Ruthenium complexes with cooperative PNP ligands: Bifunctional catalysts for the dehydrogenation of ammonia-borane

Article abstract of DOI:10.1002/anie.200805108

(Chemical Equation Presented) G"Ru"vy reactivity: The new ruthenium(II) complex having cooperative PNP enamido ligand A reversibly activates two equivalents H₂ under reversible hydrogenation of amido (B) and amino (C) complexes. B exhibits the

Full text of DOI:10.1002/anie.200805108

Angewandte
Chemie
DOI: 10.1002/anie.200805108
Ammonia–Borane Dehydrogenation
Ruthenium Complexes with Cooperative PNP Ligands: Bifunctional
Catalysts for the Dehydrogenation of Ammonia–Borane**
Martina Kꢀß, Anja Friedrich, Markus Drees, and Sven Schneider*
The quest for alternative energy sources has sparked consid-
erable interest in chemical hydrogen storage.[1] A promising
approach is the reversible hydrogenation and dehydrogen-
ation of small molecules, such as alcohols or ammonia–borane
[2]
ꢀ
(H3B NH3). In particular, the latter has been proposed
because of its high hydrogen to mass ratio.[3] Although
transition-metal-catalyzed homogenous hydrogenations are
widespread reactions in synthetic organic chemistry and
industrial processes, efficient catalysts for dehydrogenation
are surprisingly rare. Only recently, few homogeneous and
colloidal transition-metal catalysts were reported for the
release of up to 2.8 equivalents of H2 from ammonia–borane
under mild conditions.[4]
Scheme 1. Synthesis of amido complex 6 (PNPH =HN(CH2CH2PiPr2)2).
Bifunctional catalysts bearing cooperative amino ligands,
which are involved in the catalytic cycle via reversible
chemical transformations, were successfully introduced by
Noyori et al. for the hydrogenation and transfer hydrogena-
tion (TH) of polar double bonds.[5,6] According to the
principle of microscopic reversibility the dehydrogenation
of polar functional groups could be possible with this catalyst
class, however such bifunctional catalysts have not been
utilized for these reactions to date.[7] Herein we present new
RuII compounds having PNP amido chelate ligands, which can
undergo reversible hydrogenation/dehydrogenation reactions
both at the N functionality and the ethylene backbone. The
reactivity of the ruthenium complexes is utilized for the
homogeneous catalytic dehydrogenation of ammonia–borane
with unprecedented activities.[7]
result from the deprotonation of an intermediate hydrido-
chloro-imino complex at the acidic a position relative to the
imino group.[9] The novel ligand of enamido complex 4
represents an aliphatic analog of [RuH(CO){NC5H3-
(CHPiPr2)(CH2PiPr2)}] which was reported by Milstein et al.
to be obtained upon deprotonation of a pyridine-based PNP
pincer complex.[10] The reversibility of the ligand backbone
dehydrogenation was demonstrated by the reaction of 4 with
H2. The resulting amino complex 5 underwent the partial
elimination of H2 during workup, and could therefore not be
obtained analytically pure. However, evacuation of the solid
crude-product under dynamic vacuum at room temperature
gives analytically pure, dark red, and highly air sensitive
amido complex 6 in yields around 85% over five steps.
Whereas 6 quantitatively adds hydrogen in solution under H2
atmosphere, 6 slowly releases H2 under argon at room
temperature over several days, demonstrating that the
heterolytic H2 activation reactions shown in Scheme 2 are
trans-[RuCl2(PMe3)(PNPH)] (3) is prepared almost quan-
titatively in two steps starting from [{RuCl2(p-cymene)}2] (1)
(Scheme 1).[8] Upon reaction with 3.3 equivalents of KOtBu,
the dark green, highly air sensitive complex 4 was isolated in
95% yield. The reaction was rapid and no intermediates were
detected by using 31P NMR spectroscopy. When less than
three equivalents of KOtBu was used, only incomplete
conversion of 3 was observed. The formation of 4 can be
attributed to deprotonation of the N center of 3 and
subsequent b-hydride elimination. Finally, 4 would then
[*] M. Kꢀß, A. Friedrich, Dr. M. Drees, Dr. S. Schneider
Department Chemie, Technische Universitꢀt Mꢁnchen
Lichtenbergstraße 4, 85748 Garching b. Mꢁnchen (Germany)
Fax: (+49)892-891-3473
Scheme 2. Hydrogenation/dehydrogenation equilibria between amino
(5), amido (6), and enamido (4) complex.
E-mail: sven.schneider@ch.tum.de
[**] The authors are grateful to Prof. W. A. Herrmann for generous
support and to Dr. F. Kraus and Dr. G. Raudaschl-Sieber for
recording the powder diffractogram and MAS-11B NMR spectrum.
This work was funded within the Emmy-Noether program of the
German Research Council (DFG, SCHN950/2-1).
reversible. To the best of our knowledge, the reversible
hydrogenation/dehydrogenation of ethylene bridges in amido
chelate complexes, which are frequently used as hydrogena-
tion catalysts, has not been directly observed.[11]
The chemical shifts (d = ꢀ8.00 and d = ꢀ8.52 ppm) and
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200805108.
2J(H,P) coupling constants (2J(H,P) = 16.7–24.3 Hz) of the
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two hydride signals of 5 exclude a configuration in which the
hydrides are in a trans position to the amino or a phosphine
ligand.[11] The meridional coordination of the PNPH pincer
results from the trans-dihydride configuration. For enamido
complex 4, the hydride chemical shift (d = ꢀ31.68 ppm) and
the 2J(H,P) coupling constant for the PMe3 ligand (2J(H,P) =
47 Hz) differ significantly from amido complex 6 (d =
ꢀ24.18 ppm; 2J(H,P) = 54 Hz), suggesting considerably dif-
ferent coordination geometries. Because of their high sol-
ubility we were unable to grow single crystals of either 4 or 6,
therefore DFT models of the two complexes were generated
(Figure 1).[8] Whereas the ruthenium center of enamide 4
Figure 1. Minimized structures of 6 (left) and 4 (right) from DFT
calculations (B3LYP/6-31+G**; H atoms except hydrides and vinylene
protons are omitted). Selected bond angles of 6: N-Ru-PMe3 159.88;
N-Ru-HHydrid 120.98; Me3P-Ru-HHydrid 79.38; and 4: N-Ru-PMe3 172.68;
N-Ru-HHydrid 102.58; Me3P-Ru-HHydrid 84.68.
exhibits a slightly distorted square-pyramidal coordination
polyhedron, amido complex 6 can best be described as a
trigonal-bipyramidal having a Y-shaped distortion. The con-
siderably smaller H-Ru-PMe3 angle in 6 explains the NMR
spectroscopic results. Such Y-shaped coordination geometry
as that found in 6, are typically found for five coordinate d6-
complexes having a strong p-donating ligand.[12] Therefore,
the molecular structure of 4 can be an indication of weaker
N!Ru p interaction resulting from dehydrogenation of the
ligand backbone, indicating the delocalization of the free
electron pair of the nitrogen donor into the vinylene group.[13]
Amido complex 6 was used in the dehydrogenation of
ammonia–borane (Scheme 3).[8] Upon addition of the catalyst
Figure 2. Time course (above) and logarithmic plot (below) for ammo-
nia–borane (AB) dehydrogenation catalyzed by 6 in THF at room
temperature. Straight lines were obtained by linear regression analysis
of the data points.
The addition of Hg to the reaction affected neither the
activity nor the H2 yield. To the best of our knowledge
complex 6 marks the most active known homogeneous
catalyst for ammonia–borane dehydrogenation.[7] A powder
diffractogram, and MAS-11B NMR and IR spectra of the
precipitate are in agreement with the formation of a polymer
dehydrocoupling product (BH2NH2)n.[4i] The small amounts of
borazine detected in the reaction solution by using 11B NMR
methods explains the H2 yield of slightly over 1 equivalent.
Additional information about the mechanism was
obtained by running the reaction with deuterated substrates.
By using 0.1 mol% catalyst loadings large kinetic isotope
ꢀ
to a THF solution of H3B NH3 (0.54m) at room temperature,
ꢀ
ꢀ
effects (KIE) of 2.1 (D3B NH3), 5.2 (H3B ND3), and 8.1
ꢀ
ꢀ
(D3B ND3) were found relative to H3B NH3 dehydrogen-
Scheme 3. Catalytic dehydrogenation of ammonia–borane.
ation.[8] In comparison, KIEs of 1.7 (D3B NH3), 2.3 (H3B
ꢀ
ꢀ
ꢀ
ND3), and 3.0 (D3B ND3) were reported for ammonia–
vigorous H2 evolution was instantaneous and the formation of
a white precipitate was observed . Even with very small
catalyst loadings (0.01 mol% 6) high catalytic activities were
observed (Figure 2). Pseudo first-order rate constants of
0.013 sꢀ1 (0.1 mol% 6) and 0.0021 sꢀ1 (0.01 mol% 6) were
obtained from logarithmic plots (turnover frequency = 13–
21 sꢀ1). Whereas catalyst loadings of 0.1 mol% 6 produce
slightly more than 1 equivalent of H2, 0.01 mol% of 6
produced 0.83 equivalents of H2 (turnover number= 8300).
borane dehydrogenation using a nickel carbene complex.[4e]
The KIEs are in agreement with a concerted mechanism in
ꢀ
ꢀ
which the N H and B H bond cleavages are in the rate-
determining step, as was found for TH using Noyoriꢀs
bifunctional ruthenium catalysts.[14,15] At an early stage of
the reaction (20 mol% 6) only amino complex 5 was found by
31P NMR methods.[16] However, the slow H2 elimination from
complex 5 indicates that the spontaneous loss of H2 to
regenerate 6 cannot be of relevance for the catalytic cycle.
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Angewandte
Chemie
Chem. Soc. 2007, 129, 1844 – 1845; f) F. Cheng, H. Ma, Y. Li, J.
In conclusion we presented ruthenium(II) complexes
having PNP-pincer ligands, which can participate in the
reversible, heterolytic H2 activation with the nitrogen atom
and the ligand backbone. NMR spectroscopic results and
DFT calculations show that the novel cooperative enamido
ligand of 4 exhibits significantly different donor properties,
compared to amido analog 6. Therefore, p donation by the
amido nitrogen atom can be controlled by the reversible
chemical transformations within the pincer backbone. Amido
complex 6 shows unprecedented activity and turnover
numbers in the dehydrogenation of ammonia–borane under
mild conditions with low catalyst loadings. The present results
are in agreement with a bifunctional Noyori–Morris-type
Chen, Inorg. Chem. 2007, 46, 788 – 794; g) J. F. Fulton, J. C.
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ꢀ
[7] During the preparation of this manuscript a paper has been
published which follows the same approach. The TH catalyst
[RuCl2(R2PCH2CH2NH2)2]/KOtBu (R = iPr, tBu) exhibits com-
mechanism, having a concerted transfer of a hydride (B H)
ꢀ
and a proton (N H) from the substrate to a catalyst
species.[5,14] Therefore, our results suggest that bifunctional
catalysts could be suitable for efficient hydrogen production
from other small molecules with polar E H bonds. Initial
ꢀ
parable activities in the dehydrogenation of H3B NH3 as
complex 6. Based on the analogy to TH and DFT calculations
a bifunctional (Noyori–Morris) mechanism was proposed: N.
Blaquiere, S. Diallo-Garcia, S. I. Gorelsky, D. A. Black, K.
Fagnou, J. Am. Chem. Soc. 2008, 130, 14034 – 14035.
[8] For syntheses, characterization, catalytic protocols, and quantum
chemical details see the Supporting Information.
ꢀ
results also show that 6 exhibits good activities in the
acceptor-less dehydrogenation of alcohols.
Received: October 18, 2008
[9] V. F. Kuznetsov, K. Abdur-Rashid, A. J. Lough, D. G. Gusev, J.
Am. Chem. Soc. 2006, 128, 14388 – 14396.
Published online: December 30, 2008
[10] J. Zhang, G. Leitus, Y. Ben-David, D. Milstein, Angew. Chem.
2006, 118, 1131 – 1133; Angew. Chem. Int. Ed. 2006, 45, 1113 –
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[11] R. Abbel, K. Abdur-Rashid, M. Fraatz, A. Hadzovic, A. J.
Lough, R. H. Morris, J. Am. Chem. Soc. 2005, 127, 1870 – 1882.
[12] Y. Jean, Molecular Orbitals of Transition Metal Complexes,
Oxford University Press, Oxford, 2005.
Keywords: cooperative effects · dehydrogenation ·
p interactions · PNP ligands · ruthenium
.
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[13] The strong contribution of the vinylene carbon p-orbitals to the
ꢀ
Ru N p bond (HOMO, HOMO-2), confirm this interpretation:
see the Supporting Information.
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[16] 5 could alternatively be formed by heterolytic addition of
released H2 to 6 and is therefore not necessarily involved in the
catalytic cycle.
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