Samar O. Aljazzar

Department of Chemistry, College of Science, Princess Nourah Bint Abdulrahman University, Riyadh 11671, Saudi Arabia

Abstract One of the phenolic acids is 4-hydroxybenzoic acid (HBA) which takes the form of a white crystalline solid with a molecular formula of C7H6O3, a melting point of 214.5 ℃ and a molecular weight of 138.12 g·mol-1. It soluble in polar organic solvents like acetone and alcohols, and slightly soluble in chloroform and water. The reactions between the metal ions and the HBA were carried out under specific conditions like (molar reaction was 2∶2 (ligand to metal), reaction temperature was 60 ℃, media was neutral (pH 7), and solvent was H2O ∶MeOH (1∶1). Under these conditions, the HBA was deprotonated to form L-). The ligand L- was coordinated to the metal ions forming the metal complexation. The reaction of 4-hydroxybenzoic acid (HOC6H4CO2H; HL) with the Ni(Ⅱ), Mn(Ⅱ) and Cu(Ⅱ) ions afford metal-complexes with gross formula of [Ni2L2(NO3)2(H2O)4], [Mn2L2(NO3)2(H2O)4] and [Cu2L2(NO3)2(H2O)4], respectively. These complexes were characterized by elemental analysis (CHN), magnetic susceptibility, UV-Vis spectra, infrared (IR), and X-ray powder diffraction (XRD) techniques. The complexes of HBA are insoluble in common solvents and hence molar conductance could not be measured, but this very insolubility indicates that the complexes are neutral. Data has demonstrated that the ligand (L-) was coordinated to the metal ion by bidentate bridging carboxylate group (COO-), with an octahedral geometry. Thus, HBA is expected to act as bidentate uninegative ions and the coordination number of the metal ions is six. XRD results showed that the complexes possess uniform and organized microstructures in the nanometer range with a main diameter in the range of 11～28 nm.

Keywords 4-hydroxybenzoic acid； Metal-complex； Spectral analysis； Nanostructure

Introduction

Phenolic acids are bioactive compounds widely distributed in plant kingdom, and they consider as a group of secondary metabolites from plants and fungi. These acids are known to have antimicrobial, antioxidant, anti-inflammatory, and anti-collagenase activity[1-4]. In the environment, HBA was identified in the fruit, leaves, straw, bark, wood, and humus[5-7]. Because they have chemical functional groups (phenolic and carboxylic groups) that are found in natural organic matter, HBA considered as the first step to describe these matter[8]. The esters of HBA which known as parabens are used as preservatives in ophthalmic solutions and cosmetics[9]. HBA and some of its derivatives are used to prepare synthetic high polymers for novel liquid crystals[10-11]. The chemical interactions of HBA with Eu(Ⅲ)[8, 12], Co(Ⅱ)[13], and Zn(Ⅱ)[14]ions were reported. We aimed through this study to investigate the chemical interaction of HBA with the Ni(Ⅱ), Mn(Ⅱ), and Cu(Ⅱ) ions under the following conditions: molar ratio; 2∶2 (Ligand∶Metal), reaction temperature; 60 ℃, Media; neutral (pH 7), and solvent; MeOH∶H2O (1∶1). The obtained complexes were characterized by elemental analysis, magnetic moment, UV(Vis, IR, and XRD.

1 Materials and Methods

1.1 Materials

All solvents and chemicals used in this investigation were of analytical reagent grade and obtained from Sigma-Aldrich Co., (St Louis, MO, USA). The ligand used in this investigation is 4-hydroxybenzoic acid (HOC6H4CO2H (HBA; Figure 1); 138.12 g·mol-1; purity ≥99%). The metal nitrates used in this investigation are nickel(Ⅱ) nitrate hexahydrate (Ni(NO3)2·6H2O; 290.79 g·mol-1; purity 99.99%), manganese(Ⅱ) nitrate tetrahydrate (Mn(NO3)2·4H2O; 251 g·mol-1; purity≥97%), and copper(Ⅱ) nitrate trihydrate (Cu(NO3)2·3H2O; 241.6 g·mol-1; purity≥99%).

Fig.1 Molecular structure of 4-hydroxybenzoic acid (HBA)

1.2 Methods

1.2.1 Preparation method

A methanolic solution containing 2 mmol (20 mL) of HBA was added to a H2O solution containing 2 mol (20 mL) of a metal nitrate (Ni(NO3)2·6H2O, Mn(NO3)2·4H2O, or Cu(NO3)2·3H2O) under continuous stirring. A colored precipitate was formed when the pH of the mixture reached 7 by adding a few drops of conc. ammonium (NH3). All the mixtures were stirred for 25 minutes at 60 ℃. After cooling, the colored precipitates were collected, filtered, washed, and dried in an oven at 70 ℃.

1.2.2 Characterization methods

An elemental analyzer (model PE 2400CHN) for collecting the elemental percentage (%) of carbon, nitrogen and hydrogen. A Sherwood magnetic susceptibility balance for collecting the magnetic susceptibilities. A UV-Vis spectrometer (model UV2(Unicam) for collecting the electronic spectra in DMSO solvent over the region 200～800 nm. IR spectrometer (model Bruker FT-IR) for collecting the infrared spectra over the region 400～4 000 cm-1. An X-ray diffractometer (model Panalytical’s X’Pert PRO) for collecting the XRD spectra over the 2θrange 5°～80° (10 kV,λ=0.154 056 nm, CuKα1radiation source).

2 Results and discussion

2.1 Elemental analysis results

[Ni2L2(NO3)2(H2O)4] complex: oily green powder; C14H18N2O16Ni2(587.62 g·mol-1). Elemental data (%): found (calculated) for C, 28.44 (28.59); H, 2.95 (3.06); N, 4.98(4.76); Ni, 20.16 (19.98). [Cu2L2(NO3)2(H2O)4] complex: greenish blue powder; C14H18N2O16Cu2(597.34 g·mol-1). Elemental data (%): found (calculated) for C, 28.34 (28.12); H, 2.87 (3.01); N, 4.54 (4.69); Cu, 21.46 (21.28). [Mn2L2(NO3)2(H2O)4] complex: brown powder; C14H18N2O16Mn2(580.12 g·mol-1). Elemental data (%): found (calculated) for C, 29.16 (28.96); H, 3.25 (3.10); N, 4.65 (4.83); Mn, 18.73 (18.94).

2.2 UV-Vis spectra

The Gouy’s method was used to measure the magnetic moment (μeff) values for the synthesized complexes[15]. Theμeffvalues for [Ni2L2(NO3)2(H2O)4], [Cu2L2(NO3)2(H2O)4], [Mn2L2(NO3)2(H2O)4] were 3.1, 1.9, and 4.4 B.M., respectively. These values suggest that these complexes possess an octahedral geometry with six-coordinate chelation modes for the metal ions[16-18].

Fig.2 The UV-Vis. spectra of the Ni(Ⅱ), Cu(Ⅱ) and Mn(Ⅱ) complex

The electronic spectra of the complexes were collected over the wavelength range 200～800 nm in DMSO solvent at room temperature (Figure 2). The complexation of L-by the metal ions displayed a very strong broad band on their UV-Vis spectra. In all complexes, this band had two maximums at 290～300 nm and at 274 nm. That at 290～300 nm corresponds to then→π*transitions, while that at 274 nm corresponds to the π→π*transitions. Also, weak broad band was observed at 448 nm for [Ni2L2(NO3)2(H2O)4] complex, at 574 nm for [Cu2L2(NO3)2(H2O)4] complex, and at 402 nm for [Mn2L2(NO3)2(H2O)4] complex. These bands could be attributed to the ligand-to-metal charge transfer bands (LMCTs)[19].

2.3 FT-IR spectra

The measured IR spectra of the synthesized complexes are shown in Figure 3. Assignments of the main vibrational bands for the HBA[20], and the complexes are:

Fig.3 IR spectra of the Ni(Ⅱ), Cu(Ⅱ) and Mn(Ⅱ) complex

2.4 XRD results

The XRD and SEM techniques were used to check phase purity, crystal structure of the synthesized complexes. Figure 4 displays the XRD patterns of Ni(Ⅱ), Cu(Ⅱ), and Mn(Ⅱ) complexes collected over the 2θrange of 5°～80°. [Ni2L2(NO3)2(H2O)4] complex had one very strong diffraction line at Bragg’s angle 2θ=28.089°, and six strong lines at 17.529°, 20.606°, 24.814°, 25.707°, 29.915° and 34.084°. This complex had also eight medium strong lines at different Bragg’s angles. The XRD diffractogram of [Cu2L2(NO3)2(H2O)4] complex showed a very strong diffraction line at Bragg’s angle 2θ=26.440°, eight strong lines at 14.353°, 15.385°, 19.653°, 21.122°, 23.504°, 29.359°, 32.098° and 36.684°, and another eight medium strong lines at different Bragg’s angles. Complex [Mn2L2(NO3)2(H2O)4] had on very strong difgraction line at 2θ=27.891°, and two medium strong lines at 16.298° and 25.508°. This complex had also ten medium lines at different Bragg’s angles. The d-spacing and particle size for each complex were calculated based on the Bragg’s diffraction angle (θ) of the highest line detected in the complexes’ XRD diffractograms. For each complex, the inter-planar spacing between the atoms (d-spacing; ?) was determined using the Bragg’s law (d=λ/2sinθ)[26], where the average particle size (D; nm) was determined using the Debye-Scherrer’s law (D=0.94λ/βcosθ)[27-28]. The symbols in these two laws are:disd-spacing in ?,Dis average particle size in nm, 0.94 is Scherrer constant,λis wavelength of theKα1radiation (0.154 056 nm),θis Bragg diffraction angle in °, andβis the full-width at half-maximum of the highest line (FWHM) in rad. Table 1 tabulates the values ofβ,d, andDfor the complexes. TheDvalues were found to be ～20.8, 10.7 and 28.5 nm for Ni(Ⅱ), Cu(Ⅱ) and Mn(Ⅱ) complex, respectively. These values indicate that the complexes were nanoscale sized.

Fig.4 XRD spectra of the Ni(Ⅱ), Cu(Ⅱ) and Mn(Ⅱ) complex

Elemental, magnetic, thermal and spectral results proposed that L-coordinate to the Ni(Ⅱ), Cu(Ⅱ) and Mn (Ⅱ) ions in a bidentate bridging manner using the carboxylate group (COO-). These metal ions have six-coordinate modes, and the coordination sphere is complemented by water molecules. The synthesized complexes have the general proposed formulae [M2L2(NO3)2(H2O)4] (where M: Ni(Ⅱ),Cu(Ⅱ) or Mn(Ⅱ)) as given in Figure 5.

Fig.5 Proposed structure of the synthesized complexes (where M: Ni(Ⅱ), Cu(Ⅱ) or Mn(Ⅱ))

Table 1 The XRD spectral data for the Ni(Ⅱ), Cu(Ⅱ), and Mn(Ⅱ) complex

3 Conclusion

Spectral and thermal properties of metal-complexes formed from the reaction of HBA with the Ni(Ⅱ), Cu(Ⅱ) and Mn(Ⅱ) ions in neutral media were highlighted. The obtained complexes were characterized by elemental, magnetic, and spectral methods. Furthermore, the XRD technique was used to check phase purity, crystal structure of the formed complexes. The obtained results proposed that the ligand (L-) coordinated to the Ni(Ⅱ), Cu(Ⅱ) and Mn(Ⅱ) ions in a bidentate bridging mode using the carboxylate group (COO-), and the formed complexes have the general proposed formulae [M2L2(NO3)2(H2O)4] (where M: Ni(Ⅱ), Cu(Ⅱ) or Mn(Ⅱ)) with an octahedral geometry.