Study on the standard enthalpy of formation of 2-amino-pyrimidine derivative and their manganese complexes

Jiao Baojuan, Wei Qing, Chen Sanping, Zhang Sumin, Gao Shengli
(Shaanxi Key Laboratory of Physico-Inorganic Chemistry, Department of Chemistry, Northwest University, Xi'an Shaanxi 710069, China)

Abstract The complexes of hydrous manganese chloride with 2-amino-4, 6-dimethyl-pyrimidine (ADMP) and 2-amino-4, 6-dimethoxygenpyrimidine (AMP) were prepared through reflux in alcohol. The compositions of the complexes Mn(ADMP)₂Cl₂ and Mn(AMP)₂Cl₂ were determined by chemical and elemental analyses. The complexes were characterized by IR, XPS, ¹H NMR and TG-DTG techniques. The constant-volume combustion energies of ADMP, AMP, Mn(ADMP)₂Cl₂ and Mn(AMP)₂Cl₂,   were determined by a precise rotating bomb calorimeter at 298.15K. They were (–3664.53 ± 1.18) kJ · mol^-1, (–3340.01 ± 1.54) kJ · mol^-1, (–7043.08 ± 3.13) kJ · mol^-1, and (–6058.71 ± 3.18) kJ · mol^-1. Their standard enthalpies of combustion, , and standard enthalpies of formation, , were calculated. They were (–3666.39 ± 1.18) kJ · mol^-1, (–3339.39 ± 1.54) kJ · mol^-1, (–7043.08 ± 3.13) kJ · mol^-1, (–6053.75 ±3.18) kJ · mol^-1 and (19.09 ± 1.43) kJ · mol^-1, (–307.91 ± 1.74) kJ · mol^-1, (–669.90 ± 3.51) kJ · mol^-1, (–1659.23 ± 3.56) kJ · mol^-1.
Keywords manganese chloride, 2-amino-4, 6-dimethyl-pyrimidine, 2-amino-4, 6-dimethoxygen-pyrimidine, standard enthalpy of combustion, standard enthalpy of formation

1 EXPERIMENTAL
1.1 Reagents and experimental conditions
All chemicals and solvents were of anal. grade and used after further purification. Guanidine nitrate, diethyl malonate and acetylpropyl ketone were made in the Second Branch of The Head Reagent Factory of Shanghai; alcohol, MnCl₂ · 4H₂O, phosphorus oxychloride, triethylbenzylamine chloride, sodium methylate and K₂CO₃ were purchased from the Chemical Reagent Factory of Xi'an. ADMP and AMP were synthesized according to Ref. [7, 8]. The yield of white solid product (ADMP) is 99 %, with the melting point of 152-152.4ºC,better than 152.9-154.4 ºC in the literature [7].Referred to Ref. [9], the synthesis method of 2-amino-4, 6-dichloro-pyrimidine was ameliorated. The white crystal of AMP was prepared. The yield of AMP was beyond 80 %, and its melting point locates in the range of 97.0-97.5 ºC.
    Melting point of the compounds was measured with WRS-1A digital melting-point apparatus. Mn²⁺ content was complexometrically determined by EDTA, Cl^- by the Fajans' method and C, H, N contents were determined on a Perkin – Elmer 2400 type elemental analyzer. The IR spectra (KBr pellets) were obtained on a Bruker EQ UINOX-550 spectrophotometer in the 400 – 4000 cm^-1 region. XPS were taken on an ESCA PHI-5400 X-ray photoelectron spectrophotometer using Mgradiation X-ray source, the C_1s electron in benzene was used as the internal standard, BE = 284.6 eV, and the accuracy of the measured BE value was + 0.1 eV. The NMR spectra of the compounds were measured with a Varian Unity INOVA-400 nuclear magnetic resonance spectrometer using TMS as the reference sample and C₂D₅OD as the solvent. TG and DTG data were determined by a Perkin Elmer thermogravimetric analyzer. All TG-DTG tests were performed under a dynamic atmosphere of dry nitrogen at a flow rate of 60 mL min^-1, the heating rate was 10 deg min^-1 and sample masses approximated to 1 mg.
    The constant-volume combustion energies of the compounds were determined by a precise rotating bomb calorimeter ^[10]. The main experimental procedures were described in Ref. [11]. The initial temperature was regulated to (25.0000 ± 0.0005) ºC, and the initial oxygen pressure was 2.5 MPa. The correct value of the heat exchange was calculated according to the Linio -Pyfengdelel-Wsawa formula ^[12]. The calorimeter was calibrated with benzoic acid of 99.999 % purity. It had an isothermal heat of combustion at 298.15 K of (-26434 ± 3 ). The energy equivalent of calorimeter was determined to be (17936.01 ± 9.08) kJ · K^-1. The analytical methods for final products (gas, liquid, and solid) were the same as those in Ref. [10].
1.2 Preparation of the complexes        
For preparation of the complex b, 6.8577g (0.0557mol) of the ligand and 5.5099g (0.0278mol) of MnCl₂ · 4H₂O were weighed out, and separately dissolved in 50 mL and 25 mL of alcohol. When the solution of MnCl₂ · 4H₂O was warmed for a few minutes on a hot plate, the solution of the ligand was added dropwise into the solution of salt. Under the condition of reflux, the reaction proceeded further for 4 h. After evaporating a part of solvent, the precipitant appeared, and the reaction mixture was allowed to cool slowly to room temperature, followed by suction filtration. In order to remove the excessive ligand and salt from product, it was necessary for the reaction mixture to be rinsed thoroughly with hot alcohol and distilled water. An infrared heat lamp was employed to serve the drying of the product. Finally, the pale red product of the yield of 60 % was obtained. The complex d was prepared according to the procedures used for the complex b above, showing pale red color and the yield of 72 %. The purity of the complexes was greater than 99.9% checked by HPLC. The analytical results of the composition of these complexes are presented as follows. For b, w_i (calc.): 14.76 % Mn, 19.09 % Cl, 38.60 % C, 4.81 % H, 22.58 % N; w_i (found): 14.70 % Mn, 19.15 % Cl, 38.71 % C, 4.80 % H, 22.60 % N. For d, w_i (calc.): 12.60 % Mn, 16.28 % Cl, 33.02 % C, 4.13 % H, 19.27 % N; w_i (found): 12.55 % Mn, 16.33 % Cl, 33.05 % C, 4.18 % H, 19.37 % N. The compositions of the complexes b and d were identified as Mn(ADMP)₂Cl₂ and Mn(AMP)₂Cl₂.

(a)ADMP  (b) Mn(ADMP)₂Cl₂   (c) AMP (d) Mn(AMP)₂Cl₂
Fig.1 IR Spectra of the ligand and the complexes

2 RESULTS AND DISCUSSION
2.1 IR spectra of the complexes                
IR spectra of the ligand and the complexes were shown in Fig.1. It is obvious from the infrared spectra of the compounds that there are differences between the main characteristic absorption peaks of the complexes and those of the ligand ^[13,14]. Compared with the characteristic absorption peaks of the ligand, the characteristic absorptions of C＝N and C-N in the ring of pyrimidine, as well as those of stretching vibration and bending vibration of N-H connected with the ring, have the shifts of 15 cm^-1, 18 cm^-1, 35 cm^-1, and 40 cm^-1 for the complex b and those of 20cm^-1, 25 cm^-1, 30 cm^-1, and 40 cm^-1 for the complex d. Based on the analyses above, it is shown that for the ligand the nitrogen atom of the amino group and one of the nitrogen atoms of the pyrimidine coordinate to Mn²⁺ in a bidentate fashion. The characteristic vibrations of Cl^- in the complexes occur in the fingerprint region of 200 – 400 cm^-1, which is difficult to show in the recorder.
2.2 XPS spectra of the complexes
Consulting Ref. [15], the binding energy data of the internal shell electron for the main atoms of the compounds obtained from XPS spectra of the complexes, are listed in Table 1.

Table 1 Binding Energy Data / eV

    Four conclusions could be drawn from the analyses of the binding data. 1. Compared with the binding energy of N_1s of the amino group and that of N_1s of the pyrimidine ring, there are changes for those of the complexes, which shows that nitrogen atom of the amino group and one of the nitrogen atoms in the pyrimidine coordinate to Mn²⁺. The decrease of the binding energy could be interpreted that rather amount of feedback of d electrons existing in the coordination bond of N→Mn²⁺ leads to the increase of the electron cloud density of N_1s. 2. The binding energies of Cl^- have a distinctly changing, showing that Cl^-coordinates to Mn²⁺. 3. For the complexes b and d, the binding energy of Mn²⁺ has changes of 0.4 eV and 0.9 eV and the ranges of the differences are not wide, which illustrates a certain degree of feedback bond existing in the coordination bond. 4. Combining the analyses of XPS spectra with the results of IR analyses, it indicates that AMP has stronger coordination to Mn²⁺ than ADMP, which is credited to the strong donor strength of CH₃O-.
2. 3 ¹H NMR spectra of the compounds                 
The chemical shifts for the main group of ¹H NMR spectra of the ligand and the complexes are listed in Table 2, showing that two ligands coordinate to Mn²⁺ for the complexes.

Table 2 Chemical Shifts of the Main Groups for the compounds

2. 4 Thermostability of the complexes
Thermostability of the solid complexes was investigated by TG-DTG and the TG curves of the complexes are depicted in Fig 2. The curves of TG-DTG concerning the compounds reflect that the experimental results for the residual amount of loss of weight are in good agreement with the calculated results, and the intermediate and final products of the thermal decomposition of the complexes are identified by IR spectra as well. The thermal decomposition processes of the complexes are summarized in Table 3.
    Obviously, the complexes b and d were completely decomposed into manganese chloride after processing skeleton splitting of part of the ligand.

Fig. 2 TG-DTG curves of the complexes

Table 3 Thermoanalytical results of the complexes

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