A New Dinuclear Zinc Polymer Based on 3-Methoxy-2-hydroxybenzaldehyde:Synthesis, Structure, Spectral Characterization and Hirshfeld Surface Analysis①

YI Ming ZHAO Ru-Xia WANG Dun-Qiu XIAO Yu



A New Dinuclear Zinc Polymer Based on 3-Methoxy-2-hydroxybenzaldehyde:Synthesis, Structure, Spectral Characterization and Hirshfeld Surface Analysis①

YI Ming ZHAO Ru-Xia WANG Dun-Qiu②XIAO Yu②

(541004)

room temperature synthesis; dinuclear zinc polymer; crystal structure; luminescence; Hirshfeld surface analysis;

1 INTRODUCTION

The rational design and syntheses of novel coor- dination polymers (CPs) have achieved considera- ble progress in the field of supramolecular che- mistry and crystal engineering, owing to their fascinating structural diversities and potential applications, such as sensor technology[1], separa- tion processes[2, 3], gas storage[4, 5]luminescence[6, 7], ion exchange[8]magnetism[9–13], catalytic mate- rials[14], analytic crystal structures[15]and electro- chemiluminescence(ECL)[16]. Many complexes with novel structures and interesting physical properties have been constructed through organic ligands, which contain different functional groups, such as-donor (COOH, PO3H2, SO3H, OH)[18–20]and-donor (4,4΄-bipy, 2,2΄-bipy, 1,10-phen, Schiff base ligands)[21–23]. The design and synthesis of new coordination polymers based on the Hmhbd ligand have attracted considerable attention due to both structures and fascinating properties[24–30]. As is well known, Hmhbd ligand has been reported offour coordination modes including3:2:2:1(Scheme 1a)[24],1:1:1(Scheme 1b)[25, 26],2:1:2:1(Scheme 1c)[27–29]and4:3:2:1(Scheme 1d)[30].

Scheme 1. Coordination modes of Hmhbd

To facilitate the use CPs in these applications, many researches are focused on the generation of new structures and functionalization, as well as synthesis methods[31]. Undoubtedly, the synthesis condition is one important parameter that is invol- ved in the applicability and property determination of CPs materials. The solvothermal procedure is used most frequently to synthesize CPs and the quality of the obtained crystals is mostly suitable for crystal analysis (single-crystal X-ray diffraction). On the other hand, solvothermal synthesis of CPs is time-(days to weeks) and energy-(heating system) consuming before CPs materials can be obtained. Several new developed methods are more efficient, such as mechanochemical[32, 33]and microwave assisted methods[9, 10, 12, 18, 34]. However, these methods have special requirement for the reactors or apparatus. The solvothermal procedure is even more disadvantageous since certain starting materials are unstable at high temperature and sensitive to the reaction environment. By com- parison, the room-temperature synthesis method has the pronounced advantages of low energy cost, easy and inexpensive apparatus, and even short reaction time. In recent years, many coordination complexes, such as CPs, MOFs, clusters have been synthesized under the ambient temperature or room temperature[35–39].

Recently, Hirshfeld surface analysis as a useful tool described the surface characteristics of the crystal structures[40]. Thenormsurface is used for describing the very close intermolecular interactions in the crystals using a red-white-blue color scheme. Another important supplement for the Hirshfeld surface is the 2-D fingerprint plots. It quantitatively analyzes the nature and type of intermolecular interactions between the molecules inside the crystal[40]. Hirshfeld surface analysis and the 2-D fingerprint plot have been rapidly gaining promi- nence as a powerful technique in exploring the inter- molecular interactions of crystals[13, 16, 41-45].Herein, a dinuclear zinc ploymer [Zn2(mhbd)2(dca)2]nhas been synthesized under room temperature. Hirshfeld surface analysis and the 2-D fingerprint plot of 1 were also studied.

2 EXPERIMENTAL

2. 1 Synthesis of [Zn2(mhbd)2(dca)2]n (1)

A mixture of Zn(ClO4)2·6H2O (0.279 g, 0.75 mmol), Hmhbd (0.114 g, 0.75 mmol), NaN(CN)2(0.133 g, 1.5 mmol), and methanol (10 mL) with a pH adjusted to 7.5 by the addition of triethylamine was stirred for 30 min at room temperature. The resulting solution was left at room temperature and colorless crystals of 1 were obtained after 3 d (yield: 83 mg,. 39.16% based on Hmhbd). Anal. Calcd. (%) for C20H14N6O6Zn2: C, 42.50; H, 2.50; N, 14.86. Found (%): C, 42.45; H, 2.57; N, 14.95. IR (KBr, cm‒1): 3440 m, 2801 s, 2334 m, 2271 m, 2214 s, 1640 s, 1555 m, 1473 s, 1304 s, 1216 s, 1083 m, 965 m, 854 w, 731 m, 508 w, 417 w.

2. 2 Structure determination

The diffraction data were collected on an Agilent G8910A CCD diffractometer with graphite mono- chromated Mo-radiation (= 0.71073 Å), using thescan mode in the range of 3.62≤≤26.95°. Raw frame data were integrated with the SAINT program[46]. The structure was solved by direct methods using SHELXS-97[46]and refined by full-matrix least-squares on2using SHELXS- 97[46]. An empirical absorption correction was applied with the program SADABS[46]. All non- hydrogen atoms were refined anisotropically. All hydrogen atoms were positioned geometrically and refined as riding. Selected bond lengths and bond angles for 1 are listed in Table 1.

Table 1. Selected Bond Lengths (Å) and Bond Angles (°)

Symmetry transformation: (A),, 1;(B) 1,,

2. 3 Hirshfeld surface calculations of 1

Molecular Hirshfeld surface calculations were performed by using the CrystalExplorer program[47]. When the CIF file of 1 is read into the Crystal- Explorer program, all bond lengths to hydrogen were automatically modified to the typical standard neutron values (C–H = 1.083 Å). In this study, all the Hirshfeld surfaces were generated using a high (standard) surface resolution. The 3Dnormsurfaces were mapped by using a fixed color scale of 0.76 (red) to 2.4 (blue). The 2D fingerprint plots were displayed by using the standard 0.4～2.6 Å view with thedandddistance scales displayed on the graph axes[48].

3 RESULTS AND DISCUSSION

3. 1 Description of the crystal structure

Fig. 1. Structure of 1, symmetry codes: (a) –, –, 1–; (b) 1–, –, –. All hydrogen atoms were omitted

Complex 1 constructs double chains through double1,5-dca bridges. It must be noted that the distances of Zn(1)···Zn(2a) and Zn(1)···Zn(2b) (symmetry code: (b) 1 –, –, –z) in the chain are 8.060(1) and 8.190(1) Å, respectively. The 1D chains further formed 2D layer through C–H···O hydrogen bonds (C(7)–H(7C)···O(6I), 3.392 Å, C(15)–H(15C)···O(3II), 3.367 Å, symmetry codes: (I) 1 +,– 1,, (II),– 1,, Fig. 2b). The weakstacking extends the 2D layers into a 3D supramolecular framework (Fig. 2a). Herein,distances are 3.795(1) and 3.792(1) Å, respectively. It must be noted thatdistances between the C(1)-C(6) and {C(1)-C(6)}irings is 3.795(1) whiledistances between the C(9)–C(14) and {C(9)–C(14)}iirings is 3.792(1) Å (symmetry codes: (i) 1 –, –, 1 –, (ii) –, –, –. Fig. 3).

Fig. 2. 3-D network of 1 (a); 2-D layers of 1(b)

Fig. 3.interaction of 1

3. 2 Luminescent property

In this study, luminescent property of complex 1 and the free Hmhbd ligand have been investigated in DMF solvent with the concentrations of 4 × 10–6and 8 × 10–6mol·L–1, respectively, as shown in Fig. 4. Upon photoexcitation at 375 nm, the free ligand Hmhbd is green luminescent with the maximum at 500 nm predominantly assigned to*→transition luminescence. With photoexcitation at 375 nm, 1 also exhibits a green luminescent emission band at 465 nm. Å The emission at 465 nm probably originates from metal-to-ligand charge transfer (MLCT)[53]. Complex 1 represents a novel qualita- tive change of luminescence property resulted from the interaction between metal ion and ligand. The ligand Hmhbd has relatively larger-conjugated system of benzene ring and phenolato oxygen, aldehyde oxygen, and methoxy oxygen donors forming4:1:2:1-dentate coordinate to two zinc ions which benefits the charge transfer from Zn ion to mhbd ligands. At the same time, the 310electric structure of Zn ion benefits from metal-to-ligand charge transfer. As a result, the luminescence intensity of complex 1 is much higher than that of the Hmhbd ligand. In addition, the chelation of the ligand to metal ion increases their rigidity and thus reduces the loss of energy by thermal vibration decay. At last, the result indicates that the fluorescence intensity of complex 1 is forty-one times the fluorescence intensity of Hmhbd ligand. Complex 1 may be a good candidate for useful photoactive material due to its strong luminescent emissions.

Fig. 4. Luminescent of 1 and the free Hmhbd ligand

3. 3 IR spectrum property

The IR spectral data of the ligand Hmhbd and complex 1 are shown in Fig. 5. Contrast to the Hmhbd ligand, complex 1 shows three new strong characteristic stretching vibrations (2334, 2271, 2214 cm–1) which may be assigned to the1,5-1:1coordination mode of the dca bridged ligand[24]. The vibration bands at 2801 cm–1for 1 and 2844 cm–1for the Hmhbd ligand were observed, which are attributable to the saturation -CH2- stretching frequency in 1 and the Hmhbd ligand. The band at 3440 cm–1stretching vibration may be attributed to the intermolecular hydrogen bonds (C7–H7C···O6I, 3.392 Å, C15–H15C···O3II, 3.367 Å). It is signi- ficant that the band at 1653 cm–1is attributable to the carbonyl bond(C=O) of the free Hmhbd ligand[54–56]which red shifts to 1640 cm–1for 1. The results indicate that the aldehydo oxygen of the Hmhbd ligand is coordinated[6, 30]. The bond originating from the C–O stretching vibrations of the free Hmhbd ligand at 1254 cm–1exhibit red shifts to 1216 cm–1for 1, suggesting its partici- pation in chelation[6, 57]. At very low frequencies (510～440 cm–1), two weak bands at 417 and 508 cm–1were observed from Zn–N and Zn–O bonds, respectively. The IR attribution is consistent with the crystal structure determination.

3. 4 Hirshfeld surface analysis

Hirshfeld surface analysis and 2D fingerprint plots are often used to identify the types of the intermolecular interactions and the proportion of this interaction. It is a useful tool for describing the surface characteristics of the molecules in the crystals. The molecular Hirshfeld surface (norm) of complex 1 is shown in Fig. 6a. They clearly show the influences of different relationship on the intermolecular interactions of complex 1. The large and deep red spots on the 3D Hirshfeld surfaces indicate the close-contact interactions. Herein, the red spots mean the Zn–N coordination bonds.

The 2D fingerprint plotsare used for quan- titatively analyzing the nature and type of intermo- lecular interactions between the molecules inside the crystal (Fig.6b–6h). The fingerprint plots can be decomposed to highlight particular close contacts between the elements. This decomposition enables separation of contributions from different interac- tion types, which overlap in the full fingerprint. For 1, C···H interactions have the most significant contribution (26.1%) to the total Hirshfeld surface. They are reflected in the middle of scattered points in the 2D fingerprint plots. The N···H intermo- lecular interactions have 24.0% contribution to the total Hirshfeld surface. The H···H intermolecular interactions appear as an acanthosphere in the 2D fingerprint plots, which have 16.3% contribution to the total Hirshfeld surface. TheO···H intermo- lecular interactions have 10.0% contribution to the total Hirshfeld surface, including C–H···O hydro- gen bonds. In addition, the C···C intermolecular interactions have 7.6% contribution to the total Hirshfeld surface which mainly includeinteractions. It must be noted that the maximum interaction for each kind interaction is labeled in Fig.6c–6h (red ring). In general, it is obvious that the maincontacts in complex1 are C···H and N···H interactions (Fig.7).

Fig. 5. IR of 1 and the free Hmhbd ligand

Fig. 6. Hirshfeld surface mapped with dnorm (a); 2D finger print plot for 1(b–h)

Fig. 7. Hirshfeld surface calculations for 1

4 CONCLUSION

At room temperature, a dinuclear zinc polymer [Zn2(mhbd)2(dca)2]nwas synthesized which is astraightforward and energy-saving procedure to produce CPs. Complex 1 presents a double 1-Dchain linked by the dca ligand which further constructs a 2-D layer through hydrogen bonds.The 2-D layer formed a 3-D framework thoughstacking. In 1, the mhbd ligand displays a2:1:2:1-mhbd coordination mode, while the dca group does a1,5-dca coordination mode. The result indicates that the fluorescence intensity of complex 1 is forty-one times the fluorescence intensity of Hmhbd ligand. Complex 1may be a good candidate for useful photoactive material. Hirshfeld surface analysis indicates thatthe mainly contacts in complex 1 are C···H and N···H interactions.

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18 May 2017;

11 October 2017

10.14102/j.cnki.0254-5861.2011-1726

① This work was financially supported by the National Natural Science Foundation of China (Nos. 51638006 and 51569008) and the Natural Science Foundation of Guangxi Province (No. 2015GXNSFAA139240)

②Tel: +86 773 2537332, Fax: +86 773 2537332, E-mails: wangdunqiu@sohu.com(Wang D. Q.) and 657683458@qq.com(Xiao Y.)