Formate tungsten-containing

Recently, interest has been expressed in natural enzymes that effect the reduction of C02 to various products (see Section 11.4). For example, Reda et al. used a tungsten-containing formate dehydrogenase 1 enzyme derived from Syntropho-bacterjumaroxidans in the mediated electroreduction of C02 to formate [103]. The enzyme, which is either adsorbed onto the graphite electrode surface or is free in solution, was observed to reduce C02 to formate with near-100% faradaic efficiency. Although a minimal overpotential for the process was required (-0.4V of applied bias), the current densities were rather low. 

A phenomenon related to encroachment is the appearance of "tunnels." One example of such tunnels is shown in Figure 20. There appear to be many individual tunnels, each of constant diameter (200 to 400 A), with a single tungsten-containing particle at the end. The tunnels are observed when the same process is done on aluminum. They also occur whether or not H2 is added to the WF6. Therefore, it appears that SiF or SiF2 (which are gaseous) must be the product of whatever reaction is causing the tunnel formation. 

One of the tungsten-containing FDHs, namely FDHl, isolated by Syntrophobacter fumaroxidans, an anaerobic bacterium, oxidizes propionate to acetate, CO2, and six reducing equivalents [94]. The reducing equivalents are used to reduce protons to hydrogen or to reduce CO2 to formate. 

Raaijmakers H, Madeira S, Dias JM, Teixeira S, Bursakov S, Huber R, Moura JJG, Moura I, Romao MJ (2002) Gene sequence and the 1.8 A crystal structure of the tungsten-containing formate dehydrogenase from Desulfovibrio gigas. Structure 10 1261-1272... 

D. gigas formate dehydrogenase seems to be quite different in terms of subunit composition. It does not contain a y subunit and no heme c was detected (225). Also, two MGD were identified, but surprisingly, the enzyme contains tungsten instead of molybdenum. Mossbauer and EPR studies confirmed the presence of two [4Fe-4S] + + clusters with similar properties to the ones found in D. desulfuricans FDH (247). 

Graentzdoerffer A, D Rauh, A Pich, JR Andreesen (2003) Molecular and biochemical characterization of two tungsten-and selenium-containing formate dehydrogenases from Eubacterium acidamophilum that are associated with components of an iron-only hydogenase. Arch Microbiol 179 116-130. 

The formation of rings that contain a thioether linkage does not appear to be catalyzed efficiently by Ru, even when terminal olefins are present. On the other hand, molybdenum appears to work relatively well, as shown in Eqs. 30 [207] and 31 [208]. Under some conditions polymerization (ADMET) to give poly-thioethers is a possible alternative [26]. Aryloxide tungsten catalysts have also been employed successfully to prepare thioether derivatives [107,166,169]. Apparently the mismatch between a hard earlier metal center and a soft sulfur donor is what allows thioethers to be tolerated by molybdenum and tungsten. Similar arguments could be used to explain why cyclometalated aryloxycarbene complexes of tungsten have been successfully employed to prepare a variety of cyclic olefins such as the phosphine shown in Eq. 32 [107,193]. 

The symmetrical dienyne 58a was converted to a fused bicyclo [4.3.0] ring in 95% yield [17] (Eq. 27). With substrate 58c containing an unsymmetrical diene tether, two different products, 59c and 59c, were obtained in a ratio of 1 to 1 (Eq. 28). The reaction course in the formation of the different bicyclic rings is shown in Scheme 8. This dienyne metathesis is also catalyzed by tungsten or molybdenum complex 62 or 63 (Fig. 1), and a dienyne bearing terminal alkyne 58b could be cyclized to give 59b in 97% yield. 

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