Introduction The characterization of Schiff bases with a N 2 O 2 do- nor atom set has been done extensively by the scien- tific community, due to their relevance in biological activity and, more recently, by their applications on catalysis [1, 2]. Schiff base ligands are considered ‘privileged ligands’ to stabilize several different tran- sition metals in different oxidation states, and the re- sulting complexes have been successfully applied in catalytic reactions control, mainly due to their high activity, selectivity and enantioselectivity [3–5]. We have been also involved in the study of the thermochemical properties of several molecules of this class of tetradentate ligands derived from conden- sation of salicylaldehyde and β-diketones with aliphatic alkyldiamines [6, 7], as well as of some of their metallic complexes [8, 9]. In the present study, we report a new contribution for the energetical char- acterization of N 2 O 2 tetradentate Schiff bases result- ing from condensation of salicylaldehyde, acetyl- acetone or benzoylacetone with 1,2-cyclohexane- diamine, yielding N,N’-bis(salicylaldehydo)cyclo- hexanediimine, N,N’-bis(acetylacetone)cyclohexane- diimine or N,N’-bis(benzoylacetone)cyclohexane- diimine, respectively, whose structural formula are represented in Fig. 1. The enthalpies of formation of these three crys- talline compounds and their enthalpies of sublima- tion, at T=298.15 K, have been derived from static bomb calorimetry and from Knudsen effusion tech- nique, respectively. These results yield the standard molar enthalpies of formation of the gaseous species, which are interpreted in terms of the structural char- acteristics of the compounds. Experimental Synthesis and characterization of compounds The compound N,N’-bis(salicylaldehydo)cyclohexa- nediimine, C 20 H 22 N 2 O 2 (H 2 salch), was prepared ac- cording a literature method [10], by refluxing 0.05 mol of salicylaldehyde (Merck-Schuchard, puri- 1388–6150/$20.00 Akadémiai Kiadó, Budapest, Hungary © 2007 Akadémiai Kiadó, Budapest Springer, Dordrecht, The Netherlands Journal of Thermal Analysis and Calorimetry, Vol. 87 (2007) 1, 291–296 THREE N 2 O 2 LIGANDS DERIVED FROM THE CONDENSATION OF 1,2-CYCLOHEXANEDIAMINE WITH SALICYLALDEHYDE, ACETYLACETONE AND BENZOYLACETONE A new contribution to the energetical characterization of Schiff bases Maria D. M. C. Ribeiro da Silva * , N. R. M. Araújo, A. L. R. Silva, L. C. M. da Silva, N. P. S. M. Barros, J. M. Gon¸alves and M. A. V. Ribeiro da Silva Centro de Investiga¸±o em Química, Departamento de Química, Faculdade de Ci Ãncias, Universidade do Porto Rua do Campo Alegre, 687, 4169-007 Porto, Portugal The standard (p 0 =0.1 MPa) molar enthalpies of formation, at T=298.15 K, in the gaseous phase, for three tetradentate Schiff bases involving a N 2 O 2 set, N,N’-bis(salicylaldehydo)cyclohexanediimine (H 2 salch), N,N’-bis(acetylacetone)cyclohexanediimine (H 2 acacch) and N,N’-bis(benzoylacetone)cyclohexanediimine (H 2 bzacch), were determined from their enthalpies of combustion and sublimation, obtained by static bomb calorimetry in oxygen and by the Knudsen effusion technique, respectively. The results are compared with identical parameters for related compounds previously studied, resulting from the condensation of salicyl- aldehyde or β-diketone with aliphatic diamines. Keywords: enthalpy of formation, enthalpy of sublimation, N,N’-bis(acetylacetone)cyclohexanediimine, N,N’-bis(benzoylacetonecyclohexanediimine, N,N’-bis(salicylaldehydo)cyclohexanediimine, thermochemistry Fig. 1 Structural formulae for N,N’-bis(salicylaldehydo)cyclo- hexanediimine (H 2 salch), N,N’-bis(acetyl- acetone)cyclohexanediimine (H 2 acacch) and N,N’-bis(benzoylacetone)cyclohexanediimine (H 2 bzacch) * Author for correspondence: mdsilva@fc.up.pt