Activation of Methanes by Doubly Charged Metal Dication Complexes in the Gas Phase

Joseph K Koka, Lifu Ma, Anthon

Abstract

A doubly charged metal dication complexes [Mn(Pyridine)4]2+ ions were formed and further activated with methane in the gas phase. [Mn(Pyridine)CH4]2+, [Mn(Pyridine)2CH4]2+, [Mn(Pyridine)3CH4]2+, [MnPy2Me2]2+ [MnPy3Me2]2+ and [Mn(Pyridine)4CH4]2+ were successfully identified after the experimental analysis. The average pyridine binding energy, calculated at the different levels of theory of zero point energy are in realistic agreement. The binding energies of methane decrease drastically with increasing number of pyridine ligands in the order of [Mn(Pyridine)CH4]2+>[Mn(Pyridine)2CH4]2+>[Mn(Pyridine)3CH4]2+> [Mn(Pyridine)4CH4]2+. The optimised structure of [Mn(Pyridine)4(CH4)]2+ indicated C1 symmetry with the methane-manganese ion distance approximately r=2.00 Å. DFT calculated binding energy of methane with Mn2+ was 15.13 kJ/mol while the binding energy calculated on the potential energy curve (PEC) was 17.37 kJ/mol. This higher interaction energy calculated from the PEC was due to the fact that the charge +2 was assumed for the metal ion in the calculations, but at the optimised geometry the DFT calculated actual charge on the metal ion was (Z=1.43). The calculated binding energy of methane with manganese pyridine dication complex ion was drastically overestimated on the PEC by approximately 15% compare to DFT.