Probing the Magnetic Anisotropy of Co(II) Complexes Featuring Redox-Active Ligands
Probing the Magnetic Anisotropy of Co(II) Complexes Featuring Redox-Active Ligands
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DOI：
10.1021/acs.inorgchem.0c01812
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发表时间：
2020-11-16
影响因子：
4.6
通讯作者：
Fiedler, Adam T.
中科院分区：
文献类型：
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作者：
Kumar, Praveen;SantaLucia, Daniel J.;Fiedler, Adam T.
具有大磁各向异性（也称为零场分裂，ZFS）的配位配合物在磁性材料领域具有潜在的应用，包括单分子磁体（SMM）。先前的研究探讨了配位数和几何形状在控制高自旋 (S = 3/2) Co(II) 配合物的磁各向异性和 SMM 行为中的作用。基于这些努力，目前的工作研究了配体氧化态和结构畸变对五配位 Co(II) 配合物的自旋态和 ZFS 参数的影响。本研究中包含的五个配合物 (1-5) 具有通式 [Co(Tp(Ph2))(L-X,L-Y)](n+) (X = O, S; Y = N, O; n = 0 或 1)，其中 TpPh2 是蝎形配体氢三(3,5-二苯基吡唑基)硼酸盐(1-)，L-X,L-Y 是二齿二氧杂环戊烯型可以达到多种氧化态的配体。本文使用的具体L-X，L-Y配体是4，6-二叔丁基取代的邻氨基苯酚酯和邻氨基苯硫酚酯（分别为1和2）、邻亚氨基半醌酸酯和邻半醌酸酯基团（分别为3和4）以及邻亚氨基苯醌（5）。单晶 X 射线衍射显示，每个复合物都呈现出扭曲的三角双锥几何形状。直流（dc）磁化率实验证实，具有闭壳配体（1、2和5）的配合物具有S = 3/2基态，负D值（易轴各向异性）分别为-41、-78和-30 cm(-1)。对于 3 和 4，Co(II) 中心和邻（亚氨基）半醌酸根配体之间的反铁磁耦合导致 S = 1 基态，同样表现出非常大的负各向异性 (-100 > D > -140 cm(-1))。值得注意的是，使用远红外磁光谱 (FIRMS) 直接测量每种复合物的 ZFS。结合高频和场电子顺磁共振 (HFEPR) 研究，这些技术为配合物 1、2 和 5 提供了精确的自旋哈密顿参数。使用 CASSCF/NEVPT2 方法进行的多参考从头计算表明，这些 Co(II) 配合物的强负各向异性主要源于 Tp(Ph2) 配体施加的收缩导致的赤道面扭曲。 3 和 4 中钴(II)-自由基交换相互作用进一步放大了这种效应。
Coordination complexes that possess large magnetic anisotropy (otherwise known as zero-field splitting, ZFS) have possible applications in the field of magnetic materials, including single molecule magnets (SMMs). Previous studies have explored the role of coordination number and geometry in controlling the magnetic anisotropy and SMM behavior of high-spin (S = 3/2) Co(II) complexes. Building upon these efforts, the present work examines the impact of ligand oxidation state and structural distortions on the spin states and ZFS parameters of pentacoordinate Co(II) complexes. The five complexes included in this study (1-5) have the general formula, [Co(Tp(Ph2))(L-X,L-Y)](n+) (X = O, S; Y = N, O; n = 0 or 1), where TpPh2 is the scorpionate ligand hydrotris(3,5-diphenyl-pyrazolyl)borate(1-) and L-X,L-Y are bidentate dioxolenetype ligands that can access multiple oxidation states. The specific L-X,L-Y ligands used herein are 4,6-di-tert-butyl substituted o-aminophenolate and o-aminothiophenolate (1 and 2, respectively), o-iminosemiquinonate and o-semiquinonate radicals (3 and 4, respectively), and o-iminobenzoquinone (5). Each complex exhibits a distorted trigonal bipyramidal geometry, as revealed by single-crystal X-ray diffraction. Direct current (dc) magnetic susceptibility experiments confirmed that the complexes with closed-shell ligands (1, 2, and 5) possess S = 3/2 ground states with negative D-values (easy-axis anisotropy) of -41, -78, and -30 cm(-1), respectively. For 3 and 4, antiferromagnetic coupling between the Co(II) center and o-(imino)semiquinonate radical ligand results in S = 1 ground states that likewise exhibit very large and negative anisotropy (-100 > D > -140 cm(-1)). Notably, ZFS was measured directly for each complex using far-infrared magnetic spectroscopy (FIRMS). In combination with high-frequency and -field electron paramagnetic resonance (HFEPR) studies, these techniques provided precise spin-Hamiltonian parameters for complexes 1, 2, and 5. Multireference ab initio calculations, using the CASSCF/NEVPT2 approach, indicate that the strongly negative anisotropies of these Co(II) complexes arise primarily from distortions in the equatorial plane due to constrictions imposed by the Tp(Ph2) ligand. This effect is further amplified by cobalt(II)-radical exchange interactions in 3 and 4.