Mechanistic Study of the Oxidation of a Methyl Platinum(Ⅱ) Complex with O_2 in Water: Pt~ⅡMe-to-Pt~ⅣMe and Pt~ⅡMe-to-Pt~ⅣMe_2 Reactivity

摘要

The mechanism of oxidation by O_2 of (dpms)- Pt~ⅡMe(OH_2) (1) and (dpms)Pt~ⅡMe(OH)~- (2) [dpms = di(2- pyridyl)methanesulfonate] in water in the pH range of 4-14 at 21 ℃ 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~ⅡMe] 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~ⅡMe] 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~ⅣMe_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~ⅣMe(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.