English

• 0.   Introduction

  Searching and characterizing transition metal complexes are one of the most interesting and challenging research areas^[1-3]. These compounds are usually composed of one or several metallic atomic centers which bonded with organic ligands^[4-8]. They can be viewed as platforms for evaluating the bonding mode of specific organic liquid with the metal. Among different kinds of ligands, carboxylate ligand can be arguably recognized as one of the most studied ones^[9-12]. They have been widely used for fabricating a large number of metal complexes as well as frameworks, especially for early transition metals^[13-17]. Among them, 1D complexes have drawn special attention not merely because of their potential applications, but also for their value as a model for understanding the interaction of ligands with metal^[18-19]. However, it is still a challenge to synthesize 1D complex in a controllable manner. Taking cobalt as an example, only a few 1D complexes are available^[20-21], either by multidentate carboxylic^[22], thiolate^[23], or other kinds of ligands^[24-25]. We disclose here two new members of 1D Co(Ⅱ) complexes protected by 2-nitro benzoate (2-nba^-).

  On the other hand, to further enrich the architectures of metal complexes, introducing a second ligand can often be effective^[26]. Researchers use two or more kinds of ligands, such as carboxylic acid and pyridine, and many other combinations to explore new complexes^[27-30]. The second one which usually bears different coordination modes with metal can function as auxiliary ligands to stabilize the complex. Optionally, they also competitively bond with metal so that the original bonding pattern will be significantly altered^[31]. As a result, new complexes and bonding modes may arise. Herein, we report a typical example of this mechanism. By simply adjusting the amount of 2-nitrobenzoic acid (2-nbaH), two 1D Co complexes [Co(H₂O)(2-nba)₂]_n (Co-1) and [Co₃(2-nba)₄(acac)₂]_n (Co-2) featuring different local coordination modes were isolated. And to prove the utility of our synthetic method, we took a step further to synthesize a Zn complex [Zn₂(2-nba)₄]_n (Zn-3), which is also in one dimension.

  1.   Experimental

  1.1   Materials and measurement

  2-nbaH (98%), tris(acetylacetonato)cobalt(Ⅲ) (Co(acac)₃, 98%), and zinc(Ⅱ) acetylacetonate (Zn(acac)₂, 99%) were purchased from Shanghai Meryer Chemical Technology Co., Ltd (China). Toluene solvent was purchased from Sinopharm Shanxi Co., Ltd. All chemicals and solvents were used as received.

  The content of the Co or Zn in complexes Co-1, Co-2, or Zn-3 was determined using an inductively coupled plasma mass spectrometry (ICP-MS) spectrometer PE NexION 350. Elemental analyses were performed on a Perkin Elmer 2400 Ⅱ Elemental Analyzer. Fourier transforms infrared (FTIR) spectra were collected on Nicolet iS5 with samples prepared as KBr pellets. The UV-Vis diffuse reflectance spectroscopy (UV-Vis DRS) of the powder samples was obtained on a Purkinje IS19-2 spectrometer.

  1.2   Synthesis

  1.2.1   Synthesis of Co-1

  2-nbaH (0.050 g, 0.3 mmol) and Co(acac)₃ (0.021 g, 0.06 mmol) were mixed and dissolved in 3 mL of toluene. The mixture was sealed in a glass tube. It was then subjected to heat treatment in an oven at 100 ℃ for 4 d. After the reaction, the mixture was cooled to room temperature. The flaky single crystals were obtained, which were analyzed by single-crystal X-ray crystallography. ICP-MS for Co in C₁₄H₁₀CoN₂O₉ (409.17): Calcd. 14.40%; Obsd. 13.67%. Elemental Anal. Calcd. for C₁₄H₁₀CoN₂O₉(%): C, 41.10; N, 6.85; H, 2.46. Found(%): C, 41.22; N, 6.78; H, 2.51. Selected IR (KBr, cm^-1): 3 031 (m), 2 932 (w), 2 878 (w), 1 943 (w), 1 863 (w), 1 800 (w), 1 604 (w), 1 492 (w), 1 455 (w), 1 381 (w), 1 080 (w), 1 026 (w), 732 (s), 694 (m), 461 (m).

  1.2.2   Synthesis of Co-2

  The synthesis of Co-2 is similar to that of Co-1, except that the amount of 2-nbaH was reduced to 0.1 mmol. ICP-MS for Co in C₃₈H₃₀Co₃N₄O₂₀ (1 039.45): Calcd. 17.01%; Obsd. 16.46%. Elemental Anal. Calcd. for C₃₈H₃₀Co₃N₄O₂₀(%): C, 43.91; N, 5, 40; H, 2.91. Found(%): C, 43.97; N, 5.35; H, 3.16. Selected IR (KBr, cm^-1): 3 078 (m), 3 031 (m), 2 931 (w), 2 878 (w), 1 942 (w), 1 863 (w), 1 800 (w), 1 604 (w), 1 492 (w), 1 461 (w), 1 377 (w), 1 080 (w), 1 032 (w), 725 (s), 694 (m), 462 (m).

  1.2.3   Synthesis of Zn-3

  Preparation as for Co-1, while Zn(acac)₂ was used instead of Co(acac)₃. ICP-MS for Zn in C₂₈H₁₆Zn₂N₄O₁₆ (795.19): Calcd. Zn 16.44%; Obsd. 16.70%. Elemental Anal. Calcd. for C₂₈H₁₆Zn₂N₄O₁₆(%): C, 42.29; N, 7.05; H, 2.03. Found(%): C, 42.35; N, 6.97; H, 2.11. Selected IR (KBr, cm^-1): 3 031 (m), 2 932 (w), 2 878 (w), 1 942 (w), 1 863 (w), 1 800 (w), 1 736 (w), 1 604 (w), 1 495 (w), 1 536 (w), 1 292 (w), 1 249 (w), 1 074 (w), 1 026 (w), 725 (s), 689 (m), 684 (m), 472 (m), 462 (m).

  1.3   Crystal structure determination

  Data collection for the complexes was carried out on a Rigaku Oxford Single Crystal Diffractometer using Mo Kα radiation (λ=0.071 073 nm) at 293 K. Absorption corrections were applied by using the program CrysAlis^Pro (multi-scan). The structure was solved and refined using Full-matrix least-squares based on F² with programs SHELXS-97 and SHELXL-97 within OLEX2^[32]. All non-hydrogen atoms were refined anisotropically and the positions of C—H hydrogen atoms were calculated theoretically. The H atoms of water molecules were located by electron density and constrained by geometrical parameters during the refinement procedure. O atom of the —NO₂ group in Co-2 was refined as disordered. The two moieties were restrained to have similar distances with the N atom. The parameters of the crystal data collection and refinement of complexes Co-1, Co-2, and Zn-3 are summarized in Table 1.

  Table 1

  

  Table 1.  Crystallographic data and structure refinement details for Co-1, Co-2, and Zn-3

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  Parameter	Co-1	Co-2	Zn-3
  Empirical formula	C14H10CoN2O9	C38H30Co3N4O20	C28H16Zn2N4O16
  Formula weight	409.17	1 039.45	795.19
  Temperature / K	293(2)	293(2)	293(2)
  Crystal system	Monoclinic	Triclinic	Monoclinic
  Space group	P21/c	P1	P21/c
  a / nm	0.731 84(3)	0.997 59(10)	1.112 37(5)
  b / nm	1.899 43(7)	1.025 65(7)	1.318 66(5)
  c / nm	1.172 84(5)	1.086 74(9)	2.072 28(8)
  α / (°)		79.587(7)
  β / (°)	103.675(4)	68.323(9)	98.151(4)
  γ / (°)		89.439(7)
  Volume / nm3	1.584 12(11)	1.014 29(16)	3.009 0(2)
  Z	4	1	4
  Dc / (g·cm-3)	1.716	1.702	1.755
  μ / mm-1	1.139	1.303	1.682
  F(000)	828.0	527.0	1 600.0
  Crystal size / mm	0.3×0.1×0.05	0.35×0.1×0.02	0.5×0.4×0.1
  2θ range for data collection / (°)	7.152-59.216	6.576-57.378	7.204-59.326
  Reflection collected	18 295	8 761	33 898
  Independent reflection	3 919 (Rint=0.030 8)	4 534 (Rint=0.055 1)	7 523 (Rint=0.037 9)
  Data, restraint, parameter	3 919, 0, 246	4 534, 13, 307	7 523, 0, 451
  Goodness-of-fit on F2	1.084	1.036	1.041
  R1, wR2 [I > 2σ(I)]	0.042 8, 0.089 0	0.062 0, 0.106 2	0.036 5, 0.072 8
  R1, wR2 (all data)	0.060 2, 0.095 9	0.091 0, 0.119 4	0.057 9, 0.080 9
  Largest diff. peak and hole / (e·nm-3)	360, -360	550, -430	370, -330

  2.   Results and discussion

  2.1   Structure description of complexes Co-1 and Co-2

  To synthesize Co-1, 2-nbaH and Co(acac)₃ were mixed and dissolved in toluene, then the mixture was sealed in a glass tube and placed in a 100 ℃ oven for 4 d, after which the single crystal of Co-1 was obtained. It was then analyzed by single-crystal X-ray crystallography. The structure of Co-1 is shown in Fig. 1a. It has an infinite linear chain structure. Every neighboring two Co²⁺ ions are bonded with two 2-nba^- ligands which are roughly perpendicular to each other. For three adjacent Co²⁺ ions, there are four 2-nba^- ligands, with each pair of them pointing in opposite directions. They are aligned in such a manner most likely to avoid the steric hindrance. Besides four 2-nba^- ligands, each Co²⁺ ion was also connected by two bridging aqua ligands. In all, every Co²⁺ ion is hexa-coordinated with six oxygen atoms to form an octahedron. And it is slightly distorted due to the deviation of the Co—O bond lengths (Fig. 1b). It is also interesting that the nitro groups of 2-nba^- ligands bonding with the same two Co²⁺ ions point in opposite directions. This makes all the nitro groups in the upper area in Fig. 1a align themselves in the same direction, while the other half are arranged oppositely. Such an arrangement can avoid the unfavorable interaction of two neighboring nitro groups (Fig. 1c). The distance between two Co²⁺ ions of Co-1 is 0.3659 nm, significantly longer than common Co—Co bonds^[33-36]. Therefore, there is no metal-metal bonding in Co-1.

  Figure 1

  

  Figure 1.  (a) Crystal structure of Co-1; (b) Coordination environment of the Co²⁺ ion in Co-1; (c) A view along the a-axis

  All Co²⁺ ions are hexa-coordinated (yellow octahedrons); H atoms are omitted for clarity.

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  It is unanticipated that the original ligand of the precursor Co(acac)₃ was completely replaced by the 2-nba^- ligands in Co-1, given that acac^- is a fairly strong ligand^[37]. With this in view, we made attempts to address the possibility to retain part of the acac^- ligands in the product. And by reducing the amount of 2-nba^- to one-third, another 1D Co complex (Co-2) was successfully isolated (Fig. 2a). From Fig. 2 we can see that despite the structural similarity of Co-1 and Co-2, their differences are still outstanding. The most notable one is the participation of acac^- as a second ligand and its significant influence on the coordination of Co. Unlike Co-1 where all Co²⁺ ions have the same coordination environment, Co²⁺ ions of Co-2 can be divided into two types. One-third of them are found in a similar octahedron geometry as in Co-1 (Fig. 2b), while the other two-thirds are penta-coordinated with O atoms, shaping into trigonal bipyramids (Fig. 2c). The bonding of acac^- with Co²⁺ is responsible for the unusual penta-coordinated Co²⁺ ions. On one hand, acac^- functions solely as chelating ligands in the precursor (i.e., Co(acac)₃), where one Co²⁺ ion is coordinated with three acac^- ligands. In Co-2, however, acac^- also functions as a bridging ligand. Each binding site of acac^-, namely the O atom, bonds with two neighboring Co²⁺ ions. Also, there are no other kinds of bridging oxygen. As mentioned, two-thirds of the Co²⁺ ions are penta-coordinated. In this type of coordination mode, as shown in Fig. 2c, three vertices out of five of the trigonal bipyramids are from two closely arranged acac^- ligands. The rigidity of acac^- ligands which require the local O—Co—O motif to be roughly in a plane is supposed to be responsible for this unusual bonding pattern. Viewing along the a-axis (Fig. 2d), the Co²⁺ ions are not precisely in a line, quite different from Co-1 (Fig. 1c). This can be ascribed to the non-uniform coordination environment of Co. We also noted that the acute angle between 2-nba^- ligands of Co-2 is smaller than that of Co-1. By comparing Fig. 1c and 2d, one possible explanation is that acac^- takes up more room and pushes 2-nba^- ligands closer^[38-39].

  Figure 2

  

  Figure 2.  (a) Crystal structure of Co-2; Coordination environment of (b) hexa-coordinated Co(Ⅱ) and (c) penta-coordinated Co(Ⅱ); (d) A view along the a-axis

  One-third of the Co²⁺ ions are hexa-coordinated (yellow octahedrons) and two-thirds of the Co²⁺ ions are penta-coordinated (cyan trigonal bipyramids); H atoms are omitted for clarity.

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  2.2   Non-covalent interactions in Co-1 and Co-2

  Although Co-1 and Co-2 have different local structures, the stacking of their 1D chains both rely on the non-covalent π-π interactions of the benzene rings^[40-42]. As shown in Fig. 3a, there are two orientations of π-π interaction in Co-1 (as indicated by black dashed lines). The two interaction distances are the same (0.365 1 nm, Table 2). That is to say, there are also two orientations of the Co-1 chains. They are arranged in such a manner that the two kinds of chains can interlace with each other and maximize their interactions (Fig. 3a)^[43]. On the other hand, there is only one kind of π-π interaction in Co-2 as shown in Fig. 3b. And such interaction is less in number than that of Co-1, due to the coordination of acac^-.

  Figure 3

  

  Figure 3.  (a) Cell stacking diagram along the a-axis of Co-1; (b) Cell stacking diagram along the b-axis of Co-2

  The non-covalent π-π interactions of the benzene rings are indicated by black dashed lines; H atoms are omitted for clarity.

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  Table 2

  

  Table 2.  Geometrical parameters of the π-π interactions for complexes Co-1 and Co-2

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  Complex	DCg-Cga / nm	DCg-ringb / nm
  Co-1	0.365 1	0.355 9
  Co-2	0.379 2	0.331 8
  a Centroid distance between the centroids of the rings; b Vertical distance from the centroid to the ring.

  2.3   Structure description of complex Zn-3

  To further validate the applicability of our method for other transition metals, Zn(acac)₂ was employed as a precursor. And as expected, another 1D Zn complex Zn-3 was successfully obtained (Fig. 4). Along the zigzag-shaped chain, every four Zn²⁺ ions can be viewed as a repeating unit. This unit can be further divided into two subgroups (Fig. 4a). Each subgroup is composed of two Zn²⁺ ions and three 2-nba^- ligands, which form a commonly seen paddle wheel structure. Between every two subgroups, there is another 2-nba^- ligand bridging them (Fig. 4b). It means 2-nba^- is the only kind of ligand present in Zn-3. Every Zn²⁺ ion is tetrahedrally coordinated with four O atoms. At this stage, however, we failed to obtain a Zn chain structure where acac^- is involved (Fig. 4b and 4c). Notwithstanding, the results mentioned above indicate that our synthetic method has the potential to apply to other metals as well^[30].

  Figure 4

  

  Figure 4.  (a) Crystal structure of Zn-3; (b) Coordination environment of the Zn²⁺ ion in Zn-3; (c) A view along the b-axis

  Each Zn²⁺ ion is tetrahedrally coordinated with four O atoms (green tetrahedrons); H atoms are omitted for clarity.

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  2.4   UV-Vis DRS spectra of the complexes

  The UV-Vis DRS spectra of Co-1, Co-2, and Zn-3 are shown in Fig. 5. Co-1 and Co-2 had strong absorption in the UV and visible light region, corresponding to the color of both are purple, and there were obvious absorption peaks at 579 nm. Zn-3 is a colorless crystal. The absorption was weak in the visible region, and Zn-3 had obvious absorption in the UV region.

  Figure 5

  

  Figure 5.  UV-Vis DRS spectra of Co-1, Co-2, and Zn-3 in the solid state

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  3.   Conclusions

  Three 1D Co(Ⅱ) and Zn(Ⅱ) complexes are reported in this paper. It is proved that the competitive coordination of acac^- with 2-nba^- can significantly change the coordination mode of Co²⁺ and thus lead to the generation of new 1D Co(Ⅱ) complexes. Two-thirds of the Co²⁺ ions of the complex [Co₃(2-nba)₄(acac)₂]_n are penta-coordinated. Our synthetic strategy was also successfully applied to synthesize a new 1D Zn(Ⅱ) complex.

  
  Acknowledgements: We express our gratitude to the National Natural Science Foundation of China (Grants No.21972080, 21503123, and 21871167), Sanjin Scholar, Shanxi "1331 Project" Key Innovative Research Team for financial support. We are also grateful for the help from the Shanxi University of Scientific Instrument Center. Declaration of competing interest: All authors of this paper declare that they have no competing interests.
  
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  Figure 1  (a) Crystal structure of Co-1; (b) Coordination environment of the Co²⁺ ion in Co-1; (c) A view along the a-axis

  All Co²⁺ ions are hexa-coordinated (yellow octahedrons); H atoms are omitted for clarity.

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  Figure 2  (a) Crystal structure of Co-2; Coordination environment of (b) hexa-coordinated Co(Ⅱ) and (c) penta-coordinated Co(Ⅱ); (d) A view along the a-axis

  One-third of the Co²⁺ ions are hexa-coordinated (yellow octahedrons) and two-thirds of the Co²⁺ ions are penta-coordinated (cyan trigonal bipyramids); H atoms are omitted for clarity.

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  Figure 3  (a) Cell stacking diagram along the a-axis of Co-1; (b) Cell stacking diagram along the b-axis of Co-2

  The non-covalent π-π interactions of the benzene rings are indicated by black dashed lines; H atoms are omitted for clarity.

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  Figure 4  (a) Crystal structure of Zn-3; (b) Coordination environment of the Zn²⁺ ion in Zn-3; (c) A view along the b-axis

  Each Zn²⁺ ion is tetrahedrally coordinated with four O atoms (green tetrahedrons); H atoms are omitted for clarity.

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  Figure 5  UV-Vis DRS spectra of Co-1, Co-2, and Zn-3 in the solid state

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  Table 1.  Crystallographic data and structure refinement details for Co-1, Co-2, and Zn-3

  Parameter	Co-1	Co-2	Zn-3
  Empirical formula	C14H10CoN2O9	C38H30Co3N4O20	C28H16Zn2N4O16
  Formula weight	409.17	1 039.45	795.19
  Temperature / K	293(2)	293(2)	293(2)
  Crystal system	Monoclinic	Triclinic	Monoclinic
  Space group	P21/c	P1	P21/c
  a / nm	0.731 84(3)	0.997 59(10)	1.112 37(5)
  b / nm	1.899 43(7)	1.025 65(7)	1.318 66(5)
  c / nm	1.172 84(5)	1.086 74(9)	2.072 28(8)
  α / (°)		79.587(7)
  β / (°)	103.675(4)	68.323(9)	98.151(4)
  γ / (°)		89.439(7)
  Volume / nm3	1.584 12(11)	1.014 29(16)	3.009 0(2)
  Z	4	1	4
  Dc / (g·cm-3)	1.716	1.702	1.755
  μ / mm-1	1.139	1.303	1.682
  F(000)	828.0	527.0	1 600.0
  Crystal size / mm	0.3×0.1×0.05	0.35×0.1×0.02	0.5×0.4×0.1
  2θ range for data collection / (°)	7.152-59.216	6.576-57.378	7.204-59.326
  Reflection collected	18 295	8 761	33 898
  Independent reflection	3 919 (Rint=0.030 8)	4 534 (Rint=0.055 1)	7 523 (Rint=0.037 9)
  Data, restraint, parameter	3 919, 0, 246	4 534, 13, 307	7 523, 0, 451
  Goodness-of-fit on F2	1.084	1.036	1.041
  R1, wR2 [I > 2σ(I)]	0.042 8, 0.089 0	0.062 0, 0.106 2	0.036 5, 0.072 8
  R1, wR2 (all data)	0.060 2, 0.095 9	0.091 0, 0.119 4	0.057 9, 0.080 9
  Largest diff. peak and hole / (e·nm-3)	360, -360	550, -430	370, -330

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  Table 2.  Geometrical parameters of the π-π interactions for complexes Co-1 and Co-2

  Complex	DCg-Cga / nm	DCg-ringb / nm
  Co-1	0.365 1	0.355 9
  Co-2	0.379 2	0.331 8
  a Centroid distance between the centroids of the rings; b Vertical distance from the centroid to the ring.

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