Chemistry Reference
In-Depth Information
Figure 6.5 Equilibrium coordination of substrates to 6.
tions are quite remarkable, they fall considerably short of the upper limit. With formyl
morpholine, acetonitrile and trimethyl phosphate, the rate accelerations due to Lewis
acid activation are 2
10
2
-fold respectively. Other metal ions that
are less Lewis acidic (Table 6.2) than Co(
III
) should provide somewhat smaller rate
accelerations for the hydrolysis reactions.
The efficiency of Lewis acid activation depends not only on the reactivity of the
bound substrate but also on the equilibrium constant for coordination of the substrate.
The equilibrium constants for binding of an amide, a nitrile and phosphates to Co(
III
)
complexes have been measured (Figure 6.5). Formyl morpholine binds to
6
with an
equilibrium constant of 0.4
M
-1
[35]. Binding of acetamide to
6
could not be detected.
The steric effect of the methyl group is expected to significantly lower the binding of
acetamide compared with that of formyl morpholine.
The equilibrium constant for binding of acetonitrile to
6
(2.5
M
-1
) [41] is comparable
to that for binding of formyl morpholine to
6
even though acetonitrile is about ten
orders of magnitude less basic than the amide [42]. The tight binding of acetonitrile
may be in part due to the lack of steric interactions between the nitrile and the metal
complex. In nitrile coordination, the sp-orbital on the nitrogen is expected to interact
with the d-orbital on the metal in a linear arrangement. In amide coordination, the
sp
2
-orbital on the carbonyl oxygen is expected to interact with the d-orbital on the
metal. In the coordinated complex, the formyl hydrogen should be positioned towards
the metal complex since it is smaller than the amide nitrogen.
The equilibrium constant for binding dimethyl phosphate (2.8
M
-1
) [43] to
6
is greater
than those for binding formyl morpholine and acetonitrile to
6
. Favorable electrostatic
interaction between the metal ion and the coordinating oxygen of dimethyl phosphate
appears to counter any possible steric interactions. The exact value of the equilibrium
constants for binding of substrates to metal ions and complexes will depend on the
substrate, the metal and the ligand surrounding the metal. The examples in Figure 6.5
10
5
-, 2
10
6
- and 4
