The Nature of the Bridging Anion Controls the ... - ACS Publications

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The Nature of the Bridging Anion Controls the Single-Molecule Magnetic Properties of DyX4M 12-Metallacrown‑4 Complexes Thaddeus T. Boron, III,*,† Jacob C. Lutter,‡,§ Connor I. Daly,‡ Chun Y. Chow,§ Andrew H. Davis,† Arunpatcha Nimthong-Roldán,∥,# Matthias Zeller,∥,⊥ Jeff W. Kampf,§ Curtis M. Zaleski,*,‡ and Vincent L. Pecoraro*,§ †

Department Department § Department ∥ Department ‡

of of of of

Chemistry, Chemistry, Chemistry, Chemistry,

Slippery Rock University, Slippery Rock, Pennsylvania 16057, United States Shippensburg University, Shippensburg, Pennsylvania 17257-2200, United States University of Michigan, Ann Arbor, Michigan 48108-1005, United States Youngstown State University, Youngstown, Ohio 44555-0001, United States

S Supporting Information *

ABSTRACT: A family of DyX4M(12-MCMnIII(N)shi-4) compounds were synthesized and magnetically characterized (X = salicylate, acetate, benzoate, trimethylacetate, M = NaI or KI). The bridging ligands were systematically varied while keeping the remainder of the MC-geometry constant. The type of monovalent cation, necessary for charge balance, was also altered. The dc magnetization and susceptibility of all compounds were similar across the series. Regardless of the identity of the countercation, the Dy(Hsal)4M 12-MC-4 compounds were the only compounds to show frequency-dependent ac magnetic susceptibility, a hallmark of single-molecule magnetism. This indicates that the nature of the bridging ligand in the 12-MCMnIII(N)shi-4 compounds strongly affects the out-of-phase magnetic properties. The SMM behavior appears to correlate with the pKa of the bridging ligand.

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INTRODUCTION The field of single-molecule magnets (SMMs) has dramatically expanded within the past 20 years, covering a wide breadth of interesting coordination chemistry complexes.1−7 Transition metal-based SMMs derive their properties from an anisotropic barrier to spin relaxation U, given by the formula U = S2|D|, for integer spins S, and U = (S2 − 1/4)|D| for half-integer spins.8−10 The value D, the magnetoanisotropy of the system, must be negative to ensure the largest spin term S will be the ground spin state in the double potential energy well. The interest in developing SMMs has been their promise to deliver new spintronic devices.11 To be feasible spintronic materials, SMMs must have a large thermal energy barrier. There are two potential avenues to accomplish this goal: one is to focus on building large spin complexes; the second is to focus on increasing anisotropy. One way to merge these strategies is to incorporate lanthanide ions into transition metal complexes to form mixed 3d/4f complexes or to prepare single ion magnets using only lanthanide ions. Lanthanides ions have the largest spin values and have large amounts of magnetoanisotropy due to the relativistic effects of their 4f electrons. Mixed 3d/4f complexes have a great potential to serve as SMMs; however, the purposeful and structurally controllable synthesis of these complexes is still an area in need of improvement.12,13 One family of compounds that is able to control the molecular anisotropy through geometric constraints of 3d/4f complexes is a family of metallamacrocycles known as metallacrowns.14−19 © XXXX American Chemical Society

Metallacrowns are often thought of as inorganic analogues of crown ethers20−36 and have been especially fruitful in the field of SMMs for their ability to organize metals in a predictable geometry.15,28,31 We have particular interest in working with MnIII-based metallacrowns as high-spin MnIII ions have an S = 2 spin state as well as a magnetoanisotropy vector aligned with the Jahn−Teller axis. Using the ligand salicylhydroximate, we have the ability to isolate selectively MnIII ions in a planar geometry with all of the MnIII ions’ Jahn−Teller distortions aligned in one direction, maximizing the probability of aligning the MnIII ions’ magnetoanisotropy vectors.37 We have already shown two examples of metallacrowns utilizing this strategy that display slow-magnetic relaxation, a hallmark of SMM behavior.14,37 Herein, we report a new series of DyX4M 12-MCMnIII(N)shi-4 (X = acetate (OAc), salicylate (sal), benzoate (ben), trimethylacetate (TMA); shi3− = salicylhydroximate; and MI = NaI, KI) complexes that show interesting magnetic behavior, including the loss of SMM behavior upon changing the identity of the bridging carboxylate ligand. These complexes are very similar to the Mn(OAc)212-MCMnIII(N)shi-4 complex, which showed slow magnetic relaxation in both the solid state and when dissolved in N′N′-dimethylformamide (DMF), as well as hysteresis in the solid state.37 By substituting a DyIII ion for the pre-existing MnII into the central cavity, one should be able to Received: July 28, 2016

A

DOI: 10.1021/acs.inorgchem.6b01832 Inorg. Chem. XXXX, XXX, XXX−XXX

chemical formula formula weight (g/mol) space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) temp (K) λ (Å) ρcalc μ (mm−1) Z R1 [I > 2σ(I)] R1 (all) wR2 [I > 2σ(I)] wR2 (all)

2 DyNaMn4C71H78N9O32 1974.67 Cc 23.492(5) 24.270(5) 14.322(3) 90 91.621(4) 90 8162(3) 100(2) 0.71073 1.607 1.601 4 0.0573 0.0751 0.1408 0.1492

1

DyKMn4C73H84N10.5O33.5 2065.87

Cc 23.2718(16) 24.3346(4) 14.6609(3) 90 90.907(6) 90 8295.99(8) 85(1) 1.54178 1.583 10.767 4 0.0352 0.0367 0.0969 0.0988

C2/c 16.1208(13) 16.3127(13) 23.4413(18) 90 95.3460(10) 90 6137.63(8) 85(2) 0.71073 1.757 2.160 4 0.0328 0.0373 0.0854 0.0882

DyKMn4C51H63N9O25 1623.46

320

Table 1. Crystallographic Data for the DyIIIX4M [12-MCMnIIIN(shi)-4] Compounds

Pn 14.1657(10) 16.5718 (10) 16.7075 (11) 90 95.929(1) 90 3901.1(4) 85(2) 1.54178 1.587 10.816 2 0.0372 0.0389 0.0974 0.0993

P1̅ 12.9657(13) 16.0684(16) 17.4114(16) 89.412(5) 88.862(5) 73.308(5) 3473.9(6) 100(2) 0.71073 1.675 1.687 2 0.0243 0.0331 0.0502 0.0544

5 DyNaMn4C71H79N9O29 1927.69

DyNaMn4C54H78N10O30 1752.51

420 6

Pn 14.1334(18) 16.628(2) 16.764(2) 90 96.893(2) 90 3911.2(9) 100(2) 0.71073 1.613 1.712 2 0.0292 0.0315 0.0745 0.0768

DyKMn4C68H75.20N8O29.60 1899.52

7

P21/n 19.1240(12) 18.9136(11) 26.0448(17) 90 99.528(4) 90 9290.5(10) 100(2) 1.54178 1.489 9.285 4 0.0583 0.0854 0.1399 0.1569

DyNaMn4C64.22H96.22N9.41O28.59 1862.82

Inorganic Chemistry Article

B


Article

Inorganic Chemistry Table 2. Structural Feature Comparison of DyX4M 12-MC-4a compd

av. adjacent Mn−Mn distance (Å)

av. cross cavity Mn−Mn distance (Å)

av. cross cavity Oox−Ooxb distance (Å)

av. cross cavity Ocar−Ocarc distance (Å)

av. Dy−Ooxb MPd distance (Å)

av. Dy−Ocarc MPd distance (Å)

Dy−Mn MPd distance (Å)

Dy−Me distance (Å)

Me−Ooxb MPd distance (Å)

1 2 3 4 5 6 7

4.63 4.57 4.63 4.61 4.61 4.63 4.60

6.54 6.47 6.54 6.52 6.52 6.55 6.51

3.72 3.68 3.71 3.71 3.69 3.70 3.73

3.95 3.96 4.00 4.08 3.96 3.92 3.92

1.53 1.51 1.59 1.59 1.57 1.57 1.58

1.14 1.15 1.07 1.03 1.11 1.08 1.05

1.85 1.90 1.86 1.94 1.82 1.87 1.94

3.91 3.55 3.80 3.50 3.52 3.82 3.52

2.38 2.04 2.21 1.90 1.95 2.25 1.94

a

Schematics of the measured values are available in the Supporting Information. box = oxime oxygen of the MC plan. ccar = carboxylate oxygen atoms of the MC plane. dMP is the mean plane as calculated by Mercury using the indicated atoms. eNaI or KI. DyK3Mn4C65H73N9O31 [FW = 1975.883 g/mol]: C, 39.512; H, 3.724; N, 6.38. Found: C, 39.87; H, 3.91; N, 6.74. (Dy(OAc)4Na[12-MCMnIIIN(shi)-4](H2O)·6DMF) (4, Dy(OAc)4Na 12MC-4). Previously reported. 20 Yield: 45%. Anal. Calcd for DyNaMn4C54H78N10O30 [FW = 1752.51 g/mol]: C, 37.01; H, 4.49; N, 7.99. Found: C, 37.23; H, 4.52; N, 7.84. (Dy(ben)4Na[12-MCMnIIIN(shi)-4]·5DMF·4H2O) (5, Dy(ben)4Na 12MC-4). Four millimoles of MnCl2·4H2O (0.792 g) and 8 mmol of sodium benzoate (1.153 g) were dissolved in 16.0 mL of DMF. In another beaker, 4 mmol of H3shi (0.612 g) and 0.5 mmol of Dy(NO3)3·5H2O (0.219 g) were dissolved in 15.0 mL of DMF. When the MnCl2 and Na(O2C7H5) solution was red, it was added to the Dy(NO3)3 and H3shi solution. The solution was stirred overnight and vacuum filtered the next day. The filtrate was allowed to slowly evaporate, and X-ray quality crystals were isolated 10 months later. Yield: 60.8%. Anal. Calcd for DyNaMn4C71H79N9O29 [FW = 1927.693 g/mol]: C, 44.24; H, 4.13; N, 6.54. Found: C, 44.12; H, 4.07; N, 6.33. (Dy(ben)4K[12-MCMnIIIN(shi)-4](H2O)4·4DMF·1.6H2O) (6, Dy(ben)4K 12-MC-4). Two millimoles of Mn(OAc)2·4H2O (0.490 g) were dissolved in 10 mL of DMF, resulting in a dark orange solution. In a separate beaker, 0.125 mmol of Dy(NO3)3·5H2O (0.055 g), 2 mmol of H3shi (0.306 g), and 4 mmol of potassium benzoate (0.641 g) were dissolved in 5 mL of DMF and 5 mL of methanol, resulting in a colorless liquid with some of the potassium benzoate remaining undissolved. The manganese acetate solution was then added to this solution resulting in a dark brown solution, which was stirred overnight. The next day the solution was gravity filtered, and a precipitate was not isolated. The filtrate was then allowed to slowly evaporate, and X-ray quality dark brown/black plate-like crystals were recovered after 15 weeks. Yield: 27.5%. Anal. Calcd for DyKMn4C68H75.20N9O29.60 [FW = 1899.52 g/mol]: C, 43.00; H, 3.99; N, 5.90. Found: C, 42.98; H, 3.84; N, 6.04. (Dy(TMA)4Na[12-MCMnIIIN(shi)-4](H2O)2.59(DMF)1.41·4DMF·0.59H2O) (7, Dy(TMA)4Na 12-MC-4). Two millimoles of Mn(OAc)2·4H2O (0.490 g) was dissolved in 10 mL of DMF, resulting in a dark orange solution. In a separate beaker, 0.125 mmol of Dy(NO3)3·5H2O (0.055 g), 2 mmol of H3shi (0.306 g), and 4 mmol (based on an assumption of three waters of hydration) of sodium trimethylacetate (0.713 g) were dissolved in 10 mL of DMF, resulting in a cloudy, white mixture. The manganese acetate solution was then added to this solution, resulting in a dark brown solution, which was stirred overnight. The next day the solution was gravity filtered to remove a dark brown precipitate, which was discarded. The filtrate was then allowed to slowly evaporate, and X-ray quality dark brown/black plate-like crystals were recovered after 5 weeks. Yield: 6.33%. Anal. Calcd for DyNaMn4C64.22H96.22N9.41O28.59 [FW = 1862.82 g/mol]: C, 41.41; H, 5.21; N, 7.08. Found: C, 41.31; H, 5.03; N, 7.00. X-ray Crystallography. Crystals used for single-crystal X-ray diffraction were taken from the mother liquor and were not dried. Samples 1 and 5 were collected on a Rigaku AFC10K Saturn 944+ CCD-based X-ray diffractometer equipped with a low temperature device and a Micromax-007HF Cu-target microfocus rotating anode (λ = 1.54178 Å) operated at 0.2 kW power (20 kV, 10 mA) at 85(1) K. Rigaku d*trek images were exported to CrysAlisPro for processing and

retain 5 unpaired electrons on the central atom while introducing significant anisotropy, which is unavailable with the MnII analogue.37 In this report, we examine the impact of three structural modifications engendered by these molecules. First, we assess changing the central ion from a transition metal ion with high spin to a lanthanide with equally high spin but also single-ion anisotropy. Next, we examine the importance of the ligands bridging the ring MnIII and the central lanthanide. Finally, we assess the impact on the magnetism of diamagnetic ions placed on the opposite face of the DyIII ion.

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EXPERIMENTAL SECTION

Materials and Syntheses. Manganese(II) acetate tetrahydrate (Mn(OAc)2·4H2O), manganese(II) chloride tetrahydrate (MnCl2· 4H2O), salicylic acid (H2sal), sodium benzoate (Na(O2C7H5)), sodium trimethylacetate hydrate (NaO2CC(CH3)3), and N,Ndimethylformamide (DMF) were purchased from Sigma-Aldrich and used as received. Sodium hydroxide (NaOH), potassium hydroxide (KOH), and sodium salicylate (Na(O3C7H5)) were obtained from Fischer Scientific and used as received. Salicylhydroxamic acid (H3shi), potassium benzoate (K(O2C7H5)), and dysprosium nitrate pentahydrate (Dy(NO3)3·5H2O) were purchased and used as received from Alfa Aesar. Methanol (ACS grade) was purchased and used as received from BDH Chemicals. (Dy(Hsal)3.5(OAc)0.5K[12-MCMnIIIN(shi)-4](DMF)1.5(H2O)3.5·5DMF (1, Dy(Hsal)4K 12-MC-4/Dy(Hsal)3(OAc)K 12-MC-4). Four millimoles of Mn(OAc)2·4H2O (0.980 g), 2.7 mmol of KOH (0.151 g), and 4 mmol of H2sal (0.052 g) were dissolved in 15.0 mL of DMF. In another beaker, 4 mmol of H3shi (0.612 g) and 0.5 mmol of Dy(NO3)3·5H2O (0.219 g) were dissolved in 16.0 mL of DMF. When the Mn(OAc)2, KOH, and H2sal solution turned red, it was added to the Dy(NO3) and H3shi solution and stirred overnight. The following day, the solution was vacuum filtered and the filtrate left to slowly evaporate. After 6 months, green prism crystals of X-ray quality were isolated. Yield: 71.9%. Anal. Calcd for DyKMn4C73H84N10.5O33.5 [FW = 2065.87 g/mol]: C, 42.59; H, 4.10; N, 7.12. Found: C, 42.22; H, 4.13; N, 6.93. (Dy(Hsal) 4 Na[12-MC Mn III N(shi) -4](H 2 O) 3 (DMF)·4DMF) (2, Dy(Hsal)4Na 12-MC-4). Two millimoles of Mn(OAc)2·4H2O (0.490 g) were dissolved in 10 mL of DMF, resulting in a dark orange solution. In a separate beaker, 0.125 mmol of Dy(NO3)3·5H2O (0.055 g), 2 mmol of H3shi (0.306 g), and 4 mmol of sodium salicylate (0.640 g) were dissolved in 10 mL of DMF, resulting in a light pink solution. The manganese acetate solution was then added to this solution, resulting in a dark brown solution, which was stirred overnight. The next day the solution was gravity filtered to remove a dark green/ brown precipitate, which was discarded. The filtrate was then allowed to slowly evaporate, and X-ray quality cubic dark brown/black crystals were recovered after 5 weeks. Yield: 25.5%. Anal. Calcd for DyNaMn4C71H78N9O32 [FW = 1974.67 g/mol]: C, 43.19; H, 3.98; N, 6.38. Found: C, 42.54; H, 3.97; N, 6.28. (Dy(OAc)4K[12-MCMnIIIN(shi)-4]·5DMF) (3, Dy(OAc)4K 12-MC-4). Previously reported.20 Yield: 5.9%. Anal. Calcd for C


Article

Inorganic Chemistry

Figure 1. Single-crystal X-ray structures of DyIII(X)4M 12-MC-4 are shown with thermal ellipsoids at 50% probability level. For DyIII(Hsal)4K 12MC-4 and DyIII(Hsal)3(OAc)K 12-MC-4, the ligand connecting Mn1 to Dy1 is disordered and modeled as 50/50 salicylate/acetate. Hydrogen atoms and lattice solvents have been omitted for clarity. Color scheme: aqua = DyIII, dark purple = KI, light purple = NaI, orange = MnIII, gray = carbon, red = oxygen, blue = nitrogen. corrected for absorption. The structures were solved and refined with the Bruker SHELXTL (Version 2014/6) software package. All nonhydrogen atoms were refined anisotropically with the hydrogen atoms placed in a combination of idealized and refined positions. For compounds 2, 6, and 7, a mineral oil coated crystal was mounted on a MicroMesh MiTeGen micromount and transferred to the diffractometer. All data were collected at 100 K. For 2 and 6, data were collected using a Bruker AXS SMART APEXII CCD X-ray diffractometer using graphite-monochromated Mo Kα radiation (λ = 0.71073 Å). For 7, data were collected using a Bruker AXS X8 Prospector CCD diffractometer with Cu Kα radiation (λ = 1.54178 Å) with an IμS microsource and a laterally graded multilayer (Goebel) mirror for monochromatization. For all three compounds, data collection and cell refinement were performed using APEX2 and SAINT embedded in APEX2, respectively.38 The data were scaled and corrected for absorption with SADABS as built into APEX2.38 Space groups were assigned using XPREP within the SHELXTL suite of programs.39,40 The structures were solved using direct methods with SHELXS-97 and refined using least-squares refinements based on F2 with SHELXL-2013 and the graphical interface SHELXLE.41−43 Additional crystallographic data and experimental parameters are provided in Table 1, and the individual CIFs of each new compound are available in the Supporting Information. Important bond distances are provided in Table 2. Physical Measurements. The dc (compounds 1−6) and ac (compounds 2−7) magnetic measurements were taken on powdered samples that were mulled in eicosane to prevent torqueing of the sample in high applied magnetic fields on a Quantum Design MPMSXL7 SQUID magnetometer with a working temperature range of 2.0− 300 K. The ac magnetic susceptibility measurements of 1 were collected on a Quantum Design MPMS SQUID magnetometer at Michigan State University − Department of Physics and Astronomy. Samples were ground using a small mortar and pestle, transferred to a gelatin capsule of known mass, and weighed. A small amount of melted eicosane was added and left to solidify. The entire capsule was then weighed, and a piece of tape was applied to hold the capsule together. The capsule was then inserted into a clear plastic drinking straw. Holes were punched into the straw to allow air to escape. Diamagnetic corrections were applied for the sample holder from previous direct measurements, and molar diamagnetic susceptibilities were calculated from Pascal’s constants. The variable field dc magnetization measurements of the samples were collected at the University of Michigan on powdered samples mulled in eicosane measured at either 2.0 or 5.0 K with field increasing

from 0 to 55 000 Oe. Susceptibility was measured on the powdered samples mulled in eicosane at an applied field of 2000 Oe and increasing temperatures from 5.0 to 300.0 K for samples 1, 2, 3, and 5. For samples 4 and 6, susceptibility was measured on powdered samples at an applied field of 1000 Oe and decreasing the temperature from 300 to 2.0 K. The ac susceptibility measurements were collected on powdered samples mulled in eicosane from 10.0 to 2.0 K with a 3.5 Oe drive field and no applied external field. The field was oscillated at frequencies from 10 to 997 Hz. The ac susceptibility of 7 was collected with a 3.0 Oe drive field oscillated at 1400 Hz frequency and no applied external field. Temperatures were decreased from 15 to 2.0 K.

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RESULTS Structural Results. The syntheses of the DyX4M [12MCMnIII(N)shi-4] (hereafter DyX4M 12-MC-4) compounds are very similar to previously published 12-MC-4 analogues such as the Ln(OAc)4Na 12-MC-4.20 As in the syntheses of most metallacrown compounds, the careful control of stoichiometric amounts is vital. In addition, the DyX4M 12-MC-4 compounds are structurally similar to the Ln(OAc) 4Na 12-MC-4 compounds. Therefore, the key structural differences between the newly prepared metallacrowns will be highlighted. All of the presented 12-MC-4 compounds feature four MnIII ions coordinated by triply deprotonated shi3− ligands in the equatorial plane. The DyIII ion is coordinated in the central cavity on one face, while a monovalent cation is coordinated on the other. Examining charge balance, there are four carboxylate ligands (Hsal−, OAc−, ben−, or TMA−) and four shi3− ligands, giving a total of 16 negative charges. With four MnIII ions and one DyIII ion, 15 of these negative charges are accounted for. The last charge is neutralized by the K+ or Na+ ion. Thus, the monovalent cation is electrostatically bonded to the metallacrown, providing charge balance. The oxidation states of the manganese and dysprosium ions are supported by bond valence sum calculations44,45 and average bond lengths (Table S1). In compounds 2−7, all four carboxylates are the same; however, compound 1 is slightly different. For compound 1, there is disorder regarding one of the bridging carboxylate anions. For the three carboxylate anions that bridge Mn2, Mn3, and Mn4 to the central DyIII, a salicylate is found at 100% occupancy, but for the carboxylate that bridges Mn1 to the D


Article

Inorganic Chemistry DyIII ion, a salicylate and acetate anion are disordered with a 50/50 occupancy ratio. This results from the synthesis including acetate as a starting compound. As both 12-MC-4 compounds are found within the same crystal lattice, it is impossible to separate the two structure types. Interestingly, the substitution of salicylate for acetate does not change the structural properties of the MnIII site significantly as compared to the other three MnIII sites of the MC. For all of the DyX4M 12-MC-4 structures, two faces were formed (Figure 1). The one face is composed of the bridging carboxylate ligand and DyIII ion, while the other face includes the alkali counterion and any coordinated solvent molecules. This feature has been previously observed in the other 12-MC4 complexes.20,25 The average cross-cavity diameter, measured from oxime oxygen to oxime oxygen, ranges from 3.68 Å for Dy(Hsal)4Na 12-MC-4, to 3.73 Å for Dy(TMA)4Na 12-MC-4, (Table 2). This narrow range highlights the structural similarities between these molecules. The structural similarity between the DyIII compounds matches previously published results for a series of ErIII-based 12-MC-4 complexes with differing carboxylate ligands (Hsal, ben, and TMA).46 Two variations to the MC ring are present (Figure 2). In the Dy(TMA)4Na 12-MC-4, Dy(Hsal)4M 12-MC-4, and Dy-

pointing down toward the monovalent cations, forcing the oxime oxygens upward toward the DyIII ion. The Dy(ben)4M 12-MC-4 are not nearly as bowled; these structures are more ruffled than the other structures. The phenyl rings of the salicylate rings twist slightly as compared to the other DyX4M 12-MC-4 compounds. The impact of the bowl or ruffled shape can be seen in the angles of the MnIII Jahn−Teller axes and the centroid comprised by the oxime oxygens. The angles at which the Jahn−Teller axes point from the center of the MC range from 99.65° to 104.99°, with an average angle of 101.59° (Figure S3, Table S3). This angle can only be achieved if the ring is concave or ruffled because a perfectly planar MC should orient the Jahn−Teller axes orthogonally. The central DyIII ion, due to its size (radius = 1.207 Å),47 sits at a range of 1.51−1.59 Å above the mean plane comprised of the four oxime oxygens (Table S2), with the acetate versions having the DyIII ion furthest from the MC oxygen atoms. Interestingly, the distance the DyIII ion sits from the oxime mean plane (OoxMP) seems to be highly dependent on the identity of the bridging ligand. This subsequently results in the DyIII ion being closer to the acetate oxygen atoms than the other derivatives. The central DyIII ion is always eightcoordinate, bound to the four oxime oxygens from the MC ring and four carboxylate oxygens from the bridging ligands. This orientation forces the DyIII ion to take on pseudo-D4d symmetry. The average skew angles are all approximately 45°, supporting this assignment (Figure S4, Tables S5−S11). As may be expected, the distance the monovalent cation sits from the MC oxygen mean plane depends on the identity of the cation. The larger KI cation is forced to sit further from the MC plane than the smaller NaI cation (Table 2). The average distance that the KI ion sits from the oxime MP is 2.28 Å, while the NaI ion sits 1.96 Å from the oxime MP. On average, the KI is also separated from the DyIII ion more (3.84 Å) than the NaI ion is from the DyIII ion (3.52 Å). In the Dy(Hsal)4K 12-MC4/Dy(Hsal)3(OAc)K 12-MC-4 example, the KI ion is 9coordinate; four sites originate from the MC oxime oxygens, four additional sites from solvents coordinated to the MnIII ions, and the last site from a solvent water or DMF. In all of the other examples, the monovalent cation is 8-coordinate, with four coordination sites originating from the MC oxime oxygens and the remaining four sites from solvent molecules. Magnetic Properties. dc Magnetic Properties. The dc variable-field magnetization data are similar for complexes 1−6 (Table 3, Figure 3a, Figures S5−S10). For Dy(Hsal)4K 12MC-4/Dy(Hsal)3(OAc)K 12-MC-4, the magnetization value

Figure 2. Overlay of Dy(ben)4Na 12-MC-4 (red) and Dy(OAc)4Na 12-MC-4 (gray) demonstrate the bowling and ruffling found in the MC structures. For the overlay, the difference of the four atom oxime oxygen planes has been minimized between the two structures.

(OAc)4M 12-MC-4 compounds, the metallacrown ring takes a bowled shape with the phenyl ring from the shi3− ligands

Table 3. Comparison of the dc and ac Magnetic Properties of the DyX4M 12-MC-4 Complexes

compd Dy(Hsal)4K 12-MC4/Dy(Hsal)3(OAc) K 12-MC-4 Dy(Hsal)4 Na 12-MC-4 Dy(OAc)4K 12-MC-4 Dy(OAc)4 Na 12-MC-4 Dy(ben)4Na 12-MC-4 Dy(ben)4K 12-MC-4 Dy(TMA)4 Na 12-MC-4

magnetization maximum at 55 000 Oe (cm3 G mol−1)

χT magnetic susceptibility at 5 K (cm3 K mol−1)

χT magnetic susceptibility at 300 K (cm3 K mol−1)

in-phase χ′T magnetic presence of a frequencysusceptibility extrapolated to 0 dependent out-of-phase magnetic 3 −1 K (cm K mol ) susceptibility signal?

43 310

11.56

25.95

10.31

yes

39 510

12.22

25.02

11.96

yes

40 630 39 800

11.17 10.77

25.75 25.10

9.41 9.54

no no

39 660 39 940 N/A

12.59 11.73 N/A

23.76 24.76 N/A

12.55 10.88 11.24

no no no

E


Article

Inorganic Chemistry

Figure 3. Magnetization and dc susceptibility were measured on compounds 1−6 under the indicated conditions.

of 12.59 cm3 K mol−1 at 5.00 K. For Dy(ben)4K 12-MC-4, the susceptibility at 300 K was 24.76 cm3 K mol−1, which is also slightly less than anticipated. The susceptibility remained relatively constant until 110 K, at which point it began to steadily decrease, reaching a susceptibility of 11.73 cm3 K mol−1 at 5.00 K. ac Magnetic Properties. For the Dy(Hsal)4K 12-MC-4/ Dy(Hsal)3(OAc)K 12-MC-4, the in-phase susceptibility decreased linearly from 10 to 2.00 K for all frequencies (Figure S17). Extrapolating the in-phase susceptibility to 0 K led to a value of 10.31 cm3 K mol−1. For Dy(Hsal)4Na 12-MC-4, the in-phase susceptibility linearly decreased from 10 to 2.00 K for all frequencies (Figure S18). Extrapolating the in-phase susceptibility to 0 K gave a value of 11.96 cm3 K mol−1. For Dy(OAc)4K 12-MC-4, the in-phase susceptibility again decreased linearly from 10 to 2.00 K for all measured frequencies (Figure S19). Extrapolating the in-phase susceptibility from 4.5 to 0 K led to a value of 9.41 cm3 K mol−1. Similar behavior was observed in the in-phase for Dy(OAc)4Na 12-MC-4; the susceptibility decreased linearly from 10 to 2.00 K at all frequencies. Extrapolating from 4.5 to 0 K led to a 0 K susceptibility of 9.54 cm3 G mol−1 (Figure S20). For Dy(ben)4Na 12-MC-4, the in-phase susceptibility decreased linearly from 10 to 2.00 K for all frequencies (Figure S21). Extrapolating the in-phase susceptibility to 0 K from 2.00 K led to an in-phase susceptibility of 12.55 cm3 K mol−1. For Dy(ben)4K 12-MC-4, the in-phase susceptibility decreased linearly from 10 to 2.00 K at all frequencies (Figure S22). Extrapolating the in-phase susceptibility to 0 K from 2.00 K leads to an in-phase susceptibility of 10.88 cm3 K mol−1. The in-phase susceptibility of Dy(TMA)4Na 12-MC-4 also decreased linearly from 15.0 to 2.0 K at 1400 Hz (Figure S23). Extrapolating the in-phase susceptibility to 0 K leads to an in-phase susceptibility of 11.24 cm3 K mol−1. It was found that the Dy(Hsal) 4 K 12-MC-4/Dy(Hsal)3(OAc)K 12-MC-4 (Figure 4) and Dy(Hsal)4Na 12MC-4 (Figure 5) showed frequency dependence. However, no maxima were observed at the measured frequencies, preventing the investigation of the dynamics of the magnetic relaxation process. The other DyX4M 12-MC-4 compounds did not show frequency dependence in the out-of-phase down to 1.8 K (Table 3, Figures S24−S28).

increased linearly until 10 000 Oe, reached an inflection point, then continued to increase. At 55 000 Oe, the magnetization reached a value of 43 310 cm3 G mol−1. For Dy(Hsal)4Na 12MC-4, similar behavior was observed: the magnetization increased linearly until 10 000 Oe, then increased at a slower rate. The maximum magnetization observed was 39 510 cm3 G mol−1 at 55 000 G. For Dy(OAc)4K 12-MC-4, the magnetization increased linearly to 11 000 Oe before increasing at a slower rate. The magnetization reached a maximum of 40 630 cm3 G mol−1 at 55 000 G. The Dy(OAc)4Na 12-MC-4 structure’s magnetism closely mimicked the KI analogue; however, the magnetization reached a maximum of 39 800 cm3 G mol−1. For Dy(ben)4Na 12-MC-4, the magnetization increased linearly to 11 000 Oe and then slowly increased until 55 000 G, reaching a maximum of 39 960 cm3 G mol−1. Similar properties were observed for Dy(ben)4K 12-MC-4; the maximum magnetization was 39 640 cm3 G mol−1. The dc variable temperature magnetic susceptibility data are also similar for complexes 1−6 (Figure 3b, Figures S11−S16). For Dy(Hsal)4K 12-MC-4/Dy(Hsal)3(OAc)K 12-MC-4, the susceptibility at 300 K was 25.95 cm3 K mol−1, which is very close to the expected value for four noninteracting MnIII and one DyIII ions (χMT = 26.17 cm3 K mol−1). It remained at this value until 145 K, at which point the susceptibility linearly decreased to 105 K. At 50 K, the susceptibility decreased dramatically, reaching a value of 11.56 cm3 K mol−1 at 5.00 K. Similar behavior was observed for Dy(Hsal)4Na 12-MC-4; the susceptibility at 300 K was 25.02 cm3 K mol−1, which is slightly smaller than expected, and remained fairly constant to 110 K. Again at 50 K, the susceptibility then dramatically decreased, reaching a minimum of 12.22 cm3 K mol−1 at 5.00 K. For Dy(OAc)4K 12-MC-4, the susceptibility at 300 K was 25.75 cm3 K mol−1, which is less than the sum of noninteracting ions. The susceptibility linearly decreased until about 115 K. At 60 K, the susceptibility decreased markedly, reaching a minimum of 11.17 cm3 K mol−1 at 5.00 K. Similar susceptibility properties were observed for Dy(OAc)4Na 12-MC-4; at 300 K, the susceptibility was 25.10 cm3 K mol−1 and remained relatively constant until approximately 85 K before quickly decreasing until it reached a susceptibility of 10.77 cm3 K mol−1 at 5.00 K. For Dy(ben)4Na 12-MC-4, the susceptibility at 300 K was 23.76 cm3 K mol−1, which was less than expected for noninteracting ions. This remained relatively constant until 95 K. A dramatic decrease started at 55 K, reaching a minimum F


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Inorganic Chemistry

compound due to the relativistic spin−orbit coupling and intrinsic anisotropy associated with lanthanide ions. However, we also incorporated a diamagnetic monovalent cation that is coordinated to the MC ring electrostatically. This substitution could modify the electron density available to coordinate the DyIII ion, leading to interesting magnetic behavior. We have also modified the bridging ligand between the ring MnIII ions and the central DyIII ion, which could also impact the observed magnetic properties. During the synthesis of these MCs, it was noted that different counterions occurred on the face opposite the central LnIII ion, raising the question of what role, if any, these diamagnetic counterions have on the magnetic properties. It was clear that chemically, the diamagnetic ion provided charge balance. The four MnIII and one DyIII ions provided a 15+ charge. The four shi3− ligands and four X− ligands provided a 16− charge. Thus, the bound cation balanced the remnant negative charge. It should be noted that the corresponding cation was not needed to balance the charge in the Mn(OAc)2 12-MC-4 structure. To study what, if any, role the diamagnetic counterion played on the magnetic properties, the Dy(Hsal)4M 12-MC-4, Dy(OAc)4M 12-MC-4, and Dy(ben)4M 12-MC-4 series with M = KI and NaI were prepared. To supplement our study of the role of the bridging ligand, an additional sample type, Dy(TMA)4Na 12-MC-4, was also prepared. Initially, we hypothesized that the slight structural differences between the MCs might affect the magnetic properties (Figure 6). For instance, the Dy(Hsal)4M 12-MC-4 and Dy(OAc)4M 12-MC-4 adopted bowled shapes, with the oxime oxygens pointing toward the LnIII ion and the counterion on the opposite face, while Dy(ben)4M 12-MC-4 adopted more of a ruffled geometry. Examining the dc magnetic data, it was apparent that the structural changes of the MC did not affect the magnetic properties (Table 3). From the ac-susceptibility, it was found that only the salicylate derivatives showed slowmagnetic relaxation. If a bowled geometry was the only requirement for SMM-behavior, then the acetate derivatives should have also shown slow-magnetic relaxation. As this is not the case, it appears that geometric changes to the MC ring do not correlate to the onset of SMM-like behavior. We next turned our attention to what impact the monovalent cation had on magnetic properties. Because of its smaller ionic radius, the NaI ion was able to reside closer to the oxime oxygen mean plane (OoxMP) than the KI ion.47 This slight difference could perturb the electronics of the MC ring; more electron density would be directed toward the NaI than the KI ion due to electrostatic interactions. This should manifest itself in different

Figure 4. Out-of-phase susceptibility of Dy(Hsal)4K 12-MC-4/ Dy(Hsal)3(OAc)K 12-MC-4 showed frequency dependence with zero applied external field.

Figure 5. Out-of-phase susceptibility of Dy(Hsal)4Na 12-MC-4 showed frequency dependence with zero applied external field.

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DISCUSSION We have presented several different modifications of the basic 12-MC-4 structure type that has been previously reported.20,23,25,46,48 If we set MnII(OAc)2 12-MC-4 as our reference point, we have substituted an isotropic high-spin MnII ion for an anisotropic DyIII ion. It was anticipated that incorporating a DyIII ion should result in a more paramagnetic

Figure 6. Mn(OAc)212-MC-4 (center), which shows slow-magnetic relaxation, can be modified to include YIII and W(CN)83− (left) or DyIII, MI, and Hsal (right), and still show SMM-like behavior. This demonstrates the tremendous versatility of the 12-MC-4 framework. G


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Inorganic Chemistry

extent that the two ligand fields run parallel can provide information on deformations. The distances between the oxygen planes of the MC ring and the ligands were all approximately 2.66 Å. The DyIII was always positioned closer to the carboxylate oxygen ligand mean plane than the oxime oxygen mean plane. The edges of the squares composed of the ligands varied slightly across the series; the average length of the edge of the carboxylate oxygen square was 2.82 Å, while the oxime oxygen square was slightly smaller at 2.61 Å. Taking the ratio between the distance separating the two oxygen MPs and the average edge length as a measure of compression, all of the MCs demonstrate some degree of compression (Table 4), and the values fall in a very narrow

Dy−OoxMP distances. However, the Dy−Oox MP distances for all of the structures remained fairly constant regardless of counterion (average 1.56 Å). Additionally, the magnetic properties of the NaI and KI analogues were very similar as well (Table 3). This indicated that the monovalent cation’s identity did not dramatically affect SMM-like properties. This finding is different from what Song and co-workers recently published,48 for Y(OAc)4 12-MC-4 and Y(OAc)4W(CN)8 12-MC-4 compounds. The Y(OAc)4 12-MC-4 did not display slow magnetic relaxation due to antiferromagnetic coupling between the MnIII ions. Antiferromagnetic coupling was also observed in MnII(OAc)2 12-MC-4 (Figure 6) and Li{Li(Cl)2[12-MC-4]}.37 However, Y(OAc)4W(CN)8 12MC-4 had paramagnetic properties at low temperature and even showed SMM-like behavior. The source of this behavior was credited with incorporating W(CN)83−, which not only introduced a paramagnetic WV cation, but also modified the coupling between the MnIII ions from antiferromagnetic to ferromagnetic. Unlike the MCs described in this article, the W(CN)83− was not electrostatically coordinated to the MC ring, rather it was directly coordinated to the MnIII ions through the cyanide ligands. Cyanide is a strong-field ligand, which can dramatically change the ligand field splitting parameters of the MnIII ions, especially when compared to ligand field splitting that would result from the coordination of water or DMF. Additionally, direct coordination to the MnIII ions through the cyanide pushed the WV ion 4.578 Å from the OoxMP. This is roughly double the distance that the KI is located from the OoxMP, making any electrostatic interactions in Song’s compound significantly weaker. Because the compounds presented here lack cyano-bridges, we expected, and observed, antiferromagnetic coupling, similar to that observed in Mn(OAc)2 12-MC-4 or Y(OAc)4 12-MC-4, to dominate the magnetic properties of the ring MnIII ions in DyX4M 12-MC-4. We then assessed the impact of the DyIII coordinated environment on the SMM-behavior of the DyX4M 12-MC-4 complexes. Coronado and co-workers found that skew angles and compression ratios of about eight coordinate LnIII ions with D4d symmetry encapsulated in polyoxometallates and in bisphalocyanato-based complexes correlated to observed SMMlike behavior.1,6,49 Measuring the skew angles (the angle between the coordinated upper and lower oxygen atoms) and the ratio between the height of the square antiprism and its length could parametrize distortion from ideal D4d symmetry. The key finding was that compressed square antiprisms correlated to later LnIII ions, demonstrating SMM behavior, while elongated antiprisms favored earlier LnIII demonstrating SMM-like behavior.1,49,50 Because of the structural similarities around the DyIII ion, a similar analysis for these complexes seemed warranted. Approximating the two oxygen ligand planes above and below the DyIII as squares, the ratio of the distance between the two planes (OcarboxylateMP−OoximeMP length) and the sides of the squares (Oedge length) gives an indication of the degree of elongation (ratio greater than 1) or contraction (ratio less than 1) of the prism. Measuring the angle between the corners of the squares and the central lanthanide, known as the skew angle, also indicates the type of prism. If the skew angle is 45°, the coordination geometry about the metal ion is an ideal square antiprism and has D4d symmetry, whereas if the skew angle is 0°, the geometry is an ideal square prism and has D4h symmetry. If the angle varies between these two extremes, the degree of deformation from the ideal can be observed. Last, the

Table 4. Compression Ratios of the DyX4M 12-MC-4a compression ratio Dy(Hsal)4K/Dy(Hsal)3(OAc)K Dy(Hsal)4Na Dy(OAc)4K Dy(OAc)4Na Dy(ben)4Na Dy(ben)4K Dy(TMA)4Na

0.98 0.98 0.98 0.96 0.99 0.98 0.97

a

The degree of compression is calculated by dividing the distance separating the ligand plane from the MC oxime oxygen plane by the averaged edge length of the oxygen ligand squares.

range. None of the reported MCs, however, are as compressed as the POMs reported by Coronado.1,49 However, changes in the compression ratio have been suggested as structural factors that can affect the magnetic properties of LnIII containing compounds.50 DyIII is an oblate ion, and is favored in axially elongated coordination environments.50 All of the DyX4M 12Mc-4 structures show similar degrees of axial compression. If axial compression alone controlled the onset of SMM properties, it may be expected that a certain threshold for the onset of SMM properties could be found. This is not the case here. The skew angles for the DyX4M 12-MC-4 were approximately 45°, indicating that they are close to the ideal skew angle of a square antiprism (Tables S5−S11). The exception to this observation was Dy(TMA)4Na 12-MC-4. This structure had skew angles that were approximately 5° greater or smaller than 45°. The most likely explanation is that the bulky trimethyl groups created steric hindrances, forcing the angles to contort (Figure 7). Taken together, the similarity of the compression ratios and of the skew angles for the DyX4M 12MC-4 complexes indicates that the coordination environment of the DyIII should have little impact on the appearance or disappearance of SMM behavior of these compounds.

Figure 7. Bridging ligands, their pKa values, and accumulated electronegativity (AEN) values are given. H


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Inorganic Chemistry

The pKa of salicylic acid (2.93) is smaller than that of benzoic (4.20), acetic acid (4.77), or trimethylacetic acid (5.01). This indicates that salicylate is the most electron-withdrawing of the group, with benzoate, acetate, and trimethylacetate having similar electron-withdrawing properties. This could affect the strength of the exchange coupling between the ring MnIII metal ions and the central DyIII ion. The pKa correlates nicely with the observed out-of-phase behavior: Dy(Hsal)4M 12-MC-4 showed dramatically different out-of-phase properties as compared to Dy(OAc)4M 12-MC-4, Dy(ben)4M 12-MC-4, or Dy(TMA)4Na 12-MC-4. As the out-of-phase properties of Dy(Hsal)4K 12-MC-4 and Dy(Hsal)4Na 12-MC-4 differed slightly, it could be that the counterion “fine tunes” the magnetic properties based on its interactions with the oxime oxygen ring. Because only the Dy(Hsal) 4M 12-MC-4 compounds show SMM behavior, it can be assumed that the acetate/salicylate disorder in compound 1, Dy(Hsal)4K 12MC-4/Dy(Hsal)3(OAc)K 12-MC-4, does not dramatically influence the SMM properties.

The complete structural analysis indicates that there are differences between the MCs presented in this series; however, these differences are very slight. Thus, the magnetic properties of the MCs should not be influenced greatly by the structural properties of the MCs. Instead the chemical identify of the various components, the bridging carboxylate ligand, should affect the magnetic behavior of the MCs. The dc magnetization and susceptibility properties of all of the DyX4M 12-MC-4 are very similar (Table 3). As is often the case for DyIII-containing compounds, there is not a plateau in the dc magnetization, making it difficult to model. The dc magnetic susceptibilities of the compounds at 300 K (25.95−23.76 cm3 K mol−1) were close to those expected for four noninteracting MnIII ions (g = 2.00) and one DyIII ion (g = 4/3) (26.17 cm3 K mol−1). All DyX4M 12-MC-4 compounds also displayed similar trends as the temperature decreased, a decrease in magnetic susceptibility, commonly associated with antiferromagnetic coupling between the MnIII ions37,48 and/or depopulation of the Stark levels of the DyIII ion. The magnetic susceptibility values from the in-phase ac magnetic susceptibility values observed for the structures corroborated the dc magnetic data (Table 3). The extrapolated in-phase susceptibility values were similar across the series and generally described as smaller than expected for the present cations if they were noninteracting. The values do not lead to a clear antiferromagnetic coupling scheme between the ions, and due to the presence of the highly anisotropic DyIII ion, it is difficult to model the coupling scheme. While the identity of the bridging carboxylate ligand did not significantly alter the dc magnetic properties and in-phase ac magnetic susceptibility properties of the different MCs, the same cannot be said for the out-of-phase ac magnetic susceptibility measurements. Examining the out-of-phase ac magnetic susceptibility, it became clear that the bridging ligand played a larger role on the magnetic behavior. Only the Dy(Hsal)4M 12-MC-4 complexes showed slow magnetic relaxation. Once again the identity of the alkali metal ion is not critical. The Dy(OAc)4M 12-MC-4, Dy(ben)4M 12-MC-4, and Dy(TMA)4Na 12-MC-4 did not show onset of slow magnetic relaxation. The electronic properties of the bridging ligands may explain the observed SMM-like behavior. Boukhvalov and co-workers have proposed that the accumulated electronegativity (AEN) of the ligands could be the source of the perturbation on SMM behavior.51 Boukhvalov and co-workers found that as the AEN increased in Mn12 species, the density of states for the Mn 3d and O 2p moved closer together, enhancing ferromagnetic exchange.51 This observation and the calculated values could help provide some sense to the out-of-phase properties observed in the DyX4M 12-MC-4 complexes. However, there is one major weakness with using AEN values for the investigated bridging carboxylates. Accumulated electronegativity values do not take into account induction effects nor the electronic effects of aromatic rings. Trimethyl acetate has the largest AEN value, which is slightly larger than the AEN value of salicylate (Figure 7). If AEN was the major trend correlated to SMM-like behavior, one may expect Dy(TMA)4Na 12-MC4 to show similar SMM-like behavior as was found in the Dy(Hsal)4M 12-MC-4 compounds; this was not the case. Therefore, for MCs, it appears that pKa values, which also correlate to electron density, are better predictors for SMM-like behavior.

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CONCLUSION The 12-MCMnIIIN(shi)-4 framework has proven remarkably versatile with regards to being employed as molecular magnets (Figure 6). Incorporating isotropic, but high-spin MnII as the central ring metal in the Mn(OAc)2 12-MC-4 compound revealed that the 12-MC-4 ring oriented the anisotropy vectors of the MnIII ions perpendicular to the MC plane and parallel to each other.37 It was hoped that utilizing this optimized geometry with different central metals would increase the blocking temperature. Song and co-workers found that using diamagnetic YIII and W(CN)83− as central metals induced SMM-behavior as well as modifed the superexchange pathways between MnIII ions in the ring.48 We now have furthered this framework by incorporating an intrinsically anisotropic and high-spin DyIII ion into the central cavity as well as diamagnetic monovalent cations for charge balance.20 Using the same framework, we have also demonstrated that it is possible to rationally and predictably modify the bridging ligand between the DyIII central ion and the ring MnIII ions. It was found that changing the bridging ligand in the DyX4M 12-MC-4 compounds resulted in unique magnetic behavior in the ac out-of-phase susceptibility that appears correlated to bridging ligand pKa. Salicylate-based derivatives, having the lowest pKa of bridging ligands investigated, demonstrated SMM-like behavior, regardless of the identity of counterion investigated (M = KI, NaI). This is not a trivial observation; if the diamagnetic cation dramatically altered the magnetic properties, it could make grafting future metallacrowns on surfaces a difficult endeavor, prevent the possibility of electrostatically binding two metallacrowns together around a large dication, and also make the selection of these countercations a nontrivial decision. Combined with Song and co-workers’ findings, it appears the nature of bridging ligands in the MC family can dramatically affect the magnetic properties of the 12-MC-4. Using cyanobridges rather than simple electrostatic interactions can modify the nature of the superexchange pathways of the MC ring. Lowering the pKa of bridging ligands can perturb the interactions between the MnIII and central DyIII. These interactions provide chemists with new opportunities to fundamentally control and alter the magnetic properties of the 12-MC-4 compounds. We anticipate additional MC-based molecular magnets to become prevalent in the literature based on their versatility. I


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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b01832. Tables summarizing important structural parameters and figures indicating how these values were calculated; magnetic susceptibility, magnetization, and ac magnetic susceptibility (PDF) X-ray data of compounds 1, 2, 5, 6, and 7 (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. Present Addresses ⊥

Department of Chemistry, Purdue University, West Lafayette, Indiana 47907-2084, United States. # Department of Chemistry, Boston University, Boston, Massachusetts 02215, United States. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS T.T.B. acknowledges SRU-CHES Student-Faculty research grant and SRU-Summer Undergraduate Research Experience for funding. T.T.B. acknowledges the Chateaubriand Fellowship for funding leading to these results. C.M.Z. acknowledges the Undergraduate Research Fund at Shippensburg University and the Pennsylvania State System of Higher Education FPDC grant 2014-SU-04 for financial support. V.L.P. acknowledges NSF-grant CHE-1361779 for general funding. V.L.P. acknowledges the Blaise Pascal Chair for funding leading to these results. The research leading to these results has received funding from the European Community’s Seventh Framework Programme (FP7/2007-2013) under grant agreement no. 611488 (Marie Curie IRSES “Metallacrowns” project). Acknowledgement is made for funding from NSF grant CHE-0840456 and DMR 1337296 for X-ray instrumentation and NSF-MRI program grant CHE-1040008 for SQUID instrumentation. We thank Reza Loloee at Michigan State University Department of Physics and Astronomy for use of the SQUID magnetometer. Talal Mallah is thanked for useful discussion regarding this manuscript.

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REFERENCES

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Inorganic Chemistry

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