Engineering New Metal-Organic Frameworks Built from Flexible Tetrapyridines Coordinated to Cu(II) and Cu(I)

A series of new metal-organic frameworks have been constructed by the coordination of Cu(II) and Cu(I) with pentaerythrityl tetrakis(4-pyridyl) ether (1 = PETPE), a flexible tetradentate ligand. Networks derived from Cu(OOCCH3)(2), Cu(NO3)(2), and CuBF4 proved to have different topologies (diamondoid, PtS, and SrAl2, respectively). This reflects (1) the ability of PETPE (1) to adopt diverse conformations and (2) the varied geometries of complexes of Cu(II) and Cu(I). Extended PETPE (2), a tetrapyridine with phenyl spacers inserted into the pentaerythrityl core of PETPE (1), yielded an expanded version of the PtS network derived from simple PETPE (1) and Cu(NO3)(2). However, increases in the ability of the network to accommodate guests were largely offset by interpenetration of independent networks. Attempts to thwart interpenetration by converting ligand 2 into methyl-substituted derivative 3 led to the construction of networks with alternative topologies. In particular, the reactions of ligand 3 with both Cu(II) and Cu(I) yielded isostructural Pt3O4 networks, despite the preference of the two oxidation states for coordination spheres with different geometries. Together, these observations demonstrate that PETPE (1) and related compounds are useful ligands for constructing metal-organic frameworks, with a distinctive ability to accommodate a single metal in different oxidation states, as well as to adapt to a metal in a single oxidation state but with different counterions or secondary ligands.