Dithiocarbamate complexes molybdenum

A selection of molybdenum dithiocarbamate complexes in oxidation states +5... 

The chemistry of molybdenum dithiocarbamate complexes has come a long way since the early work of Malatesta (767), and between 1978 and 2003 there were over 300 publications in this area, accessible oxidation states ranging from 0 to +6. 

Figure 277. Examples of heterometallic mercury-molybdenum dithiocarbamate complexes...

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). 

One approach to limit dimer formation in model complexes involves the use of bulky ligands and weakly coordinating solvents. Holm and coworkers [196,198-201] have studied oxygen atom transfer reactions of 2,6-bis(2,2-diphe-nyl-2-thioethyl)pyridinate [2] molybdenum oxo complexes. In contrast to the structurally similar dithiocarbamate molybdenum complexes, the tendency of the 2,6-bis(2,2-dipheny 1-2-thioethy l)pyridinate MoIV monoxo complex to undergo di-... 

Table 5.15. Protective tribofilm formation from degradation of molybdenum dithiocarbamate (MoDTC), molybdenum dithiophosphate (MoDDP) and molybdenum amine-ester complex (MoAC) directly and in combination with ZDDPs or sulfur compounds...

The various types of compound and their mechanisms of action have been reviewed by Mitchell . He concluded that the question of the extent to which the molybdenum additives decompose to produce molybdenum disulphide, if at all, was not yet resolved and certainly depended on the type of compound and the operating conditions. There was little doubt that molybdenum dithiocarbamates form molybdenum disulphide, probably by decomposition at hot spots caused by asperity interactions. It was less certain that molybdenum dithiophosphates form molybdenum disulphide, although mixtures of zinc dialkyidithiophosphate and molybdenum complexes had been shown to do so . 

It is not clear when dithiocarbamates were first prepared, but certainly they have been known for at least 150 years, since as early as 1850 Debus reported the synthesis of dithiocarbamic acids (1). The first synthesis of a transition metal dithiocarbamate complex is also unclear, however, in a seminal paper in 1907, Delepine (2) reported on the synthesis of a range of aliphatic dithiocarbamates and also the salts of di-iTo-butyldithiocarbamate with transition metals including chromium, molybdenum, iron, manganese, cobalt, nickel, copper, zinc, platinum, cadmium, mercury, silver, and gold. He also noted that while dithiocarbamate salts of the alkali and alkali earth elements were water soluble, those of the transition metals and also the p-block metals and lanthanides were precipitated from water, to give salts soluble in ether and chloroform, and even in some cases, in benzene and carbon disulfide. 

Transition metal dithiocarbamate complexes were probably first prepared in 1907 by Delepine (2) and over the following century, dithiocarbamate complexes of all the transition elements have been prepared and in a wide range of different oxidation states (Table 1). Perhaps most impressive is their ability to stabilize molybdenum in seven oxidation states ranging from - -6 to 0 (Fig. 24). 

Another widely utilized method involves the oxidative-addition of thiuram disulhdes to metal centers (Eq. 15) a reaction that has been reviewed by Victoriano (25, 170). Dithiocarbamate complexes of a number of transition metals can be prepared via this method, with examples at vanadium (171-173), molybdenum (174-180) and tungsten (181-183) being particularly prevalent. 

Figure 84. Examples of molybdenum (VI) imido bis(dithiocarbamate) complexes.

Carmona and co-workers (943-945) detailed the synthesis and strucmre of molybdenum(II) complexes bearing an agostic acyl ligand. Thus, addition of dithiocarbamate salts to [MoCl(CO)(PMe3)3 t -C(0)CH2X ] (X = H, SiMe3)... 

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