OXYGEN-ATOM TRANSFER, COUPLED ELECTRON-PROTON TRANSFER, AND CORRELATED ELECTRON-NUCLEOPHILE TRANSFER-REACTIONS OF OXOMOLYBDENUM(IV) AND DIOXOMOLYBDENUM(VI) COMPLEXES
Lj. Laughlin et Cg. Young, OXYGEN-ATOM TRANSFER, COUPLED ELECTRON-PROTON TRANSFER, AND CORRELATED ELECTRON-NUCLEOPHILE TRANSFER-REACTIONS OF OXOMOLYBDENUM(IV) AND DIOXOMOLYBDENUM(VI) COMPLEXES, Inorganic chemistry, 35(4), 1996, pp. 1050-1058
The oxo-Mo(IV) complexes LMoO(S(2)PR(2)-S,S') [L = hydrotris(3,5-dimet
hylpyrazol-1-yl)borate; R = Me, Et, Pr-i, Ph] were prepared by reactin
g MoO(S(2)PR(2))(2) and KL in refluxing toluene. The dioxo-Mo(VI) comp
lexes cis-LMoO(2)(S(2)PR(2)-S) (R = Pr-i, Ph) were prepared by oxidati
on 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 firs
t-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(S2PP
r2); 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) fo
r 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 activat
ion 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 deg
rees C, all the above complexes catalyzed the oxidation of PPh(3) by M
e(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 th
e dioxo-Mo(V) anions [LMoO(2)(S(2)PR(2-)S)](-), which were slowly conv
erted to the analogous oxothio-Mo(V) complexes [LMoOS(S(2)PR(2-)S)](-)
. Dioxygen reacted with [LMoOS(S2PPr2)](-) to produce the oxothio-Mo(V
I) 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 o
f 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.