Octahedral Fe(III) and Ni(II) complexes of a bidentate 3-(3,4-dichlorophenyl)-1-(4-(2-hydroxynaphthalen-1-yl)diazenyl)phenyl)prop-2-en-1-one azo-chalcone: Spectroscopic,electronic,and biological insights

Abstract

The novel azo-chalcone ligand RD was synthesized via Claisen–Schmidt reaction followed by diazotization/coupling with β-naphthol. Fe(III) and Ni(II) complexes (FeRD, NiRD) were characterized as neutral octahedral species ([Fe(RD)₂(H₂O)Cl] and [Ni(RD)₂(H₂O)₂]) by molar conductivity (FeRD: 10.87 µS cm² mol⁻¹; NiRD: 10.12 µS cm² mol⁻¹). Fourier transform infrared spectroscopy confirmed bidentate coordination via phenolic O (FeRD: 3437 cm⁻¹; NiRD: 3455 cm⁻¹) and azo N (FeRD: 1511 cm⁻¹; NiRD: 1518 cm⁻¹). Magnetic moments (FeRD: 1.87 B.M.; NiRD: 3.21 B.M.) and UV-Vis transitions (FeRD: 435 nm; NiRD: 550 nm) validated geometries. Mass spectra (FeRD: m/z 998.842; NiRD: m/z 985.423) and elemental analysis (FeRD: Fe 6.14% calc/5.57%; NiRD: Ni 6.42% calc/5.94%) supported 1:2 stoichiometry. Density functional theory revealed FeRD’s optimal electronic profile: smallest ΔE (1.55 eV), highest electrophilicity (ω = 9.05 eV), and softness (σ = 0.65 eV⁻¹). Biological assays showed FeRD’s superior activity (K. pneumoniae: 95%, E. coli: 22 mm inhibition) versus NiRD (90%, 21 mm) and RD (50%, 10 mm). Molecular docking confirmed strongest TyrRS binding for FeRD (–8.70 kcal mol⁻¹).

Keywords

azo-chalcone ligand RD DFT docking nickel (II) complex octahedral iron (III)

Introduction

Azo chalcones have (–CO–CH=CH–), (N=N), II-isomers, and cis-trans isomer; trans-isomer is known as the most familiar isomer in azo chalcones. Consequently, it is continuously used in the dyeing industry,¹ pharmaceutical industries,² and have biological effectiveness such as anti-cancer “anti-tumor breast tumor cells,”³ anti-microbial “against Bacillus subtilis and Staphylococcus aureus,” and anti-oxidant “Fe (II) ion-chelating activity.”⁴ The azo formation process results from the reaction of Ar-NH₂ with (NaNO₂+HCl) to afford –N=N-salts then link with Nu⁻ such as 2- and 4-hydroxybenzene and aminobenzene. These materials can be detected through (N=N), which has significant biological activity, giving them special importance, such as anti-(cancer, viral, inflammatory, microbial, bacterial, fungal, and tubercular) properties. Furthermore, azo composites are widely utilized in the manufacture of drugs for example HIV—restrict viral copying, involving Evans blue and Congo red.^5–8 The binding of –N=N– dyes to the reverse transcriptase and protease enzymes of this virus are believed to be responsible for this effect. There are several materials that have shown antimicrobial and pesticide properties due to the bearing of the N=N-group, -N=N-imine, -N=N-pyrzoline, and pyrimidine derivatives. Furthermore, -N=N-compounds are used as a dye in many commercially available colorants.^9–11Azo-benzene compounds with lively efficient groups such as –CHO– and CH₃CO are produced by the very noteworthy reaction, -N=N-linkage, highly active aromatic rings which are linked by using an electron-loving substitution Rn., and -N=N-compounds have enormous publications in widely circulated literary journals. Claisen–Schmidt reaction (crossed-aldol Rn.) is considered as an effective tool for α,β-C₆H₅CH=CHCOPh. This reaction plays a significant role in synthetic organic chemistry.

Phenyl methyl ketone and phenyl carbaldehyde, or their derivatives, contain acetyl and aldehyde groups in their structures, respectively. They can undergo this reaction in the sodium hydroxide (NaOH) states of ethanol (EtOH) to produce chalcone. The -N=N-materials which have terminal -COCH₃ reacts with numerous Ar-carbaldehyde materials through Claisen–Schmidt condensation to yield CO-CH=CH-compound ascribed to a -N=N-component called azo-chalcone structure. This can successfully lead to the creation of a novel C–C bond, which is of great importance in the field of organic synthesis. Chalcone compounds have several names such as Me-styryl ketone and E-chalcone benzal acetone. One of the most significant and distinctive features of chalcone is its (-CO−CH=CH-) group.^9–11

Current work is focused on the design, preparation, and validation of the novel Azo Chalcones. This is accomplished through two main procedures. The first procedure is the reaction between p-aminoacetophenone and 3,4-dichlorobenzaldehyde which occurs under basic conditions and at low temperatures in a condensation step to produce a chalcone derivative. The second procedure, this product, which contains an aromatic amine group, reacts with sodium nitrate and hydrochloric acid (NaNO₂ + HCl) to form an azo compound. A coupling process then occurs with phenols (β-naphthol) under basic conditions, producing new azo chalcones, as shown in Schemes 1 and 2. These compounds are then used as new raw materials for producing novel complexes when reacted with metal salts.

Scheme 1.

Designing of 1-(p-aminophenyl)-3-(o, p-dichlorophenyl)-2-propen-one-1.

Scheme 2.

Designing of 3-(o,p-dichlorophenyl)-1-(4-(o-hydroxynaphthalen-1-yl)diazenyl)phenyl)-2-en prop-I-one compound RD.

Methodology

Reagents

All the materials and reagents used were of analytical reagent (AR) grade and commercially available in pure form without further purification. The reagents included 3,4-dichlorobenzaldehyde (C₇H₄Cl₂O, ⩾99%), p-aminoacetophenone (C₈H₉NO ⩾99%), sodium nitrite (NaNO₂, ⩾99%), and 2-naphthol (C₁₀H₈O, ⩾99%). In addition, absolute ethanol (C₂H₅OH, ⩾99.9%) was obtained from BDH and used as spectroscopically pure organic solvents.

Assembly of the RD (ligand) in two steps

Widespread procedure for synthesis of 1-(p-amino-phenyl)-3-(m, p-dichlorophenyl)-2-propen-I-one compounds 3

0.03 moles of 3,4-dichlorobenzaldehyde and 0.03 moles of p-aminoacetophenone were added together. Then, the mixture of substances was deliquesced in the minimum possible volume of EtOH (80 mL) and 0.03 moles of NaOH, which is also deliquesced in EtOH (10 mL) and H₂O (15 mL). After that, they were mixed together. Then Rn. mixture was stirred for 3 h in cold therapy to precipitate the compound 3 (see Scheme 1).¹²

Widespread procedure for synthesis of 3-(3,4-dichlorophenyl)-1-(4-(o-hydroxynaphthalen-1-yl)diazenyl)phenyl)-2-propen-1-one compound RD

The diazotization process of 1-(p-aminophenyl)-3-(o,p-dichlorophenyl)-prop-2-en-1-one (compound 3) is explained below. 0.02 moles and excess of HCl (solvent and reactant) and 0.025 moles of NaNO₂ were added. All the components of the reaction are stirred for 30 min in an ice bath at (0–5 °C). After the azo formation process is completed, 2-naphthol is dissolved in NaOH solution as equivalent weights and then added to the azo compound. This step is called the coupling process. With continued stirring and low temperature, the azo dye, with its distinctive bright crimson color, is formed. After that, the dye is filtered, washed by H₂O, dried, purified, and then recrystallized using ethyl alcohol in 83% yielding compound (see Scheme 2).

3-(3,4-dichlorophenyl)-1-(4-(o-hydroxynaphthalen-1-yl)diazenyl)phenyl)-2-propen-1-one compound RD: Yield (83%); m.p. 221–223 °C; IR (cm⁻¹): 3370 (OH), 3077 (ArH), 1668 (C=O), 1562, (-N=N-)791 (C–Cl); ¹H NMR (400 MHz, DMSO-d₆) δ (ppm): 9.68 (OH), 8.04 (d, 1H, Ph-CH=CH-CO), 7.61 (d, 1H, Ph-CH=CH-CO), 8.02–7.21(m, 13H, CHarom.), Elemental analysis for C₂₅H₁₆Cl₂N₂O₂ (447.3), (Calcd./Found); C, 67.13/67.01; H, 3.61/3.57; N, 6.26/6.10 (Scheme 2).

Assembly of the iron-RD and nickel-RD complexes

An H₂O solution (30 mL) of the corresponding M-salt was ready. The M-salt media was then dropwise added, drop by drop and slowly, to a preheated alcoholic (ethanol) medium (30 mL at 65 °C) containing the RD ligand. All the components of the reaction were refluxed at 85:95 °C with continuous magnetic stirring for several hours (5 h) to ensure a complete Rn. After cooling, the formed solid complexes were washed thoroughly with EtOH to soluble unreacted materials or impurities. Finally, the purified complexes were dried under ambient conditions. The overall synthetic pathway is outlined in Scheme 3.

Scheme 3.

Designing of FeRD and NiRD.

DFT calculation

The constructed RD, Iron-RD, and Nickel-RD molecules were geometrically optimized with the help of the B3LYP functional^13–15 with the 6-311G(d,p) basis set for elements such as O, N, H, and C, and LANL2DZ habitual for metal centers^12,16,17 with ORCA 5.0.¹⁸ After that, the boundary molecular orbitals were calculated, and the main chemical indices of the molecules were evaluated, namely HOMO, LUMO, ΔE, η, ω, and σ, where HOMO is the highest occupied molecular orbital, LUMO is the lowest unoccupied molecular orbital, ΔE is energy gap, μ is chemical electron potential, η is chemical hardness, ω is electron affinity index, and σ is softness.^19,20

Biological activity of the FeRD and NiRD complexes

The laboratory evaluated the complex compounds and the active linker as antibacterial agents, following experimental procedures. Further details can be found in the relevant sections.²¹

Molecular docking

Auto-Dock was used as the software to perform molecular docking on the chemical structure of each synthesized compound.²² This process is crucial for investigating and analyzing the binding interactions between the studied compounds, RD, Iron-RD, and Nickel RD, and the tyrosyl-tRNA synthetase (TyrRS) from Staphylococcus aureus) PDB ID: 1KZN.^23–27 The target protein’s three-dimensional structure was acquired from the Protein Data Bank (https://www.rcsb.org/structure/1KZN). Preparation of the substrate involves dehydration and removal of any heteroatoms and co-crystalline bonds, go after by the attachment of Coleman charges and polar H-atoms to strengthen the protein to Imitate docking. The ligand shaping was equipped according to earlier decided manner.²⁸ The docking grid box was identified with axis coordinates (X = 19.69, Y = 19.59, Z = 43.04) including the effective site, to appreciate binding properly.

Outcomes analysis and discussion

RD ligand characterization

Chalcone offshoots, which have the NH₂-group, are beneficial and effective basic materials to afford important parent structures for new organic synthesis. Thus, when 1-(4-aminophenyl)ethan-1-one was blended in cryotherapy with o,p-dichlorophenyl carbaldehyde in the existence of NaOH in EtOH for 3 h, the ethanoyl group (COCH₃) was bonded to the aldehyde group (CHO) to produce 1-(p-aminophenyl)-3-(o, p-dichlorophenyl)prop-2-en-1-one (compound 3) (see Scheme 1).²⁹

It has been intended that the presence of the NH₂-group in compound 3 is considered as a precious chance for azo-production when it is treated with NaNO₂ and HCl. This step is followed by its reaction with various phenols to produce an industrially suitable dye.

Therefore, stirring and cooling a mixture of compound 3 and treating it with HCl and NaNO₂ produced an azo compound. This azo compound was followed by coupling with 2-naphthol to produce 3-(2,4-dichlorophenyl)-1-(4-(-(o-(OH)naphthalen-I-yl)diazenyl)phenyl)prop-II-en-I-one (Scheme 2).

Upon investigating the ¹H NMR of 3-(o,p-dichlorophenyl)-1-(p-(o-hydroxynaphthalen-1-yl)diazenyl)phenyl)prop-2-en-1-one compound 4, it can be settled that all the predicted signals for the planned structure are contemporary, as the expected signals appear with the associated cleavage pattern, 13H, CH-aromatic showed multiple signals in the region of 7.21 to 8.02 ppm, a double signal at 7.61 ppm, corresponding to 1H, CO-CH, and a double signal at 8.04 ppm for 1H, CH-ph. Finally, a single signal appears at 9.68 ppm, corresponding to 1H, the ph.-OH group.

Also, the IR spectrum of compound 4 contains CO and its captivation peak shows at 1668 cm⁻¹ and captivation band at 3370 cm⁻¹ for the stretching of OH group.

Characterization of the FeRD and NiRD complex compounds

Molar conductivity

The EtOH (1 mM) conductivity values for both FeRD and NiRD complexes are very low: 10.87 and 10.12 µS cm² mol⁻¹, respectively, suggesting that they are non-electrolytes. This further suggests that these complexes are present as neutral species in solution, consistent with the proposed structures for these complexes: [Fe((RD)₂(H₂O)(Cl)] and [Ni((RD)₂(H₂O)₂)]. The lack of significant ionization for these complexes supports that the ligand and water are tightly bound together as a molecule, without dissociating into ions, further emphasizing the robustness of this intact octahedral framework as a solution species.

FTIR

The FTIR results for the RD ligand and its FeRD and NiRD complexes clearly demonstrate the coordination mode of the ligand with the metals. For the free ligand, there is a broad absorption band centered at 3370 cm⁻¹ assigned to the phenolic hydroxyl (-OH). The azo (-N=N-) absorption band is observed at 1562 cm⁻¹. However, when the ligand coordinates with the metals, the hydroxyl absorption band shifts to higher energies for both complexes. For the FeRD complex, it is observed to be at 3437 cm⁻¹, while in the NiRD complex, it increases to 3455 cm⁻¹.

Also, it shifts to lower wavenumbers; specifically, it appears at 1511 cm⁻¹ for FeRD and at 1518 cm⁻¹ for NiRD, indicating coordination with azo nitrogen. Bands due to M-O stretching are observed at 531 cm⁻¹ for FeRD and 328 cm⁻¹ for NiRD, whereas bands due to M-N stretching are observed at 501 cm⁻¹ and 507 cm⁻¹, respectively, confirming that RD coordinates as a bidentate ligand via phenolic-O and azo-N atoms for both metal complexes.

UV-Vis spectra and effective magnetic moment

The UV-Vis spectral details are illustrated in Figure 1. It includes the electronic behavior of the RD ligand and its respective complexes with Fe and Ni in the presence of light. The free ligand shows a peak at 305 nm due to the n→π* transition. The complex with the metal ion, FeRD, shows a peak at 435 nm due to the ⁶A_1g (F) → ⁴E_g (G) transition, indicating that the complex has an octahedral arrangement of the iron center. On the other hand, the complex with the metal ion and the ligand, NiRD, shows a peak at 550 nm due to the ³T₁ (F) → ³T₁ (P) transition, indicating a tetrahedral arrangement of the nickel center in the complex. These observations confirm the difference in the structures of the metal complexes.

Figure 1.

(a) UV-Vis spectra and (b) stoichiometry of the RD ligand and its iron-RD and nickel-RD complexes.

The availability of the accurate magnetic moment data for the FeRD and NiRD complexes can provide valuable insight into their electronic arrangements as well as their shapes too. For the FeRD arrangement, the calculated µ_eff is 1.87 Bohr magnetons, which is consistent with the presence of a low-spin Iron(III) ion that is associated with one unpaired electron as depicted by the electronic arrangement of d⁵ (t_2g⁵). Conversely, the calculated µ_eff of the NiRD arrangement is 3.21 Bohr magnetons, which is consistent with the presence of a Ni(II) ion that is associated with two unpaired electrons as dictated by the electronic arrangement of d⁸ (t_2g⁶ e_g²).

Stoichiometry

The above stoichiometric analysis, presented in Figure 1 in relation to the RD ligand and its FeRD and NiRD complexes, confirms that the ratio of metal to ligand is 1:2 in all cases. The FeRD complex is given by the formula [Fe(RD)₂(H₂O)(Cl)], consistent with two units of the RD ligand coordinating with the metal ion, which is accompanied by one H₂O molecule and one Cl⁻. The NiRD complex may be described using the formula [Ni(RD)₂(H₂O)₂], confirming that two units of the ligand coordinate with Ni²⁺ and that two H₂O units are present. These formulations confirm that binding occurs in a bidentate fashion for the given RD ligand.

Mass spectra

MS data, Figure 2, for the FeRD and NiRD complexes show molecular ion peaks at m/z 998.842 and 985.423, in the specified order. These values closely match the theoretical M.Wt. of 1001.923 for [Fe(RD)₂(H₂O)(Cl)] (C₅₀H₃₂Cl₅FeN₄O₅) and 987.331 for [Ni(RD)₂(H₂O)₂] (C₅₀H₃₄Cl₄N₄NiO₆). This close agreement confirms the proposed molecular formulas and supports the stoichiometry previously established.

Figure 2.

MS of FeRD and NiRD complex substances.

Elemental analysis

EA data, Table 1, of the RD ligand’s FeRD and NiRD metal complexes exhibit close agreement with the calculated values, affirming the anticipated molecular formulations. For the FeRD complex, the found percentages are C (60%), H (3.98%), N (6.44%), and Fe (6.14%), compared to the theoretical amounts of C (60%), H (3.22%), N (5.59%), and Fe (5.57%). Similarly, the NiRD complex shows found values of C (61.24%), H (3.90%), N (6.35%), and Ni (6.42%) closely matching the calculated values of C (60.82%), H (3.47%), N (5.67%), and Ni (5.94%). These petty deviances remain inside adequate investigational restrictions, supporting the stoichiometry of the synthesized complexes.

Table 1.

EA: found (computed) of the FeRD and NiRD complexes.

		FeRD	NiRD
EA Found (calc.)	C	60.42 (59.94)	61.24 (60.82)
	H	3.98 (3.22)	3.90 (3.47)
	N	6.44 (5.59)	6.35 (5.67)
	M	6.14 (5.57)	6.42 (5.94)

Thermal analysis

The thermal degradation profiles, Supplemental Figure S1 and Table 2, of the FeRD and NiRD complexes demonstrate two well-defined decomposition stages, supporting their structural integrity and thermal stability. For FeRD, the primary main mass loss occurs between 215 °C and 495 °C with a DTG peak at 350 °C, agreeing to 60.085% (calc. 60.645%) attributed to the injury of a C₃₀H₂₀Cl₅O₃ fragment. The remaining residue is 40.254% (calc. 39.664%), consistent through the creation of a C₂₀H₁₂FeN₄O₂ intermediate. The second stage, from 495 °C to 710 °C with a DTG peak at 585 °C, shows a mass loss of 34.954% (calc. 34.073%), attributed to C₂₀H₁₂N₄O₂ degradation, leaving a final residue of 5.952% (calc. 5.606%), corresponding to metallic iron.

Table 2.

Thermal degradation data of the FeRD and NiRD complex substances.

	TG (°C)	DTG (°C)	Mass loss (%)			Residue (%)
	TG (°C)	DTG (°C)	Calc.	Found	Assig.	Calc.	Found	Assig.
FeRD	215-495	350	60.645	60.085	C₃₀H₂₀Cl₅O₃	39.664	40.254	C₂₀H₁₂FeN₄O₂
FeRD	495-710	585	34.073	34.954	C₂₀H₁₂N₄O₂	5.606	5.952	Fe
NiRD	215-510	320	59.701	60.421	C₃₀H₂₂Cl₄O₄	40.493	40.886	C₂₀H₁₂N₄NiO₂
NiRD	510-715	610	34.537	35.088	C₂₀H₁₂N₄O₂	5.987	6.384	Ni

The NiRD complex shows a similar pattern. The first decomposition step occurs between 215 °C and 510 °C through a DTG crowing at 320 °C, showing a 60.421% mass loss (calc. 59.701%), ascribed to the elimination of C₃₀H₂₂Cl₄O₄. The resulting intermediate residue is 40.886% (calc. 40.493%), corresponding to C₂₀H₁₂N₄NiO₂. The sequel, beginning 510 °C to 715 °C and DTG at 610 °C, accounts for a 35.088% loss (calc. 34.537%), leading to a residual mass of 6.384% (calc. 5.987%), assigned to elemental nickel.

The near match stuck experimental and theoretical values approves the thermal decomposition pathway and supports the suggested compositions and coordination environments of both complexes.

DFT calculations

Molecular modeling of the RD, FeRD, and NiRD was fulfilled via DFT calculations amid B3LYP at 6-311G(d,p) and LanL2dz for (C, H, N, O) and metal atoms, respectively, exhibited in Figure 3. Maximizing the benefit of FeRD and NiRD complexes resulted in octahedral geometries throughout the Nickel(II) center, shown in Figure 3. The octahedral geometry was established by computed bond angles intently aligning with the ideal angles for an octahedron, as described in Supplemental Table S1.

Figure 3.

3D and HOM-LUMO representation of the RD, FeRD, and NiRD.

The principal players involved for the given molecule constitute the boundary molecular orbitals. The HOMO and LUMO show us the reactivity of the molecule and the stability of the molecule toward changes. Negative values correspond to stability, whether the HOMO and LUMO energy values are considered individually or collectively. Figures showing the HOMO and LUMO energy of the products considered in the paper are given in Table 3. Figure 3 illustrates the HOMO and LUMO of the given molecules, showing the negative and positive regions as characterized by the colors red and green, respectively. In addition to the HOMO and LUMO energy values, other values that characterize the principal players of the given molecules include the energy difference value ΔE, the electrochemical potential value μ, the stability of the molecule toward changes represented by the η value, the electron passion value ω, and the softness value σ.

Table 3.

Calculated HOMO, LUMO, energy gap (ΔE), ionization potential (IP), electron affinity (EA), electronegativity (χ), chemical potentials (μ), chemical hardness (η), softness (σ), and electrophilicity index (ω).

	HOMO	LUMO	∆E	IP	EA	χ	μ	η	σ	ω
RD	–4.80	–1.97	2.84	4.80	1.97	3.38	–3.38	1.42	0.35	4.04
FeRD	–4.52	–2.97	1.55	4.52	2.97	3.74	–3.74	0.77	0.65	9.05
NiRD	–4.20	–2.30	1.90	4.20	2.30	3.25	–3.25	0.95	0.53	5.58

RD possesses a HOMO energy of −4.80 eV, indicating a moderate electron-donating ability. In its complexes, the HOMO levels are raised somewhat: FeRD at −4.52 eV and NiRD at −4.20 eV, indicating an increased nucleophilicity. Higher HOMO values signify better possibilities of interaction with biological targets that are electrophilic in nature. Of these, NiRD possesses the highest HOMO and thus should exhibit the highest nucleophilic interaction possibility and probably improved bioactivity.

Among them, the LUMO energy for FeRD is the lowest at −2.97 eV, followed by NiRD at −2.30 eV, and the highest for RD at −1.97 eV. A lower LUMO increases the electron-accepting capability, thus playing a vital role in redox-sensitive biological systems. These facts account for FeRD having the deepest LUMO energy and, therefore, being the best electron acceptor, which should help it to provide very strong predicted biological activity. Complexation greatly decreases the energy gap. RD’s moderate ΔE of 2.84 eV dropped to 1.90 in NiRD and further decreased to 1.55 in FeRD. A smaller ΔE tends to improve electro-mobility and effectiveness. Thus, FeRD will be highly biologically reactive due to the least ΔE value, which aligns with the enhanced molecular level interactions. Ionization potential (IP) shows the same trend as that of HOMO. The highest IP is present in RD, with 4.80 eV, while FeRD and NiRD show somewhat lower values, amounting to 4.52 eV and 4.20 eV, correspondingly. Lower IP means easier electron loss, what is definitely a plus in the biologically oxidational-prone media. NiRD, with the lowest IP, probably would take part in redox reaction most easily, hence possessing the highest potential toward biological interaction. Electron affinity shows the maximum value for FeRD with 2.97 eV, followed by NiRD with 2.30 eV and RD at 1.97 eV. High EA is important to accept electron density from nucleophilic biomolecules. FeRD, with its strong EA, suggests a better capacity for such biological engagements.

Electronegativity varies from FeRD with an electronegativity of 3.74 eV to RD with an electronegativity of 3.38 eV; the lowest is for NiRD with an electronegativity of 3.25 eV. A higher electronegativity indicates a higher tendency toward pulling the electrons toward itself. However, a higher electronegativity may also limit the flexible movement of a molecule.³⁰ FeRD strikes a nice balance as it remains reactive while holding tight to the biological targets. Its higher electronegativity accounts for its relevance to biological chemistry. All compounds synthesized appear to exhibit negative chemical potential; hence, thermodynamics is in balance. Among these compounds, the least negative μ is for RD with −3.38 eV; the second least negative is for NiRD with −3.25 eV, while the most negative is for FeRD with −3.74 eV. The compounds with the least negative μ appear to be more inclined to donate electrons, which in turn make them more reactive. Of course, the more negative μ for FeRD is somewhat restricted by the tendency toward higher electron affinity.

Regarding the hardness values, RD has the highest hardness of 1.42 eV, while NiRD has a hardness of 0.95 eV; subsequently, the lowest hardness among the profiles is seen in FeRD at 0.77 eV. Softer structures are normally desirable for boosting the reactivity and the extent of adaptability to the constantly changing environment in biology.³¹ The low hardness of FeRD thus validates its high edge in bioactivity by virtue of its higher polarizability and increased softness. The lower the hardness of a profile, the higher the softness value; in this regard, the order of softness values follows the same pattern as the hardness values. Thus, the high softness of 0.65 eV⁻¹ is seen in FeRD, while the second highest profile in the list of soft values corresponds to NiRD at 0.53 eV⁻¹; finally, the lowest hardness translates to the lowest soft value of 0.35 eV⁻¹ in the RD profile. Softer structures normally have higher interactions with their environments; hence, the relatively higher soft value of 0.65 eV⁻¹ in the FeRD profile validates the higher edge in biological activity. Notably, the relatively higher soft value of 0.65 eV⁻¹ in the FeRD profile corresponds to a relatively higher value of the electrophilicity index of the RD ligand itself is only moderately reactive, but it is definitely enhanced in its biological potential upon complexation with iron and nickel. Among these, FeRD shows outstanding performance: low ΔE, high softness, high electron affinity, and excellent electrophilicity indicate that it is the most biologically active. In comparison, NiRD also exhibited remarkable activity, characterized by high nucleophilic strengths but relatively weaker electrophilic property. Overall, the predicted sequence of biological activities is FeRD, followed by NiRD, and then RD.

In the molecular electrostatic potential (MEP) maps of Figure 4, we see how this voltage is distributed throughout the molecule’s surface. The electron-rich areas are those with negative charges (the red colors), while the electron-deficient areas are those representing positive charges (the blue colors). In all these maps for RD, FeRD, and NiRD compounds, there are negative charges near the bonds with nitrogen and oxygen. This explains a large quantity of probable electrons for faster and stronger responses with electron-accepting materials. There is also a positive charge component that deals with biological waste.

Figure 4.

Molecular electrostatic potential (MEP) map of RD, FeRD, and NiRD.

Biological activity

Looking at the results of the free RD ligand alone, there was hardly any denting of the microbial defenses, with nice, uniform 10 mm inhibition rings seen with all the strains (see Supplemental Table S2). But once the ligand binds to a metal to form a complex, the antimicrobial punch is a lot bigger. FeRD leads the pack with a very clear, broad-spectrum effect: 17 mm against S. aureus, 17 mm against B. subtilis, 19 mm against K. pneumoniae, and 22 mm against E. coli. NiRD is close behind with 16 mm, 17 mm, 18 mm, and 21 mm against the same microbes, respectively. These results suggest a much stronger interaction of these metal complexes with the microbial membranes. Once again, the softer nature of the metal complexes likely helps in these interactions by virtue of the smaller energy gaps between the HOMO/LUMO orbitals themselves. To top it all off, with all the strains of microbes, FeRD consistently gives the best results with the largest inhibition zones.

However, in terms of overall biological activity in comparison with R Alone and R Complexes, while the activity of RD alone is modest, reaching only 40% to 50% at most, reflecting its weak ability to restrain microbial growth.The increase in activity upon complexation is considerable. The activity of the iron RD complexes is superior, standing at 77.27% for S. aureus, 85.00% for B. subtilis, 95.00% for K. pneumoniae, and 88.00% for E. coli, approaching that of chloramphenicol, albeit at the same concentration. The activity of the nickel complexes is somewhat lower, although still considerable: 72.73%, 85.00%, 90.00%, and 84.00%, respectively. It is apparent that the activity of the iron complexes is always superior.

The results have demonstrated that the binding of metal ions to the ligand enhances the ligand’s antimicrobial potential. Both FeRD and NiRD have surpassed the ligand in all the chosen aspects. Among the metal-peptide conjugates, FeRD has proved to be the strongest against bacteria and fungi. This is due to the metal ion’s electronic properties, as it has a narrow energy gap, high electron affinity, and excellent electrophilicity. Similarly, NiRD has done well in comparison to the ligand but not as well as FeRD. On the other hand, RD has demonstrated the least biological activity. The predicted biological activity has the following order: FeRD > NiRD > RD.

It is seen that the antimicrobial and antifungal activities of RD, FeRD, and NiRD match very well with their electronic properties. As mentioned earlier, RD possesses the lowest HOMO (−4.80 eV) and the highest band gap with hardness of 1.42 eV, indicating low biological activity due to low electron transfer and high stability. However, upon coordination with Fe and Ni, their biological activities increased significantly due to low band gaps of 1.55 eV and 1.90 eV, high softness of 0.65 eV⁻¹ and 0.53 eV⁻¹, and high electron affinity of 2.97 eV and 2.30 eV. As far as the activity is concerned, FeRD was found to possess the highest electrophilicity index of 9.05 eV, reflecting potential capability for biological activity with the highest activity percentages than the others for all the bacterial strains. Moreover, FeRD possesses the largest inhibition zone compared to the others. Among the remaining compounds, NiRD follows closely with relatively higher HOMO, high softness, and high electrophilicity due to moderate values of other parameters. In brief, compared to the others, higher biological activity is connected with low ΔE values, high softness, and high electrophilicity, confirming that indeed FeRD > NiRD > RD.

SAR interpretation

The observed variations in antimicrobial activity of FeRD and NiRD complexes might be attributed to the distinct effects of these ligands on the electronic structures of the complexes. Coordination of Cl⁻ or H₂O to the central metal influences the electron density distribution, ligand-field strength, and lipophilicity of the complexes. Chloride ligands transfer more electrons to the central metal, which increases π-back bonding to the overall ligand scheme. This enhances the stabilization of the frontier orbitals by reducing the HOMO-LUMO gap. As a result, charge transfer interactions with biological molecules would be facilitated, accompanied by increased lipophilicity, which enhances membrane penetration. H₂O, on the other hand, being a weak field ligand, increases the polarity of the complex while reducing the electron delocalization effects, which in turn might lower the overall activity of the interaction. This observation matches the enhanced activity of the halogenated complex. As such, it can be concluded that the activity trend follows an overall structure–activity relationship dictated by ligand field strength, charge distribution, and overall polarity.

Compared to those related systems, it is evident that the current RD ligand and FeRD and NiRD complexes possess similar coordination properties while displaying improved electronic and biological properties. It merits special discussion that the FeRD complex has an optimized HOMO–LUMO energy difference of 1.55 eV and an electrophilicity index value of 9.05 eV. This is indicative of the better charge transfer ability of the current azo metal complexes than those of similar chemistry reported so far in the literature. This draws an analogy with the optimized antimicrobial activity and good binding affinity with the target protein, TyrRS, through the docking studies (−8.70 kcal mol⁻¹).

All these FeRD and NiRD complexes can be considered extended from our previously reported FeIII and NiII azo-chalcone complexes by our group. Similar coordination patterns are followed, but differences are observed in terms of electronic and biological behavior. Modification of the ligand system reduces the energy gap between HOMO and LUMO and augments electrophilicity. FeRD has been recorded to have decreased energy (1.55 eV) and increased electrophilicity (9.05 eV), signifying increased antimicrobial behavior compared to other such complexes. A systematic extension of these complexes indicates changes in ligands that influence metal-centered reactivity.

Molecular docking

The PDB entry 1KZN defines the crystal structure of the enzyme tyrosyl-tRNA synthetase, otherwise known as TyrRS, which is found in the bacterium Staphylococcus aureus, a contender that is frequently implicated as the causative bacterium in hospital-acquired infections. From a scientific perspective, the entry 1KZN defines a drug target that is a prime subject of choice in computer-based studies of the binding of new candidates to define their potential as antimicrobial agents; the targeted enzyme, tyrosyl-tRNA synthetase, underscores its critical position as the driver of the incorporation and subsequent binding of the amino acid tyrosine to its cognate tRNA. This process is significant and sine qua non to the viability of the bacterium, and its blockade will terminate protein synthesis, making the proposed target compellingly attractive as a candidate to exploit as the basis of antimicrobial compounds. From the perspective of antifungal therapy, the 1KZN entry also defines a potentially useful candidate because all aminoacyl-tRNA synthetases, including the targeted enzyme TyrRS, continue to show significant conservation of function and critical position in the cell regardless of whether the organism in question is bacterial or of the fungal kingdom while the conventional antifungal targets of choice, including those of the ergosterol biosynthesis pathway, may not offer the same potential as pursuing the organism’s protein synthesis pathway as a therapeutically viable approach, particularly as resistance threatens to limit the utility of the majority of the antifungal agents that can at present be deployed to limit their proliferation. Because the active sites defined by the analogous enzymes in microbes differ substantially from those found in humans, the 1KZN entry defines an attractive use as it will result in lower toxicity to the host as against the microbe.

To ascertain the reliability of our docking protocol, a re-docking exercise was performed using the native ligand co-crystallized with the 1KZN protein. This re-docking exercise was considered an essential step to confirm the accuracy of our docking protocol. As seen in Supplemental Figure S2, the re-docked ligand corresponds exactly with the co-crystallized ligand. There was a minor difference between the predicted binding conformation and the actual binding conformation, thus proving the precision of our docking protocol, which can be determined by superimposing our docking results, as depicted in Supplemental Figure S2.

Supplemental Figure S3 showed the three-dimensional interactions between the target protein 1KZN and the RD, FeRD, and NiRD that were discovered by molecular docking analysis, and the data were tabulated in Supplemental Table S3.

The ligand RD interacts with the enzyme TyrRS with a moderate binding affinity of −7.40 kcal mol⁻¹. Here, the interaction is dominated by the presence of two strong hydrogen bonds with ASN46 residues with a distance of 2.86 Å and 2.92 Å. In addition, the ligand RD interacts with the enzyme with a number of hydrophobic contacts with residues like THR165, ALA47, ILE78, VAL71, VAL43, VAL167, and ILE.

The greater binding energy of the FeRD complex (−8.70 kcal mol⁻¹), in comparison to the free RD, implies a greater interaction of the complex. This is mainly due to the hydrogen bond with PRO79 at a distance of 3.58 Å. This hydrogen bond is likely to play a significant role in the docking of the complex. In addition, the complex also experiences hydrophobic interactions with ALA47, ALA86, VAL43, VAL120, VAL167, and ILE78 residues at varying distances ranging from 3.7 to 5.1 Å.

NiRD binds with TyrRS with an affinity of −8.40 kcal mol⁻¹, being slightly lower than that of FeRD, although it binds better than free RD. A notable aspect of NiRD is that it possesses several electrostatic interactions with ARG76 and GLU50, measuring between 3.66 and 4.34 Å. It has hydrophobic interactions with ALA53, ILE78, ILE90, ALA47, PRO79, and ALA86, measuring as much as 5.38.

Overall, the metal coordination was found to improve binding affinity compared to the ligand itself. The FeRD and NiRD compounds were seen to be more effective than RD. Among the FeRD and NiRD compounds, FeRD showed a higher binding affinity of −8.70 kcal mol⁻¹ than NiRD, with a binding affinity of −8.40 kcal mol⁻¹. They were followed by RD with a binding affinity of −7.40 kcal mol⁻¹. The metal coordination was seen to modulate the interacting type and binding modes by improving the fitting of the ligand to the pocket of TyrRS. This may improve their potential as inhibitors of TyrRS and their overall antibacterial activities. In conclusion, the relative binding affinities of the RD compounds are as follows: FeRD > NiRD > RD. Here, metal coordination was seen to improve their interaction with TyrRS.

Conclusion

FeRD and NiRD exhibit octahedral geometries with bidentate RD coordination (phenolic O, azo N), validated by spectroscopy, magnetism, and DFT. FeRD demonstrates superior bioactivity (95% vs K. pneumoniae; 22 mm vs E. coli) driven by its optimal electronic profile: lowest ΔE (1.55 eV), deepest LUMO (–2.97 eV), highest electrophilicity (ω = 9.05 eV), and softness (σ = 0.65 eV⁻¹). NiRD shows enhanced nucleophilicity (HOMO: –4.20 eV) but moderate electrophilicity (ω = 5.58 eV), yielding slightly lower activity (90%, 21 mm). The activity trend (FeRD > NiRD > RD) arises from metal-induced electronic modulation: reduced ΔE (RD: 2.84 eV → NiRD: 1.90 eV → FeRD: 1.55 eV), increased softness (RD: 0.35 eV⁻¹ → FeRD: 0.65 eV⁻¹), and frontier orbitals enhance biomolecular interactions. Thermal analysis (Fe residue: 5.952% calc/5.606%; Ni: 6.384% calc/5.987%) and molecular docking (FeRD: –8.70 kcal mol⁻¹) further confirm structural integrity and target affinity. The obtained results indicate that metal coordination enhances the electronic properties and biological activity of the RD ligand, suggesting that metal complexation is an effective strategy for improving functional performance.

Supplemental Material

sj-doc-1-chl-10.1177_17475198261437358 – Supplemental material for Octahedral Fe(III) and Ni(II) complexes of a bidentate 3-(3,4-dichlorophenyl)-1-(4-(2-hydroxynaphthalen-1-yl)diazenyl)phenyl)prop-2-en-1-one azo-chalcone: Spectroscopic, electronic, and biological insights

Supplemental material, sj-doc-1-chl-10.1177_17475198261437358 for Octahedral Fe(III) and Ni(II) complexes of a bidentate 3-(3,4-dichlorophenyl)-1-(4-(2-hydroxynaphthalen-1-yl)diazenyl)phenyl)prop-2-en-1-one azo-chalcone: Spectroscopic, electronic, and biological insights by Faleh Zafer Alqahtany, Raafat A El-Eisawy, Antar A Abdelhamid, Majidah Alsaeedi, Anas Alfarsi, Manal M Alzahrani, Saeed S Samman, Mansour Alsarrani, Abdullah Ahmed A Alghamdi, Sarah A Alghamdi, Othman A Farghaly and Aly Abdou in Journal of Chemical Research

Footnotes

ORCID iD

Raafat A El-Eisawy

Ethical considerations

Not applicable.

Consent to participate

Not applicable or applicable.

Consent for publication

Not applicable.

Author contributions

Faleh Zafer Alqahtany: Conceptualization, Methodology, Investigation, Writing – Reviewing and Editing. Raafat A El-Eisawy: Conceptualization, Methodology, Investigation, Writing – Reviewing and Editing. Antar A Abdelhamid: Conceptualization, Methodology, Investigation, Writing – Reviewing and Editing. Majidah Alsaeedi: Conceptualization, Methodology, Investigation, Writing – Reviewing and Editing. Anas Alfarsi: Conceptualization, Methodology, Investigation, Writing – Reviewing and Editing. Manal M Alzahrani: Conceptualization, Methodology, Investigation, Writing – Reviewing and Editing. Saeed S Samman: Conceptualization, Methodology, Investigation, Writing – Reviewing and Editing. Mansour Alsarrani: Conceptualization, Methodology, Investigation, Writing – Reviewing and Editing. Abdullah Ahmed A Alghamdi: Conceptualization, Methodology, Investigation, Writing – Reviewing and Editing. Sarah A Alghamdi: Conceptualization, Methodology, Investigation, Writing – Reviewing and Editing. Othman A Farghaly: Conceptualization, Methodology, Investigation, Writing – Reviewing and Editing. Aly Abdou: Conceptualization, Methodology, Investigation, Writing – Reviewing and Editing.

Funding

The authors received no financial support for the research, authorship, and/or publication of this article.

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Data availability statement

The corresponding author is willing to answer any questions you may have about further details and all data are available under order.

Supplemental material

Supplemental material for this article is available online.

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