Dioxomolybdenum(VI) Complexes with Pyrazole Based Aryloxide Ligands: Synthesis, Characterization and Application in Epoxidation of Olefins

Abstract

Synthesis, characterization, and epoxidation chemistry of four new dioxomolybdenum(VI) complexes [MoO2(L)2] (1–4) with aryloxide-pyrazole ligands L = L1–L4 is described. Catalysts 1–4 are air and moisture stable and easy to synthesize in only three steps in good yields. All four complexes are coordinated by the two bidentate ligands in an asymmetric fashion with one phenoxide and one pyrazole being trans to oxo atoms, respectively. This is in contrast to the structure found for the related aryloxide-oxazoline coordinated Mo(VI) dioxo complex 5. This was confirmed by the determination of the molecular structures of complexes 1–3 by X-ray diffraction analyses. Compounds 1–4 show high catalytic activities in the epoxidation of various olefins. Cyclooctene (S1) is converted to its epoxide with high activity, whereas the epoxidation of styrene (S2) is unselective. Internal olefins (S3 and S4) are also acceptable substrates, as well as the very challenging olefin 1-octene (S5). Catalyst loading can be reduced to 0.02 mol % and the catalyst can be recycled up to ten times without significant loss of activity. Supportive DFT calculations have been carried out in order to obtain deeper insights into the electronic situation around the Mo atom.

Dinuclear MoV Complexes with Thiophenolate‐oxazoline Ligands: Synthesis, Characterization, and Exceptional Activity in Catalytic Olefin Epoxidation

“…TBHP, CHCl 3 , 50 °C) at 0.1, 0.01, and 0.001 mol‐% catalyst loading (Table ). As previously observed for other Mo catalysts, catalytic activities and selectivities were significantly decreased for these more challenging substrates compared to cyclooctene , , , . Both complexes showed low activity with 1‐octene at 0.1 mol‐% catalyst loading, and no activity at all for lower catalyst loadings.…”

“…The majority of investigated Mo complexes have the general formula [MoO 2 L n ], in which L is usually an anionic ligand of varying denticity and charge (e.g., mono‐, bi‐, or tetradentate; mono‐ or dianionic) to afford six‐coordinate, neutral molybdenum(VI) complexes . Over the past few years, we and others have begun to systematically explore the chemistry and catalytic reactivity of two classes of anionic, O,N‐bidentate ligands in Mo and Re chemistry, namely, phenolate‐pyrazole (pyz) and phenolate‐oxazoline (oz; Figure ) ligands , . Both, [MoO 2 (pyz) 2 ] complexes and their phenolate‐oxazoline counterparts [MoO 2 (oz) 2 ], showed high activities in the epoxidation of cyclooctene, reaching turnover frequencies (TOF) of 5000 and 4800 h –1 , respectively, which rank both of them among the most active homogeneous Mo epoxidation catalysts according to a review published in 2013 .…”

“…Over the past few years, we and others have begun to systematically explore the chemistry and catalytic reactivity of two classes of anionic, O,N‐bidentate ligands in Mo and Re chemistry, namely, phenolate‐pyrazole (pyz) and phenolate‐oxazoline (oz; Figure ) ligands , . Both, [MoO 2 (pyz) 2 ] complexes and their phenolate‐oxazoline counterparts [MoO 2 (oz) 2 ], showed high activities in the epoxidation of cyclooctene, reaching turnover frequencies (TOF) of 5000 and 4800 h –1 , respectively, which rank both of them among the most active homogeneous Mo epoxidation catalysts according to a review published in 2013 . Recently, we have begun to explore the chemistry of the thiophenolate analogue of the oz ligand, that is, the ligand S‐Phoz ( 1 ; Figure and Scheme ) .…”

Oxomolybdenum(VI) complexes with glycine bisphenol [O,N,O,O′] ligand: synthesis and catalytic studies

“…oxidation reactions of alkenes and sulfides with H 2 O 2 or tBuOOH [7,[27][28][29][30][31][32][33][34]. oxidation reactions of alkenes and sulfides with H 2 O 2 or tBuOOH [7,[27][28][29][30][31][32][33][34].…”

Dioxidomolybdenum(VI) and –tungsten(VI) complexes with tetradentate amino bisphenolates as catalysts for epoxidation

“…Oxidative cleavage of the C=C double bond in styrene is commonly observed under epoxidation conditions, and only a few selective catalysts have been reported in literature. [9,[38][39][40] In case of S3, the eight catalysts show large differences with respect to the activity. The highest activity was displayed by Mo1a with a conversion to 1-octene oxide of 71% after 24 h. Complex Mo2b displayed an induction period with no activity in the first 7 h, after which it became active and showed a conversion of 27% after 24 h. With the other six complexes, the reactions started without any induction periods and followed a very similar time-conversion profile reaching conversions between 32 and 47% after 24 h. [33] In present study, Mo1 -Mo4 were generally less active than those complexes, which carry a dangling OH group in the side-arm.…”