Reduction by the Solvent

Several examples exist on the reduction of metallic salts by organic solvents. Probably the most popular one has been ethanol, which was long used by Toshima and co-workers for the preparation of metal nanoparticles such as Pt, Pd, Au or Rh (suitable 

Dimethylsulfoxide (DMSO) has also been used in the preparation of silver nanoparhcles. Very stable capped nanoparhcles of average diameters close to 4.4 nm and with a narrow size distribuhon were prepared by the addition of silver 2-ethylhexanoate to DMSO in the presence of trisodium citrate [40]. It was proposed that reduchon took place through the formahon of a silver-DMSO complex precursor, with both DMSO and trisodium citrate acting as reducing agents. 

The polyol process, which has been used widely to prepare anisotropic silver nanoparhcles (see Sechon 3.4.1.2), has also been used to prepare stable dispersions of spherical silver nanopartides [41]. In general, silver nitrate is reduced by ethylene glycol in the presence of PVP. In a recent report by Kim et al, the experimental set-up in the polyol process was found to be important. For example, the precursor injection rate and temperature ramping rate were found to be key factors in the produchon of monodisperse silver nanopartides [42]. 

---

The method given is similar to those described elsewhere -7 but has the advantages that the solvent pair used for recrystallization enables large well-formed single crystals suitable for spectroscopic studies to be grown very easily, and that this method is generally applicable to the isolation of bis- or tris(czs-l,2-dicyano-l,2-ethylenedithiolato) complexes which are not subject to reduction by the solvent. 

The excess oxidant is removed by warming for a few minutes. Ag(II) undergoes reduction by the solvent at a rate proportional to the square of the Ag(Il) concentration, and inversely proportional to the Ag(I) concentration. The decomposition is believed to proceed in a series of reactions involving Ag(III), H2O2, HO2, and a Ag " -H02 complex. Alternatively, the solution can be titrated potentiometrically with Fe(II) two inflections are observed, the first due to reduction of excess Ag" " and the second to the reduction of permanganate, dichromate, or Ce(IV). 

Actually, it is recognized that two different mechanisms may be involved in the above process. One is related to the reaction of a first deposited metal layer with chalcogen molecules diffusing through the double layer at the interface. The other is related to the precipitation of metal ions on the electrode during the reduction of sulfur. In the first case, after a monolayer of the compound has been plated, the deposition proceeds further according to the second mechanism. However, several factors affect the mechanism of the process, hence the corresponding composition and quality of the produced films. These factors are associated mainly to the com-plexation effect of the metal ions by the solvent, probable adsorption of electrolyte anions on the electrode surface, and solvent electrolysis. 

An unopened bottle of chromous chloride solution exploded after prolonged storage [1]. This was most likely caused by internal pressure of hydrogen developed by slow reduction of the solvent water by the powerfully reducing Cr(II) ion [2],... 

Polyurethanes are frequently used in anticorrosion coatings. The R D drive of most coatings and resin producers is the reduction of the solvent content of the paint and resin systems. One method of achieving the environmental aims demanded by legislation in the PU coatings industry is to use oxazolidines which are suitable as latent hardeners to enable production of single-pack moisture activated PUs. The isocyanate chosen for this research is tetramethylxylene diisocyanate, which has been available commercially only since 1988. 19 refs. 

The given inequalities are no longer satisfied and additional measures are required. To trigger an event (excess over design limits), at least two failures are necessary. Consequently the causal chain detailed under II a) and II b) must be interrupted by one measure in each case. These measures must be effective independently of one another. The same applies in respect of the measures concerning the failure mode which leads to a reduction in the solvent quantity D. 

In general, the rates of reduction by the ammonium salts are slower than those attained under normal conditions with the lithium salts, but the use of a non-ethereal solvent can be an advantage. Quaternary ammonium aluminium hydrides reduce ketones and amides effectively to alcohols and amines. Nitriles are also reduced to amines, whereas haloalkanes and arenes are reductively dehalogenated to give hydrocarbons in high yield [3]. 

Roughly speaking the ( -factor describes how deeply a particle is drained by the solvent a deep draining causes a reduction of the hydro dynamically effective sphere radius and c becomes small, if on the other hand only a shallow draining is possible increases and can become much larger than R. ... 

A bromide was introduced in the reaction instead of a fluoride in performing the anodic oxidation of a-stannyl ethers in dibromomethane solvent with tetrabutyl-ammonium perchlorate as the electrolyte (Scheme 19) [28]. The bromide ion was generated by the reduction of the solvent at the cathode of an undivided cell. 

We come to the conclusion that in the radical cmions of the dinitrocompounds one of the nitro-groups is excluded from contributing to the spin density distribution, most probably by fast protonation by the solvent. In this way a reduction process is started. That such reactions may play a significant role in the consecutive processes of 3,5-dinitroanisole radical anion is substantiated by the formation of 3,3 -dimethoxy-5,5 -dinitroazoxybenzene upon electrolysis of 3,5-dinitroanisole in acetonitrile-water (1 1). 

Similarly reductions with metal hydrides, metals and other compounds may give predominantly one isomer. The stereochemical outcome depends strongly on the structure of the ketone and on the reagent, and may be alfected by the solvents. 

The effect of steric hindrance can be nicely demonstrated in the reduction of two bicyclic ketones, norcamphor and camphor. The relatively accessible norcamphor yielded on reduction with complex hydrides predominantly (the less stable) endo norborneol while sterically crowded camphor was reduced by the same reagents predominantly to the less stable exo compound, isobor-neol [837], From the numerous examples shown it can be deduced that the stereoselectivity increases with increasing bulkiness (with some exceptions), and that it is affected by the nucleophilicity of the reagent and by the solvent. 

The first intermediate to be generated from a conjugated system by electron transfer is the radical-cation by oxidation or the radical-anion by reduction. Spectroscopic techniques have been extensively employed to demonstrate the existance of these often short-lived intermediates. The life-times of these intermediates are longer in aprotic solvents and in the absence of nucleophiles and electrophiles. Electron spin resonance spectroscopy is useful for characterization of the free electron distribution in the radical-ion [53]. The electrochemical cell is placed within the resonance cavity of an esr spectrometer. This cell must be thin in order to decrease the loss of power due to absorption by the solvent and electrolyte. A steady state concentration of the radical-ion species is generated by application of a suitable working electrode potential so that this unpaired electron species can be characterised. The properties of radical-ions derived from different classes of conjugated substrates are discussed in appropriate chapters. 

It was also observed, in 1973, that the fast reduction of Cu ions by solvated electrons in liquid ammonia did not yield the metal and that, instead, molecular hydrogen was evolved [11]. These results were explained by assigning to the quasi-atomic state of the nascent metal, specific thermodynamical properties distinct from those of the bulk metal, which is stable under the same conditions. This concept implied that, as soon as formed, atoms and small clusters of a metal, even a noble metal, may exhibit much stronger reducing properties than the bulk metal, and may be spontaneously corroded by the solvent with simultaneous hydrogen evolution. It also implied that for a given metal the thermodynamics depended on the particle nuclearity (number of atoms reduced per particle), and it therefore provided a rationalized interpretation of other previous data [7,9,10]. Furthermore, experiments on the photoionization of silver atoms in solution demonstrated that their ionization potential was much lower than that of the bulk metal [12]. Moreover, it was shown that the redox potential of isolated silver atoms in water must... 

Moreover, almost in all the early steps, the redox potential of the clusters, which decreases with the nuclearity, is quite negative. Therefore the growth process undergoes another competition with a spontaneous corrosion by the solvent and the radiolytic protons, corrosion which may even prevent the formation of clusters, as mostly in the case of nonnoble metals. Monomeric atoms and oligomers of these elements are so fragile to reverse oxidation by the medium that H2 is evolved and the zerovalent metal is not formed [11]. For that reason, it is preferable in these systems to scavenge the protons by adding a base to the solution and to favor the coalescence by a reduction faster than the oxidation [53]. 

---