Tuning the Structure of Lariat Crown Ethers for Ion-Selective Electrodes: Significant Shifts in Sodium/Potassium Selectivity

Abstract

Lariat crown ethers (LCEs) offer the ability to bind selectively alkali metal ions. The presence of a coordinating functionality in the sidearm chain provides a means to further tune the selectivity of LCEs. A series of four LCEs based on a 15‐crown‐5 structural motif were synthesized with variations in the coordinating functionality of the sidearm and the substitution at the carbon pivot atom. Poly(vinyl chloride) liquid‐membrane ion‐selective electrodes were prepared with each of the four ionophores, and their response patterns to various alkali metals were determined. The results of this experimentation revealed that the coordinating ability of the sidearm is more important than the substitution at the geminal group at the pivot atom. It was also demonstrated that the change of a single atom in the sidearm structure, to induce coordinating ability to the sidearm, alters the selectivity between sodium and potassium by roughly five orders of magnitude.

Competing Non-covalent Interactions in Alkali Metal Ion−Acetonitrile−Water Clusters

“…This immediately suggests that the ion-water interaction is progressively weakened going from the [0,4] to the [3,1] species. With Na + , the ν asym transition shifts from 3725 cm -1 in the [0,4] cluster ion to 3742 cm -1 in the [3,1] cluster ion. The change in the ν sym transition is not as dramatic, but is still detectable: the frequency shifts from 3645 to 3654 cm -1 going from [0,4] to [3,1].…”

“…With Na + , the ν asym transition shifts from 3725 cm -1 in the [0,4] cluster ion to 3742 cm -1 in the [3,1] cluster ion. The change in the ν sym transition is not as dramatic, but is still detectable: the frequency shifts from 3645 to 3654 cm -1 going from [0,4] to [3,1]. These trends are identical with those of K + .…”

Ionophore-based ion-selective potentiometric and optical sensors

“…For instance, while most macrocycles containing hard oxygen centers demonstrate selectivity toward alkaline and alkali earth cations, crowns containing softer sulfur atoms tend to interact with softer species like transition-metal ions. Macrocycle selectivity can also be tuned by adding side arm functionalities, or "lariats", to the molecular frame, which can lend stabilizing interactions with various cations or alter binding through steric blockage of the crown cavity [4,107,108,109]. Of equal importance to acid-base chemistry, however, is the overall size or diameter of the binding area.…”

Re-evaluation of the mechanism of cytotoxicity of dialkylated lariat ether compounds

“…25 Lariat ethers have also been reported to bind alkali cations and behave as ionophores in bulk liquid membranes and ion-selective electrodes. [26][27][28][29] The dialkylated diaza(18-crown-6) ethers, a subset of the lariat class, have been reported to have toxic activity towards prokaryotic and eukaryotic cells. 20 Evidence from toxicity and depolarization assays initially suggested that these compounds behave as ion carriers.…”

“…Binding to ionophores requires at least a partial substitution of water molecules in the hydration sphere by ions to achieve an ionophore-like transport mechanism. Thus, some degree of ion selectivity is expected, [26][27][28][29] as observed in the case of valinomycin, a natural carrier-type ionophore that is extremely selective for K + over Na + 39,40 and does not transport N-methyl-D-glucamine (NMDG + ). When depolarization assays were performed in the presence of different cations, valinomycin promoted DiSC 3 (5) release only when K + was added to the external solution, but not when Na + or NMDG + was added ( Fig.…”