"Soft" oxidative coupling of methane to ethylene: Mechanistic insights from combined experiment and theory.

The oxidative coupling of methane to ethylene using gaseous disulfur (2CH4 + S2 → C2H4 + 2H2S) as an oxidant (SOCM) proceeds with promising selectivity. Here, we report detailed experimental and theoretical studies that examine the mechanism for the conversion of CH4 to C2H4 over an Fe3O4-derived FeS2 catalyst achieving a promising ethylene selectivity of 33%. We compare and contrast these results with those for the highly exothermic oxidative coupling of methane (OCM) using O2 (2CH4 + O2 → C2H4 + 2H2O). SOCM kinetic/mechanistic analysis, along with density functional theory results, indicate that ethylene is produced as a primary product of methane activation, proceeding predominantly via CH2 coupling over dimeric S-S moieties that bridge Fe surface sites, and to a lesser degree, on heavily sulfided mononuclear sites. In contrast to and unlike OCM, the overoxidized CS2 by-product forms predominantly via CH4 oxidation, rather than from C2 products, through a series of C-H activation and S-addition steps at adsorbed sulfur sites on the FeS2 surface. The experimental rates for methane conversion are first order in both CH4 and S2, consistent with the involvement of two S sites in the rate-determining methane C-H activation step, with a CD4/CH4 kinetic isotope effect of 1.78. The experimental apparent activation energy for methane conversion is 66 ± 8 kJ/mol, significantly lower than for CH4 oxidative coupling with O2 The computed methane activation barrier, rate orders, and kinetic isotope values are consistent with experiment. All evidence indicates that SOCM proceeds via a very different pathway than that of OCM.

La[N(SiMe3)2]3-Catalyzed Deoxygenative Reduction of Amides with Pinacolborane. Scope and Mechanism.

Tris[N,N-bis(trimethylsilyl)amide]lanthanum (LaNTMS) is an efficient and selective homogeneous catalyst for the deoxygenative reduction of tertiary and secondary amides with pinacolborane (HBpin) at mild temperatures (25-60 °C). The reaction, which yields amines and O(Bpin)2, tolerates nitro, halide, and amino functional groups well, and this amide reduction is completely selective, with the exclusion of both competing inter- and intramolecular alkene/alkyne hydroboration. Kinetic studies indicate that amide reduction obeys an unusual mixed-order rate law which is proposed to originate from saturation of the catalyst complex with HBpin. Kinetic and thermodynamic studies, isotopic labeling, and DFT calculations using energetic span analysis suggest the role of a [(Me3Si)2N]2La-OCHR(NR'2)[HBpin] active catalyst, and hydride transfer is proposed to be ligand-centered. These results add to the growing list of transformations that commercially available LaNTMS is competent to catalyze, further underscoring the value and versatility of lanthanide complexes in homogeneous catalysis.

Unexpected Precatalyst σ-Ligand Effects in Phenoxyimine Zr-Catalyzed Ethylene/1-Octene Copolymerizations.

Recent decades have witnessed intense research efforts aimed at developing new homogeneous olefin polymerization catalysts, with a primary focus on metal-Cl or metal-hydrocarbyl precursors. Curiously, metal-NR2 precursors have received far less attention. In this contribution, the Zr-amido complex FI2ZrX2 (FI = 2,4-di- tert-butyl-6-((isobutylimino)methyl)phenolate, X = NMe2) is found to exhibit high ethylene polymerization activity and relatively high 1-octene coenchainment selectivity (up to 7.2 mol%) after sequential activation with trimethylaluminum, then Ph3C+B(C6F5)4-. In sharp contrast, catalysts with traditional hydrocarbyl ligands such as benzyl and methyl give low 1-octene incorporation (0-1.0 mol%). This unexpected selectivity persists under scaled/industrial operating conditions and was previously inaccessible with traditional metal-Cl or -hydrocarbyl precursors. NMR, X-ray diffraction, and catalytic control experiments indicate that in this case an FI ligand is abstracted from FI2Zr(NMe2)2 by trimethylaluminum in the activation process to yield a catalytically active cationic mono-FIZr species. Heretofore this process was believed to serve only as a major catalyst deactivation pathway to be avoided. This work demonstrates the importance of investigating diverse precatalyst monodentate σ-ligands in developing new catalyst systems, especially for group 4 olefin polymerization catalysts.

Synthesis, Characterization, and Thermal Properties of N-alkyl ß-Diketiminate Manganese Complexes.

A series of N, N'-dialkyl-ß-diketiminato manganese(II) complexes was synthesized and characterized by single crystal X-ray diffraction, UV-vis and FTIR spectroscopy, and then assayed for volatility, thermal stability, and surface reactivity relevant to vapor-phase film growth processes. Bis( N, N'-dimethyl-4-amino-3-penten-2-imine) manganese(II), 1, and bis( N- N'-diisopropyl-4-amino-3-penten-2-imine) manganese(II), 2, specifically, emerge as the most promising candidates, balancing volatility (sublimation temperatures < 100 °C at 100 mTorr) with coordinative unsaturation and reactivity, as revealed by rapid release of ligand in the presence of a silica surface. Good correlation is observed between buried volume calculations and relative surface reactivity data, indicating that metal availability resulting from sterically open ligand alkyl substituents increases surface reactivity. The thermal stability, volatility, and reactivity exhibited by these compounds render them promising precursors for the growth of manganese oxide films via vapor-phase growth processes.

Thermodynamic Strategies for C-O Bond Formation and Cleavage via Tandem Catalysis.

To reduce global reliance on fossil fuels, new renewable sources of energy that can be used with the current infrastructure are required. Biomass represents a major source of renewable carbon based fuel; however, the high oxygen content (∼40%) limits its use as a conventional fuel. To utilize biomass as an energy source, not only with current infrastructure, but for maximum energy return, the oxygen content must be reduced. One method to achieve this is to develop selective catalytic methods to cleave C-O bonds commonly found in biomass (aliphatic and aromatic ethers and esters) for the eventual removal of oxygen in the form of volatile H2O or carboxylic acids. Once selective methods of C-O cleavage are understood and perfected, application to processing real biomass feedstocks such as lignin can be undertaken. This Laboratory previously reported that recyclable "green" lanthanide triflates are excellent catalysts for C-O bond-forming hydroalkoxylation reactions. Based on the virtues of microscopic reversibility, the same lanthanide triflate catalyst should catalyze the reverse C-O cleavage process, retrohydroalkoxylation, to yield an alcohol and an alkene. However, ether C-O bond-forming (retrohydroalkoxylation) to form an alcohol and alkene is endothermic. Guided by quantum chemical analysis, our strategy is to couple endothermic, in tandem, ether C-O bond cleavage with exothermic alkene hydrogenation, thereby leveraging the combined catalytic cycles thermodynamically to form an overall energetically favorable C-O cleavage reaction. This Account reviews recent developments on thermodynamically leveraged tandem catalysis for ether and more recently, ester C-O bond cleavage undertaken at Northwestern University. First, the fundamentals of lanthanide-catalyzed hydroelementation are reviewed, with particular focus on ether C-O bond formation (hydroalkoxylation). Next, the reverse C-O cleavage/retrohydroalkoxylation processes enabled by tandem catalysis are discussed for both ether and ester C-O bond cleavage, including mechanistic and computational analysis. This is followed by recent results using this tandem catalytic strategy toward biomass relevant substrates, including work deconstructing acetylated lignin models, and the production of biodiesel from triglycerides, while bypassing the production of undesired glycerol for more valuable C3 products such as diesters (precursors to diols) in up to 47% selectivity. This Account concludes with future prospects for using this tandem catalytic system under real biomass processing conditions.

Scandium-Catalyzed Self-Assisted Polar Co-monomer Enchainment in Ethylene Polymerization.

Direct coordinative copolymerization of ethylene with functionalized co-monomers is a long-sought-after approach to introducing polyolefin functionality. However, functional-group Lewis basicity typically depresses catalytic activity and co-monomer incorporation. Finding alternatives to intensively studied group 4 d0 and late-transition-metal catalysts is crucial to addressing this long-standing challenge. Shown herein is that mono- and binuclear organoscandium complexes with a borate cocatalyst are active for ethylene + amino olefin [AO; H2 C=CH(CH2 )n NR2 ] copolymerizations in the absence of a Lewis-acidic masking reagent. Both activity (up to 4.2×102  kg mol-1 ⋅h-1> atm-1> ) and AO incorporation (up to 12 % at 0.2 m [AO]) are appreciable. Linker-length-dependent (n) AO incorporation and mechanistic probes support an unusual functional-group-assisted enchainment mechanism. Furthermore, the binuclear catalysts exhibit enhanced AO tolerance and enhanced long chain AO incorporation.

Low-Temperature Atomic Layer Deposition of MoS2 Films.

Wet chemical screening reveals the very high reactivity of Mo(NMe2 )4 with H2 S for the low-temperature synthesis of MoS2 . This observation motivated an investigation of Mo(NMe2 )4 as a volatile precursor for the atomic layer deposition (ALD) of MoS2 thin films. Herein we report that Mo(NMe2 )4 enables MoS2 film growth at record low temperatures-as low as 60 °C. The as-deposited films are amorphous but can be readily crystallized by annealing. Importantly, the low ALD growth temperature is compatible with photolithographic and lift-off patterning for the straightforward fabrication of diverse device structures.

Monomeric platinum(II) hydroxides supported by sterically dominant α-diimine ligands.

The use of two new highly sterically bulky α-diimine ligands for the stabilization of neutral, monomeric platinum(II) hydroxo complexes is described. Halide abstraction from LPtCl(2) complexes of these ligands in the presence of water, followed by deprotonation of the cationic aquo complex, leads to LPt(OH)Cl and LPt(OH)(2). The latter can be reprotonated with HNTf(2) to yield a highly fluxional hydroxoaquoplatinum(II) cation.

1,2-Bis{[3,5-bis-(2,6-diisopropyl-phen-yl)phen-yl]imino}-acenaphthene toluene monosolvate.

In the title compound, C(72)H(80)N(2)·C(7)H(8), the acenaphthene ring system is essentially planar, with a maximum deviation of 0.041 (3) Å. The benzene rings bonded to the the N atoms are essentially parallel, forming a dihedral angle of 0.80 (11)°, and these rings form dihedral angles of 87.49 (9) and 88.25 (10)° with the mean plane of the acenaphthene ring system. The methyl C atoms of three of the isopropyl groups are disordered of two sets of sites of equal occupancy.

N,N'-Bis[3,5-bis-(2,6-diisopropyl-phen-yl)phen-yl]butane-2,3-diimine.

The title mol-ecule, C(64)H(80)N(2), lies on an inversion center wherein the central butane-diimine fragment [N=C(Me)-C(Me)=N] is essentially planar [maximum deviation = 0.002 (2) Å] and its mean plane forms a dihedral of 70.88 (10)° with the attached benzene ring. In the symmetry-unique part of the mol-ecule, the dihedral angles between the benzene ring bonded to the N atom and the other two benzene rings are 89.61 (6) and 82.77 (6)°.

Orthogonal tandem catalysis.

Tandem catalysis is a growing field that is beginning to yield important scientific and technological advances toward new and more efficient catalytic processes. 'One-pot' tandem reactions, where multiple catalysts and reagents, combined in a single reaction vessel undergo a sequence of precisely staged catalytic steps, are highly attractive from the standpoint of reducing both waste and time. Orthogonal tandem catalysis is a subset of one-pot reactions in which more than one catalyst is used to promote two or more mechanistically distinct reaction steps. This Perspective summarizes and analyses some of the recent developments and successes in orthogonal tandem catalysis, with particular focus on recent strategies to address catalyst incompatibility. We also highlight the concept of thermodynamic leveraging by coupling multiple catalyst cycles to effect challenging transformations not observed in single-step processes, and to encourage application of this technique to energetically unfavourable or demanding reactions.

Silver ion enhanced C-H activation in Pt(II) hydroxo complexes.

Reacting neutral Pt(ii) hydroxo compounds (NN)Pt(R)(OH) (R = OH, Ph, and Me, NN = bulky terphenyl diimine) with silver bis(trifluromethanesulfonyl)imide produces new hydroxo complexes with the silver binding through the Pt-OH bonds as determined by (195)Pt NMR and X-ray analysis (R = OH). These complexes were found to activate the aromatic C-D bonds of C6D6 at significantly enhanced rates relative to the silver free hydroxo complexes. Mechanistic studies for R = Ph are consistent with a homogeneous pathway that is bimolecular (ΔH(‡) = 17(2) kcal mol(-1) and ΔS(‡) = -25(6) e.u. and ΔG = 10(2) kcal mol(-1)), first order in [Pt] and substrate.

Reversible insertion of carbon dioxide into Pt(II)-hydroxo bonds.

The reactivity of three monomeric diimine Pt(II) hydroxo complexes, (NN)Pt(OH)R (NN = bulky diimine ligand; R = OH, ; R = C6H5, ; R = CH3, ) towards carbon dioxide has been investigated. Insertion into the Pt-OH bonds was found to be facile and reversible at low temperature for all compounds; the reaction with bis-hydroxide gives an isolable κ(2)-carbonato compound , with elimination of water.