We report the isolation, characterization, and reactions of the unsaturated complex L tBu Co (L tBu = bulky β-diketiminate ligand). The unusual slipped kN,η 6 -arene binding mode in L tBu Co interconverts rapidly and reversibly with the traditional k 2 N,N 0 ligation mode upon binding of Lewis bases, making it a "masked" two-coordinate complex. The mechanism of this isomerization is demonstrated using kinetic studies. L tBu Co is a stable yet reactive synthon for low-coordinate cobalt(I) complexes and is capable of cleaving the CÀF bond in fluorobenzene.H emilabile ligands, which contain a Lewis basic moiety that can reversibly dissociate from a metal, create transient coordinative unsaturation that can be used for bond activation and catalysis. 1 Notable examples include catalysts for olefin metathesis and cross-coupling reactions. 2 In this communication, we report that a bulky β-diketiminate ligand on cobalt(I) undergoes a novel isomerization that allows it to behave as a hemilabile ligand. Though β-diketiminates are represented in thousands of metal complexes, 3 this is the first example of this new binding mode. We show that the ligand isomerization is rapidly reversible and that it provides a "masked two-coordinate" cobalt(I) center for ligand binding and for activation of a strong CÀF bond.The addition of tetrahydrofuran (THF) to a solution of L tBu CoNNCoL tBu [L tBu = 2,2,6,6-tetramethyl-3,5-bis(2,4,6-triisopropylphenylimido)hept-4-yl] 4 in C 6 D 6 gives an immediate color change from brown to dark-green. This color change corresponds to the appearance of a set of signals in the 1 H NMR spectrum for a new species, L tBu Co(THF) (1), in quantitative yield. On a preparative scale, 1 can be produced from the reaction of L tBu CoCl with KC 8 in THF under Ar, which gives 1 as a dark-green crystalline solid in 64% yield. Complex 1 (Figure 1 left) has pseudo-C 2 symmetry in the solid state. The threecoordinate cobalt atom is planar, as the sum of the NÀCoÀN and NÀCoÀO bond angles is 359.7(2)°. The THF ligand is bent slightly toward one aryl arm of the β-diketiminate ligand, with NÀCoÀO angles of 135. Dissolution of 1 in C 6 D 6 under Ar gives a dark-orange solution whose 1 H NMR spectrum shows peaks from 1 plus a number of additional resonances. Evaporation of the volatile materials and redissolution in fresh C 6 D 6 leads to an 1 H NMR spectrum that contains very little 1. The displacement of THF with aromatic solvents can be used for the synthesis of the unknown species on a gram scale. Thus, 1 was dissolved in toluene, and two cycles of solvent removal and addition of more toluene removed all of the THF. Crystallization from a concentrated pentane solution at À45°C under Ar gave L tBu Co (2) in 72% yield as brown crystals.The molecular structure of 2 (Figure 1 right) shows a cobalt atom and a single β-diketiminate ligand with no additional donors. In contrast to the typical k 2 N,N 0 binding mode for L tBu , the β-diketiminate ligand in 2 is bound to cobalt in a kN,η 6 -arene mode in which the co...

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Adsorption and hydrolysis of xylan polysaccharides extracted from Miscanthus biomass are demonstrated, using surface-functionalized MCN (mesoporous carbon nanoparticle) materials that comprise weak-acid sites, at a pH corresponding to biomass extract. Extracted xylan polysaccharides consist of a peak molecular weight of 2008 g/mol according to GPC (gel-permeation chromatography), corresponding to approximately 15 xylose repeat units, and, a calculated length of 7 nm and radius of gyration of 2.0 nm based on molecular dynamics simulations. A highly active material for the adsorption and depolymerization of xylan is a hydrothermally treated sulfonated MCN material, which consists of 90% weak-acid sites. In spite of the large polysaccharide size relative to its 1.6 nm pore radius, this material adsorbs up to 76% of xylan strands from extract solution, at a weight loading of 29% relative to MCN. Starting with a 9.7% xylose yield in Miscanthus extract, this material hydrolyzes extracted xylan to xylose, and achieves a 74.1% xylose yield, compared with 24.1% yield for the background reaction in acetate buffer, at 150°C for 4 h. Catalytic comparisons with other MCN-based materials highlight the role of confinement and weak-acid surface sites, and provide some correlation between activity and phenolic OH acid-site density. However, the lack of a directly proportional correlation between weak-acid site density and catalyzed hydrolysis rate signifies that only a fraction of weak-acid surface sites are catalytically active, and this is likely to be the sites that are present in a high local concentration on the surface, which would be consistent with previously observed trends in the hydrolysis catalysis of chemisorbed glucans on inorganic-oxide surfaces.The depolymerization of biomass-derived polymers into their monomeric components represents a grand challenge for enabling renewable approaches to fuels and chemicals. 1 We have recently demonstrated the hydrolysis of chemisorbed glucans that are grafted on inorganic-oxide surfaces, as an example of general-acid catalysis involving weak-acid OH defect sites in water. 2−4 We also demonstrated the rapid (transport time scale was faster than could be measured) adsorption of long-chain polysaccharides consisting of (1→4)-β-D-glucans to the interior surface of mesoporous carbon nanoparticle (MCN) materials, in spite of these polysaccharides being several-fold larger in radius of gyration than the MCN pore radius. 5 The affinity between the polysaccharide and the MCN surface was observed to increase with the molecular weight of the polymer, in a manner that was consistent with cumulative weak aromaticcarbohydrate interactions enthalpically driving adsorption, as observed previously in glycoproteins. 5 Based upon all of these data, we hypothesized that MCN materials may be able to catalyze the depolymerization of polysaccharides directly from biomass by relying exclusively on weak-acid surface sites. We rationalized such an approach to biomass-derived polysaccharide hydrolysis by ...

To assist understanding of combustion processes, we have investigated reactions of methylidyne (CH) with acrolein (CH2CHCHO) using the quantum Monte Carlo (QMC) and other computational methods. We present a theoretical study of the major reactions reported in a recent experiment on the subject system. Both DFT and MP2 computations are carried out, and the former approach is used to form the independent-particle part of the QMC trial wave function used in the diffusion Monte Carlo (DMC) variant of the QMC method. In agreement with experiment, we find that the dominant product channel leads to formation of C4H4O systems + H with leading products of furan + H and 1,3-butadienal + H. Equilibrium geometries, atomization energies, reaction barriers, transition states, and heats of reaction are computed using the DFT, MP2, and DMC approaches and compared to experiment. We find that DMC results are in close agreement with experiment. The kinetics of the subject reactions are determined by solving master equations with the MultiWell software suite.

Second- and third-row (typically precious metals) transition metal complexes are known to possess certain electronic features that define their structure and reactivity and are usually not observed in their first-row (base metal) congeners. Can these electronic features be conferred onto first-row transition metals with the aid of noninnocent and/or very high-field ligands? In this research, the impact upon methane C-H bond activation was modeled using the dipyridylazaallyl (smif) supporting ligand for late, first-row transition metal (M) imide, oxo, and carbene complexes (M = Fe, Co, Ni, or Cu; E = O, NMe, or CMe2). Density functional theory calculations suggest that the combination of smif with iron and the oxo activating ligand is the most energetically favorable complex for methane C-H activation. A change in the preferred transition state for methane C-H activation from [2+2] addition to hydrogen atom abstraction was observed upon going from Fe to Cu and for Fe as compared to precious metals. Contrary to expectations, it was the imide ligand rather than the dipyridylazaallyl ligand that was found to possess redox "noninnocent" characteristics.