Lanthanoid Template Isolation of the α‑1,5 Isomer of Dicobalt(II)-Substituted Keggin Type Phosphotungstates: Syntheses, Characterization, and Magnetic Properties

A new
series of heterometallic 3d–4f sandwich type phosphotungstates,
[Ln­{PCo2W10O38(H2O)2}2]11– (Ln = SmIII, EuIII, GdIII, TbIII, DyIII, HoIII, ErIII, TmIII, YbIII, and LuIII, denoted 1a–10a, respectively),
have been synthesized by a one-pot reaction
procedure on reacting the dilacunary K14[P2W19O69(H2O)]·24H2O precursor
with Ln­(NO3)3·nH2O and Co­(NO3)2·6H2O in an aqueous
potassium chloride solution. All the compounds
were isolated as potassium salts and further characterized with different
analytical techniques such as single-crystal X-ray diffraction, Fourier
transform infrared spectroscopy, high-resolution electrospray ionization
mass spectrometry, elemental analysis by inductively coupled plasma
atomic emission spectroscopy, magnetic measurement, and thermogravimetric
analysis. Single-crystal X-ray diffraction analysis of the compounds
reveals that all these compounds are isostructural and crystallized
in the orthorhombic crystal system in space group Iba2. The polyanions contain the α-1,5 isomer of dicobalt-substituted
α-Keggin phosphotungstate, which sandwiched lanthanoid
cation and formed novel heterometallic dimer species. The temperature-dependent
magnetic susceptibilities of 1a, 2a, 4a, and 7a–10a indicate the dominant contribution
of the ferromagnetic interaction between CoII and CoII within the cluster, while the antiferromagnetic interaction
between CoII and LnIII dominates for 3a, 5a, and 6a. The isothermal magnetizations
of 1a–10a show a gradual increase in magnetization
at low fields
and do not reach saturation even at 50 kOe.

一系列新型异金属3d-4f夹心型磷钨酸盐（heterometallic 3d–4f sandwich type phosphotungstates），通式为[Ln{PCo2W10O38(H2O)2}2]11–（其中Ln分别为SmIII、EuIII、GdIII、TbIII、DyIII、HoIII、ErIII、TmIII、YbIII及LuIII，依次记为1a~10a），通过一锅法反应（one-pot reaction procedure）合成：将双缺位前驱体K14[P2W19O69(H2O)]·24H2O与Ln(NO3)3·nH2O、Co(NO3)2·6H2O在氯化钾水溶液中反应得到。
所有化合物均以钾盐形式分离，并通过多种分析手段进行表征，包括单晶X射线衍射（single-crystal X-ray diffraction）、傅里叶变换红外光谱（Fourier transform infrared spectroscopy）、高分辨电喷雾电离质谱（high-resolution electrospray ionization mass spectrometry）、电感耦合等离子体原子发射光谱元素分析、磁性能测试及热重分析（thermogravimetric analysis）。
单晶X射线衍射分析结果表明，所有化合物均为同构体，结晶于正交晶系（orthorhombic crystal system）的空间群（space group）Iba2。该多阴离子包含二钴取代α-Keggin型磷钨酸盐（α-Keggin phosphotungstate）的α-1,5异构体，其夹心包裹镧系阳离子（lanthanoid cation），形成了新型异金属二聚体物种。
对1a、2a、4a及7a~10a的变温磁化率测试结果显示，团簇内CoII与CoII之间的铁磁相互作用（ferromagnetic interaction）占主导；而对于3a、5a及6a，CoII与LnIII之间的反铁磁相互作用（antiferromagnetic interaction）占据主导地位。
1a~10a的等温磁化曲线（isothermal magnetizations）显示，在低场下磁化强度随场强逐渐升高，即使在50 kOe的磁场下仍未达到饱和。