A metallocavitand functioning as a container for anions: Formation of noncovalent linear assemblies mediated by a cyclodextrin-entrapped NO 3- ion

Article abstract of DOI:10.1002/anie.200604564

(Figure Presented) Containing the negativity: A cyclodextrin-based complex substituted with two phosphine groups is able to fully embrace medium sized anions such as BF₄^- (see picture), NO₃ ^-, AcO^-, or X^- (X = Cl, Br, I). Besides conventional electrostatic cation...anion interactions, complex formation has been shown to involve weak interactions with the lipophilic host environment.

Full text of DOI:10.1002/anie.200604564

Angewandte
Chemie
DOI: 10.1002/anie.200604564
Anion Inclusion
A Metallocavitand Functioning as a Container for Anions: Formation
of Noncovalent ÀLinear Assemblies Mediated by a Cyclodextrin-
Entrapped NO3 Ion**
Laurent Poorters, Dominique Armspach,* Dominique Matt,* Loïc Toupet, and Peter G. Jones
Much effort is currently invested in the design and develop-
ment of metallated container molecules. Complexes of this
type constitute valuable objects for studying the influence of a
second coordination sphere on the properties of a metal
center[1] and for achieving catalytic reac-
pairs of electrons on both phosphorus atoms are directed
towards the CD axis.[6] Reaction of 1 in CH2Cl2/THF with
AgBF4 afforded complex 2 quantitatively [Eq. (1)]. All NMR
data are consistent with a C2-symmetric chelate complex and
tions in a confined environment.[2] An
increasingly prominent field concerns the
use of metallocavitands for anion com-
plexation.[3] For example, transition-
metal ions that are bound through p in-
teractions to the outside walls of calix-
[n]arenes were shown to be capable of
enhancing the receptor properties of
these cone-shaped molecules towards
anions.[4] A different approach consists
of generating coordination cages through
metal-templated self-assembly, thereby
allowing anions to be retained within
the cavity through weak interactions with
the metal center.[5] Surprisingly, no exam-
ple of anion encapsulation with dual binding to a complexed
metal ion and an appended cavity has so far been reported.
Moreover, the degree of guest encapsulation is rather small in
the anion “inclusion” complexes reported to date. We now
report the first example of a metallated cyclodextrin that is
able to fully encapsulate medium-sized inorganic anions.
The present study was carried out with the cyclodextrin
(CD) derivative 1, a rigidified diphosphine in which the lone
indicate the presence of an undistorted, circular-shaped CD
torus. These features were confirmed by an X-ray diffraction
study (Figure 1), which revealed the presence of two closely
related molecules (a and b) in the unit cell. Diphosphine 1 was
initially designed as a trans chelator,[6] and it is hardly
surprising that its bite angle, as observed in complex 2 in
the solid state, is rather large (1528 on average). Remarkably,
À
the BF4 counterion lies deep inside the cavity with two
fluorine atoms coordinated to the silver center, which adopts
a pseudotetrahedral stereochemistry (Ag-F: 2.598(3) and
2.620(3) Š in a; 2.534(3) and 2.570(3) Š in b).
[*] Dr. L. Poorters, Dr. D. Armspach, Dr. D. Matt
Laboratoire de Chimie Inorganique MolØculaire
Institut de Chimie UMR 7177 CNRS
UniversitØ Louis Pasteur
67008 Strasbourg Cedex (France)
Fax: (+33)3-9024-1749
E-mail: darmspach@chimie.u-strasbg.fr
dmatt@chimie.u-strasbg.fr
Dr. L. Toupet
Groupe Mati›re CondensØe et MatØriaux, UMR 6626 CNRS
UniversitØ de Rennes 1
35042 Rennes Cedex (France)
Prof. Dr. P. G. Jones
Institut für Anorganische and Analytische Chemie
TU Braunschweig
Hagenring 30, 38106 Braunschweig (Germany)
[**] We thank the Agence Nationale de la Recherche for financial
support (ANR Watercat).
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
Figure 1. Molecular structure of 2 showing the encapsulated BF4À ion.
For clarity, a single molecule of the unit cell is shown.
Angew. Chem. Int. Ed. 2007, 46, 2663 –2665
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2663

Communications
Careful examination of the structures shows that while
carried out on 7 revealed that the nitrate ion is fully included
in the cavity, with two oxygen atoms coordinated to the
tetrahedrally coordinated silver atom (Figure 3). As in 3,
additional, noncovalent binding occurs between the anion
(here through the coordinated oxygen atoms) and an
H5 atom of each P-substituted glucose unit (DdH-5 = +
0.38 ppm).[11] Remarkably, unlike the structures of 2 and 3
in which the CD units arrange themselves in a typical fishbone
structure, the CD units of 7 are stacked on top of each other to
form nitrate-containing tubes. The observed solid-state
organization is clearly anion-templated and involves mainly
hydrogen-bonding interactions between the protruding
oxygen atom of the nitrate ligand at the secondary face and
a meta-hydrogen atom of each PPh ring (average
distance 2.77 Š). The para-H atom of one of the two
PPh units also interacts with the NO3À ion (CH···O:
2.83Š). The distance between two successive silver
ions is only 11.70 Š (versus 14.16 Š in 2 and 14.10 Š in
3).
each coordinated F atom interacts with three inwardly
oriented H5 atoms, weak interactions also occur between
the other F atoms and some H5 as well as H3atoms (shortest
CH···Fnoncoord distances: 2.72, 2.80, and 2.86 Š in a; 2.77, 2.84,
and 2.97 Š in b). Interestingly, structure 2 represents the first
example of a BF4À ion that behaves as an F,F-chelating ligand
in a mononuclear transition-metal complex.[7] The BF4À ion
could not be displaced by acetonitrile. Note that a cyclo-
dextrin silver(I) complex able to bind nitrile ligands inside the
cavity has already been reported.[8]
To assess the ability of the [Ag(1)]+ ion to act as a receptor
towards other anions, complex 2 [Eq. (2)] was treated with a
Finally, it is worth mentioning that 31P NMR
spectroscopy is very effective at discriminating
between the different anions mentioned in this
study, thus making [Ag(1)]+ in particular a good
solution of NaCl in H2O. This reaction led readily to the
À
chloro complex 3. As expected for entrapped M Cl moieties,
the 1H NMR spectrum of 3 shows that two symmetrically
sited H5 atoms, both belonging to a P-linked glucose unit,
have undergone
a significant low-field shift (Dd = +
0.69 ppm), a good indication for the presence of noncovalent
bonds between the cavity wall and the anion. In the solid state
À
(Figure 2), the shortest Cl···H C separations are those
involving the C(35’) and C(15’)-bonded H5 atoms (2.96 and
2.99 Š, respectively, in one of the two cocrystallizing mole-
cules of the unit cell). Although these separations are larger
À
than those found in other CDs with M Cl bonds, they remain
[9]
À
À
typical for weak M Cl···H C interactions.
The coordinated chloride atom could be reversibly
displaced in water with bromide or iodide anions, thereby
resulting in 4 and 5, respectively. In keeping with the presence
of larger halide atoms in these complexes, the variation in the
chemical shift of the H5 atoms that interact with the halide is
larger than that observed for 3 (Dd = + 0.80 and + 1.03ppm,
respectively, versus 1).
On extending our investigations to oxygen-containing
anions we observed that [Ag(1)]+ is also able to host a single
CH3CO2 or NO3À ion to form complexes 6[10] and 7,
À
respectively. The addition of an excess of NaBF4 to these
complexes regenerated 2. As for the complexes described
above, in both molecules a single H5 signal (2H) in the NMR
spectrum is significantly affected by the presence of the anion
inside the cavity. A single-crystal X-ray diffraction study
Figure 2. Molecular structure of 3: side view (top) and view from the
secondary face (bottom).
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Angew. Chem. Int. Ed. 2007, 46, 2663 –2665

Angewandte
Chemie
Keywords: anion inclusion · cavitands · cyclodextrins ·
.
noncovalent interactions · phosphanes
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Figure 3. X-ray structure of 7 showing the encapsulated NO3À ion.
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(CDCl3) spectra.
anions. The present study provides a rare example of anion
encapsulation in a funnel-shaped complex and illustrates the
fact that a number of CH X bonds may contribute to the
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combination with dative anion–metal and electrostatic inter-
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À
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[10] A spatial proximity between the Me group and the H3protons of
the P-bridged glucose units was established by a ROESY NMR
experiment.
Experimental Section
Full experimental details including X-ray structural data are given in
the Supporting Information. CCDC-272302, 625926, and 601087 (2, 3,
and 7, respectively) contain the supplementary crystallographic data
for this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
[11] This complex as well as 2 are weak electrolytes in CH3CN
(208C).
Received: November 8, 2006
Published online: March 9, 2007
Angew. Chem. Int. Ed. 2007, 46, 2663 –2665
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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