Construction of endo-functionalized two dimensional metallacycles via coordination-driven self-assembly

Article abstract of DOI:10.1021/jo9019607

(Figure Presented) The synthesis of three endofunctionalized two-dimensional supramolecular metallacycles including two [2+2] rhomboids (5 and 6) and a [3+3] hexagon (7) is reported. The resulting self-assembled supramolecular structures, containing sever

Full text of DOI:10.1021/jo9019607

pubs.acs.org/joc
Construction of Endo-Functionalized Two Dimensional Metallacycles via
Coordination-Driven Self-Assembly
Liang Zhao,* Koushik Ghosh, Yao-Rong Zheng, and Peter J. Stang*
Department of Chemistry, University of Utah, 315 South 1400 East, Room 2020, Salt Lake City, Utah 84112
stang@chem.utah.edu
Received September 11, 2009
The synthesis of three endofunctionalized two-dimensional supramolecular metallacycles including
two [2 + 2] rhomboids (5 and 6) and a [3 + 3] hexagon (7) is reported. The resulting self-assembled
supramolecular structures, containing several nitrobenzyl moieties at their interior surface, have been
1
fully characterized by multinuclear NMR (31P and H) and electrospray ionization mass spectro-
metry. A significant C-H O hydrogen bonding between the nitrobenzyl acceptor and the edge
3 3 3
molecules of the supramolecular architecture is observed in the small rhomboid 5 and this interaction
gradually decreases upon the enlargement of the resulting polygonal structures from a small
rhomboid 5 through a large rhomboid 6 to a hexagon 7. Molecular modeling with the MMFF force
field gives a possible conformation of each self-assembly in different solvents and shows that the
hydrophilic nitrobenzyl moiety prefers to be buried in the cavity of the resulting polygonal structures
in nonpolar solvents, thus forming hydrogen bonds with the peripheral component building units.
Introduction
has proven to be a powerful tool in the synthesis of well-defined
multimetallic architectures with increasing structural complex-
ity based on metal-ligand interactions.2 These architectures
were spontaneously generated by simply mixing appropriately
designed complementary building units (acceptor and donor) in
a suitable solvent. The shape of the donor and acceptor building
blocks is dominated by the turning angle formed between the
bonding sites of the individual components. With the further
introduction of various functional moieties on the tectons,
a wealth of decorated supramolecular polygons and poly-
hedra can be potentially employed as precursors of electronic,3
Metallosupramolecular chemistry has been a remarkable re-
search area in the realm of supramolecular chemistry since the
1980s.1 Over the past few decades, metal-directed self-assembly
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8516 J. Org. Chem. 2009, 74, 8516–8521
Published on Web 10/16/2009
DOI: 10.1021/jo9019607
r
2009 American Chemical Society

Zhao et al.
JOCFeatured Article
catalytic,2i,4 and photophysical materials5 and used for mole-
cular recognition and encapsulation.2g,6
azobenzene units,16 perfluoroalkyl chains,17 and polymeriz-
able methyl methacrylate units.18 However, the synthesis of
endofunctionalized two-dimensional metallacyclic complexes
has not been reported up to now. Herein we report the
synthesis of two [2 + 2] rhomboids and a hexagon with endo-
functionalized architectures via coordination-driven self-as-
sembly between di-Pt(II) acceptors and an endofunctionalized
bipyridyl donor ligand.
To date, three methods have been successfully employed to
incorporate functionalities into supramolecular metal-organ-
ic assemblies. First, the functional moieties can be incorpo-
rated into the edge or corner of a building block. In this way,
numerous functional moieties including porphyrin,7 diaza-
crown ether,8 cavitand,9 carborane,10 and chiral metallocor-
ners7b,11 have been introduced into the supramolecular archi-
tectures. Furthermore, covalently grafting a functional moiety
on the exterior surface of an angle-directed building block has
resulted in a large number of discrete supramolecular metal-
organic assemblies peripherally functionalized with dendri-
mers,12 ferrocene,3d,13 crown ethers, and pseudorotaxanes.14
Lastly, the interior surface of self-assembled suprastructures
can also be decorated via covalent attachment of a functional
moiety on the concave side of a directional building block.
Fujita and co-workers have synthesized a variety of three-
dimensional endofunctionalized M12L24 cuboctahedra, which
are interiorly decorated by oligo(ethylene oxide) chains,15
Results and Discussion
Synthesis of 120° Endo-Functionalized Donor Ligand. As
shown in Scheme 1, the new endofunctionalized 120° donor
ligand was synthesized by use of 4-hydroxy-3,5-diiodobenzoic
acid as the starting material, which was protected as an ester
and subsequently reacted with 4-nitrobenzyl bromide to pro-
duce the endofunctionalized diiodo complex. Sonogashira
coupling of this diiodo complex with 4-ethynylpyridine in the
presence of catalytic Pd(PPh3)2Cl2 afforded the desired endo-
functionalized 120° donor ligand 1 in a reasonable yield (63%).
Self-Assembly and NMR Studies. The endofunctionalized
polygonal structures (5-7) were prepared by use of two
different 60° phenanthrene (2 and 3) and a 120° ketone (4)
di-Pt(II) ligand as acceptors and donor ligand 1 (Scheme 2).
The small self-assembled [2 + 2] rhomboid 5 was made by
mixing the donor ligand 1 with acceptor 2 in a 1:1 ratio in
CD2Cl2. The 31P{1H} NMR spectrum of 5 showed a single
peak at 12.58 ppm with concomitant 195Pt satellites, upfield
shifted by roughly 6.5 ppm compared with the 60° phenan-
threne acceptor ligand 2 (δ = 19.09 ppm) as a result of the co-
ordination of the pyridine rings (Figure 1). This indicates that
only one discrete supramolecular structure exists in the result-
ing solution. In the proton NMR spectrum of 5, the R- and β-
pyridyl hydrogen signals both experience significant downfield
shifts compared with their chemical shifts in the precursor
building block 1, which are associated with the loss of electron
density upon coordination by the nitrogen lone pair to the
platinum metal centers (Figure 2). It is notable that the doublet
peak at δ=8.65 ppm corresponding to the R-protons of the
pyridyl ring in 1 is split into a pair of doublets (R0 and R00) upon
self-assembly. This is due to restricted rotation around the
Pt-N coordination bond analogous to previous observations
and reports.19 On the other hand, the rather large chemical shift
difference between the two pyridine R-protons (Δδ=0.8 ppm)
in 5 is much larger than the differences (approximately 0.2 ppm)
in several previously reported rhomboid, triangle, and rectan-
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3 3 3
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SCHEME 1. Synthesis of 120° Endo-Functionalized Donor Ligand 1
SCHEME 2. Molecular Structures of Donor (Red) and Acceptor (Blue) Building Blocks and Their Self-Assembly into [2 + 2] Rhomboids
5 and 6 and [3 + 3] Hexagon 7
the proton NMR comparison between the nitrobenzyl moiety
in precursor ligand 1 and the product 5, wherein the NMR
signals attributed to the phenyl group of the nitrobenzyl moiety
undergo a downfield shift by 0.11-0.18 ppm, indicating the
loss of electron density upon hydrogen bonding.
D2O, indicating that the C-H O hydrogen bonding is
3 3 3
indeed weakened. The signal of the β-pyridyl hydrogen of 5
in CD2Cl2 is split into two doublets with a chemical shift
difference of 0.1 ppm (β0 and β00). This may arise from the
formation of hydrogen bonding between the β hydrogen atoms
of the pyridine ring and the nitro-D2O hydrogen bonding
cluster.
Furthermore, the addition of a small amount of D2O to
the CD2Cl2 solution of 5 (CD2Cl2/D2O=50:1) to replace the
C-H O hydrogen bonding with the stronger O-H
O
The electrospray ionization mass spectrum (ESI-MS) of 5
exhibited two charge states at m/z=3241.9 and 1039.4 cor-
responding to [M - NO3]+ and [M - 3NO3]3+ of the [2 + 2]
rhomboid, respectively, which are in good agreement with
their theoretical distributions (Figure 3a).
3 3 3
3 3 3
interaction between the water molecule and the nitrobenzyl
moiety. As shown in Figure 2d, the R-protons (δ = 9.53 ppm)
of the pyridine ring of self-assembled rhomboid 5 in CD2Cl2
experience a Δδ = 0.14 ppm upfield-shift upon the addition of
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FIGURE 1. Comparison of the 31P {1H} NMR spectra (in CD2Cl2) of the 60° di-Pt(II) acceptor 2 and the self-assembled [2 + 2] rhomboid 5.
FIGURE 2. Comparison of representative 1H NMR spectra of the aromatic portion of (a) the 60° di-Pt(II) acceptor 2 in CD2Cl2, (b) the 120°
donor ligand 1 in CD2Cl2, (c) the self-assembled [2 + 2] rhomboid 5 in CD2Cl2, and (d) 5 in CD2Cl2/D2O, displaying the characteristic shifts of
proton signals associated with the donor and acceptor units upon coordination.
To further investigate the interaction between the encap-
sulated endonitrobenzyl group and self-assembled struc-
tures, we next utilized the larger 60° building block 3 to
react with 1. Mixing 1 and 3 in a 1:1 ratio in a mixed solvent
of acetone-d6 and CD2Cl2 yielded a homogeneous pale
yellow solution of 6. The 31P{1H} NMR spectrum showed
the appearance of a doublet at δ=16.98 ppm with concomi-
tant 195Pt satellites (Figure 4a), likely due to the proximity of
the endofunctionalized nitrobenzyl group to a PEt3 group in
rhomboid 6 which results in a different environment for the
two phosphorus nuclei. The doublet corresponding to the
R-protons of the pyridyl ring in precursor ligand 1 is split into
a triplet upon the formation of the larger [2 + 2] rhomboid 6
(Figure 4b). Furthermore, four doublet signals correspond-
ing to the hydrogen atoms of the nitrobenzyl group, in
contrast to just two doublets in self-assembly 5, are observed
in the proton NMR spectrum of 6, indicating that this
nitrobenzyl group interacting with the peripheral edge of
the rhomboid structure leads to nonequivalent environments
for each of its hydrogen atoms. The formation of the [2+2]
rhomboid structure in 6 is further confirmed by electrospray
ionization mass spectrometry. As listed in Figure 3b, the
peaks resulting from the rhomboid self-assembly minus
- 2+
triflate anions are found: [6 - 2CF3SO3
and [6 - 3CF3SO3
isotopically resolved and in good agreement with their
]
(m/z 1725.3)
]
- 3+ (m/z 1100.4). All observed peaks are
corresponding theoretical distributions.
When a different 120° building block 4 is employed to
react with acceptor 1 in a 1:1 ratio in CD2Cl2, a [3 + 3] self-
assembled hexagon 7 is generated. Only one sharp 31P NMR
peak at 13.6 ppm is observed for 7 (Figure S6 in Supporting
Information), upfield shifted by almost 8.3 ppm, as com-
pared with the starting acceptor ligand 4 (δ=21.9 ppm), as a
result of the coordination of the pyridine moiety. In the
NMR spectrum of 7, the R- and β-proton NMR signals of the
pyridine ring each display a downfield-shifted doublet, in
contrast to a pair of doublets and a triplet in rhomboid
structures 5 and 6, respectively. Two discernible doublet
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FIGURE 3. Theoretical (top, red) and experimental (bottom, blue) ESI-MS results for self-assembled (a) [2 + 2] rhomboid 5, (b) [2 + 2]
rhomboid 6, and (c) [3 + 3] hexagon 7.
FIGURE 4. (a) 31P NMR and (b) partial 1H NMR spectra of self-assembled [2 + 2] rhomboid 6 in CD2Cl2.
signals for the nitrobenzyl moiety are observed as well,
showing that this moiety hardly interacts with the peripheral
building components upon the formation of the larger
[3 + 3] hexagon. Well-resolved peaks for the [3 + 3] hexagon
7 were collected by ESI-MS at m/z 2595.9 and 1223.4,
corresponding to [M - 2OTf]2+ and [M - 4OTf]4+, respec-
tively (Figure 3c). Their isotopic distributions are also in
good agreement with the theoretical distributions. These
mass spectral results, together with the multinuclear NMR
studies, confirm the self-assembly of the discrete supra-
molecular hexagon 7.
Molecular Force Field Modeling. Our attempts to crystal-
lize the above three polygonal structures have so far been
unsuccessful. We have therefore used molecular force field
simulations to investigate the structural details of the supra-
molecular architectures 5-7. In the self-assembly 5, model-
ing structures in different solvents display various
conformations. As shown in Figure 5, the nitrobenzyl groups
FIGURE 5. Computational models of [2 + 2] rhomboid 5 in
different solvents.
in the computed structures of 5 in the gas phase or in a
nonpolar solvent such as CHCl3 are along the edge like
cavities of the self-assembled rhomboid because of the
hydrophilic nature of the nitrobenzyl moiety. This confor-
between the R-hydrogen atoms of the pyridine ring and the
nitrobenzyl group. Likewise, the calculated structures of 6
and 7 in CHCl3 illustrate that the nitrobenzyl groups in each
self-assembly cling to the edge of the resulting supramole-
cular architectures (Figure S7).
In conclusion, we have successfully prepared three endo-
functionalized two-dimensional supramolecular architec-
tures by the self-assembly of a new 120° dipyridyl donor
ligand (1) with three different di-Pt(II) acceptors. The en-
capsulated nitrobenzyl groups are found to interact with the
mation leads to the favorable formation of C-H
O
3 3 3
hydrogen bonding in the [2 + 2] self-assembled rhomboid
5. In contrast, the calculated results of 5 in polar solvents
(octanol and water) show that the nitrobenzyl groups prefer
to protrude into the center of the rhomboid structure, thus
lowering the strength of the C-H O hydrogen bonding
3 3 3
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edge of the resulting polygons, which are characterized by
multinuclear NMR, ESI mass spectrometry, and molecular
force field modeling.
2 (5.82 mg, 5.00 μmol) were weighed accurately into a glass vial.
To the vial was added 0.7 mL of CD2Cl2 solvent, and the
reaction solution was then stirred at room temperature for 24
h to yield a homogeneous yellow solution. The solution was
transferred into the NMR tube to collect 1H and 31P NMR
spectra. Solid product was obtained by removing the solvent
Experimental Section
1
Methods and Materials. The organoplatinum acceptor li-
gands 2, 3, and 4 were synthesized according to the literature
methods.20a,21
under vacuum. Yield: 96%. H NMR (CD2Cl2, 300 MHz): δ
9.53 (d, 4H, J = 5.7 Hz), 8.92 (s, 4H), 8.72 (d, 4H, J = 5.7 Hz),
8.41 (s, 4H), 8.35 (d, 4H, J = 8.7 Hz), 7.92 (d, 4H, J = 8.7 Hz),
7.81-7.77 (t, 8H, J = 5.7 Hz), 7.60 (m, 12H), 5.85 (s, 4H), 3.99
(s, 6H), 1.38 (m, 48H), 1.10-1.20 (m, 72H). 31P {1H} NMR
Preparation of Dipyridine Ligand 1. To a solution of 4-hy-
droxy-3,5-diiodobenzoic acid (2.0 g, 5.13 mmol) in methanol
(100 mL) was added 10-15 drops of concentrated H2SO4. The
solution was refluxed overnight. The mixture was cooled, and
the solvent was evaporated. The residue was recrystallized twice
from MeOH/H2O to yield a white crystalline solid of methyl
4-hydroxy-3,5-diiodobenzoate (1.90 g, 91.7%). Mp over 250 °C
(dec). 1H NMR (CDCl3, 300 MHz) δ 8.36 (s, 2H), 6.12 (s, 1H),
3.89 (s, 3H). 13C NMR (CDCl3, 75 MHz) δ 164.2, 157.4, 141.1,
126.3, 81.8, 52.6. MS (EI) m/z 404.84 (M þ H)þ. Anal. Calcd for
C8H6I2O3: C, 23.79; H, 1.50. Found: C, 24.07; H, 1.66.
1
(CD2Cl2, 121.4 MHz): δ 12.50 (s, JPt-P = 2714.0 Hz). Anal.
Calcd for C134H174N10O22P8Pt4: C, 48.70; H, 5.31; N, 4.24.
Found: C, 48.89; H, 5.68; N, 4.39.
Self-Assembly of Rhomboid 6. To a 0.2 mL CD2Cl2 solution of
the medium organoplatinum 60° acceptor 3 (4.243 mg, 3.06
μmol) in a vial was added a 0.2 mL CD2Cl2 solution of 4-(4-
nitrobenzyloxy)-3,5-(bis(4-pyridyl)ethynyl)benzoate 1 (1.50 mg,
3.06 μmol). This process was repeated three times with acetone-d6
instead of CD2Cl2 (3 ꢀ 0.15 mL) to complete transfer of
the donor to the acceptor. The reaction mixture was stirred
at ambient temperature for 3 h to produce a clear pale
yellow solution. The solution was transferred into the NMR tube
for 1H and 31P NMR spectra collection. The solid product
was obtained by removing solvent under vacuum. Yield: 94%.
1H NMR (CD2Cl2/acetone-d6 = 1:1, 300 MHz): δ 8.95 (t, 8H,
J = 5.7 Hz), 8.54 (s, 4H), 8.33 (s, 4H), 8.26 (m, 4H), 7.93
(quadruple, 4H), 7.83-7.85 (m, 12H), 7.71 (s, 4H), 7.55 (d, 4H,
J = 8.1 Hz), 5.80 (s, 4H), 3.95 (d, 6H), 1.95-1.97 (m, 48H),
1.26-1.32 (m, 72H). 31P {1H} NMR (CD2Cl2/acetone-d6 = 1:1,
The mixture of 4-hydroxy-3,5-diiodobenzoate (0.808 g, 2
mmol), K2CO3 (2.36 g, 17 mmol), and 4-nitrobenzyl bromide
(0.475 g, 2.2 mmol) in 30 mL of dry acetone was refluxed for 16 h
and then filtered. The filtrate was evaporated to give a pale-
yellow residue. The residue was purified by column chromato-
graphy on silica gel with a mixed solvent (ethyl acetate/hexane =
1:4) as eluent. The pale-yellow crystalline solid of methyl 3,5-
diiodo-4-(4-nitrobenzyloxy)benzoate was obtained in a yield of
80.7% (0.87 g). Mp 104-106 °C. 1H NMR (CDCl3, 300 MHz) δ
8.48 (s, 2H), 8.32 (d, 2H, J = 8.7 Hz), 7.82 (d, 2H, J = 8.7 Hz),
5.15 (s, 2H), 3.93 (s, 3H). 13C NMR (CDCl3, 75 MHz) δ 164.0,
160.7, 148.1, 143.2, 141.6, 130.1, 128.7, 124.0, 90.6, 73.0, 53.0.
MS (EI) m/z 540.88 (M þ H)þ. Anal. Calcd for C15H11I2NO5: C,
33.42; H, 2.06; N, 2.60. Found: C, 33.73; H, 2.33; N, 2.51.
A 100 mL Schlenk flask was charged with methyl 3,5-diiodo-
4-(4-nitrobenzyloxy)benzoate (0.270 g, 0.5 mmol), 4-ethynyl-
pyridine hydrochloride (0.280 g, 2 mmol), bis(triphenylphos-
phine) palladium(II) dichloride (90 mg, 0.13 mmol), and copper-
(I) iodide (25 mg, 0.13 mmol) under a stream of nitrogen.
Freshly distilled triethylamine (10 mL) and tetrahydrofuran
(15 mL) were added to the flask via syringe, and the reaction
mixture was stirred overnight at 60 °C. The solvent was then
evaporated, and the resulting residue was extracted with ethyl
acetate over water. The organic phase was washed with brine
and dried over anhydrous MgSO4. Purification by silica gel
column chromatography with eluent (ethyl acetate/methanol =
10:1) yields the pale yellow powder of methyl 4-(4-nitroben-
zyloxy)-3,5-(bis(4-pyridyl)ethynyl)benzoate 1 (154 mg, 63%).
1
121.4 MHz): δ 16.98 (d, JPt-P = 2324.2 Hz). Anal. Calcd for
C146H174F12N6O22P8Pt4S4: C, 46.77; H, 4.68; N, 2.24. Found: C,
46.79; H, 4.92; N, 2.09.
Self-Assembly of Hexagon 7. The donor ligand 4-(4-
nitrobenzyloxy)-3,5-(bis(4-pyridyl)ethynyl)benzoate 1 (2.45 mg,
5.00 μmol) and the 120° acceptor 4 (6.71 mg, 5.00 μmol) were
added to separate glass vials. To the vials containing the donor
was added 0.2 mL of CD2Cl2, and the resulting solution was
transferred to the acceptor vial charged with 0.2 mL of CD2Cl2.
This process was repeated thrice (3 ꢀ 0.15 mL) to ensure
quantitative transfer of the donor to the acceptor. The reaction
solution was then stirred at ambient temperature for 4 h to yield a
homogeneous yellow solution. The solution was transferred into
the NMR tube to collect 1H and 31P NMR spectra. Solid product
was obtained by removing the solvent under vacuum pump.
Yield: 95%. 1H NMR (CD2Cl2, 300 MHz): δ 8.73 (d, 12H, J =
5.1 Hz), 8.37 (s, 6H), 8.24(d, 6H, J = 8.7Hz), 7.90 (d, 6H, J = 8.7
Hz), 7.75 (d, 12H, J = 6.0 Hz), 7.58-7.52 (m, 24H), 5.79 (s, 6H),
3.97 (s, 9H), 1.37 (m, 72H), 1.10-1.21 (m, 108H). 31P {1H} NMR
1
Mp 157-160 °C. H NMR (CDCl3, 300 MHz) δ 8.65 (d, 4H,
1
J = 6.0 Hz), 8.26 (s, 2H), 8.24 (d, 2H, J = 8.7 Hz), 7.74 (d, 2H,
J = 8.4 Hz), 7.30 (d, 4H, J = 6.0 Hz), 5.57 (s, 2H), 3.96 (s, 3H).
13C NMR (CDCl3, 75 MHz) δ 204.8, 165.1, 150.3, 136.3, 130.6,
128.3, 126.7, 125.4, 124.0, 117.0, 92.6, 88.5, 74.6, 52.9. MS (EI)
m/z 490.23 (M þ H)þ. Anal. Calcd for C29H19N3O5: C, 71.16; H,
3.91; N, 8.58. Found: C, 70.82; H, 4.02; N, 8.45.
(CD2Cl2, 121.4 MHz): δ 13.62 (s, JPt-P = 2641.9 Hz). Anal.
Calcd for C204H261F18N9O36P12Pt6S6: C, 44.62; H, 4.79; N, 2.30.
Found: C, 44.95; H, 5.13; N, 2.21.
Acknowledgment. P.J.S. thanks the NIH (Grant GM-
057052) for financial support.
Self-Assembly of Rhomboid 5. The dipyridyl donor ligand
methyl 4-(4-nitrobenzyloxy)-3,5-(bis(4-pyridyl)ethynyl)benzo-
ate 1 (2.45 mg, 5.00 μmol) and the organoplatinum 60° acceptor
Supporting Information Available: Spectroscopic character-
ization of compound 1 and assemblies 5, 6, and 7. Molecular
modeling results of [2 þ 2] rhomboid 6 and [3 þ 3] hexagon 7.
This material is available free of charge via the Internet at http://
pubs.acs.org.
(21) (a) Stang, P. J.; Persky, N. E.; Manna, J. J. Am. Chem. Soc. 1997, 119,
4777. (b) Yang, H.-B.; Ghosh, K.; Arif, A. M.; Stang, P. J. J. Org. Chem.
2006, 71, 9464.
J. Org. Chem. Vol. 74, No. 22, 2009 8521