On the Origin of Reactivity Enhancement/Suppression upon Sequential Ligation: [Re(CO)x ]+ /CH4 (x=0-3) Couples.

The thermal gas-phase reactions of rhenium carbonyl complexes [Re(CO)x ]+ (x=0-3) with methane have been explored by using FT-ICR mass spectrometry complemented by high-level quantum chemical calculation. While it had been concluded in previous studies that addition of closed-shell ligands in general decreases the reactivity of metal ions, the current work provides an exception: the previously demonstrated inertness of atomic Re+ towards methane is completely changed upon ligation with CO. Both [Re(CO)]+ and [Re(CO)2 ]+ bring about efficient dehydrogenation of the hydrocarbon under ambient conditions. However, addition of a third ligand to form [Re(CO)3 ]+ completely quenches the reactivity.