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

Five new polyoxometalate-based metal organic frameworks, [Cu-4(ppz)(4)(PO4)].[CuBr2] (1), [Cu-4(ppz)(4)(MoO4)2(H2O)].2H(2)O (2), [Cu(Hppz)(Mo2O7)] (3), [Cu-2(ppz)(2)(Mo3O10)] (4), and [Cu-6(ppz)(6)(PMo3(V)Mo(9)(VI)O(40))] (5) (Hppz = 2-(1H-pyrazol-3-yl)pyrazine), have been hydrothermally synthesized and fully characterized, in which a planar Cu-2(ppz)(2) dimer acts as a secondary building unit (SBU) and provides up to six potential binding sites. In virtue of the strong JahnTeller (JT) effect of Cu(II) ions, the potential binding sites in the Cu-2(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 15 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 [Cu-4(ppz)(4)(PO4)](+) with 1D channels filled by linear CuBr2 anions, where the Cu-2(ppz)(2) SBUs and PO4 groups act as nodes and the outward CuN bonds and oxo bridges act as linkers. 2 possesses a complicated trinodal (3,4)-connected topological framework, in which mu 3-MoO4 and Cu-2(ppz)(2) groups act as 3- and 4-connected nodes and the outward CuN bonds and double mu 2-MoO4 groups act as linkers. Differing from 1 and 2, compound 3 shows a 2D organicinorganic hybrid sheet constructed by a [Mo2O7](2) ribbon of cyclic hexameric edge-shared [MoO6]-octahedra and Cu-2(ppz)(2) SBUs. Similar to 3, compound 4 also shows 2D organicinorganic hybrid sheets constructed by zigzag [Mo(3)O10](2) ribbons and Cu-2(ppz)(2) SBUs, and adjacent sheets are extended by weak MoN bonds into the 3D network. 5 has a 3D 6-connected pcu topological framework constructed by Cu-2(ppz)(2) SBUs and [PMo12O40](6) anions, which also can be viewed as cationic of the NbO topological [Cu-6(ppz)(6)](6+) network with cavities filled by [PMo12O40](6) anions. Magnetic measurements show that there is strong antiferromagnetic coupling within the Cu-2(ppz)(2) dimer and only one unpaired electron within the three-electron reduced Keggin anion.