Rhodium nitrone complexes: Efficient catalyst for carbonylation of methanol

Article abstract of DOI:10.1016/S1381-1169(01)00338-7

Newly synthesized rhodium(I) carbonyl complexes of the type [Rh(CO)₂ClL] (1), where L = α,N-diphenylnitrone (a), α-styryl-N-phenylnitrone (b), N,N″-diphenyldinitrone (c), and α-(2-furyl)-N-phenylnitrone (d), acted as efficient catalyst precursors (maximum turn over number (TON) 1200-1400/min) for carbonylation of methanol to produce acetic acid and its ester at 150 ± 10°C and 20 bar. When the reaction was carried out for 30 min, the precursor complex 1a-d showed 48, 51, 54, and 52% conversion of methanol, respectively, with corresponding TON 691, 734, 777, and 748. Under the same experimental condition, the well-know catalyst precursor [Rh(CO)₂I₂]^- generated in situ from [Rh(CO)₂Cl]₂ showed only 30% conversion with a TON 430. Thus, the reaction time increased with increasing Ton. For a particular reaction time, the complexes 1a-d showed higher TON over the species [Rh(CO)₂I₂]^-, which might be attributed to higher stability of the complexes. The higher conversion or TON shown by the complexes 1b-d over 1a might be attributed to the possible chelate formation on the complexes through O, C=C(1b) or O, O(1c and 1d) donors of the ligands leading to higher stability of the intermediate complex formed during the catalytic reaction.

Full text of DOI:10.1016/S1381-1169(01)00338-7

Journal of Molecular Catalysis A: Chemical 178 (2002) 283–287
Letter
Rhodium nitrone complexes: efficient catalyst
for carbonylation of methanol
Pankaj Dasa, Monalisa Boruahb, Nandini Kumaria, Manab Sharmaa,
Dilip Konwarb, Dipak K. Duttaa,∗
Material Science Division, Regional Research Laboratory (CSIR), Jorhat 785 006, Assam, India
a
b
Organic Chemistry Division, Regional Research Laboratory (CSIR), Jorhat 785 006, Assam, India
Received 11 April 2001; accepted 17 August 2001
Abstract
Newly synthesized rhodium(I) carbonyl complexes of the type [Rh(CO)2ClL] (1), where L = ␣,N-diphenylnitrone (a),
␣-styryl-N-phenylnitrone (b), N,Nꢀ-diphenyldinitrone (c) and ␣-(2-furyl)-N-phenylnitrone (d), act as efficient catalyst pre-
cursors (maximum turn over number (TON) 1200–1400 per 90 min) for carbonylation of methanol to produce acetic acid and
its ester at temperature 150 10 ◦C and pressure 20 bar. © 2002 Elsevier Science B.V. All rights reserved.
Keywords: Rhodium; Nitrones; Catalyst; Carbonylation of methanol; Acetic acid
1. Introduction
alysts, particularly in carbonylation reaction [8–12].
Since the first report [8], the species [Rh(CO)2I2]−
is used as preferred commercial catalyst for carbony-
lation of methanol to acetic acid. Research activities
are continued to modify the catalyst either by incor-
porating different types of ligands in the metal com-
plex species [9–12] or by changing the metal center
[13–15]. Recently, BP Chemicals has commercial-
ized an iridium iodide based catalyst system (Cativa
process) for carbonylation of methanol to acetic acid
[14].
As a part of our work [16,17], i.e. the effects of
various ligands on rhodium catalyzed carbonylation
of methanol, we report in the present communi-
cation the synthesis of rhodium(I) complexes con-
taining different types of nitrones (Fig. 1) and their
application as catalyst precursors for carbonylation of
methanol.
The chemistry of nitrones is well established with
respect to its applications as catalysts as well as
reagents for organic synthesis [1–4], but their tran-
sition metal chemistry is not much developed. Lit-
erature survey reveals [5–7] that only a few nitrone
complexes of first row transition metals have so far
been reported, where nitrones are coordinated to metal
through oxygen donor. To the best of our knowledge,
till today there is no report on the rhodium nitrone
complex. Recently, rhodium metal complexes with
different types of donor ligands have gained much
attention because of their potential applications as cat-
∗ Corresponding author. Tel.: +91-376-370121x529;
fax: +91-376-370011.
E-mail address: dipakkrdutta@yahoo.com (D.K. Dutta).
1381-1169/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved.
PII: S1381-1169(01)00338-7

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P. Das et al. / Journal of Molecular Catalysis A: Chemical 178 (2002) 283–287
Fig. 1. Schematic presentation of different nitrones.
2. Experimental
2.1.2. Synthesis of complexes 1dꢀ
A red colored compound was formed on storing
the orange-red complex 1d in a desiccator for about
10 days. The product was thoroughly washed with
hexane–petroleum ether mixture.
Analytical data for the complexes are as follows:
1a: color (deep red); yield (93%); anal. calcd. for
C15H11ClNO3Rh in %: C (46.03), H (2.81), N (3.58);
found: C (45.93), H (2.76), N (3.49).
2.1. General methods
Elemental analyses were performed using Perkin-
Elmer 2400 elemental analyzer and IR spectra
were recorded in KBr disc on a Perkin-Elmer
System 2000 FT IR spectrophotometer. The 1H
(300.13 MHz) NMR spectra were recorded in CDCl3
1b: color (reddish yellow); yield (95%); anal. calcd.
for C17H13ClNO3Rh in %: C (48.92), H (3.11), N
(3.35); found: C (48.76), H (3.21), N (3.44).
1c: color (reddish brown); yield (91%); anal. calcd.
for C16H12ClN2O4Rh in %: C (44.23), H (2.76), N
(6.46); found: C (44.34), H (2.65), N (6.57).
1d: color (orange-red); yield (90%); anal. calcd. for
C13H9ClNO4Rh in %: C (40.94), H (2.36), N (3.67);
found: C (40.73), H (2.45), N (3.71).
solution on
a Bruker DPX-300 spectrophoto-
meter and chemical shift were reported relative to
SiMe4 as an internal reference. The carbonylation
reaction was carried out in a 150 ml teflon-coated
reactor (Laboratory High Pressure Reactor, model
HR-100, Berghof, Germany) fitted with a pres-
sure gauge and the reaction products were analyzed
by GC (Chemito 8510, FID). Nitrones [18] and
[Rh(CO)2Cl]2 [19] were prepared by literature
methods.
1dꢀ: color (red); yield (80%); anal. calcd. for
C12H9ClNO3Rh in %: C (40.73), H (2.54), N (3.96);
found: C (40.81), H (2.57), N (3.93).
2.1.1. Synthesis of complexes 1a–d
[Rh(CO)2Cl]2 (0.0515 mmol) was dissolved in
CHCl3 (40 cm3) and to this solution, a stoichiometric
quantity (Rh:L = 1:1) of the respective nitrone ligand
(a–d) (0.108 mmol) was added. The reaction mixture
was stirred for 2 h at room temperature under nitrogen
atmosphere. The solution was then evaporated to dry-
ness. The solid compound thus obtained was washed
several times with petroleum ether and finally dried
over CaCl2 in a desiccator.
2.2. Carbonylation of methanol
MeOH (3.1 ml, 0.076 mol), MeI (1 ml, 0.006 mol),
H2O (1 ml, 0.055 mol) and the complex 1 (0.045 mmol)
were charged into the reactor. The reactor was purged
with CO gas for about 5 min and then pressurized
with CO gas (6 bar, 0.036 mol at about 30 ◦C). The
pressure in the reactor was maintained at about 20 bar

P. Das et al. / Journal of Molecular Catalysis A: Chemical 178 (2002) 283–287
285
by increasing the temperature to 150 10 ◦C (reaction
time 30–90 min).
[20–22]. IR spectra also show a strong ␯(NO) stretch-
ing band in the range 1160–1200 cm−1 (Table 1),
which is about 25–40 cm−1 lower compared to the
corresponding free ligands value indicate formation
of rhodium–oxygen bond. The 1H NMR spectra of all
the complexes show (Table 1) a set of multiplates in
the range δ 6.8–7.7 ppm assigned to the aromatic pro-
ton and other characteristic resonances for different
types of –CH protons. Except 1d all the complexes
are very much stable in air.
For recycling experiment, the orange colored solu-
tion obtained after carbonylation reaction (90 min run)
was evaporated to dryness under reduced pressure to
get a dark brown solid mass. With this solid mass as
catalyst the carbonylation reaction was repeated by
maintaining the same experimental conditions as de-
scribed above.
It has been observed that the dicarbonyl complex 1d
on storing in a desiccator for about 10 days undergoes
a decarbonylation reaction resulting a monocarbonyl
complex [Rh(CO)Cl{␣-(2-furyl)-N-phenylnitrone}]
(1dꢀ) (Scheme 2) as indicated by a single terminal
␯(CO) value at 2000 cm−1. The ␯(COC) band occurs
at 1025 cm−1 which is about 15 cm−1 lower com-
pared to that of the complex 1d indicating chelate
formation through furyl oxygen atom. The chelation
of the ligand is further substantiated by 1H NMR
spectroscopy (Table 1) where the signal due to the
–CH proton attached to furyl oxygen atom is shifted
to downfield compared to that of the non-chelated
3. Results and discussion
3.1. Synthesis and characterization of complexes
The dimeric complex [Rh(CO)2Cl]2 undergoes
bridge splitting reaction with two molar equivalent
(Rh:L = 1:1) of nitrone ligands (a–d) to produce
complexes of the type [Rh(CO)2ClL](1a–d) (L =
nitrones) (Scheme 1). The IR spectra of the complexes
1a–d exhibit two almost equally intense terminal
␯(CO) bands in the range 2000–2080 cm−1 (Table 1)
indicating cis disposition of the two carbonyl groups
Scheme 1.
Table 1
IR and NMR spectral data of the complexes
Complexes
IR (cm−1
)
1H NMR (δ in ppm)b
␯(CO)
␯(NO)a
1a
1b
1c
1d
2000, 2070
2005, 2070
2020, 2080
2020, 2060
1170
1165
1160
1200
7.0–7.6 (m, 10H, C6H5), 8.2 (s, 1H, CH)
6.8–7.4 (m, 10H, C6H5), 7.6 (d, 2H, CH), 7.8 (d, 1H, CH N)
7.1–7.7 (m, 10H, C6H5), 8.4 (s, 2H, CH)
6.9–7.4 (m, 5H, C6H5), 7.9 (s, 1H, CH), 6.3 (s, 1H, C4H3O),
7.6 (s, 1H, C4H3O), 7.7 (s, 1H, C4H3O)
=
1dꢀ
2000
1200
6.8–7.3 (m, 5H, C6H5),7.9 (s, 1H, CH), 6.4 (s, 1H, C4H3O),
7.5 (s, 1H, C4H3O), 8.2 (s, 1H, C4H3O)
a Free ligand ␯(NO) value: 1200 (a); 1195 (b); 1185 (c); 1240 (d).
b m: multiplate; s: singlet; d: doublet.

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P. Das et al. / Journal of Molecular Catalysis A: Chemical 178 (2002) 283–287
Scheme 2.
complex 1d. In the complex 1dꢀ the CO is at trans to
nitrone oxygen atom because high electron withdraw-
ing nature of CO prefer a more electron donor group
at its trans position [23].
ysis of the product reveals that when the reaction was
carried out for 30 min, the precursor complex 1a–d
show 48, 51, 54 and 52% conversion of methanol,
respectively, with corresponding turn over number
(TON) 691, 734, 777 and 748. Under the same exper-
imental condition the well-known catalyst precursor
[Rh(CO)2I2]− generated in situ from [Rh(CO)2Cl]2
[24] show only 30% conversion with a TON 430. It
can be seen from Table 2 that as the reaction time
increases the TON also increases. The complexes
3.2. Carbonylation of methanol
The results of batch carbonylation of methanol to
acetic acid and its ester in presence of complexes 1a–d
as catalyst precursors are shown in Table 2. GC anal-
Table 2
Time resolved data of carbonylation reaction of methanola
Complex
Time (min)
Conversion (%)
Acetic acid (%)
Methyl acetate (%)
TONb
[Rh(CO)2Cl]2
30
60
90
30
43
64
4
9
12
26
34
52
430
618
921
1a
1b
1c
1d
30
60
90
90c
48
61
83
82
12
20
43
46
36
41
40
36
691
878
1195
1180
30
60
90
90c
51
67
91
89
14
28
55
56
37
39
36
33
734
964
1310
1281
30
60
90
90c
54
71
96
95
14
27
55
60
40
44
41
35
777
1022
1382
1367
30
60
90
90c
52
63
87
85
12
23
37
38
38
40
50
47
748
907
1252
1223
a Temperature: 150 10 ◦C; pressure: 20 bar.
b TON: mole of product per mole of catalyst.
c Recycled.

P. Das et al. / Journal of Molecular Catalysis A: Chemical 178 (2002) 283–287
287
1a–d after 90 min reaction time show a maximum
conversion of 83, 91, 96 and 87% respectively with
corresponding TON 1195, 1310, 1382 and 1252.
After completion of carbonylation reactions (90 min
run) the catalyst has been recovered. On recycling
the catalyst for second time almost the same amount
of conversion has been found (Table 2) indicating
longer durability as well as stability of the catalyst.
It has been observed that for a particular reaction
time the complexes 1a–d show higher TON over the
species [Rh(CO)2I2]−, which may be attributed to
higher stability of the complexes. The higher conver-
sion or TON shown by the complexes 1b–d over 1a
may be attributed to the possible chelate formation in
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Acknowledgements
The authors are grateful to the Director, Regional
Research Laboratory, Jorhat, Assam, India, for his
kind permission to publish the work. The Department
of Science and Technology (DST), New Delhi is also
acknowledged for a financial grant. One of the authors
(P. Das) thanks CSIR, New Delhi, for the award of
Senior Research Fellowship (SRF).
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