Self-Assembled Nanoreactors - Cluster for Molecular Chemistry

1476 Chemical Reviews, 2005, Vol. 105, No. 4 Vriezema et al.
of catalysts. Their precursors can be mixed at the
molecular level, thereby allowing the creation of
homogeneous solid-state systems.
The sol-gel process is a versatile method for the
preparation of heterogeneous catalysts that behave
as homogeneous catalysts. The advantages of a
homogeneous system, that is, high activities and
selectivities, can be combined with the advantages
of a heterogeneous system, that is, facile separation
and recovery of the catalyst. 284 To ensure sufficient
mobility, flexible spacers are used to link the catalytic
centers to the solid support. These materials can be
formed under mild conditions, in that way allowing
the incorporation of sensitive catalysts. The versatility
in the synthesis allows the preparation of solgels
with organic acids and bases, or with metal
complexes as the reactive centers. Sol-gel catalysts
have been successfully employed in a vast series of
reactions, generally with modest to excellent results.
284
An interesting finding is that sol-gel materials can
encapsulate biologic components, while keeping them
in a fully functional state. In this way, not only
proteins, enzymes, and antibodies have been entrapped
within ceramic matrixes, but also DNA and
RNA, and even living cells. 285,286 Interestingly, the
structural integrity of the cells remained intact, and
molecular recognition, catalysis, and reproduction
capabilities were preserved as well. Applications of
such hybrid systems can be found in the fields of
catalysis, sensors, separation technology, diagnostics,
and electronics.
Zeolites are microporous crystals that consist of
aluminosilicate tetrahedrons, which form a secondary
structure of channels, cavities, and pores. In the
petrochemical industry, zeolites are applied on a
large scale in cracking processes. Inside the cavities
of zeolites, catalytically active metal complexes that
are larger than the pores can be incorporated. These
types of zeolite/guest complexes are called ship-in-abottle
complexes. 287 Such complexes can be prepared
in two different ways: (i) assembly of the metal
complex within the cavity by externally adding the
metal ions and the ligands, and (ii) synthesis of the
zeolite around the preformed metal complex, also
referred to as the build-bottle-around-ship approach.
A drawback of the first method is the presence of free
ligands and metal ions, resulting in ill-defined systems.
Depending on the type of zeolite, the cavities
can be used to host large molecules such as bis-
(salicylidene)ethylenediamine (salen), 288 phthalocyanine
(Figure 32), 287,289 and porphyrin. 290
A range of reactions can be performed using shipin-a-bottle
complexes, that is, oxidations, 291 hydrogenations,
292 and acid-catalyzed isomerization and
disproportionation reactions. 293
Recent advances in organic crystal engineering,
particularly the rational design of complex solid
architectures through supramolecular preorganization,
have renewed interest in topochemical reactions.
294-299 Although not many catalysis experiments
have been carried out with these organic
crystals, it is worth mentioning the possibility in this
review. Organic analogues to zeolites and mesoporous
Figure 32. Molecular model showing Cu-phthalocyanine
encapsulated inside zeolite Y. (Reproduced with permission
from ref 287. Copyright 2004 Wiley.)
sieves were developed by Miller et al. 300 They made
catalytically active, nanoporous organic materials
based on cross-linked lyotropic liquid crystals and
demonstrated their use as efficient catalysts in
Knoevenagel condensations. Dewa et al. prepared
hydrogen-bonded solid hosts from 9,10-bis(3,5-dihydroxyphenyl)anthracene
and demonstrated that they
catalyze the Diels-Alder reaction of acrylaldehyde
and 1,3-cyclohexadiene. 301 Liquid crystals and organic
crystals have also been applied as constrained
geometries for controlled polymerizations, resulting
in interesting polymeric architectures. 302
4. Biomacromolecular Nanoreactors
A new approach to the formation of nanostructures
has emerged from studies at the interfaces between
biology, chemistry, and materials science. 303 A number
of groups are rapidly expanding the repertoire
of biological scaffolds for nanochemistry. Reviews
such as the ones from Niemeyer, 304 Young et al., 303
Belcher et al., 305 and Kirshenbaum et al. 306 describe
thoroughly these new approaches, which use biological
molecules and assemblies from them for applications
in nanoscience. The current interest in these
biosystems stems from the fact that they display a
high degree of organization, are often easy to modify,
and occur in well-defined self-assembly motifs. 307
In this part of the review, it is shown that organized
biomolecular architectures can serve as nanoreactors
or nanotemplates for crystallizations and
other reactions. Furthermore, it is shown that protein
capsids can be modified with new chemical functionalities
and can be used as templates for nanoscale
constructions. Finally, it is discussed how self-assembled
protein cages can provide spatially welldefined
host systems.
4.1. Protein Cages
Nature has developed a variety of proteins that
function as carriers or storage devices for metal ions
and minerals. The iron storage protein ferritin is
probably the most intensively studied and best
understood example. In this protein, the mineral is
sequestered within one single molecule, which has a