Nd2O3, Cr2O3, and V2O3 nanoparticles were prepared by calcining the precursor materials that are novel mixed ligand complexes: [Nd(BDC)(ADMPY)(OAc)].H2O, [Cr(BDC)(ADM PY)Cl].H2O, and [V(BDC)(ADMPY)Cl].H2O, where BDC = 1,4-benzenedicarboxylic acid and ADMPY = 2-amino-4,6-dimethyl pyrimidine. The generated compounds were examined through several techniques such as elemental analysis (C.H.N), UV-Vis spectroscopy, thermal analysis (thermogravimetric, differential thermogravimetry, and differential thermal analysis), FT-IR spectra, X-ray diffraction (XRD), scanning electron microscope (SEM), and transmission electron microscope (TEM). The TEM micrographs showed that neodymium oxide nanoparticles assumed agglomerated platelet-like particles, with particle sizes around 30.16 nm, while chromium oxide NPs showed solid block material with compact density and fewer pores with nearly spherical shape and 56.12 nm size. The vanadium oxide NPs were an agglomeration of small spherical nanoparticles of 28.4 nm size. The antimicrobial properties of the samples were assessed using two strains of Gram-positive bacteria, two strains of Gram-negative bacteria, and one strain of yeast. The antimicrobial results demonstrated that a large spectrum of activity characterizes the tested compounds because they are active on Gram-positive and Gram-negative bacteria, especially on Gram-positive strains. The antioxidant activity of prepared compounds was assessed by scavenging free radicals of DPPH. Metal oxide NPs also showed promising results as antioxidants.

1. Introduction

Antimicrobial resistance is a general health concern because it reduces the efficiency of antimicrobial drugs, which will increase morbidity and mortality as well as health-care costs [1]. Furthermore, free radicals and reactive oxygen species (ROS) are reasoned a several persistent and degenerative illnesses by inflicting oxidative damage to cell molecules [2]. The immoderate generation of ROS in the human physique may additionally result in oxidative stress [3, 4]. Nowadays, searching for novel materials has become critical to overcoming these states. Nanotechnology, since its inception, has had a tremendous impact on the physical, chemical, terrestrial, and biological sciences and has significantly developed recently [5, 6]. This development has made scientists and researchers particularly interested in nanoparticles because they have a large specific area compared to their size, allowing them to interact with vital elements on the surface of live cells [7]. Metal oxide NPs are one of the most widely used NMs [8]. Compared with organic nanomaterials, metal oxide nanoparticles as inorganic nanomaterials are more applicable because they provide superior hardness, lower toxicity, higher stability, and selectivity [9]. There are various methods for preparing metal oxides, such as the sonochemical method [1015], sol-gel synthesis [1618], coprecipitation method [19, 20], solvothermal method [2123], pyrolysis technique [24, 25], and laser ablation method [2628]. Calcination has a significant influence on the nanostructure and optical characteristics of nanoparticles. It has the ability to control the size of nanoparticles with a tight size distribution [29]. Calcination also enhances the crystalline, which aids in the removal of impurities from the samples [30]. Rare earth oxide nanoparticles (Nd2O3 NPs) have received a lot of attention as optical and magnetic nanomaterials for various applications because of their distinctive optical properties. All lanthanides share the contractual nature of the protected Nd 4f orbitals, which has a profound impact on their physical properties [31]. It has been used as a catalyst, protective coating, and photonic application [32, 33]. Chromium oxide nanoparticles (Cr2O3 NPs) have also received much attention for their value in science and technology. Because chromium has varied stable oxidation states, it can form diverse types of oxides [34]. Particular emphasis has been placed on the composition and properties of chromium oxide (Cr2O3) for its importance in specific applications such as green pigment [35], corrosion-resistant [36], and catalysts [37]. Vanadium oxides have been known as energy storage materials in lithium-ion batteries (LIB) due to their structural properties. It exists in varied compositions based on the oxidation states of the vanadium ion [38, 39]. Vanadium trioxide (V2O3) has good ionic intercalation properties as well as a high theoretical lithium storage capacity compared to that of V2O5 [40]. In addition to its high specific capacity, V2O3 has other advantages such as abundant raw material sources and low toxicity [41]. 1,4-benzenedicarboxylic acid and its coordination complexes have great attention in many studies due to their chemical and biological activities [42]. It has bifunctional carboxyl groups and exhibits many coordinate modes; it is common in the preparation of coordination polymers [43, 44]. And it has diverse great features of the compound, which are attributed to their ability to chelate with metal ions [45]. Pyrimidine is the base heterocyclic ring of an essential group of vastly studied materials due to its existence in living systems. Pyrimidines and their derivatives have apparent biological activity and have been used in a variety of fields, from medical to industrial applications [46]. Compared to pyridine bases, the presence of more than one heteroatom in pyrimidine plays an influential role in its coordination chemistry, making it preferable for biological systems [47]. The current paper reports the synthesis of Nd2O3, Cr2O3, and V2O3 NPs by calcination of new metal complexes, characterization, and study of their biological activity.

2. Experimental

2.1. Material

The chemicals outlined below were of analytical grade and were used as received were used without being purified further. 1,4-benzenedicarboxylic acid was purchased from Acros, while 2-amino-4,6-dimethyl pyrimidine, neodymium (III) acetate hydrate, chromium (III) chloride hexahydrate, vanadium (III) chloride, and solvents (ethanol, methanol, dimethyl sulfoxide (DMSO), and acetone nitrile) from Sigma-Aldrich.

2.2. Preparation of Metal Complexes
2.2.1. [Nd(BDC)(ADMPY)(OAc)].H2O

This coordination complex was prepared by dissolving neodymium acetate hydrate (4.08 gm) in 15 mL acetone nitrile, then adding it slowly to a solution of BDC (2 gm of H2BDC in ethanol 20 mL, distilled water 20 mL, and 0.001 M NaOH) with stirring. A methanol/distilled water (10 mL) mixture containing ADMPY (1.48 gm) was added with continued stirring for three days. After being refluxed, the mixture was left to cool to room temperature. Lilac precipitate was filtered, rinsed with distilled water and EtOH, and dried over anhydrous CaCl2.

2.2.2. [Cr(BDC)(ADMPY)Cl].H2O and [V(BDC)(ADMPY)Cl].H2O

The mixed ligand complex of 1,4-benzenedicarboxylic acid and 2-amino-4,6-dimethyl pyrimidine with Cr(III) was prepared via dissolving CrCl3.6H2O (3.20 gm) in 20 mL of distilled water, Then, with stirring, added it dropwise to the BDC solution (2 gm H2BDC in 20 mL ethanol, 20 mL distilled water, and 0.001 M NaOH). After that, 1.48 gm of ADMPY was dissolved in 15 mL of methanol and 15 mL of water and then added to the mixture with constant stirring for three hours. After reflux, the mixture was left to cool to room temperature. The filtered greenish-blue precipitate was washed with distilled water and ethanol before drying over anhydrous CaCl2. For V(III) complex, 1.89 gm of VCl3 was measured; the same method was followed; and the resulting precipitate was pale brown.

2.3. Preparation of Metal Oxide Nanoparticles

The metal oxide nanoparticles were prepared by direct calcination of Nd(III), Cr(III), and V(III) complexes. The complex powders were placed into a crucible, set in an electric oven, and then calcined at 750°C for three hours, 550°C for two hours, and 600°C for two hours, respectively. The resulting Nd2O3, Cr2O3, and V2O3 (light blue, green, and dark gray) nanoparticles (NPs) were washed with EtOH to remove any residual impurities and dried in air, followed by grinding to get fine particles. These were used for different characterizations.

2.4. Characterization of Compounds

Stoichiometric analyses (C.H.N) were performed using a Vario EL elemental analyzer; the physical measurements were measured as reported earlier [48].

2.5. Microbial Strains and Culture Media

In this work, different metal oxide NPs (Nd2O3, Cr2O3, and V2O3) in addition to Nd(III) and Cr(III) complexes were tested against Gram-positive and Gram-negative bacterial strains to give insight into their broad-spectrum effect. The used pathogenic strains were two Gram-positive strains “Staphylococcus aureus ATCC 25923 (S1) and Micrococcus luteus NCIMB 8166 (S4)” and two Gram-negative strains “Escherichia coli ATCC 35218 (S5) and Salmonella typhimurium ATCC 14028 (S10).” The antifungal activity was evaluated against a pathogenic reference strain of the yeast Candida albicans ATCC 90028 (9C). The strains were grown in nutrient broth (Oxoid) at 37°C for 24 hours, and they were cultivated on nutrient agar (Oxoid) at 37°C for 24 hours. The yeast strain was grown in Sabouraud Chloramphenicol broth (Oxoid) at 25°C for 24 hours and cultivated on Sabouraud Chloramphenicol agar (Oxoid) for 24 h at 37°C. The different strains are listed in Table 1.

2.5.1. Antimicrobial Activity

The antimicrobial activity of the complexes and MONPs was tested using the agar disk diffusion method [49]. Before the test, 50 mg of each extract were dissolved in 1 mL of a solution of dimethylsulphoxide “DMSO” (5%). The strains were cultured for 24 hours at 37°C in Mueller–Hinton (MH) broth (Oxoid), with suspensions adjusted to 0.5 McFarland standard turbidity. After that, 100 μl of each precultured suspension was spread onto MH agar plates. Sterilized filter paper discs (their diameter about 6 mm) were imbibition impregnated with 20 μl of each extract and placed on agar. The treated plates were set at 4°C for 1 hour and then incubated at 37°C for 24 h. After incubation, the diameter of the inhibition zone (clear halo) around the discs was measured. A duplicate of each sample was performed.

2.6. Antioxidant Activity
2.6.1. DPPH Radical Scavenging Assay

According to the method of Mahdhi et al. [50], the free radical scavenging influence of the extracts was evaluated as follows: 1 ml of the sample (5 mg/ml) and 3 ml of a methanol solution of DPPH (2,2-diphenyl-1-picrylhydrazyl) (300 μm) were mixed together. The mixture was vortexed and incubated for 30 min at 25°C. The absorbance of the solution was measured at 517 nm. As a standard, ascorbic acid was employed. The below equation was applied to calculate the DPPH inhibitory ratio:

3. Result and Discussion

The neodymium (III), chromium (III), and vanadium (III) metal complexes were synthesized via the reacting 1,4-benzenedicarboxylic acid and 2-amino-4,6-dimethyl pyrimidine in stoichiometric proportions. These compounds were prepared and used as a starting material for Nd2O3, Cr2O3, and V2O3 NPs by the calcination method. The metal complexes are stable in air and insoluble in most organic solvents, while it is partially soluble in dimethylsulphoxide “DMSO.”

3.1. Elemental Analyses (C.H.N)

The quantitative elemental analysis for the complexes of the new mixed ligands showed a significant agreement between the experimental and theoretical values of carbon, oxygen, and nitrogen, which confirms the truth of the added ratios of M:L1:L2 (1:1:1), as well as the proposed formulas of these complexes. The results of these analyses are recorded in Table 2.

3.2. Molar Conductivity Measurement

The molar conductivity of the coordination compounds was measured at room temperature in dimethylsulphoxide using 10−3M solutions of complexes; the results showed that all compounds had low conductivity values that were evidence that they were nonionic (Table 2).

3.3. Infrared Spectra

The most salient feature in the IR spectra of Nd(III), Cr(III), and V(III) complexes (Table 3) was the existence of two strong bands in the 1,556–1,558 and 1,374–1,394 cm−1 region, referred to the asym (COO) and sym (COO) stretching vibrations of the BDC ligand [45]. The absence of the characteristic band of carboxylic acids at 1,715–1,680 cm−1 indicates the complete deprotonation of the dicarboxylic acid molecule [51]. The separation value of Δν ≤ 192 cm−1 indicates a bidentate form of coordination for the carboxylate group [45]. The δ(O-C-O) in-plane appears vibration at 728–748 cm−1 [52]. Furthermore, the frequencies of asym (NH2), sym (NH2), (NH2), and C-NH2 occurring in free ADMPY at 3,312, 3,391, 1,628, and 1,250 cm−1, respectively, were found to have no shift in all complexes; this indicates that the amino group does not participate in the bonding [53, 54]. The ν(C=N) spectra appear as two bands at 1,574–1,577 and 1,648–1,653 cm−1 compared to the free bond (1,570 cm−1); these changes indicate the unequal mode of the N-atoms of the heterogeneous cycle in the compound, which indicates that the coordination mode is monodentate [53, 55]. Also, ν(C=C) and ν(C-N) bands appear at 1,476–1,478 cm−1 and 1,317–1,320 cm−1, respectively [53]. These results are consistent with the coordination of ADMPY through the heterocycle nitrogen atom in complexes. The stretching vibration of ν(CH3) for all complexes appeared at 2,985–2,989 cm−1 [53]. The spectra of complexes also showed bands in the 3,438–3,447 and 505–510 cm−1 range, which is attributed to νOH in lattice and wagging water, respectively [52]. The spectrum of neodymium (III) complex appears as bands at 1,568 and 1,339 cm−1, which are assigned to the acetate group, with separation value Δν = 229 cm−1, indicating monodentate mode [56]. Additionally, Cr(III) and V(III) complexes show bands at 418 and 420 cm−1 corresponding to metal chloride [57]. M-oxygen and M-nitrogen bonding appear in spectra at 552–570 and 469–472 cm−1 regions, respectively [58]. FT-IR spectra of metal complexes are shown in Figure 1.

3.4. Electron Spectra

Electronic spectra of metal complexes and MONPs were recorded using DMSO as solvent (Table 4). The spectra showed two characteristic bands in the ranges of 33,445–34,014 and 40,000–40,816 cm−1, assignable to π ⟶ π and n⟶π transitions within the BDC and ADMPY [46, 59, 60]. Furthermore, the f-f transition was found in the spectrum of the Nd(III) complex due to the transitions within 4f levels, which are usually forbidden but may become allowed after 5s2 5p6 electrons remove the degeneracy of 4f orbitals. The transition at 17,575 cm−1 corresponds to transition from the 4I9/24G5/2 suggesting their octahedral structure [61]. Also, there are distinct bands attributed to the d-d transitions of the chromium (III) and vanadium (III) complexes. For spectra of Cr(III) complex, three bands at 26,525, 20,450, and 18,248 cm−1 attributed to the 4A2g4T1g (P), 4A2g4T1g (F), and 4A2g4T2g (F) transitions, respectively, in agreement with octahedral geometry [62], while V(III) complex observe two bands at 19,268 and 14,728 assignable to the 3T1g3T1g (P) and 3T1g3T2g (F), respectively, indicating their octahedral geometry [63]. On the other hand, the spectra in Figures 24 showed a characteristic absorption peak for metal oxide nanoparticles, which was matching with the literature. The spectrum appears as bands at 247, 391, and 246 nm attributed to Nd2O3, Cr2O3, and V2O3, respectively [6466].

3.5. Magnetic Moment

The measurements of magnetic susceptibility in the solid-state display that the metal complexes are paramagnetic at 25°C. The magnetic moment of the Nd(III) complex was 3.70 B.M. [67], while the Cr(III) complex was 3.56 B.M. [68]. And the V(III) complex was 2.54 B.M. [69], all of which were in agreement with the values reported for octahedral complexes (Table 4; Figures 58).

3.6. Thermal Analysis

The thermal data of complexes were listed in Table 5, and the steps were represented in Scheme 1.

3.6.1. [Nd(BDC)(ADMPY)(OAc)].H2O

The Nd(III) complex undergoes a stepwise decomposition in four steps of weight losses in the temperature ranges 51–140°C, 142–200°C, 202–303°C, and 305–550°C (Figure 9 and Scheme 1). The first observed mass loss was attributed to a lattice water molecule (calc. 3.54%, found 3.10%). The DTG peak occurs at 105°C, and an exothermic peak arises at 107°C in DTA. The second step indicates the release of acetate (calc. 11.60%, found 10.76%). The DTG curve exhibits this stage at 147°C and the related DTA exothermic peak at 149°C. The observed mass loss of the third step agrees with the loss of the ADMPY decomposition (calc. 24.21%, found 24.68%; DTG midpoint at 247°C). For this step, the DTA curve displays a broad exothermic peak at 249°C. The fourth step reveals the decomposition of the BDC molecule (calc. 32.27%, found 28.76%), with a corresponding DTG peak at 410°C and a broad exothermic peak at 413°C in DTA. The final product is assigned to 1/2 neodymium (III) oxide (calc. 33.08%, found 32.70%).

3.6.2. [Cr(BDC)(ADMPY)Cl].H2O

For the thermal behavior of the Cr(III) complex, the thermogram indicates four distinct mass loss stages at 54–122°C, 124–212°C, 214–382°C, and 384–550°C. The first step is consistent with the release of the lattice water molecule (calc. 4.58%, found 3.66%). The DTG peak at 100°C corresponds to an exothermic peak in the DTA curve at 103°C. The second step shows a mass loss corresponding to chloride anion (calc. 9.02%, found 8.10%), with a DTG midpoint at 155°C associated with an exothermic peak at 157°C. The third and fourth stages relate to the release of ADMPY and BDC (calc. 73.14%, found 69.32%). Related DTG peaks at 379 and 400°C were merged in the DTA curve into one broad step centered at around 403°C. The final product is 1/2 Cr2O3 (calc. 19.34%, found 18.92%).

3.6.3. [V(BDC)(ADMPY)Cl].H2O

For V(III) complex, the TG curve is characterized by four decomposition steps at four steps. These steps occur in the temperature ranges 40–128°C, 130–226°C, 228–329°C, and 331–550°C. Elimination of the lattice water molecule (calc. 4.60%, found 4.12%) was observed at the first step. This step is related to a DTG peak at 77°C with an exothermic at 79°C in the DTA curve. The second step indicates the released chlorine ion (calc. 9.05%, found 8.76%). The corresponding DTG peak was seen at 198°C, with an exothermic peak at 200°C in the DTA curve. The third mass loss is compatible with the decomposition of the ADMPY ligand (calc. 31.44%, found 30.55%). The DTG curve shows this stage as a rise at 298°C, related to an exothermic peak at 301°C in the DTA curve. The fourth step corresponds to BDC decomposition (calc. 41.90%, found 36.74%). DTG peak at 405°C is associated with a broad exothermic peak in the DTA curve at 407°C. The residual part agrees with the formation of 1/2 V2O3 (calc. 19.13%, found 20.83%).

3.7. XRD  Study

The XRD pattern of the synthesized Nd(III), Cr(III), and V(III) complexes is shown in Figures 1012, while Figures 1315 show the XRD pattern of Nd2O3, Cr2O3, and V2O3 NPs on the scale of 2θ = 10–80. The sharp, strong peaks prove the products were well crystallized, while the broadening in the patterns indicates that the particles are of nanoscale [70]. The average particle size of the synthesized compounds was calculated by Scherrer’s equation. Crystal structure data for complexes and MONPs are listed in Tables 6 and 7.

3.8. Electron Microscope (SEM and TEM)

Changes in the size and morphology of the nano-metal oxides during the synthesis process were assessed using SEM. The morphologies of three MONPs are shown in Figures 1618. Nd2O3 NPs clearly show agglomerated platelet-like particles, with particle size around 30.16 nm, while Cr2O3 NPs show solid block material with compact density and fewer pores with nearly spherical shape and 56.12 nm size. The V2O3 NPs were an agglomeration of small spherical nanoparticles in 28.4 nm size. On the other hand, TEM images (Figures 1921) coincided nicely with the SEM images. Nd2O3 displayed a heterogeneous structure with an aggregation of particles, with more platelet-like particles of about 26.9 nm. As for the Cr2O3 NPs, some dispersed nanoparticles can be seen clearly in the TEM image with a subspherical shape with a diameter of 53.2 nm. Spherical shapes and agglomeration of particles can be seen clearly in the V2O3 NPs image, with a size of 21.32 nm. These results are well in agreement with the XRD analysis.

3.9. Antimicrobial and Antioxidant Assays

The antimicrobial results summarized in Table 8 and Figures 2225 demonstrate that a large spectrum of activity characterizes the tested compounds because they are active on Gram-positive and Gram-negative bacteria, especially on Gram-positive strains. Except for metal oxide nanoparticles, other compounds are characterized by a variable activity depending on the strain with the zone of inhibition, which varies between 1 and 2 cm. As summarized in Table 8, the results demonstrated a promising antioxidant activity for Nd2O3, Cr2O3, and V2O3 nanoparticles with a DPPH scavenging percentage ranging between 6.0 and 52%. The evaluated extracts also revealed an antifungal activity against Candida albicans except for Cr(III) complex, which is still inactive.

The data in Table 8 showed that the Nd2O3, Cr2O3, and V2O3 nanoparticles displayed perfect activity against bacterial and fungal strains. The Gram-positive bacteria cell consists of a thick wall; it is a peptidoglycan molecule. Since peptidoglycans are molecules that have a negative charge, they link positive ions emitted from metal oxide NPs in the liquid growth medium. Gram-negative bacteria such as E. coli may allow more positive ions to penetrate the plasma membrane, but they are typically less resistant to antibiotics and antibacterial treatments than Gram-positive bacteria [71].

Metal oxide NPs require connecting with bacterial cells to reach their antibacterial duty. The accepted shapes of contact contain electrostatic attraction [72], van der Waals forces [73], receptor-ligand [74], and hydrophobic interactions [75]. MONPs then cross the bacterial membrane and collect along the metabolic pathway, affecting the cell membrane’s shape and function. Afterward, NPs interact with the basic components of bacterial cells, such as DNA, lysosomes, ribosomes, and enzymes, leading to oxidative stress, heterogeneous alterations, changes in cell membrane permeability, electrolyte balance disorders, enzyme inhibition, protein deactivation, and changes in gene expression [7678]. The most frequently proposed mechanisms are oxidative stress [7982]. Finally, it may be concluded that Gram-negative bacteria have more antibacterial activity than Gram-positive bacteria due to their membrane shape. There are differences in the membrane structure of Gram-positive and Gram-negative bacteria, the most notable of which is the thickness of the peptidoglycan layer. Antibacterial activities of metal oxide NPs are principally attributed to their ability to adhere to bacteria due to their opposite electrical charges, which result in a decrease of the bacterial cell wall. Increased metal ion concentration on the surface disrupts the cell wall and allows metal ions to seep into the cells. The cell wall or membrane is injured in this method. The current study found that the produced MONPs caused bacterial mortality in Gram-positive and Gram-negative strains. C. albicans inhibition zone was largest in V2O3 and Cr2O3 NPs, respectively.

4. Conclusion

In this study, platelet Nd2O3, near-spherical Cr2O3, and spherical V2O3 NPs were prepared successfully by direct calcination of their mixed ligand complexes. The characterizations of metal complexes and MONPs revealed their formation of them. All complexes have octahedral geometry. XRD analysis of the complexes and their metal oxides NPs indicated that they have been crystalline and have various crystal systems: cubic for Nd(III) and Cr(III) complexes, hexagonal for V(III) complex, Nd2O3 NPs, and Cr2O3 NPs, monoclinic for V2O3 NPs. According to TEM, the average particle size of MONPs is about 26.9, 53.2, and 21.32 nm for Nd2O3, Cr2O3, and V2O3 NPs, respectively, which agree with SEM and XRD results. All compounds showed an excellent antimicrobial effect (except for Cr(III) complex). MONPs also showed promising results as antioxidants. Overall, calcination was an inexpensive method that can be applied to synthesize other metal oxide NPs to produce better size and morphology with high purity and crystalline. The present study suggests that MONPs have microbiological and antioxidant activity; they can be explored further for biomedical applications.

Data Availability

All data have been included within the article.

Conflicts of Interest

The authors declare that they have no conflicts of interest.