OXYGEN-ATOM TRANSFER SYSTEMS IN WHICH THE (MU-OXO)DIMOLYBDENUM(V) COMPLEX-FORMATION DOES NOT OCCUR - SYNTHESES, STRUCTURES, AND REACTIVITIES OF MONOOXOMOLYBDENUM(IV) BENZENEDITHIOLATO COMPLEXES AS MODELS OF MOLYBDENUM OXIDOREDUCTASES

Two monooxomolybdenum(IV) complexes with dithiolene-like benzenedithio late ligands, (NEt(4))(2)[(MoO)-O-IV(Ph(3)Si-bdt)(2)] (1) (Ph(3)Si-bdt 3-(triphenylsilyl)-1,2-benzenedithiolato) and (NEt(4))(2)[(MoO)-O-IV( Ph(3)Si-tdt)(2)] (2) (Ph(3)Si-tdt = 5-(triphenylsilyl)-3,4-toluenedith iolato were synthesized as models of reduced state molybdenum oxidored uctase. The MoOS4 cores of 1 and (NEt(4))2[(MoO)-O-IV(tdt)(2)] (4) (td t= 3,4-toluenedithiolato) have a square pyramidal geometry similar to that of (NEt(4))(2)[(MoO)-O-IV(bdt)(2)] (3) (bdt = I,2-benzenedithiola to) (Boyde et al., 1986). The substituent effect of the triphenylsilyl group was detected in UV/vis spectra and in electrochemical propertie s. In the UV/vis spectra the band energy and extinction coefficients o f 1 and (both with a Ph(3)Si- group) are displaced to lower energy and slightly stronger (about 2,000 M(-1) cm(-1)) than 3 and 4, respective ly. The Mo(IV)/Mo(V) redox potential of P shows a negative shift (-0.0 3 V) compared with that of 3. These differences are due to the direct or indirect effect of the electron donation from the phenyl group pi o rbital and the p pi orbital of sulfur atom. In contrast tb 1, 3, and 4 , only the complex, 2; that has the most bulky ligand, shows a quasi-r eversible couple at +0.52 V (vs SCE) assignable to the Mo(V)/Mo(VT) re dox process. The bulkiness of the ligand clearly contributes to the st abilization of the Mo(V)/Mo(VI) redox process of 2. In the O atom tran sfer reaction of the monooxomolybdenum(IV) benzenedithiolate complex w ith Me(3)NO, the corresponding dioxomolybdenum(VI) benzenedithiolate c omplex and Me(3)N are formed quantitatively. This indicates the absenc e of (mu-oxo)dimolybdenum(V)complex formation in the present reaction controlled by the unique electronic properties and the strength of che lation of the dithiolato ligands.