Progressive Transformation between Two Magnetic Ground States for

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Progressive Transformation between Two Magnetic Ground States for One Crystal Structure of a Chiral Molecular Magnet Li Li,† Sadafumi Nishihara,†,‡,§ Katsuya Inoue,*,†,‡,§ and Mohamedally Kurmoo*,§,∥ †

Department of Chemistry, Graduate School of Science, Hiroshima University, 1-3-1, Kagamiyama, Higashi Hiroshima, Hiroshima 739-8526, Japan ‡ Institute for Advanced Materials Research, Hiroshima University & Natural Science Center for Basic Research and Development, 1-3-1, Kagamiyama, Higashi Hiroshima, Hiroshima 739-8526, Japan § Center for Chiral Science, Hiroshima University, 1-3-1, Kagamiyama, Higashi Hiroshima, Hiroshima 739-8526, Japan ∥ Institut de Chimie de Strasbourg, Université de Strasbourg, CNRS-UMR 7177, 4 rue Blaise Pascal, 67008 Strasbourg Cedex, France S Supporting Information *

ABSTRACT: We report the exceptional observation of two different magnetic ground states (MGS), spin glass (SG, TB = 7 K) and ferrimagnet (FI, TC = 18 K), for one crystal structure of [{MnII(D/LNH2ala)}3{MnIII(CN)6}]·3H2O obtained from [Mn(CN)6]3− and D/La m i n o a l a n in e , i n c o n t r a s t t o o n e M G S f o r [ { M n I I ( L NH2ala)}3{CrIII(CN)6}]·3H2O. They consist of three Mn(NH2ala) helical chains bridged by MIII(CN)6 to give the framework with disordered water molecules in channels and between the MIII(CN)6. Both MGS are characterized by a negative Weiss constant, bifurcation in ZFC-FC magnetizations, blocking of the moments, both components of the ac susceptibilities, and hysteresis. They differ in the critical temperatures, absolute magnetization for 5 Oe FC (lack of spontaneous magnetization for the SG), and the shapes of the hysteresis and coercive fields. While isotropic pressure increases both Tcrit and the magnetizations linearly and reversibly in each case, dehydration progressively transforms the FI into the SG as followed by concerted in situ magnetic measurements and single-crystal diffraction. The relative strengths of the two moderate MnIII−CN−MnII antiferromagnetic (J1 and J2), the weak MnII−OCO−MnII (J3), and Dzyaloshinkii−Moriya antisymmetric (DM) interactions generate the two sets of characters. Examination of the bond lengths and angles for several crystals and their corresponding magnetic properties reveals a correlation between the distortion of MnIII(CN)6 and the MGS. SG is favored by higher magnetic anisotropy by less distorted MnIII(CN)6 in good accordance with the Mn−Cr system. This conclusion is also born out of the magnetization measurements on orientated single crystals with fields parallel and perpendicular to the unique c axis of the hexagonal space group.

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INTRODUCTION

produced several additional novel properties not encountered in magnetic materials.4−9 Magnets crystallizing in chiral space groups have an additional possibility of nonlinear but chiral organization of moment directions,10−17 which has resulted in the direct observation of theoretically predicted chiral solitons4d,18 and skyrmions19 for some metallic systems. The eventual physics depends on the crystal symmetries where those with cubic, trigonal, and hexagonal space groups are more likely to develop the above novelties than the orthorhombic ones.20 Following our directed efforts over numerous past years in the development of chiral magnets containing isotropic MnII (d5, S = 5/2, L = 0) and weakly anisotropic CrIII (d3, S = 3/2, L = 3), we have decided to elevate the magnetic anisotropy by replacing CrIII with the more anisotropic MnIII (d4, S = 2/2, L = 1).21 Here, we report the syntheses, crystal structures, and

Magnetic materials based on metallic elements and their oxides have been a major source of magical amazement and also controversies that have taken many years to be scientifically resolved.1 Understanding of such unknowns can be put to the developments of the knowledge acquired about the alignment of the moments within these materials, the forces between several magnets, and the evaluation of the energies involved in the preference of the ordering of moments using well-defined Hamiltonians.2 Whatever the terms responsible for the longrange magnetic order (LRO) are, one of the major activities in contemporary studies of magnetism is the involvement of the Dzyaloshinskii−Moriya (DM) interaction introduced by the spin and orbital anisotropies of the chemical constituents.3 New molecular materials belong to another set where the moments are derived from localized spin-states and the local anisotropies of the moment carriers can be chosen at the start in the chemical syntheses, and the materials developed so far have © XXXX American Chemical Society

Received: December 22, 2015

A

DOI: 10.1021/acs.inorgchem.5b02956 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

synthesized as described in the literature.26 Caution! Cyanidecontaining compounds are toxic and should be handled very carefully in only small amounts. Synthesis of [{Mn(L-NH2ala)}3{Mn(CN)6}]·3H2O.10 Crystals of D1 were grown by slow diffusion of K3[Mn(CN)6] (0.15 mmol) in H2O/ iso-propanol (1:1) into an aqueous solution containing MnCl2·4H2O (0.2 mmol), D-aminoalanine hydrochloride (D-NH2alaH.HCl, 0.3 mmol), and KOH (0.6 mmol) under an argon atmosphere at room temperature. After several days, black hexagonal cross-sectioned needle crystals of [{Mn(D-NH2ala)}3{Mn(CN)6}]·3H2O (D1) suitable for X-ray analyses were obtained. Characteristic IR data (KBr, cm−1): ν(O−H), 3526 vs; ν(N−H), 3332 s, 3270 s; ν(C−H), 2968 s, 2932 s, 2885 s; ν(CN), 2134 s, 2116 s, 2085 s, 2042 s; δ(C−H), 1470− 1300 s; ν(C−O), 1220−1100 s; ν(C−N), 1160−960 s; δ(N−H), 950−770 s (Figure S1). Synthesis of [{Mn(D-NH2ala)}3{Mn(CN)6}]·3H2O. Crystals of L1 were obtained in a similar way as for D1 but starting with L-NH2alaH· HCl. The crystal shape, size, and quantity were comparable to those of D1, and the IR bands are at the same energies and comparable intensities. It is worth noting that when the racemic DL-aminoalanine hydrochloride was used in place of the chiral ones, segregation of the enantiomers proceeded to separate crystals of D1 and L1. Synthesis of [{Mn(L-NH2ala)}3{Cr(CN)6}]·3H2O. Crystals of L2 were obtained by slow diffusion similar to those for D1.10 K3[Cr(CN)6] (0.15 mmol) in H2O/iso-propanol (1:1) was allowed to diffuse into an aqueous solution containing MnCl2·4H2O (0.2 mmol), L-NH2alaH· HCl (0.3 mmol), and KOH (0.6 mmol) under an argon atmosphere at room temperature. After several days, dark orange hexagonal crosssectioned needle crystals of [{Mn(L-NH2ala)}3{Cr(CN)6}]·3H2O (L2) suitable for X-ray analyses were obtained. Characteristic IR data (KBr, cm−1): ν(O−H), 3445 vs; ν(N−H), 3337 s, 3271 s; ν(C−H), 2932 s, 2887 s; ν(CN), 2134 s, 2041 s; δ(C−H), 1470−1300 s; ν(C−O), 1220−1100 s; ν(C−N), 1160−960 s; δ(N−H), 950−770 s. Physical Measurements. The X-ray diffraction intensities were collected for selected single crystals of both D1 and L1 using Bruker diffractometers−SMART-APEX II with a CCD and D8-QUEST with a CMOS area detector. Both employed graphite-monochromated Mo Kα X-ray radiation (λ = 0.71073 Å). Data reduction was made using SAINT and absorption correction by SADABS.27 The structures were solved by direct methods and refined by full-matrix least-squares against F2 of all data using SHELX-97.28 All the atoms were refined with anisotropic thermal parameters except for hydrogen atoms. Elemental analyses for C, H, and N were carried out using a PerkinElmer series II CHNS/O Analyzer 2400 (Table S1). TG-DTA measurements were performed on powder samples using a SII EXSTAR TGA operating under dry nitrogen with heating from 293 to 773 K at a rate of 5 K/min. Infrared spectra were recorded on a JASCO FT/IR-660 PLUS by transmission through KBr pellets in the range 400−4000 cm−1. X-band (∼9 GHz) EPR spectra were recorded using a Bruker ELEXSYS E500 in the field range from 2400 to 4400 Oe (Figure S2). The measurements of magnetization were carried out using Quantum Design MPMS-5S and MPMS-2 SQUID magnetometers. The magnetic field can be varied from −5 to 5 T and the temperature in the range 2−300 K. The alternate current (ac) susceptibility measurements were carried out in the frequency range of 1−1500 Hz with an oscillating magnetic field of 3 Oe by use of a Quantum Design MPMS-7XL SQUID magnetometer. The magnetization measurements for D1-SG (10.42 mg) and L1-FI (2.41 mg) in 5 Oe under a hydrostatic pressure up to 8.6 kbar were carried out using a piston−cylinder-type Cu−Be cell. The superconducting critical temperature of a piece of Sn (99.9999%) was used to monitor the pressure in the low temperature regime. (Further details are given in the SI.)

magnetic properties of the unknown optical enantiomers [{MnII (D-NH 2ala)} 3{Mn III(CN) 6 }]·nH2 O and [{MnII(LNH2ala)}3{MnIII(CN)6}]·nH2O, where NH2ala is aminoalanine (Scheme 1) and their reported analogue, [{Mn II (LScheme 1. Molecular Structure of Aminoalanine (D- or LNH2alaH) and Its Bonding Involvement within the MnII Chain

NH2ala)}3{CrIII(CN)6}]·3H2O, is a reference,10,22 as well as their dehydrated and rehydrated forms. In addition, we report the magnetic properties under isotropic pressures. In the course of this work, we observed a striking difference in the magnetic critical temperature and the absolute magnetization for different batches of crystals of the same materials having topologically the same crystal structure. We therefore undertook a thorough crystallographic study of the crystals from each batch, but the structures were topologically all the same though with slight minor differences which at first sight appear to be within the experimental resolution. This was followed by the measurements of magnetization on the same single crystals used for crystallography in two cases. After close examination of the crystallographic parameters, certain trends start to surface that are significantly larger than the standard errors. The most prominent one is the contraction (∼0.02 Å) of the MnIII(CN)6 octahedron along the c axis. We therefore explored the pressure dependence of the magnetic properties and found a tendency for the SG toward the FI while the Curie temperature of the latter increases. We further speculated that the magnetic properties might be related to the amount of water in the lattice given that the difference in behavior appears to be related to the period of the year the syntheses were performed, which has a large variation of temperature in Japan. Therefore, we studied the crystal structures and the magnetic properties upon dehydration and rehydration. The correspondence between crystallography and magnetic properties emerges, and a correlation was then established. The intermediate samples have the corresponding parameters between these limits. We note that it is the first time two ground states, SG and FI, have been observed for topologically the same crystal structure and have controllable progressive transformation between these two states. It contrasts with Co3(OH)2(C4O4)2· 3H2O,23 which is an antiferromagnet (AF) when hydrated and transformed to a ferromagnet (F) at almost the same transition temperature upon dehydration and does not have progressive intermediates. Due to the complexity of the results and to ease comprehension for the readers, we will present the crystal structures first with some comments of the magnetism and then develop in the following the magnetic properties observed before discussing the effects of pressure24 and dehydration25 and finally conclude with structure−magnetic property relationships.

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RESULTS AND DISCUSSION Syntheses. The differences in magnetic behaviors mentioned above were first thought to be due to different crystallographic phases, and thus crystals from the three

EXPERIMENTAL SECTION

General Information. All chemicals and solvents used during the syntheses of 1 and 2 were of reagent grade and commercially available. The starting materials K3[Mn(CN)6] and K3[Cr(CN)6] were B


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Inorganic Chemistry Table 1. Selected Crystallographic Data for the Eight Crystal Structures Determineda a, Å c, Å V, Å3 R1 wR2 Flack factor C4−N3, Å C5−N4, Å N2−Mn1−N4, deg N3−Mn1−O1, deg C5−Mn2−C5, deg C4−Mn2−C5, deg C5−N4−Mn1, deg N4−C5−Mn2, deg

D1-SG

L1-SG

L1-SG*

L1-IM1

L1-IM2

L1-FI′

L1-FI″

L1-FI*

13.5263(9) 8.0567(5) 1276.57(14) 0.0190 0.0564 0.036 (18) 1.152(3) 1.154(3) 96.61(7) 89.99(8) 95.63(9) 173.80(9) 143.4(2) 172.0(2)

13.5068(8) 8.0676(5) 1274.62(13) 0.0179 0.0518 0.036 (14) 1.156(2) 1.153(2) 96.58(6) 90.12(7) 95.58(7) 173.82(7) 143.49(16) 171.94(17)

13.5114(14) 8.0589(8) 1274.1(2) 0.0184 0.0536 0.040 (16) 1.156(3) 1.155(2) 96.65(7) 90.36(7) 95.84(8) 173.71(8) 143.53(18) 172.00(19)

13.5324(5) 8.0668(3) 1279.33(8) 0.0225 0.0533 0.022 (18) 1.156(3) 1.151(3) 96.40(7) 89.71(8) 95.93(10) 173.52(10) 143.8(2) 172.0(2)

13.5293(12) 8.0707(8) 1279.4(2) 0.0248 0.0609 0.03 (3) 1.148(5) 1.148(4) 96.34(11) 89.54(12) 95.87(16) 173.57(16) 144.2(3) 171.6(4)

13.5274(12) 8.0726(8) 1279.3(2) 0.0238 0.0618 0.04 (2) 1.145(4) 1.151(3) 96.03(9) 89.42(9) 96.33(11) 173.20(12) 145.0(2) 171.4(3)

13.5329(16) 8.0816(10) 1281.8(3) 0.0247 0.0652 0.06 (2) 1.144(4) 1.152(3) 95.79(9) 89.33(10) 96.48(12) 173.15(12) 145.2(3) 171.5(3)

13.5214(11) 8.0686(6) 1277.53(18) 0.0266 0.0552 0.04 (2) 1.151(4) 1.147(3) 96.11(9) 89.53(9) 96.13(12) 173.43(12) 144.7(2) 171.4(3)

a T = 293 K; crystal system = hexagonal; space group = P63; Z = 6. D1-SG and L1-SG are the single crystal from D- and L-forms of the SG. L1-FI′ and L1-FI″ are two different single crystals of the L-form of the FI. L1-SG* and L1-FI* are the same single crystals used for the single crystal magnetic measurements of SG and FI, respectively. L1-IM1 and L1-IM2 represent the two intermediate phases between SG and FI.

smaller ionic radius of Mn3+. Figure 1 shows the braiding of three helical MnII−NH2ala chains, their connections to a central MnIII(CN)6, and the projection of the structure of L1 along the c axis. Framework Structure. The key feature of this structure, which is similar to those of the MnII−CrIII enantiomers,10 is the aminoalanine behaving like an oxalate and imposing its chirality through five (Mn1O1C1C2N1) and six (Mn1O2C1C2C3N2) membered ring chelates in forming the helical Mn−NH2Ala chain. Three of these chains are interlocked, without any supramolecular interaction, to form triply braided helical tubes. Finally, three single helices of three adjacent tubes are then strongly bonded on their outside walls by the MnIII(CN)6 to generate the chiral 3D network (see Table 1, Scheme 1, and Figure 1). The remaining two chains of each tube are connected to symmetry related MnIII(CN)6. Two noncoordinated waters sit disorderly in channels of ∼4 Å diameter within the helical tubes. The third water molecule sits in the space between two MnIII ions along the c axis. Geometry of the MnIII(CN)63−. Since two different magnetic phases exist and the structures are topologically the same, the crystallographic data of eight single crystals of 1 from different batches prepared under slightly different conditions have been studied for comparison (Tables 1 and S2−S4). All eight single crystals of 1 show high crystallinity judging by the quality of the diffraction spots, the thermal parameters, and the resulting small R values that give more reliability of the high precision in the crystallographic data. Although the measured lattice parameters of the crystals are very close, we noticed that several bond lengths and bond angles are slightly different.21 The three crystals from different batches behaving as SG have the same parameters within experimental error, and so do the three crystals of FI, but their differences are larger than the standard deviations, demonstrating that the subtle structural differences between the two phases are real though small. Interestingly, the intermediate phases have average values for the crystallographic data, and the magnetization data are also in between those of SG and FI. The most remarkable differences between the two phases are (a) a ∼0.01 Å difference of the bond length of one CN bridge and (b) a ∼1.5° difference of the bond angle of C(5)−N(4)− Mn(1), which are several times the deviation; both are

batches (one of D1 and two of L1) were examined with negative results contrary to expectation. Second, we thought that the speed of crystal growth might be the determining parameter, so we performed a rapid synthesis giving mainly powder (D1-RS) whose magnetic properties behave closely to those of D1-SG and L1-SG. Third, taking note that the crystals of D1-SG and L1-SG were prepared during the summer months (August−September) of Japan and those of L1-FI during the winter months (December−March), we undertook crystal growing at two different temperatures of 5 and 0 °C (L1-IM1 and L1-IM2, respectively). These crystals have topologically the same structure but their magnetic properties, e.g., critical temperature and FC-magnetization (H = 5 Oe), were intermediate between those of L1-SG and L1-FI. Interestingly, there is no difference for the L2 sample of the Mn−Cr system to those reported.10 Crystal Structures. The crystal structures have been studied at 293 K on numerous single crystals, D1-SG, L1-SG, L1-IM1, L1-IM2, and L1-FI of the Mn−Mn system and one of L2 of the Mn−Cr system (Tables 1, S2−S4). Diffraction intensity data for the same D1-SG and L1-FI crystals were collected at 293 K when they had been subjected to dehydration in a flow of heated nitrogen gas and after being rehydrated in the air. The crystal of L2 was further studied as a function of temperature following the sequence (293 → 103 → 203 → 293 → 403 → 293 → 423 → 293 K) and after being rehydrated in the air at 293 K. Full data collections for L2 were obtained at these temperatures, and the cell parameters were monitored at every 10 K during the process. All the X-ray crystal structure determinations have been carried out at 293 K for direct comparison without having to worry about thermal effects. The results confirm that both D1 and L1 crystallize in the chiral hexagonal space group, P63, and their structures are exact mirror images of each other, as reported for D2 and L2 (Figure S3).10 We have redetermined the crystal structure of L2 under the same conditions for comparison. The crystal morphologies are all the same, and the enantiomers are not optically distinguishable. The crystallographic data for D1, L1, and L2 of different syntheses are listed in Table 1. Within the experimental precision, their crystal lattice parameters are the same except for the slight contraction of the volume of D1 and L1 compared to L2 because of the C


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Inorganic Chemistry

Figure 2. Compression of the [Mn(CN)6]3−. Note the elongation of the thermal ellipsoid of the nitrogen atoms along the c axis.

to 403 K for 1 h. After being cooled again to 293 K, intensity data were collected. The refinement qualities were similar to those of the original crystals indicating hardly any deterioration of the crystals or their mosaics upon temperature cycling, and the overall structures are identical except for minor changes in distances and angles and in the occupancy of the O3, O4, and O5 atoms from the lattice water molecules. The occupancies of these atoms were allowed to refine freely. According to the thermogravimetry measurement, the crystal loses almost half of the lattice water up to ∼400 K (Figure S4). Hence the formula for the original and dehydrated phases were set as C5H9Mn1M0.33N4O3 and C5H8Mn1M0.33N4O2.5 (M = Mn or Cr), respectively. And the molecular formula of dehydrated crystal from the X-ray analysis is very close to C5H8Mn1M0.33N4O2.5. The crystallographic data before and after dehydration are shown in Tables S5 and S7. After removing the crystal water, both the a and c axes are slightly decreased, and as a result the unit cell volume at 293 K decreases by ∼0.8% and ∼1.9% for 1 and 2, respectively. After leaving the dehydrated crystals under ambient conditions for several months, they reabsorb water to the original level that indicates the dehydration process is reversible in a single-crystal-to-single-crystal manner. Magnetic Properties. The magnetic properties have been studied on powder samples and on aligned single crystals of 1 (Figures 3, S5; Table S9). To our surprise, two distinct magnetic ground states at low temperature were observed, where one set of crystals behaves like spin glass (SG) and the other set behaves as a ferrimagnet (FI), and furthermore, some batches display properties in between those limits (Table 2). Both limiting phases and the intermediates crystallize in the same chiral hexagonal space-group P63, and the crystal lattice parameters are closely similar. The magnetic properties of 2 have been studied on a powder sample for comparison. Spin Glass. The product of the magnetic susceptibility χm and temperature of D1-SG in an applied magnetic field of 1 kOe decreases from 300 K to a minimum at 115 K, and on further cooling it increases gradually to a maximum at around 10 K. The inverse susceptibility versus temperature is linear in the range from 150 to 300 K as expected for the Curie−Weiss law with a Curie constant of 13.7(4) cm3 K mol−1 and Weiss temperature θ of −15.9(7) K. The negative Weiss constant indicates a considerable antiferromagnetic interaction. The Curie constant is slightly less than the theoretical value of 14.125 cm3 K mol−1 assuming g = 2 for MnII and MnIII. The isothermal magnetization at 2 K increases with a continuous curvature to 11.2 μB at 50 kOe, which is close to that for antiparallel alignment of the moments of three MnII and one MnIII (15−2 = 13 μB). It is still not saturated. It displays a hysteresis loop with a coercive field of 750 Oe and a

Figure 1. Crystal structure of L1 showing (a) three helical MnIINH2Ala chains (in blue, green, red) to form a tube housing the disordered water molecules (yellow), (b) three single helices from three adjacent tubes bonded to MnIII(CN)6 (cyan) through CN bridges, and (c) projection along the c axis showing three interlocked networks shown in b. Hydrogen atoms and water molecules were omitted for clarity.

concentrated on the cyanide bridge between magnetic ions. The slight shrinkage of the cyanide bond results in a compression of the [Mn(CN)6]3− along the c axis, and consequently, the bond angle C(4)−N(5)−Mn(1) becomes larger. Other bond lengths and angles in this fragment do not change. The MnIII(CN)6 adopts an asymmetric octahedron with the Mn being offset from the center and compressed along the c axis, where the bond lengths of MnIII−C(4) and MnIII− C(5) are 1.9788(16) Å and 1.9943(16) Å (Table S3), respectively. While the distance between the planes containing the three C4 and C5 carbon atoms are relatively constant, those containing the N3 and N4 nitrogen atoms vary by 0.03 Å (Figure 2). The two isosceles triangles defined by the three N3 atoms are rotated by as much as 5% with respect to that of the N4 atoms. These concerted distortions lift the degeneracy of the orbitals in a similar way to the uniaxial Jahn−Teller distortion. Dehydration and Rehydration Effects. The structural and magnetic properties after dehydration were studied on three single crystals selected from the same batches of D1-SG, L1-FI, and L2 (Tables S5−S8). The L2 crystal was temperature cycled from 293 to 100 and 423 K as mentioned above without noticeable damage. The crystals D1-SG and L1-FI were measured at 293 K to confirm the phase and then were heated D


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Inorganic Chemistry

Figure 3. Magnetic properties of the powder samples of D1-SG (left) and L1-FI (right): ZFC-FC dc-susceptibility in 5 Oe (top), ac-susceptibility as a function of frequencies (middle), and isothermal magnetization at 2 K (bottom).

Table 2. Magnetic Characteristics for D1-SG, L1-RS, L1-SG, L1-IM1, L1-IM2, L1-F, and L2 Ca Θb Tcritc Msatd H Ce MREMf

D1-SG

L1-RS

L1-SG

L1-IM1

L1-IM2

L1-FI

L2

13.65 −15.9 7.5 11.20 750* 0.58

13.60 −17.2 7.5 11.08 800 0.62

13.60 −16.7 8.0 11.49 330 1.2

13.07 −12.5 11.0 10.90 1120 1.59

12.95 −13.1 13.0 10.80 1500 1.69

13.21 −8.0 18 11.80 2000* 4.50

14.81 −17.1 34.5 12.02 50 0.95

a Curie constant/emu K mol−1. bWeiss constant/K. cCritical temperature/K based on FC magnetization in 5 Oe. dSaturation magnetization/μB at 2 K in 50 kOe. eCoercive field/Oe at 2 K (*from single crystal data). fRemanant magnetization/μB at 2 K.

remnant magnetization of 0.58 μB. The near zero magnetization after ZFC suggests a multidomain structure may be present. The very smooth and rounded form of the hysteresis would suggest that there is a progressive rotation of the moments, in contrast to a linear dependence if the ground state is ferrimagnetic. Similar magnetic behaviors have been observed for the L1 crystals. The zero-field-cooled (ZFC) and field-cooled (FC) magnetizations in the temperature range 2 to 30 K have been measured under a fixed external magnetic field set between 5 and 5000

Oe. A bifurcation is observed for measurement in low fields where both the ZFC and FC data display a peak at 7.5 K as shown for those measured in 5 Oe. We note that both decrease below the bifurcation temperature, suggesting there is no spontaneous magnetization. The slope is larger for the ZFC than for the FC data. The presence of such a difference would suggest that the crystals are fragmented into domains in agreement with the observation of a hysteresis. With increasing field, there are three effectsOne is the increase of the peak maximum temperature as it becomes more rounded. The E


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Inorganic Chemistry

Figure 4. Magnetic properties of an oriented single crystal of D1-SG (left) and of L1-FI (right): ZFC-FC dc-susceptibility in 5 Oe (top) and isothermal magnetization at 2 K (bottom).

positions of the maxima in χ′ and χ″ increase by ∼0.5 and ∼0.8 K, respectively, when the frequency is increased from 1 to 1000 Hz. The presence of the imaginary part is consistent with the existence of domain walls in a multidomain structure, in line with the presence of a bifurcation in ZFC-FC and hysteresis. However, the Cole−Cole plots for different temperatures do not follow a simple Debye relaxation behavior (Figure S6). An attempted fit of the frequency dependent data to the Arrhenius law (ν = ν0 exp(Ea/kBTf), where ν0 is the zero frequency constant and Ea the activation energy) gives ν0 = 7.8 × 1044 Hz and Ea = 6520 K, which are both physically unrealistic, and thus this compound cannot be considered as a superparamagnet30 or single-chain magnet31 where the Arrhenius law does indeed hold. Also, the sharp increase of the peak just above the blocking temperature is not consistent with the expected behavior of these two types of magnetism as well as the higher the frequency the higher is χ″. These variations lead us to suppose a spin-glass MGS for this compound.32 But we must state that such a highly crystalline molecular material is different to the metallic spin-glass systems where the moment carriers are diluted and crystallographically disordered.33 To test the occurrence of spin-glass, a microcrystalline sample of L1 was prepared by rapid mixing of the two solutions, K3[Mn(CN)6] (0.15 mmol) in H2O/iso-propanol (1:1) and MnCl2·4H2O (0.2 mmol), L-NH2alaH·HCl (0.3 mmol), and KOH (0.6 mmol) in water, under an argon atmosphere at room temperature. The solid formed was collected after 6 h. Its magnetic properties were similar to those for D1-SG. Since it was not suitable for single crystal X-ray analysis, we did not pursue with other characterization. It suffices to add that the rapid synthesis at room temperature leads to a material behaving as a SG. Ferrimagnet. Measurements of the magnetic properties on both powder samples and a single crystal of L1-FI have been

second is a lowering of susceptibility, and the third is the lowering of the bifurcation temperature to below 2 K at 5000 Oe. The first two are related to the increase of the correlation length with field, and the third may be indicating the tendency toward a single domain. One of the most important observations is the weak dependence of the susceptibility (assuming the linearity of M versus H) on field. This observation eliminates the possibility of a canted antiferromagnetic ground state, which may be considered as the most plausible.29 Given that the measurements so far have been inconclusive of the ground state of this peculiar material, we made measurement on a single crystal embedded in PMMA to prevent any torque motion in search for (a) the magnetic axes (easy and hard) and (b) the magnetic dimensionality of the system, where both are expected to be driven by the magnetic anisotropy of the MnIII ion (Figure 4). The temperature dependence of the ZFC-FC magnetizations along the c axis and ab plane show similar behaviors, but the absolute value is quite different where (χ∥c/χ∥ab)max ∼ 4, which indicates gc ∼ 2gab before the blocking temperature. It is to be noted that for both orientations the ZFC and FC magnetizations display a bifurcation at 7.5 K as seen for the powder sample. Also, the three effects noted for the powder, as the field is increased, are similarly observed for the single crystal along the two directions. The isothermal magnetization at 2 K is slightly higher for H∥c than for H∥ab, suggesting that the c axis is the easy axis, while ab is the hard plane. Temperature dependence of the in-phase (χ′) and out-ofphase (χ″) ac susceptibilities on a powder sample was measured using an oscillating magnetic field of 3 Oe with increasing temperature after cooling the sample in zero-field. Both components of the ac susceptibilities exhibit a peak near 7.5 K, and each display a weak frequency dependence where the F


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Inorganic Chemistry

Figure 5. Top: ZFC-FC magnetizations for L1-IM1, L1-IM2 in field of 5 Oe. Bottom: Isothermal magnetizations at 2 K.

ferrimagnetic long-range order. The peak χ′m is at 15.6 K and increased to 16.0 K by varying the frequency from 1 to 1500 Hz. On cooling, χ′m and χ″m exhibit a rapid increase of magnetization followed by a shoulder before tailing slowly below 10 K. These changes are indications of the formation of domain structures.34 We note the weakening of χ′m and χ″m as a function of frequency as commonly observed for ordered magnets as for example for the ferrimagnet [MnII(S/Rpn)(H2O)][MnIII(CN)6]·2H2O (pn =1,2-diaminopropane).21 Intermediates. In an attempt to mimic the winter conditions, we performed two syntheses by controlling the temperature at 5 and 0 °C in a fridge. Although good quality crystals suitable for X-ray crystallography were obtained at both temperatures, their magnetic properties were not the same as the FI. After studying them, we thought the unavoidable vibration from the fridge motor led to moderately rapid crystallization and failed to reproduce the synthesis of FI. Interestingly, the magnetic measurements show the transition temperatures, and the magnetization values of these samples were intermediate between those of the SG and FI (Figure 5). We named these two intermediate phases L1-IM1 and L1-IM2, respectively. The high temperature susceptibilities of these samples were similar to those found for D1-SG and L1-FI. The ZFC-FC magnetizations of L1-IM1and L1-IM2 were found intermediate to those of D1-SG, L1-SG, and L1-FI. They differ from those of D1-SG and L1-SG by the rounded peaks in both ZFC and FC instead of sharp peaks and to those of L1-FI where the FC data display a peak unlike leading to saturation for the latter. The isothermal magnetizations at 2 K were also intermediate in the shape and coercive fields. In terms of a sequence for all of these compounds judging from the critical transition temperatures and FC magnetization at the peak, it is D1-SG < L1-RS < L1SG < L1-IM1 < L1-IM2 < L1-FI. Pressure Effects. As the crystal structure determinations suggest the two MGS may be driven by the distortion of the MnIII(CN)6, we performed the magnetization measurements as a function of pressure. Using a Cu−Be pressure cell of 8 mm diameter, we were able to tune the pressure from 1 bar to ∼9 kbar on powdered samples held in Daphne oil (Figure 6). FC magnetizations in 5 Oe applied field were measured for D1-SG and L1-FI. We anticipated that pressure can tune the distortion of MnIII(CN)6 in line with the X-ray data as well as tuning the exchange interactions. Thus, a transformation between the two MGS may be observed. But these anticipations were not fully realized, though a tendency of the SG being transformed partially to the FI was observed which was reversible. Thus, on increasing the pressure form 1 bar to 8.3 kbar for D1-SG, the

carried out under similar conditions as above. The high temperature χmT data in 1 kOe behave similar to those of D1SG (Figure 3), which decreases from room temperature to a minimum at 100 K and increases sharply to a maximum at around 11.5 K. Fitting the susceptibility to the Curie−Weiss law in the range from 150 to 300 K (χm−1−T plot) gives a Weiss temperature of θ = −8(1) K, which is about half that of the SG (−15.9(7) K), and a Curie constant of 13.21(6) cm3 K mol−1, which is close to the theoretical value of 14.125 cm3 K mol−1, which indicates there is no change of the valence of the manganese between the two phases. The ZFC-FC magnetization has a distinctly different behavior to that of the SG. It has a close to zero magnetization after ZFC that suggests a multidomain structure is present. There is a broad maximum in the ZFC and a bifurcation with the FC at a higher temperature of 18 K, while the FC data show no decrease in magnetization on cooling. To be noted is the sharp change of magnetization at 18 K that is the signature of an ordered magnet, that is, a ferrimagnet in the present case, and the much larger magnetization (some 20 times) than the SG. The isothermal magnetization at 2 K increases rapidly to 11.8 μB at 50 kOe, which is close to that for antiparallel alignment of the moments of 3MnII and 1MnIII (15−2 = 13 μB). Magnetic hysteresis was observed, but the coercive field cannot be determined because of the motion of particles in the sample. To prevent torque motion in the search for (a) the magnetic axes and (b) the correct coercive field, we also made measurements on one selected single crystal of L1-FI embedded in PMMA (Figure 4). The temperature dependence of the FC magnetizations along the c axis and ab plane has a strong and abrupt rise in magnetizations below 18 K featuring the long-range magnetic ordering. This is confirmed by the remanant magnetization (MREM) that becomes very small just above 16.5 K. The values of magnetization in field H∥c-axis and H∥ab-plane are profoundly different. In addition, the isothermal magnetization at 2 K is slightly higher for H∥c than for H∥ab, which suggests that the c axis is the easy axis while ab is the hard plane. However, the magnetizations for H∥c and H∥ab just above TCurie are almost similar, indicating less anisotropy than that observed for the SG. Magnetic hysteresis was observed in both c axis and ab plane magnetization with a coercive field Hc of 2000 Oe. The rapid saturation along the easy axis confirmed the ferrimagnetism with possible collinear moments. The ac susceptibilities were measured using a 3 Oe oscillating magnetic field. Both the in-phase χ′m and the out-of-phase χ″m signals show a rapid increase below 18 K, indicating the G


Article

Inorganic Chemistry

needed under ambient conditions in air is very long, on the order of several months. It is also evident that the as-prepared samples are all intermediates given that TB for D1-SG can be reduced further from 7.5 to 6.5 K and its magnetization lowered by 60% upon dehydration for 10 h. These values remain the same for 15 h at 400 K. The isothermal magnetizations show smaller hysteresis loops (i.e., lower coercive fields and remanant magnetization), but they reach the same magnetization at the high field of 50 kOe for all degrees of dehydration. This demonstrates that the less water there is in the crystal, the more glassy magnetic behavior it shows. The amount of crystal water may change the compression of the [MnIII(CN)6] and alter the magnetic anisotropy. Magnetic Properties of L2. Some of the magnetic properties of 2 have been reported previously and are confirmed in the present work (Figure 8).10 The χmT of L2 measured in an applied field of 1 kOe behaves similarly to that of D1 and L1, where it decreases from 300 K to a minimum at 145 K, and with further cooling it increases sharply to a maximum at ca. 27 K. The susceptibility obeys the Curie− Weiss law in the range from 150 to 300 K with a Weiss temperature θ = −17.1(9) K. The negative Weiss constant indicates antiferromagnetic interaction. The Curie constant of 14.81 cm3 K mol−1 is very close to the theoretical value of 15.00 cm3 K mol−1 (g = 2). The ZFC-FC magnetizations in 5 Oe confirm the long-range ordering at TC = 35 K. The isothermal magnetization at 2 K increases to 12.02 μB at 50 kOe, which is close to that expected for a ferrimagnet with the moments of three MnII antiparallel to that of one CrIII (15 − 3 = 12 μB). No magnetic hysteresis can be observed because the two high spin MnII and CrIII ions are magnetically isotropic. Unlike 1, no other magnetic phase has been found for 2. Therefore, the different magnetic behaviors for 1 should be related to the magnetic anisotropy originating from the spin−orbit coupling of MnIII and, consequently, operation of the antisymmetric DM interaction. Dehydrating the sample at 400 K in a flow of He does not have a large effect on the Curie temperature, but the FC magnetization tends to lower values with increasing the treatment time. The hysteresis is hardly affected, but the saturation value is slightly lowered, which may arise from a slight increment of the g value of CrIII accompanying the contraction along the c axis of the unit cell upon dehydration mentioned above. Structure-Magnetic Properties Relationship. Considering first the connectivity of the moment carriers within the structure, one can define three next-neighbor exchange couplings (Figure S7), J1 (MnIII−CN−MnII, ∼5.05 Å), J2 (MnIII−CN−MnII, ∼5.21 Å), and J3 (MnII−OCO−MnII, ∼5.93 Å) for all compounds. Second, the strong ligand field of the six cyanides acting on the MnIII will result in a low spin state S = 1, while that of the four oxygen and two nitrogen atoms will generate a high spin S = 5/2 for MnII. The heavily distorted octahedra of the MnII can also generate a small magnetic anisotropy.35 Third, the MnIII is prone to a Jahn− Teller distortion and is expected to have an orbital contribution leading to a sizable magnetic anisotropy in contrast to a weaker one for CrIII in 2. Considering the bond multiplicities of the bridging ligands, the number of intervening atoms forming the bridges, and the distances between the metal centers, we expect the order of magnitude to be J1 > J2 ≫ J3. From the observed ferrimagnetic

Figure 6. (a) Temperature dependence of the FC magnetization (H = 5 Oe) of (top) D1-SG at different pressures between 1 bar and 8.3 kbar and (bottom) L1-FI from 1 bar to 8.6 kbar. (re-1 bar means release of pressure to 1 bar.) Insets show the pressure dependence of the critical temperatures. The sharp increase below 4 K is due to the superconducting Sn used to monitor the pressure.

sharp peak in its magnetization at TB became slightly rounded as those of the intermediate samples, TB increases from 7.5 to 9.5 K, and the value of magnetization at TB also increases. Varying the pressure from 1 bar to 8.7 kbar on a sample of L1FI, the magnetization at saturation does not change, but TC of the ferrimagnet increases from 18 to 25 K and is completely reversible on releasing the pressure. The critical temperatures of the two phases show linear dependence of pressure (Figure 6). The failure to cause a complete transformation may be due to the pressure being isotropic or it was not enough. Dehydration Effects. Failing to cause a complete transformation between the two MGS by pressure, we explored the possibility that dehydration may then exert a uniaxial contraction of the framework and therefore control the MGS. Therefore, both field and temperature dependences of the magnetizations were measured for D1-SG, L1-IM1, and L1IM2 after the samples have been kept at 400 K in a flow of He in the SQUID magnetometer for different periods up to 15 h (Table S10). L1-FI was not studied for the lack of sample due to the difficulty in reproducing the synthesis. The data are shown in Figure 7. There are three clear indications that the effect of dehydration is transforming all three samples progressively to the SG state. First, the magnetizations at the maxima are lowered as a function of the time of the dehydrating process. Second, the rounded shape of the magnetization peaks becomes sharp. Third, the critical temperature is lowered. Although the rehydration reverts these processes, the time H


Article

Inorganic Chemistry

Figure 7. ZFC-FC magnetizations in 5 Oe (left) and isothermal magnetization at 2 K (right) for powder samples of D1-SG (top), L1-IM1 (middle), and L1-IM2 (bottom) as a function of period of heating at 400 K in a flow of He.

Figure 8. ZFC-FC magnetizations for a magnetic field of 5 Oe at different treatment times (left) and the corresponding isothermal magnetization at 2 K (right).

of 3MnII contributing 15 μB and the other one of MnIII or CrIII with 2 or 3 μB, respectively (Figure S8). The values of the Weiss temperature becomes more negative on going from FI to SG, and dehydration decreases it further upon prolonging the period of heating at 400 K. This suggests that the exchange interactions are subtly changing relative to one another as may

nature of the temperature dependence magnetization and the values of the saturation moment below the critical temperatures, we can expect that the first two are antiferromagnetic and the latter can be ferromagnetic but weak (see later). It is therefore appropriate to consider that all the compounds consist of two magnetic antiparallel sublatticesone made up I


Article

Inorganic Chemistry

NH2ala)}3{CrIII(CN)6}]·3H2O. An extensive X-ray structure study of numerous crystals reveals that a subtle structural change involving the uniaxial distortion of MnIII(CN)6 and a rotation of the octahedron are responsible. Magnetic properties of several powder samples prepared under different conditions and of single crystals as a function of temperature, field, acfrequency, external pressure and period of dehydration highlight the following: (a) The two MGS are not related to the chirality. (b) There is short-range order ferrimagnetism. (c) The long-range ordering is hypothesized to be associated with the competition of one weak next-neighbor interaction, J3(MnII−O−C−O−MnII), and the antisymmetric DM exchange interaction originating from the orbital contribution of the MnIII(CN)6. (d) DM can be tuned by chemical pressure through dehydration as a function of time and not by isotropic external pressure. The absence of such behavior for the Mn−Cr system is that DM remains weak under all manipulations. A clear structure−magnetic properties relationship has been established between the critical transition temperature (TCurie or TBlocking) and the absolute magnetization in 5 Oe below that temperature as a function of the MnII−NC angle. This work has demonstrated how one subtle structural difference can have a large influence on the magnetic properties and also the importance of the antisymmetric DM exchange interactions in controlling the magnetic ground state not only between two fixed states but progressively.

be expected by the quite small changes in angles involved in the MnII−NC−MnIII linkage and the compression and rotation within the MnIII(CN)6 octahedron. However, J1 and J2 establish the short-range ferrimagnetism while J3 and DM control the long-range order (LRO) and the direction of the moments. Given that the latter two parameters are possibly of the same magnitude, the competition between them is likely to govern the overall MGS. We argue that when DM is small compared to J3 a sole stable ferrimagnetic order prevails, but when they are of similar magnitude the DM interaction will reorient the moment directions and leads to a multidimensional high degenerate spin-glass state. This is in good agreement with the crystallography study (Table S11) where the MnIII(CN)6 is more distorted for the FI than the SG, confirming the distortion lifts the degeneracy of the orbitals and less distortion results in high degeneracies and multiple minima potential surface and therefore creates the spin-glass state. This hypothesis is further justified as the degree of disorder of the MnIII(CN)6 can be separated into domains, and a continuous spectrum of MGS between SG and FI can exist, which is observed for the intermediate compounds. Furthermore, the large U33 compared to those of U11 and U22 is another indication of the range of the distortion that may exist in these compounds. The progressive dehydration of the more ferrimagnetic compounds into the spin-glass one is another clue in the jigsaw. However, in such high crystalline state materials, the presence of impurity can be easily precluded, and thus the subtle structural differences must be crucial for the change of the magnetism. The structure−magnetic properties relationship is shown graphically by plotting the maximum M/H values and the bifurcation temperature versus the C(5)−N(4)−Mn(1) angle as a collective representation of the distortion (Figure 9).

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b02956. Additional experimental procedures, IR, TGA, EPR, structure figures, and magnetism data (PDF) Crystallographic data (CIF) Crystallographic data (CIF) Crystallographic data (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by (a) a Grant-in-Aid for Scientific Research (S) (No. 25220803) “Toward a New Class Magnetism by Chemically controlled Chirality,” (b) the Center for Chiral Science in Hiroshima University (the MEXT program for promoting the enhancement of research universities, Japan), and (c) the JSPS Core-to-Core Program, A. Advanced Research Networks. L.L. acknowledges a JASSO student fellowship from MEXT, and M.K. is funded by the CNRS, France.

Figure 9. Correlation of (M/H)max values and the bifurcation temperatures with C(5)−N(4)−Mn(1) angles.

The compression of the [Mn(CN)6]3−, which causes the change in the geometry of the Jahn−Teller ion Mn3+ and thus changes the orbital anisotropy and reduces the asymmetric DM exchange interaction which competes with the J3 in stabilizing the different MGS. The absence of two such MGS for L2 due to the small orbital effect indirectly confirms the above hypothesis.

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CONCLUSION The rare and unique occurrence of two very different magnetic ground states, spin-glass and ferrimagnet, for basically one framework structure has been observed for the chiral [{MnII(D or L-NH2ala)}3{MnIII(CN)6}]·3H2O but not for [{MnII(L-

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Progressive Transformation between Two Magnetic Ground States for

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