Self-Assembled Pyridine-Dipyrrolate Cages

An inherently nonlinear pyridine dipyrrolate ligand, namely 2,6-bis­(3,4-diethyl-5-carboxy-1H-pyrrol-2yl)­pyridine (compound 1), is able to distinguish between different zinc­(II) cation sources, namely Zn­(acac)2 and Zn­(OAc)2, respectively. This differentiation is manifest both in terms of the observed fluorescent behavior in mixed organic media and the reaction chemistry. Treatment of 1 with Zn­(acac)2 gives rise to a cage dimer, cage-1, wherein two molecules of compound 1 act as double bridging units to connect two individual cage subunits. As inferred from X-ray crystallographic studies, this cage system consists of discrete zinc dimers with hydroxide bridges that, with the assistance of bound DMF solvent molecules, serve to fix the geometry and orientation of the pyridine dipyrrolate building blocks. When a different zinc source, Zn­(OAc)2, is used to carry out an ostensibly similar complexation reaction with compound 1, an acetate-bridged 1D abacus-like cage polymer is obtained as inferred from X-ray diffraction analysis. This extended solid state structure, cage-2, contains individual zinc dimer cage submits and appears stabilized by solvent molecules (DMF) and the counteranion (acetate). Rod-like assemblies are also observed by DLS and SEM. This construct, in contrast to cage-1, proved fluorescent in mixed organic media. The structure of the ligand itself (i.e., in the absence of Zn­(II)) was confirmed by X-ray crystallographic analysis and was found to assemble into a supramolecular polymer. Conversion to a dimer form was seen upon the addition of TBAOAc. On the basis of the metric parameters, the structures seen in the solid state are stabilized via hydrogen bonding interactions involving solvent molecules.