Hydrolysis of ammonia borane catalyzed by aminophosphine-stabilized precursors of rhodium nanoparticles: Ligand effects and solvent-controlled product formation

Article abstract of DOI:10.1002/chem.201003543

Releasing hydrogen: Aminophosphine complexes of rhodium are excellent systems for the hydrolysis of H₃N-BH₃. The release of H₂ in THF/H₂O mixtures generates NH₃ and B(OH)₃ and is based on the sequential catalytic dehydrogenation of H₃N-BH_3-n(OH)_n and hydrolysis of H ₂N=BH_2-n(OH)_n (n=0-2, see scheme). Copyright

Full text of DOI:10.1002/chem.201003543

DOI: 10.1002/chem.201003543
Hydrolysis of Ammonia Borane Catalyzed by Aminophosphine-Stabilized
Precursors of Rhodium Nanoparticles: Ligand Effects and Solvent-Controlled
Product Formation
Marco Fetz, Roman Gerber, Olivier Blacque, and Christian M. Frech*[a]
The switch from carbon-based fuels to nonpolluting alter-
natives,[1] such as hydrogen,[2] which is clean, has a higher
energy content per mass compared with petroleum, and can
readily be used in fuel cells in vehicles or in portable elec-
tronic devices, is highly desirable. The use in fuel cells, how-
ever, requires safe and lightweight H2 storage or a highly ef-
ficient and easy to control “on-board” hydrogen genera-
tion.[2,3] A promising approach to safe handling of hydrogen-
based fuels is the reversible hydrogenation and dehydrogen-
ation of small molecules, such as alcohols or ammonia
tion is extremely fast and was found to be based on the se-
À
quential dehydrogenation of H3N BH3Àn(OH)n (n=0–2)
and hydrolysis of H2N=BH2Àn(OH)n (n=0–2) to form NH3
and B(OH)3 and, thus,[11] allows the release of up to
À
3.0 equivalents of H2 from H3N BH3 in THF/H2O mixtures
(Scheme 1). On the other hand, B-(cyclodiborazanyl)amino-
Scheme 1. Rhodium-catalyzed, sequential dehydrogenation, and hydroly-
[3,4]
À
borane (H3N BH3),
of which the latter is of particular in-
À
sis of NH3 BH3 in THF/H2O mixtures to form NH3 and B(OH)3.
terest, because of the high hydrogen-to-mass ratio
À
(19.6 wt%). Because the thermal dehydrogenation of H3N
BH3 and derivatives thereof exhibit high kinetic barriers,[5–7]
transition-metal complexes have been developed for this re-
action.[8,9] Even though transition-metal-catalyzed hydroge-
nation reactions are well-known in synthetic organic chemis-
try as well as industrial processes, only a few systems have
borohydride, borazine, and polyborazylene were formed
when the catalytic dehydrogenation was performed in pure
THF, that is, in the absence of H2O. This is strikingly differ-
ent than [{RhClACHTNUTRGNEUNG(cod)}2], which is almost inactive under an-
hydrous reaction conditions (see below).
À
been reported to efficiently release H2 from H3N BH3 (oc-
The addition of approximately 1.1 equivalents of 1,1’,1’’-
(phosphanetriyl)tripiperidine to [{(cod)Rh(Cl)}2] in THF
under N2 at ambient temperature yielded the chloro[1,1’,1’’-
casionally up to 3.0 equiv) under mild conditions.[10]
We report herein the performance of simple, inexpensive,
and easily prepared rhodium-based chlorocycloocta-1,5-
diene aminophosphine complexes in the catalytic generation
(phosphanetriyl)tripiperidine]
complex
[RhClACHTUNGTRENNUNG(cod){P-
(NC5H10)3}] (1) within about 30 min (Scheme 2).[15]
À
of H2 from H3N BH3 at 258C in air. Rhodium aminophos-
phine complexes of the type [RhCl
A
N
-
ACHTUNGTRENNUNG(C6H11)n}] were assumed to be ideal candidates for this pro-
cess, because aminophosphines degrade in the presence of
water and, thus, promote the formation of rhodium nanopar-
ticles, which are known to efficiently catalyze the hydrolysis
[9a,10i–k]
À
of H3N BH3.
Moreover, modifications to the amino-
phosphine units were expected to allow (water-induced)
complex degradation and, hence, controlled H2 generation.
We demonstrate that the rhodium aminophosphine com-
plexes indeed serve as stable precursors to nanoparticles
and show that slight modifications to the ligand and/or the
reaction conditions allow controlled nanoparticle formation
(and hence, the generation of H2). The catalytic H2 genera-
Scheme 2. Synthesis of chloro[1,1’,1’’-(phosphanetriyl)tripiperidine](cy-
cloocta-1,5-diene)rhodium (1).
Addition of pentane to the reaction mixture leads to pre-
cipitation of 1. Filtration and subsequent extraction of resid-
ual ligand, by using pentane, afforded analytically pure 1 in
an almost quantitative yield.[16] The 31P{1H} NMR spectrum
of 1 shows a broad signal at d=21.1 ppm due to the 1,1’,1’’-
(phosphanetriyl)tripiperidine ligand. In addition to the sig-
nals due to the olefinic protons of cycloocta-1,5-diene at d=
[a] M. Fetz, R. Gerber, Dr. O. Blacque, Dr. C. M. Frech
Department of Inorganic Chemistry
University of Zꢀrich, 8057 Zꢀrich (Switzerland)
Fax : (+41)44-635-46-92
E-mail: chfrech@aci.uzh.ch
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201003543.
4732
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Chem. Eur. J. 2011, 17, 4732 – 4736

COMMUNICATION
1
5.82 and 3.55 ppm, the H NMR spectrum exhibits resonan-
which the latter correspond to turn over frequencies (TOFs)
of about 36000 hÀ1 (Figure 1).[20]
ces that are assigned to the residual aliphatic hydrogen
atoms as well as those of the aminophosphine ligand. The
13C{1H} NMR spectrum confirmed the proposed structure.
À
The same trend was noticed by varying the H3N BH3
concentrations (entries 6–11, Table 1).[16,21] Whereas com-
Chloro
octa-1,5-diene)rhodium (2), chloro(cycloocta-1,5-diene)[1-
(dicyclohexylphosphanyl)piperidine]rhodium (3), and
ACHTUNGTRENNUNG[1,1’-(cyclohexylphosphanediyl)dipiperidine](cyclo-
ACHTUNGTRENNUNG
chloro(cycloocta-1,5-diene)(tricyclohexylphosphane)rhod-
ium (4) were prepared accordingly.[17] X-ray diffraction stud-
ies were successfully performed on 1 (Scheme 2) and 2.[16]
Complex 1 is an excellent precatalyst for the release of di-
À
hydrogen from H3N BH3 and nowadays belongs to one of
the most active systems for this process. For example, com-
plex 1 efficiently promotes the sequential dehydrogenation
À
and hydrolysis of H3N BH3 and, thus, the release of
À
3.0 equivalents of H2 from H3N BH3 solutions (0.5m; THF/
H2O 95:5) in the presence of 1.0 mol% catalyst (entry 1,
Table 1). The system remains highly active after the hydroly-
À
Table 1. H2 release from a solution of H3N BH3 (THF/H2O) to form
NH3 and B(OH)3 catalyzed by 1 at 258C in air.[27]
À
Figure 1. Volumetric monitoring of the H2 release from H3N BH3 solu-
À
Entry
Cat.
[mol%]
Time
[min]
Equiv
H2
H3N BH3
THF/H2O
[v/v]
tions (THF/H2O 95:5) catalyzed by different loadings of 1.[16]
A
R
[m]
ACHTUNGTRENNUNG
1
2
3
4
5
6
7
8
1.0
0.5
0.2
0.1
0.05
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.6
5.2
8.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
0.5
0.5
0.5
0.5
0.5
0.1
0.25
0.5
1.0
2.0
4.0
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
95:5
95:5
95:5
95:5
plete conversions were achieved within approximately 2 min
in the presence of 1.0 mol% catalyst in 0.25m solutions of
10.2
4.8
1.9
1.6
1.2
1.1
1.0
8.4
2.1
1.8
1.4
1.6
1.8
4.4
4.2
9.8
95:5
À
H3N BH3 (THF/H2O mixtures with a ratio of 9:1), a drop
90:10
90:10
90:10
90:10
90:10
90:10
in activity (ꢀ5 min were required for full conversions) was
À
noticed in 0.1m solutions of H3N BH3. On the other hand,
9
faster conversions were obtained when more concentrated
10
11
12
13
14
15
16
17
18
19
20
3.0
3.0
À
H3N BH3 solutions were applied. Whereas 1.2 and 1.1 min
were required to fully dehydrogenate 1.0m and 2.0m solu-
ꢀ1.5
100:0[a]
98:2
97:3
À
tions of H3N BH3, respectively, only 1.0 min was needed for
3.0
À
3.0
3.0
3.0
3.0
a complete H2 release when a 4.0m solution of H3N BH3
96:4
was applied (Figure 2).
90:10
80:20
50:50
10:90
0:100
Also the amount of H2O present in the reaction mixtures
showed an effect on the catalytic performance of 1 (entries 1
3.0
ꢀ2.2
ꢀ2.2
[a] Reaction products are B-(cyclodiborazanyl)aminoborohydride, bora-
zine, and polyborazylene.[24]
sis is completed and the catalysis is resumed upon addition
À
of more H3N BH3—even after several days. The reaction
products are NH3 Àand B(OH)3, of which the latter is in equi-
librium with BO2 and other borate species.[18] The forma-
tion of H2 (and NH3) was confirmed by 1H NMR spectrosco-
py and analysis with a GC equipped with a thermal conduc-
tivity detector (TCD). The same yields, but slightly retarded
conversions were observed when the catalyst loadings were
lowered (compare entries 1–5, Table 1). For example, where-
À
as the release of 3.0 equivalents of H2 from H3N BH3 was
achieved within about 5 min in the presence of 0.2 mol% of
1, approximately 8 min were required with 0.1 mol% cata-
lyst, and approximately 10 min with 0.05 mol% catalyst, of
À
Figure 2. Volumetric monitoring of the H2 release from H3N BH3 (THF/
[16]
À
H2O 90:10) catalyzed by 1 at different H3N BH3 concentrations.
Chem. Eur. J. 2011, 17, 4732 – 4736
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4733

C. M. Frech et al.
and 12–20, Table 1). Whereas almost identical activities
were noticed by increasing the water content from 2 to
20%,[22] only a slight decrease in activity was obtained when
the amount of water was increased further. Thus, reactions
performed in pure H2O exhibit the lowest rates. Moreover,
catalysis stopped after about 10 min and the release of ap-
proximately 2.2 equivalents of H2. On the other hand, com-
not have an effect on neither the conversion rates nor on
the amount of hydrogen generated (in any of the reactions
performed):[29] 1) In all the reactions examined, sigmoidal-
shaped kinetics were observed, which is characteristic of
metal-particle formation and autocatalytic surface growth
that can lead to soluble, monodisperse nanoclusters or in-
soluble bulk-metal formation.[31] 2) The UV/Vis spectrum of
the reaction mixtures exhibit a continuous absorption with a
steep rise in absorbance at shorter wavelengths—a clear in-
dication of the presence of rhodium nanoparticles.[32] 3) A
black solution was instantly formed after the addition of sol-
plex 1 also shows (in contrast to [{RhClACTHNUTRGEN(UNG cod)}2]) an excellent
performance under anhydrous conditions.[23] For example,
when the dehydrogenation reactions were carried out in
pure THF, approximately 1.5 equivalents of H2 were re-
À
À
leased from H3N BH3 in about 8 min (Figure 3). Moreover,
utions of H3N BH3 in THF/H2O to the solutions of 1–4 in
THF, of which a solid with metallic appearance (insoluble in
common organic solvents and water) precipitated after the
reaction had run to completion. The filtrate was inactive in
À
the catalytic dehydrogenation of H3N BH3, whereas cataly-
sis was resumed upon addition of more substrate to the pre-
cipitate. 4) The addition of H2O to solutions of 1 in THF
lead to instant complex degradation, accompanied by the
formation of a black solid with metallic appearance, which
À
is active in the catalytic dehydrogenation of H3N BH3. The
filtrate on the other hand, showed no activity. 5) The steric
bulk and the s-donor strength of 1,1’,1’’-(phosphanetriyl)tri-
piperidine, 1,1’-(cyclohexylphosphanediyl)dipiperidine, 1-(di-
cyclohexylphosphanyl)piperidine, and tricyclohexylphos-
phane are almost identical,[33] implying similar catalytic ac-
tivities to be a molecular mechanism. However, whereas 2
operates at a similar level of activity compared to 1, a drop
in activity was noticed for 3. A further (significant) retarda-
tion was found for the phosphine analogue—the most stable
compound of this series[16]—as expected when nanoparticles
are involved in the catalytic cycle.[17]
À
Figure 3. Volumetric monitoring of the H2 release from H3N BH3 cata-
lyzed by 1 with different THF/H2O ratios.[16]
whereas NH3 and B(OH)3 have been generated in the pres-
ence of H2O, B-(cyclodiborazanyl)aminoborohydride (d=
À24.2, À11.2, and À5.2 ppm), borazine (d=30.8 ppm), and
polyborazylene (d=28.3 ppm) were reaction products under
anhydrous reaction conditions.[24] The signals due to bora-
zine and polyborazylene became dominant after some time.
However, when H2O was added to the reaction mixtures in-
stant release of additional H2, accompanied by the forma-
tion of NH3 and B(OH)3 was obtained, implying that the
borane species formed are hydrolyzed (in a first step) to
The following experimental observations indicate a cata-
lytic mechanisms based on the sequential dehydrogenation
À
of H3N BH3Àn(OH)n (n=0–2) and hydrolysis of H2N=
BH2Àn(OH)n (n=0–2): 1) The reaction products of the cata-
À
lytic dehydrogenation of H3N BH3 in the presence of H2O
are NH3 and B(OH)3, of which the latter is in equilibrium
with BO2 and other borate species.[18] 2) The catalytic dehy-
À
À
drogenation of H3N BH3 in the absence of H2O leads to the
release of ꢀ1.5 equivalents of H2 and is accompanied by the
formation of B-(cyclodiborazanyl)aminoborohydride, bora-
zine, and polyborazylene,[24] which implies that rhodium
nanoparticles are susceptible to dehydrogenation of both
À
À
H3N BH2(OH) and H3N BH(OH)2, respectively, which are
susceptible to dehydrogenation in the presence of rhodium
nanoparticles.[25]
À
H3N BH3 and H2B=NH2. However, the addition of H2O to
The same trends, but successively lower conversion rates
were obtained when 1,1’,1’’-(phosphanetriyl)tripiperidine
was substituted by 1,1’-(phosphanetriyl)dipiperidine, 1-
(phosphanetriyl)piperidine, and tricyclohexylphosphane,
which allows the efficiency of the H2 release to be con-
trolled by simple modifications to the aminophosphine
ligand.[16] The phosphine-based system, however, shows by
far the lowest catalytic activity, under all the reaction condi-
tions applied.
these reaction mixtures led to the instant release of another
1.5 equivalents of H2 and the formation of NH3 and
B(OH)3.[25] 3) The dehydrogenation of H3N BH3 performed
À
in D2O generates approximately 82% H2 and only about
1
18% HD (as indicated by H NMR spectroscopy) and, thus,
excludes mechanisms involving rhodium-catalyzed dissocia-
[25,34]
À
À
tion and hydrolysis of H3N BH3 and H3N BH2(OH).
Expectedly, no reaction occurred (neither in the absence
À
nor in the presence of H2O) when Me3N BH3 was applied
The following experimental observations indicate the in-
volvement of rhodium nanoparticles in the catalytic
cycles,[28] even though the presence of metallic mercury did
in dehydrogenation reactions. On the other hand,
ꢀ3.0 equivalents of H2 was released within 3.6 min in the
À
presence of 1.0 mol% of 1 when iPr2HN BH3 (0.5m; THF/
4734
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Chem. Eur. J. 2011, 17, 4732 – 4736

Rhodium-Catalyzed Hydrolysis of Ammonia Borane
COMMUNICATION
H2O 20:1) was used as the dihydrogen source. These obser-
vations confirm catalytic mechanisms based on the sequen-
tial dehydrogenation and hydrolysis of aminoboranes.[25]
Keywords: aminophosphines
dehydrogenation · dihydrogen · hydrolysis · rhodium
·
ammonia boranes
·
À
Indeed, dehydrogenation of Me2HN BH3 in the absence of
H2O leads to the release of approximately 1.0 equivalents of
À
[1] F. H. Stephens, V. Pons, R. T. Baker, Dalton Trans. 2007, 2613.
[2] C. W. Hamilton, R. T. Baker, A. Staubitz, I. Manners, Chem. Soc.
Rev. 2009, 38, 279.
[3] H. W. Langmi, G. S. McGrady, Coord. Chem. Rev. 2007, 251, 925.
[4] T. B. Marder, Angew. Chem. 2007, 119, 8262; Angew. Chem. Int. Ed.
2007, 46, 5826.
H2 and the formation of (Me2N BH2)2 (d=4.7 ppm, t,
1JHB =110 Hz). Diborazane (Me2HNBH2Me2NBH3, d=1.4
(t, 1JHB =108 Hz) and À14.3 ppm (q, 1JHB =94 Hz)) was
found to be an intermediate.[35] The addition of H2O to
these reaction mixtures led to the generation of additionally
about 2 equivalents of H2 and was accompanied by the for-
mation of NHMe2 and B(OH)3. The dehydrogenation of
À
À
[5] Pyrolysis of H3N BH3 has been widely investigated; H3N BH3 lib-
erates H2 in a sequence of reactions between 137 and 4008C that
reaches 19.6 wt% of the initial mass.[6] H3N BH3 also undergoes
À
À
Me2HN BH3 in D2O generates about 1.0 equivalents of H2
and about 2.0 equivalents of HD—observations that confirm
the proposed mechanism further.
In conclusion, complex 1 is a new, simple, and easy-to-pre-
pare aminophosphine rhodium complex, which nowadays
belongs to the most active systems for the dehydrogenation/
acid-catalyzed hydrolysis to produce B(OH)3 along with H2.[7]
[6] a) J. S. Wang, R. A. Geanangel, Inorg. Chim. Acta 1988, 148, 185;
b) G. Wolf, J. Baumann, F. Baitalow, F. P. Hoffmann, Thermochim.
Acta 2000, 343, 19.
[7] a) G. E. Ryschkewitsch, J. Am. Chem. Soc. 1960, 82, 3290; b) H. C.
Kelly, V. B. Marriott, Inorg. Chem. 1979, 18, 2875.
[8] A. Friedrich, M. Drees, S. Schneider, Chem. Eur. J. 2009, 15, 10339.
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Soc. 2003, 125, 9424; b) R. J. Keaton, J. M. Blacquiere, R. T. Baker,
J. Am. Chem. Soc. 2007, 129, 1844, and references therein.
À
À
hydrolysis of H3N BH3 (or Me2H BH3). H2 (3.0 equiv) is
À
released from solutions of H3N BH3 in THF/H2O within
5 min in the presence of 1.0 mol% of 1 at 258C in air. The
system remains highly active after the reaction has run to
completion and catalysis is resumed upon addition of more
[10] a) C. A. Jaska, K. Temple, A. J. Lough, I. Manners, Chem. Commun.
2001, 962; b) T. J. Clark, C. A. Russell, I. Manners, J. Am. Chem.
Soc. 2006, 128, 9582; c) F. Cheng, H. Ma, Y. Li, J. Chen, Inorg.
Chem. 2007, 46, 788; d) J. F. Fulton, J. C. Linehan, T. Autrey, M. Ba-
lasubramanian, Y. Chen, N. K. Szymczak, J. Am. Chem. Soc. 2007,
129, 11936; e) D. Pun, E. Lobkovsky, P. J. Chirik, Chem. Commun.
2007, 3297; f) A. Staubitz, A. Presa Soto, I. Manners, Angew. Chem.
2008, 120, 6308; Angew. Chem. Int. Ed. 2008, 47, 6212; g) M. Kꢂß,
A. Friedrich, M. Drees, S. Schneider Angew. Chem. 2009, 121, 922;
Angew. Chem. Int. Ed. 2009, 48, 905; Angew. Chem. Int. Ed. 2009,
48, 905; h) N. Blaquiere, S. Diallo-Garcia, S. I. Gorelsky, D. A.
Black, K. Fagnou, J. Am. Chem. Soc. 2008, 130, 14034; i) S. Caliskan,
M. Zahmakiran, S. Oezkar, Appl. Catal. B 2010, 93, 387; j) D.
Feyyaz, M. Zahmakiran, S. Oezkar, Appl. Catal. A 2009, 369, 53;
k) M. Zahmakiran, S. Oezkar, Appl. Catal. B 2009, 89, 104; l) T. W.
Graham, C.-W. Tsang, X. Chen, R. Guo, W. Jia, S.-M. Lu, C. Sui-
Seng, C. B. Ewart, A. Lough, D. Amoroso, K. Abdur-Rashid,
Angew. Chem. 2010, 122, 8890; Angew. Chem. Int. Ed. 2010, 49,
8708.
À
H3N BH3. The same trends, but successively lower conver-
sion rates were obtained when 1,1’,1’’-(phosphanetriyl)tripi-
peridine was exchanged with 1,1’-(phosphanediyl)dipiperi-
dine, 1-(phosphanyl)piperidine, and tricyclohexylphosphane,
of which the phosphine-based system exhibit (under all con-
ditions applied) by far the lowest activity. The H2 generation
is based on the sequential catalytic dehydrogenation of
H3N BH3Àn(OH)n (n=0–2) and hydrolysis of H2N=
BH2Àn(OH)n (n=0–2). Thus, the reaction products are NH3
and B(OH)3, of which the latter was found to be in equilib-
rium with BO2 and other borate species. On the other
hand, B-(cyclodiborazanyl)aminoborohydride, borazine, and
polyborazylene were formed when the dehydrogenation re-
actions were carried out under anhydrous reaction condi-
tions and, thus, demonstrates that the product formation can
be controlled by the solvent used. Overall, the aminophos-
À
À
[11] The following concerns may be problematic when NH3 and B(OH)3
are formed: 1) The pH-dependent solution–vapor equilibriums be-
tween NH4OH and NH3(g).[12] Gaseous NH3 in a H2 stream has to
be removed before reaching the fuel cell.[13] 2) Regeneration of
borate products require that a large DH is overcome.[14]
phine-based rhodium complexes of the type [RhCl
ACHTUNGTRENN(UNG cod)(P-
A
CHTUNGTRENNUNG
[12] a) M. E. Jones, J. Phys. Chem. 1963, 67, 1113; b) T. A. Wilson, Univ.
Ill. Eng. Exp. Sta. Bull. 1925, 146, 47; c) A. Mittasch, E. Kuss, H.
Schlueter, Z. Anorg. Allg. Chem. 1926, 159, 1.
[13] R. Halseid, P. J. S. Vie, R. Tunold, J. Power Sources 2006, 154, 343.
[14] R. M. Mohring, Y. Wu, AIP Conf. Proc. 2003, 671, 90.
[15] a) J. L. Bolliger, O. Blacque, C. M. Frech, Angew. Chem. 2007, 119,
6634; Angew. Chem. Int. Ed. 2007, 46, 6514; b) B. Marciniec, P. Blaz-
ejewska-Chadyniak, M. Kubicki, Can. J. Chem. 2003, 81, 1292.
[16] For details see the Supporting Information.
and convenient systems for the catalytic H2 generation from
À
H3N BH3, which—in contrast to [{RhCl
(cod)}2]—shows
high catalytic performances in the presence, as well as in the
absence, of H2O, that is, under all reaction conditions ap-
plied. Nevertheless, whether aminoboranes will find an ap-
plication as lightweight H2 storage in, for example, fuel cells
will finally depend on the development of a facile and eco-
À
nomic process to convert B(OH)3, BO2 , or other borate
[17] a) J. L. Bolliger, C. M. Frech, Chem. Eur. J. 2010, 16, 4075; b) J. L.
Bolliger, C. M. Frech, Chem. Eur. J. 2010, 16, 11072.
species and/or other dehydrogenation products, such as bor-
azine, and polyborazylene, into BH3.[36]
À
[18] NH3 BH3 dissolves in THF/H2O mixtures to form stable colorless
solutions with pH 9.1. The 11B NMR spectra exhibit a quadruplet at
d=À23.2 ppm with a coupling constant of 1JBH =90 Hz. After the re-
actions have run to completion, the 11B NMR resonance signal at
d=À23.9 ppm disappeared and a single low-field-shifted resonance
at dꢀ20 ppm appeared. High-field-shifted signals were detectable
by increasing the pH of these solutions.[19]
Acknowledgements
This work was supported by the University of Zurich and the Swiss Na-
tional Science Foundation (SNSF).
[19] M. Chandra, Q. Xu, J. Power Sources 2006, 156, 190.
Chem. Eur. J. 2011, 17, 4732 – 4736
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4735

C. M. Frech et al.
[20] Note that catalyst loadings <0.05 mol% are not appropriate, as cat-
alysis stopped after about 5 min and the release of approximately
2.5 equivalents of H2. The reason for this behavior is yet unknown.
with coupling constants of 1JHB =98 Hz in the 11B NMR spectrum.
However, its identity is unknown. For an 11B{1H} NMR spectrum
with a 11B NMR inset, see the Supporting Information.
À
À
[21] Higher H3N BH3 concentrations (>0.5m) may be desirable for
[25] Notably, instead of catalytic dehydrogenation of H3N BH(OH)2
practical applications. This is possible, but requires a larger amount
and formation of H2N=B(OH)2, followed by hydrolysis and forma-
À
of H2O. Thus, for comparisons between different concentrated
tion of NH3 and B(OH)3, dissociation and hydrolysis of H3N
À
H3N BH3 solutions, THF/H2O mixtures with a H2O content of 10%
BH(OH)2 (e.g., borazine slowly hydrolyzes in water to NH3 and
were used. Two opposite trends have been observed: 1) the higher
B(OH)3) cannot be excluded completely.[26]
À
the H3N BH3 concentration, the faster the conversions and 2) an in-
creasing amount of water retards the catalysis (see below).
[26] H. Watanabe, T. Totani, T. Yoshizaki, Inorg. Chem. 1965, 4, 657.
[27] It should be mentioned that the measured gas volume sometimes
exceeded the theoretical value from 3.0 equivalents of H2. Possible
explanations may include slight variations in temperature and/or the
accuracy of the volumetric measurements in general.
[28] For a comprehensive review article on the problem of distinguishing
true homogeneous catalysis from soluble or other metal-particle het-
erogeneous catalysis under reducing conditions see: J. A. Widegren,
R. G. Finke, J. Mol. Catal. A 2003, 198, 317.
À
[22] An aliquot of 2% H2O of a THF/H2O solution of H3N BH3 (0.5m)
correspond to approximately 2.4 equivalents of H2O relative to
H3N BH3; this amount is required to fully hydrolyze H2N=
BH2Àn(OH)n and, hence, to fully convert H3N BH3 into NH3 and
B(OH)3.
À
À
[23]
A
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
[29] This is in striking contrast to the reactivity observed for [{RhCl-
À
ꢁ20% were applied (complete conversions of H3N BH3 into NH3
ACHTUNGTNER(NNUG cod)}2], for which catalysis efficiently stopped when metallic mercu-
and B(OH)3 were achieved in less than 1 min). Complex 1 is far
ry was present.[30]
more active when THF/H2O mixtures with a H2O content of 10%
[30] C. A. Jaska, I. Manners, J. Am. Chem. Soc. 2004, 126, 9776.
[31] For details, see: a) M. A. Watzky, R. G. Finke, J. Am. Chem. Soc.
1997, 119, 10382; b) J. A. Widegren, M. A. Bennett, R. G. Finke, J.
Am. Chem. Soc. 2003, 125, 10301.
or less were used.[16] For example, whereas full conversion of H3N
À
BH3 (2.0m) into NH3 and B(OH)3 was achieved in about 1 min in
THF/H2O (20:1) mixtures in the presence of 1.0 mol% of 1, 1 h was
required for the release of 3.0 equivalents of H2 with [RhCl
(cod)]2.
[32] J. A. Creighton, D. G. Eadon, J. Chem. Soc. Faraday Trans. 1991, 87,
3881.
[33] M. L. Clarke, D. J. Cole-Hamilton, M. Z. Slawin, J. D. Woolins,
Incomplete conversions were noticed when the amount of water was
reduced further. Moreover, whereas approximately 1.5 equivalents
of H2 were released after 8.4 min in pure THF with 1, catalysis
stopped after about 2 min and the release of only about 0.2 equiva-
Chem. Commun. 2000, 2065.
À
[34] A catalytic mechanism involving dissociation of NH3 BH3 and hy-
lents of H2 with [{RhCl
N
drolysis of BH3 was recently proposed.[19] However, although cata-
[24] The identity of the borane species formed was confirmed by 11B and
11B{1H} NMR spectroscopy. The chemical shifts and coupling con-
stants compare well with those given in the literature: W. J. Shaw,
J. C. Linehan, N. K. Szymczak, D. J. Heldebrant, C. Yonker, D. M.
Camaioni, R. T. Baker, T. Autrey, Angew. Chem. 2008, 120, 7603;
Angew. Chem. Int. Ed. 2008, 47, 7493. Nevertheless, it is important
to note that the 11B{1H} NMR spectrum showed an additional singlet
signal at d=À14.3 ppm, which exhibit a quartet pattern in the
11B NMR spectrum with a coupling constant of 1JHB =98 Hz; this
would indicate the formation of a boron-containing compound with
BH3 units, similar to ammonia borane, which gave rise a singlet
signal at d=À21.9 ppm in the 11B{1H} NMR spectrum and a quartet
lytic dissociation and hydrolysis of H3N BH3 and H3N BH2(OH)
À
À
À
can be ruled out, dissociation and hydrolysis of H3N BH(OH)2
cannot be excluded completely (see also reference [25]).
[35] A further 11B NMR signal due to Me2N=BH2 (d=36.8 ppm (t,
1JHB =124 Hz)) was detected. However, this signal diminished with
time due to dimerization. The borane species formed were all identi-
fied by comparing the chemical shifts to literature data.[8,9a]
[36] N. C. Smythe, J. C. Gordon, Eur. J. Inorg. Chem. 2010, 509.
Received: December 8, 2010
Published online: March 17, 2011
4736
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