DMSO, stability constant determinations

We recently incorporated the ruthenium(II) bipyridyl moiety into acyclic, macrocyclic, and lower rim caUx[4Jarene structural frameworks to produce a new class of anion receptor capable of optical and electrochemical sensing (226, 253. 254). Stability constant determinations in DMSO using H NMR titration techniques demonstrated that these acyclic receptors (131 and 132) form strong complexes with chloride and dihydrogen phosphate anions (stronger than with analogous monopositive cobaltocenium based receptors). The ruthenium ion is dipositive and hence the electrostatic interactions are particularly favorable. The 4,4 -substituted ruthenium bipyridyls were observed to bind anions more... 

Values of the stability constant of silver(I) monensin in a range of non-aqueous solvents have recently been determined (log.K, 25 °C) 494 MeOH, 8.1 0.1 propylene carbonate, 15.0 0.1 DMF, 9.94 0.05 MeCN, 8.6 0.1 DMSO, 5.37 0.05. The value of K increased by 10 orders of magnitude on going from DMSO to propylene carbonate. For the aprotic solvents, K was observed to increase in the same order in which the solvation of the free silver ion decreased. The formation rates were practically diffusion controlled ( 1010M 1 s"1) in methanol, acetonitrile and DMF. 

The ability of this peptide to bind cations could be demonstrated by the upheld shift observed for the cation signals in the NMR spectrum on addition of a salt of, for example, the n-butyltrimethylammonium ion (BTMA+) to a solution of 2 in 0.2% d6-DMSO/CDCl3 [12]. This shift is a good indication of the interactions between the cation and 2, and it enabled us to determine a stability constant of 300 m 1 for the complex formed. The maximum chemical shift, Admax, observed for, for example, the N-methyl signal of the cation amounts to only —0.05 ppm, however, and is thus significantly smaller than the shift usually associated with com-... 

Indium Halides.—Compositions of halogeno- and thiocyanato-complexes of In in DMSO have been determined potentiometrically at 25 °C, and stability constants have been calculated.566... 

The mixed solvent dimethylsulphoxide-ethanol (20/80% v/v) provides better solubility of salts than pure DMSO. Metal ions select ligands according to the series DMSO >H20 >EtOH. The solvated ion Cu2+ in the mixed solvent reacts with TMPyP( + 4) about five times more slowly toward the low ionic strength limit than does hydrated Cu2+ in aqueous solution (6). Whereas thiocyanate is oxidized by Cu2+ in water, Cu(NCS)+ is a well-defined complex in the mixed solvent. The stability constant has been determined to iC = 8 ( 0.2) M i (0.02 < 7c <0.05). Catalysis by NCS is quite remarkable, as shown in Fig. 23. 

Proton NMR titrations in DMSO-de at 298 K were conducted in order to determine the stability constants of receptors 12a, 12b and 12c with a range of anions, added as their tetrabutylammonium salts. The strongest association constants were obtained for the 20-membered macrocycle 12b with the most significant increase in affinity between the macrocycles being observed in the binding of chloride. Enlargement from the 18-membered macrocycle... 

Table 2 Stability constants (ATa, M ) for anion binding by fluorinated calixpyrroles 2-4 and /neso-octamethylcalix[4]pyrrole 1 in CH3CN or DMSO as determined by UC analysis at 30 °C using the corresponding tetrabutylammonium salts as the anion source."...

The tweezer-type receptor 43 that was developed by Kilbum and coworkers contains a disnbstitnted guanidinium group in the middle of the chain which was expected to bind to the terminal carboxylate gronp of peptidic guests (Scheme 22). The two peptide arms that are arranged in a parallel fashion serve to induce substrate selectivity. To test this idea, 43 was incubated in aqueous sodium borate buffer (pH 9.2, 16.7% DMSO) with a 1000-member library of tripeptides attached to a TentaGel resin via the amino terminus. Mainly hydrophobic amino acid residues were incorporated into these tripeptides to ensure that receptor substrate interactions are largely due to hydrophobic interactions. Receptor 43 was found to bind to about 3% of the library members and showed 95% selectivity for Val at the carboxylate terminus of the tripeptides and 40% selectivity for Glu(OfBu) at the amino terminus. A stability constant of 4 x lO M- was determined for the complex between 43 and Z-Glu(OrBu)-Ser(OfBu)-Val-0 in 16.7% DMSO/water (1 mM sodium borate buffer, pH 9.2) by means of isothermal titration microcalorimetry. 

The Ag+ ion is labile. Even with cryptands, which react sluggishly with most labile metal ions, Ag reacts with a rate constant around 10 M s (in dmso). The higher stability of Ag(I) complexes compared with those of the main groups I and II resides in much reduced dissociation rate constants. Dissociation tends to control the stability of most metal cryp-tand complexes. Silver(I) is a useful electron mediator for redox reactions since Ag(I) and Ag(II) are relatively rapid reducers and oxidizers, respectively. Silver(I) thus promotes oxidation by sluggish, if strong, oxidants and catalyses a number of oxidations by S20 in which the rate-determining step is... 

A common technique for measuring the values has been to employ species that produce anions with useful ultraviolet (UV) or visible (vis) absorbances and then determine the concentrations of these species spectropho tome trie ally. Alternatively, NMR measurements could be employed, but generally they require higher concentrations than the spectrophotometric methods. A hidden assumption in Eq. 5 is that the carbanion is fully dissociated in solution to give a free anion. Of course, most simple salts do fully dissociate in aqueous solution, but this is not necessarily true in the less polar solvents that are typical employed with carbanion salts. For example, dissociation is commonly observed for potassium salts of carbanions in DMSO because the solvent has an exceptionally large dielectric constant (s = 46.7) and solvates cations very well, whereas dissociation occurs to a small extent in common solvents such as DME and THE (dielectric constants of 7.2 and 7.6, respectively). In these situations, the counterion, M+, plays a role in the measurements because it is the relative stability of the ion pairs that determines the position of the equilibrium constant (Eq. 6). 

Acidity constants for ionization of weak carbon acids in water caimot be determined by direct measurement when the strongly basic carbanion is too unstable to exist in detectable concentrations in this acidic solvent. Substituting dimethyl-sulfoxide (DMSO) for water causes a large decrease in the solvent acidity because, in contrast with water, the aprotic cosolvent DMSO does not provide hydrogenbonding stabilization of hydroxide ion, the conjugate base of water. This allows the determination of the pfC s of a wide range of weak carbon acids in mixed DMSO/water solvents by direct measurement of the relative concentrations of the carbon acid and the carbanion at chemical equilibrium [3, 4]. The pfC s determined for weak carbon acids in this mixed solvent can be used to estimate pfC s in water. 

Diphenylmethane is the conjugate acid of the diphenylmethyl carbanion, and the equilibrium acidity constants (Ka) have been measured both directly and indirectly in the gas phase and in solution [3]. The most extensive investigations of the effect of structure on acidity for carbon acids have been carried out in DMSO using a carbon indicator method to determine relative acidities and this scale was anchored with potentiometric measurements to provide an absolute scale of acidities [3, 43]. A summary of relevant pKg values for various carbon acids is shown in Table 2. The data in Table 2 are especially relevant for considering the reactivity of 1,1-diphenylmethyl carbanionic species as initiators in anionic polymerization. In general, an appropriate initiator for a given monomer is an anionic species that has a reactivity (stability) similar... 

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