English

• In recent decades, the coordination polymers have been widely explored as an important family of compounds in terms of high diversity of structures^[1-4], and remarkable functional properties, such as sensing^[5-6], gas sorption^[7-8], and catalysis^[9-11]. This is also governed by some features of metal ions and derived compounds, such as low cost and abundance, bio- and photoactivity, as well as rich coordination chemistry toward a broad spectrum of ligand types^[12-14]. Among them, aromatic carboxylic acids stand out as the most commonly applied linkers for assembling coordination polymers and derived materials^{[4, 10-11]}. A considerable focus of current research is devoted to the design of novel carboxylate linkers with multiple potential sites for coordination, followed by their exploration of the synthesis of metal-organic architectures.

  In pursuit of our general research line on probing various commercially available but still rarely used polycarboxylic acids as linkers for designing functional coordination polymers^{[10-11, 15-16]}, in the current work we have chosen a phenylpyridine-dicarboxylic acid: 5-(2-carboxyphenyl)nicotinic acid (H₂cpna). The organic compound can act as a versatile semi-flexible linker for the synthesis of coordination polymers. The selection of the ligand has relied on the following considerations. (1) The ligand has four O-donors and one N_pyridine site for potential coordination to metal modes. (2) The dicarboxylic acid is stable under hydrothermal synthetic conditions, and semi-flexible due to the presence of C—C bonds between pyridyl and phenyl rings. It can exhibit different degrees of deprotonation and a variety of coordination modes. (3) The H₂cpna ligand results in diverse structures with excellent luminescent and magnetic properties^[17-22]. However, only an H₂cpna-based network with catalytic properties has been reported^[23]. Thus the present study gave us a good opportunity to develop this field.

  Therefore, based on the above reasons, we designed and synthesized two Ni(Ⅱ) and Zn(Ⅱ) coordination polymers based on a phenylpyridine-type dicarboxylate ligand. Herein, we report the syntheses, crystal structures, and catalytic and anti-wear properties of two Ni(Ⅱ) and Zn(Ⅱ) coordination polymers, {[Ni(μ₃-cpna)(μ-dpea)_0.5]·H₂O}_n (1) and {[Zn(μ₃-cpna) (μ-dpey)_0.5]·H₂O}_n (2), constructed from phenylpyridine-type dicarboxylate ligand and 1, 2-di(4-pyridyl)ethane (dpea)/1, 2-di(4-pyridyl)ethylene (dpey).

  1.   Experimental

  1.1   Reagents and physical measurements

  All chemicals and solvents were of AR grade and used without further purification. The contents of carbon, hydrogen, and nitrogen were determined using an Elementar Vario EL elemental analyzer. IR spectrum was recorded using KBr pellets and a Bruker EQUINOX 55 spectrometer. Thermogravimetric analysis (TGA) data were collected on a LINSEIS STA PT1600 thermal analyzer with a heating rate of 10 ℃·min^-1. Powder X-ray diffraction (PXRD) patterns were measured on a Rigaku-Dmax 2400 diffractometer using Cu Kα radiation (λ=0.154 06 nm); the X-ray tube was operated at 40 kV and 40 mA; the data collection range was between 5° and 45°. Solution ¹H NMR spectra were recorded on a JNM ECS 400M spectrometer.

  1.2   Synthesis of compound 1

  A mixture of NiCl₂·6H₂O (0.048 g, 0.20 mmol), H₂cpna (0.049 g, 0.20 mmol), dpea (0.037 g, 0.20 mmol), NaOH (0.016 g, 0.40 mmol), and H₂O (10 mL) was stirred at room temperature for 15 min, and then sealed in a 25 mL Teflon-lined stainless steel vessel, and heated at 160 ℃ for 3 d, followed by cooling to room temperature at a rate of 10 ℃·h^-1. Green block-shaped crystals of 1 were isolated manually, and washed with distilled water. Yield: 53% (based on H₂cpna). Anal. Calcd. for C₁₉H₁₅NiN₂O₅(%): C 55.66, H 3.69, N 6.83; Found(%): C 55.47, H 3.67, N 7.05. IR (KBr, cm^-1): 3 564w, 3 505w, 3 068w, 1 606m, 1 554s, 1 528s, 1 453m, 1 426m, 1 401s, 1 315w, 1 281w, 1 229w, 1 174 w, 1 140w, 1 099w, 1 080w, 1 062w, 1 028w, 931w, 901 w, 864w, 838w, 785m, 767w, 722w, 703w, 681w, 632w.

  1.3   Synthesis of compound 2

  A mixture of ZnCl₂ (0.027 g, 0.20 mmol), H₂cpna (0.069 g, 0.20 mmol), dpey (0.036 g, 0.20 mmol), NaOH (0.016 g, 0.40 mmol), and H₂O (10 mL) was stirred at room temperature for 15 min, then sealed in a 25 mL Teflon-lined stainless steel vessel, and heated at 160 ℃ for 3 d, followed by cooling to room temperature at a rate of 10 ℃·h^-1. Colorless block-shaped crystals of 2 were isolated manually, washed with distilled water, and dried. Yield: 48% (based on H₂cpna). Anal. Calcd. for C₁₉H₁₄ZnN₂O₅(%): C 54.89, H 3.39, N 6.74; Found(%): C 55.06, H 3.41, N 6.71. IR (KBr, cm^-1): 3 050w, 2 930w, 1 613s, 1 546m, 1 509w, 1 438m, 1 382s, 1 289w, 1 248w, 1 211w, 1 177w, 1 133w, 1 073w, 1 021m, 987w, 901w, 875w, 834w, 785m, 744w, 688w, 644w.

  These compounds are insoluble in water and common organic solvents, such as methanol, ethanol, acetone, and DMF.

  1.4   Structure determinations

  The data of two single crystals with dimensions of 0.07 mm×0.05 mm×0.04 mm (1) and 0.06 mm×0.05 mm×0.04 mm (2) were collected at 293(2) K on a Bruker SMART APEX Ⅱ CCD diffractometer with Mo Kα (λ=0.071 073 nm). The structures were solved by direct methods and refined by full-matrix least-square on F² using the SHELXTL-2014 program. All non-hydrogen atoms were refined anisotropically. All the hydrogen atoms (except for the ones bound to water molecules) were placed in calculated positions with fixed isotropic thermal parameters included in structure factor calculations in the final stage of full-matrix least-squares refinement. The hydrogen atoms of water molecules in 1 were located by different maps and constrained to ride on their parent O atoms. Some highly disordered solvent molecules in 2 were removed using the SQUEEZE routine in PLATON^[24]. The final number of solvent molecules was estimated from the data of elemental and TG analyses. A summary of the crystallography data and structure refinements for 1 and 2 is given in Table S1 (Supporting information). The selected bond lengths and angles for compounds 1 and 2 are listed in Table S2. Hydrogen bond parameters of compound 1 are given in Table S3.

  1.5   Catalytic the Knoevenagel condensation reaction of aldehydes

  In a typical test, a suspension of aromatic aldehyde (0.50 mmol, benzaldehyde as a model substrate), malononitrile (1.0 mmol), and catalyst (typically x_cat=2.0%) in methanol (1.0 mL) was stirred at room temperature. After the desired reaction time, the catalyst was removed by centrifugation, followed by an evaporation of the solvent from the filtrate under reduced pressure to give a crude solid. This was dissolved in CDCl₃ and analyzed by ¹H NMR spectroscopy for quantification of products (Fig.S1). To perform the recycling experiment, the catalyst was isolated by centrifugation, washed with methanol, dried at room temperature, and reused. The subsequent steps were performed as described above.

  1.6   Preparation and friction test of oil samples with compound 2

  Compound 2 (10 mg), which was ground into a powder with a mortar, was added in poly-α-olefine synthetic lubricant (PAO10, 10 mL) whose viscosity at 100 ℃ is 10.1 mm²·s^-1. The mixtures were stirred at 85 ℃ for 24 h and ultrasonic dispersed for 1 h to obtain the oil sample (2/PAO10).

  A high-frequency oscillating tribometer (UMT-TRIBOLAB) was utilized to determine the friction and wear behaviors of oil samples under simulated operating conditions. In the testing configuration, the GCr15 steel ball with a diameter of 10 mm was selected as the static upper specimen, and the GCr15 steel block with a diameter of 25 mm and thickness of 8 mm as the reciprocating lower specimen. Before tests, GCr15 steel balls and the GCr15 steel blocks were ultrasonically cleaned in acetone for 10 min. A dosage of 0.1 mL of oil was dropped on the steel block and covered the entire surface with the micro-syringe before each friction test. The testing temperatures were set at 25 ℃ by the software of the tribometer management program, and controlled by a thermocouple which was fitted to just below the holder. All the friction tests were run for 30 min under a consistent load of 200 N (the Hertzian contact pressure of ca. 1 602 MPa) at a reciprocating stroke of 2 mm and a frequency of 25 Hz.

  2.   Results and discussion

  2.1   Description of the structure

  2.1.1   Structure of compound 1

  Single-crystal X-ray diffraction analysis reveals that compound 1 crystallizes in the monoclinic space group P2₁/c. The asymmetric unit of 1 contains one crystallographically unique Ni(Ⅱ) ion, one μ₃-cpna^2- block, one μ-dpea moiety, and one lattice water molecule. As shown in Fig. 1, the Ni1 ion is six-coordinated by four carboxylate O atoms and one N atom from three individual μ₃-cpna^2- blocks and one N atom from the dpea ligand, constructing a distorted octahedral {NiN₂O₄} geometry. The Ni—O bond lengths range from 0.204 0(2) to 0.223 8(2) nm, whereas the Ni—N bonds vary from 0.205 5(2) to 0.206 3(2) nm; these bonding parameters are comparable to those found in other reported Ni(Ⅱ) compounds^{[15, 19-20]}. In 1, the cpna^2- ligand adopts the coordination mode Ⅰ (Scheme 1), in which two deprotonated carboxylate groups show a bidentate mode. The dihedral angle of the pyridyl and phenyl rings in the cpna^2- is 60.45°. The dpea moiety takes a bridging coordination fashion. The μ₃-cpna^2- and μ-dpea blocks connect adjacent Ni(Ⅱ) ions to form a 3D framework (Fig. 2). The 3D framework is built from the 4-linked Ni1, 3-linked μ₃-cpna^2- nodes and 2-connected μ-dpea linkers (Fig. 3). It can be defined as a 2-nodal net with an ins topology and point symbol of (6³)(6⁵.8).

  Figure 1

  

  Figure 1.  Drawing of the asymmetric unit of compound 1 with a 30% probability of thermal ellipsoids

  H atoms and lattice water molecules are omitted for clarity; Symmetry codes: A: -x, y-1/2, -z+1/2; B: -x+1, y-1/2, -z+1/2; C: -x, -y+1, -z.

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  Scheme 1

  

  Scheme 1.  Coordination modes of cpna^2- ligands

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  Figure 2

  

  Figure 2.  Three-dimensional framework of compound 1 viewed along the b-axis

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  Figure 3

  

  Figure 3.  Topological representation of a 2-nodal framework with an ins topology viewed along the b-axis

  Green: Ni1 nodes; Gray: centroids of 3-linked μ₃-cpna^2- nodes; Dark blue: centroids of 2-connected μ-dpea linkers.

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  2.1.2   Structure of compound 2

  The asymmetric unit of compound 2 possesses one crystallographically independent Zn(Ⅱ) ion, one μ₃- cpnaa^2- block, one dpey ligand, and one lattice water molecule. As shown in Fig. 4, the Zn1 ion is four- coordinated and adopts a distorted trigonal-pyramidal {ZnN₂O₂} geometry formed by two carboxylate O atoms and one N atom from three individual μ₃-cpna^2- blocks and an N atom of dpey. The Zn—O distances range from 0.195 3(2) to 0.196 2(2) nm, whereas the Zn—N distance is 0.203 1(2)-0.204 6(2) nm; these bonding parameters agree with those observed in other Zn(Ⅱ) compounds^{[4, 12, 15]}. In 2, the cpna^2- block also acts as a μ₃-spacer (mode Ⅱ, Scheme 1), in which the carboxylate groups exhibit the monodentate modes. In the cpna^2- ligand, the phenyl and pyridine rings are not coplanar, with a dihedral angle of ca. 65.05°. The auxiliary dpey ligand takes a bridging coordination fashion. The neighboring Zn(Ⅱ) centers are linked by μ₃-cpna^2- and μ-dpey blocks, giving rise to a 3D framework (Fig. 5). This compound discloses a 3D structure composed of the 4-linked Zn1 nodes, 3-linked μ₃-cpna^2- nodes, and 2-connected μ-dpey linkers (Fig. 6). It is a new topology and a point symbol of (6³.10³)(6³).

  Figure 4

  

  Figure 4.  Drawing of the asymmetric unit of compound 2 with a 30% probability of thermal ellipsoids

  H atoms and lattice water molecules are omitted for clarity; Symmetry codes: A: -x+1/2, y-1/2, z; B: -x+1, y-1/2, -z+3/2; C: -x, -y+1, -z+2.

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  Figure 5

  

  Figure 5.  Three-dimensional framework of compound 2 viewed along the b-axis

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  Figure 6

  

  Figure 6.  Topological representation of a 3, 4-connected framework with a new topology viewed along the b-axis

  Cyan: 4-connected Zn1 nodes; Gray: centroids of 3-connected μ₃-cpna^2- nodes; Dark blue: centroids of 2-connected μ-dpey linkers.

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  2.2   Thermal stability

  To determine the thermal stability of compounds 1 and 2, their thermal behaviors were investigated under a nitrogen atmosphere by TGA. As shown in Fig. 7, 1 lost its one lattice water molecule in a range of 82-113 ℃ (Obsd. 4.2%, Calcd. 4.4%), followed by the decomposition at 255 ℃. For 2, there was a release of one lattice water molecule between 87 and 134 ℃ (Obsd. 4.1%, Calcd. 4.3%), whereas a dehydrated sample remained stable up to 238 ℃.

  Figure 7

  

  Figure 7.  TGA curves of compounds 1 and 2

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  2.3   Catalytic Knoevenagel condensation reaction

  Considering the potential of different M(Ⅱ) coordination compounds to act as catalysts in the Knoevenagel condensation reaction^{[11, 25-27]}, we probed the compounds 1 and 2 as heterogeneous catalysts in this reaction using assorted aldehydes with malononitrile. As a model substrate, benzaldehyde was treated with malononitrile at 25 ℃ in a methanol medium to form a 2-benzylidene malononitrile product (Scheme 2, Table 1). The influence of different reaction parameters (i.e., reaction time, solvent type, catalyst loading and recycling, and substrate scope) was studied.

  Scheme 2

  

  Scheme 2.  Catalytic the Knoevenagel condensation reaction of benzaldehyde with malononitrile (model substrate)

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  Table 1

  

  Table 1.  Catalytic Knoevenagel condensation reaction of benzaldehyde with malononitrile

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  Entry	Catalyst	Time/min	xcata/%	Solvent	Yb/%
  1	2	10	2.0	CH3OH	55
  2	2	20	2.0	CH3OH	70
  3	2	30	2.0	CH3OH	80
  4	2	40	2.0	CH3OH	88
  5	2	50	2.0	CH3OH	95
  6	2	60	2.0	CH3OH	100
  7	2	60	2.0	H2O	99
  8	2	60	2.0	C2H5OH	97
  9	2	60	2.0	CH3CN	86
  10	2	60	2.0	CHCl3	65
  11	2	60	1.0	CH3OH	94
  12	1	60	2.0	CH3OH	88
  13	Blank	60		CH3OH	21
  14	ZnCl2	60	2.0	CH3OH	32
  15	H2cpna	60	2.0	CH3OH	27
  a Catalyst loading; b The yield (Y) was calculated by 1H NMR spectroscopy: Y=nproduct/naldehyde×100%.

  Compound 2 revealed the highest activity, resulting in a 100% conversion of benzaldehyde to 2-benzylidene malononitrile (Table 1 and Fig.S1). Compound 2 was used to research the influence of different reaction parameters. The yield was accumulated with a yield increase from 55% to 100% on prolonging the reaction from 10 to 60 min (Table 1, Entry 1-6). The influence of catalyst loading was also investigated, revealing a product yield growth from 94% to 100% on increasing the catalyst loading (x_cat) from 1.0% to 2.0% (Entry 6 and 11). In addition to methanol, other solvents were tested. Water, ethanol, acetonitrile, and chloroform were less suitable (65%-99% product yields, respectively).

  In comparison with 2, compound 1 is less active, resulting in a maximum product yield of 88% (Entry 12, Table 1). It should be highlighted that under similar reaction conditions, the Knoevenagel condensation reaction of benzaldehyde with malononitrile was significantly less efficient in the absence of the catalyst (only 21% product yield) or when using H₂cpna (27% yield) or ZnCl₂ (32% yield) as catalysts (Entry 13-15, Table 1).

  Different substituted benzaldehyde substrates were used to study the substrate scope in the Knoevenagel condensation of benzaldehyde with malononitrile. These tests were run under optimized conditions (x₂=2.0%, CH₃OH, 25 ℃, 60 min). The corresponding products were obtained in the yields varying from 41% to 100% (Table 2). Benzaldehydes containing a strong electron-withdrawing group (e.g., nitro, and chloro substituent in the ring) revealed the best efficiency (Entry 2-5, Table 2), which can be explained by an increased electrophilicity of substrates. The benzaldehydes possessing an electron-donating functionality (e.g., methyl or methoxy group) led to lower product yields (Entry 7 and 8, Table 2).

  Table 2

  

  Table 2.  Knoevenagel condensation reaction of various aldehydes with malononitrile catalyzed by compound 2^a

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  Entry	Substituted benzaldehyde substrate (R-C6H4CHO)	Yb/%
  1	R=H	100
  2	R=2-NO2	100
  3	R=3-NO2	100
  4	R=4-NO2	100
  5	R=4-Cl	100
  6	R=4-OH	41
  7	R=4-CH3	97
  8	R=4-OCH3	75
  a Reaction conditions: aldehyde (0.5 mmol), malononitrile (1.0 mmol), x2=2.0%, and CH3OH (1.0 mL) at 25 ℃; b Calculated by 1H NMR spectroscopy.

  Finally, the recyclability of the catalyst 2 was tested. After each reaction cycle, the catalyst was separated via centrifugation, washed with CH₃OH, dried in air at 25 ℃, and reused in the next cycle. The obtained results prove that compound 2 preserves the activity for at least five reaction cycles (the yields were 100%, 100%, 98%, and 96% for the second to fifth run, respectively). Besides, the PXRD patterns confirm that the structure of 2 was maintained (Fig.S2), despite the appearance of several additional signals or widening of some peaks. These alterations might be expected after a few catalytic cycles and are explained by the presence of some impurities or a decrease in crystallinity.

  2.4   Friction and wear test

  The use of inorganic additives is a novel method for improving the operational efficiency of engines. Lubricating additives can swiftly permeate the friction area to prevent direct contact between the surfaces of the friction pair^[28]. On a damaged friction surface, they are more likely to precipitate or form a protective coating. Inspired by the fact that zinc dialkyl dithiophosphate (ZDDP) is an excellent anti-wear additive commonly used in engine oils, Zinc(Ⅱ) coordination polymer (2) was used as an additive in PAO10 to evaluate the anti-wear performance.

  The friction coefficient curves versus sliding time for PAO10 and 2/PAO10 samples at 25 ℃ are shown in Fig. 8. The friction coefficient of PAO10 increased sharply at 300 s, stabilizing at approximately 0.12. For 2/PAO10, the friction coefficient curve was steady. The friction coefficient was lower than that of PAO10 in all test times and not over 0.83. This indicates that the addition of compound 2 reduces the friction coefficient of the lubricating oil, which will be a potential application as an additive in lubricating oil.

  Figure 8

  

  Figure 8.  Friction curves versus sliding time for PAO10 and 2/PAO10

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  3.   Conclusions

  In summary, we have synthesized two Ni(Ⅱ) and Zn(Ⅱ) coordination polymers 1 and 2 based on a phenylpyridine-dicarboxylate ligand. Compounds 1 and 2 disclose 3D frameworks. Compound 2 exhibited an effective catalytic activity in the Knoevenagel condensation reaction at room temperature. Meanwhile, compound 2 showed an effective anti-wear activity in PAO10, which is a potential application as an additive in lubricating oil.

  Supporting information is available at http://www.wjhxxb.cn

  
  
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  Figure 1  Drawing of the asymmetric unit of compound 1 with a 30% probability of thermal ellipsoids

  H atoms and lattice water molecules are omitted for clarity; Symmetry codes: A: -x, y-1/2, -z+1/2; B: -x+1, y-1/2, -z+1/2; C: -x, -y+1, -z.

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  Scheme 1  Coordination modes of cpna^2- ligands

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  Figure 2  Three-dimensional framework of compound 1 viewed along the b-axis

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  Figure 3  Topological representation of a 2-nodal framework with an ins topology viewed along the b-axis

  Green: Ni1 nodes; Gray: centroids of 3-linked μ₃-cpna^2- nodes; Dark blue: centroids of 2-connected μ-dpea linkers.

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  Figure 4  Drawing of the asymmetric unit of compound 2 with a 30% probability of thermal ellipsoids

  H atoms and lattice water molecules are omitted for clarity; Symmetry codes: A: -x+1/2, y-1/2, z; B: -x+1, y-1/2, -z+3/2; C: -x, -y+1, -z+2.

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  Figure 5  Three-dimensional framework of compound 2 viewed along the b-axis

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  Figure 6  Topological representation of a 3, 4-connected framework with a new topology viewed along the b-axis

  Cyan: 4-connected Zn1 nodes; Gray: centroids of 3-connected μ₃-cpna^2- nodes; Dark blue: centroids of 2-connected μ-dpey linkers.

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  Figure 7  TGA curves of compounds 1 and 2

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  Scheme 2  Catalytic the Knoevenagel condensation reaction of benzaldehyde with malononitrile (model substrate)

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  Figure 8  Friction curves versus sliding time for PAO10 and 2/PAO10

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  Table 1.  Catalytic Knoevenagel condensation reaction of benzaldehyde with malononitrile

  Entry	Catalyst	Time/min	xcata/%	Solvent	Yb/%
  1	2	10	2.0	CH3OH	55
  2	2	20	2.0	CH3OH	70
  3	2	30	2.0	CH3OH	80
  4	2	40	2.0	CH3OH	88
  5	2	50	2.0	CH3OH	95
  6	2	60	2.0	CH3OH	100
  7	2	60	2.0	H2O	99
  8	2	60	2.0	C2H5OH	97
  9	2	60	2.0	CH3CN	86
  10	2	60	2.0	CHCl3	65
  11	2	60	1.0	CH3OH	94
  12	1	60	2.0	CH3OH	88
  13	Blank	60		CH3OH	21
  14	ZnCl2	60	2.0	CH3OH	32
  15	H2cpna	60	2.0	CH3OH	27
  a Catalyst loading; b The yield (Y) was calculated by 1H NMR spectroscopy: Y=nproduct/naldehyde×100%.

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  Table 2.  Knoevenagel condensation reaction of various aldehydes with malononitrile catalyzed by compound 2^a

  Entry	Substituted benzaldehyde substrate (R-C6H4CHO)	Yb/%
  1	R=H	100
  2	R=2-NO2	100
  3	R=3-NO2	100
  4	R=4-NO2	100
  5	R=4-Cl	100
  6	R=4-OH	41
  7	R=4-CH3	97
  8	R=4-OCH3	75
  a Reaction conditions: aldehyde (0.5 mmol), malononitrile (1.0 mmol), x2=2.0%, and CH3OH (1.0 mL) at 25 ℃; b Calculated by 1H NMR spectroscopy.

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