Non-nanogold catalysis of carbon monoxide oxidative amination

Article abstract of DOI:10.1021/ja065706t

Bulk metallic gold particles (～1000 nm) catalyze the reaction of CO, O₂, and primary amines (R-NH₂) to give ureas (RNH)₂C=O. Isocyanates (R-N=C=O) are identified as intermediates in the reactions and are shown to react with primary and secondary amines to give ureas under the conditions of the gold-catalyzed reactions. Although many recent studies indicate that nanosized particles of gold are required for the catalytic oxidation of CO, the results presented in this paper show that bulk gold is capable of catalyzing the oxidative amination of CO under mild conditions (45 °C, 1 atm CO and O₂). Copyright

Full text of DOI:10.1021/ja065706t

Published on Web 10/20/2006
Non-Nanogold Catalysis of Carbon Monoxide Oxidative Amination
Bolin Zhu and Robert J. Angelici*
Ames Laboratory and Department of Chemistry, Iowa State UniVersity, Ames, Iowa 50011-3111
Received August 7, 2006; E-mail: angelici@iastate.edu
In recent years, numerous papers have described the use of
Scheme 1
supported nanogold particles for catalysis of the reaction of CO
2 2
and O to form CO
, as well as a variety of other reactions.1
-3
Several factors control the activities of nanogold catalysts: (a) the
size of the Au particle (less than 5 nm), (b) the metal oxide support,
(
c) the oxidation state of the Au, and (d) the method of catalyst
1-7
8
preparation and pretreatment.
We recently reported bulk gold
catalysis of the oxidative amination of isocyanides to give carbo-
diimides according to eq 1. The catalyst was gold powder with a
Table 1. Gold Metal-Catalyzed Reactions of Primary Amines with
CO and O2 According to eq 3a
Products
entry
amine/mmol
urea (%)
isocyanate (%)
3
particle size of approximately 1 µm (10 nm). On the basis of studies
1
2
3
4
5
6
7
8
9
n-BuNH2 (0.5)
s-BuNH2 (0.5)
t-BuNH2 (0.5)
PhNH2 (0.5)
p-MeC6H4NH2 (0.5)
o-MeC6H4NH2 (0.5)
(n-Pr)2NH (0.5)
PhNH2/(n-Pr)2NH (0.5/0.5)
n-BuNH2/(n-Pr)2NH (0.5/0.5)
46
12
2
<1, trace
9
of the adsorption of isocyanides on the Au powder, identification
0
0
3
3
1
of the carbodiimide product, and kinetic studies, we proposed a
mechanism (Scheme 1) in which the rate-determining step (a) is
amine attack on the adsorbed isocyanide carbon. Intermediate A
or B then reacts by transferring hydrogen atoms to O
presumed H O byproduct. Step (a) is analogous to the well-known
attack of amines on isocyanide ligands in transition metal com-
plexes.10 It is also known11 that the CtO ligand, which is
isoelectronic with isocyanides, is attacked by amines (eq 2) when
21
24
11
0
2
to give the
2
19
b
c
3, PhNCO
0, n-BuNCO
trace, n-BuNCO
d
39 , 5
41d, 25e
10
n-BuNH /PhNH (0.5/0.5)
2
2
2
, PhNCO
ν(CO) is greater than
a
At 45 °C in 5 mL of MeCN solvent for 24 h with 1.00 g of Au powder;
b
c
CO and O2 total pressure ) ∼1 atm. PhNH(CdO)N(n-Pr)2. (n-
Bu)NH(CdO)N(n-Pr)2. (n-BuNH)2CO. e PhNH(CdO)NH(n-Bu).
d
to balloons containing an approximately 2.5:1 ratio of CO and O
2
(
∼1 atm total pressure) were inserted into the septum covering the
2
opening of the tube; the amounts of CO and O were in excess as
approximately 2000 cm- . Although CO is not detectably
1
compared with that of the amines. Products of these reactions (Table
1) were identified by their GC-mass spectra, and the yields were
determined by GC using authentic samples of the products as
calibrants. Under the standard reaction conditions (1.00 g of Au
powder, 45 °C, 24 h, MeCN solvent), the straight chain primary
2
1c
adsorbed on bulk gold at room temperature, CO adsorbed at low
temperatures on bulk12 Au or on Au nanoparticles gives ν(CO)
values of approximately 2100 cm-1, which suggests that CO
adsorbed on Au should be susceptible to attack by amines. If the
reaction of CO with primary amines were to follow the pathway
13
amine n-BuNH gives the highest yield (46%) of the urea product;
for the reaction of isocyanides with primary amines and O
and Scheme 1), one would expect the product to be an isocyanate
OdCdN-R), which in many cases would react with another
primary amine molecule to give the final urea product (eq 3).
Herein, we report that this new
2
(eq 1
the branched chain amine (s-BuNH ) gives a much lower yield
2
(12%); and t-BuNH gives a very low yield (2%). Aniline and
2
(
p-MeC H NH give 21-24% yields, while o-MeC H NH gives a
6
4
2
6
4
2
lower yield (11%). The above results indicate that the highest yields
are achieved by the most basic, least sterically hindered aliphatic
amines.
We propose that the mechanism (Scheme 2) for reaction 3 is
similar to that (Scheme 1) for the reaction of isocyanides with
amines and O (eq 1). This proposal is based on evidence for the
2
reaction of CO does occur, and it is catalyzed by bulk gold
powder (∼1000 nm particles) that does not have nanosized
dimensions, under mild temperatures (45 °C) and at atmospheric
intermediacy of the isocyanate (R-NdCdO) in reaction 3. Of
course, this mechanism requires that secondary amines not give
urea, which is observed in the reaction of (n-Pr)
7). On the other hand, when equimolar PhNH
reacted with CO and O , a 19% yield of the mixed urea PhNH(Cd
O)N(n-Pr) is obtained, but diphenylurea ((PhNH) CdO) is not
2
NH (Table 1, entry
pressures of the CO and O
2
gases.
2
and (n-Pr) NH are
2
The reactions were performed by stirring (magnetic stir bar) 1.00
g of shiny gold powder with 5 mL of a solution containing 0.50
mmol of the primary amine in a glass tube. Syringe needles attached
2
2
2
formed (entry 8). This result is consistent with the mechanism in
14460
9
J. AM. CHEM. SOC. 2006, 128, 14460-14461
10.1021/ja065706t CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
Scheme 2
supported on an anion exchange styrene divinylbenzene polymer
(
Amberlite IRA-400). However, these reactions were conducted
under much more strenuous conditions (175 °C, 4.0 MPa (40 atm)
of CO and 1.0 MPa (10 atm) of O ) than were used in the current
2
studies with gold powder. On the basis of this report, it appears
that nanogold is less active as a catalyst in reaction 3 than bulk
gold.
Considering the high catalytic activity of bulk gold, we explored
the possibility that its less expensive congeners would also be
catalysts of reaction 3. However, when 1.0 g of commercial Ag
(2-3.5 µm) or Cu (1-5 µm) powder was stirred with n-BuNH
2
and CO/O under the conditions in Table 1, the n-BuNH did not
2
2
react and no urea product was detected. Although the primary
purpose of this communication is to note the high catalytic activity
of bulk gold in an oxidative reaction of CO, the application of these
results to a practical synthesis of ureas would make more efficient
use of the gold metal by supporting it on high surface area materials;
this is currently being explored.
which Ph-NdCdO is formed initially, and it reacts faster with
(
n-Pr)
2
NH than with PhNH
2
, a known trend (R
2
NH > RNH
2
>
14
NH
3
> PhNH
2
)
in isocyanate reactions with amines. We also find
that when 0.10 mmol of PhNCO is reacted in the absence of gold
with 0.50 mmol of both (n-Pr) NH and PhNH under the conditions
in Table 1, the only product is PhNH(CdO)N(n-Pr) . Also
consistent with the intermediacy of an isocyanate is the reaction of
equimolar n-BuNH and (n-Pr) NH, which gives (entry 9) a ratio
39/5 ) 7.8) of the n-BuNH(CdO)N(n-Pr) /(n-BuNH) CdO
2
2
2
Acknowledgment. This research was supported by the U.S.
Department of Energy, Office of Science, Office of Basic Energy
Sciences, Division of Chemical Sciences under Contract W-7405-
Eng-82 with Iowa State University.
2
2
(
2
2
products, which is nearly the same as the ratio (7.2) of products
obtained from the reaction of 0.10 mmol of n-BuNdCdO with
0
2 2
.50 mmol of both n-BuNH and (n-Pr) NH in the absence of gold
Supporting Information Available: Experimental procedures for
the catalytic reactions. This material is available free of charge via the
Internet at http://pubs.acs.org.
under the standard reaction conditions. Also consistent with an
isocyanate intermediate is the observation (entry 10) that the reaction
of equimolar n-BuNH
O (41%) that is higher than that of the mixed urea product
PhNH(CdO)NH(n-Bu) (25%), while no (PhNH) CdO is produced
because of the weakly nucleophilic character of PhNH as compared
with n-BuNH in its reaction with the Ph-NdCdO intermediate.
Further evidence for the formation of isocyanates as intermediates
in reaction 3 is the detection of small amounts of the isocyanates
in reactions of the anilines (entries 4-6, 8, and 10).
2 2 2
and PhNH gives a yield of (n-BuNH) Cd
References
2
(
1) (a) Bond, G. C.; Thompson, D. T. Appl. Catal. A: Gen. 2006, 302, 1-4.
b) Thompson, D. T. Top. Catal. 2006, 38, 231-240. (c) Bond, G. C.;
Thompson, D. T. Catal. ReV. Sci. Eng. 1999, 41, 319-388.
2
(
2
(2) (a) Chen, M. S.; Goodman, D. W. Catal. Today 2006, 111, 22-33. (b)
Kung, M. C.; Costello, C. K.; Kung, H. H. Catalysis 2004, 17, 152-165.
(
(
(
3) Hutchings, G. J.; Haruta, M. Appl. Catal. A: Gen. 2005, 291, 2-5 and
other articles in this issue.
4) Janssens, T. V. W.; Carlsson, A.; Puig-Molina, A.; Clausen, B. S. J. Catal.
Bulk gold catalysis of the oxidative amination of carbon
2006, 240, 108-113.
monoxide is surprising in view of the long-known low catalytic
activity of gold powder for the oxidation of CO.1
5) (a) Miller, J. T.; Kropf, A. J.; Zha, Y.; Regalbuto, J. R.; Delannoy, L.;
-3,15
We were,
Louis, C.; Bus, E.; van Bokhoven, J. A. J. Catal. 2006, 240, 222-234.
(
b) Weiher, N.; Bus, E.; Delannoy, L.; Louis, C.; Ramaker, D. E.; Miller,
therefore, concerned that the powdered gold used in these studies
contained some nanogold particles, which were the actual catalyst.
J. T.; van Bokhoven, J. A. J. Catal. 2006, 240, 100-107.
(6) Overbury, S. H.; Schwartz, V.; Mullins, D. R.; Yan, W.; Dai, S. J. Catal.
2006, 241, 56-65.
Because it is known1
-3,15
that nanogold catalyzes the reaction of
even below room temperature, we studied
this reaction under the conditions in Table 1; however, no CO
detectable if the yield were even only 0.1%) was produced, which
suggests that nanogold is not involved in the catalysis of reaction
. To check for possible soluble catalytic gold species, a reaction
of n-BuNH , CO, and O with 1.0 g of Au powder under the
standard conditions was stopped after 6 h when the urea yield was
3%. The reaction solution was decanted from the Au powder and
put under the standard CO/O atmosphere. Reheating the solution
to 45 °C and stirring the solution for 15 h gave no additional urea
beyond the 13% produced in the presence of the Au powder). This
(
7) Casaletto, M. P.; Longo, A.; Venezia, A. M.; Martorana, A.; Prestianni,
2 2
CO and O to form CO
A. Appl. Catal. A: Gen. 2006, 302, 309-316.
2
(8) Lazar, M.; Angelici, R. J. J. Am. Chem. Soc. 2006, 128, 10613-10620.
(
9) (a) Robertson, M. J.; Angelici, R. J. Langmuir 1994, 10, 1488-1492. (b)
Shih, K.-C.; Angelici, R. J. Langmuir 1995, 11, 2539-2546. (c) Ontko,
A. C.; Angelici, R. J. Langmuir 1998, 14, 1684-1691. (d) Ontko, A. C.;
Angelici, R. J. Langmuir 1998, 14, 3071-3078.
(
3
(
10) Michelin, R. A.; Pombeiro, A. J. L; Guedes da silva, M. F. Coord. Chem.
2
2
ReV. 2001, 218, 75.
(11) Angelici, R. J. Acc. Chem. Res. 1972, 5, 335-341.
(12) (a) Chen, M.; Cai, Y.; Yan, Z.; Goodman, D. W. J. Am. Chem. Soc. 2006,
1
1
28, 6341-6346. (b) Gottfried, J. M.; Schmidt, K. J.; Schroeder, S. L.
2
M.; Christmann, K. Surf. Sci. 2003, 536, 206-224.
(13) Lemire, C.; Meyer, R.; Shaikhutdinov, Sh. K.; Freund, H.-J. Surf. Sci.
2004, 552, 27-34.
(
(
(
14) Entelis, S. G.; Nesterov, O. V. Russ. Chem. ReV. 1966, 35, 917.
15) Sanchez, A.; Abbet, S.; Heiz, U.; Schneider, W.-D.; H a¨ kkinen, H.; Barnett,
R. N.; Landman, U. J. Phys. Chem. A 1999, 103, 9573-9578.
result indicates that soluble species (ligated nanogold particles or
Au complexes) are not the catalytically active species.
An earlier report16 showed that the reaction of anilines with CO
(16) Shi, F.; Deng, Y. J. Catal. 2002, 211, 548-551.
and O
2
to give diaryl ureas was catalyzed by nanogold particles
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J. AM. CHEM. SOC.
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VOL. 128, NO. 45, 2006 14461