Structure and properties of [Fe4S4{2,6-bis(acylamino)benzenethiolato-S}(4)](2-) and [Fe2S2{2,6-bis(acylamino)benzenethiolato-S}(4)](2-): Protection of the Fe-S bond by double NH center dot center dot center dot S hydrogen bonds

Iron-sulfur clusters containing a singly or doubly NH...S hydrogen-bonded arenethiolate ligand, [Fe4S4(S-2-RCONHC(6)H(4))(4)](2-) (R = CH3, t-Bu, CF3), [Fe4S4{S-2,6-(RCONH)(2)C6H3}(4)](2-), [Fe2S2(S-2-RCONHC(6)H(4))(4)](2-) (R = CH3, t-Bu, CF3), and [Fe2S2{S-2,6-(RCONH)(2)C6H3}(4)](2-), were synthesized as models of bacterial [4Fe-4S] and plant-type [2Fe-2S] fenedoxins. The X-ray structures and IR spectra of (PPh(4))(2)[Fe4S4{S-2,6-(CH3-CONH)(2)C6H3}(4)]. 2CH(3)CN and (NEt(4))(2)[Fe2S2{S-2,6-(t-BuCONH)(2)C6H3}(4)] indicate that the two amide NH groups at the o,o'-positions are directed to the thiolate sulfur atom and form double NH...S hydrogen bonds. The NH...S hydrogen bond contributes to the positive shift of the redox potential of not only (Fe4S4)(+)/(Fe4S4)(2+) but also (Fe4S4)(2+)/(Fe4S4)(3+) in the [4Fe-4S] clusters as well as (Fe2S2)(2+)/(Fe2S2)(3+) in the [2Fe-2S] clusters. The doubly NH...S hydrogen-bonded thiolate ligand effectively prevents the ligand exchange reaction by benzenethiol because the two amide NH groups stabilize the thiolate by protection from dissociation.