4-Bromopyridine-Induced Chirality in Magnetic MII-[NbIV(CN)8]4

Article pubs.acs.org/crystal

4‑Bromopyridine-Induced Chirality in Magnetic MII-[NbIV(CN)8]4− (M = Zn, Mn, Ni) Coordination Networks Takuro Ohno,† Szymon Chorazy,†,‡ Kenta Imoto,† and Shin-ichi Ohkoshi*,† †

Department of Chemistry, School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan Faculty of Chemistry, Jagiellonian University, Ingardena 3, 30-060 Krakow, Poland

‡

S Supporting Information *

ABSTRACT: The introduction of 4-bromopyridine (4-Brpy) to a selfassembled MII-[NbIV(CN)8] (M = 3d metal ion) coordination system results in the formation of three-dimensional cyanido-bridged networks, {[MII(4-Brpy)4]2[NbIV(CN)8]}·nH2O (M = Zn, n = 1, 1; M = Mn, n = 0.5, 2; M = Ni, n = 2, 3). All these compounds are coordination frameworks composed of octahedral [MII(4-Brpy)4(μ-NC)2] complexes bonded to square antiprismatic [NbIV(CN)8]4− ions bearing four bridging and four terminal cyanides. 1 and 2 crystallize in the chiral I4122 space group as the mixture of two enantiomorphic forms, named 1(+)/1(−) and 2(+)/2(−), respectively. The chirality is here induced by the spatial arrangement of nonchiral but sterically expanded 4-Brpy ligands positioned around MII centers in the distorted square geometry, which gives two distinguishable types of coordination helices, A and B, along a 4-fold screw axis. The (+) forms contain left handed helices A, and right handed helices B, while the opposite helicity is presented in the (−) enantiomers. On the contrary, 3 crystallizes in the nonchiral Fddd space group and creates only one type of helix. Half of them are right handed, and the second half are left handed, which originates from the ideally symmetrical arrangement of 4-Brpy around NiII and results in the overall nonchiral character of the network. 1 is a paramagnet due to paramagnetic NbIV centers separated by diamagnetic ZnII. 2 is a ferrimagnet below a critical temperature, Tc of 28 K, which is due to the CN−-mediated antiferromagnetic coupling within Mn−NC−Nb linkages. 3 reveals a ferromagnetic type of NiII−NbIV interaction leading to a ferromagnetic ordering below Tc of 16 K, and a hysteresis loop with a coercive field of 1400 Oe at 2 K. Thus, 1 is a chiral paramagnet, 3 is a nonchiral ferromagnet, and 2 combines both of these functionalities, being a rare example of a chiral molecule-based magnet whose chirality is induced by the noninnocent 4-Brpy ligands.

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multiferroicity,40,45 and magnetization induced second harmonic generation (MSHG).39,44,46,47 Thus, there is a strong interest in the searching for novel families of chiral molecule-based magnets serving as promising candidates for such unique magnetochiral, magneto-optical, and magneto-electric properties.49 A considerable number of chiral molecule-based magnets has been already presented.22−48 They are generally obtained by the application of chiral organic ligands, or counterions directly inducing the chirality of the magnetic network by coordination to metal centers,27−34,41−43 or by the influence through the noncovalent interactions within the supramolecular network,35−40 respectively. The alternative approach of the spontaneous resolution of enantiopure magnets is relatively rare, and its detailed mechanism is usually unexplained.22−26,44−48 Among the chiral magnets, the intensive research has been lately devoted to cyanido-bridged bimetallic frameworks exhibiting a diversity of magnetic properties, including magnetic ordering, slow magnetic relaxation, and

INTRODUCTION Molecule-based magnets constructed of magnetically active metal complexes or organic radicals arouse the continuously increasing scientific interest in the solid state chemistry, physics, and materials science.1−7 Special attention is paid to the implementation of additional physical property into the magnetically ordered systems giving remarkable subgroups of multifunctional molecule-based magnets, including luminescent magnets,8−12 photomagnets,13−17 microporous magnets,18 magnetic sponges,19−21 or chiral and polar magnets.22−48 The introduced functionality can simply coexist with the intrinsic magnetic nature of the compound, or it can modify the magnetic properties like in magnetic sponges, revealing the dehydration driven tuning of magnetic characteristics.18−21 Alternatively, the introduced physical property can strongly interact with the magnetism leading to extraordinary cooperative effects opening the new physics of the solid state matter.49 The most fruitful case is the combination of chirality and a long-range magnetic ordering which resulted in the observation of various extraordinary phenomena such as magnetization enhanced magnetic circular dichroism (MCD) coexisting with natural circular dichroism (NCD),31,43 magnetization enhanced magnetochiral dichroism (MChD),23,26,38 © XXXX American Chemical Society

Received: April 25, 2016 Revised: June 11, 2016

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DOI: 10.1021/acs.cgd.6b00626 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

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charge transfer or spin transitions, often successfully combined with other functionalities, including chirality.3,5,50−52 In this context, octacyanidometallates, [MIV/V(CN)8]4−/3− (M = Nb, Mo, W, Re) are of a special interest due to their high eight number of cyanide ligands and stereochemical flexibility enabling the construction of various chiral motifs and coordination topologies.53−61 Following this prediction, several chiral magnetic coordination molecules and polymers built of complexes of 3d/4f metal ions and octacyanidometallates were reported.41−48,54−61 Within these compounds, a long-range magnetic ordering was detected in (i) {[CoII(H2O)2][CoII(pyrimidine)(R-1-(4-pyridyl)ethanol)]2[WV (CN) 8 ]2}· 7.5H2O ferromagnet with Tc of 19 K,41 {[CoII(H2O)]2[CoIII(R1-(4-pyridyl)ethanol)2][WIV(CN)8][WV(CN)8]}·5H2O ferromagnet with T c of 11 K, 42 {[Mn II (R-/S-mpm) 2 ] 2 [NbIV(CN)8]}·4H2O (mpm = α-methyl-2-pyridine-methanol) ferrimagnets with Tc of 23.5 K,43 all bearing the chiral ligands,41−43 and (ii) {[MnII(H2O)2][MnII(pyrazine)(H2O)2][Nb IV (CN) 8 ]}·4H 2 O ferrimagnet with T c of 48 K, 46 {[MnII(H2O)(urea)2]2[NbIV(CN)8]} ferrimagnet with Tc of 43 K,47 {[MnII2(4,4′-bpdo)(H2O)4][NbIV(CN)8]}·6H2O (4,4′bpdo = 4,4′-bipyridyl-N,N′-dioxide) antiferromagnet with Tc of 15 K,48 all crystallizing spontaneously in the chiral space groups.46−48 Only for the two-dimensional MnII(mpm)-NbIV ferrimagnet the structural characterization of both molecular enantiomorphs was reported.43 In addition, very lately, we prepared the three-dimensional cyanido-bridged framework, {[FeII(4-Brpy)4]2[NbIV(CN)8]}· 2H2O (4-Brpy = 4-bromopyridine) which was obtained by the spontaneous resolution crystallizing as the equimolar mixture of (+) and (−) enantiomorphs.62 In the as-synthesized form, this assembly did not reveal the magnetic ordering as the FeII‑HS to FeII‑LS spin crossover effect was observed in the temperature range of 110−130 K, and the resulting low temperature phase was only a paramagnet. However, after the irradiation by 473 nm light at low temperature the photoinduced ferrimagnetically ordered state with Tc of 15 K could be produced. In addition, the another ferrimagnetic state with Tc of 12 K is reversibly generated by the subsequent irradiation by 785 nm light. This material was a first example of chiral molecule-based photomagnet, and the remarkable cross-effect of a 90-degree photoswitching of SHG polarization plane was also found. This great achievement encouraged us to focus greater attention to the bimetallic [NbIV(CN)8]-based coordination networks employing the 4-Brpy ligand which was concluded to be noninnocent in the induction of chirality in the above-mentioned FeII−NbIV photomagnet. In this context, we aimed at the preparation of novel chiral molecule-based magnets especially those revealing separately crystallizing two enantiomorphic forms which are desirable in the aspect of magnetochiral effects. Therefore, we present here the syntheses, crystal structures, optical and magnetic properties of three novel cyanido-bridged networks, {[M I I (4-Brpy) 4 ] 2 [NbIV(CN)8]}·nH2O (M = Zn, n = 1, 1; M = Mn, n = 0.5, 2; M = Ni, n = 2, 3) which exhibit the identical coordination topology but different detailed structural features, including the chiral or nonchiral character, and magnetic nature, depending on the 3d metal ion: chirality with paramagnetism for 1, chirality with ferrimagnetism for 2, and centrosymmetricity with ferromagnetism for 3. The magnetic properties are explained based on the intrinsic character of the related 3d metal centers, and the crucial role of 4-Brpy ligand in the induction of structural chirality is discussed in detail.

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EXPERIMENTAL SECTION

Materials. With the exception of K4[NbIV(CN)8]·2H2O, which was synthesized according to the published procedure,63 all reagents were purchased from commercial sources (Sigma-Aldrich, Wako Pure Chemical Industries, Ltd.) and used without further purification. General Synthetic Procedure. The aqueous solution of K4[NbIV(CN)8]·2H2O (5 mL, 98.7 mg, 0.2 mmol) was dropped to a mixed aqueous solution (145 mL) of 4-bromopyridine hydrochloride (0.778 g, 4 mmol), potassium hydroxide (0.225g, 4 mmol), and chloride of 3d metal ion: ZnIICl2, MnIICl2·4H2O, or NiIICl2·6H2O (0.4 mmol). The excess of organic ligand was used to increase the yield of the synthesis and to avoid the appearance of various impurities, including the crystals of purely inorganic networks, {[MII(H2O)2]2[NbIV(CN)8]}·nH2O (M = 3d metal ion).53 The polycrystalline powder of the respective compound, {[MII(4-Brpy)4]2[NbIV(CN)8]}· nH2O (M = Zn, n = 1, 1; M = Mn, n = 0.5, 2; M = Ni, n = 2, 3), appeared immediately after mixing the reagents. The product was collected by the suction filtration, washed with water, and dried on the air. The composition of the resulting air-stable material was determined by elemental analysis. The single crystals were obtained from the mixed water or water−ethanol solutions of 3d metal salt, and 4-bromopyridine hydrochloride (4-Brpy·HCl) with potassium hydroxide, both slowly reacting with a water solution of K4[NbIV(CN)8]· 2H2O within the slow diffusion method. The single crystalline samples, including two enantiomorphs of chiral 1 and 2, were characterized only structurally, while all the other physical measurements were performed on the powder samples, 1(powder), 2(powder), and 3(powder), which were proven to be isostructural with the respective single crystals by the powder diffraction method using the Rietveld analysis. Details for 1. Pale yellow powder, named 1(powder), revealed the composition{[ZnII(4-Brpy)4]2[NbIV(CN)8]}·H2O. Yield, 245.0 mg, 71% (based on Nb). ICP/MS metals and standard CHN elemental analysis. Anal. Calcd for Zn2Nb1Br8C48H34N16O1 (1, MW = 1713.8 g· mol−1): Zn, 7.63%; Nb, 5.42%; C, 33.64%; H, 2.00%; N, 13.08%. Found: Zn, 7.49%; Nb, 5.58%; C, 33.45%; H, 2.12%; N, 13.01%. IR (nujol, cm−1): CN stretching vibrations, 2165(s), 2117(s). The single crystals were obtained from the ethanol solution of ZnII(NO3)2·2H2O, a mixed aqueous solution of 4-Brpy·HCl with KOH, and a water solution of K4[NbIV(CN)8]·2H2O. Yield for single crystals was ca. 5% (based on Nb). It is worth noticing that zinc(II) nitrate was used for the synthesis of the single crystals of 1 instead of zinc(II) chloride applied in the preparation of 1(powder). The chloride salt was used in the precipitation method due to the resulting high yield, while the nitrate precursor was applied in the slow diffusion method as it produced the better quality crystals. Details for 2. Yellow powder, noted as 2(powder), revealed the composition {[MnII(4-Brpy)4]2[NbIV(CN)8]}·0.5H2O. To increase the yield and to avoid the oxidation of MnII, L-ascorbic acid (0.176 g, 1 mmol) was added together with the MnII salt. Yield, 210.2 mg, 61% (based on Nb). ICP/MS metals and standard CHN elemental analysis. Anal. Calcd for Mn2Nb1Br8C48H33N16O0.5 (2, MW = 1683.9 g· mol−1): Mn, 6.52%; Nb, 5.52%; C, 34.24%; H, 1.98%; N, 13.31%. Found: Mn, 6.34%; Nb, 5.17%; C, 34.15%; H, 2.01%; N, 13.30%. IR (nujol, cm−1): CN stretching vibrations, 2155(s), 2115(s). The single crystals were prepared from the mixed aqueous solution of 4-Brpy· HCl, MnIICl2·4H2O, KOH, and sodium L-ascorbate slowly reacting with a water−ethanol solution (1:1, v/v) of K4[NbIV(CN)8]·2H2O. Yield for single crystals was ca. 40% (based on Nb). Details for 3. Brown powder, named 3(powder), identified with the composition {[NiII(4-Brpy)4]2[NbIV(CN)8]}·2H2O. Yield, 221.8 mg, 65% (based on Nb). ICP/MS metals and standard CHN elemental analysis. Anal. Calcd for Ni2Nb1C48H36N16O2 (3, MW = 1718.4 g·mol−1): Ni, 6.83%; Nb, 5.41%; C, 33.55%; H, 2.11%; N, 13.04%. Found: Ni, 6.83%; Nb, 5.31%; C, 33.41%; H, 2.10%; N, 13.05%. IR (nujol, cm−1): CN stretching vibrations, 2164(s), 2116(s). The single crystals were obtained from the mixed aqueous solution of 4-Brpy·HCl, KOH, NiIICl2·6H2O, and 1,3-diaminopropane, slowly reacting with a water solution of K4[NbIV(CN)8]·2H2O. Yield for B



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Table 1. Crystal Data and Structure Refinement of 1(+)/(−), 2(+)/(−), and 3 compound formula formula weight [g·mol−1] T [K] crystal system space group unit cell a [Å] b [Å] c [Å] V [Å3] Z calculated density [g·cm−3] absorption coefficient [mm−1] F(000) crystal size [mm × mm × mm] Θ range [deg] hkl indices

observed reflections symmetry−independent reflections Rint completeness [%] refined parameters GOF on F2 Flack x-parameter final R indices largest diff peak/hole [e·Å−3]

1(+)

1(−)

2(+)

2(−)

3

Zn2Nb1C48H32 Br8N16 1695.82 296(2) tetragonal I4122

Zn2Nb1C48H32 Br8N16 1695.82 296(2) tetragonal I4122

Mn2Nb1C48H32 Br8N16 1674.96 293(2) tetragonal I4122

Mn2Nb1C48H32 Br8N16 1674.96 293(2) tetragonal I4122

Ni2Nb1C48H32 Br8N16 1682.50 296(2) orthorhombic Fddd

20.3837(13) 20.3837(13) 14.0079(5) 5820.2(8) 4 1.935 6.552 3252 0.16 × 0.14 × 0.12 3.16−27.426 −26 < h < 26 −26 < k < 24 −18 < l < 18 27919 3307

20.3534(7) 20.3534(7) 13.9824(3) 5792.4(4) 4 1.945 6.584 3252 0.29 × 0.18 × 0.10 3.165−27.471 −23 < h < 26 −26 < k < 26 −18 < l < 18 45968 3306

20.6168(12) 20.6168(12) 14.0220(4) 5960.1(7) 4 1.867 6.017 3212 0.33 × 0.29 × 0.18 3.125−27.471 −26 < h < 26 −26 < k < 25 −18 < l < 17 27757 3405

20.599(2) 20.599(2) 14.0190(8) 5948.4(14) 4 1.870 6.029 3212 0.40 × 0.37 × 0.34 3.127−23.241 −22 < h < 22 −22 < k < 22 −15 < l < 15 19190 2138

13.9302(9) 26.7898(15) 30.6688(16) 11444.5(11) 8 1.953 6.484 6472 0.11 × 0.11 × 0.08 3.042−23.253 −15 < h < 15 −29 < k < 27 −34 < l < 33 17927 2066

0.0653 99.7 174 1.094 0.056(6) R[F2 > 2σ(F2)] = 0.0415 wR(F2) = 0.0739 0.383/−0.533

0.0660 99.7 174 1.245 0.078(5) R[F2 > 2σ(F2)] = 0.0527 wR(F2) = 0.0833 0.731/−0.724

0.0933 99.7 175 1.056 0.132(6) R[F2 > 2σ(F2)] = 0.0415 wR(F2) = 0.0728 0.544/−0.480

0.1086 99.6 174 1.070 0.047(8) R[F2 > 2σ(F2)] = 0.0392 wR(F2) = 0.0578 0.558/−0.511

0.0481 99.8 172 1.069 R[F2 > 2σ(F2)] = 0.0376 wR(F2) = 0.0719 1.097/−1.127

single crystals was ca. 60% (based on Nb). It is interesting to note that 1,3-diaminopropane was here found to efficiently slow down the crystallization, avoiding the quick precipitation of small, poorly shaped crystals. This is presumably caused by the preliminary coordination of this ligand to NiII which is followed by the slow subsequent exchange by the cyanides of [Nb(CN)8]4−, and 4-Brpy present in large excess, which together results in the formation of the high quality single crystals of 3. Crystal Structure Determination. Single crystal X-ray diffraction analyses of 1(+)/(−), 2(+)/(−), and 3 were performed using a Rigaku R-AXIS Rapid imaging plate area detector equipped with graphite monochromated MoKα radiation. The measurements were executed at ambient temperature on the single crystals protected by a Paratone N oil, and mounted on Micro Mounts holder. The crystal structure was solved using SHELXS-97, and refined by a full-matrix least-squares method using SHELXL-2014/7.64 The refinement procedure was performed using partially Crystal Structure crystallographic software package, and partially WinGX (ver. 1.80.05) integrated system. All non-hydrogen atoms were found independently, and refined anisotropically. The hydrogen atoms belonging to pyridine rings were calculated based on the idealized positions and refined using a riding model. Because of the possible large structural disorder, the positions of solvent water molecules could not be determined. Structural diagrams were prepared using Mercury 3.5.1. software. CCDC reference numbers for the crystal structures of 1(+)/(−), 2(+)/(−), and 3 are 1472545, 1472544, 1472546, 1472543, and 1472547, respectively. The details of the crystal data and structure refinement are presented in Table 1. The powder X-ray diffraction patterns of 1(powder), 2(powder), and 3(powder) were measured in the 2Θ range of 10−70° on a RIGAKU Ultima IV diffractometer equipped with the CuKα radiation. The related crystal structures were found by the Rietveld analysis using a RIGAKU PDXL software, taking

the positions of the atoms from the single crystal studies as the initial model for the further refinement. The details of the crystal data and structure refinement for powder samples are shown in Tables S2, S5, and S8. Physical Techniques. Elemental analyses of metals (Mn, Ni, Zn, Nb) were performed using Agilent 7700 ICP-MS, while those of C, H, and N elements was based on the microanalytical method. Infrared spectra were measured on the powder samples mixed with paraffin oil (nujol) using JASCO FT/IR-4100 spectrometer. UV−vis-NIR diffuse reflectance spectra were performed on the powder samples mixed with BaSO4 using Shimadzu UV-3100PC spectrometer. Magnetic properties were performed using a Quantum Design MPMS-XL SQUID magnetometer. The magnetic data were corrected for the diamagnetic contributions using Pascal constants. Calculations. Continuous shape measure analysis for the determination of the geometry of eight-coordinated [NbIV(CN)8]4− complexes were performed using a SHAPE software ver. 2.1.65,66

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RESULTS AND DISCUSSION

Structural Studies. The results of X-ray diffraction analyses for powder forms, 1(powder), 2(powder), 3(powder), and the single crystals of 1(+), 1(−), 2(+), 2(−), and 3, are shown in Figures 1, S1−S10, and Tables 1, S1−S10. All of the compounds consist of the three-dimensional cyanido-bridged {[MII(4-Brpy)4]2[NbIV(CN)8]} (M = Zn, Mn, Ni) networks. These coordination frameworks are built of the nearly octahedral [MII(4-Brpy)4(μ-NC)2] complexes bridged by cyanides to the [NbIV(CN)8]4− metalloligands bearing four bridging and four terminal cyanide ligands (Figure 1). C



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Figure 1. Crystal structures of 1(+)/(−), 2(+)/(−), and 3: the three-dimensional cyanido-bridged networks along c (1, 2) and a (3) axes (top), the coordination spheres of Nb and 3d metal centers (middle), and the helical structures along 4-fold (1, 2) or 2-fold (3) screw axes (bottom). Red and orange lines in the bottom represent the helices composed of M3d−NC−Nb moieties. The structural views of 1 and 2 presented above are taken from the crystal structures of 1(+)/(−). Except for the small differences in bond lengths and angles, the respective views of 2(+)/(−) are identical (Figure S4).

1(+) and 1(−) crystallize in the chiral I4122 space group of a tetragonal crystal system (Table 1). Their three-dimensional cyanido-bridged network is constructed of the molecular building unit containing [ZnII(4-Brpy)4(μ-NC)2] complex of a nearly octahedral geometry, and [NbIV(CN)8]4− moiety of an almost ideal square antiprism geometry (Figure 1, Tables S1 and S10). Thus, each ZnII center coordinates four N atoms of 4-Brpy, and two N atoms of bridging cyanides in the trans configuration. Four 4-Brpy ligands are aligned around the ZnII centers in the corners of a distorted square, and the pyridine rings of the neighboring ligands within these square are arranged closely perpendicular to each other. However, the pyridine rings situated in the opposite corners are not ideally parallel to each other, as the Brpy(N3)−Brpy(N3) and Brpy(N4)−Brpy(N5) torsion angles in 1(+) are 151.8° and 170.8°, respectively, significantly deviated from the 180° angle characteristic of a parallel arrangement (Figure 1, left middle, and Table S1). Similar respective torsion angles of 152.7° and 171.0° were found for 1(−). Because of this nonidentity in the spatial arrangement of 4-Brpy ligands around ZnII centers, 1(+)

and 1(−) reveal two types of helical structures along 4-fold screw axes (Figure 1, left bottom, and Figure S1). The 4-Brpy ligands and cyanide bridges create the coordination helices in the channels along the c crystallographic axis, and it is easy to distinguish their two types: type A with closer distances between 4-Brpy ligands within the smaller channels, and type B of broader channels and longer distances between pyridine rings. For 1(+), the type A helices are left-handed, while the broader helices of type B are right handed which is the source of the structural chirality. 1(−) exhibits right handed helices of type A and left handed helices of type B. Consequently, the crystal structure of 1(−) is as a mirror image of 1(+), and they can be considered as a pair of molecular enantiomers. The low values of Flack parameters, 0.056(6) and 0.078(5), for 1(+) and 1(−), respectively, confirm the enantiopure character of both compounds. The channels in the three-dimensional crystal structures of 1(+)/(−) are filled by 4-Brpy ligands (Figure S1), but they contain also some amount of weakly bonded water molecules, as suggested by elemental analysis showing the presence of one water molecule per {Zn2Nb} unit. However, D



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(3) color due to the rather poor absorption properties in the visible range. On the contrary, they reveal the strong UV absorption related to the multiple electronic transitions within [NbIV(CN)8]4− anions.43,63 1 exhibits the broad absorption band centered below 300 nm together with the shoulder at 345 nm, and the weaker band at 430 nm with the shoulder at 460 nm. 2 reveals similar absorption, that are the broad maximum at 310 nm with the shoulder around 350 nm, and the lower energy bands at 435 and 470 nm. In the case of 3, the analogous UV absorption with bands at 300, 355, 440, and 475 nm are accompanied by the weak broad peaks at 565 and 920 nm. These spectra can be interpreted in terms of ligand-tometal charge transfer (LMCT) transitions (300−400 nm) and l ig an d - fie l d ( L F ) t r a n s i t i o n s ( 4 0 0 −5 0 0 n m ) o f [NbIV(CN)8]4−.43,63 The additional vis-NIR absorption in 3 can be ascribed to the spin-allowed d−d 3A2(3F) → 3T1(3F) and 3A2(3F) → 3T2(3F) transitions of octahedral NiII of 3d8 valence configuration, observed at 565 and 920 nm, respectively.61,67 Analogous absorption was not detected in 2 or 1, as high spin MnII (3d5) centers reveal only the spinforbidden and, thus, very weak transitions in this range, while no d−d bands are expected for ZnII of the 3d10 valence configuration. Despite the chiral character, 1(powder) and 2(powder) do not exhibit the signal of natural circular dichroism (NCD) related to the natural optical activity as they consist of the equimolar mixture of two enantiomers, 1(+)/(−) and 2(+)/(−), respectively, showing the opposite signals in NCD spectra. Attempts to mechanically separate the enantiomorphic crystals in the amount sufficient for the measurement of NCD spectra were unsuccessful due to the similarity in their morphology; thus, these studies are not presented here. Magnetic Properties. The magnetic properties of the powder samples of 1(powder), 2(powder), and 3(powder) were investigated using a SQUID magnetometry technique, and the results are shown in Figures S12−S14, 2−3, and 4−5, respectively. The room temperature value of the molar magnetic susceptibility−temperature product, χMT of 1 is 0.36 cm3 mol−1 K for {ZnII2NbIV} unit which is in a good agreement with the value of 0.38 cm3 mol−1 K expected for the magnetically isolated NbIV (S = 1/2, g = 2.0) combined with two diamagnetic ZnII centers. Upon cooling, the χMT is stable in the broad temperature range of 10−300 K and decreases slightly below 10 K reaching 0.34 cm3 mol−1 K at 2 K (Figure S12). This indicates that 1 is a simple paramagnet originating from the nonzero spin of NbIV centers which are magnetically isolated by the diamagnetic ZnII resulting in the long Nb−Nb distances of ca. 10.8 Å. The very weak antiferromagnetic Nb− Nb interactions can be the reason for the tiny decrease of the χMT at the lowest temperatures. The temperature dependence of the inverse molar magnetic susceptibility was fitted according to the Curie−Weiss law in the low temperature range below 100 K, and the obtained values of Curie constant, C, and Weiss constant, Θ, are 0.367(2) cm3 mol−1 K and −0.05(2) K, respectively (Figure S12, the inset). The small negative Θ value points to the very weak, almost negligible, antiferromagnetic interaction between NbIV centers. The direct-current (dc) magnetic characteristics of 2 are shown in Figure 3. The χMT value reaches room temperature 8.6 cm3 mol−1 K for {MnII2NbIV} unit which is slightly lower than the value of 9.1 cm3 mol−1 K expected for the uncoupled two MnII (high spin d5, S = 5/2, g = 2.0) and one NbIV (S = 1/

the crystal data collected at room temperature indicate only very fuzzy residual electron density in the interstitial space. The powder X-ray diffraction measurement followed by the Rietveld analysis was executed for 1(powder), and the obtained results proved its perfect isostructurality with the enantiomers, showing also the good purity of the powder sample used in all physical studies (Figures S2−S3, Tables S2−S3). Except for the differences in bond lengths and angles related to the larger size of MnII than ZnII, 2(+) and 2(−) are isostructural to 1(+) and 1(−), respectively (Figures 1 and S4). They are three-dimensional networks built of octahedral [MnII(4-Brpy)4(μ-NC)2] complexes, and [NbIV(CN)8]4− ions of a closely ideal square antiprism geometry (Tables S4 and S10). The pyridine rings of 4-Brpy arranged on the opposite sites of MnII complexes are not situated within the same plane, as the Brpy(N3)−Brpy(N3) and Brpy(N4)−Brpy(N5) torsion angles are 154.8°, 172.7°, and 154.9°, 172.8°, for 2(+) and 2(−), respectively (Figure 1, middle, Figure S4, and Table S4). This deviation from the parallel arrangement results in the creation of two types of helices, A and B, along 4-fold screw axes (Figures 1, left bottom, and Figures S4−S5). For 2(+), the type A helices are left-handed, and the helices of type B are right handed when 2(−) exhibits the natural opposite helicity. Thus, the structure of 2(−) is a perfect mirror image of 2(+). The Flack parameters of 0.132(6) and 0.047(8) of 2(+) and 2(−), respectively, prove their enantiopure character. The free space in the coordination frameworks of 2(+)/(−) is filled by the organic ligands, and the small amount of weakly bonded water molecules, a half of water molecule per {Mn2Nb} unit. The powder X-ray diffraction analysis performed for the respective polycrystalline sample, 2(powder), indicated that the powder form, used in optical and magnetic studies, is isostructural to the single crystalline materials and reveals a high purity (Figures S6−S7, Tables S5−S6). In contrast to 1 and 2, the NiII-containing 3 crystallizes in the orthorhombic crystal system, in the centrosymmetric Fddd space group (Table 1). Its crystal structure still consists of the three-dimensional network based on the octahedral [NiII(4Brpy)2(μ-NC)2] complexes bridged by cyanides with the [NbIV(CN)8]4− moieties of a distorted square-antiprism geometry (Figure 1, right panel, Tables S7 and S10). 4-Brpy ligands are aligned around the NiII center in the corners of a distorted square with the closely perpendicular arrangement of the neighboring pyridine rings. In addition, the pyridine rings situated in the opposite corners are parallel to each other, as visualized by the exact 180° torsion angles between the related pyridine rings, Brpy(N3)−Brpy(N3) and Brpy(N4)−Brpy(N4). Because of the parallel arrangement of oppositely arranged pyridine rings, 3 reveals only one type of helical structure along 2-fold screw axis (Figure 1, right bottom, and Figure S8). Half of these helices is right handed, and the second half is left handed, which result in the disappearance of the chiral character of 3. The channels of the cyanido-bridged network of 3 are occupied by the 4-Brpy ligands and weakly bonded water molecules as confirmed by the elemental analysis showing two H2O molecules per {Ni2Nb} unit. The powder form, 3(powder), investigated by powder X-ray diffraction method, including Rietveld analysis, was found to be isostructural with the single crystal form and exhibits a high purity (Figures S9−S10, Tables S8−S9). Optical Properties. The UV−vis-NIR absorption spectra of 1(powder), 2(powder), and 3(powder) are shown in Figure S11. All of the compounds reveal weak yellow (1, 2) or brown E



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Figure 3. Alternate-current (ac) magnetic properties of 2(powder): temperature dependence of the out-of-plane (χM″) component of ac complex molar magnetic susceptibility at various indicated frequencies ( f = 50, 100, 500, 1000 Hz) of ac magnetic field (Hac = 3 Oe, Hdc = 0 Oe).

saturated at a small field of 3 kOe. The saturation magnetization of 9.1 μB is very close to the value of 9.0 μB calculated for the antiparallel arrangement of MnII and NbIV magnetic moments within the whole coordination framework represented by the {MnII2NbIV} unit (MAF = 2 × gMn × SMn−gNb × SNb), and it is much smaller than the magnetization of 11 μB expected for the parallel arrangement of MnII and NbIV magnetic moments (MF = 2 × gMn × SMn+gNb × SNb). This indicates antiferromagnetic coupling within the Mn−NC−Nb linkages, similar to other reported MnII−[Nb IV (CN) 8] materials (Table S11).43,46−48 The maximum of χMT at 22 K with a large value of 348 cm3 mol−1 K can be assigned to the onset of the long-range ferrimagnetic ordering in 2. The transition to the ferrimagnetic state is nicely observed in the field-cooled magnetization (FCM) curves measured in the 2−60 K range for a few small magnetic fields (Figure 2b). The FCM signals abruptly increase below 30 K with a further saturation at lower temperatures. The critical temperature (Tc) of the magnetic phase transition was found to be 28 K, as determined by the analysis of the first derivative of FCM plots in the function of temperature (Figure 2b, the inset). Despite the presence of a ferrimagnetic state, 2 does not reveal the magnetic hysteresis loop in the M−H plot even at a very low temperature of 2 K which is due to weak magnetic anisotropy of both MnII and NbIV magnetic centers.43,46−48 The paramagnet−ferrimagnet phase transition in 2 was also confirmed by the alternate-current (ac) magnetic studies shown in the temperature dependences of the complex molar magnetic susceptibility measured at zero dc field, with the ac field of 3 Oe at various frequencies (Figure 3 and S13). Both the in-plane (χ′M) and out-of-plane (χ″M) components of molar magnetic susceptibility reveal the fast increase of the signal below 28 K, followed by only a weak further increase toward the lower temperatures. These signals are positioned at the identical temperatures for various frequencies of ac magnetic field, indicating the occurrence of a typical longrange magnetic ordering below Tc of 28 K. Because of a threedimensional character of 2 network, the value of critical temperature can be applied in the estimation of the value of average magnetic coupling constant between MnII and NbIV

Figure 2. Direct-current (dc) magnetic properties of 2(powder): (a) temperature dependence of the molar magnetic susceptibility− temperature product at H = 2 kOe, (b) field-cooled magnetization (FCM) curves at various indicated low magnetic fields with their first derivatives in the inset, and (c) the molar magnetization−magnetic field curve at T = 2 K measured in the sequence +50 kOe → −50 kOe (red points) and −50 kOe → +50 kOe (blue points).

2, g = 2.0). This can suggest that that even at room temperature the magnetic interactions between spin centers operate. In fact, the χMT gradually increases upon cooling, first slowly in the 50−300 K range, and later fast, reaching the maximum of 348 cm3 mol−1 K at 22 K (Figure 2a). This is followed by the decrease of the χMT product to 51 cm3 mol−1 K at 2 K. Such behavior is typical for the three-dimensional cyanido-bridged MnII−NbIV networks revealing antiferromagnetic coupling leading to the long-range ferrimagnetic ordering.43,46−48 The antiferromagnetic type of cyanide-mediated interactions is indicated by the field dependence of the magnetization measured at 2 K (Figure 2c). The magnetization exhibits an abrupt increase with the increasing field, becoming almost F



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Figure 5. Alternate-current (ac) magnetic properties of 3(powder): temperature dependence of the out-of-plane (χM″) component of ac complex molar magnetic susceptibility at various indicated frequencies ( f = 1, 10, 100, 1000 Hz) of ac magnetic field (Hac = 3 Oe, Hdc = 0 Oe).

where ZMn and ZNb are the numbers of the nearest magnetic neighbors bridged to Mn and Nb, respectively, SMn and SNb are the spin quantum numbers of Mn and Nb, respectively, and kB is the Boltzmann constant (0.695 cm−1 K−1). Using the set of parameters of 2, that are ZMn = 2, ZNb = 4, SMn = 5/2, and SNb = 1/2, the critical temperature of 28 K gives the coupling constant of JMnNb = −8.1 cm−1, which is comparable with the properties of other MnII−NbIV coordination polymers built of octahedral MnII and square antiprismatic [NbIV(CN)8]4− (Table S11).70,71 Direct-current magnetic properties of 3 are presented in Figure 4. The room temperature χMT product is 2.7 cm3 mol−1 K per {NiII2NbIV} unit which is a good agreement with the range of 2.6−2.8 cm3 mol−1 K calculated for the uncoupled two NiII (S = 1, g = 2.1−2.2) and one NbIV (S = 1/2, g = 2.0).70,72,73 On decreasing the temperature, χMT slowly increases to around 30 K, and abruptly below this point, indicating the appearance of cyanide-mediated NiII−NbIV magnetic coupling of the presumably ferromagnetic character. The χMT product reaches the maximum of 87 cm3 mol−1 K at 13 K which is followed by the fast decrease to the value of 18 cm3 mol−1 K at 2 K (Figure 4a). The expected positive sign of the NiII−NbIV magnetic coupling constant, J NiNb , related to the ferromagnetic interaction, is confirmed by the field dependence of the magnetization at 2 K (Figure 4c). The molar magnetization saturates at the high magnetic fields at the value of 5.3 μB which is within the range of 5.2−5.4 μB expected for the parallel arrangement of NiII and NbIV magnetic moments taking into account the {NiII2NbIV} unit (MF = 2 × gMn × SMn+gNb × SNb), and it is much higher than the range of 3.2−3.4 μB related to the antiferromagnetic correlation within the Ni−NC−Nb linkages (MAF = 2 × gMn × SMn−gNb × SNb). This indicates the presence of NiII−NbIV coupling of a ferromagnetic type. The ferromagnetic interactions in 3 lead to a long-range ordering whose onset is visible in the maximum of χMT at 13 K, and the further decrease of the signal presumably related to the saturation of magnetization in the ordered state. The phase transition to the ferromagnetic state is documented by the fieldcooled magnetization (FCM) plots showing the abrupt increase of the signal below 17 K (Figure 4b). The critical temperature

Figure 4. Direct-current (dc) magnetic properties of 3(powder): (a) temperature dependence of the molar magnetic susceptibility− temperature product at H = 2 kOe, (b) field-cooled magnetization (FCM) curves at various indicated low magnetic fields with their first derivatives in the inset, and (c) the molar magnetization−magnetic field curve at T = 2 K together with the detailed insight into the hysteresis loop in the inset.

centers, JMnNb. It can be performed within the molecular field theory considering only the superexchange interaction between nearest neighbor metal ion sites, i and j, depicted by the constant Jij, within the Hamiltonian equation of H = −Jij·Si·Sj where Si, Sj stand for the spin values of metal sites, i and j, respectively.68,69 In such a simplified treatment, broadly used in the three-dimensional magnetic Prussian Blue Analogs,68 the magnetic coupling constant of 2, JMnNb, is related to the critical temperature, Tc by eq 1: Tc =

Z MnZ Nb |JMnNb | SMn(SMn + 1)S Nb(S Nb + 1) 3kB

(1) G



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Because of the presence of transition metal cations combined with paramagnetic [NbIV(CN)8]4− anions, the reported coordination frameworks exhibit interesting magnetic properties. Only 1 is a simple paramagnet due to the diamagnetism of ZnII. 2 is a molecule-based ferrimagnet with the critical temperature of 28 K, while 3 shows a long-range ferromagnetic ordering below 16 K, and the magnetization−field hysteresis loop with the significant coercivity of 1400 Oe at 2 K is observed. Therefore, among the presented compounds, 2 is the most valuable functional material, being a rare example of a chiral molecule-based magnet (Table S11), which chirality appeared due the chiral resolution induced by the noninnocent 4-bromopyridine ligands. This compound can be obtained in the monocrystalline form which makes it an attractive object in the further investigation of magnetochiral, magneto-optical, and magneto-electric properties. The challenge remains, however, for the preparation of the separately crystallized enantiomorphic forms as MnNb grows as the mixture of enantiomers which could be only distinguished by the thorough singlecrystal X-ray diffraction analysis. This will presumably demand the application of the chiral pyridine derivatives used at least as the accompanying ligand. Nevertheless, our work proved that the insertion of bulky pyridine-based ligands, such as various bromo- or iodopyridines, into the bimetallic M(3d)− [M′(CN)8] (M′ = NbIV, MoV, WV) magnetic systems can be the efficient synthetic route toward the novel chiral moleculebased magnets prepared in the single-crystalline forms, strongly required in the future discoveries and deeper understanding of extraordinary magnetochiral and magneto-optical cross-effects. Research along these lines is in progress in our laboratory.

(Tc) of the magnetic phase transition is 16 K, as found from the positions of the maxima of the first derivative of FCM plots in the function of temperature (Figure 4b, the inset). This Tc is the highest among reported NiII-[NbIV(CN)8] magnets (Table S11). As expected for a ferromagnet, 3 reveals the magnetic hysteresis loop in the magnetization−magnetic field plot with a significant coercive field of 1400 Oe at 2 K (Figure 4c). The paramagnet−ferromagnet phase transition in 3 is also confirmed by the temperature dependences of the real (χ′M) and imaginary (χ″M) part of the complex molar magnetic susceptibility measured at various frequencies of the ac field (Figure 5 and S14). Both the χ′M−T and χ″M−T plots contain the sharp maxima centered at ca. 16 K whose positions are not dependent on the frequency indicating a typical long-range magnetic ordering with Tc of 16 K. Using the methodology described above for 2, the value of Tc of 3 can be used in the estimation of the average magnetic coupling constant between NiII and NbIV centers, JNiNb. Employing a molecular field theory, the relation between Tc and JNiNb is presented by eq 1 using the parameters ZNi, SNi, and JNiNb, instead of ZMn, SMn, and JMnNb, respectively, where ZNi is the number of the nearest magnetic neighbors bridged to Ni, SNi is the spin quantum number of Ni, and the other parameters as described for 2. Using the set of parameters of 3, which are ZNi = 2, ZNb = 4, SNi = 1, and SNb = 1/2, the critical temperature of 16 K results in the magnetic coupling constant of JNiNb = +9.6 cm−1. The positive sign and the value of the intermetallic magnetic interaction in 3 is close to +8.1(3) cm−1 presented in the only report on the threedimensional NiII−[NbIV(CN)8]4− ferromagnet.70

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CONCLUSIONS In summary, we have prepared novel three-dimensional cyanido-bridged coordination polymers, {[MII(4-Brpy)4]2[NbIV(CN)8]}·nH2O (M = Zn, n = 1, 1; M = Mn, n = 0.5, 2; M = Ni, n = 2, 3), composed of octahedral complexes of three different 3d metal ions with 4-bromopyridine (4-Brpy) ligands, and square antiprismatic octacyanidoniobate(IV) moieties. 1 and 2 crystallized in the chiral I4122 space group, and two monocrystalline enantiomorphs, named (+) and (−), were structurally characterized. They were found to differ in the directions of the rotation of coordination helices detected along the 4-fold screw axes. We distinguished two helical structures differing in the spatial arrangement of 4-Brpy ligands, showing the closer ligand−ligand distances in helices A, and the relatively broader separation of pyridine rings in helices B. In 1(+) and 2(+), the helices of type A were left handed, the B helices right handed, while the opposite rotations of helices were observed in 1(−) and 2(−). Thus, we have proved that the chirality of 1 and 2 is induced by the spatial arrangement of the nonchiral but sterically expanded 4-Brpy ligands. 4-Brpy differs from other monodentate aromatic ligands, such as pyrazole or aminopyridine, which were embedded to the analogous bimetallic cyanido-bridged frameworks giving the nonchiral derivatives (Table S11). On the contrary, 3 revealed the very similar three-dimensional network but crystallized in the centrosymmetric Fddd space group with only one type of the coordination helices which the one-half is right handed, and the second half is left handed. It showed that the additional role in the induction of chirality is played by the size of the 3d metal center coordinating the 4-Brpy ligands which is the smallest for NiII when compared with FeII, MnII, or even ZnII, all giving the chiral structures.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.6b00626. Additional structural views and detailed structure parameters of 1(+)/(−), 2(+)/(−), and 3. Results of powder X-ray diffraction studies, including the Rietveld analyses of 1(powder), 2(powder), and 3(powder). Results of continuous shape measure analysis of [Nb(CN)8]4− ions. UV−vis-NIR diffuse reflectance spectra. Magnetic properties of 1(powder) and additional magnetic characteristics of 2(powder) and 3(powder). Comparison of magnetic properties of 2 and 3 magnets with other reported [Nb(CN)8]-based molecule-based magnets revealing long-range magnetic ordering (PDF) Accession Codes

CCDC 1472543−1472547 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. H



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ACKNOWLEDGMENTS This work was financed by the Japan Society for the Promotion of Science (JSPS) within the Grant-in-Aid for Specially Promoted Research, Grant No. 15H05697. S.C. is grateful for the support of the Foundation for Polish Science within a START 2015 fellowship.

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