The hydrogenation of molecules with polar bonds catalyzed by a ruthenium(ii) complex bearing a chelating N-heterocyclic carbene with a primary amine donor

Article abstract of DOI:10.1039/c0cc02664f

The complex [RuCp*(C-NH₂)(py)]PF₆ bearing a chelating N-heterocyclic carbene with an NH₂ group (C-NH₂) is an active catalyst for the H₂-hydrogenation of ketones, an epoxide, ester and ketimine in basic solution. A maximum turnover frequency of 17600 h^-1 is achieved under mild reaction conditions (25°C) and economical use of hydrogen (8 bar) while the TOF of a related complex with a phosphine-amine ligand is much smaller.

Full text of DOI:10.1039/c0cc02664f

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The hydrogenation of molecules with polar bonds catalyzed by a
ruthenium(II) complex bearing a chelating N-heterocyclic carbene
with a primary amine donorwz
Wylie W. N. O, Alan J. Lough and Robert H. Morris*
Received 19th July 2010, Accepted 3rd September 2010
DOI: 10.1039/c0cc02664f
The complex [RuCp*(C–NH2)(py)]PF6 bearing a chelating
N-heterocyclic carbene with an NH2 group (C–NH2) is an active
catalyst for the H2-hydrogenation of ketones, an epoxide, ester
and ketimine in basic solution. A maximum turnover frequency
of 17 600 hꢀ1 is achieved under mild reaction conditions (25 1C)
and economical use of hydrogen (8 bar) while the TOF of a
related complex with a phosphine–amine ligand is much smaller.
(Fig. 1).9,10 Complex 1b has good activity in the transfer
hydrogenation of acetophenone in 2-propanol.9 This encouraging
result prompted us to design other ruthenium(II)-based catalyst
systems with such a chelating C–NH2 ligand. Herein we
present the synthesis and catalytic activity of the ruthenium(II)
complex 2 bearing a C–NH2 and an electron rich penta-
methylcyclopentadienyl ligand (Cp*). The P–NH2 analogue
3a was also prepared for comparison. The related complex
RuCp*(k2-PPh2CH2CH2NH2)Cl (3b), was prepared in situ by
Ikariya and co-workers.6a,11
A powerful concept in the design of homogeneous hydrogenation
catalysts is the heterolytic splitting of dihydrogen1 across a
transition metal–amido bond.2 This provides a bifunctional
metal–hydride and protic amine grouping for the efficient,
selective reduction of polar bonds to produce valuable alcohols
and amines.2 Ruthenium(II) complexes bearing a phosphine–
amine (P–NH2) ligand that catalyze efficiently the hydrogenation
of ketones,3 imines,4 esters,5 and other polar bonds6 may
utilize this mechanism.
We previously showed that the transfer of the C–NH2
ligand from 1a to [Ru(p-cymene)Cl2]2 in acetonitrile yielded
1b in moderate yield.9 Thus, a transmetalation reaction of 1a
and 1.5 equiv. of RuCp*(cod)Cl (cod = 1,5-cyclooctadiene) in
acetonitrile, and subsequent workup in tetrahydrofuran
(THF) and excess pyridine afforded complex 2 in 64% yield
as oxygen-sensitive orange-red needles (Scheme 1). Of note,
the use 2 equiv. of RuCp*(cod)Cl and subsequent workup in
THF and toluene mixtures afforded crystallization of small
The notion of replacing a phosphine with an N-heterocyclic
carbene (NHC) donor is attractive with the promise of a
reduction in the toxicity of catalyst precursors and contaminants
in the hydrogenated products. We are interested in the design
of catalysts with chelating primary amine and NHC (C–NH2)
ligands (donor-functionalized NHC)7 that resemble those of
the P–NH2 analogues to achieve this goal of greener chemistry.
This is synthetically challenging since free NHC are known to
react with amines.8 We have reported the synthesis of a
C–NH2 ligand by the reduction of a nitrile-functionalized
imidazolium salt in the presence of NiCl2 yielding complex
1a (Fig. 1).9 Of note, only a few transition metal complexes
with a C–NH2 ligand have been structurally characterized
12
amounts of [RuCp*(Z6-toluene)]PF6 as a side product.
Complex 3a was synthesized from the reaction of 2-(diphenyl-
phosphino)benzylamine13 and RuCp*(cod)Cl in dichloro-
methane. Subsequent halide abstraction with AgPF6 and
addition of excess pyridine at ambient temperature afforded
a moderately oxygen-sensitive yellow solid in 67% yield
(Scheme 1).
Diagnostic NMR features include the Ru–Ccarbene
resonance at 198.0 ppm in the 13C NMR spectrum of complex
2 in acetone-d6 and the PPh2 peak at 44.7 ppm as a singlet in
the 31P{1H} NMR spectrum of 3a in CD2Cl2. These complexes
demonstrate fluxional behaviour at ambient temperature
(25 1C) due to restricted rotation of the Ru–Npyridine bond.
Cooling these samples to ꢀ40 1C allows the observation of the
aromatic protons, as well as the diastereotopic protons for the
CH2 linker between the primary amine group and the phenylene
spacer (see ESIz).
Fig. 1 Examples of metal complexes bearing a chelating N-hetero-
cyclic carbene with a primary amine donor.
Davenport Laboratory, Department of Chemistry,
University of Toronto, 80 St. George Street, Toronto,
Ontario M5S 3H6, Canada. E-mail: rmorris@chem.utoronto.ca
w This article is part of a ChemComm ‘Hydrogen’ web-based themed
issue.
z Electronic supplementary information (ESI) available: Experimental
procedures in the synthesis of complexes 2 and 3a and details for
catalysis. CCDC 784228 (3a) and 784229 (2). For ESI and crystallo-
graphic data in CIF or other electronic format see DOI: 10.1039/
c0cc02664f
Scheme 1 Synthesis of ruthenium(II) complexes 2 and 3a.
c
8240 Chem. Commun., 2010, 46, 8240–8242
This journal is The Royal Society of Chemistry 2010

Complex 2 and 3a were unambiguously characterized by
X-ray diffraction studies (Fig. 2 and 3). Both complexes adopt
a piano stool geometry about the ruthenium centre, with the
corresponding chelating and pyridine ligands. The Ru–Ccarbene
bond (2.03(1) A) in 2 is shorter than that of complex 1b
(2.092(5) A),9 but in the expected range of analogous
RuCp*(NHC)L2 complexes.14 The Ru–PPh2 bond length is
similar to those reported by Ikariya and co-workers.15
Table 1 The hydrogenation of acetophenone catalyzed by complex 2
in the presence of KOtBua
Entry P(H2)/bar C/B/S ratiob Conversionc (%/min) TOFd/hꢀ1
1
2
2
8
8
8
–/Ar
1/8/2515
1/8/2515
1/8/11500
1/8/2515
1/8/1200
16/60
30/20
29/30
46/4
99/120
98/30
97/90
98/10
82/120
3110
10 300
7240
17 600
1270
3e
4f
5g
Complex 2, when reacted with potassium tert-butoxide in
THF, is an efficient catalyst for the H2-hydrogenation of
acetophenone (Table 1). Full conversion to 1-phenylethanol
is achieved within 30 min under 8 bar of H2 in THF at 25 1C
with a catalyst to base to substrate (C/B/S) ratio of 1/8/2515.
On the other hand, full conversion is achieved in 10 min under
similar catalytic conditions when 2-propanol was used with a
TOF of 17 600 hꢀ1. Complex 2 also catalyzed the transfer
hydrogenation of acetophenone in 2-propanol at 25 1C, but
53/30
a
Reactions were carried out in a 50 mL Parr hydrogenation reactor in
THF (6 mL) at 25 1C unless otherwise stated; potassium tert-butoxide
b
c
was used as base. C/B/S: Catalyst/base/substrate. Conversions
were determined by GC and were reported as an average of two
d
runs. Determined from the slope of the linear portion of [alcohol] vs.
e
time plot. No solvent was used. 2-Propanol was used as solvent.
f
g
Conducted under an argon environment and 2-propanol was used as
solvent.
with a smaller TOF value (Table 1, entry 5). By comparison,
complex 3a is a poor precatalyst which when activated (C/B/S =
1/8/200) produces a maximum conversion of 8% in 4 h under
25 bar of H2 in THF at 50 1C. The related complex 3b
catalyzed the hydrogenation of acetophenone under 1 atm of
H2 in 2-propanol at 50 1C (Ru/KOH/substrate = 1/1/100)
with 16% conversion in 1 h.11 In terms of selectivity and broad
substrate scope (vide infra), complex 2 is superior to these and
a few other ruthenium(II) complexes with NHC ligands that
have been reported to catalyze the hydrogenation of ketones
and aldehydes14b,16 and it is comparable to or better than
many P–NH2 systems of ruthenium(II).4,17
Fig. 2 ORTEP diagram of [RuCp*(C–NH2)(py)]PF6, (2) depicted
with thermal ellipsoids at 30% probability. The counter anions and
most of the hydrogens have been omitted for clarity. Only one
asymmetric unit is shown. Selected bond distances (A) and bond
angles (deg): Ru(1a)–C(1a), 2.03(1); Ru(1a)–N(3a), 2.195(7);
Ru(1a)–N(4a), 2.156(8); Ru(1a)–C(12a), 2.176(9); C(1a)–Ru(1a)–N(3a),
91.9(3); N(3a)–Ru(1a)–N(4a), 90.9(4); C(1a)–Ru(1a)–N(3a), 81.3(3).
Some results of the H2-hydrogenation of substituted
acetophenones are listed in Table 2. In general, TOF values
decrease with increasing donor ability at the 40 position of the
aryl group on the ketone. On the other hand, an increase in
substituent bulk on the acyl group gave variable TOF values.
Pinacolone is effectively hydrogenated but not 1R-(ꢀ)-Fenchone
(Table 3, entries 1–5).
In the case of ketone hydrogenation, a sigmoidal type
conversion curve was observed with a variable induction
period (10–30 min) depending on the ketone of interest. The
mercury test18 was employed in the hydrogenation of
40-chloroacetophenone to test for the possibility of catalysis
Table 2 The hydrogenation of substituted acetophenones catalyzed
by complex 2 in the presence of KOtBua
Entry
R
Conversion (%/min)
TOF/hꢀ1
1
2
3
4
5
20-Chloro-
30-Chloro-
40-Chloro-
40-Bromo-
40-Methoxy-
46/40
41/15
45/15
18/15
42/15
99/60
99/20
99/25
99/30
99/25
2400
8050
7800
6970
5180
Fig. 3 ORTEP diagram of [RuCp*(P–NH2)(py)]PF6, (3a) depicted
with thermal ellipsoids at 30% probability. The counter anions and
most of the hydrogens have been omitted for clarity. Selected
bond distances (A) and bond angles (deg): Ru(1)–P(1), 2.3140(8);
Ru(1)–N(1), 2.188(2); Ru(1)–N(2), 2.171(2); Ru(1)–C(9), 2.186(3);
P(1)–Ru(1)–N(1), 84.89(7); N(1)–Ru(1)–N(2), 87.35(9); P(1)–Ru(1)N(2),
95.72(7).
a
Reactions were carried out in a 50 mL Parr hydrogenation reactor in
THF (6 mL) at 8 bar of H2 pressure at 25 1C. Potassium tert-butoxide
was used as base. The C/B/S ratio was 1/8/1500.
c
This journal is The Royal Society of Chemistry 2010
Chem. Commun., 2010, 46, 8240–8242 8241

Table 3 The hydrogenation of organic molecules with polar bonds
catalyzed by complex 2 in the presence of KOtBua
catalyst for the hydrogenation of a variety of polar bonds
under mild conditions in basic medium.
NSERC Canada is thanked for a Discovery Grant to
R.H.M. NSERC Canada and the Ministry of Education
of Ontario are thanked for graduate scholarships to
W.W.N.O.
Notes and references
1 (a) G. J. Kubas, Chem. Rev., 2007, 107, 4152–4205;
(b) R. H. Morris, Can. J. Chem., 1996, 74, 1907–1915; (c) M. Ito
and T. Ikariya, Chem. Commun., 2007, 5134–5142.
Entry
Substrate
Conversion (%/min)
TOF/hꢀ1
2 (a) T. Ikariya, K. Murata and R. Noyori, Org. Biomol. Chem.,
2006, 4, 393–406; (b) S. E. Clapham, A. Hadzovic and
R. H. Morris, Coord. Chem. Rev., 2004, 248, 2201–2237.
3 R. Guo, A. J. Lough, R. H. Morris and D. Song, Organometallics,
2004, 23, 5524–5529.
4 K. Abdur-Rashid, R. W. Guo, A. J. Lough, R. H. Morris and
D. T. Song, Adv. Synth. Catal., 2005, 347, 571–579.
5 (a) L. A. Saudan, C. M. Saudan, C. Debieux and P. Wyss, Angew.
Chem., Int. Ed., 2007, 46, 7473–7476; (b) W. Kuriyama, Y. Ino,
O. Ogata, N. Sayo and T. Saito, Adv. Synth. Catal., 2010, 352,
92–96.
6 (a) M. Ito, M. Hirakawa, A. Osaku and T. Ikariya, Organo-
metallics, 2003, 22, 4190–4192; (b) M. Ito, A. Sakaguchi,
C. Kobayashi and T. Ikariya, J. Am. Chem. Soc., 2007, 129,
290–291.
7 (a) X. Cui and K. Burgess, Chem. Rev., 2005, 105, 3272–3296;
(b) A. T. Normand and K. J. Cavell, Eur. J. Inorg. Chem., 2008,
2781–2800; (c) R. Corberan, E. Mas-Marza and E. Peris, Eur. J.
Inorg. Chem., 2009, 1700–1716.
1
2
3
4
5
6
g
h
i
j
k
l
m
n
n
o
p
75/5
99/15
13 400
1110
8100
10 400
0
0
67
209
838
0
49/60
42/5
58/5
0/60
1/60
9/120
10/120
48/60
0/60
99/120
99/15
99/15
1/120
1/120
7b
8c
9c,d
10e
11e
13/180e
23/180
78/120
0/120
28/60
43/120
247
a
Unless otherwise stated, all reactions were carried out in a 50 mL
Parr hydrogenation reactor in THF (6 mL) at 8 bar of H2 pressure and
25 1C. Potassium tert-butoxide was used as base. The C/B/S ratio was
b
1/8/1500. 2-Propanol was used as solvent. Branched to linear alcohol
c
ratio: 89 : 11. Tridecane was used as an internal standard for GC
d
analysis. Reaction was carried out at 25 bar of H2 pressure and
e
1
8 D. Enders, K. Breuer, G. Raabe, J. Runsink, J. H. Teles,
J. P. Melder, K. Ebel and S. Brode, Angew. Chem., Int. Ed. Engl.,
1995, 34, 1021–1023.
50 1C. Conversion determined by H-NMR spectroscopy.
9 W. W. N. O, A. J. Lough and R. H. Morris, Organometallics, 2009,
28, 6755–6761.
10 L. G. Bonnet, R. E. Douthwaite, R. Hodgson, J. Houghton,
B. M. Kariuki and S. Simonovic, Dalton Trans., 2004,
3528–3535.
11 M. Ito, M. Hirakawa, K. Murata and T. Ikariya, Organometallics,
2001, 20, 379–381.
12 W. W. N. O, A. J. Lough and R. H. Morris, Acta Crystallogr.,
Sect. E: Struct. Rep. Online, 2010, 66(10), m1264.
by ruthenium nanoparticles. The catalytic activity was not
perturbed during the course of the reaction (see ESIz). Catalysis
conducted in 2-propanol did not have an induction period.
We postulate that the product alcohol might auto-catalyze
the heterolytic splitting of dihydrogen by acting as a proton
shuttle.1c,11,19
The H2-hydrogenation of other polar bonds catalyzed by
complex 2 with base was also investigated. Thus, benzaldehyde
and N-(benzylidene)aniline were not hydrogenated, and
N-(1-phenylethylidene)aniline was hydrogenated to its tertiary
amine in 43% conversion within 2 h (Table 3, entries 6, 10
and 11). On the other hand, complex 2 catalyzed the hydro-
genolysis of styrene oxide in 2-propanol at 25 1C with a TOF
of 67 hꢀ1 to produce phenylethanol with branched to linear
ratio of 89 : 11 (Table 3, entry 7). A similar branched to linear
alcohol ratio was observed when 3b was used as a catalyst, yet
with a smaller TOF value (32 hꢀ1).6a
13 J. P. Cahill, F. M. Bohnen, R. Goddard, C. Kruger and
P. J. Guiry, Tetrahedron: Asymmetry, 1998, 9, 3831–3839.
14 (a) W. Baratta, E. Herdtweck, W. A. Herrmann, P. Rigo and
J. D. Schwarz, Organometallics, 2002, 21, 2101–2106;
(b) C. Gandolfi, M. Heckenroth, A. Neels, G. Laurenczy and
M. Albrecht, Organometallics, 2009, 28, 5112–5121; (c) A. P. da
Costa, J. A. Mata, B. Royo and E. Peris, Organometallics, 2010,
29, 1832–1838.
15 M. Ito, A. Osaku, C. Kobayashi, A. Shiibashi and T. Ikariya,
Organometallics, 2009, 28, 390–393.
16 (a) V. L. Chantler, S. L. Chatwin, R. F. R. Jazzar, M. F. Mahon,
O. Saker and M. K. Whittlesey, Dalton Trans., 2008, 2603–2614;
(b) J. P. Lee, Z. F. Ke, M. A. Ramirez, T. B. Gunnoe,
T. R. Cundari, P. D. Boyle and J. L. Petersen, Organometallics,
2009, 28, 1758–1775.
17 (a) W. L. Jia, X. H. Chen, R. W. Guo, C. Sui-Seng, D. Amoroso,
A. J. Lough and K. Abdur-Rashid, Dalton Trans., 2009,
8301–8307; (b) M. L. Clarke, M. B. Diaz-Valenzuela and A. M.
Z. Slawin, Organometallics, 2007, 26, 16–19; (c) M. B. Diaz-
Valenzuela, S. D. Phillips, M. B. France, M. E. Gunn and
M. L. Clarke, Chem.–Eur. J., 2009, 15, 1227–1232.
18 C. A. Jaska and I. Manners, J. Am. Chem. Soc., 2004, 126,
9776–9785.
The homogenous hydrogenation of esters is usually challenging.
Most ruthenium(II) catalysts require a high temperature
and H2 pressure with low substrate loadings to achieve
conversion to the alcohol.5a,17b,20 Complex 2 catalyzed the
hydrogenation of methyl benzoate to benzyl alcohol and
methanol at 25 1C and 8 bar H2 with a TOF of 209 hꢀ1, or
at 50 1C and 25 bar H2 with a TOF of 838 hꢀ1 and appreciable
substrate loading (C/B = 1/1500) (Table 3, entries 8 and 9).
No other side products were observed. The precatalyst trans-
RuCl2(k2-PPh2CH2CH2NH2)2, provides comparable activity
but at higher temperature (100 1C) and H2 pressure (50 bar).5a
In summary, we have presented the synthesis and catalytic
activity of complexes 2 and 3a. Complex 2 provides an active
19 A. Hadzovic, D. Song, C. M. MacLaughlin and R. H. Morris,
Organometallics, 2007, 26, 5987–5999.
20 (a) M. C. van Engelen, H. T. Teunissen, J. G. de Vries and
C. J. Elsevier, J. Mol. Catal. A: Chem., 2003, 206, 185–192;
(b) J. Zhang, G. Leitus, Y. Ben-David and D. Milstein, Angew.
Chem., Int. Ed., 2006, 45, 1113–1115; (c) S. Takebayashi and
S. H. Bergens, Organometallics, 2009, 28, 2349–2351.
c
8242 Chem. Commun., 2010, 46, 8240–8242
This journal is The Royal Society of Chemistry 2010