Introduction

Salen-like complexes have been continuously developed in different fields due to their special three-dimensional structures and unique electronic structures [1,2,3,4,5]. Later, the salamo-like ligands with a N2O2 coordination environment were obtained by modifying the salen-like ligands, which have the advantage of introducing more negatively charged oxygen atoms into the diamine to enhance its stability. Therefore, salamo-like complexes have enriched the types of complexes in structure, have been greatly supplemented in the field of supramolecular chemistry and have played an indispensable role in the field of coordination chemistry [6,7,8,9,10,11]. These metal complexes are used as catalysts [12], magnetic materials [13,14,15,16,17], nonlinear optical materials [18,19,20], luminescence materials [21,22,23,24,25,26,27,28,29,30], molecular identifiers [31,32,33], supramolecular architectures [34,35,36,37], dye and pigments [38], have application in biological fields [3] among other applications, and have great future prospects. In recent years, through the study of the fluorescent properties of metal complexes with different structures, it has been found that for many metal complexes, the interaction between the metal ions and the ligands changes the electronic excited state of the metal center, so that these metal complexes show many interesting fluorescent properties [39,40,41,42].

In this paper, two new dinuclear CoII and ZnII complexes were synthesized by the reactions of H2L with Co(OAc)2·4H2O and Zn(OAc)2·2H2O, respectively; the Hirshfeld surfaces analyses and fluorescence properties of complexes 1 and 2 were investigated.

Experimental

Materials and methods

4-(N,N’-Diethylamino)salicylicaldehyde (99%) and 7-hydroxy-4-methylcoumarin (98%) were purchased from Alfa Aesar (New York, NY, USA) and used without further purification. All other reagents and chemicals are from Tianjin Chemical Reagent Factory. The melting points were measured via an X4 microscopic melting point meter from Beijing Taike Instrument Co., Ltd. Carbon, hydrogen and nitrogen elemental analysis data are obtained from the GmbH VarioEL V3.00 automatic elemental analyzer. Elemental analysis of CoII and ZnII was detected by IRIS ER/S-WP-1 ICP atomic emission spectrometer (Berlin, Germany). UV/Vis spectra were measured by a Shimadzu UV-2550 spectrometer (Hitachi, Tokyo, Japan). The fluorescence spectra were measured on an F-7000 FL spectrophotometer (Hitachi, Tokyo, Japan). The infrared spectra were measured with a VERTEX-70 FT-IR spectrophotometer (Bruker, Germany), and the sample was prepared as KBr (500–4000 cm−1). The single-crystal X-ray structures were determined on a Bruker APEX-II CCD diffractometer.

Preparation of H2L

H2L was prepared according to similar method previously reported [43]. 1,2-Bis(aminooxy)ethane and 8-formyl-7-hydroxy-4-methylcoumarin were prepared according to previous literature [44, 45].

4-(N,N’-diethylamino)-2-[O-(1-ethyloxyamide)]oximephenol: 4-(N, N’-diethylamino) salicylicaldehyde (385.6 mg, 2.00 mmol) was dissolved in a methanol solution (25 mL) and slowly added to a methanol solution (50 mL) of 1,2-bis(aminooxy)ethane (184.3 mg, 2.00 mmol) for 4 h. The above solution was heated at 55 °C for about 5 h. After cooling to room temperature, the red-brown oil obtained by distillation under reduced pressure was separated by column chromatography using ethyl acetate/chloroform (1:20, V/V), and 78.5 mg white solid 4-(N,N’-diethylamino)-2-[O-(1-ethyloxyamide)]oximephenol was gained by distillation under reduced pressure.

H2L: a above-obtained 4-(N,N’-diethylamino)-2-[O-(1-ethyloxyamide)]oximephenol (262.21 mg, 1.00 mmol) in ethanol (25 mL) was added to a solution of 4-methyl-7-hydroxy-8-formylcoumarin (204.17 mg, 1.00 mmol) in ethanol (30 mL). The mixed solution was heated to ca. 80 °C and refluxed for 4–5 h. The solution was cooled to room temperature to obtain a white precipitate H2L, filtered and dried, as given in Scheme 1. Yield 51.7%. m.p.148–149 °C. Anal. Calcd. for C24H27N3O6 (%): C, 63.56; H, 6.00; N, 9.27. Found: C, 63.72; H, 5.89; N, 9.08.

Scheme 1
scheme 1

Synthetic route to H2L

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Preparation of complex 1

A solution of Co(OAc)2·4H2O (2.50 mg, 0.01 mmol) in ethanol (5 mL) was added to a solution of H2L (4.53 mg, 0.01 mmol) in acetone (5 mL) at room temperature. The color of the mixture turned to red-brown immediately, and then, the mixture was stirred at room temperature for about 30 min. The mixture was filtered off, and the filtrate was allowed to stand at room temperature for about 1 week. The solvent was partially evaporated, and a transparent red block single crystal suitable for X-ray crystallographic analysis was obtained. Anal. Calcd. for C48H52Co2N6O13 (1038.84) (%): C, 55.50; H, 5.05; N, 8.09; Co, 11.35. Found: C, 51.72; H, 5.01; N, 7.95; Co, 11.12.

Preparation of complex 2

A solution of Zn(OAc)2·2H2O (2.20 mg, 0.01 mmol) in ethanol (5 mL) was added to a solution of H2L (4.53 mg, 0.01 mmol) in dichloromethane (5 mL) at room temperature. The mixture is colorless, and then, the mixture was stirred at room temperature for about 30 min. The mixture was filtered off, and the filtrate was allowed to stand at room temperature for about 1 week. The solvent was partially evaporated, and a transparent colorless needle-like single crystal suitable for X-ray crystallographic analysis was obtained. Anal. Calcd. for C48H52Zn2N6O13 (1051.73) (%): C, 54.82; H, 4.98; N, 7.99; Zn, 12.43. Found: C, 54.98; H, 5.02; N, 7.84; Zn, 12.27.

Crystal structure determinations of complexes 1 and 2

X-ray diffraction data of complexes 1 and 2 were collected at 100.01(10) K, use a BRUKER SMART APEX-II CCD diffractometer with graphite monochromated Cu-Ka and Mo–Ka radiations (λ = 1.54178 Å and λ = 0.71073 Å), respectively. The LP factor and semiempirical absorption corrections were applied to the intensity data. Then, the structures were solved using SHELXS-1997 [46] and by a full matrix least-squares technique based on F2 using SHELXL-2014 and SHELXL-2015 [47]. The anisotropic thermal parameters are assigned to all non-hydrogen atoms. Contributions to scattering due to these highly disordered solvent molecules were removed using the SQUEEZE routine of PLATON; the structures were then refined again using the data generated. Crystal data and experimental parameters relevant to the structure determinations with respect to complexes 1 and 2 are listed in Table 1.

Table 1 Crystal data and structure refinement parameters for 1 and 2
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Results and discussion

Salamo-like complexes 1 and 2 have been appropriately prepared and characterized by IR, UV–Vis and fluorescence spectroscopy, X-ray crystallography and Hirshfeld surfaces analyses.

FT-IR spectra

Infrared spectra of H2L and its complexes 1 and 2 exhibited different bands in 4000–500 cm−1 region (Table 2). The IR spectral details of H2L and its complexes 1 and 2 are given in Fig. 1. The O–H stretching band of H2L is found at ca. 3417 cm−1 that belongs to the phenolic hydroxyl groups [8]. However, these bands are not observed in complexes 1 and 2, indicating that the phenolic O–H groups of H2L are fully deprotonated. New O–H stretching vibration bands in complexes 1 and 2 are observed at around 3437 and 3442 cm−1 corresponding to crystallizing water molecules [26]. The C=N stretching vibration band of H2L is observed at ca. 1634 cm−1, while those of complexes 1 and 2 are found at ca. 1617 and 1621 cm−1. The typical C=N stretching frequencies are moved to lower frequencies, showing that the CoII and ZnII atoms are coordinated to the oxime nitrogen atoms [9]. The typical Ar–O stretching band of H2L emerges at approximately 1292 cm−1, and after coordination, the stretching bands of complexes 1 and 2 shift to low wave numbers, at approximately 1242 and 1241 cm−1, showing that the Co–O and Zn–O coordinate bonds have been formed [10]. In addition, the stronger absorption band of H2L at 1728 cm−1 is attributed to the stretching vibration of the C=O double bond of coumarin; the C=O stretching vibration of complexes 1 and 2 compared to H2L appears at about 1733 and 1735 cm−1 and is attributed to the formation of M–N/O after coordination between H2L and the metal. New moderate intensity M–O vibrational bands in complexes 1 and 2 appear at 520 and 513 cm−1 [48, 49].

Table 2 The major FT-IR spectra of H2L and its complexes 1 and 2 (cm−1)
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Fig. 1
figure 1

The IR spectra of H2L and its complexes 1 and 2

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UV–Vis spectra

UV–Vis spectra of the free ligand H2L and its complexes 1 and 2 in 2.5 × 10−5 mol/L−1 ethanol solution are shown in Fig. 2.

Fig. 2
figure 2

UV–Vis spectra of H2L and its complexes 1 and 2

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The absorption spectra of H2L and its complexes 1 and 2 are given in Fig. 2. In the UV–Vis absorption spectrum of the free ligand H2L, absorption peaks appear at approximately 291, 336 and 368 nm, respectively. The absorption peak at 289 nm is attributed to the π–π* transition of the benzene ring, the second peak at 336 nm could be assigned to the n–π* transition of the C=N bond in the ligand, and the last absorption peak at 368 nm is assigned to the π–π* transition to the carbonyl group in coumarin. Compared with the free ligand H2L, complexes 1 and 2 have obvious absorption peaks at approximately 360 and 366 nm, which are assigned to L → M charge-transfer (LMCT) transitions [50].

Descriptions of the crystal structures

Selected bond lengths and angles are listed separately in Table 3. The corresponding hydrogen bonding interactions of complexes 1 and 2 are summarized in Table 4.

Table 3 Selected bond lengths (Å) and angles (º) for 1 and 2
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Table 4 Hydrogen bonding interactions (Å, °) for 1 and 2
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For complexes 1 and 2, all MII atoms are located at the N2O2 site of the deprotonated ligand (L)2− moiety with two atoms (O6 and O12) from the other salamo-type ligand (L)2− moiety, and the two MII atoms are connected in a M–O–M fashion through phenol oxygen atoms, and the crystal structures feature two-fold symmetry. At the same time, the two MII atoms of complexes 1 and 2 are penta-coordinated and adopt slightly twisted trigonal bipyramidal geometries (Co1 and Zn2) and square pyramid configuration (Co2 and Zn1) geometries (Figs. 3b and 5b) [44], which are deduced by calculating the values of τCo1 = 0.57, τCo2 = 0.45, τZn1 = 0.45 and τZn2 = 0.59, respectively [46]. From the calculation results, it can be seen that the τ values of Co1 and Zn2 are both greater than 0.5, forming trigonal bipyramidal geometries, and the τ values of Co2 and Zn1 are less than 0.5, forming square pyramid configurations. X-ray crystal structure measurement shows that complexes 1 and 2 are asymmetric binuclear structures and crystallize in the orthorhombic system, space group Pbca. The structure consists of two MII atoms (M1 and M2), two completely deprotonated (L)2− units, and one lattice water molecule, and forms an extremely rare 2:2 (molar ratio of L2−:MII atom) crystal structure which is different from the structures of 1:1 [19, 22, 51], 2:3 [7, 30, 52], 2:4 [39], 3:3 [19, 22], 4:4 [19], 1:3 [53], 2:6 [54], 1:6 [55] and 1:7 [56] (L:M2+) reported earlier. The molecular structures of complexes 1 and 2 are shown in Figs. 3a and S1a. In the coordination environment of Co2 and Zn1, two phenolic oxygen atoms (O9, O12 or O6, O12) and oxime nitrogen atoms (N4, N5 or N1, N2) are in the cis positions with each other (Figs. 3b and S1b). Due to the long distance between the two MII atoms, there is no interaction between them.

Fig. 3
figure 3

a Molecular structure and atom numbering of 1 with 30% probability displacement ellipsoids (hydrogen atoms are omitted for clarity). b Coordination polyhedrons for CoII ions

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In addition, the supramolecular structures of complexes 1 and 2 are similar. In the crystal structure of complex 1, there are two groups of intramolecular hydrogen bonds and six groups of intermolecular hydrogen bonds; these intermolecular hydrogen bonding interactions form a 3D supramolecular structure, as shown in Table 4 and Fig. 4 [57, 58]. For the crystal structure of complex 2, there are two sets of intramolecular hydrogen bonds and five sets of intermolecular hydrogen bonds as shown in Fig. S2 [59,60,61]. The 3D supramolecular structure of complex 2 is similar to complex 1.

Fig. 4
figure 4

a View of the intramolecular hydrogen bonds and b 3D supramolecular structure of complex 1

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Hirshfeld surfaces analyses

The Hirshfeld surfaces [62,63,64] of complex 1 are mapped over (a) dnorm, (b) the shape index and (c) di in Fig. 5. As shown in Fig. 6, the 2D fingerprint chart shows different intermolecular interactions. In Fig. 5, Hirshfeld surface analysis was performed on complex 1 [65], which shows the surfaces mapped on dnorm, the shape index and di. The bright red area on the depression of Hirshfeld surface indicates the presence of O···H interactions in complex 1, and light red spots indicate C–H···O interactions, while other raised blue areas indicate the presence of C···H, H···H and C···C interactions in complex 1. The Hirshfeld surfaces mapped with dnorm, shape index and di of complex 2 are similar to those of complex 1 because the complexes 1 and 2 are isomorphic. Figure 6 shows the generated 2D fingerprints [66], corresponding to O···H, C···H and H···H interactions on the Hirshfeld surface of complexes 1 (a) and 2 (b), respectively. To provide context, the outline of the full fingerprint is shown in gray, and the blue area shows individual contact. For complexes 1 and 2, the proportions of O∙∙∙H/H∙∙∙O, C∙∙∙H/H∙∙∙C and H∙∙∙H interactions in each molecule are similar to the total surface, while the differences in fingerprint plots of complexes 1 and 2 are caused by different geometrical parameters of disordered Et-groups in these two structures. Because of the existence of these hydrogen bonds, complexes 1 and 2 can be stabilized.

Fig. 5
figure 5

Hirshfeld surfaces are mapped with dnorm, shape index and di of complex 1

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Fig. 6
figure 6

Fingerprint plot of 1 and b 2: full and resolved into full and resolved into O···H, C···H and H···H contacts showing the percentages of contacts contributed to the total Hirshfeld area of molecule

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Fluorescence properties

The fluorescence spectra of H2L and its complexes 1 and 2 were measured in ethanol (2.5 × 10−5 M) at room temperature, as shown in Fig. 7. At an excitation wavelength of 350 nm, the ligand H2L has a broad absorption peak at approximate 431 nm, which is attributed to π–π* intraligand transition [67]. Compared with the ligand H2L, the absorption peak intensities of complexes 1 and 2 at 431 nm are significantly different. Compared with the ligand H2L, the fluorescence intensity of complex 1 is significantly reduced, indicating that CoII ions have fluorescence quenching properties. On the other hand, the fluorescence intensity of complex 2 was significantly enhanced due to the coordination of the ligand (L)2− units with ZnII ions and enhanced the coplanarity of the conjugated system.

Fig. 7
figure 7

Emission spectra of H2L and its complexes 1 and 2

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Conclusion

In this paper, two new dinuclear complexes were synthesized via H2L and Co(OAc)2·4H2O and Zn(OAc)2·H2O, respectively. A series of structural characterizations were performed for complexes 1 and 2. The X-ray single-crystal structure determinations indicate that both CoII and ZnII atoms in complexes 1 and 2 are penta-coordinated. The coordination configurations of the two metal atoms in complexes 1 and 2 are different: one is a twisted trigonal bipyramid configuration, and the other is a twisted square pyramid configuration. The Hirshfeld surfaces and 3D fingerprints can explain the atom-to-pair contact of crystals, which can quantify the intermolecular interactions. In addition, the coordination of CoII and ZnII ions with the ligand (L)2− units results in fluorescence quenching and enhancement, respectively.

Supplementary material

Crystallographic data have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication, CCDC Nos. 1975065 and 1975064 for complexes 1 and 2, respectively. Copies of the data can be obtained free of charge on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (Telephone: +44-01223-762910; Fax: +44-1223-336033; E-mail: deposit@ccdc.cam.ac.uk). These data can be also obtained free of charge at www.ccdc.cam.ac.uk/conts/retrieving.html.