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Field-induced single molecule magnet behavior of a three-dimensional Dy(III)-based complex
Journal Pre-proofs Field-induced single molecule magnet behavior of a three-dimensional Dy(III)-based complex Shixiong She, Xingye Gu, Yan Yang PII: DOI: Reference:
S1387-7003(19)30821-4 https://doi.org/10.1016/j.inoche.2019.107584 INOCHE 107584
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Inorganic Chemistry Communications
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12 August 2019 13 September 2019 14 September 2019
Please cite this article as: S. She, X. Gu, Y. Yang, Field-induced single molecule magnet behavior of a threedimensional Dy(III)-based complex, Inorganic Chemistry Communications (2019), doi: https://doi.org/10.1016/ j.inoche.2019.107584
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Field-induced single molecule magnet behavior of a three-dimensional Dy(III)-based complex Shixiong She*, Xingye Gu, Yan Yang College of Chemical Engineering, Qinghai University, Xining 810016, P. R. China ABSTRACT A novel three-dimensional Dy(III)-based complex [Dy(H2-DHBDC)0.5(DHBDC)0.5(C2H5OH)(H2O)]n·H2O (H4-DHBDC=2,5-dihydroxy-1,4-terephthalic acid) (1) was fabricated using a hydrothermal method. Such complex was examined by single-crystal X-ray diffraction analysis and then characterized by elemental analysis, IR spectroscopy, thermogravimetric analysis, powder X-ray diffraction as well as magnetic properties. The results of single-crystal structure analysis suggested that complex 1 consisted of three-dimensional framework structure and crystallized in the monoclinic system, space group P21/c. The DyIII ion in 1 adopted eight-coordination mode with a triangular dodecahedron. The results of magnetic property investigations suggested that the coordination polymer exhibited slow magnetic relaxation behaviors under 1500 Oe dc field. Furthermore, the frequency dependence of out-of phase (χ″) ac signals for complex 1 displayed two maxima at low temperature, indicating the presence of two relaxation processes. Keywords: Single-molecule magnet; Dysprosium; Crystal structure; Relaxation behavior; 3D coordination polymer Since their discovery in the 1990’s, single-molecule magnets (SMMs) have aroused rising attention in chemistry and materials fields for not only their intriguing architectures and functional modification, but also their potential applications in the field of quantum computer, molecular spintronic, magnetic switching and ultrahigh-density information storage at the molecule level[1-4]. Among the recently investigated results, Dy(III)-based single-molecule magnets (DyIII-SMMs) is noteworthy for their significantly high magnetic anisotropy of the spin-orbital coupled Kramer’s doublet ground state[5-10]. Great efforts were made to optimize the effective energy barrier (Ueff) and up-regulate blocking temperature (TB). In 2016, the group of Zheng et al. reported a monometallic dysprosium complex [Dy(OtBu)2(py)5][BPh4] with the anisotropy barrier Ueff =1815 K[11]. In 2017, Tong groups achieved a landmark finding with a complex [(CpiPr5)Dy(Cp*)][B(C6F5)4], displaying a hysteresis opening temperature above liquid nitrogen temperature[12]. Nevertheless, since these organometallic complexes are on the whole air-sensitive or water-unstable, their real applications are fully limited. Besides, the values of Ueff and TB for most Dy-SMMs remain unsatisfactory. Furthermore, among these DyIII-SMMs, only a small part displays 3D metal-organic frameworks (MOFs)[13-17]. Despite this, DyIII-MOFs has aroused more attention in recent years for its interesting structures and unique properties. In DyIII-MOFs, it is likely to *Corresponding author Tel:+86-971-6334202; E-mail: [email protected]
fine-tune the magnetic properties by reducing QTM or regulating and building the coordination environment. In general, the rational design of 1D, 2D and/or 3D complexes with locally large magnetic anisotropy and strong intrachain magnetic interactions is one of the noticeable challenges in the SMM field[18-23]. Besides, except for the magnetic anisotropy and intrachain couplings of DyIII ions, many factors (e.g., the strength of crystal-field, the coordination environment and the symmetry of the coordination geometry of DyIII) may affect the relaxation behaviors as well [24-31]. Given this, a novel three-dimensional Dy(III)-based complex with better symmetry and eight-coordination mode [Dy(H2-DHBDC)0.5(DHBDC)0.5(C2H5OH)(H2O)]n·H2O (H4-DHBDC=2,5-dihydroxy-1,4-terephthalic acid) is reported in this paper[32]. The results of magnetic property investigations indicated that the coordination polymer exhibited slow magnetic relaxation behaviors under 1500 Oe dc field with an effect energy of 36.11 K. Pale yellow crystals of 1 were fabricated through the reaction of Dy(NO3)3·6H2O with 2,5-dihydroxy-1,4-terephthalic acid and NaOH in the solvent containing EtOH and H2O. Crystallographic data collection and refinement parameters for complex 1 were listed in Table S1 (Supporting Information). Other parameters (selected bond lengths and selected bong angles) for the new complex are listed in Table S2 (Supporting Information). The results of single-crystal X-ray diffraction analysis demonstrated that complex 1 consisted of a three-dimensional framework structure and crystallized in the monoclinic system, space group P21/c with Z=4. Fig.1a shows that the minimal asymmetric structural unit contained one DyIII ion, half H2-DHBDC2- ligand, half DHBDC4- ligand, one EtOH and a H2O molecular. The DyIII ion was eight-coordinated by five carboxyl oxygen atoms (O2A, O3, O5C, O4B and O5B) and one phenoxide oxygen atom (O6) from four ligands, one oxygen atom (O7) from the EtOH molecular and the last oxygen atom (O8) from H2O molecular. The coordinated geometry of DyIII for complex 1 was calculated using SHAPE 2.1 software, indicating that the DyIII ion was located in triangular dodecahedron coordinating environments (TDD-8, D2d, Fig.1b) since the minimum value for the CShM parameter was 1.8116 (Table S3-S4). The distances of Dy-O bands were 2.368 Å on average, ranging from 2.241(9)-2.591(10) Å, which are consistent with the previously reported DyIII-MOFs. The angles for O-Dy-O were in the range of 51.9(3)°-150.5(4)°. The organic ligand, as an oxygen donor, is a good choice for constructing Dy-MOFs. In the complex, because there were two forms of proton loss in organic ligands, i.e. H2-DHBDC2- and DHBDC4-, the ligand acted as a μ4 bridge in two coordination modes. The one coordination mode only adopting two carboxylate groups showed two bidentate bridging coordination modes μ2-η1:η1, and the other adopted two carboxylate groups and one phenoxide group, in which the former showed μ2-η1:η2 mode and acted as monoatomic oxygen bridge between DyIII ions. Accordingly, the neighboring DyIII ions were doubly bridged by one syn, syn-carboxylate group and syn-anti-carboxylate group, respectively, forming a one-dimensional chain structure (Fig.1c) with the nearest distance of DyIII·∙∙DyIII in the 1D chain being 4.6007Å, which was longer than other reported complexes. Since the magnetic dilution could be achieved through large linkers and avoid QTM, it may be beneficial for the compound to exhibit the SMM behavior. Furthermore, given the centrosymmetric nature of the ligand, the 1D chains were then joined by means of two H2-DHBDC2- and DHBDC4- ligands, displaying a 3D framework (Fig.1d).
(a)
(c)
(b)
(d)
Fig.1 (a) Molecular structure of complex 1, all of the hydrogen atoms and solvent molecules were omitted for clarity. (b) The coordination polyhedron of the DyIII ion in complex 1. (c) 1D structure of complex 1. (d). 3D framework of complex 1.
The PXRD patterns of phase purity for complex 1 were recorded (Fig. S1). The diffraction curves are similar to the simulated patterns, revealing that the obtained compound exhibited strongly crystallinity, and there existed no impurity crystal phase. Furthermore, the thermal stability of complex 1 was tested on a temperature gradient from ambient temperature to 1450 °C at a heating rate of 5 K/min in a nitrogen atmosphere. As shown in the TG curve (Fig. S2), the complex had removed H2O and EtOH molecular before 170°C and then solvent-free phase was stabilized to 200°C. Lastly, it began to lose considerable weight, due to the breakage of the coordination bond of the complex and the decomposition of the host skeleton. The variable-temperature direct-current (dc) susceptibility of complex 1 was measured under an external magnetic field of 1000 Oe at the temperature from 2 to 300 K, with the plot of χm T versus T for complex 1 shown in Fig. 2. At ambient temperature the χm T value is 13.58 cm3 mol K-1, which is slightly lower than the theoretical value of 14.17 cm3 mol K-1 for one non-coupled DyIII (S=5/2, g=4/3, 6H15/2, C = 14.17 cm3 mol K-1)[33, 34]. Upon cooling, the values of complex 1 remained almost constant at temperature 100-300 K. Subsequently,
as the temperature further fell to about 50 K, the χm T values gradually decreased and then sharply dropped to a minimum of 10.48 cm3 K mol-1 at 2.0 K. This behavior might be attributed to the crystal field effect, especially the thermal depopulation of the Stark sublevels of DyIII in complex 1 and/or possible weak magnetic interactions between DyIII ions. Besides, the magnetization (M) versus H for complex 1 were also collected in the temperature range of 2.0-8.0 K. Fig. 3 suggests that at each temperature, the value of M increased monotonically with the rise in the value of H. Different temperatures dispaly analogous curved forms, and the final maximum values of H were about 5.48 μB for each temperature, lower than theoretical saturated value of 10 μB for one non-coupled DyIII at 6H [35, 36]. Besides, the magnetic induction intensity of complex 1 under different 15/2 states field strength conditions was tested at the temperature from 2 to 8 K (Fig. S4). The test results revealed that the M vs H/T curves did not overlap, whereas they were stepped. In brief, these phenomenon revealed that the existence of magnetic anisotropy and/or low-lying excited states[11-16, 37].
Fig.2 Tempurature dependence of χm T for complex 1.
Fig. 3 Plots of M versus H for complex 1 at 2.0, 3.0, 4.0, 5.0 and 8 K, respectively.
To investigate the magnetic relaxation behavior of complex 1, alternating current (ac) susceptibility measurements were performed under a zero dc field, however, no out-of-phase (χ″) component was observed above 2 K (Fig. S5), which indicates that the magnetic interaction has quenched the tunneling [38]. A sweep test was then performed for complex 1 in the dc field range of 0~1 T and an optimum external dc field of 1500 Oe was found (Fig. S6). Thus, the ac susceptibilities of complex 1 were further measured under 1500 Oe dc field at 2-10 K within the frequency range of 1-999 Hz. Fortunately, both the in-phase (χ') and out-of phase (χ") signals of complex 1 exhibit temperature and frequency dependence (Fig. 4 and Fig. S6). As shown in Fig. 4, the signal values of χ' and χ" constantly decreased in the low temperature region when the frequency increased; and note that the χ' ac magnetic susceptibility has no peak, whereas the χ" signals were observed in high temperature zone. In the meantime, the frequency dependence of out-of phase (χ″) ac signals for complex 1 displays two maxima at low temperature, indicating the presence of two relaxation processes, and suggesting that complex 1 exhibited field-induced SMM behavior [21-25, 39] (Fig. S7). These results reveal that the field-induced relaxation comes from the decoupling by field. Namely, the chain properties in magnetism have been destroyed and the single-ion
properties appeared[38]. The Cole-Cole plots of 1 show two-semicircular shape at the temperature from 2 to 10 K (Fig. 5 left), confirming the presence of two relaxation processes. These plots can be well fitted by the extended Debye functions. The fitting parameters are listed in Table S5. The values of α1 and α2 are in the range of 0.0035-0.52 and 0.026-0.39, respectively, indicating a broad distribution of relaxation time or the occurrence of multiple relaxation processes [39-42]. The plot of ln(τ) versus T−1 is shown in Fig. 5(right), revealing the strong linear dependence of ln(τ) versus T−1 in the high temperature region, which was a dominant Orbach relaxation process. In the high temperature region, the Arrhenius law (τ = τ0 exp(Ueff/kT) is employed to extract parameters τ0 (8.89×10−6 s) and energy barrier Ueff (36.11 K). Moreover, considering the fact that the plot shows an apparent curvature at low temperatures, other relaxation (e.g., Raman and direct process) may be also present. Therefore, the data in the whole temperature range were fitted by using the equation 1/τ = AT+B+CTn +τ0−1exp(Ueff/kT), in which the Raman, Orbach and direct processes were considered. The reliable fitting results were found to be as follows: Ueff = 23.87 K, τ0 = 6.86×10−5 s, A=59.86, B=0, C=3.034×10-6 s and n=9 [7-10,33,40].
Fig.4 Temperature dependence of the in-phase (χm′) and out-of-phase (χm″) ac susceptibility data under 1500Oe dc field in the frequency range of 1-999Hz for complex 1.
Fig. 5 The plots of cole-cole (left) and plot of ln τ versus 1/T (right) for complex 1. The solid lines represent the best fits for the corresponding data.
Some specific Dy(III)-based complexes reported in recent years were selected, in which all DyIII ions exhibited the same triangular dodecahedron coordination geometry as complex 1. These complexes including [DyIIINaI(L)(CH3CO2)2]n·2CH3CN(TDD-8, CShM =3.29)[39],
[DyCu3(H2edte)3(NO3)][NO3]2·0.5MeOH (TDD-8, CShM=2.214)[40], [Dy3(μ3-OCH3)2(μ-HL1)3(NO3)3]BPh4·4CH3OH·H2O (TDD-8, CShM=1.59)[41] and [43] [Dy(pmap)3(H2O)2]·0.375CH3COOC2H5 (TDD-8, CShM =1.441) with the value of Ueff are 13.21 K, 16.2 K, 49.21 K and 39.6 K, respectively. It is widely known that larger CShM values indicate larger ideal geometric deviations, further affecting the value of Ueff. According to the above examples, it was found that the value of Ueff was larger when the value of CShM was smaller and vice versa. For compound 1, its CShM value was significantly lower than the CShM value of the pre-two complexes with lower Ueff. Accordingly, the critical reason why a 1 had a relatively high Ueff value is that the coordination geometry of DyIII in complex 1 complies with the ideal geometry more closely. Indeed, other factors (e.g., the direction of the magnetic axis and the strength of crystal-field, the coordination environment and intrachain couplings of DyIII) may also affect the relaxation behaviors[23-28, 34-40]. In conclusion, a novel Dy(III)-based complex [Dy(H2-DHBDC)0.5(DHBDC)0.5(C2H5OH)(H2O)]n·H2O was fabricated with a three-dimensional framework structure; it crystallized in the monoclinic system, space group P21/c. Furthermore, complex 1 covered a Dyш mononuclear unit in which the Dyш atoms adopted eight-coordination mode, locating in triangular dodecahedron coordinating environments. Besides, the results of magnetic property investigations suggested that the coordination polymer exhibited slow magnetic relaxation behaviors under 1500Oe dc field with an effect energy barrier of 36.11 K. The frequency dependence of out-of phase (χ″) ac signals for 1 displays two maxima at low temperature, suggesting the presence of two relaxation processes and revealing the field-induced SMM behavior of complex 1. To study the magnetic anisotropy behavior of single-molecule magnets, our laboratory will use other organic ligands and mediators to fabricate novel Dy(III)-based complexes of the new 1D, 2D and 3D structures. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Nos. 21461021) and the Open Project of State Key Laboratory of Plateau Ecology and Agriculture, Qinghai University (No. 2019-ZZ-08). Appendix A. Supplementary data CCDC No. 1946633 contains the supplementary crystallographic data for this paper. The data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Supporting information contains the supplementary data and X-ray crystallographic files (CIF) for the complex. Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/. Reference [1] J. D. Rinehart, K. R. Meihaus, J. R. Long, Observation of a secondary slow relaxation process for the field-induced single-molecule magnet U(H2BPz2)3, J. Am. Chem. Soc. 132 (2010):7572–7573. [2] Y. J. Li, Y. L. Wang, Q. Y. Liu, The highly connected MOFs constructed from nonanuclear and trinuclear lanthanide-carboxylate clusters: selective gas adsorption and
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S1387-7003(19)30821-4 https://doi.org/10.1016/j.inoche.2019.107584 INOCHE 107584
To appear in:
Inorganic Chemistry Communications
Received Date: Revised Date: Accepted Date:
12 August 2019 13 September 2019 14 September 2019
Please cite this article as: S. She, X. Gu, Y. Yang, Field-induced single molecule magnet behavior of a threedimensional Dy(III)-based complex, Inorganic Chemistry Communications (2019), doi: https://doi.org/10.1016/ j.inoche.2019.107584
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Field-induced single molecule magnet behavior of a three-dimensional Dy(III)-based complex Shixiong She*, Xingye Gu, Yan Yang College of Chemical Engineering, Qinghai University, Xining 810016, P. R. China ABSTRACT A novel three-dimensional Dy(III)-based complex [Dy(H2-DHBDC)0.5(DHBDC)0.5(C2H5OH)(H2O)]n·H2O (H4-DHBDC=2,5-dihydroxy-1,4-terephthalic acid) (1) was fabricated using a hydrothermal method. Such complex was examined by single-crystal X-ray diffraction analysis and then characterized by elemental analysis, IR spectroscopy, thermogravimetric analysis, powder X-ray diffraction as well as magnetic properties. The results of single-crystal structure analysis suggested that complex 1 consisted of three-dimensional framework structure and crystallized in the monoclinic system, space group P21/c. The DyIII ion in 1 adopted eight-coordination mode with a triangular dodecahedron. The results of magnetic property investigations suggested that the coordination polymer exhibited slow magnetic relaxation behaviors under 1500 Oe dc field. Furthermore, the frequency dependence of out-of phase (χ″) ac signals for complex 1 displayed two maxima at low temperature, indicating the presence of two relaxation processes. Keywords: Single-molecule magnet; Dysprosium; Crystal structure; Relaxation behavior; 3D coordination polymer Since their discovery in the 1990’s, single-molecule magnets (SMMs) have aroused rising attention in chemistry and materials fields for not only their intriguing architectures and functional modification, but also their potential applications in the field of quantum computer, molecular spintronic, magnetic switching and ultrahigh-density information storage at the molecule level[1-4]. Among the recently investigated results, Dy(III)-based single-molecule magnets (DyIII-SMMs) is noteworthy for their significantly high magnetic anisotropy of the spin-orbital coupled Kramer’s doublet ground state[5-10]. Great efforts were made to optimize the effective energy barrier (Ueff) and up-regulate blocking temperature (TB). In 2016, the group of Zheng et al. reported a monometallic dysprosium complex [Dy(OtBu)2(py)5][BPh4] with the anisotropy barrier Ueff =1815 K[11]. In 2017, Tong groups achieved a landmark finding with a complex [(CpiPr5)Dy(Cp*)][B(C6F5)4], displaying a hysteresis opening temperature above liquid nitrogen temperature[12]. Nevertheless, since these organometallic complexes are on the whole air-sensitive or water-unstable, their real applications are fully limited. Besides, the values of Ueff and TB for most Dy-SMMs remain unsatisfactory. Furthermore, among these DyIII-SMMs, only a small part displays 3D metal-organic frameworks (MOFs)[13-17]. Despite this, DyIII-MOFs has aroused more attention in recent years for its interesting structures and unique properties. In DyIII-MOFs, it is likely to *Corresponding author Tel:+86-971-6334202; E-mail: [email protected]
fine-tune the magnetic properties by reducing QTM or regulating and building the coordination environment. In general, the rational design of 1D, 2D and/or 3D complexes with locally large magnetic anisotropy and strong intrachain magnetic interactions is one of the noticeable challenges in the SMM field[18-23]. Besides, except for the magnetic anisotropy and intrachain couplings of DyIII ions, many factors (e.g., the strength of crystal-field, the coordination environment and the symmetry of the coordination geometry of DyIII) may affect the relaxation behaviors as well [24-31]. Given this, a novel three-dimensional Dy(III)-based complex with better symmetry and eight-coordination mode [Dy(H2-DHBDC)0.5(DHBDC)0.5(C2H5OH)(H2O)]n·H2O (H4-DHBDC=2,5-dihydroxy-1,4-terephthalic acid) is reported in this paper[32]. The results of magnetic property investigations indicated that the coordination polymer exhibited slow magnetic relaxation behaviors under 1500 Oe dc field with an effect energy of 36.11 K. Pale yellow crystals of 1 were fabricated through the reaction of Dy(NO3)3·6H2O with 2,5-dihydroxy-1,4-terephthalic acid and NaOH in the solvent containing EtOH and H2O. Crystallographic data collection and refinement parameters for complex 1 were listed in Table S1 (Supporting Information). Other parameters (selected bond lengths and selected bong angles) for the new complex are listed in Table S2 (Supporting Information). The results of single-crystal X-ray diffraction analysis demonstrated that complex 1 consisted of a three-dimensional framework structure and crystallized in the monoclinic system, space group P21/c with Z=4. Fig.1a shows that the minimal asymmetric structural unit contained one DyIII ion, half H2-DHBDC2- ligand, half DHBDC4- ligand, one EtOH and a H2O molecular. The DyIII ion was eight-coordinated by five carboxyl oxygen atoms (O2A, O3, O5C, O4B and O5B) and one phenoxide oxygen atom (O6) from four ligands, one oxygen atom (O7) from the EtOH molecular and the last oxygen atom (O8) from H2O molecular. The coordinated geometry of DyIII for complex 1 was calculated using SHAPE 2.1 software, indicating that the DyIII ion was located in triangular dodecahedron coordinating environments (TDD-8, D2d, Fig.1b) since the minimum value for the CShM parameter was 1.8116 (Table S3-S4). The distances of Dy-O bands were 2.368 Å on average, ranging from 2.241(9)-2.591(10) Å, which are consistent with the previously reported DyIII-MOFs. The angles for O-Dy-O were in the range of 51.9(3)°-150.5(4)°. The organic ligand, as an oxygen donor, is a good choice for constructing Dy-MOFs. In the complex, because there were two forms of proton loss in organic ligands, i.e. H2-DHBDC2- and DHBDC4-, the ligand acted as a μ4 bridge in two coordination modes. The one coordination mode only adopting two carboxylate groups showed two bidentate bridging coordination modes μ2-η1:η1, and the other adopted two carboxylate groups and one phenoxide group, in which the former showed μ2-η1:η2 mode and acted as monoatomic oxygen bridge between DyIII ions. Accordingly, the neighboring DyIII ions were doubly bridged by one syn, syn-carboxylate group and syn-anti-carboxylate group, respectively, forming a one-dimensional chain structure (Fig.1c) with the nearest distance of DyIII·∙∙DyIII in the 1D chain being 4.6007Å, which was longer than other reported complexes. Since the magnetic dilution could be achieved through large linkers and avoid QTM, it may be beneficial for the compound to exhibit the SMM behavior. Furthermore, given the centrosymmetric nature of the ligand, the 1D chains were then joined by means of two H2-DHBDC2- and DHBDC4- ligands, displaying a 3D framework (Fig.1d).
(a)
(c)
(b)
(d)
Fig.1 (a) Molecular structure of complex 1, all of the hydrogen atoms and solvent molecules were omitted for clarity. (b) The coordination polyhedron of the DyIII ion in complex 1. (c) 1D structure of complex 1. (d). 3D framework of complex 1.
The PXRD patterns of phase purity for complex 1 were recorded (Fig. S1). The diffraction curves are similar to the simulated patterns, revealing that the obtained compound exhibited strongly crystallinity, and there existed no impurity crystal phase. Furthermore, the thermal stability of complex 1 was tested on a temperature gradient from ambient temperature to 1450 °C at a heating rate of 5 K/min in a nitrogen atmosphere. As shown in the TG curve (Fig. S2), the complex had removed H2O and EtOH molecular before 170°C and then solvent-free phase was stabilized to 200°C. Lastly, it began to lose considerable weight, due to the breakage of the coordination bond of the complex and the decomposition of the host skeleton. The variable-temperature direct-current (dc) susceptibility of complex 1 was measured under an external magnetic field of 1000 Oe at the temperature from 2 to 300 K, with the plot of χm T versus T for complex 1 shown in Fig. 2. At ambient temperature the χm T value is 13.58 cm3 mol K-1, which is slightly lower than the theoretical value of 14.17 cm3 mol K-1 for one non-coupled DyIII (S=5/2, g=4/3, 6H15/2, C = 14.17 cm3 mol K-1)[33, 34]. Upon cooling, the values of complex 1 remained almost constant at temperature 100-300 K. Subsequently,
as the temperature further fell to about 50 K, the χm T values gradually decreased and then sharply dropped to a minimum of 10.48 cm3 K mol-1 at 2.0 K. This behavior might be attributed to the crystal field effect, especially the thermal depopulation of the Stark sublevels of DyIII in complex 1 and/or possible weak magnetic interactions between DyIII ions. Besides, the magnetization (M) versus H for complex 1 were also collected in the temperature range of 2.0-8.0 K. Fig. 3 suggests that at each temperature, the value of M increased monotonically with the rise in the value of H. Different temperatures dispaly analogous curved forms, and the final maximum values of H were about 5.48 μB for each temperature, lower than theoretical saturated value of 10 μB for one non-coupled DyIII at 6H [35, 36]. Besides, the magnetic induction intensity of complex 1 under different 15/2 states field strength conditions was tested at the temperature from 2 to 8 K (Fig. S4). The test results revealed that the M vs H/T curves did not overlap, whereas they were stepped. In brief, these phenomenon revealed that the existence of magnetic anisotropy and/or low-lying excited states[11-16, 37].
Fig.2 Tempurature dependence of χm T for complex 1.
Fig. 3 Plots of M versus H for complex 1 at 2.0, 3.0, 4.0, 5.0 and 8 K, respectively.
To investigate the magnetic relaxation behavior of complex 1, alternating current (ac) susceptibility measurements were performed under a zero dc field, however, no out-of-phase (χ″) component was observed above 2 K (Fig. S5), which indicates that the magnetic interaction has quenched the tunneling [38]. A sweep test was then performed for complex 1 in the dc field range of 0~1 T and an optimum external dc field of 1500 Oe was found (Fig. S6). Thus, the ac susceptibilities of complex 1 were further measured under 1500 Oe dc field at 2-10 K within the frequency range of 1-999 Hz. Fortunately, both the in-phase (χ') and out-of phase (χ") signals of complex 1 exhibit temperature and frequency dependence (Fig. 4 and Fig. S6). As shown in Fig. 4, the signal values of χ' and χ" constantly decreased in the low temperature region when the frequency increased; and note that the χ' ac magnetic susceptibility has no peak, whereas the χ" signals were observed in high temperature zone. In the meantime, the frequency dependence of out-of phase (χ″) ac signals for complex 1 displays two maxima at low temperature, indicating the presence of two relaxation processes, and suggesting that complex 1 exhibited field-induced SMM behavior [21-25, 39] (Fig. S7). These results reveal that the field-induced relaxation comes from the decoupling by field. Namely, the chain properties in magnetism have been destroyed and the single-ion
properties appeared[38]. The Cole-Cole plots of 1 show two-semicircular shape at the temperature from 2 to 10 K (Fig. 5 left), confirming the presence of two relaxation processes. These plots can be well fitted by the extended Debye functions. The fitting parameters are listed in Table S5. The values of α1 and α2 are in the range of 0.0035-0.52 and 0.026-0.39, respectively, indicating a broad distribution of relaxation time or the occurrence of multiple relaxation processes [39-42]. The plot of ln(τ) versus T−1 is shown in Fig. 5(right), revealing the strong linear dependence of ln(τ) versus T−1 in the high temperature region, which was a dominant Orbach relaxation process. In the high temperature region, the Arrhenius law (τ = τ0 exp(Ueff/kT) is employed to extract parameters τ0 (8.89×10−6 s) and energy barrier Ueff (36.11 K). Moreover, considering the fact that the plot shows an apparent curvature at low temperatures, other relaxation (e.g., Raman and direct process) may be also present. Therefore, the data in the whole temperature range were fitted by using the equation 1/τ = AT+B+CTn +τ0−1exp(Ueff/kT), in which the Raman, Orbach and direct processes were considered. The reliable fitting results were found to be as follows: Ueff = 23.87 K, τ0 = 6.86×10−5 s, A=59.86, B=0, C=3.034×10-6 s and n=9 [7-10,33,40].
Fig.4 Temperature dependence of the in-phase (χm′) and out-of-phase (χm″) ac susceptibility data under 1500Oe dc field in the frequency range of 1-999Hz for complex 1.
Fig. 5 The plots of cole-cole (left) and plot of ln τ versus 1/T (right) for complex 1. The solid lines represent the best fits for the corresponding data.
Some specific Dy(III)-based complexes reported in recent years were selected, in which all DyIII ions exhibited the same triangular dodecahedron coordination geometry as complex 1. These complexes including [DyIIINaI(L)(CH3CO2)2]n·2CH3CN(TDD-8, CShM =3.29)[39],
[DyCu3(H2edte)3(NO3)][NO3]2·0.5MeOH (TDD-8, CShM=2.214)[40], [Dy3(μ3-OCH3)2(μ-HL1)3(NO3)3]BPh4·4CH3OH·H2O (TDD-8, CShM=1.59)[41] and [43] [Dy(pmap)3(H2O)2]·0.375CH3COOC2H5 (TDD-8, CShM =1.441) with the value of Ueff are 13.21 K, 16.2 K, 49.21 K and 39.6 K, respectively. It is widely known that larger CShM values indicate larger ideal geometric deviations, further affecting the value of Ueff. According to the above examples, it was found that the value of Ueff was larger when the value of CShM was smaller and vice versa. For compound 1, its CShM value was significantly lower than the CShM value of the pre-two complexes with lower Ueff. Accordingly, the critical reason why a 1 had a relatively high Ueff value is that the coordination geometry of DyIII in complex 1 complies with the ideal geometry more closely. Indeed, other factors (e.g., the direction of the magnetic axis and the strength of crystal-field, the coordination environment and intrachain couplings of DyIII) may also affect the relaxation behaviors[23-28, 34-40]. In conclusion, a novel Dy(III)-based complex [Dy(H2-DHBDC)0.5(DHBDC)0.5(C2H5OH)(H2O)]n·H2O was fabricated with a three-dimensional framework structure; it crystallized in the monoclinic system, space group P21/c. Furthermore, complex 1 covered a Dyш mononuclear unit in which the Dyш atoms adopted eight-coordination mode, locating in triangular dodecahedron coordinating environments. Besides, the results of magnetic property investigations suggested that the coordination polymer exhibited slow magnetic relaxation behaviors under 1500Oe dc field with an effect energy barrier of 36.11 K. The frequency dependence of out-of phase (χ″) ac signals for 1 displays two maxima at low temperature, suggesting the presence of two relaxation processes and revealing the field-induced SMM behavior of complex 1. To study the magnetic anisotropy behavior of single-molecule magnets, our laboratory will use other organic ligands and mediators to fabricate novel Dy(III)-based complexes of the new 1D, 2D and 3D structures. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Nos. 21461021) and the Open Project of State Key Laboratory of Plateau Ecology and Agriculture, Qinghai University (No. 2019-ZZ-08). Appendix A. Supplementary data CCDC No. 1946633 contains the supplementary crystallographic data for this paper. The data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Supporting information contains the supplementary data and X-ray crystallographic files (CIF) for the complex. Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/. Reference [1] J. D. Rinehart, K. R. Meihaus, J. R. Long, Observation of a secondary slow relaxation process for the field-induced single-molecule magnet U(H2BPz2)3, J. Am. Chem. Soc. 132 (2010):7572–7573. [2] Y. J. Li, Y. L. Wang, Q. Y. Liu, The highly connected MOFs constructed from nonanuclear and trinuclear lanthanide-carboxylate clusters: selective gas adsorption and
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