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(C6H3-2-CMe=NOH-5-R)Cl( Me2S=O)] (1, R = H (a), MeO, Me, F, and Cl) undergo deoxygenation of dimeth yl sulfoxide (DMSO) in methanol in the presence of HCl to afford the Pt(IV) dimethyl sulfide complexes fac-[Pt-IV(C6H3-2-CMe=NOH-5-R)Cl-3(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 voltammety, UV-vis, and H-1 NMR spectr ometry techniques at 40-60 degrees C in the presence of HCl, LiCl, and NaCl O4. The conversion of 1 into 2 does not occur intramolecularly and involves two time-resolved phases which were studied independently. The first is th e substitution of chloride for DMSO to afford the anionic reactive complexe s cis-[Pt(C6H3-2-CMe=NOH-5-R)Cl-2](-) (1(Cl)), which are involved in the ac id-promoted interaction with free DMSO in the second phase. The formation o f 1(Cl) follows the usual two-term rate law k(obs1) + k(s) + k(Cl)[LiCl], t he kcl-driven pathway being negligible for the electron-rich complex with R = MeO. Thus-generated complexes 1(Cl) in contrast to their precursors 1, a re more susceptible to oxidation, and the irreversible peak for 1(Cl) E(p1) , is observed ca. 300 mV more cathodically compared to that of i. The secon d phase is acid-catalyzed and at low LiCl concentrations follows the rate e xpression k(obs2)[H+](-1) = k(10)' + k(10)[LiCl]. The complexes with the el ectron-withdrawing substituents R react faster, and there is a linear corre lation between log k(10) and E(p1). The first-order in the acid is discusse d in terms of two kinetically indistinguishable mechanisms involving the ra te-limiting either electron transfer from Icl to protonated DMSO (mechanism 1) or insertion of the S=O bond of free DMSO into the platinum-hydride bon d of the reactive hydride complex of Pt(IV), cis-[Pt(C6H3-2-CMe=NOH)(H)Cl-2 ], to afford a {Pt-SMe2-OH} fragment. Its protonation by HCl and dissociati on of water fives the final product 2 (mechanism 2). H-1 NMR evidence is pr esented for the formation of the hydride species on protonation of a Pt(II) complex, whereas a density functional study of the two mechanisms indicate s that mechanism 2 is less energy demanding. The system studied is viewed a s a functioning mimetic of the Mo-dependent enzyme DMSO reductase because o f several common features observed in catalysis.