Mechanistic Variants in Methane Activation Mediated by Gold(I) Supported on Silicon Oxide Clusters.

Cationic gold has been frequently identified as a suitable reactive species for activating methane in condensed-phase studies. However, it is far from clear how the coordination site manipulates the activity of such species. Herein, by anchoring Au on silicon oxide cluster supports of variable sizes, the site-specific methane activation by Au -O has been clarified by mass spectrometry in conjunction with quantum chemistry calculations. An unexpected mechanistic switch in C-H activation was identified for the cluster anions Au(SiO ) O (n=1-3) that selectively activate one of the four C-H bonds of methane with different reaction efficiencies: a low efficiency was observed for the two-fold-coordinated gold ion (Au ), which was anchored on an AuSiO or AuSi O cluster, through an oxidative addition mechanism (a homolytic process), and high efficiency was observed for the one-fold-coordinated gold ion (Au ), which was supported on an AuSi O cluster, through Lewis acid/base pairs mechanism (Au ⋅⋅⋅O , a heterolytic process). Fine regulation of the 5d orbital level of the Au atom by the oxygen ligands accounted for the mechanistic difference between Au and Au species. The mechanistic understanding of the reactivity of Au -O at a strictly molecular level can be used to clarify the dissimilar activity of gold anchored on different oxide supports.