Introduction

Construction of desirable coordination architectures including their spatial arrangement is a far-reaching issue and, not coincidentally, resulting in a great advance of task-specific molecular functions [1,2,3,4,5,6,7] such as molecular separation, small molecular adsorption, molecular containers, ion exchangers, chemo-recognition, and hetero-catalysis [7,8,9,10,11,12,13,14]. In particular, their catalytic efficiencies have been highly dependent on solvent systems, molecular dimensions, solubility, local geometry around metal centers, counteranions, etc. [15,16,17,18,19]. Among various organic reactions, transesterification reaction is of an important mission in the field of mass production of polyesters, glycerols, and biodiesels in both academic and industrial laboratories [20,21,22]. Thus, some zinc(II) coordination compounds have been extensively examined for appropriate Lewis acidity and homogeneous catalysis of the transesterification reactions [23, 24]. Such transesterification reactions using zinc(II) complexes could be affected by various factors: halides bound to Zn(II) ion, discrete and infinite structures [17, 25], and trace water in the reaction system.

To our knowledge, this research presents a landmark transesterification catalysis effect on the space arrangement of 2D zinc(II) coordination polymers. In this context, we herein report the results on structural properties of 2D Zn(II) coordination polymers along with their heterogeneous catalysis for the transesterification reaction of phenyl acetate with methanol.

Experimental

Materials and physical measurements

All chemicals including zinc(II) perchlorate, zinc(II) trifluoromethanesulfonate, 3-bromopyridine, 2,6-dihydroxynaphthalene, triethylamine, and phenyl acetate were purchased from Sigma-Aldrich, and were used without further purification. Elemental microanalyses (C, H, N) were performed on crystalline samples at the KBSI Pusan Center using a Vario-EL III analyzer. Infrared spectra were obtained on a Nicolet 380 FT-IR spectrophotometer using samples prepared as KBr pellets. 1H (300 MHz) spectra were recorded on a Varian Mercury Plus 300. Thermal analyses were performed under N2 at a scan rate of 10 °C/min using a PerkinElmer-TGA-DSC 4000.

Preparation of naphthalene-2,6-diyl-diisonicotinate (L)

Triethylamine (6.40 mL, 50 mmol) in chloroform (10 mL) was slowly added to a mixture of 2,6-dihydroxynaphthalene (1.60 g, 10 mmol) and isonicotinoyl chloride (3.92 g, 22 mmol) in chloroform (120 mL). The reaction mixture was refluxed for 12 h. The solution was washed with distilled water several times. The chloroform layer was dried over MgSO4 and filtered. Evaporation of the chloroform solvent gave white solid. Yield: 87%. m.p. 246 °C. Anal. Calcd for C22H14N2O4 (%): C, 71.35; H, 3.81; N, 7.56. Found: C, 72.10; H, 3.98; N, 7.51. IR (KBr, cm−1): 1739 (vs), 1608 (w), 1560 (w), 1513 (w), 1411 (w), 1324 (w), 1274 (vs), 1207 (s), 1143 (s), 1128 (w), 1091 (w), 1064 (w), 939 (w), 900 (w), 752 (m), 700 (m), 640 (w). 1H NMR (CDCl3, 300 MHz, δ): 8.90 (d, 3J = 5.28 Hz, 4H), 8.07 (d, 3J = 5.28 Hz, 4H), 7.94 (d, 3J = 8.80 Hz, 3H), 7.77 (s, 2H), 7.42 (d, 2H). 13C NMR (CDCl3, 75 MHz, δ): 163.91, 150.91, 148.26, 136.66, 131.96, 129.52, 123.26, 121.80, 118.73.

Preparation of [ZnL2(H2O)2](ClO4)2·H2O

A chloroform solution (5 mL) of L (3.7 mg, 0.01 mmol) was carefully layered onto a water/acetone solution (4.5 mL of acetone and 0.5 mL of H2O) of zinc(II) perchlorate (3.7 mg, 0.01 mmol). After 4 days, transparent crystals suitable for X-ray single crystallography were obtained in a 56% yield. m.p. 297 °C. Anal. Calcd for C44H34Cl2N4O24Zn (%): C, 46.40; H, 3.01; N, 4.92. Found: C, 46.57; H, 3.18; N, 5.03. IR (KBr pellet, cm−1): 1739(s), 1560(w), 1508(w), 1419(w), 1272(vs), 1207(m), 1145(s), 1120(vs), 1087(s), 1062(w), 904(w), 808(w), 752(w), 636(w), 482(w).

Preparation of [ZnL2(CF3SO3)2]

A tetrahydrofuran solution (3.0 mL) of L (3.7 mg, 0.01 mmol) was carefully layered onto a tetrahydrofuran solution (3.0 mL) of zinc(II) trifluoromethanesulfonate (3.6 mg, 0.01 mmol). After 3 days, colorless crystals suitable for X-ray single crystallography were obtained in a 60% yield. m.p. 330 °C. Anal. Calcd for C46H28F6N4O14S2Zn (%): C, 50.03; H, 2.56; N, 5.07. Found: C, 50.43; H, 2.82; N, 5.03. IR (KBr pellet, cm−1): 1743(s), 1619(w), 1560(w), 1511(w), 1419(w), 1276(vs), 1205(s), 1130(m), 1089(w), 1060(w), 1035(m), 902(w), 848(w), 752(w), 646(m), 480(w).

Transesterification catalysis

In order to scrutinize the transesterification catalytic effects of the 2D coordination polymers, [ZnL2(H2O)2](ClO4)2·H2O, [ZnL2(CF3SO3)2], Zn(ClO4)2, and Zn(CF3SO3)2 have been employed as a catalyst of the transesterification reaction of phenyl acetate with methanol. In the present research, the transesterification of phenyl acetate (68 mg, 0.5 mmol) at 50 °C with an excess amount of methanol was accomplished. For instance, phenyl acetate (68 mg, 0.5 mmol) and each catalyst (32–33 mg, 0.03 mmol) were stirred in methanol (5 ml), and warmed up to 50 °C. The catalytic process was monitored by reference to the 1H NMR spectra.

X-ray single crystallography

All X-ray crystallographic data were collected on a Bruker SMART automatic diffractometer with a graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) and a CCD detector at − 25 °C. The thirty-six frames of two-dimensional diffraction images were collected and processed to obtain the cell parameters and orientation matrix. Data integration and reduction were undertaken with SAINT and XPREP [26]. Absorption effects were corrected by the multi-scan method using SADABS [27]. The structures were solved by the direct method and refined by full-matrix least squares techniques (SHELXL 2014/07) [28, 29]. The non-hydrogen atoms were refined anisotropically, and hydrogen atoms were placed in calculated positions and refined using a riding model. The crystal parameters and procedural information corresponding to the data collection and structure refinement are listed in Table 1.

Table 1 Crystal data and structural refinements for and [ZnL2(H2O)2](ClO4)2·H2O and [Zn(CF3SO3)2L2]
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Results and discussion

Synthetic aspects

The new ligand, naphthalene-2,6-diyl-diisonicotinate (L), was synthesized by the reaction of 2,6-dihydroxynaphthalene with isonicotinoyl chloride according to the literature method [30]. Self-assembly of ZnX2 (X− = ClO4− and CF3SO3−) with L produced transparent crystals consisting of 2D coordination frameworks as shown in Scheme 1. The 2D structure of [ZnL2(CF3SO3)2] was simply packed, whereas that of [ZnL2(H2O)2](ClO4)2·H2O was packed in an interpenetration mode in the crystalline state. The self-assembly reaction was initially carried out in the 1:2 mol ratio of ZnX2/L, but the reaction was not so significant to the formation of the 2D networks. The crystalline products are quite stable under the aerobic condition, and are insoluble in water and general organic solvents such as acetone, chloroform, acetonitrile, and dissociated in strong polar solvents such as dimethyl sulfoxide and N,N-dimethylformamide. The compositions and structures were confirmed by elemental analyses, IR, thermal analysis (Figs. S1–S5), and single crystal X-ray diffraction. The characteristic strong IR band at 1087 cm−1 of [ZnL2(H2O)2](ClO4)2·H2O and 1276 cm−1 of [ZnL2(CF3SO3)2] was found to correspond to ClO4− and CF3SO3−, respectively.

Scheme 1
scheme 1

Synthetic procedure and catalytic effects

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X-ray crystal structures

The crystal structures are shown in Fig. 1, and their relevant bond lengths and angles are listed in Table 2. For [ZnL2(H2O)2](ClO4)2·H2O, the local geometry of the zinc(II) ion is an octahedral arrangement with two water molecules in transposition (O–Zn–O = 180.0°) and four pyridine units in propeller fashion building the basal plane. Each linker ligand connects two zinc(II) ions defining 2D network with the edges of a [Zn(II)]4 rhombus unit (Zn···Zn = 37.843 Å; Zn···Zn = 21.484 Å; 76-membered ring). The most interesting feature is that the crystal structures show the occurrence of the 2D interpenetrated packing (dihedral angle = 36.21°) in the solid state (Fig. S9). The ClO4− anion exists as a simple counteranion (Zn···O = 4.617 Å), and instead water molecules are coordinated to the central Zn(II) ion in a transposition (Zn–O = 2.160(3) Å). Solvate water molecule is positioned in a long distance (Zn···O = 4.165 Å). For [ZnL2(CF3SO3)2], the ligand acts as a spacer between two octahedral zinc(II) ions with four pyridine moieties defining a 2D network with the edges of a [Zn(II)] square (Zn···Zn = 30.699 Å; Zn···Zn = 30.699 Å; 76-membered ring). The CF3SO3− anions exist as a coordination mode (Zn-OSO2CF3 = 2.191(3) Å) in transposition rather than counteranions. Inner disordered tetrahydrofuran was squeezed. In the [ZnL2(CF3SO3)2] case, the guest accessible void volumes were calculated as 31.0% (1833.2/5910.1 Å3) for all crystals, removed electrons (474e−/8 ≒ 60e−) were calculated by PLATON [29]. The 2D network of [ZnL2(CF3SO3)2] has a simple packing, consisting of  abab… layers instead of such an interwoven structure. Thus, their packing motifs are quite different owing in part to the metallophilicity of anions (Fig. 2).

Fig. 1
figure 1

Perspective view of octahedral coordination geometry around Zn(II) sites of [Zn(CF3SO3)2L2] (a), crystal packing in the space-filling mode (b), and packing in an interpenetration mode (c)

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Table 2 Selected bond lengths (Å) and angles (°) [ZnL2(H2O)2](ClO4)2·H2O and [Zn(CF3SO3)2L2]
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Fig. 2
figure 2

Perspective view of octahedral coordination geometry around Zn(II) sites of [ZnL2(H2O)2](ClO4)2·H2O (a), crystal packing in the space-filling mode (b), and packing in a simple mode (c)

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Transesterification catalysis

In order to scrutinize the packing and counteranion effects of the present 2D coordination polymers on transesterification catalysis, the coordination polymers were employed as heterogeneous catalysts for the reaction of phenyl acetate with methanol. To date, many useful catalysts for transesterifications under mild reaction conditions have been developed [25, 31]. [ZnL2(H2O)2](ClO4)2·H2O (0.05 mmol) showed a significant catalytic effect on the transesterification of phenyl acetate (0.5 mmol) at 50 °C with an excess amount of methanol (Fig. 3 and Table 3). The catalysis was monitored by reference to the 1H NMR spectra (Fig. S4). The catalysis finished within 45 h, whereas the catalysis using [ZnL2(CF3SO3)2] proceeded by only 48% for 45 h. Such a different catalytic activity can be explained by the difference in packing, counteranion, or coordination environment. The catalytic reaction using [ZnL2(H2O)2](ClO4)2·H2O, moreover, is more effective than that of several heterogeneous Brønsted-acid solids such as porous zeolites [32], acid-leached natural kaolinite [33], sulfated SnO2 [34], and trimetallic zinc(II) complexes [25]. Furthermore, the control catalytic reactions using authentic Zn(ClO4)2 and Zn(CF3SO3)2 salts were attempted in this research. The catalyses using Zn(ClO4)2 and Zn(CF3SO3)2 exhibited a reverse trend relative to [ZnL2(H2O)2](ClO4)2·H2O and [ZnL2(CF3SO3)2], indicating that the counteranion is not so significant in the catalytic reaction. Thus, the catalytic effect on the transesterification reaction shows the order of [ZnL2(H2O)2](ClO4)2·H2O > Zn(CF3SO3)2 > Zn(ClO4)2 > [Zn(CF3SO3)2L2]. In particular, [ZnL2(H2O)2](ClO4)2·H2O efficiently survives after catalysis, and thus [ZnL2(H2O)2](ClO4)2·H2O was found to be a good recyclable heterogeneous catalyst (Fig. 4). The recyclable catalysis showed 85% activity. The catalytic effects of L (Fig. S5) and the same reaction in the presence of trace water (Fig. S6) were also studied for comparison. The catalysis with trace water, as expected, proceeded slowly. The mechanism of heterogeneous-zinc(II)-ion-catalyzed transesterification probably involves electrophilic activation of the carbonyl moiety, specifically by binding of the zinc(II) to the carbonyl oxygen [31]. Thus, the vacant sites and the Lewis acidity of the zinc(II) center, accordingly, play important roles in the transesterification catalytic reactions. The significant heterogeneous catalytic activity can be explained by the density of Zn(II) ions and the stability of the 2D interpenetrated coordination polymers relative to general simple 2D coordination polymers. It seems that the catalytic effects, as plotted for the present case, are strongly dependent on the packing mode. For the reaction system, ethanolysis is much slower than methanolysis; for example, ethanolysis reaches to 20% after 80 h at 50 °C, 100% 80 h at 70 °C in (Fig. S7).

Fig. 3
figure 3

Plot showing transesterification catalysis % conversions of phenyl acetate with methanol using [ZnL2(H2O)2](ClO4)2·H2O (solid blue line), Zn(ClO4)2 (dotted blue line), [Zn(CF3SO3)2L2] (solid red line), Zn(CF3SO3)2 (dotted red line) at 50 °C. (Color figure online)

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Table 3 Transesterification catalysis reaction conditions and % conversions
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Fig. 4
figure 4

Plot showing catalytic % conversions of transesterification of phenyl acetate with methanol using [ZnL2(H2O)2](ClO4)2·H2O (solid blue line = first run; dashed blue line = second recycled crystals) in methanol at 50 °C. (Color figure online)

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Conclusion

Self-assembly of ZnX2 (X− = ClO4− and CF3SO3−) with a new ligand L produced colorless single crystals consisting of 2D coordination polymers with the same topology, but their packing structure is different, the simple 2D and interpenetrated 2D packing structure. The 2D network crystals have been used in the heterogeneous catalysis of phenyl acetate with methanol, indicating that the particular 2D packing mode is an important factor in the transesterification catalysis reaction. This result clearly shows what we believe to be the first study into molecular packing effects of catalysts on the general heterogenous transesterification catalytic reaction. More systematic studies, for example on the synthesis of related ligands, are in progress. Further experiments on the packing structure will provide more detailed structural information on the catalytic effects of the coordination polymers.

Supplementary data

IR spectra, 1H NMR spectra, and TGA curves of [ZnL2(H2O)2](ClO4)2·H2O, [ZnL2(CF3SO3)2] were available in Supplementary material. Crystallographic data for the structure reported here have been deposited with the Cambridge Crystallographic Data Centre (Deposition No. CCDC 1892785-1892786 for [ZnL2(H2O)2](ClO4)2·H2O, [ZnL2(CF3SO3)2].), respectively. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.