Self-assembly of Pt(II) spherical complexes via temporary labilization of the metal-ligand association in 2,2,2-trifluoroethanol

Article abstract of DOI:10.1021/ja2059236

Thermodynamically controlled platinum(II) spherical complexes were synthesized via temporary labilization of inert Pt(II)-pyridine bonds by the addition of the strong hydrogen-bond donor 2,2,2-trifluoroethanol (TFE), which weakens the pyridine-metal inter

Full text of DOI:10.1021/ja2059236

COMMUNICATION
pubs.acs.org/JACS
Self-Assembly of Pt(II) Spherical Complexes via Temporary
Labilization of the MetalÀLigand Association in 2,2,2-Trifluoroethanol
Daishi Fujita, Asana Takahashi, Sota Sato, and Makoto Fujita*
Department of Applied Chemistry, School of Engineering, The University of Tokyo, and CREST, Japan Science and Technology Agency
(JST), 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan
S Supporting Information
b
Scheme 1. Schematic Representation of the Self-Assembly of
Pt(II)12L24 Complex 2 with the Weakly Acidic, H-Bond-
Donating Solvent TFE
ABSTRACT: Thermodynamically controlled platinum(II)
spherical complexes were synthesized via temporary labili-
zation of inert Pt(II)Àpyridine bonds by the addition of the
strong hydrogen-bond donor 2,2,2-trifluoroethanol (TFE),
which weakens the pyridineÀmetal interaction. The plati-
num complex was stably trapped after removal of TFE
and showed higher acid durability than its palladium
counterpart.
he switching of kinetically inert metalÀligand interactions
T
into labile ones under certain external stimuli is termed
temporary labilization (eq 1).1,2 When inert coordination bonds
are temporarily labilized, desired structures can be self-assembled
from metal and ligand components under thermodynamic con-
trol and then turned into inert (stable) frameworks by “switching
off” the labilization.
Kinetically inert Pt(II)Àpyridine coordination bonds have
previously been labilized by salt-mediated nucleophilic
activation1 or photochemical activation2 to give interlocked or
cage structures that can otherwise be self-assembled only with
analogous kinetically labile Pd(II)Àpyridine bonds. However,
these labilization methods are not suitable for practical use
because the removal of the salt is troublesome and photoirradia-
tion requires unconventional apparatus that limits practical
synthesis and reaction scale-up. In addition, these methods are
applicable only to self-assembly from a small number of metal
and ligand components (typically, n < 10). When n is larger, these
methods are no longer effective, and intractable mixtures are
formed as a result of insufficient bond labilization, although there
are some exceptions involving the self-assembly of Pt(II) com-
plexes with large n.3 Here we report that a strong H-bond-donor
solvent, 2,2,2-trifluoroethanol (TFE),4,5 considerably labilizes
Pt(II)Àpyridine bonds, allowing the formation of even giant,
multicomponent Pt(II)12L24 spheres from Pt(II) ions and pyr-
idine ligands (L) (Scheme 1). The volatile TFE solvent (bp
78 °C) is easily removed, affording kinetically trapped stable
Pt(II) assemblies. We demonstrate that the Pt(II)12L24 sphere,
whose structure is shown in Figure 1, is much more stable than its
Pd(II) counterpart. Interestingly, when the ligand was made
Figure 1. Crystal structure of Pt(II)12L24 complex 2. H atoms and the
two SbF6À counterions that are present at the apical positions of each Pt2+
have been omitted for clarity.
relatively flexible with acetylene spacers, the temporary labiliza-
tion resulted in the trapping of a Pt(II)6L12 cube rather than a
Pt(II)12L24 sphere.
TFE exhibits weakly acidic character and a H-bond donating
ability comparable to that of phenol4 and forms a stable 1:1
acidÀbase complex with pyridine.5 We anticipated that the
Pt(II)Àpyridine coordination bond could be labilized in TFE
because dissociated pyridine ligands are stabilized by strong
H-bonds with TFE. Thus, ligand 1 (7.0 μmol), which forms a
Pd(II)12L24 sphere upon complexation with Pd(II) ions,6 was
treated with Pt(NO3)2 (5.6 μmol) in 60:40 TFE/DMSO mixed
1
solvent (700 μL). Time-dependent H NMR analysis revealed
that the spectrum gradually became simpler, and convergence
Received: June 25, 2011
Published: August 03, 2011
r
2011 American Chemical Society
13317
dx.doi.org/10.1021/ja2059236 J. Am. Chem. Soc. 2011, 133, 13317–13319
|

Journal of the American Chemical Society
COMMUNICATION
Scheme 2. Schematic Representation of the Self-Assembly of
the MnL2n-type Complexes Derived from Ligand 3
Figure 2. 1H NMR (500 MHz, 300 K) observation of the self-assembly
of Pt(II)12L24 complex 2 in (aÀc) 60:40 TFE/DMSO and (dÀf)
DMSO. Shown are spectra of (a, d) free ligand 1 and the the mixture
(b, e) 12 and (c, f) 24 h after complexation with Pt(NO3)2. The structure
converged only in TFE/DMSO. It should be noted that signal broad-
ening for giant Pt(II) complexes is inevitable because of their tumbling
motion, which is slow on the NMR time scale.
Figure 4. CSI-TOF mass spectrum of Pt(II)6L12 cube 4 (SbF6À salt).
The inset shows expanded spectra of the n = 10 peak (observed and
simulated).
afforded high-quality data from which the Pt(II)12L24 spherical
shell was confirmed. No unusual bond lengths or angles were
observed, and almost perfect square-planar orientation of the
four pyridyl groups around each Pt(II) center was confirmed.
Importantly, these structural features of 2 are essentially the same
as those of Pd(II)12L24 spheres with the same shell framework,
whose crystal structures have been determined previously.6
The remarkable durability of 2 toward acidic solution is
worthy of note. The NMR spectrum of 2 hardly changed upon
addition of excess nitric acid (480 equiv) to a DMSO solution of
2 (Figure 3a). In contrast, the Pd(II) analogue could not tolerate
acidic conditions and immediately decomplexed into a unassign-
able mixture of oligomers because of the labile nature of the
Pd(II)Àpyridine coordination (Figure 3b).
Figure 3. 1H NMR spectra (500 MHz, DMSO-d6, 300 K) of (a) Pt
complex 2 and (b) its Pd analogue in DMSO upon the addition of excess
DNO3.
into a single product was observed after 24 h (Figure 2b,c). The
considerable broadening of the signals was attributed to the
tumbling motion of a giant product that was slow on the NMR
time scale.7 The diffusion-ordered spectroscopy (DOSY) NMR
spectrum also indicated the formation of a single product with a
diffusion coefficient of 6.31 Â 10À11 m2/s, which is comparable
to that of the Pd(II)12L24 sphere formed from Pd(II) ions and
ligand 1.8 This structural convergence was not observed without
TFE: in DMSO only, the NMR spectrum of an initially formed,
unassignable mixture remained unchanged over 24 h (Figure 2e,f).
Addition of another protic solvent such as EtOH or acetic acid
was ineffective.
When the extended ligand 3 was employed, we unexpectedly
observed the self-assembly of Pt(II)6L12 cube 4 rather than a
Pt(II)12L24 sphere analogous to 2, though the same ligand gave
the the Pd(II)12L24 sphere (5) quantitatively (Scheme 2).8,10
The formation of cube 4 was confirmed by CSI-TOF-MS and 1D
and DOSY NMR spectroscopy (see the Supporting In-
formation). After counterion exchange with SbF6À, CSI-TOF-
MS revealed a series of [M À n(SbF6À)]n+ peaks (n = 6À10) for
4 (Figure 4). All of the m/z values of these peaks agreed with the
formulas of 4 (SbF6À salt) (e.g, m/z calcd for [M À 9(SbF6À)]9+,
621.7547; found, 621.7552]. High acid durability (toward
DNO3) was again observed for cube 4 (Figure S5 in the
Supporting Information).
The Pt(II)12L24 composition of 2 (SbF6À salt) was confirmed
by cold-spray ionization time-of-flight mass spectrometry (CSI-
TOF-MS)9 from a series of [M À n(SbF6À)]n+ peaks (n =
10À15), each of which was further resolved into an isotopic
distribution pattern consistent with theoretical simulations (see
the Supporting Information). The rigid shell framework of
Pt(II)12L24 was revealed by X-ray crystallographic analysis
(Figure 1). Single crystals were obtained by slow diffusion of
isopropyl acetate vapor into a DMSO solution of 2 (SbF6À salt).
Because of the severe disorder of solvent molecules and coun-
terions in the large void of the crystals, the diffraction with a
conventional X-ray diffractometer was too weak to be solved.
However, high-flux, low-divergence synchrotron X-ray irradiation
We assume that the M6L12 cube structure11 exists as a
metastable local-minimum structure on the potential surface
for the self-assembly process. Even in the presence of TFE, the
Pt(II)Àpyridine interaction is much stronger than the Pd-
(II)Àpyridine interaction. Thus, the Pt(II)6L12 cube is trapped
at the local minimum and not further converted into the
Pt(II)12L24 structure, whereas the Pd(II) counterpart is not deeply
13318
dx.doi.org/10.1021/ja2059236 |J. Am. Chem. Soc. 2011, 133, 13317–13319

Journal of the American Chemical Society
COMMUNICATION
trapped in the cube structure and smoothly slips down to
the minimum on the potential surface corresponding to the
Pd(II)12L24 structure 5.
In summary, we have demonstrated the self-assembly of giant
multicomponent Pt(II)nL2n complexes via the temporary labili-
zation of Pt(II)Àpyridine bonds in TFE. This conventional
method offers practical applications for the construction of a
variety of Pt(II)-linked coordination assemblies that efficiently
self-assemble in TFE but are frozen after simple removal of the
volatile TFE solvent.
’ ASSOCIATED CONTENT
S
Supporting Information. Synthesis, characterization
b
data of the compounds, crystallographic data (CIF), and addi-
tional figures. This material is available free of charge via the
Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
mfujita@appchem.t.u-tokyo.ac.jp
’ ACKNOWLEDGMENT
This work was supported by KAKENHI of MEXT. The
synchrotron X-ray crystallography experiments were performed
at SPring-8 and KEK. We thank Prof. Kari Rissanen for helpful
discussions on crystallography.
’ REFERENCES
(1) Fujita, M.; Ibukuro, F.; Yamaguchi, K.; Ogura, K. J. Am. Chem.
Soc. 1995, 117, 4175–4176.
(2) (a) Yamashita, K.; Kawano, M.; Fujita, M. J. Am. Chem. Soc. 2007,
129, 1850–1851. (b) Yamashita, K.; Sato, K.; Kawano, M.; Fujita, M.
New J. Chem. 2009, 33, 264–270.
(3) (a) Olenyuk, B.; Whiteford, J. A.; Fechtenkotter, A.; Stang, P. J.
Nature 1999, 398, 796–799. (b) Olenyuk, B.; Levin, M. D.; Whiteford,
J. A.; Shield, J. E.; Stang, P. J. J. Am. Chem. Soc. 1999, 121, 10434–10435.
(4) Middleton, W. J.; Lindsey, R. V., Jr. J. Am. Chem. Soc. 1964,
86, 4948–4952.
(5) Kivinen, A.; Murto, J.; Liljequist, S.; Vaara, S. Acta Chem. Scand.,
Ser. A 1975, 29, 911–918.
(6) Tominaga, M.; Suzuki, K.; Kawano, M.; Kusukawa, T.; Ozeki, T.;
Sakamoto, S.; Yamaguchi, K.; Fujita, M. Angew. Chem., Int. Ed. 2004,
43, 5621–5625.
(7) The 1H NMR signals of the Pd sphere were much sharper than
those of 2, indicating that the local motions of the ligands in the Pd
sphere are faster because of the weaker coordination field of the Pd(II)
centers.
(8) Sato, S.; Iida, J.; Suzuki, K.; Kawano, M.; Ozeki, T.; Fujita, M.
Science 2006, 313, 1273–1276.
(9) (a) Sakamoto, S.; Fujita, M.; Kim, K.; Yamaguchi, K. Tetrahedron
2000, 56, 955–964. (b) Yamaguchi, K. J. Mass Spectrom. 2003,
38, 473–490.
(10) Tominaga, M.; Suzuki, K.; Murase, T.; Fujita, M. J. Am. Chem.
Soc. 2005, 127, 11950–11951.
(11) Suzuki, K.; Tominaga, M.; Kawano, M.; Fujita, M. Chem.
Commun. 2009, 1638–1640.
13319
dx.doi.org/10.1021/ja2059236 |J. Am. Chem. Soc. 2011, 133, 13317–13319