Spin-state ordering in hydroxo-bridged diiron(III)bisporphyrin complexes.

We report the synthesis, structure, and spectroscopic characterization of 1,2-bis[μ-hydroxo iron(III) 5-(2,3,7,8,12,13,17,18-octaethylporphyrinyl)]ethane with PF6(–) and SbF6(–) counteranions. The two iron centers are nonequivalent with admixed intermediate spin state (S = 3/2 with a minor contribution of S = 5/2) on each metal both in the solid and in solution. The molecules are compared with previously known μ-hydroxo complexes with other counterions, such as I3(–), BF4(–), and ClO4(–), which demonstrates that the nature of the counterion can affect the spin-state ordering dramatically. To understand how the spin-state ordering is affected by external perturbations, we also have done a comprehensive computational study. The calculations show that subtle environmental perturbations affect the spin-state ordering and relative energies and are likely to be the root cause of the variation in spin-state ordering observed experimentally.