Transition metal substituted aluminium oxide clusters such as MAl7O12+ (M=Fe, Co) are gas phase models for so-called “single atom catalysts” which have attracted increasing attention in recent years. Here, we deal with C–H bond activation by supported metal oxide catalysts and study the structure and reactivity of MAl7O12+ clusters towards methane. Comparison of calculated IR spectra (DFT, TPSSh functional) with the measured IR photodissociation spectra provides evidence that the substitution of Fe for Al in Al8O12+ (Figure) is isomorphous, but accompanied by a change of the oxidation state on the transition metal, which converts the reactive terminal Al−O bond with an oxygen radical site into a non-reactive terminal M=O double bond.. For M=Fe, the DFT predictions for the singlet and triplet states agree with experiments for both the IR spectrum and the reactivity towards methane, but these are not the lowest energy spin states. The lowest energy state is the quintet state for which the predicted IR spectrum does not agree with experiment. The results for [Al7CoO12]+ are in line with those of [Al7FeO12]+. This shows that Kohn-Sham density functional theory (DFT) is not predictive with regard to the relative stability of different spin states. Multi-reference wave-function calculations are required to correctly predict the lowest energy structure, but these cannot be converged for a system of this size. It is not sufficient to include into the active space the 3d states on M, and 2p states on the terminal O, the 2p states of the three other O atoms that are also bonded to the transition metal atom M cannot be ignored.

Effect of Fe and Co substitution on the structure, spectra and reactivity of Al8O12+ cluster cations: A challenge for quantum chemistry

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