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Feb 10, 2017 - Bond valence sum calculations and bond lengths indicate the presence of CoII centers in compound 1, mixed valent Co centers (CoII/CoIII...
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Modulating Magnetic Refrigeration through Structural Variation in CoII/III−GdIII Clusters Javeed Ahmad Sheikh and Abraham Clearfield* Department of Chemistry, Texas A&M University, College Station, Texas 77843-3255, United States S Supporting Information *

ABSTRACT: Three heterometallic aggregates, [(CoII)2(GdIII)2(tBuPO3)2(O2CtBu)2(HO2CtBu)2(NO3)4]·NEt3 (1), [(CoII)2(CoIII)2(GdIII)3(μ3-OH)2(tBuPO3)2(O2CtBu)9(deaH)2(H2O)2] (2), and (CoIII)2(GdIII)5(μ2-OH)(μ3OH)2(tBuPO3)2(O2CtBu)10(HO2CtBu)(deaH)2]·MeOH (3), were successfully isolated in reactions of [Co2(μ-OH2)(O2CtBu)4]·(HO2CtBu)4, Gd(NO3)3· 6H2O, tBu-PO3H2, and diethanolamine (deaH3) by varying the stoichiometry of the reactants and/or changing the solvent. The structures of the final products were profoundly affected by these minor changes in stoichiometry or a change in solvent. The metal−oxo core of these complexes displays a hemicubane or a defective dicubane-like view. Bond valence sum calculations and bond lengths indicate the presence of CoII centers in compound 1, mixed valent Co centers (CoII/CoIII) in compound 2, and only CoIII centers in compound 3 as required for the charge balances and supported by the magnetic measurements. Magnetic studies reveal significant magnetic entropy changes for complexes 1−3 (−ΔSm values of 28.14, 25.06, and 29.19 J kg−1 K−1 for 3 K and 7 T, respectively). This study shows how magnetic refrigeration can be affected by anisotropy, magnetic interactions (ferro- or antiferromagnetic), the metal/ligand ratio, and the content of GdIII in the molecule.

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starting materials.8 Therefore, we employed diethanolamine (deaH3) as a co-ligand along with the phosphonic acid in the synthesis. This was done to exploit the coordination preferences of the 3d and 4f ions toward different donor atoms and also with the intention of getting CoIII in situ from CoII starting material. In our very recent work, we have reported two CoIII−GdIII cages employing phosphonic acid and diethanolamine (deaH3) ligands.8c This was the first example in which amino polyalcohols were used along with the phosphonates followed by the study presented here. Thus, this synthetic strategy should be quite useful in the construction of high-nuclearity complexes with interesting magnetic properties such as magnetic refrigerants and single-molecule magnets (SMMs). Herein, we report the synthesis, structural, and magnetic studies of three new heterometallic Co−Gd cage compounds, [(Co II ) 2 (Gd III ) 2 ( t BuPO3 ) 2 (O 2 C t Bu) 2 (HO 2 C t Bu) 2 (NO 3 ) 4 ]· NEt 3 (1), [(Co II ) 2 (Co III ) 2 (Gd III ) 3 (μ 3 -OH) 2 ( t BuPO 3 ) 2 (O2CtBu)9(deaH)2(H2O)2] (2), and (CoIII)2(GdIII)5(μ2-OH)(μ3-OH)2(tBuPO3)2(O2CtBu)10(HO2CtBu)(deaH)2]·MeOH (3). Bond valence sum (BVS) calculations9 and bond lengths indicate the presence of CoII centers in compound 1, mixed valent Co centers (CoII/CoIII) in compound 2, and only CoIII centers in compound 3 as required for the charge balance and supported by the magnetic measurements. Magnetic studies

INTRODUCTION Polynuclear paramagnetic cage complexes have emerged as an active area of research in the recent past because of their immense technological applications.1 Complexes containing 3d and 4f ions are steadily being explored for interesting magnetic properties such as single-molecule magnet (SMM)2 behavior and magnetic refrigeration. Magnetic refrigeration is based on the magnetocaloric effect (MCE)3 that in turn relies on the change in entropy of a material in a magnetic field. Magnetic refrigerants have been proposed to achieve very low temperatures in the sub-kelvin range and replace the expensive and rare helium-3 in some ultra-low-temperature cooling applications. Molecules with negligible anisotropy and a high-spin ground state are ideal candidates for the MCE. Additionally, the presence of ferromagnetic interactions between the metal centers can be favorable. Most of the compounds for this application often involve the isotropic GdIII ion (S = 7/2), and accordingly, a number of homo- and heterometallic (Gd-3d) cages are now well documented in the literature.4 Phosphonates of the general formula RPO32− have been well explored as bridging ligands in the construction of molecular metal clusters and cages.5 In particular, Winpenny et al. have prepared a number of 3d−4f polymetallic complexes using phosphonates, which show significant MCEs.6 Amino polyalcohol ligands have also widely been used in the synthesis of 3d/3d−4f cages of variable nuclearity and interesting magnetic properties.7 We and others have found that use of aminopolyalcohol ligands in some cases results in CoIII from CoII © XXXX American Chemical Society

Received: December 8, 2016

A

DOI: 10.1021/acs.inorgchem.6b02398 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 1. Crystal Data and Structural Refinement for Complexes 1−3 formula formula weight T (K) wavelength (Ǻ ) space group crystal system a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z Dcalcd (g cm−3) μ (mm−1) F(000) θmin, θmax (deg) no. of reflections collected no. of unique reflections R1, wR2 [I ≥ 2σ(I)] goodness of fit (GOOF) on F2

1

2

3

C40H84Co2Gd2N6O26P2 1559.43 100(2) 0.71073 P21/n monoclinic 12.2514(5) 11.1087(5) 23.5042(10) 90.00 102.143(2) 90.00 3127.3(2) 2 1.656 2.745 1572 1.74, 29.73 8897 6713 0.0382, 0.1250 0.834

C61H117Co4Gd3N2O32P2 2159.98 100(2) 0.71073 P1̅ triclinic 12.59(3) 14.48(4) 25.48(6) 99.76(4) 97.78(4) 105.15(5) 4339(19) 2 1.653 3.117 2166 1.91, 24.55 14522 8629 0.0818, 0.1850 0.944

C76H139Co2Gd5N2O37P2 2635.92 100(2) 0.71073 P21/n monoclinic 12.826(3) 30.399(7) 28.885(8) 90.00 93.113(16) 90 11246(5) 4 1.557 3.291 5229 0.973, 27.91 26919 17484 0.0616, 0.1988 1.163

filtrate was kept at room temperature for slow evaporation. Within 3− 4 days, purple block-shaped single crystals suitable for X-ray analysis were formed. The crystals were collected by filtration, washed with cold CH3CN, and dried in air (yield of 49% based on Co2). Elemental analysis: Calcd (found) for C40H84Co2Gd2N6O26P2: C, 30.80 (29.64); H, 5.42 (4.81); N, 5.38 (4.91). [(CoII)2(CoIII)2(GdIII)3(μ3-OH)2(tBPO3)2(O2CtBu)9(deaH)2(H2O)2] (2). This compound was synthesized following a procedure similar to that described for 1, but Co2 was scaled to double (100 mg, 0.1 mmol). Purple block-shaped crystals suitable for X-ray diffraction were collected by filtration after 4 days (yield of 54%, based on Co2). Elemental analysis: Calcd (found) for C61H117Co4Gd3N2O32P2: C, 33.91 (33.24); H, 5.45 (4.89); N, 1.29 (1.16). [(Co III ) 2 (Gd III ) 5 (μ 2 -OH)(μ 3 -OH) 2 ( t BuPO 3 ) 2 (O 2 C t Bu) 10 (HO 2 C t Bu)deaH)2]·MeOH (3). This compound was synthesized in a 1:1 CH3CN/ MeOH mixture following a procedure similar to that used for 1, but Gd(NO3)3·6H2O was scaled to double (90 mg, 0.2 mmol) . The solution was filtered, and the filtrate was kept at room temperature for slow evaporation. Within 3−4 days, purple block-shaped single crystals suitable for X-ray analysis were formed. The crystals were collected by filtration, washed with cold CH3CN, and dried in air (yield of 43% based on Co 2 ). Elemental analysis: Calcd (found) for C76H139Co2Gd5N2O37P2: C, 34.58 (32.84); H, 5.30 (5.61); N, 1.06 (0.94).

reveal significant magnetic entropy changes for complexes 1−3 (−ΔSm values of 28.14, 25.06, and 29.19 J kg−1 K−1 for 3 K and 7 T, respectively).

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EXPERIMENTAL SECTION

X-ray Crystallography. Data for the compounds were collected at 100 K on a Bruker Smart Apex 2 CCD diffractometer with Mo Kα (λ = 0.71073 Å) radiation using a cold nitrogen stream (Oxford). Data reduction and cell refinements were performed with SAINT,10 and the absorption correction program SADABS11 was employed to correct the data for absorption effects. Crystal structures were determined by direct methods and refined with full-matrix least squares (SHELXTL97)12 with atomic coordinates and anisotropic thermal parameters for all non-hydrogen atoms. The structures of complexes 2 and 3 contain solvent accessible voids; hence, the SQUEEZE13 module of the program suite PLATON14 was used to generate a fresh reflection file. The crystallographic data are summarized in Table 1 (CCDC entries 1507505−1507507). Materials and Methods. All the complexes were synthesized from the starting materials, [Co2(μ-OH2)(O2CtBu)4]·(HO2CtBu)4] (Co2 hereafter), which was made by a literature method.15 Other reagents were used as received from Sigma-Aldrich without any further purification. Magnetic susceptibility and magnetization measurements were taken on a Quantum Design SQUID-VSM magnetometer. The measured values were corrected for the experimentally measured contribution of the sample holder, while the derived susceptibilities were corrected for the diamagnetism of the samples, estimated from Pascal’s tables.1a Direct current magnetic measurements were performed with an applied field of 1000 G in the temperature range of 2−300 K. BVS calculations were performed following the procedure described by Liu and Thorpe.9 Elemental analyses were performed on an Elementar vario Microcube elemental analyzer. Thermogravimetric analysis (TGA) experiments were performed on a TGA Q500 TA instrument. Synthesis. [(CoII)2(GdIII)2(tBuPO3)2(O2CtBu)2(HO2CtBu)2(NO3)4]· NEt3 (1). Co2 (50 mg, 0.05 mmol) was dissolved in CH3CN (8 mL), and deaH3 (11 mg, 0.1 mmol) was added to the solution while it was being stirred followed by the addition of tBu-PO3H2 (14 mg, 0.1 mmol) and NEt3 (40 mg, 0.4 mmol). Finally, Gd(NO3)3·6H2O (45 mg, 0.1 mmol) was added, and the final mixture was further stirred overnight at room temperature. The solution was filtered, and the

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RESULTS AND DISCUSSION Synthetic Aspects. Reactions of the cobalt pivalate dimer with phosphonic acid, deaH3, and Gd(NO3)3·6H2O in CH3CN or a CH3CN/MeOH mixture at different stoichiometries and under ambient conditions generated three heterometallic Co− Gd cages. The structures of the final products were profoundly affected by these minor changes in stoichiometry or a change in solvent. TG analysis of all the complexes reveals a weight loss of around 5% up to 150 °C that may be assigned to the coordinated and noncoordinated solvent molecules for 2 and 3. Further weight loss of around 10−15% in the temperature range of 150−250 °C probably corresponds to the aromatic ligands, after which the complexes decompose completely (Figures S1−S3). B

DOI: 10.1021/acs.inorgchem.6b02398 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Structural Description. Complex 1 (Co2Gd2) crystallizes in the P21/n space group and is a tetranuclear heterometallic cage comprising a total of two CoII centers and two GdIII centers (Figure 1).

Figure 3. Ball and stick model showing the molecular structure of 2 in the crystal. The color code is the same as in Figure 1.

Figure 1. Ball and stick model showing the molecular structure of 1 in the crystal. Color code: purple, gadolinium; blue, cobalt; orange, phosphorus; red, oxygen; cyan, nitrogen; gray, carbon. Hydrogen atoms have been omitted for the sake of clarity.

The metal centers of this tetranuclear core are primarily linked by two phosphonate ligands in 4.221 coordination mode16 (Figure 2 and Figure S4). Peripheral ligation is

Figure 4. Core structure of 2. The color code is the same as in Figure 1.

The metal centers at the core are primarily linked by two μ3OH groups and two phosphonate ligands in 4.221 coordination mode11 (Figure 4 and Figure S4). The pivalate and [deaH]2− ligands surround the periphery of the complex in addition to two coordinating water molecules. The oxygen atoms of the pivalate ligands are seen either bridging the two metal centers in a 2.11 bridging mode or connected to only one metal center in a 1.11 mode (Figure S4). The doubly deprotonated [deaH]2− ligands show the 3.221 bridging mode (Figure S4). This leads to all the GdIII centers having an O8 coordination environment and featuring distorted square antiprismatic geometry (Figure S5). Of the four cobalt centers, three have octahedral geometry with O5N (Co4 and Co5) or O6 (Co7) coordination environments, and the fourth (Co6) features square pyramidal geometry with an O5 coordination environment (Figure S5). The Gd centers and two of the four cobalt centers (Co4 and Co5) form hemicubane-like units that are further linked to two more cobalt centers through bridging phosphonate ligands (Figure 4). The core of the molecule also resembles a basket and can be described as comprised of defective dicubanes (Figure S6). The average CoIII−O/CoIII−N and CoII−O bond distances are 1.90 and 2.02 Å, respectively. The average Gd−O bond distance is 2.37 Å. The average Co−O−Gd and Gd−O− Gd bond angles are 102.5° and 111°, respectively.

Figure 2. Core structure of 1. The color code is the same as in Figure 1.

provided by four pivalate groups and four nitrate anions, thus completing the coordination sphere of each ion. This finally results in both the cobalt ions being tetrahedral with O4 coordination and GdIII ions being nonacoordinated (O9) featuring distorted tricapped trigonal prismatic geometry (Figure S5). In addition to this, a triethylamine molecule was also found in the crystal structure. The average Co−O and Gd−O bond distances are 1.98 and 2.45 Å, respectively. The average Co−O−Gd bond angle is 109.1°. Complex 2 (Co4Gd3) crystallizes in the P1̅ space group and is a heptanuclear heterometallic cage comprising a total of four cobalt centers and two GdIII centers (Figure 3). Of the four cobalt centers, two are in the +2 oxidation state (Co6 and Co7) and the remaining two (Co4 and Co5) are in the +3 oxidation state (Figure 4) as shown by the BVS calculations (Table S1) and supported by bond lengths and magnetic measurements. C

DOI: 10.1021/acs.inorgchem.6b02398 Inorg. Chem. XXXX, XXX, XXX−XXX

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Magnetic Properties. The variable-temperature directcurrent (dc) magnetic susceptibility data of complexes 1−3 were collected in the temperature range of 2−300 K under a field of 0.1 T and are shown in the form of χMT (where χM is the molar magnetic susceptibility) as a function of temperature. The dc magnetic studies (Figure 7) for complexes 1−3 show room-temperature χMT values of 23.50, 29.41, and 39.74 cm3

Complex 3 (Co2Gd5) crystallizes in the P21/n space group and is a heptanuclear heterometallic cage comprising a total of two CoIII centers and five GdIII centers (Figure 5). The metal

Figure 5. Ball and stick model showing the molecular structure of 3 in the crystal. The color code is the same as in Figure 1.

centers of this heptanuclear core are primarily linked by two μ3OH groups, a μ2-OH group, and two phosphonate ligands in 4.221 coordination mode (Figure 6 and Figure S4).11 The

Figure 7. Temperature dependence of χMT measured at 0.1 T for complexes 1−3.

mol−1 K, respectively, that are in good agreement with the expected values of 19.5, 27.37, and 39.37 cm3 mol−1 K, respectively, considering the single-ion anisotropy and orbital contribution of tetrahedral and octahedral CoII ions for 1 and 2, respectively.17,18 As the temperature is lowered, the χMT products for complexes 1 and 3 increase as the temperature decreases to 60 K followed by a sharp decrease, suggesting the presence of both ferro- and antiferromagnetic interactions between the metal centers (Figures S8 and S9). Plots of 1/χM versus T were deduced for both the complexes and fitted with the Curie−Weiss equation (Figures S10 and S11). From the fitting of the plot, C = 23.43 cm3 mol−1 K and θ = 1.76 K for complex 1 and C = 39.55 cm3 mol−1 K and θ = 2.50 K for complex 3. These values of θ also indicate the presence of ferromagnetic interactions in both complexes. For complex 2 with a decrease in temperature, the χMT values decrease very gradually (from 300 to 50 K) before a sharp decrease occurring below 50 K, reaching 23.73 cm3 mol−1 K at 2 K. The gradual decline at higher temperatures may be attributed to the presence of intramolecular antiferromagnetic interaction in the molecule or depopulation of the spin orbit states of CoII.18 Magnetization measurements at low temperatures (Figures S12−S14) show saturation values of 21.12, 25.89, and 33.94 NμB at 7 T for complexes 1−3, respectively. These are close to the calculated values of 20, 27, and 35 NμB, respectively. Entropy changes (−ΔSm) for 1−3 were calculated from the magnetization data at various fields and temperatures using the Maxwell equation:19

Figure 6. Core structure of 3. The color code is the same as in Figure 1.

periphery of the molecule is surrounded by a sheath of 11 pivalate ligands and two [deaH]2− ligands. The [deaH]2− ligands bridge the metal centers in 3.221 mode, and the pivalate ligands show two different bridging modes (2.11 and 1.11). This leads to octahedral geometry for both the cobalt centers with O5N coordination. Of the five GdIII centers, one has O9 coordination (Gd5) with a distorted trigonal prismatic geometry and the remaining four (Gd1−Gd4) have O8 coordination featuring distorted square antiprismatic geometry (Figure S5). The GdIII centers at the core form a cagelike structure and are connected to the two cobalt centers in a hemicubane-like view (Figure S7). The average CoIII−O/ CoIII−N and Gd−O bond distances are 1.90 and 2.39 Å, respectively. The average CoIII−O−Gd and Gd−O−Gd bond angles are 101.5° and 108.3°, respectively.

ΔSm(T )ΔH =

∫ [∂M(T , H)/∂T ]H dH(H

= magnetic field)

(1)

The resulting values (plots of −ΔSm vs T) increase gradually with a decrease in temperature from 9 to 2 K (Figure 8). The D

DOI: 10.1021/acs.inorgchem.6b02398 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 8. Temperature dependencies (3−10 K) of the magnetic entropy change (−ΔSm) for complexes 1−3 (from left to right, respectively) as obtained from magnetization data.

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highest values of −ΔSm of 28.14 (1), 25.06 (2), and 29.19 J kg−1 K−1 (3) were obtained at 3 K and ΔH = 7 T. These results clearly show the effect of the anisotropy of CoII ions, the presence or absence of ferromagnetic interactions, the metal/ligand ratio, and the gadolinium content of the cage on MCE. The higher MCE value for complex 1 can be attributed to the presence of ferromagnetic interactions and a large metal/ ligand ratio. For complex 2, the presence of dominant spin orbit coupling of CoII centers or antiferromagnetic interactions should result in slightly less MCE. On the other hand, the highest MCE in complex 3, of those of the three complexes, is justified by the larger Gd ratio in addition to the ferromagnetic interactions. The maximal entropy for a molecule is calculated as ∑nR ln(2S + 1) = 6.92R, 9R, and 10.39R, which correspond to 36.89, 34.64, and 32.58 J kg−1 K−1 for 1−3, respectively. The differences between the theoretical and observed values can be attributed to the anisotropic nature of CoII ions and/or the presence of antiferromagnetic interactions between the paramagnetic metal centers. Corresponding volumetric entropy changes are 61.08, 57.25, and 50.72 mJ cm−3 K−1 for 1−3, respectively.

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

Abraham Clearfield: 0000-0001-8318-8122 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS A.C. thanks the R. A. Welch Foundation (Grant A0673) and the National Science Foundation (Grant DMR-0332453) for financial support.

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REFERENCES

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CONCLUSION We have reported the synthesis, structural, and magnetic properties of three polynuclear heterometallic Co−Gd phosphonate deaH3 cages. We noticed that the structures of these cages are strongly affected by minor changes in the stoichiometry and/or a change in solvent. Magnetic studies signify the effect of the anisotropy, metal/ligand ratio, and Gd content in the core of the molecules on MCE. Following this synthetic approach, we are at present trying to explore highnuclearity cages with other lanthanides under different reaction conditions.

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AUTHOR INFORMATION

Corresponding Author

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b02398. Crystallographic data of 1 (CIF) Crystallographic data of 2 (CIF) Crystallographic data of 3 (CIF) BVS calculations, TGA plots, crystallographic figures, and magnetic plots (PDF) E

DOI: 10.1021/acs.inorgchem.6b02398 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.6b02398 Inorg. Chem. XXXX, XXX, XXX−XXX