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A study is carried out in which the tetrameric 'box' [Cr(mu-Cl){mu-OSi(super t)Bu3}]4, prepared from CrCl2(THF)2 and Na[OSi(super t)Bu3], is readily cleaved by donors to afford square-planar Cr(II) species and is converted to Y-shaped [{(super t)Bu3SiO}Cr{mu-OSi(super t)Bu3}2]Na.C6H6 and ultimately into [(silox)3Cr][Na(dibenzo-18-crown-6)]. The tetrameric box was found to serve as a valuable starting material for [(silox)3Cr](super -).

Research has been conducted on the transition metal complexes. The structural characterization of these complexes has been carried out via the use of the conformational searching technique and the study of technetium with chemical motifs is presented.

Research has been conducted on the metal-containing complexes' computational design. The de novo structural prediction, a pyramidal approach employing various computational techniques such as molecular mechanics in generating the metal-containing complex complete structure, has been examined and is discussed by the authors.

The modified table with new calculation for hydrogen physisorption energies of organic linkers (OL) model for dicarboxylated ring site is presented.

The study explains the synthesis, as well as the reactivity of the [[[(silox).sub.2]MO=NR].sub.2]Hg (R = [super t]Bu, [super t]amyl; silox = O[Si.sup.t][Bu.sub.3]) compounds. These complexes are found to exhibit excellent thermal stability.

The reactivity of low-coordinate transition metal complexes is investigated where the d(super 2) group 6 (silox)(sub 2)Mo=N(super t)Bu(M = Cr, Mo, W) species are sought as lower-symmetry analogues to (silox)(sub 3)M {M = V, Nb-L [L = 4-picoline, PMe(sub 3)], Ta; silox = (super t)Bu(sub 3)SiO}. The calculations suggest that the model is a fundamental approximation for the dimolybdenum pi-bond.

Mechanistic assessments based on kinetic and isotopic methods combined with density functional theory are used to probe the diverse pathways by which C−H bonds in CH4 react on bare Pd clusters, Pd cluster surfaces saturated with chemisorbed oxygen (O*), and PdO clusters. C−H activation routes change from oxidative addition to Habstraction and then to σ-bond metathesis with increasing O-content, as active sites evolve from metal atom pairs (*−*) to oxygen atom (O*−O*) pairs and ultimately to Pd cationlattice oxygen pairs (Pd2+−O2−) in PdO. The charges in the CH3 and H moieties along the reaction coordinate depend on the accessibility and chemical state of the Pd and O centers involved. Homolytic C−H dissociation prevails on bare (*−*) and O*- covered surfaces (O*−O*), while C−H bonds cleave heterolytically on Pd2+−O2− pairs at PdO surfaces. On bare surfaces, C−H bonds cleave via oxidative addition, involving Pd atom insertion into the C−H bond with electron backdonation from Pd to C− H antibonding states and the formation of tight three-center (H3C···Pd···H)⧧ transition states. On O*-saturated Pd surfaces, C− H bonds cleave homolytically on O*−O* pairs to form radical-like CH3 species and nearly formed O−H bonds at a transition state (O*···CH3 •···*OH)⧧ that is looser and higher in enthalpy than on bare Pd surfaces. On PdO surfaces, site pairs consisting of exposed Pd2+ and vicinal O2−, Pdox−Oox , cleave C−H bonds heterolytically via σ-bond metathesis, with Pd2+ adding to the C− H bond, while O2− abstracts the H-atom to form a four-center (H3Cδ−···Pdox···Hδ+···Oox)⧧ transition state without detectable Pdox reduction. The latter is much more stable than transition states on *−* and O*−O* pairs and give rise to a large increase in CH4 oxidation turnover rates at oxygen chemical potentials leading to Pd to PdO transitions. These distinct mechanistic pathways for C−H bond activation, inferred from theory and experiment, resemble those prevalent on organometallic complexes. Metal centers present on surfaces as well as in homogeneous complexes act as both nucleophile and electrophile in oxidative additions, ligands (e.g., O* on surfaces) abstract H-atoms via reductive deprotonation of C−H bonds, and metal−ligand pairs, with the pair as electrophile and the metal as nucleophile, mediate σ-bond metathesis pathways. [ABSTRACT FROM AUTHOR]

Kinetic and isotopic data and density functional theory treatments provide evidence for the elementary steps and the active site requirements involved in the four distinct kinetic regimes observed during CH4 oxidation reactions using O2, H2O, or CO2 as oxidants on Pt clusters. These four regimes exhibit distinct rate equations because of the involvement of different kinetically relevant steps, predominant adsorbed species, and rate and equilibrium constants for different elementary steps. Transitions among regimes occur as chemisorbed oxygen (O*) coverages change on Pt clusters. O* coverages are given, in turn, by a virtual O2 pressure, which represents the pressure that would give the prevalent steady-state O* coverages if their adsorption-desorption equilibrium was maintained. The virtual O2 pressure acts as a surrogate for oxygen chemical potentials at catalytic surfaces and reflects the kinetic coupling between C-H and O═O activation steps. O* coverages and virtual pressures depend on O2 pressure when O2 activation is equilibrated and on O2/CH4 ratios when this step becomes irreversible as a result of fast scavenging of O* by CH4-derived intermediates. In three of these kinetic regimes, C-H bond activation is the sole kinetically relevant step, but occurs on different active sites, which evolve from oxygen-oxygen (O*-O*), to oxygen-oxygen vacancy (O*-*), and to vacancy-vacancy (*-*) site pairs as O* coverages decrease. On O*-saturated cluster surfaces, O*-O* site pairs activate C-H bonds in CH4 via homolytic hydrogen abstraction steps that form CH3 groups with significant radical character and weak interactions with the surface at the transition state. In this regime, rates depend linearly on CH4 pressure but are independent of O2 pressure. The observed normal CH4/CD4 kinetic isotope effects are consistent with the kinetic-relevance of C-H bond activation; identical 16O2-18O2 isotopic exchange rates in the presence or absence of CH4 show that O2 activation steps are quasi-equilibrated during catalysis. Measured and DFT-derived C-H bond activation barriers are large, because of the weak stabilization of the CH3 fragments at transition states, but are compensated by the high entropy of these radical-like species. Turnover rates in this regime decrease with increasing Pt dispersion, because low-coordination exposed Pt atoms on small clusters bind O* more strongly than those that reside at low-index facets on large clusters, thus making O* less effective in H-abstraction. As vacancies (*, also exposed Pt atoms) become available on O*-covered surfaces, O*-* site pairs activate C-H bonds via concerted oxidative addition and H-abstraction in transition states effectively stabilized by CH3 interactions with the vacancies, which lead to much higher turnover rates than on O*-O* pairs. In this regime, O2 activation becomes irreversible, because fast C-H bond activation steps scavenge O* as it forms. Thus, O* coverages are set by the prevalent O2/CH4 ratios instead of the O2 pressures. CH4/CD4 kinetic isotope effects are much larger for turnovers mediated by O*-* than by O*-O* site pairs, because C-H (and C-D) activation steps are required to form the * sites involved in C-H bond activation. Turnover rates for CH4-O2 reactions mediated by O*-* pairs decrease with increasing Pt dispersion, as in the case of O*-O* active structures, because stronger O* binding on small clusters leads not only to less reactive O* atoms, but also to lower vacancy concentrations at cluster surfaces. As O2/CH4 ratios and O* coverages become smaller, O2 activation on bare Pt clusters becomes the sole kinetically relevant step; turnover rates are proportional to O2 pressures and independent of CH4 pressure and no CH4/CD4 kinetic isotope effects are observed. In this regime, turnover rates become nearly independent of Pt dispersion, because the O2 activation step is essentially barrierless. In the absence of O2, alternate weaker oxidants, such as H2O or CO2, lead to a final kinetc regime in which C-H bond dissociation on *-* pairs at bare cluster surfaces limit CH4 conversion rates. Rates become first-order in CH4 and independent of coreactant and normal CH4/CD4 kinetic isotope effects are observed. In this case, turnover rates increase with increasing dispersion, because low-coordination Pt atoms stabilize the C-H bond activation transition states more effectively via stronger binding to CH3 and H fragments. These findings and their mechanistic interpretations are consistent with all rate and isotopic data and with theoretical estimates of activation barriers and of cluster size effects on transition states. They serve to demonstrate the essential role of the coverage and reactivity of chemisorbed oxygen in determining the type and effectiveness of surface structures in CH4 oxidation reactions using O2, H2O, or CO2 as oxidants, as well as the diversity of rate dependencies, activation energies and entropies, and cluster size effects that prevail in these reactions. These results also show how theory and experiments can unravel complex surface chemistries on realistic catalysts under practical conditions and provide through the resulting mechanistic insights specific predictions for the effects of cluster size and surface coordination on turnover rates, the trends and magnitude of which depend sensitively on the nature of the predominant adsorbed intermediates and the kinetically relevant steps. [ABSTRACT FROM AUTHOR]

Abstract: Rate measurements, density functional theory (DFT) within the framework of transition state theory, and ensemble-averaging methods are used to probe oxygen selectivities, defined as the reaction probability ratios for O* reactions with CO and CH4, during CH4–O2 catalysis on Pt and Rh clusters. CO2 and H2O are the predominant products, but small amounts of CO form as chemisorbed oxygen atoms (O*) are depleted from cluster surfaces. Oxygen selectivities, measured using 12CO–13CH4–O2 reactants, increase with O2/CO ratio and O* coverage and are much larger than unity at all conditions on Pt clusters. These results suggest that O* reacts much faster with CO than with CH4, causing any CO that forms and desorbs from metal cluster surfaces to react along the reactor bed with other O* to produce CO2 at any residence time required for detectable extents of CH4 conversion. O* selectivities were also calculated by averaging DFT-derived activation barriers for CO and CH4 oxidation reactions over all distinct surface sites on cubo-octahedral Pt clusters (1.8nm diameter, 201 Pt atoms) at low O* coverages, which are prevalent at low O2 pressures during catalysis. CO oxidation involves non-activated molecular CO adsorption as the kinetically relevant step on exposed Pt atoms vicinal of chemisorbed O* atoms (on *–O* site pairs). CH4 oxidation occurs via kinetically relevant C–H bond activation on *–* site pairs involving oxidative insertion of a Pt atom into one of the C–H bonds in CH4, forming a three-centered HC3–Pt–H transition state. C–H bond activation barriers reflect the strength of Pt–CH3 and Pt–H interactions at the transition state, which correlates, in turn, with the Pt coordination and with CH3 * binding energies. Ensemble-averaged O* selectivities increase linearly with O2/CO ratios, which define the O* coverages, via a proportionality constant. The proportionality constant is given by the ratio of rate constants for O2 dissociation and C–H bond activation elementary steps; the values for this constant are much larger than unity and are higher on larger Pt clusters (1.8–33nm) at all temperatures (573–1273K) relevant for CH4–O2 reactions. The barriers for the kinetically relevant C–H bond dissociation step increase, while those for CO oxidation remain unchanged as the Pt coordination number and cluster size increase, and lead, in turn, to higher O* selectivities on larger Pt clusters. Oxygen selectivities were much larger on Rh than Pt, because the limiting reactants for CO oxidation were completely consumed in 12CO–13CH4–O2 mixtures, consistent with lower CO/CO2 ratios measured by varying the residence time and O2/CH4 ratio independently in CH4–O2 reactions. These mechanistic assessments and theoretical treatments for O* selectivity provide rigorous evidence of low intrinsic limits of the maximum CO yields, thus confirming that direct catalytic partial oxidation of CH4 to CO (and H2) does not occur at the molecular scale on Pt and Rh clusters. CO (and H2) are predominantly formed upon complete O2 depletion from the sequential reforming steps. [Copyright &y& Elsevier]

Examines the semiempirical quantum mechanics use to predict the spin state of transition metal complexes. Context of De novo structural prediction; Feasibility of the SEQM structures for method efficiency; Reliance on the chemical properties and to develop parametrizations.