The inherent single-molecule magnet character of trivalent uranium.
The inherent single-molecule magnet character of trivalent uranium.
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DOI：
10.1002/anie.201208015
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发表时间：
2013-03
影响因子：
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通讯作者：
F. Moro;D. P. Mills;S. Liddle;J. van Slageren
中科院分区：
文献类型：
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作者：
F. Moro;D. P. Mills;S. Liddle;J. van Slageren
SMM通常基于过渡金属簇，但是最近显著的注意力集中在单个和多个镧系元素离子的络合物上，因为最低的Russell-Saunders多重峰的晶体场分裂产生大的磁各向异性。[5-11]这些各向异性是高弛豫势垒的原因，因此磁弛豫较慢。然而，很难在多核镧系元素SMM内实现良好隔离的高自旋基态，因为价4f轨道具有有限的径向延伸，并且通常与配体轨道在能量上不相容。这些固有的4f轨道性质主要是离子相互作用，并导致与相邻自旋中心的弱磁交换耦合，只有极少数例外。[7，12]原则上，锕系元素，特别是铀，具有使这些离子成为构建SMM的理想候选者的特性。这是因为，与镧系元素相比，铀表现出增强的晶体场分裂，[13，14]以及增加的共价性，后者使多核系统中的显着自旋耦合成为可能[15]，因此可以设想更强的磁交换和各向异性。这一前提是实现最近的几个密切相关的单离子吡唑硼酸铀（III）配合物，显示SMM行为的报告。[14此外，两种镎络合物显示出缓慢的磁化弛豫。[15迄今为止报道的所有铀（III）SMM彼此密切相关的事实提出了三价铀中SMM行为对配位球的组成及其对称性有多敏感的问题。在所有已发表的例子中，SMM行为在外场中比在零场中更明显，这表明磁化的量子隧穿在缩短弛豫时间方面起着重要作用。然而，原则上，低对称性晶体场分量不能诱导磁化的隧穿，因为铀（III）是Kramers半整数角动量离子。此外，核自旋I = 0 o f 238 U不能诱导隧穿的
SMMs are often based on transition-metal clusters, but significant attention has recently focused on complexes of single and multiple lanthanoid ions, because the crystal-field splitting of the lowest Russell–Saunders multiplet engenders large magnetic anisotropies. [5–11] These anisotropies are responsible for high relaxation barriers and therefore slow magnetic relaxation. However, well isolated high-spin ground states are difficult to achieve within polynuclear lanthanoid SMMs because the valence 4f orbitals have limited radial extension and are usually energetically incompatible with ligand orbitals. These inherent 4f orbital properties give predominantly ionic interactions and results in weak magnetic exchange coupling with neighboring spin centers, with very few exceptions. [7, 12] In principle, actinoids, and in particular uranium, possess properties that render these ions ideal candidates from which to construct SMMs. This is because, compared to the lanthanoids, uranium exhibits enhanced crystal field splitting, [13, 14] as well as increased covalency, the latter enabling significant spin couplings in polynuclear systems, [15] and therefore both stronger magnetic exchange and anisotropies can be envisaged. This premise was realized recently with reports of several closely related single-ion pyrazolylborate uranium(III) complexes which were shown to display SMM behavior. [14, 16–20] In addition, two neptunium complexes were shown to display slow relaxation of the magnetization. [15, 21] The fact that all uranium(III) SMMs reported to date are closely related to each other raises the question as to how sensitive SMM behavior in trivalent uranium is to the composition of the coordination sphere and its symmetry. SMM behavior in all published examples is much more pronounced in an external field than in zero field, which suggests that quantum tunneling of the magnetization plays a significant role in shortening the relaxation times. However, in principle, low-symmetry crystal field components cannot induce tunneling of the magnetization, because uranium(III) is a Kramers half-integer angular momentum ion. Also the nuclear spin I = 0o f 238 U cannot induce tunneling of the