Supramolecular architectures from ammonium-crown ether inclusion complexes in polyoxometalate association: Synthesis, structure, and spectroscopy

Abstract

The tetrahedral (T_d) molecular cation ammonium ion (NH₄⁺) gets incorporated with the smaller-cavity crown ethers through three N⁺-H···O hydrogen bonding interactions, while the fourth N-H bond is projected outward from the crown ether cavity. A hydrogen bonding acceptor, such as a polyoxometalate anion, if placed in suitable position in the crystal lattice, can interact with the ammonium-crown ether supramolecular complex involving the fourth N-H bond donation. This article describes supramolecular association of ammonium-crown ether host-guest complexes with polyoxometalate anions in five crystalline solids, formulated as [NH₄(B18C6)][Bu₄N][Mo₆O₁₉] (1), [NH₄(B18C6)]₄[SiW₁₂O₄₀]·2CH₃CN (2), [NH₄(DB18C6)]₂[Mo₆O₁₉]·4H₂O (3), [NH₄(DC18C6)]₂[Mo₆O₁₉] (4), and [NH₄(DB30C10)]₂[Mo₆O₁₉]·CH₃COOH (5), where B18C6 = benzo-18-crown-6, DB18C6 = dibenzo-18-crown-6, DC18C6 = dicyclohexyl-18-crown-6, and DB30C10 = dibenzo-30-crown-10. Single crystal X-ray structural investigations on these solids confer P? space symmetry (No. 2) for the compounds 1-2; C2/c space symmetry (No. 15) for the compound 3; P2₁/n space symmetry (No. 14) for the compound 4, and C2/m (No. 12) space symmetry for the compound 5. Careful examination of the supramolecular interactions between the different molecular fragments show direct contact between the ammonium cation and the polyoxometalate anion (N⁺-H···O) in the crystal structure of the compounds 1, 2, and 4. Lattice water molecules have taken the role of hydrogen bonding acceptor (N⁺-H···O) in the crystal structure of compound 3, while the larger and flexible crown ether DB30C10 has wrapped around the ammonium cation in the case of compound 5. Thus, there is no direct supramolecular interaction between the ammonium cation and the polyoxometalate anion (N⁺-H···O) in the crystal structures of compounds 3 and 5, where the ammonium-crown ether cations are associated with the polyoxometalate anions only via C-H···O hydrogen bonding interactions. Detailed spectroscopic (IR, ¹H NMR, and UV-visible) analyses have been included in comprehending the supramolecular association between the inclusion complexes and polyoxometalate anions. The X-ray powder diffraction analyses have been performed to scrutinize the phase purity of the solids.