Dalton Transactions Dynamic Article Links Cite this: Dalton Trans., 2011, 40, 2822 www.rsc.org/dalton PAPER Complexes of copper(II) with 3-(ortho-substituted phenylhydrazo)pentane-2,4-diones: syntheses, properties and catalytic activity for cyclohexane oxidation† Maximilian N. Kopylovich, a Andreia C. C. Nunes, a Kamran T. Mahmudov, a Matti Haukka, b Tatiana C. O. Mac Leod, a Lu´ ısa M. D. R. S. Martins, a,c Maxim L. Kuznetsov a and Armando J. L. Pombeiro* a Received 3rd November 2010, Accepted 21st December 2010 DOI: 10.1039/c0dt01527j Reactions of copper(II) with 3-phenylhydrazopentane-2,4-diones X-2-C 6 H 4 –NHN C{C( O)CH 3 } 2 bearing a substituent in the ortho-position [X = OH (H 2 L 1 ) 1, AsO 3 H 2 (H 3 L 2 ) 2, Cl (HL 3 ) 3, SO 3 H (H 2 L 4 ) 4, COOCH 3 (HL 5 ) 5, COOH (H 2 L 6 ) 6, NO 2 (HL 7 ) 7 or H (HL 8 ) 8] lead to a variety of complexes including the monomeric [CuL 4 (H 2 O) 2 ]·H 2 O 10, [CuL 4 (H 2 O) 2 ] 11 and [Cu(HL 4 ) 2 (H 2 O) 4 ] 12, the dimeric [Cu 2 (H 2 O) 2 (m-HL 2 ) 2 ] 9 and the polymeric [Cu(m-L 6 )] n ] 13 ones, often bearing two fused six-membered metallacycles. Complexes 10–12 can interconvert, depending on pH and temperature, whereas the Cu(II) reactions with 4 in the presence of cyanoguanidine or imidazole (im) afford the monomeric compound [Cu(H 2 O) 4 {NCNC(NH 2 ) 2 } 2 ](HL 4 ) 2 ·6H 2 O 14 and the heteroligand polymer [Cu(m-L 4 )(im)] n 15, respectively. The compounds were characterized by single crystal X-ray diffraction (complexes), electrochemical and thermogravimetric studies, as well as elemental analysis, IR, 1 H and 13 C NMR spectroscopies (diones) and ESI-MS. The effects of the substituents in 1–8 on the HOMO–LUMO gap and the relative stability of the model compounds [Cu(OH)(L 8 )(H 2 O)]·H 2 O, [Cu(L 1 )(H 2 O) 2 ]·H 2 O and [Cu(L 4 )(H 2 O) 2 ]·H 2 O are discussed on the basis of DFT calculations that show the stabilization follows the order: two fused 6-membered > two fused 6-membered/5-membered > one 6-membered metallacycles. Complexes 9, 10, 12 and 13 act as catalyst precursors for the peroxidative oxidation (with H 2 O 2 ) of cyclohexane to cyclohexanol and cyclohexanone, in MeCN/H 2 O (total yields of ca. 20% with TONs up to 566), under mild conditions. Introduction 3-Phenylhydrazopentane-2,4-diones are azoderivatives of b- diketones (ADB) (Scheme 1) 1 potentially with a wide range of applications, 2 e.g. they can be used as optical recording media and Scheme 1 a Centro de Qu´ ımica Estrutural, Complexo I, Instituto Superior T´ ecnico, TU Lisbon, Av., Rovisco Pais, 1049–001, Lisbon, Portugal. E-mail: pombeiro@ist.utl.pt b University of Joensuu, Department of Chemistry, P.O. Box 111, FIN-80101, Joensuu, Finland c ´ Area Departamental de Engenharia Qu´ ımica, ISEL, R. Conselheiro Em´ ıdio Navarro, 1959-007, Lisboa, Portugal †Electronic supplementary information (ESI) available. CCDC reference numbers 798948–798954. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c0dt01527j spin-coating films 3 or be applied for further organic synthesis. 4 ortho-Hydroxy substituted ADB were effectively applied for the selective determination of copper(II) and iron(III) in various indus- trial and natural objects. 5 It was assumed that these compounds can exist in several tautomeric forms, and that the tautomeric equilibria can play an important role for applications e.g. as bistate molecular switches 6 or regulation of the selectivity of analytical reactions. 5b The hydrazo ⇋ enol-azo tautomeric transitions are connected with a p-electron delocalization within a so-called “resonance assisted hydrogen bond” (RAHB). 6a,7 Usually, in the solid phase all the structurally studied ADB are stabilized in the hydrazo form, 6,8 while in solution the investigated unsymmetrical ADB can exist as a mixture of enol-azo and hydrazo tautomers. 8e In spite of the above interest in ADB compounds, their coordination chemistry still remains underdeveloped, although some complexes e.g. with copper(II), nickel(II) or sodium(I) have been reported 8b,c and we have synthesized 8d,e a few copper(II) compounds with ortho-hydroxy substituted phenylhydrazo-b- diketones containing fused six- and five-membered metallacycles (Scheme 2a). We can anticipate that the five-membered metalla- cycle is under a higher tension than the six-membered cycle and that the complexes with a six-membered ring may be more stable. 2822 | Dalton Trans., 2011, 40, 2822–2836 This journal is © The Royal Society of Chemistry 2011 Downloaded by Universidade Tecnica de Lisboa (UTL) on 16 March 2011 Published on 08 February 2011 on http://pubs.rsc.org | doi:10.1039/C0DT01527J View Online