An alternative strategy to an electron rich phosphine based carbonylation catalyst

Article abstract of DOI:10.1039/b301674a

The complexes [Rh(CO)Cl(2-Ph₂PC₆H₄COOMe)], 1, and trans-[Rh(CO)Cl(2-Ph₂PC₆H₄COOMe) ₂], 2, have been synthesized by the reaction of the dimer [Rh(CO)₂Cl]₂ with 2 and 4 molar equivalents of 2-(diphenylphosphino)-methyl benzoate. The complexes 1 and 2 show terminal v(CO) bands at 1979 and 1949 cm^-1 respectively indicating high electron density at the metal centre. The molecular structure of the complex 2 has been determined by single crystal X-ray diffraction. The rhodium atom is in a square planar coordination environment with the two phosphorus atoms trans to each other; the ester carbonyl oxygen atom of the two phosphine ligands points towards the rhodium centre above and below the vacant axial sites of the planar complex. The rhodium-oxygen distances (Rh ... O(49) 3.18 A Rh ...) and the angle O(19) ... Rh ... O(49) 179° indicate long range intramolecular secondary Rh ... O interactions leading to a pseudo-hexacoordinated complex. The complexes 1 and 2 undergo oxidative addition (OA) reactions with CH₃I to produce acyl complexes [Rh(COCH₃)ClI (2-Ph₂PC₆H₄COOMe)], 4, and trans-[Rh(COCH₃)ClI(2-Ph₂PC₆H₄COO-Me )(2-Ph₂PC₆H₄COOMe)], 5, and the kinetics of the reactions reveal that the complex 1 undergoes faster OA reaction than that of the complex 2. The catalytic activity of the complexes 1 and 2 in the carbonylation of methanol were higher than that of the well known species [Rh(CO)₂I₂]^- and the complex 1 shows higher activity than 2.

Full text of DOI:10.1039/b301674a

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An alternative strategy to an electron rich phosphine based
carbonylation catalyst†
Dipak K. Dutta,*‡a J. Derek Woollins,b Alexandra M. Z. Slawin,b Dilip Konwar,a Pankaj Das,a
Manab Sharma,a Pravat Bhattacharyyab and Stephen M. Aucottb
a Material Science Division, Regional Research Laboratory (CSIR), Jorhat, 785006, Assam,
India
b School of Chemistry, University of St. Andrews, Fife, Scotland, UK KY16 9ST
Received 11th February 2003, Accepted 13th May 2003
First published as an Advance Article on the web 28th May 2003
The complexes [Rh(CO)Cl(2-Ph2PC6H4COOMe)], 1, and trans-[Rh(CO)Cl(2-Ph2PC6H4COOMe)2], 2, have been
synthesized by the reaction of the dimer [Rh(CO)2Cl]2 with 2 and 4 molar equivalents of 2-(diphenylphosphino)-
methyl benzoate. The complexes 1 and 2 show terminal ν(CO) bands at 1979 and 1949 cmϪ1 respectively indicating
high electron density at the metal centre. The molecular structure of the complex 2 has been determined by single
crystal X-ray diffraction. The rhodium atom is in a square planar coordination environment with the two phosphorus
atoms trans to each other; the ester carbonyl oxygen atom of the two phosphine ligands points towards the rhodium
centre above and below the vacant axial sites of the planar complex. The rhodium–oxygen distances (Rh ؒ ؒ ؒ O(49)
3.18 Å; Rh ؒ ؒ ؒ O(19) 3.08 Å) and the angle O(19) ؒ ؒ ؒ Rh ؒ ؒ ؒ O(49) 179Њ indicate long range intramolecular
secondary Rh ؒ ؒ ؒ O interactions leading to a pseudo-hexacoordinated complex. The complexes 1 and 2 undergo
oxidative addition (OA) reactions with CH3I to produce acyl complexes [Rh(COCH3)ClI(2-Ph2PC6H4COOMe)],
4, and trans-[Rh(COCH3)ClI(2-Ph2PC6H4COO-Me)(2-Ph2PC6H4COOMe)], 5, and the kinetics of the reactions
reveal that the complex 1 undergoes faster OA reaction than that of the complex 2. The catalytic activity of the
complexes 1 and 2 in the carbonylation of methanol were higher than that of the well known species [Rh(CO)2I2]Ϫ
and the complex 1 shows higher activity than 2.
Introduction
Results and discussion
Carbonylation of methanol to ethanoic acid by Monsanto’s
species, [Rh(CO)2I2]Ϫ, is one of the most successful applications
of homogeneous catalysis in industry.1–2 Considerable effort has
been devoted to improve the catalyst by incorporating different
ligands3–11 in the metal complex species. In particular, hemi-
labile phosphines8–11 are of special interest because of their
multifunctional behaviour, which is believed to stabilize the
species through chelate formation. OA reactions are very
important in respect of catalysis particularly of complexes
containing tertiary phosphines functionalized with other donor
groups. Because of the electronic and steric effects, such ligand
can play important roles in catalysis.12 In general, it is known
that the tendency of a metal complex to undergo oxidative
addition reaction increases by (i) increasing the donor capacity
of the ligand i.e. by increasing the electron density on the cen-
tral metal atom, which increases the nucleophilicity; and (ii)
decreasing the bulkiness of the ligand. We have examined one
ligand, 2-(diphenylphosphino)methyl benzoate, where the
tertiary phosphine is functionalized with an electron with-
drawing –COOMe group, which should reduce the donor
capacity of phosphorus and consequently reduce the electron
density on the central metal of the complex. However, in our
present study, we have found surprisingly high electron density
The reaction of the chloro-bridged dimer [Rh(CO)2Cl]2 in
CH2Cl2 with two molar equivalents of 2-(diphenylphosphino)-
methyl benzoate, (2-Ph2PC6H4COOMe), ligand proceeds
rapidly with the evolution of CO gas to yield the yellow
chelated monocarbonyl complex 1. The complex 1 shows a
single terminal ν(CO) band at 1979 cmϪ1, which is the char-
acteristic region for such a rhodium()–carbonyl complex and is
at significantly lower frequency compared to the analogous
rhodium hemilabile phosphine complexes (Table 1).13–22 This
low ν(CO) value suggests a high electron density on the
rhodium centre. The ester carbonyl group of the complex 1
shows the ν(COO) band at 1638 cmϪ1, which is about 80 cmϪ1
lower than the free ligand value (1717 cmϪ1) indicating chel-
ation.23,24 The complex 1 shows a characteristic 31P-{1H} spec-
trum with a doublet at δ 45.1 (d, JRh–P 162 Hz) and H NMR
spectra with phenylic multiplet δ 7.1–8.2 (m, 14 H, Ph) and
methyl singlet δ 4.1 (s, 3 H, –COOCH3). Thus, the 31P-{1H}
NMR spectrum shows a downfield shift of about 48 ppm
from that of the free ligand value (δ Ϫ3.2 ppm) indicating
coordination through the phosphorus donor. On the other
hand, when the dimer [Rh(CO)2Cl]2 in CH2Cl2 is allowed to
react with 4 molar equivalents of 2-Ph2PC6H4COOMe) ligand,
a pale yellow complex 2 is obtained. The complex 2 surprisingly
shows the terminal ν(CO) band at 1949 cmϪ1, which is signif-
icantly lower in frequency compared to analogous rhodium
hemilabile phosphine complexes (Table 1) and also than that of
the well known complex trans-[Rh(CO)Cl(PPh3)2] (ν(CO) =
1964 cmϪ1).25 This low ν(CO) value of the complex 2 is
attributable to the formation of two Rh ؒ ؒ ؒ O secondary bonds
(Fig. 1) by which the ester oxygen atoms donate electron density
at the metal centre, which in turn donates electron density to
the antibonding π* orbital of CO and consequently lowers the
bond order of CO. Complex 2 shows characteristic NMR
1
1
on the rhodium centre of [Rh(CO)Cl(2-Ph2PC6H4COOMe)], 1,
and [Rh(CO)Cl(2-Ph2PC6H4COOMe)2], 2, which leads to
higher catalytic activity towards the carbonylation reaction of
methanol. Here we also report the OA reaction of CH3I with
the complexes 1 and 2 and their kinetic behaviour.
† This paper is dedicated to the memory of Prof. Noel McAuliffe in
recognition of his contribution to phosphine chemistry and friendship.
‡ E-mail: dipakkrdutta@yahoo.com
D a l t o n T r a n s . , 2 0 0 3 , 2 6 7 4 – 2 6 7 9
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 3
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Table 1 Comparison of ν(CO) of the complexes [Rh(CO)Cl(P∩X)] and [Rh(CO)Cl(P∼X)2] for different hemilabile phosphine ligands
[Rh(CO)Cl(P∩X)]
PX (X = O, S, N)
PX
[Rh(CO)Cl(P∼X)2]
PX (X = O, S, N)
PX
ν(CO)/cmϪ1 a
Ref.
ν(CO)/cmϪ1 a
Ref.
Ph2P(CH2)2SMe
Ph2P(CH2)2SEt
Ph2P(CH2)2SPh
2-Ph2PC6H4SMe
1990
1985
2000b
1998d
1990b
1992b
1985
1995
1990b
1990b
2005
1990
1979
13
14
15
8
16
9
10
10
16
16
17
18
(C6H11)2PCH2CH2OMe
Ph2PCH2SPh
Ph2P(CH2)2SPh
2-Ph2PC6H4CHO
But 2PCH2C(O)Ph
Ph2P(3-C5H4N)
2-Ph2PC6H4COOMe
4-Ph2PC6H4COOMe
1953
1957
1964
1971
1951c
1982
1949
1977
19
15
15
20
21
22
Ph2P(CH2)2OEt
Ph2PCH2P(S)Ph2
Ph2P(CH2)P(O)Ph2
Ph2P(CH2)2P(O)Ph2
Ph2P(CH2)P(O)(OPh)2
Ph2P(CH2)P(O)(OiPr)2
2-Ph2PC6H4N(Me)2
Ph2P(CH2)2C5H4N
2-Ph2PC6H4COOMe
This work
This work
This work
a In KBr, unless otherwise stated. b In CH2Cl2. c In Nujol. d Medium not mentioned.
Table 2 Crystal data and structure refinement details for complex 2
Formula
Formula weight
C41H34ClO5P2Rh
806.98
Temperature/K
Crystal system
Space group
293(2)
Triclinic
P1
Z
a/Å
b/Å
c/Å
1
9.2330(18)
9.5526(18)
11.657(2)
109.932(3)
108.346(3)
96.835(4)
0.692
α/Њ
β/Њ
γ/Њ
µ(Mo-Kα)/mmϪ1
Reflections collected
Independent reflections [R(int)]
Final R1, wR2 [I > 2σ(I )]
4406
3566 [0.0618]
0.0476, 0.0963
The thermal stability of the complex 2 in CD2Cl2/D-toluene
in the temperature range ϩ 80 to Ϫ80 ЊC was studied by 31P
NMR spectroscopy. The 31P NMR spectrum of the complex
2 shows a doublet at δ 35.9 ppm and the coupling constant is
1JRh–P = 136.5 Hz (CD2Cl2) at 30 ЊC, which marginally lowers
Fig. 1 Molecular structure of the complex 2 showing atomic labeling.
spectra26,27 31P-{1H} NMR spectrum with a doublet at δ 35.9
:
(d, 1JRh–P 136 Hz) and 1H NMR with phenylic multiplet δ 7.1–
8.3 (m, 14 H, Ph) and methyl singlet δ 3.7 (s, 3 H, –COOCH3).
Similar to complex 1, the complex 2 shows a 31P-{1H} NMR
spectrum with a downfield shift of about 39 ppm from that of
the free ligand (δ Ϫ3.2 ppm) indicating coordination through
the phosphorus donor. In order to ascertain the influence of the
position of the substituted COOCH3 on the phenylic ring of the
triphenylphosphine ligand, we have also prepared the complex
trans-[RhCOCl(4-Ph2PC6H4COOMe)2], 3, which shows the
terminal ν(CO) band at 1977 cmϪ1 and the ester ν(COO) band
at 1717 cmϪ1 indicating non-formation of the secondary bond.
Thus, the ν(CO) absorption of the complexes 2 and 3 clearly
indicate that in the former complex the metal centre is much
richer in electron density and hence expected to show higher
nucleophilicity. Miller and Shaw12 demonstrated that OA
reaction of the complex trans-[IrClCO{PMe2(2-MeOC6H4)}2]
with CH3I is much faster (100-fold) compared to those com-
plexes containing para or unsubstituted phosphine ligands. The
higher activity is attributed to the electron donation from the
methoxy oxygen by forming an Ir ؒ ؒ ؒ O bond and thereby
increasing the nucleophilicity of Ir and thus resulting in a
lower activation energy for the OA reactions. Recently, Cole-
Hamilton et al.4 reported a few electron rich complexes of the
type [Rh(CO)X(PEt3)2] (X = Cl, Br, I) having ν(CO) ca. 1960
cmϪ1, which showed high catalytic activity in carbonylation
of methanol, and postulated that electron rich centres play a
significant role in improving the rate of the reaction.
1
to JRh–P = 131.5 Hz at 60 ЊC and on further lowering the tem-
perature, the broadening is observed. With the increase of
temperature to ϩ80 ЊC, there is no remarkable change in the
peak position and coupling constant value (1JRh–P = 138.5 Hz
(D-toluene)). Thus, it indicates that the complex 2 remains
stable in solution in the temperature range ϩ 80 to Ϫ60 ЊC.
Single crystal X-ray structure
Some suitable crystals of the complex 2 were grown by slow
diffusion of diethylether into a CH2Cl2 solution of the complex.
The crystal structure was determined by single-crystal X-ray
diffraction studies. The crystal data are given in Table 2. A
perspective view of the complex molecule and numbering
scheme are shown in Fig. 1. Selected bond lengths and bond
angles are listed in Table 3. The rhodium atom lies in an
approximately square planar environment formed by the two
phosphorus donors, Rh, C (of CO) and Cl atoms. The ester
carbonyl oxygen atom of the two phosphine ligands points
towards the rhodium centre above and below the vacant axial
sites of the planar complex. The observed rhodium–oxygen
distances (Rh ؒ ؒ ؒ O(49) 3.18 Å; Rh ؒ ؒ ؒ O(19) 3.08 Å) and the
angle O(19) ؒ ؒ ؒ Rh ؒ ؒ ؒ O(49) 179Њ indicate a long range
intramolecular secondary Rh ؒ ؒ ؒ O interactions leading to
a pseudo-hexacoordinated complex. A similar type of long
range interaction was earlier reported28 for a palladium
D a l t o n T r a n s . , 2 0 0 3 , 2 6 7 4 – 2 6 7 9
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1
1
Table 3 Selected bond lengths (Å) and angles (Њ) for complex 2
centred at δ 47.0 with JRh–P 172 Hz and H NMR spectrum
with δ 7.0–8.1 (m, 14H, Ph); 4.2 (s, 3H, COOCH3) and 2.3 (s,
3H, COCH3). Thus, the 31P-{1H} NMR spectrum in the
oxidized product shows a downfield shift by about 2 ppm
compared to the parent complex 1, which agree with previous
observations.29 The complex 5 shows a 31P-{1H} NMR
spectrum as doublets centred at δ 46.1 (d, 1JRh–P 162 Hz); δ 39.5
Rh(1)–C(30)
Rh(1)–P(1)
Rh(1)–O(19)
Rh(1)–O(49)
C(30)–O(30)
1.8690(11) Rh(1)–P(2)
2.331(5)
2.4084(11)
1.19(3)
2.327(5)
Rh(1)–Cl(1)
C(19)–O(19)
C(49)–O(49)
3.080 (12)
3.184 (12)
1.0989(11)
1.18(2)
1
1
C(30)–Rh(1)–P(1)
P(1)–Rh(1)–P(2)
P(1)–Rh(1)–Cl(1)
O(30)–C(30)–Rh(1)
C(7)–P(1)–Rh(1)
C(13)–P(1)–Rh(1)
C(43)–P(2)–Rh(1)
91.6(7)
179.2(3)
87.30(19) P(2)–Rh(1)–Cl(1)
156(2)
122.2(6)
114.5(6)
112.4(7)
C(30)–Rh(1)–P(2)
C(30)–Rh(1)–Cl(1)
88.8(7)
174.8(6)
92.34(19)
108.8(6)
111.9(6)
122.7(6)
(d, JRh–P 150 Hz); H NMR δ 7.1–8.3 (m, 14H, Ph), 4.0 (s,
3H, COOCH3 chelated), 3.8 (s, 3H, COOCH3) and 2.3 (s,
3H, COCH3). Thus, the 31P-{1H} NMR spectra in the
oxidized product show a downfield shift compared to the parent
complex 2.
C(1)–P(1)–Rh(1)
C(31)–P(2)–Rh(1)
C(37)–P(2)–Rh(1)
A kinetic study for the OA reaction of CH3I with the
complexes 1 and 2 was carried out in order to understand the
steric and electronic effect of the ligand. Fig. 2a shows a plot
of absorbance against time for the disappearance of the
terminal ν(CO) band of the complex 1 (1979 cmϪ1) and
formation of a new terminal ν(CO) band at around 2061 cmϪ1,
which is a characteristic band30,31 for Rh() complex 1Ј
(Scheme 1) along with acyl ν(CO) band at around 1677 cmϪ1 for
the complex 4 (Scheme 1). The decay of the terminal ν(CO)
band of the complex 1 indicates that the entire course of the
reaction proceeds in an exponential manner. The growth of the
band at around 2061 cmϪ1 suggests that the concentration
of the intermediate (1Ј) increases up to a period of about 10
minutes and then it starts decreasing in a manner similar to that
of the decaying curve of the complex 1. The rate of growth
of the band around 1677 cmϪ1 reveals that up to a period of
10 minutes, the rate of formation of the acyl complex 4 is slow
and then it increases sharply up to a period of about 25
minutes. A similar type of kinetics was also observed for the
complex of 2-diphenylphosphino anisole where the Pd ؒ ؒ ؒ O
distance was 3.17 Å. Another long range interaction for the
complex trans-[IrCOCl(PMe2(2-MeOC6H4)2] where an Ir ؒ ؒ ؒ O
secondary bond with the methoxy oxygen was reported.12
CCDC reference number 189598.
See http://www.rsc.org/suppdata/dt/b3/b301674a/ for crystal-
lographic data in CIF or other electronic format.
OA reactions and their kinetics
The OA reactions of the complexes 1 and 2 with excess CH3I
yield complexes [Rh(COCH3)ClI(2-Ph2PC6H4COOMe)], 4, and
trans-[Rh(COCH3)ClI(2-Ph2PC6H4COO-Me)(2-Ph2PC6H4-
COOMe)], 5, respectively through their corresponding
unisolable alkylrhodium() intermediates 1Ј and 2Ј (Scheme 1).
The 31P-{1H} NMR spectrum of the complex 4 shows a doublet
Fig. 2 The absorbance of ν(CO) against time for the different
carbonyl species: decay (ꢀ) of the terminal ν(CO) band for the
complexes 1 and 2; variation of intensity of terminal ν(CO) bands (×)
for the unisolable intermediates 1Ј and 2Ј; and the growth of the acyl
ν(CO) bands (᭜) of the complexes 4 and 5.
Scheme 1 Syntheses of complexes 1 and 2 and their OA reactions with
MeI.
D a l t o n T r a n s . , 2 0 0 3 , 2 6 7 4 – 2 6 7 9
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Table 4 Results of carbonylation reaction of methanol
Catalyst precursor
Time/h
Total conversiona (%)
Methyl acetateb (%)
Ethanoic acidb (%)
TONc
0.5
1.0
1.5
0.5
1.0
1.5
0.5
1.0
1.5
0.5
1.0
1.5
30
48
64
71
77
95
36
69
82
37
57
76
26.0
38.5
52.0
32.6
60.8
28.7
30.6
59.6
30.9
27.1
34.0
16.1
4.0
9.5
12.0
37.4
16.2
65.3
5.4
9.4
51.1
9.9
493
790
1052
1168
1267
1563
609
1135
1349
592
d
[Rh(CO)2I2]Ϫ
1
2
[Rh(CO)Cl(PPh3)2]
23.0
59.9
938
1228
a Conversion = {[CO consumed (mol)]/[CO charged (mol)]} × 100. CO consumption was determined from analysis of products by GC. b Yields of
methyl acetate and acetic acid were obtained from GC analyses. c TON = [amount of product (mol)]/[amount of catalyst (Rh mol)]. d Formed from
added [Rh(CO)2Cl]2 under the catalytic condition.
complex 2 (Fig. 2b) where the rate of decay of intensity of the
terminal ν(CO), (1949 cmϪ1) band is low up to a period of
10 minutes and then increases sharply till 75 minutes. The
change in absorbance of the terminal ν(CO) band at around
2062 cmϪ1 shows that initial concentration of the intermediate
2Ј increases up to a period of about 15 minutes and attains a
maximum and then it decrease slightly followed by a steady
state up to 75 minutes and thereafter declines to a minimum.
This steady state may indicate an equilibrium between the
unisolable intermediate 2Ј and the complex 2. During the period
of 15 to 75 minutes of reaction, a sharp decrease of the
terminal ν(CO) band for the complex 2 and sharp increase of
acyl ν(CO) band for the complex 5 were observed. Therefore, it
appears that a certain concentration (critical) of the unisolable
intermediate is required to be reached for accelerating the
reaction. Similar observations were also made by us in our
recent work.32
The presence of two bulky COOCH3 groups on the phos-
phine ligands sterically restricts the path of OA reaction of
CH3I to the Rh centre of the complex 2. On the other hand, the
secondary Rh ؒ ؒ ؒ O interaction makes the Rh centre more
nucleophilic, which facilitates the OA reaction33 but tends to
retard the migratory insertion due to a strong Rh–C bond.34 As
a result, both the OA and migratory insertion steps are slow to
attain equilibrium. Such an equilibrium was not observed in
case of the complex 1, perhaps because of the absence of steric
hindrance resulting in faster reaction kinetics.
catalytic activity than the unfunctionalized phosphine and this
higher activity may be due to higher electron density on the
central metal by the chelate formation through ester oxygen
donors of the ligand. From the electronic point of view
(indicated by the ν(CO) value), 2 should show higher catalytic
activity than 1, but in practice, the reverse situation was
observed. Similar behavior has already been observed (vide
supra) and was attributed to steric and electronic factors. Thus
the steric factor due to two bulky –COOCH3 group on the two
phosphine ligands in 2 sterically restrict the path of MeI
addition, which is the rate determining step for carbonylation
of methanol. On examining the catalytic reaction mixture by IR
spectroscopy at different time intervals, and at the end of the
catalytic reactions, multiple ν(CO) bands were obtained that
matched well with the ν(CO) values of solution containing a
mixture of the parent rhodium() carbonyl complexes 1 and 2
and rhodium() acyl complexes 4 and 5. Thus, it may be
inferred that the ligands remained bound to the metal centre
throughout the entire course of the catalytic reactions.
The importance of steric effects of ortho substituted bi-
dentate phosphines in polyketone catalysts has been neatly
demonstrated by Pringle and co-workers.37 We believe this work
demonstrates the potential for ortho substituents to provide
electron density to the metal centre via secondary coordination
with effects that are as profound as those associated with major
R group manipulation in phosphines.
Experimental
Catalytic activity of complexes 1 and 2 for carbonylation of
methanol
General procedure
The results of batch carbonylation of methanol to acetic acid
and its ester in the presence of complexes 1, 2, trans-[Rh(CO)-
Cl(PPh3)2] and [Rh(CO)2I2]Ϫ as catalyst precursors are shown in
Table 4. The GC analyses of the products reveal that as the
reaction time increases from 0.5 to 1.5 h, the total conversion as
well as Turn Over Number (TON) for the different catalyst
precursors increases irrespective of the complexes. The max-
imum conversion of methanol with corresponding maximum
TON were observed over the entire catalytic reaction period for
the complex 1 and the highest conversion (95%) and TON
(1563) were observed for a 1.5 h reaction period (Table 4).
The reagents were procured from Aldrich and Lancaster. All
solvents were distilled under N2 prior to use. All operations
were carried out under oxygen-free nitrogen atmosphere using
standard Schlenk technique. Microanalyses were performed by
the St. Andrews University service (Chemistry Department).
FT-IR spectra (4000–400 cmϪ1) were recorded using a Perkin
1
Elmer 2000 spectrometer in CHCl3 and as KBr discs. The H
NMR (270 MHz) and 31P-{1H} NMR (109.36 MHz) spectra
were recorded in CDCl3 solution on a Jeol Delta 270 MHz
Spectrometer. The carbonylation reactions of alcohol were
carried out in a 100 cm3 teflon coated high pressure reactor
(HR-100 Berghof, Germany) fitted with a pressure gauge and
the reaction products were analyzed by GC (Chemito 8510,
FID). The X-ray crystallography data were collected at room
temperature using Mo-Kα radiation with a SMART system
and the structure refinements were done by full matrix least
square on F 2 using SHELXTL 97 computer program. The
Under the same experimental conditions, the well-known
35,36
catalyst precursor [Rh(CO)2I2]Ϫ generated in-situ
from
[Rh(CO)2Cl]2 shows 64% conversion with a TON 1052 while
that the complex 2 and trans-[Rh(CO)Cl(PPh3)2] exhibit 82%
and 76% conversions respectively with corresponding TON
1349 and 1228. The data in Table 4, in general, reveal this order
of efficiency of the catalytic activity: complex 1 > complex 2 >
trans-[Rh(CO)Cl(PPh3)2] > [Rh(CO)2I2]Ϫ. It therefore becomes
obvious that the functionalized phosphine show higher
¯
space group was selected as P1 rather than P1 from the distri-
bution of intensity statistics {mean |E×EϪ1| = 0.784 [expected
0.968 centrosymmetric and 0.736 non-centrosymmetric]}.
D a l t o n T r a n s . , 2 0 0 3 , 2 6 7 4 – 2 6 7 9
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Preparation
Synthesis of complex [Rh(COMe)ClI(2-Ph2PC6H4COOMe)-
(2-Ph2PC6H4COOMe)], 5
Ligands 2-Ph2PC6H4COOCH3 and 4-Ph2PC6H4COOCH3 were
prepared by a literature method.38 The starting complex
[Rh(CO)2Cl]2 was prepared by a known method.39
0.112 g (0.138 mmol) of complex 2 was dissolved in 10 cm3
CH2Cl2 and to this 6 cm3 CH3I was added. The reaction mixture
was then stirred at room temperature under nitrogen atmos-
phere for about 12 h and the solvent was evaporated under
vacuum. The yellow-reddish colored compound so obtained
was washed with diethyl ether and stored over silica gel in a
desiccator. Yield 0.106 g. [Found (calc. for C42H37ClIO5P2Rh):
C, 54.12 (53.17), H, 3.81 (3.90).] Selected IR data (KBr): 1672
[ν(COO)/cmϪ1]; 1717 [ν(CO)acyl/cmϪ1]; NMR data: 31P-{1H}
Synthesis of the complex [Rh(CO)Cl(2-Ph2PC6H4COOMe)], 1
[Rh(CO)2Cl]2 0.032 g (0.082 mmol) was dissolved in 10 cm3
CH2Cl2 and to this 0.053 g (0.164 mmol) of the ligand in 10 cm3
CH2Cl2 was added dropwise with constant stirring under
nitrogen atmosphere. The reaction mixture was stirred at room
temperature for about 1 h and the solvent was evaporated under
reduced pressure in a rotavapor to obtain a yellow solid. The
compound so obtained was washed with diethyl ether and
stored over silica gel in a desiccator. Yield 0.071 g, 83.5%.
[Found (calc. for C21H17ClO3PRh): C, 51.02 (51.84), H, 3.88
(3.49).] Selected IR data (KBr): 1979 [ν(CO)/cmϪ1], 1638
[ν(COO)/cmϪ1]. NMR: 31P-{1H) NMR δ 45.1 (d, 1JRh–P 162 Hz);
1H NMR δ 7.1–8.2 (m, 14 H, Ph), 4.1 (s, 3 H, –COOCH3).
1
1
1
δ 46.1 (d, JRh–P 162Hz); δ 39.5 (d, JRh–P 150 Hz); H NMR
δ 7.1–8.3 (m, 14H, Ph), 4.0 (s, 3H, COOCH3 chelated), 3.8
(s, 3H, COOCH3) and 2.3 (s, 3H, COCH3).
Reaction kinetics
The OA reactions of complexes 1 and 2 with CH3I were
monitored by using IR spectroscopy in a solution cell (1.0 mm
path length). 10 mg (mmol) of complexes 1 and 2 were added to
1 cm3 of neat CH3I at room temperature. An aliquot (0.5 ml) of
the reaction mixture was transferred by syringe into the IR cell.
Then the kinetic measurement were made by monitoring the
simultaneous decay of the terminal ν(CO) of the complexes in
the range 1925–2100 cmϪ1 and growth of the acyl ν(CO) of
the complex 4 and 5 in the range 1600–1750 cmϪ1. A series
of spectra were taken at regular intervals. The OA reactions of
CH3I with complexes 1 and 2 were found to be concentration
dependent on the complexes as well as on CH3I. Therefore, in
order to provide a pseudo-first order condition, the reaction
was carried out in a large excess of CH3I.
Synthesis of the complex trans-[Rh(CO)Cl(2-Ph2PC6H4-
COOMe)2], 2
[Rh(CO)2Cl]2 0.031 g (0.079 mmol) was dissolved in 10 cm3
CH2Cl2 and to this 0.102 g (0.318 mmol) of the ligand 2-Ph2-
PC6H4COOCH3 in 10 cm3 CH2Cl2 was added dropwise with
constant stirring under nitrogen atmosphere. The reaction
mixture was stirred at room temperature for about 1.5 h and the
solvent was evaporated under reduced pressure in a rotavapor
to obtain a light yellow solid. The compound so obtained
was purified by washing with diethyl ether. Suitable crystals
of the complex were developed by slow diffusion of (C2H5)2O
into a CH2Cl2 solution of the complex. Yield 0.108 g,
81.2%. [Found (calc. for C41H34ClO5P2Rh): C, 60.55 (61.02),
Carbonylation of methanol using complexes 1 and 2 as catalyst
precursors
H, 3.98 (4.22).] Selected IR data (KBr): 1949 [ν(CO)/cmϪ1];
1
1715 [ν(COO)/cmϪ1]; NMR data: 31P-{1H} δ 35.9 (d, JRh–P
=
0.099 mol of CH3OH, 0.016 mol of CH3I, 0.055 mol of H2O,
0.054 mmol of the complexes 1 or 2 were taken in the reactor
and then pressurized with CO gas (20 bar at room temperature,
0.080 mol). The reaction vessel was then placed into the heated
jacket of the autoclave and the reactions were carried out at
130 5 ЊC (corresponding pressure 35 2 bar) with variation
of reaction time. The products were collected and analyzed
by GC. The recycle experiments were done by maintaining
the same experimental conditions as described above with the
dark brown solid mass as catalyst obtained by evaporating the
carbonylation reaction mixture under reduced pressure.
136 Hz); 1H NMR δ 7.1–8.3 (m, 14 H, Ph), 3.7 (s, 3H,
COOCH3).
Synthesis of the complex trans-[Rh(CO)Cl(4-Ph2PC6H4-
COOCH3)2], 3
[Rh(CO)2Cl]2 0.026 g (0.066 mmol) was dissolved in 10 cm3
CH2Cl2 and to this 0.085 g (0.264 mmol) of the ligand 4-Ph2-
PC6H4COOCH3 in 10 cm3 CH2Cl2 was added dropwise with
constant stirring under nitrogen atmosphere. The reaction
mixture was stirred at room temperature for about 90 min and
the solvent was evaporated under reduced pressure in a
rotavapor to obtain a light yellow solid. The compound so
obtained was purified by washing with diethyl ether. Yield 0.091
g, 81.98%. [Found (calc. for C41H34ClO5P2Rh): C, 60.41 (61.02),
H, 3.96 (4.22).] Selected IR data (KBr): 1977 [ν(CO)/cmϪ1];
Acknowledgements
We are grateful for the financial support by the UK-India
Science & Technology Research Fund. Thanks are also due
to DST, New Delhi, OST, UK, RRL Jorhat (CSIR) and the
University of St. Andrews, UK.
1
1723 [ν(COO)/cmϪ1]; NMR data: 31P-{1H} δ 30.72 (d, JRh–P
127 Hz); 1H NMR δ 7.2–7.9 (m, 14 H, Ph), 3.7 (s, 3H,
COOCH3).
References
Synthesis of the complex
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