A series of heterospin complexes based on lanthanides and pyridine biradicals: synthesis, structure and magnetic properties

Abstract

The combination of anisotropic Ln^III ions and organic biradicals results in six isomorphous complexes, namely, [Ln(hfac)₃(NITPymbis)₂]₂ (Ln = Pr(1), Nd(2), Sm(3), Gd(4), Tb(5), Dy(6); hfac = hexafluoroacetylacetone; NITPymbis = (1,3-bis-(1′-oxyl-3′-oxido-4′,4′,5′,5′-tetramethyl-4,5-hydro-1H-imidazol-2-yl) pyridine). Single-crystal X-ray diffraction studies reveal that each of them is composed of two crystallographically independent mononuclear complexes, in which one NITPymbis molecule acts as a bidentate ligand coordinated to the same Ln^III ion through its two NO groups to form a tri-spin system. In all six complexes, the coordination number around the lanthanide ion is eight, and the polyhedron is a 4,4-bicappped trigonal prism. DC magnetic studies show that the Ln(III) ions interact antiferromagnetically with the directly bonding nitronyl nitroxide radicals in complexes 1–3, whereas for complexes 4–6, the Ln(III) ions interact ferromagnetically with the radicals. Especially, for complex 4, intramolecular ferromagnetic interaction between the two radicals is observed with j_Rad–Rad = 10.05 cm⁻¹, and at low temperature, intermolecular ferromagnetic interaction also plays an important role. Both complexes 5 and 6 exhibit frequency-dependent ac magnetic susceptibilities at low temperature, suggesting the possible presence of slow magnetic relaxation.

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Introduction

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 purpose in this field is the synthesis of whole families of systems, which provides a clear indication of the patterns of magnetic interaction and allowed for the rationalization of the magnetic coupling mechanism.^1a,6 Recently, lanthanide-based systems is becoming increasingly interested, due to their large number of unpaired f-electrons and large intrinsic magnetic anisotropy.^7,8 Consequently, lanthanide ions, especially heavy lanthanide ions, have become good candidates for the construction of SCMs and SMMs.^9–11 However, because the effective shielding by the outer-shell electrons and the unquenched orbital angular momentum of the rare-earth metal ions, which makes the nature of the magnetic interactions between Ln(III) ions and other spin carries remain unclear, and has become an area of research interest.¹² Therefore, synthesizing more lanthanide complexes and systematically studying their magnetic nature is becoming more and more significant.

For designing new lanthanide complexes, the choice of ligand is very important. Nitronyl nitroxide radicals which can not only act as spin carriers but also as bridging ligands attract much attention since the discovery of the first SCM by D. Gatteschi's group.¹³ The use of organic radicals has proved to be an attractive route to obtain magnetically coupled mixed 4f-organic radical heterospin systems,^2e,14 and a few lanthanide compounds involve organic radicals have been reported to exhibit single-molecule^10c,15 and single-chain magnets behavior.^11b,13,16

Compared with the single radical ligands, bis-nitronyl nitroxide radicals have two unpaired elections, in which one unpaired electron is delocalized on each imidazole ring. Aiming to develop new 2p–4f based materials and study the magnetic interaction of lanthanides and radicals, we designed a bis-nitronyl nitroxide radicals based on pyridine ring, named NITPymbis = (1,3-bis-(1′-oxyl-3′-oxido-4′,4′,5′,5′-tetramethyl-4,5-hydro-1H-imidazol-2-yl) pyridine), its structure is shown in Scheme 1. Up till now, only a few examples¹⁷ about transition metal complexes (Ni, Co, Zn, Mn, Cu) with such radical (NITPymbis) have been reported, which are all mononuclear systems containing one metal ion and four radicals. Herein we synthesized a series of isomorphic mononuclear tri-spin lanthanide complexes based on radical NITPymbis with the formula of [Ln(hfac)₃(NITPymbis)₂]₂ (Ln = Pr(1), Nd(2), Sm(3), Gd(4), Tb(5), Dy(6), hfac = hexafluoroacetyl acetonate, NITPymbis = 1,3-bis-(1′-oxyl-3′-oxido-4′,4′,5′,5′-tetramethyl-4,5-hydro-1H-imidazol-2-yl) pyridine).

Scheme 1

The static magnetic results suggested that there are antiferromagnetic interactions between Ln(III) (Pr(III), Nd(III) or Sm(III)) ions and the radicals in complexes 1–3, whereas the Gd(III), Tb(III) or Dy(III) ions interact ferromagnetically with the radicals in complexes 4–6. In addition, dynamic magnetic studies showed that 5 and 6 exhibit frequency-dependent ac susceptibility at low temperature, which suggest SMMs behavior.

Experimental details

Materials and physical measurements

All of the reagents used in the syntheses were of analytical grade, except that the solvents used were dried (heptane over sodium, CH₂Cl₂ over CaH₂ and CHCl₃ over P₂O₅) and distilled prior to use. Ln(hfac)₃·2H₂O was synthesized according to the method in the literature.¹⁸ 2,6-Pyridinedicarbaldehyde used to synthesis the nitronyl nitroxide radicals was prepared according to the literature procedures.¹⁹ The ligand NITPymbis was prepared by condensation of 2,3-bis(hydroxylamino)-2,3-dimethylbutane with 2,6-pyridinedicarboxaldehyde, followed by oxidation according to the literature method.²⁰ 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 zero static 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 TM, which were equipped with graphite monochromatic Mo-Ka 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.²¹ There are some disordered fluorine atoms in all the six complexes, which 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 isotropically, and hydrogen atoms were located and refined anisotropically. Crystallographic data for the two compounds are listed in Table 1. ESI.†

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

	1	2	3
Formula	C75H77F36N10O20Pr2	C75H76F36N10Nd2O20	C75H76F36N10O20Sm2
Mr	2353.18	2409.94	2422.16
Crystal system	Triclinic	Triclinic	Triclinic
Space group	P1̄	P1̄	P1̄
a (Å)	12.3286(2)	12.3461(17)	12.2813(15)
b (Å)	12.8944(2)	12.9126(18)	12.8859(16)
c (Å)	31.501(1)	31.521(4)	31.337(4)
α (°)	89.093(2)	89.120(3)	89.214(2)
β (°)	80.0933(15)	80.018(3)	80.045(2)
γ (°)	75.503(2)	75.287(3)	75.205(2)
V (Å^3)	4774.1(16)	4784.6(12)	4720.2(10)
Z	2	2	2
ρcalc (g cm^−3)	1.673	1.673	1.704
μ (mm^−1)	1.148	1.213	1.373
F (000)	2398	2400	2408
θ range (°)	3.26–25.01	1.73–25.01	1.32–25.01
GOF on F^2	1.025	1.073	1.093
R1/wR2 [I > 2σ(I)]	0.0486, 0.1082	0.0627, 0.1343	0.0636, 0.1334
R1/wR2 (all data)	0.0704, 0.1210	0.0780, 0.1428	0.0803, 0.1413

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	4	5	6
Formula	C72H69ClF36Gd2N10O20	C75H76F36N10O20Tb2	C75H76Dy2F36N10O20
Mr	2428.32	2439.30	2446.45
Crystal system	Triclinic	Triclinic	Triclinic
Space group	P1̄	P1̄	P1̄
a (Å)	12.2289(18)	12.228(2)	12.2513(3)
b (Å)	13.0026(19)	12.857(3)	12.9027(3)
c (Å)	31.399(5)	31.381(6)	31.3549(8)
α (°)	91.003(4)	89.34(3)	89.4667(19)
β (°)	101.201(3)	80.44(3)	80.287(2)
γ (°)	104.313(3)	75.18(3)	75.152(2)
V (Å^3)	4734.3(13)	4713.2(17)	4719.3(2)
Z	2	2	2
ρcalc (g cm^−3)	1.703	1.719	1.722
μ (mm^−1)	1.557	1.630	1.713
F (000)	2400	2420	2424
θ range (°)	1.62–25.01	1.32–25.01	3.27–25.01
GOF on F^2	1.238	1.047	1.143
R1/wR2 [I > 2σ(I)]	0.0924, 0.1943	0.0768, 0.1777	0.0593, 0.1293
R1/wR2 (all data)	0.1039, 0.1991	0.1187, 0.1983	0.0752, 0.1397

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

	1 Pr	2 Nd	3 Sm
Ln–O(hfac)	2.427(7)–2.455(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)

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	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)

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Preparation of complexes of 1–6

All of the six complexes were obtained by dissolving Ln(hfac)₃·2H₂O (0.1 mmol) (Ln = Pr(1), Nd(2), Sm(3), Gd(4), Tb(5), Dy(6)) in boiling n-heptane (20 mL). After stirring for 1 h, the solution was cooled to 60 °C, to which NITPymbis (0.1 mmol) in CH₂Cl₂ (5 mL) was added. The resulting solution was stirred with refluxing for 30 min and then the solution was cooled to room temperature. After filtrating, the resulting solution was stored in a refrigerator at 0–4 °C for a few days to give blue–violet crystals, which were suitable for X-ray analysis.

[Pr(hfac)₃(NITPymbis)₂]₂·C₇H₁₆ (1). Yield 0.042 g, 35%. C₇₅H₇₇F₃₆N₁₀O₂₀Pr₂ (2404.28): calcd for C 37.46, H 3.23, N 5.83; found: C 37.10, H 3.04, N 5.56%. IR (KBr pellet): 2366(w), 1654(s), 1534(m), 1353(w), 1259(m), 1210(s), 1144(vs), 1083(vs), 984(s), 864(m), 659(m), 528(m) cm⁻¹.

[Nd(hfac)₃(NITPymbis)₂]₂·C₇H₁₆ (2). Yield 0.041 g, 34%. C₇₅H₇₆F₃₆N₁₀O₂₀Nd₂ (2409.94): calcd C 37.34, H 3.15, N 5.81; found: C 37.08, H 3.02, N 5.65%. IR (KBr pellet): 2368(w), 1648(s), 1352(w), 1257(s), 1198(s), 1087(vs), 986(s), 867(m), 620(m), 531(m) cm⁻¹.

[Sm(hfac)₃(NITPymbis)₂]₂·C₇H₁₆ (3). Yield 0.040 g, 34%. C₇₅H₇₆F₃₆N₁₀O₂₀Sm₂ (2422.16): calcd for C 37.16, H 3.14, N 5.78; found: C 36.95, H 2.88, N 5.62%. IR (KBr pellet): 2365(w), 1654(s), 1531(s), 1355(m), 1256(s), 1193(s), 1144(vs), 1082(vs), 983(s), 863(m), 798(w), 651(w), 530(m) cm⁻¹.

[Gd(hfac)₃(NITPymbis)₂]₂·0.5C₇H₁₆·0.5CH₂Cl₂ (4). Yield 0.048 g, 40%. C₇₂H₆₉ClF₃₆Gd₂N₁₀O₂₀ (2428.32): calcd C 35.60, H 2.84, N 5.77; found: C 36.01, H 2.98, N 5.56%. IR (KBr pellet): 2372(w), 1651 (s), 1531(s), 1354(m), 1257(s), 1202(s), 1152(vs), 1083(vs), 985(s), 869(m), 807(m), 659(m), 545(m) cm⁻¹.

[Tb(hfac)₃(NITPymbis)₂]₂·C₇H₁₆ (5). Yield 0.051 g, 41%. C₇₅H₇₆Tb₂F₃₆N₁₀O₂₀ (2439.30): calcd for C 36.93, H 3.14, N 5.74; found: C 37.02, H 2.97, N 5.52%. IR (KBr pellet): 2370(w), 1656 (vs), 1540(s), 1350 (m), 1258 (vs), 1208 (vs), 1146 (vs), 1082(vs), 986(s), 862(m), 803(m), 661(m), 540(m) cm⁻¹.

[Dy(hfac)₃(NITPymbis)₂]₂·C₇H₁₆ (6). Yield 0.049 g, 40%. C₇₅H₇₆Dy₂F₃₆N₁₀O₂₀ (2446.46): calcd. C 36.81, H 3.31, N 5.73; found: C 37.12, H 3.17, N 5.78%. IR (KBr pellet): 2366(w), 1651(vs), 1555(s), 1349(m), 1257(vs), 1200 (vs), 1145 (vs), 1080(vs), 984(s), 865(m), 798 (m), 660 (m), 544(m) cm⁻¹.

Results and discussion

Crystal structure

Single-crystal X-ray diffraction analyses reveal that the molecular structures of 1–6 are isomorphous and belong to triclinic P1̄ space group. Therefore, only the structure of complex 1 will be briefly described. In 1, the asymmetric unit is composed of two crystallographically independent [Pr(hfac)₃(NITPybis)] moieties, and a heptane molecule (Fig. 1a). To maintain clarity and simplicity, the solvent molecule is not considered. As shown in Fig. 1a, each Pr^III ion lies in the centre of a dodecahedral coordination environment, completed by eight oxygen atoms from three bidentate β-diketonate coligands and one bidentate NITPymbis biradical ligand. The Pr–O(rad) (nitroxide) distances are 2.404(6) and 2.460(6) Å, respectively. The Pr–O(hfac) bond lengths are in the range of 2.427(7)–2.455(6). These bond distances are comparable to those of the reported Ln(hfac)₃ with nitronyl nitroxides.^13–16 The coordinated N–O bond lengths of the nitronyl nitroxide radicals are in the range of 1.264(9)–1.305(9) Å, indicating the existence of the nitronyl nitroxide radical. The nitrogen atom in NITPymbis is uncoordinated to the Pr(III) ion, because the distance between them is 3.266 Å. The O_rad–Pr–O_rad bond angles range from 86.7(2) to 90.5(2)°. Here the two five membered heterocyclic rings and the pyridine ring show twist angles range from 22.7(2)° to 32.9(3) °.

Fig. 1 Simplified view of the crystal structure of 1. Fluorine, hydrogen, and some carbon atoms are omitted for clarity. (b) C_2v-symmetry polyhedral of praseodymium atoms.

It is necessary to analyze the geometry accurately because the local anisotropy of the magnetic ion is strongly affected by the coordination geometry of the metallic center. Eight-coordinate geometry is mostly taken as the D_2d-dodecahedron (DD), C_2v-bicapped trigonal prism (TP) and D_4d-square antiprism (SAP). The semiquantitative method of polytopal analysis is applied.²² The relevant dihedral angles calculated for the six complexes along with the values for the ideal polyhedral are summarized in Table 3 (for complex 1) and Table S1–S5 (in ESI,† for complexes 2–6). In complex 1, the δ₁ and δ₂ values, which represent the planarity of the squares, range from 0.683 to 26.38°. The δ₃ and δ₄ values for the triangular surfaces, together with the φ values, are close to the angles (48.2, 48.2 and 14.1°) of an ideal TP polyhedron (Fig. 1b). Similar to complex 1, the geometries of the center lanthanide ions of the other five complexes are also close to the ideal TP polyhedron (Fig. S6–S10†).

Table 3 δ (°) and φ (°) values for complex 1

	Pr1		Pr2		DD	TP	SAP
a A[BC]D is the dihedral angle between the ABC plane and the BCD plane.b A–B–C–D is the dihedral angle between the (AB)CD plane and the AB(CD) plane, where (AB) is the center of A and B.
δ1	O9 [O10 O1] O3^a	0.816	O11 [O13 O19] O20^a	0.683	29.5	0.0	0.0
δ2	O6 [O5 O8] O7^a	18.477	O16 [O17 O15] O18^a	26.38	29.5	21.8	0.0
δ3	O6 [O10 O8] O3^a	38.984	O16 [O13 O15] O20^a	51.827	29.5	48.2	52.4
δ4	O9 [O5 O1] O7^a	49.719	O11 [O17 O19] O18^a	39.659	29.5	48.2	52.4
φ1	O8–O1–O6–O9^b	21.856	O15–O19–O16–O11^b	8.201	0.0	14.1	24.5
φ2	O5–O10–O7–O3^b	12.207	O17–O13–O18–O20^b	20.928	0.0	14.1	24.5

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The shortest distance between the uncoordinated N–O group is 7.557 Å, and the shortest Pr⋯Pr distance is 12.211 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 bonds interactions C–H⋯F to generate three dimensional networks with H⋯F distances in the range of 2.38–2.65 Å, which is comparable to the literature^10c,23 of the Ln(hfac)₃ complexes.

Fig. 2 Packing diagram of PrNITPymbis. (Pink dot line, C11–H11⋯F21, C56–H56⋯F14, C49–H49B⋯F10A, C11A–H11A⋯F21B, C49A–H14BA⋯F17, C46A–H46BA⋯F16, C47A–H47BA⋯F16, C51A–H51BA⋯F16).

Static magnetic properties

The effects of spin–orbit coupling and the crystal field are the two key factors that influence the magnetism of lanthanide complexes. For a Ln(III) ion, the 4fⁿ configuration is split into ^2S+1L_J spectroscopic levels by interelectronic repulsion and spin–orbit coupling.^1d,15a,24 Due to the crystal field perturbation, each of these states is further split into Stark sublevels. For most of the Ln(III) ions, the energy separation between the ^2S+1L_J ground state and the first excited state is so large that only the ground state is thermally populated at room and low temperatures. The 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. 3–7.

Fig. 3 χ_MT vs. T (□) and χ_M vs. T (○) plots for 1. The solid lines represent the theoretical values based on the corresponding equations.

Fig. 4 χ_MT vs. T (□) and χ_M vs. T (○) plots for complex 2 (left) and 3 (right).

Fig. 5 χ_MT vs. T (□)and χ_M vs. T (○) plots for complex 4 (left), the solid lines represent the theoretical values based on the corresponding equations. Field dependence of the magnetization at 2 K for complex 4 (right).

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

Fig. 7 Field dependence of the magnetization at different temperatures for complex 5 (left) and 6 (right).

Static magnetic properties for complex 1. At room temperature, the value of χ_MT is 2.28 cm³ K mol⁻¹ (Fig. 3), in good agreement with the expected value (2.35 cm³ K mol⁻¹) for an uncoupled system of one Pr(III) ion (³H₄, g = 4/5) and two organic radicals (S = 1/2, 0.375 cm³ K mol⁻¹). Upon cooling, χ_MT value gradually decreases and reaches a minimum of 0.53 cm³ K mol⁻¹ at 2 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 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 Pr(III) ion and two radicals (eqn (3)). The Pr(III) ion may be assumed to exhibit a splitting of the m_j energy levels (Ĥ = ΔĴ_z²) in an axial crystal field and χ_Pr can be described by eqn (1).^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 (4) is introduced to simulate the magnetic interactions between all the paramagnetic species in the system.

χ_total = χ_Pr + 2χ_rad (3)

The observed χ_MT data were well reproduced (Fig. 3) by using the above approximate eqn (1)–(4), giving the best fitting parameters of g = 0.82, Δ = 3.77 cm⁻¹, zJ′ = −0.13 cm⁻¹ for complex 1. 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 2 and 3. Variable-temperature magnetic susceptibilities for complexes of 2 and 3 are studied and shown in Fig. 4. The observed room-temperature χ_MT values are 2.36 and 1.10 cm³ K mol⁻¹ for Nd(III) and Sm(III) mononuclear complexes, respectively. Both the values are very close to the expected values of 2.39 and 1.22 cm³ K mol⁻¹ for an uncoupled system of one Ln(III) ion (Nd(III) or Sm(III)) plus two radicals. Upon cooling, the χ_MT value of 2 continuously decreases and reaches a minimum of 0.42 cm³ K mol⁻¹ at 2 K. While on decreasing the temperature, the χ_MT value of 3 decreases gradually and begins to decrease more quickly at 20 K till to reach a value of 0.44 cm³ K mol⁻¹ at 2.0 K.

A strictly theoretical treatment of magnetic properties for the present system is an arduous task because of the large anisotropy due to the contribution of the spin–orbit coupling for the Nd(III) and Sm(III) ions.

Static magnetic properties of complex 4. The temperature dependence of magnetic susceptibilities for 4 in the 2–300 K range is studied and shown in Fig. 5 (left). The observed room-temperature χ_MT value is 8.63 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 steadily increases and begins to increase more sharply at 35 K till to reach a peak value of 13.05 cm³ K mol⁻¹ at 2 K. The overall magnetic behaviour indicates ferromagnetic interactions between the Gd^III ion and nitroxide radicals. 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_Rad1·Ŝ_Gd+Ŝ_Rad2·Ŝ_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. Assuming that radicals and Gd(III) have the same g value,²⁵ the magnetic data were analyzed by the following approximate treatment eqn (5).^14b,26 To reproduce the rapid increase of the χ_MT product at low temperature, it is necessary to introduce intermolecular interaction based on the mean-field approximation (zj′, eqn (6)), the fitting curve in 0–50 K range is shown in Fig. S11.†

When the g_Gd and g_rad were fixed as 2.00,²⁷ the observed χ_MT data were well reproduced, giving the best fitting parameters of J_Rad–Gd = 2.04 cm⁻¹, j_Rad–Rad = 10.05 cm⁻¹, and zj′ = 0.038 cm⁻¹. The positive values of J_Rad–Gd and j_Rad–Rad indicate the ferromagnetic interactions between Gd(III) and the radicals, and also between the two intramolecular radicals. The ferromagnetic interaction between Gd(III) and the radical is very common in the similar Gd(III)–radical complexes,^{1c,3e,14b,26b,28} while the positive value for j_Rad–Rad has not been reported. In addition, the positive zj′ value indicates a very weak intermolecular ferromagnetic interaction at low temperature, this weak ferromagnetic interaction has also been reported in the GdCu₂'s systems with zj′ = 0.00044 cm⁻¹.²⁹

The ferromagnetic interaction between the Gd(III) ion and the radicals is considered to involve the empty 5d and 6s orbitals of the Gd(III) ion,²⁷ a fraction of the unpaired electrons of radical ligand transfer into these empty orbitals. According to the Hund's rule, the 4f electrons are expected to be aligned parallel to the 5d and 6s electrons, thus leading to ferromagnetism. While the superexchange of the two radicals through the empty 6s and 5d orbitals of the Gd(III) ion may be the reason for the ferromagnetism interaction between the terminal radicals.^7a,30 The ferromagnetic interaction leads to an S = 9/2 spin ground state which is also confirmed by magnetization versus field measurements at 2 K (Fig. 5, right). This phenomenon further confirms the ferromagnetic interaction between the Gd(III) ion and the coordinated radicals.

Static magnetic properties of complex 5 and 6. The direct current (DC) magnetic susceptibility data of 5 and 6 are shown in Fig. 6. For 5, the value of χ_MT at 300 K is 13.01 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 increases slowly till 50 K, and then shows an abrupt increase to a maximum value of 14.31 cm³ K mol⁻¹ at 10 K. For 6, the value of χ_MT at room temperature is 15.52 cm³ K mol⁻¹, which is also slightly higher 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, χ_MT almost remains constant to approximately 40 K. Below 40 K, χ_MT increase dramatically to reach a peak value of 18.15 cm³ K mol⁻¹ at 3 K, and then drops to 17.79 cm³ K mol⁻¹ at 2 K. The observed ferromagnetic interaction in the low-temperature range for complex 5 and 6 is agreement with those reported similar Ln^III radical complexes in the literature^5a,15,16a,31 and has been interpreted as the result of a spin polarization mechanism of the radicals' unpaired electrons on the Ln^III empty orbitals.

The field dependences of magnetization (M) for complex 5 and 6 have been determined at 2–5 K in the range of 0–70 kOe (Fig. 7). For 5, the field-dependent magnetization values below 5 K shows a rapid increase in the magnetization at low magnetic fields. At higher fields, M increases up to 7.49 Nβ at 2 K and 70 kOe, which does not reach the expected 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). The high-field variation and the nonsuperposition on the magnetization values of 2–5 K imply the presence of low-lying excited states and a certain extent magnetic anisotropy, which corresponds to the reported results.^15,32 For 6, M increases up to 8.30 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), most likely owing to the crystal-field effect on the Dy^III ions. The nonsuperposition on the M versus H/T curves at different temperatures also indicates the presence of a significant magnetic anisotropy and/or low lying excited states in the system.^{10b,11a,15,32}

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 with an oscillation of 3 Oe. The data obtained show that the out-of-phase (χ′′) ac susceptibilities at low temperature for both the two complexes are frequency-dependent (Fig. S12†), there is a possibility of these systems to be SMMs.

Conclusions

In conclusion, we have successfully obtained six lanthanide complexes based on pyridine-functionalized bis-nitronyl nitroxide radicals. The results show that all of the six complexes have similar 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 six complexes are studied, and the results show that there are antiferromagnetic interactions between the paramagnetic ions (Ln(III) and radicals) in 1, 2, and 3 and ferromagnetic interactions in 4, 5 and 6. 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 NITPymbis in the complex (ferromagnetic interaction between the Gd(III) and the radicals, and also ferromagnetic interaction between the two intramolecular radicals). In addition, at low temperature, intermolecular ferromagnetic interaction also plays an important role for complex 4. Furthermore, the frequency dependence of the ac susceptibility shows that complexes 5 and 6 exhibits slow relaxation of the magnetization at low temperature.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (21371133), and the Natural Science Fund of Tianjin, China (12JCZDJC27600).

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Footnote

† Electronic supplementary information (ESI) available. CCDC 990176, 990177 and 1035067–1035070. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4ra15074k

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