INTRODUCTION

Recently, transition metal ions [1, 2] have attracted considerable attention because of their interesting molecular structures when prepared in conjunction with co-ligands [3, 4]. Moreover, multinuclear metal complexes also are of continual interest of the chemists because their functionalities, connectivity, porosities and structural variations make them promising materials for applications in the field of magnetism, materials, biology sensors, and catalysis [5-10].

Generally, the common strategy of synthesizing multinuclear transition metal complexes is completed by appropriate organic ligands, bridging anions and co-ligands. However, the structure and nuclearity of these types of complexes are affected by various factors, such as charge, coordination number and geometry of metal ion, shape and size of ligands, flexibility, denticity and HSAB behavior.

In this context, salen-based ligands can serve as a better candidate to construct the architects of multinuclear transition metal complexes. However, among the derivatives of salen-based ligands, salamo-based ligands exploited firstly by Nabeshima group [11] are more excellent candidates. This is because salamo-based ligands introduce strong electronegative oxygen atoms into the C=N group of salen-based ligands, which makes the chemical properties of salamo-based ligands more stable than salen-based ligands [12-14]. A great variety of salamo-based ligands have been used to synthesize not only structurally different but also quite interesting mononuclear [15], polynuclear [16] and heteropolynuclear metal complexes [17]. The above complexes were found to possess wide applications in the field of catalysis [18-20], electron transport processes [21], organic catalysis, bioinorganic chemistry [10], luminescent properties [22], molecular recognitions [12] and magnetism [23]. Thiocyanate anion (SCN–) is well known flexidentate co-ligand that is simultaneously coordinate with transition metal ions in versatile coordination modes [24-27]. Therefore, the combination of salamo-based ligands with multiple coordination sites and flexidentate co-ligand creates conditions for the synthesis of structurally novel complexes with diverse structures.

In the present work, a trinuclear Co(II) complex [Co3(L)2(NCS)2] was successfully synthesized by using a salamo-based ligand H2L with multiple coordination sites. The Co(II) complex was characterized by single crystal X-ray diffraction. The luminescent property of the Co(II) complex was studied. In addition, investigation of Hirshfeld surfaces and 2D finger print plots was devoted to analyze carefully non-covalent supramolecular interactions.

EXPERIMENTAL

Materials and general methods. 3-Methoxysalicylaldehyde (99%) was purchased from Alfa Aesar. The solvents required for other experiments were purchased from Tianjin Chemical Reagent Factory. Use the GmbH VarioEL V3.00 automatic element analyzer for C, H and N analysis. Elemental analysis of Co(II) was performed using IRIS ER/S·WP-1 ICP atomic emission spectrometer. The 1H NMR spectra were measured on a Bruker AV 500 MHz spectrometer. The melting points were measured by a micro melting point instrument manufactured by Beijing Tektronix Instrument Co., Ltd. FTIR spectra were conducted by KBr (500-4000 cm–1) and CsI (100-500 cm–1) pellets on a VERTEX70 FTIR spectrophotometer (Bruker, Billerica, MA, USA). UV-Vis absorption and fluorescence spectra were determined using Shimadzu UV-3900 (Shimadzu, Japan) and Hitachi F-7000 (Hitachi, Tokyo, Japan) spectrometers, respectively. The 1 cm quartz cuvette and Xe lamp were used as light source to record the fluorescence spectra. The X-ray single crystal structure was measured on a Bruker Smart Apex CCD diffractometer. Hirshfeld surface analysis of the Co(II) complex was performed using the Crystal Explorer program were all made according to similar methods previously reported.

Crystallographic data collection and refinement. X-ray diffraction data of the Co(II) complex was collected on a Bruker D8 Venture diffractometer with the graphite-monochromatized CuKα (λ = 0.71073 Å) radiation. The complex cooled in a nitrogen gas cryostream to 173 K. Data collection was performed with the APEX2 software package and data reduction was performed using the SAINT V8.38A programs [28]. The multiscan program SADABS [29] was used for absorption correction. The coordinates of the non-hydrogen atoms were refined anisotropically, whereas hydrogen atoms were included in the calculation isotropically but not refined. The crystal structure of the Co(II) complex was solved by the direct method and refined by the full-matrix least-squares method used the SHELXL2018/3 programs implemented in Olex2 program [30]. Selected data of parameters and refinement of the Co(II) complex are summarized in Table 1.

Synthesis of H2L. The salamo-based ligand H2L was synthesized by following reported method [31]. H2L is a multisite coordination ligand that contains six coordinating sites in the form of two phenolic oxygen atoms, two oxime nitrogen atoms and two methoxy oxygen atoms. Yield 79% (571.2 mg). Found (%): C 60.16, H 5.38, N 7.55. C18H20N2O6. Calculated (%): C 59.99, H 5.59, N 7.77. 1H NMR (500 MHz, CDCl3): δ, ppm 9.73 (s, 1H), 8.26 (s, 1H), 6.91(dd, J = 7.8 Hz, 1.7 Hz, 1H), 6.85 (t, J = 7.8 Hz, 1H), 6.82 (dd, J = 7.8 Hz, 1.8 Hz, 1H), 4.48 (s, 4H), 3.91 (s, 6H).

Synthesis of the Co(II) complex. The synthetic route of the Co(II) complex is shown in Scheme 1. A methanol solution (3 mL) of Co(OAc)2·4H2O (1.5 mmol, 2.86 mg) was added to the acetonitrile solution (1 mL) of the ligand H2L (1 mmol, 3.62 mg) with constant stirring. Then a methanol solution (1 mL) of potassium thiocyanate (1 mmol, 1.00 mg) was added to the mixture. Finally, add a little triethylamine to the mixed solution, and the color of the solution changes from yellow to brown instantly. The resulting solution was stirred further for 30 min. The brown solution was then filtered and kept in a vial at room temperature. Dark brown block-shaped single crystals were then collected after several weeks. Yield: 38% (1.92 mg). Found (%): C 47.19, H 3.32, Co 17.37, N 8.21. [Co3(L)2(NCS)2] (C38H36Co3N6O12S2). Calculated (%): C 45.21, H 3.59, Co 17.51, N 8.32.

Table 1 Crystallographic Data and Refinement Parameters for the Co(II) Complex
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Scheme 1
scheme 1

Synthesis of the Co(II) complex.

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RESULTS AND DISCUSSION

IR spectra. The IR spectrum of the Co(II) complex was analyzed and systematically compared with that of the corresponding free ligand H2L in the 4000-400 cm–1 region (Fig. 1). In the IR spectrum of the ligand, the appearance of a broad band around 3167 cm–1 indicates the presence of O–H stretching vibration [32-34] of phenolic hydroxyl group of the free ligand H2L. In present case, a strong absorption band at about 1606 cm–1 in the IR spectrum of the Co(II) complex was observed which are attributed to ν(C=N) stretching frequency [35, 36]. An intense band near at 1598 cm–1 for the Co(II) complex is shifted to a lower energy value compared to that of H2L, which further confirms coordination binding mode of the oxime nitrogen atoms with the Co(II) centers. In addition, a characteristic Ar–O stretching frequency for H2L was observed at about 1260 cm–1, while that of the Co(II) complex appeared at about 1247 cm–1. The appearance of a strong absorption band ν(C≡N) around 2071 cm–1, a weak absorption peak ν(C≡S) at 845 cm–1 and a weak absorption peak δNCS at 461 in the IR spectrum of the Co(II) complex clearly indicates the presence of NCS– anions with N as the coordination atoms [3, 37], which is also evident from the crystal structure determination.

Fig. 1
figure 1

The IR spectra of H2L and its corresponding Co(II) complex.

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Further, the far-infrared spectrum of the Co(II) complex in the range of 500-100 cm–1 was also obtained so as to identify the bonds of Co–O and Co–N frequencies. The band at around 461 cm–1 in the Co(II) complex can be attributed to νCo–O, while the band at 528 cm–1 is assigned to νCo–N [16].

UV–Vis absorption spectral analyses. The UV-Vis spectra of H2L and its Co(II) complex were measured in methanol solution (1·10–5 mol/L) at room temperature. (Fig. 2a).

The free ligand exhibits intense absorption bands at approximately 227 nm and 271 nm in the UV region which are presumably due to the π–π* type transitions of the aromatic rings, whereas the free ligand displays intense absorption band at 316 nm presumably attributed to the π–π* transitions of C=N bonds [38]. It is clear that the corresponding absorption band of the Co(II) complex has bathochromically shifted to approximately 234 nm, 273 nm and 341 nm, respectively, which may be due to the structural environment of H2L and the perturbation of the intra-ligand π–π* transition by the Co(II) ions which are nearly comparable with previously reported Co(II) complexes [20].

Fig. 2
figure 2

UV-Vis spectra of H2L and its Co(II) complex (nm) (a), UV-Vis spectra of H2L with increasing concentrations of Co(OAc)2·4H2O in methanol (b).

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To examine formation reaction of the Co(II) complex, there executed UV-Vis titration in the range of 220-450 nm by the gradual addition of 1·10–3 mol/L methanol solution of Co(OAc)2·4H2O to a 1·10–5 mol/L methanol solution (2 mL) of H2L (Fig. 2b).

The absorption intensities of the bands of H2L at 271 nm and 316 nm kept on decreased, while new absorption band centered at approximately 341 nm appeared with increasing intensity with an increase in the concentration of Co(OAc)2·4H2O is shown in Fig. 2b. Furthermore, three isosbestic points were found at approximately 254 nm, 312 nm and 331 nm in the UV-Vis titration spectra, which indicated that an equilibrium between H2L, Co(OAc)2·4H2O and the Co(II) complex in the solution was reached. When H2L and Co2+ ions concentration ratio reached 2:3, the titration reached the end point [20]. This implied a 2:3 (H2L:Co2+) type complex formed which is also confirmed from single crystal X-ray structure.

Crystal structure of the Co(II) complex. The X-ray crystal structure determination revealed that the Co(II) complex crystallizes in the monoclinic space group P21/c. The Co(II) complex consists of three Co(II) atoms which was assembled with the support of two fully deprotonated ligand (L)2– units and co-ligand NCS– anions (Fig. 3). In addition, it can be clearly found that the OAc– anion is not involved in coordination from the X-ray crystal structure of the Co(II) complex, which is different from the common structures of the trinuclear cobalt(II) complexes reported previously [39]. Table 2 provides the main bond parameters of the Co(II) complex.

Notably, both of the terminal Co1 and Co3 atoms in the Co(II) complex are penta-coordinate and not hexa-coordinate as common Co(II) complexes as previously reported [39], whose coordination geometry is completed by two oxime nitrogen atoms (Co1: N1 and N2, Co3: N4 and N5) and two phenoxo oxygen atoms (Co1: O1 and O5, Co3: O11 and O7) from the fully deprotonated ligand (L)2– units, while the apical position is occupied by one nitrogen atom (Co1:N3, Co3:N6) from isothiocyanate anions. The Addison parameter τCo1 and τCo3 (τ = (β – α)/60, β and α are the largest angle and the second largest angle, respectively.) for the penta-coordinated Co1 and Co3 atoms are 0.59 and 0.68, respectively, indicating the geometries are significantly distorted trigonal bipyramidal [40]. At the same time, the axial bond length also well confirms this result, and the bond length parameters are shown in Table 1. The center Co2 atom is hexa-coordinated, whose coordination geometry is completed by six O atoms (O1, O5, O2, O11, O7 and O12) from the two fully deprotonated ligand (L)2– units in which one of the two methoxy O atoms in the ligand (L)2– unit is not coordinated. The Co2 atom has a distorted octahedral geometry.

Fig. 3
figure 3

Representation of the X-ray crystal structure of the Co(II) complex. (Hydrogen atoms have been omitted for clarity) (a); coordination configurations for Co(II) atoms of the Co(II) complex (b); perspective view of only Co(II) atoms with coordinating atoms, the coordinated co-ligand NCS– anions are shown for clarity (c).

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Table 2 The Main Bond Distances and Angles in the Co(II) Complex
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A detailed examination of the crystal structure of the Co(II) complex disclosed the presence of extensive C–H⋯π, π⋯π stacking and hydrogen bond interactions.

Fig. 4a shows two linking complexes which is formed due to a pair of the significant C–H⋯π (C29–H29B–Cg11) interactions. The hydrogen atom H29B attached to carbon atom C29 is involved in intermolecular C–H⋯π interaction with phenyl ring Cg11 (Cg11: C20–C21–C22–C23–C24–C25). The intramolecular C–H⋯π (C18–H18B–Cg12) and π⋯π (Cg10–Cg11) stacking interactions of the Co(II) complex were observed in Fig. 4b. The hydrogen atom H18B attached to carbon atom C18 is involved in intramolecular C–H⋯π interaction with phenyl ring Cg12 (Cg12: C31–C32–C33–C34–C35–C36). The π⋯π stacking interaction between the two neighboring aromatic rings (Cg10 and Cg11) was found with a centroid-to-centroid distance of 3.957(2) Å. The hydrogen atom H9A attached with C9 and H29A attached with C29 are also engaged in intramolecular hydrogen bond (Fig. 4c) with the nitrogen atoms N3 and N6 of NCS– anions. Hydrogen bond distances and angles for the Co(II) complex are given in Table 3.

Fig. 4
figure 4

The viewpoint of intermolecular C–H⋯π interactions of the Co(II) complex (a); the intramolecular C–H⋯π and π⋯π stacking interactions of the Co(II) complex (b); the viewpoint of intramolecular hydrogen bonds of the Co(II) complex (c).

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Table 3 Hydrogen Bond Lengths and Angles in the Co(II) Complex
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Luminescent properties. The emission spectral behaviors of the ligand H2L and its corresponding Co(II) complex were studied at normal room temperature using MeOH solution in the range of 350-550 nm. The fluorescence spectra of the free ligand H2L and its Co(II) complex are clearly represented in Fig. 5a.

Upon excitation wavelength at ca 330 nm, the free ligand H2L demonstrated an intense fluorescent emission peak centered mainly at approximately 403 nm which should be assigned to the intra-ligand π–π* transition whereas the emission maxima for the Co(II) complex appeared at approximately 397 nm. Further a very careful investigation from emission spectral curves of H2L and its Co(II) complex revealed that the Co(II) complex exhibits hypsochromically shifted with respect to the free ligand H2L mainly assigned to the L → M charge transfer [41]. For such single salamo-based complexes, the crystal structure is basically the same as the solution structure, and there will be no dissociation [39, 42].

In addition, the fluorescence intensities of the Co(II) complex is about 4-fold lower compared to that of H2L from the spectra, which probably may be due to N/O-donor centers coordination with the Co(II) atoms.

The fluorescence titration of H2L (1·10–5 mol/L) in 2 mL methanol solution is carried out by the gradual addition of Co(OAc)2·4H2O (1.0·10–3 mol/L) (Fig. 5b). With the increase of Co2+ ion concentration, the fluorescence intensity of the Co(II) complex weaken linearly until quenching, and the emission peak has hypsochromically shifted as the fluorescence intensity weaken. It most probably due to the formation of the Co(II) complex after coordination via N,O-donor centers with Co(II) atoms [40].

Hirshfeld surface analysis. Supramolecular interactions of the Co(II) complex were investigated using Hirshfeld surface analysis. Calculations of Hirshfeld surfaces [43, 44] were performed using the CrystalExplorer (version 17.5) software package [45]. The Hirshfeld surfaces of the Co(II) complex mapped over curvedness, dnorm and shape index are illustrated in Fig. 6.

The C–H⋯O interactions represent the closest contacts in the structure and can be viewed as red spots on the dnorm surface and other visible spots on the surface correspond to H⋯H contacts. Furthermore, it is clear that the X-ray crystal structure of the Co(II) complex do not exhibit any intermolecular π⋯π stacking interaction since there is no evidence of the adjacent red and blue triangles on the shape index surfaces.

Fig. 5
figure 5

Fluorescence spectra of H2L and its corresponding Co(II) complex (a); fluorescence spectra of H2L with increasing concentrations of Co(OAc)2·4H2O in methanol (b).

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Fig. 6
figure 6

Hirshfeld surfaces mapped with curvedness, dnorm and shape index of the Co(II) complex; the surfaces are shown as transparent to allow visualization of the orientation and conformation of the functional groups in the molecules.

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CrystalExplorer software package also was commonly used to construct fingerprint plots analysis [46] which employed successfully to illustrate intermolecular interaction patterns and the relative contributions. Fingerprint plots for the Co(II) complex and relevant surface patches associated with the specific contacts in the dnorm showed in Fig. 7.

Fig. 7
figure 7

Fingerprint plots: All (middle) and decomposed plots corresponding to all contacts involved in the structure. The relative contributions of various intermolecular contacts to the Hirshfeld surface area of the title structure are displayed by the schematic illustration.

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For the Co(II) complex the highest contribution comes from the H⋯H/H⋯H interactions (44.1%) which shows the stability of the crystal. At the top left (C–H donor) and bottom right (π acceptor) of the fingerprint plots, there are characteristic “wings” which are identified as a result of C⋯H (or C–H⋯π) interactions (20.6%). In the Co(II) complex, due to the existence of co-ligand NCS– anions, S⋯H/H⋯S interactions (18.6%) have the third largest contribution with a wing like structure. The intermolecular O⋯H/H⋯O interactions (8.0%) are represented by a pair of distinct spikes in the bottom area of the fingerprint plots.

DFT studies. Density functional theory (DFT) calculations were performed using Gaussian09 [47], Revision D.01 (basis set: B3LYP, Standard basis: SDD). The HOMO and LUMO contour surfaces of the Co(II) complex show the electron distributions between H2L and Co(II) atoms. As shown in Fig. 8, the electron distribution of the Co(II) complex occurs around the phenoxido oxygen and oxime nitrogen groups of H2L and NCS– anions. This means that interaction between Co(II) atoms and H2L takes place in the N2O2 core and NCS– anions.

In the Co(II) complex, the EHOMO and ELUMO of α spin state in the frontier molecular orbital energy are –4.8381 eV and –2.3742 eV, respectively. The EHOMO and ELUMO of β spin state are –4.8128 eV and –2.2035 eV, respectively. The HOMO–LUMO energy separation can be used to analyze the kinetic stability of molecules [48]. Because the energy is not conducive to the transition of electrons provided in the low-lying HOMO to the high-lying LUMO, the large energy gap value of HOMO–LUMO may be related to high kinetic stability and low chemical reactivity [49, 50]. The ΔE (ΔE = ELUMO – – EHOMO) of α and β spin state of the Co(II) complex are 2.4639 eV and 2.6093 eV, respectively. Compared with the previously reported ΔE of ligand H2L [51], the ΔE of the Co(II) complex is smaller. This shows that the Co(II) complex have good kinetic stability and chemical activity.

Fig. 8
figure 8

HOMO–LUMO energy transition diagram of the Co(II) complex.

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CONCLUSIONS

To investigate the synthesis, structural characterization, spectroscopic properties, Hirshfeld surface analysis and luminescent properties of the Co(II) complex, first multisite coordinated salamo-based ligand H2L was prepared. H2L was then reacted with Co(OAc)2·4H2O in presence of NCS– anions in 2:3:2 molar ratios to produce trinuclear Co(II) complexes [Co3(L)2(NCS)2]. Investigation of luminescent properties in methanol solution revealed that the Co(II) complex exhibits reduced emission over the ligand H2L. Apart from, short contacts and different types of supramolecular interactions in the trinuclear Co(II) complex are quantified on the basis of Hirshfeld surface and 2D finger print plot analyses. DFT calculation well confirmed that the Co(II) complex has better kinetic stability and chemical reaction activity than the ligand H2L.