The mixed ligand complexes of Co(II), Ni(II), Cu(II) and Zn(II) with coumarilic acid/1,10-phenanthroline

Abstract

The coumarilate (coum⁻) and 1,10-phenanthroline (phen) mixed ligand complexes of Co(II) (1), Ni(II) (2) Cu(II) (3) and Zn(II) (4) were synthesized and structural characterizations were performed by using elemental analysis, magnetic susceptibility, solid-state UV–Vis, FTIR spectra, thermoanalytical TG-DTG/DTA and single-crystal X-ray diffraction methods. The Co(II) and Ni(II) complexes are salt-type compounds, and they have two moles phen ligands bound as bidentate, two moles aqua ligands in coordination sphere and two moles anionic coum⁻ ligand outside of the coordination unit as the counter-ion of the molecular structure. At the same time, the Co(II) and Ni(II) complexes have five moles of aqua ligands as hydrated water outside of the molecules. It was obtained that Cu(II) and Zn(II) complex structures contain one mole of phen ligand, two moles coordinated (coum⁻) ligand and a one-mole aqua ligand, and the molecules (Cu(II) and Zn(II)) have fivefold structure and obey square pyramidal geometry. Thermal decomposition of each complex started with dehydration (the first removal is the dehydration of complex Co(II) and Ni(II) as removal of hydrate–aqua molecules), and then, the decomposition of organic parts was observed. The thermal dehydration of the complexes takes place in one (Cu(II) and Zn(II)) or two (Co(II) and Co(II)) steps. The decomposition mechanism, thermal decomposition steps and thermal stability of the investigated complexes provide useful data for the interpretation of their structures. The final decomposition products were found to be metal oxides. Some biological applications (antifungal/antibacterial) were performed using structurally characterized compounds.

Introduction

Oxygen-containing heterocyclic compounds are well known to exhibit important biological properties such as antiarrhythmic, spasmolytic, antiviral, anticancer, antifungal and anti-inflammatory activity [1,2,3,4,5,6,7]. Today, many of the natural or synthetic derivatives with high oxygen content have pharmaceutical applications [8, 9]. Benzofuran rings and their derivatives from these compounds are also found in our lives in various fields [10]. Coumarilic acid is one of these derivatives and formed from by linking of the 2-furan carboxylic acid ring with the benzene ring (Scheme 1a).

Scheme 1

The structure of a coumarilic acid, b 1,10-phenanthroline molecules

Its general name is benzofuran-2-carboxylic acid also called as coumarilic acid. The theoretical formula of the compound is C₉H₆O₃ with a molecular weight of 162.14 g/mol, and its melting point ranges from 193 to 196 °C. Coumarilic acid is a derivative of coumarin and found in different food sources: fruits, herbs and vegetables [11]. The benzo[b] furan system, as an important pharmacophore, can be found in many compounds and isolated from natural sources. It is also used as a basic part of recently synthesized medicines (such as amiodarone and bergapten) [12, 13].

Although there are many literature data about coumarilic acid complexes of various metal cations, studies on a mixed ligand of coumarilic acid and metal cations are very limited [14, 15]. In the course of the studies, mixed ligand complexes containing single-liganded coumarilic acid and coumarilic acid–nicotinamide ligands were synthesized and examined. The presence of a wide range of physiological and biological effects of coumarilic acid and the ligands used is important for both medical and inorganic chemistry and increases the importance of this study.

1,10-Phenanthroline selected as the neutral ligand of counter-ligand complexes, as shown in Scheme 1b, is the name used to denote heterocyclic ring systems. These heterocyclic rings are formed by the replacement of –CH groups in the phenanthrene ring system by –N=groups. These ring systems are termed as 4,5-diazaphenanthrene, 1,5-diazaphenanthrene and 1,8-diazaphenanthrene.

1,10-Phenanthroline has a planar heterophasic structure and forms stable complexes with transition metals. These stable complexes are widely used in the design of many electronic devices such as field-effect transistors, light-emitting diodes (LED), lasers and photovoltaic batteries [16,17,18]. 1,10-Phenanthroline–copper(II) complexes and their derivatives are of great interest because of their many biological effects such as inhibition of microbicidal activity, cancer and tumor formation [19].

In this study, it was aimed to synthesize mixed ligand coordination compounds containing ligands of coumarilic acid–1,10-phenanthroline of Co(II), Ni(II), Cu(II) and Zn(II) transition metals. The structures of these synthesized complexes were characterized by elemental analysis, Fourier transforms infrared spectroscopy (FTIR), thermogravimetric analysis (TGA/DTA), single-crystal X-ray diffraction diffractometry (SC-XRD), solid-state ultraviolet–visible region spectroscopy (UV–Vis), magnetic susceptibility and melting point determination methods. Further biological experiments were applied to the complexes.

Experimental

Materials and methods

The chemicals Co(CH₃COO₂)₂·4H₂O Ni(CH₃COO₂)₂·4H₂O, Cu(CH₃COO₂)₂·H₂O, Zn(CH₃COO₂)₂·2H₂O, coumarilic acid and 1,10-phenanthroline used in the synthesis of complexes were obtained from the company Sigma-Aldrich. Elemental analysis (C, H, N) was being carried out by standard methods.

Firstly, coumarilate salt compounds with natrium cation were synthesized by 0.005 mol of coumarilic acid dissolved in a 50:50 (v/v) EtOH/H₂O solution in a beaker followed by the addition of 0.005 mol of NaHCO₃ providing CO₂ output. Then, the 1,10-phenanthroline solution (0.005 mol 25 mL H₂O) was first added to the salt solutions and mixed for 1 h in the magnetic stirrer. Finally, the metal–nitrate salts (0.0025 mol) in solid phase were appended to the main solution in 1:2 ratio concerning the coumarilate anion and mixed for 5 h at 60 °C (Scheme 2, Eq. 2).

Scheme 2

Synthesis reaction of metal/coumarilate/1,10-phenanthroline coordination compounds

The final solution was allowed to crystallize at atmospheric pressure by standing at room temperature. The crystals precipitated in approximately 20–30 days were collected by filtration. The metal/ligand/ligand ratio were considered as 1:2:2 for Co(II) and Ni(II) compounds and 1:2:1 for Cu(II) and Zn(II) compounds obtained.

Magnetic susceptibility measurements were taken at room temperature using a Sherwood Scientific MXI model Gouy magnetic balance. Infrared spectra were recorded in the 4000–400 cm⁻¹ region with a PerkinElmer Spectrum One FTIR spectrophotometer using KBr pellets. (Approximately 0.03 grams of material was used for each pellet.) Thermal analyses (TG, DTA) were performed by the Shimadzu DTG-60H system, in a dynamic nitrogen atmosphere (100 mL min⁻¹) at a heating rate of 10 °C min⁻¹, in platinum crucibles as the sample vessel, using a-Al₂O₃ as a reference.

Structural characterizations of these synthesized complexes were characterized by elemental analysis, Fourier transforms infrared spectroscopy (FTIR), thermogravimetric analysis (TGA/DTA), single-crystal X-ray diffraction diffractometry (SC-XRD), solid-state ultraviolet–visible region spectroscopy (UV–Vis–NIR), magnetic susceptibility and melting point determination methods. The biologic activation studies of the elucidated molecules have been studied in cell culture medium.

The antimicrobial effects of the elucidated molecules have been studied in Mueller–Hinton agar (MHA) medium as biological applications. Five different microorganisms were used in this study. These microorganisms were obtained from the culture collection of Molecular Biology and Genetics Department at Hitit University. One fungus (Candida albicans ATCC 10231), two gram-positive bacteria (Enterococcus faecalis ATCC 29212 and Staphylococcus aureus ATCC 6538) and two gram-negative bacteria (Pseudomonas aeruginosa ATCC 27853 and Escherichia coli ATCC 25922) were used as microorganisms.

The substances to be antimicrobial affected (0.001 g/10 mL) were prepared and passed through microfilters with a porosity of 0.45 μm and sterilized. 25-μL disks were adsorbed from the materials prepared using 10% DMSO (0.001 g substance/10 mL of 10% DMSO). 10% DMSO was used as a control. It was incubated for 24 h at 37 °C, and the inhibition zone diameters around the disks after incubation were measured as mm ± SD (standard deviation). Ampicillin and fluconazole antibiotic disks were used to compare the results with as standard.

Results and discussion

The elemental analysis data, decomposition points, magnetic susceptibility, colors and yield calculations of mixed ligand metal complexes are listed in Table 1.

Table 1 Analytical data of metal complexes

FTIR analysis

The some important FTIR stretching and bending peaks of the complexes are summarized in Table 2, and the infrared curves are shown in Fig. 1. As the ligand of coumarilic acid in the synthesized complexes is bound to metal cations, a shift toward the right side is observed in the absorption bands corresponding to the functional groups. In the case of IR spectra, the strong and broadband observed around 3600–2900 cm⁻¹ comes from the presence of the –OH group in the constructions of the complexes. The C=O group derived from the carboxylic acid group in the metal complexes gives stretching pulses of 1634 cm⁻¹ for the Co(II) complex, 1608 cm⁻¹ for the Ni(II) complex, 1611 cm⁻¹ for the Cu(II) complex and 1616 cm⁻¹ for the Zn(II) complex. The aromatic C=C tensile vibration is observed at 3060–3064 cm⁻¹. The aromatic C–H stretches in the complexes give stretching vibrations between 3373 and 3447 cm⁻¹. Carboxylic acid COO⁻ asymmetric and symmetrical absorption bands correspond to stretching vibrations in the 1555–1560 cm⁻¹ and 1387–139 cm⁻¹ areas, respectively. The absorption bands corresponding to the metal–oxygen (M–O) bonds forming the basis of the complexes were observed for aqua ligands at 433 cm⁻¹ in Co(II), 427 cm⁻¹ in Ni(II), 428 cm⁻¹ in Cu(II) and 433 cm⁻¹ in Zn(II) complexes, respectively. Also, just two complexes have (M–O) bonds stretching vibrations for the coumarilate ligand, Cu(II) and Zn(II) complexes, so coumarilate ligands were coordinated to metal cations in these compounds. But in the other two complexes are the salt type, so the anionic coumarilate ligands did not coordinate to metal, Co(II) and Ni(II), cations. The vibrations of the metal–nitrogen (M–N) bonds in the complex are 588 cm⁻¹ for the Co(II) complex, 583 cm⁻¹ for the Ni(II) complex, 576 cm⁻¹ for Cu(II) complex and 588 cm⁻¹ for Zn(II) complex.

Table 2 FTIR spectra of metal–coumarilate–1,10-phenanthroline mixed ligand complexes

Fig. 1

The FTIR curves of complexes, for Co(II) is (1), for Ni(II) is (2), for Cu(II) is (3) and for Zn(II) is (4)

Thermal properties

The curves of TG, DTG and DTA for thermal characterization of complexes are shown in Fig. 2a–d. The details of the decomposition steps are listed in Table 3.

Fig. 2

Thermal analysis curves of molecules a Co(II) complex, b Ni(II) complex, c Cu(II) complex and d Zn(II) complex

Table 3 Thermoanalytical data (TG-DTG/DTA) for the metal complexes

When the Co(II) mixed ligand complex is examined on the DTG curve, thermal degradation occurs in four steps. The initial temperatures of the decays were determined as 40, 70, 237 and 332 °C. In the first decay step, it is thought that a molar water ligand has been removed from the structure.

$$[{\text{Co}}({\text{C}}_{12} {\text{H}}_{8} {\text{N}}_{2} )_{2} ({\text{H}}_{2} {\text{O}})_{2} ]({\text{C}}_{9} {\text{H}}_{5} {\text{O}}_{3} )_{2} \cdot 5{\text{H}}_{2} {\text{O}}_{{({\text{s}})}} \mathop{\longrightarrow}\limits^{{40 - 69\,^\circ {\text{C}}}}\,[{\text{Co}}({\text{C}}_{12} {\text{H}}_{8} {\text{N}}_{2} )_{2} ({\text{H}}_{2} {\text{O}})_{2} ]({\text{C}}_{9} {\text{H}}_{5} {\text{O}}_{3} )_{2} \cdot 4{\text{H}}_{2} {\text{O}}_{{({\text{s}})}} + {\text{H}}_{2} {\text{O}}_{{({\text{g}})}} .$$

(R.1)

At the temperature range of 70–147 °C, the remained six moles (2 mol coordination and 4 mol hydrated water) of the aqua ligands are completely separated from the structure at the 93 °C dehydration step.

$$[{\text{Co}}({\text{C}}_{12} {\text{H}}_{8} {\text{N}}_{2} )_{2} ({\text{H}}_{2} {\text{O}})_{2} ]({\text{C}}_{9} {\text{H}}_{5} {\text{O}}_{3} )_{2} \cdot 4{\text{H}}_{2} {\text{O}}_{{({\text{s}})}} \mathop{\longrightarrow}\limits^{{70 - 147\,^\circ {\text{C}}}}\,[{\text{Co}}({\text{C}}_{12} {\text{H}}_{8} {\text{N}}_{2} )_{2} ]({\text{C}}_{9} {\text{H}}_{5} {\text{O}}_{3} )_{{2({\text{s}})}} \cdot 6{\text{H}}_{2} {\text{O}}_{{({\text{g}})}} .$$

(R.2)

Two moles of 1,10-phenanthroline ligand and two moles of the coumarilate ligand are degraded in the temperature range of 237–863 °C. As a decomposition product, there remains a black CoO compound.

$$[{\text{Co}}({\text{C}}_{12} {\text{H}}_{8} {\text{N}}_{2} )_{2} ]({\text{C}}_{9} {\text{H}}_{5} {\text{O}}_{3} )_{{2({\text{s}})}} \mathop{\longrightarrow}\limits^{{237 - 863\,^\circ {\text{C}}}}{\text{CoO}}_{{({\text{s}})}} + 2{\text{C}}_{12} {\text{H}}_{8} {\text{N}}_{{2({\text{decomp}})}} + 2{\text{C}}_{9} {\text{H}}_{5} {\text{O}}_{{3({\text{decomp}})}} .$$

(R.3)

The DTG curve complex of the Ni(II) mixed ligand complex 54, 97, 260; 459, 644 and 831 °C, corresponding to the maximum temperatures. In the first decay step (at 37–70 °C), 1 mol of water ligand is moving away from the structure.

$$[{\text{Ni}}({\text{C}}_{12} {\text{H}}_{8} {\text{N}}_{2} )_{2} ({\text{H}}_{2} {\text{O}})_{2} ]({\text{C}}_{9} {\text{H}}_{5} {\text{O}}_{3} )_{2} \cdot 5{\text{H}}_{2} {\text{O}}_{{({\text{s}})}} \mathop{\longrightarrow}\limits^{{37 - 70\,^\circ {\text{C}}}}\,[{\text{Ni}}({\text{C}}_{12} {\text{H}}_{8} {\text{N}}_{2} )_{2} ({\text{H}}_{2} {\text{O}})_{2} ]({\text{C}}_{9} {\text{H}}_{5} {\text{O}}_{3} )_{2} \cdot 4{\text{H}}_{2} {\text{O}}_{{({\text{s}})}} + {\text{H}}_{2} {\text{O}}_{{({\text{g}})}} .$$

(R.4)

Similar to Co(II) complex, at the temperature range of 71–164 °C, all remaining aqua ligands (6 mol) are completely got away from the structure as a decomposition step.

$$[{\text{Ni}}({\text{C}}_{12} {\text{H}}_{8} {\text{N}}_{2} )_{2} ({\text{H}}_{2} {\text{O}})_{2} ]({\text{C}}_{9} {\text{H}}_{5} {\text{O}}_{3} )_{2} \cdot 4{\text{H}}_{2} {\text{O}}_{{({\text{s}})}} \mathop{\longrightarrow}\limits^{{71 - 164\,^\circ {\text{C}}}}\,[{\text{Ni}}({\text{C}}_{12} {\text{H}}_{8} {\text{N}}_{2} )_{2} ]({\text{C}}_{9} {\text{H}}_{5} {\text{O}}_{3} )_{{2({\text{s}})}} + 6{\text{H}}_{2} {\text{O}}_{{({\text{g}})}} .$$

(R.5)

Two moles of 1,10-phenanthroline ligand and two moles of coumarilate ligand decay in the temperature range 235–877 °C, leaving the medium. As a decay product, there remains a black NiO compound.

$$[{\text{Ni}}({\text{C}}_{12} {\text{H}}_{8} {\text{N}}_{2} )_{2} ]({\text{C}}_{9} {\text{H}}_{5} {\text{O}}_{3} )_{{2({\text{s}})}} \mathop{\longrightarrow}\limits^{{235 - 877\,^\circ {\text{C}}}}{\text{NiO}}_{{({\text{s}})}} + 2{\text{C}}_{12} {\text{H}}_{8} {\text{N}}_{{2({\text{decomp}})}} + 2{\text{C}}_{9} {\text{H}}_{5} {\text{O}}_{{3({\text{decomp}})}} .$$

(R.6)

The Cu(II) coordination compound exhibits three decomposition steps. These steps start at the 102, 191 and 212 °C, and the first degradation step can be attributed to the removal of one-mole aqua ligand in the coordination sphere from the structure.

$$[{\text{Cu}}({\text{C}}_{9} {\text{H}}_{5} {\text{O}}_{3} )_{2} ({\text{C}}_{12} {\text{H}}_{8} {\text{N}}_{2} )({\text{H}}_{2} {\text{O}})_{2} ]_{{({\text{s}})}} \mathop{\longrightarrow}\limits^{{102 - 150\,^\circ {\text{C}}}}\,[{\text{Cu}}({\text{C}}_{9} {\text{H}}_{5} {\text{O}}_{3} )_{2} ({\text{C}}_{12} {\text{H}}_{8} {\text{N}}_{2} )]_{{({\text{s}})}} + {\text{H}}_{2} {\text{O}}_{{({\text{g}})}} .$$

(R.7)

At the temperature range of 191–743 °C, two molecules of 1,10-phenanthroline ligand, which is the organic structure of coordination complex, and two moles of coumarilate ligands burn away from the medium. As the last product, there remains a black CuO compound.

$$[{\text{Cu}}({\text{C}}_{9} {\text{H}}_{5} {\text{O}}_{3} )_{2} ({\text{C}}_{12} {\text{H}}_{8} {\text{N}}_{2} )]_{{({\text{s}})}} \mathop{\longrightarrow}\limits^{{191 - 743\,^\circ {\text{C}}}}{\text{CuO}}_{{({\text{s}})}} {\text{ + 2C}}_{9} {\text{H}}_{5} {\text{O}}_{{3({\text{decomp}})}} + {\text{C}}_{12} {\text{H}}_{8} {\text{N}}_{{2({\text{decomp}})}} .$$

(R.8)

The Zn(II) complex also degrades at the three decomposition stages (114; 277 and 312 °C), as is the case in the Cu(II) complex structure. The first disruption step in the range 144–186 °C is due to the removal of one-mole aqua ligand in the coordination cage.

$$[{\text{Zn}}({\text{C}}_{9} {\text{H}}_{5} {\text{O}}_{3} )_{2} ({\text{C}}_{12} {\text{H}}_{8} {\text{N}}_{2} )({\text{H}}_{2} {\text{O}})]_{{({\text{s}})}} \mathop{\longrightarrow}\limits^{{144 - 186\,^\circ {\text{C}}}}\,[{\text{Zn}}({\text{C}}_{9} {\text{H}}_{5} {\text{O}}_{3} )_{2} ({\text{C}}_{12} {\text{H}}_{8} {\text{N}}_{2} )]_{{({\text{s}})}} + {\text{H}}_{2} {\text{O}}_{{({\text{g}})}} .$$

(R.9)

The dehydrated complex structure burns at a temperature range of 277–821 °C. In this step, the two moles of 1,10-phenanthroline and two moles of the coumarilate ligands are separated from the structure by forming possible CO₂/CO₂/NO₂/NO₂ gases as they are in other structures. Leaving behind the gray colored ZnO compound remained in the reaction crucible as the final decomposition product.

$$[{\text{Zn}}({\text{C}}_{9} {\text{H}}_{5} {\text{O}}_{3} )_{2} ({\text{C}}_{12} {\text{H}}_{8} {\text{N}}_{2} )]_{{({\text{s}})}} \mathop{\longrightarrow}\limits^{{277 - 821\,^\circ {\text{C}}}}{\text{ZnO}}_{{({\text{s}})}} + 2{\text{C}}_{9} {\text{H}}_{5} {\text{O}}_{{3({\text{decomp}})}} + {\text{C}}_{12} {\text{H}}_{8} {\text{N}}_{{2({\text{decomp}})}} .$$

(R.10)

All remaining relevant oxides were investigated with IR spectra, and the claim was supported.

X-ray diffraction analysis

Suitable crystals of C₄₂H₄₀CoN₄O₁₃ (1), C₄₂H₄₀N₄NiO₁₃ (2), C₃₀H₂₀CuN₂O₇ (3) and C₃₀H₂₀N₂O₇Zn (4) were selected for data collection which was performed on a Bruker APEX-II diffractometer equipped with graphite-monochromatic Mo-K_α radiation. The structures were solved by direct methods using SHELXS-97 [20] and refined by full-matrix least-squares methods on F² using SHELXL-97 [20] from within the WINGX [21] suite of software. All non-hydrogen atoms were refined with anisotropic parameters. The H atoms of C atoms were located from different maps and then treated as riding atoms with C–H distances of 0.93 Å. The water H atoms were located in a difference map refined freely. Molecular diagrams were created using MERCURY [22]. Supramolecular analyses were performed, and the diagrams were prepared with the aid of PLATON [23]. Details of data collection and crystal structure determinations are given in Table 4.

Table 4 Crystal data and structure refinement parameters for complexes 1–4

Complexes 1 and 2

The molecular structures of complexes 1–2, with the atom-numbering schemes, are shown in Figs. 3 and 4. The asymmetric units of 1–2 contain one M(II) ion (M(II) = Co(II) in 1 and Ni(II) in 2), two phenanthroline ligands, two non-coordinated coumarilate ligands, five non-coordinated water molecules and two aqua ligands. Each M(II) ion is coordinated by four nitrogen atoms of two different phenanthroline ligands and two oxygen atoms from aqua ligands. The coordination geometry around the M(II) cation can be described as a distorted octahedral geometry. The M–O bond lengths are 2.074 (5) and 2.091 (7) Å in 1 and 2.059 (5) and 2.063 (5) Å in 2, respectively. The M–N bond lengths are ranged between 2.140 (5)–2.157 (5) Å in 1 and 2.079 (5)–2.093 (5) Å in 2, respectively (Table 5). The 3D supramolecular network of molecules is shown in Fig. 5 for complex 1 and Fig. 6 for complex 2.

Fig. 3

The molecular structure of complex 1 showing the atom-numbering scheme

Fig. 4

The 3D supramolecular network in 1

Table 5 Selected bond distances and angles for complexes 1–4 (Å, °)

Fig. 5

The molecular structure of complex 2 showing the atom-numbering scheme

Fig. 6

The formation of R ⁸₈ (16) ring in 2

Molecules of 1 are linked by C–H···O hydrogen bonds, while molecules of 2 are linked into sheets by a combination of O–H···O and C–H···O hydrogen bonds (Table 6). In 2, strong hydrogen bonds are observed between coumarilate ligands and water oxygen atoms, with the O···O distances ranging from 2.664 (9) to 2.83 (3) Å. The combination of O–H···O hydrogen bonds produces R ⁸₈ (16) ring (Fig. 7). All of these intermolecular interactions give three-dimensional framework results.

Table 6 Hydrogen-bond parameters for complexes 1–4 (Å, °)

Fig. 7

The 3D supramolecular network in 2

Complexes 3 and 4

The molecular structures of complexes 3–4, with the atom-numbering scheme, are shown in Fig. 8. The asymmetric units of 3–4 contain one M(II) cation, one phenanthroline ligand, two coumarilate ligands and one aqua ligand. Each M(II) ion is coordinated by two nitrogen atoms of phenanthroline ligand, two oxygen atoms of two different coumarilate ligands and one oxygen atom from the aqua ligand. The coordination geometry around the M(II) cation can be described as a distorted square pyramidal geometry. The M–O_carboxyl bond lengths are 1.9367 (15) and 2.2872 (15) Å in 3 and 1.9972 (16) and 2.0316 (16) Å in 4, while the M–O_aqua bond length is 1.9864 (15) Å in 3 and 2.062 (2) Å in 4, respectively. The M–N bond lengths are 2.0032 (17) and 2.0244 (17) Å in 3 and 2.0981 (17) and 2.1543 (18) Å in 4, respectively (Table 5).

Fig. 8

The molecular structure of complexes 3–4 (M = Cu(II) in 3 and Zn(II) in 4) showing the atom-numbering scheme

Molecules of 4 are linked by a combination of O–H···O and C–H···O hydrogen bonds, while molecules of 3 are linked by C–H···O hydrogen bond (Table 6). In 4, atom C3 atom in the molecule at (x, y, z) acts as hydrogen-bond donor, via atom H3, to atom O4ⁱ, so forming a centrosymmetric R ²₂ (14) ring centered at (n + 1/2, 1/2, 1/2) [n = zero or integer] [(i) − x + 1, − y + 1, − z + 1]. Similarly, water O7 atom in the molecule at (x, y, z) acts as hydrogen-bond donor, via atom H7A, to atom O1ⁱⁱ, so forming centrosymmetric R ²₂ (8) ring centered at (n, 1/2, 1/2) [n = zero or integer] [(ii) − x, − y + 1, − z + 1]. The combination of these hydrogen bonds produces edge-fused R ²₂ (8)R ²₂ (14) rings which is running parallel to the [010] direction (Fig. 9). In 3, atom C12 atom in the molecule at (x, y, z) acts as hydrogen-bond donor, via atom H12, to atom O1ⁱ, so forming a centrosymmetric R ²₂ (14) ring centered at (1/2, 1/2, 1/2) [(i) − x + 1, − y + 1, − z + 1].

Fig. 9

The formation of edge-fused R ²₂ (8)R ²₂ (14) rings in 3

The solid-state UV–Vis spectroscopy

Electronic transition values were subtracted from the spectral patterns of the synthesized metal–coumarilate–1,10-phenanthroline mixed ligand complexes according to the visible region spectroscopy (UV–Vis) recorded at 900–200 nm (Fig. 10). The electronic transition peaks are listed in Table 7. According to these results, the d–d transitions of the Co(II) complex are seen at wavelengths of 632.21 nm (⁴T_1g → ⁴T_2g)(F) and 533.27 nm (⁴T_1g → ⁴T_1g)(P). The three spin-permuted d–d passages possessed by the Ni(II) complex were found to be 815.03 nm (³A_2g → ³T_1g)(P), 607.71 nm (³A_2g → ³T_1g)(F) and 441.26 nm (³A_2g → ³T_2g)(F) wavelengths, which indicate the splitting of the d orbitals of the Ni(II) metal cation to support the octahedral geometry.

Fig. 10

UV–Vis–NIR spectra of complexes 1–4

Table 7 The UV–Vis spectra of metal–coumarilate mixed ligand complexes

The spectrum for the Cu(II) complex is similar to the electronic transitions in other Cu(II) complexes in the literature [14, 15], but the peak intensity is higher. (This is seen when complexes of Cu(II) metal cations are compared.) So, the copper metal cation has a single electronic transition similar to the structure in the pseudo-octahedral, but that the energy difference between the bands with the transition is higher. For this reason, the transition can be attributed to the passage of the square pyramidal structure (A_1g → B_1g). The structure analyzed by single-crystal XRD results supports UV–Vis results. As shown in the magnetic susceptibility results, there is no d–d electron transition in the square pyramidal splitting which can occur when the orbitals in the last orbit of the metal cation in the Zn(II) complex, which has the diamagnetic property, are fully charged.

The absorption band seen at a low wavelength with high severity does not belong to the d–d transition, but the stronger energy metal → ligand (M → L) belongs to the charge transfer transition. Absorption bands at wavelengths of 247,38 and 311,11 nm for Co(II) complex, 249,77 and 325,41 nm for Ni(II) complex, 238,15 and 308,42 nm for Cu(II) complex, respectively, belong to metal → ligand (M → L transitions. The strong DTGs at the wavelengths of 247.17 and 311.23 nm for the Zn(II) complex belong to ligand → metal (L → M) charge transfer. All of the solid-phase data of metal cation complexes are suitable for the literature [24,25,26].

Biological applications

The antimicrobial (antibacterial, antifungal) activity of the synthesized complexes is given in Table 8.

Table 8 Some biological applications results of complexes

Co(II) complex was found to be effective on Candida albicans and also low inhibition effect on Enterococcus faecalis and Escherichia coli. Ni(II) complex was found to be effective in Candida albicans and also the low inhibitory effect on Gram-negative microorganisms (Escherichia coli and Pseudomonas aeruginosa). Cu(II) complex was found to be effective on Candida albicans and also low inhibition effect on Staphylococcus aureus, Enterococcus faecalis and Escherichia coli. Zn(II) complex was found to be effective in Candida albicans, and a low rate of inhibition was also found in Staphylococcus aureus, Escherichia coli and Pseudomonas aeruginosa.

Conclusions

The mixed ligand complexes of cobalt (II), nickel (II), copper (II) and zinc (II) metal cation were synthesized with coumarilic acid and 1,10-phenanthroline ligands. The ratio of metal/coumarilate/1,10-phenanthroline ligands was determined as 1:2:2 in Co(II) and Ni(II) complexes and 1:2:1 in Cu(II) and Zn(II) complexes. The Co(II) and Ni(II) cations in the structures have completed their octahedral sphere with two moles bidentate bonded 1,10-phenanthroline and two moles aqua as a ligand by entering the coordination sphere. The both of the structures include four moles hydrated water molecules but they are not isostructural, so the water molecules outside of the coordinate system are located in different spatial regions. According to the structural analysis results obtained, the complexes of 3 and 4, whose structure was identified, were determined as isostructural and their coordination modes are square pyramidal. The complexes of 1 and 2 are salt type, and two moles coumarilate anions are settled outside of the coordination spheres that are counter-ions of metal cations. 1 and 2 are similar to each other because their thermal degradation steps are similar to each other, whereas structures 3 and 4 are isostructures, the same. The biological activation studies of the elucidated molecules have been studied in cell culture medium. Synthesized molecules have been found to have higher activity, especially against Candida albicans fungus. It has been observed that these activities are close to the effect of gentamicin and fluconazole drugs present in the market.