The “Missing Link”: The Gas-Phase Generation of Platinum-Methylidyne Clusters PtnCH+ (n=1, 2) and Their Reactions with Hydrocarbons and Ammonia

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Abstract
Electrospray ionization (ESI) of tetrameric platinum(II) acetate, [Pt4(CH3COO)8], in methanol generates the formal platinum(III) dimeric cation [Pt2(CH3COO)3(CH2COO)(MeOH)2]+, which, upon harsher ionization conditions, sequentially loses the two methanol ligands, CO2, and CH2COO to form the platinum(II) dimer [Pt2(CH3COO)2(CH3)]+. Next, intramolecular sequential double hydrogen-atom transfer from the methyl group concomitant with the elimination of two acetic acid molecules produces Pt2CH+ from which, upon even harsher conditions, PtCH+ is eventually generated. This degradation sequence is supported by collision-induced dissociation (CID) experiments, extensive isotope-labeling studies, and DFT calculations. Both PtCH+ and Pt2CH+ react under thermal conditions with the hydrocarbons C2Hn (n=2, 4, 6) and C3Hn (n=6, 8). While, in ion–molecule reactions of PtCH+ with C2 hydrocarbons, the relative rates decrease with increasing n, the opposite trend holds true for Pt2CH+. The Pt2CH+ cluster only sluggishly reacts with C2H2, but with C2H4 and C2H6 dihydrogen loss dominates. The reactions with the latter two substrates were preceded by a complete exchange of all of the hydrogen atoms present in the adduct complex. The PtCH+ ion is much less selective. In the reactions with C2H2 and C2H4, elimination of H2 occurs; however, CH4 formation prevails in the decomposition of the adduct complex that is formed with C2H6. In the reaction with C2H2, in addition to H2 loss, C3H3+ is produced, and this process formally corresponds to the transfer of the cationic methylidyne unit CH+ to C2H2, accompanied by the release of neutral Pt. In the ion–molecule reactions with the C3 hydrocarbons C3H6 and C3H8, dihydrogen loss occurs with high selectivity for Pt2CH+, but in the reactions of these substrates with PtCH+ several reaction routes compete. Finally, in the ion–molecule reactions with ammonia, both platinum complexes give rise to proton transfer to produce NH4+; however, only the encounter complex generated with PtCH+ undergoes efficient dehydrogenation of the substrate, and the rather minor formation of CNH4+ indicates that CN bond coupling is inefficient.

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