Molecular and macromolecular engineering with viologens as building blocks: Rational design of phosphorus-viologen dendritic structures

Article abstract of DOI:10.1002/ejoc.201101376

The design of symmetric or asymmetric diversely functionalized monomer, dimer, or trimer viologens and the design of phosphorus building blocks allows the synthesis of unique types of mix phosphorus-viologen dendrimers. Strategies for the preparation of d

Full text of DOI:10.1002/ejoc.201101376

SHORT COMMUNICATION
DOI: 10.1002/ejoc.201101376
Molecular and Macromolecular Engineering with Viologens as Building
Blocks: Rational Design of Phosphorus–Viologen Dendritic Structures
Nadia Katir,[a,b] Jean Pierre Majoral,*[a,b] Abdelkrim El Kadib,*[a] Anne-Marie Caminade,[b]
and Mostapha Bousmina[a,c]
Dedicated to the memory of Dr. Robert Wolf[‡]
Keywords: Viologens / Dendrimers / Dendrons / Phosphorus / Synthesis design
The design of symmetric or asymmetric diversely function-
alized monomer, dimer, or trimer viologens and the design of
phosphorus building blocks allows the synthesis of unique
types of mix phosphorus–viologen dendrimers. Strategies for
the preparation of dendrimers of generations 0 and 1 are pre-
sented as are those for various dendrons. These methods ex-
hibit broad applicability, high efficiency and usability, and
are compatible with the presence of various functional
groups (aldehyde, cyano, phenol, phosphonate, brominated
groups).
bit peculiar spectroscopic features in both their dicationic
and radical cation forms.[2] Moreover, viologens are known
to give strong donor–acceptor complexes with electron-do-
nating species. These behaviors were exploited by several
authors describing the formation of host–guest complexes
and the electrochemical properties of symmetric and asym-
metric dendrimers incorporating viologen units.[3,4] In a
completely different field it can be also mentioned that
structure–activity relationships of a series of viologen units
and dendrimers as antiviral agents were recently reported,
pointing out an unexpected biological aspect of viologen
dendrimers as novel HIV-1 replication inhibitors.[5] Up to
now, viologen units were incorporated at the focal point,[4,6]
or at the core, the branches, and the periphery[7] of a very
few types of purely organic dendrimers.[8]
Similarly, phosphorus dendrimers, and especially those
we are developing, are extremely versatile macromolecules
with unique properties and applications. These dendrimers
allow the design of a new class of biosensors displaying
high efficiency both in terms of their sensitivity and sta-
bility.[9] Phosphorus dendrimers bearing two-photon ab-
sorption (TPA) chromophores at the core, the branches, or
the surface constitute a new class of “soft” nontoxic and
biocompatible fully organic chromophores exhibiting ex-
ceptional one- or two-photon brightness outperforming the
best quantum dots without suffering from the drawbacks
of the latter (blinking, toxicity, insolubility).[10] Phosphorus
dendrimers allow the template preparation of polyelectro-
lyte multilayer nanotubes based only on charged dendri-
mers and displaying an excellent detection limit for DNA
hybridization up to 10–18 mol in certain cases.[11] Phos-
phorus dendrimers with phosphonic end groups (poly-
Introduction
Dendrimers are a class of well-defined monodisperse
nanostructured macromolecules with a modifiable multi-
valent surface. A key advantage of dendrimers emanates
from the multiplicity and additive effects that can be
achieved by manipulating the densely packed end groups,
thus allowing a vast number of applications in different
fields ranging from biomedical applications to material sci-
ence, catalysis, etc.[1] The nature of the internal branches as
well as that of the core strongly also influences the behavior
and the properties of dendrimers, as frequently demon-
strated.[1]
The different types of dendrimers that contain photo-
active and/or electroactive moieties such as the 4,4Ј-bipyr-
idinium ion (viologen) are attracting interest due to the
properties of these dicationic species. Indeed, viologens are
well-known electroactive compounds that undergo two suc-
cessive one-electron reversible reduction processes and exhi-
[a] INANOTECH Institute of Nanomaterials and
Nanotechnology – MAScIR (Moroccan Foundation for
Advanced Science, Innovation and Research), ENSET,
Avenue de l’Armée Royale, Madinat El Irfane, 10100 Rabat,
Morocco
E-mail: a.elkadib@mascir.com
[b] Laboratoire de Chimie de Coordination du CNRS,
BP 44099, 205 Route de Narbonne, 31077 Toulouse cedex 4,
France
Fax: +33-5-61553003
E-mail: majoral@lcc-toulouse.fr
[c] Hassan II Academy of Science and Technology Rabat,
Rabat, Morocco
[‡] Deceased November 3, 2011.
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201101376.
Eur. J. Org. Chem. 2012, 269–273
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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N. Katir, J. P. Majoral, A. El Kadib, A.-M. Caminade, M. Bousmina
SHORT COMMUNICATION
anionic dendrimers) appear as new nanotools promoting
an anti-inflammatory and immunosuppressive activation of
human monocytes and thus prove to be good candidates
for innovative anti-inflammatory immunotherapies.[12]
Polycationic phosphorus dendrimers have a strong anti-
prion activity reducing prion replication both in vitro and
in vivo; these dendrimers did not bear any specific thera-
peutic agents.[13]Among other unique properties of phos-
phorus dendrimers,[14] their key roles as catalysts in some
reactions of industrial interest with a so-called “strong den-
dritic effect”[15] and their utility in the synthesis of so-called
“organic–inorganic hybrid materials” should be noted.[16]
In light of these examples on the properties and applica-
tions of both viologens and phosphorus dendrimers, which
are far from exhaustive, it appeared to us of great interest
to develop new methodologies for the preparation of den-
drimers by mixing viologen units and phosphorus linkages
into the same framework. The goal was not to propose a
strategy that would be one more synthetic pathway to den-
drimers with a limited extension, but to propose a large
panel of possibilities to build and tailor the dendrimers
through multistep synthesis and by using a variety of origi-
nal building blocks, “mixed” dendrons and dendrimers, and
related macromolecules, which, taking into account the
properties briefly reported above, would be fruitful nano-
objects for new applications.
Scheme 1. Synthesis of 4,4Ј-bipyridin-1-iums 1a–3a, 1b–3b, and 4
and symmetrical viologens 5a, 6a, 5b, and 6b.
We report herein (i) the synthesis of symmetric or asym-
metric new viologen monomers, dimers, and trimers; (ii) the
design of original dendrimers of generation 0 and 1 bearing
both phosphorus linkages and viologen units by using con-
vergent syntheses and different strategies; and (iii) the elab-
oration of several dendrons. In all cases, the solubility and
functionalization of these original systems can be modu-
lated by allowing the preparation of diversely multifunc-
tionalized dendritic structures.
Scheme 2. Synthesis of asymmetric viologens 7a–9a and 7b–9b.
Trihydrazidophosphane sulfide 10 and hexahydrazidocy-
clotriphosphazene 11 were intensively used in our hands as
a core for the preparation of phosphorus dendrimers incor-
Results and Discussion
In practice, the synthetic sequence of all these methodol- porating viologen units. They can be readily prepared by
ogies was initiated by the single alkylation of 4,4Ј-bipyridine substitution reactions of the corresponding tri- or
with various halogenated reagents leading selectively to hexachloro derivatives with methylhydrazine in the presence
water-soluble 4,4Ј-bipyridin-1-ium bromides 1a–3a and 4. of a base.[17] Condensation of 10 or 11 with 3 or 6 equiv. of
The corresponding hydrosoluble symmetrical [4,4Ј-bipyr- dialdehyde viologen 5b proceeds smoothly at room tem-
idine]-1,1Ј-diiums 5a–6a were obtained by a double alkyl- perature overnight to give selectively generation-0 dendri-
ation reaction involving two equivalents of the brominated mers 12 and 13 (soluble in organic solvents) bearing three
aldehyde or phenol. Anion exchange with KPF6 allowed or six aldehyde end groups in excellent yields; only one of
the formation of 1b–3b, 5b, and 6b from 1a–3a, 5a, and 6a, the two aldehyde groups of 5b reacted with each hydrazido
respectively, which are soluble in organic solvents such as unit and no cross-coupling resulting in the formation of in-
acetonitrile and acetone (Scheme 1). Asymmetric viologens soluble material was observed (Scheme 3). This is sup-
1
were prepared through alkylation of 2b, 3b, or 4 with 4- ported by analysis of the H and 13C NMR spectra reveal-
(bromomethyl)benzaldehyde at 78–80 °C over 12–24 h; the ing the presence of a singlet for the remaining aldehyde
resulting water-soluble monomers 7a, 8a (anion = PF6– and groups (10.07, 191.09 ppm for 12) and by electrospray mass
Br–), and 9a (anion = Br–) were then treated with NH4PF6 spectrometry (m/z = 2051.6 [M – PF6]). The same reactions
–
to give asymmetric compounds 7b–9b (anion = PF6 ) pres- were conducted with asymmetric viologen 9b and either 10
enting good solubility in organic solvents (Scheme 2).
or 11: dendrimers 14 and 15 bearing two different phos-
270
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© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2012, 269–273

Design of Phosphorus–Viologen Dendritic Structures
phorus units (cyclotriphosphazene unit as core and phos-
phonate as end groups) were thus quantitatively obtained
(Scheme 3).
Scheme 3. Synthesis of generation-0 dendrimers 12–15.
In order to access higher multivalent functionalities and
higher dendrimer generations, several strategies were elabo-
rated. The first access to a dendrimer of generation 1 by
mixing phosphorus linkages and viologens involves, as a
preliminary step, the preparation of trifunctionalized violo-
gens: this can be done through double alkylation of 1,3,5-
tris(bromomethyl)benzene, selectively performed on mono-
mer 1b, 3b or 4. Indeed, disubstitution with 2 equiv. of
monomers 1b, 3b and 4 afforded diviologens 16a–18a in
good yields, which, after treatment with NH4PF6, gave rise
to polycationic species 16b–18b. The presence of a remain-
ing CH2Br function in 16b and 18b allows a second reaction
with pyridine–pyridinium monomer 1b leading to the for-
mation of asymmetric trisviologens 19 and 20 in satisfac-
tory yields (from 53% for 19 to 70% for 20, Scheme 4).
These trifunctionalized trisviologens contain an aldehyde
function and either two cyano or two phosphonate groups.
They can be used as building blocks for the formation of
generation-1 dendrimers. For example, condensation of 19
or 20 with 11 leads directly to generation-1 dendrimers 21
and 22 (Scheme 5).
Scheme 4. Synthesis of asymmetric bisviologens 16a–18a and trisvi-
ologens 19 and 20.
These unprecedented types of dendrimers are equipped
with hexahydrazidocyclotriphosphazene linkage at the focal
point, trisubstituted benzene units as branching points, 18
viologens within the dendritic structure, 12 phosphonate (or
cyano) end groups, and 36 PF6 anions. The same type of
experiment can be performed with 10 instead of 11
(Scheme 5): in this case, generation-1 dendrimers 23 and 24
are also formed in high yields and were fully characterized
by NMR and IR spectroscopy and mass spectrometry (see
the Experimental Section and Figure 1 for NMR assign-
ment).
Scheme 5. Synthesis of generation-1 dendrimers.
Figure 1. Numbering used for NMR assignments.
Eur. J. Org. Chem. 2012, 269–273
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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271

N. Katir, J. P. Majoral, A. El Kadib, A.-M. Caminade, M. Bousmina
SHORT COMMUNICATION
18.1 Hz, 3JH,H = 7.3 Hz, 24 H, CH2-CH2P), 3.30 (br. s, 18 H, CH3-
Conclusions
3
N), 4.02–4.12 (m, 48 H, OCH2-CH3), 4.90 (dt, JP,H = 14.8 Hz,
3JH,H = 7.3 Hz, 24 H, CH2-CH2P), 5.85 (s, 12 H, C4-CH2), 5.90 (s,
New symmetric and asymmetric viologen monomers, di-
mers, and trimers have been shown to be versatile building 36 H, C11-CH2, C13-CH2, C15-CH2), 7.46 (d, 3JH,H = 7.7 Hz, 12 H,
3
H3), 7.69 (d, JH,H = 7.7 Hz, 12 H, H2), 7.74 (s, 18 H, H12, H14,
blocks for the synthesis of a number of polycationic dendri-
mers incorporating linear and (or) cyclic phosphorus units.
In all cases, divergent points of each generation are different
and can be cyclotriphosphazene, thiophosphotrihydrazido,
or di- or trisubstituted benzene linkages. These different
ways of preparation of mixed phosphorus–viologen-con-
taining dendrimers illustrated the potentiality of these
methodologies for the design of new types of dendritic
structures. Several properties and applications of these den-
3
H16), 7.78 (br. s, 6 H, CH=N), 8.46 (d, JH,H = 5.1 Hz, 72 H, H6,
3
H9, H18, H21), 9.04 (d, JH,H = 5.5 Hz, 72 H, H5, H10, H17, H22)
3
ppm. 13C{1H} NMR (101 MHz, CD3CN): δ = 15.67 (d, JP,C
=
5.9 Hz, OCH2-CH3), 26.53 (d, 1JP,C = 140.6 Hz, CH2-CH2P), 31.91
(br. s, CH3-N), 56.61 (CH2-CH2P), 62.39 (d, 2JP,C = 6.4 Hz, OCH2-
CH3), 63.57 (C11-CH2, C13-CH2, C15-CH2), 64.31 (C4-CH2), 127.09
(C2), 127.00, 127.44, 127.53 (C6, C9, C18, C21), 129.81 (C3), 131.77
(C12, C14, C16), 132.39 (C1), 135.00 (C11, C13, C15), 136.45 (br. d,
3JP,C = 14.8 Hz, CH=N), 137.77 (C4), 145.59, 145.89, 146.14 (C5,
dritic structures are under active investigation as is the ex- C10, C17, C22), 150.32, 150.41, 150.55 (C7, C8, C19, C20) ppm.
31P{1H} NMR (162 MHz, CD3CN): δ = 17.29, 23.63 ppm. MS
tension of these methods to the design of dendrimers of
higher generations.
(ESI): m/z = 1809.8 [M – 6PF6]6+
.
Supporting Information (see footnote on the first page of this arti-
cle): Synthesis and characterization of 1a–3a, 4, 5a–9a, 5b–9b, 12,
14, 15, 16a–18a, 16b–18b, 19, 20, 23, and 24.
Experimental Section
13: To a solution of 5b (0.25 g, 0.365 mmol) in acetonitrile (10 mL)
was added a solution of 11 (0.025 g, 0.06 mmol) in acetonitrile
(3 mL). This mixture was stirred overnight. The solvent was re-
Acknowledgments
1
moved in vacuo. An orange solid was obtained (0.25 g, 95%). H
Thanks are due to the Moroccan Foundation for Advanced Sci-
ence, Innovation and Research (Rabat, Morocco) and to Centre
National de la Recherche Scientifique (CNRS) (France) for finan-
cial support.
NMR (300 MHz, CD3CN): δ = 3.29 (s, 18 H, CH3-N), 5.79 (s, 12
3
H, C11-CH2), 5.93 (s, 12 H, C4-CH2), 7.45 (d, JH,H = 8.4 Hz, 12
3
H, H3), 7.68 (d, JH,H = 8.1 Hz, 24 H, H13, H2), 7.72 (br. s, 6 H,
3
CH=N), 8.03 (d, JH,H = 8.4 Hz, 12 H, H12), 8.37–8.43 (m, 24 H,
3
H6, H9), 8.83–8.96 (m, 12 H, H5), 9.00 (d, JH,H = 7.1 Hz, 12 H,
H10), 10.07 (s, 6 H, CHO) ppm. 13C{1H} NMR (75 MHz,
CD3CN): δ = 31.94 (CH3-N), 64.12 (C11-CH2), 64.44 (C4-CH2),
127.12 (C2), 127.47, 127.64, 127.69 (C6, C9), 129.79 (C3), 129.84
(C13), 130.33 (C12), 132.22 (C1), 137.43 (C4, C11), 137.76 (CH=N),
138.48 (C14), 145.52, 145.87 (C5, C10), 150.31, 150.56 (C7, C8),
192.14 (CHO) ppm. 31P{1H} NMR (121 MHz, CD3CN): δ =
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1695 (C=O), 1637 (C=N) cm–1.
21: To a solution of 19 (0.09 g, 4.98 10–2 mmol) in acetonitrile
(10 mL) was added 11 (0.0034 g, 8.395ϫ10–3 mmol). The mixture
was stirred overnight. The solvent was removed in vacuo to yield
1
(0.90 g, 96%) as an orange solid. H NMR (300 MHz, CD3CN): δ
3
= 3.29 (br. d, JP,H = 8.9 Hz, 18 H, CH3-N), 5.85 (s, 12 H, C4-
CH2), 5.89 (s, 36 H, C11-CH2, C13-CH2, C15-CH2), 5.93 (s, 24 H,
C23-CH2), 7.36–7.51 (m, 12 H, H3), 7.57–7.70 (m, 36 H, H24, H2),
7.69 (br. s, 6 H, CH=N), 7.76 (s, 18 H, H12, H14, H16), 7.84 (d,
3JH,H = 8.5 Hz, 24 H, H25), 8.37–8.50 (m, 72 H, H6, H9, H18, H21),
8.96–9.10 (m, 72 H, H5, H10, H17, H22) ppm. 13C{1H} NMR
(75 MHz, CD3CN): δ = 31.92 (br. d, 2JP,C = 9.6 Hz, CH3-N), 63.57
(C11-CH2, C13-CH2, C15-CH2), 63.73 (C23-CH2), 64.29 (C4-CH2),
113.41 (C26), 118.02 (CN), 127.02 (C2), 127.44, 127.54, 127.64 (C6,
C9, C18, C21), 129.67 (C3), 129.93 (C24), 131.79 (C12, C14, C16),
132.32 (C1), 133.26 (C25), 134.95, 135.03 (C11, C13, C15), 136.20 (br.
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Design of Phosphorus–Viologen Dendritic Structures
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Received: September 20, 2011
Published Online: December 2, 2011
Eur. J. Org. Chem. 2012, 269–273
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