NMR spectroscopy, in situ

This example should illustrate that in situ NMR spectroscopy can be a powerful tool with which to study catalysts dissolved in ionic liquids, if the signals of the metal complex can be detected in sufficient intensity independently from the signals of the ionic liquid. 

In addition to in situ NMR spectroscopy, other methods such as in situ IR spectroscopy, EXAFS, and electrochemistry should be very useful for the investigation of active catalytic species in ionic liquids. However, far too little effort has been directed to this end in recent years. 

For the elucidation of chemical reaction mechanisms, in-situ NMR spectroscopy is an established technique. For investigations at high pressure either sample tubes from sapphire [3] or metallic reactors [4] permitting high pressures and elevated temperatures are used. The latter represent autoclaves, typically machined from copper-beryllium or titanium-aluminum alloys. An earlier version thereof employs separate torus-shaped coils that are imbedded into these reactors permitting in-situ probing of the reactions within their interior. However, in this case certain drawbacks of this concept limit the filling factor of such NMR probes consequently, their sensitivity is relatively low, and so is their resolution. As a superior alternative, the metallic reactor itself may function as the resonator of the NMR probe, in which case no additional coils are required. In this way gas/liquid reactions or reactions within supercritical fluids can be studied... 

High-pressure in-situ NMR spectroscopy have been reported about reactions of carbon monoxide with cobalt complexes of the type, [Co(CO)3L]2. For L=P(n-C4H9)3, high pressures of carbon monoxide cause CO addition and disproportionation of the catalyst to produce a catalytically inactive cobalt(I) salt with the composition [Co(CO)3L2]+[Co(CO)4] . Salt formation is favoured by polar solvents [13],... 

In aprotic solvents, chain transfer occurs exclusively by fl-H elimination, unless a protic acid or water is present in the reaction mixture, in which case protonolysis may occur. Indirect evidence (for example, M, and M measurements) proves that P-H chain transfer in aprotic solvents is slower than methanolysis in protic solvents with comparable structures of the Pd" catalyst [5f, 17, 20, 21]. This effect and the possibility of using well-defined catalysts have remarkably favored the use of in situ NMR spectroscopy for the detection of intermediates during CO/copolymerisation in organic solvents. 

Using in situ NMR spectroscopy, Brookhart has also studied the activation barriers for the migratory insertion steps corresponding to chain growth in ethene/ CO copolymerisation catalysed by dppe-derived nickel(II) complexes [4a]. Activa-... 

By means of in situ NMR spectroscopy combined with deuterium incorporation experiments, van Leeuwen has elucidated the mechanism of termination by protonolysis, showing that the fl-chelates are in equilibrium with their enolate form by a p-H elimination/hydride migration process (Scheme 7.19). The enolate intermediates are regioselectively protonated at the C2 carbon atom by either MeOH or H2O to give Pd-OMe or Pd-OH and keto terminated copolymer. The enolate formation has been reported to be rate determining in the chain transfer [19]. 

Nuclear magnetic resonance (NMR) spectroscopy is the most widely used spectroscopic technique in synthetic chemistry [1], One main reason for the dominance of NMR is its versatility—by variation of only a few experimental parameters, a vast number of different NMR experiments can easily be performed, giving access to very different sets of information on the substance or the reaction under investigation. Today, NMR is dominant in structure elucidation, and in situ NMR spectroscopy can conveniently give insight into chemical reactions under real turnover conditions (in contrast to, e.g., x-ray crystallography, which can only provide a solid-state snapshot of a molecular conformation). 

For the past decade, the major thrust of one of the authors has been the elucidation of reaction mechanisms in heterogeneous catalysis on zeolites, metal oxides and other materials. The primary experimental tool has been in situ NMR spectroscopy, and this is increasingly coupled with theoretical calculations carried out by the other author. 

An important feature of solution phase measurements is the ability to record data while the reaction is progressing. In situ NMR spectroscopy has been a mainstay for popular forms such as 1H NMR spectroscopy for many years, but in recent years, it has been demonstrated that it is also possible for the quadrupolar halogens.28 31... 

Nuclear magnetic resonance (NMR) spectroscopy — Nuclear magnetic resonance (NMR) spectroscopy of atoms having a nonzero spin (like, e.g., H, 13C) is an extremely powerful tool in structural investigations in organic and inorganic chemistry. Beyond structural studies atoms observable with NMR can also be used as probes of their environment. Thus NMR may be employed for in situ spectroelectrochemical studies [i]. Cell designs for in situ NMR spectroscopy with electrochemical cells are scant. Because of the low sensi-... 

In-situ IR measurements overcome the problems mentioned for in situ NMR spectroscopy. The information that we obtain from vibrational spectroscopy is far less detailed, however, than that from NMR. The concentration of the catalyst may be equal to the one used in practical catalytic systems. Secondly, autoclaves have been equipped with IR cells, either as flow cells or via real in-situ monitoring in the Moser cell (see below), which allows one to work with gaseous reactants. In the following we will mention a (very) few examples of complexes that may be intermediates in the hydroformylation reaction observed with these two techniques. 

The kinetic studies were carried out with a constant catalyst concentration of 2 mol% and monitored by in situ NMR spectroscopy. The reaction rate increased... 

The noninvasive nature of NMR spectroscopy combined with the chemical specificity of the NMR method provides direct access to the distribution of various chemical constituents for the histochemistry of plant materials in situ NMR spectroscopy can be used to identify the major constituents, and chemical-.shift imaging can be used to spatially localize them. The latter can be applied to localize aromatics, carbohydrates, as well as water and fat or oil in plant samples. The suitability of many fresh fruits and living plants to be studied by NMR imaging results in a variety of applications in agriculture and food science [Mcc I, Mcc2]. 

The mechanistic study on the regioselectivity of the HRP-catalyzed oxidative polymerization was performed by using in situ NMR spectroscopy [41, 42], In the polymerization of 8-hydroxyquinoline-5-sulfonate, the 2, 4, and 7-positions were involved in the oxidative coupling with the order of preference being 7 > 2 > 4. The polymerizability of phenols via HRP catalysis was evaluated by the initial reaction rate [43]. Phenols with electron-donating groups were consumed much... 

NMR spectroscopy is a routine method in all synthetic laboratories. In situ NMR spectroscopy is, therefore, a natural first choice when it comes to the characterization of catalysts dissolved in ionic liquids. However, this method suffers in particular... 

In the last decade, in situ NMR spectroscopy has found an increasing application for the investigation of chemical reactions heterogeneously catalyzed by acidic and basic zeolites. Using this method, it is possible to study the formation of adsorbate complexes, intermediates, and reaction products inside the zeolite crystals. An excellent review of in situ NMR spectroscopic studies of chemical reactions in zeolites is given by Haw [335]. Furthermore, the reader is referred to papers published by Kfinowski [54, 58], Ivanova and Derouane [336], and Pfeifer and Ernst [4]. 

The occurrence of carbenium ions as reaction intermediates is strongly supported by the observation that the isotopic exchange can be totally suppressed in the presence of carbon monoxide. Furthermore, trapping of the intermediate carbenium ions by CO and water has been observed by in situ NMR spectroscopy, when iso-butane, water, and CO reacted on HZSM-5 zeolite to form pivalic acid.I l Regarding the low conversion. 

Fig. 5.119. Schematic drawing of an electrochemical cell for in situ NMR spectroscopy [649]. A counter electrode connection B reference electrode joint C counter electrode D platinum black working electrode E working electrode connection F purge connection...

M. Hunger, In situ NMR spectroscopy in heterogeneous catalysis, Catal. Today, 2004, 97, 3-12. 

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