Geometry optimizations used the TPSS density functional , the def2-SVP-basis set on carbon and hydrogen atoms, the def2-TZVP-basis set on heteroatoms and Grimme's D3 dispersion correction with Becke-Johnson damping. True minima were obtained for all models discussed in the main text; as is noted in the Supporting Information in more detail, for the extended models 1 Large and 2 Large in the energetically destabilised low spin states, frequency calculations either showed a significant imaginary mode of -500 cm -1 in which the iron ion is displaced from the plane, or could not be converged.