Ionic Exchange in p-Sulfonatocalix[4]arene-Mediated Formation of Metal–Ligand Complexes

Abstract

The effect of alkali and transition metal cations in the formation of host-guest complexes with the water-soluble p-sulfonatocalix[4]arene (SC4) was studied using 2-chloropyridine and Na(+) and Cu(2+) as model guest and model cations, respectively. The results obtained from isothermal titration calorimetry and NMR experiments provide evidence for the formation of 1:1:1 ternary complexes for both cations with Cu(2+) showing positive cooperativity and Na(+) negative cooperativity. The formation of ternary complexes comprising transition metal cations has been scarcely explored but present high potential for devising catalytic systems/models or for enhancing the stability and selectivity of SC4 complexes. Because transition metal cations are usually present in solution together with other SC4 countercations (e.g., Na(+)), a general binding model that considers the dynamic formation of all possible complexes (including ionic exchange between ternary complexes) is presented. This model allows the optimization of the conditions required to selectively form target complexes.

Sulfonatocalixarene Counterion Exchange Binding Model in Action: Metal‐Ion Catalysis Through Host‐Guest Complexation

“…On the other hand, both SC6 and SC8 form host-guest complexes of moderate affinity with 1 (10 2 M À 1 range), which is expected based on the lower affinity of these receptors for neutral aromatic species. [28,40,41] As already discussed above, the k 1 values seems to correlate with the pK a of the ionizable phenols group of the SCn and thus the rate acceleration affect is higher for SC8.…”

“…However, it must be stressed that for the concentrations of SCn used here, SC6 and SC8 exist in solution as 1 : 1 and 1 : 2 sodium complexes and that different stoichiometric complexes containing 1, SCn and Na + are likely to occur. [28,[36][37][38][39] Therefore k must be considered as an apparent rate constant that results from the sum of the rate constants of the different complexes averaged by their relative mole fraction.…”

“…Noteworthy, the association constant for the binding of 1 towards SC4 : Ni 2 + (K T2 = 610 M À 1 ) is almost two times higher than K 1 while the opposite effect is observed for the ternary complex with sodium cations (K T2 = 150 M À 1 ) in agreement with positive and negative cooperativity phenomena already documented for the SC4 binding with other pyridine and azoalkanes guests in the presence of transitions and alkaline metal cations, respectively. [25,28] The cooperative formation of the ternary 1: Ni 2 + : SC4 complex can be proposed to be main responsible for the observed rate acceleration in the hydrolysis of 1. Although the k 2 cannot be obtained independently, the observation of a linear dependence in the k obs vs [Ni 2 + ] plot implies that K 1:Ni � 1 and therefore k 2 � 0.019 s À 1 .…”

“…[11] This approach can be addressed through the functionalization of synthetic macrocyclic receptors with polydentate (often nitrogen-based) ligands for metal ion-binding [11] or by using macrocyclic receptors such as p-sulfonatocalix[n] arenes (SCn) or cucurbiturils which comprise binding pockets decorated with negatively charged or electronegative carbonyl groups, respectively, that in favorable conditions may accommodate the organic substrate and the metal cation in close proximity through the cooperative formation of heteroternary metal cation:substrate:receptor complexes. [25][26][27][28][29] In addition to their ability to form heteroternary complexes comprising a metal cation and an organic guests, SCn comprise phenols groups that offer the possibility of general base or nucleophilic participation in the catalyzed reaction. To test this hypothesis and evaluate for the first time the formation of heteroternary SCn complexes for catalysis, we selected the Ni 2 + catalyzed hydrolysis of 2,4-dinitrophenyl picolinate (1) as model reaction (Scheme 1).…”

Novel mixed 1D–2D lanthanide coordination polymers based on p-sulfonatocalix[4]arene and 4,4′-bipyridine-N,N′-dioxide where p-sulfonatocalix[4]arene acts as a guest

“…Supramolecular chemistry of calixarenes offers a convenient approach to producing functional materials by directional and reversible non-covalent interactions, 1,2 with special attention to cation⋯π, C-H, and π⋯π interactions 3,4 including hydrogen bonds, 5,6 host-guest recognition, 1 donor-acceptor interactions, 7,8 and metal-ligand interactions. 9 Owing to the π-electron-rich hydrophobic cavity and the robust hydrophilic nature of the upper and lower rims, [10][11][12] calixarenes have drawn much attention in the burgeoning area of supramolecular chemistry 13 and coordination chemistry 14,15 due to its host enwrapping ability of small molecules and as a ligands, developing coordination complexes or polymers with various metal ions.…”