A Chiral and Polar Single-Molecule Magnet - ACS Publications

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Article Cite This: Inorg. Chem. 2019, 58, 7236−7242

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A Chiral and Polar Single-Molecule Magnet: Synthesis, Structure, and Tracking of Its Formation Using Mass Spectrometry Wen-Jing Lang,† Mohamedally Kurmoo,*,§ and Ming-Hua Zeng*,†,‡

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Department of Chemistry and Pharmaceutical Sciences, Key Laboratory for the Chemistry and Molecular Engineering of Medicinal Resources, Guangxi Normal University, Guilin, 541004, P. R. China ‡ Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials, Ministry of Education Key Laboratory for the Synthesis and Application of Organic Functional Molecules & College of Chemistry & Chemical Engineering, Hubei University, Wuhan, 430062, P. R. China § Institut de Chimie de Strasbourg, CNRS-UMR7177, Université de Strasbourg, 4 rue Blaise Pascal, 67070 Strasbourg Cedex, France S Supporting Information *

ABSTRACT: The solvothermal reaction of cobalt(II) sulfate with S,S-1,2-bis(1-methyl-1H-benzo[d]imidazol-2-yl)ethane1,2-diol, (H2L), neutralized with triethylamine (Et3N) in a mixture of methanol and water (2:1), resulted in triangular red crystals of [CoII7(L)3(SO4)3(OH)2(H2O)9]·4H2O·3CH3OH (Co7). It is formed of chiral and polar clusters crystallizing in the R3 space group. Co7 consists of apex-shared asymmetric dicubane units where all of the metals adopt an octahedral coordination and the three ligands wrap diagonally around the unit. One end of the cluster is bonded by six water molecules and the other end by three monodentate sulfates. The head-totail packing through extended H-bonds leads to polar chains. The ligand has lost two protons, adopts a cis-conformation, and is coordinated to five metals around the waist of the dicubane. Electrospray ionization mass spectrometry (ESI-MS) of solutions of the reaction as a function of time reveals the possible step-by-step assembly process of the cluster: the initial product [CoII(HL)(SO4)]2− combines with CoSO4, forming [CoII2(HL)(SO4)2]2−, and then, upon addition of Et3N, dimerizes through a [OH]− bridge to [CoII4(HL)2(OH)(CH3OH)2(SO4)3]− followed by capture of one Co2+ and one CoSO4 to form [CoII6(L)2(OH)(CH3O)(SO4)4]2− before eventually binding to CoL to form [CoII7(L)3(OH)2(SO4)4]2−. These results allow us to propose a possible process for the formation of Co7, which is a good example for chiral multidentate chelating ligandcontrolled assembly of clusters. Magnetization measurements as a function of the temperature, field, and ac-frequency reveal ferromagnetic coupled moments and single-molecule magnetism (SMM).

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chiral ligand with the desired coordination functions.17−19 Hence, knowledge of the formation of the clusters in the solution chemistry is also desirable.18,20 We undertook a search for chiral single-molecule magnets in view of the recent attention that has been devoted to the design and synthesis of polynuclear chiral clusters and the lack of large clusters of cobalt(II) (Table S1). Up to now, only a few chiral cobalt clusters higher than seven have been characterized.14,18,19 Notably, introducing chirality in SMM structures provides a great platform in functionalizing them with an optical probe, and for the search of duality of functions, some examples had been reported with transition metals.19,22 Among them, the number of chiral Co(II) clusters acting as SMM is rather low.19,21,22 Therefore, it is necessary to synthesize such materials for further studies. The most prominent known clusters of cobalt(II) exhibiting single-

INTRODUCTION The search for multifunctional materials is now focusing on polynuclear transition-metal clusters.1−3 These activities are amplified by the ease of chemical tuning and design at the molecular level and through supramolecular interactions to the crystalline solids.1−4 This is more evident in the field of molecule-based magnetism where coordination complexes containing few magnetic centers, and also several with only one center, having the appropriate electronic and magnetic characteristics have been found to behave as magnets with the memory effect.4−10 By paying attention to the design and synthesis, an ever increasing number of coordination clusters capable of multifunction with electric, magnetic, optical, and structural responses have been reported.11−14 In this respect, chiral clusters form an ideal family.12−15 Chirality is characteristic of chemistry and biology, which by introduction into molecular magnetic materials is a desirable strategy for the assembly of multifunctional molecule-based magnets.1,12−19 The departing point in this strategy is the conception of a © 2019 American Chemical Society

Received: January 28, 2019 Published: May 15, 2019 7236

DOI: 10.1021/acs.inorgchem.9b00269 Inorg. Chem. 2019, 58, 7236−7242

Article

Inorganic Chemistry molecule magnetism (SMM) are those with the Co4-cubane, Co4-butterfly, and Co7-brucite disk structures.5,23,24 Only few are known with the Co7 apex fused dicubane as the structure reported here.4,21,25 In view of elucidating the process of formation of clusters from the raw materials, mass spectroscopy has grown into a powerful technique in deciphering the self-assembly process in solution.18,20 In a previous work, our choice of S,S-1,2-bis(1Hbenzimidazol-2-yl)-1,2-ethanediol (S,S-H4 bzimed) as a chiral ligand in the reaction with Co(NO3)2·6H2O and NaN3 successfully built chiral Co16 and Co4. Subsequently, we studied the chiral Co16 cluster assembly process from building blocks and successfully identified step-by-step substitution of the inner N3− bridges by either CH3O− or OH− by ESI-MS. The magnetic susceptibility of the former suggests superparamagnetic behavior. Here, our choice for the present study is its chiral methyl derivative S,S-1,2-bis(1-methyl-1H-benzo[d]imidazole-2-yl) ethane-1,2-diol which when reacted with CoSO4·7H2O gave one unique cluster, where the sulfates are asymmetrically coordinated to the clusters, which crystallize in a chiral and polar space group (R3). Electrospray ionization mass spectrometry (ESI-MS) reveals the possible step-by-step assembly process of the chiral cluster. It behaves as a singlemolecule magnet.

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Figure 1. Structure of Co7 (Co1, green; Co2, blue; Co3, purple; S, yellow; O, red; N, pale blue; C, gray; H, light gray). (a) View along the c-axis of a polar heptanuclear unit with sulfate on one end and water on the other. (b) The core with selected atom-labeling scheme. Some hydrogen atoms and guest molecules are omitted for clarity.

number six are bridged by oxygen atoms. The two cubanes are not symmetry related; that is, the two ends of each cube are made of two crystallographically independent cobalt atoms, Co1 and Co3, which thus resulted in the polar unit (Figure 1b). Each side has a Co3(OH) face where the octahedra are edge-sharing as in the brucite layer. The two (Co1)3(OH) and (Co3)3(OH) end units are connected by a central cobalt atom (Co2) through the oxygen atoms on the opposite face of the OH, but they are rotated with respect to each other, leading to the chiral nature of the structure and looking like a “Star of David”. The asymmetry is enforced further by the coordination of six water molecules to the outer face of Co3(OH) but by three water molecules and three apical connected sulfates to the other Co1(OH) face (Figure 1). Each ligand adopts a pseudo-cis-conformation around the central C−C and provides two nitrogen and two alkoxide oxygen atoms for coordination, which can chelate to five metals (Figure S3b). The cluster is asymmetric and assumes a polar arrangement of the disposition of the coordinating atoms. Overall, it looks like a rocket with a cylindrical body consisting of the Co octahedral with the sulfate as the exhausts and the aromatic moieties of the organic ligands acting as the stabilizing wings. These clusters are stacked along the c-axis and form a hexagonal lattice in the ab-plane (Figure 2a). Between the propeller blades of the ligands, the noncoordinated water and methanol molecules alternate in forming a trigonal spiral staircase with short H-bonds forming water−methanol pairs (Figure 2b). Another water molecule (O5W) is located in

EXPERIMENTAL SECTION

Materials and Measurements. Due to the common information of our previous works, the source of materials, synthesis of ligands, and the instrumentation used for measurements are given in the Supporting Information. Synthesis. To H2L (0.5 mmol, 161 mg) in CH3OH (10 mL) was added CoII(SO4)·7H2O (1 mmol, 280 mg) in H2O (5 mL), giving a heavy red precipitate after 5 min of stirring at room temperature. Addition of triethylamine (0.1 mL) partially dissolves the precipitate into a purple suspension that was sealed in a 25 mL Teflon-lined stainless steel autoclave and heated at 80 °C for 24 h. After the autoclave was cooled to room temperature at a rate of 10 °C·h−1, the red triangular shaped crystals that were formed (Figure S1) were washed several times with and stored in methanol and water. The synthesis is reproducible. Only a single phase is obtained which was confirmed by PXRD. Yield 246 mg, 85% (based on Co). Elemental analyses: Calcd for [Co7(C18H16N4O2)3(SO4)3(OH)2(H2O)9]·4H2O· 3CH3OH: C 33.79, H 4.38, N 8.30; Found (%): C 32.29, H 4.21, N 8.25. Selected IR (KBr cm−1): 3345(s), 3150−2850(w), 1659(m), 1488(m), 1330(m), 1082(s), 1075(s), 889(m), 828(m), 753(m), 605(m), and 426(w) (Figure S2). Alternatively, the reaction in capped glass sample bottles at 80 °C for 1 day gave clusters of crystals in similar yields. The latter samples were used only in PXRD and ESIMS measurements. Co7 was not obtained in the absence of sulfate.

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RESULTS AND DISCUSSION Crystal Structures. The compound crystallizes in the chiral and polar space group R3 of the trigonal system (Table S2). It consists of close-packing of the clusters, [Co7(L)3(SO4)3(OH)2(H2O)9], with considerable supramolecular contacts and solvents sitting in the space in between (Figure 1). The asymmetric unit contains three independent cobalt atoms (two of full occupancy and the other of onethird), one organic ligand, one sulfate, and two OH (one-third occupancy each) and three independent coordinated H2O as well as two H2O (one-third occupancy each) and a methanol solvent of crystallization (Figure S3a). The structure of the inner core of the cluster can be described as two cubanes fused at one apex where the seven cobalt atoms of coordination

Figure 2. (a) Packing diagram of Co7 in the ab-plane (Co1, green; Co2, blue; Co3, pink polyhedron). (b) A chiral chain of a water− methanol H-bonded pair as solvents between the clusters. Some hydrogen atoms are omitted for clarity. 7237

DOI: 10.1021/acs.inorgchem.9b00269 Inorg. Chem. 2019, 58, 7236−7242

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Inorganic Chemistry

departure of the noncoordinated solvents followed by a further stable state to 625 K where it is decomposed (Figure S5b). The high decomposition temperature is higher than the operating temperature of the ESI-MS, indicating that the results of the latter can be quite reliable. Compounds of 3d transition metals having the dicubane building units are quite sparse. There are only a few examples with independent dicubane (Table S5): 3 of Co(II), 3 of Ni(II), 4 of Cu(II), and 11 of Zn(II). Of the magnetic metals, Co7(OH)8(oxalate)3(piperazine)3 is a three-dimensional framework through oxalate and piperazine bridges, and consequently, they exhibit long-range antiferromagnetic ordering (TNeél = 26 K) which is driven by antiferromagnetic coupling between ferromagnetic Co7 clusters.25 The other three are isolated clusters, albeit with some weak supramolecular interactions, which behave as single-molecule magnets; the cobalt compounds have higher blocking temperatures than the nickel one. In this series, only one example of chiral and polar structure as the present case is reported.21 Electrospray Ionization Mass Spectrometry. The crystal of Co7 was partially dissolved in CH3OH, and the MS spectra of the solution were obtained in negative and positive modes under an in-source energy of 0 eV (Figure S6). All of the major peaks of the ESI-MS of Co7 can be assigned by their m/z values and isotopic distributions. In negative mode, the parent cluster fragment [CoII7(L)3(OH)2(SO4)4]2− has the highest intensity at m/z = 895.35. It is the whole Co7 cluster with an additional SO42− but all of the solvent molecules absent. In addition, other parent cluster fragments were observed at m/z = 902.37 ([Co II 7 (L) 3 (OH)(CH 3 O)(SO 4 ) 4] 2−) and 909.38 ([Co II 7(L) 3(CH3 O) 2(SO 4 ) 4] 2−) where the inner OH− bridges are replaced step by step by CH3O−. Some lower m/z species are also present at m/z 476.02 ([CoII(HL)(SO4)]−), 505.07 [CoII2CoIII(L)2(SO4)(OH)(CH3O)2(H2O)]2−, and 794.26 ([NaCoII(HL)2(OH)2(H2O)2]−) and their intensities are less than 2%, suggesting that the Co7 is relatively stable under ionization conditions. However, far from our expectation, some unexpected higher nuclear species were also observed, such as the ones at m/z 972.81, 1275.01, and 1325.50, which respectively match the following: [CoII8(L)3(OH)2(SO4)5]2−, [CoII8(L)4(SO4)5(H2O)7(CH3OH)6]2−, and [CoII8(L)5(SO4)4(H2O)9(CH3OH)]2− (Table S6 and Figure S7). The results prompted us to imagine that the smaller precursor fragment produced by the ESI-MS ionization process may undergo extended assembly processes, resulting in a variety of high nuclearity ions. In the positive mode, we do not find the parent cluster fragment; the highest intensity at m/z 351.61 is [H2CoII(HL)2]2+. Other fragments were observed, such as {Co(HL)}, {Co2 (L)}, {Co3 (L) 3}, {Co 4 (L) 3}, {Co4(L)4}, and {Co5(L)4}. It is important to note that Co7 is relatively stable in the negative mode compared to the positive mode, probably due to the capture of SO42− to enhance cluster stability. Therefore, we track the possible formation of the Co7 cluster in the negative mode from the mother liquor at different reaction times. In turn, six solutions of Co7 were prepared to investigate the formation process by ESI-MS (Figures S8 and S9). Following the addition of H2L to CoSO4 in the absence of Et3N, two prominent peaks were observed at m/z 476.02 ([CoII(HL)(SO4)]−) and 630.90 ([CoII2(HL)(SO4)2]−), suggesting that Co(HL) is the basic building unit for the construction of the cluster. By adding Et3N, new fragments were observed at m/z

cavities formed at the head-to-tail of the clusters within the stack (Figure 3).

Figure 3. Four types of H-bonds (blue bonds) existing between head (green, Co1)-to-tail (purple, Co3) packing, highlighting for clarity in three regions, and the intramolecular π−π interaction of the benzimidazole observed in Co7.

Numerous H-bonds are found within the cluster and between the clusters and the solvents of crystallization which lie in the 2.6−2.9 Å range. They are all located at the head-totail of adjacent clusters within a stack which form a Co−S−O cage (Figure S4). The water molecule (O5W) sits in this cage and is connected to the two hydroxyl groups of adjacent Co1(OH) and Co3(OH) at 2.67 and 2.75 Å, respectively. The sulfates make both intra- and intermolecular H-bonds where the O3W···O8 and O3W···O6 are 2.85 and 2.76 Å for the former and the O1W···O8 and O2W···O6 are 2.61 and 2.71 Å for the latter. It is additionally bonded to the methanol (O9) at 2.73 Å. H-Bonded intramolecular water pairs (O1W···O2W) are found at 2.79 Å. The lattice water molecule (O4W) is connected to the cluster through O1W−Co (2.77 Å) and to the methanol (O9) at 2.71 Å. These H-bonds add collectively to the stability of the compound and direct the stacking of the cluster into the chain formation (Figures 3 and S4 and Table S4). The benzimidazole moieties within each cluster are twisted so that the aromatic regions are closely overlapped at an average distance of 3.3 Å in a bond-over-ring fashion (Figure 3). There is no intermolecular interaction between chains. While the strong supramolecular interactions are localized within the stacks of the clusters, the crystalline structure is held together by weaker ones involving the solvents of crystallization. The Co−O and Co−N distances are all normal and lie within the experimentally observed range. The Co···Co distances are in the range 3.0−3.3 Å. The Co···Co distances which are relevant to the magnetism are normal to those bridged by two oxygen atoms. However, the angles can vary in the 97−104° range, which can favor either ferromagnetic or antiferromagnetic coupling depending on which side of the Goodenough−Kanamori crossover angle each pair sits (Table S3). Thus, there can be subtle changes in the magnetism which are dependent on these angles.26 Therefore, we expect the magnetic exchange interactions, J(Co1−Co2) and J(Co2− Co3), to be ferromagnetic, as has been found for several examples. It is important to note that DSC measurements indicate no phase transition between 100 and 300 K (Figure S5a). Thermogravimetry indicates a stability up to 350 K before the 7238

DOI: 10.1021/acs.inorgchem.9b00269 Inorg. Chem. 2019, 58, 7236−7242

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Inorganic Chemistry 712.84 ([CoII6(L)2(OH)(CH3O)(SO4)4]2−), 895.35 (the parent cluster [CoII7(L)3(OH)2(SO4)4]2−), and 1246.95 ([CoII4(HL)2(OH)(CH3OH)2(SO4)3]−). In such a short period of time, the parent cluster ion of Co7 had appeared, indicating that this complex cluster Co7 may be assembled quickly by fragments 1, 2, 3, and 4. After 0.5 h of reaction at 80 °C, all of these fragments already exist. After 1 h, the related fragments 3 and 4 disappeared in solution. After 12 h, fragments 2, 3, 4, and 5 disappeared from solution (Figures 4

chelating ligands, in contrast, the coordination capacity of sulfate is not as good as that of the ligands; sulfate may only simply participate in the terminal coordination and as a counteranion. Although at first sight the nucleation building unit Co(HL) has several possible forms and extensions that can complicate the self-assembly process, the ligand has two methyls leading the coordinating atoms to pointing on one side. Hence, we observed the structure and fragments suggest possibly the multidentate chelated of chiral ligand coordination driven orientation recognize step-by-step assembly process. The elucidation of the possible formation process of Co7 may thrive in the design and synthesis of coordination clusters, control of their structures, and finally their physical and chemical properties.18,20 The MS study of the mother liquor was complemented by a study of the PXRD of the solids formed (Figure 5). The PXRD

Figure 5. PXRD patterns of the solid products at different steps of the formation: stirring for 5 min at 25 °C in the absence of Et3N and in the presence of Et3N; treated at 80 °C for 0.5, 1, 5, and 12 h. They are compared to that simulated from the single crystal diffraction data.

Figure 4. (a) Key parts of the negative-mode ESI-MS spectra of solutions at different steps: stirring for 5 min at 25 °C in the absence of Et3N and in the presence of Et3N; treated at 80 °C for 0.5, 1, 5, and 12 h. (b) Proposed step-by-step assembly process.

results confirm those from the MS study which suggest the Co7 cluster is initially formed as soon as the Et3N is added which initiated first the neutralization of the acidic protons of the diol of the ligand and provided the hydroxide needed to form the Co3(OH) building unit. The solidification seems to produce a highly crystalline state of Co7, as judged by the highly resolved Bragg reflections even at fairly high angles. The good correspondence between the observed PXRD and that simulated from the single-crystal data indicates the principal crystalline product is the supramolecular network of Co7. The increasing intensity with time suggests the increased quantity of Co7 in the solid, while the lack of other reflections may be interpreted as the other products not being crystalline. Furthermore, the sharpening of the Bragg reflections with time means an increase in crystallinity. The complementarity of the results of MS and PXRD together with those of elemental analyses suggests the Co7 is the unique product. Magnetic Properties. The red crystals (31.1 mg) were wrapped in cling film and positioned on a brass holder for magnetization measurements using a Quantum Design VSMSQUID instrument at Nanyang Normal University (China).

and S11). In contrast, fragment 5 becomes stronger in sediment, indicating that all of the Co7 separated out from the solution by crystallization (Figure S10 and Table S7). The species present in the solutions are also present in the sediments, indicating that the fragments forming the solid exist in solution, and it is thus possible to follow the reaction and provide a mechanism for the formation of the polynuclear cluster. Fortunately, fragments 2, 3, and 4 only exist in solution compared with the single crystal MS, which makes it possible to study the formation of Co7 (Figure 4 and Table S6). The following is the possible formation process of Co7: fragment 1 captures a CoSO4, forming 2; then, two fragments 2 fused by one μ2-OH group from added base form 3, followed by capture of a CoSO4 fused by μ3-OH and one μ3-CH3O and the ligand lose one proton to form 4. Fragment 4 binds to CoL to eventually form 5 (Figure 4b). Throughout the assembly process, since this is a structure coated by three multidentate 7239

DOI: 10.1021/acs.inorgchem.9b00269 Inorg. Chem. 2019, 58, 7236−7242

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Inorganic Chemistry

shape to retain the symmetry and the applied field progressively forces them to be parallel at saturation. This is similar to the case of helimagnets and some chiral magnets.12,13,19 If this is the case, then the chirality of the structure may impose this behavior. Although the structure appears simple, it is not so simple to model the magnetic behaviors of the compound. First, there are three independent moment carriers per molecule which increase the number of exchange couplings (J); there are two pairs of three equivalent J12 and J23 between the central Co2 and the outer ones, Co1 and Co3, respectively, and two sets of three J33 (Co3−Co3) and J11 (Co1−Co1). In addition, one has to consider the single-ion anisotropies (D and E) of each individual independent moment carrier and their spin−orbit coupling parameters (Figure S13). Thus, it is an intractable task to extract an accurate and unique solution. However, the susceptibility and isothermal magnetization data for the compound suggest that dominant ferromagnetic exchange couplings are present for this geometrical arrangement. The ac-susceptibilities for different frequencies as a function of temperature reveal a behavior akin to those of a singlemolecule magnet where there is considerable loss shown by the nonzero imaginary component below 4 K (Figure 7). This is

The susceptibility was measured upon cooling from 300 to 2 K in an applied dc-field of 100 Oe. AC-susceptibilities as a function of temperature (2−10 K) and Cole−Cole as a function of frequency (1−1000 Hz) at fixed temperatures (1.8−3 K) were measured in zero dc-field and an oscillating field of 3 Oe. Isothermal magnetizations were measured at 2 K. The product of the magnetic susceptibility and temperature (χmT) of Co7 exhibit a gradual increase from 300 K peaking with a broad maximum at ca. 100 K before gradually decreasing to ca. 30 K. Below ca. 15 K, it undergoes a sharp decrease (Figure 6). This behavior is common for

Figure 6. Temperature dependence of χmT for Co7 (Hdc = 100 Oe) (inset: field dependence of the magnetization for Co7 at 2 K).

ferromagnetic coupling between nearest neighbors of cobalt(II). Below 100 K, spin−orbit coupling competes with the ferromagnetic coupling driving the moment to lower values, and below 15 K, the moments on each magnetic center within the molecular unit of the crystal structure start to orient and finally block. The Curie−Weiss fit of the susceptibility data above 150 K gives a Curie constant of 21.60 emu K/mol and a Weiss temperature of +9.89 K (Figure S12). The Curie constant, 3.09 emu K/mol of Co, is consistent with that expected for a Co(II) d7 ion, and the positive Weiss temperature confirms the dominant ferromagnetic coupling between nearest neighbors of the Co7 cluster. The isothermal magnetization at 2 K of Co7 exhibits a sharp rise at the low field and a weak linear dependence at the high field (Figure 6, inset). The behavior at 2 K is not that expected by the Brillouin function for a high spin molecule assuming here Seff = 7/2 for seven ferromagnetically coupled Co(II). Extrapolation of the data taken at 2 K gives a magnetization of ca. 7 NμB which will be consistent with an Seff = 7/2 molecule assuming g = 2. Therefore, the linear dependence of the magnetization at higher field may be associated with the population of the higher energy Kramers doublets. The saturation magnetization is not attained in 70 kOe; the value is nearly 14 NμB. It is slightly lower than the expected 16.8 NμB for seven Co(II), each at saturation value assuming 2.4 NμB per Co(II).23 For the linear dependence of the magnetization at elevated fields, the moments are not collinear. This will mean that the moments are possibly aligned in a fan

Figure 7. Temperature dependence of the in-phase (χm′) and out-ofphase (χm″) ac-susceptibilities for different frequencies in zero dc-field for Co7.

confirmed by the frequency dependence at fixed temperatures (Figure S14) and also by the dependence of the imaginary part of the susceptibilities versus the real one in the Argand diagram (Cole−Cole plot). An experiment performed by sweeping the frequency of the oscillating ac-field at fixed temperature shows an Argand diagram (Figure S14) for temperatures between 1.8 and 3.0 K (Figure S15a). The single semicircular shape suggests that a simple Debye model applies and the analyses result in a relative narrow distribution of relaxation times (α = 0.12−0.29), in agreement with those observed for Co(II)based SMM. The temperature dependent relaxation times were analyzed assuming a thermally activated process following the Arrhenius law (τ = τ0 exp(Ueff/kBT)). The energy barrier and extrapolated relaxation time were Ueff = 9.4 K and τ0 = 4.8 × 10−6 s for Co7 under zero dc-field (Figure S15b). 7240

DOI: 10.1021/acs.inorgchem.9b00269 Inorg. Chem. 2019, 58, 7236−7242

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Inorganic Chemistry A search for “chiral magnet” in SciFinder results in numerous examples of both oxides and molecular systems. The field is now dominated by development in the area of Skyrmions, which are magnetic domains with a handedness of the orientation of moments of the magnetic carriers.27 In contrast, the search for “polar magnet” results in fewer examples. The majority are oxides of metals. They were exploited for their dielectric properties and, in particular, as multiferroics. And, that for “chiral and polar magnet” gave only a handful and among them only one molecular in nature. The oxide examples are DyCrWO6 (Pna21),28 MnSb2O6 (P321),29 and PbVO3 (P4/mm),30 and the molecular compound is the series MII3(O2CH)5Cl(H2O), where M = Fe, Co, or Ni,31 crystallizing in both P31 and P32 space groups. As expected, neutron diffraction of CoII3(O2CH)5Cl(H2O) found a chiral ferrimagnetic order of the moments in zero field where the periodicity is three cobalt along the c-axis while the moments are tilted by ∼14.7° to the c-axis. Two of the three independent chiral chains have their orientation along (0001) and the other along (000-1) giving a final ferrimagnetic state. With respect to this search, Co7 (R3) appears to be the second chiral and polar dicubane single-molecule magnet to date; the first was reported by Vaz et al. for Co7(OH)8(tta)6(ROH)6, where tta = 4,4,4-trifluoro-1-thienoyl-2,4-butanedionate and ROH = n-butanol also crystallize in the R3 space group and has similar magnetic properties to that of Co7.21 Interestingly, when ROH = isopropanol, the crystals are centrosymmetric (R-3c) and the magnetic is almost identical, suggesting the magnetism is not affected by the polarity of the structure.

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Accession Codes

CCDC 1888163 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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Corresponding Authors

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

Mohamedally Kurmoo: 0000-0002-5205-8410 Ming-Hua Zeng: 0000-0002-7227-7688 Author Contributions

The experiments were performed by W.-J.L. and M.K. under the direction of M.K. and M.-H.Z. The paper was written by all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Science Foundation for Distinguished Young Scholars of China (No. 21525101), the National Natural Science Foundation for China (No. 21571165), the NSF of Guangxi Province (2014GXNSFFA118003, 2017GXNSFDA198040), the BAGUI talent and scholar program (2014A001), the Project of Talents Highland of Guangxi Province, and the NSF of Hubei Province innovation group project (2017CFA006). We thank the Department of Chemistry, Nanyang University, for the use of their SQUID magnetometer and W.-X. Zhang, X.-F. Ma, and X.-L. Chen for their help with some measurements. M.K. is funded by the CNRS-France.

CONCLUSION

In summary, we successfully designed and synthesized a chiral and polar Co7 cluster by choice of the organic ligand and inorganic salt and revealed its structure by crystallography and its stability and possible stepwise assembly process by mass spectrometry. ESI-MS revealed it is the multidentate chelating ability of the chiral ligand coordination which drives the orientation of the metal during the possible stepwise assembly process. This information may be a benefit to rational design and construction of coordination clusters with targeted magnetic properties in the future. The nearest neighbor magnetic exchange is ferromagnetic in each case, and a very noticeable spin−orbit coupling is evident for the cobalt compound below 100 K and displays single-ion anisotropy effects at low temperatures. Slow magnetic relaxation, as seen by an imaginary ac-susceptibility and strong frequency dependence fitting to a single-Debye relaxation rate, is observed. The compound is a good single-molecule-magnet candidate for physicists to study the magneto-chiral dichroism and dielectricity.

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AUTHOR INFORMATION

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REFERENCES

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.9b00269. Synthesis, listing of crystallographic data, plots of TGA, PXRD, DSC, ESI-MS, magnetic, and dielectric data (PDF) 7241

DOI: 10.1021/acs.inorgchem.9b00269 Inorg. Chem. 2019, 58, 7236−7242

Article

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DOI: 10.1021/acs.inorgchem.9b00269 Inorg. Chem. 2019, 58, 7236−7242