Isolation and characterization of an iodide bridged dimeric palladium complex in carbonylation of methanol

Article abstract of DOI:10.1016/j.jorganchem.2005.01.010

Palladium-catalyzed carbonylation of methanol in presence of iodide promoters was investigated. Iodide bridged palladium dimeric complex, [PPh ₃CH₃]₂[Pd₂I₆] was isolated from the carbonylation reaction mixture and characterized using X-ray crystallography. Reaction mechanism was proposed based on IR and UV spectroscopic characterizations of catalytic species involved in the catalytic cycle. The isolated dimeric palladium species, [Pd₂I ₆]^2- underwent carbonylation to give monomeric species [PdI₃CO]^- at atmospheric pressure of carbon monoxide. It was also observed that PPh₃ plays an important role to avoid catalyst deactivation at higher temperatures. Turnover frequency (TOF) of 1052 h ^-1 was achieved using Pd(OAc)₂-HI-PPh₃ catalyst system at 175°C.

Full text of DOI:10.1016/j.jorganchem.2005.01.010

Journal of Organometallic Chemistry 690 (2005) 1677–1681
www.elsevier.com/locate/jorganchem
Note
Isolation and characterization of an iodide bridged dimeric
palladium complex in carbonylation of methanol
a
Sunil S. Tonde , Ashutosh A. Kelkar , Mohan M. Bhadbhade ,
a
b
a,
*
Raghunath V. Chaudhari
a
Homogeneous Catalysis Division, National Chemical Laboratory, Homi Bhabha Road, Pune 411 008, India
b
Center for Materials Characterization (X-ray Facility), National Chemical Laboratory, Pune 411 008, India
Received 14 October 2004; accepted 6 January 2005
Available online 1 February 2005
Abstract
Palladium-catalyzed carbonylation of methanol in presence of iodide promoters was investigated. Iodide bridged palladium
dimeric complex, [PPh3CH3]2[Pd2I6] was isolated from the carbonylation reaction mixture and characterized using X-ray crystallog-
raphy. Reaction mechanism was proposed based on IR and UV spectroscopic characterizations of catalytic species involved in the
catalytic cycle. The isolated dimeric palladium species, [Pd2I6]2À underwent carbonylation to give monomeric species [PdI3CO]À at
atmospheric pressure of carbon monoxide. It was also observed that PPh3 plays an important role to avoid catalyst deactivation at
higher temperatures. Turnover frequency (TOF) of 1052 hÀ1 was achieved using Pd(OAc)2–HI–PPh3 catalyst system at 175 ꢀC.
ꢁ 2005 Elsevier B.V. All rights reserved.
Keywords: Methanol carbonylation; Palladium catalyst; Dimeric palladium; UV; IR spectroscopy; Reaction mechanism
1. Introduction
tions (150–200 ꢀC temperature and 3–6 MPa pressure).
In 1996, Iridium/iodide catalyzed process named Cativa
process was announced by BP Chemicals [4]. The Cativa
process is operated at lower water concentration but
gives high reaction rates, improved selectivity based on
carbon monoxide and minimum liquid by-products.
Ru, Ni, Pd, Pt were also found to be active for carbon-
ylation of methanol [5]. Carbonylation of methanol with
palladium catalyst, patented by Shell, used the severe
operating conditions (182 ꢀC, 11 MPa CO) with sulfo-
lane as a solvent and nitrogen-containing ligands to give
high activity (0.34 kg/L/h) [5d]. Palladium and platinum-
catalyzed carbonylation of methyl iodide has been re-
ported by Maitlis and coworkers [5c], where the activity
was observed to be very low (turnover frequency
(TOF) = 7 hÀ1). These authors have proposed a cata-
lytic cycle containing [PdI3CO]À as an intermediate spe-
cies based on analogy with Pt-catalyzed carbonylation
of methanol, but no direct evidence for this species
Acetic acid is one of the most important commodity
chemicals used in the manufacture of synthetic fibers,
resins, acetic anhydride, vinyl acetate and as a solvent
in polyester fiber production [1]. Carbonylation of meth-
anol is the preferred route for the production of acetic
acid after the successful development of cobalt catalyst
by BASF in 1960s [2]. Iodide promoted cobalt catalyst
operated at high temperatures (250 ꢀC) and high pres-
sures (68 MPa), giving 90% selectivity to the product
acetic acid based on methanol. Rhodium-catalyzed io-
dide promoted carbonylation of methanol, discovered
by Monsanto in 1970s [3], gave a high selectivity
(>99% based on methanol) at milder operating condi-
*
Corresponding author. Tel.: +91 20 5893260/+91 20 25893163;
fax: +0091 20 5893260.
E-mail address: rvc@ems.ncl.res.in (R.V. Chaudhari).
0022-328X/$ - see front matter ꢁ 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2005.01.010

1678
S.S. Tonde et al. / Journal of Organometallic Chemistry 690 (2005) 1677–1681
was reported [5c]. In this paper, we demonstrate that
conditions for the Pd(OAc)2–HI–PPh3 catalyst can
be optimized to give high TOF (in the range of
1052 hÀ1). Further, we report characterization of a
[PPh3CH3]2[Pd2I6] complex as an intermediate species
formed during carbonylation of methanol. The iodide
bridged dimeric Pd-complex catalyst was isolated from
the reaction mixture for the first time and characterized
using single crystal X-ray analysis.
at the cost of conversion maintaining the catalytic TOF
(Table 1, run #5b). Further, when gas phase was ana-
lyzed by gas chromatography, other gaseous compo-
nents like CO2, CH4 were traced below 1% of the total
gas phase, indicating minor tendency of the catalyst sys-
tem towards water gas shift reaction. Other palladium
catalysts like PdCl2, Pd(acac)2 and [Pd(Pyca)(P-
Ph3)(OTs)] [6] were examined for methanol carbonyl-
ation using TsOH and LiI as promoters at 4% water
concentration and were found to give activity compara-
ble to Pd(OAc)2 (Table 1, runs #6–8).
2. Results and discussion
In order to increase the catalytic activity of
Pd(OAc)2–HI catalyst system, higher temperatures were
employed. When the reaction was carried out at 145 ꢀC
and 10% water concentration using Pd(OAc)2 as a cata-
lyst and HI as a promoter, CO intake of the reaction
was found to slow down after 45 min (ꢀ17% conversion
of methanol) indicating deactivation of the catalyst.
Deactivation of the catalyst seems to occur significantly
at higher temperatures (>115 ꢀC) and a considerable
amount of palladium black was found precipitated when
the reactor was discharged. (Table 2, run #3b). How-
ever, at higher temperatures, addition of PPh3 with
Pd(OAc)2 and HI allowed the palladium catalyst to re-
main in solution as an active catalyst. At 145 ꢀC, 2.7
times increase in conversion with 9 times increase in
TOF was observed by addition of PPh3 (40 equiv. to
Pd) to Pd(OAc)2–HI catalyst system without any cata-
lyst precipitation at the end of reaction (Table 2, runs
#3b and 4). Further increase in temperature in
Pd(OAc)2–HI–PPh3 catalyst system increased the con-
version levels and TOF significantly without catalyst
precipitation. At 175 ꢀC, the highest TOF was observed
as 1052 hÀ1. Thus, Pd-catalyst gave the high activity by
selecting optimum operating conditions.
Preliminary catalytic reactions were carried out at
115 ꢀC and 5.4 MPa pressure of carbon monoxide using
Pd(OAc)2 as
a catalyst, toluene-4-sulphonic acid
(TsOH) and an alkali metal iodide as promoters in pres-
ence of water and 2-butanone as a solvent. Effect of LiI,
NaI and KI was examined with TsOH using Pd(OAc)2
at 4% water concentration. The results are summarized
in Table 1. Alkali metal iodides LiI, NaI and KI as pro-
moters showed comparable activity giving a TOF in the
range of 23–25 hÀ1 at 4% water concentration (Table 1,
runs #1–3). A significant effect of water concentration
was observed. An increase in water concentration from
4% to 20% doubled the catalytic TOF, when KI was
used as a promoter in presence of TsOH and Pd(OAc)2
(Table 1, run #4). At lower water concentration (4%),
the precipitation of a white salt, KOTs, was observed
when the reactor was discharged, unlike at higher water
concentration due to its solubility in water. Comparable
activity of alkali metal iodide promoters and the precip-
itation of alkali metal salts of TsOH at low water con-
centration, liberating HI in the solution encouraged us
to develop a catalytic system with HI as the only pro-
moter for Pd-catalyzed carbonylation of methanol.
When the carbonylation reaction was carried out using
Pd(OAc)2 as a catalyst and HI as a promoter at 20%
water concentration, marginal increase in selectivity
was achieved over Pd(OAc)2–KI–TsOH catalyst system
An attempt was made to isolate a catalytic intermedi-
ate [PPh3CH3]2[Pd2I6] from the reaction mixture of
Pd(OAc)2–HI–PPh3 catalyst system (see details in Sec-
tion 3) and the intermediate was characterized using sin-
gle crystal X-ray diffraction (Fig. 1) [7]. In order to
Table 1
Effect of promoters and water concentration in palladium-catalyzed carbonylation of methanol to acetic acid
Run no.
Water, v/v (%)
Catalyst
Promoter
Conversion (%)a
Selectivity A (%)a
TOF (hÀ1)
1
4
4
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
PdCl2
LiI
NaI
KI
KI
HI
LiI
LiI
LiI
97.25
97.06
97.07
92.86
78.80
96.37
97.36
93.84
64.99
78.89
73.96
89.86
94.83
67.74
67.55
72.54
23
25
24
50
50
23
24
23
2
3
4
4
20
20
4
5b
6
7
4
Pd(acac)2
Pd(Pyca)(PPh3)(OTs)
8
4
Catalyst: 0.1 mmol; Promoter: 10 mmol; TsOH: 10 mmol; Substrate (CH3OH): 31.2 mmol; Solvent (2-butanone): 21 mL; Temperature: 115 ꢀC; PCO
5.4 MPa; Time: 10 h; A: acetic acid.
:
a
Calculated based on GC analysis (see Supplementary material).
Without TsOH.
b

S.S. Tonde et al. / Journal of Organometallic Chemistry 690 (2005) 1677–1681
1679
Table 2
Effect of temperature on Pd(OAc)2–HI–PPh3 catalyzed carbonylation of methanol
Run no.
Temperature (ꢀC)
Time (h)
Conversion (%)a
Selectivity (%)a
A
TOF (hÀ1
)
B
1
115
130
145
145
145
160
175
2
1
2
1
1
1
1
20.54
26.77
17.28
46.46
47.52
62.34
64.77
17.10
21.23
15.43
28.45
25.42
54.39
61.33
56.66
52.77
80.31
57.46
52.42
42.45
33.74
100
260
59
2
3b
4
533
565
863
1052
5c
6
7
Pd(OAc)2: 0.05 mmol; PPh3: 2 mmol; HI: 5 mmol; Substrate (CH3OH): 100 mmol; H2O: 10% (v/v); Solvent (2-butanone): 20 mL; PCO: 5.4 MPa; A:
acetic acid; B: methyl acetate.
a
Calculated based on GC analysis (see Supporting information).
Without PPh3.
Isolated compound [PPh3CH3]2[Pd2I6] as catalyst.
b
c
investigate the mechanism of carbonylation of methanol
using Pd-catalysts, intermediate species generated
through stoichiometric reactions of catalyst, promoters
and carbon monoxide were characterized using UV
and IR spectroscopy. These stoichiometric reactions
were based on three different catalyst systems viz.,
Pd(OAc)2–alkali metal iodide–TsOH, Pd(OAc)2–HI,
and [PPh3CH3]2[Pd2I6] and their reactions with carbon
monoxide. It was noted that all the catalyst systems
studied followed the same mechanism and involve
[Pd2I6]2À and [PdI3CO]À as catalytic intermediates.
When the isolated complex [PPh3CH3]2[Pd2I6] was
dissolved in 2-butanone and examined by UV–Vis spec-
troscopic analysis at room temperature, the characteris-
tic absorption at 342 nm was noted (Fig. 2(a), m) that
for the solutions containing Pd(OAc)2–HI in 2-butanone
and Pd(OAc)2–TsOH–NaI in 2-butanone (Fig. 2) and it
was observed that the formation of the species [Pd2I6]2À
was consistent in all of the cases. Olsson [9] has already
reported the formation of dimeric palladium species,
[Pd2I6]2À from [PdI4]2À in organic solvents. When car-
bon monoxide was bubbled through the solution con-
taining [Pd2I6]2À, and analyzed by UV spectroscopy,
the characteristic peak at 342 nm disappeared meaning
that the [Pd2I6]2À was converted to the active Pd–CO
species by reaction with carbon monoxide (Figs. 2(b)
and (d)). In presence of carbon monoxide, the dimeric
4
was assigned for the dimeric palladium species [Pd2I6]2À
,
e
which is in good agreement with the literature [8]. Sim-
ilar UV–Vis spectroscopic observations were recorded
3
c
I6
I1
I4
2
Pd2
Pd1
a
1
d
b
I5
I2
I3
0
300
350
400
450
Fig. 1. X-ray structure of [Pd2I6]2À anion Bond lengths (A) and angles
(ꢀ): I(1)–Pd(1) 2.5898(5), I(1)–Pd(2) 2.5910(5), Pd(2)–I(5) 2.5830(5),
Pd(2)–I(6) 2.5897(5), Pd(2)–I(2) 2.5995(5), I(2)–Pd(1) 2.6074(5), Pd(1)–
I(3) 2.5923(5), Pd(1)–I(4) 2.5986(5); Pd(1)–I(1)–Pd(2) 94.500(14), I(5)–
Pd(2)–I(6) 93.042(15), I(5)–Pd(2)–I(1) 174.958(15), I(6)–Pd(2)–I(1)
90.178(14), I(5)–Pd(2)–I(2) 91.242(14), I(6)–Pd(2)–I(2) 173.629(15),
I(1)–Pd(2)–I(2) 85.878(14), Pd(2)–I(2)–Pd(1) 93.881(14), I(1)–Pd(1)–
I(3) 176.574(15), I(1)–Pd(1)–I(4) 89.847(14), I(3)–Pd(1)–I(4)
93.405(14), I(1)–Pd(1)–I(2) 85.737(14), I(3)–Pd(1)–I(2) 91.055(14),
I(4)–Pd(1)–I(2) 175.037(14).
˚
Wavelength, nm
Fig. 2. UV spectroscopic analysis of Pd-catalysts in 2-butanone at
room temperature: (a) 0.5 · 10À4 M palladium solution using the
isolated complex, [PPh3CH3]2[Pd2I6] in 2-butanone; (b) CO was
bubbled for 5 min in the solution (a); (c) 1 · 10À4 M palladium
solution using Pd(OAc)2 in 2-butanone with 4 equiv. of HI per Pd; (d)
CO was bubbled for 5 min in the solution (c); (e) 1.5 · 10À4
M
palladium solution using Pd(OAc)2 in 2-butanone with 4 equiv. of NaI
and TsOH each per Pd.

1680
S.S. Tonde et al. / Journal of Organometallic Chemistry 690 (2005) 1677–1681
anionic species [Pd2I6]2À was converted to the mono-
meric palladium species [PdI3CO]À, as evidenced by IR
spectroscopic analysis at room temperature, which
showed a strong carbonyl frequency at 2094 cmÀ1, con-
sistent with the literature [10] (see Supplementary mate-
rial, Figure 1). Maitlis and coworkers [5c] also have
proposed the formation of [PdI3CO]À, as an active cat-
alytic species in the carbonylation of methyl iodide, but
the direct evidence for this species was not shown. In
order to check the stability of [PdI3CO]À species at
the reaction conditions, the isolated complex,
[PPh3CH3]2[Pd2I6] was dissolved in 2-butanone, heated
at 145 ꢀC, pressurized with 5.4 Mpa CO and stirred
for 30 min. IR analysis of the reaction mixture after
cooling to room temperature and reducing the pressure
to 1 atmosphere, showed the characteristic peak of
[PdI3CO]À (mCO = 2094 cmÀ1) without any change. This
indicates that the palladium catalyst would be stable
even at higher temperature. A possible reaction mecha-
nism has been proposed in Scheme 1, which involves
[PdI3CO]À as an active catalytic species formed from
[Pd2I6]2À generated in situ in the carbonylation reac-
tion mixtures (in Pd(OAc)2–alkali metal iodide–
TsOH, Pd(OAc)2–HI and Pd(OAc)2–HI–PPh3 catalyst
systems).
on Bio Rad FTS 175C spectrophotometer. Solvent 2-
butanone and reactant methanol were distilled prior to
use. [Pd(Pyca)(PPh3)(OTs)] was prepared as reported
elsewhere [6]. The following materials were used as
received from their commercial sources: Pd(OAc)2,
Pd(acac)2 and PdCl2 from Aldrich Chemicals (Milwau-
kee, WI), 57% HI from Fluka Chemicals, LiI, NaI,
KI, TsOH, PPh3 and CH3I from SD Fine Chemicals
(Mumbai, India).
3.1. Isolation of [PPh3CH3]2[Pd2I6] from reaction
mixture
A solution from a set of reactions at 145 ꢀC
(Pd(OAc)2–HI–PPh3 catalyst system) was allowed to
evaporate at room temperature, from where black crys-
talline solid was obtained. The solid was washed with
water and diethyl ether and then dried in vacuum. Black
crystals were grown from dimethyl ketone and charac-
terized by X-ray crystallography [7] (Fig. 1) as
[PPh3CH3]2[Pd2I6], a iodide bridged dimeric palladium
complex. 13C NMR (50.32 MHz, acetone-d6): d (ppm)
136.1, 134.4, 134.2, 131.4, 131.1, 121.5, 119.7, 9.6, 8.5;
31P NMR (81.02 MHz, acetone-d6): d (ppm) 27.62.
Anal. Calc. for C38H36P2Pd2I6: C, 29.80; H, 2.30; I,
49.80. Found: C, 29.82; H, 2.50; I, 50.0.
3. Experimental
4. Conclusion
Carbonylation reactions were carried out in a 50 mL
Parr autoclave provided with digital temperature and
pressure display. Liquid reactants, products and gaseous
components were analyzed using HP 6890 Gas Chro-
matograph [11]. UV spectroscopic analysis was done
on PerkinElmer Lambda 25 UV/VIS Spectrometer
[12]. IR analysis of samples in 2-butanone were obtained
In conclusion a dimeric [PPh3CH3]2[Pd2I6] species
was isolated and characterized using single crystal X-
ray diffraction during carbonylation of methanol. Under
carbonylation reaction conditions the dimeric palladium
species forms monomeric [PdI3CO]À species. Palladium-
catalyzed carbonylation of methanol with Pd(OAc)2–
HI–PPh3 catalytic system showed high catalytic activity
(TOF = 1052 hÀ1) at 175 ꢀC.
[Pd2I6]2-
CO
-CO
I
CH3OH + HI
CH3I + H2O
_
Acknowledgement
CO
Pd
S.S.T. thanks Council of Scientific and Industrial Re-
search (CSIR), India for fellowship.
HI +
I
I
CH3COOH
H2O
Appendix A. Supplementary material
_
_
CH3
CO
Pd
COCH3
CO
I
Formulas for calculations of conversion and selectiv-
ities, FTIR spectra of carbonylation of [Pd2I6]2À to
[PdI3CO]À (Fig. 1), Crystallographic data for the struc-
tural analysis has been deposited with the Cambridge
Crystallographic Data Centre, CSD No. 414452, for
compound [PPh3CH3]2[Pd2I6]. Supplementary data
associated with this article can be found, in the online
version at doi:10.1016/j.jorganchem.2005.01.010.
I
Pd
I
I
I
I
I
I
CO
Scheme 1. Mechanism of Pd-catalyzed carbonylation of methanol.

S.S. Tonde et al. / Journal of Organometallic Chemistry 690 (2005) 1677–1681
1681
˚
ꢀ
group P1, a = 10.0013(17), b = 12.674(2), c = 18.552(3) A,
References
a = 101.540(3)ꢀ, b = 90.930(3)ꢀ, c = 103.252(3)ꢀ, V = 2237.9(7)
3
A , Z = 2, Dc = 2.269 g cmÀ3, l (Mo Ka) = 5.039 mmÀ1, T =
˚
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