Chemistry Reference
In-Depth Information
Spherical
(F
-
, Cl
-
, Br
-
, I
-
)
trigonal planar
(CO
3
2-
, NO
3
-
)
linear
(CN
-
, N
3
-
, SCN
-
, OH
-
)
square planar
(squarate)
tetrahedral
(PO
4
3-
, SO
4
2-
)
octahedral
(Fe(CN)
6
4-
, PF
6
-
)
Fig. 1 Typical anion shapes.
Designing of neutral anion receptors, capable of anion binding by
hydrogen bonds, continues to be an area of active research.
3
There are
two factors responsible for binding anity of receptor, namely the
number and strength of hydrogen bond donors, and their proper spatial
arrangement. Among the most commonly used hydrogen bonding motifs
are amides,
4
thioamides,
5
ureas,
6
thioureas,
7
and heterocyclic NH's.
8
The
latter have an advantage over other hydrogen bond donors lacking
hydrogen bond acceptor properties, thus excluding unfavorable intra-
molecular hydrogen bonds which will hamper the binding anity of
anion receptor. It is believed that the more acidic the NH group, the
stronger hydrogen bond ability it has. The acidity of the pyrrole ring can
be enhanced not only by electron withdrawing substituents but also
by delocalization by the benzene ring, as in indole. It is evidenced by the
pK
a
value of pyrrole (23.0) and indole (20.9).
9
Gale and coworkers
10
prepared anion receptors - bis-amidopyrrolyl-
methanes of type 1, which were able to bind several anionic guests in
demanding solvents.
1
H NMR titration method was used to determine
the association constants of the receptor 1 (Fig. 2) with various anions in
a DMSO-water (95 : 5 v/v) mixture. The receptor 1 binds fluoride and
benzoate with host/guest 1 : 1 stoichiometry, and with significant binding
constants (7560 and 354 M
1
, respectively). A substantial improvement of
binding abilities was achieved by replacing of amidopyrrole by ami-
doindole binding block.
11
An anion receptor 2 (Fig. 2), based on diin-
dolylmethane, formed the 1 : 1 stoichiometry complexes with several
anions. Its titration with benzoate anion gave binding constant 2060 M
1
(in a DMSO-water (95 : 5 v/v) mixture), roughly six times higher value than
for 1; it is worthy to notice that both receptors 1 and 2 possess similar
patterns of hydrogen bond donors. Further improvement was accom-
plished by increasing number of hydrogen bond donors (receptor 3,
Fig. 2), what allowed for binding anions in methanol.
12
Chiral recognition is one of the most important and dicult fields of
the modern supramolecular chemistry. Distinguishing between two en-
antiomeric species is a very subtle process that crucially relies on the
perfect geometrical fit of the receptor molecule.
13
The great importance
of chiral recognition may be summed up in the assertion that, ''life itself
is chiral''. Most of the biologically important molecules, from amino
acids to sugars, hormones, etc., are chiral and found in leaving
organisms in optically pure form. All life on Earth uses
L
-amino acids and
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