Preparation and use of DMF-stabilized iridium nanoclusters as methylation catalysts using methanol as the C1 source

Article abstract of DOI:10.1039/c6cc09279a

We report methylations of alcohols and anilines catalyzed by DMF-stabilized Ir nanoclusters using methanol as the C1 source. The DMF-stabilized Ir nanoclusters were prepared in one step and have diameters of 1-1.5 nm. They react in a borrowing-hydrogen reaction and are efficient methylation catalysts (TON up to 310?000).

Full text of DOI:10.1039/c6cc09279a

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DOI: 10.1039/C6CC09279A
Graphical Abstract
DMF-stabilized Ir nanoclusters showed an efficient catalytic activity in the
β-methylation of alcohols using methanol as the C1 source

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DOI: 10.1039/C6CC09279A
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Preparation and use of DMF‐stabilized iridium nanoclusters as
methylation catalysts using methanol as the C1 source
Kei Oikawa, Satoshi Itoh, Hiroki Yano, Hideya Kawasaki and Yasushi Obora*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
We report methylations of alcohols and anilines catalyzed by DMF‐ alcohols to give alcohol products.8d However, the methylation
stabilized Ir nanoclusters using methanol as the C1 source. The of secondary alcohol starting materials has not yet been
DMF‐stabilized Ir nanoclusters were prepared in one step and have reported. To achieve this, the development of a highly reactive
diameters of 1–1.5 nm. They react in a borrowing‐hydrogen catalyst is required.
reaction and are efficient methylation catalysts (TON up to
310,000).
Metal nanoclusters (M NCs) are proposed to be highly active
catalysts owing to their large surface areas and corner/edges
sites compared with those of bulk metals.9 To avoid
aggregation of the M NCs to bulk metal, external stabilizers (e.g.,
functionalized polymers, dendrimers, and metal oxides) and
reductants (e.g., NaBH4) are generally considered essential in a
typical M NC synthesis.10
As an alternative methodology, the solution synthesis of N,N‐
dimethylformamide (DMF)‐stabilized metal NCs (<2 nm) and
nanoparticles (NPs, >2 nm) of Au, Pt, Pd, and Cu have been
reported as surfactant‐free stable M NCs.11 Other than DMF,
this protocol does not require any external additives; DMF
serves as a solvent, reductant, and stabilizer in the metal NP
synthesis. The obtained Pd NCs and Cu NPs showed high
catalytic activity in cross‐coupling reactions, including Suzuki–
Miyaura, Mizoroki–Heck, and Ullmann couplings.11
Transition‐metal‐catalyzed C–C bond formation through
borrowing‐hydrogen (or hydrogen‐autotransfer) reactions has
attracted much attention for the manufacture of bulk and fine
chemicals.1 In particular, methylation is vital for the synthesis of
biologically active molecules2 Conventional methylation uses
iodomethane, diazomethane, dimethyl sulfate and dimethyl
carbonate as the common methylating reagent.3 However,
some reagents are dangerous as they have explosive or
corrosion properties.
The development of a green and
sustainable methylation process that uses methanol, an
abundant and renewable C1 source, is therefore desired by the
chemical industry.4
It is well known that Ir, Ru, and Rh complexes are efficient
catalysts for the dehydrogenation of alcohols, and these
Herein, we present the solution synthesis of DMF‐stabilized Ir
complexes have been utilized in
‐ or ‐alkylations involving
NCs and their use as a catalyst in the
using methanol as the C1 source.
‐methylation of alcohols
borrowing‐hydrogen reactions.5 Our group has developed Ir‐
catalyzed borrowing‐hydrogen reactions for various substrates
using alcohols as the alkylation reagents.6 However, it has been
difficult to apply methanol as the alkylation reagent owing to its
high dehydrogenation energy compared with that of other
alcohols, such as ethanol and higher alcohols.7
Recently, the transition‐metal‐catalyzed dehydrogenation of
methanol and its use in C1 alkylation has been developed.8
Donohoe,8a,b Andersson,8c and our group8e have reported the
‐
methylation of ketones using methanol. Among the previously
reported studies, Beller has reported the methylation of
Fig. 1 Left: Transmission electron microscopy image of DMF‐stabilized Ir
nanoclusters (NCs). Bar length: 2 nm. Right: Dynamic light scattering spectrum of Ir
NCs.
a. Department of Chemistry and Materials Engineering, Faculty of Chemistry,
Materials and Bioengineering, Kansai University, Suita, Osaka 564‐8680, Japan.
Fax: +81‐6‐6339‐4026; Tel: +81‐6‐6368‐0876; E‐mail: obora@kansai‐u.ac.jp.
b. †Electronic Supplementary Informaꢀon (ESI) available: Experimental procedures
and compound characterization data. See DOI:
The DMF‐stabilized Ir NCs are simply prepared according to the
following one‐step procedure. A solution of 0.5 mL of 0.1 M
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1‐3 | 1
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aqueous IrCl3 was added to 50 mL of DMF at 140 °C, and the
Journal Name
The results showed that the DMF properly proVtieewctAertdiclethOenlinIer
DMF solution was heated at 140 °C in a flask fitted with a reflux metal and weight loss began at around 1D00OI°:C10..1039/C6CC09279A
condenser for 10 h.
On heating the solution, a portion of the protective DMF is
The resulting orange solution of Ir NCs in DMF was expected to be liberated from the Ir metal, generating Ir NCs
photoluminescent, as observed for another M NCs.11 The with partially open sites that could act as active catalysts in the
maximum emission wavelength was around 450 nm with UV methylation reaction.
excitation at 350 nm (Fig. S1‐2, ESI†).
Initially, we examined the Ir‐NC‐catalyzed ‐methylation of 1‐
High resolution transmission electron microscopy (HR‐TEM) phenyl‐1‐ethanol (1a) with methanol (Table 1). The reaction of
and showed that the diameters of the NCs were about 1–1.5 nm 1a (1 mmol) with methanol (2 mL) was performed in the
(Fig. 1 and Fig S3 (ESI†)). To identify the binding layers presence of Ir NCs (0.1 mol%) and Cs2CO3 (1 mmol) as the base
surrounding the Ir NCs, 1H NMR and FT‐IR analyses of the NCs at 150 °C for 24 h under Ar. Ir NCs were more effective than
were performed (Figures in the ESI†). In the 1H NMR spectrum, other Ir complexes for the
‐methylation, and gave the desired
the signal at 8.15 ppm, assigned to the formyl proton of DMF product in 68% total yield with high alcohol selectivity (entry 1).
binding to the Ir NCs, was shifted to lower field compared with Under these conditions, we did not observe any metal
that in free DMF (7.92 ppm) (Fig. S4, ESI†). The IR spectrum aggregation or precipitation from the Ir NCs solution after the
contains a peak at around 1670 cm−1, which corresponds to the reaction. Use of IrCl3 (0.1 and 5 mol %), which was the starting

(C=O) vibration of DMF and suggests that the DMF molecules complex for the synthesis of the DMF‐stabilized Ir NCs, resulted
interact with the Ir NCs (Fig. S7, ESI†).
in lower yields, and the ketone product (3a) was preferentially
obtained (entries 2‐3).
Table 1. Screening of reaction conditionsa
Table 2. Ir NC‐catalyzed ‐methylation of alcohols and N‐methylation of aniline
derivatives with methanola
Ir (cat.)
Alcohols
Base
or
Entry
[Ir]
Base
Conv.
(1a) [%]
Total yield [%]b,c
Selectivity (2a : 3a)d
+
MeOH
Methylated product
150 °C, 24 h
Anilines
1
Ir NCs
IrCl3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
KOH
>99
48
68 (85 : 15)
16 (46 : 54)
16 (33 : 67)
21 (28 : 72)
18 (- : >99)
25 (25 : 75)
53 (87 : 13)
52 (87 : 13)
31 (75 : 25)
n.d.
Methylated product
Total yield [%]a / Selectivity (2 : 3)b
2
3e
4e
5e,f
6e
7
IrCl3
49
[IrCl(cod)]2
[IrCl(cod)]2/PPh3
[Cp*IrCl2]2
Ir NCs
48
44
2a
2b
2c
2d
87 (94 : 6)
94 (87 : 13)
93 (>99 : - )
90 (69 : 32)
44
>99
>99
59
8
Ir NCs
KOtBu
K2CO3
none
2e
2f
2gc
2hc
72 (>99 : - )
9
Ir NCs
87 (94 : 6)
87 (97 : 3)
94 (>99 : - )
10
11
12
13g
14h
15h,i
Ir NCs
26
none
Cs2CO3
KOH
30
n.d.
none
60
9 (>99 : -)
3 (48 :52)
quant [87]c (94 :6)
24 (94 : 6)
2ic
2jc,d
82 (>99 : - )
2kc
54 (>99 : - )
78 (>99 : - )
Ir NCs
Cs2CO3
Cs2CO3
Cs2CO3
11
Ir NCs
>99
57
Ir NCs
5ae
78
5be
80
5ce
87
aReaction conditions: 1a (1 mmol) was allowed to react with methanol (2 mL),
redissolved Ir NCs (0.1 mol%) and Cs2CO3 (1 mmol) at 150 °C for 24 h. bGC yield
based on 1a used. cThe numbers in the square bracket show isolated yields. dThe
numbers in parentheses show the selectivity for the alcohol and ketone products.
aConditions: same as Table 1 in entry 12 unless otherwise noted. All yields are
isolated yields. bThe numbers in parentheses show the selectivity for the alcohol
(2) and ketone (3) products. cFor 48 h. Cs2CO3 (1 mmol). Anilines (4) (1 mmol),
h
eIr catalyst (5 mol%) was used. fPPh3 (10 mol%) was used. gAt 100 °C. Cs2CO3 (3
d
e
mmol) was used. iIr NCs (0.001 mol%) for 48 h.
methanol (2 mL), Ir NCs (0.1 mol%) and Cs2CO3 (0.5 mmol) at 150 °C for 24 h.
Thermogravimetric analysis–differential thermal analysis (TG‐
DTA) was measured to confirm the thermal stability of the Ir NCs
(Fig. S8, ESI†).
[Cp*IrCl2]2 and [IrCl(cod)]2, known to be a good catalyst for
hydrogen‐transfer reactions, including

‐methylation,8d also
2 | J. Name., 2012, 00, 1‐3
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resulted in lower yields and preferentially gave the ketone 310,000 was achieved for the N‐methylation of aniline (
View Articl4e aO)nlitnoe
5a by lowering the catalyst loading (Table S2, ESI ).
†
14
DOI: 10.1039/C6CC09279A
product (3a) (entries 4‐6).
The poisoning test in the presence of mercury (5 equiv) with
1a was carried out under the conditions using the Ir NCs (entry
Furthermore, to examine the mechanism for the generation of
as intermediate in this reaction, reaction of acetophenone
3
1), which gave the methylated products in 33% yield (2a
72:28) with 81% conversion, whereas only trace amount of NCs catalyst. As a result, 2a was obtained as major product
2a
3a with low conversion occurred with IrCl3 catalyst (under along with the formation of 3a (Scheme S1, ESI†). In addition,
the conditions of entry 2). when the reaction of propiophenone with paraformaldehyde
:
3a
=
with methanol was carried out under these conditions using Ir
/
These results indicate that the DMF‐stabilized Ir NCs serves as was performed, 2a was obtained exclusively (Scheme S2 and
dominant active catalyst and the DMF molecules would remain Table S3 in ESI†). These results indicate that acetophenone (i.e.
bound to the iridium clusters during the catalytic cycle.
ketones 3) and formaldehyde (from methanol) would pertain to
Other bases, for example KOH, KOtBu, and K2CO3, gave the this reaction as intermediates.
desired product in lower yields than the reaction with Cs2CO3
(entries 7‐9).
Furthermore, reactions of benzophenone/styrene with
methanol were performed by using the Ir NCs catalyst system,
Recently, transition‐metal‐free alkylations have been the corresponding hydrogenated products, benzhydrol/
reported.12 We therefore examined the reaction without Ir NCs ethylbenzene were obtained in high yields (Scheme S3, ESI†).
and confirmed that the reaction did not proceed (entries 11‐12). We thus concluded that the Ir NCs serves as efficient catalyst for
Temperature was an important factor for this reaction, and the hydrogen transfer reactions.
optimum temperature was 150 °C. These results agree with the
TG‐DTA data showing that the Ir NCs are stable up to 100 °C
(entry 13).
On the basis of these results, we propose a mechanism for the

‐methylation of alcohols using methanol (Scheme 1). The
transformation begins with the oxidation of the alcohol to the
A clear improvement in the yield was observed when an ketone/aldehyde and oxidation of methanol to formaldehyde
increased amount of Cs2CO3 (3 mmol) was used (entry 14). with the generation of an Ir NCs‐hydride species. Subsequently,
The ‐methylation reaction proceeded with low catalyst loading, an aldol‐type reaction of the resulting ketone/aldehyde with

and the dimethyl product was obtained in 24% yield when 0.001 formaldehyde gives the enone. Finally, the desired alcohol
mol% Ir NCs was used as the catalyst. Furthermore, we achieved product is obtained through hydrogenation of the unsaturated
a turnover number (TON) of 48,000 for this reaction, and thus bonds in the enone by the Ir NCs hydride species. To indicate
concluded that Ir NCs have high catalytic activity (entry 15).
‐Methylations of various secondary alcohols were 2a to 3a under these conditions between the Ir NCs and IrCl3
performed under the optimized conditions (Table 2). Reaction catalysts. As a result, conversion of 2a to 3a was found to be
of 1‐phenyl‐1‐ethanol derivatives 1b 1c, and 1d with methanol very low (10 %) with the Ir NCs, whereas 26% conversion to 3a
afforded the corresponding methylation products 2b
2c, and 2d was observed with IrCl3 (Scheme S3, ESI†). These results indicate
characteristics of the Ir NCs, we examined the transformation of

,
,
in 94%, 93%, and 90% yield, respectively. Through these results, that the Ir NCs possess advantage to the selective formation of
we found that the reaction could be adapted for compounds 2a (in preference to 3a) in this types of reactions.
with electron‐donating and ‐withdrawing groups on the phenyl
ring. Reaction of 1‐(2‐naphthyl)ethanol 1e gave product 2e in
excellent yield. When 1‐phenylpropanol 1f, which contains one
reaction point, was used, monomethylation product 2f was
obtained in excellent yield.
Next, ‐methylations of primary alcohols were investigated.
However, it was necessary to adjust the amount of base used
because the acidity of the hydrogen atom was different.13 2‐
phenylethanol 1g gave desired product 2g in high yield and with
good selectivity. Reactions of 2‐arylethanols containing
electron‐donating or ‐withdrawing groups gave the
corresponding products in high yields. Interestingly, all products
from the primary alcohols were obtained with higher selectivity
than those obtained from the secondary alcohols. This is
probably a result of the different stabilities of the products due
to conjugation. However, aliphatic alcohol such as n‐decanol
was not suitable under these conditions.
To expand the scope of this methylation reaction using
methanol as the C1 source, we next performed N‐methylations
of primary anilines (Table S1; optimization results shown in the
ESI†). Reactions of primary anilines with electron‐donating and
‐withdrawing groups gave the corresponding products in
Scheme 1. A plausible‐methylation pathway
In summary, we have successfully synthesized DMF‐stabilized
Ir NCs and developed a methylation reaction using methanol as
the C1 source. From TEM and DLS analyses, we found that the
particle size of the DMF‐stabilized Ir NCs was 1–1.5 nm. 1H‐NMR,
TG‐DTA, and FT‐IR results indicated that the surface of the Ir NCs
was protected by DMF molecules. In addition, the Ir NCs have
high catalytic activity towards ‐ and N‐methylations, and an
extremely high TON of 310,000 was achieved for the N‐
methylation of primary amines by lowering the catalyst loading
excellent yields (Table 2, 5a‐c). To our delight, a high TON of
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to 0.0001 mol% (10‐4 mol%). The reaction is widely applicable to
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
Further studies towards a detailed understanding of the overall
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