New Lower Rim Calix(4)arene Derivatives with Mixed Pendent Arms and Their Complexation Properties for Alkali-Metal Cations. Structural, Electrochemical, and Thermodynamic Characterization

Abstract

The synthesis and characterization ( 1 H NMR) of two new lower rim calix(4)arene derivatives containing mixed pendent arms, namely, 5,11,17,23-tetrakis(1,1-dimethylethyl)-25,27-bis[2-(methylthio)ethoxy]-26,28bis[(pyrid-2-ylmethyl)oxy]calix(4)arene (1) and 5,11,17,23-tetrakis(1,1-dimethylethyl)-25,27-bis[2-(methylthio)ethoxy]-26,28-bis[(pyrid-3-ylmethyl)oxy]calix(4)arene (2), are reported. Transfer thermodynamic parameters of these ligands to various solvents show that these macrocycles undergo selective solvation in nonaqueous media. As far as the alkali-metal cations are concerned, 1 H NMR measurements reflect that 1 in CD 3 CN discriminates against the largest cations (K + , Rb + , and Cs + ) but interacts with Li + and Na + . These interactions appears to be more pronounced with the pendent arms containing the pyridyl ring (through the ethereal oxygens and nitrogen donor atoms) than with those containing the (methylthio)ethoxy arms (only ethereal oxygens are involved). No interaction takes place between its isomer 2 and alkali-metal cations in CD 3 CN. The complexation of the lithium and the sodium complexes was established through conductometric titrations. In all cases investigated 1:1 (ligand: metal cation) complexes are formed in acetonitrile and in benzonitrile. On the basis of the semiquantitative information provided by conductance measurements regarding the strength of complexation of 1 and the sodium cation, the sodium monoacetonitrile complex of 1 was isolated and its structure determined by X-ray diffraction methods. Thus, the cation was found in the hydrophilic cavity while the hydrophobic cavity hosts a molecule of acetonitrile. The crystallographic results confirm the key role played in the chelating process by the ethereal oxygens and the pyridyl nitrogens at the 2-position in the terminal pyridyl groups, as found in solution. The thermodynamics of complexation of 1 and alkali-metal cations (Li + and Na + ) in two dipolar aprotic solvents is interpreted in terms of the solution thermodynamics of the reactants and the product. It is the availability of these data that allows interpreting the different enthalpic and entropic contributions resulting from cation, solvent, and ligand effects. Final conclusions are given.

Solvation Effect on Complexation of Alkali Metal Cations by a Calix[4]arene Ketone Derivative

“…The alkali metal cation binding of several low-rim derivatives was studied in benzonitrile. 24,29,33 The stability constants obtained in this solvent were quite similar to those determined in acetonitrile. Calorimetric investigations indicated this to be a consequence of interesting enthalpy−entropy compensation.…”

“…Calix[4]­arene-based ligands prefer smaller cations (Li + , Na + ), whereas larger macrocycles are more suitable for Rb + and Cs + complexation. , In general, stability constants of the complexes with tertiary calixarene amides are higher than those with the corresponding secondary amides and simple ketone and ester derivatives. ,− Complexation is strongly influenced by solvation of reactants and complexes. ,− Much lower affinities of calixarene ketones, amides, and esters toward alkali metal cations in methanol compared to acetonitrile have been attributed to intermolecular hydrogen bonding with the solvent molecules. − However, the solubilities of calixarenes in methanol and acetonitrile were comparable in some cases (standard Gibbs energies of macrocycle transfer, Δ t G ° , among these solvents amounted to only a few kJ mol –1 ) ,, As it turned out, differences in standard thermodynamic parameters of cation complexation were predominantly a consequence of the favorable complex transfer from methanol to acetonitrile. , This was ascribed, at least partly, to concomitant inclusion of the acetonitrile molecule into the hydrophobic cavity of the complex, − as the affinity of macrocycles toward methanol was found to be considerably lower. ,,, Investigations of calixarene complexation properties in other solvents are less common. The alkali metal cation binding of several low-rim derivatives was studied in benzonitrile. ,, The stability constants obtained in this solvent were quite similar to those determined in acetonitrile. Calorimetric investigations indicated this to be a consequence of interesting enthalpy–entropy compensation. ,, According to Danil de Namor et al, who studied complexation of Na + with ketone calix[4]­arene derivative in N , N -dimethylformamide, the macrocycle affinity toward the cation in this solvent was more than 5 orders of magnitude lower than in both acetonitrile and benzonitrile.…”

“…The reason why the receptor binds Na + in DMF is because of the most favorable ligand–cation interactions realized in the case of this cation. This can be explained by its almost perfect fit into the binding sites of lower-rim calix[4]­arene derivatives. ,,,, …”

Coordination chemistry of calix[4]arene derivatives with lower rim functionalisation and their applications

“…De Namor et al have synthesised two calix [4]arene derivatives containing mixed pendent arms [68]. They carried out the transfer thermodynamic experiments of these ligands to various alkali metal ions out and showed that the calix [4]arenes undergo selective solvation in nonaqueous solvents.…”

Lower-Rim Substituted Calixarenes and Their Applications

“…Methylthioethoxy and pyridyl 2 methyl oxy groups were introduced at the lower rim of calix[4]arene to give rise to bis derivatives of both types 1 and 2 [45] (Figure 14). …”