Chemistry Reference
In-Depth Information
that different metal ions will exhibit different binding kinetics and thermodynamics.
A pertinent demonstration of this was recently provided by Chiper et al. (2007), who
investigated the possibility of using gel permeation chromatography (GPC) to analyze
different metal-containing MSPs. Prior investigations into the GPC analysis of labile
MSPs, for example, Zn
II
-terpy systems, revealed that fragmentation of the polymeric
aggregate occurs as it passes down the column. Metallopolymers that contain more
kinetically inert metal complexes (e.g., Ru
II
-terpy) within the backbone can be suc-
cessfully analyzed by this technique because their kinetically inert nature makes them
less like MSPs and more like covalent polymers. Chiper and colleagues prepared a
range of metal -terpy MSPs (Co
II
,Fe
II
, and Ni
II
) that, based on available binding con-
stant data, should fall between the Zn
II
and Ru
II
systems in terms of binding stability.
Initial GPC studies were carried out on mono-end-capped terpy-poly(ethylene
oxide) (PEO) metal complexes and using a range of sample concentrations. In the
Co
II
and Fe
II
systems, lowering the concentration resulted in a detectable shift
to macromonomer in the GPC whereas the Ni
II
complexes were stable across the
range of concentrations investigated (0.0011-0.0221 mM). This is consistent
with the stronger binding strength of terpy to Ni
II
than Co
II
or Fe
II
. Of course,
binding kinetics may also play a role in determining the stability of a complex as it
passes down the column. Further GPC studies on Ni
II
complexes of a ditopic
terpy-PEO macromonomer revealed an average degree of polymerization of
28 units, suggesting that this MSP was robust enough to survive GPC analysis.
From a supramolecular viewpoint, one possible advantage of using metal -ligand
interactions (e.g., vs. hydrogen bonding) is their relative insensitivity to the presence
of water. This has allowed a number of studies of MSPs in aqueous environments. For
example, Vermonden and coworkers (2003) utilized short ethylene oxide spacers
between pyridine-2,6-dicarboxylate end groups (2) to prepare MSPs in aqueous sol-
utions and studied the effects of concentration, temperature, and metal -ligand stoi-
chiometry in these systems (Fig. 7.5). As was also shown in the Rehahn system,
decreasing the concentration of a 1:1 complex of 2
.
Zn(ClO
4
)
2
increased the fraction
of rings. In addition, they showed that increasing temperature increased the fraction
Figure 7.5 Water-soluble MSPs, based on a 2,6-pyridine dicarboxylate end-capped
oligo(ethylene oxide) ditopic monomer (2a and 2b) and Zn
II
ions prepared by Vermonden
and coworkers (2003).
