Synthesis and catalytic performance of ruthenium complexes ligated with rigid: O -(diphenylphosphino)aniline for chemoselective hydrogenation of dimethyl oxalate

Article abstract of DOI:10.1039/c6ra00320f

A series of new ruthenium complexes with rigid ligand o-(diphenylphosphino)aniline, including [(PPh₃)(o-PPh₂C₆H₄NH₂)RuCl₂]₂ (1), (o-PPh₂C₆H₄NH₂)₂RuCl₂ (2), [(o-PPh₂C₆H₄NH₂)₂(o-PPh₂C₆H₄NH)Ru]⁺Cl^- (3), Ph₃P(η²-H₂)Ru(μ-H)(μ-o-PPh₂C₆H₄NH)₂RuH(PPh₃) (4), (o-PPh₂C₆H₄NH₂)(o-PPh₂C₆H₄NH)RuCl(CO) (5), (o-PPh₂C₆H₄NH₂)(o-PPh₂C₆H₄NH)RuH(CO) (6), and [(o-PPh₂C₆H₄NH)₂Ru(CO)]₂ (7) were synthesized and employed as catalysts for chemoselective hydrogenation of esters. Among them, complexes 1, 2, and 5 exhibited excellent performance in hydrogenation of dimethyl oxalate to methyl glycolate, in comparison with the ruthenium complexes with a flexible aminophosphine ligand, such as (Ph₂P(CH₂)₂NH₂)₂RuCl₂, (Ph₂P(CH₂)₃NH₂)₂RuCl₂, and (o-Ph₂PC₆H₄CH₂NH₂)₂RuCl₂, under identical conditions. Complexes 1 and 2 also displayed good activities in the hydrogenation of other aliphatic and cyclic esters. The catalytic mechanism of hydrogenation was discussed according to the results of NMR spectroscopic studies and control experiments.

Full text of DOI:10.1039/c6ra00320f

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Synthesis and catalytic performance of ruthenium
complexes ligated with rigid o-
Cite this: RSC Adv., 2016, 6, 45512
(diphenylphosphino)aniline for chemoselective
hydrogenation of dimethyl oxalate†
Xiaolong Fang, Chunyan Zhang, Jin Chen, Hongping Zhu* and Youzhu Yuan*
A
series of new ruthenium complexes with rigid ligand o-(diphenylphosphino)aniline, including
+
ꢀ
[
(PPh
3), Ph
o-PPh
3
)(o-PPh
2
C
6
H
4
NH
)Ru(m-H)(m-o-PPh
NH )(o-PPh NH)RuH(CO) (6), and [(o-PPh
2
)RuCl
2
]
2
(1), (o-PPh
2
C
6
H
4
NH
2
)
2
RuCl
2
(2), [(o-PPh
2
C
H
6 4
NH
)(o-PPh
Ru(CO)] (7) were synthesized and
2
)
2
(o-PPh
2
C
6
H
4
NH)Ru] Cl
2
(
3
P(h -H
2
2 6
C
H
4
NH)
2
RuH(PPh
3
) (4), (o-PPh
2
C
6
H
4
NH
2
C H NH)RuCl(CO) (5),
2 6 4
(
2
C
6
H
4
2
C H
2 6 4
2
C H
6 4
NH)
2
2
employed as catalysts for chemoselective hydrogenation of esters. Among them, complexes 1, 2, and 5
exhibited excellent performance in hydrogenation of dimethyl oxalate to methyl glycolate, in comparison
with the ruthenium complexes with a flexible aminophosphine ligand, such as (Ph
2 2 2 2 2 2
P(CH ) NH ) RuCl ,
Received 6th January 2016
Accepted 2nd May 2016
(
Ph P(CH NH RuCl , and (o-Ph PC CH NH RuCl , under identical conditions. Complexes 1 and 2
2
2
)
3
2
)
2
2
2
6
H
4
2
2
)
2
2
also displayed good activities in the hydrogenation of other aliphatic and cyclic esters. The catalytic
mechanism of hydrogenation was discussed according to the results of NMR spectroscopic studies and
control experiments.
DOI: 10.1039/c6ra00320f
www.rsc.org/advances
6
7
Grey et al. and Matteoli et al. used ruthenium-hydride
Introduction
anions and ruthenium-cluster complexes coordinated with
Selective hydrogenation of esters to alcohols is an important phosphine ligands as homogeneous catalysts, respectively, for
transformation from both conceptual and practical perspec- ester hydrogenation. However, these complexes exhibited low
1
ꢁ
tives. For example, the chemoselective hydrogenation of activities and required drastic reaction conditions (180 C and
dimethyl oxalate (DMO) to methyl glycolate (MG) and further to 200 bar H ) to achieve complete conversion. Subsequently,
2
8
ethylene glycol (EG) is a key step in the process so-called “coal to Elsevier et al. developed an in situ catalyst system of Ru(acac)
/
3
2
EG”. Compared with stoichiometric reactions with hydride MeC(CH
2
PPh
2
)
3
/Zn (acac ¼ acetylacetonate); this catalyst was
ꢁ
reagents (e.g., LiAlH
4
or NaBH
4
), catalytic hydrogenation with used for the DMO hydrogenation at 100 C and 70 bar H for 16
2
9
dihydrogen (H ) inhibits the production of metal salt waste and h and generated 94% EG. Milstein et al. reported the rst
is more economical and environmentally benign. Therefore, example of homogeneous non-activated aromatic and aliphatic
2
1,3
catalytic hydrogenation with H is widely accepted and adopted. ester hydrogenation by using pincer-type ruthenium complex
2
ꢁ
at 115 C. Saudan et al.
10
Industrially, this process is commonly accomplished by using a (Scheme 1) under 5.3 bar of H
heterogeneous supported metal catalysts at relatively high demonstrated the outstanding performance of ruthenium
2
1b,4
reaction temperatures and
H
2
pressures.
In contrast, complexes b and c (Scheme 1) for ester and lactone reduction,
molecularly-dened organometallic complexes are usually which contain bidentate N,P-chelate and tetradentate
0
0
considered to be more active at lower reaction temperatures and P,N,N ,P -chelate ligands, respectively. Since then, scientists
H2 pressures, which might be benecial for getting higher worldwide have paid much attention on homogeneous ester
3,5
selectivity to the desired products.
hydrogenation and developed several other efficient cata-
5,11,12
lysts.
As we know, the ligands used in these catalysts are
State Key Laboratory of Physical Chemistry of Solid Surfaces, National Engineering
Laboratory for Green Chemical Productions of Alcohols-Ethers-Esters, iChEM,
College of Chemistry and Chemical Engineering, Xiamen University, Xiamen, Fujian,
3
61005, China. E-mail: yzyuan@xmu.edu.cn; hpzhu@xmu.edu.cn
†
Electronic supplementary information (ESI) available: Experimental results,
NMR spectra of complexes 1–7 and 9 and CIF data of compounds 1–2 and 4–7.
CCDC 1443566–1443570 and 1468780. For ESI and crystallographic data in CIF
or other electronic format see DOI: 10.1039/c6ra00320f
Scheme 1 Selected homogeneous catalysts.
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relatively exible and the complexes with so structures are
1
1a,13
preferred for the hydrogenation of esters.
14
o-(Diphenylphosphino)aniline is an N,P-chelate ligand
containing an NH group. Aer ligating with ruthenium, the
2
complexes thus formed contain a rigid ve-member chelate
15
ring. They have been applied to the conversion of nitroarene
into secondary amines and tertiary amines by using primary
15a,b
alcohols as sources of hydrogen and N-alkylation groups.
However, the application of these complexes in the hydro-
genation of esters is rarely reported. In this work, we have
synthesized and characterized a series of new o-(diphenyl-
phosphino)anilinoruthenium complexes. The catalytic results
manifested that the rigid o-(diphenylphosphino)anilinor-
uthenium complexes with proper structure conguration could
show high performance for the hydrogenation of esters.
Results and discussion
Synthesis of ruthenium(II) complexes
Complexes [(PPh )(o-PPh C H NH )RuCl ] (1, 87% isolated
3
2
6
4
2
2 2
Fig. 1 Synthesis and X-ray structure of complex 4.
yield) and (o-PPh C H NH ) RuCl (2, 92% isolated yield) were
2
6
4
2 2
2
prepared by ligand substitution reactions of RuCl
2
(PPh
3
)
3
and
ꢁ
o-PPh
2
C
6
H
4
NH
2
in toluene at 100 C, with the molar ratios of
Finally, the reaction of 5 with K[HBsBu
3
] successfully produced
these two complexes setting to 1 : 1 and 1 : 2, respectively. The
complex (o-PPh C H NH )(o-PPh C H NH)RuH(CO) (6) (Fig. 2).
2
6
4
2
2
6
4
reaction of (PPh
afforded complex [(o-PPh
3
)
3
RuHCl with 3 equiv. of o-PPh
2
C
6
H
4
NH
2
ꢀ
ꢁ
Notably, heat treatment at 70 C converted 6 into [(o-PPh C -
2
6
+
2
C
6
H
4
NH (o-PPh NH)Ru] Cl
2
)
2
2 6 4
C H
H NH) Ru(CO)] (7) by eliminating one H molecule (Fig. 2).
4
2
2
2
(3, 77% isolated yield) under identical conditions. Similarly,
complex (o-PPh C H NH )(o-PPh C H NH)RuCl(CO) (5, 86%
2
6
4
2
2
6
4
Characterization of ruthenium(II) complexes
isolated yield) was prepared from the reaction of (PPh ) -
3
3
The complexes 1–7 were characterized by NMR and IR spec-
troscopy and CHN elemental analysis, of which 1–2 and 4–7 were
further studied by X-ray crystallography. X-ray structure analysis
conrmed that 1 was a dimer in solid state (Fig. S1 in ESI†). In
RuHCl(CO) and 2 equiv. of o-PPh
The reactions of 1, 2 and 5 with hydride reagent K[HBsBu
could readily afford corresponding ruthenium hydride
2 6 4 2
C H NH (Scheme 2).
3
]
ꢁ
complexes. Treatment of 1 with K[HBsBu
to room temperature yielded complex Ph
PPh C H NH) RuH(PPh ) (4) with 85% isolated yield (Fig. 1).
3
] in THF from ꢀ75 C
2
solution, 1 can dissociate into (PPh )(o-PPh C H NH )RuCl
2
3
P(h -H
2
)Ru(m-H)(m-o-
3
2
6
4
2
1
5d
ꢁ
(
1a), as indicated by the variable-temperature (25 to ꢀ75 C)
2
6
4
2
3
3
1 1
P{ H} NMR studies (Fig. S7 in ESI†). The mononuclear
complex 2 possesses a structure conguration comparable with
The formation of 4 might proceed through the dihydride
intermediate (o-PPh C H NH )Ru(H) (PPh ) (4a), similar to the
2
6
4
2
2
3
18
0
0
19
those of Noyori's N,P-chelate and P,N,N ,P -chelate ruthenium
complexes (Fig. S2 in ESI†).
transformation of dihydride complexes (PPh
3
)
2
(cydn)Ru(H)
2
2
(
(
cydn ¼ (R,R)-cyclohexyldiamine) and (R-binap)(tmen)Ru(H)
16,17
tmen ¼ NH
2
CMe
2
CMe
2
NH
2
).
The reaction of 2 with
K[HBsBu
3
] under similar condition produced a mixture of
complexes, but isolation of the pure complex was unsuccessful.
Scheme 2 Synthesis of complexes 1–3 and 5.
Fig. 2 Synthesis and X-ray structure of complexes 6–7.
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The complex 4 exhibited an asymmetric dinuclear structure
˚
(
Fig. 1). The terminal Ru–H bond length [1.65(6) A] was close to
2
˚
that of Ru–h -H [1.68(7) A (av)] but shorter than that of Ru–m-H
1.77(5) A (av)]. In the H NMR spectrum, resonances at
2
1
˚
[
d ꢀ12.27, ꢀ8.38, and ꢀ8.22 ppm are assigned to the protons of
2
Ru–m-H, Ru–H, and Ru–h -H
2
, respectively. These resonance
values are comparable with those of previously reported related
1
6c,20
moieties.
The IR bands for the bonds of these moieties were
ꢀ1
found at 2113, 1956, and 1901 cm
.
The complexes 5 and 6 both had two N,P-chelates by
o-PPh C H NH and o-PPh C H NH at ruthenium (Fig. S4 in
2
6
4
2
2
6
4
˚
ESI† and 2). The Ru–H bond length in 6 was 1.67(2) A, close to
the corresponding value in 4. The complex 5 was insoluble in
organic solvent and was only subjected to solid-state P NMR Fig. 3 Comparison of H NMR spectra for reactions of 7 with H
3
1
1
2
(I) and
ꢁ
D
2
(II) recorded in C
6
D
6
at 10 C (dNH/NH2, 5.5 to 2.0 ppm; dRuH, ꢀ11.0 to
and IR spectral analyses. The complex 6 was soluble in aromatic
1
ꢀ12.7 ppm).
hydrocarbons and was characterized by solution NMR ( H and
3
1
1
P{ H}) spectroscopy together with solid-state IR spectrometry.
1
The H NMR spectrum of 6 exhibited a proton resonance at
d ꢀ12.14 ppm, assignable to the Ru–H, which gave an IR
in a similar manner to that of 6. Nonetheless, a reaction of 1
with K[HBsBu ] in D atmosphere was carried out. By
comparing the H NMR data of the obtained complex and 4
(Fig. S10 in ESI†), a partially D/H-exchange occurred.
ꢀ1
3
1
2
vibration of Ru–H bond at 2190 cm
.
2 6 4
Compound 7 was a dimer with two o-PPh C H NH ligands
1
2
served as a m-k (N):h (N,P)-type bridge (Fig. 2). However,
complex 7 lacked symmetry probably because of the equatorial/
2 6 4
axial location difference among the four o-PPh C H NH ligands Catalytic hydrogenation of DMO
around each ruthenium center. The solution NMR spectra
The complexes 1–5 and 7 were rstly applied in DMO hydro-
displayed two groups of data for the CO carbon resonances and
ꢁ
genation. Under the conditions at 100 C and 50 bar H in THF
1
13
1
31
1
2
four groups of data for the H, C{ H}, and P{ H} resonances
solvent, both 1 and 2 could catalyze the reaction to give MG in
excellent yield (entries 1 and 2, Table 1). Moreover, 5 yielded
a quantitative conversion and a 99% yield aer 3 h (entry 6). The
performance of these rigid complexes were better than those of
of the four o-PPh C H NH ligands.
2
6
4
Kinetic studies on transformation between 6 and 7
2 2 3 2 2 2
the exible ruthenium complexes b, (Ph P(CH ) NH ) RuCl
The thermal conversion of 6 into 7 under elimination of one H
2
(
d), or (o-Ph PC CH NH RuCl (e) (46%, 16%, and 49%
2
6
H
4
2
2
)
2
2
molecule prompted us to investigate the possibility of reversing
the reaction of 7 and H to produce the original complex 6. This
process is also considered in catalytic H
yield of MG within 4 h, entries 9–11), which have conguration
similar to 2 and are good catalysts for the hydrogenation of
2
2
hydrogenation reac-
At a reaction temperature of 10 C, exposure of the
16a,b,21
ꢁ
tions.
C
6
D
6
solution of 7 to H
2
for 56 h expectedly led to the complete
a
Table 1 Selective catalytic hydrogenation of DMO to MG
1
31
1
formation of 6, as traced by H and P{ H} NMR spectra (Fig. S8
and S9 in ESI†). Therefore, while the complex 6 eliminated one
H
2
molecule through the Ru(H) ) NH
complex 7 underwent H addition through the as-formed Ru–
NH unit to be transformed into 6, exhibiting heterolytic split-
ting of H . H elimination/addition switched between 6 and 7
2
unit to form 7, the
2
2
2
b
c
c
Entry Catalyst NaOMe/Ru Time (h) Conv. (%) Yield of MG (%)
was realized by monitoring the reaction temperature. To
understand this reaction process in detail, we performed
1
2
3
4
5
6
7
8
9
1
1
1
2
3
4
4
5
7
7
b
d
e
10
10
10
0
10
10
0
10
10
10
10
1
1
1
20
20
3
3
3
4
4
97
97
0
97
97
0
a reaction using 7 with D under the same condition as that of 7
2
1
with H . Fig. 3 combines the H NMR data of this reaction with
2
97
100
100
86
99
46
17
50
92
96
99
86
99
46
16
49
that of the reaction of 7 and H
reaction completion, RuH proton resonance remained present
with NH and NH resonances, although under low integral
2
to 6. By comparison, aer
2
intensity. This nding revealed that in the nal formation of 6,
the reaction may have occurred through an alternative switch
0
1
between 7 and 6 under D
carrying out the reaction at 10 C (Scheme S1 in ESI†).
In the presumed 4a-mediated production of 4, 4a also indeed
2
/DH addition/elimination despite
4
ꢁ
a
DMO (7.57 mmol) in THF (10 mL) was hydrogenated by the catalyst at
00 C; the molar ratio of DMO to ruthenium was 200. Molar ratio.
Determined by GC.
ꢁ
b
1
formed the Ru(H) ) NH
2
unit through the metathesis reaction
molecule
c
of 1 and K[HBsBu ] and was able to eliminate one H
3
2
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10,18a,22
other esters, ketones and amines.
Under the same 10 equiv. to 20 equiv., and improved conversion by 92% was
conditions, 3 showed no activity (entry 3). The results indicate obtained (entry 40 in Table 4, vide infra). The inuence of
the superiority of rigid o-(diphenylphosphino)anilinor- NaOMe amount on the catalytic activity was found for the DMO
uthenium complexes with proper structure for the chemo- conversion into MG. Taking the results into account, we per-
selective hydrogenation of DMO to MG.
formed the reaction for EG production by DMO hydrogenation.
Without using NaOMe, the hydrogenation with complex 4 However, the reaction produced only 4% yield of EG but 94%
led to 97% conversion and 92% yield in a relatively longer time yield of MG by 2/NaOMe/DMO (1/20/200) within 16 h (entry 26
of 20 h (entry 4). When combined with NaOMe, improved result in Table 3). At this stage, when we reduced the DMO/Ru ratio to
was obtained (100% conversion and 96% yield in 20 h, entry 5). 100/1 and prolonged the reaction time to 36 h, conversion into
Similar performance was also obtained using complex 7 (entries EG at a yield of 97% was achieved (entry 29). In comparison, the
7
–8).
1/NaOMe/DMO (1/40/200) system required a higher tempera-
ꢁ
We then studied the effect of reaction conditions on the ture of 140 C to gain 94% yield of EG within 36 h (entry 25).
performance of complex 2 for the DMO hydrogenation. As listed
in Table 2, 2 could convert DMO into MG easily at either low H
Compared with DMO, MG was a less activated ester because
of the loss of one ester substituent. Mostly due to this elec-
8,13
2
pressure (entries 12–13) or mild temperature (entries 14–21). tronic characteristic, the reaction conditions were more severe
Especially, 86% yield of MG was afforded at room temperature for the conversion of MG into EG or DMO into EG than those for
aer 24 h (entry 21). The amount of NaOMe used signicantly converting DMO into MG. Increasing the reaction temperature
inuenced DMO transformation in a volcano-type trend (entries could be an effective approach for the former conversion, as
8
b
1
(
6–20), and 40 equiv. of NaOMe over 2 gave the optimal result shown in previous reports. Meanwhile, using more NaOMe
ꢁ
81% MG yield at 40 C in 1 h). NaOMe not only promoted the greatly promoted the deprotonation of the NH or NH group
2
24
ꢀ
ꢀ
23
Cl to H metathesis in the presence of H
ated the deprotonation of the NH or NH
2
,
but also acceler- and then increased the catalytic reactivity toward either DMO or
group. The latter MG. Finally, MeOH was produced as one of the products during
2
24
function has been studied in detail by Bergens et al. for the hydrogenation of DMO. The amount of MeOH that accumu-
hydrogenation of amide and imide carbonyls. Finally, 2 was lated in this closed reaction system may greatly inuence its
investigated to catalyze the hydrogenation of DMO in 2000 kinetic conversion. Apparently, this inuence was greater for
molar equiv. relative to 2. An excellent result with 98% MG yield conversion into EG than that into MG because the former
was obtained within 16 h (entry 22), indicating the high underwent an additional hydrogenation process. Reducing the
performance of 2 for this chemoselective hydrogenation.
amount of DMO relative to the catalyst is benet for conversion
into EG.
Catalytic hydrogenation of DMO to EG by complexes 1 and 2
Catalytic hydrogenation of other esters by complexes 1 and 2
The results in Tables 1 and 2 indicated that DMO could be
converted into MG as the major product by using the o- We used 1 and 2 for the hydrogenation of other esters. In
(
diphenylphosphino)anilinoruthenium complexes under the general, the catalytic activities for these reactions by 2 were
ꢁ
conditions of 100 C and 50 bar H
2
. Under these conditions, better than those by 1 (Table 4). Moreover, 2 showed slightly
negligible activity was observed for converting MG (200 equiv.) better activity for quantitative conversion of methyl lactate into
into EG by using 2 and 10 equiv. of NaOMe. With increased 1,2-propanediol (entry 41) than that of MG into EG (entry 40).
ꢁ
temperature to 120 C, MG started to be transformed into EG, This result suggested the electronic effect of the Me group
but only 25% conversion was achieved within 8 h. At this attached onto the substituent of MG on the reduction of the
temperature, the NaOMe amount was further increased from adjacent ester group. In methyl pyruvate hydrogenation, methyl
a
Table 2 Selective catalytic hydrogenation of DMO to MG by 2 under other reaction conditions
b
b
ꢁ
c
c
Entry
Catalyst
NaOMe/Ru
DMO/Ru
T ( C)
P (bar)
Time (h)
Conv. (%)
Yield of MG (%)
12
13
14
15
16
17
18
19
20
21
22
2
2
2
2
2
2
2
2
2
2
2
10
10
10
10
5
10
15
40
80
10
10
200
200
200
200
200
200
200
200
200
200
2000
100
100
60
60
40
40
40
40
40
20
20
50
50
50
50
50
50
50
50
50
1
3
1
2
1
1
1
1
1
74
98
81
98
31
35
44
81
65
86
98
74
98
81
97
31
35
44
81
65
86
98
25
100
24
16
a
b
c
DMO (7.57 mmol) in THF (10 mL) was hydrogenated by the catalyst. Molar ratio. Determined by GC.
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a
Table 3 Catalytic hydrogenation of DMO to MG and/or EG
b
c
c
c
Entry
Catalyst
DMO/Ru
Time (h)
Conv. (%)
Yield of MG (%)
Yield of EG (%)
23
24
25
26
27
28
29
1
1
1
2
2
2
2
100
100
100
200
100
100
100
36
72
36
16
16
24
36
100
100
100
100
100
100
100
31
25
0
94
51
20
0
61
68
94
4
47
75
97
d
a
c
ꢁ
b
DMO (3.49 mmol) in THF (10 mL) was hydrogenated by the catalyst at 120 C; the molar ratio of NaOMe to ruthenium was 20. Molar ratio.
Determined by GC. At 140 C.
d
ꢁ
lactate was obtained under mild conditions (entries 32 and 42). and 43). The result indicate that the complexes 1 and 2 can be
Increase the temperature and H pressure, both the ketone and also useful for the hydrogenating the other carbonyl derivatives.
2
ester groups of methyl pyruvate were hydrogenated (entries 33 Excellent performances were achieved for transforming cyclic
esters to diols (entries 44–47). However, low activities were
found for phenyl group-containing methyl phenylacetate and
a
Table 4 Catalytic hydrogenation of other esters to alcohols
methyl benzoate. The phenyl group, especially in the latter,
probably exerted a signicant steric inuence in consideration
of the rigid ve-member chelate ring on the catalyst structure.
Similar results of steric inuence have been observed in reac-
11a,b,n
tions involving sterically hindered substrates.
Proposed hydrogenation mechanism
As mentioned above, the complexes 1, 2, and 5 were active in the
DMO hydrogenation with the aid of NaOMe. In contrast, the
complexes 4 and 7 were catalytically active without NaOMe.
b
b
Entry
Ester
Catalyst
Conv. (%)
Yield (%)
ꢀ
ꢀ
NaOMe is the Cl to H metathesis reagent during the
23
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
I
II
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
62
82
100
82
96
80
93
100
39
17
92
100
100
100
97
100
92
100
68
15
61
80
100e
reaction, so the ruthenium hydride complex 4a (or 6) might be
the active state of 1 (or 5). The inactivity of 3 was probably due to
no available space for ruthenium to generate the active hydride
group. This result suggested the importance of Ru–H group in
the catalytic reaction.
c
d
III
III
IV
V
VI
VII
VIII
IX
I
82
92
80
84
95
35
9
The reversible transformation of 6 and 7 clearly indicates
a cooperative function exists between metal and o-PPh
NH ligand. In order to clarify the specic function of NH
group in o-PPh NH during the catalytic cycle, we chose
the non-NH -group-containing ligand o-PPh NMe and
prepared complexes (PPh )(o-PPh C H NMe )RuCl (8) and (o-
2 6 4
C H -
2
2
2
C
6
H
4
2
92
100
100e
C
6
H
4
2
II
2
2
c
d
25
III
III
IV
V
VI
VII
VIII
IX
3
2
6
4
2
2
98
88
99
91
95
63
12
PPh C H NMe ) RuCl (9). Both 8 and 9 were examined for the
2 6 4 2 2 2
reaction, but no catalytic activities were found under similar
conditions. The complete shutdown of the catalysis activity
aer NMe
2
group substitution either in 1 or 2 indicates that the
10,11b
NH moiety is indispensible in the catalytic reaction.
2
Based on these observations, we proposed a bifunctional
mechanism for the hydrogenation of DMO to MG (Scheme 3). At
a
Ester (7.57 mmol) in THF (10 mL) was hydrogenated under 50 bar H
2
ꢁ
ꢀ
+
and 120 C for 8 h; the molar ratio of NaOMe to ruthenium was 20 and rst, the active species like 4a (or 6) transfers the H /H
b
c
that of ester to ruthenium was 200. Determined by GC. Performed at
equivalents into the C]O bond of the ester group through an
outer-sphere interaction. Then, a hemiacetal forms and
arranges into MeOC(O)CHO by eliminating one MeOH
ꢁ
4
0 C and 10 bar H
2
for 1 h; the molar ratio of ester to ruthenium was
d
e
1
000. Methyl lactate. 1,2-Propanediol.
45516 | RSC Adv., 2016, 6, 45512–45518
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Acknowledgements
This work was supported by the National Basic Research
Program of China (2011CBA00508), the National Nature Science
Foundation of China (21403178, 21473145, 21473142, and
2013B019), and the Science Foundation of Ministry of Educa-
tion of China (20110121130002) and the Program for Innovative
Research Team in Chinese Universities (IRT_14R31).
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Scheme 3 Proposed mechanism for the reduction of DMO to MG.
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