Mechanistic Insights into Alkane Metathesis Catalyzed by Silica-Supported Tantalum Hydrides: A DFT Study

Alkane metathesis transforms small alkanes into their higher and lower homologues. The reaction is catalyzed by either supported d0 metal hydrides (M = Ta, W) or d0 alkyl alkylidene complexes (M = Ta, Mo, W, Re). For the silica-supported tantalum hydrides, several reaction mechanisms have been proposed. We performed DFT-D3 calculations to analyze the viability of the proposed pathways and compare them with alkane hydrogenolysis, which is a competitive process observed at the early stages of the reaction. The results show that the reaction mechanisms for alkane metathesis and for alkane hydrogenolysis present similar energetics, and this is consistent with the fact that the process taking place depends on the concentrations of the initial reactants. Overall, a modified version of the so-called one-site mechanism that involves alkyl alkylidene intermediates appears to be more likely and consistent with experiments. According to this proposal, tantalum hydrides are precursors of the alkyl alkylidene active species. During precursor activation, H2 is released and this allows alkane hydrogenolysis to occur. In contrast, the catalytic cycle implies only the reaction with alkane molecules in excess and does not form H2. Thus, the activity for alkane hydrogenolysis decreases. The catalytic cycle proposed here implies three stages: (i) β-H elimination from the alkyl ligand, liberating ethene, (ii) alkene cross-metathesis, allowing olefin substituent exchange, and (iii) formation of the final products and alkyl alkylidene regeneration by olefin insertion and three successive 1,2-CH insertions to the alkylidene followed by α abstraction. These results relate the reactivity of silica-supported hydrides with that of the alkyl alkylidene complexes, the other common catalyst for alkane metathesis. © 2017 American Chemical Society

Monoaryloxide Pyrrolide (MAP) Imido Alkylidene Complexes of Molybdenum and Tungsten That Contain 2,6-Bis(2,5-R[subscript 2]-pyrrolyl)phenoxide (R = i-Pr, Ph) Ligands and an Unsubstituted Metallacyclobutane on Its Way to Losing Ethylene

We report the synthesis of Mo and W MAP complexes that contain O-2,6-(2,5-R[subscript 2]-pyrrolyl)[subscript 2]C[subscript 6]H[subscript 3] (2,6-dipyrrolylphenoxide or ODPP[superscript R]) ligands in which R = i-Pr, Ph. W(NAr)(CH-t-Bu)(Pyr)(ODPP[superscript Ph]) (4a; Ar = 2,6-disopropylphenyl, Pyr = pyrrolide) reacts readily with ethylene to yield a metallacyclobutane complex, W(NAr)(C[subscript 3]H[subscript 6])(Pyr)(ODPP[superscript Ph]) (5). The structure of 5 in the solid state shows that it is approximately a square pyramid with the WC[subscript 4] ring spanning apical and basal positions. This SP′ structure, which has never been observed as an actual intermediate, must now be regarded as an integral feature of the metathesis reaction.National Science Foundation (U.S.) (CHE-1111133)National Science Foundation (U.S.) (Center for Enabling New Technologies through Catalysis (CENTC) CHE-0650456)National Science Foundation (U.S.) (Grant CHE-0946721

Reactivity of Metal Carbenes with Olefins: Theoretical Insights on the Carbene Electronic Structure and Cyclopropanation Reaction Mechanism

Present work addresses the reactivity of several phenyl-substituted metal–carbene complexes with 4-methylstyrene by means of density functional theory OPBE simulations. Different paths that lead to cyclopropanation were explored and compared to the olefin metathesis mechanism. For this purpose, we chose four different catalysts: (i) the Grubbs second-generation olefin metathesis catalyst, (ii) a Grubs second-generation-like complex, in which ruthenium is replaced by iron, and (iii) two iron carbene complexes (a piano stool and a porphyrin iron carbene) that experimentally catalyze alkene cyclopropanation. Results suggest that the nature of the applying mechanism is very sensitive to the coordination around the metal center and the spin state of the metal–carbene complex. Cyclopropanation by open-shell metal–carbene complexes seems to preferentially proceed through a two-step radical mechanism, in which the two C–C bonds are sequentially formed (path C). Singlet-state carbenes proceed either through a direct attack of the olefin to the carbene (path D) when the formation of the metallacycle is not feasible or through a reductive elimination from the metallacyclobutane when this intermediate is accessible both kinetically and thermodynamically (path B)

Reactivity of Metal Carbenes with Olefins: Theoretical Insights on the Carbene Electronic Structure and Cyclopropanation Reaction Mechanism

Present work addresses the reactivity of several phenyl-substituted metal–carbene complexes with 4-methylstyrene by means of density functional theory OPBE simulations. Different paths that lead to cyclopropanation were explored and compared to the olefin metathesis mechanism. For this purpose, we chose four different catalysts: (i) the Grubbs second-generation olefin metathesis catalyst, (ii) a Grubs second-generation-like complex, in which ruthenium is replaced by iron, and (iii) two iron carbene complexes (a piano stool and a porphyrin iron carbene) that experimentally catalyze alkene cyclopropanation. Results suggest that the nature of the applying mechanism is very sensitive to the coordination around the metal center and the spin state of the metal–carbene complex. Cyclopropanation by open-shell metal–carbene complexes seems to preferentially proceed through a two-step radical mechanism, in which the two C–C bonds are sequentially formed (path C). Singlet-state carbenes proceed either through a direct attack of the olefin to the carbene (path D) when the formation of the metallacycle is not feasible or through a reductive elimination from the metallacyclobutane when this intermediate is accessible both kinetically and thermodynamically (path B)

Dinitrogen dissociation on an isolated surface tantalum atom

Both industrial and biochemical ammonia syntheses are thought to rely on the cooperation of multiple metals in breaking the strong triple bond of dinitrogen. Such multimetallic cooperation for dinitrogen cleavage is also the general rule for dinitrogen reductive cleavage with molecular systems and surfaces. We have observed cleavage of dinitrogen at 250 degrees C and atmospheric pressure by dihydrogen on isolated silica surface-supported tantalum(III) and tantalum(V) hydride centers [(equivalent to Si-O)(2)Ta-III-H] and [(equivalent to Si-O)(2)(TaH3)-H-V], leading to the Ta-V amido imido product [(equivalent to SiO)(2)Ta(equivalent to NH)(NH2)]: We assigned the product structure based on extensive characterization by infrared and solid-state nuclear magnetic resonance spectroscopy, isotopic labeling studies, and supporting data from x-ray absorption and theoretical simulations. Reaction intermediates revealed by in situ monitoring of the reaction with infrared spectroscopy support a mechanism highly distinct from those previously observed in enzymatic, organometallic, and heterogeneous N-2 activating systems