Transition Metal Single-Molecule Magnets: A {Mn31} Nanosized

Communication Cite This: J. Am. Chem. Soc. 2017, 139, 15644-15647

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Transition Metal Single-Molecule Magnets: A {Mn31} Nanosized Cluster with a Large Energy Barrier of ∼60 K and Magnetic Hysteresis at ∼5 K Parisa Abbasi,† Kevan Quinn,† Dimitris I. Alexandropoulos,† Marko Damjanović,‡ Wolfgang Wernsdorfer,‡,§,⊥ Albert Escuer,∥ Julia Mayans,∥ Melanie Pilkington,*,† and Theocharis C. Stamatatos*,† †

Department of Chemistry, Brock University, 1812 Sir Isaac Brock Way, St. Catharines, Ontario L2S3A1, Canada Institute of Nanotechnology, Karlsruhe Institute of Technology (KIT), Hermann-von-Helmholtz-Platz 1, D-76344 Eggenstein-Leopoldshafen, Germany § Institut Néel, CNRS, BP 166, 25 avenue des Martyrs, 38042 Grenoble Cedex 9, France ⊥ Physikalisches Institut, Karlsruhe Institute of Technology (KIT), Wolfgang-Gaede-Str. 1, D-76131 Karlsruhe, Germany ∥ Departament de Quimica Inorgànica i Orgànica, secció Inorgànica and Institut de Nanociencia i Nanotecnologia (IN2UB), Universitat de Barcelona, Martí i Franqués 1-11, 08028 Barcelona, Spain ‡

S Supporting Information *

affording compounds exhibiting slow relaxation of magnetization with varying energy barriers and magnetic dynamics.2,6,7 Polynuclear MnIII-containing cluster compounds with nuclearities as high as {MnIII84}8 have been the protagonists despite the fact that only two families have demonstrated exciting SMM properties to-date. These include the ubiquitous family of {MnIII8MnIV4} SMMs with Ueff values as large as 74 K,2,9 and the {MnIII6}-oximato clusters that possess a record energy barrier of 86 K.10 Although higher-nuclearity Mn clusters have been prepared that are structurally very impressive,4,5,11 their SMM properties have always been rather modest when compared to the smaller {Mn12} and {Mn6} series of complexes. This is not surprising since, from the perspective of rational design, it is extremely difficult -if not impossible- to simultaneously control both the spin ground state and the magnetic anisotropy of these systems.12 When developing a successful design strategy for the synthesis of transition metal clusters, the organic chelating/ bridging ligands must be carefully considered. 2(Hydroxymethyl)pyridine (hmpH, Scheme S1) belongs to the family of pyridyl alchohol ligands that have been invaluable for the discovery of large 3d-metal clusters, high-spin molecules and SMMs.13 We have recently initiated a new program of research aimed at the development of small, potentially chiral chelating ligands for the discovery of complexes with unprecedented structural motifs and novel magnetic and/or electronic properties. In this context, our first target ligand, α-methyl-2pyridine-methanol (mpmH, Figure 1) possesses similar coordination features to hmpH, but offers subtly different steric and electronic effects and is unexplored in the field of 3d-cluster chemistry. We report herein the use of mpmH for the assembly of a new, mixed-valence {MnII2MnIII28MnIV} cluster with an unprecedented nanosized structure and a large energy barrier of ∼60 K for reversal of the magnetization. In addition, field dependent

ABSTRACT: The first {Mn31} cluster (1) has been prepared from carboxylate ions and the chelating/bridging ligand α-methyl-2-pyridine-methanol. Compound 1 possesses a unique nanosized structural topology with one of the largest energy barriers reported to-date for highnuclearity 3d-metal clusters. Single-crystal magnetic hysteresis studies reveal the presence of hysteresis loops below 5 K, one of the highest temperatures below which molecular hysteresis has been observed for 3d-based SMMs.

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ingle-molecule magnets (SMMs) are 0-D systems displaying slow magnetization relaxation that results from a combination of an appreciable ground state spin, S, together with significant “easy-axis”-type magnetic anisotropy.1 This leads to an energy barrier to the magnetization reversal, whose maximum (U) is equal to S2|D| or (S2 − 1/4)|D| for integer and half-integer S values, respectively, where D is the zero-field splitting parameter.2 In recent years, the field of SMMs has entered a new era with the discovery of mono- and oligonuclear lanthanide complexes that exhibit extremely large energy barriers and blocking temperatures due to the significant magnetic anisotropy associated with 4f ions.3 Nevertheless, the family of polynuclear first-row transition metal clusters comprise the largest class of SMMs reported to-date.4,5 Experimentally, these SMMs exhibit superparamagnet-like properties that are characterized by the presence of frequency-dependent out-ofphase ac signals, as well as magnetic hysteresis.1,4,5 Although such properties can also be ascribed to classical magnets, this family of coordination complexes are considered to be true nanoscale particles that straddle the boundary between the classical and quantum world, as evidenced by their ability to undergo quantum tunneling of their magnetization (QTM).6 SMMs based on Jahn−Teller (JT) axially distorted octahedral MnIII ions have dominated the field for the past 2 decades © 2017 American Chemical Society

Received: September 21, 2017 Published: October 20, 2017 15644

DOI: 10.1021/jacs.7b10130 J. Am. Chem. Soc. 2017, 139, 15644−15647

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Journal of the American Chemical Society

The molecular structure of 1 (Figure 1, top) consists of a mixed-valence (MnII2MnIII28MnIV) centrosymmetric {Mn31} cluster with a closed cage-like conformation. Mn6 and Mn(10,10′) are the MnIV and MnII ions, respectively, whereas all the remaining metal ions are assigned to MnIII. The metal ions are connected by 18 μ4-O2−, 6 μ3-O2−, 2 μ-OH−, together with 24 MeO− groups. The latter are solely bound to the MnIII ions and they are arranged in three classes: 6 are μ3- and 14 μbridging, whereas the remaining four are terminally coordinated to Mn(13,13′,16,16′). The BVS values for the terminally bound MeO− groups are 1.77 and 1.82, in support of their deprotonated form. Additional bridging ligation is provided by the deprotonated alkoxido arms of two η1:η2:μ mpm− ligands, as well as ten η1:η1:μ and four η1:η2:μ3 PhCO2− groups. Finally, the dangling O atoms of two monodentate PhCO2− groups are hydrogen bonded to the OH− ions. All Mn ions are sixcoordinate with distorted octahedral geometries except for the five-coordinate Mn(13,13′) ions which possess square pyramidal geometry (τ = 0.08).17 The JT axes of the MnIII ions are axial elongations and the majority of them were found to be parallel to each other (Figure S1). Because the molecular anisotropy is the vector sum of the single-ion values, and this sum should not be zero given the orientations of the 26 MnIII JT axes, a D ≠ 0 could be expected (vide inf ra).13 The {Mn31} core (Figure 1, bottom left) can be conveniently described as a consecutive array of edge-sharing {Mn4(μ4-O)} tetrahedra and {Mn3(μ3-O)} triangles that are linked to each other via bridging O2− and MeO− groups. An alternative description of the {Mn31} core can be derived by dissecting it into seven parallel layers of four types with an ABCDCBA arrangement (Figure 1, bottom right). Layers A and B are simple MnIII monomeric and {MnIII4} butterfly subunits, respectively, attached to each other through a μ4-bridging O2− ion. Layer C is a {MnIIMnIII5} cluster comprising three edge-sharing {Mn3} triangles. Finally, layer D consists of a {MnIII8MnIV} rod-like structure that can be further seen as a central, planar disk-like {MnIII6MnIV} unit,18 with two additional MnIII ions above and below the {Mn7} disk (Figure S2). The layers are held together and bridged to each other by a combination of bridging oxido and methoxido groups. The space-filling plot (Figure S3) shows that 1 has a nearly spherical conformation with a diameter of ∼24 Å, defined by the longest C···C distance, excluding the H atoms. The shortest Mn···Mn distance between neighboring {Mn31} clusters in the crystal is 8.540(1) Å (Figure S4). Variable-temperature direct-current (dc) magnetic susceptibility measurements were performed on a freshly prepared and analytically pure microcrystalline sample of 1 in the temperature range 2−300 K in an applied field of 0.2 kG (0.02 T). The data are shown as a χMT vs T plot in Figure S5. The value of the χMT product at 300 K is 74.48 cm3·mol−1·K, lower than the value of 94.63 cm3·mol−1·K (calculated with g = 2) expected for 2 MnII, 28 MnIII, and a MnIV non-interacting ion. The χMT product slightly decreases in the 300−160 K region, reaching a minimum of 71.83 cm3·mol−1·K, and then increases more sharply to reach a value of 83.09 cm3·mol−1·K at 35 K, before dropping rapidly to a value of 54.15 cm3·mol−1·K at 2 K. The shape of the curve suggests an overall ferrimagnetic system, where both antiferroand ferromagnetic exchange interactions are likely present within 1. This is not surprising given the many different subunits present in 1, the variety of bridging ligands and the different Mn oxidation states.19 The molecular structure of 1 is simply too complex to assign a spin ground state value with any certainty. Furthermore, with the exception of the MnIII···MnIV inter-

Figure 1. Complete structure of 1 (top), its metal−oxygen core (bottom left), and the four types of constituent layers (bottom, right) along the crystallographic c-axis. H atoms are omitted for clarity. A linedrawing of mpmH is also shown. Color scheme: MnII yellow, MnIII blue, MnIV olive green, N green, O red, C gray.

studies reveal the presence of hysteresis loops at temperatures below 5 K, among the highest temperatures observed for 3dbased SMMs to-date. The {Mn31} compound is (i) one of the largest 3d-metal clusters reported in the literature,6 (ii) the second largest Mn cluster containing an odd-number of metal ions14 and the second cluster with a nuclearity of 31,15 and (iii) the highest-nuclearity SMM reported that possesses entirely resolved out-of-phase peaks at T < 7 K and a large Ueff. Racemic mpmH was prepared via the NaBH4 reduction of 2acetylpyridine (see ESI).16 The reaction of Mn(O2CPh)2·2H2O, rac-mpmH and NEt3 in a 2:1:2 ratio in MeOH gave a dark red solution that, upon evaporation at room temperature for 20 days, afforded dark brown rod-like crystals of [Mn31O24(OH)2(OMe)24(O2CPh)16(rac-mpm)2] (1) in 30% yield. The oxidation states of the 16 crystallographically independent Mn ions and the formula of 1 was confirmed by inspection of the bond lengths and angles, bond valence sum (BVS) calculations (Table S2), and charge balance considerations. 15645


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recorded for 1 at different temperatures below 8 K, deviate significantly from the usual semicircular shape observed previously in the majority of 3d-SMMs,2 and are indicative of a large distribution of relaxation times. Indeed, the data were fit using a generalized Debye model22 and the resulting α values were in the range 0.12−0.54, indicating a large distribution of relaxation times (Table S3).23 Furthermore, the mixing of various S states may suggest the possible presence of multiple relaxation processes as well. Single-crystal hysteresis studies on 1, using a micro-SQUID apparatus, were further performed to confirm that the complex displays SMM behavior. The field was aligned parallel to the mean easy-axis of magnetization using the transverse field method.24 The resulting magnetization vs dc field data at different temperatures and a fixed field sweep rate of 0.140 T·s−1 are shown in Figure 3 (top), and at different scan rates and a

actions, most of the exchange interactions are expected to be weak, antiferromagnetic and of similar magnitude, which means that extensive spin frustration effects are likely in effect.20 Nevertheless, for a previously reported {MnII6MnIII18MnIV} compound, containing a central {MnIII6MnIV} disk-like unit akin to 1, it was established that the spin of this unit is S = 21/2 and that it made a significant contribution to the overall spin ground state of the system.21 The χMT value expected for an S = 21/2 system is 60.38 cm3·mol−1·K, close to the χMT value of 1 at ∼2 K. Reduced magnetization studies on 1 at various fields and low temperatures (Figure S6) clearly suggest the presence of lowlying excited states and/or magnetic anisotropy, as the various isofield lines do not superimpose onto the master curve expected for a well-isolated spin ground state. This result is consistent with the isolation of a high-nuclearity metal cluster with a large density of spin states, for which the dc magnetic data cannot be fit adequately with a model that assumes only population of the ground state. Given these challenges, we turned our attention to alternating-current (ac) magnetic susceptibility measurements as a means of determining the ground state of 1 and studying its magnetization dynamics in the absence of any external dc field. The ac studies were performed in a 4.0 G ac field oscillating at 30 different frequencies. Extrapolation of the χM′T signal (Figure 2, top) from above 6 down to 0 K gave a value of ∼70− 75 cm3·K·mol−1, suggestive of an S = 23/2 ground state (χM′T(S=32/2) = 71.85 cm3·K·mol−1 for g = 2). Below ∼6 K, there is a frequency-dependent decrease in χM′T followed by the appearance of frequency-dependent χM″ signals (Figure 2, bottom) consistent with the superparamagnetic-like slow relaxation of an SMM. The Cole−Cole plots (Figure S7)

Figure 3. (top) Magnetization (M) vs dc field hysteresis loops for a single-crystal of 1 at the indicated temperatures and a fixed field sweep rate of 0.140 T·s−1; (bottom) Arrhenius plot of the relaxation time (τ) vs 1/T using the data obtained from the ac susceptibility and dc magnetization decay measurements; (inset) magnetization (M) vs time decay plots in zero dc field. The dashed line is the fit of data; see the text for the fit parameters. The magnetization is normalized to its saturation value, MS.

constant T = 3 K in Figure S8. Below ∼5 K, hysteresis loops are visible and their coercivities increase with decreasing temperature and increasing field scan rates, as expected for SMM behavior. Thus, 1 is a new SMM with a blocking temperature of ∼5 K, above which no hysteresis is observed. Although the hysteresis loops do not show any clear steps characteristic of QTM behavior, this is typical for very high-nuclearity SMMs because of step-broadening effects that arise from the presence

Figure 2. Temperature dependence of the in-phase (as χM′T, top) and out-of-phase (χM″, bottom) ac magnetic susceptibilities in zero dc field for 1, measured in a 4.0 G ac field oscillating at the frequencies 1−1488 Hz (30 frequencies in total). The solid lines are guides only. 15646



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of low-lying excited states, intermolecular interactions, and other distributions or ligand disorders.3−8,10,13,21 Following these results, magnetization vs time decay data were collected on a single-crystal of 1 to assess the magnetization relaxation dynamics, and the results are shown in Figure 3 (inset). These data, together with the ac χM″ vs T data, were used to calculate the relaxation rates (1/τ) at different temperatures and construct an Arrhenius plot (Figure 3, bottom) based on the equation: τ = τ0exp(Ueff/kT), where k is the Boltzmann constant and τ0 is the pre-exponential factor. The fit to the thermally activated region above ∼0.2 K gave Ueff = 58 K and τ0 = 3 × 10−8 s. The observed Ueff is the highest barrier reported to date for a very high-nuclearity 3d-metal cluster that is also comparable in magnitude with the current record-holders in 3d-SMMs, namely Mn6 and Mn12.2,7,9,10 At about 0.5 K and below, the relaxation becomes temperature independent, as expected for relaxation by ground-state QTM. Thus, we can conclude that although 1 is comparable in size with smaller magnetic nanoparticles, it still possesses the quantum properties of a molecular species. Therefore, 1 could be potentially considered as a model system to further explore the boundary between SMMs and magnetic nanoparticles. In summary, we have reported the synthesis, structure, and magnetic properties of one of the highest nuclearity 3d-metal clusters isolated to-date.6,14,25 This complex was assembled by employing the chelating/bridging ligand mpmH together with carboxylate ions. The {Mn31} mixed-valence complex is not only structurally novel due to its layered topology and nanosize dimensions, but also exhibits SMM properties with one of the largest energy barriers yet obtained in the field of molecular 3dmetal nanomagnets. Further studies concerning the coordination chemistry of the chiral and racemic forms of this ligand together with 3d- and/or 4f-metal ions are in progress and will be reported in due course.

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REFERENCES

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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/jacs.7b10130. Various synthetic, spectroscopic, structural and magnetism details (PDF) Crystallographic data (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*[email protected] *[email protected] ORCID

Melanie Pilkington: 0000-0002-9274-8512 Theocharis C. Stamatatos: 0000-0002-9798-9331 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by NSERC-DG (Th.C.S, M.P), ERA (Th.C.S), CFI (M.P) and Brock University (Chancellor’s Chair for Research Excellence; Th.C.S). A.E. and J.M. acknowledge financial support from Ministerio de Economiá y Competitividad, Project CTQ2015-63614-P. W.W. acknowledges the Alexander von Humboldt foundation. 15647


Transition Metal Single-Molecule Magnets: A {Mn31} Nanosized

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