Kumar Praveen, SantaLucia Daniel J, Kaniewska-Laskowska Kinga, Lindeman Sergey V, Ozarowski Andrew, Krzystek J, Ozerov Mykhaylo, Telser Joshua, Berry John F, Fiedler Adam T
Department of Chemistry, Marquette University, Milwaukee, Wisconsin 53201, United States.
Department of Chemistry, University of Wisconsin-Madison, Madison, Wisconsin 53706, United States.
Inorg Chem. 2020 Nov 16;59(22):16178-16193. doi: 10.1021/acs.inorgchem.0c01812. Epub 2020 Nov 3.
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 ( = 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 (-) have the general formula, [Co(Tp)()] (X = O, S; Y = N, O; = 0 or 1), where Tp is the scorpionate ligand hydrotris(3,5-diphenyl-pyrazolyl)borate(1-) and are bidentate dioxolene-type ligands that can access multiple oxidation states. The specific ligands used herein are 4,6-di--butyl substituted -aminophenolate and -aminothiophenolate ( and , respectively), -iminosemiquinonate and -semiquinonate radicals ( and , respectively), and -iminobenzoquinone (). 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 (, , and ) possess = 3/2 ground states with negative -values (easy-axis anisotropy) of -41, -78, and -30 cm, respectively. For and , antiferromagnetic coupling between the Co(II) center and -(imino)semiquinonate radical ligand results in = 1 ground states that likewise exhibit very large and negative anisotropy (-100 > > -140 cm). 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 , , and . Multireference 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 ligand. This effect is further amplified by cobalt(II)-radical exchange interactions in and .
具有大磁各向异性（也称为零场分裂，ZFS）的配位络合物在磁性材料领域有潜在应用，包括单分子磁体（SMMs）。先前的研究探讨了配位数和几何结构在控制高自旋（S = 3/2）Co(II)络合物的磁各向异性和SMM行为中的作用。在这些工作的基础上，本研究考察了配体氧化态和结构畸变对五配位Co(II)络合物的自旋态和ZFS参数的影响。本研究中包含的五种络合物（-）具有通式[Co(Tp)(L)]（X = O，S；Y = N，O；n = 0或1），其中Tp是三（3,5 - 二苯基吡唑基）硼酸根（1 - ）螯合配体，L是可以具有多种氧化态的双齿二氧戊烯型配体。本文中使用的具体L配体是4,6 - 二叔丁基取代的 - 氨基酚盐和 - 氨基硫酚盐（分别为L₁和L₂）、-亚氨基半醌和 - 半醌自由基（分别为L₃和L₄）以及 - 亚氨基苯醌（L₅）。单晶X射线衍射表明，每种络合物都呈现出扭曲的三角双锥几何结构。直流（dc）磁化率实验证实，具有闭壳层配体（L₁、L₂和L₅）的络合物具有S = 3/2基态，其负值（易轴各向异性）分别为 - 41、-78和 - 30 cm⁻¹。对于L₃和L₄，Co(II)中心与 - （亚氨基）半醌自由基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）直接测量了每种络合物的ZFS。结合高频和高场电子顺磁共振（HFEPR）研究，这些技术为络合物L₁、L₂和L₃提供了精确的自旋哈密顿参数。使用CASSCF/NEVPT2方法的多参考计算表明，这些Co(II)络合物的强负各向异性主要源于Tp配体施加的限制导致的赤道平面畸变。这种效应在L₃和L₄中通过钴（II） - 自由基交换相互作用进一步放大。
