Probing the Slow Relaxation of Magnetization of a Square Planar Cobalt Complex with Doublet Ground State

The observation of slow relaxation of magnetization in low‐spin square planar cobalt complexes is exceedingly rare, likely due to the synthetic challenges of stabilizing such geometries, along with the complexities introduced by hyperfine interactions and spin‐orbit coupling. Additionally, accurately characterizing the ground‐state electronic configuration of these complexes remains a significant challenge. In this article, we report a unique and rare square planar cobalt complex, [Co(L1⋅⁻)₂] (1), where the coordination sites are occupied by the phenanthroiminoquinone (L1). The molecular structure of complex 1 was determined using single‐crystal X‐ray diffraction studies. A structurally analogous nickel complex, [Ni^II(L1⋅⁻)₂] (2), was also synthesized and characterized. Detailed DC magnetic susceptibility measurements of 2 reveal strong antiferromagnetic exchange interactions between the radical centers, rendering it diamagnetic. For cobalt complex 1, this strong antiferromagnetic coupling results in a doublet ground state, as corroborated by X‐band EPR measurements (at 5 K) conducted on both polycrystalline and frozen solution samples. To gain deeper insights into the electronic structure of the cobalt ion in 1, a comprehensive suite of experimental and theoretical investigations was conducted, including X‐ray diffraction, DC magnetic studies, X‐band EPR, UV‐Vis‐NIR spectroscopy, and ab initio calculations. These studies collectively indicate that the cobalt ion in 1 exists in a divalent low‐spin state. Furthermore, the observed slow relaxation of magnetization for the doublet state of 1 highlights its potential as an ideal candidate for designing spin‐based molecular qubits.