Analysis of the synergistic effect of carbonylplatinum complexes on the iridium-catalysed carbonylation of methanol to acetic acid

Article abstract of DOI:10.1002/ejic.200500931

High-pressure NMR mechanistic investigations into the promoting role of dicarbonyldiiodoplatinum complex in the iridium-catalysed carbonylation of methanol to acetic acid are consistent with iodide abstraction from the [IrI₃(CH₃)(CO)₂]^- resting state species by [PtI₂(CO)₂], unbolting the rate-limiting migratory CO insertion step. A new [Ir-Pt] heterodimetallic complex, which presumably represents the key species involved in the abstraction of I ^-, has been characterised by NMR in tandem with FAB-MS. Wiley-VCH Verlag GmbH & Co. KGaA, 2006.

Full text of DOI:10.1002/ejic.200500931

SHORT COMMUNICATION
DOI: 10.1002/ejic.200500931
Analysis of the Synergistic Effect of Carbonylplatinum Complexes on the
Iridium-Catalysed Carbonylation of Methanol to Acetic Acid
Samuel Gautron,[a,b] Nicolas Lassauque,[a] Carole Le Berre,[a] Philippe Serp,[a]
Laurent Azam,[a] Roberto Giordano,[a] Gábor Laurenczy,[c] Daniel Thiébaut,[b] and
Philippe Kalck*[a]
Keywords: Homogeneous catalysis / Carbonylation / Iridium / Platinum / Reaction mechanism
High-pressure NMR mechanistic investigations into the pro-
moting role of dicarbonyldiiodoplatinum complex in the irid-
ium-catalysed carbonylation of methanol to acetic acid are
consistent with iodide abstraction from the [IrI3(CH3)(CO)2]–
resting state species by [PtI2(CO)2], unbolting the rate-limit-
lic complex, which presumably represents the key species
involved in the abstraction of I–, has been characterised by
NMR in tandem with FAB-MS.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
ing migratory CO insertion step. A new [Ir–Pt] heterodimetal- Germany, 2006)
addition of CH3I affording [IrI3(CH3)(CO)2]– (2), is largely
Introduction
enhanced.[2] The subsequent step, the migratory CO inser-
tion leading to an Ir–acetyl species is so slow[3] that a pro-
cess based on iridium cannot be suitable for industrial ap-
plications. In 1994, Watson et al. discovered that addition
of ruthenium promoters to the iridium catalyst resulted in
a dramatic increase in catalytic activity,[4] and very recent
reports[5,6] have shown that this is due to the abstraction of
an iodo ligand from 2, allowing the rapid migratory inser-
tion of CO into the Ir–CH3 bond. The authors have pro-
posed an intermediate [Ir–Ru] heterodimetallic complex,
which has not been detected. In 1998, we patented the pro-
moting effect of platinum,[7] which gives performances com-
parable to the iridium/ruthenium system in terms of turn-
over frequencies (2500 h–1 for Ir–Pt and 2800 h–1 for Ir–
Ru, respectively, expressed with regard to Ir). For the two
dimetallic systems, maximum activities are found for water
concentrations close to 5 wt.-%, whereas the corresponding
value approaches 14 wt.-% for the Monsanto process.
We envisaged that in both cases the catalytic cycles could
present some common features. Thus, we undertook investi-
gations on the [Ir–Pt] system in order to obtain a deeper
insight into the nature of the intermediates that are formed
and are responsible for the iodide abstraction from 2. At
Carbonylation of methanol is the major route to the pro-
duction of acetic acid, and the worldwide industrial manu-
facture associated with this homogeneous catalytic process
reaches 6·106 tons/year [Equation (1)]. This reaction in-
volves a complex mixture of reactants consisting of water,
acetic acid, methyl iodide, methyl acetate, hydroiodic acid,
and traces of propionic acid, from which methanol is absent
as it is immediately converted by HI into CH3I. Many ef-
forts have been aimed at the substitution of iridium for rho-
dium since the former tolerates low levels of water during
the reaction, saving operating costs in the separation step
of water from acetic acid.
(1)
Moreover, in the active species H[IrI2(CO)2] (1) the metal
centre is more nucleophilic than that in the homologous
rhodium precursor used in the Monsanto process,[1] so that
the rate of the first step of the catalytic cycle, the oxidative
[a] Laboratoire de Catalyse, Chimie Fine et Polymères, Ecole Na- first, we decided to study the catalytic complex mixtures
tionale Supérieure des Ingénieurs en Arts Chimiques et Techno-
by high-pressure NMR spectroscopy under conditions that
logiques (ENSIACET),
closely resemble those used in the industrial process. Rel-
evant pieces of information arise from the use of 13C-en-
riched reactants, such as 13CO, 13CH313COO13CH3, and
13CH3I in 13C and 195Pt NMR spectroscopy. Thus, we were
able to focus on every part of the catalytic cycle, using a
procedure that leads to the detection of the transition metal
complexes bearing carbonyl and methyl or acyl ligands. We
118 Route de Narbonne, 31077 Toulouse Cedex4, France
E-mail: Philippe.Kalck@ensiacet.fr
[b] ACETEX, AATF1, Usine de Pardies,
B. P. 17 Pardies, 64150 Mourenx, France
[c] Ecole Polytechnique Fédérale de Lausanne, Institut des Sci-
ences et Ingénierie Chimiques, BCH-LCOM,
1015 Lausanne, Switzerland
Supporting information for this article is available on the
WWW under http://www.eurjic.org or from the author.
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P. Kalck et al.
SHORT COMMUNICATION
present here our first mechanistic results based on high- methyliridium complex H[Ir(13CH3)I3(13CO)2] (2) together
1
pressure NMR (13C, H, 195Pt) experiments. Moreover, in with small amounts of the acyl species H[Ir(13CO13CH3)-
order to characterise the short-lived complexes involved in I3(13CO)2] (4) (see Table 1). As previously proposed by For-
this process, we used the bis(triphenylphosphoranylidene) ster from IR observations,[2] we confirm that under these
[PPN]+ stabilising counterion and made observations at conditions the resting state is the methyl complex 2, which
room temperature by NMR in tandem with FAB-MS. The is characterised by signals at δ = 157.1 (s, CO), –15.2 [q,
new heterodimetallic species [IrI(CH3)(CO)2(µ-I)2PtI2(CO)]– 1J(C,H) = 141 Hz, CH3] ppm. These NMR values are very
has been identified and its evolution monitored in solu- close to those for [PPN][Ir(CH3)I3(CO)2], which was char-
tion.
acterised by X-ray crystallography.[10] Small amounts of
The iridium complex 1 alone, initially studied by For- CO2 were also detected, resulting from the water gas shift
ster[2] and Pannetier et al.,[8] is known to slowly catalyse the side reaction (WGSR). In addition, some CH4 was detected.
carbonylation of methanol to acetic acid. In mixtures sim- Evidence of occurrence of the WGSR was provided by the
ilar to those employed industrially,[9] under 3 MPa and at presence of H[IrHI3(CO)2] and H[IrI4(CO)2], as shown by
95 °C, we observed by high-pressure 13C NMR spec- the relevant NMR spectroscopic data. The low activity of
troscopy (100.6 MHz) the spontaneous formation of the 2 was also shown by the low consumption of 13CO and
Table 1. NMR spectroscopic data for the species involved in the methanol carbonylation reaction.
Complex
13C NMR (δ/ppm)
Products
13C NMR (δ/ppm)
PPN[IrI2(CO)2] (1)
H[IrHI3(CO)2]
H[IrI4(CO)2]
169.9 (s, CO)
156.0 (s, CO)
151.3 (s, CO)
AcOH
AcOMe
MeI
177.3 (s), 21.1 (q)
172 (s), 52 (q), 21 (q)
–25.0 (q)
H[IrI3Me(CO)2] (2)
[IrI2Me(CO)3] (3)
H[IrI3(COMe)(CO)2] (4)
H[PtI3(CO)] (6)
157.1 (s, CO), –15.2 (q, JC,H = 141 Hz, CH3)
146.9 (s, CO), –22.3 (q, JC,H = 140 Hz, CH3)
203.4 (s, COMe), 160.7 (s, CO), 50.6 (q, JC,H = 140 Hz, CH3)
156.8 (s, d, JPt–C = 1649 Hz, CO)
154.5 (s, d, JPt–C = 1843 Hz, CO)
MeOH
CO2
CO
49.3 (q)
125 (s)
185.4 (s)
–4.2 (quint)
CH4
[PtI2(CO)2] (5)
Figure 1. High-pressure 13C NMR of the [Ir] system (a) and the [Pt–Ir] system (b).
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[Ir–Pt]-Catalysed Carbonylation
SHORT COMMUNICATION
13CH3I.[11] Thus, in the absence of any stabilising counter- from [Pt2I2(µ-I)2(CO)2] under CO, led to a significant in-
ion, we have established the nature of the catalytic species crease in the carbonylation rate. In batch experiments using
that present the longest life-times.
a Hastelloy reactor, we obtained a carbonylation rate
Subsequently, our studies focused on the accelerating ef- (based on the transformation of AcOMe into acetic acid)
fect of platinum promoters on the iridium system. Under of 16 molL–1 h–1 with iridium alone, compared to
3 MPa (13CO/12CO = 1:3) and at 95 °C in a high-pressure 35 molL–1 h–1 after addition of [PtI2(CO)]2 (Pt/Ir = 3:7).
NMR sapphire tube, addition to the catalytic iridium mix- 13C NMR spectra led to observe the change in the concen-
ture of [PtI2(CO)2] (5), which is the stable species formed tration of the organic compounds during the catalytic runs
Figure 2. 13C NMR spectroscopic data for the [Ir–Pt]/methanol carbonylation system.
Figure 3. Proposed catalytic cycle for the [Ir–Pt] system.
Eur. J. Inorg. Chem. 2006, 1121–1126
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1123

P. Kalck et al.
SHORT COMMUNICATION
and particularly the intensity ratio between the signals of does not catalyse the carbonylation reaction. Finally, re-
acetic acid and methyl iodide. We have charged two high- ductive elimination of acyl iodide from 4 regenerates 1 (Fig-
pressure NMR tubes with IrI3.4
,
13CH3I (10wt.-%), AcOMe ure 3).
(20wt.-%), H2O (6wt.-%) and AcOH (64wt.-%) under
In order to obtain a clear view of this dimetallic process,
30 bar of a mixture of 13CO/12CO, one tube containing only we decided to decrease the reaction rate by performing the
IrI3.4 salt (a) and the other containing the mixture of IrI3.4 experiments under ambient conditions and by using the
and [PtI2(CO)]2 (b). After 3 h at 86 °C, we can compare [PPN]+ stabilising counterion under nitrogen. A number of
(Figure 1) the relative intensity ratios of the two signals of 1H NMR (250 MHz, CD2Cl2, 25 °C) spectra were recorded
13CH3I/AcOH and conclude that a larger amount (around while varying the Ir/Pt molar ratio. Addition of [Pt2I2(µ-I)2-
two times) of 13CH3I was consumed[11] with the [Ir–Pt] sys- (CO)2] to a dichloromethane solution of 2-PPN provided
tem than with the iridium catalyst only. Under these condi- several signals at δ = 2.36, 2.28, 2.14, 2.09, 1.90 and
tions, we observed the presence of H[PtI3(CO)] (6) and the 1.85 ppm (Figure 4). At low platinum concentrations (Ir/Pt
signals corresponding to the iridium complexes 2 and 4 in = 9.5:1), the signal at δ = 2.14 ppm is assigned to unreacted
significant amounts[12] (Figure 2).
2-PPN. We may tentatively assign the peak at δ = 2.09 ppm
The role of platinum is mainly to assist the formation of to the solvated neutral species [IrI2(CH3)(CO)2(solv)], in
the species [IrI2(CH3)(CO)3] (3), in which the electron which the coordinated solvent could be CD2Cl2. Indeed,
density on the iridium atom is reduced; migratory CO inser- iridium() complexes containing coordinated CH2Cl2 have
tion can then occur more rapidly to produce 4. In separate been reported previously.[13] At higher platinum concentra-
experiments we checked that platinum indeed acts as a real tions (Ir/Pt = 5.3:1), significant amounts of the two isomers
co-catalyst, since application of 3 MPa of CO exerted on 6 of [Ir2I2(µ-I)2(CH3)2(CO)4] were detected at δ = 1.90 and
provides 5 again, whereas such a platinum species alone 1.85 ppm.[10]
Figure 4. Detection of [IrI(CH3)(CO)2(µ-I)2PtI2(CO)]– and [Ir2I4(µ-I)(CH3)2(CO)4]– by tandem NMR and FAB/MS experiments:
top (Ir/Pt = 9.5:1), bottom (Ir/Pt = 5.3:1).
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[Ir–Pt]-Catalysed Carbonylation
SHORT COMMUNICATION
Scheme 1. Reaction route leading to Ir2-bridged species through the intermediate formation of an [Ir–Pt] dimetallic moiety (from 2 and
[Pt2I2(µ-I)2(CO)2]).
1
As already stated, two additional H NMR signals ap-
Experimental Section
peared at δ = 2.36 and 2.28 ppm, whose relative intensities
Synthesis of the Iridium Complexes: The synthesis of the iridium
vary with the Ir/Pt molar ratio. In order to assign these two
complexes has been previously reported.[10]
signals, for the given Ir/Pt ratios, we recorded 1H NMR
Synthesis of the Platinum Complexes
spectra in tandem (analysed from the same sample) with
[PtI2(CO)2] (5) and [PtI2(CO)]2 (5Ј): [Pt(CO)2I2] was prepared by
addition of PtI2 (2 g, 4.45 mmol) in an autoclave containing hexane
(125 mL). The mixture was pressurised with CO (12 bar) and
stirred at 75 °C for 1 h. The solution was cooled in the reactor
under CO (1 bar). 1.69 g (80% yield) of a red solid was obtained
and dried under reduced pressure, which corresponds to [PtI2(CO)]2.
Under CO, this compound leads to [PtI2(CO)2] in 100% yield.
FAB/MS experiments. Representative examples are shown
in Figure 4. For an Ir/Pt = 5.3:1 molar ratio, two anionic
complexes were detected at m/z = 1121 and 1161. The for-
mer molecular peak corresponds to the anionic heterodime-
tallic complex [IrI(CH3)(CO)2(µ-I)2PtI2(CO)]–. The latter is
consistent with the formulation [Ir2I4(µ-I)(CH3)2(CO)4]–,
which was previously suggested as an intermediate in the
[Ir–Ru] system.[5] In a second representative run, for an Ir/
Pt = 9.5:1 molar ratio, the peak at m/z = 1121 in the FAB/
MS spectra, as well as the signal at δ = 2.36 ppm in the
1H NMR spectra disappeared nearly completely. Thus, the
signal at δ = 2.28 ppm can be unambiguously assigned to
the complex [Ir2I4(µ-I)(CH3)2(CO)4]–. Additionally, 13C
[PtI (CO)] : IR (CH Cl ): ν = 2111 (CO) cm–1. 13C NMR
˜
2
2
2
2
1
(CD2Cl2): δ = 154.5 (s, JPt–C = 1843 Hz, CO) ppm. 195Pt NMR
(CD2Cl2): δ = –5328 (1JPt–C = 1843 Hz) ppm. [PtI2(CO)2]: IR
(CH Cl ): ν = 2127 (CO) cm–1. 13C NMR (CD2Cl2): δ = 166 (s,
˜
2
2
1JPt–C = 1466 Hz, CO) ppm. 195Pt NMR (CD2Cl2): δ = –5790
(1JPt–C = 1466 Hz) ppm.
[PPN][PtI3(CO)] (6-PPN): [PtI2(CO)]2 (1 g, 1.05 mmol) was dis-
NMR spectra reveal the signals of [Ir2I4(µ-I)(CH3)2(CO)4]– solved in dichloromethane (50 mL), and HI (0.26 g, 2.10 mmol)
was added to give rapidly, at room temperature, the complex
H[PtI3(CO)]. Then, [PPN]Cl (1.20 g, 2.10 mmol) was added to the
solution. The solvent was removed under vacuum to give a yellow
product. Crystallisation of this compound in CH2Cl2/hexane (1:1)
gave yellow crystals of [PPN][PtI3(CO)] (1.08 g, 45%). IR
at δ = 154.68 ppm (s, CO), –14.83 ppm (q, JC,H = 141 Hz,
CH3) and two new carbonyl signals that correspond to
[IrI(CH3)(CO)2(µ-I)2PtI2(CO)]–. The CO ligand coordi-
nated to Pt gives a signal at δ = 154.9 ppm (s, CO) with its
satellites characterised by a doublet (1JPt,C = 1650 Hz, CO).
The other carbonyl signal at δ = 155.4 ppm (no coupling
(CH Cl ): ν = 2075 (CO) cm–1. 13C NMR (CD2Cl2): δ = 152.8 (s,
˜
2
2
1JPt–C = 1759 Hz, CO) ppm. 195Pt NMR (CD2Cl2): δ = –5460
constant) is assigned to the two CO ligands coordinated to
(1JPt–C = 1759 Hz). MS (FAB–): m/z = 603.
1
the iridium atom. The quadruplet at δ = –14.2 (q, JC,H
=
141 Hz, CH3) ppm shows no coupling between 13C and
195Pt and is presumably due to the CH3 ligand coordinated
to the iridium atom. The 195Pt NMR spectra show a new
Acknowledgments
The “Ministère de la Recherche et de la Technologie” is gratefully
acknowledged for supporting an “Equipe de Recherche Technolo-
gique” (ERT 007), in our laboratory at ENSIACET. Financial sup-
port from NSF Switzerland 200020-105335/1 (to G. L.) is also
gratefully acknowledged.
1
signal at δ = –5462 (s, + satellites d, JPt,C = 1650 Hz, CO)
ppm, together with a signal of [PtI3(CO)]– at δ = –5460 (s,
1
+ satellites d, JPt,C = 1759 Hz, CO) ppm. Scheme 1 sum-
marises the reaction pathway for the formation and the
evolution of the mixed [Ir–Pt] complex. Under CO, the [Ir–
Pt] system leads very quickly to the two complexes 3 and 6.
This work presents circumstantial evidence that the main
role of the platinum promoter during the iridium-catalysed
carbonylation of methanol is to abstract an iodo ligand
from [IrI3(CH3)(CO)2]– (2) through the formation of the
heterodimetallic [IrI(CH3)(CO)2(µ-I)2PtI2(CO)]– intermedi-
ate species.
[1] The Monsanto process implies rhodium in a mixture contain-
ing water (14 wt.-%) at 180 °C and under 3 MPa.
[2] T. W. Dekleva, D. Forster, Adv. Catal. 1986, 34, 81.
[3] J. M. Pearson, A. Haynes, G. E. Morris, G. J. Sunley, P. M.
Maitlis, J. Chem. Soc., Chem. Commun. 1995, 1045.
[4] G. J. Sunley, D. J. Watson, Catal. Today 2000, 58, 293; C. J. E.
Vercauteren, K. E. Clode, D. J. Watson, European patent,
616997, 1994.
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P. Kalck et al.
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[5] A. Haynes, P. M. Maitlis, J. E. Morris, G. J. Sunley, H. Adams,
P. W. Badger, C. M. Bowers, D. B. Cook, P. I. P. E. Elliott, T.
[10] S. Gautron, R. Giordano, C. Le Berre, J. Jaud, J.-C. Daran, P.
Serp, P. Kalck, Inorg. Chem. 2003, 42, 5523.
Ghaffar, H. Green, T. R. Griffin, M. Payne, J. M. Pearson, [11] As CH3I is formed by reaction of HI with methyl acetate, we
M. J. Taylor, P. W. Vickers, R. J. Watt, J. Am. Chem. Soc. 2004,
126, 2847.
monitored the decrease of 13CH3I, whereas 12CH3I is produced
when using unlabelled methyl acetate.
[6] R. Whyman, A. P. Wright, J. A. Iggo, B. T. Heaton, J. Chem.
Soc., Dalton Trans. 2002, 5, 771.
[12] Complex 4 is observed here when adding the platinum pro-
moter, since the rate-determining step is accelerated by the I–
abstraction process.
[13] R. H. Crabtree, The Organometallic Chemistry of the Transition
Metals, 3rd ed., Wiley-Interscience, New York, 2001, p. 109.
Received: October 20, 2005
[7] C. Le Berre, P. Serp, P. Kalck, L. Layellon, D. Thiebaut, French
patent 9813954, 1998.
[8] D. Brodzki, B. Denise, G. Pannetier, J. Mol. Catal. 1977, 2,
149.
[9] IrI3, IrI4 (0.05 mol), AcOH (64 wt.-%), H2O (6 wt.-%), AcOMe
(20 wt.-%), 13CH3I (10 wt.-%).
Published Online: February 2, 2006
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