1,1-mixed bimetallic

The functionalized alkylzinc species 19S is smoothly converted to the corresponding nitrile 196 in 67% yield. Interestingly, whereas the reaction of tosyl cyanide with benzylic bromide produces selectively 1-methylbenzonitrile in 76% yield, the cyanation of the corresponding copper reagent furnishes benzyl cyanide as sole product in 80% yield (Scheme 7.59) [142]. The reaction of 1,1-mixed bimetallics of magnesium and zinc, such as 197 [143] prepared by the addition of allylzinc bromide to (Z)-octenylmagnesium bromide with tosyl cyanide produces the nitrile 198 in 93% yield (Scheme 7.59) [142]. 

The first closo metaHaborane complexes prepared (159) were the nickelaboranes [< /(9j 0-( q -C H )Ni(B22H22)] and closo-l]l- r]-Q ]) -l]l-53i] pri Q [55266-88-1] (Fig. 13). These species are equivalent to closo-C ]]ri ][ i closo-Q, p5 2 by tbe electron-counting formaUsm. The mixed bimetallic anion [ /(9j (9-(Tj -C H )2CoNi(B2QH2Q)] and other related species were reported later (160). These metallaboranes display remarkable hydrolytic, oxidative, and thermal stabiUty. 

Fig. 8 Structure of some mixed bimetallic allenylidene complexes...

Figure 5 shows a plot of TOFs for the partial hydrogenation of 1,3-COD by PdRh DENs compared to TOFs for physical mixtures of Pd and Rh monometallic DENs as a function of mol % Rh. As the mol % of Rh in the bimetallic DENs was increased, an increase in the TOF was observed that was greater than that of the physical mixtures. Importantly, the average particle size and distribution did not change as the mol % Rh increased, which was used to rule out the possibility that the TOF enhancement was a consequence of a systematic decrease in particle size. This allows for the conclusion that the bimetallic DENs are truly intimately mixed bimetallic nanoparticles and that a synergistic effect is responsible for the catalytic rate enhancement. 

A complementary study evaluated composition effects on dendrimer-templated PtCu nanoparticles [23]. Although Cu-CO bands were not observed1, a similar red shift in the Pt-CO stretching frequency to the PtAu system was observed, indicating the presence of well-mixed bimetallic nanoparticles throughout the composition range. Infrared spectroscopy of CO adsorbed on both the PtAu and PtCu catalysts showed that the shifts in the CO stretching frequency upon Cu or Au incorporation were small relative... 

Mixed bimetallic reagents possess two carbon-metal bonds of different reactivity, and a selective and sequential reaction with two different electrophiles should be possible. Thus, the treatment of the l,l-bimetailic compound 15 with iodine, at — 78"C, affords an intermediate zinc carbenoid 16 that, after hydrolysis, furnishes an unsaturated alkyl iodide in 61% yield [Eq. (15) 8]. The reverse addition sequence [AcOH (1 equiv), —80 to — 40 C iodine (1 equiv)] leads to the desired product, with equally high yield [8]. A range of electrophile couples can be added, and the stannylation of 15 is an especially efficient process [Eq. (16) 8]. A smooth oxidation of the bimetallic functionality by using methyl disulfide, followed by an acidic hydrolysis, produces the aldehyde 17 (53%), whereas the treatment with methyl disulfide, followed by the addition of allyl bromide, furnishes a dienic thioether in 76% yield [Eq. (17) 8]. The addition of allylzinc bromide to 1-octenyllithium produces the lithium-zinc bimetallic reagent 18, which can be treated with an excess of methyl iodide, leading to only the monomethylated product 19. The carbon zinc bond is unreactive toward methyl iodide and, after hydrolysis, the alkene 19... 

The unique selectivity of reagents 1 and 2 can be explained by the higher selectivity of the C—Cu bond compared to the C—Zn bond toward electrophiles. These mixed bimetallic reagents arc useful for preparation of polyfunctional products. 

Freiser and coworkers developed a method to generate by FTICR-MS mixed bimetallic cations via reactions of a bare metal cation with a neutral, volatile transition metal carbonyl like Fe(CO)s, followed by consecutive CID of the... 

It is well known that the metalalkoxides M(OR) are widely used as the precursors for metal oxides in thermo decomposition or hydrolysis processes. In reference to lanthanoids this method offers opportunities for obtaining homogeneously mixed bimetallic and trimetallic REM oxides of high purity [114, 115, 117, 144]. Besides catalytic processes these materials are often applied in engineering, specifically for the preparation of ceramics. 

Petit-Ramel MM, Khalil I (1974) Mixed bimetallic complexes II. Determination of the stability constants ofyttrium citrates of the bimetallic copper yttrium citrate. Bull Soc Chim (Fr) 1259-1263... 

The "mixed" bimetallic complex RhCl(PBu Pr2)2(u-Cl)2PtCl(PBu Pr2) in which the phosphines are axial and trans shows J(Rh,P) = 95.6 Hz [128], Many of the molecules mentioned represent new and interesting molecular types, e.g. XXII, which contains a rhodium-mercury bond, and we shall return to these at a later point. 

Almost all common metals and structural steels are liable to corrode in seawater. Regulations have to be followed in the proper choice of materials [16], In addition, there is a greater risk of corrosion in mixed constructions consisting of different metals on account of the good conductivity of seawater. The electrochemical series in seawater (see Table 2-4), the surface area rule [Eq. (2-44)] and the geometrical arrangement of the structural components serve to assess the possibility of bimetallic corrosion (see Section 2.2.4.2 and Ref. 17). Moreover the polarization resistances have considerable influence [see Eq. (2-43)]. The standards on bimetallic corrosion provide a survey [16,17]. 

In 1987 at the Weira River, four Kaplan turbines of 2.65 m diameter in two power stations were cathodically protected. The turbines were of mixed construction with high-alloy CrNi steels and nonalloyed ferrous materials with tar-EP coating. Considerable corrosion damage occurred prior to the introduction of cathodic protection, which was attributed to bimetallic corrosion and the river s high salt content of c(CT) = 0.4 to 20 g L... 

It follows from (1) that the more negative metal, at a bimetallic junction, can be subject to more aggravated attack because of this lowered cathodic polarisation. In part, this may result from the greater ease of replenishment of dissolved oxygen under conditions where the ratio of surface area to electrolyte volume is very high. Rosenfel d has also produced evidence to show that rapid convective mixing in the condensed layer, under conditions of lowered relative humidity which permit rapid evaporation, further hastens the arrival of dissolved oxygen at the cathode and results in an additional... 

The effect of precursor-support interactions on the surface composition of supported bimetallic clusters has been studied. In contrast to Pt-Ru bimetallic clusters, silica-supported Ru-Rh and Ru-Ir bimetallic clusters showed no surface enrichment in either metal. Metal particle nucleation in the case of the Pt-Ru bimetallic clusters is suggested to occtir by a mechanism in which the relatively mobile Pt phase is deposited atop a Ru core during reduction. On the other hand, Ru and Rh, which exhibit rather similar precursor support interactions, have similar surface mobilities and do not, therefore, nucleate preferentially in a cherry model configuration. The existence of true bimetallic clusters having mixed metal surface sites is verified using the formation of methane as a catalytic probe. An ensemble requirement of four adjacent Ru surface sites is suggested. 

In order to verify the presence of bimetallic particles having mixed metal surface sites (i.e., true bimetallic clusters), the methanation reaction was used as a surface probe. Because Ru is an excellent methanation catalyst in comparison to Pt, Ir or Rh, the incorporation of mixed metal surface sites into the structure of a supported Ru catalyst should have the effect of drastically reducing the methanation activity. This observation has been attributed to an ensemble effect and has been previously reported for a series of silica-supported Pt-Ru bimetallic clusters ( ). 

Methanatlon Studies. Because the most effective way to determine the existence of true bimetallic clusters having mixed metal surface sites Is to use a demanding catalytic reaction as a surface probe, the rate of the CO methanatlon reaction was studied over each series of supported bimetallic clusters. Turnover frequencies for methane formation are shown In Fig. 2. Pt, Ir and Rh are all poor CO methanatlon catalysts In comparison with Ru which Is, of course, an excellent methanatlon catalyst. Pt and Ir are completely inactive for methanatlon In the 493-498K temperature range, while Rh shows only moderate activity. 

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