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.