Ruthenium(II) Complexes of 4′-(Aryl)-2,2′:6′,2′′-terpyridyl Ligands as Simple Catalysts for the Transfer Hydrogenation of Ketones

Article abstract of DOI:10.1002/ejic.201800585

A series of cationic [Ru(L)(PPh₃)₂Cl]⁺ (1–3) and neutral [Ru(L)(PPh₃)Cl₂] (4–6) Ru^II complexes were synthesized by reacting [RuCl₂(PPh₃)₂] with 4′-(aryl)-2,2′:6′,2′′-terpyridyl-based ligands (L1–L3) with various aryl groups (tolyl, phenyl and 4-fluorophenyl). The synthesized Ru^II complexes were unambiguously characterized by various spectroscopic techniques such as FTIR and multinuclear NMR spectroscopy as well as HRMS. The neutral complexes (4–6) were also structurally characterized by single-crystal X-ray diffraction studies. Photophysical and electrochemical studies of the Ru^II complexes were performed to elucidate the effects of the 4′-aryl substituents of L1–L3. These Ru^II complexes show good catalytic activities in the transfer hydrogenation (TH) of ketones with a wide substrates scope in 2-propanol under reflux. An optimization study revealed that the neutral Ru^II complexes are better catalysts than the cationic Ru^II complexes for TH reactions. Finally, [Ru(L1)(PPh₃)₂H]⁺ (7) with a [Ru^II–H] functionality was successfully synthesized and isolated and is proposed as the catalytically active species. A control experiment with the [Ru^II–H] complex in the absence of base was performed to establish the mechanism for the catalytic TH of ketones.

Full text of DOI:10.1002/ejic.201800585

A Journal of
Accepted Article
Title: Ruthenium(II) Complexes of 4′-(aryl)-2,2':6',2''-Terpyridyl Ligands
as Simple Catalysts for Transfer Hydrogenation of Ketones
Authors: Apurba Maity, Amit Sil, and Sanjib Kumar Patra
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10.1002/ejic.201800585
European Journal of Inorganic Chemistry
FULL PAPER
Ruthenium(II) Complexes of 4'-(aryl)-2,2':6',2''-terpyridyl Ligands
as Simple Catalysts for Transfer Hydrogenation of Ketones
Apurba Maity,[a] Amit Sil,[a] Sanjib K. Patra*[a]
Abstract: A series of cationic [Ru(L)(PPh3)2Cl]+ (1-3) and neutral
[Ru(L)(PPh3)Cl2] (4-6) Ru(II) complexes have been synthesized by
reacting [RuCl2(PPh3)2] with 4'-(aryl)-2,2':6',2''-terpyridyl based
ligands (L1-L3) by varying the aryl groups (tolyl, phenyl and 4-
fluorophenyl). The synthesized Ru(II) complexes have been
unambiguously characterized by various spectroscopic tools such as
FTIR and multinuclear NMR as well as HRMS analyses. The neutral
complexes (4-6) have also been structurally characterized by single
crystal X-ray diffraction studies. Photophysical and electrochemical
studies of the Ru(II) complexes have been carried out to understand
the substituent effect of the 4′-aryl group of the ligands L1-L3. These
Ru(II) complexes show good catalytic activity in transfer
hydrogenation (TH) of ketones with a wide substrates scope in
refluxing isopropanol. Optimization study reveals that the neutral
Ru(II) complexes act as better catalysts over cationic Ru(II)
complexes for TH reactions. Finally, [Ru(L1)(PPh3)2H]+ (7) having
[Ru(II)-H] functionality has been successfully synthesized, isolated
and proposed as the catalytically active species. The controlled
experiment by [Ru(II)-H] complex in the absence of base has been
carried out to establish the mechanism of catalytic TH of ketones.
osmium(II),[5] rhodium(I/III)[6] and iridium(I/III)[7] as catalysts
for the TH of ketones is a fast growing research field.
However, ruthenium(II) complexes have been paid much
more interest as these are robust and highly efficient in
general.[1c,8] In the past few years, great improvements of
ruthenium(II)-catalyzed TH have been witnessed in several
aspects including the development of the diversity of
ligands. In this area, Noyori and co-workers have
developed
Ru(II)
complexes
containing
N-
tosylethylenediamine or β-amino alcohol based ligands
exhibiting efficient catalytic activity in the asymmetric
transfer hydrogenation of ketones and imines.[9] After their
pioneering work, a significant number of similar ligands and
their transition metal complexes have been documented.[10]
In this regard, Baratta et al. have reported a series of
cyclometalated
Ru(II)
complexes
containing
2-
(aminomethyl)pyridine ligands as efficient catalysts for the
transfer hydrogenation of ketones.[11] In some cases, the
presence of a N-H functionality as an accelerating group
offers high reactivity for TH reaction.[11g,m,o,p] Although, there
are several Ru(II) catalysts featuring no acceleration
functionality have also been known for the TH of ketones.[12]
An extensive exploration of NNN tridentate pincer ligands
based ruthenium catalysts for TH of ketones was seen over
the past few years (scheme 1).[1c,13] Yu et al. synthesized
several NNN pincer type ligands and its corresponding
ruthenium(II) complexes for the TH of ketones with or
without acceleration functionality.[14] For example, air and
Introduction
Catalytic transfer hydrogenation (TH) of unsaturated
organic compounds has become
a reliable reduction
strategy to access various hydrogenated products over
other conventional methods of reduction. It is the most
useful catalytic method for hydrogenation not only because
it is a clean and environmentally benign process but also
due to its easy manipulation, relatively low cost and wide
substrate scope.[1] Use of 2-propanol as hydrogen source
for TH reaction of ketones leads to an equilibrium reaction
which can be shifted to the alcohol product by employing 2-
propanol as solvent. One of the biggest concerns in
industrial transfer hydrogenation is to replace the use of
NaBH4 and LiAlH4 facilitating the hazardous free workup,
extending the substrate scope, reducing the amount of side
products and enhancing the reaction efficiency. In this
regards, exploration of various types of ligands and their
transition metal complexes are strongly desired for transfer
hydrogenation. The development of transition metal
complexes of cobalt(II),[2] nickel(II),[2a,3] iron(II),[2a,4]
moisture-stable ruthenium(II) complex
A exhibited excellent
transfer hydrogenation of ketones in refluxing isopropanol
due to the hemilabile unsymmetrical coordinating
environment and NH functionality.[14f] Complex
B, bearing
(trifluoromethyl)-pyrazolyl functionality converted ketones to
their corresponding alcohols in maximum of 99% yield.[14d]
Room temperature transfer hydrogenation of ketones and
aldehydes was achieved in 2-propanol by using
ruthenium(II) complex
C
as catalyst under aerobic
condition.[14k] Ruixiang Li and coworkers developed a
[a]
A. Maity, A. Sil, Dr. S. K. Patra
Department of Chemistry
Indian Institute of Technology Kharagpur
Kharagpur 721302 (India)
E-mail: skpatra@chem.iitkgp.ac.in
Scheme 1. Representative examples of recently reported NNN pincer ligands
Supporting information for this article is given via a link at the end of
the document.
based ruthenium(II) complexes as catalysts for TH reactions
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10.1002/ejic.201800585
European Journal of Inorganic Chemistry
FULL PAPER
symmetrical pyridyl-2,6-pyrazolyl ligand based ruthenium(II)
complex (D) which showed good to excellent catalytic
activity in transfer hydrogenation of wide substrate of
ketones.[13e] Song and coworkers also reported symmetrical
studies aiming to develop simple and economical Ru(II)
catalysts for transfer hydrogenation of ketones.
Results and Discussion
Ru(II) complex
transfer hydrogenation of ketones with wide substrate
scope.[13c] Additionally, complex
was proved to be an
improved catalyst than its corresponding cationic analogue.
In another report by Pizzano and coworkers, asymmetric
transfer hydrogenation of N-aryl imines derived from
(E) and observed almost quantitative
Synthesis and characterization of Ru(II) complexes:
simple and synthetically ease 4'-(aryl)-2,2':6',2''-terpyridines
L1-L3) have been selected as ligands by varying the aryl
group (tolyl, phenyl and 4-fluorophenyl) to tune their
electronic properties. The synthesis of the ligands, L1-L3
A
E
(
,
acetophenones was carried out by using the catalyst
F
was performed following Krӧhnke method affording 55-60%
yield respectively as shown in Scheme 2.[28] The synthesis
involves condensation of p- tolyldehyde with two equivalent
of acetylpyridine in ethanol leading to the formation of the
central pyridine ring in the presence of ammonia as a base.
The formation of ligands was confirmed by 1H, 13C{1H} NMR
bearing both a pybox (2,6-bis(oxazoline)pyridine) and a
monodentate phosphite ligand.[13b] For the Ru(II) complex
G, the use of 6,6′-dihydroxy terpyridine (dhtp) as a rigid
bifunctional ligand provided the directing effect of proton
transfer events with substrates coordinated to the metal
centre.[13h] However, the catalyst was found to be highly
sensitive towards bulky ketones, giving no TH product for
benzophenone. Recently, Kundu and coworkers reported
bifunctional Ru(II)-(phenpy-OH) [phenpy-OH: 2-(2-pyridyl-2-
and mass spectrometry (ESI+). Cationic (
) ruthenium(II) complexes were synthesized by reacting
1-3) and neutral
(4-6
an equimolar amount of ligands and RuCl2(PPh3)3 under
argon atmosphere to realize the role of electronic effect of
tpy ligands as varied by different 4′-aryl groups and also the
charge of complexes on catalytic process. Refluxing in
methanol followed by the addition of NaPF6 afforded the
ol)-1,10-phenanthroline] complex
H that catalyzed a diverse
range of substrates of ketones and nitriles using 2-propanol
as hydrogen source.[13a]
Design and development of versatile ligands are the
key issues to attain highly active transition metal complexes
as catalysts. Nitrogen-containing heterocyclic ligands and
their transition metal complexes have been widely studied
in coordination chemistry, homogeneous catalysis and
organic synthesis.[15] Over the past few years, NNN ligands
such as Pybox,[16] 2,6-bis(imino)pyridines[17] 2,6-bis-
(benzimidazole-2-yl)-pyridine[15d] and 2,2':6',2''-terpyridines
(tpy)[18] have been paid much more attention in coordination
chemistry, homogeneous catalysis, functional materials and
physical chemistry due to their tunable properties and
potential applications. Easily functionalizable terpyridyl
based N-donor ligands with its tunable electronic character
may function as an ideal alternative of organophosphine
ligands. Tpy ligand has been considered as an unusually
strong π-acceptor relative to other N-donors and is also
oxidatively and thermally robust.[18e,19] The advantageous
properties of the tpy based ligands allow the chemists for
exploring it in variety of catalytic processes such as
cationic Ru(II) complexes,
1-3 in 79-86% yield. Use of
toluene as refluxing solvent produced neutral complexes in
84-87% yield (Scheme 2). Both the cationic and neutral
ruthenium(II) complexes were found to be stable in air and
moisture. Multinuclear NMR analyses of all the Ru(II)
complexes are in well agreement with their expected
chemical structures. The 1H NMR signal for the terpyridyl-
CH (6, 6′′) protons adjacent to the pyridyl-N in free ligand L1
appear at 8.68 ppm and that for the cationic complex
1 and
neutral complex are shifted downfield to 9.03 and 9.47
4
ppm respectively, as a result of ligand coordination to the
Ru(II) centre. The similar trend in the downfield shift is also
observed for the other cationic and neutral complexes. The
13C{1H} NMR spectra of all the ligands (L1
-L3) display a
signal at 136.9-137.1 ppm for the pyridyl-CH (6, 6′′)
adjacent to pyridyl-N. After coordination to Ru(II), the signal
is shifted to 155.1-155.7 ppm. In 31P{1H} NMR spectra (in
CDCl3), signal for 4-6 appears at 41.4-41.7 ppm indicating
that the environment around the PPh3 ligand is identical for
asymmetric
cyclopropanation,[18e,20]
oxidation
and
dehydrogenation of alcohols,[21] co-oligomerization of
alkenes,[22] allylic alkylation,[23] hydrosilylation,[24] Negishi
coupling,[25] and rearrangement of oxaziridines.[26]
all the neutral complexes. For the cationic complexes (1-3)
the single resonance appears at 20.0-20.2 ppm, revealing
the presence of two identical PPh3 ligands around Ru(II)
centre. The formation of the cationic and neutral Ru(II)
complexes is further confirmed by HRMS (ESI+) and CHN
analyses. For the cationic complexes 1-3, the respective
peaks (m/z) at 984.1983, 970.1848 and 988.1804 for [M-
PF6]+ are observed, whereas for the neutral complexes (4-6)
the characteristic signals attributed to
Although a great potential of terpyridine family has
been implicated in a variety of catalytic processes, they
have been exploited as ligands for the development of
catalysts for the transfer hydrogenation of ketones to a
limited extent.[8l,13h,27] In this work, neutral and cationic
ruthenium(II) complexes of 4'-(aryl)-2,2':6',2''-terpyridine
ligands have been synthesized and characterized by
multinuclear NMR analyses and X-ray crystallographic
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10.1002/ejic.201800585
European Journal of Inorganic Chemistry
FULL PAPER
Scheme 2. Synthetic route for the cationic (1-3) and neutr
a
a
l (4-6) ruthenium(II) complexes.
electrochemical characterization of alll the Ru(II) complexes
) were performed to monitor the eelectronic effect of their
substituent at 4' position. Optical propperties of all the Ru(II)
complexes were studied in
CH2Cl2 at 1×10-5
(1-6
1-
6
C
M
concentration (Figure S37 in supporrting information). The
cationic complexes exhibit a broad aabsorption band in the
visible region at λmax of 490 nm with εmax
M-1cm-1, attributing to the metal to lligand charge transfer
(MLCT) transition. Interestingly, the neutral complexes
show a significant bathochromic shift by 50-52 nm than that
of the cationic Ru(II) complexes. For all the Ru(II)
ε
in the order of 104
e
Figure 1. Simulated and experimental isotopic distribu
molecular ion peak.
t
t
ion pattern for the
.
complexes, a second set of absorpttion bands in the UV
region (λmax of 270-325 nm) apppears which may be
assigned to the spin-allowed π–π* trransition involving the
terpyridyl moiety.[18a,29] Cyclic voltammetry analysis of the
Ru(II) complexes was carried out in CH2Cl2 using n-
Bu4NPF6 (0.1 M) as supporting electrrolyte, Pt-disc working
[M-Cl]+ are obtained at 721.1061, 708.092
respectively. Experimental and
distribution pattern for the molecular ion peakk are in perfect
agreement as depicted in Figure 1. The tructure of the
neutral complexes was further confir
9
9
and 726.0850
simulaated isotopic
s
m
electrode, Pt-wire counter electrode nd Ag/AgCl reference
a
a
4
-6
ed by single
electrode. All the cationic complexes 11-3 show one-electron
crystal X-ray crystallographic studies (Figu
rre 2). The six-
coordinated Ru(II) center formed the disttorted pseudo-
Table 1. Important bond distances and bo
complexes 4-6.
n
n
d
angles of neutral Ru(II)
octahedral environment in the solid state foor all the three
neutral complexes. The Ru-N distances or the central
4
5
6
f
f
Bond Distances (Å)
Ru1-N1
Ru1-N2
Ru1-N3
Ru1-P1
Ru1-Cl1
Ru1-Cl2
pyridine ring (N2) of complexes are shorter than for the two
terminal pyridine rings (N1 & N3). The Cl1-Ru1-Cl2 angles
are in the range of 87.66(6)˚-87.91(7)˚ in all the neutral
Ru(II) complexes, indicating that the two cchloride ligands
are in the cis position. The PPh3 ligand adopts a nearly
orthogonal position with respect to the NNNN plane of tpy
2.072(6)
1.945(6)
2.047(6)
2.293(2)
2.461(2)
2.460(2)
2.
1.
2.
2.
2.
2.
.
0
9
056(5)
942(5)
063(6)
289(2)
464(18)
463(17)
2.074(2)
1.946(6)
2.080(7)
2.314(2)
2.479(2)
2.447(2)
.
.0
2
.4
Bond Angles (˚)
Cl1-Ru1-Cl2
P1-Ru1-N1
P1-Ru1-N2
P1-Ru1-N3
P1-Ru1-Cl1
P1-Ru1-Cl2
Torsional angle (˚)
C7-C8-C16-C17
C9-C8-C16-C21
87.69(7)
92.61(19)
93.50(19)
91.66(18)
179.08(7)
93.10(8)
877.66(6)
900.66(16)
922.41(16)
933.03(16)
1776.92(6)
944.76(6)
87.91(7)
93.30(17)
92.20(18)
91.56(17)
178.46(8)
92.33(7)
ligand as manifested by P1-Ru1-N1, P1-R
uu1-N2 and P1-
Ru1-N3 bond angles which are in the rang
ee of 90.66(16)˚-
93.30(17)˚, 92.20(18)˚-93.50(19)˚ and 91.56(17)˚-91.66(18)˚
(Table 1) respectively. The aryl and the te pyridyl groups
are not perfectly coplanar as manif sted by the
corresponding torsional angles. Photoophysical and
r
ee
-15.31
-12.45
311.60
2299.96
17.13
18.02
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10.1002/ejic.201800585
European Journal of Inorganic Chemistry
FULL PAPER
Figure 2. Molecular structures of neutral complexes 4-6 with hydrogen atom omitted for clarity. The thermal ellipsoid are drawn at 40% of probability.
reversible oxidation wave with E1/2 value in the range of
0.94-0.96 V due to the Ru(II)/Ru(III) process as shown in
Figure 3. Similar electrochemical behavior was also
Presumably, the electron-rich neutral catalyst as indicated
by the cyclic voltammetric studies, exhibiting lower
reduction potential by 0.31-0.32 V (E1/2) compared to the
cationic ones (vide supra), facilitates the reduction.[7g,9i]
observed for all the neutral complexes 4-6 with significantly
lower reduction potential with E1/2 in 0.63-0.64V. The
positive charge in the cationic complexes and the two π-
acceptor PPh3 ligands make the Ru(II) centers relatively
electron deficient than that of the neutral complexes.
Over the period of
acetophenone reached to 66% and 84% of isolated yield
using the complexes and respectively. To see the effect
of various substitutions at 4'-position of terpyridyl unit on the
catalytic process, complexes and were employed
as catalysts under the same reaction condition (entry 2, 3, 5
and 6 in Table 2). For both the cationic and neutral
complexes, no significant alteration of reaction yields was
observed, revealing the independency of functionalization
of ligands on the catalytic process. In entry 7, the reaction
2
hours, the TH reaction of
1
4
Consequently, the cationic Ru(II) centers in complexes (
) exhibit higher reduction potential than that of neutral
complexes ( ).
1
-
2,
3,
5
6
3
4-6
using catalyst
4 was deliberately quenched after 1 hour
where only 63% of the starting material was converted to
the 1-phenylethanol. From this observation, it can be
concluded that the optimum reaction time to achieve
maximum yield is 2 hours. A significant decrease in yield
was observed when the molar ratio of base was decreased
(Table 2, entry 8). The reaction was carried out in air
(Table 2, entry 9) and over the period of 4 hours, only 49%
yield could be achieved, indicating that inert atmosphere is
Figure 3. Cyclic voltammograms of Ru(II) complexes (1-6) in CH2Cl2 using
TBAPF6 as supporting electrolyte, Pt disc working electrode, and Ag/AgCl
reference electrode. Scan rate at 100 mV/s.
Table 2. Optimization of the reaction condition for the transfer
hydrogenation of acetophenone.[a]
Entry
1
2
3
4
5
6
Cat.
1
2
3
4
5
6
4
4
base
Time, h
Yield, %[b]
66
64
67
84
81
80
63
67
Transfer hydrogenation (TH) of ketones by the
synthesized Ru(II) complexes: In our initial studies, TH of
acetophenone was chosen as the model reaction.
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
-
2
2
2
2
2
2
1
2
4
4
Complexes
1 and 4 were tested as catalysts in refluxing
isopropanol to screen the reaction condition. The reaction
was carried out at 0.5 mol% of catalyst loading with a molar
ratio of 200/20/1 for ketone/base/catalyst in the presence of
NaOH as a base under an argon atmosphere. As observed
7
8[c]
9[d]
10
4
4
49
N.R.[e]
[a] Reaction condition: ketone 2.0 mmol; iPrOH = 20 mL; ketone/base/cat.
from the Table 2 (entries, 1 and 4), the neutral complex
acts as a better catalyst than the cationic complex
4
= 200/20/1; N2 atmosphere; 82 °C. [b] Isolated yield. [c] Ketone/base/cat. =
200/10/1. [d] Reaction in air. [e] No reaction.
1
.
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10.1002/ejic.201800585
European Journal of Inorganic Chemistry
FULL PAPER
necessary to keep the catalyst active. No product was
found in the absence of base (Table 2, entry 10),
suggesting that presence of base is crucial to the relative
success of the reaction. For comparison, the effect of
maximum isolated yield than their corresponding analogues
bearing substituents at meta position (Table 4, entries 8, 9
vs. 11, 12) as increasing the steric hindrance around the
keto group in the substrates retarded the reaction rate.
various bases on the reaction in the presence of neutral
Although a contradictory result was observed for 2-
t
ruthenium(II) catalyst
4, NaOH, KOH, BuOK, K2CO3 and
methoxyacetophenone achieving 77% yield over a period of
3 hours, whereas 71% isolated yield was observed for 3-
methoxyactophenone after 4 hours (Table 4, entry 7 vs 10).
NaHCO3 were used as the bases (Table 3). After 2 hours of
reaction time, the corresponding alcohol from
acetophenone was achieved with yield of 84%, 82% and
78% by using the bases NaOH, KOH and tBuOK
respectively. While using K2CO3 and NaHCO3, relatively
lower yield of 71% and 65% was observed under same
reaction condition. So it can be concluded that strong bases
like NaOH, KOH and tBuOK act as better reaction promoter.
So based on the above results, the molar ratio of 200/20/1
for ketone/base/catalyst, 0.5 mol% of neutral ruthenium(II)
Table 4. Transfer hydrogenation of various ketones catalyzed by 4 [4]
.
Entry
1
Ketone
Time, h
2
Yield, %b
TON
166
83
2
3
4
5
6
1.5
1.5
1.5
5
78
86
84
75
81
156
172
168
150
162
complex
4 as catalyst and NaOH as the base in refluxing
isopropanol under a nitrogen atmosphere were chosen as
t
the optimized reaction condition. However, KOH and BuOK
also worked well as bases under the same reaction
condition.
Table 3. Optimization of bases for the transfer hydrogenation of
acetophenone catalyzed by Ru(II) complex 4.[a]
Yield, %[b]
3
Entry
Cat.
Base
Time, h
1
2
3
4
5
4
4
4
4
4
NaOH
KOH
2
2
2
2
2
84
82
78
71
65
7
3
77
78
83
71
81
80
81
83
72
154
156
166
142
162
160
162
166
144
tBuOK
K2CO3
NaHCO3
8
3
[a] Reaction condition: ketone, 2.0 mmol; iPrOH = 20 mL; ketone/base/cat.
= 200/20/1; N2 atmosphere; 82 °C. [b] Isolated yield.
9
2.3
4
This optimized reaction condition was followed to other
ketones in the transfer hydrogenation to access other
hydrogenated products (Table 4). For this purpose, various
substituted acetophenone, aryl alkyl ketones, aliphatic
cyclic ketones, α,β-unsaturated ketones and diones were
10
11
12
13
14
15
2
assessed for transfer hydrogenation reaction in
a
1.8
2.5
2
systematic way. Reaction time for every case was
optimized to achieve maximum isolated yield. The
electronic effect in TH reactions is very obvious, in which
acetophenone bearing electron–withdrawing substituents
such as chloro, bromo and fluoro reacts efficiently in 1.5
hours (Table 4, entries 2, 3, 4) with a TON value in the
range of 156-172. On the other hand, the electron-donating
methyl or methoxy substituents (Table 4, entries 5, 6) make
the ketone substrates more electron rich and therefore, they
need longer reaction time to achieve higher yield. Figure 4
demonstrates the relationship between the electronic effect
of the substrates and the reaction yield by giving the
positive slope, supporting the fact that the substrates with
electron withdrawing group favour the TH than the
substrates with electron donating group. Notably, 2'-
substituted acetophenone needed longer time to gain
4
16
17
3
8
75
53
150
106
18
8
56
112
[a] Reaction condition: ketone, 2.0 mmol;
200/20/1; Argon atmosphere; 82 °C; [b] Isolated yield.
iPrOH = 20 mL; ketone/base/cat. =
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10.1002/ejic.201800585
European Journal of Inorganic Chemistry
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ketone group was also obtained in 21% yield (Table 5, entry
1). Benzaldehyde responded very weakly under this
reaction condition (Table 5, entry 2). Transfer
hydrogenation of unsymmetrical diones was also examined
under the same reaction condition. Interestingly, selective
reduction of the more active ketone keeping the ester group
intact was observed (entry 3 and 4 in Table 5).
Transfer hydrogenation reaction mechanism: Based
upon the related reported literature[14d-f,30] it can be
proposed that in situ generated [Ru-H] complexes act as
catalytically active species in most of the Ru(II)-catalyzed
transfer hydrogenation reactions. For this reason, isolation
of catalytically active Ru(II)-H complex was attempted.
Figure 4. Plot demonstrating the electronic effect of the substituents at 4-
position of acetophenone derivatives in transfer hydrogenation (For each of
the cases, yields were obtained after 1.5 h.)
While treating complex
propanol, the corresponding dark brown colored
[Ru(L1)(PPh3)2(H)]+ complex (
) was successfully isolated
in 70% yield (Scheme 3). 1H NMR (CDCl3) confirmed the
presence of Ru(II)-H functionality by showing
1 with K2CO3 in refluxing 2-
7
This result attributed to the fact that in comparison to the
resonance effect, the inductive effect of the substituents is
more effective in influencing reduction rates. Methoxy group
has -I effect which is most effective when substituted at
ortho position, resulting highest conversion of product in
comparatively less time.[14f] Aryl ketones like 1-
acetonaphthone and benzophenone reacted smoothly with
a TON value of more than 160 to give the desired products
in 81-83% yield in 2.5 and 2 hours respectively (Table 4,
entry 13, 14). Heterocyclic ketones also showed good
reactivity (TON 144-150 after 4 and 3 hours for entry 15
and 16 respectively) under the same reaction condition
(Table 4, entry 15, 16). Transfer hydrogenation of aliphatic
cyclic ketones was also carried out to obtain moderate
yields after prolong reaction time (Table 4, entry 17, 18).
a
characteristic triplet resonance at -6.15 ppm with J value of
24 Hz (Fig. 5a). 31P NMR also confirmed the formation of
hydride complex by showing
a
doublet with perfect
agreement in JP-H value of two-bond P-H coupling (H-
Ru(II)-PPh3). The formation of the complex was confirmed
2
7
by HRMS analysis, showing a molecular ion peak at
950.2402 [M-PF6]+. Simulated and experimental isotopic
distribution pattern for the molecular ion peak of
perfect agreement (Fig. 5b). Complex was tested as
catalyst under the same reaction condition as in Table 2,
entry 1 without base and it was observed that complex did
7 are in
1
1
not produce any transfer hydrogenated product of
acetophenone. As anticipated, under the optimized reaction
For our curiosity and to extend the substrate scope,
α,β-
unsaturated ketone, 1-phenyl-3-p-tolylprop-2-en-1-one was
employed as substrate for TH reaction with the Ru(II)
catalyst. Over a period of 8 hours, both the alkene and keto
group had undergone reduction to give a major product in
52% yield (Supporting information).
A minor product
resulting from the selective reduction of alkene bond over
Table 5. Extended substrate scope with catalyst 4 [a]
.
Scheme 3. Isolation of the catalytically active [Ru(II)-H] species.
Entry
1
ketones
Time,
h
8
products
Yield,
%
21
TON
166
[b]
52
2
6
4
34
70
68
140
3
4
Scheme 4. Base free TH using Ru(II)–H catalyst 7.
6
43
86
[a] Reaction condition: substrate, 2.0 mmol;
iPrOH = 20 mL;
substrate/base/cat. = 200/20/1; Argon atmosphere, 82 °C; [b] Isolated yield.
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10.1002/ejic.201800585
European Journal of Inorganic Chemistry
FULL PAPER
Figure 5. a) 1H NMR (600 MHz, CDCl3) spectrum of
for the molecular ion peak of
7
(* = residual CHCl3, # = H2O, ¤ = H grease,
∆
= TMS). b) Experimental and simulated isotopic distribution
7
obtained from HRMS study.
base. The reaction furnished the desiredd product in 68% of yield
only after 2 hours (supporting informationn).
condition complex
7 catalyzed the transfer hydrogenation of
acetophenone in the absence of NaOH, resulting the
desired product in 63% yield over a period of 18 hours
(Scheme 4). However, in the presence of NaOH the
precatalyst
1 furnished the TH reaction in 2 hours (Table 2,
entry 1). Developing base free transfer hydrogenation
protocol by transition metal catalyst is the recent trend in
this area.[7g,31] In this regard, complex
7
acted as base free
catalyst for the transfer hydrogenation of acetophenone.
This result suggests that complex acts as precatalyst for
1
the transfer hydrogenation of acetophenone. In the
presence of base it is readily converted to [Ru(II)-H]
complex which is the catalytically active species.
Prompted by the above experimental result and the
reported literature, a plausible mechanism i
intermediate as the catalytically active species is proposed as
shown in scheme 5.[13a,13c,14d-f] The transfe
hydrogenation
mechanism utilizes 2-propanol both as solve t and hydrogen
nnvolving a Ru-H
r
n
n
donor. For example the cationic complex 1 reacts with 2-
propanol in presence of base to produce Ru(II)-alkoxide A, then
upon β-H elimination it forms
intermediate 7 and release of acetone is occurr
of ketone substrates to the metal centre (B) and followed by the
a
Ru-H catalytically active
e
ed. Coordination
insertion to the Ru-H bond affords Ru(II)-al
undergoes base-mediated alcohol metathesi
species A and complete the catalytic cycle
desired alcohol products. Thus, an inner-sp
considering complex 1 as precatalyst is establi
step of the catalytic cycle, exchange of the alko
another alcohol is accelerated in the prese
k
oxide C which
to regenerate
, producing the
s
s
,
hhere mechanism
Scheme 5. Proposed mechanism of TH of ketone by the cationic Ru(II)
catalyst.
s
s
hed. In the last
ide product with
n
ce of base.[14g]
x
x
n
Hence it is not surprising for observing longer reaction time to
yield 1-phenylethanol in 63% yield catalyze
without using NaOH base. To prove, we co
hydrogenation of acetophenone by 7 as catalysst in presence of
dd
by complex 7
n
n
ducted transfer
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10.1002/ejic.201800585
European Journal of Inorganic Chemistry
FULL PAPER
Conclusions
Lambda spectrometer using CDCl3 unless otherwise mentioned.
Spectra were internally referenced to residual solvent peaks (δ =
7.26 ppm for proton and δ = 77.23 for carbon (middle peak) in
CDCl3. All coupling constants (J) are given in Hz. The HRMS mass
spectometry was recorded in ESI+ mode (70 eV) in Waters mass
spectrometer (Model: Xevo-G2QTOF). The absorption spectra were
collected using a Shimadzu (Model UV-2450) spectrophotometer.
FTIR spectra were recorded in Spectrum-BX (Perkin Elmer) on KBr
pallet. Cyclic voltammetric studies were performed on a BASi
Epsilon electrochemical workstation in CH2Cl2 with 0.1 M tetra-n-
butylammonium hexafluorophosphate (TBAPF6) as the supporting
electrolyte. The working electrode was a BASi Pt-disk electrode, the
reference electrode was Ag/AgCl and the auxiliary electrode was a
In conclusion, versatile archetypical tridentate (NNN) terpyridyl
supported Ru(II) cationic and neutral complexes have been
synthesized and characterized successfully. Functionalization at
4-̍ position of terpyridyl unit was varied in metal complexes to
modify the electronic effect. These ruthenium complexes,
featuring no acceleration functionality (such as -NH), show good
catalytic activity in transfer hydrogenation (TH) of ketones with a
substrate to catalyst ratio of 200:1 in refluxing isopropanol which
acts as hydrogen source. Optimized reaction condition was
employed to electron-rich/poor, diaryl, cycloaliphatic ketones,
dione and α,β-unsaturated ketones. This substrate scope study
reveals that the electron-poor ketones react at faster rate than
the electron-rich ketones, whereas the ortho substituted
aromatic ketones react slowly than the para substituted ones
except the methoxy substituted ketones. Neutral Ru(II)
complexes exhibited better catalytic activity than cationic Ru(II)
complexes because of having lower reduction potential (E1/2) by
0.30 V, facilitating reduction of ketones. Most importantly, the
catalytically active species [Ru(II)-H] complex (7) from 1 was
isolated and characterized unambiguously to understand the
mechanism of transfer hydrogenation. As expected, the complex
7 can act as TH catalyst under base free condition. Further
development of Ru(II) catalyst of the tailored terpyridyl ligands
for more efficient TH and hydrosilylation is underway.
Pt-wire. The ferrocene/ferrocenium couple occurs at E1/2
=
+0.51(70)
V versus Ag/AgCl under the same experimental
conditions.
X-ray data collections and refinement. The dark reddish brown
block shaped single crystals of complexes 4-6 suitable for X-ray
crystallography, were obtained by layering hexanes on DCM
solution of Ru(II) complexes. Single-crystal X-ray data was
collected on a Bruker-APEX-II CCD X-ray diffractometer equipped
with an Oxford Instruments low-temperature attachment. Data were
collected at 100(2)
K using graphite-monochromated Mo Kα
radiation (λα = 0.71073 Å). The frames were indexed, integrated,
and scaled using the SMART and SAINT software package,[33] and
the data were corrected for absorption using the SADABS
program.[34] Pertinent crystallographic data for complexes 4-6 is
Experimental Section
summarized in Table S3 (Supporting information). Complex
crystallizes in the monoclinic P21/n whereas and crystallize in
P21/c space groups. Two independent molecules of complexes
and were located in the asymmetric unit with negligible
4
5
6
General considerations. Unless otherwise mentioned all
chemicals were of analytical grade, obtained from commercial
sources (Sigma aldrich, Alfa aesar, Spectrochem) and were used
without further purification. Reactions for the preparation of Ru(II)
complexes as well as all the catalytic reactions were conducted
under an argon atmosphere by using standard Schlenk techniques.
The glasswares were oven-dried (at 180 oC) and cooled under
vacuum. Solvents were purified by following the standard protocol.
Substrates such as 1-phenyl-3-p-tolylprop-2-en-1-one and methyl 5-
(4-methoxyphenyl)-5-oxopentanoate were synthesized as described
in the literature (Supporting information). Acetyl pyridine, 4-
fluorobezaldehyde, 4-methylbenzaldehydeand benzaldehyde were
purchased from Spectrochem while all the substrates were obtained
either from Avra or spectrochem. RuCl3·xH2O was acquired from
Arora Matthey Ltd. PPh3 was purchased from Spectrochem and
purified by recrystallization from absolute ethanol followed by
sublimation. Ru(PPh3)3Cl2 was synthesized following the procedure
described in the literature.[32] Silica gel (60–120 mesh) used for
column chromatography, was purchased from Merck. Eluting
systems for column chromatography purifications were determined
by thin layer chromatography (TLC) analysis. TLC plates were
visualized under UV light (254 nm). Solvents were evaporated
under reduced pressure using a rotary evaporator.
5
6
differences in their metrical parameters. CCDC 1588786-1588788
contain the supplementary crystallographic data for this paper.
These data can be obtained free of charge from The Cambridge
Crystallographic
Data
Centre
via
www.ccdc.cam.ac.uk/data_request/cif. The structures were solved
by SHELXT[35] and refined with SHELXL[36] using Olex2 program.[37]
The molecular structure was generated by using ORTEP-3 for
Windows Version 2.02.[38] Because of the failure to identify the
solvent molecules for 5, SQUEEZE option in PLATON program was
used to remove the unidentified intensities from the overall intensity
data.[39] The hydrogen atoms were included in geometrically
calculated positions in the final stages of the refinement and were
refined according to the typical riding model. All non-hydrogen
atoms were refined with anisotropic thermal parameters.
Synthesis and characterization
General procedure for the synthesis of cationic ruthenium(II)
complexes. In an oven dried Schlenk flask, a mixture of 4'(aryl)-
2,2':6',2''-terpyridine (0.1 mmol), RuCl2(PPh3)3 (0.096 g, 0.1 mmol )
and degassed MeOH (12 mL) were added and heated to reflux for
24 hrs under argon atmosphere. The reaction mixture was turned to
wine red clear solution with few solid impurities. After cooling to
room temperature the reaction mixture was filtered through a bed of
1H (600 MHz, 400 MHz), 13C{1H} (150 MHz, 100 MHz), 31P{1H} (162
MHz), 19F{1H} (376 MHz) NMR spectra were obtained from Bruker
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10.1002/ejic.201800585
European Journal of Inorganic Chemistry
FULL PAPER
celite which was washed several times by dry MeOH. Then NaPF6 (
0.25 mmol) was added to the concentrated solution of the filtrate
and was kept in the deep freeze for overnight to precipitate a red-
brown fine crystalline solid. The solid was filtered off, washed with
diethyl ether (3×10 mL) and dried under reduced pressure to furnish
analytically pure compounds.
126.4, 128.4, 129.4, 129.9, 130.1, 130.2, 132.5, 133.1, 136.8,
144.9, 155.6, 157.5, 158.2, 163.4. FTIR (KBr, cm-1): 840.2 (ꢀꢁPF6,
stretching). HRMS (ESI+, m/z): [M-PF6]+ calcd for C57H44ClFN3P2Ru
988.1727, found 988.1804. anal. calc. for C57H44ClF7N3P3Ru: C,
60.40; H, 3.91, N, 3.71. Found: C, 59.42, H, 3.87, N, 3.77. UV-Vis
λmax: 492 nm (εmax, M-1cm-1 = 1.22×104).
Complex 1. According to the general procedure for the synthesis of
cationic ruthenium(II) complexes, 4'(4-methylphenyl)-2,2':6',2''-
terpyridine (0.032 g, 0.1 mmol) and RuCl2(PPh3)3 (0.096 g, 0.1
mmol ) were refluxed in degassed MeOH (12 mL) and then addition
of NaPF6 (0.042 g, 0.25 mmol) afforded cationic ruthenium(II)
General procedure for the synthesis of neutral ruthenium(II)
complexes. A mixture of RuCl2(PPh3)3 (0.096 g, 0.1 mmol), 4'(aryl)-
2,2':6',2''-terpyridine (0.1 mmol) and 10 mL toluene was charged
into a oven dried Schlenk flask under argon atmosphere. The
reaction mixture was heated to reflux for 20 hrs and then cooled to
ambient temperature. The resulting mixture was then filtered to
afford a solid which was washed with diethyl ether (5×10 mL) and
dried in vacuum to give analytically pure product.
complex
1
which was washed with diethyl ether (3×10 mL) to
(0.097 g)
furnish analytically pure cationic ruthenium(II) complex
1
in 86% yield. 1H NMR (CDCl3, 400 MHz): δ 2.46 (s, 3H, Me-H),
3
7.04-7.08 (m, 14H), 7.17-7.21 (m, 18H), 7.42 (d, 2H, JH-H=8Hz),
Complex 4. According to the general protocol for the synthesis of
neutral ruthenium(II) complexes, a mixture of RuCl2(PPh3)3 (0.096
g, 0.1 mmol), 4'(4-methylphenyl)-2,2':6',2''-terpyridine (0.032 g, 0.1
mmol) and 10 mL toluene was heated to reflux for 20 hrs and then
cooled to ambient temperature to form a deep reddish violet solid.
The resulting mixture was then filtered, washed with diethyl ether
3
7.62-7.64 (m, 4H, Py-H), 7.71 (t, JH-H=8 Hz, Py 2H), 7.91 (d, 2H,
3
3JH-H=8Hz, Py-H), 9.03 (d, 2H, JH-H =6Hz, Py-H). 31P{1H} NMR
(CDCl3, 162 MHz): δ 20.2 (s, PPh3), -143.6 (septet, PF6). 19F{1H}
1
NMR (CDCl3, 376 MHz): δ 72.8 (d, JF-P=710.6 MHz). 13C{1H} NMR
(CDCl3, 100 MHz): δ 21.6, 120.4, 122.8, 126.4, 127.2, 128.4, 129.9,
130.2, 130.3, 130.7, 133.1, 133.2, 133.5, 136.8, 140.7, 146.1,
155.6, 157.3, 158.3. FTIR (KBr, cm-1): 842.4 ( ꢀꢁ PF6, stretching).
HRMS (ESI+, m/z): [M-PF6-PPh3]+ calcd for C40H32ClN3PRu
722.1066, found 722.1055. anal. calc. for C58H47ClF6N3P3Ru: C,
61.68; H, 4.19, N, 3.72. Found: C, 61.62, H, 4.14, N, 3.87. UV-Vis
λmax: 493 nm (εmax, M-1cm-1 = 1.1×104).
(5×10 mL) and dried in vacuum to give analytically pure product
5
(0.066 g) in 87% yield. 1H NMR (CDCl3, 600 MHz): δ 2.46 (s, 3H,
Me-H), 7.05 (d, 8H), 7.14 (d, 4H, J=6 Hz), 7.29-7.32 (m, 9H ), 7.49
(d, 2H, 3JH-H=6Hz, Py-H), 7.63 (s, 2H, Py-H), 7.75 (br s, 2H, Py-H),
3
9.47 (d, 2H, JH-H=6Hz, Py-H). 31P{1H} NMR (CDCl3, 162 MHz) δ
41.7 (s, PPh3); 13C{1H} NMR (CDCl3, 150 MHz): δ 21.5, 119.1,
121.5, 126.5, 127.0, 127.9, 128.0, 129.1, 130.4, 132.0, 132.3,
133.0, 133.1, 135.2, 139.7, 155.3, 159.1, 159.9 HRMS (ESI+, m/z):
[M-Cl]+ calcd for C40H31ClN3PRu 721.0988, found 721.1066. anal.
calc. for C40H32Cl2N3PRu: C, 63.41; H, 4.26, N, 5.55. Found: C,
Complex 2. According to the general procedure for the synthesis of
cationic ruthenium(II) complexes, 4'(phenyl)-2,2':6',2''-terpyridine
(0.031 g, 0.1 mmol) and RuCl2(PPh3)3 (0.096 g, 0.1 mmol ) were
brought together in degassed MeOH (12 mL) under refluxing
condition and then NaPF6 (0.042 g, 0.25 mmol) was added to
59.95, H, 4.20, N, 5.16. UV-Vis λmax: 541 nm (εmax, M-1cm-1
=
furnished cationic ruthenium(II) complex
2
(0.088 g, 79% yield). 1H
1.48×104).
NMR (CDCl3, 400 MHz): δ 7.06-7.08 (m, 16H), 7.17-7.21 (m, 14H),
7.51-7.54 (m, 2H), 7.61-7.64 (m, 4H), 7.70-7.73 (m, 5H), 7.92 (d,
2H, 3JH-H =4Hz, Py-H), 9.03 (d, 2H, 3JH-H =4Hz, Py-H). 31P{1H} NMR
(CDCl3, 162 MHz): δ 20.1 (s, PPh3), -143.5 (septet, PF6). 13C{1H}
NMR (CDCl3, 150 MHz): δ 114.3, 120.7, 122.8, 126.4, 127.4, 128.4,
128.7, 129.9, 130.0, 130.1, 130.3, 133.1, 136.8, 139.5, 146.1,
155.6, 157.4, 158.2 FTIR (KBr, cm-1): 844.6 ( ꢀꢁ PF6, stretching).
HRMS (ESI+, m/z): [M-PF6]+ calcd for C57H45ClN3P2Ru 970.1821,
found 970.1848. anal. calc. for C57H45ClF6N3P3Ru: C, 61.38; H,
4.07, N, 3.77. Found: C, 59.60, H, 3.83, N, 3.80. UV-Vis λmax: 493
nm (εmax, M-1cm-1 = 1.2×104).
Complex 5. According to the procedure for the synthesis of neutral
ruthenium(II) complex RuCl2(PPh3)3 (0.096 g, 0.1 mmol),
4
,
4'(phenyl)-2,2':6',2''-terpyridine (0.031 g, 0.1 mmol) and 10 mL
toluene were reacted to produce a red-brown solid which was
washed with diethyl ether (5×10 mL) and dried under vacuum to
yield analytical pure neutral ruthenium complex
6 (0.062g, 84%
yield). 1H NMR (CDCl3, 600 MHz): δ 7.0-7.03 (m, 8H), 7.10-7.13 (m,
4H), 7.25-7.28 (m, 6H), 7.43 (s, 2H, Py-H), 7.56-7.60 (m, 6H), 7.74
3
3
(d, 2H, JH-H =6Hz, Py-H), 9.43 (d, 2H, JH-H=6Hz, Py-H); 31P{1H}
NMR (CDCl3, 162 MHz) δ 41.6 (s PPh3); 13C{1H} NMR (CDCl3, 150
MHz): 119.1, 121.7, 126.5, 127.2, 127.8, 129.1, 129.4, 129.6,
131.9, 132.2, 133.0, 135.3, 137.5, 143.1, 155.1, 159.0, 159.9.
HRMS (ESI+, m/z): [M-Cl]+ calcd for C39H30ClN3PRu 708.0909,
found 708.0929. anal. calc. for C39H30Cl2N3PRu: C, 62.99; H, 4.07,
N, 5.65. Found: C, 59.92, H, 4.14, N, 5.36. UV-Vis λmax: 542 nm
(εmax, M-1cm-1 = 1.55×104).
Complex 3. According to the procedure for the synthesis of cationic
ruthenium(II) complexes, RuCl2(PPh3)3 (0.096 g, 0.1 mmol) and
4'(4-fluorophenyl)-2,2':6',2''-terpyridine (0.033 g, 0.1 mmol) were
reacted in degassed MeOH (12 mL) under refluxing condition and
followed by the anion exchange by NaPF6 (0.042 g, 0.25 mmol)
produced cationic ruthenium(II) complex
3
(0.091 g) in 80% yield.
Complex 6. According to the general protocol for the synthesis of
1H NMR (CDCl3, 400 MHz): δ 7.05-7.08 (m, 12H), 7.17-7.21 (m,
neutral ruthenium(II) complex 4, RuCl2(PPh3)3 (0.096 g, 0.1 mmol),
18H), 7.29-7.32 (m, 4H), 7.62 (s, 2H, Py-H), 7.70-7.76 (m, 4H, Py-
4'(4-fluorophenyl)-2,2':6',2''-terpyridine (0.033 g, 0.1 mmol) and 10
mL toluene were refluxed to yield a deep violet solid which was
washed with diethyl ether (5×10 mL) and dried under vacuum to
3
3
H), 7.94 (d, JH-H=4Hz, 2H, Py-H), 9.04 (d, 2H, JH-H=4Hz, Py-H).
31P{1H} NMR (CDCl3, 162 MHz) δ 20.0 (s, PPh3), -143.5 (septet,
PF6). 13C{1H} NMR (CDCl3, 150 MHz): δ 117.0, 117.2, 120.4, 122.9,
furnish analytical pure neutral ruthenium complex
7 (0.065 g) in
This article is protected by copyright. All rights reserved.

10.1002/ejic.201800585
European Journal of Inorganic Chemistry
FULL PAPER
85% yield. 1H NMR (CDCl3, 600 MHz): δ 7.01-7.04 (m, 7H), 7.12-
7.13 (m, 6H), 7.29-7.31 (m, 6H), 7.55-7.59 (m, 6H), 7.76 (d, 2H, 3JH-
H=6Hz, Py-H), 9.45 (d, 2H, JH-H=6Hz, Py-H); 31P{1H} NMR (CDCl3,
UGC, while AS acknowledges IIT KGP for doctoral
fellowship.
3
162 MHz): δ 41.4 (s, PPh3). 19F{1H} NMR (376MHz, CDCl3): δ 111.6
(s, aryl-F). 13C{1H} NMR (CDCl3, 150 MHz): δ 116.6, 116.7, 118.9,
121.6, 126.6, 127.9, 128.9, 129.2, 131.9, 132.2, 133.0, 135.3,
141.9, 155.2, 158.9, 160.0, 162.8, 164.5. HRMS (ESI+, m/z): [M-Cl]+
calcd for C39H29ClFN3PRu 726.0815, found 726.0850. anal. calc. for
C39H29Cl2FN3PRu: C, 61.50; H, 3.84, N, 5.52. Found: C, 58.96, H,
3.81, N, 5.16. UV-Vis λmax: 541 nm (εmax, M-1cm-1 = 1.48×104).
Synthesis of [Ru(II)-H] active species (7). In a 100 mL Schlenk
Keywords: Transfer hydrogenation • Catalysis • Ruthenium•
Terpyridine • Ketone
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flask purged with argon, 50 mg complex
1 (0.044 mmol) was added
,
3
to the degassed 4 mL of isopropanol. Next 61 mg of K2CO3 (0.44
mmol) was added to the reaction mixture and allowed to stir for 12
hours under refluxing condition. After cooling down to the room
temperature, all the volatiles were removed under reduced
pressure. Then 5 ml of C6H6 was added and filtered. The filtrate
Wu, J. Xiao, Chem. Commun. 2007, 2449–2466. h) C. M. Eichenseer,
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[2]
[3]
,
–
,
–
was concentrated to afford dark brown coloured complex
7 in 70%
Rösler, J. Obenauf, R. Kempe, J. Am. Chem. Soc. 2015
,
–
of yield (35 mg). 1H NMR (CDCl3, 600 MHz): δ -6.15 (t, 2JH-P=24 Hz,
1H), 2.47 (s, 3H, Me-H), 7.09-7.11 (m, 28H), 7.20-7.22 (m, 10H),
7.36 (br s, 6H), 8.10 (br s, 2H, Py-H); 31P NMR (CDCl3, 202.4 MHz):
8001.
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,
4
–
2
δ 51.5 (d, JP-H=24.3 Hz, PPh3), -144.1 (septet, PF6). 13C{1H} NMR
,
,
–
.
2016
,
45
14
,
,
(CDCl3, 150 MHz): δ 21.5, 119.2, 122.6, 125.7, 127.3, 128.4, 128.9,
129.5, 130.6, 132.6, 133.8, 134.0, 134.5, 140.1, 145.0, 152.7,
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the mixture of a ketone (2.0 mmol), complex
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[5]
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European Journal of Inorganic Chemistry
FULL PAPER
Entry for the Table of Contents
FULL PAPER
Transfer hydrogenation catalysis:
Key Topic*
Neutral
complexes
and
cationic
of simple
Ru(II)
and
Apurba Maity,[a] Ammit Sil,[a] Sanjib K.
Patra*[a]
synthetically ease 4′-(aryl)-2,2':6',2''-
terpyridyl ligands have been utilized
to
catalyze
the
transfer
Page No. – Page NNo.
hydrogenation of wide range of
ketones. The catalytically active
[Ru(II)-H] complex has been isolated
and characterized unambiguously to
understand the mechanism of
transfer hydrogenation.
Title: Ruthenium((II) Complexes of 4'-
(aryl)-2,2':6',2''-terrpyridyl Ligands as
Simple Catalysts ffor Transfer
Hydrogenation of ketones.
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