Hydroformylation activity of multinuclear rhodium complexes coordinated to dendritic iminopyridyl and iminophosphine scaffolds

Article abstract of DOI:10.1016/j.jorganchem.2010.10.048

The synthesis of poly(propyleneimine)-iminopyridyl and iminophosphine rhodium(I) metallodendrimers, with rhodium coordinated to monodentate (N-donor) and chelating, heterobidentate (P,N) moieties respectively located on the periphery, has been accomplished in order to evaluate their potential as hydroformylation catalysts. Related mononuclear complexes were obtained in a similar manner to model the multinuclear complexes. The multinuclear rhodium(I) complexes were found to be effective catalyst precursors in the hydroformylation of 1-octene, achieving higher conversions, faster reaction rates and slightly enhanced catalytic activity when compared with analogous mononuclear rhodium complexes. Hydroformylation reactions using the tetra- and octanuclear rhodium complexes generally show a chemoselective formation of aldehydes, together with a small amount of isomerisation products.

Full text of DOI:10.1016/j.jorganchem.2010.10.048

Journal of Organometallic Chemistry 696 (2011) 2003e2007
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Hydroformylation activity of multinuclear rhodium complexes coordinated
to dendritic iminopyridyl and iminophosphine scaffolds
*
Nathan C. Antonels, John R. Moss, Gregory S. Smith
Department of Chemistry, University of Cape Town, Private Bag, Rondebosch 7701, South Africa
a r t i c l e i n f o
a b s t r a c t
Article history:
The synthesis of poly(propyleneimine)-iminopyridyl and iminophosphine rhodium(I) metal-
lodendrimers, with rhodium coordinated to monodentate (N-donor) and chelating, heterobidentate (P,N)
moieties respectively located on the periphery, has been accomplished in order to evaluate their
potential as hydroformylation catalysts. Related mononuclear complexes were obtained in a similar
manner to model the multinuclear complexes. The multinuclear rhodium(I) complexes were found to be
effective catalyst precursors in the hydroformylation of 1-octene, achieving higher conversions, faster
reaction rates and slightly enhanced catalytic activity when compared with analogous mononuclear
rhodium complexes. Hydroformylation reactions using the tetra- and octanuclear rhodium complexes
generally show a chemoselective formation of aldehydes, together with a small amount of isomerisation
products.
Received 30 June 2010
Received in revised form
12 October 2010
Accepted 21 October 2010
Keywords:
Rhodium
Hydroformylation
Poly(propyleneimine)
Metallodendrimers
Iminophosphine
Iminopyridyl
Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction
a unique macromolecular environment for the coordinated cata-
lysts that can influence the regio- and chemoselectivities of
The rhodium-catalysed hydroformylation of alkenes is one of
the most important and largest industrial and commercial appli-
cations involving the use of a homogeneous transition metal cata-
lyst. Hydroformylation catalysts containing rhodium are said to be
more stable and require milder reaction conditions than other
metal-catalysed hydroformylation reactions, and generally give
high catalytic rates and better chemoselectivity towards the
formation of aldehyde products. Notably, problems arise when
separation of the catalyst from the products is required. In an
attempt to overcome this limitation, researchers have immobilised
organometallic and inorganic catalysts on various scaffolds,
including inorganic supports [1e5], polymers [6e9] and den-
drimers [10e13].
Metallodendrimers are branched, monodisperse, macromolec-
ular constructs containing metals either in the core, interspersed
through the dendritic framework or at the periphery. Advantages to
using metallodendrimers with catalysts on the periphery include
their increased multivalency (higher local concentration) which
allows for enhanced reaction rates and the possibility to separate
the catalysts from the reaction medium through precipitation or
ultrafiltration using membrane reactors. Dendrimers provide
a catalytic reaction. It is through these properties that dendrimers
are thought to combine the advantages of both homogeneous and
heterogeneous catalysis.
A large number of metallodendrimers are now known and
numerous reviews on the applications of dendrimers in catalytic
reactions have been published [10,11,14e17]. Various examples of
dendritic scaffolds have been used to immobilize homogeneous
catalysts, including carbosilanes [18], polyamidoamines [4],
polypropyleneimines [19e24] and polyarylethers [12,13], to name
a few. More pertinent to our research are poly(propyleneimine)
dendrimers, and particularly to this study, the peripheral func-
tionalisation of this scaffold with rhodium complexes. Well-defined
rhodium metallodendrimers based on a polypropyleneimines are
quite rare. Busetto and van Leeuwen synthesized a series of poly
(propyleneimine) dendimers up to the fifth generation with
alkoxycarbonylcyclopentadienyl complexes of rhodium(I) bonded
to the periphery of this dendrimer [25]. One of the earliest rhodium
metallodendrimers based on a poly(propyleneimine) scaffold and
employed in hydroformylation catalysis was synthesized by Reetz
and co-workers [26]. They utilized the DAB poly(propyleneimine)
dendritic support and functionalized it with chelating diphenyl
phosphine ligands at the termini to give a multiphosphine func-
tionalised dendrimer. [Rh(COD)BF4] (COD ¼ 1,5-cyclooctadiene)
was reacted with the diphosphane dendrimers and the resulting
* Corresponding author. Tel.: þ27 21 6505279; fax: þ27 21 6505195.
E-mail address: gregory.smith@uct.ac.za (G.S. Smith).
0022-328X/$ e see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2010.10.048

2004
N.C. Antonels et al. / Journal of Organometallic Chemistry 696 (2011) 2003e2007
rhodium(I) metallodendrimers evaluated in the hydroformylation
of 1-octene. Quantitative conversion of the olefin to aldehyde was
observed with an n:iso ratio of 60:40 and turnover frequency of
360 hꢀ1, comparable to that of monomeric analogues.
a yellow solid. Yield 74%. M.p.: 170 ꢁC (decompose, without
melting). 1H NMR (300 MHz, CDCl3): 1.36 (br s, 4H, CH2), 1.54 (br s,
8H, CH2), 1.78 (br s, 48H, CH2 and COD-CH2), 2.45 (br s,
68H, eCH2Ne and COD-CH2), 3.60 (br s, 16H, eCH2N]), 4.13 (br
s, 32H, COD-CH), 7.54 (br s, 16H, Arpyr), 8.17 (br s, 8H, imine), 8.74(br
s, 16H, Arpyr). 13C {1H} NMR (100 MHz, CDCl3): 28.02, 30.77, 51.49,
52.01, 59.58, 80.62, 122.67, 143.80, 151.27, 157.77. IR (NaCl cells,
CH2Cl2, cmꢀ1): n(imine, C]N) 1646 (m), n(pyr, C]N) 1612 (m). Calculated
for C104H148N10Rh8Cl8.2.5CH2Cl2: C, 50.55; H, 6.07; N, 8.39%. Found:
C, 50.28; H, 6.25; N, 8.13%.
Following our previous research on the synthesis of various
Schiff-base modified poly(propyleneimine) metallodendrimers
with applications as Heck cross-coupling, ethylene oligomerisa-
tion/polymerisation and norbornene polymerization catalysts, we
decided to explore the synthesis of a series of mono- and multi-
nuclear rhodium(I) complexes based on a iminopyridyl- and imi-
nophosphine-poly(propyleneimine) scaffolds. In this paper, we
report on the evaluation of the catalytic performance of these
rhodium complexes, choosing to study the hydroformylation
reaction using 1-octene as a model substrate.
2.4. Synthesis of mononuclear rhodium complex 3
A solution of propyl-pyridin-4-ylmethylene-amine (0.0601 g,
0.40 mmol) in dichloromethane (10 cm3) was syringed dropwise
into a stirring solution of [RhCl(COD)]2 (0.0947 g, 0.19 mmol) in
tetrahydrofuran (10 cm3) in a Schlenk tube. The reaction proceeded
for 1 h at room temperature after which the solvent was removed.
The yellow solid was washed with diethyl ether (5 cm3), then
n-pentane(5 cm3) anddried invacuo. Yield ¼ 58%. M.p.:124e127 ꢁC.
1H NMR (300 MHz, CDCl3): 0.87 (t, 3H, CH3), 1.66 (m, 2H, CH2), 1.77
(m, 4H, COD), 2.43 (m, 4H, COD), 3.55 (m, 2H, eCH2N]), 4.11 (m, 4H,
COD), 7.53 (d, 2H, Arpyr), 8.14 (s,1H, imine), 8.72 (d, 2H, Arpyr).13C {1H}
NMR (100 MHz, CDCl3): 11.96, 23.96, 31.09, 63.83, 80.55, 122.91,
144.27,151.52,157.59. IR (NaCl cells, CH2Cl2, cmꢀ1): n(imine, C]N) 1646
(m), n(pyr, C]N) 1612 (m). Calculated for C17H24N2RhCl: C, 51.85; H,
6.39; N, 7.11%. Found: C, 51.04; H, 6.12; N, 6.93%.
2. Experimental
2.1. General remarks
All reagents were purchased from Aldrich and used as received.
DAB-Am-8 polypropylenimine octaamine was purchased from
Symo-Chem. [RhCl(CO)2]2 was purchased from Strem Chemicals.
Ligands L1 and L2 [27], [RhCl(COD)]2 [28], (acetylacetonato)dicar-
bonylrhodium(I) [29] and complexes 4e6 [30] were prepared
according to the literature methods. The NMR spectra were recor-
ded on a Varian Unity 400 spectrometer (1H: 400 MHz, 13C:
100 MHz) or Varian Mercury 300 (1H: 300 MHz, 13C: 75 MHz) at
ambient temperature unless stated otherwise. Infrared (IR)
absorptions were measured on a Perkin-Elmer Spectrum One FT-IR
spectrometer in dichloromethane using NaCl solution cells. Micro-
analyses were carried out using a Fisons EA 110 CHNS elemental
analyser. For certain dendrimers, the analyses are outside accept-
able limits, and are ascribed to the encapsulation of solvent mole-
cules and other inorganic salts by dendritic compounds. Melting
points were determined using a Kofler hot stage microscope (Rie-
chart Thermover) and are corrected.
2.5. Hydroformylation reaction procedure
The reactions were conducted in a 10 cm3 stainless steel auto-
clave equipped with a magnetic stirrer. The autoclave was charged
with toluene (6 cm3), 1-octene (6.38 mmol) and rhodium complex
(0.0028 mmol) resulting in a substrate to rhodium ratio of 2279:1.
The autoclave was then flushed five times with syngas (CO:H2, 1:1)
to purge by displacement. The autoclave was then pressurized and
heated to the desired syngas pressure and temperature respec-
tively. Samples were taken every 2 h and analyzed using gas
chromatography (GC) with n-dodecane as internal standard. GC
analyses were conducted using a Varian 3900 gas chromatograph
equipped with a HP-PONA column for quantification. The identity
of isomeric octenes and aldehydes was validated using GCMS on an
Agilent Technologies 5973 Network Mass Selective Detector
equipped with an identical column. Mass spectra were analyzed
against the Wiley and NIST02 databases.
2.2. Synthesis of metallodendrimer 1
A solution of L1 (0.0660 g, 0.090 mmol) in dichloromethane
(10 cm3) was syringed dropwise into a stirring solution of [RhCl
(COD)]2 (0.103 g, 0.20 mmol) in dichloromethane (10 cm3) in
a Schlenk tube. The reaction was stirred at room temperature for
16 h after which the volume was reduced to approximately 5 cm3.
Diethyl ether was added to precipitate the product 1 as a yellow
solid. The solid was filtered and washed with diethyl ether (5 cm3)
and then n-pentane (5 cm3). Yield 70%. M.p.: 133e137 ꢁC. 1H NMR
(300 MHz, CDCl3): 1.29 (broad s, 4H, CH2), 1.83 (broad m, 24H, CH2,
COD), 2.48 (broad s, 28H, eCH2Ne, COD), 3.59 (broad s,
8H, eCH2N]), 4.18 (broad s,16H, COD), 7.50 (d, 8H, Arpyr), 8.21(s, 4H,
imine), 8.75 (d, 8H, Arpyr). 13C {1H} NMR (100 MHz, CDCl3): 25.10,
28.16, 30.85, 51.53, 53.99, 59.65, 80.40,122.68,143.90,151.36,157.63.
IR (NaCl cells, CH2Cl2, cmꢀ1): n(imine, C]N) 1646 (m), n(pyr, C]N) 1612
(m). Calculated for C72H120N10Rh4Cl4.1.5CH2Cl2: C, 49.41; H, 5.81; N,
7.84%. Found: C, 49.48; H, 6.06; N, 7.38%.
3. Results and discussion
3.1. Synthesis of rhodium(I) iminopyridyl- and iminophosphine-
functionalised metallodendrimers
It is well known that the influence of ligands plays an important
role in hydroformylation reactions. With this in mind, we synthe-
sized a series of first (G1)- and second (G2)-generation dendrimers
with monodentate and chelating, bidentate endgroups by simple
Schiff-base reactions of the commercially available DAB-(NH2)n
(n ¼ 4 (G1); n ¼ 8 (G2)) dendrimers with the appropriate aldehyde.
The known iminopyridyl dendritic ligands (L1, L2) [27] were
reacted with [RhCl(COD)]2 to produce the tetranuclear (1) and
octanuclear (2) monodentate rhodium(I) metallodendrimers,
respectively (Fig. 1).
2.3. Synthesis of metallodendrimer 2
[RhCl(C8H12)]2 (0.139 g, 0.28 mmol) was transferred to
a nitrogen-purged Schlenk tube and dissolved in dry dichloro-
methane (10 cm3). A solution of L2 (0.104 g, 0.070 mmol) in dry
dichloromethane (10 cm3) was added dropwise to the stirring [RhCl
(C8H12)]2 solution. The solution was stirred for 24 h and the volume
of the solution was reduced to approximately 5 cm3 in vacuo.
Anhydrous n-pentane was added to precipitate the product 2 as
The rhodium metallodendrimers (1,2) were isolated as yellow
solids in moderate to good yields and are stable at room temper-
ature. Generally, the IR spectra for 1 and 2 show a wavelength shift
of the C]N absorption band of the pyridyl ring towards higher

N.C. Antonels et al. / Journal of Organometallic Chemistry 696 (2011) 2003e2007
2005
DAB-
dendr
N
*
N
Cl
Cl
N
Rh
Rh
N
N
N
N
N
N
N
N
DAB-
dendr
*
N
N
N
Rh
n
Rh
Cl
Cl
n = 4 (L1)
n = 8 (L2)
1
Cl
Cl
Rh
Rh
N
N
N
Cl
Cl
N
N
Cl
N
Rh
Rh
Rh
N
N
N
N
N
N
N
N
N
N
N
N
N
N
Rh
Cl
N
N
Rh
Cl
3
N
Rh
Cl
N
Rh
Cl
2
Fig. 1. Mono-, tetra- and octanuclear rhodium(I) iminopyridyl complexes.
wavenumbers (from 1598 cmꢀ1 to 1612 cmꢀ1) as compared to the
free ligand. The absorption band due to the imine functionality
remains unchanged, indicative of selective coordination of rhodium
to the nitrogen of the pyridyl ring and not the imine nitrogen. This
is further confirmed in the 1H NMR spectra for complexes 1 and 2,
showing a singlet in the range 8.17e8.21 ppm for the imine proton,
remaining relatively unchanged, when compared with the NMR
spectrum of the free, uncomplexed dendritic ligand. An analogous
mononuclear compound 3 (Fig. 1) was also synthesized from the
known compound propyl-pyridin-4-ylmethylene-amine and [RhCl
(COD)]2, as a model compound for comparison, to examine the
catalytic effect of a mononuclear compounds versus a multinuclear
complex. Spectroscopic data for 3 are similar to that observed for
the rhodium(I) metallodendrimers (1,2), providing evidence of
selective metal coordination to the pyridyl nitrogen and not the
imine nitrogen for the respective metallodendrimers. As part of this
catalytic study, chelating, heterobidentate [P,N] iminophosphine
rhodium(I) poly(propyleneimine) metallodendrimers (4,5) were
also prepared (Fig. 2). The synthesis and characterisation of these
rhodium(I) metallodendrimers (4,5) and the mononuclear complex
(6) were reported previously [30].
3.2. Hydroformylation of 1-octene using rhodium(I)
complexes (1e6)
In order to evaluate the catalytic performance of the rhodium(I)
complexes (1e6), we chose to study the hydroformylation of
1-octene as a model substrate. The conversion of 1-octene, moni-
tored by GC, is almost quantitative after 8h using complexes 1e6
(Table 1), under mild reaction conditions (P(CO/H2) ¼ 30 bar;
T ¼ 75 ꢁC). The rhodium catalysts containing the monodentate
iminopyridyl functionality give very similar reaction rates. The
metallodendrimers (4,5) containing the chelating, bidentate
Table 1
Hydroformylation of 1-octene catalysed by complexes 1e6.
Rh(I)
Conversion %b % Aldehyde Isomerisation n:isoc TOF
(hꢀ1
e
Complexa
)
1
2
3
4
5
6
98
98
98
98
98
98
99
64
58
70
64
73
80
88
92
36
32
30
36
27
20
12
8
2.7
2.6
2.5
2.6
1.5
2.7d
2.3
0.9
179
162
195
179
204
223
248
259
DAB-
dendr
N
*
N
Rh PPh2
CO
Cl
Rh
CO
PPh2
Cl
RhCl(PPh3)3
Rh(CO)2(acac) 99
n
a
Rh ¼ 2.80 ꢂ 10ꢀ3 mmol, P ¼ 30 bar CO:H2 (1:1), T ¼ 75 ꢁC, t ¼ 8 h.
Substrate ¼ 6.38 mmol.
6
b
c
n = 4 (4)
n = 8 (5)
Determined at 2 h.
d
e
Determined at 4 h.
Fig. 2. Mono-, tetra- and octanuclear rhodium(I) iminophosphine complexes.
TOF ¼ (mol product/mol cat.) ꢂ hꢀ1
.

2006
N.C. Antonels et al. / Journal of Organometallic Chemistry 696 (2011) 2003e2007
Table 2
Table 3
Hydroformylation results obtained using various pressures.a
Regioselectivity data for complexes 1e6a.
Rh(I)
complexes
Entry
P (bar)
% 1-Octene
conversion
% Aldehyde
% Iso-octenes
Entry
Rh(I) Complexes
n:iso
1
2
3
4
5
6
7
G1-iminopyridyl, (1)c
73:27
74:26
72:28
72:28
60:40
73:27
48:52
1
3
4
6
1
2
3
4
5
6
7
8
9
5
10
30
5
10
30
5
10
30
5
10
30
1
91
98
2
18
98
4.4
57
98
16
82
98
0
6
64
0
7
70
0
7
64
1
8
80
100
94
36
100
93
30
100
93
36
99
91
G2-iminopyridyl, (2)c
Mononuclear iminopyridyl, (3)c
G1-iminophosphine, (4)c
G2-iminophosphine, (5)c
Mononuclear iminophosphine, (6)b
c
Rh(acac)(CO)2
a
P ¼ 30 bar, T ¼ 75 ꢁC.
b
c
Regioselectivity calculated at 4 h.
Regioselectivity calculated at 2 h.
10
11
12
20
The hydroformylation of 1-octene under these favoured condi-
a
Sampled after 8 h. T ¼ 75 ꢁC.
tions (P ¼ 30 bar; T ¼ 75 ꢁC) was chemoselective toward the forma-
tion of aldehydes (n þ iso) as major products (60e80%) as detected
by GCeMS, with a moderate amount of isomerised octenes still
obtained (Fig. 3). It was observed that the isomerisation of 1-octene
decreases with an increase in pressure. No hydrogenated products
were observed during the catalytic reactions.
iminophosphine ligands showed a contrasting influence in the
reaction rate. For the second-generation metallodendrimer (5), an
increase in reaction rate is observed over the first-generation
metallodendrimer (4), with 80% of the substrate reacted after 2 h.
Generally, the rhodium complexes (1e6) all show moderate
catalytic activity (Table 1) with TOF values ranging between 162
and 223 hꢀ1 and compare favourably with Rh(CO)2(acac) used as
a catalyst under the same conditions. The iminopyridyl complexes
(1e3) display a decreasing trend in catalytic activity on going from
the mononuclear (3) to the second-generation complex (2). This is
attributed to the formation of colloidal rhodium and the possible
agglomeration of rhodium nanoparticles, inhibiting hydro-
formylation activity. The iminophosphine rhodium metal-
lodendrimers (4e6) appear to show a slight positive dendritic
effect with an increase in activity observed on going to the higher
generation.
The hydroformylation of 1-octene using complexes 1e6 is
temperature and pressure dependent. A systematic study on the
influence of temperature and syngas pressure was carried out at
various temperatures ranging from 30 ꢁC to 75 ꢁC, and pressures
ranging from 5 bar to 30 bar. Generally, at temperatures below
75 ꢁC (P ¼ 5e30 bar), no conversion of the substrate, 1-octene, is
observed. At 75 ꢁC (5 bar), hydroformylation of 1-octene gave
poor conversions producing predominantly isomerised octenes
(Table 2; entries 1, 4, 7, 10). Upon increasing the syngas pressure
to 30 bar at 75 ꢁC, the hydroformylation of 1-octene using 1, 3, 4
and 6 gave considerably higher conversions (Table 2; entries 3, 6,
9, 12) and reached a near maximum, compared to that at 5 or
10 bar after 8 h.
In terms of regioselectivity, the complexes (1e6) all show
a slightly enhanced selectivity towards the linear aldehyde, nona-
nal (Table 3). The n:iso ratio of aldehydes for the metal-
lodendrimers is similar to that of the related mononuclear model
compounds, suggesting that the dendrimers do not have a signifi-
cant influence over the catalytic behaviour around the rhodium
environment. However, these show far better regioselectivity
towards the linear aldehyde when compared with Rh(acac)(CO)2
under the same conditions.
4. Conclusions
We have synthesized first- and second-generation rhodium(I)
poly(propyleneimine) metallodendrimers, containing either mon-
odentate iminopyridyl or chelating bidentate iminophosphine
ligands. The catalytic potential of the complexes was evaluated in
the hydroformylation of 1-octene, achieving high conversions of
the substrate under mild conditions, and showing moderate values
for catalytic activity. The dendrimeric rhodium(I) complexes give
the same selectivity as their mononuclear model analogues,
producing mainly aldehydes (slightly enhanced linear products)
and a small amount of isomerisation products.
Acknowledgements
We gratefully thank the University of Cape Town, the National
Research Foundation (NRF) of South Africa, and the NRF-DST Centre
of Excellence in Catalysis (c*change) for financial support. We thank
the AngloPlatinum Corporation and Johnson Matthey for the kind
donation of rhodium salts.
100
80
73
80
70
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64
61
60
40
20
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