Ion pair compounds

Complex 1, initial hydride 2a, its dihydrogen-bonded complex 3, contact ion pair 4, solvent-separated ion pair. Compounds 4 plus 5 are transformed to complex 6 after H2 elimination. 

Sakai et al. determined procaine by a colorimetric method that was proposed for the assay of procaine on the basis of solvent extraction [39]. Tetrabromophenolphthalein ethyl ester anion was added to an aqueous solution containing procaine, and the extract took on a red color (absorbance maximum of the extract at 580 nm). The optimal pH range for extraction of the drug from the aqueous solution was found to be 8-9. Procaine was found to form a 1 1 associated ion pair compound with the reagent in 1,2-dichloroethane. 

Dynamic reorganization of ion-pairs has also been investigated by Sekiguchi and coworkers, who reduced 55 with lithium to produce the dilithium ion-paired compound, 56 (equation 99)44. Lithium-6 NMR consisted of two lines of equal intensity at S — 0.38 and —0.66 (referenced to LiCl in methanol). The former resonance was assigned to externally solvated 6Li+ while the latter was more consistent with 6Li+ tightly bound to one face of the conjugated dianion. At low temperature the exocychc and endocyclic 13C resonances of 56 (cf. 57) each consist of a 1 1 doublet, which shows that the contact bound lithium is unsymmetrically sited with respect to the plane of the dianion. 

The methods based on complex formation may be classified into two groups on the basis of the nature of the complex species formed (i) Metal complexes with the analyte anion as ligand (ii) Ion-pair compounds with the analyte anion as counter ion. 

The use of a sensor with a solid-phase reactor in the same cell minimizes the analyte dispersion. The solid bed (resin or a gel ion exchange) contains the immobilized reagent or can be used to retain the analyte, either directly or after forming a complex or an ion-paired compound, which is retained. These... 

As with most complexation and drug solubility situations, pH b a critical variable. Cocaine base b not soluble in water, and if the drug b in thb form rather than a soluble salt, no reaction occurs. Acid b needed to ensure that the cocaine b in the water-soluble ionic form to allow for the formation of a complex. The color b the result of an ion-pair compound formed from the cationic cocaine and the anionic cobalt complex. As with all amine bases, such as ammonia, the base becomes protonated in acidic solution. The pKg of the base determines the ratio of the protonated, ionized form to the neutral form. It is possible to add too much HQ, because cobalt forms a water-soluble pink complex with chloride [CoCy . The pH can also influence the type of complex and ion pair formed. Under acidic conditions, the ion pair favored b [Co(cocaine)2l(SCN)2 (which b pinkbh and soluble in water), while in the neutral-to-basic ranges, the ion pair b assigned the structure [cocaine-H ]2 [Co(SCN)4] (which b a blue solid and soluble in chloroform). The important points of the cobalt thiocyanate reaction with cocaine are summarized in Figures 7.24r-7.26. 

Related dinickel(II) p-N bridging compounds with NHC ligands were reported by Hillhouse et al. in 2009 (Scheme 10.31) [40]. [Ni(I)(p-Cl)(l Pr)]2 underwent one-electron oxidation by MesNj to afford a dinickel(II) bridging imide [ Ni(ll)(l Pr)(Cl) 2(p-NMes)] 66, in which one of the chlorides can be abstracted by NaBAr, to produce the dinickel(ll) ion pair compound 67. The Ni(ll)-Ni(ll) bond lengths in 66 and 67 are 2.5767(15) and 2.2911(8) A, respectively (Entries 5 and 6, Table 10.3). Complex 66 can also undergo one-electron reduction of the [(Ni(ll)Cl 2(p-NMes)] core, to afford a mixed-valent Ni(I)-Ni(II) compound 68, which upon treating with a one-electron oxidant gives 67 as the product. It is noteworthy that 67 and 68 bear similar core structures, but the Ni-Ni bond... 

For reversed-phase HPLC, mixtures of methanol (or acetonitrile) and aqueous buffer solutions are preferred for the chromatography of flavin compounds. Addition of ion pair compounds (such as triethylammonium chloride or hexanesulfo-nates) can modify the retention behavior of ionic compounds (riboflavin phosphates, FAD) significantly (ion pair HPLC). 

Liu G-X, Huang R-Y, Ren X-M (2008) Four novel ion-pair compounds consisting of 7,7,8,8-tetracyanoquinodimethane (TCNQ) radical syntheses, crystal structures and properties. J Mol Struct 891 11-18... 

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