Two Interpenetrated Cobalt(II) MetalâOrganic Frameworks with Guest

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Two interpenetrated cobalt(II) metal-organic frameworks with guest dependent structures and field-induced single-ion magnet behaviors Le Shi, Dong Shao, Hai-Yan Wei, and Xin-Yi Wang Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b00714 • Publication Date (Web): 13 Jul 2018 Downloaded from http://pubs.acs.org on July 13, 2018

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Crystal Growth & Design

Two interpenetrated cobalt(II) metal-organic frameworks with guest dependent structures and field-induced single-ion magnet behaviors Le Shi,† Dong Shao,† Hai-Yan Wei,*,§ and Xin-Yi Wang*,† †

State Key Laboratory of Coordination Chemistry, Collaborative Innovation Center of

Advanced Microstructures, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing, 210023, China. §

Jiangsu Key Laboratory of Biofunctional Materials, School of Chemistry and

Materials Science, Nanjing Normal University, Nanjing, 210023, China.

ABSTRACT: Two cobalt(II) metal-organic frameworks with two-fold vertically interpenetrated

(4,4)

grids,

namely,

[Co(bpg)2(SCN)2]·3MeOH

(1)

and

[Co(bpg)2(SCN)2]·2DMF (2) (bpg = meso-α,β-bi(4-pyridyl) glycol) were prepared under different conditions. The two complexes crystalize in different space groups (orthorhombic Pccn for 1, and tetragonal P421c for 2) with different guest molecules. Although their structures are very similar, different guest molecules subtly change the coordination environments of the CoII centers, the inter-grid supramolecular interactions (hydrogen bond), and the dihedral angles of those two sets of interpenetrated grids. Magnetically, these modifications of the structures lead to the changes on their magnetism, especially the dynamic magnetic properties at low temperatures. Both compounds exhibit slow magnetic relaxation under an external field. For 1, we couldn’t observe the peak maxima of the ac susceptibilities in our

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magnetometer at the lowest temperature and highest frequency because the relaxation time is never slow enough. In contrast for compound 2, the estimated energy barrier is significantly higher than that of 1, leading to much slower magnetic relaxation. The difference of magnetic properties between 1 and 2 illustrates the role of the MOFs on the development of the tunable SIMs and the prominent guest effect in the MOF based SIMs. INTRODUCTION Molecular nanomagnets, especially single-molecule magnets (SMMs) of high energy barriers (Ueff) for the magnetization reversal and high blocking temperatures (TB), have been extensively pursued in the molecular magnetism community.1-3 After early studies on the high spin complexes such as polynuclear manganese clusters,4,5 current efforts in this field have been focused on the single-ion magnets (SIMs) with one single metal center and very high intrinsic magnetic anisotropy.6,7 An important feature of the SIMs relies on the fact that their magnetic anisotropy can be modified or even carefully designed through the regulation of the coordination environment. Along this line, the Ueffs and TBs of the SIMs have both been dramatically increased in the lanthanide-based SIMs. The highest Ueff has now over 1000 cm-1 (1277 cm-1), and the highest blocking temperature (60 K) is approaching the boiling point of liquid nitrogen.8,9 Besides the lanthanide SIMs, SIMs with 3d metal centers have also aroused a growing interest. After the first FeII SIM in 2010,10 a large amount of 3d SIMs with different metal centers have been reported.11-31 Among all these SIMs, the Co(II) complexes are of special interest. For this Kramers ion, slow magnetic


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relaxation can always be observed under a dc field, provided the existence of significant magnetic anisotropy.15,16 This character makes the design of the CoII SIMs somehow easier and has resulted in a range of CoII SIMs, not only in the structurally isolated single-ion systems,13-25 but also in the extended coordination polymers, such as in the one-dimensional,32-34 two-dimensional (2D),35-39 and even three-dimensional (3D) metal-organic frameworks (MOFs).40,41 Actually, studies of the SMM properties in the MOFs materials could lead to great chances to systematically tune the magnetic properties of SMMs.42,43 On one hand, SMMs, even those with rather large volumes, can be loaded into the cavity of the MOFs. The confinement impact of the MOFs on their SMM properties can be investigated.44 On the other hand, the majority of the current studies use the appropriate spacer ligands to connect the magnetic nodes (SIMs or SMMs), which can be either mononuclear or polynuclear 3d,37,45 4f,46,47 or 3d-4f metal centers,48 to form the extended frameworks. In these magnetic MOFs, long spacer ligands not only provide the appropriate ligand field for the magnetic anisotropy, but also serves to modulate the magnetic interaction between the SMMs or SIMs. In addition, by taking advantage of the porosity of the MOFs, these materials provided a new platform to modify the local coordination structures of the metal centers and therefore the slow magnetic relaxation behaviors of the SMMs by the guest molecules located in the cavity of the MOFs. Successful examples include [Co(bpeb)2(NCS)2]·nG (G = 1,2-dichlorobenzene, thianthrene, toluene, and pyrrole guest

molecules),37

Dy2(INO)4(NO3)2·2S

(S

=

DMF,

MeCN)47

and

{[NH2(CH3)2]2[NiDy2(HCOO)2(abtc)2]}.48 The magnetic behaviors of the SMMs,



including the magnetic anisotropy, energy barriers and the relaxation pathways, can be finely tuned by incorporation of different guest molecules. These studies provides new aspects for the investigation of structure-property relationships of the SMMs.17 However, although there are now plenty of MOF-based SMMs and SIMs, successful tuning of their SMM behaviors by different guest molecules are limited to the examples mentioned above. Herein, we reported two MOF-based CoII SIMs [Co(bpg)2(NCS)2]·solvent (solvent = 3MeOH (1) and 2DMF (2)). The linear bridging ligand bpg was chosen for two reasons. I) This long and non-conjugated ligand should lead to an effective separation of the paramagnetic metal centers with negligible magnetic coupling and retain the SIM property of the individual CoII center. II) The two OH groups can be used to introduce hydrogen bonds, which has been found to be of great significance in the switchable materials, such as the spin crossover systems.49 Both 1 and 2 are of 2D frameworks of (4,4) topology bridged by bpg ligands, which are further vertically double-interpenetrated into 3D structures. Magnetically, both compounds exhibit field-induced SIM behavior, where the details of the magnetic behavior can be modulated by different guest molecules. EXPERIMENTAL SECTION Chemicals used in this work were commercially available and used as received. Syntheses of the two complexes were described below. Details of the physical measurements and X-ray crystallography on these two complexes can be found in the Supporting Information. Crystal data, selected bond lengths and bond angles of 1 and 2 are listed in Table 1 and Table S1. The continuous shape measures calculation


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results (CShMs) for 1 and 2 using Shape 2.050 are provided in Table S2. The crystallographic data for these two complexes have been deposited with the CCDC numbers 1834430 and 1834431. [Co(bpg)2(NCS)2]··3MeOH (1): Co(SCN)2 (17.5 mg, 0.1mmol) and bpg (43.2 mg, 0.2 mmol) were dissolved in 10 mL of H2O, and vigorously stirred for 12 h at room temperature. Pink powders were formed, filtrated, and dried in the air. The powders were then re-dissolved in 5 mL of methanol. The suspension was filtrated to give a clear pink solution. Slow vapor diffusion of diethyl ether into the solution gave pink single crystals of 1 after several days. Yield: ~12 mg, 34%. Anal. Calcd for C29H36CoN6O7S2: C, 49.50; H, 5.16; N, 11.94%. Found: C, 49.68; H, 5.11; N, 11.89%. IR (KBr, cm-1): 3396(vs), 3068(w), 2885(w), 2093(vs), 1645(w), 1614(vs), 1563(w), 1419(s), 1339(w), 1221(s), 1192(w), 1063(s), 1017(s), 842(w), 611(s), 550(w), 470(w). [Co(bpg)2(NCS)2]··2DMF (2): CoCl2·6H2O (71 mg, 0.3 mmol) and NaSCN (48 mg, 0.6 mmol) were dissolved in 5 mL of distilled water, then bpg (130 mg, 0.6 mmol) dissolved in 5 mL of hot DMF was added and vigorously stirred for half an hour. The suspension appeared was filtrated to give the pink solution. Pink crystals of 2 were obtained after slow evaporation. Yield: ~40 mg, 35% based on CoCl2·6H2O. Anal. Calcd for C32H38CoN8O6S2: C, 50.99; H, 5.08; N, 14.87%. Found: C, 50.50; H, 5.36; N, 15.06%. IR (KBr, cm-1): 3381(s), 3067(w), 2885(w), 2086(vs), 1662(vs), 1614(vs), 1564(w), 1498(w), 1387(s), 1338(s), 1241(s), 1194(w), 1102(w), 1063(s), 1049(s), 1016(w), 853(w), 843(s), 814(w), 793(w), 749(w), 663(w), 611(s), 545(w), 469(w),



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404(w).

Table 1. Crystal data and structure parameters for 1 and 2. 1

2

Formula

C29H36CoN6O7S2

C32H38CoN8O6S2

Formula weight

703.69

753.75

Crystal system

orthorhombic

tetragonal

Space group a /Å b /Å

Pccn 13.4096(8) 14.8618(9)

P421c 14.7740(13) 14.7740(13)

c /Å V /Å3 Z T /K ρcalcd /g cm−3 µ(Mo–Kα) /mm–1 F (000) Rint data/restraints/parameters R1 / wR2 (I > 2σ(I) ) R1/ wR2 (all data) GOF on F2

18.2210(10) 3631.3(4) 4 296(2) 1.287 0.636 1468 0.0532 3203 / 335 / 293 0.0765 / 0.2041 0.0954 / 0.2212 1.059

17.219(3) 3758.4(8) 4 296(2) 1.332 0.619 1572 0.0858 3854 / 58 / 248 0.0683 / 0.2118 0.0863 / 0.2339 1.079

RESULTS AND DISCUSSION The phase purities of 1 and 2 were verified by powder X-ray diffraction patterns (Fig. S1). We noticed that the intensities of some PXRD peaks are of slight differences, which may be caused by the different orientation of the powder samples. To check their thermal stability, we performed the TGA analyses (Fig. S2). We can see both complexes lost lattice solvent molecules easily and the solvent molecules were completely removed at around 220 °C. The 12.2% weight loss of 1 and 19.3% weight loss of 2 is consistent with the calculated weight loss of 13.6% and 19.4% corresponding to the removal of three methanol molecules in 1 and two DMF


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molecules in 2. In addition, the decomposition of both compounds starts at around 220°C, suggesting that the desolvated frameworks are unstable at high temperatures. Crystal Structure Descriptions. Single crystal X-ray diffraction revealed that the structures of 1 and 2 are very similar despite of their different space groups (orthorhombic Pccn for 1, and tetragonal P421c for 2). In both complexes, the asymmetric unit contains half a Co2+ ion, two half bpg molecules, one thiocyanate anion and uncoordinated guest molecules (one and a half MeOH for 1 and one DMF for 2) (Fig. S3). The coordination geometry of each Co2+ ion is a slightly distorted octahedron formed by 6 nitrogen atoms, 2 of which are from the SCN- anions occupying the axial positions, and the rest 4 are from four bpg ligands occupying the equatorial planes (Fig. 1 a, b). The Co-N bond distances are Co-Nbpg = 2.158(4) and 2.169(9) Å and Co-NNCS = 2.089(8) Å for 1, and Co-Nbpg = 2.168(6) and 2.176(3) Å and Co-NNCS = 2.095(2) Å for 2, respectively. As for the N-Co-N bond angles, they are very close to the ideal 180 or 90° of an octahedron (see details in SI, Table S1). All these data indicate that the coordination environments of the Co2+ center are only slightly compressed along the Co-NNCS axial position. The deviation from the ideal octahedron of Oh symmetry can be determined by the CShM values, which are 1.197 and 0.337 for 1 and 2, respectively (Table S2). As we can see, the CShM value of 2 is significantly smaller than that of 1, suggesting a much regular geometry of the Co2+ ion in 2. From their structures, we can see that the bpg molecules are all disordered in both complexes. In 1, there are two different bpg ligands in the structure, one of which



(denoted as bpg-A) shows severer disorder than the other (denoted as bpg-B) with the whole molecules occupying two positions. For the other bpg ligand, the disorder can only be found in the middle part of the ligand, namely the (OH)HC–CH(OH) part, which is the same situation as those two symmetric dependent bpg ligands in compound 2. Regardless of the degree of disorder, four distinct positions of the hydroxyl groups can be found in all these bpg ligands.

Fig. 1 The local coordination structures of CoII centers in 1 (a) and 2 (b), and perspective view of the rhombic grids for 1 (c) and 2 (d). The four OH groups of the bpg ligands are due to the disorder of the ligand.


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Fig. 2 1D channels along the c-axis in 1 (a) and 2 (b); 2-fold interpenetrated 3D frameworks for 1(c) and 2(d). The Co2+ ions were bridged by the linear bpg ligands to form extended 2D (4,4) frameworks (Fig. 1c and 1d). The Co2+ ions in the layer are well-isolated in complexes 1 and 2 with the shortest intralayer Co··· ···Co ··· distances of 13.511(7) Å for 1 and 13.537(3) Å for 2. Although it appears that there exist large open windows (of the size of 18.221(1) × 20.017(3) Å for 1 and 17.218(9) × 20.893(6) Å for 2, respectively) in the 2D layer as depicted in Fig. 1c and 1d, the 2D layers of both compounds are interpenetrated to each other, resulting in a much smaller open channels. As we can see clearly in Fig. 2, the two interpenetrated layers are both parallel to the c axis, running along the (1, 1, 0) and (1, -1, 0) planes. Due to the different symmetry of the crystals, the two interpenetrated layers in 2 are exactly perpendicular, while they titled from each other with a dihedral angle of 84.119o for 1.



As depicted in Fig. 2a and 2b, the channel along the c axis is of the size 13.409(7) × 14.861(9) Å for 1 and 14.774(1) × 14.774(1) Å for 2, respectively. This interpenetration leads to the shortest interlayer Co···Co distances being 9.110(5) Å for 1 and 8.544(6) Å for 2. Although these distances are significantly shorter than those intralayer Co···Co distances, they are still quite large and should lead to negligible dipole-dipole magnetic interactions between the Co2+ ions. (Fig. S4).

Fig. 3 Hydrogen bonding interactions between the two interpenetrated frameworks (dashes) for 1 (a) and 2 (b). The two interpenetrated frameworks were depicted in blue and red colours. Analysis of the structures revealed that the OH groups in the bpg ligands might be of significant importance to the generation of the interpenetrated structures of both complexes by forming distinct OH···S H-bonds between these OH groups in one grid


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and the sulphur atoms of the thiocyanate groups from the interpenetrated grid. In addition, the different hydrogen bond patterns in 1 and 2 might be responsible for their different structural features (the two interpenetrated frameworks are titled for 1 and perpendicular for 2). As we can see in Fig. 3, the sulphur atoms in 1 have short H-bond interactions (S1···O1A = 3.257(1) Å and S1···O1B = 3.221(4) Å) with only O1A and O1B atoms from the bpg-A ligands (the one with lesser disorder), while there are no short H-bond interactions between S1 and the oxygen atoms (O2A and O2B) from the bpg-B ligands. In contract, in compound 2, the sulphur atoms form H-bonds with the oxygen atoms from both bpg ligands (S1···O1B = 3.072(1) Å and S1···O2B = 3.095(4) Å. Due to this different H-bond pattern, we can see that the interaction between the two interpenetrated frameworks in 1 is more asymmetric compared to that in compound 2, which might lead to the tilt of the two interpenetrated grids. Of course, abundant H-bond interactions between the frameworks and the guest molecules in 1 and 2 as depicted in Fig. S5 and Table S1 might also contribute to their different structures. Magnetic Properties. Direct-current (dc) magnetic susceptibility data of both complexes from 2 to 300 K measured under 1 kOe dc field were depicted in Fig. 4 as χMT versus T plots. At 300 K, the χMT values (2.74 and 2.79 cm3mol-1K for 1 and 2) are in the reasonable range of values of 2.7 - 3.4 cm3mol-1K. These values are significantly larger than 1.875 cm3mol−1K for an octahedron spin-only CoII ion. This fact indicates a clear orbital contribution and large spin-orbital coupling for a high-spin CoII center of a 4T state. As the temperature decreases from 300 K, χMT



values for both complexes decrease slowly above 100 K, and then more quickly to the minima of 1.71 and 1.72 cm3mol-1K at the lowest temperature for 1 and 2. This decrease of the χMT values upon cooling should be mostly caused by the single-ion magnetic anisotropy of the isolated CoII center rather than the antiferromagnetic coupling. The magnetic interactions between the CoII centers in both 1 and 2 should be negligible, either from the superexchange coupling through the long, non-conjugated bpg ligand or from the dipolar magnetic coupling between the adjacent CoII ions.37 Furthermore, we have measured the field dependent magnetizations of 1 and 2 at 2.0 - 5.0 K and the results are also shown in Fig. 4. At the highest field of 70 kOe, the largest values of magnetization (Ms = 1.87, 1.86 and 1.80 µB for 1 and 2.30, 2.29 and 2.21 µB for 2) are well below the expected value (Ms = 3.0 µB for g = 2.0) (Fig. 4, inset). The lack of magnetic saturation at 70 kOe, together with the non-superimposable reduced magnetization plots, suggest the presence of magnetic anisotropy in compounds 1 and 2 (Fig. S6).


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Fig. 4 Temperature dependence of χMT(T) curves at 1 kOe. Inset: The M(H) plots at different temperatures. To obtain the parameters of the magnetic anisotropy of complexes 1-2, we used the PHI51 program to analyze their dc magnetic data. The χMT and the M(H) curves depicted in Fig. 4 were simultaneously fitted using the following spin Hamiltonian: 1 /3 μ g ⋅ ⋅

(1)

In this Hamiltonian, the parameters D and E are the axial and rhombic zero-field splitting parameters and the other symbol have their usual meanings. The best fits gave D = 64.2(1) cm-1, E = -0.43(3) cm-1, gxy = 2.47(4), gz = 2.38(3) for complex 1 and D = 67.5(2) cm-1, E = -2.32(3) cm-1, gxy = 2.48(1), gz = 2.26(3) for complex 2. The values of g and the positive D values clearly indicate the easy-plane magnetic anisotropy of both complexes, being rather common in many CoII-based SIMs reported in the literatures.15



Fig. 5 a) Plots of the frequency-dependent χ″ data of 2 at different dc magnetic fields at T = 1.8 K; b) Magnetic relaxation time at different magnetic fields and its fitting by the sum of direct and QTM processes (see text). As we have mentioned in the Introduction part, slow magnetic relaxation under a dc field can be observed in the Kramers CoII compounds as long as a significant magnetic anisotropy is present. Thus, we measured the alternating-current (ac) magnetic susceptibilities of both complexes to investigate their single-ion magnetic behaviors. The measurements were carried out firstly with no external dc field and the results are shown in Fig. S7. Both compounds were found to have fast relaxation of the magnetization, which leads to no observable out-of-phase χ″ signals. This behavior is most likely caused by the presence of fast magnetic relaxation processes. The direct relaxation and the quantum tunneling of magnetization (QTM) process


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between the Ms = ± 1/2 levels can be very fast as observed in the majority of CoII-based SIMs. An external static field was then applied during the ac measurements, hoping to be able to slow down the magnetic relaxation by suppressing the QTM effects. Obviously, when a small dc field is applied, χ″ ac signals were immediately detected. To determine the optimum dc fields for 1-2, different static fields were applied during the ac measurements. As depicted in Fig. S8, for compound 1, although there are now obvious ac signals, no peaks can be observed above 2 K with frequency up to 1 kOe. This means that the magnetic relaxation is still too fast for the frequency limit of our SQUID. In contrast for 2, apparent peak maxima developed with Hdc = 200 to 4000 Oe (Fig. 5a). The magnetic relaxation time (τ) under different dc fields was then extracted from the analysis of the Cole-Cole plots (χ″ versus χ´, Fig. S9) by the generalized Debye model.52 As we can see from the plot of the τ vs. H (Fig. 5b), the τ has a maximum value at Hdc = 1500 Oe. Stronger external dc fields (H > 1500 Oe) accelerate the magnetic relaxation again (Fig. 5b). The decrease of the relaxation time upon larger magnetic field is very likely because of the direct relaxation process as it is proportional to the H4 for the Kramers CoII ion. Thus, the 1500 Oe dc field was selected as the optimized value to perform further ac measurements under variable temperatures and frequencies (Fig. 6, Fig. S10). For better comparison, the same measurements were performed for 1 under the same external magnetic field of 1500 Oe. Semicircle Cole-Cole plots below 3.6 (for 1) and 4.0 K (for 2) (Fig. 7a, Fig. S11) were fitted again to obtain the details of the relaxation time τ. From these α values of τ (α = 0.01-0.09 for 1 and 0.10–0.22 for 2, Table S3),



we can see that the magnetic relaxation time in both compounds is in a narrow distribution. We have to point out that for 1, as there are no observable peaks in the ac plots, the obtained τ values are only rough estimations. From these relaxation time values, Arrhenius plots, lnτ vs. 1/T, were plotted for both 1 and 2 and analyzed to extract the relaxation parameters. For compound 1, a linear fit of τ using Arrhenius law ! "#$$ / % & led to the reversal barrier of magnetization Ueff = 3.5 cm-1 (5.0 K), with the characteristic relaxation time τ0 = 2.8 × 10−5s (Fig. S12). This low Ueff value is consistent with the fact that there are no peaks observed in all the ac data of 1.

Fig. 6 Frequency-dependent ac susceptibilities measured with Hdc = 1500 Oe. As for compound 2, the same fit of the Arrhenius plot (red line in Fig. 7b) above 2.8 K gave Ueff = 10.6 cm-1 (15.3 K) and τ0 = 9.7 × 10−6s. As we can see from the Arrhenius plot, the bending of the ln(τ) points below 2.8 K suggests the existence of multiple relaxation processes. Furthermore, as observed in most of the CoII SIMs, the obtained energy barrier of 2 from the Arrhenius fit above 2.8 K is smaller than the energy differences (2D) between the ground ± 3/2 levels to the excited ± 1/2 levels.


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This indicates that the magnetic relaxation processes should involve multiple relaxation pathways, including the direct, QTM, Raman and Orbach processes. The expression of relaxation time considering the above four mechanisms should read eq 2

'() & +*- , .& / ! "#$$ /% & ,

(2)

Where the first two terms are expressions corresponding to the field-dependent processes (direct and QTM) with A, B1, and B2 coefficients. The third term corresponds to a Raman process with coefficients C and n, and the last term to a thermally activated Orbach process. At this point, we want to point out that the Raman process is actually also field-dependent.53 However, to avoid over-parameterization, we treat the C coefficient as field independent. At very low temperature, we can neglect the processes involving two phonons (Raman and Orbach mechanisms), and analyze the magnetic relaxation using only QTM and direct processes. Thus, their coefficients A, B1, and B2 can be extracted by fitting the field-dependent τ measured at 1.8 K using the first two terms ( '() &

* +, - ,

). The best fit (Fig. 5b) gives

the coefficients A = 13 s-1K-1kOe-2, B1 = 239 s-1 and B2 = 0.468 s-1K-2. Then, by fixing these obtained coefficients of direct and QTM processes in eq 2 to avoid overparameterization, all these τ data points in Fig. 5b were fitted, giving C = 163.5 K-2s-1, n = 2.6, τ0 = 5.9 × 10−6s, Ueff = 14.5 cm-1 (20.9 K). As the cobalt(II) ions in compounds 1 and 2 have very similar coordination geometries, it is expected that they might give rise to similar magnetic properties. However, our results clearly revealed that although they have quite similar dc magnetic properties, their dynamic magnetic properties at low temperatures are of



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significant differences. The magnetic relaxation for 1 is much faster than that for 2: no peaks in the ac data can be observed in our SQUID, and the estimated energy barrier of 1 is also much lower than that of 2. This result indicates the importance of the guest molecules located in the open channels on the subtle magnetic relaxation properties. First of all, the Co2+ centers in 1 and 2 are separated efficiently (Co···Co = 9.110(5) and 8.544(6) Å) by the long bpg ligands, which prevents the efficient dipole-dipole coupling between the spin centers. Thus, the change of the coordination environments of the Co2+ center by the guest molecules should be the main cause of their difference magnetic properties. On one hand, different guest molecules slightly modify the coordination environments of the CoII centers, as can be seen from their different CShM values. On the other hand, the supramolecular interactions, including the hydrogen bonds between the two-fold interpenetrating frameworks and also between the frameworks and the guest molecules can be largely changed by the guest molecules. These differences might be related to the different magnetic relaxation behaviors of compounds 1 and 2, as they might lead to different relaxation pathways. This behavior has also been reported previously by Cano et al,37 where different guest molecules also induce some slight distortions of the Co2+ coordination environment and different dynamic magnetic properties. Considering the porous metal-organic frameworks of both 1 and 2, we hope to be able

to

dynamically

tune

their

structures

and

magnetic

properties

by

replacing/exchanging the guest molecules. However, although the guest molecules can be removed from the structure by heating as can be seen from the TGA results,


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the frameworks collapse to the amorphous phases. In addition, the crystals dissolved in the pure MeOH or DMF solvents, which preclude the guest exchange experiments. Further attempts will be performed in the future.

Fig. 7 a) Cole-Cole plots for 2 with the fits to the generalized Debye model. b) Plot of lnτ vs T-1 for 2. The red line shows the Arrhenius fitting to a single Orbach process, and green line shows the fit considering all four processes.

Conclusions To conclude, we reported in this work two new cobalt(II) MOF based SIMs with two-fold vertically interpenetrated (4,4) frameworks. By changing the guest molecules included, their structures and magnetic properties, especially the magnetic



dynamics, can be modified. Structurally, different guest molecules result in subtle modification of the local environments of the CoII centers and also the hydrogen bond interactions between the two interpenetrated grids, which changes the dihedral angles between the two sets of grids. Magnetically, the changes of the structures lead to the modification of their magnetic properties, especially their dynamic magnetic properties. Although both compounds show field-induced slow magnetic relaxation, the magnetic relaxation is significantly slower in one compound. This work illustrates the role of the MOFs on the development of the tunable SIMs. Efforts to construct more MOF based SIMs with exchangeable guest molecules are devoted in our lab. ASSOCIATED CONTENT Supporting Information The Supporting Information is avaiable free of charge on the ACS Publication website at DOI:******. Details of physical measurements and X-ray crystallography, additional tables for structural parameters, thermogravimetric analysis results, PXRD patterns, additional figures for structrures and magnetic properties (PDF). AUTHOR INFORMATION

Corresponding Author

* E-mail: [email protected]; [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT ACS Paragon Plus Environment

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For Table of Contents Use Only

Two interpenetrated cobalt(II) metal-organic frameworks with guest dependent structures and field-induced single-ion magnet behaviors Le Shi, Dong Shao, Hai-Yan Wei,* and Xin-Yi Wang*

Two cobalt(II) MOF-based SIMs with two-fold vertically interpenetrated (4,4) grids have been synthesized and characterized. Different guest molecules lead to subtle changes of the coordination environments of the CoII centers and the dihedral angles between the two interpenetrated grids. These changes modify their magnetic properties, especially their dynamic magnetic properties at low temperatures.


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Two Interpenetrated Cobalt(II) MetalâOrganic Frameworks with Guest

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