Ground State and Excited State Tuning in Ferric Dipyrrin Complexes Promoted by Ancillary Ligand Exchange

Three ferric dipyrromethene
complexes featuring different ancillary ligands were synthesized by
one electron oxidation of ferrous precursors. Four-coordinate iron
complexes of the type (ArL)­FeX2 [ArL = 1,9-(2,4,6-Ph3C6H2)2-5-mesityldipyrromethene] with X = Cl or tBuO were prepared and found to be high-spin (S = 5/2), as determined by superconducting quantum interference
device magnetometry, electron paramagnetic resonance, and 57Fe Mössbauer spectroscopy. The ancillary ligand substitution
was found to affect both ground state and excited properties of the
ferric complexes examined. While each ferric complex displays reversible
reduction and oxidation events, each alkoxide for chloride substitution
results in a nearly 600 mV cathodic shift of the FeIII/II couple. The oxidation event remains largely unaffected by the ancillary
ligand substitution and is likely dipyrrin-centered. While the alkoxide
substituted ferric species largely retain the color of their ferrous
precursors, characteristic of dipyrrin-based ligand-to-ligand charge
transfer (LLCT), the dichloride ferric complex loses the prominent
dipyrrin chromophore, taking on a deep green color. Time-dependent
density functional theory analyses indicate the weaker-field chloride
ligands allow substantial configuration mixing of ligand-to-metal
charge transfer into the LLCT bands, giving rise to the color changes
observed. Furthermore, the higher degree of covalency between the
alkoxide ferric centers is manifest in the observed reactivity. Delocalization
of spin density onto the tert-butoxide ligand in
(ArL)­FeCl­(OtBu) is evidenced
by hydrogen atom abstraction to yield (ArL)­FeCl and HOtBu in the presence of substrates containing
weak C–H bonds, whereas the chloride (ArL)­FeCl2 analogue does not react under these conditions.