Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai, 400076, India.
Current address: RIKEN Center for Computational Science, 7-1-26, Minatojima-minami-machi, Chuo-ku, Kobe, 650-0047, Japan.
Chem Asian J. 2019 Dec 13;14(24):4696-4704. doi: 10.1002/asia.201901140. Epub 2019 Sep 20.
Mononuclear four coordinate Co complexes have drawn a great deal of attention as they often exhibit excellent single-ion magnet (SIM) properties. Among the reported complexes, the axial zero-field splitting parameter (D) was found to vary drastically both in terms of the sign as well as strength. There are various proposals in this respect such as structural distortions, heavier atom substitution, metal-ligand covalency, tuning secondary coordination sphere, etc. that are expected to control the D values. To assess the importance of structural distortions vs. heavier atom substitution effect, here we have undertaken detailed theoretical studies based on the ab initio CASSCF/NEVPT2 method to estimate zero-field splitting parameters for twelve complexes reported in the literature. Our test set includes the {Co X } (where X=O, S, Se) core structure where the D value was found to vary from +19 to -118 cm . Based on the structural variation, we have classified the complexes into three types (I-III) where type I complexes were found to exhibit the largest negative D value as desired for SIMs. The other two types (II and III) of complexes have been found to be inferior with respect to type I. The secondary coordination sphere was also found to influence D, as substitution on the secondary coordination sphere atom was found to significantly alter the magnitude of D values. Particularly, two structural parameters, namely, the dihedral angle between the two ligand planes and the X-Co-X polar angle were found to heavily influence the sign and strength of D values. Our analysis clearly reveals that these structural factors are much more important than the heavier atom substitution, or metal-ligand covalency. A large variation in the D and E/D values among these complexes despite possessing a very close structural similarity offers an exquisite playground for a chemist to design and develop new-generation Co -based SIMs.
单核四配位 Co 配合物因其通常表现出优异的单离子磁体 (SIM) 性质而引起了广泛关注。在报道的配合物中，轴向零场分裂参数 (D) 的符号和强度都有很大的变化。在这方面有各种建议，如结构变形、重原子取代、金属-配体共价键、调节次级配位球等，预计这些因素可以控制 D 值。为了评估结构变形与重原子取代效应的重要性，我们在这里采用基于从头算 CASSCF/NEVPT2 方法的详细理论研究，来估算文献中报道的 12 个配合物的零场分裂参数。我们的测试集包括 {Co X }（其中 X=O、S、Se）核心结构，其中 D 值的变化范围从 +19 到 -118 cm-1。基于结构的变化，我们将配合物分为三类（I-III），其中 I 类配合物表现出最大的负 D 值，符合 SIM 的要求。另外两种类型（II 和 III）的配合物在与 I 型相比时表现不佳。次级配位球也被发现会影响 D 值，因为次级配位球原子的取代会显著改变 D 值的大小。特别是，两个结构参数，即两个配体平面之间的二面角和 X-Co-X 极性角，被发现对 D 值的符号和强度有很大的影响。我们的分析清楚地表明，这些结构因素比重原子取代或金属-配体共价键更为重要。尽管这些配合物具有非常相似的结构，但它们的 D 和 E/D 值存在很大的差异，为化学家设计和开发新一代基于 Co 的 SIMs 提供了极好的机会。
