Exploiting Miraculous Atmospheric CO2 Fixation in the Design of

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Exploiting Miraculous Atmospheric CO2 Fixation in the Design of Dysprosium Single-Molecule Magnets Haiquan Tian, Lang Zhao, and Jinkui Tang Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b01612 • Publication Date (Web): 16 Jan 2018 Downloaded from http://pubs.acs.org on January 21, 2018

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Crystal Growth & Design

Exploiting Miraculous Atmospheric CO2 Fixation in the Design of Dysprosium Single-Molecule Magnets Haiquan Tian,*,‡ Lang Zhao,† Jinkui Tang*,† †

State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, P. R. China ‡ School of Chemistry and Chemical Engineering, School of Materials Science and Engineering, School of Pharmacy, and Shandong Provincial Key Laboratory of Chemical Energy Storage and Novel Cell Technology, Liaocheng University, Liaocheng 252000, P. R. China ABSTRACT: A system of CO32--bridged polynuclear dysprosium complexes has been complemented with the emerging of a key component, namely, [Dy6(μ4-CO3)3(μ3-OH2)(spch)6(MeOH)6(H2O)3]·4MeOH·3H2O (3), and thus four dysprosium(III) clusters (1-4) spontaneously fixed one, two, three and four atmospheric CO2 molecules, respectively, have been successfully assembled. Compound 1 is a Dy6 SMM (1) based on Dy3 triangles, with one CO32- group being derived from atmospheric CO2. The incorporation of an ortho-methoxy substituent into the ligand enables access to double-CO32bridged Dy6 SMM (2), where two CO32- groups occupy the two bases of the triangular prism. The trapping of three CO2 molecules of the atmosphere results in the formation of compound 3, where the three CO32- groups reside on the lateral faces of a triangular prismoid. Finally, a Dy8 SMM (4) with four CO32- bridges on the four lateral faces of the square prismoid can be isolated by converting four CO2 molecules. The magnetic investigations reveal that all four complexes exhibit single-molecular magnet (SMM) behavior with a gradual transition from the multiple to single relaxation process observed in their relaxation of the magnetization. This synthetic approach represents an efficient method to develop novel CO32--bridged lanthanide clusters through spontaneous fixation of atmospheric CO2 for magnetic dynamic studies.

INTRODUCTION Chemical fixation of Carbon dioxide has always been an exciting research field, which not only capture them to mitigate the global warming but also convert them to apply in synthetic and materials chemistry.1-4 Retrospectively, Carbonate anion as a kind of bridging ligand possesses flat D3h molecular symmetry and low steric hindrance leading to the ability that can connect together from two to nine metal centers, with up to 18 different coordination motifs, so that it is widespreadly used as templating agent or building block to mediate the process of formation of a diverse range of topologies.5-9 Up to now, several hundred studies reporting on carbonate complexes based on the fixation for atmospheric CO2 under mild conditions reveal the interesting physcial properties of assembling cyclic and cage-type metal systems.10-13 In fact, among this family, metal-organic frameworks (MOFs)14-16 and 17-19 single-molecular magnets (SMMs) have indisputably dominated the scene as two significant branchs. Especially, SMMs as a newly burgeoning subject was risenin the world until 1990s.20 Though the time is short (1993-2017), the study in this field has been of increasing interest because of their potential practical applications in high-speed computers and high-density magnetic storage devices with the promise of a revolution in information technology.21-30 Despite a large number of new molecules, such as 3d,31-33

3d-4f,34-36 4f37,38 metal complexes containing CO2 and its derivatives(e.g. carbonate, alkyl carbonate and carbamate,

Scheme 1. Four SMMs derived from the reactions of H2spch or H2opch ligands with different dysprosium(III) salts.

etc)39-41 have been reported in the last several decades or so. However, we could also become conscious of the fact that spontaneous fixation of atmospheric CO2 is relatively rare due to its low concentration in air and its high thermodynamic stability. Largely speaking, in the field of coordination chemistry, the “rules-based” design of

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coordination compounds by exploring organic ligands, metal particles, inorganic anion templates, and reaction conditions, has been a really hard research project for scientists. In particular, thematerials with inorganic anion templates has not yet appeared in a rhyming fashion, mainly because this process is often serendipitous, rather than a direct result of strategic design. The role of the anion in dictating the outcome of self-assembly reactions lies in the following aspects: (1) The appropriate size and shape of anion templates can not only participate in constructing of molecular topologies, but also bolt into the central cavity of compounds based on some directive function in synthesis process;42,43 (2) The intramolecular interaction of multitudinous complexes with excellent physical properties (optics, electricity, magnetics, catalysis, etc) can be influenced by suitable choices of anion templates.44,45 For SMMs, there are two periods in its development for the past two decades with the first wave of transition-metal SMMs46-50 and the second one of lanthanide SMMs.51-56 The recent advances show that lanthanide-based SMMs have yielded many high energy barriers (U) resulted from the large intrinsic magnetic anisotropy (D) of individual metal ions,57 although the weak or negligible magnetic exchange coupling allows fast relaxation pathways that mitigate the full potential of these species. Thus, the selection of suitable bridging group is crucial for assembling new lanthanide cluster with interesting magnetic properties due to the difficulty in promoting magnetic interactions because of the contracted nature of 4f orbitals. In this regard, the most remarkable result is the exceptionally strong magnetic exchange induced through the diffuse spin of an N23radical-bridge in dinuclear Ln2 SMMs.58,59

Scheme 2. The cores of compounds 1-4 highlighting the 2characteristic topologies directed by CO3 bridges derived from atmospheric CO2 fixation.

In the previous studies, we have taken advantage of two multidentate Schiff-base ligands derived from the condensation of pyrazine-2-carbohydrazide and salicylic aldehyde (H2spch), or o-vanillin (H2opch), to construct coordination dysprosium(III) clusters. The reaction of H2spch ligand with DyCl3·6H2O and triethylamine in a mixture of methanol and dichloromethane produces Dy6 SMM (1)60 based on Dy3 triangles, with a less common η2:η2-µ3-CO32- carbonate bridge being derived from

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atmospheric CO2. However, the incorporation of an ortho-methoxy substituent into the H2spch ligand, enables access to double-CO32- bridged trigonal prism Dy6 SMM (2)61 and quadruple-CO32- bridged square prismoid Dy8 SMM (4),62 by employing different salts Dy(OAc)3·6H2O and DyCl3·6H2O, respectively, concomitant with spontaneous atmospheric CO2 fixation. A closer look into the structures and reaction conditions of this system reveal interesting results. As is evidenced in Schemes 1 and 2, one, two and four atmospheric CO2 molecules have been respectively captured into compounds 1, 2 and 4 with the exclusive CO32- form, where two CO32- groups occupy the two bases of the triangular prism of 2 and four CO32- groups are on the four lateral faces of the square prismoid of 4. Now, the question comes. If the CO32- groups reside on the lateral faces of a triangular prism or prismoid as those in 4, then can we trap three CO2 molecules of the atmosphere in such polynuclear lanthanide system? Inspired by these exciting results and noting that 2 and 4 were obtained by the reactions of same ligand H2opch with different salts DyCl3·6H2O or Dy(OAc)3·6H2O, we intend to further explore the possibility of obtaining new aggregates by varying the metal salts in the H2spch system. Herein we report the successful assembly of a novel trigonal prismoid hexanuclear cluster, [Dy6(μ4-CO3)3(μ3-OH2)(spch)6(MeOH)6(H2O)3]·4MeOH·3 H2O (3) that traps three atmospheric CO2, using H2spch and Dy(OAc)3·6H2O. As part of our continuing interests in dysprosium SMMs, we present here an efficient strategy to design lanthanide cluster that behaves as SMMs by taking advantage of miraculous fixation of atmospheric CO2.

EXPERIMENTAL SECTION Materials and measurements. All reagents and solvents were of A.R. Grade and were used without further purification. Elemental analysis (C, H, and N) were performed with a Perkin-Elmer 2400 analyzer. FTIR spectra were recorded with a Perkin-Elmer Fourier transform infrared spectrophotometer using the reflectance technique (4000-300 cm-1). Samples were prepared as KBr disks. Magnetic susceptibility measurements were carried out on a Quantum Design MPMS-XL7 SQUID magnetometer equipped with a 7 T magnet. The direct current (dc) measurements were collected with an external magnetic field of 1000 Oe in the temperature range 1.9-300 K, and the alternating-current (ac) measurements were carried out in a 3.0 Oe ac field oscillating at different frequencies from 1 to 1500 Hz. The experimental magnetic susceptibility data are corrected for the diamagnetism estimated from Pascal’s tables63 and sample holder calibration. Synthesis of Ligand. (E)-N'-(2-hydroxybenzylidene) pyrazine-2-carbohydrazide18 and (E)-N'-(2-hyborxy-3methoxybenzylidene)pyrazine-2-carbohydrazide19 were prepared according to the methods reported in the literatures.


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Synthesis of the complexes 1, 2 and 4: These three compounds were prepared by following the procedures reported previously.[60-62] However, the details of the experiments will be described briefly in order to get insight into the synthetic strategy to isolate this series of clusters. The reaction of DyCl3·6H2O (56.5 mg, 0.15 mmol) and the H2spch (36.3 mg, 0.15 mmol) in 15 ml CH3OH/CH2Cl2 (1:2 v/v), followed by treatment of Et3N (0.6 mmol) generates compound 1 ([Dy6(μ3-OH)3(μ3-CO3)(μ-OMe)(spch)6(MeOH)4(H2O)2]·3 MeOH·2H2O) after one week undergoing the slow evaporation of the solvent in the air. Compound 4 ([Dy8(μ4-CO3)4(opch)8(H2O)8]·10MeOH·2H2O) could be synthesized by replacing H2spch with H2opch ligand. For compound 2 ([Dy6(OAc)3(μ3-CO3)2(opch)5(Hopch) (MeOH)2]·4H2O·5MeOH·EtOH), was assembled by slowly adding Dy(OAc)3·6H2O into the solution of the H2opch ligand with other things being unchanged. Synthesis of the complex 3: The solution of Dy(OAc)3·6H2O (53.3 mg, 0.15 mmol) and the H2spch (36.3 mg, 0.15 mmol) in 15 ml CH3OH/CH2Cl2 (1:2 v/v) was stirred with Et3N (0.6 mmol) for 8 h. The resultant yellow solution was left unperturbed to allow the slow evaporation of the solvent. Yellow single crystals, suitable for X-ray diffraction analysis, were formed after one week. Yield: 31 mg (28%, based on metal salt). Elemental analysis (%) calcd for C93H140Dy6N24O52: C, 32.78, H, 4.32, N, 9.87: found C, 33.02, H, 4.58, N, 9.56. IR (KBr, cm-1): 3374(w), 1608(vs), 1544(m), 1470(s), 1441(m), 1416(w), 1342(s), 1398(w), 1247(w), 1200(w), 1153(m), 1127(w), 1062(w), 1028(m), 977(w), 893(m), 799(w), 759(m), 656(w), 503(w),421(w). X-ray Crystallography. Crystallographic data and refinement details for compound 3 are summarized in Table S1. Crystallographic data were collected on a Bruker Apex II CCD diffractometer with graphite monochromated Mo-Kα radiation (λ = 0.71073A˚) at 296(2) K. The structure was solved by direct methods and refined on F2 by full-matrix least squares using SHELXS-97 and SHELXL-97 programs.64 The locations of Dy atoms were easily determined, and O, N and C atoms were subsequently determined from the difference Fourier maps. All non-hydrogen atoms were refined with anisotropic thermal parameters. The H atoms were introduced in calculated positions and refined with fixed geometry with respect to their carrier atoms. Details for the crystallographic data and reﬁnement are summarized in Table S1 and selected bond distances and angles are listed in Table S2. CCDC-945883 (3) 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.

RESULTS AND DISCUSSION The successful isolation of four dysprosium(III) clusters spontaneously fixed one, two, three and four atmospheric CO2 molecules presents us a unique opportunity to explore new synthetic strategy to design novel lanthanide

clusters by taking advantage of fixation of atmospheric CO2. The hydrazone ligand (E)-N'-(2-hydroxybenzylidene) pyrazine-2-carbohydrazide (H2spch; Scheme 1) was derived from the condensation of salicylic aldehyde and pyrazine-2-carbohydrazide in methanol, following a reported procedure by ourselves.60 This ligand provides N, O, N, O-based multichelating sites that are especially favorable for the formation of lanthanide complexes. Only one example of coordination complexes is synthesized with H2spch.60 By reacting this ligand with DyCl3·6H2O, we were able to isolated a hexanuclear dysprosium(III) compound (1) where three different binding modes for the H2spch ligand were observed in its di-deprotonated forms. Interestingly, compound 1 shows a single CO32carbonate bridge being derived from atmospheric CO2. However, simply replacing DyCl3·6H2O by Dy(OAc)3·6H2O in the reaction system generates a triple-CO32bridged hexanuclear dysprosium(III) compound [Dy6(μ4-CO3)3(μ3-OH2)(spch)6(MeOH)6(H2O)3]·4MeOH·3 H2O (3) in 28% yield, concomitant with atmospheric CO2 fixation.

Figure 1. Top: the molecular structure of compound 3. Hydrogen atoms and lattice solvents are omitted for clarity. Color scheme: Turquiose Dy, red O, blue N. Bottom: the 12+ [Dy6(μ3-OH2)(μ4-CO3)3] core. Color code: Turquiose Dy, red O, black C. Symmetry codes: A: 1-y, 1+x-y, z; B: -x+y, 1-x, z.

Structure Descriptions. The single X-ray crystallography studies reveal that compound 3 crystallizes in the trigonal R-3 space group (Figure 1 top and Table S1) and contains a hexanuclear [Dy6(μ3-OH2)(μ4-CO3)3]12+ core resembling a beautiful crown, as depicted in Figure 1 bottom. Three CO32- ligands adopt a rare 4.221 (Harris notation65) fashion as the delicate and exquisite pattern of its side, while six



dysprosium(III) ions are just like the jewels embedding in around of the crown. It is noteworthy that a single μ3-OH2 bridge caps the Dy3 triangle (Dy1Dy1ADy1B) at the top of crown consolidating the Dy6 construction. This Dy3 triangle can be identified as an equilateral triangle based on three equal Dy···Dy bond lengths (4.0315(5) Å) and Dy···Dy···Dy angles (60.000(7)°), which have never reported in a large number of relevant Dy3 triangles. Strictly speaking, the Dy6 structure should be considered as resulting from the symmetrical linkage by three carbonato ligands and one μ3-OH2 capped ligand of three petals of the asymmetric Dy2 units wrapped by two terminal spch2- ligands. Each Dy2 petal consists of two dysprosium(III) ions, two spch2- ligands, two methanol molecules and one water molecule. The asymmetric dinuclear unit is composed of one NO8 coordinate environment dysprosium(III) ion with a distorted mono-capped square-antiprismatic and one N2O6 coordinate environment (see Figure 4) dysprosium(III) ion with a distorted dodecahedral environment bridged by carboxy oxygen atom of one spch2- ligand and one oxygen atom of CO32- ligand, with the Dy···Dy distance equal to 3.9756(4) Å and two Dy-O-Dy angles of 112.544(155)° and 113.256(155)°. Two polydentate Schiff-base ligands surround the Dy2 petal and show two different binding modes in its di-deprotonated forms (Scheme S1). In each Dy2 petal, the Dy-O and Dy-N bond lengths are in the range of 2.2442(50)-2.5037(56) Å and 2.5021(51)-2.5680(36) Å, respectively. Two methanol molecules and one water molecule successively occupy the remaining coordination sites of Dy2 petal. Finally, three Dy2 units are splendidly bridged together to form a crown by three CO32- ligands and one μ3-OH2 capping ligand. Additionally four non-coordinated methanol molecules and three water molecules of crystallization are located in the crystal lattice.

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of the acs net (49·66),66 as shown in Figure 2. The shortest intermolecular Dy···Dy distance is 8.8153(8) Å. In particular, the source of three CO32- bridged ligands mentioned above has been identified by charge balance requiring three dianions in these sites and careful consideration of the X-ray diffraction data. Moreover, the IR stretching bands characteristic of coordinated carbonate are observed in the range of 1541-1562 and 1333-1346 cm-1 (see Figure 3 for details), respectively. Therefore, it appears that atmospheric CO2 has been incorporated into the structure, as seen for carbonate complexes of other metal ions.5,60-62

Figure 3. IR spectra of crystalline sample of compounds 1-4. Inset: the IR stretching bands characteristic of coordinated carbonate.

After the successful acquisition of 3, we were able to summarize herein the synthesis, structure, and magnetic properties of compounds 1-4 with the aim to explore novel synthetic strategy to design new lanthanide SMMs by make use of spontaneous atmospheric CO2 fixation. As is evidenced in Scheme 1, compounds 1 and 4 were obtained by reaction of DyCl3·6H2O with H2spch and H2opch, respectively, while compounds 2 and 3 were synthesized by using Dy(OAc)3·6H2O. As depicted in Scheme S1, a total of six H2spch ligands show three different binding modes with its di-deprotonated form in compound 1, while only two kind of binding modes can be observed with the same ligand in compound 3. In compound 2, both mono-deprotonated keto and di-deprotonated enol form ligands adopt same binding modes. In compound 4, all H2opch ligands are in the di-deprotonated forms and display only one binding mode.

Figure 2. Illustration showing the hydrogen-bonding interactions (light orange lines) and the circular hexagon packing arrangement of the molecules in compound 3.

The structure may also be viewed as a 3D network with the hexanuclear molecules being the node and connected to six others via the strong intermolecular hydrogen-bonging interactions to generate a rare example


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Crystal Growth & Design 20 K might suggest a competition between ligand field effect and possible weak ferromagnetic interactions between the dysprosium(III) ions.69 In particular, The χMT product of compound 4 gradually decreases with lowering temperature, reaching a minimum value of 97.8 cm3 K mol-1 at 30 K, which is mainly ascribed to the same effect as mentioned above. The χMT value then increases sharply to a maximum of 127.1 cm3 K mol-1 at 2 K, which obviously suggests the presence of intramolecular ferromagnetic interactions between the metal centers.53,70

Figure 4. Coordination polyhedra observed in compounds 1-4.

It is worth noting that the formation of compounds 1-4 involves the capture of one, two, three and four atmospheric CO2 molecules, being transformed into the CO32- by the mechanism similar to the carbonic anhydrase in the assembly process. In this case, CO32- ligands adopt three 3.22 (1), 3.222 (2) and 4.221 (3 and 4) fashions according to Harris notation connecting the dysprosium(III) centers with various topologies (Scheme S2). Such rich coordination modes are ascribed to the wide range of bond lengths from 2.3150(41) to 2.5086(57) Å for O-Dy interactions between the dysprosium(III) centers and the CO32- ligands in compounds 1-4. The coordination geometries of each dysprosium(III) center are distorted, as shown in Figure 4 and Table S1.33 These CO32--bridged dysprosium(III) compounds herein described would provide a valuable reference for researching of the “rules-based” coordination materials based on the unpredictable inorganic anion template, which templates further aggregation and growth of cluster compounds. Direct current (dc) magnetic susceptibility data for 1-4 were collected in the temperature range 2 – 300 K under an applied field of 1000 Oe. The plot of χMT vs. T, where χM is the molar magnetic susceptibility, is shown in Figure 5. At room temperature, the χMT values of 84.6 and 85.6 cm3 K mol-1 for compounds 1 and 2 are in good agreement with the expected value of 85.02 cm3 K mol-1 for six uncoupled dysprosium(III) ions (S = 5/2, L = 5, 6H15/2, g = 4/3). However, for the same hexanuclear compound 3, The observed χMT value at 300 K of 86.3 cm3 K mol-1 is slightly higher than the expected value, moreover, the value of 105.8 cm3 K mol-1 is lower than the expected value of 113.36 cm3 K mol-1 for eight uncoupled free ions in compound 4. In the temperature range 300 – 50 K, with the temperature decreases, the χMT products show similar tendency of gradual decline, which are mainly ascribed to the thermal depopulation of magnetic energy levels spilt by the crystal field.67 Below 50 K, these three plots have made the sharp declines to reach the minimum values of 68.4 cm3 K mol-1 (1), 66.7 cm3 K mol-1 (2) and 70.8 cm3 K mol-1 (3) at 2 K, owing to the possible intramolecular antiferromagnetic coupling and partially the effect of magnetic anisotropy. The shoulder for 2 observed around

Figure 5. Temperature dependence of the χMT product at 1000 Oe for compounds 1-4.

The magnetizations of the four complexes from zero dc field to 70 kOe at different temperatures are shown in the inset of Figure 6, with the corresponding maximum values reached at 1.9 K of 30.9 (1), 40.6 (2), 33.8 (3) and 42.6 (4) μB. These values are lower than the expected saturation values of 60 μB (compounds 1-3) and 80 μB (compound 4) based on the six or eight noninteractingdysprosium(III) ions, which are most likely derived from significant anisotropy and the crystal-field effect at the dysprosium(III) ion that eliminates the16-fold degeneracy of the 6H15/2 ground state.33,52,71 At higher fields, the variation and non-superposition of the the M vs. H/T data on a single master curve suggests the presence of a significant magnetic and/or low lying excited states in compounds 1-4.71-73 In addition, it is worth noting that a butterfly-shaped hysteresis loop can be detected by using a traditional SQUID magnetometer for 4.



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Figure 6. Magnetization M versus applied field H between 1.9 and 5 K for compounds 1-4.

Alternating current (ac) susceptibility measurements were carried out on polycrystalline samples of 1-4 at low temperatures under a zero dc field to investigate the dynamics of the magnetization. Both the temperature and frequency dependences of the out-of-phase (χ″) ac signal reveal the onset of slow relaxation of magnetization that are typical for SMM behavior, as depicted in Figure 7 and S1-4. The χ″(v) isotherms of compounds 1-4 exhibit a well-organized picture of the peaks from high- to low-frequency, indicating the presence of a gradual transition from multiple to single relaxation process of the magnetization. Thus, four complexes are divided into two groups with multiple slow relaxation species (1, 3) and single relaxation species (2, 4) based on relaxation mechanism analysis (see below).

Figure 7. Frequency dependence of the out-of-phase ac susceptibility of compounds 1-4 under zero-dc field.

First of all, as depicted in Figure 8, for compounds 1 and 3, Cole-Cole (Argand) plots in the temperature range of 1.9 – 7.0 K (1) and 1.9 – 5.0 K (3) are made using the generalized Debye expression, in which a series of curves show asymmetric semicircles. The α values obtained range from 0.22 to 0.27 (1) and from 0.25 to 0.31 (3), indicating a relatively wide distribution of relaxation times.47,72,73 The magnetization relaxation times (τ) derived from the frequency-dependence measurements are plotted as a function of 1/T in Figure 9. To quantify the relaxation barrier, two relaxation regimes are clearly visible with a transition between them corresponding to an energy gap (∆) of 5.6 and 49.1 K and a pre-exponential factor (τ0) of 4.2 × 10-5 and 1.6 × 10-6 s for the low- and high-temperature domain, respectively, for 1. Similarly, two bisecting lines can be obtained in compound 3 when modeling the behavior with Arrhenius plots (Figure 9) corresponding to ∆ of 5.4 and 186.8 K and τ0 of 2.7 × 10-5 and 1.1 × 10-9 s, below and above 9 K, respectively. At high temperatures, these pre-exponential factors are consistent with the expected τ0 of 10-6-10-11 s for a SMM, while at low temperatures, the thermal relaxation is crippled by QTM, resulting in large τ0 values.

Figure 8. Cole-Cole (Argand) plots for compounds 1-4 obtained using the ac susceptibility data. The solid lines correspond to the best fit obtained with a generalized Debye model.

For compounds 2 and 4, both the temperature and frequency dependence measurements for ac susceptibilities are carried out and the results show the appearance of single relaxation peak below 26 K (2) and 24 K (4), as shown in Figures 7 and S1-4. The χ'T values show a similar trend of the sharp decline at around 15 K (2) and 18 K (4) for the same frequency at 1200 Hz, which are completely consistent with emerging peaks of the in-phase (χ') and out-of-phase (χ'') signal when the magnetization is blocked by the anisotropy barriers. τ was extracted from the frequency-dependent data between 1.9 and 24 K and the Arrhenius plot obtained from these data is given in Figure 8. Below 3 K, two temperature-independent regimes are observed with characteristic times of 0.07461 (2) and 0.20149 s (4), suggesting the quantum tunnelling of the magnetization becomes dominant (i.e. faster than the thermal-activated relaxation). Above 8.0 K, the relaxation follows the thermally activated Orbach mechanism when thermal relaxation becomes dominant with ∆ of 56.7 and 72.4 K and τ0 of 6.6 × 10-6 and 2.1 × 10-6 s, respectively. Argand diagrams exhibit a series of faultless semicircular arcs in relatively wide temperature range of 1.9-7.0 K (4) and 6.0-10.0 K (2), which were perfectly fitted by the generalized Debye model with the values of α range from 0.06 to 0.11 (2) and from 0.08 to 0.13 (4), further indicating a very narrow distribution of relaxation time constants.23,56,61,62


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–1

captured and the poison of the template determines the final topology of the structures. Two CO32- groups occupy the two bases of the triangular prism resulting in Dy6 compound 2, while three CO32- groups residing on the lateral faces of a triangular prismoid leads to the isolation of Dy6 compound 3 and finally, four CO32- bridges on the four lateral faces of the square prismoid results in Dy8 compound 4. In particular, all four compounds show SMM behavior, and slow magnetic relaxation process displays a gradual transition from the multiple to single relaxation process. The above synthetic approach illustrated in this work represents an efficient method to develop novel CO32--bridged lanthanide SMMs by taking advantage of miraculous fixation of atmospheric CO2 with the promise of reducing greenhouse gases.

Figure 9. Magnetization relaxation time, lnτ, versus T for compounds 1-4 under zero-dc field. The solid line is fitted with the Arrhenius law (bottom).

ASSOCIATED CONTENT

In addition, as depicted in Figure 9, a gradual transition from the multiple to single process for the dynamics of the magnetization is observed for these compounds, which is most likely due to the different structures directed by the CO32- bridges.

Binding modes of the H2spch, H2opch and CO32- ligands (Scheme S1-S2), Magnetic measurements (Figures S1-S4), Crystallographic data and refinement details (Table S1), and selected bond lengths and angles (Table S2). This material is available free of charge via the Internet at http://pubs.acs.org.

Supporting Information

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected], [email protected]. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT

Figure 10. Plots of magnetization (M) versus direct current magnetic field (H) for compounds 1-4 at 1.9 K.

We thank the National Natural Science Foundation of China (Grants 21525103, 21521092 and 21331003) for financial support. J.T. gratefully acknowledges support of the Royal Society-Newton Advanced Fellowship (NA160075).

REFERENCES

CONCLUSION Reaction of Dy(OAc)3·6H2O and polydentate Schiff-base H2spch in the presence of triethylamine resulted in the assembly of new hexanuclear compound templated by triple-CO32− groups, introduced via spontaneous fixation of atmospheric CO2. Magnetic investigation reveals that the triple-CO32- bridged Dy6 compound exhibits SMM behavior with two relaxation regimes (∆(1): 5.4 K, τ0: 2.7 × 10-5 s; ∆(2): 186.8 K, τ0: 1.1 × 10-9 s) below 24.0 K. Moreover, with the emerging of this new member, a series of dysprosium(III) clusters (1-4) spontaneously fixed one, two, three and four atmospheric CO2 molecules, respectively, have been complemented. A closer look into the structures of this system reveal that the CO32- anion template derived from the atmospheric CO2 directs the assembly of the clusters and the topologies of the resulting structures depend on the abundant coordination modes and the number of the CO32- ligands. Specifically, for compounds 2-4, the number of CO2

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For Table of Contents Use Only Exploiting Miraculous Atmospheric CO2 Fixation in the Design of Dysprosium Single-Molecule Magnets ,‡

†

,†

Haiquan Tian,* Lang Zhao, Jinkui Tang*

Four dysprosium(III) clusters spontaneously fixed one, two, three and four atmospheric CO2 molecules, respectively, have been assembled representing an efficient method to develop novel CO32--bridged lanthanide clusters for magnetic studies.


Exploiting Miraculous Atmospheric CO2 Fixation in the Design of

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