Unveiling the Mechanisms of Catalytic Oxidation Reactions Mediated by Oxo-Molybdenum Complexes: A Computational Overview

Abstract

Mo(VI) complexes MoO2X2L2 (X=halide or Me, L neutral ligand) behave as catalysts for olefin epoxidation in the presence of t-butylhydroperoxide (TBHP). The active species results from OH activation of TBHP, which protonates one oxo group and leads to a seven coordinate complex, with a new OOR ligand. It was found that several Mo(II) complexes Cp'Mo(CO)3X (Cp'=C5R5, Cp* or C5H5, Cp) acted as precursors for the same reactions and the resulting Cp'MoO2X could also oxidize sulfides and sulfoxides, both with TBHP and H2O2 as oxidants. A review of the reaction mechanisms proposed for these reactions, by us and some other authors, and based on computational studies is given in this work. More than one active species can be found for the CpMoO(OH)( 1 -OOR)X intermediate, opening several competitive pathways. They differ by the O-H···O hydrogen bond formed between OH and one oxygen of the OOR ligand. This complex can also further react with oxidant to afford a peroxide complex CpMoO( 2 -O2)X, which can also promote oxidation reactions. The activation energies depend on hydrogen bond assistance, so that the Cl and Me derivatives of CpMoO2X behave differently (the peroxide complex has only been found active with Me), and on the steric constraints, more obvious when comparing Cp with Cp*. The preferred mechanism will thus depend on the specific substituents, but energy barriers are comparable.

Metal oxide-triazole hybrids as heterogeneous or reaction-induced self-separating catalysts

“…As found for 1, self-precipitation of the starting octamolybdate salts took place after the completion of reaction took place. Several mechanistic hypotheses have been put forward in the literature (mainly based on gas-phase density functional theory) for olefin epoxidation with hydroperoxides in the presence of oxoperoxomolybdenum species; reaction mechanisms may involve O-atom transfer from a g 2 -O 2 peroxo ligand [51][52][53] or a hydroperoxo (or alkylperoxo) ligand [52,54,55] to the olefin (which may or may not be coordinated to the metal center).…”

Molybdenum(II) catalyst precursors in olefin oxidation reactions

“…It was a breakthrough to find out that these Mo(VI) complexes could be obtained in situ by addition of TBHP to the Mo(II) [CpMo(CO) 3 X] precursors [32,33] (Chart 1) and used directly. Kinetic experiments gave much insight on the mechanism, [34] which was extensively studied by DFT methods, by some of us, [12,13,35] and other groups. [36,37,38] A detailed report on olefin epoxidation promoted by the complex 3 (or analogues, with Cl instead of CH 3 , for instance) can be found in a previous publication.…”

“…[36,37,38] A detailed report on olefin epoxidation promoted by the complex 3 (or analogues, with Cl instead of CH 3 , for instance) can be found in a previous publication. [13] Complex 3 by itself is not a catalyst. It complex 3C can be formed from 3 with an energy barrier ~50 kcal.mol -1 (∆G), while the calculated barrier to form 3A is only ~20 kcal.mol -1 , the conversion to 3B is ~6-7 kcal.mol -1 , and the transformation 3A to 3C is 22 kcal.mol -1 (the ranges of values result from calculations with different X or Cp).…”

“…It complex 3C can be formed from 3 with an energy barrier ~50 kcal.mol -1 (∆G), while the calculated barrier to form 3A is only ~20 kcal.mol -1 , the conversion to 3B is ~6-7 kcal.mol -1 , and the transformation 3A to 3C is 22 kcal.mol -1 (the ranges of values result from calculations with different X or Cp). [13] The peroxo complex 3C should exist in much smaller amounts and it is not a competitive catalytic species for olefin epoxidation, though it is for sulfide and sulfoxide oxidation. [39] It should be added that further reaction between complex 3C and TBHP afford another potential active species, but it catalyzes epoxidation with very high barriers.…”

“…8 We have also devoted much attention to the computational approach to reaction mechanisms, having proposed specific mechanisms for the oxidation reactions catalyzed by Mo(VI). 11,12,13 In this account we also address this problem.…”