Inorganica Chimica Acta 321 (2001) 49 – 55 www.elsevier.nl/locate/ica Thermodynamic studies on the complexation of cobalt(II) with nitrogen donor ligands in dimethyl sulfoxide Clara Comuzzi, Michela Grespan, Pierluigi Polese, Roberto Portanova, Marilena Tolazzi * Dipartimento di Scienze e Tecnologie Chimiche, Uniersita ` di Udine, Via Cotonificio 108, I -33100 Udine, Italy Received 14 February 2001; accepted 25 May 2001 Abstract Co(II) complex formation with the N-donor ligands n -butylamine (n -but), diethylenetriamine (dien), N,N-dimethyldiethylene- triamine (dmdien) and N,N,N,N,N-pentamethyldiethylenetriamine (pmdien) has been studied at 298 K in the aprotic solvent dimethyl sulfoxide (dmso) in an ionic medium set to 0.1 mol dm -3 with NEt 4 ClO 4 in anaerobic conditions. Potentiometric, spectrophotometric and calorimetric measurements have been carried out to obtain the thermodynamic parameters for the systems investigated. Only mononuclear CoL j 2 + complexes are formed ( j =1–3 for n -but; j =1, 2 for dien and dmdien; j =1 for pmdien) where the triamines act prevalently as terdentate agents. All the complexes are enthalpy stabilized whereas the entropy changes counteract the complex formation. The results are discussed in term of different basicities and steric requirements of both the ligands and the complexes formed. Preliminary information on the different affinities of the complexes for dioxygen is reported. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Cobalt(II); Amines; Complexes; Thermodynamics; Dimethylsulfoxide 1. Introduction Complexes of a number of transition metals able to bind, transport and activate small molecules such as dioxygen, carbon monoxide, carbon dioxide, nitric ox- ide and sulfur dioxide, both natural (metallo-proteins and -enzymes) and synthetic, are very important in chemical, biochemical, biological, environmental and industrial fields [1 – 15]. The activation of these small molecules usually involves a direct or indirect interac- tion with the transition metal centre, normally of the first row, situated in a very specific coordination envi- ronment [14]. In this context it is well known that Co(II) complexes both in the solid state and in solution, are able to bind dioxygen and nitric oxide and they find large applica- tions in the catalytic oxidation of organic substrates in mild conditions and in several industrial processes [7,10,15]. In addition Co(II) complexes are very impor- tant in biology because they represent one of the most successful classes of synthetic oxygen carriers, charac- terized by a broad range of dioxygen affinities [4,5,7,11]. Moreover, Co(II) and other metal complexes play an important role in the industrial cleaning of gaseous effluents from power plants through the sug- gested simultaneous absorption of nitric oxide and sul- fur dioxide [8,14,15]. So far, great interest has been paid to complexation studies of these metal ions in aqueous solution [4,5,7,11] and minor attention has been devoted to the behaviour of such complexes in non aqueous solvents where they can be used extensively as oxidation catalysts [7,8,10]. As part of a research program on the study of molecular systems for oxygen transport and activation, our group started a study on the thermodynamics of complex formation of Co(II) with linear and cyclic polyaza ligands and of their oxygenation in non aqueous solutions with the aim to design complexes suitable for practical applications and to seek for the best solvents to obtain a good reversibility of the * Corresponding author. Tel.: +39-0432-558852; fax: +39-0432- 558803. E-mail address: tolazzi@dstc.uniud.it (M. Tolazzi). 0020-1693/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII:S0020-1693(01)00513-8