PHOTOREDUCTION OF DIARYL DISULFIDES BY QUADRUPLY BONDED DIMOLYBDENUM AND DITUNGSTEN COMPLEXES

The quadruple metal-metal bonded complexes, M(2)Cl(4)(dppm)(2) (M = Mo , W; dppm = bis(diphenylphosphino)methane), photoreact with disulfides in nonaqueous solvents to yield M(2)(III) addition products. Excitati on (lambda(exc) > 435 nm) of the dimolybdenum photoreagent in the pres ence of PhSSPh affords Mo2Cl5(dppm)(2)(SPh). Whereas the MO(2)(III) ed ge-sharing bioctahedron is only accessible by photochemistry, W2Cl4(dp pm)(2) is thermally and photochemically oxidized by PhSSPh to W2Cl4(dp pm)(2)(SPh)(2), but reaction to the W-2(III) edge-sharing bioctahedron is greatly accelerated by light. The photolysis quantum yield for the Mo2Cl4(dppm)(2) photochemistry increases for excitation wavelengths t o the blue of 436 nm (phi(p)(436) = 0.01 phi(p)(405) = 0.11, phi(p)(36 0) = 0.23) and asymptotically p p approaches a maximum at wavelengths less than 320 nm phi(p)(313) = 0.27). The action spectrum red shifts b y similar to 50 p nm when the photoreagent is W2Cl4(dppm)(2). The wave length dependence of M(2)Cl(4)(dppm)(2) photochemistry and the presenc e of long-lived intermediates in the transient absorption spectra of M (2)X(4)(PP)(2) complexes (M = Mo and PP = dmpm = bis(dimethylphosphino )methane; M = W and PP = dppm) suggest that reactivity is derived from metal-localized excited states lying to higher energy of the delta de lta excited state.