Ruthenium Catalyzed Dehydrogenation of Alcohols and Mechanistic Study

Article abstract of DOI:10.1021/acs.inorgchem.9b02691

We synthesized pyridylamine ligated arene-Ru(II) complexes and employed these complexes for the catalytic acceptorless dehydrogenation of primary alcohols to carboxylic acids. All the synthesized complexes [Ru]-1-[Ru]-10 are characterized using several spectro-analytical techniques, and the structures of complexes [Ru]-1, [Ru]-2, and [Ru]-5 are determined using single crystal X-ray crystallography. Efficient catalytic conversion of primary alcohols to potassium carboxylates or carboxylic acids is achieved in toluene with the quantitative release of hydrogen gas. The studied protocol for carboxylic acid synthesis with hydrogen generation is also employed for a wide range of substrates, including aliphatic alcohols, aromatic alcohols, and heteroaromatic alcohols, to obtain respective carboxylic acids in good yields (up to 86%). The studied arene-Ru catalysts also exhibit superior catalytic activity for the bulk reaction to achieve a turnover number of 1378. Moreover, extensive mass investigations are also performed to elucidate the mechanistic pathway by identifying the crucial catalytic intermediates, including aldehyde and diol coordinated Ru species under the catalytic and controlled reaction conditions.

Full text of DOI:10.1021/acs.inorgchem.9b02691

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Ruthenium Catalyzed Dehydrogenation of Alcohols and Mechanistic
Study
Mahendra K. Awasthi and Sanjay K. Singh*
Catalysis Group, Discipline of Chemistry, Indian Institute of Technology Indore, Simrol, Indore 453552, India
S
* Supporting Information
ABSTRACT: We synthesized pyridylamine ligated arene-
Ru(II) complexes and employed these complexes for the
catalytic acceptorless dehydrogenation of primary alcohols to
carboxylic acids. All the synthesized complexes [Ru]-1−[Ru]-
10 are characterized using several spectro-analytical techni-
ques, and the structures of complexes [Ru]-1, [Ru]-2, and
[Ru]-5 are determined using single crystal X-ray crystallog-
raphy. Efficient catalytic conversion of primary alcohols to
potassium carboxylates or carboxylic acids is achieved in
toluene with the quantitative release of hydrogen gas. The
studied protocol for carboxylic acid synthesis with hydrogen
generation is also employed for a wide range of substrates,
including aliphatic alcohols, aromatic alcohols, and heteroaromatic alcohols, to obtain respective carboxylic acids in good yields
(up to 86%). The studied arene-Ru catalysts also exhibit superior catalytic activity for the bulk reaction to achieve a turnover
number of 1378. Moreover, extensive mass investigations are also performed to elucidate the mechanistic pathway by
identifying the crucial catalytic intermediates, including aldehyde and diol coordinated Ru species under the catalytic and
controlled reaction conditions.
(COD)]OTf, respectively having NHC (2-((3-methyl-2,3-
dihydro-1H-imidazol-1-yl)methyl)pyridine) and N,P (2-((di-
tert-butylphosphino)methyl)pyridine) ligands, participate in
the catalytic acceptorless dehydrogenation of benzyl alcohol to
benzoic acid (98% yield in 40 h) in the presence of a base
(KOH) in toluene at 120 °C.8b Furthermore, the Rh-based
complexes containing diolefin amido tridentate ligands are also
explored for the catalytic dehydrogenation of alcohol using
acceptors such as O2/DMSO, 1-hexene, and cyclohexanone at
room temperature.9a−c Recently, acceptorless dehydrogenation
of alcohols using Ru-based metal complexes was also explored
using the tridentate pincer-type ligand (PNN10a and PNP10b),
the N-heterocyclic carbene (NHC)10c ligand, and the
bidentate N,N-type11 ligand. Milstein et al. reported that in
the Ru complex having a PNN-type tridentate pincer ligand,
the PNN ligand undergoes dearomatization to form a
catalytically active species for the dehydration of alcohols in
an alkaline aqueous solution under reflux conditions for 18
h.10a Bera et al. explored using a Ru-hydride catalyst having
N,N-type bidentate ligands for the catalytic dehydrogenation
of alcohols in an aqueous medium, albeit under a highly basic
condition (18.5 mmol of NaOH), at 110 °C.11 The only half-
sandwich Ru-NHC complex [(p-cymene)RuCl2(IiPr)] (where
IiPr = 1,3-diisopropyl-2,3-dihydro-1H-imidazole) is reported
by Madsen et al., which is used for the catalytic alcohol
INTRODUCTION
■
Synthesis of carboxylic acids by oxidation of primary alcohols
using a metal-based1−5,6a oxidant is well established, but this
process needs either strong, toxic oxidants or TEMPO with a
stoichiometric amount of sodium hypochlorite, making it a less
atom efficient process.6b Alternatively, carboxylic acid can also
be produced by an alcohol dehydrogenation process.7−11 An
alcohol dehydrogenation process is an atom economic process
that works by generating hydrogen gas (a potential fuel) along
with the desired carboxylic acid, thus providing an opportunity
to explore alcohols as potential hydrogen storage compounds.
Interestingly, compared to the well explored oxidative
syntheses of carboxylic acids using alcohols, catalytic
dehydrogenation of alcohols (such as benzyl alcohol) has
been explored only in past decade using Ir-,8 Rh-,9 and Ru-
based10−13 metal complexes. Moreover, alcohol dehydrogen-
ation reactions are explored either in the presence of high base
concentration in water or in organic solvents at high
temperature (≥110 °C) (Scheme 1).7−13
Yamaguchi et al. reported a Cp*Ir(III)-based dicationic
complex, which has a strong electron-donating N-heterocyclic
carbene (NHC) ligand and α-hydroxypyridine moiety-based
bidentate ligand.8a They have shown that an α-hydroxypyr-
idine moiety plays an important role in the activation of
secondary and primary alcohols to dehydrogenate ketones and
acids, respectively, in neutral water at 140 °C.8a Recently,
Williams et al. reported that other Ir(III) complexes [Ir(2-
Received: September 6, 2019
t
PyCH2(C4H5N2))(COD)]OTf and [Ir(2-PyCH2PBu2 )-
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.9b02691
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Inorganic Chemistry
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Scheme 1. Literature Reported Active Catalysts for the Dehydrogenation of Alcohol to Carboxylic Acid
Scheme 2. Synthesis of Complexes [Ru]-1−[Ru]-10
dehydrogenation reaction in the presence of an additive (PCy3
and HBF4) at 110 °C in toluene for 6 h.12 Peng et al.
investigated dehydrogenation of neat alcohol using Ru
complexes [Ru(L)X(CH3CN)2]+ (L = 2,6-bis(benzimidazol-
2-yl) pyridine, X = Cl, OTf) having N,N,N-tridentate ligands,
albeit at a high temperature (150 °C) and in the presence of
CsOH.13 Apart from noble metal-based catalysts, recently, Liu
et al. investigated Mn(I)-based complexes having PNP-type
pincer ligands for the catalytic acceptorless dehydrogenation of
primary alcohols to carboxylic acids at 160 °C.14
Therefore, it is evident that most of the Ru-based catalysts
explored so far for alcohol dehydrogenation requires either
highly basic conditions, additives, or high temperatures. In the
recent past, we have also developed several arene-Ru(II)-based
catalysts for the acceptorless dehydrogenation of hydrazine
hydrate and formic acid, and we found that ligands have a
crucial role in tuning the catalytic activity.15 Herein, we
synthesized several arene-Ru(II) complexes containing biden-
tate pyridylamine-based ligands, and we explored the accept-
orless dehydrogenation of aromatic and aliphatic alcohols to
carboxylic acids in toluene. Moreover, molecular structures of
[Ru]-1, [Ru]-2, and [Ru]-5 are also authenticated by X-ray
crystallography. Further, via a mass investigation, we sought to
elucidate the mechanistic pathway by identifying several crucial
catalytic intermediate species under the catalytic and
controlled reaction conditions.
RESULTS AND DISCUSSION
■
Synthesis and Characterization of Arene-Ru Com-
plexes. N-substituted pyridylamine ligands, N-(pyridin-2-
ylmethyl)propan-2-amine (L2), N-(pyridin-2-ylmethyl)-
propan-1-amine (L3), N-(pyridin-2-ylmethyl)butan-1-amine
(L4), and 3-methyl-N-(pyridin-2-ylmethyl)butan-1-amine
(L5), were synthesized following literature reports,16 except
2-(aminomethyl)-pyridine (L1), which is commercially avail-
able. The ligands L1−L5 are treated with [{(η6-arene)-
B
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Figure 1. X-ray crystal structures of complexes (a) [Ru]-1, (b) [Ru]-2, and (c) [Ru]-5 with 30% ellipsoid probability. Counteranions are omitted
for the sake of clarity.
Table 1. Crystal Structure Refinement Data for [Ru]-1, [Ru]-2, and [Ru]-5
[Ru]-1
[Ru]-2
C15H20Cl2N2Ru
400.30
[Ru]-5
empirical formula
formula wt
C12H14ClF6N2PRu
467.74
C17H24ClF6N2PRu
537.87
color
yellow block
0.23 × 0.17 × 0.13
monoclinic
P-21/n
293(2)
red block
0.26 × 0.22 × 0.18
monoclinic
P-21/c
golden yellow block
0.23 × 0.18 × 0.13
triclinic
P-1
293(2)
crystal size (mm)
crystal system
space group
T (K)
293(2)
λ (Å), radiation
a (Å)
0.71073, Mo Kα
8.9736(8)
0.71073, Mo Kα
11.3850(2)
0.71073, Mo Kα
7.5042(3)
b (Å)
c (Å)
α (deg)
14.2623(12)
12.1118(11)
90.00
10.4973(3)
13.8742(4)
90
12.0015(5)
13.2949(5)
115.000(4)
β (deg)
95.125(7)
96.421(2)
94.297(3)
γ (deg)
V (Å3)
90.00
1543.9(2)
90
92.318(3)
1078.68(8)
1647.73(7)
Z
4
4
2
ρcalcd. (g/cm3)
F (000)
2.012
920.0
1.606
800.0
1.656
540
no. of data collected/unique data
data/restraints/parameters
final R indices [I > 2σ(I)]
R indices (all data)
goodness of fit
15913/3789 [R(int) = 0.0820]
3789/0/208
R1 = 0.0484, wR2 = 0.1354
R1 = 0.0581, wR2 = 0.1232
1.039
17041/3965 [R(int) = 0.0377]
3965/0/183
R1 = 0.0275, wR2 = 0.0631
R1 = 0.0237, wR2 = 0.0643
0.973
9410/4982 [R(int) = 0.0357]
4982/0/257
R1 = 0.0445, wR2 = 0.1157
R1 = 0.0522, wR2 = 0.1268
0.919
RuCl2}2] in acetonitrile under an argon atmosphere to obtain
the monometallic (η6-arene)Ru(II) complexes [Ru]-1−[Ru]-
10 in good yields (Scheme 2). All the synthesized complexes
displayed high stability in air and moisture and were highly
soluble in acetone. The spectro-analytical analysis of the
synthesized complexes corroborated the proposed structures.
X-ray suitable crystals of [Ru]-1, [Ru]-2, and [Ru]-5 were
grown using slow diffusion of diethyl ether into a methanol/
acetone (9:1 v/v) solution of the complexes. ESI-mass of all
the synthesized complexes displayed prominent mass peaks
corresponding to the cationic mononuclear species with the
general formula [(η6-C6H6)RuCl(κ2-L)]+ and [(η6-C10H14)-
RuCl(κ2-L)]+, where L is pyridylamine ligands L1−L5 (see the
Experimental Section).
at the aminic nitrogen (Namine), a significant upfield shift is
observed for the ortho C−H proton of the pyridine ring from
9.21 ppm in [Ru]-1 to 9.06−9.08 ppm in [Ru]-2−[Ru]-5.
Moreover, the Ru(II) coordinated η6-C6H6 ring protons
resonated as a singlet in the range of 5.90−5.93 ppm,
suggesting the equivalence of all the carbons of the benzene
ring. Analogous to (η6-C6H6)Ru complexes, in [(η6-C10H14)-
RuCl(κ2-L)]+ complexes, [Ru]-6−[Ru]-10 also exhibited an
upfield shift in the resonance of the ortho C−H of pyridine
ring from 9.12 ppm in [Ru]-6 to 8.99−9.03 ppm in [Ru]-7−
[Ru]-10, with the introduction of the alkyl substituents at the
aminic nitrogen (Namine). The Ru(II) coordination with the p-
cymene is also confirmed by the presence of peaks for the
aromatic proton in the range of 5.88−5.73 ppm, the methyl
proton in the range of 1.95−1.89 ppm, and the methine and
methyl protons of the isopropyl group, respectively in the
ranges of 2.72−2.07 and 0.98−1.38 ppm for [Ru]-6−[Ru]-10.
In the 1H NMR spectra of the complexes [Ru]-1−[Ru]-10,
the ortho C−H of pyridylamine resonated at a downfield
region of 9.21−8.99 ppm in comparison to those of free
ligands (L1−L5) (8.50−8.29 ppm),16 which is consistent with
the coordination of pyridylamine ligands with the (η6-
arene)Ru(II) moiety.17 Upon introducing an alkyl substituent
In the 31P NMR spectrum, the counteranion PF6 for
−
complexes [Ru]-1−[Ru]-10 resonated at −144 ppm.16
Further, the molecular structures of complexes [Ru]-1, [Ru]-
C
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2, and [Ru]-5 are also authenticated by X-ray crystallography
(Figure 1 and Table 1). The important bond lengths, bond
angles, and torsion angles are shown in Table 2 (Tables S1 and
was evaluated using benzyl alcohol (1a) as a model substrate at
110 °C in toluene using 1.1 equiv of KOH (Tables 3 and S3).
Table 3. Screening of Catalysts [Ru]-1−[Ru]-5 for Benzyl
a
Table 2. Important Bond Parameters of the Complexes
[Ru]-1, [Ru]-2, and [Ru]-5
Alcohol Dehydrogenation
[Ru]-1
[Ru]-2
[Ru]-5
Bond lengths (Å)
Ru−Ct
1.675
1.684
1.680
Ru−Cavg
Ru−Npy
Ru−Namine
Ru−Cl
2.186
2.193
2.168
entry
catalyst
temperature (°C)
isolated yield (1b) (%)
2.083(4)
2.123(4)
2.3923(12)
1.478(6)
2.1033(17)
2.1926(18)
2.4308(6)
1.504(3)
2.090(3)
2.138(3)
2.4007(10)
1.490(5)
b
1
2
3
4
5
6
7
[Ru]-1
[Ru]-2
[Ru]-3
[Ru]-4
[Ru]-5
[Ru]-1
[Ru]-1
110
110
110
110
110
100
90
94 (1.9)
b
88 (1.6)
90
91
92
72
39
Namine−C6
Bond Angles (deg)
Npy−Ru−Namine
Npy−Ru−Cl
Namine−Ru−Cl
Npy−Ru−Ct
Namine−Ru−Ct
Cl−Ru−Ct
76.19(14)
84.23(11)
84.19(11)
131.64
132.12
129.34
77.98(7)
76.42(12)
85.61(9)
84.28(9)
129.06
133.52
129.29
84.77(5)
93.48(5)
130.28
128.61
126.14
a
Reaction condition: 1a (1 mmol), catalyst (2 mol %), KOH (1.1
equiv), toluene (2 mL), 6 h. H2 gas released (in mmol).
b
Torsion Angles (°)
28.5(5) 27.4(3)
Typically, dehydrogenation of alcohols to carboxylic acids
involves two steps. First, alcohols are converted to an aldehyde
with the release of 1 equiv of H2 gas and finally diols, generated
from an aldehyde with the aid of a base, followed by
dehydrogenation to carboxylic acid with the release of an
additional 1 equiv of H2 gas. Therefore, with the release of 2
equiv of hydrogen gas and the quantitative yield of carboxylic
acid, alcohol dehydrogenation is considered an atom economic
reaction. Preliminary investigation inferred that among the
arene-ruthenium precursors, [{(η6-C6H6)RuCl2}2], [{(η6-
C10H14)RuCl2}2], and [{(η6-C6Me6)RuCl2}2], the [{(η6-
C6H6)RuCl2}2] exhibited a higher conversion for benzyl
alcohol dehydrogenation at 110 °C in toluene (Table S3).
Therefore, further exploration of the catalytic benzyl alcohol
dehydrogenation process was performed using [(η6-C6H6)-
RuCl(κ2-L)]+ ([Ru]-1−[Ru]-5). Moreover, the catalytic
performance of analogous [(η6-C10H14)RuCl(κ2-L)]+ ([Ru]-
6−[Ru]-10) was also explored. Our results inferred that
among the complexes [Ru]-1−[Ru]-10 explored for the
benzyl alcohol dehydrogenation process, [(η6-C6H6)RuCl(κ2-
L)]+ ([Ru]-1−[Ru]-5) exhibited higher catalytic activity,
wherein [Ru]-1 outperformed others and achieved a 94%
yield (>99% conversion) of benzoic acid with a quantitative
yield of H2 gas in 6 h (Table 3, entries 1−5, and Table S3).
The amount of gas produced during the reaction is measured
by a water displacement method and identified as hydrogen by
GC-TCD (Figures 2 and S1).
The trend of the catalytic activity for [Ru]-1−[Ru]-5
complexes is observed as [Ru]-1 > [Ru]-5 > [Ru]-4 > [Ru]-3
> [Ru]-2. Interestingly, the observed trend in the catalytic
activity is in good correlation with the basicity of the aminic
nitrogen of the complexes [Ru]-1−[Ru]-5.18 Complex [Ru]-2,
having an N-isopropyl-substituted pyridylamine ligand (L2),
exhibited the lowest activity (benzoic acid, yield 81%),
attributed to the lower basicity of Namine and the steric
crowding at the α-carbon.18 Furthermore, the activity increases
with an increase in the carbon chain length or branches in
complexes [Ru]-3−[Ru]-5. However, the marginal increase in
benzoic acid yield from 90% [Ru]-3 to 92% [Ru]-5 may be
substantiated by the fact that with the increase in alkyl carbon
chain length or branches the basicity of Namine increases, but it
N
amine-C5−C6-Npy
27.2(5)
S2). The complexes [Ru]-1 and [Ru]-2 were crystallized in a
monoclinic crystal system with P21/n and P21/c space groups,
respectively, whereas the complex [Ru]-5 was crystallized in a
triclinic crystal system with a P1 space group. Consistent with
̅
̅
̅
the NMR and mass results, the crystal structure of the
complexes [Ru]-1, [Ru]-2, and [Ru]-5 confirmed the piano
stool-type geometry of these complexes, where the η6-C6H6
ring is placed at the apex of the stool and the legs of the stool
are occupied by the κ2-pyridylamine and chloro ligands. The
bond angle from the benzene ring centroid (Ct) to each of the
ligands is more than 120°, which is consistent with the
formation of piano stool geometries for these complexes. For
complex [Ru]-1, the Ct−Ru−Npy, Ct−Ru−Namine, and Ct−
Ru−Cl bonds angles are found to be 131.64°, 132.12°, and
129.34°, respectively.16,17 Similarly for [Ru]-2 and [Ru]-3, the
respective bond angles are more than 120°, also suggesting the
piano stool-type geometry for these complexes. The centroid
(Ct) to Ru distance for [Ru]-2 (1.684 Å) and [Ru]-5 (1.680
Å) is longer than those of [Ru]-1 (1.675),17b inferring a
possible steric effect due to the alkyl substituents at the aminic
nitrogen. The Ru−Cl bond lengths for [Ru]-1, [Ru]-2, and
[Ru]-5 are 2.3923, 2.4308, and 2.4007 Å, respectively.17 The
Ru−Npy bond lengths (2.0830−2.1033 Å) are shorter than the
Ru−Namine bond lengths (2.1230−2.1926 Å), which is
consistent with the sp2 hybridized Npy and the sp3 hybridized
N
amine. Moreover, the Ru−Npy and Ru−Namine bond lengths in
[Ru]-1 are shorter than those in [Ru]-2 and [Ru]-5,
presumably due to the strong coordination of L1 with the
Ru center. The Ru−Namine bond lengths in [Ru]-1, [Ru]-2,
and [Ru]-5 are 2.1230, 2.1926, and 2.1380 respectively,
inferring that upon introducing the alkyl group at the Namine
the Ru−Namine bond length also increases. The unusually long
Ru−Namine bond in [Ru]-2 is typical of isopropyl-substituted
nitrogen atoms.18
Catalytic Dehydrogenation of Alcohols to Carboxylic
Acids. The catalytic performance of the synthesized complexes
[Ru]-1−[Ru]-10 along with their precursors for the accept-
orless alcohol dehydrogenation to carboxylic acid/carboxylate
D
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(Table 4, entries 3 and 4). Further, p-substituted benzyl
alcohols (6a−9a) also transformed to their corresponding
carboxylic acids (6b−9b) in good yields (Table 4, entries 5−
8). Among several p-substituted benzyl alcohols explored for
this reaction, those having p-methyl (8a) and p-methoxy (9a)
substituents exhibited a higher yield of the corresponding
carboxylic acid (86% for 8b and 80% for 9b). Notably, p-
chloro- and p-bromo-substituted benzyl alcohols (6a and 7a)
displayed moderate to good yields for the corresponding p-
halogen-substituted benzoic acid (6b and 7b). The lower yield
of p-bromobenzoic acid (7b) is presumably due to the cleavage
of the p-Br/C bond under the catalytic reaction conditions
(Table 4, entry 6). [Ru]-1 can also catalyze the dehydrogen-
ation of phenyl ethyl alcohol (10a), albeit with a lower yield of
the desired phenyl acetic acid (10b) (Table 4, entry 9). 2-
Naphthoic acid (11b) is also obtained with a 65% yield from 2-
naphthyl methanol (11a) (Table 4, entry 10). Notably,
pyridine-2-methanol (12a) is also efficiently converted into
pyridine-2-carboxylic acid (12b) in an 85% yield (Table 4,
entry 11).
Bulk Reaction for the Catalytic Alcohol Dehydrogen-
ation Reaction. Further, our catalytic system can also be
applied to the gram scale synthesis of carboxylic acids; 10
mmol of benzyl alcohol (1a) is dehydrogenated to benzoic
acid (1b) in the presence of 1.1 equiv of KOH in toluene, and
0.97 g of benzoic acid is obtained after 9 h, along with the
release of ca. 16 equiv of H2 gas over [Ru]-1 catalyst with a
turnover number (TON) of 400 (Figure 3). Further, a 62%
(TON 620) yield of benzoic acid (1b) is obtained over 0.1 mol
% [Ru]-1 catalyst in 24 h under the catalytic reaction
conditions (Table S6).
Figure 2. Hydrogen gas generation profile for benzyl alcohol
dehydrogenation to benzoic acid over [Ru]-1 and [Ru]-2 catalysts.
Reaction condition: 1a (1 mmol), catalyst (2 mol %), KOH (1.1
equiv), toluene (2 mL), 110 °C.
may also increase the steric hindrance at the Namine. From the
respective single crystal X-ray crystallographs, in considering
the Ru−Namine bond lengths in complexes [Ru]-1, [Ru]-2, and
[Ru]-5, the Ru−Namine bond is the shortest (2.123 Å) for
[Ru]-1 and the longest (2.1926 Å) for [Ru]-2. The observed
trend suggests a higher basicity of the Namine in [Ru]-1
compared to [Ru]-2 and [Ru]-5, which is well in accordance
with the observed trend in the catalytic activity for alcohol
dehydrogenation. To further optimize the reaction condition,
the catalytic dehydrogenation of 1a to 1b is investigated over
the high performing catalyst [Ru]-1 using various bases and
varying reaction temperature (Tables S4 and S5). Results
showed that the base had a significant effect on the catalytic
activity, where a strong base (NaOH and KOH) facilitated a
higher conversion of 1a, while a weak base (K2CO3, Na2CO3,
and NaHCO3) resulted in the observed poor conversion
(Table S4). The results also showed that decreasing the
reaction temperature to 100 °C and 90 °C also decreased the
yield of the product to 72% and 39%, respectively (Tables 3
and S5). The estimated activation energy was calculated to be
17.3 kcal/mol for the dehydrogenation of 1a to 1b over the
[Ru]-1 catalyst (Figure S2). Moreover, performing the
reaction using benzyl alcohol (1a) and the [Ru]-1 catalyst in
the absence of base in toluene revealed the formation of only
aldehyde with a 6% conversion of 1a in 6 h, suggesting the
crucial role of a base in the alcohol dehydrogenation reaction
(Figure S3).
Substrate Screening. Further, the generality and the
scope of the superior catalyst for the [Ru]-1 catalyst were
further investigated for the acceptorless dehydrogenation of
other substrates under the optimized reaction condition
([Ru]-1 (2 mol %), KOH (1.1 equiv), toluene (2 mL), 110
°C). A wide range of substrates, including aliphatic alcohols
(2a−5a), p-substituted benzyl alcohols (6a−9a), (hetero)-
aromatic alcohols (12a), and other alcohols (10a−11a), are
efficiently transformed to their respective carboxylic acid
products in moderate to good yields (up to 86%). The lower
yield of iso-butyric acid (2b, 20%) can be attributed to the
lower boiling point of iso-butyl alcohol (2a) (Table 4, entry1).
However, the n-butyl alcohol (3a, boiling point 117−119 °C)
was converted efficiently to n-butyric acid (3b) with an
isolated yield of 80% (Table 4, entry 2). 2-Methoxyethanol
(4a) and 2-(methylamino)ethanol (5a) are also transformed to
their respective carboxylic acids (4b and 5b) in good yields
The high catalytic activity (TON 620) of the (arene)Ru(II)
pyridylamine-based complex is of significant importance, and
the obtained results are superior to or on par with several
present reports (Table S7) for alcohol dehydrogenation. The
only previous report using the (arene)RuNHC complex [(p-
cymene)RuCl2(IiPr)] in toluene needs 300 mol % KOH and a
catalytic amount of PCy3 and HBF4 as additives at 110 °C.
However, Williams et al. reported an efficient formation of
benzyl alcohol to benzoic acid in 98% yield using 0.2 mol %
t
[Ir(2-PyCH2PBu2 )(COD)]OTf catalyst, but the reaction
takes 40 h at 120 °C.8b On the other hand, with [Ru]-1
catalyst, an 80% yield of benzoic acid can be obtained for a
shorter duration (9 h) and lower temperature (110 °C) in
toluene (TOF 44 h−1). Moreover, a TON as high as 1378
(TOF 57 h−1) is also achieved over [Ru]-1 catalyst for a bulk
reaction of benzyl alcohol (1a).
Mechanism. To elucidate the possible pathway for the
dehydrogenation of alcohol to carboxylate as catalyzed by (η6-
C6H6)Ru(II) pyridylamine-based complexes, several controlled
experiments and an extensive mass investigation are conducted
to identify the catalytic intermediate species involved in the
alcohol dehydrogenation reaction (Figures 4−6 and S5−S7).
Treating the complex [Ru]-1 with KOH (1 equiv) in water
gave a clear wine red solution, which is analyzed as [(η6-
C6H6)Ru(κ2-L1)]+ [Ru]-1A showing a dominant mass peak at
m/z 287. Analogously, mass analysis of the catalytic reaction
mixture in toluene also showed the presence of a mass peak at
m/z 287 ([Ru]-1A), suggesting the involvement of this species
in the catalytic reaction. Notably, an intense wine-red color
appeared upon addition of p-methylbenzyl alcohol (8a) to the
reaction mixture containing [Ru]-3 catalyst and KOH at room
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a
Table 4. Substrate Scope for Alcohol Dehydrogenation When Using a [Ru]-1 Catalyst
a
Reaction conditions: 2a−12a (1 mmol), catalyst (2 mol %), KOH (1.1 equiv), toluene (2 mL), 110 °C, 6 h.
temperature (Figure S4), and catalytic reaction condition in
toluene.
appeared. Mass analysis of the reaction mixture inferred the
presence of an aldehyde coordinated Ru species with a
dominant mass peak at m/z 407 for [(η6-C6H6)Ru(κ2-L1)(p-
methylbenzaldehyde)]+. An analogous, aldehyde coordinated
Ru species [(η6-C6H6)Ru(κ2-L3)(p-methylbenzaldehyde)]+
(m/z 449) is also observed upon treatment of [Ru]-3 with
p-methyl benzaldehyde in toluene or water. Notably, gentle
heating (85 °C) of the above reaction mixture resulted in the
disappearance of the mass peak at m/z 449, and a new peak
Notably, alcohol coordinated catalytic species are the most
obvious species in such catalytic reactions.8b For instance, p-
methoxy benzyl alcohol coordinated Ir species is reported to
formed by treating the Ir(COD)NHC complex with p-
methoxy benzyl alcohol in the presence of KOH in C6D6 at
room temperature.8b On the other hand, upon addition of an
excess of p-methyl benzaldehyde in an aqueous solution of
[Ru]-1 in the presence of KOH, an intense brown-red color
F
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3A) and the aldehyde coordinated species {(η6-C6H6)Ru(κ2-
L3)(benzaldehyde)}+ ([Ru]-3D) (Figure S5). These results
suggest that the η6-C6H6-to-Ru bond remains intact under the
catalytic reaction condition. To further authenticate this, we
conducted a controlled reaction using a stoichiometric ratio
(3:1 molar ratio) of benzyl alcohol and [Ru]-3 catalyst in the
presence of 1 equiv of KOH in toluene (2 mL) at 110 °C.
Mass spectral analysis (Figure 6 and Figure S6) of the reaction
mixture obtained after 1 h clearly evidenced the presence of
prominent peaks corresponding to the aldehyde coordinated
species {(η6-C6H6)Ru(κ2-L3)(benzaldehyde)}+ ([Ru]-3D) at
m/z 435.1 along with the species [Ru]-3A ({(η6-C6H6)Ru(κ2-
L3)}+ m/z 329.1). Further, the 1H NMR spectrum (in
methanol-d4) of the above reaction mixture also evidenced the
presence of the (η6-C6H6)Ru-based species, which is consistent
with the mass studies (Figure S7). These findings further
authenticated the intactness of the η6-C6H6-to-Ru bond under
the catalytic reaction condition of benzyl alcohol dehydrogen-
ation over the studied (η6-C6H6)Ru-pyridylamine catalysts.
Since no induction period is observed for the catalytic benzyl
alcohol dehydrogenation, it can be anticipated that various (η6-
C6H6)Ru-based catalytic species (such as [Ru]-3A and [Ru]-
3D), observed during the initial hours of the catalytic
reactions, are presumably the crucial catalytic species involved
in the alcohol dehydrogenation reaction. Though our findings
clearly suggest that the arene ring of (η6-C6H6)Ru is not lost
under the catalytic dehydrogenation reaction conditions, we
cannot completely exclude the displacement of the arene
ring.19
Figure 3. Hydrogen generation profile for gram scale dehydrogen-
ation of benzyl alcohol (1a) to benzoic acid (1b) over [Ru]-1 catalyst.
Reaction conditions: 1a (10 mmol), [Ru]-1 (0.2 mol %), KOH (1.1
equiv), toluene (10 mL), 9 h, 110 °C.
appeared at m/z 467 corresponding to the diol coordinated Ru
species ([Ru]-1D) (Figure 5).
Further, the involvement of these (η6-C6H6)Ru intermediate
species is also investigated for the alcohol dehydrogenation
reaction performed under real catalytic conditions ([Ru]-3
catalyst (0.02 mmol), KOH (1.1 mmol), and benzyl alcohol
(1a) (1 mmol) at 110 °C in toluene (2 mL)). The mass
analysis of the reaction aliquots collected at 30 min and 1 h
showed the presence of several (η6-C6H6)Ru-based catalytic
intermediate species, including {(η6-C6H6)Ru(κ2-L3)}+ ([Ru]-
Earlier reports showed that the alcohol dehydrogenation
may go through a Tishchenko-like reaction pathway, but we
could not observe any traces of ester formation in our catalytic
reaction condition. Notably, aromatic aldehydes readily
undergo disproportionation to benzyl alcohol and carboxylate
in the presence of KOH.12 Under our dehydrogenation
condition, only 8% benzyl alcohol is observed during the
Figure 4. (a) ESI-MS analysis in water with [Ru]-1 and KOH and (b) ESI-MS analysis in water with [Ru]-1, KOH, and p-methylbenzaldehyde.
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Figure 5. (a) ESI-MS analysis in water before heating with [Ru]-3, KOH, and p-methylbenzaldehyde and (b) ESI-MS analysis in water after
heating with [Ru]-3, KOH, and p-methylbenzaldehyde and gentle heating (85 °C).
Figure 6. ESI-MS analysis of the reaction aliquot for benzyl alcohol dehydrogenation after heating at 110 °C for 1 h under controlled reaction
conditions using the catalyst [Ru]-3 (0.1 mmol), benzyl alcohol (0.3 mmol), and KOH (0.3 mmol) in toluene (2 mL).
reaction of benzaldehyde with catalyst [Ru]-1, suggesting the
possibility of a Cannizzaro reaction. However, it should be
noted that the dehydrogenation of aldehyde to carboxylic acid
occurred at a faster rate, as ca. 92% of benzoic acid formation is
observed within the initial 1 h while over the [Ru]-1 catalyst.
Moreover, in contrast to the earlier reports under our reaction
condition, dehydrogenation immediately begins and ca. 8.2
equiv of H2 gas (approximately 80% conversion) is achieved
(Figure 7 and Figures S8 and S9). Analogously, a 95% isolated
yield of p-methyl benzoic acid with an equivalent amount of H2
gas is generated from p-methyl benzaldehyde over the [Ru]-1
catalyst (Figure S10).
On the basis of the experimental and mass investigations, a
proposed pathway for the catalytic alcohol dehydrogenation is
illustrated in Scheme 3. We believe that the [Ru] precatalyst is
transferred to the active [Ru]-A catalytic species in the
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undergoes dehydrogenation to regenerate [Ru]-A species.
Upon coordination with aldehyde, the [Ru]-A species further
generated a Ru-aldehyde species ([Ru]-D), which later is
transformed to a diol coordinated species [Ru]-E. The diol in
the [Ru]-E species undergoes β-hydride elimination to release
1 equiv of H2 gas and carboxylate to regenerate the catalytic
active species [Ru]-A.
CONCLUSIONS
■
We synthesized several pyridylamine ligated (η6-arene)Ru(II)
cationic complexes [Ru]-1−[Ru]-10 and confirmed the
structures of three complexes by single crystal X-ray diffraction.
We employed these complexes to achieve efficient acceptorless
dehydrogenation of benzyl alcohol (1a) to benzoic acid (1b)
with the quantitative release of 2 equiv of H2 gas, in toluene at
110 °C. Results inferred that the Namine basicity and steric
crowding at the Namine influenced the overall catalytic activity
of the studied pyridylamine-(arene)-Ru(II) complexes, where
the picolylamine ligated (η6-C6H6)Ru(II) complex ([Ru]-1)
displayed superior catalytic performance. The optimized
catalytic protocol is also employed for a wide range of
substrates, including aliphatic, aromatic, and heteroaromatic
alcohols, to obtain the respective carboxylic acids in good
yields (up to 86%). Extensive mass spectral investigations
evidenced the presence of several important organometallic
species, such as aldehyde and diol coordinated (η6-arene)Ru
species, under the catalytic and controlled reaction conditions.
Moreover, the highly active [Ru]-1 catalyst also exhibited
superior catalytic performance for the gram/bulk scale catalytic
reaction to achieve a turnover number of 1378, which is
superior or on par with several active Ru and Ir-based
complexes available in the literature.
Figure 7. Hydrogen generation from benzaldehyde to benzoic acid
using [Ru]-1 catalyst. Reaction conditions: benzaldehyde (1 mmol),
catalyst (2 mol %), KOH (1.1 equiv), toluene (2 mL), 1 h, 110 °C.
Scheme 3. A Proposed Mechanistic Pathway for the
Dehydrogenation of Alcohol
EXPERIMENTAL SECTION
■
Materials. All experiments were carried out using high purity
chemicals purchased from Sigma-Aldrich, TCI, and Alfa Aesar, unless
otherwise specified, and ligand L1 was purchased from Alfa Aesar.
The precursors [{(η6-C6H6)RuCl2}2], [{(η6-C10H14)RuCl2}2], and
[{(η6-C6Me6)RuCl2}2] are synthesized following the previously
reported method.20
General Procedure for the Catalytic Alcohol Dehydrogen-
ation Reaction over Pyridylamine-Based (η6-Arene)Ru(II)
Complexes. Typically, alcohol (1.0 mmol), catalyst (0.02 mmol),
and KOH (1.1 mmol) are mixed in dry toluene (2 mL) in a two-
necked round-bottom flask equipped with a condenser and an argon
balloon. The reaction mixture is stirred at 110 °C for 6 h in an oil
bath. After the mixture reaches the desired reaction duration, the
solvent is removed under reduced pressure, and the residue obtained
is dissolved in deionized water (20 mL) and acidified with 1 M HCl.
The acidified solution is extracted with ethyl acetate (3 × 10 mL).
The organic phase is subsequently separated and dried over Na2SO4,
and the organic solvent is removed under a vacuum. For carboxylate
salts, after the catalytic reaction, solvent is removed under reduced
pressure, and the remaining residue is dissolved in methanol, filtered,
and dried to yield the pure carboxylate salt.
General Procedure for the Synthesis of (η6-C6H6)Ru(II)
Complexes [Ru]-1−[Ru]-5 and (η6-C10H14)Ru(II) Complexes
[Ru]-6−[Ru]-10. Complexes [Ru]-1−[Ru]-10 are synthesized by
treating N-substituted pyridylamine (L1−L5) (0.59 mmol) with the
respective precursor [{(η6-arene)RuCl2}2] (0.29 mmol) in acetoni-
trile (10 mL) at room temperature for 1 h under an argon
atmosphere. Later, the solvent is completely dried and dissolved in
5 mL of ethanol, followed by the addition of excess NH4PF6. The
resulting solution is stirred for 30 min at room temperature. Adding
10 mL of water to the reaction mixture resulted in the precipitation of
presence of KOH. The first step involved the coordination of
an alcohol with the [Ru]-A species to yield [Ru]-B, which
undergoes β-hydride elimination to yield aldehyde and a Ru-
hydride species ([Ru]-C). Further, the [Ru]-C species
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the desired complexes, which is washed with water (2 × 5 mL) and
diethyl ether (3 × 10 mL) and finally dried in air.
C6H6)RuCl(κ2-L5)]+ (L5 = 3-methyl-N-(pyridin-2-ylmethyl)butan-1-
amine) [M]+ 393.0607, found 393.0860.
Synthesis of [(η6-C6H6)RuCl(κ2-(N,N)-2-(Aminomethyl)-pyridine)]-
PF6 ([Ru]-1). Complex [Ru]-1 is synthesized following the above
general procedure, using 2-(aminomethyl)pyridine (L1) (0.59 mmol)
and [{(η6-C6H6)RuCl2}2] (0.29 mmol). A yellow solid is obtained.
Yield: 65%. 1H NMR (400 MHz, DMSO-d6): δ (ppm) 9.21 (d, 1H, J
= 4 Hz), 7.95 (t, 1H, J = 8 Hz), 7.55 (d, 1H, J = 8 Hz), 7.50 (t, 1H, J1
= 4 Hz, J2 = 8 Hz), 5.90 (s, 6H), 4.14 (t, 2H, J1 = 8 Hz, J2 = 4 Hz).
13C NMR (100 MHz, DMSO-d6): δ (ppm) 161.49, 154.80, 139.19,
Synthesis of [(η6-C10H14)RuCl(κ2-(N,N)-2-(Aminomethyl)-
pyridine)]PF6 ([Ru]-6). Complex [Ru]-6 is synthesized following the
above general procedure, using 2-(aminomethyl)pyridine (L1) (0.59
mmol) and [{(η6-C10H14)RuCl2}2] (0.29 mmol). A yellow solid is
obtained. Yield: 60%. 1H NMR (400 MHz, DMSO-d6): δ (ppm) 9.12
(d, 1H, J = 4 Hz), 7.96 (t, 1H, J = 8 Hz), 7.55 (d, 1H, J = 8 Hz), 7.51
(d, 1H, J = 4 Hz), 5.91 (d, 1H, J = 4 Hz), 5.85 (d, 1H, J = 4 Hz), 5.73
(d, 1H, J = 8 Hz), 5.67 (d, 1H, J = 8 Hz), 4.21−4.10 (m, 2H), 2.72
(br, 1H), 1.95 (s, 3H), 1.12 (d, 6H, J = 8 Hz). 13C NMR (100 MHz,
DMSO-d6): δ (ppm) 161.22, 154.54, 139.13, 124.86, 121.14, 103.04,
98.01, 84.80, 83.05, 82.26, 81.70, 52.03, 30.22. 22.31, 21.42, 17.63.
124.66, 121.16, 84.53, 52.10. 31P NMR (161.97 MHz, DMSO-d6): δ
6
(ppm) −144.18 (sep, PF6). MS (ESI): m/z calculated for [(η
-
C6H6)RuCl(κ2-L1)]+ (L1 = 2-(aminomethyl)pyridine) [M]+
322.9883, found 322.9986.
31P NMR (161.97 MHz, DMSO-d6): δ (ppm) −144.18 (sep, PF ).
6
6
MS (ESI): m/z calculated for [(η -C10H14)RuCl(κ2-L1)]+ (L1 = 2-
Synthesis of [(η6-C6H6)RuCl(κ2-(N,N)-N-(Pyridin-2-ylmethyl)-
propan-2-amine)]PF6 ([Ru]-2). Complex [Ru]-2 is synthesized
following the above general procedure, using N-(pyridin-2-ylmethyl)-
propan-2-amine (L2) (0.59 mmol) and [{(η6-C6H6)RuCl2}2] (0.29
(aminomethyl) pyridine) [M]+ 379.0510, found 379.0459.
Synthesis of [(η6-C10H14)RuCl(κ2-(N,N)-N-(Pyridin-2-ylmethyl)-
propan-2-amine)]PF6 ([Ru]-7). Complex [Ru]-7 is synthesized
following the above general procedure, using N-(pyridin-2-ylmethyl)-
propan-2-amine (L2) (0.59 mmol) and [{(η6-C10H14)RuCl2}2] (0.29
1
mmol). A pale yellow solid is obtained. Yield: 68%. H NMR (400
MHz, DMSO-d6): δ (ppm) 9.06 (d, 1H, J = 4 Hz), 7.99 (d, 1H, J = 8
Hz), 7.63 (d, 1H, J = 8 Hz), 7.54 (t, 1H, J1 = 8 Hz, J2 = 4 Hz), 5.93 (s,
6H), 4.33 (d, 2H, J = 8 Hz), 3.92−3.87 (m, 1H), 1.41 (d, 2H, J = 4
Hz), 1.28 (d, 2H, J = 4 Hz). 13C NMR (100 MHz, DMSO-d6): δ
(ppm) 160.48, 154.72, 139.13, 124.93, 121.58, 85.28, 58.71, 56.62,
23.66, 20.14. 31P NMR (161.97 MHz, DMSO-d6): δ (ppm) −144.18
1
mmol). A yellow solid is obtained. Yield: 63%. H NMR (400 MHz,
DMSO-d6): δ (ppm) 9.00 (d, 1H, J = 4 Hz), 8.00 (t, 1H, J = 8 Hz),
7.65 (d, 1H, J = 8 Hz), 7.56 (t, 1H, J1 = 8 Hz, J2 = 4 Hz), 5.91 (d, 1H,
J = 4 Hz), 5.82 (s, 2H), 5.71 (d, 1H, J = 4 Hz), 4.32−4.25 (m, 2H),
3.84 (br, 1H), 2.09 (m, 1H), 1.98 (s, 3H), 1.38 (d, 3H, J = 4 Hz),
1.31 (d, 3H, J = 4 Hz), 1.09 (d, 3H, J = 8 Hz), 1.00 (d, 3H, J = 8 Hz).
13C NMR (100 MHz, DMSO-d6): δ (ppm) 159.97, 154.86, 139.17,
6
(sep, PF6). MS (ESI): m/z calculated for [(η -C6H6)RuCl(κ2-L2)]+
(L2 = N-(pyridin-2-ylmethyl)propan-2-amine) [M]+ 365.0354, found
365.0436.
125.09, 121.79, 105.28, 96.56, 86.19, 84.92, 82.41, 80.38, 58.60, 56.18,
30.45, 23.42, 21.75, 21.64, 20.26, 17.38. 31P NMR (161.97 MHz,
DMSO-d6): δ (ppm) −144.19 (sep, PF6). MS (ESI): m/z calculated
for [(η6-C10H14)RuCl(κ2-L2)]+ (L2 = N-(pyridin-2-ylmethyl)propan-
2-amine) [M]+ 421.0981, found 421.0909.
Synthesis of [(η6-C6H6)RuCl(κ2-(N,N)-N-(Pyridin-2-ylmethyl)-
propan-1-amine)]PF6 ([Ru]-3). Complex [Ru]-3 is synthesized
following the above general procedure, using N-(pyridin-2-ylmethyl)-
propan-1-amine (L3) (0.59 mmol) and [{(η6-C6H6)RuCl2}2] (0.29
1
Synthesis of [(η6-C10H14)RuCl(κ2-(N,N)-N-(Pyridin-2-ylmethyl)-
propan-1-amine)]PF6 ([Ru]-8). Complex [Ru]-8 is synthesized
following the above general procedure, using N-(pyridin-2-ylmethyl)-
propan-1-amine (L3) (0.59 mmol) and [{(η6-C10H14)RuCl2}2] (0.29
mmol). A pale yellow solid is obtained. Yield: 64%. H NMR (400
MHz, DMSO-d6): δ (ppm) 9.08 (d, 1H, J = 4 Hz), 8.01 (t, 1H, J = 8
Hz), 7.61 (d, 1H, J = 8 Hz), 7.54 (t, 1H, J1 = 8 Hz, J2 = 4 Hz), 5.93 (s,
6H), 4.38−4.26 (m, 2H), 3.52 (br, 1H), 3.16 (br, 1H), 1.78 (br, 1H),
1.6 (br, 1H), 0.96 (t, J = 8 Hz). 13C NMR (100 MHz, DMSO-d6): δ
(ppm) 159.70, 154.92, 139.17, 124.97, 121.43, 85.00, 62.01, 59.08,
21.77, 11.33. 31P NMR (161.97 MHz, DMSO-d6): δ (ppm) −144.18
(sep, PF6). MS (ESI): m/z calculated for [(η6-C6H6)RuCl(κ2-L3)]+
(L3 = N-(pyridin-2-ylmethyl)propan-1-amine) [M]+ 365.0354, found
365.0449.
1
mmol). A pale yellow solid is obtained. Yield: 72%. H NMR (400
MHz, DMSO-d6): δ (ppm) 9.03 (d, 1H, J = 4 Hz), 8.00 (t, 1H, J = 8
Hz), 7.62 (d, 1H, J = 4 Hz), 7.56 (t, 1H, J1 = 8 Hz, J2 = 4 Hz), 5.86 (t,
2H, J = 4 Hz), 5.83 (d, 1H, J = 4 Hz), 5.75 (d, 1H, J = 4 Hz), 4.35−
4.32 (m, 2H), 3.43−3.28 (br, 2H), 2.61 (br, 3H), 1.90 (s, 2H), 1.74−
1.63 (m, 2H), 1.10−1.04 (dd, 6H, J = 8 Hz), 0.96 (t, 3H, J = 8 Hz).
13C NMR (100 MHz, DMSO-d6): δ (ppm) 159.45, 154.88, 139.22,
Synthesis of [(η6-C6H6)RuCl(κ2-(N,N)-N-(Pyridin-2-ylmethyl)-
butan-1-amine)]PF6 ([Ru]-4). Complex [Ru]-4 is synthesized
following the above general procedure, using N-(pyridin-2-ylmethyl)-
butan-1-amine (L4) (0.59 mmol) and [{(η6-C6H6)RuCl2}2] (0.29
125.13, 121.62, 105.14, 96.20, 85.54, 83.84, 82.46, 81.60, 61.12, 59.00,
30.53, 22.15, 21.41, 21.13, 17.40, 11.25. 31P NMR (161.97 MHz,
DMSO-d6): δ (ppm) −144.19 (sep, PF6). MS (ESI): m/z calculated
for [(η6-C10H14)RuCl(κ2-L3)]+ (L3 = N-(pyridin-2-ylmethyl)propan-
1-amine) [M]+ 421.0981, found 421.0919.
1
mmol). A pale yellow solid is obtained. Yield: 71%. H NMR (400
MHz, DMSO-d6): δ (ppm) 9.08 (d, 1H, J = 4 Hz), 7.99 (t, 1H, J = 8
Hz), 7.61 (d, 1H, J = 8 Hz), 7.55 (t, 1H, J1 = 4 Hz, J2 = 8 Hz), 5.93 (s,
6H), 4.44−4.27 (m, 2H), 3.56−3.53 (m, 1H), 3.23−3.19 (m, 1H),
1.75−1.62 (m, 2H), 1.39 (t, 2H, J1 = 4 Hz, J2 = 8 Hz), 0.94 (t, 1H, J1
= 8 Hz, J2 = 4 Hz). 13C NMR (100 MHz, DMSO-d6): δ (ppm)
159.71, 154.93, 139.21, 125.03, 121.49, 85.04, 62.09, 57.19, 30.45,
19.17, 13.86. 31P NMR (161.97 MHz, DMSO-d6): δ (ppm) −144.17
(sep, PF6). MS (ESI): m/z calculated for [(η6-C6H6)RuCl(κ2-L4)]+
(L4 = N-(pyridin-2-ylmethyl)butan-1-amine) [M]+ 379.0510, found
379.0701.
Synthesis of [(η6-C10H14)RuCl(κ2-(N,N)-N-(Pyridin-2-ylmethyl)-
butan-1-amine)]PF6 ([Ru]-9). Complex [Ru]-9 is synthesized
following the above general procedure, using N-(pyridin-2-ylmethyl)-
butan-1-amine (L4) (0.59 mmol) and [(η6-C10H14)RuCl2]2 (0.29
1
mmol). A yellow solid is obtained. Yield: 60%. H NMR (400 MHz,
DMSO-d6): δ (ppm) 9.03 (d, 1H, J = 8 Hz), 8.00 (t, 1H, J = 8 Hz),
7.62 (d, 1H, J = 8 Hz), 7.56 (t, 1H, J = 8 Hz), 5.88 (d, 2H, J = 8 Hz),
5.83 (d, 1H, J = 8 Hz), 5.75 (d, 1H, J = 8 Hz), 4.32 (m, 2H), 3.47−
3.30 (br, 2H), 2.61−2.59 (m, 1H), 1.89 (s, 3H), 1.67 (m, 2H), 1.40
(m, 2H), 1.08 (t, 6H, J = 8 Hz), 0.94 (t, 3H, J = 8 Hz). 13C NMR
(100 MHz, DMSO-d6): δ (ppm) 159.43, 154.86, 139.20, 125.12,
121.62, 105.23, 95.94, 85.71, 83.83, 82.40, 81.57, 61.21, 56.99, 30.55,
30.10, 22.14, 21.08, 19.61, 17.33, 13.78. 31P NMR (161.97 MHz,
DMSO-d6): δ (ppm) −144.19 (sep, PF6). MS (ESI): m/z calculated
for [(η6-C10H14)RuCl(κ2-L4)]+ (L4 = N-(pyridin-2-ylmethyl)butan-
1-amine) [M]+ 435.1138, found 435.1072.
Synthesis of [(η6-C6H6)RuCl(κ2-(N,N)-3-Methyl-N-(pyridin-2-
ylmethyl)butan-1-amine)]PF6 ([Ru]-5). Complex [Ru]-5 is synthe-
sized following the above general procedure, using 3-methyl-N-
(pyridin-2-ylmethyl)butan-1-amine (L5) (0.59 mmol) and [{(η6-
C6H6)RuCl2}2] (0.29 mmol). A pale yellow solid is obtained. Yield:
1
62%. H NMR (400 MHz, DMSO-d6): δ (ppm) 9.08 (d, 1H, J = 4
Hz), 7.99 (t, 1H, J = 8 Hz), 7.61 (d, 1H, J = 8 Hz), 7.55 (t, 1H, J1 = 4
Hz, J2 = 8 Hz), 5.93 (s, 6H), 4.45−4.37 (m, 1H), 4.34−4.28 (m, 2H),
3.60−3.52 (m, 1H), 3.25−3.22 (m, 1H), 1.73−1.62 (m, 2H), 1.59−
1.50 (m, 1H) 0.95 (t, 6H, J = 8 Hz). 13C NMR (100 MHz, DMSO-
d6): δ (ppm) 159.67, 154.92, 139.20, 125.01, 121.47, 85.03, 62.13,
55.86, 37.12, 30.69, 25.77, 22.53. 31P NMR (161.97 MHz, DMSO-
d6): δ (ppm) −144.18 (sep, PF6). MS (ESI): m/z calculated for [(η6-
Synthesis of [(η6-C10H14)RuCl(κ2-(N,N)-3-Methyl-N-(pyridin-2-
ylmethyl)butan-1-amine)]PF6 ([Ru]-10). Complex [Ru]-10 is
synthesized following the above general procedure, using 3-methyl-
N-(pyridin-2-ylmethyl)butan-1-amine (L5) (0.59 mmol) and [{(η6-
C10H14)RuCl2}2] (0.29 mmol). A yellow solid is obtained. Yield: 63%.
1H NMR (400 MHz, DMSO-d6): δ (ppm) 9.02 (d, 1H, J = 4 Hz),
J
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8.00 (t, 1H, J = 8 Hz), 7.62 (d, 1H, J = 8 Hz), 7.56 (t, 1H, J = 8 Hz),
5.88 (t, 2H, J = 4 Hz), 5.82 (d, 1H, J = 4 Hz), 5.75 (d, 1H, J = 4 Hz),
4.33 (m, 2H), 3.50 (br, 1H), 3.30 (br, 1H), 2.61 (m, 2H), 1.89 (s,
3H), 1.70−1.54 (m, 4H), 1.08 (t, 6H, J = 8 Hz), 0.94 (t, 6H, J = 8
Hz). 13C NMR (100 MHz, DMSO-d6): δ (ppm) 159.41, 154.85,
139.20, 125.12, 121.62, 105.31, 95.77, 85.84, 83.79, 82.37, 81.62,
61.29, 55.65, 36.77, 30.57, 25.60, 22.58, 22.37, 22.18, 21.02, 17.30.
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31P NMR (161.97 MHz, DMSO-d6): δ (ppm) −144.19 (sep, PF ).
6
6
MS (ESI): m/z calculated for [(η -C10H14)RuCl(κ2-L5)]+ (L5 = 3-
+
methyl-N-(pyridin-2-ylmethyl)butan-1-amine) [M]
found 449.1229.
449.1294,
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ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.9b02691.
General information and materials, selected bond
lengths, screening criteria, mechanistic investigations,
and dehydrogenation data (PDF)
(10) (a) Balaraman, E.; Khaskin, E.; Leitus, G.; Milstein, D. Catalytic
Transformation of Alcohols to Carboxylic Acid Salts and H2 Using
Water as the Oxygen Atom Source. Nat. Chem. 2013, 5, 122−125.
(b) Choi, J.; Heim, L. E.; Ahrens, M.; Prechtl, M. H. G. Selective
Conversion of Alcohols in Water to Carboxylic Acids by in situ
Generated Ruthenium Trans Dihydrido Carbonyl PNP Complexes.
Dalton Trans. 2014, 43, 17248−17254. (c) Malineni, J.; Keul, H.;
Accession Codes
CCDC 1865341, 1872151, and 1872164 contain the
supplementary crystallographic data for this paper. These
data can be obtained free of charge via www.ccdc.cam.ac.uk/
data_request/cif, or by e-mailing data_request@ccdc.cam.ac.
uk, or by contacting The Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44
1223 336033.
̈
Moller, M. A Green and Sustainable Phosphine-Free NHC-
Ruthenium Catalyst for Selective Oxidation of Alcohols to Carboxylic
Acids in Water. Dalton Trans. 2015, 44, 17409−17414.
(11) Sarbajna, A.; Dutta, I.; Daw, P.; Dinda, S.; Rahaman, S. M. W.;
Sarkar, A.; Bera, J. K. Catalytic Conversion of Alcohols to Carboxylic
Acid Salts and Hydrogen with Alkaline Water. ACS Catal. 2017, 7,
2786−2790.
(12) Santilli, C.; Makarov, I. S.; Fristrup, P.; Madsen, R. J.
Dehydrogenative Synthesis of Carboxylic Acids from Primary
Alcohols and Hydroxide Catalyzed by a Ruthenium N-Heterocyclic
Carbene Complex. J. Org. Chem. 2016, 81, 9931−9938.
(13) Dai, Z.; Luo, Q.; Meng, X.; Li, R.; Zhang, J.; Peng, T. Ru(II)
Complexes Bearing 2,6-Bis(benzimidazole-2-yl)pyridine Ligands: a
New Class of Catalysts for Efficient Dehydrogenation of Primary
Alcohols to Carboxylic Acids and H2 in the Alcohol/CsOH System. J.
Organomet. Chem. 2017, 830, 11−18.
(14) Shao, Z.; Wang, Y.; Liu, Y.; Wang, Q.; Fu, X.; Liu, Q. A general
and efficient Mn-catalyzed acceptorless dehydrogenative coupling of
alcohols with hydroxides into carboxylates. Org. Chem. Front. 2018, 5,
1248−1256.
(15) (a) Awasthi, M. K.; Tyagi, D.; Patra, S.; Rai, R. K.; Mobin, S.
M.; Singh, S. K. Ruthenium complexes for catalytic dehydrogenation
of hydrazine and transfer hydrogenation reactions. Chem. - Asian J.
2018, 13, 1424−1431. (b) Patra, S.; Awasthi, M. K.; Rai, R. K.; Deka,
H.; Mobin, S. M.; Singh, S. K. Dehydrogenation of Formic Acid
Catalyzed by Water-Soluble Ruthenium Complexes: X-ray structure
and mechanistic investigations. Eur. J. Inorg. Chem. 2019, 2019,
1046−1053.
AUTHOR INFORMATION
Corresponding Author
*E-mail: sksingh@iiti.ac.in.
■
ORCID
Sanjay K. Singh: 0000-0002-8070-7350
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors thank the IIT Indore, SERB (DST), New Delhi,
and CSIR, New Delhi, for the financial support. M.K.A. thanks
IIT Indore and INSPIRE-DST for his fellowship. SIC, IIT
Indore is acknowledged for instrumentation facilities. The
authors thank Dr. Shaikh M. Mobin for providing single crystal
X-ray data for the complexes.
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