Molybdenum complexes reaction rates

The catalyst is preliminarily oxidized to the state of the highest valence (vanadium to V5+ molybdenum to Mo6+). Only the complex of hydroperoxide with the metal in its highest valence state is catalytically active. Alcohol formed upon epoxidation is complexed with the catalyst. As a result, competitive inhibition appears, and the effective reaction rate constant, i.e., v/[olefin][ROOH], decreases in the course of the process due to the accumulation of alcohol. Water, which acts by the same mechanism, is still more efficient inhibitor. Several hypothetical variants were proposed for the detailed mechanism of epoxidation. 

The molybdenum-hydroperoxide complex (Step 3) reacts with the olefin in the rate-determining step to give the epoxide, alcohol, and molybdenum catalyst. This mechanism explains the first-order kinetic dependence on olefin, hydroperoxide, and catalyst, the enhanced reaction rate with increasing substitution of electron-donating groups around the double bond, and the stereochemistry of the reaction. 

The postulated hydroperoxide-molybdenum complex indicates that there should be a steric and an electronic effect by the alkyl and aryl groups of the hydroperoxide. The steric effect is important in the transition state. The electronic effect will influence the rate of complex formation and the epoxidation reaction. Some data (Table VII) were obtained for these effects, but a clear distinction or evaluation of their role cannot be made at this time. 

To probe hydroperoxide reactivity in these systems we studied the reaction of tert-butyl hydroperoxide in the presence of [C5H5V(CO)4]. In contrast to the rhodium(I) and molybdenum complexes, [C5H5V-(CO)4] catalyzed the rapid decomposition of tert-butyl hydroperoxide to oxygen and tert-butyl alcohol in both toluene and TME (Table II). When reaction was done by adding the hydroperoxide rapidly to the vanadium complex in TME, no epoxide (I) was produced. However, when the TME solution of [C5H5V(CO)4] was treated with a small amount (2-3 times the molar quantity of vanadium complex) of tert-butyl hydroperoxide at room temperature, a species was formed in situ which could catalyze the epoxidation of TME. Subsequent addition of tert-butyl hydroperoxide gave I in 13% yield (Table II). This vanadium complex also could catalyze the epoxidation of the allylic alcohol (II) to give tert-butyl alcohol and IV (Reaction 14). Reaction 14 was nearly quantitative, and the reaction rate was considerably faster than with TME. 

Electrochemical methods have been extensively used to characterize model oxo-molybdenum compounds (Sections IV and V). Electrochemistry provides a convenient method for generating reactive molybdenum complexes in situ (see Sections V.B and C) and for investigating the reaction rates and possible reaction mechanisms of transient molybdenum complexes. 

Catalysis by metal ions is reported for molybdenum and zirconium but not for a number of other cases. With chromium(III), where complexing is well established, a marked decrease in reaction rate occurs . 

The yields were found also to increase in the presence of phosphines, particularly trimethyl or tributyl phosphine. After all the improvements of the catalyst and reaction conditions the system became by far the most active of known non-biological catalytic systems for the reduction of dinitrogen at ambient temperature and pressure. The specific activity (the rate of N2 reduction per mole of the complex) reached and even exceeded that of nitrogenase. Up to 1000 turnovers relative to the molybdenum complex can be observed at atmospheric pressure and more than 10 000 turnovers at elevated N2 pressures. 

The kinetics of ADMET with complex 6 were compared to those of complex 2 by measuring the volume of ethylene liberated from ADMET reactions over time [35], Obtaining an approximate second order rate constant from the DP versus time curves, it was found that molybdenum complex 2 polymerizes 1,9-decadiene 24 times faster than ruthenium complex 6 (Tab. 6.1). 

The direct oxidation of propylene by molecular oxygen is a low-selective reaction. The propylene oxide yield can be raised by limiting the conversion rate to a low value, about 10 to 15 per cent, by using more selective catalysts, or by achieving co-oxidation with a more oxidizable compound than propylene (acetaldehyde, isobutyraldehyde etc.). Many patents have been Hied concerning this process, but without any industrial implementation. Among them is the liquid phase oxidation of propylene on a rare earth oxide catalyst deposited on silica gel (USSR), or in the presence of molybdenum complexes in chlorobenzene or benzene (JFP Instiiut Francois du Petrole. Jefferson ChemicalX vapor phase oxidation on modified silver catalysts (BP British Petroleum IFP, or on ... 

V-substituted nitrogenases as well as active nitrogenases with no metal but iron (see Section 8.22.4). It is also difficult to explain the huge differences in reaction rate and product distribution upon mutation of residues near the iron belt but distant from the molybdenum atom. Neither of these problems is fatal to the molybdenum model, because a few octahedral vanadium and iron N2 complexes produce ammonia upon protonation. In such models, distant belt-area mutations presumably cause a change in the hydrogen bonding network around the Mo site. Thus it is tenable to hold that all nitrogenases react with N2 at the octahedral metal site in the M cluster. 

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