Thermochemical Parameters of 3-Chloroaniline Complexes of Certain Bivalent Transition Metal Bromides

Complexes of general formula [MBr2(3-clan)n] (where M is Fe, Ni, Cu or Zn; 3-clan = m-chloroaniline; n is 1.5 or 2) were prepared and their characteristic properties, such as capillary MP; C, H, N, Br and metal contents; TG/DTG and DSC curves; and IR and electronic spectra were determined. By calorimetric measurements in solution, the values of some thermodynamic parameters of the complexes were determined. From these values, the standard enthalpies of the metal-nitrogen coordinated bonds were calculated, and the standard enthalpies of the formation of the gaseous phase complexes were estimated.


Introduction
The standard enthalpies of formation are fundamental characteristics of complexes.Thermodynamic data in the literature concerning the standard enthalpies of formation of coordinated bonds in complexes are limited.Additional compounds that divalent transition metal salts form with the aniline ligand and its derivatives are mentioned in the literature .In a previous article [11], complexes formed by 4-chloroaniline with several divalent transition metal bromides were studied.In this work, the complexes formed by 3-chloroaniline with bromides of iron(II), nickel(II), cooper(II) and zinc(II) were studied.The thermodynamic characterization of these compounds is important because the finding concerning thermodynamic properties can be used to determine their applications in catalyses and in chromatographic separation of metal ions.Calorimetric measurements were performed to measure the strength of the metal-nitrogen coordinated bonds.Several correlations of the thermodynamic parameters published in the literature for aniline and 4-chloroaniline complexes with the parameters of the 3-chloroaniline complexes prepared in this study were also obtained.It is expected that the ligand 4-chloroaniline forms a weaker coordinated bond than 3-chloroanilne due to the inductive effect of the electron with-drawing chloro atom in the phenyl ring.The effect is stronger in the para-position than that in the meta-position.Aniline must form the stronger bond, because it does not have any substitution in the phenyl ring.The enthalpy of formation of the complexes derived from the gaseous-phase metal ions, bromide ions and 3-chloroaniline has been determined.

Reagents
The purity of 3-chloroaniline (98%, Aldrich) was improved by the method of Riddick [27].The anhydrous metal dibromides used in the synthesis were of analytical grade.Solvents were distilled and stocking over Linde 4 Å molecular sieves before using.

Experimental Procedure
Complexes were obtained by reacting the metal dibromides and 3-chloroaniline in solution.Hot ethanol was used as a solvent with a molar ratio salt/ligand of 1/4.The following is an example of the preparations: 1.10 g of NiBr 2 (5.03 × 10 −3 mol) was dissolved in 20 mL of hot ethanol (343 K), and 2.57 g (2.13 mL, 20.13 × 10 −3 mol) of 3-chloroaniline was poured into the solution of the salt, slowly and dropwise.The stirring was maintained after P. O. DUNSTAN, A. M. KHAN 164 the complete addition of the ligand.A green solid was formed which was filtered and washed with 60 mL of petroleum ether divided in three portions.The compound obtained was maintained in a vacuum over several hours.It was stocked in a desiccator over CaCl 2 .The chemical analysis confirmed the contents proposed by the assumed stoichiometries.Microanalytical procedures [28] were used for the determination of C, H and N contents.The bromine contents were determined by gravimetric analysis [29].The metal contents were complexometrically determined using 0.01 M ethylenediaminetetraacetic acid solution [30].Samples of the compounds in a KBr matrix were used to obtain the IR spectra.For the ligand, a film of it between KBr plates was used.The region was from 4000 to 400 cm −1 and a Perkin Elmer 1600 series FTIR Spectrophotometer was used.A UV-Vis-NIR Varian-Cary SG spectrophotometer was used to record the spectra of the compounds in the region of 350 -2000 nm using a standard reflectance attachment to obtain the spectra of the solid compounds.TG/DTG and DSC curves were recorded in an argon atmosphere in a Du Pont 951 analyzer.The mass of the samples was initially between 6.37 and 8.79 mg (TG/DTG) and from 2.65 to 4.47 mg (DSC).A heating rate of 10 K•min −1 was used from 298 to 678 K (DSC) and from 298 to 1248 K (TG/DTG).The calibration for temperatures was made with metallic aluminum as a standard (mp 933.49K).The equipment performed the calibration for mass automatically.The DSC calibration was conducted with metallic indium as a standard (mp = 438.85K, ).For the calorimetric study of the compounds, an LKB 8700-1 precision calorimeter was used.The temperature of the measurements was 298.15  0.02 K.A thin-walled ampoule that contained reactant was broken in a glass reaction vessel filled with (100.00 mL) of calorimetric solvent [31].The accuracy of the equipment was determined as previously reported [31,32].Three to six replicate measurements were made on each compound and the uncertainty intervals are twice the standard deviations.The experimental deviations of the dissolution measurements were between (0.7% -2.8%).

Complex Characterization
All complexes were solids.The interaction of MnBr 2 and CoBr 2 with 3-chloroaniline did not lead to the formation of compounds with a definite stoichiometry.The outcomes of the synthesis, capillary mp, colors, appearance, and elemental contents are presented in Table 1.

Infrared Spectra
Some absorption bands of the compounds are presented in Table 2.The band attributed to the stretching vibration of the NH group ( (N-H) ) in the free ligand is shifted towards lower frequencies in the coordinated ligand.This is observed when aniline and aniline derivatives coordinates to the metal ions [33,34].Figure 1 presents the IR spectra of the Cu(II) adduct.

Thermal Studies
The (TG/DTG) curves of the Cu and Zn complexes showed the elimination of all the ligand in three steps of mass loss, with the elimination of all the bromine content together with part of the metal content in the third stage of mass loss.The curve of the complex of Fe showed the mass of the ligand being eliminated in three stages with part of the bromine content being eliminated in the third stage together with part of the ligand content followed by the elimination of part of the bromine content in a fourth stage of mass loss.The curve of the complex of Ni showed the elimination of all the ligand in two stages with the elimination of part of the bromine content in a third stage of mass loss.The TG/DTG curves showed a residue that is the metal content plus part of the bromine content for the complexes of Fe and Ni.This residue is part of the metal content for the complexes of Cu and Zn. Figure 2 presents the TG/DTG curve of the Ni(II) complex.The DSC curves of the complexes show endothermic peaks that are consistent with the elimination of part of the ligand.Figure 3 presents the DSC curve of the    Ni(II) complex.Table 3 shows the thermal data obtained for the complexes.Discrepancies between the calculated and observed mass losses are due to losses through the TG curve between one step and the following step of mass loss.

Electronic Spectra
The complex of Ni showed the metal ions in the center of pseudo-octahedral units [NiL 2 Br 4 ] 2− and [NiLBr 5 ] 3− forming a three-dimensional polymer.This was con- cluded from the values of the calculated ligand field parameters and the positions of the absorption bands [35,36].The complex of Fe showed the metal ions in the center of pseudo-octahedral units [FeL 2 Br 4 ] 2− and [FeLBr 5 ] 3− forming a three-dimensional polymer.This was concluded from the values of the calculated ligand field parameter and position of the absorption band [37].
The Cu complex showed the metal ion in the center of pseudo-tetrahedrons [CuL 2 Br 2 ].This was concluded from the appearance and position of the broth absorption band at (15,862 cm −1 ) [37].Table 4 presents the attribution of the absorption bands and the calculated values of the ligand field parameters for the compounds.

Calorimetric Measurements
The solution enthalpy measurements of the compounds were made accordingly to the following reactions [38]: According to Hess'law for the reactions (1)-( 4), the standard enthalpy of reaction ( r H  ) of the following reaction: is equal to: , because the final state of reactions ( 2) and ( 3) is the same and  4 H  = 0, and also because the dilution of solution b into solution c has no thermal effect.
obtained for the dissolution enthalpies: ( 1 H o ) for the dissolution of MBr 2 (solution a); ( 2 H o ) for the dissolution of the ligand in the former solution, with respect to the stoichiometry of the ligand in the later solution, respecting the stoichiometry of the complex (solution b); and ( 3 H o ) for the dissolution of the complexes (solution c).This table presents the uncertainty intervals as twice the standard deviation of the means of (3 -6) replicate measurements.Using thermochemical cycles [38] and the calculated values for the enthalpies of reaction (5) ( r H o ), the thermochemical parameters for the complexes were estimated.Table 6 presents the values obtained for these enthalpies.

Discussions
The standard enthalpy of decomposition is defined by the equation: where .Table 6.Summary of the thermochemical results (kJ•mol −1 ) for the complexes.

 
where The standard enthalpy of reaction in the gaseous phase is defined by the equation: where As the complexes decomposed on heating, the enthalpies of sublimation of the adducts were estimated [39].The standard enthalpies of the metal(II)-nitrogen bonds are obtained from reaction (8) [40]: . Table 6 presents the values obtained for all of the enthalpies.The formation enthalpies of the complexes in the gaseous phase according to reaction (9): are equal to: Table 7 presents the values obtained for these enthalpy values.The acidity order was obtained based on the  r H  or D values for the complexes.Complexes of  [43] of the same stoichiometry, the basicity order is: an > 3-clan (for CuBr 2 ) and an > 3-clan (for ZnBr 2 ).As a whole, the basicity order is: an > 3-clan > 4-clan (for CuBr 2 and ZnBr 2 ).Using t he D values the order is: an > 4-clan > 3-clan (for CuBr 2 and ZnBr 2 ).The expected order would be: an > 3-clan > 4-clan because the inductive effect of the electron withdrawing chloro atom, which causes the diminution of the electronic density in the aromatic ring and of the electronic density available for the nitrogen atom linked to the ring.The chlorine atom withdraws more electronic density in the para-position than in the meta-position.This means that the D parameter is better than the  r H  parameter for determining the basicity order.This could be due to the contribution of another kind of interaction in  r H  like hydrogen bonding (Cl-----H) in the 3-clan addut.

Conclusion
Solid complexes were obtained through the interaction of 3-chloroaniline with bromides of the first row of divalent transition metals.These complexes were decomposed upon heating.The determination of the enthalpies of solution of the adducts, salts and ligand leads to the estimation of the energies of the metal ion-nitrogen coordinated bonds.The values were between 82 and 119 kJ•mol −1 .As a whole, the basic strength of 3-chloroaniline with respect to aniline and 4-chloroaniline is an >3-clan > 4-clan, as it would be expected by the inductive effect of the electron withdrawing chlorine atom, which is stronger in the paraposition than that in the meta-position.

Table 5
presents the values

Table 3 . Thermal analysis of the compounds.
a Residue at 1243 K. b (standard deviation errors).

Table 5 . Enthalpies of dissolution at 298.15 K.
he standard lattice enthalpy is defined by a Experimental data available after applying for.

Table 7 . Auxiliary data and enthalpy changes of the ionic complex formation process in the gaseous phase (kJ•mol −1 ).
[42] 2 > NiBr 2 and ZnBr 2 > CuBr 2 .The basicity order was obtained based on the  r H  values of the complexes of 3-clan and 4-clan[42]of the same stoichiometry.The basicity order is: 3-clan > 4-clan (for CuBr 2 ) and 3-clan > 4-clan (for ZnBr 2 ).Comparing the  r H  values for complexes of aniline (an)