TpRe formation

Because of its importance and frequent occurrence, the process of carbon formation and its structure have been widely studied, and many physical techniques (including recently positron-emission tomography ) have been deployed. Of these, temperature-programmed methods (oxidation, TPO reaction with hydrogen, TPRe) are the simplest and most informative. TPO can distinguish between carbon on the metal, which is relatively easily oxidised, from carbon on the support, which is less reactive. Admixture of the sample with a Pd/Si02 catalyst ensures that the effluent contains only carbon dioxide, and no monoxide. The use of the TPRe strictly requires estimation of the methane (and possibly other alkanes) that emerges, since because the H/C ratio in the carbon is unknown, the... 

Photolytic alkyl migration to a Re-oxo has also been observed. TpRe(OEt)(Cl)(py) was prepared by the photolysis of TpRe( = 0)(Cl)(Et) in the presence of pyridine. In contrast to the analogous aryl migration, the formation of the Re(III) ethoxide system is thought to occur through the formation of ethyl radical. For example, photolysis of TpRe( = 0)(Cl)(Et) in the presence of PhSH produces TpRe( = 0)(Cl)(SPh) and ethane, which is consistent with initial bond homolysis of the Re-C bond of Re-CH2CH3. 

Isolable transition metal complexes containing hydride and terminal oxo ligands are rare however, Tp Re( = 0)(H)X (X = Cl, H or OTf) and TpRe( = 0)(H)Cl have been synthesized, isolated and characterized. Reactions of Tp Re( = 0)(H) OTf (12) with unsaturated substrates (e.g., ethene, propene or acetaldehyde) result in insertion of C = C or C = 0 bonds into the Re-H bond to yield Tp Re( = 0)(R) (OTf) (R = ethyl or propyl) or Tp Re( = 0)(0Et)(0Tf) (Scheme 6). Oxidation of 12 with pyridine-iV-oxide or DMSO produces Tp Re( = 0)3, acid and free pyridine or dimethylsulfide, respectively. A likely mechanism involves initial oxidation of 12 to produce [Tp Re( = 0)2H][0Tf] (13) followed by the formation of Tp Re( = 0) (OH)(OTf) (14) via a 1,2-migration of the hydride to an oxo ligand (Scheme 6). Reaction of 14 with a second equivalent of oxidant in the presence of base yields Tp Re( = 0)3 (15). Direct deprotonation of 13 is noted as less likely than the pathway shown in Scheme 6 due to the lack of precedent for acidity of related rhenium hydride systems. 

Consistent with the experimental observations, calculated enthalpies starting from TpRe(CO)2 and CpRe(CO)2 reveal that the formation of TpRe(CO)2(CH3)H is endothermic by 6.4kcalmol while the formation of... 

The addition of aldehydes (e.g., acetaldehyde, benzaldehyde or 3-formylfuran) to fy -furan or // -2-methylfuran complexes of the type TpRe(CO)( BuNC)( -furan) gives three dihydrofuran products (Scheme 46). The product 47 comes from anti-addition of the Lewis acid modified aldehyde to C3 of the // -furan followed by formation of a 1,3-propene dipole. Rotation around the former C3-C4 of furan positions the aldehydic oxygen for nucleophilic attack on the positive end of the dipole and hence the new furan with the acetyl group syn to the metal. This pathway... 

The direct carburization of WO3 with a CH4/4H2 mixture through a TPRe led to the formation of a mixture of hex WC and fee P-W2C with a surface area of 49 m /g (14). Leclercq and co-workers (27) studied the preparation of tungsten carbides by TPRe with a CH4/H2 mixture. They found out that carburization occurs in two distinct steps W2C is formed in the first step at about 650°C with 20% CH4 in H2, whereas the formation of WC occurs only at higher temperatures. They also showed that direct carburization of WO3 by CH4/H2 does not take place, but that the carburization occurs via the initial reduction of WO3 to W metal. 

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