Ruthenium Bis(σ-B-H) aminoborane complexes from dehydrogenation of amine-boranes: trapping of H2B-NH2

Article abstract of DOI:10.1002/anie.200905970

One small step for ammonia-borane: The simplest elementary aminoborane compound H₂BNH₂, which results from dehydrogenation of ammonia-borane, has been trapped by a ruthenium complex fragment leading to the isolation of a bis(σ-B-H) aminoborane complex. The analogous H ₂BNHMe and H₂BNMe₂ complexes were also prepared. (Picture: ruthenium complex; Ru purple, P orange, N blue, B brown, H white.)

Full text of DOI:10.1002/anie.200905970

Communications
DOI: 10.1002/anie.200905970
Reaction Intermediates
ꢀ
Ruthenium Bis(s-B H) Aminoborane Complexes from
Dehydrogenation of Amine–Boranes: Trapping of H2B NH2**
ꢀ
Gilles Alcaraz,* Laure Vendier, Eric Clot, and Sylviane Sabo-Etienne*
ꢀ
Ammonia–borane has attracted considerable interest
recently as a potential hydrogen source and storage material
owing to its high hydrogen storage capacity.[1] Homogeneous
transition-metal-catalyzed dehydrogenation of ammonia–
boranes leads to dihydrogen release; however, reversible
storage from the resulting polymeric materials remains a
major problem.[2] The nature of the transition metal complex
used to catalyze the dehydrogenation of the amine–borane
ing a B N unit are restricted to two cationic rhodium
examples, as reported by Weller et al. They are amine–
1
2
ꢀ
borane adducts with h - and/or h -H3B NRR’H bonding
modes (R,R’ = H, Me; Scheme 1).[7,8]
With this work in mind, and following our recent findings
on the isolation of bis(s-B H) complexes from the reaction of
ꢀ
dihydrogenoboranes with the bis(dihydrogen) complex
[RuH2(h2-H2)2(PCy3)2],[9,10] we herein report the synthesis of
ꢀ
ꢀ
family H3B NR3ꢀnHn (n = 1–3) (I) has a direct impact both on
the first “true” bis (s-B H) aminoborane ruthenium com-
kinetics and dehydrocoupling routes.[3] However, the succes-
sive elementary steps leading to dehydrocoupling are still not
well understood, and the question of the BH/NH bond
activation mechanism is a very active area of research.[4]
plexes.[11] Stabilization of the simplest aminoborane units
ꢀ
H2B NMenꢀ2Hn (n = 1–2) results from the stoichiometric
dehydrogenation of the amine–borane precursors by the
ruthenium complex.
B H bond activation of a tertiary amine–borane adduct is
Room-temperature reaction of [RuH2(h2-H2)2(PCy3)2] in
toluene with the amine–boranes 1a–c proceeds with gas
evolution. After workup, the resulting solids, which analyzed
as [RuH2(h2:h2-H2B-NR1R2)(PCy3)2] (2), were isolated as
white powders (2a: 77%, 2b: 60%, 2c: 64%; Scheme 2) and
fully characterized by NMR and X-ray diffraction crystallog-
raphy for 2a.
ꢀ
1
ꢀ
achieved in the h -H3B NR3 Shimoi-type complexes in the
case of chromium, tungsten, and manganese (Scheme 1).[5] In
1
ꢀ
Scheme 1. Representative examples of a) h -H3B NR3 Shimoi and
1
2
ꢀ
b) h - and h -bis(H3B NH2Me) Weller complexes.
Scheme 2. Synthesis of 2 by dehydrogenation of amine–boranes 1a–c.
such a case, the substitution pattern at the nitrogen atom
prevents further dihydrogen release. Thus, these complexes
ꢀ
can be regarded as early intermediates in amine–borane B H
As a representative example, we will describe the NMR
spectroscopic properties of 2a, but similar features are
observed for compounds 2b–c and are reminiscent of the
spectroscopic data reported for the ruthenium complexes
[RuH2(h2:h2-H2B-R)(PCy3)2] (R = tBu, Mes).[9] The 1H NMR
spectrum of 2a in the hydride region in C6D6 at 298 K exhibits
a broad singlet at d = ꢀ6.80 ppm and a more shielded triplet
(JPH = 24.8 Hz) in a 1:1 integration ratio at d = ꢀ11.85 ppm.
The triplet collapsed into a singlet upon phosphorus decou-
pling, whereas the singlet sharpened upon boron decoupling.
A 1D TOCSY 1H{11B} experiment with a selective excitation
at d = ꢀ6.80 ppm showed correlations with the triplet and a
deshielded singlet at d = 2.34ppm, which were assigned to the
NH2 protons (see the Supporting Information, Figure S1).
The 31P{1H} NMR spectrum shows a sharp singlet at d =
77.43 ppm, and a broad signal is observed in the 11B{1H}
NMR spectrum at d = 46 ppm. This 11B signal is highly shifted
from the resonance of the starting ammonia–borane (d =
activation processes.[6] Starting from precursors I that are
likely to undergo dehydrogenation, known complexes retain-
[*] Dr. G. Alcaraz, Dr. L. Vendier, Dr. S. Sabo-Etienne
CNRS, LCC (Laboratoire de Chimie de Coordination)
205 route de Narbonne, 31077 Toulouse (France)
and
Universitꢀ de Toulouse, UPS, INPT
31077 Toulouse (France)
Fax: (+33)5-6155-3003
E-mail: gilles.alcaraz@lcc-toulouse.fr
sylviane.sabo@lcc-toulouse.fr
Homepage: http://www.lcc-toulouse.fr/lcc/spip.php?article434
Dr. E. Clot
Institut Charles Gerhardt, Universitꢀ Montpellier 2, CNRS 5253
cc 1501 Place Eugꢁne Bataillon, 34095 Montpellier (France)
[**] We thank the CNRS and the ANR (programme blanc ANR HyBoCat
2009) for support.
ꢀ21.6 ppm)[12] and slightly downfield from those of free
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200905970.
[13]
ꢀ
monomeric aminoboranes (H2B NR2 d = 35–36 ppm).
918
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2010, 49, 918 –920

Angewandte
Chemie
ꢀ
The X-ray structure of 2a was determined at 110 K
(Figure 1 and Table 1). The quality of the measurement
allows us to be confident on the location of the hydrogen
atoms around the metal.[14] The ruthenium atom is in a
pseudo-octahedral environment with the phosphines in axial
positions. The four hydrogen atoms (H1–4) surrounding the
metal, the boron, the nitrogen, and the NH2 hydrogen atoms
increase of the B H bond distances (1.25(2) and 1.22(3) ꢀ)
[16]
ꢀ
compared to 1.15(3) and 1.18(3) ꢀ in H3B NH3.
DFT(B3PW91) calculations on the model compound
[Ru(H)2(H2BNH2)(PMe3)2] (3) were carried out to character-
ize the mode of coordination of H2BNH2. Four different
isomers were located on the potential energy surface (see the
Supporting Information, Figure S3). The most stable isomer is
ꢀ
ꢀ
are all located in the equatorial plane. The Ru B distance
(1.956(2) ꢀ) is shorter than the sum of the covalent radii
(2.09 ꢀ) and similar to the distances previously reported for
the bis(s-B H) adduct 3a, the model for 2a. The p adducts of
H2BNH2, 3c and 3d, are significantly less stable (DE =
19.0 kcalmolꢀ1, 3c; DE = 14.3 kcalmolꢀ1, 3d). The fourth
[9,15]
ꢀ
ꢀ
the bis(s-B H) borane ruthenium complexes.
The most
isomer, 3b, features both coordination of NH2 and of a s-B H
striking features are the shortening of the B N distance
(1.396(3) ꢀ) with respect to that of ammonia–borane
(1.58(2) ꢀ as determined by neutron diffraction),[16] and the
bond and lies 11.4 kcalmolꢀ1 above 3a. Therefore, the bis(s-
ꢀ
ꢀ
B H) adduct 3a corresponds to the most stable mode of
coordination of aminoborane to [Ru(H2)(PMe3)2]. Moreover,
the calculated energy variations for the reaction
[Ru(H)2(H2)2(PMe3)2] + H3BNH3!2a + 3H2 are indicative
of both an exothermic (DH = ꢀ2.5 kcalmolꢀ1) and an exer-
ꢀ1
ꢀ
gonic (DG = ꢀ15.7 kcalmol ) transformation. The s-B H
bonds thus compete efficiently with H2 for coordination to
ruthenium and dissociation of three dihydrogen molecules
entropically drives the reaction toward facile formation of 3a.
As can be seen from Table 1, the optimized geometry
(DFT/B3PW91) is in excellent agreement with the exper-
imental structure, and comparison with the free computed
ꢀ
H2BNH2 compound shows an elongation of the B H bonds as
expected. The trigonal planar geometry of both boron and
ꢀ
nitrogen atoms and the B N distance reflect the expected
ꢀ
multiple Nsp2 Bsp2 bond character in monomeric aminobor-
anes owing to the p donation of the nitrogen lone pair into the
vacant p atomic orbital on boron. It is worth noting that the
ꢀ
ꢀ
B N and B H distances are very similar to those found in two
=
agostic complexes [Cr{HBN(SiMe3)2CH CHCMe3}(CO)4]
2
ꢀ
and [RuH2{h -H B(NiPr2)CH2PPh2}(PCy3)2] as reported
ꢀ
ꢀ
recently by Braunschweig and our group (B N/B H:
1.4034(16) ꢀ/1.257 ꢀ,[17] and 1.404(9) ꢀ/1.23(7) ꢀ,[18] respec-
Figure 1. X-ray crystal structure of [RuH2(h2:h2-H2BNH2)(PCy3)2] (2a).
Ellipsoids set at 50% probability; phosphine hydrogen atoms omitted
for clarity.
tively). All these features are in full agreement with the
[19]
ꢀ
trapping of H2B NH2, the first formal elementary step of
dehydrogenation of ammonia–borane [Eq. (1)].
H3BꢀNH3 ! H2BꢀNH2 þ H2
ð1Þ
Table 1: Selected geometrical parameters for the experimental (2a, R=
Cy) and calculated structures (3a, R=Me) of [RuH2(h2:h2-H2BNH2)-
(PR3)2] and the computed structure of H2BNH2.
In conclusion, compounds 2a–c are the first examples of
ꢀ
“true” bis(s-B H) monomeric aminoborane complexes. They
ꢀ
2a
3a
H2B NH2
result from the simple metal-assisted dehydrogenation of
amine–boranes and can be regarded as intermediates in the
dehydrogenation process generally leading to polyaminobor-
anes. Remarkably, the bis-s coordination mode enables the
stabilization of monomeric aminoboranes in the coordination
sphere of a metal, including the simplest and elusive proto-
Ru–B
1.956(2)
2.3215(5)
2.3131(5)
1.396(3)
1.54(2)
1.55(2)
1.71(2)
1.73(3)
1.25(2)
1.960
2.307
2.311
1.403
1.612
1.612
1.811
1.811
1.323
1.323
Ru–P1
Ru–P2
B–N
Ru–H1
Ru–H2
Ru–H3
Ru–H4
B–H3
B–H4
1.390
ꢀ
typical H2B NH2 elementary unit.
1.198
1.198
1.22(3)
Experimental Section
2a–c: Two to four equivalents of amine–boranes 1a–c were added to a
toluene solution of [RuH2(h2-H2)2(PCy3)2] (typically 300 mg) at room
temperature, and the reaction was monitored by 31P NMR spectros-
copy until consumption of all the starting ruthenium complex was
observed. After removal of the solvent, the residue was dried under
vacuum, dissolved in pentane, and filtered. After evaporation of the
solvent, compounds 2a–c were isolated as air-sensitive white solids
P1-Ru-P2
H3-B-H6
P1-Ru-B
P2-Ru-B
Ru-B-N
155.066(19)
120.3(16)
102.90(7)
102.03(7)
179.4(2)
156.76
127.01
100.60
102.64
178.85
121.9
[a] Distances in ꢂ and angles in degrees.
Angew. Chem. Int. Ed. 2010, 49, 918 –920
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
919

Communications
(2a: 77%, 2b: 60%, 2c: 64%). In the case of 1a, the resulting toluene
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solution was filtered after reaction before evaporation of the solvent.
Selected data for 2a–c: 2a: 31P{1H} NMR (C6D6, 298 K,
161.975 MHz): d = 77.44 ppm. 11B{1H} NMR (C6D6, 298 K,
128.377 MHz): d = 46 ppm (br). 1H NMR (C6D6, 298 K,
400.130 MHz): d = 1.2–2.5 (m, 68H, NH2 + Cy), ꢀ6.80 (br s, 2H,
BH2), ꢀ11.85 ppm (t, 2H, JPH = 24.8 Hz, RuH2). IR (Nujol): n˜ = 3484
and 3394 (s, NHs,as), 1971 and 1923 (m br, RuH), 1833 and 1785 (m br,
RuHB), 1583 cmꢀ1 (s, NHbend). Elemental analysis (%) calcd for
C36H72BNP2Ru: C 62.41; H 10.48; N 2.02; found: C 62.80; H 10.34;
N 2.05.
2b: 31P{1H} NMR (C6D6, 298 K, 202.537 MHz): d = 78.50 ppm.
11B{1H} NMR (C6D6, 298 K, 160.525 MHz): d = 45.7 ppm. 1H NMR
(C6D6, 298 K, 500.330 MHz): d = 2.675 (d, 3H, 3JHH = 5 Hz, CH3), 2.48
(br, 1H, NH), 1.2–2.4 (m, 66H, Cy), ꢀ6.80 (br s, 2H, BH2),
ꢀ11.99 ppm (t, 2H, JPH = 24.4 Hz, RuH2). Elemental analysis (%)
calcd for C37H74BNP2Ru: C 62.87; H 10.55; N 1.98; found: C 63.04;
H 9.49; N 2.11.
[5] Y. Kawano, K. Yamaguchi, S. Miyake, T. Kakizawa, M. Shimoi,
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[6] During the preparation of this manuscript, we became aware of
the fact that Weller et al. had disclosed the isolation, through
1
ꢀ
four h -H B bonds, of a dinuclear rhodium complex with a
ꢀ
bridging Me3N BH3 moiety and two amine-stabilized boryl
ligands. Herein, the B H oxidative addition of Me3N BH3 at the
metal center is a progression in the process of B H activation,
which is a significant finding in the case of an inner-sphere
dehydrogenation mechanism. Personal communication and
A. B. Chaplin, A. S. Weller, Angew. Chem. 2010, DOI: 10.1002/
ange.200905185; Angew. Chem. Int. Ed. 2010, DOI: 10.1002/
ange.200905185.
ꢀ
ꢀ
2c: 31P{1H} NMR (C6D6, 298 K, 161.975 MHz): d = 79.34 ppm.
11B{1H} NMR (C6D6, 298 K, 128.377 MHz): d = 47.7 ppm (br).
1H NMR (C6D6, 298 K, 400.127 MHz): d = 2.80 (s, 6H, NMe2), 1.2–
2.5 (m, 66H, Cy), ꢀ6.89 (br s, 2H, BH2), ꢀ12.17(t, 2H, JPH = 24.6 Hz,
RuH2). Elemental analysis (%) calcd for C38H76BNP2Ru: C 63.32;
H 10.63; N 1.94; found: C 63.69; H 9.63; N 1.75.
ꢀ
CCDC 751581 (2a) contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge from
The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.
uk/data_request/cif.
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Keywords: aminoboranes · ammonia–borane · boron ·
dehydrogenation · ruthenium
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