Proton-exchange reaction, metal

It has been possible to exchange Li in LiNb03 by protons to obtain HNb03. Such alkali metal-proton exchange reactions are common in layered oxides (e.g., H2Ti30y and HLaNbOj). 

Yong MacDonald (1998) show that upon apparent completion of metal sorption, measurements of the equilibrium pH of the system generally showed a reduction below initial pH. This reduction in pH was attributed to the resultant effect of the many reactions in the system. These reactions included the release of hydrogen ions by metal/proton exchange reactions on surface sites, hydrolyses of metals in the soil solution, and precipitation of metals. It was apparent that more detailed information was needed to distinguish between surface and solution reactions responsible for release of hydrogen ions. However, it was evident that if surface complexation models are to be used, the relationship between metal adsorption and proton release needs to be established. That is, net proton release or consumption is due to all the chemical reactions involving proton transfer. 

The proposed mechanism for this catalytic asymmetric hydrophosphonylation is shown in Figure 35. The first step of this reaction is the deprotonation of dimethyl phosphite by LPB to generate potassium dimethyl phosphite. This potassium phosphite immediately coordinates to a lanthanoid to give I due to the strong oxophilicity of lanthanoid metals. The complex I then reacts (in the stereochemistry-determining step) with an imine to give the potassium salt of the a-aminophosphonate. A proton-exchange reaction affords the product... 

Heterobimetallic catalysis mediated by LnMB complexes (Structures 2 and 22) represents the first highly efficient asymmetric catalytic approach to both a-hydro and c-amino phosphonates [112], The highly enantioselective hydrophosphonylation of aldehydes [170] and acyclic and cyclic imines [171] has been achieved. The proposed catalytic cycle for the hydrophosphonylation of acyclic imines is shown representatively in Scheme 10. Potassium dimethyl phosphite is initially generated by the deprotonation of dimethyl phosphite with LnPB and immediately coordinates to the rare earth metal center via the oxygen. This adduct then produces with the incoming imine an optically active potassium salt of the a-amino phosphonate, which leads via proton-exchange reaction to an a-amino phosphonate and LnPB. 

B). The non-integral rate with respect to pH (A) but nearly integral rate with respect to log is common in mineral dissolution reactions. A schematic representation of the four steps proposed to explain the data in (A) and (B) is presented in (C). The first three steps are surface proton exchange reactions and are followed by the rate-determining detachment of the metal from the surface. Adapted from Furrer and Stumm (1986). 

We laid most importance on the question of wether it is possible to metallize a ring carbon atom, because this would confer a considerable potential in applications. Insertion of donor substituents for stabilizing catalyst intermediates by intramolecular coordination or for controlling stereoselective insertion reactions are only two aspects for further studies. The metal-proton exchange occurs already by reaction of 6 with w-butyllithium in THF at -yS C, proved by a subsequent trapping reaction with chlorotrimethylsilane. 

Several approaches have been used to prepare transition-metal-thiolate complexes. The most common synthetic route involves the metathesis reaction of a metal halide with an alkali metal thiolate salt. For instance, [(DPPE)Pd(Ar)(S-t-Bu)] was obtained by treatment of the (DPPE) palladium aryl iodide complex with sodium fprf-butyl thiolate (Equaticm 4.106). Additionally, these compoimds have been formed by proton exchange reactions of thiols with M-C (Equation 4.107), ° M-W and M-O, bonds (Equation 4.108). Finally, the oxidative addition of RSH, or alkyl and aryl disulfides - is a useful way... 

Hydrolysis and proton-exchange reactions of each metal-oxidation state. 

Nagypal, I. and Fabian, I. (1982) NMR relaxation studies in solution of transition metal complexes. V. Proton exchange reactions in aqueous solutions of VO -oxalic acid, -malonic acid systems. Inorg. Chim. Acta, 61, 109-113. 

Benzeneselenol as a representative selenol is a colorless liquid of greater acidity than benzenethiol (p a = 5.9 (PhSeH) 6.5 (PhSH)). The bond energy of Se-H is 73 kcal/mol, is smaller than S-H (87 kcal/mol) [82]. These properties may contribute to the efficiency in the oxidative addition of selenols to low-valent transition metals, ligand-exchange reaction between high-valent transition metal complexes and selenols, and protonation process of carbon-metal bonds. Indeed, several transition metal complexes catalyze the highly selective hydrothiolation of alkynes and allenes. 

Alkynyl anions are more stable = 22) than the more saturated alkyl or alkenyl anions (p/Tj = 40-45). They may be obtained directly from terminal acetylenes by treatment with strong base, e.g. sodium amide (pA, of NH 35). Frequently magnesium acetylides are made in proton-metal exchange reactions with more reactive Grignard reagents. Copper and mercury acetylides are formed directly from the corresponding metal acetates and acetylenes under neutral conditions (G.E. Coates, 1977 R.P. Houghton, 1979). 

In this section, we summarize the kinetic behavior of the oxygen reduction reaction (ORR), mainly on platinum electrodes since this metal is the most active electrocatalyst for this reaction in an acidic medium. The discussion will, however, be restricted to the characteristics of this reaction in DMFCs because of the possible presence in the cathode compartment of methanol, which can cross over the proton exchange membrane. 

Although the redox reaction mechanisms of iridium oxide are still not clear, most researchers believe that the proton exchange associated with oxidation states of metal oxides is one of the possible pH sensing mechanisms [41, 87, 100, 105], During electrochemical reactions, oxidation state changes in the hydrated iridium oxide layer are... 

LA represents Lewis acid in the catalyst, and M represents Bren sled base. In Scheme 8-49, Bronsted base functionality in the hetero-bimetalic chiral catalyst I can deprotonate a ketone to produce the corresponding enolate II, while at the same time the Lewis acid functionality activates an aldehyde to give intermediate III. Intramolecular aldol reaction then proceeds in a chelation-controlled manner to give //-keto metal alkoxide IV. Proton exchange between the metal alkoxide moiety and an aromatic hydroxy proton or an a-proton of a ketone leads to the production of an optically active aldol product and the regeneration of the catalyst I, thus finishing the catalytic cycle. 

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