A hydridoirida-β-diketone as an efficient and robust homogeneous catalyst for the hydrolysis of ammonia-borane or amine-borane adducts in air to produce hydrogen

Article abstract of DOI:10.1039/c0dt00091d

The first homogeneous metal-catalysed hydrolysis of ammonia-borane or amine-borane adducts for hydrogen generation, using a stable organometallic complex [IrHCl{(PPh₂(o-C₆H₄CO))₂H}], is reported.

Full text of DOI:10.1039/c0dt00091d

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A hydridoirida-b-diketone as an efficient and robust homogeneous catalyst for
the hydrolysis of ammonia–borane or amine–borane adducts in air to produce
hydrogen†
Roberto Ciganda,a Mar´ıa A. Garralda,*a Lourdes Ibarlucea,a Elena Pinillab and M. Rosario Torresb
Received 8th March 2010, Accepted 18th June 2010
First published as an Advance Article on the web 9th July 2010
DOI: 10.1039/c0dt00091d
The first homogeneous metal-catalysed hydrolysis of
ammonia–borane or amine–borane adducts for hydro-
gen generation, using a stable organometallic complex
[IrHCl{(PPh2(o-C6H4CO))2H}], is reported.
up to three equivalents of hydrogen, using a hydridoirida-b-
diketone.18 Metalla-b-diketones can be considered as acylhydrox-
ycarbene complexes stabilised by intramolecular hydrogen bonds
between the acyl and the hydroxycarbene moieties.19
The hydridoirida-b-diketone [IrHCl{(PPh2(o-C6H4CO))2H}]
(1)18 is an efficient homogeneous catalyst for hydrogen generation
from hydrolysis of AB in THF–H2O mixtures, at room temperature
in the presence of air, and releases stoichiometric amounts of
hydrogen according to eqn (1).
The search for new fuels1–2 has made the safe storage of hydrogen
in a chemical compound, and its deliverance upon demand,
the subject of recent intensive research. Ammonia–borane is an
attractive candidate as a potential hydrogen storage material for
transportation applications. It contains 19.6 weight% in hydrogen,
and is thermally and water stable at room temperature.2 Reports
on the dehydrogenation reaction or dissociation and hydrolysis
of ammonia–borane and amine–borane adducts, promoted by
transition-metal catalysts under mild conditions, have appeared in
the literature only recently. Growing efforts are being devoted to
searching for new metal catalysts, optimising their catalytic activi-
ties and recovering the hydrogen storage material in order to reach
practical applications, such as hydrogen generation for portable
fuel cells.2–3 The homogeneous dehydrocoupling of ammonia–
borane (AB), methylamino–borane (MeAB) or dimethylamino–
borane (DMAB), catalysed by Ir,4 Ti,5 Rh,6 or Ru7 may efficiently
afford up to one equivalent of hydrogen gas under an inert
atmosphere. A nickel carbene complex is able to release more
than two equivalents of H2 from AB in benzene–diglyme solution
under argon.8 Theoretical studies have suggested that electron
delocalisation involving the carbene may be essential for low en-
ergy H transfers in this catalytic AB dehydrogenation.9 Hydrolytic
reactions can afford up to three equivalents of hydrogen per mol of
starting material. The generation of H2 from AB by the hydrolysis
route is catalysed by first row transition metals, supported or
as nanoparticles.10–12 Efficient water/air-stable Ni nanoparticles
have been reported recently.13 Noble-metal heterogeneous systems,
which often require an inert atmosphere, also allow fast H2 release
from AB at room temperature.14–16 Efficient regeneration methods
of AB from the dehydrogenation products or from the borate salts
have also been reported.17
1
H3N −BH3 + 2H2O ⎯⎯⎯⎯⎯⎯→NH4+ + BO2− +3H2
(1)
298K,THF−H
O
2
The rate of hydrogen release is strongly dependent on the
solvents ratio (Fig. S1, ESI†). The best results were obtained for
mixtures THF–H2O = 1 : 1, so that with an initial AB concentration
of 0.46 M and a 0.5 mol% catalyst loading, the hydrolysis of
AB and the release of 2.73 equivalents of hydrogen gas, 87%
of the maximum hydrogen content that could be produced, was
observed within 19 min. At the end of the reaction, mixtures of
complex 1, the related dihydridoirida-b-diketone [IrH2{(PPh2(o-
C6H4CO))2H}] (2) and other non-identified species could be recov-
ered from the solution upon evaporation of THF, as confirmed
by NMR spectroscopy (Fig. S2, ESI†). The transformation of
1 into 2 promoted by bases has been reported.20 When using
higher amounts of water, initial complete dissolution of 1 and H2
evolution occurred until the formation of a precipitate prevented
hydrogen release after 12 min if THF–H2O = 4 : 6, or after 9 min
if THF–H2O = 3 : 7. This precipitate was identified as 2 by NMR
(Fig. S3, ESI†).
These observations suggest the dehydrogenation reaction being
homogeneous. Performing the catalytic reaction in the presence
of Hg led to similar results and this is a further indication that
the process is homogeneous.6,16 After completion of the catalytic
reaction, the borate products from the hydrolysis were identified by
1
11B{ H} NMR. The disappearance of the starting AB resonance
at -23.2 ppm and the presence of a singlet at 16.1 ppm, which we
assign to borate formed in the reaction, were observed. It has been
reported that the chemical shift of borates can be dependent on
the conditions of their preparation and that equilibrium processes
and rapid exchange between different borate species may occur.15
We describe now the first homogeneous metal-catalysed hydrol-
ysis of ammonia–borane or amino–boranes to efficiently produce
aFacultad de Qu´ımica de San Sebastia´n, Universidad del Pa´ıs Vasco, Apdo.
1072, 20080, San Sebastia´n, Spain. E-mail: mariaangeles.garralda@ehu.es;
Fax: 34 943 015270; Tel: 34 943 018207
1
The 11B{ H} NMR spectrum of a sample prepared in THF–H2O =
bDepartamento de Qu´ımica Inorga´nica, Laboratorio de Difraccio´n de Rayos
X, Facultad de Ciencias Qu´ımicas, Universidad Complutense, 28040, Madrid
1 : 1 mixtures by mixing equimolar amounts of boric acid and an
aqueous ammonia solution exhibited a single peak at 15.4 ppm
(Fig. S4, ESI†). The rate of hydrogen release depends also on the
catalyst loading. As shown in Fig. 1, with the same initial AB
concentration, a 0.25 mol% catalyst loading increases the time
required to afford 2.73 equivalents of hydrogen to at least 50 min,
† Electronic supplementary information (ESI) available: Experimental
procedures. NMR spectra of the hydrolysis products and of the catal-
ysed reactions. CCDC reference number 776710 (3). For ESI and
crystallographic data in CIF or other electronic format see DOI:
10.1039/c0dt00091d
7226 | Dalton Trans., 2010, 39, 7226–7229
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Plausible mechanisms considered to account for the heteroge-
neous catalysed hydrolysis of NH3BH3 include the establishment
of interactions between the AB molecule and the metal particle
surface to form activated complex species, to which attack by a
H2O molecule readily leads to concerted dissociation of the B–N
bond and the hydrolysis of the resulting BH3 intermediate.10 More
recently, the involvement of boron-to-metal hydrogen transfer to
form a transient M–H bond, similar to that proposed in the metal-
catalysed borohydride hydrolysis, has been suggested.12,22
The cleavage of the B–N bond, as a consequence of the
interaction of AB with the catalyst, may afford the amine. Metalla-
b-diketones are known to react with ammonia or amines to afford
metalla-b-ketoimines;19 therefore we have studied the reaction of
1 with amines. The reaction of 1 with methylamine (see Scheme 1)
in dry THF leads to the hydridoirida-b-ketoimine 3, which reacts
with water to afford the hydridoaminodiacyl complex 4. The
reaction of 1 with methylamine in a THF–H2O = 1 : 1 mixture leads
directly to 4. On the other hand, 1 fails to react with NEt3 in dry
THF. After 24 h reaction, only unreacted 1 could be recovered. The
reaction of 1 with NEt3 in a THF–H2O = 1 : 1 mixture affords the
known dimer acyl-bridged species [Ir2H2(PPh2(o-C6H4CO))2(m-
PPh2(o-C6H4CO))2].20
Fig. 1 Hydrogen release from H3NBH3 using 0.25 mol% (ꢀ), 0.5 mol%
(᭺), 0.75 mol% (ꢀ), and 1.0 mol% (᭹) of 1 as catalyst in THF–H2O =
1 : 1 mixtures.
while a 0.75 mol% or a 1 mol% catalyst loading reduces the time
to ca. 13 or 10 min respectively.
Complex 1 is also able to hydrolyse amine–borane adducts in
THF–H2O = 1 : 1 mixtures to produce the quantitative release
of three equivalents of hydrogen gas as shown in Fig. 2. In the
absence of 1, AB and also amine–borane adducts in a THF–H2O =
1 : 1 mixture remain unaltered after 2 h. With initial substrate
concentration of 0.46 M, DMAB is hydrolysed completely within
5 min when using a 0.5 mol% loading of 1 or after 4 min when
using a 1 mol% catalyst loading. The more encumbered tert-
butylamino–borane (TBAB) requires an activation period and
longer reaction times. With initial substrate concentration of
0.46 M, a 2 min activation period and a further 24 min (0.5
mol% catalyst loading) or 4 min activation period and a further
8 min (1 mol% catalyst loading) are needed to allow the release
of all three equivalents of hydrogen gas. The kinetic profile of the
hydrogen generation suggests the formation of an intermediate
that builds up and later falls.21 However, no analytical technique
was available to follow the concentration of the substrate under
atmospheric pressure and intermediates could not be detected. On
the other hand complex 1 shows very low activity to hydrolyse the
triethylamine–borane (TEAB) adduct. After 120 min the release
of only 8% (0.5 mol% catalyst loading) or 19% (1 mol% catalyst
loading) of three equivalents of hydrogen gas is observed. This
observation suggests that a suitable catalytic activity requires the
presence of a NH functionality in the amino–borane substrate.
Nevertheless steric hindrance effects could also be responsible for
this low activity.
Scheme 1 Reaction of 1 with methylamine.
The formation of 3 and 4 was demonstrated spectroscopically
1
and that of 3 also by X-ray diffraction analyses. The 31P{ H}
NMR spectrum of 3 shows two doublets at 30.8 and 15.0 ppm
2
1
respectively with J(P,P) of 7 Hz, the 13C{ H} NMR spectrum
shows two doublets at 243.9 and 221.8 ppm respectively due to acyl
and imino groups trans to phosphorus atoms (2J(P,C) of 106 and
103 Hz respectively) and the 1H NMR spectrum shows a hydride
resonance in the high field region, at -20.67 ppm, as a triplet due
2
to J(P,H) of 15 Hz, which agrees with the hydride being trans
to chlorine and cis to both phosphines, and a low field singlet at
13.2 ppm that supports the presence of fairly strong N ◊ ◊ ◊ H ◊ ◊ ◊ O
hydrogen bonds19 and the existence of an irida-b-ketoimine. The
1
31P{ H} NMR spectrum of 4 shows two doublets at 30.5 and
2
1
25.7 ppm respectively with J(P,P) of 5 Hz, the 13C{ H} NMR
spectrum shows two doublets at 239.9 (2J(P,C) of 102 Hz) and
213.9 (2J(P,C) of 5 Hz) ppm respectively due to acyl groups trans
1
or cis to phosphorus atoms and the H NMR spectrum shows a
Fig. 2 Hydrogen release using 0.5 mol% of 1 as catalyst in THF–H2O =
1 : 1 mixtures from: AB (᭺); DMAB (ꢀ); TBAB (ꢀ) or TEAB (ꢁ).
hydride resonance in the high field region, at -7.94 ppm, as doublet
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of doublet due to 2J(P,H)trans of 130.0 Hz and 2J(P,H)cis of 20.0 Hz,
which agrees with the structure shown in Scheme 1.
with air, and to the isotopic effect making the reaction with H2O
faster than with D2O.22 The amount of H2 decreases relative to
HD while the reaction progresses. Signals for complex 1 were not
observed, instead the appearance of a triplet at d = -9.07 ppm
(2JP–H = 13 Hz) occurs, due to coupling to two phosphorus atoms
Fig. 3 shows an ORTEP view of complex 3 with the labelling
scheme of the asymmetric unit.23 The geometry about the metal
atom is a distorted octahedral with four positions occupied by
P–C of two bidentate ligands bonded between them by an enolic
hydrogen bond, and the other two positions occupied by one
hydride ligand and by a Cl atom mutually trans. The Ir–P distances
indicate a larger trans influence for the acyl group than for the
imino group. The Ir–C distances comprising the chelate ligands
are equivalent and similar to those in previously reported irida-b-
diketones.18 The C1–O1 distance in 3 agrees with those reported for
other metalla-b-diketones.18,19 The C20–N1 bond length (1.310(7)
1
31
as indicated by H{ P} NMR. This suggests the presence of an
iridium species (A) containing a hydride trans to the carbon atom
of an acyl group and cis to two phosphorus atoms. The intensity
of this resonance decreases upon progression of the catalytic
reaction, most likely due to H/D exchange. In accordance with
1
this observation the 31P{ H} NMR spectra show two resonances
at d = 7.2 (s) and 6.4 (s) ppm, which remain unaltered during the
hydrolysis and correspond to the two phosphorus atoms mutually
cis of A, as confirmed by selective phosphorus decoupled 1H NMR
experiments (Fig. S6, ESI†). These resonances appear as singlets
due to unobservable coupling constants. On completion of the
reaction the appearance of another species B is observed. In the
absence of AB the complete transformation of A into B occurs
and this transformation may be accelerated by opening the tube,
whereupon hydrogen is lost. Species B is most likely to be closely
A) evidences a substantial double bonding in the N–Ccarbene bond,24
˚
and the angles involving N1 confirm the formation of an imino
˚
fragment. The O ◊ ◊ ◊ N distance, 2.735(7) A, is in accordance with
a strong hydrogen bridge bond, and the O1–H2–N1 angle, 153.3◦,
is consistent with a nearly linear O ◊ ◊ ◊ H ◊ ◊ ◊ N bridge.
1
31
1
related to 1; its spectroscopic features (d H -21.31 ppm; d P{ H},
18.7 ppm) are quite alike to those of 1, and we believe that it can
afford 1 and 2 upon evaporation of THF.
As shown in Scheme 2, the formation of A can be rationalised
taking into account that in protic solvents the displacement of
chloride from 1 is easier. This may allow the interaction of AB
with 1, with cleavage of the B–N bond favoured by the formation
of an irida-b-ketoimine, whose hydrolysis could afford species A,
containing a hydroxycarbene moiety, and able to get involved in
the cleavage of another AB molecule. The cleavage of the B–N
bond would allow the hydrolysis of the borane to afford H2 and
borate. This proposal is consistent with the more encumbered
TBAB requiring an activation period. As indicated previously,
NEt3 fails to undergo ketoimine formation. According to this,
Et3NBH3 would not allow the formation of species A.
Fig. 3 ORTEP view of complex 3 showing the atomic numbering
(25% probability ellipsoids) and the intramolecular hydrogen bond. The
hydrogen atoms except two and the label of some C atoms have been
omitted for clarity. Selected bond lengths (A) and angles ( ): Ir–P1,
2.307(2); Ir–P2, 2.343(1); Ir–C1, 2.042(6); Ir–C20, 2.043(6); Ir–CL1,
2.512(2); Ir–H1, 1.43; C1–O1, 1.237(7); C20–N1, 1.310(7); O1 ◊ ◊ ◊ N1,
2.735(7); C20–Ir–C1, 92.9(2); C20–Ir–P1, 174.8(2); C1–Ir–P1, 81.9(2);
C1–Ir–P2, 171.3(2); C20–Ir–P2, 79.72; P1–Ir–P2, 105.53(5); N1–C20–C21,
119.4(5); C20–N1–C39, 128.6(6); O1–H2–N1, 153.3.
◦
˚
To obtain information on the course of the catalytic reaction,
a multinuclear NMR study of the catalytic hydrolysis of NH3BH3
in a THF-d8–D2O mixture was carried out (Fig. S5, ESI†).
Scheme 2 Formation of A from 1 during the catalytic hydrolysis of AB.
1
After 7 min the 11B{ H} NMR spectrum presents a singlet at
In conclusion, we have reported on the first homogeneous metal-
catalysed hydrolysis of ammonia–borane or amino–boranes to ef-
ficiently produce hydrogen. Further studies to clarify mechanistic
details of these reactions are under way.
d = -25.2 ppm due to unreacted AB along with a single peak
at d = 17.2 ppm due to the formation of B–O bonds. As the
reaction progresses the amount of AB decreases and that of
borate increases. On completion of the reaction (ca. 100 min)
the resonance due to AB disappears, revealing its complete
consumption. Borane intermediates could not be observed. The
1H NMR spectra indicate the gradual disappearance of the quartet
centred at d = 1.37 ppm (1JB–H = 92 Hz) due to AB and the
formation of HD at d = 4.52 (t) ppm (1JD–H = 43 Hz) and of
H2 at d = 4.55 (s) ppm. The appearance of H2 is due to the
unavoidable presence of light water vapor in heavy water in contact
Acknowledgements
We thank MICINN (CTQ2008-2967/BQU), UPV and
Diputacio´n Foral de Guipuzcoa for financial support. We
are grateful to Dr J. I. Santos for his generous help in conducting
NMR spectra.
7228 | Dalton Trans., 2010, 39, 7226–7229
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◦
˚
˚
˚
˚
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3
-3
3389.5(4) A , Z = 4, Dc = 1.609 Mg m , m(Mo-Ka) = 0.71073, no. of
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Dalton Trans., 2010, 39, 7226–7229 | 7229
©