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Kinetics of Alkali Metal Complex Formation by Bicyclic Cryptates.

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Kinetics of Alkali Metal Complex Formation by
Bicyclic Cryptates
By Karsten Henco, Burkhard Tummler, and Gunter Maassr]
The diazapolyoxa macrobicyclics (cryptands) synthesized
by Lehn et al. have stability constants up to 10'1 mol-I
and are therefore the strongest complexing agents presently
known for alkali metal ions in aqueous solution[']. The kinetics
of complex formation has so far only been examined for
the c2.2.21 cryptand, the methods of potentiometry, 'H-NMR
and 23Na-NMRspectroscopy merely yielding overall dissociation rate constants for the complexes of [2.2.2][21.
I, n
[2.2.1]:
m =
[2.2.2]:
m = n
=
cavity of the ligand so difficult that the rate of association
is drastically reduced.
The two complexes having the greatest stability, the sodium
complex of the c2.2.11 cryptand and the potassium complex
of [2.2.2] (both with lgK=5.4) were examined at pH=6.3
in cacodylate buffer and at pH = 7.5 in Tris buffer. The ligands
exist in the mono- and diprotonated form at the chosen pH
values. In the temperature jump experiments two relaxation
times were always observed: the fast relaxation (10 to l o o p )
is assigned to proton transfer in the acid-base system buffercryptand, and the slow relaxation (10 to 100ms) to the binding
of the metal ion to the monoprotonated ligand. Analysis of
the concentration and pH dependence of the latter relaxation
time shows that binding of the metal ion may precede or
succeed protolysis, i. e. complex formation may proceed by
the dissociative or the associative mechanism[3]:
= 0
(41
1
[ crHM]'+
k12
( I ) crH+ + M+
We here report studies on the kinetics of complexation
of nonprotonated and singly protonated cryptands with
sodium and potassium ions. For this purpose we employed
the temperature jump relaxation procedure which permits
determination of the rate constants of both association and
dissociati~n[~].
The relaxation processes were registered as
absorptions between 225 and 250 nm as functions of the cryptand concentration. In the chosen wavelength range the free
nonprotonated ligand absorbs much more strongly than its
mono- and di-protonated forms and than the complex formed
by the ligand and the metal ion.
In the basic range (pH = 12.5) a concentration-dependent
relaxation time was found for recombination of metal ion
and unprotonated ligand, corresponding to the bimolecular
k
[crM]+
kz I
+
H+ ( 2 )
32$k
cr
+
H+
+
M+
(3)
c r = Cryptand
In the neutral pH region the intermediate state ( 3 ) of
the free nonprotonated ligand is populated only very slightly.
On the other hand, we could deduce from the saturation
behavior of the slow relaxation at higher concentrations of
salt and ligand that the intermediate state (4) is present
in larger than negligible concentrations in solution. This
complex [crHM]'+ has not yet been detected by NMR spectroscopy[']. The _overalLrate constants for the forward and
reverse reaction k and k are listed in Table 1.
Table 1. Rate constants ofalkali metal complex formation of bicyclic cryptates.
Numerical evaluation yields the rate constants k12=9 x 1041
mol-' s-' and k21 = 1.4 x
for the Nat/[2.1.1] system
and kl2=3x1O71 mol-' s-' and k 2 1 = 2 x 1 0 3 ~ - 'for the
K+/[2.2.1] system. We interpret the experimental finding by
assuming that the diffusion-controlled encounter of metal ion
and ligand is followed by stepwise replacement of the water
molecules in the inner hydration shell of the metal ion by
the coordination sites of the ligand. It should be emphasized
that unimolecular conformational transitions such as the
exolendo conformational equilibria discussed by Lehn et al.
do not represent the rate-determining step of complexation.
Interestingly, K recombines about one hundered times faster
with C2.2.11 than does Na+ with [2.1.1], although the ratios
of
the
ionic
to
cavity
radius
are
equal
( r ~ ~ + 1/ =r r2~~+ / r 2 2 1 =
1.2)[']. The rate constant measured for
the K+/[2.2.1] system is comparable with the value found
for binding of alkali metal ions to sterically unhindered polydentate ligands such as ethylenediaminetetraacetic acid
(EDTA)C41. On the other hand, the lower flexibility and the
greater bond angle opening in the case of the smaller C2.1.11
cryptands render optimal adaptation of the metal ion to the
+
[*] Prof. Dr. G. Maass, Dipl.-Biochem. K. Henco, DipL-Biochem. B. Tummler
lnstitut fur Physiologische Chemie und Klinische Biochemie, Abt. Biophysikalische Chemie der Medizinischen Hochschule
Karl-Wiechert-Allee 9, D-3000 Hannover 61 (Germany)
538
Rate consta_nts
k [I m o l - ' s ~ ' ]
k [s-'1
Ligand
Cation
Stability
constantIgK
A
[2.1.1]
[2.2.1]
[2.2.1]
[2.2.2]
Na'
Na+
K+
3.2
5.40
3.95
5.4
( 9 + 1 ) lo4
( 6 f 2 ) 106
( 3 f i ) 10'
( 2 f l ) 106
K+
(1.4-tO.1) loz
18f2
(2
103
9+3
The values measured for the [2.2.2] complex are of the
same order of magnitude as the constants published by Lelin
et a1.[2J.
Received: April 20, 1977 [Z 742 IE]
German version: Angew. Chem. 89, 567 (1977)
CAS Registry numbers:
[2.1.1],31250-06-3; [2.2.1],31364-42-8; [2.2.2],23978-09-8; Na', 17341-25-2;
K', 24203-36-9
[lJ 8. Dietrich, J . M . Lehn, J . P . Sauuage, Tetrahedron Lett. 1969, 2885;
J . M . Lehn, J . P. Sauuage, J. Am. Chem. SOC.97, 6700 (1975).
[2] J. M. Lehn, Struct. Bonding 16, 1 (1973); J . M. Lehn, J . P . Souuoge,
8.Dietrich, J. Am. Chem. SOC.92, 2916 (1970); J. M . Ceraso, J . L.
Dye, ibid. 95, 4432 (1973).
[3] M. Eigen, L. De Maeyer in A . Weissberger: Techniques of Chemistry,
Vol. VI/2. Wiley, New York 1974, Chapter 3.
[4] H . Diebler, M. Eigen, G . Ilgenfritz, G . Maass, R . Winkler, Pure Appl.
Chem. 20,93 ( 1 969).
Angew. Chem. Int. Ed. Engl. 16 ( 1 9 7 7 ) No. 8
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cryptates, complex, metali, bicyclic, formation, alkali, kinetics
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