Polyaminoborane main chain scission using N-heterocyclic carbenes; Formation of donor-stabilised monomeric aminoboranes

Article abstract of DOI:10.1039/c3cc44373f

The reaction of N-heterocyclic carbenes with polyaminoboranes [MeNH-BH ₂]_n or [NH₂-BH₂]_n at 20 °C led to depolymerisation and the formation of labile, monomeric aminoborane-NHC adducts, RNH-BH₂-NHC (R = Me or H); a similar NHC adduct of Ph₂NBCl₂ was characterized by single crystal X-ray diffraction.

Full text of DOI:10.1039/c3cc44373f

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Polyaminoborane main chain scission using
N-heterocyclic carbenes; formation of
donor-stabilised monomeric aminoboranes†
Cite this: Chem. Commun., 2013,
49, 9098
Received 10th June 2013,
Accepted 12th August 2013
Naomi E. Stubbs, Titel Jurca, Erin M. Leitao, Christopher H. Woodall and
Ian Manners*
DOI: 10.1039/c3cc44373f
www.rsc.org/chemcomm
The reaction of N-heterocyclic carbenes with polyaminoboranes MeNH2ꢀBH3,8,9 was conducted by heating polymer to 70 1C for
[MeNH–BH2]n or [NH2–BH2]n at 20 8C led to depolymerisation and 19 h in both strongly donating (THF) and weakly donating
the formation of labile, monomeric aminoborane–NHC adducts, (toluene) solvents. Reaction progress was monitored by 11B NMR,
Q
RNH–BH2–NHC (R = Me or H); a similar NHC adduct of Ph2N BCl2 electrospray ionisation mass spectrometry (ESI-MS) and gel
was characterized by single crystal X-ray diffraction.
permeation chromatography (GPC). Analysis of the thermo-
lysed polymer in THF by 11B NMR revealed that, as well as
As a result of their potential applications in hydrogen storage1–3 decomposition to methyl borazine, [MeN–BH]3 (d(11B) 32.3 ppm,
and transfer4–7 and as precursors to new inorganic materials,8–12 d, 1JBH = 135 Hz) (ca. 10% of post-thermolysis product distribu-
amine–boranes (RR0NHꢀBH3, R, R0 = H, alkyl or aryl) have been tion), depolymerisation to lower molecular weight oligomers
the recent focus of intense interest.13–16 These species can be [MeNH–BH2]x (d(11B) ꢁ6.6 ppm, br) was suggested to occur by
dehydrogenated/dehydrocoupled by either thermal or catalytic the observed sharpening of the broad 11B NMR peak charac-
routes to yield an array of products, including aminoboranes, teristic of the high molecular weight material (Fig. S4, ESI†). ESI
RR0NQBH2, cycloborazanes, [RR0N–BH2]x (x > 1) and borazines, mass spectrometry analysis of the reaction mixture confirmed
[RN–BH]3. Catalytic dehydrocoupling of primary amine– the continued presence of polymer with a distribution of
boranes and ammonia–borane can yield high molecular weight peaks with a mass difference of m/z = 43, corresponding to
polyaminoboranes, [RNH–BH2]n which are isoelectronic to [MeNH–BH2] repeat units (Fig. S5, ESI†). However, GPC analysis of
polyolefins.8,9,17–19 Several catalyst systems such as IrH2(POCOP) the isolated polymer confirmed a substantial reduction in molar
(POCOP = k3-1,3-(OPtBu2)2C6H3),8,9,18 and also Cr, Mn,19 mass (Mn = 66 000 Da, PDI = 2.03, Fig. S6, ESI†). Thermolysis of
Rh9,20,21 and Fe complexes,22 and skeletal Ni17 have been [MeNH–BH2]n in toluene at 70 1C for 19 h gave similar results to
shown to be active for this transformation. Polyaminoboranes those in THF.
are of potential interest as precursors to BN ceramics,23 as
Next, we explored the influence of Lewis bases on the
piezoelectric materials24 as well as other applications.25 These stability of [MeNH–BH2]n. Treatment of [MeNH–BH2]n with one
unusual main group polymers are intriguing materials and equiv. of tri-cyclohexylphosphine (PCy3) in THF led to no obvious
their polar boron–nitrogen main chain might be expected to change by 11B NMR spectroscopy (Fig. S10, ESI†), even at elevated
undergo fragmentation by heterolytic cleavage processes. As temperatures up to 50 1C for 22 h. In contrast, analogous
part of our investigations of the properties of these materials addition of one equiv. of tri-n-butylphosphine, P(nBu)3 to
we have examined the stability of poly(methylaminoborane) [MeNH–BH2]n in THF at 20 1C led to some clear depolymerisation
1
[MeNH–BH2]n both thermally, and in the presence of neutral into MeNH2ꢀBH3 (d(11B) ꢁ18.8 ppm, q, JBH = 89 Hz) (ca. 10%)
Lewis bases. Herein we report on our preliminary results in and a trace of bis(methylamino)borane, (MeNH)2BH (d(11B)
this area.
27.8 ppm, d, br) after 22 h (Fig. S11, ESI†). Reaction of the
An initial investigation of the thermal stability of a sample strong nitrogen base 4-dimethylaminopyridine (DMAP) with
of [MeNH–BH2]n (Mn = 190 000 Da, PDI = 1.21) formed [MeNH–BH2]n after 24 h at 20 1C in THF was even more
by the Ir-catalysed dehydrogenation of methylamine–borane, extensive and 11B NMR showed, in addition to [MeNH–BH2]3orx
(d(11B) ꢁ5.3 ppm, br) (ca. 70%), the formation mainly of
1
both MeNH2ꢀBH3 (d(11B) ꢁ18.9 ppm, JBH = 97 Hz) (ca. 10%)
School of Chemistry, University of Bristol, Bristol, BS8 1TS, UK.
and [MeN–BH]3 (d(11B) 30.2 ppm, d, br) (ca. 10%) (Fig. S13,
ESI†).
Clearly strong donor ligands such as P(nBu)3 and DMAP are
E-mail: ian.manners@bristol.ac.uk; Tel: +44 (0)117 928 7650
† Electronic supplementary information (ESI) available: Experimental details,
NMR (1H, 11B, 13C, 31P) spectroscopy, ESI and CI mass spectrometry, GPC traces
and crystal structure data. CCDC 953007. For ESI and crystallographic data in CIF
or other electronic format see DOI: 10.1039/c3cc44373f
capable of cleaving the B–N main chain in polyaminoboranes
c
9098 Chem. Commun., 2013, 49, 9098--9100
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To provide further evidence of the product assignments we
also investigated the reactions of IPr with monomeric amino-
borane iPr2NQBH2, cyclic diborazane [Me2N–BH2]2, and the
amine–borane MeNH2ꢀBH3. In all cases NHC-stabilised amino-
boranes were identified as the major products (Table S1, ESI†).
In the case of [Me2N–BH2]2, this cyclic species exists in an
equilibrium with its monomeric form, Me2NQBH2,29 and it is
postulated that IPr can only bind with the monomer thereby
explaining the long reaction time and elevated temperature
needed to enable quantitative conversion. Furthermore, it was
observed that aminoborane–NHC adducts can be formed using
amine–boranes, such as MeNH2ꢀBH3 as long as two or more
equivalents of NHC are present; one to act as a hydrogen
acceptor27 and the second for adduct formation.
Scheme 1 Reaction of aminoborane (top) and amine–borane (bottom) with IPr
to yield an IPr stabilized aminoborane adduct (IPr = 1,3-bis-(2,6-diisopropylphenyl)-
imidazol-2-ylidene).
but these nucleophiles were not able to form detectable
Further characterisation of the aminoborane–carbene adducts
adducts with the resulting B–N fragments. In an attempt to was thwarted by the labile, dative B–C bond, leading to dissocia-
achieve this,26 we treated polyaminoborane [MeNH–BH2]n with tion to the aminoborane and carbene on attempted isolation.
one equiv. per repeat unit of the N-heterocyclic carbene (NHC) To reduce the lability, we explored NHC adduct formation with
1,3-bis-(2,6-diisopropylphenyl)-imidazol-2-ylidene (IPr) at 20 1C an aminoborane that possessed phenyl groups on nitrogen and
in THF (Scheme 1). This led to the quantitative formation chlorine atoms on boron. We anticipated that the resulting
of a new species as observed by 11B NMR after 10 min. (d(11B) increase in Lewis acidity at boron as a result of the reduced
ꢁ17.2 ppm, t, 1JBH = 90 Hz) (Fig. S14, ESI†). The structure of this p-donation from nitrogen and the presence of electronegative Cl
product was assigned as the aminoborane–NHC adduct MeNH– substituents would lead to a substantially more robust C–B
BH2–IPr based partly on comparative NMR data with that for donor–acceptor interaction. The reaction of Ph2NQBCl2 with
the borane adduct of this species, BH3–NH(CH3)–BH2–IPr, one equiv. of IPr was conducted in THF at 20 1C where
recently reported by Rivard and co-workers, which possesses a new species was observed by 11B NMR spectroscopy at
an internal boron environment with a broad 11B NMR peak at d(11B) 1.2 ppm (s) (Fig. S37, ESI†). Suitable crystals of the
d ꢁ14.0 ppm.27,28 Further evidence for the assigned structure product for
a single-crystal X-ray diffraction study were
was provided by in situ ESI mass spectrometry which gave a obtained from a saturated toluene solution over several days
molecular ion peak at m/z = 432 (intensity = 35%) and a peak at at 20 1C (for the determined molecular structure see Fig. 1 and
m/z = 389 (intensity = 100%) corresponding to the loss of the Fig. S42, ESI†). The structure confirmed the product to be the
aminoborane MeNHQBH2 (Fig. S18, ESI†). Unfortunately, desired adduct Ph2N–BCl2–IPr (1) with a distorted tetrahedral
attempts to isolate MeNH–BH2–IPr by crystallization and preci- boron center (B1) bound to a carbene center (C1) and a
pitation led to dissociation and the regeneration of IPr (Fig. S19, distorted trigonal nitrogen (N3), with C1–B1 and B1–N3 bond
ESI†). Analogous treatment of [MeNH–BH2]n with one equiv. of lengths of 1.653(4) Å and 1.510(4) Å respectively. The distance
1,3-bis-(2,4,6-trimethylphenyl)-imidazol-2-ylidene (IMes) (THF, between sp3 hybridised B1 and sp2 hybridised N3 is within those
20 1C, 10 min) led to the formation of a similarly labile product,
1
MeNH–BH2–IMes (d(11B) ꢁ17.0 ppm (t, JBH = 90 Hz) (Fig. S20,
ESI†) (ca. 60% yield) which was also supported by ESI mass
spectrometry with the observation of a peak in situ at m/z = 348
for the molecular ion (intensity = 5%) as well as a peak at
m/z = 305 (intensity = 100%) corresponding to the loss of
MeNHQBH2 (Fig. S21, ESI†).
In an effort to investigate the generality of these observa-
tions we also explored the reaction of IPr with poly(ammonia–
borane) [NH2–BH2]n in THF where the B–N polymer was formed
by the Ir-catalyzed dehydrogenation of NH3ꢀBH3.8,9 However,
perhaps due to the insolubility of poly(ammonia–borane) in
common organic solvents, the reaction was substantially slower
than for the methyl analogue. Nevertheless, after 24 h 11B NMR
revealed quantitative conversion to NH2–BH2–IPr with a peak
1
observed at (d(11B) ꢁ19.5 ppm, t, JBH = 88 Hz) (Fig. S22, ESI†).
This was further supported by chemical ionisation (CI) mass
spectrometry with the observation of a peak in situ at m/z =
417 for the molecular ion (intensity = 24%) and a peak at m/z =
Fig. 1 Molecular structure of compound Ph2N–BCl2–IPr (1) determined by X-ray
389 (intensity = 100%) corresponding to the loss of NH2QBH2
(Fig. S23, ESI†).
diffraction with all non-H atoms as 50% thermal ellipsoids, and with all hydrogens
and solvent (toluene) removed for clarity.
c
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encumbered adducts, with the goal of promoting polymer
regeneration. We are also exploring the consequences of the
potential recyclability of polyaminoborane materials under
mild conditions.
Notes and references
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Scheme 2 Reaction of iPr2N–BH2–IPr with B(C6F5)3 to regenerate iPr2NQBH2
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removal of the NHC by B(C6F5)3 from the less sterically
2006, 12, 4900.
c
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9100 Chem. Commun., 2013, 49, 9098--9100