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^I on silicon oxide cluster supports of variable sizes, the site-specific methane activation by Au^I -O_x 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(SiO₂ )_n 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^I, 2f ), which was anchored on an AuSiO₃ ^- or AuSi₂ O₅ ^- cluster, through an oxidative addition mechanism (a homolytic process), and high efficiency was observed for the one-fold-coordinated gold ion (Au^I, 1f ), which was supported on an AuSi₃ O₇ ^- cluster, through Lewis acid/base pairs mechanism (Au^I, 1f ⋅⋅⋅O^2- , 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^I, 2f and Au^I, 1f species. The mechanistic understanding of the reactivity of Au^I -O_x at a strictly molecular level can be used to clarify the dissimilar activity of gold anchored on different oxide supports.