Assembly of Organic–Inorganic Hybrid Supramolecular Materials Based on Basketlike {M⊂P6Mo18O73} (M = Ca, Sr, Ba) Cage and Transition-Metal Complex

Eight members of basketlike mixed-valent compounds, formulated as {H3O}2{FeIII(2,2′-bpy)3}6{Sr⊂P6MoV4MoVI14O73}2·9H2O (1), {H2(4,4′-bpy)}5{[Ni­(4,4′-bpy)­(H2O)3]2[Ni­(H2O)2]­[Sr⊂P6MoV2MoVI16O73]2}·12H2O (2), {Cu2(bim)4(H2O)}2{[Cu­(bim)2]2[Cu­(bim)­(H2O)]2Cu­(H2O)2­[Sr⊂P6MoV3MoVI15O73]2}·16H2O (3), {H3O}4{[Cd­(phen)2]2Sr­(H2O)5­[Sr⊂P6MoV4MoVI14O73]}·H2O (4), {Cu­(bim)2} {[Cu­(bim)2]2[Cu­(Hbim)­(H2O)2]­[Ca⊂P6MoV3MoVI15O73]}·9H2O (5), {[Cu­(2,2′-bpy)­(H2O)]4­[Ca⊂P6MoV2MoVI16O73]}·4H2O (6), {[Cu­(2,2′-bpy)­(H2O)]4­[Sr⊂P6MoV2MoVI16O73]}·8H2O (7), and {[Cu­(2,2′-bpy)­(H2O)]4­[Ba⊂P6MoV2MoVI16O73]}·8H2O (8), have been hydrothermally synthesized and structurally characterized via elemental analysis, thermogravimetry (TG), infrared (IR) spectroscopy, ultraviolet–visible light (UV-vis) spectroscopy, X-ray photoelectron spectroscopy (XPS), and single-crystal X-ray diffraction (XRD). The structural analysis shows that all the compounds contain basket-shaped polyoxoanions [M⊂P6MoVnMoVI18–nO73](6+n)– (M = Ca, Sr, Ba; n = 2, 3, or 4) (abbreviated as {P6Mo18O73}) as building units, which consists of a tetravacant γ-Dawson-type­{P2Mo14} unit and a “handle”-shaped {P4Mo4} segment encapsulating a single alkaline-earth metals cation M2+ in the central cavity. Compound 1 presents a zero-supported basketlike structure decorated by three discrete {Fe­(2,2′-bipy)3} fragments. Compound 2 is monosupported by {Ni­(4,4′-bipy)­(H2O)3} subunit. Compound 3 and 4 are bisupported by copper complexes and cadmium complexes, respectively. Compound 5 exhibit a trisupported basket cluster modified with two {Cu­(bim)2}2+ and one {Cu­(Hbim)­(H2O)}3+ cations. Compounds 6–8 are isostructural clusters exhibiting similar tetra-supported structures. Compound 1, 2, and 4 represent the first basket-like hybrids containing transition-metal iron, nickel, and cadmium, respectively. In addition, compounds 2, 6, 7, and 8 are observed for the first time as a two-electron-reduced basket form. The compounds display good electrocatalytic activity upon the reduction of nitrite, and fluorescent properties.