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 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)3 to 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.

Direct Partial Oxidation of Methane Catalyzed by an In Situ Generated Active Au(III) Complex at Low Temperature in Ionic Liquids

“…The transition state (TS D ‡ ) bearing a C−H activated CH 4 was obtained by the QST3 method and found to have a sixmembered ring consisting of an activated CH 4 , the Au III center, and two oxygen atoms in the CF 3 SO 3 − ligand. This transition state has a free energy barrier of 26.7 kcal/mol from structure B. Six-membered metal cycle with activated Csp3-H structures have also been reported on metal atoms such as Pt, 79 Ir, 18 and Ti 21,80 H-bonding effect is considered to be minor without affecting the relative energies of the species in the reaction profiles. In addition to the five-coordinated structure, as described in Figure 3, monovalent cation structures in the C−H activation process were all calculated by DFT calculation.…”

A comparative study of inter- and intramolecular C–H aminations: mechanism and site selectivity

“…DFT calculations were performed with the Gaussian 09 package, 41 using the M06L pure functional mode, 42 which has been found to be accurate for the energies and geometries of related C-H functionalization systems including multiple pathways. 37,38,43,44 In terms of the mechanism and selectivity of amination reactions, the steric effect is an important inuencing factor, and sometimes even a governing factor. 3b,37, 45 The catalyst Rh II,II 2 (esp) 2 , involving two bulk esp ligands, would give rise to a considerable steric effect.…”

Electrophilic RhI catalysts for arene H/D exchange in acidic media: Evidence for an electrophilic aromatic substitution mechanism

“…In addition, these electrophilic catalysts are readily poisoned by water (or other Lewis bases) and can suffer from product inhibition [33]. More recently, main group elements and transition metal complexes have been shown to activate and functionalize hydrocarbons in less acidic, trifluoroacetic acid [35][36][37][38][39][40][41].…”