Nicolas Dietl

Thermal Hydrogen‐Atom Transfer from Methane: The Role of Radicals and Spin States in Oxo‐Cluster Chemistry

Hydrogen-atom transfer (HAT), as one of the fundamental reactions in chemistry, is investigated with state-of-the-art gas-phase experiments in conjunction with computational studies. The focus of this Minireview concerns the role that the intrinsic properties of gaseous oxo-clusters play to permit HAT reactivity from saturated hydrocarbons at ambient conditions. In addition, mechanistic implications are discussed which pertain to heterogeneous catalysis. From these combined experimental/computational studies, the crucial role of unpaired spin density at the abstracting atom becomes clear, in distinct contrast to recent conclusions derived from solution-phase experiments.

Room-Temperature CH Bond Activation of Methane by Bare [P4O10].+†

No need for a metal: A combination of mass spectrometry and computational studies (density functional theory and coupled-cluster methods) shows that [P(4)O(10)](.+) is the first polynuclear nonmetal oxide cation that is capable of activating the C--H bond of methane at room temperature (see picture). This process represents a further example in the reactivity of oxygen-centered radicals.

Diatomic [CuO]+ and Its Role in the Spin-Selective Hydrogen- and Oxygen-Atom Transfers in the Thermal Activation of Methane†

The activation of methane and its subsequent conversion into more valuable feedstocks at ambient conditions is regarded as one of the major challenges in contemporary catalysis. In this context, two different transformations are of particular interest. The first one concerns the oxidative coupling of methane (OCM) to the C2 hydrocarbons ethane and ethylene using metal oxide based catalysts in heterogeneous catalysis; 2] the second process is the selective oxidation of methane to methanol, which is performed in nature by the methane monoxygenase (MMO) metalloenzymes. Soluble MMO (sMMO) contains a well-characterized doubly oxygenbridged di-iron cluster; in contrast, the reactivity of particulate MMO (pMMO), after a long controversy about the nature of its active site, has been shown to depend on copper. A useful approach to investigate model systems for oxygen-containing catalysts takes advantage of state-of-theart gas-phase experiments conducted in a mass spectrometer, in conjunction with computational studies; this combined experimental and theoretical approach provides insight into the elementary steps of these reactions at a molecular level and, thus, permits us to unravel detailed mechanistic aspects. For example, the efficient gas-phase activation of methane at room temperature has been demonstrated to be brought about by a variety of systems, including transitionand maingroup-metal oxides as well as some selected nonmetal oxides and mixed metal/nonmetal oxides; based on these studies, a rather detailed understanding of the intriguing mechanistic aspects has been arrived at. With respect to biological relevance, it was demonstrated twenty years ago that bare [FeO] is capable of activating methane at room temperature. The now well-established concept of two-state reactivity (TSR), which also proved important in describing the mechanisms of metalloenzymemediated reactions, is in fact based on a detailed analysis of the gas-phase reactions of this simple, diatomic reagent [FeO]. Yet, only recently has a complete description of the gas-phase conversion of methane to methanol by [FeO] been achieved; this elucidation was based on advanced gas-phase spectroscopy combined with rather high-level calculations. Further, while the detailed nature of the active copper oxide species in pMMO had been under debate for quite some time, 4, 12] bare [CuO] was predicted a decade ago to be a suitable, if not extremely powerful, candidate to mediate the methane to methanol conversion. 14] However, no gasphase experiments with bare [CuO] have been reported to date. The ligated cation [Cu(O)(phen)] (phen = 1,10-phenanthroline) brings about activation of small hydrocarbons, that is, propane or butane, but it is not powerful enough to attack the thermodynamically strong and kinetically inert C H bond of methane. Owing to the relatively low dissociation energy D0(Cu + O) = 130 kJ mol , it proved rather difficult to produce sufficient amounts of [CuO] to probe its reactivity in bond-activation processes, and various attempts to generate this cationic metal oxide by, for example, electrospray ionization mass spectrometry failed. 15] Thus, [CuO] is to date the only bare transition-metal oxide cation of the first row whose reactivity towards methane has not been experimentally investigated. Herein we present our results on 1) the successful formation of gaseous [CuO] and 2) its reactivity towards methane at thermal conditions. Briefly, [CuO] is generated by laser desorption/ionization from isotopically pure copper Cu targets, suitable for the laser-vaporization/ionization source of an FT-ICR mass spectrometer in the presence of a He/N2O plasma (for details about the instrumental setup, see the Experimental Section). As shown in Figure 1, [CuO] brings about efficient activation of methane at room temperature both by hydrogen abstraction [Eq. (1)] and by oxygenatom transfer [Eq. (2)]. Furthermore, the open-shell product cation [CuOH]C itself also homolytically cleaves the C H bond of a second methane molecule, thus giving rise to the formation of a closed-shell water complex [Eq. (3)].

Catalytic Redox Reactions in the CO/N2O System Mediated by the Bimetallic Oxide-Cluster Couple AlVO3+/AlVO4+†

Exhaustive studies: The exact reaction pathway of catalytic conversion of automobile exhaust gases, such as N 2O and CO, into N 2 and CO 2 is still not completely understood. Studying this reaction at room temperature using the bimetallic oxide cluster couple AlVO 3 +/AlVO 4 + in the gas phase shows that the active M-O t . site is located at the Al-bound and not the V-bound oxygen atom (see scheme, Al pink).

Structure and chemistry of the heteronuclear oxo-cluster [VPO4]•+: a model system for the gas-phase oxidation of small hydrocarbons.

The heteronuclear oxo-cluster [VPO4](•+) is generated via electrospray ionization and investigated with respect to both its electronic structure as well as its gas-phase reactivity toward small hydrocarbons, thus permitting a comparison to the well-known vanadium-oxide cation [V2O4](•+). As described in previous studies, the latter oxide exhibits no or just minor reactivity toward small hydrocarbons, such as CH4, C2H6, C3H8, n-C4H10, and C2H4, while substitution of one vanadium by a phosphorus atom yields the reactive [VPO4](•+) ion; the latter brings about oxidative dehydrogenation (ODH) of saturated hydrocarbons, e.g., propane and butane as well as oxygen-atom transfer (OAT) to unsaturated hydrocarbons, e.g. ethene, at thermal conditions. Further, the gas-phase structure of [VPO4](•+) is determined by IR photodissociation spectroscopy and compared to that of [V2O4](•+). DFT calculations help to elucidate the reaction mechanism. The results underline the crucial role of phosphorus in terms of C-H bond activation of hydrocarbons by mixed VPO clusters.

Thermal Homo‐ and Heterolytic CH Bond Activation of Ethane and Propane by Bare [P4O10].+: Regioselectivities, Kinetic Isotope Effects, and Density Functional Theory Based Potential‐Energy Surfaces

Mechanistic studies of regioselective bond activation, for example, primary versus secondary C H bonds in hydrocarbons, constitute a classical pursuit in physical organic and computational chemistry. For homolytic C H bond cleavages, brought about by halogen atoms or hydroxyl radicals, varying preferences have been noticed previously in favor of the weaker secondary C H bond. This holds also true in the thermal gas-phase reaction of bare [NiX] (X=F, Cl, Br) with propane [Eq. (1)].

Gas-Phase Reactions of Cationic Vanadium-Phosphorus Oxide Clusters with C2Hx (x=4, 6): A DFT-Based Analysis of Reactivity Patterns

The reactivities of the adamantane-like heteronuclear vanadium-phosphorus oxygen cluster ions [VxP4−xO10].+ (x=0, 2–4) towards hydrocarbons strongly depend on the V/P ratio of the clusters. Possible mechanisms for the gas-phase reactions of these heteronuclear cations with ethene and ethane have been elucidated by means of DFT-based calculations; homolytic C–H bond activation constitutes the initial step, and for all systems the P–O. unit of the clusters serves as the reactive site. More complex oxidation processes, such as oxygen-atom transfer to, or oxidative dehydrogenation of the hydrocarbons require the presence of a vanadium atom to provide the electronic prerequisites which are necessary to bring about the 2e− reduction of the cationic clusters.

Mechanistic Aspects of Gas‐Phase Hydrogen‐Atom Transfer from Methane to [CO].+ and [SiO].+: Why Do They Differ?

The reactivity of the two diatomic congeneric systems [CO](·+) and [SiO](·+) towards methane has been investigated by means of mass spectrometry and quantum-chemical calculations. While [CO](·+) gives rise to three different reaction channels, [SiO](·+) reacts only by hydrogen-atom transfer (HAT) from methane under thermal conditions. A theoretical analysis of the respective HAT processes reveals two distinctly different mechanistic pathways for [CO](·+) and [SiO](·+), and a comparison to the higher metal oxides of Group 14 emphasizes the particular role of carbon as a second-row p element.