Molecular architecture via coordination: Self-assembly of nanoscale hexagonal metallodendrimers with designed building blocks

Article abstract of DOI:10.1021/ja063377z

The first self-assembly of nanoscale metallodendrimers that have a hexagonal cavity as a core via the directional-bonding approach is reported. All metallodendrimers were characterized by multinuclear NMR (¹H and ³¹P), mass spectrome

Full text of DOI:10.1021/ja063377z

Published on Web 07/18/2006
Molecular Architecture via Coordination: Self-Assembly of Nanoscale
Hexagonal Metallodendrimers with Designed Building Blocks
Hai-Bo Yang,*,† Neeladri Das, Feihe Huang, Adam M. Hawkridge, David C. Muddiman, and
†
†
‡
‡
,
†
Peter J. Stang*
Department of Chemistry, UniVersity of Utah, 315 South 1400 East, RM 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
Received May 15, 2006; E-mail: stang@chem.utah.edu
Dendrimers that have a regularly branched three-dimensional
architecture have emerged as one of the most striking topics within
supramolecular chemistry since the mid-1980s. Recently, self-
Scheme 1. Synthesis of [G0]-[G3] 120° Dendritic Linkers
1
assembly of dendrimers to provide well-defined nanoscale archi-
tectures has been investigated by employing electrostatic, hydrogen-
bonding, metal-ligating, and other noncovalent interactions.2
Among various types of self-assembled dendrimers, metalloden-
drimers are quite attractive due to their potential applications in
,3
4
5
6
catalysis, chiroptical sensing, biological function mimicking, and
light harvesting.7
Recently, cavity-cored dendrimers have received considerable
attention because of their elaborate structures and potential ap-
8
plications in delivery and recognition. For instance, molecular
hexagons functionalized with Percec-like first generation dendrons9
and cavity-cored dendrimers formed via metal nanoparticle (MNP)-
cored strategy8 have been reported. However, until now there are
few examples of metallodendrimers that possess cavities with
f
8d,e
predesigned shape as their core. Most organometallic dendrimers
reported previously employ metal atoms, organic skeletons, or metal
nanoparticles as a core.3
b,8a
phenol to give 2 (Scheme 1). 3 was made by palladium-mediated
coupling of 2 with 4-ethynyl pyridine in reasonable yield (66%).
After ester hydrolysis and etherification, the [G0]-[G3] 120°
precursors 5a-d substituted with Fr e´ chet-type dendrons were
obtained.11
Heating the [G0]-[G3] 120° donors 5a-d with an equimolar
amount of the 120° di-platinum acceptor 6 in a 10:1 (v/v) acetone-
Figure 1. Graphical representation of the self-assembly of hexagonal
metallodendrimers.
d
6
/D
2
O mixture overnight resulted in the [3+3] hexagonal metal-
lodendrimers in excellent yields, respectively (Scheme 2). Multi-
nuclear NMR ( H and P) analysis of [G0]-[G3] assemblies 7a-d
1
31
Highly symmetric supramolecular polygons have been success-
fully generated via the directional-bonding approach.10 Metallo-
dendrimers having a cavity with predetermined shape, geometry,
and symmetry as a core can be synthesized efficiently by the proper
choice of a dendritic donor precursor or Pt-containing acceptor
subunit with predefined angles and symmetry. Herein, we report
the first self-assembly of nanoscale metallodendrimers that have a
hexagonal cavity as a core using the aforementioned strategy (Figure
exhibited very similar characteristics, which suggested the formation
31
1
of discrete, highly symmetric species. The P { H} NMR spectra
of [G0]-[G3] assemblies displayed a sharp singlet (ca. 18.5 ppm)
shifted upfield from the starting platinum acceptor 6 by ap-
proximately 5.5 ppm. This change, as well as the decrease in
195
coupling of the flanking Pt satellites (ca. ∆J ) -195 Hz), is
1
consistent with back-donation from the platinum atoms. In the H
NMR spectrum of each assembly, the hydrogen atoms of the
pyridine rings exhibited small shifts downfield (R-H, 0.34-0.41
ppm; â-H, 0.43-0.50 ppm) due to the loss of electron density that
occurs upon coordination of the pyridine-N atom with the Pt(II)
1). These novel hexagonal metallodendrimers containing a void
space have been prepared in excellent yield (>95%) by the
combination of 120° dendritic donor subunits (substituted with
Fr e´ chet-type dendrons) and 120° di-Pt(II) acceptor angular linkers
in 1:1 stoichiometric ratio.
The synthesis of [G0]-[G3] 120° donor building blocks 5a-d
commenced with acylation of commercially available 2,3-dibromo-
31
1
metal center. The sharp NMR signals in both the P and H NMR
along with the solubility of these species ruled out the formation
of oligomers.
The structures of the hexagonal metallodendrimers have also been
confirmed by ESI mass spectrometry and elemental analysis. In
the ESI mass spectra of [G0] and [G1] assemblies 7a and 7b, peaks
†
University of Utah.
North Carolina State University.
‡
10014
9
J. AM. CHEM. SOC. 2006, 128, 10014-10015
10.1021/ja063377z CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
Scheme 2. Self-Assembly of [G0]-[G3] 120° Dendritic Linkers
a-d with Di-platinum Acceptor 6
hexagonal cavity with an internal radius of approximately 1.6 nm
as the core, whereas the outer radius is about 2.8 nm.
5
The coordination-driven self-assembly described here demon-
strates the ready formation of discrete nanoscopic metallodendrimers
by simple combination of predesigned dendritic donor and di-Pt-
(II) acceptor building blocks, which enriches the library of
metallodendrimers. This strategy provides rapid access to supramo-
lecular metallodendrimers with a hexagonal cavity as a core that
may have potential applications in host-guest chemistry. We are
currently extending this idea to other two-dimensional structures
such as rhomboids, squares, and triangles, and even three-
dimensional architectures such as trigonal prisms and trigonal
bipyramids.
Acknowledgment. P.J.S. thanks the NIH (GM-57052) and the
]2+ (m/z
3
NSF (CHE-0306720) for financial support. We also thank the NSF
attributable to the loss of nitrate counterions, [M - 2NO
2267.7 7a, m/z ) 2586.3 7b), where M represents the intact
(CHE-9708413) and the University of Utah Institutional Funds
)
Committee for funding the Micromass Quattro II mass spectrometer.
D.C.M. and A.M.H. thank the W. M. Keck Foundation and NCSU
for generous financial support.
assemblies, were observed. These peaks were isotopically resolved
and they agree very well with the theoretical distribution. The ESI
mass spectrum of [G2] assembly 7c showed two charged states at
4+
m/z ) 1580.6 and 1252.0, corresponding to the [M - 4NO
3
]
Supporting Information Available: General detailed experimental
procedures and characterizations data for all metallodendrimers, ESI/
MS of metallodendrimers 7a-d, and ESI-FTI-ICR/MS of 7d. This
material is available free of charge via the Internet at http://pubs.acs.org.
5+
and [M - 5NO
3
]
species, respectively, and their isotopic
resolution is in good agreement with the theoretical distribution.
5+
3
The isotopic resolution of the peak at m/z ) 1761.5 [M - 5NO ]
in ESI-FT-ICR mass spectrum of [G3] assembly 7d is in excellent
agreement with the theoretical distribution (Figure 2).
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