Page 237 - Academic Press Encyclopedia of Physical Science and Technology 3rd Organic Chemistry

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Encyclopedia of Physical Science and Technology EN011A-543 February 12, 2002 12:40

524 Organic Macrocycles

TABLE VII Silica Gel-Supported Macrocyclic Separation
System Results from Superfund Site
Influent Treated
(Source) effluent
concentration concentration a Recovery Eluent
Metal (mg/L) (mg/L) (%) purity (%)
Cu 180 <0.02 >99 96–98
Fe 994 <0.1 >99 99.5
Al 270 <1 >99 b 97–98
Zn 554 <0.05 >99 99–99.5
Mn 194 <1 >99 75–80 b
Cd 2 <0.02
As 0.3 <0.1
a All below detection by the analytical methods used.
FIGURE 8 Chiral pyridino-18-crown-6 macrocyclic host attached b Some Al slipped past the Al system but was recovered with the Mn.
to a solid support (silica gel).
Total Al recovery was >99%.
heating a mixture of silica gel and crown ether-containing
to allow them to be marketed as concentrated solutions for
ethoxy silanes. These solid-supported macrocycles are
further refining.
currently finding success in the selective separation of
There will be increased emphasis in the future on de-
metal cations. The macrocycle is permanently bound to
veloping macrocyclic ligands that are highly selective
the solid support and cannot be removed unless the silica
for particular target cations and anions in many matri-
gel is destroyed, as in concentrated aqueous base. This
ces and conditions. The impetus for these developments
provides avenues for reusable separation systems.
will be increased environmental awareness and the need
Equilibrium constants for the association of metal
for greater cost effectiveness in the separation and purifi-
cations with the silica gel-bound crown ethers are compa-
cation procedures.
rable to log K (H 2 O) values for the association of the same
cations with unbound ligands. These bound macrocycle
C. Supramacrocycles: Self-Assembly
systems can completely retain the desired metal ions while
and Molecular Machines
allowing the other metal ions to pass through the system.
This method of using solid supported macrocyclic ligands Tremendous advances in synthetic methodologies and
to remove, separate, and concentrate specific metal ions macrocyclic structure exegesis have allowed the creation
from aqueous solutions has been commercialized for use and characterization of macrocycles and surpramacrocy-
in many analytical, environmental, metallurgical, precious cles which, until recently, were sequestered in the minds
metal, nuclear, and industrial operations. For example, this of creative chemists. Examples of molecular topology, in-
molecular recognition technology has been used in a pilot tertwining, and self-assembly are ubiquitous in the nat-
plant to separate and purify desired metal ions at a su- ural world. For example, the double helix, lipids, vi-
perfund site in Butte, Montana. The composition of metal ral capsids, and the secondary, tertiary, and quaternary
contaminants in the water at this site includes Al, As, Cd, structures of many biomolecules all posses varied levels
Ca, Cu, Fe, K, Mg, Mn, Ni, Na, Pb, Zn, and Si. The an- of molecular topology, intertwining, and self-assembly.
ions are chloride and sulfate. The pH of this solution is Macrocyclic chemistry of the 1970s has fathered the
approximately 3 and the reduction potential is approxi- supramolecular chemistry of the 1980s and 1990s. It
mately 620 mV. The pH and ion concentrations exceed is now a new millennium and it is becoming feasible
discharge regulations as promulgated by the U.S. Envi- to construct large (nanometer to millimeter dimensions)
ronmental Protection Agency (EPA) Gold Book standards and intricate supramolecular entities with machine-like
for pristine water supplies by large factors, often several properties in order to investigate the processes central
orders of magnitude. Table VII illustrates that this envi- to nature’s forms and functions. Rotaxanes, pseudorotax-
ronmentally friendly solid supported macrocyclic system anes, and catenanes (Fig. 9) are interesting examples of
developed by IBC Advanced Technologies, Inc., of Amer- this vast supramolecular class because the relative posi-
ican Fork, Utah, was capable of achieving high metal re- tions of their component parts can be induced to change as
covery and effluent water qualities that meet EPA Gold a result of an external stimulus. In most cases this mechan-
Book and/or regulatory drinking water standards. The tar- ical mobility occurs between well-defined states that can
get metals were recovered without the addition of any be switched on or off by external stimuli such as chemical,
undesirable ion to the system and were of sufficient purity photochemical, electrochemical, or electrical energy.