Fluorides, stability constants

Munze (19) has used a Born-type equation to calculate stability constants of In(III) and An(III) complexes of carboxylates as well as other ligands which agreed well with experimental values. His approach was modified by allowing the dielectric constant to be a parameter (the "effective" dielectric constant, De) in an analysis of fluoride complexation by M(II), M(III) and M(IV) cations (20). A value of De = 57 was found satisfactory to calculate trivalent metal fluoride stability constants which agreed with experimental values for Ln(III), An(III) and group IIIB cations (except Al(III). Subsequently, the equation was used... 

Previous assessments of lanthanide fluoride stability constants appropriate at 25 C and zero ionic strength indicate that logp/J is on the order of 4 or less for all lanthanides. The works of Walker and Choppin (1967) and Bilal and co-workers (Bilal et al. 1979, Bilal and Becker 1979, Bilal and Koss 1980, Bilal 1980, Bilal and Becker 1980, Becker... 

Complexes with the Fluoride Ion. For the compilation of stability constants (Table IV) of complexes with F , we have used, when needed, thermodynamic parameters (K, AH) pertaining to the dissociation of hydrofluoric acid as given by Smith and Martell (77) or extrapolated from their selection. 

Open chain polyamine ligands have been widely studied. Often the coordination of zinc is compared with other first row transition metals and factors, such as behavior across a pH range, studied. The protonation patterns and stability constants are of particular interest. Octahedral zinc tris(ethylenediamine) structures have been characterized by X-ray diffraction with a number of different counter anions.94 The X-ray structure of zinc tris(ethylenediamine) with fluoride counter ions reveals extensive hydrogen bonding.95... 

Fluoride ion selective spectrometry was used to determine the stability constants for zinc fluoride complexes in water at 25 °C, giving values j3i[ZnF(aq)]+ = 3.5 0.1 and /l2[ZnF2(-aq)] = 3.8 0.5.643 These results demonstrate that the complexation of fluoride is very weak and in aqueous chemistry no species beyond ZnF+ is of much importance. Organotitanium fluorides have been used as matrices for trapping molecular ZnF2 and MeZnF. 4... 

Manifestation of specificity in maximal rate constants rather than in stability constants is illustrated particularly well by the cyclohexaamylose-accelerated release of fluoride ion from Sarin. Although the inclusion complex of (rate acceleration is much larger for the (—)-enantiomer. Specificity is equally dramatic in... 

The stepwise formation of fluoro complexes in aqueous solution has been extensively studied in recent years by standard techniques (192). The introduction of the LaF3 electrode has enabled fluoride activities to be measured directly, even with the "insoluble fluorides such as CaF2 or PbF2 (34, 81). Values obtained depend on the salts added to maintain constant ionic strength, and care must be taken in making comparisons since not all results are extrapolated to zero strength (15). Key references to work in this field can be gleaned from standard compilations of stability constants. 

There are a limited number of fluorescent sensors for anion recognition. An outstanding example is the diprotonated form of hexadecyltetramethylsapphyrin (A-7) that contains a pentaaza macrocydic core (Figure 10.31) the selectivity for fluoride ion was indeed found to be very high in methanol (stability constant of the complex 105) with respect to chloride and bromide (stability constants < 102). Such selectivity can be explained by the fact that F (ionic radius 1.19 A) can be accommodated within the sapphyrin cavity to form a 1 1 complex with the anion in the plane of the sapphyrin, whereas Cl and Br are too big (ionic radii 1.67 and 1.82 A, respectively) and form out-of-plane ion-paired complexes. A two-fold enhancement of the fluorescent intensity is observed upon addition of fluoride. Such enhancement can be explained by the fact that the presence of F reduces the quenching due to coupling of the inner protons with the solvent. 

Fluoride ISEs have found important applications in the study of equilibria in which fluoride ions participate, especially in determination of the stability constants of fluoride coordination compounds [18, 29, 40,41, 53a, 71, 101,... 

Ammonium plutonium(iv) fluorides were obtainedby reaction of Pu with NH F in HF. The stability constants for the formation of BkCF and EsCP are very similar to those reported for AmCP", and it was concluded that the monochloro-complexes of tervalent actinides up to Es are weak and of the outer-sphere type. " ... 

A great number of anionic complexes containing four, five or six halide ions have been characterized. However, in aqueous solutions containing V111 ions and F-, the potentiometric technique using a fluoride selective electrode has shown no indication of any complex other than VF2+ (stability constant log / = 5.00 0.03 at 25 °C in 1.0 M NaC104 solution).295... 

Initial ll NMR titration data performed in DMS0-c/ /5%H20 revealed that whilst all other anions (chloride, bromide, hydrogen sulphate and benzoate) could be fitted to a 1 1 binding model, association constants could not be reliably calculated for fluoride and dihydrogen phosphate due to the sharp curve obtained. Consequently for these anions, titrations were repeated in a more competitive solvent mix of DMS0/25%H20, providing stability constants with 19 of 114 and 234 M 1 and with 20 11 and 20 M"1 respectively. 

In order to overcome this oxidation problem the synthesis of 21 and 22 was achieved in which two methyl groups occupy the positions bridging the two pyrrolic groups (Figure 20).16 Although compounds 21 and 22 were found to be stable, less stable complexes with anions were formed than with the first generation dipyrrolylmethane systems. However, despite this reduction in affinity, the stability constants for these receptors could only be calculated in DMSO-c/6/5% H20 for fluoride (124 and 89 M 1), dihydrogen phosphate (1092 and 81 M"1), benzoate (1092 and 81 M"1) calculated for receptors 21 and 22 respectively. 

Recognition of fluoride in aqueous media is particularly difficult due to the strongly hydrated nature of the anion. Shinkai and co-workers have demonstrated that ferrocene-boronic acid 27 acts as a selective redox sensor for fluoride which operates in H20 [23]. The favourable interaction between boron and fluoride (a hard acid and hard base, respectively) generates a stability constant of 700 M"1 for the fluoride-ferrocenium complex. Stability constants for both the bromide and chloride complexes are <2 M"1. 

Sessler and co-workers have recently prepared complexes 48 and 49 which are capable of selectively sensing fluoride, giving stability constants for fluoride of 12,000 and 54,000 M"1, respectively, by UV-vis spectrophotometry in DMSO... 

Other groups have subsequently reported anion receptors that work on the same principle. For instance, an Eu(III) complex of the bis-bipyridinephen-ylphosphine oxide ligand 86 made by Ziessel and co-workers is able to sense anions by luminescence enhancement in acetonitrile, with stability constants which follow the trend fluoride>acetate>chloride>nitrate [61]. Tsukube and co-workers have investigated the properties of the Eu(III) and Tb(III) complexes of the chiral ligand 87 [62]. Anion binding was assessed by profiling luminescence enhancement in acetonitrile, and it was found that the different metal centres provided different selectivities. The emission at 548 nm of the Tb(III) complex was increased by 5.5 times in the presence of 3 equivalents of chloride compared to 2.2 for nitrate and 1.1 for acetate. Conversely the emission at 618 nm of the Eu(III) complex was increased 8.3 times by 3 equivalents of nitrate, 2.5 times for chloride and 1.0 times for acetate. Stability constants were not reported. 

In equation (3), Kes is the thermodynamic stability constant, as defined elsewhere [31], Inspection of Table 3 shows that all the calix[4]pyrrole receptors (except 8-aajSjS, 22 and 24) in non-aqueous solvents at a given temperature are more selective for the fluoride anion relative to other anions (X- = Cl-, Br , I-, HSOJ and H2POJ). 

Some of the factors contributing to the stability of these complexes are now highlighted. Thus, the lower stability constants of 13 for fluoride and chloride relative to 12 [26,40] in dichloromethane may be attributed to the electron-donating ability of the eight C-rim methoxy groups which reduces the acidity of the pyrrole NH protons and hence its anion binding ability. The possibility of ion-pair formation in this solvent (low permittivity) cannot be excluded. 

Attempts to correlate stability constant data for 1 and 2 in acetonitrile with the chemical shift changes, A<5, of the pyrrole proton led to straight lines (Fig. 5). These findings clearly demonstrate the selective behaviour of these receptors for the fluoride anion. 

Calix[4]pyrroles are versatile ligands to the extent that the composition of the anion receptor complex is solvent dependent. This chapter has been concerned with the affinity of calix[4]pyrrole for the fluoride anion. It was therefore considered of relevant to focus attention on the steps required for the derivation of reliable thermodynamic data. It is indeed the ratio between stability constants which defines quantitatively the selectivity factor. Thus, representative examples are given to demonstrate selectivity in terms of anion, receptor and solvent. The key role played by solvation in the complexation of these receptors with the fluoride anion is unambiguously demonstrated in the variations observed in the stability constants, enthalpies and entropies of complexation of these systems in the various solvents (Table 2). One convenient measure to assess solvation is through the thermodynamics of transfer of product and reactants from one... 

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