Ruthenium-catalyzed dehydrogenation of ammonia boranes

Article abstract of DOI:10.1021/ja804235t

The dehydrogenation of ammonia borane (AB) and methylammonia borane (MeAB) is shown to be catalyzed by several Ru-amido complexes. Up to 1 equiv of H₂ (1.0 system wt %) is released from AB by as little as 0.03 mol % Ru within 5 min, and up to 2 equiv of H₂ (3.0 system wt %) are released from MeAB with 0.5 mol % Ru in under 10 min at room temperature, the first equivalent emerging within 10 s. Also, a mixture of AB/MeAB yields up to 3.6 system wt % H₂ within 1 h with 0.1 mol % Ru. Computational studies were performed to elucidate the mechanism of dehydrogenation of AB. Finally, it was shown that alkylamine-boranes can serve as a source of H₂ in the Ru-catalyzed reduction of ketones and imines. Copyright

Full text of DOI:10.1021/ja804235t

Published on Web 10/03/2008
Ruthenium-Catalyzed Dehydrogenation of Ammonia Boranes
Nicole Blaquiere, Sarah Diallo-Garcia, Serge I. Gorelsky, Daniel A. Black, and Keith Fagnou*
Centre for Catalysis Research and InnoVation, Department of Chemistry, UniVersity of Ottawa, 10 Marie Curie,
Ottawa, Ontario, K1N 6N5, Canada
Received June 4, 2008; E-mail: keith.fagnou@uottawa.ca
Ammonia borane (AB), which has a very high hydrogen content
by weight (19.6%), is attracting attention as a means of achieving
efficient chemical hydrogen storage.1,2 Heating AB in the solid
state,3 ionic liquids4 or the presence of acid5 has been shown to
induce the release of >1.0 equiv of H2 over the course of several
hours, and transition metal complexes of Rh,6 Ni,7 and Ir8 have
been shown to catalyze H2 release under mild conditions. Notably,
the Ni catalysts described by Baker release up to 2.8 equiv of H2,7
an observation that has prompted a more detailed evaluation of
their reaction mechanism.9a When considering strategies for de-
hydrogenation, AB is often compared to ethane.10 Computational
predictions provide little mechanistic support for this analogy,
however,9,11 and in terms of local dipoles and reactivity AB may
better be compared to methanol. Drawing from this analogy, we
have discovered that ruthenium catalysts, originally developed for
alcohol redox processes,12 will induce the release of 1 equiv of H2
from AB (up to 1.0 system wt %) within 5 min at room temperature
with as little at 0.03 mol% Ru. Furthermore, 2 equiv of H2 is rapidly
Figure 1. H2 release from AB with 0.1 mol % catalyst 1-6 after activation
released from highly concentrated (11 M) MeAB solutions at 22
°C (ambient temperature), providing 3.0 system wt % hydrogen
release. We have also found that by using an AB/MeAB mixture
at 50 °C, up to 3.6 system wt % H2 can be released with 0.1 mol
% Ru.
with KOtBu.
An evaluation of several alcohol oxidation/reduction catalysts
provided promising leads with complexes 1 to 6 (Figure 1).13 In
these studies, the precatalysts (0.1 mol%) were activated via
treatment with KOtBu (3 mol%)13c in THF under inert atmosphere
to generate the active ruthenium amide species prior to the addition
of AB to the reaction vessel.14 Complexes 1 and 2 are particularly
reactive even at 0.03 mol% Ru.14 Optimal outcomes are observed
at high [AB] (5.0 M), an important parameter for maximal
gravimetric H2 density. In reactions with AB, we note the formation
of an insoluble precipitate, which has been identified as the same
polymeric aminoborane species obtained by Goldberg8 and Man-
ners.15
Figure 2. H2 release from MeAB with 0.5 mol % 1 after activation with
KOtBu.
The dehydrogenation of methylammonia-borane (MeAB) was
also evaluated.6 Under conditions similar to those employed for
AB dehydrogenation, 0.5 mol% 1 induced the release of up to 2.0
equiv of H2 from MeAB in ∼10 min (Figure 2).16 These reactions
are characterized by a very rapid release of the first equivalent of
H2 (occurring in less than 10 s), generating poly(N-methylami-
noborane).15 This is followed by a slower release of the second
equivalent over ∼10 min.17 Since no insoluble precipitate is
observed, more concentrated conditions (11 M [MeAB]) may be
employed, increasing the system % H2 to 3% (1.9 equiv of H2
released). To further maximize the gravimetric H2 density,
AB-MeAB mixtures were evaluated.18 Initial results using a 1:1
AB/MeAB mixture at 50 °C indicate that 3.6 system wt % H2
release can be achieved with 0.1 mol% 1 under solvent-free
conditions.16
As a preliminary probe into the reaction mechanism, the viability
of a reaction pathway similar to that involved in alcohol oxidation/
reduction was evaluated by density functional theory calculations
at the B3LYP level (Scheme 1).16,19 Ru-catalyzed alcohol oxidation
has been shown to involve an outer sphere hydrogen transfer
mechanism between the alcohol and an amide ligand on the Ru
metal.12,13,20 Along a similar pathway, we determined that AB
binding to Ru-hydride 7 is energetically favored, giving 8 (a species
very close in structure to known ruthenium borohydride hydrogena-
9
14034 J. AM. CHEM. SOC. 2008, 130, 14034–14035
10.1021/ja804235t CCC: $40.75
2008 American Chemical Society

C O M M U N I C A T I O N S
Scheme 1. Catalytic Cycle for Dehydrogenation of AB with the Ru Catalyst in THF and Gas Phase from DFT Calculationsa16
a Black and blue lines indicate different conformers of the P,N ligands of the Ru species. Reported values are in kcal/mol.
Scheme 2. Reduction of Ketones and Imines Using PyrAB as the
Source of H2
References
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from these calculations involves the formation of dihydrogen
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H2 to regenerate catalyst 7. The intermediacy of a species similar
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We have also determined that AB-ketone/imine transfer hydro-
genation may be achieved.22 Superior outcomes are observed with
precatalyst 3, which is less reactive than 1 in hydrogen release
processes. Use of pyrrolidine borane (PyrAB) in conjunction with
1.5 mol% precatalyst 3 and 3 mol% KOtBu efficiently induces the
hydrogenation of several organic substrates (Scheme 2).16 In each
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(16) See Supporting Information for experimental and computational details.
(17) 1H, 13C, and 11B NMR revealed the presence of a complex reaction mixture.
See Supporting Information.
Significant challenges remain in the development of chemical
hydrogen storage. The full potential of AB will only be attained
by increasing the amount of H2 released to well over 1.0 equiv
and by solving the important problem of regenerating AB from
the waste products;23 however, the exceptionally low catalyst
loadings, mild reaction conditions, rapid release of H2, and high
concentration achievable by this system represent a new opportunity
for approaching hydrogen storage/release goals.24
(18) Lane, C. F. U. S. DOE Hydrogen Program; 2007 Annual Progress Report,
Section IV.B.5j, 2007.
(19) Homogenous catalysis was confirmed, as the addition of mercury to the
reaction medium had no effect on H2 evolution.
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B. L.; Shahriari-Zavareh, H.; Stoddart, J. F.; Williams, D. J. Chem.
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alkenes, see: (c) Clark, T. J.; Lee, K.; Manners, I. Chem.sEur. J. 2006,
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(23) For theoretical studies on the properties of AB and its decomposition
products, see: Caetano, M. R.; Gerbrand, C. J. Chem. Phys. 2007, 126,
184703. For studies on regeneration of AB: (b) Hausdorf, S.; Baitalow, F.;
Wolf, G.; Mertens, F. O. R. L. Int. J. Hydr. Energy 2008, 33, 608. (c)
Sneddon, L. G. U.S. DOE Hydrogen Program; 2007 Annual Progress
Report,Section IV.B.5e, 2007.
Acknowledgment. NSERC, the University of Ottawa, and the
Research Corporation are thanked for financial support. Kanata
Chemical Technologies (Dr. K. Abdur-Rashid) is thanked for the
donation of catalysts. Drs. A. A. Mommers, C. W. Kazakoff, and
P. M. Mayer are thanked for mass spectrometry assistance, and
Prof. T. K. Woo is thanked for use of computing facilities funded
by the CFI. We thank Prof. R. Tom Baker for helpful discussions.
Supporting Information Available: Experimental procedures,
additional information, computational details, and DFT structures.
This material is available free of charge via the Internet at http://
pubs.acs.org.
(24) Just prior to the submission of this manuscript we became aware of similar
work being performed independently at Kanata Chemical Technologies (Dr.
Abdur-Rashid) using similar catalysts.
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