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

Orthometalated aryl oxime complexes cis -( C,S )-[Pt II (C 6 H 3 -2-CMe NOH-5-R)Cl(Me 2 S O)] ( 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 (C 6 H 3 -2-CMe NOH-5-R)Cl 3 (Me 2 S)] ( 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 NaClO 4 . 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(C 6 H 3 -2-CMe NOH-5-R)Cl 2 ] - ( 1 Cl ), which are involved in the acid-promoted interaction with free DMSO in the second phase. The formation of 1 Cl follows the usual two-term rate law k obs1 = k s + k Cl [LiCl], the k Cl -driven pathway being negligible for the electron-rich complex with R = MeO. Thus-generated complexes 1 Cl, in contrast to their precursors 1, are more susceptible to oxidation, and the irreversible peak for 1 Cl, E ( p 1), 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 k obs2 [H + ] - 1 = k 10 ‘ + k 10 [LiCl]. The complexes with the electron-withdrawing substituents R react faster, and there is a linear correlation between log k 10 and E ( p 1). The first-order in the acid is discussed in terms of two kinetically indistinguishable mechanisms involving the rate-limiting either electron transfer from 1 Cl to protonated DMSO (mechanism 1) or insertion of the S O bond of free DMSO into the platinum−hydride bond of the reactive hydride complex of Pt(IV), cis -[Pt(C 6 H 3 -2-CMe NOH)(H)Cl 2 ], to afford a {Pt−SMe 2 −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.