Reaction of a rhodium(I) carbonyl complex of a para-dimethylaminophenyl substituted diphosphine with methyl iodide and hydrogen iodide

Article abstract of DOI:10.1016/j.ica.2009.05.064

Reaction of [Rh(CO)₂](μ-Cl)]₂ with bis-1,2-(di{4-dimethylaminophenyl)phosphino-ethane (L) gives the monomeric Rh(I) complex of type cis-[RhCl(L)(CO)] that was separated from a side product of type [Rh(L)₂]Cl, and character

Full text of DOI:10.1016/j.ica.2009.05.064

Inorganica Chimica Acta 362 (2009) 4263–4267
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Note
Reaction of a rhodium(I) carbonyl complex of a para-dimethylaminophenyl
substituted diphosphine with methyl iodide and hydrogen iodide
Gareth W. Lamb a, Dave Law b, Alexandra M.Z. Slawin a, Matthew L. Clarke a,
*
a School of Chemistry, University of St. Andrews, EaStCHEM, St. Andrews, Fife KY16 9ST, UK
b BP Chemicals Ltd., Saltend, Hull, HU12 8DS, UK
a r t i c l e i n f o
a b s t r a c t
Article history:
Reaction of [Rh(CO)2](l-Cl)]2 with bis-1,2-(di{4-dimethylaminophenyl)phosphino-ethane (L) gives the
Received 20 February 2009
Received in revised form 22 May 2009
Accepted 28 May 2009
monomeric Rh(I) complex of type cis-[RhCl(L)(CO)] that was separated from a side product of type
[Rh(L)2]Cl, and characterised by X-ray crystallography. This complex reacts with methyl iodide at high
temperature to give the Rh(III) acetyl complex, [Rh(I)2(C(O)Me)(L)], which was also structurally charac-
terised by X-ray crystallography. There is no sign of quaternisation of the dimethylamino groups under
these conditions. This complex is soluble in organic solvent and insoluble in the polar media used in
methanol carbonylation (AcOH/H2O/MeOH). However, in the presence of HI, this complex is readily sol-
uble in AcOH/H2O/MeOH, in contrast to [Rh(I)2(C(O)Me)(dppe)] and most other Rh-acetyl complexes of
diphosphine ligands.
Available online 6 June 2009
Keywords:
Rhodium
Methanol carbonylation
Oxidative addition
Water-soluble ligands
Phosphines
Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction
cated synthetic routes that may prove difficult or too expensive for
the production of large quantities of a diphosphine ligand, but
The control of the solubility properties of transition metal cata-
lysts is of critical importance to developing homogeneous catalysts
with well-defined behaviour and in the separation and recycling of
homogeneous catalysts. There is an extensive body of work on the
synthesis of water-soluble transition metal complexes, generally
using phosphine ligands modified with polar substituents [1]. In
the case of the carbonylation of methanol, reactions are generally
carried out in mixtures containing methanol, acetic acid, water,
hydrogen iodide and/or methyl iodide. In the course of studies at
BP aimed at developing improved Rh–phosphine catalysts for
methanol carbonylation [2], it became necessary to investigate
the chemical stability of Rh–phosphine catalysts since these gener-
ally degrade to various phosphorus compounds and [Rh(CO)2I2]À
under catalytic conditions [3,4]. In some cases, the measurement
of the stability of such complexes was complicated by the poor sol-
ubility of Rh–phosphine complexes in the polar media. From an
application perspective, low solubility would also significantly
hamper the recycle of catalysts during the distillation of the acetic
acid product. It therefore seemed of significant interest to investi-
gate the Rh chemistry of some more polar phosphine ligands with
a view to establishing some design principles for soluble, stable,
and productive Rh/phosphine-based catalysts for methanol car-
bonylation. Many water-soluble ligands are made by quite compli-
amine-containing phosphines are generally readily synthesised
and can potentially be quaternised to give water-soluble ammo-
nium salts [5,6]. Previous studies by Toth and co-workers have
shown that Rh complexes of chiral diphosphines containing a ter-
tiary amine group will react with HBF4 or Me3OBF4 to quaternise
the amine substituent and impart water solubility [6]. Methanol
carbonylation uses both acidic and methyl iodide promoters any-
way, hence a particularly efficient solution to improving solubility
in acetic acid/water/methanol would be the use of a rhodium com-
plex of an amine-containing phosphine that might be quaternised
under the methanol carbonylation reaction conditions. In this note,
we report the synthesis of a rhodium(I) complex of a bis-1,2-(di{4-
dimethylaminophenyl)phosphino-ethane and its reactions with
methyl iodide in the presence and absence of acids. The crystal
structures of two of the complexes produced are also described.
2. Experimental
2.1. General
All manipulations were performed using standard Schlenk line
techniques under nitrogen atmosphere unless stated otherwise.
Commercially available starting materials and solvents were ob-
tained from the Aldrich Chemical Company or Strem Chemicals.
Microanalyses were carried out by the University of St. Andrews
microanalytical service. NMR spectra were recorded on Bruker
* Corresponding author. Tel.: +44 (0)1334 463850; fax: +44 (0)1334 463808.
E-mail address: mc28@st-andrews.ac.uk (M.L. Clarke).
0020-1693/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2009.05.064

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G.W. Lamb et al. / Inorganica Chimica Acta 362 (2009) 4263–4267
Avance 300 instruments operating at 300 MHz for 1H, 121.4 MHz
for 31P NMR. Chemical shifts are reported in ppm with 1H 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 sol-
vent for a particular spectrum is given in the parentheses. 13C and
31P NMR spectra were recorded with broad-band proton decou-
pling. IR spectra were recorded using a Perkin Elmer Spectrum
GX FT-IR system. All solids were analysed as KBr disks. Mass spec-
troscopy data was obtained from the EPSRC National Mass Spec-
troscopy Service Centre, Swansea. In addition to detecting the
ions shown, all Rh complexes showed excellent agreement be-
tween calculated and expected isotope patterns. [RhCl(CO)(dppe)]
was prepared according to the literature [7].
The combined THF solutions were dried over anhydrous sodium
sulfate, reduced to 50 cm3 and the crude product precipitated with
methanol (80 cm3). Additional product was obtained by the further
addition of methanol to the solution.
Yield: 32% (824 mg, 1.443 mmol). HRMS (ES) Found 571.3115
[MH]+; C34H44N4P2 requires 571.3114 [MH]+. 31P{1H} NMR
(CD2Cl2): d À17.68 (s). 1H{31P} NMR (CDCl3): d 7.10 (8H, CH–Ar,
m), 6.55 (8H, CH–Ar, m), 2.82 (24H, NCH3, s), 1.84 (4H, CH2, s).
1
13C{1H} NMR (CD2Cl2): d 151.12 (CN, s), 134.00 (CH–Ar, d, JP-C
1
1
9.95 Hz), 133.87 (CH–Ar, d, JP-C 9.95 Hz), 124.92 (C-Ar, d, JP-C
1
1
4.98 Hz), 112.61 (CH–Ar, d, JP-C 3.32 Hz), 112.56 (CH–Ar, d, JP-C
1
3.32 Hz), 47.47 (CH3, s), 31.53 (CH2-Ar, d, JP-C 3.32 Hz).
2.4. Synthesis of [Rh(dppe-NMe2)CO(Cl)], 2
[Rh(CO)2(l
-Cl)]2 (30 mg, 0.077 mmol) was dissolved in 5 cm3 of
dry, degassed THF. The ligand (88 mg, 0.154 mmol) was also dis-
2.2. X-ray crystallography
solved into 25 cm3 of dry, degassed THF and was added dropwise
to the [Rh(CO)2(l-Cl)]2 and stirred for 4 h. After this time a bright
X-ray crystallography data were collected at 93 K by using a
Rigaku MM007 High brilliance RA generator and a Saturn CCD sys-
yellow ppt had formed and was filtered under argon then washed
with ice-cold pentane.
tem using Mo K
a
radiation. Intensities were corrected for Lorentz-
Yield: 84% (95.2 mg, 0.129 mmol). IR (KBr), v(CO)/cmÀ1: 1987.
polarisation and for absorption. The structures were solved by di-
rect methods. In (2) the Cl and CO were disordered over both posi-
tions. The CO was refined isotropically with fixed thermal
parameters and distance constraints. Two waters in six locations
were refined isotropically. All other non-hydrogen atoms were re-
fined anisotropically. In (4) there is a disordered quarterweight
CH2Cl2 molecule refined isotropically subject to distance con-
straints. The COMe group also shows signs of disorder. All hydro-
gen atoms (except those on 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). The following crystal structures have been deposited at
the Cambridge Crystallographic Data Centre and allocated the
deposition numbers CCDC 720965 and 720966 (see Table 1).
LSIMS
(FAB).
Found
708.1
[M-CO]+,
701.2
[M-Cl]+;
C35H44ClN4OP2Rh requires 708.2 [M-CO]+, 701.2 [M-Cl]+. HRMS
(FAB). Found 708.1779 [M-CO]+; C34H44ClN4P2Rh requires
708.1771. 31P{1H} NMR (CD2Cl2): d 67.15 (dd, JRh–P 157.4 Hz, JP-
1
2
35.6 Hz), 45.69 (dd, JRh–P 124.7 Hz, JP-P 35.6 Hz). 1H{31P} NMR
1
2
P
(CD2Cl2): d 7.49 (8H, CH–Ar, m), 6.61 (8H, CH–Ar, m), 2.89 (12H,
NCH3, s), 2.88 (12H, NCH3, s), 2.21 (2H, CH2, m), 1.93 (2H, CH2,
1
m). 13C{1H} NMR (CDCl3): d 191.38 (CO, dd (app.), JRh-C 102.8 Hz,
1
2
2JP-C 6.6 Hz), 152.44 (CN, s), 134.90 (CH–Ar, dd, JP-C 12.7 Hz, JRh-
1
2
1.11 Hz), 134.59 (CH–Ar, dd, JP-C 12.7 Hz, JRh-C 1.11 Hz),
119.73 (C-Ar, d, JP-C 57.50 Hz), 117.66 (C-Ar, d, JP-C 47.55 Hz),
112.41 (CH, d, JP-C 17.1 Hz), 112.27 (CH, d, JP-C 17.7 Hz), 40.65
(CH3, s), 31.53 (CH2, m), 26.47 (CH2, m).
C
1
1
1
1
2.5. Synthesis of [Rh(dppe-NMe2)(COMe)(I)2], 4
2.3. Synthesis of ligand dppe-NMe2, 1
About 1.5 cm3 methyl acetate and 0.5 cm3 methyl iodide were
added to
a
5.0 cm3 BiotageTM microwave vial containing
Adapted from the literature [8]. A solution of n-butyl-lithium
(115 cm3, 1.18 mol dmÀ3, 0.136 mol) in hexane was added drop-
wise over 90 min to a stirred solution of p-Me2NC6H4Br (27.1 g,
0.136 mol) in diethyl ether (400 cmÀ1) at RT. The mixture was then
[Rh(dppe-NMe2)CO(Cl)] (15 mg, 0.013 mmol). The reaction mix-
ture was then heated to 140 °C for 10 min. High quality X-ray crys-
tals were then formed by the slow evaporation of this solution.
Yield: 83% (16.4 mg, 0.0169 mmol). IR (KBr), v(CO)/cmÀ1: 1702.
LSIMS (ES). Found 969.9 [M]+; C36H47I2N4OP2Rh requires 970.0
[M]+. HRMS (FAB). Found 843.1324 [M-I]+; C36H47IN4OP2Rh re-
quires 843.1324. Anal. Calc. for C36H47I2N4OP2Rh: C, 44.56; H,
4.88; N, 5.77. Found: C, 44.61; H, 4.98; N, 5.34%. 31P{1H} NMR
stirred for
a
further 15 min and cooled to À70 °C before
Cl2PCH2CH2PCl2 (5.0 cm3, 0.031 mol) in diethyl ether was added
dropwise over 90 min (always cooler than À30 °C). The mixture
was then allowed to warm to RT and stored for 12 h. The reaction
mixture containing a white ppt was quenched with water (40 cm3)
and a yellow solid filtered off from both phases. This was redis-
solved in THF (400 cm3) and combined with a solution obtained
by dissolving the remaining solid in the reaction vessel in THF.
1
(CD2Cl2): d 69.04 (d, JRh–P 139.6 Hz). 1H{31P} NMR (CD2Cl2):d
1
1
7.54 (4H, CH–Ar, d, JP-H 8.96 Hz), 7.13 (4H, CH–Ar, d, JP-H
1
8.96 Hz), 6.61 (4H, CH–Ar, d, JP-H 8.96 Hz), 6.50 (4H, CH–Ar, d,
1JP-H 8.96 Hz), 2.92 (12H, NCH3, s), 2.88 (12H, NCH3, s), 2.66 (3H,
COCH3, s), 1.98 (1H, CH2, m), 1.56 (1H, CH2, m), 1.29 (2H, CH2, m).
Table 1
Crystallographic data for complexes 2 and 4.
3. Results and discussion
2
4
Formula
T (K)
Wavelength (Å)
Crystal system
Space group
a (Å)
b (Å)
c (Å)
b (°)
C35H44ClN4O1P2RhÁ2H2O
93(2)
0.71073
monoclinic
C2
25.6933(18)
8.1263(6)
21.3538(15)
98.610(4)
4
C36.H47I2N4OP2RhÁ0.25CH2Cl2
The main purpose of this work was to conclusively establish if
the introduction of para-dimethylaminophenyl substituents into
phosphine ligands would be a suitable approach to the in situ qua-
ternisation of the ligand, making it soluble in the very polar media
encountered in methanol carbonylation. Since most Rh–phosphine
complexes are currently rapidly decomposed under the conditions
of methanol carbonylation, we choose to use the dppe analogue,
93(2)
0.71073
monoclinic
C2/c
34.391(3)
10.3769(7)
26.060(2)
112.234(4)
8
Z
bis-1,2-(di{4-dimethylaminophenyl)phosphino-ethane
1
since
R1 (F2)
0.0904
0.0888
dppe ligands give unusually stable Rh complexes, even if they

G.W. Lamb et al. / Inorganica Chimica Acta 362 (2009) 4263–4267
4265
are not especially good ligands with regard to catalytic productiv-
ity [9]. The reaction of ligand 1 with [Rh(coe)2(-Cl)]2 did not give
any of the desired chloride bridged species, [Rh(1)(-Cl)]2 due to
the formation of what is ascribed as a cationic bis-chelate complex
of type [Rh(L)2]Cl {dP = 56, JP-Rh = 133 Hz, FABMS = 1243.8 (M+)}.
The reaction of ligand 1 with [Rh(CO)2](l-Cl)]2 was also compli-
cated by the formation of a bis-chelate complex under some condi-
tions, revealing the strong co-ordinating strength of this ligand.
However, reaction of [Rh(CO)2(l-Cl)]2 and ligand 1 in cold THF, fol-
[7,11] but the most important conclusion is that there is no sign
of quaternisation of the dimethylamino groups, and hence complex
4 has similar solubility to the dppe analogue (soluble in organic
solvents such as CH2Cl2, or CHCl3 and insoluble in water). Most
Rh–phosphine complexes decompose at least partially under the
reaction conditions used to produce 4, and even a Rh complex of
triphos has been shown to quaternise one of the phosphine groups
under the conditions used for the synthesis of 4 [4b]. However, the
rigid dppe structure imparts high chemical stability and clearly
even very hot methyl iodide is not sufficiently electrophilic to qua-
ternise the amine substituents (see Fig. 2 and Scheme 2).
The reaction of the Rh(I) complex 2 with HBF4 gave a mixture of
two main species as revealed by a broadened 31P{1H} NMR spec-
trum from which no real structural information could be gained.
In any case, treatment of this mixture of Rh(I) complexes with car-
bon monoxide (5 bar) and methyl iodide at 120 °C results in oxida-
tive addition and migratory insertion to give a single species
detected by 31P NMR and IR spectroscopy that has very similar
chemical shift, coupling constants and carbonyl stretching bands
as complex 4 but was highly soluble in water/methanol. This spe-
cies could not be isolated in what is proposed to be a protonated
form of complex 4 due to its solubility in this polar media, but
the data obtained is sufficient to show that a single stable species
that is soluble in polar media can form under these strongly acidic
conditions and that this has spectral properties that are very sim-
ilar to acetyl complex 4. In addition, if an autoclave is charged with
complex 2, HBF4 and MeI in MeOH/H2O and heated at 120 °C at
5 bar CO pressure, CO uptake is observed and the only species pres-
ent at the end of 1 h of reaction is the water/methanol soluble com-
plex that displays similar spectroscopic data as acetyl 4 as above.
The organic products were not analysed, since we did not envisage
or design this simple dppe analogue to be of interest as a catalyst
(since this backbone is not suited to methanol carbonylation).
However, this experiment does suggest that a single species that
is soluble in polar media can form and remain stable under cata-
lytic conditions, whether the complex observed at the end of the
reaction is a resting state or actual intermediate in the catalyst cy-
cle is not known. A separate experiment in which the pure com-
1
lowed by washing of the yellow precipitate formed with cold THF
and cold pentane allows the desired complex 2 to be prepared in
high yield and purity. Measuring the position of the CO stretch
on Rh(Cl)(CO)Ln complexes is frequently used as a measure of li-
gand donor strength for monodentate phosphines [10]. Unfortu-
nately, the data set for cis-[RhCl(CO)(P^P)] complexes is rather
small, since many ligand adopt bridging bidentate co-ordination
modes. Nonetheless, the infrared spectrum shows a CO stretch
for the carbonyl group at 1987 cmÀ1 which is very significantly
shifted downfield relative to all other similar complexes (e.g.
2006 cmÀ1 observed in the IR spectrum of known [Rh(dppe)CO(Cl)]
[7] that was prepared for this work). This significant difference re-
flects that ligand 1 is highly electron donating, and can therefore be
expected to be a very strong chelate ligand (see Scheme 1).
Crystals suitable for X-ray diffraction were grown from THF. A
representation of the structure is shown in Fig. 1. Examples of X-
ray structure determinations of monomeric rhodium carbonyl
chloride diphosphine complexes are scarce in the literature and
the structure of the analogous Rh complex of the unsubstituted
dppe analogue has not been reported preventing the most relevant
comparison. The complex shows some minor deviations from idea-
lised square planar geometry most likely associated with the small
bite angle of the dppe type ligand.
When complex 2 is reacted with an excess of methyl iodide at
140 °C for 10 min, the formation of a new Rh acetyl complex is ob-
served as the sole product. Slow evaporation of this solution under
a stream of nitrogen results in the formation of crystals suitable for
X-ray diffraction studies. The spectroscopic data for this complex
are very similar to the analogous complex [Rh(I)2(COMe)(dppe)]
N
N
N
N
Rh2(CO)4Cl2
P
P
P
P
Rh
THF
OC Cl
84 %
N
N
N
N
1
2
[Rh(CO)2 (µ−Cl)]2
MeOH
+
N
N
N
N
P
P
Ar
Ar
Ar
P
P
Rh
+
Rh
Ar
P
OC Cl
P
N
N
N
N
2
Product ratio = 1:4
3 Ar = C6H4-p-NMe2
Scheme 1. Synthesis of Rh(I) carbonyl complex of bis-1,2-(di{4-dimethylaminophenyl)phosphino-ethane.

4266
G.W. Lamb et al. / Inorganica Chimica Acta 362 (2009) 4263–4267
Fig. 1. X-ray structure analysis of [Rh(dppe-(NMe2)4)CO(Cl)], 2. Selected bond lengths (Å): Rh–P(1) = 2.297(3), Rh–P(2) = 2.317(3), Rh–C = 1.866(4), C–O = 1.114(5), Rh(1)–I(1)
= 2.7407(12), Rh(1)–I(2) = 2.7385(10). Selected angles (°) C–Rh–Cl = 101.0(13) P(1)–Rh–P(2) = 84.74(15).
Fig. 2. X-ray structure analysis of [Rh(dppe-(NMe2)4)COMe(I)2], 4. Selected bond lengths (Å): Rh–P(1) = 2.280(3), Rh–P(2) = 2.309(3), C–O = 1.206(8). Selected angles (°) I(1)–
Rh–I(2) = 92.13(3), P(1)–Rh–P(2) = 85.75(10).
plexes 4 and [RhI2(C(O)Me)(dppe)], 5 were suspended in H2O/
N
N
AcOH/MeOH (45:40:15) showed neither complex to be soluble.
On the other hand, these complexes were readily dissolved in
CH2Cl2. However, when a sample of complex 4 was treated with
50 equivalents of HI in the same H2O/AcOH/MeOH solvent mixture,
86% of complex 4 dissolved (as measured against an external stan-
dard of OP(n-Bu)3 dissolved in C6D6 in a glass capillary tube that
was previously calibrated against the saturated solution of the rel-
evant unprotonated Rh complexes in CH2Cl2). When the same pro-
cedure was applied to [RhI2(C(O)Me)(dppe)], none of the complex
dissolved. Similar results were obtained when HBF4 were used as
acid. This therefore suggests that although Rh complexes of ligand
1 are not quaternised by methyl iodide, the very strong acid HI
does change the solubility of the complexes such that they are sol-
uble in very polar media. Protonation of the dimethylamino group
2
2
COMe
P
P
excess MeI
P
P
Cl
I
Rh
Rh
CO
I
140 ºC, 10 mins
93 %
2
N
2
N
2
4
Scheme 2. Synthesis of a Rh(III)-acetyl complex from complex 2.

G.W. Lamb et al. / Inorganica Chimica Acta 362 (2009) 4263–4267
4267
Table 2
Differences in the solubility of complex 4 and 5.
Solvent
Additive
Solubility of complex 5a
Solubility of complex 4a
CH2Cl2
None
None
HBF4(50 eq.)
HI (50 eq.)
7.7 Â 10À3
<9 Â 10À7
<9 Â 10À7
<9 Â 10À7
7.7 Â 10À3
<9 Â 10À7
5.8 Â 10À3
6.6 Â 10À3
H2O/AcOH/MeOH (45:40:15)
H2O/AcOH/MeOH (45:40:15)
H2O/AcOH/MeOH (45:40:15)
The estimate of solubility is calculated against an external standard of P(O)n-Bu3 dissolved in C6D6 in a glass capillary tube using 31P{1H} NMR spectroscopy in mol dmÀ3
.
a
7.7 mol dmÀ3 is derived from the amount of complexes 4 and 5 that can be readily dissolved in CH2Cl2 at room temperature. <9 Â 10À7 mol dmÀ3 refers to the estimated lower
limit of detection by this method, and is therefore ꢀ0% solubility.
seems almost certainly the origin of this solubility change (see Ta-
ble 2).
free of charge from The Cambridge Crystallographic Data Centre
via www.ccdc.cam.ac.uk/data_request/cif.
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.ica.2009.05.064.
4. Conclusions
References
A Rh(I) complex of ligand 1 of type [RhCl(L)CO] has been prepared
andcharacterised;thepositionof COstretchintheIRsystemis lower
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being a very strongly electron-donating ligand. This complex has
been shown to react with methyl iodide to give a Rh(III) acetyl com-
plex that was fully characterised, with no evidence for quaternisa-
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complex changes its solubility properties significantly in the pres-
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Acknowledgements
We would like to thank the EPSRC and BP for an industrial CASE
studentship, the EPSRC National Mass Spectroscopy Service, Mela-
nya Smith and Thomas Lebl from the NMR services department at
the University of St. Andrews. The authors would also like to thank
Dr Bruce Williams and Dr. Glenn Sunley for useful discussions.
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Appendix A. Supplementary material
(b) K.G. Moloy, J.L. Petersen, Organometallics 14 (1995) 2931.
CCDC 720965 and 720966 contain the supplementary crystallo-
graphic data for complexes 2 and 4. These data can be obtained