Cooperative halogen bonding and polarized π-stacking in the formation of coloured charge-transfer co-crystals

Article abstract of DOI:10.1039/c8nj00693h

Red co-crystals are formed between the matched complementary electron rich halogen bond acceptors, isomeric 4-(N,N-dimethylamino)phenylethynylpyridines and the electron poor halogen bond donor, 1-(3,5-dinitrophenylethynyl)-2,3,5,6-tetrafluoro-4-iodobenzen

Full text of DOI:10.1039/c8nj00693h

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Cooperative halogen bonding and polarized
p-stacking in the formation of coloured
charge-transfer co-crystals†
Cite this:DOI: 10.1039/c8nj00693h
b
Chideraa I. Nwachukwu,a Zachary R. Kehoe,b Nathan P. Bowling,
a
Erin D. Speetzenb and Eric Bosch
*
Red co-crystals are formed between the matched complementary electron rich halogen bond
acceptors, isomeric 4-(N,N-dimethylamino)phenylethynylpyridines and the electron poor halogen bond
donor, 1-(3,5-dinitrophenylethynyl)-2,3,5,6-tetrafluoro-4-iodobenzene. The red 1 : 1 cocrystals exhibit
strong halogen bonding and strong p-stacking. The Nꢀ ꢀ ꢀI distances range from 2.80 to 2.85 Å and the
C–Iꢀ ꢀ ꢀN angles are between 169.9 and 175.81. In all four structures the donor and acceptor molecules
are alternately p-stacked with the centroid to centroid distances between the dinitrophenyl moiety and
the dimethylaminophenyl moiety ranging from 3.61 to 3.73 Å. The calculated p–p stacking binding
energy is ꢁ22.24 kcal molꢁ1 for the complex between 4-[4-(N,N-dimethylamino)phenylethynyl]pyridine
and 1-(3,5-dinitrophenylethynyl)-2,3,5,6-tetrafluoro-4-iodobenzene while the calculated halogen bond
Received 8th February 2018,
Accepted 27th March 2018
DOI: 10.1039/c8nj00693h
binding energy between the same couple is ꢁ7.97 kcal molꢁ1
.
rsc.li/njc
complexes, formed between an electron donor and an electron
acceptor, have found application in a variety of areas notably
Introduction
The cooperative interplay between non-covalent interactions is molecular electronics and photonics. Segregated stacking of
ubiquitous and important in both solutions and crystalline donor molecules and acceptor molecules (ꢀ ꢀ ꢀ–D–D–Dꢀ ꢀ ꢀ and
solids that include neutral or charged organic moieties. Halogen ꢀ ꢀ ꢀ–A–A–Aꢀ ꢀ ꢀ) is a necessity for metallic conductivity at room
bonding, an attractive non-covalent interaction that involves the temperature while semiconductor properties are common with
attraction of a Lewis base to an electropositive region, has been mixed stacks of alternating donor and acceptor molecules
employed as a controlling design feature across many disciplines (ꢀ ꢀ ꢀ–D–A–D–A–ꢀ ꢀ ꢀ).5 The formation of CT complexes as discrete
including supramolecular chemistry, crystal engineering, mole- entities along organic and organometallic reaction pathways
cular recognition, anion recognition, medicinal chemistry and has been demonstrated.6 A particularly elegant example of the
synthetic organic and polymer chemistry.1 The positioning of application of the role of coloured p-stacked CT complexes in
neighbouring electron withdrawing substituents enhances the synthesis, was the CT complex directed synthesis of rotoxanes.7
electropositive region, the s-hole, on the halogen bond donor While hexafluorobenzene forms colourless p-stacked cocrystals
leading to stronger interactions. Consequently, polyfluorohalo- with
a
wide variety or arenes,8 m-dinitrobenzene forms
alkanes and arenes are widely used to direct cocrystallization coloured co-crystals with electron rich amino arenes.9 It is thus
through halogen bonding.1 Nitro substituted iodobenzenes and reasonable to incorporate cooperative halogen bonding and
charge assisted halogen bond donors have also been employed.2,3 charge-transfer complexation into p-stacked cocrystals. In a
The interplay between conventional hydrogen bonding and study of the cocrystallization of bromo- and iododinitrobenzenes
halogen bonding has been explored over the past several years.4 with tetramethyl-p-phenylenediamine, p-stacking was observed
In this manuscript we will explore systems that probe cooperative with mixed stacks of donor and acceptor without halogen
halogen bonding and p–p complexation. Charge-transfer (CT) bonding.10 The p-stacking interaction was calculated to be
6–12 kcal molꢁ1 more favourable than halogen bonding in these
cocrystals. In a separate study of cocrystals formed between
diaminonaphthalene and tetrahalo-1,4-benzoquinones, halogen
bonding in the cocrystal formed with tetraiodobenzoquinone
resulted in segregated stacks of donor and acceptor whereas the
a Department of Chemistry, Missouri State University, 901 South National Avenue,
Springfield, MO, 65897, USA. E-mail: ericbosch@missouristate.edu
b Department of Chemistry, University of Wisconsin-Stevens Point,
2001 Fourth Avenue, Stevens Point, WI, 54481, USA
tetrabromo- and tetrachloroquinone cocrystals did not include
halogen bonds and mixed stacks of donor and acceptor
† CCDC 1820039–1820042. For crystallographic data in CIF or other electronic
format see DOI: 10.1039/c8nj00693h
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molecules were observed.11 Our study was initiated after we observed
that the yellow-orange co-crystal formed between 3,5-dinitro-1-iodo
benzene and N,N-dimethylaminopyridine featured both strong
halogen bonded and p-stacks of alternating donor and acceptor
molecules. Herein we report the deliberate formation of cocrystals
featuring halogen bonds and p-stacking.
Results and discussion
Preliminary results
The asymmetric unit of the 1 : 1 cocrystal A, formed between
3,5-dinitro-1-iodobenzene and 4-(N,N-dimethylamino)pyridine,
features a strong halogen bond with a NꢀꢀꢀI distance of 2.8937(1) Å,
82% of the sum of the van der Waals radii,12 and C–IꢀꢀꢀN angle of
175.27(6)1 (Table 2). Fig. 1 shows the asymmetric unit of the
halogen bonded cocrystal. The two nitro groups are coplanar with
the benzene ring, with maximum distance of oxygen below the
plane of the benzene ring of 0.154(3) Å, and the dimethylamino
group is coplanar with the pyridine ring with the maximum
distance of the methyl group above the plane of the pyridine ring
of 0.134(4) Å.13 The dihedral angle between the two planes defined
by the two aromatic compounds is 14.93(7)1 and the packing
features asymmetric offset p-stacks of alternating 3,5-dinitro-1-
iodobenzene and 4-(N,N-dimethylamino)pyridine molecules with
two distinct p–p interactions. The distance between the centroids
of the dinitroiodobenzene and dimethylamino pyridine rings is
3.657(2) and 3.718(2) Å with slippage of 1.55 and 1.78 Å respectively.
There are two non-conventional hydrogen bonds between
nitro oxygen atoms and pyridyl hydrogen atoms with C–Hꢀ ꢀ ꢀO
distances of 2.48 and 2.57 Å and C–Hꢀ ꢀ ꢀO angles of 150.0 and
165.51 respectively that support the formation of planar sheets
of the cocrystals A shown in Fig. 2.14
Fig. 2 Packing of the cocrystal A shown along the a axis (a) and b axis (b).
the halogen bond donor, tetrafluoroiodobenzene, from the
p-acceptor, dinitrobenzene. Similarly the complementary
halogen bond acceptor, pyridine, was connected to the p-donor
N,N-dimethylaminobenzene with an ethynyl group. Our initial
target molecules were thus 1-(3,5-dinitrophenylethynyl)-2,3,5,6-
tetrafluoro-4-iodo benzene, 1, and 4-[4-(N,N-dimethylamino)-
phenylethynyl]pyridine, 2.
Molecular electrostatic potential calculations were per-
formed on 1 and 2 with the geometry constrained to the planar
conformation before geometry minimization prior to calculat-
ing the electrostatic potential energy surface.15 For the halogen
bond donor 1 the maximum value of the electrostatic potential
associated with the s-hole on the iodine atom is 180.6 kJ molꢁ1
.
Thus, based on the s-hole on the iodine atom, 1 appears to be a
better halogen bond donor than 1-iodo-3,5-dinitrobenzene and
1,4-diiodo-2,3,5,6-tetrafluorobenzene for which similar calcula-
tions yielded maximum values of the electrostatic potential
associated with the s-hole on the iodine atoms of 165.9 and
168.9 kJ molꢁ1 respectively.16 The positive electrostatic potential
spreads over both aromatic rings with a significant p-hole17 of
174.3 kJ molꢁ1 in the center of the dinitro substituted benzene
ring confirming the p-acceptor nature of the dinitrophenyl
moiety (Fig. 3). It is noteworthy that the p-hole located at the
centroid of 1-iodo-3,5-dinitrobenzene at 104.0 kJ molꢁ1 is signifi-
cantly weaker than that calculated for 1 reflecting the electron
withdrawing effect of the 2,3,5,6-tetrafluoro-4-iodophenylethynyl
moiety on the p-hole on the nitrosubstituted benzene ring. The
minimum value of the electrostatic potential associated with the
pyridyl nitrogen atom in 2 is ꢁ201.0 kJ molꢁ1. This is lower than
4-(N,N-dimethylamino)pyridine, which has a minimum value of
the electrostatic potential associated with the pyridyl nitrogen
atom of ꢁ220.5 kJ molꢁ1, presumably due to the more remote
location of the electron donating N,N-dimethylamino group.
Nevertheless we expected halogen bonding between the 1 and
2 to be similar to that between 1-iodo-3,5-dinitrobenzene
and 4-(N,N-dimethylamino)pyridine. The negative electrostatic
potential is spread across both aromatic rings with a minimum
of ꢁ66.7 kJ molꢁ1 in the dimethylaminophenyl ring and a
Molecular design, electrostatic potential calculations and
synthesis
The focus of our design was to separate, but maintain, the pyridine–
iodobenzene halogen bonding interaction and the dinitrobenzene–
dimethylaminoarene interaction that we reasoned was responsible
for the charge transfer coloration. The first iteration of this
design principle, described here, uses an alkyne to separate
Fig. 1 Asymmetric unit of the halogen bonded cocrystal
A formed
between 3,5-dinitro-1-iodobenzene and 4-(N,N-dimethylamino)pyridine
showing atom labelling. Displacement ellipsoids of non-H atoms are
drawn at the 50% probability level and H atoms are shown as circles of
arbitrary size. The halogen bond is shown as a dashed line.
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Fig. 4 Photographs of individual colorless crystals of 1 (A); red cocrystals
B (B); and fine crystalline precipitate of cocrystal C (C).
Fig. 3 Molecular electrostatic potential plots of 1 and 2 shown on the
Fig. 5 Asymmetric unit of the halogen bonded cocrystal
B formed
same scale in kJ molꢁ1
.
between 1 and 2 with displacement ellipsoids of non-H atoms drawn at
the 50% probability level and H atoms are shown as circles of arbitrary size.
The halogen bond is shown as a dashed line.
minimum of ꢁ90.3 kJ molꢁ1 located on the alkyne as shown in
Fig. 3. The minimum value of the electrostatic potential on the
pyridine N atoms of the regioisomers 3-[4-(N,N-dimethylamino)-
phenylethynyl]pyridine, 3, and 2-[4-(N,N-dimethylamino)phenyl-
ethynyl]pyridine, 4, are ꢁ200.5 and ꢁ215.8 kJ molꢁ1 respectively.
It is noteworthy that the negative electrostatic potential in the
dimethylamino substituted benzene ring has a minimum of
ꢁ75.3 and ꢁ81.9 kJ molꢁ1 for 3 and 4 respectively. The syntheses
of molecules 1–4 are outlined in Scheme 1. Thus, Sonogashira
coupling of 1-bromo-3,5-dinitrobenzene with trimethylsilylacetylene
(TMSA) followed by base deprotection gave 1-ethynyl-3,5-dinitro-
benzene in good yield. Subsequent coupling with an excess of
1,4-diiodotetrafluorobenzene (Scheme 1) yielded 1 in low yield.
Sonogashira coupling of 4-ethynyl-N,N-dimethylbenzeneamine
with 4-iodopyridine yielded 2 in good yield. The regioisomers of 2,
3 and 4, were similarly prepared by coupling with 3-iodopyridine
and 2-bromopyridine respectively in good yield.
complex as shown in Fig. 5. The halogen bond has a Nꢀ ꢀ ꢀI
distance of 2.798(2) Å with C–Iꢀ ꢀ ꢀN angle of 172.4(1)1. The two
methyl carbons are slightly below the plane of the dimethyl-
amino phenyl ring, 0.035(5) and 0.142(5) Å, and one nitro group
is slightly twisted with the oxygen atoms 0.184(5) above and
0.152(5) Å below the plane of the benzene ring. The two
molecules 1 and 2 are slightly twisted with dihedral angles of
16.17(5) and 13.57(5)1 between the pyridyl and dimethylamino-
phenyl rings, and the tetrafluoroiodobenzene and the dinitro-
benzene, respectively. The packing features asymmetric offset
p-stacked molecules 1 and 2 with overlap between the dinitro-
phenyl and dimethylaminophenyl groups and the tetrafluoro-
iodophenyl and pyridyl groups as shown in Fig. 6. The centroid
to centroid distances between the dinitrophenyl ring and the
dimethylaminophenyl rings is 3.695(2) and 3.747(2) Å with
slippage of 1.33 and 1.65 Å and the centroid to centroid distance
between the tetrafluoroiodophenyl ring and the pyridyl groups
is 4.256(2) and 3.589(2) Å with slippage of 2.70 and 1.31 Å
respectively (Table 4). Given the slightly twisted conformations
of 1 and 2 it is not surprising that the crystal packing is complex.
There are two C–Hꢀ ꢀ ꢀF interactions between adjacent molecules
with Hꢀ ꢀ ꢀF distances of 2.48 and 2.43 Å for H3ꢀ ꢀ ꢀF3 and H10ꢀ ꢀ ꢀF1
and C–Hꢀ ꢀ ꢀF angles of 127.9 and 143.2 for C3–H3ꢀ ꢀ ꢀF3 and
C10–H10ꢀ ꢀ ꢀF1 respectively.18 A bifurcated C–Hꢀ ꢀ ꢀO contact to a
nitro group has H12ꢀ ꢀ ꢀO3 and H12ꢀ ꢀ ꢀO4 distances of 2.79 and
2.80 Å respectively.
Cocrystallization, structural, spectral and computational
analysis
A uniform mass of red rod-shaped crystals, cocrystal B, was
formed on slow evaporation of a dilute solution containing
equimolar amounts of the colourless components 1 and 2 in
dichloromethane (Fig. 4). The X-ray structure revealed that, as
expected, the molecules 1 and 2 formed a 1 : 1 halogen bonded
Scheme 1 Synthetic scheme for donor and acceptor molecules 1–4
used in this study.
Fig. 6 Packing of cocrystal B viewed along the b axis.
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Fig. 7 Asymmetric unit of the halogen bonded cocrystal
C formed
between 1 and 3 with displacement ellipsoids of non-H atoms drawn at
the 50% probability level and H atoms are shown as circles of arbitrary size.
The halogen bond is shown as a dashed line.
Fig. 9 Asymmetric unit of the halogen bonded cocrystal D formed
between 1 and 4 with displacement ellipsoids of non-H atoms drawn at
the 50% probability level and H atoms are shown as circles of arbitrary size.
The halogen bond is shown as a dashed line.
Similar slow evaporation of separate equimolar solutions of
each of the pyridyl regioisomers 3 and 4 with halogen bond
donor 1 also resulted in red cocrystals, C and D respectively.
Each cocrystal featured a 1 : 1 ratio of components with a strong
halogen bond. In cocrystal C (Fig. 7) both individual components
are essentially planar. One nitro group in 1 is coplanar with the
benzene while the second nitro group is slightly twisted with
oxygen atoms 0.199(9) Å above and oxygen 0.219(9) Å below the
plane of the benzene. The dihedral angle between the planes
defined by the tetrafluoroiodobenzene and the dinitrobenzene
rings is 2.3(2)1. The dihedral angle between the planes defined
by the dimethylaminophenyl and pyridyl rings 5.6(2)1. Surprising
though the two molecules are not coplanar with a dihedral angle
of 31.7(1)1 between the planes defined by the pyridyl and
tetrafluoroiodobenzene rings. Thus the iodine atom is 0.631(9) Å
out of the plane defined by the pyridine ring and the C–IꢀꢀꢀN bond
angle is slightly bent at 169.9(2)1. The offset p-stacking, shown
in Fig. 8, also features mixed stacks in which the dinitrophenyl
stacks between diaminophenyl groups similar to that observed
in cocrystal B.
Fig. 10 Packing of the cocrystal D shown along the a axis.
defined by the pyridyl and tetrafluoroiodobenzene rings and
the iodine atom is only 0.27(1) Å out of the plane defined by
the pyridine ring and the C–Iꢀ ꢀ ꢀN bond angle is 175.8(3)1. The
offset p-stacking also features mixed stacks in which the
dinitrophenyl stacks between dimethylaminophenyl groups
very similar to that observed in cocrystal C (Fig. 10). The weak
non-conventional C–Hꢀ ꢀ ꢀO and C–Hꢀ ꢀ ꢀF hydrogen bonds in the
structures of C and D are collected in Table 3.
The visible absorption spectra of the red crystalline solids
B–D were recorded along with the spectra of the essentially
colorless individual components (1–4) and the contribution of
the individual components subtracted to provide the visible
absorbance unique to each individual cocrystal. These differ-
ence spectra in Fig. 11 all feature a broad absorbance centered
around 600 nm: specifically 593 nm for cocrystal B, 574 nm for
cocrystal C and 602 nm for cocrystal D. These broad absorption
bands are typical CT absorption bands observed with p-stacked
arene donors and acceptors. In particular, m-dinitrobenzene
formed a red CT cocrystal (3 : 1) with 1,3,5-tris(4-aminophenyl)-
benzene.19 We were unable to perform successful solution
phase UV-visible studies of the complexation due to solubility
issues at the concentrations needed.
In cocrystal D (Fig. 9) the components are almost planar.
One nitro group in 1 is slightly twisted with the oxygen atoms
0.255(14) Å above and 0.300(14) Å below the plane of the
benzene. The dihedral angle between the planes defined by
the two benzene rings is 7.8(4)1 and the dihedral angle between
the planes defined by the dimethylaminophenyl and pyridyl
rings 5.2(3)1. The components 1 and 4 are slightly twisted from
planarity with a dihedral angle of 10.1(3)1 between the planes
Fig. 8 Packing of the cocrystal C shown along the a axis.
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the cocrystal A. Despite this, the molecular design clearly
facilitates concomitant cooperative p-stacking and halogen
bonding. Future computational studies of these and closely
related matched complementary electron rich halogen bond
acceptors and electron poor halogen bond donors will target
the elucidation of the relative contributions of electrostatics,
charge transfer, induction and dispersion to both the halogen
bonding and the p-stacking interactions.17
Conclusions
The simple systems presented here confirm that cooperative
halogen bonding and donor–acceptor p-stacking in cocrystalliza-
tion is a viable concept. We plan to explore the incorporation of
alternate donor and acceptor molecules as well as flexible spacers
and functionalized spacers between the two interacting groups.
Experimental
Synthesis
Fig. 11 Visible absorption spectra of the cocrystals B, C and D after
subtraction of the normalized contribution of the individual components.
All solvents and reactants were used as purchased. All NMR
spectra were recorded using CDCl3 as solvent with Me4Si
as internal standard. All NMR coupling constants are given
in Hz.
Computational study of the relative halogen bond and charge
transfer binding energies
The crystal structures clearly show both halogen bonding and
1-(3,5-Dinitro-phenylethynyl)-2,3,5,6-tetrafluoro-4-iodo-benzene, 1.
charge transfer complexation suggesting an interplay and com- 1-Ethynyl-3,5-dinitrobenzene (0.298 g, 1.50 mmol), bis(triphenyl-
petition between the two interactions (eqn (1)).
phosphine)palladium(II) chloride (60 mg, 0.09 mmol), copper iodide
(20 mg, 0.10 mmol) and an excess of 1,4-diiodotetrafluorobenzene
(1.25 g, 3.12 mmol) in a mixture of NEt3 (5 mL) and THF (2 mL)
were reacted at 55 1C for 3 h. The reaction mixture was cooled,
½1; 2ꢂ
Ð 1 þ 2 Ð ½1; 2ꢂ
(1)
halogen bonded
p-stacked
In order to address the interplay between halogen bonding and diluted with CH2Cl2 and washed with brine. The organic layer
charge transfer complexation we calculated the relative binding was dried over Na2SO4, filtered and the solvent evaporated. The
energies of the halogen bonding interaction and the p-stacking crude product was then purified by flash column chromatography
interaction for three of the complexes. The coordinates taken on a silica gel. After flash column chromatography (hexane/EtOAc =
from the X-ray structures were used as the starting point for the 30 : 1), compound 1 was obtained as an off-white solid (0.128 g,
optimization of the structures. While there is a single unique 0.27 mmol, 18%). M.p.: 166.1–169.0 1C. 1H NMR (400 MHz): d 9.08
halogen bonding interaction in each X-ray structure, each (1H, t, J 2.3, Ph), 8.73 (2H, d, J 2.3, Ph). 13C NMR (100 MHz): d 148.6,
structure features two unique p–p interactions hence two 147.1 (ddt, J 260, 14, 4.5), 146.1 (ddm, J 259, 17.7), 131.5, 125.2,
independent initial geometries to minimize. For cocrystal A, 119.2, 108 (t, J 23), 96.6 (t, J, 4), 79.6 (t, J, 4), 75.8 (t, J 28). 19F NMR
the two optimized p–p structures starting from the two unique (400 MHz): d ꢁ133.54 (m, 2F), ꢁ118.77 (m, 2F). Found: C, 35.65;
geometries optimized to mirror images of each other. For N, 5.35; H, 1.15. Calc. for C14H3F4IN2O4: C, 36.1; N, 6.0; H, 0.65.
cocrystal B, the two optimized p–p structures are essentially
4-(4-Dimethylaminophenylethynyl)pyridine, 2. 4-Iodopyridine
identical. For cocrystal C, the two p-stacking structures did (0.42 g, 2.06 mmol), bis(triphenylphosphine)palladium(II) chloride
optimize to slightly different structures resulting in two slightly (43 mg, 0.06 mmol), copper iodide (18 mg, 0.09 mmol), 4-ethynyl-
different values of the p–p binding energy. The binding energy N,N0-dimethylaniline (0.298, 2.06 mmol) in a mixture of NEt3
for the p-stacked pair, 4-(N,N-dimethylamino)pyridine and (4 mL) and CH2Cl2 (4 mL) were reacted at room temperature for
1-iodo-3,5-dinitrobenzene, is ꢁ12.41 kcal molꢁ1 compared to 24 h. The crude product was isolated as above to yield, after flash
the halogen bond binding energy of ꢁ7.75 kcal molꢁ1. The column chromatography with hexane/EtOAc = 4 : 1, compound 2
halogen bond binding energy calculated for cocrystals B and C as an off-white solid (0.33 g, 1.49 mmol, 72%). M.p.: 195.2–
1
are similar, or slightly lower, than that of A (Table 4). In contrast, 195.8 1C. H NMR (400 MHz): d 8.58 (2H, d, J1,2 5.9, Pyr), 7.42
the p–p binding energy is significantly enhanced for larger (2H, d, J1,2 9.0, Ph), 7.33 (2H, d, J 5.9, Pyr), 6.66 (2H, d, J 9.0, Ph),
p-systems at ꢁ18.78 to ꢁ19.71 kcal molꢁ1. Therefore p-stacking 3.01 (6H, s, NMe2). 13C NMR (100 MHz): d 150.9, 149.8, 133.4,
is favored, relative to halogen bonding, for the larger p-systems 132.6, 125.4, 111.9, 108.7, 96.2, 85.4, 40.3. Found: C, 80.03, N, 12.6;
by more than 11 kcal molꢁ1 as compared to 4.66 kcal molꢁ1 for H, 6.35. Calc. for C15H10N2: C, 81.05; N, 12.1; H, 6.35.
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3-(4-Dimethylaminophenylethynyl)pyridine, 3. 3-Iodopyridine Electrostatic potential calculations
NJC
(0.74 g, 3.61 mmol), bis(triphenylphosphine)palladium(II) chloride
(60 mg, 0.09 mmol), copper iodide (20 mg, 0.10 mmol),
4-ethynyl-N,N0-dimethylaniline (0.51 g, 3.51 mmol), NEt3 (4 mL)
and THF (6 mL) were reacted at room temperature for 24 h. The
crude product was isolated as above to yield, after flash column
chromatography with hexane/EtOAc = 4 : 1, compound 3 as a
slightly yellow solid (0.63 mg, 2.84 mmol, 81%). M.p.: 133.8–
134.1 1C. 1H NMR (400 MHz): d 8.73 (1H, d, J 1.6, Pyr), 8.48 (1H,
dd, J 4.7, J 1.6 Hz, 1H), 7.75 (1H, dt, J 7.8, J 2.0, Pyr), 7.42 (2H,
d, J 9.0 Hz, Ph), 7.24 (1H, ddd, J 7.8, 4.7, 0.8, Pyr), 6.66 (2H, d,
J 9.0, Ph), 3.00 (s, 6H, NMe2). 13C NMR (100 MHz, CDCl3):
d 150.2, 150.6, 147.9, 138.2, 133.0, 123.1, 121.6, 112.0, 109.3,
94.3, 84.3, 40.4. Found: C, 80.1; N, 12.25; H, 6.3. Calc. for
C15H10N2: C, 81.05; N, 12.1; H, 6.35.
2-(4-Dimethylaminophenylethynyl)pyridine, 4. 2-Bromopyridine
(0.26 g, 1.61 mmol), bis(triphenylphosphine)palladium(II) chloride
(25 mg, 0.036 mmol), copper iodide (15 mg, 0.079 mmol), 4-ethynyl-
N,N0-dimethylaniline (0.23 g, 1.57 mmol) in NEt3 (3 mL) were
reacted at 40 1C for 72 h. The crude product was isolated as above
to yield, after flash column chromatography with hexane/EtOAc =
4:1, compound 4 as an off-white solid (0.25 mg, 1.13 mmol, 72%).
M.p.: 100.1–101.2 1C. 1H NMR (400 MHz): d 8.58 (1H, m, Pyr), 7.63
(1H, td, J 7.6, J 2.0, Pyr), 7.48 (m, 3H, 2H Ph and 1H Pyr), 7.16
(1H, ddd, J 7.6, 5.9, 1.2, Pyr), 6.65 (2H, d, J 9.0, Ph), 3.00 (6H, s,
NMe2). 13C NMR (100 MHz): d 150.7, 150.1, 144.5, 136.1, 133.5,
126.8, 122.1, 111.9, 108.9, 91.3, 87.3, 40.3. Found: C, 80.2; N, 12.0;
H, 6.45. Calc. for C15H10N2: C, 81.05; N, 12.1; H, 6.35.
The geometry of molecules A–D were minimized with the
two aromatic rings constrained in planar conformations and
the electrostatic potential energy surface calculated using the
Spartan’10 molecular modelling program with density func-
tional theory (DFT) at the B3LYP/6-311+G** level.15 Minimal
differences were obtained when the geometry corresponding to
the crystal structures were used.
X-ray crystallography
Experiments were carried out at 100 K with Mo Ka radiation
using a Bruker APEX-II CCD diffractometer. Crystal data, data
collection and structure refinement details are summarized in
Table 1.20 Absorption was corrected for by multi-scan methods.
Refinement was with 0 restraints.21 All H atoms were located
in difference maps. The methyl and phenyl hydrogens were
treated as riding atoms in geometrically idealized positions
with C–H = 0.95 (aromatic) and 0.98 Å (methyl) and Uiso(H) =
kUeq(C), where k = 1.5 for the methyl hydrogens and 1.2 for the
phenyl hydrogens. The program X-Seed was used as a graphical
interface.22 The correct absolute configuration for the mole-
cules of cocrystal C in the crystal selected for data collection
was determined by the Flack ꢃ parameter of 0.015(9) by
classical fit to all intensities and calculated using 2286 quotients
[(I+) ꢁ (Iꢁ)]/[(I+) + (Iꢁ)].23 Cocrystal D formed as thin rod shaped
crystals and after data collection from several crystals followed
by refinement the program TwinRotMat was run using the
PLATON interface.13 A TWIN law was determined and included
in the refinement.
Preparation of cocrystals
For cocrystal B, 0.020 mol of 1 and 2 were weighed into a small
screw cap vial and 1 mL of dichloromethane added to form a
Computational study of interaction energies
clear yellowish solution. The vial was loosely capped and the For each of the crystalline complexes A, B and C the coordinates
solvent allowed to evaporate over the course of several days of the halogen bonded dimer and the two unique p–p charge
during which time a mass of red rod-shaped cocrystals formed. transfer complexes were extracted and fully optimized in the
Cocrystals C and D were similarly prepared.
gas-phase using the M062X24 functional with the LANL2DZdp25–29
Table 1 Crystal data and structural refinement for cocrystals A–D
A
B
C
D
Formula
Mr
Crystal system
Space group
Temp. (K)
a (Å)
C13H13IN4O4
416.17
Monoclinic
P21/c
C29H17F4IN4O4
688.36
Monoclinic
P21/c
C29H17F4IN4O4
688.36
Orthorhombic
P212121
100
7.2434(5)
7.9376(5)
47.215(3)
90
2714.6(3)
4
C29H17F4IN4O4
688.36
Monoclinic
P21/c
100
100
100
7.1573(3)
16.8141(7)
12.5916(5)
99.905(1)
1492.73(11)
4
24.1533(13)
6.8223(4)
16.8260(9)
105.600(1)
2670.5(3)
4
7.6541(10)
13.8231(18)
26.027(3)
90.058(2)
2753.8(6)
4
b (Å)
c (Å)
b (1)
V (Å3)
Z
m (mmꢁ1
)
2.17
1.27
1.25
1.23
Crystal size (mm)
Collected reflections
Unique reflections
No. of parameters
Rint
0.35 ꢃ 0.15 ꢃ 0.10
0.35 ꢃ 0.08 ꢃ 0.03
0.27 ꢃ 0.22 ꢃ 0.11
0.34 ꢃ 0.07 ꢃ 0.08
19 222
31 716
34 837
35 822
3298
201
1
0.018, 0.042
1.09
1820039
6030
381
0.044
0.031, 0.073
1.03
5977
381
0.053
0.037, 0.073
1.16
6181
382
0.031
0.040, 0.113
1.06
R[F2 4 2s(F2)], wR(F2)
Goodness of fit
CCDC
1820040
1820041
1820042
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Table 2 Geometric parameters from crystal structures for A–D
DNPꢀ ꢀ ꢀDAPb
DNPꢀ ꢀ ꢀDAP
perp. dist.c/
slippage (Å)
TFIPꢀ ꢀ ꢀPYRd
centroid-to-centroid
dist. (Å)
Xꢀ ꢀ ꢀN dist.a
C–Xꢀ ꢀ ꢀN
angle (1)
centroid-to-centroid
dist. (Å)
TFIPꢀ ꢀ ꢀPYR perp.
Cocrystal
(Å, %)
dist./slippage (Å)
A
B
C
D
2.894(2), 82.0
2.798(2), 79.3
2.842(5), 80.5
2.893(3), 82.0
175.3(1)
172.4(1)
169.9(2)
175.7(2)
3.657(1), 3.718(1)e
3.695(2), 3.747(2)
3.614(3), 3.746(3)
3.775(5), 3.886(5)
3.25/1.68
3.21/2.71
3.25/1.87
3.38/1.69
—
—
3.589(2), 4.260(2)
3.673(3), 3.675(3)
3.752(3), 3.907(3)
3.35/1.33
3.31/1.59
3.38/1.62
a
b
c
% of the sum of the van der Waals radii. DNP = dinitrophenylring, DAP = N,N-dimethylaminophenyl/pyridine ring. Shortest perpendicular
d
e
distance to the centroid of one of the two rings. TFIP = tetrafluoroiodophenylring, PYR = pyridyl ring. Distance between the centroids of the
1-iodo-3,5-dinitrobenzene and 4-N,N-dimethylaminopyridine rings.
Labsphere. The powdered compounds were spread over white
Table 3 Hydrogen-bond geometry (Å, 1) for A–D
Millipore filter and placed in the sample holder. The normalized
Cocrystal
D–Hꢀ ꢀ ꢀA
D–H
Hꢀ ꢀ ꢀA
2.57
2.48
Dꢀ ꢀ ꢀA
3.501(2)
3.333(2)
D–Hꢀ ꢀ ꢀA
165.5
150.0
spectra of each of the individual components were digitally
subtracted from the normalized spectra of the cocrystals and
the resultant difference spectrum.
A
C2–H2ꢀ ꢀ ꢀO1i
0.95
0.95
C5–H5ꢀ ꢀ ꢀO3ii
B
C
C3–H3ꢀ ꢀ ꢀF3i
0.95
0.95
0.95
0.95
2.48
2.43
2.79
2.80
3.153(3)
3.246(3)
3.468(3)
3.747(3)
127.9
143.2
128.9
172.4
C10–H10ꢀ ꢀ ꢀF1ii
C12–H12ꢀ ꢀ ꢀO3iii
C12–H12ꢀ ꢀ ꢀO4iii
Conflicts of interest
C2–H2ꢀ ꢀ ꢀF3i
0.95
0.95
0.98
0.98
0.98
2.41
2.51
2.59
2.61
2.59
3.342(6)
3.286(6)
3.226(7)
3.564(7)
3.496(7)
169.0
139.0
122.8
164.8
153.5
There are no conflicts to declare.
C12–H12ꢀ ꢀ ꢀF1ii
C14–H14Aꢀ ꢀ ꢀO4iii
C14–H14Bꢀ ꢀ ꢀO3iv
C14–H14Cꢀ ꢀ ꢀO3v
Acknowledgements
D
C13–H13ꢀ ꢀ ꢀF2
C4–H4ꢀ ꢀ ꢀO2i
C1–H1ꢀ ꢀ ꢀO4ii
0.95
0.95
0.95
2.57
2.63
2.65
3.337(6)
3.519(6)
3.478(6)
137.5
156.1
146.5
We thank the National Science Foundation for financial support of
this research (CHE1606556), the Missouri State University Provost
Incentive Fund that funded the purchase of the X-ray diffracto-
meter, and the Missouri State University Graduate College for
funding CIN. We thank Dr Nikolay N. Gerasimchuk (MSU) for help
Symmetry codes. Cocrystal A: (i) xꢁ1, y, zꢁ1; (ii) 1ꢁx, yꢁ1/2, 3/2ꢁz.
Cocrystal B: (i) x, 3/2ꢁy, zꢁ1/2; (ii) 1ꢁx, 1ꢁy, ꢁz; (iii) xꢁ1, y, zꢁ1.
Cocrystal C: (i) x, yꢁ1, z; (ii) 1ꢁx, 1/2+y, 3/2ꢁz; (iii) 1/2ꢁx, 1ꢁy, 1/2+ z;
(iv) 1/2ꢁx, 2ꢁy, 1/2+z; (v) 1ꢁx, yꢁ1/2, 3/2ꢁz. Cocrystal D: (i) 1+x, 1/2ꢁy,
1/2+z; (ii) x, yꢁ1, z.
¨
measuring the solid-state visible spectra and Lauri Kivijarvi for
help photographing the crystals. This work used the Extreme
Science and Engineering Discovery Environment (XSEDE) resource
Comet at the San Diego Supercomputing Center through
allocation wis158. XSEDE is supported by the National Science
Foundation grant number ACI-1548562. Additional computational
resources were provided by NSF-MRI awards #CHE-1039925 and
#CHE-0520704 through the Midwest Undergraduate Computa-
tional Chemistry Consortium-MU3C.
basis set on all atoms and the corresponding pseudopotential
on iodine. All structures were verified to be local minima
through vibrational frequency analysis. Counterpoise calcula-
tions were carried out according to the procedure of Boys and
Bernardi.30 All calculations were carried out using revision D.01
of the Gaussian 0931 suite of programs made available through
XSEDE (Table 4).32
Solid state spectra
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