Dioxomolybdenum(VI) and ‐tungsten(VI) Complexes with Multidentate Aminobisphenol Ligands as Catalysts for Olefin Epoxidation

Abstract

The synthesis of four molybdenum and tungsten complexes bearing tetradentate tripodal amino bisphenolate ligands with either hydroxyethylene (1a) or hydroxyglycolene (1b) substituents is reported. The molybdenum dioxo complexes [MoO2L] (L = 2a, 2b) and tungsten complexes [WO2L] (3a, 3b) were synthesized using [MoO2(acac)2] and [W(eg)3] (eg = 1,2‐ethanediolato, ethylene glycolate), respectively, as precursors. All complexes were characterized by spectroscopic means as well as by single‐crystal X‐ray diffraction analyses. The latter reveal, in all cases, hexacoordinate complexes in which the hydrogen atom of the hydroxy group is involved in hydrogen bonding with one of the metal oxo groups. In the case of the glycol substituent, the ether oxygen atom is coordinated to the metal whereas the hydroxy group remains uncoordinated. The complexes were tested as catalysts in the epoxidation of cyclooctene under eco‐friendly conditions, using an aqueous solution of H2O2 as the oxidant and dimethyl carbonate (DMC) as solvent or neat conditions, as substitutes for chlorinated solvents. Molybdenum complexes 2a and 2b showed good catalytic activity using H2O2 without added solvent, and tungsten complexes 3a and 3b showed very high activity in the epoxidation of cyclooctene using H2O2 and DMC as solvents.

Dioxomolybdenum(VI) and -Tungsten(VI) Amino Bisphenolates as Epoxidation Catalysts

“…W1 had some activity for the selective epoxidation of S3 but practically no activity for other olefins. The selectivity with substrates S1 and S3 using structurally related catalysts equals with the earlier studies by us [13] as well as other groups. [21,35,36] Mo1 and W1 were also tested as catalysts for epoxidation of S1 and S3 using aqueous hydrogen peroxide as oxidant instead of TBHP solutions.…”

“…Similarly, the W-Oaryloxide, W-N and W-OMeOH distances are comparable with previous results. [13,20,29,30] In general, W1 is isostructural with the equivalent molybdenum compound with only negligible differences in the bonding parameters [19]. …”

“…Considering these important biological functions, many examples of molybdenum-and tungsten-containing model compounds for OAT have been studied [6][7][8][9][10]. Similarly, a number of artificial dioxomolybdenum(VI) and -tungsten(VI) complexes have been prepared for catalytic applications in important industrial processes, for example olefin epoxidation [11][12][13]. Such complexes can be also considered as soluble molecular models for metal oxide catalysts [14].…”

Dioxomolybdenum(VI) Complexes with Salicylamide Ligands: Synthesis, Structure, and Catalysis in the Epoxidation of Olefins under Eco‐Friendly Conditions

“…Nevertheless, the results obtained using H 2 O 2 as oxidant are competitive with those obtained by similar catalytic systems. For example, using the MoO 2 L [L = dianionic aminobisphenolate ligand]/H 2 O 2 catalytic system, the group of Mösch‐Zanetti selectively obtained cyclooctene oxide in 45 % yield after 20 h at 50 °C in the presence of DMC as solvent, while the same reaction gave the epoxide in quantitative yield under solvent‐free conditions …”

Synthetic utility of metal catalyzed hydrogen peroxide oxidation of C-H, C-C and C=C bonds in alkanes, arenes and alkenes: Recent advances

“…Molybdenum complexes showed good catalytic activity using H 2 O 2 without added solvent, and tungsten complexes showed very high activity. 160 , various kinds of organic substrates such as alkenes, sulfides, arenes, and alkanes were selectively converted into the corresponding oxygenated products in moderate to high yields. For example 1-octene gave epoxide in 85% preparative yield and cyclooctene was oxidized to epoxide in 89% yield while selectivity was above 99% in both reactions.…”