Chemistry Reference
In-Depth Information
H
H
H
H
H
H
H
H
H
H
H
N
N
N
N
N
N
N
N
M
M
M
M
N
N
N
N
N
N
N
N
H
H
H
H
H
Figure 4.39
Possible dispositions of the secondary amine hydrogen atoms in a coordinated macrocyclic tetraamine.
Each isomer, formed only on coordination, exhibits slightly different physical properties.
equivalent to one of those shown, as they are equated simply by inverting the macrocycle.
In effect, these isomers equate to a different chirality set for nitrogen donors, fixed upon
coordination; each secondary amine RR
HN group, when coordinated, forms RR
HN M,
and then, with four nonequivalent groups covalently bound around the tetrahedral nitrogen,
the N centre is chiral. They exist in addition to any other sources of isomerism and
chirality in the molecule. Although the N-based isomers may have slightly different physical
properties, interconversion between these isomers can be readily achieved by raising the
solution pH, which promotes N-deprotonation and exchange, leading to formation of the
thermodynamically most stable N-based isomer. Usually, this subtle N-based isomerism
tends to be ignored, as it is a level of complication too far for most.
Macrocycles carrying
pendant groups
also capable of binding metal ions produce the
opportunity to 'wrap up' metal ions better (Figure 4.40). These 'molecular wrappers' have
pendant groups that can come on or off, so they behave as 'hinged lids'. The pendant groups
may be of any type, and carry any form of potential binding group - amine, carboxylic
acid, thiol, alcohol, pyridine and others. These groups may themselves be further elaborated
(a)
(b)
(c)
(d)
X
L
-2X
L
L
L
L
L
L
L
M
M
(e)
L
L
L
+2X
X
L
Figure 4.40
Simple macrocycles (a) may be augmented with pendant group(s) attached to either a heteroatom (b)
or a carbon atom (c) of the ring. Those with two pendant groups (d) offer better opportunities for
'wrapping up' octahedral metal ions, as the two pendant arms can supply additional donors that, by
being linked to the ring, enhance entrapment by 'capping' the metal, as shown in (e).
