Probe molecules phosphines

It is often said that the property of acidity is manifest only in the presence of a base, and NMR studies of probe molecules became common following studies of amines by Ellis [4] and Maciel [5, 6] and phosphines by Lunsford [7] in the early to mid 80s. More recently, the maturation of variable temperature MAS NMR has permitted the study of reactive probe molecules which are revealing not only in themselves but also in the intermediates and products that they form on the solid acid. We carried out detailed studies of aldol reactions in zeolites beginning with the early 1993 report of the synthesis of crotonaldehyde from acetaldehyde in HZSM-5 [8] and continuing through investigations of acetone, cyclopentanone [9] and propanal [10], The formation of mesityl oxide 1, from dimerization and dehydration of... 

Another possibility for characterizing zeolite acid sites is the adsorption of basic probe molecules and subsequent spectroscopic investigation of the adsorbed species. Phosphines or phosphine oxides have been quite attractive candidates due to the high chemical shift sensitivity of 31P, when surface interactions take place [218-222]. This allows one to obtain information on the intrinsic accessibility and acidity behavior, as well as the existence of different sites in zeolite catalysts. 

A remarkable application of phosphines by Grey and coworkers for acid site characterization is the use of diphosphines with alkyl chain spacers of different length between the phosphine moieties. Based on careful NMR analysis and appropriate loading levels with diphosphines, the Al distribution can be determined [223, 224], The idea behind this tool is that the phosphine groups will be proto-nated, when they are close to an acid site in the zeolite structure. Protonation of both phosphine groups in one probe molecule will only occur, when the distance between the two acid sites is compatible with the molecular dimension of the diphosphine. 

Phosphines (continued) oxides, 32 366-367 synthesis of, 25 94 as probe molecules, 42 170,172 (Phosphinoalkyl)phosphonium salts, 42 480 Phosphobetaine, 25 88... 

As examples of probe molecules directly introduced onto solid acids, we consider ketones and aldehydes, amines and other nitrogen-containing compounds, phosphines, and molecules that form multiple hydrogen bonds. 

In addihon to organic phosphines and phosphine oxides, other probe molecules which have been successfully employed in the study of Br0nsted sites include alkanenitriles [211-213] ( C and NMR), acetone [167] and CO [214] ("C NMR). 

The basic probe molecules of primary interest in this chapter are pyridine, ammonia, i-butylamine, phosphines, and phosphine oxides. All possess several nuclei amenable to solid-state NMR experiments, and, as we shall see, this multinuclear perspective is useful in extracting chemical insights into the nature of the surface. Finally, it is important to point out the early, pioneering contributions of Ian Gay 181] to this area of NMR spectroscopy. It was those initial experiments that demonstrated the potential utility of solid-state NMR methcxis to address important questions in the area of surface chemistry. 

Other useful classes of basic probe molecules used to examine silica, alumina, and silica-alumina surfaces (as well as zeolite systems) include small organic phosphines and phosphine oxides, which rely on the highly convenient P nuclide (/ = 1/2, 100% natural abundance). As Lunsford and coworkers demonstrated for zeolites [89], the P NMR signal of trimethylphosphine is a useful probe for Bronsted acid sites on surfaces. The basis for this approach is the formation of R3P -H B( ) sites at surface Bronsted acid sites, H-B(. ... 

Ousters on the outade surface of a zeolite can sometimes be distinguished from those within the cages by reactions with probe molecules of various sizes, whereby some are small enough to enter the pores and some are too large to enter. The probes most commonly used are phosphines, as they react with many metal carbonyls. The products are typically characterized by infrared spectroscopy. 

Similarly, reaction of tris(dibenzylideneacetone)dipalladium with two equivalents of f-butylisocyanide per Pd atom, followed by addition of OFCOT, leads to an analogous ring-closed product 99a. Reaction of [Pd(PPh3)4] with OFCOT afforded the phosphine analogue 99b (174). No attempts have been made to probe any further chemistry of these molecules. 

The 31P H NMR spectra of a number of heteronuclear gold cluster compounds are found to be deceptively simple and NMR studies have been used as a probe of the behavior of these species in solution. This is especially true of the higher nuclearity clusters, which often exhibit spectra that are much simpler than would be predicted on the basis of their solid-state structures. For example, [Pt(H)(PPh3)(AuPPh3)7]2+, which adopts the solid-state structure illustrated in Fig. 9 (137) in which the phosphine ligands occupy several different chemical environments within the molecule, shows only two resonances in the 31P 1H NMR spectrum. These are in a ratio of 7 1 and exhibit satellites due to coupling to the central platinum nucleus as Fig. 10 illustrates. 

An exception to the straightforward correspondence between C shifts in zeolites (or other catalysts) and solution values occurs when the structure of the compound is significantly perturbed on the catalyst. The most common example is protonation equilibria on acidic catalysts. Indeed, there have been a number of reports of the use of protonation shifts of amines 151,521, phosphines (151, and phosphine oxides (531 as probes of catalyst acidity. Similar effects are (x ca-sionally encountered in in situ experiments when a basic molecule is formed as an intermediate or product. An interesting case is the conversion of acetone to hydrocarbons on zeolites, which may involve the intermediacy of diacetone alcohol, mesityl oxide, phorone, and isophorone—all ketones. The chemical shifts of the carbonyl carbons of all these species in acidic zeolites were found to be up to 10 ppm downfield of the corresponding values in reference compilations. Furthermore, although the chemical shifts of the olefinic carbons a to the carbonyl were in reasonable agreement with values for CDCI solutions, the resonances of the olefinic carbons p to the carbonyl were very broad and shifted 20-30 ppm downfield 54. ... 

By analogy to the use of small bases (amines, phosphines, phosphine oxides, NiO) as probes of acidic sites on surfaces, one might expect that small acidic molecules could serve as probes of basic sites on surfaces. For this purpose BR compounds show some promise, based on the "B nuclide (/ = 3/2, 80.4% natural abundance) (P. Marchetti and G. E. Maciel, unpublished results). 

Intrusion of gas phase probes metal carbonyl clusters internal or external location of metal carbonyl cluster in zeolite Large phosphine molecule cannot diffuse through zeolite aperture into the zeolite cages to react with encaged carbonyl dusters effective for highly reactive carbonyl clusters. 

When a liquid crystal is reoriented in an external field the observed intensity of the fluorescence of impurity molecules dissolved in the liquid crystal is changed. For example, if the dye brilliant phosphine is dissolved in the nematic phase of p-n butoxybenzoic acid the intensity of its fluorescence increases by a factor of 3 when a field is applied [178]. The use of a fluorescent probe (stilbene dye) for investigating the kinetics of the Prederiks effect is described in [56]. Fluorescence polarization measurements allow us to obtain information on liquid crystal electronic spectra [179] and order parameter [180]. 

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