Catalytic hydrogenation of C{double bond, long}O and C{double bond, long}N bonds via heterolysis of H2 mediated by metal-sulfur bonds of rhodium and iridium thiolate complexes

Article abstract of DOI:10.1016/j.jorganchem.2009.02.018

Coordinatively unsaturated rhodium and iridium complexes having a bulky thiolate, [Cp*M(PMe₃)(SDmp)](BAr^F₄) (1a: M = Rh; 1b: M = Ir; Dmp = 2,6-(mesityl)₂C₆H₃, Ar^F = 3,5-(CF₃

Full text of DOI:10.1016/j.jorganchem.2009.02.018

Journal of Organometallic Chemistry 694 (2009) 2820–2824
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Journal of Organometallic Chemistry
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Catalytic hydrogenation of C@O and C@N bonds via heterolysis of H2 mediated
by metal–sulfur bonds of rhodium and iridium thiolate complexes
b
b
a,
*
Mayumi Sakamoto a, Yasuhiro Ohki a, , Gerald Kehr , Gerhard Erker , Kazuyuki Tatsumi
*
a Department of Chemistry, Graduate School of Science and Research Center for Materials Science, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8602, Japan
b Organisch-Chemisches Institut, Universität Münster, Corrensstraße 40, D-48149 Münster, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 31 January 2009
Received in revised form 18 February 2009
Accepted 18 February 2009
Available online 28 February 2009
Coordinatively unsaturated rhodium and iridium complexes having
a
bulky thiolate,
F
(1a: M = Rh; 1b: M = Ir; Dmp = 2,6-(mesityl)2C6H3, ArF = 3,5-(CF3)2C6H3), cat-
*
[Cp M(PMe3)(SDmp)](BAr4
)
alyzed the hydrogenation of benzaldehyde, N-benzylideneaniline, and cyclohexanone, under 1 atm of H2
*
at low temperatures. In these catalytic reactions, the M–H/S–H complexes [Cp M(PMe3)(H)(HSDmp)]-
(BArF4
) (2a: M = Rh; 2b: M = Ir) generated via H2 heterolysis by 1a or 1b were suggested to transfer both
M–H hydride and S–H proton to substrates. The catalytic reactions were terminated by the dissociation of
H-SDmp from the metal centers of 2a and 2b that occurs at ambient temperature under H2 atmosphere.
Ó 2009 Elsevier B.V. All rights reserved.
Keywords:
Rhodium
Iridium
Thiolate
H2 activation
Hydrogenation
1. Introduction
Fe(l
-StBu)3Ni{SC(NMe2)2}Br [6]. Also we have reported some
metal–sulfur complexes as functional models, which are capable
of splitting H2 in a heterolytic manner [7]. For example, the half-
sandwich rhodium and iridium complexes having a bulky SDmp
Heterolytic cleavage of H2 by transition metal complexes has
been useful for catalytic hydrogenation of aldehydes, ketones,
and imines [1,2]. A representative class of such hydrogenation cat-
alysts is the ruthenium–amide complexes, in which both ruthe-
nium and amide nitrogen atoms participate in the heterolysis of
H2 [3]. While these complexes efficiently activate H2, biological
hydrogen activation is mediated by metal–thiolate complexes in
hydrogenases. The active site of [NiFe] hydrogenase consists of a
thiolate-bridged (carbonyl/cyano)iron–nickel complex [4], which
has been postulated to use both metal and cysteinyl sulfur atoms
in the reaction with H2. The recent studies on [NiFe] hydrogenase
suggested that the nickel center is a plausible binding site of H2,
and that a cysteinyl sulfur bound to nickel possibly accepts a pro-
ton generated from the heterolysis of H2 [5]. Thus the heterolysis of
H2 by [NiFe] hydrogenase may occur in a similar manner to that
mediated by ruthenium–amide catalysts.
*
thiolate (Dmp = 2,6-(mesityl)2C6H3) [8], [Cp M(PMe3)(SDmp)]
(BArF4 (1a: M = Rh; 1b: M = Ir; ArF = 3,5-(CF3)2C6H3), were found
)
to promote the heterolysis of H2 (1 atm) at low temperatures
(À50 to À20 °C), giving rise to the M–H/S–H complexes
F
*
[Cp M(PMe3)(H)(HSDmp)](BAr4
)
(2a: M = Rh; 2b: M = Ir) (Scheme
1) [7e]. Since complexes 2a and 2b contain a hydride (M–H) and
a proton (S–H) derived from H2, we sensed that these reactions
could be applied to catalytic hydrogenation reactions. Herein we
report the hydrogenation of C@O and C@N groups of benzalde-
hyde, N-benzylideneaniline, and cyclohexanone, catalyzed by 1a
and 1b under 1 atm of H2 at low temperatures.
2. Results and discussion
The unique function of metal–thiolate complex in [NiFe]
hydrogenase prompted us to investigate into the structural and
reaction models. As structural analogues of the active site of [NiFe]
hydrogenase, we have synthesized a series of thiolate-bridged (CO/
CN)Fe–Ni and (CO)3Fe–Ni complexes such as (PPh4)[(CN)2(CO)2-
Coordinatively unsaturated thiolate complexes 1a and 1b medi-
ate the heterolysis of H2 under mild conditions. This is an advan-
tage for catalytic reactions, because most of the precedent H2
heterolysis by thiolate complexes require rigorous conditions such
as high-pressure of H2 and/or the presence of external protons [9],
preventing their application to hydrogenation catalysts. Whereas
the heterolysis of H2 by 1a or 1b proceeds under mild conditions,
the resultant M–H/S–H complexes 2a and 2b were found to be
unstable [7e]. The metal centers of 2a and 2b readily liberate
H-SDmp under H2 atmosphere at room temperature, giving rise
Fe(l-pdt)Ni(S2CNEt2)] (pdt = 1,3-propanedithiolate) and (CO)3-
* Corresponding authors.
E-mail addresses: ohki@mbox.chem.nagoya-u.ac.jp (Y. Ohki), i45100a@nucc.cc.
nagoya-u.ac.jp (K. Tatsumi).
0022-328X/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2009.02.018

M. Sakamoto et al. / Journal of Organometallic Chemistry 694 (2009) 2820–2824
2821
BArF
BArF
4
4
Rh-hydride species
M
M
1 atm H2
~ r.t.
1 atm H2
SH
S
H
or
+
PMe3
S
PMe3
H
-50 ºC (1a)
-20 ºC (1b)
BArF
4
1a (M = Rh)
1b (M = Ir)
Ir
2a (M = Rh)
2b (M = Ir)
PMe3
H
H
H
3
Scheme 1. Heterolysis of H2 mediated by 1a and 1b.
to a complex mixture of rhodium-hydride species or a trihydride
complex 1a serves as a good catalyst for hydrogenation of benzal-
dehyde, the iridium congener 1b was found to be less active. Com-
plex 1b was almost inactive at À50 °C for hydrogenation of
benzaldehyde (entry 3), because the H2 activation by 1b is very
slow at this temperature. At À20 °C, complex 1b showed some cat-
alytic activity, whereas the longer reaction time was needed (entry
4). The hydrogenation reactions of N-benzylideneaniline and cyclo-
hexanone were also attempted in a similar manner (entries 5–8),
and the yields of N-phenylbenzylamine and cyclohexanol were
66% and 53% yields, respectively, in the presence of 1a as the cat-
alyst (entries 5 and 7). The lower catalytic activity of 1b was also
the case for the hydrogenation of these substrates. The yield of
N-phenylbenzylamine was 15% by using 1b as the catalyst (entry
6), and attempts for the hydrogenation of cyclohexanone by 1b
were unsuccessful (entry 8).
The lower catalytic activity of 1b was not only because of slow
H2 activation, but also due to side reactions. After the hydrogena-
tion of cyclohexanone by 1b, the catalytically inactive trihydride
complex 3 and H-SDmp were formed. This result indicates the
low reactivity of Ir–H/S–H complex 2b toward cyclohexanone
and the facile liberation of H-SDmp from 2b in the presence of
H2. The side reaction found in the hydrogenation of benzaldehyde
by 1b was different, and the NMR spectrum indicated the
F
*
complex of iridium [Cp Ir(PMe3)(H)3](BAr4
)
(3) (Scheme 1) [10].
Therefore, the following hydrogenation reactions were conducted
at low temperatures. Anticipating that the M–H/S–H complexes
2a and 2b transfer both M–H and S–H hydrogen atoms to sub-
strates, a stoichiometric reaction between 2a and benzaldehyde
was examined. Under a nitrogen atmosphere, one equiv of benzal-
dehyde was added to a CD2Cl2 solution of 2a at À50 °C. The 1H
NMR spectrum of this reaction mixture revealed the regeneration
of 1a and the quantitative formation of benzylalcohol, completing
formal turnover of the catalytic hydrogenation (Scheme 2).
Motivated by the stoichiometric reaction between 2a and benz-
aldehyde, we set out the catalytic hydrogenation (Table 1). Expo-
sure of H2 (1 atm) to a CD2Cl2 solution of benzaldehyde and
catalytic amount of 1a (2 mol%) at À50 °C resulted in the quantita-
tive formation of benzylalcohol (entry 1). In contrast, a complex
mixture of rhodium-hydride species, formed from 2a and H2 at
room temperature, did not show any catalytic activity (entry 2),
suggesting that the thermally unstable Rh–H/S–H complex 2a is
necessary for this catalytic reaction. The Rh–H/S–H hydrogen
atoms in 2a are transferred to benzaldehyde, possibly via forma-
tion of six-membered metallacycle consisting of Rh–H, S–H, and
C@O groups (Scheme 3) as proposed for the hydrogenation medi-
ated by the ruthenium–amide complexes [3]. While the rhodium
formation of free H-SDmp and
a phenyl carbonyl complex
BArF
BArF
BArF
4
O
4
4
H
OH
H
Rh
H
Rh
Rh
1 atm H2
S
H
PMe3
+
H
S
PMe3
S
PMe3
CD2Cl2
-50 °C
CD2Cl2
-50 °C
under N2
1a
2a
1a
Scheme 2. Proton/hydride transfer from 2a to benzaldehyde.
Table 1
Hydrogenation reactions catalyzed by 1a and 1b.a
Entry
Substrate
Catalyst
Temperature (°C)
Time (h)
Yield (%)b
Product
1
2
3
4
1a
À50
À50
À50
À20
À50
À20
24
24
24
48
>98
n.d.d
<2
O
OH
Rh-hydridec
H
H
N
1b
1b
H
15
5
6
1a
1b
24
48
66
15
HN
OH
7
8
1a
1b
À50
À20
24
48
53
O
n.d.d
a
Standard conditions: 2 mol% catalysts, 1 mL CD2Cl2, 1 atm H2.
b
c
Determined by 1H NMR with reference to the internal standard {(CH3)3Si}4Si (1 wt%).
Obtained from the reaction of 1a with H2 (1 atm) at room temperature.
Not detected.
d

2822
M. Sakamoto et al. / Journal of Organometallic Chemistry 694 (2009) 2820–2824
BArF
coordinatively unsaturated rhodium and iridium complexes 1a
4
and 1b under 1 atm of H2 at low temperatures. These reactions in-
volve the heterolysis of H2 to form the M–H/S–H complexes 2a and
2b, which may transfer both M–H hydride and S–H proton to sub-
strates in a concerted manner as proposed for the hydrogenation
mediated by ruthenium–amide complexes (Scheme 3). Whereas
the yields of N-phenylbenzylamine and cyclohexanol were not suf-
ficient and the catalytic activity of 1b was low, this study may sug-
gest a new utility of the metal–sulfur bond in thiolate complexes.
OH
H
Rh
H
S
PMe3
H2
1a
BArF
BArF
4
4
Rh
H
Rh
Dmp
S
H
PMe3
PMe3
S
H
H
3. Experimental
H
Ph
O
2a
3.1. General procedures
All reactions and manipulations were performed under a nitro-
gen atmosphere using a glove box and standard Schlenk tech-
niques. CD2Cl2 was dried by CaH2 and distilled prior to use.
Hexane and CH2Cl2 were purified by the method of Grubbs and
coworkers [12], where the solvents were passed over columns of
activated alumina and supported copper catalyst supplied by Han-
sen & Co. Ltd. The 1H, 13C{1H}, and 31P{1H} NMR spectra were ac-
quired on a JEOL ECA-600. The proton and carbon signals were
referenced to the residual signals of CD2Cl2. The 31P{1H} NMR
chemical shifts are relative to the external reference of 85%
H3PO4. Infrared spectrum was recorded on a JASCO FT/IR-410 spec-
trometer. ESI-MS spectrum was obtained from Micromass LCT
TOF-MS spectrometer. Elemental analysis of 4 was recorded on a
LECO-CHNS-932 elemental analyzer where the crystalline samples
were sealed in silver capsules under nitrogen. X-ray diffraction
data of 4 was collected on a Rigaku AFC8 equipped with a CCD area
O
H
Scheme 3. A possible mechanism for hydrogenation.
detector by using graphite-monochromated Mo Ka radiation.
3.2. Reaction of 2a with 1 equiv. of benzaldehyde
A Schlenk tube with a stirring bar was charged with 1a
(30.4 mg, 0.02 mmol) and CD2Cl2 (0.6 mL with 1 wt% {(CH3)3Si}4Si
as the internal standard). After freeze–pump–thaw cycles, the tube
was filled with 1 atm of H2 at À80 °C, and the tube was kept at
À50 °C with stirring. The color of solution changed from purple
to green, indicating the dominant formation of 2a (the yield of
2a was 89% under a similar condition [7e]). The Schlenk tube
was filled with N2 after freeze–pump–thaw cycles, and one equiv
*
Fig. 1. Structure of the cationic part of [Cp Ir(PMe3)(Ph)(CO)][BAr4F
] (4) with
thermal ellipsoids at the 50% probability level. All hydrogen atoms and BArF4 anion are
omitted for clarity. Selected bond distances (Å) and angles (°): Ir–P 2.3084(18), Ir–C1
1.875(5), C1–O 1.132(6), Ir–C2 2.109(5), P–Ir–C1 89.1(2), C1–Ir–C2 89.3(2), C2–Ir–P
86.22(19).
of benzaldehyde (25 lL of 0.8 M CD2Cl2 solution, 0.02 mmol) was
added to this solution at À50 °C. The solution was stirred for 6 h
under a N2 atmosphere. The color of the reaction mixture turned
to purple, and the 1H and 31P{1H} NMR revealed the conversion
of benzaldehyde to benzylalcohol (>98%), regeneration of 1a
(86%), and the formation of decomposed rhodium-hydride species
(14%) and H-SDmp (14%).
F
*
[Cp Ir(PMe3)(Ph)(CO)](BAr4
)
(4), the structure of which was con-
firmed by an X-ray analysis (Fig. 1). The cationic part of 4 is known,
and the OTf salt of 4 has been prepared from the reaction of
*
3.3. General procedure for the catalytic hydrogenation
Cp Ir(PMe3)(Me)(OTf) with benzaldehyde [11]. Since this reaction
is accompanied by the liberation of methane, benzaldehyde may
be deprotonated by the Ir–Me groups to generate the Ir–C(O)Ph
species. Similarly, the phenyl and carbonyl ligands in 4 are likely
derived from benzaldehyde, and the formation of 4 may involve
the deprotonation from benzaldehyde and the decarbonylation
from the resultant Ir–C(O)Ph species. As the SDmp ligand in 1b
serves as a base for the heterolysis of H2, this ligand may deproto-
nate from benzaldehyde. Other possibilities involve the liberation
of H-SDmp from 2b followed by deprotonation of benzaldehyde
by Ir–H, or the protonation of the SDmp ligand in 1b by benzylal-
cohol to generate an Ir-alkoxide species which may uptake a pro-
ton from benzaldehyde.
In a glove box, a Schlenk tube with a stirring bar was charged
with 1a or 1b (0.02 mmol) and CD2Cl2 (0.5 mL, with {(CH3)3Si}4Si
as the internal standard). After freeze–pump–thaw cycles, the tube
was filled with 1 atm of H2 at À80 °C (1a) or À40 °C (1b), and sub-
strate (1 mmol in 0.5 mL CD2Cl2) was added under H2 atmosphere.
The reaction mixture was stirred at À50 °C (1a) or À20 °C (1b) un-
der H2, and was warmed to room temperature after catalysis. The
yields of the catalytic products were determined by the 1H NMR.
The rhodium-hydride species used in Table 1.entry 2, was gen-
erated from the reaction of a CD2Cl2 (0.5 mL with {(CH3)3Si}4Si) of
1a (0.02 mmol) with 1 atm of H2 at room temperature for 24 hrs.
The color of the solution turned from purple to dark green, and
the 1H and 31P{1H} NMR showed the conversion of 1a to uncharac-
In summary, we have demonstrated the hydrogenation of benz-
aldehyde, N-benzylideneaniline, and cyclohexanone, catalyzed by

M. Sakamoto et al. / Journal of Organometallic Chemistry 694 (2009) 2820–2824
2823
Table 2
terizable rhodium-hydride species and H-SDmp [7e,13]. This mix-
ture was cooled and used for the catalytic reaction.
*
Crystal data and structure refinement for [Cp Ir(PMe3)(Ph)(CO)](BArF4) (4).
4
3.4. Characterization of [Cp Ir(PMe3)(Ph)(CO)](BArF4) (4) obtained
*
Formula
Formula weight
Crystal color, habit
Crystal dimensions (mm)
Crystal system
Space group
a (Å)
b (Å)
c (Å)
C52H41OIrF24 PB
1371.86
from the reaction in Table 1, entry 4
colorless, block
0.30 Â 0.10 Â 0.05
Monoclinic
P2/c (No. 13)
18.611(3)
12.612(2)
23.741(4)
107.041(2)
5327.8(16)
4
According to Table 1, entry 4, complex 1b was treated with
benzaldehyde and H2 (1 atm) at À20 °C. After 48 h, the 1H and
31P{1H} NMR spectra of the reaction mixture exhibited the signals
for [Cp Ir(PMe3)(Ph)(CO)](BAr4F) (4, 37% based on the internal stan-
*
dard {(CH3)3Si}4Si), H-SDmp (37%), and 2b (63%). Single crystals of 4
suitable for X-ray diffraction and elemental analysis were obtained
by diffusing this solution into hexane at room temperature under a
nitrogen atmosphere. 1H NMR (CD2Cl2): d 7.71 (bs, 8H, o-H of ArF),
7.56 (bs, 4H, p-H of ArF), 7.13 (m, 5H, Ph), 1.95 (d, JPH = 2.0 Hz,
b (°)
V (Å3)
Z
Dcalc (g cmÀ3
)
1.71
26.65
55.0
Total: 41317
l
(Mo K
a
) (cmÀ1
)
Max 2h (°)
Number of reflections measured
15H, Cp ), 1.63 (d, JPH = 10.6 Hz, 9H, PMe3). 31P{1H} NMR (CD2Cl2):
*
d À35.6 (s, PMe3). 13C{1H} NMR (CD2Cl2): d 167.5 (d, JPC = 11.6 Hz,
CO), 162.3 (q, JBC = 50.2 Hz, ipso-C of ArF), 139.8, 131.0 (s, o-C of
Ph), 135.4 (s, o-C of ArF), 129.4 (q, JFC = 34.7 Hz, m-C of ArF), 126.3
(s, m-C of Ph), 125.2 (q, JFC = 274.1 Hz, CF3), 122.2 (d, JPC = 9.7 Hz,
ipso-C of Ph), 118.0 (s, p-C of ArF), 104.2 (s, C5(CH3)5), 16.3 (d,
JPC = 42.5 Hz, PMe3), 9.7 (s, C5(CH3)5). ESI-MS (CH2Cl2): m/z = 509.1
Unique: 12168 (Rint = 0.069)
12168
707
17.21
0.0538
0.143
1.069
Number of observations (all reflections)
Number of variables
Reflection/parameter ratio
R1 (I > 2r
(I))a
wR2 (all reflections)b
GOF on F2c
(4+). IR (KBr pellet): 2042 ( CO) cmÀ1. Anal. Calc. for C52H42PF24OBIr:
m
a
R1
wR2 = [(
GOF = [
=
R
||Fo| À |Fc||/
R|Fo|(I > 2r(I)).
b
c
C, 45.49; H, 3.08. Found: C, 45.49; H, 3.22%.
R
(w(|Fo| À |Fc|)2/
R
wFo2))1/2 (all reflections).
R
w(|Fo| À |Fc|)2/(No À Nv)]1/2 (where No = number of observations,
Nv = number of variables).
3.5. Degradation of 1a in the presence of excess benzaldehyde at room
temperature
Group ‘‘Complex Functional Systems in Chemistry” Münster-Na-
goya) for a graduate externship.
The reaction of 1a (30.4 mg, 0.02 mmol) with benzaldehyde
(106.2 mg, 1 mmol) in CD2Cl2 at room temperature gave a brown
solution. The 1H and 31P{1H} NMR showed the formation of free
Appendix A. Supplementary material
*
H-SDmp and an uncharacterizable rhodium species having Cp
and PMe3. 1H NMR (CD2Cl2): d 1.72 (d, JPH = 3.1 Hz, Cp ), 1.50 (d,
*
CCDC 715530 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. Supplementary data associated with this article
can be found, in the online version, at doi:10.1016/j.jorganchem.
2009.02.018.
JPH = 11.0 Hz, PMe3). 31P{1H} NMR (CD2Cl2):
d
3.29 (d,
JRhP = 157.9 Hz, PMe3). After the catalytic hydrogenation of benzal-
dehyde (Table 1, entry 1), the same rhodium species was detected
as the degradation product at room temperature.
3.6. X-ray structural determination
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Acknowledgment
This research was financially supported by Grant-in-Aids for
Scientific Research (Nos. 18GS0207 and 18064009) from the Min-
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