Catalytic applications of magnetic nanoparticles functionalized using iridium N-heterocyclic carbene complexes

Article abstract of DOI:10.1039/c5nj00803d

A synthetic modular methodology allows the preparation of catalytic materials based on magnetic nanoparticles with iridium N-heterocyclic carbene (NHC) complexes. Imidazolium salts containing a ketone/aldehyde as a pendant functional group are the key species prepared. The condensation reaction of the Cp?IrNHC-CHO compound with magnetic nanoparticles containing amine groups on the surface yields the covalent anchoring of the iridium complex to the surface of the magnetite. The catalytic properties have been evaluated in transfer hydrogenation. The iridium complexes and the material are active in the reduction of ketones using isopropanol as the solvent and hydrogen donor. The catalytic results reveal that the catalytic activity of the material and the molecular complex are equivalent. We have not observed any change in activity due to the support. The recyclability properties of the magnetic material have been evaluated. The results show that the catalyst activity is maintained for two runs. This work describes a simple methodology for anchoring molecular complexes on the surface of magnetic nanoparticles.

Full text of DOI:10.1039/c5nj00803d

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Catalytic applications of Magnetic nanoparticles function
D
a
OI
l
:
i
1
z
0.
e
1039/C5NJ00803D
d
using iridium N-Heterocyclic carbene complexes
a
a
a,b
a,*
Diego Iglesias, Sara Sabater, Arturo Azua, Jose A. Mata
aDepartamento de Química Inorgánica y Orgánica, Institute of Advanced Materials
INAM), Universitat Jaume I, Avda. Sos Baynat s/n, 12071, Castellón (Spain).Telf. +34
64387516 Fax. +34 964387522
(
9
bDepartment of Chemistry, George Washigton University, Science & Engineering Hall,
suite 4000, 800 22nd Street, NW, Washington, DC 20052.
Abstract
A synthetic modular methodology allows the preparation of catalytic materials based
on magnetic nanoparticles with iridium-N-heterocyclic carbene (NHC) complexes. The
preparation of imidazolium salts containing a ketone/aldehyde as pendant functional
groups are the key species. The condensation reaction of the Cp*IrNHC-CHO
compound with magnetic nanoparticles containing amine groups on the surface yields
the covalent anchoring of the iridium complex to the surface of the magnetite. The
catalytic properties have been evaluated in transfer hydrogenation. The iridium
complexes and the material are active in the reduction of ketones using isopropanol as
solvent and hydrogen donor. The catalytic results reveal that the catalytic activity of
the material and the molecular complex is equivalent. We have not observed any
change in activity due to the support. The recyclability properties of the magnetic
material have been evaluated. The results show that the catalyst activity is maintained
for two runs. This work describes a simple methodology for the anchoring of molecular
complexes on the surface of magnetic nanoparticles.
Keywords. Magnetic nanoparticles, Transfer hydrogenation, Supported catalysis,
Iridium catalysts, N-heterocyclic carbenes.
Electronic Supplementary Information (ESI) available: General procedures, Mass
Spectroscopy, X-Ray diffraction studies, High resolution electron microscopy,
Powder XRD and FTIR spectroscopy. See DOI: 10.1039/x0xx00000x
1

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DOI: 10.1039/C5NJ00803D
Introduction
1
The green chemistry definition proposed by Anastas and Warner as “the utilization of
a set of principles that reduces or eliminates the use or generation of hazardous
substances in the design, manufacture and application of chemical products” has
encouraged the scientific community to look for efficient ways to separate
2
homogenous catalysts from the reaction media and their subsequent recycling. The
use of magnetic nanoparticles (MNPs) as efficient supports for catalysts has become a
3
subject of intense investigation. The MNPs offer advantages in the development of
clean and sustainable green processes as they are non-toxic, 4 easy-accessible, 5
6
retrievable and reusable. In the last years, two seminal manuscripts describing the
immobilization of ruthenium catalyst have been described (Chart 1). The Noyori type
catalyst was immobilized over MNPs using a phosphoric acid-substituted BINAP (2,2'-
bis(diphenylphosphino)-1,1'-binaphthyl). The MNPs-Ru-BINAP chiral material catalyzed
the asymmetric hydrogenation of aromatic ketones in quantitative yields and excellent
enantioselectivities. The system could be recycled by magnetic decantation and reused
7
for up 14 times without loss of activity and enantioselectivity. The second generation
Hoveyda-Gurbbs catalyst has been immobilized on surface modify magnetite
nanoparticles by a divergent synthetic approach. The catalytic system is high effective
for self- and cross-metathesis reactions of methyl oleate and could be easily separated
by magnetic attraction.8
Chart 1 Noyori and Hoveyda-Grubbs catalysts immobilization strategies
The main advantages of homogeneous catalysis are selectivity and easy tuning of
properties by ligand modification. In contrast, the main drawback is catalyst separation
9
from the final reaction product. Whilst for many processes small amount of metals is
not a problem, just traces of metals become really important in the pharmaceutical
industry. Using heterogeneous catalyst systems, separation is not a problem, as they
2

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can be easily removed by simple filtration. The problem arises mostly from selectivity.
Over the last decade the use of magnetic nanoparticles functionalized with transition
1
0
metal for applications in catalysis has expanded considerably. Most of the catalytic
materials obtained lack of a general synthetic method for the preparation of different
metal complexes. In this work we describe a modular approach method for
development of catalytic materials based on magnetic nanoparticles decorated with
metal-N-heterocyclic carbene (NHC) complexes.11 The metal-NHC based complexes
have demonstrated their wide scope application in catalysis, and their usefulness in
1
2
the synthesis of complex natural products and valuable molecules. The NHC ligands
1
3
confer a high stability to the metal and act as spectator ligands. These important
characteristics make NHC ligands good candidates for the preparation of reusable
magnetite based-catalysts.
In the course of our research we have developed efficient catalysts based on
1
4
metal-NHC complexes for transfer hydrogenation. On the basis of previous results we
now described the synthesis of magnetic nanoparticles functionalized with iridium N-
heterocyclic carbene complexes (NHCs) for the preparation of advanced catalytic
materials that would be reusable and retrievable from the reaction mixture by simple
decantation using an external magnet.
Experimental
Materials and methods
All manipulations were carried out under aerobic conditions unless otherwise stated.
The 4′-(Imidazol-1-yl)acetophenone and Fe
from commercial suppliers. The imidazolium salt 1 , 4-(chloromethyl)benzaldehyde
and functionalized Fe
magnetic nanoparticles17 were prepared according to
3
O
4
magnetic nanoparticles were purchased
1
5
16
O
3 4
literature procedures. Anhydrous solvents were dried using a solvent purification
system (SPS M BRAUN) or purchased from purchased from commercial suppliers and
degassed prior to use by purging with dry nitrogen and kept over molecular sieves.
NMR spectra were recorded on Varian Innova spectrometers operating at 300 or 500
1
13
MHz ( H NMR) and 75 and 125 MHz ( C NMR), respectively, using CDCl as solvent at
3
room temperature unless otherwise stated. Elemental analyses were carried out in an
EA 1108 CHNS-O Carlo Erba analyzer. Electrospray mass spectra (ESI-MS) were
recorded on a Micromass Quatro LC instrument, and nitrogen was employed as drying
3

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and nebulizing gas. Infrared spectra (FTIR) were recorded on a JASCO FTIR-6200
spectrometer with aspectral window of 4000-500 cm-1. High resolution transmission
electron microscopy images (HRTEM) and high-angle annular dark-field HAADF-STEM
images of the samples were obtained using a Jem-2100 LaB6 (JEOL) transmission
electron microscope coupled with an INCA Energy TEM 200 (Oxford) energy dispersive
X-Ray spectrometer (EDX) operating at 200 kV. The crystallographic structure of the
samples was analyzed by X-ray powder diffraction (XRD) using a Bruker D4-Endeavor
diffractometer with a Cu-K radiation with a wavelength of 0.1542 nm. The
determination of the metal loading was done by ICP–MS Agilent 7500 CX. The
digestion of the samples was carried out under reflux of a mixture of concentrated
nitric and hydrochloric acids (3:1) for 12 h. The digestion of the samples after the
recycling experiments were carried out after several washes with milli-Q water (10 x 10
mL).
Crystallography
X-ray studies: Diffraction data was collected on a Agilent SuperNova diffractometer
equipped with an Atlas CCD detector using Mo-K radiation (= 0.71073 Å). Single
crystals were mounted on a polymer tip in a random orientation. Absorption
1
8
corrections based on the multiscan method were applied. The structures were solved
by direct methods in SHELXS-97 and refined by the full-matrix method based on F2
with the program SHELXL-97 using the OLEX software package.19
Synthetic procedures
Synthesis of 2. To a solution of 4′-(imidazol-1-yl)acetophenone (476 mg, 2.56 mmol) in
methanol (5 mL) was added 1-Bromobutane (555 μL, 5.17 mmol) in a high pressure
tube. The mixture was stirred at 100 ºC for 12h. After removal of the volatiles under
reduced
pressure
the
viscous
precipitate
was
crystallized
from
dichloromethane/diethyl ether. The oily residue was filtered and washed with diethyl
ether (3 x 5 mL) to afford the imidazolium salt 2 as a hygroscopic white solid. Yield 415
1
mg (50%). H NMR (300 MHz, CDCl
3
): δ 11.10 (s, 1H, NCHN), 8.16 (s, 1H, CHim), 8.05 (s,
), 2.56 (s, 3H, COCH ), 2.03 –
). C NMR (300
), 137.9, 136.9, 136.0, 130.1, 124.9, 122.6, 120.9 (CHPh
), 33.5 (CH ), 27.0 (COCH ), 21.2 (CH ), 14.8 (CH ). Not a proper
4
H, CHPh), 7.81 (s, 1H, CHim), 4.53 (t, J = 7.3 Hz, 2H, NCH
.88 (m, 2H, CH ), 1.39 – 1.26 (m, 2H, CH ), 0.91 (t, J = 7.3 Hz, 3H, CH
): δ 196.4 (COCH
CHim), 51.7 (NCH
2
3
1
3 20
1
2
2
3
MHz, CDCl
3
3
,
2
2
3
2
3
elemental analysis could be obtained due to the oily character of the imidazolium salt.
+
Electrospray MS (Cone 20V) (m/z, fragment): 243.0 [M-Br] . Electrospray Ms. (Cone
4

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+
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2
0V) (m/z, fragment): 243.14 [M-Br] . HRMS ESI-TOF-MS (positive mode): [M-Br]
DOI: 10.1039/C5NJ00803D
monoisotopic peak 243.1494; calc. 243.1497, ε 1.2 ppm.
r
Synthesis of 3. In a high pressure tube Cesium Carbonate (246 mg, 0.75 mmol) was
added to a solution of the imidazolium salt 1 (105 mg, 0.32 mmol) and [Cp*IrCl (100
]
2 2
mg, 0.13 mmol) in acetonitrile (5 mL). The mixture was heated at 100 °C for 4 hours.
After cooling, the mixture was filtered through a pad of celite and the solvent was
removed under reduce pressure. The crude solid was purified by crystallization from a
dichloromethane/hexane mixture giving a yellow powder. Yield 140 mg (95 %). 1H
NMR (300 MHz, CDCl
3
): δ 8.32 (s, 1H, CHPh), 7.57 (d, 3JH-H = 1.9 Hz, 1H, CHPh), 7.37 (s,
H, CHim), 7.16 (d, 3JH-H = 1.9 Hz, 2H, CHPh), 7.02 (s, 1H, CHim), 3.90 (s, 3H, NCH
), 2.62 (s,
): δ 198.3 (COCH ), 167.2
carbene), 150.8 (CPh-Ir), 142.1, 133.3, 134.8, 123.0, 122.8, 115.4, 110.3 (CHPh, CHim),
1.3 (CCp*), 36.9 (NCH ), 26.8 (COCH ), 9.7 (CH3,Cp*). Anal. Calcd. For C22 OClIr
1
3
3
1
3
1
H, COCH
3
), 1.93 (s, 15H, Cp*). C { H} NMR (300 MHz, CDCl
3
3
(C
9
3
3
H
26 2
N
(
(
[
562.1): C, 47.00; H, 4.66; N, 4.98. Found: C, 46.95; H, 4.54; N, 4.96. Electrospray MS
+
Cone 20V) (m/z, fragment): 568.1 [M-Cl+MeCN] . HRMS ESI-TOF-MS (positive mode):
+
M-Cl] monoisotopic peak 527.1680; calc. 527.1675, ε
r
0.9 ppm.
Synthesis of 4. In a round bottom flask, covered with aluminum foil, silver oxide (31.4
mg, 0.14 mmol) and the imidazolium salt of 2 (78.0 mg, 0.24 mmol) were dissolved in
dichlorometane (20 mL). The mixture was stirred at room temperature for 6 hours.
Then, [Cp*IrCl
2
2
] (100 mg, 0.13 mmol) was added and the reaction was maintained at
room temperature for 12 hours. The mixture was filtered through a pad of celite and
the solvent was removed under reduce pressure. The crude solid was purified by
crystallization from a dichloromethane/hexane mixture giving an orange powder. Yield
1
): δ 8.32 (s, 1H, CHPh), 7.63 (d, 3JH-H = 7.2 Hz,
1
1
4
1
25 mg (86 %). H NMR (300 MHz, CDCl
3
H, CHPh), 7.39 (s, 1H, CHim), 7.17 (d, 3JH-H = 8.0 Hz, 1H, CHPh), 7.05 (s, 1H, CHim), 4.42 –
.39 (m, 1H, NCH
.63 – 1.60 (m, 2H, CH
H} NMR (300 MHz, CDCl
38.0, 134.7, 124.1, 121.1, 105.3, 109.9 (CPh, CHim), 91.2, (CCp*), 50.1 (NCH
2
), 4.19 – 4.15 (m, 1H, NCH
), 1.46 – 1.42(m, 2H, CH
): δ 198.3 (COCH
2
), 2.63 (s, 3H, COCH
3
), 1.79 (s, 15H, Cp*),
), 0.99 (t, 3JH-H = 7.3 Hz, 3H, CH
). C
13
2
2
3
{1
), 166.2 (Ccarbene), 150.6 (CPh-Ir), 142.1,
3
3
1
2
), 33.3 (CH
2
),
5

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2
7.2 (COCH
3
), 20.2 (CH
2
), 13.9 (CH
3
) 9.7 (CH3.Cp*). Anal. Calcd. For C25
H
32
N
2
OClIr (604.2):
DOI: 10.1039/C5NJ00803D
C, 49.70; H, 5.34; N, 4.64. Found: C, 49.31; H, 5.48; N, 4.76. Electrospray MS (Cone 20V)
+
+
(
m/z, fragment): 610.2, [M - Cl+MeCN] . HRMS ESI-TOF-MS (positive mode): [M-Cl]
monoisotopic peak 569.2145; calc. 569.2142, ε 0.5 ppm.
r
Synthesis of 5. To a solution of 4-(chloromethyl)benzaldehyde (480 mg, 3.10 mmol) in
dry THF (8 mL) was added 1-methylimidazole (300 μL, 3.76 mmol). The mixture was
stirred at 60 ºC for 72h. After removal of the volatiles under reduced pressure the
viscous precipitate was extracted with dichloromethane (3 x 10 mL). The imidazolium
1
salt 5 was obtained as colorless oil. Yield 305 mg (41%). H NMR (300 MHz, CD
3
CN): δ
1
0.02 (s, 1H, CHO), 9.87 (s, 1H, NCHN), 8.08 (d, 3JH-H = 8.0 Hz, 2H, CHPh), 7.70 (d, 3JH-H
.0 Hz, 2H, Ph), 7.67 (s, 1H, CHim), 7.54 (s, 1H, CHim), 5.64 (s, 2H, NCH ), 3.89 (s, 3H,
CN): δ 192.4 (CHO), 140.7, 137.0, 136.7, 130.1,
=
8
2
1
3
1
NCH
3
). C { H} NMR (75 MHz, CD
3
1
29.2, 124.1, 122.3 (CHPh, CHim, NCHN), 52.0 (NCH
2
), 36.1 (NCH
3
). Electrospray MS
+
+
(
Cone 20V) (m/z, fragment): 201.4. [M-Cl] . HRMS ESI-TOF-MS (positive mode): [M-Cl]
monoisotopic peak 201.1023; calc. 201.1028, ε 2.0 ppm.
r
Synthesis of 6. Following the same procedure as described for complex 3 using Cesium
Carbonate (165 mg, 0.51 mmol), the imidazolium salt 5 (72 mg, 0.30 mmol) and
[
Cp*IrCl
2
]
2
(100 mg, 0.13 mmol). Complex 6 was obtained as an orange solid. Yield 101
1
mg (70 %). H NMR (300 MHz, CD
3
CN): δ 9.93 (s, 1H, CHO), 8.02(s, 1H, CHPh), 7.37 (d,
3JH-H = 7.6 Hz,1H, CHPh), 7.29 (m, 3H, CHPh, CHim), 4.96 – 4.79 (m, 2H, NCH
), 3.89 (s, 3H,
2
), 2.01 (m, 15H, Cp*). 13C { H} NMR (75 MHz, CD
1
CN): δ 194.7 (CHO), 157.3
NCH
3
3
(
C
carbene), 149.9 (CPh-Ir), 147.7, 144.8, 136.2, 125.7, 123.5, 122.8, 121.7 (CHPh, CHim),
1.3 (CCp*), 57.4 (NCH ), 37.3 (NCH ), 9.6 (CH3,Cp*). Anal. Calcd. For C22 OClIr
562.1): C, 47.00; H, 4.66; N, 4.98. Found: C, 47.21; H, 4.73; N, 4.99. Electrospray MS
9
2
3
H N
26 2
(
(
+
+
Cono 20V) (m/z, fragment): 527.4. [M-Cl] . HRMS ESI-TOF-MS (positive mode): [M-Cl]
1.0 ppm.
monoisotopic peak 527.1680; calc. 527.1675, ε
r
6

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Synthesis of nano-Fe
3
O
4
-dopa@Cp*IrNHC (7). In a high pressure tube nano-Fe
3
O -
4
dopa (570 mg) was added to a solution of the iridium complex 6 (30 mg, 0.05 mmol) in
toluene (5 mL). The mixture was heated at 100 °C for 20 hours. After cooling, the solid
was separated using an external magnet, washed with toluene (3 x 5 mL) and
dichlorometane (2 x 5 mL) and dried under reduced pressure. The nano-Fe
3
4
O -
dopa@Cp*IrNHC magnetic material (7) was obtained as a dark brown solid.
Catalytic studies. General procedure for transfer hydrogenation: In a high pressure
tube were added the ketone (0.5 mmol), Cs CO (1 equiv.), anisole as internal
2
3
reference standard (0.5 equiv.), catalyst (0.5 mol%) and 3 mL of iPrOH used as solvent
and hydrogen donor. The tube was closed and the mixture was heated at 100 ºC for 24
h. After cooling to room temperature the catalyst was separated using an external
magnet and the solution was filtered using celite and analyzed. Yields were
determined by GC analyses and products were identified according to spectroscopic
data of commercially available compounds. The recycling experiments were carried
out under identical reaction conditions. After completion of each run, the reaction
mixture was allowed to reach room temperature and the solid catalyst was separated
using an external magnet, washed with iPrOH (4 x 5 mL), dried and reused in the
subsequent run.
Results and discussion
Synthesis and characterization: The imidazolium salts 1 and 2 were prepared by
alkylation of 4′-(imidazol-1-yl)acetophenone using iodomethane or 1-bromobutane
1
5
according to literature procedures. The reaction of the imidazolium salts 1 or 2 under
basic conditions (Cs CO ) in the presence of [Cp*IrCl afforded the orthometallated
2
3
]
2 2
iridium complexes 3 and 4 in high yields, 95% and 86% respectively (Scheme 1). Cp*Ir
complexes bearing an N-heterocyclic carbene ligand with phenyl or benzyl wingtip
groups have a great tendency to orthometallate by C-H activation, most probably due
2
1
to a chelate-assisted process. The imidazolium salt 5 was obtained by alkylation of 1-
methylimidazole using 4-(chloromethyl)benzaldehyde. The reaction of the imidazolium
salt 5 under the reaction conditions previously described afforded the six-member
7

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cyclometallated iridium complex 6 (Scheme 1). The iridium complexes obtained (3 – 6)
are air stable in the solid state and solution. The characterization of the molecular
complexes was performed by NMR, ESI/MS, HRMS and elemental analysis. The 1H
NMR confirms that the NHC ligand metallation has occurred by the disappearance of
1
3
the C2-H acidic proton. The C NMR confirms that metallation has occurred by the
characteristic C-carbene signal at  167.2 (complex 3), 166.2 (complex 4) and 157.3
(complex 6). The nature of the iridium complexes was further analyzed by means of
electrospray mass spectrometry (ESI/MS) based on the mass/charge relation and the
isotopic pattern distribution. Positive ion electrospray mass spectra analysis show base
+
+
peaks for [M-Cl+MeCN] at m/z = 568.2 for 3, [M-Cl+MeCN] at m/z = 610.2 for 4 and
+
[
M-Cl] at m/z = 527.4 for 6. High resolution mass spectrometry (HRMS) confirmed the
exact mass with relative errors lower than 2 ppm.
Scheme 1. Synthesis of Cp*IrNHC complexes.
The molecular structure of complex 3 was confirmed by single crystal X-ray diffraction.
Figure 1). Single crystals were obtained by slow diffusion of hexane into a high
(
concentrated solution of 3 in dichloromethane. Complex 3 crystallizes in the achiral
space group P-1 and contains a single molecule in the asymmetric unit. The geometry
of the complex is the expected three-legged piano-stool with dihedral angles close to
9
0 degrees except the C(3) - Ir(1) - C(10) of 77.44(18)° due to the chelate coordination
8

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of the NHC-phenyl ligand. The C-C orthometallated NHC-phenyl ligand forms a five-
member ring in a planar conformation. The iridium distances are Ir(1)- C(3) 2.000(4) Å,
Ir(1) - C(10) 2.055(4) Å and Ir(1) - C(12) 2.4583 (4) Å and lie in the expected range for
Cp*Ir(NHC) complexes.
Figure 1. Molecular diagram of complex 3. Ellipsoids are at 50% probability level.
Hydrogen atoms omit for clarity. Selected bond lengths [Å] and angles [º]: Ir(1) - C(3)
2
-
.000(4), Ir(1) - C(10) 2.055(4), Ir(1) - Cl(2) 2.4583(10), C(3) - Ir(1) - C(10) 77.44(18), C(3)
Ir(1) - Cl(2) 91.17(12), C(10) - Ir(1) - Cl(2) 91.98(12).
Scheme 2. Condensation reaction of Cp*IrNHC-CHO (6) with the nano-Fe
3
O -dopa
4
The magnetite nano-Fe
3
O
4
functionalized with dopamine (nano-Fe
3
4
O -dopa) was
synthesized according to Varma’s method.17 The Cp*Ir molecular complex (6) was
covalently anchored on the surface of the magnetic nanoparticles by condensation
(Scheme 2). The reaction of the orthometalated iridium complex bearing an NHC-CHO
functionalized ligand (6) with the amine groups of the nano-Fe
3
O
4
-dopa afforded the
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nano-Fe
3
O -dopa@Cp*IrNHC magnetic material (7). After 20 h reaction the material
DOI: 10.1039/C5NJ00803D
4
was washed thoroughly with toluene and dichlorometane. The exact iridium content
on 7 was determined by ICP-MS analysis. The results obtained accounted for a 2.7 wt%
of the molecular complex 6 in the material 7. The characterization of the materials was
performed using XRD, FTIR and HRTEM (See supplementary data for details). The
HRTEM analysis of 7 shows the presence of Fe
range 20 – 30 nm (Figure 2). The HRTEM analysis before and after the functionalization
with the iridium complex reveals that shape and size of the Fe nanoparticles are
preserved after the condensation reaction (Compare Figure 2 and Figures S1 – S3).
3
O
4
spherical nanoparticles of a size
O
3 4
Figure 2. HRTEM images at different magnification showing the spherical morphology
of the nano-Fe -dopa@Cp*IrNHC (7).
O
3 4
Scheme 3. Condensation reaction of Cp*IrNHC-CHO (6) with benzylamine.
In order to confirm that the condensation reaction is the responsible for the anchoring
of the iridium complex on the surface of the nano-Fe
3
4
O -dopa, we performed a control
experiment (Scheme 3). In this experiment the Cp*IrNHC-CHO complex (6) was
condensed with benzylamine under exactly the same reactions conditions. The
characterization of the condensed iridium complex (8) was performed by means of
ESI/MS and HRMS. Positive ion electrospray mass spectra analysis after the
+
condensation reaction reveals the presence of two intense peaks for [M-Cl] at m/z =
5
27.2 and 617.2. These two peaks correspond to the presence of complexes 6 and 8
respectively, just confirming that the condensation reaction is not quantitative. High
1
0

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resolution mass spectrometry (HRMS) confirmed the exact mass of complex 8 with
relative errors inferior to 1 ppm. When the same experiment was carried out using the
Cp*IrNHC-COCH
3
complexes (3) and (4), we observed that no condensation reaction
occurred.
The catalytic activity of the nano-Fe
3 4
O
-dopa@Cp*IrNHC (7) was evaluated in transfer
hydrogenation.22 The Cp*Ir type complexes have extensively been evaluated as
homogeneous catalysts in the reduction of ketones and imines under transfer
2
3
hydrogenation conditions. The reactions were carried out using iPrOH as solvent and
hydrogen donor in the presence of stoichiometric amounts Cs CO and using a catalyst
loading of 0.5 mol%, under aerobic conditions and using regular solvents. (Table 1).
The catalytic activity of the nano-Fe -dopa@Cp*IrNHC (7) was compared with the
2
3
O
3 4
molecular iridium complex (6). In both catalytic systems the addition of base is
required for the formation of the catalytic active species, as has previously been
2
4
described. In the development of this work, we wanted to use complex 8 as an
analogue for the magnetite-immobilized complex and all our efforts persuade the
isolation and purification of the complex. Unfortunately complex 8 was not obtained
pure enough for catalyst testing and complex (6) was used instead. For comparative
purposes, a table in the supplementary information shows the catalytic activity of
different Cp*Ir complexes in the reduction of acetophenone by transfer hydrogenation
(Table S7).
In a first experiment the catalytic activity of the nano-Fe
3
4
O -dopa@Cp*IrNHC
(7) and the molecular iridium complex (6) was compared in the transfer hydrogenation
of acetophenone (Figure 3). The comparative reaction profile shows that there is not
any significant difference in activity between the molecular complex and the
supported counterpart.
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Figure 3. Comparative reaction profile in the transfer hydrogenation of acetophenone
using the molecular catalyst (6) and the nano-Fe -dopa@Cp*IrNHC (7). Reaction
conditions: 0.5 mmol of acetophenone, Cs CO (1 Eq), catalyst (0.5 mol %), 3 mL of
O
3 4
2
3
iPrOH for 24 hours at 100 ºC. Yields determined by GC analyses using anisole as
internal standard.
The molecular complex 6 is active in the reduction of ketones yielding the
corresponding alcohols in quantitative yields after 24 hours reaction at 100 ºC. The
same catalytic activity was obtained when using the nano-Fe
3
4
O -dopa@Cp*IrNHC (7)
material under the same reaction conditions and metal amount. This is an indication
that the support does not have any influence in the catalytic process. When the
reaction was carried out without catalyst or using nano-Fe
3
4
O -dopa as catalyst the
yields were less than 5% (entries 1, 2 and 5). These experiments reveal that the
Cp*IrNHC fragment is the responsible of catalytic process. Acetophenone was
converted into 1-phenyl ethanol in 90% yield by using catalyst 6. When the same
reaction was carried out using nano-Fe
3
4
O -dopa@Cp*IrNHC (7) the yield of 1-phenyl
ethanol was 93% (entries 3 – 4). Introduction of different electron
withdrawing/donating groups does not have much influence in the catalytic outcomes
(entries 6 – 9). When increasing the sterics of substrates the molecular complex 6 gave
slightly better results than the catalytic material (entries 10 – 13). Both catalytic
systems are moderately active in the reduction of aliphatic ketones (entries 14 – 15). A
list of comparative Cp*Ir catalysts activity in transfer hydrogenation of acetophenone
can be found in table S7.
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2

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DOI: 10.1039/C5NJ00803D
Table 1 Transfer Hydrogenation using the molecular complex 6 and the NHC-Ir-MNP
(
7).a
Entry
1
Catalyst
-
Substrate
Yield b
< 5%
2
3
4
5
6
7
8
Fe
3
O
4
dopa
< 5%
90
6
7
-
93
< 5%
98
6
7
6
75
75
9
7
72
1
1
1
1
0
1
2
3
6
7
6
7
96
85
99
90
1
1
4
5
6
7
99
99
[
a] Reactions were carried out with 0.5 mmol of substrate, Cs
2
CO (1 Eq), catalyst (0.5
3
mol %), 3 mL of iPrOH for 24 hours at 100 ºC. [b] Yields determined by GC analyses
using anisole as internal standard.
In view of the results obtained in the transfer hydrogenation, we decided to explore
the catalyst stability by exploring the catalyst recovery and reuse. Recycling
experiments of nano-Fe
3
O -dopa@Cp*IrNHC (7) were carried out using acetophenone,
4
cyclohexanone, p-bromoacetophenone and p-methoxyacetophenone (Figure 4). The
reaction conditions were the same as those described in the transfer hydrogenation
process described in Table 1. After completion of each run (24 h.), the reaction mixture
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DOI: 10.1039/C5NJ00803D
was allowed to reach room temperature, and the solid catalyst was separated using
and external magnet (Figure S8), washed with iPrOH (4 x 5 mL), dried and reused in the
subsequent run. Following this methodology the nano-Fe -dopa@Cp*IrNHC (7) was
O
3 4
reused for two times without any decrease of activity in the case of acetophenone.
Then, the catalytic activity decrease to 50% yield and is maintained up to the fifth run.
In the case of the other substrates, the catalyst activity is maintained for two runs, and
then a dramatic decrease in activity was observed, except in the case of the p-
methoxyacetophenone were after the first catalytic run the catalyst activity is
completely lost. These results suggest a deactivation of the molecular iridium complex
anchored on the surface of the nano-Fe
dependent. In order to increase the recyclability of the nano-Fe
7) we performed a recycling experiment using acetophenone under inert conditions.
The results were the same as using aerobic conditions and the catalyst activity drops to
0% after the second catalytic run. Then we analyze the catalytic material by means of
HRTEM analysis (Figure 5). The results show that after three catalytic runs the size and
3
4
O -dopa. The deactivation process is substrate
3 4
O -dopa@Cp*IrNHC
(
5
morphology of the nano-Fe
observed.
3
4
O are preserved and no degradation of the material is
Figure 4. Recycling experiments in transfer hydrogenation using Cp*IrNHC@NPsFe
O
3 4
(
7) as catalyst. Reaction conditions: 0.5 mmol of substrate, Cs CO (1 Eq), catalyst (0.5
2
3
mol %), 3 mL of iPrOH for 24 hours at 100 ºC for each run. Yields of the corresponding
alcohols obtained were determined by GC analyses using anisole as internal standard.
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Figure 5. HRTEM images at different magnification of nano-Fe
3
4
O -dopa@Cp*IrNHC (7)
after three catalytic runs using acetophenone as substrate.
The poor recyclability of the nano-Fe
3
4
O -dopa@Cp*IrNHC (7) could be attributed to
the attachment of the molecular complex 6 by an imine formation. In principle, the
imine functionality is hydrolytically unstable and liable to break under basic conditions.
In order the check this hypothesis the amount of iridium leaching was analyzed by
ICP/MS analysis in the recycling experiment using acetophenone as substrate. The
results reveal that the amount of iridium is negligible in five consecutive runs, just
indicating that decoordination of the molecular complex 6 by imine hydrolysis is not
occurring. These results are supported by other examples previously developed by our
1
4f
and others23h,
25
group
where imines are not hydrolyzed under transfer
hydrogenation conditions using strong bases.14b, 14d, 26 The transfer hydrogenation of
imines using iPrOH as solvent and hydrogen donor in the presence of stoichiometric
amounts Cs
2
CO
3
has been successfully achieve with iridium and rhodium N-
heterocyclic carbene complexes without any detectable hydrolysis product
formation.25f
Other important factor related to supported catalysis is the so called
2
7
“
boomerang effect” or “release and catch” catalytic systems. This effect consists in a
release-and-return of the molecular complex form the support, therefore implying that
the catalytic process is carried out homogeneously and then the catalytic active
species are “caugh” at the end of the reaction by the support. The imine hydrolysis of
the material nano-Fe
molecular complex 6) and the amine (nano-Fe
equilibrium reaction and could go back to the condensation product (nano-Fe
3
O
4
-dopa@Cp*IrNHC (7) could yield the corresponding aldehyde
(
O
3 4
-dopa). Note that this process is an
3
4
O -
dopa@Cp*IrNHC (7)). A hot filtration experiment was realized in order to determine if
the boomerang effect is governing the catalytic process. The hot filtration experiment
was carried out using the general conditions described in table 1 using acetophenone
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as substrate and nano-Fe
3
O -dopa@Cp*IrNHC (7) as catalyst. After 6h reaction, the
DOI: 10.1039/C5NJ00803D
4
catalyst was separated from the solution by using an external magnet at 100 ºC. At this
point, an aliquot was analyzed by GC and the yield of 1-phenyl ethanol was 38%. The
filtrate was further heated at 100 ºC for 18 h and then analyzed. The results reveal that
no further alcohol is formed (GC final yield is maintained in 38%). This experiment
suggests the absence of catalytic active species in solution due to imine hydrolysis.
Cp*Ir type complexes have been used as homogeneous catalysts in the
28
reduction imines under transfer hydrogenation conditions. The presence of an imine
in the supported catalyst nano-Fe -dopa@Cp*IrNHC (7) could lead to the
O
3 4
hydrogenation of the imine producing the modification of the parent catalyst. The
formation of the corresponding amine should not modify the catalytic properties of
the system. To check the possibility of imine hydrogenation a control experiment was
carried out using N-benzylideneaniline as model substrate. The reaction was carried
out using iPrOH as solvent in the presence of stoichiometric amounts Cs
2
CO and a
3
catalyst loading of 0.5 mol%. The results reveal that the N-benzylideneaniline is not
hydrogenated to the corresponding amine just indicating that the iridium catalyst is
not active in the hydrogenation of imines under this reaction conditions. We have also
observed that the N-benzylideneaniline is recovered quantitatively, indicating that no
hydrolysis has occurred.
Based on the previous experiments the most plausible mechanism for the poor
recyclability of the supported nano-Fe
3
4
O -dopa@Cp*IrNHC (7) catalyst in transfer
hydrogenation could be attributed to the formation of inactive iridium species that are
not catalytically active. The nature of this species is unknown but they are attached to
the magnetite and no release to solution has been observed. The situation is similar to
the catalyst activity/deactivation procedure observed for the molecular complexes.
Development of highly active and stable molecular catalysts should increase the
possibility of recycling and reuse of catalytic systems based on supported magnetite
catalysts.
Conclusions
We have described a synthetic modular methodology that allowed us to obtain
catalytic materials based on magnetic nanoparticles with iridium-N-heterocyclic
carbene (NHC) complexes. Imidazolium salts containing a ketone/aldehyde as pendant
functional group have been synthesized. Metallation of the imidazolium salts leads to
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molecular Cp*iridium complexes. The condensation reaction of the Cp*IrNHC-CHO
with magnetic nanoparticles containing amine groups on the surface yields the
covalent anchoring of the iridium complex to the surface of the magnetite. The iridium
complexes and the material are active in the reduction of ketones under transfer
hydrogenation conditions. The catalytic results reveal that the catalytic activity of the
material and the molecular complex is equivalent. We have not observed any change
in activity due to the support. The recyclability properties of the magnetic material
have been evaluated. The results show that the catalyst activity is maintained for two
runs. This work describes a simple methodology for the anchoring of molecular
complexes on the surface of magnetic nanoparticles by the formation of an imine.
Further research into the preparation of new catalytic materials following the same
methodology but with the formation of an amine or amide to improve the attachment
with the support is currently underway in our laboratory.
Acknowledgements
We thank the financial support from the Ministerio de Ciencia e Innovación of Spain
(
CTQ2011-24055/BQU). We thank the ‘Generalitat Valenciana’ for a fellowship (S.
Sabater). The authors are grateful to the ‘Serveis Centrals d’Instrumentació Científica
SCIC)’ of the Universitat Jaume I.
(
Notes and references
§
X-ray crystallographic data has been deposited at the Cambridge Crystallographic
Data Centre CCDC 1057089.
1
2
.
.
P. T. Anastas and J. Warner, Green Chemistry: Theory and Practice, Oxford
University Press, Oxford, 1998.
(a)R. B. N. Baig and R. S. Varma, Chem. Commun., 2013, 49, 752; (b)A. Schaetz,
O. Reiser and W. J. Stark, Chem. Eur. J., 2010, 16, 8950; (c)M. B. Gawande, P. S.
Branco and R. S. Varma, Chem. Soc. Rev., 2013, 42, 3371.
3
.
(a)D. Astruc, Nanoparticles and Catalysis, Wiley-VCH Verlag GmbH & Co. KGaA,,
Weinheim, Germany, 2008; (b)L. M. Rossi, M. A. S. Garcia and L. L. R. Vono, J.
1
7

New Journal of Chemistry
Page 18 of 20
View Article Online
DOI: 10.1039/C5NJ00803D
Braz. Chem. Soc., 2012, 23, 1959; (c)K. V. S. Ranganath, J. Kloesges, A. H.
Schaefer and F. Glorius, Angew. Chem. Int. Ed., 2010, 49, 7786.
(a)R. S. Varma, Green Chem., 2008, 10, 1129; (b)R. B. N. Baig and R. S. Varma,
Green Chem., 2012, 14, 625; (c)B. R. Vaddula, A. Saha, J. Leazer and R. S. Varma,
Green Chem., 2012, 14, 2133.
4
5
6
.
.
.
(a)J. M. Perez, R. Cano, M. Yus and D. J. Ramon, Synthesis, 2013, 45, 2768; (b)R.
Cano, D. J. Ramon and M. Yus, Tetrahedron, 2011, 67, 5432; (c)V. Polshettiwar
and R. S. Varma, Org. Biomol. Chem., 2009, 7, 37.
(a)B. Baruwati, V. Polshettiwar and R. S. Varma, Tetrahedron Lett., 2009, 50,
1
215; (b)H. M. R. Gardimalla, D. Mandal, P. D. Stevens, M. Yen and Y. Gao,
Chem. Commun., 2005, 4432; (c)J. Liu, X. Peng, W. Sun, Y. Zhao and C. Xia, Org.
Lett., 2008, 10, 3933; (d)P. D. Stevens, J. D. Fan, H. M. R. Gardimalla, M. Yen
and Y. Gao, Org. Lett., 2005, 7, 2085; (e)S. Sa, M. B. Gawande, A. Velhinho, J. P.
Veiga, N. Bundaleski, J. Trigueiro, A. Tolstogouzov, O. M. N. D. Teodoro, R.
Zboril, R. S. Varma and P. S. Branco, Green Chem., 2014, 16, 3494; (f)M. B.
Gawande, A. K. Rathi, J. Tucek, K. Safarova, N. Bundaleski, O. M. N. D. Teodoro,
L. Kvitek, R. S. Varma and R. Zboril, Green Chem., 2014, 16, 4137; (g)S. E. Garcia-
Garrido, J. Francos, V. Cadierno, J.-M. Basset and V. Polshettiwar,
ChemSusChem, 2011, 4, 104.
7
8
.
.
A. G. Hu, G. T. Yee and W. B. Lin, J. Am. Chem. Soc., 2005, 127, 12486.
Z. Yinghuai, L. Kuijin, N. Huimin, L. Chuanzhao, L. P. Stubbs, C. F. Siong, T.
Muihua and S. C. Peng, Adv. Synth. Catal., 2009, 351, 2650.
9
1
.
S. C. Tsang, V. Caps, I. Paraskevas, D. Chadwick and D. Thompsett, Angew.
Chem. Int. Ed., 2004, 43, 5645.
0.
(a)R. Cano, D. J. Ramon and M. Yus, J. Org. Chem., 2011, 76, 5547; (b)R. Cano,
M. Yus and D. J. Ramon, ACS Catal., 2012, 2, 1070; (c)R. Cano, M. Yus and D. J.
Ramon, Tetrahedron, 2012, 68, 1393; (d)R. Cano, M. Yus and D. J. Ramon,
Chem. Commun., 2012, 48, 7628; (e)E. Nehlig, B. Waggeh, N. Millot, Y.
Lalatonne, L. Motte and E. Guenin, Dalton Trans., 2015, 44, 501; (f)F. Zhang, J.
Jin, X. Zhong, S. Li, J. Niu, R. Li and J. Ma, Green Chem., 2011, 13, 1238.
A.-H. Lu, E. L. Salabas and F. Schuth, Angew. Chem. Int. Ed., 2007, 46, 1222.
1
1.
1
8

Page 19 of 20
New Journal of Chemistry
View Article Online
1
2.
(a)D. Bourissou, O. Guerret, F. P. Gabbai and G. Bertrand, Chem. Rev., 2000,
DOI: 10.1039/C5NJ00803D
1
00, 39; (b)M. Poyatos, J. A. Mata and E. Peris, Chem. Rev., 2009, 109, 3677;
(c)W. A. Herrmann and C. Kocher, Angew. Chem. Int. Ed., 1997, 36, 2163.
1
1
3.
4.
(a)D. J. Nelson and S. P. Nolan, Chem. Soc. Rev., 2013, 42, 6723; (b)O. Schuster,
L. Yang, H. G. Raubenheimer and M. Albrecht, Chem. Rev., 2009, 109, 3445.
(a)A. Azua, J. A. Mata, E. Peris, F. Lamaty, J. Martinez and E. Colacino,
Organometallics, 2012, 31, 3911; (b)S. Sabater, J. A. Mata and E. Peris, Chem.
Eur. J., 2012, 18, 6380; (c)A. Azua, J. A. Mata and E. Peris, Organometallics,
2
1
1
2
011, 30, 5532; (d)A. Zanardi, J. A. Mata and E. Peris, Chem. Eur. J., 2010, 16,
0502; (e)A. Zanardi, J. A. Mata and E. Peris, J. Am. Chem. Soc., 2009, 131,
4531; (f)M. Albrecht, R. H. Crabtree, J. Mata and E. Peris, Chem. Commun.,
002, 32.
1
5.
W. A. Herrmann, L. J. Goossen and M. Spiegler, J. Organomet. Chem., 1997,
47, 357.
5
1
1
1
1
6.
7.
8.
9.
R. Fiammengo, K. Musílek and A. Jäschke, J. Am. Chem. Soc., 2005, 127, 9271.
R. B. N. Baig and R. S. Varma, Chem. Commun., 2012, 48, 2582.
R. C. Clark and J. S. Reid, Acta Crystallogr. A, 1995, 51, 887.
(a)G. M. Sheldrick, Acta Crystallogr. A, 2008, 64, 112; (b)O. V. Dolomanov, L. J.
Bourhis, R. J. Gildea, J. A. K. Howard and H. Puschmann, J. Appl. Crystallogr.,
2
009, 42, 339.
2
2
0.
1.
A. C. Albeniz and R. H. Crabtree, Abstr. Pap. Am. Chem. Soc., 1991, 202, 384.
(a)R. Corberan, M. Sanau and E. Peris, Organometallics, 2006, 25, 4002; (b)M.
Viciano, M. Feliz, R. Corberan, J. A. Mata, E. Clot and E. Peris, Organometallics,
2
007, 26, 5304; (c)R. Corberan, M. Sanau and E. Peris, J. Am. Chem. Soc., 2006,
28, 3974.
1
2
2
2.
3.
(a)J. S. M. Samec, J. E. Bäckvall, P. G. Andersson and P. Brandt, Chem. Soc. Rev.,
006, 35, 237; (b)S. Gladiali and E. Alberico, Chem. Soc. Rev., 2006, 35, 226;
c)R. Noyori, M. Yamakawa and S. Hashiguchi, J. Org. Chem., 2001, 66, 7931.
2
(
(a)R. Corberan and E. Peris, Organometallics, 2008, 27, 1954; (b)R. Maity, S.
Hohloch, C.-Y. Su, M. van der Meer and B. Sarkar, Chem. Eur. J., 2014, 20, 9952;
(c)J. Campos, U. Hintermair, T. P. Brewster, M. K. Takase and R. H. Crabtree,
ACS Catal., 2014, 4, 973; (d)U. Hintermair, J. Campos, T. P. Brewster, L. M. Pratt,
1
9

New Journal of Chemistry
Page 20 of 20
View Article Online
DOI: 10.1039/C5NJ00803D
N. D. Schley and R. H. Crabtree, ACS Catal., 2014, 4, 99; (e)N. D. Schley, S.
Halbert, C. Raynaud, O. Eisenstein and R. H. Crabtree, Inorg. Chem., 2012, 51,
2313; (f)O. Eisenstein and R. H. Crabtree, New J. Chem., 2013, 37, 21; (g)S.
1
Sabater, M. Baya and J. A. Mata, Organometallics, 2014, 33, 6830; (h)X.-H. Zhu,
L.-H. Cai, C.-X. Wang, Y.-N. Wang, X.-Q. Guo and X.-F. Hou, J Mol Catal a-Chem,
2
014, 393, 134.
(a)A. Prades, R. Corberan, M. Poyatos and E. Peris, Chem. Eur. J., 2008, 14,
1474; (b)M. Albrecht, J. R. Miecznikowski, A. Samuel, J. W. Faller and R. H.
2
2
4.
5.
1
Crabtree, Organometallics, 2002, 21, 3596.
(a)D. Gnanamgari, A. Moores, E. Rajaseelan and R. H. Crabtree,
Organometallics, 2007, 26, 1226; (b)S. Gulcemal, A. G. Gokce and B. Cetinkaya,
Inorg. Chem., 2013, 52, 10601; (c)F. E. Fernandez, M. Carmen Puerta and P.
Valerga, Organometallics, 2012, 31, 6868; (d)S. Zhou, S. Fleischer, K. Junge, S.
Das, D. Addis and M. Beller, Angew. Chem. Int. Ed., 2010, 49, 8121; (e)G. Z.
Wang and J. E. Backvall, J. Chem. Soc.-Chem. Commun., 1992, 980; (f)J. R.
Miecznikowski and R. H. Crabtree, Polyhedron, 2004, 23, 2857.
2
2
6.
7.
(a)J. M. Perez, R. Cano, M. Yus and D. J. Ramon, Eur. J. Org. Chem., 2012, 4548;
(b)L. Tang, H. Sun, Y. Li, Z. Zha and Z. Wang, Green Chem., 2012, 14, 3423.
(a)M. Ahmed, A. G. M. Barrett, D. C. Braddock, S. M. Cramp and P. A.
Procopiou, Tetrahedron Lett., 1999, 40, 8657; (b)M. Gruttadauria, F. Giacalone
and R. Noto, Green Chem., 2013, 15, 2608; (c)J. S. Kingsbury, J. P. A. Harrity, P.
J. Bonitatebus and A. H. Hoveyda, J. Am. Chem. Soc., 1999, 121, 791; (d)H.
Clavier, S. P. Nolan and M. Mauduit, Organometallics, 2008, 27, 2287; (e)H.
Clavier, F. Caijo, E. Borre, D. Rix, F. Boeda, S. P. Nolan and M. Mauduit, Eur. J.
Org. Chem., 2009, 4254.
2
8.
(a)O. Saidi and J. M. J. Williams, in Iridium Catalysis, ed. P. G. Andersson, 2011,
vol. 34, pp. 77; (b)C. Wang, B. Villa-Marcos and J. Xiao, Chem. Commun., 2011,
4
7, 9773.
2
0