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• After the discovery of the first single-ion magnet [TbPc₂]^- (Pc^2-=pthalocyanine dianion)^[1], lanthanide based complexes have gained momentum in the area of molecular magnetism as they exhibit slow relaxation of magnetization at low temperatures^[2-6]. Thanks to their unquenched orbital angular momentum and inherent magnetic anisotropy, the number of lanthanide-based single-molecule magnets (SMMs) reported to-date is increasing exponentially^[7-14]. Such molecules hold tremendous application potential in high-density data storage and/or quantum computation due to their magnetic moment bistability. By using geometric design principles to minimize electronic repulsions between the electron densities of the lanthanide ions and the ligands, researchers have prepared SMMs with extremely high barriers and magnetic hysteresis as compared to previous examples^[15]. Especially, a breakthrough has been made by Layfield and co-workers in Dy-SIMs, exhibiting magnetic hysteresis at temperatures up to 80 K and a high effective energy barrier of 1 541 cm^{-1 [16]}. While most examples of the rare-earth SMMs are based on the oblate terbium and dysprosium ions (and particularly the latter^[17-19]), emerging work has revealed that prolate ions can also engender SMM behavior, with the erbium(Ⅲ) ion being the main choice for such systems^[20-22]. The first reported erbium(Ⅲ) mononuclear SMM is the organometallic complex [(Cp)Er(COT)](Cp=penta-methylcyclopentadienide; COT=cyclooctatetraenide), reported by Gao and co-workers, which exhibits two relaxation processes with energy barriers ΔE_eff/k_B of 197 and 323 K and a butterfly hysteresis loop as high as 5 K^[23]. The lack of SMM behavior in the majority of the Er(Ⅲ) complexes and the presence of large T_B values in some other Er(Ⅲ)complexes have attracted our attention.

  Existing researches point out that different environments of the magnetic centers generating various crystal fields usually bring about distinct dynamic magnetic relaxation processes; even subtle changes of the coordination pattern can drastically affect the overall magnetic properties of SMMs. It is well-known that the structural formation of a target complex is often perturbed by rational changes of the synthetic conditions such as solvent effect, pH, coun-terions^[24-31]. Of particular concern is the tunable solvent effect that usually motivates variable coordination behaviors of metal cations, even in a same reaction system, which can modulate the coordination microe-nvironment and the geometric symmetry of the metallic core^[32]. Herein, two monometallic erbium complexes, [Er(bpad)₃]·CH₃OH·H₂O (1) and [Er(bpad)₂(H₂O)₂]NO₃·3H₂O (2), have been isolated by changing the solvent ratio, with which the Er(Ⅲ) ion is surrounded, leading to a nona-coordinated monocapped square antiprism (C_4v) and octa-coordinated biaugmented trigonal prism (C_2v) geometries, respectively. In addition, the magnetic properties of complexes 1 and 2 have been investigated.

  1.   Experimental

  1.1   Materials and methods

  The Hbpad ligand was synthesized following a previous reference^[33]. Other chemicals were reagent grade and commercially available. Fourier transform infrared (FT-IR) spectra were recorded in a range of 400~4 000 cm^-1 using KBr pellets on an EQUINOX55 FT/IR spectrophotometer. Elemental analysis (carbon, hydrogen and nitrogen) was implemented on a PerkinElmer 2400 CHN analyzer. Magnetic measure-ments were accomplished using a Quantum Design MPMS-XL7 SQUID magnetometer on polycrystalline samples (restrained in eicosane to prevent torquing at high fields). The diamagnetic corrections for the complexes were estimated using Pascals constants.

  1.2   Synthesis of [Er(bpad)₃]·CH₃OH·H₂O (1)

  A mixed CH₃OH/H₂O solution (15 mL, 2:1, V/V) of triethylamine (0.007 mL, 0.05 mmol) and Hbpad (0.146 g, 0.6 mmol) was stirred for 1 h, and then Er(NO₃)₃·6H₂O (0.091 g, 0.2 mmol) was added. The mixture above was allowed to be stirring for 5 h and shelved after filtration. Yellow crystals of 1 were isolated after 4 days (Yield: 72%, based on Er³⁺). Anal. Calcd. for C₄₀H₃₉ErN₁₂O₅(%): C, 51.38; H, 4.20; N, 17.98. Found(%): C, 51.42; H, 4.25; N, 18.02. IR (KBr, cm^-1): 3 442 (w), 1 633 (m), 1 560 (s), 1 521 (s), 1 472 (s), 1 187 (s), 1 436 (m), 1 362 (s), 1 165(w), 1 029(w), 805 (w), 713 (m).

  1.3   Synthesis of [Er(bpad)₂(H₂O)₂]NO₃·3H₂O (2)

  Complex 2 was prepared by the similar way of 1, while the ratio of CH₃OH to H₂O is altered to be 1:2 (V/V). Pale yellow crystals of 2 were obtained after 5 days (Yield: 68%, based on Er³⁺). Anal. Calcd. for C₂₆H₃₂ErN₉O₁₀(%): C, 39.14; H, 4.04; N, 15.80. Found(%): C, 39.18; H, 4.07; N, 15.75. IR (KBr, cm^-1):3 436 (w), 1 635 (s), 1 585 (s), 1 565 (s), 1 521 (s), 1 479(s), 1 436 (m), 1 393 (s), 1 168 (m), 1 063 (m), 912 (w), 743 (m), 710 (s).

  1.4   Crystallographic data collection and refinement

  Single-crystal X-ray diffraction data for complexes1 and 2 were collected on a Bruker SMART APEX CCD diffractometer equipped with graphite-mono-chromated Mo Kα radiation (λ=0.071 073 nm). Data processing and absorption corrections were acco-mplished using SAINT and SADABS^[34]. The structures were solved by direct methods and refined against F ² by full-matrix least-squares with SHELXTL program^[35]. All non-hydrogen atoms were refined with anisotropic thermal parameters. All hydrogen atoms were positioned geometrically and refined using a riding model. Crystallographic data and refinement parameters are listed in Table 1, while selected interatomic distances and angles for complexes 1 and 2 are given in Table S1 (Supporting information).

  Table 1

  

  Table 1.  Crystallographic data and refinement parameters for 1 and 2

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  Complex	1	2
  Empirical formula	C40H39ErN12O5	C26H32ErN9O10
  Formula weight	935.09	797.87
  Crystal system	Monoclinic	Monoclinic
  Space group	C2/c	P21/n
  a / nm	3.492 68(18)	0.885 17(8)
  b / nm	1.158 68(8)	2.541 6(2)
  c / nm	2.234 55(14)	1.428 89(13)
  β / (°)	112.979(4)	105.107 0(10)
  V / nm3	8.325 4(9)	3.103 6(5)
  Z	8	4
  Dc / (g·cm-3)	1.492	1.708
  μ / mm-1	2.074	2.774
  Crystal size / mm	0.21×0.20×0.19	0.30×0.25×0.13
  Temperature / K	293	296
  Rint	0.041 6	0.022 5
  θ range / (°)	1.27~25.01	1.68~25.01
  Total, unique reflection	20 903, 7 342	15 598, 5 471
  Final R indices [I > 2σ(I)]	R1=0.036 3, wR2=0.096 0	R1=0.025 8, wR2=0.056 3
  R indices (all data)	R1=0.061 1, wR2=0.121 5	R1=0.038 6, wR2=0.063 0

  CCDC: 1478582, 1; 1478579, 2.

  2.   Results and discussion

  2.1   Crystal structure

  X-ray determination suggests that both complexes are mononuclear structures. Complexes 1 and 2 belong to the monoclinic C2/c and P2₁/n space groups, respectively. The Er(Ⅲ) site in the asymmetric unit of 1 is nona-coordinated by three O atoms and six N atoms from three tridentate bpad^- ligands (Fig. 1). The average Er-O and Er-N bond lengths are 0.236 1 and 0.254 6 nm, respectively(Table S1). The shortest Er…Er distance between neutral [Er(bpad)₃] molecules is 0.955 9 nm. Being compared with 1, a tridentate bpad^- ligand are substituted by two coordinated H₂O molecules, forming the configuration of 2. Accordingly, the metal center in 2 is bonded by two tridentate bpad^- anions and two coordinated H₂O units, thus completing the ErN₄O₄ coordination group (Fig. 1). The average Er-O and Er-N bond lengths are 0.236 5 and 0.247 3 nm, respectively. The closest Er…Er separation between two isolated molecules in 2 is 0.697 1 nm. Evidently, the steric hindrance of H₂O in 2 is far less than the bpad^- ligand in 1, giving rise to the shorter intermolecular distance in 2 compared to that in 1. Hydrogen bonds and π-π interactions coexist in 1 and 2, yielding a three-dimensional supramolecular structure (Fig.S1).

  Figure 1

  

  Figure 1.  Crystal structures of 1 (a) and 2 (b) and local coordination geometries of the Er(Ⅲ) ions for 1 (c) and 2 (d)

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  To ascertain the precise geometries around the metallic centers and the degree of the distortion from the ideal model for both complexes, the geometric spheres of Er(Ⅲ) cations are calculated by using the SHAPE 2.1 program^[36] based on the structural para-meters, and the typical geometric polyhedrons are depicted in Fig. 1. As listed in Table S2, the calculated values illuminate that the Er(Ⅲ) ion in 1 is best described as a monocapped square-antiprism (C_4v) with moderate distortions from the ideal geometry. By contrast, the Er(Ⅲ) nodes in 2 represent a biaugmented trigonal prism (C_2v).

  2.2   Magnetic studies

  In order to understand the consequence of this ligand architecture on the local anisotropy of Er(Ⅲ) ion, magnetic susceptibility measurements were performed with a SQUID magnetometer on freshly prepared ground polycrystalline samples of 1 and 2 sealed under N₂ to prevent sample degradation. Direct current magnetic susceptibility measurements were conducted in a temperature range of 1.8~300 K under an applied dc field of 0.1 T (Fig. 2). The room temperature χ_MT values of 11.58 and 11.37 cm³·K·mol^-1 for 1 and 2, respectively, are in good agreement with the theoretical value of 11.48 cm³·K·mol^-1 for a mononuclear Er(Ⅲ) (⁴I_15/2, S=3/2, L=6, g_J=6/5) complex. Upon decrease of the temperature, the χ_MT product remained nearly constant then decreased gradually between 100 and 50 K. Below 50 K, there was a sharp decrease reaching the minimums of 5.25 and 5.58 cm³·K·mol^-1 for 1 and 2 at 1.8 K, respectively. Such behavior is resulted from significant anisotropy as seen in some highly anisotropic complexes or weak antiferromagnetic dipolar coupling between the mono-tnuclear groups^[37-38]. Isothermal magnetization curves at 1.8 K exhibited saturation for 1 and 2 with values of 4.77Nβ and 4.44Nβ (Fig.S2), respectively, which prominently deviate from the saturated value of 10Nβ. The non-saturation as well as the non-superimposition of iso-temperature lines in the M vs H/T plot also confirm the presence of significant magnetic anisotropy in 1 and 2 (Fig. 2, Inset). Therefore, alternating-current magnetic susceptibility measurements for complexes 1 and 2 were carried out under 0 dc field in a range of 1.8~20 K and a frequency of 1 000 Hz (Fig.S3). Unfortunately, no out-of-phase ( χ_M″) signal was observed until the temperature dropped to 1.8 K, suggesting a fast quantum tunneling of magnetization (QTM) through the spin-reversal barrier and the inexistence of SMM behavior in the absence of extra field. To gain insight into the relationship between configuration and magnetic relaxation behavior, structural and magnetic parameters of some Er(Ⅲ) complexes are summarized in Table S3^{[23, 39-47]}. It is observed that the ligand field design plays a crucial role for the observance of SMM behavior in Er(Ⅲ) complexes. The sandwich-type ligand geometry leads to a higher symmetry environ-ment around the Er(Ⅲ) sites, thus exhibiting a superior magnetic property. The case with low-coordinated Er(Ⅲ) center may also act as a potential candidate for high-performant SMMs. In contrast, the seven or eight coordinated Er ions adverse to the zero-field slow magnetic relaxation.

  Figure 2

  

  Figure 2.  Plots of _χMT vs T for 1 (a) and 2 (b)

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  3.   Conclusions

  This work reports the synthesis of two mononuclear Er(Ⅲ) complexes by means of solvent effect and the characterizations of them. For 1, the coordination geometry of Er(Ⅲ) core in N₆O₃ motif could be best identified as a monocapped square-antiprism (C_4v), while the configuration of Er(Ⅲ) ion in 2 is surrounded by a biaugmented trigonal prism (C_2v) with N₄O₄ group. Magnetic studies unveil that the performance of slow magnetic relaxation is absent in these systems, owing to the possible QTM which can usually be efficiently suppressed by a DC field. The related follow-up tests are under way. The impetus for the case presented herein is to exploit the potential of synthesizing Er-based single-molecule magnets, further studies following this guideline are in progress.

  
  Supporting information is available at http://www.wjhxxb.cn
  
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• 

  Figure 1  Crystal structures of 1 (a) and 2 (b) and local coordination geometries of the Er(Ⅲ) ions for 1 (c) and 2 (d)

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  Figure 2  Plots of _χMT vs T for 1 (a) and 2 (b)

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  Table 1.  Crystallographic data and refinement parameters for 1 and 2

  Complex	1	2
  Empirical formula	C40H39ErN12O5	C26H32ErN9O10
  Formula weight	935.09	797.87
  Crystal system	Monoclinic	Monoclinic
  Space group	C2/c	P21/n
  a / nm	3.492 68(18)	0.885 17(8)
  b / nm	1.158 68(8)	2.541 6(2)
  c / nm	2.234 55(14)	1.428 89(13)
  β / (°)	112.979(4)	105.107 0(10)
  V / nm3	8.325 4(9)	3.103 6(5)
  Z	8	4
  Dc / (g·cm-3)	1.492	1.708
  μ / mm-1	2.074	2.774
  Crystal size / mm	0.21×0.20×0.19	0.30×0.25×0.13
  Temperature / K	293	296
  Rint	0.041 6	0.022 5
  θ range / (°)	1.27~25.01	1.68~25.01
  Total, unique reflection	20 903, 7 342	15 598, 5 471
  Final R indices [I > 2σ(I)]	R1=0.036 3, wR2=0.096 0	R1=0.025 8, wR2=0.056 3
  R indices (all data)	R1=0.061 1, wR2=0.121 5	R1=0.038 6, wR2=0.063 0

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•