Highly efficient homogeneous and heterogenized ruthenium catalysts for transfer hydrogenation of carbonyl compounds

Article abstract of DOI:10.1039/c4ra04136d

[Ru(acac)₂(CH₃CN)₂] was synthesized from [Ru(acac)₃] by refluxing it with zinc powder in ethanol-acetonitrile mixture. The prepared catalyst was characterized by FT-IR, ¹H and ¹³C NMR techniques. The silica supported catalyst ('SiO ₂'-NH₂-Ru^II) was prepared by stirring [Ru(acac)₂(CH₃CN)₂] with 'SiO ₂'-NH₂ in a 1:1 mixture of CH₂Cl ₂-toluene for 3 days at room temperature. It was characterized by FT-IR, SEM, solid state NMR, ICP and BET surface area methods. The transfer hydrogenation reaction of carbonyl compounds was carried out separately using [Ru(acac)₂(CH₃CN)₂] and 'SiO ₂'-NH₂-Ru^II as catalysts. The reaction conditions were optimized with different solvents, bases, catalyst amounts and temperatures using acetophenone as a model system. The scope of the reaction was extended to various substituted aryl ketones including heteroaryl ketones. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4ra04136d

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Highly efficient homogeneous and heterogenized
ruthenium catalysts for transfer hydrogenation of
carbonyl compounds†
Cite this: RSC Adv., 2014, 4, 27955
a
b
a
S. Ganesamoorthy,ab P. Jerome, K. Shanmugasundaram and R. Karvembu
*
[Ru(acac)2(CH3CN)2] was synthesized from [Ru(acac)3] by refluxing it with zinc powder in ethanol–
1
acetonitrile mixture. The prepared catalyst was characterized by FT-IR, H and 13C NMR techniques. The
silica supported catalyst (‘SiO2’–NH2–RuII) was prepared by stirring [Ru(acac)2(CH3CN)2] with ‘SiO2’–NH2
in a 1 : 1 mixture of CH2Cl2–toluene for 3 days at room temperature. It was characterized by FT-IR, SEM,
solid state NMR, ICP and BET surface area methods. The transfer hydrogenation reaction of carbonyl
compounds was carried out separately using [Ru(acac)2(CH3CN)2] and ‘SiO2’–NH2–RuII as catalysts. The
reaction conditions were optimized with different solvents, bases, catalyst amounts and temperatures
using acetophenone as a model system. The scope of the reaction was extended to various substituted
aryl ketones including heteroaryl ketones.
Received 5th May 2014
Accepted 16th June 2014
DOI: 10.1039/c4ra04136d
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strongly coordinating ones or by other substrates. The exper-
imentally determined catalytic activity of the complexes
1. Introduction
correlates well with the ligand eld stabilization energy as a
function of the number of d electrons.22 The lower the stabi-
lization energy of the catalyst, the higher is its catalytic
activity. A number of highly active ruthenium(II) complexes
has been developed and used recently23,24 for transfer hydro-
genation of carbonyl compounds, but their practical applica-
tions in pharmaceutically important processes are very oen
hindered by their working cost as well as difficulties in
removing residual metals from the desired product.25 To avoid
such critical issues, the inherent approach is to heterogenize
the homogeneous catalyst on to the solid support. The cova-
lent attachment of catalyst on silica surface is the most
appropriate method to prepare stable and cost effective het-
erogenized catalyst. The peculiar properties of silica such as
ordered structures, very high surface areas, and uniform, large
and tunable pore diameters make it as a cost effective solid
support for graing the catalyst.
Transition metal-catalyzed transfer hydrogenation of
hydrogen acceptors by hydrogen donors has been recognized
as a unique pathway in catalysis since it can be executed with
experimental simplicity.1–4 The conventional reduction routes
oen require a high hydrogen pressure and cumbersome
reducing reagents.5,6 Transfer hydrogenation uses economi-
cally viable and environmentally benign hydrogen sources like
isopropanol and formates;7,8 more over its operational safety is
better than that of traditional hydrogenation. It is a comple-
mentary protocol to the classical Meerwein–Ponndorf–Verley
reduction/Oppenauer oxidation,9,10 where a stoichiometric
amount of aluminium iso-propoxide is used. Generally trans-
fer hydrogenation is catalyzed by transition metal complexes
containing N-heterocyclic carbenes (NHCs),11 arenes,12 phos-
phines,13–15 tridentate pyridine-based framework,16 tetra-
dentate chiral NNPP17–20 and pendant ether groups as ligands,
but the essential ligand feature for catalytic activity is not
denite. In recent years metal complexes stabilized by neutral,
non-aqueous ligands are being recognized as active catalysts.
Among them the most promising class of compounds are
those incorporating easily dissociable solvent ligands.21 The
coordination of labile ligands could be easily replaced by more
2. Experimental
2.1. Materials
All the reagents used were of chemically pure and analar grade.
Commercial grade solvents were distilled according to the
standard procedures and dried over molecular sieves before
use. Fumed silica (0.014 mm) and 3-aminopropyltriethoxysilane
were purchased from Sigma Aldrich and used without further
purication. [Ru(acac)3], [Ru(acac)2(CH3CN)2] and amine func-
tionalized silica (‘SiO2’–NH2) were prepared according to the
literature procedures.26–28
aDepartment of Chemistry, National Institute of Technology, Tiruchirappalli – 620
015, India. E-mail: kar@nitt.edu
bSynthetic Chemistry, Syngene International Ltd., Biocon Park, Bommasandra
Industrial Area, Bommasandra-Jigani Link Road, Bangalore – 560 099, India
† Electronic supplementary information (ESI) available: Characterization details
of catalysts and catalytic products, and gas chromatograms. See DOI:
10.1039/c4ra04136d
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2.2. Immobilization of [Ru(acac)2(CH3CN)2] on to amine
1-p-Tolylethanol. 1H NMR (400 MHz, CDCl3): d (ppm) 1.50 (d,
J ¼ 6.40 Hz, 3H), 2.33 (s, 3H), 4.89 (q, J ¼ 6.40 Hz, 1H), 7.18 (d, J
¼ 8.00 Hz, 2H), 7.28 (d, J ¼ 7.60 Hz, 2H).
functionalized silica (‘SiO2’–NH2–RuII)
[RuII(acac)2(CH3CN)2] (100 mg) and ‘SiO2’–NH2 (1 g) were stirred
at room temperature in a 1 : 1 mixture of dry dichloromethane
and toluene (20 mL) for 72 h. The mixture was ltered to obtain
solid which was washed several times with dichloromethane
and subjected to Soxhlet extraction using acetone, followed by
ethanol for about two days to ensure leaching of all unbound Ru
complex. The solid was dried at 100 ꢀC under vacuum for 6 h to
afford a brown color powder (1.03 g, 0.089 wt% Ru) (Scheme 1).
1-(4-Methoxyphenyl)ethanol. 1H NMR (400 MHz, CDCl3): d
(ppm) 1.46 (d, J ¼ 6.40 Hz, 3H), 3.79 (s, 3H), 4.82 (q, J ¼ 6.40 Hz,
1H), 6.88 (t, J ¼ 8.80 Hz, 2H), 7.29 (t, J ¼ 8.40 Hz, 2H). 13C NMR
(100 MHz, CDCl3): d (ppm) 25.03, 55.26, 55.45, 72.09, 113.70,
113.80, 113.84, 126.68, 127.61, 138.12, 163.52, 90.
1-(Thiophen-3-yl)ethanol. 1H NMR (400 MHz, CDCl3): d
(ppm) 1.60 (d, J ¼ 6.40 Hz, 3H), 5.12 (q, J ¼ 6.40 Hz, 1H), 6.96 (d,
J ¼ 8.00 Hz, 2H), 7.22–7.24 (m, 1H). 13C NMR (100 MHz, CDCl3):
d (ppm) 25.23, 66.18, 123.15, 124.37, 126.63, 149.90.
2,3-Dihydro-1H-inden-2-ol. 1H NMR (300 MHz, CDCl3): d
(ppm) 1.81 (bs, 1H), 2.92 (dd, J ¼ 3.18, 16.32 Hz, 2H), 3.22 (dd, J
¼ 5.85, 16.34 Hz, 2H), 4.67–4.73 (m, 1H), 7.18–7.19 (m, 2H),
7.25–7.28 (m, 2H). 13C NMR (100 MHz, CDCl3): d (ppm) 42.66,
73.19, 125.00, 126.67, 140.81.
2.3. Catalyst characterization
The morphology and Ru content of the catalyst were analyzed
using JEOL JSM-6701F eld emission scanning electron
microscope (FE-SEM). FT-IR spectra were obtained on a Perkin
Elmer Spectrum 100 FT-IR spectrophotometer as KBr pellet in
the frequency range of 4000–400 cmꢁ1. The BET surface area
was estimated using ASAP 2020 system. The Ru content in the
catalysts was measured by Perkin Elmer Optima 5300 DV
inductively coupled plasma optical emission spectrometer (ICP-
OES). 13C and 29Si CP-MAS solid state NMR spectra were
recorded using Bruker DSX-300 solid state NMR spectrometer.
1
Octan-2-ol. H NMR (400 MHz, CDCl3): d (ppm) 0.88 (t, J ¼
6.80 Hz, 3H), 1.18 (d, J ¼ 6.00 Hz, 3H), 1.30–1.45 (m, 10H), 3.74–
3.82 (m, 1H). 13C NMR (100 MHz, CDCl3): d (ppm) 14.04, 22.59,
23.43, 25.72, 29.30, 31.82, 39.36, 68.12.
Heptan-2-ol. 1H NMR (400 MHz, CDCl3): d (ppm) 0.90 (t, J ¼
6.80 Hz, 3H), 1.19 (d, J ¼ 6.40 Hz, 3H), 1.29–1.36 (m, 4H), 1.39–
1.47 (m, 4H), 3.76–3.85 (m, 1H). 13C NMR (100 MHz, CDCl3): d
(ppm) 14.02, 22.63, 23.46, 25.44, 31.84, 39.33, 68.18.
2.4. Transfer hydrogenation of carbonyl compounds
Carbonyl compound (1 equiv.), [Ru(acac)2(CH3CN)2] (2 mol%)
or ‘SiO2’–NH2–RuII (0.02 mol%) and NaOH (1 equiv.) were stir-
red in isopropanol (10 mL) at 80 ꢀC. Aer the requisite time, the
reaction mixture was cooled to room temperature. The product
was extracted with ethyl acetate, dried over anhydrous sodium
sulphate and analyzed by GC. The extract was concentrated and
chromatographed on a silica gel column with n-hexane–ethyl
acetate (4 : 1) as an eluting solvent to give the corresponding
alcohol.
2.5. Product analysis
Gas chromatograph is equipped with 5% diphenyl and 95%
dimethyl siloxane, Restek capillary column (60 m length, 0.32
mm dia) and a ame ionization detector (FID). The initial
column temperature was increased from 60 to 150 ꢀC at the rate
of 10 ꢀC minꢁ1 and then to 220 ꢀC at the rate of 40 ꢀC minꢁ1. N2
was used as a carrier gas. The temperatures of the injection port
In the case of heterogeneous catalysis, the catalyst was
separated by centrifugation aer completion of the reaction.
The product was extracted from centrifugate using ethyl acetate.
The extract was dried over anhydrous sodium sulphate,
concentrated and chromatographed on a silica gel column with
n-hexane–ethyl acetate (4 : 1) as the eluting solvent to afford the
desired alcohol. The recovered catalyst was washed thoroughly
with water, ethanol and toluene. It was dried under vacuum
before reuse.
Scheme 1 Preparation of ‘SiO2’–NH2–RuII.
27956 | RSC Adv., 2014, 4, 27955–27962
Fig. 1 29Si CP-MAS solid state NMR spectrum of ‘SiO2’–NH2–RuII.
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and FID were kept constant at 150 and 250 ꢀC, respectively at 2920 and 2977 cmꢁ1 are assigned to symmetric –CH and
during product analysis.
–CH3 stretching vibrations. The 1H NMR spectrum of the
complex showed a singlet around 1.7–1.8 ppm, which has been
attributed to the methyl protons present in the coordinated
acetylacetone ligands. In addition, the complex showed another
singlet at 5.2 ppm, which has been assigned to methine proton
3. Results and discussion
3.1. Catalyst characterization
3.1.1. Homogeneous catalyst [Ru(acac)2(CH3CN)2]. [Ru(a- of acetylacetonate. The resonance at 2.64 ppm is due to the
cac)2(CH3CN)2] was characterized by FT-IR and NMR spectral methyl protons of coordinated acetonitrile ligands. The 13C
techniques. It exhibited a band at 2257 cmꢁ1 in its FT-IR NMR spectrum of the complex showed expected resonances.
spectrum, which corresponds to the fundamental n2 –C^N The signals observed in the range 27.6–27.7 ppm have been
stretching frequency. The signals at 1510 and 1564 cmꢁ1 are assigned to the methyl carbons of acetylacetonate ligands. The
characteristic of coordinated acetylacetone ligands. Two bands resonances due to CO carbons appeared at 183.8 and 185.4
ppm. The methyl carbon of acetonitrile and acetylacetonate g-
CH showed signals at 4.3 and 99.4 ppm respectively.
3.1.2. Heterogenized catalyst (‘SiO2’–NH2–RuII). The newly
prepared heterogenized catalyst was characterized by different
analytical techniques. SEM images are shown in the ESI,† which
indicate the morphology change on silica surface during the
synthesis of heterogenized catalyst. ICP-OES conrms that the
Ru loading is approximately 0.089% per weight of the total silica
mass (8.82 ꢂ 10ꢁ6 mol gꢁ1 of Ru on silica). A BET surface area of
unfunctionalized silica was 225 m2 gꢁ1; whereas Ru anchored
silica showed a surface area of 207 m2 gꢁ1. The expected
decrease in the surface area results from the attachment of Ru
complex on to the surface of silica via silyloxy linkages. The 29Si
CP-MAS NMR spectrum showed two peaks that indicate the
presence of two different forms of silicon. The peaks at ꢁ110.19
and ꢁ66.06 ppm are characteristic of Q-type [Si(Oꢁ)4] and T-type
[R1–Si–(Oꢁ)3] silicates respectively. During the graing process
of 3-aminopropyl-1-triethoxysilane, out of three ethoxy groups,
only one or two underwent reaction in some cases and the
unreacted ethoxy group(s) might have contributed to the split in
the peak at ꢃꢁ66 ppm (Fig. 1).33 In the 13C CP-MAS NMR
Fig. 2 13C CP-MAS solid state NMR spectrum of ‘SiO2’–NH2–RuII.
spectrum, the intense signals at 21.94, 33.21 and 54.04 ppm
Table 1 Effect of solvent, base and catalyst amount on transfer hydrogenation of acetophenonea
Entry
Solvent/hydrogen donor
Catalyst (mol%)
Base (equiv.)
Temperature (ꢀC)
Time (h)
GC yield (%)
1
2
3
4
5
6
7
8
HCOOH
H2O
IPA–H2O (3 : 1)
IPA–H2O (3 : 1)
IPA–H2O (3 : 1)
IPA–H2O (3 : 1)
IPA
IPA
IPA
IPA
IPA
IPA
5
5
5
5
5
5
5
5
5
5
2
1
NEt3 (2)
90
90
80
80
80
80
80
80
80
80
80
80
10
10
10
5
5
5
15
10
10
10
10
10
55
45
67
37
38
13
70
45
94
64
98
74
HCOONa (2)
NaOH (2)
KOH (2)
KOtBu (2)
Na2CO3 (2)
KOH (2)
Na2CO3 (2)
NaOH (2)
NaOH (0.5)
NaOH (1)
NaOH (1)
9
10
11
12
a
Acetophenone (1 equiv.), base, [Ru(acac)2(CH3CN)2] or ‘SiO2’–NH2–RuII, solvent (10 mL), IPA ¼ isopropanol.
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were assigned to three carbons of propyl moiety, which acetylacetonate. The coordinated acetonitrile ligands did not
conrmed the attachment of amino propyl groups on to the show any signal due to shielding effect (Fig. 2).34
silica. A signal at 175.4 ppm indicates the CO resonance of
Table 2 Transfer hydrogenation of carbonyl compounds by [Ru(acac)2(CH3CN)2]a
Entry
1
Carbonyl compound
Product
Time (h)
Yieldb (%)
7
99
2
3
9
8
94
98c
4
5
6
7
6
4
4
6
99c
91c
87
94
8
9
99c
Carbonyl compound (1 equiv.), NaOH (1 equiv.), [Ru(acac)2(CH3CN)2] (2 mol%), isopropanol (10 mL), 80 ꢀC. b Yield is determined by GC with area
normalization. c Isolated yield.
a
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The satisfactory conversion was achieved within 10 h when
isopropanol was utilized as a hydrogen donor. It was found that
isopropanol–water mixture was not effective (Table 1, entries 3–
6), while 98% conversion was achieved using isopropanol alone
as solvent and also as hydrogen donor (Table 1, entry 11). Of
different bases studied, sodium hydroxide gave better conver-
sion (Table 1, entries 7–9). The transfer hydrogenation of
3.2. Ruthenium-catalyzed transfer hydrogenation of
carbonyl compounds
3.2.1. Optimization of reaction conditions. Initially
transfer hydrogenation reaction was optimized with
[Ru(acac)2(CH3CN)2] using acetophenone as a model substrate.
Very narrow conversion was observed with NEt3–HCOOH or
HCOONa as a hydrogen donor at 90 ꢀC (Table 1, entries 1 and 2).
Table 3 Transfer hydrogenation of carbonyl compounds by ‘SiO2’–NH2–RuIIa
Entry
1
Carbonyl compound
Product
Time (h)
24
Yieldb (%)
98c
2
3
18
16
99
99c
4
5
26
21
93
95
6
7
16
16
92c
88c
8
20
91c
a
Carbonyl compound (1 equiv.), NaOH (1 equiv.), ‘SiO2’–NH2–RuII (0.02 mol%), isopropanol (10 mL), 80 ꢀC. b Yield is determined by GC with area
normalization. c Isolated yield.
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acetophenone at 80 ꢀC with 2 mol% of catalyst, 1 equiv. of have shown greater than 87% yield. The substrates with elec-
sodium hydroxide and isopropanol afforded an excellent tron withdrawing group provided the desired alcohol in good
conversion of 98%. The transfer hydrogenation of various yield within 4 h (Table 2, entries 4–6). Acetophenones bearing
substrates was studied under optimized conditions.
an electron donating group required longer reaction time to
For heterogeneous catalytic system, except catalyst quantity reach high yield (Table 2, entries 2, 3 and 7). The electronic
all other variables are unchanged. The 0.02 mol% of heteroge- effect is obvious, that electron withdrawing groups favoured the
neous catalyst was identied as ideal quantity for productive transfer hydrogenation reaction. The reaction scales well,
conversion. Further reduction of catalyst amount led to affords a high isolated yield and utilizes relatively less amount
extended reaction time.
3.2.2. Extension of scope. With optimal reaction condi- maximum yield of 99% over a period of 9 h (Table 2, entry 8).
tions, transfer hydrogenation of ketones catalyzed by In recent times the heterogenized catalysts attracted much
of catalyst.35,36 Interestingly 1-(furan-3-yl)ethanone achieved a
[Ru(acac)2(CH3CN)2] (2 mol%) was studied in reuxing iso- attention due to several advantages such as ease of work up,
propanol (Table 2). Aer 4 to 9 h, most of the ketones tested isolation of the nal product without contamination, recovery
Table 4 A comparison between homogeneous and heterogenized ruthenium catalysts: transfer hydrogenation of carbonyl compounds by
[Ru(acac)2(CH3CN)2] (2 mol%)/‘SiO2’–NH2–RuII (0.02 mol%)a
Entry
1
Carbonyl compound
Product
Time (h) homo/het
16/26
Yieldb (%) homo/het
89/88c
2
3
7/15
99c/99
76/66c
10/24
4
5
12/24
12/14
87/88
78/82
6
10/21
97/97c
Carbonyl compound (1 equiv.), [Ru(acac)2(CH3CN)2] (2 mol%)/‘SiO2’–NH2–RuII (0.02 mol%), isopropanol (10 mL), 80 ꢀC. b Yield is determined by
a
c
GC with area normalization. Isolated yield.
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and reuse of expensive and toxic catalyst.29–32 The substrate was ltered and the reaction was allowed to proceed in the
scope of heterogenized catalyst (0.02 mol%) was extended to ltrate (Fig. 3). Even aer extended period, there was no further
various ketones. 1-(2-Naphthyl)ethanone was converted into the reaction occurred in the ltrate, whereas original reaction pro-
corresponding alcohol in 24 h with excellent isolated yield of ceeded to completion. Thus it is evident that no trace of Ru
98% (Table 3, entry 1). The substrates with electron with- metal leaches out from the supported catalyst.
drawing group (Table 3, entries 2 and 3) at ortho or para position
3.2.4. Reusability. Aer completion of the reaction, the
showed excellent yield of 99%. These results are in agreement heterogenized catalyst was ltered and slurry was washed with
with the earlier reports on ruthenium catalyzed transfer water and ethanol. The recovered catalyst was dried under
hydrogenation of ketones.8,37 1-Cyclohexylethanone exhibited vacuum at 80 C for 4 h. It was reused up to 6 times without
ꢀ
slow conversion (Table 3, entry 5). However an additional signicant loss of catalytic activity (Fig. 4). At 6th run the
amount of catalyst was not used to speed up the reaction. 2- conversion remained at 90%, indicating that ‘SiO2’–NH2–RuII
Indanone underwent an excellent transformation to give 92% of has an ability to consider as reusable catalyst. The Ru wt%
the desired product (Table 3, entry 6). Linear aliphatic ketones (0.087) of the recovered catalyst is comparable to that of fresh
were smoothly reduced to secondary alcohols aer extended catalyst. The FT-IR spectrum of the recovered catalyst showed
reaction time (Table 3, entries 7 and 8).
shi in the stretching frequency values compared to the fresh
To compare the catalytic performance of homogeneous catalyst, which indicates the replacement of coordinated
system with that of heterogenized system, a set of aromatic acetonitrile by water during the workup of transfer hydrogena-
ketones were subjected to transfer hydrogenation under iden- tion reaction (Fig. 5).
tical conditions (Table 4). In both the systems, alcohols were
obtained in excellent yield (>97%). Since the catalyst was het-
erogenized by exchanging one of the acetonitrile ligands by a
surface-bound basic donor ligand, extended reaction time is
inevitable. It was understood that the additional electron
density supplied by a base (ligand) reduces the catalytic activity
to certain extend.38 Heterogeneous transfer hydrogenation of 40-
aminoacetophenone gave a lower yield of 66% (Table 4, entry 3).
The denounced catalytic activity may be due to the coordination
of amine substrate to the Ru centre. Interestingly hetero-
aromatic ketones showed a better yield of 97% (Table 4, entry 6).
All the above results indicate that the efficiency of hetero-
genized system is comparable to that of parent homogeneous
catalytic system except the prolonged reaction hours. However,
very less loading of Ru should be noted, which made the het-
erogenized catalyst superior.
3.2.3. Heterogeneous nature of the catalyst. The hot
ltration test was performed to assure the durability of the
heterogenized catalyst. A portion of reaction mixture from the
reduction of acetophenone at approximately 50% conversion
Fig. 4 Reusability of ‘SiO2’–NH2–RuII for transfer hydrogenation of
acetophenone.
Fig. 3 Hot filtration test for transfer hydrogenation of acetophenone. Fig. 5 Comparison of IR spectra of (a) fresh and (b) recovered catalyst.
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13 S. Cabrera, O. G. Mancheno, R. G. Arrayas, I. Alonso,
P. Mauleon and J. C. Carretero, Pure Appl. Chem., 2006, 78,
257.
14 T. J. Colacot, Chem. Rev., 2003, 103, 3101.
15 C. Madrigal, A. Garcia-Fernandez, J. Gimeno and E. Lastra,
J. Organomet. Chem., 2008, 693, 2535.
16 J. I. VanderVlugt and J. N. H. Reek, Angew. Chem., Int. Ed.,
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17 A. A. Mikhailine, M. I. Maishan, A. J. Lough and R. H. Morris,
J. Am. Chem. Soc., 2012, 134, 12266.
4. Conclusion
We have prepared and characterized homogeneous and
heterogenized ruthenium catalysts consisting of acetonitrile as
labile ligand. Their catalytic activity towards transfer hydroge-
nation has been studied and compared. Both the catalysts
showed excellent activity. To the advantage, 0.02 mol% of het-
erogenized catalyst was sufficient for efficient conversion. This
simple protocol is safe and economically viable for transfer
hydrogenation of carbonyl compounds. Moreover the hetero-
genized catalyst was efficiently recycled and found to be a
reusable catalyst.
18 J. F. Sonnenberg, N. Coombs, P. A. Dube and R. H. Morris,
J. Am. Chem. Soc., 2012, 134, 5893.
19 P. O. Lagaditis, A. J. Lough and R. H. Morris, J. Am. Chem.
Soc., 2011, 133, 9662.
20 A. A. Mikhailine and R. H. Morris, Inorg. Chem., 2010, 49,
11039.
Acknowledgements
S.G. and K.S. thank Syngene International Ltd., Biocon Park,
Bangalore, India for encouragement. P.J. and R.K. thank
Department of Science and Technology, Government of India
for nancial support.
21 F. R. Silvana and E. K. Fritz, Chem. Rev., 2009, 109, 2061.
¨
¨
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