Title : Menage a Trois Single atom Catalysis Mass Spectrometry and Computational Chemistry Helmut Schwarz
Abstract:
We shall present selected examples of gas-phase reactions which are of timely interest for the catalytic 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 spectroscopic studies and computational chemistry – mechanistic aspects of as well as the elementary steps involved in the making and breaking of chemical bonds are revealed.
Examples to be discussed include inter alia: (i) Metal-mediated carbon-carbon bond formation; (ii) low- temperature, catalytic oxidation of CO; or (iii) the coupling of NH3 and CH4 to produce HCN. Of particular importance are the reactions of "bare" metal-carbene complexes, when generated in the gas phase and exposed to thermal reactions under (near) single-collision conditions. In addition to the well-known metathesis and cyclopropanation processes, they exhibit rather unique reactivities. For example, at room temperature the unligated [AuCH2]+ complex brings about efficient C?C coupling with methane to produce C2Hx (x = 4, 6), or the couple [TaCH2]+/CO2 gives rise to the generation of the acetic acid equivalent CH2=C=O. Entirely unprecedented is the thermal carbon-atom extrusion from halobenzenes (X = F, Cl, Br, I) by [MCH2]+ (M = La, Hf, Ta, W, Re, and Os) and its coupling with the methylene ligand to deliver C2H2 and [M(X)(C5H5)]+. Among the many noteworthy C?N bond forming processes, the formation of CH3NH2 from [RhCH2]+/NH3, the generation of CH2=NH + from [MCH2]+/NH3 (M = Pt, Au), or the production of [PtCH=NH2]+ from [PtCH2]+/NH3 are of particular interest. The latter species are likely to be involved as intermediates in the platinum-mediated, large-scale production of HCN from CH4/NH3 (DEGUSSA process). In this context, a few examples are presented that point to the operation of co-operative effects even at a molecular level. For instance, in the coupling of CH4 with NH3 by the heteronuclear clusters [MPt]+ (M = coinage metal), platinum is crucial for the activation of methane, while the coinage metal M keeps control on the branching ratio between the C?N bond forming step and unwanted soot formation.
It will be shown that mass-spectrometry based studies on 'isolated' reactive species provide an ideal arena for probing experimentally the energetics and kinetics of a chemical reaction in an unperturbed environment at a strictly molecular level, and thus enable the characterization of crucial intermediates that have previously not been within the reach of conventional condensed-phase techniques. Clearly, these investigations open the door to a widely uncharted territory of chemistry, a field in which "each atom counts"
