Tetrahedral iron centers

Fig. 4. (a and b) Topologically distinct arrangements of protein ligands to tetrahedral iron centers. 

Reaction of K[Fe4S3(NO)7] with KOH in molten Ph2S2 as solvent leads to the isolation in modest yield of [Fe(NO)2(SPh)2] X-ray analysis of the (Et4N)+ salt confirmed the presence of the monomeric anion with linear Fe-N-O groups and an approximately tetrahedral iron center (104a). 

In the diagram shown in Figure 6.9, a sulfur atom has been inserted at the cluster s center in agreement with current thinking.24 The M center is attached to protein side chains at only two locations, cysteine 275 (at Fel) and histidine 442 (at Mo), and is unusual in having three-coordinate irons rather than the normal four-coordinate tetrahedral iron coordination sphere.29b The molybdenum forms... 

Iron-sulfur proteins serve predominantly as electron carriers (28,29). The best understood examples are those proteins with IFe, 2Fe, and 4Fe centers. The environment of the mononuclear iron center, rubredoxin, is shown in structure C (17). It consists of a distorted tetrahedral array of sulfur atoms from cysteine residues at nearly equal distances from the iron atom. Crystal structures are available for 2Fe-2S ferredoxins from Spindina plantensis (19) and Aphanothece sacrum (20). A representation of the geometry of this site is given in structure D. The 2Fe-2S core is anchored to the polypeptide by ligation to 4 cysteine sulfur atoms, yielding distorted tetrahedral geometry for both iron atoms. Crystal... 

Mononuclear octahedral/trigonal bipyramidal iron centers are found in either the ferric or the ferrous oxidation state (Whittaker etal., 1984 Arciero et ai, 1983). Because the iron may participate directly in catalysis as either a Lewis acid or base, only one state is the active form for a given enzyme. Transient redox changes may occur during turnover, but the enzyme returns to its initial condition. In contrast the tetrahedral mononuclear iron proteins appear to function primarily as electron transfer agents and therefore change oxidation state with a single turnover. 

Ferredoxins and high-potential iron proteins (HiPlPs) involve clnsters of two, three, and four tetrahedral Fe centers bridged by sulfido S donors. The coordination sphere at Fe is completed by thiolate cysteine hgands. 

The tris(mercaptophenylimidazolyl)borate iron and cobalt complexes [(Tmph)2M] (M = Fe, Co) have been synthesized by reaction of (Tmph)Tl with MI2.156 Structural characterization by X-ray diffraction demonstrates that the potentially tridentate Tmph binds through only two sulfur donors in these sandwich complexes and that the tetrahedral metal centers supplement the bonding by interactions with two BH groups. Comparison of the structures of [(Tmph)2M] with those of related tris(pyrazolyl)borate counterparts indicates that the Tm favors lower primary coordination number in divalent metal complexes. 

While copper and iron Lewis acids are the most prominent late transition metal Diels-Alder catalysts, there are reports on the use of other chiral complexes derived from ruthenium [97,98],rhodium [99],andzinc [100] in enantioselective cycloaddition reactions, with variable levels of success. As a comparison study, the reactions of a zinc(II)-bis(oxazoline) catalyst 41 and zinc(II)-pyridylbis(ox-azoline) catalyst 42 were evaluated side-by-side with their copper(II) counterparts (Scheme 34) [101]. The study concluded that zinc(II) Lewis acids catalyzed a few cycloadditions selectively, but, in contrast to the [Cu(f-Bubox)](SbFg)2 complex 31b (Sect. 3.2.1), enantioselectivity was not maintained over a range of temperatures or substitution patterns on the dienophile. An X-ray crystal structure of [Zn(Ph-box)] (01)2 revealed a tetrahedral metal center the absolute stereochemistry of the adduct was consistent with the reaction from that geometry and opposite that obtained with Cu(II) complex 31. 

---