Quaternary phosphonium cations

In recent years, a new class of ILs based on tricaprylammonium chloride (Ali-quat 336) has been investigated. By a simple replacement of the chloride anion, several new ILs have been generated and tested for their applicability, for instance, as extracting agents for heavy metals [641]. However, difficulties arise in the course of the analysis of Aliquat-based ILs as Aliquat 336 is a mixture of quaternary ammonium cations with different alkyl chain lengths rather than a well-defined pure compound. This property makes the quantification of the Aliquat cation and hence all of its products more comphcated. On the other hand, ILs based on quaternary phosphonium cations are already in use for a varietjr of applications. So far, no apphcable LC characterization and/or purity determination of these two classes of ILs was mentioned in literature. In 2008, Stojanovic et al. [642] introduced a reversed-phase liquid chromatographic method with serially coupled diode array detection (DAD) and charged aerosol detection for... 

Matsumiya M, Suda S, Tsunashima K, Sugiya M, Kishioka S, Matsuura H (2008) Electrochemical behaviors of multivalent complexes in room temperature ionic liquids based on quaternary phosphonium cations. Journal of Electroanalytical Chem 622(2) 129-135... 

Recently, RTILs based on quaternary phosphonium cations (Fig. 2) have been receiving a great deal of attention as potential substitutes of the corresponding... 

Figure 3 shows the typical synthetic route to prepare the TFSA anion based phosphonium RTILs. We have chosen triethylphosphine (P(C2Hs)3) and tii(n-bulyl)phosphine (P( -C4H9)3) as commercially available starting materials since these two phosphines can afford relatively small quaternary phosphonium cations. The preparation process of the phosphonium RTCLs includes two steps nucleophilic addition reactions of trialkylphosphines to alkyl halides to form precursor phosphonium halides and then aqueous ion exchange reactions of the precursor phosphonium halides with lithium bis(trifluoromethylsulfonyl)amide (Li-TFSA) to obtain TFSA anion based phosphonium RTBLs. 

Tsunashima K, Ono Y, Sugiya M (2011) Physical and electrochemical characterization of ionic liquids based on quaternary phosphonium cations containing a carbon-carbon double bond. Electrochim Acta 56 4351 355... 

Aliquat 336 and quaternary phosphonium cation [QP] with 2-(methylthio)benzoate [MTBA] and [TS] have been apvplied as extracting agents for PH+ from chloride solution. The extraction efficiency tends to diminish with decreasing viscosity of IL in the following order [QP][MTBA] (100%) > [A336][rS] (85%) > [QP][TS] (76%) > [A336][MTBA] (40%) (Stojanovic et al., 2010). Thus, it proves that not only functionality appended to the anion but also physico-chemical properties of IL play an important role in extraction of metal ions. 

Table 17.4 shows cations and anions of ILs recently used for lithium-ion battery studies. There is often more than one acronym that is used in literature. For a recent review on room-temperature dicyanamide ionic liquids based on quaternary phosphonium cations see Ref. [526]. 

The first catalysts utilized in phase transfer processes were quaternary onium salts. In particular, benzyltriethylammonium chloride was favored by Makosza (7 ) whereas Starks utilized the more thermally stable phosphonium salts (6,8). In either case, the catalytic process worked in the same way the ammonium or phosphonium cation exchanged for the cation associated with the nucleophilic reagent salt. The new reagent, Q+Nu , dissolved in the organic phase and effected substitution. 

Reactions of ruthenium catalyst precursors in carboxylic acid solvents with various salt promoters have also been described (170-172, 197) (Table XV, Expt. 7). For example, in acetic acid solvent containing acetate salts of quaternary phosphonium or cesium cations, ruthenium catalysts are reported to produce methyl acetate and smaller quantities of ethyl acetate and glycol acetates (170-172). Most of these reactions also include halide ions the ruthenium catalyst precursor is almost invariably RuC13 H20. The carboxylic acid is not a necessary component in these salt-promoted reactions as shown above, nonreactive solvents containing salt promoters also allow production of ethylene glycol with similar or better rates and selectivities. The addition of a rhodium cocatalyst to salt-promoted ruthenium catalyst solutions in carboxylic acid solvents has been reported to increase the selectivity to the ethylene glycol product (198). 

Guenin, E., Herve, A.C., Floch, V., Loisel, S., Yaouanc, J.J., Clement, J.C. et al (2000) Cationic phosphonohpids containing quaternary phosphonium and arsonium groups for DNA transfection with good efficiency and low cellular toxicity.Angew. Chem. Int. Edit.,39,629-631. 

Quaternary phosphonium and arsonium groups have been investigated by Clement as alternative positively-charged headgroups in the synthesis of cationic phospho-nolipids [75, 76] (see Fig. 4) and lipophosphoramidates [83, 84],... 

Oniu m Cations. Major families of ILs are composed of quaternary onium cations such as imidazolium, pyridinium, ammonium, phosphonium, sulfonium cations and so on. As described above, the fact that most ILs are composed of organic cations is attributed to weaker electrostatic interaction among component ions. There have been several reports on the effect of cation structure on the Tm of ILs. The relationship between Tm and the basic structure of onium cations is important in developing a protocol to prepare low melting ILs. 

Even if anionic chaotropes are the most popular neoteric IPRs, polarizable cations such as sulfonium and phosphonium reagents showed single selectivity toward polarizable anions their behavior was rationalized on the basis of their chaotropicity. Probe anion retention generally increases in the order of tributylsulfonium < tetrabutylammonium < tetrabutylphosphonium. Interestingly, retention was found to be influenced by the kosmotropic/chaotropic character of both the IPR and the probe anion [93] and this confirms the peculiarities of hydrophobic ion-pairing. Quaternary phosphonium salts provided increased selectivity compared to ammonium in the IPC of heavy metal complexes of unithiol [112]. 

The effect of temperature on ion transfer across the water-nitrobenzene interface was studied for a series of six quaternary ammonium and phosphonium cations and two anions using cyclic voltammetry and equilibrium impedance measurements [115]. Standard entropies (A S ) and enthalpies (A iT ) of ion transfer have been evaluated from the experimentally accessible reversible half-wave potential ( "572 and standard Gibbs energy of transfer (A G ),... 

A problem in interpreting the effect of different counterions on the mechanical properties of ionic polymers is the difficulty in evaluating how cation-anion interactions are changing from counterion to counterion. For example, metal counterions differ in ionicity as well as in size and valence, and they can have a partially covalent character. In contrast, quaternary phosphonium ions have a number of desirable characteristics that make them particularly attractive as model systems for the study of counterion effects. They have an essentially full positive charge on the heteroatom so partially covalent interactions, fractional charge transfer between the counterion and anion, and hydrogen bonding do not come into play. Furthermore, with quaternary ions there is no possibility of the tautomerism that can occur with nonquatemary ammonium or phosphonium counterions. 

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