52, Disulfide, molybdenum complexes

While all of the transformations detailed above take place at low-valent metal centers, a double sulfur-carbon bond cleavage process has also been postulated to occur at the molybdenum(VI) center (527, 528, 839). Thus, thermolysis of organic isocyanates with [Mo02(S2CNR2)2] affords imido-disulfide complexes, [Mo(NR )(S2)(S2CNR2)2] (Fig. 289), in moderate yields. The fate of the aminocarbyne fragment remains unknown and further, since monosulfido... 

SoHd lubricants ate added to help control high friction characteristics in high speed or heavy-duty appHcations where high temperatures are generated. Molybdenum disulfide [1317-33-5] M0S2, may be used alone or in a complex compound formed by grinding with fine natural graphite, and zinc sulfide [1314-98-3] ZnS. Other compounds include calcium fluoride, cryoHte [15096-52-3] Na AlF, rare-earth oxides, and metal sulfides, eg, iron, antimony, or zinc (see LUBRICATION AND LUBRICANTS). 

Catalysts based on molybdenum disulfide, M0S2, and cobalt or nickel as promoters are used for the hydrodesulfurization (HDS) and hydrodenitrogenadon (HDN) of heavy oil fractions [48,49]. The catalyst, containing at least five elements (Mo, S, Co or Ni, as well as O and A1 or Si of the support), is rather complex and represents a real challenge for the spectroscopist. Nevertheless, owing largely to research in the last twenty years, the sulfided C0-M0/AI2O3 system is one of the few industrial catalysts for which we know the structure in almost atomic detail [49, 50],... 

I) Simple Monomeric Molybdenum Complexes. The first report on the synthesis of simple Mo dithio complexes appeared in 1971 when Mo(IV)(R2Dtc)4 complexes were obtained by CS2 insertion into Mo(NR2)4 complexes (68). Shortly after this report, the synthesis of these complexes from MoC14 and NaR2Dtc in acetonitrile was reported (88). A variety of tetrakis R2Dtc complexes of Mo(IV) also were obtained by the oxidative decarbonyla-tion of Mo(CO)6 with tetraalkyl thiuram disulfide (481,482, 609). 

In addition to the dioxide MoO and the disulfide. MoS . example of leLravalem molybdenum compounds include the tetrachloride. MoClj. and teurabromide. MoBr. both of which are hydrolyzed in hot l-l-O. Other tetravalent compounds include a few of the complexes. 

A catalytic asymmetric oxidation of mono-, di-, and tri-substituted alkenes using a chiral bishydroxamic acid (BHA) complex of molybdenum catalyst in air at room temperature leads to good to excellent selectivity. It has been suggested that the Mo-BHA complex combines with the achiral oxidant to oxidize the alkene in a concerted fashion by transfer of oxygen from the metal peroxide to the alkene.78 The chiral BHA-molybdenum complex has been used for the catalytic asymmetric oxidation of sulfides and disulfides, utilizing 1 equiv. of alkyl peroxide, with yields up to 83% and ees up to 86%. An extension of the methodology combines the asymmetric oxidation with kinetic resolution providing excellent enantioselectivity (ee = 92-99%).79... 

Bis(azidomethyl)oxetane, 1896 Bis(azidothiocarbonyl) disulfide, 1012a Bis(benzeneazo) oxide, 3484 Bis(jj-benzene)chromium(0), 3505 Bis(benzene)chromium dichromate, 3845 Bis(benzenediazo) sulfide, 3485 Bis(jj-benzene)iron(0), 3506 Bis(benzeneiron)—fulvalenediyl complex, 3828 Bis(jj-benzene)molybdenum(0), 3508... 

Of the rather limited number of tetravalent molybdenum and tungsten derivatives known, wre shall mention here only molybdenum disulfide (M0S2), which occurs as the ore molybdenite and the octacyano complexes Mo(CN)i 4 and W(CN) 4 which, along with their pentavalent counterparts (p. 333), represent two of the very few examples of octa-covalency. 

Molybdenum disulfide is known to stabilize polyarylate. It was postulated that two mechanisms may be responsible for this process the formation of coordination complexes between carboxyl groups and molybdenum disulfide and the reaction with oxygen (antioxidative effect). 

It is thus seen that the interaction of molybdenum disulfide with its ambient surroundings in a tribological context involves complexities that have not been fully resolved. It has not been established why the friction of rubbed M0S2 should be adversely affected by adsorbed water vapor. Speculations range from the formation of intercalation structures... 

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