Sulfur molybdenum clusters

More recendy, molecular molybdenum-sulfur complexes and clusters have been used as soluble precursors for M0S2 in the formulation of lubricating oils for a variety of appHcations (70). Presumably, the oil-soluble molybdenum—sulfur-containing precursors decompose under shear, pressure, or temperature stress at the wear surface to give beneficial coatings. In several cases it has been shown that the soluble precursors are trifunctional in that they not only display antifriction properties, but have antiwear and antioxidant characteristics as weU. In most cases, the ligands for the Mo are of the 1,1-dithiolate type, including dithiocarbamates, dithiophosphates, and xanthates (55,71). 

Table 5 shows HDS product distributions over several catalysts prepared by using the molybdenum-nickel cluster 2. Sulfur content in decane was adjusted to 5.0 wt% in these experiments. MoNi/NaY was found to be more active than MoNi/Al203. It is to be noted that during the high temperature pretreatment the original cluster structure would have been changed. However, the high activity of the MoNi/NaY catalyst for benzothiophene HDS is probably due to the formation of active sites derived from this particular mixed metal cluster. 

Table 3.4 lists values for A Eq and for some important oxidation and spin states found in bioinorganic molecules. Data are taken from reference 24 and from Table 1 of reference 25 for hemoglobin, myoglobin, and the picket-fence porphyrin model compound, FeTpivPP(l-Melm).25 The myoglobin and hemoglobin model compounds are discussed in Section 4.8.2. Reference 26 provides the Table 3.4 data on iron sulfur clusters found in many bioinorganic species.26 The unusual iron-sulfur and iron-molybdenum-sulfur clusters found in the enzyme nitrogenase are discussed more fully below and in Chapter 6. 

It is recalled that in Chapter 9, Section 2, the electrochemical behaviour of the FeMo cofactor from FeMo-nitrogenase, was reported. It possesses a heteronuclear iron-molybdenum-sulfur (MoFe7S9) cluster, which has similarities with the above discussed iron-sulfur proteins. 

Iron-molybdenum-sulfur clusters will be considered in Section 62.1.14, which deals with aspects of the nitrogen cycle. 

Formic acid also reduces stable iminium functionalities (but not in general carbonyl groups). Acridi-nium ions are also readily reduced. This process bears some outward resemblance to the enzymatic transformation of formate to carbon dioxide by formate dehydrogenase, although a molybdenum/sulfur cluster is probably the hydride acceptor in the enzyme. ... 

Cluster compounds, chiral, 27 191 molybdenum-sulfur, 27 47 transition metal, 27 182... 

One of the earliest successful applications of EXAFS to probe a me-talloenzyme was the study of the molybdenum site of nitrogenase. Studies were made on both the C. pasteurianum and A. vinelandii MoFe-proteins and on isolated FeMoco (116). These studies showed definitively that molybdenum is present as part of a polynuclear cluster containing sulfur and iron, with Mo—S and Mo—Fe distances of —2.36 and —2.72 A, respectively. This work inspired the successful development of many chemical systems containing Mo—Fe—S clusters, and XAS studies of these systems strengthened the basis for the interpretation of corresponding data for the natural system. The most accurate picture of the molybdenum site of FeMoco currently available involves a coordination of about three oxygen (or nitrogen), sulfur, and iron atoms at —2.10, —2.37, and —2.70 A, respectively (117). 

Methods of preparing molybdenum(V) complexes with M02O4 cores have been published. However, those of sulfur-bridged molybdenum(V) complexes with M02O3S and M02O2S2 cores remain difficult. Molybdenum-sulfur cluster compounds attract much attention since almost all molybdenum enzymes contain both molybdenum and sulfur, being always accompanied by iron. 

Cyclopentadienyl molybdenum-sulfur compounds are useful synthons for the preparation of Mo—M—S (e.g. M = Fe (see Section 36.6.2), Co, Ni) clusters " in reactions with metal carbonyls. However, the principal interest in molecules of this class has arisen because of the reactivity of the Mo—S system, primarily in respect of the making and breaking of S—H... 

Despite the lack of success in FeMoco synthesis, the quest for the FeMoco has motivated the study of numerous iron-molybdenum-sulfur clusters. " MoFe3S4 cubanes represent half of the FeMoco, and therefore have been extensively studied. Catecholates are often used to model the homocitrate ligand tetrachlorocatecholate (CU-cat) is common, but other adds are also used. ... 

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