Syntheses and Crystal Structures of a Series of Novel Helical

Jul 2, 2008 - Synopsis. A series of unprecedented examples of one-dimensional infinite helical structure are reported. 1 consists of chiral polymer ch...
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Syntheses and Crystal Structures of a Series of Novel Helical Coordination Polymers Constructed from Flexible Bis(imidazole) Ligands and Metal Salts Yan Qi,† Feng Luo,† Stuart R. Batten,‡ Yun-Xia Che,*,† and Ji-Min Zheng*,†

CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 8 2806–2813

Department of Chemistry, Nankai UniVersity, Tianjin 300071, P. R. China, and School of Chemistry, P.O. Box 23, Monash UniVersity, Clayton, Victoria, 3800, Australia ReceiVed October 29, 2007; ReVised Manuscript ReceiVed March 18, 2008

ABSTRACT: This paper reports eight novel one-dimensional (1D) helical chain polymers consisting of metal salts and flexible bis(imidazole) ligands, formulated as [Co(L1)2(bimb)] (1), [Cu2(bimb)2Cl2] (2), [Co(L2)2(bimb)] (3), [Co(L3)(bix)Cl] (4), [Co(L4)2(bix)] (5), [Co(L5)2(bimb)] (6), [Ni(L6)2(bimb)] (7), and [Ni(L7)2(bimb)] (8) (bimb ) 1,4-bis(imidazol-1-yl)-butane, bix ) 1,4-bis(imidazol1-yl-methylene)-benzene, HL1 ) 3,5-dinitrobenzoic acid, HL2 ) 4-methoxylbenzoic acid, HL3 ) benzoic acid, HL4 ) 4-methylbenzoic acid, HL5 ) 3-nitrobenzoic acid, HL6 ) 4-nitrobenzoic acid and HL7 ) 4-chlorobenzoic acid), which have been obtained via hydrothermal reaction and characterized by single-crystal X-ray diffraction, elemental analysis, IR and thermogravimetric analyses. 1 consists of chiral polymer chains assembled from achiral components due to the presence of left-handed helices. 2 is a very unusual racemic compound based on perpendicular left- and right-handed helical chains. 3 contains two distinct types of helical chains, one with an almost flat, zigzag type geometry and the other showing a distinct tubular motif. 4 is also a racemic compound with tubes larger than those in 3. 5-8 show rare meso-helical structures. This series of complexes provides new examples of 1D infinite helical structures for flexible bis(imidazole) ligands. Introduction Helical structures, the foundation of the genetic code, have attracted great current interest in coordination chemistry not only for their ubiquitous appearance in nature, a typical example being the DNA molecule, but also for their practical implications in multidisciplinary areas,1 such as optical devices, biomimetic chemistry, asymmetric catalysis chemistry, and structural biology. Many chemists have made great contributions to this field, and much effort has been exerted on the rational design and synthesis of artificial helical polynuclear metal complexes and coordination polymers.2 Well documented covalently bonded single-, double-, triple-, and even quadruple-stranded metal helicates as well as circular and cylindrical helical structures have been obtained to date.3 The most important feature of helix is its chirality. However, in most cases, racemates are usually encountered when organic ligands have no intrinsic chirality. There are a limited number of examples of achiral molecules including simple metal salts which crystallize in chiral space groups.4 So far, it is not well understood how homochiral packing of helices in crystals can be induced, and the precise mechanism needs further investigation. Some chemists propose that the chirality is induced by sterogenic centers,5 atropisomerism,6 noncovalent supramolecular interactions (hydrogen bonds, π-π interactions, etc.),7 or spontaneous chiral resolution upon crystallization without any enantiopure chiral auxiliary.8 Therefore, it seems to be rather difficult to reliably assemble chiral metal-organic helices from achiral components. In addition, meso-helical motifs are also rare, although meso-helices are widespread in nature, such as the tendrils of a variety of plants. To date, only a few one-dimensional (1D) meso-helical coordination polymers have been characterized, which can be regarded as a three-dimensional (3D) presentation of a lemniscate.9 * Corresponding author: Tel: +86-22-23508056; fax: +86-22-23508056; e-mail: [email protected]. † Nankai University. ‡ Monash University.

On the other hand, the exploitation of long flexible bidentate ligands as a helical component has until recently remained limited,10 attributed to the fact that long ligands usually result in large voids which may result in interpenetrated structures, two outstanding examples of which are bimb (1,4-bis(imidazol1-yl)-butane) and bix (1,4-bis(imidazol-1-yl-methylene)-benzene). With these ligands, a large number of beautiful interpenetrated networks of ingenious design have been constructed.11,12 However, these results do not mean that other types of entanglements cannot form in the presence of such ligands. In fact, suitable conformations of flexible ligands may give rise to helical topologies when they bind to metal ions. Therefore, it is still important to explore promising synthetic routes for the construction of helical metal-organic complexes via combination of long flexible ligands and metal cations. Further research is necessary to enrich and develop this field. Inspired by the aforementioned considerations, we aimed to couple our interest in obtaining new frameworks with the desire to achieve helical structures via linking transitional metal cations with flexible imidazole-based ligands. In this work, we selected the flexible 1,4-bis(imidazol-1-yl)-butane (bimb) and 1,4bis(imidazol-1-yl-methylene)-benzene (bix) ligands (Scheme 1). Because the mixing of metal salt and bimb or bix solution usually leads to a precipitation, making it difficult to grow suitable crystals for diffraction studies, a hydrothermal approach was adopted and eight novel 1D helical chain networks were successfully synthesized, namely, [Co(L1)2(bimb)] (1), [Cu2(bimb)2Cl2] (2), [Co(L2)2(bimb)] (3), [Co(L3)(bix)Cl] (4), [Co(L4)2(bix)] (5), [Co(L5)2(bimb)] (6), [Ni(L6)2(bimb)] (7), and [Ni(L7)2(bimb)] (8) (HL1 ) 3,5-dinitrobenzoic acid HL2 ) 4-methoxylbenzoic acid, HL3 ) benzoic acid, HL4 ) 4-methylbenzoic acid, HL5 ) 3-nitrobenzoic acid, HL6 ) 4-nitrobenzoic acid and HL7 ) 4-chlorobenzoic acid) (Scheme 1). All compounds are characterized by elemental analysis, IR spectrum, thermogravimetric analyses, and X-ray crystallography. This

10.1021/cg701061q CCC: $40.75  2008 American Chemical Society Published on Web 07/02/2008

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Crystal Growth & Design, Vol. 8, No. 8, 2008 2807 Scheme 1

Table 1. Crystallographic Data and Structure Refinement Summary for Complexes 1-8 1 formula formula weight T/K crystal system space group a /Å b /Å c /Å β /deg V /Å3 Z D /g/cm3 F(000) reflns collected/unique GOF R1 [I > 2σ(I)] wR2

formula formula weight T/K crystal system space group a /Å b /Å c /Å β /deg V /Å3 Z D /g/cm3 F(000) reflns collected/unique GOF R1 [I > 2σ(I)] wR2

2

3

4

C24H20CoN8O12 C20H28Cl2Cu2N8 C26H28CoN4O6 C21H19ClCoN4O2 671.41 578.48 551.45 453.78 293(2) 298(2) 298(2) 298(2) monoclinic monoclinic monoclinic monoclinic P21 P21/c Pc P21/c 11.024(2) 11.275(2) 18.399(4) 10.908(2) 11.295(2) 8.8409(18) 15.088(3) 12.664(3) 22.040(4) 15.945(5) 24.678(8) 15.038(3) 96.15(3) 128.952(19) 132.003(18) 100.02(3) 2728.5(9) 1236.0(5) 5091(2) 2045.6(7) 4 2 8 4 1.634 1.554 1.439 1.473 1372 592 2296 932 26767/11896 [R(int) ) 0.0518] 11863/5530 [R(int) ) 0.0378] 48558/11594 [R(int) ) 0.0883] 19145/4600 [R(int) ) 0.0875] 1.018 1.072 1.046 1.020 0.0452 0.0390 0.0537 0.0491 0.0935 0.0996 0.1023 0.0983 5

6

7

8

C30H28CoN4O4 567.49 293(2) monoclinic P21/c 11.549(2) 12.806(3) 21.281(6) 116.56(2) 2815.2(11) 4 1.339 1180 26465/6364 [R(int) ) 0.0437] 1.050 0.0393 0.0976

C24H22CoN6O8 581.41 298(2) monoclinic P2/c 12.301(3) 6.6095(13) 16.694(6) 112.97(2) 1249.7(6) 2 1.545 598 11641/2854 [R(int) ) 0.0471] 1.062 0.0345 0.0921

C24H22N6NiO8 581.19 298(2) monoclinic C2/c 25.709(5) 6.8939(14) 16.300(3) 121.44(3) 2464.8(8) 4 1.566 1200 11567/2812 [R(int) ) 0.0581] 1.036 0.0437 0.0922

C24H22Cl2N4NiO4 560.07 298(2) monoclinic C2/c 21.308(4) 6.7964(14) 16.577(3) 96.71(3) 2384.2(8) 4 1.560 1152 10602/2684 [R(int) ) 0.1228] 0.759 0.0371 0.0637

work highlights the rich coordination chemistry of flexible bis(imidazole) ligands. Experimental Section Materials and General Procedures. Solvents and starting materials for synthesis were purchased commercially and used as received. The ligands bimb and bix were prepared according to reported procedures.13 IR spectra were recorded as KBr pellets on a Nicolet Magna-FT-IR 560 spectrometer in the 4000-400 cm-1 region. Elemental analyses for C, H, and N were performed on a Perkin-Elmer 240 analyzer. The thermogravimetric analyses were investigated on a standard TG analyzer under a nitrogen flow at a heating rate of 5 °C/min for all measurements. X-ray Crystallographic Measurements for 1-8. Suitable single crystals of 1-8 were selected and mounted in air onto thin glass fibers. Accurate unit cell parameters were determined by a least-squares fit of 2θ values, and intensity data were measured on a Rigaku R-axis rapid IP area detector with Mo KR radiation (λ ) 0.71073 Å) at room

temperature. The intensities were corrected for Lorentz and polarization effects as well as for empirical absorption based on a multiscan technique.14 All structures were solved by direct methods and refined by full-matrix least-squares fitting on F2 by SHELX-97.15 All nonhydrogen atoms were refined with anisotropic thermal parameters. Crystallographic data have been deposited with the Cambridge Crystallographic Data Centre (deposition numbers 659536-659543). These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Crystallographic data for the eight compounds are summarized in Table 1 and selected bond lengths and angles are listed in Table S2, Supporting Information. Synthesis. [Co(L1)2(bimb)] (1). A mixture of CoCl2 · 6H2O (0.24 g, 1.0 mmol), HL1 (0.43 g, 2.0 mmol), and bimb (0.19 g, 1.0 mmol) was dissolved in 8 mL of distilled water. The pH value was then adjusted to 6.0 with 1 M NaOH solution. The resulting mixture was then transferred and sealed in a 25 mL Teflon-lined stainless steel vessel, and then heated at 150 °C for 2 days. After the reactor was slowly

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Figure 1. View of the local coordination environment of Co atom. The dashed lines indicating the weak interactions between O and Co atoms.

Figure 2. Views of (a) the 1D left-handed helical structure of 1, and (b) adjoining parallel helical chains containing either Co1 (blue) or Co2 (red).

Qi et al. 1657 (m), 1529 (s), 1466 (s), 1364 (s), 1243 (s), 1110 (s), 956 (s), 846 (s), 793 (m), 731 (m), 660(m). [Co(L2)2(bimb)] (3). The process was similar to 1 except that HL1 was replaced by HL2 (0.29 g, 2.0 mmol). Purple block-shaped crystals of 3 were obtained in yield 45% (based on Co). Elemental analysis (%): Calcd. for (3): C 56.63, H 5.12, N 10.16. Found: C 56.59, H 5.18, N 10.20. IR (KBr): ν (cm-1) ) 3126 (m), 3062 (m), 2946 (m), 2838 (m), 1606 (s), 1549 (s), 1464 (m), 1415 (s), 1306 (s), 1246 (s), 1177 (s), 1103 (s), 851 (s), 768 (m), 662 (m). [Co(L3)(bix)Cl] (4). CoCl2 · 6H2O (0.24 g, 1.0 mmol), HL3 (0.25 g, 2.0 mmol), and bix (0.24 g, 1.0 mmol) were placed in a Teflon-lined stainless steel vessel. The mixture was sealed and heated at 150 °C for 3 days, and then cooled to room temperature. Purple block-shaped crystals of 4 were obtained in 48% yield (based on Co). Elemental analysis (%): Calcd. for (4): C 55.58, H 4.22, N 12.35. Found: C 55.62, H 4.16, N 12.29. IR (KBr): ν (cm-1) ) 3106 (m), 3027 (m), 2957 (m), 2880 (m), 1596 (s), 1560 (s), 1522 (s), 1405 (s), 1239 (s), 1105 (s), 1086 (s), 724 (s), 684 (s), 655 (m). [Co(L4)2(bix)] (5). The purple block-shaped crystals of 5 were obtained by adopting the same synthetic procedure as 4 except HL4 (0.27 g, 2.0 mmol) was used instead of HL3, and the pH value was adjusted to 6.0 by 1 M NaOH solution. Yield: 45% (based on Co). Elemental analysis (%): Calcd. for (5): C 63.49, H 4.97, N 9.87. Found: C 63.45, H 4.92, N 9.85. IR (KBr): ν (cm-1) ) 3100 (m), 3027 (m), 2985 (m), 2849 (m), 1609 (s), 1590 (s), 1534 (s), 1416 (m), 1239 (s), 1173 (m), 1112(s), 1084 (s), 860 (s), 734 (s), 665 (m). [Co(L5)2(bimb)] (6). The process was similar to 1 except that HL1 was replaced by HL5 (0.33 g, 2.0 mmol). Purple block-shaped crystals of 6 were obtained in 54% yield (based on Co). Elemental analysis (%): Calcd. for (6): C 49.58, H 3.81, N 14.45. Found: C 49.54, H 3.86, N 14.49. IR (KBr): ν (cm-1) ) 3081 (m), 2953 (m), 2863 (m), 1625 (s), 1605 (s), 1573 (s), 1530 (s), 1474 (m), 1416 (s), 1348 (s), 1232 (s), 1112 (s), 1095 (s), 830 (s), 739 (m), 721 (s), 655 (m). [Ni(L6)2(bimb)] (7). A mixture of Ni(OAc)2 · 4H2O (0.25 g, 1.0 mmol), HL6 (0.33 g, 2.0 mmol), and bimb (0.19 g, 1.0 mmol) in 8 mL of distilled water was stirred for 30 min. and then the pH was adjusted to 5.5 by addition of 1 M NaOH solution. After the sample was stirred for another 30 min, the mixture was transferred to a Teflon-lined stainless steel vessel, and heated at 160 °C for 3 days. The reaction system was then cooled to room temperature. Green block-shaped crystals of 7 were obtained in 52% yield (based on Ni). Elemental analysis (%): Calcd. for (7): C 49.60, H 3.82, N 14.46. Found: C 49.63, H 3.86, N 14.50. IR (KBr): ν (cm-1) ) 3083 (m), 3056 (m), 2935(m), 2839(m), 1675 (m), 1615 (s), 1563 (s), 1539 (m), 1512 (s), 1488 (m), 1428 (s), 1342 (s), 1109 (m), 1086 (m), 875 (m), 839 (s), 817 (m), 723 (s), 654 (m). [Ni(L7)2(bimb)] (8). The process was similar to 7 except that HL6 was replaced by HL7 (0.31 g, 2 mmol). Green block-shaped crystals of 8 were obtained in 48% yield (based on Ni). Elemental analysis (%): Calcd. for (8): C 51.47, H 3.96, N 10.00. Found: C51.44, H 3.99, N 10.02. IR (KBr): ν (cm-1) ) 3097 (m), 3077 (m), 2948 (m), 1676 (m), 1590 (s), 1540 (s), 1463 (m), 1415 (s), 1331 (m), 1113 (s), 1091 (s), 856 (s), 822 (m), 772 (s), 654 (m).

Results and Discussion Figure 3. View of the local coordination environments of the Cu atoms in 2. cooled to room temperature, purple block-shaped crystals were filtered off, washed with distilled water, and dried in air. Yield: 62% (based on Co). Elemental analysis (%): Calcd. for (1): C 42.93, H 3.00, N 16.69. Found: C 42.89, H 3.04, N 16.75. IR (KBr): ν (cm-1) ) 3092 (m), 2950 (m), 2866 (m), 1623 (vs), 1577 (s), 1535 (s), 1456 (m), 1408 (s), 1340 (s), 1240 (m), 1091 (s), 1070 (s), 838 (m), 654 (m). [Cu2(bimb)2Cl2] (2). CuCl (0.10 g, 1.0 mmol), bimb (0.19 g, 1.0 mmol), and DMF (8 mL) was put into a 25 mL Teflon-lined stainless steel vessel. The mixture was sealed and heated at 160 °C for 3 days, then cooled to room temperature. Light green block-shaped crystals of 2 were obtained in 51% yield (based on Cu). Elemental analysis (%): Calcd. for (2): C 41.52, H 4.88, N 19.37. Found: C 41.59, H 4.83, N 19.42. IR (KBr): ν (cm-1) ) 3089 (s), 3034 (m), 2924 (m), 2856 (m),

Crystal Structure of [Co(L1)2(bimb)] (1). Single-crystal X-ray structural analysis revealed that 1 crystallized in the asymmetrical monoclinic space group P21, and consists of onedimensional (1D) left-handed helical chains. As shown in Figure 1, the asymmetric unit consists of two crystallographically independent CoII atoms, four L1 ligands, and two bimb ligands, in which both CoII atoms exhibit distorted tetrahedral geometry. The Co1 and Co2 centers have identical coordination environments, both coordinated by two oxygen atoms from two L1 ligands and two nitrogen atoms from two different bimb ligands. The Co-O bond lengths range between 1.992(2) and 2.003(2) Å, while the Co-N bond lengths vary from 2.001(3) to 2.018(3) Å, which are in the normal range.11 Both L1 ligands exhibit monodentate coordination mode and the two uncoordinated carboxylic oxygen atoms have weak interactions with Co atoms (Co1-O2 ) 2.645(3) Å for Co1 and Co2-O16 ) 2.697(3) Å

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Crystal Growth & Design, Vol. 8, No. 8, 2008 2809

Figure 4. Views of the 1D left- (blue) and right-handed (pink) helical chains of 2 in perpendicular arrangement (a); rectangular channels constructed by left- and right-handed helical chains (b).

for Co2). There are two kinds of independent bimb spacers, both of which act as bidentate ligands to bridge two adjacent Co atoms. Both bimb ligands exhibit cis configurations (Scheme 1) with the dihedral angles between the two pairs of imidazole rings being 75.72° and 73.71°. For the previously reported coordination frameworks based on bimb, the cis geometry is rare, with only one such example known in the literature.16 The bimb ligands bridge the Co atoms into parallel helical chains running along the 21 axes in the a direction (Figure 2). Crystallographically there are two types of chains: one containing the Co1 atoms, the other containing the Co2 atoms. However, both types are chemically and geometrically similar. The helical chains are decorated with L1 ligands on one side, and the pitch of the helices is 11.024 Å. The most salient structural feature of 1, however, is the chiral coordination polymer assembled from achiral components due to the presence of a left-handed helix. [Cu2(bimb)2Cl2] (2). Single-crystal X-ray structural analysis shows that 2 contains helical 1D polymer chains with an interesting packing motif. The asymmetric unit (Figure 3) consists of two crystallographically independent Cu(I) atoms, two chlorine anions, and two bimb ligands, in which the two CuI atoms have similar coordination geometries with slight differences in bond lengths and angles. Each CuI cation is threecoordinated by two nitrogen atoms from two bimb ligands and a chloride anion. The Cu-N bond lengths range from 1.893(3) to 1.912(3) Å, while the Cu-Cl bond lengths are 2.5365(12) and 2.4961(12) Å. In contrast to those in 1, the two types of bimb both show twisted trans conformation (Scheme 1) with the dihedral angles between the two pairs of imidazole rings being 61.02° and 68.46°. The Cu · · · Cu separations across the bimb bridges are 13.410 and 12.782 Å, respectively. The bimb spacers connect neighboring CuI(1) and CuI(2) cations to form a 1D infinite single helical chain. The pitch of each helix contains two Cu(I) ions and two bimb ligands and spans a distance of 26.183 Å, while in 1 only one Co(II) ion and one bimb ligand form the repeating pitch. In the packing arrangement of 2, left- and right-handed helical chains are present in equal numbers (giving an overall racemic compound) but run perpendicular to each other (Figure 4a). The chains line up in layers, in which each chain has the same handedness. These layers then alternate with layers containing chains with of the opposite hand, and the chains in each layer are orientated at an inclined angle to those in the adjoining layers (Figure 4b). This results in the formation of infinite rectangular channels running perpendicular to each layer. However, the channels are too small to contain guest molecules. The final helical structure

Figure 5. View of the local coordination environment of Co atom. The dashed lines indicating the weak interactions between O and Co atoms.

is further stabilized by Cl · · · H-C (2.661-2.842 Å) hydrogen bonds (Table S2, Supporting Information). Adjoining helical chains are connected by these hydrogen bonds, generating an overall 3D supramolecular network. To the best of our knowledge, such a racemic compound based on separate layers of perpendicular left- and right-handed helical chains is unprecedented. [Co(L2)2(bimb)] (3). Complex 3 also contains a helical structure, but in this case it is based on two distinct types of helical chains. As illustrated in Figure 5, two kinds of Co(II) centers are present in the asymmetric unit: one is five-coordinate with an irregular geometry, while the other is fourcoordinate with a distorted-tetrahedral geometry. The coordination sphere around the five-coordinate Co1 cation is composed of three oxygen atoms from two L2 ligands (Co1-O ) 2.039(2)-2.370(3) Å) and two nitrogen atoms from two bimb ligands (Co1-N1 ) 2.061(3) Å and Co1-N3 ) 2.042(3) Å). The carboxylic groups of two L2 ligands coordinate to Co1 with either a bidentate chelating or a monodentate coordination mode. The remaining carboxylic oxygen atom has a weak interaction with Co1 (Co1-O5 ) 2.425(3) Å). The other Co(II) center (Co2) adopts a tetrahedral geometry with two oxygen atoms of two L2 ligands (Co2-O7 ) 2.008(2) Å and Co2-O10 ) 2.014(2) Å) and two nitrogen atoms of two bimb ligands (Co2-N5 ) 2.042(3) Å and Co2-N7 ) 2.028(3) Å). Both L2 ligands exhibit monodentate coordination modes and the two uncoordinated carboxylic oxygen atoms also have weak interac-

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Figure 6. Views of (a) the first type of covalently bonded helices in 3; (b) the second type of covalently bonded helices in 3; and (c) the Co1 (blue) and Co2 (red) helices viewed end-on.

Figure 7. View of the local coordination environment around the Co atom. The dashed lines indicate the weak interaction between an O atom and the Co atom.

tions to the Co2 atom (Co2-O8 ) 2.438(5) Å and Co2-O9 ) 2.790(9) Å). There are also two types of bimb spacers, which bridge between the Co atoms. Both exhibit trans geometries with the dihedral angles between pairs of imidazole rings being 19.16° and 16.8°. Two adjacent Co atoms are separated by bimb with the distances of 13.865 and 13.441 Å, respectively. Co1 ions are bridged by the bimb ligand containing N1 into leftand right-handed helices running along crystallographic 21 axes in the b direction (Figure 6a). The pitch of the helix is 15.088 Å. The other bimb ligands to which N5 belong link the Co2 ions and form similar helical enantiomers with the same pitch (Figure 6b). Notably, the sizes of two helical chains constructed by Co1 and Co2 are different (Figure 6c). The one containing Co1 is much narrower, showing an almost flat zigzag type motif, while the Co2 helix has more of a spiral tube motif. This is due to the different conformations of the bimb ligands. [Co(L3)(bix)Cl] (4). Similar to 3, 4 also crystallizes in the centrosymmetric space group P21/c, leading to a racemic compound. In contrast to 3, however, only one kind helix was observed due to the presence of another flexible bis(imidazole) ligand, bix, in place of bimb. A view of 4 with atom labeling is shown in Figure 7, and the coordination geometry around the Co(II) center could be described as a distorted tetrahedron, composed of two nitrogen atoms from two bix ligands (Co-N ) 2.004(3) and 2.012(3) Å), one carboxylic oxygen atom from a L3 ligand (Co-O ) 1.991(2) Å), and one chlorine atom (Co-Cl ) 2.2893(10) Å). Similar to 1 and 3, the uncoordinated

Figure 8. Perspective and space-filling views of (a) the 1D helical structure of 4; and (b) the tubular motif shown by the helices, with the width being 6.97 × 11.18 Å.

carboxylic oxygen atoms also show a weak interaction to the Co atom (Co-O ) 2.505(2) Å). Only one type of bix ligand is observed, which assumes the trans conformation with two imidazole groups extending to opposite sides of the benzene ring and bridges two adjacent Co atoms with a separation of 12.550 Å. Left- and right-handed helical chains along the b axis are formed (Figure 8a). The pitch of the helix is 12.664 Å. A helical tube with a width of 6.97 × 11.18 Å, that is much larger than those in 3 (Figure 8b), is formed. This may be because the bix ligand is more rigid than bimb. [Co(L4)2(bix)] (5), [Co(L5)2(bimb)] (6), [Ni(L6)2(bimb)] (7), and [Ni(L7)2(bimb)] (8). Single-crystal X-ray structural analyses reveals that 5-8 are rare meso-helical structures, significantly different from the structures discussed earlier. The metal cations in 5, 7, and 8 are six-coordinate, and their asymmetric units are shown in Figure 9a. Each metal atom (M) is in a distorted octahedral coordination sphere, {MN2O4}, defined by four oxygen atoms from two carboxyl groups which adopt bichelating coordination modes (M-O ) 2.080(2)-2.247(2) Å) and two nitrogen atoms from two bis(imidazole) ligands (M-N ) 2.026(2)-2.0693(18) Å). The Co atom in 6 has a

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Figure 9. (a) View of the local coordination environment of metal atom in 7 (M ) Ni). Compounds 5 and 8 have similar local geometries, but with L7 in place of L6 (8), or L4 in place of L7 and bix in place of bimb (5, M ) Co). (b) View of the local coordination environment of Co in 6. The dashed lines indicate weak interactions between O and Co atoms.

Figure 10. View of (a) the 1D meso-helical structure of 6 (a); and (b) two neighboring interlaced meso-helical chains viewed from the side (left) and along the c axis (right). Compounds 5, 7, and 8 show similar motifs.

four-coordinate environment, bonded to two oxygen atoms (Co-O ) 1.9844(14) Å) from two carboxyl groups and two nitrogen atoms (Co-N ) 2.0183(17) Å) from two bimb molecules (Figure 9b). The uncoordinated carboxylic oxygen atoms also have weak interactions with the Co atoms (Co-O ) 2.699(2) Å). In all compounds, neighboring metal centers are bridged together via trans bis(imidazole) ligands, affording extraordinary meso-helical chains with M · · · M distances ranging from 13.371 to 13.860 Å. These chains contain both left- and right-handed helical loops in each chain, and display a “∞” shape, as shown in Figure 10a. The pitches range from 12.806 to 16.694 Å. Neighboring chains are arranged as illustrated in Figure 10b. To date, only a few meso-helices have been reported.9 5-8 represent unprecedented examples among imidazole-based coordination polymers. In addition, it is interesting that four complexes have similar meso-helical structures in spite of their different crystal space groups and ligand sets. Crystals of 5 and 6 belong to the monoclinic space group P21/c and P2/

c, respectively, while 7 and 8 both crystallize in the monoclinic space group C2/c. Thermogravimetric Analyses. To study the stability of these polymers, thermogravimetric analyses (TGA) of complexes 1-8 were performed (Figure 11). The TGA curve of these complexes indicates that the framework of compounds 1, 2, 3, 6, and 8 all began to collapse from 265 °C, while the compounds 4, 5, and 7 are more stable up to 320 °C, where the decomposition of the framework starts. In compound 2, the rapid weight loss from 265 to 445 °C corresponds to the loss of the bimb ligand. The observed weight loss of 65.88% is in agreement with the calculated one (65.74%). At about 650 °C, it decomposed completely and the remaining residue is presumed to be Cu2O (calcd: 24.12%; found: 23.34%). The resulting residue of compounds 3 and 6 remain as CoO (calcd: 13.60%, found: 14.56% for 3; calcd: 12.91%, found: 14.62% for 6) after the complete decomposition of the organic ligands. In compound 8, a rapid weight loss of 60.85% from 265-435 °C corresponds

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groups besides coordinated functional groups may be a feasible way for the formation of various architectures. Finally, the coordination geometry of the metal cations also plays a crucial role in tuning the types of helical structure. In this paper, the transition metal cations adopt various coordination geometries, including three-, four-, five-, and six-coordinate. In summary, the successful isolation of this series of new complexes not only provides the first examples of 1D infinite helical structures for long flexible bis(imidazole) ligands but also further confirms the conformational diversity possible for flexible ligands. This work may give an impetus to the further exploration of metal-bis(imidazole) complexes with helical characters. Figure 11. Thermogravimetric analyses (TGA) curve of compounds 1-8.

to one L7 anion and one bimb ligand (calcd: 61.62%) and the remaining residue is presumed to be NiO (calcd: 13.39%; found: 13.04%). However, other compounds keep decomposing until 750 °C. Conclusion We have successfully prepared eight different types of 1D helical metal-organic frameworks from the appropriate combination flexible bis(imidazole) ligands and metal salts, which further enrich the coordination chemistry of these ligands. 1 is comprised of 1D homochiral polymer chains assembled from achiral components due to the presence of only left-handed helices. 2 shows an unusual 3D supramolecular structure containing perpendicular left- and right-handed helical chains, which crystallizes in the noncentrosymmetric monoclinic space group Pc. The Cl · · · H-C interactions likely play important roles in forming and stabilizing the observed structure. In 3 and 4 left- and right-handed helices are obtained in equal amounts. It is interesting that 3 has two types of helical chains, one of which has an almost flat zigzag-like arrangement, while the other has a more tubular cross-section, but smaller than those in 4 owing to the fact that bimb is more linear than bix. In addition, 5-8 all create extraordinary meso-helical structures, despite their different ligand sets and space groups, which are less common in helical molecules. Overall, the structures reported here show that the steric geometry of the flexible bis(imidazole) ligands has a significant influence on the resulting structure and, in particular, the generation and nature of helical motifs. The flexible nature of the spacers allows the ligands to bend and rotate when it coordinated to metal atoms, resulting in numerous possible conformations, though mainly cis and trans, as shown in Scheme 1. Furthermore, it is evident that the distance between the two coordinating nitrogen atoms and the dihedral angles between the two imidazolyl groups in the ligand are quite variable (Table S3, Supporting Information), even if the bimb ligands all assume trans conformations. On the other hand, the aromatic carboxylates as auxiliary exhibit either monodentate or bidentate chelating coordination modes according to the coordination geometry of metal cations. However, they are quite different due to the different coexistent groups in the benzene ring (Figure 1). The existence of these groups can provide different spatial effects on the construction of the architecture even though it is not involved in coordination with metal centers, which have profound effects on the construction of coordination polymers with different helical characters. Thus, considering coexistent noncoordinated

Acknowledgment. This work was supported by the National Natural Science Foundation of China (50572040). Supporting Information Available: Crystallographic data for compounds 1-8 in the crystallographic information file (CIF) format; tables of bond lengths and angles, conformation of bis(imidazole) ligand; Cl · · · H-C hydrogen bond distances and angles; figures of complexes 7 and 8. This information is available free of charge via the Internet at http://pub.acs.org.

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