First example of an ice-like water hexamer boat tape structure in a supramolecular organic host

Article abstract of DOI:10.1039/b600348f

A T6(2) tape of hydrogen bonded water molecules in boat cyclohexane conformation resides in the channel structure of a dibromophloroglucinol (DBPG) host; water escapes at 40-90°C but is readily re-absorbed by the sponge-like apohost. The Royal Society of

Full text of DOI:10.1039/b600348f

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First example of an ice-like water hexamer boat tape structure in a
supramolecular organic host{
Binoy K. Saha and Ashwini Nangia*
Received (in Cambridge, UK) 10th January 2006, Accepted 7th March 2006
First published as an Advance Article on the web 17th March 2006
DOI: 10.1039/b600348f
A T6(2) tape of hydrogen bonded water molecules in boat
aqueous methanol.{ The asymmetric unit (space group C2/c)
cyclohexane conformation resides in the channel structure of a
dibromophloroglucinol (DBPG) host; water escapes at 40–
90 uC but is readily re-absorbed by the sponge-like apohost.
contains 0.5 molecule of DBPG (with O1 phenol and C4 atoms
residing on the 2-fold rotation axis) and two water molecules (O3
and O4). Four DBPG molecules are hydrogen bonded via water
…
…
…
˚
molecules (O2–H O3 2.67 A, O3–H O2 2.81 A) and Br Br
˚
A proper theoretical, structural, spectroscopic and thermody-
namicunderstanding offinite andinfinite waterclustersisacentral
researchprobleminchemistry.1 Itcomesas no surprisethatstudies
on water were ranked in the top ten breakthroughs of 2004 by
Science.2 The primary aim of structural studies on water clusters is
to accurately characterize finite ring motifs (e.g. tetramers,
pentamers, hexamers, dodecamers, etc.), 1D chains, helices and
tapes, and 2D sheet-like water motifs by X-ray crystallography3
for a detailed insight into related patterns in Nature and/or that
control biological processes. The dominant category of hexamer
water clusters occur in different six-membered ring conformations
(Scheme 1) within an energy of 0.8 kcal mol21: chair # cage #
book , prism , boat.4 There is no example of an ice-like four-
coordinated water hexamer tape structure in the boat conforma-
tion to our knowledge. Intercalated water sheet structures in
chair–boat and boat conformations have been published.5 We
report the first example of a cyclohexane boat conformation water
tape in dibromophloroglucinol tetrahydrate, DBPG?4H2O.
Interestingly, and in contrast to several previously published
hydrate structures wherein the authors did not simultaneously
reportthe anhydrous form,6 wedisclosetheX-raycrystalstructure
of anhydrous DBPG and reversible loss/uptake of water from its
sponge-like channel framework.
˚
˚
contacts (3.45 A) to form a rectangular grid of 8.0 6 4.5 A voids
in the ab-plane (Fig. 1a). Aromatic rings of adjacent layers stack
…
˚
with excellent p p overlap at a 3.42 A inter-planar distance to
make channels along the c-axis. Hydrogen bonded water molecules
reside in the tubular framework of the DBPG host connected via
…
water phenol H bonding. The H atom of phenol O1 is disordered
over two locations by crystallographic 2-fold symmetry and one of
the H atoms of water O4 has 0.5 occupancy (H4B, H4C). The
intricate H bond network of water molecules along [001] is shown
in Fig. 1b. Three structural features of the water cluster deserve
discussion. (1) O3 and O4 water molecules are H bonded in a
hexamer of boat cyclohexane conformation. The four O atoms
that constitute the bottom of the boat are nearly coplanar,
˚
deviating by only 0.25 A from the mean plane of O3–O4–O3–O4
atoms. (2) Such hexamers are connected at the O4–O4 bond to
form a linear tape of tetra-coordinated boat water clusters for the
first time (Fig. 1c), similar to the H bond network along the c-axis
in hexagonal ice (Ih) (see Fig. 1d). (3) The edge O atoms pointing
towards each other in the high-energy boat hexamer of water
molecules are stabilized by cooperative H bonds with the phenol
…
˚
host (O1–H O4 2.77 A).
Infantes et al.7 classified water clusters as ring (R), chain (C),
tape (T), layer (L) motifs, etc. The infinite tape motif in the present
hydrate structure has the notation T6(2), namely a tape of
6-membered rings with 2 shared water molecules. The seven
examples of T6(2) water topology7a reported in the 2002 version of
the Cambridge Structural Database (CSD)8 have chair cyclohex-
Crystals of the composition DBPG?4H2O were obtained by the
slow evaporation of a solution of dibromophloroglucinol in
ane conformation. However,
a very recent (2005) critical
evaluation of water oligomers by Mascal et al.9 shows that out
of 31 examples of T6(2) clusters, 7 structures have water molecules
in the boat conformation (CSD refcodes ODIMAP, BELFED,
WUXLOQ, PIZPET, PIZPIX/01 and PIZPOD). Six of these hits
are metal–organics and one is an organic host structure. However,
water H atoms are located in only 2 crystal structures, ODIMAP
and PIZPIX01, but both these were determined at room
temperature.10 We have collected reflections on a DBPG?4H2O
crystal at 298 K and 100 K to confirm that there is no phase
transition and water H atom positions are assigned from the
difference electron density map at 100 K. The T6(2) boat water
tape in the DBPG host is unique among the 7 CSD refcodes. (1)
Our structure has tetra-coordinated water molecules, as in ices
Ih and Ic, in contrast to a mixture of 4-coordinated and
Scheme 1 Computed energies (MP2/aVTZ level) of the five lowest
energy (relative to chair, in kcal mol21) water hexamer structures (ref. 4).
School of Chemistry, University of Hyderabad, Hyderabad 500 046,
India. E-mail: ashwini_nangia@rediffmail.com; Fax: +91 40 2301 1338
{ Electronic supplementary information (ESI) available: Synthesis of
DBPG, crystallographic data, and H bond parameters. See DOI: 10.1039/
b600348f
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Chem. Commun., 2006, 1825–1827 | 1825

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Fig. 1 (a) Rectangular voids between DBPG molecules in the ab-plane and stacking of aromatic rings with little offset generate channels down the c-axis.
(b) Hydrogen bonding between O3 and O4 water molecules in the water hexamer tape structure bonded to phenol O1 and O2 atoms along the tubular host
framework. (c) The boat cyclohexane conformation of T6(2) water tape in DBPG?4H2O. (d) The T6(2) tape (red) highlighted in the structure of hexagonal
ice Ih (blue). Water O atoms are located at the nodes and H bonds are represented as lines in the 3D network.
3/5-coordinated waters in the reported structures. The 1D water
tape in ODIMAP is highly distorted compared to the tetrahedral
geometry in Ih. Gillon et al.11 surveyed the CSD for hydration
motifs in molecular crystals: 3-coordinated water is the most
common (38.40%) followed by 4-coordinated water (27.45%), or
the so-called Walrafen pentamer.1a (2) Whereas the T6(2) tapes are
coordinated to other water molecules in CSD structures, only
water molecules that are part of the boat cyclohexane cluster
propagate the tape motif in DBPG tetrahydrate. The water cluster
boat geometry shows the expected variation with temperature.{
…
˚
˚
The mean O O distance of 2.75 A at 100 K (2.77 A at 298 K) is
˚
closer to the distance in hexagonal ice (2.75 A) than liquid water
˚
(2.85 A), whereas O–O–O angles vary from 92 to 108u compared
to the mean value of 109.5u in the tetrahedral arrangement of
water neighbors in ice.
Fig. 2 DSC and TGA thermograms of DBPG?4H2O. The major
endothermic peak at 81.6 uC is due to water loss consistent with TGA
data. The endotherm at 97.7 uC is a phase transition of the water-filled
channel structure to the anhydrous apohost, which starts melting at
y140 uC (small endotherm) and then DBPG completely melts at 170.8 uC.
DSC/TGA data were recorded at a temperature ramp of 10 uC min21
The evolution of water from the channel structure of DBPG
was analyzed by thermal gravimetric analysis and differential
scanning calorimetry (TGA, DSC plots in Fig. 2). There is a 19.5%
weight loss between 40 and 90 uC in DBPG?4H2O (calc. = 20.2%)
consistent with the broad endothermic peak at 81.6 uC. The
enthalpy for water release is 29.7 kJ mol21, which means 10.8 kJ
mol21 per hydrogen bond. This value is lower than the H bond
energy in water helices3i and a (H2O)17 cluster12 but about half that
in crystalline ice polymorphs (y22 kJ mol21).13 The second
endotherm at 97.7 uC is a phase transition of the channel structure
to the anhydrous form since this thermal event in the DSC trace is
not concomitant with a weight loss in the TGA data. DBPG melts
at 170–171 uC. Whereas there are several examples of water
clusters trapped in organic and metal–organic host lattices, the
structure of the apohost is seldom reported at the same time
because of their high tendency to include water molecules and not
crystallize in a guest-free form.6 A rare example of a water sheet
structure and clathrate hydrate was recently reported.14
under dry N2 purge of 150/50 mL min21
.
Crystallization of DBPG from CHCl3–EtOAc afforded an
anhydrous crystalline form (space group Pbcn) exhibiting a
pseudo-hexagonal arrangement of molecules (Fig. 3a) via phenol
Fig. 3 (a) Crystal structure of anhydrous DBPG. (b) The type II inter-
halogen interaction in the anhydrous form and (c) the type I contact in the
hydrate.
…
…
˚
O–H O H bonds (2.92, 3.12 A) and Br Br interactions (3.49 A);
˚
however, there is no significant p–p stacking. Here too, the
1826 | Chem. Commun., 2006, 1825–1827
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Notes and references
{ DBPG?4H2O: monoclinic, C2/c, a = 16.0309(12), b = 10.6848(8), c =
3
˚
˚
7.3246(5) A, b = 111.4500(10)u, V = 1167.71(15) A , Z = 4, R1 = 0.0198,
wR2 = 0.0502, T = 100 K. DBPG: orthorhombic, Pbcn, a = 5.4011(5), b =
3
˚
˚
12.8974(13), c = 10.9478(11) A, V = 762.63(13) A , Z = 4, R1 = 0.0298,
wR2 = 0.0794, T = 298 K. Structure solution and refinement were carried
out with Bruker SHELXTL. See ESI for further crystallographic details
and crystal data of DBPG?4H2O at 298 K.{ CCDC 294904–294906. For
crystallographic data in CIF or other electronic format see DOI: 10.1039/
b600348f
§ Ref. 3i and unpublished crystal structures in this series of compounds.
1 (a) R. Ludwig, Angew. Chem., Int. Ed., 2001, 40, 1808; (b) J. M. Ugalde,
I. Alkorta and J. Elguero, Angew. Chem., Int. Ed., 2000, 39, 717; (c)
D. C. Clary, D. M. Benoit and T. van Mourik, Acc. Chem. Res., 2000,
33, 441; (d) U. Buck and F. Huisken, Chem. Rev., 2000, 100, 3863; (e)
G. A. Jeffrey, An Introduction to Hydrogen Bonding, OUP, Oxford,
1997.
2 The News Staff, Science, 2004, 306, 2013.
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Eur. J., 2005, 11, 4643 (tetramer, chain, sheet); (b) B. -Q. Ma, H.-L. Sun
and S. Gao, Chem. Commun., 2004, 2220 (pentamer); (c) Y.-C. Liao,
Y.-C. Jiang and S.-L. Wang, J. Am. Chem. Soc., 2005, 127, 12794
(hexamer); (d) U. Mukhopadhyay and I. Bernal, Cryst. Growth Des.,
2005, 5, 1687 (hexamer); (e) X.-M. Zhang, R.-Q. Fang and H.-S. Wu,
Cryst. Growth Des., 2005, 5, 1335 (hexamer tape); (f) L. J. Barbour,
G. W. Orr and J. L. Atwood, Nature, 1998, 393, 671 (10-mer cluster); (g)
B.-Q. Ma, H. -L. Sun and S. Gao, Chem. Commun., 2005, 2336 (book-
shaped hexamer); (h) B. Sreenivasulu and J. J. Vittal, Angew. Chem., Int.
Ed., 2004, 43, 5769 (helix); (i) B. K. Saha and A. Nangia, Chem.
Commun., 2005, 3024 (helix); (j) K. Raghuraman, K. K. Katti,
L. J. Barbour, N. Pillarsetty, C. L. Barnes and K. V. Katti, J. Am.
Chem. Soc., 2003, 125, 6955 (18-mer cluster); (k) P. S. Sidhu,
K. A. Udachin and J. A. Ripmeester, Chem. Commun., 2004, 1358
(chain); (l) R. Luna-Garcia, B. M. Damia´n-Murillo, V. Barba, H. Ho¨pfl,
H. I. Beltra´n and L. S. Zamudio-Rivera, Chem. Commun., 2005, 5527
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and P. L. Raston, J. Am. Chem. Soc., 2001, 123, 7192 (octamer); (n)
M. Zuhayra, W. U. Kampen, E. Henze, Z. Soti, L. Zsolnai, G. Huttner
and F. Oberdorfer, J. Am. Chem. Soc., 2006, 128, 424 (tetramer).
4 M. Losada and S. Leutwyler, J. Chem. Phys., 2002, 117, 2003. The
relative energies vary slightly depending on the level of calculation and/
or the spectroscopic method used (ref. 1).
Fig. 4 Powder X-ray diffraction plots. (a) Anhydrous DBPG (simu-
lated); (b) expulsion of water from the tetrahydrate (experimental); (c)
rehydration of the anhydrous form with water vapor (experimental); (d)
DBPG?4H2O (simulated).
molecule resides on the 2-fold rotation axis and phenol protons are
disordered over two sites with 0.5 occupancy. The packing fraction
of DBPG?4H2O is 68.3% with water occupying 35.8% of the
crystal volume; the DBPG packing fraction is 73.7%. Stronger H
bonding in the tetrahydrate structure (see ESI for distances at
298 K{) compared to the anhydrous form is the reason for water
inclusion despite a lower packing fraction in the hydrate crystal.
Whereas the inter-halogen geometry15 is of type I in the hydrate
structure, polarization-induced type II contacts are present in the
anhydrous form (Fig. 3b, c). The greater significance of halogen
bonding over hydrogen bonding in anhydrous DBPG compared
to the tetrahydrate structure is consistent with ongoing experi-
ments.§ The halogen atom plays a significant role in these strong H
¯
bonded structures because diiodophloroglucinol (P1) has a
5 P. Rodr´ıguez-Cuamatzi, G. Vargas-D´ıaz and H. Ho¨pfl, Angew. Chem.,
Int. Ed., 2004, 43, 3041; K.-M. Park, R. Kuroda and T. Iwamoto,
Angew. Chem., Int. Ed. Engl., 1993, 32, 884.
6 A guest-free form of TATM was recently reported. P. S. Sidhu,
G. D. Enright, K. A. Udachin and J. A. Ripmeester, Chem. Commun.,
2005, 2092. Water chains in the TATM host are reported in ref. 3k.
7 (a) L. Infantes and S. Motherwell, CrystEngComm, 2002, 4, 454; (b)
L. Infantes, J. Chisholm and S. Motherwell, CrystEngComm, 2003, 5,
480.
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9 M. Mascal, L. Infantes and J. Chisholm, Angew. Chem., Int. Ed., 2006,
45, 32.
10 F. Seela, T. Wiglenda, H. Rosemeyer, H. Eickmeier and H. Reuter,
Angew. Chem., Int. Ed., 2002, 41, 603 (ODIMAP); A. Michaelides,
S. Skoulika, E. G. Bakalbassis and J. Mrozinski, Cryst. Growth Des.,
2003, 3, 487 (PIZPIX01).
11 A. L. Gillon, N. Feeder, R. J. Davey and R. Storey, Cryst. Growth Des.,
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J., 2004, 10, 1366.
13 M. Henry, ChemPhysChem, 2002, 3, 607.
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and L. J. Wright, Chem. Commun., 2005, 4047.
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Cryst. Growth Des., 2005, 5, 887.
different crystal packing.
Dehydration of the water-filled channels at 90 uC for 30 min at
0.2 Torr gave a microcrystalline solid whose powder XRD pattern
matches with simulated peaks from the Pbcn X-ray crystal
structure (Fig. 4). When the dehydrated material was left overnight
in a chamber saturated with water vapour it completely converts to
the tetrahydrate form (PXRD, TGA). Thus, the organic host
DBPG functions like a supramolecular sponge:16 it readily uptakes
moisture and releases the guest under relatively mild conditions
(,100 uC). The IR spectra (KBr) of anhydrous and hydrated
DBPG show similar broad peaks for the OH stretch in the range
3000–3500 cm21 due to H bonded phenol and water aggregates,
making it difficult to identify the peaks from water clusters
(compare with hexagonal ice at 3220 cm21 and liquid water at
3280 cm21). Cyclic water hexamers are prototypical structural
motifs in ice polymorphs Ic, Ih, proton-disordered ice II, and bulk
water. Cubic ice is stable below 2120 uC but undergoes a phase
transition to normal hexagonal ice above 280 uC. The absence of
a phase transition in DBPG?4H2O between 100 and 298 K
provides a 1D ice-like structural motif for variable-temperature
diffraction and spectroscopy experiments.
16 The word ‘sponge’ has been used previously for a host lattice. M. P.
Byrn, C. J. Curtis, Y. Hsiou, S. I. Khan, P. A. Sawin, R. Tsurumi and
C. E. Strouse, J. Am. Chem. Soc., 1990, 112, 1865. We feel that the word
is appropriate here because DBPG exhibits facile uptake and loss of
water like a sponge.
A.N. thanks the DST for funding (SR/S5/OC-02/2002) and
B.K.S. thanks the CSIR for a fellowship. DST and UGC are
thanked for the X-ray CCD diffractometer and the UPE program.
This journal is ß The Royal Society of Chemistry 2006
Chem. Commun., 2006, 1825–1827 | 1827