Tetranitro- and tetraamino-dibenzo[18]crown-6-ether derivatives: complexes for alkali metal ions, redox potentials, crystal structures, molecular sorption, and proton conducting behaviours

Abstract

Tetranitro-dibenzo[18]crown-6 (1) and tetraamino-dibenzo[18]crown-6 (2) are ion-recognition electron acceptor and electron donor molecules, respectively. The electron-accepting ability of 1 is lower than that of o-dinitorobenzene, whereas the electron-donating ability of 2 exceeds that of tetrathiafulvalene, a typical electron donor. The oxidation potential of 2 is drastically reduced by di-protonation at the two –NH₂ sites under acidic conditions. Thermally annealed 1 exhibits gate-opened gas adsorption–desorption behaviors for CO₂ at 200 K and N₂ at 77 K. The central cavity of 1 can bind alkali metal ions (M) to form M(1) complexes for M = Na⁺, K⁺, Rb⁺, and Cs⁺; these crystallize into monomers (M = Na⁺ and K⁺), dimers (M = Rb⁺), and one-dimensional polymers (M = Cs⁺) in single crystals of [M(1)I]·nCH₃CN. Multiple M⁺⋯O₂N– axial coordination at the M⁺ site generates dimer and polymer networks in solids. Single crystals of [Na(2)(H₂SO₄)₃]·nH₂O and [K(2)(H₂SO₄)₃]·nH₂O were obtained by crystallization of 2 in the presence of Na⁺ and K⁺, respectively, in dilute H₂SO₄. The single crystals formed an ionic channel in the one-dimensional array of ⋯[(Na)_0.25(2)]⋯H₂O⋯[(H₃O)_0.25(2)]⋯H₂O⋯, and the mixed protonated state of HSO₄⁻ and H₂SO₄ formed an O–H⋯O hydrogen-bonding network with the aid of additional H₂O molecule occupation around the ionic channels. In contrast, the two oxygen atoms of HSO₄⁻ were axially coordinated to the K(2) complex, which isolated them from each other, and HSO₄⁻ and H₂O occupied [K(2)(H₂SO₄)₃]. These two hydrated crystals showed a proton conductivity (σ_H⁺) of ∼5 μS cm⁻¹ at 310 K, while in the dehydrated crystals the σ_H⁺ values disappeared; furthermore, the motional freedom of the polar HSO₄⁻ was observed in the Debye-type dielectric relaxations.