Hans H. Cornehl

The performance of density‐functional/Hartree–Fock hybrid methods: Cationic transition‐metal methyl complexes MCH+3 (M=Sc–Cu,La,Hf–Au)

Hybrid methods, including a mixture of Hartree–Fock exchange and density functional exchange‐correlation treatment have been applied to the cationic methyl complexes MCH+3 of the first and third‐row transition metals (M=Sc–Cu,La,Hf–Au). Bond dissociation energies and optimum geometries obtained with the ‘‘Becke‐Half‐and‐Half‐Lee–Yang–Parr’’ and ‘‘Becke‐3‐Lee–Yang–Parr’’ functionals and from calibration calculations employing quadratic configuration interaction with single and double excitations and with a perturbative estimate of triple excitations are reported. A comparison of the results for the 3d‐block species to earlier high‐level ab initio calculations and experimental data is carried out in order to assess the reliability of hybrid methods as a practical tool in organometallic chemistry. Furthermore, the bond dissociation energies of the cationic 5d‐block transition‐metal methyl complexes, many of which have not been investigated so far, are predicted.

Hydrocarbon activation by “bare” uranium cations: formation of a cationic uranium-benzene complex from three ethylene units☆

The ability of the atomic uranium cation U+ to activate a variety of saturated and unsaturated hydrocarbon molecules is investigated. Both CH and CC bond activation processes proceeds at markedly higher kinetic efficiencies compared with the lower congener Nd+ from the lanthanide series. Formation of a cationic uranium-benzene complex occurs in three consecutive dehydrogenation reactions between U+ and ethylene. The mechanism of this metal-mediated cyclotrimerization is enlightened by kinetic measurements, collision-induced dissociation experiments and ion-molecule reactions of the intermediate species.

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, Er 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 π-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.