Efficient carbonylation of methanol catalyzed by rhodium(I) cyclooctadiene complexes with triphenylphosphinechalcogenide ligands

Article abstract of DOI:10.1246/cl.2002.766

Rhodium(I) complexes of the type, [Rh(COD)ClL] (COD = 1,5-cyclooctadiene and L = Ph₃PO, Ph₃PS and Ph₃PSe) have been synthesized. The complexes show higher efficiency as catalyst for carbonylation of methanol to acetic acid and methyl acetate at 130°C and 15 bar pressure than the industrially used species [Rh(CO)₂I₂]^-.

Full text of DOI:10.1246/cl.2002.766

766
Chemistry Letters 2002
Efficient Carbonylation of Methanol Catalyzed by Rhodium(I) Cyclooctadiene Complexes
with Triphenylphosphinechalcogenide Ligands
Pankaj Das, Pratap Chutia, and Dipak Kumar Duttaꢀ
Material Science Division, Regional Research Laboratory (CSIR), Jorhat 785 006, Assam, India
(Received March 6, 2002; CL-020214)
Rhodium(I) complexes of the type, [Rh(COD)ClL]
(COD ¼ 1,5-cyclooctadiene and L ¼ Ph3PO, Ph3PS and
Ph3PSe) have been synthesized. The complexes show higher
efficiency as catalyst for carbonylation of methanol to acetic acid
and methyl acetate at 130 ꢁC and 15 bar pressure than the
industrially used species [Rh(CO)2I2]À.
Rhodium catalyzed carbonylation of methanol is a commer-
cially important route for the production of acetic acid and methyl
acetate. Since the first report,1 the species [Rh(CO)2I2]À, has been
used as preferred commercial catalyst. Research activities are
continued to replace the existing rhodium catalyst either by
incorporating different types of electron donating ligands in the
metal complex species2 or by changing the metal center.3
Recently, BP chemicals has introduced4 a new catalytic process
for acetic acid synthesis in which rhodium is replaced by iridium
(Cativa process).
Figure 1. The TON with corresponding time of carbonylation reaction
of methanol in the presence of (a) [Rh(CO)2I2]À; (b)
As a part of our work5 i.e., the effects of various ligands on
rhodium catalyzed carbonylation of methanol, we report here
synthesis and catalytic activity of rhodium(I) complexes contain-
ing 1,5-cyclooctadiene and triphenylphosphinechalcogenide
ligands.
[Rh(COD)Cl(PPh3O)];
[Rh(COD)Cl(PPh2Se)]
Moles of product
Moles of catalyst xreaction time
(c)
as
[Rh(COD)Cl(PPh2S)];
catalyst precursors.
(d)
TON ¼
Moles of product have been calculated from
GC analysis.
The dimeric complex, [Rh(COD)Cl]2, undergoes bridge
splitting reaction with two equivalent of triphenylphosphine-
chalcogenide in dichloromethane under refluxing condition for
30 min to yield complexes of the type [Rh(COD)ClL]
(L ¼ Ph3PO, Ph3PS and Ph3PSe). The IR spectra of the
complexes show ꢀ(P=O), ꢀ(P=S) and ꢀ(P=Se) bands at 1170,
600 and 540 cmÀ1 respectively which are 30, 33 and 18 cmÀ1 less
compared to the corresponding free ligands indicating complexa-
tion through chalcogen donor. The complexes have also been
characterized6 by elemental analysis, 1H, 13C and 31P NMR
spectroscopy.
precursors may be attributed to the high nucleophilicity of the
rhodium center caused by the presence of (i) COD group which
has a lesser ꢁ-acceptor capacity compared to CO group and (ii)
Ph3PX (X=O, S and Se) group, which are stronger donor than
iodide. It is interesting to note that the complexes containing
phosphinesulfide and phosphineselenide ligand show much
higher activity compared to that containing phosphineoxide
ligand. This higher activity may be attributed to the soft nature of
Rh(I) which interacts strongly with soft S or softer Se atom and
thus provides more electron density on the rhodium centre as
compared to hard oxygen. It may be mentioned here that the
present complexes [Rh(COD)ClL] (except L ¼ PPh3O) showed
much higher catalytic activity for carbonylation of methanol even
under milder reaction conditions and within shorter reaction time
compared to our earlier work5a where the complexes of the type
[Rh(CO)2ClL] (L ¼ PPh3O, PPh3S, PPh3Se) exhibited a max-
imum conversion of 72–75% methanol at 150 ꢁC and 20 bar
pressure within a period of 1.5 h. It is clear (Figure 2) that with the
progress of the reaction, the selectivity of the catalyst changes i.e.
initially the major product was methyl acetate but near the end of
the reaction acetic acid was the major yield (in our earlier work5a
methyl acetate was the major yield). Among the complexes, the
one containing phosphineoxide ligand shows the lowest AcOH :
MeOAc ratio upto about 50 min reaction time and thereafter the
ratio changes steeply to the highest value during 60 min. After
completion of reaction (60 min run) the catalysts have been
recovered and on recycling9 for second time, almost same amount
of conversion was obtained indicating longer durability as well as
The results of batch carbonylation of methanol7 to acetic acid
and methyl acetate in the presence of complexes [Rh(COD)ClL]
as catalyst precursors (eq 1) are shown in Figure 1 and 2.
The Figure 1 indicates that for any reaction time, the
complexes [Rh(COD)ClL] show higher turn over number (TON)
compared to the well known species, [Rh(CO)2I2]À generated in-
situ from the added [Rh(CO)2Cl]2.2c;8 With the progress of
reaction time, the TON of the catalyst increases and at 60 min,
maximum TON of 453, 782 and 766 with corresponding
conversion of 58, 100 and 98% have been obtained for the
complexes containing phosphineoxide, phosphinesulfide and
phosphineselenide, respectively. Such a high TON shown by
the complexes [Rh(COD)ClL] over [Rh(CO)2I2]À as catalyst
Copyright Ó 2002 The Chemical Society of Japan

Chemistry Letters 2002
767
2
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3
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M. Maitlis, G. J. Sunley, M. J. Baker, and A. Haynes, J. Chem. Soc.,
Chem. Commun., 1998, 1023. c) J. Yang, A. Haynes, and P. M.
Maitlis, J. Chem. Soc., Chem. Commun., 1999, 179. d) M. Cheong,
R. Schmid, and T. Ziegler, Organometallics, 19, 1973 (2000).
Chem. Ind., (London), 1996, 483.
a) P. Das, D. Konwar, P. Sengupta, and D. K. Dutta, Trans. Met.
Chem., 25 426 (2000). b) P. Das, D. Konwar, and D. K. Dutta,
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178, 283 (2002).
4
5
Figure 2. The ratio of acetic acid and methyl acetate with time for
carbonylation of methonal in the presence of (a) [Rh(CO)2I2]À; (b)
[Rh(COD)Cl(PPh3O)];
[Rh(COD)Cl(PPh3Se)] as catalyst precursors.
(c)
[Rh(COD)Cl(PPh3S)];
(d)
6
Analytical and spectroscopic data of the complexes,
[Rh(COD)Cl(PPh3O)]: yield: 98%; Anal. Calcd for
C
26H27ClPORh: C, 59.54; H, 5.15%; Found: C, 59.41; H, 5.13%.
IR (KBr/cmÀ1): 1170 (ꢀ PO). NMR (300 MHz, in CDCl3, ꢂ), 1H:
7.70–7.27 (m, C6H5), 4.22 (-CH=CH), 2.49 (-CH2-C); 13C{1Hg:
133.13–128.40 (m, C6H5), 78.76 (-CH=CH), 30.11
(-CH2-C); 31P{1Hg: 29.81. [Rh(COD)Cl(PPh3S)]: Yield: 99%;
Anal. Calcd for C26H27ClPSRh: C, 57.77; H, 5.00%; Found: C,
57.91; H, 5.02%. IR (KBr /cmÀ1): 600 (ꢀ PS). NMR (300 MHz, in
CDCl3, ꢂ), 1H: 7.75–7.41 (m, C6H5), 5.30 (-CH=CH), 2.48
(-CH2-C); 13C{1Hg: 132.44–128.41 (m, C6H5), 78.57 (-CH=CH),
30.85 (-CH2-C); 31P{1Hg: 43.87. [Rh(COD)Cl(PPh3Se)]: Yield:
98%; Anal. Calcd for C26H27ClPSeRh: C, 53.15; H, 4.59%; Found:
C, 53.03; H, 4.62%. IR (KBr/cmÀ1): 540 (ꢀ PSe). NMR (300 MHz,
in CDCl3, ꢂ), 1H: 7.76–7.26 (m, C6H5), 4.23 (-CH=CH), 2.46
(-CH2-C); 13C{1Hg: 134.87–128.66 (m, C6H5), 78.79
(-CH=CH), 33.10 (-CH2-C); 31P{1Hg: 31.81 (d, JPse ¼ 364 Hz).
Catalytic experiment: MeOH (3.1 ml, 0.077 mol); MeI (1 ml,
0.006 mol); H2O (1 ml, 0.055 mol) and the complex
7
[Rh(COD)Cl(PPh3O)]
[Rh(COD)Cl(PPh3S)]
(24mg,
(25 mg,
0.046 mmol)
0.046 mmol)
or
or
Scheme 1.
[Rh(COD)Cl(PPh3Se)] (27 mg, 0.046 mmol) was 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 15 bar by increasing the
temperature to 130 ꢁC.
stability of the catalyst.
A possible pathway (Scheme 1) has been proposed for the
above mentioned reaction where a Rh(III) alkyl complex (A) and
Rh(III) acyl complex (B) act as intermediates. The acyl species
(B) on subsequent reductive elimination of MeCOI gives back the
parent complex and thus completes the cycle. This mechanism is
similar to the cycle for catalysis by [Rh(CO)2I2]À.10 The above
mechanistic route is proposed on the basis of separately done
model reaction of [Rh(COD)Cl(Ph3PS)] with MeI, which leads to
the formation of [Rh(COD)ClI(Me) (Ph3PS)](A).11 The complex
A in methanol reacts with CO gas (6 bar) at 130 ꢁC and 15 bar
8
9
D. Forster, J. Am. Chem. Soc., 98, 846 (1976).
Catalyst recycle experiment: recycle experiments were done by
maintaining the same experimental condition as described in
Ref. 7.
10 a) D. Forster, Adv. Organomet. Chem., 17, 255 (1979). b) A.
Haynes, B. E. Mann, G. E. Morris, and P. M. Maitlis, J. Am. Chem.
Soc., 115, 4093 (1993).
11 [Rh(COD)ClI(Me)(PPh3S)]: Yield: 68%; Anal. Calcd for
C
27H30ClIPSRh: C, 47.50; H, 4.39%; Found: C, 47.61; H, 4.37%;
IR (KBr/cmÀ1): 593 (ꢀ PS); NMR (300 MHz, in CDCl3, ꢂ), 1H:
7.82–7.36 (m, C6H5), 5.18 (-CH=CH), 2.43 (-CH2-C), 1.91
(-CH3); 13C{1Hg: 131.97–127.90 (m, C6H5), 78.01 (-CH=CH),
31.01 (-CH2-C), 11.1(CH3); 31P{1Hg: 44.26.
pressure for 1 h converts to
[Rh(COD)ClI(COMe)(Ph3PS)](B).12
a rhodium acyl complex
The authors are grateful to the Director, Regional Research
Laboratory Jorhat, India, for his kind permission to publish the
work. The DST, New Delhi is also acknowledged for a financial
grant. One of the authors (PD) thanks CSIR, New Delhi, for the
award of SRF.
12 [Rh(COD)ClI(COMe)(PPh3S)]: Yield: 72%; Anal. Calcd for
C
28H30ClIPO-SRh: C, 47.32; H, 4.22%; Found: C, 47.20; H,
4.20%; IR (KBr/cmÀ1): 595 (ꢀ PS); 1700 (ꢀ CO); NMR (300 MHz,
in CDCl3, ꢂ), 1H: 7.76–7.27 (m, C6H5), 5.07 (-CH=CH), 2.46
(-CH2-C), 3.01 (-COCH3); 13C{1Hg: 132.07–128.01 (m, C6H5),
78.49 (-CH=CH), 30.87 (-CH2-C), 209.10 (-CO), 37.10
(-CH3); 31P{1Hg: 44.06.
References and Notes
1
F. E. Paulik and J. F. Roth, Chem. Commun., 1968, 1578.