Iron complexes sulfur ligands

In summary, the fragmentary evidence at hand suggests that sulfur was the primordial iron complexing material and that this ligand system was selectively modified by stepwise transition to a mixed sulfur-nitrogen and sulfur-oxygen and, finally, to an all-oxygen type of coordination. 

A large number of studies devoted to metal-sulfur centers are motivated by the occurrence of such arrangements at the active site of various metalloenzymes [1-13]. Mononuclear complexes with Mo=0 func-tion(s) and possessing sulfur ligands in their coordination sphere have been extensively investigated since they can be seen as models of the active site of enzymes such as nitrate- and DM SO reductases or sulfite- and xanthine oxidases [1-4]. On the other hand, a large variety of mono-, di-, and polynuclear Mo—S centers have been synthesized in order to produce functional models of the Mo-nitrogenase since the exact nature (mono-, di- or polynuclear) of the metal center, where N2 interacts within the iron-molybdenum cofactor (FeMo—co) of the enzyme is still unknown [4-8]. 

Quantitative conversion of the iron in succinate dehydrogenase to this form is possible if additional cysteine is added to the reaction mixture. It is probable that not enough cysteinyl sulfur ligands are available for complex formation without addition of the extra cysteine some of the nitrosyl complex does form without any cysteine addition in these systems. 

The syntheses of iron isonitrile complexes and the reactions of these complexes are reviewed. Nucleophilic reagents polymerize iron isonitrile complexes, displace the isonitrile ligand from the complex, or are alkylated by the complexes. Nitration, sulfonation, alkylation, and bromina-tion of the aromatic rings in a benzyl isonitrile complex are very rapid and the substituent is introduced mainly in the para position. The cyano group in cyanopentakis(benzyl isonitrile)-iron(ll) bromide exhibits a weak "trans" effect-With formaldehyde in sulfuric acid, benzyl isonitrile complexes yield polymeric compositions. One such composition contains an ethane linkage, suggesting dimerization of the transitory benzyl radicals. Measurements of the conductivities of benzyl isonitrile iron complexes indicate a wide range of A f (1.26 e.v.) and o-o (1023 ohm-1 cm.—1) but no definite relationship between the reactivities of these complexes and their conductivities. 

The crystal structure of this molecule has been determined. The iron is found in a distorted octahedral coordination, and the zinc is tetrahedrally coordinated. The Znl2 unit shares the Et2Dtc sulfur atoms trans to the RNC groups with the iron (Fig. 32). Attempts to use the Fe(Et2Dtc)3 complexes as ligands for Znl2 have not been successful (439). 

Szacilowski K, Chmura A, Stasicka Z. Interplay between iron complexes, nitric oxide and sulfur ligands Structure, (photo)reactivity and biological importance. Coord Chem Rev 2005 249 2408-36. 

The electronic coupling has been calculated for each of the flavin electron transfer complexes described in this chapter and is described below, with the exceptions of CPR, TMADH and PDR. For these three proteins, the two redox cofactors are in direct van der Waals contact, either between the C-7 and C-8 methyl groups of two flavins (CPR), or between the flavin C-8 methyl and a cysteinyl sulfur ligand to the iron-sulfur center (TMADH and PDR). In these cases the coupling between the redox centers should be maximal and the electron transfer rates should depend only on the driving foree and reorganization energy for the electron transfer proeesses. 

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