Nature of Cp*MoO₂⁺ in Water and Intramolecular Proton-Transfer Mechanism by Stopped-Flow Kinetics and Density Functional Theory Calculations

Abstract

A stopped-flow study of the Cp*MoO3- protonation at low pH (down to zero) in a mixed H2O-MeOH (80:20) solvent at 25 degrees C allows the simultaneous determination of the first acid dissociation constant of the oxo-dihydroxo complex, [Cp*MoO(OH)2]+ (pKa1 = -0.56), and the rate constant of its isomerization to the more stable dioxo-aqua complex, [Cp*MoO2(H2O)]+ (k-2 = 28 s-1). Variable-temperature (5-25 degrees C) and variable-pressure (10-130 MPa) kinetics studies have yielded the activation parameters for the combined protonation/isomerization process (k-2/Ka1) from Cp*MoO2(OH) to [Cp*MoO2(H2O)]+, viz., DeltaH++= 5.1 +/- 0.1 kcal mol-1, DeltaS++ = -37 +/- 1 cal mol-1 K-1, and DeltaV++ = -9.1 +/- 0.2 cm3 mol-1. Computational analysis of the two isomers, as well as the [Cp*MoO2]+ complex resulting from the dissociation of water, reveals a crucial solvent effect on both the isomerization and the water dissociation energetics. Introducing a solvent model by the conductor-like polarizable continuum model and especially by explicitly inclusion of up to three water molecules in the calculations led to the stabilization of the dioxo-aqua species relative to the oxo-dihydroxo isomer and to the substantial decrease of the energy cost for the water dissociation process. The presence of a water dissociation equilibrium is invoked to account for the unusually low effective acidity (pKa1' = 4.19) of the [Cp*MoO2(H2O)]+ ion. In addition, the computational study reveals the positive role of external water molecules as simultaneous proton donors and acceptors, having the effect of dramatically lowering the isomerization energy barrier.

A Computational Study of the Olefin Epoxidation Mechanism Catalyzed by Cyclopentadienyloxidomolybdenum(VI) Complexes

“…[ 47] In that case, we found that the barrier could be dramatically reduced by the proton relay action of a water molecule, which is the reaction solvent, because this allows a reduced angular distortion. In CHCl 3 , the solvent molecules cannot assure a proton relay mechanism, but the same role may be exerted by additional ROOH molecules, and also by the corresponding ROH co-product, once this has started to form, as shown in Scheme 6.…”

“…[46] Although the dinuclear compound may exert the same mechanistic function as [Cp*MoO 2 Cl] (the Cl ligand being replaced by the oxidobridged Cp*MoO 3 group), only the cationic complex is likely to exert a catalytic function among the charged species, because the water ligand can dissociate rather easily [47] and the resulting coordination site may be used for the oxidant activation. Useful background information comes from our recent computational study of hydration and proton transfer processes for the [Cp*MoO 2 ] + (aq) system.…”

“…Useful background information comes from our recent computational study of hydration and proton transfer processes for the [Cp*MoO 2 ] + (aq) system. [47] Therefore, we decided to examine the mechanism of the olefin epoxidation process by both Bergmans [Cp*MoO 2 It should also be noted that in a recent contribution, Colbran et al have shown that a (perarylcyclopentadienyl)molybdenum(VI) dioxido complex, although catalytically active in cyclooctene epoxidation by TBHP, decomposes to a more active non-cyclopentadienyl-containing catalyst as the reaction proceeds. [48] However, an equivalent protonolysis is not necessarily associated also with the more robust [45] Cp* À Mo bond.…”

“…+ system: This investigation was prompted by our knowledge of the speciation of compound [Cp* 2 Mo 2 O 5 ] in an aqueous medium, [46,47] and by our recent finding that the compound catalyzes cyclooctene epoxidation by TBHP in the presence of water. [41,42] À .…”

“…[47] The electron richness of the anionic complex is likely to preclude H 2 O 2 (or TBHP) activation and olefin epoxidation catalysis, but the cationic complex, which is isoelectronic with compound 1, could lead to epoxidation catalysis. Therefore, we repeated the study described above using this cationic system as a catalyst.…”