EXAFS molybdenum oxides

These examples would seem to indicate that the molybdenum atom, that for a long time was considered to be the specific site of dinitrogen coordination, is of little importance. It should be borne in mind, however, that the X-ray structure of the protein was obtained in the resting state. As noted, under such conditions, the Mo atom is assigned oxidation state IV and has a saturated coordination, hence not able to further coordinate nitrogen. EXAFS studies on the active protein indicate a Mo coordination different to that determined by X-ray diffraction. One hypothesis considers the dissociation of the homocitrate, induced by addition of electrons, that would leave vacant coordination sites which could then be saturated by the nitrogen molecule. 

Detailed studies were conducted by infrared, TPD, XPS but also by more sophisticated techniques such as CP-MAS solid state NMR or EXAFS, on the various steps by which molybdenum can be deposited on alumina supports starting from [Mo(CO)e]. Indeed, thin films of molybdenum or of its oxides have wide application as gas sensors or solar cell catalysts. 

The Mo(IV) aqua ion was first reported by Souchay et al. as a monomeric species in 1966 (93), and the Mo3044+ core structure has been confirmed by single-crystal X-ray structure analyses, as described in this review, 95Mo NMR (94), EXAFS structure analysis (95), and 180-labeling experiments (96), after the appearance of many contradictory reports (97). Information on the reduction products of the Mo(IV) aqua ion is also available (98), Richens and Sykes have summarized the preparation of the different aqua ions of molybdenum in oxidation states II to V (99). 

Metal-oxygen and metal-metal distances have been established for many oxo- and hydroxo-bridged dimers and polynuclear cations, for example (57)220 and several of type (58).217 Early work was on solids, but recently X-ray diffraction studies have established the geometries of a few such species in solution. EXAFS has proved valuable in establishing the geometry of the various di- and tri-nuclear forms of the various oxidation states of molybdenum in aqueous solution.23... 

Figure 9 The coordination geometry around molybdenum as suggested from the x-ray crystallographic studies of formate dehydrogenase (Escherichia coli) (a) oxidized form (b) reduced form. The dotted line and Se—S bond distance are suggested from EXAFS [124] but are not seen in the x-ray structure [111,124],...

FIGURE 13 Graded oxide nanoparticle. Mo02 was oxidized with air at 723 K to give a core-shell structure of molybdenum dioxide and possibly molybdenum trioxide that was identified by its different electron energy loss spectrum. No structural description of the highly disordered and catalytically relevant outer oxide shell could be determined, either with XRD or with TEM, (or even with EXAFS spectroscopy) as the signals are dominated by the core structure. 

This chemistry may be relevant to the nature and function of the molybdenum center of E. coli formate dehydrogenase. The Mo K-edge EXAFS of both the oxidized and reduced form of this enzyme were found to be very similar, each inolving a des-oxo-molybdenum site with four Mo S bonds at 2.35 A, (probably) one Mo O bond at 2.1 A, and one Mo—Se interaction at 2.62 A. The Se K-edge EXAFS showed clear evidence for a S Se contact of 2.19 A, presumably indicative of a novel seleno-sulfide ligand to the molybdenum (121). 

Early Mo K-edge EXAFS data for sulfite oxidase showed that the oxidized resting state contains a [Mo 02] unit, whereas the fully reduced enzyme possesses a [Mo O] unit (8). Both units are ligated by two or three sulfur atoms and possibly additional N or O ligands. Two of the coordinated sulfur atoms are presumably provided by molyb-dopterin, as shown in 2. However, similar EXAFS results would be expected if the molybdenum atom were bound to thiolate groups of the protein itself... 

Recently, Mo K-edge EXAFS data have been obtained at low pH/high Cl and high pH/low Cl for each of the three oxidation states of the molybdenum center of sulfite oxidase (69) by poising the potential with redox dyes (78). This latter EXAFS study provides strong evidence that one chloride ion binds to the Mo(IV) and Mo(V) states of the enzyme at low pH/high Cl", but that Cl" does not appear to bind to the Mo(VI) state of the enzyme. Combination of the EXAFS and EPR data for sulfite oxidase yields the minimal structures for the molybdenum center shown in Fig. 6. 

Further support for an Mo=0 group in the reduced states of xanthine oxidase was provided by Mo K-edge EXAFS studies of the Mo(V) state of xanthine oxidase inhibited with pyridine-3-carboxaldehyde and of the Mo(IV) state of alloxanthine-inhibited enzyme (102). Both of these inhibited species showed clear evidence for an Mo=0 group (1.70 A), but neither exhibited an Mo=S distance of 2.15 A, as is observed for the oxidized state of functional xanthine oxidase (8,68,100-102). Comparison of the EPR parameters for model compounds containing the [Mo =0] and [Mo =S] groups also favors the presence of an Mo=0 group in the Mo(V) states of xanthine oxidase (Section IV.C.2) (110). Section VI presents a molecular mechanism for xanthine oxidase that combines the wealth of EPR and EXAFS data available for the various forms of the enzyme (17,64,107-109) with recent developments in model molybdenum chemistry (107-109, 111-113). 

Sulfite oxidase is responsible for the physiological oxidation of sulfite to sulfate. The molybdenum K-edge EXAFS results achieved (109) for this enzyme are the clearest such data and the interpretation achieved represents a prototype for other oxomolybdoenzymes. The molybdenum site has been investigated in each of its three accessible oxidation states [(VI), (V), and (IV)] as a function of pH and chloride concentration. The molybdenum(VI) is coordinated to two oxo groups. 

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