Bruno G. M. Rocha

Supported gold nanoparticles as reusable catalysts for oxidation reactions of industrial significance

The efficient single‐pot oxidative functionalisation of alkanes and alcohols under mild conditions was catalysed by Au nanoparticles supported on Al2O3, Fe2O3, ZnO and TiO2. The obtained materials were tested for cyclohexane oxidation under mild conditions (60 °C, atmospheric pressure) using an environmentally friendly oxidant (H2O2). The materials were also tested in the oxidation of benzyl alcohol and methylbenzyl alcohol in the presence of tert‐butylhydroperoxide as the oxidant under microwave irradiation. With regard to cyclohexane oxidation, all materials were highly selective towards the formation of cyclohexanol and cyclohexanone. No traces of byproducts were detected under the optimised conditions. Au on Fe2O3 led to the best results (13.5 % yield). This system showed an interesting almost exclusive formation of cyclohexanol at 4 h reaction time. Catalyst recycling was tested in up to five cycles, and the catalyst maintained almost the original level of activity after three cycles with no significant leaching. With regard to oxidation of benzyl alcohol and methylbenzyl alcohol, all materials were highly selective towards the formation of benzaldehyde or acetophenone, respectively. No traces of byproducts were detected. Addition of Au increased alcohol conversion from 5 (TiO2) to 91 % (Au/TiO2). The recycling of Au/TiO2 was tested in up to 10 cycles, and the catalytic activity remained high in the first four cycles.

V(IV), Fe(II), Ni(II) and Cu(II) complexes bearing 2,2,2-tris(pyrazol-1-yl)ethyl methanesulfonate: application as catalysts for the cyclooctane oxidation

Water-soluble compounds [VOCl2{CH3SO2OCH2C(pz)3}] (pz = pyrazol-1-yl) 1, [FeCl2{CH3SO2OCH2C(pz)3}] 2, [NiCl2{CH3SO2OCH2C(pz)3}] 3 and [Cu{CH3SO2OCH2C(pz)3}2](OTf)24 were obtained by reactions between the corresponding metal salts and 2,2,2-tris(pyrazol-1-yl)ethyl methanesulfonate, CH3SO2OCH2C(pz)3. They were isolated as air-stable solids and fully characterized by IR, FTIR, NMR (for 2), EPR (for 1), ESI-MS(+/−), elemental analysis and (for 4) single-crystal X-ray diffraction. In all, half- (1–3) or full-sandwich (4), compounds the C-scorpionate ligand shows the N,N,N-coordination mode. 3 and 4 appear to provide the first examples of a Ni(II) and a full-sandwich Cu(II) compound respectively, bearing that scorpionate ligand. Compound 3 is the first Ni(II) tris(pyrazol-1-yl)methane type complex to be applied as catalyst for the oxidation of alkanes. Compounds 1–4 exhibit catalytic activity for the peroxidative (with aq. H2O2) oxidation, in water/acetonitrile medium and under mild homogeneous conditions, of cyclooctane to the corresponding alcohol and ketone (yields up to ca. 27%). The effect of the presence of additives, such as nitric acid or pyridine, was studied.

Simple soluble Bi(III) salts as efficient catalysts for the oxidation of alkanes with H2O2

A simple catalytic system based on a soluble bismuth(III) salt, Bi(NO3)3/H2O2/HNO3/CH3CN + H2O, exhibits pronounced activity towards the homogeneous oxidation of inert alkanes with the yield of oxygenate products up to 32% and TON up to 112. The experimental selectivity parameters and kinetic data together with theoretical DFT calculations indicate that the reaction occurs via a free radical mechanism involving the formation of the HO˙ radicals which directly react with alkane molecules. The mechanism of the HO˙ generation (which is the rate limiting step of the whole process) includes the substitution of a water ligand for H2O2 in the initial aqua complex [Bi(H2O)8]3+, hydrolysis of the coordinated H2O2, second H2O-for-H2O2 substitution and the homolytic HO–OH bond cleavage in complex [Bi(H2O)4(H2O2)(OOH)]2+ (6). The relatively low overall activation energy for this process (ca. 20 kcal mol−1) is accounted for by the high lability and acidity of the Bi aqua complexes and tremendous activation of coordinated H2O2 in 6 towards homolysis.

Oxidation of olefins with H2O2 catalysed by salts of group III metals (Ga, In, Sc, Y and La): epoxidation versus hydroperoxidation

The catalytic activity of aqua complexes of the group III metals [M(H2O)n]3+ (M = Ga, In, Sc, n = 6; M = Y, n = 8; M = La, n = 9) towards the oxidation of olefins with H2O2 was investigated in detail by theoretical (DFT) methods. It was predicted and then confirmed in a preliminary experiment that these complexes formed from simple soluble salts in aqueous medium are able to efficiently catalyse the olefin oxidation. The reaction occurs via two competitive reaction channels which are realized concurrently, i.e. (i) hydroperoxidation of the allylic C atom(s) via a radical Fenton-like mechanism involving HO˙ radicals and leading to alkyl hydroperoxides ROOH and (ii) epoxidation of the CC bond through a one-step mechanism involving oxygen transfer from the hydroperoxo ligand in an active catalytic form [M(H2O)n−k(OOH)]2+ (M = Ga, In, Y, La, k = 2; M = Sc, k = 1) to the olefin molecule and leading to epoxides and/or trans-diols. Other concerted and stepwise mechanisms of the epoxidation were also considered but found less favourable.

Platinum Complexes with Chelating Acyclic Aminocarbene Ligands Work as Catalysts for Hydrosilylation of Alkynes

This work describes the preparation of a series of platinum–aminocarbene complexes [PtCl{C(N=Ca(C6R2R3R4R5CONb))=N(H)R1}(CNR1)]a–b (8–19, 65–75% isolated yield) via the reaction of cis-[PtCl2(CNR1)2] (R1 = Cy 1, t-Bu 2, Xyl 3, 2-Cl-6-MeC6H34) with 3-iminoisoindolin-1-ones HN=Ca(C6R2R3R4R5CONbH) (R2–R5 = H 5; R3 = Me, R2, R4, R5 = H 6; R3, R4 = Cl, R2, R5 = H 7). New complexes 17–19 were characterized by elemental analyses (C, H, N), ESI+-MS, Fourier transform infrared spectroscopy (FT-IR), one-dimensional (1H, 13C{1H}), and two-dimensional (1H,1H correlation spectroscopy (COSY), 1H,13C heteronuclear multiple quantum correlation (HMQC)/1H,13C heteronuclear single quantum coherence (HSQC), 1H,13C heteronuclear multiple bond correlation (HMBC)) NMR spectroscopy, and authenticity of known species 8–16 was confirmed by FT-IR and 1H and 13C{1H} NMR. Complexes 8–19 were assessed as catalysts for hydrosilylation of terminal alkynes with hydrosilanes to give vinyl silanes, and complex [PtCl{C(N=Ca(C6H3(5-Me)CONb))=N(H)(2-Cl-6-MeC6H3)}{CN(2-Cl-6-MeC6H3)}]a−b (18) showed the highest catalytic activity. The catalytic system proposed operates at 80–100 °C for 4–6 h in toluene and with catalyst loading of 0.1 mol %, enabling the reaction of a number of terminal alkynes (PhC≡CH, t-BuC≡CH, and 4-(t-Bu)C6H4C≡CH) with hydrosilanes (Et3SiH, Pr3SiH, i-Pr3SiH, and PhMe2SiH). Target vinyl silanes were prepared in 48–95% yields (as a mixture of α/β isomers) and with maximum turnover number of 8.4 × 103. Hydrosilylation of internal alkynes (PhC≡CPh, Me(CH2)2C≡C(CH2)2Me, and PhC≡CMe) with hydrosilanes (Et3SiH, PhMe2SiH) led to the corresponding trisubstituted silylated alkenes in 86–94% yields. Initial observations on the mechanism of the catalytic action of platinum–ADC catalysts 8–19 suggested a molecular catalytic cycle.

Cobalt(II) complexes with pyridine and 5-[(E)-2-(aryl)-1-diazenyl]-quinolin-8-olates: synthesis, electrochemistry and X-ray structural characterization

Abstract Nine new cobalt(II) compounds, trans-[Co(LPAQ)2(Py)2] (1), trans-[Co(LPAQ)2(3-MePy)2] (2), trans-[Co(LMeAQ)2(Py)2] (3), trans-[Co(LOMeAQ)2(Py)2] (4), trans-[Co(LOEtAQ)2(Py)2]·2(H2O) (5), trans-[Co(LCAQ)2(Py)2] (6), trans-[Co(LBAQ)2(Py)2] (7), cis-[Co(LBAQ)2(3-MePy)2] (8a) and trans-[Co(LBAQ)2(3-MePy)2]·2(3-MePy) (8b) (primary ligand: LXAQ = substituted 5-[(E)-2-(aryl)-1-diazenyl]quinolin-8-olate; secondary ligands: Py = pyridine, 3-MePy = 3-methylpyridine), have been synthesized and characterized by elemental analysis, IR and UV-vis spectroscopy. Magnetic measurements of the cobalt compounds were performed in solution by 1H NMR spectroscopy using the Evans’ method while their redox properties were studied by cyclic voltammetry. Single-crystal X-ray diffraction analysis of the compounds revealed their octahedral geometries and trans configuration, except for 8a, which has a cis configuration. Intermolecular noncovalent interactions were detected, π···π interactions in 5, C – H···π interactions in 2 and C – H···π edge-to-face (T-shaped) arrangements in 3, 4, 6, and 7. Graphical Abstract