English

• 0.   Introduction

  In recent years, the design and hydro(solvo)ther-mal in situ syntheses of metal-organic coordination plymers have attracted great interest in the field of coordination chemistry and organic chemistry not only because of their intriguing architectures and topologies, but also for that they have shown a variety of potential applications in catalysis, magnetism, luminescence and gas absorption^[1-8].Commpared with the traditional synthesis method, hydrothermal and solvothermal method could create more chances for in situ synthese of ligands due to the reaction conditions of high temperature and high pressure^[9-12].At the same time, the hydro(solvo)thermal method has demonstrated increasing success in providing alternative pathways to crystalline complexes with in situ synthesized ligands which are difficult to obtain by routine synthetic methods.

  Following our interest in the exploration of novel and poorly investigated multicarboxylic acids for the design of coordination polymers^[13-17], recently, we began to construct coordination polymers by use of the advantage of in situ ligand reaction.On the basis of current research on in situ ligand reaction, the carboxylate-based ligands can be in situ generated from the CN-containing ligands precursor^{[7, 18-19]}.Thus, the precursor we selected is 5-(2, 3-dicyanobenzoxy)nicotic acid (Hdbna) due to its following characteristics: (1) it contains one carboxylate and two CN groups, which can form a tricarboxylate ligand through in situ ligand reaction (Scheme 1); (2) its corresponding acid, 5-(2, 3-dicarboxylphenoxy)nicotic acid (H₃dpna), still remain largely unexplored in the construction of coordination polymers^[20].

  Herein, we report the synthesis, crystal structure, and catalytic properties of Mn(Ⅱ) coordination polymer with H₃dpna ligands.

  Scheme 1

  

  Scheme 1.  H₃dpna ligand formed via in situ reaction

  ]

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  1.   Experimental

  1.1   Reagents and physical measurements

  All chemicals and solvents were of AR grade and used without further purification.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[Mn(μ₃-Hdpna)(2, 2′-bipy)]_n(1)

  A mixture of Mn Cl₂·4H₂O (0.040 g, 0.2 mmol), Hdbna (0.053 g, 0.2 mmol), 2, 2′-bipy (0.031 g, 0.2mmol), Na OH (0.016 g, 0.4 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.Yellow blockshaped crystals were isolated manually, and washed with distilled water.Yield: 55% (based on Hdbna).Anal.Calcd.for C₂₄H₁₅Mn N₃O₇(%): C 56.26, H 2.95, N8.20;Found(%): C 56.48, H 2.93, N 8.16.IR (KBr, cm^-1): 1 688w, 1 598m, 1 553s, 1 470s, 1 437s, 1 412m, 1 371m, 1 301w, 1 256m, 1 210w, 1 148w, 1 062w, 1 017w, 976w, 905w, 844w, 819w, 794w, 761m, 736w, 716w, 695w, 629w.

  The compound is insoluble in water and common organic solvents, such as methanol, ethanol, acetone and DMF.

  1.3   Structure determination

  The single crystal with dimensions of 0.24 mm×0.21 mm×0.20 mm was collected at 293(2) K on a Bruker SMART APEX Ⅱ CCD diffractometer with Cu Kα radiation (λ=0.154 178 nm).The structure was solved by direct methods and refined by full matrix least-square on F²using the SHELXTL-2014 program^[21].All non-hydrogen atoms were refined anisotropically.All the hydrogen atoms were positioned geometrically and refined using a riding model.A summary of the crystallography data and structure refinement for 1 is given in Table 1.The selected bond lengths and angles for compound 1 are listed in Table 2.Hydrogen bond parameters of compound 1 are given in Table 3.

  Table 1

  

  Table 1.  Crystal data for compound 1

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  Formula	C24H15MnN3O7		F(000)	522
  Formula weight	512.33		θ range for data collection / (°)	3.606~65.998
  Crystal system	Triclinic		Limiting indices	-9 < h < 7, -12 < k < 13, -15 < l < 15
  Space group	P1		Reflection collected, unique (Rint)	6 931, 3 547 (0.076 5)
  a / nm	0.784 98(8)		Dc / (g·cm-3)	1.617
  b / nm	1.130 30(13)		μ / mm-1	5.592
  c / nm	1.290 01(11)		Data, restraint, parameter	3 547, 0, 317
  α/ (°)	73.119(9)		Goodness-of-fit on F2	1.015
  β / (°)	79.443(8)		Final R indices [I≥2σ(I)] R1, wR2	0.067 5, 0.149 9
  γ /(°)	75.309(10)		R indices (all data) R1, wR2	0.119 4, 0.199 3
  V /nm3	1.052 0(2)		Largest diff. peak and hole / (e·nm-3)	512 and -888
  Z	2

  Table 2

  

  Table 2.  Selected bond distances (nm) and bond angles (°) for compound 1

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  Mn1—O1	0.219 4(4)	Mn1—O2	0.230 7(4)	Mn1—O6A	0.208 5(4)
  Mn1—N1B	0.222 7(5)	Mn1—N2	0.225 9(5)	Mn1—N3	0.228 5(5)
  O6A—Mn1—O1	106.66(18)	O6A—Mn1—N1B	100.3(2)	O1—Mn1—N1B	90.28(18)
  O6A—Mn1—N2	90.0(2)	N2—Mn1—O1	158.02(19)	N1B—Mn1—N2	101.0(2)
  N3—Mn1—O6A	161.3(2)	O1—Mn1—N3	91.28(18)	N1B—Mn1—N3	84.60(19)
  N2—Mn1—N3	71.3(2)	O6A—Mn1—O2	93.6(2)	O1—Mn1—O2	58.29(16)
  O2—Mn1—N1B	148.27(18)	O2—Mn1—N2	107.48(18)	O2—Mn1—N3	91.34(19)
  Symmetry codes: A: x, y+1, z; B: x+1, y, z.

  Table 3

  

  Table 3.  Hydrogen bond parameters of compound 1

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  D—H…A	d(D—H) / nm	d(H…A) / nm	d(D…A) / nm	∠DHA / (°)
  O4—H4…O7	0.082 0	0.159 6	0.240 9	170.89

  CCDC: 2025016.

  1.4   Catalytic cyanosilylation of aldehydes

  In a typical test, a suspension of an aromatic aldehyde (0.50 mmol, 4-nitrobenzaldehyde as a model substrate), trimethylsilyl cyanide (1.0 mmol) and catalyst(molar fraction: 3%) in dichloromethane (2.5 mL) was stirred at room temperature.After a 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 solid was dissolved in CDCl₃ and analyzed by ¹H NMR spectroscopy for quantification of products (Fig.S1, Supporting information).To perform the recycling experiment, the catalyst was isolated by centrifugation, washed with dichloromethane, dried at room temperature, and reused.The subsequent steps were performed as described above.

  2.   Results and discussion

  2.1   Description of the structure

  X-ray crystallography analysis reveals that compound 1 crystallizes in the triclinic system space group P1.As shown in Fig. 1, the asymmetric unit of 1 bears one crystallographically unique Mn(Ⅱ) atom, one μ₃-Hdpna^2- block, and one 2, 2′-bipy.The Mn1 center is six-coordinated and forms a distorted octahedral{Mn N₃O₃}geometry.It is completed by three carboxylate O and one N atoms from three μ₃-Hdpna^2- blocks and two N atoms from the 2, 2′-bipy moiety.The Mn—O and Mn—N bond distances are 0.208 5(4)~0.230 7(4)nm and 0.222 7(5)~0.228 5(5) nm, respectively; these are within the normal ranges observed in related Mn(Ⅱ) compounds^[15-17].In 1, the Hdpna^2- ligand adopts the coordination mode Ⅰ (Scheme 2) with two deprontonated carboxylate groups being monodentate or bidentate.In the Hdpna^2- ligand, a dihedral angle (between two aromatic rings) and a C—O_ether—C angle are 69.75° and 116.43°, respectively.The μ₃-Hdpna^2- ligands connect Mn1 atoms to give a 2D sheet (Fig. 2).This sheet is composed of the 3-connected, topologically equivalent Mn1 and μ₃-Hdpna^2- nodes (Fig. 3).The resulting net can be described as a uninodal 3-connected layer with an hcb (Shubnikov hexagonal plane net/(6, 3)) topology and point symbol of (6³).

  Figure 1

  

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

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  Scheme 2

  

  Scheme 2.  Coordination mode of Hdpna^2- ligand in compound 1

  ]

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  Figure 2

  

  Figure 2.  Perspective of 2D sheet along a and b axes in 1

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  Figure 3

  

  Figure 3.  Topological representation of uninodal 3-connected metal-organic layer with hcb (Shubnikov hexagonal plane net/(6, 3)) topology viewed along c axis

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  2.2   TGA analysis

  To determine the thermal stability of compound 1, its thermal behavior was investigated under nitrogen atmosphere by TGA.As shown in Fig. 4, compound 1 did not contain solvent of crystallization or H₂O ligands and remained stable up to 312 ℃, followed by a decomposition on further heating.

  Figure 4

  

  Figure 4.  TGA curve of compound 1

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  2.3   Catalytic cyanosilylation of aldehydes

  Given the potential of manganes(Ⅱ) coordination compounds to catalyze the organic reactions^[22-23], we explored the application of 1 as a heterogeneous catalyst in the cyanosilylation of 4-nitrobenzaldehyde as a model substrate to give 2-(4-nitrophenyl)-2-((trimethyl-silyl)oxy)acetonitrile.Typical tests were carried out by reacting a mixture of 4-nitrobenzaldehyde, trimethylsilyl cyanide (TMSCN) and a Mn catalyst in dichloromethane at room temperature (Scheme 3, Table 4).Such effects as reaction time, catalyst loading, solvent composition, catalyst recycling and finally substrate scope were investigated.

  Scheme 3

  

  Scheme 3.  Mn-catalyzed cyanosilylation of 4-nitrobenzaldehyde (model substrate)

  ]

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  Table 4

  

  Table 4.  Mn⁃catalyzed cyanosilylation of 4⁃nitrobenzaldehyde with TMSCN

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  Entry	Catalyst	Time / h	Catalyst loading / % (n/n)	Solvent	Yield* / %
  1	1	1	3.0	CH2Cl2	29
  2	1	2	3.0	CH2Cl2	53
  3	1	4	3.0	CH2Cl2	62
  4	1	6	3.0	CH2Cl2	69
  5	1	8	3.0	CH2Cl2	75
  6	1	10	3.0	CH2Cl2	78
  7	1	12	3.0	CH2Cl2	82
  8	1	12	2.0	CH2Cl2	63
  9	1	12	4.0	CH2Cl2	84
  10	1	12	3.0	CH3CN	71
  11	1	12	3.0	THF	63
  12	1	12	3.0	CH3OH	75
  13	1	12	3.0	CH3Cl	77
  14	blank	12	—	CH2Cl2	3
  15	MnCl2	12	3.0	CH2Cl2	7
  16	H3dpna	12	3.0	CH2Cl2	6
  * Calculated by 1H NMR spectroscopy: nproduct/naldehyde×100%.

  Upon using compound 1 as the catalyst (molar fraction: 3%), a high conversion of 82% of 4-nitrobenz-aldehyde into 2-(4-nitrophenyl)-2-((trimethylsilyl)oxy) acetonitrile was reached after 12 h in dichloromethane at room temperature (Table 4, Entry 7).Upon extending the reaction time further to 18 h, only a slight increase in the product yield to 83% occurred.Moreover, no other products were detected, and the yield of this product was considered the conversion of 4-nitrobenzaldehyde (Fig.S1).

  We also compared the activities of catalyst 1 in the reactions of other substituted aromatic and aliphatic aldehydes with trimethylsilyl cyanide, and the corresponding cyanohydrin derivatives were produced in yields ranging from 43% to 82% (Table 5).Aryl aldehydes bearing strong electron-withdrawing substituents (e.g., nitro and chloro) exhibited the higher reactivities(Table 5, Entry 2~5), which may be related to an increase in the electrophilicity of the substrate.Aldehydes containing electron-donating proups (e.g., methyl) showed lower reaction yields (Table 5, Entry 7), as expected.An ortho-substituted aldehyde showed lower reactivity, possibly as a result of steric hindrance.

  Table 5

  

  Table 5.  Cyanosilylation of various aldehydes with TMSCN catalyzed by compound 1^a

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  Entry	Substituted benzaldehyde substrate (R—C6H4CHO)	Product yieldb / %
  1	R=H	52
  2	R=2-NO2	72
  3	R=3-NO2	78
  4	R=4-NO2	82
  5	R=4-Cl	54
  6	R=4-OH	48
  7	R=4-CH3	43
  a Reaction condition: aldehyde (0.5 mmol), TMSCN (1.0 mmol), catalyst 1(molar fraction: 3.0%) and CH2Cl2(2.5 mL) at room temperature; b Calculated by 1H NMR spectroscopy.

  To examine the stability of 1 in the cyanosilylation reaction, we tested the recyclability of this heterogeneous catalyst.For this purpose, upon completion of a reaction cycle, we separated the catalyst by centrifugation, washed it with CH₂Cl₂, and dried it at room temperature before its further use.The catalytic system mained the high activity in at least five consecutive cycles (the yields were 80%, 80%, 79% and 78% for second to fifth run, respectively).According to the PXRD data (Fig. 5).the structure of 1 was essentially preserved after five catalytic cycles.

  Figure 5

  

  Figure 5.  PXRD patterns for 1

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  A possible catalytic cycle for the cyanosilylationreaction catalyzed by a Mn coordination polymer is proposed in Scheme 4.It can involve dual activation of thecarbonyl and TMSCN by the Mn(Ⅱ) centre and a ligatedcarboxylate group, respectively, followed by the formation of the C—C bond leading to cyanohydrin^[24-25].

  Scheme 4

  

  Scheme 4.  Mechanism for the Mn-catalyzedcyanosilylation reaction

  ]

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  3.   Conclusions

  In summary, we have synthesized one Mn(Ⅱ) coordination polymer 1 based on a tricarboxylate ligandgenerated by in situ reaction.Compound 1 shows a 2D sheet structure, and it exhibits a higher catalytic activityin the cyanosilylation at room temperature.

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

  Scheme 1  H₃dpna ligand formed via in situ reaction

  ]

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

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  Scheme 2  Coordination mode of Hdpna^2- ligand in compound 1

  ]

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  Figure 2  Perspective of 2D sheet along a and b axes in 1

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  Figure 3  Topological representation of uninodal 3-connected metal-organic layer with hcb (Shubnikov hexagonal plane net/(6, 3)) topology viewed along c axis

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  Figure 4  TGA curve of compound 1

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  Scheme 3  Mn-catalyzed cyanosilylation of 4-nitrobenzaldehyde (model substrate)

  ]

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  Figure 5  PXRD patterns for 1

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  Scheme 4  Mechanism for the Mn-catalyzedcyanosilylation reaction

  ]

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  Table 1.  Crystal data for compound 1

  Formula	C24H15MnN3O7		F(000)	522
  Formula weight	512.33		θ range for data collection / (°)	3.606~65.998
  Crystal system	Triclinic		Limiting indices	-9 < h < 7, -12 < k < 13, -15 < l < 15
  Space group	P1		Reflection collected, unique (Rint)	6 931, 3 547 (0.076 5)
  a / nm	0.784 98(8)		Dc / (g·cm-3)	1.617
  b / nm	1.130 30(13)		μ / mm-1	5.592
  c / nm	1.290 01(11)		Data, restraint, parameter	3 547, 0, 317
  α/ (°)	73.119(9)		Goodness-of-fit on F2	1.015
  β / (°)	79.443(8)		Final R indices [I≥2σ(I)] R1, wR2	0.067 5, 0.149 9
  γ /(°)	75.309(10)		R indices (all data) R1, wR2	0.119 4, 0.199 3
  V /nm3	1.052 0(2)		Largest diff. peak and hole / (e·nm-3)	512 and -888
  Z	2

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  Table 2.  Selected bond distances (nm) and bond angles (°) for compound 1

  Mn1—O1	0.219 4(4)	Mn1—O2	0.230 7(4)	Mn1—O6A	0.208 5(4)
  Mn1—N1B	0.222 7(5)	Mn1—N2	0.225 9(5)	Mn1—N3	0.228 5(5)
  O6A—Mn1—O1	106.66(18)	O6A—Mn1—N1B	100.3(2)	O1—Mn1—N1B	90.28(18)
  O6A—Mn1—N2	90.0(2)	N2—Mn1—O1	158.02(19)	N1B—Mn1—N2	101.0(2)
  N3—Mn1—O6A	161.3(2)	O1—Mn1—N3	91.28(18)	N1B—Mn1—N3	84.60(19)
  N2—Mn1—N3	71.3(2)	O6A—Mn1—O2	93.6(2)	O1—Mn1—O2	58.29(16)
  O2—Mn1—N1B	148.27(18)	O2—Mn1—N2	107.48(18)	O2—Mn1—N3	91.34(19)
  Symmetry codes: A: x, y+1, z; B: x+1, y, z.

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  Table 3.  Hydrogen bond parameters of compound 1

  D—H…A	d(D—H) / nm	d(H…A) / nm	d(D…A) / nm	∠DHA / (°)
  O4—H4…O7	0.082 0	0.159 6	0.240 9	170.89

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  Table 4.  Mn⁃catalyzed cyanosilylation of 4⁃nitrobenzaldehyde with TMSCN

  Entry	Catalyst	Time / h	Catalyst loading / % (n/n)	Solvent	Yield* / %
  1	1	1	3.0	CH2Cl2	29
  2	1	2	3.0	CH2Cl2	53
  3	1	4	3.0	CH2Cl2	62
  4	1	6	3.0	CH2Cl2	69
  5	1	8	3.0	CH2Cl2	75
  6	1	10	3.0	CH2Cl2	78
  7	1	12	3.0	CH2Cl2	82
  8	1	12	2.0	CH2Cl2	63
  9	1	12	4.0	CH2Cl2	84
  10	1	12	3.0	CH3CN	71
  11	1	12	3.0	THF	63
  12	1	12	3.0	CH3OH	75
  13	1	12	3.0	CH3Cl	77
  14	blank	12	—	CH2Cl2	3
  15	MnCl2	12	3.0	CH2Cl2	7
  16	H3dpna	12	3.0	CH2Cl2	6
  * Calculated by 1H NMR spectroscopy: nproduct/naldehyde×100%.

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  Table 5.  Cyanosilylation of various aldehydes with TMSCN catalyzed by compound 1^a

  Entry	Substituted benzaldehyde substrate (R—C6H4CHO)	Product yieldb / %
  1	R=H	52
  2	R=2-NO2	72
  3	R=3-NO2	78
  4	R=4-NO2	82
  5	R=4-Cl	54
  6	R=4-OH	48
  7	R=4-CH3	43
  a Reaction condition: aldehyde (0.5 mmol), TMSCN (1.0 mmol), catalyst 1(molar fraction: 3.0%) and CH2Cl2(2.5 mL) at room temperature; b Calculated by 1H NMR spectroscopy.

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