Angewandte
Communications
Chemie
that the structure of this material should better be described
as a polyborazylene (IV), that is, a poorly-defined macro-
molecular network of partially fused borazine rings.[13b,i,m,p] In
fact, the provided NMR data are remarkably similar to those
of a polyborazylene sample prepared through a different
route.[15]
Thermolysis of Mannersꢀ poly(N-methylaminoborane) (I,
R = CH3) in solution resulted in depolymerization to give
a mixture of low-molecular-weight oligomers and N,N’N’’-
trimethylborazine. The formation of a poly(iminoborane) has
not been described.[4d] In the 1980s, Paetzold and co-workers
reported on the isolation of waxy materials for which
a constitution of linear poly(iminoborane)s (II, R = R’ =
alkyl) was proposed.[16] These materials were obtained upon
the generation of monomeric iminoboranes[17] in the gas
phase and subsequent trapping at ꢀ1968C. The products were
found to be insoluble in common organic solvents, so their
structural characterization was limited. Minor amounts of the
corresponding borazines (III) additionally formed could be
separated by extraction. The given assignment was based on
elemental analysis and mass spectrometry data and on the
observation that the compound identified as [EtBNEt]n
transformed into hexaethylborazine upon heating above
1508C. An analogous transformation of the derivative
[nPrBNnPr]n, however, was not achieved.[16a]
Herein, we present the synthesis and comprehensive
characterization of a processable oligo(iminoborane) with
well-defined microstructure, composed of a chain of 12–14
BN units on average. For this purpose, the unwanted
competing side reaction of potential BN monomers to give
their cyclic trimers, that is, the respective borazine derivatives
(III), seemed to be a major hurdle to overcome. We chose
a strategy by which possible pathways leading to III should be
prevented. For this purpose, we introduced an ethylene bridge
to link the nitrogen atoms of the monomer. This steric
constraint proved effective to preclude any borazine forma-
tion. Related approaches have been followed previously by
Neilson and Retta, but, to the best of our knowledge, the
identification of a poly- or an extended oligo(iminoborane)
was not achieved in these studies.[18]
Scheme 2. a) Synthesis of oligo(iminoborane) 4 and end-capped deriv-
ative 4’. b) Synthesis of model compound 7 (Oct=n-C8H17).
in intensity. One broad signal remained in the 11B{1H} NMR
spectrum for the bulk boron atoms of the oligomer 4, which in
the course of the reaction was shifted slightly upfield to d =
31 ppm. In the 1H NMR spectrum, a common signal appeared
for the protons of the ethylene bridge, centered at d =
3.32 ppm. Small peaks at d = 3.00–3.25 and 3.45–3.65 ppm
remained, which we assign to the ethylene protons of the rings
at the chain ends. The proton resonance for the SiMe3 end
group was detected at d = 0.15 ppm, and concomitant for-
mation of the volatile condensation byproduct Me3SiCl was
1
evidenced by its H resonance at d = 0.45 ppm. Quantitative
evaluation of the signals over time revealed that conversion of
the reactive groups leveled off at about 85%, which may be
associated with a rate reduction due to a marked increase in
the viscosity of the solution. An estimation of the degree of
polymerization after 14 days by using Carothersꢀ equation
yielded DPn ꢁ 7. Then, Me3SiNMe2 (6 mol%) was added to
ꢀ
deactivate the B Cl end groups of 4. The end-capped product
4’ was purified by adding the mixture to an excess amount of
anhydrous acetonitrile, which resulted in separation of 4’ from
solution. This afforded oligo(iminoborane) 4’ as a highly
viscous amber fluid in 83% yield.
Recently, we presented a novel class of organic–inorganic
hybrid polymers comprised of alternating diimidoborane and
para-phenylene units.[11] The synthesis thereof was achieved
The hydrolytically sensitive oligomer 4’ was characterized
by multinuclear NMR spectroscopy, including 1H DOSY
(Figure S28), mass spectrometry (MS), elemental analysis, gel
permeation chromatography (GPC), dynamic light scattering
(DLS), small-angle X-ray scattering (SAXS), differential
scanning calorimetry (DSC), and thermogravimetric analysis
(TGA), as well as FTIR and UV/Vis spectroscopy. The GPC
trace suggested molecular-weight averages of Mn = 1800 and
Mw = 1900, respectively (Figure 1a), which is consistent with
the presence of a chain of about 6–7 repeat units, correspond-
ing to 12–14 catenated BN units, on average. DLS gave
a hydrodynamic radius (Rh) of 2.2 nm for particles of 4’ in n-
pentane (Figure 1b). The SAXS measurements revealed that
4’ adopts a slightly anisotropic structure with a small aspect
ratio in n-pentane. The distance distribution indicates an
ellipsoidal structure (prolate) with axes of inertia of 1.7 and
5.0 ꢂ 1.0 nm and a radius of gyration (Rg) of 0.8 nm (Fig-
ure 1d). Fitting to a worm-like chain model gave a rigid rod
with 2.7 nm for the extended chain length together with
using
a
facile polycondensation process through Si/B
ꢀ
exchange with silazane Si N bond cleavage. For the current
study, we adopted this approach: we employed 1,3-bis(trime-
thylsilyl)-1,3,2-diazaborolidine 1 for polycondensation with
dichloroborane 2 (Scheme 2a). n-Octyl side chains (herein
denoted as Oct) at boron were chosen in order to impart
solubility. Additionally, we prepared compound 7 as a molec-
ular model system (Scheme 2b).
Monitoring of the reaction of 1 with 2 in CD2Cl2 at
1
ambient temperature by 11B{1H} and H NMR spectroscopy
showed that both 1 and 2 were immediately consumed with
initial selective formation of 3 (see Figure S27 in the
Supporting Information). Even after short reaction times,
the spectra revealed that further oligomerization had oc-
11
ꢀ
curred. The B resonance for the B Cl end groups of the
growing chain (at around 44.5 ppm) continuously decreased
2
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
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