Ring-Shaped Polyoxometalates Possessing Multiple 3d Metal Cation Sites: [{M2(OH2)2}2{M(OH2)2}4P8W48O176(OCH3)8]16– (M = Mn, Co, Ni, Cu, Zn)

Abstract A ring-shaped polyoxometalate (POM), [P8W48O184]40– (P8W48), formed by condensation of four hexavacant lacunary α-Dawson-type [α-P2W12O48]14– units, is an attractive superlacunary compound possessing a large cavity (approximately 10 Å). Although the introduction of metal species into the cavity was attempted in aqueous media using the K+/Li+ salt of P8W48, we encountered several challenges: excess metal species are required, and K+ and/or Li+ remain in the cavity. Therefore, controlling the number and arrangement of the introduced metal species proved difficult. In this study, we overcame the aforementioned problems by developing a novel method for introducing metal species in organic media by employing the organic-solvent-soluble tetra-n-butylammonium (TBA) salt of P8W48 (I, TBA12H28[P8W48O184]·30H2O). We began by synthesizing compound I via a cation exchange reaction of the K+/Li+ salt of P8W48 with TBABr in aqueous nitric acid. Single crystals (I′, TBA14H26­[P8W48O184]·20H2O·2CH3NO2) were obtained by recrystallizing I from a nitromethane/toluene mixture in the presence of excess TBAOH. The bond valence sum values of I′ showed that protons were located on the oxygen atoms inside the ring, and thus highly reactive sites capable of reacting with metal species were selectively generated within the ring. When I was reacted with 8 equiv of 3d metal species (Mn2+, Co2+, Ni2+, Cu2+, or Zn2+) in an acetonitrile/1,2-dichloroethane mixture in the presence of TBAOH and methanol, a series of POMs with eight metal sites (IIM, TBA14H2­[{M2(OH2)2}2­{M­(OH2)2}4­P8W48O176­(OCH3)8]·nH2O·mCH3CN, where M = Mn, Co, Ni, Cu, Zn) was synthesized. In IIM, two edge-sharing bis­(square-pyramidal) {(μ-O)2(M–OH2)­(μ3-O)2(M–OH2)­(μ-O)2} and four isolated M­(OH2)2 sites were present within the cavity. During the metal introduction, two α-P2W12 units were converted into γ-P2W12 units, and unique α,γ,α,γ-type P8W48 frameworks were obtained.