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

http://pubs.acs.org/doi/abs/10.1021/acs.organomet.5b00849

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 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)₃to 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 d¹⁰ 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.