Molybdenum radical

Radical recombination. The results p>ertaining to many of the reactions in Scheme I will now be presented, starting with (a). When a solution of the metal dimer is subjected to a laser flash, -25% of the dimer is dissociated in a typical experiment a 20 iM solution of [CpM(CO)3l2 (M = Mo, W) yields about 25 pM of the dimer. A slightly different procedure was used for M = Cr, but with comparable results. The recombination of the radicals occurs over about 300 ps and follows second-order kinetics. A typical experiment for the molybdenum radical and the fit to second-order kinetics is shown in Figure 1. The rate constants (fc/lO L mol s ) in acetonitrile at 23 °C arc Cr 0.27, Mo 2.16, and W 4.7. In other organic solvents the rate constants are comparable to these, reflecting the relatively small differences in viscosity. In aqueous solution (C5H4C02 )Mo(CO)3 has it/10 L mol s = 3.0. 

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. ... 

Application of the Marcus equation for electron transfer affords the electron exchange rate of the molybdenum radical/anion couple. The value is fcge = 3 X 10 L mol s. The high value argues that very little nuclear reorganization is needed to add an electron to the SOMO of the 17e radical. 

A similar case of concurrence of one-electron transfer and nucleophilic addition is observed in the thermal ion-pair annihilation of CpMo(CO)3 anion with (dienyl)Fe(CO)3+ cations [84, 179]. Thus, spontaneous electron transfer (A et) occurs upon mixing of ( / -cyclohexadienyl)Fe(CO)3 with CpMo(CO)3 in acetonitrile to afford the transient 19-electron iron radical and the 17-electron molybdenum radical which both rapidly dimerize (Eq. 51). 

Recently, Stair and coworkers [10, 11] developed a method to produce gas-phase methyl radicals, and used this to study reactions of methyl groups on Pt surfaces [12] and on molybdenum oxide thin films [13]. In this approach, methyl radicals are produced by pyrolysis of azomethane in a tubular reactor locat inside an ulttahigh vacuum chamber. This method avoids the complications of co-adsorbcd halide atoms, it allows higher covraages to be reached, and it allows tiie study of reactions on oxide and other surfaces that do not dissociate methyl halides effectively. 

In the original rapid-freezing work on xanthine oxidase (53) it was found that in experiments employing about 1 mole of xanthine per mole of enzyme and an excess of oxygen, the time sequence of appearance of the various EPR signals was molybdenum (V), followed by flavin semi-quinone radical (FADH), followed by iron. This suggested that the electron transfer sequence might be ... 

We have not so far mentioned the Phase III increase in the Rapid signal (Fig. 5). It seems (67) that Phase II represents over reduction of molybdenum to Mo(IV), possibly by substrate radicals (see Section V H). The system then comes towards thermodynamic equilibrium by interaction between reduced active enzyme molecules and oxidized inactive ones (67, cf. 64). As Mo(IV) of the former is oxidized to Mo(V), during Phase III, so iron or flavin of the inactive enzyme is reduced. Later, in Phase IV, molybdenum of the inactive enzyme is reduced also to give the Slow signed. Alloxanthine, which as noted above, forms a stable complex with Mo(IV), seems to abolish both the slow phase in the 450 nm bleaching of the enzyme by xanthine and the Phase III increase in Rapid signal (91). 

The question about the competition between the homolytic and heterolytic catalytic decompositions of ROOH is strongly associated with the products of this decomposition. This can be exemplified by cyclohexyl hydroperoxide, whose decomposition affords cyclo-hexanol and cyclohexanone [5,6]. When decomposition is catalyzed by cobalt salts, cyclohex-anol prevails among the products ([alcohol] [ketone] > 1) because only homolysis of ROOH occurs under the action of the cobalt ions to form RO and R02 the first of them are mainly transformed into alcohol (in the reactions with RH and Co2+), and the second radicals are transformed into alcohol and ketone (ratio 1 1) due to the disproportionation (see Chapter 2). Heterolytic decomposition predominates in catalysis by chromium stearate (see above), and ketone prevails among the decomposition products (ratio [ketone] [alcohol] = 6 in the catalytic oxidation of cyclohexane at 393 K [81]). These ions, which can exist in more than two different oxidation states (chromium, vanadium, molybdenum), are prone to the heterolytic decomposition of ROOH, and this seems to be mutually related. 

The resulting radicals are not usually observed, but thermal desorption products indicate the nature of the surface intermediates. Molybdenum(V) dispersed on silica also gives rise to 0 and O2 ions when exposed to N2O and O29 respectively. The 0 ion on this surface may be used to activate methane and ethane in a catalytic cycle which leads to their partial oxidation. 

Elimination to yield alkenes can be induced thermally or by treatment with acids or bases (for one possible mechanism, see Figure 3.39) [138,206]. Less common thermal demetallations include the thermolysis of arylmethyloxy(phenyl)carbene complexes, which can lead to the formation of aryl-substituted acetophenones [276]. Further, (difluoroboroxy)carbene complexes of molybdenum, which can be prepared by treating molybdenum hexacarbonyl with an organolithium compound and then with boron trifluoride etherate at -60 °C, decompose at room temperature to yield acyl radicals [277]. 

The condensation step is catalyzed by the glycosyltransferase SpcF. Further enzyme-catalyzed oxidation is probably needed for the introduction of the hemiketal linkage between cyclitol and sugar units. The SpcY enzyme, which has a similar counterpart, HygY, among the /lyg-cluster encoded proteins (see Section 2.2.4.3.1), is a candidate enzyme for this reaction. SpcY is a member of the radical SAM superfamily of proteins and relatives of SpcY have been found before all in connection with molybdenum-cofactor biosynthesis but to our knowledge no details of the mechanism involved is known for those. 

The second manufacturing method for propylene oxide is via peroxidation of propylene, called the Halcon process after the company that invented it. Oxygen is first used to oxidize isobutane to r-butyl hydroperoxide (BHP) over a molybdenum naphthenate catalyst at 90°C and 450 psi. This oxidation occurs at the preferred tertiary carbon because a tertiary alkyl radical intermediate can be formed easily. 

An inner-sphere hydrogen atom abstraction from the alcohol by a peroxo metal complex, thus forming a coordinated ketyl radical [(CH3)2—C —O—V(0)(00H)]" , has been proposed for the aerobic oxidation of alcohols catalyzed by peroxidic molybdenum and vanadium derivatives (Scheme 16). While in the case of Mo-catalyzed reaction the H2O2 produced is quantitatively converted to products (ketone and H2O), in the vanadium mediated process, hydrogen peroxide accumulates . In this latter case, the direct involvement of a vanadium monoperoxo species has been substantiated by ESI-MS data. 

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