Synthesis and structure of nickel and copper complexes containing the N-allyl-o-hydroxyacetophenoniminato ligand and the application of copper complex as catalyst for aerobic alcohol oxidations

• Published in: Inorganica Chimica Acta Vol. 455; pp. 638 - 644
• Main Authors: Conejo, María del Mar, Ávila, Paloma, Álvarez, Eleuterio, Galindo, Agustín
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier B.V 30-01-2017; Elsevier Science Ltd
• Subjects: Benzaldehyde; Benzyl alcohol; Catalysis; Catalysts; Catalytic activity; Chemical synthesis; Coordination compounds; Copper; Copper compounds; DFT; Ionic liquids; Ligands; Nickel; Octanol; Oxidation; Substrates; Toluene; X-ray diffraction; Ionic liquids; Nickel; Oxidation; DFT; Catalysis; Copper
• ISSN: 0020-1693; 1873-3255
• DOI: 10.1016/j.ica.2016.04.051

Complex [Cu(Lallyl)2] is a selective catalyst, in combination with TEMPO, for the oxidation of benzylic-type alcohols using air at atmospheric pressure and room temperature. The use of ionic liquids as solvents gave good aldehyde yields and allowed catalyst recycling. [Display omitted]
•The novel compound HLallyl was prepared by a solvent-free synthetic procedure.•The first structurally characterised complexes with the Lallyl ligand are reported.•Cu-complex is a good catalyst for the aerobic oxidation of benzylic-type alcohols.•The use of ionic liquids as solvents allowed the catalyst recycling.
The ligand precursor 2-(1-(allylimino)ethyl)phenol, HLallyl, was synthesized by condensation of o-hydroxyacetophenone and allylamine, in the absence of solvent, and was analytically and spectroscopically characterized. The reaction of compound HLallyl with nickel acetate and copper acetate in the presence of NEt3 afforded the corresponding complexes [M(Lallyl)2] (M=Ni, 1; Cu, 2), which were structurally characterized by X-ray diffraction methods. A four-coordinate square planar environment around the metal centre was found for both compounds, in which the two N-allyl-o-hydroxyacetophenoniminato ligands, Lallyl, act as bidentate N,O-chelators and lie in a pseudo-trans conformation. Copper complex [Cu(Lallyl)2], 2, was tested as catalyst, in combination with TEMPO (2,2,6,6-tetramethylpiperidine-N-oxyl radical), for the aerobic oxidation of benzyl alcohol as model reaction. Importantly, by using air at atmospheric pressure and room temperature, good yields of the corresponding aldehyde were observed after 18h in toluene as solvent. After the optimization of the reaction conditions, the study was extended to other alcohol substrates and good catalytic activity was found for benzylic-type alcohols, while low-medium yield was found for 1-octanol. Interestingly, the use of ionic liquids [C4mim]PF6 and [C8mim]PF6 (C4mim=1-butyl-3-methylimidazolium; C8mim=1-octyl-3-methylimidazolium) as reaction solvents gave comparable aldehyde yields in the Cu/TEMPO-catalyzed oxidation of benzylic-type alcohols and this allowed the catalyst recycling. In fact, benzaldehyde yields remained approximately constant after three catalytic cycles in [C4mim]PF6, but from fourth cycle a small drop in activity was detected.