Design, Synthesis and Self-Assembly of Functional Amphiphilic Metallodendrimers

Article abstract of DOI:10.1002/open.201900298

A new family of alkynylated, amphiphilic dendrimers consisting of amidoamine linkers connected to 5,5′-functionalized 2,2′-bipyridine cores has been developed and evaluated in the formation of metallodendrimers of different generations and in self-assembly protocols. A convergent synthetic strategy was applied to provide dumbbell-shaped amphiphilic dendrimers, where the 2,2′-bipyridine cores could be coordinated to Fe^II centers to afford corresponding metallodendrimers. The ability of the metallic- and non-metallic dendritic structures to self-assemble into functional supramolecular aggregates were furthermore evaluated in aqueous solution. Spherical aggregates with sizes of a few hundred nanometers were generally produced, where controlled disassembly of the metallodendrimers through decomplexation could be achieved.

Full text of DOI:10.1002/open.201900298

DOI: 10.1002/open.201900298
Full Papers
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Design, Synthesis and Self-Assembly of Functional
Amphiphilic Metallodendrimers
Kitjanit Neranon,[a] Laura Alberch,[a] and Olof Ramström*[a, b, c]
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Dedicated to Prof. Jean-Marie Lehn on the occasion of his 80th birthday
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A new family of alkynylated, amphiphilic dendrimers consisting
of amidoamine linkers connected to 5,5’-functionalized 2,2’-
bipyridine cores has been developed and evaluated in the
formation of metallodendrimers of different generations and in
self-assembly protocols. A convergent synthetic strategy was
applied to provide dumbbell-shaped amphiphilic dendrimers,
where the 2,2’-bipyridine cores could be coordinated to FeII
centers to afford corresponding metallodendrimers. The ability
of the metallic- and non-metallic dendritic structures to self-
assemble into functional supramolecular aggregates were
furthermore evaluated in aqueous solution. Spherical aggre-
gates with sizes of a few hundred nanometers were generally
produced, where controlled disassembly of the metalloden-
drimers through decomplexation could be achieved.
1. Introduction
actions are generally required for dendrimer self-assembly,[28]
for example, H-bonding, π-π interactions, donor-acceptor
interactions, or metal-ligand coordination.[29] Therefore, appro-
priate amphiphilic dendritic structures consisting of hydrophilic
and hydrophobic segments, able to assemble through van der
Waals’ interactions and solvophobic effects, have witnessed
great interest as building blocks, and have been employed to
create a variety of functional materials.[26] In addition, reversible
binding that can be controlled by external stimuli is of primary
importance in dynamic assembly applications, where non-
covalent interactions and reversible covalent bonds are key to
achieve the desired effects.[30–32] In this context, metal coordina-
tion occupies an established position for building discrete and
well-defined structures with controlled formation and disassem-
bly profiles.[33,34] Supramolecular metal coordination complexes
thus provide a useful foundation for self-assembly protocols,
where a large variety of coordination geometries can be
employed, with bond strengths ranging from very labile
coordination to covalent bond energies.[29] In principle, these
features enable tunable and controllable formation of aggre-
gates that are assembled through metal coordination, amena-
ble for a wide variety of applications.[26,34]
Dendrons and dendrimers are well-defined, branched molecular
structures with typically wedge-like or spherical shapes. These
highly tunable structures are generally prepared via divergent
or convergent syntheses, where especially the multiple end
groups at the peripheries can be straightforwardly
functionalized.[1–3] Dendrimer surface modification improves
many properties such as improved solubilities and increased
reactivities,[4–6] as well as enhanced structural stabilities.[7] This
facilitates many potential applications, including liquid crystal
formation,[7] catalysis,[8,9] biosensing/biorecognition,[10,11] light-
emitting materials,[12–14] and targeted delivery.[15–19] Various
biomolecular structures have furthermore been conjugated to
dendrimer surfaces for various biomedical applications.[20–23]
Dendritic molecules have moreover been pursued in the
fabrication of supramolecular self-assemblies.[24,25] Assembly of
the primary units into larger and more complex entities thus
leads to a wide range of potential applications, for example, in
the
fields
of
drug
delivery,
biomedicine
and
nanotechnology.[26,27] However, adequate supramolecular inter-
Despite tremendous achievements in generating self-
assembled structures based on, for example, amphiphilic
effects, H-bonding or metal coordination, the development of
[a] Dr. K. Neranon, Dr. L. Alberch, Prof. Dr. O. Ramström
Department of Chemistry, Royal Institute of Technology
Teknikringen 36, S-10044 Stockholm, Sweden
[b] Prof. Dr. O. Ramström
dendrimers
as
building
blocks
for
well-organized
Department of Chemistry, University of Massachusetts Lowell
One University Ave, Lowell, MA 01854, USA
E-mail: olof_ramstrom@uml.edu
supramolecular architectures remains challenging. Examples
include assembly of oligo(amidoamine) dendrimers,[35] and
prototypic peptide-based amphiphilic dendrimers showing
similarities with globular proteins.[28] This has been addressed in
the present study, where we describe the design and develop-
ment of a new family of amphiphilic dendrimer structures
based on 2,2’-bipyridine cores functionalized with amidoamine
branches carrying multiple alkynylated aryl ether end-groups
(Figure 1a). The resulting dumbbell-shaped amphiphilic struc-
tures were subsequently evaluated for self-assembly into func-
tional supramolecular nanostructures controlled by metal
complexation with FeII-ions (Figure 1b).
[c] Prof. Dr. O. Ramström
Department of Chemistry and Biomedical Sciences
Linnaeus University, SE-39182 Kalmar, Sweden
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/open.201900298
An invited contribution to a Special Collection dedicated to Functional Su-
pramolecular Systems
©2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
This is an open access article under the terms of the Creative Commons
Attribution Non-Commercial NoDerivs License, which permits use and dis-
tribution in any medium, provided the original work is properly cited, the
use is non-commercial and no modifications or adaptations are made.
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Figure 1. Structures of amphiphilic dendrimers and overview of synthetic
pathways to supramolecular aggregates.
2. Results and Discussion
2.1. Synthesis
Figure 2. Synthesis of hydrophilic amidoamine structures (D1G1 and D1G2).
The amphiphilic dendrimers were designed as indicated in
Figure 1, composed of three different elements: 1) 2,2’-
bipyridine cores; 2) hydrophilic amidoamine branches; and 3)
alkynylated aryl ether end groups. This results in rod-like,
dumbbell-shaped dendrimers with either eight or 16 alkyne
groups that can be used for further functionalization, e.g.,
through alkyne-azide cycloaddition. In addition, the 2,2’-bipyr-
idine core structures enable metal coordination for metal-
lodendrimer formation. A convergent synthesis method was
adopted, where the dendron branches were first produced,
functionalized with aryl ether group, and subsequently con-
jugated to the 2,2’-bipyridine cores. This modular strategy
enables the synthesis of a variety of structures in short time,
also applicable to other core structures. The metallodendrimers
could finally be produced from the dendritic structures through
metal complexation.
Preparation of the internal, hydrophilic amidoamine den-
dron branches were achieved through a traditional sequence
based on repeated conjugate addition and amidation.[36] One of
the amino groups of ethylenediamine (1) was thus first Boc-
protected to give compound 3, from which double conjugate
addition to acrylic ester 4 yielded compound 5. This diester was
subsequently allowed to react with ethylenediamine 1 to
Figure 3. Synthesis of hydrophobic exterior structure AK.
provide diamide structure D1G1 (generation
1 dendron).
Dendrons of generation 2 (D1G2) could then be synthesized by
repeating the conjugate addition and amidation steps of
structure D1G1. Both dendron generations were obtained in
excellent yields as presented in Figure 2.
Synthesis of the alkynylated, hydrophobic exterior started
from esterification of dihydroxybenzoic acid 7 giving ester 8.
This ester was then deprotonated and subjected to reaction
with propargyl bromide (9) to provide compound 10. Finally,
hydrolysis of ester 10 resulted in structure AK in good yield
(Figure 3).
Figure 4. Synthesis of amphiphilic dendrons generation 1 and 2.
provide dendrons D1G1AK and D1G2AK dendrons in excellent
yields (Figure 4). Finally, amide coupling of both dendron
structures D1G1AK and D1G2AK with core element 2,2’-
bipyridine-5,5’-dicarboxylic acid (13) using (benzotriazol-1-
yloxy)tris(dimethylamino)phosphonium hexafluorophosphate
(BOP) as coupling agent in dimethylformamide provided the
non-metallated (series I) amphiphilic dendrimers of generation
1 and 2 in moderate yields (Figure 5).
Amide coupling of the hydrophilic interior structures (D1G1
and D1G2) with the alkynylated hydrophobic exterior molecule
(AK) gave Boc-protected amphiphilic dendrons of generation 1
(11) and 2 (12), respectively. These were then deprotected to
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Figure 5. Synthesis of amphiphilic dendrimers of series I, (D1G1AK)2BPy and (D1G2AK)2BPy.
The resulting amphiphilic series I-dendrimers (D1G1AK)2BPy
and (D1G2AK)2BPy were subsequently subjected to coordina-
tion with FeII-ions (metal/ligand 1:3) resulting in both metal-
lodendrimer (series II) complexes (D1G1AK)2BPy-Fe and
(D1G2AK)2BPy-Fe) in excellent yields (Figure 6). Complex for-
mation between the dendrimers and FeII resulted in purple
properties were investigated. The structures were thus eval-
uated for their abilities to form large, functional structures
through reversible aggregation in aqueous solution. The
processes were followed using dynamic light scattering (DLS),
transmission electron microscopy (TEM), and scanning electron
microscopy (SEM).
structures, of which the generation
1
structure showed
The results are displayed in Figure 8, clearly showing
aggregate formation from all dendritic structures. The obtained
amphiphilic dendrimers (series I) and their metal coordination
complexes (series II) thus self-assembled into smaller spherical
aggregates ranging in size of approximately 125, 200, 155, and
190 nm (DLS), and 100–130, 80–150, 100–200, and 150–200 nm
(TEM), for the four different structures (D1G1AK)2BPy,
(D1G2AK)BPy, (D1G1AK)2BPy-Fe, and (D1G2AK)2BPy-Fe, re-
spectively. SEM showed somewhat broader distributions with
especially deep color. The recorded electronic spectra also
displayed remarkably intense charge-transfer (MLCT) bands in
the visible region (500–600 nm), in analogy to typical transition-
metal bipyridine complexes (Figure 7).[37,38]
Self-assembly. Having successfully synthesized the targeted
dendritic structures of series I, (D1G1AK)2BPy and
(D1G2AK)2BPy, and the metallodendrimers of series II,
(D1G1AK)2BPy-Fe and (D1G2AK)2BPy-Fe, their self-assembly
Figure 6. Synthesis of metallodendrimers of series II, (D1G1AK)2BPy-Fe and (D1G2AK)2BPy-Fe.
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of the resulting supramolecular self-assemblies. This is seen by
the size increase of the aggregated structures as a consequence
of the larger dendrimers in those two series. In addition,
increased sizes of the metallodendrimer assemblies in compar-
ison to their non-metallic counterparts were produced. The
metallodendrimers also displayed different properties in terms
of morphology, where TEM indicated solid generation 1-
structures and hollow generation 2-spheres. Further, from
inspection of the SEM micrographs (Figure 8), the presence of
concave features (white arrows) in some particles of
(D1G2AK)2BPy-Fe indicates the possible formation of hollow
spherical structures. The proportion of broken spheres is also
very low, suggesting that the dendrimeric spheres are robust
and difficult to break. This indicates that aggregation of the
dendritic structures is dynamically reversible,[39] resulting in the
formation of spheres in aqueous solution prior to vacuum
treatment during SEM analysis.[40,41]
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Figure 7. (a) Schematic representation of metallodendrimer formation and
subsequent self-assembly into aggregates; photographic images and
absorption spectra in MeOH of (b) (D1G1AK)2BPy and (D1G1AK)2BPy-Fe;
and (c) (D1G2AK)2BPy and (D1G2AK)2BPy-Fe.
Supramolecular disassembly. Due to the intriguing self-
assembly results, especially regarding the hollow nanostruc-
tures obtained with the generation
2 metallodendrimer
assemblies of 100–150, 100–200, 150–500, and 200–800 nm. In
all cases, larger aggregates were also seen.
(D1G2AK)2BPy-Fe, the disassembly behavior of the spheres was
further studied. Both metallodendrimer preparations,
(D1G1AK)2BPy-Fe and (D1G2AK)2BPy-Fe, were thus selected for
evaluation of controlled disassembly in the presence of an
external stimulus. Based on the metallodendrimer FeII-bipyridine
coordination, competitive binding of ethylenediaminetetraace-
tate (EDTA) towards iron(II) would thus trigger the
supramolecular aggregates to disassemble into the basic
dendritic (series I) ligands, thereby changing the expression of
the overall system. The process was followed by absorption
spectroscopy, but could also be directly observed by the naked
eye (Figure 9). Interestingly, the two metallodendrimer systems
displayed different behavior in response to the metal complex-
ation competitor. The absorption spectra obtained of the
system based on metallodendrimer (D1G1AK)2BPy-Fe demon-
strated a rapid, but partial, decrease of the absorption intensity
of the MLCT band over the first 30 min, after which time only
minor changes occurred over 30 h (Figure 9a).
The generation 1-structure of series I ((D1G1AK)2BPy)
displayed the smallest size with a low degree of larger
assemblies (Figure 8 far left), whereas the analogous generation
2-structure ((D1G2AK)2BPy) showed a broader distribution with
assemblies up to approximately 850 nm (Figure 8 middle left).
In both cases, the particles showed core-shell morphologies by
TEM. Interestingly, the generation
1
metallodendrimer
((D1G1AK)2BPy-Fe) showed a bimodal distribution with a
relatively large amount of aggregates around 970 nm in size
(Figure 8 middle right). In contrast, the larger metalloden-
drimers exhibited a narrower distribution, with larger aggre-
gates of approximate 600 nm diameter (Figure 8 far right).
This unique self-assembly behavior, where both the average
particle size and distribution varies between the dendritic
structures, is effectuated by building block generation and
metal complexation. The balance between the hydrophilic
amidoamine segments and the lipophilicity of the alkynylated
aryl ether segments thus plays a significant role in the control
In the case of the system based on metallodendrimer
(D1G2AK)2BPy-Fe, on the other hand, the MLCT absorption
Figure 8. Self-assembly of dendritic structures; a) TEM images; b) SEM images; c) DLS histograms.
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through the alkyne groups at the dendrimer periphery, giving
access to a wide range of structures with various properties.
Thus, these multifunctional metallodendrimers constitute an
important step toward tunable nanomaterials, offering a means
to future applications of functional materials.
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Experimental Section
Materials. Triethylamine (Et3N) was distilled over calcium hydride
under inert atmosphere, and stored under nitrogen. Anhydrous
solvents used in air- and moisture sensitive reactions were dried
using a solvent dispensing system (Glass-contour, Inc.) by passing
over alumina columns. Water used for supramolecular assembly
was obtained from a Millipore Milli-Q system with at least 18.2 MΩ
resistivity. Other commercially available compounds and solvents
were of HPLC, certified ACS, or reagent grade, and used as received
unless otherwise specified. Air- and moisture sensitive reactions
were carried out in flame-dried, septum-capped flasks under an
atmospheric pressure of nitrogen.
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Figure 9. Absorption spectra of (a) (D1G1AK)2BPy-Fe; (b) (D1G2AK)2BPy-Fe;
and (c) Photographic images of (I) (D1G1AK)2BPy-Fe, (II) (D1G1AK)2BPy-Fe
+EDTA, (III) (D1G2AK)2BPy-Fe, and (IV) (D1G2AK)2BPy-Fe+EDTA in meth-
anol (1.3×10À 5 M).
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Instrumentation. H and 13C NMR spectra were recorded on Bruker
AscendTM 400 or Bruker DMX 500 spectrometers at 298 K. Chemical
shifts (δ) are reported in parts per million (ppm) with (residual)
solvent as internal reference. J-values are given in hertz (Hz). Thin
layer chromatography (TLC) was performed on pre-coated Croma-
tofolios AL Silica gel 60 F254 plates (Merck). Flash column
chromatography was carried out on silica gel 60, 0.040–0.063 nm
(SDS). Size exclusion column chromatography was performed using
Sephadex G-10 columns with methanol as eluent. Electrospray
mass spectrometry was performed in positive mode on a Finnigan
LCQ Advantage Max LC-MS/MS spectrometer (Thermo Fisher
Science). Matrix-assisted laser desorption ionization with time-of-
flight (MALDI-TOF) MS spectra were recorded on a Bruker Ultraflex
TOF/TOF instrument using 2,5-dihydroxybenzoic acid as matrix.
Dynamic light scattering (DLS) measurements were performed on a
intensity progressively decreased as a function of time, reaching
nearly complete decomplexation over 30 h (Figure 9b). More-
over, the photographic images in Figure 9c clearly show the
metallodendrimer color quenches after exposure to EDTA over
30 h. These results are indicative of stronger complexes
between the smaller generation 1-dumbbell dendrimers and
iron(II), likely owing to a higher degree of steric hindrance of
the larger generation 2-dendrimers around the metal core. The
results also support the findings from microscopy, inasmuch as
the generation 1-metallodendrimers displayed a more solid
appearance, thereby being more resistant to ligand exchange,
whereas the hollow generation 2-dendrimers were more
accessible to decomplexation.
°
Malvern Zetasizer Nano ZS at 20 C with a backscattering detector
°
at 173 . TEM measurements were performed on a Hitachi H-7650
transmission electron microscope. SEM measurements were per-
formed on a field emission scanning electron microscopy with Zeiss
Ultra-55 scanning electron microscope. UV absorption spectra were
recorded using a Cary 300Bio spectrophotometer.
3. Conclusions
We have developed a new class of amphiphilic, dendritic
structures based on a modular approach involving three design
elements. The structures thus include 2,2’-bipyridine core
structures, amenable to transition metal complexation, linked to
internal amidoamine branches of different generation, which
are capped with alkynylated aryl ether groups. The structures
could be straightforwardly synthesized following a convergent
strategy, and showed efficient and branch-dependent metal
complexation with iron(II). All structures formed supramolecular
aggregates in aqueous solution, showing distinct differences
with respect to dendrimer size and metal complexation.
Spherical structures of either core-shell, solid or hollow nature
were thus observed by microscopy. The metallodendrimers
were furthermore responsive to an external stimulus, where
decomplexation and associated structural rearrangement oc-
curred in the presence of competitive complexation ligands.
These results show that metal coordination not only enables
the construction of flexible supramolecular dendrimers, but also
possesses the advantage of controlled disassembly and poten-
tial release of contained molecules. The structures further
provide the opportunity for straightforward modification
Synthesis of Amphiphilic Dendrimers
tert-Butyl (2-aminoethyl)carbamate (3).[42] To a solution of ethyl-
enediamine 1 (5.51 mL, 82.47 mmol) in dichloromethane (40 mL) at
°
0 C was added dropwise a solution of di-tert-butyl dicarbonate 2
(3.00 g, 13.74 mmol) in dichloromethane (80 mL). The mixture was
stirred overnight at ambient temperature. Following evaporation to
dryness under reduced pressure, the resulting solid was redissolved
in distilled water (40 mL), filtered, and subsequently extracted with
dichloromethane (2×40 mL). Evaporation of the solvent and drying
afforded product 3 as a white solid (1.80 g, 82%). 1H NMR
(500 MHz, CDCl3) δ: 5.31 (br, 1H), 3.03 (t, J=6.0 Hz, 2H), 2.66 (t, J=
6.0 Hz, 2H), 1.31 (s, 9H), 1.19 (s, 2H). 13C NMR (126 MHz, CDCl3) δ:
156.3, 79.0, 43.1, 41.6, 28.3.
Diethyl
3,3’-((2-((tert-butoxycarbonyl)amino)ethyl)azanediyl)di-
propanoate (5). A methanol solution (4 mL) of N-Boc-ethylenedi-
amine 3 (1.00 g, 6.26 mmol) was added dropwise to a stirred
solution of ethyl acrylate 4 (2.67 mL, 25.05 mmol) in methanol
°
(4 mL) at 0 C. The reaction mixture was allowed to reach room
temperature and was kept stirred for 2 d. The volatiles were
removed by evaporation and the resulting product dried under
vacuum for 3 d, providing compound 5 as a viscous pale yellow oil
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(2.22 g, 98%). H NMR (500 MHz, CDCl3) δ: 5.06 (br, 1H), 4.08 (q, J=
drying under vacuum provided compound 8 as a white solid
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7.0 Hz, 4H), 3.12 (t, J=5.0 Hz, 2H), 2.70 (t, J=7.0 Hz, 4H), 2.47 (t, J=
5.0 Hz, 2H), 2.36 (t, J=7.0 Hz, 4H), 1.38 (s, 9H), 1.20 (t, J=7.0 Hz,
6H). 13C NMR (126 MHz, CDCl3) δ: 172.7, 156.1, 78.8, 60.4, 53.1, 49.1,
38.1, 32.7, 28.3, 14.2; MS (ESI-MS, positive mode): Calculated for
C17H32N2O6: [M+Na]+ m/z=383.22. Found: [M+Na]+ m/z=383.28.
(2.81 g, 84%). H NMR (500 MHz, CD3OD) δ: 6.92 (d, J=2.0 Hz, 2H),
6.47 (t, J=2.0 Hz, 1H), 3.85 (s, 3H). 13C NMR (126 MHz, CD3OD) δ:
167.2, 158.3, 131.5, 107.3, 106.7, 51.0.
Methyl 3,5-bis(prop-2-yn-1-yloxy)benzoate (10).[46,47] Methyl 3,5-
dihydroxybenzoate 8 (256 mg, 1.52 mmol) was dissolved in anhy-
drous DMF (15 mL) and treated with TBAI (112 mg, 0.30 mmol) and
powdered K2CO3 (842 mg, 6.09 mmol). The reaction mixture was
stirred for 5 min, and then treated with propargyl chloride (0.41 mL,
tert-Butyl
(2-bis(3-((2-aminoethyl)amino)-3-oxopropyl)amino)
ethyl)carbamate (D1G1).[43] To a cold solution (0 C) of ethylenedi-
amine 1 (5.70 mL, 85.49 mmol) in methanol (10 mL), a methanol
solution (10 mL) of ethyl ester-terminated amidoamine dendron 5
(2.05 g, 5.70 mmol) was added dropwise within 1 h. The resulting
reaction mixture was allowed to reach room temperature and was
°
°
4.6 mmol). The reaction mixture was stirred at 60 C for 18 h, then
°
quenched at 0 C with saturated NH4Cl (aq.) solution (15 mL), and
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then extracted with diethyl ether (3×15 mL). The combined organic
phase was washed with brine (15 mL), dried over Na2SO4, and
concentrated to dryness. Purification by column chromatography
using 20% EtOAc-hexane afforded compound 2 as a white solid
kept stirred for
evaporation, after which the excess ethylenediamine
5
d. The reaction solvent was removed by
was
1
removed by azeotropic distillation with toluene/methanol, followed
by vacuum distillation to remove the remaining toluene. The
desired product was further dried under vacuum for 2 d to afford
product D1G1 as a viscous amber oil (2.10 g, 95%). 1H NMR
(500 MHz, CDCl3) δ: 7.59 (br, 2H), 6.42 (br, 1H), 3.29 (t, J=5.5 Hz,
4H), 3.14 (br, 2H), 2.93 (br, 4H), 2.85 (t, J=5.5 Hz, 4H), 2.68 (t, J=
5.5 Hz, 4H), 2.50 (br, 2H), 2.33 (t, J=5.5 Hz, 4H), 1.40 (s, 9H). 13C NMR
(126 MHz, CDCl3) δ: 173.0, 156.4, 79.1, 50.2, 49.0, 45.4, 41.1, 40.9,
33.9, 28.5
1
(344 mg, 93%). H NMR (400 MHz, CDCl3): δ 7.30 (d, J=2.5 Hz, 2H),
6.81 (t, J=2.5 Hz, 1H), 4.72 (d, J=2.5 Hz, 4H), 3.91 (s, 3H), 2.54 (t, J=
2.5 Hz, 2H). 13C NMR (126 MHz, CDCl3): δ 166.6, 158.6, 132.3, 109.0,
107.6, 78.1, 76.1, 56.2, 52.5.
3,5-Bis(prop-2-yn-1-yloxy)benzoic acid (AK).[48] To a stirred solution
of ester 10 (3.00 g, 12.28 mmol) in methanol (10 mL) was slowly
added an aqueous solution (5 mL) of NaOH (2.95 g, 73.70 mmol).
After stirring at room temperature for 18 h, methanol was removed
using a rotary evaporator, and water (30 mL) was added. The
resulting solution was carefully acidified with 2 M HCl and the
precipitated product was extracted with dichloromethane (30×
3 mL). Drying over Na2SO4, removal of the solvent by evaporation,
and drying under vacuum afforded acid AK as a greenish yellow
Diethyl 11-(2-((tert-butoxycarbonyl)amino)ethyl)-bis(3-ethoxy-3-
oxopropyl)-8,14-dioxo-4,7,11,15,18-pentaazahenicosane-1,21-dio-
ate (6). A methanol solution (3 mL) of D1G1 (0.50 g, 1.29 mmol)
°
was slowly added to a cold solution (0 C) of ethyl acrylate 4
(1.10 mL, 10.32 mmol) in methanol (3 mL). The mixture was allowed
to reach room temperature and was kept stirred for 2 d. The
volatiles were removed by evaporation and the resulting product
dried under vacuum for 3 d to provide product 6 as a sticky, pale
1
solid (2.35 g, 83%). H NMR (500 MHz, CDCl3) δ: 7.36 (d, J=2.0 Hz,
2H), 6.87 (t, J=2.0 Hz, 1H), 4.74 (d, J=2.0 Hz, 4H), 2.56 (t, J=2.5 Hz,
2H). 13C NMR (126 MHz, CDCl3) δ: 158.6, 109.4, 108.3, 77.9, 76.1, 56.2.
1
yellow oil (1.00 g, 98%). H NMR (500 MHz, CDCl3) δ: 7.01 (br, 2H),
tert-Butyl (2-(bis(3-((2-(-bis(prop-2-yn-1-yloxy)benzamido)ethyl)
amino)-3-oxopropyl)amino)ethyl)carbamate (11). Acid AK (0.59 g,
2.57 mmol) and BOP (1.47 g, 3.34 mmol) were mixed and dissolved
in dry dimethylformamide (8 mL) after which Et3N (1.08 mL,
7.72 mmol) and compound D1G1 (0.50 g, 1.29 mmol) were succes-
sively added. The resulting solution was stirred at ambient temper-
ature for 16 h. After removal of the solvent by evaporation, the
resulting solid was dissolved in dichloromethane (2×30 mL), and
the combined organic phase washed with water (2×10 mL) and
brine (10 mL). Drying over Na2SO4 and evaporation of solvent
5.44 (br, 1H), 4.03 (q, J=7.0 Hz, 8H), 3.59 (t, J=6.5 Hz, 4H), 3.21 (t,
J=6.5 Hz, 4H), 2.88 (t, J=5.5 Hz, 2H), 2.68 (t, J=7.0 Hz, 8H), 2.47 (t,
J=5.5 Hz, 4H), 2.41 (t, J=6.5 Hz, 4H), 2.33 (t, J=7.0 Hz, 8H), 2.26 (t,
J=5.5 Hz, 2H), 1.33 (s, 9H), 1.16 (t, J=7.0 Hz, 12H). MS (ESI-MS,
positive mode): Calculated for C37H68N6O12: [M+Na]+ m/z=
811.4793. Found: [M+Na]+ m/z=811.48.
tert-Butyl (16-amino-10-(3-((2-aminoethyl)amino)-3-oxopropyl)-3-
(3-((2-(bis(3-((2-aminoethyl)amino)-3-oxopropyl)amino)ethyl)
amino)-3-oxopropyl)-dioxo-3,7,10,14-tetraazahexxandecyl)
carbamate (D1G2).[44] Ethyl ester-terminated amidoamine dendron
6 (1.08 g, 1.38 mmol) was dissolved in methanol (5 mL) and then
yielded
a crude product, which was purified using column
chromatography (eluent: DCM-MeOH, 9.5:0.5 v/v), yielding the
desired product 11 as a pale yellow solid (0.47 g, 45%). 1H NMR
(500 MHz, CDCl3) δ: 7.72 (br, 2H), 7.63 (br, 2H), 7.00 (s, 4H), 6.70 (s,
2H), 5.43 (br, 1H), 4.67 (d, J=2.5 Hz, 8H), 3.52 (br, 4H), 3.45 (br, 4H),
3.11 (br, 2H), 2.62 (br, 4H), 2.55 (s, 4H), 2.46 (br, 2H), 2.29 (br, 4H),
1.38 (s, 9H). MS (ESI-MS, positive mode): Calculated for C43H52N6O10:
[M+Na]+ m/z=835.3643. Found: [M+Na]+ m/z=835.18.
°
slowly added to a cold solution (0 C) of ethylenediamine 1
(1.84 mL, 27.50 mmol) in methanol (5 mL) within 1 h. The resulting
solution was allowed to reach room temperature and was kept
stirred for 5 d. The reaction solvent was evaporated by evaporation,
after which the excess ethylenediamine 1 was removed by an
azeotropic distillation with toluene/methanol, followed by vacuum
distillation to remove the remaining toluene. The resulting product
was further dried under vacuum for 3 d to provide product D1G2
N,N’-(((’-((2-Aminoethyl)azanediyl)bis(propanoyl))bis(azanediyl))
bis(ethane-2,1-diyl))bis(3,5-bis(prop-2-yn-1-yloxy)benzamide)
(D1G1AK). Boc-protected amidoamine dendron 11 (0.27 g,
0.33 mmol) was dissolved in dichloromethane (1 mL), to which
solution trifluoroacetic acid (TFA) (0.51 mL, 6.66 mmol) was added
1
as a viscous amber oil (1.12 g, 96%). H NMR (500 MHz, CDCl3) δ:
7.95 (br, 2H), 7.60 (br, 4H), 7.47 (br, 1H), 3.33–3.24 (m, 12H), 3.16 (t,
J=5.5 Hz, 2H), 2.89 (t, J=5.5 Hz, 2H), 2.84 (t, J=5.5 Hz, 8H), 2.74–
2.65 (m, 12H), 2.53 (t, J=5.5 Hz, 4H), 2.41–2.31 (m, 12H), 2.10 (br,
8H), 1.42 (s, 9H).
°
dropwise at 0 C. After stirring at room temperature for 18 h, the
volatiles were removed by evaporation. The residue was redissolved
in dichloromethane (10 mL), and the resulting solution washed
with an aqueous solution of Na2CO3 (25%, 3 mL), and dried over
anhydrous MgSO4. The solvent was removed by evaporation and
the resulting solid dried under vacuum for 3 d, providing product
Methyl 3,5-dihydroxybenzoate (8).[45] To a stirred solution of 3,5-
dihydroxybenzoic acid 7 (3.08 g, 19.98 mmol) in methanol (50 mL),
concentrated H2SO4 (2 mL) was slowly added. The resulting solution
was heated to reflux for 1 d, after which it was allowed to cool
down to ambient temperature, and the solvent was removed by
evaporation. The crude product was extracted with ethyl acetate
(2×50 mL), and the combined organic layer washed with saturated
aqueous NaHCO3-solution, and dried over Na2SO4. Evaporation and
1
D1G1AK as a viscous amber solid (0.24 g, 99%). H NMR (500 MHz,
CDCl3) δ: 8.13 (t, J=5.0 Hz, 2H), 7.82 (t, J=5.0 Hz, 2H), 7.06 (d, J=
2.0 Hz, 4H), 6.67 (t, J=2.0 Hz, 2H), 4.63 (d, J=2.0 Hz, 8H), 3.46 (t, J=
5.5 Hz, 4H), 3.39 (t, J=5.5 Hz, 4H), 2.72 (t, J=5.0 Hz, 2H), 2.59 (t, J=
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5.5 Hz, 4H), 2.54 (t, J=2.5 Hz, 4H), 2.42 (t, J=5.0 Hz, 2H), 2.30 (t, J=
5.5 Hz, 4H). MS (ESI-MS, positive mode): Calculated for C38H44N6O8:
[M+Na]+ m/z=735.3118. Found: [M+Na]+ m/z=735.38.
(0.06 mL, 0.42 mmol) and compound D1G2AK (0.22 g, 0.14 mmol)
were successively added. The resulting solution was stirred at
ambient temperature for 16 h. After removal of the solvent by
evaporation, the residue was dissolved in dichloromethane (2×
30 mL) and the solution subsequently washed with water (2×
10 mL) and brine (10 mL). Drying over Na2SO4, evaporation to
dryness, and purification using column chromatography (eluent:
DCM-MeOH, 9:1 v/v), yielded the desired product (D1G2AK)2BPy as
1
2
3
4
5
6
7
8
9
tert-Butyl
(9-(3-((2-(-bis(prop-2-yn-1-yloxy)benzamido)ethyl)
amino)-3-oxopropyl)-16-(9-(3-((2-(3,5-bis(prop-2-yn-1-yloxy)
benzamido)ethyl)amino)-3-oxopropyl)-1-(3,5-bis(prop-2-yn-1-
yloxy)phenyl)-1,6,13-trioxo-2,5,9,12-tetraazapentadecan-15-yl)-1-
(3,5-bis(prop-2-yn-1-yloxy)phenyl)-1,6,13-trioxo-2,5,9,12,16-pen-
taazaoctadecan-18-yl)carbamate (12). Acid AK (0.33 g, 1.42 mmol)
and BOP (0.63 g, 1.42 mmol) were mixed and dissolved in dry
dimethylformamide (6 mL), to which solution Et3N (0.46 mL,
3.26 mmol) and D1G2 (0.30 g, 0.35 mmol) were successively added.
The resulting solution was stirred at ambient temperature for 16 h.
After removing the solvent by evaporation, the resulting residue
was dissolved in dichloromethane (2×30 mL), and the solution
washed with water (2×10 mL) and brine (10 mL), and dried over
Na2SO4. The solvent was removed by evaporation, and the resulting
solid purified using column chromatography (eluent: DCM-MeOH,
9:1 v/v) to give the desired product 12 as a bright yellow solid
1
a pale yellow solid (0.10 g, 42%). H NMR (500 MHz, CD3OD) δ: 9.08
(s, 2H), 8.45 (d, J=8.0 Hz, 2H), 8.30 (d, J=8.0 Hz, 2H), 7.03 (d, J=
2.0 Hz, 16H), 6.75 (t, J=2.0 Hz, 8H), 4.74 (d, J=2.5 Hz, 32H), 3.46–
3.39 (m, 44H), 3.18 (br, 8H), 2.99 (t, J=2.5 Hz, 16H), 2.82–2.70 (m,
24H), 2.51–2.30 (m, 28H). HRMS (MALDI-TOF MS, positive mode):
Calculated for C180H204N30O38: [M+H]+ m/z=3394.5031. Found: [M
+H]+ m/z=3395.64.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
Synthesis of Metallodendrimers
Dendrimeric complex (D1G1AK)2BPy-Fe. 2,2’-bipyridine-dendrimer-
ic ligand (D1G1AK)2BPy (0.058 g, 0.035 mmol) and iron(II)
perchlorate hydrate (0.003 g, 0.012 mmol) were dissolved in
dichloromethane:methanol (5 mL, 1:1, v/v) and the mixture was
stirred at room temperature for 1 h. The precipitated purple
product was purified by centrifugation from methanol 3 times.
Drying under vacuum afforded product (D1G1AK)2BPy-Fe as a
1
(0.25 g, 41%). H NMR (500 MHz, CDCl3) δ: 7.90–7.61 (m, 10H), 7.08
(s, 8H), 6.70 (s, 4H), 6.59 (br, 1H), 4.66 (d, J=2.0 Hz, 16H), 3.51–3.44
(m, 20H), 3.18–3.13 (m, 4H), 2.65 (br, 8H), 2.56 (t, J=2.0 Hz, 8H), 2.51
(t, J=6.0 Hz, 2H), 2.46 (t, J=5.5 Hz, 4H), 2.30–2.26 (m, 12H), 2.21 (t,
J=6.0 Hz, 2H), 1.39 (s, 9H). 13C NMR (126 MHz, CDCl3) δ: 173.9,
167.6, 158.7, 136.3, 106.9, 105.4, 78.1, 76.1, 56.1, 50.6, 41.0, 39.2,
34.3, 28.5. MS (ESI-MS, positive mode): Calculated for C89H108N14O20:
[M+H]+ m/z=1693.7943. Found: [M+H]+ m/z=1693.46.
1
deep purple solid (0.054 g, 92%). H NMR (500 MHz, CDCl3/CD3OD,
8:1) δ: 9.20 (s, 6H), 8.47 (d, J=8.0 Hz, 6H), 8.38 (d, J=7.5 Hz, 6H),
6.98 (br, 24H), 6.65 (br, 12H), 6.55 (br, 30H), 4.62 (br, 48H), 3.92–2.08
(m, 144H). 13C NMR (126 MHz, CDCl3/CD3OD, 8:1) δ: 163.3, 160.2,
141.9, 131.9, 115.0, 111.0, 109.7, 106.1, 104.5, 82.8, 60.1, 56.4, 35.8,
32.9, 21.7. HRMS (MALDI-TOF MS, positive mode): Calculated for
C264H276FeN42O54: [M+H]+ m/z=4954.96. Found: [M+H]+ m/z=
4955.54.
Amphiphilic dendron D1G2AK. Boc-protected amidoamine den-
dron 12 (0.24 g, 0.14 mmol) was dissolved in dichloromethane
(0.9 mL), to which trifluoroacetic acid (TFA) (0.22 mL, 2.83 mmol)
°
was added dropwise at 0 C. After stirring at room temperature for
18 h, the volatiles were removed by evaporation. The residue was
redissolved in dichloromethane (10 mL), washed with an aqueous
solution of Na2CO3 (25%, 3 mL), and the organic phase dried over
anhydrous MgSO4. Removal of the solvent by evaporation and
drying under vacuum for 3 d provided product D1G2AK as a
viscous pale yellow oil (0.22 g, yield: 99%). 1H NMR (500 MHz,
CDCl3) δ: 7.64–7.97 (m, 10H), 7.09 (d, J=2.5 Hz, 8H), 6.72 (t, J=
2.5 Hz, 4H), 4.66 (d, J=1.5 Hz, 16H), 3.51–3.43 (m, 20H), 3.12–3.05
(m, 4H), 2.55–2.50 (m, 22H), 2.33–2.17 (m, 14H). 13C NMR (126 MHz,
CDCl3) δ: 158.7, 107.0, 106.8, 78.1, 76.0, 56.1, 29.7. MS (ESI-MS,
positive mode): Calculated for C84H100N14O18: [M+H]+ m/z=
1593.7418. Found: [M+H]+ m/z=1593.76.
Dendrimeric complex (D1G2AK)2BPy-Fe. 2,2’-bipyridine-dendrimer-
ic ligand (D1G2AK)2BPy (0.04 g, 0.012 mmol) and iron(II)perchlorate
hydrate (0.001 g, 0.004 mmol) were dissolved in dichloromethane:
methanol (5 mL, 1:1, v/v) and the mixture was stirred at room
temperature for 1 h. The precipitated purple product was purified
by centrifugation from methanol 3 times. Drying under vacuum
afforded product (D1G2AK)2BPy-Fe as a purple complex (0.037 g,
90%). 1H NMR (400 MHz, CDCl3/CD3OD, 8:1) δ: 9.09 (s, 6H), 8.71 (br,
6H), 8.68 (br, 6H), 8.28–7.67 (m, 12H), 7.02 (br, 48H), 6.66 (br, 24H),
6.54 (br, 15H), 4.63 (d, J=4.0 Hz, 96H), 3.43–2.45 (m, 360H). 13C NMR
(100 MHz, CDCl3/CD3OD, 8:1) δ: 168.0, 158.6, 136.2, 107.0, 105.3,
78.2, 76.1, 56.1, 39.2, 36.7, 36.6. HRMS (MALDI-TOF MS, positive
mode): Calculated for C540H612FeN90O114: [M+H]+ m/z=10237.44.
Found: [M+H]+ m/z=10237.64.
Amphiphilic dendrimer (D1G1AK)2BPy. Dicarboxylic acid 13
(0.04 g, 0.16 mmol) and BOP (0.18 g, 0.42 mmol) were mixed and
dissolved in dry dimethylformamide (4 mL), to which solution Et3N
(0.13 mL, 0.96 mmol) and compound D1G1AK (0.23 g, 0.32 mmol)
were successively added. The resulting solution was stirred at
ambient temperature for 16 h. After removal of the solvent by
evaporation, the residue was dissolved in dichloromethane (2×
30 mL) and the solution subsequently washed with water (2×
10 mL) and brine (10 mL). Drying over Na2SO4, evaporation to
dryness, and purification using column chromatography (eluent:
DCM-MeOH, 9:1 v/v), yielded the desired product (D1G1AK)2BPy as
a yellow solid (0.12 g, 45%). 1H NMR (500 MHz, CDCl3) δ: 9.20 (s,
2H), 8.43 (d, J=8.0 Hz, 2H), 8.40 (d, J=8.0 Hz, 2H), 8.15 (br, 2H), 7.62
(br, 4H), 7.34 (br, 4H), 7.03 (d, J=2.0 Hz, 8H), 6.68 (t, J=2.0 Hz, 4H),
4.64 (d, J=2.5 Hz, 16H), 3.58 (br, 4H), 3.44 (br, 8H), 3.35 (br, 8H),
2.67 (br, 8H), 2.62 (br, 4H), 2.52 (t, J=2.5 Hz, 8H), 2.33 (br, 8H). MS
(ESI-MS, positive mode): Calculated for C88H92N14O18: [M+H]+ m/z=
1633.6792. Found: [M+H]+ m/z=1634.36.
Acknowledgments
This research work was in part supported by the People
Programme (Marie Curie Actions) of the European Union’s Seventh
Framework Programme FP7/2007–2013/ under REA grant agree-
ment no 264645, the Carl Trygger Foundation, and the Swedish
Institute.
Conflict of Interest
Amphiphilic dendrimer (D1G2AK)2BPy. Dicarboxylic acid 13
(0.02 g, 0.07 mmol) and BOP (0.08 g, 0.18 mmol) were mixed and
dissolved in dry dimethylformamide (3 mL), to which solution Et3N
The authors declare no conflict of interest.
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Full Papers
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Keywords: amphiphiles · metallodendrimers · self-assembly ·
coordination · nanomaterials · supramolecular systems
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Manuscript received: September 30, 2019
Revised manuscript received: November 20, 2019
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