Binding sites on the outside of metallo-supramolecular architectures; engineering coordination polymers from discrete architectures

Article abstract of DOI:10.1039/b403749a

Aggregation of metallo-supramolecular architectures through additional coordination is explored by introducing metal-binding units onto the outside of the supramolecular architectures. This is achieved within the framework of our imine-based approach to supramolecular architecture, by replacing the pyridylimine units with pyrazylketimine units. An advantage of the design is that it retains the ease-of-synthesis which characterises our imine-based approach. Silver(I) complexes of three pyrazylketimine ligand systems are described. The complexes demonstrate that introducing pyrazine donor units does indeed allow higher-order assembly of the distinct supramolecular architectures into engineered coordination polymers. Two distinct types of aggregation are observed. In the first, the donors on the outside of one architecture bind to the metals of another to link the units into a polymeric array. In the second type, the donors on the outside of the architectures bind to separate metal centres which are themselves not part of the architectures, and these separate metal centres link the units to form the macromolecular array. The weaker donor nature of the pyrazine nitrogens (compared to pyridine) also introduces an additional element into the design; higher coordination numbers are favoured and this can lead to arrays with higher connectivity than those observed in the discrete pyridylimine architectures.

Full text of DOI:10.1039/b403749a

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Binding sites on the outside of metallo-supramolecular
architectures; engineering coordination polymers from discrete
architectures†
Mirela Pascu, Floriana Tuna, Edyta Kolodziejczyk, Gabriel I. Pascu, Guy Clarkson and
Michael J. Hannon*
Centre for Supramolecular and Macromolecular Chemistry, Department of Chemistry,
University of Warwick, Gibbet Hill Road, Coventry, UK CV4 7AL.
E-mail: M.J.Hannon@warwick.ac.uk
Received 11th March 2004, Accepted 2nd April 2004
First published as an Advance Article on the web 21st April 2004
Aggregation of metallo-supramolecular architectures through additional coordination is explored by introducing
metal-binding units onto the outside of the supramolecular architectures. This is achieved within the framework of
our imine-based approach to supramolecular architecture, by replacing the pyridylimine units with pyrazylketimine
units. An advantage of the design is that it retains the ease-of-synthesis which characterises our imine-based
approach. Silver() complexes of three pyrazylketimine ligand systems are described. The complexes demonstrate
that introducing pyrazine donor units does indeed allow higher-order assembly of the distinct supramolecular
architectures into engineered coordination polymers. Two distinct types of aggregation are observed. In the first,
the donors on the outside of one architecture bind to the metals of another to link the units into a polymeric array.
In the second type, the donors on the outside of the architectures bind to separate metal centres which are themselves
not part of the architectures, and these separate metal centres link the units to form the macromolecular array.
The weaker donor nature of the pyrazine nitrogens (compared to pyridine) also introduces an additional element
into the design; higher coordination numbers are favoured and this can lead to arrays with higher connectivity than
those observed in the discrete pyridylimine architectures.
actions5 or hydrogen-bond interactions6 are used to assemble
Introduction
pyridylimine metallo-supramolecular architectures into higher-
The design of complex molecular architectures using supra-
order structures. Herein we extend this approach by exploring
molecular methodology is a topic of much current interest and
the use of metal–ligand interactions to aggregate such
has afforded a variety of different nanoscale structures with
architectures.14
dimensions greater than those associated with covalent syn-
thetic approaches.1 The increase in size and in architectural
Molecular design
complexity are clear benefits of the approach, but nevertheless
the approach does rely, at least in part, on covalent synthesis
to construct the molecular building blocks which are then
assembled through the supramolecular interactions. Simplify-
ing (or minimising) the covalent aspects of the synthesis is
central to the ability to build up large supramolecular species
rapidly. In this context we have championed the use of pyr-
idylimine ligands to assemble complex metallo-supramolecular
arrays in one-pot reactions (either with or without solvent)
from commercial aldehydes, amines and metal salts.2–10 Freed
from extensive covalent synthetic procedures (our covalent
synthesis is a simple condensation reaction), we have been able
to systematically explore the design features of such arrays,
developing routes to encode the precise microstructure of the
supramolecular architectures,3 and importantly to explore the
properties of the supramolecular arrays.4
To investigate metal-based aggregation, our approach is to
incorporate additional donor groups into the covalent ligand
framework such that when the architecture is assembled, these
additional donors point out of the architecture and are avail-
able to link the distinct architectures into larger arrays. To
achieve this we chose to replace the pyridine of our pyridylim-
ine ligands with a pyrazine. In effect, this adds an additional
nitrogen opposite the pyridylimine binding site and pointing
out of the architecture. Our design approach is illustrated
schematically in Fig. 1. The advantage of adding the additional
functionality at the pyridylimine unit, rather than within a
spacer group, is that it allows us to explore the effect of the
While the covalent synthetic steps may be minimised, they
nevertheless limit the dimensions of the building blocks and
thus of the supramolecular architecture constructed. To move
to larger arrays, one approach that we5–6,11,12 and others13 are
developing is the use of multiple recognition events in concert.
Thus in an initial supramolecular event the covalent building
blocks are assembled into a distinct supramolecular archi-
tecture, and then, in a second supramolecular event, this
supramolecular architecture is assembled into a higher-order
array. We have recently described examples where π–π inter-
† Electronic supplementary information (ESI) available: Fig. S1: View
showing the herringbone style packing of the π-stacked columns in the
structure of ligand La. See http://www.rsc.org/suppdata/dt/b4/
b403749a/
Fig. 1 Schematic representation illustrating the design approach
investigated herein in which pyrazylketimines are used in place of
pyridylimines to afford additional aggregation sites on the exterior of
metallo-supramolecular units.
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aggregation unit within a range of different ligand systems that
give rise to various different architectures. Three such ligand
systems are investigated herein.
tion mixture on standing and X-ray crystallography confirms
the expected ligand structure (Fig. 3). In the solid-state struc-
ture, the ligand is essentially planar and is π-stacked into
columns, such that the imine unit of one ligand lies above the
pyrazine ring of another. The ligand adopts a trans configur-
ation about the central N–N bond and the pyrazine N1
nitrogen is similarly trans to the imino nitrogen as would be
anticipated (minimising proton–proton interactions while
maximising conjugation) and as observed in pyridylimine
systems.10,18 The π-stacked columns are packed together in a
herringbone fashion (see Fig. S1, ESI†).
Previously, in a polypyridyl ligand system which supports
metallo-supramolecular box formation, we have attached
thioether groups to the back of a pyridine ring and demon-
strated that these groups can then be used to aggregate the box
units through metal coordination.11 The aggregation site in
these pyrazylketimine systems will be different since the aggre-
gation site is now contained within the same aromatic ring
as one of the donors which assembles the architecture. Con-
sequently the aggregation site has the potential to influence,
and/or be influenced by, the architecture-forming site.
Pyrazines themselves are well known dinucleating ligands
and, as with many dinucleating N-heterocyclic ligands, have
attracted much attention from crystal engineers. Of particular
relevance to the work described herein, the coordination of
pyrazine to silver() gives a wide range of different possible
structures in which the silver exhibits coordination numbers in
the range two to six.15–17
Results and discussion
Fig. 3 Crystal and molecular structure of the ligand La illustrating the
π-stacked columns of ligands. Hydrogens are omitted for clarity.
Ligand La and its silver(I) complexes
The ligand La was reacted with one equivalent of silver()
hexafluorophosphate to afford a white solid, crystals of which
were obtained from acetonitrile by slow diffusion of tetrahydro-
furan. The crystal structure reveals a complex of 1 : 1 stoichio-
metry containing dinuclear double-helical units (Fig. 4).
To initiate our studies, we explored the system in which the two
pyrazylketimine units are directly linked through the imino
nitrogens without a spacer between the binding sites (La). We
have made a number of detailed explorations of the corre-
sponding pyridylimine ligands bearing different substituents at
the imino carbon.7–9 We found that when these ligands bound
d10 monocations a library of different solution architectures
was possible, including planar dimers, double-helices, triple-
helices and circular helicates of varying nuclearity. The pres-
ence of substituents at the imino carbon enforces ligand
twisting and thus adoption of helical architectures. In the
dimeric double-helical complexes, the silver() centres appear
dissatisfied with the coordination environment offered by the
ligand and were observed to form additional long interactions
to anions or solvent molecules (Fig. 2).8,9 Indeed, the con-
straints of the ligand lead to significant distortion from tetra-
hedral coordination, creating vacant sites for such interaction.
Such architectures were thus attractive since the additional
pyrazine nitrogens from other units might fulfil this additional
coordination role in place of the anions/solvent. This made the
La ligand system the starting point for our investigations.
These double-helical units comprise two silver() centres each
attached to two didentate pyrazylketimine binding units from
two different ligands. The ligands wrap around the metal–metal
axis giving rise to the double-helical architecture on the outside
of which are located the monodentate pyrazine nitrogens. These
double-helical structural units are analogous to those that we
have characterised for the corresponding pyridylimine ligand
systems.7–9 However, the additional pyrazine nitrogens do have
the desired aggregation effect and link these double-helical
units together by coordinating to the silver() centres of
adjacent units. There are two such interactions to each helix,
both to the same metal centre. The other metal centre interacts
with an acetonitrile solvent molecule. The interactions are long
(Ag ؒ ؒ ؒ Npz 3.32 Å; Ag ؒ ؒ ؒ NMeCN 3.46 Å) as observed pre-
viously for solvent and anion interactions in the correspond-
ing pyridylimine systems.7–9,19 The silver() centres within the
cations are separated by 4.42 Å. The twisting of the ligand
strands (which is essential for helicate formation) takes place
primarily about the N–N bond between the binding units
(torsion angle 98Њ), and the pyrazylketimine units are close to
planar (dihedral angles in the range 10–15Њ). The Ag–Npz and
Ag–Nim distances within the helices are similar to those
observed in the pyridylimine systems, with the bonds to the
imino nitrogens slightly longer than those to the pyrazines due
to the constraints of the structure.
The interactions between the double-helical units link them
into a two-dimensional network. These two-dimensional planes
are stacked together creating square channels in which the hexa-
fluorophosphate anions reside (Fig. 5) making short contacts
to the pyrazine and methyl protons and the acetonitrile.
The complex has poor solubility, having some solubility in
acetonitrile and nitromethane, but being only very poorly sol-
uble in solvents such as acetone and dichloromethane. This is in
contrast with the corresponding discrete pyridylimine com-
plexes and is consistent with the solid-state polymeric structure
described herein. We were able to obtain weak 1H NMR spectra
in acetonitrile and nitromethane solutions. At room temper-
ature a single set of peaks were observed in each case. As we
have previously shown this is unrevealing, being consistent
Fig. 2 Schematic representation of the interaction of an anion with
the double-helical cation in the analogous pyridylimine system. Solvent
molecules are also observed to interact in a similar fashion.
The ligand La is readily prepared by condensation of two
equivalents of pyrazinemethylketone with one equivalent of
hydrazine. Crystals of the ligand were obtained from the reac-
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Fig. 4 Structure of the planes of inter-connected double-helical cations in the crystal structure of complex {[Ag2(La)2][PF6]2ؒMeCN}n. The central
cation has been shaded to emphasise the double-helical architecture. Hydrogens are omitted for clarity.
Fig. 5 Packing of the cation layers (which run vertically) and the resulting channels filled with hexafluorophosphate anions in the complex
{[Ag2(La)2][PF6]ؒMeCN}n. Hydrogens are omitted for clarity.
either with a single solution species or with multiple species
interconverting rapidly on the NMR timescale as observed in
the corresponding pyridylimine complex.7–9 At lower temper-
ature (248 K in CD3NO2 and 238 K in CD3CN) the peaks
remained sharp but in nitromethane some small additional
peaks were observed in the baseline. The FAB mass spectrum
shows Ag2L2 peaks, however, we could observe no complex
peaks in electrospray (possibly consistent with polymeric solu-
tion species). It is therefore unclear whether in solution the
complex consists of dinuclear-double helicates or rather (as for
the pyridylimine complex) a rapidly interconverting library
comprising a double-helical dimer, with circular helical trimers
and possibly tetramers.
The formation of a library of architectures would seem more
likely from the pyridylimine chemistry and further support for
this comes from a recent paper from Dong and co-workers
(which appeared during the preparation of this publication).20
The focus of that paper is the design of crystalline organic–
inorganic hybrid networks and describes “the first example of
an extended . . . 3D network containing . . . organic, organic/
inorganic hybrid and inorganic components”. The paper
describes two additional silver() complex salts of ligand La
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Fig. 6 View illustrating that the cationic network in {[Ag5(La)3(NO3)3][Ag(NO3)3]2ؒ3CHCl3}n reported in ref. 18 represents an aggregated analogue
of the trimeric circular helicate that we have previously reported in ref. 8. The strands of the circular helicate in the bottom left corner of the diagram
have been coloured to highlight the circular helical nature of the components. Coordinated nitrates, anions, encapsulated solvent and hydrogens have
been omitted for clarity.
with different anions. Although the focus of that paper lies
elsewhere, the structures are clearly pertinent to the design
approach that we are exploring. Consequently we have exam-
ined these structures in greater detail to explore whether they
are also aggregates of the specific architectures that we have
characterised for the pyridylimine ligands.
superficially similar to the structure of the hexafluoro-
phosphate salt that we report herein. Indeed the helical units
are again aggregated through coordination of the external
nitrogens of one unit to the silver() centres of another unit.
However, the connectivity of the helicate units is different and
this leads to a quite different macromolecular structure. In the
PF6 salt we have described herein, two pyrazines at the same end
of the helix form connection points and coordinate to the
silver() centres of adjacent helicates. By contrast, in Dong’s
SbF6 structure two pyrazines at opposite ends of the helix form
connection points. Each silver() centre is five-coordinate,
bound to a single monodentate pyrazine, and the bond to the
monodentate pyrazine (∼2.6 Å) is consequently shorter than
that in our hexafluorophosphate salt.
Our analysis of the cationic network in Dong’s structure
{[Ag5(La)3(NO3)3][Ag(NO3)3]2ؒ3CHCl3}n reveals that it repre-
sents an aggregated analogue of the trimeric circular helicate
that we have proposed as a component of the solution library
for the silver() complex of the pyridylimine ligand7 and which
we have structurally characterised for the corresponding
copper() complex.8 This is illustrated in Fig. 6. The trinuclear
circular helicate is formed from three ligands wrapping
around three silver() centres, affording pseudo-tetrahedral co-
ordination at each silver() centre. The circular helical archi-
tectures themselves are very similar to the cuprous analogue8
that we have previously described. In this silver() structure,
chloroform solvent molecules are encapsulated into the cavities
formed by the aromatic rings on the upper and lower faces of
the circular helicate, just as hexafluorophosphate anions and
diethyl ether solvent are located in these cavities in the corre-
sponding copper() pyridylimine trinuclear circular helicate.8
The coordination environment in the trinuclear circular helicate
structure does not afford the additional coordination sites
(filled with anions or solvent) present in the dinuclear double-
helical structures. With such sites not available for aggregation,
the nature of the aggregation is consequently different. Rather
than the external nitrogens of one unit coordinating to the
silver centre of another unit, the monodentate pyrazine donors
which point out from these trinuclear units bind to additional
silver() centres and silver() nitrate units which form four and
two links, respectively, and link the circular helicates into a 3D
array.
Silver(I) complex of ligand Lb
To explore the approach further, we investigated the ligand sys-
tem in which a phenylene unit separates the binding domains
(Lb). Reaction of silver() with the corresponding bis-pyridylim-
ine ligand21 can, depending on the anion, give rise either to
tetranuclear grid structures (similar to those reported by
Youinou et al.22 and by Lehn and co-workers23) or, more fre-
quently, linear polymers analogous to those we have described
with a 1,5-naphthyl spacer.10 In these structures, the silver()
centres are bound to two pyridylimine units. However (in con-
trast to the double-helical structures described for La and its
pyridylimine analogue7–9) the silver environment is close to
tetrahedral and does not have open sites to which additional
donors might coordinate. We anticipated that, for Lb, the diden-
tate pyrazylketimine sites should give rise to similar structures
to those observed with the corresponding pyridylimine ligand
but that the monodentate pyrazyl nitrogens located on the
Our analysis of the cationic network in Dong’s second struc-
ture {[Ag2(La)2][SbF6]2ؒCH2Cl2}n reveals that it represents an
aggregate of double-helical architectures. This is illustrated in
Fig. 7. In so far as it is an aggregate of double-helices, it is
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Fig. 7 View illustrating that the cationic network in the structure {[Ag2(La)2][SbF6]2ؒCH2Cl2}n reported in ref. 18 represents an aggregate of double-
helical units. Each double-helicate has been coloured to highlight the nature of the components. Anions, solvent and hydrogens have been omitted for
clarity.
Fig. 8 View of the complex {[Ag(Lb)][BF4]}n illustrating the brickwork-like network associated with the first type of silver() centre. Hydrogens,
anions and the second silver() sites are omitted for clarity. The three ligands about one silver() centre are shaded to illustrate the coordination and
connectivity.
exterior would form additional coordinate interactions to other
distinct metal centres (rather than to vacant sites on metals
supporting the architectural motifs) as seen for the trinuclear
circular helicate complex of La. Although this is indeed the
basic principle of what is observed the lower donor ability of
the pyrazine (compared to pyridine) itself introduces further
elements of complexity.
Reaction of the ligand Lb with silver() acetate followed by
precipitation as the tetrafluoroborate anion and recrystallis-
ation from acetonitrile by the slow diffusion of diethyl ether
afforded crystals suitable for X-ray characterisation. The struc-
ture reveals a complex of 1 : 1 stoichiometry containing two
distinct silver() centres, one coordinated only to didentate
pyrazylketimine units and the other only to monodentate
pyrazine donors. Dealing with each silver() centre in turn:
The first silver() centre is coordinated to three didentate
pyrazylketimine units. With pyridylimines the usual co-
ordination number is four and this higher coordination number
(six) reflects the lower donor ability of pyrazine. This is also
reflected is the Ag–Npz bond lengths of ∼2.5 Å which are about
10% longer than those to pyridine in pyridylimine com-
plexes. The increase in number of ligands coordinated to the
metal leads to a two-dimensional polymeric network with a
brickwork-like net structure (rather than a one-dimensional
chain observed with the pyridylimines10,21). This is shown in
Fig. 8.
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Fig. 9 Space filling views of the complex {[Ag(Lb)][BF4]}n illustrating the networks associated with the second type of silver() centre. Hydrogens,
anions and the first silver() sites are omitted for clarity. (a) illustrating the rhombohedral cubic nature with different ligands shaded in different
colours; (b) illustrating the triple interpenetration of the rhomboidal cubic networks with the three nets shaded in different colours
The second silver() centre also has coordination number
six and is bound to six monodentate pyrazines. The con-
sequence of this is the formation of a three-dimensional net-
work of cube-like architecture (Fig. 9). Although the angles at
the metal are very close to 90Њ, the monodentate pyrazine
donors are not located along the long axis of the ligand but
instead located on the rings in a position ortho to this axis.
Consequently the sides are not located perpendicular at the
corners of the cube and thus a network based on rhombohedral
cubes results.
pyrazines per silver from each of two nets) and thereby link the
nets together.
(ii) As three interpenetrating rhombohedral cuboid networks
which are connected together by silver() centres which are each
attached to three strands, one from each of the three interpene-
trating networks.
The tetrafluoroborate anions reside in small voids in this
three-dimensional network and form short contacts to various
protons on the ligand strands. Using description (i) of the
structure, one anion lies between the brickwork-like nets and
forms short contacts to pyrazine and methyl hydrogens, while
the other lies within the nets forming short contacts to
phenyl and methyl hydrogens. The three pyrazylketimine
ligands are oriented in a fac conformation and within the
two-dimensional brickwork-like nets the chirality at adjacent
silver() centres alternates, rendering the net achiral. The
pyrazylketimine unit is approximately planar (torsion angle 2Њ)
while the phenyl group is twisted through 60Њ with respect to
that plane.
For the d10 monocations silver() and copper() coordination
numbers of two and four are more usual, and in consequence
with bis-monodentate ligands adamantoid networks (co-
ordination number four) or linear chains (coordination number
two) are more common. However a variety of different net-
works have been observed for pyrazine with silver()15–17 and
among these is one example of a cubic network containing
six-coordinate silver centres.15 Despite the great number of
metallo-organic coordination frameworks that have been pre-
pared by crystal engineers, three-dimensional cubic metallo-
organic coordination networks remain rare. This is somewhat
surprising since a cubic network is the logical consequence of
interacting all six sites of an octahedral metal centre with a
linear bis-monodentate ligand such as 4,4Ј-bipyridine; Within
metallo-organic networks formed by octahedral centres, anions
and/or solvent molecules frequently coordinate to the metal,
thereby lowering the extent of network connectivity.24 Rect-
angular-cube shaped hetero-ligand frameworks have been pre-
pared by linking two-dimensional square grids either through
coordination or other non-covalent interactions.25
The complex shows poor solubility (consistent with the
polymeric structure) except in strongly coordinating solvents
such as acetonitrile, in which electrospray MS indicates that the
polymeric structure fragments.
Silver(I) complex of ligand Lc
Within three-dimensional network structures, network inter-
penetration is common, since the voids created by the network
are frequently large.26 Indeed the extent of interpenetration is
governed by the length of the bis-monodentate ligand. In
Ciani’s prototypical {Ag(pz)3(SbF6)}n cubic metallo-organic
framework, the cavities are small and filled by the anions; inter-
penetration is not observed. In the rhombohedral cuboid net-
work described herein the more extended ligand leads to large
voids in the network which are filled by interpenetration of
three distinct networks (Fig. 7(b)).
Both types of silver() centres (and the networks they give rise
to) operate on the same ligands and so the overall structure may
be described either:
(i) as planes of two-dimensional brickwork-like net struc-
tures separated by planes containing silver() centres which bind
the monodentate pyrazines from each brickwork-like net (three
Finally we decided to explore coordination to the ligand system
which contains 4,4Ј-diarylmethane as a spacer (Ld). The bis-
pyridylimine analogue of Ld gives dinuclear double-stranded
species with either helical (rac-) or box (meso-) conform-
ation,2a,3a and containing four-coordinate pseudo-tetrahedral
silver() centres. The ligand structure can support triple-helix
formation with metal ions which bind six donors.2a Con-
sequently a series of linked triple-helices (if the silver is six
coordinate) or double-stranded structures (if the silver is four
coordinate) might be anticipated with Ld.
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It did not prove possible to prepare and isolate the ligand Ld
directly; a mixture of starting materials and ligands was
obtained even on extended reflux with molecular sieves. We
have previously encountered similar problems with a bis-
pyridylimine ligand containing a binapthyl spacer, but were able
to prepare the complexes directly without isolating the ligand.2b
We applied the same approach for this ligand. Solutions of two
equivalents of acetylpyrazine and one equivalent of 4,4Ј-
methanedianaline were mixed and heated and then the solution
treated with one equivalent of silver() tetrafluoroborate to
afford a silver() tetrafluoroborate salt, crystals of which were
obtained from acetonitrile by slow diffusion of tetrahydro-
furan. The structure reveals that in this case the silver() has
found an alternative way to satisfy its coordination require-
ments; the complex contains not ligand Ld but rather a ‘half-
ligand’ Lc which consists of a pyrazylketimine group at one end
with an aniline group at the other. The same initial ligand mix-
ture treated with octahedral metal ions afforded triple-helical
complexes of the expected Ld thereby confirming that Ld is
available in the solution but has not been selected in this silver()
complex.27
The structure of this 1 : 1 complex of Lc reveals each ligand
to be bridging three silver() centres and binding as a didentate
pyrazylketimine, a monodentate pyrazine and a monodentate
aniline. Each silver() centre is bound to three ligands; one as a
didentate pyrazylketimine, one as a monodentate pyrazine and
the third as a monodentate aniline. The silver() centres are five-
coordinate with the fifth coordination site being a longer con-
tact to the oxygen of a thf solvent molecule (Ag ؒ ؒ ؒ O 2.65 Å).
The structure consists of planes of networks which extend in
two dimensions. Two ligands bridge two metals through the
monodentate pyrazine and monodentate aniline groups
(Fig. 10) giving a dinuclear double-stranded box-like struc-
ture2a,3 with the ligands arranged in a head-to-tail configur-
ation.3b This box unit then has two pyrazylketimine units on the
outside which link the boxes together to give the polymeric
array. Alternatively the structure may be viewed as linear chains
formed by coordination through the pyrazylketimine and mon-
odentate pyrazine, and that these chains are linked into a 2D
network through coordination to the aniline.
The two-dimensional network sheets are packed together
with the thf solvents interdigitated and forming a thf layer sep-
arating the coordination planes. The tetrafluoroborate anions
are packed towards the edges of the planes by (but not in) the
thf layer that separates the planes. They form a number of short
contacts to thf, pyrazine, and phenyl hydrogens but the most
striking contacts are two hydrogen bonds from the aniline
amino hydrogens to two fluorines of the anion (N–H ؒ ؒ ؒ F
2.09, 2.15 Å; N–H ؒ ؒ ؒ F 165, 161Њ; N ؒ ؒ ؒ F 2.98, 3.03 Å).
As with the other polymeric complexes herein, this complex
shows poor solubility in most solvents. Although the complex
appears to show some limited solubility in acetonitrile, NMR
and electrospray MS indicate that the acetonitrile coordinates
to the silver() ion causing complex degradation to give a
mixture of Lc and starting materials.
Fig. 10 Views of the complex {[Ag(Lc)(thf )][BF4]}n illustrating the
network. Hydrogens and anions are omitted for clarity: (a) illustrating
the planes terminated by the thf layers, with two ligands shaded to
illustrate the box structures; (b) the view perpendicular to view (a) and
with the carbons of the thf removed for clarity.
Conclusion
The results obtained demonstrate that by introducing pyrazine
donor units we can indeed induce higher-order assembly of
the supramolecular architectures observed with pyridylimine
ligands. Two distinct types of aggregation can occur. In the
first, the donors on the outside of one architecture bind to the
metals of another architecture to link the units into a poly-
meric array. In the second type, the donors on the outside of the
architectures bind to separate metal centres which are them-
selves not part of the architectures, and these metals link
the units to form the macromolecular array. Both modes are
observed in the silver() complexes of ligand La, with the
double-helical units being aggregated by the first mode and the
trinuclear circular helicates aggregated by the second mode.
The second mode is associated with a higher ratio of metal :
ligand as observed in the stoichiometry of the complex contain-
ing the trinuclear circular helical units. Indeed it seems apparent
that in this way (for the La ligand system) the metal : ligand
stoichiometry can affect which architecture (double-helix or
trinuclear circular helix) is selected from the available library of
architectures as the building unit for the macromolecular array.
For the two double-helical complexes of La the connectivity of
the helicate units is different and this leads to quite different
macromolecular structures. In these two structures not only are
the anions different but the solvents from which the salts were
D a l t o n T r a n s . , 2 0 0 4 , 1 5 4 6 – 1 5 5 5
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crystallised also differ. In particular, it is striking that in the
hexafluorophosphate salt, the silver() centre at one end of the
helix is coordinated to an acetonitrile solvent molecule and is
not involved in the aggregation. By way of contrast, the hexa-
fluoroantimonate salt was crystallised from (and contains) the
non-coordinating solvent dichloromethane. Consequently the
silver() centres at both ends of the helix are available for and
engage in the aggregation. Coordinating solvents may therefore
be able to play a significant role in determining the aggregation
pattern.
Within crystal-engineered coordination polymers, anion
effects are common when the anion is coordinated to the metal
centre.28 Effects of non-coordinating anions, while less com-
mon, can result in switching between different coordination
polymer motifs.29 Such anion effects relate both to the ability of
the anions to form C–H ؒ ؒ ؒ X contacts (the number, spatial
distribution and nature of their hydrogen bond acceptors) and
to the sizes of the anions in relation to the voids in the struc-
tures. However, with current knowledge, prediction of potential
structures and of how they might be selected by a given anion
(or solvent) is not yet feasible. For such a system, guidelines for
architecture control are most likely to arise through experi-
mental observation. In this instance, both of the observed
macromolecular structures assembled from double-helical units
afford tube-like channels in which the anions reside, with the
number and pattern of the F ؒ ؒ ؒ H contacts being similar in
both cases.
While the pyrazines do lead to the desired aggregation being
achieved, the weaker donor nature of the pyrazine nitrogens
(compared to pyridine) introduces an additional element into
the design, since higher coordination numbers are favoured. For
the two double-helical complexes of La this feature reinforces
the design, providing the increase in coordination sites required
to link the units directly without further metal centres. How-
ever, since the nature of the discrete architecture is determined
by the metal–ligand interactions, the nature of the architecture
can also be affected by the switch to pyrazine. This is the case
for the Lb ligand system in which the expansion in the co-
ordination number (to six) leads to an increase in the connectiv-
ity of the array and the formation of a 2D brickwork-like net
rather than a 1D linear chain as seen in the corresponding
pyridylimine system. The pyrazine nitrogens on the outside of
this brickwork-like 2D-net structure are then connected to
separate metal centres which link the units to form the macro-
molecular array, giving aggregation through the second mode.
This structure shows further similarities to the aggregate of
trinuclear circular helices in that both contain the pyridylimine-
type architectural motif (albeit with increased connectivity in
the case of Lb) linked by planes of connecting silver() centres.
While the trinuclear circular helical La structure represents
an aggregate of a discrete architecture, the Lb structure is in
effect an aggregate of a metallo-supramolecular coordination
polymer.
normally accessible with the pyridylimines, as illustrated for Lb.
This is the subject of continuing studies in our laboratories.
Experimental
General
All starting materials were purchased from Aldrich and used
without further purification. Infrared spectra were recorded
with a Perkin Elmer Paragon 1000 FTIR spectrometer from
KBr pellets. NMR spectra were recorded on Brüker DPX 300
and 400 instruments using standard Brüker software. FAB
mass spectra were recorded by the Warwick mass spectrometry
service on a Micromass Autospec spectrometer using 3-nitro-
benzyl alcohol as matrix. Electrospray ionisation (ESI) analyses
were performed by the EPSRC National Mass Spectrometry
Service Centre, Swansea on a Micromass Quatro (II) in positive
ionisation mode. Samples were loop injected into a stream of
water–methanol (1 : 1). Nebulisation was pneumatically
assisted by a flow of nitrogen through a sheath around the
capillary. Capillary (ionising) voltage ϩ 3.5 kV; source voltage
20 V. Microanalyses were conducted on a Leeman Labs CE44
CHN analyser by the University of Warwick Analytical
Service.
Syntheses
Preparation of La. To a stirred solution of acetylpyrazine
(0.244 g, 2 mmol) in methanol was added dropwise hydrazine
monohydrate (0.05 g, 1 mmol) and a few drops of glacial acetic
acid. The mixture was heated under reflux for 4 h and then
cooled to room temperature. Orange crystals precipitated from
the solution on standing and were collected by vacuum fil-
tration and washed with methanol (0.2 g, 83%). X-Ray quality,
orange crystals were obtained by slow evaporation of the
solution.
Mass spectrum (FAB): m/z 241 [M ϩ H]ϩ.
Elemental analysis: calc. (%) for C12H12N6: C: 59.9, H: 5.0, N:
34.9; found: C: 59.8, H: 5.0, N: 34.8.
1H NMR (300 MHz, CDCl3, 298 K): δ 9.46 (1H, s, H3), 8.58
(2H, m, H5, H6), 2.37 (3H, s, CH3).
IR: ν 3047 (m), 1611 (m), 1571 (m), 1466 (s), 1397 (m), 1363
(vs), 1301 (m), 1158 (m), 1101 (m), 1046 (m), 1012 (vs), 849 (s),
757 (w), 653 (m) cmϪ1
.
Coordination of La to silver(I). Care was taken to exclude light
during the following procedure: Ligand La (0.024 g, 0.1 mmol)
and silver() hexafluorophosphate (0.025 g, 0.1 mmol) were
stirred in methanol for 30 min. The resulting yellow precipitate
(0.074 g, 75%) was collected by vacuum filtration and washed
with methanol. X-Ray quality, pale yellow crystals were
obtained by slow diffusion of tetrahydrofuran into a solution
of the complex in acetonitrile.
Mass spectrum (FAB) m/z 841 [Ag2L2(PF6)], 696 [Ag2L2].
Elemental analysis: calc. (%) for [Ag2(C12H12N6)2](PF6)2ؒ
H2O: C: 28.7, H: 2.6, N: 16.7; found: C: 28.9, H: 2.6, N: 16.6.
1H NMR (400 MHz, CD3NO2, 298 K): δ 9.11 (1H s, H3), 8.85
(1H d, J = 2.5 Hz, H5/6), 8.69 (1H, dd, J = 2.5, 1.5 Hz, H5/6), 2.29
(3H, s, CH3).
The observation of the half-ligand Lc complex was un-
expected and might be ascribed to the lower donor ability of
pyrazine being offset by conversion of one end of the ligand to
an amine donor, although the instability of the ligand itself
(and of the complex in acetonitrile) implies that the situation
may be more complex.
When comparing the metallo-aggregation investigated herein
with the H-bond and π–π aggregation approaches that we have
previously investigated, it is clear that the design of aggregates
is conceptually simpler when two different types of interactions
are used in concert (one type for the assembly of the archi-
tectures and another for the aggregation)5,6 or when the
external aggregation site is removed from the architecture
assembly site.11 However, the effect of electronically linking the
aggregation and architecture assembly sites as in the design
herein does also have potential benefits by allowing con-
struction (and subsequent aggregation) of architectures not
IR: ν 3647 (w), 1614 (m), 1571 (m), 1405 (m), 1371 (m), 1319
(s), 1163 (m), 1128 (m), 1051 (s), 1034 (m), 831 (vs), 736 (w),
cmϪ1
.
Preparation of Lb. To a stirred solution of acetylpyrazine
(0.244 g, 2 mmol) in methanol was added dropwise 1,4-phenyl-
enediamine in methanol (0.104 g, 1 mmol) and a few drops of
glacial acetic acid. The mixture was heated under reflux for 3 h
and then cooled to room temperature. A yellow solid precipi-
tated from the solution on standing and was collected by
filtration and washed with methanol (0.268 g, 85%).
Mass spectrum (FAB): m/z 317 [M ϩ H]ϩ.
D a l t o n T r a n s . , 2 0 0 4 , 1 5 4 6 – 1 5 5 5
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Table 1 Crystallographic data and details of refinement
La
{[Ag2(La)2][PF6]2ؒMeCN}n
{[Ag(Lb)][BF4]}n
{[Ag(Lc)][BF4]ؒthf}n
Empirical formula
Mr
C12H12N6
240.28
C26H27Ag2F12N13P2
1027.29
C18H16AgBF4N6
511.05
C23H26AgBF4N4O
569.16
T /K
Crystal system
Space group
180(2)
Monoclinic
P21/n
180(2)
Orthorhombic
Pbcn
180(2)
180(2)
Monoclinic
P21/c
Trigonal
¯
P3
a/Å
b/Å
c/Å
4.4272(16)
7.440(3)
17.810(6)
90.344(9)
586.6(4)
2
15.1092(4)
15.3225(4)
15.7434(12)
90
3644.8(3)
4
10.3804(11)
10.3804(11)
16.099(3)
90
1502.3(3)
3
18.4389(13)
9.8630(6)
13.5869(9)
106.578(2)
2368.2(3)
4
β/Њ
U/Å3
Z
Dc/g cmϪ3
1.360
0.090
1.872
1.265
1.695
1.060
1.596
0.906
µ/mmϪ1
Reflections collected
Independent reflections (Rint
Parameters
3516
1411 (0.0402)
83
0.877
R1 = 0.0497,
wR2 = 0.1236
17193
4507 (0.0492)
292
0.812
R1 = 0.0357,
wR2 = 0.0933
7601
1780 (0.1133)
164
1.003
R1 = 0.0561,
wR2 = 0.1048
11833
4151 (0.1737)
326
0.987
R1 = 0.0700,
wR2 = 0.1430
)
Goodness-of-fit on F 2
Final R indices [I > 2σ(I )]
Elemental analysis: calc. (%) for C18H16N6: C: 68.3, H: 5.1, N:
26.6; found: C: 68.2, H: 5.1, N: 26.5
X-Ray crystallography
Crystallographic data are collected in Table 1. Crystal data for
the compounds were collected with a Siemens-SMART-CCD
diffractometer30 equipped with an Oxford Cryosystem Cryo-
stream Cooler.31 Refinements used SHELXTL.32 Systematic
absences indicated the appropriate space groups, shown to be
correct by successful refinements. The structures were solved by
direct methods with additional light atoms found by Fourier
methods. Hydrogen atoms were added at calculated positions
and refined using a riding model with freely rotating methyl
groups. Anisotropic displacement parameters were used for all
non-H atoms; H-atoms were given isotropic displacement
parameters equal to 1.2 (or 1.5 for methyl hydrogen atoms)
times the equivalent isotropic displacement parameter of the
atom to which the H-atom is attached.
1H NMR (300 MHz, CD2Cl2, 298 K): δ 9.49 (2H, s, H3), 8.63
(2H, m, H5, H6), 6.93 (4H, s, HPh), 2.42 (6H, s, CH3).
IR: ν 2989 (w), 1635 (s), 1571 (w), 1519 (w), 1494 (s), 1470
(m), 1404 (s), 1362 (s), 1298 (m), 1228 (m), 1153 (m), 1100 (s),
1044 (s), 1014 (s), 969 (w), 850 (vs), 749 (m), cmϪ1
.
Coordination of Lb to silver(I). Care was taken to exclude light
during the following procedure: Ligand Lb (0.054 g, 0.17 mmol)
and silver() acetate (0.028 g, 0.17 mmol) in methanol were
heated under reflux for 30 min, cooled to room temperature,
filtered through Celite and treated with methanolic ammonium
tetrafluoroborate. The resulting orange precipitate was col-
lected by vacuum filtration and washed with methanol (0.056 g,
64%). X-Ray quality, pale orange crystals were obtained by
slow diffusion of diethyl ether into a solution of the complex in
acetonitrile.
CCDC reference numbers 233558–233561.
See http://www.rsc.org/suppdata/dt/b4/b403749a/ for crystal-
lographic data in CIF or other electronic format.
Mass spectrum (FAB): m/z 425 [AgL]ϩ.
Mass spectrum (ES; MeCN) m/z 425 [AgL]ϩ, 317 [LH]ϩ.
Elemental analysis: calc. (%) for [Ag(C18H16N6)(BF4)ؒH2O]n:
C: 40.9, H: 3.5, N: 15.9; found: C: 41.0, H: 3.1, N: 15.9
1H NMR (400 MHz, CD3CN, 298 K): δ 9.39 (1H s, H3), 8.73
(1H d, J = 2.5 Hz, H5/6), 8.65 (1H, dd, J = 1.5, 0.75 Hz, H5/6),
6.95 (2H, s, HPh), 2.40 (3H, s, CH3).
{[Ag2(La)2][PF6]2ؒMeCN}n. The asymmetric unit contains
one ligand and two half silvers, a disordered PF6 and half an
acetonitrile solvent molecule. The PF6 is disordered about the
equator over two positions with nearly equal occupancy. The
acetonitrile lies on special position (4c) as do both silver ions.
IR: ν 2996 (w), 1632 (m), 1448 (m), 1401 (m), 1373 (m),
1304 (m), 1220 (m), 1173 (w), 1122 (m), 1045 (vs), 858 (m),
{[Ag(Lb)][BF4]}n. The asymmetric unit contains half a ligand
with two silvers and two BF4 groups on special positions. Ag1
lies on the three-fold inversion axis, and Ag2 lies on a three-fold
axis. B10 lies on a three-fold axis and B20 lies on three-fold axis.
F21 lies on the three-fold inversion axis and the B20 BF4 is
disordered in two positions about F21 along the three-fold axis.
The B10 BF4 was very disordered and the fluorines were
modelled over two positions (occupancy 7 : 3) with one of the
fluorines of the minor component (F14) lying on the three-fold
axis (position 3d).
749 (w), cmϪ1
.
Preparation of Lc and coordination to silver(I). To a stirred
solution of acetylpyrazine (0.041 g, 0.30 mmol) and ground 3Å
dried molecular sieves in methanol was added dropwise bis-
(4-aminophenyl)methane (0.033 g, 0.15 mmol) in methanol and
a few drops of glacial acetic acid. The mixture was heated under
reflux for 24 h. The molecular sieves were removed by filtration
and the filtrate treated with silver() tetrafluoroborate (0.029 g,
0.15 mmol) and stirred for 30 min. The resulting yellow precipi-
tate (0.051 g, 68%) was collected by vacuum filtration and
washed with methanol. X-Ray quality, pale yellow crystals were
obtained by slow diffusion of tetrahydrofuran into a solution
of the complex in acetonitrile.
{[Ag(Lc)][BF4]ؒthf}n. The asymmetric unit contains a mono
pyridine imine ligand with a free amine end, a silver, a dis-
ordered BF4 and a disordered THF. The THF disorder was
modelled as two related envelope forms that share the some
O001 C003 and C004. These were fixed at 50 : 50 occupancy.
The fluorines of the BF4 were disordered over two positions
Mass spectrum (FAB): m/z 411 [AgL]ϩ.
Elemental analysis: calc. (%) for [Ag(C19H18N4)(BF4)ؒ
0.5MeOH]n; C: 45.6, H: 3.9, N: 10.9: found: C: 46.1, H: 3.9, N:
10.6
(occupancy 9 : 1) with the minor component refined
isotropically.
IR: ν 3305 (w), 1612 (m), 1512 (s), 1440 (m), 1409 (m),
1372 (m), 1313 (m), 1224 (m), 1178 (m), 1128 (m), 1051 (vs),
La. The asymmetric unit contains half a molecule as the
molecule lies on an inversion centre between the two nitrogens
of the aza group.
831 (m), 764 (w), cmϪ1
.
D a l t o n T r a n s . , 2 0 0 4 , 1 5 4 6 – 1 5 5 5
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13 See, for example: R. F. Carina, G. Bernardinelli and A. F. Williams,
Acknowledgements
Angew. Chem., Int. Ed., 1993, 32, 1463; E. Breuning, U. Ziener, J.-M.
Lehn, E. Wegelius and K. Rissanen, Eur. J. Inorg. Chem., 2001, 1515;
Z. Qin, M. C. Jennings and R. J. Puddephatt, Chem. Commun.,
2001, 2676; S-Y. Yu, T. Kusukawa, K. Biradha and M. Fujita, J. Am.
Chem. Soc., 2000, 122, 2665; R. W. Saalfrank, I. Bernt and
F. Hampel, Chem. Eur. J., 2001, 7, 2770; E. C. Constable, A. J.
Edwards, P. R. Raithby and J. V. Walker, Angew. Chem., Int. Ed.,
1993, 32, 1465; M. Vázquez, A. Taglietti, D. Gatteschi, L. Sorace,
C. Sangregorio, A. M. González, M. Maneiro, R. M. Pedrido and
M. R. Bermejo, Chem. Commun., 2003, 1840.
This work was supported by an EU Marie Curie Individual
Fellowship (F. T.; MCFI-2000-00403) and EU Marie Curie
Training Site Fellowships in Supramolecular and Macro-
molecular Chemistry (M. P.; HPMT-GH-01-00365-11; G. I. P.;
HPMT-GH-01-00365-10; E. J. K.; HPMT-GH-01-00365-14).
We thank EPSRC and Siemens Analytical Instruments for
grants in support of the diffractometer and the EPSRC
National Mass Spectrometry Service Centre, Swansea for
recording the electrospray mass spectra.
14 For examples of supramolecular helicates aggregated in covalent
macromolecular structures, see: A. Lavalette, J. Hamblin, A. Marsh,
D. M. Haddleton and M. J. Hannon, Chem. Commun., 2002,
3040; H. Houjou, Y. Shimizu, N. Koshizaki and M. Kanesato,
Adv. Mater., 2003, 15, 1458.
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