Spectroscopic and TDDFT studies of N-phenyl-N′-(3-triazolyl)thiourea) compounds with transition metal ions

2.1 General procedures

Phenyl isothiocyanate, 2-amino-1,3,4-triazole, pyridine, metal perchlorates (Co(ClO₄)₂·6H₂O, Ni(ClO₄)₂·6H₂O, and Cu(ClO₄)₂·6H₂O) and organic solvents were purchased from the commercial sources and used as received. The thiourea ligand was prepared by following the previously reported method. (Mansour, 2018) Potassium bromide pellets of the ligand and complexes were subjected to IR analysis using Jasco FTIR 460 plus. Micro-elemental analyses were determined on an Automatic Analyzer CHNS, Vario EL III Elementar. Electronic absorption spectra were recorded on OPTIZEN POP automate spectrophotometer. The magnetic susceptibility values of the paramagnetic complexes were determined using Sherwood scientific magnetic balance using Gouy method. (Hilal and Fredericks, 1954) The molar conductivity values were obtained by digital Jenway 4310 (cell constant is 1.02). X-band EPR measurements were executed on solid samples at 298 K using a Bruker EMX spectrometer. The magnetic modulation frequency was 100 kHz, and the microwave power was set to 0.201 mW. The g-values were recorded by referencing to a diphenylpicrylhyrdazyl (DPPH) sample with g = 2.0036. The modulation amplitude was fitted at 4 Gauss while the microwave frequency was verified as 9.775 GHz. Thermogravimetric analysis (TGA) was executed under dinitrogen atmosphere (20 mL min⁻¹) in platinum crucible with a heating rate of 10 °C min⁻¹ using a Shimadzu DTG-60H simultaneous DTG/TG apparatus.

2.2 Synthetic procedures

To a round flask charged with H₃L ligand (2 mmol, 436 mg), and acetone (20 mL), 5 mL aqueous solution of metal perchlorate (1 mmol; Co(ClO₄)₂·6H₂O (365 mg), Ni(ClO₄)₂·6H₂O (366 mg) and Cu(ClO₄)₂·6H₂O (370 mg) was added and then the pH of the reaction mixtures was adjusted at around 8.0 by adding few drops of ammonia. Then the solutions were heated to reflux for 4 h, whereupon precipitation occurred. The solid products were collected by filtration, washed with diethyl ether, and dried under vacuum. Stock solutions (10⁻³ M) of complexes 1–3 (Scheme 1) in DMF were prepared and their molar conductance values were recorded and compared to the reported data for 1:1 (65–90 Ω⁻¹ cm² mol⁻¹) and 1:2 (130–170 Ω⁻¹ cm²mol⁻¹) electrolytes in the same solvent at the same temperature. (Mansour, 2014) The conductance values of 1–3 were found to be in the range of 13.8–41.2 Ω⁻¹ cm²mol⁻¹ reflecting the non-ionic nature of the complexes and rules out the probability of presence of the perchlorate ion(s) in the secondary coordination sphere.

• [Co(H₂L)₂]·0.5H₂O (1): Brown, yield 75% (759 mg, 1.5 mmol). IR (FTR, cm⁻¹): ν = 3399 (br, NH), 3211 (br, NH), 3024 (w, CH), 1563 (w, –S–C = N–^thiol/CC), 1520 (w, CC), 1487, 1397, 1310 (m, $C\ddot{-}N$ /C $\ddot{-}$ S/C–H^β), 1257, 1173, 1083 (br, C–N/C $\ddot{-}$ S), 754 (m, C $\ddot{-}$ S). C₁₈H₁₆CoN₁₀S₂·0.5H₂O: C 42.86, H 3.40, N 27.77, found C 43.22, H 3.44, N 27.92. UV/Vis (DMF, 10⁻⁴ M, nm): 260, 280, 570. ESI-MS (positive mode, acetone): 496.0558 {M + 1}⁺ (M: molecular mass). Effective magnetic moment, μ_eff (μ_B, 298 K): 2.79. Molar conductance (DMF, 10⁻³ M, Ω⁻¹cm² mol⁻¹): 41.2.

• [Ni(H₂L)₂] (2): Buff, yield 79% (788 mg, 1.57 mmol). IR (FTR, cm⁻¹): ν = 3386 (br, NH), 3228 (br, NH), 3030 (w, CH), 1594 (m, –S–C = N–^thiol), 1530 (w, CC), 1494, 1396, 1311 (m, $C\ddot{-}N$ /C $\ddot{-}$ S/C–H^β), 1209, 1086 (br, C–N/C $\ddot{-}$ S), 747 (m, C $\ddot{-}$ S). C₁₈H₁₆NiN₁₀S₂: C 43.66, H 3.26, N 28.28, found C 43.22, H 3.28, N 27.92. UV/Vis (DMF, 10⁻⁴ M, nm): 255, 295, 370, 445. ESI-MS (positive mode, acetone): 494.0935 {M + 1}⁺. μ_eff (μ_B, 298 K): 0.78. Molar conductance (DMF, 10⁻³ M, Ω⁻¹cm²mol⁻¹): 13.8.

• [Cu(H₂L)₂]·2H₂O (3): Dark-brown, 76% (812 mg, 1.51 mmol). IR (FTR, cm⁻¹): ν = 3399 (br, NH), 3299 (br, NH), 3026 (w, CH), 1572 (w, –S–C = N–^thiol/CC), 1493, 1361, 1313 (m, $C\ddot{-}N$ /C $\ddot{-}$ S/C–H^β), 1259, 1096 (br, C–N/C $\ddot{-}$ S), 743 (m, C $\ddot{-}$ S). C₁₈H₁₆CuN₁₀S₂·2H₂O: C 40.33, H 3.76, N 26.13, found C 40.32, H 3.44, N 25.85. UV/Vis (DMF, 10⁻⁴ M, nm): 300, 335, 390, 405, 470, 620. ESI-MS (positive mode, acetone): 501.1330 {M + 1}⁺. μ_eff (μ_B, 298 K): 1.44. Molar conductance (DMF, 10⁻³ M, Ω⁻¹ cm²mol⁻¹): 20.3.

2.3 DFT/NBO/TDDFT calculations

Full geometry optimization of the proposed four-coordinated conformations of 1–3, in the ground state, and their vibrational analyses were carried out by Becke 3-parameter (exchange) Lee–Yang–Parr (B3LYP) functional, (Becke, 1993) and def2-SVP basis set using Gaussian03 package (Frisch et al., 2003). For time dependent density functional theory (TDDFT) calculations, the 30 singlet excited states were considered and calculated at CAM-B3LYP (with long range correction) (Yanai et al., 2004)/def2-SVP level of theory. Dimethyl sulfoxide was incorporated in the TDDFT calculations using the polarized continuum model as implemented in Gaussian03. Natural population analyses were executed using NBO method at B3LYP/def2-SVP level of theory. Local minimum structures, electronic spectra, and frontier molecular orbitals were visualized by Gaussview03 (Frisch et al., 2000).

2.4 Antibacterial activity

The antibacterial activities of the functionalized thiourea ligand and its coordination compounds against cultures of Staphylococcus aureus and Escherichia coli microbes were assessed using a modified Kirby-Bauer disc diffusion method (Abdel-Ghani and Mansour, 2012) at the concentration of 20 mg mL⁻¹, while keeping the final DMSO concentration to a maximum of 0.5% and serially diluted 1:2 fold for 8 times. The toxicity of the compounds was compared to the standard tetracycline. In this assay, the centrifuged pellets of bacteria from a 24 h old culture comprising of approximately 10⁴–10⁶ CFU mL⁻¹ (CFU: colony forming unit) were distributed on the surface of Mueller-Hinton Agar plates (evaluated for composition and pH). Then, the wells were contaminated with 10 mL of prepared inocula to get 10⁶ CFU mL⁻¹. Petri plates were made by flowing 100 mL of seeded nutrient agar. Plates were covered and incubated at 37 °C for 18 h. DMSO (0.1 mL) was used as control under the same conditions for each microorganism, subtracting the diameter of the inhibition zone obtained with the solvent from that observed in each case. The antibacterial activities were determined as a mean of three replicates.

3.1 Synthesis and spectral characterization

The transition metal complexes 1–3 (Scheme 1) were prepared by heating to reflux the slightly alkaline reaction mixtures of the metal perchlorates and H₃L, in 75% (v/v) acetone–water mixture, where complexes [M(H₂L)₂] (M = Co(II) (1), Ni(II) (2) and Cu(II) (3)) were collected by filtration. Their proposed structures were deduced from elemental analysis, TGA, UV/Vis, IR, ESR, magnetic and conductivity measurements. The vibrational spectrum of the free ligand (Fig. S1) shows two broad bands at 3381, and 3207 cm⁻¹ assigned to the ν(NH)^Ph and ν(NH)^{triazole−exo} modes as well as a group of bands in the range of 1570–700 cm⁻¹ having a contribution from –NH–C = S group as follows: 1540 (β(N–H)/β(C = C)) I), 1385 (ν( $\mathit{C}\ddot{-}\mathit{N}$ )/ν(C = S)/β(C–H)) II), 1018 (ν(C–N)/ν(C–S)) III), and 837 cm⁻¹ (δ(C = S) IV) (Mansour, 2016; Mansour, 2018). The ν(NH)^{triazole−endo} could not be identified owing to the tautomeric structure of the triazole ring. The absence of ν(S–H) mode at around 2570 cm⁻¹ in the IR spectrum of the free ligand as well as the presence of a signal at δ 177.0 ppm in the ¹³C NMR spectrum confirm the existence of H₃L in the thione form in both the solution and solid states. Coordination of H₃L to the metal ion induced considerable spectral changes in the vibrational ranges of 1630–1300, and 900–700 cm⁻¹. A new band assigned to ν(S–C = N-^thiol) at 1594 cm⁻¹ in the spectrum of 2 or allocated for ν(S–C = N-^thiol)/ν(CC) at 1563 and 1572 cm⁻¹ in the spectra of 1, and 3 is observed, which may be attributed to enolization of –NH–C = S group into –N = C–SH, ionization of SH group, and coordination to metal ion (Fig. S1). The shift of ν(NH)^Ph mode to higher wave numbers (3386–3399 cm⁻¹) rules out the role of the phenyl-NH group in the metal complex formation. The participation of the triazole nitrogen in the chelation inhibits the tautomeric behaviour within the moiety and leads to grown of a new band in the range of 3299–3211 cm⁻¹ assigned to the ν(NH)^{triazole−endo} mode. IR studies indicated that the H₃L coordinates to Mⁿ⁺ via C–S⁻, and triazole nitrogen.

The electronic absorption spectrum of the free ligand, in DMF, (Fig. S2) displays two electronic transitions at 271, and 310 nm (Mansour, 2018). In the highest energy region of the electronic spectrum of 1, two prominent bands corresponding to the intra-ligand transitions are observed at 260 and 280 nm. In addition, a broad shoulder is seen at around 570 nm in the spectrum of 1 (Fig. S2), which may be assigned to ⁴A₂→⁴T₁(P) (ν₃) in tetrahedral Co(II) geometry (Kowalkowska-Zedler et al., 2020; Vašák et al., 1981). The effective magnetic moment of 1 is 2.79 μ_B. Generally, octahedral Co(II) complexes exhibit μ_eff values around 4.7 μ_B owing to the large orbital contribution, while the square-planar complexes show values around 1.7 μ_B corresponding to presence of an unpaired electron. Tetrahedral Co(II) complexes have μ_eff values usually equivalent to the population of three unpaired electrons that may be affected by orbital contribution. The observed μ_eff of 1 further complements the electronic findings of the distorted tetrahedral field. The μ_eff value of 2 is 0.78 μ_B suggesting a highly probability of presence of a mixture of square-planar/tetrahedral (Al-Awadi et al., 2006; Kandil et al., 2002) or existence of Ni(II) ion in a flattened tetrahedral field. By assuming the presence of a mixture of the two four-coordinated geometries, the tetrahedral percentage (N_t) in the proposed mixture is 5.6% according to the following relation, N_t = [(μ_eff)²/(3.3)²] × 100, where μ_eff value for ideal tetrahedral Ni(II) complexes is 3.3 μ_B. The electronic spectrum of 2 shows four transitions at 255, 295, 370, and 445 nm (Fig. S2). The two highest energy bands at 255, and 295 nm are assigned to π–π* transitions within the ligand framework. The shoulder at 370 nm is allocated for LMCT (S(σ) → Ni(II)) and the broad band at 445 nm is assigned to a combination of LMCT (N(σ) → Ni(II))/ ${\mathit{d}}_{{\mathit{z}}^2}\to {\mathit{d}}_{{\mathit{x}}^2-{\mathit{y}}^2}$ (¹A_1g→ ¹B_1g) transitions (Royer et al., 1972). For Cu(II) complex 3, the μ_eff (1.44 μ_B) is slightly lower than the expected value for the regular spin only value of d⁹ metal ion revealing the probability of presence of weak antiferromagnetic interactions between the neighbouring species (Mansour, 2013). The electronic absorption spectrum of 3 (Fig. S2), in DMF, is characterized by four transitions in the visible region at 390, 405, 470, and 620 nm. Generally, square-planar Cu(II) coordination compounds are characterized by a broad band at around 650 nm corresponding to the unresolved d-d transitions, while tetrahedral complexes have no electronic transitions in the 500–1000 nm region (Sreekanth and Prathapachandra-Kurup, 2003). The transitions at 405, 470, and 620 nm may be assigned to LMCT (S(σ) → Cu(II))) (Dhara et al., 2004), LMCT(N(π) → Cu(II)) (Sreekanth and Kurup, 2003) and ${\mathit{d}}_{\mathit{x}\mathit{z}}/{\mathit{d}}_{\mathit{y}\mathit{z}}/{\mathit{d}}_{{\mathit{z}}^2}\to {\mathit{d}}_{{\mathit{x}}^2-{\mathit{y}}^2}$ (²B_1g → ²E_1g and ²B_1g → ²A_1g (Lever, 1984; Garg et al., 1993) within the square-planar field. Therefore, in the solution state, Cu(II) ion of 3 may be surrounded by four ligands, which are arranged in a planar form.

The thermal decompositions of complexes (1–3) were examined in the temperature range from the ambient up to 1000 °C, under pure dinitrogen atmosphere, using a heating rate of 10 °C/min (Fig. S3). For 1, the decomposition starts at about 50 °C and proceeds through three complicated thermal steps maximally at 79, 283, and 489 °C. The first step is accompanied by a mass loss of 1.67% (calcd. 1.79%) corresponding to desorption of 0·.5H₂O. The thermal steps bring the total mass loss up to 65.45% (calcd. 66.20%) leaving carbonized CoS₂ (Mansour, 2014). In the case of 2, no thermal decomposition is observed below 130 °C revealing the absence of any volatile solvents and/or water of hydration. The thermogravimetric analysis of 2 shows four stages maximally at 242, 416, 488, and 782 °C. For 3, the mass loss has been taken in four thermal steps maximally at 77, 189, 238, and 484 °C. The first thermal stage up to 110 °C, with a mass loss of 6.52% (calcd. 6.71%), may be attributed to desorption of two hydrated water molecules. It seems that compounds 2 and 3 are hardly to be decomposed in the temperature range up to 1000 °C under an inert atmosphere with a total mass loss of 54.61% and 58.22%, respectively, as no plateau is reached and no stable Ni(II) or Cu(II) residue may be proposed to be existed at 1000 °C. This behaviour has been reported with similar complexes under the same experimental conditions (Mansour, 2016; Faraglia et al., 1990).

The solid-state X-band EPR spectrum of Cu(II) complex 3 (Fig. 1), at the room temperature is axial with two well defined ${\mathbf{g}}_{\Vert }=$ 2.30 ( ${\mathbf{A}}_{\Vert }$  = 129 × 10⁻⁴ cm⁻¹) and ${\mathbf{g}}_{\perp }=$ 2.05 values and ${\mathbf{g}}_{\Vert }>$ ${\mathbf{g}}_{\perp }>$ ${\mathbf{g}}_{\mathbf{e}}$ (2.0023) indicating that the unpaired electron may be present in the ${\mathit{d}}_{{\mathit{x}}^2-{\mathit{y}}^2}$ orbital. The degree of the distortion from the tetrahedral geometry can be deduced from the f value ( ${\mathbf{g}}_{\Vert }/{\mathbf{A}}_{\Vert }$ ), where f = 105–135 cm is recognized for the idea square-planar geometry and f > 135 cm indicates the presence of a distorted tetrahedral geometry (Sundaravel et al., 2009). It is worthy mention that the f value of 3 is 178 cm suggesting presence of Cu(II) ion in the tetrahedron field in the solid state, and this has been further supported by quantum chemical calculations (vide infra).

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Fig. 1

EPR spectrum of Cu(II) complex, 3.

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The spectroscopic, magnetic, and analytical data of the complexes, prepared from the reaction of H₃L, and metal perchlorate salts in a slightly basic media, indicated the formation [M(H₂L)₂] compounds. Searching in the literature showed that the reaction between H₃L and metal chloride salts afforded compounds of the type [M(H₃L)_nCl₂] (M = Co(II), Ni(II), and Cu(II); n = 1 and 2). (Salman, 2001) In comparison, changing of the experimental conditions e.g. pH of the medium, and/or using of metal salts of weakly coordinating anions e.g. ClO₄⁻ affect the coordinating ability of our tribasic ligand, and increase the probability of the departure of the anions of the metal salts from the coordination spheres of the complexes compared to the published complexes, which were prepared from the same ligand, but with different metal salts and experimental conditions.

3.2 DFT/NBO analysis

In the previous section, the analytical and spectroscopic data indicated that compounds 1–3 are present in a ''flattened'' tetrahedron field, in which the thiourea ligand coordinates to the title metal ions, as a mono-negatively bidentate ligand, via C–S⁻, and triazole nitrogen. However, there is still two questions that have not been experimentally answered as follows: which nitrogen of the triazole ring is involvement in the complex formation (N(1) vs. N(3))? and which geometrical isomer (cis- vs. trans-) is the local minimum structure of the complex?. To address these two questions, full geometry optimizations of the proposed structures of 1–3, in the ground state, were executed at B3LYP/def2-SVP level of theory. As shown in Table 1, conformations I, and II have been assigned to the trans- and cis-(H₂L, H₂L) arrangements of N(3)-coordinated ligands, while conformation III has been designated to the structure in which N(1) atom of the triazole is involved in the complex formation. The resulting geometries were checked as local minima via the harmonic frequency analysis (Table 2). The conformations I and III of complex 1 show imaginary frequencies and consequently they cannot be classified as local minima structures.

Table 1 The optimized proposed structures of 1–3, obtained at B3LYP/def2-svp level of theory (Conformations I–III are shown from up to down).

1	2	3

Conformation II has no imaginary vibrations as well as lower sum of electronic and zero-point energy values and consequently it may be characterized as a local minimum structure of the synthesized Co(II) complex. In the case of the Ni(II) complex 2, we did not find any imaginary modes in the three optimized structures and thus the sum of the electronic and zero-point energy values was compared. As shown in Table 2, conformation I, incorporating two N(3)-coordinated ligands in a trans-arrangement, is the local minimum structure of the nickel complex. For 3, only conformations II and III show no imaginary frequencies. Conformation III of the Cu(II) complex is characterized as a local minimum structure because it has a lower energy value among the three optimized structures. To conclude, the complexes adapt three different conformations (Fig. 2) based on the data of the quantum chemical calculations.

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Fig. 2

Structures of the local minima structures of 1–3 based on the data of geometry optimization and vibrational analyses.

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Selected bond lengths and bond angles of all the optimized conformations of 1–3 are given in Table 3. Compounds 1–3 contain metal ion in a distorted tetrahedral geometry, in which the two thiourea ligands bound M²⁺ iso-energetically as the bonds (M–N and M–S) are equivalent. To get an insight into the distortion of the tetrahedral geometry, the four coordinated geometry index, τ₄ (Yang et al., 2007), calculated as follows, (τ₄ = (360–(α + β))/141) (where α and β are the two largest bond angles around the central metal ion), can be applied to evaluate the distortion between the ideal four coordinated geometries; tetrahedral (τ₄ = 1) and square-planar (τ₄ = 0). As shown in Table 3, the τ₄ values are in the range of 0.13–0.45. The local minimum structures of complexes 1–3, based on the previously mentioned vibrational analysis and ground-state energy comparison, have τ₄ values of 0.35, 0.28, and 0.45 indicating that the geometries of the complexes may be considered as an intermediate between the square planar and tetrahedral. In the case of conformation I of compound 2 (Fig. 2), the triazole NH group participates in intramolecular hydrogen bond with C–S–M via H–N…S with distance of 2.30922 Å, and angle of 121.1 °.

The calculated vibrational spectra of the stable conformations of 1–3 are shown in (Fig. S4). In general, the calculated wavenumbers are expected to be higher than the experimental finding because of the basis set truncation and the neglection of the electron correlation and mechanical anharmonicity especially in the range of the bending modes. (Rauhut and Pulay, 1995) To compensate these limitations, scaling methods should be introduced. Uniform scaling (0.96) was introduced for B3LYP/def2-SVP method. The scaled IR modes of NH, C = N, M–S and M–N bonds are found in the ranges of 3424–3427, 1560–1581, 356–361, and 331–342 cm⁻¹.

The natural charge, nature of bonding, type of hybridization and strength of the bonds (M–N and M–S) of the local minimum structures of 1–3 were deduced from the natural bond orbital (NBO) analysis of Weinhold and co-workers (Reed et al., 1988) and second order perturbation theory analysis of Fock Matrix in NBO Basis obtained at B3LYP/def2-SVP level of theory. The electronic arrangement of the metal center in cobalt(II) complex 1 is [Ar]4s^0.393d^7.584p^0.565 s^0.01, with a natural charge of 0.46222e. The occupancies of the 3d-orbitals are as follows: $d_{\mathit{xy}}$ ^0.98828 $d_{\mathit{xz}}$ ^1.16688 $d_{\mathit{yz}}$ ^1.64564 $d_{x^2-y^2}$ ^1.85639 $d_{z^2}$ ^1.92636. The σ(Co–S) bond is formed from sp^6.41d^0.01 hybrid on S atom (a mixture of 13.48% s, 86.40% p and 0.12% d) and sp^3.25d^2.29 hybrid on Co (a mixture of 32.58% s, 25.13% p and 42.29% d) and is polarized towards S atom (74.79%). Triazole nitrogen bounds metal center via a lone pair interaction. Both the σ(Co–S16) → σ*(Co–S24) and σ(Co–S24) → σ*(Co–S16) interactions are equal with E² value of 1.32 kcal mol⁻¹ (E²: second order interaction energy). The triazole interactions are also equal and the lone pair interaction LP(1)N19 with σ*(Co–S24) has E² value of 1.73 kcal mol⁻¹. The natural charge of Ni in 2 is 0.44184e. The electronic configuration of Ni(II) ion in 2 is [Ar]4s^0.413d^8.634p^0.515 s^0.01 and the occupancies of 3d orbitals are $d_{\mathit{xy}}$ ^1.95706 $d_{\mathit{xz}}$ ^1.97954 $d_{\mathit{yz}}$ ^1.95604 $d_{x^2-y^2}$ ^0.82553 $d_{z^2}$ ^1.91507. The σ(Ni–S) bond is created from sp^7.50d^0.01 hybrid on S atom (comprised of 11.76% s, 88.13% p and 0.11% d) and sp^4.05d^3.06 hybrid on Ni (a mixture of 12.33% s, 49.90% p and 37.76% d) and is polarized towards S atom (77.51%). The σ(Ni–S) bond is more polarized than σ(Co–S) bond because of the difference in the electronegativity between the two metal ions. The E² values of σ(Ni–S16) → σ*(Ni–S24) and LP(1)N19 → RY*(1)Ni are 3.81 and 0.36 kcalmol⁻¹, respectively. Compared to 1, the larger the E² values of 2, the more intensive is the interaction between Ni(II) ion and coordination sites.

For 3, the electronic arrangement of Cu is [Ar]4s^0.423d^9.464p^0.49, the valence electrons are 10.37780 and Rydberg electrons are 0.00506. This agrees well with the natural charge on Cu ion (0.62021e), which corresponds to the difference between 28.37979e and the total number of electrons in the free copper(II) ion. The occupancies of Cu-3d subshells are $d_{\mathit{xy}}$ ^1.62771 $d_{\mathit{xz}}$ ^1.97958 $d_{\mathit{yz}}$ ^1.89038 $d_{x^2-y^2}$ ^1.98588 $d_{z^2}$ ^1.98142. The interactions of the nitrogen and suphur coordination sites with Cu(II) ion may be considered as charge transfer. The E² values of LP(1)(S16) → RY*(1)Cu and LP(1)N18 → RY*(1)Cu are 0.09 and 0.23 kcalmol⁻¹, respectively.

3.3 TDDFT

The nature of the electronic absorption transitions shown in the experimental UV/Vis spectra of the local minimum structures of 1–3 has been investigated by executing time-dependent density functional theory (TDDFT) calculations. The lowest 30 singlet–singlet spin allowed excitation states were computed using CAM-B3LYP functional with long range interaction and def2-SVP basis set. The computed wavelengths including d-d transitions and their assignments are given in Table S1. In 250–800 nm range, the calculated spectrum of 1 shows two characteristic transitions at 735, and 486 nm corresponding to HOMO–4(β)/ HOMO–5(β) → LUMO(β) and HOMO–4(α) → LUMO + 2(α), respectively. As shown in Fig. 3, the bands at 486 and 735 nm have the nature of d–d transitions.

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Fig. 3

Computed electronic spectrum of 1 at CAM-B3LYP/def2-SVP and frontier molecular orbitals of some selected transitions.

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The TDDFT spectrum of 2 displays two main transitions at 387 and 306 nm corresponding to HOMO → LUMO and HOMO–6 → LUMO, respectively. As shown in Fig. 4, both HOMO and HOMO–6 orbitals are contained upon d(Ni), while LUMO is composed of π* of phenyl ring. Thus, the transitions observed in the calculated spectrum of 2 are MLCT. The calculated spectrum of 3 shows three main transitions at 398, 317, and 269 nm corresponding to HOMO(α) → LUMO + 1(α)/HOMO(β) → LUMO + 2(β), HOMO–4(β) → LUMO(β) and HOMO(β)/ HOMO–2(β) → LUMO(β). HOMO and HOMO–2 orbitals with β-spin are mainly S(π)/ligand π-orbitals (Fig. 5), whereas LUMO(β) orbital is coming from d(Cu)/S(π*)/Triazole N(π*). Thus, the transition at 398 nm is LMCT/ LLCT. As shown in Fig. 5, the transition at 317 nm has a nature of d-d/MLCT.

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Fig. 4

Computed electronic spectrum of 2 at CAM-B3LYP/def2-SVP and frontier molecular orbitals of some selected transitions.

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Fig. 5

Computed electronic spectrum of 3 at CAM-B3LYP/def2-SVP and frontier molecular orbitals of some selected transitions.

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3.4 Antibacterial activity

Recently, screening efforts have showed an unexpectedly high hit rate for coordination and organometallic compounds (9.9%) compared with the purely organic compounds (0.87%) submitted to the same assays. (Frei et al., 2020) Minimum inhibitory concentrations (MICs) of the tested transition metal-based compounds (Mⁿ⁺ = Mn, Co, Zn, Ru, Ag, Ir, and Pt) against both Gram-positive and Gram-negative bacteria were observed, with activities down to the nanomolar range against methicillin resistant S. aureus (MRSA). The susceptibility of Staphylococcus aureus, a gram-positive bacterium, and Escherichia coli, a gram-negative microbe, towards the free ligand and its transition metal complexes was assessed at 20 mgmL⁻¹. The potency of the compounds was compared with the standard tetracycline. The free thiourea functionalized ligand exhibits similar moderate toxicity to the tested bacteria. The possible mechanism of action of thiourea compounds functionalized with triazole moiety may be attributed to the inhibition of bacterial biofilm formation. (Stefańska et al., 2016) Coordination of the ligand to either Co(II) or Cu(II) did not alter the potency, while chelation of Ni(II) ion gives rise to inactive compound (Fig. 6). The inactivity of 2 compares well with those of Pd(II) and Pt(II) complexes of the same ligand. (Mansour, 2018) The difference between the activities of the synthesized compounds may be attributed to the variation of the lipophilicity values, where the diffusion of the compound is low and consequently, they cannot inhibit the growth of the microbe. (Anacona et al., 2019)

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Fig. 6

Representative and comparison of the susceptibilities of S. aureus and E. coli bacteria towards the thiourea ligand and its complexes. DMSO was used as a control and its inhibition zone was withdrawing from that of each case.

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