Hydrogen isotope exchange in PtII–thiazole complexes

Abstract

Hydrogen–deuterium exchange of the carbon-bound protons of thiazole coordinated to Pt^II in complexes 1, 2 and 3, has been measured in aqueous buffer solutions, at 60 °C, by ¹H NMR spectroscopy. Analysis of the rate data indicates that Pt^II enhances exchange of C(2)–H by a factor of ca. 10⁶, with respect to the neutral substrate, but is ca. 10³–10⁴ times less effective compared to H⁺ or Et⁺. Exchange of C(5)–H is enhanced ca. 6 × 10³ by Pt^II relative to the neutral heterocycle; an effect much less than that for C(2)–H exchange. We also report the first quantitative measurement of C(4)–H exchange in a thiazole moiety, as observed in 1 under moderate conditions (pD 8.69 and 60 °C, k₂= 2.54 dm³ mol^–1s^–1). The C(5)–H exchange process in complex 1 is moderately faster than C(4)–H exchange, a result opposite to that expected on the basis of an inductive/field effect by Pt^II. This is ascribed to the enhanced stabilization of the carbanionic intermediate at C(5) by the adjacent sulfur atom which is absent in the carbanion at C(4). This factor overrides the inductive/field effect of Pt^II, and accounts for the order of reactivity observed in this study, C(2)–H C(5)H > C(4)–H.