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.