Novel anion binding selectivity trends exhibited by new dinuclear rhenium(I), ruthenium(II) and osmium(II) bipyridyl cleft-type receptors

Article abstract of DOI:10.1039/a607086h

Novel dinuclear rhenium(I), ruthenium(II) and osmium(II) bipyridyl cleft-type receptor systems exhibit remarkable selectivity for dihydrogenphosphate over halide anions dictated by the nature of the bridging linkage and the transition-metal Lewis-acidic centre.

Full text of DOI:10.1039/a607086h

View Article Online / Journal Homepage / Table of Contents for this issue
Novel anion binding selectivity trends exhibited by new dinuclear rhenium(i),
ruthenium(ii) and osmium(ii) bipyridyl cleft-type receptors
Paul D. Beer,*a† Simon W. Dent,a Gerald S. Hobbsa and Trevor J. Wearb
a Inorganic Chemistry Laboratory, South Parks Road, Oxford, UK OX1 3QR
b Kodak Ltd, Headstone Drive, Harrow, Middlesex, UK HA1 4TY
Novel dinuclear rhenium(I), ruthenium(II) and osmium(II)
bipyridyl cleft-type receptor systems exhibit remarkable
selectivity for dihydrogenphosphate over halide anions
dictated by the nature of the bridging linkage and the
transition-metal Lewis-acidic centre.
1 equiv. of diamine in dichloromethane solution afforded the
new ‘cleft’-type bipyridyl (bpy) ligands 3–6 in excellent yields
(Scheme 1). The free ligands were chelated to the positively
charged [M(bpy)2]2+ fragment (M = Ru, Os) unit by reaction
with either 2 equiv. of cis-[Ru(bpy)2Cl2]·2H2O14 or cis-
[Os(bpy)2Cl2]15 in ethanol–water–acetic acid. The mixed
ruthenium–osmium receptor, Ru–6b–Os, was prepared by
reaction of ligand 6b with 1 equiv. of cis-[Ru(bpy)2Cl2]·2H2O,
followed by 1 equiv. of cis-[Os(bpy)2Cl2]. The crude products
were purified by column chromatography on Sephadex LH-20
and converted to the hexafluorophosphate salts with an excess
of aqueous NH4PF6 (Scheme 2). The rhenium(i) receptor, Re–
6b–Re, was prepared by reaction of the free ligand 6b with 2
equiv. of [Re(CO)5Br] in thf and purified by recrystallisation
from acetone–diethyl ether (Scheme 3).‡
Anions are of crucial importance in many essential chemical
and biochemical processes and therefore their binding and
recognition is presently the subject of intensive research.1 To
date, a number of research groups have designed and syn-
thesised anion binding reagents that use either favourable
hydrogen-bonding interactions alone,2,3 or hydrogen-bonding
interactions in conjunction with electrostatic forces4–6 to
coordinate and, in some instances, sense anions. Examples of
the latter which have been particularly successful in this respect
are amide containing cobaltocenium7,8 and ruthenium(ii) poly-
pyridyl molecules.9–11 In an effort to enhance selective anion
recognition through the design of new cooperative polymetallic
Lewis-acidic systems, we report here the synthesis of novel
dinuclear transition-metal bipyridyl, cleft-type receptors whose
remarkable selectivity for the dihydrogenphosphate anion over
halide anions is profoundly influenced by the nature of the
bridging linkage, and the electronic character of the transition-
metal Lewis-acidic centre.
2
Proton NMR titration experiments with Cl2 and H2PO4
anions were carried out in (CD3)2SO solution and revealed
significant downfield shifts in the receptors’ amide and 3,3A-bpy
protons. For example, the amide protons of Ru–6b–Ru were
perturbed by 0.93 ppm and the 3,3A-bpy protons by 0.84 ppm
after the addition of an excess of tetrabutylammonium dihy-
drogenphosphate. In all cases the resulting titration curves (for
example, Fig. 1) suggest complexes with a 1:1 receptor:anion
stoichiometry are formed in solution. Stability constants were
calculated from the titration data using the EQNMR computer
program16 and the values are presented in Table 1.§
The reaction of 4-carboxy-4A-methyl-2,2A-bipyridine12 or
4-carbonyl-4A-ethoxycarbonyl-2,2A-bipyridine13 with thionyl
chloride afforded the acid chlorides 1 and 2 respectively in
quantitative yields. The condensation of 2 equiv. of 1 or 2 with
Clearly the nature of the bridging linkage has a dramatic
influence on the anion binding strength and selectivity these
dinuclear cleft-type receptors display. Of particular note is the
O
Cl
O
O
N
R
N
H
N
H
R
2
+
H2N
NH2
N
N
N
N
N
X
1 X = Me
X
X
2 X = CO2Et
NEt3
CH2Cl2 or thf
(1) [Ru(bpy)2Cl2]•2H2O
or
O
O
[Os(bpy)2Cl2]
R
(2) NH4PF6
N
H
N
H
N
N
N
N
O
O
N
R
N
N
N
H
H
N
N
N
N
N
N
N
X
X
M2+
M2+
N
N
N
R =
X
X
–
]
[PF6
4
3 X = CO2Et
4 X = CO2Et
5 X = CO2Et 6a X = CO2Et
b X = Me
M = Ru or Os
Scheme 1
Scheme 2
Chem. Commun., 1997
99

View Article Online
2
remarkable increase in the magnitude of stability constant of
with H2PO4 and is Cl2 selective, the 2,2A-dimethyl-1,3-
propane-bridged analogue, Ru–6a–Ru forms an extremely
strong and selective complex with H2PO42. Comparing Ru–6a–
Ru and Ru–6b–Ru, it is also of interest to note there is a
significant 4A-bipyridyl substituent effect, with H2PO42 anions
favouring the ester containing receptor and chloride the methyl
containing one.
2
H2PO4 that results from simply altering the linkage from m-
phenylene Ru–3–Ru (K = 55 m21) to p-phenylene Ru–4–Ru
(K = 4320 m21), with the halide stability constant values
remaining virtually unchanged. In addition, whereas the ethyl-
bridged receptor Ru–5–Ru forms an extremely weak complex
Table 1 also demonstrates that the nature of the bipyridyl-
coordinated transition-metal centre strikingly dictates the
strength and degree of anion recognition selectivity. With both
O
O
N
H
N
H
2
H2PO4 and Cl2 anionic guest species the stability constants
N
N
N
N
increase in magnitude in the order Re–6b–Re < Ru–6b–Ru <
Ru–6b–Os < Os–6b–Os. From electrostatic considerations it is
perhaps not surprising the neutral bis-rhenium(i) receptor forms
the relatively weaker anion complexes. However, what is
unexpected is the dramatic increase in magnitude of stability
constant for the osmium-containing receptors, especially in
respect to H2PO42 complexation. This observation suggests the
(bpy)2Os2+ moiety is a particularly efficient Lewis-acidic centre
to incorporate into this new cleft-type class of transition-metal
bipyridyl amide anion receptor.
Me
O
Me
[Re(CO)5Br]
thf
O
Br
Br
N
H
N
H
We thank Kodak Ltd for a studentship (S. W. D.) and Johnson
Matthey for the generous loan of ruthenium and osmium
salts.
OC
OC
N
N
N
N
CO
CO
Re
Re
Footnotes
CO
CO
Me
Me
† E-mail: beer%server.icl.@ox.ac.uk
http://www.ncl.ox.ac.uk/icl/beer.html
‡ All new compounds were fully characterised by elemental analysis, 1H
and 13C NMR and FABMS.
Scheme 3
bpy-3 proton
¶ The addition of chloride or dihydrogenphosphate anions to (CD3)2SO
solutions of [M(bpy)3)][PF6]2 (M = Ru, Os) had no effect on the 1H NMR
spectrum. The metal-free bipyridyl ligands exhibited extremely weak and
insignificant interactions with both chloride and dihydrogenphosphate
anions in (CD3)2SO.
10.5
10.0
9.5
amide proton
bpy-3′ proton
References
1 B. Dietrich, Pure Appl. Chem., 1993, 65, 1457.
2 A. M. Kelly-Rowley, V. M. Lynch and E. V. Ansyln, J. Am. Chem. Soc.,
1995, 117, 3438.
3 P. A. Gale, J. L. Sessler, V. Kra´l and V. Lynch, J. Am. Chem. Soc., 1996,
118, 5140.
9.0
8.5
4 P. D. Beer, C. A. Dickson, N. C. Fletcher, A. J. Goulden, A. Grieve,
J. Hodacova and T. J. Wear, J. Chem. Soc., Chem. Commun., 1993,
828.
8.0
5 V. Kra´l, A. Andrievsky and J. L. Sessler, J. Am. Chem. Soc., 1995, 117,
2953.
6 N. Ohata, H. Masuda and O. Yamauchi, Angew. Chem., Int. Ed. Engl.,
1996, 35, 531.
7 P. D. Beer, C. Hazelwood, D. Hesek, J. Hodacova and S. E. Stokes,
J. Chem. Soc., Dalton Trans., 1993, 1327.
7.5
7.0
5
0
1
2
3
4
6
–
Molar equivalents of H PO
2
4
8 P. D. Beer, M. G. B. Drew, A. R. Graydon, D. K. Smith and S. E. Stokes,
J. Chem. Soc., Dalton Trans., 1995, 403.
9 P. D. Beer, S. W. Dent and T. J. Wear, J. Chem. Soc., Dalton Trans.,
1996, 2341.
10 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.
11 M. S. Goodman, V. Jubian and A. D. Hamilton, Tetrahedron Lett., 1995,
36, 2551.
Fig. 1 1H NMR titration curve of Ru–6b–Ru with NBu4H2PO4 in
(CD3)2SO
Table 1 Stability constant data for dinuclear cleft-anion receptors in
(CD3)2SO
2 a,b
K (Cl2)a,b
m21
/
K (Br2)a,b
m21
/
K (H2PO4 ) /
Receptor
m21
12 D. G. McCafferty, B. M. Bishop, C. G. Wall, S. G. Hughes,
S. L. Mecklenberg, T. J. Meyer and B. W. Erickson, Tetrahedron, 1995,
51, 1093.
13 G. Ve´riot, J.-P. Dutasta, G. Matouzenko and A. Collet, Tetrahedron,
1995, 51, 389.
14 B. P. Sullivan, D. J. Salmon and T. J. Meyer, Inorg. Chem., 1978, 17,
3334.
15 P. A. Lay, A. M. Sargeson and H. Taube, Inorg. Synth., 1986, 24,
291.
Ru–3–Ru
Ru–4–Ru
Ru–5–Ru
Ru–6a–Ru
Re–6b–Re
Ru–6b–Ru
Ru–6b–Os
Os–6b–Os
25
55
70
245
120
310
440
825
45
40
60
55
4320
10
19700
1820
15480
22150
> 30000
170
c
—
220
c
—
—
c
16 M. J. Hynes, J. Chem. Soc., Dalton Trans., 1993, 311.
a Average for amide and bpy protons. b Errors estimated to be < 10%. c Not
calculated.
Received, 17th October 1996; Com. 6/07086H
100
Chem. Commun., 1997