The potentiometric titration method has been used to study the equilibria of cationic in sytems formed by substituted pyridine N-oxides in the polar, non-aqueous solvents acetone and methanol. For comparison, the systems with trimethylamine N-oxide as a representative of aliphatic amine N-oxides and pyridine representing parent heterocyclic amines were also studied. The cationic heteroconjugation constants, i.e. the equilibrium constants for conjugation reactions between free and protonated N-bases leading to the formation of unsymmetric BHB'+ cations, were determined in experimental systems with and without proton transfer. It was found that there were significant differences in the values of the cationic heteroconjugation constants determined in these two acid-base systems. The proton-transfer reactions limit and even preclude the determination of the cationic heteroconjugation constants. On this basis it was concluded that the heteroconjugation constants should be determined in systems without proton transfer. In such systems, in the amphiprotic solvent methanol, cationic heteroconjugation was ascertained in all substituted pyridine N-oxide systems, the values of heteroconjugation constants being relatively low (logarithms of their values of the order of 2-2.5), and only negligible in systems involving trimethylamine N-oxide. A more pronounced tendency towards cationic heteroconjugation of the [OHO]+ type was observed in the aprotic protophobic acetone, where heteroconjugation constants were determined for all amine N-oxide systems studied including those containing protonated trimethylamine N-oxide as a proton donor. However, the values of the cationic heteroconjugation constants were found to be, in methanol likewise, relatively low (log KBHB'+ of the order of 2-3). On the contrary, a greater extent of cationic heteroconjugation equilibria was observed in methanol than in acetone in the case of systems containing pyridine, i.e. [NHO]+ type bridges formed by amine N-oxides and heterocyclic amines. In methanol the heteroconjugation constants turned out to be determinable for all such systems studied (logarithms of the equilibrium constants being of the same order as for N-oxide systems), whereas in acetone the hetero constants were indeterminable for all systems.
The crystal structure of tetraethylammonium tetrachloridoferrate(III) was determined. The crystals are hexagonal, space group P6 3 mc, a = 8.198 (1) The asymmetric unit of [(C 2 H 5 ) 4 N][FeCl 4 ] consists of half each of a tetraethylammonium cation and a tetrachloridoferrate(III) anion. Tetrachloridoferrate(III) ion adopts almost ideal tetrahedral geometry. Structural characterization of the compound is supplemented by the results of its magnetic susceptibility measurements and electron paramagnetic resonance (EPR) spectra. Magnetic measurements of a powdered [(C 2 H 5 ) 4 N][FeCl 4 ] sample gave a negative Weiss constant of -1.57, which suggests antiferromagnetic coupling. The susceptibility curve of [(C 2 H 5 ) 4 N][FeCl 4 ] against temperature exhibits a maximum, indicating the presence of antiferromagnetic ordering with a Neel temperature of approximately 2.9 K.
Acidity constants have been determined potentiometrically for a variety of conjugate acids of substituted pyridine N-oxides in N,N- dimethylformamide ( dmf ) and dimethyl sulfoxide ( dmso ). The pKa values in these solvents varied in the same direction and correlated with the pKa values of these species in water and in the protophobic aprotic solvent acetonitrile. Further, a linear relationship has been established between the pKa values in the two protophilic aprotic solvents under study. The most basic substituted pyridine N-oxides exhibited a weak tendency towards cationic homoconjugation in dmf , whereas in the more basic dmso the homoconjugation equilibrium was not established for any of the heterocyclic N-oxides. The phenomenon of cationic homoconjugation was also not observed with pyridine as a representative of heterocyclic amines, both in dmf and dmso. This finding complies with the results obtained in other polar aprotic solvents.
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