English

• 0.   Introduction

  Recently, coordination polymers (CPs) have been an area of a wide range of studies owing to their potential applications in the fields of heterogeneous catalysis, gas separation, ion exchange, and luminescence, but also to their variety of architectures and topologies^[1-10]. Several excellent reviews have witnessed the great progress of this field in the last decades. Among these investigations, structural diversities were exhibited by changing the synthetic strategy of CPs. It is known that the choice of metal ions and organic ligands is crucial for the construction of CPs. The flexibility, the number of ligand coordination sites, and metal ions are the main factors that influence the final network topology^[1-10]. Particularly, the N-donor ligands grab attention due to their versatile conformations according to the restrictions imposed by the coordination geometry of the metal ion^[7-10]. Generally, semirigid ligands such as Ⅴ-shaped ligands allow them to rotate when coordinated to central metal ions, which makes it possible to construct interpenetration, cavities, helical structures, and other novel motifs with unique architectures.

  In our previous work, we successfully synthesized a series of entangled CPs using N-donor ligand bis(4-(1H-imidazol-1-yl)phenyl)methanone (bipmo)^[9-10]. As part of our research on coordination chemistry with Ⅴ-shaped ligands, we report here two CPs based on two kinds of Ⅴ-shaped bidentate ligands (bipmo, bppmo) and 2,6-naphthalenedicarboxylic acid (Scheme 1), namely, {[Cd(bipmo)(NDC)]·1.75H₂O}_n (1), {[Cd(bppmo)(NDC)H₂O]·H₂O}_n (2), where H₂NDC=2,6-naphthalenedicarboxylic acid, bppmo=bis(4-(pyridin-4-yl) phenyl)methanone. The complexes were characterized by X-ray crystallography, bond valence sum calculations, IR spectra, and elemental analyses. The effects of Ⅴ-shaped ligands on the final topologies have been discussed in detail. In addition, the luminescent properties of bipmo, bppmo, and complexes 1 and 2 have also been investigated.

  Scheme 1

  

  Scheme 1.  Structure of H₂NDC, bipmo, and bppmo

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  1.   Experimental

  1.1   Materials and methods

  All solvents, Cd(NO₃)₂·4H₂O, and 2,6-naphthalenedicarboxylic acid were purchased from commercial sources and used without further purification. Ligand bipmo was synthesized according to the literature^[9]. Elemental analyses (EA) were performed on an Elementar Vario EL Ⅲ elemental analyzer. The IR spectra were recorded on a Bruker Vector 22 spectrophotometer with KBr pellets in the 4 000-400 cm^-1 region. The luminescence spectra were measured on a Hitachi F-4600 fluorescence spectrometer.

  1.2   Synthesis of bppmo

  Potassium carbonate (9.66 g, 70 mmol) was suspended in a degassed solution of 1,4-dioxane (120 mL) and water (80 mL). Bis(4-bromophenyl)methanone (3.40 g, 10 mmol), 4-pyridyl boronic acid (6.76 g, 55 mmol), and Pd(PPh₃)₄ (100 mg) were then added and the reaction mixture was heated at 90 ℃ under nitrogen for 4 d. The solvent was evaporated until the volume was approximately 120 mL, precipitating a white compound which was filtered after cooling to r.t., washed with EtOH, and then with water. Yield: 67.5%. Anal. Calad. for C₂₃H₁₆N₂O(%): C, 82.12; H, 4.79; N, 8.33. Found(%), C, 81.94; H, 4.58; N, 8.49.¹H NMR (CDCl₃, 500 MHz): δ 8.77 (s, 2H), 8.00-7.80 (dd, J₁=10 Hz, J₂=90 Hz, 4H), 7.62 (d, J=5 Hz, 2H). IR (KBr, cm^-1): 3 020(m), 1 641(s), 1 589(s), 1 539(m), 1 481(w), 1 398(s), 1 294(s), 1 276(m), 1 188(m), 1 149(m), 1 072(m), 991(m), 933(m), 865(m), 815(s), 761(s), 730(m), 676(m), 628(m), 553(w), 470(m), 428(w).

  1.3   Synthesis of complex 1

  A mixture of Cd(NO₃)₂·4H₂O (30.8 mg, 0.1 mmol), bipmo (31.4 mg, 0.1 mmol), and H₂NDC (21.6 mg, 0.1 mmol) was dissolved in 5 mL of DMF-H₂O (3∶2, V/V) solution. The final mixture was placed in a Parr Teflon-lined stainless steel vessel (15 mL) under autogenous pressure and heated at 105 ℃ for two weeks. Colorless block single crystals suitable for X-ray data collection were obtained. Yield: 42% based on bipmo. Anal. Calad. for C₃₁H_23.50CdN₄O_6.75(%): C, 55.37; H, 3.52; N, 8.33. Found(%), C, 55.31; H, 3.45; N, 8.45. IR (KBr, cm^-1): 3 134(s), 3 028(m), 1 664(m), 1 639(m), 1 606(m), 1 552(m), 1 519(m), 1 493(m), 1 400(s), 1 359(m), 1 333(w), 1 305(m), 1 280(m), 1 249(m), 1 182(w), 1 120(m), 1 064(m), 960(w), 929(w), 858(w), 804(m), 790(m), 769(w), 740(w), 671(w), 648(w), 621(w), 520(w), 484(w), 451(w).

  1.4   Synthesis of complex 2

  A mixture of Cd(NO₃)₂·4H₂O (30.8 mg, 0.1 mmol), bppmo (33.6 mg, 0.1 mmol), and H₂NDC (21.6 mg, 0.1 mmol) was dissolved in 6 mL of DMF-H₂O (3∶3, V/V) solution. The final mixture was placed in a Parr Teflon-lined stainless steel vessel (15 mL) under autogenous pressure and heated at 95 ℃ for a week. Yellow single crystals suitable for X-ray data collection were obtained. Yield: 46% based on bppmo. Anal. Calad. for C₃₅H₂₆CdN₂O₇(%): C, 60.14; H, 3.75; N, 4.01. Found(%), C, 60.01; H, 3.57; N, 4.14. IR (KBr, cm^-1): 3 263(m), 1 660(m), 1 600(s), 1 554(s), 1 489(m), 1 400(s), 1 352(s), 1 288(m), 1 269(m), 1 188(w), 1 134(w), 1 097(w), 1 070(w), 1 002(w), 929(m), 823(m), 792(s), 678(w), 555(w), 486(m), 455(m).

  1.5   Crystal structure determination

  Single crystal X-ray diffraction data were collected on a Bruker Smart-1000CCD diffractometer equipped with graphite monochromated Mo Kα radiation (λ=0.071 073 nm) at 293 and 150 K for complexes 1 and 2, respectively. The corrections for Lp factors were applied. Absorption corrections were applied using the SADABS program^[11]. The structures were solved by direct methods^[12] with the SHELXTL (version 6.10) program^[13] and refined by full-matrix least-squares techniques on F² with the SHELXTL^[13]. All non-hydrogen atoms were refined anisotropically and the hydrogen atoms bonded to carbon atoms were generated geometrically. The water hydrogen atoms were located in difference maps and refined with O—H distance of 0.085(2) nm and H…H distance of 0.135(2) nm as restraints. Crystallographic data and structure refinements for complexes 1 and 2 are listed in Table 1. Selected bond distances and angles for 1 and 2 are summarized in Table 2.

  Table 1

  

  Table 1.  Crystallographic data and refinement parameters for complexes 1 and 2

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  Parameter	1	2
  Formula	C31H23.50CdN4O6.75	C35H26CdN2O7
  Formula weight	672.44	698.98
  Temperature/K	293(2)	150(2)
  Crystal system	Triclinic	Triclinic
  Crystal size/mm	0.26×0.22×0.14	0.33×0.29×0.22
  Space group	P1	P1
  a/nm	1.081 32(6)	0.828 79(6)
  b/nm	1.165 87(6)	1.294 47(8)
  c/nm	1.330 78(7)	1.486 95(11)
  α/(°)	108.770(2)	104.813(2)
  β/(°)	112.527(2)	103.773(3)
  γ/(°)	98.135(2)	105.377(2)
  V/nm3	1.398 40(13)	1.404 70(17)
  Dc/(g·cm-3)	1.597	1.653
  Z	2	2
  μ/mm-1	0.837	0.835
  F(000)	679	708
  Unique reflection	5 020	5 768
  Observed reflection [I > 2σ(I)]	4 463	4 490
  Number of parameters	388	410
  GOF	1.022	1.026
  Final R indices [I > 2σ(I)]	0.029 3, 0.082 1	0.033 5, 0.107 9
  R indices (all data)	0.035 6, 0.085 3	0.041 3, 0.122 0

  Table 2

  

  Table 2.  Selected bond lengths (nm) and bond angles (°) of complexes 1 and 2

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  1
  Cd1—O3	0.221 2(2)	Cd1—O2	0.233 9(2)	Cd1—N1	0.223 9(2)
  Cd1—O1	0.237 9(2)	Cd1—N4#3	0.225 2(2)	Cd1—O4	0.278 3(2)
  O3—Cd1—N1	122.49(9)	N4#3—Cd1—O2	102.12(8)	O3—Cd1—N4#3	111.08(9)
  O3—Cd1—O1	98.95(9)	N1—Cd1N4#3	95.26(9)	N1—Cd1—O1	85.21(8)
  O3—Cd1—O2	92.32(9)	N4#3—Cd1—O1	143.45(9)	N1—Cd1—O2	131.87(8)
  O2—Cd1—O1	54.93(7)
  2
  Cd1—O3	0.224 3(2)	Cd1—N1	0.234 3(3)	Cd1—O1	0.224 6(2)
  Cd1—N2#1	0.246 7(3)	Cd1—O6	0.232 8(2)	Cd1—O4	0.258 4(2)
  Cd1—O2	0.277 1(2)
  O3—Cd1—O1	122.67(9)	O6—Cd1—N2#1	165.27(9)	O3—Cd1—O6	98.95(9)
  N1—Cd1—N2#1	88.18(9)	O1—Cd1—O6	99.16(9)	O3—Cd1—O4	53.91(8)
  O3—Cd1—N1	148.58(9)	O1—Cd1—O4	167.00(9)	O1—Cd1—N1	88.31(9)
  O6—Cd1—O4	93.83(8)	O6—Cd1—N1	79.12(9)	N1—Cd1—O4	94.74(8)
  O3—Cd1—N2#1	87.97(9)	N2#1—Cd1—O4	79.68(8)	O1—Cd1—N2#1	87.80(9)
  Symmetry codes: #1: -x-1, -y, -z; #2: -x+2, -y+2, -z-1; #3: -x, -y+2, -z+1 for 1; #1: -x+2, -y+1, -z+1; #2: -x, -y+2, -z; #3: -x-1, -y+1, -z+1 for 2.

  2.   Results and discussion

  2.1   Crystal structure of the complexes

  Complexes 1 and 2 were prepared from the reaction of Cd(NO₃)₂·4H₂O, H₂NDC, and Ⅴ-shaped ligands (bipmo or bppmo) in a molar ratio of 1∶1∶1 under solvothermal conditions. To investigate the coordination chemistry of the novel frameworks with the Ⅴ-shaped ligands, Cd(Ⅱ) salts, and H₂NDC, numerous parallel experiments were carried out by varying the ratios of the solvents, the reaction materials, and the reaction temperatures. However, other experiments only gave powder forms.

  These complexes were characterized by IR, EA, and single-crystal X-ray diffraction analyses. The IR spectra of 1 and 2 were consistent with their structural characteristics as determined by single-crystal X-ray diffraction. Asymmetric C=O stretching modes were visible at 1 664 cm^-1 (for 1) and 1 660 cm^-1 (for 2). Medium intensity bands in the range of 1 639-1 400 cm^-1 (for 1) and 1 600-1 400 cm^-1 (for 2) can be ascribed to stretching modes of the aromatic rings within the ligands.

  X-ray diffraction analyses reveal that both 1 and 2 are 2D layered structures. Therefore, we will restrict our description to complex 1 with pertinent details mentioned for complex 2, where appropriate.

  Complex 1 crystallizes in a triclinic P1 space group (Fig. 1). As shown in Fig. 1a, the asymmetric unit of 1 contains one Cd²⁺ ion, one NDC^2- anion, one bipmo ligand, one and three-fourth lattice water molecules. The Cd²⁺ ion is six-coordinated by two carboxylate O atoms from two NDC^2- anions and two N atoms from two bipmo ligands in an octahedral geometry. The carboxylate groups of the NDC^2- anion adopt μ₁-η¹∶η¹-coordination mode, and the corresponding bond angles around the Cd²⁺ are in a range of 54.93(7)°-143.45(9)°. The Cd—O bond lengths fall in a range of 0.221 2(2) to 0.278 3(2) nm, and Cd—N bond lengths are 0.223 9(2) and 0.225 2(2) nm, respectively. Both Cd—N and Cd—O bond lengths are well-matched to those observed in similar complexes^[14-16]. Two bipmo ligands coordinate to the cadmium ion through their terminal imidazole groups, forming a ring of [Cd₂(bipmo)₂]_n with a Cd…Cd distance of 1.576 nm. On the other hand, two carboxylate groups of the NDC^2- ligand coordinate to two Cd²⁺ ions with a bidentate mode, giving rise to an infinite chain [Cd₂(bipmo)₂(NDC)]_n extending along the a-direction. Finally, the chains are further connected and form a single 2D structure through the NDC^2- ligands with the same orientation between adjacent chains (Fig. 1b). Simultaneously, this arrangement leads to the formation of parallel hexagons with side lengths of 1.576, 1.342, and 1.338 nm. Interestingly, the two 2D structures are interwoven through the NDC^2- ligands passing through the [Cd₂(bipmo)₂]_n rings, resulting in a 2D interpenetrated structure (Fig. 1c).

  Figure 1

  

  Figure 1.  Crystal structure of complex 1: (a) coordination environments (the hydrogen atoms bonded to carbon atoms are omitted for clarity; 30% ellipsoid probability; symmetry codes: #1: -x-1, -y, -z; #2: -x+2, -y+2, -z-1; #3: -x, -y+2, -z+1); (b) one of the single 2D net; (c) view of 2-fold interpenetrating 2D architecture; (d) simplified structure of the 2D architecture

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  To simplify the rather intricate structure of complex 1, the Cd(Ⅱ) ion can be simplified as a 3-connected node, the ligand bipmo and NDC^2- anion can be reduced to bridges, and the structure of 1 can be represented as a 3-connected net with a point symbol of {6³}. The most striking feature of 1, that is, a pair of identical 2D single nets is interlocked with each other, thus directly leading to the formation of a 2-fold interpenetrated 2D architecture with {6³} topology (Fig. 1d).

  The asymmetric unit of complex 2 contains one Cd²⁺ ion, one bppmo ligand, one deprotonated H₂NDC ligand, one coordinated water molecule, and one lattice water molecule. As shown in Fig. 2a, each Cd²⁺ ion is seven-coordinated by four carboxylate oxygen atoms from two different deprotonated NDC^2- ligands, one N atom from one bppmo ligand at the basal positions, and one O atom from one coordinated water and one N atom from two bipmo molecules at the apical positions to form a distorted pentagonal bipyramidal geometry. The Cd—N bond lengths are 0.234 3(3) and 0.246 7(3) nm, respectively, while Cd—O bond lengths vary from 0.224 3(2) to 0.277 1(2) nm. These values are similar to previously reported Cd—O bond lengths of 0.218 8(3)-0.275 7(6) nm and Cd—N bond lengths of 0.222 2(4)-0.250 4(9) nm^[14-17]. Compared to parallel hexagons (1.576, 1.342, and 1.338 nm) in complex 1, similar parallel hexagons with side lengths of 1.814, 1.372, and 1.334 nm are observed in complex 2 (Fig. 2b). The difference between the corresponding side lengths of the two hexagons in 1 and 2 is 0.238, 0.030, and 0.004 nm, respectively. The largest difference arises from the side lengths constructed by Ⅴ-shaped ligands, resulting in the different folds of penetration in 1 and 2. One of the most captivating features of complex 2 is its 3D 3-fold interpenetrating framework, which arises from the interlocking arrangement of three identical 2D frameworks (Fig. 2c and 2d).

  Figure 2

  

  Figure 2.  Crystal structure of 2: (a) coordination environments (the hydrogen atoms bonded to carbon atoms are omitted for clarity; 30% ellipsoid probability; symmetry codes: #1: -x+2, -y+1, -z+1; #2 -x, -y+2, -z; #3 -x-1, -y+1, -z+1); (b) one of the single 2D net; (c) view of complex 2 having a 3-fold interpenetrated net structure; (d) simplified structure of the 2D architecture

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  2.2   Bond valence sum calculations

  The bond valence sums, as depicted in Eq.1, are frequently employed for predicting the coordination number or oxidation state in solids, relying on the metal-ligand bond lengths in the crystal structure. Eq.2 is utilized to calculate S_ij from the observed bond lengths r_ij.

  $ z_j=\sum S_{i j} $

  (1)

  $ S_{i j}=\exp \left[\left(r_0-r_{i j}\right) / B\right] $

  (2)

  where S_ij represents the bond valence and r_ij denotes the observed bond distance. The parameters r₀ and B are empirically determined and obtained from the literature^[18-19]. By utilizing the bond lengths listed in Table 2, we obtained a calculated bond valence sum (BVS) of 2.038 for complex 1, which shows excellent agreement with the oxidation state of the metal atom (Cd²⁺). However, when excluding the bond Cd1—O4, the BVS decreased to only 1.945. Similar results can be obtained for complex 2. The value of S_ij was 1.894, whereas it is 1.798 when excluding the bond Cd1—O2.

  2.3   Luminescence properties

  Currently, luminescent CPs containing d¹⁰ metal atoms are generating considerable attention due to their diverse functionalities and potential applications in areas such as photocatalysis, fluorescent sensors, biomedical imaging, and electroluminescent devices^[20-22]. In the solid state at room temperature, the luminescent properties of bipmo, bppmo, 1, and 2 were investigated (Fig. 3). The free bipmo and bppmo ligands exhibited a primary emission peak at 447 nm (λ_ex=407 nm) and 472 (λ_ex=431 nm), respectively, which may be ascribed to the π*→n or π*→π transitions^[23]. Complexes 1 and 2 displayed wide emission bands with maximum peaks at 498 nm (λ_ex=414 nm) and 458 nm (λ_ex=412 nm), respectively. Compared to the free bipmo ligand, the peak of complex 1 red-shifted by 47 nm, which may be assigned to the charge-transfer transitions between ligands and metal ion centers^[23]. The peak of complex 2 blue-shifted by 14 nm as compared with the pure bppmo ligand. This emission band may be caused by ligand-to-metal charge transfer (LMCT) of bppmo ligand or metal-perturbed intraligand charge transfers of NDC^2- ligand^[24]. The above observation implies that complexes 1 and 2 have demonstrated promising characteristics as potential candidates for fluorescent materials.

  Figure 3

  

  Figure 3.  Solid-state emission spectra of ligands bipmo and bppmo, and complexes 1 and 2 at room temperature

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  3.   Conclusions

  In summary, two new CPs based on d¹⁰ metal ions, H₂NDC, and the bipmo/bppmo ligand have been synthesized under solvothermal conditions. These complexes exhibit interesting 2D structures that are influenced by the lengths and flexibility of the ancillary ligands. Furthermore, their luminescence behavior indicates that complexes 1 and 2 could be suitable candidates for potential fluorescent materials.

  
  
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  Scheme 1  Structure of H₂NDC, bipmo, and bppmo

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  Figure 1  Crystal structure of complex 1: (a) coordination environments (the hydrogen atoms bonded to carbon atoms are omitted for clarity; 30% ellipsoid probability; symmetry codes: #1: -x-1, -y, -z; #2: -x+2, -y+2, -z-1; #3: -x, -y+2, -z+1); (b) one of the single 2D net; (c) view of 2-fold interpenetrating 2D architecture; (d) simplified structure of the 2D architecture

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  Figure 2  Crystal structure of 2: (a) coordination environments (the hydrogen atoms bonded to carbon atoms are omitted for clarity; 30% ellipsoid probability; symmetry codes: #1: -x+2, -y+1, -z+1; #2 -x, -y+2, -z; #3 -x-1, -y+1, -z+1); (b) one of the single 2D net; (c) view of complex 2 having a 3-fold interpenetrated net structure; (d) simplified structure of the 2D architecture

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  Figure 3  Solid-state emission spectra of ligands bipmo and bppmo, and complexes 1 and 2 at room temperature

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  Table 1.  Crystallographic data and refinement parameters for complexes 1 and 2

  Parameter	1	2
  Formula	C31H23.50CdN4O6.75	C35H26CdN2O7
  Formula weight	672.44	698.98
  Temperature/K	293(2)	150(2)
  Crystal system	Triclinic	Triclinic
  Crystal size/mm	0.26×0.22×0.14	0.33×0.29×0.22
  Space group	P1	P1
  a/nm	1.081 32(6)	0.828 79(6)
  b/nm	1.165 87(6)	1.294 47(8)
  c/nm	1.330 78(7)	1.486 95(11)
  α/(°)	108.770(2)	104.813(2)
  β/(°)	112.527(2)	103.773(3)
  γ/(°)	98.135(2)	105.377(2)
  V/nm3	1.398 40(13)	1.404 70(17)
  Dc/(g·cm-3)	1.597	1.653
  Z	2	2
  μ/mm-1	0.837	0.835
  F(000)	679	708
  Unique reflection	5 020	5 768
  Observed reflection [I > 2σ(I)]	4 463	4 490
  Number of parameters	388	410
  GOF	1.022	1.026
  Final R indices [I > 2σ(I)]	0.029 3, 0.082 1	0.033 5, 0.107 9
  R indices (all data)	0.035 6, 0.085 3	0.041 3, 0.122 0

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  Table 2.  Selected bond lengths (nm) and bond angles (°) of complexes 1 and 2

  1
  Cd1—O3	0.221 2(2)	Cd1—O2	0.233 9(2)	Cd1—N1	0.223 9(2)
  Cd1—O1	0.237 9(2)	Cd1—N4#3	0.225 2(2)	Cd1—O4	0.278 3(2)
  O3—Cd1—N1	122.49(9)	N4#3—Cd1—O2	102.12(8)	O3—Cd1—N4#3	111.08(9)
  O3—Cd1—O1	98.95(9)	N1—Cd1N4#3	95.26(9)	N1—Cd1—O1	85.21(8)
  O3—Cd1—O2	92.32(9)	N4#3—Cd1—O1	143.45(9)	N1—Cd1—O2	131.87(8)
  O2—Cd1—O1	54.93(7)
  2
  Cd1—O3	0.224 3(2)	Cd1—N1	0.234 3(3)	Cd1—O1	0.224 6(2)
  Cd1—N2#1	0.246 7(3)	Cd1—O6	0.232 8(2)	Cd1—O4	0.258 4(2)
  Cd1—O2	0.277 1(2)
  O3—Cd1—O1	122.67(9)	O6—Cd1—N2#1	165.27(9)	O3—Cd1—O6	98.95(9)
  N1—Cd1—N2#1	88.18(9)	O1—Cd1—O6	99.16(9)	O3—Cd1—O4	53.91(8)
  O3—Cd1—N1	148.58(9)	O1—Cd1—O4	167.00(9)	O1—Cd1—N1	88.31(9)
  O6—Cd1—O4	93.83(8)	O6—Cd1—N1	79.12(9)	N1—Cd1—O4	94.74(8)
  O3—Cd1—N2#1	87.97(9)	N2#1—Cd1—O4	79.68(8)	O1—Cd1—N2#1	87.80(9)
  Symmetry codes: #1: -x-1, -y, -z; #2: -x+2, -y+2, -z-1; #3: -x, -y+2, -z+1 for 1; #1: -x+2, -y+1, -z+1; #2: -x, -y+2, -z; #3: -x-1, -y+1, -z+1 for 2.

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