Cyano-Bridged Homochiral Heterometallic Helical Complexes

Dec 3, 2010 - Synthesis, Structures, Magnetic and Dielectric Properties. Xiao-Dan ... homochiral [Ni(SS-L)]2+/[Ni(RR-L)]2+ cations are alternately bri...
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DOI: 10.1021/cg101314j

Cyano-Bridged Homochiral Heterometallic Helical Complexes: Synthesis, Structures, Magnetic and Dielectric Properties

2011, Vol. 11 302–310

Xiao-Dan Zheng,† Yan-Long Hua,† Ren-Gen Xiong,‡ Jia-Zhen Ge,‡ and Tong-Bu Lu*,† †

MOE Key Laboratory of Bioinorganic and Synthetic Chemistry, State Key Laboratory of Optoelectronic Materials and Technologies, and School of Chemistry and Chemical Engineering, Sun Yat-Sen University, Guangzhou 510275, China, and ‡Ordered Matter Science Research Center, Southeast University, Nanjing 211189, China Received October 6, 2010; Revised Manuscript Received November 7, 2010

ABSTRACT: The reaction of the chiral macrocyclic compound [Ni(SS-L)](ClO4)2 with Na2[Fe(CN)5NO] in acetonitrile/water gave {[Ni(SS-L)][cis-Fe(CN)5NO]}n (Δ-2) and {[Ni(SS-L)][trans-Fe(CN)5NO]}n (Λ-3), and the reaction of chiral [Ni(RRL)](ClO4)2 with Na2[Fe(CN)5NO] gave the corresponding {[Ni(RR-L)][cis-Fe(CN)5NO]}n (Λ-2) and {[Ni(RR-L)][trans-Fe(CN)5NO]}n (Δ-3), while the reaction of racemic [Ni(rac-L)](ClO4)2 with Na2[Fe(CN)5NO] generated five supramolecular isomers of one achiral {[Ni(rac-L)][cis-Fe(CN)5NO]}2 (meso-1) and four homochiral Δ-2/Λ-2 and Δ-3/Λ-3 (L = 5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane). In meso-1, a pair of enantiomers of [Ni(SS-L)]2þ and [Ni(RR-L)]2þ are bridged by two [Fe(CN)5NO]2- through two cis cyano groups to form a [2 þ 2] type of discrete molecular square. In Δ-2/Λ-2, the homochiral [Ni(SS-L)]2þ/[Ni(RR-L)]2þ cations are alternately bridged by [Fe(CN)5NO]2- anions through two cis cyano groups to produce one-dimensional (1D) homochiral right-handed and left-handed helical chains of Δ-2 and Λ-2, respectively. In Δ-3/ Λ-3, however, the homochiral [Ni(RR-L)]2þ/[Ni(SS-L)]2þ cations are alternately bridged by [Fe(CN)5NO]2- anions through two trans cyano groups to produce 1D homochiral right-handed and left-handed helical chains in Δ-3 and Λ-3, respectively. Their thermal stability, CD spectra, magnetic and dielectric properties were also investigated.

Introduction In recent years, the design and synthesis of chiral helical compounds are active fields of crystal engineering and material science, not only because of their intriguing variety of architectures1 but also owing to their potential applications in nonlinear optical materials,2 asymmetric catalysis and separation,3,4 ferroelectrics,5,6 and chiral magnets.7-9 Some approaches are successful for the construction of chiral helical compounds using chiral10,11 or achiral12,13 building blocks. In our previous work,11,12 efforts have been made for the design and construction of homochiral helical structures using the racemic macrocyclic compound [Ni(rac-L)](ClO4)2 and its enantiopure [Ni(SS-L)](ClO4)2/[Ni(RR-L)](ClO4)2 (L = 5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane) as building blocks, and a series of one-dimensional (1D) helical chains have been constructed. The correlation between the helicity of 1D chains and the chirality of the building blocks has been investigated.11,12 However, the effect of bridging ligands on the helicity of 1D chains was neglected. Indeed, the bridging ligands play an important role in inducing the helicity of the 1D chain; for example, the dicyanometalate [Ag(CN)2]- or [N(CN)2]- connects [Ni(SS-L)]2þ to generate a 1D right-handed helical chain, while the tetracyanometalate [Ni(CN)4]2- bridges [Ni(SS-L)]2þ to form a 1D left-handed helical chain. Therefore, the influence of bridging ligands on the helicity of 1D chain is worthwhile to be studied. As a continuation of our research, [Fe(CN)5NO]2- was chosen as a bridging ligand to react with racemic [Ni(rac-L)]2þ for the construction of helical chains. By carefully controlling the experimental conditions, spontaneous resolution occurred, *To whom correspondence should be addressed. Fax: þ86-20-84112921. E-mail: [email protected]. pubs.acs.org/crystal

Published on Web 12/03/2010

generating four new heterometallic homochiral helical compounds of {[Ni(SS-L)][cis-Fe(CN)5NO]}n (Δ-2), {[Ni(RR-L)][cis-Fe(CN)5NO]}n (Λ-2), {[Ni(RR-L)][trans-Fe(CN)5NO]}n (Δ-3) and {[Ni(SS-L)][trans-Fe(CN)5NO]}n (Λ-3), together with a meso compound of {[Ni(rac-L)][cis-Fe(CN)5NO]}2 (meso-1). The magnetic and temperature dependence dielectric properties of meso-1, Δ-2, and Δ-3 were investigated. Experimental Section Materials and General Methods. The macrocyclic ligand (L) and its nickel(II) complex were prepared according to the literature method,14 and separated as the racemic form of rac-L. The enantiopure complexes [Ni(RR-L)](ClO4)2 and [Ni(SS-L)](ClO4)2 were prepared according to the previously reported methods.12a All of the other chemicals are commercially available and used without further purification. Elemental analyses were determined using an Elementar Vario EL elemental analyzer. The IR spectra were recorded in the 4000-400 cm-1 region using KBr pellets and a Bruker EQUINOX 55 spectrometer. Thermogravimetric analysis (TGA) data were collected on a Netzsch TG-209 instrument with a heating rate of 10 °C min-1. The solid state (KCl pellets) circular dichroism (CD) spectra were recorded on a JASCO J-810 spectropolarimeter. The X-ray powder diffraction (XRPD) measurements were recorded on a RIGAKU D/MAX 2200 VPC diffractometer. Magnetic susceptibility data were collected in the 2-300 K temperature range with a Quantum Design SQUID Magnetometer MPMSXL-7 and an applied field of 1T. A correction was made for diamagnetic contributions prior to data analysis. Temperaturedependent dielectric constants (ε) were measured using an automatic impedance Shimadzu SSP-10A analyzer with frequencies of 500-1 MHz, and pellet samples were made through high pressure of 10.0 MPa; electrodes were made by imitation of the parallel-plate capacitor. Caution!. Perchlorate salts of metal complexes with organic ligands are potentially explosive. They should be handled with care and prepared only in small quantities. r 2010 American Chemical Society

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Table 1. Crystal Data and Structure Refinements for 1-3 compound formula fw crystal syst space group crystal size (mm) a (A˚) b (A˚) c (A˚) β (°) V (A˚ 3) Z/Dc/g cm-3)/μ max/min reflns collected unique reflns (Rint) GOF R1,a wR2b (I > 2σ(I)) R1,a wR2b (all data) Flack parameter a

meso-1 3 (3H2O)n C42H78N20Fe2Ni2O5 1172.28 monoclinic C2/c 0.45  0.36  0.34 27.226 (2) 13.882 (1) 18.032(1) 123.734(1) 5667.9(7) 8/1.374/1.214 0.7676/0.6973 16798 5500(0.0582) 1.092 0.0559, 0.1369 0.0801, 0.1535

Δ-2 3 (2H2O)n C21H40N10FeNiO3 595.19 hexagonal P61 0.34  0.12  0.06 19.2199(6) 19.2199(6) 15.8463(7) 90 5069.4(3) 6/1.168/1.020 0.8258/0.7699 14771 3989(0.0797) 1.084 0.0461, 0.0997 0.0795, 0.1286 0.11(3)

Λ-2 3 (2H2O)n C21H40N10FeNiO3 595.19 hexagonal P65 0.35  0.09  0.08 19.2182(3) 19.2182(3) 15.8420(4) 90 5067.17(17) 6/1.170/1.021 0.7701/0.7165 40750 7684(0.0567) 1.062 0.0359, 0.0748 0.0486, 0.0831 0.004(12)

Δ-3 3 (H2O)n C21H38N10FeNiO2 577.13 trigonal P3121 0.45  0.16  0.08 10.2422(6) 10.2422(6) 23.3430(16) 90 2120.7(2) 3/1.356/1.215 0.910/0.7925 5480 2364 (0.0721) 1.095 0.0469, 0.0905 0.1114, 0.1298 0.17(4)

Λ-3 3 (H2O)n C21H38N10FeNiO2 577.13 trigonal P3221 0.48  0.15  0.09 10.2272(6) 10.2272(6) 23.3035(14) 90 2110.9(2) 3/1.362/1.220 0.910/0.7925 12995 3198(0.0370) 1.036 0.0279, 0.0646 0.0309, 0.0659 -0.019(13)

R1 = Σ||Fo| - |Fc||/Σ|Fo|. b wR2 = [Σ[w(Fo2 - Fc2)2]/Σw(Fo2)2]1/2, where w = 1/[σ2(Fo)2 þ (aP)2 þ bP] and P = (Fo2 þ 2Fc2)/3.

Scheme 1

{[Ni(SS-L)][cis-Fe(CN)5NO]}n (Δ-2) and {[Ni(RR-L)][cis-Fe(CN)5NO]}n (Λ-2). A water solution (4 mL) of Na2[Fe(CN)5NO] (32 mg, 0.12 mmol) was layered with an acetonitrile solution (20 mL) of [Ni(SS-L)](ClO4)2/[Ni(RR-L)](ClO4)2 (54 mg, 0.1 mmol) in the dark. After about 1 week, pale-red needle-shaped crystals of Δ-2 3 (2H2O)n/Λ-2 3 (2H2O)n formed. The crystals were filtrated off and dried in air. Yield: 18 mg, 30.7% for Δ-2 3 (2H2O)n, and 16 mg, 27.3% for Λ-2 3 (2H2O)n. Anal. Calcd for C21H40N10FeNiO3 (Δ-2 3 (2H2O)n): C, 42.38; H, 6.77; N, 23.53. Found: C, 42.71; H, 6.49; N, 23.68%. IR (KBr, cm-1): 3435, 3269, 3225, 2967, 2935, 2169 (νCN, coordinated), 2140 (νCN, uncoordinated), 1924 (νNO), 1637, 1453, 1369, 1171, 1083, 1036, 968, 817, 659, 519. Anal. Calcd for C21H40N10FeNiO3 (Λ-2 3 (2H2O)n): C, 42.38; H, 6.77; N, 23.53. Found: C, 42.60; H, 6.61; N, 23.75%. IR (KBr, cm-1): 3429, 3266, 3231, 2960, 2936, 2173 (νCN, coordinated), 2136 (νCN, uncoordinated), 1918 (νNO), 1634, 1455, 1360, 1178, 1079, 1042, 968, 816, 657, 519. {[Ni(RR-L)][trans-Fe(CN)5NO]}n (Δ-3) and {[Ni(SS-L)][transFe(CN)5NO]}n (Λ-3). After the filtration of Δ-2 3 (2H2O)n/Λ-2 3 (2H2O)n referred to above, the mother liquid was evaporated slowly in the air. One month later, orange-red prism-shaped crystals of Λ-3 3 (H2O)n/Δ-3 3 (H2O)n formed at the bottom of the beaker. Yield: 15 mg, 25.6% for Λ-3 3 (H2O)n, and 13 mg, 22.2% for Δ-3 3 (H2O)n. Anal. Calcd for C21H38N10FeNiO2 (Λ-3 3 (H2O)n): C, 43.70; H, 6.64; N, 24.27. Found: C, 43.95; H, 6.89; N, 24.11%. IR (KBr): 3535, 3278, 3227, 2972, 2935, 2172 (νCN, coordinated), 2136 (νCN, uncoordinated), 1916 (νNO), 1485, 1446, 1375, 1169, 1083, 1047, 979, 813, 660, 518 cm-1. Anal. Calcd for C21H38N10FeNiO2: C, 43.70; H, 6.64; N, 24.27. Found: C, 43.85; H, 6.68; N, 24.30%. IR (KBr): 3537, 3281, 3225, 2973, 2935, 2170 (νCN, coordinated), 2134 (νCN, uncoordinated), 1913 (νNO), 1480, 1451, 1377, 1163, 1078, 1045, 976, 812, 658, 519 cm-1. {[Ni(rac-L)][cis-Fe(CN)5NO]}2 (meso-1). Using [Ni(rac-L)](ClO4)2 instead of [Ni(SS-L)](ClO4)2/[Ni(RR-L)](ClO4)2 in a procedure analogous to that detailed for the preparation of Δ-2/Λ-2 generated crimson block-shaped crystals of meso-1 3 3H2O. Yield: 9 mg, 15.4%. Anal. Calcd for C42H78N20Fe2Ni2O5 (meso-1 3 3H2O): C, 43.03; H, 6.71; N, 23.89. Found: C, 43.30; H, 6.92; N, 23.78%. IR

(KBr): 3596, 3279, 3213, 2969, 2936, 2187 (νCN, coordinated), 2147(νCN, uncoordinated), 1900 (νNO), 1626, 1460, 1368, 1171, 1082, 1029, 965, 618, 661, 517 cm-1. The crystals of Δ-2 3 (2H2O)n/ Λ-2 3 (2H2O)n and Δ-3 3 (H2O)n/Λ-3 3 (H2O)n also formed later together with meso-1, and meso-1, Δ-2/Λ-2, Δ-3/Λ-3 could be separated manually by the shapes of the crystals. Determination of Crystal Structures. Single-crystal X-ray diffraction data for five compounds were collected on a Rigaku RAXISRAPID Imaging Plate diffractometer with graphite monochromated Mo KR radiation (λ = 0.71073 A˚) at 153(2) K. The numerical absorption corrections were applied using the program of ABSCOR.15 The structures were solved using direct methods, which yielded the positions of all non-hydrogen atoms. These were refined first isotropically and then anisotropically. All of the hydrogen atoms of the ligands were placed in calculated positions with fixed isotropic thermal parameters and included in the structure factor calculations in the final stage of full-matrix least-squares refinement. The hydrogen atoms of the water molecules in meso-1 and Δ-3/Λ-3 were located in the different Fourier maps and refined isotropically, and the hydrogen atoms of the water molecules in Δ-2/ Λ-2 were not added. All calculations were performed using the SHELXTL system of computer programs.16 The details of crystallographic data for the five compounds are summarized in Table 1, and the selected bond lengths and angles are listed in Supporting Information (Table S1).

Results and Discussion Preparation Chemistry. During the reaction of [Ni(racL)](ClO4)2 with K2[Fe(CN)5NO] in a molar ratio of 1:1 in MeCN/H2O (5:1), spontaneous resolution occurred to produce two pairs of 1D homochiral helical chains of Δ-2/Λ-2 and Λ-3/Δ-3, as well as a centrosymmetric dimer of meso-1 (see Scheme 1). meso-1, Δ-2/Λ-2, and Λ-3/Δ-3 can be easily separated manually by the different shapes of the crystals. As shown in Figure S1 (Supporting Information), meso-1

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Figure 1. (a) The molecular square in meso-1, and (b) the molecular packing arrangement of meso-1 along the c axis, showing the intermolecular hydrogen-bonding interactions (H atoms are omitted for clarity).

formed as crimson block-shaped crystals, Δ-2/Λ-2 formed as pale-red needle-shaped crystals, and Λ-3/Δ-3 isolated as orange-red prism-shaped crystals. Interestingly, by increasing the concentration of water in MeCN/H2O (e1:2), meso-1 did not isolate, and only Δ-2 and Λ-2 were incipiently obtained, and Δ-2 and Λ-2 could be totally converted to prism-shaped crystals of Λ-3 and Δ-3 when the solution was left for a long time (more than 3 weeks). Further more, Λ-3 and Δ-3 can also be obtained by evaporating the mother liquid after removing the crystals of Δ-2 and Λ-2. Though spontaneous resolution has been found in many cases,12,13 it still cannot be predicted and the products are normally a racemic mixture, even though each crystal is an enantiopure. Thus, using the homochiral building blocks to get the enantiopure products seems to be more valuable. For this purpose, homochiral [Ni(SS-L)](ClO4)2 and [Ni(RRL)](ClO4)2 were used instead of [Ni(rac-L)](ClO4)2 and the enantiopure compounds of Δ-2 (Λ-3) and Λ-2 (Δ-3) were successfully obtained. Likewise, in a lower ratio of MeCN/ H2O (e1:2), the reaction of [Ni(SS-L)](ClO4)2/[Ni(RR-L)](ClO4)2) with K2[Fe(CN)5NO] incipiently produced the palered needle-shaped crystals of Δ-2/Λ-2, which could be totally

converted to orange-red prism-shaped crystals of Λ-3/Δ-3 in solution. The IR spectra of all the five compounds show two absorption bands near 2180(7) and 2143(5) cm-1, corresponding to the bridging and terminal cyano groups of [Fe(CN)5NO]2anion. The strong bands around 1910(10) cm-1 are attributed to the NO stretching vibration of [Fe(CN)5NO]2- anion, and the absorption values in the five compounds are lower than that found in sodium salt (1940 cm-1). Similar results have also been observed in other nitroprusside compounds.17 Crystal Structure of meso-1. In meso-1, a pair of enantiomers of [Ni(SS-L)]2þ and [Ni(RR-L)]2þ are bridged by two [Fe(CN)5NO]2- through the cis cyano groups to form a [2 þ 2] type of discrete molecular square (Figure 1a). The assembly of equal amount of enantiomers with opposite chirality induces the crystallization of meso-1 in the centrosymmetrical space group C2/c. In meso-1, each Ni(II) displays a distorted octahedral geometry by coordinating with four nitrogen atoms from the folded macrocyclic ligand L, and two nitrogen atoms from two [Fe(CN)5 NO]2-. The adjacent molecular squares are combined together via the weak intermolecular hydrogen bonding interactions

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Figure 2. (a) Structural segments of {[Ni(SS-L)][Fe(cis-CN)5NO]}n in Δ-2 (left) and {[Ni(RR-L)][Fe(cis-CN)5NO]}n in Λ-2 (right). (b) 1D right-handed helical chain along the c axis in Δ-2 (left), and 3D framework with 1D right-handed open channels in Δ-2 (middle top); 1D lefthanded helical chain along the c axis in Λ-2 (right), and 3D framework with 1D left-handed open channels in Λ-2 (middle bottom). (H atoms and water molecules are omitted for clarity.)

[O(1W) 3 3 3 N(8)a = 3.088(8), O(1W) 3 3 3 N(8) = 3.115(7) and N(2) 3 3 3 N(6)b = 3.059(5) A˚; a = -x, -y þ 1, -z; b = x þ 1/2; -y þ 3/2, z þ 1/2] (Figure 1b). Crystal Structures of Δ-2 and Λ-2. Δ-2 and Λ-2 are a pair of supramolecular stereoisomers crystallizing in chiral space groups P61 and P65, with absolute structure parameters of 0.11(3) and 0.004(12), respectively. The asymmetric unit of Δ-2 consists of one [Ni(SS-L)]2þ cation and one [Fe(CN)5NO]2- anion (Figure 2a). The Ni(II) shows a similar coordination environment to that in meso-1. In contrast to meso-1, [Fe(CN)5NO]2- anions alternately connect the homochiral [Ni(SS-L)]2þ cations through the cis NC-FeCN bridges to form a 1D right-handed helical chain along the 61 axis (Figure 2b, left). Moreover, each right-handed

helical chain connects with three adjacent chains through interchain hydrophobic interactions, resulting in a 3D homochiral framework with 1D right-handed helical channels (Figure 2b, middle top). In Λ-2 however, the connections of homochiral [Ni(RR-L)]2þ cations with [Fe(CN)5NO]2- anions produce a left-handed helical chain along the 65 axis (Figure 2b, right), and the left-handed helical chains are connected through the interchain hydrophobic interactions to generate a 3D homochiral framework with 1D lefthanded helical channels (Figure 2b, middle bottom). The channels are filled with lattice water molecules. Crystal Structures of Δ-3 and Λ-3. Δ-3 and Λ-3 also belong to a pair of supramolecular stereoisomers crystallizing in chiral space groups P3121 and P3221, with absolute structure

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Figure 3. (a) Structural segments of {[Ni(RR-L)][Fe(trans-CN)5NO]}n in Δ-3 (left) and {[Ni(SS-L)][Fe(trans-CN)5NO]}n in Λ-3 (right). (b) 1D right-handed helical chain along the c axis (left), and the interchain hydrogen bonding interactions between the adjacent chains (middle top) in Δ-3; 1D left-handed helical chain along the c axis (right), and the interchain hydrogen bonding interactions between the adjacent chains in Λ-3 (middle bottom). (c) 3D structures of Δ-3 (left) and Λ-3 (right).

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parameters of 0.17(4) and -0.019(13), respectively. The asymmetric unit of Δ-3 contains one [Ni(RR-L)]2þ and one [Fe(CN)5NO]2- (Figure 3a) as that of Λ-2. However, in contrast to the left-handed helicity of Λ-2, [Fe(CN)5NO]2anions alternately connect the homochiral [Ni(RR-L)]2þ cations through the trans NC-Fe-CN bridges to form a 1D right-handed helical chain along the 31 axis (Figure 3b, left). Furthermore, the adjacent chains with the same helicity are connected through interchain hydrogen bonding interactions (N(1) 3 3 3 O(1W) = 3.016(9) A˚, O(1W) 3 3 3 N(5)a = 2.931(7) A˚, a = -x, -1 - x þ y, 1/3 - z), resulting in a homochiral 3D structure of Δ-3 (Figure 3b, middle top, and Figure 3c, left). In Λ-3, the alternate connections of [Ni(SSL)]2þ cations through trans NC-Fe-CN bridges of [Fe(CN)5NO]2- produce a left-handed helical chain (Figure 3b, right), and the adjacent homochiral helical chains are linked through the interchain hydrogen bonding interactions to generate a homochiral 3D structure of Λ-3 (Figure 3b, middle bottom, and Figure 3c, right). In contrast to Δ-2/ Λ-2, there are no channels formed in Δ-3/Λ-3. It is interesting to note that Δ-2 and Λ-3 show the same compositions of {[Ni(SS-L)][Fe(CN)5NO]}n with the opposite helicity caused by different connecting modes of [Fe(CN)5NO]2-. The connections of [Fe(CN)5NO]2- with [Ni(SS-L)]2þ through the cis-NC-Fe-CN bridges generate the right-handed helical chains in Δ-2, while the connections of [Fe(CN)5NO]2- with [Ni(SS-L)]2þ through the trans-NC-Fe-CN bridges generate the left-handed helical chains in Λ-3, demonstrating the different connecting modes of bridging ligands will also affect the helicity of 1D helical chains. XRPD, CD Spectra, and Thermal Analyses. X-ray powder diffraction (XRPD) measurements were used to check the phase purity of the five compounds. As shown in Figure S2 (Supporting Information), all the peaks displayed in the measured patterns closely match those in the simulated patterns generated from the single-crystal diffraction data, indicating single phases of meso-1, Δ-2/Λ-2, and Δ-3/Λ-3 are formed. In order to confirm the chiral nature of Δ-2/Λ-2 and Δ-3/Λ-3, solid state CD spectra were measured at room temperature. As shown in Figure 4, the bulk crystals of Δ-2 and Λ-3 in the solid state show similar Cotton effects, with one positive Cotton effects at 450 nm and two negative Cotton effects at 550 and 640 nm, respectively, while the bulk crystals of Λ-2 and Δ-3 in the solid state show opposite Cotton effects at the same wavelengths. The Cotton effects of these compounds are close to the building block of [Ni(SSL)](ClO4)2 or [Ni(RR-L)](ClO4)2,12a demonstrating that the displayed Cotton effects of Δ-2 and Λ-2, as well as Δ-3 and Λ-3, originate from the chiral macrocyclic ligand. The slight difference of the Cotton effects may be attributed to the different structures. As Δ-2/Λ-2 and Δ-3/Λ-3 are supramolecular stereoisomers which exhibit similar thermal properties, only the TGA measurements of meso-1, Δ-2, and Δ-3 were taken. As shown in Figure 5, the water molecules in meso-1 and Δ-2 began to be lost at room temperature, while the water molecules in Δ-3 began to be lost around 100 °C, and all three compounds were stable up to 240 °C and then began to decompose upon further heating. Magnetic Properties of meso-1, Δ-2, and Δ-3. The magnetic properties of meso-1, Δ-2, and Δ-3 are similar. As shown in Figure 6, the observed χmT values at room temperature are 1.24, 1.17, and 1.31 cm3 mol-1 K for meso-1, Δ-2, and Δ-3, respectively (all the data were calculated with per [(NiL)Fe(CN)5NO] unit; the [FeII(CN)5NO]2- group is

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Figure 4. The solid CD spectra of (a) Δ-2/Λ-2 and (b) Δ-3/Λ-3.

Figure 5. The TGA curves for meso-1 (green), Δ-2 (blue), and Δ-3 (red).

diamagnetic). These values are slightly larger than the spinonly values of 1.00 cm3 mol-1 K for one octahedral Ni(II) ion (S = 1, g = 2.0), which is probably due to the spin-orbit coupling interactions. The χmT values slowly decrease with decreasing temperature to reach minimum values of 1.15 cm3 mol-1 K at 12 K, 0.84 and 0.96 cm3 mol-1 K at 2 K for meso-1, Δ-2, and Δ-3, respectively. This behavior suggests an

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Figure 6. Temperature dependence of χm (b) and χmT (O) for (a) meso-1, (b) Δ-2, and (c) Δ-3 (inset: χm-1 vs T plots, the solid lines are the best-fit curves).

antiferromagnetic coupling between the adjacent Ni(II) ions through the diamagnetic [Fe(II)(CN)5NO]2- anion. Upon further cooling, the χmT values of meso-1 increase up to a maximum of 1.17 cm3 mol-1 K at 9 K and then decrease again with decreasing temperature to reach 1.03 cm3 mol-1 K at 2 K. This effect may be related to the spin canted antiferromagnetic behavior18 with a weak ferromagnetic ordering taking place below the Neel temperature. The magnetic susceptibilities of meso-1 above 2 K, Δ-2 and Δ-3 above 2.4 K, obey the Curie-Weiss law, with a negative Weiss constant (θ) of -1.703, -2.138, and -2.681 K for meso-1, Δ-2, and Δ-3, respectively. This fact also indicates a weak antiferromagnetic coupling between the adjacent Ni(II) ions in these compounds. The larger |θ| value of Δ-3 is due to the stronger antiferromagnetic coupling between the

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Figure 7. Temperature dependence of dielectric constants and dielectric loss for (a) meso-1, (b) Δ-2, and (c) Δ-3 from -170 to 170 °C at different frequencies.

adjacent Ni(II) ions through linear NC-Fe-CN bridges (C-Fe-C angle = 170.2°). Dielectric Properties of meso-1, Δ-2, and Δ-3. Temperature dependence dielectric constants (ε1) and dielectric loss (D = ε2/ε1) at different frequencies of meso-1, Δ-2, and Δ-3 are shown in Figure 7, and they all display similar dielectric properties. It is interesting to note that the dielectric constants of meso-1 at the temperature range of 50-170 °C are frequency and temperature dependent, which decrease along with increasing frequency, and increase smoothly with increasing temperature. The dielectric loss measurements reveal the same trend. The dielectric behaviors of meso-1, Δ-2, and Δ-3 are similar to those of perovskite-related oxide CaCu3Ti4O1219 and the homochiral coordination polymers reported recently.20 Such features would support the presence of an orderly polarization process under the highfrequency alternating electric field (HAEF) rather than a temperature process.19,20 To the best of our knowledge,

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compounds Δ-2 and Δ-3 are the first examples of cyanobridged helical coordination polymers exhibiting such dielectric behaviors. Conclusions In summary, five cyano-bridged compounds of meso-1, Δ-2/Λ-2, and Δ-3/Λ-3 were obtained by the reactions of corresponding [NiL]2þ with [Fe(CN)5NO]2-. Spontaneous resolution occurs during the reaction of [Ni(rac-L)]2þ with [Fe(CN)5NO]2- to give four homochiral helical supramolecular stereoisomers of Δ-2/Λ-2 and Δ-3/Λ-3, in which they all display 1D helical structures. The connecting modes of CNFe-CN bridge of [Fe(CN)5NO]2- play an important role during the formation of helical chains. The alternately connecting of [Fe(CN)5NO]2- to [Ni(SS-L)]2þ/[Ni(RR-L)]2þ through the cis-NC-Fe-CN bridges generate the right-handed and left-handed helical chains of Δ-2/Λ-2, while the alternately connecting of [Fe(CN)5NO]2- to [Ni(SS-L)]2þ/[Ni(RR-L)]2þ through the trans-NC-Fe-CN bridges generate the helical chains of Λ-3/Δ-3 with opposite helicity. Compounds meso-1, Δ-2, and Δ-3 display weak antiferromagnetic couplings and interesting dielectric properties. The results presented here indicate that the different connecting modes of the bridging ligands will affect the helicity of 1D helical chains.

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Acknowledgment. This work was supported by NSFC (20625103, 20831005, and 20821001) and 973 Program of China (2007CB815305). (10)

Supporting Information Available: The selected bond distances and angles, photograph of the crystals, XRD patterns in PDF format; and X-ray crystallographic files in CIF format. This information is available free of charge via the Internet at http://pubs. acs.org.

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