Speciation and kinetics related to catalytic carbonylation in the presence of cis-[Ir(CO)2I2]P(C6H5)4 under CO and H2 pressures

Article abstract of DOI:10.1021/ic000269j

The differences in the reactivities of the square-planar complexes cis-[Rh(CO)₂I₂]^- (1) and cis-[Ir(CO)₂I₂]^- (2), involved in the catalytic carbonylation of olefins, are investigated, with P(C₆H₅)₄⁺ as the counterion, by ambient- and high-pressure NMR and IR spectroscopy. Under an elevated pressure of CO, 1 and 2 form the [M(CO)₃I] complexes with the equilibrium constants K(Ir) ? 1.8 x 10^-3 and K(Rh) ? 4 x 10^-5. The ratio K(Ir)/K(Rh) close to 50 shows that, under catalytic conditions (a few megapascals), only complex 1 remains in the anionic form, while a major amount of the iridium analogue 2 is converted to a neutral species. The oxidative addition reactions of HI with 1 and 2 give two monohydrides of different geometries, mer,trans-[HRh(CO)₂I₃]^- (3) and fac,cis-[HIr(CO)₂I₃]^- (4), respectively. Both hydrides are unstable at ambient temperature and form, within minutes for Rh and within hours for Ir, the corresponding cis-[M(CO)₂I₂]^- (1 or 2) and [M(CO)₂I₄]^- (5 or 6) species and H₂. When an H₂ pressure of 5.5 MPa is applied to a nitromethane solution of complex 2, ca. 50% of 2 is transformed to cis-dihydride complexes. The formation of cis,cis,cis-[IrH₂(CO)₂I₂]^- (8a) is followed by intermolecular rearrangements to form cis,trans,cis-[IrH₂(CO)₂I₂]^- (8b) and cis,cis,trans-[IrH₂(CO)₂I₂]^- (8c). A small amount of a dinuclear species, [Ir₂H(CO)₄I₄](x-) (9), is also observed. The formation rate constants for 8a and 8b at 262 K are k₁/²⁶² = (4.42 ± 0.18) x 10^-4 M^-1 s^-1, k_-1/²⁶² = (1.49 ± 0.07) x 10^-4 s^-1, k₂/²⁶² = (2.81 ±0.04) x 10^-5 s^-1, and k_-2/²⁶² = (5.47 ± 0.16) x 10^-6 s^-1. The two equilibrium constants K₁/²⁶² = [8a]/([2][H₂]) = 2.97 ± 0.03 M^-1 and K₂/²⁶² = [8b]/[8a] = 5.13 ± 0.10 show that complex 8b is the thermodynamically stable addition product. However, no similar H₂ addition products of the rhodium analogue 1 are observed. The pressurization with H₂ of a solution containing 2 and 6 give the monohydride 4, the dihydrides 8a and 8b, the dinuclear complex 9, and the two new complexes [Ir(CO)₂I₃] (10) and [HIr(CO)₂I₂] (11). The reactions of the iridium complexes with H₂ and HI are summarized in a single scheme.

Full text of DOI:10.1021/ic000269j

Inorg. Chem. 2000, 39, 4137-4142
4137
Speciation and Kinetics Related to Catalytic Carbonylation in the Presence of
cis-[Ir(CO)2I2]P(C6H5)4 under CO and H2 Pressures
Raphae1l Churlaud, Urban Frey, Franc¸ois Metz,† and Andre´ E. Merbach*
Institut de Chimie Mine´rale et Analytique, Universite´ de Lausanne,
BCH, CH-1015 Lausanne, Switzerland
ReceiVed March 9, 2000
The differences in the reactivities of the square-planar complexes cis-[Rh(CO)2I2]- (1) and cis-[Ir(CO)2I2]- (2),
involved in the catalytic carbonylation of olefins, are investigated, with P(C6H5)4+ as the counterion, by ambient-
and high-pressure NMR and IR spectroscopy. Under an elevated pressure of CO, 1 and 2 form the [M(CO)3I]
complexes with the equilibrium constants KIr ≈ 1.8 × 10-3 and KRh ≈ 4 × 10-5. The ratio KIr/KRh close to 50
shows that, under catalytic conditions (a few megapascals), only complex 1 remains in the anionic form, while
a major amount of the iridium analogue 2 is converted to a neutral species. The oxidative addition reactions of
HI with 1 and 2 give two monohydrides of different geometries, mer,trans-[HRh(CO)2I3]- (3) and fac,cis-[HIr-
(CO)2I3]- (4), respectively. Both hydrides are unstable at ambient temperature and form, within minutes for Rh
and within hours for Ir, the corresponding cis-[M(CO)2I2]- (1 or 2) and [M(CO)2I4]- (5 or 6) species and H2.
When an H2 pressure of 5.5 MPa is applied to a nitromethane solution of complex 2, ca. 50% of 2 is transformed
to cis-dihydride complexes. The formation of cis,cis,cis-[IrH2(CO)2I2]- (8a) is followed by intermolecular
rearrangements to form cis,trans,cis-[IrH2(CO)2I2]- (8b) and cis,cis,trans-[IrH2(CO)2I2]- (8c). A small amount
of a dinuclear species, [Ir2H(CO)4I4]x- (9), is also observed. The formation rate constants for 8a and 8b at 262
K are k2162 ) (4.42 ( 0.18) × 10-4 M-1 s-1, k-2612 ) (1.49 ( 0.07) × 10-4 s-1, k2262 ) (2.81 ( 0.04) × 10-5 s-1
,
and k262 ) (5.47 ( 0.16) × 10-6 s-1. The two equilibrium constants K1262 ) [8a]/([2][H2]) ) 2.97 ( 0.03 M-1
-2
and K2262 ) [8b]/[8a] ) 5.13 ( 0.10 show that complex 8b is the thermodynamically stable addition product.
However, no similar H2 addition products of the rhodium analogue 1 are observed. The pressurization with H2 of
a solution containing 2 and 6 give the monohydride 4, the dihydrides 8a and 8b, the dinuclear complex 9, and the
two new complexes [Ir(CO)2I3] (10) and [HIr(CO)2I2] (11). The reactions of the iridium complexes with H2 and
HI are summarized in a single scheme.
Introduction
conditions in CD2Cl2 (Scheme 1) shows the presence of the
hydride complex mer,trans-[HRh(CO)2I3]- (3), formed by the
The square-planar complexes cis-[Rh(CO)2I2]- (1) and cis-
[Ir(CO)2I2]- (2) are important intermediates in catalytic cycles
for industrial carbonylations of alcohols to form carboxylic
acids.1-7 Carbonylations of alcohols higher than ethanol produce
mixtures of linear- and branched-chain carboxylic acid isomers
for both rhodium and iridium systems. The isomerizations occur
through hydride-olefin species.3,5,6 Under the catalytic condi-
tions used for alcohol carbonylations, the alkenes also transform
into carboxylic acids.5,8 A recent study, by Roe et al., on the
carbonylation of ethene using rhodium as the catalyst under mild
oxidative addition of HI to 1.9 The nonsaturated hydride [HRh-
(CO)2I2] (3a), in equilibrium with 3, is proposed to react with
ethene to form an acyl species, which will form the propanoic
acid. Olefin carbonylation reactions have not received as much
attention as alcohol carbonylation reactions, especially in the
presence of iridium catalysts.8-12
The goal of this work is to understand the speciation and the
kinetics of olefin carbonylation in the presence of the iridium
complex 2 and to compare these with the behavior of the
rhodium analogue 1 in the absence and presence of different
components (CO, olefins, and HI) that participate in the catalytic
* Corresponding author. Tel: +41-21-692-38 71. Fax: +41-21-692-
38-75. E-mail: andre.merbach@icma.unil.ch.
cycle.9 We have chosen P(C6H5)4 as the counterion, but it
+
† Permanent address: Rhodia Recherches, Centre de Recherches de Lyon,
85 rue des Fre`res Perret, BP 62, 69192 St. Fons, France.
(1) Forster, D. J. Chem. Soc., Dalton Trans. 1979, 1639.
(2) Haynes, A.; Mann, B. E.; Morris, G. E.; Maitlis, P. M. J. Am. Chem.
Soc. 1993, 115, 4093.
should be stressed that the equilibria and kinetic behaviors of
our model systems depend also on the nature of the counterion
(e.g., Na+ or PPN+). This study, followed by NMR and IR
spectroscopy, is performed under a high pressure of CO to
increase the dissolved gas concentrations and to displace the
(3) Forster, D.; Dekleva, T. W. J. Chem. Educ. 1986, 63, 204.
(4) Fulford, A.; Hickey, C. E.; Maitlis, P. M. J. Organomet. Chem. 1990,
398, 311.
(5) Ellis, P. R.; Pearson, J. M.; Haynes, A.; Adams, H.; Bailey, N. A.;
Maitlis, P. M. Organometallics 1994, 13, 3215.
(6) Maitlis, P. M.; Haynes, A.; Sunley, G. J.; Howard, M. J. J. Chem.
Soc., Dalton Trans. 1996, 2187.
(9) Roe, D. C.; Sheridan, R. E.; Bunel, E. E. J. Am. Chem. Soc. 1994,
116, 1163.
(10) Hershman, A.; Morris, D. E.; Forster, D. (Monsanto Co.). French Patent
2,280,622, 1975.
(7) Paulik, F. E.; Hershman, A.; Knox, W. R.; Roth, J. E.; Monti, D.
(Monsanto Co.) U.S. Patent 3,769,329, 1973.
(8) Mizoroki, M.; Matsumoto, T.; Ozaki, A. Bull. Chem. Soc. Jpn. 1979,
52, 479.
(11) Forster, D.; Hershman, A.; Morris, D. E. French Patent 2,280,586,
1975.
(12) Forster, D.; Hershman, A.; Morris, D. E. Catal. ReV.sSci. Eng. 1981,
23, 89.
10.1021/ic000269j CCC: $19.00 © 2000 American Chemical Society
Published on Web 08/15/2000

4138 Inorganic Chemistry, Vol. 39, No. 18, 2000
Churlaud et al.
before and after each spectral acquisition by a substitution technique
with a platinum resistor.20 For the medium gas pressure work (up to
2.0 MPa of 13CO and 5.5 MPa of H2), 10 mm (outer diameter) NMR
sapphire tubes were used.18 The parameters for the acquisition of 13C
(1H) NMR spectra were as follows: spectral width 21 (4-15) kHz;
32-128 K (32-128 K) data points; pulse length 10 (7) µs; exponential
line broadening 1 (0) Hz; 0.5-2 K (0.5-2 K) scans. All 13C NMR
spectra were obtained with 13C-enriched carbon monoxide, except where
stated.
Scheme 1. Proposed Rhodium Catalytic Cycle for Ethylene
Carbonylation5,9
IR Measurements. The IR spectra were recorded on a Perkin-Elmer
FTIR 2000 spectrometer using a high-pressure cell.21 Due to an intense
absorption of free CO under pressure, no signals could be detected
between 2100 and 2200 cm-1
.
Data Treatment. The analysis of data using the appropriate
equations was accomplished with the nonlinear least-squares-fitting
program Scientist.22 The reported errors correspond to one standard
deviation.
Results and Discussions
Reactivities of cis-[M(CO)2I2]P(C6H5)4 (M ) Rh, Ir) with
CO, Olefins, and HI. In solution, under a nitrogen atmosphere,
the iridium(I) complex 2 is quite stable and decomposes only
at temperatures higher than 340 K in CDCl3 or 380 K in CD2-
Cl2 or CD3NO2. Under the same conditions, the rhodium(I)
complex 1 is unstable, showing partial decomposition at room
temperature after several hours. Olefins, CO, and HI are
reactants in olefin carbonylation processes catalyzed by cis-
[Rh(CO)2I2]-. Therefore, the interactions of these reactants with
2 were studied.
equilibria to favor the observation of possible intermediates.
The study is extended to high pressure of dihydrogen not
because H2 enters into the reaction equations but rather because
it is a byproduct of the carbonylation process, produced through
the water-gas shift reaction from H2O and CO.
Experimental Section
Chemicals and Solutions. Methanol (Fluka, >99.8%), n-hexane
(Merck, >95%), [Rh(CO)2Cl]2 (Strem Chemical, >95%), [Ir(CO)3Cl]n
(Strem Chemical, >95%), P(C6H5)4I (Fluka, >95%), chloroform-d
(CDCl3, Armar, 99.8 atom % D), methylene-d2 chloride (CD2Cl2,
Armar, 99.6 atom % D), nitromethane-d3 (CD3NO2, Armar, 99.5 atom
% D), CO (Carbagaz, 99.997%), and carbon-13-enriched carbon
monoxide (Cambridge Isotope Laboratories, 99 atom % 13C) were used
without further purification. HI was purified before use by distillation
according to a literature procedure, but without adding hypophosphorous
acid as a stabilizer.4
Previously, we investigated the exchange between free and
coordinated CO in complexes 1 and 2 by high pressure 13C
NMR spectroscopy in dichloromethane. The study pointed to a
limiting associative, A, mechanism with second-order CO
exchange rate constants of 850 × 103 and 99 × 103 L mol-1
s-1 at 298 K, respectively.19 In this previous study, a color
change from yellow to red was observed when a CH2Cl2 solution
of 2 was pressurized with CO up to 1.5 MPa. We could estimate
the formation constant of the red species, [Ir(CO)3I], formed
by iodide substitution, using UV-visible spectroscopy. In the
present study, we investigated the effect of higher CO pressures
(up to 28 MPa) by IR spectroscopy in CHCl3. The low-pressure
spectra confirmed the formation of [Ir(CO)3I], which had already
been identified by its two characteristic CO stretches at 2046
The complexes cis-[Rh(CO)2I2]P(C6H5)4,4,13 cis-[Ir(CO)2I2]P(C6H5)4,14
and cis-[Ir(CO)2I4]P(C6H5)415,16 were synthesized according to methods
described in the literature. The purities of the three complexes were
checked by IR and NMR comparison to literature data.2,15,17 The 13C-
enriched square-planar complexes of rhodium(I) and iridium(I) were
each obtained by pressurizing a solution of the complex in a sapphire
tube18 twice with 1.0 MPa of 13CO, shaking the tube for 1 min to
solubilize the gas more rapidly in solution, then removing the CO
pressure, and, finally, removing free CO by pressurizing twice with
1.0 MPa of N2. The 13C enrichment of the two square-planar complexes
could be performed according to this procedure because, as we have
shown in a preceding study, the exchange of CO between free CO and
1 (or 2) is fast.19
and 2073 cm-1 1,23
This allowed the determination of its
.
formation constant, KIr ≈ 1.8 × 10-3 (KIr ) [Ir(CO)3I][I-]/
[cis-Ir(CO)2I2-][CO]), in CHCl3. A further increase of the
pressure caused the appearance of two new CO stretches at 2010
and 2090 cm-1 due to an unknown species. Simultaneously,
there was a strong decrease of the two bands of 2 at 1970 and
2048 cm-1: 35% of complex 2 was converted at 15 MPa. Under
the same pressure, less than 4% of 1 (CO stretches at 1988 and
2059 cm-1) was converted into [Rh(CO)3I] (one CO stretch at
2087 cm-1, the second one at 2061 cm-1 being obscured by
the CO stretch of 1).24 We then estimated the formation constant,
KRh ≈ 4 × 10-5, of this species in CHCl3. This result shows a
difference in CO affinity for complexes 1 and 2 by a factor of
KIr/KRh ≈ 50.
NMR Measurements. The 1H and 13C NMR spectra were recorded
on a Bruker ARX 400 spectrometer with a narrow-bore cryomagnet
1
(9.4 T; 400.18 and 100.63 MHz, respectively). The H and 13C NMR
chemical shifts, δ(1H) and δ(13C), were referenced to TMS and
measured with respect to the solvent (1H and 13C shifts: CDCl3, 7.24
and 77.0 ppm; CD2Cl2, 5.32 and 53.8 ppm; CD3NO2, 4.33 and 62.8
ppm) at all temperatures. The temperature was controlled to within
(0.2 K using a Bruker B-VT 2000 unit and was measured ((1 K)
The reactivity of 2 toward olefins (either substitution or
oxidative addition) was also checked. In the absence or presence
of a CO pressure, no reactions with ethylene, hexene, butadiene,
(13) Vallarino, L. M. Inorg. Chem. 1965, 4, 161.
(14) Piraino, P.; Faraone, F.; Pietropaolo, R. Inorg. Nucl. Chem. Lett. 1973,
9, 1237.
(15) Forster, D. Synth. Inorg. Met.-Org. Chem. 1971, 1, 221.
(16) Forster, D. Inorg. Chem. 1972, 11, 473.
(17) Forster, D. Inorg. Chem. 1969, 8, 2556.
(18) Cusanelli, A.; Frey, U.; Richens, D. T.; Merbach, A. E. J. Am. Chem.
Soc. 1996, 118, 5265.
(19) Churlaud, R.; Frey, U.; Metz, F.; Merbach, A. E. Inorg. Chem. 2000,
39, 304.
(20) Ammann, C.; Meier, P.; Merbach, A. E. J. Magn. Res. 1982, 46, 319.
(21) Laurenczy, G.; Lukacs, F.; Roulet, R. Anal. Chim. Acta 1998, 359,
275.
(22) Scientist, Version 2.0; MicroMath, Inc.: Salt Lake City, UT, 1995.
(23) Schrod, M.; Luft, G.; Grobe, J. J. Mol. Catal. 1983, 22, 169.
(24) Morris, D. E.; Tinker, H. B. J. Organomet. Chem. 1973, 49, C53.

Olefin Carbonylation in Presence of cis-[Ir(CO)2I2]-
Inorganic Chemistry, Vol. 39, No. 18, 2000 4139
2
Table 1. Chemical Shifts, δ, Coupling Constants, J, and IR Data for All Complexes of This Study
2
2
2
1
ν(CO) (cm-1
)
ν(Ir-H) (cm-1
)
1
1
13
13
13
complex
δH (ppm)
δ
CO (ppm)
J H- H (Hz)
J
C (Hz)
J H- C (Hz)
C-
1a
183.8
1988, as
2059, s
2
171.4
1970, as
2048, s
2080
39,b,c
4d
-10.4
-11.7
177.9
156.0
3.8
2.3
n.d.k
2160e
2107e
2051e
5f
6
169.7
150.6
2084
2065
2110
8ag
-10.1 (a)
-14.3 (b)
161.1 (b)
164.7 (a)
4.6
1.8
57.9 (a-a)
6.2 (a-b)
3.1 (b-a)
4.0 (b-b)
2015
n.d.k
2164
2135
8bg
-16.4
171.2
164.7
0.9h
5.5
2033
2119
2139
8cg
9g
-11.3
-15.6
5 or 7
5 or 7
1.3
41.9, 5, or 7
23.5, n.d.k
161.3i
162.3j
10g
11g
157.2
163.7
-16.8
5.1
a
2
c
2
2
f 2
13
) 72.3 Hz. b In CD2Cl2. J H-
) 3.8 Hz; J
) 50.5 Hz. d In CDCl3. e In CHCl3.
J
) 53.4 Hz. g In CD3NO2.
103
C- Rh
13
103
1
103
13
103
J
C- Rh
Rh
C- Rh
2JH-D ) 0.9 Hz. i Terminal. j Bridged. k n.d. ) not determined.
h
Scheme 2. Reactions among Different Iridium Complexes
in the Presence of HI and H2
and CF2dCH2 were observed for 2 in CD2Cl2 and CDCl3.
Similarly, no reaction with hexene was observed for 1 in CDCl3.
The olefin fixation on the rhodium complex 1 (Scheme 1) is
known to proceed through the hydride species 3 formed by the
oxidative addition of HI. When 1 equiv of HI is added to a
solution of 13C-enriched iridium analogue 2 in CDCl3 at ambient
temperature, we observe the formation of the monohydride
fac,cis-[HIr(CO)2I3]- (4). The 1H NMR spectrum shows a triplet
2
1
13
at -11.7 ppm ( J H- ) 2.3 Hz) which is characteristic of a
C
hydride ligand coupled to two equivalent CO ligands (Table
1). In the 13C NMR spectrum, a doublet due to the presence of
one hydride is present at 156 ppm. The IR spectrum in CHCl3
shows three vibrational bands at 2158 (br, ν(Ir-H)), 2113
(i, ν(CO)), and 2058 (i, ν(CO)) cm-1 in agreement with the
literature.1,10,11 Roe et al. showed that HI reacted with the
rhodium analogue, 1, to form the hydride mer,trans-[HRh-
(CO)2I3]- (3) at 253 K.9 The geometries of the two hydrides 3
and 4 are different: the two CO ligands are in trans and cis
positions, respectively. Roe et al. found that when a solution of
3 was cooled to 193 K, a new hydride, [HRh(CO)2I2] (3a),
formed upon the elimination of an iodide.9 Under these
conditions, we found no iridium hydride analogue. However,
the possible iridium analogue [HIr(CO)2I2] (11) could be
assigned as one of the minor products in the reaction of a
mixture of cis-[Ir(CO)2I2]- and cis-[Ir(CO)2I4]- with H2 (see
Scheme 2). Moreover, Forster et al. found that this hydride,
[HIr(CO)2I2], is formed from [HIr(CO)3I2] upon removal of a
CO in CH2Cl2 by flushing N2.11 The fully NMR-characterized
iridium hydride 4 is stable for hours at room temperature but
rapidly reacts according to eq 1 when the temperature is raised.
evaporation from CD2Cl2, can reversibly give 4 after addition
of solvent and pressurization with H2. A more detailed study
of this back-reaction will be discussed below. Roe et al. found
The mixture of the resulting products 2 and 6, obtained by

4140 Inorganic Chemistry, Vol. 39, No. 18, 2000
Churlaud et al.
that the rhodium hydride 3 reacts already at 273 K (eq 2).9 The
structures of the [M(CO)2I4]- complexes, 5 and 6, are also
different for rhodium and iridium. For the iridium complex 6,
the cis geometry is the thermodynamically stable form; the trans
isomer can be synthesized, but under a CO atmosphere, an
isomerization to the cis configuration was observed.5,25,26 The
rhodium compound 5 was found to have a trans geometry,3,5,26
but a cis geometry could also be found in solids, depending on
the counterion.
We have shown that the square-planar complexes 1 and 2
react differently toward some of the entering components (CO
and HI) of the catalytic cycle proposed by Roe et al. for
rhodium.9 Indeed, under 15 MPa of CO, while 35% of complex
2 is transformed into the [Ir(CO)3I] species, less than 4% of 1
forms the analogous [Rh(CO)3I] species. The reactivities with
HI are also different, oxidative additions of HI to 1 and 2
producing two structurally different hydrides: fac,cis-[HIr-
(CO)2I3]- and mer,trans-[HRh(CO)2I3]-. A further difference
is found in the reactivities of these two hydrides with olefins.
While the rhodium hydride complex 3 reacts with ethylene to
form an acyl complex,9 the iridium hydride 4 is unreactive.
Reactivities of cis-[M(CO)2I2]P(C6H5)4 (M ) Rh, Ir) and
cis-[Ir(CO)2I4]P(C6H5)4 with H2. H2 does not explicitly appear
in the rhodium catalytic cycle (Scheme 1); in other words, it is
not an entering or leaving constituent. Nevertheless, H2 can play
an important role, as it is produced either by hydride decom-
position (eq 1) or by a water-gas shift reaction in the presence
of complex 6.1 We first investigated the reactions of H2 with 2
and 6 independently. The addition reaction of H2 with 2 was
performed in CD3NO2 using a high pressure of H2. The 1H and
13C NMR spectra (Figure 1) recorded after applying 5.5 MPa
of H2 ([H2] ) 0.12 M) to a solution of 2 at room temperature
indicate that around 50% of the starting complex is converted
into different hydride complexes (eq 3). If H2 is added from
1
Figure 1. 13C (top) and H (bottom) NMR spectra of a solution of
cis-[Ir(13CO)2I2]- solution (3.2 × 10-2 M) in CD3NO2 under 5.5 MPa
of H2 at 298 K: major complex 8b and minor complexes 8a and 9.
one side of the square plane of 2, the first dihydride complex
that is formed must be 8a (two enantiomers) and all other
isomers are formed through either inter- or intramolecular
rearrangements. Compound 8a was only formed as a minor
product of the observed new dihydride complexes (Figure 1).
The addition reaction of H2 with 2 was also followed by high-
pressure (14.5 MPa of H2) IR spectroscopy at ambient temper-
ature (Figure 2). As soon as H2 is present in the solution, the
Figure 2. IR spectra of a 1.1 × 10-2 M solution of 2 in CH3NO2
under an N2 atmosphere and 14.5 MPa of H2 (after 0, 5, 10, and 20
min) at 298 K.
and 2164 cm-1, respectively, are attributed to 8a in agreement
with the order of appearance of the signals of the species in the
NMR study. The second ν(CO) stretching band of 8a should
be superposed on the νas(CO) band of 2. The NMR chemical
shifts, coupling constants, and IR data for 8a are given in Table
1.
two CO stretching bands of 2 (1970 (νas) and 2048 (νs) cm-1
)
decrease with time and some other signals appear, in both the
carbonyl and the hydride regions. The first three appearing new
vibration bands ν(CO), ν(Ir-H), and ν(Ir-H), at 2015, 2135,
The chemical shifts, coupling constants (Figure 1), and IR
data (Figure 2, IR time-dependent spectra) for the major product
8b, appearing after 5 min, are also reported in Table 1. The
determination of the structure of 8b, using the NMR data, was
(25) Kumbhar, A. S.; Padhye, S. B.; Kelkar, A. A.; Patil, R. P.; Chaudhari,
R. V.; Puranik, V. G.; Dhaneshwar, N. N.; Tavale, S. S. Polyhedron
1996, 15, 1931.
(26) Haynes, A.; McNish, J.; Pearson, J. M. J. Organomet. Chem. 1998,
551, 339.
2
1
13
not straightforward. Even if the value of the J H- coupling
C
constant indicated that the hydride and the carbonyl ligands are
cis to each other, three structures are still possible: 8b (H cis

Olefin Carbonylation in Presence of cis-[Ir(CO)2I2]-
Inorganic Chemistry, Vol. 39, No. 18, 2000 4141
These NMR data are compatible with the structure suggested,
having one hydride and two CO bridging ligands.
Very recently, parahydrogen enhanced NMR spectroscopy
at low pressure (P ) 0.3 MPa, in benzene-d6) has enabled
Hasnip et al. to detect isomers 8a and 8b of the cis-dihydride
[IrH2(CO)2I2]-; however, the third isomer 8c and the dinuclear
species 9 were not observed.31 Furthermore, in the parahydrogen
study, the monohydride complex [IrH(CO)2I3]-, which may have
formed according to eq 1 in the presence of a small amount of
6, was observed and a trans CO structure was assigned, different
from the cis CO structure of 4 found in nitromethane.
To obtain quantitative kinetic and thermodynamic information
about the formation and rearrangement of 8a and 8b, we
1
performed a high-pressure (5.5 MPa of H2) H NMR study at
262 K. The model used to adjust the experimental data is
presented in eq 4. The reversibility of these reactions is easily
Figure 3. 1H and 13C NMR spectra of 8c obtained from a ≈ 3.5 ×
10-2 M solution of 2 in CD3NO2 under 5.0 MPa of H2 at 253 K:
spectrum a, natural-abundance 13C; spectra b-d, 99%-enriched 13C.
and CO trans) and two complexes with the hydrogens trans and
with the carbonyls cis or trans to each other. Two NMR
techniques were used to distinguish if the hydrogens were cis
or trans: JH-D coupling constant27-29 and longitudinal relax-
2
observed by the disappearance of complexes 8a and 8b after
flushing of the solution with N2. Equations 5-7 were fitted to
ation time T1 measurements.30 For a cis-dihydride, the values
2
of JH-D are between 2 and 3 Hz, and for a trans-dihydride,
this value is near 0 Hz. The measured value of 0.9 Hz for 8b
does not permit a clear assignment. The T1 temperature
dependence of the hydride signal of 8b fitted to the relaxation
equations yielded a dH-H value of 2.18 ( 0.01 Å. This value is
close to the calculated value of 2.26 Å (based on an average
dIr-H value of 1.60 Å from crystal data) for two cis hydrides.
These T1 NMR data together with the two observed IR ν(Ir-
H) stretches confirm the structure of 8b given in eq 3. We should
note that, during the 2JH-D measurement, HD slowly converted
to H2 and D2. Therefore, it was not necessary in this case to
d[2]/dt ) d[H2]/dt ) -k1[2][H2] + k-1[8a]
d[8a]/dt ) -k-1[8a] - k2[8a] + k1[2][H2] + k-2[8b] (6)
d[8b]/dt ) -k-2[8b] + k2[8a] (7)
(5)
the time-dependent concentrations of complexes 8a and 8b, and
the obtained rate constants are k2162 ) (4.42 ( 0.18) × 10-4
M-1 s-1, and k2-612 ) (1.49 ( 0.07) × 10-4 s-1, k2262 ) (2.81 (
0.04) × 10-5 s-1, and k-2622 ) (5.47 ( 0.16) × 10-6 s-1. This
2
allows one to determine the two equilibrium constants K1262
)
use HD pressure to obtain the JH-D value but was sufficient
[8a]/([2][H2]) ) 2.97 ( 0.03 M-1 and K2262 ) [8b]/[8a] ) 5.13
( 0.10, which show that complex 8b is effectively the more
stable of the cis-dihydrides [IrH2(CO)2I2]-.
just to pressurize with a mixture of H2 and D2. To explain this
reaction, we propose that the rearrangement from 8a to 8b
proceeds via an intermolecular exchange of the hydrides.
At low temperature (253 K), we observed a new H2 addition
complex, 8c (Figure 3), which disappeared with time at 269 K.
The structure of 8c could be determined in nitromethane using
1H and 13C NMR spectroscopy. The NMR data are given in
The rhodium(I) square-planar analogue 1 shows no reaction
with H2 at ambient temperature, but modest reactivity could be
observed at 350 K using the parahydrogen technique.31 The
higher extent of formation of dihydrides from iridium(I) than
from rhodium(I) should favor olefin hydrogenation reactions.
We therefore reacted dihydrogen (5.5 MPa) at 327 K with
hexene in the presence of small amounts of cis-[Ir(CO)2I2]-
(10:1 ratio). The reaction was followed by 13C{1H} NMR
spectroscopy, which revealed that, after 3 days, hexene was
quantitatively converted to hexane. During this reaction, no
species with bound olefin were detected in either aliphatic or
carbonyl 13C-enriched regions. This reaction can compete with
olefin carbonylation when 2 is used as the catalyst.
To complete the study of the back-reaction of eq 1, we
pressurized the octahedral iridium(III) complex 6 with H2 and
could detect no reaction.
Reactivity of a Mixture of cis-[Ir(CO)2I2]P(C6H5)4 and cis-
[Ir(CO)2I4]P(C6H5)4 with H2. We have shown that Ir(III)
monohydride 4 reacts according to eq 1 to form 2 and 6 and
that this reaction can be made fully reversible by pressurizing
with H2. We now present the results of a study of the back-
2
1
13
Table 1. The J H-
value of 41.9 Hz indicates the presence
C
trans
of a CO trans to H. Three other coupling constants of 5, 5, and
7 (or 5, 7, and 7) Hz were observed, but as the two homonuclear
coupling constants were identical, no assignments could be
made. The three dihydride complexes (8a-c) observed in this
study and formed by addition of H2 to the square-planar complex
2 and further isomerizations all have a cis geometry for their
hydride ligands. No dihydrides with a trans geometry could be
detected.
A small amount of a new dinuclear compound [I2(OC)Ir(µ2-
CO)2(µ2-H)Ir(CO)I2]x- (9), was also detected. The values of
the chemical shifts and the coupling constants are given in Table
2
1
13
1. The J H-
value of 23.5 Hz is between the values of the
C
trans
coupling constants found for CO trans to H (42-58 Hz for 8a
and 8c) and for CO cis to H (4-6 Hz for 4, 8a, 8b, and 8c).
(27) Heinekey, D. M.; Oldham, W. J., Jr. Chem. ReV. 1993, 93, 913.
(28) Kubas, G. J.; Ryan, R. R.; Swanson, B. I.; Vergamini, P. J.;
Wasserman, H. J. J. Am. Chem. Soc. 1984, 106, 451.
(29) Kubas, G. J. Acc. Chem. Res. 1988, 21, 120.
(30) Hamilton, D. G.; Crabtree, R. H. J. Am. Chem. Soc. 1988, 110, 4126.
(31) Hasnip, S.; Duckett, S. B.; Taylor, D. R.; Barlow, G. K.; Taylor, M.
J. Chem. Commun. 1999, 889.

4142 Inorganic Chemistry, Vol. 39, No. 18, 2000
Churlaud et al.
The reactions of the iridium complexes with H2 and HI are
summarized in Scheme 2.
The proposed intermediate 12 was not observed but should
be involved in the formation of fac,cis-[HIr(CO)2I3]- from cis-
[Ir(CO)2I2]-, cis-[Ir(CO)2I4]-, and H2.
Conclusions
With the use of ambient- and high-pressure NMR and IR
spectroscopy, the main differences in the catalytic cycles of the
carbonylation of olefins in the presence of the iridium complex
2 and in the presence of the rhodium analogue 1 have been
presented. Under elevated pressures of CO, 1 and 2 form the
corresponding tricarbonyl halides M(CO)3I by substitution of
an iodide by CO, with the equilibrium constants KIr ≈ 1.8 ×
10-3 and KRh ≈ 4 × 10-5. The ratio KIr/KRh close to 50 shows
that, under catalytic conditions (a few megapascals), only
complex 1 remains in the anionic form, while a major amount
of the iridium analogue 2 is converted to a neutral species.
The oxidative addition reactions of HI with 1 and 2 give two
monohydrides of different geometries, mer,trans-[HRh(CO)2I3]-
(3) and fac,cis-[HIr(CO)2I3]- (4), respectively. Both hydrides
are unstable at ambient temperature and form, within minutes
for Rh and within hours for Ir, the corresponding cis-
[M(CO)2I2]- (1 or 2) and [M(CO)2I4]- (5 or 6) species and H2.
Applying a high pressure of H2 (H2 is produced in situ during
the carbonylation reaction) at ambient temperature to a ni-
tromethane solution containing complex 2 first formed the cis-
dihydride 8a, followed by intermolecular isomeration to form
8b and the newly observed complex 8c. However, no similar
H2 addition products of the rhodium analogue were observed.
Very small quantities of H2 addition products were previously
observed by Hasnip et al. in benzene at 350 K. These major
differences between rhodium and iridium help to explain their
different activities in the olefin carbonylation catalytic cycle.
Applying a pressure of H2 to a solution containing 6 and an
excess of 2 forms, in addition to the already described complexes
4, 8a, 8b, and 9, the two new complexes 10 and 11. All these
observations are summarized in Scheme 2.
Acknowledgment. We thank Rhoˆne-Poulenc and the Swiss
National Science Foundation for financial support. We are also
grateful to Dr. Gabor Laurenczy for performing the high-
pressure IR studies.
Figure 4. 1H (top), 13C (middle), and 13C{1H} (bottom) NMR spectra
of a solution of 2 (2.5 × 10-2 M) and 6 (7.9 × 10-3 M) in CD3NO2
under 5.5 MPa of H2 at 298 K after 29 h.
reaction of eq 1, but with an excess of 2 under the following
conditions: a solution containing 2.50 × 10-2 mol L-1 of 2
and 7.90 × 10-3 mol L-1 of 6 with 5.5 MPa of H2 in CD3NO2.
After more than 1 day, the 1H, 13C, and 13C{1H} spectra (Figure
4) show as products the expected hydride 4, the starting
complexes (2 and 6), the previously observed complexes (8a,
8b, and 9) due to the excess of 2 and the two new complexes
10 and 11. Complex 10 shows one 13C NMR signal at 157.2
Supporting Information Available: Experimental concentrations
of complexes 8a and 8b as functions of time, obtained from a solution
of 2 under 5.5 MPa of H2 at 262 K (Table S1), IR spectra of solutions
of cis-[Ir(CO)2I2]- and cis-[Rh(CO)2I2]- in CHCl3 under different CO
pressures at ambient temperature (Figure S1),1H NMR spectra of 8b
(H2 addition product) and 8b* (HD addition product), obtained from a
solution of 2 in CD3NO2 under 3.2 MPa of HD at 298 K, showing the
1
2JH-D coupling constant of 8b* (Figure S2, top), H NMR T1 of the
2
1
13C
hydride 8b as a function of temperature, obtained from a solution of 2
in CD2Cl2 under 5.0 MPa of H2 (Figure S2, bottom), experimental and
adjusted concentrations of complexes 8a and 8b as functions of time,
obtained from a solution of 2 under 5.5 MPa of H2 at 262 K (Figure
S3), NMR spectra of 9, obtained from a solution of cis-[Ir(13CO)2I2]-
in CD3NO2 under 5.5 MPa of H2 at 298 K (Figure S4), and NMR
spectra of a cis-[Ir(13CO)2I2]-/hexene solution in CD3NO2 under 5.5
MPa of H2 at 327 K after 5 min, 5 h, and 3 days (Figure S5). This
material is available free of charge via the Internet at http://pubs.acs.org.
ppm and has no hydride ligand, since no J H- coupling
1
constant and no corresponding H NMR signal are observed.
This complex should contain only two magnetically equivalent
CO moieties, and therefore we suggest the formula [Ir(CO)2I3].
The 1H NMR spectrum of the minor product 11 shows a triplet
2
at -16.8 ppm ( J H- ) 5.1 Hz) and the 13C NMR spectrum
1
13
C
a doublet at 163.7 ppm, indicating two equivalent CO ligands
and a unique hydride ligand, compatible with the formula [HIr-
(CO)2I2], already suggested for a compound formed under
different conditions.11
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