Mo -cysteine complex

Model systems composed of Mo complexes and Fe-S clusters can give facile N2 reduction to NH3 under mild conditions. The Fe-S cluster is considered to act only as an electron transfer catalyst from reducing agents, such as BH4 and S2042, to the Mo complexes at which the reduction occurs [147-149]. Similarly the reduction of N2H4 to NH3 can be achieved by a catalyst of sodium molybdate, L-cysteine, and NaBH4 with a turnover number of 4.2 NH3 mol/(Mo-cysteine complex moljhr (148). 

Figure 3. ESR spectra of Mo-cysteine complexes in borate buffer (pH 9.6) at 77 K (A), cysteine + MoCls (B), cysteine + K3MoCl6 or (A) + NaBH4...

Figure 2. The cocatalytic effects of various charge carriers on acetylene reduction by the Mo(V)-cysteine complex (0.42 mmol) and NaBH4 (1.0 mmol), plotted against the reduction potentials (V vs. SHE) of the corresponding additives.

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. 

Ligand Effect. In Mo complexes containing cysteine-related ligands as potential catalysts, structural variations in the ligand have... 

Table I. Acetylene Reduction by Mo-Complex Systems of Cysteine and Its Related Ligands0...

Figure 1. Catalytic activity of Co(II)-complex catalysts as compared with corresponding Mo catalysts (A) Co-cysteine (O) Co-cyste amine ...

The oxidation of thiols in the form of L-cysteine, penicillamine, and thioglycollic acid by [Mo(CN)g] in aqueous acidic solution also formed disulfides as final products 111). The reactions show a second-order substrate dependence, and the rates are found to decrease with increasing hydrogen ion concentration. This is attributed to the deprotonation of the —SH and —COOH groups in these thiols prior to electron transfer. The reactions are interpreted in terms of outer-sphere activation. An explanation for the second-order dependence on thiol concentration involves ion association between the cyano complex and a protonated form of the thiol, followed by reaction of this complex with a second thiol molecule. 

---