Synthesis of [RuX(CO)(dppp)(NN)]Cl (X = H, Cl; NN = en, ampy) complexes and their use as catalysts for transfer hydrogenation

Article abstract of DOI:10.1021/om4004785

The monocarbonyl hydride complexes [RuH(CO)(dppp)(NN)]Cl (dppp = Ph ₂P(CH₂)₃PPh₂; NN = ethylenediamine (en), 1; NN = 2-aminomethylpyridine (ampy), 3) have been isolated in high yield by reaction of RuHCl(CO)(PPh₃)(dppp) with en and ampy, respectively, in toluene at reflux. The chloride complexes [RuCl(CO)(dppp)(NN)]Cl (NN = en, 2; NN = ampy, 4) have been obtained quantitatively by dissolution of 1 in CH ₂Cl₂ and 3 in CHCl₃, through substitution of the hydride with a chloride and isomerization. Treatment of 4 with NaOiPr in 2-propanol at room temperature cleanly leads to the hydride complex 3. Complexes 1-4 have been proven to be robust and productive catalysts for the transfer hydrogenation of alkyl aryl, diaryl, dialkyl, and cyclic ketones using 0.2-0.004 mol % of catalyst and in the presence of 2 mol % of NaOiPr, affording TOF values up to 2.5 × 10⁵ h^-1.

Full text of DOI:10.1021/om4004785

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Synthesis of [RuX(CO)(dppp)(NN)]Cl (X = H, Cl; NN = en, ampy)
Complexes and Their Use as Catalysts for Transfer Hydrogenation
Shuanming Zhang, Salvatore Baldino, and Walter Baratta*
̀
Dipartimento di Chimica, Fisica Ambiente, Universita di Udine, Via Cotonificio 108, I-33100 Udine, Italy
S
* Supporting Information
ABSTRACT: The monocarbonyl hydride complexes [RuH-
(CO)(dppp)(NN)]Cl (dppp = Ph2P(CH2)3PPh2; NN =
ethylenediamine (en), 1; NN = 2-aminomethylpyridine
(ampy), 3) have been isolated in high yield by reaction of
RuHCl(CO)(PPh3)(dppp) with en and ampy, respectively, in
toluene at reflux. The chloride complexes [RuCl(CO)(dppp)-
(NN)]Cl (NN = en, 2; NN = ampy, 4) have been obtained
quantitatively by dissolution of 1 in CH2Cl2 and 3 in CHCl3,
through substitution of the hydride with a chloride and
isomerization. Treatment of 4 with NaOiPr in 2-propanol at
room temperature cleanly leads to the hydride complex 3. Complexes 1−4 have been proven to be robust and productive
catalysts for the transfer hydrogenation of alkyl aryl, diaryl, dialkyl, and cyclic ketones using 0.2−0.004 mol % of catalyst and in
the presence of 2 mol % of NaOiPr, affording TOF values up to 2.5 × 105 h−1.
pyridine ligand, which undergoes an aromatization/dearomati-
zation process.18 Incidentally, in 2004 we described mono-
carbonyl cyclometalated ruthenium complexes of type A
(Figure 2), containing N−H bidentate nitrogen ligands which
efficiently catalyze the TH of ketones.19
INTRODUCTION
■
The search for new, more efficient transition-metal catalysts is
of crucial importance for the preparation of high-value-added
organic compounds. Ligand design plays a key role for
achieving catalysts with high productivity and selectivity for
industrial applications. Among the transition metals, ruthenium
has attracted a great deal of attention on account of its high
performance and versatility for a large number of catalytic
organic transformations.1 The catalytic reduction of the
carbonyl bond via hydrogenation (HY)2 and transfer hydro-
genation (TH)3 is widely accepted in industry as a cost-
effective and environmentally benign way for the production of
a number of organic products.4 The Noyori catalysts trans-
RuCl2(PP)(1,2-diamine)5 (PP = diphosphine) and (η6-arene)-
RuCl(TsNCHPhCHPhNH2)6 (Ts = SO2C6H4CH3), displaying
the N−H function,7 have paved the way for the development of
new more efficient catalysts for the HY and TH of carbonyl
compounds.8 In the past decade, ruthenium monocarbonyl
complexes have been intensively investigated on account of
their high catalytic performance for several organic trans-
formations, including HY of carboxylic and carbonic acid
derivatives,9 alcohol dehydrogenation,10 and borrowing hydro-
gen reactions.11 In addition to the catalysts developed by
Robinson [Ru(OCOCF3)2(CO)(PPh3)2]12 and later by
The derivative bearing 2-aminomethylpyridine (ampy) was
found to be more active with respect to the ethylenediamine
(en) complex, with TOF values up to 6.3 × 104 h−1. The
diphosphine ampy complexes cis-RuCl2(PP)(ampy)20 (B) are
among the most active catalysts for the ketone TH (TOFs up
to 5 × 105 h−1) and for the HY of bulky substrates. These
complexes also show activity in dehydrogenation and
racemization of alcohols.20c On account of their easy
deactivation, they are usually employed with a catalyst loading
of 0.1 mol % or higher. Therefore, the development of new
robust carbonyl Ru catalysts, containing active ligands, such as
those with an N−H function or a pyridine ring, represents a
crucial target for achieving clean catalytic organic trans-
formations.
We describe herein the preparation and characterization of
monocarbonyl ruthenium complexes of formula [RuX(CO)-
(PP)(NN)]Cl (X = H, Cl; dppp = Ph2P(CH2)3PPh2; NN = en,
ampy), containing H and Cl and the N−H function, which in
the presence of a base show high catalytic activity for the TH of
ketones.
13
Hulshof [Ru(μ-OCOC2F4OCO)(CO)(diphosphine)]2 rele-
vant examples are those reported by Milstein,14 Gusev,15 and
Saito (Takasago catalyst)16 (Figure 1).
RESULTS AND DISCUSSION
■
Synthesis of the Complexes [RuX(CO)(dppp)(NN)]Cl (X
= H, Cl; NN = ampy, en). The monocarbonyl ruthenium
The presence of one CO ligand stabilizes the Ru(II) hydride
complexes, leading to highly productive catalysts with a low
tendency to decarbonylate substrates, a known pathway of
catalyst deactivation.16,17 For the Milstein complexes the high
activity has been ascribed to the active role played by the
Received: May 28, 2013
© XXXX American Chemical Society
A
dx.doi.org/10.1021/om4004785 | Organometallics XXXX, XXX, XXX−XXX

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Figure 1.
hydride 1 is significantly more reactive than 1′, on account of
the presence of the phosphine ligand, which exerts a stronger
trans influence to the hydride with respect to the amine.25 The
31P{1H} NMR spectrum of 2 shows one singlet at δ 30.8,
whereas the 13C{1H} NMR signals of the CO and ethylenedi-
amine ligands appear as a triplet at δ 199.7, with 2J(C,P) = 15.6
Hz, and a singlet at δ 46.8, respectively. These results indicate
that the substitution of the hydride with the chloride in 1
occurs with a concomitant isomerization, affording complex 2
with the CO ligand trans to Cl.
Similarly to the synthesis of 1, treatment of RuHCl(CO)-
(PPh3)(dppp) with ampy in toluene at reflux (12 h) leads to
the cationic complex [RuH(CO)(dppp)(ampy)]Cl (3), which
was isolated in 84% yield (Scheme 2).
Figure 2.
The 31P{1H} NMR spectrum of 3 shows two doublets at δ
42.6 and 12.8 with 2J(P,P) = 22.7 Hz, whereas in the 1H NMR
spectrum the resonance of the ruthenium hydride appears as a
doublet of doublets at δ −5.23 with 2J(H,P) = 113 and 19.2 Hz,
which are values similar to those of 1. The bidentate ampy
ligand shows one N−H proton at δ 3.87 and the ortho pyridine
proton at δ 8.38, shifted to low field with respect to that of the
free ligand. The CO and Ru−H infrared absorbances are at ν
1944 and 1853 cm−1, respectively, very close to those of 1,
suggesting that the CO ligand is trans to the NH2 function.
Similarly to 1, dissolution of 3 in chlorinated solvents leads to
the corresponding chloride complex [RuCl(CO)(dppp)-
(ampy)]Cl (4) by hydride substitution and isomerization
(Scheme 2). Complex 4 is quantitatively obtained from 3 and
CHCl3 at room temperature overnight, while the reaction of 3
with CH2Cl2 at reflux requires 3 days for complete conversion
into 4. The 31P{1H} NMR spectrum of 4 shows two doublets at
δ 35.4 and 22.7 with 2J(P,P) = 38.2 Hz, which is a value higher
than that of the hydride 3.23 The 13C NMR signal of CO is a
doublet of doublets at δ 203.6 with 2J(C,P) = 15.5 and 14.2 Hz,
complex RuHCl(CO)(PPh3)(dppp) has been prepared by
reaction of the RuHCl(CO)(PPh3)3 precursor with the
21
diphosphine dppp, according to the procedure reported in the
literature.22 The reaction of RuHCl(CO)(PPh3)(dppp) with
ethylenediamine (1.1 equiv) in toluene at reflux (4 h) affords
the cationic complex [RuH(CO)(dppp)(en)]Cl (1) in the
presence of a small amount of the isomer 1′ (<5%), as inferred
from NMR measurements (Scheme 1).
The 31P{1H} NMR spectrum of 1 in CD2Cl2 shows two
doublets at δ 44.2 and 14.9 with a relatively small 2J(P,P) value
of 21.2 Hz, consistent with the presence of two nonequivalent
cis phosphorus atoms bound to a Ru−H moiety.23 The hydride
1
ligand gives rise in the H NMR spectrum to a doublet of
doublets at δ −5.86 with 2J(H,P) = 113 and 18.9 Hz, indicating
that the phosphorus atoms are trans and cis with respect to the
hydride. The IR carbonyl stretching of 1 is at ν 1948 cm−1,
while the absorbance at 1854 cm−1 is for Ru−H. The minor
isomer 1′ displays in the 31P{1H} NMR spectrum two doublets
at δ 45.5 and 22.6 with 2J(P,P) = 38.4 Hz, whereas the hydride
signal is at δ −13.18 with 2J(H,P) = 25.2 and 20.0 Hz,
consistent with the presence of a hydride with two cis
nonequivalent phosphorus atoms. Complex 1 cleanly reacts
with chlorinated solvents, such as CHCl3 and CH2Cl2, leading
to the chloride complex [RuCl(CO)(dppp)(en)]Cl (2)
(Scheme 1).24 With CH2Cl2 quantitative conversion of 1 into
2 was achieved at 40 °C after 15 h. It is worth noting that the
1
consistent with a fac Ru(PP)(CO) arrangement. In the H
NMR spectrum (CD2Cl2) one N−H proton resonates at δ
4.13, whereas the second N−H is shifted significantly to low
field at δ 8.16, as established by a 1H−1H COSY experiment,24
suggesting an NH···Cl hydrogen bond. The IR CO stretching
absorbance is at 1974 cm−1, similar to that of 2 (ν 1975 cm−1),
in agreement with a CO trans to the chloride. In the solid state,
Scheme 1
B
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Scheme 2
Scheme 3
complexes 1−4 are not air sensitive, whereas in solution the
chloride complexes 2 and 4 are significantly less sensitive than
the hydride species 1 and 3. Attempts to prepare the analogous
bipyridine Ru carbonyl derivative by treatment of RuHCl-
(CO)(PPh3)(dppp) with 2,2′-bipyridine in toluene at reflux
(24 h) failed, leading to the recovery of the starting materials.
Since hydride complexes are the key species involved in the
catalytic hydrogenation reactions, we studied the reactivity of
the chloride complex 4 with a base in alcohol media, i.e. under
the conditions in which the catalysis takes place. Treatment of 4
with NaOiPr (2 equiv) in 2-propanol at room temperature (1
h) affords the hydride 3 as the only product, as inferred from
NMR measurements (Scheme 2). This reaction is likely to
occur through chloride substitution and formation of the Ru
alkoxide, which leads to the hydride 3 via β-elimination and
isomerization of the complex, in line with the reaction of
trans,cis-RuHCl(PPh3)2(ampy) with NaOiPr which gave cis,cis-
Ru(H)2(PPh3)2(ampy) as an intermediate species.20a The clean
and reversible conversion of 4 into 3 is ascribed to the presence
of the CO ligand, which stabilizes the Ru hydride. Attempts to
observe deprotonation of the NH2 function in basic media by
NMR spectroscopy failed. Dissolution of the hydride 3 in
methanol-d4 leads to two 31P{1H} NMR doublets at δ 47.4 and
results in the formation of the chloride 4 in the presence of
uncharacterized diphosphine complexes.27
Catalytic Transfer Hydrogenation. The ruthenium
complexes 1−4 have been studied in the TH of ketones in 2-
propanol and under basic conditions (Scheme 3). The hydride
diamine derivative 1 (0.1 mol %) catalyzes the reduction of
acetophenone 5 to 1-phenylethanol (94% after 40 min) under
reflux conditions and in the presence of NaOiPr (2 mol %)
with a TOF of 2.5 × 103 h−1 (Table 1). Using the chloride 2,
quantitative reduction was attained in 40 min with a lower rate
(TOF = 1.5 × 103 h−1).
An increase of speed was achieved with the ampy hydride 3
(0.2 mol %), which led to the quantitative formation of 1-
phenylethanol in 20 min with a TOF value of 1.0 × 104 h−1.
Table 1. Catalytic TH of Acetophenone 5 with Complexes
1−4 in the Presence of NaOiPr (2 mol %)
a
catalyst amt of Ru (mol %) time (min) conversion (%) TOF (h−1
)
1
2
3
3
3
3
4
0.1
40
80
94
98
98
98
96
60
99
2.5 × 103
0.1
1.5 × 103
1.0 × 104
1.2 × 104
1.1 × 104
3.6 × 103
4.0 × 103
0.2
20
0.1
30
2
13.7 with J(P,P) = 20.3 Hz. Addition of KOtBu (5 equiv) at
0.01
0.004
0.1
80
room temperature does not significantly affect the resonance at
δ 47.4, whereas the second peak at δ 13.6 appears as a
210
40
26
2
2
pseudoquartet with J(P,P) = J(P,D) ≈ 20 Hz, suggesting
the formation of the corresponding deuteride complex
[RuD(CO)(dppp)(ampy)]Cl, in agreement with the disap-
pearance of the Ru−H signal at δ −4.85 (2J(H,P) = 118 and
19.5 Hz). Finally, in CD2Cl2 the reaction of 3 with KOtBu
a
The conversion and TOF (moles of acetophenone converted into
alcohol per mole of catalyst per hour at 50% conversion) were
determined by GC analysis. Conditions: T = 82 °C, substrate 0.1 M in
2-propanol.
C
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Interestingly, a similar rate was observed at lower catalyst
loading. With 0.1 mol % of 3, the substrate 5 was reduced to
the corresponding alcohol in 30 min (TOF = 1.2 × 104 h−1),
whereas with 0.01% of 3, 96% conversion was achieved in 80
min (TOF = 1.1 × 104 h−1), indicating that 3 is a productive
catalyst. Conversely, at 0.004 mol % of 3 only 60% of the
alcohol was observed in 3.5 h. It is worth noting that, by
performing the TH of 5 with 3 (0.1 mol %) in air, 72%
conversion was achieved in 30 min (TOF = 2 × 103 h−1),
whereas quantitative formation of 1-phenylethanol is attained in
3 h.
Finally, the chloride ampy complex 4 (0.1 mol %) catalyzes
the TH of 5 with complete conversion in 40 min and TOF =
4.0 × 103 h−1. The most active system, 3, has been proven to
efficiently catalyze the TH of alkyl aryl, diaryl, dialkyl, and cyclic
ketones. Using 0.2 mol % of 3, the bulky ketone iPrCOPh (6)
was quantitatively reduced to the corresponding alcohol (98%)
in 24 min with a TOF of 7.5 × 103 h−1, a value slightly lower in
comparison to that of acetophenone (Table 2).
noting that, without transition-metal complexes, acetophenone
is quantitatively reduced in 1 day in 2-propanol at reflux in the
presence of NaOH (34 mol %), as described by James.28
Complex 3 (0.1 mol %) in the absence of base is not active in
the TH of 5 in 2-propanol at reflux (Table 3).
Table 3. Influence of NaOiPr on TH of Acetophenone 5
with 3
amt of 3
(mol %)
amt of NaOiPr
time
(h)
conversion
(%)
TOF
a
(equiv)
(h−1
)
0.1
0.1
0.1
0.1
0.1
0.01
0
1
1
0
70
95
94
98
96
2.5
4
1.9 × 102
1.0 × 103
2.5 × 103
1.2 × 104
1.1 × 104
2
5
1
20
200
0.5
1.5
a
The conversion and TOF (moles of acetophenone converted into
alcohol per mole of catalyst per hour at 50% conversion) were
determined by GC analysis. Conditions: T = 82 °C, substrate 0.1 M in
2-propanol.
Table 2. Catalytic TH of the Ketones 5−13 with Complex 3
in the Presence of NaOiPr (2 mol %)
With 1 equiv of NaOiPr the catalyst 3 shows a moderate
activity, affording 1-phenylethanol (70% conversion) in 2.5 h
(TOF = 190 h−1). When the amount of base is increased,
namely NaOiPr/3 = 2, 5 and 20, a faster reduction occurs with
TOF values of 1000, 2500, and 12000 h−1, respectively.
Employment of a lower amount of 3 (0.01 mol %) with 200
equiv of NaOiPr gives complete conversion of MeCOPh in 1.5
h with TOF = 1.1 × 104 h−1 (Table 3). A high rate in TH is
achieved when 3 (0.01 mol %) is activated by reaction of
NaOiPr (20 equiv) in 2-propanol at reflux (5 min). Subsequent
addition of the substrate 5 leads to 86% of the alcohol in 6 min.
Conversely, when the complex 3 is added to a basic 2-propanol
solution of ketone at reflux, a lower conversion (64%) is
achieved in 10 min. This indicates that the hydride 3 requires a
base to form the catalytically active species, which does not
deactivate at high temperature. It is likely that under catalytic
conditions in the presence of an excess of base, the cationic
carbonyl complex 3 is deprotonated by NaOiPr, affording the
neutral Ru−H amide complex RuH(CO)(dppp)(2-
PyCH2NH). It is worth noting that in basic alcohol media
the Noyori catalyst (η6-arene)RuCl(TsNCHPhCHPhNH2)6
and the pincer complexes RuCl(CNN)(diphosphine)29 lead to
neutral Ru−H amine species. Conversely, the trans-RuCl2(PP)-
(1,2-diamine) system leads to Ru−H amine and Ru−H amide
species.5b
Preliminary results show that the complexes [RuX(CO)-
(dppp)(NN)]Cl catalyze the acceptorless dehydrogenation and
racemization of alcohols.20c,30 With 0.4 mol % of 3, 1-tetralol
was converted into 1-tetralone (50%, 12 h) in tert-butyl alcohol
and toluene (1/1 in volume) at 130 °C (bath temperature) in
the presence of KOtBu (0.8 mol %), whereas 60% conversion
was achieved with 2 and KOtBu (4 mol %). Complex 3 (1 mol
%) was found to racemize (R)-1-phenylethanol quantitatively in
the presence of KOtBu (2 mol %) at 70 °C (1 h), while at 0.5
mol % of 3 the reaction requires 2.5 h.
time
conversion
(%)
TOF
a
ketone amt of catalyst (mol %)
(min)
(h−1
)
5
0.2
20
24
20
150
10
30
300
10
3
98
98
98
99
97
93
99
99
99
99
94
1.0 × 104
7.5 × 103
4.6 × 103
8.8 × 103
1.5 × 104
4.2 × 103
3.3 × 102
7.5 × 103
6
0.2
7
0.1
8
0.2
9
0.1
10
11
12
13
13
13
0.1
0.1
0.1
b
0.1
n.d.
0.01
0.004
4
2.5 × 105
7.5 × 104
90
a
The conversion and TOF (moles of ketone converted into alcohol
per mole of catalyst per hour at 50% conversion) were determined by
GC analysis. Conditions: T = 82 °C, substrate 0.1 M in 2-propanol.
On account of the high reaction rate, the TOF value could not be
b
determined.
With 0.1 mol % of 3, 3-methoxyacetophenone 7 is converted
to alcohol in 20 min (98%). The diaryl ketone 8 is efficiently
reduced to benzhydrol (99%, 2.5 h) with a relatively high TOF
(8.8 × 103 h−1). The aliphatic ketones 2-nonanone (9) and 3-
heptanone (10) have efficiently been reduced in 10 and 30 min,
the substrate 9 displaying the highest rate (TOF = 1.5 × 104
h−1). The unsaturated aliphatic ketone hex-5-en-2-one 11
undergoes chemoselective reduction at the CO bond (99%
conversion) after 5 h, with no hydrogenation or isomerization
of the CC bond (TOF = 3.3 × 102 h−1). It is worth noting
that cyclohexanone 12 and cyclopentanone 13 give complete
conversion to the corresponding alcohols in 10 and 3 min (0.1
mol % of 3), respectively, the substrate 13 being significantly
more quickly reduced than 12. As a matter of fact, 13 leads to
cyclopentanol with 0.01 and also 0.004 mol % of 3 in 4 and 90
min, respectively, achieving surprisingly high TOF values (2.5
× 105 and 7.5 × 104 h−1) (Table 2). Conversely, at a lower
loading of 3 (0.002 mol %) incomplete conversion has been
observed (45% in 4 h), suggesting a deactivation of the catalyst.
The influence of the base on the catalytic activity of the
ruthenium hydride 3 has also been investigated. It is worth
These results indicate that the Ru carbonyl complexes 1−4
efficiently catalyze the TH of ketones, affording complete
conversion with 0.2−0.004 mol % loading of catalyst. The ampy
derivatives 3 and 4 display a higher rate with respect to the
ethylenediamine complexes 1 and 2. In addition, the hydride
complexes 1 and 3 show higher rates in comparison to the
D
dx.doi.org/10.1021/om4004785 | Organometallics XXXX, XXX, XXX−XXX