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Relevant academic research and scientific papers

Selective direct hydroxylation of benzene to phenol with hydrogen peroxide by iron and vanadyl based homogeneous and heterogeneous catalysts

A series of first row transition metal complexes with Schiff base ligands or with readily available acetylacetonate ligands were screened as homogeneous catalysts in the challenging direct hydroxylation of benzene to phenol under mild conditions. Phenol was the main product of the oxidation of benzene in acetonitrile at 50°C using hydrogen peroxide as the oxidant. The Fe(II/III) complexes with Schiff base ligands were the best homogeneous catalysts using only 1% mol based on the substrate with very high selectivities. All the Fe(II/III) complexes were more selective towards phenol than the VO(IV) complexes. The Fe(II/III) Schiff base complexes were more active than the readily available Fe(II/III) acetylacetonate complexes, whereas the opposite was observed for the VO(IV) complexes. The room temperature reaction gave a significantly lower phenol yield. Due to their easy anchoring, the readily available Fe(II), Fe(III) and VO(IV) acetylacetonate complexes were immobilized onto economical porous supports, hexagonal mesoporous silica and activated carbon, which were previously functionalized with amine groups. An Fe(III) Schiff base complex was also anchored onto these materials activated with a NaOH solution. The materials were characterized by elemental analysis, ICP-AES, FTIR spectroscopy, adsorption isotherm analysis at 77 K and TG analysis, showing that both porous materials were conveniently functionalized and that the metal complexes could be effectively anchored onto these materials. All the heterogeneous catalysts prepared were active and selective in the direct oxidation of benzene to phenol with higher yield and selectivity than the original metal salts or complex in homogeneous phase. For the immobilized Fe(II/III) acetylacetonate complexes, a significant improvement in the phenol yields was observed in comparison with the corresponding homogeneous phase catalysts, showing that the incorporation of nitrogen atoms on the metal coordination sphere improves the catalytic activity. The heterogeneous catalysts could also be recycled by simple filtration and re-used in at least three more consecutive cycles without significant loss of catalytic activity. Phenol yields comparable to or higher than the best reported in the literature were obtained.

Electroactive Co(iii) salen metal complexes and the electrophoretic deposition of their porous organic polymers onto glassy carbon

This paper reports the CO2 electroreduction properties of three bis-bromo Co(iii) salen metal complexes and their Porous Organic Polymers (POPs) as a platform for using the salen core as a multi-electron reducing agent. Although Co(iii) salen metal complexes have been studied extensively for their chemical catalysis with CO2, their electrochemical behaviour, particularly their reduction, in the presence of CO2 is much less explored. The discrete Co(iii) complexes enabled the reduction of CO2 to CO in faradaic efficiencies of up to 20%. The reductive electrochemical processes of Co(iii) salen complexes are relatively unknown; therefore, the mechanism of reduction for the complexes was investigated using IR and UV-Vis-NIR spectroelectrochemical (SEC) techniques. The discrete bis-bromo salen complexes were incorporated into POPs with tris-(p-ethynyl)-triphenylamine as a co-ligand and were characterised using solid state NMR, IR, UV-Vis-NIR and Field Emission Scanning Electron Microscopy (FE-SEM). The POP materials were electrophoretically deposited onto glassy carbon under milder conditions than those previously reported in the literature. Direct attachment of the POP materials to glassy carbon enabled improved solid state electrochemical analysis of the samples. The POP materials were also analysed via SEC techniques, where a Co(ii/i) process could be observed, but further reductions associated with the imine reduction compromised the stability of the POPs.

Oxidation of cyclohexane by transition-metal complexes with biomimetic ligands

This work reports the catalytic activity, in homogeneous phase, of transition-metal complexes of the first-row (V(IV), Mn(III), Fe(III) Co(III) and Cu(II)) with biomimetic Schiff base ligands with N2O2 coordination sphere, as well as an N4 (Fe(II)), in the room-temperature oxidation of cyclohexane using environmentally benign reagents: hydrogen peroxide (30 wt%) as the oxygen source and acetonitrile as the solvent. Nitric acid is also used as promoter of the oxidation reaction. The structure of the ligands is confirmed by FTIR, 1H NMR and high-resolution ESI mass spectrometry. The corresponding transition metal complexes are characterized by elemental analysis, high resolution ESI mass spectrometry, FTIR and UV-vis. Cyclohexanone and cyclohexanol are the main products of the oxidation of cyclohexane, obtained when the following complexes are used as homogeneous catalysts in only 1 mol% based on the substrate: VO(IV), Fe(III) and Cu(II) complexes with the N2O2 Schiff base, new Fe(II) complex with the Schiff base with N4 coordination sphere and commercial [VO(acac)2] with O4 coordination sphere. The Fe(III) complex with N2O2 Schiff base ligand ([Fe(salhd)Cl]) is the homogeneous catalyst with highest activity, which could be further enhanced by the addition of methyl electron donating groups to the N2O2 Schiff base aldehyde fragment (reaching 46% oxygenate yields and 45 turnover numbers). Cyclooctane and n-hexane could also be oxidized to the corresponding ketones and alcohols with higher turnover numbers than cyclohexane by the Fe(III) complex with N2O2 Schiff base ligand.