Coordination of aminoborane, NH2BH2, dictates selectivity and extent of H2 release in metal-catalysed ammonia borane dehydrogenation

Article abstract of DOI:10.1039/b809190k

In situ ¹¹B NMR monitoring, computational modeling, and external trapping studies show that selectivity and extent of H₂ release in metal-catalysed dehydrogenation of ammonia borane, NH₃BH₃, are determined by co

Full text of DOI:10.1039/b809190k

View Article Online / Journal Homepage / Table of Contents for this issue
COMMUNICATION
www.rsc.org/chemcomm | ChemComm
Coordination of aminoborane, NH2BH2, dictates selectivity and extent of
H2 release in metal-catalysed ammonia borane dehydrogenationw
Vincent Pons,a R. Tom Baker,z*a Nathaniel K. Szymczak,b David J. Heldebrant,b
John C. Linehan,b Myrna H. Matus,c Daniel J. Grantc and David A. Dixon*c
Received (in Berkeley, CA, USA) 30th May 2008, Accepted 28th October 2008
First published as an Advance Article on the web 12th November 2008
DOI: 10.1039/b809190k
In situ 11B NMR monitoring, computational modeling, and
external trapping studies show that selectivity and extent of
H2 release in metal-catalysed dehydrogenation of ammonia
borane, NH3BH3, are determined by coordination of reactive
aminoborane, NH2BH2, to the metal center.
Over the last few years, the scientific community has expressed
an increasing interest in ammonia borane (NH3BH3, AB, 1)
due to its remarkable hydrogen storage potential (19.6 wt%
H).1 While AB dehydrogenation in the solid state2 and in
solution3 has been studied previously in some detail, new
reports have shown that metal-complex-catalysed reactions
allow greater extent of hydrogen release at lower tempera-
tures.4–9 The observation of different products and rates
obtained for various catalysts prompted us to investigate the
Scheme
1 Selectivity in metal catalysed AB dehydrogenation.
(POCOP = 2,6-(tBu2POCH2)C6H3, NHC = N-heterocyclic carbene,
mechanism(s) involved in determining the selectivity in AB
dehydrogenation (Scheme 1).
cod = 1,5-cyclooctadiene).
Manners and others have shown that metal-catalysed de-
hydrogenation of alkylamineboranes RR0NHBH3 affords
In contrast, while aminodiborane, B2H5(NH2),14 has been
observed as a minor by-product of metal-catalysed AB de-
hydrogenations,15 discussion of product selectivity has focused
on formation of borazine, PB, and the cyclic pentamer,
(BH2NH2)5 (Scheme 1). If AB dehydrogenation follows simi-
lar pathways as those observed for alkylamineboranes, is
monomeric aminoborane, NH2BH2, formed? While direct
observation is unlikely (preparation in low-temperature ma-
trices showed oligomerisation even below ꢀ155 1C),16 we now
employ in situ monitoring to provide indirect evidence for its
formation.
Monitoring AB dehydrogenation by 11B NMR spectro-
scopy at 25 1C using 10 mol% Ni(NHC)2 reveals three
prominent resonances at ꢀ5.3 (doublet, JB–H = 114 Hz),
ꢀ11.3 (triplet, JB–H = 104 Hz) and ꢀ24 ppm (quartet,
JB–H = 93 Hz) (Fig. 1). Further reaction at 60 1C in the
NMR probe showed simultaneous decay of these signals and
i
dialkylaminoborane RR0NBH2 (R, R0 = Pr, Cy)4,10–12 that
in some cases undergoes oligomerisation to cyclic dimers (R,
R0= Me) or trimers (R = H; R0 = Me). For primary
amineboranes and parent AB, further dehydrogenation yields
borazines and even B–N linked borazine (polyborazylene, PB).
We have reinvestigated this chemistry at different tempera-
tures in several solvents using in situ 11B NMR monitoring.y
Dehydrogenation of more sterically demanding primary ami-
t
neboranes, RNH2BH3 (R = Bu, Cy), using [Rh(cod)Cl]2 as
precatalyst reveals, in addition to oligomeric cyclic amino-
boranes, (RNH–BH2)n (n = 1–3), formation of both diamino-
borane, HB(NRH)2 (25 ppm), and aminodiborane,
B2H5(NHR) (ꢀ18 ppm) (Fig. S1 and S2, ESIw). Analogous
products were also observed for secondary amineboranes (R
n
= Me, R0 = Bu).13 These products result presumably from
metal-mediated B–N bond cleavage (Scheme 2).
a Chemistry Division, MS J582, Los Alamos National Laboratory,
Los Alamos, NM 87545, USA. E-mail: rbaker@uottawa.ca;
Fax: +1 (613) 562-5613; Tel: +1 (613) 562-5698
b Fundamental Sciences Directorate, Pacific Northwest National
Laboratory, Richland, Washington, 99354, USA
c Department of Chemistry, The University of Alabama, Shelby Hall,
Tuscaloosa, Alabama, 35487-0336, USA
w Electronic supplementary information (ESI) available: In situ 11B
NMR spectra, experimental details of trapping reactions and compu-
tational studies. See DOI: 10.1039/b809190k
z Current address: Department of Chemistry, University of Ottawa,
Ottawa, Ontario, Canada, K1N 6N5.
Scheme 2 Alkylamineborane B–N bond cleavage products.
Chem. Commun., 2008, 6597–6599 | 6597
ꢁc
This journal is The Royal Society of Chemistry 2008

View Article Online
low barrier of 5.0 kcal molꢀ1 (Fig. S6C and D, ESIw). Finally,
this trimer chain can rearrange in an exothermic process with a
small barrier (o6 kcal molꢀ1) to form the ring compound
B2N2H7NH2BH3 (NH2BH3 side chain bonded to B) as observed.
The other ring compound, B2N2H7BH2NH3, which is higher
in energy (G3MP2 calculations: DDH298 = 2.3 kcal molꢀ1
,
DDG298 = 2.7 kcal molꢀ1) was never formed in any of our
simulations. The H2 elimination barriers that have been found so
far for the isolated molecules are comparable to the B–N bond
energy in BH3NH3 suggesting a potential role for solvent or
coordination to the catalyst.19 These results are consistent with
the DFT B3LYP/6-311+G(2d,p) calculations of Nutt and
McKee.22 Further work is in progress to understand the detailed
thermochemistry and reaction pathways of 2 as well as more
details on its generation.3b Preliminary results show no change
after heating diglyme solutions of 2 for 24 hours at 70 1C,
whereas addition of rhodium catalyst to these solutions yields PB
and borazine even at 25 1C.
Fig. 1 Evolution of the 11B{1H} NMR spectra of AB dehydrogena-
tion (400 MHz, diglyme, 5 mol% Ni(NHC)2) K PB;Eborazine; * 2; #
NHC–BH3. Spectrum A was recorded after 12 h reaction time at
25 1C, B after 10 min equilibration in the NMR probe at 60 1C, and
C–E at 60 1C after additional intervals of 20, 40 and 120 min.
To further support our mechanistic proposal involving
formation of reactive NH2BH2 in metal-catalysed AB de-
hydrogenation, we performed external trapping reactions with
cyclohexene. Using [Rh(cod)Cl]2 as precatalyst and a large
excess of cyclohexene in diglyme solvent at 25 1C, the expected
resonances for borazine and PB were not observed. Instead, a
23
Scheme 3 Trapping of NH2BH2 by AB. G3MP2, B3LYP/DZVP2
broad signal observed at 48 ppm is assigned to Cy2BNH2
gas phase enthalpies at 298 K. The highly accurate CCSD(T)/CBS
(complete basis set) result for the first step is ꢀ5.1 kcal molꢀ1 20
arising from cyclohexene hydroboration using both BH bonds
of aminoborane. Remarkably, use of Ir(POCOP)H2 under the
same reaction conditions still afforded (BH2NH2)5 as the sole
product. However, at 60 1C, a mixture of Cy2BNH2 and
insoluble pentamer was observed. In keeping with our propo-
sal, the same Ir-catalysed reaction in the absence of cyclo-
hexene at 60 1C now forms a significant amount of borazine
along with (BH2NH2)5. Finally, in the presence of 20 equivalents
of cyclohexene–AB and 10 mol% Ni(NHC)2, NH2BH2 is not
trapped at 25 1C, PB and borazine are still observed after the
reaction goes to completion (Table S1, ESIw). Based on the
trapping results using rhodium and iridium catalysts, we can
propose that formation of cyclic (BH2NH2)n species results
from strong coordination of aminoborane to the iridium
center; partial dissociation at 60 1C results in some trapping
(Scheme 4).24 Details of the proposed metal-mediated
dehydrocyclisation are currently under investigation.
.
formation of borazine and PB (28–35 ppm). After completion
of the reaction, two weak signals remain at ꢀ11 and ꢀ35 ppm.
They are assigned respectively to a small amount of amino-
borane oligomer and NHC–BH3.
Using {1H–11B} HETCOR NMR spectroscopy, we were
able to assign the three types of BH resonances to intermediate
2 shown in Scheme 3.17 The IR spectrum shows broad NH
(3263 cmꢀ1) and BH stretches at 2361 and 2371 cmꢀ1. Given
the highly reactive nature of aminoborane, this intermediate 2,
B-(cyclodiborazanyl)aminoborohydride, may be formed by
trapping reactions with AB (Scheme 3).18
Calculations at the B3LYP/DZVP2 level were performed to
map out possible reaction pathways. In complementary results
to those found previously for BH3 and AB,19 NH2BH2 can
either weakly catalyse the unimolecular loss of H2 from AB or
the BH2 of NH2BH2 interacts with AB in a complex transition
state leading to H2 loss (Fig. S6A, ESIw). In both cases, two
NH2BH2 molecules are formed and the chain structure
BH3NH2BHNH2 can only be formed by an unfavorable
high-energy pathway (there is a slightly higher energy struc-
ture where the BH3 forms a bridging H to the BH group, Fig.
S6B, ESIw).20 The BH3NH2BHNH2 chain can be generated in
a slightly exothermic reaction by the dimerisation of the two
weakly interacting NH2BH2 molecules with a low barrier of
10.5 kcal molꢀ1. We found no evidence for formation of the
other chain isomer NH3BH2NHBH2 in the dimerisation pro-
cess even though it is only 2.3 kcal molꢀ1higher in energy at
the G3MP2 level.21 The BH3NH2BHNH2 chain can then
catalyse H2 loss from AB leading to formation of NH2BH2
which subsequently adds to the BH3NH2BHNH2 chain to
form an NH2BH2 trimer (with a BHB terminal bond) with a
While AB dehydrogenation has appeared at first glance to
be more complex than that of alkylamineboranes, we have
Scheme 4 Metal-catalysed AB dehydrogenation mechanism.
ꢁc
This journal is The Royal Society of Chemistry 2008
6598 | Chem. Commun., 2008, 6597–6599

View Article Online
now shown that the diversity of dehydrogenation products
from AB can be attributed to the high reactivity of amino-
borane. Further catalyst development in our laboratories
focuses on efficient and exclusive release of aminoborane, rapid
conversion of intermediate 2 to borazine and subsequent
cross-linking to polyborazylene.25
9 D. Pun, E. Lobkovsky and P. J. Chirik, Chem. Commun., 200731),
3297–3299.
10 M. Shimoi, K. Katoh, M. Uruichi, S. Nagai and H. Ogino, Spec.
Publ.-R. Soc. Chem., 1994, 143, 293–296.
11 T. J. Clark, C. A. Russell and I. Manners, J. Am. Chem. Soc., 2006,
128, 9582–9583.
12 Y. Jiang and H. Berke, Chem. Commun., 200734), 3571–3573.
13 I. G. Green, K. M. Johnson and B. P. Roberts, J. Chem. Soc.,
Perkin Trans. 2, 1989, 1963–1972.
14 H. I. Schlesinger, D. M. Ritter and A. B. Burg, J. Am. Chem. Soc.,
1938, 60, 2297; K.-K. Lau, A. B. Burg and R. A. Beaudet, Inorg.
Chem., 1974, 13, 2787.
15 While the known compound HB(NH2)2 (D. C. Reuter,
L. R. Thorne and W. D. Gwinn, J. Phys. Chem., 1982, 86(24),
4737–45) has not been observed, further evidence for B–N bond
cleavage is provided by formation of (POCOP)IrH(BH4), see ref. 7.
16 H. A. McGee, Jr and C. T. Kwon, Inorg. Chem., 1970, 9, 2458–2461.
We thank the Department of Energy’s Office of Science’s
Basic Energy Sciences Division (DEFG02-05ER15719-AO)
and its Office of Energy Efficiency and Renewable Energy
for support through the Chemical Hydrogen Storage Center of
Excellence. We also acknowledge Prof. Larry G. Sneddon and
Robert Butterick, III for 11B NMR chemical shift calculations.
Notes and references
17 Crude samples of
2 were obtained from Ni-catalyzed AB
y Caution should be exercised in managing pressure build-up in
these reactions. They were typically conducted in evacuated J-Young
NMR tubes, in regular NMR tubes with minimally perforated
rubber septa, or in sealed polyether ether ketone cells designed for
pressure work.
dehydrogenation reactions after filtration, solvent removal and
washing with toluene to remove PB. DFT GIAO calculations of
the 11B NMR chemical shifts of 2 performed by R. Butterick, III
and L. G. Sneddon show BH, BH2 and BH3 signals at ꢀ4.4, ꢀ11.2,
ꢀ23.8 ppm, respectively.
18 (a) V. Pons, R. T. Baker, D. M. Camaioni and J. Li, Abst. INOR
595, 2008 Spring National ACS Meeting; (b) We note that buildup
of 2 during AB dehydrogenation using [Rh(cod)Cl]2 precatalyst is
not observed. Nevertheless, 11B NMR spectra of AB dehydrogena-
tion using FeH(CH2PMe2)(PMe3)3 and Ni(PMe3)4 catalysts show
its formation (see supplementary material in ref. 8).
1 F. H. Stephens, V. Pons and R. T. Baker, Dalton Trans., 2007,
2613–2626.
2 (a) G. Wolf, J. Baumann, F. Baitalow and F. P. Hoffmann,
Thermochim. Acta, 2000, 343, 19–25; (b) M. G. Hu,
R. A. Geanangel and W. W. Wendlandt, Thermochim. Acta, 1978,
23, 249–255; (c) V. Sit, R. A. Geanangel and W. W. Wendlandt,
Thermochim. Acta, 1987, 113, 379–382; (d) P. A. Storozhenko,
R. A. Svitsyn, V. A. Ketsko, A. K. Buryak and A. V. Ul’yanov,
Zh. Neorg. Khim., 2005, 50, 1066–1071.
3 (a) J. S. Wang and R. A. Geanangel, Inorg. Chim. Acta, 1988, 148,
185–190; (b) W. J. Shaw, J. C. Linehan, N. K. Szymczak,
D. J. Heldebrant, C. Yonker, D. M. Camaioni, R. T. Baker and
T. Autrey, Angew. Chem., Int. Ed., 2008, 47, 7493–7496.
4 C. A. Jaska, K. Temple, A. J. Lough and I. Manners, J. Am. Chem.
Soc., 2003, 125, 9424–9434.
19 M. T. Nguyen, V. S. Nguyen, M. H. Matus, G. Gopakumar and
D. A. Dixon, J. Phys. Chem. A, 2007, 111, 679–690.
20 M. H. Matus, K. Anderson, D. M. Camaioni, S. T. Autrey and
D. A. Dixon, J. Phys. Chem. A, 2007, 111, 4411–4421.
21 L. A. Curtiss, P. C. Redfern, K. Raghavachari, V. Rassolov and
J. A. Pople, J. Chem. Phys., 1999, 110, 4703–4709. The G3(MP2)
method has been benchmarked for borane amines as described in
ref. 20.
22 W. R. Nutt and M. L. McKee, Inorg. Chem., 2007, 46, 7633–7645.
23 G. E. Coates and J. G. Livingstone, J. Chem. Soc., 1961,
1000–1008.
5 Y. Chen, J. L. Fulton, J. C. Linehan and T. Autrey, J. Am. Chem.
Soc., 2005, 127, 3254–3255.
24 While AB reacts with cyclohexene at 60 1C (but not at 25 1C), this
side reaction is slow enough to be neglected in the metal-catalysed
trapping of NH2BH2 and formation of Cy2BNH2.
25 Efficient catalysis of cross-linking will prevent build-up of volatile
borazine that could contaminate the H2 stream and poison the fuel
cell catalyst.
6 R. J. Keaton, J. M. Blacquiere and R. T. Baker, Prepr. Symp.-Am.
Chem. Soc., Div. Fuel Chem., 2006, 51, 644–645.
7 M. C. Denney, V. Pons, T. J. Hebden, D. M. Heinekey and
K. I. Goldberg, J. Am. Chem. Soc., 2006, 128, 12048–12049.
8 R. J. Keaton, J. M. Blacquiere and R. T. Baker, J. Am. Chem. Soc.,
2007, 129, 1844–1845.
ꢁc
This journal is The Royal Society of Chemistry 2008
Chem. Commun., 2008, 6597–6599 | 6599