Comparative Kinetics and Mechanism of Oxygen and Sulfur Atom Transfer Reactions Mediated by Bis(dithiolene) Complexes of Molybdenum and Tungsten

Although the kinetics and mechanism of metal-mediated oxygen atom (oxo) transfer reactions have been examined
in some detail, sulfur atom (sulfido) transfer reactions have not been similarly scrutinized. The reactions [MIV(O-p-C6H4X‘)(S2C2Me2)2]1- + Ph3AsQ → [MVIQ(O-p-C6H4X‘)(S2C2Me2)2]1- + Ph3As (M = Mo, W; Q = O, S) with
variable substituent X‘ have been investigated in acetonitrile in order to determine the relative rates of oxo versus
sulfido transfer at constant structure (square pyramidal) of the atom acceptor and of atom transfer at constant
structure of the atom donor and metal variability of the atom acceptor. All reactions exhibit second-order kinetics
and entropies of activation (−25 to −45 eu) consistent with an associative transition state. At parity of atom acceptor,
k2S (0.25−0.75 M-1s-1) > k2O (0.023−0.060 M-1s-1) with M = Mo and k2S (4.1−66.7 M-1s-1) > k2O (1.8−9.8
M-1s-1) with M = W. At constant atom donor and X‘, k2W > k2Mo with reactivity ratios k2W/k2Mo = 78−184 (Q =
O) and 16−89 (Q = S). Rate constants refer to 298 K. At constant M and Q, rates increase in the order X‘ = Me
≲ OMe < H < Br < COMe < CN; increasing electron-withdrawing propensity accelerates reaction rates. The probable
transition state involves significant Ph3AsQ···M bond-making (X‘ rate trend) and concomitant As−Q bond weakening
(bond energy order As−O > As−S). Orders of oxo and sulfido donor ability of substrates and complexes are
deduced on the basis of qualitative reactivity properties determined here and elsewhere. This work complements
previous studies of the reaction systems [MIV(O-p-C6H4X‘)(S2C2Me2)2]1-/XO where the substrates are N-oxides and
S-oxides and k2W > k2Mo at constant substrate also applies. The reaction order of substrates is Me3NO > (CH2)4SO
> Ph3AsS > Ph3AsO. This research provides the first quantitative information of metal-mediated sulfido transfer.