Promotion of Iridium-Catalyzed Methanol Carbonylation: Mechanistic Studies of the Cativa Process

Article abstract of DOI:10.1021/ja039464y

The iridium/iodide-catalyzed carbonylation of methanol to acetic acid is promoted by carbonyl complexes of W, Re, Ru, and Os and simple iodides of Zn, Cd, Hg, Ga, and In. Iodide salts (LiI and Bu₄NI) are catalyst poisons. In situ IR spectroscopy shows that the catalyst resting state (at H₂O levels ≥ 5% w/w) is fac, cis-[Ir(CO)₂I ₃Me]^-, 2. The stoichiometric carbonylation of 2 into [Ir(CO)₂I₃(COMe)]^-, 6, is accelerated by substoichiometric amounts of neutral promoter species (e.g., [Ru(CO) ₃I₂]₂, [Ru(CO)₂I₂] _n, InI₃, GaI₃, and ZnI₂). The rate increase is approximately proportional to promoter concentration for promoter: Ir ratios of 0-0.2. By contrast anionic Ru complexes (e.g., [Ru(CO) ₃I₃]^-, [Ru(CO)₃I₃] ^-, [Ru(CO)₂I₄]^2-) do not promote carbonylation of 2 and Bu₄NI is an inhibitor. Mechanistic studies indicate that the promoters accelerate carbonylation of 2 by abstracting an iodide ligand from the Ir center, allowing coordination of CO to give [Ir(CO)₃I₂Me], 4, identified by high-pressure IR and NMR spectroscopy. Migratory CO insertion is ca. 700 times faster for 4 than for 2 (85 °C, PhCl), representing a lowering of ΔG? by 20 kJ mol ^-1. Ab initio calculations support a more facile methyl migration in 4, the principal factor being decreased π-back-donation to the carbonyl ligands compared to 2. The fac, cis isomer of [Ir(CO)₂I ₃(COMe)]^-, 6a (as its Ph₄As⁺ salt), was characterized by X-ray crystallography. A catalytic mechanism is proposed in which the promoter [M(CO)_mI_n] (M = Ru, In; m = 3, 0; n = 2, 3) binds I^- to form [M(CO)_mI_n+1] ^-H₃O⁺ and catalyzes the reaction HI _(aq) + MeOAc → Mel + HOAc. This moderates the concentration of HI_(aq) and so facilitates catalytic turnover via neutral 4.

Full text of DOI:10.1021/ja039464y

Published on Web 02/17/2004
Promotion of Iridium-Catalyzed Methanol Carbonylation:
Mechanistic Studies of the Cativa Process
Anthony Haynes,*,† Peter M. Maitlis,*,† George E. Morris,*,‡ Glenn J. Sunley,*,‡
Harry Adams,† Peter W. Badger,† Craig M. Bowers,† David B. Cook,†
Paul I. P. Elliott,† Talit Ghaffar,† Helena Green,† Tim R. Griffin,† Marc Payne,‡
Jean M. Pearson,† Michael J. Taylor,§ Paul W. Vickers,† and Rob J. Watt‡
Contribution from the Department of Chemistry, UniVersity of Sheffield, Sheffield, S3 7HF, U.K.,
BP Chemicals Ltd, Hull Research and Technology Centre, Saltend, Hull, HU12 8DS, U.K., and
BP Chemicals Ltd, Chertsey Road, Sunbury-on-Thames, Middlesex, TW16 7LL, U.K.
Received November 6, 2003; E-mail: a.haynes@sheffield.ac.uk; sunleyg@bp.com
Abstract: The iridium/iodide-catalyzed carbonylation of methanol to acetic acid is promoted by carbonyl
complexes of W, Re, Ru, and Os and simple iodides of Zn, Cd, Hg, Ga, and In. Iodide salts (LiI and Bu4NI)
are catalyst poisons. In situ IR spectroscopy shows that the catalyst resting state (at H2O levels g 5%
w/w) is fac,cis-[Ir(CO)2I3Me]-, 2. The stoichiometric carbonylation of 2 into [Ir(CO)2I3(COMe)]-, 6, is
accelerated by substoichiometric amounts of neutral promoter species (e.g., [Ru(CO)3I2]2, [Ru(CO)2I2]n,
InI3, GaI3, and ZnI2). The rate increase is approximately proportional to promoter concentration for promoter:
Ir ratios of 0-0.2. By contrast anionic Ru complexes (e.g., [Ru(CO)3I3]-, [Ru(CO)2I4]2-) do not promote
carbonylation of 2 and Bu4NI is an inhibitor. Mechanistic studies indicate that the promoters accelerate
carbonylation of 2 by abstracting an iodide ligand from the Ir center, allowing coordination of CO to give
[Ir(CO)3I2Me], 4, identified by high-pressure IR and NMR spectroscopy. Migratory CO insertion is ca. 700
times faster for 4 than for 2 (85 °C, PhCl), representing a lowering of ∆Gq by 20 kJ mol-1. Ab initio calculations
support a more facile methyl migration in 4, the principal factor being decreased π-back-donation to the
carbonyl ligands compared to 2. The fac,cis isomer of [Ir(CO)2I3(COMe)]-, 6a (as its Ph4As+ salt), was
characterized by X-ray crystallography. A catalytic mechanism is proposed in which the promoter [M(CO)mIn]
(M ) Ru, In; m ) 3, 0; n ) 2, 3) binds I- to form [M(CO)mIn+1]-H3O+ and catalyzes the reaction
HI(aq) + MeOAc f MeI + HOAc. This moderates the concentration of HI(aq) and so facilitates catalytic
turnover via neutral 4.
Introduction
iridium/iodide catalyst, which now operates on four plants
worldwide. Promoters, which enhance the catalytic activity, are
One of the most successful industrial applications of homo-
geneous transition metal catalysis is the carbonylation of
methanol to acetic acid.1 The global annual manufacturing
capacity for acetic acid is ca. 8 million tonnes, about 80% of
which is based on methanol carbonylation technology. In the
last 40 years, there have been three major advances in the
catalyst system used commercially, descending group 9 of the
periodic table from cobalt2 to rhodium3,4 and, most recently,
iridium. The rhodium-based process was the basis of most new
acetic acid manufacturing capacity after its commercialization
by Monsanto in 1970. In 1996, BP Chemicals announced a new
methanol carbonylation process, Cativa,5 based upon a promoted
key to the success of the iridium-based process.6 In this paper,
we combine the results of catalytic studies with mechanistic
and theoretical investigations of key steps in the catalytic cycle,
to identify the mechanism by which the promoters act.
The Cativa Process. The key features of the Cativa process
have been reported elsewhere7-10 and will be summarized here
to place the mechanistic studies in context. A broad range of
conditions are accessible for the Ir catalyst without precipitation
of IrI3 occurring.11 By contrast, the precipitation of RhI3 at low
pCO can be problematic in the product purification stage of
(5) Chem. Br. 1996, 32, 7. Chem. Ind. (London) 1996, 483.
(6) Garland, C. S.; Giles, M. F.; Sunley, J. G. Eur. Pat. Pub. 0643034, 1995.
Garland, C. S.; Giles, M. F.; Poole, A. D.; Sunley, J. G. Eur. Pat. Pub.
0728726, 1996. Baker, M. J.; Giles, M. F.; Garland, C. S.; Rafeletos, G.
Eur. Pat. Pub. 0749948, 1996. Baker, M. J.; Giles, M. F.; Garland, C. S.;
Muskett, M. J.; Rafeletos, G.; Smith, S. J.; Sunley, J. G.; Watt, R. J.;
Williams, B. L. Eur. Pat. Pub. 0752406, 1997.
† University of Sheffield.
‡ BP Chemicals Ltd, Hull Research and Technology Centre.
§ BP Chemicals Ltd, Sunbury-on-Thames.
(1) Howard, M. J.; Jones, M. D.; Roberts, M. S.; Taylor, S. A. Catal. Today
1993, 18, 325. Yoneda, N.; Kusano, S.; Yasui, M.; Pujado, P.; Wilcher, S.
Appl. Catal. A 2001, 221, 253.
(2) Hohenschutz, H.; von Kutepow, N.; Himmle, W. Hydrocarbon Process
1966, 45, 141. von Kutepow, N.; Himmle, W.; Hohenschutz, H. Chem.-
Ing.-Tech. 1966, 37, 383.
(3) Paulik, F. E.; Roth, J. F. Chem. Commun. 1968, 1578.
(4) Roth, J. F.; Craddock, J. H.; Hershman, A.; Paulik, F. E. Chemtech 1971,
600. Eby, R. T.; Singleton, T. C. Appl. Ind. Catal. 1971, 483.
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Stud. Surf. Sci. Catal. 1999, 121, 61.
(8) Sunley, G. J.; Watson, D. J. Catal. Today 2000, 58, 293.
(9) Jones, J. H. Platinum Metals ReV. 2000, 44, 94.
(10) Haynes, A. Educ. Chem. 2001, 38, 99.
(11) Clode, K. E.; Watson, D. J.; Vercauteren, C. J. E. Eur. Pat. Pub. 0616997,
1994. Clode, K. E. Eur. Pat. Pub. 0786447, 1997.
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J. AM. CHEM. SOC. 2004, 126, 2847-2861
2847

A R T I C L E S
Haynes et al.
involve fundamental steps similar to those in the well-established
rhodium system:17 oxidative addition of MeI to Ir(I), followed
by migratory CO insertion to form an acetyl complex which is
the precursor to acetic acid. On the basis of spectroscopic and
kinetic observations, Forster proposed that two different (but
linked) cycles exist,14 one involving neutral iridium complexes,
and the other predominantly anionic species (Scheme 1). Similar
neutral and anionic cycles were also proposed for the competing
water gas shift reaction. At low concentrations of water and
iodide, the “neutral cycle” operated, with [Ir(CO)3I] (9) as the
resting state and rate-determining oxidative addition of MeI to
[Ir(CO)2I] (8). At higher iodide concentrations the “anionic
cycle” predominated, and the catalyst resting state was fac,cis-
[Ir(CO)2I3Me]- (2). Under these conditions, the rate of catalysis
increased with CO pressure but was inhibited by ionic iodide.
It was proposed that the rate-determining step involves dis-
sociative substitution of I- by CO in 2, followed by migratory
CO insertion in the tricarbonyl 4. This contrasts with the
rhodium sytem, in which oxidative addition of MeI to
[Rh(CO)2I2]- is rate determining. Model kinetic studies18,19 have
shown that addition of MeI to [Ir(CO)2I2]- (1) is much faster
(ca. 100 times) than to the rhodium analogue.
We have communicated preliminary data concerning the
reactivity of the anionic iridium methyl species 2 and the first
detection of neutral 4.20,21 In this paper we present the results
of in situ spectroscopic, kinetic, mechanistic, and theoretical
studies aimed at determining the mechanism by which promoters
enhance the activity of an iridium catalyst. Our results show
that a key feature of all the promoters is their ability to accept
an iodide ion and facilitate conversion of 2 into 4, which
undergoes substantially faster migratory insertion.
Figure 1. Batch autoclave data8 for iridium and iridium/ruthenium
catalysts: effect of water concentration on carbonylation rate at ca. 30%
(w/w) MeOAc, 8.4% (w/w) MeI, 1950 ppm Ir, 28 barg total pressure, and
190 °C. The data plotted are listed in the Supporting Information (Table
S1).
the Rh-based process. The greater stability of the iridium catalyst
can be ascribed to stronger metal-ligand bonding for the third-
row metal, which inhibits CO loss from the Ir center.
The rate of iridium-catalyzed carbonylation displays a rather
complicated dependence on a range of process variables such
as pCO, [MeI], [MeOAc], and [H2O]. The catalytic rate displays
a strong positive dependence on [MeOAc] but is zero order in
[MeI] above a limiting threshold and independent of CO partial
pressure above ca. 10 bar. Figure 1 illustrates a rate profile with
respect to [H2O] which shows that maximum activity is achieved
at ca. 5% w/w H2O.12
A range of compounds can enhance the activity of the iridium
catalyst. The promoters fall into two categories: (i) carbonyl
or halocarbonyl complexes of W, Re, Ru, and Os; (ii) simple
iodides of Zn, Cd, Hg, Ga, and In. None of the promoters show
any detectable activity in the absence of iridium catalyst,
although ruthenium has been reported previously to catalyze
methanol carbonylation, albeit under harsh conditions and with
relatively low selectivity.13 For a promoter:Ir mole ratio of
5:1 the carbonylation rate is enhanced by factors of 2.6
([Ru(CO)4I2]); 2.3 ([Os(CO)4I2]); 1.2 ([Re(CO)5Cl]); 1.1
([W(CO)]6); 1.4 (ZnI2, HgI2); 1.8 (CdI2); 1.5 (GaI3); and 1.8
(InI3) (Supporting Information Table S2). A ruthenium promoter
(added as [Ru(CO)4I2]) is effective over a range of water
concentrations (Figure 1). By contrast, ionic iodides such as
LiI and Bu4NI are both strong catalyst poisons, reducing activity
by factors of ca. 2 and 3, respectively, when added in a 1:1
molar ratio with Ir.
Results and Discussion
Batch Autoclave Catalytic Reactions. In addition to the data
summarized in the Introduction, further studies have been
performed to determine how the catalytic rate depends on
process variables and additives. In batch autoclave tests methyl
acetate is used as the substrate, together with water at an
appropriate concentration. This mimics the process conditions
in which the standing concentration of methanol is low due to
esterification by the acetic acid cosolvent. The overall reaction
occurring in these batch studies can therefore be represented as
MeOAc + CO + H2O f 2 AcOH
(1)
Mechanism of Iridium-Catalyzed Methanol Carbonyla-
tion. Iridium/iodide catalysts for the carbonylation of methanol
were first identified by Monsanto,3 and elegant mechanistic
studies were reported by Forster.14 Others have also investigated
iridium-based catalysts.15,16 The catalytic cycle is thought to
Figure 2 compares the effects of Ru and In promoters with
the effect of LiI over a range of additive:Ir ratios. The
carbonylation rate is strongly inhibited at high concentrations
of ionic iodide, but the effect can be partially suppressed by
increasing the CO pressure. Notably, the promotional effect of
(12) The water concentration at which the maximum rate is attained varies with
[MeOAc], [MeI], and [CO].
(17) Forster, D.; Singleton, T. C. J. Mol. Catal. 1982, 17, 299. Forster, D. AdV.
Organomet. Chem. 1979, 17, 255. Maitlis, P. M.; Haynes, A.; Sunley, G.
J.; Howard, M. J. J. Chem. Soc., Dalton Trans. 1996, 2187. Haynes, A.;
Mann, B. E.; Gulliver, D. J.; Morris, G. E.; Maitlis, P. M. J. Am. Chem.
Soc. 1991, 113, 8567. Haynes, A.; Mann, B. E.; Morris, G. E.; Maitlis, P.
M. J. Am. Chem. Soc. 1993, 115, 4093.
(13) Jenner, G.; Bitsi, G. J. Mol. Catal. 1987, 40, 71. Kelkar, A. A.; Kohle, D.
S.; Chaudhari, R. V. J. Organomet. Chem. 1992, 430, 111. Braca, G.;
Paladini, L.; Sbrana, G.; Valentini, G.; Andrich, G.; Gregorio, G. Ind. Eng.
Chem. Prod. Res. DeV. 1981, 20, 115. Braca, G.; Sbrana, G.; Valentini,
G.; Andrich, G.; Gregorio, G. J. Am. Chem. Soc. 1978, 100, 6238. Braca,
G.; Sbrana, G.; Valentini, G.; Cini, M. J. Mol. Catal. 1982, 17, 323. Gautier-
Lafaye, J.; Perron, R.; Leconte, P.; Colleuille, Y. Bull. Soc. Chim. Fr. 1985,
353.
(18) Bassetti, M.; Monti, D.; Haynes, A.; Pearson, J. M.; Stanbridge, I. A.;
Maitlis, P. M. Gazz. Chim. Ital. 1992, 122, 391.
(19) Ellis, P. R.; Pearson, J. M.; Haynes, A.; Adams, H.; Bailey, N. A.; Maitlis,
P. M. Organometallics 1994, 13, 3215.
(14) Forster, D. J. Chem. Soc., Dalton Trans. 1979, 1639.
(15) Brodzki, D.; Denise, B.; Pannetier, G. J. Mol. Catal. 1977, 2, 149. Mizoroki,
T.; Matsumoto, T.; Ozaki, A. Bull. Chem. Soc. Jpn. 1979, 52, 479.
(16) Matsumoto, T.; Mizoroki, T.; Ozaki, A. J. Catal. 1978, 51, 96.
(20) Pearson, J. M.; Haynes, A.; Morris, G. E.; Sunley, G. J.; Maitlis, P. M. J.
Chem. Soc., Chem. Commun. 1995, 1045.
(21) Ghaffar, T.; Adams, H.; Maitlis, P. M.; Sunley, G. J.; Baker, M. J.; Haynes,
A. Chem. Commun. 1998, 1023.
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Promotion of Ir-Catalyzed Methanol Carbonylation
A R T I C L E S
Scheme 1. Anionic and Neutral Cycles Proposed by Forster for Iridium-Catalyzed Methanol Carbonylation (adapted from Ref 14)
In Situ Spectroscopic Studies of Catalyst Species. The
metal carbonyl complexes present during batch catalytic reac-
tions were probed in situ using high-pressure infrared (HPIR)
spectroscopy. Selected spectra are provided in the Supporting
Information (Figures S1-S4). In the absence of additives the
catalyst precursor, H2IrCl6, was rapidly converted into iridium
iodocarbonyl species after injection into the reaction solution,
which had been allowed to reach the required temperature and
pressure (190 °C, 22 barg). For reactions conducted at relatively
high [H2O] (i.e. g ca. 5% w/w), the predominant (>95%)
iridium species was the anionic methyl complex 2 (ν(CO) 2100,
2047 cm-1), which can be regarded as the “active” form of the
catalyst. A weak band at 1975 cm-1 indicated the presence of
a small amount of [Ir(CO)2I2]-, 1. As the catalytic reaction
Figure 2. Batch autoclave data: effect of [Ru(CO)4I2], InI3, and LiI on
carbonylation rate (190 °C, 1500 rpm). Total pressures 22 and 40 barg
correspond to CO partial pressures 7.5 and 25.5 bar, respectively. Autoclave
charge: MeOAc (648 mmol), H2O (943 mmol), AcOH (1258 mmol), MeI
(62 mmol), H2IrCl6 (1.56 mmol) plus additive if required. Carbonylation
rate measured at 50% conversion of MeOAc.
proceeded, the concentration of 2 decreased and ν(CO) bands
appeared at 2112 and 2065 cm-1, characteristic of cis-[Ir-
(CO)2I4]- (11), which is regarded as an “inactive” catalyst. At
50% methyl acetate conversion, the ratio of “active” 2 to
“inactive” 11 was approximately 1:1. The buildup of 11 as the
reaction proceeds is caused by the decrease in water concentra-
tion according to eq 1, which lowers the rate of reduction of 11
to 1 in the water gas shift cycle. For reactions conducted at
very low water concentrations ((i.e., [H2O] < ca. 4% w/w),
bands due to [Ir(CO)3I] (9) (2070, 2049sh cm-1) and [Ir(CO)3I3]
(13) (2186w, 2132 cm-1) were observed,7 consistent with the
operation of the “neutral cycle” under these conditions. During
reactions conducted in the presence of LiI or Bu4NI (equimolar
with Ir) infrared spectroscopy shows the catalyst to remain
predominantly (g90%) as the “active” species 2 at 50% MeOAc
conversion, with buildup of 11 occurring only later in the
[Ru(CO)4I2] (Ru:Ir ) 2:1) is removed when an equimolar
quantity of LiI is also added (equivalent to addition of Li[Ru-
(CO)3I3]). A corollary of this is that the inhibiting effect of
LiI is negated by the addition of an equimolar amount of
[Ru(CO)4I2].
The effect of a ruthenium promoter was also examined over
the range 170-200 °C (Supporting Information Table S3).
Eyring plots of the data gave the apparent activation parameters
listed in Table 1. Addition of a ruthenium promoter has a rather
subtle effect, with a slightly higher ∆Hq being offset by a less
negative ∆Sq, resulting in a lowering of ∆Gq(190 °C) by less
than 2 kJ mol-1. The Ir and Ir/Ru catalysts exhibit larger ∆Hq
but less negative values of ∆Sq than those for Rh catalysts,22
consistent with a different rate equation.
(22) Dekleva, T. W.; Forster, D. AdV. Catal. 1986, 34, 81. Smith, B. L.; Torrence,
G. P.; Murphy, M. A.; Aguilo´, A. J. Mol. Catal. 1987, 39, 115. Hjortkjaer,
J.; Jensen, V. W. Ind. Eng. Chem., Prod. Res. DeV. 1976, 15, 46.
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A R T I C L E S
Haynes et al.
Table 1. Activation Parameters Derived from Eyring Plots of Kinetic Data Measured for Ir-Catalyzed Carbonylation and Stoichiometric
Carbonylation of Iridium Methyl Complexes 2 and 4
q
q
reaction
promoter (promoter:Ir)
solvent
∆H /kJ mol-1
∆S /J mol-1 K-1
Ir-catalyzed carbonylationa
Ir-catalyzed carbonylationa
Ir-catalyzed carbonylation16
2 + CO f 6a + 6b
2 + CO f 6a
AcOH
AcOH
33% MeOH/PhCOMe
PhCl
25% MeOH/PhCl
PhCl
PhCl
PhCl
PhCl
1% MeOH/PhCl
83 ((6)
96 ((3)
34 ((2)
152 ((6)
33 ((2)
90 ((5)
104 ((10)
108 ((9)
89 ((3)
73 ((2)
-73 ((12)
-40 ((6)
-195 ((4)b
82 ((17)
-197 ((8)
-63 ((12)
-19 ((28)
-8 ((24)
-36 ((8)
-79 ((6)
Ru (2:1)
2 + CO f 6a + 6b
2 + CO f 6a + 6b
2 + CO f 6a + 6b
4 + CO f 7
Ru (1:13)
InI3 (1:10)
3d (1:10)
4 + CO f 7
a Conditions as in Figure 2 with 40 barg total pressure. b Value of -246 J mol-1 K-1 given in ref 20 was wrongly calculated from the data in ref 16.
Figure 3. Series of in situ HPIR spectra for the reaction of 2 with CO (5.5 barg) in PhCl at 93 °C.
reaction. Thus iodide salts appear to lower the activity of 2 rather
than convert it into an inactive iridium complex.
obtained for the reaction in PhCl is shown in Figure 3. The
two ν(CO) bands of 2 at 2093 and 2040 cm-1 decay and are
replaced by new absorptions due to 6 at 2105 (m), 2057 (s),
and 1670 (m, br) cm-1. An additional weak absorption at 1966
cm-1 is assigned to a small amount of 1, presumably formed
by reductive elimination of MeI from 2.
1H and 13C HPNMR spectra for the reaction of [Ir-
(13CO)2I3(13CH3)]- with 13CO (42 barg) in CD2Cl2 at 80 °C
revealed the slow formation of two isomers of [Ir(13CO)2I3-
(13CO13CH3)]- in a ca. 1:1 ratio.24 The observed chemical shifts
and coupling constants are listed in the Supporting Information
(Table S8). A singlet for each isomer in the terminal CO region
indicates equivalent CO ligands, and the absence of coupling
between terminal and acetyl carbonyl resonances shows that
neither isomer of 6 has acetyl trans to CO. The two isomers
are therefore assigned as the fac,cis and mer,trans complexes,
6a and 6b (Scheme 2).25 The unusual relative intensities of
product ν(CO) bands in the IR (Figure 3) are explained by the
Prior to injection of the iridium catalyst, a ruthenium promoter
(added as [Ru(CO)4I2]) was present (at 19 barg and 190 °C) as
a mixture of ruthenium(II) iodocarbonyl species [Ru(CO)2I2-
(sol)2] (ν(CO) 2053, 1996 cm-1), [Ru(CO)3I2(sol)] (2126, 2068,
2012 cm-1), and [Ru(CO)3I3]- (2107, 2041(sh) cm-1). After
injection of the iridium catalyst (with the conditions given in
Figure 2, and Ru:Ir ) 2:1), [Ru(CO)3I3]- became the dominant
ruthenium species. At higher ruthenium concentrations (e.g.,
Ru:Ir ) 10:1) the IR band intensities indicated a greater
proportion of neutral ruthenium complexes (both before and after
injection of the iridium catalyst). The strong ν(CO) bands of
ruthenium carbonyl complexes hinder the observation of iridium
species. However, in situ HPIR studies for an indium-promoted
catalyst (InI3:Ir 2:1) showed iridium speciation very similar to
that observed in the absence of promoter, with 2 being converted
into 11 as the reaction progressed.23
Model Reaction Studies: Carbonylation of 2. The reaction
of 2 with CO (eq 2) was identified in Forster’s original study14
as the rate-determining step in the anionic cycle for catalytic
carbonylation.
(23) An additional band at 2053 cm-1 is tentatively assigned to a trans dicarbonyl
isomer of 2. The intensity of this band was higher when the In:Ir ratio was
increased from 2:1 to 10:1.
(24) Additional Ir methyl and acetyl species observed on prolonged heating are
assigned as mixed halide species formed by halide exchange with the
solvent. The Ir-Me species were assigned on the basis of exchange
experiments using Bu4NCl (Vickers, P. W. Ph.D. Thesis, University of
[Ir(CO)2I3Me]- (2) + CO f [Ir(CO)2I3(COMe)]- (6) (2)
Sheffield, 1997) as [Ir(CO)2Cl2IMe]- (Me trans to I) δ 13C -2.91, 157.9,
1
δ
1H 1.71, JH-C 139 Hz; [Ir(CO)2Cl2IMe]- (Me cis to I) δ 13C -9.30,
In situ HPIR and HPNMR spectroscopy show that in weakly
polar aprotic solvents the reaction proceeds slowly even at
elevated temperatures (>80 °C). A typical set of IR spectra
155.2, 158.7, δ 1H 1.94, 1JH-C 139 Hz. The two acetyl species have δ 13C
198.3, 196.3, 43.6, 39.4, JC-C 33 Hz in each case; δ 1H 2.74, JH-C 130
1
1
2
1
2
Hz, JH-C 6 Hz, 2.67, JH-C 123 Hz, JH-C 7 Hz.
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Promotion of Ir-Catalyzed Methanol Carbonylation
A R T I C L E S
Figure 4. Series of IR spectra recorded in situ during the reaction of 2 with CO (5.5 barg) in PhCl-MeOH (3:1 v/v) at 33 °C.
Scheme 2. Proposed Mechanism for Carbonylation of 2 to Give
observed in neat CD2Cl2 and is assigned as 6a on the basis of
the IR spectrum. On warming to 60 °C (10 min), weak signals
due to the trans isomer grew in, suggesting that isomerization
6a f 6b can occur.27 Also observed after warming to 60 °C
were 13CH313CO2CD3 and the iridium hydride [Ir(CO)2I3D/H]-
resulting from methanolysis of iridium acetyl species according
to eq 3.
6a and 6b
[Ir(13CO)2I3(13CO13CH3)]- + CD3OD/H f
[Ir(13CO)2I3D/H]- + 13CH313CO2CD3 (3)
When [Ir(13CO)2I3(13CH3)]- was carbonylated with unlabeled
CO in 25% MeOH/PhCl, 1JC-C doublets in the 13C NMR spectra
of 6a and 6b indicated doubly labeled 13CO13CH3 groups,
showing that the methyl group migrates to a CO ligand already
coordinated to Ir in 2.
Careful choice of reaction conditions (see Experimental
Section) enabled the isolation of 6 as an analytically pure solid,
which 1H NMR showed to contain 6a and 6b in a ca. 9:1 ratio.
A crystal from this sample was selected for X-ray diffraction,
the structure obtained being that of 6a (Figure 5). The complex
has distorted octahedral geometry with the acetyl ligand leaning
away from the two cis iodide ligands to give a bond angle with
the trans iodide of 173°. The plane of the acetyl ligand is twisted
ca. 30° relative to the approximate mirror plane of the Ir(CO)2I3
fragment. The Ir-I bond trans to acetyl is longer by ca. 0.1 Å
than those trans to CO due to the strong trans influence of
acetyl.
Kinetics of Carbonylation of 2. In situ HPIR spectroscopy
was used to follow the exponential decay of the 2093 cm-1
band of 2. Observed first-order rate constants are reported in
the Supporting Information (Tables S4-S6). In PhCl solvent
the reaction rate was found to be independent of CO pressure
above a threshold of ca. 3.5 barg. Variable-temperature kinetic
data were measured over the range 80-122 °C, and an Eyring
plot of these data gave the activation parameters listed in Table
1. Addition of Bu4NI (equimolar with Ir) lowered the observed
coincidence of the low-frequency antisymmetric ν(CO) modes
of 6a and 6b at 2057 cm-1 26
.
Carbonylation of 2 was much more facile on addition of
methanol, and convenient reaction times were accessible at much
lower temperatures (ca. 30-50 °C). A typical set of IR spectra
for the reaction in PhCl-MeOH is shown in Figure 4. In this
case, the two terminal ν(CO) bands of the product have similar
intensities, indicating selective formation of the fac,cis isomer
6a. At the high end of the temperature range employed, a band
was also observed to grow at 1746 cm-1, due to formation of
MeOAc by methanolysis of 6a.
In situ HPNMR spectroscopy showed that the reaction
of [Ir(13CO)2I3(13CH3)]- with 13CO (6.3 barg) in CD2Cl2-
1
CD3OD (3:1) at 25 °C gave a major product with H and 13C
resonances that correspond well with one of the isomers
(25) Isomers 6a and 6b were also identified in the NMR spectra of products
recovered after carbonylation of 2 in PhCl (80 psig, 100 °C). 1H NMR
(CDCl3): δ 2.77, 3.04 (COCH3). 13C NMR (CDCl3): δ 49.6, 50.5 (COCH3),
δ 196.9, 197.8 (COCH3), and δ 151.5, 160.5 (Ir-CO).
(26) Similarly, fac,cis-[Rh(CO)2I3(COMe)]- has ν(CO) at 2114 (s) and 2081
(s) cm-1, while mer,trans-[Rh(CO)2I3(COMe)]- has ν(CO) at 2141 (vw)
and 2084 (s) cm-1. The low-frequency ν(CO) bands of cis and trans
[M(CO)2I4]- (M ) Rh, Ir) are also nearly coincident (Haynes, A.; McNish,
J.; Pearson, J. M. J. Organomet. Chem. 1997, 551, 339).
1
1
(27) Another (unidentified) minor acetyl species showed δ H 2.70 (dd, JH-C
128.4 Hz, JH-C 5.4 Hz), δ 13C 203.9 (d, JC-C 32 Hz, COCH3, COCH3
signal presumed hidden by solvent peaks) and 162.9 (Ir-CO).
2
1
9
J. AM. CHEM. SOC. VOL. 126, NO. 9, 2004 2851

A R T I C L E S
Haynes et al.
Figure 5. X-ray crystal structure of 6a. Ph4As+ counterion omitted. Selected
bond lengths (Å): Ir-C(25) 1.890(6), Ir-C(26) 1.903(5), Ir-C(27) 2.193-
(5), Ir-I(1) 2.6939(7), Ir-I(2) 2.7115(7), Ir-I(3) 2.8281(7), O(1)-C(25)
1.118(7), O(2)-C(26) 1.121(6), O(3)-C(27) 1.208(6), C(27)-C(28) 1.375-
(8). Selected bond angles (deg): C(25)-Ir-C(26) 96.5(2), C(25)-Ir-C(27)
91.4(2), C(26)-Ir-C(27) 90.2(2), C(27)-Ir-I(1) 85.20(12), C(27)-Ir-
I(2) 95.34(12), I(1)-Ir-I(2) 92.688(13), C(27)-Ir-I(3) 172.93(12), O(1)-
C(25)-Ir 178.5(6), O(2)-C(26)-Ir 177.7(5), O(3)-C(27)-C(28) 127.8(5),
O(3)-C(27)-Ir 111.5(4), C(28)-C(27)-Ir 120.7(4).
rate constant by a factor of ca. 2, similar to the effect of iodide
salts in the catalytic reactions.28
In a preliminary communication we reported that carbon-
ylation of 2 to 6 is dramatically accelerated by the presence of
methanol.20 For example, addition of 1% MeOH (v/v) results
in a ca. 104-fold rate enhancement at 33 °C (based on the
extrapolated rate constant in PhCl using activation parameters
measured at higher temperatures). Higher methanol concentra-
tions gave an approximately linear increase in rate. Figure 6
shows the dependence of rate on CO pressure and added
Ph4AsI in 3:1 PhCl-MeOH. In the absence of iodide salt a
saturation dependence on pCO is found. Substantial inhibition
is caused by Ph4AsI (2 molar equiv/Ir), but this decreases as
the CO pressure is raised. The behavior of the model reaction
closely resembles the kinetics of the catalytic carbonylation
system (Table S2 and Figure 2).
Figure 6. Plots of kobs for carbonylation of 2 in PhCl-MeOH (3:1 v/v) at
33 °C: (a) effect of CO pressure in the absence or presence of Ph4AsI
(2 molar equiv per Ir); (b) effect of [Ph4AsI] at 5.5 barg CO.
of 6a as the kinetic product in PhCl-MeOH can be explained
if 5a is trapped by coordination of a methanol solvent molecule,
preventing isomerization to 5b. The mixture of isomers observed
at high temperature may be under thermodynamic control, with
isomerization occurring via loss of iodide or CO or via a
tricarbonyl 7.30-32
The accelerating effect of protic solvents such as methanol
can be understood in terms of their ability to solvate an iodide
anion. The large negative ∆Sq found at high [MeOH] likely
results from strong solvation of the dissociating iodide ligand
due to H-bonding interactions (eq 4). Alcohols also promote
the reaction of 2 with phosphites to give acetyl complexes,
[Ir(CO)L2I2(COMe)] (L ) (P(OR)3),33 and protic solvents
facilitate other reactions that involve halide dissociation from
a metal complex.34
Variable-temperature kinetic measurements in PhCl-MeOH
gave quite different activation parameters (Table 1) from those
in neat PhCl, with a much lower ∆Hq partly offset by a large
negative ∆Sq. Higher alcohols gave smaller rate enhancements,
and a plot of ln kobs vs the alcohol’s Taft R value29 shows a
good linear correlation, indicating that the alcohol’s hydrogen
bond donor ability controls the rate. By contrast, addition of a
polar aprotic donor solvent such as MeCN had only a small
effect.
Mechanism of the Unpromoted Carbonylation of 2.
Scheme 2 illustrates a mechanism for carbonylation of 2
consistent with the kinetic data presented above. Dissociative
substitution of iodide by CO gives 4, for which spectroscopic
data indicate a fac tricarbonyl structure (vide infra). Theoretical
calculations indicate that migratory insertion in 4 leads directly
to 5a, which has a square pyramidal geometry with an apical
acetyl ligand (consistent with acetyl’s large trans influence).
Coordination of iodide to 5a would give 6a, whereas prior
isomerization to 5b would lead to 6b. The selective formation
[Ir(CO)2I3Me]- (2) + n MeOH h
[Ir(CO)2I2Me] (3) + (MeOH)n‚‚‚I- (4)
Other migratory insertion reactions are promoted by good
donor solvents35 which coordinate to the vacant site created by
(30) In the Rh system, oxidative addition of acetyl iodide to [Rh(CO)2I2]- gives
fac,cis-[Rh(CO)2I3(COMe)]-, which isomerizes to the mer, trans geometry
at room temperature (ref 32). The mer, trans isomer is formed directly
2-
from reaction of [Rh(CO)I3(COMe)]2 with CO (ref 31).
(28) Due to the low solubility of Ph4AsI in PhCl, the effect of iodide salt was
studied using Bu4NI and the Bu4N+ salt of 2. In the absence of added
iodide salt Bu4N+[2] was carbonylated marginally slower than Ph4As+[2].
(29) Kamlet, M. J.; Abboud, J. L. M.; Taft, R. W. Prog. Phys. Org. Chem.
1981, 13, 485.
(31) Adams, H.; Bailey, N. A.; Mann, B. E.; Manuel, C. P.; Spencer, C. M.;
Kent, A. G. J. Chem. Soc., Dalton Trans. 1988, 489.
(32) Howe, L.; Bunel, E. E. Polyhedron 1995, 14, 167.
(33) Haynes, A.; Pearson, J. M.; Vickers, P. W.; Charmant, J. P. H.; Maitlis, P.
M. Inorg. Chim. Acta 1998, 270, 382.
9
2852 J. AM. CHEM. SOC. VOL. 126, NO. 9, 2004

Promotion of Ir-Catalyzed Methanol Carbonylation
A R T I C L E S
alkyl migration, but in the case of 2 the ability to solvate an
iodide anion appears more important. Strong coordination by
solvent (e.g., MeCN) to Ir will also inhibit formation of the
tricarbonyl 4 (vide infra).
Effect of Promoters on the Carbonylation of 2. In light of
the beneficial effects of various metal compounds on catalytic
carbonylation (vide supra), we tested the effects of some of these
compounds on the kinetics of carbonylation of 2. The effects
of ruthenium and indium complexes were studied in detail to
compare the two categories of promoter. As in the absence of
promoters, acetyl products 6a and 6b were formed in a ca. 1:1
ratio in PhCl. Kinetic data are reported in the Supporting
Information (Tables S5 and S6). Addition of the neutral
ruthenium(II) iodocarbonyl complexes, [Ru(CO)3I2]2, [Ru-
(CO)4I2], or [Ru(CO)2I2]n (Ru/Ir molar ratio <0.2), was found
to give substantial rate enhancements (by factors of 15-20 for
a Ru:Ir ratio of 1:13 at 93 °C, PhCl). Indium and gallium
triiodides and zinc diiodide gave accelerations very similar to
the neutral ruthenium species.36,37 By contrast, addition of
anionic ruthenium(II) species [Ru(CO)3I3]- or [Ru(CO)2I4]2-
(as their Bu4N+ salts) did not lead to any appreciable promotion
or inhibition. This behavior reflects that found in the catalytic
system where the promotional effect of [Ru(CO)4I2] was negated
by the presence of an equimolar quantity of LiI (equivalent to
addition of Li[Ru(CO)3I3]). The effectiveness of neutral, but
not anionic Ru complexes indicates that the ability to accept
an iodide ligand is a key property of the promoter.
Figure 7. Plots of kobs for carbonylation of 2 vs [promoter] in PhCl (at 5.5
barg and 93 °C unless stated otherwise).
scale, a dimeric neutral iridium methyl complex, [Ir(CO)2I2-
Me]2 (3d), was isolated and characterized spectroscopically and
by X-ray crystallography.38,39
NMR spectroscopy showed the presence of two isomers of
3d in CD2Cl2 solution in the approximate ratio 5:2, consistent
with the data reported by Gautron et al.39 Molecular weights
determined for solutions of 3d by osmometry were 1001 in PhCl
(consistent with the dimeric structure; MW 1034) and 538 in
MeCN, indicating bridge cleavage by the coordinating solvent.
This was confirmed by the observation of ν(CN) at 2338 cm-1
for [Ir(CO)2(NCMe)I2Me] in 3% v/v MeCN-CHCl3, along with
a shift of ν(CO) to 2126 and 2080 cm-1. Addition of 2 equiv
of I- to the dimer quickly regenerates the anion, 2. Similarly,
InI3 was found to abstract iodide from the acetyl complex 6a
to give [Ir(CO)2I2(COMe)]214 (ν(CO) (CH2Cl2) 2124, 2081, and
1712 cm-1), and addition of Bu4NI resulted in rapid conversion
back to 6a.
The reaction of 2 with [Ru(CO)3I2]2 resulted in quite
complicated IR spectra. On mixing CH2Cl2 solutions of [Ru-
(CO)3I2]2 and 2 (to attain eqimolar [Ir] and [Ru]), the ν(CO)
bands due to both starting complexes decayed slowly, to be
replaced by new absorptions at 2119 sh, 2103 s 2074 w, 2060
sh, 2048, and 2038 m. The spectrum is consistent with the
presence of some [Ru(CO)3I3]- and 3d, but the additional
absorptions are not due to known Ir or Ru complexes. The
reaction of 3d and Bu4N[Ru(CO)3I3] in CH2Cl2 resulted in a
similar IR spectrum after several minutes. The new product is
tentatively assigned as a mono-iodide-bridged Ir-Ru dimer 15
(Scheme 4), which would be expected to exhibit up to five
ν(CO) bands. 13C NMR measurements on this system support
the formation of a mixed dimer of this sort.40 Evidence for a
The effect of promoter concentration on the rate of carbon-
ylation of 2 is illustrated graphically in Figure 7. The effect is
proportional to [promoter] at low concentrations, but some
leveling-off is evident at higher [promoter]/[Ir] ratios, particu-
larly for [Ru(CO)3I2]2. The graph shows that the promoters all
give similar rate enhancements at a given temperature and
[promoter]/[Ir] ratio. Increasing CO pressure was found to have
little effect on the reaction rate in the presence of promoter.
The activation parameters (Table 1) derived from Eyring plots
show a substantial lowering of ∆Hq and a change from positive
to negative ∆Sq on addition of promoter. In mixed PhCl-MeOH
solvent systems (25-50% MeOH, v/v) significant promotion
was evident only on addition of larger quantities of [Ru(CO)3I2]2
(e.g., Ru:Ir ) 2:5).
Stoichiometric Iodide Transfer Reactions. Complex 2 was
found to react cleanly with a small excess of GaI3 or InI3 in
CH2Cl2, to give a product with ν(CO) bands (2118 and 2074
cm-1) consistent with formation of a neutral species. The
reaction of 2 with zinc diiodide also gave some of the same
product, but did not go to completion, perhaps due to solubility
limitations. From the reaction of 2 with InI3 on a preparative
(34) Bennett, M. A.; Crisp, G. T. Organometallics 1986, 5, 1792. Bennett, M.
A.; Crisp, G. T. Organometallics 1986, 5, 1800. Romeo, R.; Minniti, D.;
Lanza, S. Inorg. Chem. 1980, 19, 3663. Kelm, H.; Louw, W. J.; Palmer,
D. A. Inorg. Chem. 1980, 19, 843. Treichel, P. M.; Vincenti, P. J. Inorg.
Chem. 1985, 24, 228. Blum, O.; Milstein, D. J. Am. Chem. Soc. 1995,
117, 4582.
(35) Mawby, R. J.; Basolo, F.; Pearson, R. G. J. Am. Chem. Soc. 1964, 86,
3994. Butler, I. S.; Basolo, F.; Pearson, R. G. Inorg. Chem. 1967, 6, 2074.
(36) Despite the promotional effect of [W(CO)6] on the catalytic process, neither
[W(CO)6] nor [Cr(CO)6] had any effect on the rate of carbonylation of 2.
Although group 6 metal hexacarbonyls are known to react with iodide salts
on heating to give [M(CO)5I]- (ref 37), it appears that iodide abstraction
by these reagents is not efficient under the conditions of our experiments.
Aluminum triiodide was also tested, but found not to be effective.
(37) Abel, E. W.; Butler, I. S.; Reid, J. G. J. Chem. Soc. 1963, 2068. Palitzsch,
W.; Beyer, C.; Bo¨hme, U.; Rittmeister, B.; Roewer, G. Eur. J. Inorg. Chem.
1999, 1813.
(38) Since our preliminary communication (ref 21), an X-ray structure of 3d
has also been reported independently (ref 39). The two structures show no
significant differences. Crystallographic data for our structure are listed in
the Supporting Information.
(39) Gautron, S.; Giordano, R.; Le Berre, C.; Jaud, J.; Daran, J.-C.; Serp, P.;
Kalck, P. Inorg. Chem. 2003, 42, 5523.
(40) 13C NMR data have recently been reported for 15, formed by reaction of
[Ru(CO)4I2] with 2. Under 6 bar CO 15 is cleaved to give 4 and
[Ru(CO)3I3]-, demonstrating iodide transfer from Ir to Ru. Whyman, R.;
Wright, A. P.; Iggo, J. A.; Heaton, B. T. J. Chem. Soc., Dalton Trans.
2002, 771.
9
J. AM. CHEM. SOC. VOL. 126, NO. 9, 2004 2853

A R T I C L E S
Haynes et al.
Table 2. Reaction Energies (kJ mol-1) Calculated at the MP2 Level (2DZ and 2DZ* bases) for Selected Ligand Substitution, Transfer, and
Interchange Processesa
∆E/kJ mol-1
reactant(s)
2 + CO
products
vacuum 2DZ
CO/Iodide Substitution
vacuum 2DZ*
PhCl 2DZ
MeOH 2DZ
4 + I-
87.1
115.9
-11.4
-27.8
Iodide Transfer
2 + [Ru(CO)3I2]
6a + [Ru(CO)3I2]
1 + [Ru(CO)3I2]
2 + InI3
3 + [Ru(CO)3I3]-
5a + [Ru(CO)3I3]-
8 + [Ru(CO)3I3]-
-55.8
-82.3
11.7
-44.5
-71.0
23.0
-48.4
-85.4b
-1.7
-35.1
-70.5b
11.6
-56.9
-83.2
6.7
-38.8
-65.1
24.8
-59.4
-83.1
8.6
-42.8
-66.5
25.2
-
3 + InI4
-
6a + InI3
5a + InI4
-
1 + InI3
8 + InI4
Iodide/CO Interchange
5.3
2 + [Ru(CO)4I2]
6a + [Ru(CO)4I2]
1 + [Ru(CO)4I2]
4 + [Ru(CO)3I3]-
7 + [Ru(CO)3I3]-
9 + [Ru(CO)3I3]-
0.8
8.8b
-0.1
8.0
3.6
11.7
9.9
2.1
17.8
10.7
10.8
Iodide/Solvent Interchange
2 + [Ru(OH2)(CO)3I2]
1 + [Ru(OH2)(CO)3I2]
[Ir(OH2)(CO)2I2Me] + [Ru(CO)3I3]-
[Ir(OH2)(CO)2I] + [Ru(CO)3I3]-
-7.5
2.1
-8.0b
5.2
a Solvent effects (in PhCl, ꢀ ) 5.7; MeOH, ꢀ ) 33) were calculated using the PCM method. b Single-point 2DZ* calculations on 2DZ optimized structures.
Scheme 3. Reversible Iodide Abstraction from 2 Leading to 3d
Substitution of an iodide ligand in 2 by CO to give 4 is
predicted to be highly endothermic in vacuo, but exothermic
by 11 or 28 kJ mol-1 in PhCl or MeOH, respectively. Cheong
et al. calculated the same substitution reaction to be endothermic
by 17 kJ mol-1 in PhCl.46 Solvent effects are less pronounced
for reactions where an iodide ligand is transferred between metal
centers, as the solvation energies of reactants and products are
Scheme 4. Anionic Iodide-Bridged Dimers 15 and 16
similar. Iodide transfer from 2 to [Ru(CO)3I2] or InI3 is predicted
to be exothermic (by ca. 50 and 40 kJ mol-1, respectively).
The corresponding reactions of 6a are even more exothermic,
whereas iodide transfer from 1 to [Ru(CO)3I2] or InI3 is slightly
endothermic, indicating that Ir(I) binds iodide more effectively
than Ir(III). For ligand interchange processes in which a CO or
solvent ligand from Ru is exchanged with an iodide from Ir,
similar Ir-only species was also found in the reaction between
the reaction energies are much smaller (0 ( ca.10 kJ mol-1
)
2 and 3d in CH2Cl2 or by adding substoichiometric amounts of
Bu4NI to 3d.41 In each case a product assigned as 16 is observed
than those for iodide transfer. Thus interchange of iodide and
solvent (or CO) ligands between iridium and ruthenium centers
is thermodynamically feasible.
1
(ν(CO) 2108, 2060 cm-1; δ H 2.3 in CDCl3), which exists
only in an equilibrium with 2 and 3d and cannot be isolated.
Dimers 15 and 16 are not observed in polar solvents such as
MeCN or MeNO2 and are probably not significant catalytic
intermediates.
Ab Initio Calculations for Iodide Transfer. Reaction
energies for dissociation of iodide, CO, and solvent (H2O or
MeOH) were computed by ab initio (MP2) theoretical methods
for the important Ir, Ru,42-44 and In species (Supporting
Information, Table S11). Solvent medium effects were consid-
ered by the PCM method using the dielectric constants of PhCl
(ꢀ ) 5.7) and MeOH (ꢀ ) 33).45 Selected reaction energies
derived from combination of the appropriate ligand dissociation
energies are listed in Table 2.
Reactivity of fac,cis-[Ir(CO)3I2Me] (4). The spectroscopic
characterization and reactivity of the neutral tricarbonyl 4 has
been reported in a preliminary communication.21 The complex
is generated by reaction of the dimer 3d with CO in CH2Cl2 or
PhCl. IR, 13C NMR, and 1H NMR spectroscopic data for 4 are
given in the Supporting Information (Table S8 and Figure S6).
Three ν(CO) bands of similar intensity are consistent with a
fac-tricarbonyl geometry for 4. Using 1 atm CO, less than 10%
conversion of 3d into 4 was achieved, but essentially complete
conversion occurs under 27.5 barg CO. On venting the CO and
introduction of an inert atmosphere, 4 reverts to 3d over several
minutes. When acetonitrile was used as solvent, formation of 4
was suppressed even at high CO pressures, presumably due to
cleavage of the dimer and occupation of the vacant site by
MeCN.
(41) The ν(CO) bands of 16 were also observed during reactions between 2
and InI3 or GaI3 in CH2Cl2.
(42) The preferred geometrical isomers of ruthenium iodocarbonyl complexes
(cis-[Ru(CO)4I2] (ref 43) and fac-[Ru(CO)3I3]- (ref 44)) were correctly
predicted by single-point 2DZ* calculations on the 2DZ optimized
geometries (Table S9). The five-coordinate [Ru(CO)3I2] was taken to have
a fac-Ru(CO)3 unit.
(43) Dahl, L. F.; Wampler, D. L. Acta Crystallogr. 1962, 15, 946.
(44) Zoeller, J. R. Inorg. Chem. 1986, 25, 3933. Galletti, A.; Braca, G.; Sbrana,
G.; Marchetti, F. J. Mol. Catal. 1985, 32, 291.
(45) Despite their different characters, the dielectric constants of acetic acid
(ꢀ ) 6.2) and chlorobenzene (ꢀ ) 5.7) are remarkably similar. The actual
dielectric constant of the catalytic reaction solvent will be modified by
temperature and the presence of other species, particularly water.
Isotopic labeling experiments indicated that when unlabeled
3d reacts with 13CO, the incoming 13CO ligand coordinates trans
to the methyl group. When the 13CO atmosphere was removed,
[Ir(CO)2(13CO)I2Me] was found to revert cleanly to [Ir(CO)2-
I2Me]2, with no detectable retention of isotopic label. The
reversible reaction of 3d with 13CO is therefore stereospecific,
(46) Cheong, M.; Schmid, R.; Ziegler, T. Organometallics 2000, 19, 1973.
9
2854 J. AM. CHEM. SOC. VOL. 126, NO. 9, 2004

Promotion of Ir-Catalyzed Methanol Carbonylation
A R T I C L E S
Scheme 5. Stereochemistry of Reaction of 13CO with 3da
Table 3. Calculated Activation Energies (∆Eq) and Reaction
Energies (∆Emig) for Migratory CO Insertion in 2 and 4 and
Calculated Reaction Energies (∆ECO) for Carbonylation of 2 and 4
to Give 6a and 7, Respectively (values in kJ mol-1
)
q
reactant
method
∆E
∆Emig
∆ECO
[Ir(CO)2I3Me]-, 2
MP2 2DZ*
ADF46
149.0
124.7
117.6
102.1
78.2
46.2
14.8
-10.9
16.4
-12.2
-29.3
-64.7a
a The extent of labeling in the terminal carbonyls of 7 is undetermined.
B3LYP50
MP2 2DZ*
ADF46
[Ir(CO)3I2Me], 4
-58.8a
B3LYP50
82.8
a Single-point 2DZ* calculation on 2DZ optimized structures.
and CO ligands. After passing through the transition state this
angle continues to open, to give a square pyramidal product
(14a or 5a) with an apical acetyl ligand. This is attributable to
the strong trans influence of acetyl and provides an explanation
why the kinetic products of migratory CO insertion often have
the entering ligand trans to acetyl,48 rather than cis as found in
the classic [Mn(CO)5Me] system.49
While this work was in progress, two DFT studies of the
same reactions were reported,46,50 and the computed activation
and reaction energies are summarized along with ours in Table
3. All the theoretical methods predict that methyl migration in
4 has a substantially lower activation barrier (by between 35
and 47 kJ mol-1) and is more exothermic than the corresponding
reaction of 2. Our ab initio (MP2) activation energy slightly
overestimates the experimental ∆Hq for carbonylation of 4
(89 ((3) kJ mol-1), whereas the DFT calculations underestimate
the barrier by a similar amount.51 Consideration of solvent
effects (using the PCM method) has only a marginal effect on
the activation barriers.
The lower activation barrier for migratory CO insertion in 4
has been attributed previously to the ability of a trans CO ligand
to labilize the Ir-Me bond, combined with stabilization of the
transition state by an additional π-acceptor CO ligand.46,50 The
computational data show the Ir-Me bond to be only marginally
longer (by 0.01 Å) in 4 than in 2, and estimated Ir-Me bond
dissociation energies actually indicate a stronger Ir-Me bond
in 4.52 A larger perturbation is found for the Ir-CO bond
distances cis to methyl in 2 (1.87 Å) and 4 (1.92 Å), reflecting
a decrease in π-back-donation from the Ir center in 4. This is
also apparent in the average ν(CO) values, which shift to high
frequency by 27% (experimental) and 23% (theoretical) for 4
compared with 2. Weaker back-donation from Ir in 4 depopu-
lates the CO π* orbitals,53 thereby raising the electrophilicity
of the carbonyl ligand and promoting methyl migration (which
can be regarded as an intramolecular nucleophilic attack).
Figure 8. Optimized structures (MP2/LANL2DZ) for stationary points on
the reaction coordinate for migratory CO insertion in complexes 2 and 4.
with the labeled ligand entering and leaving from the site trans
to methyl, as shown in Scheme 5.
When a solution of 4 under CO pressure was warmed above
40 °C, the ν(CO) bands of 4 were replaced by new absorptions
due to [Ir(CO)3I2(COMe)] (7), which is assigned a mer-
tricarbonyl geometry. This is confirmed by the 13C NMR
2
spectrum of [Ir(13CO)3I2(13COMe)], which displayed no JC-C
coupling for the acetyl carbonyl, consistent with all three 13CO
ligands being cis to acetyl. The spectrum of 7 generated from
the reaction of unlabeled 3d with 13CO indicated no incorpora-
tion of 13CO into the acetyl group, consistent with the mech-
anism shown in Scheme 5. On releasing the CO pressure, 7
was observed to convert slowly into [Ir(CO)2I2(COMe)]2 (ν(CO)
2123, 2091, 1708 cm-1), resulting from loss of CO.14
Pseudo-first-order rate constants for the migratory insertion
reaction of 4 in PhCl (27.5 barg CO, 44-85 °C) are listed in
the Supporting Information (Table S7), and an Eyring plot of
these data yields the activation parameters given in Table 1.
Complex 4 is carbonylated more than 700 times faster than the
anion 2 at 85 °C, representing a lowering of ∆Gq by 20 kJ
mol-1. Extrapolation to higher temperatures using the measured
activation parameters reduces the difference in rates, but even
at 180 °C, 4 is predicted to react an order of magnitude faster
than 2. Substitution of iodide by CO is also known to facilitate
methyl migration in [Fe(CO)2(PMe3)2IMe].47 Addition of small
amounts of methanol was found to accelerate the carbonylation
of 4 (see Table S7) but not to the dramatic extent found for 2.
Activation parameters calculated for the reaction in 1% MeOH-
PhCl show a modest decrease in ∆Hq and more negative ∆Sq
than in the absence of methanol (Table 1).
(48) Alonso, F. J. G.; Llamazares, A.; Riera, V.; Vivanco, M.; Granda, S.; Diaz,
M. R. Organometallics 1992, 11, 2826. Glyde, R. W.; Mawby, R. J. Inorg.
Chim. Acta 1971, 5, 317. Barnard, C. F. J.; Daniels, J. A.; Mawby, R. J. J.
Chem. Soc., Dalton Trans. 1979, 1331.
(49) Noack, K.; Calderazzo, F. J. Organomet. Chem. 1967, 10, 101. Flood, T.
C.; Jensen, J. E.; Statler, J. A. J. Am. Chem. Soc. 1981, 103, 4410.
(50) Kinnunen, T.; Laasonen, K. J. Mol. Struct. (THEOCHEM) 2001, 542, 273.
(51) The ab initio activation barrier for migratory insertion in 2 is very similar
to the experimental ∆Hq for carbonylation of 2 in PhCl, but the validity of
a direct comparison is doubtful due to the competing neutral pathway in
the experimental studies.
Ab initio calculations (at the MP2 level) have been performed
on migratory insertion in 2 and 4, and stationary points on the
reaction coordinates are depicted in Figure 8. In both transition
states, formation of the acetyl C-C bond is accompanied by
opening of the bond angle between ligands trans to the methyl
(52) The Ir-Me bond energy is estimated to be 40 kJ mol-1 stronger for 4 than
for 2 on the basis of calculated energies (before relaxation) of the homolysis
products CH3 and [Ir(CO)xIy]n- (x ) 2 or 3, y ) 3 or 2, n ) 1 or 0).
(53) In support of this, Mulliken population analysis indicates that the partial
charge on the carbon of a CO cis to methyl decreases from +0.32 in 4 to
+0.26 in 2. Also the appropriate unoccupied acceptor orbital (an out of
phase combination of π*CO with dπ) is significantly higher in energy for
2 than for 4 (0.160 and 0.039 hartrees respectively).
(47) Cardaci, G.; Reichenbach, G.; Bellachioma, B.; Wassink, B.; Baird, M. C.
Organometallics 1988, 7, 2475. Bellachioma, B.; Cardaci, G.; Macchioni,
A.; Reichenbach, G.; Foresti, E.; Sabatino, P. J. Organomet. Chem. 1997,
531, 227. Wright, S. C.; Baird, M. C. J. Am. Chem. Soc. 1985, 107, 6899.
9
J. AM. CHEM. SOC. VOL. 126, NO. 9, 2004 2855

A R T I C L E S
Haynes et al.
Scheme 6. Mechanism for Promotion of Carbonylation of 2 by
Iodide Abstractors
migratory CO insertion (for example in [Mn(CO)5Me]), by
accepting a coordinate bond from the carbonyl oxygen of the
incipient acyl ligand. An analogous intramolecular interaction
has also been invoked to explain promotion of methyl migration
in [FeW(CO)9Me]- relative to [Fe(CO)4Me]-.57 Although such
interactions might play a minor role in the promotion of
carbonylation of 2, our results show clearly that the major role
of the promoter is as an iodide acceptor.
Comparison of Kinetic Data for Model and Catalytic
Reactions. Activation parameters for stoichiometric carbon-
ylation of 2 and 4 and catalytic carbonylation of methanol are
gathered in Table 1. The activation parameters for catalytic
reactions in acetic acid are best matched by those for carbon-
ylation of 2 in PhCl in the presence of a promoter ([Ru(CO)3I2]2
or InI3). Extrapolation of the appropriate Eyring plots gives a
satisfactory prediction of catalytic rates. Although the reaction
medium (and counterion) clearly differ,45 it appears that the
catalytic reaction is modeled reasonably well by the stoichio-
metric reaction of 2 with CO. Substantially different activation
parameters were found for the model reaction in 3:1 PhCl-
MeOH. The relatively small ∆Hq and large negative ∆Sq arise
from associative assistance of iodide dissociation from 2 by
methanol (eq 4). These parameters correspond very closely with
those calculated from the catalytic data in 2:1 acetophenone-
methanol reported by Matsumoto et al.16,58 Thus the catalytic
rate at high [MeOH] is also well modeled by our rate data for
the carbonylation of 2.
Mechanism for Promotion of Carbonylation of 2. It has
been demonstrated that (i) iodide transfer from 2 to promoter
species is facile and (ii) the neutral species 4 is carbonylated
much more readily than 2. We can therefore propose a
mechanism in which an iodide ligand is initially transferred from
2 to the promoter, allowing coordination of CO to Ir to give 4.
Migratory insertion then proceeds to form 5a. Iodide transfer
from the promoter back to Ir at this point is unlikely, given
that InI3 abstracts iodide from 6a (vide supra). Instead, an iodide
ligand could be transferred between iridium centers as shown
in Scheme 6. In this scenario the additive (e.g., InI3, or [Ru-
(CO)3I2]2) behaves more like an initiator, abstracting iodide to
generate 3, as shown by the dashed arrow in Scheme 6. The
Promoters gave much larger rate enhancements for the model
reactions than for the catalytic system. For example, a promoter:
Ir ratio of 1:10 gave a 20-30-fold acceleration of the carbon-
ylation of 2 in PhCl, whereas a 10-fold excess of promoter over
Ir gave only a 2-3-fold improvement of catalytic activity
(Figure 2). The difference in solvent is undoubtedly important,
since PhCl will solvate iodide ions much less strongly than the
catalytic medium. Consistent with this, larger concentrations
of promoter were required to accelerate the carbonylation of 2
in a mixed PhCl-MeOH solvent system than in neat PhCl.
Activation parameters for the reaction of MeI with 119 can
be used to predict a second-order rate constant of ca. 11 dm3
mol-1 s-1 at 190 °C, corresponding to a pseudo-first-order rate
constant (at 0.4 M MeI) of 4.5 s-1. To achieve the observed
catalytic rate (in the absence of promoter) would require the
anion InI4 or [Ru(CO)3I3]- then acts as a spectator, with
-
promotion being propagated by an iodide “hole” introduced by
the iodide abstractor. Such a mechanism can operate if the Ir(III)
methyl and acetyl complexes have similar affinities for iodide,
such that the equilibrium 2 + 5a h 3 + 6a does not lie heavily
to the left.54
We tested this hypothesis by investigating the kinetics of
carbonylation of 2 in the presence of substoichiometric amounts
of 3d. This regime mimics the iodide abstracting effect of a
promoter such as [Ru(CO)3I2] or InI3, by introducing a propor-
tion of the neutral iridium complex, but avoids any direct
promoter-Ir interactions. The data obtained (Table S6) show
that addition of 3d causes a rate enhancement very similar to
that of other metal promoters. Variable-temperature data gave
activation parameters comparable to those obtained for [Ru-
(CO)3I2]2 and InI3 additives (Table 1). Thus the key role of the
promoter is to perturb the ratio of anionic and neutral iridium
complexes, and the effect can be mimicked by deliberate tuning
of this ratio in an Ir-only system.
-
(55) The mixed-metal cluster anions [M3Ir(CO)13
]
(M ) Ru, Os) have been
used as catalyst precursors for methanol carbonylation, but under catalytic
conditions they fragment to give [Ir4(CO)12] and [M(CO)3I3]-: Suss-Fink,
G.; Haak, S.; Ferrand, V.; Stoeckli-Evans, H. J. Mol. Catal. A-Chem. 1999,
143, 163.
(56) Butts, S. B.; Strauss, S. H.; Holt, E. M.; Stimson, R. E.; Alcock, N. W.;
Shriver, D. F. J. Am. Chem. Soc. 1980, 102, 5093. Richmond, T. G.; Basolo,
F.; Shriver, D. F. Inorg. Chem. 1982, 21, 1272.
(57) Arndt, L. W.; Bancroft, B. T.; Darensbourg, M. Y.; Janzen, C. P.; Kim, C.
M.; Reibenspies, J.; Varner, K. E.; Youngdahl, K. A. Organometallics 1988,
7, 1302.
(58) Matsumoto (ref 16) proposed rate-determining methanolysis of Ir(III)-
COMe on the basis of rate dependence on [MeOH] and isolation of
[Ir(CO)(PPh3)2I2(COMe)] on addition of PPh3 to the cooled catalytic
reaction mixture. However, ref 33 shows that P-donor ligands can induce
CO insertion in 2 to give products of this type, so isolation of an Ir-acetyl
does not show it to be the catalytic resting state. Methanolysis of 6 to give
MeOAc is slow at ambient temperature but observable at ca. 50 °C. The
much higher temperature and presence of water make it likely that solvolysis
of 6 is relatively fast under process conditions. A pre-equilibrium 2 +
CO ) 6 followed by rate-determining solvolysis of 6 would result in a
first-order dependence on [CO], which is not observed. Although 6 reverts
to 2 + CO on heating in the absence of added CO, the equilibrium favors
6 even at 5 bar CO. Therefore if solvolysis of 6 were rate-determining,
one would expect to observe it (rather than 2) as the resting state at higher
pCO.
These results argue against the participation of a more direct
interaction between the promoter and the iridium center such
as redox processes or heterodi- or poly-nuclear complexes.55
Classic studies by Shriver and co-workers56 showed that certain
Lewis acids (e.g., AlCl3) can effect dramatic accelerations of
(54) This was demonstrated by 1H NMR spectroscopy: an MeCN solution
containing equimolar 6a and 3-NCMe attained an equilibrium in which
ca. 50% of 3-NCMe was converted into 2.
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2856 J. AM. CHEM. SOC. VOL. 126, NO. 9, 2004

Promotion of Ir-Catalyzed Methanol Carbonylation
A R T I C L E S
Scheme 7. Mechanism for Promoted Iridium-Catalyzed Methanol Carbonylation
MeOAc + H3O+ + I- h MeI + H2O + AcOH (5)
concentration of 1 to be ca. 0.5 mM or 5% of the total iridium
concentration, consistent with the observation of weak IR bands
due to 1 during catalysis. Similarly the rate constant for
migratory insertion in 4 is estimated as 12 s-1 at 190 °C,
requiring the concentration of 4 to be 0.2 mM (or 2% of the
total Ir) to sustain the observed catalytic rate. The kinetics
measured for the model reactions are therefore consistent with
complexes 1 and 4 being intermediates in the catalytic cycle,
but with most of the iridium existing in the form of the resting
state 2.
Methyl migration in 2 (indicated by the dashed arrows in
Scheme 7) may make a minor (e5%) contribution to catalytic
turnover.60 Assuming that the steady state concentration of 4 is
less than 5% of the total [Ir] (since it is not observed by in situ
IR), we can estimate that methyl migration must be at least 200
times faster for 4 than for 2 at 190 °C. This would require that
∆Gq is at least 20 kJ mol-1 lower for 4 than for 2, consistent
with experimental and computational results.
Catalytic Mechanism. A catalytic mechanism consistent with
the data is shown in Scheme 7. The participating iridium
complexes are grouped into three sets, namely, Ir(I) species,
Ir(III)-methyls, and Ir(III)-acetyls. The three complexes within
each set are linked by equilibria involving I- and CO, and the
dominant route for catalytic turnover is indicated by the red
arrows. The interconversions of Ir species represented in Scheme
7 are essentially equivalent to Forster’s mechanism (Scheme
1). However, we prefer to view the catalytic process as one
rather than two cycles, with the process conditions influencing
the nature of the catalyst resting state rather than the active cycle.
We also depict the net oxidative addition of 1 + MeI f 2 as
proceeding via 3, resulting from initial SN2 attack50,59 by Ir(I).
If the water concentration is decreased below 5% w/w, the
catalytic rate diminishes and catalyst resting state shifts to the
neutral Ir(I) tricarbonyl 9. According to eq 5, low [H2O] will
afford a low [I-], which is insufficient to sustain the iridium
catalyst in a predominantly anionic form. Under these condi-
tions, the equilibria61 at the top of Scheme 7 are shifted toward
9, which accumulates due to its low nucleophilicity toward MeI
relative to 2.62
Role of Promoters. The model stoichiometric reactions show
that neutral compounds such as [Ru(CO)3I2]2 and InI3 are
capable of abstracting iodide from complex 2 and promoting
its carbonylation. In the catalytic process, the situation is
somewhat more complex, as iridium complexes are not the sole
source of iodide for the promoter to bind. HPIR spectroscopy
showed that a ruthenium promoter exists as a mixture of
[RuCO)2I2(sol)2], [RuCO)3I2(sol)], and [RuCO)3I3]- in the
absence of Ir, but that [RuCO)3I3]- becomes the dominant Ru
species after injection of the Ir catalyst. Each turnover of the
iridium cycle (Scheme 7) generates 1 molar equiv of HI
Under the conditions of highest activity, the catalyst resting
state is 2, which sits outside the main catalytic loop. Added
iodide salts will favor the equilibria leading to 2, thus lowering
the standing concentrations of 3 and 4 within the main loop
and inhibiting the carbonylation rate. Similar inhibition is caused
by raising the water concentration (above 5% w/w, Figure 1),
which will lead to an increase in [I-], according to eq 5.
Similarly, the strong positive dependence of catalytic rate on
[MeOAc]8,9 can be understood in terms of it reducing the steady
state concentration of I-.
(60) The estimated contribution of methyl migration in 2 to the catalytic activity
is based on the rate measured at 15:1 LiI:Ir, which is ca. 10% of the rate
in the absence of LiI. Since the in situ HPIR shows the Ir speciation to be
>90% and ca. 50% 2, respectively, with or without LiI, methyl migration
in 2 can account for e5% of the catalytic rate in the absence of LiI.
(61) Quantitative studies of the equilibrium 1 + CO ) 9 + I- have recently
been undertaken by Merbach and co-workers: Churland, R.; Frey, U.; Metz,
F.; Merbach, A. E. Inorg. Chem. 2000, 39, 304; 2000, 39, 4137.
(59) Fulford, A.; Hickey, C. E.; Maitlis, P. M. J. Organomet. Chem. 1990, 398,
311. Ivanova, E. A.; Gisdakis, P.; Nasluzov, V. A.; Rubailo, A. I.; Ro¨sch,
N. Organometallics 2001, 20, 1161. Griffin, T. R.; Cook, D. B.; Haynes,
A.; Pearson, J. M.; Monti, D.; Morris, G. E. J. Am. Chem. Soc. 1996, 118,
3029. Rendina, L. M.; Puddephatt, R. J. Chem. ReV. 1997, 97, 1735.
Labinger, J. A.; Osborn, J. A. Inorg. Chem. 1980, 19, 3230.
(62) The second-order rate constant for oxidative addition of MeI to neutral
[Ir(CO)2(NCMe)I] (ν(CO)/cm-1: 2078, 2006; generated by reaction of 1
with InI3 in MeCN) is ca. 200 times smaller (1.56 × 10-5 dm3 mol-1 s-1
than that for 1 (3.09 × 10-3 dm3 mol-1 s-1) (25 °C, MeCN).
)
9
J. AM. CHEM. SOC. VOL. 126, NO. 9, 2004 2857

A R T I C L E S
Haynes et al.
The presence of a Brønsted acidic H3O+ counterion for the
anionic promoter species appears to be crucial in obtaining above
normal carbonylation rates. This is demonstrated by the
observation that the promotional effect of [Ru(CO)4I2] is negated
by an equimolar quantity of LiI (equivalent to addition of
[Ru(CO)3I3]-Li+) (Table S2, Figure 2).65
Scheme 8. Catalytic Cycle for Ionic Iodide, Showing
Acid-Catalyzed Mechanism for Activation of Methyl Acetatea
Conclusions
The use of a third-row transition metal in an industrial
homogeneous catalytic process is rare; indeed relatively inert
5d metal complexes are often used as models for the more active
3d and 4d congeners. Stronger Ir-ligand bonds and a preference
for higher oxidation states lead to important differences in
behavior for Ir- and Rh-based methanol carbonylation catalysts.
The stronger metal-ligand bonding increases catalyst stability
(by reducing susceptibility to CO-loss and precipitationof IrI3)
but has a dramatic inhibitory effect on the migratory CO
insertion step of the catalytic cycle. Thus carbonylation of the
Ir(III) methyl complex 2 becomes rate determining, and com-
mercially advantageous rates are achieved only using promoters
that accelerate this step.
The role of promoters (e.g., transition metal carbonyls/
halocarbonyls or group 12/13 iodides) has been investigated in
detail. The two different classes of promoter act by a common
mechanism. Neutral promoter species (e.g., [Ru(CO)3I2]2 or InI3)
are shown to accelerate the stoichiometric carbonylation of the
catalyst resting state [Ir(CO)2I3Me]- (2) by acting as iodide
acceptors (or sources of “iodide holes”). This facilitates conver-
sion of 2 into the neutral tricarbonyl [Ir(CO)3I2Me] (4), which
is carbonylated much faster than the anion, primarily due to
increased competition for π-back-donation, which raises the
electrophilicity of the carbonyl ligands. In the catalytic process
the promoters bind iodide to form a [promoter-I]-H3O+ adduct
(e.g., [Ru(CO)3I3]-H3O+ or InI4-H3O+), which catalyzes the
reaction of HI(aq) with MeOAc. This moderates the standing
concentration of ionic iodide, thereby aiding carbonylation via
neutral intermediates. Effective promoters must have a relatively
high iodide affinity in order to scavenge iodide into a form in
which it no longer acts as a poison for carbonylation.
a A similar scheme can also be drawn for activation of methanol.
(expected to be largely dissociated as H3O+ and I-63) according
to the stoichiometry shown in eq 6.
MeI + CO + 2 H2O f MeCO2H + H3O+ + I- (6)
The onset of catalysis will perturb the standing concentration
of HI(aq) above its equilibrium value, explaining the increased
proportion of [Ru(CO)3I3]- relative to neutral ruthenium iodo-
carbonyls. The HI(aq) produced by the Ir cycle must be recycled
by reaction with methyl acetate (or methanol) to generate MeI
as shown in Scheme 7. We propose that this recycling is
accelerated by promoters such as [Ru(CO)4I2] or InI3, thereby
moderating the steady state concentration of HI(aq). In the
proposed mechanism (Scheme 8) the neutral promoter species
[Ru(CO)3I2(sol)] or InI3 scavenges HI(aq) to give [Ru(CO)3-
I3]-H3O+ or InI4-H3O+. Crucially the I- is scavenged into a
form in which it no longer acts as a poison for the key migratory
insertion reaction (as demonstrated by the model kinetic studies).
These species can act as Brønsted acid catalysts for the reaction
of HI(aq) with MeOAc, consequently lowering the standing
concentration of HI(aq) and enhancing turnover in the Ir cycle.
Consistent with this, 13C labeling experiments showed that
methyl exchange between MeI and MeOAc is promoted by [Ru-
(CO)3I3]-H3O+,64 which can be explained by conventional acid
catalysis (eqs 7 and 8).
Experimental Section
Materials. Solvents and reagents were purified by standard meth-
ods:66 Dichloromethane and acetonitrile were distilled from calcium
hydride after reflux; diethyl ether was distilled from a purple solution
of sodium/benzophenone immediately before use; methanol was dis-
tilled from magnesium methoxide (magnesium turnings (1 g), iodine
(0.1 g), and spectroscopic grade methanol (200 cm3), after reflux
(24 h)). Other reagents were used as supplied: lithium iodide, methyl
acetate, methyl iodide, 13C-methyl iodide, water, Ph4AsCl, Bu4NI, InI3,
GaI3, AlI3, ZnI2, AgBF4, I2, [Cr(CO)6], [W(CO)6] (Aldrich), acetic acid
(BP Chemicals), iridium and ruthenium chloride hydrates (Johnson
Matthey). H2IrCl6 was obtained from Johnson Matthey as an aqueous
solution and used as supplied. Standard Schlenk techniques and
glassware were used for preparative reactions.67 Nitrogen and carbon
monoxide (99.9%, Linde or BOC CP Grade) were dried through a short
(20 × 3 cm diameter) column of molecular sieves (4 Å) which was
regularly regenerated; the carbon monoxide was also passed through a
H3O+ + MeOC(dO)Me h H2O + [MeOC(OH)Me]+ (7)
[MeOC(OH)Me]+ + I- h MeI + MeCO2H
(8)
(63) The strong acid HI (pKa ca. -10) will be largely dissociated, according to
the equilibrium HI + H2O ) H3O+ + I- in the catalytic medium (aqueous
acetic acid).
(64) Scrambling of labeled methyl groups between 12CH3I and 13CH3OAc was
monitored by 1H NMR spectroscopy under conditions (168 °C, aqueous
acetic acid, CO, 28.5 barg total pressure) close to those of the catalytic
carbonylation system. The extent of 13CH3-labeling in MeOAc was
determined from the relative intensities of the 1H singlet of 12CH3OAc and
doublet of 13CH3OAc (δ 3.55, 1JCH 147 Hz). Addition of [Ru(CO)4I2] (4000
ppm Ru) reduced the half-life for methyl exchange from 144 to 55 min
(Supporting Information, Figure S5), representing an acceleration by a factor
of 2.6. At the end of the experiment, 13C NMR spectroscopy showed the
presence of [Ru(CO)4I2] and [Ru(CO)3I3]- in a ca. 1:3 ratio. In studies of
Ru/iodide-catalyzed carbonylation and homologation reactions, Braca
suggested that “[Ru(CO)3I3]-H+” can activate methyl acetate by protona-
tion: Braca, G.; Sbrana, G.; Valentini, G.; Andrich, G.; Gregorio, G. J.
Am. Chem. Soc. 1978, 100, 6238.
(65) Conversely, ruthenium can negate the poisoning effect of LiI by scavenging
I- to give [Ru(CO)3I3]+Li+.
(66) Perrin, D. D.; Armerego, W. L. F.; Perrin, D. R. Purification of Laboratory
Chemicals, 3rd ed.; Pergamon Press: Oxford, 1988.
(67) Shriver, D. F. The Manipulation of Air SensitiVe Compounds; McGraw-
Hill: New York, 1986.
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Promotion of Ir-Catalyzed Methanol Carbonylation
A R T I C L E S
short column of activated charcoal to remove any iron pentacarbonyl
impurity.68 Bulbs of 13C-enriched carbon monoxide for preparative
labeling of complexes were supplied by Cambridge Isotope Laboratories
and Aldrich. High-pressure 13CO for HPNMR experiments was supplied
by Isotec Inc. [Ru3(CO)12] was supplied by Strem or prepared from
ruthenium chloride by the method of Bruce et al.69 Salts of [Ru(CO)3I3]-
and [Ru(CO)2I4]2- were prepared by established methods.70
The methyl iodide was removed in vacuo. Recrystallization from CH2-
Cl2-Et2O gave orange crystals, which were dried in vacuo, yield 1.08
g (93%). Anal. Calcd for (C27H23AsI3IrO2): C, 31.6; H, 2.3; I, 37.1.
Found: C, 31.8; H, 2.4; I, 37.0. IR (CH2Cl2) ν(CO)/cm-1: 2098, 2046.
1H NMR (CDCl3): δ 2.1 (s, 3H, CH3), 7.6-7.9 (m, 20H, Ph4As). 13
C
NMR (CDCl3): δ 155.5 (CO) 134.9, 132.9, 131.5 (Ph, CH) 120.2 (Ph,
AsC); -16.3 (CH3). An analogous procedure was used to prepare Bu4-
N[Ir(CO)2I3Me]. 13CO-labeled Ph4As[Ir(13CO)2I3Me] was prepared by
the same method using Ph4As[Ir(13CO)2I2] as the precursor. IR (CH2-
Cl2) ν(CO)/cm-1: 2049, 1998. 13CH3-labeled Ph4As[Ir(CO)2I3(13CH3)]
was prepared by the same method using 13CH3I. 1H NMR (CDCl3): δ
Instrumentation. Routine IR spectra were recorded on either a
Perkin-Elmer 1640, a Perkin-Elmer 1710, or a Mattson Genesis Fourier
transform spectrometer using a solution cell with CaF2 windows (path
length 0.1 or 0.5 mm). High-pressure/high-temperature IR spectra were
recorded on a Perkin-Elmer 1710 Fourier transform spectrometer using
a SpectraTech cylindrical internal reflectance (CIR) cell (vide infra).
1H spectra were recorded on either a Bruker AM250 or a Bruker AC250
instrument with a Bruker B-ACS60 automatic sample changer operating
in pulse Fourier transform mode using the solvent as reference. 13C
NMR spectra were recorded on either a Bruker WH400 or a Bruker
AMX 400 Fourier transform instrument, again with solvent as the
internal standard. High-pressure NMR spectra (1H and 13C) were
measured on a JEOL 400 MHz NMR spectrometer at the BP Chemicals
Research Centre in Sunbury-on-Thames (UK) using sapphire tubes
(5 mm o.d.) supplied by Saphikon, pressure rated to 160 barg, fitted
(cemented) with a titanium valve. Osmometry measurements were made
using a Wescar 5500 vapor pressure osmometer, calibrated using benzil
solutions of known concentration in the required solvent. Elemental
analyses were determined by the University of Sheffield microanalysis
service.
Synthesis of Iridium Complexes. (a) Ph4As[Ir(CO)2I2], 1. This
compound was prepared by a variation of the method described by
Forster.71 IrCl3‚xH2O (1.64 g, 4.65 mmol, assuming x ) 3) and NaI
(10 g, large excess) were dissolved in a mixture of 2-methoxyethanol
(80 cm3) and distilled water (5 cm3) and refluxed, while a stream of
carbon monoxide was bubbled through the solution. Over the course
of 6 h the black solution turned pale yellow. Ph4AsCl (2.2 g, 5.3 mmol),
dissolved in 2-methoxyethanol (5 cm3), was added to the hot solution,
which was then filtered through Celite. Distilled water was added to
the solution until the precipitation of Ph4As[Ir(CO)2I2] commenced.
The precipitation was allowed to go to completion at -20 °C under an
atmosphere of carbon monoxide. The bright yellow precipitate was
filtered, washed with cold methanol then diethyl ether, and dried in
vacuo; yield 3.42 g (70%). Anal. Calcd for (C26H20AsI2IrO2): C, 35.3;
H, 2.3; I, 28.7. Found: C, 35.1; H, 2.3; I, 28.4. IR (CH2Cl2) ν(CO)/
cm-1: 2046, 1968. 13C NMR (CDCl3): δ 169.7 (CO) 134.8, 132.8,
131.5 (Ph, CH) 120.1 (Ph, AsC). An analogous procedure was used to
prepare Bu4N[Ir(CO)2I2], using Bu4NI in place of Ph4AsCl.
1
2.1 (d, 3H, CH3 JH-C 139 Hz). The doubly labeled compound
Ph4As[Ir(13CO)2I3(13CH3)] was prepared by the same method from
Ph4As[Ir(13CO)2I2] and 13CH3I. 13C NMR (CDCl3): δ 155.5 (d CO),
2
-16.3 (t, CH3) JC-C 1.2 Hz.
(d) [Ir(CO)2I2Me]2, 3d. Anhydrous InI3 (350 mg, 0.706 mmol) was
added to a stirred solution of Bu4N[Ir(CO)2I3Me] (500 mg, 0.564 mmol)
in CH2Cl2 (20 cm3) under nitrogen (20 °C). Stirring was continued
(1 h) while an orange powder started to precipitate. The solvent was
removed in vacuo to leave an orange-red solid residue from which pure
[Ir(CO)2I2Me] was extracted into hot cyclohexane by adding portions
of 20 cm3 until the solution remained colorless. The combined extracts
were evaporated to dryness, and the product was obtained as an orange
powder, yield 220 mg, 75%. Anal. Calcd for (C6H6I4Ir2O4): C, 7.0; H,
0.6; I, 49.1. Found: C, 7.0; H, 0.4; I, 49.1. MW: calcd 1034, found
1001 (PhCl); 538 (MeCN). IR (CH2Cl2) ν(CO)/cm-1: 2119, 2074. 1H
NMR (CD2Cl2): δ 1.94 (major), 1.88 (minor) (CH3, 2 isomers in ratio
ca. 5:2). 13C NMR (CD2Cl2): δ 152.4, 150.3, 150.0 (CO) -5.30, -8.76
(CH3). Recrystallization from hot chloroform gave crystals suitable for
a single-crystal X-ray structure determination.
(e) Ph4As[Ir(CO)2I3(COMe)], 6. Ph4As[Ir(CO)2I3Me] (0.380 g,
0.375 mmol) was dissolved in THF (20 cm3). MeOH (0.5 cm3) and
MeI (0.1 cm3) were added, and the solution was transferred to a Fisher-
Porter apparatus. The vessel was flushed twice with CO (5 barg), then
pressurized with CO to 10 barg, and warmed to 40 °C using an oil
bath. After stirring for 3 days the CO was released and the solution
transferred to a Schlenk tube and evaporated to dryness in vacuo. The
crude product was recrystallized by dissolving in CH2Cl2 (5 cm3),
layering with Et2O, and cooling to 5 °C to give yellow-orange crystals.
A suitable crystal was selected for X-ray diffraction. The remaining
crystals were separated from the solution and dried in vacuo, yield
0.299 g (76%). Anal. Calcd for (C28H23AsI3IrO3): C, 31.87; H, 2.20.
Found: C, 31.97; H, 2.05. IR (CH2Cl2) ν(CO)/cm-1: 2110, 2062, 1697,
1
1658. H NMR (CD2Cl2): δ 2.94, 2.67 (each s, total 3H, COCH3 for
(b) Ph4As[Ir(13CO)2I2]. Ph4As[Ir(CO)2I2] (400 mg, 4.5 × 10-4 mol)
was dissolved in CH2Cl2 (40 cm3) in a two-necked round-bottomed
flask under N2 which was connected to a high-vacuum line fitted with
a Toepler pump and a 1 dm3 bulb of 13C-labeled carbon monoxide.
The flask was evacuated and then opened to admit 13CO while stirring
the solution at the gas-liquid interface. At regular intervals the gas
above the solution was exchanged with fresh 13CO from the bulb and
a sample of the solution was removed by syringe for analysis by IR
spectroscopy. This was continued until the solution was approximately
95% 13CO labeled, as judged by the IR spectrum. The 13CO in the
reaction flask was transferred back to the reservoir bulb using the
Toepler pump before removing the reaction flask. The solvent was
removed and the product washed with diethyl ether and dried in vacuo.
IR (CH2Cl2) ν(CO)/cm-1: 1997, 1923. 13C NMR (CDCl3): δ 169.8
(CO) 134.9, 133.0, 131.5 (Ph, CH) 120.2 (Ph, AsC).
6a and 6b in ratio ca. 9:1), 7.6-7.9 (m, 20H, Ph4As). 13C NMR (CD2-
Cl2): δ 196.42, 198.06 (COMe, 6a and 6b, respectively), 151.96, 160.84
(Ir-CO, 6a and 6b, respectively), 131.62, 133.27, 135.18 (Ph, CH),
120.64 (Ph, C-As), 49.88, 50.91 (COCH3, 6a and 6b, respectively).
Synthesis of Ruthenium Complexes. (a) [Ru(CO)4I2]. Method 1.
Acetic acid (204.0 g, 3.40 mol), iodine (12.20 g, 48.1 mmol), and [Ru3-
(CO)12] (9.99 g, 15.6 mmol) were placed in a 300 mL zirconium
autoclave (Parr), and the autoclave was sealed. The autoclave was
pressure tested with nitrogen (30 barg) and then vented. The autoclave
was flushed three times with CO (10 barg) and pressurized with CO to
10 barg. The contents were then heated to 180 °C with stirring (1500
rpm). Once at temperature the total reactor pressure was increased to
25 barg by feeding CO to the autoclave. The reactor was held at
temperature for 1 h and then cooled. On opening the autoclave, an
orange precipitate was found to be present in the solution, on the reactor
base, and around the autoclave walls and stirrer shaft. This precipitate
was removed by filtration to give a bright orange solution. Water
(ca. 700 cm3) was added to the filtrate to precipitate an orange powder.
The orange solid was filtered off and washed with water (5 × 50 cm3)
and then dried in vacuo to yield [Ru(CO)4I2] (14.0 g, 64% yield). IR
(CH2Cl2) ν(CO)/cm-1: 2165, 2109, 2094sh, 2076.
(c) Ph4As[Ir(CO)2I3Me], 2. Ph4As[Ir(CO)2I2] (1 g, 1.1 mmol) was
dissolved in methyl iodide (10 cm3) and stirred under nitrogen for 1 h.
(68) Haynes, A.; Ellis, P. R.; Byers, P. K.; Maitlis, P. M. Chem. Br. 1992, 28,
517.
(69) Bruce, M. I.; Jensen, C. M.; Jones, N. L. Inorg. Synth. 1990, 28, 216.
(70) Colton, R.; Farthing, R. H. Aust. J. Chem. 1971, 24, 903.
(71) Forster, D. Inorg. Nucl. Chem. Lett. 1969, 5, 433.
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A R T I C L E S
Haynes et al.
Table 4. Summary of Crystallographic Data for 6a[Ph4As]
Method 2. A variation of the method used by Johnson72 was also
used. Heptane solutions of Ru3(CO)12 (100 mg, 1.56 mmol in 100 cm3)
and iodine (127 mg, 0.50 mmol in 100 cm3) were cooled (-40 °C)
using a solid carbon dioxide-acetone bath, before mixing to give
immediate formation of a brown suspension, which became more dense
as the solution was allowed to warm to room temperature. The heptane
was removed in vacuo to leave a brown powder, which exhibited an
IR spectrum consistent with a mixture of [Ru(CO)4I2] and [Ru(CO)3I2]2.
A sample (100 mg) of the mixture produced by this method was
dissolved in CH2Cl2 (15 cm3). The solution was transferred to a Fisher-
Porter tube, pressurized with CO (10 barg), and heated to 40 °C. After
one week the infrared spectrum of the solution indicated the sole
presence of [Ru(CO)4I2]. Removal of the CH2Cl2 by bubbling CO
through the solution to dryness left [Ru(CO)4I2] as a yellow powder.
IR ν(CO)/cm-1 (CH2Cl2): 2161, 2106, 2097, 2068.
(b) [Ru(CO)3I2]2. A sample (300 mg) of the mixture produced in
(a) method 2 was dissolved in CHCl3 (15 cm3), and the solution was
refluxed under N2 for 3 h, at which point the IR spectrum indicated
the sole presence of [Ru(CO)3I2]2. The solvent was removed in vacuo
to leave [Ru(CO)3I2]2 as a red-brown powder. IR ν(CO)/cm-1
(CH2Cl2): 2123, 2067.
(c) [Ru(CO)2I2]n. This compound was prepared by a variation of
the method described by Johnson.72 Ru3(CO)12 (100 mg, 1.56 mmol)
and iodine (127 mg, 0.5 mmol) were added to n-nonane (50 cm3) and
refluxed under N2 for 3 h. The resultant brown suspension was cooled
and filtered to give a rust-brown product. Yield: 148 mg (77%). IR
ν(CO)/cm-1 (n-nonane): 2060, 2000.
formula
C28H23AsI3O3Ir
fw
cryst syst
space group
1055.28
monoclinic
P21/n
color
yellow
a (Å)
b (Å)
c (Å)
12.351(3)
16.500(4)
15.971(4)
90
110.277(4)
90
R (deg)
â (deg)
γ (deg)
temp (K)
100(2)
Z
4
final R indices [I>2σ(I)]
R1 ) 0.0325
wR2 ) 0.0770
R1 ) 0.0434
wR2 ) 0.0809
1.007
R indices (all data)
GOF
reaction composition were charged to the autoclave via a liquid addition
port. The autoclave was flushed with CO, pressurized with carbon
monoxide (ca. 5 barg), and heated with stirring (1500 rpm) to the
reaction temperature. The total pressure was then raised to ap-
proximately 3 bar below the desired operating pressure by feeding
carbon monoxide from the ballast vessel. Once stable at temperature,
the catalyst was injected using an overpressure of carbon monoxide to
achieve the required pressure. The reactor pressure was maintained
constant ((0.5 bar) by feeding gas from the ballast vessel throughout
the experiment. Gas uptake from the ballast vessel was measured using
data-logging facilities throughout the course of the experiment. The
reaction temperature was maintained within (1 °C of the desired
reaction temperature by means of a heating mantle connected to a
Eurotherm control system. In addition, excess heat of reaction was
removed by means of the cooling coils. At the end of the reaction the
ballast vessel was isolated and the reactor crash-cooled by use of the
cooling coils.
Synthesis of Ph4AsI. Ph4AsCl (1.00 g, 2.39 mmol) and NaI (3 g,
20 mmol) were dissolved in methanol (50 cm3) in a round-bottom flask
and stirred (16 h). The solvent was removed in vacuo and the resulting
white solid dissolved in dichloromethane and filtered through Celite
to remove excess NaI. The solvent was removed under vacuum and
the solid recrystallized from CH2Cl2-Et2O, yield 1.01 g, 83%. Anal.
Calcd for (C24H20AsI): C, 56.5; H, 3.95; I, 24.9. Found: C, 56.4;
H,3.99; I, 25.0.
X-ray Structure Determination for 6a. Data were collected on a
Bruker Smart CCD area detector with an Oxford Cryostream 600 low-
temperature system using Mo KR radiation (λh ) 0.71073 Å). The
structure was solved by direct methods and refined by full matrix least-
squares methods on F2. Hydrogen atoms were placed geometrically
and refined using a riding model (including torsional freedom for methyl
groups). Complex scattering factors were taken from the SHELXL
program packages.73 Crystallographic data are summarized in Table 4,
and full listings are given in the Supporting Information.
Batch Carbonylation Experiments. Carbonylation experiments
were performed using a 300 mL zirconium autoclave (Parr) equipped
with a magnetically driven stirrer, liquid injection facility, and water
fed cooling coils. A carbon monoxide supply to the autoclave was
provided from a ballast vessel, feed gas being provided on demand to
maintain the autoclave at a constant pressure. The rate of carbon
monoxide consumption was used to calculate the carbonylation rate
(in mol dm-3 h-1) at 50% conversion of methyl acetate, the consumption
of 1 mol of carbon monoxide, 1 mol of methyl acetate, and 1 mol of
water being equivalent to the carbonylation of 1 mol of methanol. For
each batch carbonylation experiment the catalyst, H2IrCl6, dissolved
in a portion of an acetic acid-water mixture (7.5 g each) was charged
to the liquid injection facility. If an additive was used, this was placed
in the reactor and covered with a portion of the acetic acid charge
(10 g), and the reactor was sealed. The reactor was then pressure tested
with nitrogen and vented. The autoclave was then flushed with carbon
monoxide several times. The remaining liquid components of the
In Situ IR Spectroscopic Monitoring of Catalytic Reactions.
Catalytic reactions were monitored in situ by high-pressure IR
spectroscopy using a modified batch autoclave in which all wetted parts
were manufactured from Hastelloy B2. An autoclave head unit
comprising a Magnedrive stirrer, a pressure gauge, two liquid feed lines,
a gas feed line, and a thermocouple well was placed on a 300 cm3
base fitted with an ASI Systems Inc. Sentinel SiComp ATR probe.
The unit was sealed before transferring to a blast bay. An electrical
band heater and thermocouple were placed on the autoclave assembly
before connecting to the gas and liquid feed lines, water cooling hoses,
and overhead stirrer. The Sentinel probe was connected to an ASI
systems ReactIR 1000 series spectrometer via an optical conduit. The
gas and liquid feed inlet valves were opened, and the assembly pressure
was tested with nitrogen (32 barg). The unit was flushed with nitrogen
(2 × 20 barg pressure and vent cycles). The Sentinel probe was aligned
and a background spectrum taken before flushing the autoclave unit
with carbon monoxide (3 × 10 barg pressure and vent cycles). The
autoclave was opened to vent and a funnel connected to the liquid inlet.
Methyl iodide followed by the remainder of the autoclave charge
(including any additives) was added via the funnel before removing it
and resealing the autoclave. The iridium catalyst solution was placed
into the catalyst injector followed by an acetic acid wash. The autoclave
stirrer was switched on before pressurizing with carbon monoxide (6
barg). The assembly was heated to reaction temperature (190 °C). Once
the temperature had stabilized, the pressure in the autoclave was
adjusted to the desired initial pressure (19 barg) as was the catalyst
injector (34 barg). The ballast vessel was charged with carbon monoxide
(54 barg) before injecting the catalyst solution. After injection the
autoclave was adjusted to the desired pressure (22 barg), which was
kept constant by feeding carbon monoxide from the ballast vessel on
demand. The temperature in the autoclave was kept constant by
(72) Johnson, B. F. G.; Johnson, R. D.; Lewis, J. J. Chem. Soc. (A) 1969, 792.
(73) Sheldrick, G. M. SHELXL93, An integrated system for solving and refining
crystal structures from diffraction data; University of Gottingen: Germany,
1993. Sheldrick, G. M. SHELXL97, An integrated system for solving and
refining crystal structures from diffraction data; University of Gottingen:
Germany, 1997.
9
2860 J. AM. CHEM. SOC. VOL. 126, NO. 9, 2004

Promotion of Ir-Catalyzed Methanol Carbonylation
A R T I C L E S
controlling the flow of cooling water. Spectra were recorded automati-
cally by the ReactIr system. Spectra were recorded by accumulating
128 scans at a resolution of 4 cm-1 every 60-90 s. On completion of
the run, the ballast vessel was isolated, the heating switched off, and
the autoclave crash-cooled to below 30 °C. Once below 30 °C, the
autoclave was removed from the bay, emptied, and cleaned. Analogous
results were also obtained using a transmission HPIR cell, based on a
modified batch autoclave equipped with CaF2 windows (path length
0.1 mm) and fitted with an electromagnetic agitator, gas feeds, and a
catalyst injection system. In this case spectra were recorded using a
Nicolet 710 spectrometer with a KBr beam splitter and MCT detector.
In Situ IR Spectroscopic Monitoring of Model Reactions. Model
reactions were monitored in situ by high-pressure IR spectroscopy using
a cylindrical internal reflectance (CIR) cell comprising a batch autoclave
(Parr) modified (by SpectraTech) to accommodate a crystalline silicon
CIR rod as described by Moser.74 Spectra were recorded using a Perkin-
Elmer 1710 FTIR spectrometer fitted with an MCT detector. The cell
was placed either in an external optical bench coupled to the
spectrometer or directly in the spectrometer sample compartment. The
reaction solution was prepared by dissolving the iridium compound
Ph4As[Ir(CO)2I3Me] (typically 0.3 g, 0.29 mmol) or [Ir(CO)2I2Me]2
(typically 0.15 g, 0.145 mmol or 0.29 mmol Ir) and the appropriate
amount of any additive (see Tables S4-S7) in the required solvent
(8 cm3). The reaction solution was then filtered into the CIR cell. The
cell head was fitted to the body of the cell, ensuring that the stirrer
was firmly tightened into the cell head. The vent lines, stirrer, and gas
inlet from the cylinder were then fitted, tightened as required, and
flushed out four times with the gas to be used in the experiment,
typically carbon monoxide. The cell was then flushed with the same
gas at least four times, the last two with the stirrer on, and then slowly
filled to the required pressure. The cell was then heated to the required
reaction temperature, and spectra were recorded over the range 2200-
1600 cm-1 at regular intervals under computer control. Data analysis
was achieved using the Kaleidagraph package to obtain pseudo-first-
order rate constants by fitting exponential curves to absorbance vs time
data. All reaction rates were reproducible to (5%. The values quoted
are averages of all the runs performed.
Hessians with a single negative eigenvalue. We employed the LANL2DZ
(abbreviated as 2DZ) Gaussian basis set developed by Hay and Wadt,77
which uses a semi-core double-ú contraction scheme for the heavy
elements Ir, Ru, Rh, I, and In, with the light atoms C, O, and H being
described by the split-valence Dunning 9-5V all-electron basis. To
obtain more accurate energy estimates, an extended LANL2DZ* basis
set was used, either in a single-point calculation at the 2DZ optimized
geometry or in a full optimization. The 2DZ* basis was generated by
the addition of a single high angular momentum polarization function
to the 2DZ basis of each atom. The orbital exponents of polarization
functions used in the 2DZ* basis were 1.100 (p) for H,75 1.154 (d) for
O,78 0.600 (d) for C,78 0.250 (d) for I,79 and 1.000 (arbitrary) (d) for Ir,
Ru, and Rh. Solvent effects were considered using the polarizable
continuum model (PCM).80
Acknowledgment. This work was supported by BP Chemi-
cals Ltd (PDRF to T.G., CASE studentships to P.W.B., C.M.B.,
P.I.P.E., T.R.G., J.M.P., P.W.V.), the University of Sheffield,
The Royal Society, and EPSRC. An award of 13CO was made
by Cambridge Isotope Laboratories. Dr. A. Castro is thanked
for assistance with X-ray crystallography and Dr. S. Haak for
the results in ref 62.
Supporting Information Available: Tables of kinetic data
for catalytic and stoichiometric reactions. In situ HPIR spectra.
Tables of X-ray crystallographic data for 3d and 6a. Atomic
coordinates for optimized structures and selected reaction
energetics data from ab initio calculations. This material is
available free of charge via the Internet at http://pubs.acs.org.
JA039464Y
(75) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Gill, P. M. W.; Johnson, B.
G.; Robb, M. A.; Cheeseman, J. R.; Keith, T.; Petersson, G. A.;
Montgomery, J. A.; Raghavachari, K.; Al-Laham, M. A.; Zakrzewski, V.
G.; Ortiz, J. V.; Foresman, J. B.; Cioslowski, J.; Stefanov, B. B.;
Nanayakkara, A.; Challacombe, M.; Peng, C. Y.; Ayala, P. Y.; Chen, W.;
Wong, M. W.; Andres, J. L.; Replogle, E. S.; Gomperts, R.; Martin, R. L.;
Fox, D. J.; Binkley, J. S.; Defrees, D. J.; Baker, J.; Stewart, J. P.; Head-
Gordon, M.; Gonzalez, C.; Pople, J. A. Gaussian 94 (Revision D.4);
Gaussian, Inc.: Pittsburgh, PA, 1995.
Computational Details. Ab initio quantum mechanical calculations
using second-order Møller-Plesset (MP2) theory were performed using
the Gaussian94 suite of programs.75 Geometries of stationary points
were optimized using the Berny algorithm76 as implemented in
Gaussian94. Transition states were located by employing a reaction
coordinate method followed by a transition state search based on the
Berny algorithm and characterized by frequency calculations to give
(76) Schlegel, H. B. J. Comput. Chem. 1982, 3, 214.
(77) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270. Wadt, W. R.; Hay,
P. J. J. Chem. Phys. 1985, 82, 284. Hay, P. J.; Wadt, W. R. J. Chem. Phys.
1985, 82, 299.
(78) Huzinaga, S.; Andzelm, J.; Klobukowski, M.; Radzio-Andzelm, E.; Sakai,
Y.; Tatewaki, H. Gaussian Basis Sets for Molecular Calculations; Physical
Sciences Data Vol. 16; Elsevier: Amsterdam, 1984.
(79) The exponent value for iodine is intermediate between the values given in
(74) Moser, W. R. In Homogeneous Transition Metal Catalysed Reactions;
Moser, W. R., Slocum, D. W., Eds.; Advances in Chemistry Series, Vol.
230, American Chemical Society: Washington, DC, 1992; pp 3-18.
ref 78 for I and I-.
(80) Miertus, S.; Scrocco, E.; Tomasi, J. J. Chem. Phys. 1981, 55, 117. Miertus,
S.; Tomasi, J. Chem. Phys. 1982, 65, 239.
9
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