Ruthenium ONO-type pincer complex: Synthesis, structural characterization, and catalysis

Article abstract of DOI:10.1002/adsc.201000083

A novel nitrone-based pincer ligand was developed by a single-step synthesis from N-(tert-butyl)hydroxylamine acetate and 2,6- pyridinedicarboxaldehyde. The developed ligand allowed us to synthesize a cationic ruthenium pincer complex. A distorted octahedral coordination environment around the ruthenium center was observed. The complex showed excellent catalytic activity in transfer hydrogenation reactions with turnover numbers up to 590,000.

Full text of DOI:10.1002/adsc.201000083

UPDATES
DOI: 10.1002/adsc.201000083
Ruthenium ONO-Type Pincer Complex: Synthesis, Structural
Characterization, and Catalysis
Yao Zhang,a Xingwei Li,b,c,* and Soon Hyeok Honga,*
a
Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological
University, Singapore 637371
Fax : (+65)-6791-1961; e-mail: hongsh@ntu.edu.sg
Current address: Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, Peopleꢀs Republic
b
of China
Fax : (+86)-411-8437-9089; e-mail: xwli@dicp.ac.cn
The Scripps Research Institute, Scripps Florida, Jupiter, Florida 33458, USA
c
Received: February 1, 2010; Published online: July 6, 2010
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.201000083.
Abstract: A novel nitrone-based pincer ligand was
developed by a single-step synthesis from N-(tert-
metals will presumably be enhanced. Some examples
of transition metal complexes with nitrones as ligands
have been reported.[4] In a recent report, Yao et al.
achieved the ortho-palladation of a-phenyl-tert-butyl-
nitrone, and the dimeric palladacycles have shown
high catalytic activity with excellent turnover numbers
(TONs) in the Heck coupling reactions.[5] Very recent-
ly, we also reported 1,3-dinitrone OCO-type pincer li-
gands, such as 1 and 3, and the related Pd and Ni
complexes that were highly active for the Heck and
the Kumada cross-coupling reactions (Scheme 1).[6]
In pursuit of pincer ligands that display excellent
catalytic activity along with improved stability, a
novel ONO-type pincer ligand 6 was synthesized by
modification of the phenylide part of 1 with a pyri-
dine-based neutral N donor (Scheme 2). Herein, we
report a Ru catalyst bearing the ONO pincer ligand
6, which is highly active in the transfer hydrogenation
(TH) reaction.
butyl)hydroxylamine
acetate
and
2,6-
pyridinedicarboxaldehyde. The developed ligand al-
ACHTUNGTRENNUNG
lowed us to synthesize a cationic ruthenium pincer
complex. A distorted octahedral coordination envi-
ronment around the ruthenium center was ob-
served. The complex showed excellent catalytic ac-
tivity in transfer hydrogenation reactions with turn-
over numbers up to 590,000.
Keywords: nitrones; N,O ligands; pincer complexes;
ruthenium; transfer hydrogenation
Introduction
The development of transition metal pincer com-
plexes that display high robustness and catalytic activ-
ity has been an interesting topic in homogeneous cat-
alysis, coordination chemistry, and organic synthe-
sis.[1,2] The unique balance between stability and reac-
tivity of the pincer complexes can be controlled by
systematic ligand design and subsequent cooperative
construction of metal complexes.[2] Consequently,
there has been extensive research on the development
of various pincer ligands.
Nitrones are a series of compounds well known for
their pharmaceutical and medical values, and they are
typically used as spin traps in physiological media, an-
tioxidants, and neuroprotective agents.[3] The dipolar
nature of nitrone could impart enhanced electron
density to the O (nitrone) atom, so that the coordina-
tion bonding between O (nitrone) and transition Scheme 1. Pd and Ni OCO-type pincer complexes.
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Yao Zhang et al.
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Complex 7 has been fully characterized by standard
methods including X-ray crystallography. The NMR
spectra of complex 7 showed that it has a symmetrical
structure: two equivalent t-Bu groups (d=0.88) were
1
observed in H NMR spectrum and two equivalent
PPh3 groups (d=27.3) in 31P NMR spectrum. IR spec-
tra revealed a vC of 1591 cmÀ1 for product 7 and
=
N
1557 cmÀ1 for starting material 6. The increase of vC
=
N
is in agreement with the previous example,[4b] reflect-
ing the increase of bond order of C=N. A single crys-
tal of 7·CH2Cl2 suitable for the X-ray crystallographic
analysis was obtained by diffusing the Et2O into the
solution of complex 7 in CH2Cl2. The crystal structure
of the cation of 7 (Figure 1) features a distorted octa-
hedron.[8] The N(1) Ru(1) Cl(1) angle is slightly dis-
À
À
Scheme 2. Ru ONO-type pincer complex.
torted from the idealized 1808 to 178.14(8)8, and the
À
À
N(1) Ru(1) P(1) angle is 88.603(15)8. P(1) and
P(1A) could be considered to occupy axial positions
within the octahedron, and O(1), O(2), N(1) and
Cl(1) occupy equatorial positions. In addition, Cl(1) is
Results and Discussion
Condensation between N-(tert-butyl)hydroxylamine located in a trans position to pyridine moiety. The
À
À
acetate and 2,6-pyridinedicarboxaldehyde (5) in the Ru O(1) and Ru O(2) bond lengths are 2.056(2) and
presence of a base (NaHCO3) and a dehydrating re- 2.051(2) ꢂ. They are a little shorter than the non-
agent (Na2SO4) afforded the ONO-type pincer ligand pincer system [2.214(4) ꢂ] in a metalloporphyrin ni-
(6) in 73% yield.[6,7] Treatment of 6 and RuCl (DMPO)][9] reported by
2ACHTUNGTREN(UNNG PPh3)3 trone complex [RuACHTUNRTEGG(NNUN oep)(CO)ACHTGUNTRENNUGN
at 1108C for 8 h led to a dark red solution. Removal Richter–Addo,[4a] which indicates that the pincer
À
of the solvent and subsequent recrystallization system may enhance the Ru O
(CH2Cl2/Et2O) could lead to the isolation of the cat- pected.
ionic Ru complex 7 in 53% yield.
ACHTUNGTREN(NUNG nitrone) bond as ex-
Figure 1. Molecular structure of the cation of 7 with ellipsoids shown at 50% thermal probability. Hydrogen atoms are omit-
À
À
À
À
ted for clarity. Selected bond distances (ꢂ): Ru1 N1: 2.006(3), Ru1 O1: 2.056(2), Ru1 O2: 2.051(2), Ru1 P1: 2.3988(7),
À
À
À
À
À
À
À
Ru1 Cl1: 2.4586(8). Selected bond angles (deg): N1 Ru1 O1: 94.87(10), N1 Ru1 O2: 95.14(10), N1 Ru1 P1: 88.603(15),
À
À
À
À
À
À
À
À
O2 Ru1 O1: 169.99(9), O1 Ru1 P1: 89.692(15), P1 Ru1 P1A: 177.08(3), N1 Ru1 Cl1: 178.14(8).
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Ruthenium ONO-Type Pincer Complex: Synthesis, Structural Characterization, and Catalysis
Transfer hydrogenation (TH) reactions[10–12] of ke- (entry 3). For most of the substituted acetophenones,
tones were chosen to examine the catalytic activity of excellent yields of the corresponding alcohols were
7 (Table 1). Reduction using TH methods has been an obtained, and in some cases (entries 7, 10 and 12),
attractive transformation due to its simplicity at ambi- almost quantitative conversions were achieved. Slight-
ent pressure using an alternative source of hydrogen. ly reduced yields were observed with electron-defi-
The reactions were carried out in 2-propanol in the cient substrates. The reduction of aliphatic ketones
presence of a weak base, Cs2CO3. In some cases, a was also smoothly achieved (entries 16–18).
stronger base such as NaOH showed better activity or
Compared with the activity of previously reported
TON (entries 2, 3, and 17). When 1 mol% of 7 was pincer Ru complexes in the TH of ketone, complex 7
employed as catalyst, an excellent yield was obtained showed excellent TONs but exhibited a slower reac-
with acetophenone (entry 1). With a reduced loading tion rate.[12] We think that the dipolar nitrone-based
of catalyst, 0.0001 mol% of 7, 59% of the product was ONO pincer ligand 6 could stabilize the active cata-
obtained demonstrating
a
high 590,000 TON lytic intermediate sufficiently as shown with high
Table 1. Transfer hydrogenation of ketones catalyzed by 7.[a]
Entry Substrate
1
Product
Time [h] Yield [%][b]
Entry Substrate
10
Product
Time [h] Yield [%]
18
24
24
24
6
95
6
6
6
6
6
>99
97
2
3
4
5
73[c]
59[d]
88
11
12
13
14
>99
82
98
95
6
7
8
6
96
15
16
17
18
6
98
12
8
>99
95
24
24
24
84
70[e]
88
9
6
95
[a]
Reaction conditions: 0.5 mmol of ketone, 3 mL of 2-propanol, 0.005 mmol of 7, 0.1 mmol of Cs2CO3, 838C, unless other-
wise noted.
[b]
[c]
[d]
[e]
1
Determined by H NMR spectroscopy using 1,3,5-trimethoxybenzene as an internal standard.
0.01 mol% 7, 10 mol% NaOH.
0.0001 mol% 7, 10 mol% NaOH.
10 mol% NaOH.
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Yao Zhang et al.
UPDATES
TONs, but the initiation of electronically saturated Synthesis of 7
precatalyst 7 might require the dissociation of a phos-
A 25-mL oven-dried Schlenk tube was charged with 6
(141.9 mg, 0.51 mmol), RuCl2ACHTNUGRTNEUNG(PPh3)3 (489.0 mg, 0.51 mmol)
phine ligand that could be relatively strongly ligated
to the Ru center.
and 5 mL of toluene. The reaction mixture was heated to
1108C and stirred for 8 h. After removing the solvent, the
crude product was washed with Et2O (3ꢃ5 mL). Analytical-
ly pure 7 was obtained by recrystallization from CH2Cl2/
Conclusions
1
Et2O; yield: 258.5 mg (53%). H NMR (400 MHz, CD2Cl2):
d=7.99 (br, 10H, PPh3), 7.38–7.31 (m, 20H, PPh3), 6.89 (s,
A novel ruthenium(II) complex 7 bearing a symmetri-
cal pyridine-based ONO-type pincer ligand was syn-
thesized. A quasi-octahendral coordination environ-
ment around ruthenium(II) was observed by X-ray
crystallography. Complex 7 showed excellent catalytic
activity in transfer hydrogenation reactions of ke-
tones.
3
2H, CHnitrone), 6.65 (m, 1H, CHpry), 5.85 (d, JH,H =7.76 Hz,
2H, CHpry), 0.83 (s, 18H, CH3); 31P{1H} NMR (163 MHz,
CD2Cl2): d=28.1; 13C{1H} NMR (100 MHz, CD2Cl2): d=
150.2, 134.7, 134.6, 132.5, 132.4, 132.2, 129.9, 129.8, 128.7,
128.6, 128.5, 128.0, 74.2, 27.8; anal. calcd. for
C52H55Cl4N3O2P2Ru (1058.8): C 58.98, H 5.24, N 3.97; found:
C 58.64, H 5.05, N 4.22.
General Procedure for Transfer Hydrogenation
Experimental Section
A 25-mL oven-dried Schlenk tube was charged with aceto-
phenone substrates (0.5 mmol), 7 (0.005 mmol), Cs2CO3
(0.1 mmol) or NaOH (0.05 mmol) and 3 mL of 2-propanol.
The reaction mixture was stirred at 838C for the desired
time (6–24 h). The conversion of the reaction was detected
General Information
Unless otherwise noted, all reactions were carried out using
standard Schlenk techniques or in an argon-filled glove box.
Dichloromethane, diethyl ether and toluene were dried over
a Pure Solv solvent purification system. 2-Propanol was
dried over molecular sieves and degassed with N2. Deuterat-
ed solvents were purchased from Cambridge Isotope Labo-
ratories (CIL) and dried over molecular sieves. All the
chemicals were purchased from commercial suppliers and
used as received without further purification. NMR spectra
were recorded in CDCl3, CD2Cl2 using a JEOL ECA400
spectrometer, and TMS (tetramethylsilane) was used as a
reference. Chemical shifts are reported in ppm and coupling
constant in Hz. Elemental analyses were carried out in the
Division of Chemistry and Biological Chemistry, Nanyang
Technological University. HR-MS were obtained in the ESI
mode on a Finnigan MAT95XP GC/HR-MS system. IR
spectra were recorded on a Shimadzu IRPrestige-21 FTIR
spectrometer from 4000 to 600 cmÀ1 with 4 cmÀ1 resolution.
1
by H NMR spectroscopy using 1,3,5-trimethoxybenzene as
an internal standard.
Acknowledgements
We thank the National Research Foundation (NRF-RF2008-
05) and the Nanyang Technological University for financial
support. Dr. Yongxin Li is gratefully acknowledged for X-ray
crystallography.
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An 250-mL round-bottemed flask was charged with 2,6-pyri-
dinedicarboxaldehyde (298.4 mg, 2 mmol), N-(tert-butyl)hy-
droxylamine acetate (135.1 mg, 1 mmol), NaHCO3
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under vacuum. The crude product was purified by flash
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to 1:2 v/v) to afford 6 as a white solid; yield: 202.2 mg
(73%). 1H NMR (400 MHz, CDCl3): d=9.16 (d, JH,H
=
3
3
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