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Chiral Mononuclear Lanthanide Complexes and the Field-induced Single-ion Magnet Behaviour of Dy Analogue Shuang-Yan Lin,a, b Chao Wang,a Lang Zhao,a Jianfeng Wu,a and Jinkui Tang*a 5
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Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX DOI: 10.1039/b000000x Three pairs of homochiral mononuclear lanthanide complexes, with general formula [LnH4LRRRRRR/SSSSSS(SCN)2](SCN)2·xCH3OH·yH2O (Ln = Dy (R/S-Dy1), Ho (R/S-Ho1) and Er (R/SEr1)), have been obtained via self-assembly between chiral macrocyclic ligands and respective thiocyanates, all of which show saddle-type conformation with seven-coordinated metal ion. Magnetic measurements revealed that the Dy complex shows the field-induced single-ion magnet behaviour, which is a rarely reported seven-coordinated lanthanide-based SIM encapsulated in a macrocyclic ligand. The absolute configuration of all enantiomers was determined by single crystal X-ray crystallography and confirmed by electronic CD and VCD spectra.
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Interest in single-molecule magnets (SMMs) continues to grow motivated by potential applications in quantum computing,1 highdensity memory storage devices2 and molecule spintronics.3 A significant effort has been dedicated to the preparation and study of new systems in search for a better understanding of the various phenomena that influence SMM behaviour. These studies have led to a flood of groundbreaking results based on lanthanide SMMs thanks to their large magnetic moments and large anisotropy,4 such as the vortex arrangement of the magnetic moments in dysprosium(III) triangles,5 the recorded relaxation energy barrier in polylanthanide alkoxide cage complexes,6 as well as the highest blocking temperature in an N23– radical bridged Tb2 complex.7 Another fascinating result was the observation of SMM behaviour in complexes comprising a single anisotropic lanthanide ion, which are also called single-ion magnets (SIMs). These SIMs play a crucial role in understanding the electronic structure nature of molecular nanomagnets due to the accessible structure design and the simplification in the analysis of local anisotropy in contrast to the complexity of a polynuclear system. Indeed since the discovery of magnet-like behaviour in the phthalocyanine double-decker Ln complexes ([LnPc2]),8 some relevant examples such as the organometallic double-decker compounds9 and the Dy-β-diketones series,10 and the DyDOTA complexes4, 11 have been emerged. Among them, most of 4f-center is eight-coordinate with D4d axial ligand-field symmetry, only a few cases are observed with lower symmetric coordination sphere.12 Moreover, SMMs and SIMs, as molecule-based magnets, provide unique opportunities for the assembly of crystal structures by the intellectual selection of suitable molecular building blocks that exhibiting peculiar functionalities in addition to and/or interacting with the magnetic properties, thus leading to This journal is © The Royal Society of Chemistry [year]
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the design of advanced multifunctional materials.13 So far, it has been successfully demonstrated that some SMMs can show simultaneously other physical properties such as ferroelectricity or chirality.14 In particularly, introduction of chirality in SMMs provides a great platform for the study of the cross-effects of circular dichroism (CD), vibrational circular dichroism (VCD) and magnetochiral dichroism (MChD).15 However, the number of the case is quite limited, especially lanthanide complexes;14a, 14c, 16 thus it is still a great challenge to synthesize more chiral lanthanide compounds to explore the relationship between structure and property.
Scheme 1. Structure of the H3LRRRRRR ligand. 60
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In fact, the stereoselective synthesis of enantiopure metal complexes is still a challenging task. The most fruitful method of enantio- and diastereoselective synthesis of chiral metal complexes is based on the use of enantiopure chiral ligands,14b, 17 because such chiral ligand can transfer chiral information into the entire system through coordination bonds. In this work, we select an elastic ligand, macrocyclic ligand H3LRRRRRR/SSSSSS (shown in Scheme 1), which not only has enough space in the cavity to bind three metal ions (transition ions or lanthanide ions),18 but also can [journal], [year], [vol], 00–00 | 1
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wrap only one lanthanide ion19 through adjusting its cavity. Mononuclear complexes [LnH4L(NO3)2]19 (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) have been reported with only La, Gd, Eu complexes analyzed by X-ray analysis, but their magnetic behaviour has remained unexplored. In the context, we have focused our attention towards the design of chiral mononuclear macrocyclic lanthanide complexes by using this enantiopure chiral macrocyclic ligand and studying their magnetic properties. Therefore, three pairs of enantiopure chiral macrocyclic complexes [LnH4LRRRRRR/SSSSSS(SCN)2](SCN)2·xCH3OH·yH2O (Ln = Dy, x = 5, y = 0 (R-Dy1); Dy, x = 4, y = 1 (S-Dy1); Ho, x = 5, y = 0 (R/SHo1) and Er, x = 5, y = 0 (R-Er1); Er, x = 4, y = 1, S-Er1)) have been synthesized, of which Dy derivative shows field-induced SIM behaviour.
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Experimental Section
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General. All starting materials were of A.R. Grade and were used as commercially obtained without further purification. 2, 6diformyl-4-methylphenol, H3LRRRRRR and H3LSSSSSS were prepared according to a previously published method.20 Elemental analyses for C, H, and N were carried out on a Perkin-Elmer 2400 analyzer. IR spectra (4000-300 cm-1) and vibrational circular dichroism (VCD) (1900-1000 cm-1) spectrum were measured using KBr pellets by a fourier transform infrared spectrometer nicolet 6700. All magnetization data were recorded on a Quantum Design MPMS-XL7 SQUID magnetometer equipped with a 7 T magnet. The variable-temperature magnetization was measured with an external magnetic field of 1000 Oe in the temperature range of 2-300 K. The experimental magnetic susceptibility data are corrected for the diamagnetism estimated from Pascal’s tables and sample holder calibration. The solution circular dichroism (CD) spectra were measured on a Biologic MOS-450 spectrometer at room temperature using quartz optical cells under the following conditions: scanning speed, 1 nm/0.1 s; bandwidth: 2 nm; sensitivity standard: 1000 mdeg. The obtained spectra were corrected by subtraction from the spectrum of methanol. X-ray Crystallography. Suitable single crystals were selected for single-crystal X-ray diffraction analysis. Crystallographic data were collected at 185(2)/296(2) K on a Bruker ApexII CCD diffractometer with graphite monochromated Mo Kα radiation (λ = 0.71073 Å). The structure was solved by direct methods and refined on F2 with full-matrix least-squares techniques using SHELXS-97 and SHELXL-97 programs.21 The locations of lanthanide ions were easily determined, and O, N, C and S atoms were subsequently determined from the difference Fourier maps. Anisotropic thermal parameters were assigned to all nonhydrogen atoms. The H atoms were introduced in calculated positions and refined with a fixed geometry with respect to their carrier atoms. Crystallographic data and refinement details are given in Table 1 and the select bond lengths and angles are listed in Table S1. CCDC 972387-972392 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Synthesis of S-Dy1: A solution of H3Lssssss (0.05 mmol) in 15
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DOI: 10.1039/C4DT02889A
mL CH3OH was added 0.05 mL 1 mol·L-1 HCl aqueous solution and Dy(SCN)3·6H2O (91.6 mg, 0.2 mmol). The resulting mixture was stirred for about 2 h at room temperature and then left unperturbed to allow for slow evaporation of the solvent. The colorless stick-shaped crystals of S-Dy1 were obtained. Yield: 27 mg (41 %, based on the ligand). Elemental analysis (%) calcd for DyC53H85N10O8S4: C, 49.69, H, 6.69, N, 10.93; found C, 50.08, H, 6.50, N, 11.57. IR (KBr, cm-1): 2941 (m), 2862 (w), 2061 (vs), 1605 (w), 1474 (vs), 1451 (s), 1304 (w), 1245 (m), 1163 (w), 1065 (w), 1014 (w), 877 (m), 800 (m), 760 (w), 499 (w). Synthesis of R-Dy1: The colorless stick-shaped crystals of complex R-Dy1 were obtained by following the same procedure as that described for complex S-Dy1 except H3LRRRRRR instead of H3Lssssss. Yield: 30 mg (46 %, based on the ligand). Elemental analysis (%) calcd for DyC54H87N10O8S4: C, 50.08, H, 6.77, N, 10.81; found C, 49.65, H, 6.28, N, 10.21. IR (KBr, cm-1): 2936(s), 2859 (m), 2055 (vs), 1613 (w), 1575 (w), 1474 (s), 1450 (s), 1307 (m), 1254 (s), 1164 (w), 1099 (w), 1065 (w), 1014 (w), 877 (m), 804 (m), 763 (w), 508 (w). Synthesis of S-Ho1: The pale-orange stick-shaped crystals of complex S-Ho1 were obtained by following the same procedure as that described for complex S-Dy1 except Ho(SCN)3·6H2O (0.2 mmol) instead of Dy(SCN)3·6H2O. Yield: 24 mg (37 %, based on the ligand). Elemental analysis (%) calcd for HoC54H87N10O8S4: C, 49.99, H, 6.76, N, 10.79; found C, 49.63, H, 6.51, N, 10.32. IR (KBr, cm-1): 2943 (m), 2862 (m), 2052 (vs), 1603 (w), 1476 (vs), 1451 (s), 1304 (m), 1245 (s), 1164 (m), 1065 (w), 1015 (w), 877 (m), 801 (m), 761 (w), 501 (w). Synthesis of R-Ho1: The pale-orange stick-shaped crystals of complex R-Ho1 were obtained by following the same procedure as that described for complex S-Ho1 except H3LRRRRRR instead of H3Lssssss. Yield: 27 mg (41 %, based on the ligand). Elemental analysis (%) calcd for HoC54H87N10O8S4: C, 49.99, H, 6.76, N, 10.79; found C, 49.71, H, 6.49 N, 10.33. IR (KBr, cm-1): 2942 (m), 2862 (m), 2053 (vs), 1613 (w), 1476 (vs), 1451 (s), 1305 (w), 1246 (m), 1164 (m), 166 (w), 1016 (w), 877 (m), 802 (m), 762 (w), 501 (w). Synthesis of S-Er1: The pink stick-shaped crystals of complex S-Er1 were obtained by following the same procedure as that described for complex S-Dy1 except Er(SCN)3·6H2O instead of Dy(SCN)3·6H2O. Yield: 22 mg (34 %, based on the ligand). Elemental analysis (%) calcd for ErC53H85N10O8S4: C, 49.51, H, 6.66, N, 10.89; found C, 49.79, H, 6.58, N, 11.23. IR (KBr, cm1 ): 2941 (m), 2862(m), 2051 (vs), 1614 (w), 1477 (vs), 1451 (s), 1305 (m), 1246 (s), 1164 (m), 1065 (w), 1015 (w), 877 (m), 802 (m), 762 (w), 501 (w). Synthesis of R-Er1: The pink stick-shaped crystals of complex R-Er1 were obtained by following the same procedure as that described for complex S-Er1 except H3LRRRRRR instead of H3Lssssss. Yield: 31 mg (48 %, based on the ligand). Elemental analysis (%) calcd for ErC54H87N10O8S4: C, 49.89, H, 6.75, N, 10.78; found C, 49.53, H, 6.56, N, 10.42. IR (KBr, cm-1): 2937 (m), 2861 (m), 2056 (vs), 1613 (w), 1476 (vs), 1450 (s), 1312 (m), 1254 (s), 1163 (m), 1066 (w), 1015 (w), 878 (m), 805 (m), 762 (w), 501 (w).
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Results and Discussion Table 1. Crystal data and structural refinement parameters. Compound Empirical formula Fw (g/mol) Temperature (K) Crystal system Space group a (Å) b (Å) c (Å) α (o) β (o) γ (o) V (Å3) Z, ρcalcd (Mg/m3) F(000) Rint R1, wR2 [I > 2σ (I)] R1, wR2 (all data) GOF Flack parameter
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R-Dy1 S-Dy1 R-Ho1 S-Ho1 C54H87DyN10O8S4 C53H85DyN10O8S4 C54H87HoN10O8S4 C54H87HoN10O8S4 1295.08 1281.05 1297.51 1297.51 185(2) 296(2) 185(2) 185(2) Triclinic Triclinic Triclinic Triclinic P1 P1 P1 P1 10.2956(7) 10.409(3) 10.297(1) 10.2833(8) 11.6900(8) 11.756(3) 11.6829(11) 11.6475(8) 13.6363(9) 13.650(4) 13.5696(13) 13.6139(10) 106.886(1) 106.550(5) 106.808(2) 106.800(1) 98.239(1) 98.526(6) 98.496(2) 98.131(1) 94.427(1) 94.961(6) 94.203(2) 94.398(1) 1541.95(18) 1568.9(8) 1533.9(3) 1533.38(19) 1, 1.395 1, 1.356 1, 1.405 1, 1.405 675 667 676 676 0.0243 0.0472 0.0238 0.0310 0.0523, 0.1322 0.0722, 0.1737 0.0507, 0.1310 0.0534, 0.1284 0.0533, 0.1335 0.0931, 0.1941 0.0508, 0.1313 0.0546, 0.1298 1.049 1.026 1.034 1.030 0.001(19) 0.064(19) 0.027(14) 0.022(18)
The flexible chiral macrocycle H3L reveals to be an ideal platform for the formation of a multitude of metal complexes, such mono- and tri-nuclear lanthanide(III) complexes as well as trinuclear transition metal complexes.18-19, 22 When chelating three Ln3+ ions, the macrocyclic ligand coordinates with O3N6 in tri-deprotonated form L3- (three phenolate oxygen atoms are deprotonated), and the ligands are relatively flat; when chelating only one Ln3+ ion, four amine nitrogen atoms in the ligand are blocked by protonation and the ligand coordinates with O3N2 (three phenolate oxygen atoms are deprotonated, but four amine nitrogen atoms are protonated), because the open cavity of the macrocyclic ligand is far too large to accommodate a single Ln ion. Thus the mononuclear complexes are highly screwy. However, the case is different from the macrocycle formed by the 2,6-diformylpyridine,23 which has to wrap very tightly around the Ln3+ ion in a helical fashion in order to form a nine-coordinate complex. Therefore, the 3 + 3 macrocycle based on 2,6diformylphenol (H3L) exhibits less helical twist in its monoLn(III) complexes in comparison with the Ln(III) complexes of the 3 + 3 macrocycle derived from 2,6-diformylpyridine.
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Fig. 1 Top: Enantiomeric pair of S-Dy1 and R-Dy1; Bottom: side view of molecular structure of R-Dy1, and the coordination polyhedra for Dy1 in R-Dy1.
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R-Er1 S-Er1 C54H87ErN10O8S4 C53H85ErN10O8S4 1299.84 1285.81 185(2) 185(2) Triclinic Triclinic P1 P1 10.2722(8) 10.2345(6) 11.6628(9) 11.6666(7) 13.4833(11) 13.5492(8) 106.821(1) 107.027(1) 98.773(1) 98.881(1) 94.062(1) 94.118(1) 1516.8(2) 1516.57(16) 1, 1.423 1, 1.408 677 668 0.0314 0.0336 0.0649, 0.1650 0.0577, 0.1320 0.0670, 0.1693 0.0600, 0.1344 1.043 1.017 0.00(2) 0.028(18)
Compounds R/S-Dy1, R/S-Ho1 and R/S-Er1 crystallize in the triclinic space group P1 and contain a single, complete mononuclear complex in the asymmetric unit. The X-ray crystal structures show that, in these complexes, the Ln(III) ion is encapsulated in the chiral macrocycle, which acts as a pentadendate ligand and coordinates with three phenol oxygen atoms and two amine nitrogen atoms (shown in Fig. 1 and S1). Two SCN- ions filled the coordination sphere of metal ion in axial positions on two sides of the macrocycle with the Ln1N7/N8 distances of 2.384(12)-2.447(19) Å and the N7-Ln1-N8 angles 162.0(3)-162.8(5)o (Table S1). The seven-coordinated LnIII ions are characterized by a distorted capped trigonal prism environment, as calculated using the SHAPE 2.1 software24 (Table S2). Unlike the relatively flat macrocycle in the Dy3Mac complexes,25 macrocycle in the mononuclear macrocylic complexes reported here is highly twisted and folded with saddletype conformation. Noteworthy, four amine nitrogen atoms of the ligand are protonated to block their coordination so as to adjust the cavity of the macrocycle. This case is reminiscent of the earlier described form in [LnH4L(NO3)2],19 however, the coordination number of the metal ion is different due to different axial anions: bidendate nitrate anions complete the coordination [journal], [year], [vol], 00–00 | 3
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sphere forming nine-coordinate Ln ion in [LnH4L(NO3)2]; while in [LnH4L(SCN)2] reported here, monodendate SCN- ions coordinate in axial positions forming the seven-coordinated Ln ion. In addition, as shown in Table S1, the Ln1-N5/N6 (N atoms from macrocyclic ligand) distances of 2.499(8)-2.545(11) Å are longer than the Ln1-O distances of 2.135(14)-2.245(14) Å and Ln1-N7/N8 (N atoms from SCN- ions) distances. Both Dy3Mac and mononuclear macrocylic complexes indicate remarkable flexibility of the macrocycle H3L, which can adjust its cavity size to fit the size of the bound metal ions. Additionally, the crystal packing pictures reveal that the molecules are stacked in the same manner with only one stereoisomer along the crystallographic axes (Fig. S2). The shortest intermolecular Ln···Ln distances are 10.2956(7) Å for RDy1, 10.297(1) Å for R-Ho1 and 10.2722(9) for R-Er1, which indicate the ligands protect almost completely the lanthanide ions.
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Fig. 2 Plots of χMT vs T for compounds R-Dy1, R-Ho1 and R-Er1 measured in an applied field of 1000 Oe.
Magnetic properties. Direct-current (dc) magnetic susceptibility studies of R-Dy1, R-Ho1 and R-Er1 were performed under a field of 1000 Oe in the temperature range 2−300 K. The plots of χMT versus T are shown in Fig. 2. At room temperature (300 K), χMT products for R-Dy1, R-Ho1 and R-Er1 are 13.81, 13.64, and 11.16 cm3Kmol-1, which are all slightly smaller than the expected values for uncoupled DyIII (4f9, 6H15/2), HoIII (4f10, 5I8) and ErIII (4f11, 4I15/2) ions of 14.17, 14.07 and 11.48 cm3Kmol-1, respectively. Upon cooling in all three cases, χMT values decrease gradually and more rapidly below 50 K down to minimum values of 12.67, 12.32 and 9.61 cm3Kmol-1 at 2 K, respectively. This behaviour is mainly due to the depopulation of the Stark sublevels of the lanthanide ion.26 The magnetizations variation for the three complexes R-Dy1, R-Ho1 and R-Er1 at different applied fields determined between 1.9 and 5 K are shown in Fig. S3-S5, and the corresponding maximum values reached are 5.17, 5.68, and 5.05 μB at 1.9 K and 70 kOe, respectively. The lack of saturation of magnetization at 70 kOe can be attributed to crystal-field effects and the low lying excited states, while the non-superposition of the isofield lines in the M versus H/T plots implies the presence of significant magnetic anisotropy.
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Fig. 3 Frequency dependence of the out-of-phase ac susceptibility under various applied fields at 1.9 K for R-Dy1. The solid lines are guides for the eyes.
Fig. 4 Frequency dependence of the out-of-phase ac susceptibility for RDy1 between 1.9 and 15 K at Hdc = 200 Oe. The solid lines are guides for the eyes.
Alternating current (ac) magnetic susceptibility measurements were performed for complexes R-Dy1, R-Ho1 and R-Er1 to probe the low-temperature magnetic relaxation behaviour of the anisotropic magnetic moments. At zero external field, no out-ofphase signal (χ″) for the ac susceptibilities are observed at frequencies of up to 999 Hz and at temperatures down to 1.9 K for all of the complexes (Fig. S6). The relaxation of SIMs can be influenced by quantum effects, and the application of a dc field may probably remove the degeneracy of the ±MJ energy levels and prevent the tunneling of electrons from the +MJ state to the – MJ state.27 Therefore, the variable frequency ac susceptibility at 2.0 K was measured with application of small dc fields for checking the quantum tunneling effects for compound R-Dy1. As shown in Fig. 3 and S7, the application of a small dc field indeed slows the magnetic relaxation sufficiently to afford a nonnegligible signal in the out-of-phase ac susceptibility (χ″) for R-Dy1, with well-resolved peak under 200 Oe dc field. Therefore, ac susceptibility measurements as a function of frequency were determined again under a dc field of 200 Oe. As shown in Fig. 4 and S7, both the χ′ and χ″ signals are strongly frequencydependent below 7 K, and good peak shapes are observed with the maximum in χ″ shifting to higher frequency with increasing temperatures.
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Fig. 7 Comparison of IR (bottom) and VCD (top) spectra observed for complex R/S-Dy1 based on a pressed KBr disk of the respective compound. Fig. 5 Cole−Cole plots between 1.9−9 K for R-Dy1 (Hdc =200 Oe). The solid lines represent the best fitting with the Debye functions.
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Relaxation times were extracted from the frequency-dependent data between 1.9 and 9 K based on τ = 1/ 2πν. A plot of ln τ versus 1/T is shown in Fig. S8, and an Arrhenius fit (τ = τ0 exp(Ueff/kT)) to the data gives the effective energy barrier Ueff/k = 34.5 K and τ0 = 1.1 × 10−6 s. The relatively low thermal energy barrier may be due to the low symmetry of the ligand field around the Dy ion, leading to a reduced single-ion axial anisotropy, which is ultimately responsible for the SIM behaviour. The data plotted as Cole−Cole plots in the form of χ″ versus χ′ show asymmetrical semicircle (Fig. 5), which were fitted using the Debye models. The fit results are summarized in Table S3, α values are less than 0.25, indicating that thermally activated relaxation process has a narrow distribution of relaxation time.
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On the other hand, vibrational circular dichroism (VCD),28 the extension of electronic CD into the infrared region, is a relatively new and powerful technique to monitor the conformation of a chiral molecule. Fig. 7 and S9 show the VCD and IR spectra of R/S-Dy1, R/S-Ho1 and R/S-Er1, respectively. As expected, the IR spectra of both enantiomers are almost the same, while the VCD spectra are nearly mirror images. More important, the VCD spectra show strong signals corresponding to the IR spectrum, indicating the reliability of the spectra and the chirality of the two enantiomers. The most pronounced feature of the VCD spectra is a strong negative or positive couplet centred at 1474 cm-1 and 1450 cm-1, which is assigned to the δ(C-H) mode of methylene and methyl groups of the ligand. The band at 1450 cm-1 is also attributed to the skeleton vibrations of benzene ring. The bands at 1307 and 1254 cm-1 are assigned to δ(O-H) and ν(C-O) mode of phenol. The weak band at 1163 cm-1 corresponds to ν(C-N) of imino group.
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Fig. 6 CD spectra of R/S-Dy1, R/S-Ho1, R/S-Er1 and H3LSSSSSS/ H3LRRRRRR in MeOH (10−5 M) at room temperature.
CD and IR, VCD Spectra. The optical activity and enantiomeric nature of three pairs of complexes R/S-Dy1, R/S-Ho1 and R/SEr1 were verified by their circular dichroism (CD) spectra (Fig. 6). In methanol solution, the spectrum of R-Dy1 shows a negative Cotton effect at about λmax = 253 and 313 nm, while S-Dy1 exhibits Cotton effects of the opposite sign at the same wavelengths. The results reflect that the optical activity of these compounds originates from the chiral ligands, and demonstrate the chirality is successfully transferred from the ligand to the coordination environment of Ln centers.
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In conclusion, we have reported a rare series of enantiopure mononuclear lanthanide complex pairs obtained with the selfassembly process promoted by chiral macrocyclic ligands and thiocyanate. All of the complexes are characterized by X-ray crystallography and show similar saddle-type structures with seven-coordinated lanthanide ion. Magnetic measurements revealed that the Dy complex exhibits field-induced SIM behaviour. The mirror images of CD and VCD spectra verify that the chiral information has been transfer from each enantiopure macrocyclic ligand to the stereochemistries of the LnIII metal centres. These results are expected to shed some light on new multifunctional molecular materials for use in a new generation of miniaturized devices. More work to construct new chiral molecular-based magnet is underway in our laboratory.
Acknowledgment
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We thank the National Natural Science Foundation of China (Grants 21331003, 21241006, 21221061, and 21371166) for financial support.
Notes and references This journal is © The Royal Society of Chemistry [year]
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State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, P. R. China. E-mail: [email protected] b Key Laboratory of Photonic and Electric Bandgap Materials, Ministry of Education, School of Physics and Electronic Engineering, Harbin Normal University, Harbin 150025, P. R. China. 1.
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(a) A. Dei and D. Gatteschi, Angew. Chem. Int. Ed., 2011, 50, 1185211858; (b) M. N. Leuenberger and D. Loss, Nature, 2001, 410, 789793; (c) S. Hill, R. S. Edwards, N. Aliaga-Alcalde and G. Christou, Science, 2003, 302, 1015-1018; (d) M. Hosseini, S. Rebic, B. M. Sparkes, J. Twamley, B. C. Buchler and P. K. Lam, Light: Sci. Appl., 2012, 1, e40. 2. (a) R. Sessoli, H. L. Tsai, A. R. Schake, S. Wang, J. B. Vincent, K. Folting, D. Gatteschi, G. Christou and D. N. Hendrickson, J. Am. Chem. Soc., 2002, 115, 1804-1816; (b) M. Mannini, F. Pineider, P. Sainctavit, C. Danieli, E. Otero, C. Sciancalepore, A. M. Talarico, M.-A. Arrio, A. Cornia, D. Gatteschi and R. Sessoli, Nat. Mater., 2009, 8, 194-197; (c) M. Gu, X. Li and Y. Cao, Light: Sci. Appl., 2014, 3, e177. 3. (a) N. Papasimakis, S. Thongrattanasiri, N. I. Zheludev and F. J. Garcia de Abajo, Light: Sci. Appl., 2013, 2, e78; (b) S. Sanvito, Chem. Soc. Rev., 2011, 40, 3336-3355; (c) L. Bogani and W. Wernsdorfer, Nat. Mater., 2008, 7, 179- 186. 4. G. Cucinotta, M. Perfetti, J. Luzon, M. Etienne, P.-E. Car, A. Caneschi, G. Calvez, K. Bernot and R. Sessoli, Angew. Chem. Int. Ed., 2012, 51, 1606-1610. 5. J. Tang, I. Hewitt, N. T. Madhu, G. Chastanet, W. Wernsdorfer, C. E. Anson, C. Benelli, R. Sessoli and A. K. Powell, Angew. Chem. Int. Ed., 2006, 45, 1729-1733. 6. (a) R. J. Blagg, C. A. Muryn, E. J. L. McInnes, F. Tuna and R. E. P. Winpenny, Angew. Chem. Int. Ed., 2011, 50, 6530–6533; (b) R. J. Blagg, L. Ungur, F. Tuna, J. Speak, P. Comar, D. Collison, W. Wernsdorfer, E. J. L. McInnes, L. F. Chibotaru and R. E. P. Winpenny, Nat Chem, 2013, 5, 673-678. 7. J. D. Rinehart, M. Fang, W. J. Evans and J. R. Long, J. Am. Chem. Soc., 2011, 133, 14236–14239. 8. (a) N. Ishikawa, M. Sugita, T. Ishikawa, S. Koshihara and Y. Kaizu, J. Am. Chem. Soc., 2003, 125, 8694-8695; (b) N. Ishikawa, M. Sugita, N. Tanaka, T. Ishikawa, S. Koshihara and Y. Kaizu, Inorg. Chem., 2004, 43, 5498-5500; (c) N. Ishikawa, M. Sugita and W. Wernsdorfer, J. Am. Chem. Soc., 2005, 127, 3650-3651. 9. (a) S. D. Jiang, B. W. Wang, H. L. Sun, Z. M. Wang and S. Gao, J. Am. Chem. Soc., 2011, 133, 4730-4733; (b) M. Jeletic, P.-H. Lin, J. J. Le Roy, I. Korobkov, S. I. Gorelsky and M. Murugesu, J. Am. Chem. Soc., 2011, 133, 19286-19289. 10. (a) S. D. Jiang, B. W. Wang, G. Su, Z. M. Wang and S. Gao, Angew. Chem. Int. Ed., 2010, 49, 7448-7451; (b) N. F. Chilton, S. K. Langley, B. Moubaraki, A. Soncini, S. R. Batten and K. S. Murray, Chem. Sci., 2013, 4, 1719; (c) G.-J. Chen, Y.-N. Guo, J.-L. Tian, J. Tang, W. Gu, X. Liu, S.-P. Yan, P. Cheng and D.-Z. Liao, Chem.Eur. J., 2012, 18, 2484-2487; (d) G. J. Chen, C. Y. Gao, J. L. Tian, J. Tang, W. Gu, X. Liu, S. P. Yan, D. Z. Liao and P. Cheng, Dalton Trans., 2011, 40, 5579-5583. 11. (a) M. E. Boulon, G. Cucinotta, J. Luzon, C. Degl'innocenti, M. Perfetti, K. Bernot, G. Calvez, A. Caneschi and R. Sessoli, Angew Chem Int Ed 2013, 52, 350-354; (b) P.-E. Car, M. Perfetti, M. Mannini, A. Favre, A. Caneschi and R. Sessoli, Chem. Commun., 2011, 47, 3751–3753. 12. (a) E. Colacio, J. Ruiz, A. J. Mota, A. Rodriguez-Dieguez, S. Titos, J. M. Herrera, E. Ruiz, E. Cremades and J.-P. Costes, Chem. Commun., 2012, 48, 7916-7918; (b) F. Gao, M.-X. Yao, Y.-Y. Li, Y.-Z. Li, Y. Song and J.-L. Zuo, Inorg. Chem., 2013, 52, 6407-6416; (c) F. Gao, L. Cui, W. Liu, L. Hu, Y.-W. Zhong, Y.-Z. Li and J.-L. Zuo, Inorg. Chem., 2013, 52, 11164-11172; (d) F. Gao, L. Cui, Y. Song, Y.-Z. Li and J.-L. Zuo, Inorg. Chem., 2014, 53, 562-567. 13. (a) L. Sorace, C. Benelli and D. Gatteschi, Chem. Soc. Rev., 2011, 40, 3092-3104; (b) J. Zhang, K. F. MacDonald and N. I. Zheludev, Light: Sci. Appl., 2013, 2, e96; (c) D. Dai, J. Bauters and J. E. Bowers, Light: Sci. Appl., 2012, 1, e1.
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† Electronic Supplementary Information (ESI) available: Tables of selected bonds and angles, CShM values and relaxation fitting parameters (Tables S1-S3), figures of crystal structures (Fig. S1-S2) and magnetic measurements (Fig. S3-S8) and VCD spectra (Fig. S9). See DOI: 10.1039/b000000x/ 14. (a) D. P. Li, T. W. Wang, C. H. Li, D. S. Liu, Y. Z. Li and X. Z. You, Chem. Commun., 2010, 46, 2929-2931; (b) C. Train, M. Gruselle and M. Verdaguer, Chem. Soc. Rev., 2011, 40, 3297-3312; (c) X.-L. Li, C.-L. Chen, Y.-L. Gao, C.-M. Liu, X.-L. Feng, Y.-H. Gui and S.-M. Fang, Chem.-Eur. J., 2012, 18, 14632-14637. 15. (a) Y.-Y. Zhu, X. Guo, C. Cui, B.-W. Wang, Z.-M. Wang and S. Gao, Chem. Commun., 2011, 47, 8049-8051; (b) C. Train, R. Gheorghe, V. Krstic, L.-M. Chamoreau, N. S. Ovanesyan, G. L. J. A. Rikken, M. Gruselle and M. Verdaguer, Nat. Mater., 2008, 7, 729-734; (c) N. Hoshino, Y. Sekine, M. Nihei and H. Oshio, Chem. Commun., 2010, 46, 6117-6119; (d) W. Kaneko, S. Kitagawa and M. Ohba, J. Am. Chem. Soc., 2006, 129, 248-249. 16. (a) G. Novitchi, G. Pilet, L. Ungur, V. V. Moshchalkov, W. Wernsdorfer, L. F. Chibotaru, D. Luneau and A. K. Powell, Chem. Sci., 2012, 3, 1169-1176; (b) P.-H. Guo, J.-L. Liu, J.-H. Jia, J. Wang, F.-S. Guo, Y.-C. Chen, W.-Q. Lin, J.-D. Leng, D.-H. Bao, X.-D. Zhang, J.-H. Luo and M.-L. Tong, Chem.-Eur. J., 2013, 19, 87698773; (c) X.-L. Li, C.-L. Chen, H.-P. Xiao, A.-L. Wang, C.-M. Liu, X. Zheng, L.-J. Gao, X.-G. Yang and S. Fang, Dalton Trans., 2013, 42, 15317-15325; (d) F. Luo, Z.-W. Liao, Y.-M. Song, H.-X. Huang, X.-Z. Tian, G.-M. Sun, Y. Zhu, Z.-Z. Yuan, M.-B. Luo, S.-J. Liu, W.-Y. Xu and X.-F. Feng, Dalton Trans., 2011, 40, 12651-12655; (e) P.-H. Lin, I. Korobkov, W. Wernsdorfer, L. Ungur, L. F. Chibotaru and M. Murugesu, Eur. J. Inorg. Chem., 2011, 1535-1539; (f) L. Zhang, P. Zhang, L. Zhao, S.-Y. Lin, S. Xue, J. Tang and Z. Liu, Eur. J. Inorg. Chem., 2013, 1351-1357; (g) C.-M. Liu, D.-Q. Zhang and D.-B. Zhu, Inorg. Chem., 2013, 52, 8933-8940; (h) J. Liu, X.-P. Zhang, T. Wu, B.-B. Ma, T.-W. Wang, C.-H. Li, Y.-Z. Li and X.-Z. You, Inorg. Chem., 2012, 51, 8649-8651. 17. J. Lisowski, Inorg. Chem., 2011, 50, 5567-5576. 18. (a) M. J. Kobylka, J. Janczak, T. Lis, T. Kowalik-Jankowska, J. Klak, M. Pietruszka and J. Lisowski, Dalton Trans., 2012, 41, 1503-1511; (b) M. Paluch, K. Ślepokura, T. Lis and J. Lisowski, Inorg. Chem. Commun., 2011, 14, 92-95; (c) S. R. Korupoju, N. Mangayarkarasi, P. S. Zacharias, J. Mizuthani and H. Nishihara, Inorg. Chem., 2002, 41, 4099-4101; (d) S.-Y. Lin, C. Wang, L. Zhao and J. Tang, Chem. Asian J., 2014. 19. (a) M. Paluch and J. Lisowski, J. Alloys Compd., 2008, 451, 443-447; (b) M. Paluch, J. Lisowski and T. Lis, Dalton Trans., 2006, 381-388. 20. (a) R. R. Gagne, C. L. Spiro, T. J. Smith, C. A. Hamann, W. R. Thies and A. K. Shiemke, J. Am. Chem. Soc., 1981, 103, 4073-4081; (b) J. Gao, R. A. Zingaro, J. H. Reibenspies and A. E. Martell, Org. Lett., 2004, 6, 2453-2455. 21. (a) G. M. Sheldrick, SHELXS-97, Program for Crystal Structure Solution, University of Göttingen, Germany, 1997; (b) G. M. Sheldrick, SHELXL-97. Program for Crystal Structure Refinement, University of Göttingen, Germany, 1997. 22. (a) A. González-Alvarez, I. Alfonso, J. Cano, P. Díaz, V. Gotor, V. Gotor-Fernández, E. García-España, S. García-Granda, H. R. Jiménez and F. Lloret, Angew. Chem. Int. Ed., 2009, 48, 6055-6058; (b) M. J. Kobyłka, K. Ślepokura, M. Acebrón Rodicio, M. Paluch and J. Lisowski, Inorg. Chem., 2013, 52, 12893-12903. 23. J. Gregolinski and J. Lisowski, Angew. Chem.Int. Ed., 2006, 45, 6122-6126. 24. M. Llunell, D. Casanova, J. Cirera, P. Alemany and S. Alvarez, shape 2.1, 2013. 25. S.-Y. Lin, Y.-N. Guo, Y. Guo, L. Zhao, P. Zhang, H. Ke and J. Tang, Chem. Commun., 2012, 48, 6924-6926. 26. O. Kahn, ed., Molecular Magnetism, VCH Publishers, New York, 1993. 27. F. Habib, P.-H. Lin, J. r. m. Long, I. Korobkov, W. Wernsdorfer and M. Murugesu, J. Am. Chem. Soc., 2011, 133, 8830-8833. 28. (a) Y. Xu and C. Merten, Dalton Trans., 2013, 42, 10572-10578; (b) J. Sadlej, J. C. Dobrowolski and J. E. Rode, Chem. Soc. Rev., 2010, 39, 1478-1488.
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A rare series of enantiopure mononuclear lanthanide complex pairs were obtained by using chiral macrocyclic ligand and thiocyanate, where the CD and VCD spectra verify their enantiomeric nature. The Dy complex reveals to be a rare seven-coordinated lanthanide-based SIM encapsulated in a macrocyclic ligand.
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Chiral Mononuclear Lanthanide Complexes and the Field-induced Single-ion Magnet Behaviour of Dy Analogue Shuang-Yan Lin,a, b Chao Wang,a Lang Zhao,a Jianfeng Wu,a and Jinkui Tang*a 5
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Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX DOI: 10.1039/b000000x Three pairs of homochiral mononuclear lanthanide complexes, with general formula [LnH4LRRRRRR/SSSSSS(SCN)2](SCN)2·xCH3OH·yH2O (Ln = Dy (R/S-Dy1), Ho (R/S-Ho1) and Er (R/SEr1)), have been obtained via self-assembly between chiral macrocyclic ligands and respective thiocyanates, all of which show saddle-type conformation with seven-coordinated metal ion. Magnetic measurements revealed that the Dy complex shows the field-induced single-ion magnet behaviour, which is a rarely reported seven-coordinated lanthanide-based SIM encapsulated in a macrocyclic ligand. The absolute configuration of all enantiomers was determined by single crystal X-ray crystallography and confirmed by electronic CD and VCD spectra.
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Interest in single-molecule magnets (SMMs) continues to grow motivated by potential applications in quantum computing,1 highdensity memory storage devices2 and molecule spintronics.3 A significant effort has been dedicated to the preparation and study of new systems in search for a better understanding of the various phenomena that influence SMM behaviour. These studies have led to a flood of groundbreaking results based on lanthanide SMMs thanks to their large magnetic moments and large anisotropy,4 such as the vortex arrangement of the magnetic moments in dysprosium(III) triangles,5 the recorded relaxation energy barrier in polylanthanide alkoxide cage complexes,6 as well as the highest blocking temperature in an N23– radical bridged Tb2 complex.7 Another fascinating result was the observation of SMM behaviour in complexes comprising a single anisotropic lanthanide ion, which are also called single-ion magnets (SIMs). These SIMs play a crucial role in understanding the electronic structure nature of molecular nanomagnets due to the accessible structure design and the simplification in the analysis of local anisotropy in contrast to the complexity of a polynuclear system. Indeed since the discovery of magnet-like behaviour in the phthalocyanine double-decker Ln complexes ([LnPc2]),8 some relevant examples such as the organometallic double-decker compounds9 and the Dy-β-diketones series,10 and the DyDOTA complexes4, 11 have been emerged. Among them, most of 4f-center is eight-coordinate with D4d axial ligand-field symmetry, only a few cases are observed with lower symmetric coordination sphere.12 Moreover, SMMs and SIMs, as molecule-based magnets, provide unique opportunities for the assembly of crystal structures by the intellectual selection of suitable molecular building blocks that exhibiting peculiar functionalities in addition to and/or interacting with the magnetic properties, thus leading to This journal is © The Royal Society of Chemistry [year]
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the design of advanced multifunctional materials.13 So far, it has been successfully demonstrated that some SMMs can show simultaneously other physical properties such as ferroelectricity or chirality.14 In particularly, introduction of chirality in SMMs provides a great platform for the study of the cross-effects of circular dichroism (CD), vibrational circular dichroism (VCD) and magnetochiral dichroism (MChD).15 However, the number of the case is quite limited, especially lanthanide complexes;14a, 14c, 16 thus it is still a great challenge to synthesize more chiral lanthanide compounds to explore the relationship between structure and property.
Scheme 1. Structure of the H3LRRRRRR ligand. 60
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In fact, the stereoselective synthesis of enantiopure metal complexes is still a challenging task. The most fruitful method of enantio- and diastereoselective synthesis of chiral metal complexes is based on the use of enantiopure chiral ligands,14b, 17 because such chiral ligand can transfer chiral information into the entire system through coordination bonds. In this work, we select an elastic ligand, macrocyclic ligand H3LRRRRRR/SSSSSS (shown in Scheme 1), which not only has enough space in the cavity to bind three metal ions (transition ions or lanthanide ions),18 but also can [journal], [year], [vol], 00–00 | 1
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wrap only one lanthanide ion19 through adjusting its cavity. Mononuclear complexes [LnH4L(NO3)2]19 (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) have been reported with only La, Gd, Eu complexes analyzed by X-ray analysis, but their magnetic behaviour has remained unexplored. In the context, we have focused our attention towards the design of chiral mononuclear macrocyclic lanthanide complexes by using this enantiopure chiral macrocyclic ligand and studying their magnetic properties. Therefore, three pairs of enantiopure chiral macrocyclic complexes [LnH4LRRRRRR/SSSSSS(SCN)2](SCN)2·xCH3OH·yH2O (Ln = Dy, x = 5, y = 0 (R-Dy1); Dy, x = 4, y = 1 (S-Dy1); Ho, x = 5, y = 0 (R/SHo1) and Er, x = 5, y = 0 (R-Er1); Er, x = 4, y = 1, S-Er1)) have been synthesized, of which Dy derivative shows field-induced SIM behaviour.
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General. All starting materials were of A.R. Grade and were used as commercially obtained without further purification. 2, 6diformyl-4-methylphenol, H3LRRRRRR and H3LSSSSSS were prepared according to a previously published method.20 Elemental analyses for C, H, and N were carried out on a Perkin-Elmer 2400 analyzer. IR spectra (4000-300 cm-1) and vibrational circular dichroism (VCD) (1900-1000 cm-1) spectrum were measured using KBr pellets by a fourier transform infrared spectrometer nicolet 6700. All magnetization data were recorded on a Quantum Design MPMS-XL7 SQUID magnetometer equipped with a 7 T magnet. The variable-temperature magnetization was measured with an external magnetic field of 1000 Oe in the temperature range of 2-300 K. The experimental magnetic susceptibility data are corrected for the diamagnetism estimated from Pascal’s tables and sample holder calibration. The solution circular dichroism (CD) spectra were measured on a Biologic MOS-450 spectrometer at room temperature using quartz optical cells under the following conditions: scanning speed, 1 nm/0.1 s; bandwidth: 2 nm; sensitivity standard: 1000 mdeg. The obtained spectra were corrected by subtraction from the spectrum of methanol. X-ray Crystallography. Suitable single crystals were selected for single-crystal X-ray diffraction analysis. Crystallographic data were collected at 185(2)/296(2) K on a Bruker ApexII CCD diffractometer with graphite monochromated Mo Kα radiation (λ = 0.71073 Å). The structure was solved by direct methods and refined on F2 with full-matrix least-squares techniques using SHELXS-97 and SHELXL-97 programs.21 The locations of lanthanide ions were easily determined, and O, N, C and S atoms were subsequently determined from the difference Fourier maps. Anisotropic thermal parameters were assigned to all nonhydrogen atoms. The H atoms were introduced in calculated positions and refined with a fixed geometry with respect to their carrier atoms. Crystallographic data and refinement details are given in Table 1 and the select bond lengths and angles are listed in Table S1. CCDC 972387-972392 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Synthesis of S-Dy1: A solution of H3Lssssss (0.05 mmol) in 15
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mL CH3OH was added 0.05 mL 1 mol·L-1 HCl aqueous solution and Dy(SCN)3·6H2O (91.6 mg, 0.2 mmol). The resulting mixture was stirred for about 2 h at room temperature and then left unperturbed to allow for slow evaporation of the solvent. The colorless stick-shaped crystals of S-Dy1 were obtained. Yield: 27 mg (41 %, based on the ligand). Elemental analysis (%) calcd for DyC53H85N10O8S4: C, 49.69, H, 6.69, N, 10.93; found C, 50.08, H, 6.50, N, 11.57. IR (KBr, cm-1): 2941 (m), 2862 (w), 2061 (vs), 1605 (w), 1474 (vs), 1451 (s), 1304 (w), 1245 (m), 1163 (w), 1065 (w), 1014 (w), 877 (m), 800 (m), 760 (w), 499 (w). Synthesis of R-Dy1: The colorless stick-shaped crystals of complex R-Dy1 were obtained by following the same procedure as that described for complex S-Dy1 except H3LRRRRRR instead of H3Lssssss. Yield: 30 mg (46 %, based on the ligand). Elemental analysis (%) calcd for DyC54H87N10O8S4: C, 50.08, H, 6.77, N, 10.81; found C, 49.65, H, 6.28, N, 10.21. IR (KBr, cm-1): 2936(s), 2859 (m), 2055 (vs), 1613 (w), 1575 (w), 1474 (s), 1450 (s), 1307 (m), 1254 (s), 1164 (w), 1099 (w), 1065 (w), 1014 (w), 877 (m), 804 (m), 763 (w), 508 (w). Synthesis of S-Ho1: The pale-orange stick-shaped crystals of complex S-Ho1 were obtained by following the same procedure as that described for complex S-Dy1 except Ho(SCN)3·6H2O (0.2 mmol) instead of Dy(SCN)3·6H2O. Yield: 24 mg (37 %, based on the ligand). Elemental analysis (%) calcd for HoC54H87N10O8S4: C, 49.99, H, 6.76, N, 10.79; found C, 49.63, H, 6.51, N, 10.32. IR (KBr, cm-1): 2943 (m), 2862 (m), 2052 (vs), 1603 (w), 1476 (vs), 1451 (s), 1304 (m), 1245 (s), 1164 (m), 1065 (w), 1015 (w), 877 (m), 801 (m), 761 (w), 501 (w). Synthesis of R-Ho1: The pale-orange stick-shaped crystals of complex R-Ho1 were obtained by following the same procedure as that described for complex S-Ho1 except H3LRRRRRR instead of H3Lssssss. Yield: 27 mg (41 %, based on the ligand). Elemental analysis (%) calcd for HoC54H87N10O8S4: C, 49.99, H, 6.76, N, 10.79; found C, 49.71, H, 6.49 N, 10.33. IR (KBr, cm-1): 2942 (m), 2862 (m), 2053 (vs), 1613 (w), 1476 (vs), 1451 (s), 1305 (w), 1246 (m), 1164 (m), 166 (w), 1016 (w), 877 (m), 802 (m), 762 (w), 501 (w). Synthesis of S-Er1: The pink stick-shaped crystals of complex S-Er1 were obtained by following the same procedure as that described for complex S-Dy1 except Er(SCN)3·6H2O instead of Dy(SCN)3·6H2O. Yield: 22 mg (34 %, based on the ligand). Elemental analysis (%) calcd for ErC53H85N10O8S4: C, 49.51, H, 6.66, N, 10.89; found C, 49.79, H, 6.58, N, 11.23. IR (KBr, cm1 ): 2941 (m), 2862(m), 2051 (vs), 1614 (w), 1477 (vs), 1451 (s), 1305 (m), 1246 (s), 1164 (m), 1065 (w), 1015 (w), 877 (m), 802 (m), 762 (w), 501 (w). Synthesis of R-Er1: The pink stick-shaped crystals of complex R-Er1 were obtained by following the same procedure as that described for complex S-Er1 except H3LRRRRRR instead of H3Lssssss. Yield: 31 mg (48 %, based on the ligand). Elemental analysis (%) calcd for ErC54H87N10O8S4: C, 49.89, H, 6.75, N, 10.78; found C, 49.53, H, 6.56, N, 10.42. IR (KBr, cm-1): 2937 (m), 2861 (m), 2056 (vs), 1613 (w), 1476 (vs), 1450 (s), 1312 (m), 1254 (s), 1163 (m), 1066 (w), 1015 (w), 878 (m), 805 (m), 762 (w), 501 (w).
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Results and Discussion Table 1. Crystal data and structural refinement parameters. Compound Empirical formula Fw (g/mol) Temperature (K) Crystal system Space group a (Å) b (Å) c (Å) α (o) β (o) γ (o) V (Å3) Z, ρcalcd (Mg/m3) F(000) Rint R1, wR2 [I > 2σ (I)] R1, wR2 (all data) GOF Flack parameter
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R-Dy1 S-Dy1 R-Ho1 S-Ho1 C54H87DyN10O8S4 C53H85DyN10O8S4 C54H87HoN10O8S4 C54H87HoN10O8S4 1295.08 1281.05 1297.51 1297.51 185(2) 296(2) 185(2) 185(2) Triclinic Triclinic Triclinic Triclinic P1 P1 P1 P1 10.2956(7) 10.409(3) 10.297(1) 10.2833(8) 11.6900(8) 11.756(3) 11.6829(11) 11.6475(8) 13.6363(9) 13.650(4) 13.5696(13) 13.6139(10) 106.886(1) 106.550(5) 106.808(2) 106.800(1) 98.239(1) 98.526(6) 98.496(2) 98.131(1) 94.427(1) 94.961(6) 94.203(2) 94.398(1) 1541.95(18) 1568.9(8) 1533.9(3) 1533.38(19) 1, 1.395 1, 1.356 1, 1.405 1, 1.405 675 667 676 676 0.0243 0.0472 0.0238 0.0310 0.0523, 0.1322 0.0722, 0.1737 0.0507, 0.1310 0.0534, 0.1284 0.0533, 0.1335 0.0931, 0.1941 0.0508, 0.1313 0.0546, 0.1298 1.049 1.026 1.034 1.030 0.001(19) 0.064(19) 0.027(14) 0.022(18)
The flexible chiral macrocycle H3L reveals to be an ideal platform for the formation of a multitude of metal complexes, such mono- and tri-nuclear lanthanide(III) complexes as well as trinuclear transition metal complexes.18-19, 22 When chelating three Ln3+ ions, the macrocyclic ligand coordinates with O3N6 in tri-deprotonated form L3- (three phenolate oxygen atoms are deprotonated), and the ligands are relatively flat; when chelating only one Ln3+ ion, four amine nitrogen atoms in the ligand are blocked by protonation and the ligand coordinates with O3N2 (three phenolate oxygen atoms are deprotonated, but four amine nitrogen atoms are protonated), because the open cavity of the macrocyclic ligand is far too large to accommodate a single Ln ion. Thus the mononuclear complexes are highly screwy. However, the case is different from the macrocycle formed by the 2,6-diformylpyridine,23 which has to wrap very tightly around the Ln3+ ion in a helical fashion in order to form a nine-coordinate complex. Therefore, the 3 + 3 macrocycle based on 2,6diformylphenol (H3L) exhibits less helical twist in its monoLn(III) complexes in comparison with the Ln(III) complexes of the 3 + 3 macrocycle derived from 2,6-diformylpyridine.
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Fig. 1 Top: Enantiomeric pair of S-Dy1 and R-Dy1; Bottom: side view of molecular structure of R-Dy1, and the coordination polyhedra for Dy1 in R-Dy1.
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R-Er1 S-Er1 C54H87ErN10O8S4 C53H85ErN10O8S4 1299.84 1285.81 185(2) 185(2) Triclinic Triclinic P1 P1 10.2722(8) 10.2345(6) 11.6628(9) 11.6666(7) 13.4833(11) 13.5492(8) 106.821(1) 107.027(1) 98.773(1) 98.881(1) 94.062(1) 94.118(1) 1516.8(2) 1516.57(16) 1, 1.423 1, 1.408 677 668 0.0314 0.0336 0.0649, 0.1650 0.0577, 0.1320 0.0670, 0.1693 0.0600, 0.1344 1.043 1.017 0.00(2) 0.028(18)
Compounds R/S-Dy1, R/S-Ho1 and R/S-Er1 crystallize in the triclinic space group P1 and contain a single, complete mononuclear complex in the asymmetric unit. The X-ray crystal structures show that, in these complexes, the Ln(III) ion is encapsulated in the chiral macrocycle, which acts as a pentadendate ligand and coordinates with three phenol oxygen atoms and two amine nitrogen atoms (shown in Fig. 1 and S1). Two SCN- ions filled the coordination sphere of metal ion in axial positions on two sides of the macrocycle with the Ln1N7/N8 distances of 2.384(12)-2.447(19) Å and the N7-Ln1-N8 angles 162.0(3)-162.8(5)o (Table S1). The seven-coordinated LnIII ions are characterized by a distorted capped trigonal prism environment, as calculated using the SHAPE 2.1 software24 (Table S2). Unlike the relatively flat macrocycle in the Dy3Mac complexes,25 macrocycle in the mononuclear macrocylic complexes reported here is highly twisted and folded with saddletype conformation. Noteworthy, four amine nitrogen atoms of the ligand are protonated to block their coordination so as to adjust the cavity of the macrocycle. This case is reminiscent of the earlier described form in [LnH4L(NO3)2],19 however, the coordination number of the metal ion is different due to different axial anions: bidendate nitrate anions complete the coordination [journal], [year], [vol], 00–00 | 3
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sphere forming nine-coordinate Ln ion in [LnH4L(NO3)2]; while in [LnH4L(SCN)2] reported here, monodendate SCN- ions coordinate in axial positions forming the seven-coordinated Ln ion. In addition, as shown in Table S1, the Ln1-N5/N6 (N atoms from macrocyclic ligand) distances of 2.499(8)-2.545(11) Å are longer than the Ln1-O distances of 2.135(14)-2.245(14) Å and Ln1-N7/N8 (N atoms from SCN- ions) distances. Both Dy3Mac and mononuclear macrocylic complexes indicate remarkable flexibility of the macrocycle H3L, which can adjust its cavity size to fit the size of the bound metal ions. Additionally, the crystal packing pictures reveal that the molecules are stacked in the same manner with only one stereoisomer along the crystallographic axes (Fig. S2). The shortest intermolecular Ln···Ln distances are 10.2956(7) Å for RDy1, 10.297(1) Å for R-Ho1 and 10.2722(9) for R-Er1, which indicate the ligands protect almost completely the lanthanide ions.
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Fig. 2 Plots of χMT vs T for compounds R-Dy1, R-Ho1 and R-Er1 measured in an applied field of 1000 Oe.
Magnetic properties. Direct-current (dc) magnetic susceptibility studies of R-Dy1, R-Ho1 and R-Er1 were performed under a field of 1000 Oe in the temperature range 2−300 K. The plots of χMT versus T are shown in Fig. 2. At room temperature (300 K), χMT products for R-Dy1, R-Ho1 and R-Er1 are 13.81, 13.64, and 11.16 cm3Kmol-1, which are all slightly smaller than the expected values for uncoupled DyIII (4f9, 6H15/2), HoIII (4f10, 5I8) and ErIII (4f11, 4I15/2) ions of 14.17, 14.07 and 11.48 cm3Kmol-1, respectively. Upon cooling in all three cases, χMT values decrease gradually and more rapidly below 50 K down to minimum values of 12.67, 12.32 and 9.61 cm3Kmol-1 at 2 K, respectively. This behaviour is mainly due to the depopulation of the Stark sublevels of the lanthanide ion.26 The magnetizations variation for the three complexes R-Dy1, R-Ho1 and R-Er1 at different applied fields determined between 1.9 and 5 K are shown in Fig. S3-S5, and the corresponding maximum values reached are 5.17, 5.68, and 5.05 μB at 1.9 K and 70 kOe, respectively. The lack of saturation of magnetization at 70 kOe can be attributed to crystal-field effects and the low lying excited states, while the non-superposition of the isofield lines in the M versus H/T plots implies the presence of significant magnetic anisotropy.
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Fig. 3 Frequency dependence of the out-of-phase ac susceptibility under various applied fields at 1.9 K for R-Dy1. The solid lines are guides for the eyes.
Fig. 4 Frequency dependence of the out-of-phase ac susceptibility for RDy1 between 1.9 and 15 K at Hdc = 200 Oe. The solid lines are guides for the eyes.
Alternating current (ac) magnetic susceptibility measurements were performed for complexes R-Dy1, R-Ho1 and R-Er1 to probe the low-temperature magnetic relaxation behaviour of the anisotropic magnetic moments. At zero external field, no out-ofphase signal (χ″) for the ac susceptibilities are observed at frequencies of up to 999 Hz and at temperatures down to 1.9 K for all of the complexes (Fig. S6). The relaxation of SIMs can be influenced by quantum effects, and the application of a dc field may probably remove the degeneracy of the ±MJ energy levels and prevent the tunneling of electrons from the +MJ state to the – MJ state.27 Therefore, the variable frequency ac susceptibility at 2.0 K was measured with application of small dc fields for checking the quantum tunneling effects for compound R-Dy1. As shown in Fig. 3 and S7, the application of a small dc field indeed slows the magnetic relaxation sufficiently to afford a nonnegligible signal in the out-of-phase ac susceptibility (χ″) for R-Dy1, with well-resolved peak under 200 Oe dc field. Therefore, ac susceptibility measurements as a function of frequency were determined again under a dc field of 200 Oe. As shown in Fig. 4 and S7, both the χ′ and χ″ signals are strongly frequencydependent below 7 K, and good peak shapes are observed with the maximum in χ″ shifting to higher frequency with increasing temperatures.
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Fig. 7 Comparison of IR (bottom) and VCD (top) spectra observed for complex R/S-Dy1 based on a pressed KBr disk of the respective compound. Fig. 5 Cole−Cole plots between 1.9−9 K for R-Dy1 (Hdc =200 Oe). The solid lines represent the best fitting with the Debye functions.
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Relaxation times were extracted from the frequency-dependent data between 1.9 and 9 K based on τ = 1/ 2πν. A plot of ln τ versus 1/T is shown in Fig. S8, and an Arrhenius fit (τ = τ0 exp(Ueff/kT)) to the data gives the effective energy barrier Ueff/k = 34.5 K and τ0 = 1.1 × 10−6 s. The relatively low thermal energy barrier may be due to the low symmetry of the ligand field around the Dy ion, leading to a reduced single-ion axial anisotropy, which is ultimately responsible for the SIM behaviour. The data plotted as Cole−Cole plots in the form of χ″ versus χ′ show asymmetrical semicircle (Fig. 5), which were fitted using the Debye models. The fit results are summarized in Table S3, α values are less than 0.25, indicating that thermally activated relaxation process has a narrow distribution of relaxation time.
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On the other hand, vibrational circular dichroism (VCD),28 the extension of electronic CD into the infrared region, is a relatively new and powerful technique to monitor the conformation of a chiral molecule. Fig. 7 and S9 show the VCD and IR spectra of R/S-Dy1, R/S-Ho1 and R/S-Er1, respectively. As expected, the IR spectra of both enantiomers are almost the same, while the VCD spectra are nearly mirror images. More important, the VCD spectra show strong signals corresponding to the IR spectrum, indicating the reliability of the spectra and the chirality of the two enantiomers. The most pronounced feature of the VCD spectra is a strong negative or positive couplet centred at 1474 cm-1 and 1450 cm-1, which is assigned to the δ(C-H) mode of methylene and methyl groups of the ligand. The band at 1450 cm-1 is also attributed to the skeleton vibrations of benzene ring. The bands at 1307 and 1254 cm-1 are assigned to δ(O-H) and ν(C-O) mode of phenol. The weak band at 1163 cm-1 corresponds to ν(C-N) of imino group.
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Fig. 6 CD spectra of R/S-Dy1, R/S-Ho1, R/S-Er1 and H3LSSSSSS/ H3LRRRRRR in MeOH (10−5 M) at room temperature.
CD and IR, VCD Spectra. The optical activity and enantiomeric nature of three pairs of complexes R/S-Dy1, R/S-Ho1 and R/SEr1 were verified by their circular dichroism (CD) spectra (Fig. 6). In methanol solution, the spectrum of R-Dy1 shows a negative Cotton effect at about λmax = 253 and 313 nm, while S-Dy1 exhibits Cotton effects of the opposite sign at the same wavelengths. The results reflect that the optical activity of these compounds originates from the chiral ligands, and demonstrate the chirality is successfully transferred from the ligand to the coordination environment of Ln centers.
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In conclusion, we have reported a rare series of enantiopure mononuclear lanthanide complex pairs obtained with the selfassembly process promoted by chiral macrocyclic ligands and thiocyanate. All of the complexes are characterized by X-ray crystallography and show similar saddle-type structures with seven-coordinated lanthanide ion. Magnetic measurements revealed that the Dy complex exhibits field-induced SIM behaviour. The mirror images of CD and VCD spectra verify that the chiral information has been transfer from each enantiopure macrocyclic ligand to the stereochemistries of the LnIII metal centres. These results are expected to shed some light on new multifunctional molecular materials for use in a new generation of miniaturized devices. More work to construct new chiral molecular-based magnet is underway in our laboratory.
Acknowledgment
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We thank the National Natural Science Foundation of China (Grants 21331003, 21241006, 21221061, and 21371166) for financial support.
Notes and references This journal is © The Royal Society of Chemistry [year]
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State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, P. R. China. E-mail: [email protected] b Key Laboratory of Photonic and Electric Bandgap Materials, Ministry of Education, School of Physics and Electronic Engineering, Harbin Normal University, Harbin 150025, P. R. China. 1.
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† Electronic Supplementary Information (ESI) available: Tables of selected bonds and angles, CShM values and relaxation fitting parameters (Tables S1-S3), figures of crystal structures (Fig. S1-S2) and magnetic measurements (Fig. S3-S8) and VCD spectra (Fig. S9). See DOI: 10.1039/b000000x/ 14. (a) D. P. Li, T. W. Wang, C. H. Li, D. S. Liu, Y. Z. Li and X. Z. You, Chem. Commun., 2010, 46, 2929-2931; (b) C. Train, M. Gruselle and M. Verdaguer, Chem. Soc. Rev., 2011, 40, 3297-3312; (c) X.-L. Li, C.-L. Chen, Y.-L. Gao, C.-M. Liu, X.-L. Feng, Y.-H. Gui and S.-M. Fang, Chem.-Eur. J., 2012, 18, 14632-14637. 15. (a) Y.-Y. Zhu, X. Guo, C. Cui, B.-W. Wang, Z.-M. Wang and S. Gao, Chem. Commun., 2011, 47, 8049-8051; (b) C. Train, R. Gheorghe, V. Krstic, L.-M. Chamoreau, N. S. Ovanesyan, G. L. J. A. Rikken, M. Gruselle and M. Verdaguer, Nat. Mater., 2008, 7, 729-734; (c) N. Hoshino, Y. Sekine, M. Nihei and H. Oshio, Chem. Commun., 2010, 46, 6117-6119; (d) W. Kaneko, S. Kitagawa and M. Ohba, J. Am. Chem. Soc., 2006, 129, 248-249. 16. (a) G. Novitchi, G. Pilet, L. Ungur, V. V. Moshchalkov, W. Wernsdorfer, L. F. Chibotaru, D. Luneau and A. K. Powell, Chem. Sci., 2012, 3, 1169-1176; (b) P.-H. Guo, J.-L. Liu, J.-H. Jia, J. Wang, F.-S. Guo, Y.-C. Chen, W.-Q. Lin, J.-D. Leng, D.-H. Bao, X.-D. Zhang, J.-H. Luo and M.-L. Tong, Chem.-Eur. J., 2013, 19, 87698773; (c) X.-L. Li, C.-L. Chen, H.-P. Xiao, A.-L. Wang, C.-M. Liu, X. Zheng, L.-J. Gao, X.-G. Yang and S. Fang, Dalton Trans., 2013, 42, 15317-15325; (d) F. Luo, Z.-W. Liao, Y.-M. Song, H.-X. Huang, X.-Z. Tian, G.-M. Sun, Y. Zhu, Z.-Z. Yuan, M.-B. Luo, S.-J. Liu, W.-Y. Xu and X.-F. Feng, Dalton Trans., 2011, 40, 12651-12655; (e) P.-H. Lin, I. Korobkov, W. Wernsdorfer, L. Ungur, L. F. Chibotaru and M. Murugesu, Eur. J. Inorg. Chem., 2011, 1535-1539; (f) L. Zhang, P. Zhang, L. Zhao, S.-Y. Lin, S. Xue, J. Tang and Z. Liu, Eur. J. Inorg. Chem., 2013, 1351-1357; (g) C.-M. Liu, D.-Q. Zhang and D.-B. Zhu, Inorg. Chem., 2013, 52, 8933-8940; (h) J. Liu, X.-P. Zhang, T. Wu, B.-B. Ma, T.-W. Wang, C.-H. Li, Y.-Z. Li and X.-Z. You, Inorg. Chem., 2012, 51, 8649-8651. 17. J. Lisowski, Inorg. Chem., 2011, 50, 5567-5576. 18. (a) M. J. Kobylka, J. Janczak, T. Lis, T. Kowalik-Jankowska, J. Klak, M. Pietruszka and J. Lisowski, Dalton Trans., 2012, 41, 1503-1511; (b) M. Paluch, K. Ślepokura, T. Lis and J. Lisowski, Inorg. Chem. Commun., 2011, 14, 92-95; (c) S. R. Korupoju, N. Mangayarkarasi, P. S. Zacharias, J. Mizuthani and H. Nishihara, Inorg. Chem., 2002, 41, 4099-4101; (d) S.-Y. Lin, C. Wang, L. Zhao and J. Tang, Chem. Asian J., 2014. 19. (a) M. Paluch and J. Lisowski, J. Alloys Compd., 2008, 451, 443-447; (b) M. Paluch, J. Lisowski and T. Lis, Dalton Trans., 2006, 381-388. 20. (a) R. R. Gagne, C. L. Spiro, T. J. Smith, C. A. Hamann, W. R. Thies and A. K. Shiemke, J. Am. Chem. Soc., 1981, 103, 4073-4081; (b) J. Gao, R. A. Zingaro, J. H. Reibenspies and A. E. Martell, Org. Lett., 2004, 6, 2453-2455. 21. (a) G. M. Sheldrick, SHELXS-97, Program for Crystal Structure Solution, University of Göttingen, Germany, 1997; (b) G. M. Sheldrick, SHELXL-97. Program for Crystal Structure Refinement, University of Göttingen, Germany, 1997. 22. (a) A. González-Alvarez, I. Alfonso, J. Cano, P. Díaz, V. Gotor, V. Gotor-Fernández, E. García-España, S. García-Granda, H. R. Jiménez and F. Lloret, Angew. Chem. Int. Ed., 2009, 48, 6055-6058; (b) M. J. Kobyłka, K. Ślepokura, M. Acebrón Rodicio, M. Paluch and J. Lisowski, Inorg. Chem., 2013, 52, 12893-12903. 23. J. Gregolinski and J. Lisowski, Angew. Chem.Int. Ed., 2006, 45, 6122-6126. 24. M. Llunell, D. Casanova, J. Cirera, P. Alemany and S. Alvarez, shape 2.1, 2013. 25. S.-Y. Lin, Y.-N. Guo, Y. Guo, L. Zhao, P. Zhang, H. Ke and J. Tang, Chem. Commun., 2012, 48, 6924-6926. 26. O. Kahn, ed., Molecular Magnetism, VCH Publishers, New York, 1993. 27. F. Habib, P.-H. Lin, J. r. m. Long, I. Korobkov, W. Wernsdorfer and M. Murugesu, J. Am. Chem. Soc., 2011, 133, 8830-8833. 28. (a) Y. Xu and C. Merten, Dalton Trans., 2013, 42, 10572-10578; (b) J. Sadlej, J. C. Dobrowolski and J. E. Rode, Chem. Soc. Rev., 2010, 39, 1478-1488.
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A rare series of enantiopure mononuclear lanthanide complex pairs were obtained by using chiral macrocyclic ligand and thiocyanate, where the CD and VCD spectra verify their enantiomeric nature. The Dy complex reveals to be a rare seven-coordinated lanthanide-based SIM encapsulated in a macrocyclic ligand.
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Graphical Abstract
