A family of rare earth complexes with chelating furan biradicals: syntheses, structures and magnetic properties

Abstract

The combination of anisotropic Ln^III ions and the furan biradical results in six complexes, namely, [Ln(hfac)₃(NITFumbis)]₂ (Ln = La(1), Ce(2), Pr(3), Gd(4), Tb(5), Dy(6); hfac = hexafluoroacetylacetone; NITFumbis = (2,5-bis-(1′-oxyl-3′-oxido-4′,4′,5′,5′-tetramethyl-4,5-hydro-1H-imidazol-2-yl)furan). Compounds 1–3 are isostructural and crystallize in the P1̄ space group, while compounds 4–6 crystallize in the P2₁/c space group. For all the six complexes, the coordination number around the lanthanide ion is eight, and the polyhedron is in a distorted 4,4-bicappped trigonal prism or triangular dodecahedron coordination geometry (D_2d or C_2v symmetry) finished by three bischelate hfac⁻ ligands and one bidentate radical ligand. DC magnetic studies show that the Ln(III) ions interact antiferromagnetically with the directly bonding nitronyl nitroxide radicals in all the six complexes. Especially, for complex 4, an intramolecular antiferromagnetic interaction between the two radicals is observed with the largest J (J_Rad–Rad = −24.89 cm⁻¹) ever reported in Ln(III)-radical systems, and at low temperature, intermolecular ferromagnetic interaction also plays an important role. In addition, complex 6 exhibits field-induced single-molecule magnet (SMM) behavior.

---

Introduction

In the last few decades, molecular magnetism has attracted continuous interest because it provides genuine opportunities for both the synthesis of new compounds with unconventional properties (especially if following a rational design),^1–3 and for exploring the fundamental aspects of magnetic interactions and magneto-structural correlations in molecular systems, which will further aid the design of new molecular magnets.^4,5 One goal in this field is the synthesis of a whole family of systems, which provides a clear indication of the patterns of magnetic interactions and allowed for the rationalization of the magnetic coupling mechanism.⁶ Recently, lanthanide-based systems is becoming increasingly interested, due to their large number of unpaired f-electrons and large intrinsic magnetic anisotropy.^7–9 Consequently, lanthanides ions, especially heavy lanthanide ions, have become good candidates for the construction of SCMs and SMMs.^10–12 However, because the effective shielding by the outer-shell electrons and the rather large and anisotropic magnetic moments of the rare-earth metal ions, which makes the nature of the magnetic interactions between Ln(III) ions and other spin carries difficult to be treated, and has become an area of research interest.¹³ Therefore, synthesizing more lanthanide complexes and systematically studying their magnetic nature is becoming more and more significant.

In recent years, lanthanide compounds involving organic radicals (2p–4f) have attracted much attention, because several of these lanthanide compounds have been found to exhibit single-molecule^11d,14 and single-chain magnets behavior^12b,15 due to the Ising anisotropy of their 4f centers, and the big strength of magnetic interaction promoted organic radicals. Furthermore, the researchers showed that the properties and structures of the lanthanide-radical complexes are affected greatly by modifying the radical ligand.

In order to explore how the electron cloud density of the aromatic ring in the nitronyl nitroxide radical ligand affects the structure and magnetic character of lanthanide-radical complexes, we decided to use a new bis-nitronyl nitroxide radical based on furan ring, named NITFumbis = (1,3-bis-(1′-oxyl-3′-oxido-4′,4′,5′,5′-tetramethyl-4,5-hydro-1H-imidazol-2-yl)furan), its structure is shown in Scheme 1. Compared with pyridyl or phenyl substituted bis-nitronyl nitroxide radical,^11d,16 furyl ring is rich in electron cloud density, which will affect the strength of magnetic interaction promoted by organic radicals. Up till now, no metal complexes with such radical has been reported. Herein we synthesized a series of isomorphic mononuclear tri-spin lanthanide complexes based on radical NITFumbis with the formula of [Ln(hfac)₃(NITFumbis)₂]₂ (Ln = La(1), Ce(2), Pr(3), Gd(4), Tb(5), Dy(6), hfac = hexafluoroacetylacetonate, NITFumbis = 1,3-bis-(1′-oxyl-3′-oxido-4′,4′,5′,5′-tetramethyl-4,5-hydro-1H-imidazol-2-yl)furan). The static magnetic results suggested that there are antiferromagnetic interactions between Ln and the radicals in all of the six complexes. In addition, dynamic magnetic studies showed that complex 6 exhibit frequency-dependent ac susceptibility at low temperature, which suggest SMMs behavior.

Scheme 1

Experimental details

Materials and physical measurements

All of the reagents used in the syntheses were of analytical grade, except that the solvents used were distilled prior to use. Ln(hfac)₃·2H₂O was synthesized according to the method of the literature.¹⁷ 2,5-Furanedicarboxaldehyde used to synthesize the nitronyl nitroxide radicals was prepared by activated manganese dioxide (Across) and 2,5-bis(hydroxymethyl)furane (Aldrich) according to the literature procedures.¹⁸ The ligand NITFumbis was prepared by condensation of 2,3-bis(hydroxylamino)-2,3-dimethylbutane with 2,5-furanedicarboxaldehyde, followed by oxidation with NaIO₄ according to the literature method.¹⁹ Elemental analyses for carbon, hydrogen, and nitrogen were performed on a Perkin-Elmer 240 elemental analyzer. Infrared spectra were recorded from KBr pellets in the 4000–400 cm⁻¹ region on a Bruker TENOR 27 spectrometer. Powder X-ray diffraction measurements were recorded on a D/Max-2500 X-ray diffractometer using Cu-Kα radiation. Direct-current (dc) magnetic susceptibilities of crystalline samples were measured on an MPMS-7 SQUID magnetometer in the temperature range of 2–300 K with 1000 Oe applied magnetic field. Alternating-current (ac) susceptibilities were performed on the same magnetometer under 0 or 3000 Oe DC field with an oscillating of 3 Oe. The data were corrected for the diamagnetism of the samples using Pascal constants.

X-Ray crystallography

All crystallographic data were carried out with an Oxford Diffractometer SuperNova™, which were equipped with graphite monochromatic Mo-Kα radiation (λ = 0.71073 Å). Lorentz polarization and absorption corrections were applied. Structures were solved by direct methods with the SHELXS-97 program and refined by full-matrix least-squares techniques against F² with the SHELXTL-97 program package.²⁰ The disordered fluorine atoms in all the six complexes were refined isotropically. Some restraints are applied, such as ISOR (anisotropic parameter), DFIX (restricting the distance between two atoms). Besides fluorine atoms, all other non-hydrogen atoms were refined anisotropically, and hydrogen atoms were located and refined isotropically. Crystallographic data for all of the six compounds are listed in Table 1. CCDC-1410302 (for 1), CCDC-1410303 (for 2), CCDC-1410315 (for 3), CCDC-1410316 (for 4), CCDC-1410317 (for 5) and CCDC-1410319 (for 6), contain the supplementary crystallographic data for this paper.

Table 1 Crystallographic data and structure refinement details for 1–6

	1	2	3	4	5	6
Formula	C66H58F36La2N8O22	C66H58Ce2F36N8O22	C66H58F36N8O22Pr2	C66H58F36Gd2N8O22	C66H58F36N8O22Tb2	C66H58Dy2F36N8O22
Mr	2277.02	2279.44	2281.02	2313.70	2317.04	2324.20
Crystal system	Triclinic	Triclinic	Triclinic	Monoclinic	Monoclinic	Monoclinic
Space group	P1̄	P1̄	P1̄	P21/c	P21/c	P21/c
a (Å)	13.895 (3)	14.1078 (4)	13.9203 (4)	29.7235 (10)	29.7640 (16)	29.6112 (12)
b (Å)	17.084 (3)	17.1982(6)	17.1400 (5)	12.6890 (3)	12.6516 (6)	12.6818 (5)
c (Å)	21.297 (4)	21.4263 (5)	21.2848 (7)	23.8780 (7)	23.9061 (12)	23.9558 (10)
α (°)	108.753 (3)	108.562 (3)	68.426 (3)	90	90	90
β (°)	92.788 (3)	93.305 (2)	87.010 (3)	108.287 (3)	108.037 (1)	107.2800 (10)
γ (°)	113.849 (3)	113.769 (3)	66.321 (3)	90	90	90
V (Å^3)	4284.3 (13)	4405.4 (2)	4297.8 (3)	8551.1 (4)	8559.7 (7)	8589.9 (6)
Z	2	2	2	4	4	4
ρcalc (mg m^−3)	1.765	1.718	1.763	1.797	1.798	1.797
μ (mm^−1)	1.135	9.233	9.976	11.319	1.791	1.878
F (000)	2248	2252	2256	4552.0	4560	4568
θ range (°)	1.40–25.01	3.46–68.07	3.49–67.08	3.72–67.08	0.72–25.01	1.44–25.01
GOF on F^2	1.031	1.041	1.042	1.023	1.105	1.028
R1/wR2 [I > 2σ(I)]	0.0379, 0.0959	0.0741, 0.1870	0.0411, 0.1025	0.0732, 0.1785	0.0436, 0.0909	0.0448, 0.0953
R1/wR2 (all data)	0.0451, 0.1026	0.1051, 0.2063	0.0458, 0.1074	0.1185, 0.2168	0.0560, 0.0979	0.0599, 0.1036

---

Preparation of complexes of 1–6

All of the six complexes were obtained by dissolving Ln(hfac)₃·2H₂O (0.05 mmol) (Ln = La(1), Ce(2), Pr(3), Gd(4), Tb(5), Dy(6)) in boiling n-heptane (20 mL). After stirring for 2 h, the solution was cooled to 60 °C, to which NITFumbis (19 mg, 0.05 mmol) in CH₂Cl₂ (5 mL) was added. The resulting solution was stirred with refluxing for 30 min and then cooled to room temperature. Slowly evaporation of the final solution for about one week yield dark-blue block crystals suitable for single-crystal X-ray analysis.

[La(hfac)₃(NITFumbis)]₂ (1). Yield 0.026 g, 45%. C₆₆H₅₈F₃₆La₂N₈O₂₂ (2277.02 g mol⁻¹): calcd for C 34.81, H 2.57, N 4.92; found: C 35.22, H 2.84, N 5.08%. IR (KBr pellet): 2362(w), 1651(s), 1613(w), 1400(vs), 1262(m), 1192(s), 1142(s), 801(w), 617(m) cm⁻¹.

[Ce(hfac)₃(NITFumbis)]₂ (2). Yield 0.024 g, 42%. C₆₆H₅₈Ce₂F₃₆N₈O₂₂ (2279.44 g mol⁻¹): calcd C 34.77, H 2.56, N 4.92; found: C 35.08, H 2.66, N 5.18%. IR (KBr pellet): 2357(w), 1649(s), 1398(w), 1256(s), 1197(s), 1143(vs), 776(m), 618(m) cm⁻¹.

[Pr(hfac)₃(NITFumbis)]₂ (3). Yield 0.028 g, 48%. C₆₆H₅₈F₃₆N₈O₂₂Pr₂ (2281.02 g mol⁻¹): calcd for C 34.75, H 2.56, N 4.91; found: C 34.88, H 2.92, N 5.13%. IR (KBr pellet) 2361(w), 1651(s), 1562(w), 1502(m), 1399(vs), 1338(s), 1275(vs), 1205(vs), 1154(s), 983(s), 814(m), 660(w), 622(m), 565(m) cm⁻¹.

[Gd(hfac)₃(NITFumbis)]₂ (4). Yield 0.025 g, 43%. C₆₆H₅₈F₃₆Gd₂N₈O₂₂ (2313.70 g mol⁻¹): calcd C 34.26, H 2.53, N 4.84; found: C 34.18, H 2.65, N 4.72%. IR (KBr pellet): 2362(w), 1653(s), 1524(s), 1399(vs), 1326(w), 1262(s), 1192(vs), 1146(vs), 795(w), 649(w), 579(m) cm⁻¹.

[Tb(hfac)₃(NITFumbis)]₂ (5). Yield 0.024 g, 42%. C₆₆H₅₈F₃₆N₈O₂₂Tb₂ (2317.04 g mol⁻¹): calcd for C 34.21, H 2.52, N 4.84; found: C 34.14, H 2.68, N 4.96%. IR (KBr pellet): 2362(w), 1650(vs), 1524(s), 1401(vs), 1268(vs), 1192(vs), 1135(vs), 987(w), 789(w), 609(m) cm⁻¹.

[Dy(hfac)₃(NITPymbis)]₂ (6). Yield 0.026 g, 44%. C₆₆H₅₈Dy₂F₃₆N₈O₂₂ (2324.20 g mol⁻¹): calcd C 34.10, H 2.51, N 4.82; found: C 34.35, H 2.42, N 4.66%. IR (KBr pellet): 2361(w), 1653(vs), 1512(s), 1403(vs), 1269(vs), 1190(vs), 1157(vs), 992(vs), 795(s), 606(m) cm⁻¹.

Results and discussion

Crystal structure

Structures of 1. Single-crystal X-ray diffraction analyses reveal that the molecular structures of 1–3 belong to triclinic P1̄ space group. The asymmetric units in all the three complexes are composed of two crystallographically independent [Ln(hfac)₃(NITFumbis)] moieties, and every [Ln(hfac)₃(NITFumbis)] exhibits mononuclear tri-spin structure. The molecular structure of complex 1 as an example is shown in Fig. 1, the central La^III ion is eight coordinated by three bischelate hfac⁻ anions and two non-bridged NO groups from two separate organic radicals. The La–O(rad) (nitroxide) distances are in the range of 2.449(2)–2.503(2) Å, and the La–O(hfac) bond lengths are in the range of 2.441(3)–2.556(2) Å. The coordinated N–O bond lengths of the nitronyl nitroxide radicals are in the range of 1.294(2)–1.301(2) Å, which is a little larger than the uncoordinated N–O bonds (1.262(3)–1.268(2) Å). However, both are within the range of N–O bond length of the nitronyl radical. The O_rad–La–O_rad bond angles are 92.23(8)° and 94.53(8)°, respectively (Table 2). Here the two five membered heterocyclic rings and the furan ring show twist angles range from 22.7(2)° to 32.9(3)°. By employing the classic Continuous Symmetry Measures (CSM) method, the coordination sphere of La1 and La2 is estimated as nearly ideal D_2d triangular dodecahedron with the deviation parameter S = 1.862 or C_2v-bicapped trigonal prism with the derivation parameter S = 1.102, respectively (Table 3).²¹

Fig. 1 Simplified view of the crystal structure of 1. Fluorine, hydrogen, and some carbon atoms are omitted for clarity. (b) Polyhedral of lanthanum atoms.

Table 2 Selected bond distances (Å) and angles (°) for complexes 1–6

	1 La	2 Ce	3 Pr
Ln–O(hfac)	2.424(7)–2.480(6)	2.377(13)–2.465(4)	2.384(6)–2.433(7)
O(rad)–Ln–O(rad)	86.7(2), 90.5(2)	86.4(2), 90.6(2)	86.1(2), 90.3(2)
Ln–O(rad)	2.404(6)–2.460(6)	2.394(4)–2.426(4)	2.360(5)–2.386(5)
Ln–O(rad)–N	134.2(5)–139.1(5)	134.1(3)–137.7(3)	133.8(4)–137.7(4)
O(rad)–N	1.264(9)–1.305(9)	1.291(6)–1.297(7)	1.277(7)–1.300(7)
	4 Gd	5 Tb	6 Dy
Ln–O(hfac)	2.354(10)–2.404(7)	2.350(11)–2.382(12)	2.321(9)–2.380(9)
O(rad)–Ln–O(rad)	86.6(3), 90.7(3)	86.1(5), 90.2(5)	85.8(3), 88.8(3)
Ln–O(rad)	2.345(8)–2.374(7)	2.324(14)–2.364(13)	2.321(9)–2.349(9)
Ln–O(rad)–N	133.9(6)–138.7(7)	134.2(10)–137.7(9)	134.1(7)–137.3(7)
O(rad)–N	1.277(12)–1.307(11)	1.279(19)–1.299(16)	1.288 (13)–1.309(14)

---

Table 3 Lanthanide geometry analysis by SHAPE software

Ln(III)	D2d-DD	C2v-TP	D4d-AP	Ln(III)	D2d-DD	C2v-TP	D4d-AP
La1	1.862	2.046	4.289	La2	1.964	1.102	2.104
Ce1	1.994	1.839	4.056	Ce2	1.994	1.058	1.770
Pr1	1.804	2.018	4.130	Pr2	2.014	0.990	2.051
Gd1	2.157	0.825	1.912	Gd2	2.963	1.107	3.249
Tb1	2.017	0.709	1.743	Tb2	2.955	1.104	3.244
Dy1	2.250	0.777	1.802	Dy2	2.802	1.056	3.051

---

The shortest distance between the uncoordinated N–O group is in the 10.924 Å, and the shortest La⋯La distance is 10.121 Å, hence the two sub-units are incompact, mononuclear moiety is considered for the magnetic analysis. The packing diagram for 1 is given in Fig. 2. There is no π–π stacking interaction in the system, and weak hydrogen bond interactions C–H⋯F or C–H⋯O to generate three dimensional networks with H⋯F distances of 2.460 Å or H⋯O distances in the range of 2.448–2.484 Å, which are comparable to the literature^11d,14f,22 of the Ln(hfac)₃ complexes (Table 3).

Fig. 2 Packing diagram of LaNITFumbis. (Pink dot line, C17–H17⋯O9, C50–H50⋯O20, C17–H17⋯F33).

Structure of 4. Compound 4–6 crystallizes in space group P2₁/c with Z = 4. The asymmetric unit also contains two crystallographically independent [Ln(hfac)₃(NITFumbis)] moieties, and every [Ln(hfac)₃(NITFumbis)] exhibits mononuclear tri-spin structure with central metal ion in an LnO₈ coordination sphere. The molecular structure of complex 4 as an example is shown in Fig. 3. The Gd–O(hfac) distances range from 2.354(10) to 2.404(7) Å. In contrast to 1, the Ln–O (radical) bond lengths of compounds 4–6 are in the range of 2.345(8)–2.374(7) Å for 4, 2.324(14)–2.364(13) Å for 5, and 2.321(9)–2.349(9) Å for 6, which exhibits the phenomenon of shrinking the bond distances for heavier lanthanide ions. When applying the C_2v symmetry to the GdO₈ site, CSM method gives the Gd1 and Gd2 the minimal value of S = 0.825 or S = 1.107, respectively. Similar to 1, hydrogen bonds C11–H11B⋯F28, C38–H38⋯F23, C28–H28B⋯F7, C50–H50A⋯O3, C50–H50C⋯O9 (H⋯F distances ranging from 2.451(3) to 2.616(5) Å and H⋯O distances ranging from 2.451(3) to 2.616(5) Å) attach isolated molecules into 3D supermolecular structure (Fig. 4).

Fig. 3 Simplified view of the crystal structure of 4. Fluorine, hydrogen, and some carbon atoms are omitted for clarity. (b) Polyhedral of gadolinium atoms.

Fig. 4 Packing diagram of GdNITFumbis. (Pink dot line, C11–H11B⋯F28, C38–H38⋯F23, C28–H28B⋯F7, C50–H50A⋯O3, C50–H50C⋯O9).

Static magnetic properties

Magnetic measurements were performed on polycrystalline samples of 1–6. The phase purity of the bulk samples was confirmed by XRD analyses as shown in Fig. S7 and 8 (ESI).† Direct current (dc) magnetic susceptibilities for the six complexes were measured under 1 kOe in the 2–300 K range and the magnetic behaviors are shown in Fig. 5–8.

Fig. 5 χ_MT vs. T (○) and χ_M vs. T (□) plots for complex 1.

Fig. 6 χ_MT vs. T (○) and χ_M vs. T (□) plots for complex 2 (left) and 3 (right).

Fig. 7 χ_MT vs. T (□)and χ_M vs. T (○) plots for complex 4, and the solid lines represent the theoretical values based on the corresponding equations (left). Field dependence of the magnetization for complex 4 at 2 K (right).

Fig. 8 χ_MT vs. T (○) and χ_M vs. T (□) plots for 5 (left) and 6 (right).

Static magnetic properties for complex 1(La), 2(Ce), and 3(Pr). At room temperature, the value of χ_MT for complex 1 is 0.76 cm³ K mol⁻¹ (Fig. 5), in good agreement with the expected value (0.75 cm³ K mol⁻¹) for two organic radicals (S = 1/2, 0.375 cm³ K mol⁻¹). Upon cooling, χ_MT value gradually decreases and reaches a minimum of 0.03 cm³ K mol⁻¹ at 2 K. This behavior evidences that weak antiferromagnetic radical–radical interactions operate in the lanthanum complex. The experimental data between 2−300 K are well fitted using a dinuclear model H = −2J_Rad–RadŜ_Rad1Ŝ_Rad2, the magnetic data were analyzed by the eqn (2). The least-squares fit to the data of 1 leads to g = 2.01, J_Rad–Rad = −5.3 cm⁻¹.

Variable-temperature magnetic susceptibilities for complexes of 2 and 3 are studied and shown in Fig. 6. The observed room-temperature χ_MT values are 1.53 and 2.37 cm³ K mol⁻¹ for Ce(III) and Pr(III) mononuclear complexes, respectively. Both the values are very close to the expected values of 1.55 and 2.35 cm³ K mol⁻¹ for an uncoupled system of one Ln(III) ion (Ce(III) or Pr(III)) plus two radicals. Upon cooling, the χ_MT value of 2 continuously decreases and reaches a minimum of 0.14 cm³ K mol⁻¹ at 2 K. While on decreasing the temperature, the χ_MT value of 3 decreases gradually and reaches a minimum of 0.07 cm³ K mol⁻¹ at 2.0 K, which may be ascribed to the progressive depopulation of excited Stark sublevels of Pr^III and the weak interactions between Pr(III) ion and radicals.

To obtain a rough quantitative estimate of the magnetic interaction parameters between paramagnetic species (Ce(III) [or Pr(III) ion] and the radicals) in the mononuclear tri-spin system, we assumed that the total magnetic susceptibility χ_total is given by the sum of the isolated Ln(III) ion and two radicals (eqn (2)). The Ce(III) [or Pr(III)] ion may be assumed to exhibit a splitting of the M_J energy levels (Ĥ = ΔĴ_z²) in an axial crystal field. Thus χ_Ce and χ_Pr can be described by eqn (4) and (5), respectively.^3e,15a,23 In the expression, Δ is the zero-field-splitting parameter, g is the Lande factor. The zJ′ parameter based on the molecular field approximation in eqn (6) is introduced to simulate the magnetic interactions between all the paramagnetic species in the system. Thus, the magnetic data can be analyzed by the following approximate treatment of eqn (2)–(6).

χ_total = χ_Ln + 2χ_rad (2)

The observed χ_MT data were well reproduced (Fig. 6) by using the above approximate eqn (2)–(6), giving the best fitting parameters of g = 0.89, Δ = 5.76 cm⁻¹, zJ′ = −0.52 cm⁻¹ for complex 2 and g = 0.83, Δ = −18.92 cm⁻¹, zJ′ = −1.07 cm⁻¹ for complex 3. The negative zJ′ value is indicative of the antiferromagnetic interaction between the paramagnetic ions (Pr(III) and radicals) in the mononuclear tri-spin systems.

Static magnetic properties of complex 4. The temperature dependence of magnetic susceptibilities for 4 is studied and shown in Fig. 7. The observed room-temperature χ_MT value is 8.72 cm³ K mol⁻¹, in agreement with the expected value 8.63 cm³ K mol⁻¹ for one uncoupled Gd^III ion (⁸S_7/2, g = 2) and two organic radicals (S = 1/2). On decreasing the temperature, the χ_MT value keeps almost constant down to 100 K, then decreases slightly to reach a minimum of 8.04 cm³ K mol⁻¹ at 22 K. Upon further cooling, χ_MT increases to 8.22 cm³ mol⁻¹ K at 2 K. Such a magnetic behavior is consistent with dominant antiferromagnetic interactions. Accordingly, the system was modeled as a mononuclear tri-spin unit, and the magnetic analysis was carried out by using the spin Hamiltonian H = −2J_Rad–Gd (Ŝ_Rad1Ŝ_Gd + Ŝ_Rad2Ŝ_Gd) − 2J_Rad–RadŜ_Rad1Ŝ_Rad2, where J_Rad–Gd and J_Rad–Rad characterized the exchange interactions for radical–Gd(III) and radical–radical, respectively. At low temperature, the increase of χ_MT may be assigned to weak intermolecular ferromagnetic interaction, so the mean-field approximation (zJ′) was introduced. Assuming that radicals and Gd(III) have the same g value,²³ the magnetic data were analyzed by the following approximate treatment equations.²⁴

The observed χ_MT data at 2–300 K were well reproduced, giving the best fitting parameters of g = 2.00, J_Rad–Gd = −1.00 cm⁻¹, J_Rad–Rad = −24.89 cm⁻¹ and zJ′ = 0.004 cm⁻¹. The negative values of J_Rad–Gd and J_Rad–Rad indicate the antiferromagnetic interactions between Gd(III) and the radicals, and also between the two intramolecular radicals.

The fitting results of J_Rad–Gd is in the range of ever reported Gd^III-radicals compounds, the J_Rad–Rad is larger than the reported highest value (J_Rad–Rad = −11.89 cm⁻¹) for Ln-radical systems,^3f although much larger value for J_Rad–Rad is found in transition-metal-radical systems.²⁵ Because a spin delocalization exists from the magnetic orbital π* of NITFumbis to the empty 6s and 5d orbitals on the Ln^III ion, the antiferromagnetic interaction between the intramolecular radicals could be interpreted based on the superexchange of the two radicals through the empty 6s and 5d orbitals of the Ln(III) ion.^7c,26 The shorter bond distances of Gd(III)–O_rad (4) than La(III)–O_rad (1) may result in good overlapping of the π* electronic cloud through the empty 6s and 5d orbitals, which is the reason of the more pronounced antiferromagnetic interaction in compound 4 than in 1. The fact that Gd^III mediates the magnetic interaction more efficiently than Y^III has already been observed in the literature.²⁷ The much larger J_Rad–Rad for compound 4 than other Gd-radical complexes^{3f,16c,d,27,28} may be not only due to the good conjugation among the furyl ring and the two imidazol rings on which the radicals locate, but also due to the high electronic density of the furyl ring.

The field dependences of magnetization for complexes 4 have been determined at 2 K in the range of 0–70 kOe (Fig. 7 (right)). The experimental magnetization is compared to the theoretical magnetization given by the Brillouin function for S = 9/2 of the ferromagnetic and the S = 7/2 for antiferromagnetic state. The experimental magnetization is close to the Brillouin function for S = 7/2, which further confirms the antiferromagnetic interactions between the paramagnetic centers.

Static magnetic properties of complex 5 and 6. The direct current (DC) magnetic susceptibility data of 5 and 6 are shown in Fig. 8. For 5, the value of χ_MT at 300 K is 12.91 cm³ K mol⁻¹, which is a little higher than the expected value 12.50 cm³ K mol⁻¹ for one uncoupled Tb^III ion (⁷F₆ and g = 3/2) and two organic radicals (S = 1/2). Upon cooling, the value of χ_MT decreases smoothly till 60 K, and then shows an abrupt decrease to 10.26 cm³ K mol⁻¹ at 2 K. For 6, the value of χ_MT at room temperature is 14.18 cm³ K mol⁻¹, which is slightly lower than the expected value of 14.92 cm³ K mol⁻¹ for one uncoupled Dy^III ion (⁶H_15/2) and two organic radicals (S = 1/2). As the temperature is lowered, the χ_MT value decreases slightly until 100 K and then decays sharply to a minimum of 11.30 cm³ K mol⁻¹ at 2 K. The decrease of χ_MT at high temperature regime can be ascribed to the depopulation of the Ln^III M_J states and/or antiferromagnetic coupling between spin carriers.

The field dependences of magnetization (M) for complex 5 and 6 have been determined at 2 K in the range of 0–70 kOe (Fig. 9). For 5, the field-dependent magnetization value at 2 K shows a rapid increase in the magnetization at low magnetic fields. At higher fields, M increases up to 5.81 Nβ at 2 K and 70 kOe, which is much lower than the saturation values of 11 Nβ (9 Nβ for each Tb^III ion for J = 6 and g = 3/2, plus 1 Nβ for each organic radical). For 6, M increases up to 6.57 Nβ at 2 K and 70 kOe with the increase of the applied field, which also does not reach the expected saturation values of 12 Nβ (10 Nβ for each Dy^III ion for J = 15/2 and g = 4/3, plus 1 Nβ for each organic radical). Considering the strong spin-orbit coupling in Ln^III ions, the large gaps between experimental data and theoretical saturation values for compounds 5 and 6 can be ascribed to the presence of magnetic anisotropy and/or low-lying excited states in the systems.^{11c,12a,14,29}

Fig. 9 Field dependence of the magnetization at 2 K for complex 5 (left) and 6 (right).

Dynamic magnetic properties for 5 and 6. To examine the spin dynamics of complex 5 and 6, alternating current (AC) measurements were carried out under a zero DC field or 3000 Oe with an oscillation of 3 Oe. The imaginary component χ′′ of the complex 5 does not show any frequency-dependent phenomenon (Fig. S5, see ESI†). As for Dy^III compound, the imaginary component of ac magnetic susceptibility data displays temperature dependence with no peaks observed while the real component exhibits a steady increase down to 2 K under zero dc field (Fig. S6, see ESI†). As is well known, magnetization can reverse via quantum mechanical tunneling process in lanthanide SMMs within the lowest energy doublet, which can be suppressed by applying a static magnetic field. Here a DC field of 3000 Oe is applied to probe the dynamic behavior of AC magnetic susceptibility (Fig. 10). Consequently, well-resolved peaks emerge which suggest that the relaxation process is slowed by the external fields. The maximums of the imaginary component of the ac susceptibility are extracted for relaxation time τ. Plots of ln τ versus T⁻¹ display linear dependence indicating spin reversed by thermal activated Orbach mechanism process (Fig. 11). Arrhenius fit (τ = τ₀ exp(Δ_eff/k_BT)) gives the effect energy gap of 15 K with the pre-exponential τ₀ of 1.25 × 10⁻⁶ s, which fall in the range well for SMMs.^14,30

Fig. 10 Temperature dependence of the in-phase (top) and out-of-phase (bottom) components of the AC magnetic susceptibility for complex 6 (right) in a 3000 Oe DC field with an oscillation of 3 Oe.

Fig. 11 (a) Magnetization relaxation time, ln τ vs. T⁻¹ plot for 6 under 3000 Oe dc field. The solid line is fitted with the Arrhenius law.

Conclusions

In conclusion, a nitronyl nitroxide radical NITFumbis and six new compounds [Ln(hfac)₃(NITFumbis)]₂ have been synthesized. The results show that all of the Ln^III-complexes have analogous structures, and each of them features as monometallic tri-spin complex, and the asymmetric unit is composed of two crystallographically independent [Ln(hfac)₃(NITPymbis)] moieties. The temperature dependencies of magnetic susceptibilities for the Ln^III-complexes are studied, and the results show that there are antiferromagnetic interactions between the paramagnetic ions (Ln(III) and radicals) in all the six complexes. For Gd(III) complex 4, the fitting results of the magnetic susceptibility reveal that there are two different magnetic interactions between the Gd(III) ion and the NITPymbis ligand (antiferromagnetic interaction between the Gd(III) and the radicals, and also antiferromagnetic interaction between the two intramolecular radicals). The Dy^III compound exhibits field-induced SMM behavior with U_eff of 15 K and τ₀ of 1.25 × 10⁻⁶ s under 3000 Oe external field, while the Tb^III compound shows no out-of-phase ac signal. The results reported here demonstrated that the local symmetry of lanthanide ions is significant for magnetic anisotropy and for rationally design of new lanthanide SMMs.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (21371133).

References

1. (a) C. Benelli, A. Caneschi, D. Gatteschi, L. Pardi, P. Rey, D. P. Shum and R. L. Carlin, Inorg. Chem., 1989, 28, 272; (b) O. Kahn, Molecular Magnetism, VCH Publishers, New York, 1993; (c) C. Benelli, A. Caneschi, D. Gatteschi and R. Sessoli, J. Appl. Phys., 1993, 73, 5333; (d) M. L. Kahn, J. P. Sutter, S. Golhen, P. Guionneau, L. Ouahab, O. Kahn and D. Chasseau, J. Am. Chem. Soc., 2000, 122, 3413; (e) O. Evans and W. Lin, Chem. Mater., 2001, 13, 3009; (f) M. T. Lemaire, Pure Appl. Chem., 2004, 76, 277.
2. (a) G. St-Pierre, A. Chagnes, N. A. Bouchard, P. D. Harvey, L. Brossard and H. Menard, Langmuir, 2004, 20, 6365; (b) A. B. Descalzo, K. Rurack, H. Weisshoff, R. Martinez-Manez, M. D. Marcos, P. Amoros, K. Hoffmann and J. Soto, J. Am. Chem. Soc., 2005, 127, 184; (c) D. Luneau and P. Rey, Coord. Chem. Rev., 2005, 249, 2591; (d) M. A. Aldamen, J. M. Clemente-Juan, E. Coronado, C. Marti-Gastaldo and A. Gaita-Arino, J. Am. Chem. Soc., 2008, 130, 8874.
3. (a) T. C. Stamatatos, S. J. Teat, W. Wernsdorfer and G. Christou, Angew. Chem., Int. Ed., 2009, 48, 521; (b) R. Sessoli and A. K. Powell, Coord. Chem. Rev., 2009, 253, 2328; (c) M. Mannini, F. Pineider, P. Sainctavit, C. Danieli, E. Otero, C. Sciancalepore, A. M. Talarico, M. A. Arrio, A. Cornia, D. Gatteschi and R. Sessoli, Nat. Mater., 2009, 8, 194; (d) P. Dawid, C. Szymon and S. Olaf, Nature, 2010, 468, 417; (e) L. Sorace, C. Benelli and D. Gatteschi, Chem. Soc. Rev., 2011, 40, 3092; (f) Y. L. Wang, Y. Y. Gao, Y. Ma, Q. L. Wang, L. C. Li and D. Z. Liao, CrystEngComm, 2012, 14, 4706.
4. (a) A. Caneschi and D. Gatteschi, Prog. Inorg. Chem., 1991, 39, 331; (b) N. Koga and S. Karasawa, Bull. Chem. Soc. Jpn., 2005, 78, 1384; (c) G. Christou, Polyhedron, 2005, 24, 2065.
5. (a) G. Poneti, K. Bernot, L. Bogani, A. Caneschi, R. Sessoli, W. Wernsdorfer and D. Gatteschi, Chem. Commun., 2007, 1807; (b) P. H. Lin, T. J. Burchell, L. Ungur, L. F. Chibotaru, W. Wernsdorfer and M. Murugesu, Angew. Chem., Int. Ed., 2009, 48, 9489; (c) D. Visinescu, O. Fabelo, C. Ruiz-Perez, F. Lloret and M. Julve, CrystEngComm, 2010, 12, 2454.
6. R. D. Willet, D. Gatteschi and O. Kahn, Magneto-Structural Correlations in Exchange Coupled Systems, Reidel Publishing, Dordrecht, The Netherlands, 1983.
7. (a) A. Bencini, C. Benelli, A. Caneschi, R. L. Carlin, A. Dei and D. Gatteschi, J. Am. Chem. Soc., 1985, 107, 8128; (b) A. Bencini, C. Benelli, A. Caneschi, A. Dei and D. Gatteschi, Inorg. Chem., 1986, 25, 572; (c) C. Benelli, A. Caneschi, D. Gatteschi, L. Pardi and P. Rey, Inorg. Chem., 1990, 29, 4223; (d) C. Benelli, A. Caneschi, O. Guillou and L. Pardi, Inorg. Chem., 1990, 29, 1750; (e) C. Benelli, A. Caneschi, D. Gatteschi and R. Sessoli, Inorg. Chem., 1993, 32, 4797; (f) O. Kahn, Adv. Inorg. Chem., 1995, 43, 179; (g) J. L. Sanz, R. Ruiz, A. Gleizes, F. Lloet, J. Fans, M. Julve, J. J. Borras-Alvenar and Y. Journeaux, Inorg. Chem., 1996, 35, 7384.
8. (a) C. Lescop, E. Belorizky, D. Luneau and P. Rey, Inorg. Chem., 2002, 41, 3375; (b) N. Ishikawa, M. Sugita, T. Ishikawa, S. Y. Koshihara and Y. Kaizu, J. Am. Chem. Soc., 2003, 125, 8694; (c) N. Ishikawa, M. Sugita and W. G. Wernsdorfer, J. Am. Chem. Soc., 2005, 127, 3650; (d) Y. N. Guo, G. F. Xu, P. Gamez, L. Zhao, S. Y. Lin, R. P. Deng, J. K. Tang and H. J. Zhang, J. Am. Chem. Soc., 2010, 132, 8538.
9. (a) N. Ishikawa, J. Phys. Chem. A, 2003, 107, 5831; (b) S. Takamatsu, T. Ishikawa, S. Koshihara and N. Ishikawa, Inorg. Chem., 2007, 46, 7250; (c) J. D. Rinehart, M. Evans, W. J. Evans and J. R. Long, J. Am. Chem. Soc., 2011, 133, 14236.
10. (a) P. D. W. Boyd, Q. Li, J. B. Vincent, K. Folting, H. R. Chang, W. E. Streib, J. C. Huffman, G. Christou and D. N. Hendrickson, J. Am. Chem. Soc., 1988, 110, 8537; (b) A. Caneschi, D. Gatteschi, N. Lalioti, C. Sangregorio, R. Sessoli, G. Venturi, A. Vindigni, A. Rettori, M. G. Pini and M. A. Novak, Angew. Chem., 2001, 113, 1810; (c) D. Gatteschi and R. Sessoli, Angew. Chem., Int. Ed., 2003, 42, 268; (d) A. Mishra, W. Wernsdorfer, K. A. Abboud and G. Christou, J. Am. Chem. Soc., 2004, 126, 15648; (e) N. Ishikawa, M. Sugita and W. Wernsdorfer, Angew. Chem., Int. Ed., 2005, 44, 2931; (f) X. Y. Wang, Z. M. Wang and S. Gao, Chem. Commun., 2008, 281; (g) H. L. Sun, Z. M. Wang and S. Gao, Coord. Chem. Rev., 2010, 254, 1081; (h) J. S. Miller and D. Gatteschi, Chem. Soc. Rev., 2011, 40, 3065.
11. (a) N. Ishikawa, M. Sugita and W. Wernsdorfer, Angew. Chem., 2005, 117, 2991; (b) J. Tang, I. Hewitt, N. T. Madhu, G. Chastanet, W. Wernsdorfer, C. E. Anson, C. Benelli, R. Sessoli and A. K. Powell, Angew. Chem., Int. Ed., 2006, 45, 1729; (c) K. Bernot, J. Luzon, L. Bogani, M. Etienne, C. Sangregorio, M. Shanmugam, A. Caneschi, R. Sessoli and D. Gatteschi, J. Am. Chem. Soc., 2009, 131, 5573; (d) K. Bernot, F. Pointillart, P. Rosa, M. Etienne, R. Sessoli and D. Gatteschia, Chem. Commun., 2010, 46, 6458; (e) H. S. Ke, G. F. Xu, Y. N. Guo, P. Gamez, C. M. Beavers, S. J. Teat and J. K. Tang, Chem. Commun., 2010, 46, 6057; (f) S. D. Jiang, B. W. Wang, G. Su, Z. M. Wang and S. Gao, Angew. Chem., Int. Ed., 2010, 49, 7448.
12. (a) P. H. Lin, T. J. Burchell, R. Clérac and M. Murugesu, Angew. Chem., Int. Ed., 2008, 47, 8848; (b) R. N. Liu, L. C. Li, X. L. Wang, P. P. Yang, C. Wang, D. Z. Liao and J. P. Sutter, Chem. Commun., 2010, 46, 2566.
13. M. Estrader, J. Ribas, V. Tangoulis, X. Solans, M. Font-Bardia, M. Maestro and C. Diaz, Inorg. Chem., 2006, 45, 8239.
14. (a) J. X. Xu, Y. Ma, D. Z. Liao, G. F. Xu, J. K. Tang, C. Wang, N. Zhou, S. P. Yan, P. Cheng and L. C. Li, Inorg. Chem., 2009, 48, 8890; (b) N. Zhou, Y. Ma, C. Wang, G. F. Xu, J. K. Tang, J. X. Xu, S. P. Yan, P. Cheng, L. C. Li and D. Z. Liao, Dalton Trans., 2009, 8489; (c) X. L. Wang, L. C. Li and D. Z. Liao, Inorg. Chem., 2010, 49, 4735; (d) T. Han, W. Shi, X. P. Zhang, L. L. Li and P. Cheng, Inorg. Chem., 2012, 51, 13009; (e) X. L. Mei, Y. Ma, L. C. Li and D. Z. Liao, Dalton Trans., 2012, 41, 505; (f) X. L. Mei, R. N. Liu, C. Wang, P. P. Yang, L. C. Li and D. Z. Liao, Dalton Trans., 2012, 41, 2904; (g) P. Hu, X. F. Wang, Y. Ma, Q. L. Wang, L. C. Li and D. Z. Liao, Dalton Trans., 2014, 43, 2234; (h) L. L. Li, S. Liu, Y. Zhang, W. Shi and P. Cheng, Dalton Trans., 2015, 44, 6118.
15. (a) A. Caneschi, D. Gatteschi, N. Lalioti, C. Sangregorio, R. Sessoli, G. Venturi, A. Vindigni, A. Rettori, M. G. Pini and M. A. Novak, Angew. Chem., Int. Ed., 2001, 40, 1760; (b) L. Bogani, C. Sangregorio, R. Sessoli and D. Gatteschi, Angew. Chem., Int. Ed., 2005, 44, 5817; (c) K. Bernot, L. Bogani, A. Caneschi, D. Gatteschi and R. Sessoli, J. Am. Chem. Soc., 2006, 128, 7947; (d) T. Han, W. Shi, Z. Niu, B. Na and P. Cheng, Chem.–Eur. J., 2013, 19, 994; (e) X. L. Wang, X. Bao, P. P. Xu and L. Li, Eur. J. Inorg. Chem., 2011, 3586; (f) L. L. Li, S. Liu, H. Li, W. Shi and P. Cheng, Chem. Commun., 2015, 51, 10933.
16. (a) G. Francese, F. M. Romero, A. Neels, H. Stoeckli-Evans and S. Decurtins, Inorg. Chem., 2000, 39, 2087; (b) H. Oshio, M. Yamamoto, T. Ito, H. Kawauchi, N. Koga, T. Ikoma and S. Tero-Kubota, Inorg. Chem., 2001, 40, 5518; (c) L. Tian, Y. Q. Sun, B. Na and P. Cheng, Eur. J. Inorg. Chem., 2013, 4329; (d) S. Y. Zhou, X. Li, T. Li, L. Tian, Z. Y. Liu and X. G. Wang, RSC Adv., 2015, 5, 17131.
17. (a) Y. Z. Zheng, Y. Lan, C. E. Anson and A. K. Powell, Inorg. Chem., 2008, 47, 10813; (b) S. Kanegawa, M. Maeyama, M. Nakano and N. Koga, J. Am. Chem. Soc., 2008, 130, 3079; (c) X. L. Wang, L. C. Li and D. Z. Liao, Inorg. Chem., 2010, 49, 4735.
18. (a) N. M. Shavaleev, F. Gumy, R. Scopelliti and J. C. G. Bünzli, Inorg. Chem., 2009, 48, 5611; (b) B. Drahoš, J. Kotek, P. Hermann, I. Lukeš and É. Tóth, Inorg. Chem., 2010, 49, 3224.
19. (a) E. F. Ullmann, L. Call and J. H. Osiecki, J. Org. Chem., 1970, 35, 3623; (b) M. S. Davis, K. Morokum and R. N. Kreilick, J. Am. Chem. Soc., 1972, 94, 5588.
20. (a) G. M. Sheldrick, SHELXS-97, Program for solution of crystal structures, University of Göttingen, Germany, 1997; (b) G. M. Sheldrick, SHELXL-97, Program for refinement of crystal structures, University of Göttingen, Germany, 1997; (c) G. M. Sheldrick, SADABS, Program for area detector adsorption correction, Institute for inorganic chemistry, University of Göttingen, Germany, 1996.
21. (a) H. Zabrodsky, S. Peleg and D. Avnir, J. Am. Chem. Soc., 1992, 114, 7843; (b) M. Pinsky and D. Avnir, Inorg. Chem., 1998, 37, 5575.
22. Y. L. Wang, B. Gu, Y. Ma, C. Xing, Q. L. Wang, L. C. Li, P. Cheng and D. Z. Liao, CrystEngComm, 2014, 16, 2283.
23. M. F. Richardson, W. F. Wagner and D. E. Sands, J. Inorg. Nucl. Chem., 1968, 30, 1275.
24. (a) C. Benelli, A. Caneschi, D. Gatteschi, J. Laugier and P. Rey, Angew. Chem., Int. Ed., 1987, 26, 913; (b) S. Akine, T. Matsumoto, T. Taniguchi and T. Nabeshima, Inorg. Chem., 2005, 44, 3270; (c) Q. H. Zhao, Y. P. Ma, L. Du and R. B. Fang, Transition Met. Chem., 2006, 31, 593.
25. (a) G. Francese, F. M. Romero, A. Neels, H. Stoeckli-Evans and S. Decurtins, Inorg. Chem., 2000, 39, 2087; (b) I. A. Gass, C. J. Gartshore, D. W. Lupton, B. Moubaraki, A. Nafady, A. M. Bond, J. F. Boas, J. D. Cashion, C. Milsmann, K. Wieghardt and K. S. Murray, Inorg. Chem., 2011, 50, 3052; (c) I. A. Gass, S. Tewary, A. Nafady, N. F. Chilton, C. J. Gartshore, M. Asadi, D. W. Lupton, B. Moubaraki, A. M. Bond, J. F. Boas, S. X. Guo, G. Rajaraman and K. S. Murray, Inorg. Chem., 2013, 52, 7557.
26. (a) Y. Pei, A. Lang, P. Bergerat, O. Kahn, M. Fettouhi and L. Ouahab, Inorg. Chem., 1996, 35, 193; (b) K. Fegy, A. Lang, P. Bergerat, O. Kahn, M. Fettouhi and L. Ouahab, Inorg. Chem., 1998, 37, 4518.
27. J. P. Sutter, M. L. Kahn, S. Golhen, L. Ouahab and O. Kahn, Chem.–Eur. J., 1998, 4, 571.
28. (a) C. Lescop, D. Luneau, P. Rey, G. Bussiere and C. Reber, Inorg. Chem., 2002, 41, 5566; (b) C. Lescop, D. Luneau, E. Beloeizky, P. Fries, M. Guillot and P. Rey, Inorg. Chem., 1999, 38, 5472.
29. (a) K. Bernot, L. Bogani, R. Sessoli and D. Gatteschi, Inorg. Chim. Acta, 2007, 360, 3807; (b) Y. Ma, G. F. Xu, X. Yang, L. C. Li, J. K. Tang, S. P. Yan, P. Cheng and D. Z. Liao, Chem. Commun., 2010, 46, 8264; (c) S. Osa, T. Kido, N. Matsumoto, N. Re, A. Pochaba and J. Mrozinski, J. Am. Chem. Soc., 2004, 126, 420; (d) Y. M. Bing, N. Xu, W. Shi, K. Liu and P. Cheng, Chem.–Asian J., 2013, 8, 1412.
30. (a) C. Benelli, A. Caneschi, D. Gatteschi and L. Pardi, Inorg. Chem., 1992, 31, 741; (b) M. L. Kahn, J. P. Sutter, S. Golhen, P. Guionneau, L. Ouahab, O. Kahn and D. Chasseau, J. Am. Chem. Soc., 2000, 122, 3413; (c) J. X. Xu, Y. Ma, G. F. Xu, C. Wang, D. Z. Liao, Z. H. Jiang, S. P. Yan and L. C. Li, Inorg. Chem. Commun., 2008, 11, 1356; (d) H. X. Tian, R. N. Liu, X. L. Wang, P. P. Yang, Z. X. Li, L. C. Li and D. Z. Liao, Eur. J. Inorg. Chem., 2009, 4498; (e) Y. L. Miao, J. L. Liu, J. Y. Li, J. D. Leng, Y. C. Ou and M. L. Tong, Dalton Trans., 2011, 40, 10229; (f) R. N. Liu, C. M. Zhang, L. C. Li, D. Z. Liao and J. P. Sutter, Dalton Trans., 2012, 41, 12139.

---

Footnote

† Electronic supplementary information (ESI) available. CCDC 1410302, 1410303, 1410315–1410317, 1410319. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5ra13386f

---