TpRe synthesis

Metal fragments of the form (TpRe(CO)(L), in which L = Melm or NH3, have the ability to bind substituted benzenes in an 2-fashion. The fragment having L = Melm has seen more application because of difficulties associated with the synthesis and isolation of aromatic complexes having L = NH3. Anisoles, phenols, and naphthalenes all form thermally stable -complexes when stirred in excess (10-25 equiv.) with TpRe(CO)(MeIm)( /2-benzene) in THF. The complexes are typically isolated by precipitation from hexanes. The benzene complex can be prepared by direct reduction of the Re(III) precursor, TpRe(MeIm)(Br)2, with Na° in benzene under one atmosphere of CO [35]. 

Schrock and co-workers have described the synthesis of a rhenium neopentyli-dyne complex containing the tris(pyrazolyl)borate ligand from Re( = C Bu) (CH2 Bu)3(OTf). Treatment of this complex with excess pyridine afforded the colourless, six-coordinate rhenium neopentylidene/neopentylidyne complex Re( = C Bu)(OTf)(CH2 Bu)( = CH Bu)(py)2], which was subsequently subject to ligand substitution by Na[Tp], producing the thermally stable, 18-electron complex TpRe( = C Bu)(CH2 Bu)( = CH Bu) (Scheme 27). Rhenium is one of the three... 

In order to access a more electron-rich Re(I) system, Harman et al synthesized TpRe(CO)(PMe3)(f/ -cyclohexene). The synthesis was reported from both heating a pressure tube containing the Re(III) system TpReCl2(PMe3) in a benzene solution with 20 equivalents of cyclohexene, 10 psi CO and Na/Hg as well as using 20 equivalents cyclohexene, 1 atm of CO and The IR spectrum of TpRe... 

Scheme 32. Synthesis of TpRe(CO)(MeIm)( / -aromatic) complexes.

Djega-Mariadassou and co-workers (67) reported that a long induction period preceded the simultaneous reduction and carburization steps in the synthesis of NbC from the TPRe of niobium oxide with CH4/4H2. To avoid the long induction period, different catalysts were incorporated into the initial Nb205 and were treated with CH4/4H2 mixtures at relatively low carburization temperatures. In the presence of catalysts of methane decomposition (Ni and Rh), they were able to eliminate the induction period and fee 5-NbC with superficial carbon contamination and surface areas of 11-18 m /g were obtained. When a NiMo catalyst was mixed with Nb205 and carburized with a CH4/4H2 mixture gas, 5-NbC with a surface area of 50 m /g and large amounts of carbon contamination was produced. They also synthesized 5-NbC with surface areas of 27-49 m /g from the carbonization of niobium oxynitrides, which were prepared from the nitridation of niobium oxide with pure ammonia at 923 K. 

Green and co-workers (68) prepared 5-NbC with surface areas as high as 113 m /g from the TPRe of niobium oxide with C2H6/8.6 H2 as a reactive gas. They also synthesized 5-NbC with a surface area of 76 m /g from the carburization of niobium nitride using CH4/4H2 mixture gas. They did not report the elemental analysis data for these materials, which would give information on the carbon contamination. In most cases, the use of the nitride as a precursor or ethane as a reactant increases the surface area of the products, as well as lowers the required synthesis temperature. Nitrides can be converted readily to carbides while maintaining both the high surface area and structure. 

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