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

Authors

Praveen Kumar, Marquette UniversityFollow
Daniel J. Santa Lucia, University of Wisconsin - Milwaukee
Kinga Kaniewska-Laskowska, Gdańsk University of Technology
Sergey Lindeman, Marquette UniversityFollow
Andrew Ozarowski, Florida State University
J. Krzystek, Florida State University
Mykhaylo Ozerov, Florida State University
Joshua Telser, Roosevelt University
John F. Berry, University of Wisconsin - MadisonFollow
Adam T. Fiedler, Marquette UniversityFollow

Document Type

Article

Language

eng

Publication Date

11-16-2020

Publisher

American Chemical Society Publications

Source Publication

Inorganic Chemistry

Source ISSN

0020-1669

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 dioxolene-type ligands that can access multiple oxidation states. The specific L^X,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.

Comments

Accepted version. Inorganic Chemistry, Vol. 59, No. 22 (November 16, 2020): 16178-16193. DOI. © 2020 American Chemical Society Publications. Used with permission.

Recommended Citation

Kumar, Praveen; Santa Lucia, Daniel J.; Kaniewska-Laskowska, Kinga; Lindeman, Sergey; Ozarowski, Andrew; Krzystek, J.; Ozerov, Mykhaylo; Telser, Joshua; Berry, John F.; and Fiedler, Adam T., "Probing the Magnetic Anisotropy of Co(II) Complexes Featuring Redox-Active Ligands" (2020). Chemistry Faculty Research and Publications. 1014.
https://epublications.marquette.edu/chem_fac/1014