Chemistry Reference
In-Depth Information
(+ 2H
2
O + 2 H
+
)
(+ 2 H
2
O)
H
N
H
3
C
O
NH
2
CH
3
H
3
C
S
S
-
N
M
2+
M
2+
+ 2
S
-
N
H
S
H
3
C
O
H
3
C
H
3
C
SH
metal template reaction product
organic reaction product
Figure 6.6
An example of the normal organic reaction route compared with the template reaction in the presence
of a metal ion; a distinctly different path is followed.
Metal-directed reactions have been used to prepare a wide range of cyclic and acyclic
ligand systems. Often, they involve reaction of a coordinated amine with an aldehyde or
ketone. Reaction of a carbon acid anion with an electron-deficient site is also commonly
featured. Zinc(II)-directed condensation between an aldehyde (R CHO) and an aromatic
nitrogen heterocycle (pyrrole) has been used to prepare substituted porphyrin rings (aro-
matic tetraaza macrocycles, analogous to hemes found as iron complexes in blood) since
the 1960s. Once formed, the new ligands can be removed from the templating metal ion
and different metal ions bound to it. This usually involves one of the following: treatment
with acid to protonate the ligand and cause it to dissociate; reduction or oxidation of the
metal to an oxidation state which will not bind the ligand effectively, allowing it to be re-
moved; treatment with a strongly-binding anion (such as CN
−
) that removes the metal ion
competitively, leaving the free ligand; addition of another competing metal ion to which the
ligand binds preferentially to a solution of the templated product, causing metal exchange
(or ligand transfer to the added metal ion).
Polynuclear complexes form through self-assembly also, where both the ligand and
precursor metal complex geometry play a role in the outcome - the pieces tend to fit
together in a particular way like children's building blocks. An early example involves
the self-assembly of 4,4
-bipyridyl and [Pd(en)(ONO
2
)
2
]. The two
cis
-disposed O-bound
nitrate ions are readily substituted by the preferred pyridine nitrogen donors, but they
impose an L-shape when including the palladium, while the ligand imposes a rod-like
shape; thus an assembly of four Pd 'corner' L-shapes and four ligand 'rod' shapes creates
a square 'picture frame' shape, which is now a large cyclic molecule (Figure 6.7).
The product in the above reaction is of low solubility, and its precipitation from the
reaction solution drives further formation, leading to a good yield of the product. The
reaction is, in effect, a sequence of substitution steps, with coordinated nitrate ions each
replaced in turn by the pyridine nitrogen donors.
6.5.2
Reactions of Coordinated Ligands
It has been known for decades that a range of reactions occurs that involve chemistry of the
ligands and in which metal-ligand bond cleavage is not involved. We can regard these as
reactions of coordinated ligands. These early and deceptively simple studies provide fine
examples of chemical detective work. One of the earliest studies probed the preparation
of a H
2
O
Co(III) species from a O
2
CO
Co(III) precursor, a reaction which was seen to
