High density monolayer of diisocyanide on gold surface as a platform of supported Rh-catalyst for selective 1,4-hydrogenation of α,β- unsaturated carbonyl compounds

Article abstract of DOI:10.1039/c3cc48181f

A high density monolayer of diisocyanide on gold surface was utilized as a platform of supported Rh catalyst for selective 1,4-hydrogenation of α,β-unsaturated carbonyl compounds. The catalyst exhibited high turnover numbers in a range of 50-000 to 150-000 per Rh atom and showed steady catalyst performance over six recycle usages.

Full text of DOI:10.1039/c3cc48181f

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High density monolayer of diisocyanide on gold
surface as a platform of supported Rh-catalyst for
selective 1,4-hydrogenation of a,b-unsaturated
carbonyl compounds†
Cite this: Chem. Commun., 2014,
50, 5046
Received 25th October 2013,
Accepted 3rd March 2014
S. Jagtap,a Y. Kaji,b A. Fukuokaa and K. Hara*a
DOI: 10.1039/c3cc48181f
www.rsc.org/chemcomm
A high density monolayer of diisocyanide on gold surface was their potential application in catalysis will be increasingly
utilized as a platform of supported Rh catalyst for selective recognized. We thus envisioned the utilization of a monolayer of
1,4-hydrogenation of a,b-unsaturated carbonyl compounds. The isocyanide for catalyst preparation. Compared to commonly used
catalyst exhibited high turnover numbers in a range of 50 000 to self-assembled monolayers of alkanethiolate on gold surfaces,4
150 000 per Rh atom and showed steady catalyst performance over monolayers of isocyanide have been much less studied. Although
six recycle usages.
fundamental studies on formation of isocyanide monolayers5
as well as microscopic and spectroscopic studies6 have been
Self-assembling of organic molecules on surfaces automatically reported, its application has not been extensively explored.
forms ordered and densely packed molecular monolayers. This Immobilization of ruthenium phthalocyanine on an isocyanide
spontaneous phenomenon can be used as a low-cost method to monolayer was reported;7 however, application of the metal-
prepare functional materials for a wide range of applications immobilized monolayer was not shown. In this study, a densely-
including catalysts, sensors, photo- and electronic devices. packed monolayer of diisocyanide molecule on gold surface was
In addition to the ease in preparation as mentioned above, utilized for high-density immobilization of a rhodium complex.
another advantage of using self-assembled monolayers for The Rh-immobilized monolayer showed high catalyst turnover
catalyst preparation is its potential possibility to exhibit novel numbers in selective 1,4-hydrogenation of a,b-unsaturated
catalytic performance originated from its high density. For carbonyl compounds as well as steady catalyst performance
example, adjacent active centers on the top of a monolayer may over at least six recycle usages.
synergistically enable unique catalytic activity and selectivity.
As illustrated in Fig. 1, immersion of a gold surface (evapo-
However, such unique catalytic performance of high-density rated on glass) in a 1.0 mM solution of 4,400-p-terphenyl diiso-
monolayers has been reported in a limited number of publica- cyanide (TPDI) in CH2Cl2 for 24 h, followed by washing with
tions.1–3 Milstein et al. reported that monolayer and Langmuir– CH2Cl2, afforded [Au]–TPDI (1).5 Complexation of 1 with Rh
Blodgett films of a rhodium–bipyridine complex show high to obtain [Au]–TPDI–Rh (2) was carried out by immersion of
catalyst turnover numbers in hydrogenation of acetone as well
as a unique selectivity towards acetone in the presence of
butanone.1 We previously utilized a monolayer of alkanethiolate
on gold surface4 to prepare a high-density monolayer of rhodium–
phosphine complex for dehydrogenative silylation of alcohols
and obtained complete selectivity towards primary alcohols
over secondary alcohols.3 If more examples of advantageous
features are demonstrated with high-density monolayer catalysts,
a Catalysis Research Centre, Hokkaido University, Kita 21 Nishi 10, Kita-ku,
Sapporo, Hokkaido 001-0021, Japan. E-mail: hara@cat.hokudai.ac.jp;
Fax: +81 11 706 9139; Tel: +81 11 706 9136
b Graduate School of Science, Hokkaido University, Kita 10 Nishi 8, Kita-ku,
Sapporo, Hokkaido 060-0810, Japan
† Electronic supplementary information (ESI) available: Detailed procedures for
catalyst preparation and characterization; space-filling model of catalyst. See DOI:
10.1039/c3cc48181f
Fig. 1 Preparation of a high-density monolayer of Rh–diisocyanide on
gold surface.
5046 | Chem. Commun., 2014, 50, 5046--5048
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Table 1 Rh-catalyzed selective 1,4-hydrogenation of 2-cyclohexene-1-onea
Entry Catalyst
Yield 4 (%) Yield 5 (%) 1,4-Selectivity (%)
1
2
3
Au surface
[Au]–TPDI (1)
[Au]–TPDI–Rh (2) 77
[RhCl(CO)2]2
[RhCl(CO)2]2
BPI–RhCl(CO)2
5
0
2
0
2
15
19
4
71
—
97
35
34
87
4
8
10
26
5b
6c
Fig. 2 IR-RAS spectra of (a) [Au]–TPDI (1) and (b) [Au]–TPDI–Rh (2), and
(c) FT-IR spectrum of a 2 : 1 mixture of 4-biphenyl isocyanide (BPI) and
[RhCl(CO)2]2 (isocyanide/Rh ratio = 1) in CH2Cl2.
a
Reaction conditions: 2-cyclohexene-1-one 12 mmol, 2-cyclohexene-1-one/
b
Rh = 72 000, EtOH 120 mL, H2 2 MPa, 25 1C, 12 h. Reaction was carried
out for 20 h. Reaction was carried out for 2 h. 2 : 1 mixture of 4-biphenyl
c
isocyanide (BPI) and [RhCl(CO)2]2 was used as catalyst (isocyanide/Rh
ratio = 1).
monolayer 1 into a 5.0 mM solution of [Rh(CO)2Cl]2 in benzene
for 3 h, followed by washing with benzene.
IR-RAS measurement of [Au]–TPDI (1) revealed the stretching sluggish and no reaction, respectively, which implies that
modes of both Au surface bound (2195 cmꢀ1) and free (2122 cmꢀ1
catalytically active sites in [Au]–TPDI–Rh (2) are the immobilized
)
isocyanides as shown in Fig. 2a.5 In contrast, the IR-RAS spectrum Rh species. The hydrogenation in the presence of [RhCl(CO)2]2 as
of [Au]–TPDI–Rh (2) (Fig. 2b) showed no peak for the free a homogeneous catalyst resulted in a very low yield and low 1,4-
isocyanide whereas a new peak at 2165 cmꢀ1, which can be selectivity (entry 4). Notably, the reaction with [RhCl(CO)2]2 after
assigned to Rh-bound isocyanide, was observed. The peaks another 8 h gave no significant increase in yield (entry 5), showing
appearing at 2025 and 2091 cmꢀ1 are at similar wavenumbers severe deactivation of catalytically active species. Although
with those of a corresponding mixture of isocyanide and Rh another homogeneous control experiment using a 2 : 1 mixture
precursor (Fig. 2c), suggesting two carbonyl ligands are bound of 4-biphenyl isocyanide (BPI) and [RhCl(CO)2]2 facilitated the
to the Rh atom in cis-position in square-planer coordination. hydrogenation reaction, the 1,4-selectivity was o90% even in
These results indicate that all the free isocyanide groups on top an initial stage of the reaction where 26% of 4 was formed
of the diisocyanide monolayer (1) were coordinated to Rh in (entry 6). These experiments indicate that Rh–diisocyanide mono-
the preparation of [Au]–TPDI–Rh (2). ICP-MS measurement of layer 2 exhibits a uniquely high 1,4-selectivity. The catalyst turn-
[Au]–TPDI–Rh (2) gave a Rh concentration of 0.66 nmol cmꢀ2
.
over number obtained with [Au]–TPDI–Rh (2) was 55 000 in 12 h.
Based on this surface concentration of Rh and the assumption Increase in the reaction scale to 30 mmol of 3 afforded a catalyst
of 1 : 1 complex formation between Rh and isocyanide, a space- turnover number of 147 000 with 82% yield of 4 and 98% 1,4-
filling model of [Au]–TPDI–Rh (2) can be drawn as shown in selectivity in 12 h. This experiment demonstrates the feasibility
Fig. S1 (ESI†). Considering the sizes of TPDI molecule and the of scale-up of the present catalyst system. For practical pur-
Rh species, formation of the densely-packed monolayer in poses, chemical engineering methodologies, such as micro-
[Au]–TPDI (1) and complete Rh complexation in [Au]–TPDI–Rh fluidic systems, may further enhance the efficiency of the present
(2) could be reasonable.
catalytic system.
While abundant literature is available on the use of various
Choice of the solvent was found to be particularly important
homogeneous and heterogeneous catalysts for selective hydro- since it determines the activity and the stability of catalyst [Au]–
genation of a,b-unsaturated carbonyl compounds, there are still TPDI–Rh (2). The reactions in n-hexane9 and toluene resulted in
several problems associated with catalyst recyclability and lower lower turnover numbers of 25 000 and 19 000, respectively. The
catalyst turnover number.8 The catalytic activity of [Au]–TPDI– use of isopropanol gave further sluggish reaction, indicating that
Rh (2) was thus examined in the hydrogenation of 2-cyclohexen- a hydrogen transfer step is less likely involved in the reaction
1-one (3) as a model substrate (Table 1). Accordingly, a single mechanism. Although the use of THF afforded a relatively high
chip of [Au]–TPDI–Rh (2) (5 mm ꢁ 5 mm) was placed at the turnover of 66000, the catalyst activity of 2 completely disap-
bottom of a high-pressure reactor containing an EtOH solution peared after the second recycle run. In contrast, the catalytic
of 3 (12 mmol). Under these reaction conditions, the substrate/ performance of 2 in EtOH was maintained during repeated uses
catalyst (Rh atom) ratio is 72 000. The hydrogenation reaction without any significant loss of activity even after the fifth recycle
was conducted under 2 MPa at 25 1C for 12 h to afford run (Fig. 3). The total catalyst turnover number of 2 in EtOH over
1,4-hydrogenated product 4 in 77% yield with only 2% yield the six successive runs reached 892 000.
of further hydrogenated product 5, with no formation of
Application of [Au]–TPDI–Rh (2) for the hydrogenation of
1,2-hydrogenated product (2-cyclohexen-1-ol), so corresponding various a,b-unsaturated compounds is summarized in Table 2.
to a 1,4-selectivity of 97% (entry 3). Control experiments with a,b-Unsaturated ketones and esters were selectively converted
gold surface alone (entry 1) and [Au]–TPDI (1) (entry 2) gave very into the corresponding CQC hydrogenated products in high
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over that of the ketones. More detailed experiments to clarify the
origin of the selectivities are now under investigation in our lab.
In summary, a monolayer of diisocyanide on gold surface
was utilized to immobilize catalytically active Rh species on
the top of the monolayer. IR-RAS and ICP-MS analyses of the
Rh-immobilized surface confirmed the formation of a high-
density monolayer of Rh–diisocyanide. The Rh-immobilized
monolayer exhibited extremely high catalyst turnover numbers
and uniquely high 1,4-selectivity for hydrogenation reaction of
various a,b-unsaturated carbonyl compounds with high recycl-
ability. It can be noted that the catalyst preparation in the
present approach using high-density monolayer of metal com-
plexes has unique features against the conventional heterogeneous
and homogeneous catalyst systems. Further applications based on
the present approach by using simple wet processes are expected
to show its versatile significance in various catalytic reactions.
Prof. T. Kawaguchi and Prof. K. Shimazu are gratefully
acknowledged for preparation of gold surfaces, Prof. T. Watanabe
is acknowledged for ICP-MS measurement and Mr K. Namba for
experimental help. This work was financially supported by the
Asahi Glass Foundation, JSPS KAKENHI Grant Numbers
23105503, 25105703 and the Global COE Program (Project
No. B01: Catalysis as the Basis for Innovation in Materials
Science) from MEXT, Japan.
Fig. 3 Catalyst turnover numbers (TON) in recycled uses of [Au]–TPDI–Rh (2)
in hydrogenation of 2-cyclohexen-1-one. Reaction conditions: 2-cyclohexene-
1-one 30 mmol, 2-cyclohexene-1-one/Rh = 180 000, EtOH 120 mL, H2 2 MPa,
25 1C, 12 h.
Table 2 Hydrogenation of various a,b-unsaturated carbonyl compounds
using [Au]–TPDI–Rh (2)a
Yield 1,4-Selectivity
Entry Reactant
Product
(%)
(%)
1
77
97
Notes and references
2
3
4
5
6
78
71
81
98
99
499.5
499.5
499.5
499.5
499.5
¨
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a
Reaction conditions: a,b-unsaturated carbonyl compound 30 mmol,
b
substrate/Rh = 180 000, EtOH 120 mL, H2 2 MPa, 25 1C, 12 h. Citral
15 mmol.
yields (entries 1–6). Citral can also be converted to 1,4-hydrogenated
product, citronellal, in 65% yield with 98% 1,4-selectivity
(entry 7).
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5048 | Chem. Commun., 2014, 50, 5046--5048
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