration Hydrogenation

Monophasic separation-friendly solvent systems were investigated for the sustainable acid-catalyzed dehydration of fructose to 5-hydroxymethylfurfural (HMF). The HMF selectivity depends on both fructose conversion, temperature, and the amount of cosolvent present in the aqueous solvent mixture. Use of HMF-derived 2,5-(dihydroxymethyl)­tetrahydrofuran (DHMTHF) or low-boiling tetrahydrofuran (THF) as co-solvents results in increased selectivity (>70%) to HMF at fructose conversions of ca. 80%. Analysis of the fructose tautomer distribution in each solvent system by ¹³C NMR revealed higher furanose fractions in the presence of these and other protic (tetrahydrofurfuryl alcohol) and polar aprotic co-solvents (DMSO) relative to water alone. Formation of fructosides and/or difructose anhydrides in the presence of the co-solvents causes lower selectivity at early reaction times, but reversion to fructose and dehydration to HMF at longer reaction times results in increasing HMF selectivity with fructose conversion. In 9:1 DHMTHF:water, a 7.5-fold increase in the initial rate of HMF production was observed relative to water alone. This mixed solvent system is proposed for use in a tandem catalytic approach to continuous DHMTHF production from fructose, namely, acid-catalyzed dehydration of fructose to HMF, followed by its catalytic hydrogenation to DHMTHF