Oxidation states, molybdenum center

A set of oxygen donor atoms, providing both a and tt donation to a metal center, is not appropriate to stabilize any low oxidation state of a metal.19 This is, however, a synthetic advantage since very reactive, unstable, low-valent metalla-calix[4]arenes can be generated in situ and intercepted by an appropriate substrate. In the absence of a suitable substrate, the reactive fragment, however, can collapse to form metal-metal bonded dimers. The formation of metal-metal bonds has been, however, so far observed in the case of Group V and VI metals only. The most complete sequence so far reported has been for tungsten, molybdenum, and niobium. 

The x-ray crystallographic results for formate dehydrogenase [111,122] show that the cofactor is des(oxido) but coordinated by selenocysteine in both the MoVI and MoIV oxidation states. The lack of an oxido is consistent with the CEPT role for the cofactor with the selenocysteine [3,111,122] serving as the proton acceptor while modulating the redox behavior of the molybdenum center (Figure 23). 

M(VI) and M(IV) oxidation states. The M(V) state is generated by a one-electron reduction of the M(VI) state, or the one-electron oxidation of the M(IV) state, and occurs during the catalytic cycle—en route to the regeneration of the catalytically active state. Spectroscopic studies of the Mo—MPT enzymes, notably electron spin resonance (EPR) investigations of the Mo(V) state, have clearly demonstrated that the substrate interacts directly with the metal center (37). The first structural characterization of a substrate-bound complex was achieved for the DMSOR from Rhodobacter capsulatus DMS was added to the as-isolated enzyme to generate a complex with DMSO that was O-bound to the molybdenum (43). 

Electrochemical investigations have shown that Mo( OA r)(S 2C2R 2)21 complexes exhibit two reversible redox processes, at (versus SCE) E /2 = —1.95 and 0.10 V (R = Me Ar = C6H3-2,6-/-Pr2) and -1.74 and 0.30 V (R = Ph Ar = C6H3-2,6-/-Pr2), that are attributed to the Mo(IV/III) and Mo(V/IV) couples, respectively. The very negative potential that is required to produce the Mo(III) state of these systems clearly suggests that this oxidation state is unlikely to be accessible to corresponding molybdenum centers of the Mo MPT enzymes. 

In particular, Schrock-type catalysts suffered from extreme moisture and air sensitivity because of the high oxidation state of the metal center, molybdenum. Due to the oxophilicity of the central atom, polar or protic functional groups coordinate to the metal center, poisoning the catalyst and rendering it inactive for metathesis. Since late transition metal complexes are typically more stable in the presence of a wide range of functionalities, research was focused on the creation of late transition metal carbene complexes for use as metathesis catalysts. 

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. 

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