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 M(VI) O + Q with the biological metals M = Mo and W. Systematic variation of the electronic environment at the WIV 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-p-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- (X4,X'], 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 E1/2 and Hammett constant 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 WIV(OC6H4-p-X')(S2C2(C6H4-p-X)2)2]1- + (CH2)4SO W(VI)O(OC6H4-p-X')(S2C2(C6H4-p-X)2)2]1- + (CH2)4S 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']/kX4,H]) = rho(ax)sigma(p) and log(kX4,X']/k(H4,X']) = 4rho(eq)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 approximately 5 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.