Contrasting Mechanisms and Reactivity of Tl(III), Hg(II), and Co(III) for Alkane C鈥揌 Functionalization
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- 作者：Samantha J. Gustafson ; Jack T. Fuller III ; Deepa Devarajan ; Justin Snyder ; Roy A. Periana ; Brian G. Hashiguchi ; Michael M. Konnick ; Daniel H. Ess
- 刊名：Organometallics
- 出版年：2015
- 出版时间：November 23, 2015
- 年：2015
- 卷：34
- 期：22
- 页码：5485-5495
- 全文大小：589K
- ISSN：1520-6041
文摘
Activation and functionalization of alkane C鈥揌 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鈥揌 activation and Tl鈥揳lkyl functionalization mechanisms. The discovery of a C鈥揌 activation pathway is surprising, because TlIII often oxidizes arene C鈥揌 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鈥揌 bond activation that is lower in energy than open-shell pathways and a subsequent metal鈥揳lkyl 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鈥揌 activation pathway and transition state model provide a straightforward explanation for why TlIII(TFA)3 promotes alkane C鈥揌 bond activation but HgII(TFA)2 does not.