Crown ether-like octanuclear molybdenum(V) clusters for cation binding and gas adsorption

Octanuclear polyoxomolybdenum-based porous materials, Na₈[Mo₈O₈(μ₂-O)₈(μ₂-OH)₈(3-apz)₄]₂·26H₂O (1, 3-Hapz = 3-aminopyrazole), K₈[Mo₈O₈(μ₂-O)₈(μ₂-OH)₈(3-apz)₄]₂·7H₂O (2) and (NH₄)₄[Mo₈O₈(μ₂-O)₈(μ₂-OH)₄(3-apz)₈]·20.5H₂O (3), have been successfully synthesized by a hydrothermal method and fully characterized. X-ray structural analyses show that microporous materials 1-3 contain round pores formed by octanuclear molybdenum-oxygen groups connected sequentially with pore sizes of 4.0, 4.0, and 4.8 Å, respectively. Both 1 and 2 are composed of two {Mo₈} rings, which are connected by strong intramolecular hydrogen bonds between bridging hydroxy groups and oxygen atoms to form dimeric structures. The central pores in 1 and 2 are occupied by Na⁺ and K⁺, respectively, while they are empty in 3. This reflects the structural expansion and contraction effects induced by different cations. Through intermolecular stacking, 1-3 also exhibit channels with sizes of 14.0 × 6.4, 4.6 × 2.6, and 5.4 × 5.4 Å, respectively, which were used for the studies of gas adsorption. The results show that 1-3 can selectively adsorb CO₂ and O₂, including the empty hole in 3, while they show little or no affinity for gases H₂, N₂, and CH₄. Moreover, an additional polyoxomolybdenum-based species (Mo₈O₂₆)·4n(3-H₂apz) (4) has been obtained with protonated 3-aminopyrazole in the absence of a reducing agent, which can serve as an intermediate for the polyoxomolybdenum-based porous products.