Molybdenum complexes sulfur ligands

Complexes of molybdenum and tungsten with bidentate sulfur ligands have been investigated extensively. In recent years, the work in this field has been escalated by the impetus of designing models of such bioinorganic enzymes as nitrogenase and xanthine oxidase (125). The early work reviewed by Coucouvanis (1) dealt exclusively with the isolation of oxomolybdenum(V) and -(VI) species. 

A large number of studies devoted to metal-sulfur centers are motivated by the occurrence of such arrangements at the active site of various metalloenzymes [1-13]. Mononuclear complexes with Mo=0 func-tion(s) and possessing sulfur ligands in their coordination sphere have been extensively investigated since they can be seen as models of the active site of enzymes such as nitrate- and DM SO reductases or sulfite- and xanthine oxidases [1-4]. On the other hand, a large variety of mono-, di-, and polynuclear Mo—S centers have been synthesized in order to produce functional models of the Mo-nitrogenase since the exact nature (mono-, di- or polynuclear) of the metal center, where N2 interacts within the iron-molybdenum cofactor (FeMo—co) of the enzyme is still unknown [4-8]. 

Molybdenum complexes of polysulfide ligands are reactive and, for example, readily make and break sulfur-carbon bonds, a property undoubtedly relevant to the involvement of Mo—S... 

The electrochemical transformation of a molybdenum nitrosyl complex [Mo(NO)(dttd)J [dttd = 1,2-bis(2-mercaptophenylthio)ethane] (30) is rather interesting (119). Ethylene is released from the backbone of the sulfur ligand upon electrochemical reduction. The resulting nitrosyl bis(dithiolene) complex reacts with O2 to give free nitrite and a Mo-oxo complex. Multielectron reduction of 30 in the presence of protons releases ethylene and the NO bond is cleaved, forming ammonia and a Mo-oxo complex (Scheme 15). The proposed reaction mechanism involves successive proton-coupled electron-transfer steps reminiscent of schemes proposed for Mo enzymes (120). 

All three members of the electron transfer chain [Mo(CO)2(S2C2Me2)]" (n = 0,1, 2) were isolated and characterized. The complexes have trigonal prismatic geometries, and structural, spectral, and electrochemical properties that are consistent with the redox orbital having predominantly sulfur-ligand based character (>80%) behavior consistent also with DFT calculations. Closely related benzene- and 2,4-di-i-butyl-benzene-dithiolene complexes, [Mo(LL )(bdt)2] (L = 0, L = PPha, L = L = PMeR2 (R = Me or Ph)) were obtained by thioether S-dealkylation of appropriate dttd (22) complexes of molybdenum carbonyls and oxo species. 

Scheme 6 Formation of molybdenum complexes bearing ditbiocarboxylato ligands by sulfurization of tbe alkylidyne complexes...

In order to avoid the unwanted dimerization, steric constraints were incorporated into the used ligands. This hinders the metal centres to approach each other, so either the equilibrium is shifted to the side of the monomeric molybdenum species or the dimerization completely prevented. One of the first bulky sulfur-containing ligands that was able to successfully promote oxygen atom transfer catalysis of its molybdenum complex was developed by Berg and Holm (Scheme 4.7 BuLNS = bis(4-tert-butylphenyl)-2-pyridylmethanthiolate). ... 

---