Christian van der Linde

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)].

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.

Reactions of M(+)(H2O)n, n < 40, M = V, Cr, Mn, Fe, Co, Ni, Cu, and Zn, with D2O reveal water activation in Mn(+)(H2O)n.

Reactions of M(+)(H(2)O)(n), n < 40, M = V, Cr, Mn, Fe, Co, Ni, Cu, and Zn, with D(2)O are studied by Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. Isotopically highly enriched metals are used as applicable. Isotopic scrambling with formation of HDO is not observed for M = Cr, Fe, Co, Ni, Cu, and Zn, which indicates that these hydrated metal ions consist of a singly charged metal center and a hydration shell of intact, inactivated water molecules. In the vanadium case, HDO formation is observed in the size region where also hydroxide formation with evolution of molecular hydrogen occurs. For manganese, HDO formation occurs in the size regime n ≈ 8-20. Additional experiments show that, in this size regime, Mn(+)(H(2)O)(n) is slowly converted into HMnOH(+)(H(2)O)(n-1) under the influence of room temperature blackbody radiation. The reaction is mildly exothermic; ΔH ≈ -21 ± 10 kJ mol(-1).

Hydrated Magnesium Cations Mg+(H2O)n, n ≈ 20–60, Exhibit Chemistry of the Hydrated Electron in Reactions with O2 and CO2

Ion-molecule reactions of Mg(+)(H(2)O)(n), n ≈ 20-60, with O(2) and CO(2) are studied by Fourier-transform ion cyclotron resonance (FT-ICR) mass spectrometry. O(2) and CO(2) are taken up by the clusters. Both reactions correspond to the chemistry of hydrated electrons (H(2)O)(n)(-). Density functional theory calculations predicted that the solvation structures of Mg(+)(H(2)O)(16) contain a hydrated electron that is solvated remotely from a hexa-coordinated Mg(2+). Ion-molecule reactions between Mg(+)(H(2)O)(16) and O(2) or CO(2) are calculated to be highly exothermic. Initially, a solvent-separated ion pair is formed, with the hexa-coordinated Mg(2+) ionic core being well separated from the O(2)(•-) or CO(2)(•-). Rearrangements of the solvation structure are possible and produce a contact-ion pair in which one water molecule in the first solvation shell of Mg(2+) is replaced by O(2)(•-) or CO(2)(•-).

The [Aun,Si]−, n=1–4, potential energy surface: Competition between Au–Si and Au–Au bonding

A systematic theoretical investigation of the structure of anionic gold-silicon clusters Au(n)Si(-), n=1-4, has been carried out. The two lowest-lying spin states and all topologically possible connectivities were considered for n=1-3. For n=4, the doublet state and the majority of low-lying geometries were taken into account. Transition states connecting the local minima were identified. Calculation of the density of states of the minima shows that, while Au-Si bonds are enthalpically preferred, the low vibrational frequency of Au-Au bonds makes those entropically favored. The high symmetry of the minimum structures reduces their density of states, up to a factor of 12 for the aurosilane Au(4)Si(-). Under experimental conditions of rapid cooling and inefficient annealing, the Au(2)SiAuAu(-) isomer of C(s) symmetry can be expected to be as abundant as the aurosilane Au(4)Si(-) of T(d) symmetry in the gas phase.

Insights into Gas-Phase Structural Conformers of Hydrated Rubidium and Cesium Cations, M+(H2O)nAr (M = Rb, Cs; n = 3–5), Using Infrared Photodissociation Spectroscopy

Infrared photodissociation (IRPD) spectra of M(+)(H2O)nAr (M = Rb, Cs; n = 3-5) with simultaneous monitoring of [Ar] and [Ar+H2O] fragmentation channels are reported. The comparison between the spectral features in the two channels and corresponding energy analysis provide spectral assignments of the stable structural conformers and insight into the competition between ion-water electrostatic and water-water hydrogen bonding interactions. Results show that as the level of hydration increases, the water-water interaction exhibits the tendency to dominate over the ion-water interaction. Cyclic water tetramer and water pentamer substructures appear in Cs(+)(H2O)4Ar and Cs(+)(H2O)5Ar systems, respectively. However, cyclic water tetramer and pentamer structures were not observed for Rb(+)(H2O)4Ar and Rb(+)(H2O)5Ar systems, respectively, due to the stronger influence of the rubidium ion-water electrostatic interaction. The energy analysis, including the available internal energy and the IR photon energy, helped provide an experimental estimate of water binding energies.

Reactions of hydrated singly charged first-row transition-metal ions M+(H2O)n (M=V, Cr, Mn, Fe, Co, Ni, Cu, and Zn) toward nitric oxide in the gas phase.

Reactions of M(+) (H2 O)n (M=V, Cr, Mn, Fe, Co, Ni, Cu, Zn; n≤40) with NO were studied by Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. Uptake of NO was observed for M=Cr, Fe, Co, Ni, Zn. The number of NO molecules taken up depends on the metal ion. For iron and zinc, NO uptake is followed by elimination of HNO and formation of the hydrated metal hydroxide, with strong size dependence. For manganese, only small HMnOH(+) (H2 O)n-1 species, which are formed under the influence of room-temperature black-body radiation, react with NO. Here NO uptake competes with HNO formation, both being primary reactions. The results illustrate that, in the presence of water, transition-metal ions are able to undergo quite particular and diverse reactions with NO. HNO is presumably formed through recombination of a proton and (3) NO(-) for M=Fe, Zn, preferentially for n=15-20. For manganese, the hydride in HMnOH(+) (H2 O)n-1 is involved in HNO formation, preferentially for n≤4. The strong size dependence of the HNO formation efficiency illustrates that each molecule counts in the reactions of small ionic water clusters.

Photodissociation and photochemistry of V+(H2O)n, n = 1–4, in the 360–680 nm region

Photodissociation and photochemistry of V+(H2O)n, n = 1–4, was studied in 360–680 nm region using a Fourier transform ion cyclotron resonance mass spectrometer. The light of a high pressure mercury arc lamp was filtered using a set of bandpass filters with the center wavelengths from 360 to 680 nm in steps of 20 nm. The bandwidth of the filters, defined as full width at half maximum, was 10 nm. Photodissociation channels were attributed to loss of water molecules as well as atomic or molecular hydrogen, possibly accompanied by loss of water molecules. The most intense absorptions were red-shifted with increasing hydration. Theoretical spectra were calculated using time-dependent density functional theory. Calculations reproduced all the features of the experimental spectra, including the red shift with increasing hydration shell and the overall pattern of strong and weak absorption peaks.

Electrons Mediate the Gas-Phase Oxidation of Formic Acid with Ozone

Gas-phase reactions of CO3 (.-) with formic acid are studied using Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. Signal loss indicates the release of a free electron, with the formation of neutral reaction products. This is corroborated by adding traces of SF6 to the reaction gas, which scavenges 38 % of the electrons. Quantum chemical calculations of the reaction potential energy surface provide a reaction path for the formation of neutral carbon dioxide and water as the thermochemically favored products. From the literature, it is known that free electrons in the troposphere attach to O2 , which in turn transfer the electron to O3 . O3 (.-) reacts with CO2 to form CO3 (.-) . The reaction reported here formally closes the catalytic cycle for the oxidation of formic acid with ozone, catalyzed by free electrons.

Hydration Leads to Efficient Reactions of the Carbonate Radical Anion with Hydrogen Chloride in the Gas Phase

The carbonate radical anion CO3•- is a key intermediate in tropospheric anion chemistry. Despite its radical character, only a small number of reactions have been reported in the literature. Here we investigate the gas-phase reactions of CO3•- and CO3•-(H2O) with HCl under ultrahigh vacuum conditions. Bare CO3•- forms OHCl•- with a rate constant of 4.2 × 10-12 cm3 s-1, which corresponds to an efficiency of only 0.4%. Hydration accelerates the reaction, and ligand exchange of H2O against HCl proceeds with a rate of 2.7 × 10-10 cm3 s-1. Quantum chemical calculations reveal that OHCl•- is best described as an OH• hydrogen bonded to Cl-, while the ligand exchange product is Cl-(HCO3•). Under tropospheric conditions, where CO3•-(H2O) is the dominant species, Cl-(HCO3•) is efficiently formed. These reactions must be included in models of tropospheric anion chemistry.