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

Abstract

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.

Group Members

William A. Goddard III

Charles and Mary Ferkel Professor