Anion binding by Ag(I) complexes of urea-substituted pyridyl ligands

Article abstract of DOI:10.1039/b415818k

A series of Ag(I) complexes of ureidopyridyl ligands 1 and 2 have been prepared from oxo-anion salts. In all cases the new materials contain the AgL2⁺ cation interacting with oxo-anions via the urea moiety. The complexes containing the para ligand 2: [Ag(2)₂]CF₃SO ₃ · 2H₂O (3), [Ag(2)₂]CH ₃CO₂ · 1.33H₂O · MeOH (4) and [Ag(2)₂]NO₃ · H₂O (5), all exhibit remarkably similar chain-like structures based around a linear Ag(I) centre, despite the change in the counter-ion. A recurring R₂²(8) hydrogen-bonding ring motif between the urea group and the oxo-anion is observed in almost all cases. An exception to this trend is the anhydrous nitrate structure [Ag(2)]NO₃ (6) in which the nitrate is coordinated in a bridging position between two silver centres, which adopt distorted trigonal pyramidal geometries. Structures containing the ligand 1, [Ag(1) ₂]CF₃SO₃ · 0.5H₂O (7), [Ag(1)₂]CF₃SO₃ · H₂O · MeCN (8), [Ag(1)₂]₂SO₄ (9), [Ag(1) ₂]NO₃ · MeOH (10) and [Ag(1)₂]NO ₃ · 0.5MeOH · 0.5MeNO₂ (11), display very different geometries, although the R₂²(8) is observed to persist throughout. The most notable of these structures are 10 and 11 in which the nitrate anion is chelated within a 'pincer' arrangement by the silver complex. The nitrate anion is situated asymmetrically within the cavity of the host complex. This discrete nitrate complex persists in solution with strong nitrate binding by the [Ag(1)₂]⁺ host compared to other anions being observed.

Full text of DOI:10.1039/b415818k

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P A P E R
Anion binding by Ag(I) complexes of urea-substituted
pyridyl ligands
David R. Turner,a Benjamin Smith,a Elinor C. Spencer,a Andres E. Goeta,a Ivana
Radosavljevic Evans,a Derek A. Tocher,b Judith A. K. Howarda 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
b Department of Chemistry, University College London, 20 Gordon Street, London,
UK WC1H 0AJ
Received (in Montpellier, France) 12th October 2004, Accepted 10th November 2004
First published as an Advance Article on the web 9th December 2004
A series of Ag(I) complexes of ureidopyridyl ligands 1 and 2 have been prepared from oxo-anion salts. In all
+
cases the new materials contain the AgL2 cation interacting with oxo-anions via the urea moiety. The
complexes containing the para ligand 2: [Ag(2)2]CF3SO3 Á 2H2O (3), [Ag(2)2]CH3CO2 Á 1.33H2O Á MeOH (4)
and [Ag(2)2]NO3 Á H2O (5), all exhibit remarkably similar chain-like structures based around a linear Ag(I)
centre, despite the change in the counter-ion. A recurring R22(8) hydrogen-bonding ring motif between the
urea group and the oxo-anion is observed in almost all cases. An exception to this trend is the anhydrous
nitrate structure [Ag(2)]NO3 (6) in which the nitrate is coordinated in a bridging position between two silver
centres, which adopt distorted trigonal pyramidal geometries. Structures containing the ligand 1,
[Ag(1)2]CF3SO3 Á 0.5H2O (7), [Ag(1)2]CF3SO3 Á H2O Á MeCN (8), [Ag(1)2]2SO4 (9), [Ag(1)2]NO3 Á MeOH (10)
and [Ag(1)2]NO3 Á 0.5MeOH Á 0.5MeNO2 (11), display very different geometries, although the R22(8) is
observed to persist throughout. The most notable of these structures are 10 and 11 in which the nitrate
anion is chelated within a ‘pincer’ arrangement by the silver complex. The nitrate anion is situated
asymmetrically within the cavity of the host complex. This discrete nitrate complex persists in solution with
strong nitrate binding by the [Ag(1)2]+ host compared to other anions being observed.
offers the possibility of forming an R22(8) hydrogen-bonded
Introduction
ring (Fig. 1).10,14,15 A search of the Cambridge Structural
Database (CSD)16 reveals many structures involving urea in
this type of interaction with a variety of oxo-anions. The R22(8)
geometry of the interaction is expected from Etter’s rules since
the urea group and the oxo-anions represent, respectively, the
strongest hydrogen-bond donors and acceptors in the crystal.17
It is therefore expected that the combination of Ag(I) salts with
1 and 2 would produce systems in which the complex-anion
interaction occurs via such a motif.
The binding and detection of anionic species by synthetic
receptors is currently an area of significant interest.1–3 The
intrinsic challenges of anion binding, compared to cationic or
neutral guests, is compensated for by the relevance and benefits
of such systems in biological4 and environmental sensing5
applications. Although the majority of anion receptors have
been based around organic scaffolds, there has been a recent
and growing trend towards the use of metal-based assemblies
as host species. Hosts assembled around metal centres can be
either inert or covalent entities6 or can themselves be formed
under thermodynamic control in solution.7–10 Labile host
systems can be advantageous, in that the presence of certain
guest species may template the formation of the host and
stabilise the self-assembled complex.
We now present a simple, self-assembling host based around
a urea-substituted pyridyl ligand that shows remarkable shape
selectivity for nitrate in the solid state and in solution. We
contrast this discrete species with a variety of extended hydro-
gen-bonded polymers linked via hostÁ Á Áanion interactions. Part
of this work has previously been communicated.11
Ligands 1 and 2 were reacted with AgNO3, AgCF3SO3,
AgCH3CO2 and Ag2SO4, resulting in seven new materials.
Four species (Table 1) were obtained using ligand 2, namely
[Ag(2)2]CF3SO3 Á 2H2O
(3),
[Ag(2)2]CH3CO2 Á 1.33H2O Á
MeOH (4), [Ag(2)2]NO3 Á H2O (5) and [Ag(2)2]NO3 (6). The
new compounds were characterised by X-ray crystallography
while bulk composition was confirmed by elemental analysis.
The crystal structures of 3–5 all contain the [Ag(2)2]+ cation,
which adopts an essentially linear coordination geometry
around the Ag(I) centre. The N–Ag–N angle is slightly re-
moved from 1801 in all cases with the exact deviation depen-
dant upon the proximity of other potentially ligating solvent
groups. The linear geometry of the cationic complex gives rise
to chain-like motifs running through these three structures.
The structure of 3 involves a triflate anion interacting with
one of the urea groups via two of the sulfonate oxygen atoms in
an R22(8) motif (Fig. 2). The third oxygen atom receives a
hydrogen bond from one of the two enclathrated water mole-
cules within the structure. This water molecule itself receives
hydrogen bonds from the remaining urea group in an R21(6)
motif. The second water molecule is weakly coordinated to the
silver centre, resulting in a distortion from linearity of the
Results and discussion
Extended hydrogen-bonded systems
We have previously reported the ureidopyridyl ligands 1 and 2
in the context of solid-state networks involving hydrogen
bonds to metal-bound chloride.12 The ligand 2 has also been
used within systems displaying marked metal distortion due to
the strength of multiple hydrogen bonds.13 Urea is well-known
in its ability to interact well with oxo-anions as such a pairing
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 a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 5
90
N e w J . C h e m . , 2 0 0 5 , 2 9 , 9 0 – 9 8

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Fig. 2 The immediate environment around one [Ag(2)2]+ complex in
the structure of 3 with a weak silver-oxygen interaction, Ag(1)–O(7), of
2.685(4) A. Hydrogen bond data are shown in Table 2.
position where it bridges urea protons of one molecule and a
urea carbonyl oxygen on another. The water molecule also
interacts with the nitrate anion forming an R33(10) pattern
(Fig. 5).
A second nitrate structure was also obtained, complex 6,
which does not have any solvent enclathrated within it and
which adopts a 1 : 1 Ag : 2 stoichiometry. The only difference
between the formation conditions of these two materials was
the solvent used: acetonitrile and methanol for 5 and 6,
respectively. The structure of complex 6 is radically different
to that of the hydrated 1 : 2 structure. Rather than the more
common linear geometry the Ag(I) centre exists in a distorted
trigonal geometry coordinated to one pyridyl and two nitrato
ligands.18 This unit forms a silver–nitrate–silver coordination
polymer (Fig. 6). The polymeric chains are connected together
via ureaÁ Á Ánitrate hydrogen bonding with the R22(8) motif that
is so prevalent within this series of structures (Fig. 7). The
structure of 6 has the nitrate positioned in the correct geometry
to form a bidentate interaction but at a long Ag–O distance of
2.739(6) A. A number of other silver nitrate coordination
polymers are known with nitrate adopting both bidentate
and monodentate coordination modes.19,20
The para substitution of the urea upon the pyridyl ring in 2
allows for chain-like architectures to form readily within Ag(I)
complexes of this ligand, as seen in 3–5. However, when the
urea group is situated in the meta position, as in ligand 1, very
different structures, both from those discussed above and from
each other, are observed (Table 3), although crucially, the
ureaÁ Á Áanion R22(8) motif is retained in all but one of the
structures (that of 8). The structure of the triflate complex
[Ag(1)2]CF3SO3 Á 0.5H2O (7) is the most akin to those formed
using 2 and displays a geometry in which the ligands
are rotated 1801 away from each other around a linearly
Fig. 1 The R22(8) hydrogen bond motif that can form between urea
and an oxo-anion.
N–Ag–N axis, with an angle of 168.7(2)1. This solvent mole-
cule also bridges two urea oxygen atoms in neighbouring
molecules (Fig. 3). The sheets of [Ag(2)2]+ in the structure
are stacked above each other through multiple face-to-face
p-p interactions.
The acetate containing structure, 4, closely resembles that of
3, with the acetate anion residing in the same position as
occupied by the triflate (Fig. 4). The remaining urea site is
once more occupied by an enclathrated water molecule.
Methanol is also included within the structure and engages in
a weak interaction with the Ag(I) centre [Ag–O, 2.621(2) A],
again causing a distortion from a linear coordination geome-
try, with an N–Ag–N angle of 168.1(1)1. There is also a partial
occupancy water molecule enclathrated within the asymmetric
unit. As with the triflate complex, the cationic complexes are
p-stacked with the enclathrated water occupying a bridging
position between neighbouring molecules and also hydrogen
bonding to the acetate anions. The methanol is also involved in
hydrogen-bond donation to the acetate.
The structure of [Ag(2)2]+ in the nitrate complex 5 has the
same general form as complexes 3 and 4 and consists of a linear
Ag(2)2+ cation binding the anion to one urea group in an R22(8)
motif. Differences arise in the nature of the enclathrated
solvent in the structure. No interaction between the solvent
and the silver centre is observed. Instead, the solvent occupies a
Table 1 Crystallographic data for complexes 3–6
3
4
5
6
Formula
M
C27H30AgF3N6O7S1
747.50
C29H35AgN6O6.25
676.86
C26H28AgN7O6
642.42
C13H13AgN4O4
397.14
Monoclinic
P21/c
Crystal system
Space group
Triclinic
P-1
Triclinic
P-1
Monoclinic
C2/c
a/A
b/A
10.2106(9)
11.4626(11)
14.2081(14)
72.229(2)
89.484(2)
69.943(2)
1479.0(2)
2
9.3295(5)
12.9275(6)
13.1198(6)
83.850(2)
76.896(2)
75.806(2)
1491.86(13)
2
38.1880(16)
10.1919(4)
13.7940(5)
90
7.0590(14)
8.5990(17)
22.610(5)
90
c/A
a/1
b/1
98.755(3)
90
94.14(3)
90
g/1
U/A3
5306.2(4)
8
0.815
1368.9(5)
4
1.498
Z
m/mmÀ1
0.828
4253
0.729
6807
Unique reflections
Observed reflections
Rint
3801
3436
3113
2160
3181
0.0688
5592
0.0365
0.0201
0.0686
0.0316
0.1085
0.1814
0.0800
wR2 (all data)
R [I Z 2s(I)]
0.0814
0.0403
0.0950
0.0419
N e w J . C h e m . , 2 0 0 5 , 2 9 , 9 0 – 9 8
91

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Fig. 3 Hydrogen bonding involving enclathrated water within the
structure of 3. Some ligands are omitted for clarity. Hydrogen bond
data are shown in Table 2.
Fig.
the structure of 6. Selected distances (A): Ag(1)–O(2), 2.510(6);
6
A
silver-nitrate coordination polymer running through
Ag(1)–O(3), 2.334(7).
rated water (which is involved in a bridging position with a
urea oxygen atom) and three pyridyl CHÁ Á ÁO interactions
(Fig. 11). This arrangement makes the triflate anion hydrogen-
bond saturated.
Fig. 4 The immediate environment around one [Ag(2)2]+ unit in the
structure of 4 with a weak silver-methanol interaction, Ag(1)–O(5), of
2.621(2) A. Hydrogen bond data are shown in Table 2.
The sulfate complex [Ag(1)2]2SO4 (9) forms helical, inter-
digitating stacks in the solid-state structure (Fig. 12). These
chains are held together by virtue of long-range Ag–O inter-
actions [3.217(2) A] utilising the urea oxygen atom. The sulfate
anion, as in the previous structures, is bound to the urea
protons in an R22(8) motif on the ‘outside’ of the complex.
Each sulfate anion is surrounded by four urea donors, making
the oxygen atoms hydrogen-bond saturated. The structure
bears a close resemblance to previous sulfate-containing struc-
tures using the same ligand coordinated to octahedral metal
centres.13
coordinated silver atom, with the urea protons facing away
from the metal centre (Fig. 8). The triflate anions bridge urea
groups, once more forming both R22(8) and R12(6) rings. All of
the urea groups are therefore involved in hydrogen bonding
with triflate anions. The sulfonate group is disordered between
these two ring-forming positions. The long-range structure is
composed of discrete, helical trimers that are held together by
Ag–O and CHÁ Á ÁO interactions (Fig. 9).
A second triflate structure was obtained in which both water
and acetonitrile are enclathrated: [Ag(1)2]CF3SO3 Á H2O Á
MeCN (8), in the chiral space group P21. This difference in
the enclathrated solvent, brought about due to different reac-
tion conditions, results in a different conformation of the
Discrete structures
The most interesting complexes from an anion binding stand-
point, however, are those containing the [Ag(1)2]NO3 complex.
Two solvates of this complex were obtained, [Ag(1)2(MeOH)]-
NO3 (10), and a co-crystal, 2[Ag(1)2(MeOH)]NO3 Á 3[Ag(1)2
(MeNO2)]NO3 (11). Unlike the other structures reported in
this work, the urea hydrogen atoms are orientated ‘inwards’
+
Ag(1)2 cation and therefore very different intermolecular
interactions from those in the structure of 7. Whereas in the
structure of 7 the urea NH groups are facing away from the
central silver atom, the structure of 8 has these groups facing
inwards, although the arms are still rotated 1801 away from
each other about the N–Ag–N axis (Fig. 10). The triflate anion
within 8 shows no disorder, arising from the fact that it is held
in place by six hydrogen bonds, with each oxygen atom
receiving two. The donor groups are a urea group, with the
recurring R21(6) motif, one hydrogen bond from the enclath-
Fig. 5 The environment around one [Ag(2)2]+ unit in 5. Hydrogen
bond data are shown in Table 2.
Fig. 7 Hydrogen bonding between coordination polymer chains in 6.
Hydrogen bond data is given in Table 2.
92
N e w J . C h e m . , 2 0 0 5 , 2 9 , 9 0 – 9 8

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Table 2 Hydrogen bond data for structures 3–6
Symmetry equivalent
d(D–H)/A
d(HÁ Á ÁA)/A
d(DÁ Á ÁA)/A
+(D–HÁ Á ÁA)/1
Complex 3
N(2)–H(2N)Á Á ÁO(6)
N(3)–H(3N)Á Á ÁO(6)
N(5)–H(5N)Á Á ÁO(5)
N(6)–H(6N)Á Á ÁO(4)
O(7)–H(4O)Á Á ÁO(1)
O(7)–H(3O)Á Á ÁO(2)
O(6)–H(2O)Á Á ÁO(3)
O(6)–H(1O)Á Á ÁO(7)
Complex 4
x, y, z
x, y, z
0.91(5)
0.78(4)
0.74(4)
0.83(5)
0.81(5)
0.92(9)
0.82(6)
0.82(7)
1.94(5)
2.22(4)
2.27(4)
2.11(5)
1.94(5)
1.82(9)
2.10(6)
1.91(7)
2.814(6)
2.937(6)
2.992(5)
2.927(5)
2.747(5)
2.735(5)
2.878(5)
2.713(5)
162(4)
153(4)
163(4)
169(4)
172(5)
171(8)
159(6)
167(7)
x, y, z
x, y, z
Àx, Ày + 1, Àz
Àx, Ày, Àz + 1
Àx, Ày, Àz + 1
x, y + 1, z
N(2)–H(2N)Á Á ÁO(3)
N(3)–H(3N)Á Á ÁO(4)
N(5)–H(5N)Á Á ÁO(6)
N(6)–H(6N)Á Á ÁO(6)
O(5)–H(1O)Á Á ÁO(4)
O(6)–H(2O)Á Á ÁO(3)
O(6)–H(3O)Á Á ÁO(1)
Complex 5
x, y, z
x, y, z
0.78(4)
0.78(3)
0.79(4)
0.77(3)
0.77(4)
0.795(10)
0.75(5)
2.05(4)
2.11(3)
2.12(4)
2.40(3)
1.99(5)
2.10(2)
2.12(6)
2.826(3)
2.888(3)
2.889(3)
3.109(4)
2.747(3)
2.878(4)
2.841(3)
173(4)
173(3)
163(3)
154(3)
166(5)
164(6)
164(6)
x, y, z
x, y, z
x, y, z À 1
Àx + 2, Ày + 1, Àz
Àx + 1, Ày + 1, Àz
N(2)–H(2N)Á Á ÁO(6)
N(3)–H(3N)Á Á ÁO(5)
N(5)–H(5N)Á Á ÁO(4)
N(6)–H(6N)Á Á ÁO(3)
O(6)–H(1O)Á Á ÁO(3)
O(6)–H(2O)Á Á ÁO(1)
Complex 6
x, y, z
0.79(4)
0.73(4)
0.78(3)
0.77(3)
0.76(4)
0.88(5)
2.06(4)
2.33(4)
2.17(4)
2.18(3)
2.04(5)
1.98(5)
2.838(4)
2.977(4)
2.950(4)
2.926(4)
2.792(4)
2.858(4)
165(3)
148(4)
174(4)
163(3)
173(5)
176(5)
x + 1/2, y À 1/2, z
x, y, z
x, y, z
x + 1/2, y À 1/2, z
x, Ày, z À 1/2
N(2)–H(2N)Á Á ÁO(4)
N(3)–H(3N)Á Á ÁO(2)
x, Ày À 1/2, z + 1/2
x, Ày À 1/2, z + 1/2
0.77(9)
0.87(9)
2.24(10)
2.19(9)
3.006(10)
3.039(10)
173(9)
168(8)
and converge on a central pocket in which the nitrate anion is
positioned in an fashion that is co-planar with the ligands,
chelated between the two arms of the cationic complex. The
structures are not, therefore, hydrogen-bonded polymers akin
to the other species reported in this work, but instead are
comprised of discrete entities, with the exception of hydrogen-
bonded interactions to urea carbonyl moieties from ligated
methanol in both cases. The [Ag(1)2]+ host system displays a
remarkably good shape match for the nitrate anion.
Within the methanol solvate, 10, the solvent is bound to the
silver centre, resulting in a distorted trigonal geometry. The
anion is not positioned symmetrically within the host complex,
which would result in two R22(8) rings. Instead, the skewed
orientation allows for a CHÁ Á ÁO interaction to the nitrate, in
addition to the four hydrogen bonds from the more acidic urea
protons (Fig. 13). This off-centre position is a consistent
feature of the four independent molecules in 10 and the three
closely related independent molecules in 11, although disorder
is present in some complexes such that the nitrate can lie
towards either the left- or right-hand side of the complex.
The urea groups interact with the nitrate anion by means of
both R22(8) and R21(6) motifs, as seen in the structure of 7. The
crystal packing of 10 is also remarkable, with 3.5 independent
molecules contained in the asymmetric unit.21 The half mole-
cule displays an apparently linear AgÁ Á ÁO–N vector due to its
position on a rotation axis, although this is merely a disorder
average over two asymmetric orientations as adopted by the
other three independent molecules. The long-range structure
consists of discrete p-stacked heptamers held together by
hydrogen bonding between the methanol and urea carbonyl
(Fig. 14).
The MeOH/MeNO2 solvate 11 also displays unusual crystal
packing. The structure adopts the same space group as that of
10 (C2/c) but with two and a half independent molecules per
Table 3 Crystallographic data for complexes 7–11
7
8
9
10
11
Formula
M
C27H26.67AgF3N6O5.33
716.80
S
C28H31AgN7O6F3S
770.54
C52H52Ag2N12O8S
1220.86
Tetragonal
P-421c
17.4146(6)
17.4146(6)
8.2655(6)
90
C27H30AgN7O6
656.45
C27H27.2AgN7.6O6.6
671.63
Crystal system
Space group
Triclinic
P-1
Monoclinic
P21
Monoclinic
C2/c
Monoclinic
C2/c
a/A
b/A
7.1774(5)
16.7930(15)
19.6054(18)
107.586(3)
93.998(3)
91.066(4)
2245.2(3)
3
10.2326(11)
15.6739(17)
10.4685(11)
90
49.675(4)
13.4895(10)
33.477(3)
90
30.599(3)
13.8115(12)
32.625(3)
90
c/A
a/1
b/1
106.780(2)
90
90
90
122.132(7)
90
91.3120(10)
90
g/1
U/A3
1607.5(3)
2
0.763
2506.7(2)
2
0.892
18996(3)
28
0.799
13767(2)
20
0.792
Z
m/mmÀ1
0.810
5223
Unique reflections
Observed reflections
Rint
7300
6857
2878
2766
20860
18601
11431
9242
3996
0.0531
0.0351
0.0850
0.0350
0.0288
0.0706
0.0288
0.0613
0.2750
0.0660
0.0352
0.1261
0.0519
wR2 (all data)
R [I Z 2s(I)]
0.1419
0.0566
N e w J . C h e m . , 2 0 0 5 , 2 9 , 9 0 – 9 8
93

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Table 4 Hydrogen bond data for structures 7–11
Symmetry equivalent
d(D–H)/A
d(HÁ Á ÁA)/A
d(DÁ Á ÁA)/A
+(D–HÁ Á ÁA)/1
Complex 7a
N(2)–H(2N)Á Á ÁO(5A)
N(2)–H(2N)Á Á ÁO(5)
N(3)–H(3N)Á Á ÁO(4)
N(3)–H(3N)Á Á ÁO(5A)
N(5)–H(5N)Á Á ÁO(4A)
N(5)–H(5N)Á Á ÁO(6)
N(6)–H(6N)Á Á ÁO(6)
N(6)–H(6N)Á Á ÁO(6A)
N(8)–H(8N)Á Á ÁO(7)
N(8)–H(8N)Á Á ÁO(9)
N(9)–H(9N)Á Á ÁO(7)
Complex 8
x + 1, y + 1, z
x + 1, y + 1 ,z
x + 1, y + 1, z
x + 1, y + 1, z
x, y, z
0.88
0.88
0.88
0.88
0.88
0.88
0.88
0.88
0.88
0.88
0.88
1.91
2.15
2.19
2.20
2.10
2.10
1.98
2.13
2.22
2.36
2.04
2.72(4)
152.1
147.8
164.7
145.8
163.4
151.5
150.8
154.5
153.9
150.7
159.3
2.929(13)
3.043(16)
2.97(4)
2.96(3)
x, y, z
x, y, z
2.905(15)
2.783(12)
2.95(3)
x, y, z
x, y, z
3.033(17)
3.153(13)
2.881(16)
Àx + 2, Ày, Àz
x, y, z
N(2)–H(2N)Á Á ÁO(5)
N(3)–H(3N)Á Á ÁO(5)
N(5)–H(5N)Á Á ÁO(7)
N(6)–H(6N)Á Á ÁO(7)
C(1)–H(1)Á Á ÁO(4)
x, y, z
x, y, z
0.77(4)
0.68(4)
0.85(4)
0.80(5)
0.93
2.48(4)
2.19(4)
2.11(4)
2.23(5)
2.97
3.154(4)
2.861(4)
2.919(4)
2.974(4)
3.715(4)
2.906(5)
3.591(4)
3.296(4)
2.791(4)
146(4)
175(5)
160(4)
156(4)
138.2
x, y, z
x, y, z
x, y, z
O(7)–H(2O)Á Á ÁO(4)
C(5)–H(5)Á Á ÁO(3)
x, y, z À 1
x, y, z À 1
x, y, z À 1
x À 1, y, z À 1
0.77(5)
0.93
0.93
2.14(5)
2.78
2.45
171(4)
146.4
150.9
C(14)–H(14)Á Á ÁO(3)
O(7)–H(1O)Á Á ÁO(1)
Complex 9
0.795(4)
2.049(4)
155.3(4)
N(2)–H(2N)Á Á ÁO(2)
N(3)–H(3N)Á Á ÁO(2)
Complex 10ab
x, y, z
Ày + 1, x, Àz + 1
0.84(3)
0.77(2)
2.18(3)
2.08(3)
3.005(2)
2.844(3)
168(3)
174(2)
N(11)–H(11N)Á Á ÁO(106)
N(12)–H(12N)Á Á ÁO(106)
N(14)–H(14N)Á Á ÁO(107)
N(15)–H(15N)Á Á ÁO(108)
C(53)–H(53A)Á Á ÁO(107)
Complex 11b
x, y, z
x, y, z
x, y, z
x, y, z
x, y, z
0.88
0.88
0.88
0.88
0.93
2.21
2.16
2.08
2.06
2.52
2.995(11)
2.922(9)
2.928(10)
2.940(9)
3.317(13)
148.0
144.8
162.1
176.6
142.0
N(2)–H(2N)Á Á ÁO(7B)
N(3)–H(3N)Á Á ÁO(7B)
N(5)–H(5N)Á Á ÁO(6B)
N(6)–H(6N)Á Á ÁO(8B)
C(14)–H(14)Á Á ÁO(6B)
a
x, y, z
x, y, z
x, y, z
x, y, z
x, y, z
b
0.83(6)
0.74(5)
0.73(4)
0.77(4)
0.95
2.26(6)
2.16(6)
2.39(4)
2.15(5)
2.32
3.02(2)
2.86(3)
151(5)
158(6)
159(4)
162(4)
150.4
3.084(9)
2.89(3)
3.185(12)
NH hydrogen positions are geometrically fixed. Only data for one crystallographically independent [Ag(1)2]NO3 complex shown as an example
for brevity.
asymmetric unit. Each asymmetric unit contains one
[Ag(1)2]NO3 complex with nitromethane occupying the third
coordination site, one complex with a 50 : 50 occupancy of
nitromethane and methanol and the half complex is a methanol
complex situated on the twofold rotation axis. As with 10 the
nitrate anions are chelated by both urea groups in an offset
fashion (with some disorder), allowing for CHÁ Á ÁO as well as
NHÁ Á ÁO interactions. It is noteworthy that this discrete
ML2(NO3) unit persists with both MeOH and NO2Me co-
ligands. Moreover, the bidentate ligation of the Ag(I) centre by
NO2Me is highly unusual (Fig. 15), with only one previous
example of this coordination mode to a transition metal, also
silver(I), recorded in the CSD.22
Anion binding behaviour
The exact size and shape fit of the nitrate anion in the
structures of 10 and 11 indicates that an assembly of this type
may persist in solution. As a result we have undertaken a
Fig. 9 A discrete trimeric assembly within the structure of 7 held
together by long-range Ag–O interactions [2.851(5) and 2.967(6) A]
and weak CHÁ Á ÁO interactions.
Fig. 8 The hydrogen bonding around one disordered triflate anion
in 7. Hydrogen bond data is given in Table 4.
94
N e w J . C h e m . , 2 0 0 5 , 2 9 , 9 0 – 9 8

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Fig. 10 The asymmetric unit of 8, showing the urea groups in R21(6)
interactions with both the triflate anion and water. CH hydrogen atoms
not shown for clarity. Hydrogen bond data shown in Table 4.
Fig. 13 Close size and shape complementarity of the nitrate anion
with [Ag(1)2]+ in the structure of 10.
These results show that nitrate and particularly acetate are
both able to hydrogen bond strongly to the urea functionalities
of the ligands. Upon addition of one equivalent of acetate to a
solution of either 1 or 2 remarkable chemical shift changes of
up to Dd = 4.0 ppm are observed. Binding constants with this
guest are above 105
M
À1, the limit of detection by NMR
Fig. 11 The hydrogen bonding environment around the triflate anion
in the structure of 8. Coordinated acetonitrile omitted for clarity.
methods. This is attributable to the high basicity of the acetate
anion. The spectra with acetate present also show a splitting of
the urea NH proton signals into two species, believed to
represent two separate binding modes, possibly with a dimeric
structure forming, a phenomenon that is not observed for the
binding of nitrate or perrhenate. Ligand 2 displays stronger
anion binding than 1, which may be due to the inductive effects
of the pyridyl N atom on urea acidity.
The tweezer-like conformation of the [Ag(1)2]NO3 complex
that exists in the solid-state structures of 10 and 11 is also
observed to persist in solution. A 1H NMR titration of
[Ag(1)2]CF3SO3 with nitrate (added as the tetrabutylammo-
nium salt) clearly shows two separate binding events occurring
when following the signal of the para-pyridyl proton (Fig. 16).
Whilst the signals corresponding to the urea NH protons
steadily move downfield during the experiment, the para
pyridyl resonance displays a clear inflection point after the
addition of one equivalent of nitrate, corresponding to a
change in the binding mode. The first of these associations is
believed to result in a 1 : 1 host–guest complex analogous to the
solid-state structure with the subsequent binding event giving
rise to a 1 : 2 stoichiometry. A Job plot confirms that this is
indeed the case. The values of the association constants (K11
and K12) are 30 200 and 2900 MÀ1 for the 1 : 1 and 1 : 2
complexes, respectively. In a preliminary communication we
survey of the solution anion-binding properties of ligands 1
and 2 and their silver(I) complexes. The unbound ligands 1 and
2 themselves interact well with oxo-anions despite their lack of
1
charge. H NMR titrations were carried out in acetone on the
uncoordinated ligands, with the oxo-anions added as tetrabu-
tylammonium salts. Association constants were calculated for
1 and 2 using the program HypNMR23 (Table 5).
Fig. 12 Part of a helical chain in the structure of 9 held together by
Ag–O interactions [Ag–O = 3.217(2) A]. Hydrogen atoms omitted
for clarity.
Fig. 14 A discrete heptameric assembly within the structure of 10.
N e w J . C h e m . , 2 0 0 5 , 2 9 , 9 0 – 9 8
95

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Fig. 16 The chemical shift change of the para-pyridyl proton during
the titration of [Ag(1)2]CF3SO3 + TBA-NO3, showing two different
processes occurring.
chemical shift values of the NH protons was observed until
after the addition of one equivalent of acetate. This behaviour
is attributed to the ligation of the Ag(I) centre by the first
equivalent of acetate, unlike nitrate, which does not ligate until
an excess of the guest is present (Fig. 18). The addition of
further aliquots of TBA-acetate produced significant binding,
although there was no evidence of two separate binding events
as observed for nitrate. Hence, the behaviour of the complex
was modelled as 1 : 1 and 1 : 3 aggregates, with values of K11
Fig. 15 The [Ag(1)2(MeNO2)]NO3 complex within the structure of 11.
Selected bond lengths (A): Ag(1)–O(14), 2.623(4); Ag(1)–O(15),
2.717(4). Only one disordered nitrate position shown.
and K12 Â K13 found to be 4.97 Â 105 and 5.31 Â 106 MÀ1
,
respectively. It is speculated that either a complex in which one
acetate anion binds to each arm forms, or, the ligation of the
acetate causes a break-up of the [Ag(1)2]+ complex and the
arms act independently. The acetate anion is not of the correct
shape to template the formation of the tweezer geometry of the
complex and therefore a very different conformation may
result.
Binding of perrhenate by [Ag(1)2]+ was almost negligible, as
expected for this labile anion with diffuse charge. An upfield
shift is observed for the urea proton signals, contrary to the
downfield shifts observed for acetate and nitrate. This suggests
that the perrhenate anion is hydrogen bonded less strongly
than the triflate anion that it displaces.
Unfortunately, the solution behaviour of the [Ag(2)2]+
complex could not be ascertained due to its insolubility.
However, this complex is not expected to show cooperative
binding between the ligands due to the position of the urea
group on the pyridyl ring, as evidenced in the crystal structures
obtained.
reported slightly lower values.11 The present data is of higher
precision and was obtained at lower concentration, eliminating
precipitation problems. In addition to these two binding con-
stants that are associated with the urea ligands there is also a
third, weaker binding process that is apparent during the
modelling of the data (K13 = 550 MÀ1). This is attributed to
a direct coordination of the nitrate anion to the silver ion when
a significant excess of the guest is present. Evidence for this
process comes from the much stronger interaction of the metal
centre with acetate (vide infra).
In the presence of the triflate anion there is apparently little
or no chelation of the anion within the tweezer cavity, whereas
the presence of the nitrate anion brings about a change in
conformation, resulting in the chelated geometry observed in
the crystal structure. This structure persists until addition of
more than one equivalent of nitrate, at which point the
chelated 1 : 1 structure gradually converts to a situation in
which nitrate anions bind separately to the two ligands on
the ‘outside’ of the complex in a fashion analogous to anion
binding by free 1 (Fig. 17). Such a conformational change
would significantly alter the environment of the para-pyridyl
proton, with it facing towards the urea oxygen atom in the 1 : 1
complex and closer to a urea hydrogen atom in the 1 : 2
complex, giving rise to the observed change. The much higher
value of K11 compared to K12 supports the formation of the
chelated geometry observed crystallographically.
Conclusions
The complexes obtained with various anions using both the
[Ag(1)2]+ and [Ag(2)2]+ cations show that the urea function-
ality has a strong tendency to form R22(8) motifs with oxo-
anions. When this affinity is harnessed into a host assembly
with the correct geometry for a specific guest, as in the case of
[Ag(1)2]NO3, then significant binding persists in solution with
the binding of the templating anion markedly enhanced
Titration of [Ag(1)2]CF3SO3 with acetate produced a nota-
bly different response to that of nitrate. Very little change in the
1
Table 5 Binding constants (MÀ1) obtained via H NMR titrations for the free ligands 1 and 2 and the complex [Ag(1)2]+
[Ag(1)2]+
1
K11
2
K11
Anion
K11
K12
K13
À
NO3
CH3O2
956
4105 a
3500
4105 a
30 200c
2900
b
—
o10a
550c
4.97 Â 105 c
5.31 Â 106 cd
À
À
ReO4
o10a
o10a
o10a
o10a
a
b
Binding constants are too high/low to be accurately determined. Acetate does not form an identifiable 1 : 2 complex with [Ag(1)2]+, see
c
text. For nitrate K13 represents the ligation of the anion to the metal centre, whereas for acetate K11 represents this process, see text. Modelled as
d
simultaneous binding of two anions, i.e., K12 Â K13
.
96
N e w J . C h e m . , 2 0 0 5 , 2 9 , 9 0 – 9 8

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.
[Ag(2)2]NO3 H2O (5). Crystals grown from a solution of 2
(0.050 g, 0.22 mmol) and AgNO3 (0.038 g, 0.22 mmol) in wet
acetonitrile (5 ml) at room temperature. Anal. calcd for
C26H30N7O6Ag: C, 48.61; H, 4.39; N, 15.26%; found: C,
48.35; H, 4.37; N, 15.32%.
[Ag(2)]NO3 (6). Crystals grown from a solution of 2 (0.050 g,
0.22 mmol) and AgNO3 (0.038 g, 0.22 mmol) in methanol (5
ml) at room temperature. Anal. calcd for C13H13N4O4Ag: C,
39.32; H, 3.30; N, 14.11%; found: C, 39.25; H, 3.28; N,
14.16%.
Fig. 17 The two proposed conformations adopted by the [Ag(1)2]+
host in solution with either one anion coordinated between arms or
with the arms binding independently.
.
[Ag(1)2]CF3SO3 0.5H2O (7). Crystals grown from a solu-
tion of 1 (0.025 g, 0.11 mmol) and AgCF3SO3 (0.0094 g, 0.04
mmol) in cyclohexane–methanol (50 : 50 v/v; 5 ml) at room
temperature. Anal. calcd for C27H27N6O5.5SF3Ag: C, 45.00;
H, 3.75; N, 11.67%; found: C, 45.23; H, 3.77; N, 11.60%.
compared to the free ligand. In the presence of strongly ligating
anions such as acetate, however, the self-assembly process is
disrupted by direct binding of the anion to the metal centre.
.
.
[Ag(1)2]CF3SO3 H2O MeCN (8). Crystals grown from a
solution of 1 (0.025 g, 0.11 mmol) and AgCF3SO3 (0.0094 g,
0.04 mmol) in water–acetonitrile (50 : 50 v/v; 5 ml) at room
temperature. Anal. calcd for C29H31N7O6SF3Ag: C, 45.19;
H, 4.02; N, 12.73%; found: C, 45.26; H, 4.12; N, 12.62%.
Experimental
The syntheses of the ligands 1 and 2 have previously been
reported.12 All commercial starting materials and solvents were
used as purchased with no further purification or drying. The
syntheses of 3–11 were aimed toward the formation of crystal-
line samples and generally proceeded in good yield. Elemental
analyses were carried out at the University of Durham using an
Exeter Analytical E-440 instrument.
1H NMR titration experiments were conducted in acetone-d6
using a Varian Mercury 400-BB spectrometer with TMS as an
internal reference. An initial concentration of 0.034 mol dmÀ3
of 1 or 2 was used in all cases, with the guest anions added as
tetrabutylammonium salts in 10 ml aliquots (corresponding to
0.1 equivalents of the guest).
[Ag(1)2]2SO4 (9). Crystals grown from a solution of 1 (0.025
g, 0.11 mmol) and AgSO4 (0.011 g, 0.04 mmol) in acetonitrile–
water (50 : 50 v/v; 5 ml) at room temperature. Anal. calcd for
C52H52N12O8Ag2S: C, 51.15; H, 4.26; N, 13.77%; found: C,
51.01; H, 4.21; N, 13.83%.
.
[Ag(1)2]NO3 MeOH (10). Crystals grown from a solution of
1 (0.025 g, 0.11 mmol) and AgNO3 (0.006 g, 0.04 mmol) in
toluene–methanol (50 : 50 v/v; 5 ml) at room temperature.
Anal. calcd for C27H30N7O6Ag: C, 49.39; H, 4.57; N,
14.94%; found: C, 48.93; H, 4.27; N, 15.16%.
Synthesis
.
[Ag(2)2]CF3SO3 2H2O (3). Crystals grown from a solution
.
.
[Ag(1)2]NO3 0.4MeOH 0.6MeNO2 (11). Crystals grown
from a solution of 1 (0.025 g, 0.11 mmol) and AgNO3 (0.006
g, 0.04 mmol) in nitromethane–methanol (50 : 50 v/v; 5 ml) at
4 1C. Anal. calcd for C27H29.5N7.5O6.5Ag: C, 48.32; H, 4.40;
N, 15.66%; found: C, 48.05; H, 4.43; N, 15.48%.
of 2 (0.050 g, 0.22 mmol) and AgCF3SO3 (0.057 g, 0.22 mmol)
in water–methanol (50 : 50 v/v; 5 ml) at room temperature.
Anal. calcd for C27H30N6O7SF3Ag: C, 43.38; H, 4.05; N,
11.24%; found: C, 43.36; H, 4.05; N, 11.27%.
.
.
[Ag(2)2]CH3CO2 1.25H2O MeOH (4). Crystals grown
from a solution of 2 (0.050 g, 0.22 mmol) and AgCH3CO2
(0.037 g, 0.22 mmol) in water–methanol (50 : 50 v/v; 5 ml) at
room temperature. Anal. calcd for C29H34.5N6O6.25Ag: C,
51.53; H, 5.26; N, 12.44%; found: C, 51.55; H, 5.27; N,
12.52%.
Crystallographyw
X-Ray diffraction data were collected using either Bruker
3-circle diffractometers with SMART 1000 (3–6, 10) or Apex
(8, 9 and 11) CCD detectors or a Nonius KappaCCD diffract-
ometer (7), with monochromated Mo-Ka radiation in all cases.
All data were collected at 120 K as maintained by Oxford
Cryosystems open-flow N2 cryostats. The structures were
solved by direct methods using SHELXS-9724 and refined by
full-matrix least squares against F2 of all reflections using
SHELXL-97.25 All C–H bond lengths were set to fixed X-ray
distances, as were N–H bonds in lower quality data sets (0.880
A), and allowed to ride. Hydrogen atoms attached to oxygen
or nitrogen were located experimentally from difference Four-
ier maps, except in the cases of 7 and 10 for which the data
quality was not sufficient to permit this. All hydrogen atoms
are treated with isotropic atomic displacement parameters.
Structure 7 contains one triflate anion that has an SO3 group
disordered over two positions (50 : 50 occupancies) between
two urea groups. The half triflate per asymmetic unit exists in
the same site as the half occupancy enclathrated water mole-
cule, refined as separate parts. Structure 10 contains disordered
Fig. 18 Titration plots of [Ag(1)2]CF3SO3 with TBA-NO3 (squares)
and TBA-CH3CO2 (diamonds) following the urea NH resonance
adjacent to the pyridyl ring. The first equivalent of acetate coordinates
to the silver before binding occurs with increasing guest concentration,
whereas with nitrate binding occurs prior to coordination.
w CCDC reference numbers are 256159–256167 for 3–11. See http://
www.rsc.org/suppdata/nj/b4/b415818k/ for crystallographic data
in .cif or other electronic format.
N e w J . C h e m . , 2 0 0 5 , 2 9 , 9 0 – 9 8
97

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methanol ligands on two of the independent molecules. Struc-
ture 11 contains disordered nitrate anions (two and a half per
asymmetric unit) across two positions, centred around the
nitrogen atom, of 50 : 50 occupancies.
11 D. R. Turner, E. C. Spencer, J. A. K. Howard, D. A. Tocher and
J. W. Steed, Chem. Commun., 2004, 1352.
12 D. R. Turner, B. Smith, A. E. Goeta, I. Radosavljevic-Evans,
D. A. Tocher, J. A. K. Howard and J. W. Steed, CrystEngComm,
2004, submitted.
13 D. R. Turner, M. B. Hursthouse, M. E. Light and J. W. Steed,
Chem. Commun., 2004, 1354.
14 M. Albrecht, J. Zauner, R. Burget, H. Rottele and R. Frohlich,
Mater. Sci. Eng. C, 2001, 18, 185.
15 J. Bernstein, R. E. Davis, L. Shimoni and N.-L. Chang, Angew.
Chem., Int. Ed. Engl., 1995, 33, 143.
16 (a) CSD Version 5.25, CCDC, Cambridge, Nov. 2003; (b) F. H.
Allen, Acta Crystallogr., Sect. B, 2002, 58, 380.
17 M. C. Etter, Acc. Chem. Res., 1990, 23, 120.
18 F. A. Cotton, G. Wilkinson, C. A. Murillo and M. Bochman,
Advanced Inorganic Chemistry, John Wiley and Sons, New York,
1999.
Acknowledgements
We thank the EPSRC for studentships (DRT, ECS) and for an
advanced fellowship (JAKH). Special thanks to Dr Len
Barbour, University of Stellenbosch, South Africa, for the
25
SHELX interface program X-Seed (www.x-seed.net).26
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N e w J . C h e m . , 2 0 0 5 , 2 9 , 9 0 – 9 8