2-propanol vs glycerol as hydrogen source in catalytic activation of transfer hydrogenation with (η6-benzene)ruthenium(II) complexes of unsymmetrical bidentate chalcogen ligands

Article abstract of DOI:10.1021/om500579r

1H-Benzoimidazole on subjection to a sequence of reactions with benzyl bromide, PhECH₂Cl (E = S, Se), and elemental S or Se results in 1-benzyl-3-phenylchalcogenylmethyl-1,3-dihydrobenzoimidazole-2-chalcogenones (L1-L4), which are unsymmetrical

Full text of DOI:10.1021/om500579r

Article
pubs.acs.org/Organometallics
2‑Propanol vs Glycerol as Hydrogen Source in Catalytic Activation of
Transfer Hydrogenation with (η6‑Benzene)ruthenium(II) Complexes
of Unsymmetrical Bidentate Chalcogen Ligands
Alpesh K. Sharma, Hemant Joshi, Kamal Nayan Sharma, Pancham L. Gupta, and Ajai K. Singh*
Department of Chemistry, Indian Institute of Technology Delhi, New Delhi − 110016, India
S
* Supporting Information
ABSTRACT: 1H-Benzoimidazole on subjection to a se-
quence of reactions with benzyl bromide, PhECH2Cl (E = S,
Se), and elemental S or Se results in 1-benzyl-3-phenyl-
chalcogenylmethyl-1,3-dihydrobenzoimidazole-2-chalcoge-
nones (L1−L4), which are unsymmetrical bidentate chalcogen
ligands having a unique combination of chalcogenoether and
chalcogenone donor sites. Half sandwich complexes, [(η6-
C6H6)Ru(L)Cl][PF6] (1−4), have been synthesized by
reactions of [(η6-C6H6)RuCl(μ-Cl)]2 with the appropriate L
at room temperature followed by treatment with NH4PF6.
L1−L4 and their complexes 1−4 have been authenticated with
1
HR-MS and H, 13C{1H}, and 77Se{1H} NMR spectra. The single-crystal structures of 1−4 have been determined by X-ray
crystallography. Each L acts as an unsymmetric (E,E) or (E,E′) bidentate ligand. The Ru atom in 1−4 has pseudo-octahedral
half-sandwich “piano-stool” geometry. The Ru−S and Ru−Se bond distances (Å) respectively are 2.358(3)/2.3563(18) and
2.4606(11)/2.4737(10) (thio- and selenoether), and 2.4534(17)/2.435(3) and 2.5434(9)/2.5431(10) (thione and selone).
Catalytic activation with complexes 1−4 has been explored for the transfer hydrogenation (TH) of aldehydes and ketones using
various sources of hydrogen. 2-Propanol and glycerol have been compared and found most suitable among the sources screened.
The catalytic efficiency of other sources explored, viz. formic, citric, and ascorbic acid, is dependent on the pH of reaction
medium and is not promising. A comparative study of 2-propanol and glycerol as hydrogen sources for catalytic activation of TH
with 1−4 has revealed that with glycerol (for comparable conversion in the same time) more amount of catalyst is needed in
comparison to that of 2-propanol. The catalytic process is more efficient with 3 (where Ru is bonded with selone), followed by 1
≈ 4, and 2 showing the least activity among all four complexes. The transfer hydrogenation involves an intermediate containing a
Ru−H bond and follows a conventional alkoxide intermediate based mechanism. The results of DFT calculations appear to be
generally consistent with experimental catalytic efficiencies and bond lengths/angles.
transfer hydrogenation reactions.7 Noyori and co-workers have
studied ruthenium(II) complexes designed with several ligands,
INTRODUCTION
■
Transfer hydrogenation has an important place among organic
BINAP, ethylenediamine, or other 1,2-diamines and PR3, for
transformations. It is used for the reduction of carbonyl
catalytic transfer hydrogenation (including asymmetric) and
compounds to the corresponding alcohols, important (partic-
ularly chiral ones) in the pharmaceutical, perfume, and
proposed a mechanism involving the N−H group.8−18
agrochemical industries.1 Transfer hydrogenation (TH) avoids
Ruthenium half-sandwich complexes explored for catalytic
TH generally have benzene or p-cymene.19 The coligands used
the use of stoichiometric reducing reagents or hazardous
molecular hydrogen,2 and because of this, it has attracted
to design such complexes include pyridyl/bipyridyl li-
gands,20−22 benzimidazoles,24c NHCs,23,24a,b and triazoles.25
substantial interest.3 The TH employs a catalyst and hydrogen
The promise of metal complexes of organochalcogen ligands
source. Several hydrogen sources such as 2-propanol, formic
acid, glycerol, and cyclopentanol have been explored4,5 for this
has been demonstrated recently in the catalysis of various
chemical transformations and as single-source precursors for
purpose. Similarly, there have been many catalysts screened to
metal chalcogenide nanoparticles.26 Half-sandwich ruthenium-
activate transfer hydrogenation reactions. Among all the
(II) complexes designed using such ligands catalyze the
catalysts reported so far, those based on Ru(II), Rh(III),
oxidation of alcohols and transfer hydrogenation of ketones.27
Ir(III), and Ir(I) (TOF 24000) have been the most
successful.6a−o Other metal catalysts and organocatalysts have
The promising efficiency of these catalysts has been partially
also been reported6p for transfer hydrogenation, but many of
attributed to the strong electron-donating ability of the
them are slow and the resulting enantioselectivity (in
asymmetric TH) is not good. Ruthenium(II) complexes have
gotten a good deal of attention in the recent past for catalytic
Received: May 31, 2014
Published: July 2, 2014
© 2014 American Chemical Society
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Scheme 1. Synthesis of Ligands L1−L4 and Their Complexes 1−4
chalcogen donor site, particularly in the case of sulfur and
selenium. Ligands incorporating both hard (N) and soft donor
atoms (S, Se, Te) have been more explored to design half-
sandwich ruthenium(II) complexes for TH than have (E,E)
ligands (E = S, Se).28 To the best of our knowledge, half-
sandwich complexes of Ru(II) with bidentate ligands having
chalcogenoether and chalcogenone donor sites together have
not been explored for TH so far. It was therefore thought
worthwhile to design 1-benzyl-3-phenylchalcogenylmethyl-1,3-
dihydrobenzoimidazole-2-chalcogenone ligands (L = L1−L4),
which are unsymmetric bidentate chalcogen ligands having a
unique combination of thione or selone donor site with a
thioether or selenoether site. Half-sandwich ruthenium
complexes of these ligands have been synthesized and
characterized by single-crystal structure studies. A single-crystal
structure of a Ru complex of any selone ligand has not been
reported to date, and the present report is the first,29a whereas
one report on the structure of a Ru(II) thione complex is
known.29b The Ru(II) complexes of L1−L4 have been explored
in detail for transfer hydrogenation of carbonyl compounds
using several hydrogen sources. A comparative study of 2-
propanol and glycerol has been carried out. DFT calculations
have been found to support experimental observations. All of
these results are described in this paper.
the selenoether group at 402.6 ppm in the 77Se{1H} NMR
spectrum of L1 is at a slightly lower frequency (2.1 ppm) with
respect to that of the selenated precarbene precursor B (404.7
ppm), whereas such a signal in the 77Se{1H} NMR spectrum of
L2 appears at 396.2 ppm: i.e., at 8.51 ppm lower frequency with
respect to that of B. The selone signals in the 77Se{1H} NMR
spectra of L1 and L3 appear at 101.8 and 92.5 ppm,
respectively. The signals of the selenoether group in the
77Se{1H} NMR spectra of 1 and 2 have been found to be
shifted to higher frequency (13.4, and 112.0 ppm, respectively),
with respect to those of the corresponding free ligand. The
shifts probably arise due to coordination of L1 and L2 with Ru
via Se. In 2 the ligand is (Se,S), whereas in 1 it is (Se,Se). The
Se coordinates more strongly than S. Electron transfer to the
metal from Se is thus easier in the case of (Se,S) ligand rather
than (Se,Se) ligand, as the one Se has to compete with the
other in the latter. Thus, a high-frequency shift of large
magnitude is more likely to occur in 2, as observed
experimentally. The signal (−11.3 ppm) in 77Se{1H} NMR
spectrum of 3 due to selone has been found to be shifted to
lower frequency by 103.8 ppm with respect to that of free L3.
In complex 1 a selone peak was not observed. On coordination,
the relaxation time of Se increases, which results in a
broadening of the signal.30 The broadening of the signal has
been correlated to the coordination of ligands with metal
ions.31 Similar phenomena appear to occur in the present case,
which make the selone signal invisible. The invisibility of the
selone signal32 on coordination has been reported earlier as
well. The signals for benzimidazolium protons observed at δ
RESULTS AND DISCUSSION
■
The syntheses of L1−L4 and their complexes are summarized
in Scheme 1. Ligands L1−L4 have good solubility in common
organic solvents: viz., CHCl3, CH2Cl2, CH3OH, and CH3CN.
In contrast, complexes 1−4 are only moderately soluble in
these solvents, except for CH3CN. They have good solubility in
DMSO. The complexes 1−4 and their ligands are insensitive to
air and moisture, as they can be stored at room temperature for
several months under ambient conditions.
1
9.85 and 9.91 ppm in the H NMR spectra of B and C,
respectively, are in the range reported for such protons (due to
its high acidity) in the case of other imidazolium salts (δ 9.0−
12.0).33,34 In the H NMR spectra of ligands L1−L4, there is
1
no benzimidazolium proton signal (at δ 9.85 and 9.91 ppm)
due to the formation of thione/selone. The signal of the carbon
atom C12 of benzimidazolium has been observed in 13C{1H}
NMR spectra of L1−L4 at higher frequency (∼28.2 ppm) with
respect to those of free B and C, due to the formation of
thione/selone. The signals of CH2 (C5 and C13) in 13C{1H}
NMR spectra of complexes 1−4 have been found to be shifted
to higher frequency (∼0.6−2.7 ppm) with respect to those of
1
NMR Spectra. The H, 13C{1H}, and 77Se{1H} NMR and
mass spectra of L1−L4 and their complexes 1−4 are given in
the Supporting Information. For B and C (Scheme 1), L1−L4,
and complexes 1−4 the spectra have been found to be in
agreement with their molecular structures (Scheme 1). The
structures of 1−4 have been corroborated with their single-
crystal structures determined by X-ray diffraction. The signal of
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the corresponding free ligand (from L1−L4). This is due to the
coordination of L1−L4 with ruthenium. The signal of C12
in13C{1H} NMR spectra of complexes 1−4 has been found to
be shifted to lower frequency (∼3.4 to 27.2 ppm) with respect
to that of the corresponding free ligand. The signals (singlet) of
1
η6-benzene in H and 13C{1H} NMR spectra appear to be
shifted to lower frequency with respect to those of [(η6-
C6H6)RuCl2]2. This occurs due to substitution of Cl with S and
Se, which have relatively lower electronegativities and are
stronger donors.
In the mass spectra of ligands L1−L4 peaks appearing at m/z
480.9696, 433.0248, 433.0248, and 385.0793, respectively, may
be ascribed to [L + Na]+ (see the Supporting Information,
Figures S5−S8). In the mass spectrum of complex 3 there is a
peak at m/z 624.9558, which corresponds to its cation. In the
mass spectra of both complexes 1 and 2, a molecular ion peak
or peak of their cation was not observed. However, there is a
peak in mass spectra of both complexes at m/z 379.0709, which
corresponds to [C21H19N2Se]+ and appears to be of precursor
B of ligands.
Figure 2. Structure of the cation of 2. Bond lengths (Å): Ru(1)−Se(1)
2.4737(10), Ru(1)−S(1) 2.4534(17), Cl(1)−Ru(1) 2.4212(17),
C(14)−S(1) 1.713(6), C(6)−Se(1) 1.934(7). Bond angles (deg):
S(1)−Ru(1)−Se(1) 94.72(5), Cl(1)−Ru(1)−Se(1) 79.42(5), Cl(1)−
Ru(1)−S(1) 86.04(6).
Crystal Structures. Single crystals of 1−4 suitable for X-ray
diffraction were grown from 1/3 (v/v) methanol/acetonitrile
mixtures and subjected to crystal structure studies.
The crystal data and refinement parameters are given in the
Supporting Information (Table S1). The structures (ellipsoids
at 30% probability) of the cations of 1−4 are shown in Figures
1−4, respectively, with selected bond lengths and angles. H
Figure 3. Structure of the cation of 3. Bond lengths (Å): Ru(1)−S(1)
2.3563(18), Ru(1)−Se(1) 2.5434(9), Ru(1)−Cl(1) 2.4200(19),
N(2)−C(14) 1.357(8), Se(1)−C(14) 1.862(7), S(1)−C(6)
1.798(7). Bond angles (deg): S(1)−Ru(1)−Se(1) 93.96(5), Cl(1)−
Ru(1)−Se(1) 85.74(5), S(1)−Ru(1) − Cl(1) 80.89(7).
Figure 1. Structure of the cation of 1. Bond lengths (Å): Ru(1)−Se(1)
2.4606(11), Ru(1)−Se(2) 2.5431(10), Cl(1)−Ru(1) 2.423(2),
C(14)−Se(2) 1.876(7), C(6)−Se(1) 1.948(8). Bond angles (deg):
Se(1)−Ru(1)−Se(2) 94.62(3), Cl(1)−Ru(1)−Se(2) 85.49(5),
Cl(1)−Ru(1)−Se(1) 79.73(6).
−
atoms and PF6 anion are omitted in each figure for clarity.
L1−L4 exhibit similar bonding modes in all complexes 1−4.
The six-membered chelate ring is formed due to their
coordination with ruthenium through chalcogen atoms. In the
cation of each of the four complexes there is a pseudo-
octahedral half-sandwich “piano stool” type disposition of
donor atoms around the Ru center. The centroid of the η6-
benzene ring occupies the center of three octahedral sites,
making a triangle. The chalcogen donor atoms of L1−L4 and
chlorine complete the coordination sphere. Half-sandwich
Ru(II) complexes of ligands containing a seleno- or thioether
donor site in conjunction with a selone or thione donor group
are not known to us. The present ones complexes probably the
Figure 4. Structure of the cation of 4. Bond lengths (Å): Ru(1)−S(1)
2.358(3), Ru(1)−S(2) 2.435(3), C(14)−S(2) 1.702(10), C(6)−S(1)
1.777(11). Bond angles (deg): S(1)−Ru(1)−S(2) 93.63(10), S(1)−
Ru(1)−Cl(1) 81.08(10), Cl(1)−Ru(1)−S(2) 86.19(11).
first examples. The Ru−S(thione) distances in the cations of 2
and 4 (2.4534(17) and 2.435(3) Å, respectively) are
normal.35,36 The Ru−S(thioether) bond lengths in the cations
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Figure 5. Noncovalent C−H···F interactions in 1.
Figure 6. Noncovalent C−H···F interactions in 4.
of 3 (2.3563(18) Å) and 4 (2.358(3) Å) are in the range
(2.3548(15)−2.4156(9) Å) for those of several reported Ru(II)
complexes of morpholine- and pyrrolidine-based organosulfur
ligands.27a−c The Ru−Se(selenoether) bond length in the
cation of 1 is 2.4606(11) Å, and that of 2 is 2.4737(10) Å.
These values are consistent with the range 2.4756(10)−
2.5240(9) Å for similar bond lengths reported for the Ru(II)
complexes27a−c [RuCl(η6-p-cymene)(N-{2-(phenylseleno)-
ethyl}pyrrolidine)]+, [RuCl(η6-C6H6)(N-[2-(arylseleno)ethyl]-
morpholine)]+, and [RuCl(η6-C6H6)(N-{2-(phenylseleno)-
ethyl}pyrrolidine)]+ and Ru−Se clusters27d [Ru3(μ3-Se)(μ3-
S)(CO)7(μ-dppm)] and [Ru3(μ3-Se)(CO)7(μ3-CO)(μ-
dppm)]. The Ru−Cl bond lengths are in range 2.423(12)−
2.420(19) Å and are normal.27b The PF6 anion in complexes
1−4 has been found to be involved in C−H···F secondary
interactions, resulting in chains, as shown in Figures 5 and 6 for
complexes 1 and 4, respectively. The crystals of 1−4 also have
C−H···Cl secondary interactions (for details see the Supporting
Information, Table S5).
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Catalytic Transfer Hydrogenation. A transfer hydro-
genation reaction (Scheme 2, in which one organic molecule
transfers hydrogen to another) avoids the use of inflammable
molecular hydrogen.2b
Scheme 2. Transfer Hydrogenation of Carbonyl Compounds
The catalysis of transfer hydrogenation of aldehydes and
ketones at 80 °C has been standardized using benzaldehyde as
substrate, 3 (0.5 mol %) as catalyst, and citric acid, ascorbic
acid, formic acid, glycerol, or 2-propanol as the source of
hydrogen (Table 1). In the presence of citric acid and ascorbic
Figure 7. Time profile of the catalytic transfer hydrogenation of
benzaldehyde with complex 3 in air. Conditions: catalyst, 0.1 mol %
for 2-propanol and 0.5 mol % for glycerol; benzaldehyde, 1.0 mmol;
KOH, 0.4 mmol; solvent, 5 mL of glycerol or 2-propanol; temperature,
80 °C. Conversions were monitored with NMR.
Table 1. Screening of Proton Sources for Catalytic Transfer
Hydrogenation
a
entry
H source
conversn, %
c
1
citric acid
ascorbic acid
formic acid
glycerol
<10
7
c
2
c
3
53
90
95
1−4 a variety of aldehyde and ketone substrates with varying
substituents were studied. With glycerol only 2 and 3 were
investigated in detail. The results are given in Tables 2 and 3 for
2-propanol and glycerol, respectively.
b
4
b
5
2-propanol
a
Reaction conditions: catalyst 3 (0.5 mol %), aldehyde (1 mmol).
b
KOH (2 mL of a 0.2 M solution or 0.4 mmol), bath temperature 80
c
a
°C, reaction time 3 h. pH 3.
Table 2. Catalytic Transfer Hydrogenation in 2-Propanol
conversn, %
acid the reaction shows negligible conversion (Table 1, entries
1 and 2). However, in the case of formic acid the progress of
the reaction is dependent on the pH of the reaction mixture, as
at pH 7.0 there is no conversion, whereas at pH 4.0 and 5.0 the
conversion to alcohol is around 38%. When the pH is lowered
to 3.0, the conversion scales up to 53% (Table 1, entry 3). As
the conversions in the presence of glycerol (90%, Table 1, entry
4) and 2-propanol (95%, Table 1, entry 5) were high, they were
selected for further studies, particularly the comparative study.
In view of the claim of transfer hydrogenation promoted by
KOH/NaOH,37 control reactions of acetophenone (as its
hydrogenation is easiest) were carried out under optimum
conditions in the absence of catalyst for 3 h in both 2-propanol
and glycerol. Approximately 8% conversion to the desired
product took place in 2-propanol, whereas in glycerol the
product was not detected. In the report of NaOH promoted
transfer hydrogenation authors themselves are not fully
confident of claim.37b In the claim of KOH-promoted transfer
hydrogenation two points are important to note. First, for
ketones a reaction time of 18−24 h is required, and second, a
yield of more than 75% has not been achieved for any aldehyde
entry
substrate
benzaldehyde
1
2
3
4
1
2
3
4
5
6
7
8
93
89
85
88
91
91
88
84
90
84
82
86
88
90
86
82
95
90
88
92
93
96
90
87
93
88
87
89
90
94
87
85
4-methylbenzaldehyde
4-anisaldehyde
4-bromobenzaldehyde
cyclopentanone
acetophenone
propiophenone
4-methylacetophenone
a
Reaction conditions: catalyst, 0.1 mol %; aldehyde/ketone, 1 mmol;
KOH, 2 mL of a 0.2 M solution; bath temperature, 80 °C; reaction
time, 3 h.
With 2-propanol the conversions (Table 2) have been found
to be high in the cases of benzaldehyde (up to 95%, Table 2,
a
Table 3. Catalytic Transfer Hydrogenation in Glycerol
conversn, %
entry
substrate
benzaldehyde
2
3
or ketone.37 Thus, the role of the Ru catalyst in the present
a
1
2
3
4
5
6
7
83
72
81
80
85
81
74
90
82
86
89
91
87
83
case is significant. The time profile of both these hydrogen
sources is shown in Figure 7. The conversion almost linearly
increases up to 2 h, and thereafter the rate is slowed down.
Further optimization of the catalytic reaction with these two
hydrogen sources has been carried out using 3 as a catalyst with
benzaldehyde substrate. The optimum catalyst loading is 0.1
mol % with 2-propanol and 0.5 mol % with glycerol. The KOH
was found to be a suitable base at a temperature of 80 °C. To
understand the scope of catalytic reactions using Ru complexes
4-anisaldehyde
4-bromobenzaldehyde
cyclopentanone
acetophenone
propiophenone
4-methylacetophenone
a
Reaction conditions: catalyst, 0.5 mol %; aldehyde/ketone, 1 mmol;
KOH, 0.4 mmol; bath temperature, 80 °C; reaction time, 3 h.
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Figure 8. Frontier molecular orbitals of complexes 1−4 and their HOMO−LUMO energy gaps.
Table 4. Comparison of Selected Bond Lengths (Å) and Angles (deg) of 1−4 Determined Experimentally and Calculated by
DFT
1
2
3
4
bond angle/
length
DFT
value
bond angle/
length
DFT
value
bond angle/
length
DFT
value
bond angle/
length
DFT
value
Ru−S/Se(chalcogenoether)
Ru−Cl
2.4606(11)
2.4230(2)
2.5431(10)
2.5625
2.4737(10)
2.4212(17)
2.4534(17)
2.564
31
2.3563(18)
2.4200(19)
2.5434(9)
2.5094
2.4112
2.6325
2.358(3)
2.414(3)
2.435(3)
2.5061
2.4080
2.5615
2.420 60
2.630 48
2.418
45
Ru−S/Se(chalcogenone)
2.554
52
S/Se(chalcogenone)−Ru−S/Se(chalcogenoether)
Cl−Ru−S/Se(chalcogenoether)
Cl−Ru−S/Se(chalcogenone)
94.62(3)
79.73(6)
85.49(5)
93.86
77.74
85.23
94.72(5)
79.42(5)
86.04(6)
93.57
77.65
86.04
93.96(5)
80.89(7)
85.74(5)
93.26
93.63(10)
86.19(11)
81.08(10)
92.63
79.30
85.85
79.105
85.178
entry 1) and acetophenone (up to 96%, Table 2, entry 6) with
all of the catalysts 1−4. In case of aliphatic ketones the
conversion was up to 93% (Table 2, entry 5 for cyclo-
pentanone). The efficiency of complex 3 is somewhat greater
than those of the other three complexes (Table 2).
hydrogenation in glycerol using complexes 2 and 3 (0.5 mol %)
as catalysts in the presence of KOH as a base at a temperature
of 80 °C (Table 3). The carbonyl compound is reduced to the
corresponding alcohol while glycerol is dehydrogenated to
1
dihydroxyacetone (DHA; H NMR δ 4.4 and 3.5 ppm) and
other products,38 but all are obtained in low yield and are
difficult to separate. The low yield of dihydroxyacetone is not a
great concern because glycerol is very cheap and high recovery
of this main byproduct is not going to cut the cost of the
process very significantly. Using complexes 2 and 3 for catalytic
TH, conversions have been found to be high in the cases of
benzaldehyde (up to 90%, Table 3, entry 1) and acetophenone
(up to 91%, Table 3, entry 5). The complex 3 is somewhat
more efficient in comparison to 2 (Table 3). With glycerol as
the hydrogen source 1 mol % of the ruthenium species explored
for TH, viz. RuCl3(TPPS)3, RuCl3(TPP)3, RuCl[(p-cymene)-
TsDPEN], and [Ru(η6-arene)(NHC)CO3], is needed, which is
higher than the required loading of 2 and 3 (0.5 mol %).7f,g
Further, the reaction time for good conversion is on the order
of 24−48 h, whereas with 0.5 mol % of 2 and 3 good
conversion occurs in 3 h. The two hydrogen donors glycerol
and 2-propanol are efficient for catalytic TH with 1−4; the
latter donor is, of course, slightly better.
The comparison of performance of 1−4 as catalysts for TH
in 2-propanol with the catalysts reported in literature reveals a
mixed bag. The catalyst loading required for [RuCl2(p-
7a
cymene)]2 and [Ru(diamine)(η6-arene)(dimethylimidazole)-
7b
(Cl)]+ is 0.5 mol %, higher than those of the present
complexes for good conversion. Further, the reaction time is
long, at least 20 h. The complex [RuBr(OAc)(PPh3)(p-
aNHC)]7c as catalyst is nearly comparable to 1−4 and shows
almost similar efficiency. The Ru(II) diamide complex7d has
been used at loading somewhat lower (up to 0.01 mol %) than
that for 1−4, and the required reaction time is also short. For
the [RuCl2(PPh3)3]−ethylenediamine system the optimum
catalyst loading is 0.2 mol %.8 The optimum loadings of
complexes of [RuCl2(arene)] with ImEt−CH2CH2OEt for TH
have been reported as 0.05−0.5 mol %, which are comparable
to those of 1−4.7e
The scope of catalytic transfer hydrogenation with glycerol as
a hydrogen source and 2 and 3 as catalysts has been explored
(Table 3). Aldehydes and ketones were subjected to transfer
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Figure 9. NBO atomic charges of complexes 1−4.
The catalytic TH reactions catalyzed with 3 have been
monitored with 77Se{1H} NMR spectroscopy. The signals in
the spectra shift to higher frequency (∼17 ppm). This indicates
that probably the Ru−Cl bond is cleaved or weakened very
significantly to make a coordination site on metal center
available so that formation of an intermediate having a Ru−H
energy gap of a complex and its chemical reactivity43a,b is
expected. This is because the chemical reactivity is related to
chemical hardness, defined as the resistance to perturbation in
the electron distribution in a molecule.43b In terms of frontier
orbitals, chemical hardness corresponds to the energy gap
between the HOMO and LUMO and is approximated by
εHOMO − εLUMO, where εLUMO and εHOMO are the LUMO and
HOMO energies.43a,d The magnitude of HOMO−LUMO
energy gaps in 1−4 is not unusual: i.e., on the order of 4 eV
reported for the complexes of group VIII metal ions with
organochalcogen donors.43c The large HOMO−LUMO energy
gap makes the deformation of the electron cloud difficult, which
in turn results in less reactivity.43d The HOMO−LUMO energy
gap is lower in the case of 1/3(selone analogues) relative to 2/
4 (thione analogues) (see Figure 8). Thus, the reactivities of
complexes of selone ligands are expected to be greater than
those of the corresponding thione analogues. This is consistent
with the experimentally observed order of catalytic efficiencies
of complexes: Se > S (of course only marginal). Natural bond
orbital (NBO) analysis of atomic charge for 1−4 shows that the
charge on the Ru center of the 3 is highest (Figure 9). The
presence of high electronic charge on Ru facilitates the
formation of ruthenium hydride needed in the case of transfer
hydrogenation. Thus, selone-containing species are expected to
be more efficient for transfer hydrogenation catalysis, as the
nature of Se is soft, which may result in a higher charge on Ru
bond takes place.39 In H NMR spectra a broad singlet has
1
been noticed around −8.2 to −10.7 ppm during the course of
the catalytic reaction. A signal at this position is characteristic of
a metal hydride and indicates the formation of a Ru−H bond.40
Thus, catalytic transfer hydrogenation reactions with the
present complexes probably proceed via formation of a metal
hydride intermediate, as suggested for the conventional
mechanism41 based on metal alkoxide formation.
DFT Calculations. Density functional theory (DFT)
calculations were performed on all complexes 1−4. Their
HOMOs (highest occupied molecular orbitals) are positioned
primarily over the metal center, chalcogens (S/Se), and Cl with
some contribution from the benzimidazole ring (Figure 8). The
agreement between the experimentally observed bonding
parameters and the calculated values is reasonable42 for Ru−
E(chalcogenoether) (E = S, Se). Ru−Cl, Ru−E′(chalcoge-
none), and Ru−benzene(centroid) calculated and experimen-
tally found bond lengths may be in general considered
consistent with experimental results (Table 4). The calculated
and experimental values of bond angles are reasonably close. A
correlation to some extent between the HOMO−LUMO
3635
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Organometallics
Article
optimizations have been done without any symmetry restriction by
using X-ray coordinates of the molecule. Frequencies of all complexes
have been computed at the same level of theory to confirm that all
optimized structures are at true minima, which means they have no
imaginary frequencies. The molecular orbital plots have been
generated using the Chemcraft program package (http://www.
chemcraftprog.com). Cartesian coordinates and associated energies
are given in the Supporting Information.
in comparison to that for the sulfur analogues. A similar
observation is made experimentally.
CONCLUSION
■
1-Benzyl-3-phenylchalcogenylmethyl-1,3-dihydrobenzoimida-
zole-2-chalcogenone ligands and their four half-sandwich
complexes with (η6-benzene)RuII have been synthesized and
characterized by NMR and HRMS. The complexes were
structurally characterized by single-crystal X-ray diffraction
studies. These unsymmetrical bidentate chalcogen ligands
having a unique combination of chalcogenoether and
chalcogenone donor sites are the first examples of such ligands
and have been explored for designing half-sandwich complexes
of Ru(II). The disposition of donor atoms around Ru is a
pseudo-octahedral “piano-stool” type. These half-sandwich
complexes of ruthenium(II) have been explored for the transfer
hydrogenation of aldehydes and ketones at a moderate
temperature of 80 °C. Of the different hydrogen sources (2-
propanol, glycerol, formic acid, citric acid, and ascorbic acid)
screened for the catalytic TH reaction, 2-propanol and glycerol
were found to be the best. With complexes 2 and 3, catalytic
TH with glycerol is slightly less efficient than that with 2-
propanol. Complex 3 shows the highest activity, followed by 1
and 4, which show almost equal activity; 2 shows the least
activity of all. The catalytic transfer hydrogenation with the
present complexes probably proceeds via formation of a
ruthenium hydride as an intermediate. DFT calculations
support the experimental results, both catalytic and structural.
Complex 3, which has the lowest energy gap between the
HOMO and LUMO, shows the best catalytic activity among
the four complexes.
Chemicals and Reagents. The reported methods were used for
the synthesis of [{(η6-C6H6)RuCl(μ-Cl)}2],46 chloromethyl phenyl
selenide,47 and 1-benzyl-1H-benzoimidazole.47b Benzimidazole, benzyl
bromide, ruthenium(III) trichloride hydrate, chloromethyl phenyl
sulfide, diphenyl diselenide, sodium borohydride, and tetrabutylam-
monium bromide (TBAB) were procured from Sigma-Aldrich (St.
Louis, MO, USA) and used as received. All solvents were dried and
distilled before use by standard procedures.48 The common reagents
and chemicals available commercially within India were used.
Synthesis of 3-Benzyl-1-((2-phenylselanyl/2-
phenylsulfanyl)methyl)-3H-benzoimidazolium Chloride (B/C).
A (0.417 g, 2.0 mmol) was placed in a Schlenk tube equipped with a
magnetic stirrer, and chloromethyl phenyl selenide (0.411 g, 2.0
mmol)/chloromethyl phenyl sulfide (0.318 g, 2.0 mmol) was added.
The mixture was heated for 8 h at 120 °C under an N2 atmosphere
and thereafter cooled to room temperature. The white solid that was
obtained was washed with dry CH3CN (2 × 40 mL) and dried in
vacuo.
1
Data for Compound B. Yield: 0.788 g, 95%. H NMR (CDCl3, 25
°C vs Me4Si): δ (ppm) 5.71 (s, 2H, H13), 6.20 (s, 2H, H5), 7.27−7.23
(m, 4H), 7.42−7.36 (m,6H), 7.65−7.63 (m, 2H), 7.95 (d, 3JH−H = 7.8
3
Hz, 1H), 8.08 (d, JH−H = 7.8 Hz, 1H), 9.85 (s, 1H, H12). 13C{1H}
NMR (CDCl3, 25 °C vs Me4Si): δ (ppm) 142.1 (C12), 134.8 (C3),
134.1 (C4), 131.4 (C6), 130.9 (C11), 130.1 (C2), 129.5 (C15),
129.3(C1), 129.2 (C17), 128.6 (C16), 127.5 (C7), 127.1 (C10), 126.8
(C14), 115.2 (C8), 114.6 (C9), 50.2 (C5), 42.4 (C13). 77Se{1H}
NMR (CD3CN, 25 °C, Me2Se; δ (ppm)): 404.7.
Data for Compound C. Yield: 0.682 g, 93%. 1H NMR (CDCl3, 25
°C vs Me4Si): δ (ppm) 5.73 (S, 2H, H13), 6.14 (S, 2H, H5), 7.37−
7.24 (m, 10H), 7.70−7.62 (m, 2H), 7.95 (d, 3JH−H = 8.4 Hz, 1H), 8.08
EXPERIMENTAL SECTION
■
1
Physical Measurements. The H, 13C{1H}, and 77Se{1H} NMR
3
(d, JH−H = 7.5 Hz, 1H), 9.91 (s, 1H, H of H12). 13C{1H} NMR
spectra have been recorded on a NMR spectrometer at 300.13, 75.47,
and 57.24 MHz, respectively. The C, H, and N analyses were carried
out with a C, H, and N analyzer. X-ray diffraction data on single
crystals were collected using Mo Kα (0.71073 Å) radiation at 298(2)
K. The software SADABS44a was used for absorption correction (if
needed) and SHELXTL for space group and structure determination
and refinements.44b,c 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. The least-squares
refinement cycles were performed until the model converged. High-
resolution mass spectral measurements were performed with electron
spray ionization (10 eV, 180 °C source temperature, sodium formate
as reference compound) with the sample being taken in CH3CN. The
commercial nitrogen gas was used after passing it successively through
traps containing solutions of alkaline anthraquinone, sodium
dithionite, alkaline pyrogallol, concentrated H2SO4, and KOH pellets.
A nitrogen atmosphere, if required, was created using Schlenk
techniques. Yields refer to isolated yields of compounds, which have
purity ≥95% (established by 1H NMR). All reactions were carried out
in glassware dried in an oven, under ambient conditions, except for the
syntheses of L1−L4, which were carried under a nitrogen atmosphere.
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-09
program.45 The geometries of complexes 1−4 were fully optimized by
using the M06 hybrid functional. This functional has been shown to
give more accurate results for organometallic complexes.43f For metal
and chalcogen atoms, the LANL2DZ43g basis set was used, and for C,
H, N, and Cl atoms the 6-31G* basis set was used. Natural bond
orbital (NBO) analysis of atomic charges was carried out for all
complexes 1−4 by using the M06 functional.43h All DFT calculations
were carried out in the gas phase and at 298.15 K. Geometry
(CDCl3, 25 °C vs Me4Si): δ (ppm) 142.3 (C12), 134.1 (C4), 132.8
(C3), 131.4 (C6), 130.8 (C11), 130.2 (C2), 129.5 (C15), 129.3 (C1),
129.2 (C17), 128.5 (C16), 127.5 (C7), 127.3 (C10), 115.1 (C8),
114.6 (C9), 51.1 (C5), 50.2 (C13).
Synthesis of L1−L4. White crystalline compound B (2.07 g, 5
mmol) or C (1.83 g, 5 mmol) was placed in a 250 mL round-bottom
flask fitted with a reflux condenser and treated with dry methanol (75
mL), sulfur (0.176 g, 5.5 mmol)/selenium powder (0.44 g, 5.5 mmol),
and anhydrous K2CO3 (1.38 g, 10 mmol). The reaction mixture was
heated to reflux for 24 h. After completion of the reaction, the solvent
was evaporated off under reduced pressure on a rotary evaporator.
Dichloromethane (20 mL) and water (20 mL) were added to the
residue left. The organic layer was separated and washed thoroughly
with water (20 mL). It was dried with anhydrous Na2SO4 and filtered.
The solvent was removed under reduced pressure to give ligands L1−
L4 as white crystalline solids.
Data for Ligand L1. Yield: 2.03 g, 89%. 1H NMR (CDCl3,25 °C vs
Me4Si): δ (ppm) 5.69 (s, 2H, C13), 5.91 (s, 2H, C5), 7.61−7.64 (m,
2H), 7.38−7.31 (m, 5H), 7.28−7.22 (m, 2H), 7.12−7.05 (m, 3H),
6.85−6.82 (m, 1H), 6.85−6.82 (m, 1H). 13C{1H} NMR (CDCl3, 25
°C vs Me4Si): δ (ppm) 167.7 (C12), 136.1 (C3), 135.1 (C4), 132.9
(C6), 132.3 (C11), 129.2 (C2), 128.8 (C15), 128.6 (C1), 123.2
(C17), 127.9 (C16), 127.5 (C7), 123.6 (C10), 123.2 (C14), 110.5
(C8), 110.2 (C9), 50.3 (C5), 43.7 (C13). 77Se{1H} NMR (CD3CN,
25 °C, Me2Se): δ (ppm) 402.6, 101.8. HR-MS (CH3CN):
[C21H18N2NaSe2]+ m/z 480.9696; calculated value for
[C21H18N2NaSe2]+ 480.9695 (δ 0.3 ppm).
Data for Ligand L2. Yield: 1.71 g, 84%. 1H NMR (CDCl3, 25 °C vs
Me4Si): δ (ppm) 5.55 (s, 2H, C13), 5.80 (s, 2H, C5), 7.63−7.60 (m,
2H), 7.36−7.22 (m, 8H), 7.13−7.02 (m, 3H), 6.79−6.77 (m, 1H).
13C{1H}NMR (CDCl3, 25 °C vs Me4Si): 169.8 (C12), 136.0 (C3),
3636
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Organometallics
Article
135.4 (C4), 131.8 (C6), 131.4 (C2), 131.0 (C11), 128.7 (C15), 128.5
(C1), 128.0 (C17), 127.8 (C16), 127.5 (C7), 123.2 (C10), 122.8
(C14), 109.9 (C8), 109.6 (C9), 48.3 (C5), 41.6 (C13). 77Se{1H}
NMR (CD3CN, 25 °C, Me2Se): δ (ppm) 396.21. HR-MS (CH3CN):
[C21H18N2NaSSe]+ m/z 433.0248; calculated value for
[C21H18N2NaSSe]+ 433.0244 (δ 0.9 ppm).
NMR (CD3CN, 25 °C, Me2Se): δ (ppm) −11.3. HR-MS (CH3CN):
[C27H24N2RuSSe]+ m/z 624.9558; calculated value for
[C27H24ClN2RuSSe]+ 624.9559 (δ −0.1 ppm).
Data for Complex 4. Yield: 0.132 g, 90% Anal. Calcd for
C28H26ClF6N2PRuS2: C, 45.68; H, 3.56; N, 3.81;. Found: C, 45.51; H,
1
3.87.; N, 6.64. Mp: 205.0 °C. H NMR (CDCl3, 25 °C vs Me4Si): δ
Data for Ligand L3. Yield: 1.68 g, 82%. 1H NMR (CDCl3, 25 °C vs
Me4Si): δ (ppm) 5.66 (s, 2H, C13), 5.90 (s, 2H, C5), 7.49−7.47 (m,
2H), 7.30−7.25 (m, 7H), 7.18−7.03 (m, 5H). 13C{1H} NMR (CDCl3,
25 °C vs Me4Si): δ (ppm) 168.1 (C12), 135.0 (C4), 134.2 (C3), 132.7
(C6), 132.1 (C11), 132.1 (C2), 129.0 (C15), 129.0 (C1), 128.5
(C17), 127.8 (C16), 127.3 (C7), 123.5 (C10), 123.2 (C14), 110.6
(C8), 110.1 (C9), 51.9 (C5), 50.1 (C13). 77Se{1H} NMR (CD3CN,
25 °C, Me2Se): δ (ppm) 92.5. HR-MS (CH3CN): [C21H18N2NaSSe]+
m/z 433.0248; calculated value for [C21H18N2NaSSe]+ 433.0238 (δ
2.5 ppm).
(ppm) 5.92 (s, 2H, C13), 5.97 (6H, Ru-Ar-H), 6.02 (S, 2H, C5),
7.27−7.16 (m, 8H), 7.38−7.30 (m, 3H), 7.50−7.44 (m, 3H). 13C{1H}
NMR (CDCl3,25 °C vs Me4Si): δ (ppm) 167.1 (C12), 135.6 (C3),
132.2 (C4), 132.0 (C11), 131.7 (C6), 128.8 (C2), 128.5 (C15), 128.3
(C1), 128.2 (C17), 127.3 (C16), 123.6 (C7), 123.2 (C10), 111.2
(C8), 110.5 (C9), 87.6 (Ru-Ar-C), 50.7 (C5), 48.9 (C13).
Catalytic Transfer Hydrogenation Reaction in 2-Propanol.
Ketone (1 mmol)/aldehyde (1 mmol), KOH (2 mL of a 0.2 M
solution in 2-propanol), and one of the complexes 1−4 (0.1 mol %)
were heated under reflux (80 °C) in 10 mL of 2-propanol for 3 h.
Thereafter 2-propanol was removed with a rotary evaporator and the
product that formed was extracted with diethyl ether. The solvent
from the extract was evaporated off, resulting in a residue which was
Data for Ligand L4. Yield: 1.45 g, 80%. 1H NMR (CDCl3, 25 °C vs
Me4Si): δ (ppm) 5.51 (s, 2H, C13), 5.76 (s, 2H, C5), 7.46−7.44 (d,
3JH−H = 7.2 Hz, 2H), 7.28−7.19 (m, 8H), 7.07−7.09 (m, 2H), 7.02−
6.99 (m, 2H). 13C{1H} NMR (CDCl3, 25 °C vs Me4Si): 170.5 (C12),
134.0 (C3), 132.8 (C4), 132.3 (C6), 131.8 (C11), 131.1 (C2), 129.0
(C15), 128.7 (C1), 128.4 (C17), 127.7 (C16), 127.4 (C7), 123.2
(C10), 122.8 (C14), 110.2 (C8), 109.5 (C9), 49.9 (C5), 48.3 (C13).
HR-MS (CH3CN): [C21H18N2NaS2]+ m/z 385.0804; calculated value
for [C21H18N2NaS2]+ 385.0793 (δ −2.8 ppm).
1
analyzed with H NMR spectroscopy.
Catalytic Transfer Hydrogenation Reaction in Glycerol.
Ketone (1 mmol)/aldehyde (1 mmol), KOH (0.4 mmol) as a solid,
and one of the complexes 1−4 (0.5 mol %) were heated under reflux
(80 °C) in 10 mL of glycerol for 3 h. Thereafter the product that
formed was extracted with diethyl ether. The solvent from the extract
1
Syntheses of [(η6-C6H6)Ru(L)Cl][PF6] (1−4). To a solution of L1
(0.091 g, 0.2 mmol)/L2 (0.082 g, 0.2 mmol)/L3 (0.082 g, 0.2 mmol)/
L4 (0.072 g, 0.2 mmol) made up in CH3OH (5 mL) was added a
solution of [{(η6-C6H6)RuCl(μ-Cl)}2] (0.050 g, 0.1 mmol) in
CH3OH (5 mL). 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 mL) with a rotary evaporator. It was mixed
with solid NH4PF6 (0.032 g, 0.2 mmol), and the orange microcrystal-
line solid resulting instantaneously was filtered, washed with 5 mL of
ice-cold CH3OH, and dried in vacuo. Single crystals of each of the four
complexes were obtained from a mixture (1/4) of CH3OH and
CH3CN.
was evaporated off, resulting in a residue which was analyzed with H
NMR spectroscopy.
ASSOCIATED CONTENT
* Supporting Information
■
S
Tables, figures, and CIF and xyz files giving crystal data and
refinement details, bond lengths and angles, secondary
interaction figures and distances, NMR and mass spectra,
crystallographic data for 1 (CCDC no. 1002221), 2 (CCDC
no. 1002222), 3 (CCDC no. 1002223), and 4 (CCDC no.
1002224), bond lengths, frontier orbitals, and Cartesian
coordinates for calculated structures. This material is available
free of charge via the Internet at http://pubs.acs.org.
Data for Complex 1. Yield: 0.139 g, 84%. Anal. Calcd for
C28H26ClF6N2PRuSe2: C, 40.52; H, 3.16; N, 4.27. Found: C, 44.86; H,
1
3.32; N, 6.57. Mp: 190.0 °C. H NMR (CDCl3, 25 °C vs Me4Si): δ
(ppm) 5.50 (s, 2H, C13), 5.72 (s, 6H, Ru-Ar-H), 5.83 (s, 2H, C5),
8.53 (s, 1H), 7.88−7.78 (m, 2H), 7.73−7.68 (m, 2H), 7.44−7.38 (m,
6H), 7.31−7.26 (m, 3H). 13C{1H} NMR (CDCl3, 25 °C vs Me4Si): δ
(ppm) 140.5 (C12), 135.6 (C3), 132.3 (C4), 131.5 (C11), 130.8
(C2), 129.9 (C15), 129.7 (C1), 129.3 (C17), 128.5 (C16), 127.5
(C7), 127.3 (C10), 125.9 (C14), 114.3 (C8), 113.9(C9), 84.5 (Ru-Ar-
C) 50.9 (C5), 42.4 (C13). 77Se{1H} NMR (CD3CN, 25 °C, Me2Se): δ
(ppm) 416.0. HR-MS (CH3CN): [C21H19N2Se]+ m/z 379.0709;
calculated value for [C21H19N2Se]+ 379.0715 (δ −1.60 ppm).
AUTHOR INFORMATION
Corresponding Author
*Corresponding Author A.K.S.: e-mail, aksingh@chemistry.
iitd.ac.in, ajai57@hotmail. com; fax, +91 11 26581102; tel, +91
11 26591379.
■
Notes
The authors declare no competing financial interest.
Data for Complex 2. Yield: 0.138 g, 88%. Anal. Calcd for
C28H26ClF6N2PRuSSe: C, 42.95; H, 3.35; N, 5.53. Found: C, 41.35; H,
ACKNOWLEDGMENTS
■
1
A.K.Singh thanks the Council of Scientific and Industrial
Research (CSIR) of India (Project No. 01(2421)10/EMR-II)
and the Department of Science and Technology of India
(Project No. SR/S1/IC-40/2010) for financial support.
A.K.Sharma and K.N.S. thank the CSIR of India for JRF and
SRF, respectively. H.J. thanks the University Grants Commis-
sion (UGC) for JRF/SRF.
3.94; N, 4.22. Mp: 180 °C. H NMR (CDCl3, 25 °C vs Me4Si): δ
(ppm) 5.72 (s, 2H, C13), 5.79 (6H, Ru-Ar-H), 5.87 (s, 2H, C5),
7.61−7.43 (m, 8H), 7.39−7.21 (m, 5H), 7.01−6.98 (m, 1H). 13C{1H}
NMR (CDCl3, 25 °C vs Me4Si): δ (ppm) 158.9 (C12), 134.6 (C3),
132.9 (C4), 131.1 (C11), 130.9 (C2), 129.6 (C15), 128.8 (C1), 128.4
(C17), 127.7 (C16), 126.1 (C7), 125.3 (C10), 124.9 (C14), 111.2
(C8), 109.7 (C9), 87.0 (Ru-Ar-C), 49.1 (C5), 44.3 (C13). 77Se{1H}
NMR (CD3CN, 25 °C, Me2Se): δ (ppm) 508.2. HR-MS (CH3CN):
[C21H19N2Se]+ m/z 379.0709; calculated value for [C21H19N2Se]+
379.0708 (δ 3.8 ppm).
REFERENCES
■
(1) (a) Larock, R. C. Organic Transformations; VCH: New York,
1989l p 411. (b) Kabalka, G. W.; Verma, R. S. In Comprehensive
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Data for Complex 3. Yield: 0.127 g, 81%. Anal. Calcd for
C28H26ClF6N2PRuSSe: C, 42.95; H, 3.35; N, 5.53. Found: C, 42.63; H,
1
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3H). 13C{1H} NMR (CDCl3, 25 °C vs Me4Si): δ (ppm) 152.7 (C12),
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