An approach towards functional dendrimers based on the hydroboration reaction and spontaneous boron-nitrogen bond formation

Article abstract of DOI:10.1002/ejic.200500415

The reaction of 1,4-bis- or 1,3,5-tris(bromoboryl)benzenes 4 and 6, respectively, with 2 or 3 equiv. of 4,4′-bis(but-3″-enyl)-2, 2′-bipyridyl (2) leads to the corresponding 2,2′-bipyridylboronium cations 5 and 7 in almost quantitative yield. Using the mon

Full text of DOI:10.1002/ejic.200500415

FULL PAPER
An Approach Towards Functional Dendrimers Based on the Hydroboration
Reaction and Spontaneous Boron–Nitrogen Bond Formation
Monika C. Haberecht,[a] Michael Bolte,[a] Hans-Wolfram Lerner,[a] and Matthias Wagner*[a]
Keywords: Boranes / Dendrimers / Hydroboration / N ligands
The reaction of 1,4-bis- or 1,3,5-tris(bromoboryl)benzenes 4
and 6, respectively, with 2 or 3 equiv. of 4,4Ј-bis(but-3ЈЈ-
enyl)-2,2Ј-bipyridyl (2) leads to the corresponding 2,2Ј-bipyr-
idylboronium cations 5 and 7 in almost quantitative yield.
Using the monocation [(MeC6H4)B(Br){4,4Ј-bis(but-3ЈЈ-enyl)-
2,2Ј-bipyridyl}]+ (3Br) as a model system, it is shown that the
olefinic side-chains of such compounds are readily trans-
formed into alkyldibromoborane functionalities by hydrobor-
ation with HBBr2. Subsequent addition of 4,4Ј-dimethyl-2,2Ј-
bipyridyl leads to the formation of a branched system of three
2,2Ј-bipyridylboronium cations (9Br). The species 5 and 7 can
be regarded as generation zero (G0) 2,2Ј-bipyridylboronium
dendrimers, and the hydroboration/B–N adduct formation se-
quence offers a convenient route for the assembly of higher
generations.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2005)
cene backbone and its Diquat-like substituents,[15] acts as a
fully reversible nine-electron redox system in DMF solu-
tion.[16] In an attempt to generate larger aggregates, we ini-
tially employed 1,1Ј-diborylated ferrocenes and bridging
2,2Ј-bipyridyl derivatives (e.g. quaterpyridine) with the hope
of obtaining polymeric materials. However, macrocyclic di-
mers (e.g. D; Figure 1) always formed as the sole reaction
products irrespective of the experimental conditions ap-
plied.[17] In our second approach, we tried to link ferrocene-
1,1Ј-diylbis(2,2Ј-bipyridylboronium) cations via their boron
atoms by introducing oxygen bridges between them. In con-
trast to our expectations, no polymeric species were formed
but ansa-bridged complexes E (Figure 1) were obtained as
the result of an intramolecular reaction.[12]
We therefore developed a new concept for the prepara-
tion of macromolecules containing larger numbers of 2,2Ј-
bipyridylboronium cations and thus possessing the poten-
tial to act as electron storage media. Our synthetic method-
ology is based on a two-reaction sequence using a sponta-
neous B–N adduct formation in combination with a hydro-
boration reaction (Scheme 1).
To start, multiply borylated core units are required. Fer-
rocene and benzene are particularly well-suited candidates
since they can easily be borylated up to four (ferrocene)[18]
or up to three times (benzene).[19] In the present study, p-
(dibromoboryl)toluene together with 1,4-bis- and 1,3,5-tris-
(dibromoboryl)benzene will be employed. The other build-
ing blocks are 4,4Ј-bis(but-3ЈЈ-enyl)-2,2Ј-bipyridyl (2) and
dibromoborane, HBBr2, which may either be purchased as
its dimethyl sulfide adduct or prepared in situ from tribro-
moborane and triethylsilane[20,21] in dichloromethane solu-
tion. In order to build the desired macromolecules, we will
follow a divergent reaction sequence (Scheme 1): First, the
Introduction
Dendrimers are macromolecules with well-defined mol-
ecular architectures derived from a central atom or core
with multiple branches. They are gaining great attention
due to their potential applications in, for example, biology,
catalysis and materials science. Apart from purely organic
dendrimers, metallodendrimers[1–3] and heteroatom-based
dendrimers[4–6] represent an important area of research. In
1994, Puddephatt reported organometallic dendrimers fea-
turing (2,2Ј-bipyridyl)PtIV chelates in every generation.[7,8]
Since then, numerous dendrimers incorporating (polypyri-
dyl)metal complexes have been synthesised, mainly with the
aim of studying photoinduced energy transfer and multi-
electron redox processes.[9]
Research in our group focuses on the chemistry of main
group chelates of the 2,2Ј-bipyridyl (bipy) ligand, particu-
larly on 2,2Ј-bipyridylboronium cations A (Figure 1), which
are relatives of the organic electron acceptor Diquat (B;
Figure 1) and thus behave as reversible two-electron redox
systems.[10–15]
Cations A form spontaneously and under very mild con-
ditions upon treatment of haloboranes XBR2 (X = Cl, Br)
with 2,2Ј-bipyridyl. For example, starting from 1,1Ј,3,3Ј-tet-
rakis(bromoboryl)ferrocene, the ferrocene derivative C
(Figure 1) bearing four redox-active 2,2Ј-bipyridylboronium
units has been synthesised in almost quantitative yield.[16]
Compound C, which possesses an unusual deep-purple col-
our due to charge-transfer interactions between the ferro-
[a] Institut für Anorganische Chemie, Johann Wolfgang Goethe-
Universität Frankfurt am Main,
Marie-Curie-Straße 11, 60439 Frankfurt, Germany
Fax: +49-69-798-29260
E-mail: Matthias.Wagner@chemie.uni-frankfurt.de
Eur. J. Inorg. Chem. 2005, 4309–4316
DOI: 10.1002/ejic.200500415
© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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M. C. Haberecht, M. Bolte, H.-W. Lerner, M. Wagner
FULL PAPER
Scheme 1. Concept for the synthesis of branched 2,2Ј-bipyridyl-
boronium cations by hydroboration with dibromoborane.
for further dendrimer syntheses, and (iii) to describe our
first results regarding the synthesis of G1 dendrimers.
Results and Discussion
Syntheses
Treatment of p-(dibromoboryl)toluene (1Br)[19] with tet-
ramethylstannane or diethyl ether gives p-[bromo(methyl)-
boryl]toluene (1Me) and p-[bromo(ethoxy)boryl]toluene
(1OEt), respectively (Scheme 2). Addition of 1Br, 1Me or
1OEt to the 2,2Ј-bipyridyl derivative 2[22] in toluene or
dichloromethane at ambient temperature leads to the for-
mation of 3Br, 3Me and 3OEt, respectively, in excellent
yields. Compound 3Br, bearing a bromo substituent at the
Figure 1. 2,2Ј-Bipyridylboronium cation (A), Diquat (B), ferrocene
C bearing four 2,2Ј-bipyridylboronium substituents, macrocyclic
dinuclear ferrocene complex D, and ansa-ferrocene E.
core unit is treated with the appropriate amount of substi- boron atom, closely mimics the reaction intermediates en-
tuted 2,2Ј-bipyridyl to prepare the zeroth generation (G0) countered during the hydroboration/adduct formation cycle
of the dendrimer. Second, the G0 2,2Ј-bipyridylboronium (cf. Scheme 1). However, B–Br bonds are prone to hydroly-
cation is hydroborated with HBBr2 at its peripheral olefin sis and it was therefore desirable to have also the moisture-
substituents to generate a molecule bearing alkyldibromo- stable methyl derivative 3Me at hand. The ethoxy-2,2Ј-bipy-
borane side-chains. In the third step, addition of the func- ridylboronium cation 3OEt is described here because it gave
tionalised 2,2Ј-bipyridyl gives access to the G1 dendrimer. crystals suitable for X-ray crystal structure analysis. Most
The reaction cycle may then be repeated to attach further importantly, the bromo substituent of 3Br can be replaced
2,2Ј-bipyridylboronium cations and to obtain higher gener- by a methoxy group (3OMe; Scheme 2) when 3Br is dis-
ation dendrimers. The purpose of this paper is (i) to present solved in dry methanol in the presence of stoichiometric
an optimised procedure for the hydroboration of bipyridyl- amounts of triethylamine. This reaction proceeds cleanly
boronium cations with olefinic side-chains, (ii) to prove that and quantitatively, and thus offers a convenient way to
up to three bipyridylboronium substituents can be grouped transform the primary products of future dendrimer syn-
around a benzene ring such that it qualifies as a core unit theses into air- and moisture-stable materials.
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An Approach Towards Functional Dendrimers
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Scheme 2. Synthesis of the 2,2Ј-bipyridylboronium bromides 3Br,
3Me, 3OEt and 3OMe.
The bromo(methyl)boryl-substituted benzenes 4 and 6,
which are accessible from SnMe4 and 1,4-bis(dibromo-
boryl)benzene
or
1,3,5-tris(dibromoboryl)benzene,[19]
respectively, in a way similar to the synthesis of 1Me, readily
react with ligand 2 in dichloromethane to form the di- and
trications 5 and 7, respectively (Scheme 3). For purification,
the reaction products were precipitated from their dichloro-
methane solutions by the addition of hexane.
The crucial hydroboration step was carried out using the
bromo-substituted monocation 3Br, since it is the least
complex system available but nevertheless contains all the
functional groups of a growing dendrimer according to
Scheme 1. At the beginning we relied on HBBr2·SMe2 as
Scheme 3. Synthesis of the bis- and tris(2,2Ј-bipyridylboronium)
bromides 5 and 7.
the hydroboration reagent, but it soon turned out that the studies to a derivative of 3Br bearing methyl groups at both
in situ generation of HBBr2 from equimolar amounts of its C-9 atoms. In this case, however, C-9 becomes a chiral
triethylsilane and boron tribromide[20,21] gives better results. atom after hydroboration and a mixture of diastereomers is
In the optimised procedure, a mixture of 3Br and triethylsi- obtained.
lane in dichloromethane is added dropwise at –78 °C to a
Apart from the regioselectivity problem, the crude prod-
solution of tribromoborane in the same solvent. The reac- uct 8 is sufficiently pure for further reaction with 2,2Ј-bipyr-
tion is usually complete after warming to ambient tempera- idyls. A sample of 8 was thus treated with 4,4Ј-dimethyl-
ture. Note that 3 equiv. of BBr3 are required since 1 equiv. 2,2Ј-bipyridyl in dichloromethane and subsequently with
coordinates the bromide counterion of 3Br to form tetra- methanol/triethylamine to synthesise the G1 trication
bromoborate [BBr4]–. As expected, the hydroboration of 9OMe. Attempts to isolate the regioisomers of 9OMe by
3Br is not fully regioselective such that isomers 8t (t = ter- reversed-phase HPLC failed since a satisfactory separation
minal) and 8i (i = internal) with a BBr2 substituent attached could not be achieved.
to C-10 and C-9, respectively, are obtained [approx. ratio
As already mentioned above, tetrabromoborate is created
(1H NMR spectroscopy): 8t/8i = 4:1; see below] (Scheme 4). as a new counterion in the hydroboration reaction. This
In order to increase the regioselectivity, we extended our causes additional problems, since [BBr4]– reacts stoichio-
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FULL PAPER
the formation of contact ion pairs, an interpretation also
supported by the structure of 3OEt in the solid state in
which the two C-3–H-3 vectors act as “chelating hydrogen-
bond donors” towards the bromide ion (corresponding
H···Br contacts in the range between 2.80 and 3.00 Å (Fig-
ure 2; for the theoretical treatment of a related system, see
ref.[24]). Hydroboration of 3Br leads to the disappearance
of the signals for the olefinic protons [δ(H-9) = 5.81, δ(H-
10E) = 5.05, δ(H-10Z) = 4.99 ppm] and carbon atoms [δ(C-
9) = 135.6, δ(C-10) = 117.1 ppm] and the appearance of
new signals in the aliphatic region of the spectrum [8t: δ(H-
9) = 1.80, δ(H-10) = 1.71 ppm; δ(C9) = 26.4, δ(C-10) =
36.2 ppm]. Moreover, the 11B NMR spectrum of 8 now re-
veals two resonances possessing chemical shift values typi-
cal of four- and three-coordinate boron nuclei [δ(Bbipy) =
6.5, δ(BBr2) = 57.3 ppm; integral ratio Ϸ 1:2]. Upon ad-
dition of 2 equiv. of 4,4Ј-dimethyl-2,2Ј-bipyridyl to 8, the
signal at δ = 57.3 ppm disappears and an extremely broad
signal occurs at δ Ϸ 7 ppm, which clearly shows that all
former BBr2 groups are now tetracoordinate.[23] There are
three possible regioisomers of 9OMe, containing between
two and three different kinds of 2,2Ј-bipyridyl fragments.
1
As a consequence, both the H and the 13C NMR spectra
of 9OMe are poorly resolved and very complex. It is there-
fore not possible to assign resonances to individual atoms
Scheme 4. Hydroboration of 3Br, subsequent formation of the
branched cation 9Br, and transformation of 9Br into the moisture-
stable derivative 9OMe.
metrically with 2,2Ј-bipyridyl similar to the alkyldibromo-
borane side-chains of 8. As a consequence, the preparation
of the first generation dendrimer 9Br requires 3 rather than
2 equiv. of the ligand to achieve quantitative conversion. In
our search for more suitable counterions, we finally found
that [B(C6F5)4]– is not affected by BBr3/HSiEt3, which
makes it the counterion of choice for further dendrimer
syntheses.
NMR Spectroscopy
Figure 2. Ball-and-stick model of 3OEt in the solid state. For clar-
The 11B NMR shifts of the 2,2Ј-bipyridylboronium cat- ity, only one of the three crystallographically independent mole-
ions appear in the range between δ = 9.8 (3OMe) and
cules within the asymmetric unit is shown. Selected bond lengths
[Å], angles [°] and torsion angles [°]: B(1)–O(1) 1.416(12), B(1)–
C(31) 1.539(16), B(1)–N(11) 1.635(13), B(1)–N(21) 1.666(11),
N(11)–C(12) 1.369(10), C(12)–C(22) 1.473(11), N(21)–C(22)
6.5 ppm (3Br, 7), thus indicating the presence of tetracoor-
dinate boron nuclei.[23] Very similar H NMR spectra are
1
observed for the monocations 3Br, 3Me, 3OEt and 3OMe.
In all these compounds, the position of the H-3 resonance
is strongly dependent on the nature of the counterion and
the choice of solvent [cf. 3Me: δ(H-3) = 10.31 (Br–/CDCl3),
8.87 (Br–/CD3OD), 7.16 ([B(C6H5)4]–/CDCl3), 8.88 ppm
1.334(11); O(1)–B(1)–C(31) 114.7(8), O(1)–B(1)–N(11) 111.2(9),
O(1)–B(1)–N(21) 110.2(7), C(31)–B(1)–N(11) 114.6(8), C(31)–
B(1)–N(21) 111.9(8), N(11)–B(1)–N(21) 92.0(6), C(1)–O(1)–B(1)
123.1(8), C(12)–N(11)–B(1) 115.0(6), C(22)–N(21)–B(1) 113.9(7),
N(11)–C(12)–C(22) 108.1(7), N(21)–C(22)–C(12) 110.8(7), C(13)–
C(12)–C(22) 129.7(8), C(12)–C(22)–C(23) 127.1(8); N(11)–C(12)–
([PF6]–/CDCl3)]. This effect may therefore be attributed to C(22)–N(21) –3.4(12).
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An Approach Towards Functional Dendrimers
FULL PAPER
in this case. However, the proposed molecular structure of along the periphery of a benzene ring to attach up to three
9OMe was clearly proven by mass spectrometry.
branching subunits. Compounds 5 and 7 may therefore be
regarded as generation zero (G0) dendrimers.
In order to optimise the hydroboration reaction, the
monocation 3Br, derived from p-(dibromoboryl)toluene
(1Br) and 4,4Ј-bis(but-3ЈЈ-enyl)-2,2Ј-bipyridyl (2), was em-
ployed as a model system. Quantitative conversion of both
its olefinic side-chains into alkyldibromoboryl substituents
was achieved under mild conditions with BBr3/HSiEt3 as
hydroborating reagent. The regioselectivity of the hydrobor-
ation reaction is high (ratio of terminal vs. internal hydro-
boration Ϸ 4:1). Introduction of methyl groups into the β-
positions of the terminal alkenes eliminates the regioselec-
tivity problem (NMR spectroscopic control) but leads to an
inseparable mixture of diastereomers after hydroboration.
Apart from the fact that 8 was found to be a mixture of
isomers 8t and 8i, the sample obtained was sufficiently pure
for further derivatisation with 4,4Ј-dimethyl-2,2Ј-bipyridyl.
This reaction requires 3 rather than 2 equiv. of bipyridyl
ligand; the third equivalent is consumed by the [BBr4]–
counterion of 8. Bromo-substituted 2,2Ј-bipyridylboronium
cations are sensitive towards hydrolysis, therefore 9Br was
transformed into the air- and moisture-stable derivative
9OMe by treatment with methanol/triethylamine. The suc-
cessful synthesis of 9OMe was proven by ESI mass spec-
trometry.
Overall, the general synthetic concept outlined in this pa-
per has successfully passed its first evaluation. However, the
following modifications are required: (1) Despite numerous
efforts, we have not been able to separate the different re-
gioisomers of 9OMe by HPLC or GPC techniques. Apart
from the fact that the solubilities, polarities etc. of these
isomers are obviously similar, the alkyl(bipyridyl)boronium
cations within the dendrimer branches appear to be less
stable than the aryl(bipyridyl)boronium cations formed
with the core unit. We therefore decided to abandon the
hydroboration step in future syntheses and to look for an
alternative reaction that leads to more stable aryl(bipyridyl)-
boronium ions and does not suffer from any regioselectivity
problems. All these requirements are met by the system (tri-
methylsilyl)arene/BBr3, which gives aryldibromoboranes
and bromotrimethylsilane (in fact, this reaction has already
been employed for the synthesis of our di- and triborylated
core units[19]). Since we already have the 2,2Ј-bipyridyl de-
rivative 10 (Figure 3) at our disposal,[22] there should be no
major problem on switching from the hydroboration reac-
Mass Spectrometry
Low resolution electrospray mass spectra (ESI) in the
positive-ion mode were recorded for the G0 compounds
3Me, 3OMe, 3OEt, 5 and 7. The spectra of the 2,2Ј-bipyrid-
ylboronium salts 3Me, 3OMe, 3OEt reveal exclusively
peaks of the free monocations [3Me – Br]+, [3OMe – Br]+,
and [3OEt – Br]+ at m/z values of 381, 397 and 411, respec-
tively. A similar result was obtained for the diborylated spe-
cies 5, which gave rise to only one peak assignable to the
free dication [5 – 2 Br]2+ (m/z = 328). In the case of the
trisubstituted benzene derivative 7, two peaks with an inte-
gral ratio of 100 (m/z = 512):95 (m/z = 315) appear in the
mass spectrum, which correspond to the di- and trications
[7 – 2 Br]2+ and [7 – 3 Br]3+. An ESI mass spectrum was
also recorded for 9OMe. Again, the most intense peaks
(m/z = 465, 284) by far may be assigned to the cations
[9OMe – 2 Br]2+ and [9OMe – 3 Br]3+
.
X-ray Crystallography
¯
Single crystals of 3OEt (triclinic, space group P1) grew
slowly from an oily sample at room temperature. There are
three crystallographically independent molecules in the
asymmetric unit. Since their structure parameters are equal
within experimental error, the data of only one of the three
molecules are discussed here. As already deduced from the
NMR spectra, the compound contains a tetracoordinate
boron centre chelated by a 2,2Ј-bipyridyl ligand [B(1)–
N(11) = 1.635(13) Å, B(1)–N(21) = 1.666(11) Å; N(11)–
B(1)–N(21) = 92.0(6)°]. Chelation of the small boron atom
leads to a slight distortion of the 2,2Ј-bipyridyl moiety such
that the angles C(13)–C(12)–C(22) and C(12)–C(22)–C(23)
on the outer frame of the aromatic system are stretched to
values of 129.7(8)° and 127.1(8)°, respectively. The terminal
atoms C(19Ј), C(20Ј), C(29Ј) and C(30Ј) of the butenyl
chains are disordered. Although a model with two different
conformations has been employed, the anisotropic displace-
ment parameters of C(19Ј) and C(20Ј) are still rather large.
Conclusions
This paper reports a novel strategy for the generation tion to the Si/B exchange protocol. (2) During the hydro-
of dendritic macromolecules consisting of redox-active 2,2Ј-
bipyridylboronium cations. 1,4-Di- and 1,3,5-triborylated
benzenes are chosen as core units and a hydroboration/B–
N adduct formation sequence is suggested for the assembly
of the individual branches.
To prove the suitability of the benzene core unit, we first
synthesised the air- and moisture-stable benzene derivatives
5 and 7 bearing two and three 4,4Ј-bis(but-3ЈЈ-enyl)-2,2Ј-
bipyridylboronium substituents in their 1,4- and 1,3,5-posi-
tions. The reactions proceed cleanly and quantitatively,
which leads to the conclusion that there is sufficient space ethyl groups.
Figure 3. 2,2Ј-Bipyridyl ligand 10 bearing 2-[(trimethylsilyl)aryl)]-
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M. C. Haberecht, M. Bolte, H.-W. Lerner, M. Wagner
FULL PAPER
boration step, a [BBr4]– ion is created, which is reactive
towards 2,2Ј-bipyridyls. Since the perfluorated tetraphen-
ylborate anion [B(C6F5)4]– turned out to be inert under the
reaction conditions applied, it is advantageous to replace
the Br– ion by [B(C6F5)4]– in future syntheses {e.g. by treat-
ment with Tl[B(C6F5)4] or Ag[B(C6F5)4]}.
2), 142.3 (C-6), 140.0 (C-Arp), 135.6 (C-9), 132.2 (C-Aro), 129.4 (C-
Arm), 129.0 (C-5), 126.8 (C-3), 117.1 (C-10), 35.3 (C-7), 33.1 (C-8),
21.2 (CH3-Ar) ppm; C-Ari not observed.
3Me: A solution of 1Me (0.591 g, 3.00 mmol) in dichloromethane
(5 mL) was added dropwise, at ambient temperature, to 2 (0.793 g,
3.00 mmol) in dichloromethane (10 mL). The solvent volume was
reduced to approximately 5 mL and n-hexane added dropwise,
whereupon 3Me precipitated as a pale-yellow oil. Yield: 1.211 g
1
(88%). H NMR (400.1 MHz, CDCl3, 25 °C): δ = 10.31 (br., 2 H,
Experimental Section
3
3
H-3), 8.22 (d, JH,H = 5.9 Hz, 2 H, H-6), 7.59 (dd, JH,H = 5.9,
4JH,H = 1.4 Hz, 2 H, H-5), 7.06 (d, JH,H = 7.8 Hz, 2 H, H-Arm),
3
General Remarks: All reactions were carried out under oxygen-free
argon or nitrogen using standard Schlenk techniques. Solvents were
distilled under argon from sodium/benzophenone (toluene, n-hex-
ane, n-pentane, triethylamine, methanol) or passed through a 4 Å
molecular sieves column (CH2Cl2, CHCl3) prior to use; the synthe-
ses of p-(dibromoboryl)toluene (1Br),[19] 1,4-bis(dibromoboryl)ben-
zene,[19] 1,3,5-tris(dibromoboryl)benzene[19] and 4,4Ј-bis(but-3ЈЈ-
enyl)-2,2Ј-bipyridyl (2)[22] have been described elsewhere. NMR
spectra were recorded with Bruker AMX 250, DPX 250 or Avance
400 spectrometers. Chemical shifts are referenced to residual sol-
vent peaks (1H, 13C{1H}); 11B NMR chemical shifts are reported
relative to external BF3·Et2O. Low resolution electrospray mass
spectra (ESI) were recorded in the positive-ion mode with a Fisons
VG Platform II spectrometer.
6.91 (d, 3JH,H = 7.8 Hz, 2 H, H-Aro), 5.85 (ddt, 3JH,H = 17.1, 10.3,
3
6.7 Hz, 2 H, H-9), 5.07 (d, JH,H = 17.1 Hz, 2 H, H-10E), 5.01 (d,
3JH,H = 10.3 Hz, 2 H, H-10Z), 3.20 (t, JH,H = 7.5 Hz, 4 H, H-7),
3
2.69 (m, 4 H, H-8), 2.27 (s, 3 H, CH3-Ar), 0.61 (s, 3 H, CH3B)
ppm. 11B NMR (128.4 MHz, CDCl3, 25 °C): δ = 7.5 (h1/2 = 350 Hz)
ppm. 13C NMR (62.9 MHz, CDCl3, 25 °C): δ = 161.5 (C-4), 145.1
(C-2), 141.3 (C-6), 138.1 (C-Arp), 135.7 (C-9), 131.1 (C-Aro), 129.0
(C-Arm), 128.3 (C-5), 125.3 (C-3), 116.5 (C-10), 34.8 (C-7), 33.2 (C-
8), 20.9 (CH3-Ar), 6.9 (CH3B) ppm; C-Ari not observed. ESI-MS:
m/z (%) = 381 (100) [M+ – Br]. C26H30BBrN2 (461.25): calcd. C
67.70, H 6.56, N 6.07; found C 67.11, H 6.51, N 5.91.
3OEt: Compound 2 (2.247 g, 8.50 mmol) was dissolved in toluene
(20 mL) and a solution of 1OEt (1.929 g, 8.50 mmol) in toluene
(20 mL) was added dropwise at ambient temperature, whereupon
the reaction mixture became cloudy and the product precipitated
as a red-brown oil. The solvent was decanted and the oil washed
with n-hexane. Yield: 3.645 g (87%). 1H NMR (250.1 MHz,
1Me: Tetramethylstannane (4.699 g, 26.27 mmol) was added drop-
wise, at ambient temperature, to 1Br (6.543 g, 25.00 mmol) in n-
pentane (15 mL), and the mixture was stirred for 5 h. After the
solvent had been evaporated, the residue was kept at 50 °C in vacuo
for 3 h. Subsequent vacuum distillation gave pure 1Me as a colour-
less liquid. Yield: 3.498 g (70%). 1H NMR (250.1 MHz, CDCl3,
3
CDCl3, 25 °C): δ = 10.18 (br., 2 H, H-3), 8.27 (d, JH,H = 5.8 Hz,
3
4
2 H, H-6), 7.67 (dd, JH,H = 5.8, JH,H = 1.4 Hz, 2 H, H-5), 7.00
3
3
3
25 °C): δ = 7.99 (d, JH,H = 8.0 Hz, 2 H, H-Aro), 7.25 (d, JH,H
=
(m, 4 H, H-Ar), 5.81 (ddt, JH,H = 17.2, 10.3, 6.8 Hz, 2 H, H-9),
3
3
8.0 Hz, 2 H, H-Arm), 2.39 (s, 3 H, CH3-Ar), 1.49 (s, 3 H, CH3B)
5.02 (d, JH,H = 17.2 Hz, 2 H, H-10E), 4.96 (d, JH,H = 10.3 Hz, 2
ppm. 11B NMR (128.4 MHz, CDCl3, 25 °C): δ = 70.5 (h1/2
=
3
3
H, H-10Z), 3.17 (t, JH,H = 7.6 Hz, 4 H, H-7), 2.92 (q, JH,H
=
250 Hz) ppm. 13C NMR (62.9 MHz, CDCl3, 25 °C): δ = 146.8 (C-
Arp), 138.0 (C-Aro), 129.0 (C-Arm), 22.1 (CH3-Ar) ppm; C-Ari,
CH3B not observed.
6.9 Hz, 2 H, OCH2), 2.65 (m, 4 H, H-8), 2.23 (s, 3 H, CH3-Ar),
3
1.06 (t, JH,H = 6.9 Hz, 3 H, OCH2CH3) ppm. 11B NMR
(128.4 MHz, CDCl3, 25 °C): δ = 9.1 (h1/2 = 350 Hz) ppm. 13C
NMR (62.9 MHz, CDCl3, 25 °C): δ = 163.6 (C-4), 145.2 (C-2),
141.7 (C-6), 138.7 (C-Arp), 135.7 (C-9), 131.0 (C-Aro), 129.0 (C-
Arm), 128.5 (C-5), 126.0 (C-3), 116.8 (C-10), 58.2 (OCH2), 35.1 (C-
7), 33.3 (C-8), 21.2 (CH3-Ar), 17.4 (OCH2CH3) ppm; C-Ari not
observed. ESI-MS: m/z (%) = 411 (100) [M+ – Br]. C27H32BBrN2O
(491.27): calcd. C 66.01, H 6.57, N 5.70; found C 65.74, H 6.47, N
5.59.
1OEt: Diethyl ether (1.495 g, 20.17 mmol) was added with a sy-
ringe to 1Br (5.235 g, 20.00 mmol) in toluene (40 mL). After the
solution had been stirred at room temperature for 5 d, the solvent
was evaporated to give 1OEt as an off-white oil, which was used
without further purification. Yield: 3.993 g (88%). 1H NMR
3
(250.1 MHz, CDCl3, 25 °C): δ = 8.16 (d, JH,H = 7.8 Hz, 2 H, H-
3
3
Ar), 7.13 (d, JH,H = 7.8 Hz, 2 H, H-Ar), 4.18 (q, JH,H = 7.1 Hz,
3
2 H, OCH2), 2.16 (s, 3 H, CH3-Ar), 1.13 (t, JH,H = 7.1 Hz, 3 H,
3OMe: Compound 3Br (0.789 g, 1.50 mmol) was dissolved in dry
methanol (5 mL), treated with dry triethylamine (0.152 g,
1.50 mmol) from a syringe and the resulting reaction mixture was
stirred at ambient temperature for 2 h. The solvent was evaporated
and the residue dissolved in dichloromethane (5 mL). Addition of
a few drops of n-hexane made the product precipitate as a light
brown oil, which was isolated by decantation and washed with a
mixture of dichloromethane and n-hexane. Yield: 0.635 g (89%).
1H NMR (250.1 MHz, CDCl3, 25 °C): δ = 10.18 (br., 2 H, H-3),
OCH2CH3) ppm. 11B NMR (128.4 MHz, CDCl3, 25 °C): δ = 38.4
(h1/2 = 200 Hz) ppm. 13C NMR (62.9 MHz, CDCl3, 25 °C): δ =
137.9, 135.9, 128.9 (C-Ar), 66.0 (OCH2), 21.4 (CH3-Ar), 16.1
(OCH2CH3) ppm; C-Ari not observed.
3Br: A solution of 1Br (0.393 g, 1.50 mmol) in toluene (2.5 mL)
was added dropwise to a solution of 2 (0.397 g, 1.50 mmol) in tolu-
ene (5 mL). The reaction mixture immediately turned yellow and
3Br precipitated as a yellow oil. This oil was isolated, washed with
n-hexane and dried in vacuo to give the pure product as a yellow
foam. Yield: 0.602 g (76%). 1H NMR (250.1 MHz, CDCl3, 25 °C):
3
3
4
8.26 (d, JH,H = 5.8 Hz, 2 H, H-6), 7.67 (dd, JH,H = 5.8, JH,H
=
3
1.5 Hz, 2 H, H-5), 7.02 (d, JH,H = 8.0 Hz, 2 H, H-Arm), 6.97 (d,
3
3
δ = 10.25 (br., 2 H, H-3), 8.45 (d, JH,H = 6.0 Hz, 2 H, H-6), 7.76
3JH,H = 8.0 Hz, 2 H, H-Aro), 5.81 (ddt, JH,H = 17.1, 10.4, 6.9 Hz,
(dd, 3JH,H = 6.0, 4JH,H = 1.4 Hz, 2 H, H-5), 7.15 (d, 3JH,H = 8.3 Hz,
2 H, H-9), 5.03 (d, JH,H = 17.1 Hz, 2 H, H-10E), 4.97 (d, JH,H =
3
3
2 H, H-Aro), 7.09 (d, 3JH,H = 8.3 Hz, 2 H, H-Arm), 5.81 (ddt, 3JH,H
10.4 Hz, 2 H, H-10Z), 3.18 (t, JH,H = 7.5 Hz, 4 H, H-7), 2.86 (s, 3
H, OCH3), 2.66 (m, 4 H, H-8), 2.23 (s, 3 H, CH3-Ar) ppm. 11B
NMR (128.4 MHz, CDCl3, 25 °C): δ = 9.8 (h1/2 = 400 Hz) ppm.
3
3
= 17.1, 10.3, 6.6 Hz, 2 H, H-9), 5.05 (d, JH,H = 17.1 Hz, 2 H, H-
10E), 4.99 (d, 3JH,H = 10.3 Hz, 2 H, H-10Z), 3.20 (t, 3JH,H = 7.4 Hz,
4 H, H-7), 2.67 (m, 4 H, H-8), 2.27 (s, 3 H, CH3-Ar) ppm. 11B 13C NMR (62.9 MHz, CDCl3, 25 °C): δ = 163.7 (C-4), 145.2 (C-
NMR (128.4 MHz, CDCl3, 25 °C): δ = 6.5 (h1/2 = 400 Hz) ppm.
2), 141.7 (C-6), 138.8 (C-Arp), 135.7 (C-9), 131.0 (C-Aro), 129.0 (C-
13C NMR (62.9 MHz, CDCl3, 25 °C): δ = 164.7 (C-4), 144.6 (C- Arm), 128.6 (C-5), 126.0 (C-3), 116.8 (C-10), 50.4 (OCH3), 35.1 (C-
4314
© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
Eur. J. Inorg. Chem. 2005, 4309–4316

An Approach Towards Functional Dendrimers
FULL PAPER
7), 33.2 (C-8), 21.2 (CH3-Ar) ppm; C-Ari not observed. ESI-MS:
According to the NMR spectra the conversion was quantitative.
1
m/z (%) = 397 (100) [M+ – Br]. C26H30BBrN2O (477.25): calcd. C Major isomer 8t: H NMR (400.1 MHz, CD2Cl2, 25 °C): δ = 8.87
3
65.43, H 6.34, N 5.87; found C 64.80, H 6.33, N 5.62.
(s, 2 H, H-3), 8.60 (d, JH,H = 6.0 Hz, 2 H, H-6), 7.86 (dd, 3JH,H
=
4
3
6.0, JH,H = 1.4 Hz, 2 H, H-5), 7.27 (d, JH,H = 7.8 Hz, 2 H, H-
Aro), 7.17 (d, 3JH,H = 7.8 Hz, 2 H, H-Arm), 3.15 (t, 3JH,H = 7.6 Hz,
4 H, H-7), 2.32 (s, 3 H, CH3-Ar), 1.94 (m, 4 H, H-8), 1.80 (m, 4
H, H-9), 1.71 (m, 4 H, H-10) ppm. 11B NMR (126.8 MHz, CD2Cl2,
25 °C): δ = 57.3 (BBr2, h1/2 = 530 Hz), 6.5 (Bbipy, h1/2 = 650 Hz),
–24.1 ([BBr4]–, h1/2 = 260 Hz) ppm. 13C NMR (100.6 MHz,
CD2Cl2, 25 °C): δ = 165.5 (C-4), 144.6 (C-2), 143.7 (C-6), 140.5 (C-
Arp), 132.9 (C-Aro), 130.0 (C-5), 129.7 (C-Arm), 124.4 (C-3), 36.5
(C-7), 36.2 (C-10), 31.8 (C-8), 26.4 (C-9), 21.4 (CH3-Ar) ppm; C-
Ari not observed.
4: 1,4-Bis(dibromoboryl)benzene (2.295 g, 5.50 mmol) was dis-
solved in chloroform (15 mL) and tetramethylstannane (1.967 g,
11.00 mmol) was added dropwise at ambient temperature. The re-
action mixture was stirred overnight, the solvent was evaporated,
bromotrimethylstannane removed at 50 °C in vacuo, and the resi-
due vacuum-sublimed to give pure 4 as a colourless solid. Yield:
1
0.854 g (54%). H NMR (250.1 MHz, CDCl3, 25 °C): δ = 8.14 (s,
4 H, H-Ar), 1.55 (s, 6 H, CH3B) ppm. 11B NMR (128.4 MHz,
CDCl3): δ = 72.3 (h1/2 = 400 Hz) ppm. 13C NMR (62.9 MHz,
CDCl3, 25 °C): δ = 135.5 (C-Ar) ppm; C-Ari, CH3B not observed.
9OMe: Compound 8 (0.840 g, 0.75 mmol) was dissolved in dichlo-
romethane (5 mL) and a solution of 4,4Ј-dimethyl-2,2Ј-bipyridyl
(0.415 g, 2.25 mmol) in dichloromethane (5 mL) added dropwise
with stirring. Upon addition, the solution adopted an intense yel-
low colour. After the reaction mixture had been stirred at ambient
temperature for 2 h, the solvent was evaporated and the solid resi-
due redissolved in methanol (5 mL). Et3N (0.227 g, 2.25 mmol) was
added with a syringe, the reaction mixture was stirred for 2 h, and
the volatiles were removed in vacuo. All attempts to isolate 9OMe
from the solid residue obtained applying reversed-phase HPLC
techniques [stationary phase: ReproSil-Pur C18-AQ (10 µm,
250×20 mm); eluent: methanol or methanol/acetonitrile] failed.
However, ESI mass spectrometry proved the presence of 9OMe in
the crude product. ESI-MS: m/z = 284 [M3+ – 3 Br], 465 [M2+ – 2
Br].
5: A solution of 4 (0.431 g, 1.50 mmol) in dichloromethane (5 mL)
was added dropwise, at ambient temperature, to 2 (0.793 g,
3.00 mmol) in dichloromethane (10 mL). After the addition was
complete, the solvent volume was reduced to approximately 5 mL
and n-hexane added dropwise, which made 5 precipitate as a pale-
yellow oil. Yield: 1.084 g (89%). 1H NMR (250.1 MHz, CDCl3,
3
25 °C): δ = 9.88 (br., 4 H, H-3), 8.31 (d, JH,H = 5.9 Hz, 4 H, H-
3
4
6), 7.76 (dd, JH,H = 5.9, JH,H = 1.4 Hz, 4 H, H-5), 6.89 (s, 4 H,
3
H-Ar), 5.81 (ddt, JH,H = 17.0, 10.3, 6.6 Hz, 4 H, H-9), 5.04 (d,
3JH,H = 17.0 Hz, 4 H, H-10E), 4.98 (d, JH,H = 10.3 Hz, 4 H, H-
3
3
10Z), 3.15 (t, JH,H = 7.5 Hz, 8 H, H-7), 2.63 (m, 8 H, H-8), 0.58
(s, 6 H, CH3B) ppm. 11B NMR (128.4 MHz, CDCl3, 25 °C): δ =
7.7 (h1/2 = 580 Hz) ppm. 13C NMR (62.9 MHz, CDCl3, 25 °C): δ
= 161.8 (C-4), 145.2 (C-2), 141.4 (C-6), 135.9 (C-9), 131.5 (C-Ar),
128.6 (C-5), 125.5 (C-3), 116.9 (C-10), 35.1 (C-7), 33.4 (C-8) ppm;
C-Ari, CH3B not observed. ESI-MS: m/z (%) = 328 (100) [M2+
–
Crystal Structure Determination: C27H32BBrN2O, M = 491.27, tri-
2 Br]. C44H50B2Br2N4 (816.34): calcd. C 64.74, H 6.17, N 6.86;
found C 64.29, H 6.07, N 6.66.
¯
clinic, space group P1, a = 13.154(2), b = 14.371(2), c =
23.377(5) Å, α = 87.329(15)°, β = 87.113(15)°, γ = 63.314(12)°, V
= 3941.9(12) Å3, Z = 6, Dc = 1.242 Mgm–3, F(000) = 1536, Mo-Kα
6: Preparation and purification was achieved as described for 4.
1,3,5-Tris(dibromoboryl)benzene (2.348 g, 4.00 mmol), tetrameth-
ylstannane (2.217 g, 12.40 mmol), chloroform (20 mL). Yield:
radiation (λ
= 0.71073 Å), T = 173(2) K; crystal size
0.24×0.23×0.16 mm, ω-scan mode, measurement range (–16 Յ h
Յ 15, –17 Յ k Յ 17, –28 Յ l Յ 26, 3.58 Յ θ Յ 25.92), 15100
independent reflections, µ = 1.584 mm–1. The structure was solved
by direct methods[25] and full-matrix least-squares refinement was
carried out against F2 using the SHELXL-97 program,[26] 905 para-
meters, R1 = 0.1833 (all data), wR2 = 0.2230 (all data), max./min.
residual electron density +0.580/–0.512 eÅ–3. CCDC-271402 con-
tains the supplementary crystallographic data for this paper. These
data can be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
1
0.674 g (43%). H NMR (250.1 MHz, CDCl3, 25 °C): δ = 9.02 (s,
3 H, H-Ar), 1.34 (s, 9 H, CH3B) ppm. 11B NMR (128.4 MHz,
CDCl3, 25 °C): δ
= 72.0 (h1/2 =
350 Hz) ppm. 13C NMR
(62.9 MHz, CDCl3, 25 °C): δ = 149.0 (C-Ar) ppm; C-Ari, CH3B
not observed.
7: Compound 6 (0.863 g, 2.20 mmol) was dissolved in dichloro-
methane (10 mL) and added dropwise to a solution of 2 (1.718 g,
6.50 mmol) in dichloromethane (20 mL). The reaction mixture be-
came yellow. After the addition was complete, n-hexane was added,
1
which made 7 precipitate as a yellow oil. Yield: 2.086 g (80%). H
NMR (400.1 MHz, CDCl3, 25 °C): δ = 8.89 (br., 6 H, H-3), 8.81
(d, 3JH,H = 5.9 Hz, 6 H, H-6), 8.13 (dd, 3JH,H = 5.9, 4JH,H = 1.3 Hz,
6 H, H-5), 6.62 (s, 3 H, H-Ar), 5.80 (ddt, 3JH,H = 17.1, 10.4, 6.6 Hz,
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3
3
6 H, H-9), 5.05 (d, JH,H = 17.1 Hz, 6 H, H-10E), 5.01 (d, JH,H
=
10.4 Hz, 6 H, H-10Z), 3.07 (m, 12 H, H-7), 2.56 (m, 12 H, H-8),
0.45 (s, 9 H, CH3B) ppm. 11B NMR (128.4 MHz, CD3OD, 25 °C):
δ = 6.5 (h1/2 = 600 Hz) ppm. 13C NMR (62.9 MHz, CDCl3, 25 °C):
δ = 160.4 (C-4), 144.5 (C-2), 143.9 (C-6), 135.6 (C-9), 134.3 (C-Ar),
129.8 (C-5), 123.2 (C-3), 117.0 (C-10), 35.2 (C-7), 33.6 (C-8), 7.6
(CH3B) ppm; C-Ari not observed. ESI-MS: m/z (%) = 512 (100)
[M2+ – 2 Br], 315 (95) [M3+ – 3 Br]. C63H72B3Br3N6 (1185.45):
calcd. C 63.83, H 6.12, N 7.09; found C 63.39, H 6.08, N 6.85.
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8: An intensely yellow solution of 3Br (0.395 g, 0.75 mmol) and
Et3SiH (0.262 g, 2.25 mmol) in dichloromethane (5 mL) was added
dropwise at –78 °C to BBr3 (0.564 g, 2.25 mmol) in dichlorometh-
ane (5 mL) and the mixture then warmed to room temperature.
Addition of n-hexane (5 mL) made 8 precipitate as a pale-yellow
oil, which was isolated and vacuum-dried to give a colourless foam.
Eur. J. Inorg. Chem. 2005, 4309–4316
www.eurjic.org
© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
4315

M. C. Haberecht, M. Bolte, H.-W. Lerner, M. Wagner
FULL PAPER
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Zanello, M. Wagner, J. Chem. Soc., Dalton Trans. 2002, 1566.
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Received: May 9, 2005
Published Online: September 21, 2005
[19] M. C. Haberecht, J. B. Heilmann, A. Haghiri, M. Bolte, J. W.
Bats, H.-W. Lerner, M. C. Holthausen, M. Wagner, Z. Anorg.
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4316
© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
Eur. J. Inorg. Chem. 2005, 4309–4316