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Rational Design of a Coordination Cage with a Trigonal-Bipyramidal Shape Constructed from 33 Building Units.

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Cage Compounds
Rational Design of a Coordination Cage with a
Trigonal-Bipyramidal Shape Constructed from 33
Building Units**
Iris M. Mller* and Daniela Mller
The rapidly growing field of supramolecular coordination
chemistry can be divided into two classes according to the
products formed. The polymers are already well-studied
because of possible applications as new materials, heterogeneous catalysts, and ion-exchange materials.[1] The other class
of supramolecular entities, discrete cage molecules, are
conceived by rational design and prepared from a set of
molecular building blocks that fit together such that the
stoichiometry and symmetry elements of the product are
predestined by the starting compounds.[2] In most of the
coordination cages formed by this method the corners
correspond to the metal centers, which are linked by twofold
bridging ligands.[2?6] The use of building blocks that cover the
faces of the cage is not as common. Triangular subunits were
used to form tetrahedral, octahedral, and adamantanoid
coordination cages.[2?6] Cages with lower symmetry and
triangular faces have been synthesized only in the past few
years.[6?10] Here we report the rational design of a coordination cage with the outer shape of a trigonal bipyramid. The
characterization of an intermediate gives insight into the
reaction mechanism.
To our knowledge, the formation of a trigonal bipyramid
consisting of five metal centers as the corners and nine ligands
as twofold connecting units (M5L9) has not been achieved.
The reason may be the different coordination requirements of
the metal centers: the equatorial centers require four
coordination sites while the axial ones need only three
(Figure 1 a). Nevertheless, a few systems have been reported
in which the equatorial edges are not covered by ligands, and
coordination cages with M5L6 topology resulted (Figure 1 b).[7]
A related system is shown in Figure 1 c, d, in which three
metal centers (depicted in gray) are coordinated by two
ligands with threefold symmetry. Depending on the geometry
of the central atom or atom group (usually a phenyl ring) in
this ligand, a trigonal bipyramid or a trigonal-prismatic
system is formed. Both types of complexes have been
reported a few times.[8, 9] The use of triangular subunits leads
to two additional means of constructing a trigonal-bipyramidal cage (Figure 1 e, f). In the M5L6 system of type e again the
[*] Dr. I. M. Mller, Dipl.-Chem. D. Mller
Lehrstuhl fr Analytische Chemie, NC 4/27
Ruhr-Universitt Bochum
44780 Bochum (Germany)
Fax: (+ 49) 234-321-4420
[**] The authors thank Heike Schucht (Max-Planck-Institut fr Bioanorganische Chemie, Mlheim) for collecting data for 1 and 2.
Supporting information for this article is available on the WWW
under or from the author.
Angew. Chem. Int. Ed. 2005, 44, 2969 ?2973
DOI: 10.1002/anie.200463034
2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Figure 2. Schematic drawing of the octahedral cage [(Pd3L)8{m(bar)}12]16 .
Figure 1. Schematic drawing of different trigonal-bipyramidal (tbp)
cages and one related prismatic cage with different topologies;
a) M5L9, b) M5L6, c) M3L2 (tbp), d) M3L2 (prismatic), e) M5L6, f) M9L6.
metal centers must adopt different coordination geometries,
and again such cages have not been observed up to now.
Cages in which C3-symmetric ligands cover the six faces of the
trigonal bipyramid and the metal centers represent the edges
have been reported only twice. Cages with one and two metals
per edge form capsules with M18L6 and M15L6 topology
constructed from 24 and 21 building units, respectively.[10]
To construct larger cages with more building units, we
followed this design principle, but in addition to metal centers
for the linkage along the edges we used a second twofold
bridging ligand. One equatorial corner in the intended
trigonal bipyramid is similar to the corner in an octahedron,
which we previously constructed from the ligands tris(2hydroxybenzyl)triaminoguanidinium chloride, [H6L]Cl, and
sodium 5,5-diethylbarbiturate, NaHbar, which have threefold
and twofold symmetry, respectively (Figure 2).[5] The obvious
question is how formation of such an octahedral cage of
higher symmetry can be prevented. At the corners of the
octahedron close contacts between two aromatic protons
were observed (2.5(2) ).[5] The replacement of these hydrogen atoms with bulkier Br atoms should prevent the
formation of the octahedral cage. Therefore, we allowed
chloride, [H6Br3L]Cl,[11] to react with PdCl2 and NaHbar under
similar conditions. Dark red plates of (Et3NH)6(Et4N)6[{Pd3(Br3L)}6(m-bar)9] (1) formed; the result of the
X-ray structure analysis is shown in Figure 3.[12] As expected,
a trigonal bipyramid [{Pd3(Br3L)}6(m-bar)9]12 containing a
total of 33 building units was formed. Six [Br3L]5 units cover
the faces, and each coordinates three Pd2+ centers in a
tridentate manner. The fourth coordination site of the square-
2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Figure 3. Crystal structure of 1. a) Schematic drawing, b) space-filling
models, and c) stereoplot (protons and counterions omitted for
planar metal centers is occupied by a barbiturate dianion,
which bridges two adjacent faces.
The influence of the bromine atoms can be seen clearly at
the corners. While at the axial corners the BrиииBr distances
are all equal and between 6.20(8) and 6.24(8) , the corners
in the equatorial plane are more open with observed BrиииBr
distances of 5.15(8)?12.4(2) . The observed bond lengths
Angew. Chem. Int. Ed. 2005, 44, 2969 ?2973
and angles in both ligands in 1 remain unchanged upon
coordination, indicating that no significant distortion occurs
(see Tables S1 and S2 in the Supporting Information). The
cage anion in 1 is chiral; all ligands show the same screw
direction of the central CN6 core. Not surprisingly both
enantiomers are present in the centrosymmetric space group
C2/c. The anion in 1 has a twelvefold negative charge, and five
of the counterions (4 Et3NH+, 1 Et4N+) are present but
disordered within the cage. The inside volume of the cage is
estimated to be 1600 3, which leaves enough space for up
to 30 water molecules. Outside the cage a volume of
4500 3 (or 36 000 3/46 % of the whole unit cell) is
occupied by the remaining seven cations (2 Et3NH+, 5 Et4N+)
and up to 110 disordered water molecules.
Interestingly, in addition to the dark red plates of 1 in the
reaction mixture, bright red prismatic crystals of (Et3NH)4(Et4N)4[{Pd3(Br3L)}4(m-bar)4(Hbar)4] (2) are always found.
The result of their crystal structure analysis is depicted in
Figure 4.[13] In one way 2 can be regarded as a tetrahedral
coordination cage in which the bottom face is linked to only
one and not to all three other faces. Each face is formed by a
triangular {Pd3(Br3L)}+ unit. The three faces that form the top
of this open tetrahedron are linked by bridging bar2 ligands,
and the fourth face is also attached by a bridging barbiturate.
In this way four Pd2+ centers are not involved in the linkage,
and these metals each bond an Hbar ligand at their fourth
free coordination site. As a result the bottom face is not
flexible but fixed in its position by strong hydrogen bonds
between the protonated nitrogen atom of Hbar and the
carbonyl oxygen atom of another barbiturate (d(NиииO) =
2.88(8) ). The inside volume of the chiral cage is estimated
to be 800 3, leaving enough space for three Et4N+
counterions. Outside the cage a volume of 2100 3 (or
4200 3/31 % of the whole centrosymmetric unit cell) is
occupied by the remaining five cations (1 Et4N+ and
4 Et3NH+) and roughly 40 water molecules. All bond lengths
and angles in 2 fall within the expected ranges, and no
significant distortions are observed (see Tables S1 and S2 in
the Supporting Information).
Cage 2 can also be seen as two-thirds of an incipient
trigonal bipyramid. Only two triangular faces are missing, and
all required barbiturates are already in place. Taken together
with previous results, the structure of 2 provides insight into
possible formation mechanism. In the first step the deprotonated ligand coordinates three square-planar metal centers.
Since the free coordination sites are occupied by Hbar
ligands, the dianionic triangular building block {M3(Br3L)}2
results (see Figure 5 a). This fragment has already been
crystallized and analyzed for M = Cu2+.[11] These units then
react with each other with loss of H2bar; this type of reaction
has been confirmed under ESI conditions, again with M =
Cu2+ (see Figure 5 b).[11] Linkage of three and four of these
units with accompanying loss of H2bar leads to the open
trigonal pyramid and the coordination cage 2, respectively
(see Figure 5 c). The subsequent introduction of two more
triangular units would then lead to the formation of the
trigonal bipyramid 1.
We have succeeded in the rational, planned synthesis of
the trigonal-bipyramidal coordination cage 1 with low symmetry from a set of molecular building blocks. Based on the
structure of another cage compound, 2, and previous results
we suggest a possible formation mechanism.
Experimental Section
Figure 4. Crystal structure of 2. a) Schematic drawing and b) crystal
structure (protons and counterions omitted for clarity).
Angew. Chem. Int. Ed. 2005, 44, 2969 ?2973
Tris(5-bromo-2-hydroxybenzyl)triaminoguanidinium chloride was
prepared according to literature methods.[11] Sodium 5,5-diethylbarbiturate was purchased and used without further purification.
1 and 2: A solution of PdCl2 (48.6 mg, 0.274 mmol) and Et4NCl
(42.5 mg, 0.256 mmol) in acetonitrile (4 mL) was prepared. Solutions
of tris(5-bromo-2-hydroxybenzyl)triaminoguanidinium chloride
(64.7 mg, 0.0938 mmol) and sodium 5,5-diethylbarbiturate (27.6 mg,
0.134 mmol) in a mixture of acetonitrile and water (10:1, 2 mL each)
were added. Then triethylamine (1 mL) was slowly diffused into the
reaction mixture. After a few weeks dark red plates of 1 and red
prisms of 2 formed. The two compounds were separated manually
under the microscope, which resulted in lower yields of 30.8 mg (3.5 10 3 mmol, 23.5 % relative to NaHbar) for 1 and 40.9 mg (6.5 10 3 mmol, 38.8 % relative to NaHbar) for 2. Elemental analyses
2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
data for their influence with the SQUEEZE routine in PLATON.[16]
CCDC 258971?258972 contain the supplementary crystallographic
data for this paper. These data can be obtained free of charge from
the Cambridge Crystallographic Data Centre via
Received: December 22, 2004
Published online: April 7, 2005
self-assembly и bridging ligands и cage compounds и palladium и
solid-state structures
Figure 5. Possible reaction mechanism for the formation of coordination cages 1 and 2 see text for details.
[%] calcd for C288H378N66O45Br18Pd18и140 H2O, 1, (11 360.47): C 30.45,
H 5.84, N 8.14, Pd 16.86; found: C 30.26, H 4.99, N 8.08, Pd 17.01.
Elemental analyses [%] calcd for C208H276N48O36Br12Pd12и30 H2O, 2,
(6801.07): C 36.73, H 4.98, N 9.89, Pd 18.78; found: C 36.83, H 4.16 , N
10.02, Pd 18.45.
X-ray analysis: Intensity data were collected for 1 on a Nonius
Kappa CCD (MoKa rotating anode) and for 2 on a Siemens SMART
CCD (CuKa rotating anode), and in both cases the w-scan method was
employed. All data were corrected for Lorentz and polarization
effects. The structures of 1 and 2 were solved by direct methods
(SHELXS-97)[14] and refined by a full-matrix least-squares refinement procedure (SHELXL-97).[15] All protons were placed at geometrically estimated positions. Due to the large number of disordered
counterions and solvent molecules outside the cages and the resulting
low ratio of reflections to parameters, we decided to correct the X-ray
2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
[1] For recent reviews, see: a) S. Kitagawa, R. Kitaura, S. Noro,
Angew. Chem. 2004, 116, 2388; Angew. Chem. Int. Ed. 2004, 43,
2334; b) C. Janiak, Dalton Trans. 2003, 2781; c) B. Moulton, M. J.
Zaworotko, Chem. Rev. 2001, 101, 1629; d) S. R. Batten,
CrystEngComm 2001, 18, 1; e) M. J. Zaworotko, Angew. Chem.
2000, 112, 3180; Angew. Chem. Int. Ed. 2000, 39, 3052; f) R.
Robson, J. Chem. Soc. Dalton Trans. 2000, 3735.
[2] For recent reviews, see: a) R. W. Saalfrank, B. Demleitner, H.
Glaser, H. Maid, S. Reihs, W. Bauer, M. Maluenga, F. Hampel,
M. Teichert, H. Krautscheid, Eur. J. Inorg. Chem. 2003, 822;
b) G. F. Swieger, T. J. Malefetse, Coord. Chem. Rev. 2002, 225,
91; c) S. Leininger, B. Olenyuk, P. J. Stang, Chem. Rev. 2000, 100,
[3] For chiral cages, see: T. D. Hamilton, L. R. MacGillivray, Cryst.
Growth Des. 2004, 4, 419.
[4] a) I. M. Mller, D. Mller, C. A. Schalley, Angew. Chem. 2005,
117, 485; Angew. Chem. Int. Ed. 2005, 44, 480; b) I. M. Mller, R.
Robson, F. Separovic, Angew. Chem. 2001, 113, 4519; Angew.
Chem. Int. Ed. 2001, 40, 4385.
[5] I. M. Mller, S. Spillmann, H. Franck, R. Pietschnig, Chem. Eur.
J. 2004, 10, 2207.
[6] a) R. W. Saalfrank, H. Glaser, B. Demleitner, F. Hampel, M. M.
Chowdhry, V. Schnemann, A. X. Trautwein, G. B. M. Vaughan,
R. Yeh, A. V. Davis, K. N. Raymond, Chem. Eur. J. 2002, 8, 493;
b) D. W. Johnson, J. Xu, R. W. Saalfrank, K. N. Raymond,
Angew. Chem. 1999, 111, 3058; Angew. Chem. Int. Ed. 1999,
38, 2882.
[7] Recent papers: a) X. Sun, D. W. Johnson, D. L. Caulder, K. N.
Raymond, E. H. Wong, J. Am. Chem. Soc. 2001, 123, 2752; b) X.
Sun, D. W. Johnson, K. N. Raymond, E. H. Wong, Inorg. Chem.
2001, 40, 4504; c) C. J. Matthews, L. K. Thompson, S. R. Parsons,
Z. Xu, D. O. Miller, S. L. Heath, Inorg. Chem. 2001, 40, 4448.
[8] Recent papers: a) P. S. Mukherjee, N. Das, P. J. Stang, J. Org.
Chem. 2004, 69, 3526; b) Y. K. Kryschenko, S. R. Seidel, D. C.
Muddiman, A. I. Nepomuceno, P. J. Stang, J. Am. Chem. Soc.
2003, 125, 9647; c) C. J. Kuehl, Y. K. Kryschenko, U. Radhakrishnan, S. R. Seidel, S. D. Huang, P. J. Stang, Proc. Natl. Acad.
Sci. USA 2002, 99, 4932.
[9] Recent papers: a) C. J. Kuehl, T. Yamamoto, S. R. Seidel, P. J.
Stang, Org. Lett. 2002, 4, 913; b) C.-Y. Su, Y.-P. Cai, C.-L. Chen,
F. Lissner, B.-S. Kang, W. Kaim, Angew. Chem. 2002, 114, 3519;
Angew. Chem. Int. Ed. 2002, 41, 3371.
[10] a) K. Umemoto, H. Tsukui, T. Kusukawa, K. Biradha, M. Fujita,
Angew. Chem. 2001, 113, 2690; Angew. Chem. Int. Ed. 2001, 40,
2620; b) N. Takeda, K. Umemoto, K. Yamaguchi, M. Fujita,
Nature 1999, 398, 794.
[11] I. M. Mller, D. Mller, Eur. J. Inorg. Chem. 2005, 257.
[12] Crystal data for 1: 0.09 0.25 0.25 mm3, monoclinic, C2/c, a =
79.90(2), b = 27.828(6), c = 36.148(7) , b = 102.28(3), V =
78 534(27) 3,
1calcd = 1.495 g cm 3,
2qmax = 46.048,
0.71073 , T = 100 K, 212 440 measured reflections, 51 676
independent reflections (Rint = 0.1123), 24 895 observed reflec-
Angew. Chem. Int. Ed. 2005, 44, 2969 ?2973
tions (I > 2s(I)), m = 2.70 mm 1, empirical absorption correction,
Tmin = 0.497, Tmax = 0.785, 2354 parameters, R1(I>2s(I)) = 0.112,
wR2(all data) = 0.283, max./min. residual electron density 2.33/
1.31 e 3.
Crystal data for 2: 0.10 0.10 0.15 mm3, triclinic, P1?, a =
20.224(1), b = 21.536(1), c = 35.090(2) , a = 90.074(3), b =
g = 114.542(3)8,
V = 13 680(2) 3,
1calcd =
1.519 g cm 3, 2qmax = 135.408, l = 1.54178 , T = 100 K, 72 857
measured reflections, 36 554 independent reflections (Rint =
0.1028), 16 311 observed reflections (I > 2s(I)), m = 8.803 mm 1,
numerical absorption correction, Tmin = 0.236, Tmax = 0.416, 1697
parameters, R1(I>2s(I)) = 0.1188, wR2(all data) = 0.3258, max./
min. residual electron density 2.84/ 2.24 e 3.
G. M. Sheldrick, SHELXS-97, Program for Crystal Structure
Solution, University Gttingen, 1997.
G. M. Sheldrick, SHELXL-97, Program for Crystal Structure
Refinement, University Gttingen, 1997.
A. L. Spek, Acta Crystallogr. Sect. A 1990, 46, C34.
Angew. Chem. Int. Ed. 2005, 44, 2969 ?2973
2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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