Dicarbonylrhodium(I) complexes of benzoylpyridine ligands: Synthesis, reactivity and catalytic carbonylation reaction

Article abstract of DOI:10.1016/j.molcata.2009.11.022

The new complexes of the type [Rh(CO)₂ClL] (1a-c), where L = 2-Benzoylpyridine (a), 3-Benzoylpyridine (b) and 4-Benzoylpyridine (c) have been synthesized and characterized. Oxidative addition (OA) of 1a-c with CH₃I, C₂H₅I and C₆H₅CH₂Cl afford penta coordinated Rh(III) complexes, [Rh(CO)(CORⁿ)ClXL]{R¹ = -CH₃ (2a-c), R² = -C₂H₅ (3a-c); X = I and R³ = -CH₂C₆H₅ (4a-c); X = Cl}. Kinetic data for the reaction of 1a-c with CH₃I indicate a pseudo-first order reaction. The 1a-c exhibit high catalytic activity in the carbonylation of methanol to acetic acid and its ester and show a higher turn over number (TON = 1529-1748) than the well known commercial species [Rh(CO)₂I₂]^- (TON = 1000) under the reaction conditions: temperature 130 ± 2 °C, pressure 30 ± 2 bar and time 1 h.

Full text of DOI:10.1016/j.molcata.2009.11.022

Journal of Molecular Catalysis A: Chemical 319 (2010) 66–70
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Dicarbonylrhodium(I) complexes of benzoylpyridine ligands:
Synthesis, reactivity and catalytic carbonylation reaction
Bibek Jyoti Borah, Biswajit Deb, Podma Pollov Sarmah, Dipak Kumar Dutta∗
Materials Science Division, North-East Institute of Science & Technology (Council of Scientific and Industrial Research), Jorhat 785006, Assam, India
a r t i c l e i n f o
a b s t r a c t
Article history:
The new complexes of the type [Rh(CO)2ClL] (1a–c), where L = 2-Benzoylpyridine (a), 3-Benzoylpyridine
(b) and 4-Benzoylpyridine (c) have been synthesized and characterized. Oxidative addition (OA) of 1a–c
with CH3I, C2H5I and C6H5CH2Cl afford penta coordinated Rh(III) complexes, [Rh(CO)(CORn)ClXL]{R1 = -
CH3 (2a–c), R2 = –C2H5 (3a–c); X = I and R3 = –CH2C6H5 (4a–c); X = Cl}. Kinetic data for the reaction of 1a–c
with CH3I indicate a pseudo-first order reaction. The 1a–c exhibit high catalytic activity in the carbony-
lation of methanol to acetic acid and its ester and show a higher turn over number (TON = 1529–1748)
than the well known commercial species [Rh(CO)2I2]− (TON = 1000) under the reaction conditions: tem-
perature 130 2 ◦C, pressure 30 2 bar and time 1 h.
Received 28 October 2009
Received in revised form
28 November 2009
Accepted 30 November 2009
Available online 4 December 2009
Keywords:
Rhodium(I)
Dicarbonyl complexes
Benzoylpyridine
Oxidative addition
Carbonylation
© 2009 Elsevier B.V. All rights reserved.
1. Introduction
the use of rhodium complexes containing N-donor ligands and their
implications in catalytic carbonylation reaction have found only
The rhodium chemistry towards the activation of small
molecules like CH3I, H2, I2, etc., and their direct implications in
various catalytic reactions such as carbonylation of alcohols, hydro-
formylation of alkenes, etc., are of great importance for academic
as well as industrial interest [1–3]. The well known Monsanto’s
species [Rh(CO)2I2]− is used as catalyst precursor in the industrial
production of acetic acid from methanol [4–6]. Efforts are given to
synthesize new rhodium complexes to improve the catalytic effi-
cacy by incorporating different types of ligands into its coordination
sphere to improve the reaction parameters for the overall produc-
tion of acetic acid [7–10]. Phosphorous, being a soft donor, is used
for a long time in making metal complexes in lower oxidation states
like rhodium(I) for catalytic studies [7–9]. In the recent time, N-
containing ligands have also gained much attention in the field of
synthetic organometallic chemistry due to their binding capabili-
ties in both high and low oxidation states [11–18]. In contrast to the
P-atom, N-atom has only ␴-donor (and no ␲-acceptor) properties
due to which it imparts more ionic character to the metal ligand
bond. The N-containing ligands having strong ␴-donor capabili-
ties enhance the nucleophilicity of the Rh(I) centre which in turn
increases the catalytic activity of the complexes [13–15]. However,
limited use [13–15,19–22]. With these precedents in mind and
encouraged by effectiveness of N, O donor ligands in our earlier
works [13–15,21,22], we believed it of interest to explore the abil-
ity of related rhodium(I) carbonyl complexes of N,O donor ligands
in catalytic carbonylation reaction. Thus, in this paper we report
the synthesis and reactivity of Rh(I) complexes containing three
isomeric benzoylpyridine ligands and their behavior in catalytic
carbonylation reaction.
2. Experimental
All solvents were distilled under N2 prior to use. RhCl3·xH2O
was purchased from M/S Arrora Matthey Ltd., Kolkata, India. The
ligands were purchased from M/S Aldrich, USA and used with-
out further purification. Elemental analyses were performed on a
Perkin-Elmer 2400 elemental analyzer. IR spectra (4000–400 cm−1
)
were recorded in KBr discs and CHCl3 on a Perkin-Elmer system
2000 FT-IR spectrophotometer. The 1H and 13C NMR spectra were
recorded at room temperature (r.t.) in CDCl3 solution on a Bruker
DPX-300 Spectrometer and chemical shifts were reported rela-
tive to SiMe4 as internal standard. The carbonylation reactions of
methanol were carried out in a Parr reactor (Model: Parr - 4592,
USA) fitted with a pressure gauge and the reaction products were
analyzed 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 © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2009.11.022

B.J. Borah et al. / Journal of Molecular Catalysis A: Chemical 319 (2010) 66–70
67
2.1. Synthesis of starting material
2c: Yield: 95%; Anal. Found (calcd.) for C15H12ClINO3Rh(%): C,
34.58 (34.66); H, 2.22 (2.31); N, 2.58 (2.69); selected IR data (KBr,
The starting dimeric rhodium moiety [Rh(CO)2Cl]2 was pre-
pared by passing CO gas over RhCl3·xH2O powder at 100 ◦C in the
presence of moisture [23].
cm−1): 2077 [␯CO], 1752 [␯CO]acyl 1H NMR data (ı in ppm): ı 9.13
,
(H-1, H-4, d, Py), ı 8.15 (H-2, H-3, d, Py), ı 7.47–7.82 (5H, m, Ph) ı
3.47 (CH3, s), 13C NMR data (ı in ppm): ı 124–151 (Ph/Py), ı 185,
185.9 (
, COt), ı 204.8 (COacyl), ı 43 (CH3).
2.2. Synthesis of complexes [Rh(CO)2Cl(L)] (1a–c) Where,
L = ꢀ1-(N) coordinated, 2-Benzoylpyridine (a), 3-Benzoylpyridine
(b) and 4-Benzoylpyridine (c)
3a: Yield: 95%; Anal. Found (calcd.) for C16H14ClINO3Rh(%): C,
35.91 (36.00); H, 2.53 (2.62); N, 2.58 (2.62); selected IR data (KBr,
cm−1): 2054.8 [␯CO], 1749.9 [␯CO]acyl 1H NMR data (ı in ppm): ı
,
9.05 (H-1, d, Py), ı 8.45–8.68 (3H, m, Py), ı 7.45–7.71 (5H, m, Ph), ı
[Rh(CO)2Cl]2 0.0642 mmol (25 mg) was dissolved in
dichloromethane (10 cm3) and to this solution, a stoichiometric
quantity (Rh:L = 1:1) 0.1285 mmol (23.5 mg) of a particular ligand
was added. The reaction mixture was stirred at r.t. (∼25 ◦C) for
about 30 min and then the solvent was evaporated under reduced
pressure to obtain yellow to brick-red color solid compounds
which were washed with hexane and dried over silica gel in a
desiccator.
1.53 (CH3, t), ı 2.51 (CH2, q), 13C NMR data (ı in ppm): ı 122–159
(Ph/Py), ı 185, 184.6 (
, COt), ı 204.1 (COacyl), ı 22 (CH3), ı 57
(CH2).
3b: Yield: 93%; Anal. Found (calcd.) for C16H14ClINO3Rh(%): C,
35.93 (36.00); H, 2.59 (2.62); N, 2.53 (2.62); selected IR data (KBr,
cm−1): 2050.6 [␯CO], 1754.1 [␯CO]acyl 1H NMR data (ı in ppm):
,
ı 9.09 (H-1, d, Py), ı 9.07 (H-4, s, Py), ı 8.39–8.58 (2H, m, Py), ı
7.46–7.74 (5H, m, Ph), ı 1.68 (CH3, t), ı 2.71 (CH2, q), 13C NMR data
(ı in ppm): ı 125–160 (Ph/Py), ı 185, 186.4 (
(COacyl), ı 24 (CH3), ı 61 (CH2).
, COt), ı 203.6
Analytical data for the complexes 1a–c
1a: Yield: 95%; Anal. Found (calcd.) for C14H9ClNO3Rh(%): C,
44.48 (44.51); H, 2.31 (2.38); N, 3.62 (3.70); selected IR data (KBr,
3c: Yield: 96%; Anal. Found (calcd.) for C16H14ClINO3Rh(%): C,
cm−1): 2081.4, 2007.8 [␯CO], 1667 [␯CO]benzoyl 1H NMR data (ı in
,
35.90 (36.00); H, 2.53 (2.62); N, 2.53 (2.62); selected IR data (KBr,
cm−1): 2055.8 [␯CO], 1746.1 [␯CO]acyl 1H NMR data (ı in ppm): ı
,
ppm): ı 9.02 (H-1, d, Py), ı 8.15–8.39 (3H, m, Py), ı 7.35–7.71 (5H,
m, Ph), 13C NMR data (ı in ppm): ı 127–155 (Ph/Py), ı 187, 183
9.10 (H-1, H-4, d, Py), ı 8.70 (H-2, H-3, d, Py), ı 7.45-7.59 (5H, m, Ph),
ı 1.61 (CH3, t), ı 2.35 (CH2, q), 13C NMR data (ı in ppm) ı 125–155
(
, COt).
(Ph/Py), ı 186, 184.9 (
, COt), ı 205.2 (COacyl), ı 23 (CH3), ı 63
1b: Yield: 93%; Anal. Found (calcd.) for C14H9ClNO3Rh(%): C,
(CH2).
44.23 (44.51); H, 2.35 (2.38); N, 3.68 (3.70); selected IR data (KBr,
cm−1): 2086, 2009 [␯CO], 1665 [␯CO]benzoyl 1H NMR data (ı in
,
4a: Yield: 94%; Anal. Found (calcd.) for C21H16Cl2NO3Rh(%): C,
49.91 (50.00); H, 3.09 (3.17); N, 2.72 (2.77); selected IR (KBr, cm−1
)
ppm): ı 9.08 (H-1, d, Py), ı 9.16 (H-4, s, Py), ı 8.41–8.82 (2H, m,
data: 2071 [␯CO], 1744 [␯CO]acyl 1H NMR data (ı in ppm): ı 9.07
(H-1, d, Py), ı 8.01–8.39 (3H, m, Py), ı 7.16–7.92 (10H, m, Ph), ı
4.20 (2H, –CH2, m)Bz (Bz stands for C6H5CH2Cl), 13C NMR data (ı
Py), ı 7.32–7.76 (5H, m, Ph), 13C NMR data (ı in ppm): ı 128–154
(Ph/Py), ı 187, 185 (
, COt).
1c: Yield: 92%; Anal. Found (calcd.) for C14H9ClNO3Rh(%): C,
in ppm): ı 127–151 (Ph/Py), ı 187, 185.3 (
(COacyl), ı 60 (CH2).
, COt), ı 206.2
44.28 (44.51); H, 2.31 (2.38); N, 3.65 (3.70); selected IR data (KBr,
cm−1): 2085.3, 2010 [␯CO], 1666 [␯CO]benzoyl 1H NMR data (ı in
,
4b: Yield: 96%; Anal. Found (calcd.) for C21H16Cl2NO3Rh(%): C,
49.87 (50.00); H, 3.11 (3.17); N, 2.72 (2.77); selected IR (KBr, cm−1
data: 2076 [␯CO], 1746 [␯CO]acyl
1H NMR data (ı in ppm): ı 9.09
(H-1, d, Py), ı 9.01 (H-4, s, Py), ı 8.08–8.29 (2H, m, Py), ı 7.36–8.00
(10H, m, Ph), ı 4.31 (2H, –CH2, m)Bz (Bz stands for C6H5CH2Cl), 13
ppm): ı 9.11 (H-1, H-4, d, Py), ı 8.11 (H-2, H-3, d, Py), ı 7.43–7.83
(5H, m, Ph), 13C NMR data (ı in ppm): ı 123–152 (Ph/Py), ı 187, 182
)
,
(
, COt).
C
2.3. Synthesis of complexes [Rh(CO)(CORn)ClX(L)], R1 = –CH3
(2a–c), R2 = –C2H5 (3a–c); X = I and R3 = –CH2C6H5 (4a–c); X = Cl
NMR data (ı in ppm): ı 122–151 (Ph/Py), ı 185, 184.2 (
ı 205.6 (COacyl), ı 63 (CH2).
, COt),
4c: Yield: 95%; Anal. Found (calcd.) for C21H16Cl2NO3Rh(%): C,
49.95 (50.00); H, 3.13 (3.17); N, 2.73 (2.77); selected IR (KBr, cm−1
data: 2070 [␯CO], 1748 [␯CO]acyl
1H NMR data (ı in ppm): ı 9.12
[Rh(CO)2Cl(L)] (1a–c) 0.0529 mmol (20 mg) was dissolved in
dichloromethane (10 cm3). To this solution each of RnX (6 cm3)
(RnX = CH3I, C2H5I and C6H5CH2Cl) was added. The reaction mix-
ture was then stirred at r.t. for about 7, 12 and 15 h for CH3I, C2H5I
and C6H5CH2Cl respectively. The color of the solution changes from
yellowish red to dark reddish brown and the solvent was removed
and washed with diethyl ether and stored over silica gel in a desic-
cator.
)
,
(H-1, H-4, d, Py), ı 8.60 (H-2, H-3, d, Py), ı 7.49–8.12 (10H, m, Ph),
ı 4.54 (2H, –CH2, m)Bz (Bz stands for C6H5CH2Cl), 13C NMR data
(ı in ppm): ı 126–151 (Ph/Py), ı 188, 186.5 (
(COacyl), ı 61 (CH2).
, COt), ı 206.4
2.4. Kinetic experiment
Analytical data for the complexes 2a–c, 3a–c and 4a–c
2a: Yield: 91%; Anal. Found (calcd.) for C15H12ClINO3Rh(%): C,
34.58 (34.66); H, 2.23 (2.31); N, 2.59 (2.69); selected IR data (KBr,
The kinetic experiments of OA reaction of complexes 1a–c with
neat CH3I were monitored using FT-IR spectroscopy in a solu-
tion cell (CaF2 windows, 1.0 mm path length). In order to obtain
pseudo-first-order condition excess of CH3I relative to metal com-
plex was used. FT-IR spectra (4.0 cm−1 resolution) were scanned
in the ␯(CO) region (2200–1650 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 analyzed
off line using OriginPro 7.5 software. Kinetic measurements were
made by following the decay of lower frequency ␯(CO) band of the
complexes 1a–c in the region 2040–1950 cm−1. The pseudo-first
cm−1): 2076 [␯CO], 1750 [␯CO]acyl 1H NMR data (ı in ppm): ı 9.07
,
(H-1, d, Py), ı 8.09–8.25 (3H, m, Py), ı 7.33–7.75 (5H, m, Ph), ı 3.35
(CH3, s), 13C NMR data (ı in ppm): ı 126–156 (Ph/Py), ı 186, 184.4
(
, COt), ı 207.1 (COacyl), ı 47 (CH3).
2b: Yield: 94%; Anal. Found (calcd.) for C15H12ClINO3Rh(%): C,
34.60 (34.66); H, 2.25 (2.31); N, 2.62 (2.69); selected IR data (KBr,
cm−1): 2063 [␯CO], 1749.4 [␯CO]acyl, 1H NMR data (ı in ppm): ı 9.10
(H-1, d, Py), ı 9.16 (H-4, s, Py), ı 8.15–8.31 (2H, m, Py), ı 7.30–7.72
(5H, m, Ph), ı 3.27 (CH3, s), 13C NMR data (ı in ppm): ı 123–152
(Ph/Py), ı 188, 185.1 (
, COt), ı 206.6 (COacyl), ı 45 (CH3).

68
B.J. Borah et al. / Journal of Molecular Catalysis A: Chemical 319 (2010) 66–70
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.
free ligands show a downfield shift when they involve in com-
plex formation. These indicate that the coordination to the metal
centre in 1a–c takes place through N-donor site. The 13C NMR spec-
tra of 1a–c show characteristic resonances of terminal carbonyl
group in the range ı 182–185 ppm and multiplets in the region
ı 123–155 ppm for carbon atoms of pyridine and benzene rings.
2.5. Carbonylation of methanol using 1a–c as catalyst precursors
CH3OH (0.099 mol, 4 cm3), CH3I (0.016 mol, 1 cm3), H2O
(0.055 mol, 1 cm3) and rhodium catalyst (0.0514 mmol) were taken
into the reactor. The reactor was then purged with CO for about
5 min and then pressurized with CO gas (20 1 bar) at 25 ◦C. The
carbonylation reactions were carried out at 130 2 ◦C for 1 h under
CO pressure (30 2 bar). The products were collected and analyzed
by GC.
3.2. Reactivity of 1a–c towards various electrophiles
Carbonylation of methanol to acetic acid is considered as one
of the most important industrial process utilizing homogeneous
rhodium metal based catalyst. In this context, oxidative addition of
alkyl halide with Rh complexes is very important reaction as it is
the key step in the carbonylation reaction [2,3,13–15]. Therefore,
OA of various electrophiles were evaluated.
3. Results and discussion
The 1a–c undergo OA reactions with CH3I, C2H5I and C6H5CH2Cl
followed by migratory insertion reaction to afford five coor-
dinate Rh(III) complexes [Rh(CO)(CORn)ClXL]{R1 = –CH3 (2a–c),
R2 = –C2H5 (3a–c); X = I and R3 = –CH2C6H5 (4a–c); X = Cl}. The IR
spectra of the oxidized products 2a–c, 3a–c and 4a–c show a sin-
gle characteristics ␯(CO) absorption in the range 2050–2079 cm−1
and a broad ␯(CO) band in the range 1743–1755 cm−1, indicative
of Rh(III)-acyl complex, resulting from facile migratory insertion
in 1a–c after OA of various electrophiles. The 1H NMR spectra of
2a–c show a singlet in the region ı 3.35–3.50 ppm indicating the
formation of -COCH3 group including other characteristic bands
of the ligands. In a similar manner, the 3a–c show a triplet at
around ı 1.53–1.68 ppm for methyl and a quartet in the region ı
2.35–2.72 ppm for methylene protons of the ethyl group. In case of
complexes 4a–c, the singlet resonances corresponding to methy-
lene protons of –OCH2C6H5 appears in the range ı 4.20–4.55 ppm.
The presence of electron withdrawing phenyl group deshields the
–OCH2– proton resonances and the peaks are obtained downfield
[24]. The 13C NMR spectra of 2a–c, 3a–c and 4a–c exhibit two car-
bonyl signals in the range ı 184–188 ppm for terminal carbonyl
group and a poorly resolved slightly broad signal in the range ı
203–208 ppm for acyl carbonyl group along with other characteris-
tic peaks correspond to different carbons present in the complexes.
Depending on the stereochemical arrangement of the ligands
and alkyl halides, several hexa-coordinated alkyl intermediates are
possible during OA reactions. As most of the penta coordinated
carbonyl-Rh(III)-acyl complexes reported are square pyramidal in
nature [24–26], it is likely that all the acyl complexes would also
have a similar geometry. The presence of a single high terminal
␯(CO) value is consistent with CO group trans to a weak trans
influencing chloride [25]. On the other hand, in view of high trans
influencing nature, the acyl group favours apical position trans to
the vacant coordination site [26–28]. Thus, the most probable struc-
tures of the acyl complexes are presented in Scheme 1.
3.1. Synthesis and characterization of Rh(I) complexes
The dimeric precursor [Rh(CO)2Cl]2 reacts with various ben-
zoylpyridine ligands a–c in 1:2 mol ratio to afford the complexes
of the type [Rh(CO)2ClL](1a–c) [Scheme 1]. The molecular compo-
sition of 1a–c were well supported by elemental analysis data. The
IR spectra of 1a–c show two almost equally intense terminal ␯(CO)
bands in the range 2007–2086 cm−1 indicating two carbonyl groups
are mutually cis to one another [21,22]. The ␯(CO) bands of the ben-
zoyl substituent appear almost in the same position as that of the
corresponding free ligands a–c [␯(CO) in cm−1: 1669(a), 1665(b),
1667(c)] and hence consistent with non coordination nature of the
group of the ligands upon complexation. The 1H NMR spec-
tra of 1a–c exhibit a doublet resonance in the range ı 9.02–9.11 ppm
for H1 and multiplets in the range ı 7.32–8.83 ppm for the other
protons of pyridine and benzene rings. The 1H NMR spectra of the
The kinetic experiments of OA reaction of 1a–c with CH3I were
monitored using FT-IR spectroscopy by following the decay of lower
frequency ␯(CO) band of 1a–c in the region 2010–2007 cm−1. A typ-
ical series of spectra of 1c when reacts with CH3I at 25 ◦C are shown
in Fig. 1, in which the bands due to 1c changes and that due to 2c
grow until equilibrium is attained. The two terminal ␯(CO) bands
of 1c at 2085.3 and 2010 cm−1 were replaced by the terminal and
acyl ␯(CO) bands of 2c at 2077 and 1752 cm−1, respectively. The
spectrum of the product 2c, exhibits quite broad absorptions with
shoulders, which are due to the presence of mixtures of isomers
[14,24,28]. Absorbance versus time plots for the decay of lower
intensity ␯(CO) band (2010 cm−1) of 1c is shown in Fig. 2. A linear
fit of pseudo-first-order was observed for the entire course of the
reaction of CH3I with 1a–c as is evidenced from the plot of ln(A0/At)
versus time t. (Fig. 3). From the slope of the plot, the rate constants
were calculated and found to be 3.56 × 10−4 s−1, 2.03 × 10−4 s−1
and 0.719 × 10−4 s−1, respectively for 1a, 1b and 1c. The values of
Scheme 1. Synthesis of Rh(I) and Rh(III) complexes of benzoylpyridine ligands.

B.J. Borah et al. / Journal of Molecular Catalysis A: Chemical 319 (2010) 66–70
69
Fig. 1. Series of IR spectra {␯(CO) region} illustrating the reaction of 1c with neat
CH3I at r.t. (∼25 ◦C). The arrows indicate the behavior of each band as the reaction
progresses.
Fig. 3. Plot of In(A0/At) versus time for the OA reaction of 1c with neat CH3I at r.t.
(∼25 ◦C).
metal centre [7,29]. From the observed trend of ␯(CO) frequen-
cies of 1a–c (1c > 1b > 1a), it is clearly observed that 1a is highly
nucleophilic and more prone towards the attack of electrophiles
and shows high reactivity towards OA. Therefore, the rate of OA of
CH3I with 1a–c substantiates the order as mentioned above.
3.3. Carbonylation of methanol to acetic acid and its ester using
1a–c as the catalyst precursor
The results of carbonylation of methanol to acetic acid and its
ester in the presence of 1a–c and [Rh(CO)2Cl]2 as catalyst pre-
cursors are shown in Table 1. The precursor 1a–c show a total
conversion of 79.5, 90.9 and 80.5% of CH3OH at 130 2 ◦C and
30 2 bar CO pressure with corresponding TON of 1529, 1748 and
1548. Under the same experimental condition, the well known cat-
alyst precursor [Rh(CO)2I2]− generated in situ from [Rh(CO)2Cl]2,
show lower TON 1000 only with corresponding conversion of about
52.1%. The effect of different ligands on the efficacy of catalytic
carbonylation reaction is clearly reflected and follows the order
1b > 1c >1a > [Rh(CO)2I2]−. It is well known that higher the rate of
OA higher is the catalytic activity. However, the observed trend in
activities among 1a–c towards catalytic carbonylation reaction can
not be explained by the observed trend of the rate of OA. The lower
activity of 1a compared to 1b and 1c may be due to the steric effect
caused by the benzoyl substituent at 2-position of pyridine ring
[7,14,29] and thus the steric effect of substituent plays a dominat-
ing role over electronic effect. On examining the catalytic reaction
mixture by IR spectroscopy at different time intervals and at the end
of the catalytic reaction, multiples ␯(CO) bands are obtained that
matched well with the ␯(CO) values of solution containing a mix-
ture of the parent Rh(I) carbonyl complexes 1a–c and Rh(III) acyl
complexes 2a–c. Thus, it may be inferred that the ligands remained
bound to the metal centre throughout the entire course of the cat-
alytic reactions.
Fig. 2. Kinetic plot showing the decay of ␯(CO) bands of 1c during the reaction of
1c with neat CH3I at r.t. (∼25 ◦C).
these rate constants clearly indicate that the rate of OA of CH3I with
1a–c follows the order 1a > 1b > 1c. The observed trend of OA may
be explained in terms of nucleophilicity of the metal centre which
in turn depends on the electron donating capacity of the ligand
around the coordination sphere. The electron donating power of
the N-atom (i.e. basicity) in the ligands is affected by the presence
of electron withdrawing –CO–Ph group as shown in Scheme 2. The
presence of –CO–Ph group at the 2- and 4-positions of the pyridine
ring in their corresponding complexes 1a and 1c should reduced the
basicity of N-atom and consequently show higher ␯(CO) frequen-
cies and hence should have slower rate of OA. Thus, to explain the
observed fact one must consider some other factors like neighbor-
ing group effects, field effects, etc. The appearance of lower ␯(CO)
frequency of 1a over 1b may be due to some short of interaction
between the metal centre and the benzoyl group at the ortho posi-
tion of the ligands and hence increased in nucleophilicity at the
Scheme 2. Effect of electron withdrawing benzoyl group on the electron donating capacity of N-donor site of the benzoylpyridine ligands.

70
B.J. Borah et al. / Journal of Molecular Catalysis A: Chemical 319 (2010) 66–70
Table 1
Results of carbonylation of methanol.
Catalysts precursor
Time (h)
Acetic acida (%)
Methyl acetatea (%)
Total conversion (%)
TONb
[Rh(CO)2I2]−c
1a
1b
1c
1
1
1
1
10.3
32.3
58.8
46.3
41.8
47.2
32.1
34.2
52.1
79.5
90.9
80.5
1000
1529
1748
1548
a
Yield of methyl acetate and acetic acid were obtained from GC analyses.
TON = [amount of product (mol)]/[amount of catalysts (Rh mol)].
Formed from added [Rh(CO)2Cl]2 under catalytic condition.
b
c
4. Conclusions
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S.M. Aucott, J. Organomet. Chem. 691 (2006) 1129.
[9] G. Lamb, M. Clarke, A.M.Z. Slawin, B. Williams, L. Key, Dalton Trans. (2007)
5582.
[10] P.R. Ellis, J.M. Pearson, A. Haynes, H. Adams, N.A. Bailey, P.M. Maitlis,
Organometallics 13 (1994) 3215.
[11] J.G. Haasnoot, Coord. Chem. Rev. 131 (2000) 200.
[12] M.H. Klingele, S. Brooker, Coord. Chem. Rev. 241 (2003) 119.
[13] N. Kumari, B.J. Sarmah, D.K. Dutta, J. Mol. Catal. A: Chem. 266 (2006) 260.
[14] B.J. Sarmah, B.J. Borah, B. Deb, D.K. Dutta, J. Mol. Catal. A: Chem. 289 (2008)
95.
The three new complexes 1a–c have been synthesized and char-
acterized. The 1a–c undergo OA with electrophiles like CH3I, C2H5I
and C6H5CH2Cl to give oxidized Rh(III) complexes 2a–c, 3a–c and
4a–c. The kinetics study of 1a–c with CH3I follows pseudo-first
order reaction. The 1a–c exhibit high catalytic activity in the car-
bonylation of methanol to acetic acid and its ester and show a
higher TON (1529–1748) than the well known Monsanto’s species
[Rh(CO)2I2]− (TON = 1000).
[15] D.K. Dutta, P. Chutia, B.J. Sarmah, B.J. Borah, B. Deb, J.D. Woollins, J. Mol. Catal.
A: Chem. 300 (2009) 29.
[16] A.J. Canty, Acc. Chem. Res. 25 (1992) 83.
[17] B.D. Brown, P.K. Byers, A.J. Canty, Organometallics 9 (1990) 1231.
[18] M.I. Bruce, Angew. Chem. 89 (1975) 75.
Acknowledgements
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The authors are grateful to Dr. P.G. Rao, Director, NEIST (CSIR),
Jorhat-785006, Assam, India for his kind permission to publish the
work. Thanks are also due to the DST, New Delhi (Grant: SR/S1/IC-
05/2006) for the partial financial grant. The authors BJB, BD and
PPS thank CSIR, New Delhi, for the award of Junior Research Fel-
lowships.
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