Molecular complexes of iron violurate with Donor amines

Spectrochimica Acta Part A 55 (1999) 2745 – 2751 www.elsevier.nl/locate/saa

Letter

Molecular complexes of iron violurate with Donor amines Arafa A.M. Belal a, Laila H. Abdel-Rahman b,* a b

Department of Chemistry, Faculty of Science, Aswan, Egypt Department of Chemistry, Faculty of Science, Sohage, Egypt Received 16 October 1998; accepted 20 August 1999

Abstract The acceptor character of iron violurate complex was studied by examining the electronic, vibrational and 1H-nmr spectra of the charge transfer molecular complexes formed between the iron violurate as p-acceptor and some amines as n-donors. Elemental analysis and spectral results establishes 1:2 stoichiometry of the adducts. The study has been conducted at different temperatures. Values of DG°, DH° and DS° have been calculated from the self-consistent values of the formation constants (KCT). Ionization potentials of the donors have been calculated and the solvent effect on the KCT values is discussed. The antibacterial and antifungal effects of the molecular complexes were studied. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Iron violurate complex; Electronic spectra; Vibrational spectra; Antibacterial and antifungal effects

1. Introduction The coordinative interaction of substituted violuric acid are of great importance in elucidating various enzymatic and drug actions [1,2] and are also used as antibacterial and antifungal agents [3]. The possibility of utilizing violuric acid as an analytical reagent for the determination of many actions was studied [1]. The interaction of violuric acid with transition metals ions may form violurate salts [4] or complexes [2], where coordination presumably occurs through the oxime functions [5]. However, it has been shown that the metal * Corresponding author.

violurate complexes, which contain p-acceptor site, may be attacked by nucleophiles such as ammonia and arylamines [6,7]. In our earlier work, we suggested that the violuric acid might function as an p-acceptor during its interaction with some amines [8]. In continuation of our previous work we report in this article the results of the interaction of iron(II) violurate with some donors viz; diethylamine, triethylamine, 2aminopyridine and piperidine in non-polar solvents including chloroform, 1,2-dichloroethane or carbon tetrachloride. Ferrous ion was chosen due to good solubility character of its violuric acid complex in non-polar solvents. The thermodynamic parameters of the formed CT molecular

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was achieved using a Haake NB 22 ultrathermostat of accuracy9 0.05°C. The IR spectra of the solid complexes were recorded on Perken Elmer 599B IR spectrophotometer using KBr discs. The 1H-nmr spectra were performed on a varian EM-390 (90 MHz) spectrometer in DMSOd6 (Merk) using TMS as an internal indicator.

complexes, and the ionization potentials of the donors were also calculated. The solid molecular complexes were prepared and characterized. Moreover, the antibacterial and antifungal effect of the formed molecular complexes were studied.

2. Experimental

2.3. Thermogra6imetric analysis (TGA)

2.1. Materials and solutions

TGA was carried out at a constant heating rate of 10°C min − 1 in N2 atmosphere using a Du Pot 900 thermal analyser equipped with an automatic thermobalance.

Violuric acid (H3va) was supplied by Aldrich Chem. Co. and was used as received. Diethylamine (Dea), triethylamine (Tea), piperidine (Pip) and 2-aminopyridine (Amp) were Aldrich and Sigma reagents. 1, 2-Dichloroethane, chloroform and carbon tetrachloride were spectroscopic grade solvent products prepared in the non-polar solvents. The 1:2 [Fe(H2va)2-amine] solid molecular complexes were synthesizes by mixing ethanolic solutions of 1.0 mmol ferrous ammonium sulfate and 2.0 mmol violuric acid. A dark red colour iron violurate developed and the excess of alcohol was allowed to evaporate slowly at room temperature. Then, 2.0 mmol of each amine in 1,2dichloroethane was slowly added to the complex. The blue precipitates were formed which were filtered and dried under vacuum. The solid complexes were subjected to C, N and H analysis (Table 1).

2.4. Biological effects Antibacterial and antifungal activations of Fe(H2va)2 and Fe(H2va)2-amine molecular complexes were determined in vitro using solutions of the test compounds (ethyleneglycol) at 200 mg concentration by the paper disk plate method. 3. Results and discussion Preliminary investigation of the interaction of iron(II) with violuric acid gave red coloured solution. On addition of an amine to the violuric acid chelate the colour changed from red to blue, while the addition of an amine to free Fe(II) solution had no effect on its colour in the studied concentration range. It has been found that the Fe(H2va)2 causes moderate inhabitation of the fungal and bacterial strains, while the amine iron violurate compounds deactivate this inhabitation. This behaviour can be attributed to the presence of amines in the CT formed complexes.

2.2. Physical measurements The electronic spectra of the studied molecular complexes solutions were recorded with Shimadzu UV-Vis recording 240 spectrophotometer using 1 cm matched silica cells. The temperature control

Table 1 Elemental analysis, molecular weights, melting points and colour of 1:2 Fe(H2va)2·(H2O)2-amine molecular complexes Complex

Colour

Dec. p. (°C)

Analysis, calc. (found) (%C)

Fe(H2va)2(Tea)2(H2O)2 Fe(H2va)2(Dea)2(H2O)2 Fe(H2va)2(Pip)2(H2O)2 Fe(H2va)2(Amp)2(H2O)2

Blue Blue Blue Blue

280 285 300 250

39.61 34.92 37.64 36.50

(40.05) (35.00) (37.77) (36.60)

MW

(%H)

(%N)

6.32 5.50 5.27 3.40

18.48 20.37 19.51 23.65

(6.17) (5.36) (5.22) (3.34)

(18.40) (20.33) (19.50) (23.45)

606.431 550.327 574.347 592.287

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cated at 360 nm was ascribed to n−p transition [10]. As indicated in Fig. 1, when the solution of Fe(H2va)2 interact with the amine solution to form molecular complexes, the properties of the spectra are perturbed, hence the band at 256 nm disappeared due to the absence of phenyl ring conjugation, and the shoulder at 310 nm increased with increasing the concentration of donors. In all cases the band located at 360 nm appeared as shoulder. The spectra of the acceptor with Dea in C2H4Cl2, CHCl3 and CCl4 are similar except the position of lmax of the complex and the isosbestic points. The recorded UV absorption spectra of the molecular complexes of Fe(H2va)2 with Pip (Fig. 2) and Amp, show that at low concentration of these donors, the band at 256 nm is red shifted to 280 nm. This perturbed band of iron violurate complex can be attributed to the formation of 1:1 molecular complex. At higher concentration of the donor the results show the disappearance of

Fig. 1. Electronic absorption spectra of molecular complex solution of iron violurate with Tea in C2H4Cl2 at 25°C. [Fe(H2va)2·(H2O)2] = 6×10 − 5 mol dm − 3 (3), [Tea] =2, 4, 6, 8, 10, 12 and 14 ×10 − 5 mol dm − 3 in mixtures of 4, 5, 6, 7, 8, 10 and 12, respectively. {[H3va]= 6× 10 − 5 mol dm − 3 (1), [Fe + 2] =6 × 10 − 5 mol dm − 3 (2).}

3.1. Equilibrium studies Fig. 1 shows the UV absorption spectra of 1,2-dichloroethane solutions of pure H3va, Fe(II), Fe(H2va)2 and Fe(H2va)2-Tea molecular complex at 25°C. The concentration of the acceptor (Fe(H2va)2) was kept constant in all mixtures, but that of donor (amine) varied using the solvent as a blank. In the region scanned (200 – 400 nm) negligible absorption of the donors were detected, but the acceptor (Fe(H2va)2) displays three absorption bands in this region. The band appearing at 256 and the shoulder at 310 nm were assigned to p −p* transitions of conjugated violurate and chelate ring, respectively [9]. The third band lo-

Fig. 2. Electronic absorption spectra of molecular complex solution of iron violurate; [Fe(H2va)2·(H2O)2]= 6 ×10 − 5 mol dm − 3 (a) with Pip in C2H4Cl2 at 25°C. [Pip] =1, 1.5, 2, 2.5, 3, 3.5, 4, 5, 6, 8, 10 and 13×10 − 5 mol dm − 3 in mixtures of 1, 2, 3, 4, 5,6, 7, 8, 9, 10, 11 and 12, respectively.

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the characteristic bands of Fe(H2va)2 located at 256 and 310 nm, while the shoulder at 310 nm becomes a uniform band at 300 nm. This new perturbed band of iron violurate suggest the formation of 1:2 (A:D) complex form. The appearance of two isosbestic points at 286 and 340 nm confirm the formation of 1:1 and 1:2 in the case of Amp and Pip. The visible spectra of the mixtures of iron violurate and amines were carried out. The same concentrations of the acceptor as in the test solution were used as blank to eliminate any possible overlap that may arise from acceptor absorption spectrum. Since the previous reports indicate that ferrous violurate complex in water shows an absorption band at lmax 605 nm [11]. The band located at 600 nm which observed in case of Fe(H2va)2 with Dea, Tea and Pip is attributed to n − p* transition between the nonbonding n-orbital on the oxime nitrogen and p* orbital which is delocalized over the Fe(H2va)2 molecule [4]. The broadness of this absorption band can be considered as a further proof that the CT molecular complexes are formed [12]. However, on mixing high concentrations of Amp and Fe(H2va)2, the mixture become turbid and a solid product was isolated. Thus, no characteristic absorption band was observed for Fe(H2va)2-Amp complex in the visible region. Moreover, in the region scanned, the addition of an amine to free Fe(II) solution had no effect on the UV absorption band of Fe(II). This confirms that the cin colour in the addition of the amine to complexed Fe(II) is due to the CT complexes without Fe(II)amine coordination. Based on the above results, the formation of Fe(H2va)2-amine molecular complex can be explained as follows: in agreement with the earlier report [13], the structure of Fe(H2va)2 molecular complexes is given with hexa-coordinated Fe(II) bound via the nitroso nitrogen, enolic oxygen and 2H2O molecules. The TGA confirms the presence of coordination of two water molecules in the complex sphere. This type of bonding leads to the formation of stable five cyclic chelate rings. When the Fe(II) ion coordinates to the oxime nitrogen atom, the lone pair electrons of the nitrogen atom will be localized. Consequently, the n − p transition shows pronounced affect as

shown in Figs. 1 and 2. On the other hand, the 256 nm band (which is assigned to n− p* transition in the metal chelate system) is red shifted in the Fe(H2va)2-amine system indicating the delocalized p-electrons in CT molecular complex [14]. The formation of the Fe(II) chelate ring results in increasing the delocalization of p-electrons of the complex aromatic rings. Accordingly the n character of the acceptor Fe(H2va)2 will be pronounced. Hence the Fe(H2va)2-amine interactions is suggested to be n− p* type originating from the lone-pair electrons of the amines, since the latters are more basic (pK =10.8) than violuric acid (pK =4.57) [15]. The stoichiometries of the CT complexes are determined by using the spectrophotometric continuous variation and molar ratio methods [16]. The results indicate the formation of 2:1 ratios (D:A) in the case of Dea and Tea and both 1:1 and 2:1 in the case of Amp and Pip complexes. Accordingly, the equilibrium established in solution between Fe(H2va)2 and amines can be represented by the following equations: Fe(H2va)2 + D    Fe(H2va)2D

(1)

Fe(H2va)2 + 2D    Fe(H2va)2D2

(2)

Values of equilibrium constants (KCT) and molar absorptivities (oCT) for 1:1 molecular complexes were obtained from the absorbance (A) and initial concentration of acceptor and donor (C °D and C °A) following the procedure given before [17]. In this procedure A, CA and CD for a series of solutions are related to KCT and oCT by the following equation: (C °AC °D)/A = 1/KCToCT + (1/oCT)[(C °A + C °D)− A/oCT]

(3)

Since the Benesi-Hildebrand procedure requires separated spectra. In addition, the procedure requires the presence of one reagent in large excess over the other, the Eq. (3), rather than the BenesiHildebrand equation or its variations, were deemed appropriate because the solutions used did not have large differences between C °A and C °D. In order to obtained values of KCT and oCT from this optical data and Eq. (3), it was first necessary to find an approximate value of oCT

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Table 2 Spectral and thermodynamic parameters of 1:1 Fe(H2va)2·(H2O)2-amine molecular complexes at 25°C Donor

Solvent

lCT

liso

oCT (dm3/mol cm)

log K1

DG (kJ/mol)

DH (kJ/mol)

DS (J/mol)

Pip

C2H4Cl2 CHCl3 CCl4 C2H4Cl2 CHCl3 CCl4

280 280 280 280 280 280

275 275 275 278 275 270

24243 18789 10222 18029 16624 12861

4.390 4.350 4.050 4.270 4.201 4.146

30.86 30.61 23.22 24.36 23.97 23.56

18.55 17.05 10.31 9.15 6.73 4.31

41.30 45.46 43.33 69.12 78.12 58.54

Amp

Table 3 Spectral and thermodynamic parameters for 1:2 Fe(H2va)2·(H2O)2-amine complexes at 25°C Donor

Solvent

lCT

oCT (dm3/mol cm)

log K2

DG (kJ/mol)

Pip

C2H4Cl2

300 610 300 610 300

107460 700 94610 670 92220

8.757

49.96

9.214

136.30

8.700

49.62

10.607

130.93

8.680

49.52

11.822

126.68

315 605 315 605 315 605

93080 680 85510 670 83250 670

8.695

49.60

13.115

122.44

8.678

49.50

16.301

111.40

8.661

49.41

18.397

104.00

90100 590 74530 570 73330

8.678

49.51

11.769

126.63

8.611

49.34

16.891

108.90

CCl4

310 600 310 600 310

8.538

48.71

21.56

91.11

C2H4Cl2 CHCl3 CCl4

302 302 302

58030 55020 53150

7.367 7.336 7.300

41.970 41.855 41.653

9.771 12.161 17.022

108.10 99.64 82.65

CHCl3 CCl4 Dea

C2H4Cl2 CHCl3 CCl4

Tea

C2H4Cl2 CHCl3

Amp

using the Benesi-Hildebrand equation and then calculate values of C °A + C °D −A/oCT for each solution. In the case of 1:2 complex, the value of KCT and oCT are given by the following equation [18]: C °A2C °D/A= 1/KCToCT + (1/oCT) C °A(4C °A + C °D)

(4)

By plotting (C °AC °D)/A versus CA(4C °A + C °D) a straight line is obtained. From the slope and interception, KCT and oCT of the molecular complexes in different solvents were calculated. All calculations were carried out using a computer

DH (kJ/mol)

DS (J/mol)

program based on the formula of linear least square method. Spectrophotometric continuous variation method was also used in the determination of the apparent formation constants of the 1:1 and 1:2 complexes [19]. Tables 2 and 3 show the mean values of oCT, log K1 and log K2 obtained. It is evident from the data that KCT and oCT are markedly affected by variation of the solvent and basic strength of the donor. The results indicate that KCT increases as donor basicity increased. KCT are found to correlate with the refractive index or dielectric constant

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of the solvents. These results can be attributed to the polarization interaction of the solvent with the ground and excited states of the complex. Since the excited state of these CT complexes is more polar then the ground state, it has been predicted that increasing solvent polarity should favour stabilization of the excited state, thereby reducing the electronic transition energy of the CT molecular complex. The data indicate that the KCT and values obtained of the donors with the same acceptor are in order: Amp B TeaB DeaB Pip This behaviour is in agreement with the increasing basicity of these donors along the same sequence (pKa=11.12, 10.93, 10.73 and 6.86) for Pip, Dea, Tea and Amp, respectively [15]. The results indicate that KCT and oCT of all Fe(H2va)2-amine molecular complexes have higher values than that of H3vaamine molecular complex [8]. This behaviour can be explained on the basis that the coordination of Fe(II) with violuric acid leads to lowering of the p-electron density of the violurate ring. Hence, the interaction of n-electrons of the donor (amine) with p-orbital of acceptor is enhanced, through the formation of molecular complexes of higher stability and higher extinction coefficient.

3.2. Thermodynamic properties of the molecular complexes The KCT values of the investigated complexes were determined at different temperatures and the obtained results (Tables 2 and 3) reveals that the formed complex is better stabilized as the temperature is lowered. This indicates the exothermic nature of complex formation. The self-consistent Table 4 Ionization potential of the studied donors and numerical values of the ratio b 2/a 2 (in C2H4Cl2) Donor

Ionization potential (eV)

b 2/a 2

Pip Dea Tea Amp

9.2 9.00 9.10 9.17

0.10 0.14 0.125 0.103

values of the formation constants were used to calculate the change in free energy due to the CT complex (DG°= −RTlnK). The results were fitted to the least-squares relation 5. RT1nK = DS − DH/T

(5)

Making use of the Van’t Hoff plots of RlnK versus 1/T, the values of DS and DH were calculated and listed in Tables 2 and 3. The obtained values for DG, DH and DS are of the same magnitude as reported for strong n− p* complex [20]. These data can be supported by the determination of the ratio between the weights of the dative-bond and the no-bond configurations (b 2/a 2). This ratio can be calculated from the following relation [21]: b 2/a 2 = − DH/hn

(6)

Where hn is the energy of CT transition. The obtained values=0.12 (Table 4) are in line with those of strong CT complexes [22]. The data also agrees with the order of the basicity of the donors. Ionization potentials of the donors are calculated using the general relation between the energy of the CT transition, hn, and the ionization potential of the highest filled molecular orbitals on the donors, ID, as given by the following empirical relation: hn= aID + b

(7)

The values of constants a and b are dependent on the type of donor and acceptor [23]. Results are given in Table 4. The data are nearly the same as compared with the literature data. This means that the same donor orbital is involved in the CT with Fe(H2va)2 acceptor.

3.3. Characterization of the solid complex The results of the elemental analysis of the isolated complexes (Table 1) clearly establish the 1:2 (Fe(H2va)2:amine) stoichiometry, and the analysis exhibits two water molecules are present. Generally, the IR spectra of the molecular complexes show no major changes as compared to those of free acceptor and donors. The very broad band located at 3380–34 200 cm − 1 region in the spectra of the acceptor and acceptor donor complexes are assigned to coordination water molecules. The gNO band of the enol form of the

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acceptor at 1800 – 1900 cm − 1 are shifted to lower values, which reflect the increasing p-acceptor character of the Fe(H2va)2 complex. This behaviour is in line with n −p* CT character [24]. The n(CC) and n(NC) bands belonging to the acceptor molecule (1420 – 1500 cm − 1) are shifted to lower frequencies, while those for the Amp exhibited a remarkable shift to higher frequencies in accordance with the increased p-electron density of the acceptor ring due to charge migration from the n-amto the p-acceptor. Moreover, the stretching modes of Fe(Hv2va)2 at 1700, 1260 and 550 cm − 1 (nCO, nCNH and Fe – O, respectively) did not show measurable change. The 1H-nmr spectra of the prepared solid CT complexes provides additional evidence for n−p* nature of Fe(H2va)2-amine molecular complexes. In fact the 1H-nmr spectra of Fe(H2va)2 display a very broad signal at 10.4 ppm, which assigned to N3 – H and N1 – H protons [25]. In molecular complex this signal splits into two broad signals at 10.9 and 11.5 ppm for N3 –H and N1 –H protons, respectively [2]. This is due to the paramagnetic and dipole – dipole interaction character of the studied molecular complex. Based on the above discussion the interaction of Fe(H2va)2 as acceptor with amines (donors) in 1:1 and 1:2 ratio complexes can be represented as follows:

.

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