Potassium cation induced switch in anion selectivity exhibited by heteroditopic ruthenium(II) and rhenium(I) bipyridyl bis(benzo-15-crown-5) ion pair receptors

Article abstract of DOI:10.1039/a800356d

Cl^- and H₂PO₄^- anion selectivity properties of new heteroditopic Ru^II and Re^I bipyridyl bis(benzo-15-crown-5) receptors are remarkably dependent upon the presence of co-bound intramolecular sandwich crown ether complexed K⁺.

Full text of DOI:10.1039/a800356d

View Article Online / Journal Homepage / Table of Contents for this issue
Potassium cation induced switch in anion selectivity exhibited by heteroditopic
ruthenium(ii) and rhenium(i) bipyridyl bis(benzo-15-crown-5) ion pair
receptors
Paul D. Beer*† and Simon W. Dent
Inorganic Chemistry Laboratory, University of Oxford, South Parks Road, Oxford, UK OX1 3QR
2
Cl2 and H2PO4 anion selectivity properties of new
heteroditopic RuII and ReI bipyridyl bis(benzo-15-crown-5)
receptors are remarkably dependent upon the presence of
co-bound intramolecular sandwich crown ether complexed
K+.
over Cl2. It is noteworthy that Table 1 explicitly shows that the
bis crown ether containing receptors L1 and L3 exhibit
substantial increases in the magnitudes of stability constant for
Cl2 binding on addition of KPF6 by nearly sixfold for L3.
Interestingly no such increase in stability constant value is
displayed by receptors L2 and L4 which do not contain crown
ether binding sites. In contrast Table 2 displays a dramatic
decrease in the stability constant values for H2PO42 binding for
L1 and L3 on addition of KPF6. The important consequence of
which is the presence of K+ can in principle induce a novel
switch in the anion selectivity properties of both receptors.
Tables 1 and 2 clearly illustrate the free receptors L1, L3 are
The design of new multisite ion pair receptors that contain
covalently linked binding sites for both cations and anions is a
new topical area of coordination chemistry.1–6 In addition to
being potential selective extraction/membrane transportation
reagents for metal salts these ditopic ligand systems can be
tailored to exhibit novel cooperative and allosteric behaviour
whereby the complexation of one charged guest can influence,
through electrostatic and conformational effects, the subsequent
coordination of the pairing ion. We report here the synthesis of
new heteroditopic RuII and ReI bipyridyl bis(benzo-15-crown-
5) receptors whose selectivity towards Cl2 and H2PO42 anionic
guest species is remarkably dependent upon allosteric con-
formational and electrostatic effects of co-bound intramolecular
sandwich crown ether complexed K+ cations.
2
H2PO4 selective whereas initial coordination of K+ changes
their anion selectivity preference to Cl2. Repeating the
titrations in the presence of tenfold excess of NBu4PF6 resulted
in virtually no change in the determined stability constant
values which negates simple ion pairing as a possible explana-
tion for these positive and negative cooperative binding
effects.
The synthetic routes to preparing the bipyridyl amide
functionalised mono- and bis(benzo-15-crown-5) ligands and
3,4A-dimethoxyphenyl appended analogues are shown in
Scheme 1. Reaction with [RuCl2(bipy)2] followed by NH4PF6
and with [Re(CO)5Br], respectively, afforded the new receptors
L1–L4 in good yields (Scheme 2). The K+ cation and both Cl2
and H2PO42 anion coordination properties of the receptors were
investigated by 1H and in some instances by 13C NMR titration
experiments in (CD3)2SO solution. The addition of KPF6 to
solutions of L1 and L3 caused the crown ether methylene
carbons of the 13C NMR spectra to significantly shift upfield by
up to 0.6 ppm. Analysis of the resulting titration curves using
the computer program EQNMR7 suggested both receptors were
forming 1:1 stoichiometric intramolecular sandwich bis crown
ether complexes with K+ with respective stability constant
values of 350 and 540 m21 for L1 and L3. The relatively smaller
value of stability constant for L1 may reflect the positively
charged RuII centre electrostatically destabilising the crown
ether bound K+. Negligible shifts were observed with L2 and L4
suggesting K+ complexation only takes place at the crown ether
binding sites. Significant downfield perturbations of the
respective receptor’s amide and H3-bipyridyl protons were
0.5 equiv.
1
2
observed on the addition of NBu4Cl and H2PO4 salts to
(CD3)2SO solutions of L1–L4 indicating the amide-bipyridyl
vicinity of the receptor is the site of anion binding. In all cases
with Cl2 the titration curves suggested 1:1 complex stoichio-
H2N
i,
NHBoc
iii, 0.5 equiv. 1
ii, TFA
2
metry which was also observed with H2PO4 and L1, L3.
Precipitation problems unfortunately prevented complete
H2PO42 titration curves from being obtained with receptors L2
and L4. Where possible titrations with both anions were
repeated in the presence of 2 equiv. of KPF6 and EQNMR
determined stability constant values for all these titration
experiments are presented in Tables 1 and 2. As found
previously with related simple acyclic transition metal bipyridyl
amide derivatives8 a comparison of Tables 1 and 2 reveals L1
2
and L3 exhibit a pronounced selectivity preference for H2PO4
Scheme 1
Chem. Commun., 1998
825

View Article Online
(PF6–)4•4H2O
L5
Table 2 Stability constants for H2PO42 binding in the presence and absence
of K+ in (CD3)2SO
Receptora
Kb/m21
L1
900
60
205
35
L1 + 2 equiv. KPF6
L3
L3 + 2 equiv. KPF6
a
Precipitation problems prevented complete titration curves from being
obtained with L2 and L4 in presence and absence of KPF6. b Errors estimated
to be @10%.
K+
M = (bipy)2Ru2+, Re(CO)3Br
Fig. 1 Proposed solution structure of K+Cl2 ion pair complex of L1 and L3.
M = Ru(bipy)22+ or Re(CO)3Br
chromic shift of 7 nm. However in the presence of 1 equiv. of
KPF6 the subsequent addition of H2PO42 had very little effect
on either the emission intensity or lmax. In contrast with Cl2
a
Scheme 2
significant enhancement of intensity was observed.
In summary the anion selectivity properties of new RuII and
ReI bipyridyl bis(benzo-15-crown-5) receptors can be dramat-
ically switched via the binding of K+ cations. In the absence of
Table 1 Stability constants for Cl2 binding in the presence and absence of
K+ in (CD3)2SO
2
K+ the receptors are selective for H2PO4 over Cl2, whereas
Receptor
Ka/m21
following the formation of the intramolecular K+ bis crown
2
sandwich complex the reverse selectivity Cl2 over H2PO4 is
L1
190
660
195
165
55
300
46
55
L1 + 2 equiv. KPF6
exhibited.
L2
We thank Kodak Ltd for a studentship (S. W. D.), the EPSRC
for use of the mass spectrometry service at University College
Swansea and Johnson Matthey for a generous loan of ruthenium
salts.
L2 + 2 equiv. KPF6
L3
L3 + 2 equiv. KPF6
L4
L4 + 2 equiv. KPF6
Notes and References
a Errors estimated to be @10%.
† E-mail: paul.beer@chem.ox.ac.uk
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 J. E. Redman, P. D. Beer, S. W. Dent and M. G. B. Drew, Chem.
Commun., 1998, 231.
7 M. J. Hynes, J. Chem. Soc., Dalton Trans., 1993, 311.
8 F. Szemes, D. Hesek, Z. Chen, S. W. Dent, M. G. B. Drew, A. J. Goulden,
A. R. Graydon, A. Grieve, R. J. Mortimer, J. S. Weightman and
P. D. Beer, Inorg. Chem., 1996, 35, 5868.
9 P. D. Beer, F. Szemes, V. Balzani, C. M. Sala`, M. G. B. Drew, S. W. Dent
and M. Maestri, J. Am. Chem. Soc., 1997, 119, 11 864.
The K+ induced positive cooperativity of Cl2 binding may be
a consequence of favourable electrostatic attraction between the
crown ether bound K+ and bipyridyl amide complexed anion,
and, in addition be due to a favourable allosteric K+ induced
conformational effect. Solution formation of the 1:1 bis-benzo-
15-crown-5 potassium cation intramolecular sandwich complex
results in L1 and L3 forming a pseudo-macrocyclic preorganised
structure which enhances Cl2 recognition but disfavours the
binding of H2PO42 (Fig. 1). Of particular note is the structurally
related macrocyclic receptor L5 which exhibits remarkable
thermodynamic stability and selectivity for Cl2 and does not
bind H2PO42 phosphate at all in (CD3)2SO solutions.9
Preliminary fluorescence emission spectroscopic measure-
ments corroborate the NMR binding studies. The addition of
2
Cl2 and H2PO4 to MeCN solutions of L1 caused significant
increases of up to 140% in luminescence intensity of the MLCT
emission band (lmax = 640 nm) with a concomitant hypso-
Received in Cambridge, UK, 13th January 1998; 8/00356D
826
Chem. Commun., 1998