Planar Cu₂(ppz)₂ Dimers as SBUs for Diverse Polyoxometalate-Based Metal Organic Frameworks

Five new polyoxometalate-based metal organic frameworks, [Cu₄(ppz)₄(PO₄)]·​[CuBr₂] (<b>1</b>), [Cu₄(ppz)₄­(MoO₄)₂­(H₂O)]·​2H₂O (<b>2</b>), [Cu­(Hppz)­(Mo₂O₇)] (<b>3</b>), [Cu₂(ppz)₂­(Mo₃O₁₀)] (<b>4</b>), and [Cu₆(ppz)₆­(PMo₃^VMo₉^VIO₄₀)] (<b>5</b>) (Hppz = 2-(1<i>H</i>-pyrazol-3-yl)­pyrazine), have been hydrothermally synthesized and fully characterized, in which a planar Cu₂(ppz)₂ dimer acts as a secondary building unit (SBU) and provides up to six potential binding sites. In virtue of the strong Jahn–Teller (JT) effect of Cu­(II) ions, the potential binding sites in the Cu₂(ppz)₂ SBU can facilely be occupied by auxiliary oxygen-donor anions and/or outward pyrazine nitrogen atoms of neighboring SBUs. The auxiliary oxygen-donor anions in compounds <b>1</b>–<b>5</b> range from simple phosphate and molybdate, zonal Mo₂O₇^2– and Mo₃O₁₀^2– to a spherical three-electron reduced Keggin anion [PMo₁₂O₄₀]^6–, respectively. Compound <b>1</b> contains an unprecedented 3D (4,6)-connected cationic open framework [Cu₄(ppz)₄­(PO₄)]⁺ with 1D channels filled by linear CuBr₂^– anions, where the Cu₂(ppz)₂ SBUs and PO₄ groups act as nodes and the outward Cu–N bonds and oxo bridges act as linkers. <b>2</b> possesses a complicated trinodal (3,4)-connected topological framework, in which μ₃-MoO₄ and Cu₂(ppz)₂ groups act as 3- and 4-connected nodes and the outward Cu–N bonds and double μ₂-MoO₄ groups act as linkers. Differing from <b>1</b> and <b>2</b>, compound <b>3</b> shows a 2D organic–inorganic hybrid sheet constructed by a [Mo₂O₇]^2– ribbon of cyclic hexameric edge-shared [MoO₆]-octahedra and Cu₂(ppz)₂ SBUs. Similar to <b>3</b>, compound <b>4</b> also shows 2D organic–inorganic hybrid sheets constructed by zigzag [Mo₃O₁₀]^2– ribbons and Cu₂(ppz)₂ SBUs, and adjacent sheets are extended by weak Mo–N bonds into the 3D network. <b>5</b> has a 3D 6-connected <b>pcu</b> topological framework constructed by Cu₂(ppz)₂ SBUs and [PMo₁₂O₄₀]^6– anions, which also can be viewed as cationic of the NbO topological [Cu₆(ppz)₆]⁶⁺ network with cavities filled by [PMo₁₂O₄₀]^6– anions. Magnetic measurements show that there is strong antiferromagnetic coupling within the Cu₂(ppz)₂ dimer and only one unpaired electron within the three-electron reduced Keggin anion