Probing the Magnetic Anisotropy of Co(II) Complexes Featuring Redox-Active Ligands

Abstract

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,Y )] n+ (X = O, S; Y = N, O; n = 0 or 1), where Tp Ph2 is the scorpionate ligand hydrotris(3,5-diphenyl-pyrazolyl)borate(1−) and L X,Y are bidentate dioxolenetype ligands that can access multiple oxidation states. The specific L X,Y ligands used herein are 4,6-di-tert-butyl substituted oaminophenolate 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 singlecrystal 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.

Applying Unconventional Spectroscopies to the Single‐Molecule Magnets, Co(PPh3)2X2(X=Cl, Br, I): Unveiling Magnetic Transitions and Spin‐Phonon Coupling

“…Thus, the magnetic transitions are observable in far‐IR. Frequency‐domain spectroscopy (i. e., FIRMS) using high magnetic fields, revealing how the Zeeman effect changes the transitions between ZFS levels, is ideal to probe magnetic transitions in addition to studying IR‐active phonon excitations [13a,b,16,17c–m,36] . In the present S =3/2, D <0 case, the m S =±3/2 level can be excited to the m S =±1/2 level and spin‐Hamiltonian parameters can then be extracted from the spectra.…”

Composition and electronic structure of Mn3O4 and Co3O4 cathodes in zinc/air batteries: a DFT study

“…However, in this case and because the amount of oxygen atoms is low, the charge on the ions is lower than +1.35e and they remain in oxidation state +2. As it usual for many divalent ions, they are in a high spin state (with magnetic moment around 3µ B ), even when the geometry of coordination is not anymore tetrahedral [62].…”

The evolution of paramagnetic NMR as a tool in structural biology

“…On the other hand, relativistic CASSCF calculations, with second-order perturbation theory corrections, implemented in ORCA, have been shown to be quite reliable in predicting the magnetic properties of transition metal complexes and their temperature dependencies in a number of cases. 119,131,132 Therefore, starting from the crystallographic model of the cobalt(II) coordination cage, a structural refinement procedure was implemented so as to obtain a susceptibility tensor from ORCA in good agreement with the PCSs-derived anisotropy tensor. 133 The structural refinement was performed by adjusting, through a steepest descent search, 19 degrees of freedom among coordination bond lengths, angles and dihedral angles, selected in a way to avoid altering the structure of the ligands themselves.…”