Anion Directed Assembly of Cu(II) Complexes Formed by a Flexible Ligand①

LIANG Zhen-Ning ZHANG Zong-Yo YANG Yu DING Shu-Ping YU Zhi-Yong CAO Rui,

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Anion Directed Assembly of Cu(II) Complexes Formed by a Flexible Ligand①

LIANG Zhen-NingaZHANG Zong-Yaoa②YANG YuaDING Shu-PingbYU Zhi-YongaCAO Ruia, b

a(100872)b(710119)

Coordination polymers, consisting of metal, anion and organic ligands, have attracted much attention. The structure of coordination polymers is affected by various factors. To investigate the effects of anion, syntheses and structures of four Cu(II) complexes, namely [(CuLCl2)(PF6)]n(1), [(CuLCl2)(ClO4)]n(2), [(CuLCl3)×CH3OH]n(3) and CuLCl3(4), are reported based on a flexible ligand, 1,3-bis(2-pyridylmethyl)imidazolium (L). The ligand is accommodative to kinds of anions, and forms 1-D chains generally upon reaction with Cu(II) salts. The effect is measured by the conformational variation through the dihedral angles between different aromatic rings of the ligand. In order to further investigate the effect of anion, a protonated sample of L, namely, LH(ClO4)2×H2O (5) is also synthesized and structurally characterized, showing an intriguing hydrogen bonded helix.

copper(II), one-dimensional coordination polymer, flexible ligand, crystal structure;

1 INTRODUCTION

Coordination polymers (CPs) have gained much attention and been studied in recent years, because they have various topological structures[1], and therefore versatile properties in spectroscopy[2, 3], electronic[4], thermology[5]and potential applications in various fields, such as ion exchange[6, 7], catalysis[8, 9], gas absorption[10, 11]and magnetism[12, 13]. Those properties are directed by the structures of CPs, and the latter is determined by a mix of factors, including internal factors like metal ions and organic ligands[14], or external factors like solvents[15], reac- tion temperature[9], counter anions[15], pH of the solution[16, 17]. Among those factors, the organic ligand plays a decisive role. Rigid ligands with large-conjugated structures are generally preferred for their predictable structures and easecrystalliza- tion[18], while flexible ligands have inherent flexi- bility, showing conformational variations[19]. Among these factors, the size and shape of ligand are an internal cause[18], and many other factors affect the structure by affecting this conformation, which even lead to distinct symmetries and complicated structures.

Although efforts have been made into exploring the decisive factors of conformations of flexible ligands, yet there is still much unclear, because that conformation is easily affected, and comparison is difficult when faced with distinct structures, even different dimensions. Our group has reported a series of Ag(I) complexes using a flexible ligand, 1,3-bis(2-pyridylmethyl)imidazolium (L) salt[20]. Owing to the labile geometry of Ag(I), different Ag(I) complexes are synthesized depending on anions. Those results have also shown the ligand is accom- modative to different anions while forming similar chain structures, making it suitable to analyze the effect of anions. By using L, we herein report the syntheses and structures of four Cu(II) coordination complexes, namely [(CuLCl2)(PF6)]n(1), [(CuLCl2)(ClO4)]n(2), [(CuLCl3)×CH3OH]n(3) and CuLCl3(4). Complexes 1～3 have similar chain structures, making it possible for structural analysis on conformations and effect of anions. To further investigate the effect of the anion, a protonated sample of the perchlorate salt of the ligand namely LH(ClO4)2·H2O (5) is also synthesized. The ligand is synthesized according to previous reports. Com- plexes 1～4 and compound 5 are characterized by X-ray crystallography and elemental analysis. Complexes 1～3 are further characterized by FTIR and TG.

2 EXPERIMENTAL

2.1 Synthesis and crystallization

All reagents were purchased from commercial suppliers and used as received unless otherwise noted. Elemental analysis was carried out on an Elementar Vario III elemental analyser. Infrared spectra (2% sample in KBr pellet) were recorded using a Brüker spectrophotometer running the OPUS software.1H NMR measurements were made on a Bruker spectrometer operating at 400 MHz.Thermogravimetric analysis (TGA) was conducted on a thermogravimetric analyzer (TGA Q50, TA Instruments) from room temperature to 1073 K under nitrogen at a heating rate of 10 K·min-1.

2. 1. 1 Synthesis of ligand 1,3-bis(2-pyridyl-methyl)imidazolium in different salts

This compound is synthesized according to pre- vious reports by our group[20]. A mixture of 2-(chloro-methyl)pyridine hydrochloride (11.86 g, 72.3 mmol), imidazole (2.46 g, 36.1 mmol) and NaHCO3(9.11 g, 112 mmol) was taken up in ethanol (100 mL). Then the mixture was kept reflux for 2 d and filtered. The solvent was then removed in vacuo, and the residue was extracted with dichloromethane and dried over Na2SO4. The solution was filtered, and the removal of dichloromethane in vacuo gave brown oil that was triturated with 40 mL of tetrahydrofuran to give a brown powder, which was further washed with tetrahydrofuran (2 × 20 mL) and dried in vacuo to give 7.46 g of 1,3-bis(2-pyridylmethyl)imidazo- lium chloride (L-Cl) (yield, 72%).1H NMR (400 MHz, CDCl3):10.90 (s, 1H), 8.52 (m, 2H), 7.73 (m, 4H), 7.63 (m, 2H), 7.29 (m, 2H), 5.71 (s, 4H).

The hexafluorophosphate salt L-PF6was prepared by dissolving L-Cl (1.00 g, 3.48 mmol) in 5 mL of water. Under stirring, a large excess of NH4(PF6) (5.70 g, 34.8 mmol) was added in small potions. The precipitate formed was filtered, washed with water, iced cold ethanol and ether, and finally dried in vacuo to give 1.15 g of L-PF6(yield, 83.3%). FT-IR (cm-1):3169(w), 1595(m), 1570(m), 1477(w), 1436(m), 1382(m), 1157(m), 841(vs), 754(m), 559(s).

The perchlorate salt L-ClO4was prepared using the same way mentioned above with excess NaClO4(yield, 72.1%). FT-IR (cm-1):3116(w), 1595(m), 1571(m), 1479(w), 1436(m), 1361(w), 1166(m), 1085(vs), 995(w), 759(m), 655(m), 624(w).

2. 1. 2 Syntheses of complexes 1～4

To a light yellow solution of L-PF6(10 mg, 25 μmol) in 3 mL of acetonitrile was added CuCl2(3.38 mg, 25 μmol). Then, 3 mL methanol was added under stirringto help to dissolve the Cu(II) salt. The yellow solution gradually turned green with the dissolution ofCu(II) salt. Then, the solution was stirred for 1 hand filtered. Slow diffusion of ether into the filtrate at room temperature affords blue block crystals of 1 (yield: 6.32 mg, 47.2%). Anal. Calcd. for C15H15Cl2CuF6N4P: C, 33.95; H, 2.85; N, 10.56%. Found: C, 33.74; H, 2.22; N, 10.38%.

Complexes2～4 were synthesized using the similar method as 1 by changing the reactants. Complex 2 was synthesized with the reaction of L-Cl, CuCl2and Cu(ClO4)2·6H2O in a ratio of 2:1:1 or equivalentL-ClO4and CuCl2(yield 62.6%, 56.3%, separately). Anal. Calcd. for C15H15Cl3CuN4O4: C, 37.13; H, 3.12; N, 11.55%. Found: C, 37.04; H, 3.23; N, 11.65%. Complex 3was prepared with the reaction of equivalent L-Cl and CuCl2(yield, 70.1%). Complex 4 was synthesized using the same way as 3 while using evaporation to afford crystals instead of diffusion (yield, 18.7%). Anal. Calcd. for C15H15Cl3CuN4: C, 42.77; H, 3.59; N, 13.30%. Found: C, 42.59; H, 3.34; N, 13.28%.

2. 1. 3 Synthesis of complex 5, LH(ClO4)2·H2O

To a solution of L-ClO4(10 mg, 35 μmol) in 2 mL methanol and 0.5 mL water was added HClO4(3.52 mg, 35 μmol) and stirred for 1 h. The light yellow solution was stood at room temperature to afford light yellow block crystals of 5 (yield: 5.94 mg, 36.2%). Anal. Calcd. for C15H16Cl2N4O8: C, 39.93; H, 3.57; N, 12.42%. Found: C, 39.88; H, 3.62; N, 12.23%.

2.2 Structure refinement

Crystal data, data collection and structure refine- ment details are summarized in Table 1. The data of five single crystals with dimensions of 0.25mm × 0.20mm × 0.20mm for 1, 0.25mm × 0.25mm × 0.20mm for 2, 0.20mm × 0.20mm × 0.20mm for 3, 0.40mm × 0.20mm × 0.20mm for 4 and 0.40mm × 0.30mm × 0.20mm for 5, respectively were collected on a Bruker D8 VENTURE X-ray diffractometer performing- and-scans at 150(2) K. Diffraction intensities were measured using graphite-monochromated Moradiation (= 0.071073 nm). Data collection, indexing, initial cell refinements, frame integration and final cell refinements were accomplished using the program APEX2[21]. Data reduction and empirical absorption correction were performed using the SAINT and SADABS program[21]. The structures were solved by direct methods using SHELXS[22]and refined by full-matrix least-squares2using SHELXL[22]. All non-hydrogen atoms were refined by full-matrix techniques with anisotropic displacement para- meters. All hydrogen atoms bound to carbon, nitrogen or oxygen were included into the model at geometrically calculated positions and refined using a riding model. The isotropic displacement parameters of all hydrogen atoms were fixed to 1.2 times the U value of the atoms they are linked to (1.5 times for methyl groups). For complex 5, the H atom bound to pyridine N atom was first added geometrically withiso(H) = 1.2eq(N). The water H atoms were added according to difference Fourier maps and refined with O–H = 0.83(2) ? andiso(H) = 1.2eq(O).

Table 1. Crystal Data and Structure Refinements for Complexes 1, 2, 3, 4 and 5

3 RESULTS AND DISCUSSION

3.1 Synthesis and characterization

The chlorine salt of the imidazolium ligand Lwas synthesized in a one-pot reaction with moderate yields. Then, the ligand in different salts is prepared via ion exchange reaction. Complexes 1～4 are synthesized via direct reaction between ligand and corresponding salts. The solvent used in each reaction is different, because the solubility of the ligand in different salts is different. For example, L-Cl is highly soluble in methanol and water, and L-PF6is soluble in acetonitrile but not so soluble in methanol. Complex 2 can be synthesized in two ways either using L-Cl or L-ClO4. Complexes3 and4 are obtained from different crystallization con- ditions. Diffusion method affords complex 3 in relative higher yields, while direct evaporation of the solution gives complex 4 in a rather low yield, with crystal blocks submerged in oily products. Attempts to acquire the perchlorate salt of ligand L are failed. Alternatively, we managed to grow samples of a protonated ligand L with the addition of HClO4. The ligand Lwas characterized by1H NMR, which is in accordance with the literature reports[20, 23].

3.2 Structural descriptions

Selected bond lengths and bond angles are shown in Table 2. Complex 1, [(CuLCl2)(PF6)]n, crystalizes in monoclinic space group21/with=4. In each asymmetric unit, there is one molecule of (CuLCl2)(PF6) (Fig. 1). The Cu atom is coordinated by two Cl?and two N atoms from two different ligands to exhibit a slightly distorted square planar geometry. The Cu–Cl(1) and Cu–Cl(2) bond dis- tances are 2.2445(13)and 2.2336(12) ?[24, 25], and the Cu–N bond distances are 2.005(4)and 2.009(4) ?[26]. The ligand acts as a linker coordinating two Cu(II) cores by the pyridinyl N atom to form an infinite 1-D zigzag chain. PF6?anions are not coordinated to the metal ions. Instead, they are located aside the 1-D chain (Fig. 3). The structure of complex 1 is further confirmed by the IR spectrum, showing characteristic absorption peaks of PF6?anions at 842 and 558 cm-1[27].

Complexes2 and 3 havesimilar 1-D chain struc- tures as 1. In the structure of2, ClO4?anions are placed in the interplace of the chain other than PF6?anions and the IR spectrum shows characteristic absorption of ClO4?ions at 1090 cm-1[27]. In the structure of complex 3, there are no counter ions while the methanol solvent molecules are placed aside the chain. The Cu(II) core in3 is five-coordinated and the τ parameter[28]showing struc- tural differences between trigonal bipyramidal and square-pyramidal geometriesis 0.60, indicating a distorted trigonal bipyramidal geometry. Complex 4 has the same asymmetry unit with 3 butforms a monomer. The versatile structures of complexes3 and 4 can be attributed to the flexibility of the ligand and the variable coordination of copper atom[29, 30]. In those complexes, the flexible ligand L also has similar but different conformations.

Table 2. Selected Bond Lengths (?) and Bond Angles (°) for 1～4

Fig. 1. Crystal structure of complex 1. Hydrogen atoms are omitted for clarity. Symmetry codes: a:–1, –+1/2,–1/2; b:+1, –+1/2,+1/2

Fig. 2. Crystal structure of complex 5. Some hydrogen atoms are omitted for clarity. Hydrogen bonds are shown in orange dashed lines. Symmetry code: a: –+1,+1/2, –+1/2

Fig. 3. Zigzag 1-D chain structure of complex 1. Hydrogen atoms are omitted for clarity

In order to have a further investigation of the role of ligand L and the effect of the anion on the conformation of L, we managed to get a protonated crystal sample of L-ClO4, namely LH(ClO4)2·H2O (5). Compound 5 crystalizes in monoclinic space group21/with=4. An asymmetric unit of 5 concludes one protonated L, two perchlorate ions and one water molecule (Fig. 2). Of two pyridine N atoms from the same ligand, one N atom is protonated and forms a hydrogen bond with an O atom from the solvent water molecule. Moreover, one of the water hydrogen atoms attached to pyridine N atom from another ligand to build a helical 1-D chain. In the meantime, the other hydrogen atom bound to the same O atom forms a hydrogen bond with a perchlorate oxygen atom, with the hydrogen bond parameters shown in Table 3. The perchlorate ion with hydrogen bonds is placed inside the helical chain, while other perchlorate ions were stayed outside the chain acting as counter ions. The ligands were connected by the coordinated water molecular via hydrogen-bonding interactions. These three kinds of hydrogen bonds have significant strength[31], thus they all participate in stabilizing the structure of 5.

Table 3. Hydrogen Bond Lengths (?) and Bond Angles (°) for Complex 5

Symmetry code: a: –+1,+1/2, –+1/2

As is reported by our group, the conformation of ligand L can be concluded in three categories, namely,and[20]. They are easy to identify from each other by the dihedral angle and the relative position between the two pyridinyl planes. Precisely, the conformation can be further identified by analyzing the different dihedral angles between the pyridinyl and imidazolyl planes through the structural analysis over chain structures in complexes 1～3. As is concluded in Table 4, the dihedral angle between two pyridinyl planes vary most compared with the other ones, revealing the degree of fluctuation. The dihedral angles between the pyridine and imidazole ring planes in the five complexes range from 66.039(106)o to 89.418(78)o and especially most of these angles are larger than 80o, which means the pyridine ring is preferably to be placed in a vertical position with the imidazole ring. That configuration of the ligand might be less steric hindrance and it is more stable and easier to construct structures. It reveals that the perchlorate ions perhaps make the ligand L into a parallel configuration and show the important roles of the anions in building CPs.

Table 4. Summarization of Dihedral Angles between Pyridine Ring and Imidazole Ring, Pyridine Ring and Pyridine Ring of the Ligand for Complexes 1～5

Im, Py1 and Py2 refer to the imidazole ring and two pyridine rings of the ligand, respectively

3.3 TG analysis

The TGA curves of complex 1～3 are shown in Fig. 4. The TGA curve of complex 1 shows that it is thermally stable up to 452 K and then it starts to decompose. Complex 2 exhibits a weight loss that begins at 429 K, which corresponds to its decom- position. It is worth noting that the discontinuity observed at 504 K results from the explosive decomposition of perchlorate salts[32]. Complex 3 undergoes a two-step of weight loss of 4% between 346 and 366 K, associated with the loss of methanol molecules (calculated 7%). The deviation may be caused by the pre-drying treatment of the sample. Furthering heating leads to the gradual decom- position of the complex.

Fig. 4. TGA curvers of complexes 1, 2 and 3

4 CONCLUSION

In summary, we have synthesized four labile Cu(II) complexes using the flexible ligand 1,3- di(2-picolyl)imidazolium salts and characterized by X-ray single-crystal diffraction and FT-IR spectro- scopy. Complexes 1～3 have similar chain struc- tures but distinct shapes. By contrast, complex 4 is a monomer. The different product shows the flexi- bility of the ligand and the structural lability of the complex. In addition, compound 5 is a protonated crystal sample of L-ClO4, which forms a helical chain through hydrogen bonds. Further investiga- tion of the anions and conformations of the ligand shows that anions have notable impact on the conformation of the ligand and therefore on the formation of the complex.

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12 March 2018;

21 May 2018 (CCDC 1842192～1842196)

① This work was supported by Thousand Talents Program of China, National Natural Science Foundation of China (No. 21101170 and 21573139), Fundamental Research Funds for the Central Universities and Research Funds of Renmin University of China

. Zhang Zong-Yao, E-mail:zhangzongyaochem@ruc.edu.cn

10.14102/j.cnki.0254-5861.2011-2002