The study of magnetic properties, especially the magnetic anisotropy of iron–porphyrin complexes employing multiconfigurational methods, is quite challenging due to many strongly correlated electrons in nearly degenerate orbitals. However, a prerequisite for observing the magnetic anisotropy and slow magnetization relaxation, the zero-field splitting parameter, D, was experimentally observed decades ago for halide-based axially ligated penta-coordinate Fe(III)–porphyrins. In these complexes, the signs of D were reported mostly as positive; in a few cases, inconclusive signs of the D parameter were also mentioned. However, no ab initio calculations have been reported to shed light on this. Deciphering the electronic structure of these penta-coordinated complexes employing the complete active space self-consistent field method and N-electron valence second-order perturbation theory, we confirm the positive D values. However, a negative D value is highly desired to observe the single-molecule magnet properties without an external magnetic field, which we observed in the Fe(II)–porphyrin complexes with axial imidazole ligands instead of halide ligands. The detailed analysis of the multireference wave functions unravels the role of axial ligands in determining the sign and magnitude of the D parameters.

中文翻译：

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铁卟啉可以起到单分子磁体的作用吗？

由于近简并轨道中存在许多强相关电子，因此采用多构型方法研究铁卟啉配合物的磁各向异性，特别是磁各向异性，非常具有挑战性。然而，几十年前通过实验观察到卤化物基轴向连接五配位 Fe(III)-卟啉的零场分裂参数D是观察磁各向异性和慢磁化弛豫的先决条件。在这些复合体中， D的迹象大多呈阳性；在少数情况下，还提到了D参数的不确定迹象。然而，没有从头计算的报道来阐明这一点。利用完全活性空间自洽场法和N电子价态二阶微扰理论破译了这些五配位配合物的电子结构，确认了正D值。然而，在没有外部磁场的情况下，我们非常需要负 D 值来观察单分子磁体特性，我们在具有轴向咪唑配体而不是卤化物配体的 Fe(II)-卟啉配合物中观察到了这一点。对多参考波函数的详细分析揭示了轴向配体在确定D参数的符号和大小中的作用。