Hydroformylation of 1-octene using low-generation Rh(i) metallodendritic catalysts based on a tris-2-(2-pyridyliminoethyl)amine scaffold

Article abstract of DOI:10.1039/c2dt30856h

The synthesis and characterization of low-generation pyridylimine Rh(i) metallodendrimers is described. These metallodendrimers were obtained via a Schiff base condensation of tris-2-(aminoethyl)amine with 2- pyridinecarboxaldehyde to afford the tris-2-(2-pyridylimine ethyl) amine ligand (1). Subsequent complexation reactions with [RhCl(CO)₂]₂ and [RhCl(COD)]₂ yielded the corresponding metal-containing dendrimers containing -RhCl(CO) and -Rh(COD) moieties on the periphery. These new rhodium metallodendrimers (2 and 3) and their precursor ligand (1) are thermally stable and have been characterized using ¹H NMR, ¹³C NMR, ³¹P NMR, FT-IR spectroscopy, elemental analysis as well as mass spectrometry. The Rh(i) metallodendrimers are highly active and chemo- and regioselective in the hydroformylation of 1-octene. Aldehydes were favoured at moderate to high temperatures (95 °C and 75 °C) and pressure (30 bars), while more iso-octenes were formed at low temperature (55 °C) and pressures (5 and 10 bars). The mononuclear analogues (5 and 6) also produced more aldehydes (albeit showing catalyst decomposition at 95 °C and 75 °C, 30 bars) and these aldehydes were mostly branched.

Full text of DOI:10.1039/c2dt30856h

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PAPER
Hydroformylation of 1-octene using low-generation Rh(I) metallodendritic
catalysts based on a tris-2-(2-pyridyliminoethyl)amine scaffold†
Banothile C. E. Makhubela,a Anwar M. Jardine,a Gunnar Westmanb and Gregory S. Smith*a
Received 19th April 2012, Accepted 14th June 2012
DOI: 10.1039/c2dt30856h
The synthesis and characterization of low-generation pyridylimine Rh(I) metallodendrimers is described.
These metallodendrimers were obtained via a Schiff base condensation of tris-2-(aminoethyl)amine
with 2-pyridinecarboxaldehyde to afford the tris-2-(2-pyridylimine ethyl) amine ligand (1).
Subsequent complexation reactions with [RhCl(CO)2]2 and [RhCl(COD)]2 yielded the corresponding
metal-containing dendrimers containing –RhCl(CO) and –Rh(COD) moieties on the periphery. These
new rhodium metallodendrimers (2 and 3) and their precursor ligand (1) are thermally stable and have
been characterized using 1H NMR, 13C NMR, 31P NMR, FT-IR spectroscopy, elemental analysis as
well as mass spectrometry. The Rh(I) metallodendrimers are highly active and chemo- and regioselective
in the hydroformylation of 1-octene. Aldehydes were favoured at moderate to high temperatures (95 °C
and 75 °C) and pressure (30 bars), while more iso-octenes were formed at low temperature (55 °C)
and pressures (5 and 10 bars). The mononuclear analogues (5 and 6) also produced more aldehydes
(albeit showing catalyst decomposition at 95 °C and 75 °C, 30 bars) and these aldehydes were mostly
branched.
Introduction
Dendrimers are polymeric compounds with highly branched
structures and functionally tunable peripheral groups. They
possess low polydispersities, a high degree of molecular uni-
formity and precisely controlled structures. Since dendrimers are
built systematically, their shape is fully controllable. Dendrimers
can therefore demonstrate a variety of biomedical, photophysical
and catalytic properties, as well as tunability in size and shape.1
With the attachment of metals, metallodendrimers (Fig. 1) or
dendrimer composites (which can also be seen as a type of orga-
nometallic polymer) provide diverse characteristics for appli-
cations in catalysis and biomimetic catalysis, photoluminescent
technology, imaging contrast agents, biomedical sensors and as
therapeutic agents.1
Organometallic polymers of a linear and a dendritic (highly
branched) nature have been widely prepared for applications in
catalysis.2–6 The appeal of organometallic polymers for catalysis
lies in that, unlike organic and inorganic hybrid-polymer sup-
ported catalysts, they possess inherent well-defined active sites
that eliminate negative effects such as poor accessibility, random
anchoring, or disturbed geometry phenomena that often result in
Fig. 1 Simplistic depiction of a metallodendrimer with metals located
poor activity and selectivity in classical immobilization.
on the periphery.
aDepartment of Chemistry, University of Cape Town, Rondebosch, 7701,
Cape Town, South Africa. E-mail: Gregory.Smith@uct.ac.za
Organometallic polymers are also often soluble in reaction sol-
bDepartment of Chemical and Biological Engineering, Chalmers
vents and thus operate under true homogeneous conditions and
University of Technology, SE-412 96 Göteborg, Sweden
can be separated from solution by precipitation, or using nano-
or ultra-filtration methods.
†Electronic supplementary information (ESI) available. See DOI:
10.1039/c2dt30856h
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A large number of metallodendrimers and their catalytic inves-
tigations have been reported including those based on polycarbo-
silanes (PCS),7 polyamides,4d,8 polyarylethers (PAE)6,9 and
polypropyleneimines (PPI) supports4,5,10a to name a few. Several
first- and second-generation PPI metallodendrimers bearing four
and eight metal centres have been previously reported and used
in catalytic transformations such as carbon–carbon cross-coup-
ling, olefin polymerization and hydroformylation.4,5,10a
Hydroformylation has been widely used in industry for the
production of aldehydes as a means to accessing secondary
products such as alcohols, amines, carboxylic acids, acetals etc.
The industrial applicability of this reaction has been demon-
strated through the OXEA process (formerly Ruhrchemie/
Rhône-Poulenc) which employs a Rh/(TPPTS)3 (TPPTS = 3,3,3-
phosphinidynetris(benzenesulfonic acid) trisodium salt/P(C6H4-
SO3Na)3) water soluble catalyst in the hydroformylation of
propene and butene. Over the past decade, much effort has been
directed toward the synthesis of highly active, selective and
stable catalysts for the hydroformylation reaction, using different
transition metals.11–13 The most commonly used catalysts for
this reaction are based on rhodium due to its high activity and
selectivity under milder conditions and better stability than
cobalt.
the [M − H]+ (m/z = 412.78) ion, thus further supporting the for-
mation of ligand (1) and elemental analysis data is also in agree-
ment with the proposed structure.
The rhodium complexes (2 and 3) of the tris-2-(2-pyridyl-
imine ethyl)amine ligand (1) were obtained by reacting the
dendritic ligand with either [RhCl(CO)2]2 or [RhCl(COD)]2
respectively in DCM and DCM–MeOH (4 : 1) as outlined in
Scheme 1. Synthesis of metallodendrimer (3) proceeds via for-
mation of an intermediate bearing the chloride counter-ions.
Attempts to isolate this species were unsuccessful as this
afforded an unstable sticky red product. However, addition of
three equivalents of NaPF6 10 minutes into the reaction yielded
compound (3) as the hexafluorophosphate salt in a stable form.
The metallodendrimers (2 and 3) are soluble in chloroform (2)
and dimethylsulfoxide (DMSO) (2 and 3) and are fairly ther-
mally stable showing decomposition without melting at the onset
temperatures of 280 °C and 315 °C respectively.
The 1H NMR spectra of both metallodendrimers display broad
overlapping multiplets in the aromatic and aliphatic regions
similar to the dendritic ligand (1). Additionally, metallodendri-
mer (3) give rise to a broad singlet and multiplet at δ 4.25 and
1.83 ppm (integrating for 12 protons each) for the vCH and
CH2 functionalities of the COD co-ligands. The remaining 12
protons due to the CH2 in the COD co-ligands are incorporated
in a broad overlapping multiplet ranging from δ 2.08–3.52 ppm.
More conclusive results are seen using 13C NMR spectroscopy
wherein the spectra of metallodendrimers (2 and 3) display all
the expected signals. For compound (2), a doublet at ca. δ 181 ppm
for the carbonyl carbon along with a Rh–C coupling of 71.6 Hz
is seen (Fig. 2). There is also a signal of very low intensity at
ca. δ 189 ppm, suggesting the presence of a less favoured isomer
under solution NMR conditions. This isomer may be either
where the carbonyl co-ligands are cis or trans to the imine nitro-
gen. There are downfield shifts of the imine carbon from ca. δ
163 ppm to ca. δ 165 ppm (2) and ca. δ 174 ppm (3) thus
proving coordination of rhodium centres to the dendritic ligand.
No noticeable shifts in the signals due to the aromatic and ali-
phatic carbons are evident, thus they remain in the region of
ca. δ 122–150 ppm and ca. δ 60–46 ppm respectively.
Broad signals due to COD (ca. δ 83, 30 and 27 ppm) are also
observed in the 13C NMR spectrum of metallodendrimer (3).
The FT-IR spectra of metallodendrimers (2 and 3) clearly
display absorption bands at ≈1630 cm−1 due to the coordinated
imine moieties and no absorption at 1649 cm−1 for the uncom-
plexed ligand (1). These shifts in the pyridyl imine absorption
bands to lower wavenumbers are due to back-bonding of the
rhodium to the imine bond, thus increasing its electron density.
Notably, two carbonyl absorption bands are observed in the
spectrum for metallodendrimer (2).10b This indicates that in the
solid state, this compound exists as isomers, where the carbonyl
co-ligands (2075 cm−1 and 1996 cm−1), are cis or trans to the
imine nitrogen as also seen in 13C NMR experiments.10b
We were interested in investigating the properties and catalytic
behaviour of the less explored intermediate sized/low-generation
metallodendrimers. This manuscript discusses the synthesis and
characterization of rhodium-containing tris-2-(2-pyridylimine
ethyl)amine metallodendrimers and their application as catalyst
precursors for the hydroformylation of 1-octene.
Results and discussion
Synthesis and characterization of tris-2-(2-pyridylimine ethyl)-
amine ligand (1) and tris-2-(2-pyridylimine ethyl)amine
rhodium(I) dendrimers (2 and 3)
The tris-2-(2-pyridylimine ethyl)amine ligand (1) was syn-
thesized via a Schiff base condensation reaction of tris(2-amino-
ethyl)amine and 2-pyridinecarboxaldehyde in refluxing DCM–
MeOH (30 : 70) over 16 hours (Scheme 1). This dendritic ligand
was characterized fully using 1H NMR, 13C NMR, FT-IR
spectroscopy, elemental analysis (C, H and N) and mass
spectrometry.
The 1H NMR spectrum of ligand (1) displays very broad over-
lapping signals in the aliphatic and aromatic regions. A broad
signal assigned to the imine proton is seen at δ 8.42 ppm. The
methylene protons of the tris(2-iminoethyl)amine core resonate
in the region of δ 2.09–3.42 ppm while the aromatic protons are
observed at δ 8.57–7.15 ppm.
13C NMR spectroscopy displayed more meaningful results
though broad resonances were also observed. The aliphatic
carbons resonate at ca. δ 54–58 ppm and the imine carbon exhi-
bits a characteristic signal at ca. δ 163 ppm, while the aromatic
carbons are seen between ca. δ 121–153 ppm. FT-IR spectro-
scopic results display absorption bands at 1649 cm−1 and
1590 cm−1 due to the imine and pyridyl imine moieties respec-
tively. This data correlates well with similar first and second
generation polypropylene imine dendrimers from the litera-
ture.4a–c,10a Electron impact mass spectrometry (EI-MS) revealed
Elemental analyses (C, H and N) of 2 and 3 correspond with
the calculated values and their mass spectra confirm the struc-
tures of these metallodendrimers through exhibiting [M − CO]+
(m/z = 889.2) (2) and [M − 3PF6−]+ (m/z = 349.03) ion peaks
for compound (3) (where M = the positively charged complex
−
without the PF6 counter ion in (3)). The fragmentation pattern
of metallodendrimer (2) follows sequential loss of the carbonyl
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Scheme 1 Outline of the preparation of dendritic ligand (1) and Rh dendrimers (2 and 3).
ligands. This is observed in the mass spectrum of this compound
which exhibits peaks associated with [M − 2CO]+ (m/z = 861.2)
and [M − 3CO]+ (m/z = 833.2).
Elemental analyses results were in agreement with the pro-
1
posed formulation and the literature.4a–c The H NMR spectrum
of ligand (4) displays two signals for –CH2– in the regions
of δ 3.22 and 1.81 ppm and a signal for the propyl chain –CH3
at δ 0.95 ppm. Aromatic resonances are observed at δ
8.99–7.24 ppm. The Schiff base condensation reaction was
confirmed by the appearance of a peak in the range of δ
8.91 ppm assigned to the imine proton.
Synthesis and characterization of 2-pyridylpropylimine ligand
(4) and 2-pyridylpropylimine rhodium(I) complexes (5 and 6)
The known 2-pyridylpropylimine ligand (4) was prepared by the
reaction of 2-pyridine carboxaldehyde with propylamine at room
temperature (Scheme 2).4a–c Ligand (4) was isolated as a yellow
oil in good yield (98%) and characterized using elemental analy-
sis (C, H and N), FT-IR and 1H NMR and 13C NMR spec-
troscopy as well as mass spectrometry.
The 13C NMR spectrum for ligand (4) displays a signal for
three aliphatic carbons in the regions of ca. δ 63–11 ppm and
aromatic carbons in the range of ca. δ 155–121 ppm. The imine
carbon was the most deshielded signal in the region of ca. δ
161 ppm. For compound 4, the IR spectrum displays two
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Fig. 2 13C NMR spectrum of rhodium chlorocarbonyl dendrimer (2) conducted at 25 °C (* = DMSO-d6).
absorption bands in the region of 1650 cm−1 ν(CvN)imine and
1587 cm−1 ν(CvN)pyridyl. Electron impact mass spectrometry
(EI-MS) further confirmed the integrity of this ligand by display-
ing a peak at m/z 148 for the [M − H]+ ion.
A reaction of ligand (4) with [RhCl(CO)2]2 and [RhCl-
(COD)]2 dimers separately afford complexes 5 and 6 respectively
(Scheme 2). The products 5 and 6 were isolated in good yields
(89 and 96% respectively) as a dark purple (5) and a brown
solid (6). These complexes have been characterized using
elemental analysis (C, H and N), FT-IR, 1H NMR and 13C NMR
spectroscopy as well as mass spectrometry.
δ 8.62 ppm in complex (5) and 8.34 ppm in complex (6).
13C NMR spectra for complexes (5 and 6) also display expected
downfield shifts of imine carbon signals from ca. δ 161 ppm in
ligand (4) to ca. δ 170 ppm in complex (5) and ca. δ 172 ppm in
complex (6). In addition, a doublet due to the carbonyl carbon is
seen at δ 189 ppm along with Rh–C coupling of 73.1 Hz. These
trends are in agreement with similar Rh(I) complexes that have
been reported.14 Complex (6) displays signals at 29.83 and
85.13 ppm assigned to the aliphatic and alkene functionalities of
the COD ligand respectively.
Evidence for the coordination of the Rh centre was further
confirmed through a shift of the imine absorption band in the
FT-IR spectrum of ligand (4) (1650 cm−1) to 1609 cm−1 in
complex (5) and 1626 cm−1 in complex (6). Additionally, a
single very strong terminal carbonyl absorption band is evident
at 1970 cm−1 suggesting one preferred isomer in complex (5).
Elemental analyses results for complexes (5 and 6) are in
agreement with the calculated percentage C, H and N and
electrospray ionization mass spectra (ESI-MS) further esta-
blished the integrity of the complexes through base peaks
observed at m/z = 278.9 and 359.1 for the [M − Cl]+ and [M]+
fragments respectively (in complex (6), M = the positively
−
charged ion without PF6 counter ion). NMR spectroscopy sup-
ports coordination of ligand (4) to rhodium in a chelating
manner to form the rhodium chlorocarbonyl and cyclooctadiene
(COD) complexes.
Hydroformylation of 1-octene
The pyridylimine Rh(I) metallodendrimers (2–3) and complexes
(5–6) were employed as catalyst precursors in the hydroformyla-
tion reaction of 1-octene at 55–95 °C and 5–30 bars of syngas
1
The H NMR spectra of these complexes display an upfield
shift of the imine proton signal from δ 8.91 ppm in ligand (4) to
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Scheme 2 Outline of the preparation of 2-pyridylpropylimine (4) and Rh(I) complexes (5 and 6).
pressure (CO–H2 = 1 : 1) in toluene (10 mL). A minimal amount
of 2.87 × 10−4 mol% (metal) of each catalyst precursor was used
(catalyst–substrate = 1 : 2500) (eqn (1)).
iso-octenes (cis and trans 2- and 3-octenes) (Fig. 4a). At 10 bars
(75 °C), conversion of 1-octene increased to near complete con-
versions (99% and 98% respectively). The chlorocarbonyl-based
In general, conversions of 1-octene are excellent (>99%) with
near quantitative conversion of 1-octene observed after 8 hours.
Slight differences in catalytic rates are seen in the first 4 hours
(Fig. 3). In the first 4 hours, the COD-based system (3 and 6)
displays slightly inferior conversion to the chlorocarbonyl
counterpart (2 and 5). This suggests that the COD-based systems
take a longer period to generate the active species as compared
to catalyst precursors (2 and 5) already bearing CO on the active
site.
system (2) exhibited better selectivity for aldehydes than the
COD-based system (3) (Fig. 4b). Excellent conversion and
chemoselectivity is seen at 30 bars syngas pressure as has pre-
viously been reported with Rh(I) systems (Fig. 4c).10a,15 At this
pressure, the best regioselectivity for nonanal is observed
(Fig. 4d), thus all subsequent reactions were carried out at
30 bars syngas pressure. The mononuclear analogues (5 and 6)
also exhibit high chemoselectivity (up to 89 and 84% aldehydes
respectively (Fig. 4c)). Slightly over 50% of these aldehydes are
branched (Fig. 4c). Thus there is a sharp contrast in regio-
selectivity in comparison to the metallodendritic precursors
(2 and 3) suggesting that the dendritic effect strongly influences
regioselectivity.
Effect of pressure. Reactions carried out at 5 bars (75 °C)
indicated good to moderate conversion of 1-octene (51 to 98%)
and the products constituted low levels of aldehydes and mostly
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Effect of temperature. Temperature variation studies were
conducted at 55, 75 and 95 °C. A lower temperature (55 °C,
30 bars) sees minimal conversion of 1-octene to largely iso-
octenes and almost exclusively in the reaction carried out
using catalyst precursor (3) (Fig. 5a). There is a marked
increase in conversion of 1-octene (94–99%) when operating at
95 °C and 30 bars syngas pressure. Moreover, 1-octene is con-
verted almost quantitatively to aldehyde products (Fig. 5b).
Similarly, at 75 °C (30 bars) conversion to mostly the desired
aldehyde is observed (vide ante Fig. 4c). Therefore, 75 °C and
95 °C were established as the optimum operating temperatures
depending on the type of aldehyde (linear or branched) desired
when using catalyst precursors (2–3) and (5–6) (vide ante
Fig. 4d and vide infra Fig. 6).
Chemoselectivity, regioselectivity and turnover frequency
(TOF h−1). Overall, catalyst precursors 2 and 3 favour the for-
mation of iso-octenes at low temperature (55 °C) and pressures
Fig. 5 Effect of temperature on chemoselectivity in the hydroformyla-
tion of 1-octene using catalysts (2–3) and (5–6) at 30 bars and: (a) 55 °C
and (b) 95 °C. Average error estimate: (2) = 0.12, (3) = 0.11, (5) =
0.10 and (6) = 0.14.
Fig. 3 Percentage conversion of 1-octene over 8 hours using catalysts
(2–3 and 5–6), data collected at 75 °C and 30 bars. Average error esti-
mate: (2) = 0.11, (3) = 0.10, (5) = 0.11 and (6) = 0.18.
Fig. 4 Effect of pressure on chemo- and regioselectivity in the hydroformylation of 1-octene using catalysts (2–3) and (5–6) at 75 °C and: (a) 5 bars,
(b) 10 bars and (c–d) 30 bars. Average error estimate: (2) = 0.10, (3) = 0.11, (5) = 0.11 and (6) = 0.16.
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(5 and 10 bars) as has been previously seen with other Rh(I)
systems.12,15 At the established optimal temperatures (75 and
95 °C) and pressure (30 bars), the catalyst precursors (2–3) and
(5–6) display the best chemoselectivity towards aldehydes (vide
ante Fig. 4c and 5b) (84–100%). Careful investigations indicate
that the linear aldehyde (nonanal) is favoured at 75 °C with the
metallodendrimer-based systems (2 and 3) displaying superior
selectivity in this regard (up to 94%) (vide ante Fig. 4d). These
good selectivities for nonanal compared to mononuclear Rh(I)
systems (5 and 6) as well as those reported in the literature imply
that the dendrimer scaffold may provide a favourable microenvir-
onment for the catalytically active rhodium that drives selectivity
towards the linear product.12,16 Conversely, branched aldehydes
are favoured at 95 °C (30 bars) (Fig. 6). It is known that isomeri-
zation is dominant at higher temperatures, therefore this implies
that branched aldehydes are formed via iso-octenes.15,17,18 The
dendritic support appears to stabilize both COD- and chlorocar-
bonyl-based systems (2 and 3) since reactions carried out at
95 °C (30 bars) did not show any observable decomposition of
the catalyst after 8 hours, while the mononuclear systems decom-
posed to black species. Thus this affirms the stabilizing role of
the dendritic scaffold.
hydroformylation conditions, a feature which assists in prevent-
ing catalyst deactivation. The metallodendritic catalyst precursor
(2) performs better than the COD-based system (3), this may be
attributed to poor accessibility of the substrate to the active
rhodium centres induced by the bulky COD co-ligands thus
slightly hampering hydroformylation. Additionally, the catalyst
precursors (2, 5 and 6) outperform the common hydroformy-
lation catalyst precursor [Rh(acac)(CO)2] under similar
conditions.10a
Conclusions
Rh(I) low-generation pyridylimine metallodendrimers and their
mononuclear counterparts have been successfully prepared
and characterized using an array of analytical and spectroscopic
techniques. These Rh(I) systems were investigated as catalyst
precursors and are active in the hydroformylation of 1-octene
under mild conditions (75 or 95 °C and 30 bars). The activities
and selectivities of these catalyst precursors are affected by
temperature and pressure, with lower temperature and pressures
yielding more of the iso-octene fractions. Good selectivity for
aldehydes was achieved at 75 or 95 °C and 30 bars syngas
pressure. The metallodendritic precursors (2 and 3) influence
regioselectivity for nonanal at 75 °C while the mononuclear
systems (5 and 6) form almost equal amounts of branched and
linear aldehydes. Based on these studies, the temperature can be
varied to either 75 °C in order to obtain more nonanal or to
95 °C to obtain more branched aldehydes. The important obser-
vation of stability in the current catalyst precursors can lead to
possible reusability of these systems. Chlorocarbonyl-based
systems exhibit the best activity (TOFs), chemoselectivity and
regioselectivity in comparison to COD-based systems under
optimal conditions.
The Rh(I) catalyst precursors show good TOF values (where
TOF = (molproduct/molcatalyst × h−1) with activities observed in
the range 200–320 h−1 (Table 1). The key factors to consider
in interpreting TOF are steric bulk of the catalyst precursor or
active species stability. Metallodendrimers (2 and 3) show good
TOFs, thus the dendritic scaffold plays a stabilizing role under
Experimental
General details
4-Aminobenzyl alcohol, 2-pyridinecarboxaldehyde, COD,
sodium hexafluorophosphate (NaPF6), sodium tetraphenylborate
(NaBPh4) and tris(2-aminoethyl)amine, were purchased from
Sigma Aldrich and used as received. All solvents were obtained
commercially and [RhCl3·3H2O] was obtained from Anglo Plati-
num Coorporation. [RhCl(CO)2]2 19a and [RhCl(COD)]2 19b were
prepared according to literature procedures. Ligand (4) (2-pyridi-
nepropylimine) was prepared using a literature protocol.1i,5
Fig. 6 Regioselectivity in the hydroformylation of 1-octene using cata-
lysts (2–3) and (5–6) all at 30 bars and 95 °C. Average error estimate:
(2) = 0.12, (3) = 0.11, (5) = 0.10 and (6) = 0.14.
FT-IR spectra were recorded as KBr pellets using either a
Bruker Tensor 27 or a Perkin Elmer 100 Spectrum spec-
trometers. Elemental analyses were conducted with a Fision EA
110 CHNS Analyzer. Nuclear Magnetic Resonance (NMR)
spectra were recorded on a Varian Unity XR400 spectrometer
(1H: 400 MHz; 13C{1H}: 100 MHz; and 31P{1H}: 161 MHz) or
a Varian Mercury XR300 (1H: 300 MHz; 13C{1H}: 75 MHz;
and 31P{1H}: 121 MHz) all at ambient temperature. Melting
points were determined using a Kofler hot stage microscope
(Riechter Thermo). Electrospray Ionization (ESI) mass spectro-
metry was carried out on a Waters API Quattro Micro triple
quadrupole mass spectrometer in the positive or negative-ion
mode. Electron Impact mass spectrometry was conducted on an
Table 1 TOF for the Rh(I) catalyst precursors (2–3) and 5–6) at
30 bars and 75 °C
Catalyst
TOF (molproduct/molcatalyst × h−1) (h−1
)
2
3
5
6
320
200
273
261
259
[Rh(CO)2(acac)]a
a Ref. 10a. TOFs are calculated based on total aldehyde products
formed. Average error estimate: (2) = 0.12, (3) = 0.11, (5) = 0.10
and (6) = 0.14.
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Agilent 6890 N spectrometer. Catalysis products were analysed
using a Perkin-Elmer Clarus 580 GC.
The precipitate was isolated by filtration (suction), washed with
Et2O (30 mL) and dried under vacuum for 4 hours. The product
(3) was obtained as a reddish brown powder. Yield: (0.116 g,
65%). mp.: decomposition without melting, onset occurs at
315 °C. FT-IR (νmax/cm−1, KBr): 1628s (CvN)imine, 1596s
(CvN)pyridyl, 1572s (CvC). δH (300 MHz, DMSO-d6, 25 °C)
(ppm) = 8.91–7.16 (overlapping multiplets, 15H, Ar, CH–imine),
4.25 (br s, 12H, CH–COD) 3.52–2.08 (overlapping multiplets,
24H, CH2–amine core, CH2–COD), 1.83 (m, 12H, CH2–COD).
δC (75 MHz, DMSO-d6, 25 °C) (ppm) = 174. 90 (C–imine),
150.3–122.67 (Ar), 83.73 (CH–COD), 60.42, 46.39 (CH2–amine core),
30.24, 27.42 (CH2–COD). δP (121 MHz, DMSO-d6, 25 °C)
Synthesis of tris-2-(2-pyridylimine ethyl) amine ligand (1)
Tris(2-aminoethyl)amine (1.00 g, 5.838 mmol) and 2-pyridine-
carboxaldehyde (2.107 g, 20.514 mmol) were refluxed in DCM–
MeOH (30 : 70, 40 mL) in the presence of anhydrous mag-
nesium sulfate for 16 hours, resulting in an orange to brown
solution. The solution was filtered by gravity and the solvent
was reduced to ≈15 mL. Et2O (20 mL) was added to the concen-
trated solution and a light brown solid precipitated out. The pre-
cipitate was isolated by filtration (suction), washed with Et2O
(30 mL) and dried under vacuum for 4 hours. Ligand (1) was
obtained as a light brown solid. Yield: (1.872 g, 66%). mp.:
decomposition without melting, onset occurs at 250 °C. FT-IR
1
(ppm) = −144.24 (sept, JP–F = 712 Hz). Elemental analysis
(calculated for C48H63F18N7P3Rh3): C, 38.91; H, 4.29; N, 6.62.
Found: C, 38.69; H, 4.22; N, 6.65%. ESI-MS (m/z) = 349.03
([M − 3PF6−]+, 33%) (where M = the positively charged
complex without the PF6− counter ion).
(νmax/cm−1, KBr): 1649s (CvN)imine, 1590s (CvN)pyridyl
,
1565s (CvC). δH (400 MHz, DMSO-d6, 25 °C) (ppm) = 8.57
(br s, 3H, Ar), 8.42 (br s, 3H, CH–imine), 7.79 (br m, 3H, Ar),
7.40 (br s, 3H, Ar), 7.15 (br m, 3H, Ar), 3.42–2.09 (overlapping
multiplets, 12H, CH2). δC (75 MHz, DMSO-d6, 25 °C) (ppm) =
163.09 (C–imine), 153.21, 149.67, 136.18, 125.85, 121.22 (Ar),
58.42, 54.17 (CH2). Elemental analysis (calculated for
C24H27N7): C, 69.71; H, 6.58; N, 23.71. Found: C, 69.07; H,
6.28; N, 23.35%. EI-MS (m/z) = 412.78 ([M − H]+, 13%).
Synthesis of Rh(I) mononuclear complexes (5 and 6)
Synthesis of 2-pyridylpropylimine rhodium(I)chlorocarbonyl
complex (5). [RhCl(CO)2]2 (0.080 g, 0.206 mmol) in dichloro-
methane (5 mL) was added to a solution of 2-pyridylpropylimine
ligand (4) (0.061 g, 0.412 mmol) in dichloromethane (5 mL).
There was immediate formation of a dark purple solution. The
reaction was stirred at room temperature for 10 minutes after
which the solvent was removed by rotary evaporation to afford a
dark purple solid of (5) which was dried under vacuum
for 5 hours. Yield: (0.058 g, 89%). mp.: 213–217 °C. FT-IR
(νmax/cm−1, KBr): 1970vs (CuO), 1609s (CvN)imine, 1595s
(CvN)pyridyl. δH (400 MHz, acetone-d6, 25 °C) (ppm) = 9.18 (s,
1H, Ar), 8.91 (s, 1H, CH–imine), 8.23 (s, 1H, Ar), 8.09 (s, 1H,
Ar), 7.87 (s, 1H, Ar), 3.89 (s, 2H, CH2–propyl), 1.95 (s, 2H,
CH2–propyl), 0.96 (s, 3H, CH3–propyl). δC (75 MHz, DMSO-d6,
Synthesis of Rh(I) metallodendrimers (2 and 3)
Synthesis of tris-2-(2-pyridylimine ethyl)amine rhodium(I)-
chlorocarbonyl metallodendrimer (2). [RhCl(CO)2]2 (0.140 g,
0.363 mmol) in dichloromethane (15 mL) was added to a solu-
tion of tris-2-(2-pyridylimine ethyl)amine ligand (1) (0.100 g,
0.242 mmol) in dichloromethane (10 mL). There was immediate
formation of a dark purple precipitate. The reaction was stirred at
room temperature for 15 minutes. The precipitate was isolated by
filtration (suction), washed with Et2O (20 mL) and dried under
vacuum for 5 hours. The product (2) was obtained as a dark
purple powder. Yield: (0.173 g, 78%). mp.: decomposition
1
25 °C) (ppm) = 189.02 (d, JRh–C = 73.1 Hz, CO), 170.25
(C–imine), 154.61–123.02 (Ar), 11.18–58.22 (Aliphatic).
Elemental analysis (calculated for C10H12ClN2ORh): C, 38.18;
H, 3.84; N, 8.91. Found: C, 38.29; H, 3.57; N, 8.94%. ESI-MS
(m/z) = 278.9 ([M − Cl]+, 50%).
without melting, onset occurs at 280 °C. FT-IR (νmax/cm−1
KBr): 2075, 1996vs CuO), 1630s (CvN)imine 1605s
,
,
Synthesis of 2-pyridylpropylimine rhodium(I)1,5-cycloocta-
diene metallodendrimer (6). [RhCl(COD)]2 (0.080 g,
0.163 mmol) in dichloromethane (5 mL) was added to a solution
of 2-pyridylpropylimine ligand (4) (0.048 g, 0.325 mmol) in
dichloromethane (5 mL) dropwise. There was an immediate for-
mation of a dark red solution which was stirred at room tempera-
ture for 30 minutes. Sodium hexafluorophosphate (NaPF6)
(0.028 g, 0.163 mmol) in MeOH (3 mL) was added dropwise to
the solution and this was stirred for a further 1 hour at room
temperature. The solvent was reduced to approximately 5 mL
and the product was precipitated out with Et2O (5 mL). After
filtration and washing with Et2O, the product was obtained as a
brown solid (6) (which formed a bright red solution in
acetone) and was dried under vacuum for 5 hours. Yield:
(0.079 g, 96%). mp.: decomposition without melting, onset
(CvN)pyridyl, 1570m (CvC). δH (400 MHz, DMSO-d6, 25 °C)
(ppm) = 8.66–7.06 (overlapping multiplets, 15H, Ar, CH–imine),
3.48–2.06 (overlapping multiplets, 12H, CH2). δC (75 MHz,
1
DMSO-d6, 25 °C) (ppm) = 181.74 (d, JRh–C = 71.6 Hz, CO),
165.65 (C–imine), 154.61–122.02 (Ar), 62.33, 46.45 (CH2).
Elemental analysis (calculated for C27H27Cl3N7O3Rh3): C,
35.53; H, 2.98; N, 10.74. Found: C, 35.35; H, 2.85; N, 10.18%.
ESI-MS (m/z) = 889.2 ([M − CO]+, 96%).
Synthesis of tris-2-(2-pyridylimine ethyl)amine rhodium(I)1,5-
cyclooctadiene metallodendrimer (3). [RhCl(COD)]2 (0.089 g,
0.182 mmol) in dichloromethane (10 mL) was added to a solu-
tion of tris-2-(2-pyridylimine ethyl)amine ligand (1) (0.050 g,
0.121 mmol) in dichloromethane (10 mL). There was an
immediate formation of a dark red solution which was stirred at
room temperature for 10 minutes. Sodium hexafluorophosphate
(NaPF6) (0.050 g, 0.121 mmol) in MeOH (5 mL) was added
dropwise to the solution. A reddish brown precipitate formed
immediately and the mixture was stirred for a further 30 minutes.
occurs at 208 °C. FT-IR (νmax/cm−1, KBr): 1626s (CvN)imine
,
1595s (CvN)pyridyl. δH (300 MHz, acetone-d6, 25 °C) (ppm) =
8.87 (s, 1H, Ar), 8.34 (s, 1H, CH–imine), 8.13 (s, 2H, Ar), 7.88
(s, 1H, Ar), 4.67 (s, 4H, CH–COD), 3.47 (s, 2H, CH2–propyl),
10722 | Dalton Trans., 2012, 41, 10715–10723
This journal is © The Royal Society of Chemistry 2012

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25 °C) (ppm) = −144.22 (sept, JP–F = 711 Hz). Elemental
analysis (calculated for C17H24F6N2PRh): C, 40.49; H, 4.80; N,
5.56. Found: C, 40.99; H, 4.25; N, 5.44%. ESI-MS (m/z) =
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Hydroformylation reactions were conducted in a 90 mL stainless
steel autoclave. The autoclave was charged with toluene
(10 mL), 1-octene (715 mg, 6.37 mmol), n-decane as the internal
standard (180 mg, 1.26 mmol) and either of the Rh catalyst pre-
cursors (2, 3, 5 or 6) (2.87 × 10−3 mmol, substrate–Rh ratio =
2500 : 1). The autoclave was flushed three times with syngas
(CO–H2, 1 : 1 ratio) followed by pressurizing and heating to the
desired syngas pressure and temperature respectively. Samples
were taken every 2 hours and analyzed using gas chromato-
graphy (GC). The products were confirmed in relation to auth-
entic iso-octenes and aldehydes.
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Acknowledgements
We would like to thank the University of Cape Town, the
National Research Foundation and Department of Science and
Technology of South Africa (NRF-DST Centre of Excellence in
Catalysis c*change), the Canon Collins Trust and the
NRF-SIDA (Swedish International Development Cooperation
Agency) for financial support. A generous donation of rhodium
trichloride from Anglo American Platinum Limited is gratefully
acknowledged.
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