English

• 0.   Introduction

  In recent years, the transition-metal coordination polymers (CPs) have received much attention not only in virtue of their intriguing structures, but also for their potential applications in catalysis^[1-2], separation and storage^[3-4], optical properties^[5-7], magnetism^[8-9], etc. The self-assembly method is frequently used and important in synthesis to construct desired transition-metal CPs with rational and controllable structures. Moreover, selecting appropriate reaction conditions and reactants also plays a vital role, because many factors affect the formation, such as the metal ions, the pH value, and the molar ratio of reactants, especially the selection of organic ligands.

  To the best of our knowledge, the aromatic carboxylate ligands have been widely used in the assembly process with their special advantages, in which the carboxyl groups can coordinate to metal ions through diverse coordination modes, such as terminal monodentate, chelating to one metal atom and bridging bidentate in different fashions^[10]. Moreover, the carboxylate groups may be completely or partially deprotonated, and can act as hydrogen bond acceptors as well as hydrogen bond donors to assemble supramolecular structures; on the other hand, the existence of aromatic rings can promote the formation of π-π stacking interactions, which also play an important role in supramolecular networks. Further studies have shown that the current use of rigid carboxylic acid ligands, such as 3, 5-pyrazoledicarboxylic acid (H₃pdc), p-nitrobenzoic acid, pyrazine-2, 3-dicarboxylic acid, and pyridine-3, 4-diarboxylic acid, has proved to be a wise decision. Many compounds with interesting structures^[11-14] and properties have been synthesized. Encouraged by these factors, the rigid ligand H₃pdc/p-nitrobenzoic acid has attracted our attention, which contains dicarboxylate/ carboxylate groups and aromatic rings, providing action sites for coordination and intermolecular forces to meet the coordination requirements in the assembly process. Moreover, among the transition-metal ions, Ni²⁺ ions are widely used in the synthesis of CPs due to their potential as electrochemical, catalytic, magnetic, and fluorescent active centers, and excellent coordination abilities with N-/O-donor ligands^[15-17].

  Herein, with H₃pdc/p-nitrobenzoic acid and 1, 4-bis(imidazol-1-ylmethyl)butane (bib) as starting materials, we obtained two CPs, [Ni(Hpdc)(bib)(H₂O)]_n (1) and {[Ni(bix)₃](ClO₄)₂}_n (2), which all have 2D structures. Meanwhile, the quantum-chemical calculations of 1 are also discussed in this paper.

  1.   Experimental

  1.1   Reagents and methods

  All starting raw materials were of analytical grade and obtained from commercial sources without further purification. Elemental analysis for C, N, and H was performed on a PE 240C elemental analyzer. The IR spectra (KBr pellets) were recorded on a Varian 640 FTIR spectrometer in the 400-4 000 cm^-1 region. Thermogravimetry (TG) studies were carried out on a STA7300 analyzer under nitrogen at a heating rate of 10 ℃·min^-1. Powder X-ray diffraction (PXRD) patterns were collected in a 2θ range of 5°-50° with a scan speed of 0.1 (°)·s^-1 on a Bruker D8 Advance instrument using a Cu Kα radiation (λ=0.154 18 nm, voltage: 40 kV, current: 40 mA) at room temperature.

  1.2   Synthesis of the CPs

  CP 1: A mixture of H₃pdc (0.20 mmol, 0.034 g), Ni(NO₃)₂·6H₂O (0.10 mmol, 0.029 g), and bib (0.20 mmol, 0.038 g) was dissolved in deionized water (15 mL). After continuously stirring for 30 min, a suitable amount of triethylamine was added to the above solution to adjust the pH value to 6.05. Then the solution was sealed up in a Parr Teflon-lined stainless-steel vessel (25 mL) under autogenous pressure, heated at 140 ℃ for 168 h, and cooled to room temperature over one day. Green columnar crystals of CP 1 were collected, washed with water, and air-dried (Yield: 21%). Elemental analysis Calcd. for C₁₅H₁₈N₆NiO₅(%): C, 42.79; H, 4.31; N, 19.96. Found(%): C, 42.21; H, 4.08; N, 19.65. IR (cm^-1): 3 360w, 3 111w, 2 950w, 2 362w, 1 593s, 1 522w, 1 497w, 1 446w, 1 349s, 1 280w, 1 242 w, 1 222w, 1 155w, 1 107m, 1 086w, 1 035w, 1 018w, 1 005w, 944w, 899w, 830m, 805m, 740w, 664m, 631w, 544w, 522w.

  CP 2: A mixture of p‑nitrobenzoic acid (0.20 mmol, 0.0334 g), Ni(ClO₄)₂·6H₂O (0.20 mmol, 0.073 g), and bib (0.20 mmol, 0.038 g) was dissolved in deionized water (15 mL). After continuously stirring for 30 min, a suitable amount of NaOH and 5 mL DMF was added into the above solution to adjust the pH value to 7.14. Then the mixture was sealed up in a Parr Teflon-lined stainless-steel vessel (25 mL) under autogenous pressure, heated at 120 ℃ for 168 h, and cooled to room temperature over one day. Green columnar crystals of CP 2 were collected, washed with water, and air-dried (Yield: 19%). Elemental analysis Calcd. for C₃₀H₄₂Cl₂N₁₂NiO₈(%): C, 43.50; H, 5.11; N, 20.29. Found(%): C, 43.03; H, 4.89; N, 19.79. IR (cm^-1): 3 422 w, 3 126m, 2 952w, 1 637w, 1 522s, 1 484w, 1 443w, 1 406w, 1 373m, 1 303w, 1 281m, 1 238s, 1 097s, 937m, 870w, 834m, 772m, 743w, 728w, 668m, 637w, 623s. It is worth noting that p-nitrobenzoic acid has no coordination with Ni(Ⅱ) ions, and ClO₄^- is involved in balancing the charge.

  1.3   Determination of the crystal structures

  Single-crystal X-ray diffraction data of CPs 1 and 2 were collected at 293(2) K on a Bruker SMART APEX Ⅱ CCD diffractometer with Mo Kα (λ=0.071 073 nm). The related structures were solved by direct methods and refined by full-matrix least-squares on F² using the SHELXTL-2018 program. 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 CPs 1 and 2 is given in Table 1. The selected bond lengths and angles for 1 and 2 are listed in Table 2.

  Table 1

  

  Table 1.  Crystallography data and structure refinements for CPs 1 and 2

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  Parameter	1	2
  Empirical formula	C15H18N6NiO5	C30H42Cl2N12NiO8
  Formula weight	421.06	828.37
  Crystal system	Monoclinic	Hexagonal
  Space group	P21	R3
  a / nm	0.823 46(4)	1.393 14(7)
  b / nm	0.973 37(5)	1.393 14(7)
  c / nm	1.142 96(6)	1.724 73(12)
  β / (°)	108.738 0(10)
  Volume / nm3	0.867 56(8)	2.899 0(3)
  Z	2	3
  Dc / (g·cm-3)	1.612	1.423
  GOF	1.028	1.064
  Reflection collected, unique	3 536, 3 414	1 471, 1 317
  Rint	0.019 9	0.043 6
  R [I > 2σ(I)]	0.020 2	0.056 0
  wR	0.050 4	0.167 8

  Table 2

  

  Table 2.  Selected bond lengths (nm) and bond angles (º) for CPs 1 and 2

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  1
  Ni1—O1	0.209 00(12)	Ni1—O2A	0.205 29(11)	Ni1—O5	0.206 64(13)
  Ni1—N1	0.208 39(13)	Ni1—N3	0.206 59(14)	Ni1—N6B	0.210 50(15)
  O2A—Ni1—N3	87.67(6)	O2A—Ni1—O5	96.40(5)	N3—Ni1—O5	87.01(8)
  O2A—Ni1—N1	166.11(5)	N3—Ni1—N1	93.61(6)	O5—Ni1—N1	97.48(5)
  O2A—Ni1—O1	87.89(4)	N3—Ni1—O1	88.53(7)	O5—Ni1—O1	173.68(6)
  N1—Ni1—O1	78.32(5)	N3—Ni1—N6B	178.40(6)	O5—Ni1—N6B	91.93(7)
  N1—Ni1—N6B	85.33(6)	O1—Ni1—N6B	92.43(6)
  2
  Ni1—N1	0.213 9(2)	Ni1—N1A	0.213 9(2)
  N1A—Ni1—N1	180.00(12)	N1A—Ni1—N1	89.00(9)	N1B—Ni1—N1	91.00(9)
  Symmetry codes: A: 1-x, y-1/2, 2-z; B: x-1, y, z-1 for 1; A: 2-x, -y, 2-z; B: 1+y, 1-x+y, 2-z for 2.

  2.   Results and discussion

  2.1   Description of crystal structures

  Single-crystal X-ray diffraction shows that the 2D structure of CP 1 is constructed from [Ni₂(Hpdc)(H₂O)₂] subunits and bib linkers. The structural unit is composed of one Ni(Ⅱ) cation, one Hpdc^2- anion, one coordinated water molecule and one flexible bib ligand (Fig. 1). The central Ni(Ⅱ) cation is surrounded by three nitrogen atoms from two different bib ligands, one Hpdc^2- ligand and a pair of carboxylic oxygen atoms from two Hpdc^2- anions and one coordinated water molecule to furnish a distorted octahedral coordination geometry. The Ni—N bond distances are in a range of 0.206 59(14)-0.210 50(15) nm and the Ni—O bond lengths lie in a range of 0.205 29(11)-0.209 00(12) nm. The bond angles around the Ni(Ⅱ) cation are in a range of 78.32(5)°-178.40(6)°. In 1, one μ₃-Hpdc^2- anion and two water molecules coordinate with two Ni(Ⅱ) cations to form a [Ni₂(Hpdc)(H₂O)₂] subunit. The adjacent subunits are linked by μ₂-bib ligands to generate a 2D CP. It is noteworthy that there are thirty-two number rings in the 2D network structure (Fig. 2). Further analysis of the crystal packing revealed hydrogen-bond interactions in carboxylate oxygen atoms, pyrazole-nitrogen atom, pyrazole-carbon atom, imidazole-carbon atoms and coordinated water molecules (Table 3). Moreover, there are π-π interactions in 1 between the pyrazole ring of the Hpdc^2- ligand and the imidazole ring of the bib ligand. As shown in Fig. 2a, the centroid‑to‑ centroid distance between adjacent rings is 0.399 06(13) nm for the N1-N2-C4-C3-C2 pyrazole ring and N5-C13-N6-C14-C15 imidazole ring (Symmetry code: x-1, y, z-1). The perpendicular distance is 0.347 09(8) nm for the N1-N2-C4-C3-C2 pyrazole ring and N5-C13-N6-C14-C15 imidazole ring (Symmetry code: x-1, y, z-1), undoubtedly stabilize the structure of 1 and form a 3D supramolecular architecture (Fig. 3).

  Figure 1

  

  Figure 1.  Coordination environment (at 30% probability level) of the Ni(Ⅱ) center in CP 1

  Symmetry codes: A: 1-x, y-1/2, 2-z; B: x-1, y, z-1.

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  Figure 2

  

  Figure 2.  (a) Two-dimensional network structure and (b) polyhedral view of the 2D network of CP 1

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  Table 3

  

  Table 3.  Hydrogen bond parameters for CP 1

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  D—H···A	d(D—H) / nm	d(H···A) / nm	d(D···A) / nm	∠DHA / (°)
  N2—H2···O3A	0.078(2)	0.195(2)	0.270 1(2)	159(2)
  O5—H5A···O3A	0.075(3)	0.199(3)	0.271 92(16)	166(4)
  O5—H5B···O1B	0.080(3)	0.205(2)	0.268 78(19)	137(2)
  C5—H3···O4C	0.091(2)	0.249(2)	0.337 1(3)	165(18)
  C6—H6···O1	0.097(4)	0.256(3)	0.301 8(3)	109(2)
  C8—H8···O5	0.090(3)	0.259(3)	0.301 8(3)	110(3)
  Symmetry codes: A: 1-x, -1/2+y, 1-z; B: 1-x, -1/2+y, 2-z; C: 1-x, 1/2+y, 1-z.

  Figure 3

  

  Figure 3.  View of the 3D supramolecular architecture of CP 1 along the a-axis

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  The synthesized CP 2 appears as a green block crystal. Herein, the asymmetric unit is composed of one-sixth crystallographically Ni(Ⅱ) cation, half bib ligand, and one one-third free ClO₄^-, as shown in Fig. 4. The Ni²⁺ cation surrounded by six nitrogen atoms from different bib ligands is observed in 2. The bond lengths of Ni—N are 0.213 9(2) nm, and ∠N—Ni—N fall in a range of 89.00(9)°-180.00(12)°, indicating that Ni²⁺ ions are close to the positive octahedron. In 2, the flexible bib ligand shows trans-configuration and the dihedral angle between the two imidazole rings is 0°. In addition, six bib ligands linked the Ni^Ⅱ ions to form a 2D network with a 36-number ring (Fig. 5). More significantly, free ClO₄^- is located in voids, as listed in Fig. 6, which may be used for adsorption in the material field. It is worth noting that the structure of CP 2 is similar to our reported Co^Ⅱ complex^[18], and the latter is also a 2D network with a 36-membered ring. Further analysis of the crystal packing revealed that [Ni(bib)₃]²⁺ and ClO₄^- units form 3D supramolecular architectures through two hydrogen bonds: C1—H1A···O2 [C1···O2 0.331 68(8) nm] and C3—H3A···O1 [C3···O1 0.323 4(9) nm], undoubtedly stabilize the structure of CP 2.

  Figure 4

  

  Figure 4.  View of coordination environment (at 30% probability level) of Ni(Ⅱ) ion of CP 2

  Symmetry codes: A: 2-x, -y, 2-z; B: 1+y, 1-x+y, 2-z; C: 2-x+y, 1-x, z; D: x-y, x-1, 2-z; E: 1-y, x-y-1, z.

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  Figure 5

  

  Figure 5.  View of 2D network structure with 36-number ring along the c-axis of CP 2

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  Figure 6

  

  Figure 6.  View of 2D network structure with free ClO₄^- in voids along the a-axis of CP 2

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  2.2   IR and PXRD analysis

  In the frequency range of 400 to 4 000 cm^-1, the IR spectra of CPs 1 and 2 displayed the main inorganic and organic vibration absorption peaks (Fig.S1 and S2, Supporting information). The broad band of 3 360 cm^-1 represents —OH stretching modes within the carboxyl group of 1^[19]. The peaks observed between 1 593 and 1 349 cm^-1 for CP 1 are assigned to the stretching bands of ν_as, _COO— and ν_s, _COO—, respectively^[20]. The strong bands in a range of 740-631 cm^-1 can be attributed to the ν_C—N stretching of N-heterocyclic rings of the bib ligand in 1^[21]. The broad bands of 1 238 and 1 097 cm^-1 represent C=C stretching modes of bib ligands in 2^[19]. The strong bands in a range of 728-623 cm^-1 can be attributed to the ν_C—N stretching of the N-heterocyclic rings of the bib ligand in 2^[21].

  The phase purity of CPs 1 and 2 was determined by the comparison of their experimental PXRD patterns with the corresponding simulated patterns (calculated based on the single-crystal X-ray diffraction data) (Fig. 7). The above patterns have similar peak positions, indicating that the samples have good phase purity. The differences in intensity are in virtue of the preferred orientations of the powder samples.

  Figure 7

  

  Figure 7.  PXRD patterns of CPs 1 and 2

  Bottom: simulated; Top: experimental.

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  2.3   Thermal stability analysis

  TG curves were obtained from crystal samples to investigate the thermal stability of CPs 1 and 2. The samples were subjected to heating at a rate of 5 ℃·min^-1 under a flowing nitrogen atmosphere. As illustrated in Fig.S3, the TG curve of 1 demonstrates that the CP remained stable up to 220 ℃, after which it underwent decomposition upon further heating. Similarly, the TG curve of 2 indicates that the CP remained stable up to 235 ℃, beyond which it underwent decomposition upon further heating.

  2.4   UV spectrum

  The UV spectra for CPs 1 and 2, and ligands H₃pdc and bib have been investigated in the solid state. For the H₃pdc ligand, there was no absorption band, and bib had one absorption band at about 274 nm, while the title CPs had one absorption band at about 220 and 235 nm, respectively (Fig. 8), which should be assigned to the n→π* transition of bib and the charge transfer transition^[22].

  Figure 8

  

  Figure 8.  UV spectra of CPs 1 and 2

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  2.5   Quantum chemistry calculations

  The calculations were performed based on density functional theory (DFT) using Gaussian 16 and VASP software for the periodic system. The calculations were performed on "molecular fragments" extracted from the crystal structure of CP 1. The PBE0^[23-24] hybrid functional and the LANL2DZ basis set^[25] were employed for the NBO analysis. The generalized gradient approximation (GGA) in the scheme proposed by the Perdew-Burke‑Ernzerhof (PBE) functional was used to deal with the electronic exchange correlation.

  Table 4 presents the selected natural bond orbital (NBO) bond orders (in atomic units), Wiberg bond indices, natural electron configurations, and natural atomic charges of CP 1. Analysis of the results reveals that the electronic configurations of the Ni(Ⅱ) ion, nitrogen, and oxygen atoms are as follows: Ni(Ⅱ) ion-4s^0.263d^8.515p^0.33, nitrogen-2s^1.33-1.362p^3.60-4.11, and oxygen-2s^1.55-1.682p^5.01-5.17. Based on these findings, we can infer that the coordination between the Ni(Ⅱ) ions and nitrogen/oxygen atoms primarily occurs through the 4s, 3d, and 5p orbitals. Nitrogen atoms form coordination bonds with the Ni(Ⅱ) ion utilizing the 2s and 2p orbitals, while all oxygen atoms contribute electrons from their 2s and 2p orbitals to form coordination bonds with the Ni(Ⅱ) ion. Therefore, the Ni(Ⅱ) ion obtains some electrons from two N atoms of the bib ligand, one N atom of the Hpdc^2- ligand, two O atoms of the Hpdc^2- ligand, and one coordinated water molecule^[26-27]. Consequently, in the light of valence-bond theory, the atomic net charge distribution and NBO bond orders of 1 (Table 4) show the obvious covalent interaction between the coordinated atoms and Ni(Ⅱ) ion. The different NBO bond orders of Ni—O and Ni—N make different bond lengths for them^[27], which is consistent with the crystal structure data of 1.

  Table 4

  

  Table 4.  Selected atom net charges, electron configurations, Wiberg bond indexes, and NBO bond orders of CP 1

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  Atom	Net charge	Electron configuration	Bond	Wiberg bond index	NBO bond order
  Ni1	0.728 93	[core]4s0.263d8.515p0.33
  O1	-0.713 62	[core]2s1.682p5.02	Ni1—O1	0.301 2	0.289 7
  O2A	-0.673 95	[core]2s1.662p5.01	Ni1—O2A	0.371 3	0.312 2
  O5	-0.747 52	[core]2s1.552p5.17	Ni1—O5	0.233 7	0.253 7
  N1	-0.287 28	[core]2s1.362p3.90	Ni1—N1	0.276 6	0.328 0
  N3	-0.458 80	[core]2s1.332p4.11	Ni1—N3	0.429 0	0.378 7
  N6B	-0.466 35	[core]2s1.342p4.11	Ni1—N6B	0.394 0	0.367 7

  As depicted in Fig. 9, it is evident that the lowest unoccupied molecular orbital (LUMO) of CP 1 is predominantly composed of the Ni(Ⅱ) ion and the bib ligand. Conversely, the highest occupied molecular orbital (HOMO) primarily consists of the Ni(Ⅱ) ion and the Hpdc^2- ligand. Therefore, based on the observed molecular orbital contours, it is possible to infer the occurrence of ligand-to-ligand charge transfer (LLCT) within CP 1.

  Figure 9

  

  Figure 9.  Frontier molecular orbitals of CP 1

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  The calculated total and orbital-projected density of states (DOS) for the primitive cell of CP 1 is shown in Fig. 10. The bandgap of the material was 1.081 eV. The valence band is mainly occupied by Op and Np orbitals, while the conduction band is mainly occupied by Nid and Np orbitals. The DOS plot shows a sharp jagged shape, indicating the relative localization of electron orbitals.

  Figure 10

  

  Figure 10.  Calculated total and orbital-projected DOS for the primitive cell of CP 1

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  3.   Conclusions

  In summary, we have provided two new nickel CPs, [Ni(Hpdc)(bib)(H₂O)]_n (1) and {[Ni(bib)₃](ClO₄)₂}_n (2), by mixing Ni²⁺, 3, 5-pyrazoledicarboxylic acid (H₃pdc)/p-nitrobenzoic acid and 1, 4-bis(imidazol-1- ylmethyl)butane (bib). In CP 1, there is a 2D network constructed by six-coordinated Ni(Ⅱ) centers, bib and Hpdc^2- ligands, while a 2D network with a 36‑ membered ring is built by Ni(Ⅱ) and bib ligands in 2. Meanwhile, the quantum-chemical calculations of 1 are also discussed in this manuscript.

  
  Supporting information is available at http://www.wjhxxb.cn
  
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  Figure 1  Coordination environment (at 30% probability level) of the Ni(Ⅱ) center in CP 1

  Symmetry codes: A: 1-x, y-1/2, 2-z; B: x-1, y, z-1.

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  Figure 2  (a) Two-dimensional network structure and (b) polyhedral view of the 2D network of CP 1

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  Figure 3  View of the 3D supramolecular architecture of CP 1 along the a-axis

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  Figure 4  View of coordination environment (at 30% probability level) of Ni(Ⅱ) ion of CP 2

  Symmetry codes: A: 2-x, -y, 2-z; B: 1+y, 1-x+y, 2-z; C: 2-x+y, 1-x, z; D: x-y, x-1, 2-z; E: 1-y, x-y-1, z.

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  Figure 5  View of 2D network structure with 36-number ring along the c-axis of CP 2

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  Figure 6  View of 2D network structure with free ClO₄^- in voids along the a-axis of CP 2

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  Figure 7  PXRD patterns of CPs 1 and 2

  Bottom: simulated; Top: experimental.

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  Figure 8  UV spectra of CPs 1 and 2

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  Figure 9  Frontier molecular orbitals of CP 1

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  Figure 10  Calculated total and orbital-projected DOS for the primitive cell of CP 1

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  Table 1.  Crystallography data and structure refinements for CPs 1 and 2

  Parameter	1	2
  Empirical formula	C15H18N6NiO5	C30H42Cl2N12NiO8
  Formula weight	421.06	828.37
  Crystal system	Monoclinic	Hexagonal
  Space group	P21	R3
  a / nm	0.823 46(4)	1.393 14(7)
  b / nm	0.973 37(5)	1.393 14(7)
  c / nm	1.142 96(6)	1.724 73(12)
  β / (°)	108.738 0(10)
  Volume / nm3	0.867 56(8)	2.899 0(3)
  Z	2	3
  Dc / (g·cm-3)	1.612	1.423
  GOF	1.028	1.064
  Reflection collected, unique	3 536, 3 414	1 471, 1 317
  Rint	0.019 9	0.043 6
  R [I > 2σ(I)]	0.020 2	0.056 0
  wR	0.050 4	0.167 8

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  Table 2.  Selected bond lengths (nm) and bond angles (º) for CPs 1 and 2

  1
  Ni1—O1	0.209 00(12)	Ni1—O2A	0.205 29(11)	Ni1—O5	0.206 64(13)
  Ni1—N1	0.208 39(13)	Ni1—N3	0.206 59(14)	Ni1—N6B	0.210 50(15)
  O2A—Ni1—N3	87.67(6)	O2A—Ni1—O5	96.40(5)	N3—Ni1—O5	87.01(8)
  O2A—Ni1—N1	166.11(5)	N3—Ni1—N1	93.61(6)	O5—Ni1—N1	97.48(5)
  O2A—Ni1—O1	87.89(4)	N3—Ni1—O1	88.53(7)	O5—Ni1—O1	173.68(6)
  N1—Ni1—O1	78.32(5)	N3—Ni1—N6B	178.40(6)	O5—Ni1—N6B	91.93(7)
  N1—Ni1—N6B	85.33(6)	O1—Ni1—N6B	92.43(6)
  2
  Ni1—N1	0.213 9(2)	Ni1—N1A	0.213 9(2)
  N1A—Ni1—N1	180.00(12)	N1A—Ni1—N1	89.00(9)	N1B—Ni1—N1	91.00(9)
  Symmetry codes: A: 1-x, y-1/2, 2-z; B: x-1, y, z-1 for 1; A: 2-x, -y, 2-z; B: 1+y, 1-x+y, 2-z for 2.

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  Table 3.  Hydrogen bond parameters for CP 1

  D—H···A	d(D—H) / nm	d(H···A) / nm	d(D···A) / nm	∠DHA / (°)
  N2—H2···O3A	0.078(2)	0.195(2)	0.270 1(2)	159(2)
  O5—H5A···O3A	0.075(3)	0.199(3)	0.271 92(16)	166(4)
  O5—H5B···O1B	0.080(3)	0.205(2)	0.268 78(19)	137(2)
  C5—H3···O4C	0.091(2)	0.249(2)	0.337 1(3)	165(18)
  C6—H6···O1	0.097(4)	0.256(3)	0.301 8(3)	109(2)
  C8—H8···O5	0.090(3)	0.259(3)	0.301 8(3)	110(3)
  Symmetry codes: A: 1-x, -1/2+y, 1-z; B: 1-x, -1/2+y, 2-z; C: 1-x, 1/2+y, 1-z.

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  Table 4.  Selected atom net charges, electron configurations, Wiberg bond indexes, and NBO bond orders of CP 1

  Atom	Net charge	Electron configuration	Bond	Wiberg bond index	NBO bond order
  Ni1	0.728 93	[core]4s0.263d8.515p0.33
  O1	-0.713 62	[core]2s1.682p5.02	Ni1—O1	0.301 2	0.289 7
  O2A	-0.673 95	[core]2s1.662p5.01	Ni1—O2A	0.371 3	0.312 2
  O5	-0.747 52	[core]2s1.552p5.17	Ni1—O5	0.233 7	0.253 7
  N1	-0.287 28	[core]2s1.362p3.90	Ni1—N1	0.276 6	0.328 0
  N3	-0.458 80	[core]2s1.332p4.11	Ni1—N3	0.429 0	0.378 7
  N6B	-0.466 35	[core]2s1.342p4.11	Ni1—N6B	0.394 0	0.367 7

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