A mechanistic investigation into the elimination of phosphonium salts from rhodium-TRIPHOS complexes under methanol carbonylation conditions

Article abstract of DOI:10.1039/b803067g

Phosphine modified rhodium complexes are currently the topic of considerable research as methanol carbonylation catalysts, but often suffer from poor stability. This paper reports on an investigation into how coordination mode affects the elimination of phosphonium salts from rhodium complexes, namely [trans-RhCl(CO)(PPh₃)₂] 1, [RhCl(CO)(dppe)] 2, [RhCl(CO)(dppb)]₂3, [Rh(TRIPHOS)(CO)₂]Cl 4. These complexes are all potential pre-catalysts for methanol carbonylation. The reaction of these complexes with methyl iodide at 140 °C under both N ₂ and CO atmospheres has been studied and has revealed clear differences in the stability of the corresponding Rh(iii) complexes. In contrast to both monomeric 2 and dimeric 3 that react cleanly with CH₃I to give stable Rh(iii) acetyl complexes, 4 forms a novel bidentate complex after the elimination of the one arm of the ligand as a quaternised phosphonium salt. The structure of this complex has been determined spectroscopically and using X-ray crystallography. The mechanism of formation of this novel complex has been investigated using ¹³CH₃I and strong evidence that supports a dissociative mechanism as the means of phosphine loss from the rhodium centre is provided.

Full text of DOI:10.1039/b803067g

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www.rsc.org/dalton | Dalton Transactions
A mechanistic investigation into the elimination of phosphonium salts from
rhodium–TRIPHOS complexes under methanol carbonylation conditions†
Gareth W. Lamb,a Matthew L. Clarke,*a Alexandra M. Z. Slawina and Bruce Williamsb
Received 21st February 2008, Accepted 18th June 2008
First published as an Advance Article on the web 29th July 2008
DOI: 10.1039/b803067g
Phosphine modified rhodium complexes are currently the topic of considerable research as methanol
carbonylation catalysts, but often suffer from poor stability. This paper reports on an investigation into
how coordination mode affects the elimination of phosphonium salts from rhodium complexes, namely
[trans-RhCl(CO)(PPh3)2] 1, [RhCl(CO)(dppe)] 2, [RhCl(CO)(dppb)]2 3, [Rh(TRIPHOS)(CO)2]Cl 4.
These complexes are all potential pre-catalysts for methanol carbonylation. The reaction of these
complexes with methyl iodide at 140 ◦C under both N2 and CO atmospheres has been studied and has
revealed clear differences in the stability of the corresponding Rh(III) complexes. In contrast to both
monomeric 2 and dimeric 3 that react cleanly with CH3I to give stable Rh(III) acetyl complexes, 4 forms
a novel bidentate complex after the elimination of the one arm of the ligand as a quaternised
phosphonium salt. The structure of this complex has been determined spectroscopically and using
X-ray crystallography. The mechanism of formation of this novel complex has been investigated using
13CH3I and strong evidence that supports a dissociative mechanism as the means of phosphine loss
from the rhodium centre is provided.
Cole-Hamilton et al. have previously reported highly ac-
Introduction
tive rhodium catalysts containing electron rich phosphine PEt3
ligands.15,16 At temperatures exceeding 120 ◦C the catalyst rapidly
The carbonylation of methanol to acetic acid is one of the most
decomposes forming the phosphorus(V) species [Et3PCH3]+I−
important applications in homogeneous catalysis. Large scale
industrial processes have typically used either [Rh(CO)2(I)2]− or
[Ir(CO)2(I)2]− as catalysts with methyl iodide1–3 and ruthenium
halide scavengers as promoters.4–6 With most of the possible vari-
ables having now been explored in ‘unmodified’ catalyst systems,
considerable attention is now focussed on phosphine-modified
rhodium catalysts7–14 with the belief that they may offer the best
opportunities for eliminating side-products or allowing the use
of lower grade CO. Several BP projects have recently reported
systems based on both bidentate and potentially tridentate ligands
that show promise for methanol carbonylation in the presence of
hydrogen.7–9
However, even if a suitable phosphine can be designed to
deliver an improvement to selectivity and activity, the catalyst
and ligand will need to show long-term stability under the
forcing conditions required for industrial methanol carbonylation.
It has been known for some time that rhodium complexes of
monodentate phosphines are only stable for very short periods of
time under industrially significant conditions. Several diphosphine
systems have been reported that seem to show significantly better
stability.10–13 However, the stability of these systems has not been
studied under comparable conditions making it difficult to make
any conclusions on how ligand co-ordination mode, electronic and
steric effects impact on stability.
=
and Et3P O. It was suggested that such species might form
directly from Rh(III) species observed in catalysis such as
[RhI(CH3)(CO)(PEt3)2], [Rh(I)2(H)(PEt3)2] and [Rh(I)3(PEt3)2]
possibly without prior dissociation of the free phosphine. An
alternative mechanism involves dissociation of phosphine from
the metal centre followed by quaternisation by methyl iodide.
We have recently studied the intermediates formed during
methanol carbonylation using C4 diphosphine ligands.7 One
interesting observation that prompted the investigation de-
scribed here was the increased stability of the bridging biden-
tate complex, [RhCl(CO)(dppb)]2 over that of chelate complex,
[RhCl(CO)(BINAP)] to methyl iodide. This observation does
not easily fit with a dissociative model for phosphine loss in
these complexes, since on reaction of Rh–dppb dimer with
methyl iodide, the only species present is the crystallographi-
cally characterised acetyl species, with chelating dppb ligand,
[Rh(I)2(COCH3)(dppb)]. Interestingly no quaternisation occurs
during this transition from a bridging to a chelating co-ordination
mode, even though dissociation of one or more phosphorus
atoms from the metal centre must occur in order to achieve this
rearrangement.
In this paper, we report a study on how co-ordination mode
affects catalyst stability under methanol carbonylation conditions,
and in particular, provide evidence that supports a dissociative
mechanism as the means of phosphine loss from a tridentate
rhodium complex. Given that multi-dentate ligands are currently
being extensively studied by several groups to deliver more
stable catalyst systems, this work taken with our initial findings7
should help guide multi-dentate ligand design for genuinely stable
methanol carbonylation catalysts.
aSchool of St. Andrews, University of St. Andrews, St. Andrews, Fife, UK
KY16 9ST; Fax: 44 (0)1334 463808; Tel: 44 (0)1334 463850. E-mail:
mc28@st-andrews.ac.uk
bBP Chemicals Ltd, Saltend, Hull, UK HU12 8DS
† CCDC reference number 679097. For crystallographic data in CIF or
other electronic format see DOI: 10.1039/b803067g
4946 | Dalton Trans., 2008, 4946–4950
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species, 5a (Fig. 2) that contains two co-ordinated phosphines
and one quaternised phosphine, and therefore the Rh complexes
of TRIPHOS is less stable than either of the bidentate systems.
However, the quaternised chelate complex was itself highly stable
with no subsequent decomposition detected.
Results and discussion
In this study, we elected to compare the stability of the known
complexes that have ligands in different co-ordination modes;
monodentate [trans-RhCl(CO)(PPh3)2], the bidentate chelate com-
plex [RhCl(CO)(dppe)], bridging complex [RhCl(CO)(dppb)]2
and the tridentate complex [Rh(TRIPHOS)(CO)2]Cl (Fig. 1).17–20
These complexes were prepared via literature procedures and
◦
their stability was tested by heating to 140 C in the presence of
methyl iodide under nitrogen and carbon monoxide atmospheres
(Table 1).
Fig. 2 X-Ray structure analysis of [Rh(I)2(C(O)CH3){Ph2P(CH2)2P(Ph)-
(CH2)2(PPh2CH3)}]+I− 5a.
Using complex 4, similar results were obtained under methanol
carbonylation conditions with the only detectable TRIPHOS
complex being 5a, containing both quaternised and coordinated
phosphines. The structure of the novel TRIPHOS complex was
elucidated by spectroscopic and crystallographic methods.
The crystal structure serves to confirm the connectivity and
general structural features expected in complex 5a. The I–Rh–I
bond angle [91.79(4)◦] and the P–Rh–P bond angles [85.46(13)◦]
for the novel bidentate complex 5a are similar to that of
[Rh(I)2(C(O)CH3)(dppe)].22
Fig. 1 The rhodium–phosphine complexes investigated in this study.
In the case of the control system [trans-RhCl(CO)(PPh3)2], the
stability testing shows, as expected, decomposition to P(V) species
at higher t◦emperatures under both CO and N2 atmospheres.15
At 140 C, both dppe and dppb species show little to no sign
of decomposition with high recovery of unquaternised rhodium
acetyl complexes. Further experiments carried out at the higher
temperature of 175 ◦C for 1 h under 5 bar CO results in the
complete decomposition of dppb complexes to a mixture of
unassigned P(V) species in comparison to the dppe complex that
1
The 31P{ H} NMR spectrum of 5a contains three different
phosphorus environments, one of these being very clearly not co-
3
ordinated to Rh (d = 26.5, d, JP–P 47.2 Hz). The co-ordinated
phosphines show significant downfield co-ordination chemical
1
shifts as expected for 5-membered chelate rings23 and JP–Rh
coupling to Rh (see experimental).
Examining the mechanism behind the formation of this complex
should lead to a greater understanding of phosphine loss in
multidentate systems, which may in turn lead to the design of
more stable catalysts. A number of 13CH3I labelled experiments
were carried out in order to assess whether quaternisation of the
phosphine occurs via direct elimination from the rhodium centre
or via dissociation of the phosphine. The doubly labelled com-
plex [Rh(I)2(CO13CH3){Ph2P(CH2)2P(Ph)(CH2)2PPh213CH3}]+I−,
5b, was prepared to aid in the characterisation of complexes 5a–
5d. The 13C NMR (CDCl3) spectrum of 5b contained a singlet
at 45 ppm associated with the CO13CH3 group (a) and a doublet
1
showed a much higher degree of stability (ca. 90% of the 31P{ H}
NMR spectra can be assigned to the Rh(III) acetyl of dppe).
The TRIPHOS complex however, quantitatively forms a new
Table 1 Stability of Rh(I) carbonyl complexes under N2 and CO atmo-
spheres in the presence of methyl iodide
a
Ligand
140 ◦C under N2
140 ◦C under COb
1
at 9 ppm with a JP–C coupling of 54 Hz characteristic of the
PPh3
dppe
dppb
TRIPHOS
0
0
13CH3 group (b) present on the quaternised phosphine. This one
step reaction gave a single species that was fully characterised
(Scheme 1).
100c
94.4c
66.7d
75.5c
60.2c
66.7d
On reaction with 13CH3I (Scheme 2), the precursor
[Rh(TRIPHOS)(CO)2]Cl 4, forms [Rh(I)(13CH3)(TRIPHOS)
(CO)] 6b which may be isolated in good yield. The 13C NMR
spectra shows a clearly defined doublet of quartets relating to the
labelled methyl group (Scheme 2). When complex 6b was then
dissolved in methanol and heated to 140 ◦C in the presence of
12CH3I, formation of labelled 5c took place, alongside a small
amount of an unidentified product. The 13C NMR (CDCl3) spectra
a % of Rh–P species present with respect to an external standard of
tributylphosphine oxide after heating 0.0265 mmol of complex for 10 min
in a CH3OAc–CH3I mixture (2 mL). b % of Rh–P complex recovered as
precipitate at the end of a catalytic run at 140 ◦C under 26 bar CO. c Rh
species is assigned as [Rh(I)2(C(O)CH3)(L)] by comparison of NMR and
IR data with that reported in the literature.7,21,22 d Quantitative conversion
to complex 5a containing a single quaternised phosphine. This has been
recorded as 66.7% Rh–P species since 2 out of the 3 phosphorus atoms are
unquaternised.
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no longer associated with the metal centre. Through a series of
experiments using 13CH3I, we have provided strong evidence that,
for the TRIPHOS system, the loss of the phosphine occurs via a
simple dissociative mechanism and not through direct elimination.
This does not however, rule out the possibility that for other
ligands phosphine loss could occur via direct elimination from the
metal centre. However, we propose on the basis of these studies
along with our previous observations on the Rh–BINAP system7
that the stability of Rh–phosphine complexes to CH3I is related
to the stability of the Rh-acetyl species, rather than merely being a
reflection of co-ordination mode and typical association constants
of ligands to late transition metals. Given that multidentate ligands
are being investigated more intensively in methanol carbonylation
recently, we would suggest that a key experiment in the first stages
of a study is to investigate the stability of the Rh(III) acetyl species,
since multidentate systems can be less stable than those derived
from diphosphines. Further studies aimed towards increasing the
stability of phosphine modified rhodium carbonylation catalysts
are required.
Scheme 1 Direct formation of complex 5 and index of 13C/12C analogues.
Scheme 2 Labelled studies using 13CH3I.
of the resulting solution, shows no evidence for the 13CH3 label
in the part of the 13C spectrum where (RPPh213CH3)+ would be
observed. The inverse of the above experiment was also conducted
by heating a sample of unlabelled [RhI(12CH3)(TRIPHOS)(CO)]
6a to 140 ◦C in the presence of 13CH3I. Once again, this two-
step reaction produced a side product but the 13C NMR of this
mixture clearly shows two doublets at 9.19 ppm and 8.13 ppm with
1JP–C couplings of 54.8 and 56.6 Hz respectively, characteristic of
the 13CH3 now being part of the quaternised phosphine for both
major and minor species. All experiments in which 6b was treated
with CH3I gave these chelate species with quaternised phosphine
and no sign of tridentate species as intermediates (we were only
interested in the reactivity in the presence of CH3I, since CH3I
will be present under catalytic conditions). These results therefore
point towards a dissociative mechanism in the quaternisation of
this ligand.
Experimental
General
All manipulations were performed using standard Schlenk line
techniques under nitrogen supplied by BOC. All chemicals and
solvents were obtained through commercial sources. All complexes
used were prepared by standard literature procedures. Micro-
analyses were by the University of St. Andrews microanalytical
service. NMR spectra were recorded on Bruker Avance 300
1
instruments. Chemical shifts are reported in ppm with H and
13C NMR spectra referenced to tetramethylsilane (external). 31P
NMR spectra were referenced externally to 85% H3PO4. Proton
and carbon signal multiplicities are given as s (singlet), d (doublet),
t (triplet), q (quartet), m (multiplet), b (broad) or a combination
of them. All spectra were recorded at room temperature and the
solvent for a particular spectrum is given in the parentheses. 13C
and 31P NMR spectra were recorded with broad-band proton
decoupling. IR spectra were recorded using a Perkin Elmer
Spectrum GX FT-IR system. All solids were analysed as KBr
disks. Mass spectroscopy data was obtained from the EPSRC
National Mass Spectroscopy Service Centre, Swansea. In addition
to detecting the ions shown, all Rh complexes showed excellent
agreement between calculated and expected isotope patterns.
[trans-RhCl(CO)(PPh3)2], [RhCl(CO)(dppe)], [RhCl(CO)(dppb)]2
and [Rh(TRIPHOS)(CO)2]Cl were prepared according to litera-
ture procedures.17,18,25
In order to investigate this further we attempted the direct prepa-
ration of the rhodium acetyl complex by reacting the rhodium
acetonitrile dimer, [RhI(COCH3)(CO)(CH3CN)]2(l-I)2,24 with the
TRIPHOS ligand. This reaction gives several species that were not
separable and have therefore not been fully characterised, but this
product mixture reacts cleanly with CH3I at room temperature
1
to give complex 5a as the only species observable by 31P{ H}
NMR spectroscopy. This latter experiment could be consistent
with either mechanism (but the labelling studies demonstrate the
dissociative variant). However, it does suggest that the reason
behind the dissociation is the lower stability of tridentate Rh(III)
acetyls of TRIPHOS in addition to, or instead of, a high energy
transition state for migratory insertion in the tridentate complex,
6a/b.
General procedures for stability testing
The majority of the stability tests run under a CO atmosphere were
carried out at BP chemicals Ltd in Hull in a 300 cm3 Hastelloy
high pressure infrared (HPIR) autoclave fitted with a bursting
disc, catalyst injector, overhead MagnedriveTM stirrer, impeller
with gas sparging facility, gas inlet, high pressure (195 bar) carbon
monoxide reservoir and thermocouple.
[Rh(CO)2](l-Cl)2] (150 mg, 0.386 mmol), ligand (2 eq,
0.772 mmol) and 100 g CH3OH were stirred together for a few
minutes, which we have independently confirmed forms complexes
Conclusions
In summary, the reactivity of Rh(I) complexes of PPh3, dppe, dppb
and TRIPHOS has been studied. As expected, the coordination
mode can have a large impact on the stability of rhodium–
phosphine complexes. The use of the tridentate ligand TRIPHOS
results in the loss of one of the phosphines and favours the
formation of complex 5a containing a quaternised phosphine
4948 | Dalton Trans., 2008, 4946–4950
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1
779.01 [M − CO]+. 31P{ H} NMR (CD3OD): d 108.73 (dt, 1JRh–P
1–4. The relevant complex was added to the autoclave under
104.0 Hz, 2JP–P 2.97 Hz), 41.85 (dd, 1JRh–P 92.1 Hz, 2JP–P 2.97 Hz).
CO. The autoclave was then sealed and pressurised with carbon
◦
1H{ P} NMR (CD3OD): d 7.78 (4H, CH-Ar, m), 7.45 (4H, CH-
31
monoxide (5 bar) and heated to 100 C, controlled by the use of
a thermocouple in a thermowell in the reaction solution, whilst
stirring at 900 rpm. Using the catalyst injector, iodomethane
(22.6 g, 0.159 mol) was then added to the autoclave with an over
pressure of CO. Simultaneously, the autoclave was pressurised with
CO (27 bar) from the high pressure ballast vessel. The autoclave
was then heated to the desired temperature and the data collection
activated. The pressure was maintained at 27 bar by a pressure
regulating value which transferred CO from the high pressure
ballast to the autoclave when the pressure dropped below the set-
point. After the reaction, the autoclave was allowed to cool before
gas, liquid and solid samples were collected.
Ar, m), 7.16 (17H, CH-Ar, m), 3.51 (1H, CH2, m), 2.99 (1H, CH2,
m), 2.74 (6H, CH2, m), −0.14 (3H, CH3, s).
[RhI(13CH3)(TRIPHOS)CO]+I− 6b. Yield: 85% (106 mg,
0.131 mmol). IR (KBr), v(CO)/cm−1: 2076. LSIMS (ES) Found
779.77 [M − CO]+; C3413CH36IP3Rh requires 780.02 [M − CO]+.
31P{ H} NMR (CD3OD): d 108.9 (m, JRh–P 103.7 Hz), 42.0 (d
1
1
1
2
2
1
31
13
(dd), JRh–P 92.45 Hz, JP–P 2.97 Hz, JP– 5.77 Hz). H { P}
C
NMR (CD4OD): d 8.12–7.31 (25H, CH-Ar, m), 3.82 (2H, CH2,
13
1
13
m), 3.05 (6H, CH2, m), 0.17 (3H, CH3, d, J
135.7 Hz).
C–H
13C NMR (CD3OD): d 7.12 (13CH3, dq, JRh– 14.4 Hz, JP–
1
1
13
13
C
C
6.1 Hz).
Other stability tests carried out under CO utilised a smaller
Hastelloyautoclaveusingisolatedcomplexesandmagneticstirring
at a range of temperatures.
[Rh(I)2 (C(O)CH3 ){Ph2P(CH2 )2P(Ph)(CH2 )2 (PPh2CH3 )}]+I−
5a. CH3I (0.1 mL, 1.06 mmol) was added to a 2.0 mL BiotageTM
microwave vial containing [Rh(TRIPHOS)(CO)2]Cl prepared
from [Rh(CO)2](l-Cl)2] (15 mg, 0.039 mmol) and TRIPHOS
ligand (41.3 mg, 0.077 mmol) in 2 mL of dry degassed methanol.
The reaction mixture was then heated to 140 ◦C for 10 min.
Crystals suitable for analysis by X-ray diffraction were then formed
from the slow evaporation of this solution.
Yield: 75% (54 mg, 0.058 mmol). IR (KBr), v(CO)/cm−1:
1709. HRMS (ES) Found 948.9364 [M]+; C37H39I2OP3Rh requires
948.9353 [M]+. Anal. Calc. for C37H39I3O1P3Rh·2 H2O (as shown
in X-ray structure of crystals): C, 39.95; H, 3.90%. Found: C, 39.85;
For the stability tests run under a N2 atmosphere, a BiotageTM
microwave vial containing 0.0265 mmol of Rh(I)–phosphine
ligand complex was charged with a solution of CH3OAc–CH3I
(3 : 1) and heated at 140 ◦C for 10 min in a BiotageTM microwave.
The % of Rh–P complex present was then calculated with respect
to a calibrated external standard of tributylphosphine oxide using
1
31P{ H} NMR spectroscopy.
Safety note: These experiments were carried out using BiotageTM
equipment built to withstand pressures of up to 22 bar and
temperatures in excess of 200 ◦C. CH3I is extremely toxic and
should be used with extreme care.
1
1
H, 3.75%. 31P{ H} NMR (CDCl3): d 79.55 (dd, JRh–P 143.7 Hz,
3JP–P 47.2 Hz), 70.69 (d, 1JRh–P 134.1 Hz), 26.50 (d, 3JP–P 47.2 Hz).
X-Ray crystallography
1H{ P} NMR (CDCl3): d 7.55 (25H, CH-Ar, m), 3.75 (1H, CH2,
31
X-Ray crystallography data were collected at 93 K by using
a Rigaku MM007 High brilliance RA generator and Mer-
cury/Saturn CCD systems using Mo Ka radiation. Intensities
were corrected for Lorentz-polarisation and for absorption. The
structures were solved by direct methods. All hydrogen atoms
(except the solvent water molecules which were not located) were
refined as idealised riding geometries and structural refinements
were obtained with full-matrix least-squares based on F2 by using
the program SHELXTL Version 6.10 (Bruker).
m), 3.41 (2H, CH2, m), 3.26 (1H, CH2, m), 3.09 (1H, CH2, m),
3.06 (2H, CH2, m), 2.82 (3H, CH3, s), 2.59 (3H, CH3, s), 1.78
(1H, CH2, m), 1.52 (4H, 2xH2O, m). 13C{ H} NMR (CDCl3): d
1
211.12 (COCH3, dt, 1JRh–C 26.54 Hz, 1JP–C 4.42 Hz), 135.30–128.29
(CH-Ar), 118.80 (C-Ar, d, 1JRh–C 17.14 Hz), 117.67 (C-Ar, d, 1JRh–C
17.14 Hz), 44.99 (COCH3), 29.08 (CH2, ddd, 1JP–C 33.17 Hz, 2JP–C
2
1
2
12.16 Hz, JRh–C 2.21 Hz), 27.61 (CH2, dd, JP–C 32.07 Hz, JP–C
1
2
7.74 Hz), 21.43 (CH2, dd, JP–C 25.98 Hz, JRh–C 2.21 Hz), 20.74
(CH2, d, 1JP–C 48.65 Hz), 8.98 (CH2, d, 1JP–C 54.73 Hz).
[Rh(I)2(C(O)13CH3){Ph2P(CH2)2P(Ph)(CH2)2(PPh213CH3)}]+I−
5c. Yield: 77% (55.6 mg, 0.059 mmol). IR (KBr), v(CO)/cm−1:
Crystal data†
[Rh(I)2(C(O)CH3){Ph2P(CH2)2P(Ph)(CH2)2PPh2CH3}]+I− 5a.
1
1
3
1709. 31P{ H} NMR (CDCl3): d 78.37 (d, JRh–P 143.8 Hz, JP–P
47.0 Hz), 69.52 (d, 1JRh–P 47.0 Hz), 25.26 (dd, 3JP–P 47.0 Hz, 1JP–C
54.6 Hz). 1H NMR (CDCl3): d 8.01–7.08 (25H, CH-Ar, m), 3.75
(1H, CH2, m), 3.39 (1H, CH2, m), 3.26 (2H, CH2, m), 3.07 (2H,
CH2, m), 2.88 (3H, 13CH3, d, 1JC-H 98.4 Hz), 2.85 (1H, CH2, m),
[C37H39I2O1P3Rh]I·2H2O,
M
=
1112.23, monoclinic, ◦
a
=
˚
18.916(3), b = 11.4403(17), c = 20.007(4) A, b = 115.263(8) , V =
3
˚
3915.4(12) A , T = 93(2) K, space group P2(1)/n, Z = 4, 22382
reflections measured, 6829 unique of which 5151 were observed
2.54 (3H, CH3, d, JC-H 97.9 Hz), 1.86 (1H, CH2, m). 13C NMR
[I > 2rI] (R1 = 0.0840, wR2 = 0.1949, Rint = 0.0886).
1
(CDCl3): d 44.97 (CO13CH3, s), 9.04 (13COCH3, d, 1JP–C 54.6 Hz).
[RhI(CH3)(TRIPHOS)CO]+I− 6a20. By modification of a pro-
cedure given in ref. 20 CH3I (1 mL, 16.06 mmol) was added to a
solution of [Rh(TRIPHOS)(CO)2]Cl prepared from [Rh(CO)2](l-
Cl)2] (30 mg, 0.077 mmol) and TRIPHOS ligand (82.5 mg,
0.154 mmol) in 5 mL of dry degassed methanol. The solution
was then stirred for 24 h to obtain a bright yellow solution. The
solvent was removed under vacuum and the product isolated in
good yield.
Acknowledgements
We would like to thank the EPSRC and BP for an industrial CASE
studentship, EPSRC National Mass Spectroscopy Service as well
as Melanya Smith and Thomas Lebl from the NMR services
department at the University of St. Andrews. The authors would
also like to thank Dr Dave Law and Dr Glenn Sunley for useful
discussions.
Yield: 91% (113 mg, 0.140 mmol). IR (KBr), v(CO)/cm−1: 2075.
LSIMS (ES) Found 779.06 [M − CO]+; C35H36IP3Rh requires
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12 S. Burger, B. Therrien and G. Suss-Fink, Helv. Chim. Acta, 2005, 88,
478–486.
13 Z. Freixa, P. C. J. Kamer, M. Lutz, A. L. Spek and P. W. N. M. van
Leeuwen, Angew. Chem., Int. Ed., 2005, 44, 4385–4388.
14 C. Jimenez-Rodriguez, P. J. Pogorzelec, G. R. Eastham, A. M. Z.
Slawin and D. J. Cole-Hamilton, Dalton Trans., 2007, 4160–
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