Molybdenum complexes reductive coupling

Divalent molybdenum compounds occur in mononuclear, dinuclear, and hexanuclear forms. Selected examples are shown in Figure 6. The mononuclear compounds are mostiy in the realm of organometaUic chemistry (30—32). Seven-coordinate complexes are common and include MoX2(CO)2(PR3)2, where X = Cl, Br, and I, and R = alkyl MoCl2(P(CH3)3)4, heptakis(isonitrile) complexes of the form Mo(CNR) 2 (Fig. 6d), and their chloro-substituted derivatives, eg, Mo(CNR)3CR. The latter undergo reductive coupling to form C—C bonds in the molybdenum coordination sphere (33). 

Polyparaphenylenevinylene films have been grown electrochemically by the reductive coupling of a,a,a, a -tetrabromoxylene [39]. Deposition could be accelerated by the addition of chromium or molybdenum carbonyl complexes. The material produced appeared to be contaminated with both residual chlorine and elec-... 

Upon purification of the XDH from C. purinolyticum, a separate Se-labeled peak appeared eluting from a DEAE sepharose column. This second peak also appeared to contain a flavin based on UV-visible spectrum. This peak did not use xanthine as a substrate for the reduction of artificial electron acceptors (2,6 dichlor-oindophenol, DCIP), and based on this altered specificity this fraction was further studied. Subsequent purification and analysis showed the enzyme complex consisted of four subunits, and contained molybdenum, iron selenium, and FAD. The most unique property of this enzyme lies in its substrate specificity. Purine, hypoxanthine (6-OH purine), and 2-OH purine were all found to serve as reductants in the presence of DCIP, yet xanthine was not a substrate at any concentration tested. The enzyme was named purine hydroxylase to differentiate it from similar enzymes that use xanthine as a substrate. To date, this is the only enzyme in the molybdenum hydroxylase family (including aldehyde oxidoreductases) that does not hydroxylate the 8-position of the purine ring. This unique substrate specificity, coupled with the studies of Andreesen on purine fermentation pathways, suggests that xanthine is the key intermediate that is broken down in a selenium-dependent purine fermentation pathway. ... 

A second example of a membrane-bound arsenate reductase was isolated from Sulfurospirillum barnesii and was determined to be a aiPiyi-heterotrimic enzyme complex (Newman et al. 1998). The enzyme has a composite molecular mass of 100kDa, and a-, P-, and y-subunits have masses of 65, 31, and 22, respectively. This enzyme couples the reduction of As(V) to As(III) by oxidation of methyl viologen, with an apparent Kra of 0.2 mM. Preliminary compositional analysis suggests that iron-sulfur and molybdenum prosthetic groups are present. Associated with the membrane of S. barnesii is a h-type cytochrome, and the arsenate reductase is proposed to be linked to the electron-transport system of the plasma membrane. 

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). 

Reduction of the metal radicals. The anionic complexes CpM(CO)3" are well known species they are stable entities with 18 valence electrons. The standard reduction potential for the CpMo(CO)3 -CpMo(CO)3 couple is -0.08 V vs SSCE. The molybdenum radical is thus a mild oxidizing agent with suitable electron donors it can be reduced to the anion. For example, the radical oxidizes Fe(Ti -C5Meg)2 with a rate constant of 2.2 x 10 L moT s in acetonitrile at 23 °C. ... 

Molybdenum has a marked predilection for OAT reactions and biomimetic systems have been reviewed. OAT is a concerted two-electron process that results in the oxidation of the oxygen atom acceptor and the reduction of the donor. The most common reactions for Mo are shown in equations (10) and (11). Catalysis results from the coupling of these reactions, leading to net oxidation of X by X O (equation 12, cf. equation 6). Transformations involving oxo-Mo(VI) and desoxo-Mo(IV) complexes are recent and important developments (see Section 9). 

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