Connectivity and topology invariance in self-assembled and halogen-bonded anionic (6,3)-networks

Article abstract of DOI:10.3390/molecules22122060

We report here that the halogen bond driven self-assembly of 1,3,5-trifluorotriiodobenzene with tetraethylammonium and -phosphonium bromides affords 1:1 co-crystals, wherein the mutual induced fit of the triiodobenzene derivative and the bromide anions (halogen bond donor and acceptors, respectively) elicits the potential of these two tectons to function as tritopic modules (6,3). Supramolecular anionic networks are present in the two co-crystals wherein the donor and the acceptor alternate at the vertexes of the hexagonal frames and cations are accommodated in the potential empty space encircled by the frames. The change of one component in a self-assembled multi-component co-crystal often results in a change in its supramolecular connectivity and topology. Our systems have the same supramolecular features of corresponding iodide analogues as the metric aspects seem to prevail over other aspects in controlling the self-assembly process.

Full text of DOI:10.3390/molecules22122060

molecules
Communication
Connectivity and Topology Invariance in
Self-Assembled and Halogen-Bonded Anionic
(6,3)-Networks
Franck Meyer, Tullio Pilati, Konstantis F. Konidaris ID , Pierangelo Metrangolo
ID
and Giuseppe Resnati *
Laboratory of Nanostructured Fluorinated Materials (NFMLab), Department of Chemistry, Materials,
and Chemical Engineering “Giulio Natta”, Politecnico di Milano, Via L. Mancinelli 7, 20131 Milano, Italy;
franck.meyer@ulb.ac.be (F.M.); tullio.pilati@gmail.com (T.P.); konstantis.konidaris@polimi.it (K.F.K.);
Pierangelo.metrangolo@polimi.it (P.M.)
* Correspondence: giuseppe.resnati@polimi.it; Tel.: +39-02-2399-3032 or +39-338-8719-492
Received: 24 October 2017; Accepted: 21 November 2017; Published: 24 November 2017
Abstract: We report here that the halogen bond driven self-assembly of 1,3,5-trifluorotriiodobenzene
with tetraethylammonium and -phosphonium bromides affords 1:1 co-crystals, wherein the mutual
induced fit of the triiodobenzene derivative and the bromide anions (halogen bond donor and
acceptors, respectively) elicits the potential of these two tectons to function as tritopic modules
(6,3). Supramolecular anionic networks are present in the two co-crystals wherein the donor and
the acceptor alternate at the vertexes of the hexagonal frames and cations are accommodated in the
potential empty space encircled by the frames. The change of one component in a self-assembled
multi-component co-crystal often results in a change in its supramolecular connectivity and topology.
Our systems have the same supramolecular features of corresponding iodide analogues as the metric
aspects seem to prevail over other aspects in controlling the self-assembly process.
Keywords: halogen bonding; supramolecular chemistry; crystal engineering; anion coordination
1. Introduction
Non-covalent intermolecular interactions hold a major role in the design of supramolecular
assemblies and hence constitute a critical tool for crystal engineering [
intermolecular interactions, halogen bonding (XB) has attracted considerable interest in recent years [
and systems self-assembled under XB control have found applications in several areas, spanning
biopharmacology [ ], catalysis [ ], and materials science [ 10]. XBs display similar characteristics
1
]. Among the various
2
],
3–
5
6
,7
8–
with hydrogen bonds (HBs) in terms of their strength and high directionality [2].
XB directionality plays a particularly critical role in the design and synthesis of supramolecular
systems as it translates tectons geometry into self-assembled architecture geometry [11,12]. For instance,
when para-diiodotetrafluorobenzene, or its dibromo analogue, self-assemble with linear XB acceptors,
e.g., 4,4’-dipyridyl, linear infinite chains are formed; when meta or ortho isomers are used, zig-zag
infinite chains are obtained with angles along the chain close to 120◦ and 60◦, respectively [13
–
18].
Iodoaromatics are usually good XB donors and this is particularly true when the aromatic moiety
bears electron withdrawing groups [ ], and it can be expected that 1,3,5-trifluorotriiodobenzene (
has a tendency to work as a tritopic XB donor after a trigonal geometry and to afford (6,3) networks
when interacting with bi- or tritopic acceptors [19 20]. A search in the Cambridge Structural Database
(CSD) confirms that this may be the case [21 23]. On the other hand, calculations have shown that, as a
consequence of the charge transfer component of the interaction [19], the XB donor ability of the iodine
2
1)
,
–
atoms of
1
decreases when the number of XBs the tecton (1) is already involved in is increased [20].
Molecules 2017, 22, 2060; doi:10.3390/molecules22122060
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In other words, the formation of the third XB is energetically less favored than the second, and the
formation of the second less favored than the first one [24]. Indeed, the number of XBs given by the
XB donor
1 with usual acceptors (e.g., N atoms and anions) can be two or even one [25–27], and it
has been argued that the preferential formation of infinite chains (1D nets) rather than honeycomb
systems (2D nets) may also be associated with steric reasons, i.e., the problem to fill the potential
empty space encircled by the hexagonal frame. Moreover, the number of XBs formed by a given
acceptor on interaction with
1
may vary, e.g., if solvated co-crystals are formed [28,29]. Moreover,
when a series of compositionally related co-crystals is obtained on assembly of
1
with a series of salts
wherein the cation is the same and the anion changes, the number of XBs formed by the different
anions may vary or remain unchanged and the same may happen for the topology of the obtained
supramolecular anions [18,21]. For instance, 1 and tetra-n-butylammonium iodide [21], or bromide
and chloride [30] give 2D systems wherein both the XB donor and acceptors are tritopic; however,
when tetra-n-butylammonium thiocyanate is used [22], a 1D system is formed wherein
1 is tritopic
and the anion either mono- or bitopic.
It thus seems that the connectivity and topology landscape of halogen bonded adducts given
by the donor is quite diversified and not easy to predict, and this is particularly true when anionic
acceptors are used. We have already reported a series of adducts between and iodide salts where
both and the iodide anion are tritopic and the systematic change of the cation showed that the cation
1
1
1
size plays a major role in enabling the formation of (6,3) networks [21]. It is required that cations fit in
the cavity encircled by the hexagonal frames of the nets and the upper metric limit for the onium cation,
enabling the I···I− supramolecular synthon to form (6,3) networks that lie between the dimensions of
tetra-n-propyl- and tetra-n-butylammonium cations. For instance, the self-assembly of
1
with Et4N+I−
or Et4P+I−
(2a,b) affords co-crystals 3a,b where both 1 and the iodide ions are tritopic and form (6,3)
networks where the two modules alternate at the network nodes (Scheme 1); the same happens with
n-Pr4N+I− but not with n-Bu4N+I−.
Scheme 1. The formation of the two species (i.e., iodobenzene derivative
component (i.e., , the cation, and the anion from salts 2a ) co-crystals 3a
1,3,5-trifluorotriiodobenzene (1) with onium halides 2a–d.
1
and salts 2a
–d) and three
1
–
d
–d
via self-assembly of
As a part of an ongoing project aimed at changing the composition of multi-component and
self-assembled systems, while maintaining unmodified their supramolecular characteristics [31],
we were interested in identifying cases where the change of the anion in 3a
supramolecular characteristics described above, and decided to co-crystallize
Et4P+Br−
2c ) (Scheme 1). We reasoned that the formation of (6,3) networks 3c
,
b
preserves all the
with Et4N+Br− and
from and 2c is
1
(
,
d
,
d
1
,d

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quite likely while the hexagonal frames having bromide anions at three of their vertexes should be
smaller than analogous frames having iodide anions (I···Br− bonds are typically shorter than I···I−
bonds), the used onium cations would comfortably fit also in the smaller cavities formed by bromide
anions as tetraethyl onium cation is safely below the cation upper size limit for the formation of
(6,3) nets based on the I···I− supramolecular synthon. Here, we report that the connectivity and the
topology in the adducts 3c,d (afforded by 1 and 2c,d) is the same as in 3a,b (Figure 1).
Figure 1. Partial representation of one honeycomb network of 3c. The Et4N+ cations, sitting at the
center of the hexagonal cavities, are represented with space-filling models. The XBs are black dotted
lines. Color code: C: grey; N: blue; I: purple; F: light green; Br: olive.
The number of XBs given by the donor
1 and by iodide and bromide acceptors remains unchanged
in the four co-crystals 3a , and the same holds for the topology of the respective supramolecular
–d
anions, as the contraction of the hexagonal frames size resulting from bromide for iodide substitution
has been tolerated, and cations invariably sit in the hexagonal cavities.
2. Results and Discussion
Whitish crystalline solids (3c,d) were obtained on slow evaporation at room temperature of
equimolar solutions of and tetraethylammonium or -phosphonium bromide (2c or 2d, respectively)
1
in CH2Cl2/methanol mixtures. The melting points of these solids were quite sharp and in between
those of starting compounds, suggesting that well-defined chemical species had been obtained rather
than physical mixtures. IR spectra showed the presence of peaks of both C6F3I3 and the onium moieties,
only minor differences in peak intensities and wave numbers were observed with respect to pure
starting compounds.
1H and 19F NMR analyses in the presence of bis(2,2,2-trifluoroethyl) ether as internal standard for
peaks integration [32] revealed that the starting compound ratio is 1:1.
Single crystals X-ray analyses of 3c,d confirmed the starting compounds ratio established via the
NMR technique and revealed that both compounds crystallize in the trigonal space group R3c 1 and
the bromide anions act as tritopic trigonal XB donors and acceptors, respectively, leading to 2D anionic
networks with (6,3) topology. The bromide ions and iodobenzene moiety
1 alternate at the nodes of
the net and Et4N+ or Et4P+ cations are accommodated in the center of the hexagonal space.

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−
−
Two different I···Br contacts and C-I···Br angles are present in both 3c and 3d, and the respective
values are quite similar in the two structures. I2···Br1 distances are 329.4(5) and 330.2(5) pm in 3c
and 3d, respectively, while I1···Br1 distances are slightly longer (341.6(3) and 343.8(4) pm). These
values correspond to normalized contacts (Nc) that are quite small [33] (they are in the range 0.84–0.87),
suggesting that XBs in these structures are fairly strong. This is even more notable [19
,20] if we consider
that the electron donor ability of the bromide and the electron acceptor ability of the iodofluorocarbon
are split over three XBs. Consistent with this strength, the two different C-I···Br angles present in 3c
,d
are almost linear (they span in a value range of 172.87(5)–179.88(5)◦). The hexagonal motifs of the 2D
honeycomb networks are slightly distorted due to the deviation of the I···Br−···I angles (they vary
between 117.68(1)◦ and 124.62(1)◦) from the ideal 120◦ for a symmetric trigonal coordination.
All these features nicely parallel those in corresponding nets formed by iodide anions and
a comparison between the hexagonal frames of 3c and 3a (refcode CIZRUZ) [21] is depicted in
Figure 2. The dimensions of the hexagonal cavities are estimated by the sides of the two triangles;
one is connecting the naked Br− or I− ions and the other one the centroids of the phenyl rings of
1.
Iodide anions are larger than bromide anions and I··· I− halogen bonds are longer than the I··· Br−
ones, resulting in hexagons with slightly greater dimensions in the iodide based nets. Despite this,
the honeycomb topology is sustained, consistent with the fact that its formation is critically dependent
on the cation’s sitting in the internal space of the hexagonal frame and with the fact that the critical
cation size was found between the dimensions of n-Pr4N+ and n-Bu4N+ in honeycomb nets formed by
iodide anions. The ability of the hexagonal frames formed by bromide anions to nicely accommodate
Et4N+ and Et4P+ cations in 3c
,d is confirmed by the fact that the 2D supramolecular anions are quite
flat (Figure 3), while they become undulated when the frame/cation mismatch forces the cations to
protrude out of the hexagons [21].
Figure 2. A view along the crystallographic c axis of one hexagonal frame of 3c
(left) compared to
the corresponding frame for 3a
(
right). Semi-transparent triangles connect the naked Br− or I− ions
(greenish) and the centers of the phenyl rings of 1 (purple).
The cations’ nature affects the distance between two adjacent anionic layers as well; specifically,
the larger the cation is, i.e., the more it protrudes out of the hexagons, the greater the interlayer
separation becomes [34]. The distance between the two adjacent layers is 437.0(1) pm in 3c and
is slightly greater in 3d (458.5(1) pm). This trend has also been observed between 3a and 3b
(interlayer separations are 452.9 and 460.1 pm, respectively) and is probably due to the fact that
a phosphonium cation is larger than the corresponding ammonium cation. The interlayer separations
in 3c,d are smaller than the separation of the two supramolecular anionic nets in the co-crystal formed

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by the triiodobenzene derivative
1
with tetra-n-propylammonium iodide (554.9 pm), the largest
tetraalkylammonium iodide that affords a honeycomb net upon self-assembly with 1.
Figure 3. Two supramolecular anionic layers in the crystal structure of 3d. Semitransparent planes are
the mean square planes through the anions of a layer. Color codes as in Figure 1; brown lines are the
covalent bonds; black dotted lines are XBs; Et4P+ cations have been omitted for clarity.
Short contacts exist between the fluorine atoms of
of the methylene groups bound to the nitrogen and phosphorous atoms of 3c
1
and the partially positive hydrogen atoms
(Figure S1), and this
,
d
suggests that onium cations potentially serve as templating agents for the formation of the hexagonal
frames. These contacts can be considered as C-H···F-C hydrogen bonds and may play a role in assisting
the self-assembly of the hexagonal anionic frames. This possibility is supported by the presence of
similar HBs in related honeycomb nets [21]. The templating role of the cation is backed by the fact that,
in the honeycomb network formed by triethyl-chloromethylammonium chloride [25], the XB donor
1
is pinned in its position by the I···Cl− XBs forming the supramolecular anion, by the H···F HBs
involving the “acidic” methylene groups, and by a further XB wherein the chlorine of the N+CH2Cl
moiety is the XB donor and the belt of the iodine of 1 is the acceptor.
In order to confirm that metric aspects enable massive self-assembly of 1 and 3c,d into honeycomb
nets, powder X-ray diffraction (pXRD) analyses were carried out for 3c (Figure S5) and 3d (Figure 4).
Experimental patterns nicely match patterns simulated from single crystal analyses, and phase purity
of the crystallized compounds is thus proven.
Figure 4. Powder X-ray Diffraction patterns of 3d.
Finally, 19F NMR experiments were performed to assess the XB presence between
solution (Figure 5). Small upfield shifts were observed for the signal of upon the addition of different
onium salts, with the chemical shift changes ranging from 0.028 to 0.162 ppm (Table S1). For a given
1 and 2c,d in
1

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cation, bromide salts afforded smaller chemical shift changes than iodide salts in all cases, consistent
with observations reported for other haloperfluorocarbons/halide adducts in solution [35].
Figure 5. Variation of 19F NMR chemical shift of 1,3,5-triidotrifluorobenzene (
1) upon interaction with
some onium bromides and iodides when 10 equivalents of the onium salt was added to a 5 mM solution
of 1 in deuterochloroform.
3. Materials and Methods
3.1. General Information
The starting materials 1,3,5-trifluorobenzene, tetraethylammonium bromide, and
tetraethylphosphonium bromide were purchased from Sigma-Aldrich and used without further
purification. Melting points were determined on a Reichert instrument by observing the melting
process through an optical microscope. ATR-FTIR spectra were obtained using a Nicolet Nexus FTIR
spectrometer. Peaks frequencies, given in wave numbers, were rounded to 1 cm−1 using automatic
peak assignment.
3.2. Procedures and Compound Characterization
3.2.1. Preparation of 1,3,5-Triiodo-2,4,6-trifluorobenzene (1) [36]
KI (7.14 g, 43.56 mmol) was added slowly to a stirred mixture of periodic acid (3.30 g, 14.50 mmol)
in concentrated H2SO4 (20 mL) at 0 ◦C. The obtained dark mixture was cooled with an ice bath while
1,3,5-trifluorobenzene (1.0 mL, 9.68 mmol) was added over 25 min. The mixture was heated to 70 ◦C
for 4 h, then cooled to room temperature, poured on ice, and extracted with diethyl ether (3
×
50 mL).
The collected organic phases were washed with sat. Na2S2O3 and water, then dried with Na2SO4.
After evaporation of the solvent,
1
was recovered as a pure white powder in 80% yield. 19F NMR
(CDCl3, 235 MHz): δ −69.90 ppm; m.p.: 158–159 ◦C; FT-IR (KBr pellet, cm−1): 1561, 1401, 1325, 1048,
704, 652.
3.2.2. Preparation of Co-Crystal 3c
Co-crystals of 3c were formed by dissolving equimolar amounts of 1,3,5-trifluorotriiodobenzene
and tetraethylammonium bromide in a mixture of CH2Cl2/MeOH (1:1) and after slow isothermal
evaporation of the solvents at room temperature. m.p.: 240–242 ◦C. FT-IR (KBr pellet, cm−1): 2982,
1563, 1473, 1458, 1394, 1184, 1036, 1005, 794, 709.

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3.2.3. Preparation of Co-Crystal 3d
◦
A procedure similar to that used for 3c was employed. m.p.: 238-241 C. FT-IR (KBr pellet, cm−1):
2905, 1561, 1458, 1393, 1331, 1267, 1036, 779, 709.
3.3. Single Crystal Structure Determination
The single crystal X-ray diffraction measurement of the 3c
APEX CCD area detector diffractometer, equipped with a Bruker KRYOFLEX low temperature device,
graphite monochromator, Mo K radiation ( = 0.71069 Å) at 123 K. Cell refinement and data reduction
,d was conducted on a Bruker SMART
α
λ
were performed with a Bruker SAINT [37]. The structures were solved with SHELXS [38] and refined
with SHELX-97 [38]; absorption correction was performed based on a multi-scan procedure using
SADABS [37]. Further crystallographic details of the structures in this paper are reported in Table S2.
3.4. 19F NMR Experiments
19F NMR spectra were recorded on a Bruker ADV 500 spectrometer at 25 ◦C, CDCl3 was used as
solvent and CFCl3 as internal standard. ∆δ values reported in Figure 4 for 19F NMR chemical shift
of 1,3,5-trifluorotri−iodobenzene (
1) were obtained upon the addition of 10 equivalents of the onium
−
−
halides (n-Pr4N+Br , Et4N+Br , Et4P+Br , n-Pr4N+I−, Et4N+I−, and Et4P+I−) to 5 mM solutions of
1 in
CDCl3: δF (ppm, 5 mM sol. of 1 in CDCl3) = −69.90; ∆δF (ppm) = δ5 mM 1 − δ5 mM 1 + 10 eq. onium halide).
3.5. Powder X-ray Diffraction Analyses
The crystalline powder material of co-crystals was packed on borosilicate glass slides and
the data sets were collected on a Bruker D8 instrument at 293 K. The measurements were made
in Bragg–Brentano geometry using Johansson monochromator to produce pure CuKα1 radiation
(1.5406 Å; 45 kV, 30 mA) and a step–scan technique in the 2
θ
range of 4–40◦. The data were acquired
´Celerator detector in continuous scanning mode with a step size of
from a spinning sample by an X
◦
0.0167 using a sample dependently counting times of 90 s per step. The comparison of simulated and
experimental PXRD pattern confirms the structural uniformity of bulk co-crystal powders.
4. Conclusions
In general, different halides can assemble different structures under control of electrostatic interactions,
metal coordination, and HB [39], and the same holds when XB is the driving force of the self-assembly
processes [40
substitution in heteromeric multicomponent systems (
attraction between opposite ions (the two strongest interactions in co-crystals 3a
strengths that are quite different [42]. The triiodobenzene derivative can work as a mono-, bi-, or
,
41]. Different halides can present different coordination spheres, and bromide for iodide
) is expected to give rise to XBs and electrostatic
) endowed with
3
–
d
1
tritopic XB donor [43], and bromide anions frequently function as di-, tri-, or tetratopic acceptors [44],
but such anions can also present other XBs in their first coordination sphere, as they have been reported
to act even as octatopic acceptors [45]. In the systems described here, the tetraethyl onium cations
enable the assembly of three units of
steric and electronic levels, of all the components of 3a
1
around a bromide anion and combined matching, both at
allows to work as a planar and trigonal
–
d
1
tecton in the XB driven self-assembly of supramolecular halide networks. This behavior recalls the HB
driven self-assembly processes wherein trimesic acid or 1,3,5-triazine derivatives function as planar
and threefold tectons.
The supramolecular similarity of 3a
the ability of to function as a tritopic XB donor is robust enough to elicit the tritopic acceptor
potential of bromide anions, to tolerate non-minor differences in the strength of interactions driving
the co-crystals self-assembly, and to afford the honeycomb nets. The similarity of 3a gives rise to
–d indicates that, metric requirements being fulfilled,
1
–
d
further examples wherein the mutual induced fit among the different components of a heteromeric
co-crystal elicits their respective ability to act as tritopic tectons and affords honeycomb nets, with

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either bromide or iodide anions at the nodes [21]. The use of this heuristic principle in the design of
other multi-component systems assembly with the same topology but with different compositions is
under study.
Supplementary Materials: The following are available online: CCDC 1576571 and 1576572 contain the
supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.
cam.ac.uk/data_request/cif, or by emailing data_request@ccdc.cam.ac.uk, or by contacting The Cambridge
Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
Acknowledgments: KFK gratefully acknowledges the Politecnico di Milano for a Polimi International Fellowship
(PIF).
Author Contributions: F.M. performed the experiments; T.P. collected X-ray data, and solved and refined the
reported structures; K.F.K. wrote the paper and performed part of the experiments; P.M. conceived and designed
the experiments; G.R. conceived and designed the experiments and wrote the paper.
Conflicts of Interest: The authors declare no conflict of interest.
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Sample Availability: Samples of the compounds are not available from the authors.
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