FULL PAPER DOI:10.1002/ejic.201402352 CLUSTER ISSUE Polynuclear Copper(II) Complexes as Catalysts for the Peroxidative Oxidation of Cyclohexane in a Room-Temperature Ionic Liquid Raja Jlassi, [a,b] Ana P. C. Ribeiro, [a] M. Fátima C. Guedes da Silva,* [a] Kamran T. Mahmudov,* [a,c] Maximilian N. Kopylovich, [a] Tatiana B. Anisimova, [a] Houcine Naïli, [b] Gonçalo A. O. Tiago, [a] and Armando J. L. Pombeiro* [a] Keywords: N,O ligands / Copper / Oxidation / Ionic liquids / Homogeneous catalysis The reaction of copper(II) nitrate hydrate with sodium 2-[2- (2,4-dioxopentan-3-ylidene)hydrazinyl]terephthalate (Na 2 HL 1 ) in the absence or presence of imidazole (im) leads to the new complexes [(H 2 O)Cu 0.5 {(μ-L 1 -1κO:2κ 3 N,O',O'')Cu(H 2 O) 2 }] 2 (1) and [Cu(im)(μ-HL 1 -1κO:2κ 3 N,O,O')] 2 (2), respectively, whereas the 1D coordination polymer [Cu 2 (H 2 O) 2 (μ-H 2 L 2 - 1κ 3 N,O,O':2κO'')(μ-L 2 -1κ 3 N,O,O':2κ 2 N',O'')] n (3) is obtained from Cu(NO 3 ) 2 ·2.5H 2 O and sodium 2-[2-(4,4-dimethyl-2,6- dioxocyclohexylidene)hydrazinyl]terephthalate (Na 2 HL 2 ). All of the compounds were characterized by ESI-MS, IR spectroscopy, elemental analysis and X-ray diffraction analy- sis. Di- or tri-deprotonated ligands display different coordina- Introduction Arylhydrazones of β-diketones (AHBD) [1] have been ex- tensively used as multidentate ligands, [2] and their com- plexes possess a wide variety of useful properties. For exam- ple, they can be used as catalysts, [3] sensors or analytical reagents, [4] optical storage media [5] or spin-coating films. [6] Depending on the position and nature of the substituents in the β-diketone fragment and the aromatic moiety of the AHBD molecules and on the metal ion, different types of coordination can be achieved; this allows the design of [a] Centro de Química Estrutural, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1049-001 Lisbon, Portugal E-mail: fatima.guedes@tecnico.ulisboa.pt pombeiro@tecnico.ulisboa.pt http://cqe.ist.utl.pt/members/group5_memb.php [b] Laboratoire de Physico-Chimie de l’Etat Solide, Département de Chimie, Faculté des Sciences deSfax, Université de Sfax, BP 1171, 3000 SFax, Tunisia [c] Department of Chemistry, Baku State University, Z. Xalilov Str. 23, Az 1148 Baku, Azerbaijan E-mail: kamran_chem@yahoo.com http://bsu.edu.az/en/ Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/ejic.201402352. Eur. J. Inorg. Chem. 2014, 4541–4550 © 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 4541 tion modes and lead to distinct nuclearities of the Cu II com- plexes depending on the conditions used for the syntheses. Complexes 1–3 were successfully tested as dual-role cata- lysts for cyclohexane oxidation under acid-free conditions in the ionic liquid [1-butyl-3-methylimidazolium tetrafluoro- borate, (bmim)(BF 4 )] as well as in acetonitrile. The applica- tion of the ionic liquid (IL) increased the product yield and turnover number and decreased the reaction time, in com- parison with those for the use of the conventional acetonitrile solvent. Easy recycling of the catalyst, with preservation of almost the initial activity and selectivity, was also achieved by using the IL medium. metal complexes with the properties required for specific applications. For example, the water solubility of AHBDs and their complexes can be increased by the introduction of hydrophilic polar groups such as sulfonic, carboxy or nitro groups. [7] On the other hand, the combination of an active metal centre and an acidic promoting group in a sin- gle molecule in some instances leads to “dual-role” catalysts for alkane peroxidative oxidations under acid-free condi- tions. [8] This approach allows the simplification of the cata- lytic system and the elimination of some hazardous ingredi- ents and, therefore, it can be of significance for useful appli- cations. Therefore, we decided to modify AHBD ligands with several carboxylic groups to increase the water solubil- ity of their complexes and to introduce several acidic sites into the catalyst. Moreover, auxiliary ligands can also be used as proton-acceptor or -donor pH-tuneable sites within the complex molecules; this principle is widely followed in biological catalytic systems and was also attempted in the current study. Imidazole (im) is a widespread moiety in en- zymes and was also applied as an auxiliary ligand in this study. Copper(II) complexes can catalyze the functionalization of inert C–H bonds of hydrocarbons. [9] The oxidation of