A Spontaneous Condensation Sequence from a {Fe6Dy3} Wheel to a

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A Spontaneous Condensation Sequence from a {FeDy3} Wheel to a {FeDy} Globe 6

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Olga Botezat, Jan van Leusen, Juerg Hauser, Silvio Decurtins, Shi-Xia Liu, Paul Kögerler, and Svetlana G. Baca Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b01668 • Publication Date (Web): 27 Feb 2019 Downloaded from http://pubs.acs.org on March 5, 2019

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Crystal Growth & Design

A Spontaneous Condensation Sequence from a {Fe6Dy3} Wheel to a {Fe7Dy4} Globe Olga Botezat,†, ‡ Jan van Leusen,‡ Jürg Hauser,§ Silvio Decurtins,§ Shi-Xia Liu,§ Paul Kögerler,*,‡ and Svetlana G. Baca*,† †

Institute of Applied Physics, Academiei 5, MD2028 Chisinau, Moldova

‡

Institute of Inorganic Chemistry, RWTH Aachen University, Landoltweg 1, 52074 Aachen,

Germany §

Department of Chemistry and Biochemistry, University of Bern, Freiestrasse 3, CH-3012 Bern,

Switzerland

ABSTRACT. In the presence of dicyanamide and N-butyldiethanolamine (bdeaH2), Fe(III) and Dy(III)

precursors

self-assemble

into

the

charge-neutral

coordination

cluster

[Fe6Dy3(ib)9(bdea)6(MeO)6]∙MeOH (1) (ib: isobutyrate) featuring an approx. C3-symmetric wheel of edge-sharing FeO6 and DyO6N2 polyhedra. This is followed by a remarkable selfinduced conversion of this cluster into the highly condensed globe-shaped cluster [Fe7Dy4O4(OH)3(ib)9.25(bdea)6(NO3)0.75(H2O)] (2) in methanolic solution under ambient conditions. The transition from {Fe6Dy3} to {Fe7Dy4} coincides with a slowdown of the clusters’

ACS Paragon Plus Environment

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Crystal Growth & Design 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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magnetization dynamics, as evident from a shift of the onset of significant out-of-phase components of their magnetic ac susceptibility from 3.6 K up to 12 K (1000 Hz) in the absence of a static bias field.

INTRODUCTION Polynuclear heterometallic 3d-4f coordination clusters have attracted significant attention over past decades due to their highly versatile structural chemistry and great potentials for technological and industrial applications, especially in emerging fields such as development of “intelligent” multifunctional materials.1-3 These coordination clusters are particularly attractive while combining valuable physical properties with structural peculiarities such as large size and intriguing architectures that allow to cross boundaries between chemistry, physics and material sciences. For instance, they show fascinating magnetic phenomena4-8 such as single-molecule magnet behavior (SMM), macroscopic quantum tunneling of magnetization, and quantum coherence – all key properties needed for materials to function as quantum bits (qubits), as well as to show relevance to some fundamental physics such as high temperature superconductivity and colossal magnetoresistance. However, at the forefront of 3d-4f research, it is still necessary to gather new examples of structurally intriguing cluster compounds to study their magnetostructural correlations in the context of their magnetic relaxation dynamics. Among 3d-Ln coordination clusters that incorporate the high spin carriers such as Mn, Ni, Co, Cu or Cr ions and have a large potential of SMMs high-nuclearity, Fe-Ln clusters remain attractive targets over the recent years.9-23 An effective, commonly used strategy to construct such polynuclear heterometallic clusters relies on the identification of stable pre-designed metal-containing


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building blocks and utilization of flexible and multidentate ligands for the arrangement of these blocks into the desired topologies. Here we report the synthesis, structures and magnetic properties of two new odd-nuclearity FeIII-DyIII clusters that serendipitously self-assemble in an unusual

one-pot

reaction

containing

trinuclear

iron-oxo

carboxylate

blocks

of

[Fe3O(ib)6(H2O)3](NO3)·2(H2O)∙2(MeCN), dysprosium(III) nitrate hexahydrate and alkoxo ligands known for their excellent bridging and chelating abilities to Ln ions.

EXPERIMENTAL SECTION Materials and Methods. All manipulations were performed under aerobic conditions using chemicals and solvents as received without further purification. An oxo-centered trinuclear isobutyrate precursor [Fe3O(ib)6(H2O)3]NO3∙2(MeCN)∙2(H2O) has been prepared by using a method described elsewhere.24 IR transmission spectra were recorded in the solid state (KBr pellets) on a Perkin-Elmer Spectrum One spectrometer in the 400–4000 cm–1 range. TGA/DTA measurements were carried out with a Mettler Toledo TGA/SDTA 851 in dry N2 stream (60 mL min–1) at a heating rate of 10 K min–1 from 25 °C to 800 °C. Synthesis

of

[Fe6Dy3(ib)9(bdea)6(MeO)6]∙MeOH

[Fe7Dy4O4(OH)3(ib)9.25(bdea)6(NO3)0.75(H2O)]

(2).

A

(1) mixture

and of

[Fe3O(ib)6(H2O)3](NO3)·2(H2O)∙2(MeCN) (0.12 g, 0.12 mmol), Dy(NO3)3∙6H2O (0.02 g, 0.06 mmol), sodium dicyanamide (0.01 g, 0.11 mmol) and bdeaH2 (0.02 g, 0.2 mmol) in 12 mL MeOH was refluxed for 1 hour and then filtered. The filtrate was kept in a closed vial at room temperature. Yellow crystals of 1 suitable for single-crystal X-ray analysis were filtered off after two weeks, washed with methanol and dried in vacuum. Yield: 0.029 g (25% based on Fe). Anal. Calcd for C91H187Dy3Fe6N6O37: C, 39.23; H, 6.77; N, 3.02%. Found: C, 38.3; 38.07; H, 6.62;


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6.91; N, 3.08; 3.01%. IR data (KBr pellet, cm–1): 3440br, 2962m, 2929sh, 2873m, 1556vs, 1472s, 1422s, 1373m, 1289m, 1166w, 1096s, 1049s, 1013sh, 902m, 836w, 660sh, 565m. If the formed yellow crystals of 1 were left in the mother solution they are completely dissolved and then red-brown crystals of 2 suitable for X-ray analysis started to grow. After three months they have been filtered off, washed with methanol and dried in vacuum. Yield: 0.008 g (5% based on Fe). Anal. Calcd for C85H171.75Dy4Fe7N6.75O40.75: C, 34.15; H, 5.80; N, 3.16%. Found: C, 34.09, 34.13; H, 5.83, 5.83; N, 3.50, 3.48%. IR data (KBr pellet, cm–1): 3429br, 2960m, 2925sh, 2861m, 1588s, 1536s, 1473m, 1430s, 1374m, 1292m, 1169w, 1093m, 1033m, 897w, 604m, 558sh, 511sh. X-ray Crystallography. Diffraction data sets were collected at 100(2) K on a Bruker diffractometer with APEX II CCD detector for 1 and at 173(2) K on an Oxford Xcalibur CCD for 2 using graphite-monochromatized MoKα radiation. The structures were solved by direct methods and reﬁned by full-matrix least-squares on weighted F2 values for all reflections using SHELX suite of programs.25 All non-H atoms in the clusters were refined with anisotropic displacement parameters. H atoms were placed in fixed, idealized positions and refined as rigidly bonded to the corresponding atom. H atoms of solvent MeOH molecules could not be located in 1. Some methyl groups of isobutyrate, N-butyldiethanolamine, and methoxy groups were found to be disordered over two positions. Application of restrains provided reasonable geometrical parameters and thermal displacement coefficients. In 2, the coordinated nitrate anion shares position with the chelate isobutyrate (0.75:0.25). Crystallographic data and structure reﬁnements of 1 and 2 are summarized in Table 1 and selected bond distances are given in Table 2.


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CCDC 1854191 (1) and 1854196 (2) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from the Cambridge Crystallographic data centre via www.ccdc.cam.ac.uk/data_request/cif. Magnetic Measurements. Magnetic susceptibility data for 1 and 2 were obtained using a Quantum Design MPMS-5XL SQUID magnetometer. The polycrystalline samples were compacted and immobilized into cylindrical PTFE capsules. The dc data were acquired as a function of field (0 – 5.0 T at 2.0 K) and temperature (2.0 – 290 K at 0.1 and 1.0 T). Ac data were measured at zero static bias field from 2.0 – 50 K for frequencies between 3 – 1000 Hz. All data were corrected for diamagnetic contributions from the sample holder and the compounds (χm,dia = –1.37×10–3 cm3 mol–1 (1); –1.52×10–3 cm3 mol–1 (2)). Table 1. Crystal Data and Details of Structural Determinations for 1 and 2

formula Mr / g mol–1 T/K cryst system space group a/Å b/Å c/Å α β γ V / Å3 Z Dcalcd / Mg/m3  / mm–1 F(000) reflns collected / unique Data / restraints / parameters GOF

1 {Fe6Dy3} C91H187Dy3Fe6N6O37 2780.05 100(2) triclinic P-1 18.424(3) 19.448(3) 20.301(3) 102.016(2)° 108.799(2)° 110.864(2)° 5990.2(14) 2 1.541 2.625 2850 65236 / 21094 [Rint = 0.0874] 21094 / 573 / 1403 1.015

2 {Fe7Dy4} C85H171.75Dy4Fe7N6.75O40.75 2981.48 173(2) monoclinic P 21/c 17.0512(5) 18.7764(3) 36.6927(11) 90° 92.347(3)° 90° 11737.7(5) 4 1.687 3.427 6004 169135 / 20659 [Rint = 0.0903] 20659 / 125 / 1360 1.009


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final R indices [I > 2(I)] R indices (all data)

R1 = 0.0587, wR2 = 0.1345 R1 = 0.1121, wR2 = 0.1637

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R1 = 0.0931, wR2 = 0.2734 R1 = 0.1193, wR2 = 0.2909

Table S2. Selected Bond Distances (Å) for 1 and 2 1 Dy1O22 Dy1O20 Dy1O19 Dy1O21 Dy1O1 Dy1O18 Dy1N1 Dy1N2 Dy2O26 Dy2O23 Dy2O6 Dy2O24 Dy2O7 Dy2O25 Dy2N4 Dy2N3 Dy3O27 Dy3O30 Dy3O29 Dy3O28

2.274(6) 2.299(5) 2.320(6) 2.326(6) 2.338(7) 2.348(6) 2.660(7) 2.673(7) 2.278(6) 2.294(6) 2.330(7) 2.341(6) 2.348(6) 2.352(6) 2.660(8) 2.685(7) 2.291(7) 2.297(6) 2.311(7) 2.326(7)

Dy3O13 Dy3O12 Dy3N5 Dy3N6 Fe1O20 Fe1O21 Fe1O32 Fe1O31 Fe1O2 Fe1O3 Fe2O23 Fe2O25 Fe2O32 Fe2O31 Fe2O4 Fe2O5 Fe3O24 Fe3O26 Fe3O33 Fe3O34

Dy1O14 Dy1O28 Dy1O15 Dy1O5 Dy1O31 Dy1O39 Dy1O27 Dy1N1 Dy1O1 Dy2O36 Dy2O21 Dy2O32 Dy2O2 Dy2O4 Dy2O19 Dy2O20 Dy2O30

2.297(12) 2.325(10) 2.352(12) 2.403(9) 2.433(11) 2.458(10) 2.509(10) 2.728(13) 2.777(10) 2.266(11) 2.273(11) 2.295(11) 2.367(10) 2.379(11) 2.405(15) 2.522(12) 2.560(10)

Dy4O13 Dy4O34 Dy4O37 Dy4O11 Dy4O6 Dy4O5 Dy4O12 Dy4O27 Fe1O1 Fe1O31 Fe1O32 Fe1O7 Fe1O28 Fe1N3 Fe2O1 Fe2O6 Fe2O9

2.340(7) 2.351(7) 2.658(8) 2.676(8) 1.949(6) 1.960(6) 1.981(5) 1.994(6) 2.041(6) 2.056(6) 1.938(6) 1.966(6) 1.988(6) 1.994(6) 2.049(7) 2.050(7) 1.949(6) 1.956(6) 1.977(7) 1.994(7)

Fe3O9 Fe3O8 Fe4O27 Fe4O29 Fe4O33 Fe4O34 Fe4O11 Fe4O10 Fe5O28 Fe5O35 Fe5O30 Fe5O36 Fe5O14 Fe5O15 Fe6O19 Fe6O22 Fe6O35 Fe6O36 Fe6O17 Fe6O16

2.047(7) 2.067(9) 1.946(8) 1.956(6) 1.971(6) 2.007(8) 2.035(7) 2.070(7) 1.954(7) 1.955(6) 1.960(7) 2.004(7) 2.040(7) 2.071(8) 1.944(6) 1.966(7) 1.972(6) 2.004(7) 2.054(7) 2.070(8)

2.275(14) 2.304(12) 2.316(11) 2.366(13) 2.369(11) 2.397(10) 2.503(11) 2.572(10) 1.882(10) 2.012(10) 2.012(12) 2.023(10) 2.024(11) 2.241(14) 1.918(10) 2.021(10) 2.031(11)

Fe3O4 Fe4O6 Fe4O34 Fe4O40 Fe4O33 Fe4O10 Fe4N4 Fe5O2 Fe5O36 Fe5O35 Fe5O40 Fe5O17 Fe5N5 Fe6O3 Fe6O2 Fe6O18 Fe6O33

2.017(9) 1.922(11) 1.932(13) 2.002(12) 2.032(11) 2.081(11) 2.268(14) 1.935(10) 1.944(11) 2.001(11) 2.002(11) 2.081(13) 2.264(13) 1.931(12) 1.996(10) 2.015(13) 2.023(11)

2


6

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2.328(11) 2.352(9) 2.363(11) 2.392(11) 2.441(12) 2.451(12) 2.454(10) 2.635(10) 2.641(14)

Dy3O29 Dy3O4 Dy3O22 Dy3O23 Dy3O30 Dy3O24 Dy3O38 Dy3O3 Dy3N2

Fe2O35 Fe2O8 Fe2O27 Fe3O1 Fe3O3 Fe3O5 Fe3O2 Fe3O6

2.050(12) 2.061(10) 2.096(11) 1.995(10) 1.998(10) 1.999(10) 2.009(11) 2.013(10)

2.070(10) 2.098(11) 1.887(11) 1.999(13) 2.013(12) 2.031(12) 2.036(11) 2.199(13)

Fe6O26 Fe6O30 Fe7O3 Fe7O37 Fe7O38 Fe7O29 Fe7O25 Fe7N6

RESULTS AND DISCUSSION Synthesis and IR Characterization. The reaction of a trinuclear FeIII-oxo isobutyrate precursor, [Fe3O(ib)6(H2O)3](NO3)·2(MeCN)∙2(H2O),24 with Dy(NO3)3·6H2O, sodium dicyanamide and Nbutyldiethanolamine in methanol solution led to the formation of crystals of a nonanuclear {Fe6Dy3}-type cluster compound, [Fe6Dy3(ib)9(bdea)6(MeO)6]∙MeOH (1), in 25% yield. When the yellow crystals of 1 were left in the mother solution they completely re-dissolve, and redbrown

crystals

of

an

undecanuclear

{Fe7Dy4}-type

cluster

compound,

[Fe7Dy4O4(OH)3(ib)9.25(bdea)6(NO3)0.75(H2O)] (2), form in 5% yield (Figure 1). We note that the presence of sodium dicyanamide is mandatory for the formation of both 1 and 2 assuming that sodium dicyanamide acts as a template for the conversation of wheel-shaped cluster 1 into condensed globe-like cluster 2, and that upon isolation of 1 from the reaction solution no further crystallization of any products was observed. The infrared spectra of 1 and 2 display the OH stretching vibrations caused by the presence of hydroxo groups, solvent methanol and water molecules in the region 3449 – 3440 cm–1. The CH stretching vibrations of CH3, CH2 and ˃CH groups of isobutyrate and bdea2 ligands were observed in the range of 2962 – 2861 cm–1. These are accompanied by strong peaks in the region 1473 – 1472 cm–1 and 1374 – 1372 cm–1 arising from asymmetric and symmetric bending


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vibrations of these groups, respectively. The characteristic strong peaks observed in the region 1587 – 1536 cm–1 and 1430 – 1424 cm–1 correspond to the asymmetric and symmetric stretching vibrations of the coordinated isobutyrate groups, respectively.

Figure 1. Scheme of the reaction sequence leading to cluster compounds 1 and 2. Dy: green, Fe: yellow, O: red, N: blue spheres; C(isobutyrate): blue-gray, C(bdea): light gray, C(MeO): black lines. For 2, the FeO6 coordination environments are shown as yellow octahedra, emphasizing their degree of condensation; the nitrate ligand is highlighted by purple NO bonds, and the O site of the water ligand (orange) is encircled. Structural Description. Single-crystal X-ray diffraction reveals the {Fe6Dy3} compound 1 to crystallize in the triclinic space group P-1; the {Fe7Dy4} compound 2 crystallizes in the monoclinic space group P21/c. The {Fe6Dy3} cluster core is S6-symmetric. The six FeIII and three DyIII ions are linked by nine syn, syn - 1:1:µ isobutyrates, six aminoalcoholate (bdea2) ligands, which bind in a 2:1:2:µ3 fashion (Figure 2a), and six methoxy groups forming an almost perfect ring-type structure, whereby two adjacent FeIII ions alternate with DyIII ions, as shown in


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Figure 1. The deviations of Dy1, Dy2 and Dy3 atoms from the mean Fe1-Fe6 plane are 0.488, 0.687 and 0.349 Å, respectively (Figure S1). The six identical FeO6 coordination environments in 1 result from two O atoms of two different carboxylates, one Obdea and three OMeO sites, with Fe−O distances ranging from 1.938(6) to 2.071(8) Å (Table 2). The DyO6N2 environment originates from two bdea2 ligands and two O atoms from different bridging carboxylates (Dy−Obdea: 2.274(6) and 2.352(6) Å, Dy−Ocarb: 2.330(7) − 2.351(7) Å, Dy−N: 2.660(8) ‒ 2.685(7) Å). The nearest-neighbor Fe···Fe distances vary from 3.047(3) to 3.060(2) Å, and Fe···Dy distances from 3.354(2) to 3.374(1) Å. Interestingly, the wheel has shrunk compared to a similar {Fe6Dy3} compound based on ovanillinoxime and benzoate reported in ref. 19.

(a)

(b)

(d)

(c)

(e)


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Figure 2. Crystallographically established coordination modes of bdea2 in 1 (a) and 2 (b-e). Dy: green, Fe: yellow, O: red, N: blue, C: gray spheres; H: black lines. The core cluster structure in 2 consists of seven FeIII ions and four DyIII ions interlinked via four µ4-O and three µ3-OH groups, six ib and six bdea2 ligands forming a globe-like structure with no symmetry elements (Figure 3). This topology is slightly different from a recently reported {Fe7Dy4} pivalate cluster with triethanolamine ligands.23 One additional monodentate ib and one water ligand complete the coordination environment of Dy1, whereas chelating carboxylates complete the inner coordination of Dy3 and Dy4. In case of Dy2, a nitrate group shares the position with the chelate isobutyrate. The coordination modes of isobutyrates fall into three categories: i) coordination in a chelating mode to three DyIII ions, ii) coordination to one FeIII and one DyIII ion in a syn, syn  1:1:µ bridging mode, iii) monodentate coordination to a DyIII ion. The Fe coordination environments are distorted octahedral (NO5 for Fe1, Fe4, Fe5, and Fe7; O6 for Fe2, Fe3 and Fe6). The NO5 environments are formed by a 4-oxo group, a bridging ib (one O), two aminoalcoholate ligands (three O and one N), or by one 4-oxo and one hydroxo group, a bridging ib (one O) and a doubly deprotonated bdea2 (two O, one N) ligand. For the central Fe3 atom, the O6 set stems from four 4-oxo and two hydroxo groups, for Fe2 from three 4-oxo groups, two bridging carboxylates (two O) and a doubly deprotonated bdea2 (one O), and for Fe6 from two 4-oxo groups, two bridging carboxylates (two O) and two doubly deprotonated bdea2 (two O).


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Figure 3. A view of the globe-like metal oxo core structure of 2 and a polyhedral plot showing the complete coordination environments. The Fe−O bond distances range from 1.882(10) to 2.098(11) Å, the FeN distances from 2.199(13) to 2.268(14) Å (Table 2). Two DyIII centers (Dy1 and Dy3) possess a distorted monocapped square-antiprismatic NO8 coordination geometry arising from one 4-oxo and one hydroxo group, bridging ib (one O), two doubly deprotonated bdea2 (three O, one N) and a monodentate ib and one water molecule (two O for Dy1) or a chelating isobutyrate (two O for Dy3). Dy2 and Dy4 reside in a distorted square-antiprismatic O8 environment formed by one 4oxo and one hydroxo group, bridging ib (one O), three doubly deprotonated bdea2 (three O) and a chelating nitrate or ib group (two O, for Dy2) or chelating carboxylate (two O, for Dy4). Bond distances for DyO range from 2.266(11) to 2.777(10) Å, and for DyN they amount to 2.641(14) and 2.728(13) Å (Table S2). Nearest-neighbor Fe···Fe distances vary from 2.933(3) to 3.577(3) Å, and Fe···Dy distances from 3.911(1) to 3.977(1) Å. Notably, in 2 six bdea2 ligands that are tridentate adopt different coordination modes as shown in Figure 2b-e when coordinating to FeIII or DyIII ions.


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Magnetic Studies. The magnetic properties of 1 were investigated via both dc and ac susceptibility measurements. The dc magnetic data of {Fe6Dy3} (1) are presented as χmT vs. T and Mm vs. B curves in Figure 4a. At 290 K, the value of χmT is 63.25 cm3 K mol–1 which is below the range 65.29–68.41 cm3 K mol–1 expected for six FeIII and three DyIII non-interacting centers.26 The χmT values continuously decrease to 31.1 cm3 K mol–1 by lowering the temperature to 2.0 K. These observations reveal predominant antiferromagnetic exchange interactions within the compound besides single-ion effects of the DyIII centers (thermal depopulation of the splitted mJ substates). The molar magnetization Mm is 16.6 NA μB at 2.0 K and 5.0 T, which is well below the saturation magnetization of 60 NA μB (10 NA μB per DyIII and 5 NA μB per FeIII center). Due to the large magnetic anisotropy of DyIII centers, the magnetization reaches about half of the saturation value at 2.0 K and fields larger than 3 T for powdered samples of such isolated centers, i.e. about 3×10 NA μB/2 = 15 NA μB for three DyIII centers. (a)

(b)

Figure 4. Magnetic data of {Fe6Dy3} (1): (a) temperature dependence of χmT at 0.1 Tesla; inset: molar magnetization Mm vs. magnetic field B at 2.0 K. (b) Cole-Cole plot of out-of-phase vs. in-


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phase molar magnetic susceptibility (experimental data (symbols), least-squares fits to a generalized Debye expression (solid lines)). Therefore, and additionally considering the well-known weak exchange interaction between 3d and 4f centers as well as the structural information of two adjacent FeIII centers, the latter most likely form antiferromagnetically coupled pairs with a ground state characterized by a total spin of SFeFe = 0. Ac measurements of 1 show out–of–phase magnetic susceptibility χm signals up to 3.6 K at zero bias field (see Figures 4b, S4). We, therefore, analyze the data in terms of a generalized Debye expression27 by simultaneously fitting the χm vs. f and χmʺ vs. f data at each temperature. This results in relaxation times τ that are shown in Figure S5 as Arrhenius plot. The distribution of relaxation times (α = 0.075 ± 0.058) is close to zero, thus suggesting very few or even a single, dominating relaxation pathway. Since the semi-logarithmic representation of τ vs. 1/T is a straight line within the errors, we solely consider an Orbach relaxation process as dominant process using τ = τ0×exp(Ueff/(kBT)). The corresponding least-squares fit yields an effective energy barrier of Ueff = (3.6 ± 0.2) cm–1, and an attempt time of τ0 = (4.9 ± 0.6)×10–6 s. These values represent a rather small effective energy barrier and a commonly large attempt time in comparison to other 3d-4f SMMs.5 The magnetic dc data of {Fe7Dy4} (2) are shown in Figure 5a as the field dependence of the molar magnetization Mm at 2.0 K, and the temperature dependence of the product of molar magnetic susceptibility and temperature χmT at 0.1 and 1.0 T. The χmT vs. T curves exhibit 66.42 cm3 K mol–1 and 65.42 cm3 K mol–1 at 290 K for 0.1 and 1.0 Tesla, respectively. These values are well below the range 82.68–86.84 cm3 K mol–1 that is expected25 for seven FeIII and four DyIII non-interacting centers indicating dominant antiferromagnetic exchange interactions potentially between the FeIII centers.


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The values of χmT stay almost constant down to T ≈ 50 K, only decreasing by 1–2 cm3 K mol–1. Cooling beyond 50 K, however, reveals maxima at both applied magnetic fields indicating the presence of (weaker) ferromagnetic exchange interaction pathways. The maximum at 1.0 T is smaller in comparison to 0.1 T due to the Zeeman effect, which causes a larger split of the energy states for higher fields. At 2.0 K, the Mm vs. B curve shows a distinct slope, and Mm increases to 28.13 NA μB at 5.0 T, well below the saturation value of 75 NA μB. (a)

(b)

Figure 5. Magnetic data of {Fe7Dy4} (2): (a) molar magnetization Mm vs. magnetic field B at 2.0 K; inset: temperature dependence of χmT at 0.1 T (black circles) and 1.0 T (blue circles). (b) Cole-Cole plot of out-of-phase vs. in-phase molar magnetic susceptibility (experimental data (symbols), least-squares fits to a generalized Debye expression (solid lines)). Following the line of arguments as for the magnetization of 1, the seven FeIII centers potentially form an antiferromagnetically coupled sub-unit with a ground state characterized by a total spin of S7Fe = 5/2 or 7/2. The ac measurements of 2 show out-of-phase magnetic susceptibility χm′′ signals up to 12 K at zero bias field (see Figures 5b, S6). Analyzing the data in terms of a generalized Debye expression26 at each temperature yields the relaxation times τ shown in Figure


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S7. The distribution of relaxation times is α = 0.240 ± 0.189, therefore indicating the presence of multiple relaxation pathways. The semi-logarithmic representation of τ vs. 1/T is a straight line for T > 2.4 K. We, thus, consider an Orbach relaxation process as dominant process for this temperature range. The least-squares fit results in an effective energy barrier of Ueff = (15.0 ± 0.2) cm–1, and an attempt time of τ0 = (3.7 ± 0.2)×10–6 s. These values represent common values for 3d-4f SMMs.5 The cause of the observed deviation from an exponential behavior of τ at temperatures below 2.4 K cannot be identified due to the limited amount of data. Potentially a different relaxation process becomes dominant at these temperatures, or the errors of the Debye fit are still underestimated – considering the rather featureless curves of χm vs. f and χmʺ vs. f – even though they are much larger than for the values of τ at the other temperatures. CONCLUSION In summary, the consecutive self-assembly reaction of the trinuclear FeIII isobutyrate cluster with dysprosium(III) nitrate hexahydrate and N-butyldiethanolamine in the presence of sodium dicyanamide, possibly acting as a template, leads to the wheel-shaped cluster compound [Fe6Dy3(ib)9(bdea)6(MeO)6]∙MeOH (1) that transforms into the highly condensed cluster [Fe7Dy4O4(OH)3(ib)9.25(bdea)6(NO3)0.75(H2O)] (2) under ambient conditions. The experimental magnetic data point to the enhancement of out-of-phase magnetic susceptibility χmʺ signals in these aggregates from 3.6 K up to 12 K at zero bias field. Research on a synthetic extension to other possible {Fe6Ln3} and {Fe7Ln4} clusters is currently underway.

ASSOCIATED CONTENT


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Supporting Information. Packing diagrams for 1 and 2, additional magnetic data. (PDF) Crystallographic data (CIF) Crystallographic data (CIF). This information is available free of charge via the Internet at http://pubs.acs.org/. AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (P.K.); [email protected] (S.G.B.). Author Contributions The manuscript was written through contributions of all authors. Notes The authors declare no competing financial interests. ACKNOWLEDGMENT This work was supported by the Swiss national Science Foundation (SCOPES project IZ73Z0_152404/1) Authors thank group of Prof. Ullrich Englert for collecting the diffraction data set for compound 1. O.B. acknowledges a DAAD fellowship.

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SYNOPSIS The transition from a {Fe6Dy3} wheel cluster to a highly condensed {Fe7Dy4} globe-shaped cluster coincides with a shift of the onset of significant out-of-phase ac susceptibility components from 3.6 K up to 12 K.


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A Spontaneous Condensation Sequence from a {Fe6Dy3} Wheel to a

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