Rational design of tightly closed coordination tetrahedra that are stable in the solid state, in solution, and in the gas phase

Article abstract of DOI:10.1002/anie.200461800

(Figure Presented) No way out: Small guest cations, such as [Et ₄N]⁺ template the formation of stable and tightly closed octaanionic metallo-supramolecular tetrahedra (see picture). Since [Et ₃NH]⁺ is co-encapsu

Full text of DOI:10.1002/anie.200461800

Communications
Cage Compounds
Rational Design of Tightly Closed Coordination
Tetrahedra that are Stable in the Solid State, in
Solution, and in the Gas Phase**
Iris M. Mꢀller,* Daniela Mꢁller, and
Christoph A. Schalley
Molecular “Lego” utilizing simple building blocks with a
rationally programmed arrangement of binding sites permits
the construction of complex coordination cages by self-
assembly,[1] among them different types of coordination
[*] Dr. I. M. Mꢀller, Dipl.-Chem. D. Mꢁller
Lehrstuhl fꢀr Analytische Chemie, NC 4/27
Ruhr-Universitꢂt Bochum
44780 Bochum (Germany)
Fax: (+49)234-321-4420
E-mail: iris.m.mueller@rub.de
Priv.-Doz. Dr. C. A. Schalley
Kekulꢃ-Institut fꢀr Organische Chemie und Biochemie
Universitꢂt Bonn, 53121 Bonn (Germany)
[**] The authors thank Heike Schucht (Max Planck Institut fꢀr
Bioanorganische Chemie, Mꢀlheim) for the data collection of 1a, 2,
3,and 4 and Manuela Winter (Anorganische Chemie II, Ruhr-
Universitꢂt Bochum) for the data collection of 1b. We are grateful to
the Deutsche Forschungsgemeinschaft and the Fonds der Chem-
ischen Industrie for financial support. C.A.S. thanks the DFG for a
Heisenberg fellowship and the F.C.I. for a Dozentenstipendium.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
480
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DOI: 10.1002/anie.200461800
Angew. Chem. Int. Ed. 2005, 44, 480 –484

Angewandte
Chemie
tetrahedra (Figure 1). The oldest and most common [M4L6]
systems combine four metal centers at the corners with six
bridging ligands as edges (Figure 1a).[2] [M4L4] tetrahedral
cages, whose faces are covered by ligands with threefold
basic solution, a change to conformation 2 is observed.[6] To
examine whether metalation of 1 has the same effect, we
allowed a mixture of CdCl2 and 1 to react in acetone at 888C.
The
yellow
solid-state
crystals
structure
of the
of
the
pale
complex
resulting
[H6Br3L]2[CdCl4]·6(CH3)2CO·H2O (1b) showed no change
in conformation of 1.[8] In 1b there are two [H6Br3L]+ ligands
together with a [CdCl4]2À counterion in the asymmetric unit.
A similar reaction of CdCl2 and 1 in methanol in the
presence of Et4NCl (as a tetrahedral counterion) and Et3N as
base leads to the formation of bright yellow crystals.[9] This
time, the ligand is fully deprotonated and binds three CdCl
units. All observed bond lengths and angles at the square
pyramidal cadmium centers fall in the expected ranges (see
Supporting Information) with three coordination sites taken
by the ligand [Br3L]5À and one by a ClÀ ligand. The fifth site is
occupied by a phenolate oxygen atom of a neighboring
[(CdCl)3Br3L] building block so that a chiral cage with the
formula (Et4N)5(Et3NH)3[{(CdCl)3Br3L}4] (2), is formed as
shown in Figure 2 with each [(CdCl)3Br3L]2À unit color coded
Figure 1. Tetrahedral cage molecules with different topologies.
symmetry (Figure 1b) have been reported more rarely.[3]
Nearly all [M6L4] systems are truncated tetrahedra or
adamantanoid cages (Figure 1c) with rather wide openings
at their corners allowing encapsulated guest molecules to be
exchanged.[4] To our knowledge, only one [M6L4] tetrahedron
with an almost completely closed surface has been reported.[5]
In this case, M is a (CdO)2 four-membered ring and an
[Et4N]+ ion is securely trapped within the octaanionic capsule.
The structural characterization of such large cage com-
pounds by X-ray crystallography is often difficult owing to
rapid solvent loss and severe disorder of counterions and
solvent molecules. The introduction of heavy atoms with their
higher scattering power into the ligands should help to
improve the resolution and quality of the X-ray data. With
tris[(5-bromo-2-hydroxybenzylidene)amino]guanidinium
chloride ([H6Br3L]Cl (1)), we herein present a new ligand
suitable for the formation of chiral (although racemic), tightly
closed tetrahedral cages with the formal [M6L4] topology.
Compound 1 is easily obtained by a Schiff base reaction of
5-bromosalicylaldehyde and triaminoguanidinium chloride.[6]
Figure 2. a) Connectivity of one [(CdCl)3Br3L] unit. b) Crystal structure
of [{(CdCl)3Br3L}4]8À, hydrogen atoms and counterions are omitted for
clarity.
Diffusion of HCl into an acetonitrile solution of
1
leads to orthorhombic pale yellow crystals of
[H6Br3L]Cl·3CH3CN·H2O (1a).[7] In the solid state 1a
adopts conformation 1 (Scheme 1) which is unfavorable for
the coordination of metal centers and which is stabilized
through hydrogen bonding with the ClÀ counterions and
solvent molecules (see Supporting Information). In slightly
differently. Although the ligand [H6Br3L]+ is fully deproto-
nated and coordinating three Cd2+ centers in 2 the observed
bond lengths and angles remain at the 3s level the same as
those found in 1a and 1b. Only the propeller-like distortion
becomes more distinct, with dihedral angles of 16.9–36.48. Not
unexpectedly, this avoids close Br···Br contacts at the tightly
packed tetrahedron corners with their already short Br···Br
separations of 3.72(2)–4.89(2) ꢀ (Figure 3). The cage dimen-
sions can be determined by an imaginary inner sphere that is
defined by the four central carbon atoms (r= 4.31(8) ꢀ). The
theoretical edge length (a) is calculated to be 21.1(4) ꢀ, the
height (h) to be 17.2(3) ꢀ (Scheme 2, Table 1). The observed
edge length can be gauged by the H···Br separation (d) which
has values of 15.46(3)–15.99(3) ꢀ, so that only the ends of the
corners remain uncovered. This situation also confirms that
the [(CdCl)3Br3L] units are triangular building blocks, each of
which covers almost completely one face of the tetrahedron
leaving effectively no opening at the cage corners. In the
asymmetric unit of 2, two cages and 16 counterions can be
found, two of the [Et4N]+ ions are located in the cavities of the
two cage anions. The remaining 14 cations (six [Et3NH]+,
eight [Et4N]+) are located outside the cages with the [Et3NH]+
Scheme 1. Unfavorable and favorable conformation for the coordina-
tion of metal centers to 1.
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481

Communications
(4).[11] Again, all crystallographically observed bond lengths
and angles fall in the expected ranges and show no additional
distortion compared with 2 or 3. An [Et3NH]+ ion is captured
inside tetrahedron 4, together with a water molecule, which is
hydrogen bonded to the cation (N···O 2.66(9) ꢀ). Apparently,
the size of the guest is more important than its symmetry for
efficiently templating the tetrahedron formation.
Cage 4 is quite stable in solution. The 1H NMR spectrum
of 4 in [D6]acetone shows different signals for the [Et3NH]+
ions inside (methyl groups, dH = 0.95 ppm) and outside
(methyl groups, dH = 1.27 ppm) the cage, which indicates
that their exchange is slow on the NMR timescale.
Electrospray ionization Fourier-transform ion-cyclotron-
resonance (ESI-FT-ICR) mass spectrometry provides a soft
method for the ionization of intact tetrahedra 4 and other
metallosupramolecular assemblies.[12] When sprayed from
acetonitrile or acetone, doubly, triply, and quadruply charged
anions are generated by successively stripping off [Et3NH]+
counterions (Figure 4a). Although the spectra are complex,
they can be analyzed in detail. In addition to the
[MÀnEt3NH]nÀ ions, losses of Et3N and Et3NHCl are
observed (Figure 4b). An exact comparison of the exper-
imental spectrum with that simulated on the basis of natural
isotope abundances reveals that each of these ions is
accompanied by an ion bearing exactly one water molecule
although dry acetonitrile was used as the spray solvent. In
marked contrast, no such ions bearing water molecules are
observed in the mass spectrum of 2 obtained under the same
conditions (Figure 4c). This finding rules out incomplete
desolvation as the reason for the presence of a water molecule
in the spectra of 4 and points to its presence as a guest inside
the cage, thus providing evidence for the intact structure of
the tetrahedron even in the gas phase. Consequently, it can
safely be assumed that the much larger
Figure 3. Space-filling model of [{(CdCl)3Br3L}4]8À in 2, view towards
the corner of the cage (counterions not shown).
Scheme 2. Definition of the cage dimensions in 2.
[Et3NH]+ guest is also still inside the cage in
Table 1: Theoretical and observed dimensions [ꢄ] of the tetrahedral cages 2–4.
these ions.
Dimension[a]
2
3
4
In conclusion, a tightly closed tetrahe-
dral cage successfully self-assembles around
templating cations as long as these cations
have a suitable size. While [Et4N]+ almost
exactly fills the space inside, [Et3NH]+
leaves enough space for an additional
water molecule, which is co-encapsulated
in the cavity of the tetrahedron. The crystal
structures of several tetrahedra are in
r
a
h
4.31(8)
21.1(4)
17.2(3)
3.72(2)–4.89(2)
15.46(3)–15.99(3)
4.36(8)
21.3(4)
17.5(3)
3.77(2)–4.72(2)
15.60(3)–16.02(3)
4.36(5)
21.3(3)
17.4(2)
3.85(2)–4.56(2)
15.68(3)–15.96(3)
Br···Br
d
[a] See Scheme 2.
ions forming hydrogen bridges to the Cl ligands of the
tetrahedra (N···Cl = 3.13(2) ꢀ).
excellent agreement with data from solution and the gas
phase which provide evidence for their existence as stable,
discrete entities. It is particularly surprising, that ESI mass
spectrometry is capable of identifying the encapsulated water
molecule, which may also be interpreted as an effect of the
tightly closed surfaces of the tetrahedra, which impose a
considerable barrier for the water expulsion from the interior.
The tetrahedra are chiral, although they form as a racemic
mixture. As cations template their formation, a diastereose-
lective formation of one tetrahedron enantiomer around an
enantiopure guest cation might be possible.
To learn more about the influence of the size and
geometry of the counterion, [Et4N]+ was replaced by larger
[Ph4P]+ ions in a second reaction of CdCl2 and [H6Br3L]Cl
with Et3N performed under identical conditions. Again bright
yellow crystals were obtained which according to the crystal-
structure analysis correspond to a tetrahedral cage with the
formula (PPh4)1.33(Et3NH)6.67[{(CdCl)3Br3L}4] (3)[10] and of the
same size as 2. An [Et3NH]+ ion is located inside the cage.
Encouraged by these results, the experiment was repeated
without the addition of any tetrahedral cations. Again, bright
yellow crystals suitable for crystal-structure analysis were
obtained corresponding to (Et3NH)8[{(CdCl)3Br3L}4]·xH2O
Received: August 26, 2004
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Angewandte
Chemie
Figure 4. a) ESI-FT-ICR mass spectrum of a 100 mm acetonitrile so-
lution of 4 (F=fragment). b) Region of triply negative ions (top), spec-
trum simulated on the basis of natural isotope abundances without
(middle), and with (bottom) signals for cages containing one water
molecule. Note the good fit of the bottom simulation to the experi-
mental spectrum. c) Region of quadruply charged ions from the ESI-
FT-ICR mass spectrum of 2 (top) and simulated spectrum (bottom).
No signals for water adducts are observed.
C. Duan, D. Dang, K. Pang, Q. Meng, Chem. Commun. 2004,
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K. N. Raymond, Inorg. Chem. 2004, 43, 846; e) D. Fiedler, D. H.
Leung, R. G. Bergman, K. N. Raymond, J. Am. Chem. Soc. 2004,
126, 3674.
Keywords: cadmium · cage compounds · mass spectrometry ·
self-assembly · structure elucidation
.
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Frꢁhlich, Chem. Commun. 2003, 2854; b) R. W. Saalfrank, H.
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Angew. Chem. Int. Ed. 2005, 44, 480 –484
www.angewandte.org
ꢀ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
483

Communications
[7] Experimental details are given in the Supporting Information.
0.16 ꢃ 0.19 ꢃ 0.25 mm3, orthorhombic, Pbcn, a = 28.649(6), b =
15.133(3), c = 15.739(3) ꢀ, V= 6823(2) ꢀ3, 1calcd = 1.617 gcmÀ3
,
2qmax = 62.088, l = 0.71073 ꢀ, T= 100 K, 63876 measured reflec-
tions, 10825 independent reflections (Rint = 0.0701), 7685
observed reflections (I > 2s(I)), m = 3.673 mmÀ1
, numerical
absorption correction, Tmin = 0.460, Tmax = 0.591, 406 parameters,
R1(I > 2s(I)) = 0.0492, wR2(all data) = 0.0856, max./min. resid-
ual electron density 0.63/À0.48 eꢀÀ3 [13]
.
[8] Experimental details are given in the Supporting Information.
3
¯
0.24 ꢃ 0.18 ꢃ 0.14 mm , triclinic, P1, a = 16.437(7), b = 16.746(6),
c = 16.889(7) ꢀ, a = 80.529(9), b = 74.212(5), g = 61.36(1)8, V=
3923(3) ꢀ3, 1calcd = 1.633 gcmÀ3, 2qmax=47.508, l = 0.71073 ꢀ, T=
213 K, 18324 measured reflections, 11762 independent reflec-
tions (Rint = 0.0805), 5118 observed reflections (I > 2s(I)), m =
3.534 mmÀ1, empirical absorption correction, Tmin = 0.472, Tmax
=
0.608, 560 parameters, R1(I > 2s(I)) = 0.0767, wR2(all data) =
0.1947, max./min. residual electron density 0.76/À0.97 eꢀÀ3 [13]
.
Owing to the weak scattering power of 1b, the large number of
solvent molecules and the poor reflection to parameter ratio, all
solvent molecules as well as the phenyl groups were refined
isotropically. The bond lengths in the solvent molecules were
fixed to literature values.
[9] Experimental details are given in the Supporting Information.
0.08 ꢃ 0.14 ꢃ 0.14 mm3, triclinic, P1, a = 19.7629(1), b =
¯
31.7856(1), c = 34.1879(2) ꢀ, a = 107.491(2), b = 94.548(2), g =
96.091(2)8, V= 20227.0(2) ꢀ3, 1calcd = 1.748 gcmÀ3
,
2qmax =
50.508, l = 0.71073 ꢀ, T= 100 K, 248756 measured reflections,
73078 independent reflections (Rint = 0.0697), 50612 observed
reflections (I > 2s(I)), m = 3.819 mmÀ1
, empirical absorption
correction, Tmin = 0.597, Tmax = 0.732, 3026 parameters, R1(I >
2s(I)) = 0.0634, wR2(all data) = 0.1984, max./min. residual elec-
tron density 1.07/À1.14 eꢀÀ3 [13]
.
[10] Experimental details are given in the Supporting Information.
0.15 ꢃ 0.19 ꢃ 0.19 mm3, trigonal, R3c, a = 34.764(1), c =
94.689(1) ꢀ, V= 99102(1) ꢀ3, 1calcd = 1.664 gcmÀ3
,
2qmax =
47.748, l = 0.71073 ꢀ, T= 100 K, 140050 measured reflections,
28466 independent reflections (Rint = 0.0517), 26602 observed
reflections (I > 2s(I)), m = 3.520 mmÀ1, numerical absorption
correction, Tmin = 0.476, Tmax = 0.571, 1476 parameters, R1(I >
2s(I)) = 0.0585, wR2(all data) = 0.1553, max./min. residual elec-
tron density 1.38/À1.00 eꢀÀ3 [13]
.
[11] Experimental details are given in the Supporting Information.
0.10 ꢃ 0.19 ꢃ 0.22 mm3, monoclinic, C2/c, a = 65.484(1), b =
20.4154(4), c = 33.0347(6) ꢀ, b = 113.215(1)8, V= 40588(1) ꢀ3,
1calcd = 1.703 gcmÀ3
, 2qmax = 50.508, l = 0.71073 ꢀ, T= 100 K,
128167 measured reflections, 36682 independent reflections
(Rint = 0.0381), 27665 observed reflections (I > 2s(I)), m =
3.805 mmÀ1
,
numerical absorption correction,
Tmin = 0.470,
T
max = 0.683, 1499 parameters, R1(I > 2s(I)) = 0.0510, wR2(all
data) = 0.1630, max./min. residual electron density 0.94/
À1.10 eꢀÀ3 [13]
.
[12] a) S. Sakamoto, M. Fujita, K. Kim, K. Yamaguchi, Tetrahedron
2000, 56, 955; b) C. A. Schalley, T. Mꢂller, P. Linnartz, M. Witt,
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c) C. A. Schalley, Int. J. Mass Spectrom. 2000, 194, 11; d) C. A.
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[13] CCDC-248614–248618 contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/conts/retrieving.html (or from
the Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB21EZ, UK; fax: (+ 44)1223-336-033; or deposit@
ccdc.cam.ac.uk).
484
ꢀ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Angew. Chem. Int. Ed. 2005, 44, 480 –484