Proton complexes solution chemistry

Pyridinecarboxaldehyde, 3. Possible hydration of the aldehyde group makes the aqueous solution chemistry of 3 potentially more complex and interesting than the other compounds. Hydration is less extensive with 3 than 4-pyridinecarboxaldehyde but upon protonation, about 80% will exist as the hydrate (gem-diol). The calculated distribution of species as a function of pH is given in Figure 4 based on the equilibrium constants determined by Laviron (9). 

Inorganic solution chemistry often involves proton transfers to and from solvated metal ions as well as to and from the acids and bases that complex metal ions. Eight generalizations are presented below that attempt to summarize the insights regarding proton transfer reactions that have emerged in the past quarter century. The masterful reviews by Eigen (1) and Bell (2) provide much more extensive analysis of most of these points. 

It may have been the dramatic 1964 publication of E.S. Lewis and L. Funderburk that forced the question of hydrogen tunneling in complex solution reactions near room temperature into the consciousness of a larger scientific public, particularly in physical-organic chemistry. This article presented isotope effects for proton abstraction from 2-nitropropane by a series of substituted pyridines, and the values rose sharply as the degree of steric hindrance to the reaction increased (Fig. 1). AU the observed H/D isotope effects, from 9.6 to 24, were larger than expected from the simplest version of the so-called semiclassical theory of isotope effects (Fig. 2). 

As with uranium, the solution chemistry is complicated, owing to hydrolysis and polynuclear ion formation, complex formation with anions other than perchlorate, and disproportionation reactions of some oxidation states. The tendency of ions to displace a proton from water increases with increasing charge and decreasing ion radius, so that the tendency to hydrolysis increases in the same order for each oxidation state, th at is, Am > Pu > Np > U and M4+ > M02+ > M3+ > M02 simple ions such as Np02OH+ or PuOH3+ are known in addition to polymeric species that in the case of plutonium can have molecular weights up to 1010. 

Most of the studies on the RE(III) complexes with neutral and basic amino acids have shown that complexation takes place when pH > 6, and that RE(OH)3 precipitation forms if the pH > 8, and experiments done at pH < 7 or 7.5 are considered to be free of significant hydrolysis. However, a few studies did show that protonated complex species form at pH < 2, and the hydrolysis of RE(III) starts at pH < 6 [126,135,136]. The large discrepancies among the data could be a result of different experimental conditions [concentrations of the RE(III) and the ligands, the RE L ratios] and various computing models used, as all of the experiments and the calculations are based on Bjerrum s method [142]. More detailed and systematic studies are thus definitely needed for the solution chemistry of RE(III)-amino acid complexes. 

In this respect the solution chemistry of common anions is very different. For example with phosphate (Fig. 4.4), HP04 and H2P04 are the dominating solution species over the typically studied pH range, and fully dissociated anions occur only at extremely high pH values, which are of limited interest in adsorption studies, e.g. many common adsorbents are unstable at such a high pH (dissolution). On the other hand, the final products of hydrolysis on anions, i.e. fully protonated acid molecules are usually water soluble, thus, the applicability of surface complexation model is not limited by surface precipitation as it was discussed above for metal cations. 

Each of the complexes described in this section have been synthesised, at least initially, by the serendipitous assembly of manganese salts or pre-formed clusters of Mn with a combination of one or more flexible organic bridging ligands. These species represent the vast majority of SMMs reported to date. Mn cluster chemistry involves complicated processes in which the crystallographically characterised products are almost certainly not the only complexes present in solution at any one time and where the synthesis also involves the protonation/deprotonation, redox chemistry and structural rearrangement of many species simultaneously. Thus, mechanistic, kinetic and any other detailed studies of the reaction processes are almost impossible. 

To derive true mechanistic information from dissolution experiments, the rate laws must be expressed in terms of the concentration of all distinct metal-ligand surface complexes that vary in reactivity and stoichiometry. Distinction must be made, for example, between protons that adsorb onto the ligand and those that adsorb onto the mineral surface in addition to the ligand. These adsorbed ligands all potentially change conformation with solution chemistry and these changes would be manifested in the rate laws. 

The overall PES for nitration in the gas phase is very diffident fiom that in solution. In the gas phase, formation of the o-complex from the free reactants is a vay exothermic process with a zcto overall barrier. However, compared to solution, the deprotonation of the a-complex is a slower process. Different pathways for intramolecular proton transfo have been studied, including transfer to an oxygen as well as initial transfer to a nearby carbon [51,55,56]. We will not go into the details of these reactions, since they are of little relevance for solution chemistry. 

The extension of analytical mass spectrometry from electron ionization (El) to chemical ionization (Cl) and then to the ion desorption (probably more correctly ion desolvation ) techniques terminating with ES, represents not only an increase of analytical capabilities, but also a broadening of the chemical horizon for the analytical mass spectrometrist. While Cl introduced the necessity for understanding ion—molecule reactions, such as proton transfer and acidities and basicities, the desolvation techniques bring the mass spectrometrist in touch with ions in solution, ion-ligand complexes, and intermediate states of ion solvation in the gas phase. Gas-phase ion chemistry can play a key role in this new interdisciplinary integration. 

As can be seen by Reactions 10.1-10.4, the state of the Stern layer depends on the chemistry of the solution it contacts. As pH decreases, the numbers of protonated sites (e.g., >(w)FeOH+) and sites complexed with bivalent anions (e.g., >(w)FeS04) increase. If protonated sites dominate, as is likely under acidic conditions, the surface has a net positive charge. 

Hydride transfer reactions from [Cp2MoH2] were discussed above in studies by Ito et al. [38], where this molybdenum dihydride was used in conjunction with acids for stoichiometric ionic hydrogenations of ketones. Tyler and coworkers have extensively developed the chemistry of related molybdenocene complexes in aqueous solution [52-54]. The dimeric bis-hydroxide bridged dication dissolves in water to produce the monomeric complex shown in Eq. (32) [53]. In D20 solution at 80 °C, this bimetallic complex catalyzes the H/D exchange of the a-protons of alcohols such as benzyl alcohol and ethanol [52, 54]. 

Figure 6.1 A simple electrostatic adsorption mechanism illustrating the protonation-deprotonation chemistry of surface hydroxyl groups on oxide surfaces (which are neutral at the PZC) and the corresponding uptake of anionic or cationic complexes. Proton transfer to or from the surface can significantly affect the solution pH.

We illustrate the nomenclature introduced above in an example taken from coordination chemistry. In fact, equilibrium species of interesting complexity are commonly encountered in coordination chemistry and to a large extent coordination chemists have developed the principles of equilibrium studies. Consider the interaction of a metal ion M (e.g. Cu2+) with a bidentate ligand L (e.g. ethylenediamine, en) in aqueous solution. For work in aqueous solution the pH also plays an important role and thus, the proton concentration H (=[ff+]), as well as several differently protonated species, need to be taken into account. Using the nomenclature commonly employed in coordination chemistry, there are three components, M, L, and H. In aqueous solution they interact to form the following species, HL, H2L, ML, Mia, ML3, MLH, MLH1 and OH. (In fact, more species are formed, e.g. ML2H 1, but the above selection will suffice now.) The water molecules are usually not defined as additional components. The concentration of water is constant and its value is taken into the equilibrium constants. 

Fig. 17. Gd-aqueous proton radial distribution function for the aqueous solution of the Gd(III)(DOTP) complex (after Borel, A. Helm, L. Merbach, A.E. Chemistry - A European Journal 2001, 7, 600-610).

The question of protonation sites is one of the basic questions in the behaviour of complex organic molecules in solution, since protonated molecules are intermediates in synthetic organic chemistry, and the knowledge of protonation sites is important for the theory of reaction mechanisms of acid-catalysed reactions. It is also of fundamental importance for structural theory in general, since it is intimately connected with the concepts of mesomerism, electron density and bond polarization. 

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