Activation of C2H6 and C3H8 by Gas-Phase Mo+: Thermochemistry of Mo−Ligand Complexes

Thermochemical electronegativities of the elements

Electronegativity is a key property of the elements. Being useful in rationalizing stability, structure and properties of molecules and solids, it has shaped much of the thinking in the fields of structural chemistry and solid state chemistry and physics. There are many definitions of electronegativity, which can be roughly classified as either spectroscopic (these are defined for isolated atoms) or thermochemical (characterizing bond energies and heats of formation of compounds). The most widely used is the thermochemical Pauling's scale, where electronegativities have units of eV-1/2. Here we identify drawbacks in the definition of Pauling's electronegativity scale-and, correcting them, arrive at our thermochemical scale, where electronegativities are dimensionless numbers. Our scale displays intuitively correct trends for the 118 elements and leads to an improved description of chemical bonding (e.g., bond polarity) and thermochemistry.

Enhancement of the Catalytic Activities of Heteronuclear Bimetallic Cations for the C-H Bond Activation of Cyclohexane

Heterometallic cations NiCu⁺ and CoNi⁺ can easily induce triple dehydrogenation of cyclohexane with high yield, and monometallic cations Ni⁺ and Co⁺ only give rise to double dehydrogenation with low yield. Reaction mechanisms of the six C-H bond activations for cyclohexane are systematically investigated by comparing the difference between bimetallic cations and monometallic ones. Fragment molecular orbital analysis clearly indicates that charge transfer (CT) occurs from the occupied interacting orbital of the metallic cation to the σ*-antibonding orbital of the first, third, and fifth activated C-H bonds in transition states. The synergistic effects of heteronuclear bimetallic cations result in the destabilization of the occupied interacting orbital in bimetallic cations, which raise the reactivity of bimetallic cations and enhance the CT between catalysts and substrates. Contrary to the absence of the third dehydrogenation product in the mononuclear metallic cation catalytic reaction, a significant amount of the third dehydrogenation product is observed in the presence of heteronuclear cations (NiCu⁺ and CoNi⁺). π back-bonding between Ni of heteronuclear metallic cations and the substrate cyclohexadiene plays an essential role in lowering the energies of transition states, which accelerate the third dehydrogenation. The reasons why heteronuclear bimetallic cations are more reactive than monometallic ones are discussed in detail.

Gas-Phase Reactivity Studies of Small Molybdenum Cluster Ions with Dimethyl Disulfide

Molybdenum sulfide is a potent hydrogen evolution catalyst, and is discussed as a replacement of platinum in large-scale electrochemical hydrogen production. To learn more about the elementary steps of MoS₂ production by sputtering in the presence of dimethyl disulfide (DMDS), the reactions of Mo_x⁺, x = 1–3, with DMDS are studied by Fourier transform ion cyclotron resonance mass spectrometry and density functional theory calculations. A rich variety of products composed of molybdenum, sulfur, carbon and hydrogen was observed. Mo_xS_y⁺ species are formed in the first reaction step, together with products containing carbon and hydrogen. The calculations indicate that the strong Mo-S bonds are formed preferentially, followed by Mo–C bonds. Hydrogen is exclusively bound to carbon atoms, i.e. no insertion of a molybdenum atom into a C–H bond is observed. The reactions are efficient and highly exothermic, explaining the rich chemistry observed in the experiment.

The dehydrogenation of alcohols and hydrocarbons by atomic metal anions

The reactivity of anionic metal-carbonyl systems toward hydrocarbons, alcohols and a variety of other classes of molecules is well established in the literature. In this study we explored the reactions of atomic metal anions M(-), notably K(-), Cs(-), Co(-), Fe(-), Cu(-) and Ag(-), with alcohols, alkanes, alkenes and alkynes. All of the metal anions deprotonated the alcohols and alkynes. Also observed were the subsequent reactions of the resulting organic anions. Fe(-) and Cu(-) consistently displayed mono- and bis-dehydrogenation of primary and secondary alcohols, and alkanes, alkenes and alkynes to form MH(-) and MH(2)(-). Mechanisms for the dehydrogenation reactions are proposed and substantiated with isotopically-labelled reagents and thermochemical arguments.

Activation of propane C-H and C-C bonds by gas-phase Pt atom: a theoretical study

The reaction mechanism of the gas-phase Pt atom with C₃H₈ has been systematically investigated on the singlet and triplet potential energy surfaces at CCSD(T)//BPW91/6-311++G(d, p), Lanl2dz level. Pt atom prefers the attack of primary over secondary C-H bonds in propane. For the Pt + C₃H₈ reaction, the major and minor reaction channels lead to PtC₃H₆ + H₂ and PtCH₂ + C₂H₆, respectively, whereas the possibility to form products PtC₂H₄ + CH₄ is so small that it can be neglected. The minimal energy reaction pathway for the formation of PtC₃H₆ + H₂, involving one spin inversion, prefers to start at the triplet state and afterward proceed along the singlet state. The optimal C-C bond cleavages are assigned to C-H bond activation as the first step, followed by cleavage of a C-C bond. The C-H insertion intermediates are kinetically favored over the C-C insertion intermediates. From C-C to C-H oxidative insertion, the lowering of activation barrier is mainly caused by the more stabilizing transition state interaction ΔE^≠_int, which is the actual interaction energy between the deformed reactants in the transition state.