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Influence of a Redox-Active Phosphane Ligand on the Oxidations of a Diiron Core Related to the Active Site of Fe-Only Hydrogenase



Modulation of the Fe redox levels within the diiron dithiolato carbonyls with the coordination of a redox-active phosphane is reported. Treatment of [Fe2{μ-S(CH2)2NR(CH2)2S}(CO)6]2 and [Fe2(μ-pdt)(CO)6] with mppf affords the phosphane substituted species, [Fe2{μ-S(CH2)2NR(CH2)2S}(CO)5(mppf)]2 (1 and 2) and [Fe2(μ-pdt)(CO)5(mppf)] (3), respectively. These mppf-substituted complexes have been structurally and spectroscopically characterized. Results of the electrochemical study on 1 and 2 indicate that oxidation of the mppf-substituted complexes occurs at a potential 100 mV less than the related species ligated by the redox-inactive phosphane with approximately the same σ-donating ability. Stability of the oxidized species is improved on the electrochemical time scale. For the complex 3, three reversible oxidation events are observed. The mppf unit is oxidized at 0.16 V, which exerts an influence on the oxidation potential of the diiron core. In contrast to the PPh3 analogue, oxidation of both Fe centers is accessible at 0.47 and 0.77 V for 3. A large ΔE1/2 value suggests a substantial electron delocalization within the Fe2 core.