Catalytic redistribution and polymerization of diborazanes: Unexpected observation of metal-free hydrogen transfer between aminoboranes and amine-boranes

Article abstract of DOI:10.1021/ja208752w

Ir-catalyzed (20 °C) or thermal (70 °C) dehydrocoupling of the linear diborazane MeNH₂-BH₂-NHMe-BH₃ led to the formation of poly- or oligoaminoboranes [MeNH-BH₂]_x (x = 3 to >1000) via an initial redistribution process that forms MeNH ₂?BH₃ and also transient MeNH=BH₂, which exists in the predominantly metal-bound and free forms, respectively. Studies of analogous chemistry led to the discovery of metal-free hydrogenation of the B=N bond in the "model" aminoborane iPr₂N=BH₂ to give iPr₂NH?BH₃ upon treatment with the diborazane Me₃N-BH₂-NHMe-BH₃ or amine-boranes RR′NH?BH₃ (R, R′ = H or Me).

Full text of DOI:10.1021/ja208752w

COMMUNICATION
pubs.acs.org/JACS
Catalytic Redistribution and Polymerization of Diborazanes:
Unexpected Observation of Metal-Free Hydrogen Transfer between
Aminoboranes and Amine-Boranes
Alasdair P. M. Robertson, Erin M. Leitao, and Ian Manners*
School of Chemistry, University of Bristol, Cantock’s Close, Bristol BS8 1TS, U.K.
S Supporting Information
b
Scheme 1. Proposed Mechanism of Polymerization of (a)
ABSTRACT: Ir-catalyzed (20 °C) or thermal (70 °C)
MeNH2 BH3 and (b) MeNH2ꢀBH2ꢀNHMeꢀBH3 with 0.3
3
dehydrocoupling of the linear diborazane MeNH2ꢀBH2ꢀ
mol % IrH2POCOP
NHMeꢀBH3 led to the formation of poly- or oligoamino-
boranes [MeNHꢀBH2]x (x = 3 to >1000) via an initial
redistribution process that forms MeNH2 BH3 and also
3
transient MeNHdBH2, which exists in the predominantly
metal-bound and free forms, respectively. Studies of analo-
gous chemistry led to the discovery of metal-free hydro-
genation of the BdN bond in the “model” aminoborane
iPr2NdBH2 to give iPr2NH BH3 upon treatment with the
3
diborazane Me3NꢀBH2ꢀNHMeꢀBH3 or amine-boranes
RR0NH BH3 (R, R0 = H or Me).
3
polymerization of this species (Scheme 1a). Surprisingly, reac-
tion of the linear diborazane MeNH2ꢀBH2ꢀNHMeꢀBH3 with
0.3 mol % IrH2POCOP under the same conditions over 1.5 h
mine-boranes RR0NH BH3 (R, R0 = alkyl, aryl, or H) are
well-known main-group species that have recently become
3
yielded a spectroscopically identical product, as determined by
NMR spectroscopy and electrospray ionization mass spectro-
metry (ESI-MS) (86% yield; Mw = 97 000 Da, PDI = 1.44)
(see Figures SI-1 and SI-2 in the Supporting Information).
During this reaction, an apparent intermediate was detected by
11B NMR spectroscopy (δB = ꢀ18.7 ppm) that split into a
quartet (JBH = 94 Hz) upon proton coupling and was assigned as
A
the focus of intense interest as a result of their use in hydrogen
storage and transfer and as precursors to new materials.1,2 For
example, soluble polyaminoboranes, inorganic analogues of
polyolefins, have been prepared from the dehydrocoupling of
RNH2 BH3 (R = Me or nBu) in the presence of catalysts such as
3
IrH2POCOP [POCOP = k3-1,3-(OPtBu2)2C6H3].3,4 In addi-
MeNH2 BH3.8f The detection of this species suggested that a
3
tion, NH3 BH3 has attracted attention as a precursor to “white
3
redistribution reaction of the diborazane precursor may have
taken place (Scheme 1b). On the basis of the substrate stoichi-
ometry, such a process would also yield MeNHdBH2, the
proposed precursor to [MeNHꢀBH2]n in the dehydrocoupling
graphene”, monolayer films of hexagonal boron nitride, on Cu
surfaces.5 The increasing importance of metal-mediated pro-
cesses in these fields has led to the rapid emergence of a new area
of coordination chemistry involving amine-boranes and amino-
boranes, RR0NdBH2, as reactive ligands.6 A key challenge for
hydrogen storage applications is the rehydrogenation of “spent
fuel” dehydrogenation products.7 Recent advances in this area
of MeNH2 BH3 by IrH2POCOP.3b
3
Significantly, Burg and Randolph previously reported that
thermolysis of the unsymmetrical diborazane Me3NꢀBH2ꢀ
NMe2ꢀBH3 at 80 °C in the solid state leads to the formation
have involved the use of hydrazine to regenerate NH3 BH3
3
of Me3N BH3 and [Me2NꢀBH2]2 (Scheme 2),9 where the latter
following metal-catalyzed hydrogen release.7b
3
Linear diborazanes, RR02NꢀBH2ꢀNRR0ꢀBH3 (R, R0 = Me
or H) have recently attracted attention, partly as a consequence
of their role as intermediates in a range of metal-catalyzed
dehydrogenations of amine-boranes.8 In this paper, we report a
series of unexpected results that emerged from our recent studies
of the catalytic dehydrocoupling of a series of linear diborazanes
MeR2NꢀBH2ꢀNMeRꢀBH3 (R = H or Me).
species was presumably formed by the cyclodimerization of the
monomeric aminoborane Me2NdBH2. To explore whether this
redistribution process could occur in a catalytic manner at 20 °C,
we prepared Burg’s diborazane, Me3NꢀBH2ꢀNMe2ꢀBH3, via
the reaction of Me3N with [H2B(μ-H)(μ-NMe2)BH2] in THF
solution at ꢀ78 °C using an approach developed recently by
Shore.8h Subsequent treatment of a THF-d8 solution of Me3Nꢀ
BH2ꢀNMe2ꢀBH3 with 1 mol % IrH2POCOP (20 °C, 4 h)
indeed led to rapid redistribution of the substrate and clean
We previously showed that dehydrocoupling of MeNH2 BH3
3
in THF with 0.3 mol % IrH2POCOP at 20 °C yields high-
molecular-weight poly(methylaminoborane), [MeNHꢀBH2]n
(Mw = 160 000 Da, PDI = 2.90).3b A mechanism was tentatively
proposed that involves two metal-catalyzed steps: initial generation
of the monomeric aminoborane MeNHdBH2, and subsequent
formation of Me3N BH3 and [Me2NꢀBH2]2, which were
3
Received: September 16, 2011
Published: October 28, 2011
r
2011 American Chemical Society
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|

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Scheme 2. (i) Thermal and (ii) Catalytic Redistribution of
Me3NꢀBH2ꢀNMe2ꢀBH3
Scheme 4. (i) Thermal and (ii) Catalytic Redistribution of
Me3NꢀBH2ꢀNHMeꢀBH3
Scheme 5. Reaction of Me3NꢀBH2ꢀNHMeꢀBH3 with
iPr2NdBH2
Scheme 3. (a) Thermolysis of MeNH2ꢀBH2ꢀNHMeꢀBH3
(b) with Cyclohexene Trapping
The 11B NMR spectrum of MeNHdBCy2 consisted of a broad
1
singlet at 44.7 ppm (C6D6) that showed no splitting upon H
identified by multinuclear NMR spectroscopy (Scheme 2 and
Figure SI-3). An intermediate, the monomeric aminoborane
Me2NdBH2, was also observed in small amounts during the
catalytic redistribution as a triplet in the 11B NMR spectrum at
37.0 ppm (JBH = 130 Hz).8d
coupling. The isolation of this species strongly implicates the
presence of free MeNHdBH2 in the thermal redistribution of
the diborazane MeNH2ꢀBH2ꢀNHMeꢀBH3 and is in contrast
with observations in the Ir-catalyzed redistribution/polymeriza-
tion of the same substrate.
To probe the catalytic redistribution chemistry further, we pre-
pared another, unsymmetrical diborazane, Me3NꢀBH2ꢀNHMeꢀ
BH3. On the basis of the aforementioned results, we predicted
that Ir-catalyzed redistribution of this diborazane would yield
To provide further evidence for the proposed steps in the
Ir-catalyzed redistribution/polymerization of our diborazane,
MeNH2ꢀBH2ꢀNHMeꢀBH3, we attempted to demonstrate
the presence of the transient monomeric aminoborane
MeNHdBH2 within the reaction mixture [the other redistribu-
tion product, MeNH2 BH3, was identified by 11B NMR spec-
Me3N BH3 and oligomeric or polymeric products derived from
3
3
MeNHdBH2. Treatment of Me3NꢀBH2ꢀNHMeꢀBH3 with
1 mol % IrH2POCOP in THF solution over 6 h led to complete
consumption of the initial diborazane and formation of a mixture
troscopy (see above)]. Although MeNHdBH2 has neither been
observed directly nor isolated through trapping reactions, the
trapping of the related species NH2dBH2 as NH2dBCy2 using
excess cyclohexene has recently been described.6d,8g,10 We there-
fore explored the reaction of MeNH2ꢀBH2ꢀNHMeꢀBH3,
with 0.3 mol % IrH2POCOP (20 °C, THF, 3 h) in the presence
of 2.5 equiv of cyclohexene. Significantly, the reaction appeared
to be unperturbed by the presence of the olefin and yielded the
polyaminoborane [MeNHꢀBH2]n with no evidence for the
formation of MeNHdBCy2, which appears at δB = 44.9 ppm
in the 11B NMR spectrum in THF solution (see below and
Supporting Information). We believe that the inability to trap
MeNHdBH2 despite the formation of [MeNHꢀBH2]n indi-
cates that the aminoborane remains bound to the metal center
during both the redistribution and polymerization8a and hence
may not be free in solution for sufficiently long to allow reaction
with cyclohexene.11
containing, as expected, Me3N BH3 (51%) and oligomeric/
3
polymeric species [MeNHꢀBH2]x (28%) (see Figure SI-5).11b
Somewhat surprisingly, however, we also observed the formation
of [MeNꢀBH]3 (13%) and MeNH2 BH3 (8%)11b (Scheme 4).
3
These products are indicative of an additional mode of reactivity.
Interestingly, essentially the same product distribution was
obtained when the thermolysis of Me3NꢀBH2ꢀNHMeꢀBH3
was performed under metal-free conditions (70 °C, THF, 16 h).
This result indicated that the Ir catalyst does not play a role in the
unexpected chemistry.
Perhaps the most likely route to [MeNꢀBH]3 and MeNH2
3
BH3 involves the highly reactive aminoborane MeNHdBH2.
However, because of the transient nature of MeNHdBH2, it was
not easy to probe the reactivity of this species directly in further
studies. Instead, we therefore chose to explore the reactivity of
iPr2NdBH2, a sterically encumbered “model” aminoborane that
is stable in its monomeric form at ambient temperature.12
Following treatment of Me3NꢀBH2ꢀNHMeꢀBH3 with
iPr2NdBH2 (20 °C, THF, 18 h), analysis of the reaction mixture
by 11B NMR spectroscopy indicated partial conversion of the
reactants. The products identified showed ca. 20% hydrogenation of
To probe this postulate further, we also studied the thermo-
lysis of the diborazane MeNH2ꢀBH2ꢀNHMeꢀBH3 (70 °C,
THF, 18 h) in the absence of the Ir catalyst. Under these
conditions, in the absence of cyclohexene, the diborazane
was shown to be almost quantitatively converted to MeNH2
3
BH3 and [MeNHꢀBH2]3 together with a small amount of
the borazine [MeNꢀBH]3 (Scheme 3a). The formation of
[MeNHꢀBH2]3 is again consistent with elimination of
MeNHdBH2, which then may cyclotrimerize. In striking
contrast to the Ir-catalyzed process, repeating the metal-free
reaction in the presence of 2.5 equiv of cyclohexene yielded both
iPr2NdBH2 to iPr2NH BH3 together with significant redistri-
bution of the diborazane to yield products including Me3N BH3
3
3
(17%) and also [MeNꢀBH]3 (9%), MeNH2 BH3 (8%), and a
3
trace of [H2B(μ-H)(μ-NMeH)BH2] (2%)11b (Scheme 5). This
room temperature experiment clearly indicated that aminobor-
anes generated from the redistribution of linear diborazanes are
capable of surprising reactivity, and this can even involve
their hydrogenation. This process apparently occurs with the
MeNH2 BH3 and the trapped product MeNHdBCy2 in an
3
approximately 1:1 ratio (Scheme 3b and Figure SI-4). The latter
trapping product was characterized by multinuclear NMR spec-
troscopy and high-resolution chemical ionization MS (CI-MS).
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Scheme 6. Reaction of MeNH2 BH3 with iPr2NdBH2
3
Figure 1. 11B{1H} NMR spectrum for the reaction of iPr2NdBH2 and
NH3 BH3 after 18 h: (A) [NHꢀBH]3, (B) iPr2NH BH3, (C) NH3
3
3
3
BH3, (D) NH2B2H5.
formation of side products such as [MeNꢀBH]3 and MeNH2
3
BH3, which were also identified in the aforementioned reaction
in the presence of the Ir catalyst.
The highly unexpected observation of aminoborane hydro-
genation detected with diborazanes inspired us to conduct a
further investigation of related reactions of iPr2NdBH2 with
simpler substrates under metal-free conditions. The reaction of
Figure 2. 11B{1H} NMR spectrum of the reaction of iPr2NdBH2 and
Me2NH BH3 after 48 h: (A) iPr2NdBH2, (B) [Me2NꢀBH2]2, (C)
iPr2NdBH2 with NH3 BH3 (20 °C, THF) in either a sealed or
3
3
3
Me2NH BH3, (D) iPr2NH BH3.
open system led to a clear reaction solution from which a
colorless solid rapidly began to precipitate. Analysis of the
reaction mixture by 11B NMR spectroscopy after 18 h
(Figure 1) indicated the complete consumption of iPr2NdBH2
(δB = 34.6 ppm) to form the amine-borane hydrogenation
3
Allowing this reaction to stir for a further 30 h produced no
further conversion of iPr2NdBH2 or MeNH2 BH3. This suggested
3
that the system may be at equilibrium under the conditions
studied. Once again, the formation of MeNHdBH2 in the reaction
was confirmed by the addition of cyclohexene to the initial
reaction mixture, with 45% MeNHdBCy2 formed over 18 h.11b
product iPr2NH BH3 (δB = ꢀ21.7 ppm). A trace quantity
3
of NH3 BH3 remained in solution, with [H2B(μ-H)(μ-
3
NH2)BH2]8h and [NHꢀBH]3 identified as the only other
soluble products. However, these species accounted for a total
The reaction of iPr2NdBH2 with Me2NH BH3 (20 °C, 18 h)
3
of only ca. 50% of the initial boron present as NH3 BH3.11b The
yielded 54% hydrogenation of iPr2NdBH2 and 58% dehydro-
3
colorless precipitate from the reaction mixture was collected via
decantation but proved to be entirely insoluble in common
organic solvents. Characterization by FT-IR spectroscopy
showed both NꢀH and BꢀH stretching modes at frequencies
consistent with those of the insoluble polyaminoborane
[NH2ꢀBH2]x, which has been previously prepared via the Ir-
genation of Me2NH BH3 to furnish [Me2NꢀBH2]2 cleanly.
3
Again in this case, even though the reaction was allowed to stir for
a further 30 h, complete conversion of Me2NH BH3 to
3
[Me2NꢀBH2]2 could not be achieved, which also suggested
that the system reached an equilibrium under these conditions
(Figure 2 and Figure SI-8).
catalyzed dehydrogenation of NH3 BH3.3b
To explore the existence of an equilibrium further, the “reverse”
3
These results indicate that, remarkably, iPr2NdBH2 is quan-
titatively hydrogenated by NH3 BH3 to form iPr2NH BH3 and
reaction between iPr2NH BH3 and [Me2NꢀBH2]2 (20 °C,
3
THF) was also studied. Although notably slower, this reaction
also reached equilibrium after 190 h, and the equilibrium mixture
3
3
NH2dBH2. The transient aminoborane NH2dBH2 presumably
then spontaneously oligomerizes to form insoluble [NH2ꢀBH2]x.
The presence of the free aminoborane NH2dBH2 was confirmed
by repeating the reaction in the presence of 2.5 equiv of
cyclohexene. Under these conditions, only a small amount of
[NH2ꢀBH2]x was observed to form as an insoluble white
precipitate, and 75% conversion to NH2dBCy2 (δB = 46.9
ppm) was apparent by 11B NMR spectroscopy.13,11b
contained comparable quantities of iPr2NdBH2/iPr2NH BH3
3
and [Me2NꢀBH2]2/Me2NH BH3 (Figure SI-9). The observa-
3
tion that the degree of hydrogenation of iPr2NdBH2 with
amine-boranes appears to decrease in the order NH3 BH3 >
3
MeNH2 BH3 > Me2NH BH3 is consistent with the calculated
3
3
dehydrogenation enthalpies for the latter species.15
For comparison, we also investigated the reaction of
iPr2NdBH2 and Me2ND BD3 (20 °C, THF). Analysis by 11B
To investigate the generality of this hydrogen transfer process, we
3
also investigated the reactions of iPr2NdBH2 with MeNH2 BH3
NMR spectroscopy after 2 h was consistent with initial transfer of
3
and Me2NH BH3 at 20 °C in THF.14 The reaction of iPr2NdBH2
D2 to iPr2NdBH2 to give iPr2ND BH2D, which possessed a
3
3
and MeNH2 BH3 (20 °C, 21 h) resulted in 90% hydrogenation of
iPr2NdBH2 to iPr2NH BH3, accompanied by 75% dehydrogena-
tion of MeNH2 BH3 to form various products including [H2B(μ-
H)(μ-NHMe)BH2] (6%), [MeNꢀBH]3 (4%), and [MeNHꢀ
BH2]x (Scheme 6; also see Figures SI-6 and SI-7). The formation
of the latter would be anticipated on the basis of the isolation of
broad triplet characteristic of a BH2D group (Figures SI-10 and
3
SI-11). As expected, this reaction proceeded more slowly than that
3
with Me2NH BH3 under the same conditions, consistent with
3
3
cleavage of NꢀH/D and/or BꢀH/D bonds in the rate-determin-
ing step. In addition, scrambling of deuterium into all of the
hydrogen environments was detected as the reaction progressed.
In summary, Ir-catalyzed dehydrocoupling of diborazanes such as
MeNH2ꢀBH2ꢀNHMeꢀBH3 proceeds with redistribution and
[NH2ꢀBH2]x in the case of the reaction between NH3 BH3 and
3
iPr2NdBH2 (see above).
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subsequent polymerization/oligomerization reactions of the
aminoborane MeNHdBH2, which appears to be present as a
metal-bound species. Thermal treatment leads to similar reac-
tions in the absence of the Ir catalyst, but the aminoborane is
“free” in solution and can be trapped with cyclohexene. The
observation of other side products led to the interesting dis-
covery that the model aminoborane iPr2NdBH2 can be hydro-
genated in reactions with diborazanes and amine-boranes in the
absence of a metal catalyst. The facile, room temperature
hydrogenation of the BdN bond in aminoboranes is of funda-
mental significance16 and also has other potential implications,
such as in the regeneration of amine-borane hydrogen storage
materials and the use of BꢀN compounds in metal-free catalysis.
These results are also important with respect to the mechanistic
considerations associated with metal-catalyzed group 13ꢀ15
dehydrocoupling reactions, as to date the focus has been almost
entirely on the role of the metal. Further studies in this intriguing
area are in progress in our group.
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’ ASSOCIATED CONTENT
S
Supporting Information. Experimental data and rele-
b
vant spectra. This material is available free of charge via the
Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
Ian.Manners@bristol.ac.uk
of MeNH2 BH3 and 0.3 mol % IrH2POCOP in the presence of
3
cyclohexene, indicating that any MeNHdBH2 formed remains bound
to the Ir center as previously proposed (ref 3b). (b) Percentages refer to
the proportion of soluble boron content, as calculated via integration of
the 11B NMR spectra. See the Supporting Information for further details.
(12) Pasumansky, L.; Haddenham, D.; Clary, J. W.; Fisher, G. B.;
Goralski, C. T.; Singaram, B. J. Org. Chem. 2008, 73, 1898.
(13) Heating a THF solution of iPr2NdBH2 and cyclohexene at
70 °C in THF over 20 h produced no reaction, with only unreacted
starting material apparent upon analysis by 11B NMR spectroscopy.
’ ACKNOWLEDGMENT
A.P.M.R., E.M.L., and I.M. acknowledge the EPSRC for
funding. We thank Dr. George R. Whittell for useful discussions.
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