Oxo ligand as a reactivity switch in gas-phase ion chemistry of the lanthanides

The reactions of 'bare' as well as oxo-ligated lanthanide cations with buta-1,3-diene have been systematically investigated. Only those lanthanides with two non-f electrons in their electronic ground state (La, Ce, Gd, Lu) and those which exhibit the lowest excitation energies to such states (Pr, Tb) are able to activate butadiene as 'bare' cations. Dehydrogenation of the organic substrate, loss of ethylene and formation of a butadiene complex (only Lu+) are the only primary product channels observed, in line with an insertion-elimination mechanism. Upon addition of an oxygen ligand, the lanthanides with the lowest bond energies to oxygen, EuO+ and YbO+, preferentially react by transferring the oxygen atom to the hydrocarbon substrate. The reactive Ln(+) becomes inert upon addition of an oxygen ligand, whereas the cationic oxides LnO(+) of the unreactive lanthanides Dy, Ho, Pr and Tm activate butadiene. Besides loss of acetylene, the same products as in the reactions of 'bare' Ln(+) are obtained. However, based on a correlation of the reaction rates with the ionisation energies of LnO, a completely different mechanism is proposed for the initial activation step: following an electrophilic attack of LnO(+) on the pi-system of the diene, a cationic metalla-oxa cyclohexene is formed as the key intermediate, and this step represents a formal Diels-Alder cycloaddition with LnO(+) acting as a dienophile. The mechanism is further substantiated by additional experimental investigations on LnO(+)-isoprene as well as Ln(+)-dihydrofuran and Ln(+)-tetrahydrofuran systems.