Structure, magnetism, and theoretical study of a mixed-valence Co3IICo4III heptanuclear wheel:: Lack of SMM behavior despite negative magnetic anisotropy
Structure, magnetism, and theoretical study of a mixed-valence Co3IICo4III heptanuclear wheel:: Lack of SMM behavior despite negative magnetic anisotropy
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DOI：
10.1021/ja8029416
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发表时间：
2008-09-17
影响因子：
15
通讯作者：
Luneau, Dominique
中科院分区：
文献类型：
--
作者：
Chibotaru, Liviu F.;Ungur, Liviu;Luneau, Dominique
合成了一种混合价Co（II）/Co（III）七核轮[（Co 3Co 4 III）-Co-II（L）（6）（MeO）（6）]（LH 2 = 1，1，1-三氟-7-羟基-4-甲基-5-氮杂庚-3-烯-2-酮），用X射线单晶衍射法测定了其I型晶体结构。每个钴离子的价态通过键价和计算建立。对磁化率的温度依赖性和磁化强度的场依赖性的研究证明了化合物内的铁磁相互作用。为了了解这种Co-7轮的磁性，我们在CASSCF/CASPT 2水平上对每个钴碎片进行了从头计算，包括SO-RASSI方法中的自旋轨道耦合效应。四个钴（III）离子被发现是抗磁性的，并给出了显着的温度无关的顺磁贡献的磁化率。三个Co（II）位置上的自旋-轨道耦合导致基态和激发态Kramers双峰之间的间距近似为200 cm（-1），将Co-7轮置于弱交换极限，其中最低电子态由Co（II）位置上最低Kramers双峰之间的各向异性交换相互作用充分描述。交换相互作用的模拟线模型内完成，保持完全从头处理的磁各向异性的影响，个别钴碎片使用最近开发的方法。最近邻（J（1））和次最近邻（J（2））交换参数（分别为1.5和5.5 cm（-1））得到了磁化率和磁化强度的良好描述。远距离的钴离子之间的强铁磁相互作用产生的结果，低电子促进能量的交换桥含有钴（III）离子。计算结果表明，沿主磁轴（10.1 μ（B））的磁化强度沿着较大的值，这是两个边缘Co（II）网站上的铁磁交换相互作用和负磁各向异性的综合效果。在[（Co 3Co 4 III）-Co-II（L）（6）（MeO）（6）]中缺乏单分子磁体行为的解释是，通过从头计算证明，基态Kramers双重态的最大磁化状态之间的横向磁矩的相对大的矩阵元，以及在晶体中存在偶极横向磁场的情况下这些状态之间的相关大隧穿率。
A mixed-valence Co(II)/Co(III) heptanuclear wheel [(Co3Co4III)-Co-II(L)(6)(MeO)(6)] (LH2 = 1,1,1-trifluoro-7-hydroxy-4-methyl-5-aza-hept-3-en-2-one) has been synthesized and its I crystal structure determined using single-crystal X-ray diffraction. The valence state of each cobalt ion was established by bond valence sum calculations. Studies of the temperature dependence of the magnetic susceptibility and the field dependence of the magnetization evidence ferromagnetic interactions within the compound. In order to understand the magnetic properties of this Co-7 wheel, we performed ab initio calculations for each cobalt fragment at the CASSCF/CASPT2 level, including spin-orbit coupling effects within the SO-RASSI approach. The four Co(III) ions were found to be diamagnetic and to give a significant temperature-independent paramagnetic contribution to the susceptibility. The spin-orbit coupling on the three Co(II) sites leads to separations of similar to 200 cm(-1) between the ground and excited Kramers doublets, placing the Co-7 wheel into a weak-exchange limit in which the lowest electronic states are adequately described by the anisotropic exchange interaction between the lowest Kramers doublets on Co(II) sites. Simulation of the exchange interaction was done within the Lines model, keeping the fully ab initio treatment of magnetic anisotropy effects on individual cobalt fragments using a recently developed methodology. A good description of the susceptibility and magnetization was obtained for nearest-neighbor (J(1)) and next-nearest-neighbor (J(2)) exchange parameters (1.5 and 5.5 cm(-1), respectively). The strong ferromagnetic interaction between distant cobalt ions arises as a result of low electron-promotion energies in the exchange bridges containing Co(III) ions. The calculations showed a large value of the magnetization along the main magnetic axis (10.1 mu(B)),which is a combined effect of the ferromagnetic exchange interaction and negative magnetic anisotropy on the two marginal Co(II) sites. The lack of single-molecule magnet behavior in [(Co3Co4III)-Co-II(L)(6)(MeO)(6)] is explained by relatively large matrix elements of transverse magnetic moments between states of maximal magnetization of the ground Kramers doublet, evidenced by ab initio calculations, and the associated large tunneling rates between these states in the presence of dipolar transverse magnetic fields in the crystal.