Cation complexation selectivity

In what follows, the phenomenology of carrier transport will be briefly reviewed along with the mechanism of the Valinomycin model of carrier transport. The development of the molecular structure of Valinomycin will be considered in some detail, since the key to the dramatic selectivity of Valinomycin is thought to reside in the energetics of the molecular structure. Confidence in an understanding of the molecular structure of the Valinomycin-cation complex becomes tantamount to confidence in the presented basis of ion selectivity. 

The transmembrane potential derived from a concentration gradient is calculable by means of the Nemst equation. If K+ were the only permeable ion then the membrane potential would be given by Eq. 1. With an ion activity (concentration) gradient for K+ of 10 1 from one side to the other of the membrane at 20 °C, the membrane potential that develops on addition of Valinomycin approaches a limiting value of 58 mV87). This is what is calculated from Eq. 1 and indicates that cation over anion selectivity is essentially total. As the conformation of Valinomycin in nonpolar solvents in the absence of cation is similar to that of the cation complex 105), it is quite understandable that anions have no location for interaction. One could with the Valinomycin structure construct a conformation in which a polar core were formed with six peptide N—H moieties directed inward in place of the C—O moieties but... 

Platinum ammine complexes have been a fertile area for studying transinfluence. Table 3.21 lists data for a range of ammines showing how /(195Pt-15N) depends upon the trans-atom [153]. (A further selection of data can be found in R.V. Parish, NMR, NQR, EPR and Mossbauer Spectroscopy in Inorganic Chemistry, Ellis-Horwood, Chichester, 1991, pp. 76, 87.) Possibly the most detailed study (of complexes of tribenzylphosphine) examined over a hundred neutral and cationic complexes [154] (Table 3.22). 

Eisch s work promoted investigation into the preparation of cationic metallocene complexes of Group 4 metals. Several preparative routes to cationic group 4 metallocene complexes are illustrated in Scheme II. Catalytic activities of some selected cationic metallocene complexes for the polymerization of a-olefins are summarized in Tables 5 and 6. The catalyst systems based on these cationic complexes are just as active as M AO-activated metallocene catalysts for the polymerization of a-olefins. 

The kinetics of base hydrolysis of several complexes of the type [Co(NH3)3L3] have been examined in order to see whether the mechanism for these uncharged complexes is the same as that operating for base hydrolysis of the standard cationic complexes (75). A comparison of kinetic parameters - a small selection is given in Table II (76,77) - suggests that all cobalt(III)-nitro-amine complexes, charged and uncharged, undergo base hydrolysis by the SnICB (Dch) mechanism. 

A cationic complex, formed in situ from 5 and [Rh(COD)2]OTf, was also active in biphasic hydrogenation [14]. No preference for the fluorous phase was found for ligands containing only one perfluoroalkyl tail, but neutral and cationic complexes, containing mono- and bidentate 4a or 5, respectively, were selectively dissolved in the fluorous phase. No leaching and recycling studies were performed. 

Sr2+ so well that it was selectively extracted from a bulk sample of a barium salt (Helgeson et al., 1973a). Binding constants for metal-cation complexes of 1,3-xylyl-crown ethers [66]—[69] carrying an additional carboxylate binding... 

Exchange of complex cations. Complexation of transition metal cations with uncharged ligands such as with amines and with amino acids results in a selectivity enhancement compared to the selectivity of the aqueous metal cation (27, 65-72). Fig. 3 shows an example for the Cu(ethylenediamine) adsorption in montmorillonites of different charge density. Standard thermodynamic data for other cases are given in table IV. In all cases the free ligand concentration in equilibrium solution was... 

For unsaturated lactones containing an endocyclic double bond also the two previously described mechanisms are presumably involved and the regio-selectivity of the cyclocarbonylation is governed by the presence of bulky substituents on the substrate. Inoue and his group have observed that the catalyst precursor needs to be the cationic complex [Pd(PhCN)2(dppb)]+ and not a neutral Pd(0) or Pd(II) complex [ 148,149]. It is suggested that the mechanism involves a cationic palladium-hydride that coordinates to the triple bond then a hydride transfer occurs through a czs-addition. Alper et al. have shown that addition of dihydrogen to the palladium(O) precursor Pd2(dba)3/dppb affords an active system, in our opinion a palladium-hydride species, that coordinates the alkyne [150]. 

The hydride reacts immediately with ethene to give the expected ethyl complex selectively and quantitatively, which again is ideal for the catalytic activity. The hydride is very unstable when CO is bubbled into MeOH solution, even at low temperature [115] at room temperature it reacts immediately with ethene giving a cationic ethyl complex. In the presence of both CO and ethene, like under catalytic conditions, decomposition does not occur because the hydride reacts much faster with ethene than with CO. Once the ethyl intermediate is formed, fast insertion of CO occurs with formation of an acyl intermediate, which in turn reacts with MeOH yielding MP with quantitative regeneration of the starting hydride to continue the catalytic cycle [114,115]. The formation of the ethyl and of the acyl intermediates involves facile equi-... 

S. A. Jonker, F. Ariese, and J. W. Verhoeven, Cation complexation with functionalized 9-arylacrid-inium ions Possible applications in the development of cation-selective optical probes, Reel Trav. Chim. Pays-Bas 108, 109-115 (1989). 

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