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

## TL;DR

This paper explores how cobalt and its ions interact with certain radicals to form strong magnetic materials through noncovalent bonds.

## Contribution

The study reveals that noncovalent interactions between cobalt and odd-electron radicals lead to strong ferromagnetic coupling.

## Key findings

- Magnetic exchange coupling constants (J) calculated using DFT and CASSCF show strong ferromagnetic interactions.
- Radicals stabilize when bound to cobalt or cobalt ions, behaving as transition-state structures without metal.
- Aromaticity analysis using NICS, HOMA, and GIMIC reveals complex and multidimensional aromatic behavior.

## Abstract

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.