Chiral cyanido-bridged Mn–Nb magnets including halogen-bonds†

Takuro Ohno ^a, Koji Nakabayashi ^a, Kenta Imoto ^a, Masaya Komine ^a, Szymon Chorazy ^b and Shin-ichi Ohkoshi *^a
aDepartment of Chemistry, School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan. E-mail: ohkoshi@chem.s.u-tokyo.ac.jp
^bFaculty of Chemistry, Jagiellonian University, Gronostajowa 2, 30-387 Kraków, Poland

First published on 24th September 2018

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Functional materials composed of molecule-based magnets have drawn much attention due to their high designability based on the building block diversity of metal ions and organic ligands.¹ Molecule-based magnets exhibit photomagnetism,² luminescence,³ gas/solvent adsorption,⁴ ferroelectricity,⁵ electronic conduction,⁶ and ionic conduction.⁷ The structural features strongly affect the physical properties and functionalities. Intrinsically, porous structures make gas/solvent adsorption possible, while ferroelectricity and pyroelectricity can be observed on structures with spontaneous polarization. In addition, the electronic and ionic conduction pathways significantly depend on the structure and structural dimensionality of the compound.

Numerous molecule-based magnets with various structures have been synthesized. Among them, chiral structured magnets show prominent phenomena: multiferroicity, magnetization-induced second-harmonic generation (MSHG),⁸ magnetization-enhanced magnetochiral dichroism (MChD),⁹ and magnetization-enhanced magnetic circular dichroism (MCD) accompanied with natural circular dichroism (NCD).¹⁰ To fabricate chiral-structured molecule-based magnets, generally chiral organic ligands are applied into coordination networks or chiral noncovalent molecules are incorporated into the cavities of crystal structures.^{5b,8b,9b,10b,11} However, in rare cases, chiral magnets are obtained from achiral building blocks by spontaneous resolution towards an enantiopure compound.^{5a,8a,8c,8d,9a,9c,12} We have reported a three-dimensional cyanido-bridged Fe–Nb bimetal assembly, Fe₂[Nb(CN)₈](4-Brpy)₈·2H₂O (4-Brpy = 4-bromopyridine), which shows the photoreversible switching of the MSHG effect.^2h Although this compound has a chiral structure, it is prepared from achiral building blocks by spontaneous resolution, resulting in a mixture of (+) and (−) enantiomorphs. Additionally, the family of M–Nb bimetallic coordination networks, [M^II(4-Brpy)₄]₂[Nb^IV(CN)₈]·nH₂O (M^II = Zn, Mn, Ni), has been recently reported. In this family, metal substitution induces chiral crystal structures.^12e

Here, we present cyanido-bridged metal assemblies [Mn^II(4-Xpy)₄]₂[Nb^IV(CN)₈]·nH₂O (4-Xpy = 4-halopyridine; X = I, n = 0, 1; X = Cl, n = 0.5, 2). Both show long-range magnetic ordering. However, 1 and 2 have chiral and achiral crystal structures, respectively. Additionally, we discuss the halogen-substitution effect on the crystal structures and examine the SHG effect for the chiral magnet of 1.

Single crystals of 1 were obtained from a water–ethanol solution (1:1, v/v) of MnCl₂·4H₂O, L-ascorbic acid sodium salt, and 4-iodopyridine by slowly reacting with an aqueous solution of K₄[Nb^IV(CN)₈]·2H₂O. Single crystal X-ray diffraction measurements reveal that 1 has a tetragonal crystal system in the chiral space group of I4₁. The crystal structure has a three-dimensional coordination network composed of Mn^II and Nb^IV centers bridged by cyanides (Fig. 1). In the crystal structure of 1, [Nb^IV(CN)₈]⁴⁻ has a square antiprism coordination geometry (Table S3†) where four cyanides are bridged to pseudo-octahedral Mn^II and the other four cyanides are terminal. Mn^II is coordinated by two cyanides in axial positions and four 4-Ipy molecules in equatorial positions (Fig. 1b). Alternating connections of Mn^II and Nb^IV by cyanides form Mn^II–NC–Nb^IV–CN–Mn^II coordination helices. This configuration leads to the quadrangular-channels along the c-axis, which are occupied by 4-Ipy ligands. Two types of quadrangular-channels exist: narrow and wide (Fig. 1a and c). In addition, the coordination helices of adjacent channels have opposite handedness (Fig. 1c). Thus, the whole network structure has both wide-right-handed and narrow-left-handed helices in one enantiomer (or wide-left-handed and narrow-right-handed helices in the other enantiomer). Detailed structural parameters of both enantiomers are presented in the ESI† (Tables S1 and S2). Among the four 4-Ipy ligands coordinated to one Mn^II, two have the planes of the aromatic rings parallel to the ab plane, while the other two are aligned toward the c-axis. The iodine atoms of the two ligands aligned toward the c-axis are oriented toward the nitrogen atoms of the terminal cyanides in [Nb^IV(CN)₈]⁴⁻ with the angles of carbon(4-Ipy)–iodine(4-Ipy)–nitrogen(cyanide), ∠NIC angles = 176.5(3)° and 166.0(3)° (Fig. 1d). The I⋯N distances of 3.044(7) and 3.107(8) Å are significantly shorter than the sum of the van der Waals radii of the iodine and nitrogen atoms (3.53 Å). The short distances and angles suggest the formation of halogen bonds between the iodine atoms of the 4-Ipy ligands and the nitrogen atoms of the terminal cyanides.¹³ As shown in Fig. 1d and 2, the halogen bond clearly distorts the coordination geometry of Mn^II coordinated by 4-Ipy. This distortion produces two types of helices: wide and narrow. From this viewpoint, we examined the crystal structure of [Mn^II(4-Brpy)₄]₂[Nb^IV(CN)₈]·0.5H₂O (Mn–Nb–4-Brpy network) reported previously.^12e In the Mn–Nb–4-Brpy network, a halogen bond between 4-Brpy and the terminal cyanide is formed with Br⋯N distance of 3.087(9) Å and ∠NBrC angle of 170.8(1)° (Fig. S1†). The distance and the angle imply a weaker halogen bond than that of 1 because a long distance between halogen and nitrogen atoms and an angle that deviates from 180° weaken the halogen bond.

Thus, the lower symmetry in 1 (I4₁ space group) than that in the Mn–Nb–4-Brpy network (I4₁22 space group) is probably due to the existence of strong halogen bonds, which distort the coordination geometry of Mn^II coordinated by 4-Ipy.

Single crystals of 2 were prepared in almost the same manner as 1 except for minor changes such as the solvent condition. Single crystal X-ray analysis reveals that the crystal structure belongs to the centrosymmetric space group of Fddd, and has an achiral 3-D coordination network with a cyanido-bridged Mn^II–Nb^IV metal assembly (Fig. S2†). The coordination geometries of Mn^II and [Nb^IV(CN)₈]⁴⁻ are pseudo-octahedron and square antiprism, respectively. Four cyanides of [Nb^IV(CN)₈]⁴⁻ are coordinated to Mn^II, while the other four are terminal. For the Mn^II coordination site, four 4-Clpy molecules and two cyanides are coordinated. The cyanido-bridged Mn^II–Nb^IV metal assembly forms coordination helices with a two-fold rotation axis along the a-axis. When one helix is left-handed, the adjacent helix is right-handed. These helices with the same shape are symmetrically related and result in an achiral structure. The distance of Cl⋯N between 4-Clpy and the terminal cyanide is 3.183(2) Å and the ∠NClC angle is 166.8(1)° (Fig. 2b). The distance and the angle suggest a weaker halogen bond than that of 1. This weak halogen bonding may make it difficult to construct a distorted crystal structure with the two types of helices similar to the crystal structure of 1. The powder X-ray diffraction measurements of 1 and 2 prove that the powder samples and the respective single crystals have identical structures (Fig. S3†). The UV–vis–NIR absorption spectra of 1 and 2 are shown in Fig. S4.† Both 1 and 2 exhibit the broad and strong absorption bands below 400 nm, and the weaker band at 420–430 nm with the shoulder up to 460–470 nm. The observed peaks can be assigned to ligand-to-metal charge transfer (LMCT) transitions (200–400 nm) and ligand-field transitions (400–500 nm) of [Nb^IV(CN)₈]⁴⁻, and intra-ligand transitions of 4-halopyridines (200–300 nm).^10b In infrared spectra, CN⁻ stretching peaks are observed at 2142 cm⁻¹ and 2115 cm⁻¹ for 1, and 2157 cm⁻¹ and 2114 cm⁻¹ for 2. Vibrational modes of 4-halopyridines are observed in the 600–1800 cm⁻¹ range whereas absorption peaks between 400 and 600 cm⁻¹ are assigned to Nb–C stretching modes (Fig. S5†). Both of 1 and 2 show thermal stability up to around 400 K. Significant decreases of their weight over 400 K could be caused by decomposition with evaporation of organic ligands of 4-Ipy or 4-Clpy (Fig. S6†).

Fig. 3 and S7† show the magnetic properties of 1, including the temperature dependence of the magnetization, the molar magnetic susceptibility, χ_MT, and the field dependence of magnetization. The χ_MT value at 300 K is 8.7 cm³ mol⁻¹ K, which is close to with the expected value of 9.1 cm³ mol⁻¹ K for the {Mn^II₂Nb^IV} unit assuming S_Mn = 5/2, g_Mn = 2.0, S_Nb = 1/2, and g_Nb = 2.0. Upon cooling, there is no significant change in the χ_MT value above 100 K. However, it begins to gradually increase at 100 K. Below 50 K, the χ_MT value rapidly increases and reaches a maximum of 503 cm³ mol⁻¹ K at 17 K. After that, it quickly decreases to 92 cm³ mol⁻¹ K at 2 K (Fig. S7†). The field-cooled magnetization (FCM) plot exhibits a spontaneous magnetization at a T_C of 22 K, indicating a phase transition to a magnetically ordered state (Fig. 3a). The magnetization (M) vs. external magnetic field (H) plot at 2 K reveals a saturation magnetization (M_s) of 9.0 μ_B. This value agrees with the expected value of 9.0 μ_B in the case of antiferromagnetic coupling between Mn^II and Nb^IV (2 × g_MnS_Mn − g_NbS_Nb = 2 × 2.0 × (5/2) − 2.0 × (1/2) = 9.0 μ_B) (Fig. 3b). The results demonstrate that 1 is a ferrimagnet with antiferromagnetic superexchange couplings between the metal centers via the cyanido bridges. The almost negligible coercive field suggests the small anisotropy which is due to the lack of an orbital moment for both Mn^II and Nb^IV. The average value of the superexchange interaction, J_MnNb, can be estimated using the equation based on molecular field theory,¹⁴ |J_MnNb| = 3k_BT_C[Z_MnNbZ_NbMnS_Mn(S_Mn + 1)S_Nb(S_Nb + 1)]^−0.5, in which a Hamiltonian (Ĥ = −J_MnNbS_Mn·S_Nb) is assumed. Here, k_B is the Boltzmann constant and Z_ij denotes the number of the nearest neighbor j-site ions surrounding the i-site ion. The values of Z_MnNb, Z_NbMn, S_Mn, S_Nb, and T_C for 1, which are revealed by the crystal structure and the magnetic measurement, are 2, 4, 5/2, 1/2, and 22 K, respectively. From these parameters, J_MnNb, is calculated as −6.4 cm⁻¹ for 1, which is comparable to the previously reported value of −6.4 cm⁻¹ for {[Mn^II(pyrazole)₄]₂[Nb^IV(CN)₈]·4H₂O}_n.¹⁵

The magnetic properties of 2 were also examined by measuring the temperature-dependent magnetic susceptibility and the field-dependent magnetization. The χ_MT value at 300 K is 8.5 cm³ mol⁻¹ K for 2, which is similar to the expected value of 9.1 cm³ mol⁻¹ K suggesting the {Mn^II₂Nb^IV} unit with S_Mn = 5/2, g_Mn = 2.0, S_Nb = 1/2, and g_Nb = 2.0 (Fig. S8†). The FCM measurement reveals a spontaneous magnetization at a T_C of 28 K (Fig. 3c), and the observed M_s value of 9.2 μ_B indicates an antiferromagnetic coupling between Mn^II and Nb^IV as well as that for 1, resulting in ferrimagnetism (Fig. 3d). Based on molecular field theory, the average superexchange interaction is calculated as −8.1 cm⁻¹ for 2 using the following parameters: Z_MnNb = 2, Z_NbMn = 4, S_Mn = 5/2, S_Nb = 1/2, and T_C = 28 K.

SHG measurements for powdered samples of 1 and the previously reported Mn–Nb–4-Brpy network were performed at room temperature (Fig. S9a and b†). Fundamental light (775 nm) was irradiated by using a frequency-doubled Ti:sapphire laser. 1 and the Mn–Nb–4-Brpy network exhibit SHG light due to their noncentrosymmetric structure. The intensity of SHG light is 0.095% intensity of potassium dihydrogen phosphate (KDP) and 0.008% intensity of KDP for 1 and the Mn–Nb–4-Brpy network, respectively. In the case of 2, such nonlinear optical phenomena are quenched due to its centrosymmetric structure. The tensor element of the SH susceptibility (χ_ijk) is related to SH polarization (P_i). That is, P_i = χ_ijkE_jE_k where i, j, and k are the coordinates, and E_j and E_k are the electric field of the incident wave. Since the crystal system of 1 is tetragonal in the noncentrosymmetric space group of I4₁ (crystallographic point group: 4), SHG is permitted (see ESI†). The system of I4₁22 (crystallographic point group: 422) for the Mn–Nb–4-Brpy network allows SHG, but the number of elements in the SH susceptibility is less than that of 1. This difference in the number of elements probably results in the difference of the observed SH light intensity between 1 and the Mn–Nb–4-Brpy network.

In conclusion, we have synthesized chiral- and achiral-structured magnets of 1 and 2, respectively. The chiral structure of 1 is composed of achiral building blocks, which lead to an enantiopure compound through a spontaneous resolution process. In the crystal structure, there are two types of coordination helices, wide and narrow ones, where halogen bonds between 4-Ipy and cyanide are formed along the c-axis. Halogen bonding distorts the coordination geometry around Mn^II coordinated by 4-Ipy, forming the wide and narrow helices that lead to the chiral structure. In case of 2, the left-handed helix and the adjacent right-handed helix, which possess identical shapes, are symmetrically related. This results in the achiral structure. Both 1 and 2 show ferrimagnetism with T_C of 22 K and 28 K, respectively. Due to the chiral structure in the I4₁ space group, 1 exhibits SHG phenomena, while such nonlinear optical phenomena are quenched in 2 due to its centrosymmetric structure. The observed SH intensity for 1 is stronger than that for the Mn–Nb–4-Brpy network. This difference is attributed to fewer elements in the crystallographic term in the SH susceptibility of the Mn–Nb–4-Brpy network with the crystallographic point group of 422 than those of 1. The present results suggest that minor modifications of the building blocks such as halogen substitution may control the symmetry of the crystal structure and consequently, the nonlinear optical phenomena. To make a chiral structure in molecule-based magnets, halogen bonding as well as hydrogen bonding are worth considering. Such fine designs for molecule-based magnets can yield new developments for nonlinear optical and magneto-optical materials.

The present research was supported in part by the JSPS Grant-in-Aid for Specially Promoted Research Grant Number 15H05697, Grant-in-Aid for Scientific Research on Innovative Area Soft Crystals (area No. 2903, 17H06367), and APSA from MEXT. The Global Science course from MEXT, the Cryogenic Research Center in The University of Tokyo, and the Center for Nano Lithography & Analysis in The University of Tokyo supported by MEXT are acknowledged. T. O. and M. K. acknowledge the support of the Advanced Leading Graduate Course for Photon Science (ALPS). M. K. is grateful for a Research Fellowship for Young Scientists of JSPS (18J12325).

Conflicts of interest

There are no conflicts to declare.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Details of synthetic methods, physical techniques, the selected crystallographic data, the results of continuous shape measurement (CShM) analysis, the UV-vis and IR spectrum, the additional temperature-dependent magnetization plots, and the result of the SH light measurement. CCDC 1854369–1854374. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c8ce01353e

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