Aromí Guillem, Bouwman Elisabeth, Burzurí Enrique, Carbonera Chiara, Krzystek J, Luis Fernando, Schlegel Christoph, van Slageren Joris, Tanase Stefania, Teat Simon J
Departament de Química Inorgànica, Facultat de Química, Universitat de Barcelona, Diagonal 647, 08028 Barcelona, Spain.
Chemistry. 2008;14(35):11158-66. doi: 10.1002/chem.200801450.
A highly asymmetric Ni(II) cluster Ni(4)(OH)(OMe)(3)(Hphpz)(4)(MeOH)(3) (1) (H(2)phpz=3-methyl-5-(2-hydroxyphenyl)pyrazole) has been prepared and its structure determined by means of single-crystal X-ray diffraction by using synchrotron radiation. Variable-temperature bulk-magnetization measurements show that the complex exhibits intramolecular-ferromagnetic interactions leading to a spin ground state S=4 with close-lying excited states. Magnetization and high-frequency EPR measurements suggest the presence of sizable Ising-type magnetic anisotropy, with zero-field splitting parameters D=-0.263 cm(-1) and E=0.04 cm(-1) for the spin ground state, and an isotropic g value of 2.25. The presence of both axial and transverse anisotropy was confirmed through low-temperature specific heat determinations down to 300 mK, but no slow relaxation of the magnetization was observed by AC measurements down to 1.8 K. Interestingly, AC susceptibility measurements down to temperatures as low as 23 mK showed no indication of slow relaxation of the magnetization in 1. Thus, despite the presence of an anisotropy barrier (U approximately 4.21 cm(-1) for the purely axial limit), the magnetization relaxation remains extremely fast down to the lowest temperatures. The estimated quantum tunneling rate, Gamma>0.667 MHz, makes this complex a prime candidate for observation of coherent tunneling of the magnetization.
已制备出一种高度不对称的镍（II）簇合物Ni(4)(OH)(OMe)(3)(Hphpz)(4)(MeOH)(3) (1)（H(2)phpz = 3 - 甲基 - 5 -（2 - 羟基苯基）吡唑），并通过使用同步辐射的单晶X射线衍射确定了其结构。变温体磁化测量表明，该配合物表现出分子内铁磁相互作用，导致自旋基态S = 4且激发态紧密相邻。磁化和高频电子顺磁共振测量表明存在相当大的伊辛型磁各向异性，自旋基态的零场分裂参数D = -0.263 cm(-1)和E = 0.04 cm(-1)，各向同性g值为2.25。通过低至300 mK的低温比热测定证实了轴向和横向各向异性的存在，但在低至1.8 K的交流测量中未观察到磁化的缓慢弛豫。有趣的是，低至23 mK的交流磁化率测量未显示1中磁化缓慢弛豫的迹象。因此，尽管存在各向异性势垒（对于纯轴向极限，U约为4.21 cm(-1)），但在最低温度下磁化弛豫仍然极快。估计的量子隧穿速率Gamma>0.667 MHz，使该配合物成为观察磁化相干隧穿的主要候选物。
