Bonding characteristics mediated by saddle-shaped porphyrin deformation: A theoretical approach to the control of spin state of iron(III) porphyrins

Five-coordinate iron(III) porphyrin complexes can exist in high-spin (S = 5/2), intermediate-spin (S = 3/2), and admixed intermediate-spin (S = 5/2, 3/2) states. It has been found that weak-field axial ligands, a small core size of porphyrin macrocycle, and saddle-shaped deformation of porphyrin macrocycle induces the contribution of intermediate spin to the ground state. While the experimental ground state depends on the nature of axial ligands and porphyrin macrocycles. EHT and INDO calculations on a series of five-coordinate iron(III) porphyrin complexes in this study suggest a clear crystal-field explanation of the factors that can contribute to the stability of the intermediate-spin state. Based on our calculations, all these factors can increase the energy separation between the d(x2-y2) and d(z2) orbitals and between the d(x2-y2) and d(xy) orbitals and contribute to the relative stability of intermediate-spin state. Saddle-shaped deformation of porphyrin decreases the symmetry (C(4v)-C(2v)) of the coordination sphere and increases the probabilities of bonding interactions between metal and macrocycle. It is the number of bonding interactions of saddle-shaped metalloporphyrins that elevates the energy of d(x2-y2) orbital. On the other hand, for the same symmetry rationalization, d(x2-y2) and d(z2) orbitals are extensively hybridized and induce large electronic structure asymmetry to the saddle-shaped iron(III) porphyrin complexes. A novel concept of symmetry switch to control the spin transfer pathway that may be critical to the biological activities in nature is proposed