Dithiodipropionate and Fumarate Ni, Cu, and Zn Mixed Ligand Complexes

Abstract

Three nickel, copper, and zinc complexes with dicarboxylic acids (3,3′-dithiodipropionic acid (H₂dtdp) and fumaric acid (H₂fu)) and N-donor ligands (1,10-phenanthroline (phen), N′–methyldipropylenetriamine (mdpta), and N,N,N′,N″,N″-pentamethyldiethylenetriamine (pmdien)) were synthesized. These complexes were characterized using elemental analysis, IR spectroscopy, and single-crystal X-ray diffraction. Interestingly, [Ni(dtdp)(phen)(H₂O)₃]∙0.5H₂O (1) is a mononuclear complex, where the dtdp dianion employs only one carboxylate group for coordination to the central nickel atom. [(ClO₄)(mdpta)Cu(μ-dtdp)Cu(mdpta)(H₂O)](ClO₄) (2) is a dinuclear copper complex with a dtdp bridge and different coordination on the copper center. [{Zn(pmdien)(H₂O)}₂(μ-fu)](ClO₄)₂ (3) is a symmetric dimer with a bridging fumarate ligand. These coordination compounds were tested for their antibacterial activities on Escherichia coli, Pseudomonas aeruginosa, Staphylococcus aureus, and Enterococcus faecalis bacteria strains. All the complexes show moderate activities on the mentioned strains.

1. Introduction

Carboxylic acids are versatile ligands capable of forming coordination compounds with metals through various coordination modes [1]. Carboxylate anions are exceptionally versatile in constructing diverse coordination compounds due to their remarkable ability to coordinate in up to fifteen different modes [2]. This flexibility stems from a carboxylate group’s capacity to form anywhere from one to five coordination bonds. When dicarboxylate anions are employed, the complexity and diversity of the resulting structures increase dramatically, facilitating the formation of dinuclear, trinuclear, or polynuclear complexes, as well as coordination polymers. This versatility makes them valuable for tuning structural motifs and, consequently, the properties of the resulting complexes. These complexes have potential applications in diverse fields, including optics, magnetism, catalysis, and bioactivity. Transition metal complexes with carboxylates are extensively studied not only for their magnetic properties, but also for their potential as drugs with anti-cancer, antibacterial, and other therapeutic effects [3,4,5,6,7]. A key advantage of carboxylic acids is their ability to function as bridging ligands, enabling the synthesis of dinuclear or polynuclear complexes [8,9]. Research indicates that polynuclear complexes often exhibit superior antibacterial properties compared to their mononuclear counterparts [10]. In light of this, we have explored the synthesis of several complexes containing carboxylic acids with the aim of developing new antibacterial agents.

Tricarboxylic and dicarboxylic acids in complexes are also of special interest due to their many possibilities of coordination to central atoms [11,12]. Our recently released review focused on copper, zinc, and nickel complexes with 2,2′-thiodioacetic, 3,3′-thiodipropionic, 3,3′-dithiodipropionic, and fumaric acid in combination with nitrogen donor ligands. Aside from structural findings, the review focuses on biological properties [12]. The carboxylate anions can be bonded monodentately to one or two metal centers or through a chelating mode. With two carboxylate groups, it is more complicated as chelating coordination or a bridging mode can occur and the complexes can be dinuclear, trinuclear, tetranuclear, or polymeric [13]. 2,2′-thiodioacetic (H₂tda) and 3,3′-thiodipropionic acids (H₂tdp) have a sulfur atom in the chain that can potentially be used for coordination to the central atom, and mostly mononuclear or dinuclear Cu and Zn complexes can be formed [14,15,16,17,18,19,20]. Interestingly, mononuclear or dinuclear nickel complexes can be prepared depending on the bidentate N-N donor ligands. Thus, mononuclear complexes [Ni(bpy)(tda)(H₂O)]·4H₂O and [Ni(tda)(1,3-pn)(H₂O)]·H₂O—bpy = 2,2′-bipyridine, 1,3-pn = 1,3-diaminopropane—were prepared, whereas dinuclear complexes are formed with [(en)Ni(μ-tda)₂Ni(en)]·4H₂O and [Ni₂(μ-tda)₂(1,2-pn)₂], where ethylenediamine = en and 1,2-diaminopropane = 1,2-pn, respectively [21,22]. Interestingly, Cu thiodipropionate complexes with phen, imidazole, and benzimidazole were structurally characterized by Arici et al. [20], along with Zn thiodipropionate complexes [23,24,25], and do not involve thioether sulfur in coordination to the metal center. In comparison with tda and tdp complexes, there are plenty of structurally characterized Cu, Zn, and Ni fumarate complexes. Many of them are dimers or polymers depending on the method of preparation and nitrogen donor ligands [13,23,24,25,26,27,28].

Although mixed ligand complexes with the above-mentioned dianions of dicarboxylic acids have been synthesized and their molecular structures have even been solved, their biological study data are not often presented. Abbashadeh et al. presented the inhibition zones of [(phen)₂Cu(μ-tda)Cu(phen)](ClO₄)₂ and [Cu(phen)(tda)] on the B. subtilis, S. aureus, E. faecalis, E. coli, K. pneumoniae, and P. aeruginosa bacterial strains [29]. Buchtelova et al. prepared and proved the antibacterial properties of [Cu₂(pmdien)₂(H₂O)₂(μ-tdp)](ClO₄)₂·H₂O on E. coli, S. aureus, and MRSA [30]. Moreover, binding to DNA and ROS (reactive oxygen species) activity were studied in cancer cells. Similarly, the ROS, CT-DNA binding, and photo-induced cleavage activity of [{(phen)Cu}₂(μ-dtdp)₂]·2H₂O were reported [31]. Paul et al. reported the structure, BSA and HSA interaction, and CT-DNA binding of the Schiff base complex {[Cu₂(L)₂(fu)]·(H₂O)·(MeOH)}_n, where HL = (E)-2-((1-hydroxybutan-2-ylimino)methyl)phenol [32].

In our previous paper, we prepared and structurally characterized [Cu₂(μ-dtdp)(pmdien)₂(H₂O)₂](ClO₄)₂ [22]. The antibacterial activity of E. faecalis, S. aureus, P. aeruginosa, and E. coli was checked. This paper is a continuation of our research in this area.

2. Results and Discussion

2.1. Characterization of Complexes

The mononuclear nickel complex (1) was prepared from nickel dithiodipropionate in a similar manner as in the preparation of nickel thioacetate [21]. The addition of phen to the precursor led to its complete dissolution, and from the blue solution, crystals were obtained. As was proved by an X-ray structure analysis, sulfur atoms, as well as the second carboxylate oxygen atoms, are not involved in coordination with nickel. Copper and zinc perchlorates were the sources of the central atoms for the rest of the complexes. To the solutions of perchlorates, nitrogen ligands were added, followed by the addition of the potassium salts of the dicarboxylate acids. Complexes 2 and 3 are binuclear with the expected dicarboxylate bridges between the central atoms. The structural formulae of the studied ligands are depicted in Scheme 1.

In the IR spectrum of dithiodipropionate nickel(II) (Figure S6), very strong peaks at 1395 and 1555 cm⁻¹ can be assigned to ν_s(COO⁻) and ν_as(COO⁻) vibrations [13]. In complex 1, there are four peaks in the range that can be caused by COO- vibrations (1347 sh, 1403 vs. 1517 sh, and 1555 vs. 1575 sh) (Figure S5). The band at 1342 cm⁻¹ belongs to the C-N deformation vibration of the aromatic ring of phenanthroline. Most of the C=C and C=N stretching and deformation vibration peaks of phenanthroline are hidden behind the very intense peaks of the carboxylic group. In complexes 2 and 3 (Figure S5), the bands corresponding to the N-H deformation vibrations are in the range of 1547 to 1583 cm⁻¹, and those of the N–H stretching vibrations are in the region 3320–3260 cm⁻¹. Very strong peaks observed in spectra of 2 and 3 in the 1062–1077 cm⁻¹ range belong to ν₄(ClO₄) [11,13]. The splitting of the perchlorate peak in the spectrum of 3 observed maxima of 1096, 1044, and 1024 cm⁻¹, which can be explained as a result of a reduction in the symmetry of the perchlorate anion to C_3v or C_2v [13].

In the visible region of the UV-Vis spectra of 1 (Figure S1), two absorption peaks are present at 608 and 734 nm, which can be assigned to ³A_2g → ³T_2g(F) and ³A_2g → ¹E_g transitions, respectively. The peak of the third spin-allowed d-d transition (³A_2g → ³T_1g(P)) bellow 400 nm is hidden behind the very intense peaks in the UV region [21]. The visible region of the spectrum of 2 exhibits one broad band at 626 nm, corresponding to d-d transitions that are routinely observed in many Cu(II) complexes with distorted square pyramidal geometry [14,15]. In the visible region of the UV-Vis spectra of 3, no absorption peak was observed due to the electron configuration of Zn(II).

2.2. Structural Analysis

The crystal structures of three novel coordination compounds were determined by the single-crystal X-ray diffraction studies. Compound 1, [Ni(dtdp)(phen)(H₂O)₃]∙0.5H₂O, is a mononuclear nickel complex containing two structurally independent coordination units in the asymmetric part, which are identical in composition and coordination bonds, along with one non-coordinating water molecule (Figure 1a). In both units, the nickel atom exhibits an octahedral coordination geometry (OC-6) with a coordination number of 6 (Figure 1b).

The inner coordination spheres of six-coordinated nickel atoms consist of one chelating 1,10-phenanthroline (phen), three terminal water molecules, and one 3,3′-dithiodipropionate (dtdp) anion coordinating through one oxygen atom. The second carboxylate group of dtdp is involved solely in hydrogen bonding. The geometries of the coordination polyhedra were determined using calculations performed in the SHAPE program [33], which is based on the Continuous Shape Measures (CShM) concept [34]. The CShM value S_Q(P) represents the degree of similarity between a given structure (Q) and a reference geometry (P). It ranges from 0 to 100, where a value of 0 indicates perfect agreement with the reference geometry. Values close to 0 (typically below 1) suggest very good agreement, while increasing values represent progressively larger deviations from the ideal shape. While the theoretical upper limit for the S_Q(P) value is 100, in reality, even highly distorted chemical structures seldom exhibit values exceeding 40. In the case of compound 1, both nickel atoms adopt a geometry very close to the ideal octahedron, as evidenced by S_Q(OC-6) values lower than 1 (Table S1). The dtdp anion chains adopt the same trans–gauche–intermediate gauche/eclipsed–gauche–gauche conformation from the coordinated to the non-coordinated carboxylate group (Table S2). The supramolecular structure is stabilized by an extensive three-dimensional network of hydrogen bonds due to the large number of hydrogen bond donors and acceptors. The graph-set analysis is particularly useful in describing the hydrogen bonding patterns [35]. In this approach, hydrogen bonds are represented as edges connecting donor and acceptor atoms (nodes), allowing for a systematic classification of hydrogen bonding motifs. The general notation G_d^a(n) is used, where G denotes the graph pattern (e.g., R for ring, C for chain, etc.); a is the number of acceptors; d is the number of donors; and n represents the total number of atoms involved in the hydrogen bonding pattern. In the structure of 1, only O-H···O-type hydrogen bonds are present (Table S3), forming various cyclic motifs such as R₂²(8), R₃²(8), R₄³(10), R₅⁴(12), R₆³(12), and R₆⁴(14) (Figure S2). Additionally, an intramolecular hydrogen bond forming an S(6) motif is observed within the coordination units. Next to the hydrogen bonds, π-π stacking interactions (Figures S3 and S4) exist between phen molecules (Table S4), and they are propagated along the crystallographic [0 1 0] axis.

Compound 2, [(ClO₄)(mdpta)Cu(μ-dtdp)Cu(mdpta)(H₂O)](ClO₄), is a dinuclear copper coordination complex composed of two distinct coordination units bridged by a dtdp anion (Figure 2a). The first central atom, Cu1a, possesses a coordination number of 5 and a square pyramidal (SPY-5) coordination polyhedron, while the second central atom, Cu1b, is six-coordinated with a distorted octahedral geometry—OC-6 (Figure 2b; Table S1). The S_Cu1b(OC-6) value is higher than the corresponding values for the nickel atoms of 1 (4.273 vs. 0460 and 0.440), which indicates larger deviations from an ideal octahedron.

Both copper atoms are chelated by tridentate N′–methyldipropylenetriamine (mdpta). Cu1a is further coordinated by one water molecule and one oxygen atom from the dtdp carboxylate group, with the second oxygen atom of this group forming an intramolecular hydrogen bond with the coordinating water molecule, creating an S(6) motif. Cu1b is coordinated by a monodentate perchlorate anion and a chelating carboxylate group from dtdp. An interesting feature of 2 is the competition between chlorate anions and water molecules for positions in the inner coordination sphere of the copper centers. Despite the presence of a water molecule in the inner coordination sphere of Cu1a leading to the formation of an intramolecular hydrogen bond that further stabilizes this arrangement, a perchlorate ion is found in the inner coordination sphere of Cu1b at the expense of a water molecule. This is accompanied by a change in the coordination mode of the dtdp carboxylate group to chelating, due to its inability to form an intramolecular hydrogen bond. The observed equilibrium between water and perchlorate ions suggests that the stability constants of both coordination units are similar, allowing for the formation of a dinuclear compound built from central atoms with different coordination spheres. The dtdp anion chain adopts a conformation analogous to that observed in 1 (Table S2). The dinuclear entity carries a single positive charge, which is balanced by a perchlorate anion in the outer coordination sphere. The supramolecular structure is stabilized by hydrogen bonds, with mdpta amine groups acting as donors, forming N-H···O bonds with carboxylate groups, perchlorate anions, and water molecules, as well as N-H···S bonds with dtdp sulfur atoms (Table S3). The coordinating water molecule also forms a hydrogen bond with the non-coordinating perchlorate anion. This diverse hydrogen bonding network results in cyclic motifs such as R₂²(12), R₃³(10), and R₅⁵(16) (Figure S3).

The phase purity of complexes 1–3 was confirmed via powder X-ray diffraction, employing Rietveld analysis to compare the measured data with the calculated pattern based on the crystal structure. Data refinement was conducted using parameters derived from method used by Bergmann et al. [36], with the microstrain (k2) and crystallite size (B1) parameters refined isotropically. The preferred orientation effects were addressed using the “SPHAR8” model. The fit demonstrates good agreement with the calculated powder pattern of complexes in all studied cases (Figure 3).

Compound 3, [{Zn(pmdien)(H₂O)}₂(μ-fu)](ClO₄)₂, is a symmetrical dinuclear zinc coordination complex with a fumarate (fu) anion serving as the bridging ligand (Figure 4a). It is isomorphous to the previously reported copper compound (CSD refcode: SIXMAO [13]). The fu chain adopts a trans conformation, and the symmetry of the dinuclear molecule is evidenced by an inversion center at the bisector of the fumarate anion (the special positions d of the P2₁/c space group). The coordination sphere of the five-coordinated zinc cation consists of a tridentate chelating N,N,N′,N″,N″-pentamethyldiethylenetriamine (pmdien), a water molecule, and a monodentate carboxylate group from the fu anion. The coordination polyhedron adopts a trigonal bipyramidal geometry (TBPY-5) (Figure 4b, Table S1).

An intramolecular hydrogen bond of the S(6) type is observed between the water molecule and the non-coordinated atom of the carboxylate group, similar to those in 1 and 2. The water molecule also forms a hydrogen bond with a non-coordinating perchlorate anion. The dinuclear entity carries a double positive charge, balanced by two perchlorate anions in the outer coordination sphere.

2.3. EPR Study of [(ClO₄)(mdpta)Cu(μ-dtdp)Cu(mdpta)(H₂O)](ClO₄) (2)

To complement the structural analysis, complex 2, containing Cu(II), was studied using EPR spectroscopy (Figure 5).

The molecular structure of [(ClO₄)(mdpta)Cu(μ-dtdp)Cu(mdpta)(H₂O)](ClO₄) (2) contains two inequivalent Cu(II) centers, necessitating the use of two distinct sets of g-factors for fitting. The resulting parameters are g_⊥ = 2.06374 ± 0.00004, g_‖ = 2.1711 ± 0.0003, g′_x = 2.0772 ± 0.0001, g′_y = 2.10756 ± 0.00006, and g′_z = 2.1520 ± 0.0002, with anisotropic linewidth η_x = 0.1428± 0.0005 GHz, η_y = 0.1493 ± 0.0004 GHz, and η_z = 0.254 ± 0.002 GHz. The first Cu(II) center exhibits a normal axial spectrum with g_‖ > g_⊥ > g_e (2.0023), indicating that the unpaired electron in the ground state resides in the $d_{x^2-y^2}$ orbital, which is characteristic of an elongated octahedral or square pyramidal geometry [37]. The second Cu(II) center displays a rhombic spectrum with g′_z > g′_y > g′_x, also suggesting the presence of an unpaired electron in the $d_{x^2-y^2}$ orbital in the ground state [38]. These findings are consistent with the shapes observed in the structural analysis, specifically an elongated octahedron and a tetragonal pyramid. Given the minimal distortion of the penta-coordinated square pyramidal site and the more pronounced deformation in the hexa-coordinated octahedral site, it is inferred that the axial g-factor g is associated with the penta-coordinated site, while the rhombic g-factor g′ corresponds to the hexa-coordinated site. The splitting of g′ is likely induced by the deformation of the angle between atoms O12b and O12c, which is 152° instead of 180°, as is expected in an ideal octahedron.

2.4. Antimicrobial Activity of the Complexes

The antibacterial activity of complexes 1–3 was evaluated against both Gram-positive (E. faecalis and S. aureus) and Gram-negative (P. aeruginosa and E. coli) bacteria. The minimum inhibitory concentration (MIC) values (

Table 1. Minimal inhibition concentration (MIC) [g/L] of complexes 1–3 for different bacterial strains.

MIC [g/L]
Complex	E. coli	P. aeruginosa	E. faecalis	S. aureus
1	25	25	6.25	12.5
2	6.25	12.5	3.13	0.78
3	6.25	>25	6.25	12.5

) varied among the complexes and strains, with the complexes generally exhibiting a higher activity against Gram-positive bacteria. The Cu(II) complex (2) demonstrated the strongest antibacterial activity against all tested strains, with particularly high efficacy against S. aureus (0.78 g/L). Complex 2 is characterized by a dinuclear copper complex with distinct coordination geometries for the two copper atoms, which likely facilitates significant antimicrobial efficacy through potential synergistic interactions between the different coordination units. Dinuclear copper(II) complexes are known to possess enhanced antimicrobial properties compared to their mononuclear complexes [10], suggesting that the antimicrobial efficacy is highly dependent on the specific ligand environment and the overall structure of the complex. These findings underscore the critical role of the coordination environment and metal–ligand bonds in determining the biological activity of such complexes.

In our previous paper [22], we reported the MICs for three nickel and three copper complexes. Two of the nickel complexes, namely [Ni(tda)(phen)(H₂O)]·3H₂O and [Ni₂(μ-tda)₂(1,2-pn)₂], were active against E. faecalis with an MIC of 8.44 g/L. The third complex, [Ni(tda)(1,3-pn)(H₂O)]·H₂O, showed the same MIC (8.44 g/L) for E. coli, E. faecalis, and S. Aureus. The copper complex [Cu₂(μ-tdp)(pmdien)₂(H₂O)₂](ClO₄)₂·H₂O was active against E. faecalis and S. aureus only (MIC = 16.88 g/L). Much better results, i.e., lower MIC values, were obtained for [Cu₂(μ-fu)(pmdien)₂(H₂O)₂](ClO₄)₂ (E. coli—8.44, P. aeruginosa—2.11, E. faecalis—4.22, and S. Aureus—1.05) and [Cu₂(μ-dtdp)(pmdien)₂(H₂O)₂](ClO₄)₂ (E. coli—16.88, P. aeruginosa—1.05, E. faecalis—8.44, and S. Aureus—8.44). When we compare the MIC values with those presented in this paper, we can conclude that complex 1 with phen shows the same activity as [Ni(tda)(phen)(H₂O)]·3H₂O against E. faecalis, but 1 is active to all bacterial strains. Binuclear copper complex 2 is very similar to [Cu₂(μ-tdp)(pmdien)₂(H₂O)₂](ClO₄)₂·H₂O, where tridentate ligand pmdien is exchanged to mdpta. Complex 2 shows a lower MIC for E. faecalis (3.13) and S. Aureus (0.78), whereas the pmdien complex shows a better parameter for P. aeruginosa (1.05).

3. Materials and Methods

3.1. Chemicals and Methods

3,3′-dithiodipropionic acid (H₂dtdp), fumaric acid (H₂fu), 1,10-phenanthroline (phen), N′–methyldipropylenetriamine (mdpta), and N,N,N′,N″,N″-pentamethyldiethylenetriamine (pmdien) were supplied by Sigma-Aldrich (St. Louis, MO, USA) in ACS purity. The C, H, N, and S analyses were carried out on an EA Thermo Scientific Flash 2000. IR spectra (400–4000 cm⁻¹) were recorded with the help of an FT-IR spectrometer—Jasco FT/IR-4700 (Jasco, Easton, PA, USA)—using the ATR method. The diffuse-reflectance UV-Vis spectra (DR-UV/VIS) were obtained using a Cintra 3030 spectrometer (BGC Scientific Equipment, IL, USA) equipped with an integrated sphere assembly (barium sulfate as reference) in the range 200–800 nm. Electronic spectra were recorded at room temperature. The UV-VIS spectra of solutions of 1–3 (water:methanol = 1:1 v/v) were measured in 10 mm path length quartz cuvettes. The powder XRD patterns were measured using a Rigaku MiniFlex600 diffractometer (Rigaku, Austin, TX, USA) equipped with the Bragg–Brentano geometry and using Cu Kα radiation. EPR spectra were obtained with an Electron Paramagnetic Resonance Spectrometer (EPR) SPINSCAN X (LINEV systems), and measurements were carried out in the solid state with a polycrystalline powder sample. For the interpretation of data, programs Profex 5.2 (XPD) [39] and PHI 3.1.6 (EPR) [40] were used. For visualization, Mercury 2023.3.0 software was used [41].

3.2. Complex Preparation

3.2.1. Ni(dtdp)(H₂O)

The dithiodipropionate nickel(II) compound was prepared similarly to the published method for nickel thiodiacetate [21]. To a solution of H₂dtdp (4.2 g, 20 mmol) under stirring, a suspension of Ni(OH)₂ (1.86 g, 20 mmol) in water (250 mL) was added. The suspension was heated at 80 °C for 8 h. After filtration, the solution was left to crystallize. The light-green product was filtered off and dried in an oven at 50 °C. Yield: 5.67 g (96.6%). Anal. Calc.: C, 25.3; H, 3.5; S, 22.5. Found: C, 24.8; H, 3.7; S 22.2%.

FT-IR (ATR, cm⁻¹): 678 m, 839 w, 930 w, 1138 w, 1202 m, 1267 m, 1300 m, 1395 vs, 1555 vs, 1693 w, 3205 br.

3.2.2. [Ni(dtdp)(phen)(H₂O)₃]∙0.5H₂O (1)

The phen (0.20 g, 1 mmol) was added to a stirred solution of [Ni(dtdp)(H₂O)₃] (0.32 g, 1 mmol) in water (50 mL), and was heated at 50 °C for 2 h. After cooling, the solution was filtered, and the filtrate was left for crystallization. Blue crystals (suitable for X-ray analyses) were collected on a frit funnel, washed with water, and dried in air. Yield: 0.15 g (46%). Anal. Calc.: C, 43.1; H, 4.7; N, 5.6; S, 12.7. Found: C, 43.4; H, 4.5; N, 5.4; S 12.5%.

FT-IR (ATR, cm⁻¹): 424 w, 644 m, 672 m, 725 s, 770 w, 842 m, 1139 w, 1264 m, 1297 m, 1347 sh, 1403 vs, 1517 sh, 1555 vs, 1575 sh, 2917 sh, 3217 br.

DR-UV/VIS (λ_max, nm): 300, 605, 740.

3.2.3. [(ClO₄)(mdpta)Cu(μ-dtdp)Cu(mdpta)(H₂O)](ClO₄) (2)

The ligand mdpta (0.16 mL, 1 mmol) was added to a stirred solution of copper perchlorate hexahydrate (0.37 g, 1 mmol) in methanol (40 mL). The potassium salt of H₂tdp was added. The salt was prepared by the reaction of H₂dtdp (0.10 g) with KOH (0.05 g) in 5 mL of water. Potassium perchlorate was filtered off and the solution was left for crystallization. Dark blue crystals were obtained in a week. Yield: 0.36 g (48.0%). Anal. Calc.: C, 28.5; H, 5.7; N, 10.0; S, 7.6. Found: C, 28.2; H, 5.7; N, 9.9; S, 7.4%.

FT-IR (ATR, cm⁻¹): 487 w, 514 w, 620 s, 766 w, 927 m, 988 sh, 1015 sh, 1063 vs, 1268 w, 1395 s, 1433 m, 1466 w, 1557 s, 1597 m, 2899 m, 2925 m, 2965 m, 3148 m, 3230 m, 3268 s, 3321 s, 3501 w.

DR-UV/VIS (λ_max, nm): 280, 585.

3.2.4. [{Zn(pmdien)(H₂O)}₂(μ-fu)](ClO₄)₂ (3)

The ligand pmdien (0.2 mL, 1 mmol) was added to a stirred solution of zinc perchlorate hexahydrate (0.37 g, 1 mmol) in methanol (40 mL). The potassium salt of H₂fu, prepared as described above, was added. The formed potassium perchlorate was filtered off and the solution was left to crystallize. White crystals were obtained in a week. Yield: 0.34 g (58%). Anal. Calc.: C, 32.1; H, 6.4; N, 10.2. Found: C, 31.5; H, 6.1; N, 9.6%.

FT-IR (ATR, cm⁻¹): 438 w, 456 w, 489 w, 589 w, 623 s, 676 m, 760 w, 776 w, 800 m, 905 m, 932 s, 972 s, 1024 sh, 1044 vs, 1096 vs, 1248 w, 1309 w, 1368 sh, 1383 s, 1441 sh, 1471 s, 1525 s, 2823 w, 2885 m, 2983 w, 3133 w, 3399 s.

DR-UV/VIS (λ_max, nm): 228.

3.3. Determination of Crystal Structures

The diffraction experiment for the crystal structure determination of 1 and 3 was performed on an XtaLAB Synergy Dualflex diffractometer equipped with a Pilatus 300K detector, while that of 2 was performed on a Bruker D8 VENTURE Kappa Duo with a PHOTONIII detector. The structure was solved by intrinsic methods (SHELXT) [42] and refined by full-matrix least squares based on F² (SHELXL) [43]. The hydrogen atoms on carbon and nitrogen were fixed into idealized positions (riding model) and were assigned temperature factors of either H_iso(H) = 1.2 U_eq (pivot atom) or H_iso(H) = 1.5 U_eq (pivot atom for methyl moiety). The hydrogen atoms of the water molecule were found on a difference Fourier map and were refined under a rigid-body assumption with assigned temperature factors H_iso(H) = 1.2 U_eq (pivot atom). The outer coordination sphere chlorate anions in the structures of 2 and 3 were distorted over two positions with occupancy factors of 0.54:0.46 and 0.66:0.34, respectively. In 3, there are also three carbon atoms of pmdien (C3, C4, and C9), which are distorted over two positions (0.54:0.46). Details concerning crystal data and refinement are given in

Table 2. Detailed information about crystallographic experiments regarding studied complexes.

Compound (CCDC Number).	1 (2369177)	2 (2314973)	3 (2369178)
Empirical formula	C36H46N4Ni2O15S4	C20H48Cl2Cu2N6O13S2	C11H26ClN3O7Zn
Formula weight	1020.43	842.74	826.33
Temperature (K)	100.3 (5)	120 (2)	100.0 (1)
Crystal system	orthorhombic	monoclinic	monoclinic
Space group	Pbca	Cc	P21/c
a (Å)	22.0565 (1)	15.1199 (5)	8.1853 (1)
b (Å)	14.4105 (1)	11.2259 (4)	15.4006 (2)
c (Å)	27.4138 (1)	20.9787 (6)	14.2517 (2)
α (°)	90	90	90
β (°)	90	105.828 (1)	101.713 (2)
γ (°)	90	90	90
Volume (Å3)	8713.34 (8)	3425.80 (19)	1759.14 (4)
Z	8	4	2
ρcalc (g/cm3)	1.556	1.634	1.560
μ (mm−1)	3.481	1.586	1.584
F(000)	4240	1752	864
Crystal size (mm3)	0.469 × 0.081 × 0.051	0.166 × 0.116 × 0.108	0.427 × 0.229 × 0.207
Radiation	Cu Kα (λ = 1.54184)	Mo Kα (λ = 0.71073)	Mo Kα (λ = 0.71073)
θ range for data collect. (°)	3.224 to 78.895	2.018 to 27.482	2.645 to 31.581
Index ranges	−28 ≤ h ≤ 27,	−19 ≤ h ≤ 17,	−10 ≤ h ≤ 11,
	−18 ≤ k ≤ 16,	−14 ≤ k ≤ 14,	−22 ≤ k ≤ 20,
	−34 ≤ l ≤ 33	−27 ≤ l ≤ 27	−19 ≤ l ≤ 20
Reflections collected/independent	201405/9260	28941/7369	59565/5458
Rint	0.0734	0.0305	0.0305
Data/restraints/parameters	9260/21/592	7369/26/427	5458/14/280
Goodness-of-fit on F2	1.100	1.056	1.071
Final R indexes [I > 2s(I)]	R1 = 0.0408,	R1 = 0.0174,	R1 = 0.0297,
	wR2 = 0.1077	wR2 = 0.0426	wR2 = 0.0764
R indexes (all data)	R1 = 0.0426,	R1 = 0.0179,	R1 = 0.0366,
	wR2 = 0.1088	wR2 = 0.0429	wR2 = 0.0809
Largest diff. peak and hole (e·Å−3)	0.905/−0.725	0.319/−0.259	1.179/−0.440
Flack parameter	-	0.020 (3)	-

.

CCDC 2314973, 2369177, and 2369178 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge at https://www.ccdc.cam.ac.uk/ [or from the Cambridge Crystallographic Data Centre (CCDC), 12 Union Road, Cambridge, CB2 1EZ, UK; fax: +44(0) 1223 336 033; email: [email protected]].

3.4. Determination of Antibacterial Activity

The standard dilution method was used to determine the antimicrobial activity and minimum inhibition concentration (MIC) of the complexes. For the purpose of antimicrobial testing, the water solution of each sample was used and diluted by the culture medium (Mueller–Hinton Broth, BD Difco, France) in a geometric progression from 2 to 1024 times in microtiter plates. The culture medium was inoculated with the tested bacteria at a concentration of 5 × 10⁵ CFU/mL and incubated for 24 h at 35+/−1 °C. After 18+/−2 h, the MIC was determined according to standard testing protocols (EUCAST) as the lowest concentration of the tested substance, inhibiting the visible growth of the tested bacterial strain. Escherichia coli CCM 3954, Pseudomonas aeruginosa CCM 3955, Staphylococcus aureus CCM 4223, and Enterococcus faecalis CCM 4224 were purchased from the Czech Collection of Microorganisms (Brno, Czech Republic) and were stored in cryotubes (ITEST plus, Czech Republic) at –80 °C.

4. Conclusions

Three novel complexes of dicarboxylic acids H₂dptm and H₂fu—mononuclear Ni(II) complex 1, asymmetric dinuclear Cu(II) complex 2, and symmetric Zn(II) complex 3—were prepared. Their structures were determined by X-ray structural analysis, and complexes were characterized by a wide array of methods, including CHNS analysis, FTIR spectroscopy, UV-Vis spectroscopy, and in the case of Cu(II) complex, also EPR spectroscopy and powder X-ray diffraction.

Biological experiments revealed that minimal inhibition concentrations among the studied complexes on all bacterial lines are the lowest for complex 2. It can be assumed that a role in its antibacterial properties has synergic effects between two non-symmetric Cu(II) centers. On the opposite side, the highest inhibition concentrations are for mononuclear complex 1.

The variability of the obtained structures displays the versatile nature of the used ligands. It can be noted that these results show that using suitable N-donor ligands in combination with dicarboxylic acid can lead to interesting dinuclear systems, which can be used for the further tuning of similar systems to potentially improve their bioactivity.