Iron complexes metal-sulfur interactions

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]. 

The complexes, [Fe(py>4]X2 (X = Br, 1), have been prepared by oxidation of metallic iron with bromine or iodine in methanol. Pyridine is added to the resultant solution and the pyridine complexes precipitated.22 Interaction between metallic zinc or cadmium and an excess of arsenic pentafluoride in liquid sulfur dioxide leads to the formation of solvento-complexes containing two or four SO2 ligands depending on the experimental conditions.23/24 The reactions studied are summarized as follows, Eq.4.4 ... 

Iron-iron bonds exist in carbonyl and nitrosyl derivatives, but these are not considered in this section. Properties consistent with Fe-Fe interactions in other systems are dominated by sulfur donor ligands. The presence of Fe-Fe bonds is not unambiguous, with sulfur bridges invariably accompanying short metal-metal separations. Synthetic routes to polynuclear iron-sulfur complexes are presented for completeness without implying the cluster bonding scheme. 

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. 

Sulfur cycling is affected in a variety of ways, including UV photoinhibition of organisms such as bacterioplankton and zooplankton that affect sources and sinks of DMS and UV-initiated CDOM-sensitized photoreactions that oxidize DMS and produce carbonyl sulfide. Metal cycling also interacts in many ways with UVR via direct photoreactions of dissolved complexes and of metal oxides and indirect reactions that are mediated by photochemically-produced ROS. Photoreactions can affect the biological availability of essential trace nutrients such as iron and manganese, transforming the metals from complexes that are not readily assimilated into free metal ions or metal hydroxides that are available. Such photoreactions can enhance the toxicity of metals such as copper and can initiate metal redox reactions that transform non-reactive ROS such as superoxide into potent oxidants such as hydroxyl radicals. 

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