Ruthenium(ii) carbonyl complexes bearing CCC-pincer bis-(carbene) ligands: Synthesis, structures and activities toward recycle transfer hydrogenation reactions

Article abstract of DOI:10.1039/c3dt51161h

A new series of ruthenium(ii) carbonyl complexes with benzene-based CCC-pincer bis-(carbene) ligands, [(^RCCC^R)Ru(CO) ₂(X)]^0/+ and [(^RCCC^R)Ru(CO)(NN)] ⁺ (^RCCC^R

Full text of DOI:10.1039/c3dt51161h

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Ruthenium(II) carbonyl complexes bearing CCC-pincer
bis-(carbene) ligands: synthesis, structures and
activities toward recycle transfer hydrogenation
reactions†
Cite this: Dalton Trans., 2013, 42, 13161
Abbas Raja Naziruddin, Zhao-Jiunn Huang, Wei-Chih Lai, Wan-Jung Lin and
Wen-Shu Hwang*
A new series of ruthenium(II) carbonyl complexes with benzene-based CCC-pincer bis-(carbene) ligands,
[(RCCCR)Ru(CO)2(X)]0/+ and [(RCCCR)Ru(CO)(NN)]+ (RCCCR = 2,6-bis-(1-alkylimidazolylidene)benzene, R =
Me or nBu; X = I, Br, CH3CN, or 6-(aminomethyl)pyridine (ampy); NN = 2·CH3CN, or chelating ampy or
bipyridine), was synthesized and fully characterized. X-Ray structure determinations revealed that these
eight complexes have pseudo-octahedral configurations around the ruthenium center with the pincer
ligand occupying three meridional sites. These complexes prove to be efficient precatalysts demonstrating
very good activity and reusability for the transfer hydrogenation of ketones.
Received 3rd May 2013,
Accepted 15th June 2013
DOI: 10.1039/c3dt51161h
www.rsc.org/dalton
allow the preparation of their complexes with a variety of geo-
metries and provide a new dimension to the preparation of
Introduction
N-Heterocyclic carbenes (NHCs) have emerged as a useful catalysts with high thermal and air stabilities.1e,12 A wide array
class of ligands in the development of homogeneous catalysts of complexes coordinated to bis-, tris-, and tetra-NHCs, which
and have proven to be an efficient alternative to the vastly used act as bis-chelating, pincer, tripodal, or bridging ligands,
tertiary phosphines due to their strong σ-donating and weak along with a combination of chiral motifs may allow the prepa-
π-accepting character.1 Analogous to the phosphorus donor ration of asymmetric catalysts. In search of a rationalized
ligands, NHCs constitute a sterically and electronically tunable description to the specific coordination modes of poly-NHCs,
ligand set that support catalysis on coordination with catalyti- in addition to few recent works that describe their synthesis13
cally active metal centers. However, relative to phosphorus several reviews discuss the properties of bis- and tris-chela-
donor ligands, the principles regulating their structures and ting12b,h,j and pincer NHCs,12a,c,d,h,j,14 thus covering the chem-
properties are as yet little developed. NHCs form stable bonds istry of most poly-NHCs that are known until now.
with various metals in different oxidation states and stabilize
Ruthenium complexes with several traditional amine/phos-
the catalytically active intermediates.2 The last few decades phine donors bearing a Ru–NH linkage tend to be more
have witnessed extensive exploration of M–NHC complexes, efficient in transfer hydrogenation reactions.15 This structural
enabling their applications in wide range of organic transform- requisite for the catalyst molecule was overcome in the RuII
ations including olefin metathesis,3 hydrosilylation,4 hydro- p-cymene complex bearing a bidentate P–N chelate ligand,
formylation,5 hydrogenation,6 copolymerization,7 furan which demonstrated good activities in the absence of N–H
synthesis,8 C–C9 and C–N10 coupling reactions, as well as functionality.16 Moreover, RuII–NHC complexes have increas-
asymmetric transformations.11
ingly found applications in hydrogen transfer reactions,17 race-
Among the NHCs, poly-NHCs (ligands containing more mization of chiral alcohols,18 and asymmetric reductions.19
than one NHC branch) have attracted large attention as they Ruthenium complexes with N-functionalized chelate carbene
ligands, including those with chelating bis-NHC ligands,6d,20
have been shown to be stable and effective in the transfer
hydrogenation of ketones.
Department of Chemistry, National Dong Hwa University, Hualien 974, Taiwan,
ROC. E-mail: hws@mail.ndhu.edu.tw; Fax: +886-3-8633570; Tel: +886-3-8633577
†Electronic supplementary information (ESI) available: CIF data, crystal data
and refinement parameters for all complexes (Table S1), and ORTEP drawings of
After the first isolation and structural characterization of a
Ru–CNC-pincer complex, [Ru(CNC)Cl2(PPh3)] (CNC = 2,6-bis-
(1-alkylimidazolylidene)pyridine), by Danopoulos et al. in
2002,21 [Ru(CNC)(CO)Br2] was reported by Peris et al. in
2003.22 These complexes have been shown to be stable and
complexes 2b and 3a (Fig. S1 and S2). CCDC 919550, 908681, 908679, 919551 for
2a, 2b, 3a, 3b and 880396–880399 for 4a, 6a, 7a, and 5a. For ESI and crystallo-
graphic data in CIF or other electronic format see DOI: 10.1039/c3dt51161h
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efficient towards transfer hydrogenation reactions. Despite NH assistance, this also allows us to overcome the general
rich transfer hydrogenation activities of ruthenium complexes, structural prerequisites necessary for transfer hydrogenation
the complexes of this metal are mainly centered on the pyri- reactions.
dine-bridged pincer bis-carbene ligands.12a,d,h,i,14,21–23
Previously, CCC pincer-bis carbene complexes of IrIII were
prepared by either transmetallation of zirconium complexes24
Results and discussion
or by direct metallation using a suitable base.25 To the best of
our knowledge, a report on the first RuII complex with a CCC
pincer ligand and a terpyridine donor, describes the substitu-
Halide salts of the CCC-pincer-bis-(carbene) ligand precursors
2,6-bis-(1-alkylimidazolium-3-yl) benzene 1a and 1b, [(RCCCR-
ent dependent MLCT absorptions and redox potentials.26 In
this contribution, we present the synthesis and molecular
structures of the first ruthenium carbonyl complexes bearing a
phenyl-bridged CCC-pincer-bis-carbene ligand, [(RCCCR)Ru-
(CO)2(X)]0/+ and [(RCCCR)Ru(CO)(NN)]+ (RCCCR = 2,6-bis-(1-alkyl-
H3)]X2 (a: R = Me, X = I; b: R = nBu, X = Br), were prepared
according to literature procedures.28 Syntheses of complexes
are outlined in Scheme 1. Treatment of 1a and 1b with
[Ru(COD)Cl2]n in ethylene glycol–EtOH (1 : 2) at 165 °C for 24 h
resulted in the formation of [(RCCCR)Ru(CO)2X] complexes 2a
and 2b, respectively, in good yield (70%). On carrying out the
reaction using the more readily accessible RuCl3·nH2O in ethyl-
ene glycol or EtOH under the same conditions, a substantially
reduced yield (15%) was obtained. It was proposed that the CO
source for the formation of 2a and 2b was from the decarbony-
lation of alcohol, which was used as the solvent.22 The for-
mation of complexes 2a and 2b might involve an initial
n
imidazolylidene)benzene, R = Me or Bu; X = I, Br, CH3CN, or
6-(aminomethyl)pyridine (ampy); NN = 2·CH3CN, or chelating
ampy or bipyridine), and demonstrate their use in the transfer
hydrogenation of ketones. The best Ru–NHC catalytic system
for transfer hydrogenation reactions, presented in this work,
demonstrates very good catalytic conversions and recyclability
as compared to other closely related NHC complexes27 without
Scheme 1 Syntheses of RuII complexes bearing CCC pincer bis-carbene ligands.
13162 | Dalton Trans., 2013, 42, 13161–13171
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coordination of the NHC ligand to the ruthenium center, fol- 2004–2043 cm−1, while those of 4a, 5a, and 7a show only one
lowed by a sequence of oxidative insertion of phenylene–H and ν(CO) stretching frequency at 1897–1974 cm−1
.
a reductive elimination of the hydride. A similar mechanism
All the complexes reported in this work are air and moisture
was previously described towards the formation of RuII and stable. Single crystals suitable for X-ray diffraction analyses
RhI complexes in which the native oxidation state of the metal were obtained by slow evaporation of CH2Cl2 solution of 2a,
centers were preserved, due to
a redox-tautomerization acetone solution of 5a, and a CH2Cl2–acetone mixture of 4a, or
process.29,30 Complexes 3a and 3b, [(RCCCR)Ru(CO)2(CH3CN)]- by slow diffusion of diethyl ether into a CH2Cl2 solution of 2b,
PF6, were obtained quantitatively from direct reaction of 2a CH3CN solutions of 3a, 3b, and 7a, and a CH3CN–acetone
and 2b with AgPF6 in CH3CN at ambient temperatures. Reac- mixture of 6a. Crystallographic data and refinement para-
tion of 2a with AgPF6 in refluxing CH3CN in the presence of meters are listed in Table S1 (ESI†). Selected bond lengths and
Me3NO for 12 h afforded complex 4a, [(MeCCCMe)Ru(CO)- angles are summarized in Table 1. All eight complexes showed
(CH3CN)2]PF6. Complex 5a, [(MeCCCMe)Ru(CO)(bpy)]I, was syn- similar coordination features around the RuII center. A pincer
thesized by heating 2a in ethylene glycol at 165 °C for 1 h in ligand, two or one CO ligands, and a halide (2a and 2b) or one
the presence of bipyridine. Complex 6a, [(MeCCCMe)Ru- or two CH3CN (3a, 3b, and 4a) or an amine-donor ampy (6a) or
(CO)2(ampy)]PF6, was obtained from direct reaction of 2a with a chelating bpy/ampy ligand (5a/7a) are orthogonally arranged
2-(aminomethyl)pyridine (ampy) in CH2Cl2 at ambient temp- around the RuII core to form a pseudo-octahedral geometry.
erature for 2 h in the presence of AgPF6. Alternatively, on using The pincer ligand occupies three meridional sites with three
ethylene glycol as the solvent and heating the reaction mixture rings in a quasi-planar disposition and a bite angle Ccarbene
–
at 100 °C for 8 h in the presence of NH4PF6, complex 7a, Ru–Ccarbene ranging from 151.3 to 154.1°, which is analogous
[(MeCCCMe)Ru(CO)(ampy)]PF6, was obtained instead of 6a. The to the closely related complexes. The Ru–CPh distance was in
difference between 6a and 7a is the coordination mode of the the order of 2.00–2.05 Å. The Ru–Ccarbene distances of
ampy ligand. In 6a, only the –NH2 moiety of the ampy ligand 2.07–2.12 Å, which essentially lie in the single bond range, are
coordinates to the RuII center, while in complex 7a, ampy acts slightly longer than those in other RuII complexes with pincer
as a bidentate chelating ligand. All complexes were analytically NHC ligands (2.048–2.077 Å).12i,20c,d,21–23,31
characterized by NMR, IR spectroscopy and Mass spectro-
As shown in Fig. 1, the two CO ligands in 2a are oriented in
metry. Assignments of signals in the NMR spectra of the a mutually cis fashion with Ru–CCO distances of 1.944(6) (trans
complexes were made by comparing with other analogous to the phenyl ring with the CCO–Ru–Cphenyl angle of 174.2(2)°)
1
RuII–NHC complexes.12i,20c,d,21–23,31 The H NMR spectra of all and 1.926(8) Å (trans to iodide with the CCO–Ru–I angle of
complexes are devoid of NCHN imidazolium proton reson- 175.50(18)°), and the corresponding C–O distances are
ances, while the 13C NMR spectra exhibit resonances for carbo- 1.006(7) and 1.131(7) Å, respectively. This data is commensu-
nyl carbon atom(s) at δ ∼ 200 ppm, characteristic carbenoid rate with the CO stretching frequencies (2007 and 1949 cm−1
carbon atom(s) at δ ∼ 180 ppm, and a coordinated phenyl observed from the IR spectrum.
)
carbon atom at δ ∼ 160 ppm, confirming the coordination of
The core structure and the coordination environment of 2b
the phenyl-bridged pincer dicarbene ligand and carbonyl (Fig. S1, ESI†) are quite similar to that of 2a, depicting slight
ligand(s). The IR spectra of complexes 2a, 2b, 3a, 3b, and 6a changes in the bond parameters (Table 1), with two N-functio-
show two ν(CO) stretching frequencies at 1935–1981 and nalized nbutyl wing tips pointing toward the same direction
Table 1 Selected bond lengths (Å) and bond angles (°) of complexes
2a
2b
3a
3b
4a (A)
4a (B)
5a
6a
7a
Ru–Cphenyl
Ru–Ccarbene
2.044(5)
2.098(5)
2.102(5)
2.047(3)
2.108(3)
2.106(3)
2.106(3)
1.953(4)
1.915(4)
2.5936(5)
2.045(2)
2.116(2)
2.102(2)
2.052(2)
2.107(2)
2.107(3)
2.018(6)
2.090(5)
2.089(6)
2.039(6)
2.089(5)
2.087(6)
2.004(3)
2.093(3)
2.074(3)
2.041(2)
2.108(2)
2.114(3)
1.997(2)
2.103(2)
2.103(2)
Ru–CCO (trans to Ph)
Ru–CCO
Ru–I/Br
1.944(6)
1.926(8)
2.7671(6)
1.963(3)
1.856(2)
1.959(3)
1.863(2)
1.932(7)
1.926(8)
1.963(3)
1.857(3)
1.835(7)
1.833(3)
Ru–NCH CN
2.1245(19)
2.1012(19)
2.034(5)
2.012(5)
2.033(5)
2.018(6)
3
Ru–Nbpy/ampy
C–O
2.126(3)
2.121(3)
2.201(2)
2.1742(19)
2.1749(19)
1.131(7)
1.006(7)
152.9(2)
174.2(2)
175.50(18)
1.130(4)
1.039(5)
151.89(13)
176.28(14)
179.51(10)
1.127(3)
1.135(3)
151.31(10)
173.50(9)
1.126(3)
1.130(3)
151.57(11)
177.16(9)
1.140(8)
1.126(9)
1.128(4)
1.141(3)
151.51(10)
178.79(12)
1.155(8)
152.98(14)
1.149(3)
154.09(9)
C
C
C
C
carbene–Ru–Ccarbene
CO–Ru–Cphenyl
CO–Ru–I/Br
152.6(3)
178.7(3)
153.1(3)
178.5(3)
CO–Ru–NCH CN
177.39(8)
178.74(9)
3
N
CH CN–Ru–NCH CN
178.2(2)
178.5(2)
3
3
Cphenyl–Ru–Nbpy/ampy
CO–Ru–Nbpy/ampy
170.70(14)
172.1(2)
165.74(8)
175.11(9)
C
176.28(10)
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parameters. As shown in Fig. 3, two CH3CN ligands are
mutually trans with an average Ru–N distance of 2.023 Å,
slightly shorter than that in 3a (2.1245(19) Å), in which the
CH3CN is trans to a CO ligand.
The molecular structure of 5a, as shown in Fig. 4(a), to a
large extent resembles the previously known complex
[
nBuCNCnBu)Ru(bpy)(H2O)]2+ (where CNC is a pyridyl bridged
pincer type bis(carbene) ligand).32 Apart from the pincer
ligand, there is a chelating bipyridine that coordinates trans to
the bridging phenyl group of the pincer ligand and the carbo-
nyl ligand with Cphenyl–Ru–Nbpy and CCO–Ru–Nbpy bond angles
spanning 170.70(14) and 172.1(2)°, respectively. The bpy
ligand is almost perpendicular to the pincer ligand plane with
a dihedral angle of 88.94°. The Ru–Nbpy distances, measuring
2.126(3) and 2.121(3) Å, tend to be longer than those in
Fig. 1 ORTEP plot (30% probability ellipsoids) of 2a.
[
nBuCNCnBu)Ru(bpy)(H2O)]2+ (2.038(4) and 2.011(3) Å) due to
from the pincer ligand plane and opposite to that of the the anionic character of the CCC-pincer ligand.
bromide ligand.
The major structural features of 6a resemble those of 3a
The major structural features of 3a and 3b are also similar with slight changes in the bond parameters. As shown in
to those of 2a and 2b, as shown in Fig. S2 (ESI†) and Fig. 2, Fig. 4(b), a 2-(aminomethyl)pyridyl group substitutes the
respectively. The halide ligand had been replaced by CH3CN CH3CN ligand in 3a and coordinates to the RuII center via a
with the Ru–N distances measuring 2.1245(19) Å for 3a and –NH2 group, featuring a Ru–N distance of 2.201(2) Å.
2.1012(19) Å for 3b. The replacement of a halide ligand with a
The molecular structure of 7a, as depicted in Fig. 4(c), is
neutral CH3CN results in a cationic RuII complex with substan- analogous to 5a except that a chelating 2-(aminomethyl)pyri-
tially shorter Ru–CCO bond distances trans to the CH3CN dine (ampy) ligand has displaced the bpy ligand with the –NH2
moiety (Ru–CCO = 1.856(2) Å for 3a and 1.863(2) Å for 3b). Pre- group trans to the carbonyl and the pyridyl nitrogen trans to
dictably, positive charges on complexes 3a and 3b decrease the the phenyl ring of the pincer ligand. The two trans angles,
back-bonding and hence increase the ν(CO) stretching fre- C19–Ru1–N6 and C1–Ru1–N5, are 175.11(9) and 165.74(8)°,
quencies in comparison to those of the neutral complexes 2a respectively. The bond distances of Ru1–N6 and Ru1–N5 are
and 2b. The complex 3b displays similar bonding features as 2.1749(19) and 2.1742(19), respectively, and are longer than
n
those of 2b, with two N-functionalized butyl wing tips point- those observed in Ru1–N5/6 (bpy) bond distances in 5a due to
ing toward the same direction from the pincer ligand plane the reduced back-bonding ability of the ampy ligand, however,
and opposite to that of CH3CN ligand (in 3b) or the bromide they are shorter than that in 6a due to the chelate effect. More-
ligand (in 2b) (Fig. 2 right).
over, the dihedral angle between the pyridine ring of the ampy
The molecular structures of [(MeCCCMe)Ru(CO)(CH3CN)2]- chelating ligand and the pincer ligand plane is reduced to
PF6 (4a), Fig. 3, shows that the asymmetric unit displays two 75.58°.
independent cations with similar core structures and coordi-
All eight complexes reported herein were evaluated for cata-
nation environments. The major bonding features in complex lytic activity towards transfer hydrogenation of ketones using
4a are similar to those of 3a with slight changes in the bond 2-propanol as the hydrogen donor and NaOH as the base. The
Fig. 2 (Left) ORTEP plot (30% probability ellipsoids) of 3b. PF6 anion and co-solvent CH3CN are omitted for clarity. (Right) Orientation of two nBu groups in space.
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Fig. 3 ORTEP plot (30% probability ellipsoids) of 4a. PF6 anion and cosolvents (CH3CN and acetone) are omitted for clarity.
Fig. 4 ORTEP plot (30% probability ellipsoids) of (a) 5a, (b) 6a and (c) 7a. Anions are omitted for clarity.
standard transfer hydrogenation experiment was carried out
using 2.0 mmol of cyclohexanone and 0.4 mmol of base in
10 mL of 2-propanol with 1.0 mol% Ru-catalyst loading at
82 °C. The product yields were monitored by GC. The catalytic
results are summarized in Fig. 5. Reaction profiles show that
complexes 2a, 2b, 3a, 3b, and 4a demonstrate very good activity
towards catalytic transfer hydrogenation of ketones and quan-
titatively converted them into the corresponding alcohol
within 15 min. Complex 7a took 40 min to reach the same
result, while complexes 5a and 6a required almost 3 and 2 h,
respectively, to effect the complete conversion. Complex 4a,
bearing two labile CH3CN ligands, is expected to be an
efficient catalyst for transfer hydrogenation of ketones.
However, complexes 2a, 2b, 3a, and 3b, bearing only one labile
halide or CH3CN ligand, also showed reasonably good activi-
Fig. 5 Reaction profiles of catalytic transfer hydrogenation of cyclohexanone.
Reaction conditions: 2.0 mmol of cyclohexanone and 0.4 mmol of NaOH in
10 mL of 2-propanol with 1.0 mol% Ru-catalyst loading at 82 °C.
ties compared to that of 4a. Comparing the reaction profiles of
2a vs. 2b and 3a vs. 3b, it seems that the N-wingtip substituent
on the NHC (Me vs. nBu) does not significantly influence the
result of the catalytic conversion, due to the structural features
of 2b and 3b.
reaction, the reaction mixture was extracted in a dichloro-
In order to study the stabilizing effect offered by the cyclo- methane/water system. The organic phase was separated and
metallated pincer-bis-(carbene) and carbonyl ligands in the the aqueous layer was decanted. The residue was dried
catalytic reaction, the RuII complexes were examined after the in vacuo and was subjected to NMR and IR analyses. Results
catalytic reaction. After the completion of the catalytic revealed that the pincer ligand and the two carbonyl groups
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remain intact at the ruthenium center in 2a, 2b, 3a, 3b, and frequencies (TOF) in 3–6 h with 2a, 2b, 3a, 3b, 4a, 6a, and 7a.
6a, as confirmed by NMR and IR spectra, after the catalytic Amongst the eight new RuII–CCC-pincer-bis(carbene) com-
reactions. In the case of 6a, the 1H NMR resonances corres- plexes described herein, 2a, 2b, 3a, 3b, and 4a demonstrate
ponding to the 6-(aminomethyl)pyridine ligand disappeared a high TOF ≥ 3000 for the reduction of cyclohexanone.
after the reaction. We believe that these di-carbonyl RuII– Complex 4a shows a very high activity for the reduction of
pincer-bis(carbene) precatalysts release the halide, CH3CN, or cyclohexanone, resulting in 99% conversion with a TOF of
the NH2-coordinated ampy ligand, to generate a coordination 3300 achieved in 3 h. Nevertheless, the reduction of acetophen-
vacancy for the incoming deprotonated alkoxide moiety, to one and benzophenone occurred with 99% and 90% conver-
initiate the catalytic reaction. This also explains the low cataly- sions with a slightly longer reaction time (5 h) and relatively
tic efficiency of 6a in comparison to that of 2a, 2b, 3a, and 3b. lower TOF of 1980 and 1800, respectively. Although the cata-
During the 10 min of catalytic reaction of the complex 4a, we lytic activities of ruthenium complexes bearing traditional
observed the formation of traces of ruthenium black. Post cata- phosphine and amine donors are superior,15 the results
lytic analysis of the complex 4a revealed that only the pincer reported herein are among the best within closely related Ru–
ligand and a carbonyl group remain coordinated to the ruthe- NHC complexes6d,20–23 that lack the assistance offered by the
nium center. After the release of two CH3CN ligands, the cata- NH effect.
lytically active species might not have been stable enough and
The mechanism of the ruthenium catalyzed transfer hydro-
hence decomposed gradually during the course of the reaction. genation reaction of ketones has been previously described.33
On the other hand, we quantitatively recovered complexes 5a Compared with the literature reports,6d,20–23 the most efficient
and 7a, with a chelating bidentate bpy and an ampy ligand, precatalyst 4a contains two labile acetonitrile ligands at the
respectively, still coordinated to the ruthenium center after the ruthenium center and hence enables the generation of coordi-
catalytic reactions.
nation vacancies for the incoming deprotonated alkoxide
To investigate the effect of catalyst loading on the product moiety and the subsequent formation of the metal hydride,
yield and the tolerance of the substituent functionality on the thus contributing to the embedding matrix. In the case of
ketones, similar optimal reaction conditions were employed. complexes 2a, 2b, 3a, or 3b, where the precatalyst has only one
Table 2 summarizes the results of the catalytic transfer hydro- labile halide or CH3CN, we propose that, after the dissociation
genation of cyclohexanone, acetophenone, and benzophenone of the halide or the acetonitrile ligand, the CCC-pincer-bis
using 0.010 mol% catalytic loading after 3–6 h of reaction. (carbene)–RuII intermediate that might be in a dangling mode
Cyclohexanone, acetophenone, and benzophenone underwent could act as the active species in the catalytic cycle.33,34 Ana-
hydrogen transfer reduction efficiently with good turnover lysis of the reaction mixture after the catalytic transformation
Table 2 Catalytic transfer hydrogenation of ketonesa,b
Cat.
Substrate
Time (h)
Yield (%)
TOF (h−1
)
2a
Cyclohexanone
Acetophenone
Benzophenone
Cyclohexanone
Acetophenone
Benzophenone
Cyclohexanone
Acetophenone
Benzophenone
Cyclohexanone
Acetophenone
Benzophenone
Cyclohexanone
Acetophenone
Benzophenone
Cyclohexanone
Acetophenone
Benzophenone
Cyclohexanone
Acetophenone
Benzophenone
3
93
3100
3/5
3/5
3
3/5
3/5
3
3/5
3/5
3
3/5
3/5
3
3/5
3/5
3/6
3/6
3/6
3/5
3/5
3/5
56/94
58/92
90
48/88
53/90
97
74/99
60/89
96
65/95
58/90
99
67/99
60/90
28/94
20/88
16/85
61/97
41/85
33/77
1867/1880
1934/1840
3000
1600/1760
1767/1800
3233
2467/1980
2000/1780
3200
2167/1900
1934/1800
3300
2234/1980
2000/1800
933/1567
667/1467
533/1417
2034/1940
1367/1700
1100/1540
2b
3a
3b
4a
6a
7a
a Reaction conditions: 2.0 mmol ketone, 0.4 mmol NaOH, 0.010 mol% Ru cat., 10 mL 2-propanol, 82 °C and 3–6 h. Product yields were detected
by GC. b 18% conversion of cyclohexanone was observed in the absence of catalysts.
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confirms the absence of any dissociated imidazolium moiety, (2.65 μm film thickness) RESTEK Rtx-2887 fused silica capil-
which is attributed to the protection offered by the CCC- lary column.
pincer-bis(carbene) ligand that stabilizes the ruthenium
center, preventing its degeneration during the course of the
catalytic cycle, and thus explains the higher yield in a relatively
Synthesis of (MeCCCMe)Ru(CO)2I (2a)
[
MeCCCMe-H3]I2 (1a) (100 mg, 0.202 mmol), [Ru(COD)Cl2]n
short reaction time. A similar mechanism might operate with (113 mg, 0.404 mmol) and triethylamine (204 mg, 20.2 mmol)
the complex 6a. However, a longer time was necessary for the were mixed with ethylene glycol (0.5 mL) and EtOH (1.0 mL) in
dissociation of the NH2-coordinated ampy ligand so as to a screw capped vial. The reaction mixture was heated at 165 °C
initiate the catalytic reaction. In complexes 5a and 7a, bearing for 24 h. After cooling to room temperature, the reaction
a chelating bpy or ampy ligand, we assume that both the CCC- mixture was suspended in dichloromethane (5.0 mL) and fil-
pincer-bis(carbene) and bpy/ampy ligands in the complex tered. To the filtrate was added water and the mixture was
might also be in a dangling mode to generate the active RuII shaken well in a separating funnel and the organic layer was
intermediate for the catalytic cycle. Further work in our labora- separated and concentrated. Addition of a small amount of
tory is directed to isolate the catalytic intermediate as well as MeOH into the concentrated solution resulted in the precipi-
to attain improved catalytic efficiencies by suitably functiona- tation of the colorless product 2a. The product was collected
lizing the coordinated poly carbene ligands.
The recyclability of 3a and 5a was examined following a 0.15 mmol, 73% yield). H NMR (DMSO-d6): δ 8.20 (d, JH–H
strategy of repetitive process described in the literature (see 2.1 Hz, 2H, imidazole H), 7.54 (d, JH–H = 2.1 Hz, 2H, imidazole
experimental part).35 The precatalysts were used for seven H), 7.40 (JH–H = 7.8 Hz, 2H, phenyl-H3, H5), 7.17 (t, JH–H
by filtration and dried under reduced pressure (78 mg,
1
=
=
repetitive runs and the product yield remained constant 7.8 Hz, 1H, phenyl-H4), 3.89 (s, 6H, NCH3) ppm. 13C NMR
(≧97%) in 30 min/3 h of reaction. These promising results (DMSO-d6): δ 193.3, 193.1 (CO), 183.1 (NCN), 159.4 (Ru-CPh),
confirm the high catalytic activity and stability of the catalysts, 147.4, 124.8, 122.7, 116.4 (phenyl-C), 108.2 (imidazole-C), 37.8
and hence demonstrate their potential use for recycle-transfer (CH3) ppm. IR (KBr): νCO 2007, 1949 cm−1. Mass (MALDI):
hydrogenation reaction of ketones.
m/z = 494 (M+ − CO), 466 (M+ − 2CO), 367 (M+ − CO − I). Anal.
calcd for C16H13IN4O2Ru: C 36.86, N 10.75, H 2.51. Found: C
36.81, N 10.71, H 2.60%.
Conclusions
Synthesis of (nBuCCCnBu)Ru(CO)2Br (2b)
In summary, we have prepared the first series of ruthenium
[
nBuCCCnBu-H]Br2 (1b) (100 mg, 0.206 mmol), [Ru(COD)Cl2]n
carbonyl CCC-pincer-bis(carbene) complexes and structurally (115 mg, 0.411 mmol) and triethylamine (205 mg, 20.3 mmol)
characterized all eight complexes. We have evaluated these were mixed with ethylene glycol (0.5 mL) and EtOH (1.0 mL) in
complexes towards catalytic transfer hydrogenation reactions. a sealed tube. A similar procedure as that for the synthesis of
Complexes 2a, 2b, 3a, 3b, and 4a demonstrate good activities 2a was followed to obtain product 2b (77 mg, 0.138 mmol,
1
for the reduction of cyclohexanone, with 4a effecting 99% con- 67%). H NMR (DMSO-d6): δ 8.20 (d, JH–H = 2.1 Hz, 2H, imid-
version and a TOF of 3300 in 3 h. Complexes 3a and 5a azole H), 7.58 (d, JH–H = 2.1 Hz, 2H, imidazole H), 7.40 (JH–H
=
showed excellent recyclability for seven repetitive runs with 7.8 Hz, 2H, phenyl-H3, H5), 7.20 (t, JH–H = 7.8 Hz, 1H, phenyl-
consistently high conversions (≥97%). The rigid coordination H4), 4.22 (t, JH–H = 7.2 Hz, NCH2CH2CH2CH3, 4H), 1.87 (m,
sphere around the ruthenium center offered by the CCC- NCH2CH2CH2CH3, 4H), 1.36 (m, NCH2CH2CH2CH3, 4H), 0.93
pincer-bis(carbene) and carbonyl ligands, contributes to the (t, JH–H = 7.2 Hz, 6H, NCH2CH2CH2CH3) ppm. 13C NMR
stability of complexes and their potential application in (acetone-d6): δ 201.4 and 194.4 (CO), 184.5 (NCN), 161.7 (Ru-
recycle-transfer hydrogenation reactions.
CPh), 147.2, 125.0 and 121.6 (phenyl-C), 116.3 and 108.4 (imi-
dazole-C), 51.2, 33.4, 19.6, and 13.2 (nBu-C) ppm. IR (KBr):
νCO 2004, 1935 cm−1. Mass (MALDI): m/z = 532 (M+ − CO), 504
(M+ − 2CO), 480 (M+ − Br), 452 (M+ − Br − CO), 323 (L+). Anal.
calcd for C22H25Br N4O2Ru: C 47.31, N 10.03, H 4.51. Found: C
Experimental
General procedures, materials and physical measurements: 47.24, N 10.08, H 4.61%.
solvents and reagents were used without further purification.
1
Synthesis of [(MeCCCMe)Ru(CO)2(CH3CN)]PF6 (3a)
Solvents were purchased from Aldrich Chemical Co. H NMR
spectra were recorded on a Bruker DPX-300 spectrometer (1H, Treating 2a (100 mg, 0.192 mmol) with AgPF6 (50 mg,
299.96 MHz; 13C, 75.43 MHz) with tetramethylsilane as an 0.198 mmol) in CH3CN (2.0 mL) at r. t., for 6 h, followed by fil-
internal standard. Elemental microanalyses were performed at tration and removal of the solvent in vacuum, afforded the
the Taiwan Instrumentation Center. IR spectra were recorded product 3a as a white solid (109 mg, 0.188 mmol, 98% yield).
on a Mattson Genesis Series FT-spectrophotometer. GC ana- 1H NMR (acetone-d6): δ 8.14 (d, JH–H = 2.0 Hz, 2H, imidazole H),
lyses were performed with a GC9800 gas chromatograph 7.57 (d, JH–H = 2.0 Hz, 2H, imidazole H), 7.50 (JH–H = 7.8 Hz,
(Shanghai Kechuang Chromatograph Instrument Co.) 2H, Phenyl-H3, H5), 7.36 (t, JH–H = 7.8 Hz, 1H, phenyl-H4), 4.12
equipped with a flame ionization detector and a 10 m (s, 6H, NCH3), 2.14 (s, 3H, CH3CN) ppm. 13C NMR (acetone-d6):
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1
δ 200.0 and 197.8 (CO), 179.6 (NCN), 153.8 (Ru-CPh), 147.7, 0.171 mmol, 89% yield). H NMR (DMSO-d6): δ 9.68 (d, JH–H
=
127.2 (phenyl-C), 125.2 (CH3CN), 124.0 (phenyl-C), 117.4 and 5.1 Hz, 1H, bpy-H), 8.79 (d, JH–H = 8.1 Hz, 1H, bpy-H), 8.67 (d,
109.5 (imidazole-C), 38.3 (CH3), 1.96 (CH3CN) ppm. IR (KBr): JH–H = 8.1 Hz, 1H, bpy-H), 8.25 (d, JH–H = 8.1 Hz, 1H, bpy-H),
νCO 1972, 2043 cm−1. Mass (MALDI): m/z = 436 (M+), 395 8.21 (d, JH–H = 1.8 Hz, 2H, imidazole H), 7.95 (t, JH–H = 8.1 Hz,
(M+ − CH3CN), 367 (M+ − CO − CH3CN). Anal. calcd for 1H, bpy-H), 7.82 (d, JH–H = 7.8 Hz, 1H, bpy-H), 7.55 (d, JH–H
=
C18H16F6N5O2PRu: C 37.25, N 12.07, H 2.78. Found: C 37.31, N 7.8 Hz, 2H, phenyl-H) 7.32 (m, 3H, imidazole-H; phenyl-H),
12.02, H 2.83%.
7.25 (t, JH–H = 6.9 Hz, 1H, bpy-H), 7.03 (d, JH–H = 5.1 Hz, 1H,
bpy-H), 2.49 (s, 6H, NCH3) ppm. 13C NMR (CD3CN): δ 199.6
(CO), 186.5 (NCN), 162.9 (Ru-CPh), 155.6, 154.6, 153.7, 149.9
Synthesis of [(nBuCCPCnBu)Ru(CO)2(CH3CN)]PF6 (3b)
A similar procedure as that for the synthesis of 3a was followed (bpy-C), 148.4 (phenyl-C), 138.4, 137.4, 127.7, 126.7, 125.4
to obtain 3b as white solid (98% yield). 1H NMR (DMSO-d6): (bpy-C), 124.9 (phenyl-C), 124.1 (bpy-C), 123.8 (phenyl-C),
δ 8.19 (d, JH–H = 2.1 Hz, 2H, imidazole H), 7.65 (d, JH–H = 2.1 117.4, 109.7 (imidazole-C), 36.7 (CH3) ppm. IR (KBr): νCO
Hz, 2H, imidazole H), 7.51 (JH–H = 7.8 Hz, 2H, phenyl-H3, H5), 1931 cm−1. Mass (MALDI): m/z = 650 (M+ + I), 523 (M+), 495
7.38 (t, JH–H = 7.8 Hz, 1H, phenyl-H4), 4.44 (t, JH–H = 7.3 Hz, (M+ − CO). Anal. calcd for C25H21IN6ORu: C 46.23, N 12.94, H
NCH2CH2CH2CH3, 4H), 2.12 (s, 3H, CH3CN), 1.98 (m, 3.26. Found: C 46.36, N 12.86, H 3.40%.
NCH2CH2CH2CH3, 4H), 1.50 (m, NCH2CH2CH2CH3, 4H), 0.99
Synthesis of [(MeCCCMe)Ru(CO)2(ampy)]PF6 (6a)
(t, JH–H = 7.2 Hz, 6H, NCH2CH2CH2CH3) ppm. 13C NMR
(acetone-d6) δ 199.3 and 193.5 (CO), 178.7 (NCN), 154.5 (Ru- 2-(Aminomethyl)pyridine (ampy) (58 mg, 0.53 mmol) was
CPh), 148.4 and 128.0 (phenyl-C), 125.2 (CH3CN), 123.6 added dropwise to a dichloromethane (5 mL) solution of 2a
(phenyl-C), 118.1 and 110.2 (imidazole-C), 52.1, 34.3, 20.4, and (105 mg, 0.201 mmol) and AgPF6 (67 mg, 0.26 mmol). The
13.8 (nBu-C), 2.57 (CH3CN) ppm. IR (KBr): νCO 1981, reaction mixture was then stirred at ambient temperature for
2038 cm−1. Mass (MALDI): m/z = 520 (M+), 479 (M+ − CH3CN). 1 h. After filtration through celite, the filtrate was washed with
Anal. calcd for C24H28F6N5O2PRu: C 43.38, N 10.54, H 4.25. water and the organic layer was concentrated in vacuo. The
Found: C 43.48, N 10.70, H 4.46%.
addition of diethyl ether into the concentrated solution
resulted in the precipitation of the pale yellow product 6a. The
product was purified by washing with a small amount of ethyl
Synthesis of [(MeCCCMe)Ru(CO)(CH3CN)2]PF6 (4a)
2a (100 mg, 0.192 mmol) and AgPF6 (96 mg, 0.23 mmol) were acetate and drying in vacuo (33 mg, 0.051 mmol, 25% yield).
mixed in CH3CN (5 mL) and the reaction mixture was stirred at 1H NMR (CD3CN): δ 8.29 (d, JH–H = 5.3 Hz, 1H, ampy-H), 7.78
r.t., for 1 h. The reaction mixture was filtered through celite (d, JH–H = 2.0 Hz, 2H, imidazole H), 7.60 (t, JH–H = 7.8 Hz, 1H,
and the filtrate was refluxed with Me3NO (33 mg, 0.28 mmol) phenyl-H), 7.32 (m, 3H, phenyl-H), 7.25 (d, JH–H = 2.1 Hz, 2H,
for 12 h. After cooling the reaction mixture to r.t., the solvent imidazole-H) 7.17 (t, JH–H = 6.3 Hz, 1H, ampy-H), 6.88 (d, JH–H
=
was removed in vacuo and the residue was subjected to neutral 7.8 Hz, 1H, ampy-H), 3.87 (s, 6H, NCH3), 3.26 (t, JH–H = 6.0 Hz,
Al2O3 column chromatography using acetone as the eluent to 2H, ampy-CH2), 2.99 (br, 2H, NH2) ppm. 13C NMR (CD3CN): δ
obtain the pure product 4a (51.0 mg, 0.108 mmol, yield 56%). 199.6, 194.3 (CO), 181.6 (NCN), 157.1 (Ru-CPh), 155.5 (py-C),
1H NMR (acetone-d6): δ 8.07 (d, JH–H = 2.0 Hz, 2H, imidazole H), 148.6 (phenyl-C), 147.5, 136.8 (py-C), 127.0, 123.5 (phenyl-C),
7.47 (d, JH–H = 2.0 Hz, 2H, imidazole H), 7.38 (d, JH–H = 7.8 Hz, 122.5, 121.5 (py-C), 117.0 and 109.3 (imidazole-C), 51.0 (ampy-
2H, Phenyl-H3, H5), 7.26 (t, JH–H = 7.8 Hz, 1H, phenyl-H4), 4.15 CH2), 38.1 (CH3) ppm. IR (KBr): νCO 2033, 1975 cm−1. Mass
(s, 6H, NCH3), 1.98 (br, 3H, CH3CN) ppm. 13C NMR (acetone- (MALDI): m/z = 504 (M+), 476 (M+ − CO), 448 (M+ − 2CO), 368
d6): δ 203.3 (CO), 188.5 (NCN), 165.9 (Ru-CPh), 148.5 (phenyl- (M+ − CO − ampy), 340 (M+ − 2CO − ampy). Anal. calcd for
C), 125.3 (CH3CN), 124.1 (phenyl-C), 122.7 (phenyl-C), 116.5 C22H21F6N6O2PRu: C 40.81, N 12.98, H 3.27. Found: C 40.86, N
and 108.0 (imidazole-C), 37.8 (CH3), 2.24 (CH3CN) ppm. IR 12.89, H 3.37%.
(KBr): νCO 1974 cm−1. Mass (MALDI): m/z = 449 (M+), 421
Synthesis of [(MeCCCMe)Ru(CO)(ampy)]PF6 (7a)
(M+ − CO), 380 (M+ − CO − CH3CN), 367 (M+ − 2CH3CN), 339
(M+ − CO − 2CH3CN). Anal. calcd for C19H19F6N6OPRu: C 2-(Aminomethyl)pyridine (ampy) (230 mg, 2.11 mmol) was
38.45, N 14.16, H 3.23. Found: C 38.38, N 14.12, H 3.32%.
added to a ethylene glycol (5 mL) solution of 2a (113 mg,
0.216 mmol). The reaction mixture was heated at 100 °C for
8 h. After cooling to r.t., saturated NH4PF6 solution (2 mL) was
Synthesis of [(MeCCCMe)Ru(CO)(bpy)]I (5a)
2a (100 mg, 0.192 mmol) and 2,2′-bipyridine (bpy) (35 mg, added to precipitate the crude product 7a as a colorless solid.
0.236 mmol) were dissolved in ethylene glycol (2 mL) in a Compound 7a was further purified by washing with a small
sealed tube. The reaction mixture was heated at 165 °C for 1 h. amount of dichloromethane three times, and drying in vacuo
1
After cooling to r.t., the reaction mixture was extracted with (57 mg, 0.092 mmol, 43% yield). H NMR (CD3CN): δ 9.34 (d,
dichloromethane–water (5/5 mL). The organic layer was dried JH–H = 5.3 Hz, 1H, ampy-H), 7.85 (m, 1H, ampy-H), 7.78 (d,
over MgSO4, filtered, and concentrated. Addition of diethyl JH–H = 2.1 Hz, 2H, imidazole H), 7.44 (d, JH–H = 7.8 Hz, 2H,
ether into the concentrated solution resulted in the precipi- phenyl-H), 7.21 (m, 5H, 2 imidazole-H, 2 ampy-H, 2 phenyl-H),
tation of 5a as an orange solid. The product was collected by 3.87 (t, JH–H = 6.0 Hz, 2H, ampy-CH2), 3.36 (s, 6H, NCH3), 2.74
filtration and dried under reduced pressure (111 mg, (br, 2H, NH2) ppm. 13C NMR (CD3CN): δ 197.6 (CO), 187.1
13168 | Dalton Trans., 2013, 42, 13161–13171
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(NCN), 161.1 (Ru-CPh), 159.7, 152.4 (py-C), 148.0 (ph-C), 137.0
(py-C), 124.6, 123.2 (ph-C) and 122.1 (Py-C), 117.1 and 108.2
(imidazole-C), 51.6 (ampy-CH2), 36.3 (CH3) ppm. IR (KBr): νCO
1897 cm−1. Mass (MALDI): m/z = 476 (M+), 448 (M+ − CO).
Anal. calcd for C21H21F6N6OPRu: C 40.61, N 13.54, H 3.41.
Found: C 40.50, N 13.46, H 3.46%.
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In a standard experiment, the RuII catalyst (1.0 mol%), NaOH
i
(0.40 mmol), and 10 mL of PrOH were charged into a screw
capped vial under air, and heated in an oil bath for 20 min.
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General procedure for the recycling experiment
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as in the original reaction, and the reaction was continued for
the next run.
X-Ray crystallography
The single crystals suitable for X-ray diffraction analyses were
obtained by slow evaporation of a CH2Cl2 solution of 2a, an
acetone solution of 5a, and a CH2Cl2–acetone mixture of 4a, or
by slow diffusion of diethyl ether into a CH2Cl2 solution of 2b,
CH3CN solutions of 3a, 3b, 7a, or a CH3CN–acetone mixture of
6a. Data was obtained on an APEX II diffractometer, using
graphite monochromatic Mo Kα radiation (λ = 0.71073 Å). Data
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data, including the details of data collection and refinement,
and complete geometric information are available in CIF
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Acknowledgements
The authors acknowledge the National Science Council
(Taiwan) and National Dong Hwa University for the financial
support.
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