Synthesis of nanometer-sized, rigid, and hydrophobic anions

Article abstract of DOI:10.1002/anie.201007070

Size matters: The strategy of divergent dendronization allows for the synthesis of unprecedented large, rigid, and bulky anions (see picture). Their size, density, and chemical nature of surface can be tailored to obtain more hydrophobic, less nucleophilic, and more weakly coordinating anions. Copyright

Full text of DOI:10.1002/anie.201007070

Communications
DOI: 10.1002/anie.201007070
Dendronized Anions
Synthesis of Nanometer-Sized, Rigid, and Hydrophobic Anions**
David Tꢀrp, Manfred Wagner, Volker Enkelmann, and Klaus Mꢀllen*
To increase the size and bulkiness of molecular anions is a
major task for the preparation of more weakly coordinating
anions (WCAs),[1] which have attracted remarkable interest
because of their importance in catalysis,[2] polymerization,[3]
electrochemistry,[4] stabilization of electrophilic cationic spe-
cies,[5] ionic liquids,[6] and battery technology.[7] Among the
many different WCAs reported, tetraphenylborate and its
derivatives are some of the most frequently applied.[8] The
advantageous properties of tetraphenylborates originate from
their relatively large size as compared to more classical
parameters can be addressed and tested individually with
regard to its impact on coordination and dissociation proper-
ties.
An ethynyl-functionalized analogue of tetraphenylborate
is required to enable its rigid dendronization with polyphe-
nylenes. Tetraphenylborates are easily accessible from lith-
iation of aryl halides and conversion with boron trichloride in
diethyl ether. Following this method, ethynyl-functionalized
aryl bromide 2 was synthesized and converted into the
corresponding functionalized borate salt 3 (Scheme 1). After
À
WCAs such as BF4 and PF6À. The increase of anion size
results in a reduction of Coulomb interaction between ions of
opposite charge and thus promotes their dissociation in low-
polarity solvents.[1] Both fluorine and CF3 groups have been
introduced into tetraphenylborates, thus helping to increase
their hydrophobicity,[9] and also largely improving their
stability against protic acids and oxidants.[10] In order to
further reduce the coordination strength of tetraphenylbo-
rates, synthetic routes were proposed to increase borate size
by introduction of yet larger and bulkier ligands.[3a,11] How-
ever, if the ligand is too large and bulky, its steric demand
precludes the synthesis of the according borate.
Herein we aim to circumvent this steric limitation for the
first time by means of divergent dendritic growth. To realize
this strategy, we chose to firstly synthesize a relatively small
but functionalized tetraphenylborate, which could subse-
quently be grown bulkier and larger. The final goal is to
encapsulate the ion into a large, nonpolar, and bulky scaffold
and to thereby sterically crowd out its counterion. An ethynyl
function enables the divergent build-up of polyphenylene
dendrons by Diels–Alder cycloaddition.[12] Polyphenylenes
are ideal dendrons because they 1) are chemically very stable,
2) are hydrophobic and lack basic oxygen or nitrogen sites
that occur in other dendrons, and 3) based on their rigidity
supply the ion with a shape-persistent, noncollapsible shell.[12]
The strategy of dendronization presented here allows for
several structural parameters of the anion to be easily
modified, such as anion size (dendrimer generation), density
of the shell (degree of branching), or the chemical nature of
its surface (choice of building blocks). Each of these anion
Scheme 1. Syntheses of ethynyl-functionalized tetraphenylborates 4
and 8: a) TIPS-acetylene, [PdCl2(PPh3)2], CuI, NEt3, toluene, 08C, 16 h,
97%; b) nBuLi, BCl3, Et2O, À788C–RT, 12 h, 46%; c) TBAF, THF, RT,
2 h, 81%; d) TIPS-acetylene, [PdCl2(PPh3)2], CuI, NEt3, toluene, 808C,
16 h, 94%; e) nBuLi, BCl3, Et2O, À788C–RT, 12 h, 52%; f) TBAF, THF,
RT, 2 h, 71%.
removal of triisopropylsilyl (TIPS) groups with tetrabutylam-
monium fluoride (TBAF), the desired ethynyl-functionalized
tetraphenylborate 4 was obtained. The compound can easily
be grown in large, transparent needles of up to 2 cm length
(see the Supporting Information for crystal structure). Also, a
fluorinated analogue 8 of ethynyl-functionalized borate 4 was
synthesized because of its expected improved stability (see
the Supporting Information for crystal structure).
A common solvent for the build-up of polyphenylene
dendrimers by [4+2] Diels–Alder cycloadditions is o-
xylene.[13] However, herein it was not chosen because of its
poor dissolving power for polar compounds. Instead, all
cycloaddition reactions were carried out in diglyme (di(2-
methoxyethyl)ether), which readily dissolves borate salt 4 and
can be heated to the elevated temperatures (about 1608C)
required for thermal cycloaddition. Under these conditions,
tetracyclone was introduced to borate 4, hence yielding the
[*] D. Tꢀrp, Dr. M. Wagner, Dr. V. Enkelmann, Prof. Dr. K. Mꢀllen
Max-Planck-Institut fꢀr Polymerforschung
Ackermannweg 10, 55128 Mainz (Germany)
Fax: (+49)6131-379-350
E-mail: muellen@mpip-mainz.mpg.de
[**] We gratefully acknowledge financial support of this work by the
Deutsche Forschungsgemeinschaft (DFG) within the frame of
Sonderforschungsbereich (SFB) 625. D.T. thanks the Graduate
School of Excellence “Material Science in Mainz” (MAINZ) for a
scholarship.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201007070.
4962
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Angew. Chem. Int. Ed. 2011, 50, 4962 –4965

first-generation, rigidly dendronized borate 11. Similarly, a
perfluorinated analogue of tetracyclon[14] was used for the
synthesis of a surface-fluorinated dendronized borate 12.
Both first-generation dendronized borates 11 and 12
represent larger tetraphenylborate species than ever synthe-
sized before. Although their direct synthesis from already
dendronized ligands is sterically inhibited, a subsequent
divergent dendronization can be easily performed. As evi-
denced by their crystal structures[15] (Figure 1), the polyphen-
Figure 2. Size comparison for modelled structures of tetraphenylborate
9 and rigidly dendronized borate 17.
constant e ꢀ 4.8) or toluene (e ꢀ 2.4). It can be expected that
both increased steric shielding and surface fluorination will
promote ion dissociation in low-polarity media, which will
yield larger numbers of free charge carriers and should hence
be reflected in better conductivities of their solutions.
However, at the same time, the lower mobilities of larger-
sized dendronized borates will decrease conductivity. To
investigate the individual effects of anion size, density, and
nature of the surface on the coordination and dissociation
properties of dendronized borates, conductivities s of THF
solutions (c = 0.001 molLÀ1; T= 298 K) of TBA+ borate salts
9 to 15 were measured (Table 1).
Figure 1. Crystal structures of first generation rigidly dendronized
borate 11 (left) and its surface-fluorinated analogue 12 (right); solvent
molecules omitted for clarity; TBA+ carbon atoms in red for better
contrast.
ylene dendrons form a large, rigid, and bulky extension of the
inner borate core. The achieved increase of steric shielding
should promote ion dissociation. In case of borate 12, weak
coordinative interactions additionally should be suppressed
by its fluorinated surface.
The advantage of divergent dendritic growth was further
exploited for the build-up of yet far larger and bulkier borate
anions (see Scheme 2, borates 13 to 17). For their synthesis we
made use of the fluorinated analogue 8 of ethynyl-function-
alized tetraphenylborate 4, which proved to be much more
stable under high-temperature conditions. While fluorination
of the core (borates 13 to 17) helped to largely improve borate
stability, characterization by means of NMR spectroscopy was
complicated because of the occurrence of atropisomerism.[16]
Nevertheless, completion of cycloaddition could easily be
followed by means of MALDI-TOF mass spectrometry.
In addition to normally branched second and third
generation dendronized borates 14 and 16, highly branched
analogues 15 and 17 were synthesized to maximize the effect
of steric screening. The largest dendronized borate 17
represents a molecularly defined anion with a diameter of
more than 6 nm. To illustrate the increase in anion size, a
comparison of the modelled structures[17] of tetraphenylbo-
rate 9 and rigidly dendronized borate 17 is shown in Figure 2.
Anions of this size and bulkiness are clearly far beyond the
reach of all previously applied synthetic methods. The salts of
these anions therefore constitute a whole new class of
materials.
Table 1: Molar masses M and conductivities s of borates 9 to 15.
compound
M [gÀ1 molÀ1
]
s [mScmÀ1 [a]
]
9
561.7
921.5
21.9
34.6
24.4
36.5
23.6
26.3
27.6
10
11
12
13
14
15
2083.6
3522.9
2371.5
5415.3
8459.1
[a] Measured in THF at 298 K, c=0.001 molLÀ1 with a relative accuracy
of 0.5%.
The conductivity values slightly increase with increasing
anion size despite the substantial decrease of anion mobility
accompanied therewith. For example, dendronized borate 11
exhibits an even slightly higher conductivity than non-
dendronized borate 9, despite possessing only half of its
mobility.[18] Hence, reduced mobility of borate 11 must have
been overcompensated by an increased degree of dissociation
of its TBA+ salt in THF as compared to borate 9. The
tendency towards stronger dissociation can exclusively be
attributed to better steric shielding of charge by the hydro-
phobic dendritic shell. The same arguments apply for the
comparison of borates 10 and 12, where dissociation is further
enhanced because of surface fluorination. Finally, the obser-
A direct effect of polyphenylene dendronization of
borates is an increase of their hydrophobicity, which is
reflected in a largely increased solubility of their salts even
in solvents of low polarity such as chloroform (dielectric
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Communications
Scheme 2. Chemical structures (2D projections) of rigidly dendronized borates synthesized herein.
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Keywords: borates · dendrimers · dendronized anions ·
electrolytes · weakly coordinating anions
.
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20.6612(6) ꢉ;
103.0393(9)8, V= 7643.7(4) ꢉ3; 49311 reflections measured;
17080 unique reflections, int = 0.061, 14042 reflections
b = 12.3816(3) ꢉ;
c = 30.6700(9) ꢉ;
b =
R
observed (I > 2s(I)). Refinement on F; R = 0.054: Rw = 0.060.
ꢀ
12: C190H122Cl24F80BN; triclinic; space group P1; a =
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21.3889(8) ꢉ;
89.752(3)8; b = 107.915(4)8; g = 89.972(4)8; V= 10198.9(10) ꢉ3;
59318 reflections measured; 48148 unique reflections; Rint
0.059; 36411 reflections observed (I > 2s(I)). Refinement on
F; R = 0.0739; Rw = 0.0842.
b = 14.8083(7) ꢉ;
c = 33.5428(10) ꢉ;
a =
=
[16] The observed atropisomerism will be discussed in detail in an
upcoming publication.
[17] Spartan version ꢈ04, Wavefunction Inc., 18401 Von Karman
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[18] Diffusion coefficients D and according mobilities m = D/kBT of
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