Molybdenum complexes probes

M-F coupling constants in, 4 245 Mo, molybdenum center probes, 40 16 multinuclear, tetracyano complexes containing oxo or nitrido ligands, 40 303-304... 

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. 

As yet, no X-ray crystal structures are available for any of the molybdenum enzymes in Table I. Therefore, present descriptions of the coordination environment of the molybdenum centers of the enzymes rest primarily upon comparisons of the spectra of the enzymes with the spectra of well-characterized molybdenum complexes. The two most powerful techniques for directly probing the molybdenum centers of enzymes are electron paramagnetic resonance (EPR) spectroscopy and X-ray absorption spectroscopy (XAS), especially the extended X-ray absorption fine structure (EXAFS) from experiments at the Mo K-absorption edge. Brief summaries of techniques are presented in this section, followed by specific results for sulfite oxidase (Section III.B), xanthine oxidase (Section III.C), and model compounds (Section IV). 

Sublimation of the resulting brown residue at 50-60°, O.OOS torr onto a water-cooled probe for 3 days affords 13.6 g (84% yield) of (>i -CjH5)W(CO)2(NO). The corresponding chromium and molybdenum complexes are obtained similarly in yields of 60 and 93%, respectively. 

Adam and Lohray122 have used thianthrene 5-oxide (88) as a mechanistic probe in oxidations with transition metal peroxides. They oxidized 88 with various diperoxo complexes of chromium, molybdenum and tungsten and formulated a plausible mechanism on the basis of the products formed, 89 and 90. 

Molybdenum and tungsten complexes with three crown ether benzenedithio-lene ligands (21) have been reported (105) and the effect of alkali ion binding has been probed by CV (106). Upon binding with Li+, Na+, or K+, positive shifts in the redox potential have been observed for all complexes. This observation suggests that the tris(crown ether benzodithiolene) complexes of Mo and W may potentially be useful as sensors for alkali metal cations (106). 

The transient molybdenum(V) states of sulfite oxidase have been probed by both EXAFS and EPR spectroscopy. The EPR spectral parameters are sensitive to pH (89) and to anions in the medium (90), as shown in Fig. 5 (69). Comparison of the enzyme EPR parameters to those of known Mo(V) complexes (Section IV.B.2) shows that the large... 

The third principal application of the electron spin resonance technique is to the study of paramagnetic transition metal ions in biochemical systems. Most examples are complexes of copper, iron, manganese, chromium, cobalt and molybdenum. Other metals such as titanium, vanadium and nickel are sometimes employed as structural probes. Only four of these ions, Cu ", Mn, Gd " and VO ", are seen in ESR spectroscopy at room temperature under virtually all conditions. Therefore, they are of special importance. 

The use of ESR spectroscopy to probe Mo centres in biological systems has stimulated attempts to reproduce the ESR spectra of molybdoenzymes using well-defined chemical systems. The similarity between ESR parameters of species formed by reacting Na2[Mo04] with S-donor ligands and those of xanthine oxidase imply that in the enzyme (cysteinyl)S ligation of molybdenum may occur. However, no well-characterized Mo complex has an ESR spectrum which reproduces all the aspects of one observed for the Mo centre of an oxomolybdenum enzyme. A particular problem is the absence of superhyperfine coupling constants for well-defined Mo systems a notable exception is the observation of... 

In order to probe the tolerance of the molybdenum(O) aromatic complexes to electrophilic/acidic environments, a tandem addition sequence was attempted for the complex TpMo(NO)(MeIm)(q -naphthalene) (114) (Fig.25) [17]. In a strategy similar to that used with the TpRe(CO)(MeIm)(q -naphthalene) analog,(see below) an acetonitrile solution (-35 °C) of 114 was exposed sequentially to triflic acid, l-methoxy-2-methyl-l-trimethylsiloxypropene, and an amine base. The 1,2-dihydronaphthalene complex 116 was isolated in virtually quantitative yield. No evidence of free naphthalene or 1,4-addition product was observed. Stirring the reaction mixture with exposure to air resulted in an 80% overall yield of 2-(l,2-dihydro-naphthalen-2-yl)-2-methyl-propinoic acid methyl ester (117) following TLC purification [17]. 

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