Probing the strong magnetic exchange behaviour of transition metal–radical complexes: a DFT case study†
Attaining very large magnetic exchange coupling values for polymetallic clusters is of great interest in the area of Single-Molecule Magnets (SMMs). Direct contact of paramagnetic centers in metal–radical complexes is known to yield very large magnetic exchange coupling values, but the estimation of such magnetic coupling values for polymetallic metal–radical complexes is not straightforward. Here, five previously reported M2–radical complexes (where M = CrIII, MnII, FeII, and CoII) have been studied using density functional theory. Our calculations suggest that a very strong overlap between the metal 3d orbitals and the radical's singly occupied π* orbital is the foremost reason for reported large magnetic exchange values. Our used methodology also gives a precise estimate of magnetic coupling values for this class of complexes. Model calculations and magneto-structural correlation studies suggest the importance of structural parameters, such as the metal–N(radical) distance and metal–radical planarity, in attaining very higher magnetic exchange coupling values with small structural modification.