Oxygen atom transfer, coupled electron-proton transfer, and correlated electron-nucleophile transfer reactions of oxomolybdenum(IV) and dioxomolybdenum(VI) complexes

The oxo-Mo(IV) complexes LMoO(S(2)PR(2)-S,S') [L = hydrotris(3,5-dimethylpyrazol-1-yl)borate; R = Me, Et, Pr-i, Ph] were prepared by reacting MoO(S(2)PR(2))(2) and KL in refluxing toluene. The dioxo-Mo(VI) complexes cis-LMoO(2)(S(2)PR(2)-S) (R = Pr-i, Ph) were prepared by oxidation of the oxo-Mo(IV) complexes or by reaction of LMoO(2)Cl with NaS(2)PR(2). Oxygen atom transfers from Me(2)SO to LMoO(S(2)PR(2)) were first-order with respect to Me(2)SO and complex; the overall second-order rate constants at 40 degrees C range from 9.0(1) x 10(-5) M(-1) . s(-1) for LMoO(S(2)PMe(2)) to 2.08(5) x 10(-4) M(-1) . s(-1) for LMoO(S2PPr2); activation parameters were in the ranges Delta H double dagger = 63(1) to 73(1) kJ . mol(-1), Delta S double dagger = -88(1) to -111(1) J . K-1 . mol(-1), and Delta G double dagger = 100(2) kJ . mol(-1) for LMoO(S-2-PMe(2)) to 98(2) kJ . mol(-1) for LMoO(S2PPr2). Oxygen atom transfer from pyridine N-oxide to LMoO(S2PPr2) was also second-order with a rate constant of 1.54(5) x 10(-3) M(-1) . s(-1) at 40 degrees C, Delta H double dagger = 62(1) kJ . mol(-1), Delta S double dagger = -90(1) J . K-1 . mol(-1), and Delta G double dagger = 90(1) kJ . mol(-1). The second-order rate laws and large negative entropies of activation are consistent with associative mechanisms for the above reactions. Oxygen atom transfer from LMoO(2)(S2PPr2) to PPh(3) was first-order with respect to reactants, with an overall second-order rate constant of 2.5(3) x 10(-4) M(-1) . s(-1) at 30 degrees C. In toluene at 40 degrees C, all the above complexes catalyzed the oxidation of PPh(3) by Me(2)SO, with turnover rates of ca. 0.9 mol of PPh(3)/(mol of catalyst/h). Reduction of LMoO(2)(S(2)PR(2)) by SH- led to the generation of the dioxo-Mo(V) anions [LMoO(2)(S(2)PR(2-)S)](-), which were slowly converted to the analogous oxothio-Mo(V) complexes [LMoOS(S(2)PR(2-)S)](-). Dioxygen reacted with [LMoOS(S2PPr2)](-) to produce the oxothio-Mo(VI) complex LMoOS(S2PPr2-S). The (hydroxo)oxo-Mo(V) complexes LMoO(OH)(S(2)PR(2-)S) were formed upon reduction of LMoO(2)(S(2)PR(2)) with PPh(3) in wet (3-5 M H2O) tetrahydrofuran or upon ferrocenium oxidation of LMoO(S(2)PR(2)) in wet tetrahydrofuran. In dry solvents, LMoO(S(2)PR(2)) were oxidized to the corresponding cations, [LMoO(S(2)PR(2-)S,S')](+), which reacted with water to form LMoO(OH)(S(2)PR(2)). The Mo(V) complexes have been characterized by EPR spectroscopy.