Conspectus
“Functional
molecular systems”, discrete and self-assembled
constructs where control over molecular recognition, structure, bonding,
transport, release, catalytic activity, etc., is readily achieved,
are a topic of current interest. Within this broad paradigm, oligopyrrolic
cages have garnered attention due to their responsive recognition
features. Due to the presence of slightly polar pyrrole subunits which
can also behave as hydrogen-bonding donors, these oligopyrrolic cages
are potential receptors for various polarized species. In this Account,
we summarize recent advances involving the syntheses and study of
(1) covalent oligopyrrolic macrobicyclic cages, (2) oligopyrrolic
metallacages, and (3) oligopyrrolic noncovalently linked cages. Considered
in concert, these molecular constructs have allowed advances in applied
supramolecular chemistry; to date, they have been exploited for selective
guest encapsulation studies, anion binding and ion-channel formation,
and gas absorption, among other applications. While key findings from
others will be noted, in this Account will focus on our own contributions
to the chemistry of discrete oligopyrrolic macrocycles and their use
in supramolecular host–guest chemistry and sensing applications.
In terms of specifics, we will detail how oligopyrrole cages with
well-defined molecular geometries permit reversible guest binding
under ambient conditions and how the incorporation of pyrrole subunits
within larger superstructures allows effective control over anion/conjugate
acid binding activity under ambient conditions. We will also provide
examples that show how derivatization of these rudimentary macrocyclic
cores with various sterically congested β-substituted oligopyrroles
can provide entry into more complex supramolecular architectures.
In addition, we will detail how hybrid systems that include heterocycles
other than pyrrole, such as pyridine and naphthyridine, can be used
to create self-assembled materials that show promise as gas-absorbing
materials and colorimetric reversible sensors. Studies involving oligopyrrolic
polymetallic cages and oligopyrrolic supramolecular cages will also
be reviewed. First, we will discuss all-carbon-linked oligopyrrolic
bicycles and continue on to present systems linked via amines and
imines linkages. Finally, we will summarize recent work on pyrrolic
cages created through the use of metal centers or various noncovalent
interactions. We hope that this Account will provide researchers with
an initial foundation for understanding oligopyrrolic cage chemistry,
thereby allowing for further advances in the area. It is expected
that the fundamental design and recognition principles made in the
area of oligopyrrole cages, as exemplified by our contributions, will
be of general use to researchers targeting the design of functional
molecular systems. As such, we have structured this Account so as
to summarize the past while setting the stage for the future.