Iron compounds tetrahedral clusters

Iron Sulfur Compounds. Many molecular compounds (18—20) are known in which iron is tetrahedrally coordinated by a combination of thiolate and sulfide donors. Of the 10 or more structurally characterized classes of Fe—S compounds, the four shown in Figure 1 are known to occur in proteins. The mononuclear iron site REPLACE occurs in the one-iron bacterial electron-transfer protein rubredoxin. The [2Fe—2S] (10) and [4Fe—4S] (12) cubane structures are found in the 2-, 4-, and 8-iron ferredoxins, which are also electron-transfer proteins. The [3Fe—4S] voided cubane structure (11) has been found in some ferredoxins and in the inactive form of aconitase, the enzyme which catalyzes the stereospecific hydration—rehydration of citrate to isocitrate in the Krebs cycle. In addition, enzymes are known that contain either other types of iron sulfur clusters or iron sulfur clusters that include other metals. Examples include nitrogenase, which reduces N2 to NH3 at a MoFe S8 homocitrate cluster carbon monoxide dehydrogenase, which assembles acetyl-coenzyme A (acetyl-CoA) at a FeNiS site and hydrogenases, which catalyze the reversible reduction of protons to hydrogen gas. 

The multinuclear tetrahedral iron clusters have the potential for additional formal oxidation states. Because not all of these states have been found in proteins or model compounds, it is possible that some oxidation states may be unstable. For a given Fe S protein only one redox couple is used the other possible states appear to be excluded by restrictions of the protein structure. This selection rule is illustrated with two 4Fe 4S low-molecular-weight electron transfer proteins ferredoxin and high-potential iron protein (HiPIP). The 4Fe 4S clusters in both proteins were shown by X-ray crystallography to be virtually identical. However, the redox potential and oxidation states for the two proteins are vastly... 

Typical structural motifs for S-, Sc-, and Te-containing iron cluster compounds shown in Scheme 9 (E, E = S, Se, Te cf. Scheme 1) involve the tetrahedral monocapped species with the closed iron triangle 118 and the dicapped triiron species with the open Fcs framework 121, which can also be regarded as a square-pyramidal c2,EE skeleton with an Fcafifi basal plane 121. The protonated forms of 118 (119 and 120) and the dicapped octahedral structure 122 based on an Fc4 square are also known. Butterfly 11 and interstitial structures (14 and 15 Scheme 1) are very rare for non-first-row main group elements, and a rare example of the /44-S butterfly cluster 116 is discussed in Section... 

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