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] (1), [Cu4(ppz)4­(MoO4)2­(H2O)]·​2H2O (2), [Cu­(Hppz)­(Mo2O7)] (3), [Cu2(ppz)2­(Mo3O10)] (4), and [Cu6(ppz)6­(PMo3VMo9VIO40)] (5) (Hppz = 2-(1H-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 1–5 range from simple phosphate and molybdate, zonal Mo2O72– and Mo3O102– to a spherical three-electron reduced Keggin anion [PMo12O40]6–, respectively. Compound 1 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. 2 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 1 and 2, compound 3 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 3, compound 4 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. 5 has a 3D 6-connected pcu 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.