Field-induced slow magnetic relaxation, molecular docking and antibacterial studies of quasi-isotropic copper(II) (S = ½) systems stabilised by tetradentate (ONNO) and tridentate (NNO)-donor ligands

Abstract

A series of penta-coordinate Cu(II) complexes were synthesised and structurally characterised to explore the relationship between coordinating environment, molecular magnetism, and antibacterial activities. A dinuclear complex, [Cu₂(L₁)₂] (1), derived from an ONNO-coordinated tetradentate ligand (H₂L₁ = N,N′-bis[(3-methoxy-2-hydroxybenzylidene)]ethane-1,2-diamine), and three paramagnetic Cu(II) complexes, [Cu₂(N₃)₂(L₂)₂] (2), [Cu(SCN)(L₂)]_n (3), and [Cu(CH₃COO)(L₂)]_n (4), stabilised by a tridentate NNO-donor ligand (HL₂ = (E)-1-(pyridin-2-yldiazenyl)naphthalen-2-ol), were isolated. Dinuclear complex 2 features an asymmetric end-on μ₁,₁-azido bridge, whereas 3 and 4 exhibit end-to-end μ₁,₃-thiocyanate/acetate bridges, forming one-dimensional polymeric architectures. Single-crystal X-ray diffraction confirmed their square-pyramidal molecular geometries. Complexes 1 and 4 exhibit field-induced single-molecule magnet (SMM) behaviour, consistent with quasi-isotropic S = ½ Cu(II) centres. All complexes show χ_MT ≈ 0.4 cm³ mol⁻¹ K⁻¹ with a slight decrease below 10 K. EPR parameters support the existence of mixed orbital character d_z2/d_x2−y2 (g_x = 2.23, g_y = 2.06, g_z = 1.90) (1) and d_x2−y2 (g_∥ = 2.21, g_⊥ = 2.06) (4) in the ground states. Molecular docking analyses demonstrated complex 3 has strong binding affinities against four biologically relevant targets: B-DNA (PDB ID: 1BNA), human DNA topoisomerase I (hTOPI, PDB ID: 1SC7), Escherichia coli MenB enzyme (EC-MenB, PDB ID: 3T88), and human serum albumin (HSA, PDB ID: 4LA0), indicating its potential for target-specific activity.

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Introduction

Molecular magnetism and the exploration of intrinsic biological activity in copper(II) complexes is a fascinating and rapidly evolving field in contemporary coordination chemistry. Coordination complexes featuring bistable ground states with slow relaxation of magnetisation, characteristic of single-ion or single-molecule magnet (SIM or SMM) behaviour,¹ have attracted considerable attention from the reseachers owing to their promising implications in high-density data storage,² spintronics,³ quantum computing,⁴ and quantum sensing.⁵ Over the past few decades, molecular bistability arising from substantial magnetic anisotropy and high-spin ground states⁶ has been extensively explored, particularly in lanthanide-based systems⁷ and a few unique examples of 3d-metal ion complexes.⁸ However, recent research interest has increasingly focused on systems that display delayed relaxation of magnetisation arising from diverse mechanisms, typically observed under an externally applied magnetic field. These kinds of systems are usually generated from quasi-isotropic slow-relaxing molecules, which typically are related to ions with half-filled shells⁹ and an S = ½ ground state, such as VO(II),¹⁰ Cu(II),¹¹ Ag(II),¹² and, in specific cases, Fe(III).¹³ A common feature of these molecular systems is that their slow magnetic relaxation cannot be explained by an over-barrier mechanism, because of the absence of large magnetic anisotropy barriers and the lack of accessible excited magnetic sublevels beyond ±m = ½. As a result, in such a system, field-induced slow magnetic relaxation arises from alternative relaxation pathways, primarily ‘quantum tunnelling’ or ‘spin-phonon coupling’ processes.¹⁴

The focus of both experimental and theoretical investigations on single-ion magnets (SIMs) has gradually accelerated towards transition-metal systems since 2010, when Long and his team reported an unprecedented mononuclear Fe(II) complex exhibiting slow magnetic relaxation.¹⁵ Among the 3d metal ions, Cu(II) complexes (d⁹, S = ½) are considered magnetically isotropic in zero field splitting (ZFS). Following the pioneering work by Boča et al.,^11a,16 several mononuclear Cu(II) complexes that exhibit field-induced delayed magnetic relaxation under an applied DC field have been reported. More recently, quasi-one-dimensional polymeric Cu(II) systems¹⁷ and polyoxometalate-supported Cu(II) complexes¹⁸ showing spin–lattice relaxation have further broadened the scope of SIM chemistry by incorporating quasi-isotropic S = ½ spin systems. However, dimeric or polymeric Cu(II) complexes incorporating tetradentate Schiff base (ONNO) and non-Schiff base (NNO) ligand frameworks rarely exhibit single-molecule magnet (SMM) behaviour.

Cu(II) complexes are also notable for their rich biological activity, which originates from their geometric flexibility and redox behaviour. Cu(II) complexes stabilised by tetradentate (ONNO) and tridentate (NNO) donor ligands have promising potential to show strong antibacterial, antioxidant, and cytotoxic properties. The coordination of these types of ligands to the Cu(II) centre increases the overall lipophilicity and thermodynamic stability, enhancing cellular permeability and interaction with biomolecular targets like DNA and proteins. The redox-active Cu(II)/Cu(I) pair promotes the formation of reactive oxygen species (ROS), which causes oxidative stress in microbial and cancer cells and contributes to their biological efficacy. Salen-type (ONNO) and tridentate NNO donor Cu(II)-complexes have high DNA/protein binding and cleavage ability, as well as SOD-mimetic activity, indicating their multifunctionality.^19,20

In this context, to elucidate the underlying magnetic behaviour and bioactivity, numerous dinuclear/polymeric copper(II) complexes with two distinct ligand environments were synthesised. The coordination modes of the ligand H₂L₁ are depicted in Scheme 1(a). Here, the tetradentate ligand (ONNO donor) (H₂L₁ = N,N′-bis(3methoxysalicylidene) ethylenediamine), in its dianionic state(L₁²⁻), was employed to obtain the dimeric complex [Cu₂(L₁)₂] (1). Additionally, asymmetric end-on double μ_1,1-azido dinuclear [Cu₂(N₃)₂(L₂)₂] (2) and μ_1,3-thiocyanate/μ₂-acetate-linked polymeric [Cu(SCN)(L₂)]_n (3) [Cu(CH₃COO)(L₂)]_n (4) complexes were synthesised. Scheme 1(b) illustrates potential coordinating frameworks and the accessible electronic states of the ‘redox-active’²¹ tridentate motif of HL₂, commonly found in coordination complexes. Here, the (NNO donor) tridentate HL₂ (HL₂ = (E)-1-(pyridin-2-yldiazenyl)naphthalen-2-ol) ligand uses its monoanionic state(L₂¹⁻),²¹ to stabilise complexes 2, 3 and 4. The binding mode of the bridging ligand employed in complexes 2–4 is shown in Scheme 2.

Scheme 1 (a) Accessible coordination modes of the ligand (H₂L₁). (b) Possible coordination modes and electronic states of the tridentate motif of the ligand (HL₂).

Scheme 2 (a) Asymmetric end-on double μ_1,1-azido binding mode in 2, (b) μ_1,3-thiocyanate binding mode in 3 and (c) μ₂-syn–syn acetate binding mode observed in 4.

Complexes 1 and 4 exhibit field-induced slow magnetic relaxation, while complexes 2–4 demonstrate potential target-specific biological activity through molecular docking analyses.

Understanding the synthesis, characterisation and structural-magnatic correlations of such systems, along with their potential bioactivity, is crucial for effectively creating multifunctional magnetic and bioactive materials.

Experimental

Materials and physical measurements

All necessary chemicals, spectroscopic-grade solvents and 1-(pyridin-2-yldiazenyl) naphthalen-2-ol used as ligand HL₂ were purchased from the commercial suppliers and employed without further purification. The C, H, and N contents of the complexes were obtained from a Perkins-Elmer 2400 Series II elemental analyser. Infrared spectra (4000–400 cm⁻¹) were recorded from KBr pellets on a Bruker IFS-125 FT-IR spectrophotometer (for 1), whereas a PerkinElmer Spectrum-II FT-IR spectrophotometer is used for other complexes. ESI mass spectra were recorded on a Shimadzu LCMS 2020 mass spectrometer equipped with electrospray ionization (ESI) ion source. Electronic absorption spectra in solution were obtained on a Thermo-Fischer Evolution One spectrophotometer in the range 1100–250 nm.

The X-band EPR spectra were recorded on a Bruker Elexys E580 spectrometer. All measurements were performed over a sweep range of 12 000 G with modulation amplitude of 10 G and a microwave power attenuation of 25 dB.

Magnetic susceptibility and magnetization measurements were performed on pressed polycrystalline samples using a Quantum Design MPMS-5XL SQUID magnetometer operating in the temperature range of 2–40 K at the CCiT Magneto- chemistry Unit, University of Barcelona. Diamagnetic corrections were applied using Pascal's constants.

Syntheses

H₂L₁. (N,N′-bis[(3-methoxy-2-hydroxybenzylidene)]ethane-1,2-diamine), For the synthesis of H₂L₁, 25 mmol (1.503g) of ethylenediamine was added to 50 mmol (7.608 g) 2-hydroxy-3-methoxybenzaldehyde in 50 ml of methanol. The mixture was refluxed for 1 hour. Then the solution was cooled at room temp., and a yellow precipitate was produced. IR (KBr, ν_max/cm⁻¹): 3423(br), 2931(s), 2843(s), 1633(s), 1453(s), 1055(m), 963(m). ¹H NMR (CDCl₃, 400 MHz): δ 13.61 (S, 2H) (–OH), 8.35 (S, 2H) (–HC=N), 6.93 (dd, J = 4, 4Hz, 2H), 6.87 (dd, J = 4, 8Hz, 2H), 6.8 (t, 2H), 3.98 (t, 2H), 3.91 (s, 6H) (–OCH₃).

[Cu^II₂(L₁)₂](1). 0.25 mmol (60 mg) of Cu(NO₃)₂·3H₂O was added to 83 mg (0.25 mmol) of H₂L₁ in 20 ml of methanol. This solution was stirred for an hour and then allowed to crystallize through gradual evaporation. After one day, blue crystals appropriate for X-ray diffraction had developed. Elemental anal. (%) for C₇₆ H₈₀ Cu₄N₈O₂₀: Calcd C 54.34, H 4.80, N 6.67; Found, C 54.51, H 4.70, N 6.8. Mass spectrum (ESI, positive ion, CH₃CN): m/z 839.8, {1/2}⁺. IR (KBr, ν_max/cm⁻¹): 2835(vs), 1610(s), 1230(s), 652(s), 550(s).

[Cu^II₂(N₃)₂(L₂)₂] (2). A solution of HL₂ (50 mg, 0.2 mmol) in a mixture of MeOH (20 ml) and DCM (10 ml) was heated to 60 °C for 15 minutes. After the solution had cooled to room temperature, filtering was done. To this filtrated copper(II)(II) acetate monohydrate (40 mg, 0.2 mmol) and excess sodium azide (65 mg, 1.0 mmol) in 10 ml MeOH solution were added successively, and the resulting solution was allowed to evaporate in the air. After 2–3 days, suitable green single crystals were obtained and picked for X-ray data collection. Elemental anal. (%) for C₁₅H₁₀CuN₆O: Calcd C 50.87, H 2.83, N 23.74; Found, C 50.62; H 2.72; N 23.49. Mass spectrum (ESI, positive ion, CH₃CN): m/z 353.8, {2/2} ⁺. IR (KBr, ν_max/cm⁻¹): 2045(vs), 1355(s), 1329(s), 1252(s), 1210(s), 1152(m), 756(m).

Complexes 3 and 4 were prepared following the same procedure as for complex 2, while potassium thiocyanate (97 mg, 1.0 mmol) was used in the case of complex 3 instead of sodium azide. However, to grow suitable single crystals of 4via gradual evaporation of a dark red methanol–dichloromethane reaction mixture required more than 5–6 days.

[Cu^II(SCN)(L₂)]_n (3). Elemental anal. (%) for C₁₆H₁₀CuN₄OS: Calcd C 51.91, H 2.70, N 15.14; Found, C 51.32; H 2.52; N 15.09. Mass spectrum (ESI, positive ion, CH₃CN): m/z 311.8, {[Cu(SCN)(L₂)] − SCN}⁺. IR (KBr, ν_max/cm⁻¹): 2112 (vs), 2095 (s), 1607 (m), 1511 (m), 1363(s), 1328(m), 1152(s), 1205(s), 1181(m).

[Cu^II(CH₃COO)(L₂)]_n (4). Elemental anal. (%) forC₁₇H₁₃CuN₃O₃: Calcd, C 55.01, H 3.51, N 11.32; Found, C 54.92, H 3.32, N 11.19. Mass spectrum (ESI, positive ion, CH₃CN): m/z 311.5, {[Cu(CH₃COO)(L₂)] − C₂H₃O₂}⁺. IR(KBr, ν_max/cm⁻¹): 1610(w), 1534(m), 1336(vs), 1252(s), 1235(s), 1247(vs), 1204(vs), 1135(s), 767(m).

X-ray crystallographic data collection and refinement of the structure

For the single crystals of 1, green prism-like specimens were used for the X-ray crystallographic analysis. The X-ray intensity data was measured on a D8 Venture system equipped with a multilayer monochromator and a Mo microfocus. Powder X-ray diffraction was performed with a PANalytical X'Pert PRO MPD θ/θ powder diffractometer of 240 millimeters of radius, in a configuration of convergent beam with a focalizing mirror and a transmission geometry with flat samples sandwiched between low-absorbing films and Cu Kα radiation (λ = 1.5418 Å). On the other hand, dark green single crystals of 2, 3 and 4 were picked up with nylon loops and mounted on a Bruker AXS D8 QUEST ECO diffractometer equipped with a Mo target rotating-anode X-ray source and a graphite monochromator (Mo Kα, λ = 0.71073 Å). The final cell constants were obtained from least-squares fits of all measured reflections. The intensity data were rectified for absorption using intensities of redundant reflections. The frames were integrated with the Bruker SAINT Software package using a narrow-frame algorithm. The structure was solved using the SHELXS-97 program package and refined with SHELXL-2014/6.²² All non-hydrogen atoms were refined anisotropically. Hydrogen atoms were positioned geometrically and refined using a riding model with isotropic displacement parameters.

The X-ray crystallographic plots of all the complexes are presented in the SI section (Fig. S1–S4). The crystallographic data and refinement parameters are listed in Table S1, while selected bond lengths and bond angles for complexes 1, 2, 3, and 4 are summarised in Tables S2, S3, S4, and S5, respectively. Further crystallographic details can be found in the corresponding CIF files provided in the SI.

Results and discussion

Synthesis and characterization

The dimeric complex [Cu₂(L₁)₂] (1) was obtained by reacting Cu(NO₃)₂·3H₂O with H₂L₁ (1 : 2) in methanol. The salen-type ligand H₂L₁ was prepared in only one step by the refluxing of ethylenediamine and methanolic 2-hydroxy-3-methoxybenzaldehyde in a (1 : 1) ratio. The other μ₂-pseudo halide bridged Cu(II) complexes (2–4) were also synthesised in a single step by treating Cu(OAc)₂·H₂O with the tridentate NNO-donor ligand HL₂ and the appropriate co-ligand in a CH₃OH–CH₂Cl₂ (3 : 1) solvent mixture. Complexes 2 and 3 were isolated upon adding NaN₃ and KSCN, respectively, to the reaction medium.

IR spectra

The synthesis of the ligand H₂L₁ was confirmed by the presence of characteristic IR bands at 1633 cm⁻¹ and 3423 cm⁻¹, corresponding to the stretching vibrations of the azomethine (C=N) and phenolic –OH groups, respectively. However, in complex 1, the C=N stretching band shifted to a lower wavenumber, indicating the coordination of the azomethine nitrogen atom to the Cu(II) center. The IR spectrum of complex 2 shows a characteristic band at 2044 cm⁻¹, corresponding to the asymmetric stretching vibration of the metal-coordinated end-on azide bridge.²³ The azo (–N=N–) stretching vibrations for complexes 2–4 appear in the range 1335–1355 cm⁻¹. Complex 3 exhibits two distinct absorption bands at 2112 and 2095 cm⁻¹, confirming the μ₁,₃-bridging coordination mode of the thiocyanate ligand; these bands are attributed to the S- and N-coordination modes of the SCN⁻ group, respectively.²⁴ In complex 4, strong bands at 1336 and 1203 cm⁻¹ are assigned to the asymmetric and symmetric stretching vibrations of the acetate group, respectively. The separation between ν_sym(–COO⁻) and ν_asym(–COO⁻) (Δν = 133 cm⁻¹) is consistent with a bidentate syn–syn bridging mode of the carboxylate group.²⁵ In all three complexes (2–4), the strong stretching frequency observed in the range 1135–1181 cm⁻¹ is attributed to coordination of the deprotonated naphtholic oxygen atom of the ligand (HL₂) to the Cu(II) centre.^23–25

Electronic spectra

All the complexes exhibit significant absorption in the visible region. The UV–Vis spectra, recorded in methanol (Fig. 1), and the corresponding absorption maxima are summarised in Table 1. Complex 1 displays broad absorption maxima at 565 nm. The remaining complexes (2, 3 and 4) exhibit closely comparable profiles, featuring intense absorption bands in the ranges 430–440 nm and 560–565 nm. The broad absorption maxima observed in the 560–565 nm region is attributed to a ligand-to-metal charge-transfer (LMCT) transition from the naphtholato-O donor to the Cu(II) centre, with an incorporated contribution from Cu(II) d–d transitions, (²E_g → ²T_2g) as expected from square-pyramidal Cu(II) complexes.^12,23,26 In addition, the absorption around 380–480 nm (2–4) is assumed to be the azo (n → π*) excitation of the coordinated ligand, which is typically observed in the HL₂ coordinated metal complexes.^26b An allowed intense band observed near 335 nm for complex 2 may be assigned as the azide (N₃⁻) → Cu(II) charge transition.²⁷

Fig. 1 UV-vis absorption spectra of 1 (green), 2 (black), 3 (red) and 4 (blue) in MeOH solvent at 298 K.

Table 1 Electronic spectrum of 1, 2, 3 and 4 in methanol at 25 °C

Complexes	λ max, nm (ε, 10^4 M^−1 cm^−1)
1	565 (0.072), 370 (1.131), 281(3.076)
2	560 (0.15), 430(0.072), 335 (0.070), 280 (0.221)
3	565(0.06), 440(0.027), 335(0.031), 270(0.128)
4	560(0.2), 430(0.079), 335(0.089), 315(0.103), 275(0.177), 255(0.196)

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Structural description

[Cu₂(L₁)₂] (1). Single-crystal X-ray diffraction of 1 shows that it crystallizes in the space group P1̄. A view of the molecular structure of 1 is shown in Fig. 2. The centrosymmetric complex consists of a dinuclear copper(II) entity, where each copper(II) center exhibits a slightly distorted pentacoordinated environment afforded by a doubly deprotonated L₁²⁻ ligand. The coordination sphere of each copper center is defined by the iminic-N and phenolato-O donors of the Schiff base, which form the basal plane of the square-pyramidal environment, while the apical site is occupied by a phenoxo oxygen atom (O2A) from the neighboring [Cu(L₁)] fragment. This arrangement generates a double Cu–O–Cu bridge and results in a CuN₂O₃ chromophore.

Fig. 2 Molecular structure of complex 1.

According to the usual Jahn–Teller distortion, axial Cu–O bond length (2.708 Å) is clearly larger than the basal Cu–O/Cu–N bond distances, which are comprised between 1.919–1.951 Å. The intermetallic Cu⋯Cu distance in this dimeric structure is 3.469 Å, and the coordinated tetradentate ligand forms a planar environment with the copper(II) atom. The methoxy arms of the Schiff base do not participate in metal coordination but engage in a network of hydrogen bonds with crystallization methanol molecules, contributing to lattice stabilization.

[Cu₂(N₃)₂(L₂)₂] (2). The azide ion exhibits versatile coordination mode, functioning as a mono- or bi-dentate ligand and frequently serving as a bridging unit that can connect to four metal ions in forming transition-metal complexes. Azido groups can exhibit symmetric/asymmetric end-on (μ_1,1) or end-to-end (μ_1,3) binding modes, depending on the metal ion as well as the electronic or steric states of the other co-ligands present in the complex. Magnetic features of these types of polynuclear complexes largely depend on the bridging fashion of the azido ligand. This structural-magnetic correlation comparison between huge amounts of the structurally characterised end-on double azide bridge dimeric copper complexes has been nicely analysed by Chattopadhyay et al. in their recently published review work.²⁸

In complex 2, single-crystal X-ray analysis confirms an asymmetric end-on double μ₁,₁-azido bridge between two copper(II) centres (Fig. 3). Centrosymmetric complex 2 crystallises in the P2₁/n space group. Each Cu(II) ion adopts a distorted square-pyramidal geometry, where the basal plane is defined by three nitrogen donors and one oxygen atom. Among these, one nitrogen originates from the pyridyl (N3) and two from the azo (N2) moieties of the tridentate NNO ligand, while naphtholato-oxygen (O1) coordinates the other site. The apical position is occupied by the nitrogen atom (N4) of an azide group (Fig. 3, right). The meridionally coordinated tridentate ligand (L²⁻) forms a nearly planar environment around Cu(II), with the azido-bridged fragment orientated almost perpendicular (91.58°) to this plane. The Cu⋯Cu separation is 3.180 Å. The asymmetric nature of the double end-on bridge is evident from the unequal Cu–N(azide) bond lengths [Cu1–N4 = 2.447(4) Å and Cu1A–N4 = 1.952(4) Å], consistent with values reported for similar Cu(II)–azido dimers (≈2.5 Å). The Cu–azide–Cu (Cu(1)–N(4A)–Cu(1A)) angle is 91.87°, which is relatively small but within the typical range (>85.5°) observed for such species.²⁹ The Cu–O(naphtholate) and N=N (azo) bond lengths are 1.988 and 1.301 Å, respectively. The coordinating azide ligand itself is nearly linear, with an N–N–N angle of 176.2(5)° and N–N bond lengths of 1.201(6) and 1.154(6) Å.

Fig. 3 Molecular structure of complex 2.

[Cu(SCN)(L₂)]_n (3). On the other hand, SCN⁻, a potential ambidentate ligand with a polarisable π-bonding motif, can coordinate with metals through either the sulphur or nitrogen atom, or both. Depending on the bridging modes of the thiocyanate ligand, several Cu(II) dimers can produce a variety of supramolecular topologies that can lead to notable magnetic interaction.³⁰

Here, single-crystal X-ray diffraction analysis reveals that complex 3 crystallises in the P2₁/c space group (Fig. 4, left). Crystal packing of these polymeric structures suggested here each asymmetric unit [Cu(SCN)(L₂)] is connected via end-to-end bridging thiocyanate anion. Each Cu(II) ion in asymmetric unit [Cu(SCN)(L₂)] adopts a square-pyramidal geometry, with the basal plane defined by two nitrogen atoms (azo-N1 and pyridyl-N3), one oxygen atom (naphtholate-O1) from the tridentate NNO ligand, and one nitrogen atom (N4) from the thiocyanate anion. The apical position is occupied by the sulphur atom of a symmetry-related thiocyanate ligand, generating a μ₁,₃-thiocyanate bridge. The Cu–N and Cu–O bond lengths range from 1.934(3)–2.000(3) Å, while the N3–Cu1–N1 and N3–Cu1–N4 angles are 79.00(11)° and 97.18(12)°, respectively. The Addison parameter (τ) is calculated as 0.3566, using the formula τ = (α − β)/60, where α and β are the two largest L–Cu–L angles of the coordination sphere.³¹ This value indicates that the coordination geometry around the copper(II) center is very close to an ideal square pyramidal structure. The N–C–S fragment is nearly linear [178.5(3)°], and the Cu⋯Cu separation (6.108 Å) is longer than those typically observed in similar thiocyanate-bridged Cu(II) systems.

Fig. 4 Molecular structure of the complex 3.

[Cu(CH₃COO)(L₂)]_n (4). Another widely used bidentate bridging ligand in coordination chemistry is acetate, which connects the metal centres via both of its carboxylate oxygen atoms. The unique binding modes of acetate-bridged complexes have a significant impact on their structural and electrical properties.³²

Single-crystal X-ray diffraction reveals that complex 4 crystallises as a polymeric chain of [Cu(CH₃COO)(L₂)] units extending along the c-axis (Fig. 5). Each acetate ligand bridges two Cu(II) centres in a μ₂-η¹:η¹syn–syn mode. Each Cu(II) ion exhibits a distorted square-pyramidal geometry, with the basal plane defined by the NNO donor set of the ligand and one acetate oxygen, while a symmetry-related acetate oxygen occupies the axial site. The Cu⋯Cu separation within the dimeric unit [Cu^II₂(CH₃COO)₂(L)₂] is 4.244 Å, longer than the typical 2.6–3.5 Å observed in acetate-bridged Cu(II) dimers. The Cu–O (naphtholato) bond length (1.990 Å) is slightly shorter than that in related systems, while the average Cu–O (acetate) distance (2.425 Å) is relatively longer. The asymmetric acetate coordination is supported by unequal C–O bond lengths of 1.215 and 1.283 Å.³³

Fig. 5 Molecular structure of complex 4.

Powder X-ray diffraction (PXRD) analysis

The experimental PXRD patterns of complexes 2 and 3 are in good agreement with the simulated data, confirming phase purity and consistency with the single-crystal structures, whereas complex 4 exhibits slight deviations, likely due to minor differences in crystallinity or packing (Fig. S5).

Non-covalent interaction

According to the crystal structure analysis of 1, the unit cell consists of two asymmetric dimeric units of [Cu₂(L₁)₂], and each dimeric unit is linked to a methanol solvent molecule through hydrogen bonding. In the extended packing pattern, the two dimeric [Cu₂(L₁)₂] units are oriented in nearly perpendicular molecular planes. Furthermore, each dimeric unit engages in non-covalent interactions, leading to the formation of a distinctive staircase-type supramolecular arrangement (Fig. 6).

Fig. 6 (Left) H-bonding interactions between the dimeric units of [Cu₂(L₁)₂] moieties and solvent molecules in complex 3. (Right) Non-covalent interactions in the extended three-dimensional supramolecular network in the crystal lattice of 1.

In complex 2, the planar, alternating naphthalene moieties of two neighbouring molecules [Cu₂(N₃)₂(L₂)₂], separated by 3.847 Å, engage in pronounced π–π stacking interactions (Fig. 7, left). A weak hydrogen bond measuring 2.705 Å is observed between a hydrogen atom of the naphthalene ring and a hydrogen atom of the pyridine moiety of an adjacent molecule. Additionally, a one-dimensional zigzag chain is formed in the expanded crystal lattice through a notable intermolecular non-covalent interaction (2.658 Å) between a hydrogen atom of the naphthalene ring and the non-bridging N(6) atom of the azide group of a neighbouring molecule. This chain further extends into a cross-linked supramolecular network within the overall crystal packing (Fig. 7, right).

Fig. 7 (Left) π–π stacking interactions between the planar naphthalene moieties of neighbouring molecules in complex 2. (Right) Non-covalent interactions leading to the formation of a one-dimensional zigzag chain and the extended three-dimensional supramolecular network in the crystal lattice of 2.

Comparable non-covalent interactions are also observed in the expanded crystal structures of complexes 3 and 4. For complex 3, the μ_1,3-thiocyanate bridge in the asymmetric unit [Cu(SCN)(L₂)], upon repetition, generates a polymeric architecture. Here, the planar, alternating pyridine moieties of neighbouring molecules, separated by 4.715 Å, exhibit π–π stacking interactions. The crystal packing of 3 also reveals a significant intermolecular contact (3.308 Å) between the azo nitrogen (N1) atom and the sulphur atom of the thiocyanate bridging unit of an adjacent molecule, resulting in a one-dimensional helical network (Fig. 8).

Fig. 8 (Left) π–π stacking interactions between the planar pyridine moieties of neighbouring molecules in complex 3. (Right) Non-covalent interactions in the extended three-dimensional supramolecular network in the crystal lattice of 3.

In complex 4, the [Cu(CH₃COO)(L₂)] fragments are connected through acetate bridges in an alternating (syn–syn)–(anti–anti)–(syn–syn) coordination mode, producing a ribbon-like one-dimensional chain and forming a polymeric Cu(II) complex. The extensive acetate bridging between neighbouring asymmetric units leads to a three-dimensional supramolecular architecture in the crystal packing, where alternating asymmetric units align parallel to each other (Fig. 9).

Fig. 9 (Left) One-dimensional ribbon-like chain structure formed through acetate bridging in complex 4. (Right) Three-dimensional supramolecular network generated via non-covalent interactions in the crystal lattice of 4.

EPR spectra

EPR spectroscopy serves as a valuable tool for probing the magnetic anisotropy, exchange interactions, and electronic structure in paramagnetic Cu(II) systems. The X-band (CW) EPR spectra of complexes 1–4 were recorded at room temperature in the solid state under aerobic conditions (Fig. 9). Complex 1 displays a rhombic EPR spectrum characterised by three distinct g-values (g_x = 2.23, g_y = 2.06, g_z = 1.90), suggesting a ground state of the metal ion in this particular system composed of the combination of the d_z2 and d_x2−y2 orbitals. This behaviour correlates well with the significant geometric distortion observed in the coordination environment (Fig. 2, right). Complexes 2 and 3, in the polycrystalline solid state, show a single broad isotropic signal at g_iso = 2.12and g_iso = 2.09, respectively. Such broad resonances are commonly attributed to dipolar broadening and enhanced spin–lattice relaxation effects. None of the complexes exhibit hyperfine splitting, which is generally expected for Cu(II) (I = 3/2) ions, likely due to exchange coupling and line broadening in the solid state. However, complex 4 in the solid state shows a broad peak in which the simulation reveals a resolved axial spectrum with a splitting in the parallel region, with g_∥ = 2.21 and g_⊥ = 2.06 (Fig. 10). Hence, the complex 4 has a distorted square pyramidal geometry with similar binding modes and (g_∥ > g_⊥ > 2.0023), which shows that a single electron resides in the d_x2−y2 ground state. All the complexes have very poor solubility in common coordinating solvents, even DMF, so the EPR spectra could not be recorded in solution by adjusting the concentration, sweep range, and modulation amplitude.

Fig. 10 CW-EPR spectra of complexes 1–4 recorded in the solid state at room temperature (from top to bottom, respectively).

Magnetic properties

Static measurements. The χ_MT values for all four complexes are comparable, with values close to 0.4 cm³ mol⁻¹ K⁻¹, as shown in Fig. 11. Upon cooling, at low temperature (∼10 K), the χ_MT values remain nearly constant and exhibiting a weak decay below this temperature.

Fig. 11 χ _M T vs. T (K) plot for complexes 1–4 in the low range of temperature. Inset: reduced magnetization for 1–4. (The value corresponds to a dimeric unit for complexes 1 and 2 and for monomeric unit for 3–4).

The magnetization values under the maximum applied magnetic field of 7 T approach the expected value of 1N_μB for a mononuclear Cu(II) ion, consistent with an S = ½ ground state. Although structurally complex, 1–4 feature dimeric or one-dimensional polymeric Cu(II) centers connected through various bridging ligands; the magnetic measurements suggest that the spin carriers behave as nearly magnetically isolated. In all four systems, the copper(II) ions are linked via one axial and one equatorial coordination site, connecting the dx₂₋y₂ magnetic orbital with the non-magnetic dz₂ orbital, resulting in negligible magnetic interaction.

Dynamic measurements. To evaluate the slow relaxation of the magnetization properties of compounds 1–4, AC measurements were performed on powdered and pressed samples. No signal was observed for any one of the four compounds. When a static (DC) magnetic field was applied, complexes 1 and 4 exhibited field-induced slow relaxation of the magnetisation (Fig. S6). The in-phase (χ′, top) and out-of-phase (χ″, bottom) components show clear frequency dependence, indicative of slow magnetic relaxation (Fig. 12 and 13). Cole–Cole plots for complexes 1 and 4 (Fig. 14 (left) and 15 (left)) were fitted using the generalised Debye model to extract relaxation times, which are plotted as ln(τ) versus 1/T (Fig. 12 and 14, right).³⁴ For complex 1, two relaxation processes are observed, described by a combination of Direct (AT) and Raman (CTⁿ) mechanisms:

τ⁻¹ = AT + CTⁿ (1)

Fig. 12 In-phase (top) and out-of-phase (bottom) magnetic susceptibility at a fixed static field of 0.5 T for compound 1.

Fig. 13 In phase (top) and out-of-phase (bottom) magnetic susceptibility at a fixed static field of 0.7 T for compound 4.

Fig. 14 (Left) Cole–Cole plots for complex 1 recorded under an applied DC field, showing frequency-dependent χ″ components. (Right) Temperature dependence of the relaxation time plotted as ln(τ) vs. 1/T for complex 1. The solid line corresponds to the best fit using the combined Direct and Raman relaxation processes (eqn (1)).

The fitted parameters yielded an α value of 0.08, suggesting a narrow distribution of relaxation times. The Raman relaxation pathway was best fitted with C = 0.23 s⁻¹ K⁻ⁿ and n = 7.9. Complex 4 displayed similar behaviour, with the data also satisfactorily fitted to a combined Direct + Raman relaxation model. The corresponding parameters are C = 6.9 s⁻¹ T⁻ⁿ and n = 6.2, with α values varying between 0.24 at 1.8 K and 0.18 at 4.2 K. The relatively low Raman coefficients, compared with those typically observed for Kramers ions, may be attributed to the involvement of optical phonons in the spin–phonon coupling process (Fig. 15).³⁵

Fig. 15 (Left) Cole–Cole plots for complex 4 recorded under an applied DC field, showing frequency-dependent χ″ components. (Right) Temperature dependence of the relaxation time plotted as ln(τ) vs. 1/T for complex 4. The solid line corresponds to the best fit using the combined Direct and Raman relaxation processes (eqn (1)).

Biological study

Frontier molecular orbital (HOMO–LUMO) analysis. The chemical reactivity and stability of complexes (2–4) in comparison with 1 were analysed through frontier molecular orbital (FMO) theory. The highest occupied molecular orbital (HOMO), which represents the nucleophilic functional site, while the lowest unoccupied molecular orbital (LUMO) represents the electrophilic functional site, and the energy gap (ΔE_H–L) which represents the structural flexibility and stability of the compound. The restricted Hartree–Fock (RHF) model with single-point energy calculations was employed, and HOMO–LUMO energy plots were visualized using Avogadro–ORCA 1.2.0 software.^36,37 The calculated FMO results (Fig. 16) revealed that complex 3 exhibited the largest energy gap (5.950 eV), followed by 2 (4.676 eV), 4a (4.444 eV), and 1 (3.511 eV). According to FMO theory, a smaller HOMO–LUMO gap implies higher chemical reactivity and lower kinetic stability, whereas a larger gap indicates lower reactivity and enhanced stability.

Fig. 16 The HOMO, LUMO, and their energy gap (ΔE_H–L) of 1, 2, 3, and 4 expressed their flexibility and reactiveness profile.

Thus, 3 demonstrated the highest kinetic stability and lowest reactivity among the complexes. On the other hand, all three NNO-coordinated Cu(II) complexes (2–4) are comparatively less reactive than 1. These findings suggest that the synthesised complexes exhibit higher structural stability due to specific modifications. Overall, their unique reactivity and better kinetic stability make them promising candidates for further target-specific study of therapeutic, medicinal, or material science applications in various models.

Therapeutic potency through molecular docking study. The therapeutic potential of the metal complexes (2–4) was evaluated through molecular docking studies to assess their binding efficacy with selected biological targets. Based on the literature review, we have selected four putative target enzymes: B-DNA, human DNA topoisomerase 1 (hTOP1), the Escherichia coli MenB enzyme (EC-MenB), and human serum albumin (HSA) for the docking study. Further, metal complexes were converted to 3D ligand structures, optimized, and saved in the dot-pdb (.pdb) file format prior to docking studies.³⁶

The crystallographic protein structure of target enzymes was retrieved from the protein data bank with individual PDB IDs as follows: B-DNA (PDB ID: 1BNA), H-TOPI (PDB ID: 1SC7), EC-MenB (PDB ID: 3T88), and HAS (PDB ID: 4LA0). Further, the docking study was performed using AutoDock 4.2 software with the existing docking protocol and a user-defined grid box for each target enzyme. Individual ligands generated ten different docking conformations against each target, and the lowest energy pose (expressed in a higher negative score) was selected as the most potential docking score or binding efficacy.³⁶

The docking score (kcal mol⁻¹) of three complexes (2–4) against four drug targets was recorded, where 3 showed comparatively higher binding efficacy than 2 and 4 (Table 2). Especially, 3 demonstrated a higher binding efficacy against hTOP1 with a docking score of −9.30 kcal mol⁻¹, while it showed the lowest efficacy against EC-MenB with a docking score of −7.11 kcal mol⁻¹. Similarly, 2 and 4 showed higher efficacy against hTOP1 and HAS, with a docking score of −8.89 kcal mol⁻¹. On the other hand, 4 showed their docking score within −7 to −8 kcal mol⁻¹, against all four target enzymes (Table 2).

Table 2 Molecular docking score (kcal mol⁻¹) of complexes 2–4 against four drug targets

Complexes	B-DNA (PDB ID: 1BNA)	hTOP1 (PDB ID: 1SC7)	EC-MenB (PDB ID: 3T88)	HAS (PDB ID: 4LA0)
hTOPI, human DNA topoisomerase 1; EC-MenB, Escherichia coli MenB enzyme; HAS, human serum albumin.
2	−7.33	−8.89	−7.83	−8.89
3	−8.54	−9.30	−7.11	−7.87
4	−7.83	−7.07	−7.09	−7.83

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Further, for better understanding, we have generated the protein–ligand interaction of the least binding pose of 3 against four targets (Fig. 17). Overall, docking scores suggest that complexes may exhibit higher anticancer potency than antibacterial activity, which warrants further exploration in an experimental study.

Fig. 17 Three-dimensional protein–ligand interactions of complex 3 with four target proteins, visualized using BIOVIA Discovery Studio Visualizer for the best docking poses (lowest binding energy).

In vitro antibacterial study. Initially, the antibacterial activity of complexes 2–4 was evaluated at a concentration of 500 µg mL⁻¹, while the standard antibiotic gentamicin was tested at 10 µg mL⁻¹ against all selected bacterial strains (Table 3). Based on the zone of inhibition results, all three complexes exhibited inhibition zones within 7–8 mm, indicating weak to moderate antibacterial activity. Among them, complex 3 displayed a comparatively larger inhibition zone of 11 mm against E. coli and S. aureus, whereas complex 4 showed the lowest inhibition zone of 6 mm against P. aeruginosa. In contrast, the positive control (gentamicin) produced an inhibition zone of 15 mm against all tested bacterial strains, while the negative control (DMSO) exhibited no inhibition.

Table 3 In vitro antibacterial activity represented as the zone of inhibition (mm) for complexes at 500 µg mL⁻¹ and standard gentamicin at 10 µg mL⁻¹ against four pathogenic bacterial strains

Complexes	Escherichia coli	Klebsiella pneumoniae	Pseudomonas aeruginosa	Staphylococcus aureus
a Used as positive control. b Used as negative control.
2	09	07	08	07
3	11	08	09	10
4	07	07	06	07
Gentamicin^a	15	14	12	12
DMSO^b	—	—	—	—

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Subsequently, the concentration of the complexes was increased to 1000 µg mL⁻¹ to determine their minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) values. Consistent with the zone of inhibition results, the MIC values were found to be between 800 and 1000 µg mL⁻¹, and the MBC values were >1000 µg mL⁻¹ (Table S6). In comparison, gentamicin exhibited MIC and MBC values within the previously reported range of 5–20 µg mL⁻¹. These findings clearly indicate that the synthesized complexes possess weak antibacterial activity, suggesting the need to explore alternative biological applications in view of their unique chemical properties.

From a technical standpoint, in silico and in vitro methods represent two complementary platforms for investigating the biological potential of new compounds. The in silico approach primarily predicts possible biological activities by targeting specific enzymes or molecular sites, whereas in vitro assays evaluate the actual biological efficacy of the compounds against live pathogens comprising multiple enzymatic targets. Although we were unable to assess the anticancer potential of these complexes due to limited facilities, future studies will aim to explore diverse biological activities to provide a more comprehensive understanding of their pharmacological relevance. Together, both in silico and in vitro approaches offer significant advantages in identifying promising candidates and are widely adopted in academic and pharmaceutical research for preclinical drug screening.^37–39

Conclusions

In summary, a series of dinuclear and polymeric Cu(II) complexes incorporating ONNO- and NNO-donor ligands were successfully synthesized and structurally characterized. Detailed molecular and bonding analyses revealed that complex 1 is centrosymmetric, consisting of a dinuclear Cu(II) core where each metal centre exhibits a slightly distorted pentacoordinated geometry coordinated by the doubly deprotonated L₁²⁻ ligand. In contrast, complex 2 features an asymmetric double end-on μ₁,₁-azido bridge between Cu(II) centres, while polymeric complexes 3 and 4 are stabilized by μ₁,₃-thiocyanate and μ₂-(syn–syn)-acetate bridges, respectively. Magnetic studies indicate that complexes 1 and 4 exhibit slow relaxation of magnetization under an applied magnetic field, consistent with single-molecule magnet (SMM) behaviour. Furthermore, molecular docking studies provided valuable insights into the binding interactions of these complexes with biologically relevant targets, suggesting their potential therapeutic applications. Overall, this comprehensive investigation establishes a clear relationship between the structural architecture, magnetic properties, and biological activity of the synthesized Cu(II) complexes, highlighting their significance for future studies in molecular magnetism and bioinorganic chemistry.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting the findings of this study are available within the article and its supplementary information (SI). The data supporting this article have been included as part of the SI (figures and table mention below). Structural, magnetic data and antibacterial activity of the compounds are available in SI. Further crystallographic details can be found in the corresponding CIF files provided in the SI. Supplementary information: structural and magnetic data. See DOI: https://doi.org/10.1039/d6dt00119j.

CCDC 2496996 and 2403121–2403123 (1–4) contain the supplementary crystallographic data for this paper.^40a–d

Acknowledgements

A. S. R. expresses his deep appreciation to Dr Jaydeep Sarangi, Principal of New Alipore College, Kolkata 700053, for his unwavering support and encouragement in these research endeavors. A. S. R., S. G. and I. N. extend their sincere gratitude to the mentor, Dr Prasanta Ghosh, Associate Professor of Chemistry, R. K. Mission Residential College, Narendrapur, for his support and guidance in conducting the experiments. A. S. R. is thankful to Dr Sachinath Bera, Assistant Professor of Chemistry, SMHGGDCW, for his constructive suggestions during the preparation of the manuscript. A. S. R. gratefully acknowledged partial financial support received from the New Alipore College for research fund (NAC/2022/177/12). S. G. acknowledges CSIR-New Delhi (File Number-08/0531(13772)/2022-EMR-I). J. M. and A. E. thank the funding from MICINN (Project PGC2018-094031-B-100).

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