Tantalum species

The last decade has seen enormous developments in SS NMR, and techniques that were used only in soluhon have become available also for solid samples. Nowadays, even conformers can be identified by solid-state NMR [22]. Therefore, this section outlines these improvements as well as their application to the characterization of selected surface organometallic species. An application of EXAFS to the characterization of surface organometallic tantalum species is also shown. Structural data obtained through EXAFS (such as that in Table 11.1) are often essential for comparisons with data obtained by DFT calculations. 

Compared with the direct allylation of carbonyl groups, little has been reported on the selective conjugate allylation of enones. Allylic tantalum species, generated by the Sn-Ta exchange, is the choice of tools for performing conjugate addition of sterically hindered allylic groups (Equation (43)).140... 

Although the oxidative addition of aromatic compounds does not lead to isolable tantalum species, the niobium ones can be characterized [cf. oxidative addition of benzene to (/j5-C5H5)2Mo (page 774)].29 29 G. W. Parshall et al., J. Amer. Chem. Soc., 1970,92, 5235 1971,93, 3793. 

If efficient active sites shall be created by stabilizing the molecular species at the surface of a well-defined phase, as in the case of V0x/Ti02 - anatase catalysts, the better activity in NO SCR was achieved by cocondensation of the dopant alkoxide with the surface groups of preformed colloidal particles of the matrix precursor, followed by gelation [76]. A similar approach of modifying the matrix precursor sol with dopant molecules yielded well-dispersed tantalum species at the silica surface, efficient in selective oxidation of pyrimidine thioether [77]. 

Reaction of but-2-yne with [(RNH)(RN=)2V(OEt2)] (R = Bu 3Si) resulted 33 in displacement of the diethyl ether ligand and formation of [(RNH)(RN=)V C(Me>=C(Me)N(R))l which underwent ligand metallation to afford the ri -i-azaallyl complex (15). The synthesis and reactions of the 16-electron bis-(Ti3-allyl) tantalum species [Cp Ta( n3.phCHCHCH2)2] have been reported 34 and the X-ray crystal structure determined 35. 

The pursuit of the synthesis of FK506 and rapamycin continues to draw contributions from the carbohydrate area. The C(21)-C(34) fragment 82 of FK506 has been prepared as shown in Scheme 16. Use of a tantalum-alkyne complex to couple aldehyde 80 with alkyne 81 is an especially interesting step. The requisite low valent tantalum species is generated in situ and the Ta-alkyne complex shows reasonable levels of Qam selectivity towards aldehydes. Alkyne 81 was, in turn, prepared from D-quinic acid. 

The gases used in the CVD reactor may be either commercially available gases in tanks, such as Ar, N2, WF, SiH, B2H, H2, and NH Hquids such as chlorides and carbonyls or soflds such as Mo carbonyl, which has a vapor pressure of 10 Pa (75 mtorr) at 20°C and decomposes at >150° C. Vapor may also come from reactive-bed sources where a flowing haUde, such as chlorine, reacts with a hot-bed material, such as chromium or tantalum, to give a gaseous species. 

To return to die problem of die vaporization of die tantalum silicides, which could be transported as the tetraiodide of each element, but not as the elementary species. From these data it can be concluded that whatever die starting point in the composition range, the composition of the surface phase will tend towards Tag Sis, which is die most nearly congruently vaporizing composition. 

In the case of tantalum, the treatment under H2 at 150 °C of a mixture of [(=SiO)Ta(= CHtBu)(CH2tBu)2] and [(= SiO)2Ta(= CHtBu)(CH2tBu)] is transformed into mainly one surface species [(=SiO)2Ta-H] (Scheme 13) [52]. However, when the temperature is increased step by step to 450 °C, this surface complex is gradually converted to [(=SiO)3Ta] and (Si - H) [53]. The gradual reactivity of [(= SiO)2Ta - H] with adjacent silox-ane bridges also speaks for an heterogeneity of the sihca surface. 

The major difference in the dithiocarhamato chemistry of tantalum and niobium is the existence of the Ta(V) species Ta(R2C tc)5, whereas the comparable niobium compound is unknown. The preparative methods for Ta(R2C tc)5 involve either a CS2 insertion in Ta(NR2)j (R = Me )) or a reaction between TaClj and sodium or ammonium dithiocarbamate (R = Et (7), R2 = ( 112)4 ( ))- The structure of this interesting diamagnetic compound is not elucidated, but from infrared data the presence of one or more unidentate ligands is suggested 11,15). 

Alkylidene complexes are generally considered to be reactive intermediates but the actual surface organometallic species have never been fully characterized. However, the synthesis of silica-supported tantalum(V) carbene complexes and their characterization have been reported.332... 

The tantalum center in the Schrock methylene complex 2 is electron rich by virtue of the electron-releasing ligands coordinated to it, while the osmium and ruthenium center in the methylene 3 and difluorocarbene 4 species are electron-rich because they have d8 electron configurations in neutral complexes. 

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