Photoactivated, iron-catalyzed dehydrocoupling of amine-borane adducts: Formation of boron-nitrogen oligomers and polymers

Article abstract of DOI:10.1002/chem.201003397

Dehydrogenation chemistry with iron: The first examples of efficient iron-catalyzed dehydrocoupling/dehydrogenation of amine-boranes at ambient temperature have been reported. In the case of the primary amine-borane adduct MeNH₂·BH₃, both the polymer and/or N-alkyl borazine can be obtained (see scheme; 1=[{CpFe(CO)₂}₂]). Copyright

Full text of DOI:10.1002/chem.201003397

COMMUNICATION
DOI: 10.1002/chem.201003397
Photoactivated, Iron-Catalyzed Dehydrocoupling of Amine–Borane
Adducts: Formation of Boron–Nitrogen Oligomers and Polymers
James R. Vance, Alasdair P. M. Robertson, Kajin Lee, and Ian Manners*[a]
Metal-catalyzed dehydrocoupling/dehydrogenation of
amine–borane adducts has attracted growing attention from
the perspectives of hydrogen storage,[1] hydrogen transfer to
organic substrates,[2] and the preparation of new inorganic
polymeric materials.[3] Mechanistic interest in the associated
transformations has also recently led to the rapid emergence
of a new area of coordination chemistry associated with
amine–borane ligands with metals of the d and s block.[4,5]
Although many important advances have been made over
the last decade,[1–11] the development of convenient to
handle, cheap, and efficient catalysts of low toxicity that are
effective for a broad range of boron–nitrogen substrates is
of considerable interest. Herein we report on recent work
concerning an interesting new photogenerated iron-based
catalytic system that operates on a series of different ad-
ducts.
Treatment of the more sterically encumbered adduct
iPr2NH·BH3 (4) with 1 (5 mol%) under photoirradiation
conditions that allowed the escape of gaseous byproducts
also led to dehydrogenation but the process was significantly
slower than for 2 (15 h for almost quantitative conversion
under the same conditions; Scheme 2). In this respect, the
Scheme 2. The dehydrogenation of 4 with precatalyst 1 (5 mol%) results
in the formation of 5.
Treatment of the secondary amine–borane adduct
Me2NH·BH3 (2) with a catalytic amount (5 mol%) of
[{CpFe(CO)2}2] (1) in THF at 208C led to no detectable re-
action, as indicated by 11B NMR spectroscopy. However, a
rapid (4 h), virtually quantitative conversion to the cyclic di-
borazane, [Me2N-BH2]2, (3) was observed under photoirra-
diation conditions that allowed the escape of gaseous by-
iron system shows a much more similar behavior to that of
late-transition-metal precatalysts such as [{RhACHTUNGTRENNUNG(1,5-
cod)Cl}2],[6] rather than one of the systems containing early
transition metals such as [Cp2Ti],[9] which show higher activi-
ty towards this particularly bulky substrate than towards 2.
The activity of the photoirradiated [{CpFe(CO)2}2] (1)
precatalyst towards the primary amine–borane adduct
MeNH2·BH3 (6) was also investigated. Analysis of the prod-
ucts by 11B NMR spectroscopy after three hours of photo-
products (Scheme 1).
A significantly slower conversion
(80% after 6 h) was detected when the reaction was per-
formed in a closed system, consistent with evolved CO and/
or H2, thus competitively consuming the photogenerated,
coordinatively unsaturated active catalytic species.
AHCTUNGTRENNiGUN rradiation indicated that polyaminoborane [MeNH-BH2]n
(7)[3c] was formed in approximately 90% yield. Subsequent
characterization of the polymeric product by GPC showed
that the material has a high molecular weight (Mw =117700,
PDI=1.83). However, upon prolonged irradiation in the
presence of 1 we found that intermediate 7 lost a further
equivalent of hydrogen to form mainly N-methylborazine 8.
Indeed, after 16 hours, no residual polyaminoborane 7 was
detected and 8 was formed in 60% yield as the only identifi-
able product (Scheme 3).
Scheme 1. The dehydrogenation of 2 with precatalyst 1 (5 mol%) results
in the formation of 3.
[a] J. R. Vance, A. P. M. Robertson, K. Lee, Prof. I. Manners
School of Chemistry, University of Bristol
Cantockꢀs Close, Bristol, BS8 1TS (UK)
E-mail: ian.manners@bristol.ac.uk
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201003397.
Scheme 3. The dehydrogenation of 6 with precatalyst 1 (5 mol%) gives
initally 7, which is further dehydrogenated to give 8.
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Although many dehydrocoupling/dehydrogenation pre-
ACHTUNGTRENNUNG
Figure 1. The 11B {1H} NMR spectrum (THF) showing the reaction mix-
ture derived from 9 with 1 (5 mol%) after 3 h of irradiation. The signal
labeled with a * is assigned to polyborazylene (see reference [13]).
Analysis of the reaction mixture derived from 9 and pho-
toirradiated 1 in THF after one hour at 208C by 11B and
11B{1H} NMR spectroscopy revealed approximately 60%
conversion to the cyclolinear borane trimer (10)[13] as the
major product. After three hours, the near quantitative con-
version of 9 was detected, which lead to 10 as the major
component (ca. 65%) together with borazine (11)
(ca. 35%), which was the product of further dehydrogena-
tion (Scheme 4, Figure 1, Table 1). Unlike the case of 6, the
Table 1. Dehydrocoupling of ammonia–borane
[{CpFe(CO)2}2] (1; 5 mol%) under photoirradiation.
9
catalyzed by
Conditions[a]
Conversion[%]
Distribution of products[b]
aminoborane
trimer (10)
borazine (11)
0 h (hv)
1 h (hv)
3 h (hv)
0
60
95
0
95
65
0
5
35
[a] Photolysis experiments were carried out at 208C in THF using a
125 W medium pressure Hg lamp. [b] The distribution of products pres-
ent at the% conversion reported.
dehydrocoupling/dehydrogenation reactions using precata-
lyst 1 are very convenient for mechanistic studies and we
have performed some initial studies on the reactions with
substrate 2. During the course of the reaction a new
11B NMR signal was detected at 36.5 ppm that initially grew
and subsequently declined in intensity (Figure 2, Table 2).
This signal was particularly striking after 30 minutes and
was assigned to the dehydrocoupled monomer Me2N=BH2
(12) based on the chemical shift and the triplet-splitting pat-
tern (1JBH =125 Hz) detected in the 1H-coupled 11B NMR
spectrum of the reaction mixture. This data is similar to that
reported in the literature for this and other monomeric ami-
noboranes (for example, Et2N=BH2; 11B NMR: d=
36.6 ppm (t, br, BH2, JBH =127 Hz)).[4n,16]
Scheme 4. The dehydrogenation of 9 with precatalyst 1 (5 mol%) gives
initally 10, which is further dehydrogenated to give 11.
formation of borazine was not preceded by the formation of
polyaminoborane. In a recent paper by Baker and co-work-
ers, a mechanism for the formation of 10 and the subsequent
transformation of this species into 11 through a nickel-medi-
ated dehydrocoupling/dehydrogenation of 9 has been pro-
posed.[13] It was suggested that the transient dehydrocoupled
monomer [NH2 =BH2] reacted first with 9 and then formed
10 through successive additions.
The mechanism for the photoactivated catalytic dehydro-
coupling/dehydrogenation reactions involving 1 as a precata-
lyst is of considerable interest. This dimer is known to un-
dergo CO loss and also to cleave to afford mononuclear
products under photoirradiation conditions.[14] A potential
combination of these processes could generate several dis-
tinct coordinatively unsaturated, catalytically active species
and, at this preliminary stage, we have not identified the
true photogenerated catalyst.[15] Nevertheless, the rates of
Significantly, the linear diborazane Me2NH-BH2-NMe2-
BH3 (13), which has been implicated as a key intermediate
in Ti-,[9b] Ru-,[10] and Rh-catalyzed[6] dehydrogenations of 2
was not detected in significant amounts during this reaction.
Moreover,
separate
experiments
showed
that
[{CpFe(CO)2}2] (1) was virtually inactive as a catalyst for the
dehydrogenation of 13 under photoirradiation with negligi-
ble conversion to cyclic diborazane 3 after photoirradiation
for 21 hours (THF, 208C). Furthermore, when equimolar
mixtures of 2 and 13 were treated with [{CpFe(CO)2}2] (1;
4100
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Iron-Catalyzed Dehydrocoupling of Amine–Boranes
COMMUNICATION
Figure 2. The 11B {1H} NMR spectrum (THF) showing the reaction mix-
ture derived from 2 with 1 (5 mol%) after a) 10 min of irradiation,
b) 30 minutes of irradiation and c) 50 minutes of irradiation. Plots b and c
are increasingly offset relative to the x axis.
Figure 3. The 11B NMR spectrum (THF) showing the reaction mixture
derived from Me2NH·BD3 and 13 with 1 (5 mol%) after 4 h of irradia-
tion.
Table 2. Dehydrocoupling
[{CpFe(CO)2}2] (1; 5 mol%).
of
Me2NH·BH3
(2)
catalyzed
by
diborazane 13, our preliminary results for this iron-catalyzed
transformation strongly suggest a dominant two-stage pro-
cess in which the aminoborane 12 is generated first by the
metal-mediated dehydrogenation of 2; the cyclic diborazane
3 is then formed in a second stage through the cyclodimeri-
zation of 12 in an off-metal process.[17] This mechanism for
Fe precatalyst 1 is similar to that proposed for the photoacti-
vated [M(CO)6] (M=Cr, Mo, and W) precatalyst systems, in
which [M(CO)4] intermediates have been invoked as the
active catalytic species.[8]
Conditions[a]
Conversion [%]
Yield of products[b]
3
12
13
HBACTHUNGTRENNU(G NMe2)2
0 h (hv)
1 h (hv) (3 h)[c]
0.5 h (hv)
1 h (hv)
0
85
35
0
0
6
0
2
0
3
2
0
0
1
0
0
6
91
70
84
91
85
30
13
1
75
4 h (hv)
95
15 h (hv)
100
0
15
[a] Photolysis experiments were carried out at 208C in THF using a
125 W medium pressure Hg lamp. [b] The% yield of products present at
the% conversion reported (average of six experiments). [c] After irradia-
tion the sample was stirred for 3 h while covered in aluminum foil.
In summary, we have found that on photoirradiation,
[{CpFe(CO)2}2] (1) works as a versatile dehydrocoupling cat-
alyst towards not only secondary and primary amine–borane
adducts but also ammonia–borane. This commercially avail-
able Fe species therefore represents a rare example of an ef-
ficient amine–borane dehydrocoupling catalyst that operates
over an extensive range of substrates and at room tempera-
ture.[18] This process also represents the first example of an
efficient dehydrocoupling/dehydrogenation catalyst based
on the abundant and relatively environmentally benign first
row transition metal iron.[19] Further studies will target the
identification of the true, apparently homogeneous photo-
generated catalyst,[20] and this will potentially facilitate the
development of more active, well-characterized analogues
devoid of ligation such as CO with its toxicity implications.
In addition, of particular mechanistic interest is the iron-cat-
alyzed formation of poly(methylaminoborane) (7) from
MeNH2·BH3 (6). This transformation differs from that iden-
tified for previously reported photogenerated Group 6 car-
bonyl catalysts, which formed the N-Methyl borazine 8 di-
rectly. Recently published work,[3c] in which various transi-
tion-metal catalysts are employed, led to the tentative sug-
gestion of a mechanism for the dehydropolymerization of 6
to form 7 that involved the metal-mediated polymerization
of MeNH=BH2 as a key step. In contrast, analogy with the
5 mol%) and irradiated for four hours in THF at 208C, 2
was fully converted to 3 with 13 left unchanged. To simplify
the characterization by minimizing the overlap of the NMR
signals, this result was confirmed by 1H-coupled 11B NMR
analysis of the reaction products for the deutero-analogue
of 2, (Me2NH·BD3), in the presence of 13 with the iron pre-
catalyst 1 under photoirradiation conditions (see Scheme 5
and Figure 3).
In contrast to the mechanisms proposed for the Ti-, Ru-,
and Rh-catalyzed dehydrogenations of 2, which are believed
to proceed predominantly or at least partly through linear
Scheme 5. The attempted dehydrogenation of 13 and Me2NH·BD3 with
precatalyst 1 (5 mol%) results in the formation of [Me2N-BD2]2. Com-
pound 13 remains unreacted.
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Acknowledgements
We acknowledge the EPSRC for funding. I.M. also thanks the E.U. for a
Marie Curie Chair and the Royal Society for a Wolfson Research Merit
Award. We would also like to thank Prof. Kevin Booker-Milburn of the
University of Bristol for experimental assistance.
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Keywords: ammonia–borane · dehydrogenation · iron ·
photolysis · polymers
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Iron-Catalyzed Dehydrocoupling of Amine–Boranes
COMMUNICATION
is detected after 4 days at 208C. See the Supporting Information of
reference [11c].
Fe0 sources [Fe2(CO)9] or FeCl2/NaBH4. See the Supporting Infor-
mation for details.
[20] The true catalyst does not appear to be Fe nanoparticles or colloids.
No significant rate of dehydrogenation of 2 was detected using the
Received: November 24, 2010
Published online: March 8, 2011
Chem. Eur. J. 2011, 17, 4099 – 4103
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4103