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

Abstract

The synthesis, characterization, and epoxidation catalysis of two neutral, dinuclear molybdenum(V) complexes [{MoO(Lx)(µ‐O)}2] (x = 1: 1; x = 2: 2) bearing S,N‐bidentate thiophenolate‐oxazoline ligands (L1, L2) are described. Both complexes are formed under reduction of the metal with formation of disulfides (L1)2 and (L2)2. Each Mo center is coordinated by one bidentate thiophenolate‐oxazoline ligand and one terminal and two bridging oxido ligands. The solid‐state structures of NaL2 and complex 1 were confirmed by single‐crystal X‐ray diffraction. The five‐coordinate nature of the molybdenum atoms in 1 and 2 leads to exceptionally high catalytic activities in the epoxidation of cyclooctene. The catalyst loading can be reduced to 0.001 mol‐% (10 ppm), with a turnover number (TON) of 85000 and a maximum turnover frequency (TOF) of 62000 h–1 for 1, and a TON of 87400 and a TOF of 107400 h–1 for 2. For cyclooctene, the catalyst solutions can be recycled multiple times without significant loss of activity; however, other substrates (styrene, 1‐octene, and limonene) showed conversions with lower selectivity. The fate of catalyst 1 was examined by stoichiometric addition experiments with tert‐butyl hydroperoxide, which resulted in the formation of benzenesulfonate‐oxazoline derivative HL3 and polyoxomolybdates.

Hydrogen bond donor functionalized dioxido-molybdenum(VI) complexes as robust and highly efficient precatalysts for alkene epoxidation

“…[1,7] With the prospect to develop environmentally more friendly processes, the use of non-toxic, earth-abundant metals such as molybdenum is of key interest. [8] Especially mononuclear molybdenum(VI) dioxido complexes have proven to be highly active and selective precatalysts for the epoxidation of internal aliphatic olefins, [9,10,11] recently the substrate scope could to some extent be expanded to terminal olefins. [12,13] Furthermore, the use of environmentally benign solvents as well as oxidants remains a major objective.…”

“…[16] Dioxidomolybdenum systems on the other hand were able to catalyze the oxidation of a broad scope of substrates, albeit with varying selectivities. [9,11,17,18] Especially the use of bidentate Schiff-base ligands with donor sites led to very selective precatalysts. [19] Inspired by the work of Borovik and coworkers, [20] we expanded our library of iminophenolate ligands by introducing amide functionalities as hydrogen bond donors and investigated their coordination chemistry with the dioxidomolybdenum(VI) fragment, finding unusual C-C and C-N coupling reactions at the metal center.…”

“…This is in stark contrast to previously reported structural analogs [18] surpassing the activity of homogeneous epoxidation precatalysts known to date without additional hydrogen bond donors. [9,12,22] In addition we disclose herein the catalytic activity using H2O2 as oxidant as well as the use of environmentally benign alcoholic solvent. [23]…”

Molybdenum (VI) Complexes Containing Pyridylimine Ligands: Effect of the Imine Nitrogen Substituent in the Epoxidation Reaction

“…Applying C 6 and C 9 gave the desired product in moderate selectivities of 63 % and 58 %, respectively. In comparison to previously reported Mo(VI) complexes bearing a nitrogen‐chelating ligand, the current system C 5 exhibited a notable catalytic activity in the reaction of epoxidation under similar conditions [22,29,37] …”

“…In comparison to previously reported Mo(VI) complexes bearing a nitrogen-chelating ligand, the current system C 5 exhibited a notable catalytic activity in the reaction of epoxidation under similar conditions. [22,29,37] It was necessary to increase the catalyst loading and the reaction time to obtain the best outcome with styrene and αmethyl styrene. Using 0.1 mol% of C 5 with styrene, the best results were obtained at 12 hours of reaction, achieving 60 % of yield and 92 % of selectivity to obtain the specific epoxide.…”