1
72
E. LONGHINOTTI ET AL.
pH 9.4; above this pH, the predominant species is
[
The simple observation that the dinuclear hydroxo
3þ
2
ꢀ
Sm (BTP) (OH) ] .
2
complexes of Ln are the active species in the catalysis
of the BDNPP hydrolysis reaction is interesting from the
2
8
3
Species distribution diagrams for Tb –BTP show
þ
3
þ
similar results to those for the Sm
system. The
complex reaches a maximum
point of view of the bimetallic cooperativity presented by
1,21
þ
[
(
Tb (BTP) (OH) ]
2
some natural metalloenzymes.
dinuclear complexes in phosphate ester hydrolysis reac-
The participation of
2
5
36.5%) at pH 7.8, and at this pH there are also small
7
tions was proposed by Yatsimirsky and co-workers and
amounts of the [Tb (BTP) (OH) ] and [Tb (BTP)
2
2
2
6
2
ꢀ
ꢀ
(
OH)7] complexes. The [Tb (BTP) (OH) ] species
2
allows one of the lanthanides to promote the binding
act efficiently, while the second metal ion through as-
sociation with a nucleophilic species induces a synchro-
nic effect on the hydrolysis reaction rate. The overall
effect results in both the ability of the metal ion to
polarize P—O bonds and the placement of a nucleophilic
species in a suitable position, an effect which results
in charge, high coordination numbers and binding
directionality.
2
7
reaches its maximum at pH 8.7 with 25.6% formation,
whereas the [Tb (BTP) (OH) ] species only reaches
2
2
6
6
.9% at pH 8.3 (Fig. 5).
Figure 6 shows the formation of 47% of the [Er2
BTP) (OH) ] complex at pH 7.8. The yield of the
(
[
2
6
ꢀ
Er (BTP) (OH) ] complex is only 4.3% at pH 8.7. At
7
2
2
2ꢀ
this pH the [Tb (BTP) (OH) ] species is formed and it
2
2
7
prevails at higher pH.
The species distribution diagrams with the plot of
observed rate constant for the catalytic reaction make it
possible to identify the reactive species present in those
systems. For Sm , Tb and Er we can see that the
pH–kobs profiles are closely related to the distribution
curves for the hydroxo complexes.
The equilibrium results were used to interpret the
kinetics of the BDNPP hydrolysis reaction, where kobs
is the sum of the specific catalytic constants of each
species present in solution:
Acknowledgements
3
þ
3þ
3þ
The authors are indebted to PRONEX, CAPES and
CNPq, Brazil, for financial support of this work. The
authors are grateful to Professor A. K. Yatsimirsky for
helpful discussions.
REFERENCES
þ
kobs½Ln ðBTPÞ ꢃ
¼ k ½Ln ðBTPÞ ðOHÞ ꢃ
2
2 total
1
2
2
5
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1 4
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1
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3þ
ꢂ
9
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2.7
k1
k2
k3
k4
2.7
—
3.1
2.9
2.3
2.7
2.8
2.8
—
2.9
—
2
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3.1
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