A modular, self-assembled, separated ion pair binding system

Article abstract of DOI:10.1039/b402882a

Assembly of a silver(I) complex of a simple pyridyl ligand containing a urea derivative is templated by nitrate; analogous complexes of Ag ₂SO₄ and AgCF₃SO₃ exhibit radically different geometries.

Full text of DOI:10.1039/b402882a

A modular, self-assembled, separated ion pair binding system
a
a
a
b
David R. Turner, Elinor C. Spencer, Judith A. K. Howard, Derek A. Tocher and Jonathan W.
Steed*
a
a
Department of Chemistry, University of Durham, South Road, Durham, UK DH1 3LE.
E-mail: jon.steed@durham.ac.uk; Fax: +44 (0)191 384 4737; Tel: +44 (0)191 334 2085
Department of Chemistry, University College London, 20 Gordon Street, London, UK WC1H 0AJ
b
Received (in Columbia, MO, USA) 24th February 2004, Accepted 4th May 2004
First published as an Advance Article on the web 25th May 2004
Assembly of a silver(
containing a urea derivative is templated by nitrate; analogous
I
) complex of a simple pyridyl ligand
different metal centres. Thus in complex 3 triflate anions bridge
+
2
2
between pairs of Ag(1a)
motifs, Fig. 1a. In contrast, only the non-bifurcated R (8) motif is
2
2 1
cations exhibiting both R (8) and R (6)
2
complexes of Ag
geometries.
2 4 3 3
SO and AgCF SO exhibit radically different
2
2
2
observed for 4 with the double negative charge of the SO
4
anion
necessitating the surrounding of the anion by four urea groups
instead of two, Fig. 1b.
A holistic view of ion-binding systems requires a full consideration
of the overall charge neutrality requirement. A neutral ion receptor
must contain a priori binding sites for both anion and cation either
as a contact or separated ion pair, as exemplified by the elegant
In contrast, the nitrate complex 2 displays a much more
interesting, discrete 1 : 1 assembly in which the nitrate anion is
asymmetrically chelated by two ligands attached to the same metal
centre, Fig. 2, forming a highly complementary cavity. The metal is
approximately trigonal pyramidal with the third coordination site
occupied by methanol. The deviation from linearity of the Ag…O–
N vector is extremely interesting. Exact linearity would give two
1
,2
receptors designed by Smith and Atwood. Charged receptors
effectively replace, and compete with counter-ions, usually form-
3
,4
ing new contact ion paired host–guest systems. In both cases the
host is generally a covalently bonded organic molecule assembled
under kinetically controlled conditions in the absence of the target
2
2
12
R (8) motifs as observed intermolecularly for 4. In contrast, the
5
6
guest. Recently, work by both Gale and Loeb and ourselves has
highlighted the preparation of coordination complexes in which a
relatively inert metal cation such as Pt(II) or Ru(II) acts as a core
linking together two or more anion-binding arms bearing hydrogen
bonding functionality. These ‘semi-labile’ coordination complex
host systems are conceptually analogous to organic podands in
which the core is an arene ring, for example, and indeed we have
applied the same pyridyl-terminated arms in both contexts.7
Taking the inorganic core concept to extremes, there is no reason
why the same species may not be formed using a highly labile metal
ion in a fully thermodynamically controlled self-assembly regime.9
Thus it should be conceptually possible to replace the thermody-
namic equilibrium involving association of a covalent host and a
guest ion or ion pair with a multi-component templated thermody-
namic self-assembly in which the guest anion and cation play an
intimate part by templating the overall assembly. Such assemblies
might potentially prove more effective sensors for particular salts
than their traditional covalent counterparts since changes in colour
or electrochemical or photochemical properties should result from
different self-assemblies.
rotation away from 180° (155°, 156° or 165° in three independent
molecules) results in formation of the two motifs observed for 3 and
allows the formation of an extra CH…O interaction. The crystal
1
3
packing in 2 is also fascinating with 3.5 independent molecules.
The extra half is situated on a two-fold rotation axis and hence
exhibits an apparently linear Ag…O–N vector since all atoms lie on
the rotation axis. In fact the anisotropic displacement ellipsoids
indicate that this apparent symmetry is a disorder average of two
orientations that are similar to those observed for the other three
,8
2 3
independent molecules. The whole [Ag(1a) ](NO )·MeOH in 2 is
almost exactly planar. The solid state structure comprises discrete,
p-stacked seven-molecule aggregates (Fig. 3) held together by
OH…O interactions from the coordinated methanol to the urea
carbonyl and, in the centre, long Ag–O bonds to urea carbonyls
(2.53 Å). The heptamer is terminated by CH…O interactions.
Complexes 3 and 4 also exhibit remarkable crystal packing
arrangements. Complex 3 exists as discrete, helical trimers linked
In order to test this concept we have designed and prepared a
series of simple pyridyl–urea ligands (R = p-tolyl 1a, n-octyl 1b)
readily synthesised in 90–95% yields from 3-aminopyridine and the
appropriate isocyanate. The pyridine nitrogen atoms are capable of
binding a metal cation while the urea groups are known to bind
strongly to anions.1
0,11
Reaction of 1a with labile Ag( ) as the
I
nitrate, triflate and sulfate salts in methanol–water (50 : 50) results
in the isolation of a series of complexes of 1 : 2 stoichiometry,
2
Fig. 1 Infinite solid state arrays mediated by hydrogen bonding (a) R (8)
2
2
namely [Ag(1a)
2
](NO
3
)·MeOH 2, [Ag(1a)
2
](CF
3
SO
3
)·0.5H
2
O 3
and R (6) (bifurcated acceptor) in complex 3 and (b) non-bifurcated
1
and [Ag(1a) (SO
2
]
2
4
) 4, all of which have been characterised
interactions in 4.
crystallographically.†It is to be expected from Etter’s rules that the
urea NH donors should interact with the anions in each case since
the anion oxygen atoms are the strongest hydrogen bond acceptors.
In the case of complexes 3 and 4 these interactions result in infinite
hydrogen bonded assemblies in the solid state in which each anion
is surrounded by an array of two or four urea groups (in the case of
Fig. 2 The discrete [Ag(1a) ](NO )·MeOH in 2 showing the slight tilt of the
2
3
3
and 4 respectively) all of which come from ligands attached to
nitrate anion.
1
352
C h e m . C o m m u n . , 2 0 0 4 , 1 3 5 2 – 1 3 5 3
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4

Ag(CF
indicating 1 : 2 stoichiometry ([Ag(1a)
data give nitrate binding constants, K11, of 17000 M and K12 of
3
SO
3
). The Job plot reaches a clear maximum at 0.33
2
] : NO ), while the titration
3
2
1
2
1
1
660 M for Ag(1a)
2
in the presence of triflate. Thus, while
[
Ag(1a) ](NO ) is extremely stable, in the presence of excess
2
3
nitrate there is gradual formation of an additional 1 : 2 complex in
which each urea group interacts with a different nitrate anion.
Fig. 3 One of the discrete heptameric assemblies in 2.
+
In summary, the simple highly labile Ag –1a system is
2
templated in both the solid state and solution by NO
3
to give a
by long range Ag–O and CH…O interactions, while 4 possesses
discrete host–guest assembly. Non-complementary anions such as
linear, polar chains in which the Ag(1a)
2
units interdigitate, Fig.
2
22
CF
3
SO
3
and SO
4
do not form analogous species. This result, in
4.
a labile, thermodynamic self-assembly regime highlights the
Infinite assemblies such as found in 3 and 4 are not expected to
modularity7
,8,14–16
of hydrogen bonding pyridine based systems.
persist in solution, and indeed all species are distinctly insoluble
1
The same ligand 1a may also be used in a covalent anion binding
host.17
We thank the EPSRC for studentships (DRT and ECS) and for a
senior research fellowship (JAKH).
once crystalline. However, a series of H NMR experiments were
carried out on supersaturated samples (which are relatively slow to
nucleate) in order to probe the possibility of the discrete assembly
1
in 2 forming and persisting in solution. Fig. 5 shows the H NMR
spectrum of ligand 1a in acetone solution (A) alone, (B) following
addition of half an equivalent of NBu
addition of half an equivalent of Ag(CF SO
half an equivalent of NBu (NO ) to a solution of 1a +
as in ‘C’. The spectra clearly indicate that in the presence of nitrate
4
(NO
3
), (C) following
3
3
) and (D) addition of
Notes and references
1
2
4
3
3 3
Ag(CF SO )
†
Crystal data: for compound 2: C27
7 6
H30AgN O , M = 656.45, monoclinic,
3
a = 49.675(4), b = 13.4895(10), c = 33.477(3) Å, U = 18996(3) Å , T =
+
+
2
or Ag and a non-coordinating counter ion (NBu
4
or CF
3
SO
3
)
21
1
20(2) K, space group C2/c (no. 15), Z = 28, m(Mo–Ka) = 0.799 mm ,
there is little change in the chemical shift of 1a. However, when
57083 reflections measured, 18601 unique (R = 0.0613) which were used
int
+
2
both Ag and NO
3
are present a very marked change is observed
in all calculations. The final R1 and wR2 were 0.0660 and 0.1593 [I >
with Dd 1.2 ppm for the NH protons of 1a. This result is consistent
with solution self-assembly of complex 2. Precipitation of 2 over
periods of ca. 10 min makes NMR titration a difficult task,
however, a titration curve and Job plot were obtained by addition of
3 6
2s(I)]. For compound 3: C27H26.67AgF N O5.33S, M = 717.47, triclinic, a
=
7.1774(5), b = 16.7930(15), c = 19.6054(18) Å, a = 107.586(3), b =
3
9
3.998(3), g = 91.066(4), U = 2245.2(3) Å , T = 120(2) K, space group
¯
21
P1 (no. 2), Z = 3, m(Mo–Ka) = 0.810 mm , 7417 reflections measured,
223 unique (Rint = 0.0531) which were used in all calculations. The final
R1 and wR2 were 0.0566 and 0.1255 [I > 2s(I)]. For compound 4:
5
4 3
NBu (NO ) to solutions of 1a containing one equivalent of
C
52
H
52Ag
2
N
12
O
8
S, M
=
1220.86, tetragonal, a
=
17.4146(6), c
=
3
¯
8
1
.2655(6) Å, U = 2506.7(2) Å , T = 150(2) K, space group P42
14), Z = 2, m(Mo–Ka) = 0.892mm , 21760 reflections measured, 3055
1
c (no.
21
unique (Rint = 0.0299) which were used in all calculations. The final R1 and
wR2 were 0.0307 and 0.0749 [I > 2s(I)]. CCDC 233238, 233239 and
2
38925. See http://www.rsc.org/suppdata/cc/b4/b402882a/ for crystallo-
graphic data in .cif or other electronic format.
1
2
3
4
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Fig. 5 H NMR spectrum of ligand 1a (A) alone, (B) following addition of
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half an equivalent of NBu
equivalent of Ag(CF SO ) and (D) addition of half an equivalent of
) to a solution of 1a +
4 3
(NO ), (C) following addition of half an
3
3
1
NBu
red.
4
(NO
3
2
3 3
Ag(CF SO ). NH peaks highlighted in
C h e m . C o m m u n . , 2 0 0 4 , 1 3 5 2 – 1 3 5 3
1353