Abstract
Ionic oxidoperoxido-molybdenum(VI) complexes of the type [Cat][MoO(O2)2(pic)], with pic = N,O-chelated picolinate ligand and Cat = monocation, were prepared in high yields (82–95%) from the precursor complex [H3O][MoO(O2)2(pic)] via [H]+ cation exchange for 1-ethyl-3-methylimidazolium [EMIM]+, 1-butyl-3-methylimidazolium [BMIM]+, 1-octyl-3-methylimidazolium [OMIM]+, N-cetylpyridinium [C16Py]+, and N-methyl-N,N,N-trioctylammonium [Aliquat]+. The structure and purity of the ionic compounds were assessed by 1H and 13C NMR, FT-IR, and elemental analysis (C, H, N), and the electrochemical properties were studied by differential pulse voltammetry (DPV) and cyclic voltammetry (CV). The [Cat][MoO(O2)2(pic)] compounds showed promising catalytic epoxidation activity based on the model reaction of cis-cyclooctene with tert-butyl hydroperoxide as oxidant. The type of cation influenced the physical state of the compound and the catalytic performance.
Highlights
Oxidodiperoxido-metal complexes of the type [MO(O2)2(L)n] with M=Mo, W and L an organic Lewis base (n = 1, 2), were discovered by Mimoun et al [1] in 1969 and reported as stable complexes
In the same decade, Garah et al [31,32,34,60] found that several monoanionic complexes of the type [Cat][MoO(O2)2(L)n] ([Cat]+ = tetraorganophosphonium type cation, and L = conjugated base of 2-hydroxybenzoic acid, benzoic acid [31], pyridine-2-carboxylic acid [32], or an oxime such asethenone [60], salicylaldoxime [34] and pyridine-2-carboxaldoxime [32]) were inactive as catalysts for olefin epoxidation using H2O2 as oxidant
With the exception of ethanol used in the syntheses (96%, Valente & Ribeiro, Belas, Portugal), which was distilled before use, all other chemicals were used as received from commercial suppliers: molybdenum(VI) oxide (99.5%, Alfa Aesar, Kandel, Germany), hydrogen peroxide (30% w/v, Panreac), [EMIM][Br], [OMIM][Br] and [BMIM][Cl], N-cetylpyridinium chloride (>98%, BDH), Aliquat 336 (>97%, Sigma-Aldrich, Algés, Portugal), cis-cyclooctene (95% Alfa Aesar, Kandel, Germany), 2-trans-octene (97%, Sigma-Aldrich, Algés, Portugal), cyclododecene (96%, mixture cis/trans, Aldrich, Algés, Portugal), tert-butyl hydroperoxide (5.5 M in decane, Aldrich, Algés, Portugal), 30% aqueous hydrogen peroxide (Riedel-de-Haën), anhydrous α,α,α-trifluorotoluene (≥99%, Sigma-Aldrich, Algés, Portugal), acetonitrile, 1,2-dichloroethane (99%, Aldrich, Algés, Portugal), and undecane (≥99%, Aldrich, Algés, Portugal) as the internal standard
Summary
Zare et al [43,44] reported the first active olefin epoxidation catalyst of the type [Cat]m[MoO(O2)2(L)n] with L = phox = 2-(2 -hydroxyphenyl)-5,6-dihydro-1,3-oxazine ([Cat]+ was not specified) In these studies, TBHP was used as oxidant, as opposed to the previous catalytic epoxidation studies using the ionic complexes with H2O2 as oxidant. A set of compounds was prepared possessing L = 2-picolinate (pic) and [Cat]+ = N-cetylpyridinium [C16Py]+, N-methyl-N,N,N-trioctylammonium [Aliquat]+, 1-octyl-3-methylimidazolium [OMIM]+, 1-butyl-3-methylimidazolium [BMIM]+ or 1-ethyl-3-methylimidazolium [EMIM]+ These compounds and their synthetic precursor [H3O][MoO(O2)2(pic)] were tested as catalysts in the model epoxidation reaction of cis-cyclooctene/TBHP, at 70 ◦C, giving up to quantitative epoxide yield at 3 h reaction
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