Synthesis and structure of a copper(II) tetranuclear complex of 3-(6-methyl-2-pyridyl)-5-(2-pyridyl)pyrazole ligand Josefina Pons a, * , Francisco Javier Sanchez a , Jaume Casab o a , Angel Alvarez-Larena b , Joan Francesc Piniella b , Josep Ros a a Departament de Qu  ımica (Inorganica) Facultat de Ciencies, Unitat de Qu  ımica Inorg  anica, Universitat Aut  onoma de Barcelona, Bellaterra 08193, Barcelona, Spain b Departament de Geologia, Unitat de Cristal.lografia, Universitat Aut  onoma de Barcelona, Bellaterra 08193, Barcelona, Spain Received 17 February 2003; accepted 6 March 2003 Abstract The reaction of the ligand 3-(6-methyl-2-pyridyl)-5-(2-pyridyl)pyrazole (HL) with CuðNO 3 Þ 2  3H 2 O gives a complex of formula ½Cu 4 L 4 ðNO 3 Þ 3 ðNO 3 Þ, which has been characterised by elemental analyses, infrared and electronic spectroscopies and single-crystal X-ray diffraction. The molecular structure of this complex is tetranuclear. The Cu 2þ ions are linked to four ligands, forming a macromolecular cyclic system. The co-ordination geometries of the Cu(II) are square pyramid. The formation of this complex implies a control of nuclearity by steric hindrance. Ó 2003 Elsevier Science B.V. All rights reserved. Keywords: Copper complex; Pyrazolate bridge; Polynuclear complex; Crystal structure 1. Introduction The polymetallic systems with extended bridges are of interest to biologists and bioinorganic chemists because of their structure similar to those found for the poly- nuclear metal centres in proteins, to physics and physical inorganic chemists that investigate new magnetic mate- rials [1]. Pyrazolate-bridged binuclear and polynuclear tran- sition metal compounds have attracted interest for many years [2–4]. The pyrazole ligand can coordinate in a monodentate mode or as a bridging group between two metals. An interesting kind of pyrazole-derived li- gand can be obtained by attaching two chelating arms to the 3- and 5-positions of the pyrazole ring [5–7], that increases the number of coordination atoms. Metal complexes of some of these ligands have also been studied [1–6]. In spite of the versatile properties of these ligands in the formation of multi-metallic transition metal complexes, they usually display a tetradentate coordination mode, bridging two metal centres. The ligands 6-(3-pyrazolyl)-2,2 0 -bipyridine and 3-(2-pyridyl) pyrazole have been recently used in the preparation of grid-like tetranuclear Cu(II) complexes [8,9]. In previous studies [10–12] we reported the synthesis of a dinucleating pyrazole ligand with two pyridines as chelating arms attached to the 3 and 5 positions: 3,5- bis(2-pyridyl)pyrazole (Hbpypz) and a series of metal complexes with divalent metal ions Ni(II), Co(II), Zn(II) and Cu(II). Structural data for the Ni(II) [10] and the Cu(II) [11] complexes have shown the formation of di- nuclear complexes in which the ligand exhibits a strictly planar bridging mode of co-ordination. Munakata et al. [13] following other study, have as- sayed the reactivity of Hbpypz with Ag(I) and Cu(II). The structure of the Cu(II) complex is similar to that previously reported by us. In order to evaluate the influence of the replace- ment of the ortho H of the pyridine by a methyl group on the nuclearity of Cu(II) complex, we have synthesized a Cu(II) complex with the ligand 3-(6- methyl-2-pyridyl)-5-(2-pyridyl)pyrazole [14] (Fig. 1), with the finality of comparing the structure with that of the Cu(II) complex with 3,5-bis(2-pyridyl)pyrazole ligand [11]. Inorganic Chemistry Communications 6 (2003) 833–836 www.elsevier.com/locate/inoche * Corresponding author. Fax: +34-93-581-3101. E-mail address: josefina.pons@uab.es (J. Pons). 1387-7003/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S1387-7003(03)00117-5