Chelate and trans effect of P,O donor phosphine ligands on rhodium catalyzed carbonylation of methanol

Article abstract of DOI:10.1016/j.molcata.2011.10.023

Four complexes of the type [Rh(CO)Cl(η²-P,O-L)](1a,1b) and [Rh(CO)Cl(η¹-P-L)₂](2a,2b), where L = Ph ₂PC₆H₄-2-OCH₃(a) and Ph ₂PC₆H₄-2-CH₂OCH₃(b), have been synthesized by the reaction of [Rh(CO)₂Cl]₂ with appropriate mol equivalents of the ligands in CH₂Cl₂. The complexes show single intense ν(CO) bands in the range 1965-1989 cm ^-1 indicating the presence of terminal carbonyl groups. All the complexes have been characterized by elemental analyses, mass spectrometry, IR and multinuclear NMR (¹H, ³¹P and ¹³C) spectroscopy, and the molecular structure of the ligand b is determined by single crystal X-ray diffraction. The complexes undergo oxidative addition (OA) with excess CH₃I to afford Rh(III)-acyl complexes of the type [RhCl(COCH₃)I(L)](3a,3b) and [RhCl(COCH₃)I(L) ₂](4a,4b). The kinetic data for the OA reactions with CH₃I indicate a first order reaction and also exhibit that the rate of OA for the chelate complexes (1a and 1b) is higher than those of trans-complexes (2a and 2b). The catalytic efficiencies of 1a, 1b, 2a and 2b in carbonylation of methanol exhibit higher Turn Over Frequency (TOF) 689-1808 h^-1 than the well-known Monsanto's species [Rh(CO)₂I₂]^- (TOF = 464-1000 h^-1) under similar experimental conditions. The catalytic activities vary in the order as 1a > 1b > 2a > 2b.

Full text of DOI:10.1016/j.molcata.2011.10.023

Journal of Molecular Catalysis A: Chemical 353–354 (2012) 7–12
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Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Chelate and trans effect of P,O donor phosphine ligands on rhodium catalyzed
carbonylation of methanol
Dipak Kumar Duttaa,∗, Biswajit Deba, Guoxiong Huab, J. Derek Woollinsb
a Materials Science Division, Council of Scientific and Industrial Research, North East Institute of Science and Technology, Jorhat 785006, Assam, India
b EaStCHEM School of Chemistry, University of St. Andrews, St. Andrews, UK KY16 9ST, UK
a r t i c l e i n f o
a b s t r a c t
Four complexes of the type [Rh(CO)Cl(ꢀ2-P,O-L)](1a,1b) and [Rh(CO)Cl(ꢀ1-P-L)2](2a,2b), where
L = Ph2PC6H4-2-OCH3(a) and Ph2PC6H4-2-CH2OCH3(b), have been synthesized by the reaction of
[Rh(CO)2Cl]2 with appropriate mol equivalents of the ligands in CH2Cl2. The complexes show single
intense ꢁ(CO) bands in the range 1965–1989 cm−1 indicating the presence of terminal carbonyl groups.
All the complexes have been characterized by elemental analyses, mass spectrometry, IR and multinu-
clear NMR (1H, 31P and 13C) spectroscopy, and the molecular structure of the ligand b is determined by
single crystal X-ray diffraction. The complexes undergo oxidative addition (OA) with excess CH3I to afford
Rh(III)-acyl complexes of the type [RhCl(COCH3)I(L)](3a,3b) and [RhCl(COCH3)I(L)2](4a,4b). The kinetic
data for the OA reactions with CH3I indicate a first order reaction and also exhibit that the rate of OA
for the chelate complexes (1a and 1b) is higher than those of trans-complexes (2a and 2b). The catalytic
efficiencies of 1a, 1b, 2a and 2b in carbonylation of methanol exhibit higher Turn Over Frequency (TOF)
689–1808 h−1 than the well-known Monsanto’s species [Rh(CO)2I2]− (TOF = 464–1000 h−1) under similar
experimental conditions. The catalytic activities vary in the order as 1a > 1b > 2a > 2b.
Article history:
Received 8 July 2011
Accepted 29 October 2011
Available online 7 November 2011
Keywords:
Rhodium
Carbonyl complex
Phosphine
Oxidative addition
Carbonylation
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
have selected two typical P,O ligands viz. Ph2PC6H4-2-OCH3 and
Ph2PC6H4-2-CH2OCH3 for the synthesis of rhodium complexes.
The coordination chemistry of unsymmetrical phosphine-
phosphineoxide (P,O) ligands is of considerable interest because
of their structural novelty, reactivity and catalytic activity [1–5].
Due to the presence of two different types of donor sites in the lig-
ands, they are capable of coordinating either as a mono-dentate
(ꢀ1-P coordinated) or bi-dentate chelating (ꢀ2-P,O coordinated)
fashion depending upon the central metal atom and its envi-
ronment [3,6]. However, the chemistry of these ligands is more
fascinating as they can confer extra stability to the metal cen-
tre by chelate formation and may create vacant coordination
sites by the cleavage of the relatively weaker metal–oxygen
bond. Thus, such ligands have great impact on OA reactions
[1b,2d,7,8], which is a key step in many catalytic reactions like
the carbonylation of methanol. Since the first introduction of the
commercial species, i.e. [Rh(CO)2I2]− as an efficient catalyst for
the carbonylation of methanol to acetic acid [7a–d], considerable
efforts have been devoted to improve the catalyst by incorpo-
rating different ligands [2,7,9–11] into its coordination sphere.
As a part of our ongoing research activity [2,10], herein, we
In this paper, we report the synthesis of chelate (ꢀ2-P,O) and
trans-(ꢀ1-P) rhodium(I) carbonyl complexes and their activity in
catalytic carbonylation of methanol. The effect of chelate and
trans-complexes on the methanol carbonylation has also been
demonstrated.
2. Experimental
2.1. General
All solvents were distilled under N2 prior to use. RhCl3·xH2O was
purchased from M/S Arrora Matthey Ltd., Kolkota, India.
Elemental analyses were performed on a Perkin-Elmer 2400
elemental analyzer. IR spectra (4000–400 cm−1) were recorded as
thin film using NaCl cell on a Shimadzu IR Affinity-1 spectropho-
tometers. The 1H, 13C and 31P NMR spectra were recorded at room
temperature in CDCl3 solution on a Bruker DPX-300 Spectrometer
and chemical shifts were reported relative to SiMe4 and 85% H3PO3
as internal and external standards respectively. Mass spectra of the
complexes were recorded on ESQUIRE 3000 Mass Spectrometer.
The carbonylation reactions of methanol were carried out in a high
pressure reactor (Parr-4592, USA) fitted with a pressure gauge and
the reaction products were analysed by GC (Chemito 8510, FID).
∗
Corresponding author. Tel.: +91 376 2370081; fax: +91 376 2370011.
E-mail address: dipakkrdutta@yahoo.com (D.K. Dutta).
1381-1169/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2011.10.023

8
D.K. Dutta et al. / Journal of Molecular Catalysis A: Chemical 353–354 (2012) 7–12
1
(CH2), ı 127–143 (Ph), ı 183 (CO). 31P { H} NMR (CDCl3, ppm):
ı 28.5 [PV, d, JP-Rh = 125.3 Hz]. Elemental analyses; Found (Cald.
for C21H19ClO2PRh): C, 53.02 (53.34), H, 3.89 (4.02). Mass: 472.41
(m/z+).
2.2. Synthesis of ligands
2.2.1. Synthesis of Ph2PC6H4-2-OCH3 (a)
The ligand a was prepared by literature method [12].
Analytical data for a: 1H NMR (CDCl3, ppm): ı 3.71 (s, 3H, CH3),
ı 7.28–7.36, 6.8–6.9 (m, 14H, Ph). 13C NMR (CDCl3, ppm): ı 61.2
2.5. Synthesis of the complexes [Rh(CO)Cl(ꢀ1-P-L)2] (2a,2b)
L = Ph2PC6H4-2-OCH3 (a), Ph2PC6H4-2-CH2OCH3 (b)
(CH3), ı 125–139 (Ph), 31P{ H} NMR (CDCl3, ppm): ı −16.09 [s, PIII].
1
Elemental analyses; Found (Cald. for C19H17OP): C, 77.96 (78.08),
H, 5.78 (5.82). Mass: 292 (m/z+).
[Rh(CO)2Cl]2 [0.128 mmol, 50 mg] was dissolved in
dichloromethane (10 cm3) and to this solution, a stoichiometric
quantity of the respective ligands [0.512 mmol, 149.5 (a), 156.7
(b) mg] were added. The reaction mixture was stirred at room
temperature (25 ◦C) for about 30 min and the solvent was evap-
orated under vacuum. The yellowish to reddish brown coloured
compounds so obtained were washed with pentane as well as
diethyl ether and also recrystallized from DCM/hexane and stored
over silica gel in a desiccator.
2.2.2. Synthesis of Ph2PC6H4-2-CH2OCH3 (b)
1.0 M solution of NaOCH3, prepared from 0.60 g of metal sodium
(25.0 mmol, 1.20 equiv.) in 15 cm3 of CH3OH, was added to 5.22 g of
2-bromobenzyl bromide (20.9 mmol) and heated to reflux for 5 h.
The mixture was then cooled to room temperature and dried. The
residue was taken up with water/ethyl acetate. The organic layer
was washed with water and brine for three times and dried over
MgSO4. The organic solvent was removed to produce a colourless
oil 1-bromo-2-(methoxymethyl)benzene in 92% yield (3.70 g).
n-Butyllithium solution (7.5 cm3, 2.5 M in hexane, 18.5 mmol)
was added to a mixture of 1-bromo-2-(methoxymethyl)benzene
(3.70 g, 18.5 mmol) in 40 cm3 of THF at −78 ◦C for a period of 1 h. The
mixture was stirred for another 1 h. Then chlorophenylphosphine
(3.2 cm3, 18.5 mmol) was added to the mixture at −78 ◦C within
30 min. The mixture was further stirred at −78 ◦C for 2 h and kept
for 3 h to reach upto room temperature. The solvent was removed
in vacuum and the residue was taken up with water/ethyl acetate.
The organic layer was washed with water and brine and dried over
MgSO4. Concentrated the organic layer to 5 cm3 and purified by
silica gel (eluent by CH2Cl2) gave 2.72 g as colourless crystals after
crystallization from CH2Cl2 solution.
Analytical data for the complexes 2a, 2b are as follows:
2a: Yield: 72%; IR (Thin film, NaCl): 1970 [ꢁ(CO)] cm−1 1H NMR
.
(CDCl3): ı 3.68 (s, 6H, CH3), ı 6.85–6.95, 7.27–7.45, 7.65–7.91 (m,
28H, Ph) ppm. 13C NMR (CDCl3): ı 57.5 (CH3), ı 118–135 (Ph),
1
ı 165 (C–O), ı 187.4 (CO) ppm. 31P { H} NMR (CDCl3): ı 22.8
[PV, d, JP-Rh = 135 Hz] ppm. Elemental analyses; Found (Cald. for
C39H34ClO3P2Rh): C, 61.86 (62.36), H, 4.25 (4.53). Mass: 750.41
(m/z+).
2b: Yield: 71%. IR (Thin film, NaCl): 1965 [ꢁ(CO)] cm−1 1H NMR
.
(DMSO-D6): ı 3.18 (s, 6H, CH3), ı 4.72 (s, 4H, CH2), ı 7.39–7.63 (m,
28H, Ph) ppm. 13C NMR (DMSO-D6): ı 54.7 (CH3), ı 74.8 (CH2), ı
1
126–142.3 (Ph), ı 184.3 (CO) ppm. 31P { H} NMR (DMSO-D6): ı
24.6 [PV, d, JP-Rh = 127.2 Hz] ppm. Elemental analyses; Found (Cald.
for C41H38ClO3P2Rh): C, 62.88 (63.20), H, 4.78 (4.88). Mass: 778.41
(m/z+).
Analytical data for b: 1H NMR (CDCl3, ppm): ı 3.26 (s, 3H, CH3),
ı 4.64 (s, 2H, CH2), ı 7.17–7.32, 7.52–7.68 (m, 14H, Ph). 13C NMR
(CDCl3, ppm): ı 53.8 (CH3), ı 74.3 (CH2), ı 128–142 (Ph), 31P{ H}
1
NMR (CDCl3, ppm): ı −15.08 [s, PIII]. Elemental analyses; Found
(Cald. for C20H19OP): C, 78.43 (78.11), H, 6.20 (6.08). Mass: 306
(m/z+).
2.6. Reactivity of [Rh(CO)Cl(ꢀ2-P,O-L)] and [Rh(CO)Cl(ꢀ1-P-L)2]
with CH3I
Synthesis
of
[RhCl(COCH3)(I)(ꢀ2-P,O-L)]
(3a,3b)
and
2.3. Starting materials
[RhCl(COCH3)(I)(ꢀ1-L)2] (4a,4b), where L = Ph2PC6H4-2-OCH3
(a), Ph2PC6H4-2-CH2OCH3 (b)
[Rh(CO)2Cl]2 was prepared by passing CO gas over RhCl3·3H2O
[Rh(CO)Cl(ꢀ2-P,O-L)] or [Rh(CO)Cl(ꢀ1-P-L)2] (50 mg) was dis-
solved in CH2Cl2 (10 cm3) and each of CH3I (3 cm3) was added to it.
The reaction mixture was then stirred at r.t. for about 0.5–2 h. The
colour of the solution changed from yellowish red to dark reddish
brown and the solvent was evaporated under vacuum. The com-
pounds so obtained were washed with diethyl ether and stored
over silica gel in a desiccator.
at 100 ◦C in the presence of moisture [13].
2.4. Synthesis of the complexes [Rh(CO)Cl(ꢀ2-P,O-L)] (1a,1b),
L = Ph2PC6H4-2-OCH3 (a), Ph2PC6H4-2-CH2OCH3 (b)
[Rh(CO)2Cl]2 [0.128 mmol, 50 mg] was dissolved in
dichloromethane (10 cm3) and to this solution,
a stoichio-
metric quantity of the respective ligands [0.256 mmol, 74.75 mg
(a), 78.35 mg (b)] were added. The reaction mixture was stirred
at room temperature (25 ◦C) for about 20 min and the solvent
was evaporated under vacuum. The yellowish to reddish brown
coloured compounds so obtained were washed with pentane as
well as diethyl ether and also recrystallized from CH2Cl2/hexane
and stored over silica gel in a desiccator.
2.7. Kinetic experiment
The kinetic experiments of OA reactions of the complexes 1a,
1b, 2a and 2b with CH3I were monitored using FTIR spectroscopy
in a solution cell (NaCl windows, 0.5 mm path length). In order to
obtainpseudo-first-ordercondition, excess of CH3I relative to metal
complex was used. FTIR spectra (4.0 cm−1 resolution) were scanned
in the ꢁ(CO) region (2200–1600 cm−1) and saved at regular time
interval using spectrum software. After completion of experiment,
absorbance versus time data for the appropriate ꢁ(CO) frequencies
were extracted by subtracting the solvent spectrum and analysed
off line using OriginPro 7.5 software. Kinetic measurements were
made by following the decay of lower frequency ꢁ(CO) band of
the complexes in the region 1965–1989 cm−1. The pseudo-first-
order rate constants were found from the gradient of the plot of
ln(A0/At) versus time, where A0 is the initial absorbance and At is
the absorbance at time t.
Analytical data for the complexes 1a, 1b are as follows:
1a: Yield: 81%; IR (Thin film, NaCl): 1979 [ꢁ(CO)], cm−1 1H
.
NMR (CDCl3): ı 3.54 (s, 3H, CH3), ı 6.9, 7.27–7.43, 7.55–7.68,
7.81–7.93 (m, 14H, Ph) ppm. 13C NMR (CDCl3, ppm): ı 58.5 (CH3), ı
1
114–137 (Ph), ı 168 (C–O), ı 184.8 (CO). 31P { H} NMR (CDCl3):
ı 27.96 [PV, d, JP-Rh = 128 Hz]. Elemental analyses; Found (Cald.
for C20H17ClO2PRh): C, 51.89 (52.35), H, 3.63 (3.70). Mass: 458.41
(m/z+).
1b: Yield: 74%. IR (Thin film, NaCl): 1989 [ꢁ(CO)] cm−1 1H
.
NMR (CDCl3, ppm): ı 3.14 (s, 3H, CH3), ı 4.54 (s, 2H, CH2), ı
7.42–7.62 (m, 14H, Ph). 13C NMR (CDCl3, ppm): ı 53.5 (CH3), ı 74.2

D.K. Dutta et al. / Journal of Molecular Catalysis A: Chemical 353–354 (2012) 7–12
9
CH3
CH3
Br
O
O
BuLi
THF
Na / CH3OH
Reflux
I
Br
Li
Ph2PCl
THF
P
O
Fig. 1. X-ray crystal structure of Ph2PC6H4-2-CH2OCH3 (b). Hydrogen atoms are
˚
omitted for clarity. Selected bond lengths (A): P(1)–C(17) 1.836(2), P(1)–C(19)
Scheme 1. Synthesis of Ph2PC6H4-2-CH2OCH3 (b).
1.831(2), P(1)–C(3) 1.832(2), C(10)–C(20) 1.511(3), C(20)–O(2) 1.404(3), C(12)–O(2)
1.422(3).
2.8. Carbonylation of methanol using complexes 1a, 1b, 2a and
2b as catalyst precursors
at ı 28.0 (d, JP-Rh = 128 Hz) and 28.5 (d, JP-Rh = 125 Hz) ppm respec-
tively, which show downfield shifts compared to the free ligands [ı
−16.09 (a) and −15.08 (b) ppm] indicating the formation of Rh-P
bonds. The 1H NMR spectrum of 1a shows characteristic phenylic
multiplet in the region ı 6.90–7.93 ppm and methyl singlet at ı
3.54 ppm. However, 1b shows the phenylic multiplet in the range
ı 7.52–7.68 ppm, methylene and methyl singlet at around ı 4.54
and 3.14 ppm respectively. The appearance of upfield shifts of CH3
and CH2 protons in 1H NMR spectra and the downfield shift of
CH3OH (0.099 mol, 4 cm3), CH3I (0.016 mol, 1 cm3), H2O
(0.055 mol, 1 cm3) and catalyst (0.0514 mmol) were placed in a
50 cm3 reaction vessel. The reaction mixture was then purged with
CO for about 5 min and then pressurized with CO gas (5, 10 and
20 bar) at 25 ◦C. The carbonylation reactions were carried out at
130 2 ◦C for 1 h with corresponding gas pressure at around 15 2,
20 2 and 33 2 bar. The products were collected and analysed by
G.C.
1
phosphorus in 31P{ H} NMR spectra compared to free ligands sub-
stantiate the formation of chelate ꢀ2-P,O complex. On the other
hand, the dimeric precursor [Rh(CO)2Cl]2 in CH2Cl2 reacts with four
3. Results and discussion
3.1. Synthesis and characterization of the ligands a and b
CO
CO
OC
OC
Cl
Cl
Rh
Rh
The ligand a was synthesized by literature method [12] and b
was prepared according to Scheme 1. The ligands were character-
ized by elemental analyses, mass spectrometry and multinuclear
NMR spectroscopic techniques. The 1H NMR spectra of a and b show
characteristic phenylic multiplets in the region ı 6.8–7.68 ppm and
methyl and methylene singlets in the range ı 3.26–4.64 ppm. 31P-
Stirr.,
15 min., r.t.
Stirr.,
30 min., r.t.
DCM,
L
DCM,
2L
4L
1
{ H} NMR spectra of a and b show characteristic resonances at ı
−16.09 and −15.08 ppm, respectively corresponding to the triva-
lent phosphines. In their 13C NMR spectra, the chemical shifts
due to the phenylic and methyl carbons of a, b are found in the
range 125–142 and 53.8–61.2 ppm respectively, and the methy-
lene carbon of the ligand b is found at around 74.3 ppm. All the
spectroscopic studies as well as the elemental and mass spectro-
metric results substantiate the formation of the desired ligands. The
ligand b was also structurally characterized by single crystal X-ray
structure determination (Fig. 1).
CO
L
O
CO
Rh
Rh
(1a,1b)
Cl
Cl
P
(2a,2b)
CH3I
CH3I
O
L
I
I
Rh
Rh
Cl
3.1.1. Synthesis and characterization of 1a, 1b, 2a and 2b
P
Cl
L
The reaction of the chloro-bridged dimer [Rh(CO)2Cl]2 in CH2Cl2
with two mol equivalents of ligands (a,b) proceeds rapidly with
the evolution of CO gas to yield the yellow chelated monocar-
bonyl complex of the type [Rh(CO)Cl(ꢀ2-P,O-L)](1a,1b), where
L = Ph2PC6H4-2-OCH3(a) and Ph2PC6H4-2-CH2OCH3(b) (Scheme 2).
Elemental analyses of the complexes support the observed molec-
ular composition of 1a and 1b. The IR spectra of 1a and 1b exhibit
single intense ꢁ(CO) bands at around 1979 and 1989 cm−1 respec-
tively, indicating the presence of a terminal carbonyl group. 1a
COCH3
(4a,4b)
COCH3
(3a,3b)
PPh2
PPh2
L
=
OCH3
CH2OCH3
(a)
(b)
1
and 1b show characteristic 31P{ H} NMR spectra with doublets
Scheme 2. Syntheses and reactivity of 1a, 1b, 2a and 2b.

10
D.K. Dutta et al. / Journal of Molecular Catalysis A: Chemical 353–354 (2012) 7–12
Table 1
COCH3
CH3
Rh
Significant IR, 1H and 13C NMR data for the OA adducts.
L
CO
L
L
I
L
CH3I
Cl
CO
L
OA adducts
IR (cm−1
)
1H NMR (ppm)
COCH3
13C NMR (ppm)
(CO)acyl
Rh
Rh
L
Cl
Cl
ꢁ(CO)acyl
I
(2a/,2b/)
3a
3b
4a
4b
1694
1690
1698
1692
2.58(s)
2.51(s)
2.61(s)
2.67(s)
206(s)
(4a,4b)
(2a,2b)
205.6(s)
203.4(s)
204.1(s)
Scheme 3. Reaction of 2a and 2b with CH3I.
0.08
0.06
0.04
0.02
0.00
mol equivalents of ligands (a,b) to yield trans-[Rh(CO)Cl(ꢀ1-P-L)2]
(2a,2b) (Scheme 2). The synthesis and structural characterization
of 2a was already reported [14], however, 2b is newly synthesized
in this report. Elemental analyses and mass spectrometric results
of the complexes support the expected molecular composition of
2a and 2b. The IR spectra of 2a and 2b exhibit single intense ꢁ(CO)
bands at 1970 and 1965 cm−1 respectively, indicating the presence
of terminal carbonyl groups. The 1H NMR spectrum of 2a shows a
characteristic phenylic multiplet in the region ı 6.85–7.91 ppm and
methyl singlet at ı 3.68 ppm. However, 2b shows the phenylic mul-
tiplet in the range ı 7.39–7.63 ppm, methylene and methyl singlet
at around ı 4.72 and 3.18 ppm respectively. 2a and 2b show char-
1690 .5
2059
1972
1
acteristic 31P{ H} NMR spectra with doublet at around ␦ 22.8 (d,
JP-Rh = 135 Hz) and 24.6 (d, JP-Rh = 128 Hz) ppm respectively, which
show downfield shift compared to the free ligands, indicating the
formation of Rh-P bond. The appearance of almost similar shifts of
CH3 and CH2 protons in 1H NMR spectra and the downfield shift
2100
2050
2000
1950
1710 1695 1680 1665 1650
Wavenumber(cm-1)
of phosphorus in 31P{ H} NMR spectra in the complexes compared
1
Fig. 2. Series of IR spectra {ꢁ(CO) region} illustrating the reaction of 2a with CH3I
at 25 ◦C. The arrows indicate the behaviour of each band as the reaction progresses.
to free ligands substantiate the formation of ꢀ1-P coordinated com-
plex. The 13C NMR spectra of 1a, 1b, 2a and 2b show characteristic
resonances in the range ı 183.0–187.4 ppm respectively for the car-
bonyl carbons. The phenyl and other carbons in the complexes are
found in their respective ranges.
at 1690.5 cm−1. Absorbance versus time plots for the decay of lower
intensity ꢁ(CO) bands at 1972 cm−1 of 2a is shown in Fig. 3. A linear
fit of pseudo-first order was observed for the entire course of the
reaction of CH3I with the complex 2a as is evidenced from the plot
of ln(A0/At) versus time, where A0 and At are the absorbance at time
t = 0 and t, respectively (Fig. 4). The rate constants for the OA reac-
3.2. Reactivity of 1a, 1b, 2a and 2b with CH3I
tion of CH3I towards 1a, 1b, 2a and 2b are 3.51 × 10−3, 9.14 × 10−4
,
The complexes 1a, 1b, 2a and 2b undergo OA reactions with
CH3I followed by migratory insertion reaction to generate Rh(III)-
acyl complexes of the type [RhCl(COCH3)I(ꢀ2-P,O-L)] (3a,3b) and
[RhCl(COCH3)I(ꢀ1-P-L)2] (4a,4b) (Scheme 2). The IR spectra of
the oxidized products show broad ꢁ(CO) bands in the range
1690–1698 cm−1 characteristic of acyl carbonyl groups (Table 1).
The 1H NMR spectra of 3a, 3b, 4a and 4b display singlet resonances
in the range 2.58–2.67 ppm suggesting the formation of –OCCH3
group including other characteristic bands of the ligands. Similarly,
the 13C NMR spectra of the OA adduct exhibit bands in the range
203–206 ppm due to the formation of –COCH3 group (Table 1).
Attempts to substantiate the structures of different rhodium(I)
and rhodium(III) carbonyl complexes by X-ray crystal structure
determination was not possible because no suitable crystals could
be obtained after numerous attempts.
Kinetic measurements for the OA reaction of 1a, 1b, 2a and 2b
with excess methyl iodide were carried out using IR spectroscopy
by monitoring the changes in the lower ꢁ(CO) bands. The reaction
of 1a and 1b with methyl iodide leads smoothly to the acetyl prod-
uct, however, when the reactions of 2a and 2b were performed
with methyl iodide, a strong ꢁ(CO) band at around 2058–2062 cm−1
was observed in the IR spectrum in addition to the bands in 2a, 4a
and 2b, 4b; which are consistent with the presence of intermediate
methyl complexes 2aꢀ and 2bꢀ (Scheme 3). A typical series of IR spec-
tra is shown in Fig. 2. When the complex 2a reacts with CH3I at 25 ◦C,
immediately two new bands grow at around 1690 and 2059 cm−1
along with the shifted parent ꢁ(CO) band of 2a at 1972 cm−1. As the
reaction progresses, the bands around 1972 and 2059 cm−1 decay
and the band at 1690 cm−1 grows continuously. Finally, the former
two ꢁ(CO) bands vanish and a new broad acyl ꢁ(CO) band appears
1.12 × 10−4 and 9.44 × 10−5 S−1, which varies as 1a > 1b > 2a > 2b.
The observed values of the rate constants indicate that the rate
of OA is higher in case of chelate ꢀ2-P,O complexes than those of
trans-ꢀ1-P complexes. This may be due to the presence of bulky
OCH3/CH2OCH3 groups at the ortho position of one of the phenyl
rings of the phosphine ligands which may sterically restrict the
path of OA reaction of CH3I to the Rh centre of 2a and 2b. However,
such steric hindrance is absent in the chelate complexes (1a and
1b) resulting in faster reaction kinetics.
0.030
0.025
0.020
0.015
0.010
0.005
0.000
0
100
200
300
400
500
600
Time (seconds)
Fig. 3. Kinetic plot of showing the decay of ꢁ(CO) bands of 2a during the reaction
with neat CH3I at room temperature (∼25 ◦C).

D.K. Dutta et al. / Journal of Molecular Catalysis A: Chemical 353–354 (2012) 7–12
11
Table 2
Results of methanol carbonylation.
Catalyst precursor
[Rh(CO)2I2]− c
CO pressure at 25 ◦C ( 1 bar)
Total conv. (%)
Acetic acida (%)
Methyl acetatea
TOFb (h−1
)
5
10
20
5
10
20
5
10
20
5
10
20
5
24.1
41.4
52.1
46.9
68.8
94.0
45.8
58.3
90.8
35.8
50.2
87.4
39.8
52.3
86.5
4.0
12.3
10.3
10.1
26.1
62.1
17.7
23.5
52.1
8.7
21.3
53.7
12.7
25.1
50.0
20.1
29.1
41.8
36.8
42.7
31.9
28.1
34.8
38.7
27.1
28.9
33.7
27.1
27.2
36.5
463
796
1000
902
1323
1808
881
1121
1747
689
966
1681
766
1006
1660
1a
1b
2a
2b
10
20
a
Yield of methyl acetate and acetic acid were obtained from GC analyses after 1 h reaction time.
TOF = [amount of product (mol)]/[amount of catalyst (Rh mol)]/time (h).
Formed from added [Rh(CO)2Cl]2 under catalytic condition [15].
b
c
3.3. Carbonylation of methanol to acetic acid and ester using the
4. Conclusions
complexes 1a, 1b, 2a and 2b as the catalyst precursors
Chelate and trans-complexes of the type [Rh(CO)Cl(L)](1a,1b)
and [Rh(CO)Cl(L)2] (2a, 2b) respectively, have been synthesized and
characterized, and the structure of the ligand b was determined by
single crystal X-ray diffraction. The complexes undergo oxidative
addition with CH3I to afford acyl Rh(III) complexes. The kinetic data
for the OA reactions with CH3I indicate a first order reaction and also
exhibit that the rate of OA for the chelate complexes 1a, 1b is higher
than those of trans-complexes. The catalytic activities of 1a, 1b, 2a
and 2b in carbonylation of methanol to produce acetic acid and
its ester exhibit higher TOF (689–1808 h−1) than that of the well-
The results of carbonylation of methanol to acetic acid and
methyl acetate in the presence of 1a, 1b, 2a and 2b as catalyst pre-
cursors are shown in Table 2. The precursor complexes 1a, 1b, 2a
and 2b show total conversion of CH3OH in the range 35.8–94.0%
at 130 2 ◦C under initial CO pressure 5–20 bar (at ∼25 ◦C) for 1 h
reaction time with corresponding TOF 689–1808 h−1. From Table 2
it can be noted that as the applied CO pressure increases from
5 to 20 bar for 1a, total conversion increases from 46.9 to 94.0%
with corresponding increase in TOF from 689 to 1808 h−1. Likewise,
the complexes 1b, 2a and 2b also show a similar increasing trend
with the increase in CO pressure and maximum TOF of about 1747
(1b), 1681 (2a), and 1660 (2b) with corresponding conversions of
90.8, 87.4, and 86.5% have been obtained at 20 bar initial CO pres-
sure (∼25 ◦C). Under the similar experimental conditions, the well
known precursor [Rh(CO)2I2]− generated in situ from [Rh(CO)2Cl]2
[15] shows lower TOF compared to 1a, 1b, 2a and 2b. Thus, the
efficiency of the complexes depends on the nature of the ligands
and follows the order 1a > 1b > 2a > 2b > [Rh(CO)2Cl]2. The observed
trend is also well supported by their kinetic experiments. The
chelate complexes 1a and 1b undergo OA of methyl iodide with
higher rate than those of trans-complexes 2a and 2b, and hence the
former complexes show higher TOF compared to latter ones.
known commercial species [Rh(CO)2I2]− (TOF = ∼464–1000 h−1
)
under similar experimental conditions.
Supplementary data
CCDC 795222 contains the supplementary crystallographic
data for the ligand b. These data can be obtained free of
charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or
from the Cambridge Crystallographic Data Centre, 12 Union
Road, Cambridge CB2 1EZ, UK. Fax: +44 1223 336 033; e-mail:
deposit@ccdc.cam.ac.uk.
Acknowledgements
The authors are grateful to Dr. P.G. Rao, Director, North East Insti-
tute of Science and Technology, Jorhat-785 006, Assam, India for
his kind permission to publish the work. The Department of Sci-
ence and Technology (DST), New Delhi (Grant: SR/S1/IC-05/2006)
and The Royal Society (UK) International Joint Project 2007/R2-
CSIR (India) joint research scheme are acknowledged for the partial
financial grant. The author B. Deb thanks to CSIR, New Delhi for
providing the Senior Research Fellowship.
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
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