Tetranuclear Fe Clusters with a Varied Interstitial Ligand: Effects on the Structure, Redox Properties, and Nitric Oxide Activation.

A new series of tetranuclear Fe clusters displaying an interstitial µ4-F ligand was prepared for a comparison to previously reported µ4-O analogues. With a single nitric oxide (NO) coordinated as a reporter of small-molecule activation, the µ4-F clusters were characterized in five redox states, from FeII3{FeNO}8 to FeIII3{FeNO}7, with NO stretching frequencies ranging from 1680 to 1855 cm-1, respectively. Despite accessing more reduced states with an F- bridge, a two-electron reduction of the distal Fe centers is necessary for the µ4-F clusters to activate NO to the same degree as the µ4-O system; consequently, NO reactivity is observed at more positive potentials with µ4-O than µ4-F. Moreover, the µ4-O ligand better translates redox changes of remote metal centers to diatomic ligand activation. The implication for biological active sites is that the higher-charge bridging ligand is more effective in tuning cluster properties, including the involvement of remote metal centers, for small-molecule activation.