Magnetic Exchange Coupling in Macrocyclic Cobalt (II) Complexes: The Influence of Bridging Ligands and Choice of the Computational Methodologies

Exchange‐coupled spin states of cobalt complexes represent the key to understanding the challenging nature of the magnetism of cobalt dimer. In this study, we introduce a comprehensive investigation of the nature of magnetic super‐exchange coupling interactions between two metallic centers in a macro ligand cobalt dimer. More importantly, we provide a detailed comparison between a series of bridging ligands and study their role in the magnetic exchange coupling constant in cobalt dimer with tetraaza macrocycle ligand scaffold. By varying the types of bridging and terminal ligands, we explored the nature of magnetic coupling in 24 species of cobalt dimers, which allowed us to draw a magneto‐structure relationship and monitor the effect of bridging/terminal ligands on the overall magnetism. We employed different density functional theory (DFT) methodologies and complete active space self‐consistent field (CASSCF) in our investigation. The results show that there are comprehensible functional dependencies, and the choice of exchange‐correlation functional is critical for calculating the magnetic properties. Using the experimental values of the exchange coupling, we found that hybrid functionals with 10–20 % Hartree‐Fock exchange integrals give acceptable accuracy. Furthermore, we adjusted the percentage of HF integrals to estimate the optimum value required to be included in the hybrid functionals. Finally, we used CASSCF to gain deeper understanding of the magnetic coupling for the valence magnetic electrons and to achieve the true ground state wavefunction, which is not feasible with conventional DFT calculations. However, the CASSCF calculations poorly underestimate the magnetic coupling, which can be significantly improved by including the dynamic correlations, as implemented in the NEVPT2 methodology. This study investigates the magnetic interactions between cobalt ions in a tetraaza macrocycle ligand scaffold. The role of different types of ligands and density functional theory methods in determining the magnetic properties is explored. The study shows that the accuracy of these methods is highly dependent on the exchange‐correlation functional used, and that a multideterminant approach is needed to accurately calculate unachievable spin states. These findings provide important insights into the design of new materials with tailored magnetic properties.