Chemistry of highly reactive group 5 and 6 transition metal compounds

Trimethylphosphine complexes of tungsten and molybdenum have been used to model the coordination chemistry and reactivity that may be observed on the surface of an industrial hydrotreating catalyst. Most notably, it was observed that W(PMe₃)₄(η²-CH₂PMe₂))H is capable of (i) the unprecedented cleavage of an aromatic carbon–carbon bond, and (ii) desulfurizing thiophene, benzothiophene, and dibenzothiophene. In addition to the group 6 chemistry, the first [CCC] X₃-donor pincer ligand for a transition metal was synthesized by two consecutive cyclometalations of a terphenyl complex of the group 5 metal tantalum. Chapter 1 describes two new transformations that occur between W(PMe₃)₄(η²-CH₂PMe₂))H and haloarenes, namely the formation of (i) the alkylidene complex, [W(PMe₃)₄(η²-CH₂PMe₂)H]X (X = Br or I) and (ii) the phosphoniocarbyne complex, W(PMe₃)₃Cl₂(CPMe₂Ph). Additionally, treatment of [W(PMe₃)₄(η²-CH₂PMe₂)H]X with LiAlD₄, allows for the isolation of the isotopomer W(PMe₃)₄(η²–CHDPMe₂)H, thereby providing a means to measure the rate constant for the formation of the 16-electron species [W(PMe₃)₅] from W(PMe₃)₄(η²-CH₂PMe₂)H. Chapter 2 describes the reactivity of trimethylphosphine complexes of molybdenum with phenazine and related N-heterocycles, in order to model aspects of hydrodenitrogenation. Several new coordination modes of phenazine to molybdenum were observed. Studies also indicate that oxidative addition of H2 is promoted by (i) incorporation of nitrogen substituents into the central ring and (ii) ring fusion. Furthermore, ring fusion promotes hydrogenation of the heterocyclic ligand. Chapter 3 describes the novel aromatic carbon–carbon bond cleavage and dehydrogenation of quinoxaline by W(PMe₃)₄(η²-CH₂PMe₂)H, giving the chelating bisisocyanide complex, [κ²-C₂-C6H₄(NC)₂]W(PMe₃)₄. Chapter 4 describes the reactivity of trimethylphosphine complexes of tungsten and molybdenum with thiophenes, in order to model aspects of hydrodesulfurization. Mo(PMe₃)₄H₄ desulfurizes thiophene and benzothiophene. Moreover, W(PMe₃)₄(η²-CH₂PMe₂)H is the first tungsten complex that is capable of desulfurization of thiophene, benzothiophene and dibenzothiophene. Chapter 5 describes the reactivity of trimethylphosphine complexes of tungsten and molybdenum with furans, in order to model aspects of hydrodeoxygenation. Most notably, Mo(PMe₃)₄H₄ is capable of cleaving the C–C, C–O, and C–H bonds of furan, thereby producing propene and carbon monoxide. Chapter 6 describes the synthesis of the first [CCC] X3-donor pincer ligand for a transition metal. Specifically, addition of PMe₃ to [Arᵀᵒˡ²]TaMe₃Cl ([Arᵀᵒˡ²] = 2,6-di-ptolylphenyl) induces elimination of methane and formation of the pincer complex, [κ³- Arᵀᵒˡ’²]Ta(PMe₃)₂MeCl (Tol’ = C6H₃Me). Reduction of [κ³-Arᵀᵒˡ’²]Ta(PMe₃)₂Cl2 with KC8 in benzene gives the arene complex [κ³- Arᵀᵒˡ’²]Ta(PMe₃)2(η⁶-C₆H₆), which is the first structurally characterized benzene complex of tantalum. Deuterium labeling employing Ta(PMe₃)₂(CD₃)₃Cl2 demonstrates that the pincer ligand is generated by a pair of Ar–H/Ta–Me sigma-bond metathesis transformations, rather than by a mechanism that involves #–H abstraction by a tantalum methyl ligand. The [CCC] pincer ligand ([κ³- Arᵀᵒˡ’²]) was also synthesized on niobium. Chapter 7 describes the synthesis of a variety of other terphenyl complexes of tantalum, namely [Arᵀᵒˡ²]Ta(NMe₂)₃X (X = Me, Et, Prn, Bun, Np, BH₄, and [Arᵀᵒˡ²]), all of which cyclometalate under varying conditions to give [κ²-Arᵀᵒˡ,ᵀᵒˡ’]Ta(NMe₂)₃. The dialkyl complexes, [Arᵀᵒˡ²]Ta(NMe₂)₂R₂ (R = Me, Et, Prn, Bun, and Np) have also been synthesized from the dichloride complex, [Arᵀᵒˡ²]Ta(NMe₂)₂Cl2. The bis-neopentyl complex, [Arᵀᵒˡ²]Ta(NMe₂)₂Np₂, is not stable in solution at room temperature and converts to [κ²-Arᵀᵒˡ,ᵀᵒˡ’]Ta(NMe₂)₂Np and neopentane, of which the former isomerizes to produce [κ²-Ar*ᵀᵒˡ,ᵀᵒˡ’]Ta(NMe₂)₂Np (* indicates the new connectivity of the aryl ligand)

Enhanced Productivity of a Supported Olefin Trimerization Catalyst

Treatment of dry silica with methylaluminoxane (MAO) followed by (FI)TiCl_3 (FI = (N-(5-methyl-3-(1-adamantyl)salicylidene)-2′-(2″-methoxyphenyl)anilinato) gives a heterogeneous supported ethylene trimerization catalyst, s(FI)Ti, which exhibits productivity more than an order of magnitude higher than its homogeneous analogues. This increase in productivity is attributed to a decreased rate of catalyst decomposition, a process that is proposed to occur via comproportionation to an inactive TiIII species; immobilization retards this process. In addition, s(FI)Ti catalyzes trimerization of α-olefins with high selectivity. Based on regioisomer distributions, catalysis by s(FI)Ti involves the same active species as the previously reported homogeneous systems (FI)TiR_2Me/B(C_6F_5)_3 (R = Me, CH_2SiMe_3, CH_2CMe_3)

Cosupported Tandem Catalysts for Production of Linear Low-Density Polyethylene from an Ethylene-Only Feed

Linear low-density polyethylene (LLDPE) is produced from an ethylene-only feed over a tandem catalyst system consisting of a phenoxy–imine titanium trimerization catalyst and a silylene-linked cyclopentadienyl/amido titanium polymerization catalyst cosupported on the same methylaluminoxane/silica particles. The level of 1-hexene incorporation can be controlled by varying the ethylene pressure

Highly Selective Olefin Trimerization Catalysis by a Borane-Activated Titanium Trimethyl Complex

Reaction of a trimethyl titanium complex, (FI)TiMe_3 (FI = phenoxy-imine), with 1 equiv of B(C_6F_5)_3 gives [(FI)TiMe_2][MeB(C_6F_5)_3], an effective precatalyst for the selective trimerization of ethylene. Mechanistic studies indicate that catalyst initiation involves generation of an active TiII species by olefin insertion into a Ti–Me bond, followed by β-H elimination and reductive elimination of methane, and that initiation is slow relative to trimerization. (FI)TiMe_3/B(C_6F_5)_3 also leads to a competent catalyst for the oligomerization of α-olefins, displaying high selectivity for trimers (>95%), approximately 85% of which are one regioisomer. This catalyst system thus shows promise for selectively converting light α-olefins into transportation fuels and lubricants

Genetic Resources for the Improvement of Switchgrass (\u3cem\u3ePanicum virgatum\u3c/em\u3e L.) for Biomass and Forage

Switchgrass (Panicum virgatum L.) is an important forage and biomass species for many parts of the USA. Switchgrass can be of several ploidies. Octoploid cultivars are most often used in forage and conservation settings, while the tetraploid cultivars are mostly targeted for bioenergy end-uses, due to their higher biomass yields. Switchgrass populations also occur as upland and lowland ecotypes, and constitute different heterotic groups. Switchgrass is mostly an obligate outcrosser resulting in substantial genotypic and phenotypic variation within populations. In the last ~15 years, significant resources have been dedicated to both breeding and understanding the genomic makeup of this plant, with a focus on bioenergy. This investment has resulted in the development of elite lines as well as a considerable increase in available genetic, physiological, and biomass-related information. The United States Department of Agriculture-Agricultural Research Service has been a major player in these developments (Mitchell and Schmer, 2012; Vogel et al., 2011). With significant improvements in DNA-sequencing technologies (High Throughput Sequencing, HTS), it has become possible to undertake large-scale analysis of both the genomic and functional genomic components of switchgrass. One such undertaking by the United States Department of Energy-Joint Genomics Institute has provided a draft assembly and annotation of the switchgrass genome (www.phytozome.org). This remarkable resource has permitted a complete utilization of HTS to analyze gene expression using RNA-Seq and related bioinformatic pipelines. Large-scale studies that are performed using field-grown plants and populations with well-characterized phenotypic traits, it increases the likelihood of discovering molecular events that underpin phenomena of interest. Even though lowland tetraploid cultivars have higher biomass yields than upland tetraploid cultivars, they can suffer significant winter-kill in more northern locations (Central Great Plains of the USA). Winter-kill is associated with the loss of rhizomes and other perenniating structures resulting in a complete or partial loss of tillering ability in the following seasons. Partial attrition of tiller production serves to limit new rhizome growth in successive years. One or more cycles of winter kill will ultimately kill the plant. We are trying to understand the cellular metabolism associated with the onset of rhizome dormancy and to connect the links between tiller/leaf senescence and rhizome metabolism using field grown plants from diverse populations, HTS and RNA-Seq

Lewis Acid Promoted Titanium Alkylidene Formation: Off-Cycle Intermediates Relevant to Olefin Trimerization Catalysis

Two new precatalysts for ethylene and α-olefin trimerization, (FI)Ti(CH_2SiMe_3)_2Me and (FI)Ti(CH_2CMe_3)_2Me (FI = phenoxy-imine), have been synthesized and structurally characterized by X-ray diffraction. (FI)Ti(CH_2SiMe_3)_2Me can be activated with 1 equiv of B(C_6F_5)_3 at room temperature to give the solvent-separated ion pair [(FI)Ti(CH_2SiMe_3)_2][MeB(C_6F_5)_3], which catalytically trimerizes ethylene or 1-pentene to produce 1-hexene or C_(15) olefins, respectively. The neopentyl analogue (FI)Ti(CH_2CMe_3)_2Me is unstable toward activation with B(C_6F_5)_3 at room temperature, giving no discernible diamagnetic titanium complexes, but at −30 °C the following can be observed by NMR spectroscopy: (i) formation of the bis-neopentyl cation [(FI)Ti(CH_2CMe_3)_2]^+, (ii) α-elimination of neopentane to give the neopentylidene complex [(FI)Ti(═CHCMe_3)]^+, and (iii) subsequent conversion to the imido-olefin complex [(MeOAr_2N═)Ti(OArHC═CHCMe_3)]^+ via an intramolecular metathesis reaction with the imine fragment of the (FI) ligand. If the reaction is carried out at low temperature in the presence of ethylene, catalytic production of 1-hexene is observed, in addition to the titanacyclobutane complex [(FI)Ti(CH(CMe_3)CH_2CH_2)]^+, resulting from addition of ethylene to the neopentylidene [(FI)Ti(═CHCMe_3)]^+. None of the complexes observed spectroscopically subsequent to [(FI)Ti(CH_2CMe_3)_2]^+ is an intermediate or precursor for ethylene trimerization, but notwithstanding these off-cycle pathways, [(FI)Ti(CH_2CMe_3)_2]^+ is a precatalyst that undergoes rapid initiation to generate a catalyst for trimerizing ethylene or 1-pentene

Electronic Excited States of Tungsten(0) Arylisocyanides

W(CNAryl)_6 complexes containing 2,6-diisopropylphenyl isocyanide (CNdipp) are powerful photoreductants with strongly emissive long-lived excited states. These properties are enhanced upon appending another aryl ring, e.g., W(CNdippPh^(OMe)_2)_6; CNdippPh^(OMe)_2 = 4-(3,5-dimethoxyphenyl)-2,6-diisopropylphenylisocyanide (Sattler et al. J. Am. Chem. Soc. 2015, 137, 1198−1205). Electronic transitions and low-lying excited states of these complexes were investigated by time-dependent density functional theory (TDDFT); the lowest triplet state was characterized by time-resolved infrared spectroscopy (TRIR) supported by density functional theory (DFT). The intense absorption band of W(CNdipp)_6 at 460 nm and that of W(CNdippPh^(OMe)_2)_6 at 500 nm originate from transitions of mixed ππ*(C≡N–C)/MLCT(W → Aryl) character, whereby W is depopulated by ca. 0.4 e– and the electron-density changes are predominantly localized along two equatorial molecular axes. The red shift and intensity rise on going from W(CNdipp)_6 to W(CNdippPh^(OMe)_2)_6 are attributable to more extensive delocalization of the MLCT component. The complexes also exhibit absorptions in the 300–320 nm region, owing to W → C≡N MLCT transitions. Electronic absorptions in the spectrum of W(CNXy)_6 (Xy = 2,6-dimethylphenyl), a complex with orthogonal aryl orientation, have similar characteristics, although shifted to higher energies. The relaxed lowest W(CNAryl)_6 triplet state combines ππ* excitation of a trans pair of C≡N–C moieties with MLCT (0.21 e–) and ligand-to-ligand charge transfer (LLCT, 0.24–0.27 e–) from the other four CNAryl ligands to the axial aryl and, less, to C≡N groups; the spin density is localized along a single Aryl–N≡C–W–C≡N–Aryl axis. Delocalization of excited electron density on outer aryl rings in W(CNdippPh^(OMe)_2)_6 likely promotes photoinduced electron-transfer reactions to acceptor molecules. TRIR spectra show an intense broad bleach due to ν(C≡N), a prominent transient upshifted by 60–65 cm^(–1), and a weak down-shifted feature due to antisymmetric C≡N stretch along the axis of high spin density. The TRIR spectral pattern remains unchanged on the femtosecond-nanosecond time scale, indicating that intersystem crossing and electron-density localization are ultrafast (<100 fs)

Structural modulation and direct measurement of subnanometric bimetallic PtSn clusters confined in zeolites

[EN] Modulating the structures of subnanometric metal clusters at the atomic level is a great synthetic and characterization challenge in catalysis. Here, we show how the catalytic properties of subnanometric platinum clusters (0.5-0.6 nm) confined in the sinusoidal 10R channels of purely siliceous MFI zeolite are modulated upon introduction of partially reduced tin species that interact with the noble metal at the metal/support interface. The platinum-tin clusters are stable in H(2)over an extended period of time (>6 h), even at high temperatures (for example, 600 degrees C), which is determined by only a few additional tin atoms added to the platinum clusters. The structural features of platinum-tin clusters, which are not immediately visible by conventional characterization techniques but can be established after combination of in situ extended X-ray absorption fine structure, high-angle annular dark-field scanning transmission electron microscopy and CO infrared data, are key to providing a one-order of magnitude lower deactivation rate in the propane dehydrogenation reaction while maintaining high intrinsic (initial) catalytic activityThis work was supported by the European Union through the European Research Council (grant ERC-AdG-2014-671093, SynCatMatch) and the Spanish government through the "Severo Ochoa Program" (SEV-2016-0683). L.L. thanks the ITQ for providing a contract. The authors also thank the Microscopy Service of the UPV for the TEM and STEM measurements. The XAS measurements were carried out in the CLAESS beamline of the ALBA synchrotron. We thank Giovanni Agostini for his kind support in the analysis of XAS data. HR-HAADF-STEM measurements were performed at DME-UCA at Cadiz University with financial support from FEDER/MINECO (MAT2017-87579-R and MAT2016-81118-P). C.W.L. thanks CAPES (Science without Frontiers -Process no. 13191/13-6) for a predoctoral fellowship. The financial support from ExxonMobil for this project is also greatly acknowledged.Liu, L.; Lopez-Haro, M.; Lopes, CW.; Rojas-Buzo, S.; Concepción Heydorn, P.; Manzorro, R.; Simonelli, L.... (2020). Structural modulation and direct measurement of subnanometric bimetallic PtSn clusters confined in zeolites. Nature Catalysis. 3(8):628-638. https://doi.org/10.1038/s41929-020-0472-7S62863838Liu, L. & Corma, A. Metal catalysts for heterogeneous catalysis: from single atoms to nanoclusters and nanoparticles. Chem. Rev. 118, 4981–5079 (2018).An, K. & Somorjai, G. A. Nanocatalysis I: synthesis of metal and bimetallic nanoparticles and porous oxides and their catalytic reaction studies. Catal. 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Generation of Powerful Tungsten Reductants by Visible Light Excitation

The homoleptic arylisocyanide tungsten complexes, W(CNXy)_6 and W(CNIph)_6 (Xy = 2,6-dimethylphenyl, Iph = 2,6-diisopropylphenyl), display intense metal to ligand charge transfer (MLCT) absorptions in the visible region (400–550 nm). MLCT emission (λ_max ≈ 580 nm) in tetrahydrofuran (THF) solution at rt is observed for W(CNXy)6 and W(CNIph)_6 with lifetimes of 17 and 73 ns, respectively. Diffusion-controlled energy transfer from electronically excited W(CNIph)_6 (*W) to the lowest energy triplet excited state of anthracene (anth) is the dominant quenching pathway in THF solution. Introduction of tetrabutylammonium hexafluorophosphate, [Bun4N][PF_6], to the THF solution promotes formation of electron transfer (ET) quenching products, [W(CNIph)6]+ and [anth]^•–. ET from *W to benzophenone and cobalticenium also is observed in [Bun4N][PF6]/THF solutions. The estimated reduction potential for the [W(CNIph)6]^(+)/*W couple is −2.8 V vs Cp_(2)Fe^(+/0), establishing W(CNIph)_6 as one of the most powerful photoreductants that has been generated with visible light