Detailed Ab Initio First-Principles Study of the Magnetic Anisotropy in a Family of Trigonal Pyramidal Iron(II) Pyrrolide Complexes
Detailed Ab Initio First-Principles Study of the Magnetic Anisotropy in a Family of Trigonal Pyramidal Iron(II) Pyrrolide Complexes
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DOI：
10.1021/ic200196k
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发表时间：
2011-08-15
影响因子：
4.6
通讯作者：
Neese, Frank
中科院分区：
文献类型：
--
作者：
Atanasov, Mihail;Ganyushin, Dmitry;Neese, Frank
探索了具有轨道简并或轨道近简并基态的过渡金属络合物磁各向异性的理论、计算和概念框架。这种处理是基于完全活跃空间自洽场(CASSCF)波函数，结合N电子价微扰理论(NEVPT2)和准简并微扰理论(QDPT)来处理磁场和自旋相关的相对论效应。该方法被应用于由三吡咯甲胺配体(TPA)的几个变体提供的几乎三角锥体对称的配位场中的一系列Fe(II)络合物。近年来，这些体系作为单核单分子磁体(SMM)络合物引起了人们的极大关注。这项研究旨在确定如何微调配位场以最大化磁各向异性势垒。在三角配位场中，高自旋Fe(II)络合物采用轨道简并的E-5基态，具有较强的在态自旋-轨道耦合(SOC)。我们研究了SOC和E-5环次epsilon多模Jahn-Teller效应对TPA配体的竞争效应。这些细微的扭曲被发现对磁各向异性有显著的影响。通过严格处理d(6)组态中三重态和五重态产生的所有自旋多重态，从第一性原理预测了有效自旋哈密顿(SH)方法的参数。基于非摄动方法，我们研究了SH方法在哪些条件下是有效的，以及哪些项需要保留。最近报道的四个结构和磁性很好的体系[Fe(TPA(R))](-)(-)(R=叔丁基，Tbu(1)，甲苯基，Mes(2)，苯基，Ph(3)，和2，6-二氟苯基，DFP(4))的晶体结构中已经观察到的微小几何扭曲足以导致五个最低的和热可及的自旋子能级由S=2SH很好地描述，只要扩展一个四阶各向异性项。利用这一与实际物理相一致的最基本的参数化方法，重新解释了已报道的靶系统的磁化强度数据，发现与从头计算结果符合得很好。用配位场(角重叠)模型(从头算配位场，AILFT)对从头计算得到的多重态能量进行了非常一致的拟合。这使得可以确定成键参数，并定量地证明了D值越来越负与外围配体引起的sigma键强度变化之间的相关性。事实上，沿着系列1>；2>；3>；4，配体的sigma成键能力(因此Lewis碱性)降低。
A theoretical, computational, and conceptual framework for the interpretation and prediction of the magnetic anisotropy of transition metal complexes with orbitally degenerate or orbitally nearly degenerate ground states is explored. The treatment is based on complete active space self-consistent field (CASSCF) wave functions in conjunction with N-electron valence perturbation theory (NEVPT2) and quasidegenerate perturbation theory (QDPT) for treatment of magnetic field- and spin-dependent relativistic effects. The methodology is applied to a series of Fe(II) complexes in ligand fields of almost trigonal pyramidal symmetry as provided by several variants of the tris-pyrrolylmethyl amine ligand (tpa). These systems have recently attracted much attention as mononuclear single-molecule magnet (SMM) complexes. This study aims to establish how the ligand field can be fine tuned in order to maximize the magnetic anisotropy barrier. In trigonal ligand fields high-spin Fe(II) complexes adopt an orbitally degenerate E-5 ground state with strong in-state spin-orbit coupling (SOC). We study the competing effects of SOC and the E-5 circle times epsilon multimode Jahn-Teller effect as a function of the peripheral substituents on the tpa ligand. These subtle distortions were found to have a significant effect on the magnetic anisotropy. Using a rigorous treatment of all spin multiplets arising from the triplet and quintet states in the d(6) configuration the parameters of the effective spin-Hamiltonian (SH) approach were predicted from first principles. Being based on a nonperturbative approach we investigate under which conditions the SH approach is valid and what terms need to be retained. It is demonstrated that already tiny geometric distortions observed in the crystal structures of four structurally and magnetically well-documented systems, reported recently, i.e., [Fe(tpa(R))](-) (R = tert-butyl, Tbu (1), mesityl, Mes (2), phenyl, Ph (3), and 2,6-difluorophenyl, Dfp (4), are enough to lead to five lowest and thermally accessible spin sublevels described sufficiently well by S = 2 SH provided that it is extended with one fourth order anisotropy term. Using this most elementary parametrization that is consistent with the actual physics, the reported magnetization data for the target systems were reinterpreted and found to be in good agreement with the ab initio results. The multiplet energies from the ab initio calculations have been fitted with remarkable consistency using a ligand field (angular overlap) model (ab initio ligand field, AILFT). This allows for determination of bonding parameters and quantitatively demonstrates the correlation between increasingly negative D values and changes in the sigma-bond strength induced by the peripheral ligands. In fact, the sigma-bonding capacity (and hence the Lewis basicity) of the ligand decreases along the series 1 > 2 > 3 > 4.