A Ru-η6-arene complex as a C-based Lewis acid in the activation of hydrogen and hydrogenation catalysis

Article abstract of DOI:10.1021/ja403912n

The PBP ligand (Ph₂PC₆H₄)₂BPh was used to prepare ((Ph₂PC₆H₄) ₂B(Cl)(η⁶-Ph)RuCl (1) and subsequently [((Ph ₂PC₆H₄)₂B(η⁶-Ph)) RuCl][B(C₆F₅)₄] (2). The latter species exhibited Lewis acidity on the η⁶-Ph ring, as reaction with Cy₃P gave the donor-acceptor adduct [(Ph₂PC ₆H₄)₂B(η⁵-C₆H ₅-o-PCy₃)RuCl][B(C₆F₅)₄] (3). Steric frustration of this binding was seen with Mes₃P, and yet the combination of 2 and Mes₃P reacted with H₂ to give a 2:1 mixture of 5-o and 5-p, two isomers of [(Ph₂PC₆H ₄)₂B(η⁵-C₆H₆)RuCl] (5), along with [Mes₃PH][HB(C₆F₅)₃]. Compound 2 behaves as C-based Lewis acid and thus can also be used for catalytic hydrogenation of aldimines at room temperature via a frustrated Lewis pair mechanism.

Full text of DOI:10.1021/ja403912n

Communication
pubs.acs.org/JACS
A Ru−η6‑Arene Complex as a C‑Based Lewis Acid in the Activation of
Hydrogen and Hydrogenation Catalysis
Michael P. Boone and Douglas W. Stephan*
Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario, Canada M5S 3H6
S
* Supporting Information
The species (Ph2PC6H4)2BPh reacted with (Ph3P)3RuCl2 in
ABSTRACT: The PBP ligand (Ph2PC6H4)2BPh was used
to prepare ((Ph2PC6H4)2B(Cl)(η6-Ph)RuCl (1) and
subsequently [((Ph2PC6H4)2B(η6-Ph))RuCl][B(C6F5)4]
(2). The latter species exhibited Lewis acidity on the η6-
Ph ring, as reaction with Cy3P gave the donor−acceptor
adduct [(Ph2PC6H4)2B(η5-C6H5-o-PCy3)RuCl][B(C6F5)4]
(3). Steric frustration of this binding was seen with Mes3P,
and yet the combination of 2 and Mes3P reacted with H2
to give a 2:1 mixture of 5-o and 5-p, two isomers of
[(Ph2PC6H4)2B(η5-C6H6)RuCl] (5), along with
[Mes3PH][HB(C6F5)3]. Compound 2 behaves as C-
based Lewis acid and thus can also be used for catalytic
hydrogenation of aldimines at room temperature via a
frustrated Lewis pair mechanism.
CH2Cl2 to give an orange solution that upon isolation yielded
the zwitterion ((Ph2PC6H4)2BCl)(η6-Ph)RuCl (1) in 76%
yield. The 31P{1H} spectrum of 1 displayed a low-field
resonance at 26.4 ppm, while the 11B NMR spectrum exhibited
1
a resonance at 1.5 ppm. The H NMR spectrum showed
resonances at 6.17, 6.05, and 4.65 ppm, suggesting an arene−
metal interaction. The X-ray crystallographic data for 1
confirmed this coordination mode (see the Supporting
Information), in which the boron atom adopts a pseudo-
tetrahedral geometry and the PBP ligand binds to Ru via the
two phosphorus donors and an η6 interaction of the boron-
bound phenyl group. The present binding mode of the PBP
ligand in 1 results in a Ru−B distance of 3.221(5) Å. Earlier this
year, Peters and co-workers described similar η6 binding of the
B-bound phenyl ring to Fe(0) species.24 This zwitterionic
compound is reminiscent of ambiphilic ligand complexes of Au,
Ni, Pd, Pt, and Ru described by the Bourissou25 and Tilley26
groups as well as our group.27 In addition, it is noteworthy that
PBP ligands have also been shown by several groups to exhibit
z-type binding,28−39 while related compounds displaying B−
H−M interactions for Rh40 and Ni41 species have also been
observed.
he advent of “frustrated Lewis pairs” (FLPs) in 2006 has
T
enabled the heterolytic activation of H2 by nonmetal
systems.1 Such systems can be exploited to activate H2 and
deliver H− and H+ to a substrate, thereby effecting catalytic
hydrogenation.2−9 While much effort has focused on the range
of substrates to which this strategy can be applied, the range of
Lewis bases and acids used in work to date has largely been
limited to group-15 (P, N)/group-13 (B, Al) combina-
tions.10−16 The use of group-14 species in related chemistry
has drawn less attention. In 2007, Bertrand and co-workers17
reported the reaction of monoamino carbenes with H2, and
computations supported the role of the carbene carbon as both
a Lewis acid and a Lewis base. Our group18 and that of Tamm19
have described the use of carbenes as Lewis bases in FLP H2
activation. Efforts to exploit carbon-based Lewis acids have also
been described. In a highly creative effort by Alcarazo and co-
workers,20,21 the use of electron-deficient allenes to effect the
heterolytic cleavage of disulfides was presented. Although
The reaction of 1 with K[B(C6F5)4] resulted in halide
abstraction, affording the red product [((Ph2PC6H4)2B(η6-
Ph))RuCl][B(C6F5)4] (2) in 88% yield (Scheme 1).
Compound 2 exhibited a 31P{1H} resonance at 32.8 ppm and
11B resonances at 48.0 and −16.6 ppm. These data together
Scheme 1. Synthesis and Reactivity of 2 and 3
Muller and co-workers22 reported the activation of H2 by
̈
silylium cation/phosphine combinations, the corresponding
reactions of trityl cations with phosphine resulted in phosphine
substitution at the aryl carbon para to the central carbon of the
cation. In 2011, Arduengo and co-workers23 reported the
activation of H2 by trityl cation and a bulky di-tert-butyl
carbene, demonstrating the first example of the use of a C-
based Lewis acid for H2 activation. In this work, we exploited an
ancillary metal center to enhance the Lewis acidity of aromatic
carbons, generating a C-based Lewis acid that can participate in
FLP activation of H2 as well as act as a C-based hydrogenation
catalyst for the reduction of aldimines.
Received: April 19, 2013
© XXXX American Chemical Society
A
dx.doi.org/10.1021/ja403912n | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
The formation of 3 is perhaps surprising in view of the
presence of the trigonal-planar boron center in 2. However, this
experimental observation is consistent with the computed
lowest unoccupied molecular orbital (LUMO) of 2 (Figure 1b),
in which the major components reside on the ortho carbons of
the B-bound η6 phenyl ring, with minor components on the
boron and para carbon atoms.
The reaction of 3 with [Ph3C][B(C6F5)4] resulted in the
capture of phosphine, affording the known species [p-
Cy3P(C6H5)CPh2][B(C6F5)4]42 and regenerating 2. This
finding demonstrates that the formation of the donor−acceptor
adduct 3 is reversible, suggesting that an equilibrium governs
the formation of 3. In probing this aspect, 3 was subjected to 4
atm H2 at 45 °C for 72 h. This resulted in the formation of two
new products, 4 and 5 (Scheme 2). The 11B NMR spectrum
Figure 1. (a) POV-ray depiction of the cation of 2. H atoms have been
omitted for clarity. Atom colors: Ru, teal; P, orange; B, yellow-green;
Cl, green; C, black. (b) Depiction of the LUMO of the cation of 2.
with the X-ray data for 2 (Figure 1a) confirmed the removal of
the Cl atom from B of the PBP ligand, resulting in a Ru−B
distance of 2.647(2) Å. Despite this approach, the sum of the
angles about boron is 360.0°. This Ru−B distance in 2 is longer
than the formal Rh−B bond length of 2.306(3) Å reported for
[((Ph2PC6H4)2BPh)RhCl]2.30
Scheme 2. Reactions of 3 with H2 and 2 + PMes3 with D2
In stark contrast to the related Ni and Rh species,34−40 no
reaction occurred upon exposure of 2 to H2. However,
compound 2 did react instantaneously with Cy3P, resulting in
a color change. Subsequent workup afforded a yellow crystalline
product, [(Ph2PC6H4)2B(η5-C6H5-o-PCy3)RuCl][B(C6F5)4]
(3), in 92% yield (Scheme 1). The 31P{1H} NMR spectrum
revealed three doublets of doublets at 40.4 ppm (2JPP = 25 Hz,
4
4JPP = 7 Hz), 34.1 ppm (2JPP = 25 Hz, JPP = 9 Hz), and 23.1
4
ppm (4JPP = 9 Hz, JPP = 7 Hz), consistent with three
inequivalent and coupled phosphorus environments. The 11B
NMR spectrum showed a resonance at 35.0 ppm, inferring
retention of the three-coordinate B center in the ligand
backbone. In addition, a resonance appeared at −16.6 ppm,
contained resonances at 34.8 and −16.6 ppm. The latter peak,
together with the 31P{1H} NMR singlet at 33.5 ppm (1JPH
=
444 Hz) was consistent with the presence of [Cy3PH][B-
(C6F5)4] (4). In addition, the 31P{1H} NMR spectrum also
showed doublets at 42.3 ppm (2JPP = 22 Hz) and 39.3 ppm
(2JPP = 22 Hz) as well as a peak at 50.2 ppm. These data infer
heterolytic cleavage of H2 to afford the cation of 4 and hydride
delivery to the arene ring to give the species [(Ph2PC6H4)2B-
(η5-C6H6)RuCl] (5). The 1H NMR data revealed the presence
of two isomers of 5. Resonances at 5.20, 5.04, 4.05, 2.59, 1.90,
and 1.77 ppm were consistent with hydride addition to one of
the ortho carbons to afford 5-o, while signals at 3.97, 3.22, 3.01,
and 2.70 ppm infer hydride addition at the para position to give
5-p. Interestingly, the ratio of these two isomers varied with
reaction stoichiometry. A 1:1 Ru:P ratio afforded a mixture with
5-o:5-p = 2:1. Altering the initial Ru:P ratio to 1:10 resulted in
almost exclusively the 5-p isomer, although in this case the
reaction was much slower, taking 1 week at room temperature
to go to completion. Monitoring this mixture over time showed
no change in this ratio, demonstrating that there is a
thermodynamic barrier to interconversion of these isomers.
The analogous combination of 2 with the phosphine Mes3P
resulted in no reaction. This FLP reacted upon exposure of the
mixture to 4 atm H2 at room temperature for 4 h. The reaction
proceeded in a fashion similar to that for 3, affording two
products: the phosphonium salt [Mes3PH][B(C6F5)4] (6) and
a mixture of the Ru complexes 5-p and 5-o in a ratio of 2:1, as
identified by NMR data. Exhaustive efforts to grow crystals of 5
were unsuccessful, but exchange of the chloride on Ru in 5-o to
bromide via reaction with Me3SiBr gave crystals of 5-o-Br. X-
ray diffraction studies (Figure 3) confirmed the formulation and
demonstrated the hydride delivery to the carbon ortho to the
planar boron atom. While related complexes in which hydride is
−
consistent with the presence of the B(C6F5)4 counteranion.
These data and a crystallographic study confirmed that 3
resulted from phosphine attack at a phenyl carbon ortho to
boron (Figure 2); quaternization of C2 resulted in typical P−C
and C−C single bonds, while the other C−C distances within
the B-bound arene ring remained shorter [1.388(4)−1.444(4)
Å], consistent with η5 binding to Ru. Interestingly, the Ru−B
bond shortened significantly to 2.509(4) Å, reflecting the
increase in electron density at Ru.
Figure 2. POV-ray depiction of the cation of 3. H atoms except for
those on the η5-bound arene ring have been omitted for clarity. Atom
colors: Ru, teal; P, orange; B, yellow-green; Cl, green; C, black.
B
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Journal of the American Chemical Society
Communication
Table 1. Hydrogenation of Imines Using 2
Figure 3. POV-ray depiction of 5-o-Br. H atoms except for those on
the η5-bound arene ring have been omitted for clarity. Atom colors:
Ru, teal; P, orange; B, yellow-green; Br, red; C, black; H, white.
a
b
4 atm H2. NMR yields.
amine in 2 h. Reducing the catalyst loading slowed the
conversion (Table 1). In a similar fashion, PhCHNPh was
reduced to PhCH2NHPh under 102 atm H2 using 5 mol % 2 in
4 h (Table 1). The slower reduction of the latter imine is
attributed to its lower basicity. It is noteworthy that NMR
spectra of the reaction mixtures following reduction showed
evidence of only the products and the species 5-o, 5-p, and 2.
This is consistent with a proposed FLP-type hydrogenation
mechanism in which the imine and 2 act as an FLP to split
hydrogen, with subsequent hydride delivery to the generated
iminium cation (Scheme 3).
added to an aromatic fragment have been described, such as
[Ru(Me3tacn)(η6-C6H6)](OTf)2,43 the present reaction repre-
sents the first in which such a species is derived from the
activation of H2 and not harsh hydride sources such as
[Li][HBEt3] or LiAlH4.44−47
The corresponding reaction of 2 and Mes3P with D2 resulted
in the formation of the analogous products 5-d1 and 6-d1
1
2
(Scheme 2). In the case of 5-d1, the H, H and correlation
spectroscopy (COSY) NMR data confirmed the exclusive
formation of the isotopomer in which the deuteride occupies
the exo position on the methylene carbon of the six-membered-
ring substituent of B that is η5-bound to Ru. These data are
contrary to an intramolecular mechanism involving hydride
delivery from Ru. Preliminary kinetic data were found to be
consistent with a rate law that is first order in Ru, thus also
eliminating a bimolecular mechanism involving hydride delivery
from Ru. Thus, this heterolytic cleavage of H2 is proposed to
proceed by the action of the phosphine and the Lewis acidic
carbon in an “FLP-type” reaction. While C-based Lewis acids
have been described in FLP activations of disulfides and H2
with the trityl cation or allenes and bulky carbenes, this is the
first FLP system to effect H2 activation by means of an aromatic
C-based Lewis acid.
Scheme 3. Proposed Mechanism for the Catalytic Reduction
of Imines Using 2 and H2 (Only the 5-o Isomer of the
Intemediate Is Shown)
Density functional theory calculations were performed on 5-
o and 5-p. The energy difference is small, favoring 5-o by only
2.5 kcal/mol. In addition, the energy of ((Ph2PC6H4)2B(H)(η6-
Ph))RuCl (7) in which the hydride is located on the boron
center was computed, and 7 was found to be ∼18 kcal/mol
higher in energy than 5-o or 5-p. These data further support the
description of 3 as a C-based Lewis acid in which the “ancillary”
metal center enhances the Lewis acidity of the aromatic
carbons.
Treatment of the mixture of 5-o and 5-p with [Ph3C][B-
(C6F5)4] resulted in hydride abstraction from the aromatic ring
and regeneration of 2. This prompted an examination of 5-o
and 5-p as hydride sources for the hydrogenation of aldimines.
Mixing a stoichiometric amount of 2 with N-benzylidene-tert-
butylamine (PhCHNtBu) resulted in no reaction. However,
exposure of the mixture to H2 resulted in the formation of 5-o
and 5-p along with the amine PhCH2NHtBu. This reduction
could also be effected in a catalytic fashion. With 10 mol % 2
under 4 atm H2, the hydrogenation of PhCHNtBu was
sluggish but reached completion in 144 h at 45 °C (Table 1).
Using a higher H2 pressure (102 atm) and 5 mol % 2 greatly
accelerated the reduction of PhCHNtBu at room temper-
ature, resulting in quantitative reduction to the corresponding
In summary, coordination of an arylborane to Ru and
generation of the Ru cation in a coordinatively saturated
complex, 2, that is highly Lewis acidic at the ortho and para
carbon centers has been described. The sterically less
demanding phosphine PCy3 reacted with 2 to form the Lewis
acid−base adduct 3 via coordination to the ortho carbon; this
adduct formation was found to be reversible. Also, 3 reacted
further with H2 to effect heterolytic splitting of H2. While
mixing the bulkier phosphine PMes3 with 2 resulted in no
adduct, this combination behaved as an FLP to effect H2
splitting with hydride delivery to either the ortho or para
position of the coordinated arene. This system is a unique
example in which an aryl carbon center acts as a Lewis acid in
the FLP activation of H2 and stands in contrast to the
mechanism of hydrogenation described for Ni−PBP species.40
Finally, 2 was used for catalytic hydrogenation of imines under
H2 at room temperature with catalyst loadings as low as 1 mol
%. The further reactivity of such unusual systems in which
C
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Journal of the American Chemical Society
Communication
(26) Fischbach, A.; Bazinet, P. R.; Waterman, R.; Tilley, T. D.
Organometallics 2008, 27, 1135.
(27) Granville, S. L.; Welch, G. C.; Stephan, D. W. Inorg. Chem.
metals act as ancillary groups to enhance Lewis acidity are
currently the subject of ongoing studies.
2012, 51, 4711.
ASSOCIATED CONTENT
* Supporting Information
Synthetic, spectroscopic, and crystallographic data. This
material is available free of charge via the Internet at http://
pubs.acs.org.
■
(28) Bontemps, S.; Bouhadir, G.; Gu, W.; Mercy, M.; Chen, C. H.;
Foxman, B. M.; Maron, L.; Ozerov, O. V.; Bourissou, D. Angew. Chem.,
Int. Ed. 2008, 47, 1481.
(29) Bontemps, S.; Sircoglou, M.; Bouhadir, G.; Puschmann, H.;
Howard, J. A. K.; Dyer, P. W.; Miqueu, K.; Bourissou, D. Chem.Eur.
J. 2008, 14, 731.
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(30) Bontemps, S. B.; Gornitzka, H.; Bouhadir, G.; Miqueu, K.;
Bourissou, D. Angew. Chem., Int. Ed. 2006, 45, 1611.
(31) Sircoglou, M.; Bontemps, S.; Mercy, M.; Saffon, N.; Takahashi,
M.; Bouhadir, G.; Maron, L.; Bourissou, D. Angew. Chem., Int. Ed.
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AUTHOR INFORMATION
Corresponding Author
dstephan@chem.utoronto.ca
■
Notes
(32) Bontemps, S.; Bouhadir, G.; Miqueu, K.; Bourissou, D. J. Am.
Chem. Soc. 2006, 128, 12056.
The authors declare no competing financial interest.
(33) Sircoglou, M.; Bontemps, S.; Bouhadir, G.; Saffon, N.; Miqueu,
K.; Gu, W.; Mercy, M.; Chen, C.-H.; Foxman, B. M.; Maron, L.;
Ozerov, O. V.; Bourissou, D. J. Am. Chem. Soc. 2008, 130, 16729.
(34) Sircoglou, M.; Bontemps, S.; Mercy, M.; Miqueu, K.; Ladeira, S.;
Saffon, N.; Maron, L.; Bouhadir, G.; Bourissou, D. Inorg. Chem. 2010,
49, 3983.
ACKNOWLEDGMENTS
■
D.W.S. gratefully acknowledges financial support by the
NSERC of Canada through the award of a Canada Research
Chair.
(35) Derrah, E. J.; Sircoglou, M.; Mercy, M.; Ladeira, S.; Bouhadir,
G.; Miqueu, K.; Maron, L.; Bourissou, D. Organometallics 2011, 30,
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