Lewis acids and lewis acid-functionalized ligands in rhodium-catalyzed methyl acetate carbonylation

Article abstract of DOI:10.1021/om200341t

The application of Lewis acids and Lewis acid-functionalized ligands as activators for methyl acetate in the rhodium-catalyzed carbonylation of methyl acetate to acetic anhydride has been investigated. The reaction of methyl acetate with B(C₆F₅)₃ results in the formation of the adduct [MeOAc·B(C₆F₅)₃]. The combination of this adduct with [Rh(OAc)(CO)₂]₂ results in a transfer of the Lewis acid to the rhodium complex, rather than an anticipated oxidative addition reaction. In a second approach, novel Lewis acid-functionalized rhodium(I) complexes [Rh(CO)Cl(BPP)], [Rh(CO) ₂(BPP)]SbF₆, and [Rh(MeCN)₂(BPP)]SbF ₆ (BPP = PhB(C₆H₄PPh₂)₂) have been prepared. The lack of reactivity of [Rh(CO)₂(BPP)]SbF ₆ toward MeOAc has shown that the rhodium-boron interaction is too strong for activation of methyl acetate, and no carbonylation activity was observed under the conditions used.

Full text of DOI:10.1021/om200341t

ARTICLE
pubs.acs.org/Organometallics
Lewis Acids and Lewis Acid-Functionalized Ligands in
Rhodium-Catalyzed Methyl Acetate Carbonylation
Christopher M. Conifer,† David J. Law,‡ Glenn J. Sunley,‡ Andrew J. P. White,† and George J. P. Britovsek*,†
†Department of Chemistry, Imperial College London, Exhibition Road, London, SW7 2AY, U.K.
‡Hull Research and Technology Centre, BP Chemicals Ltd., Saltend, Hull, HU12 8DS, U.K.
S Supporting Information
b
ABSTRACT: The application of Lewis acids and Lewis acid-
functionalized ligands as activators for methyl acetate in the
rhodium-catalyzed carbonylation of methyl acetate to acetic
anhydride has been investigated. The reaction of methyl acetate
with B(C6F5)3 results in the formation of the adduct [MeOAc
B(C6F5)3]. The combination of this adduct with [Rh(OAc)-
3
(CO)2]2 results in a transfer of the Lewis acid to the rhodium
complex, rather than an anticipated oxidative addition reaction.
In a second approach, novel Lewis acid-functionalized rhodium-
(I) complexes [Rh(CO)Cl(BPP)], [Rh(CO)2(BPP)]SbF6, and [Rh(MeCN)2(BPP)]SbF6 (BPP = PhB(C6H4PPh2)2) have been
prepared. The lack of reactivity of [Rh(CO)2(BPP)]SbF6 toward MeOAc has shown that the rhodiumÀboron interaction is too
strong for activation of methyl acetate, and no carbonylation activity was observed under the conditions used.
’ INTRODUCTION
for the activation of methanol have been investigated, for example
the application of solid acids, such as zeolites,5À8 and hetero-
polyacids9 or superacids.10 These strong acids can protonate
The carbonylation of methanol and methyl acetate are
currently the most important industrial processes for the
large-scale production of acetic acid and acetic anhydride.1À3
Annual production capacity is seven million tonnes world-
wide and continues to grow, especially in Asia.4 The catalyst
system for these carbonylation reactions typically comprises a
rhodium or iridium source and an iodide cocatalyst. Acetic
acid is used as the solvent and, under the reaction conditions,
methanol is rapidly converted to methyl acetate, which there-
fore constitutes the actual substrate. Because OAcÀ forms a
better leaving group than OHÀ in the pivotal CÀO bond
cleavage reaction, methyl acetate reacts initially with iodide to
form methyl iodide, which is subsequently carbonylated and
eventually hydrolyzed to acetic acid, according to the overall
reaction in eq 1. In the absence of water, methyl acetate is
carbonylated to acetic anhydride.
methanol, which upon elimination of H2O generates a carbenium
+ 8,10
cation, CH3 .
Although these strong acids can carbonylate
methanol without the presence of iodide, the selectivities are
generally lower due to the formation of significant amounts of
dimethyl ether.9,11,12
An alternative approach would be to use Lewis acids for the
activation of methyl acetate. Lewis acids are known to be able
to activate CÀO bonds in a variety of organic and inorganic
transformations as well as in catalysis.13À19 For example, in
carbocationic olefin polymerization, BCl3 is used to activate
alkylacetate esters to generate [BCl3(OAc)]À and a carbe-
nium cation, which initiates the polymerization reaction.20 It
is also known that esters, for example, ethyl acetate, bind
reversibly via the carbonyl oxygen atom to Lewis acids such as
B(C6F5)3 to form an adduct, [EtOAc B(C6F5)3].21 These
3
findings suggest that Lewis acids might be applicable in place
of Brønsted acids for the CÀO bond activation and carbo-
nylation of methyl acetate, according to the reaction scheme
depicted in Scheme 1. The anion [B(C6F5)3(OAc)]À has
been reported previously.22
Here we have investigated the application of Lewis acids and
Lewis-acid-functionalized ligands in the catalytic carbonylation
of methyl acetate. The first part of this study describes our
investigations into the application of external Lewis acids such as
The high temperatures and strongly acidic reaction conditions
required for carbonylation reactions, combined with the use of
iodide as the cocatalyst, present significant challenges in terms of
reactor engineering and corrosion prevention. Alternative cata-
lysts that can be used under milder reaction conditions and avoid
the use of iodide could significantly lower the cost of acetic acid
and acetic anhydride production. Several alternative strategies
Received: April 21, 2011
Published: July 18, 2011
r
2011 American Chemical Society
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Scheme 1
’ RESULTS AND DISCUSSION
Synthesis of Ligands and Complexes. B(C6F5)3 was reacted
with an excess of methyl acetate in pentane to yield the
novel adduct [MeOAc B(C6F5)3], which was characterized by
3
multinuclear NMR spectroscopy, elemental analysis, and IR
spectroscopy. The ν(CdO) and ν(BÀO) bands are observed
at 1649 and 1469 cmÀ1, which are very similar to the values
reported for [EtOAc B(C6F5)3].21
3
The ligand PhB(C6H4PPh2)2 (BPP) was previously de-
scribed by Bourissou and co-workers.33 We used a
slightly different procedure for the synthesis of BPP. A
palladium coupling reaction between 1,2-iodobromobenzene
and HPPh2 in toluene resulted in o-bromophenyl dipheny-
Scheme 2
n
lphosphine,34 which was lithiated with BuLi according to the
procedure described by Harder et al.35 Half an equivalent of
PhBCl2 was reacted with the lithium compound in toluene to
yield the diphosphanyl borane ligand BPP as a white solid in
52% yield.
[RhCl(CO)2]2 was reacted with two equivalents of BPP in
dichloromethane to obtain [Rh(CO)Cl(BPP)] as a yellow solid
in 65% yield. Elemental analysis and LSIMS mass spectrometry
confirm the composition of [Rh(CO)Cl(BPP)], but 31P NMR
spectroscopy (Figure 1) shows that three separate isomers
exist in solution, which are assigned as cis, trans-A, and trans-B
(see eq 2). The 31P NMR spectrum displays two doublets
centered at 28.4 and 33.8 ppm with 1JPÀRh coupling constants
of 110.1 and 106.3 Hz, which are tentatively assigned to the
trans-B and trans-A complexes, respectively. In the case of a
similar rhodium diphosphanyl borane complex, [Rh(DPB)-
(CO)Cl] (DPB = PhB(C6H4PiPr2)2), DFT calculations have
suggested that the major trans-isomer has the phenyl group
orientated above the carbonyl ligand (like trans-A).28 The cis-
isomer of [RhCl(CO)(BPP)] is observed in the 31P NMR
spectrum as two double doublets at 54.2 and 36.3 ppm, with
1JPÀRh coupling constants that equal 146.1 and 119.2 Hz,
respectively, and a 2JPÀP value of 36.4 Hz. At 298 K in CDCl3,
the cis-isomer of [RhCl(CO)(BPP)] accounts for 65% of all
the species in solution, trans-A 29%, and trans-B 6%. 31P
NMR spectra in CDCl3 at 223 and 323 K did not show any
changes, indicating that there is no equilibrium between these
isomers.
B(C6F5)3 in the activation and carbonylation of methyl acetate.
In the second part of this study, we have investigated the
application of metal complexes with a ligand that contains
an internal Lewis acid. The coordination chemistry of ligands
with Brønsted acidic or basic functionalities and their appli-
cation in homogeneous catalysis has been an ongoing theme
in our laboratory, and we have previously reported on the use
of such ligands in the context of alkane oxidation23À25 and for
methyl acetate carbonylation.26 Our attention was drawn to
the interesting transition metal complexes developed by
Bourissou and co-workers, which contain diphosphine ligands
i
of the type PhB(C6H4PR2)2 (R = Pr, Ph) with a Lewis
acidic boron center.27À29 These ambiphilic phosphonyl
borane ligands coordinate to the metal center through the
phosphine donors and the Lewis acidic boron center. Metal-
boron interactions of this type are relatively new and are
attracting considerable attention.30 Of particular interest to this
study are the observations by Parkin and co-workers that certain
nickel and iron complexes, containing ligands with a dative
covalent metalÀboron interaction, can activate carbonÀhalide
bonds to yield complexes with halide anions bound to the boron
center.31,32 We were intrigued whether this activation strategy
could also be applied to methyl acetate activation and carbo-
nylation, as depicted schematically in Scheme 2.
Here we present the synthesis of the adduct [MeOAc
3
B(C6F5)3] and its reactivity toward [Rh(OAc)(CO)2]2 in
an attempt to activate methyl acetate using the external
Lewis acid B(C6F5)3. In addition, the synthesis and charac-
terization of new rhodium(I) complexes containing the
diphosphanyl borane ligand PhB(C6H4PPh2)2 is reported.
These complexes contain an internal Lewis acid functiona-
lity, and an investigation into their application in CÀO bond
activation and carbonylation catalysis has been carried out.
The formation of a boronÀMeOAc adduct is a common
feature in both strategies, in an attempt to achieve the
essential OÀMe oxidative addition reaction at the rhodium
center.
One equivalent of AgSbF6 was reacted with [RhCl-
(CO)(BPP)] in chloroform under a CO atmosphere to yield
[Rh(CO)2(BPP)]SbF6, which was isolated as a yellow solid
in 61% yield (eq 3). Only one isomer is observed in the
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ARTICLE
Figure 1. 31P NMR spectrum of [RhCl(CO)(BPP)] in CDCl3 at 298 K.
1
31P NMR spectrum. A doublet at 35.5 ppm with a JPÀRh
coupling constant of 119.3 Hz indicates that the phosphine
donors are in cis-conformation. Two ν(CO) bands in the IR
spectrum at 2123 and 2096 cmÀ1 confirm a dicarbonyl com-
plex, as does the double doublet in the 13C NMR spectrum at
178.7 ppm. The 1JCÀRh and 2JCÀP coupling constants are very
similar and cannot be distinguished. The parent ion peak
[M]+ is observed in the positive ESI mass spectrum at 769 m/z,
and the SbF6À counterion is identified in the negative ESI mass
spectrum at 235 m/z. A singlet is observed in the 11B NMR
spectrum at 0.45 ppm, shifted upfield from 29.6 ppm for
[RhCl(CO)(BPP)].
Figure 2. Molecular structure of [Rh(CO)2(BPP)]+. Selected bond
lengths (Å) and angles (deg): RhÀP(1), 2.3116(6), RhÀP(15),
2.3604(6), Rh B(8), 2.449(3), RhÀC(46), 1.958(3), RhÀC(47),
3 3 3
1.950(3), P(1)ÀRhÀB(8), 83.04(7), P(1)ÀRhÀP(15), 93.43(2),
B(8)ÀRhÀP(15), 78.64(6), C(46)ÀRhÀC(47), 90.18(11).
was repeated by stirring [Rh(CO)2(BPP)]SbF6 in nondeuter-
ated acetonitrile at room temperature for 16 h (eq 4), and the
product [Rh(MeCN)2(BPP)]SbF6 was fully characterized by
multinuclear NMR spectroscopy, X-ray diffraction analysis,
mass spectrometry, elemental analysis, and IR spectroscopy.
Dissolution of [Rh(MeCN)2(BPP)]SbF6 in CDCl3 allows
comparison of the NMR spectra with those of [Rh(CO)2-
(BPP)]SbF6. The phosphine donor signal in the 31P NMR
spectrum resonates at 54.3 ppm, with a 1JPÀRh of 155.5 Hz, which
is ∼19 ppm downfield relative to the signaÀl of [Rh(CO)2-
(BPP)]SbF6. The fluorine signal of SbF6 is present at
À123 ppm in the 19F NMR spectrum, and a broad singlet at
8.1 ppm is observed in the 11B NMR spectrum, slightly downfield
from the boron signal of [Rh(CO)2(BPP)]SbF6, which is at
À0.5 ppm.
An unusual spectroscopic feature of this complex is the
absence of any fluorine signals in the 19F NMR spectrum in
benzene, chloroform, and dichloromethane at room tempera-
ture. At this stage we can only speculate that given the fluor-
ophilicity of the boron center,36 this spectroscopic phenomenon
is caused by interactions of the SbF6À anion with the quadrupolar
boron center. The 19F NMR spectra at 233 and 323 K of
[Rh(CO)2(BPP)]SbF6 did not show any changes compared to
the spectrum at 298 K.
When [Rh(CO)2(BPP)]SbF6 was dissolved in d3-acetonitrile,
a reaction took place converting [Rh(CO)2(BPP)]SbF6 partially
(∼90%) to [Rh(d3-MeCN)2(BPP)]SbF6. For this complex the
SbF6À anion is observed in the 19F NMR spectrum. The reaction
Solid-State Structures. Single crystals suitable for X-ray
diffraction analysis were grown by slow diffusion of pentane
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ARTICLE
respectively. The shortening of the Rh B bond is particularly
3 3 3
interesting, as it suggests that the strength of the rho-
diumÀboron interaction can be tuned by modifying the metal-
bound ligands. A relatively weak Rh B interaction would be
3 3 3
desirable for substrate activation, as shown in Scheme 2.
Oxidative Addition Reactions. The oxidative addition of
methyl iodide at the rhodium(I) center of [RhI2(CO)2]À is
an essential and rate-determining step in the carbonylation
of methanol.1 Methyl acetate does not react directly with
rhodium(I) complexes such as [RhI2(CO)2]À, and therefore
iodide is required for the initial conversion of methyl acetate into
methyl iodide, which subsequently reacts with the rhodium
complex. In order to assess whether boranes such as B(C6F5)3
could be used to activate methyl acetate for the oxidative addition
at rhodium(I) complexes, we have investigated the reaction
between [MeOAc B(C6F5)3] and [Rh(OAc)(CO)2]2. This
3
particular rhodium complex was chosen instead of [RhI2-
(CO)2]À to avoid halide exchange reactions and the formation
of methyl iodide.
Figure 3. Molecular structure of [Rh(MeCN)2(BPP)]+. Selected
bond lengths (Å) and angles (deg): RhÀP(1), 2.2298(7), RhÀP(15),
The reaction of two equivalents of [MeOAc B(C6F5)3] with
3
2.2631(7), Rh B(8), 2.288(3), RhÀN(46), 2.104(3), RhÀN(49),
3 3 3
one equivalent of the dimeric complex [Rh(OAc)(CO)2]2 in
C6D6 was monitored by 1H NMR spectroscopy (see spectrum 3
in Figure 4). The two methyl signals at 1.07 and 3.02 ppm for
2.099(3), P(1)ÀRhÀB(8), 84.02(9), P(1)ÀRhÀP(15), 93.50(3), B(8)À
RhÀP(15), 80.36(9), N(46)ÀRhÀN(49), 87.07(11).
[MeOAc B(C6F5)3] shift to 1.61 and 3.28 ppm, which are
3
into dichloromethane solutions of [Rh(CO)2(BPP)]SbF6 and
[Rh(MeCN)2(BPP)]SbF6. The structures of [Rh(CO)2-
(BPP)]SbF6 and [Rh(MeCN)2(BPP)]SbF6 with selected bond
lengths and angles are shown in Figures 2 and 3, respectively.
For comparison, the rhodium diphosphanyl borane complex
trans-[RhCl(CO)(DPB)] ((DPB = PhB(C6H4PiPr2)2), which
was previously reported by Bourissou and co-workers, has RhÀP
bond lengths of 2.332(1) and 2.327(1) Å and a RhÀB bond
length of 2.374(3) Å.28
The rhodium center in both [Rh(CO)2(BPP)]SbF6 and
[Rh(MeCN)2(BPP)]SbF6 adopts a square-based pyramidal
coordination geometry with a fac conformation for the BPP
ligand (see Figures 2 and 3 respectively). This contrasts with the
structure of [RhCl(CO)(DPB)], which, while also having a
square-based pyramidal coordination geometry, has a mer con-
formation for the closely related DPB ligand.28 The boron center
occupies the apical site, but the potential Cs symmetry is broken
the signals for methyl acetate in C6D6. The methyl signal of
[Rh(OAc)(CO)2]2 is shifted from 1.68 ppm to 1.34 ppm.
1
Analysis of the 19F and H NMR spectroscopic data discounts
the formation of the elimination product [(AcO)B(C6F5)3]À,
which is a known anion,22 and no 1H NMR signals corresponding
to the products of a methyl acetate oxidative addition reaction are
observed. The reaction of [Rh(OAc)(CO)2]2 with two equiva-
lents of B(C6F5)3 results in a new complex with a single methyl
resonance at 1.34 ppm (see spectrum 5 in Figure 4). It appears
that the Lewis acid B(C6F5)3 binds more strongly to the rhodium
complex [Rh(OAc)(CO)2]2 than to methyl acetate, resulting
in a new complex, [Rh(OAc)(CO)2 B(C6F5)3], of an as yet
3
unknown structure.
B(C6F5)3 could form an adduct with [Rh(OAc)(CO)2]2
either by binding directly to the rhodium center or by binding
to the oxygen atoms of the carbonyl or the acetate ligands. The
coordination of Lewis acids to terminal CO ligands in metal
carbonyl complexes is well known,37À40 including examples of
B(C6F5)3 binding to the carbonyl ligands in iron(II) and
ruthenium(II) complexes.41 From the stoichiometry of the
reaction, it appears that two equivalents of B(C6F5)3 bind to a
single equivalent of [Rh(OAc)(CO)2]2 to form a new product.
in each case by a twist about the Rh B vector such that the
3 3 3
boron-bound phenyl ring sits above one of the carbonyl/
acetonitrile ligands; the associated CPhÀB(8)ÀRhÀC(47)/
N(49) dihedral angles are 11° and 12° in [Rh(CO)2(BPP)]-
SbF6 and [Rh(MeCN)2(BPP)]SbF6, respectively. This places
the centroid of the aryl ring approximately 3.20 and
3.22 Å from the centroid of the CtO and NtC triple bonds,
respectively. The RhÀP bond lengths are asymmetric, with
those to P(15) being longer than those to P(1) by 0.05 and
0.03 Å in [Rh(CO)2(BPP)]SbF6 and [Rh(MeCN)2(BPP)]SbF6,
respectively (see Figures 2 and 3, respectively). This asymmetry
is presumably associated with the twisting of the BPP ligand
discussed above; the longer RhÀP bonds are those trans to
the carbonyl/acetonitrile ligand proximal to the boron-bound
aryl ring.
Several attempts to isolate the product [Rh(OAc)(CO)2
B(C6F5)3] invariably resulted in decomposition.
3
Oxidative addition reactions of complex [Rh(CO)2(BPP)]-
SbF6 were carried out with methyl acetate and trifluoromethyl
acetate. The complex was dissolved in separate solutions of
trifluoromethyl acetate and methyl acetate in CDCl3 (4 M) and
1
heated to 70 °C. The reactions were monitored by H and
31P NMR spectroscopy over the course of seven days. No
rhodium-bound methyl or acetyl signals were observed. The
only methyl signals observed were those of trifluoromethyl
acetate and methyl acetate, suggesting that the rhodiumÀboron
bond was not cleaved to yield a boron methyl acetate or boron
trifluoromethyl acetate adduct as anticipated in Scheme 2.
The oxidative addition of methyl iodide to [Rh(CO)2(BPP)]-
SbF6 was also investigated. [Rh(CO)2(BPP)]SbF6 was dissolved
in a solution of methyl iodide in CDCl3 (4 M), and the reaction
Carbon monoxide is a stronger π-acceptor ligand than acet-
onitrile. Consequently, changing the CO ligands in [Rh(CO)2-
(BPP)]SbF6 for MeCN ligands (eq 4) results in an increase of
electron density at the metal center, which in turn results in
contractions of the RhÀP and Rh B distances by 0.08, 0.10,
3 3 3
and 0.21 Å for the RhÀP(1), RhÀP(15), and RhÀB(8) bonds,
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Figure 4. 1H NMR spectra monitoring a series of reactions between [Rh(OAc)(CO)2]2, B(C6F5)3, and [MeOAc B(C6F5)3] in C6D6 at 298 K. Spectrum
3
1: [Rh(OAc)(CO)2]2; spectrum 2: [MeOAc B(C6F5)3]; spectrum 3: [Rh(OAc)(CO)2]2 + 2 [MeOAc B(C6F5)3] f 2 [Rh(OAc)(CO)2 B(C6F5)3] + 2
3
3
3
MeOAc; spectrum 4: [Rh(OAc)(CO)2]2 + 2 [MeOAc B(C6F5)3] + 2 B(C6F5)3 f 2 [Rh(OAc)(CO)2 B(C6F5)3] + 2 [MeOAc B(C6F5)3]; spectrum 5:
[Rh(OAc)(CO)2]2 + 2 B(C6F5)3 f 2 [Rh(OAc)(CO)2 B(C6F5)3]. * = impurities: NaOAc in spectrum 1 and [B(C6F5)3 H2O] in spectrum 5.
3
3
3
3
3
1
was monitored by H and 31P NMR spectroscopy at room
temperature over seven days. The reaction resulted in multiple
unknown products, but no rhodium-bound methyl or acetyl
species were observed.
[Rh(CO)2(BPP)]SbF6 is too strong for cleavage to occur. An
interesting feature observed in the molecular structures of
[Rh(CO)2(BPP)]SbF6 and [Rh(MeCN)2(BPP)]SbF6 is a
shortening of the RhÀB bond upon substituting the carbonyl
ligands for acetonitrile ligands, which suggests that the rhodiumÀ
boron interaction can be “tuned” by ligand alterations. The
application of Lewis acid-functionalized complexes with weaker
metalÀboron interactions should be more effective in carbonyla-
tion catalysis, and these will be investigated next.
Carbonylation Experiments. A series of carbonylation ex-
periments were carried out under anhydrous conditions without
an iodide cocatalyst. A mixture of [MeOAc B(C6F5)3] and
3
[Rh(OAc)(CO)2]2 (10:1) was dissolved in dry methyl acetate
and transferred into a nitrogen-purged high-pressure reactor.
The reactor was charged with 40 bar of CO and heated for 16 h at
150 °C and at 200 °C. No methyl acetate carbonylation was
observed in any of these experiments. In another experiment,
[Rh(CO)2(BPP)]SbF6 was reacted in methyl acetate at 130 °C
under 40 bar of CO pressure for 16 h. No methyl acetate
carbonylation products could be detected.
’ EXPERIMENTAL SECTION
General Procedures. All moisture- and oxygen-sensitive com-
pounds were prepared using standard high vacuum line, Schlenk, and
cannula techniques. A standard nitrogen-filled glovebox was used for any
subsequent manipulation and storage of these compounds. Standard 1H,
19F, 31P, 11B, and 13C NMR spectra were recorded using a Bruker AV400
spectrometer. 1H NMR chemical shifts were referenced to the residual
nondeuterated solvent signal; 13C NMR chemical shifts, to the signal of
the deuterated solvent. The 31P NMR chemical shifts were referenced
to H3PO4. The 19F NMR chemical shifts were referenced to CFCl3.
’ CONCLUSIONS
The aim in this study was to use Lewis-acidic triaryl boron
compounds to weaken or cleave the OÀMe bond in methyl
acetate in order to form rhodiumÀmethyl complexes via an
oxidative addition reaction, with a view to carbonylating methyl
11B NMR chemical shifts were referenced to [F3B OEt2]. Mass spectra
3
acetate to acetic anhydride. The reaction between [MeOAc
3
were recorded using either a VG Autospec or a VG Platform II spec-
trometer. FTIR spectra were measured using a Perkin-Elmer Spectrum
GX spectrometer. Elemental analyses were performed by the Science
Technical Support Unit at The London Metropolitan University.
Solvents and Reagents. Diethyl ether and tetrahydrofuran were
dried by prolonged reflux, under a nitrogen atmosphere, over sodium
metal with a benzophenone ketyl indicator and distilled freshly prior to
use. Dichloromethane, acetonitrile, hexane, and methyl iodide were
dried over calcium hydride and distilled under nitrogen. Toluene and
pentane were dried by passing through a column, packed with commer-
cially available Q-5 reagent (13% CuO on alumina) and activated
alumina (pellets, 3 mm), in a stream of nitrogen. Methyl acetate was
dried over P2O5 and distilled under nitrogen. Acetone was dried over
B2O3 and distilled under nitrogen. Ethanol and methanol were dried
B(C6F5)3] and [Rh(OAc)(CO)2]2 did not yield an oxidative
addition product but instead resulted in the transfer of the Lewis
acid from methyl acetate to the rhodium complex, resulting in a
new adduct, [Rh(OAc)(CO)2 B(C6F5)3], which could not be
isolated or structurally characterized.
In a second part of this study, a series of novel Lewis acid-
functionalized rhodium complexes [Rh(CO)Cl(BPP)], [Rh-
(CO)2(BPP)]SbF6, and [Rh(MeCN)2(BPP)]SbF6 were synthe-
sized and fully characterized. The carbonyl ligands of [Rh(CO)2-
(BPP)]SbF6 were substituted for MeCN ligands in quantitative
yield upon reaction of [Rh(CO)2(BPP)]SbF6 with acetonitrile.
Reactions of [Rh(CO)2(BPP)]SbF6 with methyl acetate and
acetonitrile demonstrated that the rhodiumÀboron bond in
3
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V = 4309.14(11) Å3, Z = 4, Dc = 1.680 g cmÀ3, μ(Mo KR) = 1.272
mmÀ1, T = 173 K, pale brown blocks, Oxford Diffraction Xcalibur 3
42
over sodium and distilled under nitrogen. [Rh(CO)2Cl]2 and [Rh-
43
(CO)2(OAc)]2 were prepared according to published procedures.
The preparation of PhB(C6H4PPh2)2 was adapted from a procedure
diffractometer; 14 435 independent measured reflections (Rint =
described by Bourissou and co-workers.28
0.0575), F2 refinement, R1(obs) = 0.0377, wR2(all) = 0.1055, 9106
independent observed absorption-corrected reflections [|Fo| > 4σ(|Fo|),
2θmax = 65°], 557 parameters. CCDC 807789.
PhB(C6H4PPh2)2. HPPh2 (2.92 g 16.0 mmol), 1-iodo-2-bromoben-
zene (7.71 g, 16.2 mmol), and [Pd(PPh3)4] (0.1 g, 0.08 mmol) were
weighed into a glass ampule before adding NEt3 (2.8 mL, 20 mmol) and
toluene (3 mL). The ampule was sealed and heated to 80 °C. After 14 h
the brown suspension was allowed to cool to room temperature before
extracting with 6 Â 20 mL portions of toluene. Toluene was removed in
vacuo, and the resultant yellow solid redissolved into toluene (10 mL)
and filtered through a 4 cm silica pad (under a nitrogen atmosphere).
Four 20 mL portions of toluene were used to wash the product from the
silica pad. The toluene was removed in vacuo to yield Ph2P(C6H4Br) as
an off-white solid (5.1 g, 92% yield). The ESI mass spectrum and NMR
data were identical to those reported by Peters.34
[Rh(MeCN)2(BPP)]SbF6. [Rh(CO)2(BPP)]SbF6 (70 mg, 0.07 mmol)
was dissolved in acetonitrile (20 mL) and left to stir overnight. The
acetonitrile was removed in vacuo to yield the product as a yellow solid
(63 mg, 88%). Crystals suitable for X-ray diffraction analysis were grown
by slow diffusion of pentane into a dichloromethane solution of the
product. Anal. Found for C46H39N2P2F6SbBRh: C, 53.44; H, 3.75; N,
2.66. Calcd: C, 53.58; H, 3.81; N, 2.72. 31P NMR (162 MHz, CD3CN): δ
54.3 (d, 1JRhÀP = 155.5). 11B NMR (128 MHz, CDCl3): δ 8.1 (br). 19
NMR (377 MHz, CD3CN): δ À123 (superposition of sextet due to
F
121SbF6À and octet due to 123SbF6À). IR (Nujol): ν(CN), 2004 cmÀ1
.
ESI/MS+ (m/z): 795 ([M]+, 3%), 754 ([M À (MeCN)]+, 16%), 713
([M À 2(MeCN)]+, 100%), LSIMS/MSÀ (m/z): 235 ([M]À, 100%).
Crystal data for [Rh(MeCN)2(BPP)](SbF6): [C46H39BN2P2Rh]-
(SbF6), M = 1031.20, orthorhombic, P212121 (no. 19), a =
10.56293(4) Å, b = 17.51545(7) Å, c = 23.79538(10) Å, V =
Freshly dried Et2O (15 mL) was added to Ph2P(C6H4Br) (1.01 g,
n
2.96 mmol) before cooling the resultant suspension to 0 °C. BuLi
(1.3 mL, 3.11 mmol, 2.5 M) was added dropwise before allowing the
suspension to warm to room temperature. The residue was filtered and
washed with 2 Â 5 mL portions of Et2O beforedryingin vacuo. The product
Ph2P(C6H4Li) (0.7 g, 88% yield) was used without characterization.
Ph2P(C6H4Li) (0.7 g, 2.6 mmol) was suspended in freshly dried
toluene (15 mL) and cooled to À78 °C. PhBCl2 (0.206 g, 1.3 mmol) was
added dropwise by cannula to the stirring suspension (washing the flask
with 3 Â 3 mL portions of toluene). The suspension was stirred at
À78 °C for 2 h before allowing it to warm to room temperature and
stirring for a further 2 h. The supernatant was isolated by filtration before
removing the solvent in vacuo to yield the product PhB(C6H4PPh2)2 as
an off-white solid (0.410 g, 52% yield). ESI/MS+ (m/z): 611 ([M]+,
4402.49(3) Å3, Z = 4, Dc = 1.556 g cmÀ3, μ(Cu KR) = 9.095 mmÀ1
,
T = 173 K, yellow blocks, Oxford Diffraction Xcalibur PX Ultra
diffractometer; 8711 independent measured reflections (Rint
=
0.0360), F2 refinement, R1(obs) = 0.0273, wR2(all) = 0.0724, 8566
independent observed absorption-corrected reflections [|Fo| > 4σ(|Fo|),
2θmax = 145°], 580 parameters. The absolute structure of [Rh(MeCN)2-
+
(BPP)](SbÀF6) was determined by a combination of R-factor tests [R1
0.0273, R1 = 0.0731] and by use of the Flack parameter [x+
+0.024(4), xÀ = +0.976(4)]. CCDC 807790.
=
=
1
100%), 652 ([M + CH3CN]+, 70%). H NMR (400 MHz, C6D6):
[B(C6F5)3 MeOAc]. MeOAc (1 mL) was added to a stirred suspension
3
6.7À8.0 ppm (m, ArH). 31P NMR (162 MHz, CDCl3): À6.14 ppm (see
of B(C6F5)3 (88 mg, 0,17 mmol) in pentane (30 mL). After one hour the
suspension was filtered. Upon removal of the pentane the product was
isolated as a white solid (49 mg, 49%). Anal. Found for C21H6O2F15B:
C, 42.95; H, 0.99. Calcd: C, 43.04; H, 1.03. 1H NMR (400 MHz,
CDCl3): δ 4.09 (s, 3H); 2.14 (s, 3H). 19F NMR (377 MHz, CDCl3):
δ À134.44 (d, 6F, 3J = 18.8); À155.35 (t, 6F, 3J = 18.8); À162.83 (t, 6F,
3J = 18.8). 13C{1H} NMR (101 MHz, CDCl3): δ 20.6 (CH3), 58.3
Supporting Information).
[Rh(CO)Cl(BPP)]. PhB(C6H4PPh2)2 (75 mg, 0.13 mmol) was dis-
solved in dichloromethane (5 mL) and added dropwise to [RhCl-
(CO)2]2 (24 mg, 0.065 mmol) dissolved in dichloromethane (5 mL).
The resultant yellow solution was left to stir for 40 min before
concentrating the solution (1 mL) and adding pentane to precipitate a
yellow solid. The product (85 mg, 65%) was isolated by filtration and
dried in vacuo. Anal. Found for C43H33P2OBClRh: C, 66.53; H, 4.24.
Calcd: C, 66.48; H, 4.28. 31P NMR (162 MHz, CDCl3): cis (65%) δ 54.2
1
(O-CH3), 116.1 (br, C-B), 137.5 (d, JCÀF = 250.8, Ar C-F), 140.9
1
1
(d, JCÀF = 258.3, Ar C-F), 148.0 (d, JCÀF = 242.8, Ar C-F), 184.6
(CO2Me). 11B NMR (128 MHz, CDCl3): δ 4.1 (br). IR (Nujol):
1649 cmÀ1 ν(CdO), 1469 cmÀ1 ν(BÀO).
2
1
2
1
(dd, JPÀP = 36.4, JRhÀP = 146.1), 36.3 (dd, JPÀP = 36.4, JRhÀP
=
119.2); trans-A (29%) δ 33.8 (d, 1JRhÀP = 110.1); trans-B (6%) δ 28.4
Oxidative Addition Studies with Methyl Acetate and [Rh(CO)2-
(BPP)]SbF6. [Rh(CO)2(BPP)]SbF6 (5 mg, ∼5 μmol) was dissolved in a
4 M methyl acetate solution in CDCl3 (0.5 mL) before heating to 70 °C
for seven days in a sealed NMR tube. The reaction was monitored by
1H NMR and 31P NMR spectroscopy.
(d, JRhÀP = 106.3). 11B NMR (128 MHz, CDCl3): δ 29.6 (br). IR
1
(Nujol) ν(CO) 2060, 2013, 1976 cmÀ1. LSIMS/MS+ (m/z): 713 ([M
À (CO) À Cl]+, 100%), 748 ([M À (CO) + H]+, 30%).
[Rh(CO)2(BPP)]SbF6. A chloroform solution (20 mL) of [Rh(CO)-
Cl(BPP)] (0.096 g, 0.123 mmol) was added to a CO-saturated suspen-
sion of AgSbF6 (43 mg, 0.123 mmol) in chloroform (10 mL). The
resultant suspension was stirred for 6 h under an atmosphere of CO
before filtering. The filtrate was concentrated (1 mL) before precipitat-
ing the product from solution as a yellow solid (75 mg, 61%) with
pentane. The supernatant was removed by filtration, and the residue
dried in vacuo. Crystals suitable for X-ray diffraction analysis were grown
by slow diffusion of pentane into a dichloromethane solution of the
product. Anal. Found for C44H33P2O2F6BSbRh: C, 52.49; H, 3.18. Calcd:
C, 52.58; H, 3.31. 31P NMR (162 MHz, CDCl3): δ 35.5 (d, 1JRhÀP = 119.3).
11B NMR (128 MHz, CDCl3): δ À0.5 (s). 13C{1H} NMR (101 MHz,
CDCl3, J values in Hz): δ 178.7 (dd, J = 81.3, J = 58.7, Rh-CO). IR
(Nujol): ν(CO), 2123, 2096 cmÀ1. ESI/MS+ (m/z): 769 ([M]+, 40%),
713 ([M À 2(CO)]+, 100%). ESI/MSÀ (m/z): 235 ([M]À, 100%).
Oxidative Addition Studies with Trifluoromethyl Acetate and
[Rh(CO)2(BPP)]SbF6. [Rh(CO)2(BPP)]SbF6 (5 mg, ∼5 μmol) was
dissolved in a 4 M trifluoromethyl acetate solution in CDCl3 (0.5 mL)
before heating to 70 °C for seven days in a sealed NMR tube. The
reaction was monitored by 1H NMR and 31P NMR spectroscopy.
Oxidative Addition Studies with Methyl Iodide and [Rh(CO)2-
(BPP)]SbF6. [Rh(CO)2(BPP)]SbF6 (5 mg, ∼5 μmol) was dissolved in a
4 M methyl iodide solution in CDCl3 (0.5 mL) before heating to 70 °C
for seven days in a sealed NMR tube. The reaction was monitored by
1H NMR and 31P NMR spectroscopy.
Oxidative Addition Studies with [B(C6F5)3 MeOAc] and [Rh(OAc)-
(CO)2]2. [MeOAc B(C6F5)3] (5 mg, 12 μmol) and [Rh(OAc)(CO)2]2
3
3
(3.3 mg, 5.7 μmol) were dissolved in C6D6 (0.5 mL) before heating to
70 °C for seven days in a sealed NMR tube. The reaction was monitored
by 1H, 11B, and 19F NMR spectroscopy.
Crystal data for [Rh(CO)2(BPP)](SbF6): [C44H33BO2P2Rh](SbF6)
CH2Cl2, M = 1090.04, monoclinic, P21/c (no. 14), a = 18.4627(3) Å,
b = 15.58519(17) Å, c = 15.9429(2) Å, β = 110.0615(17)°,
3
Methyl Acetate Carbonylation Studies without LiI. A rhodium com-
pound (42 μmol) was dissolved in anhydrous MeOAc (38 g, 0.51 mol)
4065
dx.doi.org/10.1021/om200341t |Organometallics 2011, 30, 4060–4066

Organometallics
ARTICLE
before transferring to a nitrogen-purged, 300 mL, high-pressure reactor.
The reactor was then pressurized with CO (40 bar) and heated at 130 °C for
16 h. Upon cooling to 5 °C the CO was vented from the reactor and the
reaction mixture weighed. A 150 mg aliquot of the reaction mixture was
weighed into a vial with 50 mg of MeCN before mixing and analyzing by
1H NMR in d6-dmso. The molar composition of the reaction mixture was
determined by calculating the areas of the 1H NMR peaks relative to the
MeCN standard.
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’ AUTHOR INFORMATION
Corresponding Author
*Tel: +44-(0)20-75945863. Fax: +44-(0)-20-75945804. E-mail:
g.britovsek@imperial.ac.uk.
’ ACKNOWLEDGMENT
We are grateful to BP Chemicals Ltd. and EPSRC for financial
support. We thank Johnson Matthey for their generous loan of
RhCl3.
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