Two-point halogen bonding between 3,6-dihalopyromellitic diimides

Article abstract of DOI:10.1039/c4sc00999a

The syntheses of 3,6-dichloro-, -dibromo-, and -diiodopyromellitic diimides - ACl, ABr, and AI, respectively - have been achieved. X-Ray crystallography of single crystals of ACl and ABr unveils the formation of extensive halogen-bonding networks in the solid state as a consequence of interactions between the lone pairs on the carbonyl oxygen atoms with the σ-holes of the halogen atoms. Further, the solid-state superstructure of diiodopyromellitic diimide is characterised by the formation of associated halogen-π dimers. The co-crystallisation of ACl or ABr with a 1,5-diaminonaphthalene derivative DN yields co-crystals of a mixed-stack charge-transfer (CT) complex which are supported by an expansive hydrogen-bonded network in addition to halogen-bonded belts that bring adjacent mixed-stacks into association with each other. 2,6-Dimethoxynaphthalene (DO) proved to be an effective CT complement to AI, yielding solvent-free co-crystals with superstructures which are comprised of a 1 - 2 ratio of AI to DO. This dimeric halogen-bonding motif is reminiscent of the formation of hydrogen-bonded dimers between carboxylic acids.

Full text of DOI:10.1039/c4sc00999a

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Two-point halogen bonding between
3,6-dihalopyromellitic diimides†
Cite this: Chem. Sci., 2014, 5, 4242
Dennis Cao, Michael Hong, Anthea K. Blackburn, Zhichang Liu, James M. Holcroft
*
and J. Fraser Stoddart
The syntheses of 3,6-dichloro-, -dibromo-, and -diiodopyromellitic diimides—ACl, ABr, and AI,
respectively—have been achieved. X-Ray crystallography of single crystals of ACl and ABr unveils the
formation of extensive halogen-bonding networks in the solid state as a consequence of interactions
between the lone pairs on the carbonyl oxygen atoms with the s-holes of the halogen atoms. Further,
the solid-state superstructure of diiodopyromellitic diimide is characterised by the formation of
associated halogen-p dimers. The co-crystallisation of ACl or ABr with a 1,5-diaminonaphthalene
derivative DN yields co-crystals of a mixed-stack charge-transfer (CT) complex which are supported by
an expansive hydrogen-bonded network in addition to halogen-bonded belts that bring adjacent mixed-
stacks into association with each other. 2,6-Dimethoxynaphthalene (DO) proved to be an effective CT
complement to AI, yielding solvent-free co-crystals with superstructures which are comprised of a 1 : 2
ratio of AI to DO. This dimeric halogen-bonding motif is reminiscent of the formation of hydrogen-
bonded dimers between carboxylic acids.
Received 4th April 2014
Accepted 29th June 2014
DOI: 10.1039/c4sc00999a
www.rsc.org/chemicalscience
Herein, we report (i) the syntheses of three 3,6-dihalo-
Introduction
pyromellitic diimides AX (X ¼ Cl, Br, or I), (ii) the formation of
dimeric two-point halogen-bonded networks in the solid-state
superstructures of these three compounds, (iii) the formation of
CT co-crystals of ACl or ABr with the electron-rich naphthalene
derivative DN, and (iv) a 1 : 2 complex formed between AI and
2,6-dimethoxynaphthalene (DO). See Fig. 1 for denitions of DN
and DO.
The development of noncovalent bonding interaction motifs is
of crucial importance1 to crystal engineers and chemists alike.
As a consequence, an extensive range of hydrogen bonding2 and
aromatic CT2e,3 motifs have been identied in both the solid
state and solution phase. These and other noncovalent bonding
interactions have been further exploited in supramolecular self-
assembly processes,4 as well as in diverse applications including
molecular sensing5 and organic electronics.3e,6 The interaction
known as halogen bonding7—namely, the noncovalent bonding
interaction between a Lewis basic atom and a Lewis acidic
halogen atom—has become increasingly attractive because of
its utility as a further source of inuence which oen acts7g
orthogonally to other recognition motifs in supramolecular
systems.
We became interested in exploring derivatives of the electron
acceptor pyromellitic diimide (PMDI) in the context of our
research6d into organic CT ferroelectric co-crystals. We hypoth-
esised that, by replacing the two hydrogen atoms on the PMDI
benzenoid core with halogens, the resulting halogenated PMDI
would also become engaged in halogen-bonding interactions,
especially since the electron-decient nature of PMDI would
accentuate the Lewis acidity of the s-holes7e in the aryl halides.
Experimental section
General methods
Unless otherwise stated, compounds and solvents were
purchased from commercial vendors (4Cl and 4Br were
obtained from Sigma Aldrich and TCI Chemicals, respectively)
and were used as supplied without further purication.
Compounds DN 8 and 1I 9 were prepared following literature
methods. Nuclear magnetic resonance (NMR) spectra were
recorded at 298 K on a Bruker Avance III 500 spectrometer, with
Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, IL
60208, USA. E-mail: stoddart@northwestern.edu; Fax: +1-847-491-1009; Tel: +1-847-
491-3793
† CCDC 995177–995182. For crystallographic data in CIF or other electronic
format see DOI: 10.1039/c4sc00999a
Fig. 1 Structural formulae of the naphthalene donors DN and DO.
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a working frequency of 500 MHz for H and 125 MHz for 13C.
Chemical shis are listed in ppm on the d scale and coupling
constants are recorded in Hertz (Hz). Deuterated solvents for
NMR spectroscopic analyses were used as received. Chemical
shis are reported in d values relative to the signals corre-
sponding to the residual non-deuterated solvents (CD3COCD3:
dH 2.05 ppm, dC 29.84 ppm). X-Ray diffraction data were
obtained on Bruker Platform and Kappa diffractometers,
equipped with a MoKa (ABr) or CuKa (all other crystals) sealed-
tubesource and an APEX II CCD detector. Intensity data were
collected using u and 4 scans spanning at least a hemisphere of
reciprocal space for all structures (data were integrated using
SAINT). Absorption effects were corrected on the basis of
multiple equivalent reections (SADABS). Structures were
solved by direct methods (SHELXS)10 and rened by full-matrix
least-squares against F2 (SHELXL).10 The structures were solved
and rened using Olex2. The majority of the hydrogen atoms
were assigned riding isotropic displacement parameters and
constrained to idealised geometries. Crystallographic images
were produced using UCSF Chimera.11 Atom-to-atom distances
were measured employing Mercury.12
1
Scheme 1 Synthesis of acceptor compounds AX where X ¼ Cl, Br, or I.
(i) KMnO4/H2O/tBuOH; (ii) Ac2O/AcOH and (iii) NH4OAc/AcOH.
reaction mixture. Aer the ltrate had cooled to room temper-
ature, it was acidied with 2 M HCl, and the solvent evaporated.
The residue was dispersed in Me2CO and ltered to remove
inorganic salts. The ltrate was collected and the solvent
evaporated to yield 2Cl as a white solid (5.57 g, 79%). 13C NMR
(CD3COCD3, 125 MHz, 298 K): dC (ppm) ¼ 165.0, 136.3, 127.7.
2Br. In a similar manner to that described above for
preparing 2Cl, reaction of 1Br (43.0 g, 147 mmol) and KMnO4
(188 g, 1.19 mol total) in tBuOH/H2O (1 : 1 v/v, 500 mL) yielded
2Br as a brown solid (50.4 g, 83%). 13C NMR (CD3COCD3, 125
MHz, 298 K): dC (ppm) ¼ 165.8, 138.4, 116.5.
2I. In a similar manner to that described above for preparing
2Cl, reaction of 1I (ref. 9) (10.0 g, 25.9 mmol) and KMnO4
(30.5 g, 193 mmol total) in tBuOH/H2O (1 : 1 v/v, 100 mL) yielded
2I as a light-yellow solid (11.2 g, 85%). 13C NMR (CD3COCD3,
125 MHz, 298 K): dC (ppm) ¼ 167.4, 142.1, 91.2.
Synthetic procedures
While the syntheses of some N-substituted dihalopyromellitic
diimide derivatives and their precursors have been reported
previously,13 all of the parent diimides AX are unknown to date.
Compounds AX (X ¼ Cl, Br, or I, Fig. 1) were synthesised
(Scheme 1) following similar synthetic routes to those in the
literature.13c,d Oxidation of the appropriate 3,6-dihalodurene 1X
to the tetracarboxylic acid 2X, followed by dehydration, yielded
the dianhydride 3X. Reaction of the anhydrides with NH4OAc
led to formation of the desired dihalogenated PMDIs AX, which
precipitate out of the reaction mixture as a consequence of their
low solubilities in AcOH. During the course of the synthetic
optimisation, it was found, in agreement with the previous lit-
erature,13c,d that acetic acid was the ideal solvent because more
polar solvents such as dimethylsulfoxide (DMSO) or even
anhydrous dimethylformamide (DMF) could not be used as a
reaction solvent at elevated temperatures. Highly coloured
byproducts formed when reactions were conducted in these
solvents, presumably as a consequence of the reactivity14,15 of
the aryl halide. While N-methyl-2-pyrrolidone (NMP) could be
used as a crystallisation solvent, highly coloured byproducts
were still formed if NMP was used as a solvent for condensa-
tions between AX and primary amines.
Typical procedure for the preparation of compounds 3X
3Cl. A mixture of 2Cl (4.00 g, 12.4 mmol) and Ac2O/AcOH
(1 : 1 v/v, 20 mL) was heated under reux for 8 h. The reaction
mixture was then cooled to room temperature, and the precip-
itate isolated by ltration to yield 3Cl as an off-white solid
(1.22 g, 34%). 13C NMR (CD3COCD3, 125 MHz, 298 K): dC (ppm)
¼ 158.6, 137.0, 129.5.
3Br. In a similar manner to that described above for
preparing 3Cl, reaction of 2Br (50.0 g, 12.4 mmol) and Ac2O/
AcOH (1 : 1 v/v, 400 mL) yielded 3Br as a brown solid (24.7 g,
54%). 13C NMR (CD3COCD3, 125 MHz, 298 K): dC (ppm) ¼ 159.1,
138.6, 117.3.
3I. In a similar manner to that described above for preparing
3Cl, reaction of 2I (10.0 g, 19.8 mmol) and Ac2O/AcOH (1 : 1 v/v,
80 mL) yielded 3I as a yellow solid (5.91 g, 64%). 13C NMR
(CD3COCD3, 125 MHz, 298 K): dC (ppm) ¼ 159.8, 141.1, 89.0.
Typical procedure for the preparation of compounds AX
ACl. NH4OAc (297 mg, 3.86 mmol) and 3Cl (500 mg,
1.74 mmol) were added to AcOH (5 mL). The reaction mixture
Typical procedure for the preparation of the 2X compounds
2Cl. A mixture of 1Cl (5.00 g, 24.6 mmol), KMnO4 (20.0 g, was le to stir at room temperature for 1 h, then heated under
127 mmol), and tBuOH/H2O (1 : 1 v/v, 100 mL) was heated reux for 8 h. The reaction mixture was then cooled to room
under reux until the purple colour of the solution had dis- temperature, and the precipitate isolated by ltration and
appeared, indicating complete consumption of KMnO4. The washed with MeOH to yield ACl as an off-white solid (439 mg,
reaction mixture was then cooled to room temperature, and 88%). An NMR spectrum could not be obtained on account of
additional KMnO4 (5.00 g, 31.6 mmol) was added carefully. The the poor solubility of the compound in standard deuterated
mixture was then heated under reux for an additional 4 h, aer solvents. The structure of the product was conrmed by X-ray
which time EtOH (10 mL) was added to quench the remaining crystallography of single crystals obtained by liquid–liquid
KMnO4. The brown solids were removed by hot ltration of the diffusion of H2O into a saturated solution of ACl in NMP.
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Table 1 Crystallographic data for all crystals
ACl
ABr
AI$(H2O)$(NMP)
ACl$DN
ABr$DN
AI$(DO)2
Empirical formula
M (g molꢀ1
C10H2Cl2N2O4
285.04
C10H2Br2N2O4
373.96
C15H13I2N3O6
585.08
C56H56Cl4N8O16
1238.88
C56H56Br4N8O5
1416.72
C34H26I2N2O8
844.37
)
Temperature (K)
Crystal system
Space group
100
Monoclinic
P21/c
7.11/5.48/12.25
90/90.7/90
476.6
100
Monoclinic
P21/c
7.37/5.42/12.53
90/92.3/90
499.5
100
Triclinic
ꢀ
P1
7.22/9.49/13.28
93.2/102.2/99.8
872.5
100
Monoclinic
P21/c
6.68/22.98/8.86
90/101.6/90
1332.0
100
Monoclinic
P21/c
6.76/22.67/9.11
90/101.6/90
1368.0
100
Triclinic
ꢀ
P1
6.63/9.47/12.36
82.1/80.7/79.4
747.1
˚
a/b/c (A)
a/b/g (ꢁ)
Volume (A )
3
˚
Z
2
2
2
1
1
1
rcalc (mg mmꢀ3
)
1.986
1.54178
284
6.23–67.68
2930
2.486
0.71073
356
2.77–30.54
17 489
2.227
1.54178
556
3.42–68.01
14 244
1.544
1.54178
644
5.45–67.68
6929
1.720
1.54178
716
5.33–68.08
2462
1.877
1.54178
414
3.65–67.68
4204
m (mmꢀ1
F (000)
)
2qmax (ꢁ)
Reections collected
Data/restraints/parameters
807/0/82
1.099
1527/0/82
1.124
3131/0/252
1.097
2301/0/191
1.053
13 777/0/191
1.081
2217/0/210
1.081
Goodness-of-t on F2
Final R indices [I > 2s (I)]
R1 ¼ 0.0313
wR2 ¼ 0.0365
R1 ¼ 0.0813
wR2 ¼ 0.0844
995182
R1 ¼ 0.0216
wR2 ¼ 0.0241
R1 ¼ 0.0550
wR2 ¼ 0.0568
995181
R1 ¼ 0.0383
wR2 ¼ 0.0436
R1 ¼ 0.1057
wR2 ¼ 0.1098
995180
R1 ¼ 0.0315
wR2 ¼ 0.0329
R1 ¼ 0.0827
wR2 ¼ 0.0837
995179
R1 ¼ 0.0226
wR2 ¼ 0.0227
R1 ¼ 0.0576
wR2 ¼ 0.0576
995178
R1 ¼ 0.0209
wR2 ¼ 0.0226
R1 ¼ 0.0547
wR2 ¼ 0.0557
995177
Final R indices [all data]
CCDC no.
lter into a 10 mL test tube. Fresh Milli-Q water (5 mL) was
layered carefully on top of the NMP solution. Orange needles
were observed to grow within 1 day.
ABr. In a similar manner to that described above for
preparing ACl, reaction of NH4OAc (615 mg, 8.53 mmol) and 3Br
(1.00 g, 2.66 mmol) in AcOH (10 mL) yielded ABr as a white solid
(680 mg, 68%).
AI. In a similar manner to that described above for preparing
ACl, reaction of NH4OAc (177 mg, 2.30 mmol) and 3I (500 mg,
1.06 mmol) in AcOH (10 mL) yielded AI as a yellow solid (361
mg, 73%).
Results and discussion
Single crystals of ACl and ABr suitable for X-ray diffraction were
grown by liquid–liquid diffusion of H2O into NMP solutions.
X-Ray crystallography reveals (Table 1) that ACl and ABr crys-
tallise isostructurally in the monoclinic P21/c space group (no.
14). The superstructures are solvent-free—the molecules are
Crystallisation conditions
General procedure. Extended sonication and gentle heating packed together tightly by a plethora of close contacts. The
were required in order to dissolve the majority of the AX molecules are arranged (Fig. 2a and b) in a herringbone fashion
compounds in N-methyl-2-pyrrolidone (NMP). Once the mixture in which the imide hydrogens engage in hydrogen-bonding
was cooled to room temperature, the resulting solution was interactions with nearby carbonyl oxygen atoms ([O/N]
˚
passed through a 0.45 mm syringe lter into a 10 mL test tube. distances: 2.88 and 2.89 A for ACl and ABr, respectively). Belts of
Fresh Milli-Q water (5 mL) was layered carefully on top of the ACl and ABr molecules are formed as a consequence of signif-
NMP solution and the tubes were le in the dark.
icant two-point intermolecular halogen–oxygen-bonding inter-
Crystals AX. A mixture of pyromellitic diimide AX (X ¼ Cl, Br, actions between adjacent molecules. These contacts are
or I, 0.05 mmol) and NMP (5 mL) was crystallised following reminiscent of the hydrogen-bonded dimers formed2c between
the general procedure. Crystals were observed to grow within carboxylic acids. A search of the Cambridge Structural Database
2 days.
(CSD) with Conquest16 reveals that this dimeric halogen-
Co-crystals ACl$DN and ABr$DN. Mixtures of naphthalene bonding motif is quite rare and has not been explored as a
donor8 DN (0.09 mmol) and either pyromellitic diimide ACl or supramolecular motif. In both superstructures, the [O/X]
ABr (0.04 mmol) and NMP (5 mL) were crystallised following distances (Fig. 2c and d, red) are signicantly less than the
the general procedure. Dark needles were observed to grow summation of the expected17 van der Waals radii for [O/X],
˚
within 7 days.
which are 3.25 and 3.35 A for X ¼ Cl and Br, respectively.
Co-crystal AI$(DO)2. Pyromellitic diimide AI (10 mg, Because of the rigid geometry of the molecules, the halogen
0.02 mmol) was added to NMP (5 mL). Extended sonication and atoms are in close proximity to both oxygen atoms of the ortho
heating of the mixture was necessary in order to dissolve the imide carbonyl groups. Even though the intramolecular
majority of AI. Once the mixture was cooled to room tempera- halogen-to-oxygen distances are also less than the expected17
ture, naphthalene donor DO (4 mg, 0.02 mmol) was added to van der Waals distances, halogen bonding between them is not
the solution, and immediately a coloured precipitate was expected18 because the Lewis basic lone pairs on the oxygen
formed. The supernatant was passed through a 0.45 mm syringe atoms are not in the correct geometry to interact with the
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ꢀ
(Table 1) that AI crystallises in the triclinic P1 space group
(no. 2) with an asymmetric unit containing one molecule each
of H2O, NMP, and AI. The intermolecular [O/I] distance of
17
˚
˚
3.01 A, which is 0.47 A less than the expected van der Waals
distance, is even smaller than the [O/X] distances observed in
the solid-state superstructures of ACl and ABr. This observation
agrees with the general halogen-bonding strength trend of I > Br
> Cl. Additionally, there appear (Fig. 3c) to be [I/p] inter-
actions7j present in the superstructure. The distances between
the planes dened by the central benzenoid rings alternate
˚
between 3.47 and 3.68 A, providing evidence that pairs of AI
molecules are Peierls distorted into dimers by these [I/p]
Fig. 2 Three different views of the solid-state superstructures of ACl
(left) and ABr (right), variously portrayed in tubular and space-filling
representations. (a and b) Looking down the [1 1 0] direction reveals the
herringbone packing of the molecules. (c and d) A perpendicular view
of two AX molecules which are engaged in two-point halogen-
bonding interactions (dashed lines). Distances which are shorter than
the expected van der Waals distances are depicted in red. The inter-
molecular halogen-bonding distances [Cl/O], [Cl/Cl], [Br/O], and
[Br/Br] are (A) 3.13, (B) 3.53, (C) 3.13, and (D) 3.51 A˚, respectively. (e and
f) A view down the N–N axis of the AX molecules reveals that they are
nearly coplanar. The spheres outline the van der Waals radii of the
atoms. AX ¼ blue / Cl ¼ green / Br ¼ maroon / oxygen ¼ red /
hydrogen ¼ white.
˚
halogen atom. The [Br/Br] distance (3.51 A) in the solid-state
superstructure of ABr is also shorter than the van der Waals
7b
˚
distance (3.70 A), and falls within the denition of a Type I
halogen–halogen interaction—namely, the two [C–Br/Br]
angles which dene the interaction in question are the same
(127ꢁ) as a result of the centre of inversion in the solid-state
superstructure. In contrast, and presumably as a consequence
of the ACl molecules favouring the more signicant [O/Cl]
˚
interactions, the [Cl/Cl] distance (3.53 A) is nearly equal to the
˚
expected van der Waals distance (3.50 A). These Type I contacts
arise typically from close-packing effects and are not consid-
ered7k to be true halogen-bonding interactions, unlike the
[O/X] interactions.19 A side effect of the steric bulk of the
halogen atoms is that the PMDI molecules do not lie completely
at in the solid-state superstructure—average torsional angles
of 5.9 and 6.4ꢁ are present between the C]O and C–X bonds in
ACl and ABr, respectively. Adjacent ABr and ACl molecules are
essentially coplanar (Fig. 2e and f) with each other in a fashion
that maximises directional halogen-bonding interactions. An
intermolecular hydrogen-bonding interaction between imide
groups is also evident in the crystal superstructures, with an
Fig. 3 Three different views of the solid-state superstructure of
AI$(H2O)$(NMP) variously portrayed in tubular and space-filling
representations. (a) A view down the a axis reveals the formation of
[I/p] stacks. (b) A perpendicular view of two AI molecules which are
engaged in two-point halogen-bonding interactions (dashed lines).
The intermolecular halogen-bonding distances [I1/O2], [I2/O3], and
[I1/I2] are (A) 3.02, (B) 3.00, and (C) 3.98 A˚, respectively. (c) A view
down the N–N axis of the AI molecules reveals that the molecules
dimerise as a consequence of [I/p] interactions (dashed lines). The
two [I2/p] interactions are (D) 3.60 A˚. In (b) and (c), the solvent
molecules are omitted for the sake of clarity. Distances which are
shorter than the expected van der Waals distances are depicted in red.
The spheres outline the van der Waals radii of the atoms. AI ¼ blue / I ¼
purple / NMP ¼ white / nitrogen ¼ blue / oxygen ¼ red / hydrogen ¼
white. The NMP hydrogen atoms are omitted for the sake of clarity.
˚
[H–N/O] distance of 2.88 A for both ABr and ACl. These
interactions, which occur orthogonally to the halogen-bonding
ones, also play a role in the herringbone assembly of the crystal
structure.
Single crystals of AI suitable for X-ray diffraction were
obtained by liquid–liquid diffusion between H2O and a solution
of AI in NMP. The solid-state superstructure (Fig. 3) differs
vastly from those of ACl and ABr. X-Ray crystallography reveals
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Fig. 4 Solid-state superstructures of ACl$DN (left) and ABr$DN (right) variously portrayed in tubular and space-filling representations. Side view
of the alternating mixed-stack formed between DN and (a) ACl or (b) ABr. Looking down a mixed-stack in (c) ACl$DN and (d) ABr$DN reveals
halogen-bonding interactions between the stacks. Intermolecular halogen-bonding interactions between two AX molecules are depicted
by dashed lines. The intermolecular halogen-bonding distances [Cl/O], [Cl/Cl], [Br/O], and [Br/Br] are (A) 3.42, (B) 3.49, (C) 3.27, and (D)
3.59 A˚, respectively. The hydrogen atoms on the glycol chains are omitted for clarity. The spheres outline the van der Waals radii of the atoms.
AX ¼ blue / DN ¼ red / Cl ¼ green / Br ¼ maroon / oxygen ¼ red / hydrogen ¼ white. Aromatic and aliphatic hydrogens are omitted for the sake
of clarity.
17
˚
interactions. As a consequence of this distortion, the bend angle (3.13 A) in ABr and the predicted van der Waals distance
˚
between the imide nitrogens and the centroid of the benzenoid (3.35 A). These observations suggests that the CT from the
ring in AI is about 174ꢁ. Each imide group in a molecule of AI electron-rich DN to the electron-poor ACl and ABr decreases the
engages in intermolecular hydrogen-bonding interactions with Lewis acidity of the halogen atoms signicantly enough to
a molecule of NMP or H2O, with [H–N/O] distances of 2.81 and weaken the halogen-bonding interactions. Indeed, the adjacent
˚
2.74 A, respectively.
ABr and ACl molecules are no longer coplanar with each other
Subsequently, we attempted to co-crystallise compounds AX in the co-crystals. In ACl$DN, the plane-to-plane distance
˚
with electron-rich naphthalene DN (ref. 8) (Fig. 1). DN is char- between adjacent molecules of ACl is 1.58 A, while in ABr$DN,
˚
acterised by diethylene glycol ‘arms’ which are capable of only 1.30 A separates the planes of the ABr molecules. The
“locking” around suitable hydrogen-bonding sites in comple- hydrogen-bonding tendencies of the imide groups in ACl and
mentary molecules under the lock-arm supramolecular order- ABr are manifested by the presence of [H–N/O] interactions
ing6d (LASO) paradigm. It has been reported previously20 that a (2.82 and 2.81 A, respectively) with the terminal oxygen atoms of
˚
compound similar to DN—with oxygen atoms in place of the the DN diethylene glycol chains.
NH functions on the naphthalene core—co-crystallises with
Despite repeated attempts, no co-crystals were formed
PMDI to form a CT complex held together by a myriad of between AI and DN. All of our attempts yielded only crystals of
hydrogen-bonding interactions, similar to those found6d previ- AI$(H2O)$(NMP). An explanation for this outcome may be found
ously in LASO co-crystals. We discovered that liquid–liquid in the steric bulk of AI which prevents the formation of the CT
diffusion between H2O and solutions of ACl or ABr and DN in complex. Looking down a mixed-stack in ACl$DN or ABr$DN
NMP produced large centimetre-long crystals of ACl$DN and reveals (Fig. 4c and d) that the halogen atoms are positioned
ABr$DN, respectively, suitable for X-ray diffraction (Table 1). closely on top of the p bonds in DN, a situation which may be
Complexes ACl$DN and ABr$DN were found to crystallise iso- difficult to replicate with the large electron clouds associated
structurally (Fig. 4) in the monoclinic P21/c space group (no. 14). with iodine atoms. We posited that utilising20 a differently
The superstructures are solvent-free and are characterised by an substituted naphthalene would allow for an alternate stacking
alternating mixed-stack between ACl or ABr and DN in which geometry. Indeed, simply mixing compounds AI and 2,6-
the diethylene glycol chains form interwoven hydrogen bonds dimethoxynaphthalene (DO) in NMP resulted in the immediate
with the imide hydrogens. The dimeric halogen-bonding motif formation of a pink/orange precipitate. Careful co-crystal-
observed in the solid-state superstructures of ACl and ABr were lisation of AI with DO (Fig. 1) yielded orange needles suitable for
found to hold together adjacent mixed-stacks in these co-crys- X-ray diffraction (Table 1), an observation which conrms that
˚
tals. The [O/Cl] distance (3.42 A) in ACl$DN is, however, the altered substitution pattern of the naphthalene favours the
˚
signicantly larger than that (3.13 A) in the solid-state super- formation of a CT complex. The superstructure (Fig. 5) of the co-
structures of ACl, and also exceeds the expected17 van der Waals crystal—which is in the triclinic P1 (no. 2) space group—
ꢀ
˚
˚
distance (3.25 A). In ABr$DN, the [O/Br] distance (3.27 A) consists of mixed-stacks of AI and DO, and additional molecules
compares more favourably to both the analogous interaction of DO oriented in
a herringbone fashion between the
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weakened halogen-bonding interactions between the charge-
transfer mixed-stacks. Diiodopyromellitic diimide co-crystal-
lised with 2,6-dimethoxynaphthalene in the form of a 1 : 2
complex. Presently, we are evaluating the materials properties
of these co-crystals, while also exploring the utility of this
dimeric halogen-bonding motif in solution.7h,21
Acknowledgements
We thank Dr Amy A. Sarjeant, Charlotte L. Stern, and the
personnel in the Integrated Molecular Structure Education and
Research Center (IMSERC) at Northwestern University for their
assistance in the collection of some raw X-ray crystallographic
data. This research is part (Project 34-949) of the Joint Center of
Excellence in Integrated Nano Systems (JCIN) at King Abdul-aziz
City for Science and Technology (KACST) and Northwestern
University (NU). The authors would like to thank both KACST
and NU for their continued support of this research. D.C. was
supported by a NSF Graduate Research Fellowship; he also
gratefully acknowledges support from a Ryan Fellowship awar-
ded by the NU International Institute for Nanotechnology (IIN).
A.K.B. acknowledges Fulbright New Zealand for a Fulbright
Graduate Award and the New Zealand Federation of Graduate
Women for a Postgraduate Fellowship Award.
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