Download Frieder Jäkle FUNCTIONAL MATERIALS AND ORGANOBORON

Frieder Jäkle
Project A
FUNCTIONAL MATERIALS AND ORGANOBORON NANOSTRUCTURES
VIA CONTROLLED POLYMERIZATION METHODS
The incorporation of organoborane moieties into polymers is not only of great interest for the development of
supported Lewis acid catalysts, but also provides new opportunities for applications in the field of materials
chemistry. Our initial synthetic approach for the generation of
well-defined boron-containing polymers has relied on the
“masking” of styrenic polymers with silyl groups that can then
quantitatively be replaced with boron tribromide to give access to
the reactive polymer poly(4-dibromoborylstyrene). This unusual
functional polymer is highly versatile in that the Lewis acidity of
the individual boron sites can readily be fine-tuned through highyield substitution reactions. We thereby gain access to a family of
new boron polymers (PS-BR2; R = alkyl, aryl, alkoxy, amino
groups), including highly Lewis acidic triarylborane polymers that
are otherwise very difficult to prepare.
In contrast to most previously reported organoboron polymers, we are able to realize different polymer
architectures and place the Lewis acid groups at well-defined positions. Our approach can be directly applied to the
synthesis of random copolymers, telechelic (end-functionalized) polymers, and block copolymers containing
organoborane moieties at well-defined positions of the polymer chain.
More recently, we have also succeeded in the direct polymerization of functional polymers, thus further expanding
the scope of this chemistry. We expect these novel materials to lead us into intriguing new areas including micellar
catalysis, nano-structured materials, and even drug delivery applications in the case of B(OH)2-functionalized
polymers. Some recent projects are described in a bit more detail in the following.
I Luminescent Organoboron Polymers for Sensor Systems and Device Applications.
Molecular species such as aluminum tris(8-hydroxyquinolate) (Alq3) and related diphenylboron-quinolate (Ph2Bq)
are known to possess excellent properties as electron-conduction and light emitting components in organic light
emitting diodes (OLEDs) and related devices. The use of polymeric materials is expected to be advantageous due to
the improved processability, which facilitates device fabrication. We
found that polymer substitution reactions as outlined above provide a
straightforward route to suitable organoboron polymers, such as the
novel organoboron quinolate polymers shown here. These polymers
are highly soluble and luminescent thin films can be cast from
solution. Importantly the emission characteristics of these materials
are readily tuned by variation of the substituent X on the quinoline
moiety, covering almost the entire visible spectrum from blue to red.
We have further extended this work to block copolymers that feature organoboron quinolate chromophores as
pendant groups of one of the polyolefin blocks. In this case, direct polymerization techniques can be utilized to
successively polymerize each of the constituent blocks. Using this strategy we have prepared fluorescent
amphiphilic block copolymers and the first examples of organoboron star polymers by reversible additionfragmentation chain transfer (RAFT) polymerization of fluorescent styryl-type organoboron monomers.
Well-defined luminescent star polymers can be
obtained with a boron quinolate crosslinker and
PNIPAM-b-PS in the shell. A combination of dynamic
light scattering and TEM imaging was used to confirm
the structures. The star polymers further aggregate in
water
to
form
strongly
green-luminescent
superstructures. These polymers could prove useful for
biological imaging applications.
Application of our synthetic strategy to tricoordinate organoboron polymers results in interesting sensory materials,
which function based on the selective interaction of nucleophiles with the Lewis acidic boron centers. Coordination
of the substrate to boron triggers a response in the photophysical characteristics of suitably substituted luminescent
organoboron polymers (e.g. bithiophene and mesityl substituents on boron shown here). This response is specific to
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© Frieder Jäkle, all rights reserved
Project A
the nature of the nucleophile under investigation and can readily be verified via
spectroscopic screening. We discovered that our polymers selectively bind
fluoride and cyanide, a process that leads to a distinct change in the absorption
and emission color. Intriguingly, amplification effects were observed in the
case of fluoride binding, allowing for highly efficient detection in the low
micromolar concentration range. The amplification effect, which is more
commonly encountered for conjugated polymers (see Project B), is tentatively
attributed to through-space energy migration.
II Organoboron Polymers as “Polymeric Lewis Acids”.
We also succeeded in the synthesis of a highly Lewis acidic polymer containing pentafluorophenyl substituents on
boron, PS-B(C6 F5)2. This unusual polymer represents a supported analog of the industrially important class of
fluorinated organoborane Lewis acid catalysts. The key step in the synthesis of PS-B(C6F5)2 relies on the selective
exchange of the bromine substituents in PS-BBr2 with pentafluorophenyl copper, a novel aryl transfer reagent
developed in our laboratory (see Project D). These and related polymeric Lewis acids are promising as supported
Lewis acid catalysts, as activators for olefin polymerization, and for small molecule activation in combination with
bulky Lewis bases (so-called “Frustrated Lewis Pairs”).
Further, we study the coordination of Lewis bases to our polymeric Lewis acids with special emphasis on the
aggregation behavior with donor-functionalized polymers. Fine-tuning of the Lewis acidity of the boron centers
allows us to control the strength and reversibility of the donor acceptor bonding. For instance, triarylborane
polymers PS-BR2 (R = thienyl, C6F5) and borane polymers PS-BH2 strongly coordinate nucleophiles as exemplified
in the formation of soluble, isolable polymeric donor acceptor complexes with pyridines. A temperature-dependent
equilibrium between coordinated and non-coordinated sites on the other hand is established with weak donors. The
supramolecular assembly of polymeric Lewis acids and Lewis bases is expected to allow reversible formation of
complex polymer architectures such as block- and graft-copolymers and of cross-linked materials.
III Boronic-acid Functionalized Block Copolymers.
Polyelectrolytes and amphiphilic block-copolymers are well known for
their unique physical properties including the formation of unusual selfassembled structures in solution as well as the bulk material. In an
intriguing variation of this theme, we have prepared boronic-acid
functionalized block copolymers that are pH sensitive: at high pH they
essentially behave like ionic borate-polymers and form spherical
micelles in solution as evident from TEM analysis. However, when
dissolved at a pH close to 7, (reversible) cross-linking occurs and larger
aggregates are observed by TEM analysis. Thanks to their ability to
reversibly bind sugars and other biomolecules, they show great potential
as stimuli-responsive materials and in delivery applications.
IV Metallopolymers and Supported Catalysts.
We have also successfully converted our organoboron precursor
polymers to ionic borate and boronium-based polyelectrolytes and
amphiphilic organoboron diblock copolymers. They form very
regular micellar structures in solution based on dynamic light
scattering and TEM studies. Loading with transition metals (e.g.
Rh) leads to catalysts that are encapsulated in the core of the
micelles.
While the borate moieties are weakly coordinating in the previously
described materials, strong metal binding can be achieved with tris(1pyrazolyl)borate (Tp)-functionalized polymers. Tp complexes are
widely applied in catalysis and materials chemistry. Polytopic Tp
complexes that are anchored to polystyrene were prepared through a
carefully designed synthetic protocol that allows for selective polymer
attachment and facile control over the degree of loading with the
ligand binding sites. This polymeric ligand scaffold in turn serves as a
© Frieder Jäkle, all rights reserved
Project A
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versatile precursor to metal-containing polymers. Complexation with CpRu moieties was studied to demonstrate the
synthetic utility. In collaboration with the Sheridan group we are extending this concept to other ligand types.
Tris(2-pyridyl)borates represent a new robust and tunable ligand
family that combines the desirable complexation behavior of
“scorpionate” ligands with the
high stability of weakly
coordinating arylborate anions. Initial studies on the ligand
properties of tris(pyridyl)borates have focused on the ability of
the tris(pyridyl)borate to form complexes with M2+ metal ions
(M = Mg, Fe, Mn, Cu). As an example, treatment with FeCl2 in
THF/MeOH in the presence of NEt3 resulted in a red solid that
was purified by column chromatography and recrystallized from
toluene. Based on the NMR data, the iron complex is diamagnetic in solution at RT, indicating a low spin
configuration, which is consistent with solid state data derived from a single crystal X-ray diffraction analysis. UVvis measurements were performed in CH2Cl2 to further explore this aspect. Absorption maxima at ca. 480 and 430
tBu
nm can be assigned to M→L charge transfer
(MLCT). Given the high stability, the strongly
N
N
N
donating ability toward main group and transition
Et3N
tBu
t
2
+ FeCl2
Fe
B
B
Bu
metals, and the possibly for modular synthesis and
B
THF
N
N
N
N
ligand fine-tuning using different pyridyl
N
HN
derivatives, we anticipate broad applications of this
new ligand class in catalysis, bioinorganic chemistry, and in the field of supramolecular polymer chemistry. Current
work focuses on polymerizable derivates of the pyridylborates and related charge-neutral pyridylsilane species.
Relevant Publications:
Reviews: F. Cheng, F. Jäkle, Polym. Chem. 2011, 2, 2122-2132; F. Cheng, F. Jäkle, Chapter 3 in ACS Symposium
Series, Progress in Controlled Radical Polymerization (Eds: Kris Matyjaszewski, Brent S. Sumerlin, Nicolay V.
Tsarevsky). 2012, pp 28-38; F. Jäkle, Chem. Sus. Chem. 2011, 4, 325-326; F. Jäkle, Coord. Chem. Rev. 2006, 250,
1107-1121; Jäkle, J. Inorg. Organomet. Polym. Mater. 2005, 15, 293-307.
Topic I:
F. Cheng, E. M. Bonder, A. Doshi, F. Jäkle, Polym. Chem. 2012, 3, 596-600; F. Cheng, E. M. Bonder,
F. Jäkle, Macromolecules 2012, 45, 3078-3085; H. Kuhtz, F. Cheng, S. Schwedler, Lena Böhling, A. Brockhinke,
L. Weber, K. Parab, F. Jäkle, ACS Macro Lett. 2012, 1, 555-559; K. Parab, A. Doshi, F. Cheng, F. Jäkle,
Macromolecules 2011, 44, 5961-5967; F. Cheng, F. Jäkle, Chem. Commun. 2010, 46, 3717-3719; K. Parab, F.
Jäkle, Macromolecules 2009, 42, 4002-4007; Y. Qin, I. Kiburu, S. Shah, F. Jäkle, Macromolecules 2006, 39, 90419048; Y. Qin, I. Kiburu, K. Venkatasubbaiah, S. Shah, F. Jäkle, Org. Lett. 2006, 8, 5227-5230; K. Parab, Y. Qin, F.
Jäkle, J. Am. Chem. Soc. 2006, 128, 12879-12885; Y. Qin, C. Pagba, P. Piotrowiak, F. Jäkle, J. Am. Chem. Soc.
2004, 126, 7015-7018.
Topic II: Y. Qin, F. Jäkle, J. Inorg. Organomet. Polym. Mater. 2007, 17, 149-157; Y. Qin, C. Cui, F. Jäkle,
Macromolecules 2007, 40, 1413-1420; A. Doshi and F. Jäkle, Main Group Chem. 2006, 5, 309-318; Y. Qin, G.
Cheng, O. Achara, K. Parab, F. Jäkle, Macromolecules 2004, 37, 7123-7131; Y. Qin, G. Cheng, A. Sundararaman,
F. Jäkle, J. Am. Chem. Soc. 2002, 124, 12672-12673.
Topic III: C. Cui, E. M. Bonder, Y. Qin, F. Jäkle, J. Polym. Sci. A, Polym. Chem. 2010, 48, 2438-2445; Y. Qin,
V. Sukul, D. Pagakos, C. Cui, F. Jäkle, Macromolecules 2005, 38, 8987-8990.
Topic IV: C. Cui, R. A. Lalancette, F. Jäkle, Chem. Commun. 2012, 48, 6930-6932; Y. Qin, P. Shipman, F. Jäkle,
Macromol. Rapid Commun. 2012, 33, 562-567; C. Cui, E. M. Bonder, F. Jäkle, J. Am. Chem. Soc. 2010, 132, 18101812; C. Cui, J. Heilmann-Brohl, A. Sanchez Perucha, M. D. Thomson, H. G. Roskos, M. Wagner, F. Jäkle,
Macromolecules 2010, 43, 5256-5261; C. Cui, F. Jäkle, Chem. Commun. 2009, 2744-2746; C. Cui, E. M. Bonder,
F. Jäkle, J. Polym. Sci. A, Polym. Chem. 2009, 47, 6612-6618; C. Cui, F. Jäkle, Chem. Commun. 2009, 2744-2746;
Y. Qin, C. Cui, F. Jäkle, Macromolecules 2008, 41, 2972-2974.
© Frieder Jäkle, all rights reserved
Project A
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