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Chiral Ln (tetramethylurea)–[W CN)] coordination chains showing slow magnetic relaxation Olaf Stefanczyk, Anna M. Majcher, Koji Nakabayashi, and Shin-ichi Ohkoshi Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b01719 • Publication Date (Web): 12 Feb 2018 Downloaded from http://pubs.acs.org on February 18, 2018

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Crystal Growth & Design

Chiral LnIII(tetramethylurea)–[WV(CN)8] coordination chains showing slow magnetic relaxation Olaf Stefańczyk,† Anna M. Majcher,‡ Koji Nakabayashi† and Shin–ichi Ohkoshi†* † Department of Chemistry, School of Science, The University of Tokyo, 7–3–1 Hongo, Bunkyo–ku, Tokyo 113–0033, Japan. ‡ Institute of Physics, Jagiellonian University in Kraków, Łojasiewicza 11, 30–348 Kraków, Poland. KEYWORDS: chirality, lanthanide complexes, octacyanidometallates, slow magnetic relaxation, spontaneous resolution, optical activity.

ABSTRACT: We prepared a series of isostructural chiral cyanido–bridged zigzag chains [Ln(tmu)5][W(CN)8] (Ln = Gd, 1; Tb, 2; Dy, 3; Ho, 4; Er, 5; Tm, 6) using achiral tmu = tetramethylurea. Their chiral character was confirmed with single crystal X–ray diffraction and circular dichroism measurements. Magnetic studies show antiferromagnetic interactions within cyanido–bridged LnIII–WV pairs, and interchain ordering of net spins in 1, 4 and 5. It is worth to emphasize that Dy–, Er– and Tm–based systems combine magnetic field–induced slow magnetic relaxation and chirality. Analysis of AC magnetic data with two relaxation processes for 5 gives

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energy barrier ∆τ/kB = 1.2(3) K and relaxation time τ0 = 2.63(8)·10-2 s, and ∆τ/kB = 22(2) K and τ0 = 1.21(3)·10-8 s. Cole–Cole function fits for 3 and 6 result in ∆τ/kB = 17(1) K and τ0 = 1.68(3)·106

s, and ∆τ/kB = 5.7(3) K and τ0 = 1.53(4)·10-2 s, respectively. Slower relaxation processes have

been assigned to dipole–dipole interactions while faster ones to single ion magnet behavior of Ln(III) ions.

TEXT INTRODUCTION The research focused on design of new multifunctional molecular materials gains particular interest in the last decades.1,2 We are witnessing the rapid development of novel scientific paths of research to combine interesting magnetic phenomena (long–range magnetic ordering, magnetic coupling, spin bistability and slow magnetic relaxation)3–9 with a second useful functionality (luminescence, gas sorption, conductivity and nonlinear optical activity).10–20 Among them we can distinguish an exceptional class of chiral magnets combing nonlinear optical (NLO) activity and unique magnetic properties. The majority of chiral magnets based on two– and three–dimensional coordination polymers comprise 3d and 4f metal complexes with polycyanidometallates11–23 or tris(oxalato)metallates14,16,24–28 anions. These systems efficiently combine long–range magnetic ordering with nonlinear optical phenomena which leads to strong enhancement of magnetic circular dichroism (MCD),21,22 magneto–chiral dichroism (MChD)23 and/or magnetization–induced second harmonic generation (MSHG)24–28 signals below critical temperature. It is noteworthy to allude to another extraordinary multifunctional FeII–[NbIV(CN)8] chiral photomagnet revealing a 90 degree photoswitching of the SHG polarization plane.24 Recently, we reported several assemblies joining slow magnetic relaxation and chirality,29 long–


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range magnetic ordering and SHG,30,31 and ferrimagnetic properties with chirality and luminescence simultaneously.32 The most of these compounds owe their properties to the presence of lanthanides(III) ions, bearing intrinsic properties: luminescence, single molecule magnetic behavior or magnetic coupling due to large spin and anisotropy, and incorporation of chiral ligands transferring chirality into crystal structure.16 Nowadays scientists make a great effort to develop of new family of single molecule magnets (SMMs)33–40 and single ion magnets (SIMs)33,34,41–44 revealing not only high energy barriers and high blocking temperatures,45–47 but also other functionalities, such as ferroelectricity,48–52 chirality,48,51,52,53–58 luminescence48,49,59–63 and conductivity.64–67 Notably, the introduction of chirality in SMMs provides a great platform for the research of the cross–effect of SMM behavior and nonlinear optical effect of second harmonic generation (SHG), magnetic and natural circular dichroism (MCD and NCD, respectively)51,53–58 and magneto–chiral dichroism (MChD). Nonetheless, the number of these systems is rather low, especially concerning lanthanide assemblies.48–58 Therefore, synthesis of chiral lanthanide complexes is still highly desirable to explore the relationship between their structure and properties they exhibit. In this context we focused on the design and synthesis of new chiral magnetic materials based on lanthanide(III) complexes with achiral ligands and octacyanidotungstate(V) which reveal spontaneous resolution. Despite the fact that this approach can hardly be considered as a rational strategy,68–71 we deduced that incorporation of largely substituted amides significantly increase the probability to obtain 1–D non–centrosymmetric coordination polymers. Self–assembly in the presence of N,N–dimethylformamide (dmf) resulted in formation of centrosymmetric [GdIII(dmf)6][MV(CN)8] (MV = Mo, W)68,71 assemblies while reaction with N,N–dimethylamide


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[GdIII(dma)6][WV(CN)8]

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[GdIII(dma)5][WV(CN)8].70 Herein, we report the synthesis and comprehensive structural, optical and magnetic studies of a new series of isostructural chiral cyanido–bridged zigzag chains [LnIII(tmu)5][WV(CN)8] (Ln = Gd – Tm, 1 – 6) applying achiral tetramethylurea (tmu) ligand. We will present the details of crystal structures with a possible explanation for spontaneous structural resolution in these materials, as well as describe magnetic properties confirming slow magnetic relaxation. EXPERIMENTAL SECTION Materials and Physical Measurements. All chemicals were purchased from commercial sources and used as purchased without further purification. Sodium octacyanidotungstate(V) salt was prepared according to the published procedure.72 Elemental analysis (C, H, N) was performed on Elementar Analysensysteme GmbH: vario MICRO cube while concentrations of metals were analyzed by means of Agilent 7700 inductively coupled plasma mass spectrometer (ICP–MS). Infrared absorption spectra in KBr were recorded with a JASCO FT/IR–4100 spectrometer. Solid state UV–Vis diffuse–reflectance spectra of 1 – 6 diluted in BaSO4 was recorded with a JASCO V–670 UV–Vis spectrophotometer equipped with a ISN–723 integrating sphere accessory, converted with the Kubelka–Munk function and deconvoluted by means of the Origin 8.0 software.73 Circular dichroism spectra were collected for powder samples of 1 – 6 mixed with a drop of mineral oil (nujol) placed between two CaF2 windows using JASCO J–810 spectropolarimeter. All magnetic measurements were performed on around 15 mg samples with a drop of nujol mineral oil placed in a 30 µm thick polyethylene bag (30 x 5 mm) using a Quantum Design MPMS 5S magnetometer. All DC measurements were corrected for the


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diamagnetic contribution of the sample holders, paraffin oil and constituent atoms (Pascal’s tables).3 Synthesis of [LnIII(tmu)5][WV(CN)8] (Ln = Gd – Tm, 1 – 6). Tetramethylurea solution (1 mL) of 1 mmol Ln(NO)3·6H2O salt (Ln = Gd, 1; Tb, 2; Dy, 3; Ho, 4; Er, 5; Tm, 6) were mixed upon vigorous stirring with 1 mL yellow solution of Na3[W(CN)8]·4H2O (1 mmol) in tmu. The resulting clear light yellow solution was left in a closed vessel in the dark at 4°C for crystallization. Crystals suitable for single crystal measurement were obtained within one week. Yellow crystalline products were collected with glass Pasteur pipette, washed with small amount of cold tetramethylurea and dried on filtration paper under flow of argon. Additional information about yields and results of microelemental and ICP–MS analyses of products 1 – 6 as well as results of IR spectroscopy of 1 – 6 are collected in Supporting Information, Table S1 and S2 and Figure S1, respectively. Single Crystal X–Ray Diffraction. Data for 1 – 6 were collected at room temperature on a Rigaku R–AXIS RAPID diffractometer equipped with imaging plate area detector using graphite monochromated Mo–Kα radiation [λ = 0.71075 Å]. Single crystals covered in paratone–N oil were mounted with a 100 µm Dual Thickness Micro MountTM loop. Collected data were integrated by Rigaku RAPID AUTO. Structures were solved by direct methods using SHELXS– 97 incorporated in the CrystalStructure 4.0 crystallographic software package74 and refined using a F2 full–matrix least squares technique of SHELXL–2014/7 included in the OLEX–2 1.2 software package.75–77 Non–hydrogen atoms were refined anisotropically while hydrogen atoms were positioned with an idealized geometry and refined using a riding model. Crystal data, data collection, and refinement parameters for 1 – 6 are listed in Table 1. CCDC 1589924–1589930 contain the supplementary crystallographic data for 1 – 6 in series. These data can be obtained


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Data

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www.ccdc.cam.ac.uk/data_request/cif. The structural data presented as figures were prepared with the use of the CCDC Mercury 3.9 visualization software.78 Geometries of metal centers are estimated with the Continuous Shape Measures (CShM) analysis with the use of SHAPE v2.0 software.79,80 RESULTS AND DISCUSSIONS Crystal Structures. Single crystal X–ray diffraction studies of yellow trapezoidal prism crystals of 1 – 6 confirmed their isostructural character which are composed of cyanido–bridged zigzag chains aligned along the crystallographic a direction (Figure 1), and crystallized in the non–centrosymmetric space group I2 (Table 1). Moreover, linear relationship for a monotonic decrease in unit cell volume and cell constants with the systematic decrease in Shannon effective ionic radius81,82 of lanthanides of 1 – 6 (Supporting Information, Figure S2), consistent with Vegard’s rule,83 was observed. Structural units of 1 – 6 consist of seven–coordinated [LnIII(µ–NC)2(tmu)5]+ entities (LnIII = Gd, 1; Tb, 2; Dy, 3; Ho, 4; Er, 5; Tm, 6) revealing intermediate geometries between capped trigonal prism and pentagonal bipyramid (Supporting Information, Figure S3), which are alternately aligned with slightly deformed triangular dodecahedron [WV(CN)8]3- anions with two bridging cyanides (Supporting Information, Table S3). Selected bond lengths and angles of 1 – 6 and similar 1–D compounds are listed in Supporting Information, Table S4. The largest average LnIII–O and LnIII–N bond lengths of 2.296 and 2.508 Å, respectively, are observed for complex 1 with gadolinium(III) cation while the smallest distances of 2.240 and 2.456 Å, respectively, are present in the structure of assembly 6 with thulium(III). Distances between trivalent metal ions and donor atoms of ligands decrease linearly with the decrease of crystal radii of ions and


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correlate with the lanthanide contraction effect (Supporting Information, Figure S2). Moreover, these distances are rather short in respect to previously reported 1–D amide–based coordination compounds (Supporting Information, Table S4), although the tetramethylurea ligand incorporated in structures of 1 – 6 is more bulky than N,N–dimethylformamide (dmf) and N,N– dimethylacetamide (dma) used in other assemblies. However, sterical effect of tmu ligand is compensated by the decrease of coordination number of Ln(III) from the eight to the less frequent coordination number seven.84 Additionally, it is worth to emphasize that the sterical hindrance of tmu is crucial for the induction of chirality in lanthanide(III) complexes in 1 – 6. The LnIII–N–C and N–LnIII–N angles for 1 – 6 adopt values of ca. 174 and 147°, respectively, which do not stand out from values for other 1–D compounds with dmf, however, N–LnIII–N angles for 1 – 6 are much larger in respect to systems with dma. Similarly, the W–C and C–N bond lengths as like as C–W–C and W–C–N angles, describing structural parameters related to cyanido–bridges, do not change significantly with the decrease of trivalent ion crystal radii. The W–C and C–N distances for 1 – 6 adopt values of ca. 2.17 and 1.15 Å, respectively, meanwhile C–W–C and W–C–N angles equal ca. 137.5 and 179°, respectively. The intrachain LnIII…WV distances for 1 – 6 range between 5.813 and 5.764 Å and the shortest interchain intermetallic distance (LnIII…WV) is around 9.0 Å. The intrachain lengths are almost identical to the reference compounds while the intrachain lengths are larger by up to 0.5 Å. Moreover, assemblies 1 – 6 are rare examples of 1–D coordination polymers without crystallization solvent molecules.12,13,85 Thus, chains are very well separated and hydrogen bond networks are not observed in their structures. Additionally, compounds 1 – 6 have a pyroelectric crystal structure, in which the electric polarization is along the b axis.


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Table 1. Crystal data, data collection, and refinement parameters for 1 – 6. Formula FW (g/mol) T (K) Crystal system, space group a (Å) b (Å) c (Å) β (°) V (Å3) Z ρcalcd (g/cm-3) µ (mm-1) F(000) Radiation 2θ range (°) Index ranges

Reflections collected / unique

Data/restraints/parameters GOF on F2 R1 / wR2 (I>2σ(I)) R1 / wR2 (all data) Largest diff. peak and hole (e·Å-3) Flack / Hooft parameters

Formula FW (g/mol) T (K) Crystal system, space group a (Å) b (Å) c (Å) β (°) V (Å3) Z ρcalcd (g/cm-3) µ (mm-1) F(000) Radiation 2θ range (°) Index ranges

Reflections collected / unique

Data/restraints/parameters GOF on F2 R1 / wR2 (I>2σ(I)) R1 / wR2 (all data) Largest diff. peak and hole (e·Å-3) Flack / Hooft parameters

1 (-)

2 (-)

3 (-)

C33H60GdN18O5W 1130.09 293(2) monoclinic, I2 10.9671(3) 14.5050(3) 14.5875(18) 94.871(9) 2312.2(3) 2 1.623 3.964 1124 MoKα (λ = 0.71075) 7.048 – 54.93 -14 ≤ h ≤ 12, -18 ≤ k ≤ 18, -18 ≤ l ≤ 18 11088 / 5168 [Rint = 0.0155, Rsigma = 0.0331] 5168 / 1 / 275 0.963 0.0243 / 0.0308 0.0290 / 0.0315 1.29 / -1.53 0.007(7) / -0.016(8) 4 (-)

C33H60N18O5TbW 1131.76 293(2) monoclinic, I2 10.9499(2) 14.4884(3) 14.5692(17) 94.861(8) 2303.0(3) 2 1.632 4.076 1126 MoKα (λ = 0.71075) 7.058 – 54.928 -13 ≤ h ≤ 14, -18 ≤ k ≤ 18, -18 ≤ l ≤ 18 10879 / 5123 [Rint = 0.0173, Rsigma = 0.0319] 5123 / 1 / 275 0.956 0.0152 / 0.0242 0.0186 / 0.0248 0.85 / -0.68 0.010(6) / 0.017(6) 4 (+)

C33H60DyN18O5W 1135.34 293(2) monoclinic, I2 10.9341(3) 14.4750(3) 14.5506(18) 94.811(9) 2294.8(3) 2 1.643 4.177 1128 MoKα (λ = 0.71075) 7.07 – 54.922 -14 ≤ h ≤ 13, -18 ≤ k ≤ 18, -18 ≤ l ≤ 18 10971 / 5110 [Rint = 0.0163, Rsigma = 0.0316] 5110 / 1 / 275 0.939 0.0157 / 0.0271 0.0176 / 0.0275 0.65 / -0.83 0.005(6) / 0.003(6) 5 (-)

6 (-)

C33H60HoN18O5W 1137.77 293(2) monoclinic, I2 10.9130(6) 14.4616(8) 14.5377(19) 94.805(9) 2286.3(3) 2 1.653 4.289 1130 MoKα (λ = 0.71075) 7.078 – 54.902 -13 ≤ h ≤ 14, -18 ≤ k ≤ 18, -18 ≤ l ≤ 18 10710 / 5128 [Rint = 0.0334, Rsigma = 0.0628] 5128 / 1 / 275 1.022 0.0297 / 0.0367 0.0383 / 0.0378 1.34 / -1.77 0.032(10) / 0.046(12)

C33H60HoN18O5W 1137.77 293(2) monoclinic, I2 10.9126(2) 14.4663(3) 14.5430(17) 94.759(9) 2287.9(3) 2 1.652 4.286 1130 MoKα (λ = 0.71075) 7.078 – 54.882 -14 ≤ h ≤ 13, -18 ≤ k ≤ 18, -18 ≤ l ≤ 18 11017 / 5105 [Rint = 0.0176, Rsigma = 0.0329] 5105 / 1 / 275 1.047 0.0197 / 0.0288 0.0224 / 0.0296 0.95 / -1.15 -0.001(7) / -0.021(7)

C33H60ErN18O5W 1140.10 293(2) monoclinic, I2 10.8984(2) 14.4569(3) 14.5302(17) 94.726(9) 2281.6(3) 2 1.660 4.403 1132 MoKα (λ = 0.71075) 7.086 – 54.94 -12 ≤ h ≤ 14, -18 ≤ k ≤ 18, -18 ≤ l ≤ 18 17888 / 5105 [Rint = 0.0183, Rsigma = 0.0217] 5106 / 1 / 275 1.211 0.0168 / 0.0292 0.0179 / 0.0294 1.32 / -1.20 0.028(7) / 0.015(5)

C33H60N18O5TmW 1141.77 293(2) monoclinic, I2 10.8859(4) 14.4379(7) 14.5146(18) 94.707(9) 2273.6(3) 2 1.668 4.524 1134 MoKα (λ = 0.71075) 4.874 – 54.956 -14 ≤ h ≤ 14, -18 ≤ k ≤ 18, -18 ≤ l ≤ 18 10894 / 5004 [Rint = 0.0175, Rsigma = 0.0308] 5004 / 1 / 275 1.107 0.0183 / 0.0283 0.0204 / 0.0287 0.99 / -0.95 0.014(8) / 0.004(8)


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Figure 1. a) Structural diagrams for both chiral form of [Ho(tmu)5][W(CN)8] (4 (-) and 4 (+)) with selected atoms labeling. Thermal ellipsoids of 50% probability are shown. b) Zigzag chain structures of 4 (-) and 4 (+). c) Crystal packing along a axis. Hydrogen atoms in all figure were omitted for clarity. It is noteworthy to highlight that 1 – 6 can crystallize in two mirror–image forms, with Flack and Hooft parameters close to zero (Table 1),86–89 due to the occurrence of the specific spatial organization of ligands around lanthanide(III) centers (Figure 1). We need to introduce a novel notation for seven–coordinated complex to distinguish the forms, as in this case the revised ∆/Λ notation is not applicable.90 As it is apparent in Figure 2, the [LnIII(µ–NC)2(tmu)5] entities can be characterized by the acute angle (θ) between two mean planes incorporating LnIII, O1, O2, O2*


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and LnIII, O1, O3, O3* atoms under restriction that the oxygen atom of tmu (O1) located on 2– fold axis must be oriented towards viewer and the plane described on the LnIII, O1, W, W* atoms must be placed horizontally. Negative value of θ angle describes the (-) form while a positive one designates the (+) form. In accordance with this notation we can state that the (-) form is the preferred form in our syntheses. This phenomenon will be described in more detail in the next section. Nevertheless, the presence of two chiral forms is very interesting from the point of view of observation of nonlinear optical effects.

Figure 2. Conformations of a [LnIII(µ–NC)2(tmu)5] entities and corresponding nomenclature. Natural Optical Activity (NOA) Studies and Spontaneous Resolution in 1 – 6. The optical activities of 1 – 6 were investigated by means of natural circular dichroism (NCD) spectroscopy preceded by solid state UV–Vis diffuse–reflectance spectroscopy. All solid state UV–Vis diffuse–reflectance spectra show very similar shape and intensities of absorption peaks (Supporting Information, Figure S4). The deconvolution of spectra of 1 – 6 delivered three Gaussian components with maxima around 260, 295 and 355 nm which can be assigned to the π– π* transition of tmu as well as to ligand–to–metal charge transfer (LMCT) between CN- and tungsten(V) in [W(CN)8]3- moieties. NCD spectra (Supporting Information, Figure S5) for the (-) forms of 1 – 6 reveal two distinct positive low intense maxima about 350 and 450 nm, and hardly appreciable signals below 275 nm. Meanwhile, the NCD spectrum for the (+) form of 4 is an almost perfect mirror image in


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respect to the patterns for the (-) forms, which unambiguously proves the enantiopure nature of both compounds. Furthermore, quantitative studies of NCD signals of both forms of 4 imply that the amounts of chiral species obtained in distinct syntheses are almost equal. We were not able to determine the absolute concentration in respect to structurally characterized enantiopure single crystals due to very low signal of them. The above presented research leads to a conclusion that cyanido–bridged 1 – 6 coordination polymers reveal a unique spontaneous resolution process accompanied by chiral symmetry breaking at macroscopic level. The generation of enantiopure crystals of 1 – 6 with exceptional preference of one form can be a consequence of crystal growth exclusively occurring on the surface of the existing crystals of the same chirality, formed at a very early state of nucleation, and suppression of crystal nucleation of the opposite chirality. Magnetic properties of 1. The χMT value for GdIIIWV unit at room temperature reaches 8.25 cm3Kmol-1 (Figure 3a), which corresponds well to an isolated pair of Gd(III) (S = 7/2, gJ = 2) and W(V) (S = 1/2, g = 2). This value is almost constant with decreasing temperature down to 100 K. The fit of the Curie–Weiss model to the data in the 100 – 300 K temperature range leads to the Curie constant of C = 8.12 cm3Kmol-1 and the Weiss temperature close to 0 K. Below 100 K magnetic susceptibility decreases more abruptly, reaching a minimum of 5.86 cm3Kmol-1 at 7.3 K which proves antiferromagnetic interactions between Gd(III) and W(V) magnetic centers leading to cyanido–bridged ferrimagnetic chains. On further cooling, the magnetic signal increases sharply up to 8.98 cm3Kmol-1 at 2 K most probably due to the attendance of interchain ordering of net spins known as the correlation length. This finding is compatible with the result of M vs. H measurement at 2 K (Supporting Information, Figure S6). The magnetization increases monotonically with augmentation of magnetic field and reaches 6.6 NAµB in HDC = 50


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kOe which is closer to the value of 6 NAµB expected for antiferromagnetically coupled GdIII–WV pairs than the value of 8 NAµB corresponding to ferromagnetically coupled one. The observed magnetic properties of 1 are similar to the results of magnetic studies for other LnIII–[MV(CN)8] chains.32,70,71,91 Magnetic properties of 2 – 6. The magnetic properties of the polycrystalline samples were investigated by determining the thermal dependence of χM (the molar magnetic susceptibility per LnIIIWV unit) in the 2 – 300 K temperature range in the magnetic field of 1 kOe (Figure 3a). The χMT values at 300 K for 2 – 6 are in good agreement with χMT values calculated as a sum of contributions of the W(V) magnetic centre (SW(V) = ½ and gW(V) ≈ 2.0 leading to (χMT)W(V) = 0.375 cm3Kmol-1) and the Ln(III) centre (χMT determined from van Vleck equation) (Supporting Information, Table S5). On cooling, magnetic signals of 2 (Tb), 5 (Er) and 6 (Tm) steadily decrease reaching 5.68, 5.62 and 4.66 cm3Kmol-1 at 2 K in series (Figure 3b). This behavior corresponds to the depopulation of the MJ sublevels of lanthanide ions due to the splitting of the ground term by the ligand field and antiferromagnetic interactions through Ln(III)–NC–W(V) linkages. On the other hand, the behavior at low temperatures for 3 (Dy) and 4 (Ho) assemblies reveal a slightly different pattern in respect to 2, 5 and 6 but similar to compound 1. On decreasing temperature, the χMT signals diminish monotonically down to 11.09 and 11.26 cm3Kmol-1 at around 5 K for 3 and 4, respectively (Figure 3b). Further decrease of temperature leads to upturn of plots and gives χMT values of 12.23 and 11.44 cm3Kmol-1 at 2 K for 3 and 4, respectively. The nature of magnetic behavior can be interpreted in the same way like for 1 but it needs to be extended with the depopulation of the MJ sublevels of lanthanide ions effect. Thus, the initial


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decrease of magnetic signal on cooling is related to the antiferromagnetic interactions between Ln(III) and W(V) within chain and depopulation of the MJ sublevels of Ln(III). The magnetization M vs. external magnetic field HDC curves for 2 – 6 at 2 K are presented in Supporting Information, Figure S6. The increase of external magnetic field up to 10 kOe leads to fast augment of magnetization for all assemblies. At higher fields, the magnetic signals increase much more slowly and do not reach saturation even in fields up to HDC = 50 kOe. As a result, the obtained M values are much lower than the expected saturation values for anti– or ferromagnetically coupled LnIII–WV pairs (Supporting Information, Table S5). A potential explanation of such behavior is the presence of large anisotropy in lanthanide ions and/or in the chain structure combined with the antiferromagnetic character of interactions in Ln(III)– W(V).68,92–94


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Figure 3. χMT vs. T plots measured in external magnetic field of 1 kOe (a) and χMT vs. T plots below 50 K (enlarged) (b) for 1 – 6. Dynamic magnetic properties of 1 – 6. AC magnetic susceptibility measurements in zero DC field for 1 – 6 with AC field HAC = 3 Oe indicate that none of the compounds exhibit frequency dependent signals above 1.85 K. This could be due to either small magnetic anisotropy, fast quantum tunneling of the magnetization (QTM) or a very small thermal energy barrier. The same studies performed with application of an external magnetic field reveal full or partial suppression of the QTM relaxation process for 3 (Dy), 5 (Er) and 6 (Tm) systems in HDC fields of 1, 2 and 2.5 kOe in series. As a result, strong frequency dependence of the out–of phase susceptibility signals were observed for 3 and 5, and weaker for 6. It is noteworthy to mention that the


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antiferromagnetic interactions between Ln(III) and W(V) ions are weak, so the applied DC magnetic field at first plays a role of decoupling as well as suppressing the tunneling effect, and then leads to the lanthanide ions showing the magnetic relaxation as single-ion magnets. The dysprosium(III) assembly (3) displays two relaxation processes. Although the process at lower frequencies is hardly appreciable in χ"M(f) and in Cole–Cole plots (Figure 4a and Supporting Information, Figure S7a and S8), the process in 17 – 1500 Hz frequency range, corresponding to the faster relaxation, has been characterized by fitting a single Cole–Cole function. The resulting α parameters are within the range of 0.17 – 0.41, indicating a fairly narrow distribution of relaxation times. The relaxation times obey the Arrhenius law above 3.5 K, giving a relatively low energy barrier ∆τ/kB of 17(1) K and the relaxation time τ0 = 1.68(3)·10-6 s. These values are typical for dysprosium(III)–based SMMs.39,40 To the best of our knowledge, assembly 3 is the first example of a field–induced single ion magnet based on Dy(III) complexes with paramagnetic octacyanidometallates.12 Assembly 5 with erbium(III) ions also exhibits two slow magnetization relaxation processes which have been analyzed by means of two relaxation Cole–Cole function (Figure 4b and Supporting Information, Figure S7b and S8). The α1 and α2 parameters are 0.14 – 0.23 and 0.14 – 0.33 for slower and faster relaxation processes, respectively. The slower relaxation process changes very weakly with temperature. An Arrhenius law fit to the data obtained from Cole–Cole fits to individual AC magnetization vs. frequency data delivers the nominal value of energy barrier ∆τ/kB = 1.2(3) K and long relaxation time τ0 = 2.63(8)·10-2 s which most probably is related to dipole–dipole interactions within the crystal lattice. Meanwhile, the faster one obeys the Arrhenius law above 3 K, giving ∆τ/kB = 22(2) K and τ0 = 1.21(3)·10-8 s which are lower than the limits usual for erbium–based SMMs.39,40 The last compound, 6 with thulium(III), exhibits a single relaxation process which


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could be fitted with a single Cole–Cole function in the low frequency range (Supporting Information, Figure S7c and S9). In this case the energy barrier is very low ∆τ/kB = 5.7(3) K, and a very long relaxation time τ0 = 1.53(4)·10-2 s is observed (α in 0.20 – 0.27 range). Slow magnetic relaxation process in thulium(III) centre in 6 is highly uncommon due to the non– Kramers nature of this ion and to the difficulties to assure the coordination environment symmetry constrains for the prolate shape of electron density of MJ = ± 6.35,95 Rarely, this effect can be overcome by application of external DC magnetic field. Hence, the slow relaxation in 6 can be ascribed to dipole–dipole interactions originating from crystal lattice vibrations modifying intermetallic distances, which amplify the magnetic field produced by a dipole at a neighboring dipole and modify the relaxation parameters. The slower relaxation processes in isostructural assemblies 3 and 5 can be assigned to the dipole–dipole interactions. Moreover, the determined energy barriers and relaxation times for the slowest relaxation processes of 5 and 6 are very similar.

Figure 4. The Cole–Cole plots for 3 (a) and 5 (b). CONCLUSIONS We report a series of isostructural chiral cyanido–bridged zigzag chains [Ln(tmu)5][W(CN)8] (Ln = Gd – Tm, 1 – 6) obtained as a product of self–assembly of lanthanide(III) ions with


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octacyanidotungstate(V) in the presence of am achiral tetramethylurea ligand. These assemblies follow the Vegard’s rule, linear relationship for a monotonic decrease in unit cell parameters with the systematic decrease in Shannon effective ionic radius. Moreover, assemblies 1 – 6 are rare examples of 1–D cyanido–bridged coordination polymers with a pyroelectric crystal structure and without crystallization solvent molecules. The structural X–ray analysis revealed that 1 – 6 can crystallize in two enantiopure mirror–image forms with Flack and Hooft parameters close to zero. This effect is mainly related to the unique spatial orientation of five tmu and two CN- ligands coordinated to the lanthanide(III) centre which required the introduction of a new notation for a seven–coordinated complex to distinguish the forms. Detailed crystallographic studies of various crystals of 1 – 6 (performed for samples obtained in different syntheses) followed by nonlinear optical activity studies by means of natural circular dichroism (NCD) spectroscopy, demonstrated that all assemblies formed homogenous enantiopure single–phase products. In addition to the unique chiral structure, compounds 3 and 5 show slow magnetic relaxation under external DC fields of 1 and 2 kOe, respectively. The AC magnetic measurements revealed that the relaxation times (τ0) and the energy barriers (∆τ/kB) are 1.68(3)·10-6 s and 17(1) K for 3, and 1.21(3)·10-8 s and 22(2) K for 5. These assemblies are rare examples of field–induced single molecule magnets based on bimetallic LnIII–[MV(CN)8]3systems. Finally, we achieved to synthesize chiral octacyanidometallate–based materials showing SMM behavior. In the future we plan to investigate nonlinear effects such as second harmonic generation and ferroelectricity for these assemblies and their analogues. ASSOCIATED CONTENT Supporting Information.


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Information about yields and chemical composition of 1 – 6; Interpretation of results of IR spectroscopy; Evolution of unit cell and structural parameters in relation to the Shannon effective ion radius of lanthanides of 1 – 6 plots; Graphical presentation of lanthanide complexes geometries; Listings of SHAPE parameters calculated for the LnIII and WV centers; Selected bond lengths and angles of 1 – 6 compared with these quantities for similar 1–D compounds; Analysis of UV–Vis solid state spectra for 1 – 6; Results of optical activity measurements, and DC and AC magnetic studies. AUTHOR INFORMATION Corresponding Author *E–mail: [email protected]–tokyo.ac.jp Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The present research was partially supported by the Grant–in–Aid for Specially Promoted Research of JSPS (JSPS Kakenhi grant number 15H05697), Grants-in-Aid for Scientific Research on Innovative Areas Soft Crystals (area No. 2903, 17H06367) and Advanced Photon Science Alliance (APSA) from the Ministry of Education, Culture, Sports, Science and Technology of Japan (MEXT). The Global Science course from MEXT, the Cryogenic Research


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Center in The University of Tokyo, and the Center for Nano Lithography & Analysis in The University of Tokyo supported by MEXT are acknowledged. K. N. is thankful to the Grant–in– Aid for Challenging Exploratory Research 15K13666 from JSPS. The authors acknowledge partial support of this research by the Polish National Science Centre within the SONATA Project UMO–2015/19/D/ST5/01936. The authors thank Prof. Z. Tomkowicz for the technical support with the measurements of natural circular dichroism (NCD) spectra carried out with the equipment purchased with the financial support of the European Regional Development Fund within the Polish Innovation Economy Operational Program (contract no. POIG.02.01.00–12– 023/08). REFERENCES (1) Ouahab, L. Multifunctional Molecular Materials; Pan Stanford, Singapore, 2013. (2) Mukhopadhyay, S. M. Nanoscale Multifunctional Materials: Science and Applications; Wiley, New Jersey, 2012. (3) Kahn, O. Molecular Magnetism, VCH, New York, 1993. (4) Herrera, J. M.; Franz, P.; Podgajny, R.; Pilkington, M.; Biner, M.; Decurtins, S.; Stoeckli– Evans, H.; Neels, A.; Garde, R.; Dromzée, Y.; Julve, M.; Sieklucka, B.; Hashimoto, K.; Ohkoshi, S.;

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“For Table of Contents Only” Chiral LnIII(tetramethylurea)–[WV(CN)8] coordination chains showing slow magnetic relaxation Olaf Stefańczyk, Anna M. Majcher, Koji Nakabayashi and Shin–ichi Ohkoshi*

Chiral cyanido–bridged zigzag chains [Ln(tmu)5][W(CN)8] (Ln = Gd – Tm, 1 – 6) have been investigated by means of natural circular dichroism spectroscopy. Magnetic studies show antiferromagnetic interactions within LnIII–CN–WV linkages, and interchain ordering of net spins in 1, 4 and 5. Furthermore, Dy–, Er– and Tm–based systems combine magnetic field–induced slow magnetic relaxation and chirality.


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Figure 1. a) Structural diagrams for both chiral form of [Ho(tmu) 5][W(CN) 8] (4 (-) and 4 (+)) with selected atoms labeling. Thermal ellipsoids of 50% probability are shown. b) Zigzag chain structures of 4 () and 4 (+)). c) Crystal packing along c axis. Hydrogen atoms in all figure were omitted for clarity. 84x117mm (300 x 300 DPI)



Figure 2. Conformations of a [MII(µ–NC)2(tmu)5] entities and corresponding nomenclature. 84x39mm (300 x 300 DPI)


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Figure 3. χM(T) vs. T plots measured in external magnetic field of 1 kOe (a) and χM(T) vs. T plots below 50 K (enlarged) (b) for 1 – 6. 84x121mm (300 x 300 DPI)



Figure 4. The Cole–Cole plots for 3 (a) and 5 (b). 177x61mm (300 x 300 DPI)


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88x34mm (300 x 300 DPI)


Chiral LnIII(tetramethylurea) - ACS Publications

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