Chemistry Reference
In-Depth Information
MSPs are not just an academic curiosity. Nature uses reversible metal-ligand
interactions in a variety of ways, and inspiration for synthetic MSPs arises from
a variety of natural metal-containing biopolymers. [For example, this has led to
the field of artificial metalloenzymes, which merges protein chemistry and structure
with synthetic organometallic chemistry to access hybrid catalysts (Creus and
Ward 2007).] Metalloproteins often utilize metal binding cofactors as centers for
catalysis, electron transport, or substrate binding. An example of other biomaterials
that have attracted significant industrial interest and can be classified as MSPs
are alginate polysaccharides (ErtesvĖg and Valla 1998). These biopolymers are
found in brown seaweeds and are linear blocky copolymers of guluronic acid
(G) and mannuronic acid (M). In the presence of certain metal ions, which can
cross-link the polymers through the carboxylate functionality on the sugar repeat
units, these biopolymers form gels (Fig. 7.2). Such biomaterials hold promise
for a variety of applications including biomedical, sequestering agents, and
viscosity modifiers.
7.2. METAL-LIGAND BINDING MOTIFS
With access to a deep catalog of ligands and nearly half the periodic table offering
applicable metal ions, a vast range of metal -ligand complexes can be envisaged.
The characteristics of a particular metal -ligand supramolecular motif can change dra-
matically, depending on the combination of metal ion and ligand utilized. Significant
variations in motif structure (geometry, metal-ligand stoichiometry), dynamics
(kinetic lability), and thermodynamic stability can all easily be achieved. With
such a wide variety of potential metal-ligand supramolecular motifs available, the
researcher can choose the appropriate motif to impart the desired properties onto
the targeted material.
Some typical examples of metal -ligand supramolecular motifs and ligands uti-
lized in MSPs are provided in Figure 7.3a and b. The coordination number and geo-
metry (e.g., tetrahedral, square planar, octahedral, etc.) of the complex depends on the
choice of metal ion as well as ligand. The combination of these two components
dictates the stoichiometry of the motif. For example, 2:1 ligand/metal complexes
can be obtained with bidentate ligands bound to metal ions with a coordination
number of 4 or alternatively with terdentate ligands binding to hexacoordinate
metal ions. It is also possible to design other metal-ligand stoichiometries; for
example, 3:1 ligand/metal complexes can be obtained through the use of bidentate
ligands bound to hexacoordinate metal ions or terdentate ligands with lanthanide
ions, which prefer higher coordination numbers (8-10). Such systems will generally
be homocoordinate species, in that only one type of ligand is bound to the metal ion.
An additional level of structural diversity can be envisioned if metal -ligand motifs
can be designed that allow exclusively the formation of a single heterocoordinate
complex. In dynamic systems, this level of control is generally not easy to attain and a
combination of homo- and heterocoordination species results. There are however
classes of metal-ligand species that have been utilized as heterocoordinate motifs, for
