Mechanism of Biologically Relevant Deoxygenation of Dimethyl Sulfoxide Coupled with Pt(II) to Pt(IV) Oxidation of Orthoplatinated Oximes. Synthetic, Kinetic, Electrochemical, X-ray Structural, and Density Functional Study

Abstract

Orthometalated aryl oxime complexes cis-(C,S)-(Pt II (C6H3-2-CMedNOH-5-R)Cl(Me2SdO)) (1 ,R ) H( a), MeO, Me, F, and Cl) undergo deoxygenation of dimethyl sulfoxide (DMSO) in methanol in the presence of HCl to afford the Pt(IV) dimethyl sulfide complexes fac-(Pt IV (C6H3-2-CMedNOH-5-R)Cl3(Me2S)) (2), the composition of which was confirmed by an X-ray structural study of 2a. The mechanism of the deoxygenation coupled with the oxidation of Pt(II) to Pt(IV) was investigated using cyclic voltammetry, UV- vis, and 1 H NMR spectrometry techniques at 40-60 °C in the presence of HCl, LiCl, and NaClO4. The conversion of 1 into 2 does not occur intramolecularly and involves two time-resolved phases which were studied independently. The first is the substitution of chloride for DMSO to afford the anionic reactive complexes cis-(Pt(C6H3-2-CMedNOH-5-R)Cl2) - (1Cl), which are involved in the acid-promoted interaction with free DMSO in the second phase. The formation of 1Cl follows the usual two-term rate law kobs1 ) ks + kCl(LiCl), the kCl-driven pathway being negligible for the electron-rich complex with R ) MeO. Thus-generated complexes 1Cl, in contrast to their precursors 1, are more susceptible to oxidation, and the irreversible peak for 1Cl, E(p1), is observed ca. 300 mV more cathodically compared to that of 1. The second phase is acid-catalyzed and at low LiCl concentrations follows the rate expression kobs2(H + ) -1 ) k10' + k10(LiCl). The complexes with the electron-withdrawing substituents R react faster, and there is a linear correlation between log k10 and E(p1). The first-order in the acid is discussed in terms of two kinetically indistinguishable mechanisms involving the rate-limiting either electron transfer from 1Cl to protonated DMSO (mechanism 1) or insertion of the SdO bond of free DMSO into the platinum-hydride bond of the reactive hydride complex of Pt(IV), cis-(Pt(C6H3- 2-CMedNOH)(H)Cl2), to afford a {Pt-SMe2-OH} fragment. Its protonation by HCl and dissociation of water gives the final product 2 (mechanism 2). 1 H NMR evidence is presented for the formation of the hydride species on protonation of a Pt(II) complex, whereas a density functional study of the two mechanisms indicates that mechanism 2 is less energy demanding. The system studied is viewed as a functioning mimetic of the Mo-dependent enzyme DMSO reductase because of several common features observed in catalysis.