Dicarbonylrhodium(I) complexes of bipyridine ligands with proximate H-bonding substituents and their application in methyl acetate carbonylation

Article abstract of DOI:10.1002/ejic.201100423

A series of cationic cis-dicarbonylrhodium(I) complexes [Rh(L)(CO) ₂]SbF₆ has been prepared containing 2,2′-bipyridine ligands with proximate H-bonding substituents R in the 6- and 6′-position (R = OH, NH₂, COOEt and PO(OEt)₂). The solid-state structures have been determined by X-ray crystallography for the complexes where R = OH, NH₂ and COOEt. The molecular structures have revealed metal-metal and π-π interactions between the square-planar complexes resulting in the formation of molecular chains in the solid state. IR studies on the reaction of [Rh(bipy)(CO)₂]SbF₆ with MeI have shown the formation of a neutral complex [RhI(CO)(bipy)], which undergoes oxidative addition of MeI much faster than the cationic complex [Rh(bipy)(CO) ₂]⁺. Although all complexes show good activities for the carbonylation of methyl acetate, this is believed to be due to their instability and the formation of [RhI₂(CO)₂]^- under the reaction conditions.

Full text of DOI:10.1002/ejic.201100423

FULL PAPER
DOI: 10.1002/ejic.201100423
Dicarbonylrhodium(I) Complexes of Bipyridine Ligands with Proximate
H-Bonding Substituents and Their Application in Methyl Acetate
Carbonylation
Christopher M. Conifer,[a] David J. Law,[b] Glenn J. Sunley,[b] Anthony Haynes,[c]
John R. Wells,[c] Andrew J. P. White,[a] and George J. P. Britovsek*[a]
Keywords: Carbonylation / Rhodium / Acetic acid / Methanol / Carbon monoxide / Homogeneous catalysis
A
series of cationic cis-dicarbonylrhodium(I) complexes
IR studies on the reaction of [Rh(bipy)(CO)2]SbF6 with MeI
[Rh(L)(CO)2]SbF6 has been prepared containing 2,2Ј-bipyr- have shown the formation of a neutral complex [RhI(CO)-
idine ligands with proximate H-bonding substituents R in the
6- and 6Ј-position (R = OH, NH2, COOEt and PO(OEt)2). The
solid-state structures have been determined by X-ray crystal-
lography for the complexes where R = OH, NH2 and COOEt.
The molecular structures have revealed metal–metal and π–
π interactions between the square-planar complexes re-
sulting in the formation of molecular chains in the solid state.
(bipy)], which undergoes oxidative addition of MeI much
faster than the cationic complex [Rh(bipy)(CO)2]+. Although
all complexes show good activities for the carbonylation of
methyl acetate, this is believed to be due to their instability
and the formation of [RhI2(CO)2]– under the reaction condi-
tions.
Introduction
the concentration of free iodide in the system needs to be
carefully controlled through the use of additional pro-
moters.[8,10–14] Alternative strategies for the activation of
methanol have used strong acids such as zeolites,[15–18] het-
eropolyacids[19] and superacids.[20] These strong acids can
The carbonylation of methanol is currently the most im-
portant industrial process for the large-scale production of
acetic acid.[1,2] Annual production capacity is ca. 7 million
tons worldwide and continues to grow, especially in Asia.[3]
The first commercial methanol carbonylation plants were
operated by BASF in the 1960s, and the initial catalysts
were based on cobalt.[4] These were quickly superseded by
a rhodium-catalysed process, which was developed by Mon-
santo in the 1970s.[5,6] In the 1990s BP introduced the Ca-
tiva process based on iridium, and this is currently the pre-
ferred catalyst technology for new acetic acid plants.[7,8] All
catalyst systems require iodide as a co-catalyst for the acti-
vation of methanol (or methyl acetate) to generate methyl
iodide. Methyl iodide reacts with the transition-metal cata-
lyst by oxidative addition to form a metal–methyl bond,
which can subsequently undergo CO insertion followed by
the reductive elimination of acetyl iodide and hydrolysis to
acetic acid.[9] In the rhodium-catalysed process, the rate of
carbonylation is first order in iodide concentration due to
a relatively slow oxidative addition reaction. In contrast,
for the iridium-catalysed process, the rate of carbonylation
shows an inverse relation to the iodide concentration, and
protonate methanol, which upon elimination of H2O gener-
+ [18,20]
ates the carbenium cation, CH3
.
Although these
strong acids can carbonylate methanol without the presence
of a transition metal, the selectivities are generally lower
due to the formation of significant amounts of dimethyl
ether.[19,21,22]
The high temperatures and the strongly acidic reaction
conditions required for methanol carbonylation, combined
with the use of iodide as the co-catalyst, present significant
challenges in terms of reactor engineering and corrosion
prevention. Alternative catalysts that can operate under
milder reaction conditions could significantly lower the cost
of acetic acid production. To this end, during the last dec-
ade several groups have investigated the use of group 9
metal complexes containing a range of ligands as catalysts
for methanol carbonylation.[23] Examples are the applica-
tion of monodentate[24,25] and bidentate phosphane li-
gands[26–33] as well as cyclopentadienyl ligands[34,35] and
combinations thereof.[36] Nitrogen-based ligands such as di-
imines and pyridine ligands have also been investi-
gated.[37–39] Noteworthy, Dutta and co-workers have re-
cently reported improved catalytic behaviour of rhodium
catalysts for methanol carbonylation, when monodentate
pyridine ligands with carboxylic acid substituents were em-
[a] Department of Chemistry, Imperial College London,
Exhibition Road, South Kensington, London SW7 2AY, UK
Fax: +44-20-75945804
E-mail: g.britovsek@imperial.ac.uk
[b] Hull Research and Technology Centre, BP Chemicals Ltd.,
Saltend, Hull, HU12 8DS, UK
[c] Department of Chemistry, University of Sheffield,
Brook Hill, Sheffield, S3 7HF, UK
Supporting information for this article is available on the ployed.[40]
WWW under http://dx.doi.org/10.1002/ejic.201100423.
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FULL PAPER
The coordination chemistry of ligands with proximate the carbonylation of methyl acetate to give acetic acid or
acidic or basic functionalities and their application in acetic anhydride has been investigated. Furthermore, the
homogeneous catalysis has been an ongoing theme in our oxidative addition reaction of MeI has been studied in
laboratory. Previously, we have reported on the application greater detail for complex [Rh(1)(CO)2]SbF6, and the sta-
of such ligands in the context of alkane oxidation,[41–43] and bility of the complexes under carbonylation conditions has
very recently we have communicated our first results on been evaluated.
methyl acetate carbonylation.[44] Here we present the syn-
thesis and characterisation of a series of novel dicarbon-
ylrhodium(I) complexes [Rh(L)(CO)2]SbF6 containing 2,2Ј-
bipyridine ligands with Brønsted-acidic H-bonding substit-
Results and Discussion
uents in the 6- and 6Ј-position (see Figure 1), with a view
to activate the methyl acetate substrate in the vicinity of the
Synthesis of Ligands and Complexes
metal centre. The functionalities investigated in this study
The preparation of cationic dicarbonylrhodium(I) com-
cover a range of pKa values, from weakly acidic OH groups plexes of the type [Rh(bipy)(CO)2]+ (bipy = 2,2Ј-bipyrdine)
(cf. PhOH, pKa = 10.0) to ammonium NH3 (cf. PhNH3
,
was first reported by Reddy and Ramesh.[45] In this study,
+
+
pKa = 4.6), carboxylic acid COOH (cf. PhCOOH, pKa = the complexes [Rh(L)(CO)2]SbF6 have been prepared ac-
4.2) to moderately acidic phosphonic acid functionalities cording to a synthetic procedure developed by Oro and co-
PO(OH)2 [cf. PhPO(OH)2, pKa1 = 2.1]. In the case of 6,6Ј- workers for the synthesis of similar rhodium complexes.[46]
dihydroxy-2,2Ј-bipyridine (2), the acidic OH functionalities The starting material [Rh(cod)Cl]2 (cod = 1,5-cycloocta-
are present in the starting complex, whereas in the other diene) was treated with AgSbF6 in acetone, followed by the
complexes the acidic functionality is generated in situ, addition of the bipyridine ligand to give the intermediate
either by protonation of the amine substituents of 6,6Ј-di- complexes [Rh(L)(cod)]SbF6. These complexes were sub-
amino-2,2Ј-bipyridine (3), or by transesterification of the sequently treated with CO at room temperature to give the
esters under acidic conditions in the case of ligands 4 and series of complexes of the type [Rh(L)(CO)2]SbF6, as shown
5. The catalytic activity of the functionalised complexes for in Equation (1).
Figure 1. Cationic dicarbonylrhodium(I) complexes used as carbonylation catalysts.
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Dicarbonylrhodium(I) Complexes of Bipyridine Ligands
Complexes [Rh(1)(CO)2]SbF6 and [Rh(3)(CO)2]SbF6 to evaluate the stability of our new complexes under the
were prepared according to this method by using bipyridine carbonylation reaction conditions, solutions of the com-
(1) and 6,6Ј-diaminobipyridine (3), respectively. We recently plexes [Rh(3)(CO)2]SbF6, [Rh(4)(CO)2]SbF6 and [Rh(5)-
communicated the preparation of complex [Rh(2)(CO)2]- (CO)2]SbF6 in CD3COOD at 100 °C were monitored by 1H
SbF6 from 6,6Ј-dihydroxy-2,2Ј-bipyridine (2), which nor- NMR spectroscopy over time (see Figures S15–S19). The
mally exists as its tautomer 2,2Ј-bipyridone. This tautomer amino-substituted complex [Rh(3)(CO)2]SbF6 has a half-
was therefore converted to 6,6Ј-bis(tert-butyldimethyl- life of ca. 1 d under these conditions, decomposing slowly
siloxy)-2,2Ј-bipyridine, which is easily hydrolysed after co- to a multitude of species. Complexes [Rh(4)(CO)2]SbF6 and
ordination to the rhodium centre to give the target complex [Rh(5)(CO)2]SbF6 have half-lives of approximately 12 h and
[Rh(2)(CO)2]SbF6.[44] A number of metal complexes con- 4 h, respectively, and show the formation of ethyl acetate
taining bipyridine ligands with hydroxy substituents in vari- under these conditions, which suggests that the aforemen-
ous positions have been reported, for example 3,3Ј- and tioned transesterification does indeed take place.
4,4Ј-dihydroxy-2,2Ј-bipyridine[47,48] and also 6-hydroxy-
2,2Ј-bipyridine;[49] but, to the best of our knowledge,
[Rh(2)(CO)2]SbF6 is the first metal complex containing the
6,6Ј-dihydroxy-2,2Ј-bipyridine ligand.
Solid-State Structures
Square-planar d8 RhI complexes are known to form RhI–
RhI interactions in the solid state resulting in one-dimen-
sional molecular chains.[50,51] These metal–metal interac-
tions, together with π–π interactions between the ligands,
give rise to unusual optical and electrical properties, which
make these materials of interest for example as molecular
wires.[52,53] The solid-state structures of the rhodium(I)
complexes studied here have been analysed in detail as they
all show intermolecular interactions in the solid state.
The solid-state structure of [Rh(2)(CO)2]SbF6 was al-
ready discussed in detail in our recent communication.[44]
For comparison with the other structures, the main features
are described briefly. The structure was found to contain
five crystallographically independent molecules A–E in the
asymmetric unit; complex [Rh(2)(CO)2]SbF6-A is shown in
Figure 2 (see Table 1 for bond lengths and angles). Each
complex is nearly planar, and the close Rh···Rh separations
between 3.28 and 3.33 Å, together with Rh···Rh···Rh angles
between 172 and 178° result in molecular chains. Within
these chains there are significant π–π interactions between
the pyridyl rings of adjacent complexes, and in each case
the rings are inclined by less than 2°.
Initial attempts to prepare dicarbonylrhodium(I) com-
plexes with 2,2Ј-bipyridine-6,6Ј-dicarboxylic acid as the an-
cillary ligand were hampered by the low solubility of the
free acid in common organic solvents. We therefore resorted
to the use of the diester derivative 4, as it was anticipated
that under the acidic carbonylation conditions, transesterifi-
cation would occur. This was confirmed by monitoring the
reaction of complex [Rh(4)(CO)2]SbF6 at 100 °C in
1
CD3COOD by H NMR spectroscopy (vide infra). The re-
action of the diene complex [Rh(4)(cod)]SbF6 with CO in
CH2Cl2 resulted in the substitution of the 1,5-cycloocta-
diene ligand with CO ligands to give [Rh(4)(CO)2]SbF6.
However, unlike the substitution reactions with ligands 1–
1
3, it was observed by H NMR spectroscopy that this sub-
stitution reaction is reversible, and evaporation of the sol-
vent at the end of the reaction resulted in the loss of CO
and regeneration of the starting complex [Rh(4)(cod)]SbF6.
The complex [Rh(4)(CO)2]SbF6 could eventually be isolated
from a CO-saturated CH2Cl2 solution by precipitation with
hexane. It thus appears that the CO ligands are much more
labile in this complex compared to the complexes of ligands
1–3.
The tetraethyl ester of 2,2Ј-bipyridine-6,6Ј-diphosphonic
acid (5) was employed for the synthesis of complex
[Rh(5)(cod)]SbF6. This complex does not react with CO in
CH2Cl2 at room temperature. However, heating the com-
plex [Rh(5)(cod)]SbF6, suspended in CO-saturated toluene
at 80 °C for 5 h resulted in the formation of the dicarbonyl
complex [Rh(5)(CO)2]SbF6. All complexes were character-
ised by multinuclear NMR and IR spectrosopy, mass spec-
trometry, and microanalysis. The solid-state structures of
complexes [Rh(2)(CO)2]SbF6, [Rh(3)(CO)2]SbF6 and
[Rh(4)(CO)2]SbF6 have been determined by X-ray crystal-
lography.
Figure 2. Molecular structure of [Rh(2)(CO)2]SbF6-A, one of the
five crystallographically independent cationic complexes present in
the crystals of [Rh(2)(CO)2]SbF6.[44]
Complex Stability
The NH2-substituted complex [Rh(3)(CO)2]SbF6 shows
Typical carbonylation conditions for the Monsanto and a folded geometry whereby the {N(1),C(6),C(7),N(12)} and
Cativa process involve temperatures up to 200 °C and pres- {N(1),N(12),Rh} planes are inclined by ca. 29° (see Fig-
sures up to 50 bar, and the current catalysts, [RhI2(CO)2]– ure 3 and Table 2 for selected bond lengths and angles).
and [IrI2(CO)2]–, are stable under these conditions. In order This appears to be a consequence of the different steric
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G. J. P. Britovsek et al.
Table 1. Selected bond lengths [Å] and angles [°] for the five independent complexes A–E present in the crystals of [Rh(2)(CO)2]SbF6.
FULL PAPER
A
B
C
D
E
Rh(1)–N(1)
Rh(1)–N(12)
Rh(1)–C(15)
Rh(1)–C(16)
N(1)–Rh(1)–N(12)
N(1)–Rh(1)–C(15)
N(1)–Rh(1)–C(16)
N(12)–Rh(1)–C(15)
N(12)–Rh(1)–C(16)
C(15)–Rh(1)–C(16)
2.088(6)
2.102(6)
1.880(8)
1.900(10)
78.3(2)
177.0(3)
98.7(3)
98.8(3)
176.1(3)
84.3(4)
2.092(7)
2.082(7)
1.842(10)
1.861(9)
78.0(3)
177.2(3)
98.1(3)
99.3(3)
176.1(3)
84.6(4)
2.101(6)
2.067(6)
1.845(8)
1.853(10)
78.4(3)
175.9(3)
99.7(4)
97.6(3)
177.8(4)
84.3(4)
2.070(6)
2.064(6)
1.847(10)
1.829(10)
77.9(3)
176.5(3)
99.8(3)
98.7(4)
177.6(3)
83.5(4)
2.091(6)
2.089(6)
1.862(8)
1.843(9)
78.3(3)
176.5(3)
98.8(3)
99.5(3)
177.2(3)
83.3(4)
requirements of the NH2 vs. OH substituents. Adjacent Ci- Table 2. Selected bond lengths [Å] and angles [°] for [Rh(3)(CO)2]
SbF6.
related complexes stack through a pair of π–π interactions
between ring A in one complex and ring B in the next one
(interaction a in Figure 4) with centroid···centroid and
mean interplanar separations of ca. 3.72 and 3.40 Å, respec-
tively; the two rings are inclined by ca. 12°. These pairs of
complexes are approached by C2-related pairs and are
loosely held together by π–π interactions between rings A
and B in neighbouring pairs (interaction b in Figure 4) with
centroid···centroid and mean interplanar separations of ca.
4.09 and 3.32 Å, respectively; the two rings are inclined by
ca. 22°. The combination of these π–π interactions results
in a chain of complexes extending along the crystallo-
graphic a-axis direction. All four N–H protons in the struc-
ture of [Rh(3)(CO)2]SbF6 are involved in N–H···F hydrogen
bonds to the SbF6 anion as shown in Figure 3.
Rh–N(1)
Rh–C(15)
N(1)–Rh–N(12)
N(1)–Rh–C(16)
N(12)–Rh–C(16) 174.24(8)
2.0766(16)
1.865(2)
77.62(7)
98.99(8)
Rh–N(12)
Rh–C(16)
N(1)–Rh–C(15)
N(12)–Rh–C(15) 97.59(8)
C(15)–Rh–C(16) 85.14(9)
2.0720(15)
1.874(2)
171.13(8)
Figure 4. Part of one of the chains of π–π linked complexes along
the crystallographic a-axis direction present in the crystals of
[Rh(3)(CO)2]SbF6. The π–π interactions
a
and
b
have
centroid···centroid and mean interplanar separations [Å] of (a)
3.72, 3.40, and (b) 4.09, 3.32, respectively.
complex adopts a similar folded conformation to that
seen
for
complex
[Rh(3)(CO)2]SbF6,
with
the
Figure 3. Molecular structure of the cation in [Rh(3)(CO)2]SbF6
showing the N–H···F hydrogen bonding interactions. The N···F
and H···F distances, and the N–H···F angles [°] are (a) 3.025(2),
2.13, 172; (b) 3.121(3), 2.37, 142; (c), 3.124(2), 2.39, 139; (d)
3.077(2), 2.21, 161; (e) 3.000(2), 2.12, 166.
{N(1),C(6),C(7),N(12)} and {N(1),N(12),Rh} planes being
inclined by ca. 27 and 25° for complexes A and B, respec-
tively. Accompanying these folds are twists of the ester moi-
eties out of the pyridine plane by ca. 24, 27, 39 and 31° for
The structure of complex [Rh(4)(CO)2]SbF6 was found the O(13A), O(14A), O(13B) and O(14B) groups, respec-
to contain two crystallographically independent molecules tively. In each case, the carbonyl oxygen atom is pointing
in the asymmetric unit: [Rh(4)(CO)2]SbF6-A (shown in Fig- towards the metal centre, minimising the steric repulsion
ure 5) and [Rh(4)(CO)2]SbF6-B (in Figure S22) (bond between the metal atom and the ester groups. The two com-
lengths and angles are collected in Table 3). The cationic plexes stack in an alternating ···A···B···A···B··· fashion
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Dicarbonylrhodium(I) Complexes of Bipyridine Ligands
Table 3. Selected bond lengths [Å] and angles [°] for the two independent complexes A and B present in the crystals of [Rh(4)(CO)2]-
SbF6.
A
B
A
B
Rh–N(1)
Rh–C(21)
N(1)–Rh–N(12)
N(1)–Rh–C(22)
N(12)–Rh–C(22)
2.125(6)
1.862(9)
78.5(2)
98.0(3)
173.1(4)
2.119(8)
1.881(11)
78.1(4)
98.5(4)
172.0(4)
Rh–N(12)
Rh–C(22)
N(1)–Rh–C(21)
N(12)–Rh–C(21)
C(21)–Rh–C(22)
2.112(6)
1.872(9)
169.6(4)
95.7(3)
2.147(9)
1.867(11)
173.1(4)
99.7(4)
86.7(4)
82.8(5)
along the crystallographic a-axis direction such that the bi- Spectroscopy
pyridyl rings of adjacent complexes are on opposite sides
The structural aspects of the series of dicarbonylrhodi-
um(I) complexes discussed in the previous section have
shown that the plane of the bipyridine ligand can form an
angle with the coordination plane of the metal complex,
which appears to be caused by the steric requirements of
the substituents in the 6,6Ј-positions of the ligand.[54] The
angle θs between the two planes defined by [N,Rh,N] and
the bipyridine ligand [N,C,C,N] for all five complexes are
listed in Table 4.[55] These angles are smaller than the θs
angles observed in square-planar platinum(II) and palladi-
um(II) complexes with similar 6,6Ј-dimethyl-substituted
bipyridine ligands. For example, the θs angles in
dichlorido(6,6Ј-dimethylbipyridine)platinum(II) and -palla-
dium(II) complexes are 36.73(11)° and 38.84(9)°, respec-
tively; and similar angles have been observed in (2,9-
(Figure 6). The most notable feature of these chains are the
Rh···Rh separations of ca. 3.92 Å; the associated
Rh···Rh···Rh angles are ca. 176 and 179° at RhA and RhB,
respectively.
dimethylphenanthroline)palladium(II) complexes.[56]
A
smaller angle of 27.44(7)° was observed in the case of the
dichloridopalladium(II) complex with only one 6-methyl
substituent,[55] which is similar to the angles seen here for
complexes [Rh(3)(CO)2]SbF6 and [Rh(4)(CO)2]SbF6.
Figure 5. Molecular structure of one ([Rh(4)(CO)2]SbF6-A) of the
two crystallographically independent cationic complexes present in
the crystals of [Rh(4)(CO)2]SbF6.
Table 4. Geometric and spectroscopic properties of [Rh(L)(CO)2]-
SbF6 complexes.
[c]
1JC-Rh
Complex
θs[a]
ν(CO)[b]
(see Figure 1)
[°]
[cm–1]
[Hz]
[Rh(1)(CO)2]SbF6
(R = H)
[Rh(2)(CO)2]SbF6
2109(s), 2095(w),
2054(s), 2025(w)
2, 0, 0, 1, 2 2108(s), 2101(s),
2093(s),
69.5
70.9
(R = OH)
2052(s), 2045(s),
2031(s)
[Rh(3)(CO)2]SbF6
(R = NH2)
[Rh(4)(CO)2]SbF6
(R = COOEt)
[Rh(5)(CO)2]SbF6
[R = PO(OEt)2]
29
2097(s), 2084(w),
2035(s), 2010(w)
2097(s), 2079(w),
2041(s), 2005(w)
2096(s), 2033(s)
69.1
60.0
73.4
27, 25
[a] Angle between the 2,2Ј-bipyridine ligand and the coordination
plane defined by [N,C,C,N] and [N,Rh,N]. For complexes
[Rh(2)(CO)2]SbF6 and [Rh(4)(CO)2]SbF6 several independent mo-
lecules are observed in the solid-state structures. [b] Measured in
Nujol (s = strong, m = medium, w = weak). [c] Measured in [D6]-
acetone.
Selected spectroscopic features of the dicarbonylrhodium
complexes containing ligands 1–5 are collected in Table 4.
Infrared spectroscopy shows two strong absorptions for
each complex in the carbonyl stretching region, as expected
for square-planar cis-dicarbonyl complexes. The frequencies
for complex [Rh(1)(CO)2]SbF6 are comparable to the two
Figure 6. Part of one of the chains of complexes along the crystal-
lographic a-axis direction present in the crystals of [Rh(4)(CO)2]-
SbF6. The Rh···Rh separations a and b are both ca. 3.92 Å.
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FULL PAPER
sets of values reported for [Rh(1)(CO)2]ClO4 (2108 & complex in CH2Cl2, indicate that the reaction between
2050 cm–1 and 2103 & 2044 cm–1)[45,57] However, additional [Rh(1)(CO)2]SbF6 and MeI is not a simple oxidative ad-
bands are also observed for the complexes containing li- dition reaction. The initial decay of the reactant ν(CO)
gands 1–4, which in the case of complex [Rh(2)(CO)2]SbF6 bands is accompanied by the appearance of absorptions at
are almost as intense as the main bands (see Table 4 and 2070 and 2094 cm–1 due to terminal CO ligands, along with
Figures S12–S14). These additional absorptions are be- bands at 1711 and 1733 cm–1 that are assigned to acetyl
lieved to be due to solid-state effects such as metal–metal ligands. Over a longer time period, the bands at 2070 and
and π–π interactions, as seen in the solid-state structures.[58] 2094 cm–1 both decay (but at different rates), as does the
No apparent correlation can be observed between the θs acetyl band at 1733 cm–1, while the lower-frequency acetyl
angles and the ν(CO) values. In addition, from the last col- band continues to grow.
umn in Table 4 it can be seen that the carbon-rhodium cou-
pling constants JC-Rh appear to be independent of the an-
gle θs.
To explain these observations, the mechanism shown in
Scheme 1 is proposed, involving two pathways from the re-
actant [Rh(1)(CO)2]+ to an (acetyl)RhIII product, [RhI-
(COMe)(1)(NCMe)2]+. In route A, oxidative addition of
MeI to the reactant complex is followed by migratory CO
insertion to give an (acetyl)(carbonyl)RhIII species, that we
propose gives rise to the ν(CO) bands at 2094 and
1733 cm–1. The vacant coordination site generated by the
migratory insertion step will likely be occupied by the coor-
dinating solvent MeCN. Subsequent slow replacement of
CO by MeCN would then give [RhI(COMe)(1)(NCMe)2]+,
consistent with the observation of a sole acetyl ν(CO) band
at 1711 cm–1 for the ultimate product.
1
Oxidative Addition Studies with MeI
The reactivity of methyl iodide with the bipy complex
[Rh(1)(CO)2]SbF6 was investigated by IR spec-
troscopy.[59–61] [Rh(1)(CO)2]SbF6 was dissolved in a 3.2 m
solution of MeI in acetonitrile, and the carbonyl region of
the IR spectrum was monitored at 25 °C over 900 min (Fig-
ure 7). During the course of the reaction, the ν(CO) bands
of [Rh(1)(CO)2]SbF6 at 2099 and 2040 cm–1 decayed with a
half life of approximately 50 min. This is approximately
The ν(CO) band observed at 2070 cm–1 corresponds to
3000 times slower than the oxidative addition of MeI to the the known complex, [RhI2Me(1)(CO)], which has a ν(CO)
neutral complex [RhI(1)(CO)] in CH2Cl2, based on extrapo- value of 2071 cm–1 in CH2Cl2.[38] This (methyl)RhIII species
lation of the kinetic data at lower [MeI] values for that com- would result from oxidative addition of MeI to
plex.[38] The IR spectra in Figure 7, which were determined [RhI(1)(CO)] (Scheme 1, route B). Formation of this neu-
in acetonitrile because of the poor solubility of the ionic tral RhI complex requires replacement of a CO ligand by
Figure 7. IR spectra obtained from the reaction between [Rh(1)(CO)2]SbF6 and MeI in CH3CN at 25 °C over 900 min ([MeI] = 3.2 m).
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Dicarbonylrhodium(I) Complexes of Bipyridine Ligands
Scheme 1.
I– in the reactant complex [Rh(1)(CO)2]+, a known reaction with its weak trans effect and a decrease of the negative
reported previously by Mestroni et al.[62] The free I– neces- charge at the metal centre facilitate the migratory insertion
sary to form [RhI(1)(CO)] can be generated during the se- process.
quence of steps in route A, presuming that oxidative ad-
In an attempt to prepare the final complex [RhI-
dition of MeI occurs by the conventional SN2 mechanism (COMe)(1)(NCMe)2]SbF6 by an independent route, the di-
and/or that I– can dissociate from RhIII iodide species. An meric complex [RhI(μ-I)(COMe)(CO)(NCMe)]2, previously
alternative source of free I– is the equilibrium reaction be- prepared by Haynes and co-workers,[63] was treated with
tween MeCN and MeI to give [MeCNMe]+ and I–.[63] Once 2,2Ј-bipyridine. This resulted in the formation of a new
formed, [RhI(1)(CO)] will undergo rapid oxidative addition complex [RhI2(COMe)(1)(CO)], which was fully character-
of MeI to give the observed [RhI2Me(1)(CO)]. On the basis ised and shows ν(CO) and ν(COMe) IR bands (in Nujol) at
of the kinetics measured at 25 °C in CH2Cl2,[38] the pre- 2058 and 1694 cm–1, respectively. Although the exact ste-
dicted half life for this reaction with 3.2 m MeI is ca. 1 s, reochemical arrangement of the ligands could not be estab-
consistent with the nonobservation of [RhI(1)(CO)] [ν(CO) lished, an arrangement with the acetyl group trans to the
at 1977 cm–1 in CH2Cl2]. Haynes and co-workers found that iodido ligand is proposed, as shown in Equation (2), on the
[RhI2Me(1)(CO)] undergoes migratory CO insertion only basis of the relative trans effects of the ligands and similari-
very slowly in CH2Cl2, giving small amounts of an acetyl ties with other acetylrhodium(III) complexes. Subsequent
product.[38] Over 15 h in MeCN, the ν(CO) band at removal of one of the iodido ligands by the addition of
2070 cm–1 due to [RhI2Me(1)(CO)] decays almost com- 1 equiv. of AgSbF6 in MeCN resulted in a complex mixture
pletely (Figure 7). It is proposed that substitution of iodide of products that could not be analysed. The addition of
by MeCN gives a cationic complex that undergoes mi- 2 equiv. of AgSbF6 in MeCN also resulted in a complex
gratory CO insertion more readily due to the stronger trans mixture of products.
effect of MeCN compared to that of iodide, leading to
In summary, the reaction of [Rh(1)(CO)2]SbF6 with MeI
[RhI(COMe)(1)(NCMe)2]+, the same acetyl complex in MeCN is approximately 3000 times slower than the oxi-
formed in route A. The released iodide ligand is recycled to dative addition of MeI to [RhI(1)(CO)] in CH2Cl2. The
generate more [RhI(1)(CO)] from the starting complex.
neutral complex [RhI(1)(CO)] is formed in situ by substitu-
It should be noted that related replacements of I– and tion of a CO ligand in [Rh(1)(CO)2]+. Oxidative additions
MeCN have been observed previously for acetylrhodi- of MeI at [Rh(1)(CO)2]SbF6 and [RhI(1)(CO)], therefore,
um(III) complexes, for example between [RhI3(COMe)- occur simultaneously to yield a common acetylrhodium(III)
(CO)(NCMe)]– and [RhI2(COMe)(CO)(NCMe)2].[63] In ad- product. This is an important insight into the potential re-
dition, in the iridium-catalysed methanol carbonylation re- activity of [Rh(1)(CO)2]SbF6 during carbonylation reac-
action, I– is first substituted for CO in [IrI3Me(CO)2]– to tions as the presence of iodide will lead to the formation of
give the neutral complex [IrI2Me(CO)3], before CO mi- a neutral complex [RhI(1)(CO)], which undergoes much
gratory insertion can take place.[7,63] The absence of iodide more rapid oxidative addition. It is also possible that
Eur. J. Inorg. Chem. 2011, 3511–3522
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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3517

G. J. P. Britovsek et al.
FULL PAPER
[Rh(1)(CO)2]SbF6 and [RhI(1)(CO)] are both active species 195 °C, which is similar to the temperatures used in indus-
in carbonylation reactions, which are described in the next trial processes. In order to minimise catalyst decomposition,
section.
the catalytic activities of [Rh(1)(CO)2]SbF6 and [Rh(3)-
(CO)2]SbF6 (Runs 11–14) were determined at 130 °C. It can
be seen from Runs 11–14 that under these conditions, the
TOFs are somewhat lower but comparable to [RhCl-
(CO)2]2 (Runs 1–3).
Carbonylation Studies
Initial carbonylation reactions of methyl acetate were
carried out by using methyl iodide as the co-catalyst in a
solvent mixture of acetic acid and water (see Exp. Sect. for
details). Quantitative conversion of methyl acetate to acetic
acid can be achieved by using the benchmark catalyst pre-
cursor [RhCl(CO)2]2 under the conditions used (Run 2).
Runs 3 and 4 show that when the amount of rhodium cata-
lyst is lowered to 25 μmol, the conversion of methyl acetate
is reduced to approximately 50%. All other catalysts have
been tested under these reaction conditions (P = 30 bar, T
= 130 °C, t = 16 h), and the results in Table 5 show that
fairly similar conversions are obtained for all catalysts con-
taining ligands 1–5 compared to [RhCl(CO)2]2. In the ab-
sence of methyl iodide, no carbonylation was observed.
Stability studies in acetic acid have shown that the life-
time of the complexes decreases with an increase in the size
of the 6,6Ј-substituents at the bipyridine ligand. Further-
more, the oxidative addition experiments with MeI have
shown that the presence of iodide leads to the formation of
neutral complexes of the type [RhI(L)(CO)], which undergo
a much faster oxidative addition reaction compared to cat-
ionic complexes. Taking all results into consideration, we
conclude that under the carbonylation conditions used, in
the presence of excess iodide and CO, it is very likely that all
cationic complexes [Rh(L)(CO)2]+ studied here, are rapidly
converted to neutral complexes of the type [RhI(L)(CO)],
potentially followed by further slower substitution of the
bipy ligand to give a common active species [RhI2(CO)2]–
in all cases.
Table 5. Carbonylation of methyl acetate under aqueous condi-
tions.[a]
Run
Catalyst
Rh
[μmol]
MeI
[mmol]
Conv.[b] TOF
[%]
[h–1]
Conclusions
1
2
3
4
5
6
7
8
9
[RhCl(CO)2]2
[RhCl(CO)2]2
[RhCl(CO)2]2
58
80
24
26
24
22
25
23
24
25
25
90
80
80
90
80
80
80
80
80
80
80
73
100
48
49
41
40
42
39
35
43
47
160
160
250
240
220
230
210
210
180
220
240
A
series of cis-dicarbonylrhodium(I) complexes
[Rh(L)(CO)2]SbF6 containing 6,6Ј-disubstituted bipyridine
ligands has been synthesised. Depending on the size of the
substituents, a deviation between the plane of the bipyridine
ligand and the metal coordination plane is observed.
Metal–metal interactions as well π–π interactions are ob-
served in the solid-state structures of the complexes con-
taining ligands 2–4, which are thought to be responsible for
additional peaks observed in the carbonyl region of the IR
spectrum.
[RhCl(CO)2]2
[Rh(1)(CO)2]SbF6
[Rh(1)(CO)2]SbF6
[Rh(1)(CO)2]SbF6
[Rh(2)(CO)2]SbF6
[Rh(3)(CO)2]SbF6
[Rh(4)(CO)2]SbF6
[Rh(5)(CO)2]SbF6
10
11
[a] Conditions: solvent: AcOH (0.67 mol) and H2O (0.5 mol); sub-
strate: MeOAc (0.2 mol); P(CO) = 30 bar; T = 130 °C, t = 16 h. [b]
Conversion of methyl acetate, determined by 1H NMR spec-
troscopy.
The stability of the complexes containing ligands 3–5 un-
der conditions that are comparable to those during carbon-
ylation experiments, was investigated in CD3COOD by H
1
In order to avoid aqueous acidic conditions, the carbon- NMR spectroscopy. The stability decreases with the size of
ylation of methyl acetate to acetic anhydride was investi- the substituents in the 6,6Ј-positions and is believed to be
gated under anhydrous conditions by using methyl acetate related to the catalytic activity observed in carbonylation
as the substrate and solvent and LiI as the co-catalyst. The experiments. Whereas the more stable complexes containing
first three runs in Table 6 were carried out to confirm repro- ligands 1–3 generally show a slightly lower activity com-
ducibility. When the iodide concentration is halved, the pared to experiments with [RhCl(CO)2]2, the complexes
TOF is approximately halved (Runs 3/4 and Runs 5/6), in with ligands 4 and 5 show comparable activities. Taking all
line with a first-order dependence on the iodide concentra- these observations together, it is concluded that all com-
tion commonly observed in rhodium-based carbonylation plexes under the carbonylation conditions, where an excess
systems.[64,65] The effect of temperature is illustrated in of iodide is present, are eventually converted into the well-
Runs 6–8, whereby the TOF is dramatically increased at known carbonylation catalyst [RhI2(CO)2]–. In the case of
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Eur. J. Inorg. Chem. 2011, 3511–3522

Dicarbonylrhodium(I) Complexes of Bipyridine Ligands
Table 6. Carbonylation of methyl acetate under anhydrous conditions.[a]
Run
Catalyst
T
[°C]
Rh
[μmol]
LiI
[mmol]
Ac2O
[g]
AcOH
[g]
Conv.
[%]
TOF
[h–1]
1
2
3
4
5
6
7
8
11
12
13
14
[RhCl(CO)2]2
[RhCl(CO)2]2
[RhCl(CO)2]2
[RhCl(CO)2]2
[RhCl(CO)2]2
[RhCl(CO)2]2
[RhCl(CO)2]2
[RhCl(CO)2]2
[Rh(1)(CO)2]SbF6
[Rh(1)(CO)2]SbF6
[Rh(1)(CO)2]SbF6
[Rh(3)(CO)2]SbF6
130
130
130
130
130
130
150
195
130
130
130
130
42
42
41
42
78
78
76
36
41
41
40
41
30
30
30
15
15
7
15
16
30
30
30
30
1.8
1.2
1.3
0.7
1.7
0.5
3.1
34
0.1
0.1
0.1
0.2
0.2
0.4
0.3
0.3
0.3
0.2
0.6
2.7
0.3
0.4
0.2
0.3
4.0
3.6
3.2
2.1
4.2
1.6
7.7
76
1.0
1.4
0.9
1.3
29
27
25
15
17
7
33
660
8
11
7
10
[a] Conditions: substrate: MeOAc (0.51 mol); P(CO) = 40 bar; t = 16 h.
the less stable complexes [Rh(4)(CO)2]+ and [Rh(5)(CO)2]+,
substitution of the bipy ligands with iodido ligands is prob-
ably faster, resulting in similar activities being observed for
these complexes compared to [RhI2(CO)2]–, whereas for the
complexes containing ligands 1–3, the slower substitution
of these bipy ligands results in a reduced overall catalytic
activity. The bipyridine ligands 1–3 appear to retard the
carbonylation reaction and are therefore not suited for ap-
plication in carbonylation catalysis. A possible reason why
the cationic dicarbonylrhodium(I) complexes are less active
than [RhI2(CO)2]–, is the lower electron density at the rho-
dium(I) centre, which reduces the rate of oxidative addition
of MeI. Reactivity studies of [Rh(1)(CO)2]+ with MeI in
acetonitrile have shown that oxidative addition does occur,
but is much slower than the oxidative addition at the neu-
tral complex [RhI(1)(CO)], which is formed during the reac-
tion. Further substitution to remove the bipyridine ligands
and form the anionic complex [RhI2(CO)2]– was not ob-
served under these reaction conditions, but cannot be ex-
cluded at this stage. These results raise an important ques-
tion regarding the general applicability of ligand-substi-
tuted rhodium complexes in carbonylation catalysis, and
further studies will be needed to clarify this.
SbF6} and CCDC-748278 {for [Rh(4)(CO)2]SbF6} contain the sup-
plementary crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Solvents and Reagents: Diethyl ether and tetrahydrofuran were
dried by prolonged reflux under nitrogen in the presence of sodium
metal with a benzophenone ketyl indicator and distilled freshly
prior to use. Dichloromethane and acetonitrile were treated in a
similar manner, but by using calcium hydride as the drying agent.
Toluene and pentane were dried by passing through a column,
packed with commercially available Q-5 reagent (13% CuO on alu-
mina) and activated alumina (pellets, 3 mm), in a stream of nitro-
gen. Methyl acetate was dried with P2O5 and distilled under nitro-
gen. Acetone was dried with B2O3 and distilled under nitrogen. The
starting materials [RhCl(CO)2]2,[66] [RhCl(cod)]2,[67] [RhI2(COMe)-
(CO)(MeCN)]2,[63] 2,2Ј-dipyridone,[68] and the ligands 6,6Ј-di-
amino-2,2Ј-bipyridine (2)[42] and diethyl 2,2Ј-bipyridine-6,6Ј-bis-
(phosphonate) (5)[69] were prepared according to published pro-
cedures. The preparation of diethyl 2,2Ј-bipyridine-6,6Ј-bis(carbox-
ylate) (4) was adapted from a previously published procedure.[70]
Synthesis of Ligands
Diethyl 2,2Ј-Bipyridine-6,6Ј-bis(carboxylate) (4): A 300 mL high-
pressure reactor was charged with [PdCl2(PPh3)2] (90 mg,
0.13 mmol), 6,6Ј-dibromo-2,2Ј-bipyridine (1 g, 3.19 mmol), NEt3
(15 mL), ethanol (15 mL) and CO (10 bar). The reactor was heated
to 80 °C for 24 h before cooling to room temperature and venting
the CO. The resultant black suspension was pumped to dryness
before adding dichloromethane (50 mL) and filtering through a sil-
ica pad to afford an orange solution. The dichloromethane was
removed in vacuo and the product (0.68 g, 61%) purified on an
Et3N-deactivated silica column with a 90:10 hexane/dichlorometh-
ane solvent system. C16H16N2O4 (300.31): calcd. C 63.99, H 5.37,
N 9.33; found C 64.05, H 5.34, N 9.38. 1H NMR (400 MHz,
CDCl3): δ = 8.77 (d, 3J = 7.6 Hz, 2 H, Ar-H), 8.16 (d, 3J = 7.6 Hz,
Experimental Section
General Remarks: All moisture- and oxygen-sensitive compounds
were prepared by using standard high-vacuum line, Schlenk and
cannula techniques. A standard nitrogen-filled glove box was used
for any subsequent manipulation and storage of these compounds.
1
Standard H, 19F, 31P and 13C NMR spectra were recorded with a
1
Bruker AV-400 or DRX-400 spectrometer. H NMR spectra were
3
3
2 H, Ar-H), 7.99 (t, J = 7.6 Hz, 2 H, Ar-H), 4.5 (q, J = 7.1 Hz,
referenced to the residual nondeuterated solvent signal and 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. Mass spectra were re-
corded with either a VG Autospec or a VG Platform II spectrome-
ter. IR measurements were carried out with a Perkin–Elmer Spec-
trum RX or GX FT-IR spectrometer. In situ IR measurements
were performed with a thermostatted solution cell (CaF2 windows,
0.5 mm path length) as described previously.[38] Elemental analyses
were performed by the Science Technical Support Unit at The Lon-
don Metropolitan University. CCDC-748277 {for [Rh(3)(CO)2]-
3
4 H, CH2), 1.48 (t, J = 7.1 Hz, 6 H, CH3) ppm. 13C{1H} NMR
[101 MHz, (CD3)2CO]: δ = 15.6 (CH3), 63.1 (CH2), 125.7, 127.13,
140.27, 150.1 (Ar Cipso), 157.1 (Ar Cipso), 166.5 (CO2Et) ppm. IR
(nujol): ν = 1733 (CO) cm–1. ESI/MS+: m/z (%) = 323 [M + Na]+,
˜
100, 301 (48) [M + H]+.
6,6Ј-Bis(tert-butyldimethylsiloxy)-2,2Ј-bipyridine:
A 50 mL, two-
neck round-bottomed flask was loaded with 2,2Ј-bipyridone
(0.78 g, 4.14 mmol), tert-butyldimethylsilyl chloride (2.5 g,
16.6 mmol), a stirrer bar, fitted with a reflux condenser, and nitro-
gen was introduced to the system. Dry triethylamine was added
Eur. J. Inorg. Chem. 2011, 3511–3522
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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3519

G. J. P. Britovsek et al.
FULL PAPER
with stirring during which time a grey emulsion formed. The mix-
ture was subsequently refluxed overnight. The resulting suspension
was filtered under reduced pressure and the remaining solid washed
with a further 10 mL of triethylamine after which the solvent was
removed under reduced pressure. The resulting beige solid was dis-
solved in hot methanol, allowed to cool to room temp. and then
cooled to –20 °C to induce crystallisation. The white solid was
rapidly filtered and washed with two portions of cold methanol,
dried under vacuum, and stored in a glove box. Yield: 0.54 g (31%).
C22H36N2O2Si2 (416.71): calcd. C 63.41, H 8.71, N 6.72; found C
63.50, H 8.82, N 6.79. 1H NMR (CD2Cl2): δ = 0.42 [s, 12 H,
Si(CH3)2], 1.03 [s, 18 H, SiC(CH3)3], 6.71 (dd, J = 0.8, 8.1 Hz, 2
to form a light yellow solution and a white precipitate. The suspen-
sion was stirred for 20 min before filtering. The filtrate was added
to 6,6Ј-bis(tert-butyldimethylsilyloxy)-2,2Ј-bipyridine (0.144 g,
0.344 mmol) dissolved in deoxygenated technical grade acetone
(30 mL). The resultant dark orange solution was stirred for 30 min
before concentrating (ca. 2 mL) and adding pentane (30 mL) to
afford a yellow precipitate, which was isolated by filtration. The
yellow precipitate was dissolved in dichloromethane (15 mL), and
CO was bubbled through the solution for 30 min whilst stirring.
The solution was reduced to 2 mL before adding pentane (30 mL)
to afford the product as a green precipitate (0.162 g, 81%). The
solid was isolated by filtration, washed with pentane (2ϫ5 mL por-
H, Ar-H), 7.69 (t, J = 7.8 Hz, 2 H Ar-H), 7.92 (dd, J = 0.8, 7.5 Hz, tions), and dried in vacuo. C12H8F6N2O4RhSb (582.85): calcd. C
2 H, Ar-H) ppm. 13C{1H} NMR (CD2Cl2): δ = –4.5, 17.9, 25.6,
112.9, 113.6, 139.8, 153.7, 162.2 ppm. MS [CI]: m/z (%) = 417 (100)
[M + H]+.
24.73, H 1.38, N 4.81; found C 24.82, H 1.43, N 4.74. 1H NMR
[400 MHz, (CD3)2CO]: δ = 12.87 (br., OH), 8.17 (t, J = 8.0 Hz, 2
3
3
3
H, Ar-H), 7.96 (d, J = 8.0 Hz, 2 H, Ar-H), 7.30 (d, J = 8.0 Hz,
2 H, Ar-H) ppm. 13C{1H} NMR [101 MHz, (CD3)2CO]: δ = 114.4,
Synthesis of Metal Complexes
1
116.8, 145.8, 155.6 (Ar Cipso), 164.4 (Ar Cipso), 184.3 (d, JC-Rh
=
70.9 Hz, Rh-CO) ppm. 19F NMR [377 MHz, (CD3)2CO]: δ =
[Rh(1)(CO)2]SbF6: AgSbF6 (0.280 g, 0.82 mmol) was dissolved in
acetone (10 mL) and added to [RhCl(cod)]2 (0.204 g, 0.41 mmol)
to form a yellow solution and a white precipitate. The suspension
was stirred for 15 min before filtering. The filtrate was added to
2,2Ј-bipyridine (1; 0.127 g, 0.82 mmol) dissolved in acetone (5 mL).
The resultant orange solution was stirred for 20 min before remov-
ing the solvent in vacuo and re-dissolving in dichloromethane
(5 mL). CO was then bubbled through the solution for 30 min to
afford the product as a green precipitate (0.36 g, 80%), which was
isolated by filtration, washed with dichloromethane (2ϫ3 mL por-
tions) and dried in vacuo. C12H8F6N2O2RhSb (550.85): calcd. C
26.17, H 1.46, N 5.09; found C 26.25, H 1.49, N 5.04. 1H NMR
–123.0 (superposition of sext due to 121SbF6 and oct due to
–
–
123SbF ) ppm. IR (nujol): ν = 3324 (OH); 2108, 2052 (CO) cm–1.
˜
6
FAB/MS+: m/z (%) = 347 (100) [M]+. FAB/MS–: m/z (%) = 235
(100) [M]–.
[Rh(3)(CO)2]SbF6: AgSbF6 (0.213 g, 0.61 mmol) was dissolved in
acetone (15 mL) and added to [RhCl(cod)]2 (0.153 g, 0.31 mmol)
to form a yellow solution and a white precipitate. The suspension
was stirred for 15 min before filtering. The filtrate was added to
6,6Ј-diamino-2,2Ј-bipyridine (3; 0.116 g, 0.61 mmol) dissolved in
acetone (5 mL). The resultant solution was stirred for 20 min be-
fore removing the solvent in vacuo and re-dissolving the resultant
orange solid in dichloromethane (15 mL). CO was bubbled through
the orange/brown suspension for 30 min whilst stirring. The dichlo-
romethane suspension was concentrated (ca. 2 mL) before adding
diethyl ether (30 mL) to afford the product as a brown precipitate
(0.251 g, 73%). The solid was isolated by filtration and washed with
diethyl ether (2ϫ3 mL portions) before drying in vacuo.
C12H10F6N4O2RhSb (580.88): calcd. C 24.81, H 1.74, N 9.65;
3
[400 MHz, (CD3)2CO]: δ = 9.07 (d, J = 5.7 Hz, 2 H, Ar-H), 8.74
3
3
4
(d, J = 7.8 Hz, 2 H, Ar-H), 8.54 (td, J = 7.9, J = 1.2 Hz, 2 H,
Ar-H), 7.97 (ddd, 3J = 7.9, J = 5.7, 4J = 1.5 Hz, 2 H, Ar-H) ppm.
3
13C{1H} NMR [101 MHz, (CD3)2CO]: δ = 125.2, 129.6, 143.8,
1
156.0, 157.4 (Ar Cipso), 184.3 (d, JC-Rh = 69.5 Hz, Rh-CO) ppm.
19F NMR [377 MHz, (CD3)2CO]: δ = –122.2 (superposition of sext
due to 121SbF6– and oct due to 123SbF ) ppm. IR (nujol): ν = 2109,
–
˜
6
2054 (CO) cm–1. FAB/MS+: m/z (%) = 315 (100) [M]+. FAB/MS–:
m/z (%) = 235 (100) [M]–.
1
found C 24.95, H 1.79, N 9.56. H NMR (400 MHz, CD2Cl2): δ =
3
3
7.81 (t, J = 7.9 Hz, 2 H, Ar-H), 7.43 (d, J = 7.9 Hz, 2 H, Ar-H),
6.9 (d, 3J = 7.9 Hz, 2 H, Ar-H), 5.52 (br., 4 H, NH2) ppm. 13C{1H}
NMR [101 MHz, (CD3)2CO]: δ = 112.1, 113.3, 141.6, 153.3 (Ar
[Rh(2)(cod)]SbF6: AgSbF6 (0.131 g, 0.38 mmol) was dissolved in
acetone (10 mL) and added to [RhCl(cod)]2 (0.094 g, 0.19 mmol)
to form a light yellow solution and a white precipitate. The suspen-
sion was stirred for 20 min before filtering. The filtrate was added
to 6,6Ј-bis(tert-butyldimethylsilyloxy)-2,2Ј-bipyridine (0.158 g,
0.38 mmol) dissolved in deoxygenated technical grade acetone
(30 mL). The resultant dark orange solution was stirred for 30 min
before concentrating (ca. 2 mL) and adding pentane (30 mL) to
afford the product as a yellow solid (0.246 g, 75%). The solid was
isolated by filtration, washed with pentane (2ϫ3 mL portions),
and dried in vacuo. C18H20F6N2O2RhSb (635.01): calcd. C 34.05,
H 3.17, N 4.41; found C 33.97, H 3.20, N 4.35. 1H NMR
1
Cipso), 161.4 (Ar Cipso), 183.3 (d, JC-Rh = 69.1 Hz, Rh-CO) ppm.
19F NMR [377 MHz, (CD3)2CO]: δ = –123.4 (superposition of sext
due to 121SbF6– and oct due to 123SbF ) ppm. IR (nujol): ν = 3473,
–
˜
6
3378 (NH2); 2097, 2035 (CO) cm–1. FAB/MS+: mz (%) = 345 (100)
[M]+. FAB/MS–: m/z (%) = 235 (100) [M]–.
Crystal Data for [Rh(3)(CO)2]SbF6: [C12H10N4O2Rh]SbF6, M =
580.90, monoclinic, I2/a (no. 15),
a
=
=
12.96970(18),
93.0730(11)°, V
b
=
=
=
12.62358(16),
c
=
20.7263(3) Å,
β
3388.51(8) Å3,
Z
=
8, Dcalcd. =
2.277 gcm–3, μ(Mo-Kα)
2.646 mm–1, T = 173 K, yellow blocks, Oxford Diffraction Xcalibur
3
[400 MHz, (CD3)2CO]: δ = 11.58 (br., OH), 8.02 (t, J = 7.9 Hz, 2
3
3
3 diffractometer; 5653 independent measured reflections (Rint
=
H, Ar-H), 7.92 (d, J = 7.9 Hz, 2 H, Ar-H), 7.05 (d, J = 7.9 Hz,
0.0302), F2 refinement, R1(obs.) = 0.0210, wR2(all) = 0.0543, 5321
independent observed absorption-corrected reflections [|Fo| Ͼ
4σ(|Fo|), 2θmax = 65°], 252 parameters.
3
2 H, Ar-H), 5.34 (br., 4 H, CH), 2.39 (m, 4 H, CH2), 1.89 (q, J =
8.2 Hz, 4 H, CH2) ppm. 13C{1H} NMR [101 MHz, (CD3)2CO]: δ
= 168.5 (Ar Cipso), 156.8 (Ar Cipso), 144.8, 117.3, 115.3, 85.0 (d,
1JC-Rh = 12.5 Hz, Rh-CH), 32.2 (CH2) ppm. 19F NMR [377 MHz,
[Rh(4)(CO)2]SbF6: AgSbF6 (0.115 g, 0.333 mmol) was dissolved in
acetone (10 mL) and added to [RhCl(cod)]2 (0.082 g,
0.0.167 mmol) to form a yellow solution and a white precipitate.
The suspension was stirred for 15 min before filtering. The filtrate
was added to diethyl 2,2Ј-bipyridine-6,6Ј-biscarboxylate) (4; 0.1 g,
0.333 mmol) dissolved in acetone (5 mL) and stirred for 20 min be-
fore removing the solvent. The resultant solid was dissolved in
(CD3)2CO]: δ = –123.4 (superposition of sext due to 121SbF6 and
–
oct due to 123SbF ) ppm. IR (nujol): ν = 3375 (OH) cm–1. FAB/
–
˜
6
MS+: m/z (%) = 399 (100) [M]+. FAB/MS–: m/z (%) = 235 (100)
[M]–.
[Rh(2)(CO)2]SbF6: AgSbF6 (0.118 g, 0.344 mmol) was dissolved in
acetone (10 mL) and added to [RhCl(cod)]2 (0.085 g, 0.172 mmol)
3520
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Eur. J. Inorg. Chem. 2011, 3511–3522

Dicarbonylrhodium(I) Complexes of Bipyridine Ligands
3
3
dichloromethane (15 mL) before bubbling CO through the solution
for 2.5 h whilst stirring to give an orange/brown suspension. Pen-
tane (30 mL) was added to the suspension to yield the product as
an orange solid (0.178 g, 77%), which was isolated by filtration and
dried in vacuo. C18H16F6N2O6RhSb (694.98): calcd. C 31.11, H
10.03 (d, J = 5.7 Hz, 1 H, Ar-Hortho), 8.96 (d, J = 5.1 Hz, 1 H,
Ar-Hortho), 8.18 (t, 3J = 7.8 Hz, 2 H, Ar-Hmeta), d 8.04 (m, 2 H, Ar-
Hpara), 7.67 (m, 1 H, Ar-Hmeta), 7.58 (m, 1 H, Ar-Hmeta), 3.16 (s, 3
H, CH3) ppm. 13C{1H} NMR (101 MHz, CDCl3): δ = 217.8 (d,
1JC-Rh = 18.6 Hz, Rh-COMe), 185.3 (d, 1JC-Rh = 62.1 Hz, Rh-CO),
155.3 (Ar-Cipso), 153.8 (Ar-Cortho), 153.3 (Ar-Cipso), 151.0
1
2.32, N 4.03; found C 30.99, H 2.25, N 3.93. H NMR [400 MHz,
(CD3)2CO]: δ = 9.06 (d, 3J = 7.8 Hz, 2 H, Ar-H), 8.75 (t, 3J = (Ar-Cortho), 139.7 (Ar-Cpara), 139.2 (Ar-Cpara), 126.7 (Ar-Cmeta),
3
3
7.8 Hz, 2 H, Ar-H), 8.57 (d, J = 7.8 Hz, 2 H, Ar-H), 4.63 (q, J
126.6 (Ar-Cmeta), 123.0 (Ar-Cmeta), 123.0 (Ar-Cmeta), 52.8 (s, CH3)
= 7.1 Hz, 4 H, CH2), 1.51 (t, 3J = 7.1 Hz, 6 H, CH3) ppm. 13C{1H} ppm. IR (nujol): ν = 2058 (CO), 1694 (COMe) cm–1. ESI/MS+: m/z
˜
NMR [101 MHz, (CD3)2CO]: δ = 14.4 (C-CH3), 64.9 (C-CH2),
128.5, 129.9, 145.1, 152.3 (Ar Cipso), 157.7 (Ar Cipso), 165.3
(%) = 457 (100) [M – I]+.
Methyl Acetate Carbonylation with MeI: A 300 mL high-pressure
reactor was charged with AcOH (40 g, 0.67 mol), MeOAc (15 g,
0.2 mol), H2O (9 g, 0.5 mol), MeI (10.5 g, 74 mmol), a rhodium
compound (25 μmol) and CO (30 bar) before heating at 130 °C for
16 h. After cooling the reactor 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 MeCN (50 mg)
before mixing and analysing by 1H NMR spectroscopy in [D6]-
DMSO. The molar composition of the reaction mixture was deter-
1
(CO2Et), 182.4 (d, JC-Rh = 60.0 Hz, Rh-CO) ppm. 19F NMR
[377 MHz, (CD3)2CO]: δ = –123.3 (superposition of sext due to
–
–
121SbF6 and oct due to 123SbF ) ppm. IR (nujol): ν = 2097, 2041
˜
6
(CO); 1722 (CO) cm–1. FAB/MS+: m/z (%) = 403 (100) [M –
2 CO]+, 459 (8) [M]+. FAB/MS–: m/z (%) = 235 (100) [M]–.
Crystal Data for [Rh(4)(CO)2]SbF6: [C18H16N2O6Rh](SbF6), M =
694.99, orthorhombic, Pna21 (no. 33), a = 15.66829(19), b =
15.58784(16), c = 18.44328(18) Å, V = 4504.49(8) Å3, Z = 8 (two
1
independent complexes), Dcalcd.
= =
2.050 gcm–3, μ(Cu-Kα)
mined by integration of the H NMR peaks relative to the MeCN
16.266 mm–1, T = 173 K, yellow needles, Oxford Diffraction Xcali-
bur PX Ultra diffractometer; 8682 independent measured reflec-
tions (Rint = 0.0510), F2 refinement, R1(obs.) = 0.0315, wR2(all) =
0.0873, 7051 independent observed absorption-corrected reflec-
tions [|Fo| Ͼ 4σ(|Fo|), 2θmax = 143°], 633 parameters. The absolute
structure was shown to be a polar twin by a combination of R-
standard (see Figure S10). Carbonylation reactions without methyl
iodide as the co-catalyst were carried out in a similar way, except
that more MeOAc was used (21 g, 0.28 mol).
Methyl Acetate Carbonylation with LiI: A rhodium compound
(42 μmol) and LiI (4 g, 30 mmol) were dissolved in anhydrous
MeOAc (38 g, 0.51 mol) before transferring to a nitrogen-purged
300 mL high-pressure reactor. The reactor was then pressurised
with CO (40 bar) and heated at 130 °C for 16 h. After cooling the
reactor to 5 °C, the CO was vented from the reactor, and the reac-
tion mixture was weighed. A 150 mg aliquot of the reaction mix-
ture was weighed into a vial with MeCN (50 mg) before mixing
+
–
factor tests (R1 = 0.0359, R1 = 0.0363) and by use of the Flack
parameter [x+ = +0.490(8), x– = +0.510(8)].
[Rh(5)(CO)2]SbF6: AgSbF6 (0.080 g, 0.234 mmol) was dissolved in
acetone (10 mL) and added to [RhCl(cod)]2 (0.058 g, 0.117 mmol)
to form a yellow solution and a white precipitate. The suspension
was stirred for 15 min before filtering. The filtrate was added to
diethyl 2,2Ј-bipyridine-6,6Ј-bis(phosphonate) (5; 0.1 g, 0.234 mmol)
dissolved in dry acetone (5 mL) whilst stirring. After 20 min, the
volatiles were removed before adding toluene (25 mL) to afford a
brown solid suspension in a yellow solution. CO was bubbled
through the suspension for 30 min whilst stirring. The suspension
was then heated at 80 °C under CO for 5 h. The product was iso-
lated as a green solid (0.124 g, 49%) by filtration before washing
with toluene (2ϫ5 mL) and drying in vacuo. C20H26F6N2O8RhSb
(761.08): calcd. C 29.19, H 3.18, N 3.4; found C 26.18, H 3.09, N
1
and analysing by H NMR spectroscopy in (CD3)2SO. The molar
composition of the reaction mixture was determined by integration
of the 1H NMR peaks relative to the MeCN standard (see Fig-
ure S11). Carbonylation reactions without the iodide co-catalysts
were carried out in exactly the same way, but without LiI.
Supporting Information (see footnote on the first page of this arti-
cle): Selected NMR and IR spectra of the complexes and NMR
spectra of the carbonylation product analysis and stability mea-
surements, as well as crystallographic details for complexes
[Rh(3)(CO)2]SbF6 and [Rh(4)(CO)2]SbF6.
1
3
3.13. H NMR (400 MHz, (CD3)2CO): δ = 8.91 (d, J = 7.7 Hz, 2
3
3
3
H), 8.60 (td, JHH = 7.7, JHP = 4.4 Hz, 2 H), 8.36 (t, J = 7.7 Hz,
3
2 H), 4.36 (m, 8 H, CH2), 1.41 (t, J = 7.1 Hz, 12 H, CH3) ppm.
Acknowledgments
13C{1H} NMR (126 MHz, (CD3)2CO): δ = 16.7 (C-CH3), 65.0 (C-
CH2), 127.0, 131.6 (d, 2JC-P = 18.3 Hz, Ar C), 142.5, 154.6 (d, JC-
1
We are grateful to BP Chemicals Ltd and Engineering & Physical
Sciences Research Council (EPSRC) for financial support. We
thank Johnson Matthey for their generous loan of RhCl3.
P = 237.9 Hz, Ar C-P), 157.2 (Ar Cipso), 181.4 (d, 1JC-Rh = 73.4 Hz,
Rh-CO) ppm. 31P NMR [162 MHz, (CD3)2CO]: δ = 9.58 ppm. 19
F
NMR (377 MHz, (CD3)2CO): δ = –123.4 (superposition of sext due
to 121SbF6 and oct due to 123SbF ) ppm. IR (nujol): ν = 2096,
–
–
˜
6
2033 (CO) cm–1. FAB/MS+: mz (%) = 531 (100) [M – 2 CO]+. FAB/
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Received: April 19, 2011
Published Online: July 8, 2011
3522
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© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2011, 3511–3522