Dominique Agustin

Solvent‐Free Epoxidation of Olefins Catalyzed by “[MoO2(SAP)]”: A New Mode of tert‐Butylhydroperoxide Activation

The mononuclear molybdenum complexes [MoO2(acac)2] (1, acac=acetylacetonate), [MoO2(SAP)(MeOH)] (2), and dinuclear oxomolybdic complexes [MoO2L]2 [L=salicylideneaminophenolato (SAP, 5), salicylideneaminoethanolato (SAE, 6), salicylideneaminomethylpropanolato (SAMP, 7)] have been investigated as (pre)catalysts for the epoxidation of olefins under solvent‐free conditions, using tert‐butylhydroperoxide (TBHP, 70 % in water) as an oxidant. Complexes 6 and 7, although active, are limited by ligand hydrolysis during the catalytic process, whereas complexes 2 and 5 are not altered under catalytic conditions and yield essentially the same selectivity and activity, which is not suppressed by excess MeOH. Although these catalysts are less active than 1, their selectivity is higher (97–98 %). DFT calculations are consistent with the active form of the catalyst being the 5‐coordinate “[MoO2(SAP)]”. The oxidant is activated by forming a weak adduct stabilized by a very loose Mo⋅⋅⋅O interaction and a hydrogen bond, predisposing it to the oxygen transfer to external olefin by a mechanism closely related to Bartlett’s epoxidation with peroxyacids.

Stable heterocyclic (Schiff base) divalent Group 14 element species (M=Ge, Sn, Pb)

Abstract The synthesis and characterization of new stable divalent germanium, tin and lead homoleptic species L2M [L2=2,2′-N,N′-bis(salicylidene)ethylenediamine, M=Ge (1), Sn (2), Pb (3); (R,R)-(−)-N,N′-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexanediamine, M=Ge (4), Sn (5), Pb (6) and N-methyl-2,2′-imino-bis(8-hydroxyquinoline), M=Ge (7), Sn (8)] are described. Compounds 1–8 were obtained in good yields by alcoholysis of the MN bonds of the divalent precursors [(Me3Si)2N]2M by diols with bis(salicylidene)diamine and 2,2′-imino-bis-quinoline structures. They have been isolated as solids at ambient temperature and are monomeric. NMR, IR and UV spectra are suggestive of N⋯M intramolecular coordination. The chemistry of 1–8 is illustrated through their reactions with iodine and 1,2-benzoquinones. The X-ray structure of the L2Sn-quinone adduct reveals a distorted octahedral coordination geometry around tin with remarkably short SnN distances. Various RCHO insertion reactions into the MO bonds of 1–8 and MO cleavage reactions with organic acids and acyl halides are also described; they provide a convenient procedure for the generation of new heteroleptic divalent species.

Dioxomolybdenum(VI) and dioxotungsten(VI) complexes chelated with the ONO tridentate hydrazone ligand: synthesis, structure and catalytic epoxidation activity

Synthesis of the dioxomolybdenum(VI) complexes [MoO2(L3OMe)(EtOH)] (1), [MoO2(L4OMe)(EtOH)] (2) and [MoO2(LH)(EtOH)] (3) and dioxotungsten(VI) complexes [WO2(L3OMe)(EtOH)] (4), [WO2(L4OMe)(EtOH)] (5) and [WO2(LH)]n (6a) was carried out using [MO2(C5H7O2)2] (M = Mo or W) and the corresponding aroylhydrazone ligand H2LR (3-methoxysalicylaldehyde 4-hydroxybenzhydrazone (H2L3OMe), 4-methoxysalicylaldehyde 4-hydroxybenzhydrazone (H2L4OMe), or salicylaldehyde 4-hydroxybenzhydrazone (H2LH) in ethanol. Compounds obtained upon heating of the mononuclear complexes in acetonitrile or dichloromethane, [MO2(LR)]n (1a–6a) or [MoO2(L3OMe)]2 (1b), respectively, were also investigated. Crystal and molecular structures of the mononuclear 1, 2 and 3, polynuclear 1a·MeCN and dinuclear 1b complexes were determined by the single crystal X-ray diffraction method. Powder X-ray diffraction showed isostructurality of 1 and 4, and 2 and 5. The complexes were further characterized by elemental analysis, IR spectroscopy, TG and DSC analyses, and one- and two-dimensional NMR spectroscopy. The catalytic performances of 1–5 and 6a were investigated for epoxidation of cyclooctene using aqueous tert-butyl hydroperoxide (TBHP) as the oxidant.

Pyridoxal hydrazonato molybdenum(VI) complexes: assembly, structure and epoxidation (pre)catalyst testing under solvent-free conditions

Pyridoxal hydrazonato molybdenum(VI) complexes were prepared by the reaction of the corresponding hydrazone (H2L1 = pyridoxal isonicotinic acid hydrazone, H2L2 = pyridoxal benzhydrazone, H2L3 = pyridoxal 4-hydroxy benzhydrazone) and [MoO2(acac)2] under appropriate conditions. The complexes can be classified into three categories: mononuclear [MoO2(L1–3)(MeOH)], polynuclear [MoO2(L1–3)]n and hybrid organic–inorganic compounds with the Lindqvist polyoxomolybdate [MoO2(HL1–3)]2Mo6O19. A unique example of a cationic polymer assembly with Lindqvist anions is reported herein for the first time. The compounds were characterised by elemental, TG and DSC analyses and by spectroscopic (IR, UV-Vis, 1H, 13C NMR) techniques. The crystal and molecular structure of the pyridoxal benzhydrazone H2L2, three mononuclear complexes [MoO2(L1–3)(MeOH)], and the Lindqvist-containing compounds [MoO2(HL2)]2Mo6O19·2MeCN and (H4L1)Mo6O19 were determined by single crystal X-ray diffraction. All complexes were tested as (pre)catalysts for the epoxidation of cyclooctene under solvent-free conditions with the use of aqueous TBHP (TBHP = tert-butylhydroperoxyde) as an oxidant. Optimal results in terms of conversion, selectivity, TOF and TON were obtained at very low (pre)catalyst loadings (0.05% [Mo] vs. substrate). The influence of the Linqvist anion on catalytic performance is discussed.

Investigation of induction times, activity, selectivity, interface and mass transport in solvent-free epoxidation by H2O2 and TBHP: a study with organic salts of the [PMo12O40]3− anion

The phosphomolybdate salts Q3[PMo12O40] [Q = tetra-n-butylammonium (TBA), n-butylpyridinium (BP), cetylpyridinium (CP)] have been used as catalysts for the epoxidation of cyclooctene under organic solvent-free conditions, using H2O2 or t-BuOOH (TBHP) in water as oxidants, and compared with the BP salt of the analogous tungsten derivative, (BP)3[PW12O40]. High catalytic activities have been recorded down to very low catalyst loadings (2 ppm). The activity and selectivity depend on the nature of the oxidant and the cation. The dominant process appears to be homogeneous and this rationalizes, together with the evolution of the phase equilibria, the presence of an induction period for the epoxidation by H2O2 and the evolution of the epoxide selectivity. The recovered catalysts are reusable and exhibit equivalent reactivity.

Towards a global greener process: from solvent-less synthesis of molybdenum(VI) ONO Schiff base complexes to catalyzed olefin epoxidation under organic-solvent-free conditions

Nine Schiff base ligands derived from o-hydroxyaldehydes (2-hydroxybenzaldehyde, 2-hydroxy-3-methoxybenzaldehyde, 2-hydroxy- 1-naphthaldehyde) and nine corresponding dioxomolybdenum(VI) complexes, cis-[MoO2L(CH3OH)] or cis-[MoO2L(CH3OH)]·CH3OH and dinuclear [MoO2L]2, have been prepared using the conventional solution-based method as well as mechanochemically, by liquid assisted grinding (LAG). All products have been characterised by means of IR spectroscopy, thermal analyses and also by powder and five molybdenum complexes by single crystal X-ray diffraction. The crystal structure analysis of mononuclear complexes reveal distorted octahedral Mo(VI) coordination by ONO donor atoms from a dianionic tridentate Schiff base ligand, two oxido oxygen atoms from the MoO22+ moiety and an oxygen atom from the MeOH molecule trans to the oxido oxygen atom. Due to the trans effect of the oxido oxygen atom, Mo–O(MeOH) is the longest bond distance within the Mo coordination sphere and it expected to be the point of maximum reactivity of the complexes. All complexes have been studied as pre(catalysts) for the epoxidation of cis-cyclooctene, cyclohexene and (R)-limonene using aqueous tert-butyl peroxide (TBHP) as the oxidant and in the absence of an organic solvent.

Dioxotungsten(VI) complexes with isoniazid-related hydrazones as (pre)catalysts for olefin epoxidation: solvent and ligand substituent effects

The mononuclear dioxotungsten(VI) complexes [WO2(L3OMe)(D)] (1a and 1b), [WO2(L4OMe)(D)] (2a and 2b) and [WO2(LH)(D)] (3a and 3b) (D = EtOH (1a–3a) or MeOH (1b–3b); L3OMe = 3-methoxy-2-oxybenzaldehyde isonicotinoyl hydrazonato, L4OMe = 4-methoxy-2-oxybenzaldehyde isonicotinoyl hydrazonato, LH = 2-oxybenzaldehyde isonicotinoyl hydrazonato) were synthesized by the reaction of [WO2(acac)2]·0.5C6H5Me with the respective isoniazid-related hydrazone. The compounds were characterized by microanalysis, FT-IR and NMR spectroscopy, thermogravimetric analysis, and powder X-ray diffraction method. The crystal and molecular structures of 1a, 1b, 3a and [WO2(acac)2]·0.5C6H5Me were determined by single crystal X-ray diffraction. The structures of 1a, 1b, 3a are mononuclear and form hydrogen bonded centrosymmetric dimers. In all three complexes, the dimers are also held together by π⋯π interactions between aromatic rings. The catalytic performances (activity and selectivity) of 1a–3a and 1b–3b towards alkene epoxidation by tert-butyl hydroperoxide (TBHP) were investigated under different conditions.