Mechanistic Study of the Oxidation of a Methyl Platinum(II) Complex with O2 in Water: PtIIMe-to-PtIVMe and PtIIMe-to-PtIVMe2 Reactivit
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- 作者：Anna V. Sberegaeva ; Wei-Guang Liu ; Robert J. Nielsen ; William A. Goddard ; III ; Andrei N. Vedernikov
- 刊名：Journal of the American Chemical Society
- 出版年：2014
- 出版时间：March 26, 2014
- 年：2014
- 卷：136
- 期：12
- 页码：4761-4768
- 全文大小：500K
- 年卷期：v.136,no.12(March 26, 2014)
- ISSN：1520-5126
文摘
The mechanism of oxidation by O2 of (dpms)PtIIMe(OH2) (1) and (dpms)PtIIMe(OH)鈭?/sup> (2) [dpms = di(2-pyridyl)methanesulfonate] in water in the pH range of 4鈥?4 at 21 掳C was explored using kinetic and isotopic labeling experiments. At pH 鈮?8, the reaction leads to a C1-symmetric monomethyl PtIV complex (dpms)PtIVMe(OH)2 (5) with high selectivity 鈮?7%; the reaction rate is first-order in [PtIIMe] and fastest at pH 8.0. This behavior was accounted for by assuming that (i) the O2 activation at the PtII center to form a PtIV hydroperoxo species 4 is the reaction rate-limiting step and (ii) the anionic complex 2 is more reactive toward O2 than neutral complex 1 (pKa = 8.15 卤 0.02). At pH 鈮?10, the oxidation is inhibited by OH鈥?/sup> ions; the reaction order in [PtIIMe] changes to 2, consistent with a change of the rate-limiting step, which now involves oxidation of complex 2 by PtIV hydroperoxide 4. At pH 鈮?12, formation of a C1-symmetric dimethyl complex 6, (dpms)PtIVMe2(OH), along with [(dpms)PtII(OH)2]鈭?/sup> (7) becomes the dominant reaction pathway (50鈥?0% selectivity). This change in the product distribution is explained by the formation of a Cs-symmetric intermediate (dpms)PtIVMe(OH)2 (8), a good methylating agent. The secondary deuterium kinetic isotope effect in the reaction leading to complex 6 is negligible; kH/kD = 0.98 卤 0.02. This observation and experiments with a radical scavenger TEMPO do not support a homolytic mechanism. A SN2 mechanism was proposed for the formation of complex 6 that involves complex 2 as a nucleophile and intermediate 8 as an electrophile.