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A new family of interdimer [MnII2LnIII2]2 clusters: Syntheses, structures, and magnetic properties
Inorganic Chemistry Communications 70 (2016) 132–135
Contents lists available at ScienceDirect
Inorganic Chemistry Communications journal homepage: www.elsevier.com/locate/inoche
Short communication
A new family of interdimer [MnII2LnIII2]2 clusters: Syntheses, structures, and magnetic properties Lei Sun a,b, Hui Chen a, Chengbing Ma a, Changneng Chen a,⁎ a b
State Key Laboratory of Structural, Fujian Institute of Research in the Structure of Matter, Chinese Academy of Science, Fuzhou 350002, China University of Chinese Academy of Sciences, Beijing 100039, China
a r t i c l e
i n f o
Article history: Received 25 April 2016 Received in revised form 2 June 2016 Accepted 4 June 2016 Available online 5 June 2016 Keywords: Mn/Ln clusters Magnetic caloric effect Single molecule magnets
a b s t r a c t A new family of isostructural Mn/Ln clusters: [MnII2LnIII2(hmp)6(NO3)4(CH3OH)2][MnII2LnIII2(hmp)6(NO3)4(H2O)2] ([Ln = Gd (1), Tb(2), Dy(3), hmpH = 2-(hydroxymethyl)pyridine]) have been synthesized by the reaction of Mn(NO3)2 and Ln(NO3)3·6H2O with hmpH as ligand under solvothermal conditions. Compounds 1–3 are isostructural and possess butterfly core. That the antiferromagnetic interactions within compounds 1–3 were suggested by soild-state dc magnetic susceptibility analyses. Compound 1 displays a magnetic-caloric effect (MCE) with 21.91 J Kg−1 K−1 of the entropy change at 5 K for ΔH = 8 T. Compounds 2 and 3 exhibit frequency-dependent ac susceptibility signals suggestive of slow magnetic relaxation. © 2016 Published by Elsevier B.V.
In last decade, polynuclear 3d–4f clusters as molecule based magnets have drawn intense attention for their versatile magnetic properties which can be utilized for information storage and cryogenic technology [1], especially the 3d–4f single-molecule-magnets (SMMs) have been investigated as a focus research. The SMMs are individual molecules whose magnetic properties arise from a high energy barrier to reversal of the magnetisation leading to slow magnetization relaxation below a certain blocking temperature [2]. The materials with high ground spin state and large magnetic anisotropy are considered to be potential SMMs. Compared with traditional 3d clusters which we have mainly explored, mixing the transition metal and lanthanide ion in different proportions may modulate magnetic properties as lanthanide ion often has large ground-state spin and significant single-ion anisotropy. On the other hand, magnetic cooler materials based on the magnetic-caloric effect (MCE) have emerged as attractive candidates to replace the rare and expensive [3]. He in some ultralow temperature region [3]. The MCE is associated with the change of magnetic entropy upon variation of the magnetic field, which could be applied to cooling technique via adiabatic demagnetization [4]. It is critical to possess a large spin ground state and negligible anisotropy for an ideal molecule refrigerant. Thus, heterometallic clusters with high spin MnII (d5), GdIII (f7) ions and small ligands could be a good candidate for magnetic cooler materials. For molecule-based heterometallic magnets behaving as SMMs or magnetic refrigerants, an appropriate ligand to coordinate the 3d and 4f metal ions is important. The 2-(hydroxymethyl)pyridine (hmpH) is bridging ligand, possessing a N/O bidentate chelate which could facilitate the coordination affinities of Mn and Ln metal ions, and it has ⁎ Corresponding author. E-mail address: [email protected] (C. Chen).
http://dx.doi.org/10.1016/j.inoche.2016.06.005 1387-7003/© 2016 Published by Elsevier B.V.
been employed for the synthesis of high-nuclearity Mn clusters, such as Mn10, Mn12, Mn18, Mn21 [5]. However, only few examples of Mn/Ln clusters used hmpH as ligand have been reported to date with the four family clusters MnIII2LnIII2, MnIII2LnIII4, MnIII8LnIII4 and MnIII4CeIII2 [6]. Besides, few Mn/Ln clusters reported were yielded by the means of solvothermal reaction [7], though the metal-organic frameworks always were synthesized by this method. In the present work, solvothermal reactions of Mn(NO3)2 with hmpH, NEt3 and Ln(NO3)3·6H2O (Ln = Gd; Tb; Dy) in MeOH afforded a family of [MnII2LnIII2]2 clusters, the NEt3 is assumed to be the proton acceptor. Different reaction conditions have been explored, such as diverse solvents and reaction temperatures. The temperatures of reactions like 90 °C, 100 °C, 110 °C, 120 °C have been explored, and in the temperature of 120 °C products with the highest yield and best quality were obtained. Using benzoic acid, propionic acid and sodium formate as the ancillary ligands yielded the same crystallized products in poor quality. It is considered that the reaction system with [Mn2Ln2]2 is quite stable. Reactions with lighter Lanthanide ions (like La, Ce, Pr and Nd) and heavier lanthanide ions (like Er, Yb and Lu) afforded no crystallized products. We assumed that the radius of lanthanide ions affect the formation of compounds. Otherwise, it should be noted that the other solvents instead of MeOH were used, such as MeCN, while no crystallized products were yielded. Compounds 1–3 are isostructural, therefore the structure of compound 1 will be described in detail as representative of this series. The partially labeled structure and core of compound 1 are shown in Fig. 1. Compound 1 crystallizes in the triclinic space group P-1, and the asymmetric unit contains two similar structure units of [Mn2Gd2] with a little difference in the peripheral ligands. Each unit of Mn2Gd2 can be described as two face-sharing defected cubane units of two Mn2Gd(μ3-
L. Sun et al. / Inorganic Chemistry Communications 70 (2016) 132–135
133
Fig. 1. (Top) Partially labeled structure of compound 1. (Bottom) The core of compound 1. Color scheme: Gd, green; Mn, teal; O, red; N, blue. Hydrogen atoms and crystallization solvents have been omitted for clarity.
OR)2(μ2-OR)2, the two Mn2Gd units are bridged by two μ3-O atoms which are of two η3:η1:μ3 hmp− groups. It should be noted that this is the first μ3-O of hmp− group bridged MnII and LnIII atoms. In the core the metal atoms also held together by four μ2-O atoms of four η2:η1:μ2 hmp− groups. The structure of core can also be described as butterfly subunit with the Mn atoms at the body positions and the Gd atoms at the ring positions, which is common in M4 clusters [8]. The peripheral ligation is completed by two chelating η1:η1:μ1 NO− 3 ligands and two η1:μ1 H2O or MeOH terminal groups. The oxidation states of Mn atoms and the protonation level of O atoms were established by the metric parameters and charge balance consideration, which confirmed by bond valence sum (BVS) [9] calculations (Table S3). All four MnII atoms are six coordinate, and the Mn–O distances range from 1.999 to 2.271 Å. The Gd atom is nine-coordinate and can be described as a capped square antiprism geometry using SHAPE-analysis [10] which is formed by four oxygen atoms from two NO− 3 counterions, two bridging μ2-O from two η2:η1:μ2 hmp− ligands, one nitrogen atom and one oxygen atom from a η3:η1:μ3 hmp− group, one oxygen atom of η1:μ1 H2O or MeOH group. The Gd–O bond distances range from 2.323 to 2.576 Å and the distances of Gd–N bonds are 2.552 and 2.579 Å. The solid-state, variable-temperature dc magnetic susceptibility data of compounds 1–3 were measured in the 2–300 K temperature range with an applied field of 0.1 T. Plots of χMT versus T for compounds 1–3 are depicted in Fig. 2. The χMT versus T plots of compounds 1 and 2 show similar trends. For both compounds, the χMT product is nearly independent of the temperature within the span of 300 K to 48 K,
indicating very weak magnetic interactions. Upon lowering the temperature, it undergoes a slightly decrease, which display paramagnetic states. For compound 3, the χMT value decreases slowly upon lowering the temperature until approximately 10 K, and then rapidly decreases.
Fig. 2. The χMT vs. T plots for compounds 1–3 in a 0.1 T dc field.
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L. Sun et al. / Inorganic Chemistry Communications 70 (2016) 132–135
Fig. 3. (Top) Field-dependent magnetization at indicated temperature (2–7 K) for compound 1. (Bottom) temperature dependence of magnetic entropy change of compound 1.
The overall shape of χMT versus T curves for compounds 1–3 suggest weak and antiferromagnetic interactions. The negligible anisotropy of GdIII ion urged us to explore the MCE of compound 1. Magnetic susceptibilities of compound 1 were collected at 2–7 K in the range of 1–8 T (Fig. 3). As the MCE can be described as the Maxwell equation − ΔSm = ∫[∂M(T,H)/∂T]HdH, the maximum value of − ΔSm is 21.91 J Kg—1 K—1 at 5 K for ΔH = 8 T (Fig. 3). The value is lower than the theoretical maximum entropy of 42.22 J Kg—1 K—1 calculated according to the equation nRln(2S + 1) for four isolated MnII ions and four isolated GdIII ions. The value of − ΔSm is larger than some Mn/ Ln clusters [11], but smaller than most of the magnetic cryocooling materials ranging from 28–60.3 J Kg−1 K−1 [11]. In consideration of the SMMs research based on Tb/Dy compounds, the ac susceptibility measurements were performed on compounds 2 and 3 in the temperature range of 2.0 K to 10.0 K with a 3.0 G ac field oscillating in the scope of 311 Hz–3511 Hz and a zero direct current field.
Fig. 4. Temperature dependence of the out-of-phase (χM″) signals of the ac susceptibility under different frequency (Hz) for compounds 2 (a) and 3 (c). Plots of ln(χ″/χ′) versus T−1 for compounds 2 (b) and 3 (d), the solid lines represent the fitting in the range of 2.0–3.4 K.
L. Sun et al. / Inorganic Chemistry Communications 70 (2016) 132–135
Compounds 2 and 3 showed frequency-dependent decrease in χM′ T indicating slow magnetic relaxation, and the out-of-phase χM″ signals displayed concomitant appearance thus indicating probably SMM behaviour. However, the peak of out-of-phase signals were not observed above 2 K for compounds 2 and 3 under an applied 500 Oe dc field. So, the effective energy barrier and the magnetization relaxation time could not be determined by fitting cusps of out-of-phase susceptibility data to the Arrhenius expression. Assumed that there exist only one relaxation process in compounds 2 and 3 [11], the values of energy barrier and relaxation time can be obtained from fitting the ac susceptibility data with the Debye relationship ln(χ″/χ′) = ln(ωτ0) + Ea / KBT, giving Ea = 3.84 / 3.86 K and τ0 = 6.43 × 10−7 / 1.24 × 10−6 s respectively (Fig. 4). This calculated method has been used in many compounds [12]. Although the value of characteristic parameters are not enough accurate, the relaxation time are in the range of 10−6–10−11 s for an SMM. In summary, new examples of 3d/4f interdimer clusters have been yielded using simple metal salts by solvothermal method. The solvothermal method provides a promising alternative route to synthesize heterometallic Mn/Ln clusters. Weak antiferromagnetic interactions within the three compounds were indicated by solid-state dc magnetic susceptibility analyses. Compound 1 displays an MCE with 21.91 J Kg−1 K−1 of the entropy change at 5 K for ΔH = 8 T. Ac magnetic susceptibility investigations showed frequency-dependent out-ofphase χM″ signals, indicating slow magnetic relaxation and potential SMM behaviour for compounds 2 and 3.
(c) (d) (e) [2] (a)
[3] [4]
[5]
[6]
[7] [8]
Acknowledgements This work was supported by the National Natural Science Foundation of China (Nos. 21173219, 21303201 and 21203195).
[9] [10]
Appendix A. Supplementary data
[11]
Procedures for ligand and compound synthesis, X-ray structure determinations, selected bond lengths and bond angles for compounds 1–3 (CCDC 1444888, 1444889, 1444890). Supplementary data associated with this article can be found in the online version, at doi 10.1016/ j.inoche.2016.06.005.
[12]
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Contents lists available at ScienceDirect
Inorganic Chemistry Communications journal homepage: www.elsevier.com/locate/inoche
Short communication
A new family of interdimer [MnII2LnIII2]2 clusters: Syntheses, structures, and magnetic properties Lei Sun a,b, Hui Chen a, Chengbing Ma a, Changneng Chen a,⁎ a b
State Key Laboratory of Structural, Fujian Institute of Research in the Structure of Matter, Chinese Academy of Science, Fuzhou 350002, China University of Chinese Academy of Sciences, Beijing 100039, China
a r t i c l e
i n f o
Article history: Received 25 April 2016 Received in revised form 2 June 2016 Accepted 4 June 2016 Available online 5 June 2016 Keywords: Mn/Ln clusters Magnetic caloric effect Single molecule magnets
a b s t r a c t A new family of isostructural Mn/Ln clusters: [MnII2LnIII2(hmp)6(NO3)4(CH3OH)2][MnII2LnIII2(hmp)6(NO3)4(H2O)2] ([Ln = Gd (1), Tb(2), Dy(3), hmpH = 2-(hydroxymethyl)pyridine]) have been synthesized by the reaction of Mn(NO3)2 and Ln(NO3)3·6H2O with hmpH as ligand under solvothermal conditions. Compounds 1–3 are isostructural and possess butterfly core. That the antiferromagnetic interactions within compounds 1–3 were suggested by soild-state dc magnetic susceptibility analyses. Compound 1 displays a magnetic-caloric effect (MCE) with 21.91 J Kg−1 K−1 of the entropy change at 5 K for ΔH = 8 T. Compounds 2 and 3 exhibit frequency-dependent ac susceptibility signals suggestive of slow magnetic relaxation. © 2016 Published by Elsevier B.V.
In last decade, polynuclear 3d–4f clusters as molecule based magnets have drawn intense attention for their versatile magnetic properties which can be utilized for information storage and cryogenic technology [1], especially the 3d–4f single-molecule-magnets (SMMs) have been investigated as a focus research. The SMMs are individual molecules whose magnetic properties arise from a high energy barrier to reversal of the magnetisation leading to slow magnetization relaxation below a certain blocking temperature [2]. The materials with high ground spin state and large magnetic anisotropy are considered to be potential SMMs. Compared with traditional 3d clusters which we have mainly explored, mixing the transition metal and lanthanide ion in different proportions may modulate magnetic properties as lanthanide ion often has large ground-state spin and significant single-ion anisotropy. On the other hand, magnetic cooler materials based on the magnetic-caloric effect (MCE) have emerged as attractive candidates to replace the rare and expensive [3]. He in some ultralow temperature region [3]. The MCE is associated with the change of magnetic entropy upon variation of the magnetic field, which could be applied to cooling technique via adiabatic demagnetization [4]. It is critical to possess a large spin ground state and negligible anisotropy for an ideal molecule refrigerant. Thus, heterometallic clusters with high spin MnII (d5), GdIII (f7) ions and small ligands could be a good candidate for magnetic cooler materials. For molecule-based heterometallic magnets behaving as SMMs or magnetic refrigerants, an appropriate ligand to coordinate the 3d and 4f metal ions is important. The 2-(hydroxymethyl)pyridine (hmpH) is bridging ligand, possessing a N/O bidentate chelate which could facilitate the coordination affinities of Mn and Ln metal ions, and it has ⁎ Corresponding author. E-mail address: [email protected] (C. Chen).
http://dx.doi.org/10.1016/j.inoche.2016.06.005 1387-7003/© 2016 Published by Elsevier B.V.
been employed for the synthesis of high-nuclearity Mn clusters, such as Mn10, Mn12, Mn18, Mn21 [5]. However, only few examples of Mn/Ln clusters used hmpH as ligand have been reported to date with the four family clusters MnIII2LnIII2, MnIII2LnIII4, MnIII8LnIII4 and MnIII4CeIII2 [6]. Besides, few Mn/Ln clusters reported were yielded by the means of solvothermal reaction [7], though the metal-organic frameworks always were synthesized by this method. In the present work, solvothermal reactions of Mn(NO3)2 with hmpH, NEt3 and Ln(NO3)3·6H2O (Ln = Gd; Tb; Dy) in MeOH afforded a family of [MnII2LnIII2]2 clusters, the NEt3 is assumed to be the proton acceptor. Different reaction conditions have been explored, such as diverse solvents and reaction temperatures. The temperatures of reactions like 90 °C, 100 °C, 110 °C, 120 °C have been explored, and in the temperature of 120 °C products with the highest yield and best quality were obtained. Using benzoic acid, propionic acid and sodium formate as the ancillary ligands yielded the same crystallized products in poor quality. It is considered that the reaction system with [Mn2Ln2]2 is quite stable. Reactions with lighter Lanthanide ions (like La, Ce, Pr and Nd) and heavier lanthanide ions (like Er, Yb and Lu) afforded no crystallized products. We assumed that the radius of lanthanide ions affect the formation of compounds. Otherwise, it should be noted that the other solvents instead of MeOH were used, such as MeCN, while no crystallized products were yielded. Compounds 1–3 are isostructural, therefore the structure of compound 1 will be described in detail as representative of this series. The partially labeled structure and core of compound 1 are shown in Fig. 1. Compound 1 crystallizes in the triclinic space group P-1, and the asymmetric unit contains two similar structure units of [Mn2Gd2] with a little difference in the peripheral ligands. Each unit of Mn2Gd2 can be described as two face-sharing defected cubane units of two Mn2Gd(μ3-
L. Sun et al. / Inorganic Chemistry Communications 70 (2016) 132–135
133
Fig. 1. (Top) Partially labeled structure of compound 1. (Bottom) The core of compound 1. Color scheme: Gd, green; Mn, teal; O, red; N, blue. Hydrogen atoms and crystallization solvents have been omitted for clarity.
OR)2(μ2-OR)2, the two Mn2Gd units are bridged by two μ3-O atoms which are of two η3:η1:μ3 hmp− groups. It should be noted that this is the first μ3-O of hmp− group bridged MnII and LnIII atoms. In the core the metal atoms also held together by four μ2-O atoms of four η2:η1:μ2 hmp− groups. The structure of core can also be described as butterfly subunit with the Mn atoms at the body positions and the Gd atoms at the ring positions, which is common in M4 clusters [8]. The peripheral ligation is completed by two chelating η1:η1:μ1 NO− 3 ligands and two η1:μ1 H2O or MeOH terminal groups. The oxidation states of Mn atoms and the protonation level of O atoms were established by the metric parameters and charge balance consideration, which confirmed by bond valence sum (BVS) [9] calculations (Table S3). All four MnII atoms are six coordinate, and the Mn–O distances range from 1.999 to 2.271 Å. The Gd atom is nine-coordinate and can be described as a capped square antiprism geometry using SHAPE-analysis [10] which is formed by four oxygen atoms from two NO− 3 counterions, two bridging μ2-O from two η2:η1:μ2 hmp− ligands, one nitrogen atom and one oxygen atom from a η3:η1:μ3 hmp− group, one oxygen atom of η1:μ1 H2O or MeOH group. The Gd–O bond distances range from 2.323 to 2.576 Å and the distances of Gd–N bonds are 2.552 and 2.579 Å. The solid-state, variable-temperature dc magnetic susceptibility data of compounds 1–3 were measured in the 2–300 K temperature range with an applied field of 0.1 T. Plots of χMT versus T for compounds 1–3 are depicted in Fig. 2. The χMT versus T plots of compounds 1 and 2 show similar trends. For both compounds, the χMT product is nearly independent of the temperature within the span of 300 K to 48 K,
indicating very weak magnetic interactions. Upon lowering the temperature, it undergoes a slightly decrease, which display paramagnetic states. For compound 3, the χMT value decreases slowly upon lowering the temperature until approximately 10 K, and then rapidly decreases.
Fig. 2. The χMT vs. T plots for compounds 1–3 in a 0.1 T dc field.
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L. Sun et al. / Inorganic Chemistry Communications 70 (2016) 132–135
Fig. 3. (Top) Field-dependent magnetization at indicated temperature (2–7 K) for compound 1. (Bottom) temperature dependence of magnetic entropy change of compound 1.
The overall shape of χMT versus T curves for compounds 1–3 suggest weak and antiferromagnetic interactions. The negligible anisotropy of GdIII ion urged us to explore the MCE of compound 1. Magnetic susceptibilities of compound 1 were collected at 2–7 K in the range of 1–8 T (Fig. 3). As the MCE can be described as the Maxwell equation − ΔSm = ∫[∂M(T,H)/∂T]HdH, the maximum value of − ΔSm is 21.91 J Kg—1 K—1 at 5 K for ΔH = 8 T (Fig. 3). The value is lower than the theoretical maximum entropy of 42.22 J Kg—1 K—1 calculated according to the equation nRln(2S + 1) for four isolated MnII ions and four isolated GdIII ions. The value of − ΔSm is larger than some Mn/ Ln clusters [11], but smaller than most of the magnetic cryocooling materials ranging from 28–60.3 J Kg−1 K−1 [11]. In consideration of the SMMs research based on Tb/Dy compounds, the ac susceptibility measurements were performed on compounds 2 and 3 in the temperature range of 2.0 K to 10.0 K with a 3.0 G ac field oscillating in the scope of 311 Hz–3511 Hz and a zero direct current field.
Fig. 4. Temperature dependence of the out-of-phase (χM″) signals of the ac susceptibility under different frequency (Hz) for compounds 2 (a) and 3 (c). Plots of ln(χ″/χ′) versus T−1 for compounds 2 (b) and 3 (d), the solid lines represent the fitting in the range of 2.0–3.4 K.
L. Sun et al. / Inorganic Chemistry Communications 70 (2016) 132–135
Compounds 2 and 3 showed frequency-dependent decrease in χM′ T indicating slow magnetic relaxation, and the out-of-phase χM″ signals displayed concomitant appearance thus indicating probably SMM behaviour. However, the peak of out-of-phase signals were not observed above 2 K for compounds 2 and 3 under an applied 500 Oe dc field. So, the effective energy barrier and the magnetization relaxation time could not be determined by fitting cusps of out-of-phase susceptibility data to the Arrhenius expression. Assumed that there exist only one relaxation process in compounds 2 and 3 [11], the values of energy barrier and relaxation time can be obtained from fitting the ac susceptibility data with the Debye relationship ln(χ″/χ′) = ln(ωτ0) + Ea / KBT, giving Ea = 3.84 / 3.86 K and τ0 = 6.43 × 10−7 / 1.24 × 10−6 s respectively (Fig. 4). This calculated method has been used in many compounds [12]. Although the value of characteristic parameters are not enough accurate, the relaxation time are in the range of 10−6–10−11 s for an SMM. In summary, new examples of 3d/4f interdimer clusters have been yielded using simple metal salts by solvothermal method. The solvothermal method provides a promising alternative route to synthesize heterometallic Mn/Ln clusters. Weak antiferromagnetic interactions within the three compounds were indicated by solid-state dc magnetic susceptibility analyses. Compound 1 displays an MCE with 21.91 J Kg−1 K−1 of the entropy change at 5 K for ΔH = 8 T. Ac magnetic susceptibility investigations showed frequency-dependent out-ofphase χM″ signals, indicating slow magnetic relaxation and potential SMM behaviour for compounds 2 and 3.
(c) (d) (e) [2] (a)
[3] [4]
[5]
[6]
[7] [8]
Acknowledgements This work was supported by the National Natural Science Foundation of China (Nos. 21173219, 21303201 and 21203195).
[9] [10]
Appendix A. Supplementary data
[11]
Procedures for ligand and compound synthesis, X-ray structure determinations, selected bond lengths and bond angles for compounds 1–3 (CCDC 1444888, 1444889, 1444890). Supplementary data associated with this article can be found in the online version, at doi 10.1016/ j.inoche.2016.06.005.
[12]
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