Thermochemical parameters of 1 , 2 , 3 , 4-tetrahydroquinoline adducts of some divalent transition metal bromides

The adducts [MBr2(L)n], where M = Fe, Co, Ni, Cu or Zn; L = 1,2,3,4-tetrahydroquinoline (THQ); n = 0.75, 1 or 2 have been obtained from the interaction in hot solution of THQ with the metal(II) bromides. The compounds were characterized by melting points, elemental analysis, thermal analysis and IR spectroscopy. From calorimetric studies in solution, the standard enthalpies of formation of them and several other thermochemical parameters were determined. The mean standard enthalpies of the metal(II)-nitrogen bonds have been estimated.


INTRODUCTION
Quinoline and quinoline derivatives are known to form complexes with transition metal(II) halides .Thermochemical parameters related to the transition metal(II)-nitrogen coordinated bonds formed in these compounds are not found in the literature.In a recent article [23] it was determined the values for several thermochemical parameters of adducts of some transition metal(II) bromides with quinoline.Following with the purpose of filling the lack of information on the energy evolved in the formation of these compounds, in the present article, it is reported the calorimetric determination of the energy involved in the formation of the coordinated metal(II)-nitrogen bonds, as well as, the values of several thermochemical parameters for the compounds formed between some metal(II) bromides with tetrahidroquinoline.The knowledge of these energy values is very important for understanding the coordinated metal(II)-nitrogen bonds formed.The thermodynamic properties of the compounds eventually could be used in determining their applications in catalysis and in the chromatographic separation of the metallic ions.

EXPERIMENTAL
1,2,3,4-Tetrahidroquinoline (98%, Aldrich was purified by distillation through an efficient column and stored over Linde 4Å molecular sieves.All the anhydrous metal(II) bromides used in the preparation of the adducts were of reagent grade (99%+).Solvents used in the synthesis of the compounds and in calorimetric measurements were purified by distillation and stored over Linde 4Å molecular sieves.

Adducts Synthesis
The adducts were prepared by the interaction of metal (II) bromides and ligand in solution.It was used hot ethanol or hot methanol.It was used a molar ratio salt/ligand of 1/4 or 1/2.Following, the solvent was evaporated by using vacuum.The solid obtain was re-crystallized, washed with three portions of petroleum ether and dried in vacuum.A typical procedure is given below.

CoBr 2 -THQ
To a solution of 1.0 g of CoBr 2 (4.57mmol) in 50 mL of hot ethanol, 2.3 mL (18.29 mmol) of tetrahidroquinoline was added slowly and dropwise under stirring.After filtering and evaporation of the solvent, a green solid was obtained.This was re-crystallized from chlorophorme.The product was dried for several hours in vacuum and stored in a desiccator over calcium chloride.

Analytical and Physical Measurements
Carbon, hydrogen and nitrogen were determined by micro analytical procedures [24].Halide analysis was made by gravimetry using standard N/10 AgNO 3 aqueous solution, after the adducts were dissolved in water [25].Metal contents were determined by complexometric titration with 0.01 M EDTA solution of aqueous solution of the adducts [26].The capillary melting points of them were determined with a UNIMELT equipment from Thomas Hover.Spectra were obtained with samples in KBr matrix for the solid adducts.For tetrahidroquinoline, a film of the ligand sandwiched between KBr plates was used.A Perkin-Elmer 1600 series FT-IR spectrophotometer in the 4000 -400 cm -1 region was used.TG/DTG and DSC measurements were obtained in argon atmosphere in a Du Pont 951 TG analyzer with the sample varying in mass from 5,58 to 19,94 mg (TG/DTG) and from 2,44 to 9,09 mg (DSC) and a heating rate of 10 K•min -1 in the 298 -678 K (DSC) and 298-1248 (TG/DTG) temperature ranges.TG calibration for temperature was made with metallic aluminum as a standard (mp = 660.37o C) and the equipment carried out the calibration for mass automatically.The DSC calibration was made with metallic indium as a standard (mp = 165.73o C,  s l H  = 28.4J•g -1 ).Spectra in the 350 -2000 nm region were obtained with a UV-Vis-NIR Varian-Cary 5G spectrophotometer with a standard reflectance attachment for obtaining the spectra of the solid adducts.All the solution calorimetric measurements were carried out in an LKB 8700-1 calorimeter as described before [27].The solution calorimetric measurements were performed by dissolving samples of 2.7 -85.3 mg of the adducts or metal(II) bromides in 100 mL of 1.2 M aqueous HCl and the ligand in this last solution maintaining a molar relation salt/ligand equal to the stoichiometry of the adduct.The accuracy of the calorimeter was checked by determining the heat of dissolution of tris [(hydroxymethyl)amino] methane in 0.1 mol•dm -3 HCl.The result (-29.78  0.03 kJ•mol -1 ) is in agreement with the value recommended by IUPAC (-29.763 0.003 kJ•mol -1 ) [28].

Characterization of the Compounds
All the adducts were solids.The yields range from 22% to 60%.The capillary melting points and analytical data are summarized in Table 1.

Infrared Studies
The more important IR bands of the compounds are reported in Table 2.The spectra show shift of several bands after coordination with respect to the free ligand.Shifts to lower frequencies of the N-H stretching modes of the coordinated tetrahidroquinoline are observed.This is indicative of coordination to the metallic(II) ion through the nitrogen atom [13,29].

Thermal Studies
The themogravimetry of the compounds shows the  loss of part of the ligand in 2 -4 steps of mass loss, alone or together with the loss of part of the bromine or with part of bromine and part of the metal content in the last step of mass loss.Bromine is lost in the last step or in the two final steps of mass loss, alone or together with the mass loss of part of the metal content.A residue is left that is part of the metal content.The DSC curves are consistent with the TG data.They present endothermic peaks due to the elimination of part of the ligand or part of bromine, alone or together with the elimination of part of the metal content.They present exothermic peaks due to the decomposition of the ligand or intermediate compounds.Table 3 presents the thermoanalytical data of the adducts.

Electronic Spectra
The ligand field parameters for the cobalt adduct have been calculated according to Lever [30].Considering the number and position of the bands [31,32] and according with the magnitude of the crystal field parameters as compared with that of Bolster [33], it is concluded that two nitrogen atoms from two ligand molecules and by two bromides ions pseudo-tetrahedrally surround Co(II) ion.The ligand field parameters for the Ni(II) adduct were calculated according to Reedijk et al. [34] and Lever [30].According to the number and position of the observed bands and considering the magnitude of the crystal field parameters as compared with that of Bolster [33], it is concluded that the Ni(II) ion is pseudo-tetrahedrally surrounded by two nitrogen atoms from two ligand molecules and two bromides ions, one of which is bridging to other Ni(II) ion in a dimeric structure.This last ion is surrounded by one nitrogen atom from one ligand molecule and three bromine ions, one of which is the mentioned bridge with the first nickel ion.For the Cu(II) adduct, the electronic spectra showed a rather broad asymmetrical band with maxima at 10528 cm -1 .Its intensity and position correspond with those observe for pseudo-octahedral compounds [33], with the Cu(II) ion being surrounded by two nitrogen atoms from two ligand molecules and by four bromide ions in a bridge structure.The ligand parameters for the adduct of Fe(II) were calculated according to Bolster [33].It is concluded that one unit is formed by Fe(II) ion pseudo-octahedrally surrounded by one nitrogen atom from one ligand molecule and five bromide ions in a polymeric structure bridging with other units of Fe(II) ion surrounded by six bromide ions.Table 4 contains the band maxima assignments and calculated ligand field parameters for the adducts.

Calorimetric Measurements
The standard enthalpies of dissolution of metal(II) bromides, tetrahidroquinoline and adducts were obtained as previously reported [27].The standard enthalpies of dissolution were obtained according to the standard enthalpies of the following reactions in solution: The application of Hess' law to the series of reactions (1) -( 4) gives the standard enthalpies of the acid/base reactions ( r H  ) according to the reaction: where since the final thermodynamic state of reactions ( 2) and ( 3) is the same and  4 H  = 0. Table 5 gives the values obtained for the enthalpies of dissolution of MBr 2 ( 1 H  ), THQ into the solution of MBr 2 ( 2 H  ) and of the adducts ( 3 H  ).Uncertainty intervals given in this table are twice the standard deviation of the mean of 4 -9 replicate measurements.The thermochemical parameters were calculated for hypothetical monomeric adducts.From the values obtained for the standard enthalpies of the acid/base reactions ( r H  ) and by using appropriate thermochemical cycles [29], the following thermochemical parameters for the adducts were determined: the standard enthalpies of formation ( f H  ), the standard enthalpies of decomposition ( D H  ), the standard lattice enthalpies ( M H  ) and the standard enthalpies of the Lewis acid/base reactions in the gaseous phase ( r H  (g)).These later values can be used to calculate the standard enthalpies of the M-N bonds, being equal to D (M-N) = - r H  (g)/n [35].
The enthalpies for the process of hypothetical monomer complex formation in the gaseous phase, from metal(II) ions, bromide ions and THQ molecules can be evaluated: where Table 7 lists the values obtained for these enthalpies values.

CONCLUSIONS
The interaction of transition metal(II) bromides with tetrahidroquinoline produced solid adducts of defined stoichiometry.The calorimetric study of them determined the standard enthalpies of formation and several other thermochemical parameters.The mean standard enthalpies of metal(II)-nitrogen coordinate bonds have values from 100 to 248 KJ•mol -1 .Comparing with the values obtained for quinoline adducts of metal(II) bromides of the same stoichiomety [23], it is observed that the bonds formed by THQ are weaker than the bonds formed by quinoline.This means that the hydrogenation of the heterocycle of quinoline to get THQ leads to the weakness of the bond formed by the nitrogen atom with metal(II) ions.The basicity order of the ligands is: THQ < quinoline.Based on the  r H  values obtained for the adducts, the acidity order of the salts, for the adducts of the same stoichiometry can be established: CoBr 2 > CuBr 2 > ZnBr 2 .Using the D (M-N) values, the order is: CoBr 2 > ZnBr 2 > CuBr 2 .

Table 1 .
Melting points, yields, appearance and analytical data of the adducts.

Table 2 .
Main IR spectral data (cm -1 ) of the compounds.

Table 3 .
Thermal analysis of the compounds.

Table 4 .
Band maxima and calculated ligand field parameters for the compounds.

Table 5 .
Enthalpies of dissolution at 298.15 K.

Table 6 .
Summary of the thermochemical results (KJ•mol -1 ) for the compounds.

Table 7 .
Auxiliary data and enthalpy changes of the ionic complex formation process in the gaseous phase (KJ•mol -1 ).