Functional analogue reaction systems of the DMSO reductase isoenzyme family:: Probable mechanism of S-oxide reduction in oxo transfer reactions mediated by bis(dithiolene)-tungsten(IV,VI) complexes

The recent development of structural and functional analogues of the DMSO reductase family of isoenzymes allows mechanistic examination of the minimal oxygen atom transfer paradigm M-IV + QO --> (MO)-O-VI + Q with the biological metals M = Mo and W. Systematic variation of the electronic environment at the W-IV center of desoxo bis(dithiolene) complexes is enabled by introduction of para-substituted phenyl groups in the equatorial (eq) dithiolene ligand and the axial (ax) phenolate ligand, The compounds [W(CO)(2)-(S2C2(C6H4-rho-X)(2))(2)] (54-60%) have been prepared by ligand transfer from [Ni(S2C2(C6H4-p-X)(2))(2)] to [W(CO)(3)-(MeCN)(3)]. A series of 25 complexes [W-IV(OC6H4-p-X')(S2C2(C6H4-p-X)(2))(2)](1-) ([X4X'], X = Br, F, H, Me, OMe; X' = CN, Br, H, Me, NH2; 41-53%) has been obtained by ligand substitution of five dicarbonyl complexes with five phenolate ligands. Linear free energy relationships between E-1/2 and Hammett constant sigma(p) for the electron-transfer series [Ni(S2C2(C6H4-p-X)(2))(2)](0.1-.2-) and [W(CO)(2)(S2C2(C6H4-p-X)(2))(2)](0.1-.2-) demonstrate a substituent influence on electron density distribution at the metal center. The reactions [W-IV(OC6H4-p-X')-(S2C2(C6H4-p-X)(2))(2)](1-) + (CH2)4(S)O --> [(WO)-O-VI(OC6H4-p-X')(S2C2(C6H4-p-X)(2))(2)](1) + (CH2)(4)S with constant substrate are second order with large negative activation entropies indicative of an associative transition state. Rate constants at 298 K adhere to the Hammett equations log(k((X4,X')/k(X4,H))) = rho(ax)sigma(p) and log(k((X4,X')/lJ4.X'))) = 4(rhoeq)sigma(p). Electron-withdrawing groups (EWG) and electron-donating groups (EDG) have opposite effects on the rate such that k(EWG) > k(EDG). The effects of X' on reactivity are found to be similar to5 times greater than that of X (rho(ax) = 2.1, rho(eq) = 0.44) in the Hammett equation. Using these and other findings, a stepwise oxo transfer reaction pathway is proposed in which an early transition state, of primary W-IV-O(substrate) bond-making character, is rate-limiting. This is followed by a six-coordinate substrate complex and a second transition state proposed to involve atom and electron transfer leading to the development of the W-VI = O group. This work is the most detailed mechanistic investigation of oxo transfer mediated by a biological metal.