Organometallics ASAP
Contrasting Mechanisms and Reactivity of Tl(III), Hg(II), and Co(III) for Alkane C−H Functionalization
Samantha J. Gustafson,† Jack T. Fuller, III,† Deepa Devarajan,† Justin Snyder,† Roy A. Periana,‡ Brian G. Hashiguchi,‡ Michael M. Konnick,‡ and Daniel H. Ess*,†
† Department of Chemistry and Biochemistry, Brigham Young University, Provo, Utah 84602, United States
‡ Department of Chemistry, The Scripps Research Institute, Jupiter, Florida 33458, United States
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 TlIII(TFA)3 (TFA = trifluoroacetate). Here we present density-functional calculations which demonstrate that TlIII(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 TlIII often oxidizes arene C–H bonds through an electron transfer mechanism and the transition-metal complex CoIII(TFA)3, with similar oxidation state and ligand coordination, oxidizes alkanes via an open-shell radical mechanism. Comparison of TlIII(TFA)3to the transition-metal analogue IrIII(TFA)3 reveals that key to TlIII 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 CoIII(TFA)3 provides a low-energy open-shell decarboxylation pathway that leads to radical oxidation of alkanes, which is not available for the d10 TlIII(TFA)3 complex. The C–H activation pathway and transition state model provide a straightforward explanation for why TlIII(TFA)3 promotes alkane C–H bond activation but HgII(TFA)2 does not.
