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Published March 26, 2014 | Supplemental Material
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Mechanistic Study of the Oxidation of a Methyl Platinum(II) Complex with O_2 in Water: Pt^(II)Me-to-Pt^(IV)Me and Pt^(II)Me-to-Pt^(IV)Me_2 Reactivity


The mechanism of oxidation by O_2 of (dpms)Pt^(II)Me(OH_2) (1) and (dpms)Pt^(II)Me(OH)^− (2) [dpms = di(2-pyridyl)methanesulfonate] in water in the pH range of 4–14 at 21 °C was explored using kinetic and isotopic labeling experiments. At pH ≤ 8, the reaction leads to a C_1-symmetric monomethyl Pt^(IV) complex (dpms)Pt^(IV)Me(OH)_2 (5) with high selectivity ≥97%; the reaction rate is first-order in [Pt^(II)Me] and fastest at pH 8.0. This behavior was accounted for by assuming that (i) the O_2 activation at the Pt^(II) center to form a Pt^(IV) hydroperoxo species 4 is the reaction rate-limiting step and (ii) the anionic complex 2 is more reactive toward O_2 than neutral complex 1 (pK_a = 8.15 ± 0.02). At pH ≥ 10, the oxidation is inhibited by OH^– ions; the reaction order in [Pt^(II)Me] changes to 2, consistent with a change of the rate-limiting step, which now involves oxidation of complex 2 by Pt^(IV) hydroperoxide 4. At pH ≥ 12, formation of a C_1-symmetric dimethyl complex 6, (dpms)Pt^(IV)Me_2(OH), along with [(dpms)Pt^(II)(OH)_2]^− (7) becomes the dominant reaction pathway (50–70% selectivity). This change in the product distribution is explained by the formation of a C_s-symmetric intermediate (dpms)Pt^(IV)Me(OH)_2 (8), a good methylating agent. The secondary deuterium kinetic isotope effect in the reaction leading to complex 6 is negligible; k_H/k_D = 0.98 ± 0.02. This observation and experiments with a radical scavenger TEMPO do not support a homolytic mechanism. A S_N2 mechanism was proposed for the formation of complex 6 that involves complex 2 as a nucleophile and intermediate 8 as an electrophile.

Additional Information

© 2014 American Chemical Society. Received: February 4, 2014. Published: March 5, 2014. Publication Date (Web): March 5, 2014. This work was supported by the Center for Catalytic Hydrocarbon Functionalization, an Energy Frontier Research Center Funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Award Number DE-SC0001298.

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August 20, 2023
October 26, 2023