Communication pubs.acs.org/IC
Heterometallic CoIII−GdIII Clusters as Magnetic Refrigerants Javeed Ahmad Sheikh and Abraham Clearfield* Department of Chemistry, Texas A&M University, College Station, Texas 77843-3255, United States S Supporting Information *
The large gadolinium ratio was achieved by carefully playing with the reaction conditions. Also, we were able to get CoIII from CoII starting materials by incorporating the deaH3 ligand and exploit the coordination preferences of the 3d and 4f ions toward different donor atoms. The presence of CoIII centers in both complexes was shown by bond-valence-sum (BVS) calculations (Table S1),12 bond lengths (Table S2), and magnetic measurements. Complex 1 (Co2Gd6) crystallizes in the C2/c space group and is an octanuclear heterometallic cluster comprising a total of two CoIII centers and six GdIII centers (Figure 1). The GdIII centers
ABSTRACT: Two heterometallic CoIII−GdIII nanomagnets (Co2Gd6 and Co2Gd9) with defective dicubane-like cores were isolated from the same set of reactants by varying the reaction conditions. These are the first examples of cobalt(III)−gadolinium(III) phosphonate compounds and a rare class of compounds with large 4f ratio among the reported 3d−4f complexes. Magnetic studies reveal large magnetic entropy changes for both complexes (−ΔSm = 27.81 and 33.07 J kg−1 K−1, respectively at 3 K and 7 T).
M
agnetic refrigeration based on the magnetocaloric effect (MCE) has received immense interest in the recent past because of its proposed application in some ultralow-temperature cooling applications.1−5 Molecules built from paramagnetic metal ions (often referred as molecular nanomagnets) should be an ideal choice for such a study, provided they have a large spin ground state, S, and negligible anisotropy. Accordingly, GdIII (the f7 ion) has been explored by constructing homo- and heterometallic (Gd-3d) cages and clusters.6 Phosphonate ligands are among the various groups of ligands that have now been employed to some extent in such syntheses. In particular, Winpenny et al. have prepared a number of 3d−4f polymetallic complexes using phosphonates, which show significant MCEs.7 However, the presence of highly anisotropic CoII ions in some of these reported compounds should have an adverse effect on the MCE. A thorough literature survey shows that the use of aminopolyalcohol ligands in some cases results in CoIII from CoII starting materials.8 So, we employed diethanolamine (deaH3) as a coligand in the synthesis with the intention of getting CoIII and suppressing the anisotropy. To the best of our knowledge, this ligand has never been used along with phosphonates, and we believe that this synthetic strategy should be quite useful in the construction of magnetic refrigerants, particularly when the starting materials contain anisotropic ions (CoII in the present case). The reaction of t Bu-PO 3 H 2 , deaH 3 , [Co II 2 (μ-OH 2 )(O2CtBu)4]·(HO2CtBu)4,9 and Gd(NO3)3·6H2O under ambient and solvothermal conditions (see the Supporting Information for synthetic details) led to the isolation of these two complexes having the formulas [CoIII2GdIII6(μ3-OH)2(μ2-OH)(tBuPO3)4(tBuPO3H)2(O2CtBu)3(HO2CtBu)3(deaH)2(deaH2)4]·10H2O and [Co III 2 Gd III 9 (μ 3 -OH) 2 ( t BuPO 3 ) 4 (deaH) 4 (O 2 C t Bu) 13 (HO2CtBu)]·12H2O. A thorough survey of the Cambridge Structural Database (CSD)10 and Polynuclear Inorganic Clusters Database (PICD)11 shows no example of Co2Gd6 and Co2Gd9 complexes. These are thus a rare class of compounds with a large 4f ratio among the 3d−4f discrete complexes reported to date. © XXXX American Chemical Society
Figure 1. Ball-and-stick model showing the molecular structure of 1 in the crystal. Color code: purple, gadolinium; blue, cobalt, cyan, nitrogen; red, oxygen; gray, carbon. Hydrogen atoms are omitted for clarity.
are primarily held together by two μ3-hydroxo groups, one μ2hydroxo group, and six phosphonate ligands and then bridged to the CoIII centers with four dianionic [deaH]2− ligands (Figure 2). Two more singly deprotonated [deaH2]− ligands are found to coordinate to the two gadolinium centers at the periphery. In addition to this, peripheral ligation is provided by six pivalate groups, thus completing the coordination sphere of each ion. The metal−oxo core can be described as comprised of defective dicubanes (Figure S1). The phosphonate ligands show three different bridging modes in this compound (3.211, 3.221, and 2.210; Figure S2).13 Similarly, the deaH3 ligands also show only one bridging mode in this compound (3.321; Figure S2). The pivalate ligands bridge the metal centers in 2.11 and 1.11 coordination modes (Figure S2). This finally results in both the cobalt ions being six-coordinated with octahedral geometry and all of the GdIII ions being eight-coordinated, featuring squareReceived: June 10, 2016
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DOI: 10.1021/acs.inorgchem.6b01414 Inorg. Chem. XXXX, XXX, XXX−XXX
Communication
Inorganic Chemistry
pivalate groups, thus completing the coordination sphere of each ion. Another way of describing the core would be that the five GdIII centers at the middle are linked to a triangle comprised of two GdIII centers and one CoIII center on each side by the phosphonate ligands (Figure 4). In this compound, the phosphonate ligands show only one bridging mode (4.221; Figure S2) and the deaH3 ligands show three different bridging modes (3.310, 3.111, and 1.100; Figure S2). The pivalate ligands bridge the metal centers in 2.11 and 1.11 coordination modes. This leads to the cobalt ions being sixcoordinated with octahedral geometry. Of the nine GdIII centers, six are eight-coordinated, featuring square-antiprismatic geometry, and the remaining three are seven-coordinated, having pentagonal-bipyramidal geometry (Figure S3). The CoIII ions have O4N2 coordination and the GdIII centers display either a O8 or O7 coordination environment. The molecular structures of complexes 1 and 2 are roughly flat (Figure S6), thus making them suitable candidates for surface deposition in which a good contact between the substrate and molecule is desirable. This is important because many future applications of molecular magnets involve surface deposition. The variable-temperature direct-current (dc) magnetic susceptibility data of complexes 1 and 2 were collected in the temperature range 2−300 K under a field of 0.1 T and are shown in the form of χMT (χM is the molar magnetic susceptibility). The dc magnetic studies (Figure S7) show room temperature χMT values of 48.18 and 70.19 cm3 mol−1 K respectively, that are in good agreement with the expected values of 47.25 cm3 mol−1 K (1, six uncoupled GdIII, and g = 2) and 70.87 cm3 mol−1 K (2, nine uncoupled GdIII, and g = 2). As the temperature is lowered, the χMT products for both complexes increase to 75−100 K, followed by a sharp decrease, suggesting the presence of both weak ferro- and antiferromagnetic interactions between the metal centers (Figure S8). 1/χM versus T plots were deduced for both complexes and fitted with the Curie−Weiss equation (Figure S9). From the fitting of the plot, C = 48. 38 cm3 mol−1 K and θ = 0.13 K for complex 1 and C = 69.98 cm3 mol−1 K and θ = 0.72 K for complex 2 were obtained. These low and positive values of θ further suggest the presence of very weak ferromagnetic interactions in both complexes. Magnetization measurements at low temperature (Figures S10 and S11) show saturation values of 43.07 and 62.84 NμB at 7 T for complexes 1 and 2, respectively. These are close to the theoretical values of 42 and 63 NμB, respectively. The magnetic entropy changes (−ΔSm) for 1 and 2 were calculated at various fields and temperatures from magnetization data using the Maxwell equation:
Figure 2. Core structure of 1 in the crystal. Color code: same as that in Figure 1.
antiprismatic geometry (Figure S3). The CoIII ions have O4N2 coordination, and all of the GdIII centers display an O8 coordination environment. Removing of all of the atoms and connecting the metal centers by imaginary lines show an almost planar arrangement of the same (Figure S4). Complex 2 (Co2Gd9) crystallizes in the P1̅ space group and is an undecanuclear heterometallic cage comprising a total of two CoIII centers and nine GdIII centers (Figure 3). The five GdIII
Figure 3. Ball-and-stick model showing the molecular structure of 2 in the crystal. Color code: same as that in Figure 1.
centers at the core are primarily held together by two μ3-hydroxo groups, forming a defective dicubane-like core (Figure S5), and then bridged to four more GdIII centers by four phosphonate ligands, resulting in a butterfly-like core structure (Figure 4). These four GdIII centers are then bridged to the CoIII centers with two [deaH]2− ligands. Peripheral ligation is provided by 10
ΔSm(T )ΔH =
∫ [∂M(T , H)/∂T ]H dH
(1)
where H = magnetic field. The resulting values gradually increase with a lowering of the temperature from 9 to 2 K (Figure 5). The highest values of −ΔSm = 27.81 J kg−1 K−1 (1) and 33.07 J kg−1 K−1 (2) were obtained at 3 K and ΔH = 7 T. The magnetic entropy value observed for 2 is among the highest observed for 3d−4f discrete complexes outperformed only by a few compounds.6a,15 Its value is, however, the highest among the cobalt−gadolinium phosphonate clusters and second highest for 3d−4f phosphonate compounds, exceeded in decimal only by a Mn4Gd6P6 cage (−ΔSm = 33.7 J kg−1 K−1).7d The maximum entropy for a molecule is calculated as ∑nR ln(2S+1) = 12.47R and 18.71R, which correspond to 31.32 and 39.64 J 14
Figure 4. Core structure of 2 in the crystal. Color code: same as that in Figure 1. B
DOI: 10.1021/acs.inorgchem.6b01414 Inorg. Chem. XXXX, XXX, XXX−XXX
Communication
Inorganic Chemistry Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors are thankful to Dr. Marco Evangelisti and Dr. Amit Adhikary for helpful scientific discussion. 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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Figure 5. Temperature dependencies (3−10 K) of a magnetic entropy change (−ΔSm) for complexes 1 and 2 (from top to bottom, respectively) as obtained from magnetization data.
kg−1 K−1 for 1 and 2, respectively. The differences between the theoretical and observed values can be attributed to the presence of antiferromagnetic interactions between the paramagnetic metal centers. Corresponding volumetric entropy changes are 46.10 and 55.32 mJ cm−3 K−1 for 1 and 2, respectively. Employment of the deaH3 ligand along with phosphonate not only facilitated the isolation of polynuclear heterometallic clusters and restricted the number of cobalt centers but also led to CoIII, which, in turn, resulted in large magnetic refrigeration. This synthetic approach represents a promising route toward the design of new heterometallic clusters with interesting magnetic properties. Currently, we are trying to isolate gadolinium-only-containing complexes and other 3d−4f complexes employing this synthetic strategy.
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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.6b01414. X-ray crystallographic data in CIF format (CCDC 1475301) (CIF) X-ray crystallographic data in CIF format (CCDC 1475302) (CIF) Synthetic details, BVS calculations, bridging modes of ligands, crystallographic figures, magnetic plots, PXRD, and TGA plots (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail: clearfi
[email protected]. C
DOI: 10.1021/acs.inorgchem.6b01414 Inorg. Chem. XXXX, XXX, XXX−XXX
