Synthesis of six-component metallodendrimers via [3 + 3] coordination-driven self-assembly

Article abstract of DOI:10.1021/jo900067v

A new class of 120° dendritic di-Pt(II) acceptor subunits has been designed and synthesized, from which six-component hexagonal metallodendrimers were easily formed with 120° dendritic dipyridine donors via [3 + 3] coordination-driven self-assembly. The s

Full text of DOI:10.1021/jo900067v

importantly, the incorporation of metals endows desirable
properties to the resulting metallodendrimers. This new family
of dendrimers has been extensively explored as potential
functional materials in catalysis,4 as biological mimetics,5 and
in photo- and electrochemistry.6 Recently, dendritic metallo-
cycles have received considerable attention because of their
potential application in the recognition and delivery of guests
incorporated within their cavities.7 However, the precise control
of the size and the shape of the metallodendrimers remains a
challenge.
Synthesis of Six-Component Metallodendrimers
via [3 + 3] Coordination-Driven Self-Assembly
Hai-Bo Yang,*,† Brian H. Northrop,‡ Yao-Rong Zheng,‡
Koushik Ghosh,‡ Matthew M. Lyndon,§
David C. Muddiman,§ and Peter J. Stang*,‡
Shanghai Key Laboratory of Green Chemistry and Chemical
Processes, Department of Chemistry, East China Normal
UniVersity, 3663 North Zhongshan Road, Shanghai, China
200062, Department of Chemistry, UniVersity of Utah, 315
South 1400 East, Room 2020, Salt Lake City, Utah 84112,
and W. M. Keck FT-ICR Mass Spectrometry Laboratory and
Department of Chemistry, North Carolina State UniVersity,
Raleigh, North Carolina 27695
Coordination-driven self-assembly8 has been demonstrated
to be a powerful and facile approach to the construction of
polygons and polyhedra9 with well-designed shapes, sizes, and
symmetry. By employing such a strategy, we have previously
constructed a series of functionalized metallacycles10 including
well-defined metallodendrimers11 with various sizes and shapes.
For example, by combining predesigned 120° angular dendritic
hbyang@chem.ecnu.edu.cn; stang@chem.utah.edu
ReceiVed January 12, 2009
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A new class of 120° dendritic di-Pt(II) acceptor subunits has
been designed and synthesized, from which six-component
hexagonal metallodendrimers were easily formed with 120°
dendritic dipyridine donors via [3 + 3] coordination-driven
self-assembly. The structures of all metallodendrimers are
confirmed by multinuclear NMR, ESI-TOF-MS/ESI-FTMS,
and elemental analysis. MMFF force-field simulations indi-
cates that all metallodendrimers have a hexagonal ring with
an internal radius of approximately 1.4 nm.
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† East China Normal University.
‡ University of Utah.
§ North Carolina State University.
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3524 J. Org. Chem. 2009, 74, 3524–3527
10.1021/jo900067v CCC: $40.75 2009 American Chemical Society
Published on Web 04/03/2009

organic donors with 180° di-Pt(II) acceptors, “snowflake-
shaped” [6 + 6] metallodendrimers11b were prepared via
coordination-driven self-assembly. More recently, a new family
of metallodendritic squares has been synthesized from 4,4′-
bipyridines funtionalized with Fre´chet-type dendrons and (dp-
pp)Pt(II) or Pd(II) triflate by Schalley et al.12
According to the “symmetry interaction” model,8a,e 120°
anglular building units are required for hexagonal assemblies
whereas 108° anglar units lead to the production of pentagonal
structures. As supramolecular structures increase in size they
also tend toward increasing flexibility. For large, complex
supramolecular hexagons, small distortions away from the ideal
bond angles of the subunits can occur and make up for the
necessary 12° difference per corner between a hexagon and a
pentagon. Although no evidence for any other species, such as
pentagonal assemblies, was found in multinuclear NMR or mass
spectroscopy studies during the formation of [6 + 6] hexagonal
metallodendrimers,11b it is necessary to find alternative means
to prepare six-component hexagonal metallodendrimers that rule
out the possibility of forming pentagonal structures entirely.
In general, the shape of an individual two-dimensional
polygon is determined by the value of the turning angle within
its angular components. According to the “directional bonding”
and “symmetry interaction” models,8a,e discrete hexagonal
entities can be self-assembled via the combination of two
complementary ditopic building blocks A2 and X2, each
incorporating 120° angles between their coordination sites,
allowing for the formation of hexagonal structures of type
A2 X2 .13 This strategy, in contrast to the use of six 120° angular
FIGURE 1. Schematic and molecular structure of 120° dendritic donor
subunit 1 and 120° dendritic acceptor subunit 2.
SCHEME 1. Synthesis of 120° Dendritic Di-Pt(II) Acceptor
Subunits 2a-c
3
3
units and six 180° linear units that results in larger and more
flexible hexagonal assemblies, is able to rule out the possibility
of the formation of alternative, nonhexagonal species. Herein,
we report the synthesis, via [3 + 3] coordination-driven self-
assembly, of six-component hexagonal metallodendrimers from
the newly designed 120° di-Pt(II) acceptor 1 and 120° dipyridine
donor 2 (Figure 1), both of which are substituted with Fre´chet-
type dendrons.
The 120° dendritic di-Pt(II) acceptors 2a-c can be easily
synthesized as shown in Scheme 1. The Fre´chet-type dendrons
([G-1]-[G-3]) were introduced by a coupling reaction of 3 with
the different generation dendritic carboxylic acids.14 Compounds
4a-c were then reacted with 4 equiv of trans-PtI2(PEt3)2 to
give diiodo metal complexes 5a-c. Subsequent halogen ab-
straction with AgOTf resulted in the isolation of bistriflate salts
2a-c in reasonable yield. The 31P{1H} NMR spectra of
diplatinum acceptors 2a-c displayed singlets at 22.19, 22.65,
and 22.56 ppm, respectively, each accompanied by flanking 195Pt
satellites.
Single crystals of [G-1] di-Pt(II) diiodide complex 5a, suitable
for X-ray diffraction studies, were grown by slow evaporation
of a dichloromethane/methanol solution (1/1) at ambient tem-
perature for 2-3 days. An ORTEP representation of the
structure of 5a (Figure 2) shows that it is indeed a suitable
candidate for a 120° building unit, with the angle between the
two platinum coordination planes being approximately 124°.
The distance between the two Pt centers in 5a is ∼1.0 nm.
When [G-1]-[G-3] 120° dendritic donor subunits 1a-c were
reacted with 120° dendritic di-Pt(II) acceptors 2a-c in CD2Cl2
at room temperature, the [3 + 3] six-component hexagonal
(10) (a) Northrop, B. H.; Yang, H.-B.; Stang, P. J. Chem. Commun. 2008,
5896–5908. (b) Yang, H.-B.; Ghosh, K.; Northrop, B. H.; Zheng, Y.-R.; Lyndon,
M. M.; Muddiman, D. C.; Stang, P. J. J. Am. Chem. Soc. 2007, 129, 14187–
14189. (c) Yang, H.-B.; Ghosh, K.; Zhao, Y.; Northrop, B. H.; Lyndon, M. M.;
Muddiman, D. C.; White, H. S.; Stang, P. J. J. Am. Chem. Soc. 2008, 130, 839–
841. (d) Ghosh, K.; Yang, H.-B.; Northrop, B. H.; Lyndon, M. M.; Zheng, Y.-
R.; Muddiman, D. C.; Stang, P. J. J. Am. Chem. Soc. 2008, 130, 5320–5334. (e)
Northrop, B. H.; Glo¨ckner, A.; Stang, P. J. J. Org. Chem. 2008, 73, 1787–1794.
(f) Ghosh, K.; Zhao, Y.; Yang, H.-B.; Northrop, B. H.; White, H. S.; Stang,
P. J. J. Org. Chem. 2008, 73, 8553–8557.
(11) (a) Yang, H.-B.; Das, N.; Huang, F.; Hawkridge, A. M.; Muddiman,
D. C.; Stang, P. J. J. Am. Chem. Soc. 2006, 128, 10014–10015. (b) Yang, H.-
B.; Hawkridge, A. M.; Huang, S. D.; Das, N.; Bunge, S. D.; Muddiman, D. C.;
Stang, P. J. J. Am. Chem. Soc. 2007, 129, 2120–2129.
(12) Baytekin, H. T.; Sahre, M.; Rang, A.; Engeser, M.; Schulz, A.; Schalley,
C. A. Small 2008, 4, 1823–1834.
(13) Leininger, S.; Schmitz, M.; Stang, P. J. Org. Lett. 1999, 1, 1921–1923.
(14) Xie, D.; Jiang, M.; Zhang, G.; Chen, D. Chem.sEur. J. 2007, 13, 3346–
3353.
FIGURE 2. ORTEP diagram of 120° [G-1] di-Pt(II) diiodide complex
5a. Thermal ellipsoids are drawn to 30% probability.
J. Org. Chem. Vol. 74, No. 9, 2009 3525

FIGURE 3. Calculated (top) and experimental (bottom) ESI-TOF-MS spectra of metallodendrimers 6a (A and B) and 6b (C and D) and ESI-
FTMS spectra of metallodendrimer 6c (E).
SCHEME 2. Self-Assembly of Six-Component Hexagonal
Metallodendrimers via [3 + 3] Coordination-Driven
Self-Assembly
All results of mass studies of assemblies 6a-c have provided
strong support for the formation of hexagonal metallodendrim-
ers. In the ESI-TOF-MS spectra of [G-1] and [G-2] assemblies
6a and 6b, peaks at m/z ) 1513.0 and 1180.6 for 6a and m/z )
2149.7 and 1690.0 for 6b, corresponding to the charge states
[M - 4OTf]4+ and [M - 5OTf]5+, respectively, were observed,
and their isotopic resolutions are in excellent agreement with
the theoretical distributions (Figure 3A-D). Given the signifi-
cantly larger molecular weight (14278.7 Da) of the [G-3]
hexagonal metallodendrimer, it is more difficult to get strong
mass signals even under the ESI-TOF-MS conditions. With
considerable effort, a peak at m/z ) 2708.8 was observed in
the ESI-FTMS spectrum of [G-3]-assembly 6c, which corre-
sponds to the [M - 5OTf]5+ charge state. The peak was
isotopically resolved (Figure 3E) and agrees well with the
theoretical distribution, although overlap with the signal of a
minor unknown fragment, which could not be attributed to any
alternative polygonal supramolecules, was also observed.
metallodendrimers 6a-c, respectively, were formed (Scheme
2). 31P{1H} NMR analysis of the reaction mixtures is consistent
with the formation of single, highly symmetric species as
indicated by the appearance of a sharp singlet (ca. 16.2 ppm)
with concomitant 195Pt satellites, shifted upfield by ca. 6.4 ppm
as compared to 2a-c. This change, as well as the decrease in
coupling of the flanking 195Pt satellites (ca. ∆1JPPt ) -62 Hz
for 6a; ∆1JPPt ) -64 Hz for 6b; ∆1JPPt ) -67 Hz for 6c), is
consistent with back-donation from the platinum atom, lending
further support to the formation of the supramolecular com-
plexes. In the 1H NMR spectra of metallodendrimers 6a-c, the
ꢀ-H of the pyridine rings exhibited ca. 0.40-0.51 ppm down-
field shifts relative to uncoordinated 5a-d due to the loss of
electron density that occurs upon coordination of the pyridine-N
atom with the Pt(II) metal center. Upon stirring at 298 K for
72 h, the 31P{1H} and 1H NMR of assemblies 6a-c do not show
any significant change, demonstrating the stability of these novel
supramolecular assemblies. The sharp NMR signals in both the
31P{1H} and 1H NMR spectra (see the Supporting Information)
along with the solubility of these species ruled out the formation
of oligomers.
We have demonstrated that [3 + 3] hexagonal structures can
be self-assembled by combining three 120° pyridyl donor
building blocks and three 120° diplatinum acceptors.13 Further-
more, recent studies have indicated that the addition of functional
groups at the vertices of individual building units, such as
ferrocene and crown ether moieties, does not hinder the
formation of [3 + 3] self-assembled metallosupramolecules.13
Close examination of the ESI mass spectra of 6a-c revealed
no peaks indicating the formation or existence of [2 + 2]
rhomboidal or [4 + 4] octagonal structures. It should again be
noted that it is not possible to form polygons with an odd
number of sides (i.e., pentagon, heptagon, etc.) by combining
120° donors with 120° acceptors as they would require the direct
connection of either two acceptor or two donor moieties. The
lack of mass spectral peaks corresponding to other polygon
architectures and the singularity of each 31P NMR signal ensures
that only [3 + 3] hexagonal metallodendrimers are formed in
each self-assembly.
Mass-spectrometric studies of metallodendrimers 6a-d were
performed by ESI-TOF-MS and ESI-FTMS spectrometry, which
allows the assemblies to remain intact during the ionization
process in order to obtain the high resolution required for
unambiguous determination of individual charge states. The high
molecular weight and complex isotope splitting of such charged
multiplatinum species necessitates high-resolution mass spectral
analysis in order to determine their absolute molecular weight
and their molecularity, which reveals the absolute stoichiometry
of constituent subunits and can definitively rule out the
possibility of supramolecular pentagon assembly.
Large supramolecular coordination-compounds and flexible,
high-generation dendrimers often prove difficult to crystallize.
Likewise, all attempts to grow X-ray-quality single crystals of
hexagonal metallodendrimers 6a-c have proven to be unsuc-
cessful to date. MMFF force-field simulations were employed
to optimize the geometry of all metallodendrimers 6a-c (For
[G-1] and [G-2], see the Supporting Information). Simple space-
filling models of the simulated structures indicate that each of
6a-c have a very similar, roughly planar hexagonal ring at their
3526 J. Org. Chem. Vol. 74, No. 9, 2009

Experimental Section
General Procedure for the Preparation of Six-Component
Hexagonal Metallodendrimers 6a-c. To a 0.5 mL dichlo-
romethane-d2 solution of triflate 2a-c (for 2a, 5.40 mg, 0.00334
mmol; for 2b, 5.95 mg, 0.00291 mmol; for 2c, 5.70 mg, 0.00197
mmol) was added a 0.5 mL dichloromethane-d2 solution of the
appropriate [G-1]-[G-3] dendritic donor precursors 1a-c (for 1a,
2.0 mg, 0.00334 mmol; for 1b, 2.98 mg, 0.00291 mmol; for 1c,
3.69 mg, 0.00197 mmol) drop by drop with continuous stirring (10
min). The reaction mixture was stirred for 1 h at room temperature.
The solution was evaporated to dryness, and the product was
collected.
6a. Yield: 7.10 mg (pale yellow solid), 96%. 1H NMR (CD2Cl2,
300 MHz): δ 8.63 (d, J ) 6.3 Hz, 12H), 7.81 (d, J ) 6.3 Hz,
12H), 7.59 (s, 3H), 7.31-7.44 (m, 72H), 7.08 (s, 3H), 6.95 (s, 6H),
6.91 (s, 3H), 6.70 (s, 6H), 6.60 (s, 3H), 5.13 (s, 12H), 5.11 (s, 6H),
5.06 (s, 12H), 1.80-1.83 (m, 72H), 1.13-1.24 (m, 108H). 31P{1H}
NMR (CD2Cl2, 121.4 MHz): δ 16.12 (s,1JPt-P ) 2299.8 Hz). Anal.
Calcd for C294H330F18N6O39P12 Pt6S6: C, 53.11; H, 5.00; N, 1.26.
Found: C, 53.45; H, 5.23; N, 1.52.
FIGURE 4. Simulated molecular model of [G-3] six-component
hexagonal metallodendrimer 6c (C ) gray, O ) red, N ) blue, P )
purple, Pt ) yellow; hydrogen atoms have been removed for clarity).
6b. Yield: 8.42 mg (pale yellow solid), 97%. 1H NMR (CD2Cl2,
300 MHz): δ 8.61 (d, J ) 6.3 Hz, 12H), 7.80 (d, J ) 6.3 Hz,
12H), 7.60 (s, 3H), 7.35-7.43 (m, 132H), 7.07 (s, 3H), 6.95 (s,
6H), 6.88 (s, 3H), 6.69 (s, 30H), 6.58 (s, 15H), 5.01-5.10 (m, 78H),
1.79-1.81 (m, 72H), 1.12-1.23 (m, 108H). 31P{1H} NMR (CD2Cl2,
121.4 MHz): δ 16.18 (s,1JPt-P ) 2310.1 Hz). Anal. Calcd for
core surrounded by flexible dendrons. In the case of 6c, the
hexagonal ring-shaped metallodendrimer has an internal radius
of approximately 1.4 nm while the outer dendron radius averages
3.5 nm (Figure 4).
C462H474F18N6O63P12Pt6S6: C, 60.35; H, 5.20; N, 0.91. Found: C,
60.18; H, 5.46; N, 0.98.
1
6c. Yield: 8.83 mg (pale yellow glassy solid), 94%. H NMR
(CD2Cl2, 300 MHz): δ 8.58 (s, 12H), 7.78 (d, J ) 6.3 Hz, 12H),
7.61 (s, 3H), 7.32-7.40 (m, 252H), 7.06 (s, 3H), 6.94 (s, 6H), 6.87
(s, 3H), 6.69 (br, 78H), 6.58 (br, 39H), 4.97-5.00 (m, 174H),
1.79-1.81 (m, 72H), 1.12-1.23 (m, 108H). 31P{1H} NMR (CD2Cl2,
121.4 MHz): δ 16.15 (s,1JPt-P ) 2305.8 Hz). Anal. Calcd for
In summary, we have designed and synthesized a new class
of 120° dendritic di-Pt(II) acceptor subunits, from which six-
component hexagonal metallodendrimers can be easily formed
via [3 + 3] coordination-driven self-assembly, thus enriching
the library of dendritic metallocycles. More importantly,
compared to previous reports of the synthesis of six-component
hexagonal metallodendrimers, this study provides another highly
efficient approach to the construction of such species that
unambiguously avoids the possible formation of other polygons
caused by small distortions of the subunit bond angles. All
metallodendrimers are characterized with multinuclear NMR,
ESI-TOF-MS/ESI-FTMS, and elemental analysis. Their struc-
tural properties have been studied using MMFF force-field
simulations, which indicate that all metallodendrimers have
hexagonal rings with internal radii of approximately 1.4 nm.
This research has once again proven the versatility and modular-
ity of the directional bonding approach to self-assembly, which
surely will be employed to the synthesis of other functional
dendritic metallocycles in the future.
C798H762F18N6O111P12Pt6S6: C, 67.08; H, 5.38; N, 0.59. Found: C,
66.78; H, 5.51; N, 0.61.
Acknowledgment. P.J.S. thanks the NIH (Grant No. GM-
057052) for financial support. B.H.N. thanks the NIH (Grant
No. GM-080820) for financial support. D.C.M. and M.M.L.
thank NCBC and NCSU for generous financial support.
Supporting Information Available: Experimental proce-
dures, characterization data, copies of spectra for compounds
(2a-c, 4a-c, 5a-c, and 6a-c), simulated molecular model of
6a and 6b, and crystallographic file (CIF) of 5a. This material
is available free of charge via the Internet at http://pubs.acs.org.
JO900067V
J. Org. Chem. Vol. 74, No. 9, 2009 3527