New insights into the mechanism of oxodiperoxomolybdenum catalysed olefin epoxidation and the crystal structures of several oxo–peroxo molybdenum complexes

Abstract

[Mo(O)(O(2))(2)(L)(2)] compounds (L = pz, pyrazole; dmpz, 3,5-dimethylpyrazole) were reacted stoichiometrically, in the absence of an oxidant, with cis-cyclooctene in an ionic liquid medium where selective formation of the corresponding epoxide was observed. However, this oxo-transfer reaction was not observed for some other olefins, suggesting that alternative reaction pathways exist for these epoxidation processes. Subsequently, DFT studies investigating the oxodiperoxomolybdenum catalysed epoxidation model reaction for ethylene with hydrogen peroxide oxidant were performed. The well known Sharpless mechanism was first analysed for the [Mo(O)(O(2))(2)(dmpz)(2)] model catalyst and a low energy reaction pathway was found, which fits well with the observed experimental results for cis-cyclooctene. The structural parameters of the computed dioxoperoxo intermediate [Mo(O)(2)(O(2))(dmpz)(2)] in the Sharpless mechanism compare well with those found for the same moiety within the [Mo(4)O(16)(dmpz)(6)] complex, for which the full X-ray report is presented here. A second mechanism for the model epoxidation reaction was theoretically investigated in order to clarify why some olefins, which do not react stoichiometrically in the absence of an oxidant, showed low level conversions in catalytic conditions. A Thiel-type mechanism, in which the oxidant activation occurs prior to the oxo-transfer step, was considered. The olefin attack of the hydroperoxide ligand formed upon activation of hydrogen peroxide with the [Mo(O)(O(2))(2)(dmpz)(2)] model catalyst was not possible to model. The presence of two dmpz ligands coordinated to the molybdenum centre prevented the olefin attack for steric reasons. However, a low energy reaction pathway was identified for the [Mo(O)(O(2))(2)(dmpz)] catalyst, which can be formed from [Mo(O)(2)(O(2))(dmpz)(2)] by ligand dissociation. Both mechanisms, Sharpless- and Thiel-type, were found to display comparable energy barriers and both are accessible alternative pathways in the oxodiperoxomolybdenum catalysed olefin epoxidation. Additionally, the molecular structures of [Mo(O)(O(2))(2)(H(2)O)(pz)] and [Hdmpz](4)[Mo(8)O(22)(O(2))(4)(dmpz)(2)]·2H(2)O and the full X-ray report of [Mo(O)(O(2))(2)(pz)(2)] are also presented.

DFT Study of the Molybdenum‐Catalyzed Deoxydehydration of Vicinal Diols

“…Previously, various molybdenum catalysts (Mo(CO) 6 , [Mo-(CO) 4 12 O 40 ) have been tested and shown to have similar reactivities in spite of different oxidation states, ligands, and nuclearities. [4] Therefore, we could reasonably assume that the active catalyst consists of an oxomolybdenum complex with one or two coordinating diols and no other ligands except water.…”

“…[8,9] Other oxomolybdenum complexes have been used for epoxidations [10] and reductions of sulfoxides and alkenes. [11] Molybdenum-catalyzed reactions that have previously been studied by computational methods include oxidations, [12] hydrosilylations, [13] and reductions using either hydrogen [11h] or silanes. [14] In line with earlier work, [15][16][17] we have carried out a computational study using density functional theory (DFT) to clarify the mechanism of the molybdenum-catalyzed DODH.…”

Synthesis, structure and catalytic olefin epoxidation activity of a dinuclear oxo-bridged oxodiperoxomolybdenum(VI) complex containing coordinated 4,4′-bipyridinium

“…2). This feature is commonly found, for example, in mononuclear complexes in which the metal coordination sphere is completed by two monodentate ligands or one N,Nchelating bidentate ligand [5,7,8,10,13,15,22,24]. In 1, the organic ligand only contributes one coordinating atom, with the overall coordination sphere being completed by a 2 -bridging oxo group which is at the genesis of the dinuclear molecular unit.…”

“…Oxoperoxo-molybdenum(VI) complexes are known to contain MoO(O 2 ) 2 , MoO(O 2 ) and MoO 2 (O 2 ) fragments, although the latter is rare [5]. These motifs are present in mononuclear complexes such as [MoO(O 2 ) 2 (L) n ] [6][7][8][9][10][11][12][13][14][15], [MoO(O 2 )Cl 2 (L) n ] acyclic alkenes. A limited number of studies have focussed on the epoxidation of olefins obtained from renewable resources such as citrus and turpentine oils, plant oils and fats [12,13,[24][25][26].…”

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).…”