Assessment of the intramolecular magnetic interactions in the highly saddled iron(iii) porphyrin π-radical cations: the change from planar to saddle conformations

The intramolecular magnetic interactions in one-electron oxidized iron(iii) porphyrin pi-radical cations, [Fe(OETPP)Cl][SbCl6] (1), [Fe(OMTPP)Cl][SbCl6] (2) and [Fe(TPP)Cl][SbCl6] (3), have been compared by means of X-ray crystallography, SQUID magnetometry, cyclic voltammetry, UV-Vis spectroelectrochemical analysis, NMR spectroscopy analysis and unrestricted DFT calculations. Unlike a generally recognized antiferromagnetic coupling d(xy)(up arrow)d(xz)(up arrow)d(yz)(up arrow)dz(2 up arrow)dx(2)-y(2 up arrow)P(+)(a(2u))(down arrow) (S = 2) state via a weak bonding interaction as in (3), we have disclosed that a strong bonding interaction among iron dx(2)-y(2)and porphyrin a(2u) orbitals forms in (1) into a highly delocalized psi(pi) = [P+(a(2u)) + Fe-III(dx(2)-y(2), dz(2))] orbital that is able to accommodate two spin-paired electrons to form the psi(2)(pi)d(xy)(1)d(xz)(1)d(yz)(1), dz(21) (S = 2) ground state. Concurrently, the spin polarization effect is exerted on the paired spins in the psi(pi) orbital by magnetic induction from the remaining unpaired electrons in the iron d orbitals. The interpretation mentioned above is further verified by the diamagnetic nature of the saddled copper(ii) porphyrin pi-cation radical, Cu-II(OETPP)(ClO4) (S = 0), where the strong bonding interaction leads to the psi(2)(pi)d(xy)(2)d(xz)(2)d(yz)(2)dz(22) (S = 0) ground state but no spin polarization exists. Thus, the magnetic nature of the iron(iii) porphyrin pi-radical cation is tuneable by saddling the ring planarity.