Cooperative binding of potassium cation and chloride anion by novel rhenium(I) bipyridyl amide crown ether receptors

Article abstract of DOI:10.1039/a707680k

New rhenium(I) bipyridyl amide crown ether receptors are prepared and shown to complex K⁺Cl^- ion pairs in which crown ether complexation of the potassium cation cooperates in the binding of chloride anion.

Full text of DOI:10.1039/a707680k

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Cooperative binding of potassium cation and chloride anion by novel
rhenium(i) bipyridyl amide crown ether receptors
James E. Redman,a Paul D. Beer,*a Simon W. Denta and Michael G. B. Drewb
a Inorganic Chemistry Laboratory, University of Oxford, South Parks Road, Oxford, UK OX1 3QR
b Department of Chemistry, University of Reading, Reading, UK RG6 6AD
New rhenium(I) bipyridyl amide crown ether receptors are
prepared and shown to complex K+Cl2 ion pairs in which
crown ether complexation of the potassium cation co-
operates in the binding of chloride anion.
crown ether bound potassium cation and bipyridyl amide
complexed chloride anion.
Receptor L3 was co-crystallized with KCl and the X-ray
crystal structure of the ion-pair complex [L3KCl]2·2H2O which
has a crystallographic centre of symmetry is shown in Fig. 1 and
2.†‡
Ion pair recognition, the simultaneous binding of cationic and
anionic guest species by ditopic receptors is a new emerging
field of coordination chemistry.1–5 These multisite ligand
systems as well as being potential selective extraction/mem-
brane transportation reagents for metal salts can also be
designed to exhibit allosteric and cooperative behaviour
whereby the binding of one charged guest can influence the
subsequent coordination of the pairing ion. We have recently
shown that charged or neutral transition metal organometallic
and coordination amide containing receptors can selectively
bind and sense anionic guest species.6 The covalent attachment
of benzo crown ether metal cation recognition sites to these
anion receptors will create novel ion pair binding reagents. We
report here the synthesis and preliminary coordination chem-
istry of new neutral rhenium(i) bipyridyl amide crown ether
receptors that complex potassium cations and simultaneously
bind chloride anions with positive cooperativity which is a first
step towards the design of ion pair recognition systems.
O
O
O
O
O
O
Cl
H2N
+
NHBoc
i
NEt3
O
ii TFA
O
O
O
O
H2N
O
O
O
O
N
H
Cl
Cl
N
N
0.5 equiv.
O
O
1
Cl
O
O
O
N
N
O
O
O
O
N
H
N
H
The bipyridyl amide functionalised benzo-18-crown-6 and
3,4-dimethoxyphenyl ligands were prepared in good yields
according to Scheme 1. Reaction of each of these bipyridyl
ligands with [Re(CO)5Br] in THF produced the target receptors
L1–L5 as yellow solids (Scheme 2). The potassium cation and
chloride anion coordination properties of the receptors were
N
O
O
R
O
O
O
O
N
2
H
N
H
N
O
O
O
1
investigated by H NMR titration experiments in (CD3)2SO
O
O
solution. The addition of KPF6 to 1H NMR solutions of L2, L3
and L5 caused the crown ether methylene protons to sig-
nificantly shift to lower field. Analysis of the resulting titration
curves with the computer program EQNMR7 suggested the
respective crown ether recognition site of each receptor to form
a 1:1 complex with the potassium cation, with stability constant
values of ca. 500 dm3 mol21 for L3 and L5. Negligible shifts
were observed with L1 and L4 suggesting K+ complexation only
takes place at the crown ether binding sites. The addition of
tetrabutylammonium chloride to (CD3)2SO solutions of L1–L5
caused significant downfield perturbations of the respective
receptor’s amide and H3-bipyridyl protons by up to Dd = 0.55
ppm, indicating anion binding is taking place exclusively in the
amide-bipyridyl vicinity of the receptor. In all cases the titration
curves suggested 1:1 complex stoichiometry. The titrations
with chloride were repeated in the presence of 2 equiv. of KPF6
and EQNMR7 determined stability constant values for both
titration experiments are presented in Table 1. Table 1 clearly
shows the addition of KPF6 to the crown ether containing
receptors L2, L3 and L5 leads to a substantial increase in the
magnitude of stability constant, by nearly threefold for L2, for
the respective chloride anion receptor-complex. It is noteworthy
that receptors L1, L4, lacking crown ether moieties, do not
exhibit such an increase in stability constant in the presence of
KPF6 which suggests the origin of positive co-operativity is an
intramolecular electrostatic attraction between a simultaneously
O
O
O
O
N
H
N
H
N
N
O
O
R = Me
R = CO2Et
R
O
O
NHBoc
OMe
i
N
H
N
H
O
H2N
ii TFA
N
OMe
OMe
OMe
Cl
iii 0.5 equiv. 1
OMe
OMe
N
H
N
H
N
O
O
NHBoc
i
H2N
ii TFA
iii
2
O
O
OMe
OMe
N
H
N
H
N
N
R = CO2Et
R
Scheme 1
Chem. Commun., 1998
231

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O
O
O(200)
C(200)
OMe
OMe
N
H
N
H
N
N
OC
OC
OC
Re
Br
Cl(1)
Re
O(100)
C(100)
OMe
OMe
N(22)
H
N
H
N
O(300)
C(300)
N(11)
L1
O
O
O(93)
O(90)
O
O
O
O(87)
O(84)
O(900)
K(1)
O
O
O
O
N
H
N
H
O(62S1)
N
N
OC
O
O
OC
OC
O(96)
Re
N(63)
Cl(21)
Br
O
O
O
O
O(81)
Cl(22)
H
N
H
N
N(66)
O
O
O
O
L2
O(68)
O
O
O
O
O
N
H
N
H
N
N
O
O
OC
OC
OC
Re
Br
R = Me
R = CO2Et L5
L3
R
Fig. 1 The centrosymmetric dimer [L3KCl]2·2H2O with ellipsoids shown at
20% probability together with the atomic numbering scheme. Bond lengths
from the potassium to crown oxygens are to O(81) 2.752(13), O(84)
2.913(15), O(87) 2.76(2), O(90) 2.801(16), O(93) 2.881(13), O(96)
2.722(14) Å and to the amide oxygen O(62S1) 2.654(17) and to the water
molecule O(900) 2.753(17) Å.
O
O
OMe
OMe
N
H
N
H
N
OC
OC
OC
Re
Br
N
CO2Et
L4
Scheme 2
Table 1 Stability constants for chloride anion binding in the presence and
absence of potassium cations in (CD3)2SO
Receptor
Ka/dm3 mol21
L1
44
46
48
120
24
57
L1 + KPF6 (2 equiv.)
L2
Fig. 2 The centrosymmetric dimer [L3KCl]2·2H2O with ellipsoids shown at
20% probability
L2 + KPF6 (2 equiv.)
L3
L3 + KPF6 (2 equiv.)
College Swansea. The University of Reading and EPSRC are
gratefully acknowledged for funding towards the crystallo-
graphic image-plate system.
L4
15
L4 + KPF6 (2 equiv.)
L5
15
b
—
L5 + KPF6 (2 equiv.)
35
Footnotes and References
a
b
Errors estimated to be @5%. Very weak binding, a stability constant
* E-mail: paul.beer@icl.ox.ac.uk
could not be determined.
† Crystal data: C68H76Cl4K2N8O24Re2, M = 1981.76, triclinic, space group
P1, Z = 1, a = 11.445(12), b = 14.668(17), c = 16.328(17) Å, a
= 115.27(1), b = 90.02(1), g = 112.34(1)°, U = 2249 Å3, Dc = 1.463 Mg
m23, m = 29.69 mm21, F(000) = 988, 7037 independent reflections, 1191
with I > 2s(I) refined on F2 to R of 0.0808 for data with I > 2s(I). CCDC
182/697.
In the Re(CO)3(bipy) moieties, the environment of the metal
is octahedral with dimensions in the expected range. Each bipy
is linked via an amide group to a benzo-18-crown-6 in which is
encapsulated a potassium ion. The benzo crown has a planar
conformation with the six oxygens coplanar to within 0.10 Å
with the potassium 0.24 Å from the plane. The hexagonal
bipyramidal potassium coordination is completed by two axial
bonds to a water molecule and to an amide oxygen atom from
another substituted Re(CO)3(bipy) moiety, thus forming the
centrosymmetric dimer. The chloride anion is disordered over
two positions 1.07 Å apart with relative occupancies 56 and
44%, but each anion is hydrogen bonded to both amide nitrogen
atoms, Cl(21) to N(63) at 3.25 and N(66) at 3.41 Å and Cl(22)
to N(63) 3.12 and N(66) at 3.29 Å.
‡ A substitution of Br for Cl has occurred at the ReI centre. During the NMR
titrations no evidence of this reaction was ever seen over the course of the
experiment for ca. 2 h.
1 M. T. Reetz, C. M. Niemeyer and K. Harris, Angew. Chem., Int. Ed.
Engl., 1991, 30, 1472.
2 D. M. Rudkevich, Z. Brzozka, M. Palys, H. C. Visser, W. Verboom and
D. N. Reinhoudt, Angew. Chem., Int. Ed. Engl., 1994, 33, 467.
3 K. I. Kinnear, D. P. Mousley, E. Arafar and J. C. Lockhart, J. Chem. Soc.,
Dalton Trans., 1994, 3637.
4 P. D. Beer, M. G. B. Drew, R. J. Knubley and M. I. Ogden, J. Chem. Soc.,
Dalton Trans., 1995, 3117.
5 J. Scheerder, J. P. M. van Duynhoven, J. F. J. Engbersen and
D. N. Reinhoudt, Angew. Chem., Int. Ed. Engl., 1996, 35, 1090.
6 P. D. Beer, Chem. Commun., 1996, 689 and references therein.
7 M. J. Hynes, J. Chem. Soc., Dalton Trans., 1993, 311.
In summary, these new rhenium(i) bipyridyl amide crown
ether receptors are capable of binding K+Cl2 ion pairs in which
crown ether complexation of the potassium cation co-operates
in the complexation of chloride anion.
We thank Kodak Ltd for a studentship (S. W. D.) and the
EPSRC for use of the mass spectrometry service at University
Received in Cambridge, UK, 24th October 1997; 7/07680K
232
Chem. Commun., 1998