Reactions of platinum-carbene clusters PtnCH2+ (n=1-5) with O2, CH4, NH3, and H2O:: Coupling processes versus carbide formation

The gas-phase reactions of the platinum-carbene clusters PtnCH2+ (n = 2-5) with the substrates O-2, CH4, NH3, and H2O have been investigated by FT-ICR mass spectrometry and compared with previous results for the mononuclear homologue PtCH2+. The ion-molecule reactions of the clusters PtnCH2+ with O-2 and CH4 are similar to the corresponding processes observed for PtnCH2+. In contrast, a surprising difference evolves in the reactions with NH3 and H2O. Whereas PtCH2+ mediates carbon-heteroatoin bond formation as an attractive way toward methane functionalization, the homologous clusters fail in this regard and exclusively yield the carbide complexes PtnC+.NH3 and PtnC+.H2O, respectively. The differences in reactivities have been analyzed further by means of kinetic isotope effects, H/D-exchange reactions, and energy-dependent collision-induced dissociations of PtnCH2+. Essentially, the higher stabilities of the platinum-carbide clusters PtnC+ compared to PtC+ cause the change in reactivity. This conclusion is confirmed by the reactions of independently generated clusters PtnC+ (n = 1-5) with O-2, CH4, NH3, and H2O. The results underline the importance of carbide structures in these gas-phase reactions, thereby resembling catalyst deactivation by soot formation on heterogeneous platinum catalysts.