Rare case of a triple-stranded molecular braid in an organic cocrystal

Article abstract of DOI:10.1021/cg100577t

We describe the use of two "bent" molecules, tetrabromobisphenol- S (2) and 1,3-bis(4-pyridyl)-propane (3), to build an interesting organic cocrystal [(C₆H₂OHBr₂)₂SO ₂]?[(C₅H₄N)₂(CH ₂)₃]?MeOH (4). In the molecular structure of cocrystal 4, the cocrystallizing components (coformers) 2 and 3 assemble into a 1:1 supramolecular adduct featuring unprecedented triple-stranded molecular entanglement, called a "molecular braid", via O-H...N interactions, which is the first example of such one-dimensional entanglement in the realm of organic cocrystals.

Full text of DOI:10.1021/cg100577t

DOI: 10.1021/cg100577t
Rare Case of a Triple-Stranded Molecular Braid in an Organic Cocrystal
2010, Vol. 10
4217–4220
†
Jian Lu, Li-Wei Han,†,‡ Jing-Xiang Lin,† Tian-Fu Liu,†,‡ and Rong Cao*,†
€
†State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter,
Chinese Academy of Sciences, Fujian, Fuzhou 350002, P. R. China, and ‡Graduate School,
Chinese Academy of Sciences, Beijing 100039, P. R. China
Received April 29, 2010; Revised Manuscript Received September 7, 2010
ABSTRACT: We describe the use of two “bent” molecules, tetrabromobisphenol-S (2) and 1,3-bis(4-pyridyl)-propane (3), to build an
interesting organic cocrystal [(C6H2OHBr2)2SO2] [(C5H4N)2(CH2)3] MeOH (4). In the molecular structure of cocrystal 4, the
cocrystallizing components (coformers) 2 and 3 assemble into a 1:1 supramolecular adduct featuring unprecedented triple-stranded
3
3
molecular entanglement, called a “molecular braid”, via O-H N interactions, which is the first example of such one-dimensional
3 3 3
entanglement in the realm of organic cocrystals.
˚
(Ohydroxyl Ohydroxyl) of about 8.8 A, which is comparable to the
Cocrystals are one kind of crystalline material composed of
different molecular species that are, in their pure forms, solid
under ambient conditions, held together by virtue of noncovalent
forces.1 The crystal engineering of cocrystals has been well-
established by pharmaceutical scientists and used for means of
improving and tailoring the physicochemical properties of active
pharmaceutical ingredients (APIs) and as a consequence of
sustainable research aimed at gathering a better knowledge of
the supramolecular forces that direct and uphold crystal
structures.2,3 Furthermore, a handful of research has also ex-
tended the scopes more extensively by using a larger range of
cocrystallizing components (coformers) in order to take full
advantage of those current advancements in API cocrystals to
create exotic supramolecular architectures.4,5
The molecular mimics of topologically interesting architectures
arouse a formidable challenge for synthetic chemists. The crystal
entanglements, thanks to their topological complexity, structural
integrity, and aesthetic beauty, among others, have captured
enormous research interest, evidenced by the ever-increasing num-
ber of examples being recently documented.6 Once rare and
mysterious, some entanglement structures, such as molecular inter-
penetration, intertwining, rotaxanes, and catenanes, now become
somewhat ordinary, as summarized in recent review articles.7
On the other hand, nonconventional and intelligent molecular
entanglements have been given birth, aided by the accumulated
knowledge of the self-assembly processes and the supramolecular
self-organization based on crystal engineering.8
Very recently, two examples of attractive molecular entangle-
ments have been reported by Wang et al.9 The coordination-
driven self-assemblies of 1-D metal-organic chains that stack
into fascinating triple-stranded molecular braids were observed.
In these two cases, the formation of a “molecular braid” was
believed, thanks to both the intrinsic property of the meso-helical
chains and the assistance of appropriate intermolecular interac-
tions among the individual chains of a braid. Herein, we report a
hydrogen-bonded organic cocrystal of the “molecular braid”
realized by means of “module matching”, and it features an
assembly recognition that requires no weak interaction assistance
but is only driven by inherent close-packing. It is so far the first
known example of an organic cocrystal featuring an entangled
structure of a triple-stranded molecular braid.
3 3 3
˚
acceptor-acceptor distance (N N) of 8.5-9.5 A (according to
3 3 3
a trans-gauche (TG) or trans-trans (TT) conformation) in a BPP
molecule; the other is that the ability of the hydroxyl groups on
TBBPS to form hydrogen interactions is boosted due to the
existence of electron-withdrawing Br atoms because the H bond
strengths of specific molecular building blocks can be carefully
tuned through relatively simple covalent modifications,5a which,
we believe, may endow some superiority in directing molecular
assembly and recognition.
Solvothermal reaction of TBBPS and BPP (1:1) in MeOH
yielded light yellow crystals (cocrystal 4).10 The structure of
cocrystal 4 was determined unambiguously by single-crystal
X-ray crystallographic analysis.10 Cocrystal 4 crystallized in
monoclinic space group P21/n with a TBBPS/BPP/MeOH stoi-
chiometry of 1:1:1 (Figure 1a). The fresh crystals of 4 lose solvent
MeOH molecules in the crystal lattice instantly and become
powder sample (4a) in air.10
In the structure of cocrystal 4, the BPP molecule adopts the
trans-trans (TT) conformation, in which the N N distance
3 3 3
˚
(9.511(1) A) is very close to the actual donor-donor distance
˚
hydroxyl) of 8.995(7) A. In addition, the dihedral
(Ohydroxyl
O
3 3 3
angles of the two pyridinerings on a BPP andthe two phenylrings
on a TBBPS are 76.4° and 79.9°, respectively, which indicates that
BPP and TBBPS are geometrically matched organic modules in
the self-assembly process. The TBBPS and BPP bind to each
other in a “head-to-tail” fashion via Ohydroxyl-H N hydrogen
3 3 3
bonds, giving birth to a supramolecular meso-helical string with
alternate iteration of left-handed and right-handed half turns
(Figure 1c and d). Interestingly, the two modules, TBBPS and
BPP, that compose a half turn unit are connected by a shorter
˚
hydrogen bonding of 2.509 A, whereas the half turns of different
˚
handedness are connected by longer hydrogen bonds of 2.586 A
(Table S1 of the Supporting Information). It should be noted that
the distances of the Ohydroxyl-H N hydrogen bonds are con-
3 3 3
˚
siderably shorter than the ordinary H-bond (2.70-3.00 A), and
they are thus considered as double charge-assisted H-bonds,
comprised by strong 1/2-D Hþ A1/2- interactions (D=
3 3 3
3 3 3
H-bond donor; A = H-bond acceptor).11 Moreover, the two
transitional aromatic rings within the same half turn display a
dihedral angle of 76.0°, which matches the curvature of the
modules (76.4° and 79.9°). However, the transitional aromatic
rings between the half turns of different handedness have a
dihedral angle of 38.7°, which is considered as mismatching
segments and responsible for the formation of meso-helical
(rather than helical) strings. The bromination of TBBPS may
also have subtle influence for the formation of the meso-helical
string, due to the enhanced ability of the hydroxyl groups to form
A “bent” brominated organic molecule tetrabromobisphenol-
S,10 denoted as TBBPS (2), was synthesized to assemble with BPP
(1,3-bis(4-pyridyl)propane, 3) (Scheme 1) due to the following
two considerations: one is TBBPS has a donor-donor distance
*To whom correspondence should be addressed. E-mail: rcao@
fjirsm.ac.cn.
r
2010 American Chemical Society
Published on Web 09/15/2010
pubs.acs.org/crystal

€
Lu et al.
4218 Crystal Growth & Design, Vol. 10, No. 10, 2010
Figure 1. View of (a) the basic building unit; (b) the basic iteration pattern of the triple strings; (c) the top-view; and (d) the side-view of a
meso-helical string in cocrystal 4.
Scheme 1. Formation of the Organic Cocrystal 4
hydrogen bonds, which can be tentatively demonstrated by the
helical chain structure built by parent BPS (1) with BPP (3).12
Packing of the strings generates a 3-D matrix of triple string
layers,15,16 and 3-D structures of polycatenation based on hcb
layers. The only exception comes from a nickel coordination
polymer containing “Borromean” sql (square layer) layers.17 It is
not intricate to understand that 0-D, 2-D, and 3-D “Borromean”
arrays are easier to attain, since one essential element for the
formation of “Borromean” links is the “rings”, that is, circuits in
the structures. As to 1-D structures, only complex patterns with
intrinsic “rings”, such as molecular ladders and columns, may
meet the demands ofpossible 1-D “Borromean” entanglement, as
predicted in a recent review article by Ciani et al.18 Actually, no
1-D “Borromean” array has ever been observed in a real mole-
cular structure. As known from Alexander’s theorem, “each link
in three-dimensional spaces is ambient isotopic to a link in the
form of a closed braid”.19 A “Borromean” link can form with
“closure of a braid” by closing the top/bottom of one period of
the molecular “braid”, and it may be counted as the closest rela-
tive of the family of “Borromean” links, since a seat is currently
left vacant for one-dimensional “Borromean” entanglements.20
In summary, we address herein the cocrystallization of an
organic cocrystal (4) that exhibits an interesting structural topol-
arrays, which are interlinked by Br Osulfonyl halogen bonds
3 3 3
˚
˚
(2.983 A and 3.227 A). It is interesting to notice that the triple
entanglements of strings in cocrystal 4 arrange in a way that any
one string is placed always fully above another string and, at the
same time, below the other one (Figure 1b). In such a way, the
three strings interpenetrate each other inextricably, but if any
one string is removed, the other two separate automatically
(Figure 2). Such a feature of entanglement is reminiscent with
the fascinating “Borromean” links (Figure 3).13 A “Borromean”
link was originally defined as triple entanglements of rings taken
together inseparably, but removal of any one ring sets free the
other two, which is known not only as conceptual and physical
manifestation but also used as a symbol of “strength in unity” in
many fields since time immemorial. Rational construction of
“Borromean” molecular arrays has long been considered as one
of the most challenging goals in synthetic chemistry.14-17
So far, only a handful of synthetic compounds assembled by
means of coordinative14,15 or even rare supramolecular interac-
tions16 (argentophilic, hydrogen, and halogen interactions) ex-
hibit molecular features of “Borromean” links. The most fre-
quently encountered synthetic “Borromean” links are molecular
oligomers (0-D),5 2-D polymeric nets having hcb (honeycomb)
ogy of the molecular “braid” directed by O-H N interactions
3 3 3
andinherentclose-packing assembly. The successfulisolationof4
indicates the feasibility to create topologically fascinating supra-
molecular architectures, taking full advantage of knowledge
in the scope of API organic cocrystals. Further investigation

Communication
Crystal Growth & Design, Vol. 10, No. 10, 2010 4219
Figure 2. View of the triple-stranded entanglement in cocrystal 4.
Figure 3. Symbols of the “Borromean” link (left) and “braid” (right).
€
on the effect of halogenations on traditional organic modules
in construction of interesting supramolecular architectures is
underway.
(2) (a) Almarsson, O.; Zaworotko, M. J. Chem. Commun. 2004, 1889.
(b) Vishweshwar, P.; McMahon, J. A.; Peterson, M. L.; Hickey, M. B.;
Shattock, T. R.; Zaworotko, M. J. Chem. Commun. 2005, 4601.
(c) Vishweshwar, P.; McMahon, J. A.; Bis, J. A.; Zaworotko, M. J.
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Acknowledgment. Financial support from the 973 Program
(2006CB932903, 2007CB815303), NSFC (20731005, 20821061,
20873151, and 21001105), NSF of Fujian Province (E0520003),
and Key Project from CAS are gratefully acknowledged. J.L.
thanks Prof. Davide M. Proserpio of the University of Milano
(Italy) for helpful discussions.
(3) See references for example: (a) Walsh, R. B.; Bradner, M. W.;
Fleischman, S.; Morales, L. A.; Moulton, B.; Rodriguez-Hornedo,
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S. G.; Kuduva, S. S.; McMahon, J. A.; Moulton, B.; Walsh, R. D. B.;
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1
464w. H NMR (DMSO-d6): σ 8.187 (s, Ar) ppm. Crystal data for
˚
2: monoclinic, C2/c, a= 13.563(6) A,b= 12.075(5) A, c= 9.293(4) A,
˚
˚
3
˚
β = 91.723(6)°, V = 1521.2(1) A , Z = 4, GooF = 0.847. Final
residuals were R1=0.0297, wR2=0.0984 for 5258 independent
reflections. Synthesis of [(C6H2OHBr2)2SO2] [(C5H4N)2(CH2)3]
3
3
ꢀ
MeOH Cocrystal 4: Crystals of 4 were originally obtained from a
solvothermal reaction described as follows: a mixture of 1:1 TBBPS (2,
mp 297 °C) and 1,3-bis(4-pyridyl)propane (BPP, 3, mp 53-56 °C) was
added to 6 mL of methanol and sealed in a Teflon-lined autoclave (20
mL) and kept at 130 °C for 3 days. After slow cooling to room
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for further single crystal X-ray diffractions by locking them inside
a glass tube filled with mother liquid. Crystal data for 4 (CCDC
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(20) Mathematical connection between a “braid” and “borromean”
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˚
˚
775136): Monoclinic, P21/n, a = 9.043(1) A, b = 34.31(5) A, c =
˚
3
˚
9.524(2) A, β = 105.40(3)°, V = 2849(7) A , Z = 4, GooF = 1.065.
The final residuals were R1 = 0.0632 and wR2 = 0.1648 for 5955