Trinuclear CuII Structural Isomers: Coordination, Magnetism, Electrochemistry and Catalytic Activity towards the Oxidation of Alkanes

• Published in: European journal of inorganic chemistry 2015-08, Vol.2015 (23), p.3959-3969
• Main Authors: Sutradhar, Manas, Martins, Luísa M. D. R. S., Guedes da Silva, M. Fátima C., Mahmudov, Kamran T., Liu, Cai-Ming, Pombeiro, Armando J. L.
• Format: Article
• Language: English
• Subjects: Copper; Electrochemistry; Magnetic properties; N,O ligands; O ligands; Oxidation
• ISSN: 1434-1948; 1099-0682
• DOI: 10.1002/ejic.201500440

The reaction of the Schiff base (3,5‐di‐tert‐butyl‐2‐hydroxybenzylidene)‐2‐hydroxybenzohydrazide (H3L) with copper(II) nitrate, acetate or metaborate has led to the isomeric complexes [Cu3(L)2(MeOH)4] (1), [Cu3(L)2(MeOH)2]·2MeOH (2) and [Cu3(L)2(MeOH)4] (3), respectively, in which the ligand L exhibits dianionic (HL2–, in 1) or trianionic (L3–, in 2 and 3) pentadentate 1κO,O′,N:2κN′,O″ chelation modes. Complexes 1–3 were characterized by elemental analysis, IR spectroscopy, single‐crystal X‐ray crystallography, electrochemical methods and variable‐temperature magnetic susceptibility measurements, which indicated that the intratrimer antiferromagnetic coupling is strong in the three complexes and that there exists very weak ferromagnetic intermolecular interactions in 1 but weak antiferromagnetic intermolecular interactions in both 2 and 3. Electrochemical experiments showed that in complexes 1–3 the CuII ions can be reduced, in distinct steps, to CuI and Cu0. All the complexes act as efficient catalyst precursors under mild conditions for the peroxidative oxidation of cyclohexane to cyclohexyl hydroperoxide, cyclohexanol and cyclohexanone, leading to overall yields (based on the alkane) of up to 31 % (TON = 1.55 × 103) after 6 h in the presence of pyrazinecarboxylic acid. Three isomeric CuII complexes derived from Schiff base (3,5‐di‐tert‐butyl‐2‐hydroxybenzylidene)‐2‐hydroxybenzohydrazide and three different CuII sources are reported. Their structural, electrochemical and magnetic properties as well as catalytic activity toward the peroxidative oxidation of cyclohexane to cyclohexyl hydroperoxide, cyclohexanol and cyclohexanone under mild conditions are described.