Contrasting Mechanisms and Reactivity of Tl(III), Hg(II), and Co(III) for Alkane C–H Functionalization

Abstract

Activation and functionalization of alkane C–H bonds has historically been dominated by transition-metal complexes. Light alkanes can also be partially oxidized by sixth-row main-group compounds, such as Tl III (TFA) 3 (TFA = trifluoroacetate). Here we present density-functional calculations which demonstrate that Tl III (TFA) 3 oxidizes alkanes by closed-shell C–H activation and Tl–alkyl functionalization mechanisms. The discovery of a C–H activation pathway is surprising, because Tl III often oxidizes arene C–H bonds through an electron transfer mechanism and the transition-metal complex Co III (TFA) 3, with similar oxidation state and ligand coordination, oxidizes alkanes via an open-shell radical mechanism. Comparison of Tl III (TFA) 3 to the transition-metal analogue Ir III (TFA) 3 reveals that key to Tl III oxidation of alkanes is a moderate barrier for C–H bond activation that is lower in energy than open-shell pathways and a subsequent metal–alkyl functionalization reaction step with a very low barrier. Our calculations suggest that the high-spin ground state of Co III (TFA) 3 provides a low-energy open-shell decarboxylation pathway that leads to radical oxidation of alkanes, which is not available for the d 10 Tl III (TFA) 3 complex. The C–H activation pathway and transition state model provide a straightforward explanation for why Tl III (TFA) 3 promotes alkane C–H bond activation but Hg II (TFA) 2 does not.