Reaction with Unmodified Surface

In this section, we will describe some examples of surface reactivity, where a metal complex (being mononuclear or cluster) reacts in a chemical sense with some of the functionalities that are present on the solid used as support. The reactions involving grafted organic fragments introduced beforehand in order to play the role of ligands for the metallic entities will be treated in the last section of this chapter. 

The various possible surface reactions are (i) ion pairing usually with surface cationic centers such as Al + in alumina or zeolites, (ii) weak interaction with surface hydroxyl (M-OH) or oxo (M = O) groups via H-bonds, for example, 

The catalytic activity of grafted molecular fragments might be similar or modified when compared to the original complex. The presence of the support, via electronic or steric effects, might affect positively the selectivity. Moreover, it will favor site isolation and hinder dimerization of the complex bound to the surface. These well-defined materials allow the establishment of structure-activity relationships at the molecular level [37, 39]. The limitation of such systems is that the munber of OH groups on the surface controls the number of chemisorbed species, hence the loading of active phase. 

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It was found that reactions with unmodified catalyst proved to be much faster than those with Cnd modified catalyst, which also is quite different from the results reported for Pt-alumina-Cnd catalysts in the hydrogenation of alpha-keio esters. Reuse of catalysts resulted in almost complete loss of ee and indicates elution and absence of Cnd fi om the surface of the catalyst. 

The definitions and the kinetic treatment were described in detail by Garland and Blaser [4b]. Reversible adsorption of the modifier (adsorption constant Km) on unmodified surface platinum atoms, Pt , creates modified Pt atoms, Ptm (see Figure 1) which catalyze the reaction with the rate constant km and selectivity s In principle, two situations are possible ... 

EUROPT-1 modified in 3.4mM alkaloid gave ee = 41%(/ ) which was retained over the 5 h test period (Figure 3). This catalyst provides ee - 70%(/ ) in dichloromethane solution at 30 bar, and 65%(/ ) in ethanolic solution at 10 - 110 bar. That gas phase reaction may be less selective is to be expected from the absence of a rate enhancement. In the liquid phase, enantioselective reaction is so much faster than racemic reaction (factor 40) that the contribution to the enantiomeric excess from reaction at unmodified sites is small by comparison with that at enantioselective sites. By contrast, in the gas phase reaction, the rate of reaction at the modified surface is less than that at the mimodified surface, so the effect of reaction at racemic sites at a modified surface makes a larger contribution to the overall reaction. 

Ill (140). The ET reaction is initiated by photogenerated [Ru(bpy)3], which rapidly reduces the surface ruthenium. The [Ru(bpy)3] + is then scavenged by EDTA before it can back react with a5Ru(II)(histidine). Electron transfer to the protein metal center is then monitored spectroscopically. In the case of a heme (FeP), a fast increase in absorbance because of direct reduction of Fe(III)P by [Ru(bpy)3] is followed by a slower increase in absorbance due to reduction of Fe(III)P by the Ru(II) on the protein surface. Control flash experiments with unmodified proteins show only the fast initial increase in absorbance resulting from Fe(III)P reduction by [Ru(bpy)3]-. Such control experiments demonstrate for horse heart cytochrome c (140), azurin (93), and sperm whale myoglobin (30) that slow reduction of the heme by the EDTA radical produced in the scavenging step does not occur in competition with intramolecular ET. For C. krusei cytochrome c, however, the control experiment shows evidence for slow EDTA radical reduction of the heme after initial fast reduction by [Ru(bpy)3p (164). 

The justification for the threshold in surface pH is based on the steps of hydrogen sulfide adsorption/oxidation on unmodified carbons [87, 94], They are as follows (1) H2S adsorption on the carbon surface, (2) its dissolution in a water film, (3) dissociation of H2S in an adsorbed state in the water film, (4) and surface reaction with adsorbed oxygen. 

Her (324) prepared a series of sols modified with various amounts of aluminate. One series was made from an unmodified sol of 22 nm particles (Ludox TM) and another of 14 nm particles (Ludox HS). The coverage by aluminate ions ranged from 1.8 to 25% of the total silanol groups on the surface (assuming 8 SiOH nm ). Stabilization as an aluminate ion in the particle surface requires that each aluminum atom be surrounded by three oxygens linked to silicon, which means that no more than 25% of the surface silicon sites can be replaced by aluminum atoms. This assumes, of course, that the underlying silica surface is nonporous and not accessible to reaction with Al(OH)4" ions. 

Cyclic voltammograms of ferrocyanide/ferricyanide redox couple with the bare and the modified electrodes are shown in Figure 8. The peak currents due to the reversible electrode reaction of a Fe(CN)5 /Fe(CN>5 system on the bare Au electrode were significantly suppressed by the treatment with the disulfide-modified DNA. In contrast, the treatment with unmodified DNA made no suppression, and that with 2-hydroxyethyl disulfide (HEDS) did only a slight as seen in Figure 8. These results indicate that the surface-anchored DNA blocks the electrochemical reaction of Fe(CN) with the underlying Au electrode, due to the electrostatic repulsion between the polyanionic DNA and the anionic redox couple ions. 

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