Strong Ferromagnetic Coupling between Co and Co2+ with Odd Electron (Anti)aromatic Radicals via Noncovalent Interaction

We have aimed to understand the interaction between odd electron aromatic or antiaromatic radicals with cobalt and their dipositive ion to understand the magnetic interaction between them. Density functional theory (DFT) along with the complete active space self consistent field (CASSCF) method has been used to calculate the magnetic exchange coupling constant (J) between the radical molecules and Co/Co²⁺. The DFT-calculated J ranging from 897 to 6060 cm^–1 and 2534 to 18574 cm^–1 for the CASSCF method signifies that the odd electron (anti)aromatic-based magnetic molecules could be useful as strong low-dimensional magnetic materials. Frequency analysis reveals that some of the radicals behave as a transition-state structure in the absence of metal but become stabilized upon the addition of Co or Co²⁺. The noncovalent interaction (NCI) and electron localization function (ELF) analysis indicate that there is no covalent bonding between radicals and the metal. The absence of covalent bonding between the metal and radicals indicates direct ferromagnetic interaction between them. Aromaticity in the studied Co/Co²⁺–radical complexes has been evaluated using the nucleus independent chemical shift (NICS), harmonic oscillator model of aromaticity (HOMA), and gauge-including magnetically induced currents (GIMIC) analysis, revealing a complex, multidimensional nature of aromaticity. NICS(1) values indicated that the same ring exhibits both aromatic and antiaromatic behavior, depending on the spatial orientation of the metal center. The HOMA value shows a strong correlation with the magnetic exchange coupling constant (J), supporting a link between structural aromaticity and magnetic interaction. The aromaticity index GIMIC is not well correlated with other aromaticity indexes like NICS and HOMA. These observations highlight the need for multiple aromaticity descriptors to fully capture the complex aromatic character of these systems.