Rhodium(I) carbonyl complexes of quinoline carboxylic acid: Synthesis, reactivity and catalytic carbonylation reaction

Journal of Molecular Catalysis A: Chemical 372 (2013) 1–5

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Rhodium(I) carbonyl complexes of quinoline carboxylic acid: Synthesis, reactivity and catalytic carbonylation reaction Podma Pollov Sarmah, Dipak Kumar Dutta ∗ Materials Science Division, CSIR-North East Institute of Science and Technology, Jorhat 785006, Assam, India

a r t i c l e

i n f o

Article history: Received 6 October 2012 Received in revised form 22 January 2013 Accepted 5 February 2013 Available online xxx Keywords: Rhodium Carbonyl ligand N-donor ligand Oxidative addition Carbonylation

a b s t r a c t The dimeric rhodium precursor [Rh(CO)2 Cl]2 reacts with the ligands (L) quinoline-2-carboxylic acid (a) or quinoline-8-carboxylic acid (b) in 1:2 mole ratio to afford complexes of the type cis-[Rh(CO)2 ClL] (1a and 1b). The complexes have been characterized by elemental analysis, Mass spectrometry, FT-IR and NMR (1 H, 13 C) spectroscopy. 1a and 1b undergo oxidative addition (OA) with different electrophiles such as CH3 I, C2 H5 I and I2 to give Rh(III) complexes of the type [Rh(CO)(COR)ClIL] {R = CH3 (2a and 2b), R = C2 H5 (3a and 3b)} and [Rh(CO)ClI2 L] (4a and 4b) respectively. OA of the CH3 I with 1a forms relatively stable acyl intermediate which is evident from IR spectroscopy. The complexes 1a and 1b show higher catalytic activity for carbonylation of methanol to acetic acid and methyl acetate [Turn over number (TON) upto 1775 in 1 h] compared to that of the well known Monsanto’s species [Rh(CO)2 I2 ]− (TON = 1000 in 1 h) under the reaction conditions: temperature 130 ± 2 ◦ C, pressure 30 ± 2 bar, 450 rpm and 1 h reaction time. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Acetic acid is one of the vital chemical required for preparation of lots of specialty and important chemicals [1–3]. Methanol is one of the cheap and readily available chemicals, which can be converted to acetic acid by carbonylation reaction. About 80% of the total acetic acid is produced annually by methanol carbonylation [4]. The well known Monsanto’s process is the most widely adopted industrial carbonylation technology used for acetic acid production. The Monsanto’s process is based on Rh promoted catalytic carbonylation where [Rh(CO)2 I2 ]− is the active catalytic species and CH3 I used as co-catalyst [5–7]. But this process requires drastic reaction conditions like high pressure and temperature. Therefore, considerable efforts have been made to improve the catalysts by incorporating different ligands into its coordination sphere for better activity compared to Monsanto’s species [8–14]. So far, phosphorus containing ligands are most extensively studied, since they can stabilize low valent metal centre by both ␴-bonding and ␲-back bonding. Recently ligands containing N/N∼O donor atoms have also aroused considerable interest because of their structural novelty and catalytic activity [15–24]. N-atom is strong ␴-donor, thereby it imparts more ionic character to metal-ligand bond and makes the metal centre more electron rich. Therefore, the metal centre

∗ Corresponding author. Tel.: +91 376 2370081; fax: +91 376 2370011. E-mail addresses: [email protected], dutta [email protected] (D.K. Dutta). 1381-1169/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.molcata.2013.02.005

becomes more susceptible to oxidative addition, which is the key step in carbonylation reaction [1,2,14]. Moreover, N-atom is not susceptible to aerial oxidation unlike P-atom and thus N-donor ligands may be advantageous over P-donor ligands. In case of N∼O ligands, the oxygen atom, being hard donor, confers stability to metal at high oxidation state in the oxidative addition reaction [15]. Thus, the different hardness and donor properties of N∼O ligands may offer advantages in catalysis. As a part of our continuing research activity [8,9,14–16,23–26], we have chosen two N∼O donor ligand, quinoline-2-carboxylic acid and quinoline-8-carboxylic acid for synthesis of rhodium carbonyl complexes. The reactivities of small molecules like CH3 I, C2 H5 I, and I2 towards the complexes were evaluated. The kinetics of OA of CH3 I with the complexes was also carried out. The catalytic activity of the complexes has also been demonstrated in carbonylation of methanol for the production of acetic acid and methyl acetate. 2. Experimental 2.1. General information All operations were carried out under N2 environment. All solvents were distilled under N2 prior to use. RhCl3 ·xH2 O (Rh content 40%) was purchased from M/S Arrora Matthey Ltd., Kolkata, India. Quinoline carboxylic acid ligands (98%) were purchased from M/S Aldrich, USA and used without further purification. Dichloromethane (99%), diethyl ether (99%), hexane (99%) and methanol (99.5%) were purchased from Ranbaxy fine chemicals

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P.P. Sarmah, D.K. Dutta / Journal of Molecular Catalysis A: Chemical 372 (2013) 1–5

limited (India) and used after distilled using standard technique. Methyl iodide (99.5%), KBr (IR grade) and CHCl3 (IR grade) were purchased from M/S Merck, Germany and used without further purification. Carbon monoxide gas (99.9%) was purchased from Alchemie gases and chemicals Pvt. Ltd. (India). 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 Shimadzu IRAffiniry-1 spectrophotometer. The 1 H and 13 C NMR spectra were recorded at room temperature (r.t.) in CDCl3 solution on a Bruker DPX-300 Spectrometer and chemical shifts were reported relative to SiMe4 . 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 analyzed by GC (Chemito 8510, FID). 2.2. Starting materials [Rh(CO)2 Cl]2 was prepared by passing CO gas over RhCl3 ·3H2 O at 100 ◦ C in the presence of moisture [27].

2b: yield: 0.053 g, 76%. IR (KBr): 2065 [␯(CO)], 1734 [␯(acyl)] cm−1 . 1 H NMR (300 MHz, CDCl3 ) data (ı in ppm): ı 2.43 (3H, s, CH3 ), ı 7.85–8.93 (5H, m, Py, Ph), ı 9.36 (H-2, d, JH–H = 7.6 Hz, Py). 13 C NMR (75 MHz, CDCl3 ) data (ı in ppm): ı 46.0 (CH3 ), ı 129.8–155.0 (m, Ph, Py), ı 170.0 (COOH), ı 188 (br, CO), ı 207.6 (br, COacyl ). 3a: yield: 0.051 g, 72%. IR (KBr): 2076 [␯(CO)], 1731 [␯(acyl)] cm−1 . 1 H NMR (300 MHz, CDCl3 ) data (ı in ppm): ı 1.19 (3H, t, JH–H = 6.1 Hz, CH3 ), ı 2.87 (2H, q, JH–H = 6.1 Hz, CH2 ), ı 7.89–8.37 (4H, m, Ph), ı 8.71 (H-3, d, JH–H = 7.9 Hz, Py), ı 9.59 (H-4, d, JH–H = 7.9 Hz, Py). 13 C (75 MHz, CDCl3 ) data (ı in ppm): ı 22.1 (CH3 ), ı 56.3 (CH2 ), ı 122.7–159.1 (m, Ph, Py), ı 167.5 (COOH), ı 189 (br, CO), 205 (br, COacyl ). 3b: yield: 0.049 g, 69%. IR (KBr): 2067 [␯(CO)], 1739 [␯(acyl)] cm−1 . 1 H NMR (300 MHz, CDCl3 ) data (ı in ppm): ı 1.27 (3H, t, JH–H = 6.3 Hz, CH3 ), ı 2.98 (2H, q, JH–H = 6.3 Hz, CH2 ), ı 7.89–8.91 (5H, m, Ph), ı 9.28 (H-2, d, JH–H = 7.6 Hz, Py). (75 MHz, CDCl3 ) data (ı in ppm): ı 25.3 (CH3 ), ı 56.6 (CH2 ), ı 125.9–153.9 (m, Ph, Py), ı 169.3(COOH), ı 191 (br, CO), ı 208 (br, COacyl ). 2.5. Synthesis of [Rh(CO)ClI2 L] (4a and 4b)

2.3. Synthesis of the complexes [Rh(CO)2 ClL] (1a and 1b), L = quinoline-2-carboxylic acid (a), quinoline-8-carboxylic acid (b) About 0.257 mmol (100 mg) [Rh(CO)2 Cl]2 was dissolved in dichloromethane (10 cm3 ) and to this solution, 0.514 mmol (89 mg) of the appropriate ligand was added. The reaction mixture was stirred at r.t. for 30 min and the solvent was evaporated under vacuum. The dark purple compounds so obtained were washed with diethyl ether, recrystallised and stored over silica gel under vacuum in a desiccator. Analytical data for 1a and 1b are as follows: 1a: yield: 0.156 g, 83%. IR (KBr): 2085, 1999 [␯(CO)], 1660 [␯( COOH)] cm−1 . 1 H NMR (300 MHz, CDCl3 ) data (ı in ppm): ı 7.91–8.28 (4H, m, Ph), ı 8.39 (H-3, d, JH–H = 8.4 Hz, Py), ı 9.32 (H-4, d, JH–H = 8.4 Hz, Py). 13 C NMR (75 MHz, CDCl3 ) data (ı in ppm): ı 125.7–149.3 (m, Ph, Py), ı 169.5 (COOH), ı 186.1 (CO, 1 JRh–C = 67.7 Hz), ı 189 (CO, 1 JRh–C = 64.3 Hz). C12 H7 NClO4 Rh (367.57): cald. C 39.18, H 1.90, N 3.81; found C 39.01, H 1.81, N 3.75. MS: m/z = 367.5 [M+ ]. 1b: yield: 0.164 g, 87%. IR (KBr): 2073, 1985 [␯(CO)], 1666 [␯( COOH)] cm−1 . 1 H NMR (300 MHz, CDCl3 ) data (ı in ppm): ı 7.81–8.91 (5H, m, Py, Ph), ı 9.23 (H-2, d, JH–H = 7.6 Hz, Py). 13 C NMR (75 MHz, CDCl3 ) data (ı in ppm): ı 123.7–151.0 (m, Ph, Py), ı 167.2 (COOH), ı 186.4 (CO, 1 JRh–C = 64.7 Hz), ı 188.2 (CO, 1J Rh–C = 62.3 Hz). C12 H7 NClO4 Rh (367.57): cald. C 39.18, H 1.90, N 3.81; found C 38.96, H 1.82, N 3.71. MS: m/z = 367.7 [M+ ]. 2.4. Synthesis of [Rh(CO)(COR)ClIL] {R = CH3 , (2a and 2b), R = C2 H5 (3a and 3b)} [Rh(CO)2 ClL] (50 mg) (1a and 1b) was dissolved in dichloromethane (5 cm3 ) and each of RX (3 cm3 ) (RX = CH3 I, C2 H5 I) was added to it. The reaction mixture was then stirred at r.t. for about 6–12 h to yield 2a, 2b and 3a, 3b. The color of the solution changed from yellowish red to dark reddish brown and the solvent was evaporated under vacuum. The compounds so obtained were washed with diethyl ether and stored over silica gel in a desiccator. Analytical data for 2a, 2b, 3a and 3b are as follows: 2a: yield: 0.046 g, 67%. IR (KBr): 2073 [␯(CO)], 1726 [␯(acyl)] cm−1 . 1 H NMR (300 MHz, CDCl3 ) data (ı in ppm): ı 2.51 (3H, s, CH3 ), ı 7.93–8.39 (4H, m, Ph), ı 8.69 (H-3, d, JH–H = 7.7 Hz, Py), ı 9.64 (H-4, d, JH–H = 7.7 Hz, Py). 13 C NMR (75 MHz, CDCl3 ) data (ı in ppm): ı 45.0 (CH3 ), ı 123.9–151.3 (m, Ph, Py), ı 169.9 (COOH), ı 189 (br, CO), ı 204.0 (br, COacyl ).

0.018 mmol (0.05 g) of [Rh(CO)2 ClL] (1a and 1b) was dissolved in dichloromethane (5 cm3 ). To that solution iodine was added (0.02 mmol, 0.025 g). The reaction mixture was stirred for 6 h to generate 4a and 4b. The color of the solution changed from yellowish red to dark reddish brown and the solvent was evaporated under vacuum. Excess iodine was removed by washing several time with hexane and stored over silica gel in a desiccator. Analytical data for 4a and 4b are as follows: 4a: yield: 0.048 g, 57%. IR (KBr): 2065 [␯(CO)] cm−1 . 1 H NMR (300 MHz, d6 -DMSO) data (ı in ppm): ı 7.81–8.43 (4H, m, Ph), ı 8.77(H-3, d, JH–H = 7.2 Hz, Py), ı 9.76 (H-4, d, JH–H = 7.2 Hz, Py). 13 C NMR (75 MHz, d6 -DMSO) data (ı in ppm): ı 123.7–155.3 (m, Ph, Py), ı 171.2 (COOH), ı 187 (br, CO). 4b: yield: 0.066 g, 81%. IR (KBr): 2068[␯(CO)] cm−1 . 1 H NMR (300 MHz, d6 -DMSO) data (ı in ppm): ı 7.81–8.96 (5H, m, Py, Ph), ı 9.29 (H-2, d, JH–H = 7.1 Hz, Py). 13 C (75 MHz, d6 -DMSO) data (ı in ppm): ı 129.1–154.6 (m, Ph, Py), ı 168.7 (COOH), ı 189.2 (br, CO). 2.6. Kinetic experiments The kinetic experiments of OA reaction of complexes 1a and 1b with neat CH3 I were monitored using FT-IR spectroscopy in a solution cell (NaCl windows, 1 mm path length). In order to obtain pseudo-first-order condition, excess of CH3 I relative to metal complex 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 8 software. Kinetic measurements were made by following the decay of lower frequency ␯(CO) band of the complexes 1a and 1b in the region 1980–2000 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. 2.7. Carbonylation of methanol using complexes 1a and 1b as catalyst precursors CH3 OH (0.099 mol, 4 cm3 ), CH3 I (0.016 mol, 1 cm3 ), H2 O (0.055 mol, 1 cm3 ) and 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 (5, 10 and 20 bar) at 25 ◦ C. The carbonylation reactions were carried out at 130 ± 2 ◦ C for 1 h with CO

P.P. Sarmah, D.K. Dutta / Journal of Molecular Catalysis A: Chemical 372 (2013) 1–5

RhCl3.xH2O

2085

0.5 0.4

Absorbance

[Rh(CO)2Cl]2 L stir., 30 min, r.t

OC OC

2073 2026

CO

100oC, moisture

3

Rh 1a,1b

L Cl

0.3

1999

1726

0.2 0.1 0.0

2100

Cl OC

COCH3 L Rh I

Cl OC

2a, 2b

Cl

COC2H5 L Rh I

I OC

3a, 3b

Rh 4a, 4b

L I

L =

N a

COOH

N COOH b

Scheme 1. Synthesis and reactivity of 1a and 1b.

gas pressure 14 ± 2, 20 ± 2 and 33 ± 2 bar respectively. The products were collected and analyzed by GC. The quantitative amounts of the individual components are calculated from the peak area of respective component and comparing with total peak area of all the components. 3. Results and discussion 3.1. Synthesis and characterization of 1a and 1b The reaction of the chloro-bridged dimer [Rh(CO)2 Cl]2 in dichloromethane with two mol equivalents of the ligands, a and b affords the complexes of the type cis-[Rh(CO)2 Cl(L)] (la and 1b) [where L = a and b] (Scheme 1). Elemental analysis and mass spectrometric results of the complexes support the observed molecular composition of 1a and 1b. The IR spectra of 1a and 1b exhibit two equally intense ␯(CO) vibrations band in the range 1985–2085 cm−1 indicating the formation of cis-dicarbonyl rhodium (I) complexes. The 1 H NMR spectra of 1a and 1b show characteristic multiple resonances for the phenylic and pyridyl proton in the range ı 7.81–9.32 ppm. In the 13 C NMR spectra show two characteristics doublets at ı 186.1 (1 JRh–C = 67.7 Hz) and ı 189.0 (1 JRh–C = 64.3 Hz) ppm of 1a and, ı 186.4 (1 JRh–C = 64.7 Hz) and ı 188.2 (1 JRh–C = 62.3 Hz) ppm of 1b for the carbonyl groups coordinated to Rh metal. The signals for phenylic, pyridyl and carboxylic carbons are found in the respective ranges. Moreover, no prominent shifting of the 13 C NMR value of carboxylic group in 1a and 1b compared to the corresponding free ligand were observed, which indicates that the ligand are coordinated to metal centre through N-atom of the ligand only. 3.2. Reactivity of 1a and 1b towards different electrophiles. Activation of small molecules is one of the key steps in homogeneous catalysis. In this respect, OA of alkyl halides with rhodium metal complex is very important reaction step in the carbonylation

2000

Wavenumber (cm -1 )

1700

Fig. 1. Series of IR spectra illustrating the reaction of 1a with CH3 I at 25 ◦ C and growth of 2a.

reaction. Therefore, OA of various electrophiles like CH3 I, C2 H5 I and I2 with 1a and 1b was evaluated. The complexes 1a and 1b undergo OA with CH3 I and C2 H5 I to form Rh(III) complexes of the type [RhCl(CO)(COR)IL], where R = CH3 (2a, 2b) and C2 H5 (3a and 3b) (Scheme 1). On the other hand 1a and 1b reacts with I2 in CHCl3 to produce pentacoordinated Rh(III) complexes of the type [RhCl(CO)I2 L] (Scheme 1). FT-IR spectra of the oxidative adducts of 1a and 1b show a single band in the range 2065–2076 cm−1 , characteristic of terminal mono-carbonyl group. Moreover, a broad IR band in the range 1726–1739 cm−1 for 2a, 2b and 3a, 3b indicates the formation of acyl group. The single high value of the terminal ␯(CO) band indicates the formation of the oxidized products. Apart from the characteristics resonances for 2a, 2b and 3a, 3b the 1 H NMR, resonance in the range ı 2.51–2.98 ppm indicates the formation of acyl group. 13 C NMR for 2a, 2b and 3a, 3b show singlet peak in the range ı 189–191 and a broad peak in the range ı 204–208 ppm for terminal carbonyl group and acyl group respectively apart from other characteristic resonances. The 1 H and 13 C NMR of 4a and 4b show characteristic resonance of the phenylic and pyridyl group at slightly downfield compared to the parent compounds 1a and 1b. In addition, 13 C NMR spectra also show characteristic resonances in the range ı 187–189 ppm attributable to the presence of terminal CO group. Attempts to substantiate the structure of different rhodium(I) and rhodium(III) complexes by single crystal X-ray crystallography was not possible because, no suitable crystal could be obtained even after several attempts. 3.3. The kinetic study of OA reaction of CH3 I with 1a and 1b The kinetic experiments of OA of neat CH3 I with complexes 1a and 1b were monitored using FT-IR spectroscopy by monitoring the decay of the lower ␯(CO) band of the complexes. Fig. 1 shows a typical series of spectra of 1a during the reaction with CH3 I at 25 ◦ C, in which the bands at around 1999 and 2085 cm−1 decay and new bands grow at 2073 and a broad band 1720–1745 cm−1 until the equilibrium is attained. Finally, the two terminal ␯(CO) bands are replaced by the terminal ␯(CO) band at 2072 cm−1 and acyl ␯(CO) band at 1737 cm−1 . Absorbance versus time plots for the decay of lower intensity ␯(CO) bands at 1999 and 1985 cm−1 of 1a and 1b respectively are shown in Fig. 2. A linear fit of pseudo-first order was observed for the entire course of the reaction of CH3 I with the complexes 1a and 1b 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,

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P.P. Sarmah, D.K. Dutta / Journal of Molecular Catalysis A: Chemical 372 (2013) 1–5

0.06

0.24

1a

1b

0.05 0.04

0.16

Absorbance

Absorbance

0.20

0.12 0.08

0.03 0.02 0.01

0.04 0

500

1000

Time (sec)

1500

0.00

2000

0

500

1000

1500

Time (sec)

Fig. 2. Kinetic plot showing the decay of ␯(CO) bands with time of 1a and 1b during the reaction with neat CH3 I at r.t. (∼25 ◦ C).

2.5

2.0

2.0

ln (Ao/At)

1a

1.0

1b

1.5

ln (Ao/At)

1.5

1.0

0.5

0.5

0.0

0.0 0

500

1000 1500 Time (sec)

2000

0

500

1000

Time (sec)

1500

2000

Fig. 3. Plot of ln(A0 /At ) versus time for the OA reaction of the complex 1a and 1b with neat CH3 I at r.t (∼25 ◦ C).

Scheme 2. Carbonylation of methanol to acetic acid and methyl acetate in the presence of catalyst.

respectively (Fig. 3). From the slopes of the plots, the rate constants were calculated and found as 1.052 × 10−5 and 2.567 × 10−5 s−1 for the complexes 1a and 1b respectively. The observed values of rate constants indicate that the rate of OA is higher in case of 1b (about 2.5 times) than 1a. This may be due to the some short of interaction of carboxylic acid group with the Rh centre which may sterically restrict the OA of CH3 I group with Rh centre [26]. It is interesting to observe additional ␯(CO) band around 2026 cm−1 after addition of CH3 I to 1a, which is assigned to intermediate dicarbonyl species [Rh(CO)2 ClIL(CH3 )] formed during first step of OA [9,28]. The band depletes rapidly with the progress of the reaction time and finally disappears. 3.4. Catalytic carbonylation of methanol by 1a and 1b The catalytic activity of 1a and 1b were evaluated in carbonylation of methanol to acetic acid and methyl acetate (Scheme 2). The results are shown in Table 1. The precursor complexes show a total conversion of CH3 OH in the range 45.1–92.5% at 130 ± 2 ◦ C

under CO pressure of 14 ± 2 to 33 ± 2 bar for 1 h reaction time with corresponding TON in the range 866–1768. It has been observed for 1a, that increase in initial CO pressure from 5 bar to 20 bar enhance the conversion from 45.1% to 92.1% with corresponding TON from 866 to 1768. Similar trend has also been observed for 1b. Under the similar reaction conditions, the well known Monsanto’s species [Rh(CO)2 I2 ]− shows a total conversion in the range 20.1% to 52.1% (14 ± 2 to 33 ± 2 bar CO pressure at 130 ± 2 ◦ C) with corresponding TON of 463 to 1000 in 1 h. This indicates that the catalytic efficiency of the complexes is greatly enhanced by the incorporation of the ligands (L) into the coordination sphere of the rhodium centre. Again, in our earlier work [15], the rhodium carbonyl complex with unsubstituted quinoline showed a maximum TON of 1711 under the similar experimental condition. Thus, substitution of carboxylic acid group in the quinoline ring has increases the catalytic efficiency of the complexes. It is interesting to observe here that the both complexes show comparable catalytic efficacy though less reactivity of complex 1a was observed than 1b during OA of CH3 I. This is probably due the drastic reaction condition leading to minimize the steric restriction imparted by ligand during OA of CH3 I group with Rh centre of the complex 1a. The formation of methyl acetate as one of the products (Table 1) is due to the condensation reaction between methanol and acetic acid which is formed during reaction, and also the acetic acid catalyze simultaneously the condensation reaction [29]. The significant higher conversion

P.P. Sarmah, D.K. Dutta / Journal of Molecular Catalysis A: Chemical 372 (2013) 1–5

5

Table 1 Results of carbonylation reaction of methanol to acetic acid and methyl acetate. CO pressure at 25 ◦ C (bar)

CO pressure at 130 ± 2 ◦ C (bar)

Total conversion (%)

Acetic acid (%)a

Methyl acetate (%)a

TONb

[Rh(CO)2 I2 ]

5 10 20

14 ± 2 20 ± 2 33 ± 2

24.1 41.4 52.1

4 12.3 10.3

20.1 29.1 41.8

463 796 1000

1a

5 10 20

14 ± 2 20 ± 2 33 ± 2

45.1 58.2 92.1

10.9 20.8 9.8

34.2 37.4 82.3

866 1117 1768

1b

5 10 20

14 ± 2 20 ± 2 33 ± 2

47.6 59.4 92.5

12.2 24.1 18.9

35.4 35.3 73.6

914 1160 1775

Catalyst precursor c

a b c

Yield of methyl acetate and acetic acid were obtained from GC analyses. TON = [amount of product (mol)]/[amount of catalyst (Rh mol)]. Formed from added [Rh(CO)2 Cl]2 under catalytic condition.

to methyl acetate at 20 bar CO pressure in case of catalyst 1a compared to 1b may be due to the chelation tendency of the COOH group of the ligand a in the catalytic reaction process, which is likely to increase the acidity of carboxylic acid group than in 1b and accordingly enhance the rate of ester formation. The organometallic residue of 1a and 1b were recovered after the first catalytic run, which were identified as rhodium(III) acyl complexes. 4. Conclusion Two new complexes 1a and 1b have been synthesized and characterized. The complexes undergo OA with different electrophiles like CH3 I, C2 H5 I and I2 to afford Rh(III) complexes of the type [Rh(CO)(COR)ClIL] [R = CH3 (2a and 2b), R = C2 H5 (3a and 3b)] and [Rh(CO)ClI2 L] (4a and 4b). OA of CH3 I with 1a formed relatively stable acyl intermediate substantiated by IR spectroscopy. The catalytic activities of 1a and 1b for the carbonylation of methanol to acetic acid and its ester exhibit a higher TON (upto 1775) in 1 h compared to the well known Monsanto’s species [Rh(CO)2 I2 ]− (TON = 1000) under the similar experimental condition. Acknowledgments The authors are grateful to Dr. P. G. Rao, Director, CSIR-North East Institute of Science and Technology, Jorhat-785006, Assam, India for his kind permission to publish the work. The authors thank Dr P. Sengupta, Head, Materials Science Division, CSIR-NEIST, Jorhat, for his constant encouragement. Thanks are also due to Department of Science and Technology (DST), New Delhi (Project File No. SR/S1/IC34/2011) and CSIR, New Delhi [Project No. MLP-6000/1] for the financial support. The author P.P. Sarmah thanks to CSIR, New Delhi for providing the Senior Research Fellowship. References [1] V.H. Agreda, J.R. Zoellar (Eds.), Acetic Acid and its Derivatives, CRC Press, New York, 1992.

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