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Synthesis characterization and hydroformylation activity of 7-azaindolate-bridged dinuclear rhodium(I)phosphines with pendant polar-groups.

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Full Paper
Received: 12 March 2009
Revised: 26 July 2009
Accepted: 29 July 2009
Published online in Wiley Interscience 30 September 2009
(www.interscience.com) DOI 10.1002/aoc.1546
Synthesis, characterization
and hydroformylation activity
of 7-azaindolate-bridged dinuclear
rhodium(I)phosphines with pendant
polar-groups
Chandra Sekhar Vasama,b , Sarangapani Modema , Shravankumar Kankalaa,
Geeta Budigea and Ravinder Vaddea∗
New dinuclear Rh(I)–Phosphines of the types [Rh(µ-azi)(CO)(L)]2 (1,3–7) and [Rh(µ-azi)(L)]2 (8) with pendant polar groups, and
a chealated mononuclear compound [Rh(azi-H)(CO)(L)] (2) (where azi = 7-azaindolate, L = polar phosphine) were isolated from
the reaction of [Rh(µ-Cl)(CO)2 ]2 with 7-azaindolate followed by some polar mono- and bis-phosphines (L1 –L8 ). A relationship
between δ 31 P-NMR and ν(CO) values was considered to define the impact of polar-groups on σ -donor properties of the
phosphines. These compounds were evaluated as catalyst precursors in the hydroformylation of 1-hexene and 1-dodecene
both in mono- and biphasic aqueous organic systems. While the biphasic hydroformylations (water + toluene) gave exclusively
the aldehydes, the monophasic one (aqueous ethanol) showed propensity to form both aldehydes and alcohols. The influence
of bimetallic cooperative effects, and σ -donor and hydrophilic properties of the phosphines with pendant polar-groups
in enhancing the yields and selectivity of hydroformylation products was emphasized. In addition, when strong σ -donor
phosphine was used, the π -acceptor nature of pyridine ring of 7-azaindolate spacer was found to be a considerable factor in
facilitating the facile cleavage of CO group during hydroformylation and in supplementing the cooperative effects. Copyright
c 2009 John Wiley & Sons, Ltd.
Keywords: polar-phosphines; rhodium(I); 7-azaindolate; hydroformylation; water-soluble
Introduction
460
The industrial success of propylene hydroformylation process
catalyzed by a water-soluble triphenylphosphine trisulfonate
(TPPTS)–Rh(I) complex has opened up a new area of research
in organometallic chemistry in view of the recycling of catalysts of
expensive metals[1] . Following this line, tremendous efforts have
been made to design a variety of Rh(I)–phosphines containing
various other pendant polar groups.[2] In this continuation, the synthesis of such Rh(I)–phosphines in dinuclear architectures using
various dinucleating spacers (bridging) ligands such as thiolates
and pyrazoles has attracted considerable interest in view of the cooperative effects between two metal centers, which are proposed
to enhance the catalytic efficiency.[3] It is worth mentioning that the
cooperative effects between two distinct metal centers in numerous metalloenzymes are well established,[4] and several complexes
have been designed as structural and functional mimics.
Previously one of us reported the synthesis of some polar tertiaryphosphines (L1 , L2 and L5 , Scheme 1), and their propensity
to design catalytically efficient dinuclear Rh(I)–phosphines using
pyrazoles as spacer or bridging ligand.[5] When [Rh(µ-Cl)(COD)]2
was used as the Rh(I) precursor, a synthetic methodology depicted
in Scheme 2 was developed.[5] Indeed the phosphines L1 and
L5 [(3-carboxyphenyl)- and (2-formylphenyl)diphenylphophine)]
have produced dinuclar Rh(I)-phosphines and the phosphine L2 (2-carboxyphenyl)diphenylphosphine) has produced a
Appl. Organometal. Chem. 2009 , 23, 460–466
chelating mononuclear Rh(I)–phosphine. Specifically, the dinuclear Rh(I)–phosphines were found to be superior to mononuclear
Rh(I)–phosphine in the catalytic hydroformylation.
In view of the continuous research and developments in
the chemistry of Rh(I)–phosphines,[2,3] we are interested in
introducing different N∧ N-bidentate spacers along with a range
of polar phosphines (Scheme 2, L1 –L8 ) to see whether the
electronic properties of spacer ligand controlled the chemical
behavior of these phophines and finally the catalytic efficiency
of the resulting Rh(I)–phosphines. We noticed that the anion
of 7-azaindole [1H-pyrrolo(2,3-bipyridine)] would be an entity
with a similar bridging potential to pyrazol to act as a versatile
dinucleating spacer ligand for the transition metals. In particular,
the combination of the electron-withdrawing (or n-deficient)
pyridine ring with the electron-releasing (or n-excessive) pyrrole
ring in 7-azaindole is expected to influence both structural and
catalytic properties of the Rh(I) compounds more than simple
∗
Correspondence to: Ravinder Vadde, Kakatiya University, Chemistry,
Vidhyaranyapuri, Warangal, Andhra Pradesh, 506009, India.
E-mail: ravichemku@rediffmail.com
a Department of Chemistry, Kakatiya University, Warangal, 506009, India
b Department of Chemistry, GITAM University, Visakhapatnam, 530045, India
c 2009 John Wiley & Sons, Ltd.
Copyright 7-Azaindolate-bridged dinuclear rhodium(I)phosphines
instead of [Rh(µ-Cl)(COD)]2 (shown in Scheme 2), with the available
precursor [Rh(µ-Cl)(CO)2 ]2 , we modified the synthetic strategy.
The synthetic reactions between [Rh(µ-Cl)(CO)2 ]2 and sodium
7-azaindolate were followed by polar-group functionalized phosphines L1 –L8 ; the sequential numbering of the reactants and
products is outlined in Scheme 3. The dinuclear Rh(I)–phosphines
1,3–8 were obtained by addition of phosphines L1 , L3 –L8 ,
whereas a mononuclear Rh(I)–phosphine 2 was afforded by the
addition phosphine L2 .
Characterization
All the complexes are air-stable and diamagnetic. They are freely
soluble in polar solvents and insoluble non-polar solvents. Except
for compound 1, the remainder were all freely soluble in water
and stable for longer times. Elemental analyses, FTIR, 1 H, 13 C,
31 P-NMR and electronic spectroscopies were used to characterize
compounds 1–8. The experimental analysis data is in close
agreement with the calculated data of the proposed molecular
formulae (see the Experimental section).
FTIR and NMR Data Analysis
Scheme 1. Chemical structures of polar-group functionalized phosphines
(L1 –L8 ).
Scheme 2. Synthesis of dinuclear Rh(I)-phosphines using [Rh(µ-Cl)(COD)]2.
pyrazole or thiolate spacers. Moreover the azaindole compounds
are also of pharmacological and photophysical interest.[6]
In this manuscript, we report the synthesis, characterization and
higher-alkene hydroformylation activity of 7-azaindolate bridged
dinuclear Rh(I)–phosphines containing pendant polar groups. An
attempt has been made to evaluate the σ -donor properties of
these phosphines with respect to the electronic properties of
the polar groups present on them and the competency with
carbonyl groups. The influence of bimetallic cooperative effects
and σ -donor properties of the phosphines on the yields and
selectivity of the products of hydroformylation conducted in
aqueous-organic solvent systems are highlighted. The advantages
with 7-azaindolate-type dinucleating spacers that have both
electron-deficient and -releasing ring systems, of favoring the
hydroformylation, are also described.
Results and Discussion
Synthesis
Appl. Organometal. Chem. 2009, 23, 460–466
c 2009 John Wiley & Sons, Ltd.
Copyright www.interscience.wiley.com/journal/aoc
461
The phosphines L1 –L8 were prepared using the methods reported
by us and others.[5,7] During the synthesis of Rh(I)–phosphines,
The dinuclear compounds of the type [Rh(µ-azi)(CO)(L)]2 (1,
3–7) displayed only one single broad IR band in the region
of 1925–1965 cm−1 for two terminal ν(CO)s. This situation
indicates that for steric reasons phosphines L1 , L3 –L7 replaced
only one terminal (CO) group on each Rh(I) center of dinuclear
tetracarbonyl intermediate [Rh(µ-azi)(CO)2 ]2 (Scheme-2) to form
[Rh(µ-azi)(CO)(L)]2 (1,3–7). Since the remaining two terminal (CO)
groups were trans to each other in [Rh(µ-azi)(CO)(L)]2 (1,3–7),
only one IR band was observed.[5d] This situation also indicates
that the two terminal phosphines are located in trans positions
in [Rh(µ-azi)(CO)(L)]2 . For the case of Rh(I)–phosphine 8, since
the bis-phosphine replaced all the four CO groups, no band
corresponding to ν(CO) was detected. Instead, the two new
bands observed at ∼1750 cm−1 were ascribed to ν(C O) of
maleic anhydride moiety of respective bis-phosphine (L8 ). The
mononuclear Rh(I)–phosphine 2 also had a band at 1955 cm−1 for
ν(CO). A discussion on compound 2 is presented below.
We have noted that the ν(CO) values depended on the σ -donor
properties of the phosphines, since both the ligands interact with
the same d–π orbital of the Rh(I) metal center. Electron density
donated by a phosphine to the Rh(I) metal center are back-donated
to the π -orbital of the CO ligand and reduce the ν(CO) frequency
dependent on the degree of σ -donation by phosphine. Instead of
ν(Rh–p)s, we considered 31 P-NMR data to evaluate the σ -donor
properties of these phosphines. All the compounds 1–8 exhibited
only one doublet signal in the downfield as compared with the
free ligand spectra and specified that two phosphines in dinuclear
compounds 1,3–7 have trans arrangement. For example, the
FTIR, 13 C and 31 P-NMR spectral features of coordinated carbonyl
and a phosphine with a pendant, i.e. non-bonded, carboxyl
functional group in compound 1 can be seen Fig. 1. The δ 31 P
values (δ complex – δ free phosphine) presented in Table 1
reveal that the electronic properties of the polar group on the
phosphines determine their δ-donating ability. The phosphines
with less electronic withdrawing substituent exhibited larger
δ 31 P values, i.e. their σ -donating ability was high. Summing
up, the increasing σ -donor properties of the mono-phosphines
in the Rh(I)–phosphines with reference to the δ 31 P values were
found in the order P(CH2 CH2 COOH)3 > EtP(CH2 COOH)2 >
C. S. Vasam et al.
Scheme 3. Synthetic reactions of Rh(I)-phosphines 1–8.
Figure 1. FTIR 13 C and 31 P-NMR spectral features of Rh(I)-phosphine (1).
462
Ph2 P(C6 H4 CH2 OH) > Ph2 P(2-C5 H4 N) > Ph2 P(C6 H4 -3-COOH) >
Ph2 P(C6 H4 -2-COOH) > Ph2 P(C6 H4 -2-CHO). Following this order, as
the δ 31 P values were increased, the ν(CO) values decreased due
to the increased the back-donation to the CO ligand from Rh(I),
i.e. the more the δ 31 P, the less the ν(CO) value (see Table 1). A
relationship made between δ 31 P and the ν(CO) is also presented
in Fig. 2 to confirm this conclusion. Further, the 13 C chemical shifts
for coordinated CO groups are consistent with the Varshavski
rule[8] where the compound with a lower ν(CO) shows a higher
13 C chemical shift for CO (see the Experimental section).
www.interscience.wiley.com/journal/aoc
Regarding the identity of N-coordinated 7-azaindolate, the IR
band near 1630cm−1 corresponding to the free C N group of 7azaindole was shifted to a lower frequency of ∼1580–1600 cm−1
after coordinating to Rh(I) metal ion. This is further supported
by 13 C NMR spectra, where downfield signals observed in the
range of 157–159 ppm were assigned for the coordinated C N
of 7-azaindolate.
Except in the compound 2, the polar groups present on
phosphines did not participate in the chelation along with
the phosphorous atom. Compounds 1,3,4 have characteristic
c 2009 John Wiley & Sons, Ltd.
Copyright Appl. Organometal. Chem. 2009, 23, 460–466
7-Azaindolate-bridged dinuclear rhodium(I)phosphines
Table 1. A relationship between δ 31 P-NMR and ν(CO) of
Rh(I)–phosphines
31
1
2
3
4
5
6
7
8
Complex
P-NMR (J Rh–P)
(ppm)
35.32 (162)
34.78 (158)
52.96 (160)
23.90 (164)
26.22 (162)
42.04 (164)
40.45 (162)
50.64 (164)
Free ligand
31
P-NMR
(ppm)
−6.4
−4.7
5.02
−26.3
−11.44
−3.8
−3.2
–
δ 31 P-NMR
(ppm)
ν(CO)
(cm−1 )
41.72
39.48
47.94
50.20
37.66
45.84
43.65
–
1955
1960
1932
1925
1965
1940
1947
–
FTIR bands at 1690–1710 cm−1 for non-bonded ν(COOH) and
compound 5 at 1680 cm−1 for non-bonded ν(CHO). The hydroxyl
functionality in compound 6 was also remained pendant. For
steric reasons, the pyridyl nitrogen of phosphine of compound
7 may not approach the Rh(I). The 1 H and 13 C NMR data also
supports the non-bonding nature of polar groups (see the
Experimental section). However, the phosphine which contains
the carboxylic group in the ortho position of phenyl ring as
in (2-carboxyphenyl)diphenylphosphine (L2 ) participated in the
chelation along with the phosphorous atom and produced the
mononuclear Rh(I)–phosphine 2. The two new absorption bands
corresponding to νasym (COO) at 1560 cm−1 and νsym (COO) at
1390 cm−1 , and the 13 C-NMR signal at 200.44 ppm support the
chelation. This process is assumed to involve proton transfer from
the carboxylic moiety to the azaindolate group through rhodiumassisted oxidative addition and reductive elimination as observed
in our previous report.[5d] Nevertheless, our concept is to synthesize
dinuclear Rh(I)–phosphines with pendant (non-bonded) polarfunctional groups in water-soluble catalyst precursors. Owing the
unavailability of single crystal X-ray diffraction method at our
institution, we depended mainly on spectroscopic techniques to
characterize these Rh(I)–phosphines.
Electronic Spectra
The electronic spectrum recorded for all the complexes in acetone
exhibited two or three bands of comparable intensity in the range
520–250 nm. The bands in the visible region may be assigned to
metal–ligand charge-transfer transaction,[9] whereas the bands
observed in the ultraviolet region are assigned to intra-ligand
transitions. No d–d transitions were detected. The Rh(I) species
is strongly reducing in character, so that the d–d transitions are
usually obscured by intense charge transfer transitions.
Catalytic Hydroformylation
Appl. Organometal. Chem. 2009, 23, 460–466
Biphasic hydroformylation
The biphasic hydroformylation of 1-hexene conducted in a water
+ toluene system at 50 ◦ C has shown limited conversions of
∼20–40% even after the prolonged reaction periods (>24 h).
However this reaction produced only the C7 -aldehydes selectively.
When the temperature was raised to 80 ◦ C, the conversion of
1-hexene was increased dramatically to 70–95%. The data
presented in Table 1 indicates that at higher temperatures
also only C7 -aldehydes were produced without any alcohol
biproducts. Further, a maximum linear to branched (L : B) ratio
of 9.6 : 1 was achieved. Notably, the dinuclear catalysts 1,4,5,6
and 7 showed better catalytic hydroformylation activity than
mononuclear catalyst 2, which could be due to the cooperative
effects between two Rh(I) metal centers.[3f] This observation is
in good agreement with Poilblanc’s proposal[10] and also other
reports, including our previous observations.
It is also interesting to observe that among the dinuclear
Rh(I)–phosphine catalysts (1,4–7), those with strong σ -donor
phosphines (i.e. those phosphines contain fewer electron withdrawing substituents) have improved the yields and L : B ratios of
the hydroformylation products. As shown in Table 2, the yields and
L : B ratios of the hydroformylation reaction enhanced by dinuclear
Rh(I)–phosphine catalysts are in the order 4 > 6 > 7 > 1 > 5.
However, according to the literature reports, the phosphines
with more electron-withdrawing substituents would improve the
yields and selectivity of hydroformylation.[11] This concept is different from our observation. It may be true for mononuclear
Rh(I)–phosphines, in which the more electron withdrawing substituents on the phosphine (i.e when the phosphine become a less
σ -donating) facilitate the carbonyl (CO) group cleavage and favor
the easy addition of alkene during the hydroformylation. However,
when we talk about the back-donation in 7-azaindolate-bridged
dinuclear Rh(I) catalysts [Rh(µ-azi)(CO)(L)]2 , there is also the possibility of back-donation from Rh(I) to pyridine ring of 7-azaindolate.
The π -acceptor properties of the pyridine transfer of electron
density from Rh(I) are known.[12] Since MLCT bands are present
in the absorption spectra, we propose that the electron density
received from phosphines could be resonating or moving across
the CO-Rh(I)–pyridine vector. In this connection, the CO group
cleavage during the hydroformylation cannot be obstructed by
strong σ -donor properties of the phosphine. Furthermore, in this
instance the kinetic stability of the Rh(I) catalysts may enhance
and avoid the use of excess of phosphine in catalytic reactions. If
this is the situation, 7-azaindolate can be considered as an ideal
bridging ligand to supplement the cooperative effects between
the two metal centers and minimizes the back-donation effects
on the cleavage of CO group. In addition to this explanation, the
hydrophilic nature of phosphine appears to be another potential
reason to accelerate the biphasic hydroformylation. In our experiments, the phosphines which possess strong σ -donor properties
are highly hydrophilic. For example, the phosphine L4 in dinuclear
Rh(I)–phosphines 4 is not only the strongest σ -donator among
them all, but also more hydrophilic due to the presence of more
number of carboxyl groups.
No rhodium metal leaching was observed during the reaction,
which is an important consideration in the catalysis by expensive
metal catalysts. Except for compound 5, the elemental analysis
data of these catalyst precursors did not change after the first
catalytic cycle. The results presented in this manuscript are comparable or relatively better than many other existing reports on
c 2009 John Wiley & Sons, Ltd.
Copyright www.interscience.wiley.com/journal/aoc
463
The dinuclear compounds 1,4,5,6 and 7 and the mononuclear
compound 2 were employed as catalysts in the hydroformylation
reactions of 1-hexene and 1-dodecene. These reactions were
investigated both in biphasic solvent system of water + toluene
and a monophasic system of aqueous ethanol at a temperature
of 50 and 80 ◦ C using CO + H2 pressure of 25 atm for
a required period of time. An alkylammonium salt, namely
tricaprylmethylammonium chloride, was used as the phase
transfer agent in the biphasic systems. We also attempted to
utilize dinuclear compound 8 in these reactions, but the phosphine
moiety was hydrolyzed.
Hydroformylation of 1-Hexene
C. S. Vasam et al.
Table 2. Catalytic hydroformylation of 1-hexene in water + toluene
(1 : 1)
Table 3. Single-phase catalytic hydroformylation of 1-hexene in
aqueous ethanol (1 : 1)
Yield of isomerized
Rh(I)
Conversion of
Yields of
1-hexene
catalyst
1-hexene
C7 -aldehyde L : B ratio
Rh(I)
catalyst
1
2
4
5
6
7
1
2
4
5
6
7
88.5
73.7
96
74.8
93.4
91.3
87
70
95
72
92
90
7.1
4.6
9.6
4.8
8.5
7.8
1.5
3.7
1.0
2.8
1.4
1.3
Yield of
C7 - aldehyde
L:B
Yield of
C7 -alcohol
L:B
62
51
74
54
69
70
4.3
1.8
5.6
2.1
5.0
4.7
20
17
21
18
24
20
6.2
6.7
2.5
7.8
5.2
3.9
Catalyst = 5 × 10−3 M; substrate : catalyst 1000 : 1, T = 80 ◦ C; CO +
H2 = 25 atm.
Catalyst = 5 × 10−3 M; substrate : catalyst 1000 : 1; T = 80 ◦ C; CO +
H2 = 25 atm.
bi/monophasic hydroformylation of 1-hexene using Rh(I) catalysts
in terms of yields and L : B ratios of aldehydes.[2k – m,3a – e] When
excess phosphine was added, there was no considerable improvement in terms of the selectivity or the yields. Catalyst recovery and
recycling experiments conducted with Rh(I)–phosphine 4 using
Baricelli’s method[3] showed >87% conversion of 1-hexene even
after four cycles with similar L : B ratios.
In order to find out the nature and stability of the catalytic
species, we carried out UV–visible spectroscopy experiments
similar to some previous reports by Baricelli.[3] The UV–visible
analysis of the aqueous phase that contained Rh(I) catalyst showed
two or three bands in the range 250–520 nm. The position of
these bands was almost unchanged from the bands that were
present before the catalysis of the Rh(I) catalysts and indicates that
possibly dinuclear Rh(I) catalytic species were present throughout
the hydroformylation reaction. As mentioned before, elemental
analysis experiments on the dinuclear Rh(I) catalyst provide
additional support for this proposal.
We also conducted experiments to study the Syngas (CO +
H2 ) pressure effect in the biphasic hydroformylation using the
Rh(I)–phosphines 1 and 4. At lower pressures (CO + H2 = <25
atm), the conversion of 1-hexene to aldehydes was very low and
accompanied by considerable amounts of isomerized hexene.
At higher pressures (CO + H2 = >40 atm) the conversion
of 1-hexene to C7 -aldehydes was improved to a small extent
but accompanied by hydrogenated products of C7 -alcohols and
n-hexane. Importantly, the linear C7 -aldehyde selectivity was
decreased [L : B ratios of C7 -aldehydes; 3.1 with Rh(I)–phosphine
1 and 4.3 with Rh(I)–phosphine 4]. Further, an increase in the H2
gas pressure (doubled) alone also led to the formation of mixture
of C7 -aldehyde, C7 -alcohol and n-hexane. Again the L : B ratios of
C7 -aldehydes were reduced [2.5 with Rh(I)–phosphine 1 and 3.3
with Rh(I)–phosphine 4], but the linear C7 -alcohol was increased
(L : B = 3.9 with catalyst 1 and 5.7 with catalyst 4).
Table 4. Catalytic hydroformylation of 1-dodecene in water +
toluene (1 : 1)
Single-phase hydroformylation
464
The catalytic hydroformylation reactions of 1-hexene conducted
in a single-phase system of aqueous–ethanol (1 : 1) have shown
a propensity to form both C7 aldehydes and the C7 -alcohols
almost in the ratio of 3 : 1. The data is presented in Table 3.
Further, when compared with the biphasic hydroformylation of
1-hexene, the L : B ratios of aldehydes were also decreased. To find
a source for hydrogen that encouraged the formation of alcohols,
we conducted additional experiments by increasing the ratio of
ethanol to water (2 : 1). In this case, the ratio between aldehyde
www.interscience.wiley.com/journal/aoc
Rh(I)
catalyst
1
2
4
5
6
7
Conversion
of 1-dodecene
Yields of
C13 -aldehyde
L:B
ratio
Yield of isomerized
1-dodecene
76.4
63.9
92.1
68.6
87.5
81.2
75
60
91
66
86
80
3.5
2.4
5.4
2.9
4.6
4.0
1.4
3.9
1.1
2.6
1.5
1.2
Catalyst = 5 × 10−3 M; substrate : catalyst 1000 : 1; T = 80 ◦ C; CO +
H2 = 25 atm.
and alcohol reached 1.5 : 1. This observation substantiates the role
of ethanol as a source of hydrogen. Another interesting feature
of the single-phase hydroformylation was the lesser reaction
times to achieve maximum conversion of 1-hexene as compared
with the biphasic system (Table 3). The increased polarity of the
solvent system may be the reason for this observation. Similar
to the observations made in biphasic hydroformylation, the
performance of dinuclear catalysts was found to be superior
to that of mononuclear catalyst. No isomerized hexene content
was identified in the single-phase hydroformylation.
Hydroformylation of 1-Dodecene
The results obtained from the biphasic system (water + toluene)
are provided. As shown in Table 4, the catalytic hydroformylation
of 1-dodecene produced selectively the C13 -aldehydes without
any C13 -alcohols. However, the L : B ratios were relatively low
when compared with the results of biphasic hydroformylation of
1-hexene. Again the dinuclear Rh(I)–phosphines were found to
be effective catalysts and among them those composed of strong
σ -donors and hydrophilic phosphines achieved the maximum
yields and L : B ratios. No rhodium metal leaching was also detected
in this reaction.
Conclusion
A synthetic strategy to afford catalytically useful dinuclear
Rh(I)–phophines with pendant polar groups and 7-azaindolate
spacer is described. This method usually provides only a transisomer for dinuclear [Rh(µ-azi)(CO)(L)]2 compounds. A relationship
c 2009 John Wiley & Sons, Ltd.
Copyright Appl. Organometal. Chem. 2009, 23, 460–466
7-Azaindolate-bridged dinuclear rhodium(I)phosphines
Experimental
The solvents, organic reagents and metal salts used were of AR
grade. The solvents were dried and deoxygenated by refluxing and
stored over sodium. The phosphines were prepared according to
methods reported reported by us and others.[5,7] All the synthetic
reactions were carried out in a nitrogen atmosphere using the
Schlenk technique. Elemental analyses were performed on a
Perkin Elmer 2400 CHN analyzer. FTIR spectra were recorded
on a Nicolet-740 spectrophotometer and Perkin-Elmer 580B
spectrophotometer. Brucker WH 270 NMR and Brucker SXP-100
instruments were used to record 1 H, 13 C and 31 P-NMR spectra.
Synthesis
[Rh(µ-azi)(CO)(Ph2 P-3-C6 H4 COOH)]2
Figure 2. A relationship between δ 31 P-NMR and ν(CO) values.
Figure 3. Structural drawing of Rh(I)-phosphine (1).
Appl. Organometal. Chem. 2009, 23, 460–466
[Rh(azi-H)(CO)(Ph2 P-2-C6 H4 COO)]
FTIR (ν cm−1 ): 1986 (br Rh–CO), 1560 (asym COO), 1390 (sym COO),
1595 (ms C N), 525 (ms Rh–P). 1 H-NMR (δ ppm): 13.50 (br, NH),
6.50–8.57 (m, aryl). 13 C-NMR (δ ppm): 208.95 (CO), 157.10 (C N),
200.44 (COO), 126.70–138.95 (aryl). Anal. calcd for C27H20 N2 O3 PRh:
C, 58.49; H, 3.61; N, 5.05. Found: C, 58.17; H, 3.28; N, 4.91%.
[Rh(µ-azi)(CO){C2 H5 P(CH2 COOH)2 }]2
FTIR (ν cm−1 ): 1958 (Rh–CO), 1700 (COOH), 1595 (C N), 525
(Rh–P). 1 H-NMR (δ ppm), 3.80 (d, J ≈ 4 Hz, CH2), 3.05 (q, J ≈ 3.5
Hz, CH2 of C2 H5 ), 2.50 (t, J ≈ 4 Hz, CH3 of C2 H5 ), 6.90–8.60 (aryl).
13
C-NMR (δ ppm): 182.58 (CO), 159.15 (C N), 169.84 (COOH),
39.70–41.55 (-CH2 ), 22.60 (-CH3 ), 129.30–140.60 (aryl). Anal. calcd
for C28 H32 N4 O10 P2 Rh2 : C, 39.53; H, 3.76; N, 6.58. Found: C, 39.42; H,
3.28; N, 6.91%.
[Rh(µ-azi)(CO){P(CH2 CH2 COOH)3 }]2
FTIR (ν cm−1 ): 1950 (Rh–CO), 1710 (COOH), 1590 (C N), 520
(Rh–P). 1 H-NMR (δ ppm): 10.9 (br, COOH), 3.50–3.80 (tt, J ≈ 4.3, CH2 -CH2 ), 6.75–8.51 (m, aryl). 13 C-NMR (δ ppm): 184.73 (CO), 158.25
(C N), 171.20 (COOH), 38.65–42.40 (-CH2 -CH2 ), 129.60–139.50
(aryl). Anal. calcd for C34 H40 N4 O14 P2 Rh2 : C, 40.97;H, 4.01;N, 5.62.
Found: C, 39.80; H, 3.98; N, 5.61%.
c 2009 John Wiley & Sons, Ltd.
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465
drawn between δ 31 P-NMR and ν(CO) values can be used to
evaluate the σ -donating ability of the phosphines. The δ 31 P
values were found to depend on the electronic properties
of pendant polar-groups on the phosphines. Because of the
unavailability of single-crystal X-ray data, at our present institution
we depend mainly on spectral data to explain the structural
parameters. Based on the information gained from elemental
analysis and spectral data, a square planar geometry proposed for
these Rh(I)–phosphines is shown in Fig. 3.
The dinuclear [Rh(µ-azi)(CO)(L)]2 compounds were proved to
be effective catalysts for the hydroformylation of higher alkenes
in biphasic and monophasic aqueous organic systems than
many similar dinuclear Rh(I)–phosphines. While this reaction in a
biphasic (water + toluene) system favored the selective formation
of aldehydes, the monophasic one in more polar aqueous ethanol
has shown a propensity to form a mixture of aldehydes and
alcohols. The role of bimetallic cooperative effects, σ -donors and
the hydrophilic nature of the phosphines to improve the yields and
L : B ratios of the hydroformylation products was optimized. Most
importantly the dinucleating spacers like 7-azaindolate, those
containing electron-withdrawing, i.e. π -acceptor, pyridine ring,
are helpful in minimizing the π -back-bonding effects from Rh(I)
to CO and facilitate its cleavage during the hydroformylation.
Except for the Rh(I)–phosphine 5, the remaining catalysts are
easily recoverable and recyclable specifically from the biphasic
solvent system of water + toluene mixture. The exact nature of
catalytic species participating in the reaction is known at this stage.
To a dichloromethane solution of [Rh(µ-Cl)(CO)2 ]2 (0.194 g
0.5 mmol), 0.140 g (1 mmol) of aqueous solution of sodium
azaindolate was added followed by 0.306g (1 mmol) of (3carboxyphenyl)diphenylphosphine. The reaction mixture was
stirred for a period of 2 h until the red colored reaction mixture
instantaneously changed to light yellow with the evolution of
carbon monoide. Later the reaction mixture was concentrated
to small volume to which diethyl ether was added to crystallize
the title product as a yellow semi-crystalline solid. This solid was
filtered by suction, washed with ether and dried in vacuum: FTIR
(ν cm−1 ): 1975(Rh–CO), 1695 (COOH), 1595 (C N), 535 (Rh–P).
1
H-NMR (δ ppm): 10.8 (s, COOH), 6.75–8.54 (m, aryl). 13 C-NMR (δ
ppm): 176.40 (CO), 157.85 (C N), 180.10 (COOH), 127.32–140.15
(aryl). Anal. calcd for C54 H40 N4 O6 P2 Rh2 : C, 58.49; H, 3.61; N, 5.05.
Found: C, 57.73; H, 3.59; N, 4.92%.
The following compounds were synthesized by using the same
reaction procedures.
C. S. Vasam et al.
[Rh(µ-azi)(CO)(Ph2 P-2-C6 H4 CHO)]2
FTIR (ν cm−1 ): 1980 (Rh–CO), 1680 (CHO), 1590 (C N), 510 (Rh–P).
1 H-NMR (δ ppm): 10.60 (br, CHO), 6.70–8.55 (m, aryl). 13 C-NMR (δ
ppm): 174.30 (CO), 158.25 (C N), 174.83 (CHO), 127.40–140.85
(aryl). Anal. calcd for C54 H40 N4 O4 P2 Rh2 : C, 60.23; H, 3.71; N, 5.20.
Found: C, 59.94; H, 3.48; N, 5.11%.
[Rh(µ-azi)(CO)(Ph2 P-2-C6 H4 CH2 OH)]2
FTIR (ν cm−1 ): 1940 (Rh–CO), 3410 (OH), 1595 (C N), 525 (Rh–P).
1 H-NMR (δ ppm): 2.51 (OH), 2.80 (s, CH2), 6.75–8.59 (m, aryl).
13 C-NMR (δ ppm): 180.14 (CO), 158.55 (C N), 65.60 (-CH ),
2
127.35–139.80 (aryl). Anal. calcd for C54 H44 N4 O4 P2 Rh2 : C, 60.01; H,
4.10; N, 5.18. Found: C, 59.94; H, 4.11; N, 5.15%.
[Rh(µ-azi)(CO)(Ph2 P-2-C5 H4 N)]2
FTIR (ν cm−1 ): 1969 (Rh–CO), 1590 (C N), 505 (Rh–P). 1 H-NMR (δ
ppm): 6.75–8.56 (m, aryl). 13 C-NMR (δ ppm): 178.52 (CO), 159.75
(C N), 127.65–139.80 (aryl). Anal. calcd for C50 H38 N6 O2 P2 Rh2 : C,
58.71; H, 3.71; N, 8.22. Found: C, 58.25; H, 3.74; N, 8.14%.
[Rh(µ-azi){Ph2 PC C(PPh2 )COOCO}]2
[3]
FTIR (ν cm−1 ): 1770 and 1750 (C O), 1590 (C N), 530
(Rh–P). 1 H-NMR (δ ppm): 6.70–8.60 (aryl). 13 C-NMR (δ ppm):
162.45&160.39 (CO), 157.25 (C N), 124.37–140.86 (aryl). Anal.
calcd for C70 H50 N4 O6 P4 Rh2 : C, 61.23; H, 3.64; N, 4.08. Found: C,
60.91; H, 3.28; N, 3.97%.
Hydroformylation Reactions
A mixture of 2 mmol of the 1-hexene or 1-dodecene, 5 × 10−3
mmol of the catalyst was mixed with water + toluene (1 : 1) or
water + ethanol, transferred in to a glass-lined miniautoclave and
purged with argon. Later, the reaction mixture was treated with CO
+ H2 at 25 atm and then placed in an oil bath thermostatted at the
desired temperature. After a period of 3–5 h, the reaction solutions
were cooled and analyzed by 1 H–NMR and GC techniques.
[4]
[5]
[6]
Acknowledgments
We are gratefully acknowledging the cooperation extended by
Professor H.Schumann, Technical University-Berlin, Germany and
Dr VG Akula, Scientist IICT-Hyderabad to obtain the important
spectra and catalytic data.
[7]
[8]
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dinuclear, synthesis, group, bridge, hydroformylation, rhodium, activity, characterization, pola, phosphine, azaindolate, pendant
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