Crown ether-like discrete clusters for sodium binding and gas adsorption

Abstract

Hexanuclear polyoxomolybdenum-based discrete supermolecules Na_x[Mo^V₆O₆(μ₂-O)₉(Htrz)_6−x(trz)_x]·nH₂O (x = 0, n = 15, 1; x = 1, n = 12, 2; x = 2, n = 10, 3; x = 2, n = 49, 4; Htrz = 1H-1,2,3-triazole) have been prepared and fully characterized with different amounts of sodium cations inside and outside the intrinsic holes. Structural analyses demonstrate that they all exist a triangular channel constructed by six molybdenum–oxygen groups with inner diameters of 2.86 (1), 2.48 (2), and 3.04 (3/4) Å, respectively. Zero, one, or two univalent enthetic guest Na⁺ have been hosted around the structural centers, which reflect the expansion and contraction effects at microscopic level. Water-soluble species can serve as crown ether-like metallacycles before and after the sodium binding. Diverse nanoscale pores are further formed through intermolecular accumulations with hydrogen bonding. Gas adsorption studies indicate that 2–4 can selectively adsorb CO₂ and O₂ but have little or even no affinities toward H₂, N₂, and CH₄. Theoretical calculations corroborate the roles of Na⁺ and auxiliary ligand with different states in bond distances, molecular orbitals, electrostatic potentials, and lattice energies in these discrete clusters. The binding orders of sodium cations in 2–4 are similar with the classical crown ethers, where 2 is the strongest one with 2.226(4)_av Å for sodium cation bonded to six O atoms.