Synthesis, Structure and Magnetic Property of a Tricapped Trigonal Prismatic TbIII-Based 3d-4f Complex

A novel 3d-4f CoII/TbIII cluster [Co6Tb(Pic)6O3Cl3](H2O)6 (1) (Pic = 2-Picolinic acid) has been synthesized and characterized via X-ray crystallographic and magnetic measurements. X-ray crystallographic and continued shape measurements analysis revealed that the central Terbium (III) is coordinated in TbO9 geometry with D 3h symmetry. Magnetic studies indicated the complex presents a magnetic anisotropy with ferrimagnetic interactions.


Materials and Methods
All the materials for synthesis were obtained commercially and used without further purification. The Infrared (IR) spectra were recorded with a range of 400-4000 cm −1 on a Nicolet 5DX spectrometer (KBr pellets). Magnetic data were collected using a Quantum Design MPMS XL7 SQUID magnetometer. The diamagnetic correction was performed by a rough estimation (χd = Mr × 10 -6 cm 3 mol -1 , Mr is the molecular weight.

Description of the Crystal Structure
First, the single crystal X-ray analysis showed that 1 crystallized in the hexagonal P63 space group (table 1 left). In complex 1, six Co II ions form a large trigonal prism and a Tb III ion is situated at the center. The six Co III ions belong to three bases, in each base, the two Co III ions are connected to form an μ3oxygen atom, a μ2-chloride ion and terminal oxygen atom from water. The three bases and central Tb III ion are connected by picolinic acid ligands and μ3-oxygen atoms formed a heptanuclear trigonal prismatic Co6Tb clusters. The central Tb III ion is bridged six oxygen atoms from six pic ligands respectively and also bridged by three oxygen atoms (figure 1), which performs the distorted trigonal prism coordination geometry, and in further, the analysis from continued shape measurements (CShM) [34,35] (figure 1 right, table 1 right) confirmed it. The value from analysis indicates the closing degree of the coordination geometry to the ideal model (value: zero), thus, the non-zero values indicate that the Tb III coordination sites can be described as distorted.

Magnetic Properties
The direct-current magnetic susceptibilities has been collected in a temperature range of 300-2 K on polycrystalline samples (figure 2). The room temperature MT (M: the molar susceptibility) value (26.96 cm 3 K mol −1 ) of 1 was obtained from magnetic measurements with an applied dc magnetic field of 1000 Oe (figure 2 left). Upon cooling, the MT values of 1 keep constant in the temperature range of 300-100 K, and then decrease to the minimum gradually at 20 K due to the temperature depopulation of split crystal field sublevels from Tb III /Co II ions, from 20 K to 2 K, the MT values of 1 increased, which exhibits a ferrimagnetic coupling behaviour for complex 1.
The variation (2.0, 2.5, 3.0, 3.5 and 4.0 K) of the magnetization M under the applied dc field B of 1 was also investigated, the M vs. BT -1 data (figure 2 right) demonstrates that the isotemperature curves which is not overlaying, which confirmed the presence of low-lying excited states and/or non-negligible magnetic anisotropy for complex 1, and in further, 1 would display SMMs behaviour [20].
The alternating-current susceptibility datas of 1 (figure 3) has been carried out with ac frequencies ranging from 0.1 to 1500 Hz under an oscillating-drive field of 3 Oe. Unfortunately, we cannot observe the slow magnetic relaxation from the data of the out-of-phase susceptibilities (imaginary part), probably owing to its large quantum tunnelling of the magnetization (QTM), which makes the relaxation too fast to signal in the ac susceptibility characterization, but in nature, probably due to the distorted around the central Tb III ion of 1, which exhibit tricapped trigonal prism coordination geometry.

Conclusions
In summary, a Tb III -based 3d-4f complex with an anisotropy has been synthesized successfully. The Tb III achieved a TbO9 9-coordinate configuration presenting the distort D3 symmetry from the CShM analysis. The magnetic studies of 1 demonstrates that the SMMs behaviour is largely affected by symmetry of the central Tb III ion and the molecule, if we want to obtain the large effective energy barrier and slow relaxation of SMMs, a better control of the molecule symmetry to decrease QTM would be an effective way.