Introduction

The cyclic compounds having a heteroatom and the derivatives have been used in the field of medicine and agriculture. The coumarilic acid having benzofuran and derivatives are the examples used in these and also in various fields of our life [1]. Coumarilic acid (1-benzofuran-2-carboxylic acid) is formed by the combination of 2-furan carboxylic acid ring having benzene nucleus.

Other name for this compound is benzofuran-2-carboxylic acid having the empirical formula of C9H6O3 and the molecular mass of 162.14 g mol−1. The melting point of benzofuran-2-carboxylic acid ranges from 193 to 196 °C.

It is well known that many of the heterocyclic compounds having oxygen show significant biological properties such as antiarrhythmic, spasmolytic, antiviral, anticancer, antifungal and anti-inflammatory activity [2,3,4,5,6,7,8]. Currently, there are many pharmaceutical applications on the use of natural or synthetic derivatives [9, 10]. Although there have been numerous studies on derivatives of coumarilic acid [11,12,13], the number of studies on metal complexes has been limited. In the study conducted by Kose et al. [14], the synthesis of CoII- and ZnII-coumarilate–nicotinamide mixed ligand complexes were performed and characterized.

Drzewiecki et al. [15] have synthesized the Cu(II) complexes by electrochemical methods. Three different coumarilic acid derivatives were used for the complex formation. These are complexes of 7-acetyl-6-methoxy-3-methyl-benzo[b]furan-2-carboxylic acid (HL1), 7-acetyl-5-bromo-6-hydroxy-3-methyl-benzo[b]furan-2-carboxylic acid (HL2) and 6-acetyl-5-hydroxy-2-methyl-benzo[b]furan-3-carboxylic acid (HL3) [16,17,18].

Within the scope of the complexation reactions of coumarilic acid, the synthesis and characterization of coordination compounds having NiII, CoII, ZnII, MnII [8, 11], CuII [10], CeIII, NdIII [19], LaIII [20, 21], DyIII, ErIII, EuIII, GdIII, TbIII, SmIII [22, 23] were studied. Despite the many studies about the metal complexes of coumarilic acid, there are a limited number of studies on the crystal structures [10, 11, 14, 19]. Because the coumarilic acid has broad physiological and biological effects, it plays very important role in medicinal inorganic chemistry [24, 25].

In this study, the coordination compounds of coumarilic acid having CoII, NiII, CuII and ZnII transition metals were synthesized. The molecular structures of the complexes synthesized were determined via elemental analysis, Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TG/DTA), single-crystal X-ray diffraction diffractometry (SC-XRD), solid ultraviolet–visible spectroscopy (UV–Vis), magnetic susceptibility and melting point detection methods.

Materials and methods

The chemicals Co(CH3COO)2.4H2O, Ni(CH3COO)2.4H2O, Cu(CH3COO)2.H2O, Zn(CH3COO)2.2H2O and coumarilic acid 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. Magnetic susceptibility measurements were performed at room temperature using a Sherwood Scientific MXI model Gouy magnetic balance. IR spectra were recorded in the 4000–400 cm−1 region with a PerkinElmer Spectrum One FTIR spectrophotometer using KBr pellets. 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 α-Al2O3 as a reference.

Experimental

In the first step of the synthesis, coumarilic acid of 0.005 mol was dissolved in the solution of EtOH/H2O in 50:50 (v/v). The acetate salts of Co, Ni, Cu and Zn metals (0.0025 mol of each) were added to the solution obtained stirred in reflux at 70 °C for 36 h. In the reaction, the reference stoichiometric ratio was 1:2 (metal/ligand). The emission of CH3COOH (acetic acid) gas was observed. The heating and stirring were continued until no acetic acid odour was smelled.

The final solution was allowed to crystallize at atmospheric pressure at room temperature. The crystals were formed in approximately 2–4 weeks collected by filtration.

The metal/ligand ratio was determined as 1:2 for the compounds (Co, Ni, Cu and Zn transition metal complexes). The synthesis reaction is shown in Scheme 1.

Scheme 1
scheme 1

Synthesis reaction of metal–coumarilate coordination compounds

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Results and discussion

Elemental analysis results of metal–coumarilate complexes are given in Table 1. The results of melting points, magnetic susceptibility, colours and the yield determination are also given in the table.

Table 1 Analytical data of metal complexes
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FTIR analysis

The significant stretching and bending vibrations obtained using FTIR spectroscopy are summarized in Table 2. The coumarilic acid attached to the structure as a ligand bounded to the metal cations. The carboxyl group peaks of coumarilic acid were shifted to the right due to the coordination to the metal cations. Therefore, there are stretching vibrations observed for the complexes of CoII, NiII, CuII and ZnII originating from the carboxylic acid groups at 1591, 1593, 1599 and 1597 cm−1, respectively.

Table 2 FTIR spectra of metal–coumarilate mixed ligand complexes (cm−1)
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The strong and broaden band observed near 3350–2800 cm−1 in the spectrum is due to the presence of –OH groups. Aromatic C–H stretching vibrations in the complexes result a peak in between 3022 and 3030 cm−1. The asymmetric and symmetric absorption bands of COO− correspond to the stretching vibrations at 1549–1576 and 1398–1408 cm−1, respectively. The absorption bands corresponding to the metal–oxygen (M–O and M–O=) bindings, which is a base for complexes, provide stretching vibrations at 428 cm−1 for CoII, 429 cm−1 for NiII, 432 and 471 cm−1 for CuII and 427 and 487 cm−1 for ZnII.

Thermal analysis

Thermal analysis curves (TG, DTG and DTA) of the complexes are shown in Fig. 1a–d. The decomposition steps and leaving groups are summarized in Table 3.

Fig. 1
figure 1

Thermal analysis curves of compounds, a [Co(C9H5O3)2(H2O)4], b [Ni(C9H5O3)2(H2O)4], c [Cu(C9H5O3)2(H2O)2], d [Zn(C9H5O3)2(H2O)4]

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Table 3 Thermal analysis data of metal–coumarilate complexes
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It can be observed from the DTG plot of the CoII mixed ligand (1) complex that it goes degradation in three steps corresponding to the maximum temperatures of 128, 148 and 329; 387; 479 °C. The first degradation step is the removal of 2 of 4 mol of water bonded to the metal via coordinated covalent bond in the coordination sphere.

figure a

The other 2 mol of water remaining in the coordination sphere during the degradation step at 148 °C was completely removed at the temperature range of 134–215 °C.

figure b

The coumarilate ligands in the degradation step at 329; 387; 479 °C were removed by degradation at the temperature range of 275–767 °C. The black coloured CoO compound remained as a decomposition product.

figure c

It can be seen in the DTA curves of the complex (2) having NiII mixed ligand that it decomposes in two steps corresponding to the maximum temperatures of 168 and 342; 362; 745 °C. The first decomposition step is the removal of 4 mol of water from the structure within the coordination sphere.

figure d

The coumarilate ligands in the degradation step at 342; 362; 745 °C were removed by degradation at the temperature range of 238–780 °C. The black coloured NiO compound remained as a decomposition product.

figure e

It can be seen in the DTG curves of the complex (3) having CuII mixed ligand that it decomposes at three steps corresponding to the maximum temperatures of 63, 132; 151 and 301; 313; 715 °C. The first decomposition step was the removal of moisture from the structure at the temperature of 35–86 °C. The 2 mol of water within the coordination sphere in the decomposition step at 132; 151 °C was removed at the temperature range of 120–167 °C.

figure f

The coumarilate ligands in the degradation step at 301; 313; 715 °C were removed by degrading at the temperature range of 267–860 °C. The black coloured CuO compound remained as a decomposition product.

figure g

It can be seen in the DTG curves of the complex (4) having the ZnII mixed ligand that it decomposes in three steps corresponding to the maximum temperatures of 90, 166 and 301; 328; 378; 663; 879 °C. The first decomposition step was the removal of moisture from the structure at the temperature of 36–114 °C. The 2 mol of water within the coordination sphere in the decomposition step at 166 °C was removed at the temperature range of 116–200 °C.

figure h

The coumarilate ligands in the degradation step at 301; 328; 378; 663; 879 °C were removed by degradation at the temperature range of 281–892 °C. The grey coloured ZnO compound remained as a decomposition product.

figure i

All of the last decomposition products related metal oxides were identified by FTIR spectroscopy [26].

Single X-ray diffraction analysis

Suitable crystals of 1, 2 and 4 were selected for data acquisition, which was performed on a Bruker D8-QUEST diffractometer equipped with a graphite monochromatic Mo-Kα radiation. The structures were determined by direct methods using the software SHELXS-97 [27] and refined by full-matrix least-squares methods on F2 using the software SHELXL-97 [27] within the software WINGX [28]. All atoms except for hydrogen were refined with anisotropic parameters. The H atoms bonded to C atoms were located in simulation using a different maps and then treated as riding atoms having C–H distances of 0.93 Å. The H atoms of water were located in simulation using a different map refined freely. Molecular diagrams were created using the software MERCURY [29]. Supramolecular analyses were conducted. The diagrams were prepared with the aid of the software PLATON [30]. Details of data acquisition and crystal structure determinations are given in Table 4.

Table 4 Crystal data of [Co(C9H5O3)2(H2O)4], [Ni(C9H5O3)2(H2O)4] and [Zn(C9H5O3)2(H2O)2] complexes
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The molecular structures of complexes 1–2 are shown in Fig. 2 with the atom-numbering schema, and selected bond and bond angles are summarized in Table 5. The asymmetric units of 1–2 have one M(II) ion [M(II) = Co(II) in 1 and Ni(II) in 2], two coumarilic acid ligands and four aqua ligands. Each M(II) ion was coordinated by two oxygen atoms of two different coumarilic acid ligands and four oxygen atoms of aqua ligands. The coordination geometry around the M(II) ion M(II) ion [M(II) = Co(II) in 1 and Ni(II) in 2] can be described as the distorted octahedral. The M–Ocarboxyl bond lengths are 2.124 (6) and 2.129 (6) Å in 1 and 2.093 (12) and 2.103 (11) Å in 2. The M–O(aqua) bond lengths are ranged between 2.074 (5)–2.097 (5) Å in 1 and 2.012 (9)–2.068 (12) Å in 2.

Fig. 2
figure 2

Molecular structure of complexes 1 and 2 (M=Co(II) in 1 and Ni(II) in 2) showing the atom-numbering scheme

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Table 5 Selected bond distances and angles for complexes 1, 2 and 4 (Å, º)
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Compounds of 1–2 are attached to the sheets by a combination of O–H···O hydrogen bonds (Table 6). Strong hydrogen bonds were observed between carboxyl groups and aqua oxygen atoms, with the O···O distances ranging from 2.623 (8) to 3.089 (9) Å for complex 1 and from 2.601 (15) to 3.193 (11) Å for complex 2. The combination of O–H···O hydrogen bonds produce edge-fused R 21 (5), R 12 (6), R 22 (6) and R 22 (8) rings running parallel to the [100] direction (Fig. 2). Similarly, a combination of O–H···O hydrogen bonds produces edge-fused R 22 (12) rings running parallel to the [001] direction (Fig. 3). All of these intermolecular interactions give two-dimensional framework results. The formation of edge-fused R 22 (12) rings in 1 and 2 (Fig. 4).

Table 6 Hydrogen bond distances of complexes with 1, 2 and 4 geometry (Å, º)
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Fig. 3
figure 3

Formation of edge-fused R 21 (5), R 12 (6), R 22 (6) and R 22 (8) rings in 1 and 2

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Fig. 4
figure 4

Formation of edge-fused R 22 (12) rings in 1 and 2

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The molecular structure schema of complex 4 with atom labelling is shown in Fig. 5, and the selected bonds and bond angles are summarized in Table 5. The asymmetric unit of complex 4 contains one Zn(II) ion, one coumarilic acid and one aqua ligand. The Zn(II) ion was located in the centre of the symmetry and coordinated to four oxygen atoms of two different coumarilic acid ligands and two oxygen atoms of aqua ligands. The Zn(II) ion was coordinated by two oxygen atoms of two different coumarilic acid ligands and four oxygen atoms of aqua ligands. The coordination geometry around the Zn(II) ion can be described as the distorted octahedral. The Zn–Ocarboxyl bond lengths are 2.0170 (19) and 2.523 (2) Å, while the Zn–Oaqua bond length is 1.979 (2) Å.

Fig. 5
figure 5

Molecular structure of complex 4 showing the atom-numbering scheme [(i) −x + 1, y, −z + 1]

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The complex 4 was attached to the sheets by the combination of O–H⋯O and C–H⋯O hydrogen bonds (Table 6). The O4 atom of water in the compounds at (x, y, z) acted as hydrogen-bond donor, via atoms H2A and H2B, to atoms O2iii and O1iv, so forming R 33 (10) rings [(iii) −x + 3/2, y + 1/2, −z + 1 and (iv) −x + 1, y + 1, −z + 1]. The combination of these hydrogen bonds produces two-dimensional framework result (Fig. 6).

Fig. 6
figure 6

An infinite 2D layer in 4

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The solid-state UV–Vis spectroscopy

The electronic transition values of metal–coumarilate mixed ligand complexes synthesized in this study were derived from the spectra pattern obtained using solid-state visible spectroscopy (UV–Vis) in the wavelength interval of 900–200 nm (Fig. 7). The data obtained from the spectroscopic data are summarized in Table 7.

Fig. 7
figure 7

UV–Vis–NIR Spectra of complexes 1–4

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Table 7 UV–Vis spectra of metal–coumarilate mixed ligand complexes
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According to the solid-state UV–Vis spectroscopy results, d–d transitions attributed to CoII(1) complexes were observed at 512.35 (4T1g → 4T2g) (F) and 463.12 nm (4T1g → 4T1g) (P). The spin-allowed d–d transition belonging to NiII(2) complex was observed in the wavelengths of 817.22 nm (3A2g → 3T1g) (P), 654.76 nm (3A2g → 3T1g) (F) and 395.67 nm (3A2g → 3T2g) (F). Therefore, these transition bands show the cleavage of d orbitals of NiII metal cations supporting the octahedral geometry. The multiple absorption bands owned by CuII(3) complex were formed by band-overlapping. It has a broaden view in a wide wavelength range of 887.62–501.13 nm. In the light of these spectral data, it is thought that Cu2+ metal cations promote the “pseudo-octahedral” structure under the influence of the Jahn–Teller effect. The maximum absorption band of the broad spectrum of the CuII(3) complex corresponds approximately to the wavelength of 709.71 nm (2Eg → 2T2g). With respect to the magnetic susceptibility values, because the d orbitals in the last orbital of the metal cation in the ZnII(4) complex with diamagnetic properties were fully occupied, there was no any d–d electronic transition observed for any possible octahedral cleavage.

The absorption bands with high intensity observed in low wavelengths are not d–d transitions, but special to metal → ligand charge transfer transitions with higher energy.

While absorption bands in 252.49 nm for CoII(1) complex, 253.11 nm for NiII(2) complex, 244.79 nm CuII(3) complex are owned by metal → ligand (M → L) transitions, the intense band observed in 268.57 nm for ZnII(4) complexes is owned by ligand → metal (L → M) charge transfer. These data were suitable with the literature [31,32,33].

Conclusions

In this study, the mixed ligand coordination compounds of cobalt(II), nickel(II), copper(II) and zinc(II) transition metals and coumarilate were synthesized. According to the elemental analysis results of complexes, the metal/ligand ratio was determined as 1:2.

According to structural analysis results obtained, the complexes 1 and 2, structures of which were identified, were determined as isostructural. Co and Ni cations in the structures have completed their octahedral sphere with four water atoms bonded as a ligand by entering the coordination sphere. The existence of octahedral sphere was proved by the data obtained using both solid-state UV–Vis–NIR spectrophotometer and magnetic susceptibility test. The single crystal structure of the compound having ZnII cation was the one only identified among others. The compliance between the spectroscopic results and the structure number 3 proves that these two compounds are isostructural. The geometries of these structures were also determined as distorted octahedral. Two moles of monoanionic coumarilate ligands were coordinated to the metal in the coordination sphere. However, this coordination occurs as bidentate. Besides coumarilate ligands, two moles of water have completed the octahedral geometry by coordination with the metals. Moreover, the octahedral sphere was also proved by the data obtained using both solid-state UV–Vis–NIR spectrophotometer and magnetic susceptibility test.

The mono- and bidentate binding features of coumarilate ligands were also proved by FTIR spectra. The difference between ν(COO−)asym and ν(COO−)sym gives an idea about the binding behaviour of carboxylate whether mono- or bidentate. If the difference between these two stretching vibrations is greater than 150 cm−1, the binding is monoanionic bidentate; if smaller, the binding is monoanionic monodentate [23, 26, 33,34,35]. According to Table 2, the differences between ∆ν as-s were found in order of 144, 144, 175 and 177 cm−1. These results are strong evidence for the binding behaviour of coumarilate ligand as monoanionic monodentate for compounds 1 and 2, whereas monoanionic bidentate for compounds 3 and 4.

In the light of the thermal analysis data, the thermal decomposition of the complexes was started firstly by the elimination of aqua ligands, continued by the combustion of coumarilate which is an organic group. At the end, the elimination was ended by the remaining of the related metal oxides.

Supplementary material

Crystallographic data for the structural analysis have been deposited with the Cambridge Crystallographic Data Centre, CCDC No. 1458972 for 1, 1458973 for 2 and 1458974 for 4. Copies of this information may be obtained free of charge from the Director, CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (Fax: +44-1223-336033; E-mail: deposit@ccdc.cam.ac.uk or www: http://www.ccdc.cam.ac.uk).