Synthesis of cationic rhodium(I) and iridium(I) carbonyl complexes of tetradentate P(CH2CH2PPh2)3 ligand: An implication of steric inhibition and catalytic hydroformylation reaction

Article abstract of DOI:10.1016/j.molcata.2013.10.016

New cationic carbonyl complexes of the type [M(CO)L]Cl (1) [M = Rh (a) and Ir (b); L = P(CH₂CH₂PPh₂)₃] have been synthesized and characterized by various spectroscopic techniques including the single crystal X-ray diffraction. Both the complexes crystallize in trigonal bipyramidal symmetry with the metal at the centre. Two strong intermolecular hydrogen bonds are observed in 1a. Despite the high trans effect of the CO group, the symmetrical tetradentate phosphine ligand bonded strongly to the metal centre through all sites and thereby expelling the Cl^- ion outside the coordination sphere. The complexes are very stable and inert towards the oxidative addition (OA) of small molecules like CH₃I, C₂H₅I and I₂ at room as well as higher temperature. However, the complexes show catalytic activity towards the hydroformylation of alkene to the corresponding aldehydes under the reaction conditions: pressure 35 ± 2 bar, temperature 80 ± 2°C, 500 rpm and time 5-8 h. The yields of the aldehyde products are in the ranges 55-75%. The complexes exhibited dissociation of weaker MP bond under the above reaction conditions to form a vaccant coordination site for initiation of the catalytic cycle.

Full text of DOI:10.1016/j.molcata.2013.10.016

Journal of Molecular Catalysis A: Chemical 381 (2014) 188–193
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Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Synthesis of cationic rhodium(I) and iridium(I) carbonyl complexes of
tetradentate P(CH2CH2PPh2)3 ligand: An implication of steric
inhibition and catalytic hydroformylation reaction
Kokil Saikia, Biswajit Deb, Dipak Kumar Dutta∗
Materials Science Division, Council of Scientific and Industrial Research, North East Institute of Science and Technology, Jorhat 785 006, Assam, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 20 July 2013
Received in revised form 15 October 2013
Accepted 20 October 2013
Available online 28 October 2013
New cationic carbonyl complexes of the type [M(CO)L]Cl (1) [M = Rh (a) and Ir (b); L = P(CH2CH2PPh2)3]
have been synthesized and characterized by various spectroscopic techniques including the single crystal
X-ray diffraction. Both the complexes crystallize in trigonal bipyramidal symmetry with the metal at the
centre. Two strong intermolecular hydrogen bonds are observed in 1a. Despite the high trans effect of the
CO group, the symmetrical tetradentate phosphine ligand bonded strongly to the metal centre through
all sites and thereby expelling the Cl− ion outside the coordination sphere. The complexes are very stable
and inert towards the oxidative addition (OA) of small molecules like CH3I, C2H5I and I2 at room as well
as higher temperature. However, the complexes show catalytic activity towards the hydroformylation of
alkene to the corresponding aldehydes under the reaction conditions: pressure 35 2 bar, temperature
80 2 ◦C, 500 rpm and time 5–8 h. The yields of the aldehyde products are in the ranges 55–75%. The
complexes exhibited dissociation of weaker M P bond under the above reaction conditions to form a
vaccant coordination site for initiation of the catalytic cycle.
Keywords:
Rhodium
Iridium
Carbonyl complex
Tetradentate phosphine
Hydroformylation
© 2013 Elsevier B.V. All rights reserved.
1. Introduction
because of their lower efficiency [36–38]. The rhodium(I) and irid-
ium(I) complexes tend to undergo OA reactions which are often the
Steric and electronic properties of ligands play important roles
in the reactivity and catalytic activity of organometallic complexes
[1–13]. In general, the coordination chemistry of phosphine ligands
with transition metals is of considerable interest because of their
structural novelty, reactivity and catalytic activity [1–19]. A wide
range of transition metal carbonyl complexes of phosphine based
ligands show excellent reactivity towards small molecule acti-
vation [8–14,17–25]. The chemistry of rhodium(I) and iridium(I)
complexes of different ligands is a subject of interest due to their
rich chemistry, both in academia as well as industry. Several cat-
alytic systems like rhodium based Monsanto’s process of methanol
carbonylation [26–29], l-DOPA synthesis [30], etc. and iridium
based Cativa process [31] for production of acetic acid are a few
notable examples of the existing industrial processes. Apart from
these, rhodium complexes are also known to act as efficient cata-
lysts for hydroformylation of alkenes to corresponding aldehydes
[32–35]. Though a large number of rhodium complexes are well
documented in literature for such process [17,32–35], the studies
of iridium complexes in hydroformylation reaction are quite scanty
key step in catalytic cycles. A series of rhodium carbonyl complexes
of different mono- and multidentate ligands have been reported
by us [17–19,28,29,39] for homogeneous catalytic carbonylation
reactions. In this work, we report the synthesis and structural char-
acterization of two new cationic pentacoordinated rhodium(I) and
iridium(I) carbonyl complexes of tetradentate phosphine ligand
P(CH2CH2PPh2)3. The effect of the ligand towards the activation
of small molecules and catalytic activity in hydroformylation reac-
tions are also demonstrated.
2. Experimental
2.1. General information
All solvents were distilled under N2 prior to use. RhCl3·xH2O and
IrCl3·xH2O were purchased from M/S Arrora Matthey Ltd., Kolkata,
India. The ligand tris[2-(diphenylphosphino)ethyl]phosphine was
purchased from M/S Acros Organics, USA and was used with-
out further purification. Elemental analyses were performed on a
Perkin-Elmer 2400 elemental analyzer. IR spectra (4000–400 cm−1
)
were recorded on KBr discs and liquid cells with a Shimadzu IR
Affinity-1 FTIR spectrophotometer. 1H, 13C and 31P NMR spectra
were recorded in CDCl3 solution on a Bruker DPX-300 spectrom-
eter, and chemical shifts were reported relative to SiMe4 and
∗
Corresponding author. Tel.: +91 376 2370081; fax: +91 376 2370011.
E-mail addresses: dipakkrdutta@yahoo.com, dutta dk@rrljorhat.res.in
(D.K. Dutta).
1381-1169/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molcata.2013.10.016

K. Saikia et al. / Journal of Molecular Catalysis A: Chemical 381 (2014) 188–193
189
85% H3PO4 as internal and external standard respectively. Mass
spectra of complexes were recorded on an ESQUIRE 3000 mass
spectrometer. Thermal analyses of the complexes were carried out
using a thermal analyzer (TA instrument, model SDQ 600) simul-
taneous DTA-TGA) under argon atmosphere with a heating rate
of 10 ◦C/min. The hydroformylation reactions of alkene were car-
ried out in a high pressure reactor (Parr-4592, USA) fitted with a
pressure gauge and the reaction products were analyzed by GC
(Chemito 8510, FID).
solvent was evaporated under vacuum. The light green coloured
compounds so obtained were washed with hexane as well as
diethyl ether and recrystallized from DCM solution. The compound
was dried and stored over silica gel in a desiccator.
Analytical data for 1a: Yield: 85%; IR (KBr, cm−1): 1980 [ꢀ(CO)].
1H NMR (CDCl3, ppm): ı 2.3, 2.67 (m, 12H, CH2), ı 7.18, 7.22 (m, 30H,
Ph), 13C NMR (CDCl3, ppm): ı 30.92, 40.45 (CH2), ı 125.14–141.3
1
(Ph), ı 199.24 (CO). 31P { H} NMR (CDCl3, ppm): ı 152.6 [d,
JRh P = 82.84 Hz P/], ı 62.1 [d, JRh P = 156.94 Hz, P]. Elemental analy-
ses; Found (Cald for RhC43H42OP4): C, 60.88 (61.48), H, 5.11 (5.24).
MS: m/z = 834.6 (M+).
2.2. Synthesis of starting materials
The starting dimeric rhodium moiety [Rh(CO)2Cl]2 [40] and irid-
ium moiety [Ir(COE)2Cl]2 [41] were synthesized by following the
literature procedure.
2.4. Synthesis of the complex [Ir(CO){P/(CH2CH2PPh2)3}]Cl (1b)
[Ir(COE)2Cl]2 (100 mg) was dissolved in acetonitrile. The solu-
tion was then stirred at room temperature for about 10 min under
a N2 atmosphere. The flow of N2 was stopped and the CO gas was
passed to the solution until a clear greenish yellow solution was
obtained. A stoichiometric amount (Ir:L = 1:1) of the ligand solu-
tion in DCM was added to the solution and the mixture was stirred
for about another 30 min. The solvent was evaporated under vac-
uum. The greenish yellow coloured compounds so obtained were
2.3. Synthesis of the complex [Rh(CO){P/(CH2CH2PPh2)3}]Cl (1a)
[Rh(CO)2Cl]2 (100 mg) was dissolved in dichloromethane (DCM)
(10 cm3) and to this solution, a stoichiometric amount (Rh:L = 1:1)
of the ligand solution in DCM was added. The reaction mixture
was stirred at room temperature (25 ◦C) for about 30 min and the
Scheme 1. Synthesis of 1a and 1b.

190
K. Saikia et al. / Journal of Molecular Catalysis A: Chemical 381 (2014) 188–193
washed with hexane as well as diethyl ether and recrystallized from
DCM solution. The compounds were dried and stored over silica gel
in a desiccator.
11
10
9
Analytical data for 1b: Yield: 84%; IR (KBr, cm−1): 1965 [ꢀ(CO)].
1H NMR (CDCl3, ppm): ı 1.56, 2.17 (m, 12H, CH2), ı 7.45, 7.74 (m,
30H, Ph), 13C NMR (CDCl3, ppm): ı 22.7, 29.37 (CH2), ı 128–137.1
8
1
(Ph), ı 200.63 (CO). 31P { H} NMR (CDCl3, ppm): ı 129.96 [d,
JP P = 13.43 Hz P/], ı 36.79 [d, JP P = 13.27 Hz, P]. Elemental analy-
ses; Found (Cald for IrC43H42OP4): C, 57.32 (57.91), H, 4.29 (4.71).
MS: m/z = 952.2 (M+).
7
6
1b
1a
2.5. X-ray structural analysis
5
Single crystals of 1a and 1b suitable for X-ray crystallographic
analyses were obtained by layering DCM solution of the respective
compounds with hexane. Intensity data of 1a and 1b were col-
4
0
100 200 300 400 500 600 700 800 900
Temperature (oC)
˚
lected on a Bruker Smart-CCD with Mo K␣ radiation (ꢁ = 0.71073 A)
at 110 K. The structures were solved with SHELXS-97 and refined
by full-matrix least squares on F2 using the SHELXL-97 computer
program [42]. Hydrogen atoms were idealized by using the riding
models.
Fig. 1. TGA plots of 1a and 1b.
with the metal centre in the former complex. The 1H NMR spectra
of the complexes show characteristic resonances for methylene
and phenylic protons in the range 1.56–2.67 and 7.18–7.74 ppm
respectively. In the 13C NMR spectra of 1a and 1b, weak signals for
the carbonyl carbons appeared in the range 199.24–200.63 ppm.
The thermogravimetric analyses (TGA) data (Fig. 1) indicate that
both the complexes are thermally stable up to around 390 ◦C. The
mass loss of ∼9% at around 45 ◦C is due to the decomposition of
DCM as solvent of crystallization. Both the complexes decompose
at around 400 ◦C with mass loss of ∼50%.
2.6. Catalytic activity towards hydroformylation reaction
The hydroformylation reactions were carried out in a Parr reac-
tor. 2.5 mmol of the alkene and 0.005 mmol of the catalyst in 5 cm3
DCM were placed in a 50 cm3 reaction vessel. The reaction mixture
was purged with CO/H2 (1:1) for about 5 min. The CO/H2 pressure
was increased to 35 bar (actual pressure) at 25 ◦C in the closed reac-
tor. The hydroformylation reactions were carried out at 80 2 ◦C for
3–8 h at 35 2 bar. The reaction mixture was analyzed by GC and
products were isolated by column chromatography taking hexane
and ethyl acetate as eluents.
The structures of complexes 1a and 1b determined by single
crystal X-ray diffraction (Fig. 2) show that the metal atom occupies
the centre of a distorted trigonal bipyramidal geometry formed by
four P atoms of the ligand P4 and a C (of CO) atom. In 1a, inter-
2.7. IR study
˚
molecular hydrogen type bond (2.682 A) is observed between the
oxygen of carbonyl group and a hydrogen of phenyl ring (Fig. 3),
while such interaction is absent in 1b. The CO group occupies one
of the axial positions trans to central P atom of the ligand P4. The
P(1)-M-P(3) angles in the equatorial plane of 1a and 1b are 123.28◦
and 119.97◦ respectively which are very close to the normal value
i.e. 120◦ for the angles of the attributed due to the symmetric dis-
tribution of the ligand around the metal centre. The bulky tripodal
ligand forms capped complexes with the metal centres, leaving very
little space for other donors to attach with. Despite the high trans
effect of the CO group, the highly symmetrical phosphine ligand
bonded strongly to the metal centre through all sites, resulting the
expulsion of the Cl− ion outside the coordination sphere to gener-
ate cationic complexes. It appears (Fig. 2) that all P atoms are not
at equal distances from the central metal centre in the complexes.
The course of the hydroformylation reactions were monitored
by FTIR spectroscopy in a solution cell (CaF2 windows, 1.0 mm
path length). The reaction mixtures were scanned in the range
1550–2200 cm−1 at different time intervals to investigate the dif-
ferences in the corresponding spectra.
3. Results and discussion
3.1. Synthesis and characterization
The dimeric precursor [Rh(CO)2Cl]2 reacts with 2 mol equiv-
alents of the ligand P4 [Dimer:Ligand = 1:2] in DCM to afford the
complex [Rh(CO)P(CH2CH2PPh2)3]Cl (1a); while in situ generated
dimeric precursor [Ir(CO)2Cl]2 reacts with 2 mol equivalents of
the ligand P4 [Dimer:Ligand = 1:2] in DCM to yield the complex
[Ir(CO)P(CH2CH2PPh2)3]Cl (1b) (Scheme 1). Elemental analy-
ses and mass spectrometric data of the complexes support the
observed molecular composition of 1a and 1b. The IR spectra of
1a and 1b exhibit intense ꢀ(CO) bands at 1980 and 1965 cm−1
respectively, indicating the formation of monocarbonyl complexes.
The appearance of relatively lower ꢀ(CO) in IR spectra reveals the
presence of electron rich metal centres in both the complexes. The
˚
In 1a, the bond distance (2.287 A) of central P atom from the metal
centre is slightly smaller compared to the other three P atoms. On
˚
the other hand, the Rh P(4) bond distance (2.346 A) is the highest
among all Rh P bonds, which may dissociate preferably compared
to others during a catalytic cycle. Similarly, the Ir P(4) bond dis-
tance is the longest among all Ir P bonds in 1b, indicating that the
dissociation may likely to occur at the Ir P(4) bond.
1
31P{ H} NMR spectra of the complexes show two distinct doublets
3.2. Reactivity
at ı 152.6 (JRh P = 82.84 Hz), 62.1 (JRh P = 156.94 Hz) ppm (1a) and
ı 129.96 (JP P = 13.43 Hz), 36.79 (JP P = 13.27 Hz) ppm (1b) for two
different types of P-atoms in the complexes. The intensity ratio of
The reactivity of the complexes were evaluated towards the OA
reaction of small molecules like I2, CH3I and C2H5I at different tem-
peratures. But, the complexes were found inert under the reaction
condition, which may be due to the steric effect of the phosphine
ligand around the metal centres (Fig. 2). The bulkiness of the P4
ligand around the metal centres in both the complexes allow no
1
two chemical shifts in 31P{ H} NMR spectra are around 1:3, which
indicates that out of four phosphorus centres, three are in the
similar chemical environment. The significant downfield shift in
1a compared to 1b indicates a stronger interaction of the P-atoms

K. Saikia et al. / Journal of Molecular Catalysis A: Chemical 381 (2014) 188–193
191
Fig. 3. Intermolecular hydrogen bonding in 1a.
CO/H2
R
R
CHO
R
Catalyst
CHO
Scheme 2. Formation of aldehydes by the hydroformylation of terminal alkenes.
temperature 130 2 ◦C, pressure 30 2 bar, 450 rpm and time 1 h,
but no product could be detected, which may be due the inertness
of 1a and 1b towards the OA of CH3I, a key step of carbonyla-
tion reaction. The complexes were further investigated towards
the hydroformylation of alkenes under the reaction conditions:
35 2 bar, 80 2 ◦C, 500 rpm, 5–8 h (Scheme 2) and good results
are obtained (Table 1). The complex 1a shows better catalytic
activity over 1b, under the given reaction conditions. The reaction
products separated by column chromatography were identified
by 1H NMR spectroscopy. The reaction mixture was analyzed
by GC to determine the conversion and regioselectivity. The
hydroformylation of 1-octene yielded 71% nonanal with a regios-
electivity (n:iso) of 62:35 in the presence of 1a as catalyst, while
the complex 1b yielded 62% nananal with a regioselectivity of
55:32 (Table 1, entry 3). The bulkiness of the P4 ligand around
the metal centres in both the complexes results lower turnover
frequency (TOF) for hydroformylation of 1-octene compared to
the previously reported rhodium-based catalytic systems [43]. The
Fig. 2. Ortep diagram of 1a and 1b (thermal ellipsoids are drawn at 50% probability).
room to the incoming electrophile for coordination to the metal
centre.
3.3. Catalytic activity
The catalytic activity of the complexes was investigated
in carbonylation of methanol under the reaction conditions:
Table 1
Catalytic efficiency of 1a and 1b for hydroformylation of alkenes.a
Sl. no
Substrate
Product
Catalyst
Time (h)
Isolated yieldb (%)
Conversion(%)c
TOF (h−1
)
n
iso
n
iso
1a
1b
5
8
75
60
75
63
–
–
75
39.4
–
–
1
1a
1b
5
8
69
55
64
52
–
–
64
33
–
–
2
3
1a
1b
5
8
71
62
62
55
35
32
62
34.4
35
20
a
Reaction conditions: 2.5 mmol of the alkene, 0.005 mmol of catalyst, 35 2 bar, 80 2 ◦C, 500 rpm, 5–8 h.
Isolated yields were calculated with respect to the substrate taken after separating the product by column chromatography.
Conversion and n:iso ratio was determined by GC.
b
c

192
K. Saikia et al. / Journal of Molecular Catalysis A: Chemical 381 (2014) 188–193
0.10
4. Conclusions
Two new cationic rhodium(I) and iridium(I) carbonyl complexes
5 h
1984
(a)
of the type [M(CO)P(CH2CH2PPh2)3]Cl (M = Rh, Ir) have been syn-
thesized and structurally characterized by different spectroscopic
techniques including single crystal X-ray diffraction. Both the com-
plexes exhibit a trigonal bipyramidal structure where the CO group
occupies an axial position. The complexes were found to be unre-
active towards the oxidative addition of small molecules i.e. I2,
CH3I and C2H5I at room as well as elevated temperature. However,
both the complexes were found to be good catalysts for the hydro-
formylation reaction under the experimental conditions 35 2 bar,
80 2 ◦C, 500 rpm, 5–8 h.
3 h
0 h
0.05
0.00
1658
1726
Supplementary material
1600
1700
1800
1900
2000
2100
2200
CCDC-876707 (1a) and 876708 (1b); contains the supplemen-
tary 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.
8 h
(b)
4 h
0 h
Acknowledgements
1965
0.05
0.00
The authors are grateful to Dr. R.C. Boruah, Acting Director,
CSIR-North East Institute of Science and Technology, Jorhat, Assam,
India, for his kind permission to publish the work. Department of
Science and Technology (DST), New Delhi (Project file no. SR/S1/IC-
34/2011) is acknowledged for the financial grant. The authors K.
Saikia (SRF, UGC) and B. Deb (SRF, CSIR) are grateful to the corre-
sponding organizations for providing the fellowships.
1656
1722
Appendix A. Supplementary data
Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/
j.molcata.2013.10.016.
1600
1700
1800
1900
2000
2100
2200
Fig. 4. Series of IR spectra illustrating the progress of reaction for the substrate
cyclohexene in presence of 1a (a) and 1b (b).
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respectively show a significant change indicating formation of new
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