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 mu(4)-F ligand was prepared for a comparison to previously reported mu(4)-O analogues. With a single nitric oxide (NO) coordinated as a reporter of small-molecule activation, the mu(4)-F clusters were characterized in five redox states, from Fe-3(II){FeNO}(8) to Fe-3(III){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 mu(4)-F clusters to activate NO to the same degree as the mu(4)-O system; consequently, NO reactivity is observed at more positive potentials with mu(4)-O than mu(4)-F. Moreover, the mu(4)-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.