Transition metal complexes of a tridentate ligand bearing two pendant pyridine bases: The X-ray crystal structure of pentacoordinate copper(II) complex Kalagouda Gudasi a, * , Ramesh Vadavi a , Rashmi Shenoy a , Manjula Patil a , Siddappa A. Patil a , Munirathinam Nethaji b a Department of Chemistry, Karnatak University, Pavate Nagar, Dharwad 580 003, Karnataka, India b Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560 012, India Received 20 March 2005; received in revised form 21 July 2005; accepted 21 July 2005 Available online 31 August 2005 Abstract The synthesis of a tridentate ligand, N,N 0 -bis(2-pyridinyl)-2,6-pyridinedicarboxamide [H 2 L] is described together with its man- ganese(II), cobalt(II), nickel(II), copper(II), zinc(II) and cadmium(II) complexes which were characterized based on elemental anal- ysis, conductivity measurements, spectral, magnetic and thermal studies. The IR spectral studies of all the complexes exhibit a similar feature about the ligating nature of the ligand to the metal ions and revealed that the ligand has coordinated through the nitrogens of the deprotonated amides and the central pyridine. The two pendant pyridine nitrogens in all the complexes are proton- ated and involved in hydrogen bonding with the oxygens of amide groups. This observation is confirmed by the single-crystal X-ray crystallographic studies of copper(II) complex. The geometry around the copper atom can be viewed as a distorted trigonal bipyr- amid with s = 0.74 [structural parameter, s =(b À a)/60; where a and b are the two basal angles in a five coordinate complex]. The electrochemical study of the copper(II) complex shows single quasi-reversible redox peak [Cu(II) M Cu(I)]. The EPR spectrum of copper(II) complex exhibits rhombic pattern [g 1 = 2.0276, g 2 = 2.0926 and g 3 = 2.18]. Ó 2005 Elsevier B.V. All rights reserved. Keywords: Protonated pendant pyridines; Pyridinecarboxamides; Single crystal; Pentacoordinate copper(II) complex 1. Introduction The basic unit of peptide linkage [–C (O) NH–] group has been a part of primary structure of proteins and is of crucial importance in biological systems. Pyridinecarb- oxamides that contain this linkage acts as a multidentate ligand towards the transition metal salts [1,2] and have applications in asymmetric catalysis [3,4], molecular receptors [5,6], and platinum(II) complexes with antitu- mour properties [7,8]. In particular many derivatives of 2,6-pyridinedicarboxamides show anti-inflammatory, antipyretic and analgesic activities [9]. N,N 0 -Dipyridyl- 2,6-pyridinedicarboxamides have been reported to be useful in the treatment of atherosclerosis [9]. The crystal structure of the N,N 0 -bis(2-pyridinyl)-2,6-pyridinedi- carboxamide [H 2 L] [8] and its copper(II) complex [8] re- veals that the pendant pyridine nitrogen is directed towards the molecular cleft as shown in Fig. 2. The H 2 L upon deprotonation with sodium hydride reacts with copper(II) tetrakis(pyridine)perchlorate yielded the dimeric complex [8]. In principle, transition metal ions can form com- plexes with amide via oxygen or nitrogen atoms. Depending upon the basicity differences between oxygen 0020-1693/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2005.07.033 * Corresponding author. Tel.: +91 836 2460129 (Res.)/2215286 (Off.); fax: +91 836 2771275/2747884. E-mail address: kbgudasi@rediffmail.com (K. Gudasi). www.elsevier.com/locate/ica Inorganica Chimica Acta 358 (2005) 3799–3806