The spectroscopic and structural properties of copper(II) complexes of the novel tridentate (ONO) pyridine N-oxide ligand Hpoxap Peter Baran a, *, Roman Boc ˇa b, *, Martin Breza c , Horst Elias d , Hartmut Fuess e , Vladimı ´r Jorı ´k b , Ro ´ bert Klement c , Ingrid Svoboda e a Department of Chemistry, University of Puerto Rico, Rio Piedras, P.O. Box 23346 San Juan 00931-3346, Puerto Rico b Department of Inorganic Chemistry, Slovak Technical University, SK-812 37 Bratislava, Slovakia c Department of Physical Chemistry, Slovak Technical University, SK-812 37 Bratislava, Slovakia d Institut fu ¨r Anorganische Chemie, Technische Universita ¨t Darmstadt, D-642 87 Darmstadt, Germany e Fachbereich Materialwissenschaft, Technische Universita ¨t Darmstadt, D-642 87 Darmstadt, Germany Received 29 June 2001; accepted 15 April 2002 Abstract A tridentate ligand (Hpoxap /2-(o -hydroxyphenyliminomethyl)pyridine N-oxide) has been synthesised by the Schiff condensa- tion of 2-pyridinecarboxaldehyde N-oxide with 2-aminophenol. A series of copper(II) complexes with this ONO-donor ligand has been prepared and their spectroscopic properties (electron, vibration, ESR) were studied. The structures for two complexes have been determined by X-ray analysis. The structure of [Cu(poxap)(OAc)] consists of isolated neutral molecules in which the copper(II) atom is situated in a slightly distorted square-planar surrounding. The structure of [Cu(poxap)(H 2 O)(NO 3 )] consists of neutral molecules which are interconnected by hydrogen bonds giving rise to a ladder structure of copper(II) ions with two different CuCu distances of 5.27 and 7.41 A ˚ . The copper atoms are in a slightly distorted square-pyramidal environment with the tridentate ligand poxap  and the water molecule in the basal plane; the apical position being occupied by an oxygen atom of the nitrate anion. Magnetic susceptibility studies were undertaken and these confirm predominantly a dimeric character of the complex. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: Pyridine N-oxide ligands, copper(II) complexes; X-ray structures; Hydrogen bonding; Magnetism 1. Introduction Transition metal complexes of pyridine N-oxide (pyNO) became attractive for their interesting magnetic properties [1]. The pyridine N-oxide group is able to bridge two metal centres and often a very strong antiferromagnetic coupling is observed. Pyridine N- oxides (1) have this in common with phenolates (2) (Eq. (1)). ð1Þ Besides the bridging coordination mode, both 1 and 2 can act as monodentate O donors. The difference in charge between 1 and 2 has an important influence on the stoichiometry of the complexes thus formed. The pyridine N-oxides are neutral ligands as are the pyr- idines 3 from which they are derived. Despite that, ligands 1 and 3 differ in the nature of the donor atoms, variability of coordination modes and especially in basicity [2]. Complexes of 3d metals with pyridine N-oxide and its derivatives were intensively studied in the 60s and 70s [3]. Later some potentially bidentate chelating ligands * Corresponding authors. Tel.:  /421-2-59325/610; fax:  /421-2- 52493198 E-mail address: boca@cvtstu.cvt.stuba.sk (R. Boc ˇa). Polyhedron 21 (2002) 1561  /1571 www.elsevier.com/locate/poly 0277-5387/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII:S0277-5387(02)01020-3