Synthesis, characterization, and anion selectivity of copper(II) complexes with a tetradentate Schiﬀ base ligand Shouvik Chattopadhyay a , Michael G.B. Drew b , Ashutosh Ghosh a, * a Department of Chemistry, University College of Science, University of Calcutta, 92, A.P.C. Road, Kolkata 700 009, West Bengal, India b School of Chemistry, The University of Reading, P.O. Box 224, Whiteknights, Reading RG6 6AD, UK Received 25 March 2006; received in revised form 24 May 2006; accepted 26 May 2006 Available online 9 June 2006 Abstract A new mononuclear Cu(II) complex, [CuL(ClO 4 ) 2 ](1) has been derived from symmetrical tetradentate di-Schiﬀ base, N,N 0 -bis-(1- pyridin-2-yl-ethylidene)-propane-1,3-diamine (L) and characterized by X-ray crystallography.The copper atom assumes a tetragonally distorted octahedral geometry with two perchlorate oxygens coordinated very weakly in the axial positions.Reactions of 1 with sodium azide, ammonium thiocyanate or sodium nitrite solution yielded compounds [CuL(N 3 )]ClO 4 (2), [CuL(SCN)]ClO 4 (3) or [CuL(NO 2 )]- ClO 4 (4), respectively, all of which have been characterized by X-ray analysis.The geometries of the penta-coordinated copper(II) in complexes 2–4 are intermediate between square pyramid and trigonal bipyramid (tbp) having the Addition parameters (s) 0.47, 0.45 and 0.58, respectively.In complex 4, the nitrite ion is coordinated as a chelating ligand and essentially both the O atoms of the nitrite occupy one axial site.Complex 1 shows distinct preference for the anion in the order SCN  > N 3  > NO 2  in forming the complexes 2– 4 when treated with a SCN  =N 3  =NO 2  mixture. Electrochemical electron transfer study reveals Cu II Cu I reduction in acetonitrile solution. Ó 2006 Elsevier B.V. All rights reserved. Keywords: Copper(II); Di-Schiﬀ base; Crystal structure; Anion selection 1. Introduction Encouragement for the syntheses of the symmetrical Schiﬀ bases derived from 2-pyridyl carbonyl compounds and for the study of their physical and chemical properties is provided by their potential abilities in biological modeling application, e.g. to mimic the N 4 donor set of copper(II) in superoxide dismutase (SOD) [1–7], a copper(II)–Zn(II) enzyme which destroys the toxic superoxide by catalyzing its disproportionation into peroxide and oxygen [8–12]. The copper atom in SOD is located in the centre of a dis- torted square pyramid of ﬁve coordinating atoms, four nitrogen atoms of four histidylimidazolate residues from the base and the water molecule occupies the apical site. The hexa-coordinated Cu(II) ion with d 9 conﬁguration prefers distorted octahedral geometry which is a direct con- sequence of Jahn–Teller eﬀect [13]. Thus, with a set of four strongly and two weekly coordinating ligands, the later two always occupy the axial positions. We would like to inves- tigate if any change in geometry around Cu(II) occur when one of these weekly coordinating groups (e.g. perchlorate ion) is replaced by another anion of stronger coordinating ability. We are also interested to study whether such replacement reaction shows any preference for one anion over others when treated with a mixture of anions. To our knowledge, no study of such anion selection has been reported in the literature till date. In the present paper, we report the synthesis, character- ization and X-ray crystal structure analysis of four cop- per(II) complexes with a tetradentate symmetrical di-Schiﬀ base ligand, N,N 0 -bis-(1-pyridin-2-yl-ethylidene)-propane- 0020-1693/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2006.05.029 * Corresponding author. E-mail address: ghosh_59@yahoo.com (A. Ghosh). www.elsevier.com/locate/ica Inorganica Chimica Acta 359 (2006) 4519–4525