Long-range chain orientation in 1-D co-ordination polymers as a function of anions and intermodular aromatic interactions

Article abstract of DOI:10.1039/b006202m

The influence of anions and intermolecular aromatic interactions on the orientation of one dimensional silver(i) co-ordination polymers has been studied. Reaction of AgX with 2, 7-diazapyrene (diaz) (X = BF4~ or NO3~), l, 4-bis(4-pyridyl)butadiyne (pybut) (X = BF4~, NO3~, PF6 or MeCO₂~), 4, 4'-bipy (X = BF4~) or 1, 4-bis(4-pyridylethynyl)phenylene (pyphe) (X = PF6~) afforded products of general formula {[Ag(ligand)]X}. All of the products have been structurally characterised by single crystal X-ray diffraction confirming that they exist as one-dimensional linear chain co-ordination polymers. The arrangement of the chains with respect to each other in the solid state is discussed and evaluated in terms of the relative co-ordinating ability of the anion used and the tendency of the N-donor ligand to adopt intermolecular aromatic interactions. For the complexes of diaz the overriding force in controlling chain orientation was shown to be n-n interactions between diaz ligands on adjacent chains. In the case of the pybut complexes the most dominant forces were shown to be metal-anion interactions with aromatic n-n interactions and Ag Ag interactions playing a less influential role. In the case of {[Ag(pyphe)]PF6} Ag aromatic interactions are important in the overall arrangement of adjacent chains. The Royal Society of Chemistry 2000.

Full text of DOI:10.1039/b006202m

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Long-range chain orientation in 1-D co-ordination polymers as
a function of anions and intermolecular aromatic interactions
Alexander J. Blake,a Gerhard Baum,b Neil R. Champness,*a Simon S. M. Chung,a
Paul A. Cooke,a Dieter Fenske,b Andrei N. Khlobystov,a Dmitrii A. Lemenovskii,c
Wan-Sheung Lia and Martin Schröder*a
a School of Chemistry, The University of Nottingham, University Park, Nottingham,
UK NG7 2RD
b Institut für Anorganische Chemie, Universität Karlsruhe, Engesserstr. Geb-Nr. 30.45,
76128 Karlsruhe, Germany
c Department of Chemistry, M.V.Lomonosov Moscow State University, Moscow, 119 899,
Russia
Received 1st August 2000, Accepted 6th October 2000
First published as an Advance Article on the web 16th November 2000
The influence of anions and intermolecular aromatic interactions on the orientation of one dimensional silver()
co-ordination polymers has been studied. Reaction of AgX with 2,7-diazapyrene (diaz) (X = BF4Ϫ or NO3Ϫ),
1,4-bis(4-pyridyl)butadiyne (pybut) (X = BF4Ϫ, NO3Ϫ, PF6Ϫ or MeCO2Ϫ), 4,4Ј-bipy (X = BF4Ϫ) or 1,4-bis(4-pyridyl-
ethynyl)phenylene (pyphe) (X = PF6Ϫ) afforded products of general formula {[Ag(ligand)]X}∞. All of the products
have been structurally characterised by single crystal X-ray diffraction confirming that they exist as one-dimensional
linear chain co-ordination polymers. The arrangement of the chains with respect to each other in the solid state is
discussed and evaluated in terms of the relative co-ordinating ability of the anion used and the tendency of the
N-donor ligand to adopt intermolecular aromatic interactions. For the complexes of diaz the overriding force in
controlling chain orientation was shown to be π–π interactions between diaz ligands on adjacent chains. In the
case of the pybut complexes the most dominant forces were shown to be metal–anion interactions with aromatic
π–π interactions and Ag ؒ ؒ ؒ Ag interactions playing a less influential role. In the case of {[Ag(pyphe)]PF6}∞
Ag ؒ ؒ ؒ aromatic interactions are important in the overall arrangement of adjacent chains.
Introduction
The rapidly growing area of co-ordination polymers, polymeric
compounds based on the interaction of metal cations with
organic ligands, has given rise to a wide variety of fascinating
one-, two- and three-dimensional structures. We now need to
establish the general principles of the construction of such
compounds according to their overall structure. Recent
attempts at classification1 help to highlight the relationship
between the structure of the molecular building blocks and that
of the supramolecular entity formed via self-assembly of those
blocks. Understanding this relationship is the key to controlled
design of supramolecular structures. Thus, many examples
illustrating the factors that control the formation of co-
ordination polymers have been recognised, including the func-
tionality and denticity of ligands,2 size and nature of anions3
and/or solvent.4 However, there are relatively few systematic
studies on this subject.2e,3b,4a
Scheme
1
Weak interactions observed in one-dimensional AgI–
bipyridyl co-ordination polymers: (a) favourable head-to-tail aromatic
π–π pyridyl–pyridyl interactions, (b) less-favourable head-to-head
aromatic π–π pyridyl–pyridyl interactions, (c) Ag ؒ ؒ ؒ Ag interactions,
(d) Ag ؒ ؒ ؒ aromatic interactions.
The strategy for supramolecular design proposed by
Desiraju5 in 1995 is based on interpretation of crystal struc-
tures in terms of “supramolecular interactions” and “supra-
molecular synthons”, allowing consideration of the structure
of the “supermolecule” governed by superposition of discrete
non-covalent intermolecular contacts. We report herein studies
on AgI–bipyridyl co-ordination polymers and consider a
number of supramolecular interactions that occur in this type
of compound. These include intermolecular aromatic, Ag ؒ ؒ ؒ
Ag, Ag ؒ ؒ ؒ aromatic and Ag ؒ ؒ ؒ anion interactions and electro-
static repulsions (Scheme 1). Most information about these
types of interactions has been obtained from studies on discrete
molecular models designed to investigate the nature of specific
interactions of different types and energies.5 The exploitation
of the synergic effects of one type of interaction upon another
as part of supramolecular design is a critical challenge in
crystal engineering. We illustrate herein how combinations of
supramolecular interactions can affect the structure of 1-D
chain-like co-ordination polymers which, due to their topo-
logical simplicity, may be used as models for investigating
the influence of the different factors on the structure of the
supramolecular entity.
Results and discussion
Ligand structures
“insulated” supramolecular interactions.6 However, in
a
supramolecular array these intermolecular contacts combine
so that the overall structure is a compromise between various
The discoid aromatic molecule diaz (2,7-diazapyrene) (Scheme
2) has 16 electrons in its delocalised π-electronic system.
DOI: 10.1039/b006202m
J. Chem. Soc., Dalton Trans., 2000, 4285–4291
This journal is © The Royal Society of Chemistry 2000
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Table 1 Selected interatomic distances [Å] and angles [Њ] for complexes
1, 2,a 3,b 4,c 5,d 6,e 7 and 9f
Compound
1
2
Ag1–N1
Ag1–N2
Ag1–N1
Ag1–N1i
Ag1–O1
Ag1–O1i
2.198(4)
2.203(4)
2.215(4)
2.215(4)
2.547(4)
2.547(4)
N1–Ag1–N2
180
93.98(13)
119.1(2)
144.2(2)
119.1(2)
93.98(13)
50.5(2)
180
N1–Ag1–O1
N1–Ag1–O1i
N1–Ag1–N1i
N1i-Ag1–O1
N1i-Ag1–O1i
O1–Ag1–O1i
N1–Ag1–N1i
N1–Ag1–N2i
Fig. 1 Packing diagram of free diaz showing the “herringbone”
arrangement of the molecules.
3
4
Ag1–N1
Ag1–N1
Ag1–N2i
Ag1–N1
Ag1–N2i
Ag1–O1
Ag1–N1
Ag1–N2
2.147(3)
2.172(6)
2.176(6)
2.164(6)
2.153(6)
2.687(6)
2.185(3)
2.186(3)
177.1(3)
5
N1–Ag1–N2i
179.9(3)
6
N1–Ag1–N2
N1–Ag1–Ag1i
N2–Ag1–Ag1i
169.88(10)
97.04(8)
92.96(8)
Fig. 2 Packing arrangement of free pybut showing the “herringbone”
arrangement of the molecules.10
Ag1 ؒ ؒ ؒ Ag1i 3.1927(10)
Ag1–O1
2.636(3)
2.621(3)
2.190(3)
2.201(3)
Ag1–O1i
7
9
Ag1–N101
Ag1–N102
N101–Ag1–N102 168.18(12)
N101–Ag1–Ag2 75.49(9)
N102–Ag1–Ag2 112.12(8)
Ag1 ؒ ؒ ؒ Ag2 3.1371(5)
Ag1–O1
Ag1–O2
Ag1–N1
Ag1–N2i
2.497(3)
2.746(3)
2.152(3)
2.149(3)
N1–Ag1–N2i
177.28(12)
a Symmetry operation i: Ϫx ϩ ³, Ϫy ϩ ¹, z. b Symmetry operation i: Ϫx,
¯
²
¯
²
Ϫy ϩ 1, Ϫz ϩ 1. c Symmetry operation i: x Ϫ 1, y, z Ϫ 1. d Symmetry
operation i: x ϩ 1, y, z Ϫ 1. e Symmetry operation i: Ϫx ϩ 1, Ϫy ϩ 1,
Ϫz ϩ 1. f Symmetry operation i: x ϩ ³, Ϫy ϩ ¹, z Ϫ ¹.
Scheme
2
Ligands used for constructing one-dimensional AgI–
bipyridyl co-ordination polymers.
¯
²
¯
²
¯
²
Aromatic molecules with such structures tend to interact via
face-to-face stacking,7 the driving force for their association
being electrostatic attraction between aromatic π electrons and
a positively charged ring σ framework.8 The single crystal struc-
ture of the “free” ligand (Fig. 1) shows that this intermolecular
interaction defines the packing arrangement with an inter-
planar separation between parallel molecules of 3.450(2) Å,
and the centre of the molecules offset by 1.570(2) Å. The
herringbone packing motif observed for diaz is common for
aromatic compounds in the solid state.5
The structural isomer pybut [1,4-bis(4-pyridyl)butadiyne] has
the same number of π electrons able to participate in inter-
molecular interactions but an alternative distribution of the
electronic density within the molecule. The π electrons of the
sp-hybridised carbons of the diyne linker are unlikely to take
a significant part in any kind of intermolecular interaction in
co-ordination polymers; thus, in contrast to the discoid ligand
diaz, the groups prone to adoption of aromatic-based inter-
actions are concentrated at the ends of the ligand. We can
assume that the energy of the facial interaction increases pro-
portionally with the number of π electrons in a fused aromatic
system9 and hence the energy of face-to-face aromatic inter-
actions in pybut may be considered to be lower than for that
observed for diaz. As a result of this lower energy the molecules
of free pybut are packed less closely in the solid state with an
interplanar separation of 3.523(5) Å and the centre of the
molecules offset by 1.560(5) Å.10 As with diaz, infinite aromatic
stacks are organised in a herringbone motif (Fig. 2). As pybut
and diaz differ in shape and electron distribution these two
molecules confer different abilities to associate via aromatic
interactions. Therefore, we expect the structures of supra-
molecular arrays formed by these ligands to differ because of
the differences in the strength of π–π stacking.
Fig. 3 Arrangement of infinite chains in {[Ag(diaz)]BF4ؒMeCN}∞
1. Anions and solvent guest molecules are omitted for clarity (silver,
cross-hatched).
a linear geometry.11 The introduction of a metal centre to the
structure may be considered as addition of a new non-covalent
interaction into the supramolecular system.
Silver(I) complexes with 2,7-diazapyrene: control by facial
ꢀ–ꢀ-interactions. The influence of Ag–N co-ordination inter-
actions on the organisation of molecules in the crystal system
is easily recognised as the relative orientation of the ligand
molecules is changed from that observed for the “free” ligand.
1c
Thus, in complex 1 {[Ag(diaz)]BF4ؒMeCN}∞ (Fig. 3) linear,
coplanar ribbons of ligands are observed. It is clear that only a
linear orientation of the ligand molecules, as observed in 1 (see
Table 1) can meet the co-ordination requirements of two-co-
ordinate AgI. Despite the reorientation of the diaz molecules,
aromatic face-to-face contacts remain a dominant supra-
molecular interaction in complex 1, with an interplanar
separation between parallel diaz molecules of 3.380(5) Å
(Fig. 3) compared to 3.450(2) Å in the “free” ligand. It is
important to note that the co-ordination of diaz to the silver()
cation strengthens the facial aromatic interaction due to an
increase of both the positive charge of the σ framework and
the π-electron density on the ligand, as a result of lone pair
donation from the ligand nitrogen atoms and potential π back
donation from the silver() cations. Such polarisation of the
aromatic molecule encourages a more effective intermolecular
interaction.8 This π–π interaction results in a significant offset
of the molecules [centre–centre separation = 3.640(5) Å, offset
distance = 1.351(5) Å] and prevents close interaction of silver()
cations [Ag ؒ ؒ ؒ Ag 3.640(3) Å].12 Indeed, the approach of
Metal complex structures
Silver() complexes of exo-bidentate ligands often give rise to
1-D chain-like polymers because of the preference of AgI for
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Fig. 4 Arrangement of infinite chains in {[Ag(diaz)(NO3)]}∞ 2.
Symmetry code i: Ϫx ϩ ³, Ϫy ϩ ¹, z (silver, cross-hatched; oxygen,
right-hatched).
Fig. 6 Arrangement of infinite chains in {[Ag(4,4Ј-bipy)]BF4ؒH2Oؒ
MeCN}∞ 4. Anions and solvent guest molecules are omitted for clarity.
Symmetry code i: x Ϫ 1, y, z Ϫ 1 (silver, cross-hatched).
¯
²
¯
²
Fig. 5 Arrangement of infinite chains in {[Ag(pybut)]BF4ؒMeCN}∞ 3.
Anions and solvent guest molecules are omitted for clarity. Symmetry
code i: Ϫx, Ϫy ϩ 1, Ϫz ϩ 1 (silver, cross-hatched).
cationic centres in this case should be considered as a cause of
repulsion between chains, with the Coulombic force counter-
acting the attractive aromatic stacking. Stacks of infinite poly-
Ϫ
cationic chains form infinite sheets between which lie the BF4
Fig. 7 Co-ordination geometry of AgI in {[Ag(pybut)]NO3ؒMeCN}∞
5. Hydrogen atoms are omitted for clarity (silver, cross-hatched;
oxygen, right-hatched; nitrogen, dotted).
anions (shortest Ag ؒ ؒ ؒ F distance > 5 Å).
The crystal structure of {[Ag(diaz)(NO3)]}∞ 2 supports this
Ϫ
hypothesis (Fig. 4) as the chelating co-ordination of the NO3
anion to the AgI can be considered partially to neutralise the
positive charge within the chain, allowing the interplanar
separation to be reduced by 0.04 Å in comparison to that of
complex 1 [interplanar separation = 3.340(5) Å, centre–centre
separation = 3.749(5) Å, offset distance = 1.703(5) Å]. As a
separation. As the BF4Ϫ anions are unco-ordinated, and located
between the aromatic stacks, the metal centres have a perfect
linear geometry (Table 1). It should be noted that the related
bipyridyl ligand 4,4Ј-bipy (Scheme 2) gives the complex
{[Ag(4,4Ј-bipy)]BF4ؒH2OؒMeCN}∞ 4 which exhibits essentially
the same packing motif, and where the cationic repulsion is
minimised with the same “head-to-tail” aromatic pairing
(Fig. 6). The shortest interplanar separation is 3.456(6) Å and
the shortest interchain Ag ؒ ؒ ؒ Ag distance is 5.838(3) Å. A
recently published account of π–π stacking in metal complexes
which demonstrates that “head-to-tail” stacking is more
common is supported by numerous examples of co-ordination
compounds.8c Although water is included in the structure of 4 it
is not involved in any hydrogen bonding and therefore does not
appear to play a significant role in the overall packing
arrangement.
Ϫ
result of NO3 co-ordination, AgI adopts a strongly distorted
tetrahedral geometry with a N1–Ag–N1i angle of 144.2(2)Њ,
with the linearity observed in compound 1 altered in polymer 2
into zigzag shaped chains where neighbouring ligand molecules
are twisted with respect to each other by 17.6(2)Њ. In this way
the packing arrangement of diaz molecules in 2 resembles the
herringbone motif observed in the “free” ligand. The structures
of complexes 1 and 2 demonstrate that co-ordination of AgI
to diaz does not preclude face-to-face aromatic interactions;
indeed these are the strongest intermolecular contacts in struc-
tures containing diaz, thus preventing formation of a polymer
with a “shifted” placement of chains (Scheme 3). Therefore,
The structure of the complex {[Ag(pybut)]NO3ؒMeCN}∞
5 demonstrates the same “head-to-tail” structural pattern
observed in complex 3. Once again weak co-ordination of the
anions (Fig. 7) causes the cationic centres to be closer than in 3,
with a Ag ؒ ؒ ؒ Ag separation of 7.079(2) Å observed for 5.
Bidentate anions, such as PO2F2Ϫ and MeCO2Ϫ, have a higher
density of negative charge on their peripheral, potentially inter-
Scheme 3 Possible “shifted” placement of chains that is overcome by
the adoption of π–π interactions adopted by the diaz ligands.
Ϫ
acting, atoms than NO3 or BF4Ϫ: they are therefore able to
co-ordinate more strongly and can significantly mediate other
interchain interactions. Thus, in the case of the complex
{[Ag(pybut)]PO2F2ؒMeCN}∞ 6 each silver() cation interacts
the relatively weak potential interactions between AgI and
π electrons cannot compensate for the loss of the stronger
face-to-face aromatic contacts.
Ϫ
weakly with two PO2F2 anions with Ag–O distances of
2.636(3) and 2.621(3) Å, allowing the cations to be closer and
therefore form Ag ؒ ؒ ؒ Ag interactions at a distance 3.1927(10)
Å. Weak asymmetric bridging of silver() cations by two oxygen
donors (Fig. 8) may also enhance the interchain contact. In
addition to these Ag ؒ ؒ ؒ Ag and Ag–anion interactions, the
structure of 6 also contains “head-to-head” aromatic inter-
actions characterised by an interplanar separation of 3.394(3)
Å which bind adjacent chains together to afford infinite
“ladders” (Fig. 9) in which the silver() centres adopt a
T-shaped co-ordination geometry (Table 1). A comparison of
complexes 3 and 6 allows us to attribute their structural differ-
Silver complexes with 1,4-bis(4-pyridyl)butadiyne: control by
synergic ꢀ–ꢀ, Ag–Ag and Ag–anion interactions. As previously
noted, the rod-like ligand pybut (Scheme 2) exhibits weaker π–π
interactions than does diaz allowing alternative arrangements
of linear chains. For example, the complex {[Ag(pybut)]BF4ؒ
MeCN}∞ 3 (Fig. 5) consists of chains arranged in a parallel
fashion, where the ligand molecules are shifted relative to each
other so that the shortest interchain Ag ؒ ؒ ؒ Ag distance is
7.147(4) Å. Again the herringbone packing of the free ligand is
disrupted by silver() co-ordination. This shifted type of place-
ment can be accounted for by Coulombic repulsions between
silver() centres, and in this case the repulsion is not compen-
sated by face-to-face ligand stacking in contrast to {[Ag(diaz)]-
BF4}∞ 1. Nevertheless, the aromatic groups in 3 are involved
in face-to-face interactions in a “head-to-tail” fashion with
an interplanar separation of 3.536(6) Å (Scheme 1a). This
arrangement is energetically more favourable than a “head-to-
head” orientation (Scheme 1b), due to more effective charge
Ϫ
ences exclusively to the different co-ordinative abilities of BF4
Ϫ
and PO2F2 anions, as both anions have a similar size and
tetrahedral shape, and the complexes were prepared under the
Ϫ
same conditions (MeCN, 70 ЊC). The tetrahedral anion PO2F2
may therefore be considered a more co-ordinating analogue of
Ϫ
BF4
.
Ϫ
Ϫ
Similarly to the relationship between PO2F2 and BF4
,
Ϫ
acetate can be related to the trigonal NO3 anion. Thus, the
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Fig. 11 Arrangement of infinite chains in {[Ag(pyphe)]PF6ؒMeCN}∞
9. Ag ؒ ؒ ؒ aromatic interactions represented by dotted lines. Anions and
solvent guest molecules are omitted for clarity. Symmetry code i: x ϩ ³,
¯
²
1
1
–
–
Ϫy ϩ 2, z Ϫ 2 (silver, cross-hatched).
complexes with pyphe could be very similar to their pybut
analogues, with the role of the co-ordinating anion dominating
over weaker supramolecular forces. Although {[Ag(pyphe)]-
NO3ؒ}∞ 8 could be isolated as a microcrystalline solid we were
unable to obtain crystals of sufficient quality for single crystal
X-ray diffraction analysis. However, we were interested to
assess the impact of these weaker aromatic interactions and
therefore prepared the linear silver() cationic polymer in the
presence of less-co-ordinating PF6Ϫ anions. Thus in the absence
of AgI–anion contacts the chains adopt a different stacking
Fig.
8
Co-ordination geometry of AgI in {[Ag(pybut)]PO2F2ؒ
MeCN}∞ 6. Symmetry code i: Ϫx ϩ 1; Ϫy ϩ 1; Ϫz ϩ 1. Hydrogen
atoms are omitted for clarity (silver, cross-hatched; oxygen, right-
hatched; nitrogen, dotted; phosphorus, light random dot pattern;
fluorine, heavy random dot pattern).
motif from that observed in {[Ag(pybut)]BF4ؒMeCN}∞
3
and {[Ag(4,4Ј-bipy)]BF4ؒH2OؒMeCN}∞ 4. In {[Ag(pyphe)]-
PF6ؒMeCN}∞ 9 the silver() centres have distorted linear
co-ordination geometries (Table 1), with the chains significantly
shifted with respect to each other [shortest Ag ؒ ؒ ؒ Ag distance
5.177(3) Å; Fig. 11]. This again can be considered as an effect
of the electrostatic repulsion of the cations. However, a more
detailed analysis of the structure of complex 9 reveals only one
type of attractive interaction between adjacent chains, namely
the Ag ؒ ؒ ؒ phenylene contact with a Ag ؒ ؒ ؒ plane distance of
3.273(3) Å. This interaction therefore defines the overall
packing arrangement.
Fig. 9 Arrangement of infinite chains in {[Ag(pybut)]PO2F2ؒMeCN}∞
6. Anions and solvent guest molecules are omitted for clarity. Symmetry
code i: Ϫx ϩ 1, Ϫy ϩ 1, Ϫz ϩ 1 (silver, cross-hatched).
Related 1-D silver complexes with pyridyl-donor ligands. The
co-ordination sphere of AgI is very flexible and can adopt co-
ordination numbers between 2 and 6 and various geometries
from linear through to tetrahedral, trigonal pyramidal and
octahedral.15 This gives rise to a wide variety of 1-, 2- and
3-dimensional complexes based on silver() cations co-
ordinated to bridging bipyridyl ligands.16 The silver() cation
usually prefers a linear, two co-ordinate geometry with nitrogen
donors,11 thus encouraging the formation of 1-D polymers with
bridging bidentate ligands. However, the number of studies
of the influence of anions on a given silver() system in 1-D
polymers is limited.
Fig. 10 Co-ordination geometry of AgI in {[Ag(pybut)]MeCO2ؒ
2.5H2O}∞ 7. Hydrogen atoms are omitted for clarity (silver, cross-
hatched; oxygen, right-hatched; nitrogen, dotted).
acetate anion can effectively neutralise cationic positive charge
and is able to bridge cations in {[Ag(pybut)]MeCO2ؒ2.5H2O}∞
7 (Fig. 10), with a Ag ؒ ؒ ؒ Ag separation of 3.1371(5) Å, which
results in the same “head-to-head” ligand arrangement as
observed in complex 6.
Some examples relevant to our research have been reported
for 1-D complexes of the smallest aromatic bidentate pyridyl-
type ligand pyrazine (pyz). Both complexes {[Ag(pyz)][PF6]0.5
-
16b
16a
[OH]0.5
}
and {[Ag(pyz)]BF4}∞
with non-co-ordinating
∞
anions exhibit the same structural motif as that of {[Ag(diaz)]-
BF4}∞. The ligand molecules are involved in aromatic inter-
molecular interactions (plane to plane separation 3.56 and 3.66
Å, respectively) but the chains are not sufficiently close for
Ag ؒ ؒ ؒ Ag interactions to be adopted (separations are 3.57 and
4.28 Å, respectively). The influence of the more co-ordinating
Silver complex with 1,4-bis(4-pyridylethynyl)phenylene: con-
trol by Ag–aromatic interaction. The interaction between
cations and the π-electron system of a ligand represents another
possible driving force for supramolecular structure formation.13
There are about 130 examples of Ag–η1- and Ag–η2-arene
bonding found in the Cambridge Structural Database with a
mean Ag–arene distance of 2.82 Å.14 Structural statistics on
long range Ag–η6 interactions are less reliable (Scheme 1d), but
the importance of such interactions has previously been demon-
strated in the significant work of Hosseini and co-workers.13c
Despite the long range interaction and its relative weakness,
Ag–η6 contacts with arenes have been documented in the solid
state, including the first example of an arene–Ag sandwich
compound;14 the same work summarises structural statistics for
metal–η6 species and reports a range of Ag–centroid distances
of 2.89–3.37 Å.14
Ϫ
NO3 anion is essentially the same as observed for diaz com-
plexes: it distorts the linear geometry of silver() centres and
allows adjacent chains to be closer17 (Ag ؒ ؒ ؒ Ag distance
3.560 Å, plane to plane separation 3.436 Å). It can also be
deduced that pyrazine molecules, with a relatively small
aromatic π-electronic system, can tolerate complete loss of
aromatic interactions if they are compensated by another
supramolecular contact. For example, in the 1-D complex
{[Ag(pyz)(NO2)]}∞ no aromatic interactions are observed
between chains, rather the chain arrangement is defined by
moderate Ag ؒ ؒ ؒ Ag interactions [3.2168(3) Å].18 4,4Ј-Bipyridyl
has been found to be more structurally flexible than pybut
in silver() co-ordination polymers. Thus in {[Ag(4,4Ј-bipy)]-
The ligand pyphe [1,4-bis(4-pyridylethynyl)phenylene] can be
considered as an elongated version of pybut (Scheme 2) where
Ϫ
᎐
the p-phenylene group separates the two C᎐C bonds and pro-
NO3}∞ the NO3 anion is weakly co-ordinating [Ag ؒ ؒ ؒ O
᎐
vides a maximum potential for aromatic-based interactions at
the midpoint of the ligand. In the presence of co-ordinating
anions (e.g. PO2F2Ϫ, NO3Ϫ) it is likely that structures of
2.790(2) Å] and the orthogonal linear chains are linked via
Ag ؒ ؒ ؒ Ag contacts.12a,d This contrasts with both the structure
of {[Ag(4,4Ј-bipy)]BF4ؒH2O}∞ and with the other structural
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motifs that we have observed with other complexes of AgNO3
and linear bipyridyl ligands. The unique structural features of
{[Ag(4,4Ј-bipy)]NO3}∞ are a result of a combination of two non-
conflicting interchain interactions. Coplanar pyridyl rings stack
(plane to plane separation 3.522 Å) and are twisted by 90Њ with
respect to each other (which allows an effective charge separ-
ation) and the Ag() cations form short Ag ؒ ؒ ؒ Ag contacts
[2.970(2) Å]. This remarkable structure is perhaps a con-
sequence of the specific geometry of 4,4Ј-bipy which fits this
particular packing arrangement of chains; however, a direct
comparison of this structure with other complexes is difficult
because {[Ag(4,4Ј-bipy)]NO3}∞ was synthesized under unusual
conditions (water, 140 ЊC) differing considerably from those
applied for the syntheses of most 1-D silver() polymers.
thoroughly mixed with powdered grey selenium metal (4.8 g,
6 × 10Ϫ2 mol). The mixture was placed in a 20 cm3 flask fitted
with a gas exhaust tube then heated in a sand bath at 265 ЊC for
3 h. The temperature was increased to 300–310 ЊC for 1 h. The
product was sublimed and deposited onto the cold neck of the
reaction vessel as yellow acicular crystals. It was removed and
recrystallised from benzene. Sublimation of the product under
reduced pressure (14 mmHg) gave 1.62 g yellow crystals of pure
diaz (yield 82%). NMR, IR and elemental analysis are identical
to the previously reported values.21
{[Ag(diaz)(NO3)]}∞ 2. A solution of AgNO3 (13 mg, 7.4 ×
10Ϫ2 mmol) in MeCN (2 cm3) was layered over a solution of
diaz (15 mg, 7.4 × 10Ϫ2 mmol) in benzonitrile (2 cm3). After
5 days large colourless crystals suitable for X-ray diffraction
were formed (yield 20 mg, 73%). IR, ν/cmϪ1: 3052m, 2923w,
2852m, 1588w, 1578w, 1384vs, 1315s, 1149w, 1121w, 1035w,
905m, 831w, 791w and 713m. Calc. for C14H8AgN3O3: C 44.92,
H 2.14, N 11.23. Found: C 44.57, H 2.03, N 11.30%.
Ϫ
{[Ag(4,4Ј-bipy)]NO2}∞, in which the NO2 anion co-ordinates
at a similar distance (Ag ؒ ؒ ؒ O 2.667 Å)18 as the NO3Ϫ anion in
{[Ag(pybut)]NO3}∞ (Ag ؒ ؒ ؒ O 2.687 Å), adopts the same chain
as observed for {[Ag(4,4Ј-bipy)]BF4ؒH2O}∞ (“head-to-tail”
stacking of pyridyl rings, plane-to-plane separation 3.383 Å) as
would be expected considering the charge compensation
afforded by the anionic interaction.
{[Ag(pybut)]BF4ؒMeCN}∞ 3. A solution of AgBF4 (20 mg,
9.8 × 10Ϫ2 mmol) in MeCN (2 cm3) was added to a solution of
pybut (20 mg, 9.8 × 10Ϫ2 mmol) in MeCN (2 cm3) at 70 ЊC
(external temperature, oil bath). The resulting solution was
heated at 70 ЊC for 1 h. The temperature was then slowly
decreased to 20 ЊC over 10 h. Colourless crystals suitable for
X-ray diffraction were formed (24 mg). An additional amount
of complex 3 (11 mg) was precipitated by diethyl ether diffusion
into the reaction solution. Overall yield 80%. Complex 3 readily
loses solvent and can be completely desolvated in vacuo. IR,
ν/cmϪ1: 3027w, 1940w, 1585vs, 1539w, 1486w, 1399m, 1125–
1036vs, 814vs, 779m and 542s. Calc. for C14H8AgBF4N2
(desolvated complex): C 42.40, H 2.36, N 7.60. Found: C 42.10,
H 2.01, N 7.12%.
Conclusion
Comparison between the crystal structures of “free” ligands
and those of their complexes allows interpretation of the
influence of metal cation co-ordination on the effectiveness of
aromatic face-to-face interactions within a polymeric array.
Varying the anion within the supramolecular complex reveals
its role in determining the overall interactions between
chains. The isomeric bipyridyl ligands diaz and pybut have
very different π-electron distributions: the discoid shape of
diaz consistently favours intermolecular aromatic π–π face-to-
face interactions. Moreover, the interplanar separations of
diaz molecules may be fine-tuned by controlling metal–anion
interactions.
The elongated pybut ligand provides structural flexibility in
its complexes. The relative placement of the ligand molecules
and the orientation of the infinite cationic chains can be varied
by metal–anion interactions, co-ordinating anions allowing
closer approaches between cationic centres and the formation
{[Ag(4,4Ј-bipy)]BF4ؒH2OؒMeCN}∞ 4. The same method as
for complex 3, using AgBF4 (19 mg, 9.6 × 10Ϫ2 mmol) and 4,4Ј-
bipy (15 mg, 9.6 × 10Ϫ2 mmol), gave colourless crystals (yield
34 mg, 87%). Complex 4 readily loses solvent and can be com-
pletely desolvated in vacuo. IR, ν/cmϪ1: 3439w, 3052w, 1599s,
1532w, 1423w, 1406w, 1220w, 1070–1034vs, 805s, 627w and
533w. Calc. for C10H8AgBF4N2 (desolvated complex): C 34.19,
H 2.28, N 7.98. Found: C 33.85, H 2.02, N 7.79%.
of “head-to-head” ligand placement as seen in complexes 6 and
Ϫ
7. In the case of relatively weakly co-ordinating anions (BF4
,
NO3Ϫ) the polymer adopts an alternative chain arrangement
where ligands have a “head-to-tail” orientation and the cationic
centres are separated by over 7 Å as observed in 4 and 5.
Using rod-like ligands, which include π-electron donating
units (phenylene ring) in their spacers, allows exploitation of
Ag ؒ ؒ ؒ aromatic interactions when combined with a non-
co-ordinating anion, as exemplified by complex 9.
{[Ag(pybut)]NO3ؒMeCN}∞ 5. The same method as for
complex 3, using AgNO3 (17 mg, 9.8 × 10Ϫ2 mmol) and pybut
(20 mg, 9.8 × 10Ϫ2 mmol), gave colourless crystals (yield 31 mg,
77%). Complex 5 readily loses solvent and can be completely
desolvated in vacuo. IR cmϪ1: 3048w, 2250w, 1939w, 1584s,
1538m, 1486w, 1385vs, 1270w, 1062w, 825w, 814s, 778m, 541m
and 460m. Calc. for C14H8AgN3O3 (desolvated complex): C
44.65, H 2.13, N 11.24. Found: C 44.91, H 2.01, N 11.23%.
We believe that by interpreting the structures of co-
ordination polymers in terms of the relative energies and
directionality of weak supramolecular interactions a greater
appreciation of co-ordination polymer design can be achieved.
We are now extending this approach to supramolecular struc-
tures with higher dimensionality and topological complexity.
{[Ag(pybut)]PO2F2ؒMeCN}∞ 6. The same method as for
complex 3, using AgPF6 (25 mg, 9.8 × 10Ϫ2 mmol) and pybut
(20 mg, 9.8 × 10Ϫ2 mmol), gave colourless crystals (yield 24 mg,
Ϫ
55%). Anion PF6 was found to be hydrolysed to PO2F2Ϫ. IR,
ν/cmϪ1: 3026w, 2922w, 1939w, 1584s, 1539w, 1486w, 1398m,
1332m, 1314s, 1153s, 852m, 836m, 814s, 779m, 542m, 513m and
498m. Calc. for C16H11AgF2N3O2P: C 42.28, H 2.42, N 9.25.
Found: C 41.95, H 2.40, N 8.93%.
Experimental
All reagents (Aldrich) were used as received. Ligands pybut and
pyphe were prepared by literature methods.19,20 The synthesis of
diaz21 was modified from the reported procedure. Elemental
analyses were carried out by the University of Nottingham
microanalysis service. Infrared spectra were obtained as KBr
{[Ag(pybut)]MeCO2ؒ2.5H2O}∞ 7. A solution of AgMeCO2
(10 mg, 6.0 × 10Ϫ2 mmol) in a 1:1 mixture of water and MeCN
(2 cm3) was layered over a solution of pybut (12 mg, 6.0 × 10Ϫ2
mmol) in benzonitrile (2 cm3). Colourless crystals were
obtained (yield 15 mg, 67%). The product readily loses solvent
and can be completely desolvated in vacuo. IR, ν/cmϪ1: 3404m,
1940w, 1585s, 1557s, 1539m, 1486w, 1411s, 1405s, 815s,
779m, 650w and 542m. Calc. for C16H11AgN2O2 (desolvated
pressed pellets using
spectrometer.
a Perkin-Elmer 1600 series FTIR
Syntheses
Diaz. 1,3,6,8-Tetrahydro-2,7-dimethyl-2,7-diazapyrene (2.4 g,
1 × 10Ϫ2 mol) prepared by the literature method22 was
J. Chem. Soc., Dalton Trans., 2000, 4285–4291
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complex): C 51.75, H 2.96, N 7.55. Found: C 51.55, H 2.89, N
7.13%.
{[Ag(pyphe)]NO3ؒMeCN}∞ 8. A solution of AgNO3 (7.5 mg,
3.6 × 10Ϫ2 mmol) in MeCN (2 cm3) was added to a solution of
pyphe (10 mg, 3.6 × 10Ϫ2 mmol) in benzonitrile (2 cm3). Slow
diffusion of diethyl ether into the resulting solution gave a
colourless polycrystalline precipitate (yield 14 mg, 84%). Using
different conditions (e.g. MeCN/70 ЊC) resulted in the same
polycrystalline material. The product readily loses solvent
and can be completely desolvated in vacuo. IR, ν/cmϪ1: 3082w,
3050w, 2221m, 1584s, 1535w, 1514m, 1403m, 1385s, 1213m,
813s, 627m, 561m and 539m. Calc. for C20H12AgN3O
(desolvated complex): C 57.44, H 2.89, N 10.05. Found: C
56.98, H 2.36, N 9.68%.
{[Ag(pyphe)]PF6ؒMeCN}∞ 9. A solution of AgPF6 (9 mg,
3.6 × 10Ϫ2 mmol) in MeCN (2 cm3) was added to a solution of
pyphe (10 mg, 3.6 × 10Ϫ2 mmol) in benzonitrile (2 cm3). Slow
diffusion of diethyl ether into the resulting solution gave
colourless crystals (yield 10 mg, 48%). The product readily loses
solvent and can be completely desolvated in vacuo. IR, ν/cmϪ1
:
3080w, 3050w, 2221m, 1589s, 1538w, 1515m, 1407m, 1213m,
838vs, 627m, 561m and 539m. Calc. for C20H12AgF6N2P
(desolvated complex): C 44.64, H 2.08, N 5.65. Found: C 44.93,
H 2.25, N 5.24%.
X-Ray crystallography
Single crystal X-ray experiments were performed on a Stoe
Stadi-4 four circle diffractometer equipped with an Oxford
Cryosystems open flow cryostat23 for diaz and complexes 3–6,
an Enraf-Nonius FAST TV detector diffractometer for 2, a
Stoe IPDS image plate diffractometer for 9 and a Bruker
SMART CCD area detector diffractometer for 7, all collec-
tions used graphite Mo-Kα radiation λ = 0.71073 Å. Absorp-
tion corrections were performed by Gaussian Integration fol-
lowing refinement of the crystal morphology and dimensions
against a set of ψ scans (4 and 6) or numerical methods (3 and
5). All structures were solved using direct methods using
SHELXS 9724 and all non-hydrogen atoms located using sub-
sequent Fourier difference methods.25 In all cases hydrogen
atoms were placed in calculated positions and thereafter
allowed to ride on their parent atoms; hydrogen atoms of
acetonitrile guest molecules in complexes 4, 5, 6 and 9 were
located from ∆F syntheses; in 3 hydrogen atoms of acetonitrile
were not found because of disorder of the guest molecule. The
Ϫ
disordered bonds in the disordered BF4 anion in 4 were
restrained to have equal length. Compound 3 exhibited a dis-
order of one B–F bond modelled over two equally occupied
sites. Structure 7 has 10.5 water molecules and 7 disordered
molecules of MeOH per unit cell. Their hydrogen atoms were
not found due to the high degree of disorder in the solvent-filled
region of the structure. Crystal data for the compounds are
listed in Table 2.
CCDC reference number 186/2218.
See http://www.rsc.org/suppdata/dt/b0/b006202m/ for crys-
tallographic files in .cif format.
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