Molybdenum complexes EXAFS

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). 

It seems likely that the active site for dinitrogen binding involves the molybdenum atom. It has been established by EXAFS that the coordination sphere consists of several sulfur atoms at distances of about 235 pm. An Mo=0 double bond, so common in complexes of Mo(IV) and Mo(Vl), is not present. There are other heavy atoms, perhaps iron, nearby (—270 pm). The ultimate source of reductive capacity is pyruvate, and the electrons are transferred via ferredoxin (see page 911) to nitro-... 

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... 

A direct confirmation of this behavior is obtained by TPR [8], One can deduce that in the presence of Pt the average oxidation state of surface molybdenum ions will be lower in an operating catalyst. It is possible to postulate the existence of surface complexes of the type PtMoOx, where below 600°C x may range from 0 to 2 depending on the reaction temperature. Recent preliminary EXAFS results seems to corroborate such a picture [9J. Conversely, one can consider the surface Pt in such complexes as being more oxidized (electron deficient) than when dispersed in the absence of modifiers such as molybdena or ceria. This is found to affect the catalytic properties of Pt. A similar behavior prevails in other systems as well. For instance, it was recently reported that addition of ceria to a Pd/AlgC catalyst results in a Pd surface state which is more difficult to reduce [10]. 

Sulphite Oxidase.— The molybdenum site of sulphite oxidase has been investigated by EXAFS in its iv, v, and vi oxidation states. The highest oxidation level has two cw-M=0 bonds with strong cis-S donors which are trans to each other. Two further S donors are also proposed at longer distances (4), Similar co-ordination has been achieved in a model complex with a quadridentate ligand and shows similar EXAFS behaviour. 

Fig 2. Fourier transform of molybdenum K-edge EXAFS spectra for the (C0M04CA) complex derived from the different precursors. 

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