Can Iron-porphyrins behave as Single-molecule Magnets?

Spin-bearing metal ions encapsulated by macrocyclic ligands in porphyrin complexes make them an ideal component in molecular spintronic devices where there is scope for the logical manipulation of various magnetic properties of the molecular system. Adding an axial ligand to the planar porphyrin complexes is established to be a very useful route to alter different aspects of the magnetochemistry of the resulting complex. Magnetic anisotropy is a lesser-known avenue in this context. For a series of high-spin pentacoordinate $d^5$ Fe(III) and $d^6$ Fe(II) porphyrin complexes with varying axial ligands, the magnetic anisotropy parameters are obtained from the spin Hamiltonian formalism. The $d^5$ high spin complexes, having a net zero orbital angular momentum, possess an almost isotropic magnetic environment. The small positive zero-field splitting (2-7 $cm^{-1}$) for these complexes arises due to near proximity of the quartet excited states. This ZFS is found to increase down the group for the halide ligands owing to the decrease of the sextet-quartet energy gap. {On the other hand, possessing a triaxial anisotropic magnetic environment, the sign and the magnitude of the ZFS parameters of the $d^6$ complexes are mostly dependent on the axial ligand itself. Amongst the considered complexes, H2O and NH3 exhibit positive ZFS while the Imidazole-based ligands possess a negative sign to the ZFS parameter.