Lanthanide–Nitronyl Nitroxide Chains Derived from Multidentate

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Lanthanide−Nitronyl Nitroxide Chains Derived from Multidentate Nitronyl Nitroxides Juan Sun,† Zan Sun,† Licun Li,*,† and Jean-Pascal Sutter*,‡ †

Department of Chemistry, Key Laboratory of Advanced Energy Materials Chemistry and Tianjin Key Laboratory of Metal and Molecule-based Material Chemistry, College of Chemistry, Nankai University, Tianjin 300071, China ‡ LCC−CNRS, Université de Toulouse, Toulouse, France S Supporting Information *

[3,5-bis(1H-1,2,4-triazol-1-yl)phenyl]-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (Nit-Ph-3,5-btrz) and 2-[3,5-bis(4pyridyl)phenyl]-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (Nit-Ph-3,5-bPy) (Scheme S1) and their association with Ln ions. These two nitronyl nitroxide derivatives have remarkable features: (a) they are tetradentate ligands with two NO groups and two N donors in an arrangement allowing them to bridge metal centers, i.e. suitable for the construction of extended coordination polymers; (b) strong magnetic coupling with paramagnetic metal ions can be achieved by direct coordination of the NO groups; (c) these radical ligands are rigid ligands that are favorable for constructing a thermally stable coordination architecture. Using these multidentate nitronyl nitroxides, unique one-dimensional (1D) loop chains, {[Ln(hfac)3]2(NitPh-3,5-btrz)} n [Ln III = Gd (1) and Dy (2); hfac − = hexafluoroacetylacetonate] and {[Dy(hfac) 3]2(Nit-Ph-3,5bPy)}n (3), have been obtained. These involve a rare 4f−2p− 4f spin arrangement,2e,7,16 Both dysprosium complexes exhibit slow magnetic relaxation behavior. Compounds 1 and 2 are isomorphous and belong to the monoclinic Ia space group. As illustrated in Figures 1 and S1 and

ABSTRACT: Unprecedented lanthanide (Ln)-radical loop-chain coordination polymers were achieved using multidentate pyridyl- or triazole- substituted nitronyl nitroxide ligands. Their magnetic units consist of ferromagnetic [Ln2Radical] moieties, leading for the dysprosium(III) derivatives to slow relaxation of magnetization, which was found to be dependent on the heterocyclic ligands.

I

n the area of molecular magnetism, compounds with intriguing spin topologies such as irregular spin states have attracted considerable attention because of their unique magnetic properties.1 However, the rational design of exotic spin topologies with specific magnetic properties remains a stimulating challenge for chemists. For coordination compounds, directing the chemical topology with dedicated bridging ligands is quite obvious, but it becomes trickier when exchange interactions have to be achieved between spin centers. In this regard, the use of radicals as ligands has proven to be an appealing strategy for constructing diverse magnetic architectures2 including a long-range magnetic order system,3 spin-transitionlike compounds,4 and molecule nanomagnets.5 Recently, much attention has been paid to Ln-radical compounds with slow magnetic relaxation behavior6−10 in relation to the potential relevance in high-density data storage,11 molecular spintronics,12 and quantum computing.13 Remarkable results have been obtained using this approach; for example, a simple binuclear terbium complex bridged by a N2•3− radical exhibited magnetization blocking for a temperature (TB) as high as 20 K.7c Many Ln-radical single-molecule magnets (SMMs)/ single-chain magnets (SCMs) take advantage of the nitronyl nitroxide radical because of its high stability and ease of chemical modification.2b,6 Of interest are nitronyl nitroxide ligands functionalized with additional donor groups of coordination preferences different from those of the aminoxyl O atoms, which may be assembled around metal ions in various arrangements with novel structural features.14−16 The first Ln-radical SMM was a cyclic four-spin dysprosium(III) dimer bridged by pyridylsubstituted nitronyl nitroxide.15 However, to date, in most reported nitronyl nitroxide−metal compounds, the functionalized radical ligand includes only one functional group such as pyridyl, imidazolyl, etc. Herein we disclose novel nitronyl nitroxide ligands possessing two additional functional groups (triazole/pyridine) able to coordinate to metal ions, namely, 2© XXXX American Chemical Society

Figure 1. Simplified view of the crystal structure of 1 and 2 and schematic view of the 1D loop chain for 1 and 2.

S2, they possess a 1D structure of fused loops. The asymmetric unit consists of two Ln(hfac)3 and one Nit-Ph-3,5-btrz ligand. For 1 and 2, this ligand is coordinated to four Ln ions through two NO groups from the radical unit and two N atoms from two triazoles, in a μ4-η1:η1:η1:η1-coordination fashion. Two Ln(hfac)3 units are linked by two Nit-Ph-3,5-btrz ligands in a top-to-tail Received: April 13, 2018

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DOI: 10.1021/acs.inorgchem.8b01004 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

The modeling performed with PHI18 leads to a very good fit for J = 3.56 ± 0.04 cm−1 and g = 2.025 ± 0.002. The positive J value confirms the ferromagnetic interactions between the GdIII ion and the coordinated aminoxyl unit and is comparable to the exchange interactions reported in the literature.19 Isothermal M versus H behavior obtained at 2, 3, and 5 K for 1 (Figure 2) tends toward 15 μB at 50 kOe, which corresponds well with the expected saturation value for a spin of S = 15/2. Moreover, M versus H recorded at 2 K closely follows that calculated with the Brillouin function for S = 15/2 with g = 2, which supports the absence of any other contribution than the ferromagnetic gadolinium aminoxyl interaction. The χMT versus T curves for 2 and 3 are shown in the inset of Figure 2. At 300 K, the χMT values are 29.20 cm3 K mol−1 for 2 and 29.29 cm3 K mol−1 for 3, in agreement with the 28.72 cm3 K mol−1 expected for two LnIII ions (DyIII: 6H15/2, S = 5/2, L = 5, g = 4 /3, and C = 14.17 cm3 K mol−1) and one S = 1/2 radical in the absence of exchange. Upon lowering of the temperature, χMT gradually decreases to a value of 26.18 cm3 K mol−1 at 20 K for 2 and 26.61 cm3 K mol−1 at 28 K for 3, before increasing rapidly to a maximum of 26.89 cm3 K mol−1 at 4.5 K for 2 and 27.72 cm3 K mol−1 at 3.5 K for 3 and then decreasing to 26.41 cm3 K mol−1 for 2 and 27.61 cm3 K mol−1 for 3 at 2 K. These χMT versus T curves show a “down−up−down” behavior, where the initial decrease of χMT in the high-temperature range can be attributed to the crystal-field effect applied for DyIII, while the increase seen for the lower temperature is characteristic for ferromagnetic Dy-radical interactions.20 The M versus H behavior for 2 and 3 has been recorded at 2.0 K in the 0−70 kOe field range, revealing a very fast increase of the magnetization already for low fields (Figure S8), in agreement with a large-spin species. No hysteresis loop was observed (Figure S9). The possibility of slow magnetic relaxation for dysprosium derivatives 2 and 3 was investigated by alternating-current (ac) susceptibility measurements in a temperature range of 2−10 K. For both compounds, clear frequency-dependent out-of-phase (χ″) signals were found below 10 K in the absence of a directcurrent (dc) field (Figures S10 and S11); however, no peak maxima were observed above 2 K, likely resulting from a fast quantum-tunneling of magnetization (QTM) process. To reduce the possible QTM effect, the ac measurements were performed in the dc field, but no significant difference was observed for 2, whereas χM″ for 3 exhibited frequency-dependent peaks; the results obtained in a 4 kOe dc field are shown in Figure 3. The QTM contribution is significantly reduced, and the temperature- and frequency-dependent peaks of χM′ and χM″ are clearly observed. The related Cole−Cole plots (Figure 4) have the expected semicircular shape. Relaxation time τ and distribution width α (0.16−0.29) could be extracted between 2.0 and 4.0 K using the generalized Debye model. The resulting plot of ln τ versus 1/T exhibits an obvious curvature in the entire temperature range, which suggests that several processes may contribute to the relaxation of magnetization (Figure 4). Analysis over the whole T domain was attempted by considering Raman or multirelaxation processes. Excellent fitting was obtained when the concomitant contributions of the Orbach and QTM processes have been used according to τ−1 = τQTM−1 + τ0−1 exp(−Ueff/kBT), yielding τQTM = 1.2 × 10−4 s, Ueff/kB = 19 K, and τ0 = 8.3 × 10−8 s for 3. For the different field-induced slow relaxation behaviors observed for 2 and 3, two reasons might be invoked. First, the slightly different coordination geometries for the Dy ions are possible. They are mostly a distorted triangular dodecahedron

arrangement, thus developing a 1D coordination polymer of fused loops (Figure 1). Each Ln3+ ion is surrounded by six O atoms from three hfac− ligands [Ln−Ohfac = 2.33(1)−2.39(1) Å for 1 and 2.291(8)−2.391(9) Å for 2], one O atom from the NO group of one Nit-Ph-3,5-btrz ligand [Ln−Orad = 2.39(1) and 2.49(1) Å for 1 and 2.365(7) and 2.428(6) Å for 2], and one N atom from the triazole ring of the other radical ligand [Ln−N = 2.52(2) and 2.57(2) Å for 1 and 2.505(8) and 2.495(9) Å for 2]. The LnIII ions display triangular-dodecahedral (D2d) coordination spheres, as revealed by the SHAPE software17 (Table S6). Each nitronyl nitroxide bridges two Ln ions by its two NO groups to produce the rare 4f−2p−4f spin arrangement with a Ln···Ln distance of 8.294 Å for 1 and 8.294 Å for 2. The Ln---Ln separations through the phenyl and the two triazole rings are 12.407 and 12.305 Å for 1 and 2, respectively, while the separations across a loop are 9.240 Å for 1 and 9.238 Å for 2. The packing arrangements of complexes 1 and 2 are shown in Figures S3 and S4, and the shortest interchain Ln---Ln distances are 10.926 Å for 1 and 10.960 Å for 2. Compound 3 has a similar loop-chain structure (Figure S5) with a intraloop Dy···Dy separation of 9.935 Å and a Dy---Dy distance through the phenyl and the two pyridine rings of 13.890 Å. The Dy−Orad bond distances are 2.349(7) and 2.398(7) Å. The packing arrangement of complex 3 is shown in Figure S7, and the nearest interchain Dy---Dy distance is 10.172 Å. SHAPE analysis indicates that the lower continuous shape measurement (CShM) values were found relative to triangular-dodecahedral (D2d) and square-antiprismatic (D4d) coordination spheres for Dy1 and D2d coordination geometry for Dy2 (Table S6). For these chemical systems, the only exchange interactions expected to take place are between the nitronyl nitroxide and the two Ln centers linked to it. The temperature dependence of χMT for 1 is presented in Figure 2. The χMT product at 300 K is 16.6

Figure 2. Magnetic behavior for 1: (left) experimental (○) and calculated () χMT = f(T) behavior; (right) field dependence of magnetization. The blue line represents the theoretical behavior at 2.0 K for a S = 15/2 spin with g = 2.0. Inset: χMT versus T behavior for 2 and 3.

cm3 K mol−1, in agreement with the Curie contributions of 16.14 cm3 K mol−1 for two GdIII ions (GdIII: S = 7/2, g = 2, and C = 7.88 cm3 K mol−1) and one organic radical (S = 1/2 and C = 0.375 cm3 K mol−1). Upon cooling, the χMT value increases first gradually and then more rapidly below 50 K to reach 29.20 cm3 K mol−1 at 2 K, close to the value expected for the S = 15/2 unit resulting from ferromagnetic gadolinium aminoxyl interactions. The magnetic data of 1 were analyzed considering an exchange interaction, J, between each Gd ion and the radical unit; the Hamiltonian describing this situation is defined as ̂ SRad ̂ + SRad ̂ SGd2 ̂ ) H = − 2J(SGd1 B

DOI: 10.1021/acs.inorgchem.8b01004 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

In conclusion, three unprecedented Ln-radical loop chains have been obtained by using the designed multidentate nitronyl nitroxide radical ligands Nit-Ph-3,5-btrz and Nit-Ph-3,5-bPy. For all complexes, each radical ligand behaves as a μ4-η1:η1:η1:η1coordination mode with its two NO groups and two N atoms to link four Ln(hfac)3 units in which the rare 4f−2p−4f structural motif is observed. Compounds 2 and 3 show different fieldinduced slow relaxation of magnetization behaviors, which is tentatively assigned to the different local ligand fields of Dy ions. Despite the large spin for these Dy−Rad−Dy units, their energy gap for spin flipping remains small, which could be related to the disadvantageous orientations of the easy magnetic axes of the Dy centers. This work demonstrates that multisite nitronyl nitroxide radical ligands are very promising for achieving novel spin topologies of 2p−4f complexes.

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Figure 3. Temperature- (left) and frequency-dependent (the red solid lines represent the fitting results) (right) ac signals of χ′ and χ″ under a 4000 Oe dc field for compound 3.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b01004. Experimental details, crystal data, bond lengths and angles, lanthanide geometry analysis, packing diagram, and additional magnetic data (PDF) Accession Codes

CCDC 1835489−1835491 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

Figure 4. (left) ln τ versus 1/T plot for compound 3. (right) Cole−Cole plots of 3 from 2.0 to 4.0 K.

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(D2d) in 2, while for 3, one of the Dy ions displays a coordination polyhedron between the triangular-dodecahedral (D2d) and square-antiprismatic (D4d) geometries. Second, albeit 2 and 3 exhibit the same [NO7] coordination environments for dysprosium, and their ligand-field strengths are different owing to the different N-atom donors (triazole N atom for 2 and pyridine N atom for 3). Such variances, even small, in the symmetry and strength of the local ligand field acting on DyIII in 2 and 3 may have an effect on the anisotropy characteristics of these ions, especially the relative orientations of the easy magnetic axes of the two Dy centers and, consequently, on their magnetic relaxation behavior. The position of the magnetic axis of each Dy ion in compounds 2 and 3 was assessed using an electrostatic model with the Magellan program.21 For the two compounds, the magnetic axes of Dy1 and Dy2 have very different orientations, with angles of 80.75° (2) and 81.15° (3) between their easy axes (Figure 5). Such a relative orientation of the easy magnetic axes is not in favor of a large anisotropy in the exchange-coupled Dy2Rad unit,22 which may account for the poor SMM performance and magnetic relaxation behavior for both complexes.23

AUTHOR INFORMATION

Corresponding Authors

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

Licun Li: 0000-0001-8380-2946 Jean-Pascal Sutter: 0000-0003-4960-0579 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Key R&D Program of China (Grant 2018YFA0306002), National Natural Science Foundation of China (Grants 21773122 and 21471083), and 111 Project (B12015).

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REFERENCES

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DOI: 10.1021/acs.inorgchem.8b01004 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

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DOI: 10.1021/acs.inorgchem.8b01004 Inorg. Chem. XXXX, XXX, XXX−XXX