Gold-stabilized

However, in some cases oxidic gold species may be the active sites for CO oxidation. Gates reported that oxidic gold dispersed on La203 by using GG of Au acac complex is active at room temperature [43]. On the other hand, we have recently found that over Au/La coprecipitates calcined at temperatures below 500 K are active even at 193 K [46]. The EXAFS and XANES analyses of the active samples showed that oxidic gold stabilized by La(OH)3 is responsible for low-temperature activity. 

EXAMPLE 13.4 Change of Stability Ratio with Ionic Concentration. Colloidal gold stabilized by citrate ions and having a mean particle radius of 103 A was coagulated by the addition of NaCI04. The kinetics of coagulation were studied colorimetrically and the stability ratio W for different NaCI04 concentrations was determined (Enustun and Turkevich 1963) ... 

Biffis et al. compared the catalytic performance of polymer gel immobilized Au NPs and Au/AC, with similar sized Au NPs, for alcohol oxidations in water [172]. Gold stabilized by polymer gel is advantageous over Au/AC for the use of hydrophobic substrates such as 1-octanol and 1-phenylethanol in aqueous media, although lower selectivity was obtained in some cases. For instance, polymer microgel supported Au NPs gave 1-octanoic acid by the oxidation of 1-octanol with a selectivity of 84% at 59% conversion, whereas Au/AC gave a selectivity of 93% at 65% conversion. 

Unsupported Au nanoclusters (or those contacting an inert support material such as BN) exhibit strong size-dependent reactivity, with optimal oxidation performance typically reached < 5 nm diameter [59], For example, colloidal gold stabilized by polyvinylpyrrolidone (PVP) shows pronounced size effects in the aerobic oxidation of benzylic alcohols in water under ambient conditions [60]. Figure 2.1 illustrates this phenomenon for p-hydroxybenzyl alcohol oxidation, wherein 1.3 nm Au clusters achieve 80 % conversion, whereas 9.5 nm clusters are catalytically dead. Differential oxygen adsorption onto these gold clusters is believed to play a crucial role in regulating reactivity. 

DL Teagarden, WJ Petre, PM Gold. Stabilized prostaglandin Ei. United States Patnent 5,741,523, April 21, 1998. 

On the other hand, in the case of enynes substituted at the alkene the double addition of aldehydes is also possible, although it is not a general process. Addition of the carbonyl compound to the gold cyclopropyl carbene I-11 gives the oxonium cation 1-24. Subsequent Prins-type addition leads to the gold stabilized carbocation 1-25, which suffers nucleophilic attack by a second carbonyl compound. Then, the oxonium cation 1-26 rearranges to the final 1,3-dioxolane 1-23. 

Gold carbenoids are frequently invoked intermediates of catalytic cycles. Their rendition as gold-stabilized earbocations or gold carbenes has been debated [eqn (4.7)]. On the basis of experimental and computational studies, it now seems that the earbene depiction is irrelevant, but the discussion is not closed. ... 

The superior activity of [(IPr)Au] has been rationalized by DFT computations. The shortest Au-C bond has been found with the model IMe, whieh reveals the carbene-like behavior provided by NHC ligands to the gold-stabilized allylic cations compared to weaker electron donors (Figure 4.14). 

FIGURE 3.7 Gold stabilization of hypervalent carbon and nitrogen. 

According to the postulated reaction pathway, after the initial addition of the olefin to the activated diene, the cyclopropyl unit undergoes a ring enlargement reaction. The gold stabilized allyl cation participates then in a Nazarov-type electrocyclization leading to the final angular tricyclic compound. 

A pletliora of different SA systems have been reported in tire literature. Examples include organosilanes on hydroxylated surfaces, alkanetliiols on gold, silver, copper and platinum, dialkyl disulphides on gold, alcohols and amines on platinum and carboxyl acids on aluminium oxide and silver. Some examples and references can be found in [123]. More recently also phosphonic and phosphoric esters on aluminium oxides have been reported [124, 125]. Only a small selection out of tliis number of SA systems can be presented here and properties such as kinetics, tliennal, chemical and mechanical stability are briefly presented for alkanetliiols on gold as an example. 

Chow M K and Zukoski C F 1994 Gold sol formation meohanism role of oolloidal stability J. Coll. Int. Sc/. 165 97... 

Terril R H ef a/1995 Monolayers in three dimensions NMR, SAXS, thermal and eleotron hopping studies of alkanethiol stabilized gold olusters J. Am. Chem. Soc. 117 12 537... 

Hosteler M J ef a/1996 Monolayers in three dimensions synthesis and eleotroohemistry of w-funotionalized alkanethiolate stabilized gold oluster oompounds J. Am. Chem. Soc. 118 4212... 

Pesohel S and Sohmid G 1995 First steps towards ordered monolayers of ligand stabilized gold olusters Angew. Chem. Int. Ed. Engl. 34 1442... 

The remarkable stability of the gold complexes is due to significant metal-metal bonding. However, their isolation and structural study are remarkable and greatly contributed to our knowledge of higher-coordinate carbocations. 

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