- Room Temperature Acceptorless Alkane Dehydrogenation from Molecular σ-Alkane Complexes
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The non-oxidative catalytic dehydrogenation of light alkanes via C-H activation is a highly endothermic process that generally requires high temperatures and/or a sacrificial hydrogen acceptor to overcome unfavorable thermodynamics. This is complicated by alkanes being such poor ligands, meaning that binding at metal centers prior to C-H activation is disfavored. We demonstrate that by biasing the pre-equilibrium of alkane binding, by using solid-state molecular organometallic chemistry (SMOM-chem), well-defined isobutane and cyclohexane σ-complexes, [Rh(Cy2PCH2CH2PCy2)(η: η-(H3C)CH(CH3)2][BArF4] and [Rh(Cy2PCH2CH2PCy2)(η: η-C6H12)][BArF4] can be prepared by simple hydrogenation in a solid/gas single-crystal to single-crystal transformation of precursor alkene complexes. Solid-gas H/D exchange with D2 occurs at all C-H bonds in both alkane complexes, pointing to a variety of low energy fluxional processes that occur for the bound alkane ligands in the solid-state. These are probed by variable temperature solid-state nuclear magnetic resonance experiments and periodic density functional theory (DFT) calculations. These alkane σ-complexes undergo spontaneous acceptorless dehydrogenation at 298 K to reform the corresponding isobutene and cyclohexadiene complexes, by simple application of vacuum or Ar-flow to remove H2. These processes can be followed temporally, and modeled using classical chemical, or Johnson-Mehl-Avrami-Kologoromov, kinetics. When per-deuteration is coupled with dehydrogenation of cyclohexane to cyclohexadiene, this allows for two successive KIEs to be determined [kH/kD = 3.6(5) and 10.8(6)], showing that the rate-determining steps involve C-H activation. Periodic DFT calculations predict overall barriers of 20.6 and 24.4 kcal/mol for the two dehydrogenation steps, in good agreement with the values determined experimentally. The calculations also identify significant C-H bond elongation in both rate-limiting transition states and suggest that the large kH/kD for the second dehydrogenation results from a pre-equilibrium involving C-H oxidative cleavage and a subsequent rate-limiting β-H transfer step.
- McKay, Alasdair I.,Bukvic, Alexander J.,Tegner, Bengt E.,Burnage, Arron L.,Mart?nez-Mart?nez, Antonio J.,Rees, Nicholas H.,Macgregor, Stuart A.,Weller, Andrew S.
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supporting information
p. 11700 - 11712
(2019/08/20)
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- Probing chemistry and kinetics of reactions in heterogeneous catalysts
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Using benzene hydrogenation over Pt/SiO2 as an industrially-relevant example, we show that state-of-the-art neutron total scattering methods spanning a wide Q-range now permit relevant time-resolved catalytic chemistry to be probed directly in situ within the pore of the catalyst. The method gives access to the reaction rates on both nanometric and atomic length scales, whilst simultaneously providing an atomistic structural viewpoint on the reaction mechanism itself.
- Youngs, Tristan G. A.,Manyar, Haresh,Bowron, Daniel T.,Gladden, Lynn F.,Hardacre, Christopher
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p. 3484 - 3489
(2013/11/19)
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- An Air/water-stable tridentate N-heterocyclic carbene-palladium(II) Complex: Catalytic C-H activation of hydrocarbons via hydrogen/deuterium exchange process in deuterium oxide
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While developing novel catalysts for carbon-carbon or carbon-heteroatom coupling (nitrogen, oxygen, or fluorine), we were able to introduce tridentate N-heterocyclic carbene (NHC)-amidate-alkoxide palladium(II) complexes. In aqueous solution, these NHC-Pd(II) complexes showed high ability for C-H activation of various hydrocarbons (cyclohexane, cyclopentane, dimethyl ether, tetrahydrofuran, acetone, and toluene) under mild conditions.
- Lee, Joo Ho,Yoo, Kyung Soo,Park, Chan Pil,Olsen, Janet M.,Sakaguchi, Satoshi,Prakash, G. K. Surya,Mathew, Thomas,Jung, Kyung Woon
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supporting information; experimental part
p. 563 - 568
(2009/11/30)
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- Ionization Energies and Entropies of Cycloalkanes. Kinetics of Free Energy Controlled Charge-Transfer Reactions.
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Enthalpies and entropies of ionization (ΔH0ion and ΔS0ion) of alkylcyclohexanes, as well as cycloheptane, cyclooctane, and trans-Decalin, have been determined by charge-transfer equilibrium measurements.Values of ΔHion, in units of kcal mol-1 (or eV), range from 229.6 (9.96) for cycloheptane to 210.7 (9.14) for trans-Decalin.A major effect of alkyl substitution is observed following substitution at a site α to a tertiary hydrogen atom (as from methylcyclohexane to 1,2-dimethylcyclohexane), or following replacement of a tertiary hydrogen atom (as from methylcyclohexane to 1,1-dimethylcyclohexane).In both cases, ΔH0 ion decreases by ca. 5 kcal mol-1.Entropies of ionization are near zero for alkylcyclohexanes but range up to 5 cal deg-1 mol-1 for nonsubstituted cycloalkanes (cyclooctane).The charge-transfer reactions involving the cycloalkanes are shown to be fast processes; i.e., the sum of the reaction efficiencies (r=k/kcollision) of the forward and reverse processes is near unity.The efficiencies of these processes appear to be determined uniquely by the overall free energy change (or equilibrium constant K).Specifically, the reaction efficiencies are defined, within a factor of 2 by the relation r=K/(1+K), which can be justified by using transition-state theory applied to the decomposition of a collision complex over surfaces lacking energy barriers.These reactions are defined as intrinsically fast processes in that they are slowed only by the overall reaction thermochemistry and not by any properties or reactions of the intermediate complex.
- Sieck, L. Wayne,Mautner, Michael
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p. 3646 - 3650
(2007/10/02)
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- Ionization of Normal Alkanes: Enthalpy, Entropy, Structural, and Isotope Effects
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Enthalpies and entropies of ionization (ΔHi0, ΔSi0) of C4 to C11 normal alkanes were determined from charge-transfer equilibrium measurement between 300 and 420 K by using photoionization high-pressure mass spectrometry.Large negative ΔSi0 values are observed in C7 and larger n-alkanes, from -4.7 cal mol-1 K-1 (-19.6 J mol-1 K-1) in heptane to -13.9 cal mol-1 K-1 (-58.1 J mol-1 K-1) in undecane; in contrast, ΔSi0 of C4-C7 n-alkanes is negligible. ΔHi0 values range from 10.35 eV (997.6 kJ mol-1) (butane) to 9.45 eV (910.9 kJ mol-1) (undecane); the incremental ΔHi0 values also suggest the occurence of an effect that stabilizes C7 and higher but not the lower molecular ions.Analogy with disubstituted alkanes suggests that the negative ΔSi0 values and excess stabilization in C7 and higher alkane ions are due to constrained cyclic conformations which result from noncovalent intramolecular bonding between the terminal -C2H5 groups in the large, flexible molecular ions.These effects are more pronounced in n-alkanes than in 2-methylalkanes.Isotope effects on ΔHi0 as measured by the equilibrium constant K290 for n-CmD2m+2+ + n-CmH2m+2 ->/+ + n-CmD2m+2 are significant for ethane (k291 = 4.5) but decrease with increasing m: in propane K290 = 3.2 and in hexane and octane K291 = 1.0.However, the isotope effects in cyclic alkanes are much larger than in corresponding normal alkanes: in cyclohexane, K321 = 3.3 compared with that in n-hexane, were K320 = 1.0.
- Meot-Ner (Mautner), M.,Sieck, L.W.,Ausloos, P.
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p. 5342 - 5348
(2007/10/02)
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