Russian Chemical Bulletin, International Edition, Vol. 59, No. 4, pp. 724—732, April, 2010 724 Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 710—718, April, 2010. 1066-5285/10/5904-0724 © 2010 Springer Science+Business Media, Inc. Spectroscopic and electrochemical study of dinuclear and mononuclear copper complexes with the bidentate ligand of the 2,2´-biquinoline series S. Z. Vatsadze, a A. V. Dolganov, a A. V. Yakimanskii, b M. Ya. Goikhman, b I. V. Podeshvo, b K. A. Lyssenko, c A. L. Maksimov, a and T. V. Magdesieva a a Department of Chemistry , M. V. Lomonosov Moscow State University, Build. 3, 1 Leninskie Gory, 119992 Moscow, Russian Federation. E-mail: tvm@org.chem.msu.ru b Institute of Macromolecular Compounds, Russian Academy of Sciences, 31 Bolshoi prosp., 199004 St. Petersburg, Russian Federation c A. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, 28 ul. Vavilova, 119991 Moscow, Russian Federation The newly synthesized complex (2) of copper(I) chloride with di-n-hexyl 2,2´-biquinoline- 4,4´-dicarboxylate (L) was spectroscopically and electrochemically characterized. The X-ray diffraction study showed that the crystals of complex 2 consist of the dinuclear moieties [L 2 Cu I 2 (μ-Cl) 2 ] containing Cu 2 (μ-Cl) 2 clusters. Spectrophotometric studies and ESI-mass spec- trometric measurements showed that after the dissolution of complex 2 in acetonitrile (AN) and N-methyl-2-pyrrolidone (NMP), the solution contained not only the dinuclear complexes [L 2 Cu I 2 (μ-Cl) 2 ] but also [L 2 Cu I ]Cl, [LCu I Cl(Sol)], and [Cu I Cl(Sol)] (Sol is the solvent). The electrochemical data also confirm the conclusion that bridged dinuclear chloride complex 2 dissociates both in NMP and AN to form the tetrahedral bis-biquinoline complex [L 2 Cu I ]Cl. In solutions of complex 2 in alcohols and N,N-dimethylformamide (DMF), only [L 2 Cu I ]Cl and [Cu I Cl(Sol)] are present. In EtOH, AN, and DMF, [Cu I Cl(Sol)] undergoes disproportionation to [Cu II Cl(Sol)] and Cu 0 . Key words: crystal engineering, supramolecular chemistry, coordination compounds, 2,2´-biquinolines, X-ray diffraction study, mass spectrometry, electrochemistry, spectro- photometry. Copper coordination compounds with 1,4-diimine- type ligands have attracted interest because they can be used to study the relationship between the redox, magnet- ic, and spectroscopic properties of the complexes, on the one hand, and the coordination geometry of the metal atom, on the other hand. 1 The ability to control the above- mentioned properties by varying the coordination envi- ronment of the metal atoms and to control the coordina- tion of the complexes by changing the external conditions allows one to design aesthetically attractive and function- ally promising structures. 2,3 In particular, the reversible Cu I /Cu II redox transition can be used to influence the macroscopic solution → ← gel transitions. The ability to con- trol the solid-state structures of the complexes is a key concept of the so-called crystal engineering, whose main aim is to synthesize desired crystal structures from appro- priately chosen molecular components. 5,6 The use of bridg- ing ligands as linkages between individual molecules of solid-state complexes is an equally important problem. This mainly refers to simple acido ligands, such as halides and carboxylates. Due to the lability of the copper—acido ligand bonds, it is possible to design systems, in which reversible transitions between two or several states can occur as a result of the replacement of anions by various coordinating molecules, primarily, by solvents. 2,2´-Biquinoline (biq) is the well known 1,4-diimine- type ligand. In the studies 7—9 on the monovalent copper complexes of biquinolines with different chemical struc- tures, it was noted that these systems hold promise as catalysts for the polymerization, photocatalytic devices, and biologically active compounds. In terms of crystal engineering, copper(II) complexes with 2,2´-bipyridine (bipy), phenanthroline (phen), and their derivatives related to 2,2´-biquinoline were studied in most detail. 10,11 In these systems, the chlorine-bridged dinuclear complexes [L 2 Cu II 2 (μ-Cl) 2 ]} 2+ (L = bipy 10,12—18 or phen 11,19—24 ) containing the metal atom in a square- pyramidal coordination environment belong to the most widespread structural type. Copper complexes with ligands of the 2,2´-biquinoline series have not been studied in terms of crystal engineer- ing. We found the only characterized crystal structure of