Dichloro and dimethyl dioxomolybdenum(vi)–diazabutadiene complexes as catalysts for the epoxidation of olefins

Abstract

The dioxomolybdenum(VI) complex [MoO 2 Cl 2 {p-tolyl(CH 3 DAB)}] has been prepared in good yield by reaction of the solvent adduct MoO 2 Cl 2 (THF) 2 with one equivalent of the bidentate ligand N,N-p-tolyl-2,3-dimethyl-1,4-diazabutadiene. Treatment of the dichloro complex with the Grignard reagent CH 3 MgCl gives the dimethyl derivative [MoO 2 (CH 3) 2 {p-tolyl(CH 3 DAB)}]. The complexes are highly active and selective catalysts for the homogeneous epoxidation of cyclooctene using tert-butyl hydroperoxide (TBHP) as the oxidant. In both cases, the initial activity is ca. 175 mol mol À1 Mo h À1 and cyclooctene oxide is obtained quantitatively within 4 h. It was possible to recover the dimethyl complex at the end of the reaction and reuse it in a second run with only a small decrease in activity. The complexes are also active and selective for the epoxidation of other olefins, such as 1-octene, 2-octene, cyclododecene and (R)-(þ)-limonene, with TBHP. The catalytic production of cyclooctene oxide was investigated in detail, varying either the reaction temperature or the initial concentrations of substrate, oxidant and catalyst precursor. Kinetic studies show that the catalyst precursor-oxygen donor complex formation is first-order in TBHP and in the metal complex [MoO 2 Cl 2 {p-tolyl(CH 3 DAB)}]. A specific rate of 3.2 mol À1 dm 3 s À1 was found for catalyst formation at 25 C. Activation parameters for this reaction have also been measured (DH 6 ¼ ¼ 48 AE 3 kJ mol À1 , DS 6 ¼ ¼ À112 AE 10 J mol À1 K À1).

A Computational Study of the Olefin Epoxidation Mechanism Catalyzed by Cyclopentadienyloxidomolybdenum(VI) Complexes

“…However, even for the H system the transition-state energy of TSA C H T U N G T R E N N U N G (4-5) is very close to that of TSA C H T U N G T R E N N U N G (2-3), and entropic effects disfavor TSA C H T U N G T R E N N U N G (4-5) with respect to TSA C H T U N G T R E N N U N G (2-3), because two species have been added to the system (ROOH and ethylene) in TSA C H T U N G T R E N N U N G (4-5) and only one (ROOH) in TSA C H T U N G T R E N N U N G (2-3) (see Supporting Information). From the experimental point of view, all previous kinetic studies yield results consistent with the rate-determining step being oxygen transfer to the olefin, [50][51][52] including a recent study on a cyclopentadienyl-substituted Mo VI catalyst, [CpMoO 2 -…”

Synthesis and Structure of a Sodium Complex of an Aromatic β-Diketone and Pyrazolylpyridine

“…In all instances, recrystallisation of the crude product from chloroform/n-hexane gave a colourless microcrystalline solid with the composition [Na 4 (pypzH) 2 (BTA) 4 ]·3H 2 O (1). The number of water molecules was confirmed by carrying out thermogravimetric analysis under air (not shown).…”

“…The main findings from this work are: (i) Sodium forms a centrosymmetric hybrid tetramer with the ligands 1-benzoyl-3,3,3-trifluoroacetone and 2-(3-pyrazolyl)pyridine, (ii) the two crystallographically independent O,O-chelating BTA residues exhibit remarkably different coordination geometries, (iii) the structure contains unusual structural motifs with respect to the Na + coordination environments, culminating in the presence of an unprecedented central {Na 4 O 6 } core, (iv) crystal packing is essentially driven through geometrical aspects combined with weak C-H···π interactions. …”

“…Most of this work has centred on either high-valent d 0 transition metal complexes, mainly Mo VI , as catalysts for olefin epoxidation [2][3][4][5], or trivalent lanthanide ions for optical applications [1,6]. Ligands derived from 2-(3-pyrazolyl)pyridine (Figure 1) have proven to be particularly interesting for both applications.…”

“…We have been interested in the coordination behaviour of a broad range of neutral or anionic chelating ligands containing N and/or O donor atoms, such as β-diketones [1], cis-diols [2], oximes [2], 2-pyridyl alcohols [3], diazabutadienes [4] and pyrazolylpyridines [5,6]. Most of this work has centred on either high-valent d 0 transition metal complexes, mainly Mo VI , as catalysts for olefin epoxidation [2][3][4][5], or trivalent lanthanide ions for optical applications [1,6].…”