Dehydropolymerisation of Methylamine Borane and an N-Substituted Primary Amine Borane Using a PNP Fe Catalyst

Article abstract of DOI:10.1002/chem.202000809

Dehydropolymerisation of methylamine borane (H₃B?NMeH₂) using the well-known iron amido complex [(PNP)Fe(H)(CO)] (PNP=N(CH₂CH₂PiPr₂)₂) (1) gives poly(aminoborane)s by a chain-growth mechanism. In toluene, rapid dehydrogenation of H₃B?NMeH₂ following first-order behaviour as a limiting case of a more general underlying Michaelis–Menten kinetics is observed, forming aminoborane H₂B=NMeH, which selectively couples to give high-molecular-weight poly(aminoborane)s (H₂BNMeH)_n and only traces of borazine (HBNMe)₃ by depolymerisation after full conversion. Based on a series of comparative experiments using structurally related Fe catalysts and dimethylamine borane (H₃B?NMe₂H) polymer formation is proposed to occur by nucleophilic chain growth as reported earlier computationally and experimentally. A silyl functionalised primary borane H₃B?N(CH₂SiMe₃)H₂ was studied in homo- and co-dehydropolymerisation reactions to give the first examples for Si containing poly(aminoborane)s.

Full text of DOI:10.1002/chem.202000809

A Journal of
Accepted Article
Title: Dehydropolymerisation of methylamine borane and an N-
substituted primary amine borane using a PNP Fe catalyst
Authors: Felix Anke, Susanne Boye, Anke Spannenberg, Albena
Lederer, Detlef Heller, and Torsten Beweries
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To be cited as: Chem. Eur. J. 10.1002/chem.202000809
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Dehydropolymerisation of methylamine borane and an N-
substituted primary amine borane using a PNP Fe catalyst
Felix Anke[a], Susanne Boye[b], Anke Spannenberg[a], Albena Lederer[b,c], Detlef Heller[a] and Torsten
Beweries*[a]
Abstract:
(H3B∙NMeH2) using the well-known iron amido complex
[(PNP)Fe(H)(CO)] (PNP = N(CH2CH2Pi-Pr2)2) (1) gives
Dehydropolymerisation
of
methylamine
borane
polymerisation, a principle that is known for olefins/polyolefins
for decades.5 Commonly accepted challenges of amine borane
dehydropolymerisation include the identification of general
reaction mechanisms, the development of a set of common
performance criteria that are deployed for polymer synthesis as
well as the elucidation of structure-activity relationships. 6 The
poly(aminoborane)s by a chain growth mechanism. In toluene, rapid
dehydrogenation of H3B∙NMeH2 following first-order behaviour as a
limiting case of a more general underlying Michaelis-Menten kinetics
is observed, forming aminoborane H2B=NMeH which selectively
couples to give high-molecular weight poly(aminoborane)s
(H2BNMeH)n and only traces of borazine (HBNMe)3 by
most
frequently
studied
amine
boranes
for
dehydropolymerisation are ammonia borane, H3B∙NH3, and
methylamine borane, H3B∙NMeH2.7,8 In the latter case polymers
that are soluble in organic solvents can be obtained, which
makes mechanistic studies of the reaction much more
depolymerisation after full conversion. Based on
a series of
comparative experiments using structurally related Fe catalysts and
dimethylamine borane (H3B∙NMe2H) polymer formation is proposed
to occur by nucleophilic chain growth as reported earlier
computationally and experimentally. A silyl functionalised primary
borane H3B∙N(CH2SiMe3)H2 was studied in homo- and co-
dehydropolymerisation reactions to give the first examples for Si
containing poly(aminoborane)s.
convenient.
Also,
reactions
with
higher
substituted
dimethylamine borane, H3B∙NMe2H, can give valuable insights
into the reaction mechanism as in this case, well defined,
spectroscopically observable and stable intermediates are
formed.9
1d,4, 10
,
11
,
12
13
The synthesis of N-
and B-substituted
poly(aminoborane)s was described in the past, however, to date
apart from one example, where a thiophenyl substituted amine
borane was used11 as the substrate all reports are limited to
alkyl substituted compounds. To broaden the scope of this
chemistry it would be very interesting to also study other
Introduction
Poly(aminoborane)s represent
a comparably new class of
heteroatom-substituted
amine
boranes
for
inorganic polymers that holds promising properties for a broad
range of applications in materials chemistry, e.g. as piezoelectric
materials or as precursors to boron-based ceramics.1 In most
cases, these polymers have been prepared by transition metal
dehydropolymerisation, the synthesis of some of these
substrates was described in the past.14
Catalysts for controlled amine borane dehydropolymerisation
typically operate in a bifunctional manner, first generating the
reactive aminoborane monomer by dehydrogenation of the
amine borane, followed by coupling of this intermediate to form
oligomers and polymers. Notably, this strongly differs from olefin
polymerisation, where the monomer itself is a stable compound
and the catalyst is only involved in coordination and C-C
coupling. The stability of aminoborane monomers is strongly
dependent on the substitution pattern with larger groups or an
increased number of substituents stabilising these species.
Methylaminoborane, H2B=NMeH, has only been observed at low
temperatures3, whereas dimethylaminoborane H2B=NMe2 can
readily be observed spectroscopically at room temperature.9 As
for the mode of B-N bond formation off-metal polymerisation,
where the monomer is not coordinated to the metal centre and
metal-centred on-metal polymerisation were described.
Furthermore, for the latter scenario three pathways are
catalysed dehydropolymerisation of primary amine boranes
2
H3B∙NRH2
, however, lately, also metal-free routes were
introduced, including Brönsted acid-mediated reactions 3 and
BH2 transfer to primary amines. 4 The existing isoelectronic
relationships between alkanes and amine boranes H3B∙NRH2,
olefins and aminoboranes H2B=NRH and thus between
polyolefins and poly(aminoborane)s [H2BNRH]n raises a series
of questions of how such compounds are activated, coordinated
and ultimately formed in the presence of transition metal centres.
Control of aminoborane coordination and B-N coupling through a
catalyst would allow for the selective and well-ordered
[a]
Dr. F. Anke, Dr. A. Spannenberg, Prof. Dr. Detlef Heller, PD Dr. T.
Beweries
Leibniz-Institut für Katalyse e.V. an der Universität Rostock
Albert-Einstein-Str. 29a, 18059 Rostock, Germany
E-mail: torsten.beweries@catalysis.de
suggested,
namely
(i)
amine
borane
coordination/dehydrogenation/aminoborane insertion at the
[b]
[c]
Dr. S. Boye, PD Dr. A. Lederer
Leibniz-Institut für Polymerforschung Dresden, Hohe Str. 6, 01069
Dresden, Germany
same metal complex (resembling insertion polymerisation of
15
olefins)
,
(ii)
amine
borane
coordination/dehydrogenation/aminoborane insertion involving
chain transfer via  complexes16 and (iii) dehydrogenation and
nucleophilic chain growth via the end of a metal-coordinated
oligomer unit by two different catalyst units (bicatalyst).17
Technische Universität Dresden, 01062 Dresden, Germany
Supporting information for this article is given via a link at the end of
the document.
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Most of the known systems for dehydropolymerisation of
H3B∙NMeH2 comprise homogeneous late transition metal
catalyst1d,15,10,18, however, very recently also first examples for
highly active early transition metal complexes were reported.11,19
Catalysis
Dehydropolymerisation experiments using complex 1 were done
at room temperature under isobaric conditions with the reaction
vessel connected to an automatic gas buret21 that records H2
evolution as a product-proportional concentration. Reactions
were done using 0.00167-0.00675 M 1 in THF or toluene
(Scheme 2). We found that reactions in THF were much slower
and less selective, giving comparably large amounts of borazine
(HBNMe)3 and other B-N by-products. Thus, for the discussion
of H3B∙NMeH2 dehydropolymerisation we focus on results that
were obtained from reactions in toluene. Details of experiments
that were done in THF can however be found in the Supporting
Information.
Schneider
has
presented
Ru(II)
pincer
systems
[(PNP)Ru(H)PMe3] and [(PNHP)Ru(H)2PMe3] (PNHP
=
HN(CH2CH2Pi-Pr2)2)8d,f for H3B∙NH3 dehydropolymerisation; the
latter was discussed as a bifunctional catalyst that is involved in
H3B∙NH3 dehydrogenation and B-N coupling. The importance of
amine cooperativity for the dehydrogenation step was verified by
using the methylated complex [(PNMeP)Ru(H)2PMe3] (PNMeP =
MeN(CH2CH2Pi-Pr2)2), which showed significantly slower
turnover. A study on a similar Fe(II) system [(PNP)Fe(H)CO] (1)
was later presented by the same authors8c, in which the Fe(II)
dihydrido complex [(PNHP)Fe(H)2CO] was found the be the
resting state relevant to the dehydrogenation step. This is
suggested to be followed by Fe centred B-N bond formation.
Recently, we showed that the related Fe(II) borate complex
[(PNHP)Fe(H)(HBH3)CO] (2) is an excellent precatalyst for the
selective dehydropolymerisation of H3B∙NMeH2 (Scheme 1),
giving borazine (HBNMe)3 only at the end of the reaction by
depolymerisation of the poly(aminoborane).18d Polymer growth
kinetics pointed to the presence of a chain-growth rather than a
step-growth mechanism. Remarkably, analysis of polymers
obtained at different catalyst concentrations suggested that off-
metal polymerisation appears to be dominant.
PiPr2
1
0.5-2 mol%
H
H
THF or toluene
N
Fe CO
B
n H3B·NMeH2
N
- n H2
P
H
Me
H
iPr2
n
1
Scheme 2. Dehydropolymerisation of H3B∙NMeH2 using complex 1.
Volumetric curves for reactions in toluene at [H3B∙NMeH2] = 0.33
M (Figure 1, top) show rapid hydrogen evolution and full
H3B∙NMeH2 conversion within less than 60 minutes. GC analysis
showed that H2 was the only gaseous reaction product in all
cases.
HBH3
PiPr2
H
N
Fe CO
P
H
iPr
2
2
Me
2
1-5 mol%
depoly-
merisation
H H
N
THF or toluene
HB BH
B
n H3B·NMeH2
N
N
N
- n H2
Me
B
H
Me
Me
H
n
poly(aminoborane)
borazine
Mn = 23.000-38.000 g·mol-1
Ð = 1.6-2.2
(vs. polystyrene, RI detection)
Scheme 1. Dehydropolymerisation of H3B∙NMeH2 using complex 2.
In this contribution, we present
a study of H3B∙NMeH2
dehydropolymerisation using the well-known amido complex 120.
As an extension of our earlier work, we now give further
mechanistic insights into the dehydropolymerisation process that
allow for the development of a more general mechanistic picture
in reactions with complexes of this type. Also, we present first
examples for Si functionalised poly(aminoborane)s that can be
readily obtained using the Fe amido complex 1.
Figure 1. Top: Volumetric curve of H3B∙NMeH2 dehydropolymerisation using
complex 1 in toluene at T = 25 °C and [H3B∙NMeH2] = 0.33 M. Bottom: PMe3
and Hg poisoning experiments (T = 25 °C and [H3B∙NMeH2] = 0.33 M).
Results and Discussion
Dehydropolymerisation with Fe amido complex 1
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31P NMR monitoring shows that similarly as in earlier work by
Schneider8c and our group 22 the well-known trans-dihydride
[(PNHP)Fe(H)2CO] is the dominant species (Figure S28). This
species is known to readily form from the amido complex 1 in
the presence of H2.8c,22,23 Also, formation of the borate complex
(vmax
)
could not be reached due to limited solubility of
H3B∙NMeH2 in toluene. In our case the rate is limited to v≈vmax/2
(Table 1, entry 3). For this, KM should be in the same range as
[H3B∙NMeH2]0, which is in line with the observed values. In
previous studies simple first-order kinetics were proposed for
related systems based on linearisations of the respective
integrated rate law.8c,d, 27 We show that using a Michaelis-
Menten-type approach as a more general kinetic model, which
includes often described first-order behaviour as a limiting case,
could help to refine the interpretation of experimental data and
explain systematic deviations from ideal first-order behaviour.
2
occurs, possibly by BH3 addition to trans-dihydride
[(PNHP)Fe(H)2CO], as proposed earlier.8c Notably, formation of
[(PNHP)Fe(H)2CO] can also be seen from an instantaneous
colour change from purple, typical for 1, to pale yellow upon
addition of H3B∙NMeH2. We discount the formation of
a
heterogeneous catalyst as the active species, as addition of 0.1
mol% PMe3 or excess Hg did not affect the kinetic profile of the
reaction (Figure 1, bottom).24
NMR spectroscopic speciation experiments with equimolar
amounts of catalyst 1 and H3B∙NMeH2 showed formation of a
new set of triplet 1H resonances at -14.53 and -15.07 ppm (2JH,P
= 58.2 Hz), likely due to cis/trans isomers of an intermediate
hydride species (Figure S54). Along with this a broad signal at -
10.37 ppm could indicate the presence of a µ-BH moiety of an
aminoborane capped species 1-H2BNMeH. 11B NMR spectra of
Table 1. Compilation of kinetic data of H3B∙NMeH2 dehydropolymerisation
using complex 1 in toluene. For reactions following first-order kinetics only kobs
is given. For reactions following the described Michaelis-Menten model KM is
given.
Reaction
conditionsa
KM
[mmol/mL]
Entry
kobs [min‒1
]
k2 [min‒1
]
-
-
1
2
3
[1] = 0.00675 M
[1] = 0.00335 M
[1] = 0.00167 M
1.189(8)∙10-1 b
7.02(7)∙10-2 b
8.0(1)∙10-2 c
-
-
this new complex show
a broad doublet at -10.2 ppm
(1JB,H = 115 Hz) that collapses into a singlet upon proton
decoupling (Figure S56). Related Ru borametallacycles derived
from reactions of [(PNP)Ru(H)PMe3] with H3B∙NH3 and
H3B∙NMeH2 were described before by Schneider and showed
similar NMR spectra.9b Also, we have presented a BH3 adduct in
an earlier communication.18d
2.60(3)∙10-1
1.94(5) ∙10-1
12.46(9)
12.3(2)
[1] = 0.00167 M,
[H3B∙NMeH2] =
0.25 M
4
1.05(4)∙10-1 c
1.25(5)∙10-1 c
[1] = 0.00167 M,
[H3B∙NMeH2] =
0.17 M
Kinetic analysis using commonly applied linearisations of the
integrated rate law for the determination of k values have shown
that most of the plots clearly deviate from the expected linearity
for an assumed simple first-order reaction (Figure S17).25 This is
also reflected in VTNA analysis26 of the reaction profiles with
variations in [1], which are in agreement with a process that is
first-order in the Fe catalyst, but show minor deviations that
cannot be neglected (Figure S23). In line with this, volumetric
data obtained from reactions in toluene and THF at low catalyst
concentrations can be best described using a Michaelis-Menten
model (eq. 1, see Supporting Information for details).
5
6
1.13(3)∙10-1
5.4(2)∙10-2
8.5(1)
[1] = 0.00167 M,
T = 2 °C
1.60(9)∙10-2 c, d
5.18(7)∙10-1
a
c
b
T
=
25 °C, [H3B∙NMeH2]
=
0.33 M.
According to first-order model.
d
Calculated according to Michaelis-Menten model: kobs = (k2∙E0)/KM. An
induction period is observed for this reaction. Kinetic analysis was done for the
post-induction period region.
In an attempt to identify the nature of (Cat-S), we performed a
low temperature in situ NMR experiment using [1] = 0.00167 M
and [H3B∙NMeH2] = 0.33 M. 1H and 31P NMR spectra after
addition of fresh H3B∙NMeH2 showed a significant broadening of
the resonances of the trans-dihydride [(PNHP)Fe(H)2CO] and
k1
k2
Cat +
S
(Cat-S)
P
+ Cat
(1)
k-1
k2·E0·(S0-P)
KM + (S0-P)
k-1 + k2
k1
dP
dt
with KM
=
rate =
=
1
loss of the H coupling pattern (Figure S28). Although this could
indicate fluxional behaviour caused by formation of an adduct
between Fe dihydride and amine borane, we cannot
unequivocally assign this to formation of a new species.
Comparative volumetric studies with deuterium labelled
substrates have shown that reactions with H3B∙NMeH2 and
D3B∙NMeH2 can be described using the same kinetic model with
In contrast, volumetric curves of reactions using higher catalyst
concentrations could be conveniently fitted using a first-order
model. In this model (eq. 1), according to the fitted values for KM
and k the active catalyst (Cat) and the substrate (S) are in
equilibrium with a catalyst-substrate complex (Cat-S), which is
converted into the product (P) and releases the active catalyst
(Cat) (Table 1). For a first-order reaction as the limiting case of
Michealis-Menten kinetics, the KM values should be higher. Or in
other words, as 1/KM represents at least the upper limit of the
thermodynamic stability constant, the pre-equilibrium is far
shifted to the left (eq. 1). Saturation kinetics at high substrate
concentrations were reported before in H3B∙NMe2H
a
moderate
kinetic
isotope
effect
(KIE,
kobs(H3B∙NMeH2)/kobs(D3B∙NMeH2) = 1.5(1)). This value could
point to the presence of a bent transition state.28 Alternatively, B-
H activation could occur very early in the transition state.
Dehydropolymerisation of H3B∙NMeD2 shows a different kinetic
profile with the rate of gas evolution showing a maximum (Figure
S24). This could point to differences in the rate-determining
steps of dehydrogenation. Therefore, kobs and KIE values could
dehydrocoupling for
a
[Rh(Ph-Xantphos)]+ system.15 For
H3B∙NMeH2, similarly as in our case, this zero-order regime
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not be derived for H3B∙NMeD2. A similar problem was described
before for the related Ru complex [(PNP)Ru(H)(PMe3)].8d
is still intact and can be recharged. Notably, the same
experiment in THF was not successful as the catalyst
decomposed after the first run. Dilution of the reaction solution
by a factor of 3 (entry 6) gave slightly lower molecular weights.
Reactions at low temperature (T = 2 °C) increases molecular
weights by a factor of 2 (entry 7).
Polymer characterisation
Poly(aminoborane)s were isolated as pale yellow powders by
precipitation into cold (-78 °C) n-hexane. NMR spectroscopic
data of the polymers are in line with literature values.1d,15,18 Other
than in a previous study of our group on a dinuclear Zr
Analysis of molecular weights at different stages of the
dehydropolymerisation reaction in toluene (Figure 3) shows that
Mn remains constant at low values throughout the reaction, while
Mw shows a significant increase towards the end of the reaction.
This could be rationalised by a chain growth scenario in which
depolymerisation occurs at low substrate concentrations (i.e. at
high conversion). 11B NMR spectroscopy supports this
assumption as polymer formation occurs directly at the onset of
the reaction and short-chain oligomers were not detected
(Figure S16). A slightly different profile of molecular weights vs
conversion plots was observed by Weller in a recent study of
[(dppp)Rh]+ catalysed dehydropolymerisation of H3B∙NMeH2.18b
In this report, Mn remained constant up to high conversion (90%)
with a more pronounced increase towards the end of the
reaction. Depolymerisation was not observed and NMR spectra
of isolated polymers were similar throughout the course of the
reaction, thus prompting the authors to conclude a hybrid chain
growth/step growth mechanism.
catalyst19a 11B NMR spectra show no pronounced resonances
,
for BH3 end groups, indicating the presence of much higher-
molecular weight polymers (Figure 2). Closer inspection of the
11B NMR spectra shows that the BH2 signal is slightly shifted to
higher field by 1.5 ppm for polymers prepared using 1 compared
to those made using 2 in an earlier study (Figure S14).18d
Figure 2. 11B, 11B{1H} NMR (96 MHz, CDCl3) spectra of the isolated
[H2BNMeH]n from H3BNMeH2 dehydropolymerisation using complex
1
(2 mol%) in toluene at T = 25 °C and [H3B∙NMeH2] = 0.33 M.
Analysis of the polymers by size exclusion chromatography
(SEC, light scattering (LS) detection giving absolute molecular
weights)29 was done using sample concentrations of 2 mg∙mL-1
in THF using 0.1 wt% of tetrabutylammonium bromide, at dn/dc
values of 0.09 mL g-1.19a For some of the samples, problems with
reproducibility occurred due to low signal intensities; in these
cases, the sample concentration was increased to 4 mg∙mL-1. It
is once more noted that concentration sensitive refractive index
(RI) detection in combination with standard calibration is
commonly used for relative molecular weight determination of
these materials. However, it is known that molecular weights will
be overestimated by a factor of 3-6.10 To verify this fact for our
set of samples made using Fe catalyst 1, we applied both, RI
and LS detection for selected samples and found that molecular
weights Mn and Mw are larger by a factor of 2-10 using RI
detection and polystyrene as calibration standard (see Table S3).
Figure 3. Poly(aminoborane) growth kinetics in toluene. Reaction conditions:
0.5 mol% 1, [H3B∙NMeH2] = 0.33 M, T = 25 °C, isobaric system connected to
gas buret. Conversion was determined from volumetric data.
Table 2. SEC-LS data.
Entry
Reaction conditions a
t
[h]
Mn
[g∙mol-1] b
Mw
[g∙mol-1] b
Ð
Table
2 shows a compilation of molecular weights of
1
2
3
4
[1] = 0.00675 M
[1] = 0.00335 M
[1] = 0.00167 M
0.5
1
24300
18900
13000
15600
85400
78600
32500
41000
3.5
4.2
2.5
2.6
poly(aminoborane)s obtained using different reaction conditions.
Higher concentrations of the catalyst 1 give higher molecular
weights of the isolated polymers. Although this is significant, we
do not regard this as evidence for step-growth polymer formation,
as in previous studies, a much more pronounced increase of Mn
with higher catalyst loading was reported.11 The presence of H2
has no significant influence on the molecular weight (entries 3/4).
Notably, polymers formed in THF appear to be more uniform,
1.5
24
[1] = 0.00167 M
closed system
5
6
7
[1] = 0.00167 M
3x H3B∙NMeH2
3x1.5
2
11300
11400
27000
36550
28100
81700
3.2
2.5
3.0
giving much smaller dispersities
Ð (Table S2). Repeated
additions of H3B∙NMeH2 in toluene resulted in full consumption
of the substrate, however, molecular weights were not affected
significantly (entry 3 vs. entry 5). Polymerisation is thus not living.
However, that the reaction proceeded shows that the Fe catalyst
[1] = 0.00167 M
[H3B∙NMeH2] = 0.11 M
[1] = 0.00167 M
T = 2 °C
12
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H2B NMe2
8
[3] = 0.00335 M
48
4500
7000
1.6
Me2N BH2
+
1
a
b
0.5-1 mol%
toluene
H3B·NMe2BH2·NMe2H
toluene, T = 25 °C, [H3B∙NMeH2] = 0.33 M. absolute molecular
weights determined using light scattering detection.
+
H3B·NMe2H
- H2
H2B NMe2
+
H
B
In situ NMR spectroscopy
Monitoring of the dehydropolymerisation reaction using 11B NMR
spectroscopy shows that H3B∙NMeH2 is initially converted into
the polymer, [H2BNMeH]n ( -5 ppm, br s; Figure S16). As soon
as the substrate is fully consumed, formation of small amounts
of (HBNMe)3 is observed ( 33 ppm), which is also reflected by
the result of volumetric analysis, giving [n(H2)/n(H3B∙NMeH2)]
values close to 1.0. 11B{1H} NMR spectra show that the BH2
signal at -5 ppm is composed of two resonances, indicating the
formation of small amounts of cyclotriborazane (H2BNMeH)3.
Also, after consumption of H3B∙NMeH2 minor amounts of
diaminoborane HB(NMeH)2 ( 28 ppm) are observed (Figure
S16). Formation of such species was found before and was
attributed to metal-assisted B-N bond cleavage in amine
boranes, followed by reaction of the amine with aminoborane.30
Alternatively, Paul and co-workers computed a rearrangement
reaction of aminoborane, H2B=NH2 to yield BH3 and HB(NH2)2 to
be slightly exergonic by 3.5 kcal∙mol-1. 31 The presence of
diaminoboranes was discussed as indirect evidence for the
presence of free aminoborane.
Me2N
NMe2
Scheme 3. Dehydrocoupling of H3B∙NMe2H using complex 1.
Volumetric analysis of reactions using 1 in toluene shows that
one equivalent of H2 is produced within two hours (Figure S61).
Under these conditions, H2 release follows first-order behaviour,
most likely due to the higher solubility of the substrate compared
to H3B∙NMeH2. 11B NMR analysis of the reaction mixture
indicates that H3B∙NMe2H is quantitatively converted into the
main
product
cyclic
diborazane
(H2BNMe2)2
( 5.5 ppm). Additionally, minor amounts of aminoborane
H2B=NMe2 ( 38.2 ppm), HB(NMe2)2 ( 28.9 ppm), and, most
importantly, the linear diborazane H3B∙NMe2BH2∙NMe2H ( -12.9
and 2.1 ppm), were detected (Figure 4).34
Addition of cyclohexene to trap transient aminoborane
H2B=NMeH in toluene were not successful, indicating that either
no free aminoborane is formed or hydroboration is much slower
than B-N bond formation (Figure S42). Experiments in THF have
shown that Cy2B=NMeH ( 46 ppm) can only be detected in 11
B
NMR spectra after full consumption of H3B∙NMeH2 when also
formation of (HBNMe)3 takes place (Figure S41). This might
indicate that H2B=NMeH is not involved in the polymer formation
step, but at a later stage where also depolymerisation occurs to
form (HBNMe)3. Notably, HB(NMeH)2 is formed already at earlier
stages of dehydropolymerisation, further supporting the
ambiguous role of species of this type.3,15,18b,19b Schneider and
co-workers have shown that for H3B∙NH3 dehydropolymerisation
catalysed by 1, Cy2B=NH2 can be observed, however, the
authors only presented 11B NMR spectra at full conversion in
presence of borazine.8c
Figure 4. In situ 11B NMR spectrum (96 MHz, 298 K, 1024 scans) of
H3B∙NMe2H dehydrocoupling in toluene-d8. Reaction conditions: 1 mol% 1,
T = 25 °C and [H3B∙NMe2H] = 0.33 M.
Similarly as for reactions with H3B∙NMeH2, a broad doublet at  -
9.0 ppm (1JB,H = 110 Hz) can be assigned to an aminoborane
capped Fe amido species [(PNBH NMe P)Fe(H)CO] (1-H2BNMe2).
In previous studies, the presence of H3B∙NMe2BH2∙NMe2H was
discussed as evidence for the existence of an on-metal pathway
for B-N coupling.9b,11,34 It should however be noted that free
aminoborane could originate either from metal-based
dehydrogenation of amine borane and release from the active
species, or from dehydrogenation of linear diborazane, which
was computed to be thermodynamically favourable and
2
2
Depolymerisation experiments using
a
catalyst-free higher
32
molecular weight sample of (H2BNMeH)n have shown that
formation of (HBNMe)3 and HB(NMeH)2 only occurs in the
presence of the Fe catalyst (Figure S44). Also, as shown by an
experiment using freshly prepared (H2BNMeH)333, the latter is
readily converted into (HBNMe)3 (Figure S48). Notably, free
H2B=NMeH could only by trapped as Cy2B=NMeH for
depolymerisation reactions in THF, again suggesting that in THF
hydroboration is much faster than in toluene.
observed
in
case
of
the
related
Ru
complex
[(PNP)Ru(H)PMe3].9b
Dehydropolymerisation with N-methyl Fe amine complex 3
To evaluate the role of cooperative effects between the PNP
ligand and the Fe centre, we have synthesised the literature-
known related N-methylated complex 335 and tested this as a
catalyst for H3B∙NMeH2 dehydropolymerisation (Scheme 4). A
bifunctional dehydrogenation and B-N coupling pathway that
involves ligand cooperation should not be accessible using this
Dehydrocoupling of H3B∙NMe2H with amido complex 1
To gain further insights into the role of the aminoborane and the
role of the Fe centre for B-N bond formation, we have performed
similar reactions using H3B∙NMe2H as a model compound and 1
as the catalyst (Scheme 3).
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10.1002/chem.202000809
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complex. As a result, N-methylation could have consequences
for both, dehydrogenation rate and product selectivity.
suggesting that aminoborane might be more relevant for
depolymerisation than for polymer formation.
Workup of the B-N products was done as before by precipitation
into cold n-hexane, however, in this case significantly lower
polymer yields were observed. SEC analysis showed much
lower molecular weights for samples made using the N-
methylated catalyst 3 (Mn = 4500 g∙mol-1, Table 2). The low-
molecular-weight nature of the material made using 3 is also
reflected in NMR spectra of the isolated products, showing a
more pronounced resonance for a BH3 end group (Figure 5).
H
PiPr2
3
1 mol%
H
H
Me
toluene
N
Fe CO
B
n H3B·NMeH2
N
- n H2
P
H
Me
H
iPr2
n
3
Scheme 4. Dehydropolymerisation of H3B∙NMeH2 using complex 3.
Figure
5 shows a comparison of volumetric curves of
Dehydropolymerisation with borane capped complex 1-BH3
In a previous study, we have reported the formation of a borane
capped Fe amido complex 1-BH3 during catalysis.18d As
mentioned above, similar aminoborane species were detected
before in dehydrocoupling of H3B∙NMe2H using a related Ru
complex, however, this was found to show poor reactivity with
amine borane.9b To further evaluate the role of such species in
dehydrogenation and growth of the B-N polymer, we have
prepared 1-BH3 by addition of BH3 to complex 1 and tested this
in catalysis. Although this species is much less active than 1,
poly(aminoborane) is formed, but the dominant species is the
cyclic borazane, clearly visible by the triplet in the in situ 11B
NMR spectra (Figure 6). 11B{1H} NMR spectra of the isolated
polymer show a relatively sharp resonance for the BH2 moiety
and a pronounced resonance at -18 ppm, which could indicate
residual starting material or BH3 end groups of a short-chain
poly(aminoborane) (Figure S35).
H3B∙NMeH2 dehydropolymerisation using complexes 1 and 3.
Evidently, reactions using 3 are much slower compared to 1. To
our surprise, reactions in toluene show a long induction period,
which was not observed in any of the other cases, but is known
for reactions using the borate complex 2 in THF.18d We discount
Fe nanoparticle formation as the reason for this long induction
period as PMe3 poisoning experiments gave the same reaction
profile and full conversion.
Figure 5. Top: Volumetric curves of H3B∙NMeH2 dehydropolymerisation using
complex 3 in toluene at T = 25 °C and [H3B∙NMeH2] = 0.33 M. Bottom: 11B,
11B{1H} NMR (96 MHz, CDCl3) spectra of the isolated [H2BNMeH]n.
Complex 3 exhibits sufficient stability for NMR monitoring in
toluene. At the onset of the dehydrogenation reaction in situ 11
B
NMR spectra show virtually the same product distribution as for
complex 1 (Figure S52), namely a broad resonance at  -5 ppm
for poly(aminoborane) along with a signal at  -18 ppm for
residual H3B∙NMeH2 or the BH3 end group of a shorter-chain
polymer. Upon further dehydropolymerisation, 11B NMR spectra
become more complex, showing resonances for aminodiborane
BH2(μ-MeNH)(μ-H)BH2 ( -22 ppm), boronium species
Figure 6. Top: Volumetric curve of dehydropolymerisation of H3B∙NMeH2
using complex 1-BH3 in toluene at T = 25 °C and [H3B∙NMeH2] = 0.33 M.
Bottom: In situ 11B, 11B{1H} NMR (96 MHz, toluene-d8) spectra recorded at the
end of the reaction.
[BH2(MeNH2)2]+ (-9.0 ppm, t, JBH = 106 Hz; cf. -8.9 ppm, JBH
=
108 Hz18c) as well as hitherto unobserved signals at  0.3 and  -
1.5 ppm that could point to the presence of further boronium
compounds or polymer branching.3 To our surprise we
succeeded in detecting Cy2B=NMeH as the product of
H2B=NMeH trapping at a later stage of the reaction, again
Low activity of 1-BH3 can be traced back to the fact that no
highly active dihydride complex [(PNHP)Fe(H)2CO] is accessible
during catalysis. Instead, exclusively 1-BH3 is present even after
reaction times of 96 hours (Figure S32). Although the
mechanism of dehydrogenation using this complex is unclear, it
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10.1002/chem.202000809
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is evident from in situ 11B NMR spectra that B-N coupling occurs
less selectively. Signals at -5 <  < 5 ppm suggest the presence
of significant amounts of branched products. Also, much more
(HBNMe)3 was produced with 1-BH3 as the catalyst, further
suggesting the presence of an alternative B-N coupling pathway,
possibly without involvement of the metal complex.
That the distribution of B-N products is similar for complex 1 and
the N-methylated complex 3, but poly(aminoborane)s are
different in molecular weight, suggests that chain propagation
occurs at the metal complex. More specifically, chain growth
occurs through interaction of the growing chain with the metal
centre, but involvement of the PNP ligand is beneficial to obtain
higher molecular weight polymers. For the related Ru system
[(PNP)Ru(H)PMe3], Paul and co-workers have suggested
interaction of the aminoborane with the Ru centre and the amido
group of the ligand, followed by nucleophilic chain growth via the
end of the growing polymer chain (Scheme 6).17 Stoichiometric
experiments of 1 with H3B∙NMeH2 and H3B∙NMe2Hconfirmed the
Mechanistic considerations
Taken together, the above observations indicate that
H3B∙NMeH2 dehydropolymerisation takes place via a chain-
growth mechanism in which the Fe catalyst is bifunctional and
involved in both, dehydrogenation and B-N bond formation. This
is reflected in comparisons of in situ 11B NMR spectra and the
observed trends in molecular weights with changes in catalyst
concentration (Table 2) and at different conversions (Figure 3).
Methylation of the nitrogen backbone of the pincer ligand in
complex 3 leads to H2 release that is slower by approximately a
factor of 10-15 (Figure 5), thus suggesting that for 1 the
dehydrogenation predominantly occurs via a well-known metal-
ligand bifunctional mechanism. Mechanistic models for related
catalytically active N-alkylated pincer complexes were described
before36; a similar scenario should be operating in the herein
reported case. In a previous experimental and theoretical
investigation of hydrazine borane dehydrocoupling using
complex 1, we have proposed facile, essentially barrier-free,
activation of the Fe amido moiety by the amine borane substrate
to yield the dihydride complex [(PNHP)Fe(H)2CO].22 This
subsequently reacts with further substrate to eliminate hydrogen
and the aminoborane. Based on these data as well as the kinetic
and spectroscopic studies reported herein (vide supra), we
formation of such species [(PNBH NMeHP)Fe(H)CO] 1-H2BNMeH
2
NMe
(Figure S54) and [(PNBH
2P)Fe(H)CO] 1-H2BNMe2. Based on
2
the NMR spectroscopic observation of aminoborane-capped Fe
amido species and the found trends in Mn (Table 2) we propose
that in the herein described Fe system a similar scenario could
explain the formation of high-molecular weight polymers for 1. In
contrast, the absence of such interactions in catalysis with 3 only
allows for the formation of comparably low-molecular weight
polymers. Stoichiometric reactions of 3 with H3B∙NMeH2 (Figure
S60) show no other species than the precatalyst.
Paul et al. (2014)
Me2P
N
Me3P Ru
P
H
Me2
+ H2B=NH2
H
H
H2
B
H
H
H
H
Me2P
Me2P
B
+ H2B=NH2
B
NH2
NH2
+ H2B=NH2
N
H
B
N
Me3P Ru
N
P
Me3P Ru
N
H
suggest
a
similar scenario for the dehydrogenation of
Me
H
P
H
H
n
Me2
Me2
H3B∙NMeH2 (Scheme 5):
(i) activation of the precatalyst 1 by proton/hydride transfer
across the Fe amido bond to form the resting state
complex [(PNHP)Fe(H)2CO]
This work:
H3B·NMeH2
[Fe]
1-H2BNMeH
H
B
H
HMeN
Me
- H2
H2B=NMeH
H
B
H
H
PiPr2
Fe CO
PiPr2
MeHN
1
3
N
P
(ii) formation of
a transient catalyst-substrate complex
N
Fe
H
CO
P
between [(PNHP)Fe(H)2CO] and H3B∙NMeH2
(iii) B-H and N-H activation, followed by hydrogen release
and formation of the aminoborane. The KIE for B-H/B-D
being small supports the presence of a non-linear
transition state or B-H activation occurring very early in
the transition state.
H
iPr2
iPr2
strong cooperative
labile
 complex
ligand metal interaction
H H
H H
B
B
N
N
Me
H
n
Me
lower
H
n
H3B·NMeH2
higher
molecular weight
molecular weight
Me
(ii)
N
H
H
H
H
H
(i)
B
H
PiPr2
PiPr2
Scheme 6. Proposed poly(aminoborane) formation from Fe amido complex 1
and N-methylated Fe amine complex 3, resulting in high- and low-molecular
weight polymers.
H3B·NMeH2
H
H
PiPr2
N
Fe CO
N
Fe CO
- H2B=NMeH
H
N
Fe CO
P
P
H
H
iPr2
iPr2
P
H
(iii)
1
iPr2
Involvement of complexes of the type 1-BH3 in dehydrogenation
and B-N coupling using 1 is less likely as rates of H2 release
were much lower and product distributions were different
compared to reactions with 1 or 3. We thus conclude that 1-BH3
rather represents an off-cycle species with limited relevance to
the dehydropolymerisation catalysis using 1.
H2B=NMeH, H2
Scheme 5. Proposed mechanism of H3B∙NMeH2 dehydrogenation using
precatalyst 1.8c,22 It should be noted that this scheme does not reflect all
reaction conditions. As shown above, the equilibrium (ii) is determined by the
experimental conditions, i.e. solvent, [1] and [H3B·NMeH2].
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10.1002/chem.202000809
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Synthesis and characterisation of silyl functionalised amine
boranes
well as additional signals that we assign to aminodiborane
species (BH2)2(μ-RNH)(μ-H) ( -22.0 ppm, R = CH2SiMe3).
Based on the above described results on H3B∙NMeH2
dehydropolymerisation and Schneider’s related work on
H3B∙NH3 we have next tested the Fe amido catalyst 1 for
dehydropolymerisation of 4. Reactions in THF were very slow,
requiring 14 days for release of one equivalent of H2 (Figure
S65). Most remarkably, changing the solvent to toluene has a
dramatic effect as in this case full conversion was observed after
only 90 minutes (Figure 6, top) along with an increase in
viscosity of the reaction mixture. We thus note that complex 1 is
much more tolerant toward sterically demanding groups at the
methyl group of the amine borane than the well-studied catalyst
[(POCOP)IrH2].
Heteroatom functionalised poly(aminoborane)s are rather rare,
to the best of our knowledge, only a handful of examples have
been presented to date.11 Silicon containing B-N polymers are
promising single-source precursors that could possess new
interesting properties, e.g. for coating applications. Also,
inclusion of Si into B-N ceramics could furnish new materials
with unforeseen thermal properties. 37 , 38 We have therefore
prepared an example for a silyl substituted amine borane and
tested this for dehydropolymerisation.
H3B∙N(CH2SiMe3)H2 (4) was prepared in analogy to the
procedure established for H3B∙NMeH239 (Scheme 7), giving the
product as a white, crystalline solid that can be purified by
sublimation.
BH3·L
H3B·NRH2
R NH2
-78 °C
: R = CH2SiMe3, L = THF
4
Scheme 7. Synthesis of silyl functionalised amine borane and molecular
structure of compound 4. Thermal ellipsoids correspond to 30% probability.
1H NMR spectroscopic analysis of 4 shows the expected signals
due to SiMe3 ( 0.11 ppm), BH3 ( 1.53 ppm), CH2 ( 2.28 ppm),
and NH2 group ( 3.71 ppm). In 11B NMR, an ill-resolved quartet
can be detected at -17.5 ppm (Figure S3). Crystals suitable for
an X-ray analysis were obtained by slow cooling of a saturated
solution of 4 in diethyl ether to -30 °C. The molecular structure is
depicted in Scheme 7. As found for other amine boranes, the B-
N bond (1.604(2) Å) in compound 4 is slightly elongated
Figure 6. Top: Volumetric curve of dehydropolymerisation of 4 using complex
1 in toluene at T = 25 °C and [H3B∙NRH2] = 0.21 M. Bottom: In situ 11B NMR
spectra (96 MHz, toluene-d8) recorded after full conversion. R = CH2SiMe3.
compared to a typical B-N single bond (rcov = 1.56 Å 40
,
H3B∙NMeH2: 1.594(1) Å41).
11B NMR analysis of the reaction mixture shows a pronounced
broad signal centred at approximately  -2 ppm (Figure 6,
bottom). Additional resonances at  29 ppm and  -17 ppm
indicate the presence of HB(NHR)2 and BH3 end-groups of a
polymer or unreacted 4, respectively. Unfortunately, after
precipitation of the polymer into n-hexane, the pale yellow
precipitate was insoluble in commonly used solvents THF or
CHCl3, thus complicating further analysis by NMR (Figures S73
and S74) or SEC.
H3B∙NMeH2 and 4 show similar rates in dehydropolymerisation
reactions using 1 as the catalyst. We thus envisioned a co-
dehydropolymerisation reaction to increase the solubility of the
Si containing poly(aminoborane) (Scheme 8), an approach that
was reported to be feasible before.1d,10,11,12
Dehydropolymerisation of functionalised amine borane 4
For initial dehydropolymerisation experiments using 4 we tested
two well-studied amine borane dehydrogenation catalysts,
42
namely Brookhart’s [(POCOP)IrH2]1d,8g,13,
(POCOP = 2,6-(t-
Bu2PO)2-C6H3)
[Rh(cod)Cl]2
as
well
as
commercially
available
39, 43
(cod = 1,5-cyclooctadiene) in THF at room
temperature. Despite using comparably high catalyst loadings of
5 mol% release of less than one equivalent of H2 was found and
reactions were stopped after 24 hours (Figure S63).
[(POCOP)IrH2] is known to be less tolerant towards sterically
more demanding amine borane adducts10 and functional groups
close to the amine moiety12, which is in line with our observation
of release of only 0.4 equivalents of H2 and the presence of
residual substrate
4
in 11B NMR spectra. Furthermore,
diaminoborane HB(NHR)2 ( 28.5 ppm) and the corresponding
borazine (HBNR)3 ( 32.0 ppm, R = CH2SiMe3) were detected
(Figure S64, Table S4). When using the Rh precursor, the
dehydrogenation reaction was much faster, however, after 24
hours only approximately 0.7 equivalents of H2 were released. In
situ 11B NMR spectra show the above mentioned resonances as
H
H
H
B
H
H3B·NMeH2
B
1
1 mol%
- H2
N
r
+
N
H
R
H
n
H3B·N(CH2SiMe3)H2
Me
m
4
R = CH2SiMe3
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10.1002/chem.202000809
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ratios, the lack of signals in 11B NMR spectra in the region -5 < 
< 5 ppm and the uniform main signal for the BH2 groups suggest
the presence of a linear random polymer without significant
branching. Soluble polymer isolated from toluene solutions
shows a molecular weight (SEC, LS detection) of Mn = 11.400
Scheme 8. Co-dehydropolymerisation of H3B∙NMeH2 and 4 to yield a random
copolymer. r used to link both monomer subunits in the polymer structure is
used to indicate that the copolymer is most likely random.
As before for reactions with 4, reactions of equimolar mixtures of
H3B∙NMeH2 and 4 in toluene show full conversion, in this case
after only 30 minutes and albeit using a lower catalyst loading (1
mol% instead of 2 mol% 1; Figure 7, top). In situ 11B NMR
spectra show a broad resonance at  -5 ppm without any
detectable proton coupling, typical for a polymer. Two minor
additional signals at  29 ppm and  33 ppm indicate formation
of HB(NHR)2 and (HBNR)3, respectively (Figure S84). Reactions
in THF were again much slower and did not show full conversion,
producing only 0.6 eq. of H2 (Figure S74).
g∙mol-1 (Mw
=
14.600 g∙mol-1) with comparably narrow
polydispersity of Đ = 1.26.
As shown above for reactions with H3B∙NMeH2, a decrease in
catalyst loading results in the formation of lower-molecular-
weight polymers. We have thus repeated dehydropolymerisation
of pure 4 with less catalyst 1 in order to obtain a well-defined,
more soluble homopolymer derived from 4. Use of 0.5 mol% 1
instead of 2 mol% (vide supra) in toluene gave no full conversion
(release of 0.6 equivalent of H2, Figure S75), most likely due to
BH3 deactivation of the active dihydrido species that was
discussed before by Schneider.8c In that case, addition of an
amine was found to increase the stability of the catalyst. In a
more recent study, Weller and co-workers have shown that for
[Rh(DPEphos)]+ and Rh pincer complexes amine addition serves
to bring the catalyst on-cycle.18a,44 We have thus added NMe2Et
and found that this gives full turnover (i.e. release of one
equivalent of H2) after 14 hours. NMR analysis of the reaction
solution (Figure 8) shows that the product mixture is however
rather complex, containing (HBNR)3, HB(NHR)2, (H2BNRH)3,
and (BH2)2(μ-RNH)(μ-H) along with the polymer. A well-defined
quartet resonance for the product of BH3 trapping, H3B∙NMe2Et
is found at  -8.8 ppm. Notably, a minor resonance at  37.5
ppm indicates the presence of aminoborane H2B=NRH, which
was not observed directly before for other primary amine
boranes.
Figure 7. Top: Volumetric curve of co-dehydropolymerisation of an equimolar
mixture of H3B∙NMeH2 and
4 using complex 1 at T = 25 °C and
[H3B∙NRH2] = 0.33 M. Bottom: 11B NMR spectra (96 MHz, CDCl3) of isolated
co-polymer after co-dehydropolymerisation of H3B∙NMeH2 and 4. R = Me or
CH2SiMe3.
Figure 8. In situ 11B and 11B{1H} NMR spectra (96 MHz, toluene-d8) after
dehydropolymerisation of 4 in toluene using 0.5 mol% 1 and 10 mol% NMe2Et
at T = 25 °C and [4] = 0.33 M. R = CH2SiMe3, * = H3B∙NMe2Et .8c
Other than for N-methylpoly(aminoborane), workup proved to be
rather difficult in this case. Addition of n-hexane at -78 °C did not
lead to precipitation and only resulted in turbid solutions from
which no (filter paper) or only small amounts (5-10% yield, short
plug of Al2O3) of polymer could be isolated by filtration. To our
delight, MeCN proved to be well-suited for workup.
Concentration of the reaction solution to a minimum volume and
addition of MeCN resulted in the formation of white plates, which
were isolated and dried in vacuum (40% yield). Notably, when
changing the amine borane ratio [4]:H3B∙NMeH2 to 3:1, yields
could be improved to 60% of a random 3:1 copolymer. The
11B{1H} NMR spectrum of the isolated 1:1 co-polymer shows the
aforementioned broad resonance at  -5 ppm for the BH2 groups
of the polymer chain (Figure 7, bottom). 1H NMR analysis
reveals the expected signals for CH2SiMe3 ( 2.0 and 0.12 ppm)
and CH3 groups ( 2.23 ppm), which are in in the same range as
found for the homopolymers (Figure S78).1d 1H NMR integral
Addition of cold n-hexane to the product mixture results in
precipitation of the polymer, which could be isolated in
approximately 40% yield. Again, the 11B and 1H NMR spectra
show the expected broad signals (11B:  -7 ppm; 1H:  0.13, 1.86,
2.03, 2.93 ppm, Figure S76), supporting the assignment as a
homopolymer derived from 4. Signals for BH3 end groups of the
polymer were not observed. The 29Si{inept} NMR spectrum
shows a broad resonance at  1 ppm that is slightly shifted to
higher field compared to 4 ( 0.6 ppm). SEC analysis (LS
detection) confirmed the presence of a well-defined high-
molecular weight polymer (Mn = 61.200 g∙mol-1, Mw = 98.500
g∙mol-1, Đ = 1.6).
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2
a) D. Han, F. Anke, M.Trose, T. Beweries, Coord. Chem. Rev. 2019, 380,
260-286. ; b) H. Johnson, T. Hooper, A. Weller, in Synthesis and
Application of Organoboron Compounds, Vol. 49 (Eds.: E. Fernández, A.
Whiting), Springer International Publishing, 2015, pp. 153-220.
O. J. Metters, A. M. Chapman, A. P. M. Robertson, C. H. Woodall, P. J.
Gates, D. F. Wass, I. Manners, Chem. Commun. 2014, 50, 12146-12149.
C. A. De Albuquerque Pinheiro, C. Roiland, P. Jehan, G. Alcaraz, Angew.
Chem. Int. Ed. 2018, 57, 1519-1522.
Conclusions
Dehydropolymerisation of methylamine borane and
a silyl
3
4
5
substituted analogue using the iron amido complex 1 furnishes
high-molecular weight poly(aminoborane)s. Kinetic studies of
H3B∙NMeH2 dehydrogenation show that this process follows a
kinetic regime which can be described as a first-order process
as a limiting case of more general Michaelis-Menten-type
kinetics with the iron dihydride [(PNHP)Fe(H)2CO] as the active
species. Analysis of the molecular weights of the obtained
polymers, comparative studies using a related N-methylated
catalyst and dimethylamine borane suggest involvement of the
metal complex. We propose that polymer growth occurs via well-
precedented nucleophilic chain growth from the end of a
coordinated oligomer chain in a chain-growth scenario. Control
of the molecular weight of the polymers is possible by variation
of the catalyst concentration, reaction temperature, the solvent
as well as the structure of the Fe catalyst. Reactions in toluene
are significantly faster and much more selective for
poly(aminoborane) compared to those in THF.
J. F. Hartwig, Organotransition Metal Chemistry. From Bonding to
Catalysis, University Science Books, Sausalito, 2010.
6
7
A. L. Colebatch, A. S. Weller, Chem. Eur. J. 2019, 25, 1379-1390.
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Bhunya, T. Malakar, G. Ganguly, A. Paul, ACS Catal. 2016, 6, 7907-7934.
Selected examples: a) S. Todisco, L. Luconi, G. Giambastiani, A. Rossin,
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groups
were
not
found
to
interfere
with
the
dehydropolymerisation process when complex 1 was used in
toluene. In fact, this precatalyst was found to be superior to
other late transition metal complexes typically used in amine
borane chemistry. The presented results once more highlight the
great potential of Fe PNP complexes, not only for catalytic
hydrogenation and dehydrogenation reactions directed at the
synthesis of organic compounds and small molecule activation45,
but also for the synthesis of novel main group polymers. Future
studies on this and related systems should be directed at the
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control the molecular weights, but also the microstructure
(branching, tacticity, incorporation of further monomer units) of
the poly(aminoborane)s.
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Acknowledgements
We thank our technical and analytical staff, in particular
Benjamin Andres and Cornelia Pribbenow for assistance, as well
as Boron Specialities for a donation of methylamine borane.
General support by LIKAT is gratefully acknowledged. This work
was funded by the Deutsche Forschungsgemeinschaft (BE
4370/9-1).
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Keywords: Iron • B-N polymers • polymerisation •
dehydrogenation • SEC analysis
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Entry for the Table of Contents
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Felix Anke, Susanne Boye, Anke
Spannenberg, Albena Lederer, Detlef
Heller and Torsten Beweries*
Page No. – Page No.
Dehydropolymerisation of
methylamine borane and an N-
substituted primary amine borane
using a PNP Fe catalyst
The well-known Fe amido complex [(PNP)Fe(H)(CO)] (PNP = N(CH2CH2Pi-Pr2)2) is
a potent catalyst for the selective formation of poly(aminoborane)s from
methylamine borane and a SiMe3 substituted analogue. Mechanistic studies
support a chain growth mechanism with polymer formation via nucleophilic attack at
the terminus of a growing B-N chain.
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