A mechanistic study of transfer hydrogenation catalyzed by cyclometallated ruthenium half-sandwich complexes

Article abstract of DOI:10.1016/j.jorganchem.2018.04.028

Transfer hydrogenation of aromatic ketones catalyzed by eight cyclometallated ruthenium half-sandwich complexes, including three new complexes, was examined. The catalytic process was studied using different ratios of substrate to base and base to catalyst and using a deuterated reductant. Optimum conditions for catalysis were shown to be in the presence of higher amounts of base in refluxing isopropanol. Under these conditions, the complexes were reduced in situ to give Ru(0) nanoparticles invisible to the naked eye. The nanoparticles were characterized by TEM, DLS and XPS. The catalytic transfer hydrogenation, under conditions in which nanoparticles were generated, was found to be far greater than the transfer hydrogenation by the molecular catalyst. Complete characterization of the three new complexes, including the X-ray crystallographic characterization of these complexes was carried out.

Full text of DOI:10.1016/j.jorganchem.2018.04.028

Accepted Manuscript
A mechanistic study of transfer hydrogenation catalyzed by cyclometallated ruthenium
half-sandwich complexes
Noor U. Din Reshi, Dineshchakravarthy Senthurpandi, Ashoka G. Samuelson
PII:
S0022-328X(18)30278-X
10.1016/j.jorganchem.2018.04.028
JOM 20420
DOI:
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 5 February 2018
Revised Date: 12 April 2018
Accepted Date: 20 April 2018
Please cite this article as: N.U. Din Reshi, D. Senthurpandi, A.G. Samuelson, A mechanistic study of
transfer hydrogenation catalyzed by cyclometallated ruthenium half-sandwich complexes, Journal of
Organometallic Chemistry (2018), doi: 10.1016/j.jorganchem.2018.04.028.
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ACCEPTED MANUSCRIPT
A Mechanistic Study of Transfer Hydrogenation Catalyzed by
Cyclometallated Ruthenium Half-sandwich Complexes
Noor U Din Reshi, Dineshchakravarthy Senthurpandi, Ashoka G. Samuelson*
Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore
560012, Karnataka, India
Abstract
Transfer hydrogenation of aromatic ketones catalysed by eight cyclometallated ruthenium
half-sandwich complexes, including three new complexes, was examined. The catalytic
process was studied using different ratios of substrate to base and base to catalyst and using a
deuterated reductant. Optimum conditions for catalysis were shown to be in the presence of
higher amounts of base in refluxing isopropanol. Under these conditions, the complexes were
reduced in situ to give Ru(0) nanoparticles invisible to the naked eye. The nanoparticles were
characterized by TEM, DLS and XPS. The catalytic transfer hydrogenation, under conditions
in which nanoparticles were generated, was found to be far greater than the transfer
hydrogenation by the molecular catalyst. Complete characterization of the three new
complexes, including the X-ray crystallographic characterization of these complexes was
carried out.
Keywords:
Cyclometallation
Ruthenium half-sandwich complexes
Transfer hydrogenation
Mechanistic Studies
Heterogeneous catalysts
Nanoparticles
* Corresponding author: Ashoka G. Samuelson. E-mail address: ashoka@iisc.ac.in
Address: Department of Inorganic and Physical Chemistry, Indian Institute of Science,
Bangalore 560012, Karnataka, India.
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1. Introduction
Transfer hydrogenation, an attractive alternative for direct hydrogenation, is a
commonly used organic transformation catalysed by many transition metal complexes [1].
The mechanism of catalytic transfer hydrogenation varies with the nature of transition metals,
ligands, donors, bases, reaction media, and substrates. The reaction pathways include a
simple direct hydrogen transfer, a monohydride pathway, a dihydride mechanism and the
more complex ligand assisted or bifunctional mechanism proposed by Noyori [2]. To add to
these possibilities, transition metal catalyzed transfer hydrogenation can be accompanied by
conversion of the transition metal complex to uncharacterized elusive species of an unknown
structure or nature. Complexes of late-transition-metals, which are often used as catalysts in
the transfer hydrogenation, are susceptible to reduction to form catalytically active
heterogeneous species like bulk metals and metal colloids under the reducing conditions
employed for transfer hydrogenation [3,4]. Not too long ago, Morris and co-workers pointed
out
that
even
chelated
iron
complexes
of
the
type
trans-
[Fe(NCMe)CO(PPh2C6H4CH=NCHR-))2][BF4]2; (R = H, Ph) are reduced to give
catalytically active Fe(0) nanoparticles under the transfer hydrogenation conditions [5].
Although the formation of heterogeneous catalysts during the use of homogeneous catalysts
has been known for a long time [4,6–14], the problem remains largely underappreciated
particularly in transfer hydrogenation. It is very difficult to detect the formation of
nanoclusters by simple methods. The mercury poisoning test for example can be misleading
for metals such as Ru, Ir, and Rh which do not form amalgams [4].
One way to prevent the formation of a heterogeneous species is to use more robust systems
which do not decompose easily. Lewis used cyclometallated Ru complexes for olefin
hydrogenation and found that these complexes resist the irreversible decomposition to metal
colloids [15]. Since then, cyclometallated complexes have been extensively used in catalysis
[16] particularly in hydrogenation [15,17–20] and transfer hydrogenation [21–34]. The true
catalyst is usually presumed to be a homogeneous metal-complex catalyst due to the
robustness of these complexes which is a result of the additional stability offered to the metal
carbon bond through chelation [35]. We report here our studies on the transfer hydrogenation
of aromatic ketones catalyzed by cyclometallated Ru half sandwich complexes. Based on a
few key observations, the formation of heterogeneous catalysts was suspected. Detailed
investigations confirmed that these presumably robust cyclometallated complexes can also be
reduced in situ to form Ru(0) nanoparticles which are more catalytically active in the transfer
hydrogenation of ketones than the precursor complex.
2. Results and discussion
2.1 Synthesis and characterisation
Ruthenium complexes (1-7) were made using the precursor (P), a dimer of
dichloro(p-cymene)ruthenium(II) and the ligands L1-L7 (Scheme 1). The metal complexes 1-
4 [36], were prepared by known methods using commercially available ligands. The ligands
L5, [37] L6, [38] and L7 [39] were synthesized following literature reports. The synthesis,
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characterization and molecular structures of complexes 5, 6 and 7 are described in the
experimental section. Complex 8, [Ru(II) (η6-p-cymene) (2-phenylpyridine-
-C,N)
(PPh3)](PF6) was prepared by NH4PF6 assisted substitution of the chloride in complex 1 with
PPh3 using a known procedure [40].
Scheme 1. Preparation of complexes [36] [40].
2.2. Catalytic Studies
Reactions using complexes 1-8 as catalysts were investigated in refluxing 2-propanol
with potassium hydroxide as a base. Optimized conditions to be used for these
cyclometallated complexes were arrived at by using complex 1 and acetophenone as the test
substrate. The data, (given in supplementary information), indicated that the rate of formation
of the product improved with the amount of base used. The reaction was very slow with 5
mole % of the base and there was a proportional increase in the rate of reaction until the base
used was 25 % of the substrate. Further increase in the amount of base did not have a
commensurate increase in product formation. Based on these studies, 25 mol% of the base
was used in all other reactions unless indicated otherwise. Under the conditions used
([S]/[C]/[B] = 100/1/25, in refluxing 2-propanol, acetophenone undergoes complete
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conversion to 1-phenyl ethanol in 8 h (Entry 1, Table 1) using complex 1 as catalyst. Similar
results were obtained with complex 5.
When the TH of acetophenone catalyzed by complex 1 was carried out using K2CO3
as base, the conversion of acetophenone to 1-phenylethanol decreased to 90% (Entry 11,
Table 1). However, using KOtBu as base in the TH of acetophenone catalyzed by complex 1,
there was complete conversion of acetophenone to 1-phenylethanol in 8 h (Entry 12, Table
1).
Catalyst loading had a significant effect on the reaction rate (Entry 10, Table 1). The
reaction was moisture and air sensitive as indicated by a sharp decrease in conversion when
the reaction of acetophenone was carried out, (a) using laboratory grade 2-propanol without
drying and distillation (Entry 13, Table 1) and (b) without maintaining a dry nitrogen
atmosphere (Entry 14, Table 1).
In the test reaction, complexes 1-6 showed similar activity to that of [RuCl2(p-
cymene)]2 which was the precursor to all complexes studied (Table 1). Only two complexes,
7 and 8 (Entries 7 and 8) showed poorer activity compared to complexes 1-6. Surprisingly,
the cationic complex 8 formed by replacement of Cl- by phosphine has reduced the activity
only marginally (Entry 8, Table 1). The reduced catalytic activity of 7 might be due to its
lower solubility and as a consequence, its slower conversion to the catalytically active
species.
The transfer hydrogenation reaction of acetophenone was carried out with almost
equal efficiency by all complexes, and so the reduction of substrates having different steric
and electronic properties was tested using the best three complexes, 1, 5 and 6 (Table 2).
Most of the substrates were reduced efficiently by these complexes. 4-nitroacetophenone and
4-amino acetophenone were difficult to reduce and less than 5 % conversion was obtained
under standard catalytic conditions used with all complexes. Interestingly increasing the
complex loading to 5 mol% resulted in the formation of three products,1-(4-
nitrophenyl)ethan-1-ol (carbonyl group reduced product A), 4-aminoacetophenone (nitro
group reduced product B) and 1-(4-nitrophenyl)ethan-1-ol (both nitro and carbonyl group
reduced C) in the TH of 4-nitroacetophenone with all complexes studied. (Entry 10, Table 2).
Table 1
Transfer hydrogenation of acetophenone [S] using complex 1 - 8 as catalysts [C].
Entrya
Catalyst
Conversionb
100 (91)
100 (89)
100 (89)
100 (90)
100 (92)
100 (89)
85 (80)
1
2
3
4
5
6
7
8
1
2
3
4
5
6
7
8
89(85)
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9
100 (90)
100 (91)
9
1
10c
11d
12e
13f
14g
15
90 (81)
100(92)
69 (57)
1
1
1
70 (60)
98 (90)
1
[RuCl2(p-cymene)]2
a Ketone 0.423 mmol, [S]/[C]/[B] = 100/1/25, reflux, IPA 10 mL. B is always KOH unless specified otherwise.
b conversion based on 1H NMR analysis, after 8 hours. The values in parenthesis show the isolated yield.
c [S]/[C]/[B] = 100/5/25, isolated yield and conversion after 4 hours.
d using K2CO3 as the base [S]/[C]/[K2CO3] = 100/1/25.
e using KOtBu as the base [S]/[C]/[KOtBu] = 100/1/25.
f using LR grade 2-propanol.
g reaction done under ambient conditions.
Table 2
Transfer hydrogenation of ketones [S] using complexes 1, 5 and 6 as catalysts [C].
Conversion (%)b
Entrya
1
Substrate
Catalyst 6
Catalyst 1
Catalyst 5
100 (81)
87 (80)
85 (74)
2c
3
100 (92)
70 (60)
87 (75)
84 (71)
90 (82)
75 (60)
80 (71)
82 (70)
95 (84)
74 (61)
75 (64)
80 (71)
4
5
6c
95 (84)
93 (80)
94 (83)
7
8
94 (80)
90 (80)
91 (69)
90 (79)
85 (77)
91 (80)
80 (68)
89 (70)
85 (70)
9c
10d, e
11f
49(42)
10
52(43)
15
53(40)
13
5

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12c
13
95 (89)
71(67)
60(51)
91 (82)
94 (86)
72(66)
65(52)
70(65)
62(49)
14d
a Ketone 0.423 mmoles, [S]/[C]/[B] = 100/1/25, reflux, IPA 10 mL.
b conversion based on 1H NMR analysis, after 12 hours. The values in parenthesis show the isolated yield after
12 hours.
c conversion and isolated yield (in the brackets) after 8 hours.
d [S]/[C]/[B] = 100/5/25.
e 4-aminoacetophenone (B) and 1-(4-nitrophenyl)ethan-1-ol (C) were also formed. After 12 h Catalyst 1, 21% B
and 10 % C; Catalyst 5, 18 % B and 14 % C; Catalyst 6, 25 % B and 13 % C.
f Product not isolated
2.3 Mechanistic studies
2.3.1 Control experiments
Due to the similarity in activity of the eight complexes we suspected that the
complexes might be pre-catalysts undergoing transformation to a common species
functioning as a catalyst. A recyclability test for the transfer hydrogenation reaction of
acetophenone using complex 1 was carried out. The results (given in supplementary
information) showed nearly complete loss of catalytic activity after the third cycle. This
suggested that the catalytically active species was undergoing decomposition during the
course of the reaction. In order to gain a better understanding of the decomposition of the
catalyst, an attempt was made to identify its fate under the reaction conditions. Hence a few
reactions were carried out with larger amounts of the complex.
Complex 1 was reacted with acetophenone (S) and KOH in the ratio [1]:[S]:[KOH] =
1:3:5 in refluxing 2-propanol (Entry 1, Table 3). The amounts of the different species present
relative to the initial concentration of the complex was estimated to be 43% (complex 1),
16% (free phenylpyridine), 14% (free cymene), 40% (other cymene containing Ru
complexes) after 2.5 hours; using ferrocene as the internal standard in the 1H NMR spectrum.
Under these conditions, 49% conversion of acetophenone to the product was observed. The
presence of a hydride could be detected in the upfield region (- δ = -8.21) but it could hardly
account for < 1 % of the cymene containing ruthenium complexes present in the reaction
mixture. Further decrease in the amount of the substrate was shown to have no significant
effect on the fate of the complex decomposition (Entries 2 and 3, Table 3). Hence, it was
inferred that complex decomposition was independent of the substrate concentration.
In order to gain further insight, a reaction between complex 1, acetophenone and base
in the ratio [1]:[S]:[KOH] = 1:5:5, was carried out in labelled 2-propanol ((CH3)2C(D)(OH),
1
(99% deuteration based on H NMR) as solvent at reflux temperature. The ratio of different
1
species detected in the H NMR spectrum of the reaction mixture after 5 hours of reflux, is
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1
given in Table 3 (Entry 4). The H NMR spectrum showed 96% deuterium incorporation at
the chiral carbon atom of the product (1-phenylethanol). Interestingly, the free ligand present
is 56% deuterated at the ortho carbon of the phenyl ring.
To study the effect of the increase in the mol% of the base on decomposition,
complex 1 was reacted with KOH ([1]: [KOH] = 1: 25) in the absence of the substrate. (Entry
5, Table 3). The 1H NMR of the reaction mixture did not contain peaks corresponding to the
intact complex, any other soluble metal complex or phenyl pyridine. The spectrum showed
the presence of free cymene which could account for only 1% of the amount of complex 1
used.
Table 3
Results of various control experiments.
Entry a
No. of
equivalents
(relative to
complex 1)
used
Amount of different species ( in terms of initial amount of 1) b
Other Ru-
cymene
complexes
Total
[Ru]
S
B
OH
1
2
3
1
0
5
0
5
5
5
5
25
43
42
54
20
0
40
33
28
8
83
75
82
28
0
16
22
14
15
16
12
1
49
83
-
3
15
40 (56% D)
0
4c
5
98 (96% D)
-
0
a
Complex 1 (0.048 mmol), IPA 10 mL, reflux, 2.5 h; amounts of species estimated using ferrocene as internal
standard. b Amount of product is relative to the acetophenone used. c ((CH3)2C(D)(OH), 99% deuterated, used as
solvent, 5 h.
These observations clearly showed that the complex decomposed under catalytically
relevant conditions. The decomposition increased with greater amount of the base and there is
a complete decomposition of the complex using [1]: [KOH] = 1: 25. Since the rate of the
transfer hydrogenation reaction is also enhanced at higher amounts of the base, it was
suspected that the decomposition products might be catalytically active. One plausible
pathway for the decomposition of these complexes is the reductive elimination of the
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intermediate Ru-hydride complex to release free ligand (phenyl pyridine) (Fig. 1). Ru hydride
intermediate can be formed by the β-hydride elimination of Ru-isopropoxide complex. The
formation of ortho-deuterated 2-phenylpyridne (56%) when deuterated 2-propnaol is used as
hydrogen donor supports this hypothesis. This will generate a ruthenium(0) species. This in
turn would lead to decomposition through loss of cymene. A similar decomposition and loss
of cymene from a ruthenium(II) complex has been observed by Baiquan Wang et al. [41].
Since nearly 44% of undeuterated 2-phenylpyridne is also formed, it is quite likely
that there is an alternative route to the formation of free phenyl pyridine. The protonolysis of
the intermediates by the solvent (or adventitious water) can also generate free non-deuterated
phenylpyridine. The formation of Ru(ll)-alkoxide (or Ru(ll)-hydroxide) complexes in the
1
reaction mixture could account for the unassigned cymene resonances in the H NMR
spectra.
Fig.1. A plausible pathway for the decomposition of the complex.
The complete disappearance of cymene signals especially at high concentrations of
KOH, the high catalytic activity under those conditions, and similar catalytic activity
exhibited by complexes 1-8 could be due to the formation of the same catalytically active
species due to the decomposition pathways described above. It was hypothesized that Ru(0)
generated by the decomposition of the complex could form nanopartcles (NPs) in absence of
any supporting ligands. These NPs could be the actual catalytic species.
2.3.2 Transmission electron microscopy (TEM) and dynamic light scattering (DLS)
Hence the reaction mixture was checked for the presence of NPs using transmission
electron microscopy (TEM) and dynamic light scattering (DLS).[4] An aliquot was
withdrawn from a reaction mixture obtained by refluxing complex 1 and KOH in 2-propanol
([C]/[B]) = 1/25 after 1h. The bright field and HR-TEM images showed the presence of
agglomerated NPs (Fig. 2). The agglomerates consisted of NPs of various morphologies. The
size of agglomerates was nearly 200 nm but the size of constituent NPs varied considerably
(3-10 nm). Energy dispersive X-ray spectrum (EDS) showed the presence of Ru, K, and Cl
(Fig. 3a). Selected-area electron diffraction (SAED) spectrum indicated the amorphous nature
of the material (Fig. 3b).
The DLS and Zeta potential measurements were carried out on this aliquot after 1h.
These measurements showed the presence of particles with an average size of 200 nm (Fig.
4). The particles detected by DLS are probably the ensembles formed by aggregation of NPs.
Fig. 2. (a and b) HRTEM images. (c - f) BF images of the reaction mixture. (g) area used for STEM imaging.
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(h) Ru mapped in the area marked in panel g.
Fig. 3. (a) EDS (b) SAED pattern of the reaction mixture.
Fig. 4. Size distribution of particles as measured by dynamic light scattering.
Zeta potential of the particles was estimated to be nearly 20 mV which indicates that these
particles can easily aggregate [42].
2.3.3 XPS study
The complex 1 was reacted with KOH ([1]: [KOH] = 1: 25) in refluxing 2-propanol.
The reaction mixture changed color from red to dark brown within a few minutes of reflux.
After 1 hour of reflux, heating was stopped. The reaction mixture was apparently
homogenous while refluxing. However, a black solid started precipitating out within 30
minutes of cooling when refluxing was stopped. X-ray photoelectron spectroscopy (XPS) of
this black solid was carried out.
Fig. 5. XPS of the solid.
The XPS spectrum of the black solid showed the presence of K, Cl, S, N, O, and Ru.
Ru was present in zero oxidation state (Fig. 5, Ru (3p) core level spectrum). After
establishing the presence of Ru(0) NPs, efforts were made to determine whether or not these
are actual catalysts. No visible precipitate could be detected by naked eye while the reaction
mixture was refluxed. When the reaction mixture was filtered hot, no precipitate was
obtained. These two observations suggest that catalysis by bulk metal can be ruled out,
leaving Ru(0) NPs as the most probable active species.
2.3.4 Mercury poisoning test
Mercury poisoning is the most commonly used test to determine whether the catalyst
is heterogeneous or homogenous. This experiment was carried out following the procedure
suggested by Finke [4]. There was no change observed in the rate of reaction by the addition
of Hg (Fig 6). However, this is not a sufficient proof that the catalyst is homogeneous. The
non-poisoning can be due to the fact that Ru does not form amalgam with Hg(0) [4].
Fig. 6. Conversion profile for mercury poisoning experiment and a standard catalytic run. Acetophenone 0.423
1
mmol, [S]: [1]: [B] = 100: 1: 25, IPA 10 mL; conversion based on H NMR analysis (conversions given are
mean of two values from two experiments, error is less than (± 2)); 1g of Hg added after 15 minutes in case of
reaction B.
2.3.5 Kinetic studies
The kinetics of the reduction of acetophenone was investigated using complex 1. The
catalytic activity was very slow for 15 minutes. This induction period was followed by
vigorous catalysis. Catalytic activity, slowed down as the reaction reached completion (Fig.
7, see also supplementary information). In many cases where nanoparticles are formed in situ,
the rate profile shows a sigmoidal curvature [3] When the complex was reacted with KOH
(1: KOH = 1: 25) in refluxing 2-propanol for 15 minutes before addition of acetophenone, no
induction period was observed. This suggested that the generation of active species from 1
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when heated with KOH in refluxing 2-propanol is the most likely reason for the induction
period.
Fig. 7. Conversion profiles for the TH of acetophenone during a standard catalytic run and a run where the
catalyst is pre-formed for 15 minutes before acetophenone is added. Acetophenone 0.423 mmol, [S]: [1]: [B] =
100: 1: 25, IPA 10 mL; conversion based on 1H NMR analysis (conversions given are the mean values from two
experiments).
2.3.6 Poisoning experiments
Poisoning experiments were attempted with 1, 10-phenanthroline, or CS2 using 1 as
precatalyst [5,43]. The poison was added to the standard reaction mixture containing 1, KOH,
acetophenone and 2-propanol (1: KOH: acetophenone = 1: 25:100) after 15 minutes i.e. after
the reaction had started. The data is shown in graphical form in Fig. 8 (see also
supplementary information). When 0.5 equivalents of 1, 10-phenanthroline (relative to 1) was
added, the catalytic activity was almost completely stopped and only 15 % conversion of
acetophenone to 1-phenylethanol was observed in 8 h. Similarly, the addition of 0.5
equivalents of CS2 (relative to 1) decreased the conversion to 20% in 8 h. The drastic
decrease in the catalytic activity by the addition of sub-stoichiometric amounts of poisons
clearly suggests that NPs are active catalysts [3].
Fig. 8. Conversion profile during the TH of acetophenone in the presence of a poison, 0.5 equivalents of poison
with respect to the catalyst were added after 15 minutes of reflux.
The poisoning experiments using CS2 were also carried out with the reaction mixtures where
KOtBu and K2CO3 were used as bases in the TH of acetophenone catalyzed by complex 1.
The data given in the supplementary information suggests that the active form of the catalyst
are also nanoparticlulate when these bases are used. The CS2 poisoning experiment was also
carried using [RuCl2(p-cymene)]2 as the catalyst in the TH of acetophenone (data given in the
supplementary information). In this case also the results suggest that NPs are the active
catalysts.
2.3.7 Stabilization of Ru(0) NPs
Plausible stabilizing agents for Ru (0) NPs in the present study are phenylpyridine, 2-
1
propanol and p-cymene [44]. The H NMR of the reaction mixture obtained by refluxing
complex 1 and KOH (1: KOH = 1: 25) did not contain peaks corresponding to the free
phenylpyridine. Moreover, only 1% of p-cymene (in terms of amount of 1 used) was
1
observed in the H NMR spectrum. These observations suggest that a significant amount of
2-phenylpyridine and p-cymene are bound to Ru NPs. However, a 13C CP-MAS NMR
spectrum of the solid obtained by refluxing the complex 1 and KOH ([1]: [KOH] = 1: 25) in
2-propanol for 1 hour, followed by evacuation to remove residual organic species showed
peaks in the aromatic and aliphatic regions (Fig. 9). The peaks observed in the aromatic
region (187.33-120.02 ppm) can be attributed to 2-phenylpyridine and p-cymene bound to
Ru(0) NPs. The peaks in the aliphatic region (63.93-10.44) could be assigned to nanoparticle
bound 2-propanol and the aliphatic C atoms of p-cymene. The IR spectrum of the solid
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showed peaks corresponding to ν(C=N stretching) at 1563 cm-1 and at 1368 cm-1 (νC=C stretching
)
[45].
Fig. 9. 13C CP-MAS NMR spectrum of the solid.
3. Concluding Remarks
The excellent catalytic activity of the cyclometallated complexes [1-8] studied in the
presence of base could be attributed to the nanoparticles formed in situ by decomposition of
the molecular catalysts. The extent of decomposition increased with the increase in the mol%
of base and there was almost complete decomposition with 25 mol% of base. TEM and DLS
measurements confirmed the presence of NPs. The product formation profile with a definite
induction time, a rapid change in the color of the reaction mixture and poisoning experiments
strongly point to NPs as the actual catalysts. Although the mercury drop experiment suggests
homogeneous catalysis, all other observations point to the formation of NPs and their being
true catalysts. This also helps one understand why the catalytic activity is essentially ligand
independent. Furthermore, the recyclability test using one of the complexes indicated a
significant decrease in the activity. This may be due to the agglomeration of nanoparticles
over time. The reduction of the nitroaromatic ketone to aminoaromatic ketone and alcohol is
also suggestive of heterogeneous catalysis. The hydrogenation of aromatic nitro group has
been used in past as a test to distinguish between homogeneous and heterogeneous catalysts
[4]. The catalytic activity observed is similar to that reported for Ru(0) NPs [46]. In the light
of above discussion, it is proposed that the dominant active catalytic species in the present
case are Ru(0) NPs.
Stoichiometric reactions of the catalyst with smaller quantities of the base give insight
into the decomposition pathways that lead to Ru(0) species. Reactions with labelled isopropyl
alcohol clearly point to the formation of a ruthenium deuteride which reductively eliminates
the deuterated phenyl pyridine. Under these conditions when the quantity of base is less,
molecular catalysis by the half sandwich complexes cannot be ruled out. The leaching from
Ru(0) NPs can also release active homogeneous catalytic species transiently into the reaction
mixture [47].
Reductive elimination has been extensively studied as a major decomposition
pathway for metal-NHC complexes[48–55]. The current report suggests that this pathway is
also accessible to cyclometallated complexes. We believe that this decomposition of the
complex might be a general phenomenon in other half-sandwich complexes particularly
under reducing conditions and in presence of large amounts of base. A systematic study of
the fate of related complexes might reveal similar decomposition pathways under reaction
conditions.
4. Experimental
4.1. General Information
All reactions and manipulations were carried out under nitrogen atmosphere by using
standard Schlenk techniques in oven-dried glassware. NMR spectra were recorded on a
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1
Bruker AMX 400 spectrometer that operates at 400 MHz for H and 100 MHz for 13C.
Chemical shifts for 1H NMR spectra are reported as δ in ppm relative to the residual solvent
peak. Chemicals were obtained from commercial sources and used without further
purification. Silica gel GF was used for preparative scale TLC. The solvents were purified by
standard methods under dry nitrogen before use. Milli-Q water was used and de-gassed
before use. The reactions were monitored by thin-layer chromatography (TLC) with Merck
precoated aluminum plates. Electrospray ionization mass spectrometry (ESI-MS)
experiments were done on Agilent 6538 Ultra High Definition (UHD) Accurate-Mass Q-TOF
using standard spectroscopic grade solvents. Elemental analysis was carried out on a vario
MICRO CUBE CHNS instrument. Bright field (BF) imaging, high resolution (HR) TEM
imaging, and selected area diffraction patterns were obtained using JEOL 2100F TEM.
Energy dispersive X-ray spectroscopy (EDS) was performed using OXFORD EDS
spectrometer attached to the TEM. The operating voltage of the field emission gun (EFG) is
200 kV. FT-IR spectra were recorded using Bruker ALPHA FT-IR spectrometer. X-ray
photoelectron spectroscopy was carried out using a Kratos AXIS Ultra XPS spectrometer.
The DLS and Zeta potential measurements were carried out using Zeta Phase Analysis Light
Scattering (Zeta PALS).
4.2. Synthesis
The ligands L5, L10, L11, [37] L6, [38] L7 [39] and L12 [56] were synthesized
following literature reports from readily available starting materials. The metal complexes 1-
4 [36], 8 [40] and 9 [57] were prepared by known methods using commercially available
ligands, 2-phenylpyridine, benzo[h]quinoline, 1-phenylpyrazole, 2-phenyl-2-oxazoline, and
2, 2-bipyridine [40,57,58]. The (CH3)2C(D)(OH) was prepared by the reduction of acetone by
LiAlD4 following a literature report [59].
4.2.1. Chloro(η6-p-cymene)( 2-phenylbenzo[d]thiazole- -C,N)ruthenium(II), 5
To dichloro(p-cymene)ruthenium(II) dimer (0.16 mmol) taken in the round bottom
flask, methanol (10 mL) was added. To this solution was added, 2-phenyl benzothiazole (L5)
(0.33 mmol) followed by potassium acetate (6.4 mmol). The reaction mixture was refluxed
for 50 hours and then the solvent was removed by evacuation leaving behind a black residue
which was dissolved in dichloromethane and the mixture was filtered. The filtrate was then
added to a large volume of diethyl ether to precipitate the product, which was filtered and
1
washed with diethyl ether and dried ( 60 mg, 40%. yield H NMR (CDCl3, 400 MHz): δ =
0.755 (3H, d, J = 6.8Hz, isopropyl), 0.895 (3H, d, J = 6.8Hz, isopropyl), 2.12 (3H, s, methyl),
2.24 (1H, m, isopropyl) , 5.13 (1H, d, J = 5.2Hz,cymene), 5.37(1H, d, J = 5.6 Hz cymene),
5.72(1H, d, J=5.2 Hz, cymene),5.9 (1H,d,J=6.0Hz, cymene) 7.09 (1H, m, ArH) , 7.22 (1H, m,
ArH), 7.44 (1H, m, ArH), 7.68 (2H, m, ArH), 7.85 (1H, d, J = 7.6 Hz, ArH), 8.25 (1H, d, J =
13
7.6 Hz, ArH) , 8.37(1H, d, J = 8.0 Hz, ArH). C NMR (CDCl3, 100MHz): δ=19.4, 21.14,
23.14, 31.37, 80.91, 82.68, 90.47, 91.7, 99.6, 102.9, 121.6, 123.3, 123.4, 125.8, 126.2, 127.6,
130.6, 132.5, 139.5, 140.3, 151.5, 175.0, 183.9. Elemental analysis for C23H22ClNRuS, calcd.
(%) C, 57.43; H, 4.61; N, 2.91; S, 6.67. Found: C, 57.14; H, 4.25; N, 2.85; S, 6.34.
12

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4.2.2. Chloro(η6-p-cymene)(1-methyl-2-phenyl-1H-benzo[d]imidazole- C,N)ruthenium(II), 6
Dichloromethane (10 mL) was added to dichloro(pcymene)ruthenium(II) dimer (0.16
mmol) in a round bottom flask. To this solution were added, L6 (0.33 mmol) followed by
potassium acetate (0.38 mmol). The reaction mixture was stirred at room temperature for 20
hours. Then the solvent was evaporated and the residue left behind was dissolved in
dichloromethane and the mixture was filtered. The filtrate was then added to large volume of
diethyl ether resulting in a precipitate which was filtered and washed with diethyl ether and
1
dried. (90 mg, 60 % yield) H NMR (CDCl3, 400 MHz): δ = 0.80 (3H, d, J = 6.8Hz,
isopropyl), 0.94 (3H, d, J = 6.8Hz, isopropyl), 2.08 (3H, s, methyl), 2.31 (1H, m, isopropyl),
4.10 (3H, s, N-methyl), 5.14 (1H, d, J = 5.9 Hz, cymene), 5.36(1H, d, J = 5.9 Hz cymene),
5.70(1H, d, J=5.9 Hz, cymene),5.84 (1H,d,J=5.9 Hz, cymene) 7.07 (1H, m, ArH) , 7.21 (1H,
m, ArH), 7.40 (3H, m, ArH), 7.80 (1H, m, ArH), 7.9 (1H, d, J = 7.6 Hz, ArH), 8.33 (1H, d, J
= 7.6 Hz, ArH). 13C NMR (CDCl3, 100MHz): δ=19.42, 22.36, 23.08, 31.35, 32.41, 81.77,
82.17, 89.26, 89.77, 99.43, 101.55, 110.27, 117.49, 122.89, 123.48, 123.61, 124.80, 129.36,
134.62, 136.74, 140.99, 141.84, 158.86, 183.66. Elemental analysis for C24H25Cl1N2Ru,
calcd. (%) C, 60.31; H, 5.27; N, 5.86. Found: C, 60.16; H, 5.14; N, 5.90.
4.2.3. Iodo(η6-p-cymene)( 1-methyl-[2,2'-bipyridin]-1-ium iodide- -C,N)ruthenium(II), 7
To dichloro(p-cymene)ruthenium(II) dimer (0.16 mmol) taken in the round bottom
flask, degassed water (10 mL) was added under nitrogen atmosphere. To this mixture was
added (L7) (0.33 mmol) followed by potassium acetate (6.4 mmol). The reaction mixture was
refluxed for 50 hours and then extracted with DCM (10 x 3 mL). The organic fraction was
layered with diethyl ether to get the red crystals of 7 (64 mg. 30% yield). The yield improved
1
to 70% when 2 eq. of NaI was used as the additive. H NMR (CDCl3, 400 MHz): δ = 1.15
(3H, d, J = 6.8Hz, isopropyl), 1.21 (3H, d, J = 6.8Hz, isopropyl), 2.13 (3H, s, methyl), 2.89
(1H, m, isopropyl) , 4.85 (3H, s, N-CH3) 5.41 (1H, d, J = 6.48 Hz,cymene), 5.46 (1H, d, J =
6.4 Hz cymene), 5.61 (1H, d, J = 6.12 Hz, cymene), 5.77 (1H ,d,J = 6.15 Hz, cymene) 7.38
(1H, m, ArH) , 7.51 (1H, m, ArH), 8.0 (1H, m, ArH), 8.42 (1H, d, J = 6), 8.49 (1H, d, J = 8.6
Hz, ArH), 8.94 (1H, d, J = 7.66 Hz, ArH) , 9.44(1H, d, J = 6 Hz, ArH). Elemental analysis for
C21H24I2N2Ru, calcd. (%) C, 38.26; H, 3.67; N, 4.25; Found: C, 38.20; H, 3.59; N, 4.15.
4.3. General procedure for the catalytic transfer hydrogenation reaction
In a typical procedure, the complex (4.23 x 10-3 mmol) was dissolved in 10 mL of 2-
propanol in a 10 mL R.B flask and to this solution was added 0.423 mmol of the ketone. This
was followed by the addition of KOH (0.105 mmol.) as a solution in IPA. The resulting
solution was refluxed using an oil bath controlled by a thermostat. Small portions of the
1
reaction mixture were withdrawn at specified intervals, and H NMR spectra recorded in
CDCl3 after removal of the solvent. The conversion was calculated from the intensity ratios
of the peaks arising from the reactant and product. After the completion of reaction, the
product was separated by preparative TLC and the isolated yield was calculated. Each
experiment was done two times to check for reproducibility.
4.4. Stoichiometric reactions
13

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Three related experiments were carried out. In the first case 0.048 mmoles of the
complex was refluxed for 2.5 h with 0.24 mmoles of KOH in 10 mL of 2-propanol. In the
second experiment 0.048 mmoles of the complex was refluxed with 0.24 mmoles of KOH
and 0.048 mmoles of acetophenone for 2.5 h in 10 mL of 2-propanol. In the third experiment
0.048 mmoles of complex was refluxed with 0.24 mmoles of KOH and 0.144 mmoles of
acetophenone for 2.5 h in 10 mL of 2-propanol. In all these an aliquot was withdrawn from
the reaction mixture, removed the solvent and added CDCl3 and known amount of ferrocene.
The amounts of different species present were estimated by comparing the integrations of
peaks at 4.15 (ferrocene), 9.21 (intact complex), 8.65(free ligand), 1.5 (phenylethanol), and
2.6 (acetophenone).
4.5. Deuterated 2-propanol reaction
To a solution of complex 1 in 2-propanol (0.048 mmoles) in 5 mL) in a small Schlenk
RB, was added 0.24 mmoles of acetophenone, followed by 0.24 mmoles of KOH. The
solution was refluxed for 5 hours. An aliquot was taken from the reaction mixture. The
solvent was removed from the aliquot and then the NMR was recorded. The percentage
deuterium incorporation in the product was calculated from the integration ratios of triplet at
1.5 ppm (due to merging of CH3 of deuterated phenylethanol and the doublet from CH3 of
undeuterated phenylethanol) and the quartet at 4.8 ppm (due to C-H of phenylethanol). The
percentage deuterium incorporation in the phenylpyridine was calculated from the integration
ratios of doublet at 8.64 ppm (due to 1H on C ortho to N) and the doublet at 7.92 ppm (due to
H on ortho position of phenyl ring).
4.6. Transmission electron microscopy
Aliquots were withdrawn at different time points (30 minutes, 1 h, 2h, and 4h, 8h)
from the reaction mixture (standard catalytic conditions). These aliquots were deposited on
TEM grids as soon as these reached the room temperature. Then TEM, SAED, EDX, and
STEM were carried out. The solid that deposits overtime, when refluxing is stopped, was also
analyzed by TEM. Except for more agglomeration, the results were essentially the same as
those obtained with the reaction mixture.
4.7. Crystal Structure determination
Single crystals of suitable size have been picked under the polarizing microscope and
mounted on a glass fiber using glue for single crystal diffraction studies. Intensity data for
all the samples were collected with Bruker SMART APEX-II diffractometer (3-axis system)
with graphite monochromated (sealed) MoKα radiation (λ= 0.71073 Å). Intensities were
corrected for Lorentz-polarization effects and absorption corrections were performed
through the program SADABS [60]. The structures have been solved through direct
methods using the program SHELXS-97 [61] and refined by full-matrix least squares
refinement on F2 using the program SHELXL [62]. All non-hydrogen atoms were refined
anisotropically as independent atoms and hydrogen atoms were constrained on
14

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geometrically calculated positions and refined using a riding model using HFIX command.
Moreover, they have been treated isotropically throughout the refinement cycles and their
thermal parameter assumes the value of 1.20 times the Ueq value (1.50 for CH3 group) of the
non-hydrogen atom to which it binds. All computations have been performed using the
WinGX package and structures have been drawn by ORTEP-3 for windows program [63].
The molecular structures of complexes 5, 6, and 7 as ORTEP representations are shown in
Fig. 10. Important bond lengths and bond angles are given in Table 4. Selected
crystallographic data and the refinement parameters for the crystals are listed in the Table 5.
CCDC-1424388 (for 5), CCDC-1443903 (for 6) and CCDC-1545791 (for 7) contain
supplementary crystallographic data for the structures reported in this paper. The molecular
structure of 2 was reported in the orthorhombic space group Pcab, CCDC 1039990 by the
Dixneuf group [36]. We obtained a different polymorph of 2 in P21/n space group. CCDC-
1424387 contains the supplementary information for that.
Fig. 10. Molecular structures of 5, 6 and 7 with 50% displacement ellipsoids, all H atoms are omitted for clarity.
Table 4
Selected bond lengths and angles for complexes 5, 6 and 7.
Complex
Bond lengths in Å
Bond angles in (°)
Ru-Cl(1)
2.4175
Ru-N(1)
2.1093
2.062(8)
2.059(6)
Ru-C(11)
2.0876
2.072(10)
2.030(7)
C(11)-Ru-N(1)
77.28
77.0(4)
C(11)-Ru-Cl(1)
86.40
Cl(1)-Ru-N(1)
85.27
85.6(2)
5
6
7
2.429(3)
88.1(3)
2.7132(8) a
76.8(3)
86.65(19) a
87.91(17)
15

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Table 5
Crystallographic data for the complexes 3, 5, 6 and 7.
Crystal
Complex 2
Complex 5
Complex 6
Complex 7
identification
CCDC No.
1424387
C23 H22 Cl1 N1 Ru1
448.93
1424388
C23 H22 Cl1 N1 Ru1 S1
480.99
1443903
C25 H26 Cl4 N2 Ru1
597.35
1545791
C21 H24 I2 N2 Ru1
659.29
Empirical Formula
Molecular Weight
Crystal System
Space Group
Monoclinic
P21/n
Orthorhombic
P21 21 21
Triclinic
P-1
Monoclinic
P21/c
a = 7.956(2) Å
Unit Cell Parameters b = 16.175(4) Å
c = 30.179(8) Å
a = 7.787(2) Å
b = 10.608(3) Å
c = 24.555(6) Å
a = 7.548(5) Å,
α= 81.630(5)°
b = 11.606(5) Å,
β = 87.659(5)°
c = 14.356(5) Å,
γ = 80.352(5)°
1226.4(1)
a = 15.050(2) Å
b = 9.4733(14) Å
β = 100.767(4)°
c = 15.499(2) Å
β= 96.759(7)°
Volume(Å3)
3856.6(2)
2028.4(9)
2170.9(6)
Z
8
4
2
4
Calculated
Density(mg m-3)
F(000)
1.546
1.575
1.618
2.017
1824
976
604
1256
Crystal Size(mm3)
Crystal Habit/Color
Theta range for data
collection
0.302 x 0.192 x 0.067 0.105 x 0.092 x 0.052
0.362 x 0.245 x 0.153
0.351 x 0.254 x 0.156
Needle/Green
3.018 to 30.529°
Block/Green
1.659 to 27.563°
Needle/Green
3.069 to 29.904°.
Block/Red
3.231 to 27.592°.
Index ranges
-11 <= h <=10,
-22 <= k <=20,
-41 <= l <=42
104298/10860
-10 <= h <=10,
-11 <= k <=13,
-31 <= l <=29
31911/4624
-9<=h<=9,
-15<=k<=15,
-19<=l<=18
35340/6111
-19<=h<=19,
-12<=k<=12,
-19<=l<=20
40231/5005
Reflections
collected/Unique
Completeness to
theta = 25.242°
Refinement method
99.8 %
99.5 %
99.5
99.8 %
Full-matrix least-
squares on F2
Full-matrix least-
squares on F2
4624 / 0 / 247
Full-matrix least-
squares on F2
6111 / 0 / 293
Full-matrix least-
squares on F2
5005 / 0 / 239
Data / restraints /
parameters
10860 / 0 / 475
1.011
Goodness-of-fit on
1.110
1.100
1.029
F2
Final R indices
[I>2sigma(I)]
R1 = 0.0400, wR2 =
0.0919
R1 = 0.0880, wR2 =
0.2512
R1 = 0.0612, wR2 =
0.0861
R1 = 0.0466, wR2 =
0.0651
R1 = 0.1227, wR2 =
0.0775
R indices (all data)
R1 = 0.0486, wR2 =
0.1078
R1 = 0.1390, wR2 =
0.2722
R1 = 0.1553, wR2 =
0.1055
Largest diff. peak
and hole
0.460 and -0.718
2.208 and -1.009
2.931 and -1.441
0.995 and -0.890
( e.Å-3)
16

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Acknowledgements
N.U.R thanks Council of Scientific and Industrial Research (CSIR), New Delhi for senior
research fellowship. Authors acknowledge Department of Science and Technology (DST),
New Delhi for a research grant; NRC, Indian Institute of Science (IISc) for CP-MAS spectra
and IISc for funding the procurement of a 200kV FETEM
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Fig. 2. (a and b) HRTEM images. (c - f) BF images of the reaction mixture. (g) area used for
STEM imaging. (h) Ru mapped in the area marked in panel g.
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Fig. 3. (a) EDS (b) SAED pattern of the reaction mixture.
5
6
7
Fig. 10. Molecular structure of 5, 6 and 7 with 50% displacement ellipsoids, all H atoms are
omitted for clarity.
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