Nitric Oxide Activation by Distal Redox Modulation in Tetranuclear Iron Nitrosyl Complexes

摘要

A series of tetranuclear iron complexes displaying a site-differentiated metal center were 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 Fe^II4/Fe^II3Fe^III (−1.733 V), Fe^II3Fe^III/Fe^II2Fe^III2 (−0.727 V), and Fe^II2Fe^III2/Fe^IIFe^III3 (0.018 V) redox-couples. Complexes in all redox states were isolated, and three were characterized structurally by single crystal X-ray diffraction. Combined Mössbauer spectroscopic 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 in the cluster, and provides a strategy for storing electron and hole equivalents without affecting the oxidation state of the coordinatively unsaturated metal. The presence of an additional single ligand-binding site allowed for study of the effect of redox modulation on nitric oxide activation by an Fe^II metal center. Treatment of the clusters with nitric oxide resulted in binding of NO to the apical iron center generating a {FeNO} moiety. As with the NO-free precursors, three reversible redox events are observed electrochemically and 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, indicative of NO activation modulated by remote metal centers. The increased activation of NO is attributed to structural changes within the clusters, in particular 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 with N2O formation performed by the more electron rich cluster.