Transfer hydrogenation (ph independent) of Ketones and aldehydes in water with glycerol: Ru, Rh, and Ir catalysts with a COOH group near the metal on a (phenylthio)methyl-2-pyridine scaffold

Article abstract of DOI:10.1021/om500515z

The reactions of 2-(pyridine-2-ylmethylsulfanyl)benzoic acid (L) with [(⁵-Cp/⁶-benzene)MCl(-Cl)]², (benzene, M = Ru; Cp, M = Rh, Ir) at room temperature followed by treatment with NH ⁴PF⁶ result in a new class of water-soluble half-sandwich complexes [(⁵Cp/⁶-benzene)M(L)Cl][PF⁶] (13, respectively, for M = Ru, Rh, Ir). Their characteristic HR-MS and ¹H and ¹³C{¹H} NMR spectra have been found. The single-crystal structures of 13 have been established with X-ray crystallography. The Ru S, Rh S, and Ir S bond lengths are 2.4079(6), 2.3989(10), and 2.3637(14), respectively. Complexes 13 have been found to be efficient for catalytic transfer hydrogenation (TH) of carbonyl compounds in water with glycerol as a hydrogen donor. Glycerol has been explored for TH in water for the first time. The efficiency in water of other hydrogen sources, viz. HCOOH, citric acid, ascorbic acid, and 2-propanol, is less and/or is pH dependent. Catalysis with glycerol as a hydrogen source is pH independent and appears to be homogeneous. Higher reactivity for the Rh complex in comparison to the Ru and Ir species has been observed. DFT calculations are generally consistent with the experimental values of bond lengths and angles and catalytic reactivity order.

Full text of DOI:10.1021/om500515z

Article
pubs.acs.org/Organometallics
Transfer Hydrogenation (pH Independent) of Ketones and Aldehydes
in Water with Glycerol: Ru, Rh, and Ir Catalysts with a COOH Group
near the Metal on a (Phenylthio)methyl-2-pyridine Scaffold
Om Prakash, Hemant Joshi, Kamal Nayan Sharma, Pancham Lal Gupta, and Ajai K. Singh*
Department of Chemistry, Indian Institute of Technology Delhi, New Delhi 110016, India
S
* Supporting Information
ABSTRACT: The reactions of 2-(pyridine-2-ylmethylsulfanyl)-
benzoic acid (L) with [(η5-Cp*/η6-benzene)MCl(μ-Cl)]2, (benzene,
M = Ru; Cp*, M = Rh, Ir) at room temperature followed by
treatment with NH4PF6 result in a new class of water-soluble half-
sandwich complexes [(η5Cp*/η6-benzene)M(L)Cl][PF6] (1−3,
respectively, for M = Ru, Rh, Ir). Their characteristic HR-MS and
1H and 13C{1H} NMR spectra have been found. The single-crystal
structures of 1−3 have been established with X-ray crystallography.
The Ru−S, Rh−S, and Ir−S bond lengths are 2.4079(6), 2.3989(10),
and 2.3637(14) Å, respectively. Complexes 1−3 have been found to
be efficient for catalytic transfer hydrogenation (TH) of carbonyl
compounds in water with glycerol as a hydrogen donor. Glycerol has
been explored for TH in water for the first time. The efficiency in
water of other hydrogen sources, viz. HCOOH, citric acid, ascorbic acid, and 2-propanol, is less and/or is pH dependent.
Catalysis with glycerol as a hydrogen source is pH independent and appears to be homogeneous. Higher reactivity for the Rh
complex in comparison to the Ru and Ir species has been observed. DFT calculations are generally consistent with the
experimental values of bond lengths and angles and catalytic reactivity order.
employed the water-insoluble catalyst [RuCl2(PPh3)3] and
the phase-transfer agent TPPMS for the biphasic reduction of
aldehydes with aqueous sodium formate. Ogo and co-workers
have developed the water-soluble complexes [Ir(η5-Cp*)-
(H2O)3]2+ and [Ru(η6-C6Me6)(H2O)3]2+ for reducing alde-
hydes and ketones in water.11 The complexes of the 1,3,5-
triaza-7-phosphaadamantane (PTA) ligand with Ru(II) have
been employed in biphasic TH12 carried out in aqueous
medium. The complexes of bipyridine (bpy) and its derivatives
have been found suitable for TH in aqueous medium. Some
examples are [Ir(η5-Cp*)(bpy)(H2O)]2+,13 [Ru(η6-C6Me6)-
(bpy)(H2O)]2+,11b [Ru(η6-arene)(dhbp)Cl]+,14 (dhbp = 6,6′-
INTRODUCTION
■
Efficient catalytic protocols, eco-friendly solvents, and mini-
mum amounts of waste are considered important for chemical
processes. Thus, the use of water instead of organic solvents has
an important place in current research.1 Water as a solvent has
a number of advantages over organic solvents. It is inexpensive,
easily available, nontoxic, and safe to handle. Therefore, its use
may result in significant gains in environmental, economic, and
safety terms. Many catalytic reactions in the aqueous phase,
including transfer hydrogenation, are known.2−5 Transfer
hydrogenation, a versatile process for the reduction of carbonyl
compounds,6 viz. ketones and aldehydes, generally uses 2-
propanol as the source of hydrogen and eliminates inflammable
hydrogen gas and required pressure vessels to handle it.7
Ruthenium, rhodium, and iridium species, including half-
sandwich complexes,4,5,8 catalyze transfer hydrogenation
(TH) of the carbonyl compounds. The number of ruthenium
species used for this purpose is larger than those of rhodium
and iridium. Some of these catalysts have enabled efficient
transfer hydrogenation in water.4,5 For catalysis in water the
catalyst has to be preferably water soluble. For the design of
water-soluble organometallic complexes, the ligands surround-
́
15
dihydroxy-2,2′-bipyridyl), and [Ru(η6-arene)(dmobpy)Cl]+
(dmobpy = 4,4′-dimethoxy-2,2′-bipyridine). Complexes of
Ru(II), Rh(III), and Ir(III) with the tosylated diamine ligands
TsDPEN (N-(p-toluenesulfonyl)-1,2-diphenylethylenediamine)
and Ts-CYDN (N-(p-toluenesulfonyl)-1,2-cyclohexanedi-
amine) have been designed and employed for TH (including
asymmetric TH) in water.4b,16 Carreira and co-workers have
explored (η5-Cp*)IrIII complexes with fluorinated Ts-DPEN
ligands for catalysis of TH in water17 to understand the effect of
fluorine on catalysis. A HCOONa/HCOOH mixture has been
used as the hydrogen donor in most of the reported catalytic
ing the metal should be hydrophilic in nature. Joο and Ben
́
yei9
have reported the use of [RuCl2(TPPMS)2] (TPPMS = 4-
(diphenylphosphino)benzenesulfonic acid) for transfer hydro-
genation in aqueous medium. Sasson and Blum10 have
Received: May 14, 2014
Published: July 11, 2014
© 2014 American Chemical Society
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1
THs in water. Catalysis with this hydrogen donor is pH
dependent and many times an excess (up to 10 mmol) is
required. However, there has been no report using glycerol as
the hydrogen donor for TH in water to our knowledge. There
are some recent reports on using glycerol as a solvent and
hydrogen source in TH carried out in organic solvents.18−20
Platinum group metal complexes of organochalcogen ligands
are promising catalysts for several organic reactions, including
the transfer hydrogenation of carbonyl compounds.21 Apart
from efficiency, these catalysts are attractive due to their
insensitivity to air and moisture, solubility in various organic
solvents, and stability in solution. In view of the efficiency of
half-sandwich complexes of Ru(II), Rh(III), and Ir(III) with
organochalcogen ligands for TH21 and the fact that glycerol has
been unexplored for it in water, we herein report a new class of
stable and water-soluble half-sandwich complexes of Ru(II),
Rh(III), and Ir(III) with 2-(pyridin-2-ylmethylsulfanyl)benzoic
acid (1−3, respectively; Chart 1), which catalyze TH in
evidenced by their H NMR spectra). The elemental analyses
and multinuclear NMR, IR, and mass spectral data of 1−3 are
consistent with their structures on the basis of single-crystal X-
ray diffraction (Scheme 1).
Crystal Structures. Crystals of 1−3 of quality suitable for
X-ray diffraction were obtained by diffusion of diethyl ether
into concentrated solutions of the complexes made up in a
methanol/acetonitrile mixture (1/4 v/v). The crystallographic
and refinement data for 1−3 are summarized in the Supporting
Information (Table S1). The ligand exhibits an identical
bonding mode in all complexes 1−3: i.e., a five-membered ring
is formed on its coordination with the metal center via the
pyridyl nitrogen and sulfur. The molecular structure diagrams
of cations of 1−3 are shown in Figures 1−3, respectively, with
Chart 1
Figure 1. Molecular structure of the cation of 1. Bond lengths (Å):
Ru(1)−S(1) 2.4079(6), Ru(1)−N(1) 2.103(2), Cl(1)−Ru(1)
2.3842(6), Ru−C 2.198(2)−2.221(2). Bond angles (deg): S(1)−
Ru(1)−N(1) 80.16(6), S(1)−Ru(1)−Cl(1) 91.95(2), N(1)−Ru(1)−
C1(1) 86.67(6), O(1)−C(7)−O(2) 122.9(2).
aqueous medium using glycerol as a hydrogen source. The
catalysis is not only efficient but pH independent. The synthesis
and single-crystal structural aspects of these complexes 1−3
have also been reported in this paper. Density functional theory
(DFT) calculations have been carried out and are consistent
with experimental relative catalytic efficiencies and structural
features. These results have also been made part of the present
paper.
RESULTS AND DISCUSSION
■
The syntheses of L and its complexes are outlined in Scheme 1.
The procedure used for the synthesis of ligand L has been
reported earlier.22 The half-sandwich ruthenium(II) (1),
rhodium(III) (2), and iridium(III) (3) complexes appear to
be formed by chloro bridge cleavages of [(η6-C6H6)RuCl(μ-
Cl)]2, [(η5-Cp*)RhCl(μ-Cl)]2, and [(η5-Cp*)IrCl(μ-Cl)]2,
respectively, followed by reaction with 2-(pyridin-2-
ylmethylsulfanyl)benzoic acid (L) at room temperature,
facilitated by chloride extraction with NH4PF6. Complexes
1−3 are moderately soluble in CHCl3, CH2Cl2, and CH3OH,
but in CH3CN their solubility is good. In the presence of KOH
the three complexes are soluble in water. They appear to be
stable to air and moisture and can be stored at room
temperature for several months under ambient conditions (as
Figure 2. Molecular structure of the cation of 2. Bond lengths (Å):
Rh(1)−S(1) 2.3989 (10), Rh(1)−N(1) 2.107(3), Cl(1)−Rh(1)
2.3724(10), Rh−C 2.155(3)−2.170(4). Bond angles (deg): S(1)−
Rh(1)−N(1) 81.37(9), S(1)−Rh(1)−Cl(1) 94.53(4), N(1)−Rh(1)−
C1(1) 87.05(9), O(1)−C(7)−O(2) 121.8(4).
selected bond lengths and angles. The ellipsoids are shown at
−
the 30% probability level, and PF6 and H atoms are omitted
for clarity. In cations of all the complexes there is a pseudo-
Scheme 1. Synthesis of Ligand L and Complexes 1−3
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1). The anion PF6− has been found to be involved in O−H···F
and C−H···F secondary interactions in all of the complexes 1−
3, resulting in a three-dimensional packing framework. In
Figure 5 this is shown for complex 1. The significant secondary
interactions for the other two complexes are detailed in
Supporting Information (Figures S1 and S2 and Table S3).
Spectral Data. NMR spectral data of ligand L and its
complexes 1−3 were found to be characteristic and consistent
in the case of 1−3 with their single-crystal structures. In 1H and
13C{1H} NMR spectra of 1−3 signals of protons and carbon
atoms generally appear at higher frequencies relative to those of
the free ligand, which coordinates with Ru, Rh, and Ir in a
bidentate mode. The magnitude of the shift to higher frequency
is large for PyCH2(S) (up to 8.03 ppm) in 13C{1H) NMR
spectra and for protons attached to its carbon atoms (up to
Figure 3. Molecular structure of the cation of 3. Bond lengths (Å):
Ir(1)−S(1) 2.3637 (14), Ir(1)−N(1) 2.099(4), Cl(1)−Ir(1)
2.3791(15), Ir−C 2.145(6)−2.194(5). Bond angles (deg): S(1)−
Ir(1)−N(1) 81.78(14), S(1)−Ir(1)−Cl(1) 93.44(5), N(1)−Ir(1)−
C1(1) 85.33(13), O(1)−C(7)−O(2) 122.9(7).
1
0.65 ppm) in H NMR spectra. These observations imply that
there is a coordination of ligand L through S/N with the metal
centers. In 1H and 13C{1H} NMR spectra of 1 signals (singlet)
of η6-benzene have been found shifted to lower frequency (up
to 0.3 and 4.4 ppm, respectively) with respect to those of [η6-
octahedral half-sandwich “piano-stool” disposition of donor
atoms around Ru, Rh, or Ir. The centroid of the η6-C6H6/η5-
Cp* ring occupies nearly the center of a triangular face of an
octahedron. The sulfur, nitrogen, and chlorine atoms along
with η6-C6H6/η5-Cp* complete the coordination sphere of Ru/
Rh/Ir. The Ru−S bond length of 1 (2.4079(6) Å) is within the
range (2.3548(15)−2.4156(9) Å) in which such a bond length
of several species has been reported.21a−d,23b The Ru−C(C6H6
centroid) distance of complex 1 (1.688(1) Å) is consistent with
the values 1.685(1)−1.687(1) Å reported earlier.23b The Rh−S
bond distance in 2 (2.3989(10) Å) is consistent with the values
reported for the complexes [η5 -Cp*RhCl{2-
(phenylthiomethyl)pyridine}]PF6 (2.383(2) Å)23a and [η5-
Cp*RhCl(1,1′-(1,2-ethanediyl)bis(3-methylimidazole-2-
thione))]Cl (2.3967(11) Å).24 The Rh−C(Cp* centroid)
distance of complex 2 (1.785(0) Å) is somewhat shorter than
the reported values (1.794−1.809 Å).23 The Ir−S bond
distance of 3 (2.3637(14) Å) falls in the range (2.318(1)−
1
(C6H6)RuCl2]2. In H and 13C{1H} NMR spectra of complex
2, the signals of the η5-pentamethylcyclopentadienyl group
(singlet in 1H NMR) were found at lower frequency (maximum
shifts ∼0.12 and 2.9 ppm, respectively) with respect to those of
[(η5-Cp*)RhCl(μ-Cl)]2. This may be due to substitution of Cl
with S and N, which have relatively lower electronegativity.
Similar observations regarding shifting of signals with respect to
those of [(η5-Cp*)IrCl(μ-Cl)]2 were made in 1H and 13C{1H}
NMR spectra of 3.
HR-MS spectra of structurally analogous complexes 1−3
−
indicate that PF6 is considerably labile and consequently a
molecular ion peak in mass spectra is not observed. The peak of
fragment the [M − (H + Cl)]+ has been observed in the
spectrum of complex 2, whereas the species [M]+ has been
found in the spectra of complexes 1 and 3 (M = cation of 1−3).
The peaks of the fragments [(η6-C6H6)RuCl]+, [(η5-Cp*)-
RhCl]+, and [(η5-Cp*)IrCl]+ have also been observed in the
mass spectra of 1−3, respectively.
Catalytic Transfer Hydrogenation of Carbonyl Com-
pounds. The activity of complexes 1−3 (0.5−1.0 mol %) for
catalytic transfer hydrogenation (TH) of carbonyl compounds
in water was explored using formic acid (sodium formate as
base), citric acid, ascorbic acid, and glycerol (using KOH as a
base with all three) as hydrogen sources (Table 1). The amount
of HCOOONa/HCOOH required for TH was large, and the
catalytic conversion was pH dependent. The TH in aqueous
2.3872(10) Å) in which Ir−S bond lengths of [η5-Cp*IrCl{2-
23a
(phenylthiomethyl)pyridine}]PF6
and [η5-Cp*Ir(CO)(μ-
STol)Pt(STol)(PPh3)]25 have been reported. The Ir−C(Cp*
centroid) distance in 3 (1.805(1) Å) is consistent with the
value reported for the complex [(η5-Cp*)Ir(phpy)Cl] (1.863
Å).26 Significant hydrogen bonding occurs in the crystal
structures of complexes 1−3. Their carboxylic groups are
involved in intramolecular (O of COOH and H of Cp* or
C6H6) as well as intermolecular hydrogen bonding (see Figure
4 for 1). Another important noncovalent interaction is C−O···S
(2.648(2) Å; sum of van der Waals radii 3.32 Å) (Figure 4 for
Figure 4. Noncovalent O−H···O and C−O···S interactions in 1.
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Figure 5. Three-dimensional packing framework showing noncovalent C−H···F and O−H···O interactions in the crystal of 1. PF6 is in the
polyhedral form.
are given in Table 2. The most efficient conversion (up to 99%)
for all of the catalysts was found in the case of benzaldehyde.
Table 1. Optimization of Reaction Conditions of Transfer
Hydrogenation
a
a
Table 2. Transfer Hydrogenation of Carbonyl Compounds
b
base/amt
(mmol)
H source/amt
(mmol)
conversion
(%)
entry omplex
pH
1
1
2
3
2
2
2
2
2
2
2
2
2
2
2
2
2
HCOONa/10
HCOONa/10
HCOONa/10
HCOONa/1
HCOONa/5
HCOONa/5
HCOONa/5
KOH/1
formic acid/5
formic acid/5
formic acid/5
formic acid/1
formic acid/5
formic acid/5
formic acid/5
citric acid/2
citric acid/2
ascorbic acid/2
ascorbic acid/2
2-propanol/5
glycerol/1
2.5
2.5
2.5
2.5
2.5
5.5
10
5
92
98
88
70
98
70
62
36
42
20
30
64
99
98
99
99
2
3
4
5
6
7
8
9
KOH/1
2.5
5
10
11
12
13
14
15
16
KOH/1
KOH/1
2.5
KOH/0.1
KOH/1
2.5
10
KOH/1
glycerol/1
KOH/1
glycerol/1
KOH/5
glycerol/5
a
Conditions: 1 mmol of benzaldehyde, catalyst 1−3 0.5 mol %, 5 mL
b
of water, temperature 110 °C. Conversion determined with NMR.
medium was the most efficient and pH independent with
glycerol as a hydrogen source at 110 °C. Thus, TH in water
with glycerol has been studied in detail. The carbonyl
compounds were reduced to the corresponding alcohols,
while glycerol was dehydrogenated to give dihydroxyacetone
(DHA: 1H NMR δ 4.4 and 3.5 ppm) and other products.18 As a
model reaction, the reduction of benzaldehyde in water using
glycerol was studied. The reaction conditions were optimized
(Table 1). KOH (1 mmol) was found most suitable as a base.
Thereafter, complexes 1−3 for promoting aqueous transfer
hydrogenation reactions of several ketones and aldehydes
having other functional groups as substituents were studied
under optimum reaction conditions. The percent conversions
a
Conditions: 1 mmol of substrate, 1 mmol of KOH, 1 mmol of
b
glycerol, 5 mL of water, temperature 110 °C. Conversion determined
with NMR.
The Rh(III) complex 2 is more efficient as a catalyst relative to
the corresponding Ru(II) and Ir(III) analogues (1 and 3).
When the transfer hydrogenation reactions catalyzed with 2
were monitored with H NMR spectra, a broad singlet was
noticed around δ −10.7 ppm after 1 h. This signal is a
characteristic of metal hydride formation, which probably
1
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occurs due to M−Cl bond cleavage or its very considerable
weakening to make available on the metal center a coordination
site for formation of an intermediate containing a M−H
bond.27 Therefore, the present catalytic reactions with 1−3
probably progress via formation of alkoxide and metal hydride
intermediates.28 With a deficiency of NH groups in the system,
transfer hydrogenation catalyzed with 1−3 appears to pursue a
conventional mechanism.28 The Rh(III) complex 2 appears to
be a somewhat better catalyst than the Ru(II)/Ir(III) analogues
(1/3) (Table 2), as in case of the Ru or Ir complexes for
conversions comparable with that of the Rh analogue with the
same catalyst loading, a somewhat longer reaction time is
needed. DFT calculations (see below) are consistent with the
better efficiency of the Rh complex 2.
It is pertinent to compare the present catalytic activations
with the reported procedures carried out in water or in which
the hydrogen source is glycerol, as there is no example of the
use of glycerol as a source of hydrogen in aqueous medium to
our knowledge. Iridium(III) complexes18d,19b of NHC-based
ligands have been reported for catalytic activation of TH in
glycerol, but the catalyst loading required is 2.5 mol % higher
than those of 1−3 (0.5−1.0 mol %). The time needed for 99%
conversion is also 7 h, except in one case, where it is 1.5 h.
Several Ru(II) complexes11,14,15 have been explored for TH in
water using HCOOH/HCOONa as the hydrogen source.
Apart from mandatory pH control, the reaction time for good
conversion (>80%) in many cases is more than 12 h more than
the time required for 1−3. However, our catalytic system may
be faster than the reported systems due to temperature of 110
°C selected in the present case for TH, which is higher than the
70−90 °C used in some earlier reported catalytic systems.14−17
Ir(III) and Rh(III) complexes have been relatively less explored
for TH in water.4,5,16,17 An (η5-Cp*)IrIII complex with a
pyridine-based ligand gives 98% conversion in 12 h at 0.1 mol
% catalyst loading using water as solvent.5e The catalyst loading
is somewhat lower than that of the present Ir complex but use
of formic acid as the H source has made the protocol pH
dependent. The Rh-TsCYDN complex gives 99% conversion in
15 min when 1 mol % of catalyst is used in water with
HCOOH as a H source.4a The Rh TsDPEN complex causes
catalytic TH in water in a very short time (30 min) at a catalyst
loading of 1 mol %, but the process is pH dependent, as the
hydrogen source is HCOOH.4e
Table 3. Mercury Test for the Catalytic Transfer
Hydrogenation
a
b
entry
catalyst/Hg
time (h)
conversion (%)
c
1
1/0
5
4
6
5
4
6
98
99
95
90
95
88
d
2
1/0
e
3
1/0
c
4
1/400
1/400
1/400
d
5
e
6
a
Conditions: 1 mmol of benzaldehyde, 1 mmol of KOH, 1 mmol of
b
glycerol, catalyst 0.5 mol %, water 5 mL, 110 °C, in air. Determined
by NMR. Ru complex 1. Rh complex 2. Ir complex 3.
c
d
e
Table 4. PPh3 Poisoning Test for the Catalytic Transfer
a
Hydrogenation
b
entry
catalyst/PPh3
time (h)
conversion (%)
c
1
1/0
1/0
1/0
1/5
1/5
1/5
5
4
6
5
4
6
98
99
95
92
94
90
d
2
e
3
c
4
d
5
e
6
a
Conditions: 1 mmol of benzaldehyde, 1 mmol of KOH, 1 mmol of
b
glycerol, catalyst 0.5 mol %, water 5 mL, 110 °C. Determined by
NMR. Ru complex 1. Rh complex 2. Ir complex 3.
c
d
e
experimental results. The HOMOs (highest occupied molec-
ular orbitals) of all complexes are essentially similar and are
positioned primarily over Ru, Rh, or Ir and the benzene/Cp*
ring, with some contribution toward S, N, and Cl donor atoms.
The d orbitals of Ru(II)/Rh(III)/Ir(III) interacting with π
orbitals of η5-Cp*/η6-benzene ring and p orbitals of chlorine,
nitrogen, and sulfur atoms compose their HOMO (Figure 6).
Detailed calculated bond length and angle parameters are given
in Table 5. The concurrence between the observed and
calculated bonding parameters is better for M−Cl and M−
arene(centroid). There is some variation between calculated
and observed M−S bond distances. The calculated and
experimentally found bond angles are also reasonably close
(Table 5), except in few cases: e.g., Cl−Ru−N1.
The HOMO−LUMO energy gap (lowest unoccupied
molecular orbital) of a complex may be correlated with its
chemical reactivity.31 Generally, a large gap reflects low
reactivity. The HOMO−LUMO energy gaps of 1−3 differ
sufficiently (Figure 6). It is lowest for the Rh complex,
indicating that most likely its catalytic activity has to be greater
than those of the Ru and Ir complexes. This is consistent with
the observed catalytic efficiency of complexes 1−3 (Table 2).
Homogeneous vs Heterogeneous Catalysis in Trans-
fer Hydrogenation. When transfer hydrogenation is catalyzed
with iridium complexes in glycerol under microwave con-
ditions, formation of Ir nanoparticles (NPs) has been reported,
which affect the catalytic process.20b Thus, it is significant to
understand whether the present transfer hydrogenation
catalysis is homogeneous or heterogeneous. In order to know
whether such metal NPs are formed with the present complexes
and also have some influence on catalysis, a mercury poisoning
test29 has been executed. The presence of mercury in the TH of
acetophenone with glycerol using 1−3 as catalysts showed no
significant inhibition of conversion (Table 3). A PPh3
poisoning test has also been carried out.30 In the presence of
5 equiv of PPh3 the catalytic reaction occurred with only a 6%
decrease in percent conversion (Table 4). Thus, the catalysis is
homogeneous in nature. Furthermore, there was no visible sign
of NP formation.
CONCLUSIONS
■
The half-sandwich complexes [(η6-C6H6)Ru(L)Cl][PF6], [(η5-
Cp*)Rh(L)Cl][PF6], and [(η5-Cp*)Ir(L)Cl][PF6] (1−3,
respectively) of 2-((2-pyridyl)methylthio)benzoic acid (L)
have been synthesized and characterized by multinuclear
NMR, HR-MS, and X-ray crystallography. The compounds
are water soluble, due to the presence of a −COOH group in
the ligand. They efficiently catalyze pH-independent transfer
hydrogenation of carbonyl compounds in water using glycerol
as a hydrogen donor. Glycerol has been explored for TH in
water for the first time. The insensitivity of the three complexes
to air and moisture is an additional advantage. The catalytic
DFT Calculations. Density functional theory (DFT)
calculations were executed on all three complexes 1−3, in
order to support structure and catalytic activity related
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Figure 6. HOMO−LUMO energy gap of 1−3.
Table 5. Selected Bond Lengths (Å) and Angles (deg) of 1−3 Determined Experimentally and Optimized with DFT
1
2
3
bond length/
angle
optimized
value
bond length/
angle
optimized
value
bond length/
angle
optimized
value
Ru−S(1)
Ru−Cl(1)
Ru−N(1)
2.4079(6)
2.3842(6)
2.103(2)
1.688(1)
80.16(6)
91.95(2)
86.67(6)
122.9(2)
2.505
2.402
2.142
1.779
81.45
91.04
83.14
122.08
Rh−S(1)
Rh−Cl(1)
Rh−N(1)
2.3989(10)
2.3724(10)
2.107(3)
1.858(0)
81.37(9)
93.44(5)
85.33(13)
121.8(4)
2.555
2.410
2.179
1.785
80.22
92.88
85.88
121.96
Ir−S(1)
Ir−Cl(1)
Ir−N(1)
Ir−C
S1−Ir−N1
S1−Ir−Cl
2.3637(14)
2.3791(15)
2.099(4)
2.541
2.424
2.151
1.842
79.97
91.60
84.57
122.14
a
a
a
Ru−C
Rh−C
1.805(1)
S1−Ru−N1
S1−Ru−Cl
Cl−Ru−N1
O1−C7−O2
S1−Rh−N1
S1−Rh−Cl
Cl−Rh−N1
O1−C7−O2
81.78(14)
93.44(5)
Cl−Ir−N1
O1−C7−O2
85.33(13)
122.9(7)
a
C = centroid.
CH3CN. The C, H, and N analyses were carried out with a C, H, and
N analyzer.
efficiency is highest for the Rh complex, which is corroborated
by DFT studies. There is no evidence for NP formation in the
catalytic process, which appears to be homogeneous. The
formation of a M−H bond favors a conventional mechanism
for the present transfer hydrogenation via alkoxide formation.
The experimental bond lengths and angles are consistent with
DFT results.
Chemicals and Reagents. Thiosalicylic acid, (2-chloromethyl)-
pyridine hydrochloride, and ammonium hexafluorophosphate were
used as received. The ligand L was prepared as reported earlier.22 [(η6-
C6H6)RuCl(μ-Cl)]2,34 [(η5-Cp*)RhCl(μ-Cl)]2,35 and [(η5-Cp*)IrCl-
36
(μ-Cl)]2 were synthesized according to literature procedures.
Common reagents and chemicals available locally were used. All of
the solvents were dried and distilled before use by standard
procedures.37
EXPERIMENTAL SECTION
■
DFT Calculations. All DFT calculations were carried out at the
Department of Chemistry, Supercomputing Facility for Bioinformatics
and Computational Biology, IIT Delhi, with the GAUSSIAN-03
program.38 The geometry of complexes 1−3 was optimized at the
B3LYP39 level using an SDD basis set for the metal and S atom and 6-
31G* basis sets for C, N, Cl, and H. Geometry optimizations were
carried out without any symmetry restriction with X-ray coordinates of
the molecule. Harmonic force constants were computed at the
optimized geometries to characterize the stationary points as minima.
The molecular orbital plots were prepared using the Chemcraft
program package (http://www.chemcraftprog.com).
Physical Measurements. All reactions have been performed in
glassware dried in an oven, under ambient conditions except for the
synthesis of L. Commercial nitrogen gas was used after passing it
successively through traps containing solutions of alkaline anthraqui-
none, sodium dithionite, alkaline pyrogallol, concentrated H2SO4, and
KOH pellets. The nitrogen atmosphere, when required, was created
1
using Schlenk techniques. The H and 13C{1H} NMR spectra were
recorded at 300.13 and 75.47 MHz, respectively. The chemical shifts
are given in ppm relative to known standards. Yields refer to isolated
1
yields of compounds which have purity ≥95% (established with H
NMR). IR spectra in the range 4000−400 cm−1 were recorded as KBr
pellets. For single-crystal structures the data were collected using Mo
Kα (0.71073 Å) radiation at 298(2) K. The software SADABS32 was
used for absorption correction (if needed) and SHELXTL for space
group, structure determination, and refinements.33 Hydrogen atoms
were included in idealized positions with isotropic thermal parameters
set at 1.2 times that of the carbon atom to which they are attached in
all cases. High-resolution mass spectral measurements were performed
with electron spray ionization (10 eV, 180 °C source temperature,
sodium formate as reference compound), with sample taken in
Synthesis of Complex 1. The ligand L (0.2 mmol) and solid [(η6-
C6H6)RuCl2]2 (0.1 mmol) dissolved in CH3OH (15 cm3) were stirred
together for 8 h at room temperature. The resulting yellow solution
was filtered, and the volume of the filtrate was reduced to ∼7 cm3 with
a rotary evaporator. The concentrate was treated with solid NH4PF6
(0.2 mmol), and the resulting yellow microcrystalline solid 1 was
filtered, washed with 10 cm3 of ice-cold CH3OH, and dried in vacuo.
The single crystal of 1 was obtained by diffusion of diethyl ether into
their solutions (4 cm3) made up in a mixture of CH3OH and CH3CN
3809
dx.doi.org/10.1021/om500515z | Organometallics 2014, 33, 3804−3812

Organometallics
Article
1
(1/4). Yield: 0.102 g, ∼85%. Anal. Calcd for C19H17ClNO2RuS·[PF6]:
C, 37.73; H, 2.83; N, 2.32. Found: C, 37.32; H, 2.92; N, 2.48. Mp: 180
°C. 1H NMR (CD3CN, 25 °C vs Me4Si; δ (ppm)): 4.74−4.96 (m, 2H,
average of two runs of each catalytic reaction. The H NMR spectra
authenticating these products are reported in the Supporting
Information (Figures S12−S19).
3
Hg Poisoning Test. In a reaction vessel was placed an excess of
Hg (Hg/(Ru/Rh/Ir) 400/1). The aqueous transfer hydrogenation
reaction of benzaldehyde (1.0 mmol) with glycerol (1.0 mmol) using
1, 2, or 3 (0.5 mol %) as catalyst was carried out in the vessel under
optimum conditions. Conversion of ∼95% was observed after 4−6 h
of reaction.
PPh3 Poisoning Test. To a catalytic transfer hydrogenation
reaction mixture of benzaldehyde in water with glycerol (1.0 mmol)
was added PPh3 (5 mol %) under optimum conditions after addition
of catalyst 1, 2, or 3 (1 mol %). After 4−6 h of reaction ∼94% of the
product was obtained.
H8), 5.78 (s, 6H, Ru-Ar-H), 6.83 (d, JH−H = 7.8 Hz, 1H, H10) 7.38−
7.44 (m, 3H, H2, H11−12), 7.84−7.87 (m, 1H, H3), 7.98−8.07 (m, 2H,
H4, H13), 8.88−8.89 (m, 1H, H5). 13C{1H} NMR (CDCl3, 25 °C vs
Me4Si; δ (ppm)): 45.8 (C8), 87.8 (Ru-Ar-C), 123.8 (C10), 124.8 (C12),
128.2 (C3), 128.5 (C2), 129.8 (C6), 130.1 (C5), 131.7 (C4), 134.4
(C11), 140.1 (C1), 157.0 (C13), 159.8 (C9), 166.0 (C7 COOH). HR-
MS (CH3CN): [M]+ m/z 459.9705; calculated value for
C19H17ClNO2RuS 459.9708 (δ −0.7 ppm). IR (KBr; cm−1): 3101
(m; νC−H(aromatic)), 2923 (s; νC−H(aliphatic)), 1691 (m; νCN),
1439 (m; νCC(aromatic)), 835 (s; νP−F), 767 (m; νC−H(aromatic)).
Synthesis of Complex 2. The solid [(η5-Cp*)RhCl(μ-Cl)]2 (0.1
mmol) and ligand L (0.2 mmol) dissolved in CH3OH (15 cm3) were
mixed. The mixture was stirred for 8 h at room temperature. The
resulting orange solution was filtered, and the volume of the filtrate
was reduced (∼7 cm3) with a rotary evaporator. It was mixed with
solid NH4PF6 (0.2 mmol), and the resulting orange microcrystalline
solid was filtered, washed with 10 cm3 of ice-cold CH3OH, and dried
in vacuo. Single crystals of 2 were obtained by diffusion of diethyl
ether into its solution (4 cm3) made in a mixture (1/4) of CH3OH
and CH3CN. Yield: 0.120 g, ∼90%. Anal. Calcd for C23H26ClNO2RhS·
[PF6]: C, 41.61; H, 3.95; N, 2.11. Found: C, 41.69; H, 3.85; N, 2.21.
ASSOCIATED CONTENT
■
S
* Supporting Information
Figures, tables, and CIF and xyz files giving crystallographic and
refinement data, bond lengths and angles, secondary interaction
distances, NMR and mass spectra, and Cartesian coordinates
(CCDC Nos. 986289, 986290, and 986291, respectively, for 1−
3). This material is available free of charge via the Internet at
http://pubs.acs.org.
1
Mp: 205 °C. H NMR (CD3CN, 25 °C vs Me4Si; δ (ppm)): 1.74 (s,
15H, Rh-Cp*-H), 4.50−4.97 (m, 2H, H8), 6.92−6.95 (m, 1H, H10),
AUTHOR INFORMATION
Corresponding Author
*A.K.S.: e-mail, aksingh@chemistry.iitd.ac.in, ajai57@hotmail.
com; fax, +91 11 26581102; tel, +91 11 26591379.
■
7.37−7.50 (m, 3H, H2, H11−12), 7.89−7.92 (m, 1H, H3), 8.03−8.06
3
(m, 2H, H4, H13), 8.45 (d, JH−H = 5.4 Hz, 1H, H5). 13C{1H} NMR
(CDCl3, 25 °C vs Me4Si; δ (ppm)): 8.5 (Rh-Cp*-C) 43.5 (C8), 99.3
(Rh-Cp-C), 124.7 (C10), 125.7 (C12), 126.8 (C3), 128.3 (C2), 129.8
(C6), 130.4 (C5), 131.1 (C4), 133.0 (C11), 140.2 (C1), 154.2 (C13),
159.7 (C9), 166.3 (C7 COOH). HR-MS (CH3CN): [M − (Cl + H)]+
m/z 482.0655; calculated value for C23H25NO2RhS 482.656 (δ −0.8
ppm). IR (KBr; cm−1): 3100 (m; νC−H(aromatic)), 2974 (s;
νC−H(aliphatic)), 1686 (m; νCN), 1443 (m; νCC(aromatic)), 843
(s; νP−F), 752 (m; νC−H(aromatic)).
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors thank Professor B. Jayaram for computational
facilities. The authors also thank the Department of Science
and Technology, New Delhi, India, for the following: research
project no. SR/S1/IC-40/2010, partial financial assistance
(under the FIST program) for the single-crystal X-ray
diffractometer and mass spectral facilities at IIT Delhi. O.P.
and H.J. thank the University Grants Commission of India, and
K.N.S. thanks the Council of Scientific and Industrial Research
of India for the award of a Junior/Senior Research Fellowship.
Synthesis of Complex 3. A mixture of solid [(η5-Cp*)IrCl(μ-
Cl)]2 (0.1 mmol) and L (0.2 mmol) dissolved in CH3OH (15 cm3)
was stirred for 10 h at room temperature. The resulting yellow solution
was filtered. After workup, as described for 2, single crystals of 3 were
obtained by diffusion of diethyl ether into its solution (4 cm3) made in
a mixture (1/4) of CH3OH and CH3CN. Yield: 0.130 g, ∼86%. Anal.
Calcd for C23H26ClNO2IrS·[PF6]: C, 36.68; H, 3.48; N, 1.86. Found:
C, 36.60; H, 3.23; N, 2.01. Mp: 210 °C. 1H NMR (CD3CN, 25 °C vs
Me4Si; δ (ppm)): 1.73 (s, 15H, Ir-Cp*-H), 4.30−5.07 (m, 2H, H8),
6.96 (m, 1H, H10), 7.40−7.45 (m, 3H, H2, H11−12), 8.03−8.08 (m,
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3H, H3−4, H13), 8.47 (d, JH−H = 5.7 Hz, 1H, H5). 13C{1H} NMR
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