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 O2 of (dpms)Pt(II)Me(OH2) (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 C1-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 O2 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 O2 than neutral complex 1 (pKa = 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 C1-symmetric dimethyl complex 6, (dpms)Pt(IV)Me2(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 Cs-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; kH/kD = 0.98 ± 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.