Dimensional caging of polyiodides: Cation-templated synthesis using bipyridinium salts

Article abstract of DOI:10.1039/c0ce00860e

The potential of bipyridinium derivatives in the cation templated synthesis of polyiodides has been explored by applying the strategy of size-matching between cations and anions. Bipyridinium cations 1-4, bearing benzyl and functionalized benzyl pendants at nitrogen atoms, are able to template the selective formation of I₄^2- and I₃^- species. Thanks to the supramolecular space compartmentation induced by the benzyl pendants, the formation of I₄^2-and I ₃^- is independent of the stoichiometry adopted in the crystallization procedure. Bipyridinium cation 5, bearing methyl pendants, is unable to induce space compartmentation and different polyiodides are obtained depending on the stoichiometry used in the crystallization process as the cation-anion size-matching alone does not control the polyiodide formation.

Full text of DOI:10.1039/c0ce00860e

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PAPER
Dimensional caging of polyiodides: cation-templated synthesis using
bipyridinium salts†
a
Marcos D. Garcıa, Javier Martı-Rujas, Pierangelo Metrangolo,
b
bc
Carlos Peinador,a Tullio Pilati,dc
*
bcd
*
ꢀ
Giuseppe Resnati,
ꢀ
Giancarlo Terraneobc and Maurizio Ursinibc
*
Received 16th November 2010, Accepted 31st March 2011
DOI: 10.1039/c0ce00860e
The potential of bipyridinium derivatives in the cation templated synthesis of polyiodides has been
explored by applying the strategy of size-matching between cations and anions. Bipyridinium cations
1–4, bearing benzyl and functionalized benzyl pendants at nitrogen atoms, are able to template the
selective formation of I42ꢀ and I3ꢀ species. Thanks to the supramolecular space compartmentation
induced by the benzyl pendants, the formation of I42ꢀand I3ꢀ is independent of the stoichiometry
adopted in the crystallization procedure. Bipyridinium cation 5, bearing methyl pendants, is unable to
induce space compartmentation and different polyiodides are obtained depending on the stoichiometry
used in the crystallization process as the cation–anion size-matching alone does not control the
polyiodide formation.
2ꢀ
unstable species I4 has been postulated to be the charge-
Introduction
transfer intermediate responsible for the charge-transport
mechanism in Iꢀ/I3ꢀ-based electrolytes of dye sensitized solar
cells (DSSCs),5 following a Grotthuss-like process.6
Polyiodides are extended structures with the general formula
nꢀ
I2m+n (m and n integers > 0, n ¼ 1–4).1 They form from the
ꢀ
building blocks Iꢀ/I3 and I2, which self-assemble as a conse-
The most common approach for the systematic study of poly-
iodide species is the preparation and characterization of simple
model systems: i.e. stable polyiodide salts with known crystal
structures. One of the major drawbacks of this approach is the
lack of synthetic control over the system as a subtle and unpre-
dictable balance of a large number of parameters frequently
influences the composition and structure of solid polyiodides
formed in a given synthesis (e.g. stoichiometry of the reagents,
nature of the cation, crystallization solvent(s), etc.).1 To
quence of the well-known tendency of the heavier halogen atoms
(especially iodine) to catenate. The exact nature of the connec-
tions between these building blocks in polyiodide species has long
been debated. In a large number of cases, the connection in
polyiodides can be rationalized in terms of halogen bonding,2
with Iꢀ/I3 subunits functioning as halogen bonding-acceptors
ꢀ
(Lewis bases, electron-donors) and molecular I2 functioning as
halogen bonding-donor (Lewis acid, electron-acceptor).3
The availability of general, selective, and robust synthetic
protocols for solid polyiodides is of great importance as the study
of their properties impacts in many applicative fields due to their
peculiar electric properties spanning from conduction values
typical of insulators to those of metals.1d,4 Moreover, the
ꢀ
circumvent this problem, metal complexes containing Iꢀ and I3
ions have been extensively used in cation-templated synthesis of
higher polyiodides.1 However, the design of metal ion complex is
rather difficult and metal coordination may strongly modify the
nature of the interactions involving Iꢀ/I3ꢀ and I2 building blocks
in the polyiodide chain.7 For all of these reasons, the general and
selective preparation of organic polyiodide salts by using the
reliable principles of supramolecular chemistry and crystal
engineering is a unique and fascinating goal.
aDepartamento de Quꢀımica Fundamental, Facultade de Ciencias,
Universidade da Corun~a, Campus A Zapateira, 15071 A Corun~a, Spain.
E-mail: mdgarcia@udc.es; Fax: +34 981 167065
bCNST-IIT@POLIMI, Via Pascoli 70/3, 20133 Milan, Italy
cNFMLab, D.C.M.I.C. ‘‘Giulio Natta’’, Politecnico di Milano, Via L.
Mancinelli 7, 20131 Milan, Italy. E-mail: pierangelo.metrangolo@polimi.
it; giuseppe.resnati@polimi.it; Fax: +39 02 2399 3180; Tel: +39 02 2399
3041, +39 02 2399 3032
Long-chain alkyl ammonium cations have been successfully
used for the dimensional caging of polyiodides. In fact, the
known tendency of these cations to organize into lamellar
structures, forces the polyiodide anions to be confined in the
vicinity of the positive charges.8 Consistent with this approach,
some of the present authors have shown that 1,6-bis(trimethy-
lammonium)hexane diiodide (BTMAH$2Iꢀ) selectively encap-
sulates iodine and yields the rare polyiodide species I42ꢀ thanks to
the size-matching between the N+–N+ separation in the dication
dISTM-CNR, Universitaꢂ di Milano, Via Venezian 21, 20133 Milan, Italy
† Electronic supplementary information (ESI) available: Synthetic and
crystallization protocols, spectroscopic characterization of compounds
1–3$2Iꢀ, 1$I42ꢀ, 2–4$2I3ꢀ; synthesis and X-ray crystallographic data for
ꢀ
ꢀ
2ꢀ
2ꢀ
structures 5$2I3
,
5$Iꢀ$I3
,
5$I3ꢀ$½I8
,
5$I5ꢀ$½I8
relevant
geometrical data, extra figures and PXRD data. CCDC reference
numbers 794577–794584. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c0ce00860e
ꢁ
unit (ꢁ8.9 A) and the separation between the terminal iodine
This journal is ª The Royal Society of Chemistry 2011
CrystEngComm, 2011, 13, 4411–4416 | 4411

2ꢀ
ꢁ
atoms in the self-assembled I4 unit (ꢁ9.6 A). Moreover,
ongoing investigations of some of the authors are proving 4,
40-bipyridinium salts (i.e. viologens) as useful building blocks in
supramolecular chemistry9 and crystal engineering.10
where four bispyridinium cations are the sides and the benzyl
pendants the bases of the cage. Finally, when the N,N0-bis-
methyl-4,40-bipyridinium cation (5) is challenged with iodine,
new polymorphs of known polyiodides or mixed, highly complex
and extended polyiodides are obtained as a function of the
stoichiometry adopted in the crystallization.
We thus decided to explore the potential of viologen deriva-
tives in the cation templated synthesis of polyiodides by applying
the strategy of the size-matching between the cation and anion. A
wide diversity of viologen derivatives is easily available by
changing either the spacer connecting the two pyridine rings or
the alkyl pendants quaternarizing the pyridine nitrogen atoms.
We reasoned that the connecting spacer is crucial in tuning the
N+–N+ separation, that is, in identifying the size-matching poly-
iodide which is preferentially templated. The alkyl pendant is
crucial in allowing for tailored space compartmentation, namely
in caging the size-matching polyiodide and hampering catenation
which leads to the formation of complex and diverse polyiodides.
The (E)-1,2-bis(4,40-bypyridiumium)ethylene cation was
identified as a particularly convenient candidate for the tem-
Results and discussion
Structure of N,N0-bis(tetrafluorobenzyl)-(E)-1,2-bis(4,40-
2ꢀ
bipyridinium) ethylene (1$I4
)
Addition of a concentrated methanol solution of I2 (0.1 mmol) to
a concentrated solution of the dipyridylethylene derivative 1$2Iꢀ
(0.1 mmol) resulted in the immediate formation of a red-
brownish microcrystalline precipitate.17 Acetone was added until
the precipitate re-dissolved and the resulting homogenous system
was allowed to slowly evaporate at room temperature. After 1–2
days, dark reddish crystals were obtained which were shown by
X-ray crystallography to be 1$I42ꢀ. By using the procedure
described above, crystals with the same colour, shape and
exhibiting similar DSC thermograms were obtained when excess
iodine was used, namely when the 1$2Iꢀ : I2 molar ratio in the
methanol/acetone solution was 1 : 2 and 1 : 3.
2ꢀ
plated synthesis of the uncommon I4 anions as the intra-
1b
molecular N+–N+ separation (typically ꢁ9.3 A) matches very
ꢁ
2ꢀ
5
ꢁ
well with the expected I4 length (ꢁ9.6 A). For the templated
ꢀ
ꢁ
formation of I3 anions (ꢁ5.8 A), the smallest polyiodide, we
0
ꢁ
chose the 4,4 -bipyridinium cation (ꢁ6.9 A), the shortest viol-
ogen. Benzyl pendants were selected to quaternarize the pyridine
nitrogen atoms as a search in the Cambridge Structure Database
(CSD)11 showed that dibenzyl viologens frequently adopt
a Z-like conformation12 (where the benzyl pendants are on
opposite sides of the planar bipyridinium unit) and organize into
columns or layers,13 so that effective space compartmentation
occurs.14
The intramolecular distance between the N atoms in the
ꢁ
dication 1 is 9.323 A and templates the formation of the dianion
2ꢀ
I4 wherein the distance between the external iodine atoms is
9.321 A. The structural features of the I42ꢀ dianion in 1$I42ꢀ are
ꢁ
typical for these types of halogen-bonded adducts18 and are in
good agreement with previously reported data in other I42ꢀ salts.
The I42ꢀ anion is symmetric and nearly linear (:I–I/Iꢀ ¼ 179.11
Moving from a comparison of the crystal packings in N,N0-bis-
benzyl-4,40-bipyridinium iodide15 and in the three polymorphs of
N,N0-bis-methyl-4,40-bipyridinium iodide,16 we reasoned that
when the pendants used for quaternarizing the nitrogen give no
p–p interactions and no steric hindrance on the bipyridinium
core, space compartmentation becomes less efficient and stoi-
chiometry dependent formation of extended polyiodides is likely
to occur. The behaviour of methyl substituted viologens was thus
studied as a negative proof of the relevance of space compart-
mentation in the cation templated synthesis of polyiodides.
In this paper we describe how the N,N0-bis(tetrafluorobenzyl)-
(E)-1,2-bis(4,40-bipyridinium)ethylene cation (1) (Fig. 1)
ꢂ
ꢀ
ꢁ
(1) ), displaying an I /I2 distance of 3.2534(6) A (21% shorter
than the sum of van der Waals and Pauling radii for I and Iꢀ, 198
and 216 pm, respectively). Consistent with the strong n / s*
character of halogen bonding involving dihalogens,19 the cova-
ꢁ
lent I–I bond (2.81 A) is significantly elongated in the complex
2ꢀ
ꢁ
1$I4 , compared to molecular iodine (2.72 A).
2ꢀ
The formation of the rare I4 dianion independent of the
stoichiometry in the crystallization solution is also a result of the
effective space compartmentation provided by the dication
which adopts the expected Z-like conformation. The overall
crystal packing is characterized by the presence of alternating
2ꢀ
layers (Fig. 2). One layer is composed of bispyridinium cations
2ꢀ
templates so effectively the formation of the I4 anion that the
generation of 1$I42ꢀ is secured independent of the 1$2Iꢀ : I2 ratio
adopted in the crystallization solution. Similarly, the N,N0-bis-
benzyl-4,40-bipyridinium cations (2–4) template the formation of
and the I4
anions which are held together by strong
ꢀ
ꢀ
the I3 anion and the salts 2–4$2I3 are formed once again
independently of the starting materials’ ratios adopted in the
crystallization. In all cases anions are confined in parallelepipeds
Fig. 2 Crystal packing of the salt 1$I42ꢀ viewed approximately along the
b axis showing the effective space compartmentation. Halogen bond
indicated as dashed lines. Colour code: grey, carbon; blue, nitrogen;
green, fluorine; violet, iodine. Hydrogen atoms are not shown for clarity.
Fig. 1 Chemical diagrams of the bipyridinium derivatives 1–5.
4412 | CrystEngComm, 2011, 13, 4411–4416
This journal is ª The Royal Society of Chemistry 2011

electrostatic interactions. The other layer is composed of segre-
gated tetrafluorophenyl rings which stack via p–p interactions in
ꢁ
an offset face-to-face (OFF) fashion (dplanes ¼ 3.4093(3) A,
ꢁ
dcentroid ¼ 4.011(1) A). This allows for an effective caging of
single I42ꢀ anions by dication molecules (Fig. 3).
Bipyridinium ethylene moieties adopt a nearly coplanar
conformation and four of them are the sides of a parallelepiped
cage which is self-capped by the phenyl rings of fluorobenzyl
pendants functioning as the bases of the cage. Any short contact
between neighbouring diiodides is prevented, the shortest
ꢁ
distance between polyiodides in the layers being 6.1713(8) A.
Probably this crystal packing of the cations is robust enough to
prevent the formation of extended polyiodides when starting
from excess iodine as its disruption would be required.
Structure of N,N0-bis-benzyl-4,40-bipyridinium bis-triiodides
ꢀ
(2–4$2I3
)
Starting from 0.2 mmol of I2 and 0.1 mmol of bipyridinium
iodides 2–4$2Iꢀ and using the protocol described above for the
preparation of 1$I42ꢀ, the triiodides 2–4$2I3 were obtained in
nearly quantitative yields as dark red crystals.
ꢀ
Fig. 4 Crystal packing of the triiodides 2$2I3ꢀ (top) and 4$2I3ꢀ (bottom)
approximately along a axis. Halogen bond indicated as dashed lines.
Colour codes as in Fig. 2. Hydrogen atoms are not shown for clarity.
The substitution pattern of benzyl pendants in the three
ꢀ
bipyridinium triiodides 2–4$2I3 differs substantially from each
other and this might translate into differences in the overall
crystal packings. On the contrary, single crystal analyses of
ꢀ
2–4$2I3 reveal numerous similarities among the three packings
(two per side) contribute to the formation of the cage bases in
ꢀ
and with 1$I42ꢀ. The size-matching between the bispyridinium
cation and the triiodide anion controls effectively the packing
and allows 2–4$2Iꢀ to template triiodide formation independent
of the substitution pattern in benzyl pendants. Here too, the
crystal packing consists of a layering of cations and anions held
together by electrostatic interactions. These layers alternate with
layers composed of the benzyl groups held together by p–p
interactions in an OFF fashion (Fig. 4).17
ꢀ
2$2I3 and 3$2I3 via a net of p–p interactions in an OFF
fashion.
In 4$2I3ꢀ only two phenyls of surrounding cages (one per side)
contribute to the formation of the cage bases as a consequence of
the different interactions given by these incoming phenyl rings.
In fact, in addition to the above mentioned p–p interactions,
ꢀ
strong halogen bonds contribute to the caps formation in 4$2I3
.
The iodine atoms of two p-iodophenyl residues of neighbouring
cages are close to the outer iodines of the two I3ꢀ in the cage, and
Iꢀ/I–C halogen bonds with standard properties are formed
Pairs of triiodide anions are caged in cavities where the sides
are four 4,40-bipyridinium systems and the caps of the cage are
the phenyl rings of the benzyl pendants (Fig. 5).17 The volume of
these cages is larger than that of cages in 1$I42ꢀ and self-capping,
occurring in 1$I42ꢀ, is no more efficient. The participation of
phenyl rings bound to viologens of nearby cages is required.
Moreover, differences in the substitution patterns on phenyl
rings of dications 2–4 translate into differences in the capping
mode of cages in 2–4$2I3ꢀ. Four phenyls of surrounding cages
ꢀ
ꢀ
ꢂ
ꢁ
(I /I distance 3.629 A, I /I–C angle 176.57 ).
ꢀ
ꢀ
ꢀ
Fig. 5 A pair of I3 anions caged in 3$2I3 (top) and 4$2I3 (bottom).
Halogen bonds are indicated as dashed lines. Colour codes as in Fig. 2.
Hydrogen atoms are not shown for clarity.
Fig. 3 A I42ꢀ dianion caged in 1$I42ꢀ by four bispyridinium dications.
This journal is ª The Royal Society of Chemistry 2011
CrystEngComm, 2011, 13, 4411–4416 | 4413

Interestingly, powder X-ray diffraction (PXRD) and DSC
ꢀ
experiments showed that 2–4$2I3 are also formed when excess
of I2 is used in the crystallization (i.e., when the 2–4$2Iꢀ : I2
molar ratio in the solution was 1 : 4 and 1 : 6). Moreover, triio-
dides 2–4$2I3ꢀ were obtained when methanol solutions of 2$2Iꢀ
(1 mmol), 3$2Iꢀ and 4$2Iꢀ (0.1 mmol) were exposed to vapours of
I2 (excess) in sealed vessels.
These results prompted us to attempt the templated synthesis
of pure triiodide ions via gas–solid conditions.5,20,21 We exposed
finely ground 3$2Iꢀ to vapours of I2 (excess) in a sealed vessel.
After 3 hours, the PXRD pattern was in good agreement with
ꢀ
a partial conversion into 3$2I3 as the peaks of the pattern
simulated from single crystal data of 3$2Iꢀ were still present
(Fig. 6). A clean and complete conversion was obtained after two
days of gas–solid reaction, and one extra day in which the
obtained brown-reddish powder was left in the open air at room
temperature.17
Structure of N,N0-bis-methyl-4,40-bipyridinium polyiodides
ꢀ
Fig. 7 Top: crystal packing of 5$Iꢀ$I3 along the c axis (top) and of
2ꢀ
5$IꢀI3ꢀ, 5$2I3ꢀ, 5$I3ꢀ$½I82ꢀ, and 5$I5ꢀ$½I8
5$2I3ꢀ along a axis (bottom). Halogen bonds are shown in dashed lines.
Colour codes as in Fig. 2. Hydrogen atoms have been omitted for clarity.
On recrystallization from acetone/MeOH mixture of the
precipitate obtained upon adding a methanol solution of I2
(0.2 mmol) to a water/MeOH (1 : 10) solution of 5$2Iꢀ
alternating cationic and anionic layers (Fig. 7, top). Catenation
of triiodide anions affords loosely bound infinite polyiodide
ꢀ
(0.1 mmol), the unusual mixed salt 5$Iꢀ$I3 (Pnma) was
obtained. The packing of this crystal shows no caging of triiodide
anions. On the contrary, it is characterized by the presence of
chains as the external iodine of a triiodide unit enters the central
ꢁ
3.9390(4) A,
iodine of another triiodide (dI–I(–I)/I–I–I
¼ 89.25(1)ꢂ, 91.54(1)ꢂ).
¼
:
I–I(–I)/I–I–I
Slow diffusion of methanol into a mixture of I2 (0.2 mmol) and
5$2Iꢀ (0.1 mmol) dissolved in DMSO, produced a polymorph of
ꢀ
5$2I3 different from the polymorphs already reported in the
CSD (Fig. 7, bottom).22 Differences between the crystal packing
ꢀ
of 5$2I3 prepared by us and that reported in the literature are
considerable. For instance type-I iodide–iodine contacts23 are
ꢁ
ꢁ
3.8192(8) A long in our polymorph and 4.024 A long in the
already reported structure.
On increasing the amount of I2 in the MeOH/DMSO experi-
mental protocol described above, polyiodides containing
increased amounts of catenated I2 are obtained. Specifically,
starting from MeOH/DMSO solutions containing 0.1 mmol of
2ꢀ
5$2Iꢀ and 0.3 mmol or 0.4 mmol of I2, polyiodides 5$I3ꢀ$½I8
or 5$I5ꢀ$½I82ꢀ were isolated, respectively. In 5$I3ꢀ$½I82ꢀ, the I3
anions have quite standard arrangement and are not involved in
,
ꢀ
2ꢀ
short iodine–iodine contacts. I8 dianions adopt a usual out-
ꢀ
stretched Z-shaped conformation and consists of two I3 ions
connected through the end atoms by a rotationally disordered I2
molecule [I3ꢀ$I2$I3ꢀ]. A net of iodine–iodine interactions cate-
nate I82ꢀ dianions into infinite two-dimensional (2D) nets (Fig. 8,
2ꢀ
top). In 5$I5ꢀ$½I82ꢀ, the I8 dianions are similar to those in
5$I3ꢀ$½I82ꢀ, the disorder of the connecting I2 molecule being
ꢀ
translational rather than rotational. The I5 anions consist of
two I2 molecules bound to an iodide anion and adopt a nearly
linear conformation. Short iodine–iodine contacts further cate-
nate the external atoms of the I82ꢀ dianion to one external atom
ꢀ
ꢀ
of I5 anion and the I5ꢀ$I82ꢀ$I5 complex is formed (Fig. 8,
bottom).
Fig. 6 PXRD study of 3$2Iꢀ exposed to I2 at different times: experi-
mental pattern of the starting 3$2Iꢀ (A); pattern acquired after 3 hours
(B) and 2 days (C) of gas–solid reaction; simulated pattern of 3$2I3ꢀ from
single crystal X-ray data (D).
The obtained structures evidence that in the absence of p–p
interactions and steric hindrance associated with the pendants
quaternarizing viologen nitrogens, the size matching between the
4414 | CrystEngComm, 2011, 13, 4411–4416
This journal is ª The Royal Society of Chemistry 2011

supramolecular parallelepipeds wherein the metastable
2ꢀ
I4 anion is effectively caged. Similarly, the bis-benzyl bipyr-
idiniums 2–4$2Iꢀ template the formation of I3ꢀ anions thanks to
the cation–anion size matching and pairs of I3ꢀ anions are caged
into parallelepipeds formed by four Z-shaped bis-benzyl
bipyridinium units.
A negative proof of the importance of the synergy of cation–
anion size-matching and anion caging come from experiments
conducted with the bis-methyl dipyridinium 5$2Iꢀ. This
compound cannot give rise to space compartmentation and
different polyiodide species are formed as a function of the
experimental setup and the stoichiometry of the reacting species
in solution.
In conclusion, we have shown that if controlled cation tem-
plated synthesis of viologen polyiodides is pursued, the syner-
gistic effect of size-matching and space compartmentation can be
used as an effective strategy to develop a general and reliable
protocol where the crystal stoichiometry is independent of the
stoichiometry in the crystallization solution. The results
described in this paper demonstrate the successful exploitation of
viologen derivatives for the cation-templated synthesis of poly-
iodides species. The method we applied may be extended to the
predictable preparation of other polyhalide species. This has the
potential for significantly augmenting the knowledge on poly-
halides, as well as providing a novel approach to their design and
synthesis. Furthermore, considering the very rich electrochemical
behaviour24 and electronic properties25 of these bipyridinium
cations, their combination with polyiodide species could result in
a new class of materials with unique and interesting electronic
properties.
2ꢀ
Fig. 8 Top: ball and stick representation of 5$I3ꢀ$½I8 evidencing the
2D net formed by I82ꢀ dianions; one bipyridinium unit and one triiodide
unit are reported (sticks representation); I(17A) and I(17B) are splitted
over two positions optimized with a 66.5 and 33.5 population, respec-
tively. Bottom: crystal packing of 5$I5ꢀ$½I82ꢀ, I5ꢀ and I82ꢀ is in ball and
stick representation, bipyridinium units are reported in sticks represen-
tation; I(19A) and I(19B) are splitted over two equally populated posi-
tions. Colour codes as in Fig. 2. Hydrogen atoms have been omitted for
clarity.
cation and the anion in viologen iodides is unable to direct
unambiguously the stoichiometry of the obtained polyiodide.
The strong tendency of polyiodides to catenate prevails over
the templating ability of viologens and the stoichiometry of the
formed polyiodide is controlled by the stoichiometry in the
crystallization solution.
Acknowledgements
ꢀ
M.D.G. and C.P. thank Ministerio de Educacion y Ciencia and
FEDER (CTQ2010-16484/BQU) for financial support. M.D.G.
also thanks the Xunta de Galicia for financial support (‘pro-
grama Isidro Parga Pondal’). P.M., G.R., and G.T. thank
Fondazione Cariplo (‘‘New-Generation Fluorinated Materials as
Smart Reporter Agents in 19F MRI’’) and MIUR (‘‘Engineering
of the Self-assembly of Molecular Functional Materials via
Fluorous Interactions’’) for financial support.
Conclusions
The N+–N+ intramolecular distance in viologen-type cations can
be easily tuned by using the different extended bipyridinium
scaffolds that are easily available. This offers a unique oppor-
tunity to pursue the selective formation of a given polyiodide by
combining the metric engineering strategy (based on the cation–
anion size-matching principle) with the space compartmentation
induced by p–p stacking interactions. In this paper, we have
shown that if benzyl pendants are used to quaternarize the
pyridine nitrogens, effective supramolecular space compart-
mentation occurs and the resulting anion caging controls the
formation of the targeted polyiodides.
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The bis-benzyl bis(pyridinium)ethylene 1$2Iꢀ templates the
2ꢀ
selective formation of 1$I4 via the matching between the
intramolecular N+–N+ separation in the cation and the separa-
tion of the external iodine atoms in the dianion I42ꢀ. 1$I4 is
2ꢀ
formed independent of the amount of iodine present in the
crystallization solution thanks to the cooperation between this
size-matching and the space compartmentation produced by the
cations. The bis-benzyl bis(pyridinium)ethylene moieties adopt
a Z-like conformation and stack via p–p interactions affording
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ꢀ
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ꢁ
ꢁ
face fashion (dcentroids ¼ 3.862 A, dplanes ¼ 3.425 A) (ref. 15).
4416 | CrystEngComm, 2011, 13, 4411–4416
This journal is ª The Royal Society of Chemistry 2011