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 FeII 3 {FeNO}8 to FeIII 3 {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.