A cis-bis-mixed-carbene ruthenium hydride complex: An olefin-selective hydrogenation catalyst

Article abstract of DOI:10.1021/om300014r

The N-heterocyclic carbene C₃H₂N₂(CH ₂CH₂OMe)₂ (1; IOMe) reacts with RuHCl(PPh ₃)₃ to give RuClH(IOMe)(PPh₃)₂ (2), which reacts further with SIMes to give the cis-bis-mixed-carbene complex RuClH(IOMe)(SIMes)(PPh₃) (3). This species has been shown to be a highly effective hydrogenation catalyst that tolerates the presence of a wide range of functional and donor groups.

Full text of DOI:10.1021/om300014r

Communication
pubs.acs.org/Organometallics
A Cis-Bis-Mixed-Carbene Ruthenium Hydride Complex: An Olefin-
Selective Hydrogenation Catalyst
Clinton L. Lund, Michael J. Sgro, Renan Cariou, and Douglas W. Stephan*
Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario, Canada M5S 3H6
S
* Supporting Information
ABSTRACT: The N-heterocyclic carbene C3H2N2(CH2CH2OMe)2 (1; IOMe)
reacts with RuHCl(PPh3)3 to give RuClH(IOMe)(PPh3)2 (2), which reacts
further with SIMes to give the cis-bis-mixed-carbene complex RuClH(IOMe)-
(SIMes)(PPh3) (3). This species has been shown to be a highly effective
hydrogenation catalyst that tolerates the presence of a wide range of functional
and donor groups.
eterogeneous hydrogenation catalysis was discovered by
Sabatier in the early portion of the 20th century.1 In the
hydrogenation catalysts for styrene.23 On the basis of this and
the notion that mimicking the ligand disposition of the active
species derived from Wilkinson’s catalyst could lead to a
general hydrogenation catalyst for olefinic residues, we targeted
a cis-bis-NHC Ru hydride complex. However, it was noted that
most common bis-carbene Ru species adopt a trans-NHC
geometry.24−34 Cis coordination of carbenes to Ru can be
imposed using chelating bis-carbene23,35,36 and bis-pincer
carbene ligands,37−44 as well as tetrakis-carbene45−47 and
polyoxometalate derivatives.48 Generally, the challenge of
preparing cis-bis-carbene complexes is related to the steric
demands of the N-bound substituents. Dixneuf and co-workers
were able to overcome this, preparing cis-carbene derivatives of
a Ru carborane using the sterically unencumbered carbene
C3Me4N2.49 In a related fashion, the Whittlesey group has
studied a series of Ru cis-bis-carbene complexes employing
NHCs with N-bound isopropyl50 or cyclohexyl51 substituents.
Herein, we report a new strategy to a cis-bis-carbene hydride
complex and demonstrate that the species is an active
hydrogenation catalyst for the selective hydrogenation of olefin
residues in the presence of a wide variety of functional groups.
Our plan began with the idea of accessing a cis-bis-carbene
hydride complex via sequential addition of NHC ligands.
However, we noted Morris et al. had reported that reaction of
RuHCl(PPh3)3 with an NHC prompted C−H activation of one
of the NHC substituents.52 While this C−H activation was
avoided using the precursor RuHCl(CO)(PPh3)3, the product
was coordinatively saturated.19,20,53 Thus, we developed a
strategy exploiting a pendant labile donor to access a
monocarbene Ru hydride synthon. To this end, a precursor
to an NHC with pendant ether donors has been readily and
quantitatively prepared by refluxing a 2:1 mixture of chloroethyl
methyl ether with trimethylsilylimidazole. The isolated
imidazolium salt is subsequently treated with Ag2O to produce
the corresponding AgCl adduct of the NHC
H
1960s Wilkinson and co-workers discovered the homogeneous
hydrogenation catalyst precursor (Ph3P)3RhCl.2 These two
findings have been seminal to countless developments and
applications. Indeed, hydrogenation catalysis is the most
common transformation used in the chemical industry and is
employed in the preparation of scores of commercial targets,
including natural products and commodity and fine chemicals.3
A plethora of homogeneous catalysts have emerged since
Wilkinson’s reports. In the 1970s, Osborn and Schrock4−6 and
subsequently Crabtree and Morris7 reported the cationic
precatalysts [(COD)Rh(Ph2PCH2CH2PPh2)]+ and [(COD)Ir-
(py)(PCy3)]+, respectively. These species have also become
commonly used precursors for homogeneous hydrogenations.
While perturbations of the Rh and Ir systems for asymmetric
catalysis have been widely studied and highly successful over
the last 30 years,8 use of other transition-metal complexes has
also garnered attention. In particular, the recent work of Chirik
and co-workers9 has demonstrated the utility of Fe-based
systems for olefin hydrogenation.
In the case of Ru, the species RuHCl(PPh3)3 was reported by
Wilkinson and co-workers to be almost as active for catalytic
olefin hydrogenation catalysis as (Ph3P)3RhCl;10 however, due
to its high air sensitivity, the Ru species has not found
widespread use. Nonetheless, over the past two decades Ru-
based hydrogenation catalysts have emerged as powerful tools
for asymmetric hydrogenation of ketones and other polar
functional groups.11−18 Interestingly, it is such reactivity that
has precluded Ru hydrogenation catalysts from applications
where selectivity for olefinic residues is required.
In our efforts to develop Ru-based, olefin-selective hydro-
genation catalysts, we were stimulated by the recent findings of
the groups of Nolan and Fogg and others who showed
compounds of the form RuHCl(CO)(NHC)(PPh3) were
effective catalysts for the hydrogenation of cyclooctene and
cyclododecene and the isomerization of terminal olefins.19−22
In addition, Albrecht and co-workers demonstrated the ability
of chelating bis-carbene Ru complex cations to act as effective
Received: January 6, 2012
Published: January 18, 2012
© 2012 American Chemical Society
802
dx.doi.org/10.1021/om300014r | Organometallics 2012, 31, 802−805

Organometallics
Communication
1
C3H2N2(CH2CH2OMe)2 (=IOMe), IOMeAgCl (1). Reaction
of 1 with RuHCl(PPh3)3 and the ensuing workup afforded the
air-sensitive bright yellow precipitate 2 (Scheme 1). The
red. After workup, dark red crystals of 3 were isolated. The H
NMR spectrum of 3 revealed a doublet at −28.87 ppm
indicative of a hydride coupled to a single phosphine ligand.
The upfield shift is also consistent with the absence of a ligand
trans to the hydride. In addition, the ipso carbon shifts of the
two NHCs were observed at 189.0 and 227.6 ppm for the
IOMe and SIMes NHCs, respectively. Single-crystal X-ray
analysis of 3 revealed a five-coordinate square-pyramidal
ruthenium where the two NHCs, chloride, and phosphine
form the base of the pyramid and the hydride occupies the
apex: thus, the formulation as RuClH(SIMes)(IOMe)(PPh3)
(Figure 2). The Ru−C distances for SIMes and IOMe were
Scheme 1. Synthesis of 2 and 3
appearance of a triplet at −23.54 ppm in the 1H NMR
spectrum and a singlet at 44.61 ppm in the 31P NMR spectrum
of 2 were consistent with a hydride coupled to two phosphine
1
ligands on Ru. H and 13C NMR spectra also confirmed the
presence of the NHC. Single-crystal X-ray analysis of 2 revealed
it to be RuClH(IOMe)(PPh3)2, in which the Ru adopts a
pseudo-octahedral coordination sphere comprised of two trans
phosphine ligands, a chloride trans to the carbene donor, and a
hydride trans to a coordinated ether arm of the NHC ligand
(Figure 1). The Ru−P distances average 2.324(1) Å, while the
Figure 2. POV-ray depiction of 3: C, black; O, red; Cl, green; P,
orange; N, blue-green; Ru, salmon; H, gray. All hydrogen atoms except
the hydride are omitted for clarity.
found to be 1.967(1) and 2.069(1) Å. The trans influence of
these carbene ligands is reflected in the elongated Ru−P and
Ru−Cl distances of 2.3007(4) and 2.4560(4) Å, respectively.
The Ru−H distance was determined to be 1.47(2) Å.
Interestingly, despite the fact that the ethereal oxygen atoms
of the IOMe ligand do not coordinate to Ru, the 16-electron
compound 3 is air-stable. This stands in marked contrast to the
case for 2. The cis diposition of the carbene ligands in 3 results
in a C−Ru−C angle of 91.44(5)°. To our knowledge, this
represents the first Ru cis-bis-mixed-carbene complex. It is the
use of the IOMe ligand that permits the incorporation of SIMes
in 3, while the mismatch of the steric conflict between the
carbene substituents allows a cis orientation.
The utility of 3 as a hydrogenation catalyst was investigated
(Table 1). Use of 5 mol % of 3 as a catalyst for the
hydrogenation of 1-hexene under 4 atm of H2 was undertaken.
Monitoring this reaction revealed that initially olefin isomer-
ization of 1-hexene to 2-hexene proceeds more quickly than
reduction. For example, after 4 h, the product mixture was seen
to be a mixture of 2-hexene and hexane in a 32:68 ratio, as
determined by NMR spectroscopy. Nonetheless, quantitative
reduction of the olefin to the alkane was observed in 8 h at 45
°C. In a similar fashion, styrene was quantitatively reduced in 6
h. The disubstituted olefins cyclohexene and 2-methyl-2-butene
were reduced in 100 and 55% yields after 24 and 96 h,
respectively, while the tetrasubstituted olefin 2,3-dimethyl-2-
butene was not reduced even on prolonged exposure to catalyst
and H2. The corresponding reaction of phenylacetylene results
in quantitative reduction to ethylbenzene in 8 h. Interestingly,
Figure 1. POV-ray depiction of 2: C, black; O, red; Cl, green; P,
orange; N, blue-green; Ru, salmon; H, gray. All hydrogen atoms except
the hydride are omitted for clarity.
Ru−C and Ru−Cl distances are found to be 1.990(4) and
2.475(1) Å. The Ru−O distance of 2.369(3) Å reflects a weak
bonding interaction, while the Ru−H distance was determined
to be 1.58(4) Å. The weak Ru−O bond together with the
sterically small NHC substituents presumably account for the
striking sensitivity of 2 to air. Even under the cover of Paratone
oil, crystals of 2 turn black within minutes upon exposure to the
atmosphere at room temperature.
Subsequent reaction of compound 2 with 1 equiv of the
Lewis base 1,3-bis(2,4,6-trimethylphenyl)-2-imidazolidine
(SIMes) produced an immediate color change from yellow to
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Organometallics
Communication
a
Table 1. Hydrogenation Catalysis with 3
substrate
T (h)
product
yield (%)
1-hexene
8
6
hexane
100
100
100
55
styrene
ethylbenzene
cyclohexane
cyclohexene
72
144
8
2-methyl-2-butene
phenylacetylene
acrylonitrile
2-methylbutane
ethylbenzene
100
b
8
propionitrile
88
b
allylamine
8
propylamine
93
allyl alcohol
4
propanol
100
b
phenyl vinyl sulfone
2-vinylpyridine
1-vinylimidazole
acrylaldehyde
8
ethyl phenyl sulfone
2-ethylpyridine
85
4
100
b
8
1-ethylimidazole
propionaldehyde
2-butanone
95
8
100
100
3-buten-2-one
4
b
phenyl vinyl thioether
methyl 3-butenoate
dimethyl 2-methylenesuccinate
8
ethyl phenyl thioether
methyl butyrate
dimethyl 2-methylsuccinate
methyl 2-(acetylamino)propanoate
98
b
4
92
b
8
79
b
methyl 2-(acetamido)acrylate
8
59
a
Conditions: 0.100 mmol of substrate and 5 mol % of catalyst in CD2Cl2 or C6D5Br, 45 °C, 4 atm of H2. Yields were determined by NMR
b
spectroscopy. Quantitative reduction was observed in 24 h.
monitoring the reaction by NMR spectroscopy reveals a 50:50
product ratio of styrene and ethylbenzene after 4 h.
bis-mixed-carbene Ru hydride complex. This species has been
shown to be a highly effective hydrogenation catalyst that
tolerates the presence of a wide range of functional and donor
groups. The study of the mechanism of action of 3 and its
derivatives and the applications of selective olefin reduction in
synthetic chemistry continues to be a focus in our laboratories.
The ability of 3 to tolerate the presence of functional and
donor groups was also probed. Employing the standard
conditions of 4 atm of H2 and 45 °C, the olefinic residues in
acrylonitrile, allylamine, allyl alcohol, phenyl vinyl sulfone, 2-
vinylpyridine, 1-vinylimidazole, acrylaldehyde, 3-buten-2-one,
phenyl vinyl thioether, 3-methylbuten-2-one, 2-methylenesuc-
cinate, and methyl-2-(acetamido)acrylate were reduced. In all
these cases, quantitative reduction was seen in 24 h, although
some were complete in as little as 4 h. In addition,
hydrogenation occurred exclusively at the olefinic residue, as
the functional groups were unaffected. In this regard, prolonged
exposure of acetophenone or N-tert-butylphenylimine to the
catalyst 3 and H2 at elevated temperature led to no reaction,
further demonstrating the selectivity for olefin reduction.
Preliminary examination of alternate conditions showed that
3-methylbuten-2-one is hydrogenated to 2-butanone at 45 °C
in C6H5Br using 0.1 mol % of the catalyst 3 at 50 bar of H2
pressure in just 30 min. This corresponds to a TON of 1000
and a TOF of 2000 mol/h. This suggests that much lower
catalyst loadings are capable of high turnover frequencies when
higher pressures of H2 are used.
Despite the fact that 3 is formally a 16-electron species, it
does not react with excess pyridine. This is consistent with the
functional group tolerance 3 exhibits in hydrogenation catalysis.
The mechanism of hydrogenation is presumed to involve a
ligand rearrangement to provide for binding olefin adjacent to
the hydride, leading to subsequent insertion. Interestingly the
rate of hydrogenation of hexene is unaffected by the presence
of a 5-fold excess of PPh3. These data suggest that phosphine
dissociation is not involved in the mechanism. It is also
interesting to speculate on the role of the pendant ether arms of
IOMe. These donors could act to stabilize transiently generated
unsaturated species. The precise mechanistic details are under
study.
ASSOCIATED CONTENT
* Supporting Information
■
S
Text, tables, and CIF files giving synthetic, experimental, and
crystallographic details. This material is available free of charge
via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: dstephan@chem.utoronto.ca.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The financial support of LANXESS Inc., the NSERC of
Canada, and the Ontario Centres of Excellence is gratefully
acknowledged. M.J.S. is grateful for the award of a NSERC
postgraduate scholarship. D.W.S. is grateful for the award of a
Killam Research Fellowship (2009−2011) and a Canada
Research Chair.
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