Helmut Schwarz

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.

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

“Rollover” cyclometalation – early history, recent developments, mechanistic insights and application aspects

“Rollover” cyclometalation constitutes a special case among the well-known class of cyclometalation reactions. An overview is given that covers the very first description of this reaction type, as well as recent developments. In addition, not only condensed-phase experiments are reviewed, but also investigations based on mass spectrometric techniques, together with “in silico” studies using DFT-based calculations are considered. While the latter two methods allow for a detailed analysis of the intrinsic factors that affect the reaction mechanisms, consideration of all three regimes permits to develop a coherent mechanistic picture and to address the often noted gap between condensed- and gas-phase studies. Moreover, the quite unexpected reactivity of “rollover” cyclometalated complexes in gas-phase experiments, as well as potential applications, e.g. in synthetic procedures, are discussed in some detail.

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).

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

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.

Thermochemistry of small cationic iron–sulfur clusters

The kinetic energy dependences of the reactions of Fen+ with COS (n=2–6) and CS2 (n=2–5) are studied in a guided-ion beam tandem mass-spectrometer. The main products arise from sulfur transfer and subsequent losses of Fe atoms. In the case of CS2, this reactant also formally replaces one Fe atom of the cluster to form Fen−1CS2+ with losses of further Fe atoms at elevated energies. In addition, the kinetic energy dependences of the reactions of FenS+ (n=2–4) with Xe and CS2 are studied. The former system yields collision-induced dissociations, whereas the latter reagent effects sulfur transfer accompanied by subsequent losses of Fe atoms. Analyses of the cross sections for endothermic reactions yield the bond energies D0(Fen+–S), n=2–5, D0(SFen−1+–Fe), n=2–5, D0(SFen+−S), n=1–3, and D0(S2Fen−1+–Fe), n=2, 3, as well as the ionization energy IE(Fe2S2). These values are derived with explicit consideration of the lifetimes of the energized reaction intermediates. The binding between sulfur and the cluster core...

Thermal ammonia activation by cationic transition-metal hydrides of the first row--small but mighty.

The thermal reactions of cationic 3d transition-metal hydrides MH(+) (M=Sc-Zn, except V and Cu) with ammonia have been studied by gas-phase experiments and computational methods. There are three primary reaction channels: 1) H(2) elimination by N-H bond activation, 2) ligand exchange under the formation of M(NH(3))(+), and 3) proton transfer to yield NH(4)(+). Computational studies of these three reaction channels have been performed for the couples MH(+)/NH(3) (M=Sc-Zn) to elucidate mechanistic aspects and characteristic reaction patterns of the first row. For N-H activation, σ-bond metathesis was found to be operative.