Solvato-controlled assembly of Pd3L6 and Pd 4L8 coordination "boxes"

Article abstract of DOI:10.1002/anie.200605084

(Figure Presented) Living in a box: The dynamic self-assembly of Pd ₄L₈ and Pd₃L₆ (L = 1,2-bis[2-(pyridin-4-yl)ethynyl]benzene) box-shaped structures is controlled by the solvent (see picture; Pd yellow, C, N bl

Full text of DOI:10.1002/anie.200605084

Angewandte
Chemie
DOI: 10.1002/anie.200605084
Coordination Compounds
Solvato-Controlled Assembly of Pd3L6 and Pd4L8 Coordination
“Boxes”**
Kosuke Suzuki, Masaki Kawano, and Makoto Fujita*
Owing to the reversibility of metal–ligand coordination
bonds, molecular assemblies from metals and ligands are
often in equilibrium when their thermodynamic stabilities are
comparable.[1] The equilibrium ratio can be controlled usually
by changing conditions (concentration, temperature, etc.)[2,3]
or by adding a template molecule that selectively binds to and
stabilizes one particular assembly.[4,5] Ring numbers of cyclic
changing solvents. To control the cavity size in such a simple
way is important, as the cavities of PdII-linked boxes have
shown many interesting properties through host–guest com-
plexation.[7]
Ligand 1 was prepared by the Sonogashira coupling
reaction of 4-ethynylpyridine with 1,2-dibromobenzene.
When a mixture of ligand 1 (20 mm)and Pd(NO )2 (10 mm)
3
assemblies have been often controlled in these ways.[2,4]
A
in [D6]DMSO (1 mL)was heated for 4 h at 60 8C, the
quantitative formation of a single and highly symmetric
product was indicated by 1H NMR spectroscopy (Figure 1a).
solvent is often an important factor that controls the stability
of the assemblies, but the solvato-control of the equilibrium
has been seldom achieved because a solvent does not make a
significant difference to the relative stabilities of the equili-
brated assemblies.[6] Herein we report a drastic solvent effect
in controlling the self-assembly of two coordination com-
pounds. PdII ions and 1,2-bis[2-(pyridin-4-yl)ethynyl]benzene
(1)are assembled into a mixture of M L8 (2)and M L6 (3)
4
3
box-shaped structures (Scheme 1). We show that the ratio of
these two assemblies is almost completely controlled by
Figure 1. 1H NMR spectra (500 MHz, 300 K) ofa) complex 2
([D6]DMSO, [1]=20 mm) and b) complex 3 (CD3CN, [1]=2 mm).
The downfield shift of the signals for the product as compared
to those for ligand 1, particularly for the pyridyl a- and b-
hydrogen atoms (Dd = 0.72 and 0.45 ppm, respectively), was
attributed to metal–ligand complexation. Cold-spray ioniza-
tion mass spectrometry (CSI-MS)[9] clearly confirmed an
M4L8 stoichiometry from a series of prominent peaks of
À
[2À(NO3 )n]n+ (n = 2–5). Four intense peaks at m/z 1520.8,
À
993.0, 729.2, and 571.2 were assigned to [2À(NO3 )2]2+,
Scheme 1. The solvato-controlled assembly ofM 4L8 (2) and M3L6 (3)
structures and their interconversion. DMSO: dimethyl sulfoxide.
À
À
À
[2À(NO3 )3]3+, [2À(NO3 )4]4+, and [2À(NO3 )5]5+, respec-
tively (Figure 2a). The structure of complex 2 was unambig-
uously determined by an X-ray diffraction study (Figure 3a).
Block-shaped single crystals, suitable for X-ray diffraction
analysis, were obtained by slow diffusion of ethyl acetate into
a solution of complex 2 in DMSO. In this complex, four PdII
ions are arranged at the corners of a square, in which the
averaged adjacent and diagonal Pd–Pd distances are 9.7 and
13.7 Š, respectively. Each PdII center adopts a square-planar
[*] K. Suzuki, Dr. M. Kawano, Prof. Dr. M. Fujita
Department ofApplied Chemistry
School ofEngineering
The University ofToyko
7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656 (Japan)
Fax: (+81)3-5841-7257
À
geometry with Pd N bond lengths of 2.0 Š.
E-mail: mfujita@appchem.t.u-tokyo.ac.jp
Surprisingly, the same components (PdII ions and 1)
assembled into another structure when CD3CN was used,
instead of [D6]DMSO, as solvent. On complexation of 1
[**] This research was supported by the CREST project ofthe Japan
Science and Technology Agency (JST), for which M.F. is the principal
investigator. L: 1,2-bis[2-(pyridin-4-yl)ethynyl]benzene.
(2 mm)and Pd(NO
)
(1 mm)in CD 3CN, the 1H NMR
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
3
2
spectrum of the complex indicated the quantitative self-
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crystallographic analysis. Reliable evidence for the M3L6
structure of 3 was obtained by X-ray analysis (Figure 3c).
Three PdII ions are arranged in a regular triangle, in which the
distance between the metal centers is 8.7 Š.
II
À
Owing to the reversibility of Pd N bonds, the M4L8 and
M3L6 assemblies 2 and 3 are in equilibrium. Thus, we observed
facile interconversion between 2 and 3 by addition or removal
of solvent. Addition of CD3CN to the solution of complex 2 in
[D6]DMSO (20 mm)followed by stirring of the solution at
608C for 3 h promoted the smooth conversion of complex 2
1
into 3 as revealed by H NMR spectroscopy (Figure 4a–f).
Figure 2. CSI mass spectra ofcomplexes a) 2 and b) 3.
Figure 4. 1H NMR spectra demonstrating the structural conversion
from M4L8 to M3L6. a) M4L8 (2) in [D6]DMSO ([1]=20 mm). b–f) After
addition ofCD CN to the sample in part (a) and stirring at 608C f or
3
3 h; [D6]DMSO/CD3CN=75:25 (b), 67:33 (c), 50:50 (d), 33:67 (e),
25:75 (f). g) After removal of CD 3CN from the sample in part (f),
which corresponds to a M3L6 structure (3), and stirring at room
temperature for 3 h, the structure reverts back to that of 2 (M4L8).
Figure 3. Crystal structures of 2 (a,b) and 3 (c,d). Parts (a) and (c)
show side views (space-fill representation: Pd yellow, C, N blue,
H gray), and parts (b) and (d) show top views (stick framework:
Pd yellow, C, N blue; space-fill representation of guests: C gray, N blue,
O red, S yellow). For clarity, some ofthe solvent molecules and anions
are omitted.
Upon increasing the ratio of CD3CN, the 1H NMR signals of
complex 2 decreased while those of complex 3 increased.
Finally, at a ratio of DMSO to acetonitrile of 25:75, complex 2
was almost completely converted into 3. This solution was
taken and CD3CN was removed by evaporation. In the
resulting [D6]DMSO solution, complex 3 was completely
converted back into complex 2 after 3 h at room temperature
(Figure 4g). The switching between the two structures by
addition/removal of solvent was repeated at least three times
assembly of another highly symmetrical complex (Fig-
ure 1b).[8] From CSI-MS measurement, this complex was
revealed to be a M3L6 structure 3 (Figure 2b). By the slow
diffusion of THF into the acetonitrile solution of 3, we
successfully obtained single crystals that were suitable for
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CCDC 630654 (2)and 630653 ( 3)contain the supplementary
without forming any by-products or precipitates (see the
Supporting Information).
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.
Although there are no significant distortions in the
structures of both 2 and 3, an entropy effect should push the
equilibrium toward the M3L6 structure 3 because it assembles
from fewer components.[10] Thus, the selective formation of 3
in CD3CN is explained in terms of the entropy effect.
Meanwhile, an advantage in enthalpy, which predominates
over the entropy effect, should be considered in DMSO as
solvent to account for the selective formation of 2 therein. An
implication is provided by the crystal structure of 2, in which
two DMSO molecules and two nitrate ions form a 2:2
aggregate inside the cavity (Figure 3b). We assume that this
aggregated structure exists even in solution to some extent
and templates the formation of the M4L8 structure. Indeed,
the following experiments showed the importance of the
Received: December 16, 2006
Published online: March 6, 2007
Keywords: N ligands · palladium · self-assembly · solvent effects
.
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Experimental Section
Crystal data for 2: C188H96N23O56S14Pd4, Mr = 4447.32, pale yellow
block (0.28 ” 0.26 ” 0.22 mm3), monoclinic, space group P21/c, a =
21.232(3), b = 15.524(2), c = 35.481(5)Š, b = 97.139(2)8, V=
11604(3)Š 3, T= 80 K, Z = 2, 1calcd = 1.273, 19815 unique reflections
out of 28706 with I > 2s(I), The structure was solved by direct
methods (SHELXL-97)and refined by full-matrix least-squares
methods on F2 with 1569 parameters, 1.63 < q < 28.898, R1-
(I>2s(I)) = 0.0877, wR2 = 0.2790, GOF = 1.050, max/min residual
density 1.652/À1.479 eŠÀ3. Inside of the cavity, DMSO molecules
and water molecules are disordered with occupancies of 0.58 and 0.42,
respectively.
Crystal data for 3: C138H108N22O31.5Pd3, Mr = 2891.67, pale yellow
3
¯
needle (0.40 ” 0.30 ” 0.05 mm ), trigonal, space group P1, a = b =
19.841(5), c = 27.850(12)Š, V= 9494(6)Š 3, T= 90 K, Z = 2, 1calcd
=
1.014, 4735 unique reflections out of 17705 with I > 2s(I). The
structure was solved by direct methods (SHELXL-97)and refined by
full-matrix least-squares methods on F2 with 594 parameters, 2.18 <
q < 30.068, R1(I>2s(I)) = 0.1227, wR2 = 0.3963, GOF = 0.899, max/
min residual density 0.856/À0.916 eŠÀ3. Several solvent molecules
and nitrate anions are disordered. The bond lengths of severely
disordered molecules were chemically restrained.
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Müller, P. Linnartz, M. Witt, M. Schꢃfer, A. Lützen, Chem. Eur.
J. 2002, 8, 3538 – 3551.
when the complexation of 1 and Pd(NO3)2 was examined at 2 mm
in [D6]DMSO.
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[9] CSI-MS is quite effective for analyzing the solution structures of
metal complexes: a)S. Sakamoto, M. Fujita, K. Kim, K.
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[8] While complex 2 was prepared at 20 mm in [D6]DMSO, the
assembly of complex 3 was carried out at 2 mm in CD3CN
because of its low solubility in acetonitrile. The concentration
effect was not a dominant factor for controlling the equilibra-
tion, as we confirmed that the ratio of 2/3 was approximately 5:1
[10] The entropic advantage of an assembly from fewer components
has been discussed for equilibrations between M3L3 and M4L4
assemblies. See Refs. [2a] and [2f].
[11] Pd(OTf)2 was prepared by treating PdCl2 with AgOTf in DMSO
at 508C for 12 h and removing precipitated AgCl.
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