Bimetallic transition structure

To resolve this problem, Rickborn made an ingenious proposal that implicated the intermediacy of a bimetallic transition structure assembly v involving a bridging Znl2 molecule (Fig. 3.5). This would accommodate the needed spatial requirements of the methylene transfer process. The importance of the polymetallic re-... 

Although the rationalization of the reactivity and selectivity of this particular substrate is distinct from that for chiral ketals 92-95, it still agrees with the mechanistic conclusions gained throughout the study of Simmons-Smith cyclopropa-nations. StOl, the possibility of the existence of a bimetallic transition structure similar to v (see Fig. 3.5) has not been rigorously ruled out. No real changes in the stereochemical rationale of the reaction are required upon substitution of such a bimetallic transition structure. But as will be seen later, the effect of zinc iodide on catalytic cyclopropanations is a clue to the nature of highly selective reaction pathways. A similar but unexplained effect of zinc iodide on these cyclopro-panation may provide further information on the true reactive species. 

The alloyed or layered character of a small bimetallic cluster structure is generally quite difficult to conclude experimentally [102,179]. Even if the surface plasmon transitions of both pure metals are specific (with one possibly in the UV), the unknown spectra of alloyed or bilayered clusters are both expected in the same intermediate region. The... 

No matter what function the bimetallic catalyst fulfils, two key trends of catalyst design emerge from the above examples (1) a relatively constrained catalyst structure is often essential to ensure a beneficial interaction of both metals this most likely helps overcome the entropic penalty associated with creating an ordered bimetallic transition state during the catalytic cycle (2) for catalysts that do not contain any prearranged metal-metal bonding or bridging atom interactions, a relatively short intermetallic distance also appears necessary. Clearly, for many of the catalysts described here, much work remains to tmderstand exactly how the bimetallic structure enhances the catalysts performance. 

Scheme 3.89 Bimetallic carboalumination mechanism via a cyclic transition structure [48].

Returning briefly to CO2 as a ligand in addition to the various mono-C02 complexes referred to above, several bis(rj -CO2) transition-metal adducts are known, e.g. /ran -[Mo( ) -C02)2(PMe3)4] (5) and trans.mer-[Mo( 2-C02)2(PMc3)3(CNPr )].< 22 The first homo-bimetallic bridging-C02 complex has also been structurally characterized by X-ray analysis, viz. [(dppp)(CO)2Re(/x, j 0,0 j C)Re(CO)3(dppp)] (6) [dppp = l,3-bis(diphenyl-phosphino)propanc]. 0 3)... 

Some similar bimetallic acylamino complexes are also known with transition metal ions, e.g., with vanadium(II) [67], palladium(II) [68], and especially platinum(II) [69]. In the Cambridge Structural Database [39] only one trimetallic structure is found in which three iron(II) ions are bridged by a total number of six acylamino ligands [70]. 

Lanthanides in combination with transition metals have been shown to have a positive effect in promoting heterogeneous catalytic reactions. The bimetallic Yb—Pd catalyst obtained from the precursor (pMF)i0Yb2 Pd(CN)4]3 K on a titania surface offers improved performance over a palladium-only catalyst for the reduction of NO by CH4 in the presence of 02.99 100 The structure, shown in Figure 6, consists of two inverted parallel zigzag chains that are connected through the lanthanide atoms by trans-bridging [Pd(CN)4]2- anions.101... 

Ffirai and Toshima have published several reports on the synthesis of transition-metal nanoparticles by alcoholic reduction of metal salts in the presence of a polymer such as polyvinylalcohol (PVA) or polyvinylpyrrolidone (PVP). This simple and reproducible process can be applied for the preparation of monometallic [32, 33] or bimetallic [34—39] nanoparticles. In this series of articles, the nanoparticles are characterized by different techniques such as transmission electronic microscopy (TEM), UV-visible spectroscopy, electron diffraction (EDX), powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) or extended X-ray absorption fine structure (EXAFS, bimetallic systems). The great majority of the particles have a uniform size between 1 and 3 nm. These nanomaterials are efficient catalysts for olefin or diene hydrogenation under mild conditions (30°C, Ph2 = 1 bar)- In the case of bimetallic catalysts, the catalytic activity was seen to depend on their metal composition, and this may also have an influence on the selectivity of the partial hydrogenation of dienes. 

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