[Fe₄S₄]^<i>q</i> Cubane Clusters (<i>q</i> = 4+, 3+, 2+) with Terminal Amide Ligands

Bis(trimethylsilyl)amide-ligated iron−sulfur cubane clusters [Fe4(μ3-S)4(N{SiMe3}2)4]z (z = 0, 1−, 2−) are accessible by the reaction of FeCl(N{SiMe3}2)2(THF) (1) with 1 equiv of NaSH (z = 0), followed by reduction with either 0.25 (z = 1−) or 1 equiv (z = 2−) of Na2S as needed. The anionic clusters are obtained as the sodium salts [Na(THF)2][Fe4S4(N{SiMe3}2)4] and [Na(THF)2]2[Fe4S4(N{SiMe3}2)4]; in the solid state, these two clusters both possess a unique contact ion pair motif in which individual sodium ions each coordinate to a cluster core sulfide, an adjacent amide nitrogen, and two THF donors. The monoanionic cluster can also be prepared as the lithium salt [Li(THF)4][Fe4S4(N{SiMe3}2)4] by the reaction of 1 with 1:0.5 LiCl/Li2S. The characterization of the three-membered redox series allows an analysis of redox trends, as well as a study of the effects of the amide donor environment on the [Fe4S4] core. Bis(trimethylsilyl)amide terminal ligation significantly stabilizes oxidized cluster redox states, permitting isolation of the uncommon [Fe4S4]3+ and unprecedented [Fe4S4]4+ weak-field cores.