Helical water chains in aquapores of organic hexahost: Remarkable halogen-substitution effect on the handedness of water helix

Article abstract of DOI:10.1039/b500284b

Water helices surrounding the nano-channels of trichlorophloroglucinol and tribromophloroglucinol have different handedness, PMPMPM and PPPMMM (P = right-handed, M = left-handed), depending on halogen...halogen interactions between the host molecules. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b500284b

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Helical water chains in aquapores of organic hexahost: remarkable
halogen-substitution effect on the handedness of water helix{
Binoy K. Saha and Ashwini Nangia*
Received (in Columbia, MO, USA) 12th January 2005, Accepted 19th April 2005
First published as an Advance Article on the web 11th May 2005
DOI: 10.1039/b500284b
Water helices surrounding the nano-channels of trichloro-
phloroglucinol and tribromophloroglucinol have different
handedness, PMPMPM and PPPMMM (P 5 right-handed,
…
M 5 left-handed), depending on halogen halogen interactions
between the host molecules.
Water is the most studied and yet the least understood of chemical
Cl–PHG crystallizes as a trihydrate from EtOAc. The X-ray
species.1 Water has attracted considerable attention from struc-
crystal structure of Cl–PHG.(H2O)3 in P21/n space group{ has one
tural chemists and biologists because of its importance in gas
host and three symmetry-independent water molecules. The heavy
hydrates, its different topologies, and the important role of water
atoms are fully ordered but phenol OH and one of the water H
in the structure, conformation and function of nucleotides and
atoms are disordered,{ even at 100 K. Inversion-related molecules
peptides as well as in protein–DNA binding. The helical motif
of Cl–PHG stack at van der Waals distance in a Piedfort Unit7
is central to biology and life: apart from the structure of
…
˚
assembly (p p 3.28, 3.41 A) to form a hexahost dimer with six
phenol OH groups radiating outwards (Fig. 1a). Three phenol
OHs (O1, O2, O3) are hydrogen-bonded to crystallographically
distinct water molecules (O4, O5, O6), which are in turn H-bonded
DNA, cooperative helical assemblies are functional in transport
mechanisms, e.g., as in plasma membrane of red blood cells, renal
tubules, gramicidin A and aquaporin-1.2 The inclusion of water in
zeolite-like 3D network structures,3a,b negative thermal expansion
of water helix in an organic crystal,3c and water conduction
through the hydrophobic channel of a carbon nanotube3d are
reported in recent papers. There is a surge of interest in applying
supramolecular chemistry principles4 to study water structures,
notably the idea that the constrained microenvironment of organic
and metal–organic host lattices is an excellent solid-state medium
to isolate and analyze various hydrogen-bonded water clusters.5
These ‘‘water trapped in a host matrix’’ structures have provided
accurate and detailed information about hydrogen bonding in
diverse water clusters. However, the unusual topology or
architecture of hydrogen-bonded water is generally observed only
in a specific host lattice. Therefore, relationship between the water
topology and the host structure is difficult to delineate. Ideally, one
would like to see similar water motifs (e.g. 1D chain, 2D layer or
3D networks) in closely related host structures. To our knowledge,
there is only one such pair of structures, a (H2O)10 cluster of ice-
like water molecules in the supramolecular complex of Cu and Co
with the same ligand, reported by Atwood and coworkers.5c,d We
report in this paper infinite 1D helical chains of water molecules in
nanoporous channels of organic hexahosts trichlorophloroglucinol
and tribromophloroglucinol, Cl–PHG.(H2O)3 and Br–
…
˚
to phenol acceptor groups (O1, O3, O2; O O 2.70–2.83 A). The
second donor hydrogen of the water molecule is used in
…
H-bonding with itself to form a helical spine of Ow–H Ow
˚
hydrogen bonds (helix pitch 5 6.93 A 5 a-axis; Fig. 1b). Six such
helices of tetracoordinated water molecules surround a Piedfort
stack of Cl-PHG host molecules. The almost flat bc-layer has a
hexagonal arrangement of Cl–PHG molecules mediated via phenol
8
…
groups, water molecules, and Cl Cl contacts. The trigonal
nanotube containing the 1D helical water polymer along the a-axis
˚
…
has a pore of y4.6 A on each side. The disorder of phenol H atoms
in Cl–PHG.(H2O)3 could be due to intramolecular O–H Cl
interactions9 with the flanking chlorine atoms.
The crystal structure of Br–PHG.(H2O)3 (crystallized from
EtOAc/CHCl3, P21/c space group) shows similarities and diffe-
rences compared to Cl–PHG trihydrate. All host and water atoms,
including hydrogens, are fully ordered. The host Br–PHG
molecules adopt a similar hexagonal arrangement (triangular pore
˚
size y4.6 A) and are bonded to six water molecules via phenol OH
…
˚
groups (O O 2.72–2.98 A). However, the handedness of water
˚
helices (helix pitch 5 7.11 A 5 a-axis; Fig. 1c,d) surrounding a
…
Piedfort stack (p p 3.30, 3.37 A) of Br–PHG host molecules is
…
different (Fig. 2). The six water helices (Ow–H Ow) encircling a
˚
…
PHG.(H2O)3. Interestingly, weak halogen halogen interactions
rod of Cl–PHG host molecule are alternately right- and left-
handed (PMPMPM; P 5 plus or right-handed, M 5 minus or
left-handed) whereas their alignment in Br–PHG trihydrate is,
surprisingly, three contiguous helices of the same chirality and the
other three of opposite helicity (PPPMMM). The latter situation is
novel among helical water chains.5k The water helix has
appear to direct the handedness of water helices surrounding a rod
of Cl–PHG or Br–PHG host molecules. There are very few
examples of helical water assemblies3c,5k,5m in the infinite water
chain category.6
{ Electronic supplementary information (ESI) available: preparation of
compounds, H atom disorder, host layer structures and PXRD traces. See
http://www.rsc.org/suppdata/cc/b5/b500284b/
…
homodromic chains of Ow–H Ow hydrogen bonds (cooperative
H-bonding) in the fully ordered Br–PHG.(H2O)3 structure.§ Two
factors should be considered to understand these differences: (1)
*ashwini_nangia@rediffmail.com
3024 | Chem. Commun., 2005, 3024–3026
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Fig. 1 (a) Piedfort Unit of p-stacked Cl–PHG molecules to show six phenol OHs extending radially. Water helices are encircled in the triangular pores of
…
three Cl–PHG stacks. Six water helices surround a rod of host molecules when viewed down the a-axis. (b) Ow–H Ow hydrogen bonding in a water helix
of Cl–PHG.(H2O)3 (disordered protons are shown). (c) Homodromic H-bonds in the water helix of ordered Br–PHG.(H2O)3. (d) Spiral assembly of host
molecules (green, blue) around the right-handed water helix in Br–PHG.(H2O)3. Higher occupancy disordered H-atoms are shown in (a) and symmetry-
independent water oxygens are colored differently in (b) and (c).
…
layer because phenyl rings connected via the short Br Br contact
are tilted.{" This tilt in the phenol OH groups changes the position
of hydrogen-bonded water molecules along the a-axis and, in turn,
the helicity of the water chain. Water helices that are adjacent to
the roughly coplanar host phenyl rings and lie across longer inter-
˚
halogen contacts of y3.4 A are of opposite handedness (PM),
whereas 1D water chains that are adjacent to the tilted host phenyl
…
˚
rings and are arranged across the short Br Br contact of y3.3 A
have the same handedness (PP or MM) (Fig. 2). The present
case is the first illustration on the fascinating role of weak
…
halogen halogen interactions in directing the helical twist of
strong Ow–H Ow chains in crystal structures.11 The more
…
polarizable bromine is able to participate in a shorter inter-
halogen contact compared to the smaller chlorine atom. This
prompted us to crystallize I-PHG but it afforded an anhydrous
form whose structural details will be discussed in a full article.
Water loss in thermal gravimetric analysis (TGA, Fig. 3)
matches with the trihydrate stoichiometry of Cl–PHG.(H2O)3
(obsd. 18.8%, calc. 19.0%) and Br–PHG.(H2O)3 (obsd. 12.5%,
calc. 12.9%). The endotherm for water evolution from Cl–PHG is
at 103 uC and from Br–PHG at 77 uC (major) and 94 uC (minor)
in DSC (differential scanning calorimetry). The normalized
enthalpy for water transit (2656 and 2332 J g21) and strength
Fig. 2 Six water helices surround the Piedfort dimer of (a) Cl–PHG and
(b) Br–PHG (phenol groups are omitted for clarity). The handedness of
˚
water helices across longer inter-halogen contacts of y3.4 A is opposite
…
of O–H O hydrogen bonds in Cl–PHG.(H2O)3 and Br–
PHG.(H2O)3 are 62 and 46 kJ mol21 per water molecule (21
˚
whereas it is the same when the distance is y3.3 A. This is due to the tilt of
phenyl ring. See text for explanation.
The 1D water helix is the central self-assembly building unit in
˚
both hydrate host–guest structures. (2) Bromine is about 0.1 A
˚
larger than chlorine (van der Waals radii: Cl 1.75 A, Br 1.85 A),
˚
and also more polarizable. There are small, yet structurally
…
significant, differences in halogen halogen interactions. While
…
both Cl Cl interactions are of about the same length (3.387,
…
˚
3.443 A) one of the Br Br contacts is much shorter than the other
(3.291, 3.473 A).10 Whereas Cl–PHG host molecules are arranged
˚
Fig. 3 DSC (a) and TGA (b) of Cl–PHG.(H2O)3 and Br–PHG.(H2O)3.
Chem. Commun., 2005, 3024–3026 | 3025
in an almost flat bc-sheet Br–PHG molecules lie in a corrugated
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CCDC 257771 & 257772. See http://www.rsc.org/suppdata/cc/b5/b500284b/
for crystallographic data in CIF or other electronic format.
§ The location of water H atoms determines the homo/anti/hetero-dromic
arrangement of hydrogen-bonded helices and the placement of O atoms
along the a-axis determines their P/M handedness. The helicity of water
chains is not affected by proton disorder in Cl–PHG.(H2O)3.
" The tilting and corrugation relieve steric congestion between these
hexasubstituted benzenes.
and 15 kJ per H-bond). These values are in good agreement with
H-bonds in water chains and proteins1d,12 The higher onset
temperature and enthalpy for water release from Cl–PHG
compared to Br–PHG channel is due to stronger (shorter)
…
˚
hydrogen bonds in the former structure (O O 2.70–2.83 A vs.
˚
2.72–2.98 A).
Powder X-ray diffraction (PXRD) of the hydrate and
anhydrous material showed differences in dehydration/rehydration
behavior. Dehydration of Cl–PHG.(H2O)3 at 115 uC for 2 h under
vacuum afforded a material whose PXRD is identical to the
original powder pattern,{ showing that the host lattice is robust
enough to the loss of interstitial water. The dehydrated material
regained about two-third of its water from atmospheric moisture
within 4 h and gained the original water stoichiometry (19% weight
increase) after 24 h (TGA). Thus, Cl–PHG.(H2O)3 exhibits
‘‘organic zeolite’’-like behavior through reversible water loss and
uptake. On the other hand, PXRD trace of Br–PHG.(H2O)3 after
dehydration is significantly different. Thus, there are structural and
functional differences between these hydrate channel inclusion
structures.
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molecules in a synthetic collagen{ is similar to Fig. 1a. The helical
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small-molecule model for studying the dynamics of water transport
in aquaporin membrane proteins. Further studies on probing the
influence of inter-halogen interactions on the handedness of water
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van der Waals distance to form a hexagonal species. D. D. MacNicol
and G. A. Downing, in Comprehensive Supramolecular Chemistry,
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A.N. thanks the DST for funding (SR/S5/OC-02/2002) and
B.K.S. thanks the CSIR for a fellowship. We thank coworkers
from UoH: S. Aitipamula for DSC/TGA, L. S. Reddy for
assistance with X-ray data, and Prof. C. Bansal for PXRD. DST
and UGC are thanked for the X-ray CCD diffractometer and the
UPE program.
Binoy K. Saha and Ashwini Nangia*
School of Chemistry, University of Hyderabad, Hyderabad, 500 046,
India. E-mail: ashwini_nangia@rediffmail.com; Fax: +91 40 23011338
8 On the role on halogen atoms in lattice inclusion hosts, see B. K. Saha,
R. K. R. Jetti, L. S. Reddy, S. Aitipamula and A. Nangia, Cryst.
Growth. Des., 2005, 5, 887.
9 R. Banerjee, G. R. Desiraju, R. Mondal and J. A. K. Howard, Chem.
Eur. J., 2004, 10, 3373.
Notes and references
…
˚
10 The Br Br contact of 3.29 A is quite short. There are only 13 structures
in the Cambridge Structural Database (CSD, http://www.ccdc.cam.ac.uk,
{ Crystal data was collected on Bruker SMART APEX CCD with Mo–
…
July 2004 update, .300 000 entries) with Br Br , 3.30 A.
˚
˚
Ka radiation (l 5 0.71073 A) at 100 K. Cl–PHG.(H2O)3: C6H9Cl3O6,
Mr 5 283.48, monoclinic, P21/n, a 5 6.9261(10), b 5 16.057(2),
…
11 Two recent examples wherein weak halogen halogen and
…
halogen oxygen interactions direct self-assembly in strongly hydrogen-
3
c 5 9.9510(15) A, b 5 109.923(2), V 5 1040.4(3) A , Z 5 4,
˚
˚
bonded systems are: (a) J. N. Moorthy, R. Natarajan, P. Mal and
P. Venugopalan, J. Am. Chem. Soc., 2002, 124, 6530; (b) S. George,
A. Nangia, C.-K. Lam, T. C. W. Mak and J.-F. Nicoud, Chem.
Commun., 2004, 1202.
R1 5 0.0289, wR2 5 0.0698. Br–PHG.(H2O)3: C6H9Br3O6, Mr 5 416.86,
˚
monoclinic, P21/c, a 5 7.1076(5), b 5 9.1708(7), c 5 16.7599(12) A,
3
b 5 93.9930(10), V 5 1089.80(14) A , Z 5 4, R1 5 0.0193, wR2 5 0.0483.
˚
Intensities were corrected for absorption effects using the multi-scan
technique SADABS. All non-hydrogen atoms were refined anisotropically
and H atoms were located from difference electron density maps. The
occupancy of disordered H atoms in Cl–PHG.(H2O)3 is given in ESI.{
Structure solution and refinement was carried out with Bruker SHELXTL.
12 M. S. Pometun, U. M. Gundusharma, J. F. Richardson and
R. J. Wittebort, J. Am. Chem. Soc., 2002, 124, 2345.
13 J. Bella, M. Eaton, B. Brodsky and H. M. Berman, Science, 1994, 266,
75.
3026 | Chem. Commun., 2005, 3024–3026
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