English

• 0.   Introduction

  The design of functional coordination polymers (CPs) turned to be a hot research topic^[1-3], especially considering a huge diversity of potential applications of such complexes and derived materials in gas storage and separation^[4-6], catalysis^[7-10], molecular magnetism^[11-12], photochemistry^[13-15], and selective sensing^[16-18]. Although the assembly of CPs may depend on many parameters (e. g., types of metal ions^[10-11], linkers^{[16, 18]}, supporting ligands^[19-20], solvent^[21-22], molar ratios of reagents^[23-25] and temperature conditions^[26-27]), the nature of metal ions and organic linkers are undoubtedly the key factors that usually define the structures and functional properties of the obtained complexes. Aromatic carboxylic acids containing several COOH functionalities represent the most populated family of linkers in the research on CPs, especially given their availability and reasonable cost, thermal stability, possibility of further functionalization and multitude of coordination modes^{[22-23, 28-29]}.

  In particular, terephthalic acid is one of the sim-plest and common building blocks that is widely used for the synthesis of different CPs and MOFs (metal-organic frameworks) ^[30-33]. However, the functionalized terephthalate ligands remain significantly less investigated^[30]. For example, an introduction of two hydroxy substituents into terephthalic acid may well affect its coordination behavior and structures of the resulting complexes. Given an unexplored coordination chemistry of 2, 3-dihydroxy-terephthalic acid^[34-35], the main aim of the present work consisted in widening the family of metal(Ⅱ) coordination polymers with this dicarboxylate linker and searching for potential catalytic applications of the obtained complexes.

  Herein, we report the syntheses, crystal structures, luminescent and catalytic properties of two Mn(Ⅱ) and Cd(Ⅱ) coordination polymers constructed from the 2, 3-dihydroxy-terephthalic acid.

  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 Xray diffraction patterns (PXRD) 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, and the data collection range was between 5° and 45°. Excitation and emission spectra were recorded on an Edinburgh FLS920 fluorescence spectrometer using the solid samples at room temperature. Solution 1H NMR spectra were recorded on a JNM ECS 400M spectrometer.

  1.2   Syntheses of [M(μ₄-H₂DTA) (bipy)]_n (M=Mn (1) and Cd (2))

  Amixture of MCl₂·xH₂O (x=4 for 1 and x=1 for 2; 0.20 mmol), H₄DTA (0.040 g, 0.20 mmol), bipy (0.031 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 days, followed by cooling to room temperature at a rate of 10 ℃·h^-1. Yellow (1) or colourless (2) block-shaped crystals were isolated manually, and washed with distilled water. Yield: 51% for 1 and 42% for 2 (based on H₄DTA). Anal. Calcd. for C₁₈H₁₂MnN₂O₆ (1, %): C 53.09, H 2.97, N 6.88; Found(%): C 53.01, H 2.99, N 6.83. IR (KBr, cm^-1): 1 638w, 1 594s, 1 474m, 1 441m, 1 388s, 1 330m, 1 260m, 1 222w, 1 132w, 1 062w, 1 013w, 852w, 835w, 810m, 764m, 736w, 646w. Anal. Calcd. for C₁₈H₁₂CdN₂O₆ (2, %): C 46.52, H 2.60, N 6.03; Found (%): C 46.41, H 2.58, N 6.06. IR (KBr, cm^-1): 1 631w, 1 594s, 1 474m, 1 441m, 1 383s, 1 330m, 1 251m, 1 222w, 1 132w, 1 062w, 1 013w, 860w, 811m, 770m, 736w, 649w.

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

  1.3   Structure determination

  Two single crystals with dimensions of 0.25 mm× 0.23 mm×0.22 mm (1) and 0.23 mm×0.22 mm×0.21 am(2) were collected at 293(2) K on a Bruker SMART APEX Ⅱ CCD diffractometer with Cu Kα radiation (λ= 0.154 178 nm). The structures were solved by direct methods and refined by full matrix least-square on F² using the SHELXTL-2014 program^[36]. 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 refinements for 1 and 2 is given in Table 1. The selected bond lengths and angles for complexes 1 and 2 are listed in Table 2.

  Table 1

  

  Table 1.  Crystal data for complexes 1 and 2

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  Complex	1	2
  Chemical formula	C18H12MnN2O6	C18H12CdN2O6
  Molecular weight	407.24	464.70
  Crystal system	Orthorhombic	Orthorhombic
  Space group	Pnna	Pnna
  a/nm	0.964 95(4)	0.976 18(3)
  b/nm	1.692 31(8)	1.685 18(4)
  c/nm	1.024 19(5)	1.042 57(3)
  V/nm3	1.672 51(13)	1.715 06(9)
  Z	4	4
  F(000)	828	920
  θ range for data collection/(°)	5.048~69.827	4.988~66.524
  Limiting indices	-11 ≤ h ≤ 9, -20 ≤ k ≤ 18, -7 ≤ l ≤ 12	-7 ≤ h ≤ 11, -20 ≤ k ≤ 18, -12 ≤ l ≤ 10
  Reflection collected, unique (Rint)	5 632, 1 573 (0.068 9)	5 351, 1 494 (0.042 2)
  Dc/(g·cm-3)	1.617	1.800
  μ/mm-1	6.791	10.564
  Data, restraint, parameter	1 573, 0, 124	1 494, 0, 124
  Goodness-of-fit on F2	1.049	1.021
  Final R indices [I≥2σ(I)] R1, wR2	0.046 7, 0.097 9	0.047 5, 0.120 4
  R indices (all data) R1, wR2	0.084 9, 0.107 5	0.051 8, 0.127 3
  Largest diff. peak and hole/(e·nm-3)	333 and -433	929 and -3 169

  Table 2

  

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

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  1
  Mn1—O1	0.213 7(2)	Mn1—O1B	0.213 7(2)	Mn1—O2C	0.217 8(2)
  Mn1—O2D	0.217 8(2)	Mn1—N1	0.229 8(3)	Mn1—N1B	0.229 8(3)
  O1—Mn1—O1B	164.71(15)	O2D—Mn1—O1B	90.39(9)	O2D—Mn1—O1	80.50(9)
  O2D—Mn1—O2C	106.99(14)	O1—Mn1—N1	93.79(11)	O1—Mn1—N1B	98.68(10)
  O2C—Mn1—N1	91.57(11)	O2D—Mn1—N1	160.51(11)	N1—Mn1—N1B	70.75(16)
  2
  Cd1—O1	0.225 6(2)	Cd1—O1B	0.225 6(2)	Cd1—O2C	0.228 8(2)
  Cd1—O2D	0.228 8(2)	Cd1—N1	0.234 9(3)	Cd1—N1B	0.234 9(3)
  O1—Cd1—O1B	107.74(17)	O2D—Cd1—O1B	89.25(10)	O2D—Cd1—O1	75.84(9)
  O2D—Cd1—O2C	154.75(14)	O1—Cd1—N1	91.97(12)	O1—Cd1—N1B	158.64(11)
  O2C—Cd1—N1	96.81(11)	O2D—Cd1—N1	103.85(11)	N1—Cd1—N1B	70.04(16)
  Symmetry codes: B: -x+1/2, -y+1, z; C: x+1/2, y, -z+1; D: -x, -y+1, -z+1 for 1; B: -x+1/2, -y+1, z; C: x+1/2, y, -z+1; D: -x, -y+1, -z+1 for 2.

  CCDC: 2060258, 1; 2060259, 2.

  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 the catalyst (typically, molar fraction=3%) in dichloromethane (2.5 mL) was stirred at 35 ℃. 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 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 dichloromethane, dried at room temperature, and reused. The subsequent steps were performed as described above.

  2.   Results and discussion

  2.1   Description of structure of 1 and 2

  Complexes 1 and 2 are isostructural (Table 1) and the structure of 1 is discussed in detail as an example. Complex 1 reveals a 3D metal-organic framework with the asymmetric unit containing a Mn(Ⅱ) ion (with half occupancy), a half of μ₄-H₂DTA^2- ligand, and a half of bipy moiety. The Mn center is six-coordinated and reveals a octahedral {MnN₂O₄} environment, which is completed by four carboxylate O atoms from four individual μ₄-H₂DTA^2- blocks and a pair of N atoms from the bipy moiety (Fig. 1). The lengths of Mn—O and Mn—N bonds are 0.213 7(3)~0.217 8(3) nm and 0.229 8(3) nm, respectively; these are within the normal values for related Mn(Ⅱ) derivatives^{[11, 22, 27]}. The H₂DTA^2- block acts as a μ₄-linker via bidentate COO^- groups (Scheme 1). The intramolecular hydrogen bond (O3—H3…O2) was found. The μ₄-H₂DTA^2- blocks connect adjacent Mn1 ions to form a 3D metal-organic framework (Fig. 2). This crystal structure features an in-tricate 3D metal-organic net that is built from the 4-connected, topologically distinct Mn1 and μ₄-H₂DTA^2- nodes (Fig. 3). The resulting net can be classified as a binodal 4, 4-connected framework with a pts (PtS, coop-erate) topology and point symbol of (4².8⁴).

  Figure 1

  

  Figure 1.  Drawing of coordination environment around Mn center in complex 1 with 30% probability thermal ellipsoids

  Hydrogen atoms are omitted for clarity except those bonding to oxygen atoms; Symmetry codes: A: x, -y+1/2, -z+3/2; B: -x+1/2, -y+1, z; C: x+1/2, y, -z+1; D: -x, -y+1, -z+1

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  Scheme 1

  

  Scheme 1.  Coordination mode of H₂DTA^2- ligand in complex 1

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  Figure 2

  

  Figure 2.  Perspective of 3D metal-organic framework along a axis

  Bipy ligands are omitted for clarity

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  Figure 3

  

  Figure 3.  Topological representation of binodal 4, 4-connected metal-organic framework with a pts topology viewed along a axis

  Green balls: 4-connected Mn1, Gray: centroids of 4-connected μ₄-H₂DTA^2- nodes

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  2.2   TGA analysis

  To determine the thermal stability of complexes 1 and 2, their thermal behaviors were investigated under nitrogen atmosphere by TGA. As shown in Fig. 4, both complexes did not contain solvent of crystallization or H₂O ligands and remains stable up to 305 and 282 ℃, respectively, followed by a decomposition on further heating.

  Figure 4

  

  Figure 4.  TGA curves of complexes 1 and 2

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  2.3   Luminescent properties

  Solid-state emission spectra of H₄DTA and complex 2 were measured at room temperature (Fig. 5). The spectrum of H₄DTA revealed a weak emission with a maximum at 426 nm (λ_ex=310 nm). In comparison with H₄DTA, complex 2 exhibited a more extensive emission at 388 nm (λ_ex=310 nm). These emissions correspond to intraligand π-π * or n-π* transition of H₄DTA^{[11, 22-23]}. Enhancement of the luminescence in 2 compared to H ₄DTA can be explained by the coordination of ligands to Cd(Ⅱ); the coordination can augment a rigidity of ligand and reduce an energy loss due to radiationless decay^{[22-23, 37]}.

  Figure 5

  

  Figure 5.  Solid-state emission spectra of H₄DTA and complex 2 at room temperature

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  2.4   Catalytic cyanosilylation of aldehydes

  Given the potential of Mn(Ⅱ) and Cd(Ⅱ) complexes to catalyze the organic reactions^[38-40], we explored the application of 1 and 2 as heterogeneous catalysts in the cyanosilylation of 4-nitrobenzaldehyde as a model substrate to give 2-(4-nitrophenyl)-2-((trimethylsilyl)oxy) acetonitrile. Typical tests were carried out by reacting amixture of 4-nitrobenzaldehyde, trimethylsilyl cyanide (TMSCN), and the catalyst in dichloromethane at 35 ℃ (Scheme 2, Table 3). Such effects as reaction time, catalyst loading, solvent composition, catalyst recycling and finally substrate scope were investigated.

  Scheme 2

  

  Scheme 2.  Catalyzed cyanosilylation of 4-nitrobenzaldehyde (model substrate)

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  Table 3

  

  Table 3.  Catalyzed cyanosilylation of 4-nitrobenzaldehyde with TMSCN

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  Entry	Catalyst	Time/h	Catalyst dosagea/%	Solvent	Yieldb/%
  1	1	1	3.0	CH2Cl2	33
  2	1	2	3.0	CH2Cl2	60
  3	1	4	3.0	CH2Cl2	70
  4	1	6	3.0	CH2Cl2	78
  5	1	8	3.0	CH2Cl2	85
  6	1	10	3.0	CH2Cl2	88
  7	1	12	3.0	CH2Cl2	93
  8	2	12	3.0	CH2Cl2	35
  9	1	12	2.0	CH2Cl2	72
  10	1	12	4.0	CH2Cl2	94
  11	1	12	3.0	CH3CN	81
  12	1	12	3.0	THF	71
  13	1	12	3.0	CH3OH	85
  14	1	12	3.0	CH3Cl	87
  15	Blank	12	—	CH2Cl2	5
  16	MnCl2	12	3.0	CH2Cl2	9
  17	CdCl2	12	3.0	CH2Cl2	6
  18	H3dpna	12	3.0	CH2Cl2	7
  aMolar fraction; b Calculated by 1H NMR spectroscopy: nproduct/naldehyde×100%.

  Upon using complex 1 as the catalyst (Molar fraction: 3%), a high conversion of 93% of 4-nitrobenzaldehyde into 2 -(4-nitrophenyl)-2-((trimethylsilyl)oxy)acetonitrile was reached after 12 h in dichloromethane at 35 ℃ (Table 3, Entry 7). Upon extending the reaction time further to 18 h, only a slight increase in the prod-uct yield to 94% occurred. Moreover, no other products were detected, and the yield of this product was consid-ered to be the conversion of 4-nitrobenzaldehyde (Fig. 6).

  Figure 6

  

  Figure 6.  PXRD patterns for 1: simulated (red), before (black) and after (blue) catalysis

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  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 48% to 93% (Table 4). Aryl aldehydes bearing strong electron-withdrawing substituents (e.g., nitro and chloro) exhibited the higher reactivities (Table 4, Entries 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 4, Entry 7) as expected. An ortho -substituted aldehyde showed lower reactivity, possibly as a result of steric hindrance.

  Table 4

  

  Table 4.  Cyanosilylation of various aldehydes with TMSCN catalyzed by 1^a

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  Entry	Substituted benzaldehyde substrate (R—C6H4CHO)	Product yieldb/%
  1	R=H	60
  2	R=2-NO2	81
  3	R=3-NO2	88
  4	R=4-NO2	93
  5	R=4-Cl	63
  6	R=4-OH	54
  7	P=4-CH3	48
  aReaction conditions: aldehyde (0.5 mmol), TMSCN (1.0 mmol), catalyst 1 (Molar fraction: 3.0%) and CH2Cl2 (2.5 mL) at 35 ℃; 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. We repeated recycling catalyst 1 and the catalytic system mained the higher activity over at least five consecutive cycles (the yields were 92%, 91%, 89% and 88% for second to fifth run, respectively). According to the PXRD data (Fig. 6), the structure of 1 was essentially preserved after five catalytic cycles.

  A possible catalytic cycle for the cyanosilylation reaction catalyzed by a Mn coordination polymer is proposed in Scheme 3. It can involve dual activation of the carbonyl and TMSCN by the Mn(Ⅱ) centre and a ligated carboxylate group, respectively, followedby the formation of C—C bond leading to cyanohydrin^[41-42].

  Scheme 3

  

  Scheme 3.  Mechanism for Mn-catalyzed cyanosilylation reaction

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  3.   Conclusions

  In summary, we have successfully synthesized and characterized two new manganese and cadmium complexes by using one unexplored dicarboxylic acid as ligand under hydrothermal condition. Both complexes feature a 3D metal-organic framework structure. Besides, the luminescence and catalytic properties were also investigated and discussed. The results show that complex 1 exhibits a higher catalytic activity in the cyanosilylation at 35 ℃.

  
  Supporting information is available at http://www.wjhxxb.cn
  ^#共同第一作者。
  
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  Figure 1  Drawing of coordination environment around Mn center in complex 1 with 30% probability thermal ellipsoids

  Hydrogen atoms are omitted for clarity except those bonding to oxygen atoms; Symmetry codes: A: x, -y+1/2, -z+3/2; B: -x+1/2, -y+1, z; C: x+1/2, y, -z+1; D: -x, -y+1, -z+1

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  Scheme 1  Coordination mode of H₂DTA^2- ligand in complex 1

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  Figure 2  Perspective of 3D metal-organic framework along a axis

  Bipy ligands are omitted for clarity

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  Figure 3  Topological representation of binodal 4, 4-connected metal-organic framework with a pts topology viewed along a axis

  Green balls: 4-connected Mn1, Gray: centroids of 4-connected μ₄-H₂DTA^2- nodes

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  Figure 4  TGA curves of complexes 1 and 2

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  Figure 5  Solid-state emission spectra of H₄DTA and complex 2 at room temperature

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  Scheme 2  Catalyzed cyanosilylation of 4-nitrobenzaldehyde (model substrate)

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  Figure 6  PXRD patterns for 1: simulated (red), before (black) and after (blue) catalysis

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  Scheme 3  Mechanism for Mn-catalyzed cyanosilylation reaction

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  Table 1.  Crystal data for complexes 1 and 2

  Complex	1	2
  Chemical formula	C18H12MnN2O6	C18H12CdN2O6
  Molecular weight	407.24	464.70
  Crystal system	Orthorhombic	Orthorhombic
  Space group	Pnna	Pnna
  a/nm	0.964 95(4)	0.976 18(3)
  b/nm	1.692 31(8)	1.685 18(4)
  c/nm	1.024 19(5)	1.042 57(3)
  V/nm3	1.672 51(13)	1.715 06(9)
  Z	4	4
  F(000)	828	920
  θ range for data collection/(°)	5.048~69.827	4.988~66.524
  Limiting indices	-11 ≤ h ≤ 9, -20 ≤ k ≤ 18, -7 ≤ l ≤ 12	-7 ≤ h ≤ 11, -20 ≤ k ≤ 18, -12 ≤ l ≤ 10
  Reflection collected, unique (Rint)	5 632, 1 573 (0.068 9)	5 351, 1 494 (0.042 2)
  Dc/(g·cm-3)	1.617	1.800
  μ/mm-1	6.791	10.564
  Data, restraint, parameter	1 573, 0, 124	1 494, 0, 124
  Goodness-of-fit on F2	1.049	1.021
  Final R indices [I≥2σ(I)] R1, wR2	0.046 7, 0.097 9	0.047 5, 0.120 4
  R indices (all data) R1, wR2	0.084 9, 0.107 5	0.051 8, 0.127 3
  Largest diff. peak and hole/(e·nm-3)	333 and -433	929 and -3 169

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

  1
  Mn1—O1	0.213 7(2)	Mn1—O1B	0.213 7(2)	Mn1—O2C	0.217 8(2)
  Mn1—O2D	0.217 8(2)	Mn1—N1	0.229 8(3)	Mn1—N1B	0.229 8(3)
  O1—Mn1—O1B	164.71(15)	O2D—Mn1—O1B	90.39(9)	O2D—Mn1—O1	80.50(9)
  O2D—Mn1—O2C	106.99(14)	O1—Mn1—N1	93.79(11)	O1—Mn1—N1B	98.68(10)
  O2C—Mn1—N1	91.57(11)	O2D—Mn1—N1	160.51(11)	N1—Mn1—N1B	70.75(16)
  2
  Cd1—O1	0.225 6(2)	Cd1—O1B	0.225 6(2)	Cd1—O2C	0.228 8(2)
  Cd1—O2D	0.228 8(2)	Cd1—N1	0.234 9(3)	Cd1—N1B	0.234 9(3)
  O1—Cd1—O1B	107.74(17)	O2D—Cd1—O1B	89.25(10)	O2D—Cd1—O1	75.84(9)
  O2D—Cd1—O2C	154.75(14)	O1—Cd1—N1	91.97(12)	O1—Cd1—N1B	158.64(11)
  O2C—Cd1—N1	96.81(11)	O2D—Cd1—N1	103.85(11)	N1—Cd1—N1B	70.04(16)
  Symmetry codes: B: -x+1/2, -y+1, z; C: x+1/2, y, -z+1; D: -x, -y+1, -z+1 for 1; B: -x+1/2, -y+1, z; C: x+1/2, y, -z+1; D: -x, -y+1, -z+1 for 2.

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  Table 3.  Catalyzed cyanosilylation of 4-nitrobenzaldehyde with TMSCN

  Entry	Catalyst	Time/h	Catalyst dosagea/%	Solvent	Yieldb/%
  1	1	1	3.0	CH2Cl2	33
  2	1	2	3.0	CH2Cl2	60
  3	1	4	3.0	CH2Cl2	70
  4	1	6	3.0	CH2Cl2	78
  5	1	8	3.0	CH2Cl2	85
  6	1	10	3.0	CH2Cl2	88
  7	1	12	3.0	CH2Cl2	93
  8	2	12	3.0	CH2Cl2	35
  9	1	12	2.0	CH2Cl2	72
  10	1	12	4.0	CH2Cl2	94
  11	1	12	3.0	CH3CN	81
  12	1	12	3.0	THF	71
  13	1	12	3.0	CH3OH	85
  14	1	12	3.0	CH3Cl	87
  15	Blank	12	—	CH2Cl2	5
  16	MnCl2	12	3.0	CH2Cl2	9
  17	CdCl2	12	3.0	CH2Cl2	6
  18	H3dpna	12	3.0	CH2Cl2	7
  aMolar fraction; b Calculated by 1H NMR spectroscopy: nproduct/naldehyde×100%.

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  Table 4.  Cyanosilylation of various aldehydes with TMSCN catalyzed by 1^a

  Entry	Substituted benzaldehyde substrate (R—C6H4CHO)	Product yieldb/%
  1	R=H	60
  2	R=2-NO2	81
  3	R=3-NO2	88
  4	R=4-NO2	93
  5	R=4-Cl	63
  6	R=4-OH	54
  7	P=4-CH3	48
  aReaction conditions: aldehyde (0.5 mmol), TMSCN (1.0 mmol), catalyst 1 (Molar fraction: 3.0%) and CH2Cl2 (2.5 mL) at 35 ℃; b Calculated by 1H NMR spectroscopy.

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