Molybdothiol systems

More recently the stereochemically specific reduction of cyclopro-pene to cyclopropane and, in acid solution, to propylene has been demonstrated for the molybdothiol systems (288). This also parallels nitrogenase behavior. 

These Mo-based cyanide and insulin systems are suggested to operate similarly to the molybdothiol systems, via the formation of a coordinatively unsaturated oxo-molybdenum species as catalyst, which binds N2 side-on and reduces it by two electrons to N2H2 (see Equations 23, 24, 25). The intermediacy of N2H2 in the formation of NH3 is supported by the reduction of fumarate to succinate by these systems when operating under N2. Succinate is not produced when other substrates, for example, acetylene, are being reduced. 

In the reduction of acetylene with molybdothiol and molybdoselenol complex catalysts, the effects of structural variation in ligands, variety of coordination-donor atom, kind of transition-metal ion, and other factors have been surveyed systematically. These factors have profound effects on the catalytic activity. The Mo complexes of cysteamine (or selenocysteamine), its N,N-dimethyl derivative, and its /3-dimethyl derivative give ethylene, ethane, and 1,3-butadiene, respectively, as the major product. The Co (I I) complexes of cysteine and cysteamine show higher catalytic activity than do the corresponding Mo complexes, and the order of the activity in the donor atom, namely S >Se 0 in the Co(II) complexes is consistent with that in the Mo complex systems. On the basis of electron spin resonance (ESR) features of these Mo complex catalysts, a relationship between their ESR characteristics and catalytic activities is discussed. 

In conclusion, subtle variations in the molybdothiol-complex catalysts—ligand substitution, coordination donor atom, and central metal ion—have large effects on the catalytic activity of acetylene reduction. The results presented here may offer useful information for a design of catalysts with optimal activity in such complex systems. 

Molybdenum iw)and Tungsteriw). The discussion about the nature of molybdenum(iv) in solution continues, evidence being presented for both mononuclear and dinuclear species. Mononuclear oxomolybdate(ii) complex anions have been demonstrated as models for the reduction of CN and N2 by molybdothiol catalyst systems. ... 

The specific reduction of C2H2 to C2H4 by a molybdothiol-borohy-dride system, but not by an iron-thiol system, is in agreement with C2H2 reduction at the Mo site of N2ase (55). 

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