Columnar organization of head-to-tail self-assembled Pt4 rings

Article abstract of DOI:10.1021/ja910886g

Coordination of Pt²⁺ to a family of tunable Schiff base proligands directs the 12-component self-assembly of disk-shaped Pt₄ rings in a head-to-tail fashion. Aggregation of these S₄ symmetric Pt₄ macrocycles int

Full text of DOI:10.1021/ja910886g

Published on Web 05/14/2010
Columnar Organization of Head-to-Tail Self-Assembled Pt4
Rings
Peter D. Frischmann, Samuel Guieu, Raymond Tabeshi, and Mark J. MacLachlan*
Department of Chemistry, UniVersity of British Columbia, 2036 Main Mall, VancouVer,
British Columbia V6T 1Z1, Canada
Received December 25, 2009; Revised Manuscript Received April 16, 2010; E-mail:
mmaclach@chem.ubc.ca
Abstract: Coordination of Pt2+ to a family of tunable Schiff base proligands directs the 12-component self-
assembly of disk-shaped Pt4 rings in a head-to-tail fashion. Aggregation of these S4 symmetric Pt4
macrocycles into columnar architectures was investigated by dynamic and static light scattering, NMR
spectroscopy, powder X-ray diffraction, and transmission electron microscopy. Data from these experiments
support the formation of columnar architectures for all of the structures studied except when bulky tris(4-
tert-butylphenyl)methyl substituents were present. In this case, aggregation was limited to dimers in CHCl3
(Kdim ) 3200 ( 200 L mol-1 at 25 °C) and a thermodynamic analysis revealed that dimerization is an
entropy driven process. Columnar architectures of Pt4 rings with branched 2-hexyldecyl substituents organize
into lyotropic mesophases in nonpolar organic solvents. These new self-assembled supramolecules are
promising candidates to access nanotubes with multiple linear arrays of Pt2+ ions.
Introduction
analogues. Phosphorescent, electrophosphorescent, luminescent,
electroluminescent, vapoluminescent, and semiconducting wires
Self-assembly has emerged as a powerful technique to
organize small molecules into functional materials with minimal
effort.1 The morphology of self-assembled materials is dictated
by the geometry and chemical functionality of the monomeric
constituents.2 Disk-shaped molecules composed primarily of
aromatic units often code for the assembly of one-dimensional
fibers or noncovalent nanotubes and are being pursued as
materials for photovoltaic active layers, sensors, and molecular
wires.3 Solvophobic and weak intermolecular forces such as
hydrogen-bonding or π-π interactions dominate the self-
assembly process of these systems.
have been constructed via self-assembly of neutral or cationic
Pt2+ complexes coordinated by chelating aromatic ligands.4
Similar complexes form luminescent metallogels, chromonic
liquid crystals, nanosheets, and wheel-shaped superstructures
through a subtle balance of Coulombic, metal-metal, and/or
π-π interactions.5 The exciting properties exhibited by these
systems hold promise for development of novel materials.
Platinum-pyridyl chemistry has been extensively investigated
in the synthesis of complex molecules. Typically, dipyridyl-
based ligands are combined with cis or trans protected Pt2+
Planar coordination complexes also assemble into fibers where
anisotropic metal-metal interactions aid in assembly and impart
unique properties, often inaccessible with purely organic
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7668 J. AM. CHEM. SOC. 2010, 132, 7668–7675
10.1021/ja910886g  2010 American Chemical Society

Organization of Pt4 Rings
A R T I C L E S
Chart 1. Reported N2O2 Pt2+ Schiff Base Monomer and
Conceptual Evolution of the Monomer into a Head-to-Tail
Self-Assembling Metallocycle
a natural example of head-to-tail assembling supramolecules,
and synthetic analogues are known to exhibit columnar ag-
gregation.13
In this article, we describe a new route to Pt-pyridyl type
metallocycles using a head-to-tail synthetic strategy. Self-
recognition is achieved by incorporating a coordinating pyridyl
ligand and an open (or solvent-occupied) Pt coordination site
into the same molecule. Specifically, tetrameric metallocycles
are constructed in a selective 12-component, one-pot self-
assembly directed by tetradentate Schiff base N-ONO donor
proligands formed either in situ or prior to metalation. Su-
pramolecular aggregation of individual Pt4 metallocycles into
columnar arrays is observed and bestows liquid crystalline
properties upon the materials. These parallel columnar arrays
are exciting materials with potential for anisotropic Pt-Pt
interactions.
complexes, and highly charged metallocycles are formed.6 This
approach has become a paradigm for development of functional
polyhedra, catenanes, and other unique architectures.7 Despite
the variety of interesting structures that have emerged from this
method, significant limitations remain. For example, the high
charge of the metallocycle has prevented the observation of
stacking in solution, a property that could be used to assemble
nanotubes that include metal-metal bonding.8 Also, only high
symmetry objects are usually accessible through this method.
Neutral Schiff base platinum(II) complexes with trans-N2O2
donors, such as the monomer depicted in Chart 1, have been
reported,9 and Bosnich demonstrated that similar complexes are
Results and Discussion
We set out to construct the Pt-containing metallocycles shown
on the right-hand side of Chart 1. To test the feasibility of this
approach and to determine the expected ring size, we first
prepared model complex 1, where the pyridyl group, essential
for self-recognition, is substituted by a phenyl group. Single-
crystal X-ray diffraction of complex 1, depicted in Figure 1,
shows that the Pt2+ is complexed by the ONO Schiff base
pocket, and the fourth coordination site is occupied by an
S-bound DMSO molecule.14 Besides the expected rotation of
the peripheral phenyl group to relieve steric repulsion between
protons (normal for a biphenyl-type system), the molecule is
nearly planar. Moreover, the angle of interest for self-assembly,
shown in Figure 1a, is approximately 90°, indicating that head-
to-tail self-assembly should direct formation of cyclic tetramers
if the phenyl group is replaced by a 3-pyridyl unit.
sterically unencumbered for one-dimensional assembly.10
A
solid-state structure revealed close axial Pt · · ·Pt contacts (3.26
Å) between a platinum(II) Schiff base complex and metalated
bis(tert-pyridyl) pincer hosts, facilitated by π-π and metallo-
philic interactions.11 With the goal of assembling cycles that
may ultimately show metal-metal bonding in a columnar
orientation, we envisioned a ligand system that codes for the
head-to-tail self-assembly of disk-shaped platinum-containing
metallocycles as illustrated in Chart 1.
When a single molecule is outfitted with a donor-acceptor
pair appropriately distributed to prohibit intramolecular recogni-
tion, one end of the molecule recognizes the other end in an
intermolecular fashion. The spatial arrangement of this self-
recognition determines whether polymers or macrocycles are
isolated. Head-to-tail self-assembly facilitated in this fashion
has been used to prepare a variety of supramolecules.12 Through
hydrogen bonds and the aid of an alkali metal, G-quartets are
When a solution of model complex 1 in DMSO-d6 was titrated
with pyridine and monitored by 1H NMR spectroscopy,
resonances assigned to both free and coordinated pyridine were
evident (Figure 2). From integration of these resonances, the
equilibrium constant (Kpyr) was determined to be 35 ( 14 mol-1
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(14) X-ray crystal data for 1 grown from DMSO: C21H18.5NO3PtS, Mw
)
560.02 g mol-1, yellow needle, orthorhombic, space group P212121,
a ) 5.5909(5) Å, b ) 17.6944(17) Å, c ) 19.0577(18) Å, V )
1885.3(3) Å3, Z ) 4, R1 ) 0.0441, wR2 ) 0.1146.
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A R T I C L E S
Frischmann et al.
TOF MS analysis, however, revealed formation of not only
cyclic tetramer 4c but also aggregates up to [4c6]+. We prepared
proligand 3d with even bulkier tris(4-tert-butylphenyl)methyl
substituents and used this compound to synthesize Pt4 ring 4d.
Despite observing aggregates up to [4d5]+ in the MALDI-TOF
MS, 4d is very soluble in organic solvents. Metallocycles 4b-d
are all brightly colored yellow solids that show absorption bands
around 300-310, 425-430, and 450-455 nm (see Supporting
Information Figure S9 for spectra).
1
The H NMR spectrum of 4d in CDCl3 (see Figure 4a)
exhibits sharp resonances that were assigned by COSY and
ROESY 2D NMR spectroscopy.16 The low intensity resonances
in the spectrum that appear to be impurities exhibit strong cross
peaks with the identified resonances in the ROESY spectrum,
1
indicative of chemical exchange. A variable concentration H
NMR spectroscopic experiment revealed that the identified
resonances are in fact due to an aggregate and the low intensity
resonances are assigned to monomers. Equilibrium constants
calculated by integration of 1H NMR spectra collected between
-45 and 25 °C fit best to a monomer-dimer rather than infinite
aggregate model,17 as might be expected for such sharp
resonances (Figure 4b). At 25 °C, Kdim ) 3200 ( 200 L mol-1
and a van’t Hoff analysis (Figure 4c) gave ∆H° ) 13 ( 1 kJ
mol-1 and ∆S° ) 110 ( 30 J mol-1 K-1. This result indicates
that dimerization is enthalpy opposed but entropy favored. The
entropy increase in this process is attributed to the release of
solvating CDCl3 molecules upon dimerization.18 Extended
aggregation of 4d in the solid state, as observed by MALDI-
TOF MS, is inhibited in solution by rapid rotation of the tris(4-
tBuPh) substituents. As very bulky substituents are necessary
to limit the aggregation even to dimers, as in the case of 4d,
we conclude that macrocycle 4b with much smaller substituents
aggregates extensively in solution, explaining its broad 1H NMR
spectrum.
Because we have been unable to obtain single crystals of
4a-d, we undertook an ab initio DFT study of complex 4a to
learn more about its structure and as a model for aggregation.
Figure 5a shows the puckered conformation of 4a determined
by DFT optimization.19 This conformation exhibits a 60°
deviation from planarity, a maximum outer diameter of 2.4 nm,
and a 0.7 nm inner pore. The intermetallic Pt-Pt distances in
the macrocycle are 1.5 nm (diagonal) and 1.1 nm (edge).
Belonging to the rare point group S4, this geometry is imposed
by the 38° dihedral angle between the 3-pyridyl and phenyl rings
of the salicylidene unit.20
Figure 1. Solid-state structure of complex 1: (a) top-down view highlighting
the 90° angle that directs tetrameric self-assembly when the peripheral
phenyl group is replaced by a 3-pyridyl group; b) side-on view. C ) green,
O ) red, N ) blue, Pt ) yellow, S ) purple.
L at 25 °C. This favorable Pt2+-pyridine interaction, even in a
competing coordinating solvent, combined with the entropy gain
expected upon forming the metallocycle (from displaced solvent)
was anticipated to drive the self-assembly of 4a-d as shown
in Scheme 1.
In a one-pot experiment where proligand 3a was generated
in situ, K2PtCl4, K2CO3, 5-(3-pyridyl)salicylaldehyde, and
o-aminophenol were reacted in degassed DMSO at 150 °C for
2 h. Upon cooling of the sample, an air-stable yellow solid
precipitated that was isolated by centrifugation and washed with
water and MeOH. Solvent free matrix assisted laser desorption
ionization time-of-flight mass spectrometry (MALDI-TOF MS)
showed the selective formation of the cyclic tetramer, 4a (m/z
) 1934), with no other oligomers or ring sizes observed. Most
interestingly, the only higher mass peaks observed correspond
to sequential aggregates from dimer, [4a2]+, up to hexamer,
[4a6]+, due to columnar aggregation, as shown in Figure 3.
As Pt4 ring 4a is insoluble in all common solvents, we pursued
2-hexyldecyl substituted Pt4 ring 4b as a soluble alternative in
order to perform solution NMR spectroscopic studies. Although
4a-d may all be prepared in situ from the imine precursors
(i.e., the aldehyde and aminophenol), starting from proligands
3a-d gives better results. In this manner, cyclic tetramer 4b
was selectively synthesized (Scheme 1) and its identity con-
firmed by MALDI-TOF MS. Aggregation of Pt4 rings was again
evident, with a series of peaks observed up to m/z ) 14 168,
corresponding to [4b5]+. 1H NMR spectroscopy of 4b in CDCl3,
C6D6, and toluene-d8 produced only broad unidentifiable reso-
nances even at elevated temperatures. This broadening is
attributed to a substantial decrease in the T2 relaxation time,15
providing further evidence for strong interactions between Pt4
rings resulting in polymeric aggregation. Clearly bulkier sub-
stituents are necessary to inhibit stacking for an NMR spectro-
scopic study.
Modeling studies reveal that the puckered Pt4 rings may
organize into columns either with a single orientation (syn) or
1
(16) The H NMR spectrum of 4d in CD2Cl2 is significantly complicated
relative to the spectrum obtained in CDCl3 because of either adoption
of a lower symmetry conformation or oligomeric aggregation being
facilitated by CD2Cl2 (see Supporting Information Figure S5 for
spectra).
(17) Martin, R. B. Chem. ReV. 1996, 96, 3043–3064.
(18) Entropy-driven dimerization: (a) Frischmann, P. D.; Facey, G. A.; Ghi,
P. Y.; Gallant, A. J.; Bryce, D. L.; Lelj, F.; MacLachlan, M. J. J. Am.
Chem. Soc. 2010, 132, 3893–3908. (b) Kang, J.; Rebek, J., Jr. Nature
1996, 382, 239–241. (c) Cram, D. J.; Choi, H. J.; Bryant, J. A.;
Knobler, C. B. J. Am. Chem. Soc. 1992, 114, 7748–7765. (d)
Frischmann, P. D.; MacLachlan, M. J. Chem. Commun. 2007, 4480–
4482.
(19) Optimization with B3LYP level of theory (LanL2dZ basis sets for Pt,
6-31G* for other atoms): Yamashita, K.; Sato, K.; Kawano, M.; Fujita,
M. New J. Chem. 2009, 33, 264–270.
Pt4 ring 4c with bulky trityl substituents was synthesized
(Scheme 1). Unfortunately, 4c is also poorly soluble. MALDI-
(20) Other supramolecules with S4 symmetry: (a) Su, C.-Y.; Smith, M. D.;
zur Loye, H.-C. Angew. Chem., Int. Ed. 2003, 42, 4085–4089. (b)
Beissel, T.; Powers, R. E.; Raymond, K. N. Angew. Chem., Int. Ed.
1996, 33, 1084–1086.
(15) Evertsson, H.; Nilsson, S.; Welch, C. J.; Sundelo¨f, L.-O. Langmuir
1998, 14, 6403–6408.
9
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Organization of Pt4 Rings
A R T I C L E S
1
Figure 2. (a) Equilibrium between DMSO-bound model complex 1 and pyridine-bound model complex 2. (b) H NMR spectra of model complex 1 in
DMSO-d6 when titrated with pyridine (25 mmol L-1, 400 MHz). Equivalents of pyridine are given on the left, and the resonances integrated for thermodynamic
analysis are color-coded on top.
Scheme 1. Head-to-Tail Self-Assembly of Pt4 Rings 4a-da
Figure 3. Aggregates of Pt4 ring 4a observed with MALDI-TOF MS.
either stacking motif, but with sterically demanding substituents,
only the anti alternating, AB pattern is possible because this reduces
intermolecular interactions between the substituents. Potential for
intermolecular Pt-Pt interaction exists in the syn case.
a (i) N-ONO salicylaldimine proligand 3a-d (or corresponding amine
and aldehyde), K2CO3, and K2PtCl4 are heated in DMSO at 150 °C for
2-4 h.
Solution aggregation of Pt4 rings 4b and 4d in CHCl3 was
further investigated by dynamic and multiangle laser light
scattering (DLS and MALLS, respectively). The 1H NMR
studies of 4d described above indicated that aggregation in
with alternating orientations (anti), depending on the steric bulk
of the peripheral R group (Figure 5b). In the syn case, the repeat
unit is a simple translation along the columnar axis, whereas
the anti orientation requires the same translation, a 90° rotation
about the columnar axis and a 180° rotation perpendicular to
the columnar axis. With small substituents, the cycles may adopt
solution was limited to dimers. DLS of 4d (0.6 mg mL-1
)
showed that only aggregates of <10 nm in diameter are present,
confirming that dimerization and not infinite aggregation is
occurring. On the other hand, DLS of 4b (0.6 mg mL-1) resulted
9
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A R T I C L E S
Frischmann et al.
Figure 5. (a) DFT optimized geometry of Pt4 ring 4a. (b) Computer model
of two possible orientations for columnar aggregation. Each ring is stacked
directly on the other in the syn orientation, whereas the anti orientation
exhibits alternating AB type stacking.
Figure 4. (a) 1H NMR spectrum of 4d in CDCl3 (400 MHz) and an inset
of the chemical structure. Resonances of the peripheral, R ) tris(4-tBuPh),
aromatic rings overlap with the residual CHCl3 resonance calibrated to 7.27
1
ppm. (b) Variable temperature H NMR spectra of 4d in CDCl3 (2 mmol
L-1) from -45 to 25 °C. Monomer-dimer equilibrium is shown, and
resonances are color-coded (monomer ) red, dimer ) blue). (c) van’t Hoff
plot for dimerization of 4d in CDCl3.
in a broad peak corresponding to a hydrodynamic radius (RH)
of 60 nm (assuming spherical particles).21
To obtain information about the shape of the aggregates of
4b in solution, a MALLS study was undertaken. Columnar
aggregation of 4b was confirmed by the Kratky plot, shown in
Figure 6a, that exhibits linear angular dependence over the light
scattering intensity of the aggregates.22 A Zimm plot was also
constructed from MALLS data over a 0.4-0.8 mg mL-1
(21) Goldin, A. A. DYNALS, version 2.0. Software for particle size
distribution analysis in photon correlation spectroscopy. http://
www.softscientific.com/science/WhitePapers/dynals1/dynals100.htm
(accessed April 13, 2010).
(22) (a) Ryu, J.-H.; Tang, L.; Lee, E.; Kim, H.-J.; Lee, M. Chem.sEur. J.
2008, 14, 871–881. (b) Ryu, J.-H.; Lee, E.; Lim, Y.-B.; Lee, M. J. Am.
Chem. Soc. 2007, 129, 4808–4814. (c) Yang, W.-Y.; Lee, E.; Lee,
M. J. Am. Chem. Soc. 2006, 128, 3484–3485. (d) Kim, B.-S.; Hong,
D.-J.; Bae, J.; Lee, M. J. Am. Chem. Soc. 2005, 127, 16333–16337.
(e) Rosselli, S.; Ramminger, A.-D.; Wagner, T.; Silier, B.; Wiegand,
S.; Ha¨ussler, W.; Lieser, G.; Scheumann, V.; Ho¨ger, S. Angew. Chem.,
Int. Ed. 2001, 40, 3137–3141. (f) Bockstaller, M.; Ko¨hler, W.; Wegner,
G.; Vlassopoulos, D.; Fytas, G. Macromolecules 2000, 33, 3951–3953.
Figure 6. MALLS analysis of 4b in CHCl3: (a) Kratky plot (0.6 mg mL-1);
(b) Zimm plot (0.4, 0.6, and 0.8 mg mL-1).
concentration range and is shown in Figure 6b with extrapola-
tions to zero concentration and angle. From the Zimm analysis
we obtained an average molecular weight, Mw ) (1.2 ( 0.4) ×
107 g mol-1, radius of gyration, Rg ) 150 ( 17, and a second
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Organization of Pt4 Rings
A R T I C L E S
Figure 7. POM images observed under crossed polarizers of growing LC textures for 4b from (a) CHCl3 and (b, c) PhCl. Black indicates an isotropic phase.
virial coefficient, A2 ) (5.4 ( 0.4) × 10-4 mol mL g-2
.
LC phases as outlined in Scheme 2.29 This is the first report of
lyotropic liquid crystallinity for Pt-pyridyl metallocycles and
a rare example of a multimetallic LC.30
Although the aggregates in this system are dynamic in nature
(owing to noncovalent interactions), making these values
nonquantitative, they are still reasonably reliable for making
predictions about the system.23 In particular, spherical or random
coil aggregates exhibit Rg/RH ratios below 1.5 whereas for rigid
rod-shaped aggregates the ratio is greater.24 The Rg/RH ratio for
Pt4 ring 4b is 2.5, indicating these are highly anisotropic rigid
column/rod-shaped aggregates. Attempts to elucidate the ther-
modynamics of columnar aggregation in CHCl3 by variable
concentration UV-vis experiments were inhibited because there
is no change in the absorption spectra from 3.4 × 10-4 to 2.6
× 10-6 mol L-1, suggesting that even at very low concentrations
4b is extensively aggregated.
Organization of the Pt4 rings was also studied in the solid
state by powder X-ray diffraction (PXRD). Figure 8a-d shows
the PXRD patterns of microcrystalline 4a-d immediately after
isolation. Unsubstituted Pt4 ring 4a displays the most crystal-
linity, with several sharp peaks present in the powder pattern.
Unfortunately, there are too few peaks to obtain a space group
or even a definitive unit cell, but the best fits to the experimental
data were for tetragonal unit cells (R ) ꢀ ) γ ) 90°) with a )
b ) 2.043 or 2.890 nm and c ) 0.5-0.7 nm.31,32 This pattern
certainly does not prove a columnar stacking of the macrocycles,
but the data are consistent with such an arrangement. If the
cycles of 4a were stacked one on top of another as shown in
Figure 5 into parallel columns, the intercolumn center-to-center
distance would be about 2.1 nm and the intermacrocycle
separation in the columns would be around 0.5 nm.
We were surprised to discover that columnar aggregates of
Pt4 ring 4b organize into lyotropic mesophases upon concentra-
tion in nonpolar organic solvents. Liquid crystalline (LC)
behavior was observed in CHCl3, C6H6, PhCl, trichloroethylene,
CS2, and pyridine; however, the highest quality mesophases,
as judged by the homogeneity of the texture, are obtained in
chlorobenzene.25 Birefringence of these LCs was observed with
a polarized optical microscope (POM) upon slow evaporation
of dropcast solutions on a microscope slide, and typical POM
images of 4b are depicted in Figure 7.26 The steric bulk of Pt4
rings 4c and 4d inhibits extended columnar aggregation, and
accordingly little to no birefringence is observed for these
metallocycles in the same solvents.
Low-angle peaks are observed at 3.1, 2.9, and 3.2 nm for
4b, 4c, and 4d, respectively. These roughly correspond to the
expected intercolumnar distance for parallel aligned aggregates,
allowing for interdigitation of the peripheral substituents.
Additional low-intensity peaks at higher angle support the
presence of some order within each column.
We were unable to directly identify the phases of the lyotropic
LCs from PXRD patterns collected of samples in capillary tubes.
Most often, threadlike or Schlieren textures are observed for
4b, suggesting the adoption of a columnar nematic LC phase.27
In Figure 7c, a fan-shaped texture is observed near the isotropic
phase, potentially due to adoption of a disordered hexagonal
columnar mesophase.28 The S4 symmetry of 4b may inhibit
organization of a high fidelity hexagonal columnar LC phase
leading to a more disordered phase. In all cases, the anisotropy
of rod-shaped aggregates self-assembled from 4b leads to long-
range parallel orientation of columns in concentrated solutions
and adoption of nematic and/or disordered hexagonal columnar
(29) For reviews on columnar LCs, see the following: (a) Kato, T.; Yasuda,
T.; Kamikawa, Y.; Yoshio, M. Chem. Commun. 2009, 729–739. (b)
Laschat, S.; Baro, A.; Steinke, N.; Giesselmann, F.; Ha¨gele, C.; Scalia,
G.; Judele, R.; Kapatsina, E.; Sauer, S.; Schreivogel, A.; Tosoni, M.
Angew. Chem., Int. Ed. 2007, 46, 4832–4887.
(30) (a) Cordovilla, C.; Coco, S.; Espinet, P.; Donnio, B. J. Am. Chem.
Soc. 2010, 132, 1424–1431. (b) Coco, S.; Cordovilla, C.; Donnio, B.;
Espinet, P.; Garc´ıa-Casas, M. J.; Guillon, D. Chem.sEur. J. 2008,
14, 3544–3552. (c) Domracheva, N.; Mirea, A.; Schwoerer, M.; Torre-
Lorente, L.; Lattermann, G. ChemPhysChem 2006, 7, 2567–2577. (d)
Binnemans, K.; Lodewyckx, K.; Cardinaels, T.; Parac-Vogt, T. N.;
Bourgogne, C.; Guillon, D.; Donnio, B. Eur. J. Inorg. Chem. 2006,
150–157. (e) Bilgin-Eran, B.; Tschierske, C.; Diele, S.; Baumeister,
U. J. Mater. Chem. 2006, 16, 1136–1144. (f) Binnemans, K.;
Lodewyckx, K.; Donnio, B.; Guillon, D. Chem.sEur. J. 2002, 8,
1101–1105. (g) Nesrullajev, A.; Bilgin-Eran, B.; Kazanci, N. Mater.
Chem. Phys. 2002, 76, 7–14. (h) Nesrullajev, A.; Bilgin-Eran, B.;
Singer, D.; Kazanci, N.; Praefcke, K. Mater. Res. Bull. 2002, 37, 2467–
2482. (i) Donnio, B. Curr. Opin. Colloid Interface Sci. 2002, 7, 371–
394.
(23) Lortie, F.; Boileau, S.; Bouteiller, L.; Chassenieux, C.; Laupreˆtre, F.
Macromolecules 2005, 38, 5283–5287.
(24) (a) Runge, M. B.; Dutta, S.; Bowden, N. B. Macromolecules 2006,
39, 498–508. (b) Scha¨rtl, W. Light Scattering from Polymer Solutions
and Nanoparticle Dispersions; Springer-Verlag: Berlin and Heidelberg,
Germany, 2007.
(25) Because of low solubility of 4b in cyclohexanone, only small LC
domains were observed in this solvent prior to drying.
(31) Shirley, R. Crysfire. An InteractiVe Powder Indexing Support System;
The Lattice Press: Surrey, U.K., 2004.
(26) The critical concentration for birefringence is difficult to estimate, as
it is observed only upon evaporation. In each example the concentration
(32) The distinct peaks observed at low angle in the diffraction pattern fit
best to a tetragonal unit cell. The lack of well-defined peaks beyond
∼19° 2θ, which give information about the third parameter (c), makes
it very difficult to obtain any reliable measure of c. For unit cells
found with a ) b ) 2.043 nm, the best value of c was either 0.44 or
0.49 nm. For the unit cells found with a ) b ) 2.890 nm, the value
of c was in the 0.57-0.68 nm range.
is >1 mg mL-1
.
(27) Dierking, I. Textures of Liquid Crystals; Wiley-VCH Verlag: Wein-
heim, Germany, 2003.
(28) Saez, I. M.; Goodby, J. W.; Richardson, R. M. Chem.sEur. J. 2001,
7, 2758–2764.
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A R T I C L E S
Frischmann et al.
Scheme 2. Dynamic Assembly of Pt4 Ring 4b into Randomly Oriented Oligomers, Elongated and Oriented Columns, and Columnar Nematic
or Disordered Hexagonal Columnar Mesophases upon Concentration
Instead, we collected PXRD data from samples of 4b in CHCl3
and PhCl that were dropcast onto amorphous silicon plates and
left to dry. Diffractograms of the dried LC phases are shown in
Figure 8e,f.33 The low-angle diffraction observed for the sample
obtained from CHCl3 (with a peak at 3.1 nm) is nearly identical
to that observed in the PXRD pattern of as-isolated 4b,
suggesting that columnar alignment of the nematic phase is
being maintained upon drying. Disorder is evident from broad
peaks centered about 1.6 and 1.1 nm, preventing any further
structural elucidation. PXRD of 4b dropcast from PhCl shows
significantly more order, and a sharp peak is present at 3.5 nm.
Close inspection of the low-angle region reveals a broad peak
centered about 3.1 nm (10) followed by relatively sharp peaks
at 1.8 (11), 1.5 (20), and 1.2 nm (21), diagnostic spacing for
hexagonal columnar ordering. These observations are in agree-
ment with POM images that show 4b adopts both columnar
nematic and hexagonal columnar mesophases in PhCl. Inter-
columnar spacing is greater for the less ordered columnar
nematic phase (3.5 vs 3.1 nm). Overall, the PXRD studies of
the films dried from the lyotropic LC phases support the
retention of columnar organization in the metallocycles.
Solid-state organization of Pt4 ring 4b was also investigated
by transmission electron microscopy (TEM); representative
micrographs are shown in Figure 9. At low magnification, we
observe oblong “pill-shaped” aggregates assembled from cy-
clohexanone. High magnification of the same sample reveals
each “pill” is composed of individual stacks of Pt4 rings
organized into parallel columnar arrays, as shown in Figure 10.
The parallel columns are spaced by roughly 4 nm and extend
hundreds of nanometers. When 4b was deposited on the TEM
grid from CHCl3, bundles of randomly oriented rigid rods with
dimensions from tens to hundreds of nanometers were observed.
This is in agreement with the MALLS data that confirmed the
existence of rigid-rod shaped aggregates of 4b in CHCl3.
Micrographs of 4b deposited from C6H6 depict large flexible
fibers that are hundreds of nanometers in diameter and span
micrometers. The breadth of the rods and fibers observed for
4b cast from CHCl3 and C6H6 suggests that they are also
composed of individual columnar arrays, similar to those
observed from cyclohexanone; however, we were unable to
achieve a similar image resolution in these cases. Organization
of individual Pt4 rings to micrometer length fibers represents
an impressive hierarchical self-assembly that spans several
orders of magnitude in length, all in one pot. Only diffuse,
Figure 8. Normalized wide-angle PXRD patterns of as-prepared Pt4 rings:
(a) 4a, (b) 4b, (c) 4c, (d) 4d, (e) dried mesophase of 4b drop-cast from
CHCl3 onto amorphous silicon, and (f) dried mesophase of 4b dropcast
from PhCl onto amorphous silicon with assigned indices for hexagonal
ordering. All data are depicted from 2° to 30° 2θ.
(33) Because of the low solubility of 4b in cyclohexanone, dropcast films
were insufficiently thick for PXRD analysis.
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Organization of Pt4 Rings
A R T I C L E S
Figure 10. High magnification TEM images of Pt4 ring 4b from
cyclohexanone: (a) individual columnar arrays visible with periodic spacing
of roughly 4 nm; (b) model of columnar aggregates.
Pt-pyridyl bonding exhibit columnar organization in the solid
state, to the best of our knowledge no reports of solution
aggregation exist, a fact we attribute to Coulombic repulsion.
Neutral Pt4 rings presented here stack strongly both in the solid
state and in solution, where they form lyotropic LCs when
concentrated. By changing the substituents and the orientation
of the N-pyridyl group, we expect that diverse metallocycles
and self-assembled architectures may be prepared. We are
investigating the supramolecular chemistry of these Pt4 rings
to construct conductive nanotubes and liquid crystalline materials.
Figure 9. Low magnification TEM images of Pt4 ring 4b dried from various
solvents: (a) “pill-shaped” oblate aggregates from cyclohexanone; (b) rigid
rod-shaped bundles from CHCl3; (c) micrometer length flexible fibers from
C6H6.
Acknowledgment. We thank the Natural Sciences and Engi-
neering Research Council (NSERC) of Canada for funding (Dis-
covery grant), UBC for a graduate fellowship (to P.D.F.), Johan
Janzen for assistance with the light scattering experiments and
analysis, Joseph Hui for TEM images, and Angela Crane for DFT
calculations performed with resources provided by Westgrid &
Compute/Calcul Canada. S.G. thanks the French Ministry of Foreign
and European Affairs for a Lavoisier Postdoctoral Fellowship.
globular aggregates were present in TEM images of sterically
encumbered Pt4 ring 4d.
Conclusions
We have developed a new class of highly tunable proligands
that selectively self-assemble into novel S4 symmetric disk-
shaped metallocycles upon platinum(II) coordination. This head-
to-tail approach offers a very flexible, simple method to develop
Pt-containing macrocycles en route to nanotubes. Data from light
scattering, X-ray diffraction, and transmission electron micros-
copy support the formation of 1D columnar aggregates from
the rings. Although some molecular squares formed with
Supporting Information Available: Crystallographic informa-
tion in CIF format and details of syntheses, characterization,
structure coordinates, and calculations. This material is available
free of charge via the Internet at http://pubs.acs.org.
JA910886G
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J. AM. CHEM. SOC. VOL. 132, NO. 22, 2010 7675