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.