Synthesis characterization and hydroformylation activity of new mononuclear rhodium(I) compounds incorporated with polar-group functionalized phosphines

Article abstract of DOI:10.2478/s11532-009-0108-x

A first effort employing a range of polar-group functionalized phosphines (L₁-L₇) to design mononuclear Rh(I) compounds of [Rh(quin-8-O)(CO)(L)] (quin-8-O = 8-hydroxy quinolate) is described. The reaction of a Rh(I) precursor [Rh(μ-Cl)(CO)₂]₂ with 8-hydroxyquinoline in the presence of a base followed by phosphines (L₁-L₇) produced only a single isomer of [Rh(quin-8-O)(CO)(L)] compounds (1-7) with pendant, i.e. non-bonded, polar-groups (includes carboxyl, hydroxyl and formyl). A relationship between Δgd³¹P chemical shifts and the ν(C≡O) was derived to evaluate and explain the σ-donor properties of these phosphines with respect to the electronic properties of the polar groups and the extent of π-back-bonding to the CO group. These mononuclear Rh(I)-Phosphines were investigated as catalysts in the hydroformylation of 1-hexene and cyclohexene in aqueous two-phase and single-phase solvent systems. The Rh(I) catalysts with strong σ-donor and hydrophilic phosphines provided better yields and selectivities for the hydroformylation products, which is a reverse trend compared to literature reports. When the Rh(I) compounds contained strong σ-donor phosphines, the π-acceptor properties of the pyridine ring of 8-hydroxyquinolate were found to be beneficial for the facile cleavage of the CO group during hydroformylation, and additionally, to improve the kinetic stability of catalysts. Versita Warsaw and Springer-Verlag Berlin Heidelberg.

Full text of DOI:10.2478/s11532-009-0108-x

Cent. Eur. J. Chem. • 8(1) • 2010 • 77–86
DOI: 10.2478/s11532-009-0108-x
Central European Journal of Chemistry
Synthesis characterization and hydroformylation
activity of new mononuclear rhodium(I)
compounds incorporated with polar-group
functionalized phosphines
Research Article
Chandra S. Vasam1,2, Sarangapani Modem1, Shravankumar Kankala1,
Shanker Kanne1, Geeta Budige1, Ravinder Vadde1*
1Department of Chemistry, Kakatiya University,
506009 Warangal, India
2Department of Chemistry, GITAM University,
530045 Visakhapatnam, India
Received 12 March 2009; Accepted 14 July 2009
Abstract: A first effort employing a range of polar-group functionalized phosphines (L1-L7) to design mononuclear Rh(I) compounds of
[Rh(quin-8-O)(CO)(L)] (quin-8-O = 8-hydroxy quinolate) is described. The reaction of a Rh(I) precursor [Rh(µ-Cl)(CO)2]2 with
8-hydroxyquinoline in the presence of a base followed by phosphines (L1-L7) produced only a single isomer of [Rh(quin-8-O)(CO)(L)]
compounds (1-7) with pendant, i.e. non-bonded, polar-groups (includes carboxyl, hydroxyl and formyl). A relationship between ∆δ31P
chemical shifts and the ν(C≡O) was derived to evaluate and explain the σ-donor properties of these phosphines with respect to the
electronic properties of the polar groups and the extent of π-back-bonding to the CO group. These mononuclear Rh(I)-Phosphines
were investigated as catalysts in the hydroformylation of 1-hexene and cyclohexene in aqueous two-phase and single-phase
solvent systems. The Rh(I) catalysts with strong σ-donor and hydrophilic phosphines provided better yields and selectivities for the
hydroformylation products, which is a reverse trend compared to literature reports. When the Rh(I) compounds contained strong
σ-donor phosphines, the π-acceptor properties of the pyridine ring of 8-hydroxyquinolate were found to be beneficial for the facile
cleavage of the CO group during hydroformylation, and additionally, to improve the kinetic stability of catalysts.
Keywords: Polar-phosphines • Rh(I) catalysts • 8-hydroxyquinolate • ∆d31P vs. n(C≡O) • Hydroformylation
© Versita Warsaw and Springer-Verlag Berlin Heidelberg.
to generate more efficient Rh(I)-phosphines [3]. This
concept is now expanded to other transformations such
1. Introduction
as carbonylation, cyclotrimerisation, hydrogenation, and
reduction [4].
Amongthetransition-metalphosphines, specialattention
has been focused on the synthesis of Rh(I)-phosphines
in view of their efficacy in industrial catalysis [1]. The
first successful accomplishment of hydroformylation
of propene using a water-soluble triphenylphosphine
trisulfonate ligand advanced the organometallic
chemistry of rhodium in view of the recovery and
recycling of expensive metal catalysts [2]. Based on
this principle, significant efforts were made to explore
other phosphines with varied polar functional groups
Mononuclear Rh(I) compounds of the type [Rh(L^L)
(CO)2] (L^L = donor based chelating monoanionic
bidentate ligands such as (O^O), (N^O) and (S^O))
in stable square planar geometry systems have been
studied extensively as precursors and model compounds
in catalysis [5]. A large number of reports are also
available about the incorporation of neutral monodentate
phosphines into these precursors [5], but limited efforts
* E-mail: ravichemku@rediffmail.com
77

Synthesis characterization and hydroformylation activity of new
mononuclear rhodium(I) compounds incorporated with
polar-group functionalized phosphines
were made with hemi-labile or polar phosphines. phosphines (L1-L7) containing carboxyl, hydroxyl and
Previously, in this laboratory [6], polar phosphines and formyl groups into the [Rh(quin-8-O)(CO)2] precursor.
their chemistry with transition metal ions including Rh(I) The presence of variable functional groups at different
metal ion were studied. Incorporation of a range of positions in phosphines could be useful to evaluate the
polar phosphines (Scheme 1, L1-L7) into the N^O donor polarisability (σ-donor and π-acceptor) of all metal–
8-hydroxyqunolate (abbreviated as quin-8-O) containing ligand bonds in the coordination sphere. The catalytic
a [Rh(quin-8-O)(CO)2] precursor is of interest. The efficiency of these compounds is scrutinized in the
[Rh(quin-8-O)(CO)2] precursor was chosen because hydroformylation of two higher alkenes--1-hexene and
the pyridyl nitrogen atom of 8-hydroxyquinolate has a cyclohexene--in two-phase and single-phase aqueous
stronger trans effect than that of a phenolic oxygen--the organic systems. In this study, the influence of polar-
carbonyl group trans to the nitrogen atom in [Rh(quin- groups on the σ-donor and hydrophilic properties of
8-O)(CO)2] will be more easily substituted by phosphines the phophines (L1-L7) and the consequences on the
[5a,7]. As a result, only one mononuclear [Rh(quin-8-O) yield and selectivity of hydroformylation products were
(CO)(L)] compound will be produced, without isomers, emphasized. Moreover, the manner in which π-acceptor
and thereby, complications in respective catalytic properties of the pyridyl ring of 8-hydroxyquinolate are
reactions are avoided.
This manuscript
characterization and hydroformylation activity of
beneficial in hydroformylation was also highlighted.
describes
the
synthesis,
some new mononuclear Rh(I)-phosphines of the type 2. Experimental procedure
[Rh(quin-8-O)(CO)(L)] with pendant polar-groups, i.e.,
non-bonded polar-groups. To our knowledge this the The solvents, organic reagents and metal salts
first effort reporting the incorporation of a variety of polar used were of AR grade. The solvents were dried and
P
CH2CH2COOH
Ph
CH2CH2COOH
CH2CH2COOH
P
Ph
COOH
(L5) (4-carboxyphenyl)diphenylphosphine
(L1) Tris(carboxyethyl)phosphine
OH
CH2COOH
P
Ph
Ph
P
CH2COOH
Et
(L6) (2-carbenolphenyl)diphenylphosphine
(L2) Bis(carboxymethyl)ethylphosphine
CHO
COOH
P
P
Ph
Ph
Ph
Ph
(L7) (2-formylphenyl)diphenylphosphine
(L3) (2-carboxyphenyl)diphenylphosphine
COOH
P
Ph
Ph
(L4) (3-carboxyphenyl)diphenylphosphine
Scheme 1. Structural drawings of the tertiaryphosphines used in the present study.
78

C.S. Vasam et al.
deoxygenatedbyrefluxingandstoringthemoversodium. 2.1.3. [Rh(quin-8-O)(CO)(Ph2P-2-C6H4COOH )]
The phosphines were prepared according to methods IR (ν cm-1): 1985 (Rh-CO), 1695(COOH), 1600 (C=N),
1
previously reported [6,14]. All synthetic reactions were 525 (Rh-P). H-NMR (δ ppm) 10.95(s, COOH), 6.45-
conducted in a nitrogen atmosphere using the Schlenk 8.35(m, aryl). 13C-NMR (δ ppm): 182.41(CO), 171.30
technique.
(COOH), 157.48(C=N), 126.75-136.75(aryl) 138.65(s,
Elemental analyses were performed on a Perkin C-P). 31P-NMR (δ ppm) 35.28 (d, JRh-P 162 Hz). Anal.
Elmer 2400 CHN analyzer. IR spectra were recorded Calc. For C29H21NO4PRh: C, 59.90; H, 3.61; N, 2.41.
on a Nicolet-740 spectrophotometer and a Perkin- Found: C, 59.78; H, 3.41; N, 2.34%.
Elmer 580B spectrophotometer. A Jeol 100 MHz FT
NMR spectrophotometer and a Brucker WH 270 NMR 2.1.4. [Rh(quin-8-O)(CO)(Ph2P-3-C6H4COOH)]
spectrophotometer were used for recording 1H, 13C and IR (ν cm-1): 1967 (Rh-CO), 1700(COOH), 1590 (C=N),
1
31P-NMR spectra.
515 (Rh-P). H-NMR (δ ppm) 10.75 (s, COOH), 6.45-
8.60(m, aryl). 13C-NMR (δ ppm): 185.48(CO), 178.25
(COOH), 158.45(C=N), 126.15-138.10 (aryl), 139.85(s,
C-P). 31P-NMR (δ ppm) 39.18 (d, JRh-P 164 Hz). Anal.
2.1. Synthesis
2.1.1. [Rh(quin-8-O)(CO){P(CH2CH2COOH )3}]
[Rh(µ-Cl)(CO)2]2 (0.194 g 0.5 mmol) was dissolved Calc. For C29H21NO4PRh: C, 59.90; H, 3.61; N, 2.41.
in 15 mL of methanol. To this, a methanol solution of Found: C, 59.23; H, 3.49; N, 2.42%.
8-hydroxyquinoline (0.145 g L mmol), barium carbonate
(0.65 gms) and a dichloromethane solution of (tris- 2.1.5. [Rh(quin-8-O)(CO)( Ph2P-4-C6H4COOH)]
carboxyethyl)phosphine (0.286 g L mmol) were added IR (ν cm-1): 1975 (Rh-CO), 1690(COOH), 1595 (C=N),
drop wise over 0.5 h. The reaction mixture was stirred for 520 (Rh-P). 1H-NMR (δ ppm) 11.10 (s, COOH), 6.55-8.60
1.5 h. The reddish orange color of the reaction mixture (m, aryl). 13C-NMR (δ ppm): 184.16 (CO), 174(COOH),
was observed to turn lemon yellow with the evolution 157.65 (C=N), 125.75-139.20(aryl), 140.55 (s, C-P). 31P-
of carbon monoxide bubbles during the reaction period. NMR (δ ppm) 38.10 (d, JRh-P, 162 Hz). Anal. Calc. For
The solution volume was then reduced to about 10 mL C29H21NO4PRh: C, 59.90; H, 3.61; N, 2.41. Found: C,
under vacuum. Addition of diethyl ether (15 mL) yielded 59.05; H, 3.47; N, 2.29%.
a lemon yellow crystalline solid, which was washed
with ether and dried in vacuo: IR (ν cm-1): 1940 (Rh- 2.1.6. [Rh(quin-8-O)(CO)(Ph2P-2-C6H4-CH2OH)]
CO), 1705 (COOH), 1590 (C=N), 505 (Rh-P). H-NMR IR (ν cm-1): 1945(Rh-CO), 3450(OH), 1590 (C=N), 510
1
(δ ppm) 11.10(s, COOH), 2.90-3.40 (tt, J = ~ 4.2, -CH2- (Rh-P). 1H-NMR (δ ppm) 2.65(s, OH), 4.55(s, CH2),
CH2-), 7.05-8.55(m, aryl). 13C-NMR (δ ppm): 190.36 (s, 6.50-8.65(m, aryl). 13C-NMR (δ ppm): 187.22(CO),
CO), 170.25(s, COOH), 158.95 (s, C=N), 34.55-38.75(s, 158.25(C=N), --(COOH), 39.48 (-CH2), 126.43-
s, -CH2-CH2-), 127.36-137.89 (m, aryl), 139.17(d, C-P, 138.23 (m, aryl), 140.18(d, C-P, JC-P = 14.30 Hz). 31P-
JC-P = 13.80 Hz). 31P-NMR (δ ppm) 28.24 (d, JRh-P NMR (δ ppm) 44.35 (d, JRh-P 164 Hz). Anal. Calc. For
164 Hz). JP-Rh in Hz, measured at 40.5 MHz in CDCl3. C29H23NO3PRh: C, 61.39; H, 4.09; N, 2.47. Found: C,
Anal. Calc. For C19H21NO8PRh: C, 43.43; H, 4.00; 61.25; H, 4.18; N, 2.41%.
N, 2.66. Found: C, 42.98; H, 3.97; N, 2.59%.
The following compounds were synthesized utilizing 2.1.7. [Rh(quin-8-O)(CO)(Ph2P(C6H4CHO)]
the same synthetic procedure as applied for compound IR (ν cm-1): 1990 (Rh-CO), 1705 (CHO), 1595 (C=N), 530
1.
(Rh-P). 1H-NMR (δ ppm) 10.62(br, CHO), 6.85-8.65 (m,
aryl). 13C-NMR (δ ppm): 186.74(s, CO), 184.49(CHO),
157.61(C=N), 126.65-138.92 (m, aryl), 140.25(d, C-P,
2.1.2. [Rh(quin-8-O)(CO){C2H5P(CH2COOH)2]
IR (ν cm-1): 1946 (Rh-CO), 1590 (C=N), 1710(COOH),
J
C-P = 8.24 Hz). 31P-NMR (δ ppm) 25.12 (d, JRh-P 160 Hz).
520 (Rh-P). 1H-NMR (δ ppm), 11.56 (s, COOH), 3.80(d, Anal. Calc. For C29H21NO3PRh: C, 61.60; H, 3.71; N,
J ~ 4 Hz, CH2), 3.10 (q, J ~ 3.5 Hz, -CH2 of C2H5), 2.60 2.47. Found: C, 59.78; H, 3.82; N, 2.51%.
(t, J ~ 4 Hz, -CH3 of C2H5), 7.10-8.57(m, aryl). 13C-NMR
(δ ppm): 188.51(s, CO), 172.45 (s, COOH), 157.35(s, 2.2. Hydroformylation reactions
C=N), 38.50-42.35(s, -CH2), 22.70 (s, -CH3), 126.65- A mixture of 2 mmol of 1-hexene or cyclohexene and
137.65 (m, aryl), 139.40 (d, C-P, JC-P = 14.50 Hz). 31P- 5х10-3 mmol of the catalyst were mixed with water +
NMR (δ ppm) 56.36 (d, JRh-P 160 Hz). Anal. Calc. For toluene (1:1) or ethanol and placed into a glass lined mini
C16H17NO6PRh: C, 48.87; H, 4.32; N, 3.56. Found: C, autoclave purged with argon. Later, the reaction mixture
48.65; H, 4.68; N, 3.40%.
was treated with CO+H2 at 25 atm and then placed in an
oil bath thermostatted at the desired temperature. After
79

Synthesis characterization and hydroformylation activity of new
mononuclear rhodium(I) compounds incorporated with
polar-group functionalized phosphines
3-5 h the mixture was cooled and analyzed by 1H-NMR The experimental data of analysis are in good agreement
and GC.
with those calculated for the theoretical formulae of the
complexes (See experimental). The data indicated that
the ligands are bonded with the metal ion in a 1:1 ratio.
2.3. Catalyst recycling experiments
Catalytic recycling experiments were conducted under
anaerobic conditions similar to the method reported by 3.1. IR and NMR spectral investigations
Baricelli and coworkers [3k]. The aqueous solution of the Absorption bands at 3480 and 1380 cm-1 are due to
Rh(I) catalysts 1 and 2, and 1-hexene or cyclohexene ν(OH) and δ(OH) vibrations of free 8-hydroxyquinoline
was placed into a glass-lined stainless steel Parr [8]. The absence of these two bands in the spectra of the
autoclave (25 mL) equipped with a mechanical stirrer compounds indicate the deprotonation of the OH group
and a temperature controlled unit. Under the operation in presence of the base and the coordination to Rh(I)
conditions mentioned above (25 atm pressure, 90˚C) through the phenolic oxygen. In addition, a strong band
a fresh organic phase that contained 1-hexene or detected in the range of 1095-1110 cm-1 can be assigned
cyclohexene was introduced into the pressure reservoir to the C-O stretching frequency at the -C-O-M site. The
each time for the hydroformylation and analyzed by band near 1630 cm-1, attributed to the C=N group of
GC.
8-hydroxyquinolate, was shifted to lower frequencies
~ 1600-1580 cm-1, which indicated the completion of
chelation through coordination of the heterocyclic ring
nitrogen, i.e., the pyridyl nitrogen. This conclusion
is further supported by 13C-NMR spectra in which a
3. Results and discussion
Thesyntheticreactionsconductedandthecorresponding downfield signal was found at ~ 157.15-158.95 ppm
products obtained are illustrated in Scheme 2. The assigned to the C=N bond of the coordinated pyridyl ring
reaction of Rh(I) precursor [Rh(μ-Cl)(CO)2]2 with nitrogen.
8-hydroxyquinoline, in the presence of a base, followed
by polar phosphines (Scheme 1, L1-L7), produced single group, compounds 1-7 displayed only one absorption
isomers of Rh(I)-phosphines 1-7.
band in the region of 1930-1966 cm-1 for ν(CO) and
In regard to the coordination features of the CO
All of the compounds are freely soluble in polar- only one 13C signal in the region of 180-190 ppm for
organic solvents and insoluble in non-polar solvents, and carbonyl carbon chemical shifts, thereby indicating that
are freely water soluble except for compound 7. These only a single isomer of each of these compounds is
compounds were characterized by elemental analysis, present. Previous reports described that, in rhodium(I)
1
and by IR, H, 13C, 31P and UV-Visible spectroscopies. carbonyl phosphine complexes, a correlation was
L1
[Rh(quin-8-O)(CO){P(CH2CH2COOH)3]
[Rh(quin-8-O)(CO){EtP(CH2COOH)2]
[Rh(quin-8-O)(CO)(Ph2P-2-C6H4COOH]
[Rh(quin-8-O)(CO)(Ph2P-3-C6H4COOH]
[Rh(quin-8-O)(CO)(Ph2P-4-C6H4COOH]
[Rh(quin-8-O)(CO)(Ph2P-2-C6H4OH]
[Rh(quin-8-O)(CO)(Ph2P-C6H4CHO]
(1)
(2)
(3)
(4)
(5)
(6)
(7)
[Rh(-Cl)(CO)2]2
+
8-Hydroxyquinoline
L2
L3
+
BaCO3
L4
[Rh(quin-8-O)(CO)2]2
(generated in situ)
L5
L6
L7
Scheme 2. Synthesis of [Rh(quin-8-O)(CO)(L)] compounds (1-7)
80

C.S. Vasam et al.
drawn between 13C chemical shifts of carbonyl (CO) reveals that only one carbonyl group in [Rh(quin-8-O)
carbon and ν(CO) by altering the electronic properties (CO)2] is substituted by a phosphine. Because the pyridyl
of tertiaryphosphine in the coordination sphere [5a,9]. nitrogen of 8-hydroxyquinolate exhibits a stronger trans
This phenomenon can also describe the nature of Rh(I)- effect than the phenolic oxygen, the carbonyl group
CO bond. A recent review by Kuhl [10] provides good trans to the pyridyl nitrogen in [Rh(quin-8-O)(CO)2] was
correlation between the bonds of metal-phosphine only substituted by phosphines (L1-L7) and prevented
and metal-carbonyl complexes. The strength of the the possible formation of isomers.
Rh(I)-CO bond of compounds 1-7 was found to be
To evaluate the σ-donor properties of these
influenced by the σ-donor and π-acceptor properties phosphines, ∆δ31P values (δcomplex―δfree phosphine)
of the tertiaryphosphine, i.e., strength of the Rh(I)-P were considered (Table 1). As the electron withdrawing
bond, because both ligands interact with the same d-π nature of polar-groups on the phosphine decreased,
orbital of the Rh(I) metal ion [10]. The phosphines L1- the ∆δ31P values increased, i.e., its σ-donating ability
L7 used in this study contained carboxyl, hydroxyl and increased. With reference to ∆δ31P values (Table 1),
formyl functional groups at different positions on the aryl the increasing σ-donor properties of the phosphines
and aliphatic phosphine moieties, which influences both can be arranged in the order of P(CH2CH2COOH)3 >
electronic and steric properties of these phosphines. As EtP(CH2COOH)2 > Ph2P(C6H4CH2OH) > Ph2P(C6H4-3-
a consequence, variation can exist in their σ-donor and COOH) > Ph2P(C6H4-4-COOH) > Ph2P(C6H4-2-COOH)
π-acceptor properties. To clarify the data regarding the > Ph2P(C6H4-2-CHO). As the ∆δ31P values increased,
σ-donor and π-acceptor properties, i.e., the extent of the ν(CO) values decreased due to increased π back-
σ-donation of the phosphines in compounds 1-7, 31P- donation of excess electron density from Rh(I) to the CO
NMR experiments were monitored. For example, Fig. 1 group. The relationship between ∆δ31P and the ν(CO)
illustrates the IR, 13C and 31P-NMR spectral features values is presented in Fig. 2 to confirm this conclusion.
of a coordinated carbonyl and a phosphine with a
pendant, i.e., non-bonded carboxyl functional group, for in compounds 1-7 participated in chelation with the
compound 4, obtained as a single isomer. phosphorous atom. The absorption bands exhibited
None of the polar-groups present in the phosphines
All compounds (1-7) indicated only a single, doublet by compounds 1-5 in the region of 1700-1720 cm-1,
31P signal in the down-field region as compared to the and by compound 7 at 1690 cm-1 are ascribed to free,
spectra of the free ligands. The data are presented in i.e., uncoordinated carboxylic and formyl groups. The
Table 1. The presence of only one doublet 31P signal hydroxyl functionality on the phosphine in compound
Figure 1. IR, 13C and 31P-NMR spectral features of compound 4
81

Synthesis characterization and hydroformylation activity of new
mononuclear rhodium(I) compounds incorporated with
polar-group functionalized phosphines
Table 1. Relationship between ∆δ31P-NMR and ν(CO) of Rh(I)-phosphines.
Complex
31P-NMR
(ppm)
Free ligand
31P-NMR
(ppm)
∆δ31P-NMR
(ppm)
ν(CO)
(cm-1)
s.No
1
2
3
4
5
6
7
28.24
56.36
35.28
39.18
38.10
44.35
25.12
-26.3
5.02
-4.7
54.54
51.34
39.98
45.58
43.20
48.15
36.56
1930
1938
1966
1950
1957
1945
1975
-6.4
-5.1
-3.8
-11.44
Figure 2. Relationship between Δδ31P(ppm) and ν(CO)(cm-1) of Rh(I)-phosphines 1-7
6 also remained uncoordinated. The 1H-NMR and
3.3. Catalytic hydroformylation
13C-NMR data provided additional support for the
non-bonding nature of these polar functional groups.
Compounds1-5exhibitedacharacteristic1H-NMRsignal
at approximately 10.7-11.5 ppm and a 13C-NMR signal
at approximately 170-174 ppm for the uncoordinated
carboxylic group, whereas compound 7 exhibited a
1H-NMR signal at 10.6 ppm and 13C-NMR signal at
184.49 ppm for the uncoordinated formyl group.
The mononuclear Rh(I)-phosphines, 1, 2, 3, 5, 6 and
7, with varied polar functional groups in the phosphines
were chosen as catalysts to study the hydroformylation
of 1-hexene and cyclohexene. The reactions were
investigated both in two-phase (water + toluene)
and single phase (water + ethanol) solvent systems
at 50°C and 80°C using a CO+H2 (1:1) pressure of
25 atm for a required period of time. An alkylammonium
salt, tricaprylmethylammonium chloride, was used
as the phase transfer agent in the two-phase
hydroformylation.
3.2. Electronic spectra
Theelectronicspectrumrecordedforeachthecomplexes
in acetone exhibited two or three bands of comparable
intensity in the range of 250-520 nm . The bands in the
visible region may be assigned to metal-ligand charge-
transfer transaction [11], 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, such that d-d
transitions are usually obscured by charge-transfer
transitions.
3.4. Hydroformylation of 1-hexene
3.4.1. Two-phase Hydroformylation
The two-phase hydroformylation of 1-hexene performed
at 50°C in a water + toluene system achieved conversion
of only approximately 20-30% even after prolonged
reaction periods (>24 h). Nonetheless, only the C7-
aldehydes were produced selectively in this reaction.
In order to see the effect of temperature, an increase
82

C.S. Vasam et al.
in the temperature to 80°C improved dramatically the in the hydroformylation reaction did not demonstrate
conversion of 1-hexene up to approximately 63-95% any considerable improvement in the product yields or
after a period of 5 h (Table 2). Further, a maximum selectivities. No change in the elemental analysis values
ratio of approximately 10:1 for linear to branched (L:B) of these compounds was observed after the first catalytic
aldehydes was noted under these conditions.
cycle, indicating that these compounds are stable.
Catalyst recycling experiments were performed
Among the Rh(I)-phosphine catalysts used, those
with phosphines that are strong σ-donors with respect with Rh(I) catalysts 1 and 4 using Baricelli’s method
to ∆δ31P-NMR, i.e., which contain fewer electron [3]. Conversion of 1-hexene with Rh(I) catalyst 1 was
withdrawing polar groups on the phosphorous atom, > 90% and with Rh(I) catalyst 4 was > 83% after four
increased the aldehyde yields and L:B ratios in the order cycles (Table 2). Rh(I) catalysts 1 and 4 have similar L:B
1 > 2 > 5 > 6 > 3 > 7. On the contrary, a reverse trend is ratios.
reported in the literature [12]. The phosphines with more
electron withdrawing substituents would minimize the 3.4.2. Single-phase hydroformylation
back donation of the electron density from Rh(I) to CO The single-phase hydroformylation of cyclohexene in
and favor the facile cleavage of CO during the addition aqueous-ethanol (1:1) catalyzed by Rh(I)-phosphines
of alkene in hydroformylation. The facile cleavage of the (1, 2, 3, 5, 6 and 7) produced C7 aldehydes and C7-
CO group can accelerate the rate of hydroformylation alcohols. The data are presented in Table 3. A maximum
and increase product yields. However, in the case of conversion of 1-hexene in the range of 71-95% was
[Rh(quin-8-O)(CO)(L)], there is also a possibility of back achieved in the single-phase reaction within shorter
donation of the electron density from Rh(I) to the pyridyl reaction periods (3 h) as compared to the two-phase
ring of 8-hydroxyquinolate when Lis a strong, σ-donating hydroformylation. The accomplishment of single-phase
phosphine.The π-acceptor property of pyridine ligands is hydroformylation in a shorter time was attributed to
a recognized phenomenon [13]. Because the absorption the presence of all the reactants in a single miscible
spectra of [Rh(quin-8-O)(CO)(L)] displayed Metal- phase. The ratios between aldehydes and alcohols
Ligand Charge Transfer (MLCT) bands, it appears that were distributed between 2.5-3.5. With the exception of
the electron density received from phosphine could be compound 1, the L:B ratios of aldehydes were lowered
resonating along the CO-Rh(I)-Pyridine vector. Due to to 3. To determine the reason for over hydrogenation, an
these cooperative effects, the cleavage of the CO group additional experiment was conducted by increasing the
becomes easier during hydroformylation even in the ratio of ethanol to water (2:1). The aldehyde to alcohol
presence of a strong, σ-donating phosphine.
ratios reached approximately 2:1 as, i.e., the portion of
The hydrophilic nature of phosphines is an appealing alcohol is increased and indicated that ethanol could
characteristic that potentially influences biphasic be the possible source of hydrogen. Similarly to the
hydroformylation. For example, the phosphines L1 and observations made in biphasic hydroformylation, the
L2 of Rh(I)-phosphines 1 and 2 are not only strongly maximum yields and L:B ratios occurred for the strongly
σ-donating, but are also highly hydrophilic due to the σ-donating phosphines. No isomerized products were
presenceofmultiplecarboxylgroups. Inourexperiments, detected in single-phase hydroformylation.
the strongly σ-donating phosphines are also highly
hydrophilic in character. Addition of excess phosphine
Table 2. Catalytic hydroformylation of 1-hexene in water + toluene (1:1)
Rh(I)
Catalyst
Yield of
isomerized 1-hexene
Yields of C7-aldehyde
L:B ratio
1
2
3
4
5
6
7
94
91
83
88
87
88
63
9.9
8.0
4.7
5.6
5.3
6.1
2.8
1.6
1.9
1.3
1.2
1.5
1.1
1.4
Catalyst = 5×10-3 M, substrate/catalyst 400/1, T = 80°C, CO+H2 = 25 atm, t = 5 h
83

Synthesis characterization and hydroformylation activity of new
mononuclear rhodium(I) compounds incorporated with
polar-group functionalized phosphines
Table 3. Single-Phase Catalytic Hydroformylation of 1-hexene in aqueous Ethanol (1:1).
Rh(I)
Catalyst
Yield of
C7- aldehyde
Yield of
C7-alcohol
L:B
L:B
1
2
3
4
5
6
7
70
68
64
68
68
69
55
4.8
3.1
1.9
2.6
2.5
2.8
1.7
25
22
16
25
23
25
16
2.9
5.4
8.3
7.1
6.2
7.7
9.5
Catalyst = 5×10-3 M, substrate/catalyst 400/1, T = 80°C, CO+H2 = 25 atm, t =3 h
3.4.3. Hydroformylation of cyclohexene
4. Conclusion
The results obtained for hydroformylation in the biphasic
system (water + toluene) are presented in Table 4. This
reaction produced the cyclohexene carboxaldehyde
selectively. The Rh(I)-phosphines with strong σ-donating
and hydrophilic phosphines provided maximum yields
and L:B ratios of the hydroformylation product. However,
the conversion rates of cyclohexene were less than the
conversion rates of 1-hexene.
The synthesis of a single isomer of mono-nuclear Rh(I)-
Phosphine catalysts of the type [Rh(quin-8-O)(CO)(L)],
specifically those with pendant carboxyl and hydroxyl
polar-groups, is described. This is useful approach
to explore efficient Rh(I)-Phosphine catalysts using
phosphines other than Tris(3-sulfophenyl)phosphine
trisodium salt (TPPTS) for aqueous system based
catalysis. A relationship between ∆δ31P chemical
shifts and the ν(CO) values indicated that the σ-donor
properties of the phosphines were determined by
the electronic properties of polar substituents on the
phosphines. Based on the data obtained from IR,
1H, 13C, 31P-NMR and electronic spectra, a square-
planar geometry is proposed for the Rh(I)-phosphines
generated in this research as shown in Fig. 3.
The Rh(I)-phosphines were demonstrated to be good
catalysts for the hydroformylation of higher alkenes in
aqueous organic two-phase and single-phase systems
under mild conditions. The Rh(I)-phosphines containing
strong σ-donating hydrophilic phosphines provided
maximum yields and L:B ratios. On the contrary a
reverse trend is mentioned in the literature using strong
Table 4. Catalytic Hydroformylation of cyclohexene in water
+ toluene (1:1)
Rh(I)
Yields of
Catalyst
C7-aldehyde
1
2
3
4
5
6
7
76
69
58
64
61
65
52
Catalyst 5×10-3 M, substrate/catalyst 400/1,
CO+H2 = 25 atm, t = 5 h
T = 80°C,
O
Ph
Ph
P
Rh
N
COOH
CO
Figure 3. [Rh(quin-8-O)(CO)(Ph2P-2-C6H4-COOH)]
84

C.S. Vasam et al.
σ-donor phophines. We propose that due to the presence
of a π-acceptor pyridyl ring in 8-hydroxyquinolate, the
back-donation to the carbonyl group from the Rh(I)
metal center is proposed to be minimized and favor its
facile cleavage during hydroformylation. This research
established that the use of strong σ-donating phosphines
in [Rh(quin-8-O)(CO)(L)]-type compounds enhanced
the stability of Rh(I)-phosphines and avoided the excess
use of phosphine.
Acknowledgements
We thank Kakatiya University, Warangal for providing
financial assistance. We gratefully acknowledge the co-
operation extended by Prof. H. Schumann, Technical
University-Berlin (Germany) and Dr V.G. Akula,
Scientist IICT-Hyderabad to obtain important spectral
and catalytic data. We also thank the referees for their
comments and useful suggestions to improve the quality
of this manuscript.
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