Synthesis, structures, and magnetic properties of a series of new heterometallic hexanuclear Co₂Ln₄ (Ln = Eu, Gd, Tb and Dy) clusters

Abstract

A series of new Co–Ln heterometallic clusters formulated as [Co₂Ln₄(μ₃-OH)₂(piv)₄(hmmp)₄(ae)₂]·(NO₃)₂·2H₂O (Ln = Eu (1), Gd (2), Tb (3), Dy (4), H₂hmmp = 2-[(2-hydroxyethylimino)methyl]-6-methoxyphenol, Hae = 2-aminoethanol, Hpiv = pivalic acid) were synthesized and characterized. X-ray crystallography reveals that each of them contains a heterometallic {Ln₄Co₂} core, which is supported by two μ₃-hydroxide, four piv⁻, four hmmp²⁻ and two ae⁻ ligands. The magnetic investigation indicates that 2 exhibits weak antiferromagnetic interactions between Gd^III ions, and a large MCE value of 24.9 J kg⁻¹ K⁻¹, while 4 shows a fast relaxation of magnetization. The TGA and VT-PXRD measurements suggest that all the compounds show good thermal stability and can be stable up to about 220 °C. In addition, to address the influence of the Gd–O–Gd angles on the magnetic properties, compound 2 was compared with a series of compounds involving different bridges between Gd^III ions. The comparison reveals that the tiny difference in the Gd–O–Gd angles favors different magnetic coupling.

---

Introduction

The research on nano-sized molecular magnetic materials with a high spin ground state has witnessed in recent years flourishing progress due to their aesthetically fascinating structures and potential applications in many fields such as quantum computing, high-density magnetic information storage, molecular spintronics and low temperature magnetic refrigerant technology.¹ One special attractive goal of this research is the synthesis of molecular magnetic materials having the cryogenic magnetocaloric effect (MCE) as candidate materials for magnetic refrigerators.^1f,2 The MCE is a magneto-thermodynamic phenomenon that is characterized by the isothermal entropy change (ΔS_M) and by the adiabatic temperature change (ΔT_ad) associated with the magnetic field variation.^2,3 Compared with the traditional gas compression–expansion technology, adiabatic demagnetization is less harmful to the environment, more energy efficient, low-lying noise^2,3 and much easier to achieve very-low temperatures, even milliKelvin temperatures.⁴ These virtues make it particularly attractive to scientists, and hence much effort has been invested in research related to materials exhibiting MCE properties.⁵ Indeed, the MCE is intrinsic in any magnetic material, but only in a few cases is the entropy change sufficiently large to make them suitable for practical use. For example, gadolinium is a suitable candidate for magnetic refrigerant materials mainly because of its ⁸S_7/2 ground state which provides the largest entropy per ion, together with a negligible superexchange interaction. As a result, many Mn–Gd,⁶ Fe–Gd,⁷ Co–Gd,⁸ Ni–Gd,⁹ Cu–Gd,^9b,10 Cr–Gd¹¹ or purely Gd-based^5d,12 molecular magnetic cryocooling materials have been reported. However, most of them are coordinated by solvent molecules, which bind to the central metal ions with weaker chemical bonds compared with the coordinated organic ligands, leading to a lower thermal stability. Very recently, Zheng et al. have reported a series of 3d-Gd clusters without coordination solvent molecules by employing phosphonates as ligands,^6b,8c,d,9a which display a fascinating large magnetocaloric effect. These clusters are synthesized by a solvothermal method and their thermal stability has not yet been investigated. For a MCE material, the alignment of randomly oriented magnetic moments by an external magnetic field can result in heating, and this heat may lead to damage of the material. Therefore, good thermal stability is important for this type of material.

On the other hand, Dy clusters, in particular planar “butterfly” type Dy₄ clusters, are of particular interest to the researchers in the field of single-molecular magnets (SMMs) owing to the large magnetic anisotropy and magnetic moment of the Dy component. Over the last few years, a large number of Dy₄ butterfly clusters have been reported.¹³ However, the Dy^III ions in these compounds are mainly eight coordinate, and there are only two examples in which the coordination numbers of Dy^III ions are greater than eight.^13b,c It has been shown that the ligand field as well as the coordination geometry strongly influence the local anisotropy of the Dy^III ion, and govern the SMM behaviour.¹⁴ Therefore, it is interesting to synthesize new “butterfly” type Dy₄ clusters with other coordination geometries and explore their SMM behaviour.

The polydentate hydroxyl-rich Schiff bases, derived from the reactions of 2-hydroxybenzaldehyde or its derivatives with a range of amino alcohols, have been proven to be versatile chelating and bridging ligands which can react with metal ions to afford a number of polynuclear 3d¹⁵ or 4f¹⁶ clusters. By comparison, less work has been done in the preparation of 3d–4f clusters with these ligands.^6d,17 One of these ligands, namely 2-[(2-hydroxyethylimino)methyl]-6-methoxyphenol (H₂hmmp), in situ generated from the reaction of 2-hydroxy-3-methoxybenzaldehyde with 2-aminoethanol, has recently been used to make Mn–Ln clusters.¹⁸ However, it has not been used in the synthesis of Co–Ln clusters. Herein, we report a series of Co–Ln clusters derived from H₂hmmp, formulated as [Co₂Ln₄(μ₃-OH)₂(piv)₄ (hmmp)₄(ae)₂]·(NO₃)₂·2H₂O (Ln = Eu 1, Gd 2, Tb 3, Dy 4, Hae = 2-aminoethanol and Hpiv = pivalic acid). The thermal analysis studies show that 1–4 have a high thermal stability up to 220 °C. Magnetic properties of 1–4 are also investigated. Isothermal magnetization measurements indicate that the Co₂Gd₄ cluster exhibits a large MCE. Moreover, alternating current susceptibility measurements indicate that the Co₂Dy₄ cluster displays fast quantum tunneling relaxation.

Experimental

Materials and physical measurements

All the materials were purchased from commercial sources and used without further purification. Thermogravimetric experiments were performed using a TGA/NETZSCH STA449C instrument heated from 30 to 800 °C (heating rate 10 °C min⁻¹, nitrogen stream). Elemental analyses of C, H and N were carried out with a Vario EL III elemental analyzer. High-resolution powder XRD patterns were collected using a PANalytical X'Pert Pro (Cu-K_α radiation: λ = 1.54056 Å) in the range of 5° < 2θ < 60°. IR spectra were recorded on a Perkin-Elmer Spectrum One using KBr pellets in the range of 4000–400 cm⁻¹. Magnetic susceptibilities were measured on polycrystalline samples with a Quantum Design PPMS-9 T system. Diamagnetic corrections were made using Pascal's constants.

Synthesis of compounds 1–4

The same procedure was employed to prepare all the compounds and hence only the synthesis of 4 is described here in detail. A mixture of 2-hydroxy-3-methoxybenzaldehyde (152 mg, 1.00 mmol) and 2-aminoethanol (76 mg, 1.25 mmol) in MeCN (30 mL) was stirred and heated at 80 °C for one hour and the solution turned dark yellow. After cooling, triethylamine (405 mg, 4.00 mmol), Dy(NO₃)₃·6H₂O (456 mg, 1.00 mmol) and pivalic acid (102 mg, 1.00 mmol) were added to give a light yellow solution. After stirring for another one hour, Co(NO₃)₂·6H₂O (125 mg, 0.50 mmol) and 3-butylene glycol (270 mg, 3.00 mmol) were added and the solution turned dark red. The solution was stirred under ambient conditions for another two hours and then filtered. Orange-yellow crystals of 4 were obtained after one week, washed with cold MeCN (2 × 5 mL) and dried under vacuum (205 mg, yield 36%). Elemental analysis calcd (%) for C₆₄H₉₈N₈O₃₂Co₂Dy₄: C 34.02, H 4.37, N 4.96; Found: C 33.89, H 4.23, N 4.85. IR (KBr, cm⁻¹): 3626 (v), 3413 (v), 3220 (v), 3115 (v), 2959 (m), 2925 (m), 2867 (m), 2695 (vw), 1643 (vw), 1631 (m), 1605 (v), 1563 (s), 1538 (m), 1484 (vw), 1471 (m), 1456 (vw), 1423 (s), 1385 (m), 1325 (vw), 1297 (m), 1223 (s), 1168 (w), 1062 (m), 970 (w), 920 (vw), 894 (vw), 739 (m), 548 (m).

Compounds 1–3 were obtained by the same procedures as that described for 4, using Ln(NO₃)₃·6H₂O (Ln = Eu, Gd and Tb) in place of Dy(NO₃)₃·6H₂O. Compound 1: yield: 36%. Elemental analysis calcd (%) for C₆₄H₉₈N₈O₃₂Co₂Eu₄: C 34.67, H 4.46, N 5.05; Found: C 34.72, H 4.49, N 5.11. IR (KBr, cm⁻¹): 3620 (v), 3412 (v), 3219 (v), 3113 (v), 2959 (m), 2928 (m), 2866 (m), 2691 (vw), 1644 (vw), 1630 (s), 1605 (m), 1558 (s), 1532 (m), 1484 (vw), 1456 (vw), 1423 (v), 1384 (v), 1325 (vw), 1297 (m), 1220 (s), 1168 (w), 1060 (s), 968 (w), 916 (vw), 893 (vw), 739 (m), 545 (v). Compound 2: yield: 43%. Elemental analysis calcd (%) for C₆₄H₉₈N₈O₃₂Co₂Gd₄: C 34.34, H 4.41, N 5.01; Found: C 34.46, H 4.48, N 5.10. IR (KBr, cm⁻¹): 3622 (v), 3411 (v), 3222 (v), 3114 (v), 2959 (m), 2927 (m), 2867 (m), 2691 (vw), 1642 (vw), 1631 (s), 1605 (m), 1560 (s), 1535 (m), 1484 (vw), 1456 (vw), 1423 (v), 1385 (v), 1325 (vw), 1297 (m), 1221 (s), 1168 (w), 1061 (s), 969 (w), 917 (vw), 894 (vw), 739 (m), 546 (v). Compound 3: yield: 38%. Elemental analysis calcd (%) for C₆₄H₉₈N₈O₃₂Co₂ Tb₄: C 34.24, H 4.40, N 5.00; Found: C 34.33, H 4.46, N 5.06. IR (KBr, cm⁻¹): 3624 (v), 3412 (v), 3221 (v), 3115 (v), 2959 (m), 2926 (m), 2867 (m), 2695 (vw), 1640 (vw), 1631 (s), 1605 (m), 1561 (s), 1536 (m), 1484 (vw), 1471 (vw), 1423 (v), 1384 (v), 1325 (vw), 1297 (m), 1222 (s), 1168 (w), 1061 (s), 970 (w), 918 (vw), 894 (vw), 740 (m), 547 (v).

X-ray crystallography

Suitable single crystals of the compounds were carefully selected and glued to thin glass fibers with epoxy resin. Intensity data were collected at room temperature on a Rigaku Mercury CCD area-detector diffractometer with a graphite monochromator utilizing Mo-K_α radiation (λ = 0.71073 Å). CrystalClear software¹⁹ was used for data reduction and empirical absorption correction. The structures were solved by direct methods using SHELXTL and refined by full-matrix least-squares on F² using the SHELX-97 program.²⁰ All the non-hydrogen atoms were refined anisotropically, except for O14, C23, C28, C29 and C30 atoms in 1, O14, O15, C23, C28, C29 and C30 atoms in 2, O14, O15, C28, C29 and C30 atoms in 3 and 4. The hydrogen atoms bonded to carbon are generated geometrically (C−H 0.97 or 0.93 Å) and U(H) values are set as 1.2 times U_eq(C). Since the position of the disordered water molecules could not be resolved from the Fourier maps, PLATON/SQUEEZE²¹ was used to compensate the data for their contribution to the diffraction patterns for 1–4. The final chemical formulae of 1–4 were calculated from SQUEEZE results combined with the TGA and elemental analysis data. Crystallographic data and other pertinent information for these compounds are summarized in Table 1. Selected bond lengths (Å) and angles (°) for compounds 1–4 are listed in Table S1.† CCDC numbers for compounds 1–4 are 948361–948364.

Table 1 Crystallographic data for 1–4

	1 (Eu)	2 (Gd)	3 (Tb)	4 (Dy)
a R 1 = ∑||Fo| − |Fc||/∑|Fo|. b wR2 = [∑w(Fo^2 − Fc^2)^2/∑w(Fo^2)^2]^0.5.
Formula	C64H94N8O30Co2Eu4	C64H94N8O30Co2Gd4	C64H94N8O30Co2Tb4	C64H94N8O30Co2Dy4
Formula mass	2183.53	2202.33	2209.01	2223.33
Crystal system	Triclinic	Triclinic	Triclinic	Triclinic
Space group	P1̄	P1̄	P1̄	P1̄
a/Å	11.579(5)	11.628(4)	11.568(5)	11.576 (2)
b/Å	13.984(6)	13.997(4)	13.976(6)	14.014 (3)
c/Å	15.517(7)	15.441(5)	15.460(7)	15.380 (3)
α/°	76.066(17)	74.478(14)	75.651(15)	74.549 (9)
β/°	68.290(16)	68.117(12)	68.137(14)	67.993 (9)
γ/°	70.713(18)	69.735(12)	70.478(15)	69.888 (9)
V/Å^3	2183.0(17)	2159.8(12)	2164.5(17)	2144.8 (7)
Z	1	1	1	1
μ/mm^−1	3.277	3.479	3.675	3.895
D calcd/g cm^−3	1.659	1.693	1.695	1.727
F(000)	1080	1084	1108	1092
Reflns measured	17 096	23 180	19 203	22 834
Independent reflns	7435	9808	7584	9662
Observed reflns	6380	7955	6335	7402
R 1 ^a [I > 2σ(I)]	0.0408	0.0490	0.0475	0.0476
wR2 ^b [I > 2σ(I)]	0.1062	0.1304	0.1135	0.1209
GOF on F^2	1.096	1.077	1.099	1.012

---

Results and discussion

Syntheses, thermal stability and spectral analysis

A reaction designed to lead to the in situ formation of a Schiff base as a ligand has been employed to synthesize compounds 1–4. During the reactions, the Schiff base H₂hmmp is formed which subsequently reacts with Ln^III and Co^II ions to generate a series of new Co^II–Ln^III clusters. Considering large magnetic anisotropy of the Co^II ion, our initial aim was to obtain Co^II–Ln^III clusters. Unfortunately, the oxidation of Co^II to Co^III occurred during the reactions, resulting in the presence of Co^III ions in the final products. It should be noted that the addition of 3-butylene glycol can greatly improve the yields of the products. Although a variety of structural types and compositions have been identified so far for Co–Ln clusters,²² to the best of our knowledge, most of them are prepared in CH₃OH or CH₃OH–CH₂Cl₂ solution. As a result, one or more CH₃OH molecules are coordinated to the metal centres. In the preparation of 1–4, excess 2-aminoethanol was used to prevent the coordination of solvent molecules and we successfully obtained four Co–Ln clusters without any coordinated solvent molecules at the metal centres. Powder X-ray diffraction (PXRD) patterns measured in the solid state have been used to check the phase purity of the bulk samples. The measured PXRD patterns of 1–4 are in good agreement with the simulated ones generated from the results of single-crystal diffraction data, indicating the phase purity of the as-synthesized samples (Fig. S1†). TGA experiments were performed to determine the thermal stability of 1–4 under a nitrogen atmosphere in the temperature range of 30–800 °C. TG analyses indicate that all these compounds have high thermal stability and exhibit similar thermal behaviour (Fig. 1a and S2†). Therefore only the thermal stability of 4 was discussed in detail. The TGA curve of 4 shows a weight loss of 1.54% from 40 to 230 °C, which can be attributed to the loss of two lattice water molecules (calcd = 1.59%). And then, it begins to decompose due to the collapse of organic ligands. The high thermal stability of 1–4 was further confirmed from the temperature resolved XRD patterns, which were performed after calcination of the sample at elevated temperatures in the range of 100–240 °C (Fig. 1b and S2†). The powder XRD patterns of 4 fit well with the simulated data below 220 °C, which indicate that it remains crystalline below this temperature (Fig. 1b). At 240 °C, the long-range order of the structure was lost and the amorphous phase was formed. The thermal stability of 4 is comparable to metal–organic frameworks (MOFs) that generally decompose in the temperature range of 200–350 °C.²³ As shown in Fig. S3,† the IR spectra of these compounds are nearly identical. The absorption bands resulting from the skeletal vibrations of the aromatic ring in 1–4 were found in the range of 1400–1600 cm⁻¹. The broad peaks around 3410 cm⁻¹ indicate the presence of free water molecules. The sharp peak around 3625 cm⁻¹ suggests the presence of hydroxyl groups in these compounds. The stretching bands for –NH₂ groups appear at about 3220 and 3113 cm⁻¹. Additionally, the IR spectra of 1–4 display absorption bands at about 2960, 2925, 2867 and 1223 cm⁻¹, which can be assigned to the characteristic signals of the Bu^t groups. The above vibration bands are consistent with the single-crystal structure analyses.

Fig. 1 (a) TGA curve of 4. (b) Variable-temperature powder XRD of 4.

Crystal structure of compounds 1–4

Single crystal X-ray diffraction analysis reveals that 1–4 are isostructural to each other. Therefore, only the description of the Dy analogue will be given here. The asymmetric unit of 4 contains one Co^III ion, two Dy^III ions, one OH⁻, one ae⁻, two piv⁻, two hmmp²⁻ and one NO₃⁻ anion (Fig. S4†). The Dy1 and Dy2 atoms are both nine-coordinated in a distorted tri-capped trigonal prismatic geometry but with different coordination environments (Fig. 2a): Dy1 is coordinated by one nitrogen atom and eight oxygen atoms with the Dy1–N bond length of 2.49 Å and the average Dy1–O bond length of 2.45 Å, while Dy2 is surrounded by nine oxygen atoms with the average Dy2–O bond length of 2.43 Å. The Co1 atom has a slightly distorted octahedral arrangement with the Co1–L_N,O bond lengths ranging from 1.874(6) to 1.942(5) Å. The Dy^III ions make up a Dy₄O₂ core, in which the four Dy^III ions exhibit a planar butterfly structure. At the centre of the Dy₄ metallic core are two μ₃-hydroxide ligands, which bridge the central hinge Dy^III (Dy1 and Dy1A) to the outer wing-tip Dy^III ions (Dy2 and Dy2A). The two (μ₃-OH)⁻ ions are located above and below the Dy₄ plane by 0.859 Å (Fig. S5†). Such a Dy₄O₂ core is bridged to two Co^III ions by two μ₃-O atoms (one from an ae⁻ ligand and the other from an hmmp²⁻ ligand), generating a Dy₄Co₂O₁₂ oxo cluster core (Fig. 2b). Around the periphery of the oxo cluster are two ae⁻, four piv⁻ and four hmmp²⁻ ligands (Fig. 2c). Both of the ae⁻ ligands adopt a μ₃–η¹η³ bonding mode. The four piv⁻ ligands are all coordinated to the Dy^III ions in the syn–syn and μ–η² bonding modes, with the former bridging the hinge ions to the wing-tip ions and the latter chelating to the wing-tip ions. The hmmp²⁻ ligands exhibit two bridging modes, μ₄–η¹η²η¹η³ and μ₃–η¹η²η¹η², to connect Dy^III and Co^III ions (Scheme 1). It should be noted that the coordination geometries of Dy^III ions in 4 are all tri-capped trigonal prismatic (nine-coordinate) which are different from those of other reported Dy₄ butterfly clusters,^13,16a where the Dy^III ions are mainly in a distorted square-antiprismatic geometry (eight-coordinate).

Fig. 2 (a) Coordination polyhedron for Dy^III ions in 4. (b) The Dy₄Co₂O₁₂ oxo cluster core in 4. (c) Coordination environments for Dy^III and Co^III ions in 4.

Scheme 1 Coordination modes of the hmmp²⁻ ligand.

Magnetic properties

The variable-temperature dc magnetic susceptibilities for 1–4 were measured in the temperature range of 300–2 K under an applied field of 1000 Oe (Fig. 3a). For 1, the χ_mT value is 5.27 cm³ mol⁻¹ K at 300 K, which is much larger than the value of 0 cm³ mol⁻¹ K expected for four Eu^III ions (J = 0). Due to the presence of the thermally populated excited states of the Eu^III ion, the magnetic properties of 1 remain difficult to explain even at room temperature.²⁴ However, its χ_mT value is close to zero (0.05 cm³ mol⁻¹ K) at 2 K, indicating a nonmagnetic ground state (⁷F₀) for the Eu^III ion. This thermally populated diamagnetic ground state of 1 leads to a value of magnetization close to zero even at 8 T (Fig. 3b). Considering the nonmagnetic ground state (⁷F₀) and thermally populated excited states (⁷F_J, J = 0, 1, 2,…, 6) of the Eu^III ion, the magnetic data of 1 can be analyzed using eqn (1) which is deduced from four magnetically non-interacting Eu^III ions.²⁵

x_m = (Nβ²/3KTx)[24 + (27x/2 + 3/2)e^−x + (135x/2 − 5/2)e^−3x + (189x − 7/2)e^−6x + (405x − 9/2)e^−10x + (1485x/2 − 11/2)e^−15x + (2457x/2 − 13/2)e^−21x]/[1 + 3e^−x + 5e^−3x + 7e^−6x + 9e^−10x + 11e^−15x + 13e^−21x] (1)

where x = λ/KT and λ is the spin–orbit coupling parameter. The least-squares fitting in the whole temperature range leads to λ = 365 cm⁻¹ and R = 8.3 × 10⁻⁴ (Fig. 3a). The value of λ is comparable to other reported Eu^III complexes,²⁶ but is smaller than 370 cm⁻¹ deduced from the average value of three components arising from the ⁷F₁ state according to the spectroscopic data.²⁵ This divergence could be attributed to the different crystal field effect.

Fig. 3 (a) Temperature dependence of the χ_mT for 1–4 with an applied field of 1000 Oe. The solid line represents the best fit for 1 and 2. (b) Field dependence of magnetization of 1–4 at 2 K.

For 2, the Gd₄ displays an χ_mT value of 30.80 cm³ mol⁻¹ K at room temperature, which is in good agreement with that for four isolated Gd^III ions (31.52 cm³ mol⁻¹ K). This value remains constant as the temperature is decreased until ca. 30 K where it sharply decreases to a value of 14.87 cm³ mol⁻¹ K at 2 K. This behaviour is indicative of the occurrence of weak antiferromagnetic interactions between the Gd^III ions. In order to quantify the magnitude of the magnetic exchange within the Gd₄, the exchange mode with three coupling constants is taken into account (Fig. S6†). The fitting to the magnetic susceptibility curve was performed using the MAGPACK program²⁷ based on the spin-Hamiltonian shown in eqn (2).

H = −2J₁(S₁S₂ + S₃S₄) − 2J₂(S₂S₃ + S₁S₄) − 2J₃S₁S₃ (2)

The best fit gave the parameters g = 1.98, J₁ = −0.07 cm⁻¹, J₂ = −0.05 cm⁻¹ and J₃ = −0.029 cm⁻¹ (Fig. 3a). The agreement factor R, defined as Σ[(χ_mT)_obsed − (χ_mT)_calcd]²/Σ(χ_mT)²_obsed, is equal to 2.53 × 10⁻⁵. The very small negative J values clearly suggest the weak antiferromagnetic couplings among the Gd^III ions. The field dependence of magnetization of 2 shows a saturation value of 27.46Nβ at 2 K and H = 8 T (Fig. 3b), which is close to the expected theoretical value of 28.00Nβ.

The bridges between Gd^III ions in 2 can be divided into three types: two μ₂-O bridges (type I), three μ₂-O bridges (type II) and mixed bridges of one syn–syn carboxylate and two μ₂-O bridges (type III) (Fig. S6†). Their values of magnetic coupling are compared with those of other magnetostructurally characterized Gd^III complexes (Table 2). It seems that there exists a correlation between the Gd–O–Gd angle (or Gd⋯Gd distance) and the magnetic interaction of Gd^III ions. For example, a large Gd–O–Gd angle with type I^12c,28 or III^12c,29 favours a ferromagnetic interaction. Accordingly, the small Gd–O–Gd angles of types I and III in 2 may lead to an antiferromagnetic interaction between the Gd^III ions. It is also found that the type II bridge usually results in antiferromagnetic interactions between the Gd^III ions,^13d,30 and the smaller the Gd–O–Gd angle, the larger the antiferromagnetic interaction. This observation is in good agreement with the fact that the type II bridge in 2 which has a medium Gd–O–Gd angle shows the presence of a moderate antiferromagnetic interaction.

Table 2 Selected magnetostructural data of Gd^III complexes

Complex^a	Gd⋯Gd/Å	Gd–O–Gd/°	J /cm^−1	Bridge	Ref.
a Abbreviations used in this table: OAc = acetate, H2sal = salicylic acid, H2mal = 1,3-propanedioic acid, H4cit = citric acid, Ph2acacH = dibenzoylmethane, acacH = acetylacetone, tpac = 3-thiopheneacetate, pac = pentanoate, H3L = N[(CH2)2N=CH-R-CH=N–(CH2)2]3N (R = 1,3-(2-OH-5-Me-C6H2)), H3ppa = 6-(3-oxo-3-(2-hydroxyphenyl) propionyl)-2-pyridinecarboxylic acid, py = pyridine, hfac^− = 1,1,1,5,5,5-hexafluoroacetylacetonate anion, tpno = tetrathiafulvaleneamido-2-pyridine-N-oxide, H3mta = methanetriacetic acid. b The spin Hamiltonian is defined as H = −2JSASB.
[Gd2(OAc)6(H2O)4]·4H2O	4.206	115.5	0.03	I	28a
[Gd2(Hsal)6(H2O)2]	4.250	116.12	0.025	I	28b
[Gd2(mal)3(H2O)6]n	4.276	116.71	0.024	I	28c
[Gd(Hcit)(H2O)2]n·nH2O	4.321	118.49	0.02	I	28d
[Gd2(OAc)2(Ph2acac)4(MeOH)2]	4.128	113.65	0.019	I	12c
[Gd2(OAc)6(H2O)4]·2H2O	4.159	115.47	0.016	I	28e
[Gd4(OAc)4(acac)8(H2O)4]	4.271/4.334	114.45/117.73	0.012	I	12c
[Gd2(tpac)6(H2O)4]	4.126	112.5	−0.006	I	28f
[Gd2(pac)6(H2O)4]	4.112	113.16	−0.016	I	28f
2	3.975	112.17	−0.029	I	This work
[Gd2(L)(NO3)2]·(NO3)·1.5H2O	3.500	92.48/94.93/95.12	−0.097	II	30a
[Gd2(Hppa)2(H2ppa)Cl(py)(H2O)]	3.813	98.86/102.93/103.36	−0.021	II	30b
[Gd4(μ3-OH)2L2(acac)6]·4CH3CN	3.689	93.52/100.67/102.44	−0.02	II	13d
[Gd2(hfac)5(O2CPhCl)(tpno)3]·2H2O	3.92	102.14/103.38/105.37	−0.005	II	30c
2	3.77	96.28/99.60/104.21	−0.05	II	This work
[Gd(H2sal)(Hsal)(sal)(H2O)]n	4.187	111.85/114.29	0.019	III	29a
[Gd(OAc)3(MeOH)]n	4.055	110/112.62	0.017	III	12c
[Gd(mta)(H2O)]n·nH2O	4.065	110/112.3	0.013	III	29b
[Gd2(succinate)3(H2O)2]n·0.5nH2O	4.059	109.96/112.24	0.01	III	29c
[Gd(OAc)3(H2O)]n	4.027/4.034	106.5/108.4	−0.006	III	12c
2	3.819	104.40/105.87	−0.07	III	This work

---

The χ_mT products for 3 and 4 display room temperature values of 46.61 and 55.88 cm³ mol⁻¹ K, respectively, which are close to the expected theoretical values using the free ion approximation (47.28 cm³ mol⁻¹ K for 3 and 56.68 cm³ mol⁻¹ K for 4) for four non-interacting lanthanide ions: Tb^III (J = 6 and g = 3/2) and Dy^III (J = 15/2 and g = 4/3). For 3, the χ_mT value slowly increases from 46.60 cm³ mol⁻¹ K at 300 K to 54.62 cm³ mol⁻¹ K at about 40 K. Upon further cooling, it decreases to reach 45.16 cm³ mol⁻¹ K at 2 K. The χ_mT versus T curve shows a broad protuberance at about 40 K, indicating the presence of ferromagnetic interactions between Tb^III ions. This kind of interaction is strong enough to compensate the decrease of χ_mT that resulted from the depopulated Stark states. The ferromagnetic interaction resulting in the increase of χ_mT upon cooling may be ascribed to the dipole–dipole interaction between Tb^III ions.³¹ For 4, the χ_mT product remains roughly constant with decreasing temperature down to about 50 K, and then drops to a minimum value of 23.53 cm³ mol⁻¹ K at 2 K. The decrease of χ_mT in 4 is most likely due to a combination effect of the thermal depopulation of Stark sublevels and antiferromagnetic interactions.³² Due to the presence of the unquenched orbital moment of the Tb^III or Dy^III center, it is difficult to fit the magnetic data of 3 and 4.

The field-dependent magnetizations for 3 and 4 are nearly the same (Fig. 3b), both showing a regular increase in magnetization below about 1 T. The magnetization of 3 exhibits a plateau at high magnetic field, whereas that of 4 shows a linear increase at high magnetic field without complete saturation even at 8 T. The magnetization values of 26.36Nβ for 3 and 29.19Nβ for 4 at 8 T are lower than their theoretically derived values (36Nβ for 3 and 40Nβ for 4). Furthermore, the M versus H/T curves at low temperatures for 3 and 4 are not superposed (Fig. 4 and S7c†), indicating the presence of magnetic anisotropy. The divergence of M versus H/T curves of 4 is more apparent than that of 3, suggesting that 4 displays a more significant magnetic anisotropy.³³

Fig. 4 Plots of M vs. H/T for 4 at different temperatures below 5 K.

In view of the weak interactions as well as the isotropic nature of Gd^III ions in 2, the MCE of 2 is investigated according to the isothermal magnetization curves measured in an applied field of up to 8 T and the temperature range of 2–7 K (Fig. 5a). The magnetic entropy changes −ΔS_M can be described by the Maxwell equation as follows:³

−ΔS_m(T)_ΔH = ∫[∂M(T, H)/∂T]_HdH (3)

Fig. 5 (a) The field dependence of magnetization of 2 at 2–7 K. (b) Calculated −ΔS_M using the magnetization data of 2 at various fields (0.5–8 T).

Using eqn (3), magnetic entropy changes ΔS_m were calculated from the experimental magnetization data (Fig. 5b). It can be seen that the maximum value of −ΔS_m is 24.9 J kg⁻¹ K⁻¹ for ΔH = 8 T at 2 K, which is lower than the expected value of 30.90 J kg⁻¹ K⁻¹ according to the equation nR ln(2S + 1) for four isolated Gd^III ions. This deviation may be caused by the weak intra-cluster anti-ferromagnetic interactions in 2.^7,34

Among the 3d-Gd based magnetic cryocooling materials, a magnetic entropy change −ΔS_m larger than 24 J kg⁻¹ K⁻¹ is relatively rare (Table 3). The maximum value of −ΔS_m in 2 is larger than most of the reported Co–Gd clusters. Nevertheless, it is surpassed only by four known Co–Gd clusters, {Co₁₀Gd₄₂},^8a {Co₄Gd₁₀},^8b {Co₆Gd₈}^8c and {Co₁₆Gd₂₄},^8e with the −ΔS_m values of 41.3, 32.6, 28.6 and 28.0 J kg⁻¹ K⁻¹ for ΔH = 7 T, respectively. This is not surprising as 2 contains diamagnetic Co^III ions that reduce the magnetic density, resulting in a low magnetic entropy change.

Table 3 −ΔS_M of 2 and related 3d–Gd compounds

Compounds	−ΔSM/J kg^−1 K^−1	Ref.
Co10Gd42	41.3 (7 T)	8a
Ni10Gd42	38.2 (7 T)	8a
Ni12Gd36	36.3 (7 T)	9d
Mn4Gd6	33.7 (7 T)	6b
Co4Gd10	32.6 (7 T)	8b
Cu5Gd4	31.0 (9 T)	10a
Co6Gd8	28.6 (7 T)	8c
Mn9Gd9	28.0 (7 T)	6b
Fe5Gd8	26.7 (7 T)	7
Ni6Gd6	26.5 (7 T)	9a
Co16Gd24	26.0 (7 T)	8e
2	24.9 (8 T)	This work
Co4Gd6	23.6 (7 T)	9b
Cu6Gd6	23.5 (7 T)	10c
Co8Gd4	22.3 (7 T)	8b
Ni8Gd4	22.0 (7 T)	9b
Ni12Gd5	21.8 (7 T)	9e
Co8Gd8	21.4 (7 T)	8c
Co8Gd4	21.1 (7 T)	9b
Cu36Gd24	21.0 (7 T)	10b
Zn8Gd4	20.8 (7 T)	9b
Co8Gd8	20.4 (7 T)	8d
Co4Gd2	20.0 (7 T)	8c
Co4Gd6	19.9 (7 T)	8d
Co6Gd4	19.7 (7 T)	8b
Mn4Gd4	19.0 (7 T)	6a
Ni6Gd2	17.6 (7 T)	9c
Mn12Gd6	17.0 (7 T)	6d
Cu8Gd4	14.6 (7 T)	9b
Cu8Gd2	12.8 (7 T)	10d
Co8Gd2	11.8 (7 T)	8c
Cr2Gd2	11.4 (9 T)	11b
Cr7Gd	5.1 (7 T)	11a

---

Due to the presence of significant magnetic anisotropy in 4, both the temperature and frequency dependences of ac magnetic susceptibilities were performed under a zero dc field (Fig. 6 and S8†). As illustrated in Fig. 6 and S8,† a frequency dependence of the ac susceptibility was observed with an out-of-phase signal that is weak in intensity and does not exhibit a maximum within the experimental temperature (above 2 K) and frequency (50 to 10⁴ Hz) windows, which indicates the presence of fast relaxation via quantum tunneling (QTM) as observed in other low symmetry lanthanide based-SMMs.^22j Hence, in order to bypass any possible quantum tunneling effects, ac susceptibility measurements were further performed under applied dc fields of 5000 and 8000 Oe (Fig. S9†). It is found that 4 shows peaks in the temperature dependence of x^′′_m under applied dc fields, which is due to the suppression of the quantum tunneling of magnetization. The relaxation follows a thermally activated mechanism, giving energy barriers of ΔE/K_B = 25.2 and 32.4 K, relaxation times τ₀ = 1.3 × 10⁻⁶ and 4.2 × 10⁻⁷ s at 5000 and 8000 Oe, respectively, based on the Arrhenius law τ(T_p) = τ₀exp(Δ/T_p) with a linear correlation of 1/T_pvs. ln(2πf) (Fig. S10†). These energy barriers are comparable to those reported for other compounds exhibiting the field induced SMM behaviour.^13d,35 It should be noted that the fast quantum tunneling relaxation observed in the case of 4 may be attributed to the low symmetric coordination geometry of the local Dy^III sites as well as the weak intramolecular interactions between the adjacent Dy^III ions.^36,37

Fig. 6 Frequency dependence in the zero dc field of the (x^′′_m) ac susceptibility component at different temperatures for 4.

Conclusions

In summary, using the in situ synthetic route, we have synthesized a series of new Co–Ln heterometallic clusters, in which the {Co₂Ln₄} core is bridged by two μ₃-hydroxide, four piv⁻, four hmmp²⁻ and two ae⁻ ligands. Due to the absence of any coordinated solvent molecules at the metal centres, all these compounds have a high thermal stability and can be stable up to 220 °C. The magnetic measurements show that the Co₂Gd₄ cluster exhibits very weak antiferromagnetic interactions among the Gd^III ions, as well as a large magnetocaloric effect at low temperatures, showing a potential application in magnetic cooling technology in the very-low temperature range. Replacement of Gd^III ions with anisotropic Dy^III ions gives the Dy₄Co₂ cluster, which displays significantly fast QTM in a zero dc field that can be slowed down by application of an external dc field. Moreover, the magnetostructural study reveals that the tiny change in Gd–O–Gd angles will result in different magnetic interactions. This work will be helpful for the rational design and synthesis of highly thermally stable 3d–4f based molecular magnetic cryogenic materials.

Acknowledgements

We thank the National Basic Research Program of China (973 program, 2012CB821702), the National Natural Science Foundation of China (21233009 and 21173221) and the State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences for financial support.

Notes and references

1. (a) M. N. Leuenberger and D. Loss, Nature, 2001, 410, 789–793; (b) S. Hill, R. S. Edwards, N. Aliaga-Alcalde and G. Christou, Science, 2003, 302, 1015–1018; (c) L. Bogani and W. Wernsdorfer, Nat. Mater., 2008, 7, 179–186; (d) M. Mannini, F. Pineider, C. Danieli, F. Totti, L. Sorace, P. Sainctavit, M. A. Arrio, E. Otero, L. Joly, J. C. Cezar, A. Cornia and R. Sessoli, Nature, 2010, 468, 417–421; (e) G. E. Kostakis, S. P. Perlepes, V. A. Blatov, D. M. Proserpio and A. K. Powell, Coord. Chem. Rev., 2012, 256, 1246–1278; (f) B. G. Shen, J. R. Sun, F. X. Hu, H. W. Zhang and Z. H. Cheng, Adv. Mater., 2009, 21, 4545–4564.
2. R. Sessoli, Angew. Chem., Int. Ed., 2012, 51, 43–45.
3. (a) M. Evangelisti, F. Luis, L. J. de Jongh and M. Affronte, J. Mater. Chem., 2006, 16, 2534–2549; (b) M. Evangelisti and E. K. Brechin, Dalton Trans., 2010, 39, 4672–4676.
4. M.-J. Martinez-Perez, O. Montero, M. Evangelisti, F. Luis, J. Sese, S. Cardona-Serra and E. Coronado, Adv. Mater., 2012, 24, 4301–4305.
5. (a) F.-S. Guo, Y.-C. Chen, J.-L. Liu, J.-D. Leng, Z.-S. Meng, P. Vrabel, M. Orendac and M.-L. Tong, Chem. Commun., 2012, 48, 12219–12221; (b) G. Lorusso, M. A. Palacios, G. S. Nichol, E. K. Brechin, O. Roubeau and M. Evangelisti, Chem. Commun., 2012, 48, 7592–7594; (c) R. Sibille, T. Mazet, B. Malaman and M. Francois, Chem. – Eur. J., 2012, 18, 12970–12973; (d) L.-X. Chang, G. Xiong, L. Wang, P. Cheng and B. Zhao, Chem. Commun., 2013, 49, 1055–1057; (e) C.-B. Tian, Z.-J. Lin and S.-W. Du, Cryst. Growth Des., 2013, 13, 3746–3753; (f) C.-B. Tian, R.-P. Chen, C. He, W.-J. Li, Q. Wei, X.-D. Zhang and S.-W. Du, Chem. Commun., 2014, 50, 1915–1917.
6. (a) G. Karotsis, M. Evangelisti, S. J. Dalgarno and E. K. Brechin, Angew. Chem., Int. Ed., 2009, 48, 9928–9931; (b) Y.-Z. Zheng, E. M. Pineda, M. Helliwell and R. E. P. Winpenny, Chem. – Eur. J., 2012, 18, 4161–4165; (c) G. Karotsis, S. Kennedy, S. J. Teat, C. M. Beavers, D. A. Fowler, J. J. Morales, M. Evangelisti, S. J. Dalgarno and E. K. Brechin, J. Am. Chem. Soc., 2010, 132, 12983–12990; (d) J.-L. Liu, W.-Q. Lin, Y.-C. Chen, J.-D. Leng, F.-S. Guo and M.-L. Tong, Inorg. Chem., 2013, 52, 457–463.
7. E. Cremades, S. Gomez-Coca, D. Aravena, S. Alvarez and E. Ruiz, J. Am. Chem. Soc., 2012, 134, 10532–10542.
8. (a) J.-B. Peng, Q.-C. Zhang, X.-J. Kong, Y.-Z. Zheng, Y.-P. Ren, L.-S. Long, R.-B. Huang, L.-S. Zheng and Z. Zheng, J. Am. Chem. Soc., 2012, 134, 3314–3317; (b) E. M. Pineda, F. Tuna, R. G. Pritchard, A. C. Regan, R. E. P. Winpenny and E. J. L. McInnes, Chem. Commun., 2013, 49, 3522–3524; (c) Y.-Z. Zheng, M. Evangelisti, F. Tuna and R. E. P. Winpenny, J. Am. Chem. Soc., 2012, 134, 1057–1065; (d) Y.-Z. Zheng, M. Evangelisti and R. E. P. Winpenny, Chem. Sci., 2011, 2, 99–102; (e) Z. M. Zhang, L. Y. Pan, W. Q. Lin, J. D. Leng, F. S. Guo, Y. C. Chen, J. L. Liu and M. L. Tong, Chem. Commun., 2013, 49, 8081–8083.
9. (a) Y.-Z. Zheng, M. Evangelisti and R. E. P. Winpenny, Angew. Chem., Int. Ed., 2011, 50, 3692–3695; (b) T. N. Hooper, J. Schnack, S. Piligkos, M. Evangelisti and E. K. Brechin, Angew. Chem., Int. Ed., 2012, 51, 4633–4636; (c) A. Hosoi, Y. Yukawa, S. Igarashi, S. J. Teat, O. Roubeau, M. Evangelisti, E. Cremades, E. Ruiz, L. A. Barrios and G. Aromi, Chem. – Eur. J., 2011, 17, 8264–8268; (d) J.-B. Peng, Q.-C. Zhang, X.-J. Kong, Y.-P. Ren, L.-S. Long, R.-B. Huang, L.-S. Zheng and Z. Zheng, Angew. Chem., Int. Ed., 2011, 50, 10649–10652; (e) Z.-Y. Li, J. Zhu, X.-Q. Wang, J. Ni, J.-J. Zhang, S.-Q. Liu and C.-Y. Duan, Dalton Trans., 2013, 42, 5711–5717.
10. (a) S. K. Langley, N. F. Chilton, B. Moubaraki, T. Hooper, E. K. Brechin, M. Evangelisti and K. S. Murray, Chem. Sci., 2011, 2, 1166–1169; (b) J.-D. Leng, J.-L. Liu and M.-L. Tong, Chem. Commun., 2012, 48, 5286–5288; (c) A. S. Dinca, A. Ghirri, A. M. Madalan, M. Affronte and M. Andruh, Inorg. Chem., 2012, 51, 3935–3937; (d) H. Zhang, G.-L. Zhuang, X.-J. Kong, Y.-P. Ren, L.-S. Long, R.-B. Huang and L.-S. Zheng, Cryst. Growth Des., 2013, 13, 2493–2498.
11. (a) M. Affronte, A. Ghirri, S. Carretta, G. Amoretti, S. Piligkos, G. A. Timco and R. E. P. Winpenny, Appl. Phys. Lett., 2004, 84, 3468–3470; (b) C. A. Thuesen, K. S. Pedersen, M. Schau-Magnussen, M. Evangelisti, J. Vibenholt, S. Piligkos, H. Weihe and J. Bendix, Dalton Trans., 2012, 41, 11284–11292.
12. (a) M. Evangelisti, O. Roubeau, E. Palacios, A. Camon, T. N. Hooper, E. K. Brechin and J. J. Alonso, Angew. Chem., Int. Ed., 2011, 50, 6606–6609; (b) J. W. Sharples, Y.-Z. Zheng, F. Tuna, E. J. L. McInnes and D. Collison, Chem. Commun., 2011, 47, 7650–7652; (c) F.-S. Guo, J.-D. Leng, J.-L. Liu, Z.-S. Meng and M.-L. Tong, Inorg. Chem., 2012, 51, 405–413.
13. (a) P.-H. Lin, T. J. Burchell, L. Ungur, L. F. Chibotaru, W. Wernsdorfer and M. Murugesu, Angew. Chem., Int. Ed., 2009, 48, 9489–9492; (b) S. Xue, L. Zhao, Y.-N. Guo, R. Deng, Y. Go and J. Tang, Dalton Trans., 2011, 40, 8347–8352; (c) S. K. Langley, N. F. Chilton, I. A. Gass, B. Moubaraki and K. S. Murray, Dalton Trans., 2011, 40, 12656–12659; (d) P.-F. Yan, P.-H. Lin, F. Habib, T. Aharen, M. Murugesu, Z.-P. Deng, G.-M. Li and W.-B. Sun, Inorg. Chem., 2011, 50, 7059–7065; (e) G. Abbas, Y. Lan, G. E. Kostakis, W. Wernsdorfer, C. E. Anson and A. K. Powell, Inorg. Chem., 2010, 49, 8067–8072; (f) P.-H. Guo, J.-L. Liu, Z.-M. Zhang, L. Ungur, L. F. Chibotaru, J.-D. Leng, F.-S. Guo and M.-L. Tong, Inorg. Chem., 2012, 51, 1233–1235; (g) A. K. Jami, V. Baskar and E. Carolina Sanudo, Inorg. Chem., 2013, 52, 2432–2438.
14. (a) P. Zhang, Y.-N. Guo and J. Tang, Coord. Chem. Rev., 2013, 257, 1728–1763; (b) J. D. Rinehart and J. R. Long, Chem. Sci., 2011, 2, 2078–2085; (c) D. N. Woodruff, R. E. P. Winpenny and R. A. Layfield, Chem. Rev., 2013, 113, 5110–5148.
15. (a) C.-M. Liu, R.-G. Xiong, D.-Q. Zhang and D.-B. Zhu, J. Am. Chem. Soc., 2010, 132, 4044–4045; (b) S. Nayak, H. P. Nayek, S. Dehnen, A. K. Powell and J. Reedijk, Dalton Trans., 2011, 40, 2699–2702; (c) P.-P. Yang, X.-Y. Song, R.-N. Liu, L.-C. Li and D.-Z. Liao, Dalton Trans., 2010, 39, 6285–6294; (d) I. J. Hewitt, J.-K. Tang, N. T. Madhu, R. Clerac, G. Buth, C. E. Anson and A. K. Powell, Chem. Commun., 2006, 2650–2652; (e) H. Oshio, N. Hoshino, T. Ito, M. Nakano, F. Renz and P. Gutlich, Angew. Chem., Int. Ed., 2003, 42, 223–225; (f) L.-L. Fan, F.-S. Guo, L. Yun, Z.-J. Lin, R. Herchel, J.-D. Leng, Y.-C. Ou and M.-L. Tong, Dalton Trans., 2010, 39, 1771–1780; (g) R. Koner, S. Hazra, M. Fleck, A. Jana, C. R. Lucas and S. Mohanta, Eur. J. Inorg. Chem., 2009, 4982–4988; (h) D. Liu, Q. Zhou, Y. Chen, F. Yang, Y. Yu, Z. Shi and S. Feng, Dalton Trans., 2010, 39, 5504–5508; (i) S. S. Tandon, S. D. Bunge, J. Sanchiz and L. K. Thompson, Inorg. Chem., 2012, 51, 3270–3282.
16. (a) Y.-Z. Zheng, Y. Lan, C. E. Anson and A. K. Powell, Inorg. Chem., 2008, 47, 10813–10815; (b) H. Ke, G.-F. Xu, L. Zhao, J. Tang, X.-Y. Zhang and H.-J. Zhang, Chem. – Eur. J., 2009, 15, 10335–10338; (c) H. Ke, L. Zhao, G.-F. Xu, Y.-N. Guo, J. Tang, X.-Y. Zhang and H.-J. Zhang, Dalton Trans., 2009, 10609–10613; (d) G.-F. Xu, P. Gamez, S. J. Teat and J. Tang, Dalton Trans., 2010, 39, 4353–4357.
17. (a) J.-L. Liu, F.-S. Guo, Z.-S. Meng, Y.-Z. Zheng, J.-D. Leng, M.-L. Tong, L. Ungur, L. F. Chibotaru, K. J. Heroux and D. N. Hendrickson, Chem. Sci., 2011, 2, 1268–1272; (b) G. Wu, I. J. Hewitt, S. Mameri, Y. Lan, R. Clerac, C. E. Anson, S. Qiu and A. K. Powell, Inorg. Chem., 2007, 46, 7229–7231; (c) S. Nayak, O. Roubeau, S. J. Teat, C. M. Beavers, P. Gamez and J. Reedijk, Inorg. Chem., 2010, 49, 216–221; (d) V. Chandrasekhar, P. Bag, M. Speldrich, J. van Leusen and P. Kogerler, Inorg. Chem., 2013, 52, 5035–5044.
18. P.-P. Yang, X.-L. Wang, L.-C. Li and D.-Z. Liao, Dalton Trans., 2011, 40, 4155–4161.
19. Crystalclear version 1.36, Molecular Structure Corp. and Rigaku Corp., The Woodlands, TX, and Tokyo, Japan, 2000.
20. G. M. Sheldrick, SHELXL-97 Program for X-ray Crystal Structure Refinement, University of Göttingen, Germany, 1997.
21. A. L. Spek, PLATON-97, University of Utrecht, Utrecht, The Netherlands, 1997.
22. (a) K. C. Mondal, A. Sundt, Y. Lan, G. E. Kostakis, O. Waldmann, L. Ungur, L. F. Chibotaru, C. E. Anson and A. K. Powell, Angew. Chem., Int. Ed., 2012, 51, 7550–7554; (b) L.-F. Zou, L. Zhao, Y.-N. Guo, G.-M. Yu, Y. Guo, J. Tang and Y.-H. Li, Chem. Commun., 2011, 47, 8659–8661; (c) M. Orfanoudaki, I. Tamiolakis, M. Siczek, T. Lis, G. S. Armatas, S. A. Pergantis and C. J. Milios, Dalton Trans., 2011, 40, 4793–4796; (d) H. Xiang, Y. Lan, H.-Y. Li, L. Jiang, T.-B. Lu, C. E. Anson and A. K. Powell, Dalton Trans., 2010, 39, 4737–4739; (e) V. Chandrasekhar, B. M. Pandian, J. J. Vittal and R. Clerac, Inorg. Chem., 2009, 48, 1148–1157; (f) H. Ke, L. Zhao, Y. Guo and J. Tang, Dalton Trans., 2012, 41, 9760–9765; (g) V. Gomez, L. Vendier, M. Corbella and J.-P. Costes, Eur. J. Inorg. Chem., 2011, 53–2656; (h) V. Gomez, L. Vendier, M. Corbella and J.-P. Costes, Inorg. Chem., 2012, 51, 6396–6440; (i) V. Chandrasekhar, B. M. Pandian, R. Azhakar, J. J. Vittal and R. Clerac, Inorg. Chem., 2007, 46, 5140–5142; (j) G.-F. Xu, P. Gamez, J. Tang, R. Clerac, Y.-N. Guo and Y. Guo, Inorg. Chem., 2012, 51, 5693–5698; (k) J.-P. Costes, L. Vendier and W. Wernsdorfer, Dalton Trans., 2011, 40, 1700–1706; (l) X.-Q. Zhao, Y. Lan, B. Zhao, P. Cheng, C. E. Anson and A. K. Powell, Dalton Trans., 2010, 39, 4911–4917; (m) E. K. Brechin, S. G. Harris, S. Parsons and R. E. P. Winpenny, Dalton Trans., 1997, 1665–1666.
23. (a) D.-X. Xue, J.-B. Lin, J.-P. Zhang and X.-M. Chen, CrystEngComm, 2009, 11, 183–188; (b) H.-L. Jiang, N. Tsumori and Q. Xu, Inorg. Chem., 2010, 49, 10001–10006.
24. J. Long, F. Habib, P.-H. Lin, I. Korobkov, G. Enright, L. Ungur, W. Wernsdorfer, L. F. Chibotaru and M. Murugesu, J. Am. Chem. Soc., 2011, 133, 5319–5328.
25. M. Andruh, E. Bakalbassis, O. Kahn, J. C. Trombe and P. Porcher, Inorg. Chem., 1993, 32, 1616–1622.
26. (a) X.-J. Wang, Z.-M. Cen, Q.-L. Ni, X.-F. Jiang, H.-C. Lian, L.-C. Gui, H.-H. Zuo and Z.-Y. Wang, Cryst. Growth Des., 2010, 10, 2960–2968; (b) Q.-Y. Liu, W.-F. Wang, Y.-L. Wang, Z.-M. Shan, M.-S. Wang and J. Tang, Inorg. Chem., 2012, 51, 2381–2392; (c) Y.-L. Wang, Y.-Y. Gao, Y. Ma, Q.-L. Wang, L.-C. Li and D.-Z. Liao, J. Solid State Chem., 2013, 202, 276–281.
27. (a) J. J. Borras-Almenar, J. M. Clemente-Juan, E. Coronado and B. S. Tsukerblat, Inorg. Chem., 1999, 38, 6081–6088; (b) J. J. Borras-Almenar, J. M. Clemente-Juan, E. Coronado and B. S. Tsukerblat, J. Comput. Chem., 2001, 22, 985–991.
28. (a) J. P. Costes, J. M. Clemente-Juan, F. Dahan, F. Nicodeme and M. Verelst, Angew. Chem., Int. Ed., 2002, 41, 323–325; (b) S. T. Hatscher and W. Urland, Angew. Chem., Int. Ed., 2003, 42, 2862–2864; (c) M. Hernandez-Molina, C. Ruiz-Perez, T. Lopez, F. Lloret and M. Julve, Inorg. Chem., 2003, 42, 5456–5458; (d) R. Baggio, R. Calvo, M. T. Garland, O. Pena, M. Perec and A. Rizzi, Inorg. Chem., 2005, 44, 8979–8987; (e) L. Canadillas-Delgado, O. Fabelo, J. Cano, J. Pasan, F. S. Delgado, F. Lloret, M. Julve and C. Ruiz-Perez, CrystEngComm, 2009, 11, 2131–2142; (f) L. Canadillas-Delgado, O. Fabelo, J. Pasan, F. S. Delgado, F. Lloret, M. Julve and C. Ruiz-Perez, Dalton Trans., 2010, 39, 7286–7293.
29. (a) J. P. Costes, J. M. C. Juan, F. Dahan and F. Nicodeme, Dalton Trans., 2003, 1272–1275; (b) L. Canadillas-Delgado, T. Martin, O. Fabelo, J. Pasan, F. S. Delgado, F. Lloret, M. Julve and C. Ruiz-Perez, Chem. – Eur. J., 2010, 16, 4037–4047; (c) S. C. Manna, E. Zangrando, A. Bencini, C. Benelli and N. R. Chaudhuri, Inorg. Chem., 2006, 45, 9114–9122.
30. (a) F. Avecilla, C. Platas-Iglesias, R. Rodriguez-Cortinas, G. Guillemot, J. C. G. Bunzli, C. D. Brondino, C. Geraldes, A. de Blas and T. Rodriguez-Blas, Dalton Trans., 2002, 4658–4665; (b) D. Aguila, L. A. Barrios, F. Luis, A. Repolles, O. Roubeau, S. J. Teat and G. Aromi, Inorg. Chem., 2010, 49, 6784–6786; (c) F. Pointillart, T. Cauchy, O. Maury, Y. Le Gal, S. Golhen, O. Cador and L. Ouahab, Chem. – Eur. J., 2010, 16, 11926–11941.
31. (a) K. Katoh, H. Isshiki, T. Komeda and M. Yamashita, Coord. Chem. Rev., 2011, 255, 2124–2148; (b) K. Katoh, Y. Horii, N. Yasuda, W. Wernsdorfer, K. Toriumi, B. K. Breedlove and M. Yamashita, Dalton Trans., 2012, 41, 13582–13600; (c) Y. Wang, X.-L. Li, T.-W. Wang, Y. Song and X.-Z. You, Inorg. Chem., 2010, 49, 969–976.
32. F. Habib and M. Murugesu, Chem. Soc. Rev., 2013, 42, 3278–3288.
33. (a) J. D. Rinehart1, M. Fang, W. J. Evans and J. R. Long, Nat. Chem., 2011, 3, 538–542; (b) J. D. Rinehart, M. Fang, W. J. Evans and J. R. Long, J. Am. Chem. Soc., 2011, 133, 14236–14239.
34. P.-F. Shi, Y.-Z. Zheng, X.-Q. Zhao, G. Xiong, B. Zhao, F.-F. Wan and P. Cheng, Chem. – Eur. J., 2012, 18, 15086–15091.
35. (a) J. J. Le Roy, M. Jeletic, S. I. Gorelsky, I. Korobkov, L. Ungur, L. F. Chibotaru and M. Murugesu, J. Am. Chem. Soc., 2013, 135, 3502–3510; (b) M. Ren, D. Pinkowicz, M. Yoon, K. Kim, L.-M. Zheng, B. K. Breedlove and M. Yamashita, Inorg. Chem., 2013, 52, 8342–8348; (c) F. Habib, J. Long, P.-H. Lin, I. Korobkov, L. Ungur, W. Wernsdorfer, L. F. Chibotaru and M. Murugesu, Chem. Sci., 2012, 3, 2158–2164; (d) S. K. Langley, N. F. Chilton, L. Ungur, B. Moubaraki, L. F. Chibotaru and K. S. Murray, Inorg. Chem., 2012, 51, 11873–11881.
36. (a) K. R. Meihaus, J. D. Rinehart and J. R. Long, Inorg. Chem., 2011, 50, 8484–8489; (b) L. Zhang, P. Zhang, L. Zhao, S. Y. Lin, S. Xue, J. Tang and Z. Liu, Eur. J. Inorg. Chem., 2013, 1351–1357.
37. (a) N. Ishikawa, M. Sugita, T. Ishikawa, S. Y. Koshihara and Y. Kaizu, J. Am. Chem. Soc., 2003, 125, 8694–8695; (b) G. J. Chen, Y. N. Guo, J. L. Tian, J. Tang, W. Gu, X. Liu, S. P. Yan, P. Cheng and D. Z. Liao, Chem. – Eur. J., 2012, 18, 2484–2487.

---

Footnote

† Electronic supplementary information (ESI) available: Selected bond lengths and angles, PXRD, TGA, VT-PXRD, IR and other magnetic measurements. CCDC 948361–948364. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4qi00116h

---