Thermochemical study of 4-(dimethylamino)pyridine complexes of some bivalent metal bromides P.O. Dunstan ⇑ , Abdul Majeed Khan Instituto de Química, Universidade Estadual de Campinas, C.P. 6154-971, Campinas, São Paulo, Brazil article info Article history: Received 12 March 2013 Received in revised form 10 June 2013 Accepted 17 June 2013 Available online 27 June 2013 Keywords: Thermochemical parameters Coordinated bonds Enthalpies of solution Bivalent metal bromides Enthalpies of formation abstract Interaction of some bivalent metal bromides with 4-(dimethylamino)pyridine in ethanol solution leads to the formation of complexes of general formula: [MBr 2 (dmapy) n ] (where M = Mn(II), Fe(II), Co(II), Ni(II), Cu(II) or Zn(II); dmapy = 4-(dimethylamino)pyridine; n = 0.75, 1, 1.25, or 2). Melting point measure- ments, elemental analysis, TG/DTG and DSC curves, and IR and electronic spectroscopy characterized the complexes. The measure of the enthalpies of solution of complexes, salts and ligand leads to the determination of several thermochemical parameters by the use of thermochemical cycles. The mean standard enthalpies of the metal-nitrogen coordinated bonds and the enthalpies of complex formation in the gaseous phase also were determined. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction Pyridine and substituted pyridines form complexes with halides or pseudo-halides of the first row of divalent transition metals [1]. These complexes are used for the separation and purification of certain organic compounds. They are also used in catalysis. The complexes were extensively studied in the early sixties and seven- ties of the last century. In the literature is found their synthesis, IR and electronic spectroscopy, thermal studies, structure determina- tion and some studies in solution [1–11]. But, the energies of the metal-nitrogen coordinated bonds formed in these complexes are not found in the literature. This article studies the interaction be- tween bromides of divalent transition metals of the first row with 4-(dimethylamino)pyridine, with the main of determining the en- ergy involved in the formation of the nitrogen-metal coordinated bonds. The values of several thermochemical parameters of the complexes have been evaluated. The thermodynamic properties of these complexes can be used for interpreting the coordinated bonds formed and founding applications of the complexes in catal- ysis and in the chromatographic separation of the metallic ions. 2. Synthesis of the complexes 4-(Dimethylamino)pyridine 99 + % (dmapy) was purified by re- crystallization from ethanol. Metal(II) bromides used in the synthe- sis were of reagent grade. Solvents were purified by distillation and stored over Linde 4Å molecular sieves. The complexes were obtained from the reaction of the metal(II) bromides and 4-(dimethyl-amino)pyridine in hot ethanol, with a molar relation salt/ligand of ½. Follows a typical procedure: To a solution of 0.603 g of CuBr 2 (2.70 mmol) in 20 mL of hot ethanol, 0.656 g (5.37 mmol) of dmapy dissolved in 20 mL of hot ethanol was added, slowly and dropewise with stirring. The brown solid formed was filtered and washed with three portions of 20 mL of petroleum ether. The compound was dried for several hours in a vacuum and stored in a desiccator over calcium chloride. The stoi- chiometry of the obtained complexes was got from the elemental analysis of C, H, N, and Br and metal contents. Carbon, hydrogen and nitrogen contents were determined by microanalytical proce- dures. Bromine content was obtained by gravimetry using stan- dard 0.1 M AgNO 3 solution [12]. Metal contents were determined by complexometric titration of the aqueous solutions of the com- plexes [13]. 3. Apparatus A Perkin Elmer 1600 series Fourier Transform IR spectrophotom- eter was used for obtaining the IR spectra of the complexes. Samples of the compounds in a KBr matrix in the region of (4000 to 400 cm 1 ) were used. Spectra in the region of (350–2000 nm) were got by using a UV–Vis–Varian–Cary 5G spectrophotometer. A standard reflec- tance attachment was used for obtaining the spectra of the solid compounds. Thermogravimetric (TG/DTG) and differential scanning calorimetry measurements were got in an argon atmosphere by using a Du Pont 951 analyzer with samples from (6.67 to 14.21 mg) (TG/DTG) and from (2.94 to 7.29 mg) (DSC). The heating 0021-9614/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jct.2013.06.010 ⇑ Corresponding author. Tel.: +55 (019) 35213088; fax: +55 (019) 35213023. E-mail address: dunstan@iqm.unicamp.br (P.O. Dunstan). J. Chem. Thermodynamics 66 (2013) 44–49 Contents lists available at SciVerse ScienceDirect J. Chem. Thermodynamics journal homepage: www.elsevier.com/locate/jct