Electronic and Magnetic Properties of Ferrous Iron in a True Square-Planar Molecular Environment

The electronic and magnetic properties of ferrous iron in the iron(II)-2,3,9,10,16,17,23,24-octakis(2,6-diisopropylphenoxy)phthalocyanine (FePc^OAr), exhibiting a true square-planar molecular environment, are investigated. Inhibition of intermolecular interactions by steric substituents allows detailed investigation of the electronic structure arising from the planar geometry of the d⁶ electron configuration. Complementary magnetometry, Mössbauer, FD-FT THz-EPR (frequency-domain Fourier-transform terahertz electron paramagnetic resonance) and pNMR (nuclear magnetic resonance of paramagnetic molecules) spectroscopies show that FePc^OAr has an S = 1 ground state with large positive axial zero-field splitting (ZFS) and a strongly anisotropic g-tensor, with two g-values much larger than the free electron g-value and one smaller. Correlation between the magnetic properties and the electronic structure is provided by high-level quantum chemical calculations. The calculations indicate a nearly triply degenerate ground level, in which spin-orbit coupling mixes the isolated ³A_2g ground state with two excited ³E_g states, whose energy gaps to the ground state are almost identical. These findings provide valuable insights in the electronic structure of iron phthalocyanines and the long-standing discussion on their true electronic ground level, which has important implications for the application of this important class of complexes in catalysis and magnetic materials.