Maria Schlangen

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.

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)].

Effects of Ligands, Cluster Size, and Charge State in Gas-Phase Catalysis: A Happy Marriage of Experimental and Computational Studies

We present selected examples of gas-phase reactions which are of timely interest for the activation of small molecules. Due to the very nature of the experiments, detailed insight in the active site of catalysts is provided and—in combination with computational chemistry—mechanistic aspects of as well as the elementary steps involved in the making and breaking of chemical bonds are revealed.Graphical Abstract

Gas-Phase C−H and N−H Bond Activation by a High Valent Nitrido-Iron Dication and 〈NH〉-Transfer to Activated Olefins

A tetrapodal pentadentate nitrogen ligand (2,6-bis(1,1-di(aminomethyl)ethyl)pyridine, 1) is used for the synthesis of the azido-iron(III) complex [(1)Fe(N3)]X2 where X is either Br or PF6. By means of electrospray ionization mass spectrometry, the dication [(1)Fe(N3)]2+ can be transferred into the gas phase as an intact entity. Upon collisional activation, [(1)Fe(N3)]2+ undergoes an expulsion of molecular nitrogen to afford the dicationic nitrido-iron species [(1)FeN]2+ as an intermediate, which upon further activation can intramolecularly activate C-H- and N-H bonds of the chelating ligand 1 or can transfer an NH unit in bimolecular reactions with activated olefins. The precursor dication [(1)Fe(N3)]2+, the resulting nitrido species [(1)FeN]2+, and its possible isomers are investigated by mass spectrometric experiments, isotopic labeling, and complementary computational studies using density functional theory.

Structure of the oxygen-rich cluster cation Al2O7+ and its reactivity toward methane and water.

The oxygen-rich cluster Al(2)O(7)(+) is generated in the gas phase and investigated with respect to both its structure and its reactivity toward small, inert molecules using Fourier-transform ion cyclotron resonance (FT-ICR) mass spectrometry and DFT-based calculations. Al(2)O(7)(+) reacts with CH(4) under ambient conditions via hydrogen atom transfer (HAT), and with H(2)O a ligand exchange occurs which gives rise to the evaporation of two O(2) molecules. The resulting product ion Al(2)O(4)H(2)(+) is also capable of abstracting a hydrogen atom from both H(2)O and CH(4). As indicated in the H(2)O/2O(2) ligand exchange and supported by collision-induced dissociation (CID) experiments, two O(2) units constitute structural elements of Al(2)O(7)(+). Further insight is provided by DFT calculations, performed at the unrestricted B3LYP/TZVP level, and reaction mechanisms are suggested on the basis of both the experimental and theoretical results.

Conversion of Methane to Methanol: Nickel, Palladium, and Platinum (d9) Cations as Catalysts for the Oxidation of Methane by Ozone at Room Temperature

The room-temperature chemical kinetics has been measured for the catalytic activity of Group 10 atomic cations in the oxidation of methane to methanol by ozone. Ni(+) is observed to be the most efficient catalyst. The complete catalytic cycle with Ni(+) is interpreted with a computed potential energy landscape and, in principle, has an infinite turnover number for the oxidation of methane, without poisoning side reactions. The somewhat lower catalytic activity of Pd(+) is reported for the first time and also explored with DFT calculations. Pt(+) is seen to be ineffective as a catalyst because of the observed failure of PtO(+) to convert methane to methanol.

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.

Electronic Origins of the Variable Efficiency of Room-Temperature Methane Activation by Homo- and Heteronuclear Cluster Oxide Cations [XYO2]+ (X, Y = Al, Si, Mg): Competition between Proton-Coupled Electron Transfer and Hydrogen-Atom Transfer

The reactivity of the homo- and heteronuclear oxide clusters [XYO2](+) (X, Y = Al, Si, Mg) toward methane was studied using Fourier transform ion cyclotron resonance mass spectrometry, in conjunction with high-level quantum mechanical calculations. The most reactive cluster by both experiment and theory is [Al2O2](•+). In its favorable pathway, this cluster abstracts a hydrogen atom by means of proton-coupled electron transfer (PCET) instead of following the conventional hydrogen-atom transfer (HAT) route. This mechanistic choice originates in the strong Lewis acidity of the aluminum site of [Al2O2](•+), which cleaves the C-H bond heterolytically to form an Al-CH3 entity, while the proton is transferred to the bridging oxygen atom of the cluster ion. In addition, a comparison of the reactivity of heteronuclear and homonuclear oxide clusters [XYO2](+) (X, Y = Al, Si, Mg) reveals a striking doping effect by aluminum. Thus, the vacant s-p hybrid orbital on Al acts as an acceptor of the electron pair from methyl anion (CH3(-)) and is therefore eminently important for bringing about thermal methane activation by PCET. For the Al-doped cluster ions, the spin density at an oxygen atom, which is crucial for the HAT mechanism, acts here as a spectator during the course of the PCET mediated C-H bond cleavage. A diagnostic plot of the deformation energy vis-à-vis the barrier shows the different HAT/PCET reactivity map for the entire series. This is a strong connection to the recently discussed mechanism of oxidative coupling of methane on magnesium oxide surfaces proceeding through Grignard-type intermediates.

On the origin of the surprisingly sluggish redox reaction of the N2O/CO couple mediated by [Y2O2]+˙ and [YAlO2]+˙ cluster ions in the gas phase.

Catalytic conversion of harmful gases produced in fossil-fuel combustion or in large-scale chemical transformations, such as CO or the oxides of nitrogen into nitrogen and carbon dioxide, is of utmost importance both environmentally and economically. For example, N2O is a potent greenhouse gas with a warming potential exceeding that of CO2 by a factor of 300,1 and its role in the depletion of stratospheric ozone is well known.2 While these redox reactions are exothermic, for example ΔrH=−357 kJ mol−1 for the process N2O + CO→N2 + CO2, they do not occur directly to any measurable extent at either room or elevated temperatures because of high energy barriers that exceed the 193 kJ mol−1 for the N2O/CO couple. Catalysts are required to open-up new, energetically more favorable pathways,3 and the first example of a homogeneous catalysis in the gas phase, whereby atomic transition-metal cations bring about the efficient reduction of N2O by CO, was reported in a landmark study by Kappes and Staley,4 which was followed in the ensuing decades by numerous investigations.5 Recently, these studies addressed more specific questions, for example, “catalyst poisoning”, and these experiments revealed remarkable effects of both the cluster size and the charge state of the catalysts.6 For example, the active species of the Pt7+ cluster are Pt7+, [Pt7O]+, [Pt7O2]+, and [Pt7(CO)]+ and it has a turnover number >500 at room temperature. The adsorption of more than one CO molecule onto the Pt7+ cluster, however, completely quenches the catalytic activity. Thus, coverage effects for any cluster sizes can be studied at a strictly molecular level. Similarly, the concept of “single-site catalysts”,7 the proper characterization and identification of which constitutes one of the challenges and intellectual cornerstones in contemporary catalysis, can be probed directly in gas-phase experiments with mass-selected heteronuclear metal-oxide clusters. For example, catalytic room-temperature oxidation of CO by N2O can be mediated by the bimetallic oxide cluster couple [AlVO4]+./[AlVO3]+..8 In the presence of CO, the cluster ion [AlVO4]+. is efficiently reduced to [AlVO3]+., and if N2O is added, the reverse reaction occurs. Both processes are clean and proceed with efficiencies (ϕ) of 59 % and 65 %, respectively, relative to the collision rates. Most interestingly, the two redox reactions occur at the Al-Ot. unit of the cluster (Ot: terminal oxygen atom); bond activation involving the V—O moiety cannot compete kinetically and thermochemically. Thus, the existence and operation of an “active site” of a catalyst can already be demonstrated in a rather small heteronuclear cluster.9