Self-assembly of two distinct supramolecular motifs in a single crystalline framework

Article abstract of DOI:10.1002/anie.200353520

Copper(II) complexes based on dipyrromethene ligands self-assemble into molecular hexagons (green) and double-helical coordination polymers (blue and yellow) within the same crystalline lattice organized by the self-segregation of CF₃ groups.

Full text of DOI:10.1002/anie.200353520

Angewandte
Chemie
metal–ligand interactions, both discrete molecular species
and coordination polymers have been investigated. We are
aware of only one case in which a reaction has yielded both a
discrete complexand an extended polymeric structure from
the same building block.[7] However, we know of no examples
in which these two general topologies were found to co-exist
in a stable crystalline solid. Herein, we report the synthesis of
a self-assembling, self-complementary metal complexthat
forms a supramolecular hexagon and a double-helical coor-
dination polymer within a single crystalline lattice. Further-
more, it is proposed that aggregation of fluorine substituents
is a key feature in directing the formation of this surprising
example of supramolecular assembly.
We have previously described heteroleptic copper(ii)
complexes with dipyrromethene (dipyrrin) ligands[8] that can
form self-complementary, one-dimensional “zig-zag” coordi-
nation polymers.[9] These complexes consist of square-planar
copper(ii) centers bound by one dipyrrin ligand and one
acetylacetonate (acac) ligand. The dipyrrin ligand features a
second donor site (pyridyl nitrogen atom) that coordinates to
the axial position of a neighboring copper(ii) center, thereby
generating an extended structure. In an effort to explore the
features of these complexes that would alter the polymeric
structure, modifications of the acac ligand were pursued.
Instead of using Cu(acac)2 as the copper source, Cu(hfacac)2
(hfacac = hexafluoroacetononate) was utilized, which
resulted in a heteroleptic metal complexwith a “blocking”
hfacac ligand (Scheme 1). This change in the spectator ligand
was found to noticeably alter the complexation behavior and
supramolecular assembly of the heteroleptic compound.
The products of the reaction between the dipyrrin ligand
(4-pyrdpm = 5-(4-pyridyl)dipyrromethene, 2) and Cu(hfa-
cac)2 were found to be dependent on the component
stoichiometry (Scheme 1). In situ generation of 2 combined
with an excess of Cu(hfacac)2 results in the isolation of the
Supramolecular Chemistry
Self-Assembly of Two Distinct Supramolecular
Motifs in a Single Crystalline Framework**
Sara R. Halper and Seth M. Cohen*
Supramolecular chemistry is a rapidly growing field of
research concerned with the construction of molecular
assemblies held together by noncovalent interactions.[1,2]
Forces such as hydrogen bonding, p–p stacking, and metal–
ligand coordination have been extensively used in the syn-
thesis of supramolecular structures. These assemblies are
anticipated to find utility as new materials with novel
catalytic, magnetic, electronic, and optical properties.[3–6]
Within the realm of supramolecular complexes that utilize
[*] S. R. Halper, Prof. S. M. Cohen
Department of Chemistry and Biochemistry
University of California, San Diego
9500 Gilman Drive, La Jolla, CA 92093–0358 (USA)
Fax: (+1)858-822-5598
E-mail: scohen@ucsd.edu
[**] This work was supported by the University of California, San Diego
and a Chris and Warren Hellman Faculty Scholar award. Acknowl-
edgement is made to the donors of the American Chemical Society
Petroleum Research Fund for support of this research. This material
is based upon work supported under a National Science Foundation
Graduate Research Fellowship (S.R.H.).
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
Scheme 1. Scheme for the synthesis of 4-pyrdpm (2) and heteroleptic copper com-
plexes (3 and 4).
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DOI: 10.1002/anie.200353520
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Communications
trinuclear species 3. This compound contains two distorted
square-planar copper(ii) complexes bound to an octahedral
copper(ii) center through the meso-pyridyl nitrogen-atom
donors (Figure 1).[10] This species is not observed nor isolated
under identical reaction conditions when employing
Cu(acac)2 as the starting material.[9] When 4-pyrdpm (2) is
mixed with substoichiometric amounts of Cu(hfacac)2, the
expected mononuclear, heteroleptic complex [Cu(4-
pyrdpm)(hfacac)] (4) can be isolated by flash silica chroma-
tography in modest yield.
The solid-state structure of 4 was anticipated to yield a
linear coordination polymer similar to that found for [Cu(4-
pyrdpm)(acac)].[9] However, crystallization of 4 reveals a
surprising structure in which the compound self-organizes
into two distinct supramolecular motifs: a discrete molecular
hexagon and a helical coordination polymer (Figure 2).[10] The
supramolecular hexagon is comprised of six molecules of 4
tethered together by apical coordination of the meso-pyridyl
nitrogen atoms (Cu-Npyr 2.28 Š).[11–18] The neighboring
Cu–Cu distance is 9.72 Š, and the ring has a diameter of
Figure 1. Structural diagram of trinuclear complex 3 with copper(ii)
atom numbering scheme (ORTEP, 50% probability ellipsoids). Hydro-
gen atoms and solvent molecules have been omitted for clarity.
Figure 2. Independent crystallographic com-
ponents of 4 shown in structural (ORTEP,
50% probability ellipsoids, left) and chemi-
cal (right) representations. Molecular hexa-
gons (top) and helical coordination poly-
mers (bottom) comprise the supramolecular
structure. Hydrogen and fluorine atoms
have been omitted for clarity.
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Chemie
about 19.24 Š (based on Cu–Cu distance of opposite metal
centers on the ring), placing this among the largest neutral
molecular hexagons characterized by single-crystal X-ray
diffraction methods.[7] The cocrystallized helical coordination
polymer[19–22] is also connected by apical coordination of
pyridyl substituents (Cu-Npyr 2.30 Š) in a manner generally
consistent with related systems.[9,23]
A view along the crystallographic c axis shows the
molecular hexagons lie in line with each other making
hexagonal channels (Figure 3). The crystallographic data
Perhaps the most striking feature of 4 is the packing
arrangement of the two supramolecular motifs within the
crystalline lattice. Adjacent to each edge of the molecular
hexagons are the coordination polymers, which run along the
crystallographic c axis parallel to the hexagonal channels
(Figure 3). The packing diagram reveals that the coordination
polymers are more than simple helical chains; the strands
arrange as pairs of antiparallel double helices.
The packing of the molecular hexagons with the double-
helical coordination polymers suggests that the fluorine
groups self-segregate from other parts of the structure to
drive the formation of this elaborate assembly. The high
immiscibility and hydrophobicity of the CF3 group has been
successfully utilized in the rational design of self-assembled
peptide coiled coils.[24–26] In the structure of 4, the hfacac
ligands of the hexagon are interdigitated between the double-
helices where they contact the hfacac ligands from the
coordination polymer chains (Figure 4). The space between
the helices is packed with CF3 groups from both the hexagon
and the polymers. The hfacac groups of the helices are in turn
arranged to interact with one another, essentially creating
Figure 3. Spacefilling packing diagram for 4 (top) along the crystallo-
graphic c-axis. The hexagons are shown in green and the strands of
the double-helices are shown in orange and blue. An isolated double-
helix in 4 (bottom) viewed perpendicular to the crystallographic c axis.
indicate that these channels are empty or only occupied by
small amounts of highly disordered solvent molecules (two
À3
residual electron density peaks of ꢀ 2.0 eŠ suggest some
disordered solvent may be present).[10] The individual metal
complexes are neutral, therefore disqualifying the presence of
counterions in the pores. Drying of the crystals under vacuum
at 508C did not appear to disturb the crystal morphology and
elemental analysis of the material was consistent with the
absence of any cocrystallized solvent. Differential scanning
calorimetry (DSC) reveals a single endotherm at 1978C and
thermogravimetric analysis (TGA) shows no weight loss up to
about 2008C. DSC and TGA of related compounds have
clearly shown the presence of trapped solvent molecules[9]
thereby supporting the contention that 4 does not contain
significant amounts of solvent guests.
Figure 4. Packing diagram for 4 (top) along the crystallographic c-axis;
hexagons are shown in spacefilling and double-helices are shown in
stick form. Same view of 4 (bottom) with the hexagons removed.
Hydrogen atoms have been removed for clarity.
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Communications
[9] S. R. Halper, M. R. Malachowski, H. M. Delaney, S. M. Cohen,
Inorg. Chem. 2004, 43, 1242.
rings of CF3 groups periodically in the c-axis channels. The
hexagonal channels are therefore composed of alternating
layers of molecular hexagon “holes” and helical polymer
“walls” that remain in register with each other (inner
diameter ꢀ 15.4 Š). Ultimately, the packing of the two
supramolecular entities suggests separation of the fluorous
phases drive the exotic solid-state arrangement of the
assembly. This hypothesis is further supported by the
observation that both [Cu(4-pyrdpm)(acac)] and [Cu(4-
[10] CCDC 225998 (3) and CCDC 225999 (4) contain the supple-
mentary crystallographic data for this paper. These data can be
obtained free of charge via www.ccdc.cam.ac.uk/conts/retrie-
ving.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). Also see Supporting
Information.
[11] B. Hasenknopf, J.-M. Lehn, N. Boumediene, A. Dupont-Gervais,
A. Van Dorsselaer, B. Kneisel, D. Fenske, J. Am. Chem. Soc.
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pyrdpm)(tfacac)]
(tfacac = trifluoroacetononate)
form
simple, one-dimensional zig-zag coordination polymers.[9,23]
Ongoing studies in our laboratory are focused on the
host–guest chemistry and self-assembly of related complexes.
[12] P. J. Stang, N. E. Persky, J. Manna, J. Am. Chem. Soc. 1997, 119,
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[17] R. Takahashi, Y. Kobuke, J. Am. Chem. Soc. 2003, 125, 2372.
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[19] K. Biradha, C. Seward, M. J. Zaworotko, Angew. Chem. 1999,
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[23] S. R. Halper, S. M. Cohen, unpublished results.
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Experimental Section
[Cu3(4-pyrdpm)2(hfacac)4] (3): 5-(4-Pyridyl)dipyrromethane (1)[9,27]
(0.30 g, 1.34 mmol) was dissolved in CHCl3 (150 mL) and stirred in
an ice bath. 2,3-Dichloro-5,6-dicyano-1,4-benzoquinone (0.31 g,
1.34 mmol) was dissolved in benzene (100 mL) and added dropwise.
Cu(hfacac)2·H2O (0.64 g, 1.34 mmol) dissolved in CHCl3 (50 mL) was
added and stirred for 10 min to form the copper(ii) complex. The
reaction mixture was evaporated to dryness, and the product was
purified by column chromatography (SiO2; CHCl3 with 1% MeOH)
to afford a red solid (0.27 g, 27% yield). Elemental analysis (%) calcd
for C48H24F24N6O8Cu3·0.5CHCl3: C 38.35, H 1.63, N 5.53; found: C
38.02, H 1.27 , N 5.56. UV/Vis (CH2Cl2): lmax = 232, 304, 490 nm; IR
(film from CH2Cl2): n˜ = 1644, 1562, 1255, 1214, 1145, 1029, 1000 cmÀ1
Red plates of 3 were grown from a solution of the complexin CHCl 3
diffused with pentane.
.
[Cu(4-pyrdpm)(hfacac)] (4): 5-(4-Pyridyl)dipyrromethane (1)[9,27]
(0.30 g, 1.34 mmol) was oxidized as described for 3. Cu(hfacac)2·H2O
dissolved in 50 mL of CHCl3 was added in small increments until all of
the oxidized methene had been converted to the metal complex. The
addition was monitored by using TLC and UV/Vis spectroscopy. The
total added Cu(hfacac)2·H2O was 0.40 g (0.84 mmol, 0.63 equiv to 1).
The reaction mixture was evaporated to dryness and the product was
purified by column chromatography (SiO2; CHCl3 with 1% MeOH)
to afford a red solid (0.15 g, 22% yield). ESI-MS: m/z 490.9 [M + H]+.
Elemental analysis (%) calcd for C19H11F6N3O2Cu: C 46.49, H 2.26,
N 8.56; found: C 46.46, H 2.23, N 8.75. UV/Vis (CH2Cl2): lmax = 232,
312, 492 nm; IR (film from CH2Cl2): n˜ = 1648, 1560, 1252, 1209, 1144,
1030, 997 cmÀ1. Red-green blocks of 4 were grown from a solution of
the complexin CHCl 3 diffused with hexanes.
Received: December 12, 2003 [Z53520]
Keywords: coordination polymers · copper · fluorine ·
.
helical structures · supramolecular chemistry
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