Copper(II) azide complexes of 1-alkyl-2-(arylazo)imidazoles: Structure and magnetism Umasankar Ray a , Kamal Krishna Sarker a , Golam Mostafa b , Tian-Huey Lu c , M. Salah El Fallah d, * , Chittaranjan Sinha a, * a Department of Chemistry, Inorganic Chemistry Section, Jadavpur University, Kolkata 700032, India b Department of Physics, Jadavpur University, Kolkata 700032, India c Department of Physics, National Tsing Hua University, Hsinchu 300, Taiwan, ROC d Department de Quı ´mica Inorga ` nica, Universitat de Barcelona, Martı ´ i Franque `s, 1-11, 08028 Barcelona, Spain Received 15 February 2006; accepted 2 April 2006 Available online 27 April 2006 Abstract Tetranuclear Cu(II)-azido complexes of the composition [Cu(RaaiR 0 )(N 3 ) 2 ] 4 have been synthesized by reacting 1-alkyl-2- (arylazo)imidazole (RaaiR 0 ), Cu(OAc) 2 Æ H 2 O and NaN 3 in 1:1:2 mole proportion [RaaiR 0 ;R=H(3), Me (4); R 0 = Me (a), Et (b)]. The structure of [Cu(HaaiMe)(N 3 ) 2 ] 4 (3a) has been confirmed by an X-ray diffraction study. The tetranuclear copper core is generated by dimerisation via l 1,1 -N 3 of two dimer units. In each dimer one copper centre is five-coordinated (CuN 5 ) and the other one is six-coor- dinated (CuN 6 ). Variable temperature magnetic measurements of 3a show the occurrence of ferromagnetic (J 1 = +25.3 cm 1 ) and anti- ferromagnetic (J 2 = 4.7 cm 1 ) interactions through the double and single end-on azido bridges, respectively. Upon addition of excess ligand, a 2-methoxy ethanol solution of the tetranuclear complex fragmented into a mononuclear copper(II) complex of the formula [Cu(RaaiR 0 ) 2 (N 3 ) 2 ], and in one case, [Cu(HaaiEt) 2 (N 3 ) 2 ], the structure was established by X-ray diffraction. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Copper(II)-azide; 1-Alkyl-2-(arylazo)imidazoles; Tetranuclear; Magnetic properties 1. Introduction Polynuclear metal complexes are of contemporary inter- est [1–7]. A careful selection of ligand, metal centre and reaction conditions can confer control over the topology of the resulting framework [8–10]. Pseudohalides, espe- cially azide, are versatile polyatomic bridging units that can bind metal ions via end-on (EO, l 1,1 ) and end-to-end (EE, l 1,3 ) motifs [11] with novel magnetic properties. This leads to extreme structural and electronic versatility allow- ing ferromagnetic (F) or antiferromagnetic (AF) coupling via EO and EE bridges, respectively, and tunes magnetic behaviors via modification of the bond parameters, in particular in the bridging region [12]. Different organic blockers [12,13] of varied denticities have been used to pre- pare such molecular architectures. Multidentate Schiff base ligands [12] and polyamines [13] have generally been used to stabilize the core unit. Use of the azo function in this direction has rarely been explored [14]. We have been engaged for some time in exploring the chemistry of 1-alkyl-2-(arylazo)imidazole with transition and non-transition metal ions [14–21]. Our understanding of copper complexes with arylazoimidazole has mainly focused on the structural, spectroscopic and electrochemi- cal properties of copper(I) complexes. Attempts to synthe- size copper(II)–RaaiR 0 have failed even under aerobic conditions in the presence of Cl  or ClO 4  as counter ions, and [Cu(RaaiR 0 ) 2 ] + has been crystallized [15]. However, the presence of N 3  or NCS  in the reaction mixture has enabled Cu(II) complexes to be stabilized [16]. Towards the preparation of multinuclear copper(II) complexes via 0277-5387/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2006.04.007 * Corresponding author. Fax: +91 33 2414 6584. E-mail address: c_r_sinha@yahoo.com (C. Sinha). www.elsevier.com/locate/poly Polyhedron 25 (2006) 2764–2772