Synthesis, spectral characterization and redox studies of [1-alkyl-2-(naphthyl-b-azo)imidazole] palladium(II) catecholates Goutam Kumar Rauth, Joydev Dinda, Sk. Jasimuddin and Chittaranjan Sinha* Department of Chemistry, The University of Burdwan, Burdwan 713104, India Received 22 January 2002; accepted 09 April 2002 Abstract 1-Alkyl-2-(naphthyl-b-azo)imidazoles [b-NaiR; R ¼ Me (a), Et (b), CH 2 Ph (c)] react with Pd(MeCN) 2 Cl 2 to yield Pd(b-NaiR)Cl 2 (2), the i.r. spectra of which support the presence of a cis-PdCl 2 configuration. The complexes react with catechols in the presence of Et 3 N to yield ternary complexes [Pd(b-NaiR)(O,O)] [O,O ¼ pyrocatecholato (cat) (3),4-t-butylcatecholato (tbcat) (4), 3,5-di-t-butylcatecholato (dtbcat) (5), and tetrachlorocatecholato (tccat) (6)], which were characterized by elemental analysis, i.r. and 1 H-n.m.r. spectral data. Redox studies by cyclic voltammetry suggest the existence of four successive redox couples wherein two responses, positive to s.c.e. are due to catechol to semiquinone and semiquinone to quinone oxidation, respectively; the couples at negative to s.c.e. are referred to azo reductions. The complexes exhibit ligand-ligand charge-transfer transitions in the near-i.r. region. The band position is largely dependent upon the substitutent on the catechol frame and exhibits negative solvatochromic effects. The transition is qualitatively assigned as the HOMO (cat) fi LUMO (b-NaiR) transition. This fact is also supported by theoretical calculations using the PM3 method. Introduction As a sequel to our interest in exploring the transition metal chemistry of the azoimine system, N@NA C@NA, we have prepared ligands incorporating arylazo groups in the imidazole [1–10] heterocyclic backbone. The importance of arylazoimidazoles is reflected in publications, by our group [3–7] and by others [11]. This has encouraged us to prepare new members of this family, namely naphthylazoimidazoles [7]. The azoimine function is p-acidic and stabilizes the low valence metal redox state by promoting metal-to-ligand charge-trans- fer (MLCT) transitions [8]. The presence of r/p-donors as coligands (e.g. catecholates and their derivatives) in the complexes, where the metal ion is redox-innocent, can result in an interesting opto-electronic transition, known as a ligand-to-ligand charge-transfer (LLCT) [1, 10]. Catechols and their derivatives attract continuous attention [1, 2, 12–22] involving exploration of their different chemical, photophysical, biological, electro- chemical and medicinal properties. The primary focus has been placed on the syntheses and characterization of transition [1, 2, 12–20] and non-transition [21, 22] metal catecholato complexes. In view of the importance of imidazole, the azoimine function and catechols, herein we describe the synthesis, and spectral characterization of l-alkyl-2-(naphthyl-b-azo)imidazole-palladium(II)–cate- cholato complexes. Experimental Materials Solvents and reagents were purchased from the Sisco Research Laboratory, Mumbai, India and from E. Merck (Analytical Reagent grade), Germany. The purification of solvents, catechols, and preparation of [NB 4 ][ClO 4 ] for electrochemical work was carried out as described previously [1]. 1-Alkyl-2-(naphthyl-b-azo) imidazoles (b-NaiR) were prepared and purified by reported procedures [7]. All other chemicals and solvents were of reagent grade and were used as received. Physical measurements I.r. and u.v.–vis. Spectra were recorded on JASCO FT- IR (model 420) and UV/VIS/NIR (model V-570) spectrophotometers, JASCO International Co. Ltd., Tokyo, Japan respectively. 1 H-n.m.r. spectra were recorded on a Bruker 300 MHz FT-NMR spectrometer Bruker U.K. Ltd. Electrochemical studies were per- formed as described earlier [2]. Microanalytical data were obtained using a Perkin-Elmer 2400 CHN elemen- tal analyzer U.S.A. Theoretical calculations were carried out by the PM3 semi-empirical method as implemented in ZINDO version 3. Calculations were made from optimized geometry of the molecules. * Author for correspondence Transition Metal Chemistry 27: 88–95, 2003. 88 Ó 2003 Kluwer Academic Publishers. Printed in the Netherlands.