Magnetic Anisotropy and Relaxation in Four-Coordinate Cobalt(II) Single-Ion Magnets with a [Co^IIO₄] Core

Area of Science:

• Coordination Chemistry
• Magnetochemistry
• Computational Chemistry

Background:

• mononuclear four-coordinate cobalt(II) complexes with a [Co^IIO₄] core are of interest for their magnetic properties.
• Structural distortions in metal complexes can significantly influence their magnetic behavior.

Purpose of the Study:

• To synthesize and characterize a mononuclear four-coordinate Co(II) complex, PPN[Li(MeOH)₄][Co(L)₂] (1).
• To investigate the impact of structural distortion on the magnetic anisotropy and relaxation dynamics of the Co(II) complex.
• To explore methods for suppressing quantum tunneling of magnetization (QTM) and understand the underlying structure-property relationships.

Main Methods:

• X-ray crystallography for structural determination.
• Magnetic characterization, including alternating current (ac) susceptibility measurements.
• Theoretical calculations to analyze magnetic anisotropy parameters.

Main Results:

• Complex 1 exhibits a severely distorted [Co^IIO₄] core with specific O-Co-O bite and dihedral twist angles.
• The distortion leads to large easy-axis magnetic anisotropy (D = -104(1) cm^-1) and a transverse component (|E| = +4(2) cm^-1).
• Slow relaxation of magnetization was observed, and QTM was suppressed by applying a dc field or diluting with a Zn matrix, although dilution altered the complex's structure.

Conclusions:

• The D parameter is primarily governed by the O-Co-O bite angle in tetrahedral Co(II) complexes.
• The rhombic component of anisotropy increases with deviation of the dihedral angle from 90°.
• These findings provide valuable guidelines for designing and tuning the magnetic properties of cobalt(II) complexes.