Systematic Investigation of Controlled Nanostructuring of Mn12 Single

May 30, 2017 - Metal−organic frameworks (MOFs) as platforms for the controlled nanostructuring of single-molecule magnets (SMMs) were systematically...
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Systematic Investigation of Controlled Nanostructuring of Mn12 Single-Molecule Magnets Templated by Metal−Organic Frameworks Darpandeep Aulakh,†,∥ Haomiao Xie,‡,∥ Zhe Shen,‡ Alexander Harley,† Xuan Zhang,§ Andrey A. Yakovenko,⊥ Kim R. Dunbar,*,‡ and Mario Wriedt*,† †

Department of Chemistry & Biomolecular Science, Clarkson University, Potsdam, New York 13699, United States Department of Chemistry, Texas A&M University, College Station, Texas 77845, United States § Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States ⊥ X-ray Science Division, Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439, United States ‡

S Supporting Information *

ABSTRACT: This is the first systematic study exploring metal−organic frameworks (MOFs) as platforms for the controlled nanostructuring of molecular magnets. We report the incorporation of seven single-molecule magnets (SMMs) of general composition [Mn12O12(O2CR)16(OH2)4], with R = CF3 (1), (CH3)CCH2 (2), CH2Cl (3), CH2Br (4), CHCl2 (5), CH2But (6), and C6H5 (7), into the hexagonal channel pores of a mesoporous MOF host. The resulting nanostructured composites combine the key SMM properties with the functional properties of the MOF. Synchrotronbased powder diffraction with difference envelope density analysis, physisorption analysis (surface area and pore size distribution), and thermal analyses reveal that the well-ordered hexagonal structure of the host framework is preserved, and magnetic measurements indicate that slow relaxation of the magnetization, characteristic of the corresponding Mn12 derivative guests, occurs inside the MOF pores. Structural host− guest correlations including the bulkiness and polarity of peripheral SMM ligands are discussed as fundamental parameters influencing the global SMM@MOF loading capacities. These results demonstrate that employing MOFs as platforms for the nanostructuration of SMMs is not limited to a particular host−guest system but potentially applicable to a multitude of other molecular magnets. Such fundamental findings will assist in paving the way for the development of novel advanced spintronic devices.

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INTRODUCTION Single-molecule magnets (SMMs), also known as molecular nanomagnets, are discrete molecules of nanoscale proportions that offer potential applications in molecular spintronics, including ultrahigh density information storage systems and quantum computing.1−7 The combination of large spin ground states and strong uniaxial magnetic anisotropies results in a barrier for spin reversal, characterized by slow magnetization relaxation rates below a threshold temperature giving rise to hysteresis effects. These provide unique opportunities to observe quantum tunneling of magnetization and quantum phase interference through a potential barrier from one orientation to another attributed solely to individual molecules rather than to long-range ordering.8−10 Thus, these unique characteristics of SMM molecules enable the storage or manipulation of information through the precise orientation of molecular spins. The potential to control magnetic information on a molecular level, rather than the long-range ordering found in traditional ferromagnetic materials, has inspired the magnetic community to organize SMMs into uniform multidimensional structures. The first step in this effort is the controlled organization of these molecules, allowing them to be addressed © 2017 American Chemical Society

individually, while retaining their chemical integrity and magnetic properties during the organization process. Pioneering efforts to deposit SMMs on surfaces was pursued in 1998 by Coronado et al.11,12 by exploiting the Langmuir−Blodgett technique to achieve a preferred orientation of SMMs, with their magnetization easy axis normal to the film surface. Several research groups subsequently reported the use of both inorganic and organic two-dimensional (2D) materials as supports for the organization of SMMs, resulting in a variety of new composite materials,13,14 such as polymeric thin films,15 polycarbonate thin films,16 silicon surfaces,17 highly oriented pyrolytic graphite18 (HOPG), self-assembled monolayers19 (SAMs), gold films,20 and ethyl acrylate polymers.21 The isolation of SMMs in these composites has been welldemonstrated, and the magnetic properties of nanostructured SMMs are found to be similar to their pristine precursors. However, additional studies proposed that three-dimensional (3D) ordering of SMMs would provide practical advantages over 2D arrays, circumventing the need for 2D array organization and also negate the need to protect the SMMs Received: February 28, 2017 Published: May 30, 2017 6965

DOI: 10.1021/acs.inorgchem.7b00514 Inorg. Chem. 2017, 56, 6965−6972

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Figure 1. Crystal structures of selected SMMs in the Mn12 family: (A) [Mn12O12(O2CCH3)16(OH2)4]·2CH3COOH·4H2O;31 (B) [Mn12O12(O2CCF3)16(OH2)4]·2CF3COOH·4H2O;32 (C) [Mn12O12[(O2C(CH3)CCH2)16(OH2)4]·4CH2C(CH3)COOH·CH2Cl2;33 (D) [Mn12O12(O2CCH2Cl)16(OH2)4]·2CH2Cl2·6H2O;34 (E) [Mn12O12(O2CCH2Br)16(OH2)4]·4CH2Cl2;35,36 (F) [Mn12O12(O2CCHCl2)16(OH2)4];37 (G) [Mn12O12(O2CCH2But)16(OH2)4]·MeOH;38 and (H) [Mn12O12(O2CC6H5)16(OH2)4]·3C6H5COOH·CH2Cl2.28 Hydrogen atoms and solvent molecules are omitted for the sake of clarity. Color code: Mn4+ (green), Mn3+ (orange), Br (maroon), Cl (turquoise), F (purple), O (red), and C (gray).

applications. To overcome this problem, we proposed MOFs as potential host materials, as their long-range crystalline nature enables the use of XRD studies, including powder and singlecrystal XRD, to investigate important host−guest interactions on a molecular level. In this context, we previously reported the successful incorporation of Mn12Ac SMMs into a mesoporous MOF host, where the sufficiently large pore size and unreactive interior of the framework facilitated the insertion and preservation of SMM’s unique magnetic properties, along with a significantly enhanced thermal stability and long-range spacial order of SMM molecules.39 Subsequent work by Pardo et al. also validated this approach for single-ion magnets (SIMs).40 In continuation of our work, we herein report that this approach is not limited to Mn12Ac alone but is also applicable to other SMMs of the Mn12 family, providing incontrovertible evidence that MOFs are excellent platforms for the nanostructuration of SMMs. In this systematic study we present the optimized synthesis and structural characterization of seven nanocomposites of the composition SMM@MOF, with SMM = Mn12 derivatives, namely, [Mn12O12(O2CR)16(H2O)4] (R = CF3 (1), (CH3)CCH2 (2), CH2Cl (3), CH2Br (4), CHCl2 (5), CH2But (6), and C6H5 (7)); and MOF = CYCU-3 (Figure 2).41 The reaction conditions for SMM incorporation were carefully optimized to achieve maximum loading capacities followed by an exhaustive composite characterization including: synchrotron-based powder diffraction (SPD) with difference envelope density analysis (DED),42,43 porosity measurements (Brunauer−Emmett−Teller (BET) and density functional theory (DFT) pore size distribution), energy-dispersive X-ray analysis (EDX), thermal analyses (thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC)), and magnetic susceptibility measurements (temperature-dependent direct-current (DC), frequency-dependent alternating-current (AC), and field-dependent hysteresis). The resulting structure−

from environmental effects, potentially increasing their thermal and chemical stabilities. This approach utilized porous supports, most notably mesoporous silicas,22−25 and carbon nanotubes.26 However, all of these composite materials share a common feature, namely, the restriction of nanostructures to a shortrange order, which raises questions regarding stability and processability.27 As a result, the controlled long-range organization of SMMs with various dimensionality architectures, while remaining in a protected chemical environment, continues to present a significant challenge. Work in this vein has primarily focused on the mixed-valence dodecanuclear oxomanganese (III, IV) complex, referred to as Mn12Ac with formula [Mn12O12(O2CCH3)16(OH2)4], which behaves as a molecular nanomagnet.28−30 A large spin ground state (S = 10) accompanied by a negative axial magnetic anisotropy (D = −0.5 cm−1) results in an effective energy barrier (Ueff = 50 cm−1) for the reversal of magnetization between the lowest two states, which accounts for the observed magnetic bistability. The core of [Mn12(μ3-O)12] consists of a central [Mn4IVO4]8+ cubane unit connected to a nonplanar outer ring of eight MnIII cations bridged by eight μ3-O2− anions and coordinated to 16 peripheral carboxylate groups. Functionalizing these organic groups permits control over the molecular dimensions (Figure 1) while preserving the metal core and the SMM behavior. The synthesis of these Mn12 derivatives is straightforward, and they exhibit key SMM characteristics of the slow relaxation of the magnetization with relatively high blocking temperatures (4−6 K). Previous approaches to nanostructuring SMMs using mesoporous silicas provide no insights into detailed structure−property relationships, as their amorphous nature does not allow structural characterization through X-ray diffraction (XRD) methods. Consequently, there are currently no fundamental studies of their structure−property relationships, which is absolutely essential for optimization of these composite materials and further development for specific 6966

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solutions of 1−7 at room temperature for 12 h is the optimal loading condition. The resulting composites were carefully worked up, including a filtration step with multiple thorough washings with acetonitrile to exclude the presence of any residual SMMs on the MOF surface. The SMM loading capacities were quantified using EDX analyses (Supporting Information, Section S3). Notably, the majority of the loading capacities were found to exceed any values reported for SMM@ MOF and SMM@silica composites, with the highest being 3.13 mol % for 3@MOF; see Table 1 for a full summary of loading capacities. Table 1. List of SMM Dimensions and SMM@MOF Loading Capacitiesa Calculated from EDX Analyses

SMM@MOF composite 1@MOF 2@MOF 3@MOF 4@MOF 5@MOF 6@MOF 7@MOF

Figure 2. Schematic representation of CYCU-3 with incorporated selected SMM molecules 1−6. Structures are shown in a space-filling style with realistic size relationship between the host−guest system. Hydrogen atoms are omitted for the sake of clarity. Color code: Al (light green), Mn4+ (green), Mn3+ (orange), Br (maroon), Cl (turquoise), F (purple), O (red), and C (gray).

Mn/Al atomic ratio [%]

loading capacity

SMM dimension [nm]

Mn

Al

mol %

wt %

× × × × × × ×

26.13 22.59 27.73 13.47 16.21 10.88 23.39

73.87 77.41 72.27 87.03 83.79 89.11 76.61

2.95 2.43 3.13 1.29 1.61 1.02 2.54

25.86 17.84 24.32 12.96 15.36 9.04 23.34

1.8 1.8 1.9 1.9 1.9 2.1 2.2

nm nm nm nm nm nm nm

1.8 1.9 1.9 1.9 1.9 2.1 2.2

nm nm nm nm nm nm nm

a

Note that loading capacities were calculated from molar weights of components neglecting any non-coordinating solvent molecules (Supporting Information, Section S3).

property relationships are discussed in the context of their magnetic behavior and thermal stabilities with the focus on properties of the as-synthesized pristine SMMs versus SMM@ MOF composites.

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Structural Characterization. In general, the combination of EDX analysis, SPD with DED analyses, BET with DFT pore size analyses, and TGA with DSC analyses allowed for detailed insight into the chemical composition and physical nature of SMM@MOF composites. The integrity of MOF host structures was confirmed by the absence of additional Bragg reflections in SPD patterns of SMM@MOF composites versus as-synthesized MOF (Supporting Information, Figures S9−S15 and Le Bail fits in Figures S25−S28). Note that the change in reflection intensities at low Bragg angles is attributed to phase cancellation effects from the presence of SMM molecules within the MOF cavities. Subsequent DED analyses provided additional evidence for SMM adsorption into the pore as opposed to an external surface deposition (Supporting Information, Section S4). DEDs of 2@MOF, 3@MOF, 4@MOF, and 6@MOF are shown in Figure 3 as representative examples. Each exhibits hexagonalshaped electron densities as the primary feature within the center of the mesopores. This can be attributed to the high electron density metal core of respective SMMs. Any defined interactions with pore walls can be excluded based on the central nature of adsorption sites, and in addition, it can be concluded that only a single SMM molecule is adsorbed in the transverse direction of the mesopores. However, note that the relatively high SMM loading capacities suggest that multiple SMMs might be stacked along the vertical direction of the pores. The integrated total electron density in difference envelopes at 1.5σ was calculated as 17.28, 19.45, 6.48, and 5.55, respectively, for 2@MOF, 3@MOF, 4@MOF, and 6@MOF. The lower values in 4@MOF and 6@MOF corroborate the respective low loading capacities calculated from EDX analyses. Adsorption Properties. The mesoporosity of nanocomposites is illustrated by N 2 adsorption−desorption

RESULTS AND DISCUSSION Synthetic Aspects. Seven Mn12Ac SMM derivatives were selected as guest molecules, namely, [Mn12O12(O2CCF3)16(OH2)4]·2CF3COOH·4H2O32 (1), [Mn12O12(O2C(CH3)CCH 2 ) 16 (OH 2 ) 4 ]·4CH 2 C(CH 3 )COOH·CH 2 Cl 2 33 (2), [Mn 12 O 12 (O 2 CCH 2 Cl) 16 (OH 2 ) 4 ]·2CH 2 Cl 2 ·6H 2 O 34 (3), [Mn12O12(O2CCH2Br)16(OH2)4]·4CH2Cl235 (4), [Mn12O12(O 2CCHCl 2) 16 (OH2 ) 4]37 (5), [Mn12 O12 (O2CCH 2Bu t) 16(OH2)4]·MeOH38 (6), and [Mn12O12(O2CC6H5)16(OH2)4)]· 3C6H5COOH·CH2Cl228 (7) with molecular dimensions ranging from 1.8 to 2.2 nm. All SMMs were readily prepared by modification of the organic periphery in Mn12Ac via previously reported ligand substitution reactions (see Experimental Section). The MOF [Al(OH) (SDC)]n41 (H2SDC = 4,4′-stilbenedicarboxylic acid) or CYCU-3 with two types of one-dimensional (1D) channels, namely, hexagonal mesopores of ∼3 nm diameter and trigonal micropores of ∼1.6 nm diameter, that exhibits high thermal and chemical stability, was selected as the host framework. These attributes suggest CYCU-3 as a well-suited host system for SMM incorporation with the added advantage that the diamagnetic aluminum centers will not influence the overall SMM properties. Reaction parameters, including the solvent, temperature, and reaction times were systematically tuned to optimize the incorporation of 1−7 into the mesoporous MOF hosts. It was found that the loadings performed under mild heating (up to 50 °C) or refluxing resulted in SMM decomposition. Maximum incorporation was obtained after stirring the reaction mixture at room temperature for 12 h, with no further improvements being observed for longer reaction times. On the basis of these observations and our previous experience with Mn12Ac, it was determined that soaking the MOF in saturated acetonitrile 6967

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Figure 3. Observed structure difference envelope densities (ρΔ = ρcomposite − ρMOF, shown in violet) overlapped with a structural model of the host MOF for (A) 2@MOF; (B) 3@MOF; (C) 4@MOF; and (D) 6@MOF.

g−1 in 6@MOF, and 1.31 cm3 g−1/2118.3 m2 g−1 in 7@MOF (Table 2). At the same time, the total N2 uptake capacities are

isotherms at 77 K and pore size distributions (Figure 4A,B). Reversible type IV isotherms show a well-defined step at

Table 2. Summary of Specific Surface Areas and Pore Volumes Calculated from Nitrogen Adsorption Isotherms compound

BET surface area [m2 g−1]

N2 uptake [cm3 g−1 STP]

mesopore volume [cm3 g−1]

MOF 1@MOF 2@MOF 3@MOF 4@MOF 5@MOF 6@MOF 7@MOF

2978.3 1575.3 1135.0 1102.0 1609.6 2245.1 2074.6 2118.3

1158.6 594.7 405.0 331.6 683.6 909.5 1016.2 950.7

1.70 0.84 0.51 0.49 0.70 1.12 0.93 1.31

reduced, while the respective micropore volumes remain unchanged. Both effects can be attributed to the inclusion of Mn12 clusters into mesopores. Notably, in composites 3−6@ MOF, the formation of a new micropore is observed. This new pore of 1.2 nm in diameter can be assumed to have emerged from partial SMM decomposition during the thermal activation process, with the small-sized decomposition products potentially resulting in partial blockages of the pristine micropore. Thermal Analyses. Heating the as-synthesized MOF to 130 °C revealed a well-defined mass loss of Δmexp = 32.3% in the thermal gravimetric (TG) curve, which is in very good agreement with the calculated loss of two dimethylformamide (DMF) molecules [Δmcalc(2DMF) = 32.0%]. Following this event, the MOF starts decomposing at 350 °C (Figure S30, blue curve). When as-synthesized 1 was heated to 350 °C, a complex three-step decomposition is observed in the TG curve, leading to a combined mass loss of 66.9%, which can be attributed to the loss of water molecules present in the crystal structure of the Mn12 carboxylate, and to the partial elimination of trifloroacetic acid peripheral ligands (Figure S30, red curve). Heating composite 1@MOF under similar conditions led to a weight loss of 12.9%, which is in rough agreement with the calculated decomposition of 2.95 mol % [Δmcalc(66.9% of 2.95 mol % 1) = 16.7%] (Figure S30, black curve). The same evaluation scheme was applied for all composites yielding weight losses of 7.1% for 2@MOF, 12.2% for 3@MOF, 4.7% for 4@MOF, 9.2% for 5@MOF, 6.7% for 6@MOF, and 9.1% for 7@MOF (Figures S31−S36). These values are found to be in good agreement with those calculated for the decomposition

Figure 4. (A) N2 adsorption isotherms collected at 77 K; and (B) DFT pore size distributions by differential pore volume; both collected for the pristine MOF and composites 1−7@MOF.

intermediate partial pressures (0.2 < p/p0 > 0.5), which can be assigned to capillary condensation effects of N2 inside the mesopores. In addition, the pore size distribution derived from DFT calculations suggests the presence of a binary pore system, including micropores (1.5 nm) and mesopores (2.64 nm; Figure 4B). Upon SMM incorporation, a significant reduction in the mesopore volume along with a decrease in BET surface area is observed: from 1.70 cm3 g−1/2978.3 m2 g−1 in the pristine MOF to 0.84 cm3 g−1/1575.3 m2 g−1 in 1@MOF, 0.51 cm3 g−1/1135.0 m2 g−1 in 2@MOF, 0.49 cm3 g−1/1102.0 m2 g−1 in 3@MOF, 0.70 cm3 g−1/1609.6 m2 g−1 in 4@MOF, 1.12 cm3 g−1/2245.1 m2 g−1 in 5@MOF, 0.93 cm3 g−1/2074.6 m2 6968

DOI: 10.1021/acs.inorgchem.7b00514 Inorg. Chem. 2017, 56, 6965−6972

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Figure 5. Temperature dependence of the out-of-phase AC magnetic susceptibility at different AC frequencies for (A) Mn12Ac@MOF; (B) 1@ MOF; (C) 2@MOF; (D) 3@MOF; (E) 4@MOF; (F) 5@MOF; (G) 6@MOF; and (H) 7@MOF.

reactions of their respective SMM loadings: Δmcalc(35.9% of 2.43 mol % 2) = 6.4%; Δmcalc(45.9% of 3.13 mol % 3) = 11.2%; Δmcalc(36.9% of 1.29 mol % 4) = 4.8%; Δmcalc(56.3% of 1.61 mol % 5) = 8.6%; Δmcalc(73.8% of 1.02 mol % 6) = 6.7%; and Δmcalc(41.7% of 2.54 mol % 7) = 9.6%, respectively. Note that these calculations were performed based on molecular weights from SMM molecules neglecting any noncoordinating solvent molecules (Table S8). From this systematic thermal analysis, we can conclude that no solvent molecules are incorporated into the MOF host during the SMM inclusion process

resulting from the presence of preferred SMM loadings at the periphery of the MOF bulk crystals, thereby preventing solvent molecules from infiltrating the inner pore regime. Additional DSC measurements were performed to investigate the thermal decomposition processes of SMMs versus their respective composites. All as-synthesized SMMs exhibit a well-pronounced exothermic event in their DSC curves, a common decomposition characteristic, whereas most of their respective composites showed this event broadened and shifted to significantly higher temperatures (207 vs 303 °C for 1@ 6969

DOI: 10.1021/acs.inorgchem.7b00514 Inorg. Chem. 2017, 56, 6965−6972

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Inorganic Chemistry MOF; 178 vs 287 °C for 2@MOF; 219 vs 392 °C for 3@MOF; 187 vs 313 °C for 4@MOF; 167 vs 223 °C for 5@MOF; and 236 vs 434 °C for 7@MOF; Figures S37−S41, S43). No exothermic signal in the DSC curve of 6@MOF could be resolved due to its relatively low SMM loading capacity (Table 1 and Figure S42). Any contribution to these thermal events originating from the MOF decomposition can be excluded, as no exothermic signal can be observed for the as-synthesized MOF within the scanned temperature range. From the observed signal shifts, it can be concluded that the confinement of SMM clusters within the nanoscopic MOF cavities enhances their thermal stability. As a further consequence of this confinement a rather slow SMM decomposition as compared to the as-synthesized SMMs can be observed, which is reflected in the peak broadening of the respective exothermic signals. Similar thermal characteristics were reported for the Mn12Ac@ CYCU-3 composite.39 Host−Guest Correlations. EDX, adsorption, and DED analyses reveal that both bulkiness and polarity of peripheral SMM ligands are fundamental parameters influencing the global SMM@MOF loading capacities. Among all studied composites (Table 1), 1@MOF shows the highest loading capacity, which is no surprise given that 1 represents the smallest SMM and is composed of peripheral highly polar trifloroacetic groups inducing strong electrostatic interactions with the MOFs AlO6 metal clusters. SMMs 2 and 3 are both similar in size, but their peripheral polarities significantly differ, methacrylic groups in 2 versus more polar chloroacetic groups in 3, resulting in enhanced loading capacities in 3@MOF as compared to 2@MOF. Similar trends can be observed for 4@ MOF and 5@MOF with bromoacetic versus dichloroacetic groups, respectively. The nonpolar, bulky tert-butyl groups in 6 mitigate the loading capacity of 6@MOF to the lowest among all composites. Notably, 7@MOF shows one of the highest loading capacities resulting from potential strong intermolecular π−π stacking interactions between the peripheral benzoate ligands of 7 and the stilbene dicarboxylic linkers of the MOF. Further potential intermolecular SMM−SMM π−π stacking interactions in 7@MOF result in a more efficient SMM packing as found in 6@MOF, which is evident from their respective available mesopore volumes (0.93 vs 1.31 cm3 g−1; Table 1). Magnetic Characterization. The magnetic properties of composites 1−7@MOF were extensively investigated by probing the characteristic magnetization dynamics of Mn12 derivatives with variable-frequency and variable-temperature AC susceptibility measurements. These data were measured under zero DC field over the frequency range of 1−1000 or 1− 1500 Hz, revealing significant frequency dependence of the outof-phase AC magnetic susceptibility (χ″) as the signal maxima shifted to higher temperatures with increasing frequencies (Figure 5 and Figures S49C−S55C). This behavior is typical of slow relaxation of the magnetization known for the Mn12 family of SMMs, and thus, it can be concluded that the unique magnetic properties of SMMs 1−7 were preserved during their MOF incorporation processes. Magnetic data of the recently reported Mn12Ac@MOF composite is displayed as reference in Figure 5A.39 Notably, an additional set of less-intense χ″ peaks at lower temperatures (below 3 K) is observed for 2@MOF, 3@MOF, 5@MOF, and 6@MOF, which is attributed to the presence of Jahn−Teller isomers of respective SMMs as reported previously.33,34,37,38 Fitting the corresponding temperatures and frequencies of the observed χ″ maxima to the Arrhenius law, τ = τ0 exp(ΔE/

kBT), where τ is the relaxation time of magnetization, T is the temperature, ΔE is the activation energy, and kB is the Boltzmann constant, resulted in respective energy barriers (ΔE/kB) and pre-exponential factors (τ0) for given relaxation processes (Figures S49A−S55A). A summary of resulting parameters is presented in Table 3 along with those of the asTable 3. Summary of Selected Magnetic Parametersa of SMM@MOF Composites composite

energy barrier Ueff [K]

Mn12Ac@MOF 1@MOF 2@MOF 3@MOF 4@MOF 5@MOF 6@MOF 7@MOF

57 [70] 48 [48] 51 [66] 54 [67.2*] 53.8 [67.4*] 48 [53.6*] 62 [70.0*] 42 [53.7*]

pre-exponential factor τ0 [s] 5.2 8.9 1.4 1.5 5.5 5.5 2.4 3.1

× × × × × × × ×

10−9 10−9 10−8 10−8 10−9 10−9 10−9 10−8

[1.2 × 10−8]39 [7.4 × 10−9]32 [8.5 × 10−9]33 [4.2 × 10−9*] [8 × 10−9*] [2.6 × 10−8*] [7.2 × 10−9*] [1.4 × 10−8*]

a

Control parameters of as-synthesized SMMs are listed in square brackets (respective references are given or denoted by *, which indicates data from this work; see Figures S44−S48).

synthesized SMMs (control samples). Subtle changes in the properties of the composites versus as-synthesized SMMs are not surprising given the differences in the chemical environment of SMMs and the absence of crystallizing solvent molecules within the MOF pores. Further magnetic characterization included variable-temperature DC (Figures S49B−S55B) and field-dependent magnetization measurements (Figures S49D-S55D), with both exhibiting typical SMM behavior. In particular, a narrowing of the hysteresis loop at low fields is consistent with quantum tunneling of the magnetization, which also illustrates that the SMM properties remain largely unaffected by nanoscale confinement.

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CONCLUSIONS The hexagonal channels of the mesoporous MOF host CYCU3 have been employed as a template for the nanostructuring of seven Mn12 SMM derivatives of compositions [Mn12O12(O2CR)16(H2O)4] with R = CF3 (1), (CH3)CCH2 (2), CH2Cl (3), CH2Br (4), CHCl2 (5), CH2But (6), and C6H5 (7). The resulting nanostructured composites exhibit key SMM properties along with functional properties of the host framework. Structural characterization by synchrotron diffraction, physisorption analysis, and thermal analyses suggests the preservation of the hexagonal host structure upon SMM incorporation, while magnetic measurements revealed slow relaxation of the magnetization. These results firmly support the conclusion that the unique SMM behavior of these compounds is fully retained during the nanostructuration process. Importantly, the data reveal that the incorporated SMM molecules exhibit enhanced thermal stabilities with only a single SMM molecule being adsorbed in the transverse direction of the MOF pores. This study establishes that there are exciting possibilities for the realization of new magnetic materials based on this relatively unexplored area of nanostructured molecular magnets.

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EXPERIMENTAL SECTION

Synthesis of Composite Precursors. All reagents and solvents were used without further purifications. The precursors CYCU-341 and 6970

DOI: 10.1021/acs.inorgchem.7b00514 Inorg. Chem. 2017, 56, 6965−6972

Article

Inorganic Chemistry Mn12 derivatives 1,32 2,33 3,34 4,35 5,37 6,38 and 728 were synthesized as previously reported. As-synthesized CYCU-3 was partially activated by heating at 100 °C for 12 h in a convection oven before any use. The purities of bulk materials were confirmed by powder XRD (Figures S1−S8). See Figure S16 for full IR data. Synthesis of SMM@MOF Composites. The incorporation of 1− 7 into CYCU-3 was performed by adding 0.1 g of partially activated CYCU-3 into respective saturated SMM solutions of dry acetonitrile under a N2 atmosphere. The mixtures were stirred at room temperature in closed vials for 12 h. Note that the color of respective SMM−MOF dispersions did not fade during the composite syntheses indicating that excesses of SMMs were used for the reactions. The resulting brown powders were filtered, thoroughly washed with acetonitrile until the filtrate became colorless, and dried at room temperature. The purities of bulk materials were confirmed by powder XRD (Figures S9−S15). See Figure S17 for full IR data. General Analytical Techniques. Powder XRD data were recorded on a Bruker D2 Phaser diffractometer equipped with a Cu sealed tube (λ = 1.541 78 Å). Powder samples were dispersed on lowbackground discs for analyses. EDX analyses were performed with a Thermo NORAN System Six EDX coupled to a JEOL JSM-7400F field-emission scanning electron microscope (FESEM) set to an acceleration voltage of 15 kV and a working distance of 8 mm. TG data were recorded using a TGA Q50 from TA Instruments. All measurements were performed using platinum crucibles in a dynamic N2 atmosphere (50 mL min−1) over the range of 25−700 °C with a heating rate of 3 °C min−1. DSC data were recorded using a TGA Q20 from TA Instruments. All measurements were performed using T zero aluminum pans and a dynamic N2 atmosphere (50 mL min−1) over the range of 25−400 °C at a heating rate of 3 °C min−1. Gas adsorption isotherms for pressures in the range from 1 × 10−5 to 1 bar were measured by a volumetric method using a Micromeritics ASAP2020 surface area and pore analyzer. A preweighed analysis tube was charged with the sample, capped with a seal frit, and evacuated by heating at 150 °C under a dynamic vacuum for 12 h. The evacuated analysis tubes containing the activated samples were then carefully transferred to an electronic balance and weighed to determine the sample mass. The tubes were then transferred to the analysis port of the gas adsorption instrument. For all isotherms, warm and cold free-space correction measurements were performed using ultrahigh purity He gas (UHP grade 5.0, 99.999% pure). N2 (99.999% purity) isotherms at 77 K were measured in liquid nitrogen. The temperature and fill levels were monitored periodically throughout the measurements. Oil-free vacuum pumps and oil-free pressure regulators were used for all measurements to prevent contamination of the samples during the evacuation process or of the feed gases during the isotherm measurements. Fourier transform infrared (FT-IR) data were recorded on a Nicolet iS10 from Thermo Scientific. Synchrotron Powder Diffraction Measurements. As-synthesized composites were used for all data collections, which were recorded at 17-BM beamline at the Advance Photon Source, Argonne National Laboratory (Argonne, IL). The incident X-ray wavelength was 0.727 68 Å. Data were collected using a PerkinElmer flat panel area detector (XRD 1621 CN3-EHS) over the angular range of 1−11° 2θ. Magnetic Measurements. Magnetic measurements were performed on a Quantum Design MPMS XL SQUID magnetometor over the temperature range of 1.8−300 K. AC magnetic susceptibility data were collected with an oscillating measuring field of 2 or 5 Oe in the frequency range of 1−1000 or 1−1500 Hz, respectively. The diamagnetic contributions of the atoms and sample holders were accounted for with Pascal’s constants.

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Powder XRD data, IR data, EDX, synchrotron-based structure envelope studies, TGA and DSC curves, magnetic characterizations, additional references (PDF)

AUTHOR INFORMATION

Corresponding Authors

*Phone: +1(979) 845-5235. Fax: +1(979) 845-7177. E-mail: [email protected]. (K.R.D.) *Phone: +1(315) 268-2355. Fax: +1(315) 268-6610. E-mail: [email protected]. (M.W.) ORCID

Kim R. Dunbar: 0000-0001-5728-7805 Mario Wriedt: 0000-0003-3118-9507 Author Contributions ∥

D.A. and H.X. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge Clarkson Univ. for their generous start-up funding and the Donors of the American Chemical Society Petroleum Research Fund (56295-DNI10). Work performed at the Advanced Photon Source at Argonne National Laboratory was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. The magnetic measurements were conducted in the Department of Chemistry SQUID Facility at Texas A&M Univ. with a magnetometer obtained by a grant from the National Science Foundation (CHE-9974899). The magnetic work in this study was performed by K.R.D. group and was funded by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences of the U.S. Department of Energy through Grant No. DE-SC0012582.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b00514. 6971

DOI: 10.1021/acs.inorgchem.7b00514 Inorg. Chem. 2017, 56, 6965−6972

Article

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DOI: 10.1021/acs.inorgchem.7b00514 Inorg. Chem. 2017, 56, 6965−6972