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 lant hanides 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 forma tion of a butadiene complex (only Lu+) are the only primary product ch annels observed, in line with an insertion-elimination mechanism. Upon addition of an oxygen ligand, the lanthanides with the lowest bond en ergies to oxygen, EuO+ and YbO+, preferentially react by transferring the oxygen atom to the hydrocarbon substrate. The reactive Ln(+) becom es inert upon addition of an oxygen ligand, whereas the cationic oxide s LnO(+) of the unreactive lanthanides Dy, Ho, Pr and Tm activate buta diene. Besides loss of acetylene, the same products as in the reaction s of 'bare' Ln(+) are obtained. However, based on a correlation of the reaction rates with the ionisation energies of LnO, a completely diff erent mechanism is proposed for the initial activation step: following an electrophilic attack of LnO(+) on the pi-system of the diene, a ca tionic metalla-oxa cyclohexene is formed as the key intermediate, and this step represents a formal Diels-Alder cycloaddition with LnO(+) ac ting as a dienophile. The mechanism is further substantiated by additi onal experimental investigations on LnO(+)-isoprene as well as Ln(+)-d ihydrofuran and Ln(+)-tetrahydrofuran systems.