posted on 2015-11-11, 00:00authored byGraham de Ruiter, Niklas B. Thompson, Davide Lionetti, Theodor Agapie
A series
of tetranuclear iron complexes displaying a site-differentiated
metal center was synthesized. Three of the metal centers are coordinated
to our previously reported ligand, based on a 1,3,5-triarylbenzene
motif with nitrogen and oxygen donors. The fourth (apical) iron center
is coordinatively unsaturated and appended to the trinuclear core
through three bridging pyrazolates and an interstitial μ4-oxide moiety. Electrochemical studies of complex [LFe3(PhPz)3OFe][OTf]2 revealed three reversible
redox events assigned to the FeII4/FeII3FeIII (−1.733 V), FeII3FeIII/FeII2FeIII2 (−0.727 V), and FeII2FeIII2/FeIIFeIII3 (0.018 V) redox couples. Combined Mössbauer and crystallographic
studies indicate that the change in oxidation state is exclusively
localized at the triiron core, without changing the oxidation state
of the apical metal center. This phenomenon is assigned to differences
in the coordination environment of the two metal sites and provides
a strategy for storing electron and hole equivalents without affecting
the oxidation state of the coordinatively unsaturated metal. The presence
of a ligand-binding site allowed the effect of redox modulation on
nitric oxide activation by an FeII metal center to be studied.
Treatment of the clusters with nitric oxide resulted in binding of
NO to the apical iron center, generating a {FeNO}7 moiety.
As with the NO-free precursors, the three reversible redox events
are localized at the iron centers distal from the NO ligand. Altering
the redox state of the triiron core resulted in significant change
in the NO stretching frequency, by as much as 100 cm–1. The increased activation of NO is attributed to structural changes
within the clusters, in particular, those related to the interaction
of the metal centers with the interstitial atom. The differences in
NO activation were further shown to lead to differential reactivity,
with NO disproportionation and N2O formation performed
by the more electron-rich cluster.