Singh Saurabh Kumar, Cramer Christopher J, Gagliardi Laura
Department of Chemistry, Minnesota Supercomputing Institute, and Chemical Theory Center, University of Minnesota, 207 Pleasant Street Southeast, Minneapolis, Minnesota 55455-0431, United States.
Inorg Chem. 2020 May 18;59(10):6815-6825. doi: 10.1021/acs.inorgchem.0c00105. Epub 2020 May 5.
The electronic structures and magnetic anisotropies for compounds [An(COT)] (An = U/U, Np/Np and Pu/Pu, COT = cyclooctatetraene) are characterized using scalar relativistic density functional theory calculations and second-order perturbation theory based on a complete active space self-consistent field reference including spin-orbit coupling. The degree of participation of 5f orbitals in actinide-ligand bonding and the associated metal-ligand covalency is found to trend as U > Np ≥ Pu for both the tetra-positive and tripositive An complexes. A spin-Hamiltonian analysis indicates only weak single-molecule magnet (SMM) characteristics for [U(COT)] and [Np(COT)] complexes and no significant SMM behavior for the other complexes. The weak SMM behavior in [U(COT)] and [Np(COT)] is attributed to a subtle interplay between local symmetry and ligand-field splitting. Such a result suggests that magnetic anisotropy in 5f ions can be modulated in general by electrostatic ligand field design. In particular, σ-donor ligands oriented 180 degrees relative to one another will have a maximal influence on the 5f-orbital ligand field splitting, while π donors like cyclopentadiene and COT generate ligand field influences that have more acute angles associated with corresponding atoms on the individual ligands. These observations rationalize the differences in SMM characteristics for [U(Bc)] (Bc = dihydrobis(methylimidazolyl)borate) and [U(Bp)] (Bp = dihydrobis(methylpyrazolyl)borate) and indicate strategies to design new actinide-based SMMs with high magnetic relaxation barriers.
利用标量相对论密度泛函理论计算和基于包含自旋轨道耦合的完全活性空间自洽场参考的二阶微扰理论，对化合物[An(COT)]（An = U/U、Np/Np和Pu/Pu，COT = 环辛四烯）的电子结构和磁各向异性进行了表征。对于四价和三价An配合物，发现5f轨道参与锕系元素-配体键合的程度以及相关的金属-配体共价性趋势为U > Np ≥ Pu。自旋哈密顿分析表明，[U(COT)]和[Np(COT)]配合物仅具有弱单分子磁体（SMM）特性，而其他配合物则没有明显的SMM行为。[U(COT)]和[Np(COT)]中的弱SMM行为归因于局部对称性和配体场分裂之间的微妙相互作用。这样的结果表明，5f离子中的磁各向异性通常可以通过静电配体场设计来调节。特别是，相对于彼此呈180度取向的σ供体配体将对5f轨道配体场分裂产生最大影响，而像环戊二烯和COT这样的π供体产生的配体场影响与各个配体上相应原子的夹角更尖锐。这些观察结果解释了[U(Bc)]（Bc = 二氢双(甲基咪唑基)硼酸盐）和[U(Bp)]（Bp = 二氢双(甲基吡唑基)硼酸盐）的SMM特性差异，并指出了设计具有高磁弛豫势垒的新型锕系元素基SMM的策略。
