Solvent-cation donors

Although solvents may be classified as donor solvents (Lewis bases) and acceptor solvents (Lewis acids), most of the more widely used nonaqueous solvents are donor solvents. Some acceptor solvents, such as S02, BrF3, AsC13, or the liquid hydrogen halides, have proved to be useful in coordination chemistry.13"16 Ionization is promoted in a donor solvent by solvation of cations and in an acceptor solvent by solvation of anions. For example, arsenic(m) iodide is ionized in a donor solvent D according to the reaction... 

It was pointed out earlier that the Gibbs energy of transfer of simple monatomic ions such as Na and CU is not related in any simple way to the dielectric permittivity of the solvent. On the other hand, for the alkali metal cations is correlated with solvent basicity. A plot of this quantity for the Na ion against the solvent s donor number DN is shown in fig. 4.15. Although the correlation is not excellent, it shows a clear trend in which AtrG° becomes more negative as DN increases. This is exactly what one expects because the ability of the solvent to stabilize the cation in solution increases with increase in solvent basicity. 

Summary The synthesis of intramolecularely Jt-stabilized silylium ions such as 7-sila-norbomadienyl- and 2-silanorbomyl cations was attempted. While benzo-7-sila-norbomadienyl cation 7 could not be detected at room temperature in hydrocarbon solvents, its donor-acceptor complex 8 was identified by NMR spectroscopy. In contrast, persistent 2-silanorbomyl cations could be synthesized and characterized by NMR spectroscopy and modem quantum chemical methods. Silanorbomyl cations 5 and 25 are stable at room temperature and have a bridged stmcture in which the silicon adopts a [3+2] coordination. 

Ebert and KonopirI distinguish between donor and acceptor functions for both acids and bases. A donor acid releases solvent cations while an acceptor acid accepts solvent anions e.g. in water. ... 

When the attempt is made to reach definitions independent of a solvent they become similar to those given by Ussanovich s according to which an acid is either a cation donor or an anion acceptor, and a base either an anion donor or a cation acceptor. In this way certain redox reactions are also considered as acid-base reactions. 

The solvent system Donor solvents such as THF allowed better control of the particle size because the metal cations were partially coordinated to solvent molecules and thus prevented collapse of the nanostmcmres. In non-donor solvents (i.e. CHCI3), the cations acted as crosslinking agents that evenmally led to HPMC-insoluble materials. This problem could be avoided by adding donor co-solvents (i.e. MeOH, acetone) that stopped both the growth and the collapse of the nanospheres. 

Several other ions have been investigated but the results are not as good even the Mg + ion, which is well-known for its superior ability to complex with oxygen donors, produces lower shifts than the Ba + ion. The reasons for this apparent incoherence were highlighted by ab initio studies, which showed that, in these cases, powerful solvent—cation interactions are operative and, thus, make these metals less suitable, because of strong solvation by the solvent MeCN. The importance of the Ba +ZMeCN-c/s pair is therefore a key aspect in this method. 

Finally, none of the ionic liquids were found to be hydrogen bond acids [5], although this may well be a consequence of the salts selected, none of which had a cation that would be expected to act as a hydrogen bond donor. Earlier qualitative measurements on ionic liquid stationary phases of mono-, di-, and trialkylammo-nium salts suggest that hydrogen bond donation can be important where a potentially acidic proton is available [7-9]. More recent work, with [BMIM] salts, also indicates that these ionic liquids should be considered to be hydrogen bond donor solvents [10]. However, this has yet to be quantified. 

As well as phosphorus ligands, heterocyclic carbenes ligands 10 have proven to be interesting donor ligands for stabilization of transition metal complexes (especially palladium) in ionic liquids. The imidazolium cation is usually presumed to be a simple inert component of the solvent system. However, the proton on the carbon atom at position 2 in the imidazolium is acidic and this carbon atom can be depro-tonated by, for example, basic ligands of the metal complex, to form carbenes (Scheme 5.3-2). 

Polar aprotic solvent (Section 11.3) A polar solvent that can t function as a hydrogen ion donor. Polar aprotic solvents such as dimethyl sulfoxide (DMSO) and dimethyl-formamide (DMF) are particularly useful in Sn2 reactions because of their ability to solvate cations. 

Those in which solvent molecules are directly involved in formation of the ion association complex. Most of the solvents (ethers, esters, ketones and alcohols) which participate in this way contain donor oxygen atoms and the coordinating ability of the solvent is of vital significance. The coordinated solvent molecules facilitate the solvent extraction of salts such as chlorides and nitrates by contributing both to the size of the cation and the resemblance of the complex to the solvent. 

In conclusion, it is very likely that the influence of solvents on the change from the heterolytic mechanism of dissociation of the C —N bond in aromatic diazonium ions to homolytic dissociation can be accounted for by a mechanism in which a solvent molecule acts as a nucleophile or an electron donor to the P-nitrogen atom. This process is followed by a one- or a two-step homolytic dissociation to an aryl radical, a solvent radical, and a nitrogen molecule. In this way the unfavorable formation of a dinitrogen radical cation 8.3 as mentioned in Section 8.2, is eliminated. 

The UV spectra suggest that the equilibrium between the diazonium ion and the solvent, on the one hand, and an electron donor-acceptor complex (8.58) on the other, lies on the side of the complex. The latter may possibly exist also as a radical pair (8.60) or a covalent compound (8.59). Dissociation of this complex within a cage to form an aryl radical, a nitrogen molecule, and the radical cation of DMSO is slow and rate-determining. Fast subsequent steps lead to the products observed. 

If the nucleophilicity of the anion is decreased, then an increase of its stability proceeds the excessive olefine can compete with the anion as a donor for the carbenium ion, and therefore the formation of chain molecules can be induced. The increase of stability named above is made possible by specific interactions with the solvent as well as complex formations with a suitable acceptor 112). Especially suitable acceptors are Lewis acids. These acids have a double function during cationic polymerizations in an environment which is not entirely water-free. They react with the remaining water to build a complex acid, which due to its increased acidity can form the important first monomer cation by protonation of the monomer. The Lewis acids stabilize the strong nucleophilic anion OH by forming the complex anion (MtXn(OH))- so that the chain propagation dominates rather than the chain termination. 

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