Thermochemical Parameters of Tetramethylthiourea Adducts of Certain Metal ( II ) Bromides

Complexes of the general formula [MBr2(TMTU)n] (where M is Mn, Fe, Co, Ni, Cu, Zn or Cd; TMTU is Tetramethylthiourea; n is 0.75, 2 or 3) were obtained by the reaction of salts and ligand in solution. The bromides were selected among several other salts because they had thermochemical data in the literature. Properties as capillary melting points; C, H, N, Br and metal contents; TG/DTG and DSC curves; and IR and electronic spectra were determined. The values of several thermodynamic parameters for the complexes were found by solution calorimetry. From them, the standard enthalpies of the metal-sulphur coordinated bonds were calculated. The standard enthalpies of the formation of the gaseous phase adducts also were estimated.

In this work, complexes formed by tetramethylthiourea with bromides of Manganese(II), Iron(II), Cobalt(II), Nickel(II), Cooper(II), Zinc(II) and Cadmium(II) were studied.Thermodynamic data found in the literature concerning the standard enthalpies of formation of coordinated bonds in this kind of complexes are limited.The knowledge of the thermodynamic properties of these compounds is important to find their applications in catalysis and in the chromatographic separation of metal ions.It could be inferred the affinity order of the metallic ions for stationary-movable chromatographic phases by the knowledge of the formation enthalpies of complexes.Also, the thermochemical parameters can be used in catalysis for finding the more adequate complex to accelerate a given reaction.In this article, calorimetric measurements were made to measure the strength of the metalsulphur coordinated bonds.Correlations of the thermodynamic properties of the complexes were got.The enthalpies of formation of the complexes derived from the gaseous-phase metal ions, bromide ions and tetramethylthiourea were determined.

Reagents
Tetramethylthiourea (RP, Fluka AG Buchs SG) was purified by recrystallization from methanol (MP 75˚C -76˚C).The anhydrous Metal(II) bromides used in the synthesis of the complexes were of analytical grade.Solvents were distilled and stocking over Linde 4Å molecular sieves before using.

Experimental Procedure
The complexes were prepared by the reaction between Metal(II) bromides and tetramethylthiourea in a molar ratio salt/ligand of 1/4 in hot tert-buthyl alcohol solution or in a hot mixture ethanol/chloroform solution.Following, an example of one preparation: 1.00 g of NiBr 2 (4.58 mmol) was dissolved in 25 mL of hot tert-butyl alcohol and 2.41 g (18 mmol) of tetramethylthiourea was dissolved in 20 mL of hot tert-butyl alcohol.The later solution was poured into the solution of the salt, slowly and dropwise with stirring.The mixture was refluxed by five hours after which, the solid that formed was filtered and washed with 60 mL of petroleum ether divided in three portions.The compound obtained was maintained in a vacuum over twelve hours.It was stocked in desiccator over CaCl 2 .The chemical analysis confirmed the contains proposed by the assumed stoichiometries.Microanalytical procedures [26] were used for the determination of C, H and N contents.Gravimetric analysis [27] was used to determine the bromine contents.The metal contents were complexometrically determined by using 0.01 M ethylenediaminetetraacetic acid solution [28].Samples of the compounds in a KBr matrix were used to get the IR spectra.The region of spectra was from 4000 to 400 cm −1 and a Perkin Elmer 1600 series FTIR spectrophotometer was used.A UV-Vis-NIR spectrophotometer was used to record the spectra of the solid compounds in the region 350 -2000 nm using a standard reflectance attachment to get the spectra.TG/DTG and DSC curves were recorded in an argon atmosphere in a Du Pont 951 analyzer.The mass of the compounds was initially between 6.37 and 8.79 mg (TG/DTG) and from 4.80 to 16.14 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 conducted with metallic aluminum as a standard (MP = 933.49K).The equipment performed the calibration for mass automatically.The DSC calibration was made with metallic indium as a standard (MP = 438.85K, 1 o  s H ∆ = 28.4J•g −1 ).For the calorimetric study of the complexes, an LKB 8700-1 precision calorimeter was used at the measurements temperature of 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 [29].The accuracy of the equipment was determined as previously reported [29] [30].Three to eight replicate measurements were made on each compound and the uncertainty intervals are twice the standard deviations.The experimental deviations of the dissolution measurements stated between (1% -3%).

Complex Characterization
The interaction of MBr 2 with TMTU in solution leads to compounds of definite stoichiometry.Only in the case of FeBr 2 the compound formed had a fractional stoichiometry.The yields (Y), capillary melting points (MP), colors, appearance (A) and analytical data are reported in Table 1.

Infrared Studies
Table 2 presents the main IR bands of the complexes.A strong band is observed in the region 1097 -1148 cm −1 .This band is attributed to the C=S stretching frequency (ν CS ) [31].It is observed in the complexes relative to the free ligand, negative shifts of this frequency and positive shifts of the ν CN frequency after coordination, indicating coordination of the ligand through the sulphur atom to the Metal(II) bromide [31].Figure 1 presents the IR spectra of the Zn(II) complex.

Thermal Studies
The thermogravimetry of the complex of Mn(II) shows the loss of the ligand in three steps of mass loss.In a fourth step it loses part of the bromine content leaving a residue that is part of the bromine content plus the metal content.3 presents the thermoanalytical data for the complexes.

Electronic Spectra
The ligand field parameters for the Ni(II) complex were calculated according to Reedijk et al. [32] and Lever [33].The number and position of the observed bands and the magnitude of the crystal field parameters as compared with that of Bolster [34] indicates that the Ni(II) complex is pseudo-tetrahedral with the Ni(II) ion surrounded by two bromide ions and two sulphur atoms from two ligand molecules.The Co(II) complex shows bands attributed to pseudo-octahedral species with Co(II) ion surrounded by three bromide ions and three sulphur atoms from three ligand molecules in a dimeric structure of bridging bromide ions.The ligand field parameters were calculated according to Lever [33].The electronic spectra of Cu(II) complex shows a rather broad and symmetrical band which position according to Bolster [34] corresponds to pseudo-tetrahedral species with the Cu(II) ion surrounded by two bromide ions and two sulphur atoms from two ligand molecules.The ligand field parameters of the Fe(II) complex were calculated according to Bolster [34].It is concluded that Fe(II) ion is pseudo-octahedral with units [FeBr 6 ] 4− and [FeBr 5 L] 3− in a molar relation of 1:3 in a polymeric chain of   bridging bromide ions.The complex of Mn(II) according with the position of the absorption band [34] is pseudo-tetrahedral with the manganese ion surrounded by two bromide ions and two sulphur atoms from two ligand molecules.Table 4 contains the band maxima assignments and calculated ligand field parameters of the complexes.

Calorimetric Measurements
The standard enthalpies of dissolution of Metal(II) bromides, TMTU and complexes were measured [35].The standard enthalpies of the following reactions were obtained: The application of the Hess' law to the reactions 1 to 4 gives the standard enthalpies of reaction (∆ r H o ):

Discussions
Using the standard enthalpies of reaction (∆ r H o ) and appropriate thermochemical cycles [35], the following thermochemical parameters were got: the standard enthalpies of formation (∆ f H o ) from Equation ( 5), The enthalpy of reaction in the gaseous phase, from Equation ( 8)  ( ) 6 presents the values obtained for all these enthalpies.The formation enthalpies of the complexes in the gaseous phase, according to the Equation ( 9) 7 shows the values obtained for these enthalpies values.
The acidity order obtained based on ∆ r H o values for the complexes of the same stoichiometry is: Zn(II) > Cu(II) > Cd(II) > Ni(II) > Mn(II).Using the D (M-S) values the, order is: Cu(II) > Zn(II) > Mn(II) > Ni(II) > Cd(II).

Conclusion
Solid state complexes were obtained from the interaction in hot tert-buthyl alcohol solution of tetramethylurea with certain divalent transition metal bromides.The complexes decomposed on heating.The dissolution enthalpies were determined for complexes, salts and ligand.By using thermochemical cycles, the energies of the Metal(II)-sulphur coordinated bonds as well as the values of other thermochemical parameters were estimated.The energies of the coordinated bonds have values between 109 and 128 kJ•mol −1 .
The complex of Fe(II) shows the loss of the ligand in three steps.Part of the bromine content is lost together with part of the ligand in the third step of mass loss.Part of the bromine content is lost in the fourth and fifth steps of mass loss leaving a residue that is part of the bromine content plus the metal content.The complex of Co(II) shows the loss of ligand in three steps of mass loss.Part of the bromine content is lost together with part of the ligand in the third step.Part of the bromine content is lost in a fourth step of mass loss leaving a residue that is part of the bromine content plus the metal content.The complex of Ni(II) shows the loss of the ligand in the first step of mass loss follow by the loss of part of the bromine content in a second step of mass loss leaving a residue that is part of the bromine content plus the metal content.The complex of Cu(II) shows the loss of the ligand in the first step of mass loss together with part of the bromine content.The rest of the bromine content together with part of the metal content is lost in the second step of mass loss leaving a residue that is part of the metal content.The complex of Zn(II) shows the loss of the ligand in three steps of mass loss.The bromine content is lost together with part of the metal in the third step of mass loss.Part of the metal content is lost in the fourth step of mass loss leaving a residue that is part of the metal content.The complex of Cd(II) shows the loss of the ligand in two steps of mass loss follow by the loss of the bromine content and part of the metal content in the third step of mass loss leaving a residue that is part of the metal content.Figure2presents the TG/DTG curve of the Co(II) complex.The DSC curves of the complexes are consistent with the TG data.They present endothermic peaks due to the elimination of part of the ligand or part of the bromine content alone or together with part of the ligand.An exothermic peak is observed in the DSC curve of the Fe(II) complex due to the decomposition of the complex.Figure3presents the DSC curve of the Co(II) complex.Table

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

Table 3 .
Thermal analysis of the compounds.

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

Table 5 .
Enthalpies of dissolution at 298.15 K. observed for the enthalpies of dissolution of MBr 2 (∆ 1 H o ), for the enthalpies of dissolution of TMTU into the solution of MBr 2 (∆ 2 H o ) and that of the complexes (∆ 3 H o ).It was not possible to measure the enthalpies of dissolution of the complex of Fe(II) due to its paste consistency that made it difficult its manipulation.Uncertainty intervals given in this table are twice the standard deviations of the mean of 3 to 8 replicate measurements.
[36]he complexes decomposed on heating, the enthalpies of sublimation of the complexes were estimated[36].From Equation (8) it is got the standard enthalpies of the metal-sulphur bonds: ( ) ( )

Table 6 .
Summary of the thermochemical results (kJ•mol −1 ) for the compounds.

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